基于断裂力学和损伤理论的混凝土开裂模型

依据宏观断裂力学和细观损伤理论,研究混凝土断裂过程区对断裂韧度的影响.在双参数模型的基础上,将细观的损伤本构模型运用于描述宏观断裂过程区的应力分布,并建立改进的混凝土开裂模型.以力学实验为基础,采用ABAQUS分析软件对不同龄期、不同初始裂缝深度的混凝土梁开裂过程进行模拟.数值模拟结果表明,应力强度因子和裂缝尖端位移都随着龄期的增加而增加.但是在同一龄期,初始缝高比对应力强度因子和裂缝尖端位移的影响并不明显.采用改进模型进行的数值模拟结果与实验结果吻合良好,表明该开裂模型能准确描述混凝土的临界开裂,可以作为评价混凝土开裂与否的理论判据.     

钱塘江下沙海塘混凝土结构耐久性检测与分析

为了对已建钱塘江海塘混凝土结构的耐久性作一分析和评价,针对影响水工混凝土结构耐久性破坏（失效）的主要因素,根据钱塘江海塘下沙段所处的环境条件及混凝土结构形式等情况,制定出对裂缝、混凝土强度、碳化深度、氯离子含量以及抗渗性等的检测方案．对检测结果的分析表明,钱塘江海塘混凝土结构的耐久性良好,影响其耐久性的主要因素是表面裂缝和混凝土的抗渗性．     

Mechanical Properties and ITZ Microstructure of Recycled Aggregate Concrete Using Carbonated Recycled Coarse Aggregate

The effect of carbonation treatment and mixing method on the mechanical properties and interfacial transition zone (ITZ) properties of recycled aggregate concrete (RAC) was investigated. Properties of recycled concrete aggregate (RCA) were tested firstly. Then, five types of concretes were made and slump of fresh concrete was measured immediately after mixing. Compressive strength and splitting tensile strength of hardened concrete were measured at 28 d. Meanwhile, the microstructure of RAC was analyzed by backscattered electron (BSE) image. It was found that the water absorption ratio of carbonated recycled concrete aggregate (CRCA) was much lower when compared to the untreated RCA. Comparatively, the apparent density of CRCA was not significantly modified. The concrete strength results indicate that the mix CRAC-2 prepared with CRCA by adopting two-stage mixing approach shows the highest compressive strength value compared to the other mixes. The microstructural analysis demonstrate that the mix CRAC-2 has a much denser old ITZ than the untreated RAC because of the chemical reaction between CO₂ and the hydration products of RCA. This study confirms that the ITZ microstructure of RAC can be efficiently modified by carbonation treatment of RCA and encourages broadening the application of construction and demolition wastes.     

Effect of Isobutyl-triethoxy-silane Penetrative Protective Agent on the Carbonation Resistance of Concrete

Effect of isobutyl-triethoxy-silane penetrative protective agent on the carbonation resistance of the concrete was studied.The concrete specimens for the 28 d accelerated carbonation process were manufactured with w/c of 0.49 and 0.64,both in the presence and absence of silane and mineral admixture.The penetration of isobutyl-triethoxy-silane and the carbonation of concrete were investigated by penetration depth,carbonation depth,XRD,SEM,and pore size distribution.The results showed that concrete compactness played an important role in the silane penetration and carbonation resistance.Penetration depth of silane-treated concrete mainly depended on the compactness of the concrete,and could not remarkably change through the accelerated carbonation process.In the accelerated carbonation process,penetrative protective agent improved the carbonation resistance of the higher compactness concretes but accelerated the carbonization process of the lower compactness concretes.As penetrative protective agent penetrated along the external connectivity pores into concrete not filling the entire surface area,the inorganic film could not fully protect the Ca(OH)_2 phase from carbonation.After 28 d accelerated carbonation,fibrous hydration products disappeared and the surface holes decreased.Due to the formation of carbonized products,the porosity of the concrete surface decreased,especially in high-strength concrete.     

高性能混凝土早期开裂防治措施研究

早期开裂是当前混凝土施工应用中经常出现的质量通病,采取什么样的措施防止这种开裂成为当前研究热点.就掺入聚丙烯纤维、碳纤维、减缩剂及复合使用时混凝土的早期开裂、自收缩、抗渗性等进行了试验研究,并从扫描电镜照片进行微观结构分析.结果表明:纤维或减缩剂的加入都对混凝土早期开裂有一定抑制作用,但对混凝土抗压强度有一定降低.从综合性能看,各种措施对防止混凝土早期开裂的效果优劣次序为:减缩剂>减缩剂+聚丙烯纤维>碳纤维>聚丙烯纤维.     

Prediction of the residual strength for durability failure of concrete structure in acidic environments

According to the results of accelerated tests of acidification corrosion depth and compressive strength of concretes subjected to sulfuric acid environments, the acidification depth laws of concretes were predicted based on the grey system theory. Thus, the remaining compressive strength was calculated when the acidification depth reached the protection layer thickness of concrete structures, which indicates that the limit state of durability failure can be defined based on strength degradation, and the calculation process was illustrated by an example. The calculated results show that the remaining compressive strength values in the durability failure limit state for the concrete structures exposed to pH=2 and 3 sulfuric acid water environments and wet-dry cyclic sulfuric acid environment with pH=2 are 74%, 72%, and 80% of initial strength, respectively. The method provides references for the durability evaluation of concrete structure design under the acidic environments.     

配筋梯度混凝土梁极限承载力分析

采用高强高性能混凝土替换普通混凝土压区形成了配筋梯度混凝土受弯结构构件,该类新型结构构件可达到使相对受压区高度易于满足截面容许受压区高度的目的.结合拉压区为异强混凝土的配筋梯度混凝土梁试验,获得了极限压应变、极限变形以及纯弯段箍筋应变等,着重分析了承载能力极限状态下平截面假定的适用性以及两类混凝土的整体工作性能.提出了配筋梯度混凝土受弯结构构件正截面承载力计算方法,探索了该类梁塑性铰转动能力与曲率延性.为配筋梯度混凝土受弯结构构件承载能力极限状态计算提供了理论支持与试验依据.     

Simulation of Fatigue Stiffness Degradation in Prestressed Concrete Beams under Cyclic Loading

In order to investigate and research the fatigue cracking of prestressed concrete fatigue properties and loading and stiffness degeneration process,cyclic loading tests were carried out on six prestressed concrete beams and the stiffness degradation under fatigue was investigated. A simulation model of stiffness degradation is proposed based on the stiffness analysis of the fatigue-damaged section. The elastic modulus of damaged concrete and the effective residual area of steel were introduced as well as an adjusted three-stage concrete fatigue damage evolution model. The strip method was used to analyze concrete damage due to changing stress along the depth of the beam section. The simulation and test results were compared and a method of predicting fatigue deflection was presented based on the simulation model. The predicted results were compared with that of the neural network method. It is in good agreement for the simulation results with the test results. It is only less than 5% error for the simulation model which can reveal the two-stage degradation of prestressed concrete beams under cyclic loading. It is more precise for the simulation prediction method under proper conditions.     

沥青混凝土均匀性数字图像评价方法

为了实时、连续、定量评价沥青混凝土均匀性,以安徽省济祁高速砀山段为试验段,运用数字图像处理技术、统计学方法和单幅图像恢复物体表面形状原理对沥青混凝土摊铺状态和压实状态分别建立了基于四边静矩变异性和构造深度变异性的沥青混凝土均匀性数字图像评价方法及标准.以静矩变异系数和构造深度变异系数作为指标,评价沥青混凝土数字图像均匀性.为获取沥青混凝土表面准确的图像信息,规范了数字图像采集和预处理过程,并推荐了图像采集的光照强度和拍摄高度.研究结果表明:沥青混凝土均匀性数字图像评价方法切实可行、便捷高效,结果定量客观,较好地反映了沥青混凝土施工质量的均匀性.沥青混凝土摊铺均匀时,静矩变异系数区间范围为(0,1.5%];AC-13和AC-25沥青混凝土压实均匀时,数字构造深度变异系数区间范围分别为(0,4.0%]和(0,8.0%].以16等分图像面积区域作为沥青混凝土压实图像划分最优方法,拟合得到不同等分方式数字构造深度变异系数之间相关性方程,且相关性显著.     

早期养护时间对混凝土早期收缩的影响

目的研究早期养护时间对混凝土早期收缩的影响,并定量地阐明早期养护对减小收缩和抑制早期裂缝的重要性.方法以掺和不掺减水剂的4种混凝土为研究对象,采用自行设计的一套试验装置测量混凝土在不同早期养护时间下的早期收缩.结果不掺减水剂的混凝土,无论是始终敞开或全程密封养护,总的早期收缩变形都很小.而掺减水剂的混凝土,早期收缩显著增大,集中产生于初凝后的8h以内,且收缩率随水灰比减小急剧增大;在相同水灰比条件下,早期收缩随着早期养护时间的增加而显著减小,但在此之后养护对收缩的抑制作用逐渐减弱.结论对不掺减水剂混凝土,单从抑制收缩的角度来考虑,并不需要采取什么特别的早期养护措施.对掺减水剂混凝土,当不养护时,初凝后8 h内的收缩值远远超过混凝土开裂极限;而有效的早期养护能显著抑制这类混凝土早期收缩,因此,掺减水剂混凝土必须在初凝前开始及时养护,且确保初凝后持续8 h以上的养护时间.     

Time-dependent Early-age Behaviors of Concrete under Restrained Condition

To investigate the early-age behaviors of concrete under a restrained condition,a set of apparatus was developed.In this way,the tensile creep and other early-age properties can be investigated in depth.By measuring the modulus of elasticity of concrete,synchronous shrinkage of concrete and steel rings and free shrinkage of concrete,the deformations of concrete ring can be quantified respectively.The experimental results show the tensile stress in concrete is time-dependent,and the stress at cracking is much lower than the tensile strength at that age;the tensile creep plays an important role in relaxing the tensile stress and postponing the cracking of concrete.     

Properties and microstructures of full graded concrete containing varied impurity aggregate

We investigated mechanical properties of concretes made with impurity aggregates of different combinations. Besides the mechanisms were explored by EDS, CT, and hardness testing. The results showed that fully rust-stained and surface rust-stained sandstone aggregate had significant adverse impact on the compressive strength of concrete while sandstone aggregate had a much more obvious impact on the ultimate tension of concrete. Concrete crack was more prone to expand along surfaces and the micro-hardness of interfacial transition zone of different aggregates was ranked in decreasing trend as sandstone, slate, SR sandstone, marble, and FR sandstone. The cluster growth of long needle-like ettringite crystal and strong preferential growth trend of Ca(OH)₂ crystals would result in wider interfacial transition zone range of concretes made with fully rust-stained sandstone and marble aggregate, respectively. Therefore, the impurity aggregate content should be strictly controlled during aggregate selection.     

Influence of subsequent curing on water sorptivity and pore structure of steam-cured concrete

Steam-cured condition is found to cause larger porosity and worse properties of concrete compared with normal curing condition. For the sake of seeking effective measurements to eliminate this bad effect of steam-cured condition on concrete, the water sorptivity and pore structure of steam-cured concretes exposed to different subsequent curing conditions were investigated after steam-curing treatment. The capillary absorption coefficient and porosity of the corresponding concretes were analyzed, and their mechanisms were also discussed. The results indicate that water sorptivity and pore structure of steam-cured concrete are greatly influenced by the curing condition used in subsequent ages. Exposure steam-cured concrete to air condition has an obviously bad effect on its properties and microstructures. Adopting subsequent curing of immersing steam-cured concrete into about 20 °C water after steam curing period can significantly decrease its capillary absorption coefficient and porosity. Steam-cured concrete with 7 d water curing has minimum capillary absorption coefficient and total porosity. Its water sorptivity is decreased by 23% compared with standard curing concrete and the porosity is 9.6% lower. Moreover, the corresponding gradient of water sorptivity and porosity of steam-cured concrete both decrease, thus microstructure of concrete becomes more homogeneous.     

Performance degradation of the repeated recycled aggregate concrete with 70% replacement of three-generation recycled coarse aggregate

The feasibility of using different generations recycled coarse aggregate(RCA) on structural concrete was fully evaluated by studying the performance of the recycled coarse aggregates and their corresponding concretes, the different generations of RCA were recycled by following the repeated mode of ‘concrete-waste concrete-coarse aggregate-concrete'. Moreover, the focus was on ‘three generations' of repeated RCAs, the RCA was produced by crushing and regenerating the artificial accelerated degraded concrete, the process was designed to follow the nature degradation of the concrete with a coupling action of accelerated carbonation and bending load. The properties of x-generation(x=1, 2 or 3) of repeated RCA were systematically investigated and the compressive and splitting tensile strengths of relating structural concretes(with 70% replacement of x-generation of RCA) were studied accordingly. The results show a competent compressive and splitting tensile strength of 30 MPa at 28 th day of structural concretes with all generations of repeated RAC. And the gradual degraded performance of the repeated RCAs was observed with an increased numbers of repetition(123 generations), the overall performances of all repeated RCAs fulfill the Class Ⅲ according to Chinese Standards GB25177-2010. Our gained insight demonstrates a feasibility of using at least 3 generations of repeated RCA for the production of normal structural concrete.     

Evolution and characterization of damage of concrete under freeze-thaw cycles

To study the internal damage of concrete under freeze-thaw cycles, concrete strains were measured using embedded strain gauges. Residual strain and coefficients of freezing expansion (CFE) derived from strain-temperature curves were used to quantify the damage degree. The experimental results show that irreversible residual strain increases with the number of freeze-thaw cycles. After 50 cycles, residual strains of C20 and C35 concretes are 320με and 100με in water, and 120 με and 60 με in saline solution, respectively. In lower temperature range (-10 ℃ to -25 ℃) CFE of C20 and C35 concretes decrease by 9.82×10-6/K and 8.44×10-6/K in water, and 9.38×10-6/K and 5.47×10-6/K in saline solution, respectively. Both residual strains and CFEs indicate that during the first 50 freeze-thaw cycles, the internal damage of concrete in saline solution is less than that of concrete in water. Thus residual strain and CFE can be used to measure the frost damage of concrete.     

酸雨腐蚀对混凝土材料断裂特性的影响

为研究酸雨腐蚀环境下混凝土断裂性能的劣化情况而进行混凝土的实验室加速腐蚀实验,并通过三点弯曲试验及电测法获得了不同腐蚀程度混凝土的起裂荷载、最大荷载以及试件的弹性模量和断裂韧度的劣化情况,同时借助电镜扫描获取腐蚀混凝土裂尖的劣化样貌。在模拟酸雨环境下,混凝土裂纹尖端的腐蚀深度随着侵蚀时间的持续而逐渐增长,裂纹尖端混凝土将发生结构性的损伤,混凝土材料松弛并产生微裂隙,这将直接影响混凝土裂尖的抗裂能力。与水中浸泡试件相比,腐蚀试件的抗裂能力发生显著的退化;在不同pH值的模拟酸雨环境中,试件的起裂荷载与最大荷载都随着腐蚀时间的增加而呈现先增长而后降低的趋势;随着腐蚀程度的增加,起裂荷载则会加速劣化。混凝土断裂韧度随腐蚀时间先增大后减小;溶液pH值越低,断裂韧度的增长周期越短,最终的损失也越大。H⁺和SO₄^2-使裂尖混凝土发生复杂的物理化学变化,直接弱化混凝土的抗裂能力。混凝土断裂韧度劣化率随裂尖腐蚀深度呈现出抛物线式的发展态势。     

自密实再生混凝土氯离子渗透性能时变规律

为了研究暴露时间对自密实再生骨料混凝土(RA-SCC)抗氯离子渗透性能的影响,并比较其与天然骨料混凝土(NA-C)和自密实天然骨料混凝土(NA-SCC)的抗氯离子渗透性能的差异,将三种混凝土在NaCl溶液中进行浸泡并测定其自由氯离子扩散系数和表面氯离子浓度.结果表明:三种混凝土的氯离子扩散系数随暴露时间的衰减规律符合幂函数变化规律,当混凝土强度等级提高时,RA-SCC表面氯离子扩散系数的时间依赖指数增大,这点与NA-SCC相同而与NA-C相反;三种混凝土的表面氯离子浓度随暴露时间增长规律同样遵循幂函数变化规律,当暴露时间超过60 d时,混凝土表面的氯离子浓度增长不再显著.     

Durability of concrete subjected to the combined actions of flexural stress,freeze-thaw cycles and bittern solutions

Freeze-thaw durabilities of three types of concretesnormal portland cement concrete （OPC）, high strength concrete （HSC） and steel fiber reinforced high strength concrete （SFRHSC） were systemically investigated under the attacks of chemical solution, and combination of external flexural stress and chemical solution. Four kinds of bitterns from salt lakes in Sinkiang, Qinghai, Inner Mongolia and Tibet provinces of China were used as chemical attack solutions. The relative dynamic modulus （RDM） was used as an index for evaluating the damage degree during the course of chemical attack and stress corrosion. The experimental results show that the freeze-thaw durability of concrete is visibly reduced in the present of the flexural stress, i e, stress accelerates the damage process. In order to quantify the stress accelerated effect, a stress accelerating coefficient was proposed. The stress accelerating coefficient is closely related with the types of bitterns and the numbers of freeze-thaw cycles is. The more numbers of freeze-thaw cycles is, the greater the stress accelerating coefficient for various concretes will be. In addition, there also exists a critical ratio of external stress to the maximum flexural stress. If the stress ratio exceeds the critical one, the freeze-thaw durability of various concretes will be greatly decreased compared to the responding concretes without applied stress. The critical stress ratio of OPC, HSC and SFRHSC is 0.30, 0.40 and 0.40, respectively, indicating that HSC and SFRHSC have advantages over OPC and are suitable to use in the bittern erosion regions.     

Utilization of waste vehicle tires in concrete and its effect on the corrosion behavior of reinforcing steels

The mechanical and physical properties of concrete specimens obtained from replacing natural coarse aggregate with waste vehi-cle rubber tires at levels of 2vol%, 5vol%, 7vol%, and 10vol% were studied, and the corrosion behavior of reinforcing steels was investigated in these specimens. Corrosion rates were determined by measuring the galvanic current between steel-reinforced concrete specimens both with and without chloride addition. The change in electrode potential of reinforcing steels in these concrete specimens was measured daily for a period of 60 d in accordance with the testing method in ASTM C876. The results show that the use of waste vehicle tires in concrete in-stead of coarse aggregate decreases the mechanical strength of the specimens, and increases the corrosion rates of the reinforcing steels em-bedded in the concretes.     

Time-dependent Early-age Behaviors of Conerete under Restrained Condition

To investigate the early-age behaviors of concrete under a restrained condition, a set of apparatus was developed. In this way, the tensile creep and other early-age properties can be investigated in depth. By measuring the modulus of elasticity of concrete, synchronous shrinkage of concrete and steel rings and free shrinkage of concrete, the deformations of concrete ring can be quantified respectively. The experimental results show the tensile stress in concrete is time-dependent, and the stress at cracking is much lower than the tensile strength at that age; the tensile creep plays an important role in relaxing the tensile stress and postponing the cracking of concrete.