Preparation and construction techniques of in-tube concrete containing calcium and magnesium oxides-based expansive agent

Ensuring the compactness of the in-tube concrete without de-bonding and separation is important in guaranteeing the construction quality of concrete-filled steel tube (CFST) arch bridge and promoting the further improvement of its span. This study investigated the expansion behaviors of concretes containing compound expansive agents composed of calcium oxides-based expansive agent (EA-CaO) and magnesium oxides-based expansive agent (EA-MgO) by using the environment simulation test chamber. The compound expansive agent with the proportion of 50% EA-CaO plus 50% EA-MgO increases the expansion deformation of in-tube concrete by about 60% at the temperature rise stage and reduced the shrinkage deformation by about 21% at the temperature drop stage, which ensures that the steel tube is always subjected to the compression state. The effects of the developed compound expansive agent and some key construction techniques on the temperature history, deformation, and ultrasonic velocity of the in-tube concrete were further studied by a full-scale steel tube model test. The results indicated that the measures of applying the developed compound expansive agent, reducing the molding temperature, appropriately extending the waiting time before pumping, and taking thermal insulation curing at the temperature drop stage could improve the compactness of the CFST structures significantly. Finally, a technical scheme is proposed for the newly built Lhasa-Nyingchi Railway Zangmu Yarlung Tsangpo River Bridge. The pass rate and average excellence rate of the compactness of the in-tube concrete obtained from the ultrasonic velocities were nearly 100% and about 87%, respectively, which indicates a successful application of the technical scheme.