Every year, about 5% of the estimated 455 million cubic yards of ready mixed concrete produced in the U.S. (est. 2006) is returned to the concrete plant. The returned concrete in the truck can be handled in several different ways.
A common approach is to discharge the returned concrete at a location in the concrete plant for processing. The hardened discharged concrete can be subsequently crushed by a crusher and the coarser material can be reused as base for pavements or fill for other construction. However, it is not easy to utilize the material finer than 2 inches.
A research project was undertaken by the NRMCA Research Laboratory to study the use of crushed returned concrete, or crushed concrete aggregate (CCA), as a portion of the aggregate component in new concrete. The Ready Mixed Concrete Research and Education Foundation funded the project.
Demolishing old concrete structures, crushing the concrete, and using the crushed materials as aggregate is not new. This material is generally referred to as recycled concrete aggregates (RCA).
But RCA is different from CCA, as construction debris tends to have a high level of contamination (rebar, oils, deicing salts, and other building components). CCA, on the other hand, is prepared from concrete that has never been in service and thus likely to contain much lower levels of contamination.
The main objective of the research project was to develop technical data that will support using CCA from returned concrete and to provide guidance on a methodology for appropriate use of the material. Using CCA can help the ready-mixed concrete industry save an estimated $300 million a year in operating costs. It also will reduce landfill space by as much as 845 football fields 10 feet high every year. Using CCA could also help attain points under systems like LEED for certifying building projects for sustainable construction.
Guidance for the producer
A cost analysis was conducted to evaluate the economics of using CCA. Based on conservative cost assumptions and the measured 28-day strengths, the cost savings of the different CCA mixtures that would yield the same 28-day strength as the control mixture was calculated. Cost calculations suggest that the concrete producer saves considerably by using CCA from reduced use of virgin materials and reduced disposal costs. Assuming no specification restrictions, the producer can use CCA in the following incremental steps.
In the first step, the producer should limit using CCA to no more than 300 pounds per yard (about 10% by weight of total aggregate) in “as received” condition. Expect negligible change in concrete performance.
The second step is for the producer to separate CCA into different strength classes by diverting returned concrete to different areas at the plant. The study found that if CCA was made from returned concrete with a specified strength of 3000 psi or higher, then it could be used at a level of 900 pounds per yard.
The third and final step will be for the producer to separate CCA into different strength classes and additionally separate the CCA into coarse and fine fractions. The study found that if CCA was made from returned concrete with a specified strength of 3000 psi or higher and if the producer separated just the coarse fraction of the CCA, the producer could replace 100% of the virgin coarse aggregate, which corresponds to about 1600 pounds per yard of CCA.