粉煤灰对ASR的抑制及有效性评估

以中国快速砂浆棒法为基础，分别研究了在40，60，80℃养护条件下低钙粉煤灰对硅质砾石、沸石化珍珠岩和石英玻璃碱硅酸反应(alkali silica rection,ASR）膨胀的抑制作用。讨论了集料碱活性、养护温度对粉煤灰抑制ASR效果的影响和建立粉煤灰及其它矿物外加剂抑制ASR有效性评估方法的技术路线和问题。结果表明：粉煤灰抑制ASR的效果与集科碱活性和养护温度密切相关。一定养护温度下，集科活性越大，抑制效果越差；对特定活性集料，养护温度越高，抑制效果越差。粉煤灰对不同集料和同种集料不同养护温度下ASR膨胀抑制效果差异，主要与ASR历程有关，即与集料ASR本身特性有关。特定集料和养护条件下粉煤灰对ASR膨胀的抑制效果不能简单推广至其它种类不同的集料和养护条件。粉煤灰抑制ASR有效性应根据工程评价目的和要求分别确定。快速评估方法的结果与混凝土长期性能之间的相关性需进一步研究。     

胶砂比和集料级配对ASR膨胀的影响

为建立快速鉴定硅质集料碱活性的方法 ,以沸石化珍珠岩和硅质砾石为集料研究了胶砂比和集料级配对砂浆试体ASR膨胀的影响。结果表明 :对检测集料ASR活性而言 ,快速砂浆棒法 (ASTMC12 6 0和CSAA2 3 2 - 2 5A)所建立的集料级配和胶砂比不是最敏感条件。采用单级配集料和多胶砂比砂浆试体可以更快速、可靠地鉴定集料ASR活性     

焙烧锂辉石对碱硅酸反应的抑制效果

以试验室焙烧锂辉石(DS)为原材料,研究了在40℃和80℃养护条件下,单掺DS和双掺DS与粉煤灰(FA)取代部分水泥,对沸石化珍珠岩集料和某高活性集料M成型不同砂浆碱集料反应膨胀的影响.试验表明:碱含量为2.5%时,在40℃和80℃养护条件下,DS掺量10%对沸石化珍珠岩集料砂浆试件ASR有效抑制,90 d龄期时试件膨胀值仍小于0.1%.DS掺量10%对高活性集料M砂浆试件ASR抑制效果不大.养护温度不同膨胀值变化趋势不同.DS掺量固定,随着FA掺量的增加,对2种集料砂浆碱集料反应膨胀抑制效果越好.     

RILEM AAR-5与其它快速法对集料ASR活性检测的对比与评价

试验采用RILEM AAR-5小混凝土柱法、ASTM C1260砂浆棒快速法、CECS48压蒸法和RILEM AAR-4加速混凝土棱柱体法检测了石门坎砂岩、两河口砂岩、锦屏砂岩、舟山安山岩和硅质白云岩的碱-硅酸反应活性,并将几种试验方法的结果进行比较,以评价RILEM AAR-5对硅质集料碱-硅酸反应活性检测的适应性.结果表明,对于慢性膨胀集料锦屏砂岩,RILEM AAR-5比ASTM C1260和CECS48检测更准确;硅质集料的小混凝土柱法(RELEMAAR-5)28 d膨胀率可以很好的预测混凝土棱柱体法试验结果;硅质集料尺寸与膨胀率的大致关系为RILEM AAR-5(5～10 mm,28 d)＞ASTM C1260(0.16 ～5 mm,14 d)＞CECS48(0.16 ～0.63 mm,6 h);碱-硅酸反应除受活性组分(化学因素)影响外,活性组分在集料中的分布(构造特征)也有重要作用.     

波茨坦砂岩的微观结构和碱活性快速检测方法的适应性

为研究集料微观结构对碱活性快速检测方法中对岩石适应性的影响,采用混凝土棱柱体法、快速砂浆棒法、压蒸法、中国快速砂浆棒法研究了波茨坦砂岩的膨胀行为,并研究了膨胀后试件的微观结构.结果表明:快速砂浆棒法、压蒸法、中国快速砂浆棒法均不能正确判定波茨坦砂岩的碱活性,主要是由于这些方法使用的集料中含大量粒径太小、不能反映该砂岩特殊结构特征的颗粒;对该类岩石,除活性组分的类型、数量外,岩石的微观结构特征能够显著影响碱-集料反应发生的位置和膨胀行为.在快速法中采用能够保持岩石原有结构特征的集料粒径是正确鉴定类似波茨坦砂岩微观结构岩石及其它非均质、多矿物岩石碱活性的关键.     

LiOH抑制碱-硅酸反应膨胀及其应用研究

研究了沸石化珍珠岩混凝土在KOH,LiOH溶液中压蒸膨胀行为,通过扫描电镜和能量散射谱对产物的形貌和组成进行了分析,说明LiOH抑制碱-硅酸反应膨胀的机理主要是在集料周围形成了含锂盐的非膨胀性产物,含锂产物层的形成对活性集料起保护作用而阻止了碱的进一步侵蚀。研究了由碱-硅酸活性集料和碱-碳酸盐活性集料制成的混凝土在各种碱中的膨胀行为。结果表明：混凝土在相同摩尔浓度的碱中压蒸,在NaOH溶液中膨胀最大,在LiOH溶液中膨胀最小。在应用LiOH抑制碱-硅酸反应膨胀促进碱-碳酸盐反应膨胀的双重作用下,在Spratt细粒硅质灰岩中,少量的白云石在碱环境中可发生去白云石化作用而对膨胀有贡献,也即尽管Spratt灰岩中碱-硅酸反应是主要的,但也存在碱-碳酸盐反应。     

集料粒径与ASR膨胀关系及膨胀预测研究进展

综述了集料粒径与碱硅酸反应(ASR)膨胀关系的研究现状,指出了ASR膨胀研究存在的局限性,阐述了大粒径、多级配集料ASR膨胀规律研究的必要性.介绍了几种ASR膨胀机理及ASR膨胀预测模型,并指出基于碱硅酸反应活化能的测定预测ASR膨胀将可能成为今后研究的一个热点.     

不同养护条件下含石英玻璃地质聚合物砂浆的变形行为

为阐明典型活性组分（无定型SiO2）在地质聚合物中的作用行为和效应,探索地质聚合物体系中碱-集料反应评价方法,研究常温（23℃）和38℃湿气养护（相对湿度〉95%）、80℃在1 mol/L NaOH溶液浸泡及150℃在10%（质量分数）的KOH溶液压蒸下,含石英玻璃集料地质聚合物砂浆的变形行为,采用扫描电镜、电子散射能谱研究产物的组成和微观结构。结果表明：4种养护条件下,特别是在传统普通硅酸盐水泥（OPC）体系所规定的养护条件和龄期内,含石英玻璃集料地质聚合物砂浆没有发生有害膨胀;但是随养护条件不同,地质聚合物基体和石英玻璃可能经历不同的化学反应过程,进而导致不同的变形行为,特别是在高温且有外碱介入时,地质聚合物基体在后期会产生膨胀效应。不宜采用单一的适于OPC体系的高温、高碱快速检测混凝土碱-集料反应的检测方法来评价地质聚合物体系中的碱-集料反应行为。     

混凝土碱硅酸反应膨胀预测模型的研究进展

本文综述了混凝土碱硅酸反应(alkali-silica reaction, ASR)膨胀预测模型的研究现状,ASR对混凝土结构造成的损伤修复难度高,修复成本大,应对这类耐久性问题主要以预防为主,补救为辅,而精确的预测模型可以评估混凝土结构的实时状态,有助于抑制混凝土中的ASR。本文首先概述了混凝土ASR的过程和机理,然后详细介绍了ASR膨胀预测模型的研究现状。ASR建模过程中需要考虑反应物含量、温度、湿度、胶凝材料组成和骨料尺寸等多种因素的影响。ASR模型主要包括理论模型、结构模型(宏观模型)和材料模型(细观模型),理论模型主要用来描述ASR的化学机理和预测骨料的最劣粒径,但该模型只适用于特定类型的骨料;材料模型在材料层面上解释了受ASR影响的混凝土的劣化机理,却忽略了混凝土收缩和徐变等因素的影响;结构模型通常被用来模拟和预测混凝土结构在ASR下的力学行为,但未充分考虑碱硅酸膨胀的化学过程以及离子扩散对ASR膨胀的影响。     

碱含量对粉煤灰抑制ASR效果的影响

采用粉煤友部分等量替代水泥,用砂浆棒快速法和混凝土棱柱体法分别进行了不同掺量的粉煤灰抑制集料ASR的试验研究。结果表明：随着混凝土砂浆中的总碱含量的增加,粉煤灰对集抖ASR膨胀的抑制效果减弱。在同等条件下,用粉煤灰部分取代高碱水泥比取代低碱水泥抑制集料的ASR膨胀更有效。     

Influence of stress restraint on the expansive behaviour of concrete affected by alkali-silica reaction

The primary objective of this study was to ascertain whether the Threshold Alkali Level (TAL) of the concrete aggregates may be taken as a suitable reactivity parameter for the selection of aggregates susceptible of alkali-silica reaction (ASR), even when ASR expansion in concrete develops under restrained conditions. Concrete mixes made with different alkali contents and two natural siliceous aggregates with very different TALs were tested for their expansivity at 38 °C and 100% RH under unrestrained and restrained conditions. Four compressive stress levels over the range from 0.17 to 3.50 N/mm² were applied by using a new appositely designed experimental equipment. The lowest stress (0.17 N/mm²) was selected in order to estimate the expansive pressure developed by the ASR gel under “free” expansion conditions. It was found that, even under restrained conditions, the threshold alkali level proves to be a suitable reactivity parameter for designing concrete mixes that are not susceptible of deleterious ASR expansion. An empirical relationship between expansive pressure, concrete alkali content and aggregate TAL was developed in view of its possible use for ASR diagnosis and/or safety evaluation of concrete structures.     

Alkali-silica reactivity of large silica fume-derived particles

Pozzolanic materials, including silica fume, are commonly added to concrete to reduce expansion due to alkali-silica reaction (ASR). It has been noted, however, that commercial silica fume is not always adequately dispersed, and large agglomerates may be present. These large particles have been hypothesized to act as amorphous silica aggregates, thereby participating in an expansive reaction with the alkalis present in cement paste pore solution. If such were the case, some silica fume particles would actually aggravate expansion due to ASR rather than suppress it. The present investigation characterizes the microstructure and morphology of agglomerated and sintered silica fume particles and compares their effects on alkali-silica-related expansion. While a 5% replacement of moderately reactive sand with sintered silica fume aggregates caused significant expansion under accelerated testing conditions (modified ASTM C1260), the replacement with large agglomerates of densified silica fume decreased expansion compared with control mortar bars containing only sand. Both the sintered aggregates and the agglomerates reacted with the pore solution; one reaction was expansive, while the other was not.     

Mechanical behavior of concretes damaged by alkali-silica reaction

Alkali-silica reaction (ASR) can induce the premature distress and loss in serviceability of concrete structures. The internal crack pattern produced by ASR affects both transport and mechanical properties. Usually linear expansions are considered as indicative of the grade of damage into the material (internal crack pattern), nevertheless as diverse types of ASR have been recognized (rapid or slow reactive aggregates, fine or coarse aggregates) the effects on strength and rheological properties could be different for a same expansion. This paper compares the mechanical response of a reference concrete (without reactive aggregates) and concretes prepared with three different types of reactive aggregates, with the same mixture proportions. The first concrete incorporated 10% of a highly reactive siliceous orthoquartzite as a part of the coarse aggregate, the second included a highly reactive sand, and the third prepared with a slow reactive granitic migmatite as coarse aggregate. Concretes were moist cured at 38 °C. When linear expansions ranging between 0.11 and 0.18% took place, the stress strain behavior in compression and the load-displacement response in flexure were measured. The same tests were performed on reference concrete at different ages, between 75 and 745 days. Microscopic observations were performed on polished and thin sections in order to analyze concrete microstructure. It appears that the failure mechanism of concrete in compression is clearly affected by ASR, the shape of the stress-strain curves reflects the presence of internal fissures, showing that the capability of controlling crack propagation decreases. Differences in the crack pattern are also reflected in the shape of the load-deflection curves in tension, damaged concretes show an increased non-linearity and a more gradual softening. However, it was found that the modifications in the mechanical properties cannot be directly associated with a level of expansion, as the behavior depends on the component materials and mechanisms involved in the reaction.     

Effects of aggregate size and alkali content on ASR expansion

Attempts to model ASR expansion are usually limited by the difficulty of taking into account the heterogeneous nature and size range of reactive aggregates. This work is a part of an overall project aimed at developing models to predict the potential expansion of concrete containing alkali-reactive aggregates. The paper gives measurements in order to provide experimental data concerning the effect of particle size of an alkali-reactive siliceous limestone on mortar expansion. Results show that no expansion was measured on the mortars using small particles (under 80 µm) while the coarse particles (0.63-1.25 mm) gave the largest expansions (0.33%). When two sizes of aggregate were used, ASR-expansions decreased with the proportion of small particles. Models are proposed to study correlations between the measured expansions and parameters such as the size of aggregates and the alkali and reactive silica contents. The pessimum effect of reactive aggregate size is assessed and the consequences on accelerated laboratory tests are discussed.     

Is alkali-carbonate reaction just a variant of alkali-silica reaction ACR = ASR?

The mechanism of the alkali-carbonate reaction (ACR) has been recognized as being different from that of the more common alkali-silica reaction (ASR). However, the identification of alkali-silica gel in ACR concrete from Cornwall, Ontario, Canada by Katayama, in 1992 raised the possibility that ASR was at least playing a role in the ACR reaction. The acid insoluble residues of the ACR aggregate from Kingston, along with two other aggregates were analyzed to determine what might be contributing to the reaction. The acid insoluble residue of the ACR Kingston rock contains 96% quartz of high solubility in NaOH. Good correlation was found between the amount of quartz and expansion of concrete prisms indicating that the expansion was due mainly to an alkali-silica reaction. This conclusion is supported by observations, in 2008, by Katayama of gel in thin sections of concrete made with the Kingston aggregate. It is concluded that ACR = ASR.     

Alteration of alkali reactive aggregates autoclaved in different alkali solutions and application to alkali-aggregate reaction in concrete (II) expansion and microstructure of concrete microbar

The effect of the type of alkalis on the expansion behavior of concrete microbars containing typical aggregate with alkali-silica reactivity and alkali-carbonate reactivity was studied. The results verified that: (1) at the same molar concentration, sodium has the strongest contribution to expansion due to both ASR and ACR, followed by potassium and lithium; (2) sufficient LiOH can completely suppress expansion due to ASR whereas it can induce expansion due to ACR. It is possible to use the duplex effect of LiOH on ASR and ACR to clarify the ACR contribution when ASR and ACR may coexist. It has been shown that a small amount of dolomite in the fine-grained siliceous Spratt limestone, which has always been used as a reference aggregate for high alkali-silica reactivity, might dedolomitize in alkaline environment and contribute to the expansion. That is to say, Spratt limestone may exhibit both alkali-silica and alkali-carbonate reactivity, although alkali-silica reactivity is predominant. Microstructural study suggested that the mechanism in which lithium controls ASR expansion is mainly due to the favorable formation of lithium-containing less-expansive product around aggregate particles and the protection of the reactive aggregate from further attack by alkalis by the lithium-containing product layer.     

Coupled effects of aggregate size and alkali content on ASR expansion

This work is a part of an overall project aimed at developing models to predict the potential expansion of concrete containing alkali-reactive aggregates. First, this paper reports experimental results concerning the effect of particle size of an alkali-reactive siliceous limestone on mortar expansion. Special attention is paid to the proportions of alkali (Na₂O_eq) in the mixtures and reactive silica in the aggregate. Results show that ASR expansion is seven times larger for coarse particles (1.25-3.15 mm) than for smaller ones (80-160 μm). In mortars for which the two size fractions were used, ASR expansion increased in almost linear proportion to the amount of coarse reactive particles, for two different alkali contents. Then, an empirical model is proposed to study correlations between the measured expansions and parameters such as the size of aggregates and the alkali and reactive silica contents. Starting with the procedure for calibrating the empirical model using the experimental program combined with results from the literature, it is shown that the expansion of a mortar containing different sizes of reactive aggregate can be assessed with acceptable accuracy.     

Experimental investigation of the mechanisms by which LiNO3 is effective against ASR

Various series of experiments were carried out on cements pastes, concretes made with a variety of reactive aggregates, composite specimens made of cement paste and reactive aggregate particles, and a variety of reactive natural aggregates and mineral phases immersed in various Li-bearing solutions. The main objective was to determine which mechanisms(s) better explain(s) the effectiveness of LiNO₃ against ASR and variations in this effectiveness as well with the type of reactive aggregate to counteract. The principal conclusions are the following: (1), the pH in the concrete pore solution does not significantly decrease in the presence of LiNO₃; (2), the concentration of silica in the pore solution is always low and not affected by the presence of LiNO₃, which does not support the mechanism relating to higher solubility of silica in the presence of lithium; (3), the only reaction product observed in the LiNO₃-bearing concretes looks like classical ASR gel and its abundance is proportional to concrete expansion, thus is likely expansive while likely containing lithium; this does not support the mechanisms relating to formation of a non or less expansive Si-Li crystalline product or amorphous gel; (4), early-formed reaction products coating the reactive silica grains or aggregate particles, which could act as a physical barrier against further chemical attack of silica, were not observed in the LiNO₃-bearing concretes, but only for a number of reactive materials after immersion in 1 N LiOH at 350 °C in the autoclave (also at 80 °C for obsidian); (5), higher chemical stability of silica due to another reason than pH reduction or early formation of a protective coating over the reactive phases, is the mechanism among those considered in this study that better explains the effectiveness of LiNO₃ against ASR.     

Chemo-mechanical modeling for prediction of alkali silica reaction (ASR) expansion

The effect of the size of the aggregate on ASR expansion has already been well illustrated. This paper presents a microscopic model to analyze the development of ASR expansion of mortars containing reactive aggregate of different sizes. The attack of the reactive silica by alkali was determined through the mass balance equation, which controls the diffusion mechanism in the aggregate and the fixation of the alkali in the ASR gels. The mechanical part of the model is based on the damage theory in order to assess the decrease of stiffness of the mortar due to cracking caused by ASR and to calculate the expansion of a Representative Elementary Volume (REV) of concrete. Parameters of the model were estimated by curve fitting the expansions of four experimental mortars. The paper shows that the decrease of expansion with the size of the aggregate and the increase of the expansion with the alkali content are reproduced by the model, which is able to predict the expansions of six other mortars containing two sizes of reactive aggregate and cast with two alkali contents.     

Impact of the alkali–silica reaction products on slow dynamics behavior of concrete

Several nondestructive techniques based on acoustics are frequently used to assess the condition of engineering materials. It has been demonstrated that nonlinear acoustics is more sensitive for detecting micro-cracks. The main challenge, regarding the assessment of alkali–silica reaction (ASR) damage in concrete, remains in the efficiency of the technique to distinguish ASR from other damaging process. Based on the fact that ASR produces a swelling viscous gel, a new approach developed for finding a signature to ASR is investigated in this paper. The research was focused upon the specific behavior of ASR causing the presence of viscous gels in micro-cracks and porosity compared with mechanical damage where cracks are empty. With this approach, the concrete response to slow dynamics tests was analyzed. The Burger spring–damping model was used for interpreting the results. This research showed that the slow dynamics technique presented here can detect cracking in concrete and that the time response to an external excitation of concrete damaged by ASR is different from that of concrete mechanically damaged.