Aggregates

Concrete consists of about 60 to 75% of rocks hence a strong contributor to its properties. Physically, the rocks must be strong and durable. In the environment that the rocks reside in the hardened concrete it could be susceptible to chemical attack and thus must be tested to avert this situation. A chemical attack often result in crack formation and degradation of concrete.

Basic test for the physical is: i) The Los Angeles Abrasion test (ASTM C131) and ii) a test to determine chemically unstable minerals in the rock (ASTM C295)

The Los Angeles (L.A.) abrasion test Is a common test method used to indicate aggregate toughness and abrasion characteristics. Aggregate abrasion characteristics are important because the constituent aggregate on some projects i.e. pavements for highways and runways must resist crushing, degradation, and disintegration to retain a durable concrete.

Weathered rocks – rocks with inherent cracks in them – tend to chip off sections of the rock during mixing entrapping voids in the concrete to the detriment of the resulting concrete. Petrographic analysis is used effectively to identify the physical characteristics and furthermore, any potential deleterious minerals in the rock.