Changes in water content, especially excess moisture, in pavement layers combined with traffic loads and freezing and thawing can significantly reduce pavement service life. Failures associated with moisture are detected on roads all over the Europe. There is some evidence to suggest that water has less impact on thick and well-construced pavements than it does on thinner ones (Hall & Crovetti, 2007). It appears that in thicker pavements the effect of water may be more indirect than in thinner ones, reducing material stiffness leading to later distress.
To minimize the negative effects of moisture on pavement performance, first we have to identify the sources of infiltration of water. There are many different possible sources of water infiltration in pavement systems. Mainly, the presence of water in pavement is due to infiltration of rainwater through the pavement surfaces through joints, cracks and other defects, especially in older, somewhat deteriorated, pavements and shoulders. An important source is also migration of liquid water upwards to the freezing front. Water may also seep upward from a high groundwater table due to capillary suction or vapour movements, or it may flow laterally from the pavement edges and side ditches.
The significance of the routes of infiltration depends on the materials, climate, and topography.
Many pavement failures are the direct result of water entering the pavement courses and/or the subgrade. Water entry in the compacted unsaturated material will increase water pressure or decrease suction, and in turn, reduce the effective stress (see Chapter 9). Hence, the strength and the elastic and plastic stiffnesses of the pavement material and the subgrade will be reduced. The rate of traffic-induced deterioration of the road will increase during this time. The loss of strength and stiffness can lead, in the extreme, to rutting and other forms of surface deformation or, more commonly, to pavement edge failures. The worst situation occurs with poorly-compacted granular material (as a result of shear strength reduction) with frost susceptible soils or with cohesive swelling clays (as a result of damaging volume changes).
Water seeping through the pavement can also transport soil particles and cause erosion and pumping (i. e. transport) of fines as well as leaching of many materials (see Chapter 6). Moisture entry can also affect the performance of the surface course by causing stripping of bitumen from aggregate, layer separation between bound courses, and pothole formation (see Section 5.5). A probable mechanism of pothole formation is illustrated in Fig. 8.1. A somewhat similar failure mechanism has also been observed in slabbed concrete pavements (Roy & Johnson, 1979): 
(ii) Stage 2 Water travels along the interface between the asphalt (AC) and the compacted aggregate base
(iii) Stage 3 Asphalt lifts to allow pressure to dissipate, but also allows more water to be pumped into the crack
Fig. 8.1 Layer separation and pothole formation (from Gerke, R. J. (1979), cited by Lay (1986)). Reproduced by permission of M. G. Lay
• the fast-moving water eroded the finer materials;
• at their edges, the concrete slabs progressively lost support from the underlying layers as material was washed away;
• the deflections of the slab became greater and the erosion more rapid eventually leading to cracking of the unsupported concrete at edges and corners of slabs; and
• dirty water was seen to squirt from the joints in the pavement when the slabs were trafficked.
Pavement damage, associated with water, can be divided into moisture-caused and moisture-accelerated distresses. Moisture-caused distresses are those that are primarily induced by moisture, while moisture-accelerated are those that are initiated by different factors, but the rate of deterioration is accelerated by presence of water. Most commonly observed damages due to the moisture are:
• surface defects;
• surface deformations; and
Generally, water threatens the stability of soil and it is a particular problem for pavement structures since they are built through areas of changeable moisture quantities.
It has been generally established that the increased subgrade water content during spring results in increased deformability, i. e. a decreased bearing capacity of the pavement. These changes in bearing capacity are, in particular, obvious for silty and clayey materials.
Water contents contained in materials under flexible pavements are influenced by the amount and intensity of rainfall. Periods of long rainfall of low intensity can be more severe than concentrated periods of high intensity, since the amount of moisture absorbed by the soil is greatest under the former conditions. Also, the combined effects of rainfall and freezing temperatures determine, in part, the extent of pavement damage.
Water can penetrate into the pavement structure in several different ways as follows (Fig. 8.2):
• Seepage from the elevated surrounding soil, which depends on the hydraulic gradient and soil permeability coefficient.
• Rise and fall of the phreatic surface, which depends on the climatic circumstances and soil composition (e. g. following heavy rains there is an increase in the sub-surface water level in permeable strata, while water remaining on the surface of impermeable ground will drain away or evaporates and there is no risk of a rapid rise in sub-surface water level).
• The penetration of water through damaged pavement surfaces causes high local concentration of water in penetration areas. Under heavy traffic load, such a condition can result in significant damage if the subgrade made of changeable material is in contact with water. Even worse pavement damage can result from freezing of the structure and subgrade when soaked with water.
• The penetration of water through shoulders (if the shoulders are permeable or their surface is deformed in such a way as to allow the retention of water), which depends on the material permeability, compactness of the surface, inclination and the drainage from pavement surface. The effect is similar to the effect of water penetration through a damaged pavement surface.
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OF WATER I ABLE
Fig. 8.2 Possibilities of water movements into the pavement zone (from Moris & Gray, (1976) cited by Lay (1986)). Reproduced by permission of M. G. Lay
• Capillary rise from the foundation soil. The water rises from the foundation soil through the fine-grained soil up to the pavement structure.
• Evaporation of water from the foundation soil and its condensation under the pavement structure if the pavement structure is colder than the soil.
Moisture behaviour in a pavement can be considered as occurring in three phases:
• an entry phase — which occurs quite rapidly;
• a redistribution phase when water moves within the material in response to suction and gravity; and
• an evaporative phase when water, as vapour, leaves a material or moves to other layers. Water vapour movements can occur under temperature gradients with the water vapour travelling from a warm to cool area when it then condenses.
Water content under road pavements will vary seasonally, annually and over longer periods. Seasonal variations in water content are commonly located in the upper 1-2 m, and in the metre or so of pavement width at the edge of the pavement surface (the outer wheel path is clearly the critical zone). Significant moisture changes will only occur immediately after rainfall if very permeable layers exist.
The behaviour of the pavement related to moisture should be considered in reference to the climate, type of soil, groundwater depth and moisture concentration in the soil.