Bond Strength of Normal-to-Lightweight

7/24/2019 Bond Strength of Normal-to-Lightweight

1/8

Bond Strength of Normal-to-Lightweight Concrete Interfaces

Hugo COSTAResearch AssistantPolytechnic Institute of CoimbraCoimbra, [email protected]

Hugo Costa, born 1977, civilengineering degree (2002), MSc degree(2008) at Univ. of Coimbra. PhDstudent at Univ. of Coimbra, since2008. His research area is related tomixture design and structural behaviourof lightweight aggregate concrete.

Pedro SANTOSAdjunct ProfessorPolytechnic Institute of LeiriaLeiria, Portugal

[email protected]

Pedro Santos, born 1977, civilengineering degree (2001), MScdegree (2005), PhD degree (2009) atUniversity of Coimbra. His researchinterests include pre-casting andstrengthening and repair of RCstructures.

Eduardo JLIOFull ProfessorICIST & DECivil, IST-UTLLisbon, [email protected]

E. Jlio is a full professor at theInstituto Superior Tcnico of theTechnical University of Lisbon. Hisresearch interests are in the field ofStructural Concrete. He is memberof IABSE WC 3,fibCommission 5and ACI 364 Committee.

SummaryStructural lightweight aggregate concrete (LWAC) can be used with advantage in new structures,especially in pre-cast components, as well as in the rehabilitation of existing structures. In bothsituations, the result is a composite structural element, with parts in LWAC and others in normalweight concrete (NWC). Besides material properties, it is also important to characterize thebehaviour of LWAC-to-NWC interfaces. With this aim, an experimental study was conducted tocharacterize the shear strength and the tensile strength of NWC-to-NWC, LWAC-to-NWC andLWAC-to-LWAC interfaces and to compare results with codes predictions. A NWC and threedifferent LWAC were considered. Six different types of surface roughness were produced on theconcrete substrate and characterized with roughness parameters assessed using the 2D-LRA method,developed by the authors. Slant shear and splitting tests were adopted to quantify the bond strengthof the interface in shear and in tension, respectively.

Keywords:lightweight aggregate concrete (LWAC); interface; bond strength; roughnessparameters; codes.

1. IntroductionStructural LWAC presents low density and high performance in terms of both strength anddurability. Therefore, this is a competitive material not only for new structures, whenever reductionin dead-weight is aimed, e.g. for the pre-casting industry, but also for the rehabilitation of existingstructures. All these situations can give rise to composite structural members, consisting of parts inLWAC and parts in NWC, casted at different ages. In these cases, it is important to assess not onlythe material properties but also the behaviour of the LWAC-to-NWC interface.

The design expressions proposed by codes for concrete structures to predict the shear strength ofconcrete-to-concrete interfaces are all based on the shear-friction theory [1] and depend mainly onfour parameters: (i) compressive strength of the weakest concrete; (ii) normal stress at the interface;(iii) shear reinforcement crossing the interface; and (iv) roughness of the substrate surface. Santos[2] characterized the influence of other parameters on the behaviour of NWC-to-NWC interfaces,namely the differential shrinkage and Youngs modulus, that are not considered by current designcodes, particularly by Eurocode 2 (EC2) [3]. Differential shrinkage may not be a relevant parameterin LWAC-to-NWC interfaces (in the case LWAC is the added layer), due to improved internalcuring provided by the moisture in the interior of the lightweight aggregates (LWA) leading toreduced shrinkage [4]. However, since the Youngs modulus of LWAC can be quite different fromthat of NWC [5], depending on the strength and on the density of the adopted LWAC, differentialstiffness may influence significantly the behaviour of the interface. The strength of the binding

matrix is usually higher than the respective LWAC strength, due to the reduced strength of LWA.Nevertheless, this is also ignored by codes to predict the interface strength.


2/8

In order to characterize the shear and tensile strength of NWC-to-NWC, NWC-to-LWAC andLWAC-to-LWAC interfaces, an experimental study was conducted to evaluate the influence ofsubstrate roughness, differential stiffness and LWAC strengths of both LWA and binding matrix.First, the following concrete mixtures were designed, produced and characterized: a single NWCmixture, with a compressive strength of 50 MPa; and three LWAC mixtures, with densities andcompressive strengths ranging between 1.5 and 1.9 and 45 and 75 MPa, respectively. Then, six

different texture conditions were considered for the substrate surface: (i) smooth surface (SS);(ii) surface treated with steel wire-brush (WB); (iii) surface left free after vibration (SF); (iv)surface treated by shot-blasting (SB); (v) surface prepared by hand-scrubbing/raking (HS); (vi)surface treated by chemical deactivation (CD). These were all characterized using roughnessparameters assessed with the 2D - laser roughness analyser (2D-LRA) method, developed by theauthors [1]. Slant shear and splitting tests were adopted to quantify the bond strength of theinterface, in shear and in tension, respectively.

2. Concrete mixture design and productionThe design method used for NWC and LWAC mixtures is based on the methodology initiallyproposed by Lourenco et al. [6] and further developed by Costa et al.[7], which includes the

following steps: (i) prediction of the binding paste strength, through the Ferets expression;(ii) adjustment of the mixtures curve to the Faurys reference curve and definition of the wanteddensity; (iii) computation of the LWAC strength, considering the strength reduction taking intoaccount the types and dosages of lightweight aggregates.

2.1 Materials

In the study herein described, the following constituents were adopted for the binding paste: cementCEM II-A/L 42.5 R; fly ash addition (in the LWAC mixtures); third generation superplasticizer,polycarboxylates based; water.

The following types of aggregates were used to produce the LWAC used in the study: (i) three typesof coarse lightweight expanded clay aggregates (Leca) - structural Leca 2/4 mm (HD2/4), structuralLeca 4/12 mm (HD4/12) and structural Leca 4/10 mm (MD); (ii) two types of siliceous fine

aggregates - fine sand 0/2 mm (FS) and medium sand 0/4 mm (MS); and (iii) one type of fine Lecasand 0.5/3 mm (XS). In relation to the NWC mixture, besides FS and MS, crushed limestone 6/12mm (CL) and gravel 4/8 mm (Gr) were also used.

Granulometric analysis was performedfor all aggregates and the followingproperties were characterized for Lecaaggregates (Table 1): dry particle density,P0; dry bulk density, 0; interiormoisture, HP; absorption from naturalstate, AN; total absorption until saturatedstate, AS; and crushing strength, fCr.

2.2 Produced concrete

Different binding pastes were defined for the NWC and for the LWAC (Table 2), with differentdosages of cement (C), fly ash (FA) and admixture. Thus, these binding paste matrices resulted indifferent values of the concrete compactness and water / binder (W/B) mass ratio. Consequently, theNWC and the LWAC mixtures have different predicted values for the strength of the binding pastematrix, fbp,p, 50 and 90 MPa, respectively.

In the LWAC mixtures, the granulometric adjustment was performed and combined with theselection and pre-blending of fine and of coarse aggregates, aiming to achieve the desired

densities, , and compressive strengths, fc,p, between 45 and 76 MPa.

Table 1: Characterized properties of Leca aggregates.

Type ofLWA

P0(kg/dm3)

0(kg/dm3)

HP(%)

AS(%)

AN(%)

fCr(MPa)

HD4/12 1,37 0,76 3,0 9,5 5,6 11,7

HD2/4 1,34 0,74 9,5 11,9 1,5 11,0

MD 0,89 0,49 3,1 11,0 7,5 4,0

XS 1,02 0,58 4,2 10,2 5,1 5,3


3/8

Table 2: Concrete references, aggregates arrangement and mixtures parameters.

Concretereference

Fineaggregate

Coarseaggregate

C(kg/m3)

FA(kg/m3)

W/B fbp,p(MPa)

fc,p(MPa)

(kg/m3)

N2.4 FS+MS CL+Gr 340 0 0,50 50 50 2350H1.9 FS+MS HD4/12+HD2/4 420 84 0,27 90 76 1900H1.7 FS+XS HD4/12+HD2/4 420 84 0,27 90 67 1700H1.5 FS+XS MD+HD2/4 420 84 0,27 90 45 1500

Typical failure surfaces of the adopted concrete are presented in Fig. 1, being possible to identifythe type and distribution of the aggregates in the concrete matrix.

a) N2.4 b) H1.9 c) H1.7 d) H1.5

Fig. 1: Failure surface of the produced concrete.

2.3 Mechanical properties

For all concrete mixtures, the average value of compressive strength (fcm) was characterized incubic specimens of 150 mm edge [8], at 7, 28 and 90 days of age. The results are presented in Fig. 2,together with the corresponding hardening curve proposed by EC2 [3]. The following propertieswere also characterized at 28 days of age (Table 3): i) average tensile strength (fctm), assessed withsplitting tests using standard cylindrical specimens [8]; ii) average Youngs modulus [9] (Ecm),determined on prismatic specimens of 150150600 mm3. For each situation, three specimens were

tested.

Table 3: Measured values, at 28 days, of

tensile strength and of Youngs modulus.

Concretereference

fctm(MPa)

Ecm(GPa)

N2.4 3,89 33,4

H1.9 4,82 27,1

H1.7 3,70 20,8010

20

30

40

50

60

70

80

90

0 7 14 2 1 28 3 5 42 4 9 56 6 3 70 7 7 84 9 1

N2.4

H1.9

H1.7

H1.5

EC2

Age (days)

fcm

(MPa)

H1.5 2,55 14,6Fig. 2: Evolution of fcmwith age.

3. Interface strength

3.1 Surface roughness

The bond strength of the interface was determined in shear, with slant shear tests, and in tension,with splitting tests, at 28 days of age of the added layer. For this purpose, two types of concretesubstrate, H1.5 and N2.4, were used. On the specimens with concrete substrate N2.4, four types ofconcrete (N2.4, H1.9, H1.7 and H1.5) were added and five different roughness conditions of thesubstrate surface (Fig. 3) were adopted: SS, WB, SB, HS and CD. For specimens produced with

concrete substrate H1.5, two types of concrete were added (H1.7 and H1.9) and three differentroughness conditions of the substrate surface (Fig. 3) were considered: SS, SF and CD. For eachsituation, three specimens were tested. These were cured in water at 20 C.


4/8

The EC2 [3] proposes a qualitative assessment of the roughness of the substrate surface, only basedon a visual inspection. This leads to a classification in four classes: (1) very smooth, (2) smooth,(3) rough and (4) indented. This qualitative approach is subjective since it depends on theinspectors judgment. Santos et al. [10] proposed a quantitative classification, and therefore morerigorous, of the roughness of the substrate, using roughness parameters, namely [10]: maximumheight of peak (Rp) average height of peak (Rpm) maximum depth of the valley (Rv) average depth

of the valley (Rvm). These parameters can be assessed using a specific method and equipment, the2D- LRA, developed by Santos and Jlio [1].

a) N2.4-SS. b) N2.4-WB. c) N2.4-SB. d) N2.4-HS.

e) N2.4-CD. f) H1.5-SS. g) H1.5-SF. h) H1.5-CD.

Fig. 3: NWC and LWAC substrate surface roughness.

The 2D-LRA method gets the texture profile of the surface and computes the correspondingroughness parameters. These values were obtained for each surface condition considered in thestudy as the average of ten readings (Table 4).

3.2 Interface shear strength

The expression defined by EC2 [3] to determine the design value of the longitudinal shear strengthof the interface between concrete layers cast at different ages, Rdi, is given by:

( )cdydnctdRdi

ffcf 5,0cossin +++= (1)

being c and the cohesion and friction coefficients, respectively. According to EC2, the surfaces ofFig. 3, as well as the corresponding values of c and , are shown in Table 5.

Table 4: Roughness parameters of the considered surface conditions.

ParameterN2.4H1.5(SS)

N2.4

(WB)

N2.4

(SB)

N2.4

(CD)

N2.4

(HS)

H1.5

(SF)

H1.5

(CD)Rp(mm) 0,57 1,59 2,15 5,11 8,22 3,36 4,37Rpm(mm) 0,23 0,86 1,38 4,21 4,29 1,97 3,56Rv(mm) 2,25 2,56 4,00 4,90 9,54 2,31 3,71

Rvm(mm) 0,63 1,24 2,31 3,56 6,88 1,36 2,83


5/8

The interface shear strength was experimentally assessed with slant shear tests [11], using prismaticspecimens of 150150450 mm3with the interface at 30 degrees with the vertical (Fig. 4a). Theadopted test rate was 5,0 kN/s.

Table 5: Roughness classification and coefficients, of EC2, for the used surfaces.

Substratesurface

N2.4

H1.5(SS)

N2.4

(WB)

N2.4

(SB)

N2.4

(CD)

N2.4

(HS)

H1.5

(SF)

H1.5

(CD)EC2

ClassificationVery

smoothSmooth Rough Rough Rough Smooth Rough

c(EC2) 0,25 0,35 0,45 0,45 0,45 0,35 0,45(EC2) 0,5 0,6 0,7 0,7 0,7 0,6 0,7

Two types of failure were observed: (1) cohesive (monolithic failure of the weakest concrete),Fig. 4.b; and (2) adhesive (debonding of the interface), Fig 4.c. The first one occurred mostly inspecimens with higher interface roughness (HS, CD, SB) and the second took place mainly inspecimens with lower roughness of the interface (SS, WB, SF).

a) Test specimen. b) Cohesive failure. c) Adhesive failure.

Fig. 4. Slant shear test.

The obtained average values of the shear strength are presented in Fig. 5, for each substrateconcrete, each roughness of the interface and each added concrete.

0

5

10

15

20

25

SS WB SB HS CD

N2.4

H1.9

H1.7

H1.5

Interface roughness

Shearstrength(MPa)

0

5

10

15

20

25

SS SF CD

H1.9

H1.7

Interface roughness

Shearstrength(MPa)

a) Substrate N2.4 b) Substrate H1.5

Fig. 5: Average values of shear strength of the interface.

It was observed that the shear strength increases with the interface roughness. For N2.4 substrate,the use of LWAC with higher strength of the binding paste matrix results, generally, in higherinterface strength when compared to NWC. It was also observed that reducing the density of addedLWAC, and hence its strength and Youngs modulus, the shear strength of the interface alsodecreases.

It was also analyzed the correlation between the roughness parameters and the shear strength of theinterface. In Fig. 6, the correlation with Rvmis presented. This adjusts well to a power function,


6/8

although the amplitude depends on the concrete strength. Thus, the coefficients of cohesion andfriction, c and , can also be estimated using this function [2, 9].

0

5

10

15

20

25

0 2 4 6 8

N2.4

H1.9

H1.7

H1.5

Rvm (mm)

Shearstrength

(MPa)

0

5

10

15

20

25

0 2 4 6 8

H1.9

H1.7Shearstrength

(MPa)

Rvm (mm)

a) Substrate N2.4 b) Substrate H1.5

Fig.6: Comparison between the interface shear strength and the parameter Rvm.

When comparing the values of the interface shearstrength, experimentally obtained, with thecorresponding EC2 prediction, Ri, according to(1), for nominal values and considering thecoefficients of Table 5, it can be observed thatthis approach leads to too conservative values. Infact, experimental results are approximatelytwice the predicted values (Fig. 7).

3.3 Interface tensile strength

The tensile strength of the interface, between concretes cast at different ages, was characterizedwith splitting tests [7], using cylindrical specimens of 150 mm of diameter and 300 mm of height(Fig. 8.a), tested at a rate of 2,0 kN/s. The occurrence of two distinct failure modes was observed, asmentioned before: cohesive (Fig. 8.b) and adhesive (Fig. 8.c).

The cohesive failure was observed on specimens with N2.4 concrete substrate, with high surfaceroughness and with H1.5 added concrete. This also occurred on specimens with H1.5 concrete

substrate, with high surface roughness and with H1.9 added concrete. Interface failure was observedin the remaining situations.

a) Test specimen. b) Cohesive failure. c) Adhesive failure.Fig. 8: Splitting test.

0,0

5,0

10,0

15,0

20,0

25,0

0,0 2,5 5,0 7,5 10,0 12,5

SS

WB

SB

SF

HS

CD

Ri(MPa)

Shearstrength(MPa)

Fig.7: Comparison between the shear

strength and the EC2 prediction, Ri


7/8

In Fig. 9, the average values of tensile strength of the interface, for both types of concrete substrate,are presented, for each roughness condition of the interface surface and for each type of addedconcrete.

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

SS WB SB HS CD

N2.4

H1.9

H1.7

H1.5

Interface roughness

Tensilestrength(M

Pa)

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

SS SF CD

H1.9

H1.7

Interface roughness

Tensilestrength(MP

a)

a) Substrate N2.4. b) Substrate H1.5.Fig.9: Average values of the tensile strength of the interface.

Results revealed an increase of the tensile strength with the interface roughness. On specimens withN2.4 substrate, the use of LWAC with higher strength of the binding paste matrix results on highertensile strength of the interface, when compared to NWC, similarly to what was observed with theshear strength. Results also show that a density reduction of the added LWAC leads, in general, to areduction of the tensile strength of the interface.

Comparing the roughness parameters with the tensile strength of the interface, it can be concludedthat Rpmpresents a better correlation. This can again be adjusted to a power function (Fig. 10),although the amplitude also depends on the concrete strength.

0

1

2

3

4

5

0 2 4 6

N2.4

H1.9

H1.7

H1.5

Rpm (mm)

Tensilestrength(MPa)

0

1

2

3

4

5

0 2 4 6

H1.9

H1.7Tensilestrength(MPa)

Rpm (mm)

a) Substrate N2.4. b) Substrate H1.5.

Fig.10: Comparison between the interface tensile strength and the parameter Rpm.

4. ConclusionsBoth the slant shear and the splitting tests proved to be adequate to evaluate the strength of NWC-to-LWAC interfaces, respectively in shear and in tension.

Two failure modes were observed, cohesive (at the weakest concrete) and adhesive (debonding ofthe interface). For slant shear tests, the failure mode was mainly monolithic, for very rough surfaces(HS and CD), and mainly adhesive, for very smooth and smooth surfaces (SS and WB). For roughsurfaces, SF and SB, both failure modes were observed. For splitting tests, the failure mode was

mainly adhesive, excepting for the specimens with a very rough substrate and with significantdifferences in both density and Youngs modulus between the substrate and the added concretelayers.


8/8

Bond Strength of Normal-to-Lightweight

Documents

Transcript of Bond Strength of Normal-to-Lightweight