The Effect of Alkali-Silica Reaction on Aggregate Interlock In Reinforced Concrete and the Use of Digital Image Correlation to Monitor the Long-Term Behaviour of Concrete Structures

Abstract

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.