膨胀剂和减缩剂对预制箱梁混凝土的防裂效果研究

测试C50箱梁混凝土的力学、抗裂、变形、热学性能,采用B4Cast软件仿真分析构件混凝土的温度、应力发展规律和开裂趋势,并研究膨胀剂、减缩剂对箱梁混凝土的防裂效果。结果表明:C50箱梁混凝土抗裂性较差,早期平板总开裂面积达310 mm²/m²,圆环开裂时间约为52 h;箱梁混凝土与外部环境最大温差为16~18 ℃,在翼板两侧和底板两侧拐角处混凝土表层拉应力达到最大,分别约为0.8 MPa、0.9 MPa。膨胀剂、减缩剂对C50混凝土拌合物工作性、力学性能、热学性能影响不大,仅单掺减缩剂会使混凝土力学性能略有降低。膨胀剂、减缩剂的掺入,可降低C50混凝土平板总开裂面积68%~95%,延后圆环开裂时间105.5~227.5 h;可使自生体积收缩变形减小63%~78%,干缩减小38%。膨胀剂与减缩剂复掺,可削减箱梁混凝土温峰达4 ℃,降低最大拉应力52%,混凝土抗裂安全系数可达3.05,开裂风险明显下降。

Anti-Cracking Effects of Expansion Agent and Shrinkage-Reducing Agent on Precast Box Girder Concrete

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.