by Michael R. Brandes and Yahya C. Kurama
American Concrete Institute ACI Structural Journal May 2018
Newswise — Researchers in the Department of Civil and Environmental Engineering and Earth Sciences at the University of Notre Dame conducted a study funded by the National Science Foundation into the variability of recycled concrete aggregates (RCA) to prequalify their use in reinforced concrete structures. This testing shows that certain measurable properties of RCA, specifically water absorption and deleterious material content, such as wood and asphalt, can serve as indicators for concrete strength, stiffness, and other characteristics. Recycled concrete aggregate with high water absorption and high levels of deleterious materials can lead to lower concrete strengths and increased deformations under loading, while concrete using high-quality RCA with lower water absorption and little deleterious materials perform similarly to concrete using natural aggregates.
“We realize that the variability in RCA sources is a major hurdle for the concrete industry to overcome, but our confidence in high-quality RCA sources made us focus on efficient and economical ways to implement RCA into concrete production,” said Michael Brandes, a design engineer with Schaefer in Cincinnati, OH, and former researcher at the University of Notre Dame.
The infrastructure around us, from one lane highways to skyscrapers, has relied on concrete, a building material that is often taken for granted. In the U.S., concrete infrastructure constructed from the early to mid-1900s is rapidly approaching the end of its design lifecycle. As these structures begin to be replaced, the construction industry will again rely heavily on concrete and the materials needed for concrete production. Natural coarse aggregates such as gravel and crushed limestone are necessary for concrete production, but the widespread mining of these materials negatively affects the environment. Coupled with this increased demand for concrete and its raw materials, the replacement of existing concrete structures in the years to come will generate vast amounts of demolished concrete. Crushing and recycling of this demolished concrete as replacement for natural aggregates in new construction can potentially reduce the environmental impacts of both natural aggregate mining and demolition waste generation.
Specifically, precast concrete plants cast many structural and nonstructural concrete members daily and those that do not fully meet the required production quality specifications (such as member dimensions, color, and finish) are discarded (Fig. 1). Other sources of discarded concrete at a precast plant include over-batching or rejected batches of concrete, as well as waste from the unwanted ends (for example, extremes) of hollow-core. Recycling of all or most of this rejected concrete as RCA in new precast members can provide unique opportunities involving high-quality materials with consistent and well-known properties (Fig. 2).
“The precast industry is an ideal candidate for the use of RCA,” Brandes said, “and the prestressed concrete application provided a unique opportunity to test the performance of RCA under rigorous loading conditions.” Eighteen prestressed concrete beams with varying parameters, such as RCA source, RCA amount, and prestress level, were loaded for up to six months and subsequently tested to failure (Fig. 3). The use of RCA had little effect on the strength of the beams. However, under high aggregate replacement levels at 50 and 100 percent by volume, RCA resulted in increased beam deflections.
Actual aggregate replacement levels at precast plants will likely be considerably smaller than 50 percent by volume because of the limited amount of discarded concrete as compared with the large volumes of new precast concrete produced (Fig. 4 and Fig. 5). Thus, according to Yahya Kurama, a professor of civil engineering at the University of Notre Dame and principal investigator of the research, “Regular use of RCA at low replacement levels consistent with the volume of discarded concrete at a precast plant can result in the elimination of concrete waste from these plants, with minimal or no effects on the short-term and long-term behavior of precast-prestressed concrete structures.”
“As a result of this research, the production facilities in the precast concrete industry will be in a much stronger position to use recycled concrete aggregates” said Greg Kerkstra, President/CEO of Kerkstra Precast, Inc., where the eighteen prestressed concrete beams tested by the researchers were produced. Collaboration between the precast concrete industry and academia is key for the ultimate practical adoption of the research results. Overall, the research demonstrated that high-quality RCA produced from precast plant operations is a feasible substitute for natural coarse aggregates in prestressed concrete construction. According to Tim Skiba, Quality Assurance and Product Development Manager at Kerkstra Precast, Inc., “The ability to use discarded concrete as aggregate in new precast production can not only result in environmental benefits but also economic benefits, so this would really be a win-win situation.”
Details of this research can be found in a paper titled “Service-Load Behavior of Precast/Prestressed Concrete Beams with Recycled Concrete Aggregates,” accepted and soon to be published by the ACI Structural Journal.
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