Evaluation of Behavior and Ductility of Polymer Modified Steel Fiber Reinforced High Performance Concrete Beams

Evaluation of Behavior and Ductility of Polymer Modified Steel Fiber Reinforced High Performance Concrete Beams

Presented byNaveen R.

Under the guidance ofDr. Sadath Ali Khan Zai Associate Professor,UVCE, Bangalore University, Bangalore

An outcome of this dissertation work, a technical paper entitled Evaluation of Behavior and Ductility of Polymer Modified Steel Fiber Reinforced High Performance Concrete Beams has been presented and published in the international conference on STRUCTURAL ENGINEERING MECHANICS AND COMPUTATION(SEMC-2013) held on 2-4 September2013 at CAPE TOWN, SOUTH AFRICA.

PATHWAYINTRODUCTIONAIM AND SCOPE OF PRESENT INVESTIGATIONMATERIALS USEDEXPERIMENTAL INVESTIGATIONSTUDIES ON FLEXURAL BEHAVIOURSUMMARY AND CONSLUSIONSREFERENCES

IntroductionConcrete is the most widely used construction material in India with annual consumption exceeding 100 million cubic meters.

Conventional Portland cement concrete is found deficient in respect of Durability in severe environments (shorter service life and require maintenance), time of construction (longer release time of forms and slower gain of strength).

It is well known that conventional concrete designed on the basis of compressive strength does not meet many functional requirements such as impermeability, resistance to frost, thermal cracking adequately.

Introduction ctd..High Performance Concrete

HPC is a concrete made with appropriate materials combined according to a selected mix design and properly mixed, transported placed, consolidated, and cured so that the resulting concrete will give excellent performance in the structure in which it will be exposed and with the loads to which it will be subjected for its design life.

High performance concrete (HPC) is defined according to ACI 363-1992 as concrete, which meets special performance and uniformity requirements that cannot always be achieved by using only the conventional materials and normal mixing, placing and curing practice.

Sailent Features of HPC

Compressive strength > 60 MPa, even up to 800 MPa Water-binder ratio =0.25-0.35 ,therefore very little free water Wide range of grain sizes Reduced flocculation of cement grains Densified cement paste Low free lime content Stronger transition zone at the interface between cement paste and aggregate Discontinuous spores Less capillary porosity

Polymer ConcreteA polymer is a large molecule, or macromolecule, composed of many repeated subunits, known as monomers. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins. Polymers, both natural and synthetic, are created via polymerization of many monomers.Synthetic polymer are man made which can be classified as into thermoplastics, thermosets, elastomers and synthetic fibers. Polymer ConcreteAn elastomer is a polymer which is derived from elastic polymer, is often used interchangeably with the term rubber or synthetic rubber.

There are many types of elastomers out of which SBR (Sytrene Butadiene Rubber) is one which is mainly used in concrete because of its versatile property as Styrene Butadiene rubber latex (SBR) performes well in augmenting the mechanical behaviors of the conventional concrete.

Use of SBR-Latex in concreteSBR Latex is mainly used in the construction as a repair material because it can reduce W/C for the same workability or increase workability for the same W/C. Polymer particles as well as entrained air bubbles have a ball bearing effect in the mixture. They lubricate the mixture particles thus reducing internal friction coefficient. SBR latex due to its air entraining properties reduces bleeding considerably. This leads to an abrasion resistant surface.

Use of SBR-Latex in concreteSBR-Latex addition increases the water resisting properties of modified concrete and mortars. As water evaporates, the SBR particles stick together this is called coalescence to form an interconnected polymer network inside the cementitious matrix. This flexible network both blocks the pores and bridges micro cracks to a certain extend. Less and thinner cracks and less voids increase water impermeability.Use of SBR-Latex in concreteEffects on addition of SBR Latex to Cement Mixes1. Increases tensile, flexural and impact strengths. 2. Overall durability increases.3. Induced flexibility increases creep.4. Setting times are generally reduced. Placement time also.5. SBR addition reduces bleeding and contributes to an enhanced abrasion resistance provided that an adequate curing procedure is followed.6. SBR latex addition increases air entrainment.

Fibre Reinforced ConcreteFiber-reinforced concrete (FRC) is a concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers each of which lend varying properties to the concrete. Fibre Reinforced ConcreteCharacteristics of Fibre Reinforced ConcreteFibers are usually used in concrete to control cracking due to plastic shrinkage and to drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion, and shear resistance in concrete. The amount of fibers added to a concrete mix is expressed as a percentage of the total volume of the composite (concrete and fibers), termed "volume fraction" Fibre Reinforced ConcreteIn the present study Steel Fibres have been used. Steel fibers can:Improve structural strengthReduce steel reinforcement requirementsImprove ductilityReduce crack widths and control the crack widths tightly, thus improving durabilityImprove impact and abrasionresistanceImprove freeze-thaw resistance

Polymer Modified Steel Fibre Reinforced High Performance Concrete(PMSFRHPC) PMSFRHPC is a composite material which has superior properties resulting from the individual properties of all the materials from which it is made.Increases strain level corresponding to value of peak stress because of its properties member can absorb and dissipates large amount of energy compared to conventional concrete.

Pre-defined Plastic hinge location

Which are likely to develop in the region of maximum moment at a length of twice the depth of the flexural member.

Aim and Scope of the Present Investigation Six test beam specimens are cast with different concrete matrixes at the central region and high performance concrete at other regions.The ductile behavior of the beam under three point bending involves the formation of a plastic hinge in the beam at the location where the modified concrete matrix has been incorporated.The structural system should be so designed as to ensure that the formation of plastic hinges at suitable locations may, at worst, result in the failure of the individual elements, but will not lead to instability or progressive collapse.

Aim and Scope of the Present InvestigationInclusion of fibers in concrete matrix, and use of latex modified concrete improves the ductility of reinforced concrete elements. Modifications are made in design as to include these only at discrete locations where higher ductilitys are needed, namely the plastic hinge locations.The present investigation aims at studying the effect of SBR-Latex and Steel Fibre modification in high performance concrete on the mechanical properties like compressive strength, split tensile strength, flexural strength andAim and Scope of the Present Investigation ctd..Extending the investigation on the behaviour of flexural structural members on the following parameters such as first cracking load, Ultimate loadUltimate strain Energy absorption capacity and ductility aspect. Toughness indexCracking behaviour and spacing,

MATERIALS USED

OPC 53 Grade CementMineral Admixture (Silica Fume)GGFBSFine AggregateCoarse AggregateChemical Admixtures (Superplasticizer)Steel fibers (crimped round)Styrene Butadiene Rubber Latex PolymerSteel Reinforcement(Fe 415)Water

Test Beam SpecimenHigh Performance Concrete beam specimen (A1,A2):2 NosPolymer Modified reinforced HPC beam specimen (B1,B2): 2 NosHYSD bars having characteristic strength of 415 MPa are used for longitudinal and transverse reinforcement in all the beams.Effective Span : 2000mm.Strength of Concrete : 60 Mpa Sectional Dimension : 150 x 230 mm. 2 of 12 mm2L 8mm230mm 150mm3of 12 mmPolymer Modified Steel Fibre reinforced HPC beam specimen (C1,C2):2 NosEXPERIMENTAL INVESTIGATIONS

Experimental InvestigationsMIX 1 M60 (Cement 79% + SF 06% + GGBS 15%)MIX 2 M60 (Cement 71% + SF 09% + GGBS 20%)MIX 3 M60 (Cement 63% + SF 12% + GGBS 25%)MIX 2 was found to obtain maximum compressive strength, split tensile strength and flexural strength respectively. Hence MIX 2 was selected as the design mix and the same was modified with SBR Latex and Steel Fibres.

Experimental InvestigationsPropertiesAgeMIX1MIX2MIX3Compressive strength (N/mm2)3 Days29.0032.0025.297 Days42.0048.0038.0028 Days61.3364.0158.71Split tensile strength (N/mm2)3 Days1.341.861.277 Days1.782.171.7128 Days12.582.802.59Flexural strength (N/mm2)3 Days1.552.001.427 Days2.522.972.2028 Days1.884.433.65Properties of Modified Hardened ConcretePropertiesAgeHPC(A1,A2)PMHPC(B1,B2)PMSFRHPC(C1,C2)Compressive strength (N/mm2)3 Days32.0026.6632.337 Days48.0042.0045.3328 Days64.0155.1166.00Split tensile strength (N/mm2)3 Days1.861.631.947 Days2.172.242.6028 Days2.803.304.80Flexuralstrength (N/mm2)3 Days2.001.692.457 Days2.972.502.9328 Days4.433.134.70It can be noticed that the compressive strength of PMSFRHPC is the highest among HPC and PMHPC, similarly split tensile strength and flexural strength also increases by the modification of HPC by polymer and Steel Fibres.

Studies on Flexural Behaviour

For the condition of simply supported reinforced concrete under single point loading at the centre of the span the plastic hinge length of 460mm is modified with PMHPC and PMSFRHPC and the remaining length on either side of beam HPC was attempted.The experimental programme consists of testing three series of test beam specimens under flexural loading. Test beam specimens consist of HPC, PMHPC and PMSFRHPC designated as A1, A2, B1, B2, C1, and C2 respectively.

Test Setup

Load Deflection Behaviour Combined Load-Deflection CurvesLoad Deflection BehaviourIt is noted that B1, B2 cracked much before A1, A2 and C1, C2 at lesser loads. A1, A2 carried higher loads than B1, B2 but did not undergo large deflection which is mainly because of the brittle nature of HPC beams.C1, C2 beams sustained higher loads than A1, A2 and B1, B2 beams. In C1, C2 the first crack was seen at 19% higher load than A1 and A2 which indicates that beams with steel fibres can carry higher loads and undergo large deformation while in B1, B2 occurred at 24% lesser load.Load Deflection BehaviourAfter the yielding of steel C1, C2 beams continued to show ductility by undergoing higher deflections with non linearity without much increase in the ultimate load.

This indicates that modification of HPC with polymer and steel fibre resulted in enhanced load carrying capacities.

First Crack LoadMix BeamExperimental first crack load in KN (E)Theoretical first crack load in KN (T)Ratio (E/T)HPCA110.0014.820.67A 211.2514.820.76LMHPCB18.7513.760.64B 27.5013.760.55LMSFRHPCC112.5013.960.83C213.7513.960.91Below table shows the First Crack Load of all the test beam specimensUltimate LoadMixBeamExperimental Ultimate load in KN (E)Theoretical Ultimate load in KN (T)Ratio (E/T)HPCA158.7546.641.26A 258.7546.641.26LMHPCB 146.2546.061.00B 248.7546.061.06LMSFRHPCC173.7546.741.58C276.2546.741.63Below table shows the Ultimate Load of all the test beam specimens

Service Load, Yield Load and Ultimate LoadMixBeamService Load (PSLkN) Yield Load (PYLkN)Ultimate Load (PUL kN)HPCA 139.1245.0058.75A 239.1245.0058.75LMHPCB 135.0035.0046.25B 232.5037.5048.75LMSFRHPCC 149.1255.0073.75C 250.8457.5076.25Below table shows service load, yield load and ultimate load of all the test beam specimens

Strain DistributionThe strain behaviour of test beams is represented in the form of strain distribution curves along the depth of the beam.The Demec gauge readings were taken at four locations along the depth of the beam in the central portion. The strains at each of these points are calculated for every increment of loads these values are plotted to appropriate scale to obtain the strain distribution behaviour.Strain DistributionStrain DistributionStrain DistributionStrain DistributionStrain DistributionStrain DistributionMixBeamStrain at ultimate loadCompressive TensileHPCA11.35x10-32.08x10-3A21.45x10-32.40x10-3LMHPCB11.85x10-36.20x10-3B22.60x10-36.35x10-3LMSFRHPCC13.43x10-37.68x10-3C23.63x10-37.89x10-3The tensile and compressive strains of reinforcement and concrete respectively were measured and tabulated as below

Experimental and Theoretical Cracking MomentsMixBeamExperimental MCR (kN-m)Theoretical MCR (kN-m)Ratio of MCR Exp./TheoHPCA15.007.410.67A 25.637.410.76LMHPCB 14.386.880.64B 23.756.880.55LMSFRHPCC16.257.530.83C26.887.530.91Experimental and Theoretical Cracking Moments calculated is as shown belowTheoretical and Experimental Ultimate LoadsMixBeamExperimental Mu (kN-m)Theoretical Mu (kN-m)Ratio of MCRExp./Theo.HPCA129.3823.321.26A229.3823.321.26LMHPCB123.1323.031.00B224.4023.031.06LMSFRHPCC136.8823.091.60C238.1323.091.65Theoretical and Experimental Ultimate Loads calculated as per IS 456:2000Flexural Ductility Index

Ductility is measured by the ratio called ductility index (). In this study, the member's ductility index was evaluated by displacement expressed by the ratio of the deflection at yield and the deflection at ultimate load.

Test Beam y,mmu,mmd =u /yAvg dA13.8 7.101.871.83A23.86.801.80B14.77

9.802.052.23B24.7911.502.40C13.8714.603.773.96C23.7015.30

4.14DeflectionMixBeamDeflection at First Crack LoadDeflection atService Loadsl (mm)Deflection atYield Loady (mm)Deflection atUltimate Loadu (mm)HPCA10.622.853.807.10A20.632.953.726.80LMHPCB10.463.784.779.80B20.383.504.7911.50LMSFRHPCC10.683.253.8714.60C20.682.843.7015.30The deflection response of reinforced HPC, HPC modified latex and HPC modified with latex and steel fibre is presented here.

Toughness Index

Test Beam Toughness IndexAvg. Toughness IndexA141.4242.54A243.66B150.5748.54B246.52C174.6272.31C270.00It is defined as the ratio of area under the curve up to ultimate load to area under the curve up to first crack load.Energy Absorption Capacity Test Beam Energy

absorption

capacity(kN-mm)Average Energy

absorption

capacity(kN-mm)A1136.23133.40A2130.55B1142.32141.00B2139.46C1236.93232.50C2228.04Energy absorption capacity of the entire test specimens was calculated as area under load deflection curve. Cracking Pattern

Crack width is an important factor from the durability point of view. Below figure shows the cracking observed for all the test beam specimens.Cracking PatternThe number of cracks in A1, A2 beams was more ranging about 10 to 14 with cracks propagating to the top surface of the beams. In case of B1, B2 cracks were ranging about 8 to 9. C1 and C2 has about 10 cracks with the cracks not reaching up to the top this indicates the polymer modified steel fibre reinforced high performance concrete which is used in plastic hinge zone is more ductile as it deflected more with cracks less than the control beam and has significant energy absorption capacity.

Summary and ConclusionsThe focus of this research was to investigate the application of polymer modified steel fibre concrete at predefined plastic hinges which are likely to develop in regions of maximum moment for a length usually which is twice the depth of the beam for strengthening and improving load characteristics and seismic performance. The results of the study indicates that modification at the central portion of the test beam specimens replaced with a highly ductile composite such as Polymer Modified Steel Fiber Reinforced High Performance concrete ensures structural integrity, resulting in enhancement of ductility, toughness and energy absorption capabilities.

Summary and ConclusionsAmong the three mixes in the present investigation namely HPC, LMHPC and LMSFHPC, the compressive strength obtained at 28 days are 64.01, 55.11, 66.00 N/mm2 respectively. In case of SBR-latex modified concrete there is decrease in compressive strength. This is due to lower density of latex with regards to matrix density (mortar rheology). Moreover the combination of SBR-latex and steel fiber the compressive strength increases appreciably. Also less number of cracks can be seen in LMHPC

Summary and ConclusionsThis is because because Polymer-modified cementatious systems seals the pores and micro cracks developed during hardening of the cement matrix, by dispersing a film of polymer phase throughout the concrete. The innovative idea of modifying the plastic hinge length of HPC beams has served its purpose by the enhancement of all the properties of beam in strengthening the flexural characteristics, increase in load carrying capacity, ductility, toughness and energy absorption capacity of all the modified beams when compared to the control beam and also which in-turns leads to reduction in the cost of construction.

Summary and ConclusionsThe addition of SBR-latex with steel fiber increases the deflection value to 15.30mm (highest) as compared to 7.10mm (lowest) in case of HPC and 9.8mm in case of LMHPC, which enhances the ductile characteristics. Significant improvements are observed in first crack load for LMSFRHPC to a value of 13.75KN followed by HPC (11.25KN) and LMHPC (7.50kN) respectively.Among the parameter studied the ultimate load carrying capacity of LMSFRHPC is 76.25kN which being highest, followed by HPC (58.75kN) and LMHPC (48.75kN).

Summary and ConclusionsThe compressive and tensile strain values of LMSFRHPC are comparatively higher than LMHPC and HPC. In the beams containing fiber and SBR-latex the formation of cracks are less in number due to fiber matrix action in concrete. In case of beams without fiber the number of cracks appeared are more in number and progressed rapidly. The minor cracks closed soon after the removal of load.

Summary and ConclusionsFrom the experimental results it is observed that the concrete beam when modified at plastic hinge with fiber and latex behaved much better with regards to first crack load and ultimate load which enhanced the flexural behavior characteristics.

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