Boundary restraint effect of concrete member by steel bars during construction

•The constraints introduced in sequentially constructed structures are described.•The boundary conditions of structural components are simplified in numerical calculation model.•The importance of boundary constraint for early age cracking are discussed. There is a risk of early age cracking for cast-in situ underground concrete construction, for example the excavated tunnel, that experience hydration, shrinkage, and temperature deformation and that are subject to external constraints and adjacent components generating tensile stresses. Quantitative determination of the boundary constraints is an important baseline to assess crack stresses and to achieve crack control measures. In this paper, the confining characteristics of the boundaries of reinforced concrete and poured concrete are analysed. In particular, it deals with the constraints introduced in sequentially constructed structures by previously built parts. A spring element is used to replace all types of boundary restraints. Mechanical modelling of tangential stiffness and normal stiffness for boundary restraint is adopted to establish the restraint stiffness calculation model and is verified using a back analysis. The influencing factors of each parameter are studied and analysed. It is shown that the anchorage length and the distance of the steel bar are the main influencing factors of the boundary constraint compared to the age of the concrete and other factors. Using the results of this study, it is possible to simplify the boundary conditions of adjacent members during numerical simulations and to accurately describe the early age performance of concrete structure that provide the basis for controlling the early age cracking of underground concrete structures.