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The bitumen surfaces need some correction factors that come in different volumes.
Surface distress is Any indication of poor or unfavorable pavement performance or signs of impending failure; any unsatisfactory performance of a pavement short of failure.
Surface distress modes can be broadly classified into the following three groups:
Fracture. This could be in the form of cracking (in flexible and rigid pavements) or spalling resulting from such things as excessive loading, fatigue, thermal changes, moisture damage, slippage, or contraction.
Distortion. This is in the form of deformation (e.g., rutting, corrugation, and shoving), which can result from such things as excessive loading, creep, densification, consolidation, swelling, or frost action.
Disintegration. This is in the form of stripping. Raveling or spalling, which can result from such things as loss of bonding, chemical reactivity, traffic abrasion, aggregate degradation, poor consolidation/compaction, or binder aging.
Thus, surface distress will be somewhat related to roughness (the more cracks, distortion, and disintegration – the rougher the pavement will be) as well as structural integrity (surface distress can be a sign of impending or current structural problems).
The pavement must be repaired prior to the application of a BST.
Pavement can be tested in order to help determine what repairs should be made.
Common tests include wheel path rutting, roughness, and surface friction. If structural failure is suspected, a falling weight deflectometer test may be conducted.
Distress Correction
Once the pavement structural distress has been quantified, the distress must be corrected before constructing a BST.
Base Repair
If the distress is so severe that the subgrade has been affected, the structure must be replaced through a full-depth dig-out, which may be done in a variety of ways.
The designer must specify the area and depth of the repair for each section of the pavement.
Patching may be done (in HMA, in PCC) to repair wheel ruts, edge raveling, and delaminated or pothole areas.
Common construction practices in placing HMA for patching include the use of a spreader box, grader/blade patching, or using a paver.
Results from using any of these methods will be satisfied if they are properly performed.
To avoid chip loss, all paved repair areas must be fog sealed prior to applying BST. Fog sealing is the light application of an asphalt emulsion.
Fog seals can also be placed on a new chip seal after the final brooming to assist in aggregate retention and prevent the shedding and shelling of the new aggregate.
The fog seal gives an additional layer of waterproofing to the new wearing course.
In this type of use, the fog seal should be applied 3 to 14 days after the chip seal was placed, with brooming to be completed immediately before the fog seal is placed.
Bitumen Correction
Crack seals are used for repairing transverse and longitudinal cracks that might need bitumen correction. This is typically the use of a rubberized tar material or a mixture of sand and asphalt emulsion to fill the crack.
Cracks and joints 1/4″ or greater should be cleaned of any incompressible material including old sealant, and then sealed prior to applying a BST.
A slurry seal ought to be applied at 25% of the design life for maximum benefit in preserving the pavement structure.
Maintenance with a slurry seal must be instituted before significant pavement deterioration occurs.
Aggregate size, emulsion type, and any additives determine the classification of the slurry seal. If distress is noted, crack sealing ought to be done before applying a slurry seal.
When a slurry is placed over dry and raveling pavement, a tack coat should be done before the slurry seal.
Construction
A single layer BST is constructed in the following steps:
Surface preparation. Surface defects, such as potholes, are repaired and the existing surface is cleaned.
Asphalt material application. Typically, an asphalt emulsion is applied from a spray truck to the surface of the existing pavement.
Aggregate application. A thin aggregate cover (only one stone thick) is spread over the asphalt material before it has been set. The aggregate usually has a uniform gradation.
Aggregate embedding. A roller is used to push the aggregate into the asphalt material and seat it firmly against the underlying pavement.
Generally, about 50 percent of each aggregate particle should be embedded in the asphalt material (see Figure 4) after the final rolling.
About 70 percent of each aggregate particle will be embedded after several weeks of traffic.
It is common to place an aggregate “choke stone” on top of the uniformly graded larger aggregates after embedment.
Choke stone is essentially a finer aggregate gradation (e.g., less than 12.5 mm (1/2 inch)) used to make a more dense aggregate matrix at the level of embedment (see Figure 5). This denser matrix helps prevent excessive aggregate loss due to traffic.
BST construction depends on many factors, as mentioned in the prior sections of this article; among them are aggregate structure and gradation, binder grade and electrical charge, design process, traffic volume, and pavement structure condition.
There are many other considerations that cannot be covered in the scope of this article, including specific construction methods and equipment used.
Common problems include loss of cover aggregate, streaking, bleeding or flushing, and utility connections in the pavement. Loss of cover aggregate
Loss of the aggregate cover is probably the most common problem with a BST.
This may be due to poor gradation, dusty aggregate, binder breaking before the aggregate is laid, poor rolling of the centerline seam, and insufficient asphalt binder.
Typical solutions to these problems are to apply more asphalt binders so that the cured binder reaches 70% of the height of the aggregate.
This is called residual asphalt. Also, a clean, dust-free aggregate must be used to ensure proper asphalt binding.
Streaking is when grooves or ridges are seen on the seal coat surface. This is an undesirable situation, however, it is primarily cosmetic.
The asphalt binder distributor is the problem when observing streaking. Proper calibration of the distributor will ensure that streaking does not happen in the future.
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