Tar has been used for many years in road construction both as a binder for macadam and as a surface dressing initially on waterbound roads. Road tar is today specified in BS 761 by the following definition: ‘tar for use in road work is prepared entirely from crude tars produced wholly or substantially as a by-product in the carbonisation of coal at above 600 °C in externally heated retorts or coke ovens’.
The operation of proportioning the various aggregate sizes, adding binder and mixing the whole to produce as far as possible a homogeneous mass is a relatively simple process if a soft binder is employed; quality control is not important and only a small amount of mixed material is required. All that is required is a heated tray and several men with shovels.
The thickness design of highway pavements requires the following large number of complex factors to be considered.
(1)
The magnitude and number of repetitions of the applied wheel loads and the contact area between the tyre carrying the load and the road surface.
(2)
The stiffness, stability, durability, the elastic and plastic deformation and resistance to fatigue loading of the pavement layers.
(3)
Volumetric changes in the subgrade due to climatic changes, the deformation of the subsoil under load and the ability of the pavement layers to reduce the stress imposed on the subsoil by the wheel loads.
Concrete pavements are constructed in a variety of forms by several different construction methods. They may be reinforced or unreinforced; if reinforced the steel may take the form of individual bars or welded mesh. The slabs may contain several different types of joint or they may be unjointed or continuous. Construction may be carried out by the conventional side-form process using a concreting train with many differing units, or one of several forms of slip-form paver may be employed operating with a minimum of additional equipment.
The importance of adequate drainage was realised by Roman road builders, but after the decline of the Roman Empire the standard of highway construction in Europe declined and planned drainage became almost non-existent. With the advent of industrialisation increasing attention was paid to the removal of surface water and the lowering of the water-table beneath the pavement. In the United Kingdom the pioneer road-building of Telford and Macadam laid stress on the incorporation of a camber or crown into the road so that surface water could be quickly removed. Increasing urban development made open ditches inappropriate and led to the increasing use of road gulleys connected to road sewers as the means of removing surface water. Improvements in the quality of road materials have resulted in impervious pavements, making the problem of sub-soil usually greatest in the cuttings, where cut-off drainage is required.
A site investigation is an essential first step in the design of any highway on a new location. The information obtained by the survey assists in locating the highway to avoid adverse geological conditions and in designing earthworks, pavement thickness, drainage works and bridge foundations.
With the drainage works completed and all services and ducts crossing the carriage-way in place, the formation may be sealed to protect the subgrade from excessive moisture changes. This is because a cohesive subgrade which has become excessively dry due to evaporation during a dry construction season may swell with subsequent increases in moisture content, resulting in differential movement of the pavement. On the other hand a subgrade that has become excessively wet is difficult to compact and to overlay with sub-base material. Universal sealing of the subgrade thus will not always be the answer because it may delay evaporation and hinder the evaporation from the soil. For work in the United Kingdom the following broad recommendations have been made.1 If the subgrade is formed during the months of October to April (inclusive) then it should be protected unless the sub-base can be laid and compacted on the same day as the subgrade is prepared. If the subgrade is cohesive and is prepared during May to September (inclusive) protection should be given if the formation is likely to be exposed for more than 4 days.
The twentieth century has seen a considerable improvement in the materials and constructional techniques used for highway pavements. This has resulted in a dramatic increase in the life of a pavement from the period when an annual surface dressing was necessary to maintain the shape of the pavement to the present time when design lives of from five to twenty years are common for heavily trafficked highways.
