Bitumen tarring occurs when there is an excess of bitumen on the surface of the pavement, forming a bitumen membrane. This membrane can become shiny, sticky, and soft in hot weather.
Tarring is caused by the migration of excess bitumen from the lower layers towards the pavement surface. It is often the result of using too much bitumen in the asphalt mixture, applying an excessive amount of primer or single coat, or having a low percentage of free space in the asphalt. Additionally, the use of softer bitumen in hot weather conditions can contribute to the occurrence of tarring.
Rusting typically appears in the wheel path and is exacerbated by two factors: traffic and hot weather. Rusting can be an indication of moisture damage. When water is present, it causes the bitumen to separate from the aggregates and gradually rise to the surface. The roughness on the road surface due to tarring poses a potential risk for accidents as it reduces the road's skid resistance.
The severity of tarring can be categorized as follows:
- Low Severity: Tarring occurs in small amounts and is only noticeable for a few days during the year. Bitumen does not stick to vehicle wheels or pedestrians' feet.
- Moderate Severity: The amount of tarring is significant enough to stick to vehicle wheels and pedestrians' feet. It is noticeable for at least a few weeks during the year, appearing as black spots on the pavement surface.
- High Severity: Tarring occurs extensively and prominently. Bitumen sticks to vehicle wheels and pedestrians' feet, happening for at least a few weeks a year. In this case, the thin bitumen membrane covers almost the entire pavement surface or wheel path.
Roughness is measured by the affected square meters of the pavement surface. Corrosion cannot be easily repaired by simply removing the cause, as it is a result of the underlying pavement structure. Mild tarring typically does not resolve until it reaches a moderate severity level. Low and moderate severity tarring can be addressed by spreading fine sand. Severe tarring may require removing and replacing the top layer. Surface coatings can also be used as a temporary solution to cover the bituminous surface.
Polishing
Polishing occurs when the surface of the pavement undergoes continuous contact, primarily due to vehicle traffic. However, even the passage of pedestrians, particularly in large numbers, can contribute to partial polishing of parking lot pavements.
Polishing results in a smooth and slippery surface, which is caused by the repeated traffic loads on the sharp edges of coarse-grained stone materials. The susceptibility of aggregates to polishing is influenced by their geological source. Certain aggregates, such as limestone, are more prone to polishing compared to other stone materials. Polished aggregates have the ability to remove bitumen from their surroundings and feel smooth to the touch. This phenomenon significantly reduces the road's skid resistance. While polishing poses a significant hazard for high-speed roads, it is generally less of a concern for parking areas.
The severity of pavement surface polishing is typically not graded separately but is considered as part of the overall assessment of skid resistance. The best preventive measure against this type of damage on high-speed roads is to avoid using aggregates that are susceptible to polishing. In cases where pavements have experienced significant damage from surface material polishing, the use of surface coatings or overlays can be an appropriate repair method. An alternative solution could involve removing the existing layer and replacing it with a new layer.
Rut (wheel track)
A rut is a change in the location of the surface during which the pavement drops and changes its location in the longitudinal section of the road and along the path of the wheel. Grooving is usually also called grooving or channeling. The sagging groove causes the displacement of the asphalt mixture along the wheel track and creates a channel. Rutting is usually visible after rain, and even a small rut can be seen after rain, so the best time to evaluate the road surface is after rain.
Grooving can happen on any type of pavement, both high traffic and low traffic roads. When the grooves are covered by rainwater or ice, there is a high possibility of hydroplaning (slippage of the vehicle on water or ice) of vehicles, which will lead to safety risks.
The low percentage of free space of stone materials (VMA) are the factors affecting the rut collapse. The higher the amount of traffic load or the amount of load (in terms of equivalent individual axial loads), the more likely the effect of the above factors will be on the displacement rut. For example, an asphalt mixture containing a lot of sand that may perform well in a parking lot pavement will suffer displacement ruts in an expressway pavement.
The severity of rutting can be defined as follows:
Low severity: groove depth less than 12 mm
Medium intensity: groove depth between 12 and 25 mm
High intensity: groove depth more than 25 mm
Groove slump is measured by the depth of each groove per square meter of the road surface. The severity of the rut drop is determined by measuring the average depth of each rut along the wheel track with a feeler gauge. The rut depth values are measured at standard longitudinal intervals, for example, every 20 meters. The average depth of the groove is calculated by laying the billet across the width of the groove, measuring the depth of the groove and then using these measurements for the length of the groove, the unit of which is mm. Usually, a low-intensity groove is not noticed until it becomes a moderate groove or a high-intensity groove. Low-intensity ruts can be fixed by patching or thin asphalt coating, or by using slurry seal, or by repairing with protective asphalt. Moderate ruts are often remedied by covering or protective asphalts. A moderate or high sag groove can be executed by grinding the existing pavement to a depth lower than the groove.
Retraction (ripple)
Rollovers typically occur at highway intersections and on- and off-ramps, primarily caused by the braking action of trucks and cars. Surface deformations, such as normal swelling and frost swelling, can also contribute to displacement. In parking areas and low-traffic roads, one common type of surface deformation is the impact of tree roots. Frost swelling primarily results from the expansion of the subsoil and the upward movement of the aggregates in this layer.
In a well-drained substrate free of debris, frost-related swelling is unlikely to occur. Surface displacement and localized pavement drop can be caused by substrate settlement, asphalt pavement cuts, drainage structures, and other factors. Proper compaction of the subsoil and embankment, as well as recompaction of excavated areas within the pavement section, help prevent displacement resulting from settlement.
Displacement and undulation are manifestations of plastic movement within the asphalt mixture. In such cases, the asphalt mixture lacks sufficient stability. The low stability of the asphalt mixture can be attributed to factors such as a low percentage of free space, excessive bitumen content, crushed stone materials, excessive sand, or the use of softer bitumen than required for the specific area. A soft asphalt mixture that might perform adequately in small-scale projects can experience displacement and distortion under significant loading.
Asphalt mixtures designed for high-traffic road paving are highly suitable for intersections and their usage helps prevent the occurrence of various forms of displacement and undulation. The grading of displacement severity is relatively straightforward and can be classified as follows:
- Low Intensity: Displacement or undulation can be felt while driving but has minimal impact on driving quality.
- Moderate Intensity: Displacement or undulation can be felt while driving and affects the quality of driving.
- High Intensity: Rollover or undulation is highly noticeable, causing vehicles to slow down when approaching. Displacement and undulation are measured in terms of affected square meters of pavement surface. Surface distortions are evaluated on a case-by-case basis, and immediate attention is required when pavement swelling or settlement occurs.
The repair method for this type of damage typically involves cutting out the damaged sections and applying patches or overlays in the affected areas. Local drops or swellings are usually repaired through patching, while surface settlements eventually develop into potholes that need to be filled. If tree roots or debris in the soil are the cause of the swelling, it is necessary to remove the roots or debris before applying the patch.
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