Full Scale Accelerated Testing of Bituminous Road Pavement Mixtures

Abstract

The in-service behaviour of a standard Irish Dense Base Coarse Macadam mixture (DBC) was evaluated by using the material to overlay a road section, which was based upon a weak pavement structure. The response of the layer under a fully laden dual axle truck was examined using a series of pressure cells and asphalt strain gauges that were embedded in the test section. The section was traversed repeatedly until a network of fatigue cracks was observed on the road surface. The transverse horizontal tensile strain was found to be the most critical parameter in initiating pavement damage. INTRODUCTION Full scale test facilities offer particular advantages over simulated laboratory programmes: the effects of size, construction, substructure, boundary and loading conditions correspond more closely to the actual on-site conditions than can be simulated directly with model experiments. However, it is seldom possible to control temperature and moisture when using such facilities and consequently there are always inherent disparities when attempting to relate laboratory and in-situ test data. Recent investigations have focussed on the mechanical performance of standard bituminous mixtures using uniaxial fatigue tests [1-3] and fatigue wheel tracking laboratory tests [4]. The present paper describes a full scale accelerated loading performance test on a 20mm Dense Base Coarse Macadam [6] which overlaid a peat based forest access roads [5]. Such forest roads generally have a weak pavement structure, which consists of a surface dressed granular base and a subbase (±0.5m deep) that overlay deep peat foundations. In general, peat (a soft biogenic deposit) provides a poor foundation for roads as it is frequently weak [7] and compressible, particularly after prolonged dry periods [8]. Various damage assessment procedures are used to evaluate the structural integrity of a pavement, for example the monitoring of crack initiation and propagation and permanent deformation on the pavement surface. In-situ material properties can be evaluated by measuring surface deflections with a Falling Weight Deflectometer (FWD), while internal stresses and strains can be monitored by use of appropriate transducers that are located at critical positions within a pavement structure. Environmental factors such as the moisture level in the subgrade and the pavement temperature can also be monitored. The present research has characterised and quantified the in-situ mechanical performance of a standard Irish Dense Base Course Macadam (DBC) by measuring surface strains, crack growth and subgrade stresses at incremental levels of fatigue cycles under controlled load conditions. LAYOUT AND CONSTRUCTION OF TEST SECTION A suitable section of a forest access road was identified for the experiment. Figure 1 shows a schematic representation of the monitoring section, the dimensions of which are 3.3m (road width) by 35m (length of overlaid area). A central area in the outer wheel path was selected for instrumentation with pavement transducers, the detailed layout of which is shown in Figure 2. FWD points were marked over the test area to measure the global pavement response. Crack monitoring grids were randomly spaced in the outer wheel path over 15m along the section. A total of 19 grids were marked out to monitor cracking. Four permanent deformation cross section measurement positions were identified, two at both ends of the crack monitoring strip. The traffic counter was located at the end of the test section. The counter had to be placed as far away as possible from the crack monitoring and permanent deformation measurement positions since counter cables had to be machined into the overlay which could induce initial defects. 0m 5m 10m 15m 20m 25m 30m 35m Traffic counter Permanent deformation measurement cross section Crack monitoring area FWD deflection point Instrumented area (detail in Fig.2) 1 2 3 4 North Overlay chamfer Figure 1. Layout of experimental road pavement section. Road center 460