Andrew Dawson[7], Niki Kringos, Tom Scarpas and Primoz Pavsic

Abstract Pavement surfaces provide a key route of ingress of rain water into the pavement construction. Thus, permeability of asphaltic materials and the water ingress capacity of cracks in the pavement are very important. A range of equipment exists to determine the permeability of asphaltic mixtures both by in-situ and labo­ratory testing. Sometimes porous asphalt surfacing is provided to deliberately allow water into the pavement to limit spray from vehicles and to limit tyre-pavement noise generation. These porous surfaces can become clogged with fines, but reha­bilitating without causing premature damage is a challenge. Except for this planned acceptance of water into the pavement, water is generally undesirable as it often causes ravelling (stripping) of the asphalt whereby aggregate and binder separate. The mechanisms behind this separation are becoming better understood due to ad­vances in computational engineering and mechanical and physio-chemical testing.

Keywords Asphalt ■ cracking ■ infiltration ■ stripping ■ ravelling ■ porous asphalt ■ permeability

5.1 Introduction

The topmost layer of most pavements is comprised of a bound layer. The vast major­ity of pavements have an asphaltic surface. A far lower proportion have a Portland cement concrete (PCC) surfacing. Whilst it is usually a design aim of these surfaces that they provide an impermeable covering to all the lower pavement layers, water does penetrate such surfacings. It may do so either through intact, but not imperme­able, bound material or through cracks and joints in the surfacing. Although the em­phasis of this book is on water movement and its impact in the unbound material and subgrade layers of the pavement, some information on the movement and response in the upper, bound layers is indispensable. Apart from any other consideration, any
complete analysis of water in the road and foundation structure must consider the input conditions – which are significantly affected, perhaps even controlled, by the surface layer. In this chapter the emphasis is on asphaltic mixtures, their permeabil­ity and the damage that they suffer from water. Consideration is also given to the ingress of water through joints and cracks. Some of this may also be applicable to the ingress through construction joints and cracks in Portland concrete surfacings. In addition, some consideration is given to porous asphalt mixtures – surfacing that is designed to prevent runoff flowing over the surface.

Researchers interested in frost action in soils agree on the description illustrated in Fig. 4.3 on how ice lenses grow and cause frost heave. Nevertheless, when it
comes to the degree of water saturation of the unfrozen soil below the freezing front there are two different conceptions. Some researchers believe that the un­frozen soil is fully saturated while others believe it is unsaturated. Of course, these discrepancies in understanding lead to different explanations of the driving force of the capillary rise of water as well as different opinions on how to run laboratory experiments. Andersland and Ladanyi (2004), for example, Konrad and Morgenstern (1980) and Nixon (1991) give equations where it is obvious that full saturation is assumed for the unfrozen soil. Accordingly, Konrad (1990) refers to experiments where the specimen freezes from below and free access to water is permitted at the top. This, of course, gives full saturation of the unfrozen soil. In contrast to this view, Miller (1980) discusses frost heave as freezing of unsaturated soil and references experiments where water is fed in at the bottom of the specimen and ice lenses are fed through capillary rise in unsaturated soil. Accordingly, Penner (1957) freezes unsaturated soil and Hermansson and Guthrie (2005) present laboratory ex­periments where freezing and frost heave takes place at a height more than 0.5 m above the level of the water supply. It should also be noted that Hermansson and Guthrie (2005) describe testing where the specimen heaves significantly without addition of any external water at all. This, of course, contradicts the assumption that the soil below the freezing front is fully saturated. The expansion without addition of water is suggested to be an effect of air entering the soil.

In agreement with the laboratory experiments Hermansson (2004) described a field study where the depth to the groundwater table is 6 m. Under such a thoroughly drained condition, it is reasonable to assume that the soil is far from saturated. De­spite this Hermansson reported 80 mm of frost heaving over a period of 2 months.

The conclusions from these studies are twofold,

• Firstly, frost heave does not require full saturation; and

• Secondly, even a well drained soil might experience a significant frost heave.

In addition to the different understandings about the importance of saturation, there are also two different schools when proposing equations to describe frost heave (Hansson, 2005). One school neglects the liquid water pressure and the other one neglects ice pressure. The first school, characterised by “Miller-type” models, de­velops models describing the frost heave on a microscopic scale while the second school, characterised by “hydrodynamic” models, handles equations for the redis­tribution of water up to the freezing front, supplying the frost heave. No computer code is known that handles both processes realistically.

3.5 Conclusions

Heat transfer in soils involves convection, radiation, vapour diffusion and con­duction. For pavements, conduction is the most important factor. During warm and sunny summer days though, natural convection should not be neglected. The heat transfer is closely associated with water movements – evaporation pulls water through the soil to the evaporation surface. Freezing also drives water movement as water is drawn to the freezing front in soils which have moderate pore sizes and moderate permeability.

Frost susceptible soils always experience frost heave at freezing even if there is no saturation. Drains will lower the heave by reducing the water content but a frost susceptible soil will always hold enough water for a significant heave. Chapter 13 describes some drainage techniques that can help to address these prob­lems. Interested readers are also directed to the reports on frost and drainage, mostly in the context of seasonally frost affected roads, available from the ROADEX project (Berntsen & Saarenketo, 2005; Saarenketo & Aho, 2005).

Frost heaving of soil is caused by crystallization of ice within the larger soil voids. Ice lenses attract water to themselves by the, so-called, cryo-suction process, and grow in thickness in the direction of heat transfer until the water supply is depleted or until freezing conditions at the freezing interface no longer support further crys­tallization, see Fig. 4.3. As the freezing front penetrates deeper into the pavement, the growth of ice lenses ceases at the previous level and commences at the new level of the freezing front. At some point the heat flow will be reduced so that further freezing slows or the weight of overlying construction will impose sufficient stress to prevent further ice lens growth.

Fundamentally, the so-called cryo-suction process results from the consequences of the phase change of pore water into pore ice and the associated energy changes in the remaining pore water. As water arrives at the point of freezing, the soil skeleton expands to accommodate it while consolidation of the adjacent, unfrozen, soil may occur as water is pulled from that. The particular characteristics of the process are strongly affected by soil porosity, soil-water chemistry, stress conditions at the point of ice formation, temperature, temperature gradients in the adjacent soil(s), water availability, etc. Coussy (2005) gives a more detailed outline of the mass flow and heat flow elements that combine in the processes that are active at the freezing front.

Capillary water is crucial for frost heaving. The pavement damage range depends on the rapidity of freezing, i. e. if freezing occurs rapidly, ice lenses are distributed over a greater mass of soil, which is somewhat more favourable compared to slow freezing where the capillary inflow of water will cause a high concentration of ice lenses. Frost heave primarily occurs in soils containing fine particles (i. e. frost – susceptible soils). Clean sands and gravel are non-frost susceptible because they cannot hold significant pore suctions, so water cannot be drawn to the freezing front through them. Silty soils represent the greatest problem. For even finer soils, like clays, the pores are very small. These small pores lead to a low coefficient of per­meability which does not allow water to travel through the pores at a speed that would allow a fast capillary rise, hence ice lens formation is less.

3.4.1 Frost Heave and Spring Thaw

Frost heave occurs in roads having fine graded, so-called frost-susceptible, mate­rial, at a depth to which the freezing front reaches during the winter. A well-built road of consistent materials and cross-section can be expected to heave relatively evenly. When inconsistencies or inhomogeneities are found in the construction of the affected subgrade, then frost heave is likely to be uneven and may well cause an uneven road surface that results in reduced travelling speed and comfort.

Although such heave can be problematic, a much greater problem usually arises in spring-time when the ice that has formed in the road construction, which was instrumental in causing the frost heave, melts and results in a very high water content in the pavement and subgrade. The increased water content often means reduced bearing capacity. For this reason many countries impose spring-thaw load restrictions on low volume roads to avoid severe pavement deterioration.