This has been provided for the information of visitors to the Institute’s website.  It is a glossary of terms commonly used in asphalt technology and is presented in good faith but carries with it no warranty whatsoever.

An area set aside for vehicles to stand or park. The surface should be sufficiently hard for the vehicles to stand safely without sinking into the surface. This may or may not require a surfacing of hardcore, subbase or bound material.
This is the abbreviation for California Bearing Ratio which is a test commonly used to indicate the strength of the material below an asphalt road, runway, car park etc. It is the ability of a material to resist an applied load compared to that of a standard granular material expressed as a percentage. The smaller the CBR, the lower the strength of the material. A typical soft clay may have a CBR value of 2%; a granular subbase may have a CBR of 200%. The CBR varies according to the nature of the material itself but may also be affected by the amount of water in the material. In certain materials, the water acts a lubricant, reducing its strength. Unbound pavement layers and foundation materials are likely to have a lower strength in winter than in summer and this is taken into account in pavement structural design. The CBR can be measured in situ directly or by using surrogate tests whose results can be translated into CBR values. It can also be measured by taking samples of the soil or other material and testing in controlled conditions in a testing laboratory. An approximate value may be obtained from an engineering description of the material.
A pit filled with broken stones, or with a perforated lining of steel, concrete or plastic, below ground to take outflow from rainwater pipes, surface water gullies, land drains or small sewage disposal plants. The soakaway permits the outflow to drain away slowly into surrounding permeable ground thus minimising any negative effect on the area. Clearly to be effective, the surrounding soil must not be saturated with water. In certain circumstances, the provision of soakaways is the subject of legislation.
Hardcore is a common term used for demolition waste or poorly graded stone from quarries. It may be unprocessed and can contain large pieces of brick, concrete, plaster, natural stone and the like. It may be contaminated with wood, paper and plastic. It may be the subject of some processing prior to delivery to crush the larger elements and remove most of the contamination and excessive fine material. Hardcore is used for temporary roads on construction sites and to strengthen the existing ground prior to laying subbase so that the subbase can be properly compacted. It can form the base for domestic driveways, but may be difficult to grade to a suitable surface tolerance.
Subbase is granular material with particles of aggregate of various sizes, from 40mm down to 63 microns. The sizes can be measured and specified by sieving the material through mesh sieves of given known size, this is known as the ‘grading’. The most commonly used subbase is known as Type 1 and it is fully described in the Highways Agency Specification for Highway Works Clause 803. It is laid beneath the lowest bound pavement layer and is not normally considered as a structural layer. However, it does make a small structural contribution to the pavement. It is often laid by a different contractor to the surfacing.
Milled Pavement (aka Planings)
When a road is to be resurfaced and the existing bound paving material removed, the existing carriageway is often removed by a milling machine. The resultant material is often described as planings or milled asphalt/concrete. A milling machine consists of a rotating cylinder with teeth around the circumference. These dig into the road surface as the machine is driven forward. The depth of cut and the speed of forward movement determines the grading of the granular material that is formed. An experienced operator can produce a material that can be very close to the grading requirements for subbase. Subbase can contain planings from different sources depending upon use and the performance requirements of the layer after completion. A trafficking trial may be required. Planings are sometimes recycled back into asphalt.
Bitumen is a black adhesive compound manufactured from crude oil. When hot, it is in liquid form and when cold, it solidifies but retains some viscous and elastic properties enabling it to act as a flexible binder. It comes in a range of grades denoted by the results from the penetration test. In this test, a standard needle is pressed down into the bitumen for 20 seconds. The further it penetrates (measured in tenths of a millimetre) the softer the material. Bitumen for roads falls in the range 10 to 190.
Asphalt is the term used for all mixtures of aggregate and bitumen including Dense Bitumen Macadam and Hot Rolled Asphalt. In the USA and literature emanating from there, it is known as hot mix - the term ‘asphalt’ is used for bitumen alone.
Coated Macadam
Coated macadam is manufactured from hard aggregate with particles of all sizes, from the maximum nominal size down to 63micron ie 63 millionths of a millimetre. It is bound together with bitumen. The hardness of the bitumen used is denoted by adding a number eg DBM50 with the number indicating the hardness of the binder. The lower the number, the harder the bitumen. The grading and binder content are fully described in BS 4987. The aggregate grading creates a skeleton that resists the compressive and shear forces generated by vehicle tyres. The material is mixed hot in a special plant and must be delivered to site and compacted in situ before it cools.
Hot Rolled Asphalt
Hot Rolled Asphalt is composed of coarse aggregate surrounded by a mixture or mastic of bitumen, sand and very fine material called filler. Whereas coated macadam derives its strength from the mechanical interlock of the constituents, hot rolled asphalt strength is largely derived from the properties of the mastic. Hot rolled asphalt was the predominant surfacing material for UK main roads until about 1995. Forthis use chippings are applied to the hot-laid material and rolled in to form a skid resistant surface.
Stone Mastic Asphalt
Stone mastic asphalts (SMA) are manufactured from hard rock aggregates with particle sizes selected to leave space for a bitumen and limestone powder mixture known as ‘mastic’. They originated in Germany and are made and laid in the same way as the more common coated macadams. Some SMAs are used as the running surface on carriageways although they can also be used in sub-surfac e layers. These materials have no specification in the UK with mixtures being proprietary but approved following rigourous testing by the British Board of Agrément.
Early Life
The early life of a pavement is the time required for the initial changes in a pavement to have taken place. Most of the early life strength of a pavement is gained as the bitumen cools. This can take up to 6 hours or longer in hot summer weather. Bitumen also contains volatile oils which help it to be workable for laying. These can take up to 48 hours to evaporate, during this time the surface may be tacky to the touch and suffer surface damage under static loads or scuffing from traffic. The surface bitumen may make the surface more slippery in wet weather, a feature that will be present until the bitumen wears away, is treated to expose the aggregate, or the surface is gritted.
The pavement is the structure built on the subgrade using layers of material as shown below.  This is not to be confused with the same word in common use that refers to a footway or footpath.
Tack Coat
A tack coat is a layer of bitumen sprayed onto a surface prior to laying the next layer of asphalt.  It helps the two layers to bond together so that the pavement acts as a single entity and helps to waterproof the pavement.  Research suggests that tack coats can add measurably to the performance of a pavement.
Bond Coat
A bond coat is similar to a tack coat but is a proprietary material.  It is formulated to perform the same function as a tack coat but is designed either for a particular application or for general purpose applications albeit with enhanced properties.
Mastic Asphalt
Mastic asphlat is a mixture of bitumen, sand and very fine material called filler. The bitumen characteristics may be modified by the addition of Trinidad Lake Asphalt or polymers. It is laid hot and cools to provide a flexible, impermeable, watertight layer used for roofing, flooring, tanking and paving.
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