混凝土的收缩及防裂

混凝土由于其中水分变化、化学反应、温度变化所引起的变形均称为混凝土的体积变形，这里先介绍其中的干缩变形。此外还有因碳化作用等所引起的变形。     

补偿收缩混凝土的自收缩特性

通过对比掺与不掺膨胀剂的混凝土的早期自收缩及其胶凝材料体系的水化热和化学结合水量发展规律,研究了膨胀剂对低水胶比混凝土自收缩的补偿作用效果。结果表明:8%掺量(质量分数)的钙矾石系膨胀剂(U型)对硅酸盐水泥混凝土和粉煤灰混凝土的自收缩有一定程度的补偿效果,而对矿渣混凝土的自收缩没有补偿。在较早期,膨胀剂促进复合胶凝材料的水化,在后期,膨胀剂也促进了矿渣–水泥体系的水化,其总放热量和化学结合水量均高于未掺膨胀剂的,但抑制了硅酸盐水泥和粉煤灰–水泥体系的正常水化,使其水化放热速率降低,总放热量和化学结合水量减小。     

补偿收缩混凝土变形性能研究进展

向混凝土中掺加膨胀剂制成补偿收缩混凝土是解决混凝土收缩开裂的一种重要措施.本文针对近年来补偿收缩混凝土变形性能的研究进展进行了综述.膨胀剂种类繁多,不同膨胀剂膨胀机理各不一样,致使出现同种掺量同种补偿对象但膨胀剂不同则补偿收缩效果不同,甚至截然相反现象.另一方面,影响补偿收缩混凝土变形性能因素众多,主要有水胶比、矿物掺合料种类及掺量、膨胀剂掺量、外加剂、养护条件等,但更多时候是若干个影响因素同时出现,整体变形结果更加不确定,也会导致工程上膨胀剂应用失效现象出现.     

低水灰比混凝土的收缩及其补偿

1　概述　　“收缩”简单地说 ,就是混凝土失水造成体积缩小的现象。严格地说 ,它是三维的变形 ,但通常以线性变形表示。因为多数情况下 ,混凝土构件一个或两个方向的尺寸往往要比第三个方向小很多 ,尺寸最大的方向收缩也最大。通常所谓收缩 ,是指混凝土暴露在相对湿度小于 10 0 %的空气中产生“干燥收缩”的简称。然而由于环境的作用 ,混凝土还会产生许多其它种类的收缩变形 ,它们彼此独立地发生或者同时出现。混凝土技术的发展 ,尤其是高效减水剂等外加剂的推广应用 ,所配制出的水灰比 (水胶比 )较低而工作度良好的混凝土已不存在困难 ,这…     

大型混凝土水池防裂措施的探讨

随着焦化工程的大型化和水处理工艺的不断发展,焦化水处理水池亦日趋大型化及复杂化。对于长度和宽度均超过我国规范规定的大型混凝土水池,由于温度收缩应力易使水池开裂。 任何物体如能自由变形,不论受热膨胀或遇冷收缩,其内部不会产生应力,由于水池是支承在地基上的,对于地质条件差的场地,还必须将水池支承在桩基上。这样,相互接触的不同物体在同一温差时就会产生不同的收缩。另外,在施工时由于先施工池底,后施工池壁,因两次施工的气温不能相同,易使池底和池壁间     

粉煤灰对混凝土补偿收缩性能的影响

本文阐述粉煤灰对普通砼和补偿收缩砼的收缩膨胀性能的不同影响 ,指出掺加粉煤灰能使普通砼减少用水量 ,降低收缩值 ,并使补偿收缩砼的膨胀量降低。粉煤灰的火山灰效应对水化产物钙矾石的形貌和性能会产生影响     

钢纤维补偿收缩再生混凝土制备及性能

利用钢纤维和膨胀剂制备补偿收缩再生混凝土,采用力学试验、SEM和变形测试等方法,对不同钢纤维体积掺量和不同再生粗骨料取代率时再生混凝土性能指标进行试验与探讨.结果 表明:随着再生粗骨料取代率的增长,混凝土抗压、抗折强度呈现先减小后增大再减小的劣化变化规律;再生混凝土中掺入适量钢纤维可以提高其力学性能和改善混凝土试件的破坏特征;利用膨胀剂水化反应产物的填充效应,提高骨料界面过渡区的密实度,降低再生混凝土的自收缩,钢纤维和膨胀剂两者同时掺入再生混凝土中,可以优势互补,从而提高再生混凝土强度和变形性能.     

UEA补偿收缩混凝土在工程中的应用

介绍在钢筋混凝土结构中,采用ＵＥＡ补偿收缩混凝土及合理设置膨胀带可取消伸缩缝和后浇带,并附有工程实例。     

补偿收缩混凝土在某地下工程中的应用

本文以某地下工程为例,应用有限元分析方法证明补偿收缩混凝土是一种有效抑制混凝土收缩开裂的技术途径;在超长超大面积混凝土结构工程中,采用膨胀性能优异的补偿收缩混凝土,结合良好的混凝土质量控制及规范的浇筑养护技术,可降低混凝土结构的开裂风险,实现超长混凝土结构无缝施工技术。     

粉煤灰掺量对补偿收缩混凝土的影响

为了探究粉煤灰掺量对补偿收缩混凝土的强度和限制膨胀率的影响规律,通过对掺有不同粉煤灰掺量的补偿收缩混凝土与基准混凝土进行对比试验研究,得出了能使补偿收缩混凝土达到最大膨胀率的粉煤灰的最优掺量为30%,并得到粉煤灰掺量与补偿收缩混凝土的强度和限制膨胀率的基本规律并对此作了一定的理论分析。     

Modelling the severity of plastic shrinkage cracking in concrete

Plastic shrinkage cracking (PShC) is one of the earliest forms of cracking in concrete as it occurs within the first few hours after the concrete has been cast. Concrete elements with large exposed surfaces are especially vulnerable to PShC. Many researchers have proposed models to simulate plastic shrinkage, bleeding and to predict the occurrence of PShC. In this paper a model to predict the degree of PShC is proposed. This model, the so called PShC Severity Model, is based on the volume of water that evaporates from the concrete between the placing and the initial setting time of the concrete. This model was verified using a large number of PShC test results. A further study was also done on the effect of the addition of low volumes of synthetic micro fibres on the degree of PShC. It was found that the addition of the fibres effectively reduces the PShC Severity value.     

对高性能混凝土的认识及混凝土开裂的问题

高性能混凝土是以耐久性为显著标志，充分考虑环境友好并兼顾可持续发展的高科技混凝土，实现混凝土高性能化，不是简单的材料与配比问题，更重要的是更新观念，精心制备，强化施工。现代混凝土的开裂问题始终是影响混凝土耐久性的重要因素，基于综合分析和评价国内外相关文献，本文归纳总结了混凝土收缩裂缝的不同形式及影响因素，认为混凝土的收缩开裂是不同形式收缩导致开裂的叠加结果。为明晰各种收缩对开裂行为的影响，有必要区分不同形式的收缩开裂，进而有针对性地改善混凝土的抗裂性，同时提出了改善混凝土抗裂性能的措施和建议。     

Influence of polypropylene fiber geometry on plastic shrinkage cracking in concrete

Plastic shrinkage cracking remains a primary concern for placements with high surface/volume ratios that are subjected to early age drying. Polypropylene fiber reinforcement controls such cracking, but the exact influence of fiber diameter, length and geometry remains unknown. A test program was carried out to understand the influence of these variables. Four commercially available polypropylene fibers were investigated at dosage rates varying from 0.1% to 0.3%. A recently developed technique of plastic shrinkage testing using a fully bonded overlay was employed. In this technique, a fiber reinforced concrete overlay is cast on a fully matured subbase with protuberances and the whole assembly is allowed to dry in an environmental chamber. Cracking in the overlay is monitored with time and characterized. Results indicate that while polypropylene fibers in general are effective in controlling plastic shrinkage cracking in concrete, a finer fiber is more effective than a coarser one, and a longer fiber is more effective than a shorter one. Further, fiber fibrillations appear to be highly effective in controlling plastic shrinkage cracking.     

Influence of shrinkage-reducing admixtures on the reduction of plastic shrinkage cracking in concrete

Fresh concrete exposed to high evaporation rates is prone to plastic shrinkage cracking, especially in structures with large surface area/volume ratios. The present work shows that the reduction of the surface tension of the mixing water is an effective way for decreasing such cracking. In this study, conventional and high strength concretes with superplasticizers and shrinkage reducing admixtures (SRAs) were exposed to drying in the plastic state. Continuous monitoring of the surface displacement facilitated the identification of the different stages of plastic shrinkage cracking. Measurements of capillary pressure, settlement, internal temperature and evaporation rate were also made. The results show the effectiveness of SRAs in reducing plastic shrinkage cracking, even in high strength concrete. This is attributed to the reduction in the evaporation rate, delay of the peak capillary pressure due to the development of menisci in the pores and lower settlement.     

The influence of curing on restrained shrinkage cracking of bonded concrete overlays

The influence of seven practical curing regimes on restrained shrinkage cracking of bonded concrete overlays was investigated. The influence of the curing regimes on the individual material properties governing restrained shrinkage cracking and on the age at cracking and crack area of ring tests and composite overlay–substrate specimens was investigated for three laboratory-made mixes of differing strengths and one commercial repair mortar. The results of the experimental testing showed that curing influences all of the material properties governing restrained shrinkage cracking. Prolonged or more effective curing was shown to either delay or reduce the rate of shrinkage respectively (dependent on the curing method), increase the tensile strength and elastic modulus, and decrease the tensile relaxation. In general, prolonged or more effective curing was shown to have a positive influence on restrained shrinkage cracking by increasing the age and net age at cracking.     

Study on effects of solar radiation and rain on shrinkage, shrinkage cracking and creep of concrete

In this paper, the effects of actual environmental actions on shrinkage, creep and shrinkage cracking of concrete are studied comprehensively. Prismatic specimens of plain concrete were exposed to three sets of artificial outdoor conditions with or without solar radiation and rain to examine the shrinkage. For the purpose of studying shrinkage cracking behavior, prismatic concrete specimens with reinforcing steel were also subjected to the above conditions at the same time. The shrinkage behavior is described focusing on the effects of solar radiation and rain based on the moisture loss. The significant environment actions to induce shrinkage cracks are investigated from viewpoints of the amount of the shrinkage and the tensile strength. Finally, specific compressive creep behavior according to solar radiation and rainfall is discussed. It is found that rain can greatly inhibit the progresses of concrete shrinkage and creep while solar radiation is likely to promote shrinkage cracking and creep.     

Internal curing of high-performance concrete with pre-soaked fine lightweight aggregate for prevention of autogenous shrinkage cracking

The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m³ of saturated LWA in HPC, providing 27 kg/m³ of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.     

Effect of cracking in drying and shrinkage specimens

The significance of cracking and microcracking caused by nonuniform drying shrinkage of test specimens is analyzed. To assure that no cracks are produced by drying in load-free specimens, one must lower the environmental humidity gradually and sufficiently slowly, and use very thin specimens (about 1 mm thick). Graphs for the maximum admissible rate of change of environmental humidity, calculated from both linear and nonlinear diffusion theories, are provided. The spacing and width of parallel cracks due to drying are estimated from fracture mechanics considerations. In normal size specimens the drying cracks are usually too narrow to be visible. Drying leads to discontinuous microcracking rather than continuous macrocracks and is represented better as strain softening than as an abrupt stress drop. Shrinkage cracking can increase drying diffusivity by several orders of magnitude.     

Modeling for prediction of restrained shrinkage effect in concrete repair

A general model of autogenous shrinkage caused by chemical reaction (chemical shrinkage) is developed by means of Arrhenius' law and a degree of chemical reaction. Models of tensile creep and relaxation modulus are built based on a viscoelastic, three-element model. Tests of free shrinkage and tensile creep were carried out to determine some coefficients in the models. Two-dimensional FEM analysis based on the models and other constitutions can predict the development of tensile strength and cracking. Three groups of patch-repaired beams were designed for analysis and testing. The prediction from the analysis shows agreement with the test results. The cracking mechanism after repair is discussed.     

A new testing configuration for shrinkage cracking of shotcrete and fiber reinforced shotcrete

Shotcrete and fiber reinforced shotcrete are commonly employed to produce layers or linings with large surface area versus volume ratios. Restrained shrinkage cracking is hence an important concern. The common test set-up used for shrinkage cracking of concrete, with a ring specimen cast around a stiff steel form, is not applicable to shotcrete. A new testing configuration, consisting of a shotcrete specimen bonded to a steel I-section and angles, is therefore proposed. In this investigation, a finite element analysis was first performed to identify member sizes that provide a good compromise between the effectiveness of constraint and weight of steel members. Restrained shrinkage tests using this new configuration were performed for plain and fiber reinforced shotcrete. Despite the simplifying assumptions in the finite element analysis, the predicted degree of restraint is in reasonable agreement with test results. From the results, the proposed set-up is shown to be a practical and viable approach for investigating the shrinkage cracking behavior of shotcrete and fiber reinforced shotcrete.