Spray Applied Polymer Surface Seals
Lessons Learned Workbook
| General Conclusions In this study, it became evident there are two different types of spray applied products: sealers that add new asphalt to seal the surface, and rejuvenators that soften age-hardened asphalt to restore desired mixture mechanical properties in the upper 3/8 to 1/2 inch of the pavement surface. Sealers: Sealer emulsions may be used for any number of paving applications, including sealing HMA dense mix from water and oxygen infiltration, recoating raveling open-graded mixes, or tying down loose aggregate on newly applied chip seals. Such products are frequently formulated with polymers or other additives as needs dictate. Although not meant to soften the underlying asphalt, the new binder can serve as a sacrificial layer that has a lower stiffness than the aged asphalt on the pavement surface, thus protecting the underlying surface from further deterioration, especially raveling and top-down cracking. Unlike the clay-stabilized coal tar or asphalt-based sealer products used for driveways and parking lots, this type of sealer needs to infiltrate the surface to provide the desired sealing. In this study, mechanical tests run on thin slabs of mixture indicate that the hardening rate of asphalt near the pavement surface can be slowed with such applications. More specifically, mixture Creep Tests run in torsion mode in a research-grade Dynamic Shear Rheometer (DSR) showed that these treatments had a distinct impact on the evolving flow time (modulus) of the mix and binder in the top 3/8” as the pavement aged, but had little influence below that. Field observations after four years of service showed the treated pavements still exhibited improved impermeability to water and reduced distress. The study participants found the term "infiltrate the surface" applicable to these fog seals. If the newly applied binder lies on the surface without infiltration, tracking and friction may be a problem. In the field studies, there was little problem with tracking of the harder asphalt-based materials, but traffic was kept off of these test sections until the emulsions had broken. These sealers are also especially effective for use on new chip seals in tying down dust as well as retaining chips. Rejuvenators: The second type of fog seal products includes those meant to soften or “rejuvenate” the aged asphalt. These generally are emulsions of oils meant to replace the oxidized "maltene" fractions in the asphalt, and may again include polymers, asphalt and other additives. The mix DSR testing showed that rejuvenator emulsions can significantly soften the asphalt in the upper 3/8 inch of the pavement surface, as long as the pavement is sufficiently permeable to accept the oil down to that depth. If the pavement is impermeable, the oil may remain on the surface, leaving a surface that may track and/or exhibit poor skid resistance. The study participants found the term "penetrate into the pavement" most apt for these rejuvenator seals. A common laboratory technique for judging the depth of emulsion penetration into the pavement is to cut thin slabs of mix (nominally 0.3 inch) from various depths, extract the binder with an 85:15 blend of toluene and 95% ethanol, and evaluate the extracted binder using conventional rheological test methods. Mixture tests on thin specimens cut from the pavement surface should be used to confirm that the aging binder has in fact been rejuvenated as predicted by the extracted binder rheology, particularly if pavement permeability is low. Rejuvenator emulsions are most useful on oxidized pavements where low temperature mix properties approach critical levels for cracking. However, conventional wisdom regarding the application of rejuvenators is based upon softening the aged asphalt by using blending charts to optimize binder penetration, viscosity, or dynamic modulus (G*). If low temperature brittleness causes aged asphalt to crack, then blended properties of aged and virgin binders should be evaluated for relaxation and fracture properties at temperatures where failure occurs. It was noted that the oil-based rejuvenator emulsions without asphalt leave an oily residue on the tops of aggregates where the asphalt has been worn off, lowering friction. Tracking of oil onto unsealed sections was also observed. Thus sanding is strongly recommended for fog seal applications that include only rejuvenator oils. Other emulsions did not track noticeably after curing. However, friction numbers during the first four hours after application were typically doubled for all fog seal products, as long as angular sands were used and no loose surface sand remained. If sand is used, brooming should take place as soon as practical after the emulsion is fully cured. Supplier guidelines for some of these emulsions do not require sanding, particularly when residues are hard or are modified with polymers. The goal of the study was to determine the effectiveness of spray applied sealers. The user survey, the collected laboratory and field data, and the field observations all confirmed that a defined application strategy of fog seals is a cost-effective tool for pavement preservation. The seals appeared to protect the surface by reducing permeability to moisture for several years after application. Visual comparisons with control sections showed water running off of sealed surfaces instead of penetrating. Fog sealing decreased friction, but original skid resistance returned over time. Sanding after application increased the friction levels significantly. The portable CTM and DFT devices appear to be a reasonable surrogate for full scale friction testing. However, the friction results were higher on the Reclamite sections using both portable and full scale testing than might be predicted from a simple spinning tire field test using a rear-wheel drive pick-up truck. Rheological testing showed the high temperature modulus of binder within 3/8 inch of the surface was reduced by those sealers containing rejuvenator oils, as long as the pavement was permeable enough for the emulsion to penetrate. None of the rheological data showed significant differences in the stiffness of the binder or the corresponding dense-graded mix more than ˝-inch deep in the pavement. Of course better penetration can be expected for open-graded HMA or asphalt-rubber mixes. Mixture testing, however, was too variable to draw conclusions for any open mixes. It appears one of the roles of the spray applied seals is as a sacrificial layer to address surface cracking. The sealers were generally inexpensive and yet effective in providing protection to pavements and prolonging service life. The primary constraint to the use of sealers on dense HMA mixes appears to be a loss in friction following application. Sealers can also prolong the life of seal coats and open-graded mixes, where concerns with friction loss are considerably fewer. Specific conclusions may be found in each of the “Lessons Learned” sections on the pages of this website. Future Testing Recommendations: There were several topics beyond the scope of this project that warrant further study. These include: § Develop a simpler, more effective field permeability test applicable to fog seals. § Develop relationships between emulsion properties (surface tension, particle size, viscosity) and pavement permeability which can predict infiltration of the emulsions into the pavement surface. § Define a procedure for determining optimum application rates. § Determine if a pay item test strip can improve performance and safety. § Define sand quality: angularity, maximum moisture content. § Understand whether vehicle control on newly sealed sections is adequately predicted by IFI. § Develop and verify performance-related specifications. o Determine aged binder properties that lead to block cracking. o Define desired physical properties of the surface following application. o Define emulsion residue properties in physical or chemical terms for an emulsion purchase spec. More detailed Lessons Learned can be found on each page on this website. | downloadable Documents: (Click to open)
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| Provide Early and Sustained Pavement Preservation |