CONSTRUCTION AND PERFORMANCE OF PLANT MIXED SEAL COATS

THE TYPE OF PLANT MIX SEAL COAT DISCUSSED IS ONE IN WHICH AGGREGATE SIMILAR TO THAT IN A NORMAL CHIP SEAL IS USED (OPEN GRADING, WITH MAXIMUM SIZE ABOUT 3/8 IN.) AND THIS AGGREGATE IS MIXED IN A PLANT WITH A RELATIVELY HIGH PERCENTAGE OF AN ASPHALT CEMENT AND PLACED TO A COMPACTED DEPTH OF 5/8-3/4 INCH BY AN ASPHALT PAVER. A THICKER COAT RESULTS WITH A CHIP SEAL. THERE ARE OTHER SIGNIFICANT DIFFERENCES: (1) THE PROBLEM OF LOSS OF CHIPS AND DAMAGE TO VEHICLES IS LARGELY ELIMINATED, (2) THE SURFACE HAS EXCEPTIONAL SMOOTHNESS, (3) THE SURFACE HAS UNIFORMLY GOOD SKID RESISTANCE AND IS OF A NATURE NOT SUBJECT TO HYDROPLANING, (4) THE SURFACE HAS A VERY PLEASING, UNIFORM APPEARANCE, (5) THE SURFACE HAS GOOD DURABILITY, (6) COSTS ARE REASONABLE, AND (7) BECAUSE OF HIGH MECHANICAL STABILITY AND GREATER THICKNESS, SOME STRUCTURAL VALUE CAN BE ALLOWED IN THE PAVEMENT THICKNESS DESIGN. PLANT-MIXED SEAL COATS ARE MOST COMMONLY PLACED ON EXISTING BITUMINOUS SURFACES, BUT THEY CAN BE AND ARE PLACED WITH GOOD RESULTS ON RIGID PAVEMENTS. THE TYPICAL HOT PLANT MIXED SEAL OF THIS TYPE USES OPEN-GRADED AGGREGATE OF HIGH QUALITY, CLEAN, HARD, TOUGH, NON-POLISHING, WITH A HIGH PERCENTAGE OF CRUSHED FACES AND GOOD RESISTANCE TO THE STRIPPING EFFECTS OF WATER. ADDITIVES SUCH AS HYDRATED LIME MAY BE NECESSARY TO PROVIDE ADEQUATE RESISTANCE TO STRIPPING. MIXING WILL BE DONE AT RELATIVELY LOW TEMPERATURE WITH A RELATIVELY THICK COAT OF AN ASPHALT CEMENT. THE MIX IS PLACED WITH A REGULAR BITUMINOUS PAVER TO A COMPACTED DEPTH OF 5/8 - 3/4 INCH. EXPERIENCE WITH PLANT MIX SEAL PROJECTS ARE DISCUSSED. COSTS DEPEND ON THE COST OF THE AGGREGATE AND IT IS AN EXPENSIVE AGGREGATE TO PRODUCE - ABOUT THE SAME AS GOOD AGGREGATE IN A CHIP SEAL. COSTS AVERAGE AROUND $.25 A SQUARE YARD AND FOR COMPARISON, AN ORDINARY CHIP SEAL WOULD PROBABLY NOT COST MORE THAN $.11 PER SQ. YD. AND WOULD ALSO BE LESS THAN HALF AS THICK. MIXING TEMPERATURES ARE QUITE CRITICAL. UNDER CONDITIONS OF VERY HEAVY TRAFFIC AND CONTINUED HIGH TEMPERATURES, ADDITIONAL CONSOLIDATION CAN TAKE PLACE WHICH MAY RESULT IN THE VOIDS BEING OVERFILLED WITH ASPHALT, WITH THE EXCESS ASPHALT THEN FLUSHING TO THE SURFACE AND ELIMINATING THE NON-SKID ADVANTAGES. THIS PROBLEM MAY BE PREVENTED BY SOME REVISION IN AGGREGATE GRADING OR USE OF A LOWER PENETRATION ASPHALT. CONSTRUCTION PROCEDURES ARE DESCRIBED.