Restrained shrinkage behavior of internally-cured UHPC using calcined bauxite aggregate in the ring test and UHPC-concrete composite slab

Abstract

Ultra-high performance concrete (UHPC) can be a good candidate for bridge deck overlay or repair materials of structural members due to its superior mechanical properties or durability. However, large autogenous shrinkage characterized by UHPC may be an obstacle to extending its applications. A novel internal curing (IC) agent, calcined bauxite (CB) aggregate, has been developed to effectively reduce the autogenous shrinkage of UHPC and meantime enhance its mechanical properties. This study further examines its efficiencies in solving shrinkage issues by restrained ring test and large-scale UHPC-concrete composite slabs. In restrained ring tests, when both no fibers, the IC UHPC matrix with CB aggregate shows smaller crack width (0.25 versus 1.0 mm at 9 d) and delayed (5.2 d versus 3.6 d) onset of cracking than the normal UHPC matrix; when having fibers, the normal UHPC shows micro-cracking while no cracking was detected in IC UHPC even observed at one year. Normal UHPC overlay shows more serious delamination and curling than the IC UHPC overlay, and it also shows a hairline crack on the side surface at midspan. Early-age exposure to drying at 1 d has a detrimental effect by exhibiting the most severe delamination and curling of the UHPC overlay, and reinforcement in UHPC layer is effective in reducing its shrinkage strain.