大体积砼产生裂缝的原因及防治措施

大体积砼裂缝问题是建筑业普通关心的问题.对砼裂缝进行控制,就必须研究砼结构中裂缝产生的原因以及在实际的设计和施工过程中采取合理的、经济的措施来控制裂缝.本文通过对大体积砼裂缝成因的理论研究,提出了合理的裂缝控制的设计和施工措施.     

“UEA”补偿收缩砼控制大体积结构裂缝的应用原理

本文叙述了用“UEA”配置的补偿收缩砼在大体积结构中，控制裂缝出现的原理。掺“UEA”的补偿收缩砼，由于“UEA”的膨胀作用，可以降低温差收缩，达到控制温差裂缝的目的。     

现浇砼板裂缝原因分析与防治

本文从砼结构裂缝产生机理入手，分析了常见的楼面裂缝产生的原因及部位，并提出防治措施。     

现浇砼楼板施工裂缝的控制及处理

阐述了现浇砼楼板施工裂缝产生的机理、状况及产生的原因，并提出了应抓好砼施工过程中几个重要环节来进行裂缝控制，以及对待施工裂缝的正确态度。     

环形加热炉基础大体积砼裂缝的控制

针对目前我国工业部门应用的大体积砼环形结构的砼裂缝控制,总结探索施工技术关键和裂缝控制技术措施,并提出科学、严密、严格量化的施工组织管理要求,证实科学精心的施工对控制大体积砼裂缝起着重要作用。为推广应用此项技术提供实践及技术依据。     

马钢2250mm热轧带钢土建工程关键部位的质量控制

通过对马钢2250热轧带钢土建工程监理实践的总结，提出超长、超深砼结构和重载地坪的质量关键部位，采取加密跟踪、科学计算等事前和事中控制措施，有效解决了主轧线定位、预埋件安装、砼结构裂缝、重载地坪处理和逆作法等施工质量难题，并为类似工程提供了可借鉴的方法。     

水钢新2^#炉基础温差开裂控制措施初探

由于工程结构的实际需要，大体积砼用途十分广泛。但大体积砼内部的水化热问题，若不很好解决，砼体发生温度变形开裂后，将使大体积砼无法满足应有的功用。因此大体积砼要保证其结构的有效性，必须处理好水化热及温差应力问题，以有效控制温度变形裂缝。     

建筑物楼板裂缝成因分析及防治对策

一般在结构封顶3～6个月后,楼板陆续出现裂缝,使建设单位和用户们普遍感到不安,引起社会和工程界的广泛关注.但是,从近代科学关于砼工作的研究及大量的砼工程实践证明,砼结构裂缝是不可避免的,从我国的<混凝上结构设计规范>(GB50010-2002)规定看,其裂缝宽度在不同的环境下,不同的砼结构物,其裂缝的宽度也有所不同的控制标准,允许裂缝宽度为0.2～0.3m.     

大体积砼基础温度裂缝控制探讨

文章对华北某钢厂高炉大体积砼基础进行了温度裂缝控制计算。与实际工程进行比对、分析、探讨，提出大体积砼基础在设计中应尽可能避免结构形式复杂、截面凹突出多变，对特殊结构(如大截面的圆形结构、有阴角的凹突处)，应沿垂直方向外侧面布置环向封闭钢筋网片。同时，对大体积砼基础施工中加强养护和保温措施提供经验和教训。     

选冶化厂区砼土道路裂缝成因及防止措施

砼道路裂缝产生原因较为复杂，在道路工程建设中十分常见。本文在对金川公司选冶化厂区道路裂缝产生原因分析的基础上，指出裂缝对砼道路的危害性，有针对性地提出了选冶化厂区道路的裂缝修补及防止措施。     

Use of Composite Reinforcement to Strengthen Concrete and Air-Entrained Concrete Masonry Walls against Air Blast

Two concrete structures and three air-entrained concrete (AEC) masonry walls were subjected to two, high explosive detonations. The concrete structures were placed at a stand-off distance such that medium damage was expected. The stand-off distance of the AEC-masonry walls was reduced on each successive detonation until breaching occurred. The two concrete structures retrofitted with composite materials were subjected to air-blast loading at a stand-off distance of approximately 14.6 m. The structures were constructed such that each long side of the structure contained a wall retrofitted with a composite material and a wall left bare as a control. Both concrete structures exhibited less residual displacement on the walls strengthened with composite materials than the bare control walls.     

In Situ Performance Testing of Deteriorating Water Tanks for Durability Assessment

Reports of failure of existing concrete structures due to a lack of durability, rather than a deficiency in structural strength, has made concrete technologists, engineers, and researchers focus research on the parameters influencing durability performance with respect to time. Systematic performance monitoring, with respect to chosen durability parameters of existing concrete structures, will decide the direction of future research in this area. Inferences based on laboratory simulations and testing need to be confirmed by in situ field measurements and studies. In situ condition rating and performance monitoring surveys have been conducted by many researchers, scientists, and professional associations, and reported in literature. Inferences of few such studies are summarized and discussed. Deterioration of concrete structures constructed in recent times is observed at relatively faster rates, and has been mainly attributed to cracking. Cracking is associated with the use of faster-hydrating portland cements with increased fineness and the tricalcium silicate (C3S) content to support the high speed of modern construction. In the present research, a case study of deteriorated water tank structures situated in the semitropical region of India is presented. Some selected parameters—such as concrete cover, carbonation depth, chloride concentration, compressive strength, etc. which influence long term durability of structures—have been measured.     

Stiffness Degradation and Time to Cracking of Cover Concrete in Reinforced Concrete Structures Subject to Corrosion

Corrosion-induced cracks in reinforced concrete (RC) structures degrade the stiffness of the cover concrete. The stiffness degradation is mainly caused by the softening in the stress-strain relation in the cracked concrete. Limited efforts have been made to model the cracking and the corresponding effects on the cover concrete, despite of its importance in assessing and modeling the behavior of RC structures. This paper proposes a stiffness degradation factor to model the stiffness degradation of the cover concrete subject to cracking. The proposed factor is computed in terms of the cracking strain corresponding to the maximum opening of the concrete cracks based on an energy principle applied to a fractured RC structure. The time to cracking of the cover concrete is then determined as the time from the corrosion initiation needed by the crack front to reach the outer surface of the cover concrete. The proposed stiffness degradation factor and the method to compute the time to cracking are illustrated through two numerical examples. The times to cracking of the cover concrete that are predicted using the proposed method are in agreement with the measured values from laboratory experiments.     

Chemo-Mechanical Effects in Mortar Beams Subjected to Water Hydrolysis

Reliable predictions of the long-term behavior of concrete structures require both an accurate knowledge of the various degradation mechanisms that affect the structures over their lifetime and the use of simulation tools. Leaching due to aggressive water can be responsible for a loss of mechanical properties and stability of concrete structures. This paper focuses on coupled chemo-mechanical effects on concrete structures subjected to aggressive water inducing calcium leaching. Experimental investigations on mortar bending beams and their analysis are detailed. The response of leached mortar beams is deduced from these tests. It is found that the stiffness of mortar decreases, along with its strength, as the chemical attack progresses. The size effect on geometrically similar bending beams at various stages of degradation is studied experimentally. Due to leaching, the brittleness of the structures is increased and the fracture energy decreases.     

Numerical Modeling of Concrete Confined by Fiber-Reinforced Composites

Lateral confinement of reinforced concrete columns can significantly increase their lateral deflection capability and load-carrying capacity. While such retrofits were initially completed using steel jackets, fiber-reinforced polymer (FRP) composites have been used successfully and extensively for seismic and blast upgrades. Numerical modeling of such structures requires the use of a concrete material model that can accurately represent the volumetric behavior of concrete under triaxial stress states, to capture the interaction between concrete expansion and the resulting stress increase in the confining jacket. Test data by Suter and Pinzelli, Karbhari and Gao, and Mirmiran and Shahawy on concrete cylinders and prisms confined by aramid, carbon, and glass FRP sheets are analyzed numerically. The concrete material model used was developed for the study of the effect of blast loading on reinforced concrete structures and was verified and validated for a variety of triaxial stress paths. The numerical analyses closely reproduce the strength enhancements observed in the test specimens for various levels of confinement. The model also confirms the observed inefficiency of low levels of lateral confinement and the superior enhancement provided by circular cross sections as compared to rectangular ones.     

Shear Strengthening of Concrete Structures with the Use of Mineral-Based Composites

Rehabilitation and strengthening of concrete structures have become more common during the last 10–15?years, partly due to a large stock of old structures and partly due to concrete deterioration. Also factors such as lack of understanding and the consequences of chloride attack affect the need for rehabilitation. In addition, more traffic and heavier loads lead to the need for upgrading. Existing externally bonded strengthening systems using fiber-reinforced polymers (FRP) and epoxy as bonding agents have been proven to be a good approach to repair and strengthen concrete structures. However, the use of epoxy bonding agents has some disadvantages in the form of incompatibilities with the base concrete. It is, therefore, of interest to substitute epoxy with systems that have better compatibility properties with the base concrete, for example, cementitious bonding agents. This paper presents a study on reinforced concrete beams strengthened in shear with the use of cementitious bonding agents and carbon fiber grids, denoted as mineral-based composites (MBC). In this study it is shown that the MBC system has a strengthening effect corresponding to that of strengthening systems using epoxy bonding agents and carbon fiber sheets. Different designs and material properties of the MBC system have been tested. An extensive monitoring setup has been carried out using traditional strain gauges and photometric strain measurements to obtain strains in steel reinforcement, in FRP, and strain fields on the strengthened surface. It has been shown that the use of MBC reduces strains in the steel stirrups and surface cracks even for low load steps as compared to a nonstrengthened concrete beam.     

Effects of Mixed Corrosion, Freeze-Thaw Cycles, and Persistent Loads on Behavior of Reinforced Concrete Beams

The effects of mixed corrosion and freeze-thaw cycles on the mechanical properties of concrete prism specimens and the effects of mixed corrosion, freeze-thaw cycles, and persistent loads on the structural behavior of reinforced concrete beams were experimentally studied. A mixed solution of NaCl and Na2SO4 was used as a corrosive medium. Results show that under alternating actions of freeze-thaw and mixed corrosive agents, increasing the number of freeze-thaw cycles decreases the compressive strength and the elastic modulus of concrete and increases the compressive strain corresponding to the maximum compressive stress. The degradation of concrete material properties accelerates with the increase of water-cement ratio. For reinforced concrete beams, a 4% reduction in the loading capacity is found when these are subjected to freeze-thaw cycles and mixed corrosion only. However, if these actions are coupled with persistent loading, as expected during the service life of reinforced concrete structures in cold regions, a more rapid drop in the strength and deformation capacity of the beams is identified. The degradation is enhanced by a larger persistent-loading ratio. The significance of an accurate simulation of service conditions in the durability study of reinforced concrete structures in cold regions is highlighted.     

锈蚀植筋下新老混凝土黏结面压剪试验研究

为研究锈蚀植筋下新老混凝土的黏结性能,采用某工程锈蚀钢筋作为植筋,制备了不同植筋率（0、0.223%、0.446%、0.502%、0.893%、1.004%、1.396%、1.786%和2.792%）的新老混凝土试件.在RYL-600微机控制岩石伺服剪切流变仪上进行了不同初始静压力（0、1、2、3和4 MPa）下混凝土试件的剪切试验.对试件的剪应力-位移曲线进行了归纳总结,得到了锈蚀植筋下新老混凝土试件在外力作用下产生变形乃至破坏的演化规律;分析了锈蚀植筋下新老混凝土黏结面的抗剪强度,得到了抗剪强度随植筋率及初始静压力变化的规律.同时根据试件试验破坏结果,着重分析了不同植筋率和不同初始静压力对其破坏模式的影响.试验结论为新老混凝土力学性能的研究提供了理论依据.      

Damage Detection in Concrete by Fourier and Wavelet Analyses

The effectiveness of vibration-based methods in damage detection of a typical highway structure is investigated. Two types of full-scale concrete structures subjected to fatigue loads are studied: (1) Portland cement concrete pavements on grade; and (2) a simply supported prestressed concrete beams. Fast Fourier transform (FFT) and continuous wavelet transform (CWT) are used in the analysis of the structures’ dynamic response to impact, and results from both techniques are compared. Both FFT and CWT can identify which frequency components exist in a signal. However, only the wavelet transform can show when a particular frequency occurs. Results of this research are such that FFT can detect the progression of damage in the beam but not in the slab. In contrast, the CWT analysis yielded a clear difference between the initial and damaged states for both structures. These findings confirm the conclusions of previous studies conducted on small-scale specimens that wavelet analysis has a great potential in the damage detection of concrete. The study also demonstrates that the approach is applicable to full-scale components of sizes similar or close to actual in-service structures.     

What Is a “Massive” Concrete Structure at Early Ages? Some Dimensional Arguments

The risk of early-age concrete cracking depends on the capacity of hardening concrete to support the thermal stresses caused by the exothermic nature of the hydration process. This has been recognized for “massive” concrete structures. However, with the increasing use of high performance concretes, it is apparent that this problem also concerns traditionally “thin” structural members (columns, beams). The definition of a “massive” concrete structure, and how the structural dimension affects intensity and occurrence of chemically-induced strucctural degradation is the main focus of this paper. Based on dimensional analysis of the governing equations, a characteristic length scale, the hydration heat diffusion length, is derived; beyond this length the structure needs to be considered as “massive,” and latent hydration heat effects affect the long-term structural integrity. From experimental data of normal strength concrete and high performance concrete, it is shown that this hydration heat diffusion length of high performance concrete is of the order of ?h = 0.2 m, and lh = 0.3 m for normal strength concrete. Through a number of case studies, the relevant similarity parameters of the risk of early-age concrete cracking are identified, which allow's the monitoring of the structural performance of early-age concrete structures.