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Civil Engineering Theses Civil Engineering
Fall 12-19-2019
EFFECT OF RECYCLED CONCRETE AGGREGATES ON THE LONG-EFFECT OF RECYCLED CONCRETE AGGREGATES ON THE LONG-
TERM, ELASTIC, TOTAL DEFLECTION AND STRENGTH OF TERM, ELASTIC, TOTAL DEFLECTION AND STRENGTH OF
PRECAST PRESTRESSED HOLLOW CORE CONCRETE SLABS PRECAST PRESTRESSED HOLLOW CORE CONCRETE SLABS
Lizeth Marisol Gomez Santana University of Texas at Tyler
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Recommended Citation Recommended Citation Gomez Santana, Lizeth Marisol, "EFFECT OF RECYCLED CONCRETE AGGREGATES ON THE LONG-TERM, ELASTIC, TOTAL DEFLECTION AND STRENGTH OF PRECAST PRESTRESSED HOLLOW CORE CONCRETE SLABS" (2019). Civil Engineering Theses. Paper 16. http://hdl.handle.net/10950/2323
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EFFECT OF RECYCLED CONCRETE AGGREGATES ON THE LONG-TERM,
ELASTIC, TOTAL DEFLECTION AND STRENGTH OF PRECAST PRESTRESSED
HOLLOW CORE CONCRETE SLABS
by
LIZETH MARISOL GOMEZ SANTANA
A thesis submitted in partial fulfillment
of the requirements for the degree of
Master of Science in Civil Engineering
Department of Civil Engineering
Michael J. McGinnis, Ph.D., Committee Chair
College of Engineering
The University of Texas at Tyler
August 2019
The University of Texas at Tyler
Tyler, Texas
This is to certify that the Master's Thesis of
LIZETH MARISOL GOMEZ SANTANA
has been approved for the thesis requirement on
August 6, 2019 for the degree Master of Science in
Civil Engineering
© Copyright 2019 by Lizeth Marisol Gomez Santana
All rights reserved.
Dedication
I dedicate this thesis to the members of my family who came before me. I would like to
thank my family: my father José del Refugio Gómez Brambila, my mother Maria
Guillermina Santana García and my siblings Guillermo and Gisselt for their support. I’d
like to thank my siblings for their enthusiasm, as soon as Guillermo and Gisselt found out
I had been given admissions at UT Tyler they wanted to immediately pack up my things
and ship me off to graduate school. Thank you Guillermo and Gisselt for your support
that first difficult semester. Thank you Gisseltita for taking on an adventure with me and
helping me move to Tyler. I would also like to thank the Rivera Balderas family
especially my friends Brenda, Emmanuel and Raymundo for supporting me both
emotionally and spiritually.
Acknowledgements
I would like to thank my advisors Dr. Michael J. McGinnis, Dr. Michael V.
Gangone and Dr. Yahya C. Kurama from the University of Notre Dame for their
guidance, patience and commitment to their profession as educators. I would like to thank
Mr. Adam Reihl from Stresscore, Inc. based out of South Bend, Indiana for their support,
technical expertise and assistance in providing the test specimens. I would also like to
thank the National Science Foundation (NSF) for their support in funding this project. I
would like to express my gratitude to Dr. Gokhan Saygili who provided encouragement,
wisdom and laughter. I would also like to express my gratitude to Dr. Torey Nalbone for
his constant support and his reminders to keep calm and carry on. I would like to thank
my colleagues who assisted me during the laboratory testing: David Etheridge, Aqil
Sheraze, Alireza and Armin Yazdanshenas, Luis Mendoza Morales, Ashmita Wasti and
Prajwol Sharma.
i
Table of Contents
List of Tables ..................................................................................................................... iii
List of Figures ..................................................................................................................... v
Abstract ............................................................................................................................ xiii
Chapter 1 Introduction ...................................................................................................... 15
1.1 Introduction ............................................................................................................. 15 1.2 Organization of thesis ............................................................................................. 16 1.3 Symbols and notation .............................................................................................. 18
Chapter 2 Literature Review ............................................................................................. 21
2.1 Concrete mixtures with Recycled Concrete Aggregate (RCA) .............................. 21
2.2 Use of RCA in reinforced concrete applications .................................................... 23
2.3 Properties of recycled concrete aggregates from precast/prestressed members ..... 25 2.3.1 Use of RCA in prestressed concrete .................................................................... 27
Chapter 3 Test specimen description ................................................................................ 29
3.1 Concrete properties: mix design ............................................................................. 30 3.1.1 RCA particle size distribution .............................................................................. 33 3.2 Manufacturing and casting conditions .................................................................... 35
3.3 Concrete stiffness (Ec) ............................................................................................ 35 3.4 Specimen geometry and casting conditions ............................................................ 40
3.5 Shipping .................................................................................................................. 41
3.6 Curing conditions .................................................................................................... 42
3.7 Replacement percentage ......................................................................................... 43 Chapter 4 Long term test description ................................................................................ 44
4.1 Test matrix .............................................................................................................. 44
4.2 Slab loading and diagram pictures .......................................................................... 45 4.3 Slab layout in laboratory ......................................................................................... 45 4.4 Loading procedure .................................................................................................. 48 4.5 Instrumentation ....................................................................................................... 49
Chapter 5 Deformations under instant and long-term constant loading ........................... 52
5.1 Deflection data ........................................................................................................ 52 5.1.1 Long-term deflection ........................................................................................... 52 5.1.2 Deflection magnifier ............................................................................................ 64
5.1.3 Instant (Elastic) deflection. .................................................................................. 69 5.1.4 Inelastic deflection ............................................................................................... 71
5.2 Long-term deflections ............................................................................................. 72 5.3 Service load stress ................................................................................................... 73 5.4 Weather conditions ................................................................................................. 74 5.5 Conclusions ............................................................................................................. 78
Chapter 6 Bending test description ................................................................................... 79
6.1 Test set up ............................................................................................................... 79
ii
6.2 Equipment ............................................................................................................... 81
6.3 Load procedure ....................................................................................................... 82 6.4 Instrumentation ....................................................................................................... 82
Chapter 7 Bending test results .......................................................................................... 86
7.1 Moment capacity ..................................................................................................... 86
7.2 Development length ................................................................................................ 87 7.3 Cracking load, maximum load to failure and code capacity ................................... 88 7.4 Cracking deflection ............................................................................................... 101 7.5 Modulus of elasticity from load vs. midspan displacement curves ...................... 101 7.6 Quarter-point deflection plots ............................................................................... 102
Chapter 8 One-way shear test description ...................................................................... 115
8.1 Test set-up ............................................................................................................. 115 Chapter 9 Results of one-way shear tests ....................................................................... 119
9.1 Theoretical shear capacity and moment capacity ................................................. 119 9.2 Moment and shear demand ................................................................................... 121
9.3 Results of four-point shear tests ............................................................................ 122 Chapter 10 Punching (two-way) shear test set up ........................................................... 137
10.1 Punching shear test set up ................................................................................... 137 Chapter 11 Punching (two-way) shear results ................................................................ 141
11.1 Punching shear results ......................................................................................... 141 Chapter 12 Conclusions and future work........................................................................ 148
12.1 Discussion of results ........................................................................................... 148
12.1.1 Deflections ....................................................................................................... 148 12.1.2 Bending strength .............................................................................................. 149
12.1.3 One-way shear strength.................................................................................... 149 12.1.4 Punching (two-way) shear strength ................................................................. 149 12.2 Suggestions for future work ................................................................................ 150
References ....................................................................................................................... 151
iii
List of Tables
Table 1 Section properties, 6SC56.................................................................................... 30
Table 2 Properties of slag ................................................................................................. 30
Table 3 Particle size distribution, slag .............................................................................. 31
Table 4 Properties of fine aggregate, sand ........................................................................ 31
Table 5 Particle size distribution, sand ............................................................................. 32
Table 6 Mix design for each of the slab specimens .......................................................... 33
Table 7 Particle size distribution, RCA ............................................................................ 34
Table 8 Properties of RCA ................................................................................................ 34
Table 9 Compressive strength of cylinder samples, 7-day strength ................................. 37
Table 10 Compressive strength of cylinder samples, (psi) ............................................... 37
Table 11 Modulus of elasticity, Ec, calculated from ACI 318 code ................................. 39
Table 12 Compressive strength and modulus of elasticity from numerical models ......... 39
Table 13 Compressive strength and modulus of elasticity from numerical models,
continued ........................................................................................................................... 40
Table 14 Specimen replacement percentages of virgin aggregate with RCA .................. 43
Table 15 Test matrix of slabs for long-term load testing .................................................. 44
Table 16 Total deflection, ΔT ............................................................................................ 55
Table 17 Deflection magnifier .......................................................................................... 65
Table 18 Instant (elastic) deformation .............................................................................. 70
Table 19 Inelastic deflection ............................................................................................. 72
Table 20 Total, elastic and long-term deflection .............................................................. 73
iv
Table 21 Service load stress vs. available stress (psi) ...................................................... 74
Table 22 Development length (ld) ..................................................................................... 88
Table 23 First cracking load ............................................................................................. 91
Table 24 Maximum load to failure and code capacity ...................................................... 92
Table 25Maximum load to failure and shear demand vs. shear capacity ......................... 92
Table 26 Cracking deflection .......................................................................................... 101
Table 27 Modulus of elasticity, Ec, from load vs. midspan displacement curves .......... 102
Table 28 Beam (one-way) shear slab lengths tested ....................................................... 118
Table 29 One-way shear demand and capacity ............................................................... 124
Table 30 Moment demand and capacity ......................................................................... 124
Table 31 Negative moment demand versus negative moment capacity ......................... 126
Table 32 Maximum load to failure, two-way (punching) shear test ............................... 142
v
List of Figures
Figure 1 Hollow core slabs ............................................................................................... 29
Figure 2 Particle size distribution curve, slag ................................................................... 31
Figure 3 Particle size distribution curve, sand .................................................................. 32
Figure 4 Gradation curves of RCA sample ....................................................................... 34
Figure 5 Average compressive strength (psi) based on Table 9 ....................................... 38
Figure 6 (a) NDS 56 day (b) ND20 56 day (c) ND30 56 day (d) ND40 56 day (e) ND60
56 day ................................................................................................................................ 38
Figure 7 Modulus of elasticity of samples from Table 11 ................................................ 39
Figure 8 Prestressing strands ............................................................................................ 40
Figure 9 Slipformer ........................................................................................................... 41
Figure 10 Hollow core concrete slabs curing ................................................................... 41
Figure 11 Shipping route .................................................................................................. 42
Figure 12 Unloading from flat bed truck .......................................................................... 42
Figure 13 Brecknell electronic crane scale ....................................................................... 45
Figure 14 Slab layout in laboratory .................................................................................. 46
Figure 15. Slab layout in laboratory ................................................................................. 47
Figure 16 Laboratory slab layout ...................................................................................... 47
Figure 17 Slab layout outside ........................................................................................... 48
Figure 18 Slab loading diagram ........................................................................................ 48
Figure 19 Instrumentation for long-term loading ............................................................. 50
vi
Figure 20 Dial gauge used for verification of long-term service loading from string
potentiometers ................................................................................................................... 51
Figure 21 NDS long-term deflection ................................................................................ 55
Figure 22 ND20 long-term deflection............................................................................... 56
Figure 23 ND30 long-term deflection............................................................................... 57
Figure 24 ND40 long-term deflection............................................................................... 58
Figure 25 ND60 long-term deflection............................................................................... 59
Figure 26 NDS 1 block long-term deflection ................................................................... 60
Figure 27 NDS 2 blocks long-term deflection .................................................................. 61
Figure 28 Long-term deflection, inside slabs ................................................................... 62
Figure 29 Long-term deflection, outside slabs ................................................................. 63
Figure 30 Long-term deflection, all slabs ......................................................................... 64
Figure 31 ND20 deflection magnifier ............................................................................... 65
Figure 32 ND30 deflection magnifier ............................................................................... 66
Figure 33 ND40 deflection magnifier ............................................................................... 66
Figure 34 ND60 deflection magnifier ............................................................................... 67
Figure 35 NDS 1 block deflection magnifier .................................................................... 67
Figure 36 NDS 2 blocks deflection magnifier .................................................................. 68
Figure 37 Single load slabs, deflection magnifier ............................................................ 68
Figure 38 Rainfall data during long-term deflection testing ............................................. 75
Figure 39 Temperature and humidity data, outside slabs ................................................. 76
Figure 40 Temperature and humidity data, inside slabs ................................................... 77
vii
Figure 41. Bending test set up, ND S................................................................................ 80
Figure 42. Bending test set up diagram ............................................................................ 80
Figure 43 Bending test set-up ........................................................................................... 81
Figure 44. Enerpac hydraulic pump and pressure gauges ................................................. 82
Figure 45 Data acquisition system .................................................................................... 83
Figure 46 Model 100, data acquisition hardware .............................................................. 84
Figure 47 String potentiometer ......................................................................................... 84
Figure 48 Load cell, model Omegadyne LC8400-213-200k ............................................ 84
Figure 49 Slab and instrumentation set up ........................................................................ 85
Figure 50 Load cell calibration curve ............................................................................... 85
Figure 51 NDS 1 block, uneven surface ........................................................................... 91
Figure 52 Applied load vs. midspan displacement up to cracking for ND20 ................... 91
Figure 53 Failure loads of flexural bending test specimens ............................................. 92
Figure 54 Loading diagrams for bending tests ................................................................. 93
Figure 55 Load vs. midspan displacement of NDS slab up to cracking load ................... 93
Figure 56 Load vs. midspan displacement of ND20 slab up to cracking load ................. 94
Figure 57 Load vs. midspan displacement of ND30 slab up to cracking load ................. 94
Figure 58 Load vs. midspan displacement of ND40 slab up to cracking load ................. 95
Figure 59 Load vs. midspan displacement of ND60 slab up to cracking load ................. 95
Figure 60 Load vs. midspan displacement of NDS 1 block slab up to cracking load ...... 96
Figure 61 Load vs. midspan displacement of NDS 2 blocks slab up to cracking load ..... 96
Figure 62 Loading and re-loading load-displacement curve of NDS during loading test 97
viii
Figure 63 Loading and re-loading load-displacement curve of ND20 during loading test
........................................................................................................................................... 97
Figure 64 Loading and re-loading load-displacement curve of ND30 during loading test
........................................................................................................................................... 98
Figure 65 Loading and re-loading load-displacement curve of ND40 during loading test
........................................................................................................................................... 98
Figure 66 Loading and re-loading load-displacement curve of ND60 during loading test
........................................................................................................................................... 99
Figure 67 Loading and re-loading load-displacement curve of NDS 1 block during
loading test ........................................................................................................................ 99
Figure 68 Loading and re-loading load-displacement curve of NDS 2 block during
loading test ...................................................................................................................... 100
Figure 69 Load vs. midspan displacement averages for all slabs ................................... 100
Figure 70 NDS (a) bending test set-up (b) bending test set-up (c) cracked section left (d)
cracked section right (e) cracked section ........................................................................ 104
Figure 71 ND20 (a) bending test set-up (b) bending test cracked section left (c) bending
test cracked section right (d) bending test cracked section left (e) bending test cracked
section ............................................................................................................................. 105
Figure 72 ND30 (a) bending test set-up (b) bending cracked section left (c) bending
cracked section right ....................................................................................................... 106
Figure 73 ND40 (a) bending test set-up (b) bending test set-up (c) bending test cracked
section left (d) bending test cracked section right .......................................................... 107
ix
Figure 74 ND60 (a) bending test set-up (b) bending test cracked section left (c) bending
test cracked section right ................................................................................................. 108
Figure 75 NDS 1 block (a) bending test set-up (b) bending test cracked section left (c)
bending test cracked section right (d) bending test cracked section ............................... 109
Figure 76 NDS 2 blocks (a) bending test set-up (b) bending test cracked left (c) bending
test cracked section right ................................................................................................. 110
Figure 77 NDS deflection plots ...................................................................................... 111
Figure 78 ND20 deflection plots..................................................................................... 111
Figure 79 ND30 deflection plots..................................................................................... 112
Figure 80 ND40 deflection plots..................................................................................... 112
Figure 81 ND60 deflection plots..................................................................................... 113
Figure 82 NDS 1 block deflection plots ......................................................................... 113
Figure 83 NDS 2 blocks deflection plots ........................................................................ 114
Figure 84 One-way shear test set up NDS ...................................................................... 117
Figure 85 One-way shear test set up ND60 .................................................................... 117
Figure 86 One-way shear test set up, all other slabs ....................................................... 118
Figure 87 Cross-section of slab showing shear area ....................................................... 121
Figure 88 Example loading diagrams for one-way shear tests, NDS ............................. 122
Figure 89 Applied load at failure for slab specimens undergoing one-way shear .......... 125
Figure 90 Load vs. midspan displacement of slab NDS during one-way shear testing .. 126
Figure 91 Load vs. midspan displacement of slab ND20 during one-way shear testing 127
Figure 92 Load vs. midspan displacement of slab ND30 during one-way shear testing 127
x
Figure 93 Load vs. midspan displacement of slab ND40 during one-way shear testing 128
Figure 94 Load vs. midspan displacement of slab ND60 during one-way shear testing 128
Figure 95 Load vs. midspan displacement of slab NDS 1 block during one-way shear
testing .............................................................................................................................. 129
Figure 96 Load vs. midspan displacement of slab NDS 2 blocks during one-way shear
testing .............................................................................................................................. 129
Figure 97 NDS (a) one-way shear set-up (b) one-way shear set-up (c) one-way shear
cracked section left (d) one-way shear cracked section left (e) one-way shear cracked
section right (f) one-way shear cracked section .............................................................. 130
Figure 98 ND20 (a) one-way shear test set-up (b) one-way shear cracked section left (c)
one-way shear cracked section right (d) one-way shear cracked section underside ....... 131
Figure 99 ND30 (a) one-way shear test set-up(b) one-way shear test cracked section left
(c) one-way shear cracked section right (d) one-way shear cracked section midspan ... 132
Figure 100 ND40 (a) one-way shear test set-up (b) one-way shear test cracked section left
(c) one-way shear test cracked end of slab (d) one-way shear test cracked section right133
Figure 101 ND60 (a) one-way shear test set-up (b) one-way shear cracked section (c)
one-way shear cracked section left (d) one-way shear cracked section end (e) one-way
shear cracked section right .............................................................................................. 134
Figure 102 NDS 1 block (a) one-way shear test set-up (b) one-way shear cracked section
left (c) one-way shear cracked section right (d) one-way shear cracked section end (e) one
way shear cracked section underside .............................................................................. 135
xi
Figure 103 NDS 2 blocks (a) one way-shear test set-up (b) one-way shear cracked section
left (c) one-way shear cracked section right (d) one-way shear cracked section slab
underside ......................................................................................................................... 136
Figure 104 Steel donut used in punching shear tests ...................................................... 138
Figure 105 Punching (two-way) shear set-up ................................................................. 138
Figure 106 Punching shear test, overhead view .............................................................. 139
Figure 107 Punching (two-way) shear test set-up........................................................... 139
Figure 108 Punching shear test box formation supports ................................................. 140
Figure 109 Failure load of two-way (punching) shear.................................................... 142
Figure 110 NDS (a) punching(two-way) set-up (b) punching(two-way) set-up(c)
punching (two-way) shear cracked section, South side (d) punching(two-way) cracked
section, North side (e) punching(two-way) cracked section, South side ........................ 143
Figure 111 ND20 (a) punching (two-way) shear test set-up (b) punching (two-way) shear
cracked section (c) punching (two-way) shear cracked section ...................................... 144
Figure 112 ND30 (a) punching (two-way) shear set-up (b) punching (two-way) shear
cracked section (c) punching (two-way) shear cracked section ...................................... 145
Figure 113 ND40 (a) punching (two-way) shear test set-up (b) punching (two-way) shear
cracked section (c) punching (two-way) shear cracked section ...................................... 145
Figure 114 ND60 (a) punching (two-way) shear test set-up (b) punching (two-way)
cracked section (c) punching (two-way) cracked section end ........................................ 146
Figure 115 NDS 1 block (a) punching (two-way) shear test set-up (b) punching (two-
way) shear cracked section (c) punching (two-way) shear cracked section end ............ 146
xii
Figure 116 NDS 2 blocks (a) punching (two-way) shear test set-up (b) punching (two-
way) shear cracked section ............................................................................................. 147
xiii
Abstract
EFFECT OF RECYCLED CONCRETE AGGREGATES ON THE LONG-TERM,
ELASTIC, TOTAL DEFLECTION AND STRENGTH OF PRECAST PRESTRESSED
HOLLOW CORE CONCRETE SLABS
Lizeth Marisol Gomez Santana
Thesis Chair: Michael J. McGinnis, Ph.D.
The University of Texas at Tyler
August 2019
Concrete constitutes the largest portion of construction and demolition waste
generated each year in the United States. The Environmental Protection Agency (EPA)
estimates that the total production of construction and demolition of concrete waste
during 2015 was 381.8 million tons. In order to reduce the economic and environmental
effects of waste concrete, researchers have begun to incorporate it into concrete mixtures.
However, the inherent variability in mechanical properties between RCA sources makes
it difficult to quantify the maximum permissible replacement of natural aggregate and the
end-result properties of concrete. This project investigates the use of recycled concrete
aggregates (RCA) obtained from a precast prestressed concrete plant in the manufacture
of prestressed hollow core concrete slabs. Seven full scale slabs were tested in two ways
(1) under long term static bending loads to investigate long term deflection behavior, and
(2) under short term loading in bending, beam (one-way) shear and in punching (two-
way) shear.
The results indicate that slabs with RCA generated larger long-term service
deflections. With regards to strength testing the slabs with RCA performed just as well, if
xiv
not better, than slabs with no RCA. Overall, this testing indicates RCA is a viable option
in the efforts to help improve sustainability while maintaining a safe design.
15
Chapter 1
Introduction
1.1 Introduction
Replacing natural aggregate in concrete is a methodology undertaken to reduce
both the costs of construction and negative impact to the environment. A literature review
of this endeavor shows that recycled concrete aggregates (RCA) are a common ingredient
in road base.
RCA are of practical and economic use in the construction industry. Currently, a
large portion of RCA are used as substitutes for natural aggregates in road base (FHWA,
2004). Construction codes and standards do not provide sufficient guideline as to the use
of RCA in structural applications. Therefore, further work is needed to ascertain their
suitability in heavy structural and industrial applications. Previous work indicates that the
inclusion of RCA negatively impacts the properties of concrete specimens. However,
even with these findings RCA may remain a suitable option for structural applications.
The University of Texas at Tyler and The University Notre Dame have
collaborated on the study of RCA on various occasions. These efforts have occurred both
as collaborations between UT Tyler and Notre Dame as well as graduate work carried out
by the individual institutions. The collaborative efforts of UT Tyler and Notre Dame have
led to studies on the design of concrete mixtures with RCA (Knaack and Kurama, 2011;
Knaack and Kurama, 2013a), service load deflections of RCA concrete (Knaack and
Kurama, 2013b), creep and shrinkage (Knaack and Kurama, 2015a) and sustained service
load deflections (Knaack and Kurama, 2015c) as well as strength and stiffness impact of
16
RCA (McGinnis et al., 2017a). Furthermore, work was also done on the effect of RCA on
time dependent deformations on reinforced concrete beams (Knaack and Kurama, 2015b;
Knaack and Kurama, 2018). Work has been undertaken on the mechanical properties of
precast prestressed concrete (Brandes and Kurama 2018a; Brandes and Kurama, 2018b;
Brandes and Kurama, 2018c; Brandes and Kurama, 2016). The economic and
environmental impacts of RCA use have also been investigated (Davis et al., 2015; Davis
et al., 2016; McGinnis et al., 2017a; McGinnis et al., 2017b; Knaack and Kurama, 2018;
Azúa et al., 2019).
This study investigates the use of RCA in precast prestressed hollow core slabs
with varying percentages of natural aggregate replaced by RCA. The replacement
percentages were as follows: 0%, 20%, 30%, 40% and 60%. The precast prestressed
hollow core concrete slabs were: (1) subjected to long term loading for a period of 3
months and (2) were loaded to failure in flexural bending, beam (one-way) shear and
punching (two-way) shear. Existing literature published by the Precast/Prestressed
Concrete Institute indicates detailed guidelines for the design of hollow core concrete
slabs; however, the existing design document does not delve into the use of RCA in these
concrete specimens.
1.2 Organization of thesis
This document is organized as follows:
Chapter 2 presents a review of the previous work undertaken by members of this research
team. This material includes studies on the properties of RCA and its use in reinforced
concrete and precast prestressed concrete.
17
Chapter 3 provides a description of the properties of the concrete mixtures and aggregates
used in this study as well as manufacturing and casting conditions and curing conditions
of the test specimens.
Chapter 4 details the test matrix of variables and samples tested as well as loading
conditions, loading procedures and instrumentation used.
Chapter 5 presents the results of long-term service loading including climatic data,
service load deflections, elastic and inelastic deformations, long-term deflections as well
as service load stress calculations
Chapter 6 provides a description of the bending test set up and procedure as well as the
equipment and instrumentation used.
Chapter 7 provides the bending test results and pictures including calculations of
development length, cracking load and cracking deflections and moment capacity as well
as modulus of elasticity calculated from load vs. midspan displacement
Chapter 8 provides a description of the beam one-way shear test set up as well as
accompanying diagrams
Chapter 9 provides results of the beam one-way shear tests including theoretical shear
capacity and moment capacity and moment and shear demand
Chapter 10 presents the test set-up for the punching (two-way) shear tests as well as
accompanying diagrams
Chapter 11 presents the results of the punching shear tests.
Chapter 12 provides summary and conclusions of the work undertaken in this research
investigation.
18
1.3 Symbols and notation
a, moment arm
a’, moment arm
A, area
ARCA, water absorption of RCA
Aslag, water absorption of slag (used as natural aggregate)
cb, distance from centroid to bottom of the section
ct, distance from centroid to top of the section
DCW, demolition construction waste
dp, depth of prestressing steel
DRCA, deleterious material content of RCA
Ec, modulus of elasticity, Young’s modulus
f’c, compressive strength
f'c,Target, target compressive strength, psi
FHWA, Federal Highway Administration
fpe, effective prestress
fps, stress in prestressing strand at nominal flexural strength
I, moment of inertia
Ie, effective moment of inertia
l, effective span length, bending tests
l’, effective span length, one-way shear tests
ld, development length of prestressing strand
19
Mcapacity, mfg., moment capacity, manufacturer calculation
Mcapacity, moment capacity
Mcr theoretical, mfg., theoretical cracking moment, manufacturer calculation
Mcr theoretical, theoretical cracking moment
Mcr, cracking moment
MLOAD, moment due to applied load
Mmax, maximum moment
MSW, moment due to self-weight
MTotal, total moment
OPEN SEES
P, applied load
RCA, recycled concrete aggregate
Vci, flexure shear strength
Vcapacity, mfg., shear capacity, manufacturer calculation
Vcapacity, shear capacity
VLOAD, shear force due to applied load
VSW, shear force due to self-weight
VTotal, total shear force
β, deflection magnifier
Δel, elastic deflection
ΔLT, long-term deflection
Δmax, maximum deflection
20
ΔT, Total deflection
σb, available, available stress at the bottom of the section
σb, due to load, stress at the bottom of the section due to applied load
21
Chapter 2
Literature Review
2.1 Concrete mixtures with Recycled Concrete Aggregate (RCA)
Thus far, the use of RCA has been restricted primarily to its use in subbase
materials in transportation applications. The Federal Highway Administration (FHWA)
conducted a survey of each states’ use of RCA in road construction. This brief
investigation found that the primary use of RCA was as base material (FHWA, 2004). In
order to better utilize this material several studies have investigated the properties of
RCA from different sources in order to quantify the RCA properties and their impact on
the strength, stiffness, creep and shrinkage of RCA concrete mixtures (Knaack and
Kurama, 2013b; Knaack and Kurama, 2015a; Knaack and Kurama, 2015b; Knaack and
Kurama, 2015c; Knaack and Kurama, 2018; Brandes and Kurama, 2018a; Brandes and
Kurama, 2018b; Brandes and Kurama, 2018c). Studies have also probed the effects of
absorption, ARCA, and deleterious material content, DRCA, of RCA source material on the
performance of RCA concrete. (McGinnis et al., 2017a). Secondly, aside from studying
RCA properties, designers must determine the methodology for incorporating RCA in
concrete mix design. In a 2013 study by Knaack and Kurama, researchers probed three
different methods of incorporating RCA into concrete mixtures (Knaack and Kurama,
2013a). The three methods studied were the: (1) direct weight replacement (DWR), (2)
direct volume replacement (DVR) and (3) equivalent mortar replacement (EMR). Studies
show that these three methods produce similar results with regards to compressive
strength and modulus of elasticity of the samples; however, the workability of the
22
mixture is greatly affected by use of the equivalent mortar method (EMR). Therefore,
Knaack and Kurama (Knaack and Kurama, 2013a) opted to use the direct volume
replacement method. In this method, a given volume of natural concrete aggregate is
replaced by an equal volume of RCA. Per Knaack and Kurama (Knaack and Kurama,
2013a), the direct volume replacement is the most efficient method for RCA applications
because this method produces RCA mixtures that most mimic the workability of natural
aggregate concrete mixtures. Furthermore, this methodology is in line with standard mix
design practice (Knaack and Kurama, 2013a). Concluding that the most important
considerations for use of RCA are the pre-qualification of high-quality source material.
Researchers found that the two most important indicators of quality for RCA are the
water absorption of RCA, ARCA, and the deleterious material content, DRCA (Knaack and
Kurama, 2013a). This study produced and proposed a set of equations that may help to
predict the compressive strength of an RCA mixture given the properties, such as
absorption (ARCA) and deleterious content (DRCA), of an RCA source as well as the
replacement percentage of natural aggregate by RCA. These equations may also be used
to design a concrete mixture by back calculating the allowable values of deleterious
material by setting an allowable maximum strength loss or stiffness loss. Researchers
also identified as a barrier to full-use of RCA in concrete mixtures the effect on the secant
modulus of elasticity of the mixture that would then lead to larger service load deflections
rather than simply the compressive strength (Knaack and Kurama, 2013a; Knaack and
Kurama, 2011).
23
Furthermore, the inclusion of RCA into concrete has shown to decrease CO2
production while simultaneously increasing the amount of water needed to process and
clean the RCA. Therefore, the use of RCA offers a complicated tradeoff where CO2
emissions are reduced but water consumption increases (Azúa et al., 2019). In general,
studies show no distinct pattern in the variability of RCA properties with regards to the
geographical origin of RCA, processing and type or deleterious material present (Knaack
and Kurama, 2013a). Additionally, the size and gradation of aggregates was also found to
affect the properties of the RCA concrete mixes. Results show a marked increase in the
stiffness and compressive strength with a decrease in maximum aggregate size by moving
from ASTM #57 to ASTM #8 aggregates (McGinnis et al., 2017a) as well as decrease in
f’c and Ec with an increase in ARCA and DRCA (Knaack and Kurama, 2013a).
2.2 Use of RCA in reinforced concrete applications
Several authors have investigated the effect of RCA on normal strength reinforced
concrete mixtures. These studies show similar trends, in general: (i) RCA has higher
absorption and LA abrasion loss, lower stiffness (largely due to adhered mortar) and is
sometimes more angular than natural aggregate (NA) (McGinnis et al., 2017a), (ii) an
increase in RCA content is found to decrease the modulus of elasticity, compressive
strength and shear strength (Knaack and Kurama, 2015b) with the effect on modulus of
elasticity being the greatest, (Knaack and Kurama, 2015c) (iii) an increase in RCA
increases the creep and shrinkage (Knaack and Kurama, 2015a; Knaack and Kurama,
2013b), and lastly (iv) an increase in RCA increases service load deflections (Knaack and
Kurama, 2018).
24
Studies have probed the use of RCA in reinforced concrete. These studies have
investigated the creep and shrinkage of RCA mixtures (Knaack and Kurama, 2015a;
Knaack and Kurama, 2013b), short-term and long-term service load deflections (Knaack
and Kurama, 2018) and the effect of RCA on modulus of elasticity and compressive
strength (Knaack and Kurama, 2015c). Research conducted utilizing a time-dependent
numerical model on an open source software (OPEN SEEES) shows that the limiting
factor for usage of RCA is the large service-load deflections (Knaack and Kurama, 2018).
In another study by Knaack and Kurama, the largest negative effect of RCA was shown
to be the stiffness of the mixture while the least affected property was the compressive
strength (Knaack and Kurama, 2015c). The large decrease in stiffness is likely due to the
increased porosity and absorption and decreased mechanical resistance of RCA. The
investigations detailed above found several important trends in RCA use: (i) an increase
in percent of RCA replacement leads to an increase in instantaneous (Malešev et al.,
2010) and long-term deflection, (ii) increase in RCA content results in a large decrease in
modulus of elasticity, Ec, and tensile strength, f’t, but a mild decrease in compressive
strength, f’c (Knaack and Kurama, 2015b.)
Knaack and Kurama (2015b) also probed the effect of RCA on flexural and shear
strength (Knaack and Kurama, 2015b). In this study, the flexural and shear strength
dropped slightly with percent increase of RCA. However, the study also found that the
use of shear reinforcement could circumvent the decrease in shear strength. This study
concludes that the controlling parameter with regards to the tensile strength of concrete is
the quality of RCA and not the quantity (Knaack and Kurama, 2015b). Furthermore, this
25
study proposes that the factors affecting compressive strength and modulus of elasticity
are the RCA replacement ratio, water absorption, ARCA, and deleterious material content,
DRCA. Other studies on the use of RCA are mostly conducted outside the United States
and show that the flexural capacity of reinforced concrete beams is not greatly affected
using RCA even at 100% replacement of natural aggregates (Knaack and Kurama,
2015b). Some studies also indicate that the initiation of shear cracking occurs at lower
service loads in RCA mixtures; this phenomenon is thought to indicate a weaker RCA-
cement paste interlock (Etxeberria et al., 2007).
The performance of concrete mixtures made with high quality aggregates such as
those derived from precast operations also follow the trends indicate above: these
mixtures have lower modulus of elasticity with increase in RCA percent as well as
increased creep and shrinkage strains (Soares et al., 2014a; Limbachiya et al., 2000).
Furthermore, a study by McGinnis et al. concluded that the strength and stiffness
decreases in RCA concrete can be managed by the prequalification of aggregates before
their inclusion in RCA mixtures (McGinnis et al., 2017a). Additionally, this study also
proposed a set of design equations to estimate the effects of RCA on both strength and
stiffness; however, these equations were more complex than those proposed by Knaack
and Kurama (Knaack and Kurama, 2013a).
2.3 Properties of recycled concrete aggregates from precast/prestressed members
Several studies have probed the use of RCA for precast elements (Soares et al.,
2014a; Limbachiya et al., 2000; Pérez-Benedicto et al., 2012; Soares et al., 2014b;
López-Gayarre et al., 2015). Traditionally, the standards for precast elements and the
26
quality control measures at precast facilities account for a higher level of quality as
compared to other sources of RCA, such as demolition and construction waste (DCW).
Most of these studies discussed here cite the high quality and uniformity in precast
members as the reasoning for selection as a reliable RCA source.
In a study by Soares et al., the aggregates used have a substantial amount of
residual mortar attached to the recycled aggregate. This residual mortar leads to an
increase in the porosity of the aggregates (Soares et al., 2014a). In general, research
shows contradicting results regarding the effect of RCA on the compressive strength of
concrete when compared to a standard mixture. Limbachiya et al. found that RCA could
be included in concrete mixtures with up to a 30% replacement of natural aggregate
without negatively affecting the compressive strength of concrete (Limbachiya et al.,
2000). On the other hand, Soares et al. found that the compressive strength of concrete
for 10% and 20% replacement was lowered by, approximately 2.3%, however, for 30%,
40%, 50% and 100% replacement of natural aggregate by RCA the compressive strength
was higher by 6% (Soares et al., 2014a). This shows a lack of a clear relationship
between compressive strength and percent replacement. The use of a superplasticizer
(SP) was found to further increase the compressive strength of concrete. A 100% RCA
mixture with SP and without SP were compared. The mixture with SP had a higher
compressive strength. Findings show that this compressive strength increase is likely due
to a quickening of the hydrating process (Soares et al., 2014a).
27
2.3.1 Use of RCA in prestressed concrete
The efficacy of utilizing RCA within a precast concrete facility is that the
properties of the RCA are known (Brandes and Kurama, 2018a). In order to utilize RCA
in precast prestressed concrete members the bond strength between the prestressing
strands and the concrete must be investigated (Brandes and Kurama, 2018a; Brandes and
Kurama, 2016). There are two important points of concern regarding bonding between
prestressing strands and concrete: the chemical and mechanical bond and the bond
strength (Brandes and Kurama, 2016).
One important consideration of the mechanical bond between the concrete and the
prestressing strand is the development length. In a study by Brandes and Kurama, ASTM
A1081 tests were conducted to ascertain the chemical and mechanical properties of the
seven-wire prestressing strand (Brandes and Kurama, 2018a). The strength and stiffness
of the concrete mixtures used was compared. No significant differences were found
between strength and stiffness gain rate between the RCA and natural aggregate concrete
cylinders.
In a study by Brandes and Kurama, researchers utilized RCA made from crushed
discarded precast prestressed concrete members (Brandes and Kurama, 2018b). As noted,
the use of RCA material from these types of sources ensures the high quality of RCA
aggregates. The results of this study showed that using RCA increased the compressive
strength of concrete. Furthermore, the study compared the effects of traditional RCA
(obtained from construction demolition) to the effects of RCA from precast prestressed
sources on the properties of concrete. Results show that the concrete specimens with
28
RCA from demolition waste had greater shrinkage strains than the concrete specimens
manufactured with discarded precast prestressed concrete (Brandes and Kurama, 2018b).
Few studies have probed the use of RCA in prestressed concrete (Brandes and
Kurama, 2018a; Brandes and Kurama, 2016; Brandes and Kurama, 2018b; Brandes and
Kurama, 2018c; Gonzalez-Corominas et al., 2017). In one study by Brandes and Kurama,
investigators used RCA in precast prestressed members (Brandes and Kurama, 2018c).
This study utilizes RCA as replacement for natural aggregate in precast prestressed
production of up to 100% replacement. In general, the study found that the usage of RCA
yielded concrete mixtures with lower stiffness and larger creep and shrinkage strains
(Brandes and Kurama, 2018c). In this study, the use of RCA caused an increase in
compressive strength and a decrease in tensile strength and concrete stiffness. This
decrease in stiffness is often associated with the residual mortar paste attached to RCA
particles that is substantially less stiff when compared to the natural aggregate, in this
case crushed limestone (Brandes and Kurama, 2018c). In conclusion, this study saw a
minor decrease in the cracking shear force and initial stiffness which, in turn, led to larger
displacements during shear loading and failure (Brandes and Kurama, 2018c). On the
other hand, some studies indicated that the use of RCA has little to no effect on the
mechanical properties of precast concrete (Gonzalez-Corominas et al., 2017). These
contradicting research findings provide the space for further testing of RCA in precast
prestressed members.
29
Chapter 3
Test specimen description
This study probes the use of RCA in hollow core prestressed concrete slabs. The
test specimens used were manufactured at STRESCORE, Inc. located in South Bend,
Indiana. These hollow core concrete slabs were manufactured with varying amounts of
RCA. In this study, different concrete mixes are made by replacing 20%, 30%, 40% and
60% of the coarse aggregate volume with RCA. The hollow core concrete section is 6”
deep and 48” inches wide. There are eight 4” diameter hollow cores that span the entire
length of the slab; there are 5 prestressed strands that run through the ribs (between two
cores) of the slab. Figure 1 below shows a cross section of the hollow core prestressed
concrete slab and Table 1 shows the section properties.
Figure 1 Hollow core slabs
30
Table 1 Section properties, 6SC56
Section properties, 6SC56
A in.2 188
I in.4 764
ct, in. 3
cb, in. 3
dp, in. 4.75
Weight, psf 49
f'c,Target 6,500
3.1 Concrete properties: mix design
The concrete mixtures were made using air-cooled blast furnace (slag) provided
by Beemsterboer Aggregates based out of Gary, Indiana. These aggregates meet the
requirements of INDOT #9 (INDOT, 2019). The properties of slag including specific
gravity and absorption as well as particle size distribution curve are shown in Table 2,
Table 3 and Figure 2. The fine aggregates used in this research program are sand acquired
from the South Bend, Indiana Rieth-Riley plant. The properties of this fine aggregate are
shown in Table 4, Table 5 and Figure 3. The concrete mix designs utilized in this study
are shown in Table 6. Finally, Table 6 shows that all mixtures are made with 500 lbs of
cement while the standard mixture NDS is made with 525 lbs. This discrepancy is not
considered during analysis.
Table 2 Properties of slag
Properties of air-cooled blast furnace (Slag)
Bulk specific gravity Absorption
2.407 3.20%
31
Table 3 Particle size distribution, slag
Properties of air-cooled blast furnace (Slag)
Sieve
No.
Diameter,
mm
%
passing Specification, INDOT 9
1" 25 100% -
3/4" 19 100% 100
1/2" 12.5 64.0% 60-85
3/8" 9.5 41.3% 30-60
No. 4 4.75 10.3% 0-15
No. 8 2.36 5.5% 0-10
No. 200 0.075 1.7% 0-2.5
Figure 2 Particle size distribution curve, slag
Table 4 Properties of fine aggregate, sand
Properties of Sand
Bulk specific gravity Absorption
2.603 4.70%
0%
20%
40%
60%
80%
100%
0.010.1110100Per
cent
pas
ing s
ieve,
%
Particle size, mm
Particle size distribution curve, Slag
32
Table 5 Particle size distribution, sand
Sieve analysis, Sand
Sieve No. Diameter, mm % passing Specification
1/2" 12.5 100%
3/8" 9.5 100% <100
No. 4 4.75 99.6% 95-100
No. 8 2.36 87.7% 80-100
No. 16 1.18 71.1% 50-85
No. 30 0.6 51.3% 25-60
No. 50 0.3 20.2% 5-30.
No. 100 0.15 2.0% 0-10
No. 200 0.075 0.6% 0-3
Pan - 0.0
Figure 3 Particle size distribution curve, sand
0%
20%
40%
60%
80%
100%
0.010.1110100
Per
cent
pas
ing s
ieve,
%
Particle size, mm
Particle size distribution curve, sand
33
Table 6 Mix design for each of the slab specimens
Material ND Standard ND20 ND30 ND40 ND60
Sand 1637 lb 1637 lb 1637 lb 1637 lb 1637 lb
RCA - 282 lb 423 lb 565 lb 846 lb
Slag 1506 lb 1205 lb 1054 lb 904 lb 631 lb
Cement 525 lb 500 lb 500 lb 500 lb 500 lb
VC6100 (plasticizer) 11 oz 11 oz 11 oz 11 oz 11 oz
Air entrainer 10 oz 10 oz 10 oz 10 oz 10 oz
Water #1 10.0 gal 10.0 gal 10.0 gal 10.0 gal 10.0 gal
3.1.1 RCA particle size distribution
The RCA utilized in this study was obtained from STRESCORE, Inc. Two large
samples of aggregates were shipped from STRESCORE, Inc. to the University of Texas
Tyler in Tyler Texas where they were sent for analysis to East Texas Testing
Laboratories (ETTL). The particle distribution curve of this aggregate sample is shown in
Figure 4 and Table 7. The specific gravity, relative density and absorption of RCA are
shown in Table 8. The RCA used for the concrete mixture proportioning of the hollow
core slabs was not sieved to specific gradation it is assumed that the gradation of the
RCA used in the slabs is like that shown above. According to the literature the absorption
of natural aggregate is typically lower than that of RCA.
34
Figure 4 Gradation curves of RCA sample
Table 7 Particle size distribution, RCA
Sieve analysis, RCA
Sieve No. Diameter, mm % passing
3" 75 100.0%
2" 50 100.0%
1" 25 100.0%
3/4" 19 100.0%
1/2" 12.5 80.4%
3/8" 9.5 53.1%
No. 4 4.75 14.0%
No. 8 2.36 5.9%
No. 16 1.18 5.4%
No. 200 0.075 1.7%
Table 8 Properties of RCA
Properties of RCA
Average apparent specific gravity Average specific gravity, SSD Absorption
2.59 2.327 7.66%
0.0%
20.0%
40.0%
60.0%
80.0%
100.0%
0.010.1110100
Per
cent
pas
ing s
ieve,
%
Particle size, mm
Particle size distribution curve
35
3.2 Manufacturing and casting conditions
The concrete specimens tested in this study correspond to hollow core concrete
slab type 6SC56 manufactured by STRESCORE, Inc. based out of South Bend, Indiana.
The hollow core slabs were manufactured on February 8th, 2018. Temperatures during
casting remained below freezing at 4° F. The factory daily production report shows a
moisture content of 4%. The slabs were initially cast as 17-foot-long sections but were
cut down to 15-foot-long sections in order to fit them into the university structures
laboratory.
3.3 Concrete stiffness (Ec)
During manufacturing concrete cylinders were cast for each of the mixtures used.
The cylinders were broken at 28 days, 56 days and 129 days (shortly after the end of
large-scale tests). Cylinders were tested for tensile and compressive strength in
accordance with ASTM C39/C39M-18 (ASTM C39/C39M-18). The results of the
compressive strength tests are shown in, and Table 10 as well as in Figure 5. Figure 5
shows that the compressive strength of each concrete mixture does not rise uniformly
over time. This lack of uniformity in compressive strength is due to the lack of proper
consolidation of cylinder samples and the dryness of the concrete mixture. Many concrete
samples had excessive voids such as the NDS cylinders, shown in Figure 6, and,
therefore, yielded low compressive strengths. The compressive strength values shown in
Table 10 lower than 5,100 psi were excluded from any further analysis including the
calculation of the modulus of elasticity, Ec.
36
The compressive strength values were later used to calculate the elastic modulus of
concrete, Ec. The elastic modulus of concrete was calculated using 𝐸𝑐 = 57,000√𝑓′𝑐
(ACI 318). The elastic modulus of concrete calculated using ACI 318 is shown in Table
11 and Figure 7. Figure 7 presents the modulus values for all three tests. The elastic
modulus is highest for mixtures NDS, ND20 and ND30.
The compressive strength, f’c, and modulus of elasticity, Ec, was also calculated
using the numerical models presented by McGinnis et al., 2017a and Knaack and
Kurama, 2013a. A value of deleterious material content, DRCA, of only 1% was assumed
since the RCA was directly processed on site from discarded precast elements and did not
come from a demolition yard. These are shown in Table 12 and Table 13. The models
proposed by Knaack and Kurama, 2013a tend to underestimate both the compressive
strength and modulus of elasticity.
37
Table 9 Compressive strength of cylinder samples, 7-day strength
Mix 1 2 3 Avg.
NDS 5152 5968 6437 5852
ND20 5320 5573 4717 5203
ND30 5899 5912 6071 5961
ND40 5750 6248 6084 6027
ND60 4545 5572 5438 5185
Table 10 Compressive strength of cylinder samples, (psi)
Compressive strength of concrete cylinder samples (psi)
NDS ND30 ND60
Specimen # Specimen # Specimen #
1 2 3 Avg. 1 2 3 Avg. 1 2 3 Avg.
29
day 7,380 7,140 7,510 7,343 7,140 6,740 7,120 7,000 5,980 6,850 7,030 6,620
56
day 7,590 3,990 3,690 7,590 7,150 7,990 8,250 7,797 5,030 5,010 7,960 7,960
129
day - 8,551 6,964 7,758 6,868 7,416 7,034 7,106 7,393 8,009 6,570 7,324
ND20 ND40
Specimen # Specimen #
1 2 3 Avg. 1 2 3 Avg.
29
day 3,990 6,640 7,390 7,015 6,580 3,850 3,710 6,580
56
day 7,300 7,990 5,930 7,073 3,920 7,620 8,020 7,820
129
day 4,704 8,898 5,695 7,297 5,840 7,917 4,180 5,979
38
Figure 5 Average compressive strength (psi) based on Table 9
(a)
(b)
(c)
(d)
(e)
Figure 6 (a) NDS 56 day (b) ND20 56 day (c) ND30 56 day (d) ND40 56 day (e) ND60
56 day
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
NDS ND20 ND30 ND40 ND60
Co
mp
ress
ive
stre
ngth
, p
si
29 DAY 56 DAY 129 DAY
39
Table 11 Modulus of elasticity, Ec, calculated from ACI 318 code
Modulus of elasticity, Ec, (psi)
29 day 56 day 129 day
NDS 4,884,515 4,066,621 5,020,370
ND20 4,417,653 4,793,877 4,571,504
ND30 4,768,962 5,033,028 4,804,934
ND40 3,913,262 4,602,551 4,407,468
ND60 4,637,713 4,415,201 4,878,081
Figure 7 Modulus of elasticity of samples from Table 11
Table 12 Compressive strength and modulus of elasticity from numerical models
Knaack and Kurama, 2013a
Slab ARCA Aslag D Replacement
% Ec, psi f'c, psi
NDS 7.66% 3.20% 1.00% 0% 4,595,487 6,500
ND20 7.66% 3.20% 1.00% 20% 4,073,545 6,534
ND30 7.66% 3.20% 1.00% 30% 4,073,545 6,584
ND40 7.66% 3.20% 1.00% 40% 4,073,545 6,634
ND60 7.66% 3.20% 1.00% 60% 4,073,545 6,734
4,300,000
4,400,000
4,500,000
4,600,000
4,700,000
4,800,000
4,900,000
5,000,000
5,100,000
5,200,000
NDS ND20 ND30 ND40 ND60
Ec,
psi
29 day 56 day 129 day
40
Table 13 Compressive strength and modulus of elasticity from numerical models,
continued
McGinnis et al., 2017a from compressive strength tests, 129 days
Slab Target, f'c Grade Ec, psi f'c,
psi Ec, psi
NDS 6,500 12.5 4,595,487 6,500 5,020,370
ND20 6,500 12.5 4,810,428 7,006 4,868,915
ND30 6,500 12.5 4,827,615 7,073 4,804,934
ND40 6,500 12.5 4,844,802 7,139 4,727,393
ND60 6,500 12.5 4,879,176 7,272 4,878,081
3.4 Specimen geometry and casting conditions
The prestressed hollow core concrete slabs were manufactured through an
extrusion process. The prestressed tendons are first run throughout the entire length of the
casting yard and are attached to thick metal plate on the end of the bed as shown on
Figure 8. The concrete mix is batched and then introduced into the hopper at the top of
the slipformer as shown in Figure 9. The slabs are extruded onto a bed and allowed to
cure. During curing time, the slabs were covered with black plastic sheathing to keep
them moist as shown in Figure 10. The hollow core slabs were then removed from the
casting yard and loaded into a truck for delivery. A member of this research team was
present during casting to ensure quality.
Figure 8 Prestressing strands
41
Figure 9 Slipformer
Figure 10 Hollow core concrete slabs curing
3.5 Shipping
The specimens were transported from the manufacturing plant to the University of
Texas at Tyler structures laboratory on a flat-bed truck at 5 days curing age. The shipping
route is shown in Figure 11; Figure 12 shows the unloading process which followed the
manufacturers recommended lifting scheme to avoid cracking the slabs.
42
Figure 11 Shipping route
Figure 12 Unloading from flat bed truck
3.6 Curing conditions
The hollow core prestressed concrete slabs were left to harden and cure until 28
days at which point long term loading began. The slabs were left to cure for a period of
three weeks in their long-term loading configuration as described in section 4.3 (later
shown in Figure 16 and Figure 17). The temperature and humidity conditions were stable
within the laboratory; however, the two test specimens left to cure outside, specimens
NDS 1 block and NDS 2 blocks, were cured outside and exposed to the elements. These
two outside specimens are were exposed to large variations in temperature and rainfall
43
events. The rainfall data is for the period between initial arrival of the test specimens and
end of long-term loading and will be discussed further in section 5.4.
3.7 Replacement percentage
The replacement percentages of virgin aggregates with RCA investigated in this
study are shown in Table 14. Three standard mixture, NDS, specimens were tested; the
variables studied were different curing conditions and load levels for the three NDS test
specimens.
Table 14 Specimen replacement percentages of virgin aggregate with RCA
Specimen # Specimen name Replacement percentage
1 NDS 0%
2 ND20 20%
3 ND30 30%
4 ND40 40%
5 ND60 60%
6 NDS 1 Block 0%
7 NDS 2 Blocks 0%
44
Chapter 4
Long term test description
4.1 Test matrix
A total of seven slabs were tested in this study. Five of the slabs were placed
inside the structural laboratory while the remaining two slabs were cured and tested
outside. The slabs arrived on February 14th and were cured in-situ in what would be their
final position for long-term load testing. The test matrix showing the slab properties,
aging conditions and test configurations are shown in Table 15.
Table 15 Test matrix of slabs for long-term load testing
Mix Length Width Section type P, total load (lbs)
Testing
Environment
NDS 15 feet 46 in.
5 strand, six inch corefloor,
6SC56 4,134 Inside
ND20 15 feet 46 in.
5 strand, six inch corefloor,
6SC56 4,134 Inside
ND30 15 feet 46 in.
5 strand, six inch corefloor,
6SC56 4,134 Inside
ND40 15 feet 46 in.
5 strand, six inch corefloor,
6SC56 4,134 Inside
ND60 15 feet 46 in
5 strand, six inch corefloor,
6SC56 4,134 Inside
NDS 1 block 15 feet 46 in.
5 strand, six inch corefloor,
6SC56 3,992 Outside
NDS 2
blocks 15 feet 46 in.
5 strand, six inch corefloor,
6SC56 8,226 Outside
45
4.2 Slab loading and diagram pictures
The slabs were loaded utilizing concrete blocks measuring 46” wide and 23.5” tall
that were manufactured in the university structures laboratory. The blocks were weighted
with a Brecknell electronic crane scale with a 6,000 lb capacity with a ± 0.1% of full
scale capacity accuracy shown in Figure 13. The blocks weighed approximately ± 4000
lbs each and were placed on top of the concrete slabs and supported by two square hollow
tubes measuring ¾” x ¾” and 1/8” thick. These tubes were placed 12” to the left and right
of the center line of the slab.
Figure 13 Brecknell electronic crane scale
4.3 Slab layout in laboratory
The concrete slabs were transported into the structures laboratory and placed in
their testing position on two 3 ½” x 3 ½”x ½” angle steel sections placed at 6” from the
edge of the slab. The long-term laboratory testing included five slabs that were placed
inside the structures laboratory and two slabs placed outside. Of the three standard
concrete mixture slabs two were placed outside, one with a single load block (NDS 1
block) and one with two load blocks (NDS 2 blocks). The last standard mixture slab
46
(NDS) was placed inside the laboratory. In this study, both the curing conditions and the
long-term loading were variables studied. The indoor slabs maintained a constant load of
4134 lbs while the outside slabs were one singly loaded slab and a doubly loaded slab
with 3992 lbs and 8226 lbs loads, respectively. Here buckets of dried aggregates were
used to increase the weight of the concrete blocks to match that of the heaviest block
thereby creating similar loading conditions across all tests. The laboratory slab layout is
shown in Figure 14 - Figure 16. The outside slab layout is shown in Figure 17. In
addition, the slab loading diagram is shown in Figure 18.
Figure 14 Slab layout in laboratory
47
Figure 15. Slab layout in laboratory
Figure 16 Laboratory slab layout
48
Figure 17 Slab layout outside
Figure 18 Slab loading diagram
4.4 Loading procedure
The slabs were loaded by lifting the load blocks from the staging area in the
laboratory utilizing the double girder overhead crane. The blocks were placed on two ¾”
steel square tubes that served as shims and spanned the full width of the slabs to provide
a uniform loading condition.
49
4.5 Instrumentation
The slabs were tested utilizing two string potentiometers placed at midspan at
each side of the span with the wire attached to a 3/16” diameter steel rod. The steel rod
was glued to the top of the slabs utilizing a JB Weld epoxy. Figure 18 shows a diagram of
the slab instrumentation and set-up and Figure 19 shows pictures of the instrumentation
set-up. Additionally, a dial gauge was attached to one side each slab. The dial gauges
used are shown in Figure 20. These dial gauges were utilized as backup for long-term
deflection measurements. Further discussion on the use of these dial gauges is provided
in Chapter 6.
50
Figure 19 Instrumentation for long-term loading
51
Figure 20 Dial gauge used for verification of long-term service loading from string
potentiometers
52
Chapter 5
Deformations under instant and long-term constant loading
5.1 Deflection data
Long-term deflection of slabs is discussed in this chapter as well as discussions on
instant (elastic) deflection, inelastic deflection and long-term deflection. Long-term
deflection was monitored by two string potentiometers on either side of the concrete slab.
Each string potentiomenter was attached to a steel rod glued at the midspan as shown in
Figure 18 and Figure 19 of chapter 4. In addition, a dial gauge was attached to one side of
the slab at 6” from the center to verify the string potentiometer measurement. Each day a
picture was taken of each dial gauge to track deflection. During this experimental
program, no considerations were made for the camber of the precast prestressed hollow
core concrete slabs.
5.1.1 Long-term deflection
The slabs were monitored for long-term loading utilizing the setup described in
chapter 4 for a period of 13 weeks from March 8th to June 9th, 2018. The following
graphs in Figure 21 - Figure 27 show the long-term deflection over time for each slab.
Each slab was instrumented with two string potentiometers (one on each side) and one
dial gauge. The asterisks in the deflection plots indicate the string potentiometer and dial
gauge placed on the same side of the hollow core concrete slab. The three measurements
(taken from the two potentiometers and the one dial gauge) are averaged as follows: the
dial gauge deflection value is averaged with one potentiometer (located on the same side)
then this value is taken and averaged with the remaining potentiometer. For example, for
53
test specimen NDS the deflection values from CH16 were averaged with the dial gauge
deflection value. Then, the average of CH16 and the dial gauge deflection value is
averaged with the second potentiometer in CH7.
The individual deflection curves for each slab are shown in Figure 21 - Figure 27.
Additionally, Figure 28 shows the deflection averages for the inside slabs, Figure 29
shows the deflection averages for the outside slabs and Figure 30 shows the deflection
averages for all slabs. These figures have curves from the two string potentiometers, dial
gauge and a fourth curve showing an average of the three instruments. The four different
curves in each graph tend to follow the same curvature in each individual test. The total
deflection for each slab is shown in Table 16. The use of RCA was seen to increase the
total deflection up to 40% replacement of natural aggregate by RCA. On average, the
total deflection of the inside slabs was approximately 203 thousandths of an inch except
for slab ND40 which had a total deflection of approximately 228 thousandths of an inch
(12.39% higher than the total deflection average of the inside slabs.). Slab NDS 1 block
had a total deflection of 285 thousandths of an inch, 40.30% higher than the average of
the inside slabs. This higher total deflection might be due to manufacturing
imperfections, namely the uneven surface of the specimen. Slab NDS 2 blocks deflected
a total of 768 thousandths of an inch. This specimen had double the load of the other test
specimens, and, therefore, larger total deflection values were expected. In conclusion, the
general trend shows that the deflection of the inside slabs increases with increasing RCA
content except for ND60. Some irregularities present in the curves specifically the
54
outside specimens NDS 1 block shown in Figure 26 and NDS 2 blocks shown in Figure
27 are due to weather conditions. This will be discussed in section 5.3.
55
Table 16 Total deflection, ΔT
Total deflection, ΔT (x 10-3 inch)
Slab ΔT
NDS 187
ND20 192
ND30 206
ND40 228
ND60 202
NDS 1 block 285
NDS 2 blocks 768
Figure 21 NDS long-term deflection
0
50
100
150
200
250
28 38 48 58 68 78 88 98 108 118 128
Def
lect
ion,
(x 1
0-3
inch
)
Age of specimen (days)
CH 13 CH 16* Dial gauge* NDS avg.
56
Figure 22 ND20 long-term deflection
0
50
100
150
200
250
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n,
, (x
10
-3in
ch)
Age of specimen (days)
CH 1* CH 4 Dial gauge* ND20 avg.
57
Figure 23 ND30 long-term deflection
0
50
100
150
200
250
300
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n,
, (x
10
-3in
ch)
Age of specimen (days)
CH 1 CH 4* Dial gauge* ND30 avg.
58
Figure 24 ND40 long-term deflection
0
50
100
150
200
250
300
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n,
, (x
10
-3in
ch)
Age of specimen (days)
CH 7* CH 10 Dial gauge* ND40 avg.
59
Figure 25 ND60 long-term deflection
0
50
100
150
200
250
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n,
(x 1
0-3
inch
)
Age of specimen (days)
CH 19* CH 22 Dial gauge* ND60 avg.
60
Figure 26 NDS 1 block long-term deflection
0
50
100
150
200
250
300
350
400
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n,
(x 1
0-3
inch
)
Age of specimen (days)
CH 13* CH 16 Dial gauge* NDS 1 block avg.
61
Figure 27 NDS 2 blocks long-term deflection
0
100
200
300
400
500
600
700
800
900
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n,
(x 1
0-3
inch
)
Age of specimen (days)
CH 7 CH 10* Dial gauge* NDS 2 blocks avg.
62
Figure 28 Long-term deflection, inside slabs
0
50
100
150
200
250
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n (
x 1
0-3
inch
)
Age of specimen (days)
NDS avg. ND20 avg. ND30 avg. ND40 avg. ND60 avg.
63
Figure 29 Long-term deflection, outside slabs
0
100
200
300
400
500
600
700
800
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n (
x 1
0-3
inch
)
Age of specimen (days)
NDS 1 block avg. NDS 2 blocks avg.
64
Figure 30 Long-term deflection, all slabs
5.1.2 Deflection magnifier
The deflection magnifier was calculated for each slab by diving the deflection of
each slab by the deflection of NDS (e.g. ND20/NDS). These values are shown in Table
17 and Figure 31 -Figure 37. This value shows how quickly the deflection of each slab
progresses as compared to the progression of downward deflection of the control sample
NDS (inside slab). The deflection magnifier values in Table 17 show that at first the
progression of the slab deflection for ND20 and ND60 is slower than NDS (inside slab).
Additionally, the progression of deflection for NDS 2 blocks is more than three times as
0
100
200
300
400
500
600
700
800
28 38 48 58 68 78 88 98 108 118 128D
efle
ctio
n (
x 1
0-3
inch
)
Age of specimen (days)
NDS avg. ND20 avg. ND30 avg. ND40 avg.
ND60 avg. NDS 1 block avg. NDS 2 blocks avg.
65
fast as NDS, and, towards the end of the long -term service load testing the deflection of
NDS 2 blocks proceeds nearly three times as fast as NDS.
Table 17 Deflection magnifier
Deflection magnifier, β
ND20 ND30 ND40 ND60 NDS 1 block NDS 2 blocks
7 days 0.945 1.040 1.168 0.991 1.192 3.476
1 month 0.991 1.063 1.192 1.048 1.144 3.456
2 months 1.017 1.085 1.212 1.068 1.242 3.642
3 months 1.030 1.096 1.222 1.075 1.432 3.985
Figure 31 ND20 deflection magnifier
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
28 48 68 88 108 128
Def
lect
ion m
agn
ifie
r
Age of specimen (days)
NDS/NDS ND20/NDS
66
Figure 32 ND30 deflection magnifier
Figure 33 ND40 deflection magnifier
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
28 48 68 88 108 128
Def
lect
ion m
agn
ifie
r
Age of specimen (days)
NDS/NDS ND30/NDS
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
28 48 68 88 108 128
Def
lect
ion m
agn
ifie
r
Age of specimen (days)
NDS/NDS ND40/NDS
67
Figure 34 ND60 deflection magnifier
Figure 35 NDS 1 block deflection magnifier
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
28 48 68 88 108 128
Def
lect
ion m
agn
ifie
r
Age of specimen (days)
NDS/NDS ND60/NDS
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
28 48 68 88 108 128
Def
lect
ion m
agn
ifie
r
Age of specimen (days)
NDS/NDS NDS 1 block/NDS
68
Figure 36 NDS 2 blocks deflection magnifier
Figure 37 Single load slabs, deflection magnifier
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
4
28 48 68 88 108 128
Def
lect
ion m
agn
ifie
r
Age of specimen (days)
NDS/NDS NDS 2 block/NDS
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
28 48 68 88 108 128
Def
lect
ion m
agn
ifie
r
Age of specimen (days)
ND20/NDS ND30/NDS ND40/NDS
ND60/NDS NDS 1 block/NDS
69
5.1.3 Instant (Elastic) deflection.
The instant deflection was also recorded for each slab. The instant deflection at
application of load was calculated by graphing the deflection vs. time curve from the
beginning of the loading period and taking an average value from the first hour after the
application of load. These values were compared to the theoretical instant (elastic)
deflection values calculated using the effective moment of inertia, Ie based on ACI 318-
14 section 24.2.3.5. The values for instant deformation are shown in Table 18. The
effective moment of inertia, Ie, was calculated using the compressive strength of concrete
at the time of application of load, 28 days curing age. The compressive strength values
used for slabs NDS, NDS 1 block and NDS 2 blocks were from the same concrete
cylinder specimens given that the three concrete slabs were made from the same mix,
NDS.
The measured elastic deflections increased with an increase in RCA content. The
elastic deflection increased from NDS to ND30 and ND40; however, the performance of
ND60 is like that of NDS. Elastic deflections then increase for NDS 1 block and NDS 2
blocks. The measured elastic deflection of all test specimens with one block was on
average 122 thousandths of an inch. The average elastic deflection for slabs with RCA is
118 thousandths of an inch which is 2.6% higher than the elastic deflection of NDS (115
thousandths of an inch). The higher elastic deformation of slab ND40 and slab NDS 1
block is likely due to imperfections in the test specimen (NDS 1 block had an uneven
surface). Slab NDS 2 blocks had a measured elastic deflection that was twice as large as
the average measured elastic deflection of the slabs with one block due to doubling of the
70
load. The theoretical values for elastic deflection calculated from ACI 318-14 are quite
close to the measured values. The theoretical elastic deflection value for NDS 2 blocks is
higher than the measured value; this is due to the higher applied load.
Table 18 Instant (elastic) deformation
Instant (elastic deformation), Δel, (x 10-3 inch)
Slab String
potentiometers Dial gauge Average ACI 318-14 24.2.3.5 Difference
NDS 122 115 101 115 106 8.03%
ND20 124 101 100 107 109 -2.17%
ND30 104 160 104 118 109 7.52%
ND40 155 128 135 137 112 17.99%
ND60 111 111 109 110 112 -1.60%
NDS 1 block 206 133 119 148 106 28.25%
NDS 2 blocks 348 453 300 362 133 63.29%
71
5.1.4 Inelastic deflection
The test specimens were unloaded at the end of the long-term loading period. The
inelastic deflection was noted and recorded for each slab. Table 19 shows the inelastic
deflection for each test specimen. The inelastic deflection value was calculated using the
same procedure as that for the long-term deflection graphs. The dial gauge deflection
value was averaged with the deflection value of the potentiometer on the same side.
Then, this value was averaged with the remaining string potentiometer.
RCA was seen to seen to increase inelastic deflection up to ND40; then the
inelastic deflection decreased from ND40 to ND60. ND60 has a similar performance than
ND40. Table 19 shows that most slabs had approximately 100 thousandths of an inch of
inelastic deflection while specimen NDS 2 blocks, which had twice as much static load as
the rest of the specimens, had more than twice as much inelastic deflection, 337
thousandths of an inch. The inelastic deflection increases with an increase in RCA
content for up to 40% replacement of natural aggregate by RCA. Slabs with RCA have,
on average, had an inelastic deflection of 105 thousandths of an inch an increase of
20.7% from the inelastic deflection of NDS.
72
Table 19 Inelastic deflection
Inelastic deflection, ΔInelastic, (x 10-3 inch)
Slab String
potentiometers Dial gauge Average
NDS 96 88 69 87
ND20 123 78 99 95
ND30 92 165 82 108
ND40 137 109 120 119
ND60 99 99 93 98
NDS 1 block 165 40 156 100
NDS 2 blocks 372 230 375 337
5.2 Long-term deflections
Long-term deflection was calculated by subtracting the elastic deflection
measured averages from the total deflection presented in Table 16. Increase in RCA
content increased the long-term deflections. Furthermore, no change in long-term
deflection was seen in the performance from ND40 to ND60. The relationship between
elastic, long-term and total deflection is: ΔLT = ΔT- Δel. Long-term deflection is shown
below in Table 20. The long-term and total deflection increases with increase in RCA
content up to 40%. The average long-term deflection for slabs with RCA is 89.5
thousandths of an inch which is 24.3% higher than that of NDS; the average total
deflection of RCA slabs is 207 thousandths of an inch which is 10.7% higher than that of
NDS.
73
Table 20 Total, elastic and long-term deflection
Total, elastic and long-term deflection, (x 10-3 inch)
Slab Total deflection, ΔT Δel ΔLT
NDS 187 115 72
ND20 192 107 86
ND30 206 118 88
ND40 228 137 92
ND60 202 110 92
NDS 1 block 285 148 137
NDS 2 blocks 768 362 406
5.3 Service load stress
The available stress, stress due to service loads and ratio between stress due to
applied load and available stress (stress at cracking of section) are in Table 21. Table 21
shows that the stress due to service loads is approximately two thirds of the available
stress for all slabs except for NDS 2 blocks. NDS 2 blocks has an applied stress/available
stress ratio of 108.34%. This percentage indicates the cracking capacity of the NDS 2
blocks section was exceeded by 8.34% during service loads.
The bottom stress (due to applied loads and self-weight) is calculated as the stress
created by the self-weight moment and applied service load moment while the available
stress is the summation of the stress due to prestressing including all loses and the stress
available from the modulus of rupture of the section calculated as 7.5√𝑓′𝑐.
74
Table 21 Service load stress vs. available stress (psi)
Slab σb, due to load, psi σb, available, psi σb, due to load/σb, available, psi
NDS 926 1,389 66.68%
ND20 926 1,374 67.39%
ND30 926 1,374 67.42%
ND40 926 1,354 68.37%
ND60 926 1,356 68.28%
NDS 1 block 926 1,389 66.68%
NDS 2 blocks 1,505 1,389 108.34%
5.4 Weather conditions
Careful observation showed that the rainfall greatly affected the flexural response
of the concrete slabs. Figure 38 shows rainfall data for the Tyler Texas region during this
testing. Figure 39-Figure 40 show the temperature and humidity data for the outside and
inside slabs. The data for the inside slabs was obtained by means of a temperature and
humidity monitor. The data for the exterior slabs was obtained from the national weather
service database (Tyler Texas Weather Station) and shows rainfall data collected at the
Tyler Pounds Airport rain gauge station. This rainfall data proved to be useful in the
interpretation of the daily deflection readings. The rainfall created an interesting pattern
in the service load deflections of the outside slabs, as shown in Figure 29. On days with
large rainfall, the dial gauge readings were seen to ‘dial back’ an average of 3 to 5
thousandths of an inch (3/1000” – 5/1000”). This ‘dialing back’ corresponded to a
negative deflection upwards due to the increased moisture of the top of the concrete slabs
that led to the expansion of the top surface and thus bending of the slabs that resulted in
upward deflection at midspan.
75
Figure 38 Rainfall data during long-term deflection testing
28-Mar, 2.62
13-Apr, 0.81
21-Apr, 2.22
22-Apr, 0.334-May, 0.6
0
0.5
1
1.5
2
2.5
3
Rai
mfa
ll,
(inch
es)
Date
76
Figure 39 Temperature and humidity data, outside slabs
0
10
20
30
40
50
60
21-Mar 31-Mar 10-Apr 20-Apr 30-Apr 10-May 20-May 30-May 9-Jun 19-Jun
Deg
rees
(°C
)
High temperature (°C) Low temperature (°C)
0
20
40
60
80
100
120
21-Mar 31-Mar 10-Apr 20-Apr 30-Apr 10-May 20-May 30-May 9-Jun 19-Jun
Hum
idit
y (
% R
H)
High humidity (% RH) Low humidity (% RH)
77
Figure 40 Temperature and humidity data, inside slabs
0
5
10
15
20
25
30
21-Mar 31-Mar 10-Apr 20-Apr 30-Apr 10-May 20-May 30-May 9-Jun 19-Jun
Deg
rees
(°C
)
High temperature (°C) Low temperature (°C)
0
10
20
30
40
50
60
70
80
90
21-Mar 31-Mar 10-Apr 20-Apr 30-Apr 10-May 20-May 30-May 9-Jun 19-Jun
Hum
idit
y (
% R
H)
High humidity (% RH) Low humidity (% RH)
78
5.5 Conclusions
In conclusion, the prestressed precast hollow core concrete slabs had an increase
in elastic deflection (by 2.6%), long-term deflection (by 24.3%), inelastic deflection (by
20.7%) and total deflection (by 10.7%) when RCA was used. No trends with increasing
RCA content were noted. For elastic, inelastic and total deflection typically the 40%
RCA slab had the largest deflections and the 60% RCA slab had behavior similar to the
standard slab, NDS. Long-term deflections increased slightly with increasing RCA
replacement. Slab NDS 1 block had larger total and elastic deflection when compared to
the other five slabs loaded with a single block. This trend is likely due to the poor quality
of the specimen (uneven surface). These results show that the replacement of natural
aggregate by RCA is feasible up to 60%. The superior performance of slab ND60 versus
slab ND40 should be investigated further. The elastic deflections calculated using ACI
318-14 were comparable to the measured deflections.
The total deflection of NDS 2 block slab was substantially larger than the total
deflection of the specimens with one load block; this result was expected given the larger
service load applied. Overall, the slab with the highest replacement of RCA, ND60,
showed a comparable performance to that of the NDS slab. Lastly, the deflection
magnifiers reported here show that in general an increase in RCA content quickens the
progression of downward deflection (larger deflection magnifier values).
79
Chapter 6
Bending test description
In this study the flexural strength, one-way shear and punching shear strength of
several hollow core slabs with varying percentages of natural aggregate replaced by RCA
were studied. This chapter describes the flexural bending strength test set-up.
6.1 Test set up
The test specimens were moved into place utilizing the overhead crane and web
slings. The slabs were lifted into the air, turned 90° and positioned between the two steel
wide flange columns. The slab was placed on two W10x54 sections with the web of the
wide flange placed at 6” from the slab end. Figure 41 shows an example of the bending
test set-up for the NDS slab test and Figure 42 shows a diagram of the test set up. All
slabs were tested in bending utilizing a uniform test set up. The location of the two wide
flange supports and the position of the wide flange point loads were constant from one
slab to the next.
80
Figure 41. Bending test set up, ND S
Figure 42. Bending test set up diagram
81
6.2 Equipment
The steel frame in the structures laboratory was utilized for loading. Once the slab
was in place the centerline of the slab was aligned with the center of the load frame, the
steel wide flange sections, hydraulic pump, load cell and plates were set in place as
shown in Figure 43. The slabs were loaded utilizing a RCH603 Enerpac hollow plunger
hydraulic cylinder with a 3” stroke and a 60 ton capacity Enerpac model number
ZU4420MB electric hydraulic pump. The hydraulic pump and its attached pressure
gauges are shown in Figure 44.
Figure 43 Bending test set-up
82
Figure 44. Enerpac hydraulic pump and pressure gauges
6.3 Load procedure
Utilizing the hydraulic pump valve and gauge, the pressure was advanced a
quarter turn at a time extending the hollow cylinder upwards. The hollow cylinder
pressed against the plate, load cell and load frame creating an equal and opposite reaction
that slowly displaced the slab downwards. The hollow cylinder was advanced while
tracking the load and the deflection of the slab at midspan. If the ram had reached stroke
prior to slab failure the slab was unloaded and additional plates were positioned between
the ram and the load frame by space left by the permanent deformation. The test was then
restarted until the slab failed.
6.4 Instrumentation
The string potentiomenters utilized for the long-term loading of the slabs were
also utilized for the large-scale tests. Three steel galvanized angle clips were placed on
both sides of the slab with a 3/16” diameter screws protruding outwards. The clips were
83
located at midspan and quarterspans (45”, 90” and 135”) on both sides of the slab. Then
utilizing the metal loops on the string potentiometer wire, the instruments were attached
to the galvanized steel angles. The clips located at the center of the slab, 90”, were
monitored while loading. The string potentiometers were wired and connected to an
instrunet data acquisition system utilizing a GW Instruments, Inc. Model 100
Analog/Digital Input/Output system with 22 channels, as shown in Figure 45 and Figure
46. The string potentiometers were connected as show in Figure 47 in Channels 1 through
16. The load cell, shown in Figure 48 was also connected to the instrunet data acquisition
system in Channel 22. The slab orientation and instrumentation are shown in Figure 49.
The values were recorded as the hydraulic pressure from the pump was increased. These
values were then plotted against the load applied to track the deflection of the slabs. The
load cell utilized in this study was an Omegadyne LC8400-213-200k with a capacity of
200,000 lbs. The load cell was calibrated before the long-term testing, as shown in Figure
50. The calibration curve was utilized in the analysis of the test data.
Figure 45 Data acquisition system
84
Figure 46 Model 100, data acquisition hardware
Figure 47 String potentiometer
Figure 48 Load cell, model Omegadyne LC8400-213-200k
85
Figure 49 Slab and instrumentation set up
Figure 50 Load cell calibration curve
y = 0.9819x + 1903.4
R² = 0.9999
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000
Load
mea
sure
d (
lbs)
Applied load (lbs)
86
Chapter 7
Bending test results
The hollow core concrete slabs were tested for flexural bending strength, one-way
shear and punching (two-way) shear. This chapter presents the results of the flexural
bending tests.
7.1 Moment capacity
The moment capacity, Mcapacity, was calculated and compared to the moment
demand. The moment capacity was calculated using the strain compatibility method and
the following assumptions and values: (i) the compressive strength, f’c, values used were
the measured strength from 129 days curing strength, (ii) the effective prestress loses
were calculated using the lump-sum method of time dependent losses, here the effective
prestress after all loses is fpe = 144,000 psi, (iii) the theoretical cracking moment is
calculated as the summation of the moment necessary to reach the modulus of rupture of
concrete plus the moment necessary to reach the total stress at the bottom of the section
due to prestress (including all loses) and stress due to bending. Additionally, the
manufacturer’s moment capacity is included and compared to the calculated moment
capacity. Furthermore, it is important to note that the manufacturers calculations are
based on the following conditions: (i) the target compressive strength, f'c,Target = 6,500 psi
is used and (ii) the prestress losses are calculated using the methods stipulated in section
2.2.3 of the PCI Manual for the design of Hollow Core Slabs and walls.
87
7.2 Development length
The development length was calculated for each slab in accordance with ACI
318-14 section 25.4.8.1, and is shown in Table 22, in order to avoid strand slip. The
development length is calculated as a function of fps (the stress in the prestressing strand
at nominal flexural strength). Here fps is calculated using the strain compatibility method.
Here, the development length of the prestressing strand for each slab is approximately 59
inches compared to the available length of 78 inches. This calculation shows that strand
slip did not occur during bending tests.
88
Table 22 Development length (ld)
Slab fps, psi ld, in.
NDS 254,689 59.51
ND20 254,174 59.32
ND30 253,913 59.22
ND40 253,631 59.11
ND60 254,200 59.32
NDS 1 block 254,685 59.51
NDS 2 blocks 254,685 59.51
7.3 Cracking load, maximum load to failure and code capacity
The flexural bending tests were carried out on the entire slab length directly after
long-term loading. The slabs were unloaded from their long-term loading configurations
the day before, and therefore, recovered elastic deflection before the large-scale tests. The
bending set-up indicated in chapter 6 creates a four-point bending where the moment is
constant between the two-point loads. The maximum flexural bending load at failure is
shown in Figure 53. The load versus midspan deflection curves are in Figure 55 - Figure
61 and the load versus midspan deflection curves showing the loading and reloading of
the slabs are in Figure 62 -Figure 69. The tests were terminated when the failure load was
reached. No attempt was made to deform any of the slabs on their yield plateau the same
for every slab.
The cracking load for each slab was determined by taking the coordinates of the
last point on the straight-line portion of the load vs. midspan displacement. Figure 52
shows an example for ND20 where the cracking load was 8,930 lbs and the cracking
deflection was 0.1495 inches. The cracking moment is calculated from the cracking load
and compared to the theoretical cracking moment, Mcr theoretical. The theoretical cracking
89
moment was calculated using the total stress at the bottom of the section due to
prestressing and prestress losses. Table 23 shows the initial cracking load is highest for
NDS and thereafter decreases with an increase in RCA. ND20 and ND30 have a similar
initial cracking load with the initial cracking load of ND30 being approximately 0.71%
higher than ND20. ND40 and ND60 show a uniform decrease in initial cracking load.
Furthermore, the cracking load for slabs NDS 1 block is significantly lower than that of
the other slabs. This lower cracking load is likely due to the irregular and unfinished
surface of the NDS 1 block slab. The initial cracking load for NDS 2 blocks is
substantially lower than that of the other slabs. The lower initial cracking load could be
due to the cracks that were present at the time of load to failure due to the service loads
applied to the section.
The images in Figure 70 -Figure 76 show the flexural bending test set-up as well
as pictures of the cracked sections. The maximum load to failure, moment due to applied
load, MLOAD, and moment due to self-weight, MSW, for each slab is shown in Table 24.The
moment due to applied load is calculated using the moment arm length from a quarter
way from the right edge of the left-most support to the application of the point load, P as
shown in Figure 54.. The moment due to self-weight is calculated using the effective
length as seen in Figure 54. The summation of the moment due to self-weight and the
moment due to applied load is the total moment, MTotal. This total moment is compared to
the moment capacity, Mcapacity. The ratio of moment demand to moment capacity is also
reported in Table 24. All slabs exceeded code capacity except for NDS 1 block; this is
likely due to the uneven surface of the slab as indicated in Figure 51. RCA was seen to
90
improve the moment capacity of the hollow core concrete slabs; ND20, ND30 and ND40
had a higher moment capacity than the standard slab by 10.2%, 5.5% and 7.7%,
respectively. NDS 2 blocks also had a higher moment capacity than the standard slab. On
the other hand, ND60 performed equally as well as the standard slab, NDS. The average
MTotal /Mcapacity ratio for RCA slabs, as shown in Table 24, is 1.20 which is approximately
5.85% higher than that of NDS. The total moment was also compared to the moment
capacity as calculated by the manufacturer, Mcapacity, mfg. The Mcapacity, mfg. is about 1.96%
higher than that the code moment capacity of NDS, as shown in Table 24.
The shear capacity was also compared to the shear demand for these bending
tests. The total shear demand, VTotal, was calculated as the summation if the shear due to
applied load, VLOAD, and shear due to self-weight, VSW. The total shear demand versus
shear capacity is shown Table 25. The VTotal/ Vcapacity ratios for all slabs were well below
1.0 indicating that the test specimens all failed by the intended mode, bending. The
results of the bending tests are within the variability of the materials and methods used in
this study.
The moment capacity calculations by the manufacturer are shown in Table 24.
The moment capacity values calculated by the research team are at most 2.8% lower than
those of the manufacturers.
91
Figure 51 NDS 1 block, uneven surface
Table 23 First cracking load
First cracking, bending
Slab P, Load, lbs Mcr, kip∙ft. Mcr theoretical, kip∙ft. Ratio Mcr theoretical, mfg.,
kip∙ft.
NDS 9,083 27.25 29.85 0.91 30.04
ND20 8,930 26.79 29.43 0.91 30.04
ND30 8,993 26.98 29.25 0.92 30.04
ND40 8,228 24.68 29.03 0.85 30.04
ND60 7,998 24.00 29.46 0.81 30.04
NDS 1 block 5,332 16.00 29.85 0.54 30.04
NDS 2 blocks 6,952 20.86 29.85 0.70 30.04
Figure 52 Applied load vs. midspan displacement up to cracking for ND20
0
2,000
4,000
6,000
8,000
10,000
-0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
92
Figure 53 Failure loads of flexural bending test specimens
Table 24 Maximum load to failure and code capacity
Slab
P,
Load,
lbs
a,
inch
l,
inch
MLOAD
kip∙ft.
MSW
kip∙ft.
MTotal,
kip∙ft.
Mcapacity,
kip∙ft.
Ratio MTotal/
Mcapacity
Mcapacity, mfg.
kip∙ft.
NDS 14,351 70.50 165 42.16 4.63 46.79 41.30 1.13 42.11
ND20 15,895 70.50 165 46.69 4.63 51.32 41.13 1.25 42.11
ND30 15,116 70.50 165 44.40 4.63 49.04 41.04 1.19 42.11
ND40 15,435 70.50 165 45.34 4.63 49.97 40.94 1.22 42.11
ND60 14,287 70.50 165 41.97 4.63 46.60 41.14 1.13 42.11
NDS 1
block 11,940 70.50 165 35.07 4.63 39.71 41.30 0.96 42.11
NDS 2
blocks 15,346 70.50 165 45.08 4.63 49.71 41.30 1.20 42.11
Table 25Maximum load to failure and shear demand vs. shear capacity
Slab P, Load, lbs VTotal, kips Vcapacity, kips Ratio VTotal/ Vcapacity
NDS 14,351 8.52 19.72 0.43
ND20 15,895 9.30 20.84 0.45
ND30 15,116 8.91 20.68 0.43
ND40 15,435 9.07 20.49 0.44
ND60 14,287 8.49 19.17 0.44
NDS 1 block 11,940 7.32 21.22 0.34
NDS 2 blocks 15,346 9.02 21.22 0.43
14,351
15,89515,116 15,435
14,287
11,940
15,346
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
ND S ND 20 ND 30 ND 40 ND 60 ND S 1
Block
outside
ND S 2
Block
outside
Fai
lure
load
(lb
s)
Slab
93
Figure 54 Loading diagrams for bending tests
Figure 55 Load vs. midspan displacement of NDS slab up to cracking load
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
-0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
94
Figure 56 Load vs. midspan displacement of ND20 slab up to cracking load
Figure 57 Load vs. midspan displacement of ND30 slab up to cracking load
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
-0.3 -0.2 -0.1 0
Lo
ad (
lbs)
Midspan displacement (in)
Channel 13 Channel 4 Average
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
-0.7 -0.5 -0.3 -0.1
Lo
ad (
lbs)
Midspan displacement (in)
Channel 13 Channel 4 Average
95
Figure 58 Load vs. midspan displacement of ND40 slab up to cracking load
Figure 59 Load vs. midspan displacement of ND60 slab up to cracking load
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
-0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
-0.2 -0.15 -0.1 -0.05 0
Load
(lb
s)
Midspan displacement (in)
Channel 13 Channel 4 Average
96
Figure 60 Load vs. midspan displacement of NDS 1 block slab up to cracking load
Figure 61 Load vs. midspan displacement of NDS 2 blocks slab up to cracking load
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
-1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
-0.3 -0.2 -0.1 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
97
Figure 62 Loading and re-loading load-displacement curve of NDS during loading test
Figure 63 Loading and re-loading load-displacement curve of ND20 during loading test
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
-3 -2.5 -2 -1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
-3 -2.5 -2 -1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)
Channel 13 Channel 4 Average
98
Figure 64 Loading and re-loading load-displacement curve of ND30 during loading test
Figure 65 Loading and re-loading load-displacement curve of ND40 during loading test
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
-3 -2.5 -2 -1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
-3 -2.5 -2 -1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
99
Figure 66 Loading and re-loading load-displacement curve of ND60 during loading test
Figure 67 Loading and re-loading load-displacement curve of NDS 1 block during
loading test
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
-2 -1.5 -1 -0.5 0
Load
(lb
s)
Midspan displacement (in)Channel 13 Channel 4 Series3
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
-3.5 -2.5 -1.5 -0.5
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
100
Figure 68 Loading and re-loading load-displacement curve of NDS 2 block during
loading test
Figure 69 Load vs. midspan displacement averages for all slabs
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
-3 -2.5 -2 -1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
-0.2 -0.15 -0.1 -0.05 0
Lo
ad (
lbs)
Displacement (in)
NDS ND20 ND30 ND40 ND60 NDS 1 block NDS 2 blocks
101
7.4 Cracking deflection
The load vs. midspan deflection curves up to the cracking load are shown in
Figure 55 - Figure 61. The cracking deflection due to the cracking load is shown in Table
26. The load vs. midspan deflection curve for slab NDS 2 blocks shown in Figure 61 does
not have a distinct straight line load vs. displacement portion as the other slab load vs.
midspan displacement curves in Figure 55 - Figure 60. The NDS 2 blocks slab was
cracked due to service loads as the weight of the two blocks created bottom stress that
exceeded the available stress from the effective prestress of the tendons.
Table 26 Cracking deflection
Slab Cracking
load, lbs Cracking deflection (in.)
NDS 9,083 0.1705
ND20 8,930 0.1495
ND30 8,993 0.1560
ND40 8,228 0.1545
ND60 7,998 0.1482
NDS 1 block 5,332 0.0895
NDS 2 blocks 6,952 0.2030
All slabs were loaded to failure. Figure 62 - Figure 68 show the load vs. midspan
displacement curves. These curves show the loading and re-loading of slabs to failure.
These figures are accompanied by pictures of the cracked sections in Figure 70 -Figure
76.
7.5 Modulus of elasticity from load vs. midspan displacement curves
The modulus of elasticity, Ec, was also calculated from the straight-line portion of
the load versus midspan displacement curves shown in Figure 55 - Figure 61. Modulus of
102
elasticity is back calculated by taking the load and deflection from each point of the
straight-line portion of the load versus displacement plots and using the elastic deflection
for maximum displacement 𝛥𝑚𝑎𝑥 = 𝑃𝑎
24𝐸𝐼∙ (3𝑙2 − 4𝑎2). The equation represents the
maximum displacement based on the loading setup. These values were compared to the
Ec values calculated from the compressive strength cylinders in section 3.3. Table 27
shows a comparison between the Ec value from load vs. midspan displacement and Ec
calculated from the ACI empirical equation which is based on the compressive strength
of the cylinders. The table shows the Ec values calculated from the load vs. midspan
displacement tests are on average 33% higher than those calculated from the compressive
strength tests.
Table 27 Modulus of elasticity, Ec, from load vs. midspan displacement curves
Ec modulus of elasticity (psi)
Slab from load vs. midspan displacement curves 57,000√𝑓′𝑐 % error
NDS 7,577,383 5,020,370 33.75%
ND20 7,581,630 4,868,915 35.78%
ND30 7,551,334 4,804,934 36.37%
ND40 6,821,636 4,727,393 30.70%
ND60 6,952,796 4,878,081 29.84%
NDS 1 block 7,606,445 5,020,370 34.00%
NDS 2 blocks 7,275,630 5,020,370 31.00%
7.6 Quarter-point deflection plots
The deflection curves for all six string potentiometers used during load to failure
testing (including those at midspan) were plotted and are shown in Figure 77 - Figure 83.
Within each bending test, the deflection plots tend to follow a similar trend with the
103
curves for channel 4 and channel 13 showing the largest displacements. The curves for
channels 4 and 13 are larger than the other deflection plots because these two channels
are located at the beam midspan and therefore have the largest deformations. The curves
for the four remaining channels (located at quarter points as shown in Figure 49) tend to
cluster around the same space. This clustering of curves indicates that the deflection at
each of these points is similar. The symmetrical positioning of these string potentiometers
also accounts for the similar deflection values.
104
(a)
(b)
(c)
(d)
(e)
Figure 70 NDS (a) bending test set-up (b) bending test set-up (c) cracked section left (d)
cracked section right (e) cracked section
105
(a)
(b)
(c)
(d)
(e)
Figure 71 ND20 (a) bending test set-up (b) bending test cracked section left (c) bending
test cracked section right (d) bending test cracked section left (e) bending test cracked
section
106
(a)
(b)
(c)
Figure 72 ND30 (a) bending test set-up (b) bending cracked section left (c) bending
cracked section right
107
(a)
(b)
(c)
(d)
Figure 73 ND40 (a) bending test set-up (b) bending test set-up (c) bending test cracked
section left (d) bending test cracked section right
108
(a)
(b)
(c)
Figure 74 ND60 (a) bending test set-up (b) bending test cracked section left (c) bending
test cracked section right
109
(a)
(b)
(c)
(d)
Figure 75 NDS 1 block (a) bending test set-up (b) bending test cracked section left (c)
bending test cracked section right (d) bending test cracked section
110
(a)
(b)
(c)
Figure 76 NDS 2 blocks (a) bending test set-up (b) bending test cracked left (c) bending
test cracked section right
111
Figure 77 NDS deflection plots
Figure 78 ND20 deflection plots
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
0 360 720 1080 1440 1800 2160
Def
lect
ion (
inch
)
Time (seconds)
CH1 CH10 CH4 CH13 CH7 CH16
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
0 360 720 1080 1440 1800 2160
Def
lect
ion (
inch
)
Time (seconds)
CH1 CH10 CH4 CH13 CH7 CH16
112
Figure 79 ND30 deflection plots
Figure 80 ND40 deflection plots
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
0 360 720 1080 1440 1800
Def
lect
ion (
inch
)
Time (seconds)
CH1 CH10 CH4 CH13 CH7 CH16
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
0 360 720 1080 1440 1800 2160
Def
lect
ion (
inch
)
Time (seconds)
CH1 CH10 CH4 CH13 CH7 CH16
113
Figure 81 ND60 deflection plots
Figure 82 NDS 1 block deflection plots
0.000
0.500
1.000
1.500
2.000
2.500
3.000
0 360 720 1080 1440 1800 2160 2520 2880
Def
lect
ion (
inch
)
Time (seconds)
CH1 CH10 CH4 CH13 CH7 CH16
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
4.500
0.000 360.000 720.000 1080.000 1440.000 1800.000
Def
lect
ion (
inch
)
Time (seconds)
CH1 CH10 CH4 CH13 CH7 CH16
114
Figure 83 NDS 2 blocks deflection plots
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
0.000 360.000 720.000 1080.000 1440.000
Def
lect
ion (
inch
)
Time (seconds)
CH1 CH10 CH4 CH13 CH7 CH16
115
Chapter 8
One-way shear test description
8.1 Test set-up
Following the bending test, the specimens were moved into place for one-way
shear testing. Due to the variability in the usable length of slab left after the first test (the
bending test) three different shear test set ups were utilized as shown in Figure 84 -
Figure 86. First, the usable slab length was measured as the distance from the right edge
of the slab (the end of the slab pointing outside of the lab) to the innermost edge of the
nearest flexural crack. In most of the bending tests, the usable length for shear testing was
approximately 90”; however, some slabs only had approximately 72” of usable length.
The tested slab length was thus different for each. The midpoint of the slab length tested
was marked using chalk. Then two W10x54 wide flange sections were placed equidistant
from the test length with their outer edge of the flange placed flush to the edge of the
flexural crack on the left and the end of the slab on the right. Two W12x72 wide flange
sections were placed near the center of the tested span length as load points to distribute
the load. Here the web of the W12x72 sections were placed at 9.875 inches from the
centerline of the shear test length for the ND60 slab, at 15.50 inches from the centerline
of the NDS slab and 12 inches from the centerline of the remaining samples. The usable
length/as tested length of each slab and the distance between the point loads is indicated
for each test in Table 28. During this test, the slabs are loaded by advancing the hydraulic
pump a quarter turn at a time. Here the hydraulic cylinder proceeds to push against the
load frame and create an equal and opposite reaction that bends the slabs downwards.
116
Here the shear tests were loaded to failure. The slabs were loaded until the maximum
stroke of the hydraulic cylinder of 3 inches was reached. The slabs were then unloaded.
At this moment, the permanent deformation of the slab causes the test assembly to
separate from the load frame which allowed for additional metal plates (shims) to be
placed between the hydraulic cylinder and the load frame. The shear test is restarted and
load applied till the span failed.
117
Figure 84 One-way shear test set up NDS
Figure 85 One-way shear test set up ND60
118
Figure 86 One-way shear test set up, all other slabs
Table 28 Beam (one-way) shear slab lengths tested
Slab Usable length of slab
tested (inches) Distance between point loads (inch)
ND S 90 31
ND20 72 24
ND30 72 24
ND40 72 24
ND60 90 19.75
NDS 1 block 72 24
NDS 2 blocks 72 24
119
Chapter 9
Results of one-way shear tests
In order to facilitate the testing procedures all concrete slabs were tested in one-
way shear directly after the bending tests. A useable length of the concrete slab was used
to test the one-way shear capacity of the different specimens. The test set-ups of these
concrete slabs were described in Chapter 8 and detailed in Figure 84 - Figure 86.
9.1 Theoretical shear capacity and moment capacity
The theoretical shear capacity of the prestressed hollow core slabs was calculated
in accordance with section 22.5.8.3.1 as the greater of the flexure shear strength, Vci,
calculated by equation(s) 22.5.8.3.1a and 22.5.8.3.1b from the ACI 318-14 design code.
Here equation 22.5.8.3.1a includes provisions for the maximum moment at section due to
the applied load. The following assumptions were made: (i) the shear area is calculated as
the area of concrete above the center of the steel prestressing strands as shown in Figure
87 and (ii) the effective prestress is taken as fpe = 144,000 psi to calculate the moment
causing flexural cracking at section due to externally applied loads, Mcre. The moment
capacity, Mcapacity, of the section is calculated as described in section 7.1.
Furthermore, the manufacturers calculation of shear capacity is also included. It is
important to note that the manufacturers calculations are based on the following
conditions: (i) the target compressive strength, f'c,Target = 6,500 psi is used and (ii) the
prestress losses are calculated using the methods stipulated in section 2.2.3 of the PCI
Manual for the design of Hollow Core Slabs and walls and is approximately fps= 168,600
120
psi. Finally, due to the short development length, strand slip did occur during one-way
shear tests.
121
Figure 87 Cross-section of slab showing shear area
9.2 Moment and shear demand
During the one-way shear tests the usable length of the slab was supported by two
W10 x 54 sections as well as a third W10 x 54 section that supported the remaining
(unbroken) span of the slab as shown Figure 84-Figure 86 and Figure 88. This additional
support creates a test set-up consisting of two spans instead of a single span. However,
during the application of load the unused span lifts-off of the W10 x 54 section which
then changes the setup from two spans to one. The spacing of the point loads varied for
each slab with the spacing for ND60 being the smallest at 19.75 inches and the spacing
between the point loads for NDS being the largest at 31 inches.
The shear and moment demand due to self-weight is calculated using the effective
length and moment arm shown in Figure 88. The effective span length for test specimen
NDS is 75 inches. The moment due to the applied load is calculated using the moment
arm, a’, shown in Figure 88. The moment arm for NDS is 22 inches.
122
Figure 88 Example loading diagrams for one-way shear tests, NDS
9.3 Results of four-point shear tests
The results of the four-point one-way shear test of the hollow core concrete slabs
are shown in Figure 89. Slabs ND60 and NDS were loaded twice; therefore, the load
versus displacement curves show an unloading and reloading curve. The load versus
displacement curves for each hollow core concrete slab are shown below in Figure 90 -
Figure 96. Photos of the one-way shear test set-up and cracked sections are shown in
Figure 97 - Figure 102.
During one-way shear tests, all slabs exceeded code shear capacity and moment
capacity. Table 29 shows the measured and theoretical one-way shear capacities based on
ACI 318-14 and calculated as described in section 9.1, and
Table 30 shows the moment demand and capacity of each test specimen. All slabs failed
in shear which was the intended failure mode; RCA was seen to improve the shear
123
capacity of the hollow core slabs. The ND20, ND30 and ND40 RCA slabs performed at
73%, 59% and 85% higher than code capacity. ND60 exceeded code capacity by only
25%; therefore, NDS performed better than ND60. The ratio of VTotal/ Vcapacity is much
lower for NDS 1 block (likely due to manufacturing irregularities) and NDS 2 blocks.
The lower shear capacity of NDS 2 blocks is likely lower due to the lower slab capacity
given that the section cracked during the service loading period. The shear capacity as
calculated by the manufacturer Vcapacity, mfg. kip∙ft., is substantially higher than the total
shear and higher than the shear capacity calculated via ACI 318-14. Furthermore, NDS 1
block has a substantially lower VTotal/ Vcapacity and MTotal/ Mcapacity ratios when compared to
the other slabs due to the manufacturing errors in the slab as shown in Figure 51. Finally,
the negative moment (taken as the maximum moment generated by the self-weight) at the
middle support as shown in Figure 84-Figure 86 hand overhanging span as shown in
Figure 88, was not seen to exceed the moment capacity of the section and therefore did
not crack the overhanging span. A comparison of the negative moment capacity versus
negative moment demand is shown in Table 31. Here the negative moment capacity is the
moment necessary to create the available stress at the bottom of section (due to
prestressing and eccentricity). The results of the one-way shear tests are within the
variability of the materials and methods used in this study.
124
Table 29 One-way shear demand and capacity
Slab a',
inch
P,
Load,
lbs
VLOAD, kip
l', inch VSW, kips VTotal, kips Vcapacity, kips Ratio VTotal/
Vcapacity Vcapacity, mfg.
kip∙ft.
NDS 22.0 53,067 26.53 75 1.64 28.17 19.72 1.43 22.92
ND20 16.5 67,227 33.61 57 2.37 35.98 20.84 1.73 22.92
ND30 16.5 60,976 30.49 57 2.37 32.86 20.68 1.59 22.92
ND40 16.5 71,181 35.59 57 2.37 37.96 20.49 1.85 22.92
ND60 27.625 44,648 22.32 75 1.64 23.97 19.17 1.25 22.92
NDS 1
block 16.5 51,664 25.83 57 2.37 28.20 21.22 1.33 22.92
NDS 2 blocks
16.5 54,343 27.17 57 2.37 29.54 21.22 1.39 22.92
Table 30 Moment demand and capacity
Slab a',
inch
P,
Load,
lbs
l', inch MLOAD
kip∙ft.
MSW
kip∙ft.
MTotal,
kip∙ft.
Mcapacity,
kip∙ft.
Ratio MTotal/
Mcapacity
NDS 22.0 53,067 75 48.64 -1.13 47.51 41.30 1.15
ND20 16.5 67,227 57 46.22 -2.20 44.02 41.13 1.07
ND30 16.5 60,976 57 41.92 -2.20 39.72 41.04 0.97
ND40 16.5 71,181 57 48.94 -2.20 46.74 40.94 1.14
ND60 27.625 44,648 75 51.39 -1.52 49.88 41.14 1.21
NDS 1
block 16.5 51,664 57 35.52 -2.20 33.32 41.30 0.81
NDS 2
blocks 16.5 54,343 57 37.36 -2.20 35.16 41.30 0.85
125
Figure 89 Applied load at failure for slab specimens undergoing one-way shear
53,067
67,227
60,976
71,181
44,648
51,66454,343
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
ND S ND 20 ND 30 ND 40 ND 60 ND S 1
Block
outside
ND S 2
Block
outside
Load
(lb
s)
Slab
126
Table 31 Negative moment demand versus negative moment capacity
Slab MSW kip∙ft. Mmax neg. kip∙ft.
NDS -6.47 -15.83
ND20 -9.08 -15.83
ND30 -9.08 -15.83
ND40 -9.08 -15.83
ND60 -6.47 -15.83
NDS 1 block -9.08 -15.83
NDS 2 blocks -9.08 -15.83
Figure 90 Load vs. midspan displacement of slab NDS during one-way shear testing
0
10,000
20,000
30,000
40,000
50,000
60,000
-2 -1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
127
Figure 91 Load vs. midspan displacement of slab ND20 during one-way shear testing
Figure 92 Load vs. midspan displacement of slab ND30 during one-way shear testing
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
-1 -0.8 -0.6 -0.4 -0.2 0
Lo
ad (
lbs)
Midspan displacement (in)
Channel 13 Channel 4 Average
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
-0.8 -0.6 -0.4 -0.2 0
Lo
ad (
lbs)
Midspan displacement (in)
Channel 13 Channel 4 Average
128
Figure 93 Load vs. midspan displacement of slab ND40 during one-way shear testing
Figure 94 Load vs. midspan displacement of slab ND60 during one-way shear testing
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
-0.8 -0.6 -0.4 -0.2 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
-1.5 -1 -0.5 0
Lo
ad (
lbs)
Midspan displacement (in)
Channel 13 Channel 4 Average
129
Figure 95 Load vs. midspan displacement of slab NDS 1 block during one-way shear
testing
Figure 96 Load vs. midspan displacement of slab NDS 2 blocks during one-way shear
testing
0
10,000
20,000
30,000
40,000
50,000
60,000
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Lo
ad (
lbs)
Midpsan displacement (in)Channel 13 Channel 4 Average
0
10,000
20,000
30,000
40,000
50,000
60,000
-0.8 -0.6 -0.4 -0.2 0
Lo
ad (
lbs)
Midspan displacement (in)Channel 13 Channel 4 Average
130
(a)
(b)
(c)
(d)
(e)
(f)
Figure 97 NDS (a) one-way shear set-up (b) one-way shear set-up (c) one-way shear
cracked section left (d) one-way shear cracked section left (e) one-way shear cracked
section right (f) one-way shear cracked section
131
(a)
(b)
(c)
(d)
Figure 98 ND20 (a) one-way shear test set-up (b) one-way shear cracked section left (c)
one-way shear cracked section right (d) one-way shear cracked section underside
132
(a)
(b)
(c)
(d)
Figure 99 ND30 (a) one-way shear test set-up(b) one-way shear test cracked section left
(c) one-way shear cracked section right (d) one-way shear cracked section midspan
133
(a)
(b)
(c)
(d)
Figure 100 ND40 (a) one-way shear test set-up (b) one-way shear test cracked section
left (c) one-way shear test cracked end of slab (d) one-way shear test cracked section
right
134
(a)
(b)
(c)
(d)
(e)
Figure 101 ND60 (a) one-way shear test set-up (b) one-way shear cracked section (c)
one-way shear cracked section left (d) one-way shear cracked section end (e) one-way
shear cracked section right
135
(a)
(b)
(c) (d)
(e)
Figure 102 NDS 1 block (a) one-way shear test set-up (b) one-way shear cracked section
left (c) one-way shear cracked section right (d) one-way shear cracked section end (e)
one way shear cracked section underside
136
(a)
(b)
(d)
(e)
Figure 103 NDS 2 blocks (a) one way-shear test set-up (b) one-way shear cracked
section left (c) one-way shear cracked section right (d) one-way shear cracked section
slab underside
137
Chapter 10
Punching (two-way) shear test set up
Each hollow core concrete slab was tested to failure in punching shear
immediately after the one-way shear tests. The hollow core prestressed concrete slabs
were repositioned within the loading frame utilizing two web sling straps and the
laboratory 2-ton overhead crane. The straps were positioned carefully at each end and the
slab was hoisted in the air and turned 180°. Once again, the usable length of the slab was
measured as the distance from the interior edge of the closest flexural crack to the
unbroken edge of the slab.
10.1 Punching shear test set up
The usable length was the same for all slabs and was approximately 84 inches
except for the NDS 2 blocks which had a usable length of approximately 66 inches. The
punching shear tests were carried out by using a 1 ½ inch thick 6 inch diameter steel
donut with a 1 inch diameter hole. The steel section assembly was placed at the center of
the remaining usable length (either 42 inches or 33 inches) and centered on top of the
steel donut shown in Figure 104. The steel donut was also placed at approximately 18 1/4
inches from the center over a rib (space between two cores) without reinforcement as
shown in Figure 105. The punching shear test set up is shown in Figure 106 and Figure
107. Steel wide flange sections were placed as supports in a box formation around the
tested span to keep the span from failing due to bending as shown in Figure 108. Metal C
clamps and spring clamps were used to keep the steel sections together as shown in
Figure 110 - Figure 116.
138
Figure 104 Steel donut used in punching shear tests
Figure 105 Punching (two-way) shear set-up
139
Figure 106 Punching shear test, overhead view
Figure 107 Punching (two-way) shear test set-up
140
Figure 108 Punching shear test box formation supports
141
Chapter 11
Punching (two-way) shear results
11.1 Punching shear results
The maximum load to failure in the punching shear tests is shown in Figure 109
and Table 32. Images for these tests are shown in Figure 110 - Figure 116. The punching
load to failure results show an interesting pattern where the punching shear strength of
the slabs is highest for the ND20 and ND30 test specimens. Test specimen NDS, which is
the standard concrete mixture, has a lower punching shear strength capacity than
mixtures ND20 and ND30. Other than the data point for the NDS test specimen the
punching shear strength seems to decrease with an increase in natural aggregate
replacement past 30% (ND30). Furthermore, the data in Figure 109 shows the punching
shear strength of the NDS 1 block and NDS 2 block slabs also had relatively low
capacities as compared to the ND20 and ND30 specimens. The punching (two-way)
shear capacity of NDS 1 block is significantly lower than all the other test specimens.
This is likely due to the uneven slab surface, as mentioned before. In conclusion, the
punching (two-way) shear capacity of these test specimens varies and does not show a
clear trend. Finally, although not utilized in this study, a quick review of the literature
indicates that the other studies that undertook similar problem statements found that for
hollow slabs the design codes(such as the German DIN 1045) do not accurately estimate
the punching shear capacity of the section. However, showed that utilizing a effective
area of the concrete section versus the gross area might provide better estimations of the
punching shear capacity a study by (Schnellenbach-Held and Pfeffer, 2002). A second
142
study by Sagadevan and Rao also found that existing design codes(specifically ACI 318
2014, EN 1992-1-1 and IS 456 200) overestimated the punching shear capacity of hollow
concrete slabs. The study arrived at better results when modifying the design codes by
using effective perimeter and net area(Sagadevan and Rao, 2019).
Figure 109 Failure load of two-way (punching) shear
Table 32 Maximum load to failure, two-way (punching) shear test
Slab Punching Shear (lbs)
NDS 21,303
ND20 23,600
ND30 24,365
ND40 21,686
ND60 17,859
NDS 1 block 17,094
NDS 2 blocks 21,814
21,303
23,600 24,365
21,686
17,859 17,094
21,814
0
5,000
10,000
15,000
20,000
25,000
30,000
ND S ND 20 ND 30 ND 40 ND 60 ND S 1
Block
outside
ND S 2
Block
outside
Load
(lb
s)
Slab
143
(a)
(b)
(c)
(d)
(e)
Figure 110 NDS (a) punching(two-way) set-up (b) punching(two-way) set-up(c) punching
(two-way) shear cracked section, South side (d) punching(two-way) cracked section,
North side (e) punching(two-way) cracked section, South side
144
(a)
(b)
(c)
Figure 111 ND20 (a) punching (two-way) shear test set-up (b) punching (two-way) shear
cracked section (c) punching (two-way) shear cracked section
145
(a)
(b)
(c)
Figure 112 ND30 (a) punching (two-way) shear set-up (b) punching (two-way) shear
cracked section (c) punching (two-way) shear cracked section
(a)
(b)
(c)
Figure 113 ND40 (a) punching (two-way) shear test set-up (b) punching (two-way) shear
cracked section (c) punching (two-way) shear cracked section
146
(a)
(b)
(c)
Figure 114 ND60 (a) punching (two-way) shear test set-up (b) punching (two-way)
cracked section (c) punching (two-way) cracked section end
(a)
(b)
(c)
Figure 115 NDS 1 block (a) punching (two-way) shear test set-up (b) punching (two-way)
shear cracked section (c) punching (two-way) shear cracked section end
147
(a)
(b)
Figure 116 NDS 2 blocks (a) punching (two-way) shear test set-up (b) punching (two-
way) shear cracked section
148
Chapter 12
Conclusions and future work
The work summarized in this document presents a strong narrative of the efficacy
of RCA as a replacement for natural aggregate in precast prestressed concrete specimens.
Specimens containing 20%, 30% and 40% replacement of natural aggregate by RCA
were found to perform, at times, better than the standard mixture, NDS. Hollow core
concrete slabs ND20, ND30 and ND40 with 20%, 30% and 40% replacement of natural
aggregate by RCA, respectively were found to outperform the NDS slabs in flexural
bending capacity, one-way shear and punching shear. RCA was shown to increase elastic
deflection, long-term deflection, inelastic deflection and total deflection. ND40 had the
largest deflections and ND60 performed much like NDS.
Furthermore, the use of RCA in precast prestressed concrete specimens is also
possible due to the manufacturers ability to recycle their own waste concrete from
previous manufacturing sessions. This internal recycling and reusing of material is
beneficial and efficient because these concrete floor panels are manufactured at plants
that also produce waste concrete; therefore, this project aims to implement the use of
RCA in a controlled environment where the manufacturer is able to certify the properties
of the concrete in order to ensure quality control.
12.1 Discussion of results
12.1.1 Deflections
The total deflection of the slabs was shown to increase with an increase in RCA
content with ND40 having the highest total deflection. NDS 1 block had a substantially
149
larger total deflection when compared to the other slabs with a single load block; this was
likely due to the uneven and unfinished condition of the slab. The elastic deflection
followed a similar pattern with an increase in elastic deflection with RCA content. ACI
318-14 code provided good estimates for elastic deflection.
Amongst the two slabs cured outside, NDS 1 block had greater total and elastic
deflection when compared to the other slabs with one load block. Secondly, NDS 2
blocks had the largest elastic and total deflection as was expected of a slab with double
load.
12.1.2 Bending strength
The results of the flexural bending tests do not show a clear relationship between
RCA content and bending strength. The slabs with the highest flexural bending capacity
were ND20 and ND40 as well as NDS 2 blocks. All slabs exceeded code moment
capacity except for NDS 1 block.
12.1.3 One-way shear strength
The one-way shear strength test results show that all specimens failed in moment
except for ND40. Measured shear strengths were compared to code capacity, Stresscore,
Inc. software program and ACI 318-14; all slabs exceeded code capacity.
12.1.4 Punching (two-way) shear strength
The results of the punching shear tests are presented herein. The results of the
punching shear tests were greatly affected by the quality and curing conditions of the test
specimens. For example, test specimen NDS 1 block had many surface imperfections and
150
seemed to be ‘patched’. These imperfections are likely due to the inherently dry concrete
mixture used and the likelihood that this specimen was manufactured at the beginning of
the slipformer’s run.
12.2 Suggestions for future work
The University of Texas Tyler and The University of Notre Dame research
partnership has studied the properties of RCA, the effect of RCA on reinforced concrete
specimens (compressive strength, tensile strength, cracking mechanisms) and the effect
of RCA on precast prestressed members (service load deflections, stiffness, and other
mechanical properties). Further work should focus on other mechanical properties of
RCA as well as on-site and in-service performance of RCA specifically in precast
prestressed members. Some suggestions for further work include:
(1) in service performance of hollow core concrete slabs by means of constructing a scale
model within a laboratory space or tracking the performance of specimens used in
exterior projects
(2) investigations on the microstructural properties of RCA aggregates and concrete
mixtures to further explain and understand the performance of RCA mixtures
(3) use of Digital Imaging Correlation software to track the progression of sagging and
deflection in as-built structures and models as mentioned in (1) to assist in calculating
strains and further monitoring the effects of RCA
(4) further investigations on the punching (two-way) shear capacity of the slabs as well as
finite element modeling of the contact effective area for calculation of theoretical, or code
capacity
151
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