Interface adhesion of fresh-on-fresh cast ultra-high performance concrete–normal concrete: Effect and mechanism of pour delay and ambient humidity

Abstract

The Ultra-high performance concrete (UHPC)-normal concrete (NC) composite element in this study was created using the fresh-on-fresh casting process with NC placed over UHPC. The impact of the pour delay of 0, 25 min, 1 h, 3 h, 7 h, 10 h, and 24 h as well as the placement environment of standard curing and dry indoors conditions on the interface bond strength were investigated. The findings demonstrate that, under standard curing conditions, a suitable pour delay can not only guarantee the interface bond strength but also decrease the dispersion; While when the pour delay exceeds a certain limit, the interface bond strength decreases linearly until the final setting of UHPC. However, in the dry condition, only a short exposure time will lead to a significant decline in the interface bond strength. From the combination result of computerized tomography (CT) and backscattered electron (BSE) test, it is found that the interface bonding is due to the synergistic effect of aggregate interlocking, fiber anchoring and staggered growth of hydration products. While in a dry condition, the evaporation of water led to the formation of a thin hardening layer on the surface of UHPC, which hindered the interaction between UHPC and NC, and finally led to the rapid loss of interface bond strength.