8.2.1 Different Types of Road Structures Versus Sensitivity to Water
Road pavements are multilayer structures (see Fig. 1.5) generally comprising a surface course and one or more asphaltic or granular base layers, resting on a pavement foundation. Chapter 1 introduced the major pavement layers — the foundation, the sub-base, the pavement base and a surfacing (see Section 1.4.2).
Water permeability should normally increase from the top of the pavement (the asphalt or concrete layers) downward until about 0.7 m depth (see Chapter 5). Otherwise water would accumulate onto the low permeability layer and keep the upper layer wet; freezing of the accumulated water might then unbind the upper layer. This would decrease it’s bearing capacity and service life. An exception to these rules are the porous asphalt surfaced pavements described in Chapter 5, Section 5.7, which are designed to carry water within their thickness.
Pavement structures can be divided in four main groups:
• Thin bituminous pavements, which consist of a relatively thin bituminous surface course, resting on one or more layers of unbound granular materials. These are typically used for carrying low traffic levels.
• Thick bituminous pavements consisting of a bituminous surfacing, over one or two bituminous layers/asphaltic concrete (AC) (base) then an aggregate (sub-base). Their application is, typically, for high traffic levels. They may be considered as flexible, but they are much stiffer then the preceding pavements.
• Semi-rigid pavements comprising a bituminous surfacing over one or two layers of materials treated with hydraulic binders (e. g. concrete). This type of pavement is also appropriate for high traffic levels.
• Portland cement concrete (PCC) pavements which consist of a Portland cement concrete slab (15-40 cm thick), possibly covered with a thin bituminous surfacing, resting on a sub-base (bound or unbound), or directly on the foundation. The concrete slab can be continuous with longitudinal reinforcement, or discontinuous. Once again, this type of pavement is also appropriate for high traffic levels.
These various pavement types present different types of mechanical behaviour, and different deterioration mechanisms. However, for all structures, water plays a major role in pavement deterioration.
In thin bituminous pavements, high stresses are transmitted to the unbound granular layers and to the subgrade; and lead to permanent deformations of these layers. Because unbound layers and subgrades are sensitive to water content, the performance of these pavements is strongly dependent on variations of moisture conditions. This can lead, in particular to “edge failures”: water infiltrates from the pavement shoulders, under the edge of the pavement, leading to subsidence at road edges. As these pavements are very sensitive to moisture, impermeability of the surface course and good drainage are very important for their performance.
In thick bituminous pavements, the much lower flexibility of their bituminous base layers means that the stresses transmitted to the soil are much lower, and the risk of permanent deformations in the soil, as well as the sensitivity to the water content of the soil, are lower. The main mechanism of deterioration of these pavements is cracking due to the combined effects of traffic-induced strains and thermally-induced movements, causing high tensile stresses in the bound layers. Once the pavement is cracked, water infiltration accelerates the degradations, leading to weakening of the subgrade, attrition at the lips of the cracks, and material chipping away to form potholes (see Chapter 5). Without maintenance, deterioration can lead rapidly to total ruin.
In pavements with layers treated with hydraulic binders, the main deterioration process is generally due to reflective cracking. Thermal contraction and shrinkage in the cement treated materials create transversal cracks. These cracks generally rise up through the surfacing and appear on the pavement surface at fairly even spacing (5-15 m). These cracks tend to deteriorate and split under traffic loads. Then again, water infiltration is a major problem, leading to a deterioration of the bonding between bound layers, a decrease of the bearing capacity of the subgrade in the cracked area, thus decreasing load transfer and favouring attrition of the crack lips. On these pavements, protection against water infiltration, by using relatively thick surface courses and by sealing the shrinkage cracks, is essential;
In concrete pavements, due to the high modulus of elasticity of concrete, only very low stresses are transmitted to the foundation. Thermal cracking in concrete structures is generally controlled by transverse joints, or by the longitudinal reinforcement, producing only very fine micro-cracks. Two main types of damage are observed:
• Cracking created by excessive tensile stresses at the top or base of the slabs due to the combination of traffic loads and deformations of the slabs due to thermal gradients; and
• Reduction in bearing capacity around joints and cracks, leading to pumping phenomena. This reduction is essentially due to the presence of water at the interface between the slab and the sub-base. Under loading by vehicles, water at the interface is locally highly pressurised, high pressure gradients appear, inducing high water flow velocities which can erode the sub-base material (near a pavement edge crack or joint), reduce the bearing capacity of the support and reduce load transfer between the slabs. This is generally observed as edge or corner cracking.
For all pavements, freezing and thawing phenomena are also a major source of deterioration. In frost sensitive, fine grained soils, freezing leads to a concentration of water near the frozen zone (due to the so-called cryo-suction process — see
Section 4.6). This leads to heaving of the pavement, and then loss of bearing capacity during the thaw period. It should also be noticed that in cold climates, where winter tyres with studs are used, the wear of the surfacing of the pavement and the consecutive re-paving is often faster than fatigue or deformation damage.
