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Conventional concrete aggregate consists of sand (fine aggregate) and various sizes and shapes of gravel or stones. However, there is a growing interest in substituting alternative aggregate materials, largely as a potential use for recycled materials. While there is significant research on many different materials for aggregate substitutes (such as granulated coal ash, blast furnace slag or various solid wastes including fiberglass waste materials, granulated plastics, paper and wood products / wastes, sintered sludge pellets and others), the only two that have been significantly applied are glass cullet and crushed recycled concrete itself.
Even though aggregate typically accounts for 70% to 80% of the concrete volume, it is commonly thought of as inert filler having little effect on the finished concrete properties. However, research has shown that aggregate in fact plays a substantial role in determining workability, strength, dimensional stability, and durability of the concrete. Also, aggregates can have a significant effect on the cost of the concrete mixture.
Certain aggregate parameters are known to be important for engineered-use concrete: hardness, strength, and durability. The aggregate must be "clean," without absorbed chemicals, clay coatings, and other fine materials in concentrations that could alter the hydration and bond of the cement paste.
It is important to note the difference between aggregate and cement, because some materials have found use both as a cementitious material and as aggregate (such as certain blast furnace slags). Materials that have been researched or applied only as cement substitutes are addressed in another Technology Inventory article - Cement Substitutes.
Aggregate composed of recycled concrete generally has a lower specific gravity and a higher absorption than conventional gravel aggregate. New concrete made with recycled concrete aggregate typically has good workability, durability and resistance to saturated freeze-thaw action. The compressive strength varies with the compressive strength of the original concrete and the water-cement ratio of the new concrete. It has been found that concrete made with recycled concrete aggregate has at least two-thirds the compressive strength and modulus of elasticity of natural aggregate concrete.
Field-testing has shown that crushed and screened waste glass may be used as a sand substitute in concrete. Nearly all waste glass can be used in concrete applications, including glass that is unsuitable for uses such as glass bottle recycling. Some of the specific glass waste materials that have found use as fine aggregate are "non-recyclable" clear window glass and fluorescent bulbs with very small amounts of contaminants. Possible applications for such waste-glass concrete are bike paths, footpaths, gutters and similar non-structural work.
Lack of widespread reliable data on aggregate substitutes can hinder its use. To design consistent, durable recycled aggregate concrete, more testing is required to account for variations in the aggregate properties. Also, recycled aggregate generally has a higher absorption and a lower specific gravity than conventional aggregate.
Research has revealed that the 7-day and 28-day compressive strengths of recycled aggregate concrete are generally lower than values for conventional concrete. Moreover, recycled aggregates may be contaminated with residual quantities of sulfate from contact with sulfate rich soil and chloride ions from marine exposure.
Glass aggregate in concrete can be problematic due to the alkali silica reaction between the cement paste and the glass aggregate, which over time can lead to weakened concrete and decreased long-term durability. Research has been done on types of glass and other additives to stop or decrease the alkali silica reaction and thereby maintain finished concrete strength. However, further research is still needed before glass cullet can be used in structural concrete applications.
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Certain substitute aggregates, such as glass, can help reduce the cost of concrete, especially if aggregate is in short supply.
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Use of any recycled material helps to keep that material out of landfills. Recycling practices also can decrease the environmental impact of obtaining / manufacturing the material from virgin resources.
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Glass aggregate can improve the durability of concrete and can also make decorative finishing easier.
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Recycled concrete is widely available, though it is typically not geared toward use as a concrete aggregate substitute. Therefore prices may be high to obtain aggregate of a particular size range specification. Crushed glass aggregate is less widely available than recycled concrete, and is also not usually sold as aggregate.
Recycled concrete aggregate sells in the approximate range of $3.50 to $7 per cubic yard, depending on the specifications (size limitations) for the aggregate and local availability. This is about one half the cost of non-recycled aggregate used for construction purposes.
The allowable size of the glass aggregate largely determines its prices, which are in the vicinity of $15 to $20 per cubic yard. Smaller aggregate (e.g. "glass sand") is the more expensive.
Not Applicable
There are no standard regulations currently addressing the use of alternative concrete aggregate for engineered use / structural applications. Given a nonstructural application, codes and regulations generally do not deny use of an aggregate material that has no known deleterious effects on the finished concrete properties. Some state and local codes specifically address the use of alternative aggregate, for example the Washington State Department of Transportation. However, this should be verified on a project-by-project basis.
Not Applicable
The installation of alternative aggregate concrete is basically the same as for conventional concrete. The use of alternative aggregates addressed in this article (glass and recycled concrete) does not present a significant deviation from standard concrete mixing and application methods.
Not Applicable
Use of any recycled material helps to keep that material out of landfills. Recycling practices also can decrease the environmental impact of obtaining / manufacturing the material from virgin resources.
New concrete made from recycled concrete aggregate generally has the same properties as stone or gravel aggregate.
Glass aggregate typically acts as a crack arrestor, benefiting concrete durability, though this depends on the specific glass aggregate properties, the concrete and its end-use. Glass aggregate can allow a greater range of aesthetic/decorative options for concrete.
Some of the additional benefits of using glass as an aggregate material in concrete include:
- concrete unit cost decrease
- lowering of freight cost
- avoided landfill costs
- boosting or creating secondary markets around recycling and selling additional common types of glass
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