Early cracking risk analysis of large precast box girders based on thermo-hygro-mechanical coupling

Due to the combined effects of hydration heat, drying shrinkage, autogenous shrinkage, and external environmental conditions, large precast box girders are highly susceptible to early-age cracking, which can severely compromise the durability and service life of bridges. Research on early-age cracking of box girders remains limited, with most existing studies focusing predominantly on thermal stress while neglecting the effects of early-age shrinkage, which hinders the accurate analysis of crack causes and the development of effective prevention strategies. In this study, a computational approach was developed to simulate the coupled thermal, hygral, and mechanical behavior of large precast concrete box girders at an early age, integrating full-scale testing and numerical modeling. The evolution patterns of temperature, moisture content, strain development, and cracking risk during the early stages were systematically analyzed in detail. Research indicates that by incorporating the equivalent age of concrete and fully considering the spatiotemporal evolution of concrete material parameters, the early-age temperature field of box girders can be accurately simulated. The highest measured temperature reached 74.8℃, occurring at the center of the web at the end section. The maximum difference between simulated and measured peak temperatures was 3.78 %. At 100 h, the surface humidity of the box girder at the end and mid-span sections was recorded as 95.1 % and 93.2 %, respectively. The thinner the cross-sectional thickness of the girder, the more significant the decrease in surface humidity. Moreover, the early humidity gradient in box girders is mainly concentrated within a depth of 10 cm to 15 cm beneath the inner surface. The mid-span standard section of the girder exhibits a relatively high risk of cracking; the cracking risks at the top-web junction at the quarter and mid-span sections at 100 h are recorded as 1.28 and 1.23, respectively. The simulated cracking time as well as the high-risk cracking zones showed strong agreement with the experimental observations. Further analysis indicated that implementing internal moisture-curing measures in box girders can reduce the early-age stress at the junction of the top slab and web at the midspan section by 0.43 MPa, thereby decreasing the risk of early-age cracking by 16 %.