The purpose of this review was to capture the most advanced uses of recycled concrete aggregate (RCA) for transportation uses in the United States. This knowledge would then be transferred to all State Transportation Agencies (STA) in the United States through the issuance of this report. The report summarizes the information collected during the review of practices in five states, Texas, Virginia, Michigan, Minnesota and California. These states were selected based on their level of use and supply generated of RCA as an aggregate as well as to obtain a cross-section of the country. This report identifies the applications where the use of RCA can have engineering, economic, and environmental advantages; the barriers related to these RCA applications; and the best practices that allowed State Transportation Agencies, recycled concrete producers and contractors to overcome these barriers. The report is intended to provide the State Transportation Agencies with recommendations, guidelines and specifications for furthering the use of RCA more widely throughout the country.
The overall findings of the review team was that RCA is a valuable resource, and by proper engineering it can be used for PCC pavement, aggregate base, miscellaneous. The material is too valuable to be wasted, and landfill. Some of the best aggregates used for highway, bridge, and building construction are already in use in our highways and bridges, effective recycling is a means to re-use these materials.
The construction of highways, bridges and buildings has been increasing from the beginning of the past century, especially in areas of high population density. These facilities need to be repaired or replaced with the passing of time because their end of service life is reached or the original design no longer satisfies the needs due to the growth in population or traffic. These facts have generated two important issues. First, a growing demand for construction aggregates and, second, an increase in the amount of construction waste. Two billion tons of aggregate are produced each year in the United States. Production is expected to increase to more than 2.5 billion tons per year by the year 2020. This has raised concerns about the availability of natural aggregates and where we will find new aggregate sources.
On the other hand, the construction waste produced from building demolition alone is estimated to be 123 million tons per year. Historically, the most common method of managing this material has been through disposal in landfills. As cost, environmental regulations and land use policies for landfills become more restrictive, the need to seek alternative uses of the waste material increases. This situation has led state agencies and the aggregate industry to begin recycling concrete debris as an alternative aggregate. Commercial construction industry has been leading the reuse of this debris, but with the State Transportation Agencies (STA) recognizing the engineering , economical and environmental benefits that can be achieved for using RCA, use for highway work is on the increase.
The production of Recycled Concrete Aggregate (RCA) is similar to the production of virgin aggregates. One of the primary differences occurs in the elimination of contaminants. Light materials such as wood, joint sealants and plastics can be removed with air knives. This device removes the light contaminants with a blast of air. Reinforcing steel and dowel bars are removed with electro-magnets. Many plants are now incorporating two magnets. The first one will be located after the primary crusher, generally a jaw type of crusher. This magnet removes the large pieces of steel, such as dowel baskets. The second magnet is located after the secondary crusher, which is either a cone type crusher or jaw type. This second magnet is used to remove the small pieces of steel that became free during secondary crushing. The steel removed in this manner will also be recycled for scrap metal.
The inclusion of these additional pieces of equipment could increase the cost of crushing concrete when compared to virgin aggregates. However, factoring in the additional cost for virgin aggregate to be blasted or mined from the quarry, excavated by equipment and loaded onto either trucks or into a hopper belt to be hauled or moved to the crushing area, the costs for additional equipment become more comparable. The concrete crushing operation often charges a tipping fee for dumping on site the raw material to be crushed into RCA. This may actually make the cost difference to be a negative, where RCA is cheaper to produce. The business plans of RCA manufacturers were not part of this review, but items gleaned during the review indicate that the crushing operations are in business to make money and the areas with well developed markets are realizing profits. An additional cost savings in transportation cost is often realized since recycled aggregate is being produced in metropolitan areas, whereas quarries are generally found further and further from the core metropolitan area.
Another advancement in the use of Recycled Concrete Aggregate is the advent of the mobile crusher. Crusher units are being moved to stockpile sites in Virginia and Michigan whenever enough material is present to justify the crusher set-up. Mobile in-place crushing equipment is also coming into greater use. Mobile units have been used on the paving grade in Iowa crushing the old pavement in-place into base material. This technology is being applied on the Michigan US-41 project in the Upper Peninsula. This type of crushing operation essentially removes all of the transportation costs replacing it with only the mobilization of the crusher unit. Currently there is an NCHRP-IDEA project, concrete road recycler – hammer-anvil test rig Project 79, targeting development of in-place recycling machine.
Recycling is also entering the realm of fresh concrete that is returned to the originating concrete plant, be it for over supply or rejection. Why wait for the concrete to harden before recycling? This question is being answered in California with reclaiming and recycling units. These units are reclaiming aggregates by washing the aggregate and allowing its reuse. This technology is being furthered by the reuse of the washed paste. The paste is being dosed with admixtures to retard the set and activators upon remixing to restart the hydration process. This allows the reuse of the paste into new mixtures removing the cement from the waste stream. This has brought up new questions concerning the value of this reused paste. Current practice is to disregard it as added cement, requiring new cement at the proper content to be added to the mix. This technology has been utilized in the European market and is just starting to see practice here in the United States. This process reduces the use of fresh water by up to 40%. An article in “Concrete Products” pointed out the economical and environmental benefits of using reclamation systems that not only recycle the sand and gravel from returned concrete, but also the cementitious paste and water. The system’s operational merits are potentially applicable to all batch plants that need a cost-effective method to recycle returned concrete.
“Reclaim systems have been around for about 20 years, but I felt the technology only recently met the needs of smaller batch plant operations,” notes Harbor Ready Mix General Manager Bob Mann. The producer previously disposed of returned concrete at a nearby concrete recycling center that converted it into road base aggregate. Disposing of the returned mix, ranging from five yards up to thirty yards a day, presented a significant cost of doing business, Mann affirms. Reducing the ongoing expense was a primary incentive for having a recycling system at his plant. “We expect to save most of the $30,000 to $35,000 a year that we previously paid out to dispose of our returned concrete,” Mann says. The new system enables them to recycle virtually all of the returned mix and the grey water accumulated from flushing the mixers and washing of trucks. In addition to reducing disposal costs, the new system will help them comply with California’s increasingly stringent environmental regulations.