Words into action
For the next four days, attendees witnessed the seminar in action. A group of volunteers accepted ACPA and PCA’s request for help and placed four test strips in the Las Vegas Convention Center’s Gold Lot. The effort was coordinated by Cory Zollinger, a pavement engineer with Cemex. Each team member brought a key element to the demonstration.
Chris Welly, operations manager for Aggregates Industries-SWR Inc., provided a critical element — the mix. Aggregate Industries produced the concrete at its North Las Vegas plant, about 10 minutes from the demonstration project.
When Welly agreed to batch the mix for the RCC Live! event, he knew his team would be up to the challenge. His production and quality control teams have provided special mixes for hundreds of Las Vegas-area projects the last few years.
But like most producers, batching RCC mixes was new. Aggregate Industries had provided RCC mixes for two high-volume jobs, a dam, and a large drainage area. For those projects, the producer didn’t have to start from scratch to create a recipe.
To prepare for the World of Concrete event, Zollinger’s advisory group provided an RCC paving mix design that had been used before. The paving mix designs used the same sized coarse aggregate as previous RCC concretes, but included quite a bit more sand.
Welly then had to step up to the operational challenge of starting up a high-volume production facility to batch two to three perfect loads each day. “We had a small window of opportunity to get the mix just right,” says Welly.
To help things get started properly, Zollinger’s team met at the plant the day before World of Concrete began. The team performed a thorough pre-job review to help eliminate any problems.
As is often the case, Welly didn’t get to see the paving operation. He hung back at the plant to help out in case any last minute problems arose. But Welly knew things had gone well when he heard reports about the pavement’s clean-out following the show.
When the show ended, the clean-up contractor sent a front-end loader to muck out the recently placed RCC pavement. After a few unsuccessful scoops, the contractor had to bring a heavy pneumatic hammer to first break up the pavement. “I heard he was working way into the night, as our mix had quickly achieved a high compressive strength,” says Welly.
Supporting organic growth
Producers know that strong contractor support fosters product acceptance. So for the past few years, Zollinger has been working with contractors. Based in Houston, he has traveled the country helping contractors place concrete pavements. His efforts, while focused on RCC, include all types of concrete paving applications.
Zollinger says his role is changing. In recent months, he’s been more involved in technical questions rather than pure marketing. Even so, the engineer has a starring role in Cemex’s outreach to contractors and engineers. Zollinger has recorded several short videos on pavement, including one on how to place RCC.
Zollinger believes contractors are slowly recognizing that flexibility is needed to stay in business. “Knowing how to use asphalt pavers to place RCC gives them another opportunity to win more projects and achieve greater margins,” he says.
The recipe for success
Credit: Bryan Haraway/Getty Images
Chris Tull explains the quality testing procedures technicians are performing on the concrete mix used at RCC Live! 2013.
Roller-compacted concrete is a unique material, says Christopher Tull, engineer and owner of CRT Concrete Consulting Inc. Tull, who has been involved in hundreds of RCC projects throughout the Midwest, was a consultant for the RCC Live! event at World of Concrete 2013. In “Planning Makes Perfect,” published in TCP’s Road Building supplement
in May 2013, Tull outlined the key attributes of a good mix design for RCC:
Technically, the science of RCC exists in the overlapping region of soil mechanics, asphalt, and concrete. This mix must consist of well-graded aggregates. A good starting place for developing the recipe is the PCA “Guide for Roller-Compacted Concrete Pavements.”
The document is a thorough introduction to RCC and its many applications, with detailed overviews of RCC properties and materials, mixture proportioning, structural design, production and construction considerations, and more. PCA provides a recommended gradation in its guide specification. Many first-time RCC producers try readily available crushed aggregates. Unfortunately, the variability in gradation of standard aggregates is too high to produce consistent RCC.
Your concrete background will be helpful here. Plotting the aggregates on a coarseness factor chart is useful. For larger aggregates, the mix should plot in Zone II. For the small maximum coarse aggregate sizes used in many of today’s mixes, the mix should plot in Zone III.
The concrete and engineering industries often want to know what water/cementitious ratio should be specified for RCC. The answer is quite simple: Do not specify a w/cm. The amount of water required is based on the amount needed for the required compaction. The materials (including the cementitious) are sent to a lab where a modified proctor is performed. This test will identify the optimal water content (a percentage of the total dry weight of all of the materials) that corresponds with the maximum density. Concrete technicians must have the skills to mathematically convert the optimal water content into saturated surface dry (SSD) batch weights for a computerized concrete batch panel or pull weights for manual batching.
Having the correct moisture content (within tolerances) is not only a strength issue — it is a density issue. RCC is delivered in dump trucks so there is no opportunity to correct water content after batching. If the moisture content is too far above or below optimal, the mixture will likely have adequate strength, but it will not be physically possible to obtain the required density. RCC pavements without proper density are subject to issues such as surface raveling and freeze-thaw durability.
It will quickly become apparent that a substantial amount of the required water will come from free moisture in the aggregates. A conventional mixture receives about 25% from free water in the aggregates, while the RCC mixture can receive 35% to 40% of its total water from the aggregates.
Contact Tull at email@example.com.
Providing assistance for RCC Live! 2013 were:
- AG Peltz Co.
- Aggregate Industries U.S.
- Allen Engineering Corp.
- The Concrete Industry Management department at Middle State Tennessee University
- CRT Concrete Consulting, LLC
- Grace Construction Products
- Holcim (U.S.), Inc.
- Husqvarna AB
- Morgan Corp.
- Rapid International USA, Inc.
- Vögele AG
- Vince Hagan Co.
- W.R. Meadows Inc.