Slipform pavers spread out, form and, depending on attachments, apply an initial finish to the surface of low-slump Portland cement concrete (PCC).
The paver is pulled over the PCC, slipping over the material, hence the name. The process is similar to extrusion, but instead of the material moving, the machine itself is moving.
PCC running through a slipform paver passes through an auger spreader, a spreader plower, strike-off plate, vibrators and or tampers and a conforming plate. It then goes through the finishing screed, which produces the final shape, followed by a float or hand finishing.
These machines provide a continuous and seamless concrete pavement surface and by nature give a smooth surface, without having joints. These machines must work on a stable subgrade, since lack of a stable subgrade can cause thickness control and profile problems.
Paving in this manner also requires a steady supply of material be available. As with asphalt paving, an inconsistent pace, including stops, along a planned straight run of pavement will create noticeable mars on the surface. Therefore, it’s important to keep a consistent head of material in the machine and to coordinate truck delivery and operations at the mix site.
Mix design and weather
Mix design and weather conditions also affect the quality of the pavement. Low humidity, high heat and high winds can dry out the PCC, which requires a delicate balance between workability and firmness. If the mix dries out, it loses workability and no manner of operational expertise will be able to improve the pavement.
“Slight variations in aggregate and moisture conditions can cause problems,” says David Howrey, president of Howrey Construction in Rockwell City, Iowa. His firm is a recent winner of a 2015 Concrete Paving Award from the Iowa Concrete Paving Association (ICPA). The awards recognize projects that are evaluated on several factors, including smoothness.
“A perfect mix design on paper is easy,” Howrey says, “but getting that perfect mix design out of the mixer and through the paver and to have it act consistent is another matter. Just a slight variation of the moisture content can make it act differently.”
Howrey says he’s witnessed contractors who work as if they believe the paver should be able to create perfect results in any condition.
“In a job done this past year, we had a dirt contractor get into the concrete pavement business and he had an inexperienced guy running the paver. It was 95 degrees and he didn’t understand why he was having issues. He was having nightmares over the project because the machine wasn’t doing what he thought it should be doing. Every minute the concrete sits in those conditions, it’s hydrating.”
Steve Jackson, president of Cedar Valley Corporation in Waterloo, Iowa, agrees with Howrey on the weather issue, as his staff lists it at the top of their challenges. Cedar Valley is consistently recognized for its quality concrete pavements, winning five awards from the ICPA’s 2015 round of recognition, and a Gold Award from the American Concrete Pavement Association’s 2015 Excellence in Concrete Pavements awards.
Mix design and concrete consistency also is a top concern, but Jackson adds that poor design may be a bigger problem when it comes to smoothness.
“Everyone wants pavement that is as smooth as a glass table top,” he says. “If the geometry of the pavement design has built roughness into the pavement, a slipform paver can’t remove it.”
Quality work crews
Design is just one part of the preparation aspect of slipform paving. Jackson says his staff believes a major contributor to top-notch concrete pavements is the quality of the workforce.
“The constant theme they gave me is that superior pavement smoothness comes from attention to detail and employing quality people who care about the job they are doing,” he says. “While quality equipment is absolutely essential to getting quality pavements, quality people seem to be the top factor.”
He says the same operation that produced smooth pavements 10 years ago is the same operation that will produce superior pavements today.
Cedar Valley says the top factors contributing to quality include:
· Properly trained staff preparing the grade and padline in front of the paving crew.
· Attention to detail performing equipment guidance “set up,” with or without stringline.
· Quality equipment designed to get the level of performance you are working towards.
· Attention to detail when performing equipment set up.
· Top-notch people producing concrete, working with equipment and materials capable of delivering consistent concrete, with enough production capability to deliver the level of production you require.
“I think the equipment manufacturers are doing a fantastic job designing and producing quality equipment capable of meeting the challenges of paving superior, smooth pavements,” Jackson says. However, he believes one of the biggest challenges faced by the concrete industry is attracting and keeping a qualified workforce that wants to be engaged and proud of their work.
He also believes consistent transportation funding to be key to maintaining consistent quality pavements. “We need to have funding mechanisms in place to have enough work for our industry to allow businesses to have a consistent workload, which will allow us to keep our trained staff and purchase quality equipment.”