- Introduction
The subject of this study concerns some of the most serious pavement deficiencies: alligator cracking, bleeding, and potholes. Alligator cracking and potholes can often shorten the pavement’s lifespan. Bleeding more often impacts road safety; bleeding causes reduced friction which, in turn, can cause skidding for drivers. The case study focuses on alligator cracking, potholes, and bleeding in an effort to explain their impact on pavement, as well as how they are remedied. In an effort to better understand the detriment these issues cause, a local two-lane road was studied for evidence of distress and damage.
These defects due to pavement distress can be unsafe, as well as costly. When left unattended, the stresses deteriorate the pavement so badly construction may be the only solution; this is very expensive. The majority of faults cause cracks, as well as breaks, in the subgrade layers of the pavement. The openings allow outside moisture to seep into the pavement’s layers, causing more damage. For example, the seeping moisture can loosen the subgrade layers, reducing loading capacity over time. In many areas, during winter, thawing cycles can cause the worst damage due to the extreme excess moisture leaking into the subgrade layers of the pavement.
2. General Causes of Pavement Failures
2.1 Potholes
Potholes occur when collective faults on the pavement combine into one error. Cracks occur due to various occurrences, cross one another, and isolate sections of pavement. These isolated areas become weaker over time because, due to the separation, they do not have the benefit of load transfer. This weakness can lead to load concentration on each isolated region, slowly weakening each section. Eventually, these weakened areas cave in, or are knocked out, when vehicles pass over them. Essentially, potholes are open holes on the pavement’s surface. They have sharp edges and can be very deep (Oregonstate.edu, 2014).
2.2 Alligator failure
Alligator failure, or alligator cracking, occurs when the subgrade layer is subjected to a load weight that is beyond the pavement’s rated capacity. Cracks on the surface layer of the pavement begin to appear when its surface layer has been overloaded. As the overload continues, the surface layer begins to give way. Cracks continue to form once the pavement’s subgrade layer shifts downward, also a byproduct of loading. If the subgrade is poorly designed, or if moisture is allowed to drain into the subgrade level and effect it negatively, it can lead to alligator cracking. Alligator cracking can also occur due to stripping. Stripping, caused by chipping of the pavement’s surface, collects because of a loose bond between asphalt. When the pavement’s surface chips, it reduces the pavement’s depth below the pavement’s design parameters. This lowers the original design loads, which can also cause alligator cracks. They are typically longitudinally aligned, intersecting at angles that look like an alligator’s hide (Wolf's Asphalt Paving Blog, 2014).
2.3 Bleeding
Cause by excess asphalt in the HMA mix, bleeding is also dangerous for pavement, as well as drivers. Excess asphalt, laid on the pavement, heats up during hot weather. It then melts, forming a sticky, sometimes shiny film on the pavement’s surface. If the HMA mix preparation contains too much asphalt, and the mixture is poor, there will be little air, allowing a surplus of asphalt to seep onto the pavement’s surface. The sticky film that forms in hot weather is dangerous because, if the weather turns wet, it can cause skidding and may lead to accidents or deaths (Oregonstate.edu, 2014).
2.4 Depression
A depression happens when a part of the pavement recedes. When this happens, the pavement forms a shallow groove that can fill with water during wet seasons. Depressions are typically caused by poor compaction of the subgrade level in the pavement during construction.
2.5 Block Cracking
Block cracking is defined by rectangular division in the pavement. It is caused by contraction, as well as expansion cycles in the HMA. This occurs when there is too much rigidity in the asphalt binder; too much rigidity does not allow the pavement to extract or expand, leading to block cracking (Oregonstate.edu, 2014).
2.6 Longitudinal Cracking
Parallel to the road, longitudinal cracks form because of HMA fatigue. They can also form because of reflective cracks in the subgrade layer, top-down cracking, and weak joints within the pavement’s other layers.
2.7 Shoving
Typically caused by heavy traffic across the pavement, and a low level of maintenance, shoving can by dangerous to drivers. Occurring most often at stop signs and red lights, the action of stopping a vehicle, and starting it again creates ripples in the pavement because of the vehicle’s friction. These ripples normally appear because of a weak HMA layer, which eventually leads to too much flexibility in the pavement. It can also lead to a reduced drag-loading capacity. Shoving can also be due to deterioration of the subgrade layer because of seeping moisture.
2.8 Rutting
Traffic loading causes the subgrade level of the pavement to shift, which results in rutting. Rutting is characterized by sunken wheel tracks wherein heavy commercial vehicles have often broken the load capacity. Rutting is also a result of a poor HMA mixture, as well as bad compaction. Poor compaction creates a porous pavement that is more susceptible to gradual traffic compaction and rutting.
I focused my case study on Abu Dhabi’s Mafraq region. The Mafraq region is located East of Mohammed Bin Zayed Area and Musaffah and North of the Banyyas District. Mafrag is primarily flat and I chose it because it is one Dubai’s chief highway, roughly 30 Kl from Abu Dhabi. As an older district of Abu Dhabi, I am expecting to discover several significant road deficits. Mafraq is located at the following geographic coordinates:
Latitude: 24° 18’ 43” N, Longitude: 54° 35’ 18” E
Latitude: 24° 20’ 50” N, Longitude: 54° 37’ 48
Mafraq has roughly 650 residential and commercial homes, as well as several international hotels.
Aim
The case study’s aim is to analyze three different types of road defects stretching across a minimum of 300 m to 500 m of road near Abu Dhabi. The defects will be evaluated in the study and then classified according to international standards. The deficits in the roads will demonstrate the specific maintenance procedures by following UAE, as well as international standard maintenance codes. Any road deficits included in the designated case study areas between Mafraq International Hotel and Mafraq Hospital are classified by myself as longitudinal failures, block cracking failures, and bleeding. The deficits I found in Mafraq are illustrated in the diagrams pictured below:
3.2 Failures observed in 300m road section and their causes
3.2.1 Bleeding
Bleeding observed on the road section was only seen on the pavement’s left side. This observation indicates the cause of bleeding is only present on the left side. Due to this observation, I ascertained that the most likely cause of the bleeding was the finishing quality. I suggest that the use of an unorthodox HMA mix on this side could have caused the bleeding. Another cause could be attributed to too much compaction; this would have resulted in voids that were necessary for asphalt expansion being filled. The pavement’s bleeding can be remedied by applying sandwich seals or chip seals, depending on the severity of the problem.
3.2.2 Longitudinal Cracking
The pavement’s length is covered with longitudinal cracking. It was possibly caused by poor joints used during construction. The pavement’s sections were unable to form into one complete structure, as a result; traffic disjoints it easily. The observed cracking was no severe enough to permit major construction. Crack sealant, as well as crack raveling, could be used to seal the cracks because they were less than half an inch wide. Sealant would prevent further moisture seepage.
3.2.3 Block Cracking
I was also able to observe block cracking. It was characterized by cracks in the pavement isolating areas into block shapes. Block cracking was most likely caused by contraction and uneven expansion between the subgrade and surface layers of the pavement. Subgrade expansion could have stretched the asphalt, also causing block cracking. The block cracking was not severe; crack sealant is a viable repair solution.
3.4 Formulation of Maintenance plan for the road distresses
Decent road conditions, as well as saving money on construction, is a crucial part of regular road maintenance. Preservation of roads allows expensive construction to be alleviated, while small repairs are put in its place. It also ensures that drivers remain safe while on the road and saves them time. The section of road being used for the case study, failure was caused by poor road design, as well as poor drainage in preparation for the wet season. The maintenance plan will diminish the costs of the design deficits.
3.4.2 Maintenance of the Road Surface
Surface road maintenance is crucial; it ensures roads stay smooth, allowing for safe travel as well as water run-off. Regular surface checks are necessary; they involve examining deterioration of the pavement’s surface and subgrade layer. Pavement deterioration occurs when the pavement experiences fluid and seepage; liquid run-off is important for safety and construction. Snow, something not considered by many construction crews, also needs to be removed regularly to prevent seepage (FAO Corporate Document Repository, 2014).
As a part of the maintenance routing, regular surface checks will be performed to detect areas that need to be repaired immediately. Every six months major road construction will also be scheduled. Potholes will be filled and patched during this time.
3.4.3 Procedure of road distresses examination
- Mark road section of road to be examined.
- Warn oncoming traffic with the use of hazard signs.
- Reflectors must be on all work vehicles and employee uniforms.
- In order to schedule work, priorities on a scale of 1 to 16 will be noted with 1 being the highest priority and 1 being the lowest.
- 3.5 Comparison between local and international pavement maintenance codes
- Local and international pavement maintenance codes
3.6 Deflects Repair
3.6.1 Bleeding Repair
Local codes dictate bleeding pavements be treated with a thin overlay, as well as chip or sandwich seals. The local codes mimic the international codes. International codes insist construction uses rough sand to reduce asphalt film. The two standards differ because in severe bleeds, international codes allow removal of excess asphalt through the use of a motor grader or heater planer (Oregonstate.edu, 2014).
3.6.2 Longitudinal Repair
Longitudinal cracking will be fixed by the same standards in international and local codes. Cracks are repaired with sealant. However, international codes allow the use of raveling if the cracks do not exceed half an inch in width.
3.6.3 Block Cracking Repair
Local and international codes also both allow the use of sealant to repair block cracking. In severe cases, both codes allow for the removal of road sections.
4. Conclusion
In sum, the case study allowed me to learn about different pavement defects, their causes, and how to repair them. I have a better understanding of each distress, how they occur, and where they are most likely to occur on the pavement. As an engineer, my newfound understanding will allow me to analyze pavement deficits, but also relieve them safely and accurately. I will be able to prevent further damages, and reduce costs for construction companies, as well as cities. Furthermore, I will be able to produce better pavement designs that will not incur these typical deficits as easily. It is crucial as an engineer that I understand these processes, the deficits within the roads, as well how to remedy and avoid them.
