A new specification requirement has been issued related to segregation in dense-graded mixtures. For visually identified segregated areas, cores will be obtained and measured for in-place density. Areas having a density less than 90.0 percent Gmm will have to be removed and replaced.
The Tennessee DOT recently tightened its production tolerance for 100 percent pay on asphalt content (AC) from 0.4 to 0.3 percent. Warm-mix asphalt is now permitted on any awarded TDOT project. Lastly, the minimum total AC content for 411-D, TDOT’s standard 12.5-mm nominal maximum aggregate size (NMAS) surface mixture, has been raised from 5.3 to 5.7 percent.
In April 2011, the Missouri DOT changed the binder specification, Section 1015, to include an option to produce binder in accordance with AASHTO MP 19. About three years ago, we added a special provision that allowed modification of binders using ground tire rubber (GTR) cross-linked with transpolyoctenamer rubber (TOR) with multiple stress creep recovery (MSCR) as an option to elastic recovery that requires polymer modification. Sections S6 and S7 of the 2009 cycle on the NCAT test track include a comparison of mixtures with SBS and GTR modification. Excellent performance of the GTR-modified mixture after 18 months allowed us to move ahead and add GTR modification to the Missouri Standard Specifications. In doing so, we elected to offer the option of MP 19 for any binder. We allow PG 64-22, and Grade H and Grade V in lieu of PG 70-22 and PG 76-22, respectively. Most polymer-modified binders are now being supplied using the MP 19 protocols due to formulation changes that allow suppliers to manufacture the binders at a lower cost.
Three projects are under contract to use permeability as the measure of density. Two are completed, and the other will be completed in the first half of 2012. The falling head permeameter developed under NCAT Report 99-01 is used as the testing device. One of the basic objectives of density measurements is to produce a long-lasting, somewhat impermeable pavement. Each mixture by its composition has a particular point of reaching impermeability. Designing mixtures with permeability in mind will hopefully reduce the compactive effort required for placement. By measuring permeability, the goals are to reduce permeability in Missouri’s pavements and provide an accurate non-destructive test. The projects completed so far also used cores for density measurements and indicate the pay factors for either method are roughly equivalent. It has been found that fresh plumber’s putty provides a consistent, satisfactory seal with this method.
Our quality control/quality assurance (QC/QA) specification has been revised to shift emphasis from thickness to cross slope. It was a small pay-factor change intended to promote maintenance of our roadway crown, which we observe is diminishing from repeated overlay projects.
We have added a milling tolerance for mill-and-fill projects. The specification forces good milling equipment maintenance. Some contractors do not replace milling teeth uniformly or frequently enough to create an evenly milled surface. This can cause uneven depths between individual passes.
Language has been added to specifically allow warm-mix asphalt (WMA) technologies. This has been done by reference to a Qualified WMA Technologies List, which is essentially the NorthEast Asphalt Users Producers Group (NEAUPG) qualified technologies list. Sections of our specification have been revised to allow lower minimum deliver and mat temperatures when WMA technologies are in use.
The Texas DOT will be implementing new HMA specifications within the next few months. All the HMA specifications have been rewritten in order to improve the quality of our HMA pavements, reduce cost and be better stewards of the environment. The use of reclaimed asphalt pavement (RAP), reclaimed asphalt shingles (RAS), WMA and “substitute binders” is permitted in most of our new specifications. We have also put more incentive into our specifications for contractors to use the Paver-IR system on paving projects. In addition, we have implemented a system of using security bags to maintain custody of samples when the samples are not within Texas DOT’s custody prior to testing. This was done to address the FHWA’s concern regarding sample custody.
Stone-matrix asphalt (SMA) mixes will now be required to pass the overlay test (min. 300 cycles) prior to use.
Our new permeable friction course (PFC) specification includes a mix with a smaller nominal aggregate size as one of the options. This fine PFC mix will typically be placed in the 0.75- to 1.0-inch thickness range.