超高性能混凝土保护层厚度取值分析

Value analysis of UHPC cover thickness

从结构的安全性、适用性、耐久性、耐火性出发,分析了影响超高性能混凝土(UHPC)保护层厚度取值的主要因素,包括黏结强度、裂缝、挠度、碳化、冻融、氯离子侵蚀、硫酸盐侵蚀、耐火性。在安全性方面,基于变形钢筋黏结强度的理论计算方法,得到了UHPC构件的最小保护层厚度,并试验检验了计算结果。在适用性方面,考虑了钢纤维对混凝土构件挠度和裂缝的影响,经过验算得到了保护层厚度最低要求。在耐久性方面,分别考虑了碳化、冻融、氯离子侵蚀、硫酸盐侵蚀,借用已有公式计算出各类环境下的最小保护层厚度。在抵抗氯离子侵蚀方面,对不同环境进行了具体分析。此外,还对比分析了普通混凝土与UHPC的耐火性。最后,综合考虑安全性、适用性、耐久性、耐火性的要求,给出了UHPC保护层厚度取值的建议值。结果表明:当保护层不小于纵筋公称直径时,可保证UHPC构件的黏结强度,且最小保护层厚度可减小到普通混凝土的0.824倍以下;UHPC抵抗碳化和冻融作用的性能远优于普通混凝土,5 mm厚的保护层能够满足UHPC构件100年内使用需求;UHPC的导热能力高于普通混凝土,应该增加UHPC构件的构造措施来满足其防火要求。

From the perspective of safety,applicability,durability and fire resistance,the main factors affecting the value of thickness of ultra-high performance concrete(UHPC)cover was analyzed,including bond strength,crack,deflection,carbonization,freeze-thaw,chloride ion penetration,sulfate penetration and fire resistance.In terms of safety,the minimum cover thickness of UHPC components was calculated based on the theoretical calculation method of bond strength of deformed reinforcement,and the calculation results were verified by experiments.In terms of applicability,considering the influence of steel fiber on the deflections and cracks of concrete members,the minimum requirement of cover thickness can be obtained by checking calculation.In terms of durability,factors such as carbonation,freeze-thaw,chloride ion penetration and sulfate penetration were considered respectively,and the minimum cover thickness under various environments was calculated by using the existing formula.In terms of resistance to chloride ion penetration,different environments were analyzed in detail.In addition,the fire resistance of ordinary concrete and UHPC was compared and analyzed.Finally,considering the requirements of safety,applicability,durability and fire resistance,the recommended value of UHPC cover thickness was given.The results show that the bond strength of UHPC members can be guaranteed when the cover thickness is not less than the nominal diameter of longitudinal reinforcement,and the minimum cover thickness can be reduced to not more than 0.824 times of the ordinary concrete.The performance of UHPC in resisting carbonation and freeze-thaw is far better than that of ordinary concrete,and the cover of 5 mm thickness can meet the needs of UHPC members within 100 years.The thermal conductivity of UHPC is higher than that of ordinary concrete,and the structural measures of UHPC components should be added to meet its fire protection requirements.