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.     

Petrography study of two siliceous limestones submitted to alkali-silica reaction

This study presents the contribution of petrography to the comprehension of the alkali-silica reaction mechanism applied to two siliceous limestones. A petrography study was made on the two aggregates before reaction to define their relative proportions and types of reactive silica and to observe their distribution in the microstructure. Then a model reactor, constituted by the reactive siliceous limestone aggregate, portlandite and NaOH, was used to measure the swelling due to reaction of the silica with alkalis and the free expansion of the aggregates. The volume evolution between both aggregates was very different and could be explained by the preliminary petrographic study. It appears that the swelling of the aggregates is conditioned by the microstructure of the carbonated matrix, the quantity and the distribution of the reactive silica.     

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.     

Replacement of “coarse” cement particles by inert fillers in low w/c ratio concretes: II. Experimental validation

It has been suggested that in low water-to-cement ratio (w/c) concretes, the “coarser” cement particles could be replaced by an “inert” filler with little loss in performance in terms of hydration and strength development. This communication presents the results of an experimental study conducted to validate this hypothesis, using a coarse limestone filler and a classified cement. The cement and limestone powders were both classified with a cutoff diameter of about 30 μm. The coarse limestone was then blended with the fine cement, and water-to-solids ratio=0.3 pastes and mortars were prepared to compare to reference (original cement powder) systems. The results for chemical shrinkage for the pastes were consistent with a simple dilution of the cement by the limestone and also with the results predicted by the CEMHYD3D hydration model. In mortars, the predicted compressive strength loss in the filled system at 7 days was consistent with model predictions, and furthermore, at 56 days, no detectable difference in strength was measured. Thus, this study further supports the idea that coarse limestones could be used to replace equivalent size cement particles in low w/c concretes with little loss in hydration and strength performance.     

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.     

The use of condensed silica fume to control alkali-silica reaction — a field case study

In 1980 a sidewalk was built at Bécancour, Québec, with condensed silica fume concretes containing highly reactive aggregates. Eleven concrete mixes were used with cement quantities from 140 to 405 kg of cement per m³ and condensed silica fume substitutions varying from 10 to 40%. In spite of the great reactivity of the aggregates, the alkali-aggregate reaction is still under control. Microstructural studies of four particular concretes have been made after the first and third winters. No silicate gel has been observed in the two leaner mixes but some has been found in a few locations encircling coarse aggregate particles in the two richer mixes. The severe scaling problem observed in one of the concrete is characterized at the microstructure level by frequent unbonding of coarse aggregates and presence of converging cracks around the aggregates.     

Using limestone aggregates and different cements for enhancing resistance of concrete to sulphuric acid attack

A research program was undertaken to improve concrete's resistance against sulphuric acid attack. Six concretes were investigated, four using calcareous limestone aggregates and two using silicious aggregates. Cements used in these concretes included a portland cement, a binary cement containing ground granulated blast furnace slag, and two ternary cements containing slag and silica fume or fly ash and silica fume. All the concretes had the same water/cement ratio of 0.4, with compressive strengths in the range of 45 MPa and 58 MPa at the age of 28 days. In the experiment, concrete cylinders were immersed in 1% sulphuric acid solution and they were periodically examined for appearance, measured for mass change and tested in compression up to 168 days. The concrete using limestone aggregates and the ternary cement containing silica fume and fly ash performed the best.     

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.     

Evaluation of the wettability of mortar component granular materials through contact angle measurements

The wettability of granular materials has a direct effect on the workability of pastes, mortars and concretes containing such materials. Given this, the wetting behaviour of two Portland cements, two mineral admixtures (limestone filler and silica fume) and different sand types (siliceous, limestone and granite) was evaluated through the measurement of the contact angle between these materials and water. The results show that all the materials possess high wettability. A chemical agent, dichlorodimethylsilane, was used to modify the surface properties of a 0–1 mm siliceous sand, making it more hydrophobic and, therefore, unable to be spontaneously wetted by water. The properties of a mortar produced with the silane-treated sand were evaluated, showing improvements on workability as a result of the chemical treatment.     

Influence of the nature of aggregates on the behaviour of concrete subjected to elevated temperature

An experimental study is carried out on concretes composed of three different types of aggregates: semi crushed silico-calcareous, crushed calcareous and rolled siliceous. For each aggregate type, two water/cement ratios (W/C), 0.6 and 0.3 are studied. Aggregates and concrete specimens were subjected to 300, 600 and 750 °C heating–cooling cycles. We analyse the evolution of thermal, physical and mechanical properties of concrete in terms of behaviour and physical characteristic evolutions of aggregates with temperature. The study of thermal behaviour of aggregates showed the importance of initial moisture state for the flints. The crystallisation and microstructure of quartz play an important role in the thermal stability of siliceous aggregates. The residual mechanical behaviour of concrete varies depending on the aggregate and the influence of aggregates is also dependent on paste composition. This study allowed to better understand the influence of chemical and mineralogical characteristics of aggregates on the thermomechanical behaviour of concrete.     

Accelerated microstructural evolution of a calcium-silicate-hydrate (C-S-H) phase in pozzolanic pastes using fine siliceous sources: Comparison with historic pozzolanic mortars

Traditional pozzolanic mortars such as those from Rhodes, Greece, or Hagia Sophia, Turkey, revealed the presence of a calcium-silicate-hydrate (C-S-H) binding phase. This phase, which is similar to that found in ordinary Portland cement (OPC), is produced under the pozzolanic reaction of slaked lime with fine reactive siliceous sources at temperatures <100 °C. The traditional siliceous sources were replaced by fumed silica or tetraethyl orthosilicate (TEOS). A microstructural analysis revealed an enhanced reaction rate but similar morphologies of the resultant C-S-H phases, confirming that the reaction-limiting factor is the dissolution of the siliceous sources.     

The so-called alkali-carbonate reaction (ACR) — Its mineralogical and geochemical details, with special reference to ASR

Typical examples of so-called alkali-carbonate reaction (ACR) in the Canadian field concretes in Ontario, CSA concrete prism, RILEM concrete microbars and RILEM mortar bar containing Pittsburg aggregate, were examined petrographically based on polarizing microscopy, SEM observation and quantitative SEM-EDS analysis of the reaction products. It was revealed that ASR gel was the main product responsible for the crack formation in concretes, and that this gel had a common nature to that in the typical ASR. That is, ASR gel presented distinctive compositional trend lines, passing from low-Ca ASR gel at [Ca/Si] = 1/2-1/6, [Ca]/[Na + K] = 1.0 to the “convergent point” with [Ca/Si] = 1.3-1.8, [Ca]/[Na + K] = 100 at which chemical equilibrium is attained with CSH gel. The so-called ACR is a combination of deleteriously expansive alkali-silica reaction (ASR) of cryptocrystalline quartz, and harmless dedolomitization which produces brucite and carbonate halo. In laboratory specimens, fine dolomitic aggregate undergoes dedolomitization, and brucite and ASR gel react to form non-expansive Mg-silicate gel on the dolomite crystals. This explains why the mortar bar produces smaller expansion than the concrete microbar, and why the reaction products are so minute that they escape attention by optical microscopy. As a crystalline counterpart, mountainite is a candidate for low-Ca ASR gel, while sepiolite is one for Mg-silicate gel. Concealed ASR was detected in ACR-affected field concretes undergoing ingress of deicing salt which formed Friedel's salt and Cl-doped CSH gel. Compositions of ASR products, methods of sample preparation and analysis for correct identification of ACR, and artifacts were critically reviewed.     

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.     

同掺再生粗细骨料混凝土的力学与耐久性能研究

为了探讨同时掺入大掺量再生粗骨料和细骨料制备C40及以上强度等级再生混凝土的可行性,在C45天然骨料混凝土配合比的基础上,采用II类再生粗骨料、I类再生细骨料,以同掺再生粗细骨料质量替代率为25%、50%、75%、100%配制了4组再生混凝土,研究了再生粗细骨料替代率对再生混凝土基本力学性能和耐久性能的影响规律。结果表明:当同掺再生粗细骨料的替代率为25%时,混凝土的力学性能下降很小,替代率为50%、75%的混凝土的抗压强度分别达到C45、C40等级,替代率100%的全再生粗细骨料混凝土的28 d抗压、劈拉、轴压强度和弹性模量等力学性能指标较天然骨料混凝土降低12.0%~23.2%,并达到C35抗压强度等级。增加再生粗细骨料的替代率会降低混凝土的耐久性,但即使是全再生粗细骨料混凝土仍可获得高的耐久性,其抗碳化性能、抗氯离子渗透性、抗冻性能分别达到T-IV、RCM-IV和F300等级,说明在混凝土中同时掺用50%及以上再生粗细骨料配制C40及以上强度等级的再生混凝土是可行的。     

Reliable quantification of AAR damage through assessment of the Damage Rating Index (DRI)

This paper presents the results of the condition assessment of twenty concrete mixtures incorporating ten different reactive aggregates through the Damage Rating Index (DRI), a microscopic and semi-quantitative petrographic tool, with the aim of verifying the development of distress due to alkali-aggregate reaction (AAR) as a function of the specimen's expansion (i.e. from 0.05 to 0.30%). The DRI was found to provide a reliable assessment of the degree of damage in the concretes incorporating reactive fine or coarse aggregates. An envelope of DRI damage assessments against the expansion level of the affected materials is proposed. For all alkali–silica reactive aggregates investigated, the progress in counts and proportions of opened cracks in the aggregate particles and in the cement paste, with and without gel, as well as the crack density parameter, were found to be diagnostic petrographic features for quantifying ASR progress. Moreover, a qualitative ASR distress model in concrete was defined.     

Reaction products of densified silica fume agglomerates in concrete

Most silica fume currently used in concrete is in the dry densified form and consists of agglomerates of sizes between 10 μm and several millimeters. Many of these agglomerates may break down only partially in normal concrete mixing. Examination of various mature silica-fume-bearing concretes using backscatter mode scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis shows that such agglomerates have reacted in situ and given rise to recognizable types of reaction products filling the space within the original outline of the agglomerate. One type is “quiescent,” and usually shows no evidence of volume instability. EDX spectra indicate that the product formed within such grains is C-S-H of very low Ca/Si ratio, with modest alkali contents. Other silica fume agglomerates may undergo a distinct alkali-silica-type reaction (ASR), with the reaction product found within the original outline of the agglomerate having significantly less calcium and usually much higher alkali contents than the quiescent type. Such reacted agglomerates show evidence of local expansion, shrinkage cracking (on drying), and other features common to ASR. Both types may be found within the same concrete, sometimes in close proximity. It further appears that exposure to seawater may convert previously formed reaction products of silica fume agglomerates to magnesium silicate hydrates.     

Dual effectiveness of lithium salt in controlling both delayed ettringite formation and ASR in concretes

The influence of lithium nitrate on expansions due to delayed ettringite formation (DEF) and alkali-silica reaction (ASR) has been investigated. Effects of the lithium salt were examined in heat-cured mortars and concretes containing one or both damage mechanisms. The mortars and concretes made using reactive and/or non-reactive aggregates were subjected to heat treatment consisting of a hydration delay period of 4 h at 23 °C followed by steam-curing at 95 °C and then stored in limewater. Results showed that the lithium salt admixture was able to reduce the occurrence of deleterious expansion due to delayed ettringite formation in addition to controlling alkali-silica reaction in cementitious systems containing one or both mechanisms. In concretes made using non-reactive limestone aggregates, incorporation of lithium nitrate in a proportion of 0.74 M ratio of Li to (Na + K) was found to control delayed ettringite formation during the one-year period of this study.By analyzing the leaching properties of lithium and other alkalis from mortars during storage, it was found that a substantial amount of lithium was retained in the cementitious system in a slightly soluble form, and is expected to be responsible for reducing DEF.     

Instant-chilled steel slag aggregate in concrete - strength related properties

The properties of concretes containing instant-chilled steel slag (I.C.S.) as aggregate are presented. The I.C.S. slag possesses good physical and mechanical properties and has sufficient stability for use as a coarse aggregate in concrete. Bond tests have shown that I.C.S. slag exhibits higher interfacial bond splitting strength with cement mortat than that of limestone aggregate. The tensile splitting strength of the slag aggregate itself is higher than that of limestone. Compressive, indirect tensile and flexural strengths of I.C.S. slag concretes were greater than those of corresponding control concretes containing limestone aggregate.     

不同结构构造硅质集料的碱硅酸反应模型

正确认识不同结构构造特征集料的碱硅酸反应（ASR）特征，对判定集料碱活性和诊断工程ASR事例以及采取正确预防措施均有重要意义、早期的以单一组分的高活性集料为基础提出的ASR模型只强调集料中活性组分对ASR的作用，仅适用于特定类型的高活性集料，本工作用扫描电镜（SEM）和光学显微镜研究了石英玻璃、沸石化珍珠岩和硅质砾石反尖特征，并综合对慢膨胀型休料ASR研究的文献，提出了结构构造特征不同的硅质活活性集料的ASR基本模型：结构构造特征不同的活性休料其ASR过程及膨胀行为不同；对以无定型SiO2为活性组分的均质高活性集料，反应特征符合传统的ASR模型；对致密、多矿物慢膨胀型集料，除活性组分类型和数量外，活性组分在集料内的分布，即集料的构造特征对ASR的速度和膨胀行为也有重要影响，结构构造特征不同的活性集料其ASR过程及膨胀行为差别的关键在于膨胀的根源和限制条件不同。     

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.