INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 1, 2012

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1 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 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 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 ambilyps@serc.res.in,ckm@serc.res.in doi: /ijcser 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 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 CO 2 emissions through improvements in process technology and enhancements in process efficiency, but further improvements are limited because CO 2 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 Received on March, 2012 Published on August

2 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 129

3 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 (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 K 2 O (%) PP % FA, 20% GGBS 1:1.31:1.44: PP % FA, 20% GGBS 1:1.31:1.44: PP % FA, 20% GGBS 1:1.31:1.44: PP % FA, 20% GGBS 1:1.31:1.44: FDPP0070 FDPP1270 FDPP1870 FDPP % FA, 20%GGBS, STEEL FIBRE 0.75% 80% FA, 20% GGBS STEEL FIBRE 0.75% 80% FA, 20% GGBS STEEL FIBRE 0.75% 80% FA, 20% GGBS STEEL FIBRE 0.75% 1:1.31:1.44: :1.31:1.44: :1.31:1.44: :1.31:1.44: Specimen details Beam Geometry The test specimens are designed as per the provisions of IS (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

4 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 131

5 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 mm and the density of the mixes was kg/m 3 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. 132

6 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 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 Compressive Service load, Ultimate load, Specimen (kn) strength (MPa) PSL (kn) PUL(kN) ID Pfl Psh PP PP PP PP FDPP FDPP FDPP FDPP 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 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, 133

7 Curvature (rotation per unit length) computed from deflection at midspan as (1) Where, δ l a = Mid span deflection = Effective span of beam = 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, Where, εc εt d (3) = Average longitudinal compressive strain in at the concrete fibre at the center of the flange = Average longitudinal tensile strain at the centroid of the tension steel = 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 134

8 4.3 Load-Deflection Characteristics Figure 5: Moment curvature plot for beams with fibre 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 135

9 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 136

10 Figure 9: Variation in longitudinal average strain in CBMZ* for with fibre * CBMZ-Constant Bending Moment Zone Table 3: Modes of failure Specimen ID PP0070 PP1270 PP1870 PP2470 FDPP0070 FDPP1270 FDPP1870 FDPP2470 Failure Mode Web crushing Shear tension & Bond failure Shear tension Web crushing Web crushing Longitudinal Splitting Diagonal compression failure Diagonal compression failure Figure 10: Crack patterns and failure modes of beam specimens without fibre 137

11 5. Conclusions Figure 11: Crack patterns and failure modes of beam specimens with fibre 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 ( 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 138

12 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 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 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 , pp 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 Ambily.P.S, Madheswaran.C.K, Sharmila.S, Muthiah.S, Experimental and analytical investigations on shear behaviour of reinforced geopolymer concrete beams,, Volume , pp 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 IS: 3812:1981, Specification for fly ash for use as pozzolana and admixtures 3812(part1): IS 12089: 1987, Specification For granulated Slag For Manufacture Of Portland Slag Cement 139

13 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. 140