When structures made of concrete are demolished or renovated, concrete recycling is an increasingly common method of utilizing the rubble. Concrete was once routinely trucked 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. Crushing facilities accept only uncontaminated concrete, which must be free of trash, wood, paper and other such materials. Metals such as rebar are accepted, since they can be removed with magnets and other sorting devices and melted down for recycling elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. After crushing has taken place, other particulates are filtered out through a variety of methods including hand-picking and water flotation.
Crushing at the actual construction site using portable crushers reduces construction costs and the pollution generated when compared with transporting material to and from a quarry. Large road-portable plants can crush concrete and asphalt rubble at 600 tons per hour or more. These systems normally consist of a rubble crusher, side discharge conveyor, screening plant, and a return conveyor from the screen to the crusher inlet for reprocessing oversize materials. Compact, self-contained mini-crushers are also available that can handle up to 150 tons per hour and fit into tighter areas. With the advent of crusher attachments - those connected to various construction equipment, such as excavators - the trend towards recycling on-site with smaller volumes of material is growing rapidly. These attachments encompass volumes of 100 tons/hour and less.
Uses of recycled concrete
Smaller pieces of concrete are used as gravel for new construction projects. Sub-base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt poured over it. The US Federal Highway Administration may use techniques such as these to build new highways from the materials of old highways. Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants. Also, concrete pavements can be broken in place and used as a base layer for an asphalt pavement through a process called rubblization.
With proper quality control at the crushing facility, well graded and aesthetically pleasing materials can be provided as a substitute for landscaping stone or mulch.
Wire gabions (cages), can be filled with crushed concrete and stacked together to provide economical retaining walls. Stacked gabions are also used to build privacy screen walls (in lieu of fencing).
Use of recycled coarse aggregate in concrete
Recent statistics show that the increasing demand of construction aggregate could reach 48.3 billion metric tons by the year 2015 with the highest consumption being in Asia and Pacific. The high demand of concrete means more new building will be constructed after the demolition of old buildings, generating a large volume of C&D waste (construction waste & demolition waste) as a by product of economic growth. However, the most common way to disposal this waste is by dumping it in a landfill. Without proper maintenance, landfills can cause many environmental problems such as air pollution and water contamination. This, along with the shortage of resources caused by this growth in construction, has caused more and more countries to begin considerign the importance of C&D waste recycling.
In general, the reuse and recycle of construction waste is concentrated in the preparation of recycled aggregate for concrete. By adding a portion of recycled aggregate instead of natural aggregate coarse into the mixture, producing the recycled concrete, which can conserve energy and materials for concrete production.
Strength and durability of the recycled aggregate
According to "The Effect of Recycled Coarse," the results of experiments showed that "the recycled aggregate absorption value is 7.5% which is higher than the WSDOT specification of aggregate in 3.7% maximum. This is because of the paste on the gravel which will influence the absorption of water. Therefore, the recycled aggregate satisfies the absorption specification. Also, different portions of recycled aggregate added to the mixture will have different degrees of influence on the specific gravity, characteristic strength, compressive strength, modulus elasticity and flexural strength. They found that as the percentage of recycled aggregate added increases, the specific gravity, characteristic strength, compressive strength, modulus elasticity and flexural strength decreases. They couldn't indicate that the recycled aggregate is not good for using just by comparing these data with the natural aggregate properties. If 65% of recycled aggregate or above is used, each physical properties will drop greatly. However, if the portion of recycled aggregate adds to concrete is limited, it won't affect each property much. Based on the research and experiment test, "the maximum replacement of recycled coarse aggregate that can be used in concrete is 35%". Some experiment showed that recycled aggregate doesn't have good durability like the natural coarse aggregate but the durability can be improved by mixing it with special materials such as flying ash to produce high strength and durable concrete.
There are a variety of benefits in recycling concrete rather than dumping it or burying it in a landfill.
- Save landfill space.
- Conserve natural resources by reducing the need for gravel mining, water, coal, oil and gas.
- When used as the base material for roadways, reduces pollution from waste transport to landfills and dumps.
- Create employment opportunities.
- Drags down material and waste transport expenses.
- Recycling one ton of cement could save 1,360 gallons water, 900 kg of CO2
Lead paint contamination
There have been concerns about the recycling of painted concrete due to possible lead content. The Army Corps of Engineers' Construction Engineering Research Laboratory (CERL) and others have conducted studies to see if lead-based paint in crushed concrete actually poses a hazard. It was concluded that concrete with lead-based paint would be able to be used as clean fill without impervious cover but with some type of soil cover.
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