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Alkali-silica reaction (ASR) is a chemical reaction between the alkali hydroxides from the concrete pore solution and some siliceous mineral phases present in the aggregates used to make concrete. ASR generates a secondary product, the so-called ASR-gel, that swells upon moisture uptake, leading to induced expansion, microcracking, and reduction in the mechanical properties of the affected material. ASR is likely the most harmful damage mechanism affecting the serviceability and long-term performance of concrete infrastructure worldwide, yet its structural implications in concrete structures remain unclear. Even though shear resistance of reinforced concrete has been studied extensively by the research community due to the brittleness and danger associated with concrete shear failures, knowledge on the impact of ASR on reinforced concrete shear resistance is very limited. To fill this lack of knowledge, the effect of ASR on the aggregate interlock shear transfer mechanism in reinforced concrete was investigated. Lightly reinforced shear push-off specimens with low to moderate expansion levels were tested while recording crack kinematics. The experimental testing program allowed to decouple the deleterious effect of ASR microcracks within the reactive coarse aggregate particles and the beneficial effect of the so-called chemical prestressing. The aggregate interlock shear strength was significantly impacted, even in the case of a low expansion level for which the microcracks have theoretically not reached the cement paste yet, and surprisingly, it was not affected by prestressing. The experimental results were compared to predictions from three existing simplified aggregate interlock models which tended to overestimate the measured shear strengths. Digital image correlation (DIC) is an innovative optical measurement technique that could provide several advantages for long-term structural inspections such as remote full-field measurements. A method was proposed to correct 2D-DIC measurement errors associated with the inevitable camera movement between photographs taken during different inspections. Using the aforementioned push-off specimens, it was applied to the monitoring of shear crack kinematics and ASR expansion. The method significantly improved measurements produced from images acquired with a non-expensive hand-positioned camera equipped with a lens of normal focal length and a free to use DIC software. For ASR expansion monitoring, the measurement errors could not be reduced below a selected tolerance limit of ±0.02 mm (±0.01% strain), although increasing the measurement gauge length could potentially provide satisfactory results. On the other hand, over 99 and 96% of the measurements were within the selected tolerance limit of ±0.1 mm for the corrected crack width and slip measurements, respectively. These promising results validate the potential of the proposed method to overcome errors associated with camera movement between photographs and as such, it represents a step towards the use of the DIC technique for periodic structural inspections.
This book reviews the fundamental causes and spectrum effects of ASR. It considers he advances that have been made in our understanding of this problem throughout the world.
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.
Aggregates containing certain constituents can react with alkali hydroxides in concrete. The reactivity is potentially harmful only when it produces significant expansion. This alkali-aggregate reactivity (AAR) has two forms--alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR, sometimes called alkali-carbonate rock reaction, or ACRR). ASR is of more concern than ACR because the occurrence of aggregates containing reactive silica minerals is more common. Alkali-reactive carbonate aggregates have a specific composition that is not very common. Alkali-silica reactivity has been recognized as a potential source of distress in concrete since the later 1930s. Even though potentially reactive aggregates exist throughout North America, ASR distress in structural concrete is not common. There are a number of reasons for this: 1. Most aggregates are chemically stable in hydraulic-cement concrete 2. Aggregates with good service records are abundant in many areas 3. The concrete in service is dry enough to inhibit ASR 4. The use of certain pozzolans or slags controls ASR 5. In many concrete mixtures, the alkali content of the concrete is low enough to control harmful ASR 6. Some forms of ASR do not produce significant deleterious expansion To reduce ASR potential requires understanding the ASR mechanism; properly using tests to identify potentially reactive aggregates; and, if needed, taking steps to minimize the potential for expansion and related cracking. Alkali-carbonate reaction in concrete was not documented until 1957. Although ACR is much less common, this report also briefly reviews the mechanism, visual distress symptoms, identification tests, and control measures.
Concrete can deteriorate as a result of an interaction between alkaline pore fluids (prinicipally originating from the Portland cements) and reactive minerals in certain types of aggregates. The mechanism of deterioration is known as alkali-aggregate reaction (AAR); it can occur in a number of forms, the most common being alkali-silica reaction (ASR).
Alkali-Aggregate Reaction (AAR) problem is common in structures such as bridges, roadways, airport runways, and nuclear power plants that were built with reactive aggregate. The Alkali-Aggregate Reaction progresses with time in concrete between the alkaline cement paste and reactive amorphous silica. The reaction uses the moisture in the atmosphere and produces a gel that keeps dilating. The dilating gel causes cracks in the concrete mass thus possibly compromising the integrity of concrete. This can cause a number of issues with regard to the performance of the concrete structures caused by deteriorating concrete properties such as lowering of tensile strength, stiffness, ductility and deterioration of bond characteristics. Results from testing two squat shear walls made with normal concrete and four walls with concrete containing reactive aggregate causing alkali-silica reaction (ASR) are presented. In addition to the squat shear walls, several companion specimens were cast to evaluate the concrete material properties and perform non-destructive tests. These specimens included 21 cylinders, six modulus of rupture (MOR) beams, three expansion prisms, and six dog-bone specimens. To accel¬erate the ASR and deterioration of the concrete, the walls were stored in an environmental chamber, specially constructed with the capacity to store large specimens in a controlled high-temperature and high-humidity condition. These walls were tested in three stages under reversed cyclic lateral loads while at the same time subjected to constant axial load simulating earthquake loads. Small companion specimens revealed that ASR caused free expan¬sion of approximately 0.23%. While concrete gained compressive strength over time, its tensile strength and stiffness deteriorated significantly due to the ASR. The lateral load carrying capacity of the walls was not adversely affected. The performance of the walls, however, deteriorated significantly over time with respect to ductility and energy dissipation capacity. The absorbed strain energy capacity of the ASR shear wall at full exhaustion was approximately 25% of that of the regular concrete wall and the displacement ductility was reduced by approximately 30% due to ASR. Finally, finite element analysis technique was used to model this behaviour which gave reasonably good estimates of the experimental shear wall responses.