What is a road pavement, and how is it designed?
A pavement is an engineered structure forming the surface of a route (such as a road or footpath) to bear the load of its intended traffic. Here, we’re referring to the surface and underlying structural layers in road construction. A paved road has a surface course, which can be made from a number of materials such as asphalt concrete (commonly known as asphalt) or portland cement concrete. The term ‘composite’ is typically used to describe the entire pavement structure (e.g., an asphalt layer over a concrete base), rather than the surface material itself. The material choice for this surface course is an important design component because it provides the functional traffic surface (e.g., smoothness, skid resistance) and acts as the top layer of the integrated pavement structure, which is designed to distribute traffic loads to the underlying subgrade. In this blog, we will focus on the construction of road pavement, its design principles, and updated methods used for pavement design.
What are the different types of pavements in road construction?
In road construction, the type of pavement selected plays a critical role in determining the overall performance, longevity, and maintenance needs of the road. Pavements are engineered to withstand the weight of traffic loads, the effects of environmental conditions, and the stresses placed on the road surface over time. There are three main categories of pavements: flexible pavements, rigid pavements, and composite pavements. Each type has distinct characteristics that make it suitable for specific applications, depending on factors such as soil conditions, traffic volume, climate, and budget considerations.
- Flexible Pavements: Flexible pavements comprise several layers, and the top surface layer is typically made of asphalt for paved roads and highways. Also known as the surface course, these types of pavements are flexible in nature and can accommodate small deformations in the underlying layers along with loading of different types of traffic. Flexible pavements distribute loads to the subgrade through a combination of layers. The stress is highest at the surface and gradually decreases with depth. They are termed “flexible” because the entire pavement structure deflects under load.
- Rigid Pavements: These are pavements made of concrete slabs that possess high rigidity and structural strength. Unlike flexible pavements, rigid pavements distribute loads over a wide area due to their high stiffness and beam-like action. This significantly reduces the stress on the subgrade, which is the primary design advantage, and results in very low deflection at the pavement surface.
- Composite Pavements: Composite pavements combine the features of both flexible and rigid pavements, typically by having a layer of asphalt over a concrete base, or less commonly, concrete over an existing asphalt pavement (whitetopping). The main intention in the design of this type of pavement is to make use of the most favorable properties of both materials on one pavement structure, combining the smooth, renewable surface of asphalt with the structural strength and durability of a concrete base.
What are the components of a road pavement?
Road pavement is a composite structure comprising several layers, and each layer has a different function. These layers work together to provide structural support to carry traffic loads for a specified design life. Key functions that contribute to this include providing stability and water resistance. Asphalt (or other binders/sealers) binds aggregate layers (in flexible pavements) or seals the surface, helping to protect the road from bad weather and water infiltration. Explaining these factors is vital in understanding the whole layout and operation of a road.
- Subgrade: The subgrade is the native soil or prepared fill that has been compacted to form the foundation upon which the pavement structure is built. A subbase layer, if used, is placed on top of the subgrade. The strength and load-bearing capacity of the subgrade are critical factors that determine the performance and durability of the overlying pavement structure. Weak subgrade conditions can result in the rutting of the overlying pavement and thus reduce its lifespan. Hence it is necessary to test the soil first and take proper measures to strengthen the soil quality before constructing a pavement. Geosynthetic materials like geogrids and geocells are effective methods for strengthening the subgrade soil, often used based on site requirements and engineering design.
- Base course: The base course is a layer of material placed above the subgrade (or subbase, if present). It serves as a primary structural component of the pavement, providing a stable foundation for the upper layers and distributing the loads down to the subgrade. The strength and stability of the base course are critical to the pavement’s overall performance.
- Binder course: The binder course is a layer of bituminous material located between the base course and the wearing course. Its primary function is structural; it distributes the heavy loads from the wearing course down to the base course. Composed of larger aggregates than the surface layer, this course provides the bulk of the asphalt structure’s strength and fatigue resistance, while also bonding the wearing course to the layers below.
- Wearing course: The top course, the wearing course of the pavement, comes in direct contact with the traffic and seals the surface. It is mainly constructed out of asphalt or concrete, which is selected depending on factors like expected traffic loads, environmental conditions, and maintenance considerations. The properties of the wearing course material, like its durability and skid resistance, are key characteristics for safety and a comfortable ride. The material selection is dependent on parameters such as traffic volume, climate, and aesthetics. For instance, asphalt is usually the choice for highways and residential roads, whereas concrete is the typical option for high-traffic areas like airport runways and central boulevards.
What factors influence road pavement design?
The road pavement design process is a multilateral approach that carefully considers different variables. These variables can significantly affect pavement performance, durability, and long-term maintenance costs.
- Volume: Vehicle volume, often characterized by the cumulative number of Equivalent Standard Axle Loads (ESALs) over the design life, directly affects the thickness and material selection of the pavement. High traffic requires thicker and more durable pavements.
- Weight: Vehicle weight, especially that of heavy trucks, is a crucial attribute. These trucks put more pressure on the pavement, and thus, the pavement structure must be designed to be stronger, which may involve increasing layer thicknesses or using higher-quality materials.
- Speed: While vehicle load and volume are the primary factors for structural thickness design, vehicle speed influences other important aspects. Higher speeds affect geometric design (e.g., superelevation on curves), require higher skid resistance from the wearing course, and can increase dynamic loads at pavement joints or rough spots.
- Climate: The pavement response is influenced by the variable thermal conditions exhibited by extreme temperatures, the presence of precipitation, and freeze-thaw conditions. Hot weather sometimes softens the asphalt, leading to permanent deformation (e.g., rutting), whereas cold weather can lead to frost heave and cracking. Changes in pavement conditions can significantly impact pavement performance, and engineers must consider these climatic factors carefully.
- Soil type: The properties of the underlying soil such as its strength, moisture content, and frost heave susceptibility, affect the pavement’s design. Soils in poor condition that have not been adequately prepared or stabilized may need additional layers or treatments to provide adequate support.
- Drainage: Good drainage is needed to avoid water infiltrating and accumulating within the pavement structure and causing damage. Approaches like ditches, culverts, and subdrains should be used to remove or divert water from the pavement properly.
- Life cycle cost analysis: This method involves not just looking at the construction costs but also the life cycle of a pavement (the whole time the pavement is in service), including construction, maintenance, and rehabilitation. It helps find the most cost-effective design.
- Geographical situations: Land topography, current infrastructure, and utility locations can influence the planned pavement. For instance, steep slopes may require erosion protection. Underground utilities may also limit how deep you can get with the excavation.
- Budget: The available budget is a primary constraint that influences material selection, layer thicknesses, and the overall scope of the pavement design. Designers must work within budgetary limitations to achieve the best possible performance and longevity.
- Material Availability and Cost: The local availability and cost of construction materials significantly influence design choices. If specific high-quality materials are expensive or difficult to procure in a region, designers may need to consider alternative materials or designs that utilize locally available resources more effectively.
What are the different ways of designing road pavements?
Pavement design involves identifying the appropriate amount of materials and level of thickness required to create a durable and functional roadway. Several methods are used to accomplish this, each with its own specific benefits and drawbacks.
- Empirical methods: This method involves using historical data and past experience to design the pavement. Straightforward formulas or charts typically compute these methods using a pair of factors, such as the traffic load and the environment in which the pavement is located. Though these methods are straightforward and require relatively little data, they can be less accurate when applied to complex pavement structures or unusual conditions.
- Mechanistic-empirical models: These methods employ the theories of pavement mechanics together with empirical evidence. They investigate the stresses, strains, and deflections in the pavement structure due to the different loads. These approaches have a higher degree of reliability, especially in the case of complex designs, but will need more data and computer resources.
- FEA (Finite Element Analysis): It is a computational method that is used to numerically solve complex engineering problems by dividing the structure into a mesh of smaller elements. This technique breaks the pavement structure into a network of elements and calculates the stresses and strains (and other responses) in each component. Although FEA can yield highly accurate results for complex pavement geometries and loading conditions, it is computationally intensive and requires expertise in specialized software and numerical analysis.
Using geosynthetic materials in road pavement construction
It is crucial to use durable and stable materials while constructing road pavements as the demands on road infrastructure are increasing. Geosynthetic materials, including geogrids, geotextiles and geocells, have emerged as key components in modern pavement design.
How geosynthetics enhance pavement performance?
- Soil reinforcement: Geosynthetics like geogrids and geocells are placed within the subgrade, at the subgrade/subbase interface, or within base/subbase layers to provide extra strength. They interlock with soil or aggregate particles and form a composite that is resistant to deformation when under load.
- Prevention of lateral movement: Geosynthetics also prevent the lateral movement of aggregate or subgrade material, which helps maintain the structural integrity and prevent issues like rutting or bearing capacity failure. Confining the soil or aggregate within an area, these geosynthetics maintain subgrade or layer integrity and hence provide a stable foundation to the pavement layers above.
- Separation: Geotextiles are used as a separator between fine-grained subgrade soil and the granular base course. This prevents the layers from mixing under load, which maintains the structural integrity and drainage capacity of the base layer.
- Filtration: In drainage applications, geotextiles act as a filter, allowing water to pass into a drainage system while preventing fine soil particles from clogging it. This helps manage water within the pavement system and maintain subgrade stability.
- Even load distribution: By spreading the loads over a larger area, geosynthetics reduce stress concentrations on the subgrade and lower layers. This even distribution prevents localized failures—such as rutting or cracking—which occur when heavy traffic loads are concentrated on specific points.
- Increased pavement lifespan: Geosynthetics prevent or minimize pavement deformation and deterioration by improving structural capacity, reducing stress on underlying layers, and maintaining layer integrity, thereby enhancing the overall distribution of load and mitigating distresses.
- Reduced material and labor costs: Geosynthetic reinforcement allows the pavement layers to be made thinner because they reinforce the subgrade and distribute loads more effectively. This reduction in thickness can result in significant savings in construction materials and labor costs and, therefore, make geosynthetics cost-effective for modern pavement design.
- Sustainability: Geosynthetics enhance pavement durability and longevity which can reduce the frequency of repairs and reconstruction, thereby diminishing the associated environmental impact.
- Resource efficiency: Geosynthetics enable the reduction of the thicknesses of pavement layers, conserving natural resources since less aggregate and asphalt or concrete would be required. This efficiency not only reduces costs but also decreases the environmental footprint associated with the construction process itself.
- Adaptability to various conditions: These materials can easily adapt to the different soil and pavement types. Whether applied on flexible pavements, rigid pavements, or composite pavements, geosynthetics provide a dependable solution in performance improvement and durability in diverse applications.
Success story of Strata: Construction of internal township road with StrataWeb® for Emami
Location: Jhansi, Uttar Pradesh, India
Client: Emami Developers, Kolkata
Product used: StrataWeb® 330-100
Application: Concrete road construction
Construction of internal township road with StrataWeb® geocells for Emami
Emami Developers initiated a large residential township project near Jhansi, featuring around 20 km of internal roads. The initial plan for conventional concrete roads was found to be too costly. Strata Geosystems proposed using StrataWeb 330-100 geocells filled with M20 concrete. This solution reduced costs and increased the roads’ lifespan. The geocells provided a durable, low-maintenance structure that minimized surface cracking and improved pavement quality. The use of StrataWeb resulted in cost-effective, durable roads with a high-quality finish. They helped control cracking and provided a durable, semi-rigid pavement surface, demonstrating the effectiveness of StrataWeb in road construction.
Strata Geosystems not only focuses on the technical performance of pavements but also emphasizes sustainability and cost-efficiency. These case studies illustrate how Strata Geosystems’ geosynthetic products, like StrataWeb and StrataGrid, provide innovative and cost-effective solutions for diverse pavement construction challenges. By choosing us for your next project, you can benefit from durable, sustainable, and high-performance pavement solutions that stand the test of time. To learn more about how Strata Geosystems can enhance your pavement projects, contact us or visit our website today.