Road Structure

Introduction
The key to understanding the makeup of road structure is thinking about what roads are actually for! Different Parts of Roadway Note, the terminology used here relates to British practice. Paved road surfaces provide a means for vehicles to move along without sinking into the surface. In the days of mud track roads, vehicles (or carts then) would tend to sink into the surface. Although travel on the M25 in rush hour is often no faster than a horse and cart, at least your Ford Mondeo doesn't sink up to its axles every time it rains. Vehicles concentrate a lot of weight through a small surface area, the tyres. Paved road surfaces allow this weight to be carried, then distributed down and evened out, so that the ground under the road supports the weight without distorting. The structure of a road is actually called the "pavement." The more normal everyday use of this word, the surface next to the road we walk on, is a hangover from the Victorians. Until Victorian days, footways (which is the correct term) were mud tracks next to the road surface, or pavement. Once they started using the same rigid materials to construct the footway, people started to use the same word, pavement, to refer to it. There are two main types of pavement (road structure), flexible and rigid. Rigid roads are more complex to build, requiring more specialised equipment. The current preference within the road building industry is to flexible and composite roads (see later). Rigid roads consist of a thick concrete top surface. Where voids appear below the surface, in a flexible road, the surface will sink. This will not (normally) happen with a rigid or composite road. The danger here is that in exceptional circumstances where a large void has appeared, eventually the unsupported concrete will collapse. Composite pavements (road structure) are where a flexible layer has been added on top of the surface of a rigid road, or where a concrete layer exists below a bitumen top surface.
Flexible Carriageway Construction
When a road is built, the surface is dug-out down to the designed depth of the intended road. Preparation is carried out on the ground now exposed below (such as compaction). The road itself will then be built up above, usually consisting of four layers: -
-------------------------------------- Surface Course -------------------------------------- Binder Course -------------------------------------- Base -------------------------------------- Sub - Base -------------------------------------- Capping - - - - - - - - - - - - - - - - Sub - Grade Flexible Road Structure The sub-grade is the ground below the road layers which is exposed once the ground has been dug out ready to build the road. The top level of this is termed the formation. The capping is a layer added above the sub-grade to protect it in new constructions. In this case, the top layer of the capping will constitute the formation. The four (typically) layers of the road above are termed (bottom to top) sub-base, base (formerly known as roadbase), base (formerly known as roadbase), binder course (formerly known as basecourse) and surface course (formerly known as wearing course).
As the stress transmitted through the road structure from the vehicles above spreads and lessens with depth, stronger and more expensive materials are needed in the upper levels. Additionally, the nearer the surface, the flatter the profile must be. This is obviously because an uneven surface will be uncomfortable for vehicle occupants and will wear more quickly (each time a vehicle hits a bump, it is in effect hammering the surface). These factors are the main reasons for the layered construction of the road. Weight on any unbound material will compact it down with time, as material is forced down and fills gaps. For this reason during construction of each layer, artificial compaction is carried out. In fact, each of the layers of the road structure are usually laid in layers themselves, with further compaction taking place each time. Roads are designed in the UK to last for 40 years, with a major reconstruction in the middle, before needing to be completely reconstructed. The life time of a road can be reduced by greater than expected increase in traffic, though a certain amount of traffic growth is allowed for when the road is designed. The only factor taken into account here is the expected amount of commercial vehicles. This is because the damaging effect of an 8200kg axle load is 100 times that of a 2700kg axle load, even the latter being greatly more than the axle load of a private vehicle.
The Sub-base
The sub-base should be laid as soon as possible after final stripping to formation level, to prevent damage from rain or sun baking which could cause surface cracks. The fact that this is required when roads are constructed, emphasises the importance of backfilling excavations quickly and properly and preventing ingress of moisture when roads have been excavated for utility works. The most commonly used material for use in sub-bases is termed Type 1. This is an unbound material made from crushed rock, crushed slag, crushed concrete, recycled aggregates or well burnt non-plastic shale. It contains particles of various sizes, the percentage of each size being within a defined range. Up to 10% may be natural sand. The predefined and calculated range of material sizes contained means that once compacted, it will resist further movement within its structure. In other words, it tends not to sink with time (though it will sink if not compacted properly when laid). Other materials used for the construction of sub-bases include bituminous-bound materials and concrete and cement-bound materials, including wet-lean concrete.
Sub-base and Base materials
Again, Type 1 is most commonly used. Other materials include Type 2 and Type 3. Slag bound material used to be known as Wet Mix. It is a plant manufactured granular aggregate. It must be laid and compacted quickly, as this must take place within 6 hours of the GBS and activator components. Various other materials are less commonly used. All materials on arrival from the plant must be protected from the weather, as drying or wetting changes the composition. They must be spread evenly. They are laid in layers of 110mm - 225mm compacted thickness, the thickness of the layers being gauged by various means including pegs and lines, sight rails and a guide wire. In initial build and reinstatement, the thickness of the layers depends on the compaction plant being used. Bituminous base materials are either dense base macadam or rolled asphalt. Various concrete and cement - bound materials are used, the specifications for these being different to those applying for sub-base materials.
Surfacing
Both the surface course and binder course are included in the part of the road structure termed the surfacing. Occasionally the surfacing is laid as a single course. Normally, it is layed as two course binder and surface. The binder course helps distribute the load of traffic above onto the base course, which is usually a weaker material. It also provides a flat surface onto which the normally thinner surface course is laid. In new construction, typical thickness is between 45mm and 105mm. Thickness may vary considerably where a new binder course is laid to an existing road structure for strengthening purposes. Stone sizes used are 20, 28 or 40mm. The thicker the binder course, the larger the stone size. Materials used include open graded macadam, dense coated macadam and rolled asphalt. Surface courses are laid in a wide range of bituminous materials, ranging in thickness from 20 to 40mm. The material selected is dependent on the anticipated traffic intensity. The two most commonly used surface materials in the UK are HRA and SMA. Hot rolled asphalt is made with high fines and asphaltic cement content with crushed rock, slag or gravel added. Normal thickness is 40mm with 20mm coated chippings rolled into the surface providing better skid resistance. Stone mastic asphalt is not as susceptible to rutting as other surfaces and reduces surface noise. Normal layer thickness is between 20mm and 40mm.