At first blush, recycled concrete aggregate may seem a second-best solution because its use is being driven not by concrete performance requirements but rather by the requirement to create a more sustainable concrete. But as experience and knowledge is gained, this perception is quickly being dispelled.
Crushing old concrete to make aggregate for new concrete is increasing in popularity for several reasons.
• Owners want projects to be sustainable and LEED certified. Recycled concrete qualifies for recycled material credits, and because it usually is found locally, it qualifies for local credits.
• Concrete with recycled concrete aggregate has a smaller carbon footprint.
• Crushing old concrete to produce aggregate takes less energy than mining virgin aggregate.
• Using old concrete helps to conserve virgin aggregate supplies, which are slowly being depleted near larger cities.
When landfills located near populated areas started refusing broken concrete in the 1970s, the construction industry started crushing it to make aggregate for use as compactible fill. Wood and steel reinforcement were separated out, leaving aggregate that often included a mixture of concrete, brick, and asphalt, along with some dirt. But not all concrete gets repurposed and the disposal problem remains.
For the purpose of this article the term crushed concrete aggregate (CCA) is defined as returned concrete and recycled concrete aggregate clean enough to be used as aggregate for new concrete. Recycled concrete aggregate (RCA) refers to crushed concrete and other materials suitable for use as fill and compactible fill.
Initiatives to use CCA in concrete mixes have proceeded slowly and cautiously for many reasons. A central concern is whether CCA can have the same quality as virgin materials—a pile of demolished concrete could have aggregate from different sources as well as contaminants that could reduce the quality of the new concrete.
Characteristics of CCA concrete
CCA is more porous than virgin aggregate. Mark Wachal, senior project manager of Recycled Materials Co., Arvada, Colo., says CCA is about 10% lighter than virgin aggregate, due to its porosity and the presence of paste with lower density than traditional mineral aggregate. Thus, CCA—combined with the fine aggregate or “dust” from the crushing process—has a higher demand for water and cement, resulting in the potiental forgreater shrinkage and lower strength.
Because the crushing process produces concrete aggregate with jagged edges and dust, the resulting concrete was considered too harsh for applications requiring finishing operations—although this was later determined not to be true.
Nonetheless, a series of successful roadway demonstration projects and continuing research has helped to maintain interest in the idea of using CCA to make concrete. Wachal says his firm was contracted to remove the old Denver Stapelton Airport runway, which was crushed into RCA and CCA, and sold to paving contractors. The contractors placed mixes with up to 75% CCA and 25% virgin aggregates as part of a value engineered proposal to the Colorado DOT. His company also started its own ready-mix company, providing enough material to stock CCA for a variety of mixes. They also have performed studies using up to 100% recycled aggregate in concrete.
Current interest in CCA
Attitudes about CCA in concrete are changing slowly. The National Concrete Pavement Techniology Center (CP Tech Center), Austin, Texas, and the Federal Highway Administration (FHWA) are developing a technology deployment plan that includes strategies for educating state DOTs and industry personnel on the CCA use in new concrete paving mixtures. The Transtec Group Inc., Austin, Texas, is helping the CP Tech Center with this effort. Project manager Sabrina Garber says the document is close to completion, after which it will be sent to the FHWA for review and public release.
Bill Turley, executive director of the Construction Materials Recycling Association, Eola, Ill., adds that recycled aggregate has become less expensive than mined aggregate, making it an affordable option and not something only used for its sustainable attributes.
Recent experience with CCA
Wachal says that he quickly overcame most of his initial concerns about working with CCA concrete. The Colorado School of Mines, Golden, Colo., conducted tests on concrete made from CCA, including the fines, that gave them the confidence to proceed. After many research projects, here’s what was learned about CCA.
• Because the aggregate is lighter than mined aggregate, it is easier to place and strike off.
• Finishing with magnesium floats and steel trowels is the same as finishing with concrete containing all-virgin aggregate.
• Finishers report CCA is more “creamy” or “fat” and therefore easier to finish.
• CCA concrete has an initial set time that is a little faster, so there is less time between placing and finishing.
• CCA concrete, properly proportioned, has the same shrinkage properties as other concrete.
Wachal’s company continues to make CCA and sell it to ready-mix producers and paving contractors. The firm also uses the concrete in its own batch plants.
Cemstone Products Co., a ready-mixed concrete and aggregate producer in Mendota Heights, Minn., is producing both CCA and RCA. They have adequate space to store each material in separate piles. The CCA pile is composed of both demolition concrete and returned concrete placed in “windrows” by ready-mix trucks and allowed to get hard. The concrete then is crushed and passed through a screening operation to produce the proper-sized material. The fines created in the crushing process also pass through the sieve and aren’t removed, resulting in product ranging from dust to top-size aggregate. Petrographic analysis performed on trial batches revealed dust created in the crushing process didn’t coat the aggregate or interfere with the paste-to-aggregate bond in the way clay and tightly bonded quarry dust interfere with virgin aggregate bonds. The unhydrated particles of portland cement found in CCA (and in all hardened concrete) plus the dust created in the crushing process has a beneficial effect by strengthening the bond between the paste and aggregate. This makes it possible for Cemstone to remove 2% to 3% of the cementitious content from its mixes and still maintain all of the durability and strength properties of the concrete. The aggregate they produce has a top size of ¾ inches, meeting ASTM C33 requirements for #67 aggregate.
Cemstone’s first large project will be constructing the retaining walls for an extension of the light-rail trains between St. Paul and Minneapolis. The project will use 10,000 cubic yards of 50% CCA and 50% virgin aggregate concrete. The mix also will include supplementary cementitious materials (SCM) and recycled water. Even though the aggregate has a 5% absorption rate, the concrete will have a water-cement ratio of 0.40 to 0.45—made possible with the addition of water-reducing admixtures. Test cylinders taken from trial batches broke at 6000 psi—the same as cylinders made with all-quarried aggregate.
Cemstone is being cautious about CCA concrete, only recommending it for footings, walls, and interior floors at present. The company isn’t selling it for exterior flatwork because of concerns about popouts from deleterious material—a result of not knowing the source of all the materials. Some difficulties have been encountered in handling the materials in plants as well. If too many fine particles are present in the aggregate blend, the material will hold more water than virgin aggregates. Careful production is required.
Ozinga Bros. Inc., a ready-mix producer in Chicago, is working with the Chicago DOT on its Green Mile Project. Marty Ozinga IV, executive vice president of Ozinga’s Chicago division, says the project is intended to be a national showcase of sustainable infrastructure. Ozinga only makes CCA from concrete returned in its ready-mix trucks to the yard. The hardened concrete is crushed and graded to ASTM C33 #67. The mix used for the Green Mile Project is composed of 30% CCA and 70% virgin limestone aggregate. The mix includes a 40% replacement of portland cement with SCMs and uses recycled water from truck washout and stormwater. Ozinga says the contractor took precautions with regard to finishing and curing on the exterior flatwork panels placed a year ago but so far hasn’t seen any indication of damage due to freeze/thaw cycles. The pavement for the project will be completed this summer.
Erring on the side of caution
The traditional view held by ready-mixed concrete producers is that the properties of aggregate used in their mixes must be known and thoroughly tested, especially when concrete is used for structural purposes. This causes concern with CCA because a pile of demolished concrete could come from many different sites and contain aggregates from different sources. As a result, the aggregate is less predictable than single-source virgin aggregate mixes. CCA also can have impurities, such as paint or concentrations of chlorides, that could affect concrete properties. On the other hand, recycled concrete from a jobsite has a history that can be viewed; scaled surfaces or signs of alkali silica reaction, for example. If the concrete is in good shape after being in service for a number of years, there is less risk of incorporating potentially problematic materials into new work and good reason to believe it will perform well in the future. When CCA is produced from concrete returned in a ready-mix truck, the aggregate is a known quantity and the results will be as predictable as using virgin aggregate.
Where to go from here
If you were a third-generation concrete construction company, your grandfather wouldn’t understand the technology behind the concrete the industry is producing today. Recycling is changing this technology even more, becoming an important material as virgin aggregate becomes more scarce.
Concern for the environment has a profound effect on how the concrete industry does business. CCA concrete helps to address environmental issues, especially as more experience is gained. Demonstration projects are helping to provide experience and dispel concerns.
With all the recycled products available for making concrete—aggregate, water, and SCMs—the life-cycle advantages make it very competitive with other materials. The concrete industry is marketing the concept of concrete’s sustainability by highlighting its long service life and eventual reuse possibilities. CCA can make concrete a continuously recycled product. Unlike some recycled materials, the properties of concrete made with CCA and SCMs can be superior to those made with virgin material alone.
Kevin MacDonald is vice president of engineering services at Cemstone in St. Paul/Minneapolis.