Recycled Glass in Concrete

 Using Recycled Glass in Concrete

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.

Clean glass aggregate in concrete must be used to prevent problems 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.  This is why we see a gaining trend is glass used in Terrazzo Floors and Concrete Countertops as these are all ideal uses for Clean Glass and Recycled Glass Aggregate and will not effect the final function of the installation.

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