The bearing capacities of concrete-filled steel tubes are normally derived through experiments with small-scale specimens, but it is uncertain whether such derivations are appropriate for the much larger components used in practical engineering. This study therefore investigates the effect of different diameters (219, 426, 630, and 820 mm) on the axial compression of short concrete columns in steel (Q235) tubes. It is found that the peak nominal stress decreases with increasing specimen size and that the axial bearing capacity is determined by three separate components: the cylinder compressive strength of the concrete, the improvement in strength due to the confining effect of the steel tube, and the longitudinal strength of the steel tube. At peak load, increases in the specimen diameter reduce the hoop stresses in the steel tube, thereby reducing the strengthening effect of confinement. Vertical stress in the steel tube is increased with diameter; therefore, the axial bearing capacity of the steel tube is directly related to the specimen size. Size effect coefficients for these three aspects of bearing capacity are defined and used to develop a size-dependent model for predicting the axial bearing capacity of large, concrete-filled steel tubes. The model is then validated against experimental data.

In this article, a new retrofit method that the frame column is strengthened by asymmetric increased single lateral concrete section is proposed. The new added section could be ordinary reinforced concrete, bonded prestressed concrete, or unbonded prestressed concrete. The new added section could be connected with the column through anchor rebars, or prefabricated first, then be connected to the column through high strength bolts. The low cyclic reversed load experiment has been launched on understanding the seismic performance of one unretrofit specimen and six retrofitted specimens. The experiment has considered four main factors that is loading direction, material adopted on the new added section, different orders of unification of the retrofit column, and the process of prestress applying. Based on the experiment study, the characteristics including the failure mode, hysteresis loops behavior, bearing capacity, ductility, stiffness degradation, and energy dissipation ability are demonstrated. The bearing capacity calculation method of the retrofit column under compression–bending action is established. The calculation results and the experiment results show good agreement. Some suggestions for seismic design of the retrofit column are proposed. The experimental results show that the retrofitted column has good seismic performance and good retrofit effects. The research results could be references to the practical engineering design works.

Cracks often appear in concrete arch dams, due to the thermal stress and low tensile strength of early age concrete. There are three commonly used temperature controlling measures: controlling the casting temperature, burying cooling pipe, and protecting the surface. However, because of the difficulty to obtain accurate temperature and thermal stress field of the concrete, the rationality and economy of these measures are not assessed validly before and after construction. In this paper, a crack risk evaluation system for early age concrete is established, including distributed optical fiber temperature sensing (DTS), prediction of temperature and stress fields, and crack risk evaluation. Based on the DTS temperature data, the back-analysis method is applied to retrieve the thermal parameters of concrete. Then, the temperature and thermal stress of early age concrete are predicted using the reversed thermal parameters, as well as the laboratory test parameters. Finally, under the proposed cracking risk evaluation principle, the cracking risk level of each concrete block is given; the preliminary and later temperature controlling measures were recommended, respectively. The application of the proposed system in Xiluodu super high arch dam shows that this system works effectively for preventing cracks of early age concrete.