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Maintenance of road pavements.
Fixing pavements is in the news, particularly since an investigation by Help the Aged found that 2,300 elderly people fall on cracked pavements every day, many of them die, and others become afraid to leave their home as a result (Mobilizing the Region (April 30, 2001 )). However fixing pavements is not so simple. There are many different kinds of pavements and not al can always be fixed. The topic is vast and this essay covers just a sliver of it.
There are 18 different descriptions of road cracking and how to fix them. The essay will go through the most important of them, describing them, determining their cause, and prescribing how to repair them. Before proceeding however, a description f some of the key term are in order:
HMA -- Hot Mix Asphalt. Produced by heating the asphalt binder to decrease its viscosity and drying to remove moisture before mixing. HMA is most commonly used on highly trafficked pavements.
Patching -- this is an area of pavement that has been replaced with new material as repair of existing pavement
Substrata- the layer just under the pavement.
1. Fatigue (Alligator) Cracking
Repeated traffic loading of road or of pavement causes a mass of interconnected cracks to appear on the HMA surface (or stabilized base). In thin pavements, cracking starts under the surface (under the bottom of the HMA layer where stress is the most intense) then intensifies to the top as one or more lengthy cracks. This is known as "bottom-up" or "classical" fatigue cracking. In thick pavements, on the other hand, the crack, most times, comes from the top in areas where this is high stress form much traffic. This results in top-down cracking form asphalt binder aging. It is called alligator cracking since after repeated tire-pavement contact and subsequent cracking, the longitudinal cracks from, patterns that look like those on the back of a crocodile or alligator.
Problem: If these cracks are allowed to intensify without being fixed, they may encourage moisture, will become rough, will extend and may develop into potholes.
Possible causes may be due to the fact that the pavement loses its structural support and this may be due to aspects such as stripping on the bottom of the HMA layer; loss of base; heavier traffic than should be on this pavement; and poor construction.
Repair of the pavement necessitates that the area should be investigated to identify the cause of the cracks. Investigation should proceed by digging a pit or coring the pavement to determine whether moisture is contributing to the problem. Since sealing of the crack in this case doesn't help, fatigue crack repair is accomplished by (a) removing the cracked pavement, replacing the poor underlayers, and improving drainage of the area or (b) placing a strong HMA overlay over the pavement surface.
2. Block cracking
These are connected cracks that divide the pavement into rectangular blocks ranging in size from 1 foot to 100 feet. Larger blocks are terms "longitudinal" or "transverse cracking." This kind of cracking will sometimes also occur in non-traffic areas and is problematic since it may result in roughness of road and moisture infiltration leading to greater problems.
The possible causes are HMA shrinkage and different temperature impact on surface of road. This causes the asphalt binder to expand and contract with the changes in temperature usually due to asphalt binder aging and poor selection of asphalt binder in the first place.
Repair of such cracks on such pavements cannot always be done. Strategies depend on the extent of the crack as well as the severity. As regards small cracks, the cracks are sealed to prevent moisture from creeping in and worsening the problems. High cracks necessitate that the pavement be removed and replaced with an overlay.
3. Corrugation and shoving / (b) Depression of pavement
These are two different instances. The first is where ripples are formed on the pavement surface due usually to starting and stopping traffic action. The ripples are perpendicular to the traffic direction. The corrugation is usually caused by excessive moisture in the sub-grade and an unstable HMA layer that is caused by factors such as poor design, manufacturing, or poor mix contamination of road materials.
Repair is implemented by investigating the root of the cracks and by either removing the distorted pavement and patching it or by overlaying it with strong HMA layer.
Depression of the pavement is the opposite scenario to rippling where parts of the pavement dip down in contrast to surrounding areas. Depressed pavements usually form 'lakes' after a missive rainfall. The problem is that these pavements can cause vehicle hydroplaning and can become quire serious.
The common causes to pavement depression are pile up of frost on pavement or inadequate compaction of subgrade settlement during construction. As with all pavement situations, assessing the root problems performs repair and removing the affected pavement and replacing it with better sub-grade repair depression.
4. Longitudinal cracking
This is usually a type of fatigue cracking where the road had come into constant contact with heavy traffic and with cycles of temperature may also contribute to causing the cracks. Other causes may be poor joint construction or location. The joints are the least dense parts of the pavements; therefore these should be constructed out of the reach of traveling wheels. This is a case of top-down cracking, and, if left unresolved, may well develop into alligator cracking.
Repairs depend upon the severity and extent of the cracking with low severity cracks simply requiring that cracks be sealed to prevent further deterioration and high severity cracks requiring that the pavement be removed and replaced with a strong HMA overlay.
This is the very umpteenth level of cracks where pits are formed in the pavement penetrating from HMA layer to various layers down depending on the depth of the hole. Potholes usually occur on roads with thin HMA surfaces, hardly occurring on roads that have thicker HMA surfaces. They can cause serious damage to vehicles that travel at top speed as well as sometimes causing accidents. Their repair is performed as with other instances, by investigating roots, digging out substrate, and patching (i.e. replacing with new material to substitute the old).
Small pieces of the pavement come apart and there is subsequent progressive disintegration of the HMA layer from the surface downwards. Problems may result in loose debris on the pavement and loss of skid resistance or vehicle hydroplaning due to water colleting in raveled areas. One of the most common causes to raveling is due to dislodging of pavement by certain types of traffic such as snowplow blades or tracked vehicles.
Repair of these kinds of pavements usually consist of removing and patching or removing the damaged pavement and replacing it with an overlay.
7. Water bleeding and pumping.
Water bleeding occurs when water seeps out of joints or cracks through an extremely porous HMA layer. Pumping occurs when water is ejected from underlying layers of the pavement through cracks under the pressure of passing traffic. The causes of this circumstance may be due to a porous pavement that was the result of poor manufacturing design or poor mix during compaction. It may also be the result of poor drainage.
Repair is performed by investigation to identify the cause and if the problem is poor drainage or high water table, the sub-grade drainage is removed. If the problem, however, was attributable to poor mix, a fog seal or slurry seal is used to limit water infiltration.
The above are 7 of the most common problems of pavement cracking, their description and of how to repair them. Other problems include transverse / thermal cracking; stripping; slippage cracking; rutting; polished aggregate; joint reflection cracking and bleeding. The procedure…[continue]
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The tests found that major differences, especially in dynamic modulus, exist in the different mixtures of asphalt, and even among samples of the same mixture. The results suggest that that the constitutive makeup of paving materials interact to have a greater effect on the performance of the material than might otherwise be suspected. In addition, it was found that certain results of the dynamic modulus tests were not accurately predicted