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Returned And Recycled


Crushed Concrete

New RMC Research & Education Foundation data provides engineering rationale to support a recycling opportunity in ready mixed that could rival fly ash in tonnage terms. ìCrushed Returned Concrete as Aggregates for New Concreteî notes that 2 percent to 10 percent of the estimated 455 million cubic yards of ready mixed concrete produced in the United States annually (est. 2006) is returned to the plant. The returned product in the truck can be used in the following manner:

  1. A small quantity of returned concrete fresh material can be batched on top. Hydration stabilizing admixtures might be involved in this process.

  2. It can be processed through a reclaimer system to reuse or dispose the separated ingredients, including the process water with a hydration stabilizing admixture as needed.

  3. Returned concrete can be used for plant paving or casting of products like concrete site blocks.

  4. Returned concrete can be discharged at the plant for processing. The hardened discharged concrete can be subsequently crushed for reuse as base for pavements or fill for other construction. The separation of the crushed material can produce different useable products. In general, the finer crushed product is difficult to manage and dispose. This could be material finer than two inches and associated fines that provide a significant challenge for the ready mixed producer to dispose of.

Crushed Returned Concrete as Aggregatesî authors contend: Option 1 is probably done on a small scale and is not always practical because of restrictions by concrete specifications. Option 2 is limited to larger-volume plants in metropolitan areas and requires a significant capital investment, followed by attention to proper practice. Option 3 is limited by several factors ó there is only so much area in a plant that can be paved and the volume of block production depends on local market conditions and opportunities. Option 4 has significant potential, and it is reasonable to assume that this can be used to manage about 60 percent of all returned concrete.

With some assumptions, one can estimate that the quantity of crushed returned concrete material generated by the ready mixed industry is on the order of 30 millions tons/year, with most of that likely being diverted to landfills. If all of this material can be beneficially used as aggregate at an estimated cost of $10/ton (cost of virgin aggregates, plus reduced cost of land filling), it would represent a recuperated total cost of about $300 million/year for the industry's bottom line. Additionally, this will significantly benefit sustainable building initiatives by enhancing the considerable benefits provided by the use of concrete as a construction material. This research project addresses the use of crushed returned concrete, referred to in this report as Crushed Concrete Aggregate (CCA), as a portion of the aggregate component in new concrete.

Demolishing old concrete structures, crushing the concrete and using the crushed materials as aggregates is not new and has been researched to some extent. This material is generally referred as Recycled Concrete Aggregates (RCA). However, RCA is different from CCA as construction debris tends to have a high level of contamination (rebar, oils, and de-icing salts, for example). CCA on the other hand is prepared from concrete that has never been in service and thus likely to contain much lower levels of contamination.Principal investigators behind the RMC Foundation study note that published research on CCA reuse is limited. Their main objective is to develop technical data that will support the industry's use of CCA from returned concrete and provide guidance on a methodology for the material's appropriate use. The technical data can be used to support revisions to current industry standards and permit the use of returned concrete as crushed aggregates. In addition to the $300 million/year in operating costs, investigators report, this process will reduce landfill space consumption equivalent to 845, 10-ft.-high football fields every year.

Experimental Study

With the exception of Task 1, this research program was conducted at the Nation Ready Mixed Concrete Association Research Laboratory in College Park, Md. Work was divided into five tasks.

Task 1-Preparation of CCA at Ready Mixed Plant

The CCA was prepared at Virginia Concrete's Edsall plant. Three different concrete mixtures with target 28-day strengths of nominal 1000 psi, 3000 psi and 5000 psi were produced at the ready mixed concrete plant in January 2006. All mixtures were non-air-entrained, portland cement-only mixtures, containing a small dosage of a Type A water reducer. A small amount of integral color was added to each concrete mixture to allow for identification of the different grades. The mix was discharged on the ground using a normal process for handling returned concrete.

The concrete mixtures were tested for slump, air content, temperature, density (unit weight), and compressive strengths at various ages. The compressive strength cylinders were subjected to two curing conditions: lab moist curing and field curing near the location where the concrete had been discharged. It was felt that the latter strengths were more representative of the concrete that was crushed to make CCA. The mixture proportions and test results are provided in Table 1.

The volume of paste divided by the volume of total aggregate varies from 31 percent to 43 percent with increasing values obtained for the higher strength concrete mixtures due to the higher cement content. Paste volume refers to the volume of cement, water and air used in the concrete mixture. CCA produced by crushing this concrete will have high absorption, lower strength and durability because it contains paste. The paste specific gravity varied between 1.43 and 1.74 with increasing values obtained for the higher strength concrete mixtures due to the higher cement contents. This would suggest that CCA, particularly the finer fraction of CCA, will tend to have lower specific gravity due to the high paste content in that fraction.

The actual 56-day field cured strengths of the different classes were averaged at 1,320 psi, 3,630 psi, and 6,480 psi. However, the different classes of CCA will continue to be referred as 1,000 psi, 3,000 psi, and 5,000 psi, primarily for ease of notation. The discharged concrete was left undisturbed for 110 days, after which the concrete was processed through a crusher to produce the CCA.

The CCA was transported and stored at the NRMCA Research Laboratory for the subsequent parts of the study. Three different strength classes of CCA were included in this study to evaluate the effects of this factor on the properties of the resulting concrete. Typically CCA results from returned concrete with different design strength levels that may have been through varied levels of retempering. It is important to study the effect of initial strength of the concrete that is crushed on the performance of new concrete containing CCA. Furthermore, it was felt that if a noticeable difference in performance existed then recommendations could be developed so that the producer can make attempts to separate CCA based on the strength levels of the returned concrete. This could help toward more efficient utilization of CCA.

In addition to the CCA prepared in a controlled manner specifically for this study, CCA generated and stockpiled at the concrete producer's yard from normal practice also was evaluated. There was no control on the concrete discharged to produce this CCA, referred to as Pile 1 in this report. This evaluation provides a means of comparing the portions of the study using the controlled CCA to that generated from normal practice. As might be expected in typical operations, the characteristics of the returned concrete from which the CCA in Pile 1 are unknown, which is one factor that cannot be quantified in this portion of the study. The ready mixed concrete producer is interested to know how much of this material can be used to still produce concrete with acceptable performance.

Task 2-Characterization of CCA from Returned Concrete

Using a large-capacity sieve shaker, CCA of all three concrete grades and Pile 1 were separated into coarse and fine fractions. Aggregate tests required by ASTM C 33, Specification for Concrete Aggregates, were conducted. Other quality tests typically performed on concrete aggregates were conducted as well.

Task 3-Experimental Study of CCA in New Concrete, Phase I

Several non-air-entrained concrete mixtures were prepared with CCA and tested on the following criteria:

  1. Control mixture using virgin coarse and fine aggregates.

  2. Use CCA in ìas receivedî state at different replacement levels for virgin aggregate.

  3. Use coarse fraction of CCA (to replace virgin coarse aggregate) and a portion of the fine fraction of the CCA to replace virgin fine aggregate at different replacement levels.

Task 4-Effect of CCA on Freeze-Thaw Resistance of Concrete, Phase II

Phase II of the study was conducted primarily to evaluate the effect of CCA on air entrainment dosage, and freeze-thaw durability. Several air entrained concrete mixtures were prepared with CCA and tested under ASTM C 666.

Task 5-Slump Retention Study, Phase III

An important aspect is the slump retention capabilities of concrete mixtures considering delivery time and ambient conditions. This portion of the study evaluated the slump retention or slump loss characteristics of limited conditions with the use of CCA.

Test Results

Using a large-capacity sieve shaker, CCA was separated into coarse and fine fractions on the 4.75-mm (No. 4) sieve. Coarse fraction (by volume) was 61 percent for the 1,000-psi CCA, and about 70 percent for the other two CCAs made for this project. In comparison, Pile 1 CCA gave a very low coarse fraction of 47 percent. Two possible reasons for this were surmised upon discussing this with the concrete producer:

  1. It is likely that the returned concrete in the normal practice had higher water content due to retempering prior to discharge.

  2. It is likely that the returned concrete was disturbed and arranged in rows (but not crushed into CCA) the next day. Both of these steps can make the resulting CCA weaker and help explain the lower amount of Coarse CCA in Pile 1.

Once the CCA was separated into coarse and fine CCA fractions with the help of a large-capacity sieve shaker, the Coarse fraction (which was in four different sieve sizes) was recombined in a 3.5-cu.-ft. concrete mixer for about 15 minutes to make it homogeneous. This portion was used for all the aggregate tests for the ìCoarse Fraction,î whereas all the material passing the No. 4 (4.75-mm) sieve was used for the aggregate tests for the ìFine Fraction.î The following aggregate tests were conducted:

  • ASTM C127-04 Specific Gravity, Absorption of Coarse Aggregate, three samples
  • ASTM C128-04a Specific Gravity, and Absorption of Fine Aggregate, three samples
  • ASTM C136-05 Sieve Analysis of Fine and Coarse Aggregates, three samples
  • ASTM C117-04 Materials Finer than 75-m (No. 200) Sieve, three samples
  • ASTM C29/C29M-97(2003) Unit Weight and Voids in Aggregate, three samples
  • ASTM C131-03 LA Abrasion, three samples
  • ASTM C40-04 Organic Impurities in Fine Aggregates for Concrete, three samples
  • ASTM C1252-03 Uncompacted Void Content of Fine Aggregate, three samples
  • ASTM C88-05 Sodium Sulfate Soundness, two samples
  • ASTM D2419-02 Sand Equivalent Value of Soils and Fine Aggregate, three samples

While the control aggregates and the 1,000-psi and 3,000-psi CCA were tested for all properties, the 5,000-psi and Pile 1 CCA were tested only for those properties that are essential for establishing concrete mixture proportions.

Findings

The main findings in this research study can be summarized as follows:

  1. Use of CCA significantly benefits sustainable development by reducing the necessity of landfilling returned concrete and conserves the use of increasingly scarce good-quality virgin aggregate. Use of CCA can also potentially help reduce $300 million in annual operator costs by the U.S. ready mixed concrete industry

  2. A detailed literature search and bibliography on the effect of recycled concrete aggregate on concrete performance has been conducted as part of this study. Most of the literature is related to the use of crushed concrete from existing structures and not of crushed concrete from returned concrete that was the main focus of this project.

  3. Compared to virgin aggregate, CCA has lower specific gravity, higher absorption, higher percentage of minus 200 fines, and lower aggregate weathering potential as measured by the sulfate soundness test. Both the coarse and fine fraction of CCA meet most of the ASTM C 33 requirements for aggregates. However, not all CCA (particularly the finer fraction of CCA) meet the percentage of minus 200 fines, and sulfate soundness test. ASTM C 33 permits the use of CCA in concrete.

  4. Mixing water content of concrete containing CCA was not substantially different from that of concrete containing virgin aggregates. However, concrete containing 100 percent coarse Pile 1 CCA, representing crushed concrete from a concrete plant with no control, had much higher mixing water content.

  5. The compressive strength and elastic modulus of concrete containing CCA is lower than that of the control concrete. However, 1) the decrease in strength is not substantial, and the strength drop can be compensated for by normal mixture adjustments to achieve the desired strength; and, 2) concrete containing 100 percent coarse Pile 1 CCA had significantly lower strengths.

  6. The three concrete mixtures that were repeated on a different day showed that the batching, mixing and testing are repeatable.

  7. The addition of CCA tends to increase the average length change due to drying shrinkage slightly. The use of large amounts of Coarse CCA increased the RCP values.

  8. The use of 600 lb./cu. yd. of ìas receivedî CCA reduced the concrete's freeze-thaw durability. However, the use of 100 percent coarse 3,000-psi CCA did not reduce freeze-thaw durability, even though it did increase surface scaling of the test specimens. The use of 3,000-psi 100 percent coarse CCA to replace virgin coarse aggregate should be admissible even in concrete applications that are exposed to freeze-thaw environments. However, concrete containing CCA in the ìas receivedî condition should be evaluated for freeze-thaw resistance prior to its use.

  9. If CCA is used in the ìas receivedî condition, slump loss due to the fine fraction of the CCA tends to be an issue. When coarse CCA is used, slump loss is negligible, particularly if the CCA is kept in a moist condition prior to batching.

  10. The pressure meter (C 231) is adequate to measure the air content of concrete containing CCA accurately. If deemed necessary, comparative testing with C 231 and C 173 can be conducted; and, if the results agree, then C 231 can be continued to be used.

  11. The use of 20 percent of crushed coarse concrete aggregates in structural concrete is now a practice accepted by codes in many European countries.

  12. Based on the results of this study, the use of ìas receivedî CCA up to 10 percent by weight of the total aggregate should be permitted in most concrete applications. The concrete produced should still meet all the performance requirements for that application. In light of the European experience, for structural concrete applications, coarse CCA should be allowed to be used at 10 percent by weight of total aggregate. Greater amounts of CCA could be allowed in nonstructural applications, provided the concrete producer does the processing requirements (using >3,000-psi returned concrete to make CCA or just using the coarse fraction of CCA, for example). For increased acceptance of CCA, it is suggested that the ASTM C 94 Standard Specification for Ready Mixed Concrete include these provisions

  13. Cost calculations suggest that the concrete producer can achieve considerable savings by using CCA from reduced use of virgin materials and reduced disposal costs. The concrete producer should test the concrete containing CCA for a wide range of properties that are important for the application. If CCA will be used, the producer should adopt quality-control measures while producing the CCA. The CCA pile should be kept moist, as the CCA should ideally be maintained at a level greater than the saturated surface dry condition. CCA characterization studies, such as absorption and relative density (specific gravity), are recommended on a weekly frequency.

TABLE 1-MIXTURE PROPORTIONS, TEST RESULTS OF VIRGIN-AGGREGATE CONCRETE FROM WHICH CCA WAS PREPARED
1,000 psi 3,000 psi 5,000 psi
MATERIAL, LB./CU. YD.
Cement 282 423 600
Fine aggregate 1,625 1,645 1,453
Coarse aggregate (No. 57) 1,800 1,800 1,850
Water 267 283 283
Type A water reducer (oz/cwt.) 3.0 3.0 3.0
Vol. of paste ( incl. air) / Vol. of aggregate,% 31 39 43
Specific gravity of paste (calculated) 1.43 1.45 1.74
FRESH CONCRETE PROPERTIES
Slump, in. 5.25 4.5 4.75
Air, % 1.7 4.2 2.0
Temperature, ∞F 75 74 76
Density, lb./cu. ft. 147.4 146.6 151.7
HARDENED CONCRETE PROPERTIES COMPRESSIVE STRENGTH, PSI (LAB CURE)
7 days 810 2,290 4,950
28 days 1,330 3,200 6,800
56 days 1,640 3,390 7,410
COMPRESSIVE STRENGTH, PSI (FIELD CURE)
7 days 610 1,990 4,160
28 days 890 2,530 5,260
56 days 1,320 3,630 6,480
117 days 1,320 3,800 7,630
All strengths are average of two cylinders. Concrete was crushed at 110 days and CCA prepared.

This article was adapted from the RMC Research & Education Foundation-funded report ìCrushed Returned Concrete as Aggregates for New Concreteî (September 2007), prepared by National Ready Mixed Concrete Association's Karthik Obla, managing director, research and materials engineering; Haejin Kim, concrete research lab manager; and, Colin Lobo, senior vice president, engineering. The authors thank Soliman Ben-Barka and Stuart Sherman who conducted the test program at the NRMCA Research Laboratory, plus Godwin Amekuedi, Teck Chua, and David Imse for report review.

Full copies of the free report are available at www.rmc-foundation.org