PERFORMANCE OF HIGH STRENGTH CONCRETE MIX WITH FLY ASH

Abstract

In the past, high-strength concrete (HSC) has mainly been employed in specialist areas such as prestressed concrete, precast .concrete' products and containment. Now, HSC has increasingly been adopted for the construction of tall commercial buildings etc. due to the ease in in-situ construction even at low w/c ratio with the use of superplasticizers. Replacement of normal strength concrete by HSC can yield reductioh in consumption of reinforcing steel or reduction in cross-sectional area keeping same quantity of steel. Because of these considerations, the further development and promotion of HSC is well justified, technically and economically. There is no absolute definition of HSC. It depends upon the present technology, requirement and geographical basis. However, ACI defines HSC as cylinder strength equal to or greater than 41MPa at 28 days of age using normal weight aggregate. As HSC contains higher cement content, attempts have been made to replace cement by some inert materials without affecting the properties of HSC adversely. This reduction in cement content can ,yield, low heat of hydration which is extremely necessary for mass construction and side by side it is economical also. In the present work fly ash was chosen as an inert material to replace part of cement and its effects were studied on properties of HSC both in the fresh state as well as in the hardened state. Behavior of HSC in fresh state is more important as compared to the behavior of normal strength concrete in fresh states, as HSC has low wfc ratio and it requires heavy compaction, which is not possible without adequate workability. So workability was taken as major factor along with fly ash. Compressive, flexural and split tensile strength were taken as strength parameters to study the behavior of HSC with fly ash. Properties of the fly ash greatly affects the behavior of HSC. Adverse effect of fly ash was observed on the strength properties of HSC. All the strength parameters showed decreasing trend with the increase in fly ash content. However, small fly ash content such as 10% did not affect strength significantly but on increasing fly ash content from 10 to 25%, high reduction in strength properties was observed. Fly ash reduced the workability of HSC. Decrease in the workability was observed with the increase in fly ash content. This was may be due to coarser ( v) fly ash used in present study. Delayed gain in strength was observed for superplasticized fly ash concerete. So specification must be changed from 28 days criteria to 56 or 91 days to take into account this delayed gain strength.