Research Problem and Background
As noted in the RFP for NJDOT project 2009?05, diminishing virgin aggregate supplies make it in the best interest of both NJDOT and the concrete industry to consider the use of recycled concrete as aggregate for new PCC. For a number of years now sources of local aggregates in Southern New Jersey have been scarce leading to increased costs from the longer distances required to transport aggregates. The use of recycled concrete could provide an additional source of aggregates, conserve the resources that are available, reduce the costs associated with disposal of construction materials, and conserve the limited landfill space in the region. Other states have found that RCA provides engineering, economic and environmental benefits1. However, recycled concrete aggregates (RCA) must meet the quality needs of NJDOT if they are to be used in PCC. While all but 12 states currently (as of 2005) allow the use of recycled concrete aggregates in base aggregate, only 11 states currently permit its use in PCC1. Some states also allow its use in HMA and as miscellaneous aggregate.
RCA is produced through the crushing of concrete pavements or other waste concrete after removal of any reinforcing steel. Required gradations are produced through crushing and screening in much the same way aggregates are produced from virgin materials. To be used as aggregate, RCA requires most of the same tests performed on virgin aggregates. However, important distinctions between RCA and virgin aggregates have led state transportation officials in New Jersey and much of the rest of the country to be cautious in the adopting RCA as an acceptable aggregate for PCC. The presence of cement paste or mortar adhering to the recycled aggregates reduces density, increases porosity, and increases drying shrinkage of PCC. Additional concerns include problems with the quality of the original concrete being recycled, and the presence of contaminants. The two primary sources of stockpiled RCA for use in PCC are Class B recyclers who accept and crush concrete waste and precasters who have an interest in crushing and reusing surplus material from the end of a production run and from damaged cast pieces. These varied sources suggest there will be a greater variety in properties both within a given a stockpile source and between the various sources. These are some of the issues that must be addressed before RCA is implemented in NJDOT projects. A preliminary literature review has been provided as a part of this proposal and expansion of this work is an important component of this project.
The proposed research will identify benefits and barriers to the use of recycled concrete aggregates as aggregate in PCC through literature reviews and discussions with state transportation agencies currently using this material. Because there is an extensive body of literature already available (discussed further in the research plan), it is anticipated that laboratory experimentation will be limited. The purpose of any experimental laboratory work will be to validate the RCA product available in the region is comparable to that used in previous studies so that these prior results may be extrapolated with confidence. Laboratory work will also be used to determine the consistency of RCA stockpiles throughout the region, looking at both variability between sources and variability within a stockpile. The outcome of the research project will be documentation of best practices and an implementable guide specification and quality control procedures for the use of RCA in concrete or a recommendation not to use the material.
The proposers believe that an extensive field installation plan attempting to address the many issues or concerns about the use of RCA in PCC is likely to lead to an unnecessarily long and open?ended research project. For this reason, it is proposed that field installations included as part of this work be limited to lower risk applications that will allow the specifications and recommendations to be tested in a controlled manner. The results of these initial field installations could then be used to inform future research efforts if needed under new contracts to either expand the types of applications or address issues that arise in the initial field trials.