The shrinkage of core concrete during construction is the key reason for the separation of steel pipes and core concrete. Utilizing expansive agents during cement hydration is one of the main techniques to prevent voids between steel pipes and core concrete and increase the structural stability of concrete-filled steel tubes. The expansion and hydration properties of CaO, MgO, and CaO + MgO composite expansive agents in C60 concrete under variable temperature conditions were investigated. The effects of the calcium–magnesium ratio and magnesium oxide activity on deformation are the main parameters to consider when designing composite expansive agents. The results showed that the expansion effect of CaO expansive agents was predominant in the heating stage (from 20.0 °C to 72.0 °C at 3 °C/h), while there was no expansion in the cooling stage (from 72.0 °C to 30.0 °C at 3 °C/d, and then to 20.0 °C at 0.7 °C/h); the expansion deformation in the cooling stage was mainly caused by the MgO expansive agent. With the increase in the active reaction time of MgO, the hydration of MgO in the heating stage of concrete decreased, and the expansion of MgO in the cooling stage increased. During the cooling stage, 120 s MgO and 220 s MgO resulted in continuous expansion, and the expansion curve did not converge, while 65 s MgO reacted with water to form brucite in large amounts, leading to its lower expansion deformation during the later cooling process. In summary, the CaO and 220 s MgO composite expansive agent in the appropriate dosage is suitable for compensating for the shrinkage of concrete in the case of a fast high-temperature rise and slow cooling rate. This work will guide the application of different types of CaO-MgO composite expansive agents in concrete-filled steel tube structures under harsh environmental conditions.

The current study aims at proposing a novel and simple method for designing fluid concrete such as self-compacting concrete (SCC) with a low cementitious binder content to reduce the carbon footprint. Different testing methods regarding the packing density of aggregate mixtures are performed and compared. The W/C was determined according to the target compression strength. Slump flow spread is carried out to determine the most appropriate superplasticizer (SP) dosage and aggregate volume fractions and proportions in concrete mixtures. Furthermore, hardened performance, including compression strength and drying shrinkage of the fluid concrete, are characterized. Finally, a mix design process of fluid concrete with low cement content was proposed based on the preferred fresh and hardened properties of the concrete mixtures.

The increasing pressure of traffic congestion on socio-economic development has made the construction of cross-water transportation ever more crucial. The immersed tunnel method is among the most extensively employed. However, a critical challenge of the immersed tunnel technique is to ensure the compactness and stability of concrete during the casting process. Conventional laboratory methods face challenges in achieving large-volume concrete casting, resulting in the notable waste of human and material resources. Hence, this study employs a simulation approach to investigate the casting parameters and the fresh properties of concrete, exploring their impacts on concrete stability and compactness. The results indicate that when the surface tension of concrete exceeds 0.03 N/m, and the yield stress and plastic viscosity are 50 Pa and 50 Pa·s, respectively, the concrete exhibits excellent casting compactness. A design incorporating three large and six small outlets, paired with a casting speed of 3 cm/s, achieves superior compactness. Additionally, when the yield stress of concrete exceeds 3 Pa, there is no segregation of aggregates. In cases where segregation occurs, the thixotropic property of the cement paste contributes to a significant reduction in the velocity of aggregate segregation.

A full-scale model experiment was carried out to research the effects of the Self-Compacting Concrete (SCC) construction technologies including SCC working performances, casting speeds and the arrangement methods of vent holes on the filling quality of the steel shell immersed tube tunnel compartments. The experimental results showed that in order to guarantee the bearing capacity of structure mainly by reducing the voids as many as possible to meet the related requriements of Guide, the total area ratio of voids with the depth more than 5.0 mm in the whole top surface of compartments must be less than 5%, the initial slump flow of SCC should be improved to 670±50 mm on account of the flowability loss caused by the particle migration of coarse aggregates during pumping and at least ten vent holes distributed around the top plate of a steel shell compartment parts of which were located on the both ends of T ribs far from the pouring holes are needed, simultaneously the appropriate pouring speeds of SCC are respectively 30 m³/h and 15 m³/h when the distances from fresh concrete level to compartment roof are more and less than 20 cm.

The early age water loss of cement based materials can cause plastic shrinkage cracking and crusting and peeling which will damage the application property and durability of concrete. Through emulsion system of small molecule super amphiphilic compound, stable and compact monomolecular film evaporation retardant on the surface of concrete is being prepared, which will inhibit the water evaporation of plastic concrete and then restrain plastic shrinkage cracking. In a harsh evaporation environment, the inhibition ratio of evaporation retardant was tested, which displays 24.1% and 59.6% inhibition ratio separately when sprayed once and twice while water nearly makes no difference to the evaporation rate. Surface appearance of cement paste and concrete applied evaporation retardant shows no crusting and peeling phenomenon. Evaporation retardant can inhibit the plastic shrinkage cracking of concrete and will reduce the total cracking area per unit area more than an order of magnitude. Evaporation retardant has been successfully applied in many key projects in China where the evaporation is high and makes a great contribution to the durability of concrete structures.