Monday, May 31, 2010

Fines, Aggregate and Concrete

Concrete is composed of different sized particles. From large aggregate (rocks) to small aggregate (sand) to paste (cement). It is important that these materials be mixed in proper proportions. If there are only large stones, there will be gaps between the stones. If there is only sand, it will not be as strong. If there is only cement paste, the material would be subject to stresses and cracking on curing (cement is also more expensive).

An ideal concrete mix is mostly large aggregate, enough small aggregate to fill the gaps between the large aggregate, and enough cement paste to bind the aggregate together. A naturally occurring mixture of rock and sand -like one might dig from a creek- is usually pretty close to the ideal mix, which is pretty fortunate. Evaluation of particle size distribution is commonly done through a sieve analysis.

Large aggregate is not usually round, or spherical. Typical aggregate (crushed stone) has a longer dimension and a shorter dimension. This results in a “tip” and a “face” on an average piece of aggregate. One of the keys to high strength concrete is tip-to-face contact between aggregate.

On the scale of small aggregate, very small aggregate is referred to as “fines” (like dust). Fines are problematic to the concrete industry. The reason is that too much fines in a mix (concrete recipe) will prevent the tip-to-face intimate consolidated structure between chunks of large aggregate which is desirable for high-strength concrete. Fines “over-stuff” the interstitial sites between aggregate and prevent tip-to-face contact between large aggregate.

Concrete manufacturers go to great lengths to reduce the amount of fines in their design mixes. The result is that there is a surplus of fines produced by concrete manufacturers. Fines are considered a waste product by the concrete/aggregate industry. The composition of fines varies depending on the composition of the parent rock, so there is a broad spectrum of composition of fines across the industry, depending on where the rock is mined.

One company has realized the value of fines produced on a national basis. This company is “Scotts,” the soil and fertilizer company. They catalog, blend, mix and sell fines from across the country. They fairly dominate this market.  They have an extensive, detailed, international, comprehensive catalog of fines.

Great Scott!  Does anyone have a better use for fines? There’s a big resource out there, and it appears to be underutilized.

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