Test methods for evaluating preventive measures for controlling expansion due to alkali-silica reaction in concrete

This paper provides a critical evaluation of the various methods available for testing the efficacy of measures for preventing expansion due to alkali-silica reaction (ASR) in concrete containing deleteriously reactive aggregate. The ideal test method should be rapid, reliable and capable of determining the influence of aggregate reactivity, alkali availability and exposure conditions. None of the currently available or commonly used methods meet all of these criteria. The shortcomings of the different test methods are discussed and suggestions are made for modifying the concrete prism test and accelerated mortar bar test to make these tests more acceptable.     

Modified model of alkali-silica reaction

Experimental studies have been carried out for understanding why soft and fluid hydrated alkali silicate generated by the alkali-silica reaction (ASR) of aggregate with alkaline pore solution accumulates the expansive pressure for cracking the aggregate and the surrounding concrete. The elemental analysis of aggregate (andesite) embedded in a cement paste has revealed that the alkali silicate has no ability of generating expansive pressure unless the aggregate is tightly packed with a reaction rim. The reaction rim is slowly generated from the alkali silicate that covers the ASR-affected aggregate. Consumption of alkali hydroxide by the ASR induces the dissolution of Ca²⁺ ions into the pore solution. The alkali silicate then reacts with Ca⁺ ions to convert to an insoluble tight and rigid reaction rim. The reaction rim allows the penetration of alkaline solution but prevents the leakage of viscous alkali silicate, so that the alkali silicate generated afterward by the ASR is accumulated in the aggregate to give an expansive pressure enough for cracking the aggregate and the surrounding concrete. The ASR of very tiny aggregate such as fly ash and municipal waste incinerator bottom ash may not cause the deterioration of concrete, since the ASR is completed before the formation of reaction rims.     

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.     

Volcanic ash and pumice as cement additives: pozzolanic, alkali-silica reaction and autoclave expansion characteristics

This study reports the results of investigation to assess the suitability of volcanic ash (VA) and pumice (VP) powder to be used as cement additives. Pozzolanic activity of VA and VP was tested according to the Italian standard and found to be acceptable. The strength activity index with Portland cement and the effectiveness of VA and VP admixture in controlling alkali-silica reaction and autoclave expansion were tested according to ASTM standards. Mortar cubes were specially prepared as per ASTM standards for these studies using different mixes with varying percentages of VA and VP (0-40%) as cement replacement. The results are then compared with ASTM requirements to assess the suitability of VA or VP as cement additives.     

Different manifestations of the alkali-silica reaction in concrete according to the reaction kinetics of the reactive aggregate

The alkali-silica reaction (ASR) is caused by the presence of reactive aggregates in contact with sufficiently alkaline pore solution and a moisture level above 80%, which leads to the formation of expansive products that cause cracking and deterioration of the structures. Petrographic analysis of the ASR damaged microstructure provides information about the detection, analysis, and progress of the reaction. A stereobinocular and a polarizing optical microscope were used in order to observe and establish the relationship between ASR development in rapid or slow-reacting aggregates. The origin, mineralogical composition and fabrics of the constituent of the two types of aggregates were analyzed first. The progress of reaction was then studied on four concrete samples analyzing the different characteristics and textural patterns imprinted on the aggregates and the mortar. From the study it follows that the mineralogy and fabric of the rocks involved are responsible for different manifestations of the reaction in the aggregate and in the interfacial transition zone, causing damage that can diminish the strength and durability of concrete.     

Use of ternary blends containing silica fume and fly ash to suppress expansion due to alkali-silica reaction in concrete

This paper investigates the effects of cementitious systems containing Portland cement (PC), silica fume (SF) and fly ash (FA) on the expansion due to alkali-silica reaction (ASR). Concrete prisms were prepared and tested in accordance with the Canadian Standards Association (CSA A23.2-14A). Paste samples were cast using the same or similar cementitious materials and proportions that were used in the concrete prism test. Pore solution chemistry and portlandite content of the paste samples are reported. It was found that practical levels of SF with low-, moderate- or high-calcium FA are effective in maintaining the expansion below 0.04% after 2 years. Pore solution chemistry shows that while pastes containing SF yield pore solutions of increasing alkalinity at ages beyond 28 days, pastes containing ternary blends maintain the low alkalinity of the pore solution throughout the testing period (3 years).     

The influence of potassium-sodium ratio in cement on concrete expansion due to alkali-aggregate reaction

In concrete containing potentially reactive aggregates, deleterious alkali-aggregate-reaction (AAR) can be prevented by the use of suitable mineral admixtures or by limiting cement content and alkalis (Na₂O-equivalent) of the cement. However, the Na₂O-equivalent of cement may not always accurately define the potential of cement to cause AAR. In this study, the potential reactivity of concrete produced with cements having similar Na₂O-equivalents but different K/Na-ratios has been measured and the composition of gel has been analyzed. Additionally, pastes and mortars have been produced to study the development of pore solution composition.The expansion of the concrete mixtures shows significant differences depending on the cement used. The different K/Na-ratio present in the cements is reflected in the pore solution of pastes and mortars and in the gel present in aggregates of the concrete mixtures. As the hydroxide concentration in the pore solutions of pastes and mortars produced with the different cements is nearly identical, the difference in K/Na-ratio has to be the reason for the observed differences in concrete expansion.     

Mechanism of damage for the alkali-silica reaction

A novel mechanism for the damage induced by alkali-silica reaction (ASR) is proposed. Two reaction steps are taken into account in the mechanism: the Q₃ tetrahedrons formation by breaking up siloxane bonds and the dissolution of these Q₃ tetrahedrons. We demonstrate that the formation of Q₃ tetrahedrons in the aggregate prevails over dissolution during the swelling step. The formation of Q₃ tetrahedrons causes a swelling and a micro-cracking of the aggregate: we observe a significant increase of the aggregate pore volume. A model based on a volume balance between the aggregate expansion and the swelling of mortar bars is proposed. This model enables us to measure an amplification factor of the aggregate swelling. This amplification factor is high (about 3) and related to the stiffness of the low porosity cement paste and to the cracking propagation process.     

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.     

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.     

Mechanical approach in mitigating alkali-silica reaction

The effect of steel microfibers (SMF) on alkali-silica reaction (ASR) was investigated using two types of reactive aggregates, crushed opal and a Pyrex rod of constant diameter. Cracks are less visible in the SMF mortars compared with the unreinforced mortars. Due to crack growth resistance behavior in SMF mortar specimens, the strength loss is eliminated and the ASR products remained well confined within the ASR site. The expansion and the ASR products were characterized by microprobe analysis and inductive coupled plasma (ICP) spectroscopy. The confinement due to SMF resulted in a higher Na and Si ion concentration of the ASR liquid extracted from the reaction site. The higher concentration reduced the ASR rate and resulted in a lower reactivity of the reactive Pyrex rods in SMF mortars.     

The alkali-silica reaction in alkali-activated granulated slag mortars with reactive aggregate

The expansion of alkali-activated granulated blast furnace slag (AAS) cement mortars with reactive aggregate due to alkali-silica reaction (ASR) was investigated. The alkaline activator used was NaOH solution with 4% Na₂O (by mass of slag). These results were compared to those of ordinary portland cement (OPC) mortars. The ASTM C1260-94 Standard Test Method based on the NBRI Accelerated Test Method was followed. The nature of the ASR products was also studied by SEM/EDX. The results obtained show that the AAS cement mortars experienced expansion due to the ASR, but expansion occurs at slower rate than with OPC mortars under similar conditions. The cause of the expansion in AAS cement mortars is the formation of sodium and calcium silicate hydrate reaction products with rosette-type morphology. Finally, in order to determine potential expansion due to ASR, the Accelerated Test Method is not suitable for AAS mortars because the reaction rate is initially slow and a longer period of testing is required.     

Alkali-aggregate reaction in concrete containing high-alkali cement and granite aggregate

The paper discusses results of the research into the influence of high-alkali Portland cement on granite aggregate. The deformation of the concrete structure occurred after 18 months. The research was carried out by means of a scanning electron microscope equipped with a high-energy dispersive X-ray analyzer that allowed observation of unpolished sections of concrete bars exhibiting the cracking pattern typical of the alkali-silica reaction. Both the microscopic observation and the X-ray elemental analysis confirm the presence of alkali-silica gel and secondary ettringite in the cracks.     

Effect of the cement chemistry and the sample size on ASR expansion of concrete exposed to salt

Mortar bars and concrete prisms made with a very alkali-silica reactive limestone were stored at 38 °C in 1 M NaOH and NaCl solutions. A high-alkali (HA) cement and a low-alkali (LA) cement were used in order to evaluate the cement chemical composition on the expansion and on the chemistry of the pore water. The mortar bars immersed in 1 M NaOH presented much more expansion than mortar bars stored at 100% RH or in 1 M NaCl. The behaviour of the concrete prisms was completely different. Low expansion was obtained for concrete prisms made with the LA cement immersed for more than 5 years in 1 M NaCl solution, while the expansion was over 0.45% for concrete prisms made with the HA cement. Chemical equilibrium between the pore waters and the immersion solution was much longer to obtain for the concrete prisms (near 3 years) than for the mortar bars (less than 3 months). The results obtained in this study show that the type of sample used (mortar bars or concrete prisms) and the cement composition strongly influence the harmful effects of ASR in concrete exposed to salt.     

The effect of the silica content of silica fume on its ability to control alkali-silica reaction

The use of silica fume (SF) has been instrumental in the development and utilization of high-strength and high-performance concrete. In the interests of economics, questions have been raised regarding the possible use and effectiveness of “lower grade” SFs with SiO₂ contents less than 85%. Such materials do not meet current CSA and ASTM standards for SF. In this study, the performance of two SFs from the same U.S. plant but with different silica contents (68% and 88% SiO₂) were compared by examining the effect of the materials on the expansion due to alkali-silica reaction (ASR) and the composition of the pore solution. The mixtures tested with these procedures included 0%, 4%, 8%, and 12% SF replacement by mass of cement. Results show that the SF with lower than standard silica contents cannot control ASR at the levels of replacement examined in this program.     

Long-term effectiveness and mechanism of LiOH in inhibiting alkali-silica reaction

A practical alkali reactive aggregate-Beijing aggregate was used to test the long-term effectiveness of LiOH in inhibiting alkali-aggregate reaction (AAR) expansion. In this paper, the most rigorous conditions were so designed that the mortar bars had been cured at 80 °C for 3 years after being autoclaved for 24 h at 150 °C. At this condition, LiOH was able to inhibit long-term alkali-silica reaction (ASR) expansion effectively. Not only was the relationship between molar ratio of n(Li)/n(Na) and the alkali contents in systems established, but also the governing mechanism of such effects was studied by SEM.     

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.     

Amorphisation mechanism of a flint aggregate during the alkali-silica reaction: X-ray diffraction and X-ray absorption XANES contributions

Flint samples at different stages of the Alkali-Silica Reaction were prepared and analyzed by X-ray diffraction (XRD) and silicon K-edge X-ray absorption near edge structure techniques (XANES). The results are compared to those of measurements performed on alpha quartz c-SiO₂ and rough flint aggregate. The molar fraction of Q₃ sites is determined as a function of the time of reaction. Up to 14 h of attack, the effect of the reaction seems of little importance. From 30 to 168 h, we showed an acceleration of the effect of the reaction on the crystal structure of the aggregate resulting in an amorphisation of the crystal. During this period, the amorphous fraction increases linearly with the number of Q₃ sites. The results of the XANES confirm the amorphisation of the aggregate during the reaction and show the presence of silicon in a tetrahedral environment of oxygen whatever the time of attack.     

Measurement of alkali-silica reaction progression by ultrasonic waves attenuation

Development of non-destructive methods, developed specifically for assessing the damage induced by alkali-silica reaction (ASR) in concrete structures, is needed in order to carry out a systematic evaluation of the concrete condition. The aim of this study is to monitor the evolution of the ASR-damage in laboratory with concrete samples with ultrasonic pulse velocity and attenuation of ultrasonic waves methods. For this study, results of both methods were compared with expansion and mass variation.One reactive concrete mixture was made with reactive aggregate, and one other mixture, incorporating non-reactive aggregate, was made as a control. Specimens were kept at 38 °C in a 1 mol l^− 1 NaOH solution to accelerate the reaction. Attenuation of transmitted ultrasonic waves appeared to be more appropriate for the evaluation of ASR-damage compared with pulse velocity. The attenuation of accelerated reactive concrete cylinders increased by 90% after 1 year while it increased by 40% for the non-reactive concrete used as a control. Major part of the attenuation increase in the non-reactive concrete is due to liquid absorption.This work suggests that in-situ non-destructive techniques based on ultrasonic wave attenuation, like ultrasonic attenuation tomography, should be developed in order to evaluate the development of ASR in concrete structures. Petrographic examination confirmed that damage to concrete is associated with ASR.     

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.