Durability properties of micro-cracked ECC containing high volumes fly ash

This paper presents the durability of Engineered Cementitious Composites (ECC) that contain high percentages of Class-F fly ash (FA). ECC is a newly developed high performance fiber reinforced cementitious composite with substantial benefit in both high ductility in excess of 3% under uniaxial tensile loading and improved durability due to intrinsically tight crack width. Composites containing two different contents of FA as a replacement of cement (55 and 70% by weight of total cementitious material) are examined after 28 days of curing. Accelerated aging (exposure to continuous sodium hydroxide at 38 °C and sodium chloride solutions at room temperature) and tests of transport properties (salt ponding, rapid chloride permeability and sorptivity tests) are used to study the effect of FA on the durability of the ECC. After accelerated aging, direct tensile tests are performed to evaluate the effect of deterioration on the tensile strength, tensile strain capacity and crack width of ECCs. In addition to virgin specimens, the durability performances of mechanically loaded specimens are also tested. Test results show that both mechanically pre-loaded and virgin (without pre-loading) ECC mixtures with high volumes of FA remain durable in terms of mechanical performances after accelerated aging period, and show a tensile strain capacity of more than 2%. In terms of transport properties, micro-cracks induced by mechanical pre-loading increase the chloride transport and the sorptivity values of ECC. Moreover, increasing FA content is shown to have a negative effect especially on the transport properties of ECC tested in this study. However, the risk of water transport by capillary suction and chloride transport by diffusion in ECC, cracked or uncracked, is found to be comparable with that in normal sound concrete.     

Effect of binder content on the performance of alkali-activated slag concretes

This paper assesses the mechanical and durability performance of concretes produced using alkali silicate-activated ground granulated blast furnace slag as sole binder. Alkali-activated concretes are formulated with 300, 400 and 500 kg slag per m³ of fresh concrete, and their performance is compared with reference concretes produced using Portland cement (OPCC). Regardless of the binder content, the alkali-activated slag concretes (AASC) develop higher compressive strength than the comparable reference concretes. A higher binder content leads to increased strength in both AASC and OPCC at 28 days. However, at 90 days, the performance penalty for low binder content is more significant in the OPCC than AASC samples. Permeability, water sorption and carbonation resistance properties are also improved at higher binder contents. By controlling mix design parameters, it is possible to produce AASC with mechanical strength and durability comparable to conventional Portland cement concretes.     

生物渗透反应格栅修复甲基叔丁基醚污染地下水的模拟

渗透反应格栅技术(PRB)修复污染地下水的现场模拟是实际工程设计、过程预测及修复效果评价的有效手段.以甲基叔丁基醚(MTBE)为研究对象,在多孔介质流体动力学、传质学、生物降解动力学等基础上,建立了生物渗透反应格栅修复MTBE污染地下水的数学模型.由双柱法的实验结果可知,模型模拟结果与实验数据的平均相对误差为5.74%,说明文中所建模型能够很好地描述MTBE在反应格栅内的降解去除过程;由污染物在格栅及其附近区域的浓度分布可知,由于部分流体从格栅侧面进入,污染物在此流径上得不到充分降解,表现为生物渗透反应格栅系统处理后的地下水区域边缘处的MTBE浓度高于其中心区域浓度,该结果可用于指导现场PRB系统及其附属设施的设计和安装.     

C30西昌全高钛重矿渣骨料混凝土性能试验研究

为研究西昌高钛重矿渣骨料配制工业与民用建筑工程泵送混凝土的可行性,针对天然砂石骨料和全高钛重矿渣骨料,结合相关规范,分别进行C30混凝土配合比设计,通过对比试验研究了混凝土拌合性能、力学性能和耐久性能.研究表明对西昌高钛重矿渣骨料进行12 h的饱水预湿机制,可以显著提高混凝土的性能;西昌高钛重矿渣骨料可以替代天然骨料配制泵送混凝土,其拌合性能、力学性能、耐久性能和结构性能,符合相关规范的要求.     

Lightweight geopolymer concrete: A critical review on the feasibility,mixture design,durability properties,and microstructure

Lightweight geopolymer concretes have gained attention because of their superior durability, lower environmental impact and sustainable characteristics. They are the product of natural or artificial aggregates with low specific gravities mixed with aluminosilicate binders, and an alkaline solution. In this study, different aspects of lightweight geopolymer concretes and mortars such as environmental and economic considerations, materials and mixture, durability-related properties like permeability, chloride attacks and performance at high temperatures, thermal conductivity, and the microstructure are reviewed. This study also discusses the effect of different geopolymer binders and various alkaline activators and additives with focus on lightweight geopolymer concrete made with different lightweight aggregates. The key results from previous studies in literature pertaining mix proportions, chemical composition and properties of lightweight geopolymers are summarized and presented. The main aim is to provide an informed outlook on the advantages and drawbacks of lightweight geopolymer concretes and present a comprehensive review of the studies performed in this area.     

Development of low-pH cementitious materials for HLRW repositories: Resistance against ground waters aggression

One of the most accepted engineering construction concepts of underground repositories for high radioactive waste considers the use of low-pH cementitious materials. This paper deals with the design of those based on Ordinary Portland Cements with high contents of silica fume and/or fly ashes that modify most of the concrete “standard” properties, the pore fluid composition and the microstructure of the hydrated products. Their resistance to long-term groundwater aggression is also evaluated. The results show that the use of OPC cement binders with high silica content produces low-pH pore waters and the microstructure of these cement pastes is different from the conventional OPC ones, generating C-S-H gels with lower CaO/SiO₂ ratios that possibly bind alkali ions. Leaching tests show a good resistance of low-pH concretes against groundwater aggression although an altered front can be observed.     

Experiment research on mix design and early mechanical performance of alkali-activated slag using response surface methodology (RSM)

To enhance the quality of alkali-activated slag (AAS) materials, scientific and efficient mix design method is preferred. This paper presents an optimization of AAS materials using Response Surface Methodology (RSM). Three factors related to early strength such as modulus (n), concentration of alkali activator (CAA) and liquid–solid ratio (LSR) were investigated. Specimens with different mix ratios were prepared based on RSM design. The early mechanical performance was assessed, after 2 or 3 h of curing. Then response surface models were established and the effect law of each factor was systemically analyzed. The result shows that both n and CAA have a significant effect on the early strength, while LSR affects slightly. By adjusting the mix design parameters, the early performance of AAS can be effectively improved. This study verifies that RSM is efficient in the preparation of AAS and it can control the early strength of AAS accurately.     

Fracture Toughness of Plasma‐Sprayed Thermal Barrier Ceramics: Influence of Processing,Microstructure, and Thermal Aging

Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd₂Zr₂O₇ which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.     

复合盐侵蚀再生混凝土耐久性能退化试验研究

我国西北地区土壤及其地下水中含有大量侵蚀性离子,造成了再生混凝土结构耐久性能快速退化,严重制约着再生混凝土在主体结构中的应用。基于西北地区耐久性环境中侵蚀离子组成及区域气候环境特征,以7.5%(质量分数)MgSO₄-7.5%(质量分数)Na₂SO₄-5%(质量分数)NaCl为侵蚀介质,采用干湿交替方式,开展复合盐侵蚀再生混凝土耐久性能退化试验研究。以粉煤灰、矿渣、硅灰、偏高岭土等活性矿物掺合料取代水泥,研究矿物掺合料搭配方式对再生混凝土耐久性能影响。采用XRD、FTIR、TG-DSC及SEM、EDS等表征手段,分析侵蚀产物矿物组成、含量及微结构变化,研究复合盐侵蚀再生混凝土损伤过程。随着干湿交替次数增加,再生混凝土相对动弹性模量、质量变化率、相对抗压强度、相对劈裂抗拉强度及损伤层厚度的变化规律与矿物掺合料搭配方式有着显著的相关性。随着侵蚀离子的不断扩散,混凝土碱度降低,在表面依次形成了以水镁石、石膏及钙钒石为主要产物的致密侵蚀产物层,短暂阻碍了侵蚀离子进一步扩散。随着侵蚀产物不断堆积及无胶凝性水化硅酸镁的形成,在膨胀应力及盐结晶压力共同作用下,再生混凝土表面大量剥蚀并开裂,耐久性能快速退化。     

生物质橡胶抗撕裂剂应用性能研究

含天然生物质的橡胶抗撕裂剂组合物及其制备方法和应用。生物质抗撕裂剂组合物是一种反应性助剂,与硫化剂一起使用,可与天然橡胶及合成橡胶等固体橡胶共交联,对环境及人体污染的缺点。含生物质抗撕裂剂组合物制得的胎面胶,胶料混合过程的能耗低,挤出物尺寸稳定性好,挤出和压延胶料表面质量好,与常用的胎面胶组合物相比,在确保加工性能的前提下胎面胶物理机械性能好,抗撕裂性高,耐切割性高,可以提高轮胎的耐久性、延长轮胎寿命。     

Effect of order of mixing on morphology and thermal properties of the compatibilized PBT and ABS alloys/OMT nanocomposites

Poly(butylene terephthalate) (PBT) and acrylonitrile–butadiene–styrene terpolymers (ABS) alloys/organically modified montmorillonite (OMT) nanocomposites using terpolymers of random ethylene, methyl acrylate, and glycidyl methacrylate as the reactive compatibilizer were prepared by different melt‐mixing sequences. The microstructures were characterized by scanning electron microscopy, X‐ray diffraction, transmission electron microscopy, and high‐resolution electron microscopy. It was found that order of mixing affects the dispersion state of OMT in the alloy matrix. The crystallization behavior of PBT in the compatibilized PBT and ABS alloys/OMT nanocomposites was studied by wide angle X‐ray diffraction. It revealed that order of mixing has influence on the preferential crystal growing direction of PBT owing to the antagonistic effect of ABS and OMT. Thermogravimetric analyses and differential scanning calorimetry also showed order of mixing changes the thermal property of the compatibilized PBT and ABS alloys/OMT nanocomposites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2130–2139, 2007     

A concrete mix proportion design algorithm based on artificial neural networks

The concepts of five parameters of nominal water-cement ratio, equivalent water-cement ratio, average paste thickness, fly ash-binder ratio, grain volume fraction of fine aggregates and Modified Tourfar's Model were introduced. It was verified that the five parameters and the mix proportion of concrete can be transformed each other when Modified Tourfar's Model is applied. The behaviors (strength, slump, et al.) of concrete primarily determined by the mix proportion of concrete now depend on the five parameters. The prediction models of strength and slump of concrete were built based on artificial neural networks (ANNs). The calculation models of average paste thickness and equivalent water-cement ratio can be obtained by the reversal deduction of the two prediction models, respectively. A concrete mix proportion design algorithm based on a way from aggregates to paste, a least paste content, Modified Tourfar's Model and ANNs was proposed. The proposed concrete mix proportion design algorithm is expected to reduce the number of trial and error, save cost, laborers and time. The concrete designed by the proposed algorithm is expected to have lower cement and water contents, higher durability, better economical and ecological effects.     

Genetic algorithm in mix proportioning of high-performance concrete

High-performance concrete is defined as concrete that meets special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices. Ever since the term high-performance concrete was introduced into the industry, it had widely used in large-scale concrete construction that demands high strength, high flowability, and high durability. To obtain such performances that cannot be obtained from conventional concrete and by the current method, a large number of trial mixes are required to select the desired combination of materials that meets special performance. Therefore, in this paper, using genetic algorithm that is a global optimization technique modeled on biological evolutionary process—natural selection and natural genetics—and can be used to find a near optimal solution to a problem that may have many solutions, the new design method for high-performance concrete mixtures is suggested to reduce the number of trial mixtures with desired properties in the field test. Experimental and analytic investigations were carried out to develop the design method for high-performance concrete mixtures and to verify the proposed mix design.     

Heterotrophic/autotrophic denitrification (HAD) of drinking water: prospective use for permeable reactive barrier

This work aims to explore the use of an innovative denitrification process developed by our group for groundwater remediation. This process is coupling heterotrophic-autotrophic denitrification processes (HAD) supported by cotton and zero-valent iron (ZVI). In the experimental part, the effect of two amounts of ZVI (150 and 300 g), two nitrate (100 and 220 mg/l) and phosphate (3 and 6 mg/l) inlet concentrations on nitrate removal performance is investigated in two parallel continuous fed plug-flow reactors. The possible in-situ application of the proposed system in permeable reactive barriers (PRB) is further discussed. The HAD showed higher volumetric nitrate removal ratio (VNR) than the cotton supported heterotrophic denitrification one, and VNR increased with the amount of ZVI packed in the reactors. Ammonium production by the reductive action of iron was kept on acceptable level adjusting the contact time between water and ZVI. Iron release from ZVI decreased with time to negligible value (<0.5 mg/l) thanks to the formation of iron green rust compounds (GR). The HAD seems to be adequate for PRB systems, because both cellulose-based material and ZVI have been used in reactive trench for site remediation. Moreover, the proposed process could have the function for removal of nitrate and priority pollutants, such as chlorinated ethenes, simultaneously.     

Material performance lessons

The use of advanced technologies applying chemical and mineral admixtures, combined with conventional raw materials, can provide cost effective solutions to meet many of the challenges facing the concrete industry, especially with respect to production of cost effective durable structures, without the necessity of application of exotic new cementitious systems. The mechanisms controlling the performance of such concretes and the concepts of mix design are generally understood. Yet, in spite of this know-how, the performance on site frequently fails to meet the potential of these concretes. Some of the causes for this gap are associated with the influence of field practices which are often ignored, as well as the dependence on specifying and monitoring of concrete quality only on strength. The present paper discusses the various parameters and strategies which need and can be considered to change this state of affairs: quantifying the limitations of strength based specifications for durability, use of models and laboratory tests to optimize mix design for durability by considering expected curing effects, sensitivity of mineral additives to provide durability enhancement in different types of exposure conditions, as well as prediction of cracking which is based on quantifying the development of residual stresses in structures at early ages.     

Shear-thickening behavior of high-performance cement grouts — Influencing mix-design parameters

Rheology of concrete is of great importance to its flow performance, placement and consolidation. A full understanding of fresh concrete flow behavior can be achieved through a good understanding of paste rheology. Cement pastes exhibit a complex rheological behavior affected by several physical and chemical factors, including water-to-cement ratio (w/c), high-range water-reducer (HRWR) type and dosage, and cement characteristics. An experimental investigation was carried out to evaluate the effect of w/c, HRWR–cement combinations, and supplementary cementitious materials (SCM) on the pseudoplastic behavior of high-performance cement grouts. Grout mixtures proportioned with w/c of 0.30, 0.33, 0.36, and 0.40, various cement–HRWR combinations, and cement substitutions by 8% silica fume were investigated. The incorporation of HRWR can lower the yield stress of mixtures, thus enhancing deformability, while silica fume improves mechanical and durability performances.High-performance structural grouts are shown to exhibit shear-thickening behavior at low w/c and shear-thinning behavior at relatively higher w/c. Mixtures made with polycarboxylate HRWR acting by steric effect exhibited greater shear-thickening behavior compared to those made with polynaphthalene sulfonate-based HRWR acting by electrostatic effect. The paper discusses the effect of mixture parameters on non–linear rheological behavior of various grout mixtures prepared with different w/c, HRWR–cement combinations, and silica fume.     

The role of alkali content of Portland cement on the expansion of concrete prisms containing reactive aggregates and supplementary cementing materials

This paper presents results covering the effects of alkali content of Portland cement (PC) on expansion of concrete containing reactive aggregates and supplementary cementing materials (SCM). The results showed that the alkali content of PC has a significant effect on expansion of concrete prisms with no SCM. When SCM is used, the expansion was found to be related to both the chemical composition of the SCM and, to a lesser extent, the alkali content of the PC. The concrete expansions were explained, at least partly, on the basis of the alkalinity of a pore solution extracted from hardened cement paste samples containing the same cementing blends. An empirical relation was developed correlating the chemical composition (Ca, Si and total Na₂O_e) of the cementing blend (PC + SCM) and the alkalinity of the pore solution. Results from accelerated mortar bar test (ASTM C 1260) and a modified version thereof are also presented.     

Process based reconstruction and simulation of a three-dimensional fuel cell catalyst layer

Fundamental understanding of catalyst layer nanostructure of hydrogen polymer electrolyte membrane (PEM) fuel cells is critical for improvement in performance and durability. A process based 3D mathematical model has been developed to elucidate the effect of electrode composition, porosity and ionomer weight fraction in catalyst layers on electrochemical and nano-scale transport phenomena. Numerical reconstruction of catalyst layer random structure has been performed through a controlled random algorithm, mimicking the experimental fabrication process. Nano-scale species transport properties, e.g., Knudsen diffusion of oxygen in nano-pores and proton transport in thin-film electrolyte, have been included in the model, allowing for more rigorous study of the catalyst layer. It was found that there is a threshold in both porosity and ionomer weight fractions, below which species percolation through the random structure becomes difficult due to reduced connectivity and increased isolation. The degree of mixing or size of agglomerates has been studied and it was discovered that increasing or decreasing the agglomerate number from the optimum value reduces the electrochemically active area (ECA) and deteriorates species transport, suggesting an optimum level of stirring of the catalyst ink during catalyst layer preparation is critical.     

沿海混凝土结构氯离子对流区深度计算模型

沿海干湿交替环境下的氯离子传输方式和影响机理等问题是混凝土耐久性研究的主要内容,描述混凝土内部氯离子传输行为的计算模型通常由偏微分方程组成,求解方法较为复杂。为了简单而相对精确地分析氯离子的传输过程,本文提出了计算修正Fick定律模型中对流区深度参数Δx的新途径,通过综合考虑混凝土细观尺度组成、环境温度场变化、水分迁移和氯离子传输等因素,建立起干湿交替区域氯离子传输的计算模型,依据该模型可以较为准确地得到Δx的数值,进而对不同服役环境中的氯离子传输展开分析,根据计算结果给出考虑服役时间、日平均气温和干燥湿润时间比等参数的对流区深度Δx时变公式。这些公式提高了预测干湿交替环境中氯离子传输的修正Fick定律模型的适用性与准确性,可为干湿交替环境中的混凝土结构耐久性设计提供了简单有效的计算方法。