The Development of a Performance Specification for Granular Base and Subbase Material

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CAIT project no.: 83 RU914

Fiscal Year: 1999/2000

Status: Final

Rutgers-CAIT Author(s): Thomas Bennert, Ali Maher

External Author(s): Anthony Chmiel



This research project encompassed evaluating the performance of New Jersey Department of Transportation (NJDOT)-specified aggregates at the respective NJDOT gradation ranges (high-end, middle, and low-end) and providing guidance as how to modify the gradation ranges to provide better performance in the field.

Currently, NJDOT specifies the use of granular materials by gradation only. However, it is well known that the gradation of granular materials has a dramatic impact on its performance. Therefore, base and subbase materials were sampled from three regions in the state and evaluated under the following performance tests: permeability (falling and constant head conditions), triaxial shear strength, cyclic triaxial loading, California Bearing Ratio (CBR) and resilient modulus. Testing was also conducted on recycled asphalt pavement (RAP) and recycled concrete (RCA) to evaluate their potential use as base and subbase materials. Materials were tested at their respective natural gradations and at manufactured gradations which represented the NJDOT high, middle, and low areas of the gradation specification in order to provide guidance as how to better refine the current NJDOT gradation specification.

Testing concluded that the gradation has an impact on each material and source tested. On average, permeability increased with increasing coarse fraction and decreasing percent fines. The triaxial strength increased as coarse fraction increased; however, the permanent deformation measured from the cyclic triaxial test indicated that at the gap-graded high end of the gradation band, instability was prevalent for the rounded subbase aggregates. This is most likely due to rounded aggregate particles not interlocking during loading (The gradation of this type of material is very similar to the non-stabilized open graded base layer that the NJDOT has used in the past). The resilient modulus testing followed a similar trend.

Overall, the closer the aggregate gradation was to the middle/high side of the NJDOT gradation specification, the better the performance. The testing of the RAP, RCA, and their blends with the base material, showed that as the percentage of RAP increased in the blend, both the CBR value and permeability decreased. RAP also caused larger permanent deformations during the cyclic triaxial testing. The inclusion of RCA provided the largest CBR, largest resilient modulus, and lowest permanent deformation values. However, as the percentage of RCA increased, the blend’s permeability decreased.