More and more traffic is motoring on the nation's roadways today. The U.S. is expected to have an increase in motor vehicle traffic from 2.7 trillion in 2000 to 3.3 trillion in 2010. Truck traffic is expected to double by 2020. Every 10 years, a population increase of 30 million is expected. Suburbs are expected to grow 80% over current levels.
What does this mean for asphalt contractors? The strain on the nation's roads is going to get heavier. There are 3.9 million miles of roads in the U.S., 2.3 million of which are paved. As demand on roadways increases, federal and state governments are requiring higher standards for asphalt performance. Governments also are looking for new and improved pavements that are smooth and have a long lifespan.
In 1991, the Federal Highway Administration (FHWA) conducted its first International Technology Scanning Program. The program took senior U.S. transportation officials to other countries to see what technologies and techniques used abroad could be adopted in the U.S. The first scanning tour took place in Europe, where transportation officials encountered stone-matrix asphalt (SMA).
SMA is widely used throughout Europe. U.S. transportation officials were drawn to the mix because of its ability to resist rutting even under the heaviest of traffic loads.
"SMA is the premier hot-mix asphalt product used anywhere in the world," said Dale Decker, a consulting engineer and former vice president for research and technology at the National Asphalt Pavement Association.
According to the FHWA, 10 million tons of SMA has been placed in more than 250 projects in 25 states since SMA was introduced in the U.S. The use of SMA on roadways with heavy traffic volumes is expected to grow. In states like Maryland, Georgia, Washington, Virginia and a host of others, SMA is the preferred mix on major roadways. Maryland has laid more SMA than any other state, with 4 million tons on roads that take heavy loads. The FHWA expects SMA to last up to 20 years with minimal maintenance.
"There are SMA pavements that have been down for 10 years, and there's no rutting," said Decker. "It's performing well when done properly."
SMA resists rutting and lasts a long time by creating the perfect balance between stiffness and flexibility. It is a gap-graded mix made of angular aggregate that locks together. This aggregate will actively carry the load and give the mix flexibility. The space between the aggregate is filled with a cellulose filler made up of shredded and compressed paper fibers. The cellulose filler gives SMA its stiffness.
"We want a stiff pavement, but with enough asphalt to provide some low-temperature flexibility," said Decker. "SMA mixtures are stiff, but at the same time it is also flexible, and that's why it performs so well. Because SMA has the stone-on-stone contact of the aggregate and the cellulose fillers help hold a high asphalt content, it becomes durable."
Can you handle it?
Of course, with any asphalt, the key to making it work is how it is placed.
"It's a challenge to handle this material," said Decker. It is a challenge that contractors should expect to face more often given SMA's growing popularity.
Decker said that one of the most important things when handling SMA is to make sure the mix doesn't get segregated. "Keep the material together. Once this mix is segregated, you won't be able to do much about it. Keep the augers going on the paver as much as possible," he said.
Not only will keeping the augers moving help the material from getting segregated, but it will ensure that the material is being laid quickly. "Get the material down as soon as possible because it's going to be difficult to deal with if you don't," said Decker.
SMA needs to be compacted while hot. As it cools, it will stiffen, but if the material is not compacted to the right density the aggregate won't lock together properly.
"This is why some contractors are using dual tamping screeds for high density behind the screed," said Brodie Hutchins, marketing manager for highway-class paving equipment at Ingersoll Rand. "Achieving higher densities at the screed gives contractors more time in the tender zone for finishing rather than breakdown and finishing."
SMA requires a higher density than most other asphalts. "We're usually looking for 94% density. That's a pretty high number," said Decker.
In order to get the proper densification, compaction has to be handled very quickly. Decker suggests the compactors stay close to the paver so the material is easier to work with.
While density for SMA is high, Decker also said it is important to remember the properties of SMA when compacting it.
"The SMA mix is based on stone-on-stone contact. Rather than making the material more dense through a great deal of compaction, we're ‘seating' the aggregate particles so they lock together. There's a lot of stone in this mix. If you try to start making the material dense like you would with a dense-graded mix, you're going to break the rock in the mix. Compaction of SMA is a seating process to get the proper density of the material," said Decker.
Decker stated that contractors should expect to use less vibration on SMA than on other mixes. Vibration will be used in the breakdown position. "Use vibration with caution. If you see you're getting any breaking of rock, you've got to back off."
Contractors should not use a pneumatic tire roller on SMA. "The mix will stick to the rubber tires on the pneumatic rollers," said Decker.
Not for the old
The unique qualities of SMA have led some contractors to invest in new equipment. In the early 1990s, after the FHWA began to recommend the use of SMA, Virginia was one of the first states to put it to use. MEGA Contracting has been completing Virginia SMA projects from the beginning.
"One of the first SMA projects we did was a mill-and-fill overlay on I-95 in front of a rest area. It is still in good condition and no further maintenance has been required even though that is a heavily traveled stretch of highway," said Paul Owen Lanier II, president of MEGA Contracting.
While MEGA found success with early SMA projects, the company also discovered the material provided challenges to their equipment.
"We began placing SMA with conventional pavers and quickly found that to get the proper density we would need to employ a high-frequency roller to work closely behind the paver's screed. As a result, we found that smoothness suffered because of the high amount of compaction effort that had to be placed upon the newly laid mat," said Lanier. "The rollers tended to push the mat horizontally as well as compact it vertically. This was due to the high temperatures that were needed to achieve the specified compaction density. It was about that time we went looking at pavers with tamper bar screeds."
MEGA Contracting invested in an Ingersoll Rand Titan paver. The paver features a screed that has a high-density, double-tamping bar. The double-tamping bar and the weight of the screed allow the paver to achieve high degrees of compaction and smoothness at the screed.
An Ingersoll Rand double-tamping bar paver produced 92% compaction at the screed in a recent SMA project in Virginia.
"We found that the double-tamper bar screed gives the compaction needed from the paver. It also lays a smoother mat because of the mechanics involved in the pre-compaction of the asphalt prior to the asphalt flowing under the screed," said Lanier. "This is extremely important because Virginia, like most other states, has SMA contracts that contain an incentive clause that is determined by the smoothness of the asphalt."
"Bonuses are not just paid on compaction and density, but also on the smoothness of the asphalt," said Hutchins. "Contractors can get penalized if smoothness is not achieved."
Saving on cost is important, especially when dealing with a mix like SMA, which will typically cost 20 to 25% more than other asphalt mixes.
"SMA is more expensive because of the cellulose fibers and modified binders as well as a higher asphalt content. Then again, the lifecycle cost typically shows that SMA will have a 30% longer life than other types of asphalt. The life of the pavement offsets the cost," said Decker.
Decker said contractors who have not placed SMA should prepare to do so in the future. Contractors, he said, should also prepare to train a younger work force to work with SMA.
"People are the key component in the placement process. Fifty percent of the current work force will be eligible for retirement by 2012. The average age of construction workers is 40."
As SMA continues to grow in use, contractors who have invested in their employees, equipment and familiarity with SMA will be the best suited to meet the country's growing transportation demands.