Evaluation of Use of Crushed Hydraulic Cement Concrete (CHCC) as an Additive to Base Course/Subbase Material

About the report:

All new pavement systems constructed under the jurisdiction of the Virginia Department of Transportation (VDOT) require drainage systems. These drainage systems are constructed with perforated pipe and coarse aggregate that surrounds the pipe. Additionally, to separate the coarse aggregate from the base course and to create a filter, the drainage system also contains nonwoven geotextile that surrounds the coarse aggregate. Currently, the use of crushed hydraulic cement concrete (CHCC) as base course in new pavement systems is not allowed in VDOT projects because of the concerns associated with CHCC potentially clogging the geotextile of the drainage system to an extent that the serviceability of the filtration system is compromised. The basis of this concern is primarily due to the limited literature from the research conducted around the late 1990s. However, these studies are often inadequate in details and also present results that are contradictory and more importantly are not based on the specific geotextile used by VDOT. Recently due to the awareness of sustainable practices, there is interest throughout the U.S. to recycle CHCC and use it as base course aggregate. If allowed, this practice would result in the CHCC or blends of CHCC/virgin aggregate (V.A.) to be in direct contact with the geotextile.

Several of the VDOT district material engineers developed a research needs statement (RNS) to investigate the VDOT provision that limits the use of CHCC. The research described in this report was conducted based on that RNS to evaluate the suitability of placing CHCC produced with a gradation similar to the Virginia 21 base course aggregate against the specific nonwoven geotextile allowed in Virginia roadways for the underdrain systems. The research described herein was primarily performed by George Mason University’s Sustainable Geotransportation Infrastructure research group. The reduction in serviceability of geotextile, when placed adjacent to CHCC and blends of CHCC and V.A., were evaluated based on the gradient ratio (GR) test.  The effects of the stockpiling of CHCC before used as base course in a pavement system as it relates to changing the chemistry of CHCC were also evaluated.

The results of this study show that stockpiling CHCC does not appear to create adverse effects as it relates to using CHCC as base course adjacent to geotextile. The chemical activity of CHCC stabilizes and the potential for leaching of calcareous constituents that may precipitate in the underdrain structure appears to decrease with increasing age of stockpiled CHCC. The precipitation of calcareous tufa from CHCC leachate due to the high release of alkali metals under favorable conditions is always a possibility. As such, the potential for precipitation is also shown and characterized using geochemical analyses in this study. Based on the findings, the calcareous precipitate potential appears to be significantly reduced by blending CHCC with V.A. When tested with base material having 9% fines or less, the serviceability of the geotextile used in this study did not decrease to levels such that the system flow was impacted. However, when the fines content exceeded 9%, major reductions in aggregate/geotextile system permeabilities were observed. Therefore, the fines content of the material placed over the geotextile used in the edge drain should be limited to 9% as determined from the particles passing No. 200 U.S. sieve size. It is recommended that the findings of this study be evaluated in the field before implementing in highway construction.

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