Effect of Dry-Wet Cycles on the Transport and Mechanical Properties of Cement Mortar Subjected to Sulfate Attack

This study deals with the analysis of the detrimental effects of a“sulfate attack”on cement mortar for different dry-wet cycles.The mass loss,tensile strength,and gas permeability coefficient were determined and analyzed under different exposure conditions.At the same time,nitrogen adsorption(NAD),scanning electron microscopy(SEM),and X-ray diffraction(XRD)techniques were used to analyze the corresponding variations in the microstructure and the corrosion products.The results show that certain properties of the cement mortar evolve differently according to the durations of the dry-wet cycles and that some damage is caused to the mortars in aqueous solution.The pores fill with corrosion products,increasing the mortar specimen mass and tensile strength while reducing the permeability coefficient and pore size distribution.As corrosion proceeds,the crystallization pressure of the corrosion products increases,resulting in a 16%reduction in tensile strength from the initial value and a 2.6-factor increase in the permeability coefficient,indicating sensitivity to sulfate attack damage.Furthermore,the main corrosion products generated in the experiment are gypsum and ettringite.Application of osmotic pressure and extension of the immersion time can accelerate the erosion process.

Investigation of Sulfate Attack Resistance of Shotcrete under Dry-wet Cycles

In order to research the sulfate attack resistance of shotcrete, the sulfate attack of shotcrete in the presence and absence of steel fiber was experimentally studied by using dry-wet cycle method. Meanwhile, compared with ordinary concrete by the same mixture, the difference of sulfate attack resistance of shotcrete was studied. The experimental results showed that, with dry-wet cycles increasing, the changes of loss rate of relative dynamic elastic modulus and mass loss rate of specimens included three stages： initial descent stage, stable stage, and rapid descent stage, respectively. However, the changes of mechanical properties first increased and then decreased. Furthermore, the corrosion products of shotcrete after sulfate attack were observed by using the method of XRD, thermal analysis, and SEM, respectively, and the failure mode of shotcrete turned from ettringite destruction to ettringite-gypsum comprehensive failure. Meanwhile, the contents of ettringite and gypsum increased with increasing dry-wet cycle. Simultaneously, the stratified powders drilled from shotcrete under 150＇s dry-wet cycle were analyzed for the mineral phase composition and thermal analysis. With the drywet cycle increasing, the content of ettringite first increased and then decreased and tended to stable. However, the determination of gypsum decreased gradually and even to 0 when the depth was more than 12 mm.

The law of strength damage and deterioration of jointed sandstone after dry-wet cycles

Under the periodic rise and fall of the water level in the Three Gorges Reservoir in China,the rock mass in the ebb and flow zone of the slope is always in a state of a dry-wet cycle.In order to explore the influence of dry-wet cycle on mechanical properties of jointed sandstone,the triaxial and uniaxial compression tests of dry-wet cycle of jointed sandstone were carried out.For the experiment,four groups of samples with different numbers of joints were set up,and the jointed rock samples were subjected to 20 dry-wet cycles.Using both the triaxial compression test and the Mohr-Coulomb(M-C)rock fracture criterion,the strength envelope of the sandstone samples was fitted,and their strength degradation was further analyzed and studied.The results show that:(1)The peak intensity and elastic modulus of the sandstone samples decrease with increased number of dry-wet cycles.(2)The total deterioration of mechanical properties of intact rock samples is bigger than that of jointed sandstone samples as the number of dry-wet cycles increases.(3)With the increase of confining pressure,the peak intensity of intact sandstone samples increases much more than that of jointed sandstone samples,which indicates that joints and their numbers have obvious influence.(4)Joints and their numbers play an important role in guiding the damage effects of sandstone samples,which weaken the damage caused by dry-wet cycles.Therefore,the envelope of the M-C strength criterion of intact sandstone samples moves more than that of jointed sandstone samples.

Stability analysis of gravity anchor foundation of layered argillaceous sandstone under dry-wet cycles

To investigate the stability of gravity anchors of suspension bridges,in-situ tests of the vertical bearing capacity of the bedrock,shear resistance of the anchor-rock interface,shear resistance of the bedrock were conducted in a suspension bridge project.Under dry-wet cycles,the deterioration law of the mechanical properties of argillaceous sandstone was identified in laboratory tests:the elastic modulus,cohesion and friction of the argillaceous sandstone deteriorated significantly at first few dry-wet cycles and then declined slowly after 10 cycles,ultimately these three mechanical parameters were reduced to about 1/3,1/3,2/3 of the initial value respectively.Moreover,numerical simulation was used to restore in-situ shear tests and a good agreement was obtained.Base on the results of in-situ and laboratory tests,the stability of the gravity anchor foundation under natural conditions and drywet cycles was calculated and its failure modes were analyzed.The results demonstrated that the dry-wet cycles caused uneven settlement of the anchor foundation,resulting in more serious stress concentration in the substrate.The dry-wet cycles remarkably reduced the stability coefficient of the anchor foundation,whose failure mode shifted from overturning failure under natural conditions to sliding failure.When there was weak interlayer in the rock layer,the anti-sliding stability of the anchor foundation was affected drastically.

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.

Research on Corrosion Effect of Sulfate Ions on Concrete Under Wetting-Drying Cycle

Saline soil is widely distributed in the marine sediments along the coast of the world and the arid-semi-arid areas of the Middle East and Iraq,and calcium sulfate erosion has become one of the important factors affecting the durability of concrete in this area.In order to clarify the mechanism of sulfate ion damage to concrete,this paper mainly takes saline soil with high sulfate content in coastal area as well as arid-semi-arid area as the research object,and uses indoor geotechnical test,field test and numerical simulation to study the influence of different dry-wet cycle times on the unconfined compressive strength of concrete test blocks,and puts forward the relationship between the erosion arrival depth and time of sulfate ion in concrete,so as to predict the long-term erosion depth by using the erosion depth of sulfate ion in concrete in short time.The results show that the shorter the erosion time when the erosion reaches a certain depth,and the larger the erosion reaches when the erosion time is the same,the faster the erosion reaches the depth with the increase of erosion time.Compared with rectangular section concrete,circular section concrete penetrates faster.The results of this study can provide a reference for the durability design of concrete in saline soil sites containing sulfate.

Calculation of salt-frost heave of sulfate saline soil due to long-term freeze−thaw cycles

Based on salt-frost heave tests of sulfate saline soil under repeated freeze−thaw cycles,this paper discusses the mechanism of the salt-frost heave under long-term freeze−thaw cycles.The results show that the salt-frost heave can be restricted considerably by loads,and there is a critical load for the salt-frost heave cumulative effect.Under this load,peak values of salt-frost heave approach a constant,and the residual values become 0.There is no longer structure heave or cumulative effect of saline soil exposed to freeze−thaw cycles under the critical load.Taking cumulative effect into account in calculations of salt-frost heave,a salt-frost heave model under freeze−thaw cycles is developed.

Effects of dry-wet cycles on three-dimensional pore structure and permeability characteristics of granite residual soil using X-ray micro computed tomography

Due to seasonal climate alterations,the microstructure and permeability of granite residual soil are easily affected by multiple dry-wet cycles.The X-ray micro computed tomography(micro-CT)acted as a nondestructive tool for characterizing the microstructure of soil samples exposed to a range of damage levels induced by dry-wet cycles.Subsequently,the variations of pore distribution and permeability due to drywet cycling effects were revealed based on three-dimensional(3D)pore distribution analysis and seepage simulations.According to the results,granite residual soils could be separated into four different components,namely,pores,clay,quartz,and hematite,from micro-CT images.The reconstructed 3D pore models dynamically demonstrated the expanding and connecting patterns of pore structures during drywet cycles.The values of porosity and connectivity are positively correlated with the number of dry-wet cycles,which were expressed by exponential and linear functions,respectively.The pore volume probability distribution curves of granite residual soil coincide with the χ^(2)distribution curve,which verifies the effectiveness of the assumption of χ^(2)distribution probability.The pore volume distribution curves suggest that the pores in soils were divided into four types based on their volumes,i.e.micropores,mesopores,macropores,and cracks.From a quantitative and visual perspective,considerable small pores are gradually transformed into cracks with a large volume and a high connectivity.Under the action of dry-wet cycles,the number of seepage flow streamlines which contribute to water permeation in seepage simulation increases distinctly,as well as the permeability and hydraulic conductivity.The calculated hydraulic conductivity is comparable with measured ones with an acceptable error margin in general,verifying the accuracy of seepage simulations based on micro-CT results.

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.

The Quantification of Micro-structural Damage of Weak Muddy Intercalation in Dry-wet Cycles Combining in-situ SEM and DIP

In order to reflect truly the damage evolution mechanism of weak muddy intercalation in dry-wet cycles, two typical weak muddy intercalations were selected for dry-wet cycles. The mineral changes of specimens were analyzed via X-ray diffraction after dry-wet cycles. By combining in-situ SEM and digital image processing(DIP), the damage evolution process and damage characteristic parameters of each stage were obtained. The experimental results indicate that the hydration and dissolution of minerals can not be a determinant factor in structure damage. The micro-structural damage is due to disintegration of mineral aggregates, leading to changes in the number and size of cracks and pores. The damage degree of specimens is related to its initial structure, and the micro-structural damage intensifies and finally tends to stabilize with cycle times increased.

Dydrogesterone treatment for menstrual-cycle regularization in abnormal uterine bleeding-ovulation dysfunction patients

BACKGROUND Dydrogesterone has shown significant efficacy in treatment of irregular menstrual cycle due to abnormal uterine bleeding-ovulation dysfunction(AUB-O),but there were few relevant studies.This observational study was designed to evaluate the effectiveness of dydrogesterone for the treatment of Chinese patients with AUB-O.AIM To evaluate the effects of dydrogesterone on menstrual-cycle(MC)regularization and metabolism in the patients with AUB-O.METHODS A prospective,non-interventional,single-arm,post-marketing observational study was conducted.Chinese women aged 16 years or above with AUB-O who had been prescribed dydrogesterone were enrolled.The patients were treated with dydrogesterone 10 mg from day 16 to day 25 of each cycle,consecutively for at least 3 cycles.The main outcome was defined as the percentage of patients whose MCs returned to normal(defined as 21 d<menstrual cycle≤35 d)after three cycles of dydrogesterone treatment.RESULTS One hundred and fourteen women with AUB-O were enrolled in the present study.Of 89 patients who completed treatment,72(80.9%)achieved a regular MC at the end of the 3rd circle.The level of androgen,including testosterone and dehydroepiandrosterone sulfate,declined significantly(P=0.01 and 0.031,respectively),whereas other hormone levels remained steady.During the treatment,44/80(55.0%)subjects in the per-protocol set had reported biphasic basal body temperature.CONCLUSION Dydrogesterone therapy was effective in achieving MC regularization for Chinese patients with AUB-O.

盐冻耦合作用下水工混凝土耐久性及寿命预测

为研究西北地区冻融盐侵环境下水工混凝土的耐久性,制备了不同粉煤灰掺量的混凝土试件,以不同浓度的硫酸钠溶液作为介质开展了冻融循环试验,阐明了不同循环次数下试件的外观、质量、抗压强度和动弹模量的变化规律,基于XGBoost模型建立了混凝土寿命预测模型并对其进行了评价和验证。研究结果表明,随着冻融循环次数增加,混凝土质量、抗压强度和动弹模量逐渐减小;冻融循环次数和硫酸钠溶液浓度是影响混凝土寿命的关键因素,8%硫酸钠溶液破坏度最高,此溶液浓度下冻融150次后混凝土质损率达4.55%;粉煤灰的掺入量对混凝土耐久性有一定影响;最优粉煤灰掺量为10%,此掺量下冻融150次后混凝土质损率为3.99%;XGBoost模型在混凝土寿命预测方面具有较高的精确性和可靠性。本研究可为混凝土结构的耐久性设计和寿命预测提供参考。

干湿循环作用下吉林省西部土壤盐离子对混凝土的侵蚀破坏机理

为了解决干湿循环条件下长期的外部碳酸盐和硫酸盐侵蚀导致混凝土耐久性能显著退化的问题,进行了在干湿循环条件下混凝土内部CO_(3)^(2-)和SO_(4)^(2-)侵蚀规律和迁移规律的研究。先通过混凝土受到离子侵蚀的化学反应过程和扫描电子显微镜(SEM)实验分析并对比两种离子对混凝土侵蚀的损伤机理,后通过引用房室模型的方法分析CO_(3)^(2-)和SO_(4)^(2-)在混凝土表面与外界交界面处的传输和交换规律。结果表明:在180 d的时间内,离子侵蚀主要发生在混凝土表面0~20 mm的范围内,房室模型的预测值与实测值决定系数R^(2)在0.75左右;混凝土被侵蚀的深度与水灰比正相关,并且房室模型可以预测混凝土受到侵蚀的程度;硫酸盐和碳酸盐的化学反应生成物引起的膨胀压力导致混凝土产生裂缝而引起破坏。

冻融循环作用下粉砂质硫酸盐渍土变形特性研究

【目的】为探究南疆地区粉砂质硫酸盐渍土的变形特性,【方法】在封闭系统条件下对粉砂土进行冻融循环试验,利用扫描电镜对土体结构进行表征分析。【结果】结果显示:粉砂质硫酸盐渍土硫酸钠含量为0和1.0%时变形以融沉为主,5次循环后变形量分别为-1.30 mm、-1.02 mm;硫酸钠含量为2.0%、5.0%、8.0%时变形表现为盐胀-冻胀变形,5次循环后变形量分别为2.46 mm、8.53 mm、3.43 mm。【结论】结果表明:粉砂质硫酸盐渍土在冻融循环作用下温度和硫酸钠含量是影响土体发生冻融变形的主要因素。在低温区间土体的变形随温度的降低表现为盐胀-冻胀,在升温过程中土体先后出现冰融和盐溶变形;土体的最终变形量与硫酸钠含量的变化表现为先增后减,当硫酸钠含量为5.0%时,土体的盐-冻胀率达到最大为2.5%。运用扫描电镜可发现粉砂质硫酸盐渍土经冻融循环后在自重作用下会产生沉降,内部盐分重结晶是抑制沉降甚至造成土体膨胀的主要原因。

硫酸盐侵蚀和干湿、冻融循环下混凝土单轴受压损伤对比

为研究宁夏典型服役环境下混凝土材料的损伤特点,本文在硫酸钠侵蚀和干湿、冻融循环三因素的共同作用和耦合作用下研究混凝土棱柱体试样的不同单轴受压应力-应变特点。改变硫酸钠溶液浓度和腐蚀次数,测量试样的质量损失率、相对动弹性模量以及应力-应变曲线。结果表明:随着腐蚀次数的增加,耦合作用下质量损失率和相对动弹模均先增大后减小,共同作用下则均逐渐减小;随着硫酸钠溶液浓度的增大,两种腐蚀制度下试样的应力-应变曲线上升段斜率和峰值应力均逐渐减小,且共同作用比耦合作用劣化效果更明显;随着腐蚀次数的增加,两种腐蚀制度下试样的应力-应变曲线上升段斜率均逐渐减小,耦合作用下峰值应力先增大后减小,共同作用下则逐渐减小,两种腐蚀制度下峰值应变均逐渐增大,共同作用比耦合作用劣化更大。本文提出的两种腐蚀制度下的混凝土应力-应变曲线预测模型可为宁夏服役环境下的混凝土结构寿命预测提供依据。

硫酸盐侵蚀冻融循环下聚乙烯醇纤维钢筋混凝土的黏结性能试验研究

为了研究硫酸盐侵蚀冻融循环下聚乙烯醇(PVA)纤维钢筋混凝土的粘结滑移性能,对4种掺不同体积百分比PVA纤维混凝土进行不同次数的硫酸盐浸泡下的冻融循环实验。实验结果表明,在一定范围内,随着PVA纤维掺量的增高可以提高混凝土的抗压强度;PVA纤维可以一定程度上阻止硫酸盐对混凝土的侵入,在硫酸盐冻融初期可以增强混凝土的抗压强度;在混凝土中添加一定量百分比体积的PVA纤维可以增加钢筋混凝土粘结的峰值滑移,增加了混凝土的塑性应变,硫酸盐冻融次数的增加会减少钢筋在混凝土中的峰值滑移。

硫酸盐环境下MKPC浆体干湿循环制度设计及性能评价

为评价硫酸盐干湿循环耦合作用下磷酸钾镁水泥(MKPC)浆体的力学性能和物理性能,根据GB/T 3810.3—2016测试方法,通过真空保水和不同温度的真空烘干试验,设计出适用于MKPC浆体试件的干湿制度。遵循设计的干湿制度,测试MKPC试件在硫酸盐环境下进行不同干湿循环次数和全浸泡不同龄期的质量损失、强度变化。试验结果如下:经过反复试验,以相邻两次质量变化率<0.1%为标准,设计出MKPC试件1个干湿循环周期为72 h。MKPC试件在5%Na_(2)SO_(4)溶液干湿循环120次后其抗折强度、抗压强度较未干湿循环试件的强度分别下降24.44%、18.63%,质量损失为4.12%;在5%Na_(2)SO_(4)溶液浸泡360 d后,MKPC试件的抗折强度较未浸泡的强度下降1.23%,抗压强度较未浸泡的试件强度增加1.11%,质量损失为1.54%。因此,MKPC试件在干湿循环作用下对抗折强度变化和质量损失更加敏感。

铁尾矿粉混凝土在荷载与硫酸盐干湿循环耦合作用下的性能劣化机理

铁尾矿粉作为矿物掺合料应用于混凝土有助于固废高值化利用和节能减排,而关于铁尾矿粉混凝土抗硫酸盐侵蚀性能的研究,大多只考虑了单一条件或者加速条件环境,其在多种环境耦合下的劣化特征尚未得到探究.本文将铁尾矿粉作为矿物掺合料应用于混凝土中,以相对动弹性模量、质量损失率为性能指标,研究荷载-硫酸盐干湿循环耦合作用下铁尾矿粉混凝土劣化过程,探究铁尾矿粉掺量和荷载率对铁尾矿粉混凝土内部劣化的影响,结合扫描电镜(SEM)、X-ray衍射分析(XRD)、压汞(MIP)等微观手段揭示铁尾矿粉混凝土在耦合作用下损伤劣化机理.结果表明:适量铁尾矿粉的掺入有利于其与矿粉的协同水化作用,提高混凝土水化程度和基体密实度,减少硫酸根离子传输路径,提高混凝土抗硫酸盐侵蚀能力;荷载的施加使试块内部微裂纹宽度和数量增加,为硫酸根离子的进入提供更多渗透路径,加速铁尾矿粉混凝土损伤劣化过程;耦合作用使得铁尾矿粉混凝土内部损伤不断累积,在微观上表现为结构的疏松和裂缝的衍生扩展,在宏观上表现为力学性能的衰减和整体结构完整性的下降.

冻融循环条件下细粒硫酸盐渍土盐冻胀力学特性试验研究

硫酸盐渍土在失水或者温度降低的情况下会产生盐胀现象,严重影响结构物的正常使用.为研究细粒硫酸盐渍土盐冻胀力学特性,选取青海乐都公路段二十里铺区沿线硫酸盐渍土,采用自制试验箱,开展室内冻融循环试验,分析含盐量及冻融循环次数对法向盐冻胀力的影响.试验结果表明:冻融循环过程中,土中盐溶液向土体上表面迁移,同时析出的盐结晶和冰结晶使土样中硫酸钠溶液浓度增大,从而引起土体膨胀变形.压实系数为0.93、含水率为20%的细粒硫酸盐渍土盐冻胀力随着含盐量和冻融循环次数的增加而增大,盐冻胀力的增长趋势随着冻融循环次数的增加逐渐降低;在一个冻融循环周期内,按照盐冻胀力中盐胀占比及冻胀占比随温度变化的规律,将盐冻胀力变化分为四个阶段:盐胀阶段、盐冻胀耦合阶段、冻胀阶段、融化阶段,并测得含盐量为1.5%~4.0%的细粒硫酸盐渍土冻结温度在2~-5℃内变化,且冻结温度随着含盐量的增加而降低;盐冻胀力冻结峰值与残余值随着冻融循环次数的增加而增加,表明细粒硫酸盐渍土的盐冻胀力具有累加性,且含盐量越大累加性越强.

盐侵冻融作用下非饱和掺碱粉煤灰混凝土损伤研究

为了让碱激发粉煤灰混凝土在工程中应用更为广泛,研究了碱性激发剂对粉煤灰混凝土耐久性的影响,以及碱激发粉煤灰混凝土在不同饱和度(30%,60%,100%)与不同硫酸盐溶液浓度(0%,5%,10%)耦合作用下的耐久性变化。从碱激发粉煤灰混凝土的外观损伤、质量损失、抗压强度损失、动弹性模量损失方面来评价试件损伤情况,并结合电子显微扫描技术分析试件在硫酸盐侵蚀-冻融耦合作用下的损伤机理。结果表明:100%饱和度的试件在浓度10%的硫酸钠溶液中抗冻性最差;碱激发粉煤灰混凝土的抗压强度损失和动弹性模量损失呈现为平稳下降和加速下降两个阶段;非饱和试件的劣化程度随试件饱和度降低而减少,因此非饱和条件下耐久性更好。最后通过理论分析和室内试验建立了精度较高的不同饱和度碱激发粉煤灰混凝土在硫酸盐侵蚀-冻融耦合作用下的损伤模型。