Experimental studies on Shear behaviour of reinforced … · concrete (RGPC) flexural members...
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INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING
Volume 3, No 1, 2012
© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0
Research article ISSN 0976 – 4399
Received on March, 2012 Published on August 2012 128
Experimental studies on Shear behaviour of reinforced
Geopolymer concrete thin webbed T-beams with and without fibres Ambily P.S
1, Madheswaran C.K
2, Lakhsmanan.N
3, Dattatreya J.K
4, Jaffer Sathik S.A.
5
1- Scientist, CSIR-Structural Engineering Research Centre, Taramani, Chennai, India
2- Principal Scientist, CSIR-Structural Engineering Research Centre, Taramani, Chennai,
India
3- Project advisor (Retired), CSIR-Structural Engineering Research Centre, Taramani,
Chennai, India
4- Senior Principal Scientist (Retired), CSIR-Structural Engineering Research Centre,
Taramani, Chennai, India
5- Project Assistant, CSIR-Structural Engineering Research Centre, Chennai, India
[email protected],[email protected]
doi:10.6088/ijcser.201203013012
ABSTRACT
This paper describes the experimental studies on shear behaviour of reinforced Geopolymer
Concrete (RGPC) thin webbed Tee beams. Since T-beams are susceptible to shear, thin
webbed T-beams are taken up for the current study. Shear failures are very sudden and
unexpected, a thorough knowledge of the different modes of shear failures and the
mechanisms involved is necessary to prevent them. Not many investigations were reported on
the shear behaviour of RGPC. In the present study shear reinforcement spacing (0, 120, 180
& 240mm) was the basic test parameters for the beam specimens. Steel fibres were used for
one set of beams and the same was compared for beams without fibres. After a series of trial
mixes on geopolymer concrete, the volume of steel fibres was fixed as 0.75. All the beams
were provided with the same flexural and shear reinforcement and the beams were tested
under two point loading with shear span to depth ratio of 1.9. This paper presents the details
of the mix proportion of geopolymer concrete (GPC) mixes, preparation of RGPC beams,
testing and evaluation of structural behavior with respect to cracking, service load,
deflections at various stages and failure modes. Investigations on the shear behavior of the
reinforced concrete beams showed that the failure mechanism can be transformed from brittle
to ductile mode by addition of steel fibres.
Keywords: Geopolymer concrete, tee beams, shear behaviour , load deflection
characteristics
1. Introduction
The cement industry has been making significant progress in reducing CO2 emissions through
improvements in process technology and enhancements in process efficiency, but further
improvements are limited because CO2 production is inherent to the basic process of
calcinations of limestone. So it is essential to find a substitute material for cement which can
be eco-friendly. In 1978, Joseph Davidovitis (1) developed inorganic polymeric materials and
coined the term “Geopolymer” for it. It was discovered that various calcined clays could be
activated with alkaline solutions to produce hardened ceramic like products at room
temperature. Geopolymer is used as the binder to completely replace the ordinary Portland
cement in producing Geoploymer concrete (GPC). Geopolymer has the potential to replace
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
129
Ordinary Portland Cement Concrete (OPCC) and produce fly ash based Geopolymer Cement
Concrete (GPCC) with excellent physical properties, mechanical properties, fire resistance
and acid resistance.
Chang studied shear and bond behavior of reinforced fly ash based geopolymer concrete
beams. Shear strength calculations of geopolymer concrete beams were performed using
current Australian code provisions and analytical models available for Portland cement
concrete members. Dattatreya J.K has carried out experimental investigations on flexural
behavior of geopolymer concrete beams and concluded that the conventional RC theory
could be used for reinforced GPCC flexural beams for the computation of moment capacity,
deflection, and crack width within reasonable limits.
Geopolymer is a new construction material and the behavior of Reinforced Geopolymer
concrete (RGPC) flexural members critical in shear is of great interest for the adoption of this
new material in practice. The behaviour of reinforced GPC beam specimens critical in shear
is discussed in detail in this paper. Therefore investigations were taken up on flexural
members with shear span to depth ratio (a/d ratio-1.9), with shear reinforcement spacing of
120mm, 180mm, 240mm and without shear reinforcement in the web section were the test
parameters for beam specimens. The behaviour of the section at various stages of loading is
studied from the initial uncracked phase up to the ultimate condition at shear collapse.
Analysis of the experimental data reveals that the RGPC beams have much better load
deflection characteristics, cracking load, service load and ultimate load. The shear tension to
shear compression characterized the improved ductility. There was no failure of fibres by pull
out. The results demonstrate that the RGPC beams with fibres are well suited for resisting
shear.
2. Research Significance
One of the potential areas of application of GPCs, which provides significant value addition
to the material and helps to realize the concept of green habitat, is their utility in structural
concrete. However, the suitability of RGPC to various structural components is to be
established by large number of experimental studies. Rangan (4) et al have investigated this
aspect using fly ash (FA) based heat treated GPCs. The CSIR-Structural Engineering
Research Centre (CSIR-SERC), Chennai has developed structural grade GPCs (5,6) and
investigated its suitability for Reinforced Geopolymer Concrete (RGPC) beams critical in
flexure (7) for the first time in the country. In continuation of these studies, the shear
behaviour of RGPC was considered for investigation in the present study. Adequate shear
resistance in structural concrete members is essential to prevent shear failures which are
brittle in nature. One of the critical parameters influencing the shear capacity of beams is
shear span to depth ratio (a/d). Experimental studies were carried out on the shear behavior
RGPC beams with a shear span to depth ratio of 1.9. This paper considers RGPC beams with
different binder compositions and compressive strengths ranging from 30 to 45 MPa and
produced by ambient temperature curing. The volume of steel fibre used is 0.75%. The
comparison of shear behaviour of RGPC thin webbed T-beams with and without steel fibres
was carried out. Performance aspects such as load carrying capacity, moments, deflections,
and strains at different stages were studied. The failure modes were also recorded for all the
beams.
3. Experimental investigations
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
130
3.1 Mix details
Fly ash and Ground Granulated Blast Furnace Slag (GGBS) were used as the main binder
system in this study. Fine aggregates, coarse aggregates and AAS formed the rest of the
material system. The GPC was obtained by mixing calculated quantities of FA and GGBS,
fine aggregate, coarse aggregate with optimized Alkaline Activator solution (AAS). FA
conforming to grade 1 of IS 3812-1981(8) and GGBS conforming to IS 12089: 1987(9) were
used. A high volume FA based GPC mix with 80% fly ash and 20% GGBS and liquid binder
ratio of 0.6 were employed for all the beams. Potassium hydroxide and potassium silicate
solution was used as the alkali activator system. River sand available in Chennai was used as
fine aggregate. In this investigation locally available blue granite crushed stone aggregates of
maximum size 20mm and 12mm were used. The characterization tests of fine and coarse
aggregates were carried out as per IS 2386(part1, part2, part3) –1963(10). The mix
proportions for GPC presented in Table 1.
Table 1 GPC concrete mixes
Mix Id. GPS Composition Mix Proportions K2O (%)
PP0070 80% FA, 20% GGBS 1:1.31:1.44:0.61 25.7
PP1270 80% FA, 20% GGBS 1:1.31:1.44:0.6 25.6
PP1870 80% FA, 20% GGBS 1:1.31:1.44:0.6 25.6
PP2470 80% FA, 20% GGBS 1:1.31:1.44:0.61 25.9
FDPP0070 80% FA, 20%GGBS,
STEEL FIBRE 0.75% 1:1.31:1.44:0.61 25.7
FDPP1270 80% FA, 20% GGBS
STEEL FIBRE 0.75% 1:1.31:1.44:0.6 25.6
FDPP1870 80% FA, 20% GGBS
STEEL FIBRE 0.75% 1:1.31:1.44:0.6 25.6
FDPP2470 80% FA, 20% GGBS
STEEL FIBRE 0.75% 1:1.31:1.44:0.61 25.9
3.2 Specimen details
3.2.1 Beam Geometry
The test specimens are designed as per the provisions of IS 456-2000(9). T-beams with cross
section having flange of 270 mm x 75 mm, web of 75mm x 300mm and length of 2200 mm,
were cast. The effective span of the beam is 1850mm. The a/d ratio for the beams was fixed
as 1.9. The beams were reinforced with two 25mm diameter rods bundled at the bottom and
one 25mm diameter provided at the top of the beam were used as tensile bars and hangar bars
respectively. The 8mm diameter transverse reinforcement was provided in the beam at
120mm, 180mm, 240mm spacing throughout the span. No transverse reinforcements were
provided for beams without web reinforcement. The beams were designed to fail in shear.
The volume of steel fibre is 0.75% added to the concrete mix. The clear cover to the
reinforcement is 43mm. The geometry of the beam specimen is shown in figure 1.
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
131
Elevation
Cross section
Figure 1: Geometry of a typical beam specimen (All dimensions are in mm)
The reinforcement bars fastened with electrical strain gauges at the mid span of the
longitudinal bar and stirrups are shown in figure 2.
Figure.2 Reinforcement bars fastened with electrical strain gauges
3.3 Preparation of test specimens and curing
The coarse aggregate and sand in saturated surface dry condition were mixed with the binder
(FA and GGBS) in a 300 kg capacity tilting drum mixer for about one minute. At the end of
the dry mixing, the alkaline activator solution (AAS) was added. Then mixing was continued
for another four to five minutes till a uniform consistency was achieved. Immediately after
mixing, the fresh concrete was cast into the moulds. Prior to casting, the inner walls of
moulds were coated with lubricating oil to prevent adhesion with the concrete specimens. The
concrete was placed in the moulds in three layers of equal thickness and each layer was
vibrated until the concrete was thoroughly compacted by the needle vibrator. With each batch,
100x100x100mm cubes and prisms of size 100x100x500mm were cast. The slump and fresh
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
132
density of every batch of fresh concrete was also measured in order to observe the
consistency of the mixes. The slump values were in the range of 225-250mm and the density
of the mixes was 2390-2420 kg/m3 respectively. The specimens were demoulded after one
day and were air cured under ambient conditions in the laboratory until the test age.
3.4 Test procedure
All the specimens were white washed in order to facilitate marking of cracks. The test setup
is shown in Figure 3. Testing was carried out on a loading frame of 50 tons capacity. Before
resting the beam on reaction blocks, the beam was centered by using a plumb bob so that its
centre lies exactly under the centre of the loading head. The beam was simply supported over
a span of 1850 mm, which is considered as the effective span. The beam was supported on
the reaction blocks by a hinged plate at one end and roller plate at the other end. The beams
were tested under two point static loading. The load was applied on two points, at a distance
of 700 mm for a /d ratio 1.9, at centre to center of the load spreader.
Figure 3: Experimental setup of beam
Five dial gauges of 0.01 mm least count were used for measuring deflections, two for
measuring deflections under the load points, two for measuring deflections at center of shear
span and one in the mid span for measuring central deflection. The behaviour of the beam
was observed carefully and the crack widths were measured using a hand held microscope.
All the measurements including deflections, strain values and crack widths were recorded at
regular intervals of load until the beam failed. The failure mode of the beams was also
recorded.
4. Experimental results
FDPP series denotes the beams with fibre. The beams with fibres showed significant
variation in compressive strength, strain and moment curvature relationship in comparison
with beams without fibres. The experimental data and the detailed comparison of the
moment-curvature relations, the strain variation between them, their compressive strength
and their behaviour are discussed in detail.
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
133
To overcome the problem of workability in the mix containing fibres, a chemical admixture
was added to the mix. The dosage of the admixture was restricted to 0.5% of the binder. The
workability was thereby improved using the admixture. The first crack appeared only after 60
KN in case of the fibre beams in comparison to the 40 KN in case of beams without fibres.
The various crack patterns of the beams with and without fibres are shown in Figure 10 and
11.
4.1 Deflection at Various Load Stages
The deflection of the beams under various loads such as cracking loads, service loads
and ultimate loads have been summarized in the Table 2.
Table.2 Deflection of beams at service and ultimate loads
First crack load, PCR
(kN)
Specimen
ID
Compressive
strength (MPa) Pfl Psh
Service load,
PSL (kN)
Ultimate load,
PUL(kN)
PP0070 44.59 40 40 155 233
PP1270 36.94 40 40 158 316
PP1870 33.73 40 60 162 305
PP2470 30.39 40 40 113 170
FDPP0070 36.30 60 60 143 215
FDPP1270 38.80 63 80 167 250
FDPP1870 31.20 60 60 183 275
FDPP2470 30.30 60 60 150 225
The deflection at failure ranges from 10 to 15 mm for reinforced GPC without fibre while the
corresponding deflection for GPC beams with fibre is 15 to 20 mm. It is seen from Table 2
that the cracking load increases due to the incorporation of fibres while the service loads are
marginally different for RGPC beams with and without fibre. The incorporation of steel
fibres improves the ductility and energy absorption characteristics of geopolymer concretes.
The fibre reinforced beams failed in shear compression mode by crushing at the web flange
junction or in the top flange while beams without fibre failed by shear tension and
longitudinal splitting due to inadequate bond. This shows the improvement in bond strength
due to the incorporation of fibres. The failures modes of T beam with and without steel fibres
have been summarized in the Table 3.
4.2 Moment-Curvature Relations
Reinforced concrete structures are generally analyzed by the conventional elastic theory
(clause 22.1; IS456:2000) (9). In flexural members, this is equivalent to assuming a linear
moment-curvature relationship. This assumption is in regard to the design criteria and is
generally referred to as limit analysis. In case of analysis of the behaviour of beam specimens
experimentally, non-linear moment curvature relationship are considered.
The moment-curvature relationship for a beam critical in flexure is generally idealized as a
trilinear elasto-plastic relation. In this study, moment-curvature relations were established
from three criteria by computing the curvature (rotation per unit length) using deflection at
midspan, combination of deflection at midspan and load points and linear strain distribution
across a section). They are as discussed below,
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
134
Curvature (rotation per unit length) computed from deflection at midspan as
------------- (1)
Where, δ = Mid span deflection
l = Effective span of beam
a = Shear span of beam
Curvature computed from measured deflections at midspan and load point
------------- (2)
Where, δ = Mid span deflection
δ1, δ2 = Deflections at any two desired symmetrical locations in the
effective span
l = Distance between the two desired symmetrical locations
Curvature computed from the average longitudinal compressive and tensile strains at the
middle of the flange and centroid of the bottom reinforcement assuming a linear strain profile
across the cross section as,
------------ (3)
Where, εc = Average longitudinal compressive strain in at the concrete fibre at
the center of the flange
εt = Average longitudinal tensile strain at the centroid of the tension steel
d = Distance between the compression and tension strain locations
considered
The Figures. 4 & 5 depicts the comparison of the moment curvature relations for the
beams computed from the methods discussed.
Figure 4: Moment curvature plot for beams without fibre
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
135
Figure 5: Moment curvature plot for beams with fibre
4.3 Load-Deflection Characteristics
The load-deflection plot of GPC beams with same stirrup spacing was compared for beams
with and without fibres. In spite of the brittle mode of failure in shear the incorporation of
fibres improves the load-deflection characteristics (Figure 6 and Figure 7) and the failure
mechanism is converted from sudden shear failure to a gradual one with a corresponding
improvement in ductility.
Figure 6: Variation of midspan deflection with load for beams without fibre
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
136
Figure 7: Variation of midspan deflection with load for beams with fibre
4.3 Strain Variation
The strain variation in the compression and tension face of the beam was determined with the
help of pfender gauge. To represent the strains variations, a graph was plotted between
different loading stages and the average strain at constant bending moment zone of the beam
specimens. Figure 6 and Figure 7 shows the typical strain variation at mid span. The positive
strain value represents the tensile strain and the negative strain value indicates the
compressive strain.
From Figures 8 & 9, it is seen that the beams with 180mm and 120mm spacing have
undergone maximum compressive and tensile strain. Since the percentage of compressive and
tensile reinforcements used was similar in all the beams, the strains in all the beams are of the
similar pattern.
Figure 8: Variation in longitudinal average strain in CBMZ* for without fibre
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
137
Figure 9: Variation in longitudinal average strain in CBMZ* for with fibre
* CBMZ-Constant Bending Moment Zone
Table 3: Modes of failure
Specimen ID Failure Mode
PP0070 Web crushing
PP1270 Shear tension & Bond failure
PP1870 Shear tension
PP2470 Web crushing
FDPP0070 Web crushing
FDPP1270 Longitudinal Splitting
FDPP1870 Diagonal compression failure
FDPP2470 Diagonal compression failure
Figure 10: Crack patterns and failure modes of beam specimens without fibre
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
138
Figure 11: Crack patterns and failure modes of beam specimens with fibre
5. Conclusions
Experimental investigations were undertaken on the shear behaviour of reinforced GPC
beams consisting of thin webbed T- sections under two point static loading with and without
steel fibres. Based on the experimental investigations and analysis of test results obtained, the
following conclusions are drawn
1. GPC mixes can be developed using potassium compounds based AAS in lieu of
normally used sodium compounds.
2. The mixes had compressive strength in the range of 30 to 44 MPa after 28 days of
casting and had good workability (225-250 mm slump). The experimental flexural
strength values were lesser than that computed from the IS 456: 2000 formula i.e.,
0.7√fck. The moment at first visible crack was due to flexure in most of the cases.
The first crack load for beams without fibre was 40 kN and for beams with fibre it
was about 60 kN.
3. The failure pattern of all the beam specimens was found to be similar. At early load
stages, flexural cracks appeared in the centre portion of the beam, and gradually
spread towards the supports. As the load increased existing cracks propagated and
new cracks developed along the span. At later load stages, flexural-shear cracks
formed near the supports. These cracks propagated towards the compression zone
under increasing load. The failure occurred by the crushing of concrete in the
compression zone, notably beneath and adjacent to the loading plates. Concrete
spalling at the compression zone was observed after the ultimate load.
4. The beams without web reinforcement failed by web crushing under diagonal
compression in beams with and without fibre. However the failure of beams with
stirrups depended on the stirrup spacing and ranged from diagonal compression (with
or without flange crushing) to shear tension with longitudinal splitting. The
incorporation of steel fibres improves the ductility and energy absorption
characteristics of reinforced geopolymer concrete thin webbed T- beams.
5. Thus the structural behaviour of the RGPC beams resembled the typical behaviour of
reinforced cement concrete beams. It is found to perform adequately as structural
components. Hence the RGPC beams can be adopted in the construction of structures
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
139
such as multi-storeyed buildings, bridges, dams, etc.,
Acknowledgements
The paper is being published with the permission of the Director, CSIR-Structural
Engineering Research Centre, Chennai. The cooperation and guidance received from
Shri T.S. Krishnamoorthy, the technical staffs of Advanced Materials Laboratory of CSIR-
SERC and Structural Testing Lab are gratefully acknowledged. The author also
acknowledges M. Bhuvaneswari, M.Tech Student, Hindustan University, Chennai, India for
her support to conduct this experiment.
6. References
1. Davidovits, J. (1991), Geopolymer: Inorganic polymeric new materials, Journal of
thermal analysis, 37, pp 1633-1656.
2. Chang, E.H., Sarker, P., Lloyd, N. and Rangan, B.V. (2007), Shear Behaviour of
Reinforced Fly Ash-Based Geopolymer Concrete Beams, Proceedings of the 23rd
Biennial Conference of the Concrete Institute of Australia, Adelaide, Australia, pp
679–688.
3. Dattatreya J.K, Rajamane N.P, Sabitha D, Ambily.P.S, Nataraja M.C, Experimental
Flexural behaviour of reinforced geopolymer concrete beams, International Journal of
Civil and Structural Engineering, Volume 2 2011, pp 138-159.
4. Rangan. B. V, (2008), Development and properties of low calcium fly ash based
geopolymer concrete, Research report GC-4, Faculty of Engineering, Curtin
University of Technology, Perth, Australia.
5. Rajamane N P, Nataraja M C, N Lakshmanan, and P S Ambily, [2009],
‘Geopolymer Concrete - An Alternate Structural Concrete’, All India seminar on
concrete Dams ConcDams'09, 2-3 October, Nagpur, organised by the Institute of
Engineer (India), Nagpur Local centre and Indian Concrete institute, Nagpur centre,
pp 274-278
6. Ambily.P.S, Madheswaran.C.K, Sharmila.S, Muthiah.S, Experimental and analytical
investigations on shear behaviour of reinforced geopolymer concrete beams,
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697.
7. Dattatreya, J. K., Rajamane, N. P., and Ambily P.S.., “Structural Behaviour of
Reinforced Geopolymer Concrete Beams and Columns”, SERC Research Report, RR-
6, May 2009
8. IS: 3812:1981, Specification for fly ash for use as pozzolana and admixtures
3812(part1):2003.
9. IS 12089: 1987, Specification For granulated Slag For Manufacture Of Portland Slag
Cement
Experimental studies on Shear behaviour of reinforced Geopolymer concrete thin webbed T- beams with and
without fibres, Ambily P.S et al
International Journal of Civil and Structural Engineering
Volume 3 Issue 1 2012
140
10. IS: 2386: Part I-1963 Methods of tests for aggregates for concrete.
11. IS: 456:2000, Indian Standard Code for Plain and reinforced concrete-code of
practice, 4th Revision, BIS, New Delhi.