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Improvement in Properties of High Performance Concrete with Partial Replacement of Cement by Ground Granulated Blast Furnace Slag

INTRODUCTION

High performance concrete (HPC) is in general, cement-based concrete which meets special performance requirements with regard to workability, strength, and durability, that cannot always be obtained with techniques and materials adopted for producing conventional cement concrete. A HPC using cement alone as a binder requires high paste volume, which often leads to excessive shrinkage and large evolution of heat of hydration, besides increased cost. A partial replacement of cement by mineral admixtures (MA), such as, fly ashore ground granulated blast furnace slag (GGBS) in concrete mixes would help to overcome these problems and lead to improvement in the durability of concrete. This would also lead additional benefits in terms of reduction in cost, energy savings, ,promoting ecological balance and conservation of natural resources etc.

MATERIAL USED

Commercially available 53 grade Portland cement, GGBS(Blaine's fineness 400 squaremetre/kg, chemical composition given in Table 1),river sand (specific gravity 2.65, fineness modulus 2.60), and crushed granite aggregates (maximum size 12.5 mm, fineness modulus 5.91), were used for preparing the mixes.

MIX PROPORTIONS

First a high strength concrete (HSC) mix without any mineral admixture (HPC-OS), having a 28-day compressive strength of about 80 MPa was developed. Preliminary mix design was carried out based on the recommendations of ACI Committee. Additional trials were carried out to optimise the ingredients and to improve the workability and cohesiveness of fresh concrete by incorporation of GGBS caused a reduction in workability and increased mixing time was required. The mixes HPC-20S,HPC-30S,HPC-50S,HPC-70S were obtained by replacing 20%, 30%, 50% and 70% of the mass of cement by GGBS (Table 2).

PREPARATION OF TEST SPECIMENS

The weighed ingredients for the batch were mixed in a tilting drum type concrete mixer. The specimens were cast in steel moulds and compacted on a table vibrator. after about 24 h of casting, specimens were demoulded and placed immediately in a water tank. 

TEST RESULTS AND DISCUSSIONS

Compressive Strength

As per general international practice 100-mm cube specimens were used for compression test for testing HPC. A minimum of three specimens was used for test. It is seen from Table 3 that the compressive strengths of HPC mixes ranges from 63.2 MPa (HPC-0S) to 39.2 MPa (for HPC-70S), from 83.5 MPa to 69.8 MPa, from 93.7 MPa to 87.6 MPa, at 7 days, 28 days, and 90 days (Table 3). It is observed that GGBS reduces compressive strength at 7 days by about 14% to 38% as cement replacement level (CRL) varies from 20% to 70%[Figure 1], whereas, the strength at 28 days and 90 days, were 2.4% to 16.4%, and 0.2% to 6.5%,respectively. Thus, GGBS significantly affects early age strength up to 7 days but thereafter reduction is less and beyond 90 days GGBS based HPCs can have higher strengths.

     Reduction in compressive strength of GGBS-based HPCs at earlier ages may be attributed to factors, namely, (i) rise in effective w/c due to reduced cement content for the same quantity of water in the mix and (ii) delayed activation of GGBS. For example, effective w/c increases from 0.3 for HPC-0S to as high as 1.0 for HPC-70S. Since, GGBS acts only as filler at early ages, cement portion alone contributes to strength development.

     It is preferable to use higher compressive strength of 90 days (instead of 28-day strength), in structural design. This advantageous in structures such as, massive bridge piers, abutment walls, off shore structures. In situations where early age strength is the criterion for design, Swamy recommends an increase in binder content by 10% for CRL up to 50% and 20% CRL up to 65%, in order to obtain strengths comparable to only-cement HPCs.

CONCLUSION

1. Addition of GGBS as a partial replacement to Cement causes a reduction in compressive strength of HPCs at initial ages due to increase in effective w/c even though the sane water-binjder ratio is maintained. This can be attributed to delay in activation of hydration reaction of GGBS. At early ages (up to 7 days), GGBS acts more filler material and nucleus for precipitation of cement hydration products rather than as a cenentitious material. At later ages, HPCs with GGBS have almost same strength as that of   HPC without GGBS.

2.Ther was an increase of only 4MPa to 17.8MPa for HPCs. This could be advantageously used, for design of structure, such as, bridge piers, abutment walls and other mass concrete structural elements where early age strength is not the criterion.

3.Use of GGBS in HPCs to replace cement by 70% helps to reduce the cement content of HPCs from about 5.30 kg/cubicmetre to 160 kg/cubicmetre. Despite this large reduction in cement content,28 days and 90 days strengths are not significantly affected, while there is considerable improvement in durability properties.

          Conventional high strength concrete has a high cement content , which often leads to higher shrinkage and greater evolution of heat of hydration. A partial substitution of cement by ground granulated blast furnace slag (GGBS) eliminates these drawbacks besides enhancing durability characteristics. This paper reports the investigations carried out for a quantitative assessment of replacement levels of cement with GGBS on the compressive strength which were in the range of 70 MPa-80MPa at 28 days, and considerable imperviousness to chloride ions were obtained.

CRM  :  cement replacement material

SWA  :  saturated water absorption