NRRA Rigid Team: Missouri DOT, Compacted Concrete Pavement vs. RCC

Investigating compacted concrete has been on the short list of research projects for the National Road Research Alliance (NRRA)’s Rigid Team. Compacted concrete pavement is a new form of roller compacted concrete (RCC) pavement that has a surface finish mimicking standard concrete pavement.

Brett Trautman, Physical Laboratory Director-Construction and Materials Division-Missouri Department of Transportation (MoDOT) is the Chair of the Rigid Team. In 2018, MoDOT paved a two-mile section of road south of Scott City, Missouri, using compacted concrete pavement. During the project, Trautman inquired, “Since we are going to build it, can some of the funds that were going to be used for the research be allocated to the work being done in Missouri?” Ultimately, the Executive Committee approved the funding. 

The contractor ran a test strip in late October 2018, then the project was in full production October 24-26. Len Palek and Steve Olson, MnROAD sensor-installation experts, arrived in Missouri and were on site to instrument 3 test sections on the project. These will be monitored for long-term performance, with assistance from students and a professor from Missouri University of Science and Technology (Missouri S&T).

Roller-compacted concrete (RCC) versus compacted concrete:
The first thing to remember about this project is that it was compacted concrete pavement and NOT roller-compacted concrete (RCC). The differences are important:
• Roller-compacted concrete uses a standard paver
• Compacted concrete must use a high-density asphalt-type paver—has a tamper screed that does the compacting of the surface, thereby forgoing the need for roller compaction. A traditional paver has vibrators in the screed.

Trautman said the compacted concrete they installed did have a very light (2-ton) roller go over it, but it only did one pass. This was followed by a riding trowel and brooming to create a surface appearing like standard concrete pavement. The key to compacted concrete, he said, is the mix. He stated, “Moisture control is everything.” If there is too little moisture it won’t compact; if there’s too much, it slumps.

The two-mile section being paved was a service road that runs along Hwy 55. The test sections were in the southbound lanes and consisted of:
• A 500-foot section with the main compacted concrete mix and 15-foot joints
• A 500-foot section with the main compacted concrete mix and 12-foot joints
• A 250-foot section with a fiber-reinforced compacted concrete mix containing 5 pounds per cubic foot of synthetic macrofibers (donated by Euclid).

Missouri completed an RCC project that almost failed a few years prior, so Trautman had cautious expectations. Overall, he was satisfied with this pavement’s outcome—this project was much better. “Considering it was the [contractor’s] first time doing it, I had visions of this being a complete mess. I think the contractor’s learning curve was steep, but I think they learned as they went. I believe they had already talked about the tweaks they would make to the plant, tweaks they would make to the mix design, and other things they could do to make this next project better,” Trautman said.

Learning Moments during the paving:

• As the mix was transported from the conveyor belt to the hopper of the paver, larger pieces ended up on the far side, while the finer were landing on the side nearest the belt.
• Correctly incorporating the mix was a challenge. Some wet sections occurred once the pavement was down, and some wet areas even exhibited a bit of slump. This was designed to be a zero-slump mix.
• They used stringless paving for this project, so the paver was being guided by telestations. On the second day, it rained, so controlling the moisture in the mix was complicated. Then the paver lost its connection with the telestations. According to Trautman, when that happened the screed just dropped. 

Fortunately, they only had to remove about 70-80 feet of the project because of this glitch. 

The NRRA became involved in the research of these test sections by funding the sensors and sending a team of experts down to Missouri from MnROAD. Steven Olson, Research Project Specialist and Len Palek, MnROAD Project Engineer, drove from Minnesota down to the Missouri project and helped with sensor installation. Trautman explained that the trick with this project, is that embedded sensors typically depend on the concrete flowing around and under them during paving. Since this mix had no real flow, the sensor team had to be creative. They packed the sensors—strain gauges, joint sensors, temperature sensors—with the mix on all sides before the paver came through. All of the sensors survived and data is being collected (via modems) and stored in the MnROAD database for use by the researchers. 

Surface Treatments
After the pavement was down and rolled lightly, they applied a proprietary product called Aceit Blue—the compacting and molding additive that reduced water and stabilized the moisture of the mix from Aceit Industries. According to Trautman, the additive was an oily film that was sprayed on the compacted concrete to help them finish it. “When you look at the final surface it looks like a concrete that’s been broom finished,” he said.

They finished with a curing compound and early-entry D3 sawing. Missouri doesn’t use a sealer for concrete pavements. Overall, they achieved 98% density consistently behind the paver. 

The fiber-reinforced test section got special attention. Several weeks before the paving, Trautman’s team did some trial batching to determine the fiber dosage they would try: 4 pounds, 5 pounds, and 6 pounds per cubic yard. They decided to aim for the middle with 5 pounds. Trautman is hoping the fiber will enhance the load transfer at joints because they couldn’t use dowel bars with compacted concrete. 

The students and a professor from the Missouri S&T were on site for performance testing:
• They molded several beams and cylinders from the mix.
• MoDOT also molded some beams and cylinders, but also cut some out of the test sections.
• They will test these against the molded samples for flexural strength, residual strength, and freeze/thaw durability. 

Economic Sense
Trautman says this project is about testing compacted concrete with the hope of adding to the toolbox of options Missouri pavement engineers can draw from. The economics of compacted concrete began to get the attention of contractors when asphalt prices began increasing. Asphalt binder prices made folks take another look at compacted concrete. He said some counties and cities might find it attractive because they’ll be able to get concrete projects, but still use a smaller paver. It won’t be for every project, but it might work for certain circumstances—especially good for some interchanges that are prone to rutting.

The future of compacted concrete is still in the monitoring mode, and that’s why the research done in Missouri is so important. Currently Louisiana, which also did some pilot projects years ago, is looking to create a pooled fund for compacted concrete use in highway projects.

“This project was ground breaking for the NRRA because it was not located at MnROAD, and it demonstrates the flexibility and power of our alliance. We hope other states will look at their upcoming test sections to see how our teams can help them implement and monitor these projects. We need to export the expertise of the NRRA to all of our members and our members-to-be,” said Glenn Engstrom, Director-NRRA.

For the entire article, please go to: http://www.dot.state.mn.us/mnroad/nrra/newsletter/november2018.html