Recommended CitationVerian, K. P., N. M. Whiting, J. Olek, J. Jain, and M. B. Snyder. Using Recycled Concrete as Aggregate in Concrete Pavements to Reduce Materials Cost. Publication FHWA/IN/JTRP-2013/18. Joint Transportation Research Program, Indiana Department of Transportation and Purdue University, West Lafayette, Indiana, 2013. https://doi.org/10.5703/1288284315220
The main objective of this project was to evaluate the effects of using aggregate produced from crushed concrete pavement as a replacement for natural (virgin) coarse aggregate in pavement mixtures. A total of ten different concrete mixtures containing recycled concrete aggregate (RCA) were designed to meet the requirements of Indiana Department of Transportation (INDOT) specifications. These included three different RCA replacement levels (30%, 50% and 100% by weight of the natural coarse aggregate) and two different cementitious systems (plain system – Type I portland cement only and fly ash system – 80% of Type I portland cement and 20% of ASTM C 618 Class C fly ash). The scope of the project included the evaluation and comparison of several properties of RCA and natural aggregates, evaluation and analysis of the effects of RCA on concrete properties, and modification of aggregate gradations and mixture composition in an attempt to improve the properties of RCA concrete.
All ten mixtures were first produced in the laboratory (trial batches) and were subsequently reproduced in the commercial ready-mixed concrete plant. Each mixture produced in the ready-mixed plant was used to prepare several types of specimens for laboratory testing. The tests performed on fresh concrete included determination of slump and entrained air content. The mechanical properties of the hardened concrete were assessed by conducting compressive strength, flexural strength, modulus of elasticity and Poisson’s ratio tests.
Concrete durability was assessed using a wide array of measurements, including: rapid chloride permeability (RCP), rapid chloride migration (RCM), electrical impedance spectroscopy (EIS), surface resistivity, free shrinkage, water absorption test, freeze-thaw resistance and scaling resistance.
The test results indicated that the properties of plain (no fly ash) concrete mixtures with 30% RCA as coarse aggregate were very comparable to (in some cases even better than) those of the control concrete (0% RCA). Although mixtures with 50% RCA showed a reduction in durability and mechanical properties of up to 36%, the test results still met INDOT’s specifications requirements. The mechanical properties of plain concretes made with 100% RCA were measurably lower (16%-25%) than those of the control concrete. It should be pointed out, however, that these properties were still above the minimums required by INDOT’s specifications except for one mixture in which the w/c was increased to 0.47 to achieve workability. The use of fly ash improved the strength and durability of RCA concrete, especially at later ages. In particular, the properties of concrete with 50% RCA coarse aggregate were similar to the properties of control concrete. Similarly, the mechanical and durability properties of the mixture with 100% RCA coarse aggregate and 20% fly ash were better than those of a similar mixture prepared without fly ash. Even though, when compared to the fly ash concrete with 100% virgin aggregate the mechanical and durability properties of the 100% RCA concrete were up to 19% and 35% lower, it still met minimum requirements imposed by INDOT’s specifications.
Once the testing of the original ten types of concrete mixtures was completed, six additional concrete mixtures were developed and produced in the laboratory using aggregate with a modified gradation (with respect to the gradation of the aggregates used in the original mixtures). These mixtures were used to study whether the virgin and RCA aggregates can be used in different proportions to produce an “optimized blend” which will improve one (or more) of the concrete characteristics. The test results obtained from the six additional mixtures indicated that modifying the aggregate gradation did not have beneficial effects with respect to either compressive or flexural strength values. This failure to improve concrete strength with modified aggregate gradation may have been due, at least in part, to the quality of the source of aggregate that was used to modify the gradation.
Considering the limited scope of this study (only one source of RCA and two natural aggregate sources were used), it is recommended that the amount of RCA coarse aggregate be limited to 30% in plain concrete and to 50% in fly ash concrete to ensure the adequate quality of the pavement concrete.
recycled concrete aggregate (RCA), pavement concrete, compressive strength, flexural strength, durability, rapid chloride permeability, electrical impedance spectroscopy, shrinkage, fly ash
Joint Transportation Research Program
Indiana Department of Transportation
West Lafayette, Indiana
Date of this Version