by Mary Weaver with photos by Bill Weaver
The Southern Minnesota Construction (SMC) asphalt plant near company headquarters in Mankato, MN is an exceptionally versatile plant. The plant became a part of SMC in 2006, joining Ireland’s vertically integrated Oldcastle Company. Oldcastle also owns the nearby New Ulm Quarry. There, they quarry, crush and screen aggregate for SMC’s asphalt, providing not only aggregates, but also manufactured sand as well. Additionally, Oldcastle owns nearby natural sand plants as well, which also supply the natural sand used in SMC’s mixes.
SMC’s asphalt plant was designed and built to house both a drum plant and a batch plant. With this configuration, they are able to manufacture material using either of the two methods. During the asphalt production process, a chute feeds dried aggregate and sand ejected from the drum plant to the batch plant, for use when small amounts or special mixes are needed. Not having to fire up the drum plant separately for these projects, saves a considerably amount of energy.
The company also owns a Gencor portable drum plant, which is equipped with a Getz Rubble Hog for breaking up clumps in the RAP. The portable drum plant is used primarily for mainline paving. The company recently won an award from the Paving Association for the overall quality of our work at the Fairmont, MN Airport. “It was a key airport project.”
The SMC plant is well equipped to use both RAP and RAS (recycled asphalt shingles) in their asphalt mixes. Ground recycled shingles are a less popular recycle in asphalt because they can be difficult to handle. They are prone to clumping and binding, rather than metering smoothly onto the conveyor.
“With 2% to 5% RAS, those conveyors move slowly,” commented Quality Control Manager Kim Tolzmann. SMC’s shingles are ground and screened by a subcontractor, Dem-Con Shingle Processing (DCSP), using a Rotochopper and trommel screen. DCSP has the expertise to produce the very fine post-grind product required by MN DOT standards with a very high degree of consistency in product size and quality.
SMC has developed several innovative methods to make the ground asphalt shingles easier to work with. Before ground RAS is used, for example, it is “fluffed up” by running it through an Astec Prosizer screen plant to break up clumps.
At another location, the bin into which the RAS is dumped is lined with a smooth, slippery white plastic. A mirror is mounted above the bin, at an angle, so the loader operator can see down into the bin to determine whether the RAS is flowing through properly, or if is bridging and clumping.
Another innovation is that the company has started to mix approximately 10% sand with the RAS in hopes that the mixture will flow more smoothly through the system. “The State allows the addition of the sand, and they are using this in the metro area,” Tolzmann continued. “How it will actually work out remains to be seen.” Each ton of RAS that can be successfully worked into asphalt mixes is another ton that doesn’t reside forever in a landfill, and SMC is working out successful ways to contribute to the solution of this landfill problem.
Using RAS also saves money because of the Asphalt Cement (AC) that is retained. “We check the percentage of oil in the shingles by doing a chemical extraction before we do mix designs,” continued Tolzmann. “It usually runs 19% to 20% AC. We haven’t seen a lot of difference in the volumetric properties we test with the AC from RAS in our mixes. It might be a little stiffer, which could contribute to fatigue cracking. That’s why the State wants a certain amount of new AC. The use of RAS, however, can contribute a significant amount of fines, affecting asphalt film thickness.”
MN State specs allow 30% binder replacement from which a combination of RAP and RAS can be used, but no more than 5% of that can be from recycled shingles. “Our mixes vary from 2% to 5% RAS.” The parameters of the mix are set by the project engineer. “Within those parameters, we design our own mixes. Those formulas are sent to the state, where they are reviewed to make sure they meet all requirements. The State then issues a written report through the project engineer, set up as a design for us to follow.
The SMC asphalt plant is computerized, and the correct amount of each ingredient is automatically metered from the storage bins into each mix. The aggregate, manufactured and natural sand are conveyed on covered conveyors into the end of the drum to the right, where they’re heated and mixed until they are dry. The RAS and RAP, because their asphalt content should not be overheated, are conveyed, with the RAS dumped onto and conveyed on top of the RAP, into the left end of the drum, where the AC is blended from a line on the other side. They enter the drum near the end of the process. “After everything is mixed, a flame is shot through the barrel for aggregate drying.”
“When we start running a mix, we send a sample to the State. As we’re running the mix, someone from the county or state is with us doing checks and taking companion samples. We constantly monitor the properties of the mix, sometimes adjusting the ingredients as necessary.”
Quality control is critical. In developing their asphalt mixes, quality control personnel compress a mix into a cylinder in a machine that gyrates and applies a certain amount of force. “We weight it to determine Bulk Specific Gravity using air dry, surfaced saturated, and immersed weights for calculation. The Bulk Specific Gravity is then compared to a Theoretical Maximum Density to determine a lab designed Air Void,” he said. They target a certain amount of Air Void in the mixture, the requirements set by the State depending on the mixture’s use. Typically, the allowable Air Void is between 3% and 4%.
Many of the mixes require polymer modified AC to make them more flexible and prevent thermal cracking. “We work with a performance grade Asphalt Cement with a temperature range of 58 degrees C. to -34 degrees C. Typically between 70 to 80% of our AC must be new AC.
The amount of manufactured sand (vs. round-grained natural sand) and the amount of crushing needed for the aggregate depends on the expected traffic levels on the finished pavement. Most state designs are based on traffic loads. Heavy traffic levels require the interlocking of the angular pieces of manufactured sand as well as well- crushed aggregate for strength. This can limit the amount of RAP that can be used in the mix. “Typically, recycle has less crushing.”
The finished hot mix asphalt can be stored in two insulated, heated storage tanks at the SMC plant. There are also two unheated storage tanks on site. “We try not to store HMA overnight,” explained Tolzmann. From there, material heads to the jobsite for paving. “When paving, again we’re doing quality control testing as we pave with an air gap system nuclear density tester,” he noted.