There is a lot of attention of binder properties on the performance of asphalt pavements. Various types of polymer are added to asphalt in the drive to improve properties.
It is easy to forget about gradation in evaluating a new additive to, say, reduce tenderness and rutting. However the prime reason that polymers are added to asphalt is to change the temperature susceptibility so that the binder appears to be more viscous at the higher temperatures than neat asphalt and softer at lower temperatures than neat asphalt. These additives may make have other benefits, however gradation is extremely important. Here are some Superpave Myths:
- Rutting is solved by asphalt modification
- A Maximum Density Line drawn from 100% passing to Zero has value
- There is a “forbidden area” that the gradation curve must avoid
- Gradations that go above the “forbidden area” are good as those that go below
Rutting is a gradation problem, not a binder problem. While modifying the binder with additives may show delayed rutting in the tests, the only sure solution is to assure that the coarse aggregate carries the load. There are forest roads that are open graded mixes bound together with a CMS-2s asphalt emulsion (an emulsified cutback) that perform very well. While modifiers may slow the rutting down, the only sure thing is the strength of interlocking rocks. If asphalt is modified so that it cannot stress relieve by flowing, non-load associated cracking will surely occur.
Maximum Density Line
Another myth, in my opinion after decades of providing mix designs, is that a “maximum density” line going from the 100% passing point to zero has value. It does not. Let’s call this the false maximum density line (FMDL) as it has no relationship to the gradation in the mix designs and is thusly of no value.
The myth of the so-called “forbidden area” of the gradation is essentially silly and does not provide any benefit. It was based upon an FMDL which has no resemblance to the reality of the gradations in mix designs. It was an academic after-thought that ended up in the specifications. Aggregates with a high rugosity may very well have gradations that could go through that area yet perform very well.
Specifying Gradations above the FMDL
Such gradations will have a hump in them. That is an invitation for rutting since the large aggregate cannot interlock to provide strength. The load has to be carried by a sand mastic.
The above myths may have a nice academic feel, but are useless in solving problems in the field.
Mixes that are Strong and Compact Easily
The tool we have found to be beneficial for strong easily compacted mixes is based upon gradations in which the coarse aggregate interlocks and carries the load. We first draw a straight line from the sieve size that first retains aggregate to the -#200 on the 0.45 power graph. For this discussion, let’s call that the true maximum density line (TMDL). That line should be the specification maximum, i.e., under no circumstance should the gradation plot go over that line. The slope of the line drawn from the sieve that first retains aggregate to the #4 should be greater (coarser) than that of the TMDL. It should not be too coarse, however, in order to avoid segregation problems during construction. Ideally the slope from the – #4 to the – #200 should be close to a straight line, with a slope less that the TMDL.
If the gradation in about the # 30 sieve goes over the TMDL, the mix will be tender and tend to rut. We call such mixes “over sanded”. There was a pavement a number of years ago that was rutting during construction. By using the above principals, not only was the rutting stopped, when the principals were place into specifications, the rutting specification for the agency could be lowered below the national norm. With certain non-modified asphalts, compaction is nearly impossible with over sanded mixes.
Another problem area in the compaction curve is below the # 30 and above the #200.If there is an overabundance of some material and lack of other, there will be a “hole” in the gradation which needs to be filled, usually with a fine sand. Without the sand as a filler, additional asphalt has to be used resulting in compactions problems.
I did a Gram-Schmidt orthogonalization of gradation data and found that only about 3-4 sieve sizes were truly independent thus having only a few sieve sizes in the specification is wise, especially when statistical specifications are used. However it is also wise for those who do mix designs to have used as many sieve sizes in the gradations as practical so that problems in the -#30 to -#200 region of the gradation can be identified and fixed.
Criteria for a Rut Resistant, Easily Compacted Economical Hot Mix
First, select a gradation that meets the TMDL guide lines. Within these guidelines for, say, a ½” nominal mix, adjust the gradation and -#200 material so that the effective asphalt content is between 4-5 %, the film thickness is 7-10 microns and the VMA is no higher than 0.5% above specifications. For every 1% the VMA is above the specifications about 0.4 % additional asphalt is required. (For the nominal ½” mix, the VMA specification is higher than it should be at 14%. Hveem mixes for this mix specified by the FHWA was 13% for decades and there is no technical reason for it to be higher.)