Recycled Aggregates

Construction materials are increasingly judged by their ecological characteristics. Concrete recycling gains importance because it protects natural resources and eliminates the need for disposal by using the readily available concrete as an aggregate source for new concrete or pavement subbase layers. According to a FHWA study, 38 states recycle concrete as an aggregate base; 11 recycle it into new portland cement concrete. The states that do use recycled concrete aggregate in new concrete report that concrete with recycled concrete aggregate performs equal to concrete with natural aggregates. Most agencies specify using the material directly in the project that is being reconstructed. For a summary of the findings, click here.

Recycling of concrete is a relatively simple process. It involves breaking, removing, and crushing existing concrete into a material with a specified size and quality. See ACI 555 (2001) for more information on processing old concrete into recycled concrete aggregates. The quality of concrete with recycled concrete aggregates is very dependent on the quality of the recycled material used. Reinforcing steel and other embedded items, if any, must be removed, and care must be taken to prevent contamination by other materials, such as: asphalt, soil and clay balls, chlorides, glass, gypsum board, sealants, paper, plaster, wood, and roofing materials which can be troublesome.

Applications

In general, applications without any processing include:

1. many types of general bulk fills


2. bank protection


3. base or fill for drainage structures


4. road construction


5. noise barriers and embankments

After removal of contaminants through selective demolition, screening, and /or air separation and size reduction in a crusher to aggregate sizes, crushed concrete can be used as:

1. new concrete for pavements, shoulders, median barriers, sidewalks, curbs and gutters, and bridge foundations


2. structural grade concrete


3. soil-cement pavement bases


4. lean-concrete or econo-crete bases
and

5. bituminous concrete

Recycled Aggregate Chararacteristics

The crushing characteristics of hardened concrete are similar to those of natural rock and are not significantly affected by the grade or quality of the original concrete. Recycled aggregates produced from all but the poorest quality original concrete can be expected to pass the same tests required of conventional aggregates.

Recycled concrete can be batched, mixed, transported, placed, and compacted in the same manner as conventional concrete. Special care is necessary when using recycled fine aggregate. Only up to 10% to 20% recycled fine aggregate is beneficial. The aggregate should be tested at several substitution rates to determine the optimal rate.

Higher porosity of recycled aggregate compared to natural aggregate leads to a higher absorption. Click here for information on differences in the behavior and properties of recycled-aggregate concrete compared to concrete made with natural aggregates. It is recommended that recycled aggregates be batched in a prewetted and close to a saturated surface dry condition. To achieve the same workability, slump, and water-cement ratio as in conventional concrete, the paste content, or amount of water reducer have to be increased.

Mix Design

It is generally accepted that when natural sand is used, up to 30% of natural crushed coarse aggregate can be replaced with coarse recycled aggregate without significantly affecting any of the mechanical properties of the concrete. Replacing higher amounts will results in increased drying shrinkage, while strength and freeze-thaw resistance are not significantly affected. For more information, click here.

Often recycled aggregate is combined with virgin aggregate when used in new concrete. An example of a mix design using recycled aggregates in a pavement application is shown following table.

Sustainability

The LEED^® Green Building Rating System recognizes recycled concrete in its point system. Credit 4 (Materials and Resources) states, “specify a minimum of 25% of building materials that contain in aggregate a minimum weighted average of 20% post-consumer recycled content material, OR, a minimum weighted average of 40% post-industrial recycled content material.” Using recycled aggregates instead of extracted aggregates would qualify as post-consumer. Because concrete is an assembly, its recycled content should be calculated as a percentage of recycled material on a mass basis.

Credit can also be obtained for Construction Waste Management. It is awarded based on diverting at least 50% by mass of construction, demolition, and land clearing waste from landfill disposal. Concrete is a relatively heavy construction material and is frequently recycled into aggregate for road bases or construction fill. See Building Green with Concrete: Points for Concrete in LEED^® for more information on the LEED^® rating system.

Reference:
ACI Committee 555, Removal and Reuse of Hardened Concrete, ACI 555R-01, ACI Committee 555 Report, American Concrete Institute, Farmington Hills, Michigan, 2001, 26 pages.

 
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