A Sustainable Choice for Unbound Base.
By Cecil Jones
This article is adapted from a recently released white paper by the Construction Materials Recycling Association. It was researched and written by Cecil Jones, retired chief materials engineer at the North Carolina Department of Transportation, and is designed to provide guidelines on overcoming the objections of highway officials to using recycled aggregates as a base product. The complete document is available for free to CMRA members. –Ed.
Aggregates play a major role in the construction industry as they are the major component of roadways, bridges, airport runways, concrete buildings, drainage systems, and many other constructed facilities. Because aggregates are the major component of much of the nation’s infrastructure, their use is engineered to provide the necessary performance in place. For instance, concrete is approximately 75 percent aggregate, and the proportioning and properties are critical to the performance of the finished product.
In the past, when concrete structures reached the end of their service life or needed to be repaired or replaced, the resulting materials were considered waste and were disposed of in embankments and landfills. The costs of transporting these materials to waste areas were considered a necessary part of the replacement work. Likewise, the costs associated with the mining of new aggregates, the production of the replacement concrete, and the transportation and placement at the project were also considered a necessary part of the work. Considering these costs, along with increasing tipping fees and diminishing landfill space, many transportation agencies are moving toward recycling construction demolition debris as an aggregate with performance characteristics equal to, if not better than, virgin aggregates. The properties of the original aggregates do not change because of being a part of a concrete that has been crushed. Because of the existence of un-hydrated cement in the concrete, Recycled Concrete Aggregate (RCA)-base courses may exhibit higher strength than one with virgin aggregates.
RCA offers an alternative to wasting concrete elements that are no longer in service. A State of Practice Review by the Federal Highway Administration found that RCA is a “valuable resource” that should not be wasted. (FHWA, 2004) The origin of the concrete from which RCA is produced includes the demolition of transportation structures such as existing concrete pavement and runways, total or partial demolition of bridge structures, curb and gutter, sidewalks and drainage structures. The concrete can also be obtained from commercial sources ranging from building demolition to excess concrete returned to concrete plants that has been discharged in a waste pile and has hardened.
The production of RCA from concrete demolition debris includes several processes. The concrete material is either stockpiled on site and a mobile processing operation set up, or transported to a central crushing facility. Concrete at this stage may contain reinforcement, soil, asphalt pavement, brick or other debris. The crushing operations typically include primary crushing equipment to break down the concrete to a more manageable size and then proceed through a series of secondary crushing and screening processes. Magnetic separation systems are effective in removing and recovering reinforcing steel along with visual and manual inspection stages to remove excess amounts of other materials such as brick and asphalt pavement. Both mobile and fixed processing systems have the capability to break down and separate the crushed material into the desired sizes and gradation blends, such as aggregate base course material. The steps for a typical crushing plant to produce RCA were described in a Transportation Research Board paper and are listed below. (Donaldson, 2011)
- Demolition: Reducing concrete to a manageable size.
- Manual or Mechanical Pre-Separation: Removal of large pieces of wood, iron, brick, and other undesirable materials.
- Primary Screening: Removal of soil, gypsum, etc.
- Primary Crushing: Initial crushing operations.
- Magnetic Separation: Removal of ferrous materials.
- Secondary Screening: Initial separation into desired sizes.
- Manual or Mechanical Removal of Contaminants: Lightweight material removal such as plastic, paper and wood.
- Secondary Crushing: Final crushing of concrete.
- Washing (if required), Screening, or Air-Sifting: Final sizing and removal of remaining contaminants such as plastic, paper and wood. Resulting RCA is ready for transport to site and incorporated into the project.
Available technology allows for the production of aggregates that meet the same criteria as virgin materials, and can result in economic savings for the owner, especially when mobile crushers are used on site. Many state highway agencies and municipalities have recognized the value and performance of RCA and have included it in their specifications. RCA can be produced in the same sizes and gradations as virgin aggregates, and has been successfully used in many different applications but using it as an aggregate base allows large quantities to be used. Using RCA as a base course also accommodates more of the finer sizes generated from crushing than does producing several different sizes of clean stone.
A survey was sent to the State Materials Engineers in September 2012 requesting basic information about each state’s allowance for the use of RCA. Information was also requested about any limitations in the use of RCA for those states allowing it, and any environmental concerns they have related to it. A copy of the state’s specifications for RCA was also requested. Thirty-nine states and the District of Columbia responded to the survey.
Thirty-three of the responses indicated that RCA was allowed as an aggregate base, reinforcing the fact that RCA is a viable replacement for virgin aggregates, and seven states did not currently allow it. Two of the states not currently allowing RCA were considering allowing its use, one of which submitted a draft specification they intend to use. One additional state that is predominantly rural stated that they would consider allowing RCA if requested.
One state not allowing it cited environmental problems (pH and runoff) and durability issues with the larger particle sizes of their limestone and gravel. Another state noted that the use was stopped in the mid-1990s because of issues with clogged rodent screens at outlets of pavement drainage systems and the outwash damaged vegetation. That state has now developed a specification to address these issues and is allowing its use experimentally on one project. Figure 1 exhibits responses from the individual states reflecting whether or not the use of RCA as an aggregate base is allowed.
As might be expected, individual states approach the use of RCA with some differences. Several states require that RCA conform to the same requirements in place for virgin aggregate base. Two states (Arizona and Maine’s provisional draft specifications) allow up to 50 percent by weight blend of RCA with virgin aggregate base. Some states require that RCA be generated from concrete within the project limits, or from another state project, while other states have additional testing and storage requirements if the concrete comes from an unknown source. Several states require the producer to be certified, while others require the producer to present a certification that the RCA does not contain hazardous materials.
Most responders indicated that no environmental concerns existed. However, concerns about leachates, pH, freeze-thaw, corrosion of metal pipe, clogging of subsurface drainage caused by precipitates (tufa), asbestos, lead, and air quality existed with a few states. The details of these environmental issues are discussed below.
States having concerns about leachates stem from several areas. One of these involves the alkalinity of water passing through the RCA layers that could lead to corrosion of metal culverts and rodent grates and could also damage vegetation.
A South Dakota study noted that although pH levels could initially be high, the low permeability of base courses will minimize the transport of water through them and “is not considered a problem.” (Cooley, 2007) FHWA reported that research indicates only a small area near drainage outlets is affected, and only for a short period of time after the initial construction. (FHWA, 2004) The AASHTO Standard Specification for Reclaimed Concrete Aggregate for Unbound Soil-Aggregate Base Course designated as M 319 also addresses this in Note 2 and states that appropriate limits should be set for the use of RCA in proximity to groundwater and surface water. States having this concern could address this by not using RCA base in low, wet areas, or where the base course is allowed to daylight into the edges of embankments thereby exposing it to surface water.
Another potential leachate issue involves the possibility of precipitates (calcium carbonate) being formed that could clog filter fabrics and reduce the effectiveness of pavement subsurface drainage systems. (Van Dam, 2011)
User Guidelines presented by the Recycled Materials Resource Center summarized research that offered a series of recommendations to limit the potential of the formation of such tufa-like precipitates, noting that this problem is not universal and that many are functioning without any precipitate formation. (RMRC, 2008) Appendix X2 of AASHTO M 319 also addresses this issue. ASTM D 5101 can be used to compare the permeability of geotextiles using virgin fine grained material and using the fine grained portion of RCA base materials to determine if a measurable impact exists. One state recommended limiting RCA fines in the base, designing drainage systems to accommodate a limited quantity of crusher fines and to blend open graded RCA with virgin aggregates to reduce precipitate formation.
The existence of hazardous materials, such as lead and asbestos, is possible when the concrete is from commercial sources. Some states address this possibility by requiring a certification from the supplier indicating that the material is in compliance with appropriate state and EPA requirements.
The Maryland Department of Transportation presented a Special Provision added to Section 900.03 of their Standard Specifications with such a requirement. The Texas Department of Transportation also has a specification that addresses how nonhazardous recyclable materials, such as RCA, are addressed from an environmental regulatory perspective. The determination about allowable uses of RCA by individual state Department of Environmental Protection (DEP) agencies also varies from state to state.
For example, in one state RCA has been defined by legislation to not be subject to review or permitting from the state DEP, another state developed a compliance agreement addressing the use of RCA, and another state had RCA exempted from solid waste regulation as long as it remained on the project site. (FHWA, 2004) Consistency in regulatory procedures between states is certainly desirable and could have an impact on both availability and cost of RCA.
The majority of the state specifications for the material properties and gradations for RCA are the same as for virgin aggregates. Because of the existence of cement paste and concrete mortar on the aggregates, RCA has a lower specific gravity and higher absorption than virgin aggregates.
Most states allow up to five percent brick and five percent bituminous pavement by weight in RCA recognizing the realities involved with removing and demolishing existing concrete structures. AASHTO M 319 includes a note and Appendix X4 that provides for using excess bituminous pavement or brick in RCA. It presents three different means to address performance criteria that can be used when considering increasing the allowable percentages. The options include validation by (1) the use of CBR testing (AASHTO T 193); (2) the use of Resilient Modulus testing (AASHTO T 307); or (3) the use of a field validation using test strips or historical data.
A similar approach could also be used to consider gradations differing from the agency’s normal requirements. When testing the durability of RCA with sodium sulfate (AASHTO T 104), some materials exhibit high soundness values that may not accurately reflect the quality. Note 5 of AASHTO M 319 cautions about this and provides discussion about alternatives in both Section 6.3 and Appendix X3. The appendix offers options that include not testing along with three alternative test methods to evaluate freeze-thaw characteristics.
In addition to not testing, AASHTO T 103 (freeze-thaw in water), New York State DOT method NY703-08 (sodium chloride brine solution), and Ontario Ministry of Transportation Test Method LS-614 (sodium chloride brine solution with five freeze-thaw cycles), are listed as acceptable alternative approaches to sodium sulfate.
Some states require concrete from their own DOT furnished sources, such as existing pavements and structures, while others allow the concrete to originate from contractor provided sources, such as building demolition or commercial recyclers. Some states have different testing and stockpile management requirements depending on the source of the concrete and others do not differentiate. If a particular state has good quality aggregates throughout, it is reasonable to assume that any concrete used to produce RCA would produce a durable and quality material. On the other hand, additional testing of concrete from unknown sources would seem reasonable for a state having high variability in the quality of available aggregates.
Concrete that has exhibited alkali-silica reactivity (ASR) is acceptable for use for unbound base course using RCA, according to the Michigan Department of Transportation Manual of Practice addressing RCA. (Van Dam, 2011) Sulfate attack may be an issue with some combination of materials. Because of this, testing for sulfates in both the soils in contact with the RCA base as well as nearby surface water may be warranted. (Cooley, 2007) Knowledge about local conditions and past experience should dictate whether or not such testing is advisable.
The U.S. Geological Survey (USGS) has estimated that 1.2 billion tons of aggregates were produced in 2011. (Willett, Mineral Commodity Summaries, January, 2012) Although this is below the record high of 2 billion tons produced in 2006, it still represents a significant quantity of aggregates. The Construction Materials Recycling Association estimates that 140 million tons of concrete are recycled annually in the United States. (CMRA, 2012) It is reasonable to expect that the demand for aggregates will increase at some point in the future, although past predictions have not materialized due to recent economic trends.
The permitting and opening of new aggregate sources is time consuming, expensive and difficult to accomplish, especially in urban areas. The availability of concrete capable of allowing the consistent production of RCA in large quantities is more likely in urban areas.
As the aging infrastructure reaches the point of needing replacement, the recycling of existing concrete accomplishes the ability to provide quality aggregate while not adding to diminishing landfill space. In that regard, the use of RCA should be considered a sustainable option for use as a base course by providing a quality choice without either adding to landfill space or exerting energy and resources to the mining and production of new aggregates. In a recent Transportation Research Board paper the use of RCA as a replacement for virgin aggregate base in Florida was deemed sustainable and displayed limited impacts in the environmental, social and economic categories. (Donaldson, 2011)
A RMRC user guideline reports that the Michigan Department of Transportation stated that RCA “performs comparably or better than virgin aggregate because of the cementitious action that can still occur within the compacted base, adding more supporting strength for the highway.” (RMRC, 2008) FHWA concluded that the engineering, economic and environmental benefits of using RCA should give states reason to seriously consider using it in their respective transportation system, noting that it is the aggregate base of choice in some states. (FHWA, 2004)
Recycled Concrete Aggregate is currently being used to some degree in the vast majority of the state Departments of Transportation across the country. RCA has proven to be both a viable and valuable alternative to the use of virgin aggregates for base courses.
The performance is equal to, if not better than, virgin aggregate base course when used appropriately. Significant research has been conducted and published for over 10 years. An AASHTO specification has been in place since 2002. Potential concerns, including precipitates, leachates, durability testing and the existence of asphalt pavement and brick have been addressed. Limiting factors to states more fully adopting the use of RCA include the following:
- Uniform Application of Specifications.
Most states developed a specification based on their research and/or experience. The survey only revealed one state that referenced AASHTO M 319, which was first published in 2002 with the most recent publication date being 2010. AASHTO specifications are developed by consensus of state highway agency members and are balloted for approval prior to publication. This specification provides a sound foundation for a specification and includes notes and appendices to address issues that have been noted. The adoption of AASHTO M 329 is encouraged.
- Uniform Consideration of the Environmental Impacts.
Some variation exists in how the individual state Department of Environmental Protection agencies address use of RCA. Uniformity in how this is addressed would allow producers doing business across state lines a clear understanding of the requirements and not necessitate multiple paths to obtain approval for the same material.
- Technology Transfer Between States.
Significant time and resources could be saved by taking advantage of research that has been completed. The time and effort of conducting similar research by states in close proximity that reach similar conclusions prevents timely acceptance of any new technology, as well as potentially increasing cost due to differences across state lines.
- Advancing and Improving Existing Specifications.
A means to continually improve existing specifications is needed to help identify and address remaining concerns that state agencies have. Perhaps the establishment of an Expert Task Group could take the lead on this similar to what has been successfully implemented with other materials.
Recycled Concrete Aggregate has proven to be a quality material capable of meeting the engineering requirements for use as an aggregate base. It is also considered by most states to be an environmentally acceptable material which also provides the potential for an economical alternative to virgin aggregates. It has demonstrated the ability to successfully provide an alternative to the use of virgin aggregates as an aggregate base course.
Many states have incorporated its use into their specifications to take advantage of equal or better engineering properties, the environmentally friendly advantage of saving natural resources and reducing landfill space and realizing the potential economic benefits it offers. We encourage the states and FHWA work cooperatively with the industry to help more states reap the advantages that using RCA offers.
CMRA. (2012). Concrete Recycling Home Page. Retrieved October 13, 2012, from Concrete Recycling: http://www.concreterecycling.org
Cooley, B. E. (2007). Evaluation of Recycled Portland Cement Concrete Pavements for Base Course and Gravel Cushion Material. Pierre, S.D.: South Dakota Department of Transportation.
Donaldson, C. N. (2011). Sustainable Assessment of Recycled Concrete Aggregate (RCA) Used in Highway Construction. Washington, D.C.: Proceedings of the 90th Annual Meeting of the Transportation Research Board.
FHWA. (2004). Transportation Applications of Recycled Concrete Aggregate - FHWA State of the Practice National Review FHWA-HRT-04-024. Washington, D.C.: US DOT.
RMRC. (2008, July 28). User Guidelines for Byproducts and Secondary Use Materials in Pavement Construction. Retrieved September 14, 2012, from Recycled Materials Resource Center: http://www.rmrc.unh.edu/tools/uguidelines/rcc3.asp
Van Dam, S. T. (2011). Using Recycled Concrete in MDOT's Transportation Infrastructure - Manual of Practice. Lansing, MI: Michigan Department of Transportation.
Willett, J. C. (January, 2012). Mineral Commodity Summaries. Reston, VA: US Geological Survey.
Willett, J. C. (2010). U.S. Geological Survey Minerals Yearbook. Reston, VA: USGS.