Hot mix asphalt is superheated at the plant to temperatures of 160 C or above. It is transported in trucks that maintain high temperatures, but it must remain above approximately 120 C (depending on the exact blend and local standards) during pouring and compacting. Both the ambient temperature and the ground temperature play a role in how long hot mix asphalt remains within a proper working temperature range, from several hours during the height of summer to just a few minutes in the winter. In addition hot mix asphalt depends on a slow process of cooling and curing to achieve maximum structural strength. Cooling too quickly can undermine the asphalt and make it more likely to fail.
For these reasons, new asphalt is only poured during the warmer summer months. For emergency winter repairs, cold patch asphalt is used. This material uses a different binder that does not rely on a cycle of superheating and slow cooling to be effective. However, it also does not harden in the same way as hot mix asphalt. Cold patch repairs are generally considered temporary, except in very limited circumstances.
