Concrete Recycling

Concrete recycling can be regulated by state environmental authorities. When structures made of concrete are to be demolished, concrete recycling is a common method of disposing of the rubble. In the past, this rubble was disposed of in landfills, but with more attention being paid to environmental concerns, concrete recycling allows reuse of the rubble while also keeping construction costs down.

Concrete Recycling

Concrete debris was once routinely shipped to landfills for disposal, but recycling has a number of benefits that have made it a more attractive option in this age of greater environmental awareness, more environmental laws, and the desire to keep construction costs down. Concrete aggregate collected from demolition sites is put through a crushing machine, often along with asphalt, bricks, dirt, and rocks. To date, recycled concrete aggregate has been use in/as aggregate base course (road base), ready mix concrete, asphalt pavement, soil stabilization, pipe bedding and landscape materials. Recycling is consistent with the American Concrete Pavement Association (ACPA) econocrete concept, which encourages the use of recycled concrete for subbases, composite pavements, and full-depth pavements. The Federal Highway Administration has endorsed this program and at the present time has a national evaluation program on the recycling of old concrete pavements. Recycling of old concrete is feasible and should be considered whenever good aggregates are not available locally or when aggregate costs are excessive or where the costs of removing and wasting the old pavement are in excess of what the cost would be for recycling that material for new aggregate. 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. The quality of concrete with recycled concrete aggregates is very dependent on the quality of the recycled material used. 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. 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. Because concrete is an assembly, its recycled content should be calculated as a percentage of recycled material on a mass basis. A specialized machine train breaks the existing slab, removes reinforcing steel, grades the road base, crushes and sizes the concrete, and stockpiles the new base material onto the prepared subgrade, ready for compaction.

Recycling Concrete

Recycling concrete provides environmental benefits, as recycling concrete saves landfill space and using recycled concrete as aggregate reduces the need for gravel mining. Recycling is consistent with the American Concrete Pavement Association (ACPA) econocrete concept, which encourages the use of recycled concrete for subbases, composite pavements, and full-depth pavements. Recycling of old concrete is feasible and should be considered whenever good aggregates are not available locally or when aggregate costs are excessive or where the costs of removing and wasting the old pavement are in excess of what the cost would be for recycling that material for new aggregate. There are a variety of benefits in recycling concrete rather than dumping it or burying it in a landfill. Concrete recycling is becoming an increasingly popular way to utilize aggregate left behind when structures or roadways are demolished.

Strength of Recycled Concrete

Among other qualities, mechanical strength, low moisture permeability, and chemical and volumetric stability are necessary. Because the cement requires time to fully hydrate before it acquires strength and hardness, concrete must be cured once it has been placed. Allowing the concrete to freeze in cold climates before the curing is complete will interrupt the hydration process, reducing the concrete strength and leading to scaling and other damage or failure. High early-strength concrete is designed to hydrate faster, often by increased use of cement, which increases shrinkage and cracking. Replacing higher amounts will results in increased drying shrinkage, while strength and freeze-thaw resistance are not significantly affected. Recycled concrete can also be used as aggregates in new concrete pavement if tests of new concrete made with crushed-concrete aggregate indicate acceptable strength and durability. Sub-base gravel is used in the bottom layer of a road, designed to give the road its strength. The ultimate strength of concrete is influenced by the water-cement ratio (w/c) [water-cementitious materials ratio (w/cm)], the design constituents, and the mixing, placement and curing methods employed.

Construction with Recycled Concrete

Construction materials are increasingly judged by their ecological characteristics. Concrete is a construction material that consists of cement (commonly Portland cement), aggregate (generally gravel and sand), water and admixtures. Concrete is a relatively heavy construction material and is frequently recycled into aggregate for road bases or construction fill. Where highway reconstruction calls for the use of recycled concrete to be crushed and placed as subbase, current practice requires demolition of existing concrete, hauling the broken concrete offset for crushing, and again hauling the crushed material back to the construction site for placement. Of the various materials used in highway construction, aggregates constitute one of the major elements of cost-between 21 and 30 percent of the cost of all materials and supplies and between 10 and 14 percent of the total construction cost (excluding the cost of right-of-way and engineering). Using recycled material as gravel reduces the need for gravel miningUsing recycled concrete as the base material for roadways reduces the pollution involved in trucking material between the construction site and a remote quarry. Over 55,000 miles of freeways and highways in America are made of this material.

Concrete recycling is becoming an increasingly popular way to utilize aggregate left behind when structures or roadways are demolished. Concrete recycling is becoming an increasingly popular way to utilize aggregate left behind when structures or roadways are demolished.