Asphalt millings, on the other hand, are laid without the use of heat. This "cold mix" asphalt is placed and cured at the temperature of the surrounding environment. Once placed, the best method to finish it is to compact it using a steamroller. The surface will harden beautifully over time if properly compacted. Asphalt millings can be difficult to work with because they are abrasive. However, they are used for their abrasiveness so that drivers know they have not been worn down by traffic. Wearing out these millings would prevent them from providing some degree of safety for vehicles traveling on bituminous surfaces. Cold-mix asphalt used for milling does not require any additional materials to harden it after placement. It is compacted during construction of the road to ensure proper drainage and flow before being overlaid with another layer of asphalt or pavement material. Cold-mix asphalt is also useful in areas where hot-mix asphalt would melt in warm weather or freeze in cold weather. In general, asphalt used for milling should be a minimum quality with no more than 30% air. This allows it to be placed in a manner that produces a durable surface rather than one that will need to be replaced often. Millings can be reused after cleaning them well with a stiff brush and fresh water. Asphalt millings can be a problem area for roads because they tend to wear away faster than other types of asphalt pavement.