Use of Double Punch Test to Evaluate the Mechanical Performance of Fiber Reinforced Concrete

Abstract

Mechanical properties of fiber reinforced concrete (FRC) determined by material test methods can be used to ensure that the FRC mixture is batched properly and to give indications of performance if used in structural members. An ideal material test method for FRC should give low variability in the measurement of properties such as peak and residual strengths. ACI 318-08 uses results from the third-point bending test [1] as the performance criteria for FRC. Experimental evidence, however, has indicated that this bending type test method has potential problems in the reliability of determining the peak and residual strengths. The coefficient of variation of residual strength is typically very high and generally greater than 20%. The considerable scatter results make it difficult for quality control, particularly when such properties are intended to be used to estimate the strength of structural members. In addition, the complex test setup and the requirement of using a closed-loop servo-controlled machine often prevent its use in small laboratories. Other test methods such as the direct tensile test experience similar problems. As a consequence, these test methods are generally time consuming and expensive as they require more specimens to obtain reliable test data.

The double punch test (DPT) was originally developed, based on the plasticity theory of concrete, to determine the tensile strength of plain concrete but has rarely been used since the 1980s. This test method can be carried out by using conventional compression test machines with small capacity and simple test setup. This study investigates the feasibility of using DPT for the evaluation of mechanical properties and quality control of FRC. Experimental results showed that DPT generally gives much lower variability compared to other test methods. DPT also has the ability to identify important FRC characteristics such as strain-softening, strain-hardening, stiffness, tensile strength, residual strength, and toughness.