Recycled Aggregate Summary Report

MnDOT study summarizes the current research and available literature on recycled aggregates By Gregory J. Schaertl and Tuncer B. Edil

The production of demolition and construction waste has been increasing at a gradual rate in recent years.1 The amount of landfill available to contain this material has been decreasing, and the need to find appropriate disposal locations has been of increasing concern.2 Recycling programs offer a viable solution. The use of these materials as recycled base course in new roadway construction has become more common in the last 20 years, with some municipalities reporting as much as 400,000 tons of recycled materials used in this manner.3,4

Recycled roadway materials are typically generated and reused at the same construction site, providing increased savings in both money and time.3 It has been speculated that in some municipalities recycled materials costs less to use than conventional crushed-stone base material by as much as 30%.5 Despite the increased acceptance of recycled base materials, research concerning the mechanical properties and durability of such materials has been lacking.3,6

The most widely used recycled materials are recycled asphalt pavement (RAP) and recycled concrete aggregate (RCA). RAP is produced by removing and reprocessing existing asphalt pavement,6,7 and RCA is the product of the demolition of concrete structures such as buildings, roads and runways.2 The production of RAP and RCA results in an aggregate that is well graded and of high quality.7 The aggregates in RAP are coated with asphalt cement that reduces the water absorption qualities of the material.6 In contrast, the aggregates in RCA are coated with a cementitious paste that increases the water absorption qualities of the material.1

Production
There is some ambiguity regarding the nomenclature involved in the production of RAP. Based on the experience of the Geo Engineering Program at the University of Wisconsin-Madison, the following classification is recommended to remove ambiguity in nomenclature: RAP refers to the removal and reuse of the hot mix asphalt (HMA) layer of an existing roadway7; full depth reclamation (FDR) refers to the removal and reuse of the HMA and the entire base course layer; and recycled pavement material (RPM) refers to the removal and reuse of either the HMA and part of the base course layer or the HMA, the entire base course layer and part of the underlying subgrade implying a mixture of pavement layer materials.6 Unless specified, these three distinct recycled asphalt materials will be collectively referred to as RAP.

RAP is typically produced through milling operations, which involves the grinding and collection of the existing HMA7, and FDR and RPM are typically excavated using full-size reclaimers or portable asphalt recycling machines.6 RAP can be stockpiled, but is most frequently reused immediately after processing at the site. Typical aggregate gradations of RAP are achieved through pulverization of the material, which is typically performed with a rubber tired grinder.8

The production of RCA involves crushing the material to a gradation comparable to that of typical roadway base aggregate. Fresh RCA typically contains a high amount of debris and reinforcing steel, and the RCA must be processed to remove this debris prior to placement. The material is first crushed in a jaw crusher that breaks the steel from the material and provides an initial crushing of the concrete.7 The material is sent down a picking belt where the steel is removed from the material.2 The remaining concrete material is further crushed and screened to a predetermined gradation.7

Material Properties
The gradation of RAP can be compared to that of a crushed natural aggregate, although with a higher content of fines. The high fine content is the result of degradation of the material during milling and crushing operations. In RPM, the inclusion of subgrade materials in the recycled material also contributes to a higher instance of fines. Finer gradations of RAP are produced through milling operations compared to crushing operations.7 A breakdown of typical physical and mechanical properties of RAP is shown in Table 1.

RCA is processed exclusively through crushing operations, and is very angular in shape.7 Depending on the crushing methods, the particle size distribution of an RCA can have a wide variability, with a lower particle density and greater angularity than would normally be found in more traditional virgin base course aggregates. Residual mortar and cement paste are typically found on the surface of the RCA, as well as contaminants associated with construction and demolition debris.2 The presence of this mortar contributes to a rougher surface texture, lower specific gravity, and higher water absorption than typical aggregates.7

The self-cementing capabilities of RCA are an interesting secondary property. The crushed material exposes un-hydrated concrete that can react with water, potentially increasing the materials strength and durability when used as unbound base course for new roadway construction. It follows that service life could also be extended as a result of these properties. Although widely acknowledged, not much actual documentation has been published regarding this secondary hydration.5 Although the cause of self-cementing properties has been studied, the actual effect of such parameters as age, grade, and mix-proportions of the RCA on the overall cementitious effect has yet to be determined.1 This effect is outside the scope of this literature review. A breakdown of typical physical and mechanical properties of RCA is shown in Table 2.

Objective
The purpose of this literature review is to summarize the current state of knowledge regarding the mechanical behavior of RCA, RAP and blends of these recycled materials with traditional aggregate material. Laboratory and field investigations were considered in the scope of this review, and long-term performance issues were noted. Of particular interest was the effect the recycled material had on resilient modulus values, stress state sensitivity, and overall material degradation.

Methods for Specification
When considering a recycled material for use as an unbound base course, the two most commonly used specifications are the gradation and the moisture-density relationship of the material. The gradation of a material can provide an indication of what the permeability, frost susceptibility, and shear strength of the material might be, and is determined through the use of material screening tests.9 Screening tests are typically conducted through sieve analysis according to ASTM Standards C 117 and C 136, and AASHTO Standards T-27 and T-11. Some highway agencies and DOTs use their own screening test methods, such as Florida DOT FM1 T-027. Classification of soils is performed using the Unified Soil and AASHTO methods according to ASTM D 2487 and AASHTO M 145, respectively.

The determination of moisture-density relationships can help define the ideal density conditions that a material can achieve through compaction. Moisture-density relationships are established through compaction tests conducted according to the following standards: AASHTO T 99 Method C, AASHTO T-180 or ASTM D698, ASTM D1557. Depending on the compaction effort to be used in the field, compaction tests can be performed in standard or modified variations. The information is used to determine the optimum moisture content (OMC) and the maximum dry density (MDD) of a material.

Through testing of specimens prepared based on this data, material properties such as strength, stiffness and moisture susceptibility can be determined.6 Other aggregate classification methods involve the determination of the specific gravity, absorption and Atterberg limits of the soils. The specific gravity and absorption characteristics of a given recycled aggregate are determined using ASTM D 854, and Atterberg limits of recycled aggregates are assessed using ASTM D 4318, AASHTO T 89 and T 90.5,6

Summary of Material Gradation
Tables 3 through 5 represent the available estimated gradations of the RAP, RCA and RPM encountered in this literature review.

Tables 3 through 5 show the coefficient of variance of gradation for the RAP, RPM and RCA remains approximately 40% or lower for materials retained on the No. 8 sieve and larger. This trend continues for the RPM and RCA retained in the remaining finer sieves. However, it can be seen that for RAP aggregates finer than the No. 8 sieve, the coefficient of variance for the data noticeably increases. This is more than likely due to the large gradation values found in the sample Guthrie R1.6

If the data for this sample is removed, the resulting variances fall within the same variance. The sample Guthrie R1 was a composite taken at different locations with different equipment, and therefore the actual source for the erratic gradation of the material could not be determined.6 Gradation requirements for recycled materials vary from agency to agency. Unless indicated, the recycled materials referenced in this report passed the gradation requirements specified by the respective agencies.

Blankenagel et al5 performed gradations on material taken from demolition sources as well as from relatively new materials sampled from batch-plant overruns and haul-back material sources. Batch plant overruns refer to excess concrete produced at a batch plant but never delivered to a job site, and haul-back material refers to excess concrete delivered to a job site but returned to the batch plant. The haul-back material was found to have more medium and fine materials than the demolition material. Although Blankenagel recognizes the source of the gradation differences could be due to crushing operations, the most likely reason is probably related to the mechanical breakdown tendencies of the materials. The haul-back material would have a higher porosity and lower strength due to being more properly consolidated and cured, resulting in a greater degree of pulverization regardless of crushing techniques.

In the study conducted by Kuo2, gradations of the RCA met Florida DOT specifications. However, for specifications regarding average gradation for each sieve, the standard deviations of the ¾-in., 3/8-in., No. 4 and No. 10 sieves were all excessively high and each fell out of specification. The test would indicate that for recycled materials, these sieves might be considered more critical than the others.

Summary of Moisture-Density
The available moisture-density relationships for the RAP, RPM and RCA encountered in this literature review are shown in Tables 6 and 7. For various blends of RAP with pure aggregate, some trends were noted regarding the effect of RAP content on the MDD and OMC of a material. Guthrie et al found that an increase in RAP content led to a decrease in MDD and OMC values.6 The aggregates particles in the RAP were partially encased in asphalt, which decreased the specific gravity. It was further assumed that the partial asphalt coating reduced the aggregate water absorption potential and inter-particle friction, leading to a reduction in the required water to achieve MDD.

For various blends of RAP with pure aggregate, some trends were noted regarding the effect of RAP content on the MDD and OMC of a material. Guthrie et al found that an increase in RAP content led to a decrease in MDD and OMC values.6 The aggregates particles in the RAP were partially encased in asphalt, which decreased the specific gravity. It was further assumed that the partial asphalt coating reduced the aggregate water absorption potential and inter-particle friction, leading to a reduction in the required water to achieve MDD.

An interesting variation in the study by Kim et al14 was the use of a gyratory compaction test (GCT) instead of a proctor compaction test (PCT) to prepare RAP specimens. Comparisons with field density measurements indicated that MDD and OMC calculations determined from GCT methods were a better correlation than those determined by PCT testing. When compared to PCT results, GCT results showed a large change in MDD values and a small change in OMC values. Kim noted the effect of RAP content on the MDD and OMC of aggregate/RAP blends. As the RAP content of the material increased, the OMC of the material decreased for both the GCT and PCT prepared specimens. As with the study by Guthrie, the increase in asphalt content most likely reduced the absorption of the material, leading to the decrease in OMC. As the RAP content of the material increased, the MDD decreased for the PCT-prepared specimens and remained the same for GCT-prepared specimens.

Bennert et al3 investigated the effect of recycled content on the MDD and OMC of samples containing both RAP and RCA. The study found that as the RAP and RCA content of a material increased, the MDD of the material decreased. As was found in the Guthrie6 and Kim14 studies, the OMC of the material decreased with increasing RAP content. However, as the RCA content of the material increased, the OMC also increased.

In the study conducted by Saeed et al9, it was found that in general virgin aggregates had a higher MDD than pure (100%) RAP and RCA samples. In agreement with the study by Kim14, the MDD of the material decreased as the RAP and RCA content of recycled material/aggregate mixtures increased.

Blankenagel et al5 noted the effect of material source on the MDD and OMC of RCA. The demolition material used in his study had an OMC of 9.7% and a MDD of 1,830 kg/m3, whereas the haul-back material had an OMC of 10.6% and a MDD of 2,020 kg/m3. The haul-back material had a higher fines content, which resulted in higher MDD and OMC values than those found in the demolition material. Pore spaces are more readily filled by the increased fines, resulting in a tighter aggregate matrix. Investigations11,13 on two RPM at the University of Wisconsin-Madison indicated an OMC of 6.5 to 7.5% and a MDD of 2,162 kg/m3.

Next Issue: Methods for Design and Performance Tests.

Authors
Tuncer B. Edil is a professor and Greg Schaertl a graduate student at the University of Wisconsin-Madison. Dr. Edil can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

References
1. Poon, C. S., Qiao, X. C. and Chan, D. X. (2006). “The Cause and Influence of Self-Cementing Properties of Fine Recycled Concrete Aggregates on the Properties of Unbound Sub-Base”, Waste Management, Vol. 26, No. 10, pp. 1166-1172
2. Kuo S. S., Mahgoub, H. S. and Nazef, A. (2002). “Investigation of Recycled Concrete Made with Limestone Aggregate for a Base Course in Flexible Pavement”, Geomaterials, No. 1787, pp. 99-108
3. Bennert, T., Papp Jr, W. J., Maher, A. and Gucunski, N. (2000). “Utilization of Construction and Demolition Debris Under Traffic-Type Loading in Base and Subbase Applications”, Transportation Research Record, No. 1714, pp. 33-39
4. Nataatmadja, A. and Tan, Y. L. (2001) “Resilient Response of Concrete Road Aggregates”, Journal of Transportation Engineering, Vol. 127, No. 5, pp 450-453 5. Blankenagel, B. J. and Guthrie, W. S. (2006). “Laboratory Characterization of Recycled Concrete for Use as Pavement Base Material”, Geomaterials, No. 1952, pp. 21-27
6. Guthrie, W. S., Cooley, D. and Eggett, D. L. (2007). “Effects of Reclaimed Asphalt Pavement on Mechanical Properties of Base Materials”, Transportation Research Record, No. 2006, pp. 44-5219
7. “User Guidelines for Byproducts and Secondary Use Materials in Pavement Construction.” FHWA Report FHWA-RD-97-148, Federal Highway Administration, McLean, Virginia (2008).
8. Bejarano, M.O., Harvey, J. T., Lane, L. (2003). “In-Situ Recycling of Asphalt Concrete as Base Material in California”, Proceedings of the 82nd Annual Meeting, Transportation Research Board, Washington D.C .CD-Rom, 22 pp.
9. Saeed, A. (2008). “Performance-Related Tests of Recycled Aggregates for Use in Unbound Pavement Layers”, NCHRP Report 598, Transportation Research Board, Washington, D.C., 53 pp.
10. Li, L., Benson, C. H., Edil, T. B., Hatipoglu, B., and Tastan, O. (2007). “Evaluation of Recycled Asphalt Pavement Material Stabilized with Fly Ash”, ASCE Geotechnical Special Publication, CD-Rom, 10 pp.
11. Carmargo, F., Wen, H., Edil, T. B., and Son, Y. H. (2009). “Laboratory Evaluation of Sustainable Materials at MnRoad”, Proceedings of the 88th Annual Meeting, Paper No. 09-3160, National Research Council, Washington D.C., CD-ROM.
12. Wen, H., and Edil, T. B. (2009). “Sustainable Reconstruction of Highways with In-Situ Reclamation of Materials Stabilized for Heavier Loads”, BCR2A Conference, Champaign, Illinois
13. Wen, H., Baugh, J., and Edil, T.B. (2007). “Use of Cementitious High Carbon Fly Ash to Stabilize Recycled Pavement Materials as a Pavement Base Material”, Proceedings of the 86th Annual Meeting, Paper No. 07-2051, National Research Council, Washington D.C., CD-ROM.
14. Kim, W., Labuz, J. F. and Dai, S. (2007). “Resilient Modulus of Base Course Containing Recycled Asphalt Pavement”, Transportation Research Record, No. 2006, pp. 27-35
15. Wen, H., Warner, J., and Edil, T. B., (2008). “Laboratory Comparison of Crushed Aggregate and Recycled Pavement Material with and without High Carbon Fly 20 Ash”, Proceedings of the 87th Annual Meeting, Paper No. 08-3075, National Research Council, Washington D.C., CD-ROM.
16. Scullion, T. and Saarenekto, T. (1997). “Using Suction and Dielectric Measurements as Performance Indicators for Aggregate Base Materials”, Transportation Research Record, No. 1577, pp. 37-44.

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