An alternative approach to testing of moisture susceptibility of recycled pavement materials

Abstract

Recycling is one of the most sustainable methods of pavement construction. However, moisture entering through thin HMA surface layer(s) overlying recycled pavements has often caused their premature deterioration. This study was conducted to develop an alternative laboratory moisture conditioning and testing procedure for recycled layers, and recommend tests and structure to enhance the resistance against moisture damage. The study developed a unique way to utilize a dynamic moisture conditioning process to obtain a more realistic evaluation of the moisture resistance of recycled layers than the currently used static immersion tests. Instead of using samples of only recycled (Full Depth Reclaimed, FDR, with asphalt emulsion) layers, the study evaluated composite samples that are more representative of actual pavement structures. Composite samples were prepared by attaching HMA samples on top of FDR samples with tack coat. The samples were tested for permeability and modulus using Ultrasonic Pulse Velocity test, a nondestructive test (NDT) test. The samples were subjected to 2,000 cycles of the Moisture Induced Stress Test (MiST) and then retested for modulus. The conditioning process was able to simulate moisture damage in the composite samples, and the nondestructive test was able to detect moisture-induced damage. The samples with two HMA layers showed significantly lower loss (28%) in modulus than those with only one HMA layer with the same thickness (72%). Two layers of tack coat for the samples with the two HMA layers were more effective in reducing the ingress of water through the HMA layers into the FDR layer than the one layer of tack coat for one HMA layer samples. Use of composite samples and NDT, and the dynamic moisture conditioning process (MiST) are recommended for the design of recycled layers. The use of two separate HMA layers with intervening two tack coat layers instead of a single HMA layer with a single tack coat is recommended for the enhancement of moisture resistance of recycled pavements.