Abstract: On the basis of testing the mechanical, crack resistance, deformation and thermal properties of box girder concrete (C50), B4Cast software was used to simulate and analyze the temperature, stress development law and cracking trend of concrete, and the anti-cracking effects of expansion agent and shrinkage-reducing agent on the box girder concrete were also studied. The results show that, the box girder concrete (C50) has poor crack resistance, and the total cracking area of the early slab is 310 mm²/m², and the crack time of ring is about 52 h. The maximum temperature difference between the box girder concrete and the external environment is 16 ℃ to 18 ℃, and the tensile stress of concrete surface layer reaches the maximum at both sides of the wing or the corner of bottom plate, which is about 0.8 MPa and 0.9 MPa. The addition of expansion agent or shrinkage-reducing agent has little effect on the workability, mechanical properties and thermal properties of C50 concrete. The mechanical properties of C50 concrete are slightly reduced by only adding shrinkage-reducing agent. The incorporation of expansion agent and shrinkage-reducing agent reduces the total cracking area of slab by 68% to 95% and delay the cracking time of ring by 105.5 h to 227.5 h. The combined mixing of expansion agent and shrinkage reducing agent reduces the shrinkage deformation of concrete by 63% to 78% and dry shrinkage by 38%. With the addition of expansion agent and shrinkage-reducing agent, the temperature peak of box girder concrete structure is reduced by 4 ℃, the maximum tensile stress is reduced by 52%, and the anti-cracking safety factor of concrete reaches 3.05, which significantly reduces the risk of cracking.

Key words: box girder, expansion agent, shrinkage-reducing agent, anti-cracking, simulation analysis, anti-cracking safety factor