混凝土抗硫酸盐侵蚀性能试验研究

研究了不同水胶比混凝土试件在(20±2)℃全浸泡作用下的抗硫酸盐侵蚀性能。试验制备0. 32与0. 36两种水胶比的普通硅酸盐水泥、高抗硫水泥以及复掺矿粉和硅灰的混凝土试件,检测了试件标养28 d后的抗压强度、动弹性模量及各试件在(20±2)℃、质量分数为3%Na_2SO_4溶液中全浸泡侵蚀的强度、动弹性模量变化情况,对混凝土在(20±2)℃全浸泡侵蚀下的抗硫酸盐侵蚀性能进行了评价。结果表明,在(20±2)℃全浸泡作用下,0. 36水胶比混凝土抗硫酸盐侵蚀性能低于0. 32水胶比混凝土,抗硫酸盐侵蚀性能随着水胶比的降低而提高;混凝土复掺矿粉和硅灰后抗硫酸盐侵蚀性能较好;高抗硫水泥通过限制C_3A的含量,进而改善混凝土抗硫酸盐侵蚀性能,不一定在任何环境下都适用;混凝土连通孔隙率从侧面证实了低水胶比混凝土和复掺矿粉和硅灰混凝土抗压侵蚀系数和相对动弹性模量较高,说明低水胶比混凝土复掺矿粉和硅灰后,可以显著提高混凝土抗硫酸盐侵蚀性能。     

不同环境中矿物掺合料混凝土抗硫酸盐侵蚀性能的研究进展

硫酸盐侵蚀是混凝土耐久性研究的热点之一。矿物掺合料的掺入改变混凝土内部的组成,细化了混凝土的孔结构,对混凝土抗硫酸盐侵蚀起着重要作用。掺合料的化学组成、细度、掺量等对混凝土抗硫酸盐侵蚀均有很大的影响。外界腐蚀环境的不同,矿物掺合料混凝土抗硫酸盐侵蚀的性能也有显著的差别。大量的研究表明,在连续浸泡的硫酸盐溶液中,矿物掺合料只要掺量适当能够提高混凝土抗硫酸盐侵蚀的能力。在干湿循环与硫酸盐溶液共同作用下,矿物掺合料混凝土抗硫酸盐侵蚀的能力有所争议,有些研究表明矿物掺合料能够提高干湿循环条件下混凝土抗硫酸盐侵蚀的能力;然而有些研究结果却呈现相反的结论,这需要进一步探索。     

混凝土抗硫酸盐侵蚀性能及其离子扩散影响因素研究

为了进一步解释混凝土受硫酸盐侵蚀劣化机理,研究了不同水胶比以及养护时间对全浸泡下混凝土抗硫酸盐侵蚀性能的影响,分析了硫酸盐在混凝土中的扩散过程.试验制备了0.32、0.35、0.38这3种水胶比的普通硅酸盐水泥混凝土试件,对其质量、 动弹性模量、 抗压强度、SO42-浓度等指标进行分析.结果表明:全浸泡条件下,混凝土抗...     

硫酸盐侵蚀玄武岩纤维轻骨料混凝土力学性能研究

为增强轻骨料混凝土抗硫酸盐侵蚀性能,对不同掺量的玄武岩纤维轻骨料混凝土(BF-LAC)进行不同龄期的硫酸盐侵蚀试验,研究BF-LAC在硫酸盐侵蚀环境下的力学性能和劣化程度.采用浓度为5％的硫酸盐溶液侵蚀玄武岩纤维掺量为0％、0.1％、0.2％、0.3％的BF-LAC并每隔60 d测试抗压强度、劈裂抗拉强度、有效孔隙率和...     

水泥混凝土硫酸盐侵蚀综述

从化学反应的角度详细介绍了水泥混凝土的硫酸盐侵蚀机理及其相应的侵蚀产物,分别从内部和外部两方面阐述了硫酸盐侵蚀的因素,论述了目前抗硫酸盐侵蚀的一些方法以及这些方法的优劣性,并展望了水泥混凝土抗硫酸盐侵蚀的前景。     

混凝土硫酸盐侵蚀理论模型的研究现状及展望

综合评述了混凝土硫酸盐侵蚀理论模型的研究进展。首先总结了混凝土受硫酸盐侵蚀的劣化机理及存在的争议问题,着重介绍了两类硫酸盐侵蚀理论模型的最新研究进展,包括离子扩散反应,材料初始裂缝的影响,水泥石微观结构改变以及材料损伤演化等问题。对各模型的理论基础、适用条件及存在的不足进行了分析,并提出了硫酸盐侵蚀建模中需要考虑的关键问题。最后,对硫酸盐侵蚀理论模型今后的研究重点进行了展望。     

基于统计损伤理论的硫酸盐侵蚀混凝土本构模型研究

基于统计损伤理论及宏观试验现象,该文建立了考虑硫酸盐侵蚀影响的混凝土单轴、双轴压缩统计损伤本构模型。混凝土变形破坏被理解为细观断裂、屈服两种损伤模式的连续累积演化过程。硫酸盐侵蚀效应改变了混凝土微结构的组成成分和力学特征,进而改变了微裂纹萌生、扩展的形态以及损伤的累积演化过程,可通过改变断裂和屈服两种细观损伤机制演化过程的概率分布形态来模拟。分析结果表明:在硫酸盐侵蚀环境下,侵蚀程度的加深显著改变了混凝土细观损伤累积演化过程,最终导致混凝土宏观力学性能呈现先“强化”后“弱化”的现象。在此过程中,细观损伤演化过程呈现出明显地规律性,可由统计损伤模型中5个特征参数的变化规律表征。该文模型为复杂环境下侵蚀混凝土细观损伤过程预测和分析提供了新的方法和工具。     

硬化期受扰动混凝土的抗硫酸盐侵蚀性能

凝结硬化期间的施工扰动,可能使混凝土产生砂浆微裂缝与骨料位移,进而影响其强度及耐久性.为分析硬化期扰动对混凝土抗硫酸盐侵蚀性能的影响,采用振动台模拟工程扰动,借助超声波和扫描电子显微镜(SEM)等测试手段,系统研究了硬化期受扰混凝土抗硫酸盐侵蚀的劣化规律,探讨了其损伤劣化机理.结果表明:临近初凝(贯入阻力值为3.5～11.5 MPa)和临近终凝(贯入阻力值为19.5～28.0 MPa)的扰动对混凝土影响较小,受侵蚀后试件质量和相对动弹模量的变化规律与未受扰混凝土基本一致.硬化中期(贯入阻力值为11.5～19.5 MPa)的扰动对混凝土性能影响明显,使混凝土抗压强度降低14％,抗折强度降低20％;硫酸盐腐蚀进程中,混凝土质量在90次循环后开始出现明显下降,相对动弹模量在130次循环后出现急剧下降,经250次循环后达到0.60,此时基准混凝土的相对动弹模量仍为0.90;SEM结果表明,受扰混凝土内部微裂纹增多,结构密实性变差,硬化期扰动加速了混凝土在硫酸盐侵蚀环境下的劣化.     

硫酸盐侵蚀下石膏的形成及破坏机制研究现状

硫酸盐侵蚀是影响水泥基材料耐久性的重要因素,它不仅会缩短材料的服役寿命,甚至可能危及结构安全。在硫酸盐侵蚀过程中,钙矾石、石膏和碳硫硅钙石等侵蚀产物不断形成,从而导致材料出现膨胀、开裂、软化和剥落等不同形式的破坏。由于不同侵蚀产物的形成条件和对水泥基材料的侵蚀机理存在明显差异,而侵蚀机理是工程实践中指导预防硫酸盐侵蚀的重要依据,因此探明不同侵蚀产物的形成及稳定条件以及各侵蚀产物对材料的作用机理成为该研究课题的主要内容。 从现有研究来看,钙矾石型硫酸盐侵蚀是目前研究最为成熟的一种硫酸盐侵蚀。钙矾石是在高碱性硫酸盐溶液条件下形成的主要侵蚀产物,并且当其在狭小封闭的孔洞中生长时会导致材料发生膨胀、开裂破坏,相应的膨胀机理有吸水肿胀理论、结晶压理论和固相反应理论等。另外,在钙矾石型硫酸盐侵蚀的预防方面,发现通过控制水泥中铝酸三钙含量可有效减小因钙矾石形成而造成的膨胀危害。近年来,世界各地的研究者竞相报道了碳硫硅钙石的形成对混凝土结构造成严重破坏的工程实例,这使得碳硫硅钙石型硫酸盐侵蚀也逐渐受到重视。目前普遍认为碳硫硅钙石的形成主要导致材料出现泥化和分解现象,但其形成条件较为复杂,只在一些特殊环境下才有可能发生。 石膏是水泥基材料在硫酸盐侵蚀下形成的另一种较为常见的腐蚀产物,它的形成同样影响着水泥基材料的耐久性能。研究发现,硫酸盐溶液浓度越高,越利于形成石膏,但后来发现溶液pH值对石膏的形成及稳定影响更为显著,同时溶液温度、离子种类以及腐蚀制度等对石膏的形成也有一定影响。由于石膏的化学组成相对简单且不含铝相,因此采用普通抗硫酸盐侵蚀方法并不能有效抑制石膏的形成及破坏。石膏的形成往往伴随着水化产物的溶解脱钙,从而导致材料出现软化和剥落现象,但在石膏的膨胀问题上仍存在较大争议。 本文综述了硫酸盐侵蚀下水泥混凝土中石膏形成的影响因素,总结了石膏的生长位置及其引起的脱钙作用,最后对石膏的膨胀作用进行了相关探讨。     

硫酸盐环境中CFRP约束劣化混凝土柱的力学性能

为研究碳纤维增强树脂复合材料(CFRP)加固经硫酸盐腐蚀劣化后混凝土柱的力学性能,构建CFRP约束劣化混凝土的力学模型,采用CFRP约束普通混凝土后腐蚀和CFRP约束劣化混凝土两种工况进行试验,在试验研究基础上,引入损伤率及强度保持率作为损伤的量化指标,建立两者之间的关系;通过对CFRP约束混凝土强度及应变规律的回归分析,建立适用于硫酸盐环境下CFRP约束劣化混凝土的极限强度、应变模型及应力-应变关系模型。结果表明:随侵蚀时间增长,两种工况下CFRP约束混凝土极限强度和应变的变化规律及劣化速度均不同。预劣化的混凝土柱虽然进行了表面处理,但内部已经存在损伤,在侵蚀环境中采用CFRP加固混凝土柱时,应该考虑混凝土初始损伤的影响。      

Procedures for assessing thaumasite sulfate attack and adjacent ground conditions at buried concrete structures

In 1998 Halcrow were appointed by the Highways Agency to investigate the thaumasite form of sulfate attack at the buried concrete foundations of structures on major highways in Gloucestershire, UK. Detailed investigations were completed at 28 structures and the results used to apply a risk assessment procedure to a further ninety structures. Standard procedures for sampling, testing and classifying soil, groundwater and concrete were devised to ensure that rigorous statistical analyses of the data could be applied. The procedures were also designed to meet the then current UK guidance notes, BRE. Digest 363 [Building Research Establishment, Sulphate and acid resistance of concrete in the ground, BRE Digest 363, CRC Press, Boca Raton, 1996] and the Thaumasite Expert Group Report [Thaumasite Expert Group, The thaumasite form of sulfate attack: Risks, diagnosis, remedial works and guidance on new construction, DETR, 1999] and are compatible with the subsequent BRE Special Digest 1 [Building Research Establishment. Concrete in aggressive ground, Part 1: assessing the aggressive chemical environment, BRE Special Digest 1, CRC, Boca Raton, 2001]. Recommendations are made for applying many of the procedures that have been developed to future site investigations to enable consistency, compatibility and easier data transfer for future research on the thaumasite form of sulfate attack.     

Thaumasite sulfate attack in field and laboratory concretes: implications for specifications

The reported studies made on field elements affected by thaumasite sulfate attack are discussed, together with the reported laboratory performance of concrete and mortars immersed in sulfate solutions maintained at 5 °C. It is concluded that magnesium ions ingressing from groundwater or resulting from dedolomitization of an aggregate containing dolomite play a major role in increasing the risk of thaumasite attack and that attack can also arise from exposure to sulfuric acid bearing groundwaters, or sulfuric acid resulting from oxidation of pyrites within aggregates. Guidance is given on concrete qualities and materials to resist sulfate attack and thaumasite sulfate attack resulting from salts present in groundwater.     

Thaumasite formation in concrete and mortars containing fly ash

Due to recent reports on deterioration of concrete structures, the thaumasite form of sulfate attack has become a subject of study and close investigation. This paper investigates the formation of thaumasite in concrete and mortars containing fly ash. The results show that thaumasite formation can occur within 84 days of exposure to sulfate solutions. High volumes of fly ash can limit or promote thaumasite formation depending on the type of cement used. Thaumasite and ettringite were found among the deterioration products. However, the thaumasite formation in the specimen prepared from sulfate resisting Portland cement was not accompanied by deterioration, except by 50% fly ash addition. The mixtures of Portland limestone cement with 40% fly ash exhibited a very limited thaumasite formation while the mixtures with 50% had no thaumasite at all. It is concluded that thaumasite can also be formed in mixtures incorporating fly ash.     

The laboratory investigation of concrete affected by TSA in the UK

Procedures that have been successfully employed by Geomaterials Research Services Ltd. in the determination of the distribution of the thaumasite form of sulfate attack (TSA) in concrete samples taken from bridge and other motorway structures throughout the UK are described. Electron microprobe analysis has been used to provide confirmation of the presence of thaumasite and to investigate the distribution of sulfate compounds including gypsum and ettringite in cement paste at the surfaces of concrete affected by sulfate attack. Electron microprobe analysis has the advantage that it is capable of detecting very small quantities of thaumasite. Electron microprobe analysis used in conjunction with petrographic analysis is regarded as the most effective tool for the diagnosis of TSA and other forms of sulfate attack.

The analysis of numerous cores taken from bridge foundations throughout the UK shows that the development of TSA is often accompanied by the formation of calcium carbonate in cement paste. High levels of chloride in the cement paste of TSA damaged concrete suggest the importance of run-off moisture as a contributory factor in the development of TSA.     

The co-existence of thaumasite and ettringite in concrete exposed to magnesium sulfate at room temperature and the influence of blast-furnace slag substitution on sulfate resistance

The effects of Type I Portland cement replacement by 45% or 72% blast-furnace slag on the sulfate resistance of laboratory concretes were analyzed by microstructural investigation. The concretes investigated were stored in water or in magnesium sulfate solutions for 23 years under laboratory conditions. For those stored in water only surface layers of carbonation and decalcification were observed. Concretes exposed to sulfate solutions formed brucite, ettringite and thaumasite. Thus, thaumasite was observed to form in concretes stored under laboratory conditions. In all cases both ettringite and thaumasite were found to co-exist in the damaged zones. However, the thaumasite appears to be moving in from the exterior after initial formation of ettringite, and has not resulted in the massive destruction of the hydrated matrix as has been found elsewhere at lower temperature exposures. Slag replacement was observed to be an effective means of conferring resistance to sulfate attack. Although the concretes studied were prepared at a W/cm (water-to-cementitious materials) ratio of 0.50, the depths of attack observed were comparable to those observed in concrete prepared at w/c=0.45 using ASTM Type V (SRPC) cement alone.     

Sulfate resistance of limestone cement concrete exposed to combined chloride and sulfate environment at low temperature

Concrete durability was investigated, taking under consideration the limestone content of the cement used, as well as the effect of chlorides on concrete’s deterioration due to the thaumasite form of sulfate attack. A normal Portland cement and two Portland limestone cements (15% and 35% w/w limestone content) were used for concrete preparation. The specimens were immersed in two corrosive solutions (chloride-sulfate; sulfate) and stored at 5 ± 1 °C. Visual inspection of the specimens, mass measurements and compressive strength tests took place for 24 months. Concretes containing limestone, as cement constituent and/or as aggregate, suffered from the thaumasite form of sulfate attack, which was accompanied by brucite and secondary gypsum formation. Limestone cement concretes exhibited higher deterioration degree compared to the concrete made without limestone cement. The disintegration was more severe and rapid, the higher the limestone content of the cement used. Chlorides inhibit sulfate attack on concrete, thus delaying and mitigating its deterioration.     

Use of mineral admixtures to improve the resistance of limestone cement concrete against thaumasite form of sulfate attack

In this work the effect of mineral admixtures on the thaumasite form of sulfate attack in limestone cement concrete is studied. Additionally, the effect of the type of sand (calcareous or siliceous) and the storage temperature is investigated. Limestone cement, containing 15% limestone, was used. Concrete specimens were prepared by replacing a part of cement with the studied minerals. The specimens were immersed in a 1.8% MgSO₄ solution and stored at 5 °C and 25 °C for 3 years. A well designed concrete made with limestone cement and fly ash, blastfurnace slag or metakaolin seems to have the ability to withstand thaumasite form of sulfate attack. The addition of natural pozzolana presented only a limited improvement of concrete’s sulfate resistance. The type of the sand and its cohesion with the cement paste has a remarkable effect on the performance of concrete at low temperature. Finally, no damage was observed in the specimens exposed to sulfate solution at 25 °C.     

Thaumasite field trial at Shipston on Stour: three-year preliminary assessment of buried concretes

A combined field and laboratory trial to investigate the thaumasite form of sulfate attack (TSA) in buried concrete was instigated by BRE in 1998 following the discovery of several cases of TSA in UK below-ground construction. Two identical sets of 98 concrete specimens were buried in sulfate-bearing Lower Lias Clay at a site in Central England. The first set of specimens was excavated in June 2001, after three years. The other set will be excavated after 10 years. A further similar assemblage has been subjected in the laboratory at 5 °C to a sulfate solution which simulates the site groundwater. Each set contains specimens with a range of quality and composition, mixes including combinations of eight binder types and four aggregate types, including limestone and siliceous aggregate.

After three years, identical concretes have generally behaved similarly in the field and laboratory studies, with a range of sulfate attack from none to moderate surface attack, dependent on concrete quality and composition. XRD analysis has shown the dominant form of sulfate attack to be TSA.