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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Reclaimed concrete material (RCM) can be used as coarse and/or fine aggregate in granular base. The properties of processed RCM generally exceed the minimum requirements for conventional granular aggregates. Being a 100 percent crushed material, processed RCM aggregates "lock up" well in granular base applications, providing good load transfer when placed on weaker subgrade. The lower compacted unit weight of RCM aggregates compared with conventional mineral aggregates results in higher yield (greater volume for the same weight), and is therefore economically attractive to contractors. For large reconstruction projects, on-site processing and recycling of RCM are likely to result in economic benefits through reduced aggregate hauling costs.
RCM 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 RCM as aggregate in base or subbase applications has been accepted by many jurisdictions. Twenty states presently use RCM. They include Arizona, California, Colorado, Florida, Indiana, Iowa, Louisiana, Maryland, Massachusetts, Minnesota, Missouri, Nebraska, New Jersey, New York, North Dakota, Ohio, Pennsylvania, Rhode Island, South Carolina, and Texas.
Two highway agencies (Illinois and Pennsylvania) have specifications that directly address RCM use in granular base.(1) A number of states are conducting or have proposed research into the use of RCM as aggregate for granular base course. They include Arizona, Iowa, Louisiana, Michigan, Missouri, and Nebraska.
Some of the positive features of RCM 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 RCM aggregate can adversely affect pavement subdrainage systems and pavement performance.(2) Tufa-like (white, powdery precipitate) precipitates have been reported by a number of agencies to have clogged subdrains and blinded geotextile filters.(2) 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 RCM. However, the problem is not universal, and many pavements with RCM granular base are reported to be functioning satisfactorily without any apparent tufa formation.
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, RCM must be crushed and screened to the desired gradation using conventional aggregate processing equipment.
Where the processed RCM 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 RCM be limited to 20 percent maximum to prevent a reduction in bearing strength.(3)
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 of RCM 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 RCM that does not contain appreciable unhydrated cement or free lime should be used for granular base applications.
Additional quality control testing (leachate testing) to assess the tufa precipitate potential of RCM 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 RCM testing.(4)
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: RCM must be crushed and screened to satisfy AASHTO M147(5) and ASTM D2940(6) 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).(7)
Specific Gravity: The specific gravity of RCM aggregates (ranging from 2.0 for fines to 2.5 for coarse particles) is slightly lower than that of virgin aggregates.(7)
Stability: RCM has high friction angle, typically in excess of 40o and consequently demonstrates good stability and little postcompaction settlement.
Strength Characteristics: Processed RCM, 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.(8,9)
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.(8)
Durability: RCM aggregates generally exhibit good durability with resistance to weathering and erosion. RCM is nonplastic, and is not susceptible to frost.
Drainage Characteristics: RCM (mainly coarse fraction) is free draining and is more permeable than conventional granular material because of lower fines content.
Processed RCM aggregates generally satisfy the requirements of AASHTO M147(5) and ASTM D2940.(6) Processed RCM 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 RCM aggregates. It is recommended that the appropriate structural number for RCM aggregates should be established by resilient modulus testing.
Material Handling and Storage
The same methods and equipment used to store or stockpile conventional aggregates are applicable for RCM. However, additional care is required in stockpiling and handling RCM aggregates to avoid segregation of coarse and fine RCM.
Placing and Compacting
The same methods and equipment used to place and compact conventional aggregate can be used to place and compact RCM.
The same test procedures as used for conventional aggregate are appropriate for granular base applications when using RCM. Standard laboratory and field tests for compacted density and field measurement of compaction are given by AASHTO test methods T191(10), T205(11), T238(12) and T239.(13)
Although there do not appear to be any environmental problems associated with leachate from RCM,(14,15) some jurisdictions require that stockpiles be separated (a minimum distance) from water courses because of the alkaline nature of RCM leachate.
Where RCM 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:(7)
Further investigation of the propensity for tufa formation of RCM aggregates in granular base is needed. This should also include the development of standard methods to assess the suitability of RCM aggregates for base course applications where subdrains are used.
TRT Terms: Waste products as road materials--Handbooks, manuals, etc, Pavements, Asphalt concrete--Design and construction--Handbooks, manuals, etc, Pavements, Concrete--Design and construction--Handbooks, manuals, etc, Pavements--Additives--Handbooks, manuals, etc, Fills (Earthwork)--Design and construction--Handbooks, manuals, etc, Roads--Base courses--Design and construction--Handbooks, manuals, etc, Wastes, Environmental impacts, Recycling