How Hot Is Asphalt When Paving?
Introduction
Asphalt is widely used in construction activities such as the construction of roads, parking lots, driveways, and more. It consists of the aggregate (stone and sand), the binder (asphalt cement in most cases), and filler material. When mixed and compacted in layers, it forms a smooth and robust ground on which automobiles can move. However, converting an asphalt mix to a paved surface entails heating the mix to high temperatures. So, really, how hot is asphalt when paving?
Temperatures During Mix Production
Hot mix asphalt production starts at an asphalt plant, where the aggregate is dried and heated before being mixed with hot asphalt cement. The temperature at which the aggregate is heated will depend on the grade of the mix being prepared and may range between 200-325°F. The asphalt cement that is incorporated into it is even hotter and is usually heated to temperatures ranging from 275-425°F.
Once the hot aggregate and the hot liquid asphalt have been mixed, the temperature of the mix will get to a point that is somewhere in that full range of 200-425°F. The actual temperature during mixing is adjusted depending on the specific properties required in the final product. The high viscosity of liquid asphalt means that some of its grades have higher production temperatures to achieve the desired viscosity for workability.
Transportation and Paving Temperature
The hot mix asphalt is then transferred to trucks for distribution to the paving site and should still be at the right temperature. To avoid the mix gaining a solid state during this trip, the haul trucks should have beds that are insulated or heated in some way. However, a certain amount of heat loss cannot be avoided.
Paving Best Practices
As a rule, paving should be done when the asphalt mix is in the range of 290-320°F. In this window, the material has the right amount of heat that makes it possible to compact correctly and without being too hot to burn the workers handling it. It must also be above roughly 225°F to avoid becoming too cool before the compaction process is complete.
In large paving jobs where production rates are steady, trucks transport hot mix from the plant that may still be at the higher end of the general temperature range. Typically, the mix that comes directly from the haul trucks may be at a temperature between 300-350°F, or even higher as it feeds into the paver. Smaller jobs may take a mix that has been transported and has had more time to cool down. Crews will also use thermometers to check whether the temperature is within the targeted range before paving begins.
Compaction Process Temperatures
Following the laying and compaction of the hot asphalt mat by the paver, more rollers pass over the surface of the newly placed layer while it is still warm. Compaction assists in eliminating air voids and enhancing density but must occur prior to the temperature reducing to approximately 175-225°F. Lower down this range, the asphalt will not possess the heat-softened viscosity required for the rollers to compact it sufficiently. When the active rolling is done, it is possible for the surface temperatures to be extremely high, reaching between 240-300°F or more, especially on newly paved surfaces.
Cooling and Curing
In the case of the final rolling, traffic is still prevented from accessing the newly placed asphalt surface until it cools down sufficiently. This way the surface of the pavement is protected from damages while the asphalt is cooling and hardening. Typically, temperatures must decrease to 140°F before vehicles can begin driving on the paved surface without creating problems such as rutting from tire impressions or loss of adhesion between layers.
The second phase will be the curing of the asphalt since it will cool down to the ambient temperature. This curing can extend from several days to an entire year to ensure that the pavement reaches its maximum strength through the oxidation course. Although the ambient air cools the pavement, the temperature of the asphalt, as it cures over weeks and months, affects its hardness – one reason why paving is not done during freezing weather.
Conclusion
From start to finish, asphalt on a paving site goes through dramatic temperature shifts:
– Temperatures in the plant vary between 200-425°F.
– Many jobs use haul trucks to transport hot mix at over 300°F.
– Pavement from trucks can be as hot as 300-350°F or even more as they are directly placed from the trucks.
– The compaction must be done before the temperature goes down to approximately 225°F.
– No traffic is allowed on the surface until the temperature drops below 140°F.
It is critical to regulate and supervise these temperatures at each phase when paving with hot mix asphalt. Crews must also monitor the mix temperature as it goes through the process to make sure that it falls within the range needed to pave the surface sufficiently.