Introduction
The climatic features of plateau regions include low atmospheric pressure, large temperature variation, and intense radiation, presenting numerous challenges for concrete structures in these areas during the service life. The substantial diurnal temperature fluctuations expose concrete structures to the risk of cracking over prolonged temperature cycles, particularly notable in complex facade structures like bridge piers, where cracks often intersect and spread across the surface. In the plateau with significant temperature variations, the surface cracks that occur in bridge pier concrete do not directly impact the structural load-bearing capacity. However, if surface damage is allowed to progress, this deterioration accelerates the penetration of moisture and CO2, among other corrosive agents, into the interior of the concrete, thereby causing the durability of the concrete structure to fail.This study focuses on typical bridge pier concrete in plateau regions with significant temperature variations, employing finite element analysis and secondary development techniques to establish a temperature fatigue damage model for bridge pier concrete under such conditions. By incorporating cohesive elements to simulate the cracking behavior of bridge pier concrete under long-term temperature cycling, this research elucidates features such as the initial crack time and temperature gradient distribution in plateau regions with significant temperature variations. Furthermore, through an analysis of the fatigue stress evolution patterns and energy accumulation processes of bridge pier concrete under temperature cycling, this study aims to uncover its cracking mechanisms.
Methods
Based on the cracking mechanism of bridge piers in the high plateau environment with significant temperature variations, this study establishes a finite element model for a dual-lane circular bridge pier. The model dimensions are 7.4 m (length) × 3.0 m (width) × 9.0 m (height). The mesh consists of hexahedral elements, with element types C3D8R and cohesive elements embedded along the mesh boundaries. The concrete material properties for the bridge pier include an elastic modulus of 42 GPa, tensile strength of 2.5 MPa, poisson's ratio of 0.2, linear expansion coefficient of 1.5×10–5, density of 2 420 kg/m3, and specific heat capacity of 1 100 J/(kg·℃). Diurnal temperature variations are set at 20–40, 20–60 ℃, and 20–80 ℃, respectively. Additionally, considering the difference between the sunny and shaded sides of the bridge pier concrete, a temperature variation of 10 ℃ is defined for the shaded side. The bottom of the bridge pier concrete is fully constrained, while the top is subjected to an upper load of 9 000 kN. Importantly, a threshold value is introduced to determine whether fatigue effects on concrete need to be considered during alternating diurnal temperature cycles.
Results and discussion
Intense solar radiation and longer hours of sunshine in plateau regions lead to the appearance of randomly distributed cracks with widths less than 0.5 mm in bridge piers after 1–2 years of normal usage. The cracking mechanism of bridge pier concrete in plateau environments is as follows: the bridge pier concrete surface heats up to 50 ℃ due to thermal radiation and air convection. Heat is then transferred internally through a heat transfer process consistent with Fourier's law. The temperature gradients generated inside the bridge pier concrete, due to its low thermal conductivity, create temperature fatigue stresses, ultimately resulting in surface cracking under the influence of temperature fatigue stress.Based on finite element analysis, this study demonstrates that in plateau environments with significant temperature variations, bridge pier concrete tends to initiate cracking from the bottom and gradually extend upwards. Additionally, it is shown that the alternation between day and night results in a more complex internal temperature field within the bridge pier concrete. It is noteworthy that as the diurnal temperature variation increases, the rate of damage accumulation on the surface of the bridge pier concrete gradually increases. Furthermore, the plastic dissipation energy generated in the bridge pier concrete over the same period also increases with the rise in diurnal temperature variation, thereby revealing the cracking mechanism of the bridge pier concrete.
Conclusions
This paper takes typical bridge piers in high plateau regions as an example and, through theoretical analysis, reveals the complete process of "thermal response-stress generation-fatigue damage accumulation" cracking mechanism of bridge piers in environments with significant temperature variations. Based on the cracking principles of bridge piers in such environments, a fatigue damage model under temperature fatigue is established using finite element simulation and secondary development techniques. Concurrently, cohesive elements are utilized to simulate the cracking behavior of bridge piers under prolonged temperature cycling. Subsequently, it is demonstrated that in environments with significant temperature variations, cracks in bridge piers extend upwards from the bottom according to a certain development pattern. Moreover, with the increase in temperature variation on the bridge pier surface, both the initial crack time and damage accumulation rate gradually accelerate.