荷载与腐蚀冻融耦合作用下再生混凝土耐久性能试验

Experiment on durability of recycled concrete under coupling multi-factors of load and corrosion freeze-thaw

现有的荷载与环境多因素研究大多是通过给试块设计安装一套夹具,然后拧紧螺丝或者用千斤顶对试块施加荷载(应力),再用此带着夹具的试块进耐久性试验,此方法操作困难、繁琐,极易发生应力损失,使研究的准确性急剧降低。该文提出了一种具有操作简便、控制精准等优点的耦合作用机制,采用先加载再腐蚀冻融,且以此循环作用的方式来模拟混凝土真实工作状态,并对普通混凝土及再生混凝土在多因素耦合作用下耐久性能进行研究。该文试验加载制度分别采用0、40%、70%应力水平的重复荷载与25次腐蚀冻融交替2次,并对此耦合作用下再生混凝土与普通混凝土进行了宏观和微细观试验研究。试验结果显示:在宏观方面,随着应力水平的从0增加至70%,再生混凝土和普通混凝土的抗压强度损失率分别增加27.2%和100%,再生混凝土在多因素耦合作用下的耐久性能表现更佳,荷载会使普通混凝土和再生混凝土的劣化速度加快;在微观方面,在腐蚀冻融50次后,再生混凝土和普通混凝土的显微硬度损失率分别为8.1%和23.8%,环境扫描电镜(environmental scanning electron microscope, ESEM)结果也可以看出界面过渡区是混凝土中较薄弱的环节,且在冻融循环作用前,天然粗骨料界面过渡区处黏结强度和塑性变形能力较强,冻融循环作用后,再生粗骨料界面过渡区处抗冻融腐蚀能力更好。该结果为普通混凝土耐久性改进及再生混凝土的推广应用提供理论参考。

Recycled aggregate concrete has obtained worldwide attention as an important part of sustainability.In the aspect of durability of recycled concrete under coupling multi-factors,a number of devices were designed and installed to investigate the durability of recycled concrete.In previous studies,the concrete specimen was tightened by the screws to apply a prestress with the jack,and then specimen was put in the devices for the durability test,which was difficult to operate and prone to stress loss.In this paper,in order to better research the durability of concrete under coupling multi-factors,we designed a coupling mechanism of alternated with loading and freeze-thaw cycles in salt-solution to replace the traditional mold loading to accurately simulate the simultaneous interaction of freezing thawing,corrosion and load.The coupling protocol was designed with repetitive load and 25 time’s freeze-thaw cycles alternately 2 times in the salt-solution,in which the maximum stress level(the applied stress to 28 days compressive strength)of repetitive load is 0,40%,70%.The specific operation was that specimens were loaded repetitively for the first time,then they were freeze-thawed in the salt-solution for 25 time’s cycles,after that,they were repetitively loaded again,and freeze-thawed in the salt-solution for another 25 time’s cycles.The experiment on compressive strength and microhardness of recycled concrete and natural concrete under coupling action was carried out.In the macroscopic test,compressive strength loss of each specimen was measured under the above mentioned coupling protocols.At the same time,the microhardness across the ITZs between the coarse aggregate and the cement matrix was measured before and after 50 freeze-thawing cycles for the control specimens.The macro test results showed that the stress level of the coupling action increased from 40%to 70%,the compressive strength loss of both recycled concrete and natural concrete increased from 1.8%and 71.3%to 27.2%and 100%respectively.It is implied that the durability of recycled concrete under coupling action is better than that of natural concrete,and the loading mechanism with high stress level will accelerate the deterioration of both recycled concrete and natural concrete.The micro test results showed that the microhardness values gradually increased with the increases of distance from the interface of natural aggregate and cement paste,the microhardness losses were 8.1%and 23.8%of the interface of both recycled aggregate and natural aggregate before and after the coupling action respectively.It can be seen that the interface transition zone was the weakest part of the concrete,and the decreasing rate of microhardness of the interface transition zone in natural aggregate specimens was faster than that of recycled aggregate after 50 time’s freeze-thaw cycles in salt-solution.The environmental scanning electron microscope(ESEM)test results indicated that the bond strength and plastic deformation ability of interface of natural aggregate are better than that of recycled aggregate before freeze-thaw cycles,while the ability to resist freeze-thaw at the interface transition zone of recycled aggregate was better than natural aggregate after freeze-thaw cycles.The related results can provide an insight into the long-term performance of recycled concrete subjected to harsh environmental conditions and thus to promote practical application of recycled aggregate concrete in the future.