A general rule of thumb for paving thicker pavements is to place two or sometimes three thin lifts as opposed to one thick lift. But an award-winning parking lot paving job by Martin Marietta Materials, Greeley, CO, instead relied on the contractor’s regular practice of placing one thick lift for a surface course.
The job, which was honored by the National Asphalt Pavement Association, involved construction from the bottom up of a 15,000-sq.-yd. parking lot adjacent to a paved employee parking lot at JBS Swift & Co., a beef producer. Martin Marietta was a subcontractor on the job to GLH out of Windsor, CO. GLH handled the subgrade work, base paving and concrete work but left the final lift of paving to Martin Marietta’s Greeley operation. The contractor runs an asphalt plant that produces an average of 180,000 tons a year and employs 25 people among a paving crew, patching crew, plant operation and a small office staff.
Paving Superintendent Dave King said the job, which was completed in summer 2013, required 3 ½-in. of a warm mix asphalt atop 7 in. of base. “Generally you might do it in two but we find we have better luck with a thicker lift,” King says. “It looks better and stays hot longer so our compaction is better and requires less manipulation.”
King says the job didn’t specify warm mix asphalt but Martin Marietta suggested it and JBS Swift liked the idea. “We like the warm mix better because it saves in heating and we feel the rock coats better with warm mix than with conventional hot mix,” he says. “It’s better for the environment, and it’s better for our workers. It’s safer for them, plus it helps keep the workforce fresher because they’re not standing on 400° F hot mix all day long.”
He adds that the mix stays warmer longer, “and it’s more forgiving if you don’t get on it right away with the roller.”
Experts at One Thick Lift
“Placing a thicker lift is not something we were just trying. We lay thicker lifts quite a bit and we have gotten very good at it,” King says, adding that customers in the Denver market have bought into the approach. He says local municipalities have even gotten together and developed a spec that requires the lift thickness to be at least 2 ½ times the nominal aggregate size where a lot of specs only require twice the nominal aggregate size.
King says the company relies on a lot of 3-in. lifts because the thickness offers a number of advantages, including the fact that one lift speeds up production because one lift is placed instead of two. But generally speaking, the advantages stem from the thicker mat maintaining its heat longer which eases and improves compaction.
“A thicker lift holds heat better so you get a little more compaction time,” King says, adding that a thicker mat requires less effort to compact. He says many contractors like thinner lifts because there’s not as much shrinkage from compaction but he says that when compacting thinner lifts it’s easy to fracture soft rock in the mix.
“When you have a thicker mat, there’s more room for all the particles and aggregate to move. So on a thick mat you’re impacting more of the particles when you compact it,” he says. “Having extra thickness gives extra room and that enables the finished pavement to be denser. If you have a ¾-in. mix, and only lay a 1 ¾-in. mat, that doesn’t give you much room to manipulate the material verses a 3 ½-in. mix, for example.”
Two Pavers on the Job
On the JBS Swift job, Martin Marietta planned for a large paver, a small paver with that crew also handling hand work, three rollers and a quality control technician.
Most paving was done with a Caterpillar 1055 paver, while a LeeBoy 8500 was used for short pulls and tight areas such as between islands. King says there were more than 30 islands sprinkled throughout the job and if they had not used a second smaller paver, they would have had productivity issues that would have made it more difficult for the crew to complete the two-day job on the consecutive weekends it was scheduled.