Crushed Concrete Aggregate

Crushed concrete aggregate (CCA) is made from unused, returned concrete, and old demolished concrete that is clean enough to be used for new concrete mixes. Crushing old concrete to make aggregate for new concrete is growing in popularity for many reasons the with first being able to create a more sustainable or "green" concrete.

Recycled concrete aggregate (RCA) is concrete and other materials like pavement that is crushed into aggregate and good enough to be used as fill material and compactable fill. As landfills across the country begin to fill up and refuse broken concrete, the need to recycle concrete into a usable material became necessary.

As building with green concrete grows in popularity the need for "crushed" concrete aggregate will grow also. Owners of new building projects will want their projects to be LEED certified and using recycled concrete will add to the credits needed to qualify for this.

Characteristics of CCA concrete:

  1. CCA is more porous than regular aggregate.
  2. CCA is about 10 percent lighter than regular stone aggregate due to its porosity and the lower density cement paste.
  3. CCA combined with the fine dust from crushing has a higher demand for water and cement, which may result in greater shrinkage and lower strength.
  4. CCA contains unhydrated particles of portland cement that increase the bond to cement paste.

Companies that produce both crushed concrete aggregate and recycled concrete aggregate store the materials in separate piles. The CCA is made from both ready mix truck returned concrete and demolition concrete. The concrete is crushed and screened to produce the proper size aggregate.

The fines or cement dust produced by the crushing process also pass through the screen and is mixed with the aggregate. There have been test done on trial batches that show the dust created in the crushing process didn't coat the aggregate or inhibit the bond between the cement paste and the aggregate, it actually strengthened it.

Some facts about Crushed Concrete Aggregate:

  1. Since the aggregate is lighter than stone aggregate, it is easier to place and level off.
  2. Finishing the surface with bullfloats and steel trowels is the same as regular concrete.
  3. CCA produces a more "creamy mix" making it very finisher freindly.
  4. CCA has a slightly faster set time than regular aggregate concrete.
  5. CCA concrete properly placed, finished, and cured has the same shrinkage properties as regular concrete mixes.

A couple concerns of concrete companies crushing recycled concrete or purchasing crushed recycled concrete aggregate to be used in their concrete mixes are the impurities such as paints, sealers, and chlorides demolished concrete may contain and the fact demolished concrete may come from many different sources.

Concrete companies like to have strict control where the aggregates come from when producing high quality concrete mixes, especially where structural concrete is needed, to guarantee their customers their getting the best concrete possible. For this reason crushed concrete aggregate may not be as predictable as aggregate from a single source of virgin aggregate.

Concerns for the environment has a major effect on how concrete companies do business. CCA has helped address the environmental issues and at the same time produced a more sustainable concrete product. CCA increases the life cycle of concrete and makes it a continuously recycled material for future projects as the need for a more "green" concrete grows in demand.

Check out this GREEN CONCRETE mix design.


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