Design and performance of low shrinkage UHPC for thin bonded bridge deck overlay

Abstract

The service life of bonded overlay materials used for bridge deck rehabilitation is affected by shrinkage of the overlay and bond between the overlay and substrate. There is a growing interest in using ultra-high performance concrete (UHPC) for thin bonded overlays. However, UHPC can exhibit high autogenous shrinkage leading to cracking under restrained conditions. This study investigated the synergistic effect of using pre-saturated lightweight sand (LWS) and a CaO-based expansive agent (EA) to mitigate shrinkage of UHPC. Five non-proprietary UHPC mixtures, a latex-modified concrete (LMC), and a conventional concrete (CC) were tested for material properties, including pore-size distribution, hydration kinetics, and bond strength. The mixtures were used to cast 16 bonded overlay slab specimens measuring 1 × 2 m with overlay thicknesses of 25, 38, and 50 mm. In-situ variations of strain, temperature, and relative humidity of overlay materials as well as cracking and overlay-substrate bond strength were monitored for 27 months. Test results indicated that the optimum UHPC made with 5% EA and 35% LWS had a relatively low autogenous and drying shrinkage of 190 and -310 με, respectively, at 28 d and the highest bond strength (3.1 MPa at 28 d). Such values were −520 and −520 με and 2.35 MPa, respectively, for UHPC made without any EA and LWS, and 20 and -475 με and 2.75 MPa, respectively, for the mixture prepared with 35% LWS. No cracking was observed in UHPC overlay slab specimens, whereas the CC and LMC overlays started cracking after approximately half a year. The increase in overlay thickness reduced crack density of the CC and LMC slabs. The optimum UHPC mixture made with 35% LWS and 5% EA had positive in-situ strain of 45 and 200 με over the 27-month testing period, indicating that the overlay was in compression. Direct pull-off strength of the UHPC was higher that of the LMC and CC mixtures and failure occurred in the CC substrate compared to the interface for the CC and LMC overlays.