Crushed stone versus gravel in your concrete. What is the difference and why does it matter?
By John Hirschfeld, BSCE, ACI member
Concrete appears to be a simple material to most, but the quality and durability of concrete are heavily impacted by chemical reactions among some basic ingredients in concrete: aggregate (sand and stone or gravel), cement powder and water. The long term durability of your concrete can be significantly improved by including crushed stone rather than glacier-formed gravel . Due to chemical composition, crushed stone reacts with the cement in the concrete to form a strong, enduring chemical bond between the paste and the aggregates. Natural gravel, with its broad and varying range of mineral content, does not. Per scholars in the field of concrete the weakest link in concrete is the bond between the aggregate and the paste. To have the aggregate chemically bond with the paste is a definite advantage for the long term durability of the concrete.
While cracks naturally form during the curing of concrete, the amount of cracking in concrete is another key influencer of ultimate durability. With local crushed stone, the cracks are forced to go through the stronger aggregates due to the stronger chemical bond, and there are fewer cracks in general. The concrete is stronger, less permeable and more durable. With gravel the cracks go around the aggregate. The concrete is not as strong and it is more permeable. This has been observed in concrete remnants from concrete cylinder samples drawn during concrete pours for later strength testing. Inside the remnants, the bonding of crushed stone and cement are readily seen, as is the lack of bonding between gravel and cement.
During a concrete pour, concrete made with crushed stone, depending upon the gradation of the stone, may need more water to get the same flowability as does concrete made with gravel. Increased water in concrete typically decreases strength. Even with this extra water, because the stone/cement bond is better, concrete mixes with local crushed stone get 15 to 20% more strength versus gravel mixes using similar amounts of cement.
Over time, concrete with crushed stone enjoys significant “life-enhancement” benefits. Local crushed stone keeps the concrete more chemically balanced and thus more resistant to long term weathering attack. The biggest long term detriment to concrete is water leaching through microcracks and carrying with it cement paste, part of which is chemical solution. This process is the same process that forms stalagmites and stalagtites in caves from water slowly dissolving the limestone through which it flows and then depositing the dissolved stone once it reaches air.
In concrete, when local crushed stone is present, the water leaches equally on the cement paste and the aggregates since they are both based on the chemical calcium. When gravel is used (which typically is not all calcium based stones), the water present in the concrete only leaches at the paste, greatly increasing the rate at which the paste and ultimately the concrete deteriorates over time When this happens, the concrete literally, over tens of years, reverts back to sand and stone. At early stages of this type deterioration in structural concrete (bridges) they may fail causing harm both to people and the pocketbook.