Just as a point of reference, take a look at the chart of liquid asphalt prices and you get a sense that things have been relatively stable for the past five years, with a slight spike in 2003. In March 2006, there is a very notable increase over last year. This, combined with fuel-price increases, will make the coming construction season more of a challenge than those in recent history.
It will require adjustments to cope with the coming hardships, and these adjustments will have to be made soon. Contractors and agencies will need to move forward together to implement strategies that allow for ways to reduce the liquid asphalt requirement in hot-mix asphalt (HMA) while ensuring performance.
Up the RAP
The HMA industry has been recycling on a large scale since the 1970s oil embargo. Research on mix properties and modifications to plant equipment were quickly done in response to the asphalt shortages at that time, and within a few years, recycling became a commonplace practice. Over the years, contractors have generally stuck with having one stockpile of recycled material and feeding anywhere from 10 to 25% RAP into the mix. This produced significant cost savings, and the industry was content to remain at this level of recycling.
It is time to consider means for increasing RAP content even further. RAP is a resource rich in asphalt and aggregate, and just as we test and process virgin materials, so too should we judge the quality of RAP and process it. The sizing of RAP will help to refine its use in HMA. Having two or even three different-size stockpiles of recycled material means greater flexibility in designing mixes for specific applications. For instance, in finer surface mixes an increased amount of fine RAP can be employed, whereas a greater fraction of coarse RAP can be used in large stone mixes.
Like any other material in HMA, RAP should be engineered into the mix, not simply dumped. Understanding the asphalt content and gradation will help the mix designer integrate the recycled material in the right proportion and adjust the virgin materials. If used in large quantities, it may be advisable to extract and test the binder to see if it is highly oxidized and brittle. If it is, then the virgin asphalt may be decreased by one grade to ensure that the resulting mix is not brittle.
If a mix using all virgin materials has a required binder content of 5.5%, then a 30% RAP mix, where the RAP contains 4% asphalt, will reduce the amount of liquid binder by 1.2%. This means a contractor could reduce the quantity of liquid asphalt by 22%. On 10,000 tons of HMA, this means a reduction of 120 tons of liquid AC. At $250 per ton, this works out to a savings of $30,000. The savings with RAP go beyond just the savings on liquid asphalt. The aggregate in RAP provides just as much, if not more, money.
High RAP content mixes may pose special problems in terms of workability and compactability. While this may be aided somewhat by the use of a reduced PG grade of virgin binder, consideration might also be given to the use of additives or processes that improve workability at high temperatures.
RAP has always been a valuable commodity, and its benefit to the industry and its customers is more evident than ever before.
Rock the asphalt
It is no secret that mixes having a larger nominal maximum aggregate size require less asphalt for the same binder coating or film thickness than those that have a smaller-size aggregate. It comes down to the surface area of the aggregate.
This can be illustrated by taking a box, measuring 10 in. in each direction. If you take 1-in.-diam. balls, each one would have a surface area of 3.142 sq in., and it would take 1,000 of them to fill the space if you stacked them directly on top of each other. If you used 1?2-in. balls, each one would have a surface area of 0.785 sq in., and it would take 8,000 to fill the box. Multiplying the surface area of each ball by the number to fill the box leaves you with a total surface area of 3,142 sq in. for the 1-in. balls and 6,280 sq in. for the 1?2-in. balls. So the larger balls have a smaller total surface area than the smaller ones. An asphalt mixture will have smaller aggregate between the larger ones and the surface area will increase for each.
While the above example is an academic exercise, it is not a stretch to see how it applies to HMA. If the opportunity presents itself to use a 37.5-mm NMAS mix in a base rather than a 19-mm mix, there will be less surface area to coat with asphalt in the larger stone mix than the finer mix. To keep the same asphalt film thickness on the aggregate, less asphalt is required for large-stone mixtures.
In a 1988 NAPA publication comparing large-stone (13?4-in. NMAS) to conventional mixture (5?8-in. NMAS), it was found that the large-stone mix required 1.7% less asphalt. For 10,000 tons of HMA, this would work out to saving 170 tons of liquid asphalt, and at $250/ton there is $42,500 in cost savings.
Segregation and permeability are problems that can plague large-stone mixes. However, if the right precautions are taken in production and paving, segregation can be minimized or eliminated. For example, proper loading of dump trucks and the use of material-transfer vehicles can do much to help segregation. Proper gradation and mix design can help take care of permeability, and it may be advisable to use lower-design air voids to avoid it.
Many times, overlays are placed more to correct functional (smoothness) deficiencies than to strengthen the existing pavement. The appropriate use of thinner overlays can be a way of stretching rehabilitation dollars. The use of a 9.5-mm mix that is 11?2 in. thick will use 25% less material than a 12.5-mm mix that must be placed at 2 in. thick. Even though a finer surface mix will necessitate the use of a higher asphalt content, money will be saved.
Let’s say the 9.5-mm mix has an optimum asphalt content of 6.5% and the 12.5-mm mix has an asphalt content of 6%. The 9.5-mm mix would have a coverage of 163 lb/cu yd, and the 12.5-mm mix would have 217 lb/cu yd, if they both had a density of 145 lb/cu ft. The 9.5-mm solution uses about 18% less liquid asphalt per sq yd than the thicker 12.5-mm mix.
Some feel that finer surface mixes will potentially result in more rutting. In fact, research at NCAT has shown that finer mixes are no more prone to rutting than coarse mixes, and they also improve the ride and lessen the traffic noise. Finer surface mixes placed in thinner overlays could serve the function of thicker overlays if structural improvements in the pavement are not needed.
Right asphalt for the right occasion
Although it is not necessarily a way to use less asphalt, in many instances less of the expensive asphalt can be used. For critical situations, it may not be wise to sacrifice performance to lessen the initial cost. However, there are instances where more expensive grades of asphalt and polymer-modified asphalt are specified when they are not necessarily warranted. Layers that are deeper in the pavement structure, low-volume pavements and overlays of existing cracked pavement are all circumstances where specifications requiring premium asphalts should be reviewed and changed if necessary.
As can be seen on the historical graph of asphalt prices, premium grades in Ohio can be anywhere from $50 to $70 per ton higher than the standard grades. It also is likely that the standard grades will be somewhat easier to come by in a shortage than the more expensive grades. Using standard grades could result in a $33,000 savings on 10,000 tons of HMA.
If a particular situation does not involve high-truck-traffic surface mixtures, then question the need for premium asphalt. Slow traffic, high temperatures and cold temperatures may all be reasons to specify a better asphalt, but simply specifying a particular grade because it can be specified is not a good use of resources.
Raise the roof
Roofing shingles, especially those from manufacturers’ waste, have many of the same ingredients as HMA. This includes high-quality asphalt binder, hard fine aggregate, mineral filler, polymers and fibers. Many state agencies allow up to 5% of recycled shingles in HMA. With an asphalt binder content of 20%, the use of 5% shingles in your mix could reduce the HMA binder content by 1%.For each 10,000 tons of mix, this would save 100 tons of liquid asphalt and at $250 per ton this would save about $25,000 on mix cost.