Temperature gradient monitoring and thermal evolution of a super mass concrete structure

Bridge anchorage core concrete, a typical mass-filling marine concrete structure, faces challenges in temperature change control and crack prevention due to its special shape, continuous casting, and complicated boundary.

Based on the mass-filling concrete of the Guangxi Longmen Bridge anchorage basement (58 606 m³), this paper conducts an online monitoring and analysis of the real thermal field and stress distribution according to the evolution mechanism of the concrete temperature gradient during the pouring period. This work includes developing a temperature gradient digital monitoring system to provide feedback on the deviation from the actual value and provide a basis for timely warning and dynamically adjusted accurate temperature control, proposing the cracking control gradient index as the space and time gradient indices (a dimensionless index), and reconstructing the temperature field to the evolution of the real thermal field base on the temperature measurements in concrete, which is of great importance for the cracking control of the concrete structure.

The main study results are as followed: (1) A major challenge in concrete cracking control was investigated according to complex structural properties, the continuous casting method, high temperature, high humidity, strong wind, and a high salt mist environment. (2) The monitoring data of the temperature gradient digital monitoring system indicated a certain difference in the temperature development in the center concrete and the area near the surface. The temperature in the concrete central area underwent a rapid increase and tended to be stable, stabilised temperature range of 53.60—54.50 ℃, and the temperature increase reached 88.16%—99.34% of the adiabatic temperature increase. The temperature near the concrete surface underwent a rapid increase and a slight decrease, peaking at 52.90 ℃. (3) The threshold values of the space gradient and time gradient indices were defined as -3.00—3.00 ℃/m and 0.002 h^-1·m^-1, respectively. The temperature gradient index met the threshold requirement, the horizontal and vertical spatial temperature gradients at the stable stage were -0.15—0.14 ℃/m and 0.29—1.08 ℃/m, respectively, and the time-temperature gradient was within 0.002 h^-1·m^-1. These results indicated that the concrete heat exchange process was performed as small temperature changes in time and space. (4) The temperature field reconstructed from the monitoring data revealed that the real temperature gradient characteristic of the mass-filling concrete and isotherms was dense near the pile foundation at 96 h, then gradually became sparse, and the time-temperature and space gradients gradually became uniform and remained uniform after 144 h. (5) The evolution of the real thermal field, from a nonuniform distribution to a uniform distribution, could be divided into three stages, i.e., thermal accumulation, thermal release, and thermal transfer. The concrete internal stress simulation indicated that the maximum tensile stress occurred at the stress concentration zone along the intersection of the circumferential pile foundation and was substantially affected by environmental temperature change. The maximum tensile stress value was 1 780.0 kPa, and the corresponding safety factor was 1.03, satisfying the design requirements.

A case study shows that the temperature gradient digital monitoring system successfully supports the dynamically adjusted temperature control and effectively controls the cracking risk. These study results can be used as a reference for the cracking control of similar projects.