Alkali-aggregate Reaction Expansion Research Articles

Mesoscale Modeling Study on Mechanical Deterioration of Alkali-Aggregate Reaction-Affected Concrete.

The alkali–aggregate reaction (AAR) is a harmful chemical reaction that reduces the mechanical properties and weakens the durability of concrete. Different types of activated aggregates may result in various AAR modes, which affect the mechanical deterioration of concrete. In this paper, the aggregate expansion model and the gel pocket model are considered to represent the two well-recognized AAR modes. The mesoscale particle model of concrete was presented to model the AAR expansion process and the splitting tensile behavior of AAR-affected concrete. The numerical results show that different AAR modes have a great influence on the development of AAR in terms of expansion and microcracks and the deterioration of concrete specimens. The AAR mode of the gel pocket model causes slight expansion, but generates microcracks in the concrete at the early stage of AAR. This means there is difficulty in achieving early warning and timely maintenance of AAR-affected concrete structures based on the monitoring expansion. Compared with the aggregate expansion model, more severe cracking can be observed, and a greater loss of tensile strength is achieved at the same AAR expansion in the gel pocket model. AAR modes determine the subsequent reaction process and deterioration, and thus, it is necessary to develop effective detection methods and standards for large concrete projects according to different reactive aggregates.

Influence of coarse aggregate size on deterioration of concrete affected by alkali-aggregate reaction

The alkali-aggregate reaction (AAR) is one of the main factors to reduce the durability of concrete structures. Dam concrete contains very large aggregates, and the influence of coarse aggregate size on AAR-induced deterioration requires a deep understanding. In this paper, a mesoscale particle model of concrete was presented to simulate the AAR process and mechanical testing of AAR-affected concrete. Four types of concrete with different sizes of coarse aggregates were considered. The experimental AAR expansion of dam concrete containing aggregates up to 150 mm was used to validate the mesoscale model. The numerical simulation results show that the coarse aggregate size significantly affects the AAR expansion and the degradation of mechanical properties. Dam concrete with 150 mm aggregates exhibits smaller expansion but earlier cracks than general concrete with small coarse aggregates. The mechanical properties in terms of elastic modulus, compressive strength and splitting tensile strength are reduced more in concrete with larger aggregates at the same AAR expansion. These phenomena illustrate that it is not appropriate to assess the AAR-induced deterioration of the mechanical properties of dam concrete using an index based on concrete with 5 ∼ 20 mm aggregates.

Engineering-based finite-element approach to appraise reinforced concrete structures affected by alkali–aggregate reaction

Modelling the expansion and damage generated by alkali–aggregate reaction (AAR) in reinforced concrete structures is quite complex, yet necessary to obtain accurate predictions of the structural response of distressed members. Several AAR models have been developed to predict expansion and damage at the material (microscopic) or the structural (macroscopic) scales. However, those models tend to either neglect or overemphasise the critical physicochemical parameters of the reaction, which limits their applicability. Therefore, a new simple yet reliable finite-element approach is proposed to fill this gap. It accounts for the most important parameters affecting AAR through an engineering approach, without the need for non-technical guesses or to ‘fit’ model parameters. The proposed model is validated through the computational simulation of reinforced concrete specimens cast and monitored in the laboratory. Results show that AAR expansion was accurately simulated by accounting for the anisotropic (stress state dependent) nature of the reaction, mechanical properties deterioration and an analytical equation capable of representing AAR's free expansion. Next steps include validating the approach by simulating real structures and incorporating phenomena like leaching and combined distress mechanisms.

Experimental and finite element analysis of mechanical behavior of concrete damaged by Alkali Aggregate Reaction (AAR) and repaired with CFRP Layers

Concrete structures is affected by a deleterious reaction, which is known as Alkali Aggregate Reaction (AAR). AAR can be defined as a chemical reaction between the alkali content in the pore water solution of the cement paste and reactive forms of silica hold in the aggregate. This internal reaction produces expansion and cracking in concrete, which can lead to loss of strength and stiffness. Carbon fiber-reinforced polymer (CFRP) is one of the methods used to suppress further AAR expansion and rehabilitate and support damaged concrete structures. In this research, thirty-six cylindrical specimens were fabricated from non-reactive and reactive concrete, which contained fused silica as 7.5%. In addition, twelve concrete prisms were fabricated from non-reactive and reactive concrete in which three different percentages of fused silica are used, 5%, 7.5% and 10% of the total aggregate. This paper investigates the impact of AAR expansion on the physical and mechanical properties of concrete. It also reports the effective use of one and two CFRP layers on wrapping concrete cylinders. The experimental results show that CFRP is effective in confining damaged concrete by AAR and results in concrete strength enhancement of up to 560%. A comparison of finite element (FE) analysis using ATENA 3D software and the experimental results indicated that FE analysis is capable of modelling the behavior of AAR-damaged concrete repaired with CFRP.

Evaluation of a direct shear test setup to quantify AAR-induced expansion and damage in concrete

Alkali-Aggregate Reaction (AAR) is one of the most harmful distress mechanisms affecting the durability and serviceability of concrete infrastructure worldwide. Over the years, a number of comprehensive testing protocols have been developed to appraise the amount of damage, expansion attained to date and mechanical properties reductions of concrete affected by AAR. Promising results were found through the use of advanced mechanical and microscopic techniques such as the Stiffness Damage Test (SDT) and Damage Rating Index (DRI). However, there is still a need to develop new “simple” yet “reliable” test procedures able to quantify AAR expansion and damage. This paper presents the use of a modified shear strength setup to appraise AAR development. Concrete samples incorporating a wide range of reactive aggregates were fabricated, monitored and tested under the proposed shear setup at distinct damage degrees. Results show that AAR influences significantly the shear strength of affected concrete and thus the proposed test setup seems promising in assessing AAR expansion and damage. Yet, further data is still needed to confirm the reliability of the results gathered in this research.

Sensitivity and uncertainty analysis of AAR affected reinforced concrete shear walls

Whereas many publications report on alkali aggregate reaction (AAR) expansion or test analyses, very few address the more pressing issue of its impact on structures. Furthermore, given the uncertainties associated with characterization of the material, probabilistic based approach are not known to have been reported in the literature. This paper addresses these limitations through an analysis of a reinforced shear wall. A two-prone approach is followed: first a sensitivity analysis is performed to narrow the number of random variables (RVs) to the most relevant ones. Then, an uncertainty quantification is performed through Latin Hypercube Sampling with and without AAR expansion. Then the capacity curves (including the summarized ones to 16, 50 and 84% fractiles) are developed. Probability of non-exceedance of a specific capacity (i.e., limit state) is shown though the so-called fragility curves. It is found that in some cases AAR increases the shear capacity, while in others it decreased it. It highly depends on the initial combination of the RVs.

MESO-SCALE PARTICLE MODELING OF ALKALI-SILICA REACTION AND MECHANICAL PROPERTIES OF CONCRETE

A three-phase meso-scale model of concrete which consists of aggregate, mortar, and interface between aggregate and mortar is established by pre-processing approach and the particle flow code (PFC2D). A homogeneous aggregate expansion procedure is proposed to simulate alkali-aggregate reaction, and splitting tensile test is conducted to explore the mechanical properties of concrete subjected to the alkali-aggregate reaction. The numerical simulation shows that this model can effectively predict the expansion trend of concrete alkali aggregate reaction expansion; the splitting tensile strength of concrete subjected to the alkali-aggregate reaction fell sharply.

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.

Modeling the alkali aggregate reaction expansion in concrete

Alkali aggregate reaction affects numerous civil engineering structures and causes irreversible expansion and cracking. This work aims at developing model to predict the potential expansion of concrete containing alkali-reactive aggregates. First, the paper presents the experimental results concerning the influence of particle size of an alkali-reactive aggregate on mortar expansion studied at 0.15–0.80 mm, 1.25–2.50 mm and 2.5–5.0 mm size fractions and gives data necessary for model development. Results show that no expansion was measured on the mortars using small particles(0.15-0.80 mm) while the particles (1.25–2.50 mm) gave the largest expansions. Finally, model is proposed to simulate the experimental results by studying correlations between the measured expansions and the size of aggregates and to calculate the thickness of the porous zone necessary to take again all the volume of the gel created by this chemical reaction.

Modelling The Effects of Aggregate Size on Alkali Aggregate Reaction Expansion

Modelling The Effects of Aggregate Size on Alkali Aggregate Reaction Expansion

알칼리-골재 반응에 의한 콘크리트 포장 팽창 장기 모니터링

PURPOSES: This paper describes the expansion caused by the alkali-aggregate reaction (AAR) in concrete pavement currently in service. It also discusses the effects of joints installed to release the stress induced by the AAR expansion. METHODS: The expansion effect on concrete pavement was verified by a visual inspection and long-term measurement of the joint width of a cut-section. The behaviors of 16 newly installed joints were monitored as part of the investigation and long-term monitoring was carried out for three years after cutting. RESULTS: The behavior of a bridge was affected when AAR occurred in the connected pavement. The newly installed joints shrank in the longitudinal direction of the bridge after cutting. The width of the joints decreased over the six months after cutting. A large portion of the joint width (8.5cm) was found to have closed nine months after cutting. It had ultimately shrunk by about 92 percent when the final measurement was taken. CONCLUSIONS : The expansion of the pavement due to AAR was quantitatively described by visual inspection and the long-term monitoring of the newly cut joints. However, the width of the new joints decreased over the six to nine months after cutting. Additional research should be conducted to determine a means of controlling the expansion due to AAR in the pavement.

Modelling The Effects of Aggregate Size on Alkali Aggregate Reaction Expansion

This work aims 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 (0.5-1.0 mm) while the particles (1.0–2.0 mm) gave the largest expansions (0.217%). Two models are proposed, the first one studies the correlations between the measured expansions and the size of aggregates, the second one calculates the thickness of the porous zone necessary to take again all the volume of the gel created.

Meso‐scale particle modeling of concrete deterioration caused by alkali‐aggregate reaction

SUMMARYA meso‐scale particle model is presented to simulate the expansion of concrete subjected to alkali‐aggregate reaction (AAR) and to analyze the AAR‐induced degradation of the mechanical properties. It is the first attempt to evaluate the deterioration mechanism due to AAR using the discrete‐element method. A three‐phase meso‐scale model for concrete composed of aggregates, mortar and the interface is established with the combination of a pre‐processing approach and the particle flow code, PFC2D. A homogeneous aggregate expansion approach is applied to model the AAR expansion. Uniaxial compression tests are conducted for the AAR‐affected concrete to examine the effects on the mechanical properties. Two specimens with different aggregate sizes are analyzed to consider the effects of aggregate size on AAR. The results show that the meso‐scale particle model is valid to predict the expansion and the internal micro‐cracking patterns caused by AAR. The two different specimens exhibit similar behavior. The Young's modulus and compressive strength are significantly reduced with the increase of AAR expansion. The shape of the stress–strain curves obtained from the compression tests clearly reflects the influence of internal micro‐cracks: an increased nonlinearity before the peak loading and a more gradual softening for more severely affected specimens. Similar macroscopic failure patterns of the specimens under compression are observed in terms of diagonal macroscopic cracks splitting the specimen into several triangular pieces, whereas localized micro‐cracks forming in slightly affected specimens are different from branching and diffusing cracks in severely affected ones, demonstrating different failure mechanisms. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of glass powder on chloride ion transport and alkali-aggregate reaction expansion of lightweight aggregate concrete

The effects of glass powder on the strength development, chloride permeability and potential alkali-aggregate reaction expansion of lightweight aggregate concrete were investigated. Ground blast furnace slag, coal fly ash and silica fume were used as reference materials. The replacement of cement with 25% glass powder slightly decreases the strengthes at 7 and 28 d, but shows no effect on 90 d’s. Silica fume is very effective in improving both the strength and chloride penetration resistance, while ground glass powder is much more effective than blast furnace slag and fly ash in improving chloride penetration resistance of the concrete. When expanded shale or clay is used as coarse aggregate, the concrete containing glass powder does not exhibit deleterious expansion even if alkali-reactive sand is used as fine aggregate of the concrete.

Parametric model for the analysis of concrete expansion due to alkali—aggregate reaction

The alkali—aggregate reaction (AAR) is a chemical reaction that provokes a heterogeneous expansion of concrete and reduces important properties such as Young's modulus, leading to a reduction in the structure's useful life. In this study, a parametric model is employed to determine the spatial distribution of the concrete expansion, combining normalized factors that influence the reaction through an AAR expansion law. Optimization techniques were employed to adjust the numerical results and observations in a real structure. A three-dimensional version of the model has been implemented in a finite element commercial package (ANSYS©) and verified in the analysis of an accelerated mortar test. Comparisons were made between two AAR mathematical descriptions for the mechanical phenomenon, using the same methodology, and an expansion curve obtained from experiment. Some parametric studies are also presented. The numerical results compared very well with the experimental data validating the proposed method.

Effect of Aluminum Hydroxide on Control of Alkali-Aggregate Reaction

Alkali-aggregate reaction (AAR) is one of serious problems of concrete. It is known that fly ash, blast furnace slag, and so on can control the expansion of concrete by AAR. Natural zeolite is known as material that control AAR expansion, too. This paper shows the effect of aluminum hydroxide on control the expansion of concrete by AAR. It is possible to control the expansion of concrete by AAR when aluminum hydroxide to binder ratio is 4.0%. Especially, when a little of fly ash is combined, the effect of aluminum hydroxide is big. But, when aluminum hydroxide is used with much fly ash, the effect of aluminum hydroxide is not clear. The effect of aluminum hydroxide appears when it dissolves in concrete. In this paper, three different type of aggregate were used in order to clarify the effect of aluminum hydroxide on the expansion of concrete by AAR. The effect of aluminum hydroxide on AAR is somewhat different in the type of aggregate. However, it will clarify that aluminum hydroxide is one of an effective admixture to control the expansion of concrete by AAR. By combining aluminum hydroxide with adequate quantity of fly ash, big effect to control the expansion of concrete by AAR can be expected.

Effectiveness of LiOH in inhibiting alkali-aggregate reaction and its mechanism

A practical alkali-reactive aggreagate—Beijing Aggregate—was used to test the long-term effectiveness of LiOH in inhibiting alkali-aggregate reaction expansion. In this paper, not only the most rigorous conditions, such as the mortar bars being cured at 80°C, for 3 years after autoclaved 24 hours at 150°C, but also the 20°C long-term curing condition, were designed. At these conditions, LiOH was also able to inhibit ASR expansion long-term effectively. Not only the relationship between molar ratio of n(Li)/n(Na) and the alkali contents in systems was established, but also the governing mechanism of such effects was also studied by SEM and EDS.

Alkali-aggregate reaction suppressed by chemical admixture at 80 °C

More detailed researches of chemical admixture in inhibiting Alkali-aggregate reaction (AAR) at 80 °C curing condition were done in this paper. It seemed that AAR expansion could be long-term effectively inhibited by LiOH both at the alkali contents of 2.5% and 3.0%. In addition, there was some effect on AAR suppression of the alkali reactivity of the aggregate, the alkali content of the mortar and the added amount of the admixture. Firstly, the higher the alkali reactivity of the aggregate was, the better the inhibiting effect of LiOH showed. Secondly, to the mortars with the same aggregate, the inhibiting effect of LiOH with the same n(Li)/ n(Na) molar ratio was improved with the augmentation of alkali content. Thirdly, when added to some extent, further augmentation of LiOH in mortar was not effective in increasing the AAR inhibition obviously. In the last, direct results of lithium compounds in suppressing AAR were found by SEM and EDS and the mechanism was discussed.

A comparison of RTA T363 and ASTM C1260 accelerated mortar bar test methods for detecting reactive aggregates

Nowadays, bridge structures are designed for 100 years of service life, and care must be taken to avoid materials that would cause premature deterioration problems in the structures. This requires careful attention to structural design, selection of sound materials and manufacture of components, which would remain durable under the given environmental conditions (exposure and loading). One of the important concrete deterioration problems is the alkali–aggregate reaction (AAR), which can cause cracking and diminished strength properties in concrete, and the best method of avoiding it is to select nonreactive aggregates for the concrete or those that could safely be combined with mineral additives to suppress their reactivity. The purpose of this paper is to compare the expansion results obtained by the two test methods using a number of aggregates with known field performance and to assess the validity of their expansion limits in predicting the field performance of these aggregates. The paper also covers the comparison of the two test methods for the assessment of the effectiveness of fly ash in suppressing AAR expansion.

Numerical simulation of concrete expansion in concrete dams affected by alkali–aggregate reaction: state-of-the-art

Many concrete dams and other concrete structures in Canada, and throughout the world, are suffering from deteriorations induced by alkali–aggregate reaction (AAR) that impair the durability and serviceability, and might also affect, in the long term, the safety of the installation. Alkali–aggregate reaction produces concrete expansion, and generally leads to a loss of strength and stiffness (cracking), and generates undesirable deformations and disturbances in the equilibrium of internal forces. The expansion mechanisms in concrete affected by AAR are complex and influenced by a number of factors that are difficult to quantify. Nevertheless, advanced numerical simulation models are generally used in close conjunction with field monitoring of displacements to assist in the structural evaluation and rehabilitation of dams where AAR has been identified. A review of the physical processes that control the structural behaviour of concrete dams suffering from AAR, and numerical simulation procedures to represent AAR concrete expansion by the finite element method, is presented herein. The present state of knowledge to simulate the AAR expansion process has been found to be limited and not yet satisfactorily developed. A methodology to distribute the observed concrete expansion in proportion to the compressive stress state, temperature, moisture, and the reactivity of the concrete constituents is proposed in this paper as a first step to rationalize the numerical modelling of the AAR concrete swelling process in concrete dams. Key words: alkali–aggregate reaction, concrete dams, finite element method, dam safety.