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Suppose I need to replace old concrete on a driveway or in any similar scenario.

The usual way is to just gather all the old concrete and dispose it to a landfill, then buy cement, sand and crushed stone and prepare new concrete (or just order already mixed unset concrete). This is not very convenient - disposing concrete can cost a fortune and also buying and moving in new crushed stone is not free and one might also be concerned about environmental impact of those operations.

It would be good to try to reuse old concrete as crushed stone. The number one problem is that old concrete fragments from a driveway have lots of dirt on them - mostly all kinds of ground and also whatever could get attached to them while the driveway was in use like oil, paint, other contaminants. Obviously using dirty stones will lower concrete quality singificantly.

Is there a simple yet reliable way to clean all those concrete fragments so that they can be reused in new concrete?

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up vote 9 down vote accepted

Recycled concrete is used for several different purposes. It can be used "as is" for bulk fill / drainage / etc.

When cleaned, crushed, and graded for size, it can be used as a base underneath a new concrete pour.

Recycled concrete can also be used as aggregate within the new concrete. Several factors must be taken into account to ensure that it will not weaken the integrity of the new concrete (moisture retention, etc). Typically, up to 30% of the aggregate can be replaced with properly processed recycled concrete.

However, concrete is generally recycled in large scale plants where they can reliably clean, crush, sort, and distribute the incoming material. It is doubtful that you could do this economically in the small scale.

More recycled concrete info

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Concrete is essentially a mixture of portland cement, sand and aggregate. The strength of the concrete does not come from the cement, but from the aggregates. You want the smallest amount of cement possible -- just enough to glue the aggregate and sand together (i.e. fill the gaps).

By reusing old concrete as an aggregate, (and adding more cement to the mix) you are increasing the total cement in the concrete, and weakening your finished product.

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