Back to the front: Ready-Mix producers are clarifying how to best put returned concrete back into the appropriate product line.
There should be a balance between operational considerations and quality of concrete produced and associated economics for a specific plant or market area.
The questions that the ready mixed concrete producer should consider: Should the returned concrete be separated by strength classes? Should there be a process set up to separate crushed material into fine and coarse fractions or use the material as processed? What classes of concrete or market segments will the crushed concrete aggregate (CCA) be used in?
These decisions will depend on factors at the specific plant and location--quantity of returned concrete, availability of space, availability of processing equipment, market served by the plant and alternative options for managing returned concrete.
To evaluate the economics of using CCA some assumptions are made here:
* The 28-day compressive strength is assumed to be the controlling factor relative to a control mixture with virgin aggregate.
* To increase concrete strength by 200 psi will cost approximately $1 per cubic yard in material costs--use of admixtures and/or additional cement.
* Cost savings from the use of CCA can be due to two reasons--cost of virgin aggregates being replaced and cost savings from transportation and disposal fees of returned concrete.
* The cost of producing the CCA will involve some cost such as the use of a crusher and associated energy costs.
* It is assumed the net cost savings to the producer is at $8 per ton of CCA used.
* An additional cost will be applicable if the producer chooses to separate the CCA into coarse and fine fractions. This cost is assumed to be $2 per ton.
DOING THE MATH
Based on the cost assumptions and the measured 28-day strengths of the different mixtures, the cost savings of the different CCA mixtures that would yield the same 28-day strength as the control makes the following scenarios possible:
1. If the CCA is not separated by strength classes it is generally of no consequence to concrete performance if the use of CCA is limited to a level of 300 pounds per cubic yard (about 10 percent by weight of the total aggregate quantity). This assumes that the CCA would be of the lowest strength grade--i.e. 1,000 psi CCA. In this scenario, the cost savings to the producer is in the range of $0.66 per cubic yard.
2. If CCA is not separated by strength classes but is separated into coarse and finer fraction the optimum option is to replace 100 percent virgin coarse aggregate with coarse CCA. This will provide a cost savings of $0.31 per cubic yard, which is less than the first scenario. This also assumes that the coarse CCA is at the lowest strength grade evaluated in this study at 1,000 psi.
3. If CCA is separated by strength classes the quantity of "as received" CCA can be increased to 900 pounds per cubic yard, assuming that strength classes exceeding 3,000 psi will be used for RMC. The concrete might be crushed after about 14 days, i.e. after the returned material has achieved a minimum strength level. The savings estimated for this scenario is in the range of $2.52 per cubic yard by using 900 pounds of CCA in "as received" condition to replace virgin coarse and fine aggregate. There will still be lower grade returned concrete that will have to be managed.
4. If the CCA was separated by strength classes and additionally separated into coarse and finer fraction, based on the results of this study, up to 100 percent coarse CCA can be used to replace virgin coarse aggregate. It is assumed that strength classes exceeding 3,000 psi will be used for producing CCA. By taking all the precautions mentioned in Scenario No. 3, the estimated cost savings to the producer is in the range of $3.98 per cubic yard. Disposal of fine CCA will still need to be managed. It might be an option to use the fine CCA in a limited manner for some applications, such as for flowable fill.
The appropriate option for the ready mixed concrete producer is strongly dependent on the local costs for processing and disposal of returned concrete. If the disposal costs are higher than assumed above, a net cost savings for using CCA can be as high as $18 per ton.
Using this number, the estimated cost savings to the producer will be in the range of $3, $8.50, $6.98, and $12.03 per cubic yard for the above four scenarios, respectively.
MEETING THE NEED
In all of these considerations only strength is given priority. Even though appropriate mixture adjustments can be made to account for lower strengths due to the use of CCA other performance criteria such as shrinkage, modulus, durability, etc., may also need to be evaluated if these are pertinent for the applications for which the concrete is furnished.
Other concrete mixture adjustments may be required so that the concrete meets the performance criteria if the producer chooses to use CCA for these types of projects. Using CCA to ensure achieving other performance criteria may or may not entail higher costs. For example, if some durability aspect of concrete containing CCA is reduced then it can be adjusted by the increased use of supplementary cementitious materials that may not result in any cost increase.
The following steps or options are recommended for the concrete producer interested in using CCA in concrete:
To start with, it is recommended that producers limit their use of CCA to no more than 300 pounds per cubic yard in an "as received" condition. The producer should evaluate the effect of this on his concrete mixtures to verify that it works with his materials and processes. No attempt need to be made in trying to separate the returned concrete into strength classes or into coarse and fine fractions. In this project as compared to the control mixture the use of 1,000 psi CCA at 300 pounds per cubic yard led to negligible change in water demand, setting time, density, shrinkage, 6 percent lower elastic modulus, and 15 percent lower RCP values.
The next step is for the producer to separate CCA into different strength classes by diverting returned concrete to different areas at the plant. In most instances this step should prove cost effective compared to trying t o separate the CCA into coarse and finer fractions. Nevertheless the producer can attempt to do an experimental study like that presented here to test the performance of the CCA that is produced in his plant. Based on performance and cost structure the producer can take the appropriate decision whether to separate returned concrete into different strength classes or separate the CCA into coarse and finer fractions.
At a minimum, lower grade concrete that has been re-tempered with large quantities of water should be diverted away from the crushing process. In this example, it was found that the producer can attempt to have all the CCA with a specified strength of 3,000 psi or higher to be discharged into an area designated for processing CCA. While discharging the concrete, the truck driver should avoid the use of water to dean the concrete truck.
One option is to discharge the concrete and wash out the truck at the wash out pit. Another operational issue would be to leave the discharged concrete undisturbed for a period of at least 14 days.
The final step will be for the producer to separate CCA into different strength classes and additionally separate the CCA into coarse and fine fractions. In this scenario, the producer can divert all returned concrete with a specified strength of 3,000 psi or higher to be discharged into a designated area to produce CCA.
The producer can use 100 percent of the coarser fraction of this CCA to replace virgin coarse aggregate. This is approximately 1,600 pounds per cubic yard of CCA. In this project, the use of 100 percent Coarse 3,000 psi CCA led to negligible change in water demand, about 60 minutes lower setting times, 6 percent lower density, 25 percent lower modulus, 36 percent higher shrinkage, 77 percent higher RCP values, acceptable freeze-thaw durability but increased scaling.
The fine fraction of CCA can be used in limited quantifies or for some applications like flowable fill. Another consideration with this option is the available market for higher strength coarse CCA for use as fill material as this might prove to be a profitable use for the concrete producer.
In all situations, the producer should conduct a laboratory and field study and develop performance data on strength and other criteria such as shrinkage, durability, etc., for the CCA mixtures.
Concrete containing CCA should not be used in applications where such concrete will not be able to meet other performance criteria such as shrinkage, creep, modulus, permeability, freeze-thaw durability, etc., unless it can be documented that concrete containing CCA meets all the required performance criteria in such applications.
The CCA stockpile should be kept moist by the use of sprinklers as the CCA should ideally be maintained at a level greater than the saturated surface dry condition. It is also recommended that CCA characterization studies such as absorption, and relative density (specific gravity) should be conducted on a weekly basis.
CCA AND SUSTAINABLE DEVELOPMENT
As mentioned earlier it is estimated the beneficial use of CCA can reduce landfill space by the equivalent volume of 845 10-foot-high football fields every year. Nowadays, there is a significant interest in sustainable development.
The use of CCA in concrete significantly contributes to concepts incorporated in sustainable construction initiatives.
The U.S. Green Building Council (USGBC), through its Leadership in Energy and Environmental Design (LEED) Green Building rating system, fosters sustainable construction of buildings. Other sustainable development initiatives include the Green Highway Initiative, Green Globes and those adopted by local jurisdictions.
The use of CCA can help attain LEED Credit points in the Construction Waste Management section of the Materials and Resources category. The wording could be as follows: "Three percent by volume of all concrete for this project was returned to the ready mixed concrete production facilities used for this project. Of that amount, 100 percent was diverted from landfills by crushing the returned concrete and reusing that as crushed concrete aggregate in concrete furnished for the project."
The full report, "Crushed Returned Concrete as Aggregates in New Concrete," is available by contacting the RMC Research & Education Foundation through its Web site at www.rmc-foundation.org.
[Editor's Note: the following text is an excerpt from a study entitled "Crushed Returned Concrete as Aggregates in New Concrete," produced by the National Ready Mixed Concrete Association, Silver Spring, Md., and its RMC Research & Education Foundation.]
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|Title Annotation:||CONCRETE RECYCLING FOCUS|
|Publication:||Construction & Demolition Recycling|
|Date:||Nov 1, 2007|
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