What are flexible pavements? Flexible pavements are a set of new construction materials and paving techniques that redefine the whole nature of what a road, driveway, or parking lot is. They allow for more than just ‘one thing’ to happen – rather, they provide flexibility in the way any particular element is built.
This means that you have options when it comes to designing your site and paving it – depending on how you want to use it!
Flexible Pavements are constructed from bituminous or unbound material and are supported by flexible ribs. Flexible Pavements are not like conventional concrete and asphalt roadways; their flexibility allows for the placement of multiple roads or driveways within the same space and permits expansion.
Flexible pavements can be used in all weather conditions, they may be sunk into the ground to create a level surface, or rise above the surface so that vehicles can access higher driveways and parking areas.
What are flexible pavements?
Pavements are the construction materials used to form the surface of roads and sidewalks. The materials used to pave a roadway may vary. The choice depends on types of traffic, maintenance and cost. Concrete, asphalt and gravel pavements are the most common types of pavement materials.
Conventional concrete pavement is the most common type of concrete pavement because it has a low cost and good wear resistance, but is slower than asphalt because trucks must slow down as they move over it.
One advantage of asphalt is that it can be installed quickly using epoxy-modified bitumen instead of liquid asphalt (often called hot mix asphalt) or precast concrete forms (PCF). However, asphalt has rapid deterioration due to environmental factors such as heat, cold, salt and pollution. Low-quality asphalt pavement deteriorates faster than high-quality pavement because it is not resistant to environmental factors.
Another disadvantage of conventional concrete is its low modulus of elasticity (MOE) which limits the road’s flexibility and durability. The road is rigid and brittle when temperatures get low or when there is a significant amount of traffic on the road.
Conventional concrete pavement also has poor resistance against hydrostatic pressure (pumping water or ground water into the pavement) in addition to poor resistance against salt damage. In general, a flexible pavement allows for more stability in design with less maintenance.
Flexible pavement is composed of a bituminous material surface course and underlying base and subbase courses. As defined in the National Design Specification (NDS) for Roads and Bridges, flexible pavement is a “pavement designed to accommodate or correct for fluctuations in ground elevation by either cambering or by providing flexibility. A flexible pavement is generally supported on flexible, as opposed to rigid, supports.”
Flexible pavements can be seen as a third generation of roadway design and construction. The first generation, which is represented by rigid pavements, features supporting structures that do not move. The second generation includes structures that are flexible and expand when loaded. Examples of this include elastic membranes and concrete slabs constructed with horizontal steel beams which allow the structure to respond to thermal expansion.
Flexible pavements were developed to expand the application of asphalt pavement. They offer an alternative to conventional concrete, particularly where the ground or subgrade condition is not suitable for traditional rigid pavement applications.
Flexible pavements are commonly found in surface streets and city streets. While these kinds of areas are usually smaller than freeways and highways, they are usually quite busy due to their location.
When flexible pavements are designed on a site, they may emanate from the edge of the curb on all sides or only from one side. The number of lanes in each direction can also vary, although most cities restrict their use to two-lane roadways.
Flexible pavement is installed by pouring it into temporary forms and allowing it to cure at room temperature for about two weeks. Then it is poured into the permanent form, with asphalt used as an overlay forming the subbase course. The base and subbase can be either single or double course concrete or bituminous material.
Where required, a layer or cap may be added to provide drainage during wet weather conditions. The pavement is then equipped with a seal coat, which provides a smooth surface for low-speed traffic. Conventional joints are constructed between pavements to provide for movement.
Transverse joints provide for longitudinal movement. Lateral joints permit flexing to occur perpendicular to the roadway centerline. Flexible pavements may be used as support systems for larger geotextiles and geo-composites which will then function as waterproofing and prevent erosion in areas that are not paved.
Flexible pavements in roadways may also perform other important functions, such as providing noise barriers or creating secure areas within parking lots or other facilities without disrupting the general traffic flow of the road.
Types of pavements
There are two types of pavements based on design considerations i.e flexible pavements and rigid pavements.
Flexible pavement can be defined as the one consisting of a mixture of asphaltic or bituminous material and aggregates placed on a bed of compacted granular material of such size that interlocking of particles does not occur, and so graded that movement is possible either in the longitudinal direction or parallel to the edges of the pavement.
The mixture should be such that, when compacted, it will be uniform in thickness and at the same time offer resistance to movement on account of flexibility. The stone aggregate should be so graded as to allow free passage of water through its interstices while preventing the flow on its surface.
The peculiar feature which distinguishes flexible pavements from other types of pavements is their ability to move with respect to each other under load. This movement may occur in both longitudinal and lateral directions between these layers.
Bituminous flexible pavements are a good solution for road applications that require high strength, low-cost and environmentally friendly pavement solutions. There are some negative aspects to the flexibility of the roadway, in which the tyre movements may be amplified leading to increased damage to tyre side wall and tread regions as they travel across the pavement.
A rigid pavement is constructed from cement concrete or reinforced concrete slabs. Grouted concrete roads are in the category of semi-rigid pavements. Its performance characteristics are determined by the grading and compaction of the bedding materials on which it is laid.
It is designed to prevent movement between the various layers, and to allow traffic to operate at the highest consistent speeds possible, with the least maintenance required. Rigid pavements are often found on high-speed roadways due to their ability to handle heavy loads. They have a high resistance against load and can be used in areas with heavy traffic.
The rigid pavement has rigidity and high modulus of elasticity to distribute the load over a relatively wide area of soil. Its resistance to heat and cold is lower than flexible pavements. It also offers a longer life than flexible pavement because the weight of the vehicle body compresses the soil, thereby lessening the impact of traffic.
Rigid pavements can be used in areas with high traffic such as highways, boulevards and parking lots.
Advantages and disadvantages of flexible pavements
Now that we have known what flexible pavements are, lets take a look at the advantages and disadvantages of flexible pavements.
- Flexible pavements do not require sealing due to their ability to adapt to the environment and are therefore less affected by cold or heat.
- Resistance to cracking and potholes as they can shift with traffic and temperature, thus reducing the gap between pavement joints.
- Generally cheaper than rigid pavements, due to lower material costs, shorter construction time and low maintenance cost.
- Ability of the roadway to handle heavier loads unlike rigid pavements which have a lower modulus of elasticity (MOE).
- The functional evaluation studies can be carried out at desired intervals and the deteriorated functional condition of the road surface can be restored with a thin bituminous re-surfacing layer.
- Susceptible to oil stains and damage from other chemicals
- Potential for long-term damage to vehicle tyres. Due to the flexibility of the road, tyre movements may be amplified causing damage to the side wall and tread regions of a tyre as it travels across the pavement.
- Welding/splitting of sections under extreme leverage from movement over joints.
- It is difficult to carry out repairs of deteriorated bituminous pavements or patching of pot-holes during the rains or under wet weather conditions.
- The bituminous pavement layers get deteriorated when exposed to stagnant water due to poor drainage of surface and subsurface water.
What are flexible pavements? Flexible pavement is a type of road which is capable of shifting with traffic and temperature. The flexibility of this pavement reacts to stress from vehicle loads and therefore enables load distribution over a relatively wide area. This reaction is similar to the way in which an airplane wing deflects under load.
Flexible pavements are found at airports and have been in use for more than 40 years. Flexible pavements (also known as asphaltum macadam) form the subgrade-base-subgrade component of flexible pavements.
They are made up of a mix of bitumen, graded aggregates and water. A flexible pavement system consists of two layers, which are placed above each other. The lower layer is referred to as the subgrade, which is the foundation of the roadway. The uppermost layer is referred to as the wearing course/surface course, which is the road surface itself.
The wearing course also contains aggregate in a bituminous mixture; however, it has less aggregate than the subgrade below.