Properties of Recycled Concrete Aggregates for Airfield Rigid Pavements

Recycled concrete aggregate (RCA) was investigated as a partial and full replacement of coarse virgin aggregate and partial replacement of fine virgin aggregate in concrete for airfield pavement applications. All mixtures explored used a total cementitious content of 517 lb/yd3 and a water-to-cementitious ratio of 0.42. In addition, mixes were cast to determine the effect of supplementary cementitious materials (fly ash, silica fume, and slag) as well as chemical admixtures (set retarder and superplasticizer) on the workability and hardened properties (i.e. strength, shrinkage, and durability) of RCA concrete. Two mixing procedures were studied, the normal laboratory mixing procedure (NMP) and a two-stage mixing approach (TSMA), which initially coats the RCA in a cement slurry to improve the initial workability and interfacial transition zone between the RCA and paste. This laboratory study was divided into five stages. The first stage investigated RCA as a partial and full replacement of virgin coarse aggregate to evaluate the effect on the workability, concrete strength, and fracture properties. With the implementation of the TSMA, the RCA concrete provided an equal or higher slump relative to the virgin aggregate. Concrete containing various replacement levels of RCA under TSMA had compressive strengths statistically similar to virgin aggregate concrete with the NMP, even though the split tensile and fracture properties were reduced for the RCA concrete. At early ages the RCA concrete free shrinkage was slightly reduced relative to the virgin aggregate concrete (VAC), but at later ages 100% RCA resulted in greater shrinkage magnitude and a higher rate of shrinkage than the VAC. Stage I results verified previous research for roadways that crushed concrete aggregates even from airfield pavements can provide a viable option for new concrete airfield pavements. Due to the lower slump values noted with the RCA mixtures, the research in Stage II examined strategies to lengthen the working time of RCA mixtures and possibly ways to improve working time with higher air temperatures by increasing the concrete’s initial slump. As expected, set retarder and higher fly ash contents were found to increase the initial slump of the RCA concrete. The rate of slump loss was approximately the same for the various strategies including set retarders, Class C and F fly ash, and superplasticizers. Set retarders did not generally have an effect on the strength development unless higher percentage of fly ash replaced the cement. Overall as the fly ash content increased, the strength development was reduced even at 28 days. From the second stage, it was noticed that the RCA moisture condition significantly affected the initial slump. Stage III studied the effect of the mixing procedure (NMP versus TSMA) and initial aggregate moisture state on the virgin aggregate and RCA concrete properties. The three moisture conditions examined were saturated surface dry (SSD), partial saturation (80-85% SSD), and oven-dry. The TSMA resulted in a higher slump relative to the NMP for all initial aggregate moisture conditions. The TSMA resulted in a slightly greater mean compressive and split tensile strength for all initial aggregate conditions for both VAC and RCA concrete. VAC did produce a higher compressive and split tensile strength than RCA concrete when considering all factors analyzed. The mixing procedure didn’t significantly affect the measured free shrinkage. The mean free shrinkage was slightly greater for the RCA for all aggregate moisture states at 112 days but at earlier ages (<28 days) the RCA concrete had a lower free shrinkage. The fourth stage of the study investigated the effect of higher contents of supplementary cementitious materials (fly ash and slag) and the use of RCA fine aggregates (RFA) as a partial replacement of virgin fine aggregate. Using only the TSMA, as the RFA content increased so did the required dosage of superplasticizer or fly ash content to maintain a constant slump. A statistical analysis showed that the compressive, split tensile, and flexural strengths were affected by RFA content and amount of supplementary cementitious materials. In general, for concrete with RFA and higher supplementary cementitious materials contents, drying shrinkage strains were greater at 56 days including the rate of shrinkage relative to VAC and RCA concrete in Stage III. The final stage of mixtures incorporated coarse and fine RCA with air-entrained concrete and various replacement levels of supplementary cementitious materials. The permeability and freeze-thaw resistance of the RCA concrete was found to be adequate for concrete pavement and behaved similar to virgin aggregate concrete as long as sufficient curing time was allowed to develop strength. This research confirmed coarse and fine RCA can be successfully utilized as a full replacement of coarse aggregate and a partial replacement of fine aggregate in airfield concrete and the potential risks can be reduced by minimizing the cementitious content and utilizing a pozzolan as partial cement replacement. For airport applications, several precautionary steps must be taken to limit the workability and strength reductions and to minimize the expected higher shrinkage levels. The main benefit in the TSMA was found in its increase in the initial slump of the RCA concrete relative to the NMP with also a slight increase in strength. Increased pozzolanic replacement may be a strategy to also increase initial workability due to the expected loss in slump with RCA coarse and fine materials. Maintaining RCA aggregates near saturation levels will benefit the fresh and hardened properties relative to leaving the aggregates near an oven-dry condition especially when using a standard mixing procedure. Although shrinkage was measured to be similar to lower at earlier ages (