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INTRODUCTION
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,economicand environmental benefits that can be achieved for using RCA, use for highway work is on the increase.
Review Methodology
FHWA conducted this review on the uses recycled concrete aggregate (RCA) in highway applications with the purpose of capturing the most advanced applications and technologies. RCA is general thought of as old PCC pavement, bridge structures/decks, sidewalks, curbs and gutters that are being removed from service, steel removed, and can be crushed to a desired gradation. Commercial construction debris can be used for RCA, provided the material is cleaned of unwanted material like bricks, wood, steel, ceramics, and glass. The STAs tend to want to re-use material recovered from either state projects or known source of supply. The aggregates used to produce PCC pavement and structural PCC for buildings will have similar if not the same aggregate used. This knowledge will then be transferred to the State Transportation Agencies (STA). The present report summarizes the findings of this review including the best practices on the use of RCA and the advantages and barriers associated with these uses in highway construction. State Transportation Agencies were surveyed to determine the current uses of RCA. From the results of this survey, five states were identified as being among the highest consumers as well as large supply of RCA in the United State. The states selected were Texas, Virginia, Michigan, Minnesota, and Utah. Utah declined to participate and California was selected to replace it in the review. These states were visited in order to collect information about the state-of-the-practice on RCA uses. The following three figures depict the extent of use for recycled concrete aggregate as determined by the survey results throughout United States.
Advantages:
Transportation agencies’ experiences and research studies have shown that recycled concrete aggregate (RCA), under specific conditions, has the potential to produce strong, durable materials suitable for use in the highway infrastructure. The coarse aggregate portion of RCA has no significant adverse effects on desirable mixture proportion workability. Recycled fines, when used, are generally limited to about 30 percent of the fine-aggregate portion of the mixture. (1)
Performance:
Angularity of RCA:
1. Helps to increased structural strength in the base, resulting in improved load carrying capacity.
2. Building pads (residual cementation), provides a strong, durable platform for which to build upon
3. Better control over gradation, in this RCA is able to meet gradation and angularity requirements.
4. Potential to minimize D-cracking and ASR: D-cracking is caused by the freeze-thaw expansive pressures of certain types of aggregate, whereas Alkali Silica Reaction (ASR) is caused by the detrimental reaction between silica found in certain aggregate and the alkali (cement) paste. These forms of distress are material related and studies show that the inclusion of RCA in the concrete mix and a suitable fly ash has the potential to reduce these distresses. Freeze-thaw testing of concrete in Minnesota made with recycled D-cracking aggregate indicated a greatly reduced potential for D-cracking when fly ash was used in the mixture.(1)
Resource Conservation
Reduced land disposal and dumping:
The use of recycled concrete pavement eliminates the development of waste stockpiles of concrete. Also, since recycled material can be used within the same metropolitan area, this can lead to a decrease in energy consumption from hauling and producing aggregate, and can help improve air quality through reducedtransportation source emissions.
Conservation of virgin aggregate:
The supply of virgin aggregate in many areas of the United States is or is becoming limited. In such areas, the use of recycled aggregate is beginning to serve as an environmentally friendly and economically viable solution. Many European countries have placed a tax on the use of virgin aggregates. This process is being used as an incentive to recycle aggregates. It is noted that several states have high tipping fees for disposal of RCA, this is done to control landfill usage thus increasing the reuse of RCA.
Reduce impacts to the landscape:
The reuse of concrete demolition debris reduces unsightly stockpiles of concrete rubble, animal infestation of stockpiles, and an overall environmental improvement when re-used.
Metal recovery:
The removal of metal, steel reinforcement, is an important step in the recycling process and can take place in several stages. Contractors usually remove continuous reinforcement on the grade, whereas dowel and tie bar removal is typically done at the plant. Most crushing plants have an electromagnet to catch steel moving along the conveyor belt between the primary and secondary crushers. Salvaged steel usually becomes the property of the crushing plant and is sold as scrap metal. Wire mesh steel generally found in reinforced concrete pipe retains a large quantity of bonded concrete and usually becomes waste. (2)
Defined as inert material in Solid Waste Regulations:
Generally in the states that use RCA the environmental regulatory agencies have reviewed the material, where it is to be used, and have deemed it inert. After all it really is just broken up concrete pavement being reused as aggregate base or PCC aggregate.
Economic
Limit haul distance:
Recycled concrete is crushed and the entire aggregate product can be used as a base material according to specifications, therefore generating no waste. This can be done on the project site or at nearby recycling plants, eliminating the transportation to distant disposal sites and the hauling in of virgin aggregate. In an urban environment concrete debris is hauled to a crushing site that is generally closer to the center of the urban area then the virgin aggregate quarry. In some cases the two operations cohabitate. Industry comments were that the RCA stockpile is usually closer to the job sites in an urban environment, thus less haul distance is less fuel burnt in delivery. Production of virgin aggregate can use more fuel to crush due to larger initial size of rock needing to be crushed to desired grade.
Reduce disposal costs:
Disposal of concrete rubble and other waste construction materials by dumping or burial is a less attractive and more expensive option. Reconstruction of urban streets and expressways results in an enormous amount of waste concrete being generated and creating a massive disposal problem. Recycling can therefore alleviate some of these problems and offer savings to the owner agencies in terms of material acquisition and disposal costs.
Overall project savings:
There may be considerable project savings by using a less amount of virgin aggregate. This saving is increased by the reduction of transportation and disposal costs. Another economic benefit is the recovery of steel from the recycling process. This material usually becomes property of the contractor, who can sell as scrap metal. There is also potential for cost savings in many areas where aggregates are not locally available, and have to be hauled long distances, often 50 miles or more. Environmental impacts reduction and extending available life of landfills is also a long term benefit that can be experienced by local governments due to increased recycling of RCA.
Minimize impacts to existing roads with reduced hauling:
Using the existing concrete on grade as the source of base aggregate eliminates the importation of large volume of virgin material for reconstruction. This reduces the heavy vehicle loadings carried by the current highway system, an economic and public interest advantage to the owner agency. For instance in the VA I-66 rehabilitation contract the contractor set up a crushing plant at a visitor center that was within the project limits and crushed the old pavement for use as aggregate base material, with very little truck traffic impacting the large numbers of daily commuters.
Maintaining grade on highways:
The process of reusing the existing pavement as a base material or concrete aggregate allows the owner
agency to redesign the new pavement structure at the existing grade. This allows the continued use of many of the existing features outside of the roadway, such at guiderails, traffic signs, and bridge clearance can be maintained. Rubblization and Crack and Seat operations usually require extensive grade increases that make relocation of the highway features mandatory.