Evaluation of initial setting time of fresh concrete

Abstract

According to ASTM 403C, initial setting time of concrete is measured based on penetration resistance of mortar sieved from concrete and it is considered as the time taken to achieve a penetration resistance of 3.5MPa which is an arbitrary value. The main objective of this research is to find a test method to obtain a realistic initial setting time value in order to plan the placing of concrete to prevent formation of cold joints which can affect the performance of the structure. Mechanical properties (tensile strength and shear strength) at a purposely made cold joint was evaluated by testing concrete cubes cast in two layers with different time lags between placing of concrete in the two layers. Point of change of each strength parameter with respect to time lag was identified and corresponding time range was identified as the initial setting time range. Standard penetration resistance test was also carried out and with the use of penetration resistance vs. elapsed time graph, corresponding penetration resistance value was also identified. Although this new method is effective to identify the initial setting time of concrete, it is not possible to use as a field test since it a time consuming and complicated test. Therefore, an attempt was made to introduce a simple field test method. Ultrasonic pulse velocity is used to measure properties of hardened concrete, because pulse velocity of a concrete sample depends on its elastic modulus and density. Since properties of fresh concrete such as elastic modulus changes with time, the ultrasound pulse velocity was used to monitor the change of stiffness of fresh concrete with time. Based on the test results of the experimental investigations, it was found that the more realistic initial setting time of concrete is the onset time of the penetration resistance of mortar measured in the standard penetration test. This was further verified by a simple qualitative field test where the initial setting time is considered as the latest time that a vibrating poker can be inserted into the concrete and removed without leaving a significant trace.