experimental studies on shear behaviour of reinforced … · concrete (rgpc) flexural members...

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





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


STEEL FIBRE 0.75% 1:1.31:1.44:0.6 25.6


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.





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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





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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.





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,





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





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





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





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





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





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,

International Journal of Civil and Structural Engineering, Volume 2 2011, pp 682-

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





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.

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