Deterioration mechanism of sulfate attack on concrete under freeze-thaw cycles

The experiments of concrete attacked by sulfate solution under freeze-thaw cycles were investigated. The sulfate solution includes two types of 5% Na2SO4 and 5% MgSO4. Through the experiment, microstructural analyses such as SEM, XRD and TGA measurements were performed on the selected samples after freeze-thaw cycles. The corrosion products of the concrete were distinguished and quantitatively compared by the thermal analysis. Besides, the damage mechanism considering the dynamic modulus of elastically of concrete under the coupling effect was also investigated. The experimental results show that, under the action of freeze-thaw cycles and sulfate attack, the main attack products in concrete are ettringite and gypsum. The corrosion products exposed to MgSO4 solution are more than those to Na2SO4 solution. Furthermore, the content of gypsum in concrete is less than that of ettringite in test, and some of gypsum can be observed only after a certain corrosion extent. It is also shown that MgSO4 solution has a promoting effect to the damage of concrete under freeze-thaw cycles. Whereas for Na：SO4 solution, the damage of concrete has restrained before 300 freeze-thaw cycles, but the sulfate attack accelerates the deterioration process in its further test period.     

Deterioration of concrete in railway tunnel suffering from sulfate attack

For the sake of understanding the deterioration behavior of concrete in actual railway tunnel structures subjected to aggressive sulfate medium in practice,detailed field investigations and tested analysis on sprayed concrete linings of approximately 40-year-old railway tunnels in environments containing sulfate ion were carried out,respectively.The results show that the deterioration of concretes in the investigated area is serious,which involves complicated physicochemical process between the sulfate salt...     

硫酸盐侵蚀下混凝土灌注桩的损伤效应

为分析外部硫酸盐腐蚀对混凝土灌注桩截面损伤和承载力的影响,研究了硫酸根离子在混凝土灌注桩中的扩散反应规律并探讨了桩身结构性能退化的影响因素.基于Fick第二定律建立了柱坐标下硫酸根离子的非稳态扩散反应方程,采用数值方法得到了扩散方程求解的有限差分格式,并通过相关试验结果验证了理论模型与计算方法的正确性.根据化学反应生成的膨胀产物计算膨胀应变,并通过损伤演化函数计算桩身截面损伤和承载力,建立了桩基损伤度与桩身承载力的相关关系,对比分析了不同腐蚀因素和腐蚀时间对桩基损伤与桩身承载力的影响.研究结果表明:内膨胀应变和损伤在表层的发展速度比内层发展更为迅速,增大桩半径能显著提高桩基的耐久性和承载能力,而水灰比大于0.4时,桩基损伤增长幅度最大.因此,合理增加桩半径并采用较小的水灰比能有效减小桩身混凝土损伤,提高桩基的耐久性并减少桩身承载能力的损失.     

高渗透压-硫酸盐侵蚀下混凝土时空劣化

为了模拟研究混凝土海水环境下的侵蚀损伤及劣化规律,采用质量分数10% Na₂SO₄溶液对混凝土试样进行不同渗透压和不同时长下的室内侵蚀试验. 结合微米压痕试验、CT扫描试验和电子显微镜扫描试验,对高渗透压-硫酸盐耦合侵蚀作用下混凝土的侵蚀损伤及微观力学性能进行研究. 试验结果显示,渗透压加速了离子迁移,主要起到了促进化学侵蚀作用. 渗透压越大,混凝土化学损伤速率越快,侵蚀深度越深;骨料与砂浆胶结处是易侵蚀、易破坏的薄弱点;混凝土内部孔隙易生成水化产物,高渗透压下容易生成大量短柱状石膏晶体及细密的针状钙矾石晶体.     

Ultrasonic testing and microscopic analysis on concrete under sulfate attack and cyclic environment

The damage process of concrete exposed to sodium sulfate attack and drying-wetting cycles was investigated. The water to binder（W/B） ratio and the concentration of sulfate solution were taken as variable parameters. Through the experiment, visual change, relative dynamic modulus of elasticity（RDME） and the surface damage layer thickness of concrete were measured.Furthermore, SEM and thermal analysis were used to investigate the changing of microstructure and corrosion products of concrete.The test results show that the ultrasonic velocity is related to the damage layer of concrete. It approves that an increase in damage layer thickness reduces the compactness and the ultrasonic velocity. The deterioration degree of concrete could be estimated effectively by measuring the surface damage layer and the RDME of concrete. It is also found that the content of gypsum in concrete is less than that of ettringite in test, and some gypsum is checked only after a certain corrosion extent. When the concrete is with high W/B ratio or exposed to high concentration of sulfate solution, the content of ettringite first increases and then decreases with corrosion time. However, the content of gypsum increases at a steady rate. The content of corrosion products does not correspond well with the observations of RDME change, and extensive amount of corrosion products can be formed before obvious damage occurs.     

氯盐与硫酸盐复合侵蚀下钢筋混凝土锈裂行为

为评估海洋、盐湖等环境中钢筋混凝土结构耐久性,通过试验与理论相结合探究氯盐与硫酸盐复合侵蚀下钢筋混凝土锈裂特性。通电腐蚀5%(质量分数)NaCl、5%NaCl+5%Na₂SO₄溶液中钢筋混凝土试件,对比分析混凝土表观形貌、钢筋锈蚀特征。设计伴随的混凝土腐蚀试验,类比保护层腐蚀劣化,分析混凝土力学性能。结果表明：硫酸盐的存在改变胀裂前混凝土形貌,使得单一氯盐侵蚀下的“白须”消失,表面粉化并出现盐结晶,延长胀裂时间;复合侵蚀下钢筋锈蚀率低于单一氯盐侵蚀,二者均显著低于法拉第定律理论值;锈胀裂缝宽度与钢筋锈蚀率线性相关,硫酸盐的存在增大裂缝随钢筋锈蚀发展的速率;通电环境中,受腐蚀混凝土的抗压强度先升高后降低,劈裂抗拉强度不断降低。提出受腐蚀混凝土的抗拉强度演化经验公式。在经典锈胀模型的基础上考虑锈蚀产物对裂缝的填充作用,并将硫酸盐的影响考虑至混凝土抗拉强度、钢筋腐蚀电流密度中,建立复合侵蚀下钢筋混凝土胀裂时间预测模型,并验证了模型的有效性。     

Evaluating Deterioration of Concrete by Sulfate Attack

Effects of factors such as water to cement ratio, fly ash and silica fume on the resistance of concrete to sulfate attack were investigated by dry-wet cycles and immersion method. The index of the resistance to sulfate attack was used to evaluate the deterioration degree of concrete damaged by sulfate. The relationship between the resistance of concrete to sulfate attack and its permeability/porosity were analyzed as well as its responding mechanism. Results show that the depth of sulfate crystal attack from surface to inner of concrete can be reduced by decreasing w/c and addition of combining fly ash with silica fume. The variation of relative elastic modulus ratio and relative flexural strength ratio of various specimens before and after being subjected to sulfate attack was compared.     

硫酸盐腐蚀混凝土构件损伤检测研究

采用化学分析、微观结构观察及超声检测等技术对混凝土腐蚀厚度进行测定,结合连续损伤原理对混凝土均匀腐蚀的损伤度进行了初步定义.提出了以化学分析结果为基准对超声检测混凝土腐蚀厚度结果进行修正的思路,研究了不同腐蚀厚度混凝土的超声检测与化学分析结果之间的关系.结果表明:超声平测法检测腐蚀厚度较小时误差较大(L0＜75 mm时),对声速突变测距L0＞75 mm时检测结果与化学分析结果相近,表明超声平测法可以用来检测不同介质分界面位置;通过对超声波在不同腐蚀厚度混凝土中的传播速率变化,发现超声声速与混凝土腐蚀层厚度有关,表明随着混凝土腐蚀厚度增加其内部结构密实性降低.     

胶凝材料组成对混凝土TSA硫酸盐侵蚀的影响

研究不同水泥品种、矿物掺合料对混凝土抗TSA侵蚀性能的影响规律,采用掺30%石灰石粉填料的胶砂件,测试各砂浆在5±1℃的2%MgSO4溶液中浸泡1年后的外观破损与强度损失情况,通过X-射线衍射仪分析各砂浆的矿物成分.结果表明:不同品种水泥的抗TSA侵蚀能力由高到低依次为:硫铝酸盐水泥 硅酸盐水泥复合>硫铝酸盐水泥>抗硫酸盐水泥>普硅水泥.掺硅灰和矿渣细粉均能明显改善混凝土抗TSA侵蚀性能,且矿渣粉掺量越大效果越明显.由于粉煤灰活性低,掺量较少时可在一定程度上减缓TSA侵蚀过程,但掺量过多时反而产生不利影响.     

干湿交替环境下混凝土受硫酸盐侵蚀劣化机理

通过试验模拟干湿循环作用下混凝土受不同质量分数硫酸钠的侵蚀特点,从不同侵蚀时间后混凝土的表观特征、质量经时变化规律、混凝土中硫酸根离子分布、侵蚀深度、混凝土轴心抗压强度、劈裂抗拉强度经时退化规律、变形经时变化规律等方面全面衡量混凝土的硫酸盐侵蚀损伤特征;讨论硫酸盐溶液质量分数、侵蚀龄期对混凝土损伤累积规律的影响;根据SEM图像分析受蚀混凝土的微观结构特征,揭示了干湿循环和硫酸盐侵蚀共同作用下,不同侵蚀时期后混凝土损伤的演变机理.试验结果表明,在干湿循环作用下硫酸钠对混凝土的侵蚀损伤是侵蚀产物与硫酸盐结晶膨胀共同作用的结果;在侵蚀初期由于晶体的填充密实作用,使得受蚀混凝土的质量、强度与延性增大;随着侵蚀时间的延长,混凝土不仅受到钙矾石、石膏等侵蚀产物的膨胀损伤作用,而且叠加了干湿循环过程中Na2SO4·10H2O的结晶膨胀作用,使得混凝土损伤反复进行并不断累积,加速了混凝土的受蚀劣化速度,质量、强度逐渐降低,脆性变大;由于拉应力的叠加效应,混凝土劈裂抗拉强度对硫酸盐侵蚀损伤更敏感.     

成型压力和浸泡时间对混凝土抗硫酸盐侵蚀性能的影响

在不同的试验条件下对抗硫酸盐侵蚀进行试验,研究了成型压力和浸泡时间对混凝土抗硫酸盐侵蚀性能的影响.结果表明:按现有规范规定的成型压力取得的试验结果偏大,对评价侵蚀性能不利,采用振动成型更接近实际情况;随着浸泡时间的延长,混凝土的耐腐蚀性均有所下降,建议延长浸泡时间评价混凝土的抗腐性能.     

Durability of Concrete under Multi-damage Action

Several different experiments, including freezing-thawing,freezing-thawmg drying-wetting,and drying-wetting, in salt solution and in water respectiwely, were designed to determine the durability of concrete. The durability damage features of concrete in the above experiments were studied. It is demonstrated that the demage extent of concrete under.freezing-tluacing and freezing-thawing doing-wetting in salt solution is larger than that in water. Thus,freezing-thawing and freezing-thawing doing-wetting in salt solution are stricter and more effeetive methods to evaluate the durhility of concrete in salt-existing envirorunent in cold regions. The damage extent of concrete under freezing-thawing drying-wetting shows an ultra-superposition effect. The order of concrete durability deterioration degree in these experiments is determined. It shows that effects of multi-demulge factors are greater than those of single-damage factor.     

成型压力和浸泡时间对混凝土抗硫酸盐侵蚀性能的影响

在不同的试验条件下对抗硫酸盐侵蚀进行试验,研究了成型压力和浸泡时间对混凝土抗硫酸盐侵蚀性能的影响．结果表明：按现有规范规定的成型压力取得的试验结果偏大,对评价侵蚀性能不利,采用振动成型更接近实际情况;随着浸泡时间的延长,混凝土的耐腐蚀性均有所下降,建议延长浸泡时间评价混凝土的抗腐性能．     

复杂侵蚀环境下透水混凝土耐久性能试验研究

为适应海绵城市建设对高耐久性透水混凝土的需求,配置了低品质活性矿物掺合料透水混凝土和聚丙烯仿钢纤维（PPTF）透水混凝土两个系列。前期已进行了基本性能研究,在此基础上,继续开展复杂多因素侵蚀环境下耐久性能研究。试验采用侵蚀溶液全浸泡方式,共持续300 d,宏观上测试了抗压强度和抗折强度随侵蚀时间的劣化规律,微观上用SEM观测了胶结层的微观结构、用EDS观测了胶结层的化学组成。复杂侵蚀环境下,透水混凝土微观结构变化规律与其宏观力学性能变化规律基本相符。综合基本性能和耐久性能,择优推荐复掺低品质硅灰和低品质粉煤灰透水混凝土和掺3 kg/m³锯齿形PPTF透水混凝土。这两种透水混凝土基本性能均满足城市既有住区道路、人行道、城市广场、体育场、户外停车场、园林景观道路等轻交通路面的使用要求,同时,又表现出了良好的耐久性能。     

海水侵蚀环境下混凝土强度的研究

采用普通硅酸盐水泥和矿渣硅酸盐水泥,分别配制了水灰比在0.45～0.60范围内的混凝土试件,采用加速腐蚀试验,研究了在海水腐蚀后,其强度随时间变化的规律及原因,并为试件损伤状态定量化解析提供了试验数据.研究结果表明,在相同的试验条件下,矿渣硅酸盐水泥混凝土抗海水侵蚀性优于普通硅酸盐水泥混凝土;混凝土试件的抗蚀系数为0.8时与美国ASTM标准规定的强度损失25%的界限值相吻合.     

海水侵蚀环境下混凝土强度的研究

采用普通硅酸盐水泥和矿渣硅酸盐水泥 ,分别配制了水灰比在 0 .4 5～ 0 .6 0范围内的混凝土试件 ,采用加速腐蚀试验 ,研究了在海水腐蚀后 ,其强度随时间变化的规律及原因 ,并为试件损伤状态定量化解析提供了试验数据。研究结果表明 ,在相同的试验条件下 ,矿渣硅酸盐水泥混凝土抗海水侵蚀性优于普通硅酸盐水泥混凝土 ;混凝土试件的抗蚀系数为 0 .8时与美国ASTM标准规定的强度损失 2 5 %的界限值相吻合     

Effect of Al in slag blended cement paste under sulfate attack

Unilateral sulfate attack of cementitious materials containing 40% slag with different water to binder ratios was investigated. The results showed that the degradation of slag blended cement pastes was nearly from the corners of paste surface with cracking and spallings, water-to-binder(w/b) ratio made a significant sense to the damage that low w/b ratio led to little weight loss, less cracking and spalling damage and vice versa. Microstructural experimental results demonstrated that in the three different stages of sulfate attack, degradation of pastes was primarily associated with the migration behavior and bonding configuration of aluminum, in the early ages Al was mostly present in C-(A)-S-H, and thus, the damage of pastes hardly appeared while at later ones Al had been largely transferred from C-(A)-S-H into AFt, leading to expansive damage.     

硫酸盐腐蚀后混凝土单轴受压本构关系

为了研究硫酸盐侵蚀环境下混凝土力学性能的变化,对受硫酸盐腐蚀的普通混凝土进行单轴受压实验.通过实验室加速腐蚀的实验方法,测定不同腐蚀时期的混凝土的应力应变全曲线,研究受腐蚀混凝土应力-应变曲线上的几个特征值(峰值应力及应变、弹性模量、割线模量、拐点应力及应变、收敛点应力及应变)随腐蚀程度变化的规律,对腐蚀后混凝土的应力应变全曲线进行拟合.结果表明:随腐蚀的进行,混凝土峰值应力、弹性模量先增加后减小,而峰值应变是先稍微有所减小,而后急剧增大;该实验条件下获得了受腐蚀后混凝土应力应变曲线上各特征值与峰值应力fc′的关系式,建立了曲线上升段参数αa的数学表达式.可以以受腐蚀混凝土的强度为基本参数,分别求解受腐蚀混凝土峰值应变、弹性模量,上升段曲线拟合参数、拐点应力以及收敛点应力.从而得到受腐蚀混凝土一系列的力学参数和本构关系.     

Deterioration of dynamic mechanical property of concrete with mineral admixtures under fatigue loading

The dynamic mechanical property of concrete is one of the key parameters, which greatly influences durability of infrastructures subjected to continuous heavy loading, such as girder and track slab of high-speed railway foundation structure. This paper reports serials of experiments designed to investigate the deterioration of dynamic mechanical properties of different concretes under fatigue loading condition. Four parameters including relative dynamic elastic modulus (RDEM), relative dynamic shear modulus (RDSM), relative compressive strength (RCS) and water absorption (WA) of concrete were evaluated to assess the dynamic properties and microstructures of concretes. Results show that the fatigue stress levels and fatigue cycle durations significantly influence the dynamic mechanical properties of concrete including dynamic elastic modulus and dynamic shear modulus. Addition of proper mineral admixture can improve the dynamic mechanical characteristics of concrete and increase its resistance against the fatigue loading effect. Keeping the amount of mineral admixture in concrete constant, its dynamic mechanical property with fly ash is lower than that with fly ash and silica fume. The water absorption in concrete, which is an indirect parameter reflecting capillary porosity, increases evidently after bearing fatigue-loading. There is a close correlation between the deterioration of dynamic mechanical property and the increasing of water absorption of concrete. This indicates that the damage of microstructure of concrete subjected to fatigue loading is the indispensable reason for the decay of its dynamic mechanical performance.     

Application of image analysis based on SEM and chemical mapping on PC mortars under sulfate attack

The degradation mechanisms of cementitious materials exposed to sulfate solutions have been controversial, despite considerable research. In this paper, two methodologies of image analysis based on scanning electron microscope and chemical mapping are used to analyse Portland cement mortars exposed to sodium sulfate solution. The effects of sulfate concentration in solution and water to cement ratio of mortar, which are considered as the most sensitive factors to sulfate attack, are investigated respectively by comparing the macro expansion with microstructure analysis. It is found that the sulfate concentration in pore solution, expressed as sulfate content in C-S-H, plays a critical role on the supersaturation with respect to ettringite and so on the expansion force generated.