While portland cement concrete is one of the most versatile building materials on earth and has facilitated industrial growth in the last century, it is also one of the biggest in terms of environmental impact. Between the production of cement, mining of natural aggregates, use of increasingly scarce fresh water resources, and development of new chemical admixtures, the concrete industry is consistently one of the most environmentally demanding industries. Much of the research to date and the current state-of-practice pertaining to resource productivity in structural concrete is limited to cement conservation through the partial replacement of cement with industrial by-products (e.g., fly ash, ground granulated blast furnace slag, and silica fume). In comparison, aggregate conservation has been mostly limited to using recycled materials in non-structural applications such as sidewalks, bulk fills, erosion control, and roadway sub base even though the quality of the recycled material is often significantly higher than is required in these applications. This relates to the proposed work, which focuses on the increased use of recycled concrete aggregate (RCA) in civil infrastructure projects in the U.S., and more specifically the use of RCA in structural reinforced concrete building applications.
Natural aggregate is essential to the needs of modern society, providing material for the construction and maintenance of buildings, roadways, dams, and other parts of the infrastructure. The volume of crushed stone, sand, and gravel produced in the U.S. accounts for more than half of all mining operations and more than twice the volume of coal produced. With concrete containing about 80% aggregate by mass, construction projects utilize vast amounts of aggregates (about 3 billion tons/year in the U.S. alone). Furthermore, the production of these materials is expected to rise significantly in the near future. For example, it is estimated that the amount of stone produced in the next 20 years will equal approximately the total amount of stone produced all of the last century (approximately 36.5 billion tons) (Stanczak 2007). The mining, processing, and transport operations involving such large quantities of aggregate consume large amounts of energy and adversely affect the ecology of forested areas and riverbeds. By using recycled concrete from our existing infrastructure as a partial or full replacement to virgin coarse aggregates in new construction, it would be possible to substantially reduce the demand for new aggregates. Furthermore, virgin aggregate deposits from permitted quarries have already been depleted in many urban regions of the U.S., thus requiring aggregates to be transported over long distances. This can prove to be prohibitively expensive considering that transportation costs of local RCA sources would be a fraction of the hauling costs for natural aggregate depending upon the quarry location (Stanczak 2007; Mehta 2001). In some cases, the demolished structure may be at or near the site of the new construction, thus, allowing better quality control of the RCA and further reducing transportation costs.
About two-thirds of the construction and demolition waste in the U.S. consists of old concrete rubble (Mehta 2001). Especially in the years to come, the renovation and replacement of our nation's aging infrastructure will result in both an increase in the supply of recycled concrete and the demand for new concrete. Already in Detroit, MI for example, many recycling plants that would otherwise accept “clean” concrete material for no cost now have such large supplies of this material that they are having to charge an unloading fee. It would be in the industry’s best interest if there were alternative ways to utilize this material other than current practice.
Undeniably, recycling old concrete into quality material suitable for structural applications has significant costs, but the process has become cost effective. Financial incentives provide the needed impetus to deliver quality RCA since the value of the material is more than double when used in structural applications ($15-$20 per ton) as compared with non-structural applications ($6-$7 per ton). RCA not only offers a way to reduce landfill waste, they are also lighter weight, resulting in reduced haul costs and overall project costs. RCA may also help earn points toward Leadership in Energy and Environmental Design (LEED) credits, both for incorporating recycled materials and for construction waste management, thus, further increasing the incentives for their use.
Despite the benefits, only a small amount of RCA has been used in structural engineering projects in the U.S. The primary obstacles against their increased utilization are: