The term “tar” is often used broadly in construction, but it refers to two distinct materials: petroleum asphalt cement (bitumen) and coal tar pitch. Both are highly viscous, black residues that are solid or semi-solid at ambient temperatures. To be successfully used in applications like road paving or roofing, they must be heated significantly to transform into a fluid state. This thermal manipulation lowers the material’s resistance to flow, making it workable for mixing, pumping, and proper adhesion.
Understanding Tar Composition and Heat Needs
The need for extreme heat is a direct consequence of the material’s high viscosity at normal temperatures. Asphalt cement (derived from crude oil refining) and coal tar pitch (a byproduct of coal distillation) are composed of complex hydrocarbon molecules that resist movement unless energy is added. Thermal energy breaks down the internal friction between these molecules, allowing the material to become liquid.
A key material property is the softening point, which is the temperature at which the material starts to deform under a specific load. For typical paving-grade bitumen, this point is low, ranging from approximately \(106^{\circ}\text{F}\) to \(126^{\circ}\text{F}\) (\(41^{\circ}\text{C}\) to \(52^{\circ}\text{C}\)). This temperature indicates the material’s resistance to flow under service conditions, but it is far below the temperature required for application.
The actual working temperature is determined by the Equiviscous Temperature (EVT), the specific temperature required to achieve optimum viscosity for application. This EVT is significantly higher than the softening point and ensures the material flows correctly for mixing or bonding. The material’s chemical makeup dictates its thermal response and required EVT.
Specific Temperature Requirements for Application
The required heat differs depending on the final application, primarily categorized as hot mix asphalt for paving and hot-applied asphalt for built-up roofing. For hot mix asphalt (HMA) used in road construction, the temperature is carefully managed to ensure proper mixing with aggregate and subsequent compaction. HMA is typically delivered to the paving site at a temperature between \(275^{\circ}\text{F}\) and \(300^{\circ}\text{F}\) (\(135^{\circ}\text{C}\) and \(149^{\circ}\text{C}\)).
The material must be compacted while it is still hot enough to permit the aggregate particles to rearrange and lock together tightly. Successful compaction must generally be completed before the mix temperature drops below approximately \(185^{\circ}\text{F}\) (\(85^{\circ}\text{C}\)). The ideal mixing temperature is often determined by the equiviscous method.
For built-up roofing (BUR) systems, the application temperatures are notably higher, especially when using oxidized roofing asphalt. The Equiviscous Temperature (EVT) for hand mopping roofing asphalt typically falls between \(330^{\circ}\text{F}\) and \(445^{\circ}\text{F}\) (\(165^{\circ}\text{C}\) and \(229^{\circ}\text{C}\)). Some specialized high-temperature roofing asphalts may require application temperatures reaching up to \(525^{\circ}\text{F}\) (\(274^{\circ}\text{C}\)).
The asphalt is heated in a kettle, and the temperature must be precisely controlled to ensure the required viscosity is achieved at the point of application on the roof deck. Coal tar pitch, which is sometimes used in BUR systems, also requires high heating temperatures, though its working temperature must be strictly governed by its lower flash point relative to some roofing asphalts.
Safety Concerns and Overheating Limits
Handling and heating these materials require strict adherence to safety limits, as the high temperatures present a significant hazard. Direct contact with molten asphalt or pitch, even at temperatures just above \(120^{\circ}\text{F}\) to \(140^{\circ}\text{F}\) (\(49^{\circ}\text{C}\) to \(60^{\circ}\text{C}\)), can cause severe third-degree burns within seconds. Steam burns are also a risk, which can occur when water contacts the hot material, causing a violent expansion of superheated steam.
A primary thermal safety constraint is the material’s flash point, which is the lowest temperature at which its vapors will ignite momentarily when exposed to an open flame. For common paving-grade bitumen, the flash point is typically between \(446^{\circ}\text{F}\) and \(500^{\circ}\text{F}\) (\(230^{\circ}\text{C}\) and \(260^{\circ}\text{C}\)), while coal tar pitch can have a minimum flash point around \(374^{\circ}\text{F}\) (\(190^{\circ}\text{C}\)). Safety guidelines mandate that the material in the heating kettle should never be heated higher than \(25^{\circ}\text{F}\) (\(14^{\circ}\text{C}\)) below its minimum flash point.
Exceeding the flash point risks a fire, but overheating, even below this point, also degrades the material quality. Prolonged or excessive heating can cause the loss of lighter, volatile components, which permanently alters the material’s chemical structure. This chemical change can reduce the material’s flexibility and negatively affect its adhesive qualities, potentially leading to premature failure of the pavement or roofing system. Overheating also dramatically increases the release of hazardous fumes, necessitating proper ventilation and temperature monitoring.