Alkali-aggregate Reaction Research Articles

Developing numerical algorithm and a new program for simulating alkali aggregate reaction in mass concrete

Developing numerical algorithm and a new program for simulating alkali aggregate reaction in mass concrete

Properties and durability of coarse igneous rock aggregates and concretes

Direct tests allowed determining properties of coarse igneous rock aggregates in comparison with gravel along with freeze-thaw resistance and susceptibility to alkali-aggregate reaction. The relationship between the properties and durability of the aggregates and concretes was evaluated taking into account the results from microstructural analyses. Granodiorite, basalt and gravel were sensitive to alkali-silica reaction due to the presence of strained quartz, chalcedony and opal, respectively. The resistance of the aggregate to freezing and thawing was demonstrated to agree with the values of crushing resistance and the lowest contents of pores with diameters unsafe in terms of the freeze-thaw resistance.

Failure of coastal protection concrete armour units due to alkali–silica reaction

The 1993 Clacton Coastal Defence Works included a 1·4 km length of sloping revetment formed of precast concrete armour units. The revetment was designed for a service life of greater than 50 years, but severe cracking of the armour units necessitated its entire replacement after only 22 years. This paper describes an investigation that determined the primary cause of the armour unit deterioration to be alkali–silica reaction (ASR), which appeared to have been exacerbated by the choice of cement, leaching action and the geometry of the units. In addition, sustainable reuse of the concrete units within the rock armour fishtailed groynes of a replacement coastal protection scheme is described. This case study is of particular importance as it is notable that the failed concrete armour units complied with the current UK guidelines for prevention of ASR. As such, it demonstrates the importance of considering the potential action of leaching and the associated alkali migration when designing concrete structures to resist alkali–aggregate reactions. In particular, designers of marine/coastal structures should be aware of the potential leaching and alkali migration implications of specifying hollow concrete armour units with relatively low surface area to volume ratios.

Alkali–aggregate reaction: performance testing, exposure sites and regulations

Several laboratory performance tests for alkali–aggregate reaction (AAR) have been used worldwide for about 20 years. However, a review performed one of the present authors in 2006 concluded that none of the test methods meets all the criteria for an ideal performance test. In the last decade, research has been performed in several countries with the aim to improve current AAR performance test methods and develop alternative tests. The Rilem (The International Union of Laboratories and Experts in Construction Materials, Systems and Structures) Technical Committee (TC) 258-AAA (2014–2019) is also focusing on this topic. The main purpose of this technical committee is to develop and promote a reliable performance-based testing concept for the prevention of deleterious AAR. Strong emphasis will be placed on the implementation of the Rilem methods and recommendations as national and international standards. It is crucial to link the results from the accelerated laboratory testing to field behaviour, primarily against field exposure sites. Some results from such exposure sites are included in this paper.

Options for minimising the risk of alkali–aggregate reactions

An alkali–aggregate reaction (AAR) is one of the many deleterious processes affecting the durability of concrete infrastructure worldwide. It is the result of reactions between the alkaline pore solution in concrete and some reactive phases in the aggregate. The reactions lead to the formation of an expansive force within the concrete, which, in certain conditions, can lead to damaging expansions and cracking in the concrete. For such damaging expansion to occur, all of the following conditions must be present simultaneously: (a) a sufficiently alkaline pore solution, (b) a critical amount of reactive material and (c) a sufficient supply of water. Effective specifications to avoid damage from the above reactions are based on ensuring that at least one of the these conditions is absent. The development of the precautions against AARs around the world generally involves the determination of the necessary level of precaution required based on the aggregate's reactivity, the size of the structural element and the environment (exposure conditions), and a classification of the ‘criticality’ of the structure to build. The development and implementation of preventive actions according to the level of precaution required are then undertaken following prescriptive or performance approaches.

Rilem activities on alkali–silica reactions: from 1988–2019

Since 1988, the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (Rilem) technical committees (TCs) have been seeking to establish universally applicable test methods for assessing the alkali-reactivity potential of aggregates, and from later on, for concrete mixes. TC 106, in the years 1988 to 2001 focused on accelerated aggregate tests, and presented the findings at the International Conferences on Alkali Aggregate Reactions (ICAAR) in Kyoto in 1989, London in 1992, Melbourne in 1996 and Quebec in 2000. The successor committee TC 191-ARP in the years 2001 to 2006 also included work on diagnosis/appraisal and specification, and presented the findings at the ICAAR in Beijing in 2004. TC 219-ACS in the years 2006 to 2014 introduced work on performance testing and modelling, and presented its findings at the ICAARs in Trondheim in 2008 and in Austin in 2012. The major recommendations were published as a Rilem state-of-the-art report in 2015. In 2014, the TC 258-AAA was established, and it scheduled the completion of work on performance-based assessment for 2019. The preliminary findings will be published at the ICAAR in São Paolo in 2016.

Study of effect of electric arc furnace slag on expansion of mortars subjected to alkali-aggregate reaction

Abstract Alkali-aggregate reaction is a chemical reaction between cement alkalis and some reactive mineral present in some aggregates, leading to concrete expansion and cracking. One kind usually observed and studied in Brazil is the alkali-silica reaction, due to its fast development. There are several methods that are effective to control and mitigate this reaction, and one of them is the partial replacement of cement by mineral additions such as pozzolans like fly-ash, silica fume and slag. In this study, we propose the use of electrical steel slag as a partial replacement of cement, evaluating its effectiveness by NBR 15577:2008, employing different proportions as replacement. It seems that the electrical steel slag, despite its expansive behavior, has been effective in the control of the ASR.

Expansive chemical reactions in dams and hydroelectric projects

The various forms of expansive reactions, including alkali–aggregate reactions and internal sulfate reactions, which may occur in the concrete in hydroelectric projects and dams are characterised. The effects of chemical expansion on different dam types and hydro-project structures and their vulnerabilities to it are identified. Preliminary estimates of strain and the timing of the development of critical issues for a sampling of known cases where data are available are presented in order to give some general indications of the magnitudes of the expansion strains that have been found to begin to cause critical operational or safety issues. The effectiveness of the current testing methods is reviewed. For dams where the expansions are continuing unabated, such as where alkali resupply is a factor, at present there is no known means to stop the reaction, and the focus is on long-term management of the expansion by slot cutting or other means to accommodate the effects. In only a very few cases has replacement become necessary due to expansive chemical reactions. Usually, the deterioration in key material properties is limited. The main concerns relate to the deformations that occur as a result of the expansion, causing cracking and/or equipment misalignment.

Utilization of quarry by-products for reduction of expansion due to alkali-aggregate reaction

The replacement of normal fine aggregate with high fines limestone screenings is a technique to reduce waste from crushed stone operations, while potentially improving the durability and performance of concrete. The main objective of this research is to evaluate varying percentages of high fines limestone screenings as a partial weight replacement of reactive fine aggregate (similar to limestone sweetening) in mortars to assess reductions in the expansion due to alkali-aggregate reaction (ASTM C 227, ASTM C 1260, and modified ASTM C 1105). Fresh and hardened properties have been evaluated to assess the effects of limestone screenings on performance criteria. Environmental scanning electron microscopy (ESEM) was performed to visually observe and confirm alkali-aggregate reaction products and associated damage. From the physical testing and microstructural analysis, it was found that 50% or more of limestone screenings significantly reduce expansions due to alkali-aggregate reaction, attributed to the preference for monocarbonate formation in the presence of limestone and/or reactive silica dilution. Replacements less than 50% did not significantly reduce expansion.

Effect of colloidal nano-silica on alkali–silica mitigation

Alkali–aggregate reaction was first observed in the US during 1920s. In the last decades, several studies investigated the mechanisms of alkali–silica reaction in concrete and the extent of damage it may cause to the cementitious matrix. Research on alkali–silica reaction mitigation covers mainly the use of supplementary cementitious materials and lithium compounds. However, with possible change in coal burning regulations and fly ash shortages in some areas, it is important to investigate approaches to mitigate ASR that involves lower amounts of supplementary cementitious materials. Furthermore, current research has shown the potential enhancements to concrete microstructure through the use of nano-silica. In the current study, the effect of using nano-silica on the alkali–silica reaction will be investigated. Preliminary results show that nano-silica can help mitigate the effect of alkali–silica reaction.

Diagnosis of alkali–aggregate reaction in practice

This is a summary paper focused on the diagnosis of the alkali–aggregate reaction (AAR) in concrete in practice. Rather than broadly attributing a crack to AAR, this paper provides guidance for a practitioner who faces a real concrete structure and must determine whether the crack is being caused by AAR, or drying shrinkage, frost, overloading, sulfate attack or something else. The structure must be allowed to express what has caused its problem. This paper helps to follow a general approach and to look for specific markers to confirm or exclude AAR. Topics covered include first symptoms, desk study, follow-up inspection, structural observations, sample collection, inspection of cores, petrographic examination and additional tests such as scanning electron microscopy, electron probe microanalysis and wet-chemical analysis.

Appraising structures affected by the alkali–aggregate reaction

This paper presents the appraisal of structures affected by the alkali–aggregate reaction (AAR) and is based on the overall philosophy and principles that have already been established in a Rilem report to be published. This report is part 2 of the recommendations on diagnosis and appraisal that have been developed in the frame of the Rilem technical committee 219-ACS. It recommends that, from the start of an investigation, there should be close links between the materials science team inspecting and testing for diagnosis and the structural engineers carrying out an appraisal. This paper first describes the initial appraisal that is based on site investigations (cracking index, global deformation and environment analysis) and the laboratory investigations on the cores taken from the structure (mechanical testing, residual expansion tests) in order to obtain a severity assessment of the structure. Then it presents the structural assessment that includes the severity rating of the structural elements and the structural assessment based on a numerical modelling referring to Larive's law.

Assessing aggregates for alkali–aggregate reaction potential

The identification of alkali-reactive aggregates has been a challenge since alkali–aggregate reactions were first diagnosed. Standards, test methods and recommendations have been developed on a local and international basis. The Rilem recommendations regarding alkali–aggregate reactions are based on a methodology in which a series of laboratory tests is suggested. The assessment starts with a petrographic examination of the aggregate, followed by rapid screening tests and concrete expansion tests. This methodology has been carried out by a number of laboratories worldwide. However, some of the tests have not been totally satisfactory as sometimes the results do not correlate with each other or with field performance. A summary of the thresholds, advantages and limitations of each test according to the findings of Rilem TC 219-ACS is presented.

Effects of alkali–aggregate reaction on concrete and structures

The effects of alkali–aggregate reaction (AAR) on concrete and structures start at the microstructural level, that is, at molecular and microscopic levels, such as the formation of reaction products in concrete and associated microcracking, which ultimately leads to macro-level effects such as visual cracking, differential movements, displacement and so on. The consequences of these effects are also wide ranging, and depend on the severity of the reaction, the configuration and functions of the affected element and the exposure conditions. The effects of AAR on concrete can range from minor cracking in non-load-bearing elements in benign environments, which may not have any significant consequence, to severe cracking in sensitive structural elements, particularly those exposed to aggressive environments, which could have serious detrimental effects on the mechanical and durability properties of concrete. This paper provides an overview of the changes that occur in the mechanical and durability properties of AAR-affected concrete elements.

Types of alkali–aggregate reactions and the products formed

The alkali–aggregate reaction comprises the alkali–silica reaction (ASR) and the alkali–carbonate reaction (ACR). Reaction kinetics of the ASR depends on the grain size and crystalline structure of the reactive silicon dioxide. The reaction starts in the aggregates along the particle periphery and progresses inward. After cracking of the particle, larger amounts of reaction products are formed and are eventually extruded in the cement paste, where they fill cracks and voids. ASR products within aggregates predominantly consist of (hydrated) silicon, alkalis and calcium with characteristic atomic ratios of (Na + K)/Si ∼ 0·25 and Ca/Si ∼ 0·25. They take up additional calcium while releasing alkalis when extruded. Amorphous and crystalline reaction products occur and coexist. Expansion is the result of water adsorption by the reaction products. In ACR, harmless dedolomitisation is distinguished from deleterious reaction of fine-grained silica disseminated throughout the carbonate matrix. Further research is needed to gain more in-depth knowledge about the thermodynamics and kinetics of ASR products and the mechanisms of expansion. This should allow the establishment of a better link between concrete structures and both accelerated testing and models.

CFRP confinement of circular concrete columns affected by alkali-aggregate reaction

Alkali-aggregate reaction (AAR) is known to cause expansion and cracking in concrete, which adversely affect the mechanical properties of concrete, leading to a decrease in the service life of structures. Wrapping damaged elements with epoxy-bonded carbon fiber reinforced polymer (CFRP) is a method of rehabilitation. This paper investigates the level of expansion in concrete caused by AAR and its effects on the mechanical properties of concrete during a period of one year. The decreases in compressive strength, flexural strength, splitting tensile strength and modulus of elasticity reached 74%, 81%, 76% and 93% respectively. Twenty six reinforced and plain columns of dimensions 200mm in diameter and 500mm in height were fabricated from normal and reactive concretes. The latter contained fused silica at 7.5% of total aggregate as reactive component. The columns developed high expansion levels due to AAR in the storage period between 120 and 180days, which caused severe cracking in the affected columns. One, two and three layers of CFRP were used to confine the affected and unaffected columns. Results indicated that CFRP wrapping increased the strength and strain capacities of confined columns compared to unconfined columns. Larger number of applied CFRP layers also increased the capacity of the affected columns.

Strain development in CFRP-wrapped circular concrete columns affected by alkali-aggregate reaction

A large number of concrete bridges and dams around the world are affected by deterioration caused by the alkali aggregate reaction (AAR). Fibre reinforced polymer (FRP) is considered one of the best materials for the rehabilitation of concrete columns affected by AAR. A number of cylindrical columns of 200mm in diameter and 500mm height were fabricated from reactive concrete using fused silica and additional alkali in its ingredients. At different periods during AAR development, i.e., 7, 15, 30, 45, 60, 90 & 120days after casting, external confinement by carbon fibre reinforced polymer (CFRP) was applied. The main objective of using CFRP as a confinement material was to reduce the effect of the AAR-induced expansion of the concrete. Results indicate that the efficiency of CFRP application in reducing AAR expansion depends on the time of CFRP wrapping and on the number of CFRP layers applied to the columns.

Alkali Aggregate Reaction Effects on an Arch Dam Behaviour Including Simulation of Construction Stages and Contraction Joints

In this paper, numerical formulation of alkali aggregate reaction (AAR) is implemented into an advanced finite element program. The formulation accounts for the induced expansion, gel absorption by the cracks and degradation of mechanical properties. A thin high arch dam is modelled as the case study, where construction stages as well as peripheral and vertical joints are modelled as reported in the as-built drawings. The analyses results show that AAR evolution is not influenced considerably by modelling of the joints and construction stages, but its subsequent structural effects are sensitive to such components. It is found that AAR influences the dam’s structural reactions and the effects of joints and construction stages are important in modelling the AAR-affected dam.

Ingress of NaCl in concrete with alkali reactive aggregate: effect on silicon solubility

Abstract Enhanced damage due to the alkali–silica reaction (ASR) in concrete exposed to deicing salt (NaCl) is usually attributed to binding of chloride ions in the hydration products of cement. To balance charge, OH− ions are released into the concrete pore solution which increases alkalinity. However, during NaCl ingress a decrease in the OH− concentration of the concrete pore solution due to potassium leaching would reduce SiO2 solubility and therefore ASR damage. The present work combines expansion measurements with pore solution analysis by ICP-OES and XRD measurements on concretes and hydrated cement pastes. Solubility equilibria calculations were performed with the hydrogeochemical simulation program PHREEQC. The investigations show that the OH− concentration of the pore solution is mainly lowered by potassium leaching during NaCl ingress. The OH− concentration also decreases owing to the formation of Friedel’s salt from ettringite which is associated with the release of sulphate. Although the OH− concentration with NaCl is lower, ASR damage is intensified and the silicon concentration in the pore solution is higher. Higher silicon solubility is explained by the higher total alkali concentration which increases surface silicate solubility, the formation of an aqueous complex NaHSiO 3 0 and a higher ionic strength. These effects promote the sensitivity of silicate minerals to ASR, the formation of alkali silica gel and finally ASR damage.

Effect and limitation of free lime content in cement-fly ash mixtures

This study emphasizes the limitation of free lime content in fly ash. A detailed experimental program was carried out in order to evaluate the extent to which free lime content in fly ash can be tolerated, particularly for fly ashes with different sulfur trioxide contents. Some basic and durability properties of fly ash mixtures with varied free lime contents were considered. Four distinct types of fly ashes were obtained from two different sources, and free lime was added to obtain overall free lime contents of 5%, 7% and 10% for each type of fly ash. Water requirement, initial and final setting times, compressive strength, autoclave expansion, alkali aggregate reaction (AAR), and sulfate resistance tests of fly ash mixtures, containing various free lime contents, with two fly ash replacement percentages (20% and 40%), were conducted. Experimental results revealed that an increase in the free lime content caused an increase in water requirement. Higher free lime content also lead to faster setting times, improved compressive strength, and higher autoclave expansion. Mixtures with 20% fly ash replacement and free lime content up to 10% as well as mixtures with 40% fly ash replacement and free lime content up to 7.72% experienced autoclave expansion within the specified limit of ASTM C618. Similar trends of expansion were observed in cases of alkali-aggregate reaction and sulfate resistance tests where fly ash mixtures with high free lime led to higher expansion. In alkali-aggregate reaction testing, the mixtures with 20% fly ash replacement and free lime content up to 7.95%, as well as mixtures with 40% fly ash replacement and free lime content up to 10%, expanded less than the cement-only mixtures. The effect of added free lime was more severe in the sulfate resistance test, as fly ash mixtures tend to expand more than cement-only mixtures, especially in the case of fly ashes with very high SO3 content (>5%). Test results also revealed that it is possible to utilize 20% of tested fly ash in a mixture as binder, with SO3 content <5% and free lime content up to 10% while not compromising the basic and durability properties. In the case of 40% fly ash replacement, tested fly ashes with SO3<5% and free lime content up to 4.23% can be utilized. In the case of high SO3 content in fly ash, i.e., 8.53% and 9.44% in this study, the limit of free lime content of fly ash is reduced to 5.31% and 3.73%, for 20% and 40% fly ash replacements, respectively to satisfy the durability requirements.