【作者】 王建刚；

【导师】 张金喜；

【作者基本信息】 北京工业大学 ， 交通运输工程， 2020， 博士

【摘要】 水泥混凝土材料广泛应用于道路、桥梁、建筑、港口码头等基础设施领域,往往经受各种环境因素的侵蚀劣化作用而造成其耐久性失效。与之相似,再生混凝土由于再生骨料的特殊性而具有比普通混凝土耐久性能影响因素更多、耐久性能更差的特点。因此,本研究系统探讨多种环境因素耦合作用下的再生混凝土耐久性能衰变规律及劣化机理,并通过骨料强化来提升再生骨料品质及再生混凝土耐久性能。本研究在对在役混凝土桥梁调查分析基础上,选择碳化、干湿循环、冻融循环等环境因素,研究5种类型再生混凝土分别在单一碳化、单一干湿、单一冻融、“碳化-冻融”复合作用、“冻融-碳化”复合作用、“碳化+冻融”交替作用、“碳化+干湿+冻融”交替作用下的耐久性能,通过测试抗压强度、动弹性模量、氯离子迁移系数、碳化深度、质量变化率、吸水率等宏观指标,以及微观孔隙参数、微观形貌等,探讨了多种环境因素耦合作用下的耐久性能衰变规律及劣化机理;在此基础上,研究骨料强化对再生混凝土耐久性的改善机理,并构建再生骨料强化效果的评价体系。主要研究结论如下:“碳化-冻融”复合作用下,碳化可提高再生混凝土的抗冻性能;“冻融-碳化”复合作用下,碳化可使再生混凝土力学性能及抗氯离子渗透性能提升,而冻融可使该提升效果近似线性关系衰减。相同碳化时间与冻融次数下,再生混凝土在“碳化-冻融”复合作用下的力学性能及抗氯离子渗透性能要略优于“冻融-碳化”复合作用,碳化对冻融的影响要小于冻融对碳化的影响。冻融循环加速了再生混凝土的劣化,为CO₂气体的渗入提供了有利条件,冻融损伤越严重,碳化速率越快。“碳化+冻融”作用下再生混凝土抗压强度大于单一冻融作用下的强度,但累计交替超过35 d时,前者小于后者;而交替作用对内部损伤、氯离子迁移系数、碳化深度的劣化效果均大于单一冻融或单一碳化。增大再生骨料掺量与掺加矿物掺合料均降低再生混凝土在多因素交替作用下的耐久性能,交替作用使再生混凝土劣化失效的根源是内部微裂纹的扩展与贯通。“碳化+干湿+冻融”作用下,质量损失率不宜作为再生混凝土耐久性的评价指标,而吸水率与抗压强度、氯离子迁移系数具有良好的相关性,可以用于评价恶劣环境因素作用下再生混凝土的耐久性能。碳化、干湿作用可增大孔隙迂曲度,而冻融、2因素耦合作用、3因素耦合作用均可减小孔隙迂曲度。孔隙迂曲度越大,孔隙结构越复杂,再生混凝土耐久性能越好。基于孔隙迂曲度、大孔百分含量指标,提出了孔隙综合指数的概念,并建立了复杂环境因素作用下再生混凝土抗压强度与氯离子迁移系数预测模型。基于BP神经网络,综合考虑碳化龄期、干湿循环次数、冻融循环次数、再生骨料掺量、矿物掺合料掺量等的影响,建立了再生混凝土在恶劣环境条件下的抗压强度、氯离子迁移系数预测模型,且模型精度较高,可以对再生混凝土耐久性能进行预测和评价。再生骨料强化方面,“碳化”处理显著提升再生混凝土的力学性能与抗干缩性能,而“裹浆”处理大幅改善再生混凝土的抗氯离子渗透性能;不同骨料处理方法对界面起到强化作用的机理不同,“碳化”改善旧界面性能大于新界面,而“裹浆”刚好相反;旧界面强度与抗压强度、动弹性模量与之间均存在良好的相关性,而不同处理后的新界面对氯离子迁移系数有不可忽视的影响。同时,本研究基于“碳化”、“裹浆”分别提出的两种优化方法可进一步提升再生骨料质量与再生混凝土性能。此外,综合考虑再生骨料与再生混凝土性能,建立了骨料强化效果的多指标评价体系。本研究对再生混凝土在复杂环境因素作用下的耐久性评价具有重要参考价值,对再生混凝土耐久性设计具有一定的指导意义。同时,本研究提出的骨料强化改进方法具有实际的工程应用价值。更多还原

【Abstract】 Cement concrete is widely used in infrastructure fields such as roads,bridges,buildings,ports and wharves.It often suffers from erosion and degradation of various environmental factors,resulting in its durability failure.Similarly,due to the particularity of recycled aggregate,the durability of recycled concrete has more influencing factors than ordinary concrete,and the durability is worse.Therefore,this research systematically explores the decay law and degradation mechanism of the durability of recycled concrete under the coupling of multiple environmental factors,and improves the quality of recycled aggregate and the durability of recycled concrete through the method of aggregate strengthening.Based on the investigation and analysis of in-service concrete bridges,this study focused on environmental factors including carbonation,dry-wet cycle,and freeze-thaw cycle,and studied the durability of five types of recycled concrete in single carbonation,single dry-wet,single freeze-thaw,the combined action of carbonation and freeze-thaw（“carbonation-freeze-thaw”）,the combined action of freeze-thaw and carbonation（“freeze-thaw-carbonation”）,the alternating action of carbonation and freeze-thaw（“carbonation+freeze-thaw”）,and the alternating action of carbonation,dry-wet and freeze-thaw（“carbonation+dry-wet+freeze-thaw”）.By testing the compressive strength,dynamic modulus of elasticity,chloride migration coefficient,carbonation depth,mass change rate,water absorption and other macro indicators,as well as micro pore parameters and micro morphology,the decay rule and mechanism of durability of recycled concrete under the coupling effect of various environmental factors are discussed.On this basis,the mechanism of improving the durability of recycled concrete by studying the strengthening of recycled aggregates was studied,and an evaluation system of the strengthening effect of recycled aggregates was established.The main conclusions are as follows:Carbonation can improve the frost resistance of recycled concrete under the action of“carbonation-freeze-thaw”.Under the combined effect of“freeze-thaw-carbonation”,carbonation can improve the mechanical properties and resistance to chloride ion penetration of recycled concrete,and freeze-thaw can attenuate the improvement effect in approximately linear relationship.Under the same carbonation age and freeze-thaw times,the mechanical properties and chloride penetration resistance of recycled concrete under the combined action of“carbonation-freeze-thaw”are slightly better than that under the combined action of“freeze-thaw-carbonation”,and the effect of carbonation on freeze-thaw is smaller than that of freeze-thaw on carbonation.The freeze-thaw cycle accelerates the deterioration of the recycled concrete and provides favorable conditions for the infiltration of CO₂.The more severe the freeze-thaw damage,the faster the carbonation rate.Under the action of“carbonation+freeze-thaw”,the compressive strength of recycled concrete is greater than that under a single freeze-thaw action,but when the cumulative alternating age exceeds 35 days,the former is less than the latter.However,the degradation effects of alternating action on internal damage,chloride ion migration coefficient and carbonation depth are greater than that of single freeze-thaw or single carbonation.Increasing the content of recycled aggregate and adding the mineral admixtures can reduce the durability of recycled concrete under the action of multiple factors.The root cause of deterioration of recycled concrete under the action of alternation is the propagation and penetration of internal microcracks.Under the action of“carbonation+dry-wet+freeze-thaw”,the mass loss rate should not be used as the durability evaluation indicator of recycled concrete,while the water absorption rate has a good correlation with compressive strength and chloride ion migration coefficient,which can be used to evaluate the durability of recycled concrete under the action of harsh environmental factors.Carbonation and dry-wet can improve the pore tortuosity,while single freeze-thaw,“carbonation+freeze-thaw”and“carbonation+dry-wet+freeze-thaw”can sharply reduce the pore tortuosity.The larger the pore tortuosity and the more complex the pore structure,the better the durability of recycled concrete.Based on the index of pore tortuosity and percentage content of large pores,the concept of integrated pore index is put forward,and the prediction model of compressive strength and chloride ion migration coefficient of recycled concrete under complex environmental factors are established.Based on BP neural network and considering the influence of carbonation age,dry-wet cycles,freeze-thaw cycles,recycled aggregate content and mineral admixture content,a prediction model of compressive strength and chloride ion migration coefficient of recycled concrete under severe environmental conditions is established,which has high accuracy and can predict and evaluate the durability of recycled concrete.For the strengthening of recycled aggregate,“carbonation”treatment significantly improved the mechanical properties and the dry shrinkage resistance of the recycled concrete.However,‘‘slurry wrapping”treatment greatly enhanced the chloride ion penetration resistance.Different aggregate treatment methods had different mechanisms to strengthen the interface.The influence of‘‘carbonation”on the old interfacial zones（ITZ）was more significant than that on the new ITZ.A contrary influence of the‘‘slurry wrapping”on both old and new ITZs was observed.The old interface had an obvious effect on the compressive strength and dynamic elastic modulus,while the new interface with different treatments had a non-negligible influence on the chloride penetration resistance.Besides,in this study,two optimization methods were proposed based on the improvement of‘‘carbonation”and‘‘slurry wrapping”treatment,respectively.These optimized methods can further improve the quality of recycled aggregate and the performance of recycled concrete.Meanwhile,a multi-index evaluation system that considers the aggregate strengthening effect of recycled aggregate and recycled concrete properties was established.This study has important reference value for the durability evaluation of recycled concrete under the complex environmental factors,and also has certain guiding significance for the durability design of recycled concrete.更多还原