When pavement is built, engineers take many conditions into account – including soil conditions, location, and expected traffic levels. They then design a pavement to meet those needs for its intended service life.
A well-maintained Pavement Philadelphia can save you money in the long run. It also reduces the risk of accidents and liability.
The subgrade is the native soil layer beneath a pavement. The subgrade is designed to withstand the loads transferred through it by the pavement layers above it. The material used to build a road’s subgrade should be well-draining and strong enough to support the load of vehicles. A weak subgrade will cause the pavement to deform and crack. It may also require special drainage structures or construction to allow water to pass through it. A subgrade should be graded to a level within plus or minus 1.5 inches of the specified elevation.
In the case of a roadway, the subgrade is usually comprised of granular soils or a combination of granular and coarse aggregates. The subgrade is heavily compacted in order to withstand the stresses placed on it by the pavement layers above it.
Depending on the type of soil, the subgrade can be stabilized using various methods. In the case of granular soils, lime or cement can be added to the material to increase its stability and strength. This can be done in addition to regular compaction. In the case of cohesive soils, a geotextile can be used to prevent the mixing of soft or inadequate material with the rest of the subgrade.
The bearing capacity of the subgrade is measured by MR or CBR tests. A good indicator of a soil’s capacity to support pavement is how easily it forms a string when squeezed in the palm of your hand. A string that forms a ball but breaks into several pieces when dropped is considered high in bearing strength, while one that feels powdery and does not hold its shape or leave moisture on your hands is low in bearing strength.
A layer of material that is placed above the subgrade and below the base course is called the sub-base course. Its major role is to provide structural support, improve drainage and reduce the intrusion of fines from the subgrade into the pavement structure. It is often stabilized with lime, cement or emulsified asphalt. It can also be topped with a capping layer to improve its strength and durability.
Pavement is the durable surfacing of a path or roadway. It is typically made from gravel or crushed stone bound together with bituminous material such as asphalt (tar or asphaltic oil) or concrete and has sufficient plasticity to absorb the stresses of passing vehicles without permanent damage or buckling. It is a key component of the transportation infrastructure and is the main means by which people move around the world. Enhanced Safety: It provides a safer driving surface than unpaved roads during inclement weather and poor road conditions. Reduced Maintenance Costs: It lasts longer than unpaved surfaces and requires less frequent repairs. Reduces Pollution: Pavements are more environmentally friendly than unpaved roads because they use fewer fossil fuels and are easier to maintain than dirt roads.
The construction of flexible pavement involves a series of layers including one or more unbound granular base layer, a sub-base and an asphalt binder course. It also includes an asphalt surfacing layer that provides the final riding surface with skid resistance and water-proofing capabilities. The layers are designed to transmit wheel load stress to the underlying subgrade in a manner that is resilient and distributes the load over a larger area.
A sub-base is a layer of materials that is placed directly beneath the surface of the binder course. It provides additional load distribution and contributes to the sub-surface drainage. It can be composed of different materials such as crushed stone, crushed slag or other untreated or stabilized aggregates.
The binder layer is a mixture of aggregates and liquid asphalt which binds the stone together. This layer is applied in a thin coat that must be uniformly spread, set quickly and provide a water-proof surface. It is a critical part of the pavement because it serves to protect the lower layers from the effect of water.
Conventional layered flexible pavement is constructed with high-quality expensive materials in the top layers to resist the highest compressive stresses as well as wear and tear while low-quality cheaper material is used in the lower layers. This system can be modified by using dowel bars that transfer load across contraction joints. Rigid pavement is constructed of plain cement concrete with closely spaced contraction joints. It is normally analyzed by plate theory where it is assumed that the concrete slab is a medium thick plate which is plane before loading and remains flat after it.
The granular base layer of a pavement lays directly below the asphalt or portland cement concrete (PCC) surface layers. The granular base is an important structural component of both types of pavement structures, as it reduces the vertical compressive stress on the soil subgrade and helps to distribute wheel load stresses throughout the entire layer structure. In addition, a granular base also improves the drainage characteristics of a pavement and insulates it from frost damage.
The strength of a granular base depends on its material properties and the amount of compaction it receives. Typically, the best material is locally available, since hauling quality aggregates from long distances increases construction costs. The most common sources of granular material include sedimentary rock materials such as limestone and sand or gravel derived from natural deposits and industrial byproducts including crushed glass, furnace slag and recycled asphalt. The quality of a granular base material can be determined by physical and chemical tests, such as Los Angeles abrasion and Micro-Deval.
A good granular base will have high stability, especially in flexible pavements that transfer wheel loads stresses through grain-to-grain contact. For this reason, a dense-graded base with a mixture of particle sizes that are tightly packed together is preferred. However, open-graded granular bases that allow water to flow freely through the layer also have adequate performance in some applications.
Stability of a granular base also depends on its moisture content and drainage characteristics. The amount of fine particles in a granular material can affect the moisture content and gradation of the base, but it also has a direct effect on its drainage properties. An optimum gradation of a granular material will maximize its drainage and stability characteristics at the same time.
The granular base layer must be strong enough to resist the effects of traffic loads and provide a stable platform for construction equipment operations, as well as to prevent rutting in the lower pavement layers. This is why a granular material should be chosen that can be tested to determine its resistance to damage, such as the Los Angeles abrasion test or the Michigan Bearing Ratio.
The asphalt layer is the final and topmost layer in a pavement structure. It is the driving surface of a road and provides friction, smoothness and noise control as well as rut and shoving resistance. The asphalt layer is designed to withstand the load of heavy vehicles and the stresses exerted by them during a drive on a pavement. It is also required to have good workability to allow it to be compacted to a dense level for optimum durability.
The layer of asphalt is placed on top of the prepared runner-crush stone base. It is known as the binder course and it utilizes bigger aggregate sizes compared to the top course of a pavement. The top course is also known as the wearing course or surface course and it uses smaller aggregates compared to the binder course.
An asphalt layer needs to have high fatigue resistance, tensile strength and stiffness modulus to withstand the loads of traffic. Engineers ensure desirable levels of these properties in the asphalt material through proper mix design. However, these materials will exhibit complex behavior when subjected to stress and temperature variations. This is because the engineering properties of a material are dependent on the relative composition and the responses of different constituents are inversely proportional to each other.
For example, while fatigue strength and stiffness modulus increase with a decrease in the content of aggregates in the asphalt mixture, the shear strength will decline. This is why it is essential to use the best quality materials for all layers of a pavement in order to get the best results.
Various asphalt mixes are available for different applications and the type of mix used is based on factors like amount of traffic, climate conditions, noise reduction requirements, and pavement design life. For overlay designs, the asphalt mix should have a higher stiffness than that of the existing granular layer to avoid buckling.
The most common asphalt mixture is a hot mix or blacktop. It has been blended with an appropriate quantity of a binding agent like bentonite clay and recycled asphalt, and is heated to a liquid form in a mixer. Cold mix asphalt is a variation of this material and it stays soft in cold temperatures and repels water. It is mainly used for pothole repairs.