# The Influence of Water on the Heat Loss of Hot Mix Asphalt
## 1 Introduction  
The main parameter describing the internal structure of hot mix asphalt is its bulk density and corresponding voids. It is determined during compaction. The effectiveness of this process depends on the compaction effort, time and the temperature of the mix being compacted [1; 2; 3; 4; 5; 6]. The compaction effort and time can be relatively easily controlled by adjusting the number roller passes on the same path during a given time period. Conversely, it is most difficult to control the temperature during the process, especially when weather conditions vary [5; 7; 8]. Quick and considerable drops of temperature are a big challenge in the HMA laying process specifically in cases like thin noise reducing asphalt layers (TAL) or ultra-thin asphalt surfacing. This concerns in particular the interfacial zones (top and bottom surfaces) exposed to the direct action of external factors. The viscosity of the binder increases rapidly at these locations in the profile. With a higher than desired viscosity it is more difficult to obtain adequate compaction. Insufficient compaction increases the content of voids which can decrease the HMA's resistance to external factors. One of the factors increasing the rate of heat absorption from the interfacial zones of HMA is water. Water can appear both on the top surface (coming from precipitation or wetting of the steel wheels of rollers) and on the underside of the layer, especially when placed on a damp base of unbound material (for example compacted aggregate). The amount of heat lost due to the effect of water depends on the initial HMA temperature, on the amount of water and on the intensity of phase change processes(boiling, evaporation), which are activated by the input of energy from HMA. Additionally relevant is the water/ HMA contact location (bottom or top surface) since it defines the type of convection and water vapor flow conditions. Coming in contact with the top surface of the HMA layer water initiates the following processes:  
* Intensive boiling and evaporation in the first seconds (until the surface temperature has dropped below 100 \({}^{\circ}\)C),
* quite intensive evaporation of water (when present in small amounts) over a period of few minutes (depending on the humidity and temperature of the ambient air and also on the wind speed),
* film formation (impeding evaporation) and retarded transfer of heat by conduction from HMA to the ambient air (through the thin film of water),
* next the heat flux stabilises (heat propagates from the inside to the surface and the surface temperature is virtually constant). When HMA comes in contact with a damp substrate the following processes are initiated:  
* Intensive boiling and evaporation (in the first seconds) until the surface has cooled off (to below 100 \({}^{\circ}\)C);
* Infiltration of water vapor into the HMA layer (resulting in excessive cooling and increase of the binder viscosity);
* Slow heat propagation from the inside to the bottom of the layer without boiling and evaporation at the lower interface (bottom surface). The consequences of quick cooling are also different between the top and bottom surfaces:  
* The top surface can get more susceptible to the weather and ambient factors (water, UV radiation, temperature) due to a higher voids content;
* In the bottom zone a higher content of voids reduces the tensile stress resistance of the layer (resulting in premature fatigue distress and low-temperature cracking, etc.) [9; 10; 11]. The heat losses related to the presence of water are many times greater than the amounts of heat removed by other external factors (wind, air, substrate, elements of compacting equipment) with the greatest portion of energy consumed in the boiling process (in any form). The parameter describing the material's ability to absorb heat is the heat transfer coefficient \(\alpha\) (Figure 1). Its value can vary over a very wide range.