Influence of Steel Fiber Addition on Workability & Mechanical Behavior of High Performance Concrete

In this study, the mechanism of crack propagation in concrete at discontinuity stress region under uniaxial compression was investigated and an attempt was made to determine the microcracking behavior of high-performance concrete with steel fiber addition at 7 different fiber volume (Vf) under two aspect ratios. For this purpose, fourteen different concrete mixes with steel fiber addition for 60 and 75 aspect ratio, each changing in proportions from 0.5 - 2.0 % with 0.25 % intervals were designed based on high performance concrete principles. Microcracking behavior of the produced concretes were analyzed by selecting two stress points before the linearity, the linearity end point, and one point after the end of the linearity using the tangent points (deviation from linearity) of load – time diagram. Findings revealed that in terms of workability, with increasing Vf addition and aspect ratio, VeBe time increases exponentially; slump and compacting factor decreases linearly, and an inverse relationship exist between yield stress and slump. It is hereby suggested that in terms of workability for this kind of mixes, a combination of parameters should be considered instead of a single parameter to characterize workability. Compressive strength increases with increase in fiber addition in aspect ratio 60, while in aspect ratio 75, the increment was up to 1.0 % fiber addition level, but it was still higher than the reference specimens without fiber addition. Tensile strengths in the form of splitting and flexural tensile strength increased with fiber addition in both cases; however, the flexural strength presented results with higher strength. Area under the load – deflection diagrams increases with increase in fiber volume and aspect ratio, an indication of the improvement in toughness of the composite. Fracture energy increases with increase in fiber volume and compressive strength, an indication of high energy required in extending cracks. Characteristic length also increases showing the increased ductility of the composite. Residual tensile strength measured indicated that there was strain hardening behavior due to the fiber addition resulting in tensile strength gain instead of tensile strength loss in concrete with fiber addition. Results from the load – time diagram used to determine the end of the linear portion of the graph indicates that the end of ascending portion (linearity) lies within 85 – 91% of the ultimate strength, aggregate cracking was the dominant failure mode up to the end of linearity as oppose to bond cracks due to improvement in the matrix quality. Failure pattern similar to what is obtained in compressive loading is applicable here at linearity end point and post linearity with extensive damage perpendicular to the casting direction. The presence of the steel fiber improved the extensive failure pattern of cracks observed at aspect ratio 60 and 75 where it changes from macrocracks to a branched network of microcracks especially at higher fiber dosage. Critical width of the cracks measured at linearity end point was within the range of 0.01 mm – 0.07 mm. The findings of this study will go a long way in helping our understanding of the microcracking in concretes with fiber addition which is the next frontier in structural concrete.