MAIN TYPES OF PAVEMENT

To permit automobile traffic to travel along a road at any time of the year at high speeds and with economic fuel consump­tion, the road pavement must be of an adequate rigidity, uniformity and resistance to wear. These requirements can be satisfied by means of various combinations of pavement structural layers con­sisting of different road-building materials. The pavement service qualities, i.e., permissible speed and traffic comfort, are determined mainly by the nature of the surfacings, which can be divided into the following basic structural types, given in con­secutive order of their development.

Cement concrete and asphalt surfacings. These surfacings are of high rigidity and high resistance to loading.

The stone aggregate is thoroughly graded, so that the intersti­ces between large particles are filled with smaller chips, and the material as a whole has a minimum porosity (maximum density). Cohesion is provided by the use of cement and organic binders.

In contrast to asphalt surfacing, cement concrete surfacing has a very considerable inherent strength and temperature stability. These surfacings usually consist of separate concrete slabs, measur­ing 3- 4 m by 6 - 7 m. The slabs are separated from each other by joints which are necessary to allow for changes in length owing to temperature fluctuations. These are expansion joints which cont­ract when the slab length increases, and contraction joints which expand when the slabs shrink. Inserted into the joints are steel bars called dowels which provide for thеpossibility of small changes in slab length but which transmit vertical loads from one slab to the other, and, to a lesser degree, flexural movements.

Apart from sand, stone dust (mineral powder) is introduced into the asphalt concrete, which enters into physical and chemical reactions with the organic binding agents, resulting in the sur­facing becoming more resistant to temperature change.

The asphalt concrete surfacing is flexible and should, therefore, be laid over a solid stone base (flexible pavement).

Вituminous macadam - broken stone and gravel surfacing treated with оrganic binders. Owing to the adhesive properties of thе binders, this surfacing is highly resistant to the destructive action of traffic. Such a pavement is impervious to water.

The differences in the methods of binder introduction in the process of construction create the fundamental structural characteristics of the surfacings obtained.

A. Mixing on the road site or in special plants provides for good coating of the chippings by the binder. The amount of binder used is less with this method then when using the method of impregnation. The mixing method together with the proper se­lection of stone material grades makes possible the provision of stronger surfacings.

The positive mixing makes possible the use of chippings graded in such a way as to form a solid matrix, the density of which approaches the optimum value.

B. Impregnation is the introduction of the binder into the surfacing by means of pressure-spraying over the surface of a lightly compacted layer of uniform chippings.

After the penetration of all the bitumen into the interstices of the aggregate, the surface of the pavement is covered with fine chippings and compacted by rolling. The stability of surfacing of the impregnation is ensured mainly by the wedging action of the chippings, which takes place during the rolling process. Among the shortcomings of this process is the comparatively high consumption of binder per unit area.

C. Broken-stone surfacings and bases made of uniform size chippings (macadam). The strength of broken-stone surfacings is provided by the wedging action which takes place during rolling. The major factor determining the stability pf the surfacing is the friction developed between chippings, also the cementing action of the stone powder formed by abrasion of the chippings during rolling. The abrasion of the edges and the crushing of the stone, in addition to the penetration of mud de­posited on the surface during use of the road give rise tо the appearance of sandy, silty and clayey particles within interstices and hence to the loss of cohesion by the surfacing especially during wet seasons.

Broken-stone surfacings have a low resistance to wear under automobile traffic, since the tangential, stresses of pneumatic tyres destroy the efficiency of packing. Consequently, such pave­ments are used as an independent type of surfacing only when the traffic intensity is low. More often they are used to pro­vide the road with a base laid beneath a surfacing treated with organic binders.

Surfacings of natural gravel or artificially graded gravel mixtures.

The strength of the material is provided by grading as close­ly as possible to the optimum mixture, keeping the interstices between big particles filled with finer ones so that the material, asa whole, has the minimum porosity. Cohesion is achieved by introducing fine mud and clay particles into the mixture. In humid seasons of the year the strength of the surfacing may be reduced owing to the decrease of cohesion.

The gravel road is the cheapest form of road and the simplest from the construction point of view. It has high strength and sta­bility when does not contain am overlarge quantity of fine fractions, which make the mixture plastic in wet conditions. Pavements of local weak materials and of industrial waste products are constructed in a manner similar to the gravel type roads.

Types of pavements. Depending on the riding qua­lity road pavements are classified as high-quality, intermediate and inferior. When classifying road pavements the decisive fac­tors are the permissible traffic speed and the rate of strain accumulation in them.

Bases beneath heavy-duty surfacings must maintain the required strength throughout the year, without showing any decrease during the wet season.

Bases under the high-quality surfacings may be made of the following types, depending on the required strength of the pavement and the availability of local building materials: broken stone, gravel, blast-furnace slag, cinder and other local industri­al waste materials, of the soil used for roadbed treated with binders, i.e. bitumen, cement or lime.

The intermediate and inferior types of pavements are laid directly on the: bed soil, with the exception of broken-stone pavements, which should be laid on a base of soil treated with binders, or of slag or other local materials.