Flexible pavements are susceptible to two types of defects: corrugation and shoving. These defects are often caused by various factors such as weak sub-grade conditions, poor mixing, improper rolling, temperature effect of bitumen, and weak bottom layers.
Corrugation and shoving can cause significant damage to flexible pavements, making them unsafe for motorists. Corrugation, which is characterized by a series of waves in the pavement, can result from a lack of compaction during construction, poor bonding between layers, or excessive moisture in the sub-grade. Shoving, on the other hand, occurs when the pavement material is pushed or displaced laterally, creating a series of humps and bumps.
Several factors can contribute to the development of corrugation and shoving in flexible pavements. Weak sub-grade conditions, which can result from poor soil quality or inadequate preparation of the sub-grade, can cause the pavement to deform and shift over time. Improper rolling during construction can also lead to these defects, as can poor mixing of the asphalt binder and aggregate. The temperature effect of bitumen, which can cause the binder to soften and deform, can also contribute to the development of corrugation and shoving. Additionally, weak bottom layers can result in excessive deformation and displacement of the pavement, leading to these types of defects.
In summary, corrugation and shoving are two significant defects that can occur in flexible pavements, and their causes can be attributed to various factors. Proper construction techniques, including adequate preparation of the sub-grade, appropriate rolling, and quality control during mixing, can help prevent the development of these defects and extend the life of flexible pavements.
What is Corrugation?
Corrugation or washboarding is a phenomenon that occurs on flexible pavement. It is characterized by the formation of ripples or waves that are generally perpendicular to the direction of traffic flow. This type of pavement damage typically occurs at points where vehicles start and stop, such as intersections or areas with heavy traffic congestion. Corrugation and washboarding can significantly impact the ride quality for drivers and passengers, and can also contribute to increased wear and tear on vehicles. As a result, it is important to address this type of pavement damage promptly to ensure the safety and comfort of all road users.
What is Shoving?
Shoving is a term used to describe the bulging of a pavement surface as a result of plastic movement. This type of deformation commonly occurs at points where a vehicle applies its brakes or where the pavement meets rigid objects. These areas experience increased stress, leading to a distortion of the pavement surface. Shoving can be a significant issue for road safety and pavement durability as it can create uneven surfaces that can be hazardous for drivers and pedestrians. Therefore, it is important to monitor and address any shoving that may occur on roadways to ensure their safety and longevity.
Causes for Corrugation and Shoving
1. Weak Sub-grade Conditions
Flexible pavements can experience problems such as corrugation and shoving if the subgrade soil is of poor quality. Weak subgrade soil typically contains highly plastic clays or organic material, and it may be unable to bear heavy loads if the water table is located too close to the surface. In order to ensure the stability of the subgrade course, weak soil is typically excavated and replaced with boulders or crushed stones.
However, if the depth of excavation is not sufficient to reach a hard stratum, the stones and boulders used to replace the weak soil may sink, which can lead to the formation of ripples on the pavement surface. This issue can be particularly problematic if the pavement is meant to be used by vehicles or heavy machinery, as the uneven surface can make it difficult to navigate. To prevent this from occurring, it is important to ensure that the depth of excavation is adequate to reach a solid foundation for the subgrade.
2. Improper Rolling
During the construction of a flexible pavement, if the rolling process is not executed properly or if the compaction is inadequate, it may result in several issues. One of the most common problems is the formation of corrugations, which are typically characterized by waves or ripples on the pavement’s surface. Another issue that may occur is the shoving of the pavement.
The improper rolling process causes the layers of the pavement to be insufficiently held together in certain places. Consequently, whenever vehicles pass over these areas, the layers become disrupted, leading to the formation of waves on the pavement. This can result in a rough, uneven surface that is difficult and uncomfortable to drive on.
The shoving of the pavement is another concern that may arise due to improper rolling or inadequate compaction. In this case, the pavement’s surface can be pushed or displaced laterally, resulting in a wave-like formation in the longitudinal direction of the road. Shoving of the pavement can lead to a significant reduction in the pavement’s service life, increase the likelihood of accidents, and create discomfort for drivers. Therefore, it is crucial to ensure proper rolling and compaction during the construction of flexible pavements to avoid these issues.
3. Poor Mixing
The proper mix or gradation of the surface course in a flexible pavement is critical. If the mixing of the surface course is not done correctly, it can lead to problems such as corrugation and shoving. When the mix is defective, corrugations can form and spread to larger areas due to the constant flow of traffic or vehicular movement.
It is essential to exercise caution when mixing the surface course to prevent these issues. Inappropriate mixing can result in significant problems that could impact the pavement’s overall quality and durability. Thus, it is crucial to ensure that the surface course is mixed correctly to avoid these problems.
Corrugations, which are a result of defective mixing, can become more extensive and cause significant damage to the pavement. They can spread rapidly due to the continuous flow of traffic, leading to a larger area of damage. Therefore, it is crucial to take the appropriate steps to prevent these issues and ensure that the pavement remains stable and durable. Proper mixing of the surface course is an essential factor in achieving this goal.
4. Temperature effect
Bitumen is a commonly used material for the surface course of flexible pavements. One of the unique properties of bitumen is that its viscous state varies depending on the temperature. Therefore, it is crucial to maintain the appropriate temperature of the bitumen mix during the mixing and rolling processes. This is because the stability of the pavement after drying largely depends on the temperature during these stages.
If the temperature is too high during the mixing or rolling process, it may lead to a reduction in stability of the pavement. Additionally, the high temperature can cause waves or corrugations to form during compacting, which can compromise the integrity of the pavement surface. Therefore, it is essential to carefully monitor and control the temperature of the bitumen mix to ensure maximum stability and quality of the finished pavement.
5. Weak bottom layers
Corrugation and shoving are more likely to occur when the lower layers of a road, such as the binder course, base course, and sub-base course, are poorly compacted or have a defective grading. The continuous movement of vehicles on these layers can cause deformation and depression in the surface course, ultimately leading to the formation of corrugations.
It is important to ensure that these lower layers are properly constructed and maintained to prevent such problems from occurring. Corrugations and shoving can not only cause discomfort for drivers and passengers, but also reduce the lifespan of the road and increase maintenance costs. Proper compaction and grading of the lower layers can help to avoid these issues and ensure a smoother and more durable road surface.
Remedial Measures for Corrugation and Shoving
To address the issues of corrugation and shoving in flexible pavements, several remedial measures can be taken. One such measure is the stabilization of the subgrade soil using appropriate stabilizing agents such as lime, cement, and chemical admixtures. This can help improve the overall quality of the subgrade, especially in cases where the soil is of poor quality.
Another measure involves the removal of inadequately compacted layers and their reconstruction with proper compaction techniques. This can help ensure that the pavement layers are sufficiently dense and can withstand the loads and stresses imposed on them.
In cases where the failure is caused by a high water table, a suitable sub-surface drainage system can be installed to lower the water table. This can help prevent water from accumulating and causing damage to the pavement layers.
If the issue is related to poor mixing of the surface course, then laying another layer of surface course with proper mixing can be a suitable solution. This can help ensure that the surface course is well mixed and can provide adequate support to the upper layers of the pavement. Overall, these remedial measures can help address the issues of corrugation and shoving in flexible pavements and ensure their long-term durability and performance.