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Though roads are very simple structures at first glance, they cover vast territories and have very complex behaviour. Understanding and analyzing this complexity is one of the major challenges of civil engineering.
Depending on the type of pavement used, two main pavement families are distinguished: flexible bituminous concrete (asphalt) pavements, which are found on over 90% of the road network, and rigid cement concrete pavements, which cover 4% of the network.
Figure 1 presents a pavement section that may be suitable to both types of structures. The most appropriate choice of structure and its design depend on various factors, such as the forecast traffic intensity, soil types, climate, costs and local availability of construction materials.
Figure 1 : Example of Typical Pavement Section
Structures That Work
A flexible pavement’s life cycle is usually 15 to 20 years, while a rigid pavement ranges from 20 to 30 years. The useful life of a new flexible pavement layer is around 9 to 12 years.
Pavement conditions are assessed in terms of certain defects, which will become accentuated with use and time. To describe these defects, we usually refer to the following factors:
roughness, which defines driving comfort and in which defects are perceived as waves, i.e. jolts, hollows and bumps;
ruts or subsidence of the surface in the wheel paths;
cracks and other visible breaks in the surface.
Each roadway behaves differently depending, among other factors, on the nature of the soil, position of the water table, thickness of the layers, climate, type of roadway, its geometry, characteristics of the construction materials, traffic. All these parameters must be considered to diagnose the causes of deterioration adequately.
The understanding of roadway distress phenomena make it possible to find solutions to counter them better.
In a Northern Country
Québec temperatures vary by 60 to 70 °C. The mercury may drop to –30 °C in winter and reach 30 °C in summer. In winter, the soil freezes to a depth of 1.2 to 3.0 m, which is clearly deeper that the pavement structures, which average 90 cm.
The table below, compare the Québec context with that of Ontario, New York State and France. Two key aspects emerge from this comparison: the harshness of our climate and the extent of our road network in relation the number of inhabitants.
Table 1: State-managed Road Network
Length of the road network (km)
Number of inhabitants (millions)
Average annual precipitation (mm)
Frost duration (days/year)
147 to 218
100 to 200
10 to 100
0 to 90
Frost depth (m)
1.2 to 3
1 to 2.6
0 to 0.5
Solutions for Adapting to the Climate
Bituminous binders adapted to the climate must be used to prevent the asphalt from becoming too brittle in winter, while remaining fairly rigid in summer.
Drainage ditches along roads and raised roadways are widely used means of reducing the risk of the water table rising in the roadway. In some cases, special drainage systems can be installed, such as clean stone, synthetic membranes or underground drains which discharge the water out of the road structure.
Another typical necessity is to protect frost susceptible soils by sheltering them from the cold. Thus, the thickness of the sand and gravel layers on northern roads is more important than on roads in hot regions.
Sometimes deeper excavations are dug to remove the problem soil and replace it with another less frost susceptible soil. The soil can also be stabilized with chemicals such as lime. In some cases, an insulating layer maybe installed in the roadway to restrict frost penetration. The insulating layers may be composed of high-density polystyrenes, insulating concretes or other insulating products.
The installation of certain devices or the use of special products results in additional costs but allow substantial saving in the longer term. These means are only used if the financial gain attributable to the extension of the roadway’s useful life is advantageous in relation to the initial investment.
The Highs and Lows of Flexible Pavement
Under the effect of cold, frost gradually penetrates downward into the soil (see Figure 2A). This frost damages the structures. Under some unfavourable conditions, the water contained in unfrozen soils can be sucked towards the frost zone. This groundwater pumping then lends to the formation of ice lenses, which are reflected in frost heaving.
Figure 2 : Effects of Freeze and Thaw Cycles on a Road
A) Frost heaving
Because of their size, these lenses can lift the pavement up to 20 cm. Frost heave zones are often uneven, which explains the hollows and bumps that are bigger at the end of the winter when the frost depth is at its maximum. Frost heaving also causes the pavement to bend, causing more or less longitudinal frost cracks to appeal.
In addition, like any other material, asphalt hardens, weakens and shrinks under the effect of cold. By shrinking over long distances, the pavement is then subjected to tensile stresses that will cause it to break, producing transverse cracks. The roadway’s troubles aren’t over, because it is then time for the spring thaw, which also flows downward. A lot of the water from surface snow melt and melting ice lenses inside the pavement ends up in the thawed soil layer. This water is then trapped in the soil because of the frozen layer below it, which is impervious (see Figure 2B).
Figure 2 : Effects of Freeze and Thaw Cycles on a Road
B) Thaw weakening
The roadway then offers only 30 to 70% of its normal summer strength. This is why load restrictions are imposed on heavy vehicles to limit damage during the thaw period. This damage contribute to the gradual appearance of ruts, fatigue cracks in the wheel paths and sometimes potholes. We should note that frost heave fades away with the thaw. However, the roadway does not necessarily return to its initial position, which alters the roughness, and thus the driving comfort.
The roadway drains during the summer and the cycles repeat each year, causing more deterioration. Its surface marring can only accelerate over time, because the appearance of small defects creates additional weak zones which allow more water to penetrate and which worsen and propagate more quickly from one occasion to the next. Thus, even for a roadway, life in the north isn’t all restful.