Performance characteristics of bound pavement layers are known to be influenced by water-borne pollutants that cause changes in mechanical behaviour, ageing and degradation. With the exception of Portland cement concrete (PCC) pavements, this has not yet received much attention. Asphalt pavements are not seriously affected by inorganic pollutants, but most of the organic chemicals, including gasoline and motor oil, soften up or break down the asphalt binder leaving the asphaltic layer vulnerable to further degradation. Damage of the surface layer, due to ageing (stiffening because of ultraviolet light), traffic induced cracks and chemical degradation, opens an ingress route in the pavement system for pollutants from the surface.
In hot climates, salt can be moved by evaporating water to near the pavement surface. This may result in the expansive crystallisation of salt in voids in asphaltic mixtures (or other bound layers) just below the road pavement’s surface leading to blistering of the running surface, to cracking of the pavement and to overall degradation. Guidelines to the understanding and treatment of this issue are available (Obika, 2001). In temperate climates, salt (NaCl) seems, usually, not to be a significant contributor to damage of asphaltic materials. It may, sometimes, accelerate deterioration of poor quality materials, but it appears that it is water damage itself which is the primary cause (see Section 5.5). However, the chemistry of the water in the pores of asphaltic materials can have an important influence on whether stones and binder adhere efficiently. Greater alkalinity (i. e. higher pH) potentially
results in increased rates of moisture damage, although Calcium Hydroxide (slaked lime) dissolved in the water doesn’t have this effect even though pH rises (Little & Jones, 2003). In cold climates, salt has been implicated in causing damage. According to Hudec and Anchampong (1994) certain fine grained aggregates degrade rapidly during wetting and drying cycles and during freeze-thaw cycles especially if deicing salts have been used abun-dantly. The extensive use of chlorides has also been reported to cause accelerated pavement deterioration (Dore et al. 1997, Saarenketo, 2006).
In PCC, deterioration is related to complex processes associated with physical and chemical alteration of the cement paste and aggregates. One major chemical degradation mechanism resulting from the long-term application of the popular chemical de-icer sodium chloride (NaCl) is the dissolution of calcium hydroxide (Ca(OH)2). Another common de-icer, CaCl2, is associated with a deleterious chemical reaction with PCC. The chemical attack is accompanied by the formation of hydrated calcium oxy-chloride according to the following reaction:
3Ca(OH)2 + CaCl2 + I2H2O ^ 3CaO ■ CaCk ■ I5H2O
This reaction is considered to be disruptive to the concrete matrix because of the expansive pressures generated. Another potential detrimental effect of the application of chemical de-icing salts is increased alkali — silica reactivity (ASR), which is a distress caused by undesirable chemical reactions between alkalis in the cement paste (Na2O and K2O) and the reactive siliceous components of susceptible aggregates. The product of the reaction is expansive in the presence of moisture, destroying the integrity of the weakened aggregate particle and the surrounding cement paste. When aggregates like dolomitic limestone are used there is a possibility of alkali-carbonate reactivity, where alkalis react with carbonate aggregates. Besides these mentioned processes, there is also the possibility of external and internal sulphate attack, which can cause deterioration. Other de-icing chemicals (magnesium chloride, calcium magnesium acetate, Ca-acetate, Mg-acetate, urea, etc.) may also have damaging effect on PCC pavement layers (MTTI, 2002).
Hydraulically bound mixtures may be considered as low strength PCC. From this point of view, the effects of the pollutants (mainly de-icers and sulphate) are similar to those on PCC pavements, except that chemical degradation, deterioration and loss of strength of the hydraulically bound layer will be quicker than in PCC layers, leading to higher stress on the layers beneath and faster degradation of the pavement.
