In the last ten years, significant progress has been made in the measurement of in-situ water contents in pavements, using in particular TDR probes (see Chapter 3, Section 3.2.2). These measurements have shown that, often, significant amounts of water infiltrate in to pavements through the pavement surface and from the shoulders.
Low traffic pavements are particularly exposed to water infiltration. Examples of moisture measurements on a typical flexible pavement (6 cm thick bituminous surfacing and granular base) are shown in Figs. 8.3 and 8.4. Figure 8.3 shows that the daily variations of water content in the granular base and in the clayey subgrade (near the pavement edge) are important and strongly related with the rainfall. Figure 8.4 shows average water contents measured in the granular base, at different locations, near the centreline of the pavement and near the edge. The critical zone is clearly the pavement edge where the water content is about 2 percentage points higher than near the centreline. In this pavement, subjected to a mild oceanic climate, seasonal variations of water content are low, but they can be more important with more continental climates.
Thick bituminous or cement-treated pavements are less permeable, and water infiltrates mainly when cracking develops, thus accelerating the deterioration. In such pavements, protection against water infiltration, by proper maintenance (crack sealing, renewal of the surface course) is one of the main concerns.
Fig. 8.3 Water content variations in the granular base and subgrade of a low traffic pavement (near the pavement edge)
Fig. 8.4 Monthly average water contents in the granular base, at the centre and near the edge of the pavement