Four-point bending tests were conducted to study the flexural toughness of nano-SiO2 and hybrid fiber reinforced epoxy resin cementitious repair materials (NF-ECRM),and toughness indices I5,I10 and I20 were selected to evaluate the influence of different contents of PVA-steel hybrid fibers on the flexural toughness of NF-ECRM. The results show that PVA-steel hybrid fibers can effectively improve the flexural toughness of NF-ECRM. When the NF-ECRM specimens are mixed with 1.2% steel fibers,the toughness indices I5,I10 and I20 first increase and then decrease with the increase in the content of PVA fibers,and reach the peak value as the content of PVA fibers is 0.9%. When the specimens are mixed with 0.9% PVA fibers,the toughness indices I5,I10 and I20 of the repair materials first increase and then decrease with the increase of steel fibers content,and the overall performance reaches the best when the steel fibers content is 1.2%. The optimal combination of specimens is 0.9% PVA fibers and 1.2% steel fibers. The research findings can be applied to dam repair projects,effectively enhancing the durability and impact resistance of dam structures and extending their service life.
四点弯曲试验依据Standard Test Method for Flexural Toughness and First Crack Strength of Fiber Reinforced Concrete(ASTM C1018)[20]中的试验方法进行,试验采用9组配合比,每组配合比使用3个尺寸为400 mm(长)×100 mm(宽)×100 mm(高)的棱柱体试件。试验所用的加载仪器为1 000 kN电液伺服万能试验机,荷载采集装置为量程50 kN、精度1 N的荷载传感器,位移采集装置为灵敏度0.35 mV/mm的位移计。试验开始前调整支座跨度为300 mm,将试件成型时的侧面作为承压面置于支座上,将位移计布置在试件的跨中两侧以测定跨中挠度,取其平均值作为结果;启动试验机对试件进行预压以消除分配梁与试件的间隙,将各测点初始值归零,然后开启试验,加载速率设为0.1 mm/min,试验过程中采用的荷载传感器与位移计同步采集荷载与跨中位移。试验现场图和示意图分别如图2和图3所示。
通过扫描电子显微镜(scanning electron microscope,SEM)测试分析对NF-ECRM的微观形貌进行观测,试验采用的仪器为北京中科科仪股份有限公司生产的扫描电镜,如图4所示。试验开始前,用切割机从试件上切出长度为10 mm的试样,观测面保留为自然断裂状态,其他面均切割平整。将试样放入无水乙醇中清洗,并在测试前1 d将试样放入烘箱中烘干,之后,将经过喷金处理的试样送入电镜的真空室进行观测。
目前关于弯曲韧性的评价方法有很多,其中应用最为广泛的包括美国标准Standard Test Method for Flexural Toughness and First Crack Strength of Fiber Reinforced Concrete (ASTM C1018)[20]、日本标准Method of Test for Flexural Strength and Flexural Toughness of Fiber Reinforced Concrete (JSCE-SF4)[21]和欧洲标准Test and Design Method of Steel Reinforced Concrete(RILEM TC 162-TDF)[22]。ASTM C1018标准将一定倍数初裂点的挠度作为终点挠度,将终点挠度与初裂挠度荷载-挠度曲线下面积的比值作为韧性指数。这一方法有明确的弹塑性力学依据,能够反映纤维对水泥基材料弯曲韧性的贡献。JSCE-SF4标准利用试件的挠度变形为L/150时荷载-挠度曲线下面积计算韧性指数(L为支座跨距)。该方法计算简单,避开了初裂点对计算结果的影响,但是L/150取值太小,无法正确反映纤维水泥基材料的实际工作状态。RILEM TC 162-TDF标准]利用荷载-挠度曲线屈服点前后的面积计算等效抗弯强度,并用于评价弯曲韧性。该方法明确了纤维对水泥基材料弯曲韧性的贡献,但是只适用于高掺量和高强的纤维水泥基材料。考虑到各种方法的优缺点,并结合本文试验结果,本试验采用ASTM C1018标准。
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