Micromechanical investigation on size effect of tensile strength for recycled aggregate concrete using BFEM

In this paper, the validity and performance of base force element method (BFEM) based on potential energy principle was studied by some numerical examples. And the BFEM on damage mechanics is used to analyze the size effect on tensile strength for recycled aggregate concrete (RAC) at meso-level. The recycled aggregate concrete is taken as five-phase composites consisting of natural coarse aggregate, new mortar, new interfacial transition zone (ITZ), old mortar and old ITZ on meso-level. The random aggregate model is used to simulate the meso-structure of recycled aggregate concrete. The size effects of mechanical properties of RAC under uniaxial tensile loading are simulated using the BFEM on damage mechanics. The simulation results agree with the test results. This analysis method is the new way for investigating fracture mechanism and numerical simulation of mechanical properties for RAC.     

Influence of silane coupling agent on quality of interfacial transition zone between concrete substrate and repair materials

Silane coupling agent (SCA) was introduced as a modifying material to significantly improve the bond quality of the repaired interfacial transition zone. SCA aqueous solutions with various concentrations were used to coat the surfaces of a granite and of old concrete substrates before applying the repair materials. Both pull-off bond strength test and microstructure observation of the different repair interfacial layers were performed. The test results show that coating a concrete substrate with a SCA aqueous solution with an appropriate concentration can noticeably modify the microstructure of the interfacial transition zone, and therefore, significantly increase the bond strength.     

Effect of carbonation of modeled recycled coarse aggregate on the mechanical properties of modeled recycled aggregate concrete

The modeled recycled aggregate concrete (MRAC) which is an idealized model for the real recycled aggregate concrete (RAC) was used in this study. The MRCAs prepared with two types of old mortars were modified by an accelerated carbonation process. The effects of carbonation of MRCA on the micro-hardness of MRCA and the mechanical properties of MRAC were investigated. The results indicated that the micro-hardness of the old interfacial transition zone (ITZ) and the old mortar in the carbonated MRCAs was higher than that in the uncarbonated MRCAs, and the enhancement of the old ITZ was more significant than that of the old mortar. The compressive strength and modulus of MRACs increased when the carbonated MRCAs were utilized, and the improvement was more significant for MRAC prepared with a higher w/c. In addition, a numerical study was carried out and it showed that the improvement in strength by carbonation treatment was less obvious when the difference between the new and old mortar was larger.     

A New Method of Controlling Shrinkage Cracking in Repaired Concrete Structures Using an Interface Layer of Carbon Fiber Reinforced Cement Mortar

Bonding an overlay of new concrete onto the damaged concrete is a usual repair method. Because of the different shrinkage rate of the new and old concrete, restrained shrinkage cracks will appear in the new concrete. The cracks will reduce durability and strength of the repaired structure. A new repair method using an interface layer of carbon fiber reinforced cement mortar between new and old concrete was developed in this paper. The new method was found to be very effective in reducing shrinkage cracking of repaired beams and slabs. Comparing with normal repaired beams, the maximum observed width of the resulting cracks was decreased by up to 43%, the cumulative width was decreased by up to 78%, the cumulative length was decreased by up to 73%, and the total area of the cracks was decreased by up to 81%.     

新型聚合物水泥胶浆界面剂粘结性能及作用机理研究

采用新型聚合物水泥胶浆作为界面剂以提高新旧混凝土之间的粘结性能,通过拉拔粘结强度与劈裂抗拉粘结强度实验对5种不同类型的聚合物水泥胶浆界面剂的粘结性能进行了测试,并利用扫描电镜(SEM)分析研究了丁苯聚合物水泥胶浆的界面增强机理。实验结果表明,5种聚合物乳液中,丁苯聚合物水泥胶浆具有较好的拉拔粘结性能,当优选m(水泥)∶m(DB-1乳液)=3∶2时,其7d、28d拉拔粘结强度分别达到1.83MPa、2.41MPa,相比水泥净浆空白样分别提高了144%、96%;在劈裂抗拉粘结强度方面,水平方向浇筑时劈裂抗拉粘结强度相对较高,当聚合物水泥胶浆的优选m(水泥)∶m(DB-1乳液)=3∶2,水平浇筑时其28d劈拉粘结强度达到2.96MPa,明显高于不掺界面剂的试样以及掺加其它配比界面剂的混凝土试样;经过微观测试分析,丁苯DB-1聚合物水泥砂浆内部界面过渡区(ITZ)相比空白样明显致密,表明丁苯聚合物的加入有效填充了水泥基材料内部的宏观与微观缺陷,提高了界面过渡区的密实程度。     

The interfacial zone and bond strength between aggregates and cement pastes incorporating high volumes of fly ash

The weak transition zone between aggregate and cement paste controls many important properties of concrete. A number of studies dealing with interfacial zone are available in the literature for normal concrete and concrete containing silica fume. High-volume fly ash concrete for structural applications was developed at CANMET in the 1980s, but to date there has been no information available for interfacial zone in high-volume fly ash concrete.In this paper, the orientation index and mean size of Ca(OH)₂ crystals in the aggregate-paste interfacial zone were determined by the X-ray diffractometer. The bond strength between the aggregate and paste was also investigated. It was found that, at the age of 28 days, there was no obvious transition zone between the aggregate and cement paste incorporating high volumes of fly ash. The higher the paste strength, the higher is the bond strength.     

考虑过渡区界面影响的混凝土宏观力学性质研究

混凝土材料的宏观力学特性及破坏机理由其细观组分来决定,界面过渡区是影响混凝土断裂破坏路径及宏观力学特性的重要因素。认为界面过渡区是区别于远处砂浆基质的一层含较高孔隙率的近场砂浆材料,采用“两步等效法”得到了混凝土细观单元的等效本构关系模型。最后基于细观单元等效化方法分析了在单轴拉伸、单轴压缩及弯拉载荷条件下混凝土试件的破坏过程及宏观力学性质,探讨了界面过渡区对混凝土力学特性的影响,并与随机骨料模型分析结果进行了对比。结果表明:界面相的存在对混凝土的弹性模量、强度及残余强度等力学性质有很大影响,在对混凝土宏观力学特性及细观断裂破坏过程进行研究时不可忽略其影响。     

新老混凝土粘结面断裂损伤过程区研究

在新老混凝土粘结面断裂性能试验的基础上,建立了新老混凝土粘结面断裂破坏模式,分析讨论了界面层特性及界面微裂缝扩展对新老混凝土粘结面断裂性能的影响。建立了新老混凝土粘结面带状微裂缝断裂过程区模型,根据该模型可求出断裂过程区尺寸。结合裂缝过程区的荷载~应变试验结果,用临界断裂韧度对新老混凝土粘结断裂韧度进行了断裂过程区影响修正。     

Modeling the kinetics of corrosion in concrete patch repairs and identification of governing parameters

Patch repair is a commonly used method for rectifying localized corrosion damage in reinforced concrete members. However, the ring-anode effect, which is corrosion at the intersection between the substrate and the repaired concrete, is a commonly observed failure mechanism after patch repairs. In this study, the kinetics of the corrosion in the ring-anode zones of repaired concrete structures is investigated by numerical modeling. All simulations, in agreement with the existing experimental and in situ observations, have demonstrated the formation of a ring-anode zone in the 2–5 cm portion of the substrate from the interface between the substrate and the repaired concrete. Furthermore, the anodic current density in the ring-anode zone is found to have a peak near the repaired concrete and to asymptotically approach to the corrosion current density observed in the substrate before the patch repair. More importantly, the simulations demonstrate quantitatively how various parameters affect the ring-anode corrosion. It was found that the resistivities of substrate and repair concretes are the most significant factors that influence the magnitude of macrocell corrosion, followed by the availability of oxygen in the patch. It was observed that the ring-anode effect is a localized problem that occurs in the substrate near the repair bond line, and the size of the patch and the cover thickness are not significant factors affecting the phenomenon.     

Behaviour of patch repair of axially loaded reinforced concrete beams

Experimental investigation is conducted to assess structurally the effectiveness of patch repair in axially loaded columns. Two patch repair materials are selected with high and low modulus of elasticity. The concrete columns were patch repaired under loaded and unloaded conditions. The patch repair is structurally effective for concrete columns repaired in an unloaded state. While for concrete columns repaired in a loaded state, the patch repair is structurally effective only when additional loading is applied. The load distribution between the patch repair, concrete core and steel reinforcement depends on the modulus of elasticity and areas of these components in the composite section at the repaired zone. For patch repair to be structurally effective, it is recommended to relieve the loads before the patch repair is applied either partially or totally if constructionally possible.     

Experimental study of the interfacial transition zone (ITZ) of model rock-filled concrete (RFC)

Rock-filled concrete (RFC), a new type of concrete that was developed mainly for large scale concrete construction, has a different casting process than conventional concrete: large rocks are piled into the formwork first, then self-compacting concrete (SCC) is poured in and fill the voids of the rock skeleton under gravity due to its high flowability. One of the key issues about RFC lies in its large interfaces between the SCC and rocks. In this paper, laboratory-scale model RFC consisting of coarse aggregates (simulating rocks) and cement grout (simulating SCC) was cast to simulate RFC in construction. The effects of different factors (aggregate size, rheology of cement grout, etc.) on the properties of the interfacial transition zone (ITZ) between cement paste and aggregates of model RFC were investigated using Backscatter Electron (BSE) and nanoindentation techniques. Furthermore, by comparing the results of BSE and nanoindentation at identical regions, the relationship between porosity and elastic modulus was found to agree well with empirical formulas, bridging the microstructure with the mechanical properties of concrete.     

Characterization of the interfacial bond between old concrete substrate and ultra high performance fiber concrete repair composite

The interfacial bond characteristics between normal concrete substrate as old concrete and ultra high performance fiber concrete as repair material have been investigated. Normal concrete substrates were first subjected to different surface preparation methods prior to bonding the ultra high performance fiber concrete to form repair composites. The interfacial mechanical bond of the composites was assessed using slant shear and tensile splitting strength tests. In addition, rapid chloride permeability test was performed to ascertain the potential chloride resistance of the composites. The microstructure of the transition zone between the normal concrete and ultra high performance fiber concrete was also studied using scanning electron microscope. The results generally indicate that surface preparation of the substrate is very much required to obtain superior mechanical bond of the composites; whereby the composites with the sand-blasted substrate providing the most superior mechanical bond. The excellent bond of the composite is also evident through the rapid chloride permeability test, as well as confirms by the scanning electron microscope image of the interface. Hence, the ultra high performance fiber concrete exhibits significant potential as an excellent material for repair and rehabilitation of concrete structures.     

氯盐干湿环境下TRC与老混凝土的界面特性研究

采用双面剪切的加载方式研究了氯盐干湿循环对纤维编织网增强混凝土(Textile reinforced concrete,TRC)与普通硅酸盐混凝土界面性能的影响,并通过SEM(Scanning electron microscope)技术探究了氯盐干湿循环作用下界面的微观结构。研究发现:氯盐干湿循环下,TRC与老混凝土界面微观结构会损伤劣化;与连续氯盐浸泡相比,氯盐干湿循环对界面粘结性能的影响较大;相同干湿循环次数下,加固界面的粘结强度随着氯盐溶液浓度的增加而降低;相同氯盐溶液浓度下,加固界面的粘结强度随着循环次数的增加而降低。     

相变储能混凝土的制备与性能

膨胀珍珠岩吸附硬脂酸丁酯后用石灰石粉末改性制成相变储能骨料（PCESA）,用其等体积部分取代砂制备相变储能混凝土。测试相变储能混凝土的抗压强度和劈裂抗拉强度,通过SEM图像分析相变储能混凝土的微观形貌,用DSC测试PCESA的相变特征及相变储能混凝土的比热容。试验结果表明：相变储能混凝土中PCESA结构保持完整且其与水泥石的界面过渡区密实,相对提高了相变储能混凝土的强度;PCESA具有较好的热物理性能,添加在混凝土中增强了混凝土的储能能力;当PCESA的掺量为20vol%时,相变储能混凝土具有较好的力学性能和热物理性能,在建筑结构中使用既可以起到承重的作用又能实现节能的目的。     

纳米SiO2改性轻骨料混凝土性能

纳米SiO₂（NS）具有极强火山灰活性、晶核作用和填充效应,因此用NS改善水泥基材料性能成为众多学者研究的热点。本课题对不同掺量的NS对轻骨料混凝土强度及耐久性的改性效果进行了研究。通过测试轻骨料混凝土的力学性能（抗压和抗折）和氯离子渗透性能及利用SEM和EDS测试分析了NS对混凝土宏观和微观结构的影响。研究结果表明:在适当的掺量下,NS能够有效地提高轻骨料混凝土的力学性能,其中28 d的抗压强度和抗折强度比空白组混凝土分别提高了21.6%和46.2%。氯离子渗透的结果表明,轻骨料混凝土的抗氯离子渗透性能随着掺量的增加而呈线性增强。混凝土界面过渡区（ITZ）也发生了显著变化,其厚度减小,形貌也更加致密。ITZ的钙硅比随着NS掺量增加而减小,说明该区域内水化产物C-S-H凝胶增多,Ca（OH）₂被消耗,从而形成致密的过渡区,有利于强度提高。      

考虑材料组成特性的混凝土轴拉破坏过程细观数值模拟

为反映骨料、砂浆及其之间的界面过渡区的组合特点和材料性能,基于材料细观非均匀性和有限元方法的混凝土破坏过程细观数值模拟需进行复杂、细致的网格剖分,导致了繁重的前处理工作和可观的计算量。该文对混凝土材料细观单元材质组成的单一化假定进行改进,将内嵌界面过渡区材料的规则化单元视为一种广义复合材料单元,建立了复合型界面损伤模型。采用等效方法确定单元的复合弹性关系,通过有限元法计算单元的局部应力;用细观层次上弹性力学性能的弱化描述单元组成材料的损伤,混凝土材料的破坏过程通过单元各组分的损伤模拟。应用该复合型界面损伤模型研究了混凝土试件的单轴拉伸破坏过程,细观数值模拟结果符合混凝土试件的宏观破坏特征,表明该模型可作为分析混凝土材料破坏过程的一种有效途径。     

纳米SiO2对钢纤维/混凝土高温后力学性能及微观结构的影响

研究了掺纳米SiO₂的钢纤维混凝土(NSFC)、 钢纤维混凝土(SFRC)和普通混凝土(NC)三种材料在不同加热温度后的抗压、 劈裂和抗折强度等力学性能, 对不同温度热处理后的微观结构进行了SEM分析, 对钢纤维与过渡区界面的相结构进行了XRD分析。结果表明: 在测试温度范围内, NSFC的抗压、 劈裂和抗折强度均高于SFRC和NC的强度, 且在400 ℃时达到最大值。在常温下, NSFC的抗压、 劈裂和抗折强度较NC分别提高27.01%、 63.28%和54.12%, 400 ℃高温热处理后比NC分别高35.09%、 84.62%和87.23%; SEM分析表明, 在钢纤维与过渡区的界面处, 致密度提高, 显微硬度提高。由于固相反应, 使界面区结构发生变化, 在钢纤维表层形成扩散渗透层(白亮层), 即化合物层, 呈锯齿状, XRD分析证明, 白亮层主要由FeSi₂和复杂的水化硅酸钙组成, 从而增强了钢纤维与基体的粘结力, 提高了混凝土的高温力学性能。     

考虑界面影响的混凝土弹性模量的数值预测

提出了一种考虑界面过渡层影响的混凝土弹性模量的数值预测方法。将球形骨料与包裹它的界面过渡层作为二相复合球结构的等效颗粒,由广义自洽方法计算不同粒径骨料与界面过渡层组成复合球的有效模量。然后由等效颗粒生成的随机骨料模型建立体积表征单元,施加均匀位移边界条件,通过数值方法计算该体积表征单元中的应力和应变场,由细观力学数值均匀化方法预测体积表征单元的有效弹性模量。计算结果表明:对于不同骨料含量的混凝土,有效弹性模量的预测值与试验值非常接近,界面过渡层的厚度对混凝土的整体弹性性质有较大影响。     

Effect of corrosion layer of steel bar in concrete on time-variant corrosion rate

Corrosion current density of steel bar in concrete was measured in active corrosion process under a designed artificially controlled climate environment. The active corrosion process shows the characteristics of the time-variant corrosion rate, and the three phases of the corrosion process are presented. The corrosion rate decreases at first; this is followed by a steady state phase; finally after concrete cover cracking caused by corrosion, an ascending phase of the corrosion rate is observed. The mechanism of the time-variation characteristics is discussed based on the microstructure of the interfacial transition zone (ITZ) between steel bars and concrete at different corrosion levels. The microstructure shows that the porous interfacial transition zone gradually transforms into a dense corrosion layer composed of concrete and corrosion products due to expansion of the corrosion products. The layer is called as corrosion layer for short in this paper. The main reason for the descent of the corrosion rate is that transportation of oxygen and moisture is retarded due to the dense corrosion layer. When the equilibrium between rates of consumption and transportation of oxygen is reached, the corrosion rate tends to be steady. The concrete cover cracking offers new access for transporting oxygen and the corrosion rate speeds up.     

Multi-scale investigation of microstructure,fiber pullout behavior,and mechanical properties of ultra-high performance concrete with nano-CaCO3 particles

The mechanical properties of a fiber-reinforced concrete are closely related to the properties of the matrix, fiber, and fiber-matrix interface. The fiber-matrix bond property is mainly governed by the adhesion between the fiber and surrounding cement materials, as well as the strength of materials at the interfacial transition zone. In this paper, the effect of nano-CaCO₃ content, varying between 0 and 6.4%, by mass of cementitious materials, on microstructure development, fiber-matrix interfacial bond properties, and mechanical properties of ultra-high performance concrete (UHPC) reinforced with 2% steel fibers were investigated. The bond properties, including bond strength and pullout energy, were evaluated. Mercury intrusion porosimetry (MIP), backscattered electron microscopy (BSEM), optical microscopy, and micro-hardness testing were used to characterize the microstructure of matrix and/or interfacial transition zone (ITZ) around an embedded steel fiber. Test results indicated that the incorporation of 3.2% nano-CaCO₃ significantly improved the fiber-matrix bond properties and the flexural properties of UHPC. This was attributed to densification and strength enhancement of ITZ as observed from micro-structural analyses. Beyond the nano-CaCO₃ content of 3.2%, the fiber bond and mechanical properties of UHPC decreased due to increased porosity associated with agglomeration of the nano-CaCO₃.