Experimental and numerical investigation of moisture variations in unbound pavements with sprayed seals during drying and wetting

Abstract

Unbound pavements with sprayed seals are one of the major types of flexible pavements used worldwide. Sprayed sealing can be considered as an economical surfacing method in particularly Australia and New Zealand due to the low initial cost. However, moisture can enter the underlying pavement layers through sprayed seals as they are variably permeable, resulting in significant performance losses in underlying pavement structural layers. Hence, an in-depth understanding of how moisture varies in the base layer under different sprayed seals is essential for designing and maintaining pavement to meet the required long-term performance.This paper investigates moisture variations in unbound base layers under typical sprayed seals using experimental and numerical techniques. Pavement layers were replicated in the laboratory in columns with three different sealing conditions such as unsealed, 10 mm single-single sealed and 14 mm single-single sealed. The moisture variations in the base layers were monitored under drying and wetting conditions using moisture sensors over several months with targeted experiments. Moreover, this study characterises two typical types of sprayed seal samples collected from operating road surfaces using X-ray tomography and laboratory experiments. The experimental results show that typical single-single sprayed seals exhibit porosity in the range of 5% to 8%, leading to saturated permeability from 10−6 to 10−7 m/s. The numerical simulations show that significant drying and wetting of the base layer occurs when the pavement operates under actual climatic conditions. In addition, this study reveals that for a given seal condition, it is easier for moisture to enter the unbound base than drying out of the base layer once the moisture has entered. However, the drying rate depends on the saturation condition of the base layer before drying begins.