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Section 7: Surface Treatments

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7.1 General

For purposes of discussion in this manual, these procedures will include treatments under items 315 (fog seal), 316 (seal coat), and 350 (microsurfacing). Scrub seals have been recently introduced to Texas and can be required by special specification. Surface treatments are applied to restore texture and weatherproofing (including protection from oxidation), but do not contribute to improvement in ride or increased structural capacity. They have become an essential part of the pavement preservation program in Texas, particularly for lower volume highways. A fog seal is a light application of a diluted slow-setting asphalt emulsion to the surface of an aged (oxidized) pavement surface. Microsurfacing is a blend of emulsified asphalt, water, well-graded fine aggregate (top size < 1/2 in.), and mineral filler. Scrub seals consist of one or more applications of an asphalt emulsion that is scrubbed with a broom and covered with a single layer of aggregate. The “scrubbing” action is designed to work the emulsion into surface cracks for a better seal. Condition surveys as described in Chapter 4, “Pavement Evaluation,” should be conducted to assess whether localized repair should be conducted prior to application of the surface treatment.

Surface treatments perform best when the underlying structure is still in good shape (minimal cracking, raveling, bleeding). In addition to a lane or road-width treatment, microsurfacing has been used to fill minor rutting. Relative performance of several traditional surface treatments (asphalt rubber chip seal, polymer modified emulsion chip seal, latex modified AC chip seal, unmodified chip seal, and microsurfacing) were evaluated in the TTI study 4040, Supplemental Maintenance Effectiveness Research Program (SMERP) (TTI-4040-3). In general, chip seals are more effective in preventing reappearance of cracking than microsurfacing, with the asphalt rubber chip seal performing best overall. Microsurfacing proved to be more effective than chip seals in preventing the reappearance of bleeding. In fact, if applied incorrectly (AC application rates too high, especially in the wheelpaths), chip seals were shown to actually increase the amount of bleeding over time. While a fog seal was also applied to test sections in this study, its effectiveness was limited essentially to anti-oxidation and anti-raveling roles. Fog seals are suitable for low-volume roads which can be closed to traffic for the 4 to 6 hrs. it takes for the slow-setting asphalt emulsion to break and set. An excessive application rate may result in a thin asphalt layer on top of the original hot-mix asphalt (HMA) pavement. This layer can be very smooth and cause a loss of skid resistance. Sand should be kept in reserve to blot up areas of excess application.

As with standard HMA overlays, the existing bituminous layers should be evaluated in the lab for stripping potential prior to applying the surface treatment. A surface treatment will generally seal off the vertical escape of moisture migrating upward out of a pavement, which can set up accelerated stripping in the existing HMA layer beneath the seal.

For more information on surface treatments and seal coats, refer to the Seal Coat and Surface Treatment Manual.

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7.2 Underseals

Underseals may be applied to the surface of an existing pavement prior to the placement of a new surface lift of HMA for two primary reasons. The first is to seal the old HMA layer, which has small to moderate amounts of cracking or is otherwise permeable because of poor compaction. The other reason is to improve the adhesion or bond of the new surface lift of HMA to the existing structure. Item 316 is the governing specification. Cautions cited above regarding the stripping susceptibility of the existing HMA layers apply with application of underseals as well.

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