Flexible Pavement PDF

Flexible Pavement A flexible pavement is an assembly of several components, with asphalt being the main element, and relies on its ability to distribute traffic loads throughout its layers to the subgrade. Loads will be transferred continuously from the driving surface. ADVANTAGES OF FLEXIBLE...

Flexible Pavement A flexible pavement is an assembly of several components, with asphalt being the main element, and relies on its ability to distribute traffic loads throughout its layers to the subgrade. Loads will be transferred continuously from the driving surface. ADVANTAGES OF FLEXIBLE PAVEMENT Flexible pavement offers several advantages that make it a preferred choice for various road construction projects. Some key benefits include: Cost-effectiveness Flexible pavement construction is generally more cost-effective making it suitable for projects with budget constraints. Use of the readily available materials and simplicity of the construction process has made it more efficient construction method. The availability of bituminous materials and the ease of construction contribute to its cost advantages. Durability Flexible pavement can withstand heavy traffic loads and is designed to distribute these loads uniformly. Its flexible nature allows it to withstand dynamic forces, preventing cracks and other forms of distress. Smooth Ride Quality The layered structure of flexible pavement, combined with its flexibility, offers a smooth riding experience for vehicles. This improves driving comfort and reduces driver fatigue, especially during long journeys. This is the one of the most versatile advantages of these types of pavements. Easy Maintenance and Repairs Flexible pavements allow for relatively easy maintenance and repairs. Individual sections of the pavement can be repaired or replaced without disrupting the entire road. This reduces maintenance costs and minimizes traffic disruptions. FACTORS AFFECTING FLEXIBLE PAVEMENT PERFORMANCE Several factors influence the performance and longevity of flexible pavements. It is important to consider these factors during the design and construction process. The key factors include: Traffic Loads The magnitude and frequency of traffic loads significantly impact the performance of flexible pavement. The pavement design must consider the anticipated traffic volume and loads to ensure structural integrity. In adequate thickness, poor material performance, etc. could affect to the lifetime of the payment. Climate and Weather Conditions Climate and weather conditions, including temperature variations and rainfall, can affect the performance of flexible pavement. Proper material selection and construction techniques should account for the local climate. In addition, the ground water table and its variations could also make significant impact on the lifetime of the road. Material Properties The properties of the materials used in flexible pavement, such as aggregate characteristics and bitumen quality, influence its strength, durability, and resistance to distress. High-quality materials are essential for long-lasting performance. Required testing as stipulated in the relevant standards shall be done. Testing of aggregates, testing of the compaction of the sub-based and sub grade, testing of the strength parameters of asphalt, etc. shall be done before and during the construction as applicable. Construction Techniques The construction techniques employed during the installation of flexible pavement play a crucial role in its performance. Proper compaction, adequate layer thickness, and accurate alignment are vital for achieving the desired structural integrity. LAYERS OF A FLEXIBLE PAVEMENT 1. The Subgrade Course The deepest element of the foundation section in flexible pavements is the subgrade. The subgrade is the native soil or improved layer upon which the pavement structure is built. It serves as the foundation for the entire pavement system and provides support to the upper layers. The subgrade must have sufficient strength and stability to withstand the loads imposed by traffic. CONSTRUCTION: Generally, the materials used to construct the sub-grade course include natural soil. The construction procedure consists of uniformly spreading the natural soil layer and then compacting it at optimum moisture content. Subgrade Preparation Proper preparation of the subgrade is essential to the performance of flexible pavement. Properly leveling and compacting, and sometimes stabilizing, the soil is necessitated to develop a strong base. Moreover, 1. Ensure the compacted subgrade is able to support construction traffic. If the subgrade ruts excessively under construction traffic it should be repaired before being paved over. Left unrepaired, subgrade ruts may reflectively cause premature pavement rutting. 2. Remove all debris, large rocks, vegetation and topsoil from the area to be paved. These items either do not compact well or cause non-uniform compaction and mat thickness. 3. Treat the subgrade under the area to be paved with an approved herbicide. This will prevent or at least retard future vegetation growth, which could affect subgrade support or lead directly to pavement failure. 2. The Subbase Course The Sub-base course is provided beneath the base course and it also functions as same as base course. If the sub-grade soil is strong and stiff, then there is no need to sub-base course. The sub-base course is an unbound layer and made of a material such as crushed stone or crumpled concrete or slag. If sub-grade is weak minimum 100 mm thick sub-base course should be provided. CONSTRUCTION: The materials used to construct this layer include crushed stones, gravel, and coarse sands. The first step in constructing this layer includes spreading the selected materials over the prepared sub-grade layer maintaining the specified thickness and cross slope. The layer is compacted by the use of rollers at optimum moisture content. Its thickness ranges from 100 mm to 300 mm. NOTE: The thickness of the Sub-Base Course is dependent on the CBR of the existing adjacent ground. The lower the CBR value, the higher the thickness of the sub-Base. 3. The Base Course The base course is important layer of pavement structure and it distributes the loads from top layers to the underneath Subbase and sub-grade layers. It provides structural support for the pavement surface. It is constructed with hard and durable aggregates which may either stabilized or granular or both. The thickness of base course must be great enough to reduce the load capacity on sub-grade and Subbase courses. CONSTRUCTION: Mainly, hard crushed aggregates are used in the construction of this layer. Initially, the crushed aggregates are mixed with water in a suitable proportion in a mixing plant to produce a wet mix of macadam (WMM). (Wet Mix Macadam is a type of road construction material that is used for base and sub-base layers. It is composed of crushed aggregates, including gravel and stone dust, mixed with water and a binding agent such as bitumen emulsion or cement.) The mix is then transported to the site and spread uniformly over the sub-base course, usually with the aid of a paver. The layer is then compacted utilizing a roller and allowed to dry for at least 24 hours. Its thickness ranges from 100 mm to 300 mm. 4. Application of Prime Coat A prime coat is an application of a low viscosity cutback bitumen above the base course. It is commonly prepared by mixing bitumen ( 80%) and kerosene (20%). To make one litre of prime coat, we need 1.05 kg bitumen and 0.10 kg kerosene and burn it to an adequate temperature. After preparing the prime coat, it is spread over the base course surface and allowed to dry for about three days. It is applied at a rate of 6-9 kg per 10 sqm. The purposes of Prime Coat are: - To fill the capillary voids present in the base course. - To bind the materials into a single unit. - To harden or toughen the surface. - To develop sufficient adhesion between base and binder course. 5. The Binder Course The binder course is an intermediate, bitumen bound aggregate layer placed between the base layer and the surfacing of asphalt pavement, sometimes referred to as a levelling course. The binder course's function is to distribute the load from the surfacing into the base course, strengthening the pavement. In general, its thickness is about 50 to 100 mm. If economy is not a problem, binder course and surface course can be constructed monotonically using good quality materials with 100 to 150 mm thickness. The function of binder course is to transfer the loads coming from surface course to the base course. 6. Application of Tack Coat A tack coat is provided between the binder course and the surface course. Asphalt emulsion diluted with water is used as tack coat material. It is applied at 6.0 to 7.5 kg per 10 sqm area; this quantity may be increased to 7.5 to 10 kilograms of a non-bituminous base. 7. The Surface Course Surface course or wearing course is the top most layer of flexible pavement which has direct contact with the vehicular loads. Since it is directly in contact with traffic, good quality aggregates and high dense bitumen or asphalt is recommended for the construction of surface course. The main function of surface course is to provide skid-resistance surface, friction and drainage for the pavement. It should be water tight against surface water infiltration. The thickness of surface course generally provided is 25 to 50 mm. 8. Application of Seal Coat Seal coat is provided directly on the top of surface course to make it watertight and to provide skid resistance to the surface. Mixture of Emulsified asphalt, mineral fillers and water is used as seal coat material. Bituminous Materials Materials that are bound together with bitumen are called bituminous materials. The use of bituminous materials were initially limited to road construction. Bituminous materials are composed of bitumen, a viscous and sticky form of petroleum, with aggregates that include materials such as sand, gravel, or crushed stone. The bitumen component acts as a binding agent to keep the aggregates together to create a composite, durable and flexible surface. Sources of Bitumen Natural Bitumen or Natural Asphalts The bitumen is obtained from petroleum naturally with the help of geological forces. They are found to seen intimately connected with the mineral aggregates. They are found deposited at bitumen impregnated rocks and bituminous sands that have only a few bitumen in percentage. Refinery Bitumen This bitumen is the residual material that is left behind after the crude oil fractional distillation process. Asphalt Asphalt is useful for flexible pavement surfaces because it is flexible, durable, and can withstand several forms of different pressures (i.e. temperature and traffic loads). This flexibility allows asphalt to withstand occasional overloading of the pavement. The grading of the asphalt cement to specific regions of the country allows asphalt to adapt to its environment and withstand rutting in the summer. It is also generally inexpensive and also requires little maintenance compared to other material options. Hot Mix Asphalt Hot mix asphalt is a combination of asphalt cement, heated fine and coarse aggregate, and water. This type of asphalt is flexible, weather resistant, resilient, and impermeable. This mix is poured at high temperatures of about 150 to 177 °C and is easy to use when hot. Hot mix asphalt is ideally used in places where temperatures are above 4°C to prevent the asphalt mix from cooling quickly. It is used on pavements and roads. Its use also needs a high level of expertise. This type of mix is further classified into three different types. Dense-Graded Mix This type of hot mix asphalt is impermeable and the most used for asphalt resurfacing and repairing. This type of mix is further classified into coarse and fine-graded mixes. Fine-graded mixes are fine, with particles the size of sand. Stone-Matrix Mix This type of hot mix is highly durable and makes pavements resistant to rutting. This type of hot mix has a higher content of asphalt and is more expensive than dense-graded mix. Stone-matrix mix is ideal for use in places that handle high volumes of traffic. Open-Grade Mix Open-graded mixes comprise crushed stone and a few grains of sand. This type of mix is water-permeable. Warm Mix Asphalt The production methods of warm mix and hot mix asphalt are the same. However, warm mix asphalt is poured at lower temperatures. To spread the asphalt mix at lower temperatures, warm asphalt is mixed with additives and binders. This mix releases fewer fumes and consumes less energy during manufacture. With this in mind, warm mix asphalt is an ideal mix for areas with poor air circulation. Cold Mix Asphalt Cold mix asphalt is a combination of cold aggregate and asphalt cement mixed with water. The cold mix is then placed on a roadbed without heating and compacted. This mix cures and hardens slowly, and the pavements have a short life span. Cold mix asphalt is used in conditions where hot mix asphalt is inapplicable. Therefore, they are used as a temporary solution in freezing temperatures. The mix is mostly used to repair cracks and potholes before permanent repair can be done. Porous Asphalt Porous pavement is designed in a way that allows water to pass through the pavement. This type of asphalt is ideally used in areas prone to water pooling and flooding. The water can drain through the pavement and safely reach the stone recharge bed and eventually the soil. Pavements made from this material are durable and hard-wearing. Porous asphalt eliminates water puddles and makes it easier to manage stormwater. Due to this asphalt’s porous nature, it helps to recharge aquifers and reduces demands on the local sewer systems. Perpetual Pavement Perpetual pavements are installed in a multi-layer paving process. This type of asphalt is strong and long-lasting. The first layer is flexible and strong. This layer prevents the formation of cracks on the road’s bottom. Two more layers are added over the first layer. The driving surface is the last layer to be added and can be removed and replaced as the other two layers stay in place. Perpetual asphalt pavements are long-lasting, durable, and reduce replacement costs. Furthermore, the installation of perpetual pavements allows continuous traffic flow and takes a short time. Clearing and Grubbing: 1. Clear the roadway for debris. 2. Ensure major stumps and roots are removed. 3. Preferably, 300mm from the ground surface should be cleared. Asphalt emulsion (emulsified asphalt), like cutback asphalt, is also designated as a liquid asphalt to distinguish this product from the typical semisolid to solid asphalt binders. It is a mixture of two normally immiscible components (asphalt and water) and an emulsifying agent (usually soap). Emulsions are used because they effectively reduce asphalt viscosity for lower temperature. Defects in Asphalt Pavement Fatigue/Crocodile Cracking This type of cracking resembles the pattern of a crocodile's skin. It is caused by the repeated application of traffic loads that exceed the structural capacity of the pavement layers. Block Cracking Block cracking appears in a rectangular or square pattern in the asphalt pavement. It is usually caused by temperature changes that cause the pavement to expand and contract, eventually leading to cracking. Longitudinal Cracking Longitudinal cracks form parallel to the direction of traffic. It can be caused by inadequate construction of the paving lane joint, excessive traffic loads, temperature changes, and a lack of pavement strength (i.e. if there are existing reflection cracks beneath the surface course). Longitudinal cracking can lead to the separation of the pavement layers and the formation of potholes. Transverse Cracking This is a type of cracking that appears perpendicular to the direction of traffic. It is usually caused by temperature changes that cause the pavement to expand and contract, eventually leading to cracking. Transverse cracking can also be caused by a lack of pavement strength, moisture infiltration, and excessive traffic loads. Reflection Cracking Reflection cracks occur when cracks from the underlying pavement layers are reflected through to the surface of a new overlay. It is caused by the movement of the underlying pavement layers due to temperature changes, moisture infiltration, and aging. Reflection cracking can be prevented by ensuring that the underlying pavement layers are in good condition before applying an overlay. Edge Cracking This is a type of cracking that occurs along the edge of the pavement from the lack of support on the shoulder caused by poor drainage and weak materials. Edge cracking first appear as crescent shaped cracks that eventually expand to the edges to resemble crocodile cracking. Edge Fatigue Cracking This kind of cracking consists of a series of interconnected cracks that form along the edges of the pavement, usually near the curb or the shoulder. Edge fatigue cracking is more common in older pavements or in areas with heavy traffic loads, and can also be exacerbated by poor construction practices, such as inadequate compaction or improper joint construction. Slippage Cracking Slippage cracking typically appears as crescent or circular-shaped cracks that extend parallel to the pavement's direction of travel. This type of cracking can be attributed to low strength surface mix (i.e. those with high sand content), or a lack of bonding between the surface and the layer below. The lack of adherence could be ascribed to various factors such as the existence of debris, grease, grime, moisture, and the absence of a tack coat or prime coat while applying a thin layer of asphalt on the aggregate foundation. Corrugation and Shoving This type of plastic movement is characterised by ripples (corrugations) or abrupt waves (shoving) that form perpendicular to the direction of traffic. Corrugation occurs in areas where traffic starts and stops, while shoving occurs where the hot mix asphalt (HMA) meets a rigid object. It is typically caused by combined effects of traffic action (starting and stopping) and an unstable HMA layer. The instability of the HMA layer can be caused by various factors, such as mix contamination, poor mix design, poor HMA manufacturing, or inadequate aeration of liquid asphalt emulsions. Additionally, too much moisture in the subgrade can negatively contribute the issue. Rutting This is a surface depression which usually occurs in the wheel path. Consolidation or lateral movement of materials resulting from the pressure of traffic loads is usually the cause of this issue. Rutting can happen in the subgrade or base from inadequate design thickness, lack of compaction or moisture infiltration. A weak asphalt mixture can also cause rutting. Depressions A depression is a specific area on the pavement that has a lower elevation than the surrounding pavement and may sometimes include cracking. It is possibly caused by traffic overloading, or by foundation soil consolidation, settlement or deterioration of the lower layers. Swelling This is a type of pavement distress where a localised area of the pavement is higher than the surrounding pavement, and is not caused by shoving. The swell can appear as a protrusion on the roadway, which can either be a sudden and steep rise in a small area or a gradual and elongated wave. Both cases may be accompanied by cracking. The cause can be attributed to frost action in the subgrade, or expansion in the soil. Potholes Potholes refer to bowl-shaped holes on the pavement surface that often extend from the asphalt layer down to the base course. Water, poor drainage, traffic loading and a weak layer(s) lead to the formation of potholes. Potholes occur as a consequence of other defects, such as crocodile cracking, swelling, ravelling, or failed patching. Patches Patches involve the removal and replacement of pavement material to repair a defect or cover a utility trench. Even though patches are a repair solution, it is still considered a defect, regardless of how well it performs. If a patch fails, it can lead to further damage to the surrounding pavement, causing roughness and lowering ride quality. Stripping This issue occurs when the lower layers of the asphalt deteriorate and cause damage to the upper layers. It can be challenging to identify bottom-up stripping as it often appears as other forms of pavement distress such as rutting, shoving, corrugations, ravelling, or cracking on the surface. To confirm if stripping is the cause of the damage, a core sample is usually required. The causes of stripping may include inadequate aggregate surface chemistry, moisture damage caused by water in the asphalt, and overlays on an open- graded surface course. Such overlays are prone to stripping. Ravelling Ravelling is a condition where the surface loses coarse aggregate particles, causing the pavement to appear rough and jagged. This distress is often found in the wheelpath. The occurrence of ravelling can be attributed to various factors, such as inadequate compaction of asphalt, constructing a thin layer during cold weather, usage of contaminated or deteriorating aggregate, insufficient amount of asphalt in the mixture, or excessive heating of the asphalt mix. In almost any instance, traffic loads and water are the two most common elements that are responsible for ravelling. Polishing Polishing in asphalt pavement refers to smoothed aggregate. It is caused by the wearing down of the aggregate particles and binder over time due to traffic. Polishing reduces the pavement's skid resistance and can increase the risk of accidents, particularly in wet conditions. Bleeding Also known as flushing, bleeding is the condition where there is excess bituminous binder on the road surface, and can often occur in the wheelpath. Bleeding can manifest in various ways, from discoloration of the surface relative to the rest of the pavement, to loss of surface texture due to excess asphalt, and even to the point where the aggregate becomes obscured by a shiny, glass-like, reflective surface that may feel tacky to the touch. Bleeding possibly occurs when asphalt binder fills the voids in the aggregate during hot weather and expands onto the pavement surface. Since it does not reverse during colder weather, the binder accumulates on the surface over time. Causes of bleeding may include excessive asphalt binder in the HMA due to mix design or manufacturing, excessive application of asphalt binder during BST application, and low air void content in the HMA, which doesn't provide sufficient space for the asphalt to expand into during hot weather. Delamination Delamination in asphalt pavement occurs when the bond between the asphalt layers breaks down, causing separation between the layers. It is caused by improper surface preparation or tack coat before the application of the wearing layer. Environmental factors such as ingress of water and repeated freeze-thaw cycles can also cause delamination.