Experimental investigation on the alkali silica reaction effect on concrete strength degradation

Alkali Silica Reaction (ASR) is a significant cause of premature concrete deterioration. This internal reaction is a result of alkali hydroxides in pore solution that reacts with reactive silica found in certain aggregates. Once the silica is released from the aggregate an alkali-silica rich gel forms. ASR has been analysed for approximately 80 years, and has been a concern for the mechanical capacity of the affected structures. Under natural environment, the ASR reaction usually does take a few years to appear and show damage; Since the project time frame has been set as 8 months, the ASR reaction has to be accelerated to suit the time frame by using highly reactive aggregate, adding chemicals and placing the sample in hot water or oven. The Queensland Department of Transport and Main Roads has concerns that some of concrete bridges been affected by ASR. They are interested on the strengths (compressive and tensile) reduction cause by the reaction. In addition, they desire to know possible methods and remedies to minimise or even stop the reaction into the structure. The aggregate was first tested for reactivity using American Society for Testing and Materials (ASTM C 1260) – Standard test method for potential alkali reactivity of aggregates (mortar bar method); mortar bar (200 mm x 50 mm x 50 mm) and cylinder (200 mm x 50 mm) were casted. Regular expansion measurements were taken. Flexural strength test were conducted on the mortar bars at 155 days. Tests planned methodology: eighteen concrete cylinders (200 mm x 100 mm) were casted. Nine samples were casted according to Australian Standards and were left in the moisture and left in moisture room; and the remaining specimens were placed in sodium hydroxide (NaOH) mixture and after curing, specimens were left in the oven at high temperature (80oC); as previous researches and testings have shown moisture is an important factor to activate the alkali-silica gel, which will then expand and consequently will cause cracks on concrete structure, these cracks were measured, and then compression and indirect tensile tests were conducted. Compression strength was reduced by 2.54% at from 28 days to 80 days after casting for cylinder standard size and smaller cylinders had presented ii reduction 47.3% at 114 days. Indirect tensile strengths percentage reductions were measured at 28, 49 and 77 days: 2.28%, 7.79% and 12.09% respectively. Bending test had percentage reduction of 41.45% at 132 days. The crack will be analysed according to width and related to the stiffness of the structure and from tensile strength results the percentage of tensile lost will be recorded and associated to moisture and crack width. Overall results will be related to field structure. Those results will give a guide about the tensile capacity of their structures. Suggestions of methodology to prevent and/or mitigate ASR reaction on existing and new structures, focusing on existing structures mitigation and diagnosis