Answers to common questions about road surface, asphalt, bitumen, pavement

What is chip sealing (spray sealing in Australia)?
In Australia chip sealing is referred to as a "sprayed seal" or "tar seal" and is used on a larger percentage of roads, both rural and urban. It is cheaper than surfacing in an asphaltic concrete pavement, Chip seals are constructed by evenly distributing a thin base of hot bitumen or bitumen Emulsion onto an existing pavement and then embedding finely graded aggregate into it. The aggregate is evenly distributed over the chip seal, and then rolled into an even surface. Two functions are served by the chip seal firstly to seal surface of the road then to provide a wear surface with the chippings (Aggregate).
The introduction of polymer-modified (Plastics) in bitumen and emulsion binder has increased the chip seal's ability to prevent crack reflection and improve stone retention by improving the properties of the bitumen binder. Newer techniques use bitumen emulsion (a mixture of liquid bitumen, surfactant, and water) instead of hot sprayed pure bitumen. This has been shown to help reduce emissions of volatile organic compounds (VOCs) due to the lower solvent content generally required.

What is Hot Mix Asphalt (common term) or Asphaltic Concrete AC?
This is a manufactured product made with much the same type of ingredients as Concrete except bitumen is used to bind the mixture together instead of cement. A blend of aggregates (crushed rock), sands and fillers are passed through a heating drum to completely dry them and raise the temperature to around 160°C adding hot bitumen in the process.
There are many types of mixes for various types of applications. The most commonly used mixes are 14mm, 10mm and 7mm, referring to the largest aggregate in the mix. 14mm and 10mm are typically used on roads, 10mm and 7mm on car parks, driveways and sport courts etc. The thickness the hot mix is laid depends on the traffic loading it will be subjected to. Typically a car park is 30mm thick for lighter vehicles..More about bitumen arrow

What is Bitumen?
The most commonly used bitumen is petroleum “Refined Bitumen”. This is the residual (bottom) fraction obtained by fractional distillation of crude oil. It is the heaviest fraction and the one with the highest boiling point at 525°C. Bitumen is a mixture of organic liquids that are highly viscous, black, sticky; entirely soluble in volatile’s such as kerosene.

What is hot spray bitumen?
Bitumen is produced in a variety of grades varying from hard and brittle to soft and fluid, at normal temperatures. Many or these grades are solid like butter. To make bitumen liquid it has to be heated from between 160°C - 220°C so it can be pumped and sprayed for chip sealing for example. To temporarily reduce the viscosity cutter oils are added i.e.  Kerosene, also to aide adhesion of stone and wetting of subases.

What is Tar?
A term in road surfacing carried over from the past when Tar derived from coal was used for surface binder instead of today’s petroleum bitumen no longer broadly used mainly due to its toxicity while heated. Confusing as most road work signage in Australia will display the classic  “Tar spraying in progress sign” when in reality it is either bitumen or bitumen Emulsion being sprayed.

What is Bitumen Emulsion?
A dispersion of bitumen in water achieved by the use of suitable chemical emulsifying agents while being forced through a colloidal mil (Homogeniser) providing a shearing action. The emulsified bitumen microscopic droplets may be anionic (negative charge slow setting), cationic (positive charge fast setting) or non-ionic (neutral).
Polymer Modified Bitumen Emulsions. By the addition of polymers to the emulsifying process, when deposited in the binder film thus improving the Binder to achieve good elasticity and tenacity characteristics encourage a strong bond between binder and chippings. This permits the use of emulsions on highly stressed sights where good chipping retention is essential. Emulsions can and are being further developed to play an ever important role in an energy and environmentally conscious age.

Polymer Modified Bitumen Emulsions. By the addition of polymers to the bitumen emulsion, the deposited binder film has good elasticity and tenacity characteristics encouraging a strong bond between binder and chippings. This permits the use of emulsions on highly stressed sights where good chipping retention is essential. Emulsions can and are being further developed to play an ever important role in an energy and environmentally conscious age.

Aggregate or Chippings
Broad terms for coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete. Aggregates are a component of composite materials such as concrete and asphaltic concrete; the aggregate serves as reinforcement to add strength to the overall composite material. For chip sealing the aggregate provides the wear surface it can be coated with a bonding agent to aid adhesion. More about Aggregate arrow

Spray Seal (Chip seal) Design
Method for determining the rate of application of bitumen or bitumen emulsion and aggregate (chips) to a given surface.  Factors such as traffic volume, heavy or light vehicles, speed of traffic and condition of the surface are assessed. The aggregate (Chips) test report is also important as this provides a factor called “Average Least Dimension” which means the average height of the stone when placed on a flat surface. For example a good quality 14mm aggregate will have an ALD of around 9.4mm. This factor is important as it plays a role in determine the volume of bitumen or emulsion required. The various State Government Road Authorities have standards that these products must meet.

What is Surface Coat or Seal Coat?
This is a manufactured product with ingredients made to be compatible with mainly AC (Hot mix) pavement commonly a liquid that is thinly applied over the top by spraying, squeegee or Roller; these products also can be made in a variety of colours.