Effects of UEA and MgO expansive agents on fracture properties of concrete.

• Fracture properties of concrete containing two expansive agents were assessed. • Addition of UEA expansive agent decreased fracture performance of concrete. • Proper amount of MgO expansive agent improved fracture performance of concrete. • 2:1 was optimal ratio of UEA and MgO expansive agents for fracture performance. Through three-point bending test, the effects of united expansive agent (UEA) and MgO expansive agent on the fracture properties of concrete were investigated. The influence mechanism of the double expansive agents on the fracture properties of concrete was analyzed based on SEM and XRD microscopic experiments. The results indicated that the development tendencies of the concrete fracture parameters varied with the curing ages under different compounding ratios of UEA and MgO expansive agents. The expansive agents generally decreased the initial fracture toughness and unstable fracture toughness of concrete before 28 days, and the fracture toughness increased only when the compounding ratio of UEA and MgO expansive agents was 2:1. During 28 days to 60 days of age, the growth rates of concrete fracture toughness with expansive agents exceeded the ordinary concrete significantly. At 60 days, the initial fracture toughness and unstable fracture toughness of expansive concrete with the compounding ratio of 2:1 reached the maximum values, which were 1.12 times and 1.09 times to that of ordinary concrete, respectively. The SEM test results showed that addition of appropriate amount of MgO expansive agent reduced the bonding cracks between the hydration products and the aggregates, which improved the interfacial transition zone (ITZ) of concrete and reduced the cracks due to dry shrinkage. Based on the experimental results obtained in this study, it can be suggested that the optimal compounding ratio of UEA and MgO expansive agents is 2:1 for improvement in fracture properties of concrete. [ABSTRACT FROM AUTHOR]