荷载裂缝和养护龄期对混凝土内氯离子传输影响研究

为了分析实际荷载和氯盐侵蚀环境共同作用下开裂混凝土结构的耐久性能,通过弯曲开裂和预压开裂两种方式制备出两类开裂混凝土构件,采用RCM试验法研究了开裂混凝土构件中氯离子的传输特性,并深入分析了荷载裂缝和养护龄期对氯离子传输过程的影响.研究结果表明,裂缝的存在加剧了氯离子在裂缝位置及其周围10 mm范围混凝土内的传输进程,且裂缝对氯离子传输的影响范围随着裂缝宽度的增大而增大;从受弯开裂构件中得到的弯曲开裂试件,当最大裂缝宽度小于0.3 mm时仍可采用RCM法进行传输性能分析,而预压开裂试件的裂缝宽度大于0.12 mm时,已不宜采用RCM法进行分析;随着养护龄期的增加,预压开裂混凝土试件的氯离子传输能力在裂缝周围略有降低,在离裂缝距离超过10 mm后,氯离子传输能力降低明显.综合现有研究结果得出,利用裂缝宽度w的二次多项式函数表征两类开裂混凝土构件的氯离子扩散系数劣化特性是可行的.     

基于不同保护层厚度下混凝土构件角区锈胀裂缝宽度与钢筋锈蚀率的关系研究

采用人工通电加速锈蚀法来获得6个不同锈蚀结果的锈蚀钢筋混凝土构件,在锈蚀钢筋混凝土试件的三个锈蚀表面划分网格,用裂缝观测仪对裂缝进行观测并记录裂缝的宽度值.将钢筋混凝土试件进行破型处理,获得对应裂缝位置处的钢筋锈蚀率.针对试验获得的保护层为10 mm、20 mm、30 mm下混凝土构件角部锈胀裂缝和锈蚀率的基本数据,通过数据拟合发现不同保护层厚度下锈蚀率和锈胀裂缝宽度存在明显的线性关系.最后,通过统计回归分析,将混凝土构件角部钢筋锈蚀率与裂缝宽度及保护层厚度之间的关系整合为一个回归方程,以方便其在实际工程中的应用.     

带裂缝混凝土氯离子扩散及碳化特性试验研究

针对带裂缝混凝土的氯离子扩散作用及碳化特性进行了试验分析,利用无损裂缝制备装置,预制了不同深度、厚度及不同间距的带裂缝混凝土试件,分别进行了氯盐溶液浸泡试验及快速碳化试验,并对二者的耦合作用进行了研究.得到了裂缝宽度、深度、裂缝间距及水灰比等因素对带裂缝混凝土试件氯离子扩散作用和碳化特性的影响,且随着碳化时间的增加,带裂缝混凝土试件的氯离子扩散深度减小,氯离子扩散对混凝土的抗碳化性能起到了一定的提高作用.     

再生粗骨料对粉煤灰基地聚物混凝土碳化性能的影响

本研究分别利用0％、50％以及100％的再生粗骨料取代天然粗骨料,制备了碱激发粉煤灰基地聚物再生混凝土和普通再生混凝土.测试和分析了试件在碳化3d、7d、14 d和28 d时的碳化深度、抗压强度的变化.采用研磨法,进一步测试分析了试件表面pH值的变化,同时使用扫描电子显微镜(SEM)观察了碳化反应前后微观结构的改变.研究结果表明,相比于普通混凝土,尽管粉煤灰基地聚物混凝土的微观结构更均匀密实,但是其抗压强度受碳化影响更明显,整体呈现下降趋势;再生粗骨料对两种混凝土的抗碳化性能都将产生不利影响,但对地聚物混凝土的抗压强度影响较小.     

考虑冻融和应力影响的混凝土构件碳化试验研究

为分析冻融循环和应力水平对混凝土结构碳化深度的影响,设计并实施了不同应力钢筋混凝土试件纯碳化试验和冻融-碳化交替试验.以Papadakis的经典碳化理论模型为基础,结合冻融和应力对混凝土CO2扩散性能的影响,以混凝土中可碳化物质在碳化过程中各物质质量平衡为条件建立微分方程,使用COMSOL Multiphysics建立考虑冻融和弯压应力的钢筋混凝土试件碳化深度模型,比较数值模拟结果与试验结果.结果 表明,压应力和冻融分别会提高和降低混凝土的抗碳化能力;当混凝土受力状态由受压变为受拉时,应力-冻融耦合对混凝土的碳化由抑制转换为促进作用,确定不同冻融条件及混凝土类型下的冻融临界应力,有助于对冻融环境下混凝土碳化进行有效防护;基于COMSOL Multiphysics的有限元分析结果和实测结果吻合较好,可为冻融和应力下混凝土构件碳化深度预测分析提供依据.     

高强再生钢筋混凝土梁抗弯力学性能分析

制备了2组不同钢筋级别和再生骨料取代率的高强钢筋混凝土梁,进行了钢筋混凝土梁的三点弯曲试验,测试了钢筋应变、梁挠度、裂缝宽度等力学性能,分析了钢筋级别和再生骨料取代率对梁抗弯性能的影响规律。研究结果表明:(1)三点弯曲试验过程中,应变-荷载曲线可分为微小变形阶段、调整阶段、稳定承载阶段以及屈服阶段;(2)梁的最大裂缝宽度和挠度随竖向荷载的增大而呈现出指数式增大;(3)高强度钢筋可以有效提高混凝土梁的抗弯强度;(4)钢筋混凝土梁的抗弯性能随再生骨料取代率的增大而呈现出指数式减小。     

碳纤维复合材料棒材（C—BAR）增强混凝土梁的弯曲性能研究

本文针对碳纤维复合材料棒材（C－BAR）增强混凝土纯弯曲梁的裂缝及挠度等问题,通过对试件试验结果的分析和对比,给出了C－BAR增强混凝土受弯构件的平均裂缝间距、最大裂缝宽度和跨中最大挠度的计算公式,为工程设计和试验结果的预报提供了必要的手段。     

干湿循环作用下混凝土裂缝区域氯离子的传输状态

研究了干湿循环作用下混凝土裂缝区域的氯离子浓度分布和裂缝影响区域.制备了不同宽度横向裂缝的混凝土试件,采用干湿循环加速氯离子对混凝土的侵蚀,定期检测不同侵蚀周期试件破型后裂缝侧面以及试件上表面至钢筋方向的氯离子浓度.试验结果表明:氯离子浓度随着传输深度增加而降低,在20 mm处基本稳定.随着干湿循环的进行,在20个干湿循环周期后,裂缝宽度为0.1~0.5 mm的试件,裂缝侧面30 mm处氯离子浓度都大于0.1%,钢筋在裂缝左右30 mm范围内有锈蚀的危险.并且由于二维传输的互相影响,裂缝宽度越大,相同传输深度氯离子浓度也越高.     

碱式硫酸镁水泥钢筋混凝土梁静力荷载和疲劳性能试验研究

为了研究碱式硫酸镁水泥钢筋混凝土梁的静力和疲劳性能,分别制备了C30、C40和C50三种强度等级的碱式硫酸镁水泥(BMSC)和普通硅酸盐水泥(PO·C)钢筋混凝土梁试件.试验结果表明,钢筋混凝土梁试件跨中挠度随着静力荷载的增加而增加.C40(BMSC)和C50(BMSC)的极限承载力分别为33 kN和38 kN,相比C40(PO·C)、C50(PO·C)的19 kN和21 kN分别提高了73.7％和81％.说明,高强度等级的BMSC试件具有较高的承载能力.在疲劳荷载的作用下,试件挠度值随着疲劳次数的增加呈下降趋势.高强度等级的BMSC试件疲劳循环寿命要多于PO·C试件,且强度等级越高,越为明显.此外,BMSC试件跨中挠度小于PO·C试件.说明,BMSC试件的刚度较大,抵抗变形的能力好.BMSC试件裂缝宽度发展包括三个阶段:快速发展阶段、稳定阶段和破坏阶段.碱式硫酸镁水泥完全能够用于制作钢筋混凝土构件,且性能更加优异,本文为碱式硫酸镁水泥混凝土能够大规模应用于结构工程提供了重要理论依据.     

粘附砂浆含量对再生骨料混凝土抗碳化性能的影响

以再生粗骨料粘附砂浆含量为变量,配制C40再生骨料混凝土进行碳化对比试验,结合碳化模型以碳化深度达到钢筋表面为准则,探究粘附砂浆含量对再生骨料混凝土抗碳化性能的影响.结果 表明,碳化深度与碳化速率均随粘附砂浆含量的增加而增大,且碳化深度增幅较大,粘附砂浆含量处于35％ ～45％(质量分数)时,其碳化深度可以满足一般环境下混凝土结构设计使用年限30 a、50 a和100 a的抗碳化性能要求.     

Influence of crack initiation length on fracture behaviors of reinforced concrete

For the purpose of discussing the fracture features of high strength reinforced concrete, under the conditions of the fundamental assumptions to reinforced concrete, the calculating process of fracture parameters for three point bending beam of high strength reinforced concrete is given, and through a total of 16 specimens with the same sizes, with the different initial crack length 40 mm to 100 mm on three point bending beam of high strength reinforced concrete, it is performed to examine the calculation method. The results show that the fracture features of high strength reinforced concrete are different from normal concrete, both the initiation toughness and the unstable toughness of high strength reinforced concrete gradually increase with the crack-to-depth ratios, and the ratios of initial load to maximum load are different depending on the initial crack length, the bigger the crack-to-depth ratios, the lower the ratio, and the ductility is best to the high strength reinforced concrete.     

Effect of carbon nanotubes grown temperature on the fracture behavior of carbon fiber reinforced magnesium matrix composites: Interlaminar shear strength and tensile strength

The design of an interfacial structure is particularly important for load transfer in composites. In this paper, different amounts of carbon nanotubes (CNTs) were grafted onto the carbon fiber (CF) surface by adjusting grown temperature using injection chemical vapor deposition (ICVD). The prepared CF preform grafted with CNTs (CNTs-CF) were used to reinforce magnesium alloy by squeeze casting process. The microstructures were analyzed by means of optical microscope (OM) and scanning electron microscope (SEM), and the interlaminar shear strength (ILSS) and tensile strength of the composites were determined by double-notch shear test and tensile test. The results indicated that moderate ILSS was more conducive to improving the tensile properties of carbon fiber reinforced magnesium matrix (C_f/Mg) composites. Compared with C_f/Mg, the tensile strength of composite with CNTs increased by about 80%. For C_f/Mg composites grafted with CNTs, CNTs had the effects of delaying crack propagation and increasing energy consumption by the pull-out and bridging mechanism, which were the main reasons for improving the strength. The analysis of shear fracture surface showed that the crack propagation path can be optimized by adjusting the amounts of grafted CNTs. The presence of CNTs affects the stress distribution and consequently the crack initiation as well as the crack propagation.     

Effect of Interfacial Shear Strength on Crack-Fiber Interaction Behavior in Ceramic Matrix Composites

Zircon matrix composites uniaxially reinforced with SiC fibers were fabricated with different interfacial properties by changing the fiber coatings. The phenomenon of crack interaction with fibers and/or fiber coatings and its dependence on the interfacial properties were studied using a microindentation technique. The influence of the fiber orientation relative to the crack extension direction on the crack-fiber interaction was also investigated. Crack deflection was observed at the fiber-matrix interface in composites having low interfacial shear strength, and the crack deflection was mostly single-sided, but double-sided deflection was also observed. Crack penetration into the fiber occurred in composites with high values of the interfacial shear strength. These observations were in general agreement with the theoretical predictions of the crack deflection behavior based on the bimaterial interfaces in ceramic composites, but additional observations were made on crack deflection at multiple fiber-matrix interfaces.     

A theory for the flexural strength of steel fiber reinforced concrete

A theory is presented to predict the flexural tensile strength of concrete reinforced with short, discontinuous steel fibers randomly oriented and uniformly dispersed in a cement-based matrix. The theory is based on a dual criterion of crack control and composite mechanics. The first crack in the fibrous composite occurs due to bond slip. The fracture process consists of progressive debonding of fibers during which slow crack propagation occurs. Final failure occurs due to unstable crack propagation when fibers pull out and the interfacial shear stress reaches the ultimate bond strength. The theory is supported by test data on fiber reinforced concrete, mortar and paste.     

Toughening of Ceramics through Crack Bridging by Ductile Particles

The fracture problem for a brittle matrix reinforced by ductile particles is considered. In the usual manner it is assumed that the crack surface bridging forces provided by the unbroken particles improve the fracture toughness of the matrix. Depending on the relative strength of the interfacial bonding between the matrix and the particles, two particle force models are introduced, namely, a force that is independent of the crack opening displacement (δ) and a force that is a highly non-linear function of δ. The problem is studied for a penny-shaped or plane strain crack in an infinite medium and for a surface crack in a semi-infinite medium under plane strain conditions. The toughness improvement in the matrix is shown to depend on a dimensionless bimaterial constant representing the inherent toughness of the matrix and the yield behavior of the particles. The effective toughness of the composite medium is calculated as a function of the crack size and the bimaterial constant.     

基于细观模型的纤维增强树脂基复合材料的裂纹与强度关系研究

纤维增强树脂基复合材料是一类高比强度、高比刚度的结构材料。开展该材料的裂纹与强度关系研究具有重要的科学意义和工程价值。采用细观力学模型和有限元方法,分析Ⅰ型裂纹长度和裂尖位置对复合材料应力状态和应力强度因子的影响,进而获得启裂载荷与裂纹长度和裂尖位置的关系。     

Fracture behavior of an epoxy/aluminum interface reinforced by sol–gel coatings

The strengths of epoxy/aluminum joints reinforced with a zirconium-silicon based sol–gel adhesion promoter were investigated using an ADCB (Asymmetric Double Cantilever Beam) wedge test. The fracture energies and loci of failure of these joints were shown to depend upon the mixity of the normal and shear modes of stress acting at the crack. The ADCB geometry enabled the crack to propagate along the epoxy/aluminum interfaces so that the effect of surface pretreatment and the processing conditions of the adhesion promoter on adhesion strength could be directly evaluated. The dry strength of these joints depends on the thickness of the sol–gel film derived from different concentrations of the precursors. Thinner films are more fully crosslinked and thus give higher adhesion strengths than those obtained with thicker films. The differences in the wet strengths of the sol–gel reinforced joints for various surface pretreatments suggest that the sol–gel films are subject to moisture degradation with certain surface pretreatments. The loci of failure of many of these joints alternate between the sol–gel/aluminum and epoxy/sol–gel interfaces. This behavior is similar to that observed more generally in adhesively-bonded joints tested in DCB (Double Cantilever Beam) geometry. The brittle versus ductile behavior associated with the failure process reveals important information about how the sol–gel films affect the adhesion strength.     

Effect of Crystallites on Surface Damage and Fracture Behavior of a Glass-Ceramic

A study was conducted of the effect of crystallization on the fracture toughness, strength, and resistance to surface damage of glass-ceramic materials with a range of microstructures obtained by different heat treatments. The hardness indentation method was used as a quantitative tool to simulate mechanical surface damage. In the uncrystallized glass and in the glass-ceramic heat-treated to result in a uniform fine-grained structure, crack size increased monotonically with indentation load. In contrast, in the glass-ceramics heat-treated to result in a microstructure consisting of larger crystallites (a few micrometers) contained within a fine-grained matrix, a discontinuity in the crack size vs load curve presented evidence for crack-pinning at crack sizes which were a small multiple of the intercrystallite spacing. At the position of crack-pinning, the fracture toughness showed a discontinuous increase with increasing crack size that was attributed to crack deflection. The strength of the glass and fine-grained glass-ceramic measured in biaxial flexure decreased monotonically with indentation load. The strength at low values of indenter load of the glass-ceramic heat-treated to yield the coarser crystallites within the fine-grained matrix was independent of indentation load, indicating stable crack propagation prior to fast fracture. At the higher values of indenter load, the coarse-grained glass-ceramics exhibited a monotonic decrease in strength with increasing indentation load. The results of this study indicate that the strengthening observed on crystallization of a glass can be attributed to a combination of a decrease in flaw size achieved at a given mechanical surface treatment, an increase in fracture toughness, and a modification in the mode of crack propagation.     

Strengthening/toughening of 2D C/SiC composites by coating

SiC and SiC_w/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiC_w/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.     

不同弹性模量纤维混杂增强活性粉末混凝土的力学性能研究

为了提高活性粉末混凝土的韧性,通过掺杂不同弹性模量的纤维,制备了纤维增强混凝土。采用ASTMC1018韧性指数法,评价了增韧效果。结果表明:碳纤维能够在微观尺度上,减少混凝土中缺陷的数量,改善混凝土内部结构,增强、阻裂作用明显,基体强度较高。钢纤维在宏观尺度上,对于混凝土的阻裂作用明显,混凝土的延展性显著提高。混掺碳-钢纤维,虽然能提高基体的初裂强度,但是韧性却有所降低。