As the demand for aggregate in construction increases, recycling concrete waste as aggregate has been recognized as an environmentally sustainable solution with potential cost savings. It would allow recycling of construction and demolition waste and reduce some of the need to quarry and haul virgin aggregate. However, the use of recycled concrete aggregate (RCA) has been limited by concerns over its quality and performance, and by lack of uniform specifications as to what kind and how much of the aggregate to use in pavement construction. Studies and on-the-ground experience have shown that high levels of RCA reduce the durability of pavements. The effects of lower replacement levels have not been well documented, however.

In a recently completed study funded by the National Road Research Alliance, researchers investigated the effects of incorporating RCA at low replacement levels on the properties of concrete. Four different RCA sources with different aggregate properties were used. For each source, replacement levels of 5, 10, and 15% by volume were tested and compared to a control group with no RCA. Of the four RCA sources investigated, three had similar levels of absorption capacity and percent fines, while one source had higher levels of both properties. RCA replaced virgin aggregate of a similar gradation.

Fresh and hardened concrete properties were tested, including air content, super air meter (SAM) number, slump, workability via the box test, compressive strength, flexural strength, elastic modulus, Poisson’s ratio, coefficient of thermal expansion, surface resistivity, freeze-thaw durability, and unrestrained shrinkage. Digital image correlation was used to visualize strain fields during compression testing. Statistical analysis of the results showed that using up to 15% of an RCA with reasonable values of absorption capacity and percent fines would not negatively impact most concrete properties.

Before RCA can be included in concrete mixes, there will need to be a specification with property limits that define if a specific RCA is allowable.

This project can serve a roadmap for future research. The researchers propose the following questions that need to be answered before RCA can be widely accepted.

• What RCA properties can be used as predictors of the effect using RCA will have on concrete properties?
• Will pavements that include RCA stand up to winter road salt?
• Does this pavement require more time to harden before salt is applied?
• Will the natural variation within a pile of RCA create issues with uniform performance?
• What is the best way to estimate freeze-thaw durability?

More Information

• Research project webpage
• Final report