Choice cuts: recycled concrete aggregate can make the grade in several applications, according to the FHWA.
These states were selected based on their level of use and generated supply of RCA and 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 STAs with recommendations, guidelines and specifications for furthering the use of RCA throughout the country.
The overall findings of the review team was that RCA is a valuable resource and, with proper engineering, it can be used for PCC pavement and aggregate base. The material is too valuable to be wasted and landfilled. 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.
FHWA conducted this review on the uses of (RCA) in highway applications with the purpose of capturing the most advanced applications and technologies.
RCA is generally thought of as old PCC pavement, bridge structures/decks, sidewalks, curbs and gutters that are being removed from service. The steel is removed, and then the concrete 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 sources 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 STAs. 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, Texas, Virginia, Michigan, Minnesota and Utah were identified as being among the highest consumers as well as large suppliers of RCA. 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.
Transportation agencies' experiences and research studies have shown that (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 proportions or workability. Recycled fines, when used, are generally limited to about 30 percent of the fine-aggregate portion of the mixture.
The angularity of RCA:
1. Helps to increase structural strength in the base, resulting in improved load carrying capacity.
2. Building pads (residual cementation) provide a strong, durable platform from which to build upon.
3. Offers better control over gradation. RCA is able to meet gradation and angularity requirements.
4. Potential to minimize D-cracking and ASR (alkali silica reaction). D-cracking is caused by the freeze-thaw expansive pressures of certain types of aggregate, whereas 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.
Several factors can be considered in this category.
* Reduced land disposal and dumping: The use of recycled concrete pavement eliminates the development of waste stockpiles of concrete. Also, because 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 reduced transportation source emissions.
* Conservation of virgin aggregate: Many European countries have placed a tax on the use of virgin aggregates. This process is being used as an incentive to recycle aggregates. 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 creates 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 [often] becomes waste.
* Defined as inert material in solid waste regulations: Generally in the states that use RCA, the environmental regulatory agencies have reviewed the material and 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.
Using recycled aggregates can provide several economic benefits.
* 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 in light of the 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 a 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 less 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 it 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 impact reductions and extending available life of landfills are also long-term benefits that can be experienced by local governments because of 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 1-66 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 as guide rails and traffic signs, and bridge clearance can be maintained. Rubbleization and crack and seal operations usually require extensive grade increases that make relocation of the highway features mandatory.
RCA: AN AGGREGATE OF CHOICE
Many states (at least 38) are currently using recycled concrete aggregate as aggregate base. This material is becoming the aggregate base of choice in states such as Minnesota and California. This may be in light of the high strengths provided by RCA base material because of some bonding of the residue cementitious material in the processed aggregate.
Their specifications are making this usage less restrictive by allowing the inclusion of asphalt material.
Minnesota allows 3 percent asphalt cement by dry weight of the aggregate. This allows the inclusion of about 50 percent recycled asphalt pavement. California takes this a step further by allowing a mixture of any percentage of recycled concrete aggregate and recycled asphalt pavement. This allows the contractor to use the most economical material in any percentage combination.
The specifications in both states allow the contractor to remove a composite pavement, process it and use it without separate operations. These specifications are providing a base aggregate with superior qualities while providing economic and environmental benefits.
Editor's Note: The following excerpts are from "Transportation Applications of Recycled Concrete Aggregate: FHWA State of the Practice National Review," the Federal Highway Administration's recently finished report on barriers to the use of recycled concrete in highway projects. A copy of the complete report is at www.fhwa.dot.gov/ pavement/recycle.htm.
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|Title Annotation:||Concrete Recycling Update; Federal Highway Administration|
|Publication:||Construction & Demolition Recycling|
|Date:||Jan 1, 2005|
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