Experimental investigation on the autogenous shrinkage of steam cured ultra-high performance concrete

The ultra-high performance concrete (UHPC) has the potential to be widely used for its excellent mechanical properties and durability. However, characteristics such as high superfine particles content, free of coarse aggregate and extremely low water to binder ratio lead to high autogenous shrinkage in UHPC that could even cause cracking. In this work, the mixture parameters on the autogenous shrinkage behavior of steam cured UHPC were investigated. High performance liquid chromatography was employed to determine the sulfate concentration at different ages. The development of microstructure and hydration products were investigated by a range of analytical techniques such as X-ray diffraction analysis, scanning electron microscopy and nuclear magnetic resonance spectroscopy etc. The results showed that key factors that affected autogenous shrinkage of UHPC were steel fiber, silica fume and aggregate to cement ratio. The autogenous shrinkage mainly occurred during steam curing, and a slight expansion behavior was observed after steam curing. The microstructure observation showed that the hydration was significantly promoted by steam curing and Ca(OH)₂ can be rarely observed. A very dense microstructure and closely bonded interface between aggregate and the matrix was observed. The decrease of sulfate concentration and appearance of a weak shoulder at about 13 ppm in the ²⁷Al MAS NMR spectra of sample at 60 d indicated that ettringite formation was responsible for slight expansion after steam curing.