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。     

盐渍土腐蚀环境下微胶囊自修复混凝土抗腐蚀性能研究

为保证盐渍土地区混凝土服役寿命,区别于以往自修复材料,合成一种可以感知外界腐蚀离子并释放内部修复剂的微胶囊,以合成温度为变量控制引发剂分解速率得到了具有不同包裹率的微胶囊。选取其中包裹率、腐蚀离子响应能力较好的微胶囊,将其掺入混凝土中得到具有自修复功能的混凝土。改变微胶囊掺量测试混凝土动弹性模量及抗腐蚀能力。研究结果表明：在合成温度为75℃环境条件下的微胶囊包裹率优于65和85℃环境条件下。盐渍土-硫酸盐干湿循环腐蚀下混凝土构件动弹性模量均呈现先上升后下降的趋势,在20次干湿循环后未掺微胶囊的混凝土动弹性模量急速下降,在40次干湿循环后为掺微胶囊的混凝土内部侵入了更多的硫酸盐,试验拌制的自修复混凝土具有良好的抗硫酸盐侵蚀能力,对实际工程中建筑结构应对硫酸盐腐蚀具有十分重要的借鉴意义。     

盐雾-烘干循环作用下钢筋混凝土腐蚀特性研究

为研究我国西部盐渍土地区大气环境中钢筋混凝土腐蚀劣化过程及掺入不同种类矿物掺和料对其影响,采用4种不同混凝土配合比制备钢筋混凝土试件并进行盐雾-烘干加速侵蚀试验,对钢筋自腐蚀电位、腐蚀电流密度及混凝土相对动弹性模量、腐蚀形貌等耐久性相关参数加以分析。结果表明,掺入不同矿物掺合料对混凝土内部孔隙结构的改善效果及对钢筋混凝土抗腐蚀性能的提升作用由优到差依次是:粉煤灰、粉煤灰-矿粉、无矿物掺和料成分、矿粉,因此掺入粉煤灰不仅节约成本,而且提高了钢筋混凝土构件在西部环境中的服役寿命。     

硝酸侵蚀/冻融循环共同作用喷射混凝土耐久性能(Ⅰ):物理力学性能及孔结构变化

以寒冷地区喷射混凝土单层永久衬砌长大公路隧道为工程背景,汽车尾气中氮氧化物与水产物硝酸为冻融介质,采用快速冻融循环法,开展喷射混凝土冻融循环试验,研究了硝酸侵蚀冻融循环共同作用对喷射混凝土耐久性能的影响。以直线导线法对硝酸侵蚀/冻融循环共同作用下混凝土的孔结构进行表征,探究了共同作用喷射混凝土的冻融损伤过程。综合分析认为,硝酸中氢离子对混凝土产生化学侵蚀,硝酸根离子在冻融循环过程中产生盐冻的效果,加快了喷射混凝土冻融损伤劣化速度。共同作用喷射混凝土的抗冻性随水胶比增大而降低,随粉煤灰掺量增大先提升后降低。但随钢纤维掺量增大,喷射混凝土动弹性模量损失率和质量损失率先增大后减小,抗压强度则逐渐增大。随着冻融循环次数增多,喷射混凝土孔结构劣化,大孔径孔和微裂缝数量增大,加速了硝酸向混凝土内部扩散,抗冻性能快速下降。     

玄武岩纤维混凝土在冻融环境下的力学性能研究

为了研究玄武岩纤维混凝土在西北寒冷地区盐冻作用下,性能是否满足工程要求,对此,该文对玄武岩纤维混凝土在冻融循环作用下的力学性能进行研究,研究结果表明以下3点：1）在75次冻融循环前,素混凝土和纤维掺入量分别为0.04%、0.08%和0.12%玄武岩纤维混凝土的质量损失率均增长缓慢,在75次冻融循环后,其质量损失率快速增大。2）在50次冻融循环前,素混凝土和不同掺量玄武岩纤维混凝土的相对动弹性模量变化较小,在50次冻融循环后,其相对动弹性模量快速变小。3）在相同冻融次数条件下,素混凝土质量损失率最大,而相对动弹性模量最小,纤维掺入量为0.12%玄武岩纤维混凝土质量损失率最小,而相对动弹性模量最大。以上研究可供类似混凝土工程参考。     

聚乙烯醇纤维对盐冻混凝土抗折强度的影响

通过聚乙烯醇纤维增强混凝土(PVA-FRC)试件和素混凝土试件28 d弯曲抗折试验和300次盐冻后弯曲抗折试验,考察盐冻对混凝土试件弯曲抗折强度的损伤和纤维对盐冻前后混凝土试件弯曲抗折强度的增强作用.设置了3种纤维体积掺量的PVA-FRC试件,并在纤维体积掺量为1.5％的基础上,分别以10％硅灰和20％粉煤灰替代等量水泥试图提高PVA-FRC试件盐冻前后抗折强度.结果表明,PVA-FRC试件盐冻前后抗折强度均大于素混凝土试件盐冻前后抗折强度,纤维体积掺量为1.5％的PVA-FRC试件盐冻前后增益比最大;PVA-FRC试件盐冻后抗折强度损伤量远小于素混凝土试件抗折强度损伤量,纤维体积掺量为1.5％的PVA-FRC试件损伤量最低;硅灰和粉煤灰的掺加没有提高PVA-FRC试件盐冻前后的抗折强度,也没能降低PVA-FRC试件盐冻后抗折强度损伤量.     

纳米碳纤维增强混凝土抗冻性能试验

对六种不同纳米碳纤维掺量的72个纳米碳纤维/混凝土试件进行了慢冻融循环试验,通过测量纳米碳纤维/混凝土经不同冻融循环次数作用后的抗剥落能力、质量损失率、相对动弹性模量和抗压强度损失率,研究了纳米碳纤维掺量对纳米碳纤维/混凝土抗冻性能的影响。另外进行了纳米碳纤维/混凝土的FE-SEM试验和压汞试验,分析了纳米碳纤维对纳米碳纤维/混凝土抗冻性能的微观改性机制。结果表明:纳米碳纤维通过改善混凝土的微观形貌,细化其孔隙结构,提高其整体性和密实度,显著改善了混凝土的抗冻性能;纳米碳纤维掺量为3vol%时,纳米碳纤维/混凝土的抗冻性能最佳。同普通混凝土相比,300次冻融循环后,纳米碳纤维/混凝土的相对动弹性模量提高了33.2%,抗剥落能力显著增强;相同冻融次数下,随着纳米碳纤维掺量的增加,纳米碳纤维/混凝土相对动弹性模量和抗压强度损失率均先增大后减小,质量损失率先减小后增大。但纳米碳纤维掺量最大为5vol%时,纳米碳纤维/混凝土的抗冻性能仍优于普通混凝土;冻融循环次数越多,纳米碳纤维对混凝土抗冻性能的改善作用越显著。      

氯盐-硫酸盐共存环境中杂散电流作用下提升砂浆中氯离子结合性能的研究

长期浸泡在地下水中的地铁混凝土结构,不仅遭受氯盐、硫酸盐的双重侵蚀,而且存在杂散电流腐蚀破坏.由于孔溶液中的自由氯离子是导致钢筋锈蚀的首要因素,通过提高混凝土中氯离子的结合能力可有效降低氯离子对钢筋混凝土的危害.本工作通过选择合理的外掺料种类及掺量,提出了提高地铁工程混凝土中氯离子结合性能的最优外掺料组合,并采用电位滴定法测定结合氯离子含量,以及结合XRD、SEM和DTG等微观测试方法对其机理进行分析.结果表明:杂散电流作用下偏高岭土对氯离子结合性能的提升效果优于硅灰,粉煤灰微珠优于沸石粉,复掺10％偏高岭土、20％微珠、1.5％PVA可再分散性乳胶粉的试件中氯离子的结合性能最优,砂浆内部氯离子的化学结合能力明显提高.     

氯盐侵蚀-冻融循环耦合作用下改性糯米灰浆耐久性能增强方法

为改善传统糯米灰浆应用在砖石古建筑修复与加固中的酥碱冻害,掺加质量分数为12.5％、25％和50％的偏高岭土与1％的麻纤维,获得了一种改性糯米灰浆复合材料.以力学强度、质量损失及相对动弹性模量为依据,详细研究了改性灰浆的应用性能.利用红外光衍射、X-ray粉末衍射和扫描电镜等微观分析方法,系统探究了改性灰浆在标准养护、冻融老化及氯盐侵蚀-冻融循环耦合作用下的性能演变机理.结果表明,混掺纤维和偏高岭土能显著改善灰浆的强度、抗冻性与耐盐性.偏高岭土生成的水化凝胶C-S-H与C-A-H可固化氯离子生成弗里德尔盐以抵抗孔隙破坏,增强了灰浆在氯盐环境下的抗冻性.氯盐侵蚀-冻融循环耦合作用下耐久性指标随麻纤维的掺加而小幅增大,随偏高岭土掺加而先提升后降低.其中以在糯米灰浆中混掺1％麻纤维与25％偏高岭土的增强方法最优.     

不同取代率下再生混凝土的抗冻融性能试验

采用快冻法对9种再生混凝土进行了冻融循环试验。分析了经受100次冻融循环后不同取代率下再生粗骨料和再生细骨料对混凝土立方体抗压强度的影响;研究了每25次冻融循环后再生混凝土的棱柱体质量损失率和相对动弹性模量的变化;比较了100次冻融循环作用对不同取代率下再生混凝土抗压承载力的影响。结果表明,再生混凝土抗冻融性能有所下降,且降低幅度随再生骨料取代率的增加而加剧;100次冻融循环后,再生粗骨料取代率为50%,细骨料为天然骨料的再生混凝土,其质量损失率、相对动弹性模量衰减幅度和抗压承载力损失率均与普通混凝土接近,建议再生粗骨料掺量不高于50%的再生混凝土可在寒冷地区推广应用。     

Service life and global warming potential of chloride exposed concrete with high volumes of fly ash

Today, it remains unclear how ‘green’ concrete with high volumes of fly ash really is, especially when subject to chloride-induced corrosion. This paper presents chloride diffusion test results for high-volume fly ash and fly ash + silica fume concrete. Apparent diffusion coefficients and surface concentrations were compared with those for traditional concrete. Instantaneous chloride diffusion coefficients and ageing exponents were estimated and critical chloride contents for submerged exposure conditions were experimentally verified. The estimated time to chloride-induced steel depassivation for the two concrete types with fly ash (60 to more than 100 years) was much longer than for traditional concrete (24–32 years). As a consequence, global warming potentials (GWPs) calculated for the required concrete volume per unit of strength and service life indicate that an important reduction in greenhouse gas emissions is possible for both concrete types with high volumes of fly ash (GWP –50 to −82%).     

Structure formation of slag ash concrete on the basis of high-calcium fly ash and silica fume

Cementless slag ash concrete may be manufactured using high-calcium fly ash and silica fume as replacements for a binder and a microfiller, and incorporating slag sand from thermal power plants (TPP) as an aggregate. This concrete consists of waste products from TPP (fly ash and slag) and ferro-alloy plants (silica fume) and contains neither natural nor artificial aggregates for lightweight and heavy concretes. Silica fume (10–20% by weight of ash) and hot water together with subsequent heat treatment of concrete products or of castin situ structures binds the excess free calcium oxide present in the ash, and thus prevents deterioration of the concrete. The processes of concrete structure formation were investigated after 24 hours, 28 days, 3 and 6 months and the physico-mechanical, deformation and special properties (frost resistance, heat conductivity, protection of reinforcement from corrosion) were studied. This concrete conforms to the Russian Federation GOST requirement for use in single, two-storey buildings. The cost of the concrete is reduced by a factor of 3 compared with that of ordinary concrete.     

Property investigation of individual phases in cementitious composites containing silica fume and fly ash

In this study, nanoindentation was used to investigate the microstructures of cementitious composites containing silica fume and fly ash. With the help of scanning electron microscope, the mechanical properties (elastic modulus and hardness) of individual phases (like outer product, inner product, calcium hydroxide, remained fly ash particles, residual cement grains) in cementitious composites containing silica fume and fly ash were investigated and analyzed. Additionally, this study examined the differences between the ‘C–S–H’ phases in the different cementitious composites and provided an insight into the influence of mineral admixtures (silica fume and fly ash) on the properties of the ‘C–S–H’ phase.     

Compressive strength and microstructure of fly ash based geopolymer blended with silica fume under thermal cycle

This work aims to reveal the effects of silica fume on properties of fly ash based geopolymer under thermal cycles. Geopolymer specimens were prepared by alkali activation of fly ash, which was partially replaced by silica fume at levels ranging from 0% to 30% with an interval of 10%, by mass. Microstructure, residual strength and mass loss of fly ash based geopolymer blended with silica fume before and after exposed to 7, 28 and 56 heat-cooling thermal cycles at different target temperatures of 200 °C, 400 °C and 800 °C were assessed and compared. The experimental results reveal that silica fume addition enhances strength development in geopolymer. Under thermal cycles, the compressive strength of geopolymer decreases, and the compressive strength loss, as well as the mass loss, increases with increasing target temperature. The strength loss is the same regardless of silica fume content after thermal cycles. Microstructure analysis uncovers that pore structure of geopolymer degrades after thermal cycles. The pores of geopolymer are refined by the addition of silica fume. The incorporation of silica fume optimizes the microstructure and improves the thermal resistance of geopolymer. Silica fume increases the strength of the geopolymer and even though the strength loss is the same, the strength after heat cycle exposure is still good.     

Effectiveness of corrosion inhibiting admixture combinations in structural concrete

A long-term corrosion study was conducted to determine the effectiveness of calcium nitrite, silica fume, fly ash, ground granulated blast furnace slag, and DSS in reducing corrosion of reinforcing steel in concrete. Mixture proportions included single, double, and triple combinations of these admixtures. Non-cracked and pre-cracked slab specimens were evaluated by visual inspections, macrocell readings, half-cell potentials, and autopsies. Triple combinations of calcium nitrite, silica fume, and either fly ash or ground granulated blast furnace slag, as well as a double combination of calcium nitrite and ground granulated blast furnace slag, performed very well and are recommended in concrete mixtures exposed to severe corrosive environments. DSS outperformed the other admixtures for corrosion prevention in this study; however, it resulted in somewhat lower compressive strengths and had not been fully tested for effects on other concrete properties.     

Sulfate resistance of plain and blended cements exposed to varying concentrations of sodium sulfate

Concrete deterioration due to sulfate attack is the second major durability problem, after reinforcement corrosion. This type of deterioration is noted in the structures exposed to sulfate-bearing soils and groundwater. Though concrete deterioration due to sulfate attack is reported from many countries, the mechanisms of sulfate attack have not been thoroughly investigated, particularly the effect of sulfate concentration and the cation type associated with the sulfate ions on concrete deterioration. This study was conducted to evaluate the performance of plain and blended cements exposed to varying concentrations of sodium sulfate for up to 24 months. Four types of cements, namely Type I, Type V, Type I plus silica fume and Type I plus fly ash, were exposed to five sodium sulfate solutions with sulfate concentrations of 1%, 1.5%, 2%, 2.5% and 4%. These concentrations are representative of the sulfate concentration in highly saline soils. The sulfate resistance was evaluated by visual examination and measuring the and reduction in compressive strength. The maximum deterioration, due to sulfate attack, was noted in Type I cement followed by silica fume and Type V cements. The performance of Type V, Type I plus silica fume and Type I plus fly ash was not significantly different from each other. The enhanced sulfate resistance noted in the Type I cement blended with either silica fume or fly ash indicates the usefulness of these cements in both sulfate and sulfate plus chloride environments.     

Compressive strength and chloride resistance of self-compacting concrete containing high level fly ash and silica fume

The influence of high-calcium fly ash and silica fume as a binary and ternary blended cement on compressive strength and chloride resistance of self-compacting concrete (SCC) were investigated in this study. High-calcium fly ash (40–70%) and silica fume (0–10%) were used to replace part of cement at 50, 60 and 70 wt.%. Compressive strength, density, volume of permeable pore space (voids) and water absorption of SCC were investigated. The total charge passed in coulombs was assessed in order to determine chloride resistance of SCC. The results show that binary blended cement with high level fly ash generally reduced the compressive strength of SCC at all test ages (3, 7, 28 and 90 days). However, ternary blended cement with fly ash and silica fume gained higher compressive strength after 7 days when compared to binary blended fly ash cement at the same replacement level. The compressive strength more than 60 MPa (high strength concrete) can be obtained when using high-calcium fly ash and silica fume as ternary blended cement. Fly ash decreased the charge passed of SCC and tends to decrease with increasing fly ash content, although the volume of permeable pore space (voids) and water absorption of SCC were increased. In addition when compared to binary blended cement at the same replacement level, the charge passed of SCC that containing ternary blended cement was lower than binary blended cement with fly ash only. This indicated that fly ash and silica fume can improve chloride resistance of SCC at high volume content of Portland cement replacement.     

Interactive Effects of Fly Ash and CNI on Corrosion of Reinforced High-Performance Concrete

The interactive effects of fly ash and CNI in corrosion of reinforced concrete were investigated. A 3⁴ full factorial design was developed considering water to cement ratio, fly ash percent, CNI and cracked condition as factors. The response factors were the weight loss calculated from Linear Polarization Resistance measurements and the pit depth of the steel bars embedded in concrete. Small-scale concrete slabs containing steel reinforcement with a cover depth of 20 mm were cast for this purpose. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day during one year; after the exposure, the slabs were broken, the bars were cleaned and the pith depth measured by using SEM. Under the studied conditions, it was found that CNI alone does not provide corrosion protection of the steel reinforcement even for uncracked silica fume concrete in a 0.45 w/c ratio; however, the combination of CNI and fly ash can be useful to overcome this problem. The results indicate that low w/c ratio concrete in its crack state creates conditions suitable for the development of pitting corrosion.     

Strength and resistance to sulfate and sulfuric acid of ground fluidized bed combustion fly ash–silica fume alkali-activated composite

Fluidized bed combustion (FBC) is an environmentally friendly process for burning of coal and is used in many small factories located in urban area. The FBC fly ash is an environmental problem and needs good disposal or utilization. This research studied the strength and resistance to sulfate and acid of alkali-activated FBC fly ash–silica fume composite. The FBC fly ash was interground with silica fume (at the dosage levels of 1.5%, 3.75% and 5.0%) to make the source material homogenous with increased reactivity. Addition of silica fume enabled the adjustment of SiO₂/Al₂O₃ ratios (6.55-7.54) of composite and improved the strength and resistance to sulfate and acid of composite. The composite with 3.75% silica fume showed the optimum strength with 28-day compressive strength of 17.0 MPa. The compressive strengths of composite with 3.75% silica fume immersed in 5% magnesium sulfate solution and 3% sulfuric acid solutions were substantially higher than the control. The strength loss was from the high calcium content of FBC fly ash and incorporation of silica fume thus increased the durability of the composite.     

橡胶颗粒-钢纤维混掺对碾压混凝土抗冻性及抗冲击性能的影响

本文研究了在质量分数为25%的醋酸钾（KAc）为冻融介质的环境中,橡胶颗粒（RP）-钢纤维（SF）混掺对碾压混凝土（RCC）抗冻性及抗冲击性能的影响,获得了冻融过程中RCC的抗弯冲击性能、相对动弹性模量、质量损失、微观形貌及孔结构变化等参数。结果表明：RCC累计质量损失随冻融次数增加而增大,RP对质量损失的影响较小,而SF能较明显控制质量损失快速增长,300次冻融循环后累计质量损失仅83.94 g/m²; RP能降低RCC的总孔隙率约0.6%,而SF能有效降低总孔隙率,但冻融后期锈蚀作用会导致总孔隙率迅速增长; SF能极大RCC提高抗弯冲击性能,试验条件下抗冲击次数从3~5次水平提升至140~170次水平,而RP对冲击性能并无明显影响,冻融结束掺有SF试验组RCC初、终裂次数降幅超过70%,但终裂次数仍达到30~40次的水平,绝对韧性均随冻融次数增加逐渐降低; RCC相对动弹模量均表现出先降低后缓慢回升的规律,掺有SF试验组下降阶段相对更短且幅度更小,最大仅8%,而RP影响并不明显,但4组降低程度均在10%内。