The introduction of the Superpave Mix Design for asphalt pavements was herald as one of the greatest improvements of the 20th century by some. The effort in developing the new design parameters was scattered among some of the most skilled asphalt technologists in the world.
I was involved in the SHRP program, and have experience in actually using the Marshall, Hveem and Superpave (Gyratory) mix designs for use by various agencies in doing so have reach the conclusion that perhaps we have not gained anything with the Superpave design and perhaps the benefits do not outweigh the deficiencies. In this blog I wish to outline the variables involved and expand on them in later blogs.
It should be mentioned that many people concentrate on the properties of the asphalt as being the determining factors defining performance. While the asphalt properties, especially the low temperature ones, are indeed important, the properties of the aggregate is vital, especially its gradation and its ability to resist the stripping of the asphalt off of the aggregate by liquid and gaseous water. A case can be made that the asphalt properties are over-defined for a well made pavement, but that is for other blogs.
Mold Size. One of the benefits for some of the mix designs with large sized aggregate with mixes on the coarse side is the 6” mold. Smaller molds can result in over-estimating the asphalt requirement caused by bridging of the aggregate. This can especially be true with the 4” Marshall Design; however with the use of the 6” Marshall design, this problem has been overcome.
Record of Continuous Compaction History. In problem solving the record of the compaction by gyrations allows one to obtain much better information of how things are compacting, and to predict how other variables will affect the compaction.
Too “Academicized”. It seems to me that if the procedure would have been turned over to state technicians to proof test, the procedure would have been better.
VMA Requirement too High. The VMA requirement was taken out of the Marshall design procedure. For a nominal ½” mix that is 14%. The old requirement for VMA from the FHWA Hveem specifications was 13%, and many successful pavements have been made in that range. As a result with certain mixes it is needlessly necessary to blow out fines just to meet the VMA requirements. (In my opinion neither the VMA or voids filled are the best specification parameters. I prefer effective asphalt and film thickness but would report the VMA for information purposes.)
Gradations Allow for Tender and Rut Prone Mixes. It amazes me that the effort to control rutting is to concentrate on the properties of the asphalt and neglect the true controlling factor: the gradation. The properties of the asphalt can affect the rate of rutting, however the gradation controls the extent of rutting. No one seemed to have listened to Dick Davis (Retired from Koppers) at meetings where he explained the importance of letting the aggregate carry the load. Both my son (Dr. Michael R. Dunning) and I know how to set the gradation to greatly reduce rutting (except from studded tires) and tenderness.
Lack of Understanding of the Gradation in the – #30 + #100 Range. Some mixes need a fine sand to be added as a filler, otherwise asphalt will have to be used to fill the void.
Lack of a Measurement of Strength. I frankly don’t understand leaving out a strength measurement. Adding an indirect tensile strength would have been easy.
Insufficient Emphasis on Water Damage. Many aggregates prefer to be wetted by water than by asphalt. Even though the rocks may be coated with asphalt, if there is any break in the film, water will get inside and lift the asphalt off. Even water vapor can cause damage and at one time was measured by the Moisture Vapor Susceptibility Test. For protection, amines and lime has been used, however there are data that suggest that the protection may be transitory, the mechanism for which will be described in a later blog. There is a new product that overcomes that problem.
Another problem can be that the test criteria for measuring stripping are not severe enough.
The research done by the SHRP projects was very valuable, however some have found that the Marshall and Hveem designs have features not found with the more costly Gyratory designs. The purpose of this blog is to suggest areas of concern from one who has practical experience.