Effect of MgO-based expansive agent on strengths, volume stability, and microstructures of C80 SCC in steel tube arch

In order to meet the requirements of concrete filling, vibration, and pumping during the construction of the concrete-filled steel tube (CFST) arch bridge, the C80 expansive self-compacting concrete (SCC) is prepared using the absolute volume method. This research investigates the effects of MgO-based expansive agent (EA) and water-to-binder ratio (w/b) on the fresh properties, mechanical properties, autogenous shrinkage, and creep behaviors of expansive SCC. The microstructure of expansive SCC is analyzed by scanning electronic microscopy (SEM) and nuclear magnetic resonance (NMR). Results show that, as the MgO-based EA content increases from 5 % to 10 %, its effect on fresh properties of SCC is negligible. The addition of EA retards the hydration reaction of cement at early ages, thus delaying the strength development. However, the long-term mechanical strengths of the SCC are enhanced, due to the expansion product refines the microstructure at later ages. Moreover, the 14-day autogenous shrinkage of SCC with 10 % EA content is eliminated and shows the micro expansion (155.3 με). The microstructural analysis shows that the porosity of SCC with 10 % EA content is 1.8 % while the pore throat below 0.1 μm accounting for 55 % and the pore size above 30 μm is negligible. This research reveals the mechanism of the effects of MgO-based EA and w/b on the volume stability of the SCC, facilitating the application of expansive SCC in CFST composite structures.