钢筋非均匀锈蚀引起混凝土保护层内裂分析

根据自然环境下保护层锈胀开裂前的钢筋锈蚀形态,将锈蚀层简化为半椭圆状的非均匀分布,从而建立了钢筋非均匀锈蚀理论模型。经过求解得到了混凝土中锈胀应力理论解,并与有限元计算结果对比,验证了理论解的精确性。根据理论解可知最大周向应力在水平轴上,因此锈蚀层的发展会首先引起保护层内部水平裂纹的产生。增加钢筋直径可有效减低锈胀应力,提高结构抗锈裂的能力。与均匀锈蚀理论模型对比结果表明：均匀锈蚀要远小于非均匀锈蚀条件下的临界锈蚀层厚度,均匀锈蚀的计算结果偏于保守。因此,对锈胀问题进行理论分析时,应采用更接近真实锈蚀形态的非均匀锈蚀模型。     

Time-variation full process mode of steel bar corrosion in concrete

This paper discusses the corrosion rate variation of reinforcing steels in concrete in constant climate environment. The corrosion current density of steel bar in concrete under a constant climate environmental condition and chloride attack. The results indicate that the corrosion rate (corrosion current density) of the steel bars is time-dependent during a corrosion process, and the time-dependent process can be divided into six phases in the full lifetime. The results also show that the effects of the concrete strength and the electrical resistivity of the concrete on the variation of the corrosion rate. This study conducts the mechanism analyses about the test results based on the microstructure of the interfacial transition zone (ITZ) between the steel bars and the concrete at different corrosion levels. The growth of the corrosion layer and the corrosion cracking are the main factors influencing the corrosion process. The time-variation mode of the corrosion rate of steel bars in full process is developed.     

Comprehensive Effects of Temperature and Relative Humidity on Rebar Corrosion in Concrete

The corrosion rate of steel in concrete is highly dependent on the heterogeneous structure of the concrete and the environmental exposure conditions. Temperature and relative humidity are two important environmental factors that simultaneously influence the corrosion process and cannot be separated or isolated from each other. The interaction of temperature and relative humidity for characterizing the corrosion rate has been sufficiently studied. The resules show that the higher the temperature is, the greater the relative humidity is when rebar corrosion rate reaches the max, and as to different relative humidity, the increasing extent of rebar corrosion rate with rise in temperature is different.     

Review of Research on Damage of Corrod Reinforced Concrete

The research result shows that the corrosion of reinforcement is one of the dominating factor for decreasing the durability of reinforced concrete structures. In this paper, the significance of the research of corroded reinforced concrete is presented, and an attempt is made to integrate the latest development with regard to the mechanical behaviors of reinforcement corrosion, the damage of concrete due to corroded bars, bond relationship between corroded bars and concrete, as well as the load capacity of corroded reinforced concrete flexural member and compressive member and the seismic behavior of corroded reinforced concrete element. And the trend of its development in future is discussed as well.     

地下水强酸盐复合腐蚀环境下不同矿物掺合料对水泥砂浆耐腐蚀性能的影响

模拟了贵州地区地下水强酸盐复合腐蚀环境,系统研究了粉煤灰、矿渣粉和硅灰3种矿物掺合料单掺与复掺对水泥砂浆耐腐蚀性能,如耐蚀系数、质量变化率、氯离子相对扩散系数的影响规律,并探讨了地下水强酸盐复合腐蚀环境下水泥砂浆的腐蚀机理。结果表明,与基准砂浆相比,单掺与复掺矿物掺合料均提高了砂浆的耐蚀系数,降低了砂浆后期氯离子相对扩散系数,但增大了砂浆质量损失率。这3项指标从不同角度反映了地下水复合腐蚀环境下水泥砂浆的耐腐蚀性能。     

再生混凝土与锈蚀钢筋间的粘结性能试验研究

为了探究再生混凝土结构的耐久性能,对5组不同钢筋锈蚀率(0~9%)的再生混凝土梁式试件进行加载试验。分析不同钢筋锈蚀率对再生混凝土梁式试件的钢筋应变、局部粘结应力、粘结滑移和极限粘结应力的影响。结果表明：钢筋锈蚀率大于3%时试件底部开始有细微锈胀裂缝出现;锈蚀率越大,荷载作用下钢筋应变沿锚固位置的变化曲线越平缓;局部粘结应力沿锚固段呈现出双峰分布,峰值主要集中在加载端和自由端附近;加载端附近位置滑移现象最先发生,远离加载端滑移现象延后;随着钢筋锈蚀率的增大,极限粘结强度先增加后降低,极限荷载下的滑移值增大。     

酸雨环境下老旧砖砌体的抗压性能

为研究酸雨环境下老旧砌体抗压力学性能退化规律,实验室配置了PH值分别为1.5、2.5和3.5的3种硫酸和硝酸混合溶液来模拟酸雨环境,采用完全浸泡加速腐蚀的试验方法对砖、砂浆和砖柱进行不同程度的腐蚀,并测定其抗压强度。试验表明,随着腐蚀时间的增加,砂浆试块抗压强度先增大后降低;砖试块抗压强度随腐蚀时间的增加而降低;模拟酸雨溶液PH值越小,砂浆和砖试块抗压强度降低程度也愈大;砖柱极限承载力和弹性模量均随腐蚀时间的增加而降低,且模拟酸雨溶液PH值越小,降低程度愈大;随着腐蚀时间的增加,砖柱初始刚度逐渐减小,极限破坏位移增大;模拟酸雨溶液PH值越小,砖柱在达到峰值荷载时的变形值越小,脆性愈大;基于试验结果,建立了酸雨环境下砖柱应力应变曲线上升段统一数学表达式及酸雨侵蚀后普通烧结粘土砖砌体剩余抗压强度回归公式。     

Experimental analysis of strengthening effect of corroded reinforced concrete beams strengthened with bolted steel plates

In order to study the mechanical behavior of corroded reinforced concrete beams strengthened with bolted steel plates, this paper designed 12 reinforced concrete beams. These beams were corroded by using accelerated electrochemical corrosion method with a designed corrosion ratio of 10%. The pre-compression experiments were performed for all RC beams before strengthening and the maximum crack width was controlled as 0.2 mm. According to the thickness of concrete cover, the beams were divided into 3 groups. Each group was composed of one comparative beam and three tested beams strengthened by steel plates bolted with study according to the thickness of steel plates which were 3 mm, 4 mm and 5 mm, respectively. It was shown that the strain distributions along the height of the strengthened beams at middle-span were in good agreement with the plain section assumption basically. The serviceability performances of corroded RC beams were significantly improved and these ultimate bearing capacities increased obviously. The steel plate bolted with stud effectively reduced the crack width and the extension height of reinforced concrete beams. It was indicated that an increase of steel plates with 35 mm resulted in a decrease of deflection by 13%51% when beams had the same thickness of concrete cover and corrosion ratio. Influence of the thickness of concrete cover on the ultimate bearing capacity was not obvious.     

Influence of fine aggregate on initial shrinkage of high performance concrete

The type and percentage of sand affect the gradation of coarse fine aggregate and the amount of spare mortar in concrete, which further affects the resistance of fresh concrete to initial plastic shrinkage. Taking into account the engineering characteristics of sand in the area of Chongqing, P. R. China, three fine aggregates composed of superfine, medium, mixed and manufactured sand, are used to prepare high performance concrete (HPC). The effects of these three fine aggregates on initial plastic shrinkage crack are investigated. The results show that the HPC prepared with superfine sand possesses a higher initial shrinkage value than HPC prepared with mixed or medium sand. An optimal sand percentage is necessary to reduce the initial shrinkage cracking of HPC prepared with mixed or medium sand.     

Micro structure Characteristics of Recycled Fine Aggregate and Its Concrete

The characteristics of recycled fine aggregate (RFA) and its concrete (RFAC) were analyzed by SEM, EDXA, XRD, Micro harness and nitrogen absorption. The results show that RFA is an artificial aggregate with complex compositions, certain hydration activity and high permeability. The mineral compositions of RFA include SiO2, CaCO3 and a small quantity of C2S, which has a certain hydration activity and can form hydration productions. The compaction rate of cement paste in RFAC is low because of pores in it. There is obvious interfacial transition zone (ITZ) between RFAC and cement paste, and its width is bigger than that in ordinary concrete. In addition, the values of micro hardness of the interfacial transition zone on both sides of RFA and cement paste are lower. The porous structure of the recycled fine aggregates and defects of ITZ are main reasons for big pores in RFAC, which has bad effect on impermeability of RFAC.     

基于内聚力模型的锈蚀砼梁抗弯刚度

粘结性能退化是导致锈蚀钢筋混凝土构件力学性能下降的主要因素之一,基于锈蚀构件粘结性能实验研究成果与内聚力模型,建立了有厚度的双线性内聚力单元与分离式钢筋混凝土梁分析模型,引入粘结界面层,研究了粘结性能退化对锈蚀钢筋混凝土梁抗弯刚度的影响。结果表明,双线性内聚力单元可以有效模拟钢筋与混凝土之间的粘结机制,而根据锈蚀深度确定的有厚度的粘结单元能合理描述锈蚀程度对粘结性能的影响。数值分析结果与实验及经验公式对比,表明了文中方法的有效性和合理性。     

Simulation Analysis of Flexural Performance of Reinforced Concrete Beam in Salt-frost Environment

Considering the damage of concrete mechanical properties and bonding behavior between the steel bar and concrete after the freezing and thawing function in the numerical simulation, the nonlinear analysis on flexural performance of reinforced concrete beam which experiences different salt-frost cycles is conducted, and the evolution law of resistance performance of reinforced concrete beam is studied. It is shown that, in the salt-frost environment, the decrease of concrete mechanical properties is the main reason that causes the degeneration of RC beam on flexural performance, while the reduction of bonding property has an unobvious effect on the beam resistance performance. When the salt-frost cycles reach a certain level, the beam failure pattern would change from the under-reinforced failure to over-reinforced failure. The freeze-thaw damage of bonding property in the beam-ends anchorage zone has a certain effect on the beam resistance behavior, especially for the more serious freeze-thaw degree, the resistance performance of the beam reduces about 4% than the perfect anchor beam.     

Research on New Type Masonry Mortar for Air-entrained Concrete

To overcome the shortcoming of traditional masonry mortar for air -entrained concrete such as poor water retention, low bonding strength and liabititv to cracking, a new type masonry mortar for air-entrained concrete with better performance than that by JC890 -2001 standard was prepared. The effects of water-retentive agent, redispersible latex powder and expansive agent on masonry mortar were studied, and their modified mechanisms were discussed respectively.     

Influence of Fine Recycled Aggregate on Physical and Mechanical Properties of Concrete

Based on a series of recycle process, waste concrete can be made into fine recycled aggregate which can be used completely or partly to compound concrete as a substitute for natural sand. But the special properties of fine recycled aggregate make the performance of concrete with fine recycled aggregate different from normal concrete. Characteristics of fine recycled aggregate and its influence on physical and mechanical properties of concrete are studied. On this basis, the influence of fine recycled aggregate on gas permeation properties, chloride permeability, and carbonation resistance of concrete are also studied. It is found that fine recycled aggregate produces a lot of defects in crushing preparation. And the compressive strength, the chloride permeability and the carbonation resistance of the concrete with fine recycled aggregate as substitution show difference from that of normal concrete.     

Effect of Coarse Aggregate on Characteristics of Plastic Shrinkage Cracking of Lightweight Aggregate Concrete

The grading of lightweight aggregate (LA) and its water content has an effect on the plastic shrinkage cracking of lightweight aggregate concrete (LWAC). Test results show that the water absorption of LA and the water evaporation of the concrete increased with the increment of cubage of LA, but the area of plastic shrinkage cracking of LWAC decreased. Besides, LA with high water content is beneficial to restraining the plastic shrinkage cracking of LWAC.     

混凝土在硫酸盐溶液中的腐蚀模型

混凝土在硫酸盐溶液中的腐蚀破坏是一种常见的混凝土破坏形式,研究混凝土抗硫酸盐腐蚀破坏对于混凝土工程具有重要意义。混凝土在硫酸盐溶液中受腐蚀的过程是一个循序渐进的过程,这个过程包括固液界面的表面吸附、硫酸盐溶液的扩散过程以及硫酸盐在混凝土试件内部的化学反应和物理结晶等物理化学过程。研究表明：可以采用Fick第二扩散定律来描述硫酸盐在混凝土中的扩散过程;硫酸盐溶液中,混凝土试件的固液相界面处发生物理吸附和化学吸附,并存在表面化学反应。     

Simulation and analysis of mechanics of coal and gas delay outburst

The buried box girders in the mountain city sewage pipeline system would be turned into simple-supported girders because of the removal of the foundation caused by landslide.This change of supporting would result in the failure risk of pipeline structure.The static properties of the buried box girders with small depth-to-span ratio in simple-supported condition are experimentally analyzed.The failure pattern,shear behavior and shear lag effect of the buried box girders in simple-supported condition are studied.The shortages in the present design codes for the calculation of shear capacities of the box girders are also discussed.It is shown that under the vertical uniformly distributed load,the crack distribution of the box girders is uniform,and no major critical diagonal crack is formed,and the box girders lose the bearing capacity in the form of concrete diagonal rod crushing.As the main shear-bearing components,the stirrups devote much to shear capacity of the box girder after concrete cracking.The strains of the longitudinal rebars are affected by those factors such as the redistribution of internal force and cracks,so only few areas of the box girders show the shear lag effect.The change of supporting form leads to the premature cracking of concrete and affects the serviceability of the sewage pipeline.     

带裂缝混凝土氯离子扩散作用试验研究及数值模拟

为了准确评估带裂缝工作混凝土结构的耐久性能,针对氯离子在带裂缝混凝土中的扩散过程进行研究。提出了无损制备裂缝的方法,可高效易行地在混凝土侧面及内部产生裂缝。对带裂缝的水泥砂浆试件进行氯盐溶液浸泡试验,深入研究了单缝和双缝试件中氯离子的扩散作用,修正了氯离子的扩散系数,并对带裂缝混凝土的氯离子扩散过程进行了数值模拟。研究结果表明：氯离子会沿裂缝发展方向及垂直于裂缝发展方向扩散,随着水灰比的减小,砂浆试件的抗氯离子扩散性能明显提高,在一定范围内,当裂缝间距增大,双缝间的氯离子扩散交互影响作用明显减小,ANSYS软件的模拟结果与试验数据吻合良好。     

Theoretical Contour Line of Reinforcement Bar of Non uniform Corrosion

Most of the previous studies on corrosion of the steel bar in reinforced concrete structures is based on the assumption that the bar is uniformly corroded. The actual measurement in projects proved that most of the reinforcement bar is in non uniform corrosion state. Based on electrochemical principles of metal corrosion and combined with the real corrosion course of the bar in concrete structures, a theoretical model of the contour line of reinforcement bar due to non uniform corrosion is established, through deducing the speed of corrosion and theoretical corrosion penetration and volume changes due to corrosion of reinforcement in general air at different position and different time. At the same time, the model is verified with the results from relative references and measured data.     

The Effect of Water-Cement Ratio on the Plastic Shrinkage Cracking of Lightweight Aggregate Concrete

In this paper, the characteristic of the plastic shrinkage cracking of the lightweight aggregate concrete (LWAC) was studied under the condition of prescribed quantity of cement and aggregate and was compared with that of the normal aggregate concrete (NC) with the same proportion of materials for the LWAC. The results suggested that the water evaporation quantity in the concrete became larger with the increase of w/c, and water evaporation velocity reached maximum value around two hours. When the water-cement ratio was low, the area of the plastic shrinkage cracking of LWAC was large and when w/c was 0.40, the area reached the maximum value and then became smaller with the increase of w/c.