Direct Tensile Strength Testing at 6 Hours of Fiber Reinforced Concrete Mortar Fractions

The prime consideration in minimizing concrete cracks in the field is to maximize the early (6-hour) tensile strength development to resist the volume reduction due to rapid water loss. This paper describes a test method, which simulates field conditions, for measuring direct tensile strength soon after initial set at 6 hours. The prototype direct tensile test described presents an effort to quantify results as a measure of crack resistance. In this investigation, three different types of concrete mortar fractions were evaluated: plain, polypropylene fiber mixed in the batch, and the same fiber but roughened by intergrinding with cement for better mechanical bond. Results of tensile testing indicate that the process of intergrinding fibers with cement improves the tensile strength of similar mortar reinforced with smooth fibers by 63 percent. Comparing the ground fiber results to a plain (no fiber) mortar mixture shows almost three times higher direct tensile strength. Based on this exploratory work on early tensile strength testing, it appears to be a viable method to arrive at quantifiable values, which will lead to a better understanding of the concrete cracking phenomenon and its control.