Alkali Silica Reaction - Diagnosis

1. Hobbs (1988) recommends answering each of the following questions before pronouncing alkali-silica reaction as the original corrosion perpetrator.

• Is the map cracking or oriented cracking characteristic of that caused by ASR?
• Has expansion occurred? Measurements could be taken or the structure could be inspected for closed joints and misaligned elements.
• Is the microcracking typical of that associated with ASR? Answering this question requires cores to be cut to look for evidence of expansion, presence of gel in the voids, and damp patches. Slices of the concrete can be taken from the core, impregnated with UV fluorescent dye, and examined under UV light.
• Is there positive evidence of ASR gel? The gel could appear on the exterior surface of the structure or it could be evident in the pores of the structure.
• Is the reactive silica in the coarse or fine aggregate? Reactive silica in the fine aggregate normally produces problematic cracking.
• Were sufficient alkali in the original concrete or induced at some later time?
• Has sufficient silica from the aggregate been depleted?

2. According to Hobbs (1988), if map cracking exists in unreinforced or lightly loaded concrete, oriented cracking occurs in unreinforced or lightly loaded concrete, the macrocracks are visibly discolored, gel is exuding from the macrocracks, and there is visible expansion of the concrete, ASR is the probable cause.

3. ASTM C 856 Petrographic Examination

• Look at thin sections of affected concrete and locate deleterious components. Search for alterations, cracks in the aggregate, cement separated from the aggregate, halos or reaction rims around siliceous grains, gel, and crystal growth (Swamy, 1992 and Farny and Kosmatka, 1997).
• Uranyl-Acetate Treatment: Spray freshly exposed concrete with uranyl-acetate solution, rinse it with water, and view the specimen under ultraviolet light. Reacted particles and gel will appear as bright yellow and green spots. (Farny and Kosmatka, 1997)

4. Ultrasonic Pulse Velocity

• Good quality concrete ® velocity > 4.0 km/s
• Poor quality concrete ® velocity < 3.0 km/s

5. X-ray diffraction and infra-red spectroscopy produce charts of the minerals in the concrete.

6. Resonant frequency will change with increased deterioration in the concrete.