ARIZONA'S EXPERIENCES WITH ASPHALTIC CONCRETE FRICTION COURSES. Gene R. Morris Engineer of Research Arizona Highway Department.

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1 ;..,IJRRIS, G.R. ARIZONA'S EXPERIENCES WITH ASPHALTIC CONCRETE FRICTION COURSES by Gene R. Morris Engineer of Research Arizona Highway Department and Noe 1 R. Scott Assistant Engineer of Materials Arizona Highway Department prepared for Sub-Committee on Maintenance - to be presented at the FiftY.-Ninth Annual Meeting of the American Association of State Highway Officials at Los Angeles, California, November 9-15, 1973.

2 ARIZONA'S EXPERIENCES WITH ASPHALTIC CONCRETE FRICTION'COURSES Recently, David H. Stevens, Chairman, Maine Department of Transportation and former chairman of the Highway Research Board, stated "We have 3.7 million miles of public highway in this country and the problem we have now is not building nel1 roads, not spreading asphalt all over God's creation, but keeping our present highways satisfactory." Over the past several years the Highl1ay Research Community has placed a maj or emphasis on "keeping our roads satisfactory" or perhaps the more definitive term l10uld be "safe". In the area of pavements the research has been principally concerned wi th the frictional quality of pavements and the effect of this quality on accidents. This research has resulted in an al1areness of the concepts of hydro-planing and parameters for minimum desirable skid resistance of pavement surfaces. From these extensive research studies one pavement surface has emerged that provides both high skid resistance and some degree of preventive control to hydro-planing. This surface is kno\1ti by several names, but is generally called an open-graded plant mix seal. The use of plant mix seal coats is not a new one, but dates back to at least 1948 when California developed the procedure. Arizona's first plant mix seal coats l1ere placed about The original reason for the development of this process was to eliminate some of the inherent l1eaknesses, in normal seal coat construction. In order to examine the characteristics and performance of the plant-mixed seal coat, one should perhaps reviel1 the functions of the normal seal coat..! -, :.. The principle functions of seal coats have been outlined to include the follol1ing: -1-

3 1. To prevent entrance of moisture and air into the pavement. 2. To renew skid resistance. 3. To rejuvenate the pavement surface. 4. To improve luminosity. 5. Traffic lane demarcation. 6. To improve appearance. It is highly questionable that a normal seal coat accomplishes any,. of the above functions for an extensive period of time, and, in fact, seal coats,per se, have many inherent weaknesses. A few of these are: 1. The limited thickness of seal coats prevents any improvement in rideability, and permits existing deterioration (patches, bleeding areas, cracks) to reflect through. 2. Construction quality control is tenuous and greatly affected by climatic conditions and traffic. Achieving proper quantities of asphalt and aggregate is at best difficult. 3. Placing of seal coats almost invariably results in poor public relations because of windshield and headlight breakage, dust, and hazardous driving conditions during construction. 4. The seal coat provides no increase in the structural value of the pavement. The plant mix seal coat was first incorporated to eliminate these 1.j 1 weaknesses. Some of its principle advantages over the normal seal coat are: 1. The 5/8 to 3/4 inch thickness does provide improved rideability. 2. Construction quality control of materials is possible. The climatic conditions are not nearly as critical; for instance, rain.i shortly after placement does not result in a disaster and traffic does not create major problems. -2-

4 3. All aggregate is tightly bound eliminating windshi,eld breakage and the subsequent public relations problems. 4. The plant mix seal does add a measurable structural value to the pavement. S. Since the plant mix seal does permit renewing rideability and is not nearly so sensitive to reflection of existing pavement characteristics, rehabilitation (heater scarification, rejuvenating agents, crack filling) of existing pavement surface is possible. From research on safety and skid resistance other advantages are now apparent. Perhaps the most important of these is that the open-graded plant mix seal reduces the possible conditions that lead to hydro-planing. In addition, if proper aggregates are used, the open-graded surface provides a very high friction coefficient. Additional spin-off benefits are that paint stripes apparently last considerably longer on a plant mix seal and water splash from passing vehicles is greatly reduced. Reviewing these above advantages, it is obvious that plant mix seal coats are here to stay... Plant mix seal coats have been extensively used in the West and Rocky Mountain region, and this use has not been restricted by traffic or climatic considerations. In Arizona, for instance, open-graded plant mix seal coats are serving satisfactorily in both heavy and light traffic and in environ ",. ments ranging from the hot deserts to cold elevations of 9000 feet. Various nomenclatures have been used by the various agencies to describe this particular process. These include plant mix seal coat, open-graded seal coat, pop-corn seal, open-graded plant mix seal coat, open-graded surface course, -3-

5 and Arizona's nomenclature, asphaltic concrete finishing course (ACFC). Along Irith the Federal Highlray Administration's recognition of this process we also now have a new title, "Asphaltic Concrete Friction Course." Note that they did retain at least the initials used by Arizona of ACFC, and this designation will be used for the remainder of this report. The concern of this group is primarily with new or unusual maintenance problems that you may be facing with the use of ACFC. To understand these problems it would be useful to discuss the materials and processes involved and to review modifications that have occurred over the years. These modifications, obviously, have been implemented as corrective measures to eliminate problems creating excessive maintenance. Figure (1) shol;s aggregate gradation and materials first used in the early 1950' s as compared to 1965 specifications and to our present designs. On first glance there lvould be only minor differences betlreen the two specifications; however, an in-depth examination reveals important changes. First, note that in original usage a tack coat was not used. We have found that while on rare occasions a tack coat may not be necessary, on the vast majority of jobs it is needed and it is considered excellent insurance for all projects. Liquid grade asphalts (RC-250) are generally.;. used for tack coats. In urban areas,because of construction problems (trucking, etc),emulsions are utilized. The most important change is reflected in the gradation of the materials. Note that our 1965 design requires a substantially more single size aggregate than the Also, the percentage of fine material has been substantially reduced. This trend was emphasized even further in our present design where we have limited the passing number 8 sieve to 15% as compared to 23% in These changes in -4-

6 gradation have been made to increase the voids in the mineral aggregate, and allow the use of greater asphalt contents, thus providing increased pavement drainage and asphalt durability. The principle problems that we have e)'."perienced with ACFC have been the result of either too little or too much asphalt at time of construction. Since 1960 we have designed ACFC I,ith an average of 6 1/2 g, asphalt for conventional gravel aggregate. At specification mixing temperature of and normal usage it is possible to use this much asphalt. However, if either hot plant dryer temperatures fluctuate upwards or long hauls of materials are involved, problems are encountered with drainage of the asphalt in the truck. This, of course, resul ts in areas of the roadway having 'too much asphalt and creates conditions leading to bleeding pavements. As a result, in the past our resident engineers were reluctant to use this high asphalt content and invaribly reduced the percent of asphalt at the project level. This reduction led to mixtures that were too lean, and asphalt film thicknesses that were subject to oxidation. While these leaner mixtures perform quite well for a period of time, sooner or later the asphalt would become brittle and loss of aggregate would occur. Each of these conditions are shown in Figures (2) and (3) respectively. Our mix designs still call for 6 1/2% asphalt today, but to correct the above construction problems we reduce the mix asphalt content to 5 1/2% and require, in the construction contract,! J an additional 1% asphalt to be applied as flush coat utilizing emulsion. It should be noted that the 5 1/2% mixtures performed quite well and can be opened to traffic for a considerable length of time before flush coat is applied. In fact, mixtures containing only 3% asphalt have been placed, I and opened to traffic for short periods without any damage. This point.l has significance that will be mentioned later. -5-

7 It has been our experience that these modifications have eliminated virtually all of the unusual maintenance problems. As with any asphalt paving material, deterioration does occur and there is certain maintenance that has to be performed. One of these problems is raveling between wheel paths at intersections, stop signs, or wherever traffic must slow down or stop. This raveling is caused by oil and gas drippings. We have placed one test section of gilsonite and one test section of coal tar applied as a flush coat at the intersections in an effort to reduce this damage, and it would appear these may be successful. However, this procedure has not been implemented as normal procedure un~il further evaluation. One concern that is cited by our maintenance personnel is that they cannot duplicate appearance using normal patching material in the ACrC. " We believe that this problem,muld be eliminated if they would use the same gradation of aggregate at a low asphalt content, and provide remainder of the asphalt in as an emulsion flush. In fact, a flush coat could be applied to present a very pleasing appearance because dimensional accuracy is possible. The other maintenance procedure that one should be aware of is that of loss of aggregate occurring as the asphalt ages over a period of years. A light flush coat of emulsion, liquid grade asphalt, or rejuvenating agent selected according to pavement condition, "ill correct this condition. The need for such a flush coat is readily discernible by a pock-marked appearance of the pavement and by appearance of loose chips. The corrective treatment should be applied as soon as the surface begins to lose, i J aggregate. -6-

8 In general then, it is our opinion that maintenance of ACFC is considerably less than other surfaces. We believe that most maintenance problems have been eliminated through design and construction modifications. Until the past few years the use of ACFC has been limited almost entirely to new construction. In the past year or two several ACFC projects have been placed as a maintenance seal coat. Generally, this "ork has been in conjunction with heater scarification and pavement rejuvenation. One of the principle limitations in Arizona to the use of ACFC for maintenance operations has been that a hot plant must be available within economic haul limitations. This limits our use of ACFC, as a maintenance tool, to the Phoenix, Tucson, and Yuma areas. Earlier I mentioned that an ACFC mixture containing only 3% asphalt had been placed and the remaining asphalt flushed in, in the form of an emulsion. We are now studying the use of emulsions in place of paving grade asphalt for the initial mixture. We have prepared and tested laboratory designs of mixes in which 5% mixing grade emulsion (3% residual asphalt) was used as the binder and an experimental section utilizing this mix will be placed in the near future. The use of emulsion will eliminate the requirement of a hot plant and will permit us to place ACFC in areas not now possible. With this development we would anticipate that ACFC would replace almost entirely the current seal coat procedures now in use. Several other possible improvements in the ACFC are now in field experimental stages. rubber in the mix. One of these is the incorporation of reclaimed For several years the City of Phoenix has been experimenting with and placing hot asphalt-reclaimed rubber seal coats.. In -7-

9 , general, this process is a difficult one during construction and usually requires extensive monitoring during its early life. However, the longterm results have been dramatic. The reclaimed rubber-asphalt seal coats placed upon pavements badly cracked by fatigue or deformation after five, six, and seven years service are still effectively sealing these pavements. We hope that by combining the reclaimed rubber with the ACFC similar resuits can be obtained while eliminating the extensive construction problems. It is believed that this possibly may even provide a thin-overlay method to renew skid resistance and rideability to concrete pavements. Other parallel experiments are being conducted with the inclusion of latex into the emulsion applied as the flush coat. In conclusion, Arizona's experience with ACFC has been excellent. We have been able to engineer out any unusual maintenance problems. The surface provides excellent skid resistance and offers an economical means to renew the rideability qualities of the pavements. It is anticipated that incorporation of designs now being studied and tested will extend Arizona'S use of ACFC. -8-

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11 Fig. 2 Asphalt Segregation

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