Agenda - Council - 02/11/2020 - Laserfiche WebLink

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Air voids requirement for Super Pave mixes (2360) are stricter than the older 2340 mixes. Some of the volumetric for 2360 deal with voids in mineral aggregate which insures proper density and amount of asphalt without issues of rutting and flushing that 2340 mixes had. Asphalt Film Thickness (AFT) In late 1980's, to help reduce the rutting in the HMA the asphalt binder content was decreased, creating drier HMA mixes. The theory was that the drier mix would be more stable. However, the drier mixes were more susceptible to stripping, cracking, and other forms of water damage. These problems did not occur as frequently in the 1970's and the early 80's as it did later in the 80's. AFT is a requirement to help insure proper amount of asphalt in the HMA to deal with durability issues. AFT came about to make sure that high Recycled Asphalt Pavement (RAP) used in new HMA have enough new asphalt to perform properly. TSR TSR is tested to determine how likely the HMA is to suffer water damage. Since this study is looking at stripping i.e. water damage. It is recommended that TSR be tested on all new HMA projects. General Discussion In discussion with some of the HMA Contractors and mix Suppliers, normally they see little to no cost differences between 2340 mixes and 2360 level 2. This can be explained because suppliers are do the quality control (QC) testing for 2360 mixes daily as it is the current specification. QC testing for 2340 mixes is not, thus some extra time is needed from the supplier to review the older specification. Suppliers are also producing 2360 Level 2 & 3 mixes on most other jobs throughout their area, i.e. county and state occurring at the same time and using the similar mix designs, increases the plant production rates. Based on finding from samples and testing in Task 3, it is apparent that variability in density is a major factor in stripping under chip seal. Air voids ranged from 5% in great performing streets to over 11% on streets that are showing severe stripping. In Task 2 it was demonstrated that an asphalt mixture that has not had issues with stripping when properly constructed would strip when the air voids were greater than 8%. This shows that a better job of compaction during construction the asphalt pavement is needed. Ordinary compaction when done properly would not have had these issues. If the test strip was properly constructed and represented the true condition of the paving job then uniform density would have been achieved. The main issue with the ordinary compaction method is lack of enforcement of the established rolling pattern. The inspector needs to be present at all times to ensure that the pavement receives proper number of passes with the roller and to make sure the temperature of the mix is with in the specified requirements during compaction. The benefit of maximum density is the ability of the agency to test density in areas that may not meet minimum requirements. Many of the projects that were looked at as part of this study were originally constructed in the 1990's. At this time the 2340 specification was the current specification and required ordinary compaction methods. See Appendix A for bituminous specifications from the City of Brooklyn Center from the time the streets studied were constructed and also the cities current bituminous specification. D-3