Pavement is a key element of highway transportation. It has a variety of functions including providing a safe and smooth riding surface for commuters.
All pavements start to deteriorate after a period of time and eventually need to be repaired or replaced. Many of the features of a pavement can support sustainability objectives but there are also trade-offs and unintended consequences.
The subgrade is the layer of soil that forms a foundation for the other pavement layers. It is important that the subgrade has sufficient strength and stiffness to support the loads acting on the pavement, as well as adequate drainage. Subgrade failure can lead to rutting, surface fatigue and/or differential settlement of the asphalt and base courses. Distresses caused by volume changes of subgrade soils due to wetting and drying, freezing and thawing, and inadequate or clogged drainage systems can also cause problems.
Suitable subgrade material must be selected and thoroughly compacted. It is important that the soil particles are tightly packed together to limit the amount of moisture entering the subgrade, as the moisture will have a negative impact on the load-bearing capacity and strength of the soil. The optimum moisture content for maximum compaction and density must be determined through proper soil engineering procedures.
Many different construction methods can be used to improve the subgrade. The most common is the use of a high-speed road roller, but this method often has a negative effect on the overall stability and strength of the subgrade. In addition, the process of rolling tends to skew the sampling area, thus missing critical subsurface features and conditions. More recently, better techniques have been developed that provide more uniform subsurface evaluation and compaction. One of these is the index method, which involves dropping a specified weight on a thick-walled tube sampler to obtain an index strength value and disturbed sample of the subgrade. Although this technique has some reliability, it still evaluates only a small fraction of the total roadway alignment and is subject to errors from soil variability.
The subgrade is a critical factor in determining the long-term performance of the pavement. It must be strong enough to resist the forces applied to it, while allowing water to penetrate into the underlying granular soil for adequate drainage and stability. It is also important that the subgrade be free of voids and depressions that can trap moisture and reduce its bearing capacity. Nonuniform permanent deformations within the wheel path can contribute to surface roughness, which in turn impacts traffic flow and safety.
Binder Course
The layer that sits between the base and surface is known as the binder course and consists of bituminous-aggregate mixture. It provides additional load distribution and contributes to sub-surface drainage. It can be constructed from a number of different materials including crushed rock, gravel, slags and recycled asphalt pavement (RAP). A wide variety of HMA mixes are available for use in the construction of the binder course. In general, these HMA mixes have larger maximum aggregate sizes and are more open graded than surface course mixes. This allows the use of more economical aggregates and results in lower HMA cost than if a surface course mix were used.
In addition to providing additional support for the structural layers, the binder course also helps to provide a more uniform surface to the wearing course. This helps to reduce the likelihood of reflective cracking and premature distress of the surface course. This is the primary reason that a properly designed and implemented pavement preservation program should be able to identify distress in the wearing course while it is still confined to this layer and can be repaired before it propagates into the underlying intermediate/binder course.
Once the granular sub-grade has been placed, compacted and stabilized, the construction of the HMA layers can begin. Each of these layers serves a specific purpose in the flexible pavement structure.
The first of these is the binder course. This consists of an HMA that is mixed with a high percentage of aggregate and binds the aggregate together to form a cohesive mixture. The aggregates can be a variety of materials including crushed rock, gravel, sands and recycled asphalt pavement (RAP). These types of material are very economical and provide a good leveling surface.
This material is then placed over the reclaimed soil or prepared subgrade and finally topped with the surface course of asphalt. The thickness of the surface course is engineered to meet the traffic demands.
One of the major advantages to using this type of paving material is that it is produced at a temperature much lower than hot mix asphalt. This results in less energy usage and less fumes during paving operations.
Base Course
The base course is a layer of aggregate material placed on top of the sub-grade. It is usually made of dense bituminous macadam (dbm) or asphalt concrete (HMA). Its chief function is to distribute load over a wider area and provide a stiffer support than the sub-grade.
A properly constructed base course is a key component of the success of a pavement. It requires attentive spreading and compacting to a minimum 95% relative compaction. The quality of the base course is critical because it will have a significant influence on the structural capacity of the pavement. It must be free draining, highly resistant to erosion and able to support the wheel load stresses of traffic. It is also required that the base course not be contaminated by the fines generated during construction.
Base courses are most commonly placed under asphalt pavement roadways, but they are also used under concrete slabs and foundations. In addition, they are frequently placed under utility pipelines and other underground pipes within a pavement structure.
The type of aggregate material used is a significant factor in the performance of a base course. Aggregate must be durable, with a high CBR value and a good gradation, and must not be susceptible to damage by moisture or frost. It is recommended that the base course be made of a mix with lower asphalt content than surface courses to reduce the risk of damaging the surface finish.
A tack coat is typically applied on top of the stabilized base layers and between lifts in thick asphalt concrete surface courses to promote bonding of the various layers of the pavement. The tack coat is also sometimes used to prevent the intrusion of water into the base or surface layers.
In rigid pavements, the base course is normally not needed because the flexural strength of the concrete provides sufficient stability. However, if the base course is needed, its function is to minimize damaging effects of frost action, improve drainage, prevent pumping of fine-grained soils at joints, improve the strength and stiffness of the pavement, and expedite construction.
Surface Course
A pavement surface can be made of a wide range of materials but is typically composed of Asphalt material (an industrial material that is formed by mixing aggregates such as sand, gravel or crushed rock with binder and filler; bitumen being the most common). The function of the surface course is to provide sufficient tire friction to facilitate safe driving and to give skid resistance. It also prevents the ingress of rainwater into the underlying structure.
A properly constructed surface course should have good durability to protect the underlying layers from weather influences, heavy (freight) traffic and de-icing salt. It should also be water-draining to avoid the build up of ponds on the road.
The permeability of the surface course can be enhanced by providing good drainage through the underlying structures or by introducing permeable pavement sections. The permeability of the surface course can also be enhanced by the use of special materials.
The layer of aggregate material immediately below the base course in a flexible pavement is called the sub-base course. The primary functions of the sub-base are to provide structural support, improve the drainage and reduce the intrusion of fines from the subgrade into the base course. It may be treated with Portland cement, lime, flyash or other admixtures to increase its strength and stiffness.
It may be necessary to include a sub-base course in a flexible pavement if the quality of the sub-grade soils is poor and/or a material suitable for the base course cannot be obtained locally. The inclusion of a sub-base can help to minimize damage caused by frost action by providing insulation to the underlying soils.
The sub-base is generally a lower grade of material than the base course and consists of aggregates which may be either stabilized or unstabilized. It can be made from uncrushed or crushed stone or from HMA. In some circumstances a sub-base is not needed and the base course can be built on a stiff enough sub-grade that it does not require further strengthening. The sub-base is typically designed with a lower aggregate size than the base course and a bituminous concrete mix is used.
