Arkansas's Superpave Experience

By Jim Gee
JWGE105@ahtd.state.ar.us

How and When We Began

Suppliers were willing and specifications for all asphalt cement were changed from viscosity gradings to performance gradings in November 1995.

Binder grades were determined from climatic information and PG 64-22 was established for use statewide because reliability was 98 % for most of the state and was the lowest at about 70 % in only 4 or 5 counties.

A maximum of three grades were established.
64-22 is used on all highways except Interstate;
Interstate is specified with 76-22;
70-22 is specified for urban, slow traffic, etc.

64-22 replaced AC-30 or AC-20;
70-22 modified 1.5 to 2%,was added;
76-22 modified 3 to 4%, replaced our previously modified viscosity grade.

The Department has purchased a 2nd set of binder testing equipment.

Since the implementation of performance grade asphalt binder in November 1995 the Department has contracted for 250,000 + tons of binder.

We began superpave trial mixes utilizing aggregate sources from existing designs in our lab in January 1995; completed about 15 designs in the first six months and invited contractors and suppliers into the lab to look at what could or could not be done with existing materials.

First job was started by change order in November 1995.

In 1996, there were 3 more jobs by change order and one was let to contract in June; another in November and another in December.

In 1997, by November there were more than 30 jobs let to contract.

To date, on about 10 jobs the Contractor has elected to substitute Superpave at the same costs as a standard marshall hot mix.

By the end of 97, there was a total of nearly 50 jobs either by contract, change order or substituion ,that included Superpave.

About half of these jobs have some Superpave down;16 jobs have been completed.

To date, approximately 1.7 million tons of Superpave has been let to contract.
690,000 tons 12.5 mm;
575,000 tons 25.0 mm;
425,000 tons 37.5 mm

With the 1998 February letting, all jobs will be Superpave.

What we know about Design Practice

22 Contractors have compactors; some Pine, some Troxler.
13 Contractors have submitted Superpave Designs.

Approval of labs to do Superpave mix designs has been handled by the Department on a personal inspection and reference sample basis. 15 labs are now approved to do Superpave designs. There are about 15 more labs that are already approved to do marshall designs and that may be approved to Superpave designs.

AHTD has purchased Gyratory compactors; 95 bought 1; 96 bought 1, then bought 12; 1997 bought 12. We now have 13 Pine and 13 Troxler. In 1998, we plan to buy 12 more. This will be a total of 38 compactors and should complete our needs throughout the state.

To date, all of the design procedures performed by the Department or by the Contractor have followed the FHWA requirements.

Realizing that the FHWA (SHRP) procedure has seven levels of compaction and in order to be perfectly clear about the required gyrations for a project, the plans indicate the number of gyrations required.

The Department has agreed to allow contractors to use existing mix designs on new projects if the number of gyrations is equal to that specified or one level higher. Mix designs made for 129 gyrations can be used for 114 or 129; 160 150 or 169. This has in effect reduced the number of designs required and follows the intent of the recommendation to simplify the application of gyrations related to ESALS.

What we know about QC/QA

Contract jobs require the contractor to sample and test the materials; AHTD samples and tests the material; results of both tests are used for acceptance. Two types of gyratory compactors are being used; few problems with correlation, due to the different equipment, have shown up.

What we know about mixing and laydown

The laydown practices require greater attention in order to obtain satisfactory compaction. It is mandatory to roll the mix immediately behind the screed. The mix must be laid and rolled at the required compaction temperature. More compactive effort will be required to obtain minimum compaction.

What we know about the costs

Price comparisons from the changes in 95 in binder specifications to present Superpave jobs does not readily indicate the costs, particularly when along the way about a year ago the standard specifications for hot mix were changed to require volumetric properties for acceptance rather just gradation and asphalt cement requirements. It is difficult to analyze how much change in costs can be attributed to this change in properties and how much can be attributed to changes due to Superpave. However, from our current 97 projects we can offer the following assessments:

• The percentage of asphalt binder in Superpave surface course mixes is about the same as conventional marshall mixes (sp5.46,ml5.35); about 0.50% more in binder course (sp5.00,ml4.44); and about 0.10% less in base courses(sp4.43,ml4.54).
• 1997 prices indicate that the Superpave surface course mixes are costing about $1.40 per ton more than the standard marshall mix; Superpave binder course mix costs about $1.34 less than the standard marshall and Superpave Base costs about $3.53 less than the standard marshall mix.
• 1997 prices for performance grade binder are about $22 a ton more than 1995 viscosity grade binder.
  

What we know about performance

There have been no problems with rutting. One intersection is flushing. Two jobs have reported excessive permeability.

The intersection cores taken where it was flushing showed a higher binder content; with the number of trucks, perhaps the grade should have been bumped up.

47 cores taken from the roadway on 16 projects show that there is definitely a relationship between density, thickness and permeability and in general the superpave mix is slightly more permeable than the marshal mix.