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[ Material Description ] - [ Portland Cement Concrete ] - [ Granular Base ] - [ Embankment or Fill ]

RECLAIMED CONCRETE MATERIALUser Guideline


Granular Base

INTRODUCTION

Recycled concrete aggregate (RCA) can be used as coarse and/or fine aggregate in granular base. This process saves valuable resources and currently 38 states allow RCA to be used as a granular base. (9,10,11) The properties of processed RCA generally exceed the minimum requirements for conventional granular aggregates. Being a 100 percent crushed material, processed RCA aggregates "lock up" well in granular base applications, providing good load transfer when placed on weaker subgrade. The lower compacted unit weight of RCA aggregates compared with conventional mineral aggregates results in higher yield (greater volume for the same weight), and is therefore economically attractive to contractors. For some reconstruction projects, on-site processing and recycling of RCA are likely to result in economic benefits through reduced aggregate hauling costs.

 

PERFORMANCE RECORD

 

RCA that has been properly processed and tested for appropriate specification compliance has been widely used and has generally demonstrated satisfactory performance in granular base applications. The use of processed RCA as aggregate in base or subbase applications has been accepted by many jurisdictions. Thirty-eight states presently use RCA as a granular subbase. (11)

Two highway agencies (Illinois and Pennsylvania) have specifications that directly address RCA use in granular base. (8) A number of states have conducted research and pilot projects using RCA as an aggregate for base course. These states include Texas , Virginia , Michigan , Minnesota , California. (11)

Some of the positive features of RCA aggregates in granular base applications include the ability to stabilize wet, soft, underlying soils at early construction stages, good durability, good bearing strength, and good drainage characteristics.

There is recent evidence that the use of some unsuitable or improperly processed RCA aggregate can adversely affect pavement subdrainage systems and pavement performance. (13) Tufa-like (white, powdery precipitate) precipitates have been reported by a number of agencies to have clogged subdrains and blinded geotextile filters. (13) The tufa precipitate appears to be Portlandite from unhydrated cement and/or calcium carbonate (CaCO3), formed by the chemical reaction of atmospheric carbon dioxide with the free lime (CaO) in the RCA. However, the problem is not universal, and many pavements with RCA granular base are reported to be functioning satisfactorily without any apparent tufa formation.

Studies in Minnesota conclude in order to reduce the potential for drainage problems it is recommended that; 1. the intentional inclusion of RCA fines (#4 minus) be eliminated in unstablized foundation layers, 2. drainage systems be designed to accommodated a limited quantity of crusher fines and insoluble bases, and 3. Open graded RCA be blended with new aggregates to provide gradations needed to improve the stability and density to reduce precipitate formation. (11)

When using RCA as an aggregate base it is important that the material be shaped minimally and the base not be excessively worked in order to prevent material segregation. It is also recommended that the base be compacted under saturated conditions to mitigate the migration of fines throughout the aggregate. In some cases RCA has preformed better than virgin aggregate, research is currently taking place to see if the increase base strength can be validated in the laboratory. (11)

 

MATERIAL PROCESSING REQUIREMENTS

 

Crushing and Screening

Following the initial crushing of concrete rubble in a jaw crusher and removal of any steel by magnetic separation, RCA must be crushed and screened to the desired gradation using conventional aggregate processing equipment.

Where the processed RCA contains some reclaimed asphalt pavement (RAP), which can occur when the RCM is derived from composite pavements, it is recommended that the RAP content in the RCA be limited to 20 percent maximum to prevent a reduction in bearing strength. (14)

Storage

Where RCM is available from different sources or concrete types, it should either be blended or maintained in separate stockpiles to ensure consistent material properties.

Washing

Washing of RCA aggregates is required by some agencies ( Ohio , for example) to remove the dust as a measure to reduce potential tufa formation. To control tufa precipitate formation, only suitable RCA that does not contain appreciable unhydrated cement or free lime should be used for granular base applications.

Testing

Additional quality control testing (leachate testing) to assess the tufa precipitate potential of RCA aggregates may be necessary for granular base applications where subdrains are involved. A special procedure to identify the potential for tufa formation in steel slags was developed, which should be appropriate for RCA testing. (12)

 

ENGINEERING PROPERTIES

 

Some of the engineering properties of RCM that are of particular interest when RCM is used as a granular base material include gradation, absorption, specific gravity, stability, strength, durability, and drainage.

Gradation : RCA must be crushed and screened to satisfy AASHTO M147 (2) and ASTM D2940 (7) requirements for aggregates.

Absorption : High absorption is particularly noticeable in crushed fine material (minus 4.75 mm (No. 4 sieve)) derived from air-entrained concrete and ranges between 4 and 8 percent (compared with 2 percent or less for virgin concrete aggregates). (1)

Specific Gravity : The specific gravity of RCA aggregates (ranging from 2.0 for fines to 2.5 for coarse particles) is slightly lower than that of virgin aggregates. (1)

Stability : RCA has high friction angle, typically in excess of 40 o and consequently demonstrates good stability and little postcompaction settlement.

Strength Characteristics : Processed RCA, being a 100 percent crushed material, is highly angular in shape. It exhibits California Bearing Ratio (CBR) values ranging from 90 to more than 140 (depending on the angularity of the virgin concrete aggregate and strength of the Portland cement matrix), which is comparable to crushed limestone aggregates. (17,18)

The inclusion of asphalt-coated particles in granular base material leads to reduced bearing capacity, varying with the proportion of asphalt-coated particles. Studies in Ontario , Canada , indicate that bearing strength is reduced below that expected for granular base (using natural aggregate) when the amount of blended asphalt coated particles exceeds 20 to 25 percent. (18)

Durability : RCA aggregates generally exhibit good durability with resistance to weathering and erosion. RCA is nonplastic, and is not susceptible to frost.

Drainage Characteristics : RCA (mainly coarse fraction) is free draining and is more permeable than conventional granular material because of lower fines content.

DESIGN CONSIDERATIONS

Processed RCA aggregates generally satisfy the requirements of AASHTO M147 (10) and ASTM D2940. (7) Processed RCA is covered by conventional granular aggregate specifications in a number of jurisdictions.

Standard AASHTO pavement structural design procedures can be employed for granular base containing RCA aggregates. It is recommended that the appropriate structural number for RCA aggregates should be established by resilient modulus testing.

When using RCA as a base they high angularity of the material combined with grading to the maximum density (as recommended) creates a very rigid base. In some cases this has led to reflection cracking in the new overlying pavement. It may be necessary to apply a layer between the rigid base and the pavement. This layer can be designed to interface the stiffness between the two layers. The layer may be an asphalt lift with a high asphalt cement content. (11)

 

CONSTRUCTION PROCEDURES

 

Material Handling and Storage

The same methods and equipment used to store or stockpile conventional aggregates are applicable for RCA. However, additional care is required in stockpiling and handling RCM aggregates to avoid segregation of coarse and fine RCA, such as stock pile watering.

Placing and Compacting

The same methods and equipment used to place and compact conventional aggregate can be used to place and compact RCA.

Quality Control

The same test procedures as used for conventional aggregate are appropriate for granular base applications when using RCA. Standard laboratory and field tests for compacted density and field measurement of compaction are given by AASHTO test methods T191 (5) , T205 (4) , T238 (3) and T239. (6)

Special Considerations

Where RCA aggregates are used in granular base course applications in conjunction with subdrains, the following procedures are recommended to reduce the likelihood of leachate precipitates clogging the drainage system: (1)

  • Wash the processed RCM aggregates to remove dust from the coarse particles.
  • Ensure that any geotextile fabric surrounding the drainage trenches (containing the subdrains) does not intersect the drainage path from the base course (to avoid potential plugging with fines).

ENVIRONMENTAL CONSIDERATIONS

For RCA, environmental considerations have focused on leachability of contaminants and pH changes from RCA storage and use. Previous research conducted on the leachability Portland cement concrete used the Toxicity Characteristic Leaching Procedure (15). Although leachability results were low (15), the TCLP simulates a municipal landfill setting and not a beneficial use environment, so results would not be applicable to environmental considerations for beneficial use. More recent research employed a serial batch test (Dutch Pre-Standard NVV 5432) (16). This research concluded that well-cured Portland cement concrete released no detectable concentrations of antimony, arsenic, beryllium, cadmium, chromium, lead, mercury, nickel and selenium (16). The internal alkaline nature of concrete is well known, but can change over time with weathering and age for numerous reasons (e.g., carbonation). RCA could also be alkaline, with potential pH values and changes similar to in-place concrete. Research conducted at Washington State University found that disposing of diamond grinding concrete slurry increased soil pH from 6.3 – 7.5 to 7.6 – 9.4 in once location and from 7.1 – 7.2 to 7.1 – 8.2 in a second location (19). Research conducted by the Ohio Department of Transportation and Iowa Department of Transportation found that the pH of RCA decreased little over time (was initially greater than 11 then decreased over time but remained above 9). The Ohio research concluded that using RCA as an aggregate base in low lying or wet areas where alkaline run-off would be likely to occur could have an adverse effect on the environment (20). The Iowa report found that the high pH of the drainage water from RCA use can kill or impede grass growth at a drain outlet (21). Texas has also completed research in using RCA in mechanically stabilized earth (MSE) berms that involved thorough material characterization, pH measurements and an evaluation of use (22,23). They concluded that pH and resistivity specifications for MSE wall backfill materials should be waived for crushed concrete, concrete structures that have suffered sulfate attack should not be crushed and used as backfill in MSE walls, and MSE walls with crushed concrete backfill should include adequate drains and high permittivity filter fabrics behind the wall to avoid drainage problems (23). The potential for a pH and drainage issues leads some jurisdictions to require that RCA stockpiles be separated (a minimum distance) from water courses.

UNRESOLVED ISSUES

 

Further investigation of the propensity for tufa formation of RCA aggregates in granular base is needed. This should also include the development of standard methods to assess the suitability of RCA aggregates for base course applications where subdrains are used. Currently research is focused on some of the testing methods for RCA. It is not clear whether or not standard testing methods for virgin aggregate is applicable to RCA.

 

REFERENCES

 
  1. ACPA. Concrete Paving Technology: Recycling Concrete Pavement, American Concrete Pavement Association, Illinois , 1993.
  2. American Association of State Highway and Transportation Officials. Standard Specification for Materials, "Aggregate and Soil-Aggregate Subbase, Base and Surface Courses," AASHTO Designation: M147-70 (1980), Part I Specifications, 14th Edition,1986
  3. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T238-86, Part II Tests, 14th Edition, 1986.
  4. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Rubber-Balloon Method," AASHTO Designation: T205-86, Part II Tests, 14th Edition, 1986.
  5. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Sand Cone Method," AASHTO Designation: T191-86, Part II Tests, 14th Edition, 1986.
  6. American Association of State Highway and Transportation Officials. Standard Method of Test, "Moisture Content of Soil and Soil Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T239-86, Part II Tests, 14th Edition, 1986.
  7. American Society for Testing and Materials. Standard Specification D2940-92, "Graded Aggregate Material for Bases and Subbases for Highways or Airports," Annual Book of ASTM Standards, Volume 4.03, West Conshohocken , Pennsylvania , 1996.
  8. Collins, R. J. and S. K. Cielieski. Recycling and Use of Waste Materials and By-Products in Highway Construction, National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington, 1994.
  9. ECCO. Recycling Concrete Saves Resources, Eliminates Dumping, Environmental Council of Concrete Orgainzations, 1997.
  10. ECCO. Recycling Concrete and Masonry, Environmental Council of Concrete Orgainzations, 1998.
  11. Gonzalez, G.P. and Moo-Young, H.K. Transportation Applications of Recycled Concrete Aggregate, FHWA State of the Practice National Review, September 2004.
  12. Gupta, J. D., W. A. Kneller, R. Tamirisa, and E. Skrzypczak-Jankun. "Characterization of Base and Subbase Iron and Steel Slag Aggregates Causing Deposition of Calcareous Tufa in Drains," Transportation Research Record No. 1434, 1994.
  13. Gupta, J. D. and W. A. Kneller. Precipitate Potential of Highway Subbase Aggregates, Report No. FHWA/OH-94/004 prepared for the Ohio Department of Transportation, November 1993.
  14. Hanks, A. J. and E. R. Magni. The Use of Recovered Bituminous and Concrete Materials in Granular Base and Earth, Ontario Ministry of Transportation Report MI-137, Downsview, Ontario, 1989.
  15. Kreitch, A.J. Leachability of Asphalt and Concrete Pavements, Heritage Research Group Report, Indianapolis , Indiana , March 1992.
  16. Hilliera, S.R., Sangha, C. M., Plunkettb, B. A., Waldena, P. J., Long-term leaching of toxic trace metals from Portland cement concrete, Cement and Concrete Research, Vol. 29, p. 515-521, 1999.
  17. Petrarca, R.W. and V.A. Galdiero, "Summary of Testing of Recycled Crushed Concrete," Transportation Research Board No. 989, pp. 19-26, Washington , D.C. , 1984.
  18. Senior, S. A. S. I. Szoke, and C. A. Rogers, " Ontario 's Experience with Reclaimed Materials for Use as Aggregates," Presented at the International Road Federation Conference, Calgary , Alberta , 1994.
  19. Shanmugam, Harini, Assessment and mitigation of potential environmental impacts of Portland Cement Concrete highway grindings, Thesis, Washington State University, 2004.
  20. Ohio Department of Transportation, Recycled Materials Report, Box Test.
  21. Steffes, R., Laboratory Study of the Leachate from Crushed Portland Cement Concrete Base Material, Iowa Department of Transportation, September 1999.
  22. Ellen M. Rathje, Alan F. Rauch, Kevin J. Folliard, David Trejo, Dallas Little, Chirayus Viyanant, Moses Ogalla, Michael Esfelle, Recycled Asphalt Pavement and Crushed Concrete Backfill: State-of-the-Art Review and Material Characterization, Center for Transportation Research at The University of Texas at Austin, Texas Department of Transportation, Octboer, 2001.
  23. Rathje, E., Rauch, A., Trejo, D., Folliard, K., Viyanant, C., Esfellar, M. Jain, A., Ogalla, M., Evaluation of Crushed Concrete and Recycled Asphalt Pavement as Backfill for Mechanically Stabilized Earth Walls, Center for Transportation Research at The University of Texas at Austin, Texas Department of Transportation, March 2006.

[ Material Description ] - [ Portland Cement Concrete ] - [ Granular Base ] - [ Embankment or Fill ]

Last Update 7/28/08