52,976 research outputs found

    Relationship between Degree of Deformation in Quartz and Silica Dissolution for the Development of Alkali-Silica Reaction in Concrete.

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    This paper presents research on the influence of quartz deformation in aggregates for the development of the alkali-silica reaction in concrete and its relationship with silica dissolution. The study also compares these characteristics with the field behavior of such rocks in concrete. The paper proposes parameters to classify the different degrees of deformation of quartz. Transmission electron microscopy showed the presence of walls even in slightly deformed quartz, which indicate the presence of the internal paths available to react with the alkaline concrete pore solutions and point to the potential development of an alkali-silica reaction. The presence of the deformation bands in the quartz grains leads to the alkali aggregate reaction occurring more rapidly. The visible spectrophotometer test was performed to evaluate the dissolution potential of the different samples of deformed quartz, which confirmed that the reactivity of the quartz increases as the deformation of the crystalline structure increases. The parameters established in the present study could be verified by analyzing the behavior of reactive and innocuous aggregates from the buildings

    Diffusion-reaction model for alkali-silica reaction in concrete

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    A new diffusion-reaction model for the potentially deleterious Alkali-Silica Reaction (ASR) process in concrete is presented. The model involves three coupled diffusion processes, two in-goingand one out-goingfrom the aggregate viewpoint. Alkali (Na+ and K+) and Calcium (Ca2+) ions diffuse “inwards”, from high molar concentration sites in the pores of the cement paste phase of the concrete specimen or at its boundaries, towards the aggregate-cement paste interfaces or the inner cracks of the aggregates. The OH- ions associated with alkali and calcium ions attack certain forms of silica in the aggregates (the “reactive silica”), dissolving it in the form of silicate ions which in turn diffuse back to the cement paste phase (“outwards”). The final potentially deleterious ASR precipitation process involves those silicate ions, plus calcium and alkalis. It takes place wherever the reactants are available by precipitating silicate hydrates of two kinds (Calcium-Silicate-Hydrates –CSH or Calcium-Alkali-Silicate-Hydrates –CASH) in a proportion depending on concentrations and temperature. The diffusion-reaction equations of this process are discretized in space and time using finite differences. An example of application in 1D is presented to illustrate the capabilities to reproduce realistically the ASR process, including some novel features not usually which are not considered in the available literature, such as the role of calcium in the development of the reaction and the inherent relationship between the reaction product composition and its swelling capacity

    Contribution to diagnosis of alkali-silica reaction in a bridge structure

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    Abstract: This paper describes an examination of concrete cores drilled from an old motorway bridge that had manifested deterioration and was in need of repair. Site observations and the nature of cracking indicated that alkali-silica reaction was suspect. Cores taken from the damaged structure were analyzed by using optical petrography for potential presence of reactive phases in aggregates, and chemical analyses to estimate the residual alkali content in the concrete. Results from the test methods were consistent with existence of alkali-silica reaction damage mechanism in the concrete. The level of severity of alkali-silica reactivity appeared to be moderate. The diagnostic features determined from tests indicate with some certainty, that alkali-silica reaction might have partly or solely contributed to distress in the bridge structure

    MAT-721: ALKALI-SILICA REACTIVITY IN SOUTHWESTERN ONTARIO AGGREGATES

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    Alkali-silica reaction (ASR) occurs between alkalis in concrete and reactive silica in aggregates. This reaction results in the formation of alkali-silica gel, which fills the pore space in the cementitious matrix, leading to expansion and damage. Reactive aggregates that can cause this type of damage were identified in different locations in Southwestern Ontario. X-Ray diffraction and petrographic analysis were used to investigate the type of reactive minerals in such aggregates. In addition, the effect of using different types of cement replacement-materials, including crushed limestone powder, fly ash, silica fume, and nano-silica on ASR were investigated in cement mortars incorporating the reactive aggregates. Results indicate that the expansion of mortar bars due to alkali-silica reaction can be controlled using an adequate type and substitution level of cement replacement materials

    Alkali Silica Reaction (ASR) Mitigation in Concrete by using Lithium Nitrate

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    Expansive alkali silica gel forms due to the alkali reaction with reactive aggregates.nbsp Cracks and damage to the part of the concrete structure are the results of the formation of this expansive gel. There are several ways to minimize this reaction, out of which Lithium Nitrate compound is believed to reduce the reaction between alkalis (present in the cement) and silica (present in the aggregates). In this paper a laboratory investigation has been done with the combination of reactive aggregate, OPC and varying percentage of Lithium Nitrate to evaluate the effectiveness of Lithium Nitrate compound in controlling expansions resulting from alkali-silica reaction.nbs

    Special Report on Alkali-Aggregate Reactivity in Iowa, MLR-80-03, 1980

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    The purpose of this investigation was to obtain information relative to the alkali-silica reaction in Iowa aggregates. Of particular concern were those aggregates in southwestern Iowa thought to be potentially alkali reactive. Further, should those aggregates have proven to be alkali-reactive, at what cement alkali content could these aggregates be considered to be deleteriously reactive? If the aggregates were proven to be reactive, what types of effects might show up in a structure in which an alkali-silica reaction has occurred? Also, what environmental conditions would cause the reaction? Finally, based on the information obtained from the investigation, would it be possible to raise the cement alkali content specifications? Would the Iowa DOT eliminate the alkali content limits altogether except for cement used with reactive aggregate in the same manner as AASHTO or ASTM? Also, would there be any other side effects that might occur as the result of using high alkali-cement

    Inhibiting alkali silica reaction in concrete with lithium salts.

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    A study into the effectiveness of various lithium salts and two lithium-containing minerals to prevent damage to concrete resulting from alkali-silica reaction is presented. An accelerated testing method recently proposed by Hudec and Larbi was used as the expansion test. Length changes were measured with a double linear variable differential transformer connected to a TRS-80 Model III computer. Three reactive aggregates and one unreactive aggregate were used. Lithium carbonate, lithium chloride, lithium fluoride, and lithium hydroxide were found to be effective in reducing expansions due to alkali-silica reaction. Four other salts, lithium acetate, lithium bromide, lithium nitrate, and lithium perchlorate were not effective in reducing the expansion. The lithium-containing minerals also proved to be ineffective against alkali silica reaction. It is concluded that the effectiveness of the lithium salts against alkali-silica reaction is related to the ionic structure of lithium. (Abstract shortened by UMI.)Dept. of Geology and Geological Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1991 .B353. Source: Masters Abstracts International, Volume: 30-04, page: 1452. Thesis (M.A.Sc.)--University of Windsor (Canada), 1991

    Effects of Alkali-Silica Reaction on the Fracture Behavior of Concrete

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    With the majority of nuclear power plants in the United States approaching their operational life span, it has become important to reevaluate their durability. In partnership with other research institutions, Oak Ridge National Laboratory (ORNL) has allocated resources to identify mechanisms for degradation of structural components in these power plants. Among these degradation mechanisms, alkali-silica reaction has proven to be common. The University of Tennessee–Knoxville has partnered with the Fusion and Materials for Nuclear Systems Division of Oak Ridge National Laboratory to evaluate the effects of this reaction.Alkali-silica reaction in concrete structures has become a subject of interest in the research community as well as in the field of structural engineering. Alkali-silica reaction (ASR) is a chemical process in concrete that involves the reaction of alkaline solution with amorphous silica present in many aggregates. The alkaline solution dissolves the silica within the aggregates and forms an expansive gel product. In the presence of water, the gel expands, which can cause internal stresses and subsequent cracking within concrete. This poses long term risks on the structural integrity of reactive concrete.At the University of Tennessee–Knoxville, a controlled environment was constructed to cure and monitor alkali-silica affected concrete specimens. This environment was used to develop specimens for testing of mechanical properties and monitor gel formation and expansion over time. Traditional testing was performed to evaluate the mechanical properties and the wedge-splitting test was performed to characterize fracture behavior. This thesis also investigates the effect of micro-crack orientation on the mechanical behavior. Additionally, a computer model was developed to simulate alkali-silica formation and loading of affected specimens

    Geological study and mining plan importance for mitigating alkali silica reaction in aggregate quarry operation

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    More than 80 million tonnes of construction aggregate are produced in Peninsular Malaysia. Majority of construction aggregate are produced from granite. Developing regions of Johor Bahru, Kuala Lumpur, Penang and Selangar utilize granite aggregates. Normally it is considered aggregates as non-alkali reactive. Geological study can identify various rock types, geological structures, and reactive minerals which contribute to Alkali Silica Reaction (ASR). Deformed granites formed through faulting results in reduction of quartz grain size. Microcrystalline quartz and phyllosilicates are found in granites in contact with country rocks. Secondary reactive minerals such as chalcedony and opal may be found in granite. Alkali Silica reaction is slow chemical reaction in concrete due to reactive silica minerals in aggregates, alkalis in cement and moisture. For long term durable concrete, it is essential to identify potential alkali silica reactive aggregates. Lack of identifying reactive aggregates may result spalling, cracking in concrete and ultimately ASR can result in hazard to concrete structure. This paper deals with geological study of any aggregate quarry to identify rock type and geological structures with laboratory test –petrographic analysis and bar mortar test can identify type of aggregates being produced. Mine plan with Surpac software can be developed for systematic working for aggregate quarry to meet construction aggregate demand

    Effects of sodium ions on synthesized alkali silica reaction gels

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    The alkali-silica reaction (ASR), leading to serious structural degradation, is the chemical reaction between reactive silica presenting in aggregates and hydroxyl ions from cement paste or pore solution. Although the chemical mechanism of ASR attack has been well studied for years, the mechanism of micro scale ASR gel formation leading to macro scale expansion is still under debate. The present study aims to illustrate the performance of ASR gel by investigating the interaction between ASR gel and sodium ion from solution. In this study, ASR gels with different calcium silica (Ca/Si) ratios (0.1, 0.5, 1) are synthesized by mixing reagent Ca(OH)2 with silica fume in a sodium hydroxide solution for seven days. Afterwards, the synthesized ASR gel is immersed in sodium hydroxide solutions with different concentrations (0.1mol/L, 0.5mol/L, 1.0mol/L) for seven days. Chemical composition, structure and water content of the ASR gel before and after alkali exposure are studied by XRD, XRF and TGA. The results confirm that an ASR gel with a targeted Ca/Si ratio can be synthesized. In addition, XRD and TGA results show that part of the calcium in the ASR gel is exchanged by sodium, leading to a structural modification. In general, this study will give further comprehension of ASR gel performance under alkaline environment, and provide detailed data to investigate the interaction between ASR gel and calcium ions in an alkaline solution in the future
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