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Section 4: Principal Faults or Defects in Seal Coats and Surface Treatments

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Some of the most serious defects in seal coats and surface treatments are:

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Loss of Aggregate

There are several major causes for serious loss of cover aggregate from surface treatments and seal coats as shown in Figure 1-1:

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  • A long delay between spraying binder and spreading cover aggregate, causing the binder to become chilled and hardened.
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  • Sealing too late in the season. Seal coats tend to perform better if they are under traffic a few months prior to winter weather.
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  • Insufficient binder is provided to cement the cover aggregate into place.
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  • Selection of an improper binder for prevailing conditions.
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  • A coating of dust or film of moisture on aggregate particles affects the adhesion to the binder.
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  • Fast traffic is permitted before adhesion is fully developed.
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  • A rainstorm occurs prior to development of adhesion.
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  • Placement of too much aggregate may cause embedded aggregate to dislodge under traffic.

    Seal coat pavement surface exhibiting aggregate
loss. (click in image to see full-size image) Anchor: #i999020grtop

    Figure 1-2. Seal coat pavement surface exhibiting aggregate loss.

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Poor Adhesion or Bond to Road Surface

The complete loss of a surface treatment or seal coat happens rarely and therefore is not listed as one of the major defects; nevertheless, it can occur. Poor bond between an existing surface and a seal coat placed over it may be due to the following:

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Streaking results when alternate longitudinal strips of a surface treatment or seal coat contain different quantities of binder, due to lack of uniformity of application of the binder inch by inch across the surface. The dark streaks which exist in this condition occur when there is not enough asphalt binder to hold the cover aggregate in place, and part of the cover stone has been torn out by traffic. These dark streaks are points of weakness at which the complete seal coat or surface treatment will wear away first under traffic. Streaking can reduce skid resistance, cause vehicle steering problems, and lead to a serious reduction in the normal life expectancy.

In addition to shortening the service life, streaking can be so pronounced that it interferes with the steering of a car on the road and can cause the vehicle to weave, thereby affecting the safety of traffic.

Some of the more common causes of streaking are mechanical faults, improper or poor adjustment, and careless operation of bituminous distributors. Another frequent cause is applying the bituminous binder at too low a temperature, so that it is not fluid enough to fan out properly from the nozzles on the spray bars.

Other common causes of streaking requiring mechanical correction are:

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  • operating with a portion of the spray nozzles partially or completely clogged (faulty strainers or absence of strainers is sometimes at least partly responsible for this)
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  • using spray nozzles of different sizes, different makes, and different rates of discharge in the same spray bar
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  • operating when some of the nozzles have not been set vertically and at the proper angle in the spray bar
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  • using damaged or badly worn spray nozzles
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  • employing spray bars in which the center-to-center spacing of the nozzles is not uniform.
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Too much bituminous binder used during the construction of seal coats and surface treatments is one of the most common defects. Excess binder exudes upward onto the pavement surface and is the origin of the black and frequently sticky surface condition referred to as flushing, bleeding, or fattening up and which can lead to a loss of skid resistance. Figure 1-3 shows an example of a flushed seal coat surface.

A seal coat pavement surface exhibiting
flushing in the wheelpaths. (click in image to see full-size image) Anchor: #i999028grtop

Figure 1-3. A seal coat pavement surface exhibiting flushing in the wheelpaths.

Every element in the finished highway (width, alignment, profile) satisfies both engineering and aesthetic demands, and yet the surface is the most obvious part of the structure. A poorly designed and constructed seal coat begins its service life with a blemished appearance and a surface that may have flushed so badly that it will exhibit a loss of skid characteristics. Consequently, the finished surface satisfies neither the artistic nor the basic engineering requirements that the public has a right to expect. This manual will provide guidelines on determining the correct binder application quantities.

The application of insufficient binder leads to a loss of aggregate, because not enough binder has been applied to cement the aggregate particles into place. Sometimes the surface on which a seal coat or surface treatment is applied is so open or porous that a large portion of the binder soaks into it. Not enough binder remains on top to hold the aggregate, and it can be easily dislodged by traffic.

In general, the use of too little binder occurs less frequently than the application of too much.


Surface Treatments Defects

In the case of a surface treatment on a granular base, potholes and smaller breaks may develop over poorly bonded areas from which traffic has removed the surface treatment (Figure 1-4). Unless attended immediately by maintenance crews when they first appear and are still small, these holes in the surface treatment may quickly become so numerous and so large that it is no longer economical to attempt to restore the surface by simple patching methods. A poor bond between the binder and the granular base that results in these breaks is usually due to pockets of dust or other fine material, or to areas with excess moisture that existed in the prepared surface, or to low temperatures when the surface treatment was constructed. Attempting to use a binder that is too viscous will also contribute to this type of failure through lack of bond to the road surface.

Potholes forming due to poorly bonded surface
treatment. (click in image to see full-size image) Anchor: #XFACTHWWgrtop

Figure 1-4. Potholes forming due to poorly bonded surface treatment.

The quality of the base finish is critical to the bonding of the surface treatment to the base. Both pneumatic and steel wheel rolers are used. The pneumatic roller is used first, followed by a steel-wheel roller. The kneading action of the pneumatic roller helps the initial rolling to even-out the bladed surface. The steel wheel roller helps to get an even and less rocky surface before the prime coat is applied.

One type of base finishing known as slush rolling is sometimes used and this technique varies depending on the amount of water used. Slush rolling with excessive water can weaken the base, first by trapping water in the base and then by altering the gradation of the base due to the pumping of fines to the top. Slush rolling can build up a layer of fines on the top of the base that will hinder the penetration of the prime coat and the prime can debond from the base easily. Therefore, slush rolling is not recommended.

The trimming technique uses the subgrade trimmer to finish the base. Excess base is used to compact the base 1-2 inches above the blue-top level, and then the trimmer is used to cut it down to the required finish level. Then the trimmed surface is rolled. This eliminates the need to do slush rolling.

A prepared road base structure that is to be surfaced using the surface treatment concept should always be primed first. The prime coat plays a very important role by facilitating the bond between the surface treatment and the base layer. A well-applied prime coat can protect the base layer from adverse weather conditions and from wear due to construction and regular traffic until the surface treatment is applied. It can also either prevent or slow down the formation of dust on the surface that will have a serious negative impact on the bonding of the binder to the base.

There are three types of prime coats used by TxDOT districts: spray-applied, worked-in prime and covered prime. A spray-applied prime (Figure 1-5) utilizes an asphalt distributor to apply between 0.1 and 0.2 gal/sy of either MC-30 or AEP products.

Figure 1-6 shows a worked-in prime coat application where diluted emulsion is sprayed on the finished base, which is then covered with a thin coating of fine base material dust working the windrow with a motor grader. This process is usually repeated 2-3 times to get a total emulsion application rate of 0.2 gal/sy. The emulsions commonly used are SS-1, CSS-1h and MS-2. This leaves an asphalt-sand layer on the finished base that is approximately 1/8 in. thick.

Spray-applied prime (MC-30 or AEP). (click in image to see full-size image) Anchor: #TYHNHELQgrtop

Figure 1-5. Spray-applied prime (MC-30 or AEP).

Worked-in (cut-in) prime. (click in image to see full-size image) Anchor: #NHFKAMAPgrtop

Figure 1-6. Worked-in (cut-in) prime.

Figure 1-7 shows a covered (or inverted) prime applied on the finished base. This covered prime is similar to a surface treatment where RC-250 cutback is applied to the finished base, which is covered by spreading Grade 5 rock. This “priming” technique is particularly useful when traffic has to be let on the primed surface before the other half of the roadway is primed. This type of prime can provide 2-3 months of satisfactory service as a very temporary wearing course under favorable traffic conditions including little or no turning traffic or heavy traffic.

Covered (inverted) prime.  (click in image to see full-size image) Anchor: #FTBPVGLXgrtop

Figure 1-7. Covered (inverted) prime.

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