What is a road pavement, and how is it designed?
A pavement is the uppermost engineered structure or surface of a road that bears the load of traffic, most often vehicular, and should not be confused with a footpath. Here, we’re referring to the surface and underlying structural layers in road construction. A paved road has a the surface course, which can be made from a number of materials such as asphalt, Hot Mix Asphalt (HMA), concrete, and other mixed materials. The material choice for this surface course in pavement design is an important design component for the pavement engineering designer because it provides the primary contact surface for traffic and contributes to the overall structural performance, while the entire pavement structure (including base, subbase, etc.) 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 a number of layers, and typically the top surface layer, is often made of asphalt when it comes to paved roads or 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 wider area of the subgrade due to their high stiffness, minimizing deformation within the slab itself.
- Composite Pavements: Composite pavements combine the features of both flexible and rigid pavements. Composite pavements combine features of both, 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 flexibility (of the asphalt surface) with durability (of the 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 are primarily responsible for the two aspects of the road pavement: 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: Technically, the subgrade is the native soil or prepared fill beneath the pavement structure; it is the foundation upon which the pavement is built. The subgrade is the prepared natural ground or fill that forms the foundation of the pavement. A subbase layer, if used, is placed on top of the subgrade. Most of the time, the strength and load-bearing capacity of the subgrade decide the extent to which the overlying pavement structure will stand. 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 distributes the pressures from the upper layers (e.g., binder and surface courses) into the subgrade. The essential construction materials for base courses are granular materials such as gravel or crushed stone. These materials have high strength and a very long composition life, which can help them absorb the traffic loads and minimize settlement under the road pavement. Factors such as traffic volume, climate, and soil condition determine the choice of material. It is very important to provide a stable surface for the base course because this layer distributes the load.
- Binder course: The binder course is a layer of bituminous material, such as asphalt or bituminous concrete. This interlocks with the base course below and the wearing course above. Its primary functions are to provide structural strength to the pavement, distribute loads from the wearing course to the base course, and provide a smooth surface for the application of the wearing course. It also contributes to the overall moisture resistance of the asphalt layers.
- 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 thickness of the materials should be increased.
- Speed: Vehicle speed can influence dynamic loading, particularly at pavement discontinuities, and is considered in some design aspects, but the primary factors for structural design are load magnitude and repetitions.. Speed limits and design speeds are also crucial factors that engineers need to consider during the design process.
- 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 its faster deterioration (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.
- Long-term costs: The fundamental principles behind designing a road surface should include the total costs of repairs and maintenance. Factors such as maintenance frequency, material costs, and labor requirements should be considered by the designer.
- 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 only require a little data and are straightforward, they are sometimes less accurate in complicated 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 may result in very accurate answers and be able to solve large and complex pavement geometry and loading conditions, it will be time-consuming and require various skills in programming 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 adding 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.
- Moisture control and separation: Geosynthetics, like geotextiles, act to prevent fine particles of soil from migrating into the aggregate layers. This function of separation is very instrumental in maintaining the drainage characteristics and structural integrity in the pavement by preventing contamination of coarser layers and helping to manage water within the pavement system.
- 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 fall concentrated on certain 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 prevented cracking and provided a flexible yet sturdy 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.