Additional materials provided by the Southeast Cement Association and Site-Prep, Inc. of NC.
A portion of the original pavement design on the State Route 5 widening and relocation project in Smyrna, South Carolina, has been replaced with a plant-mixed Cement Stabilized Aggregate Base Course (CSABC) that is delivering an economical and structurally competent alternative to traditional materials used in pavement design. This product has been used extensively by the South Carolina Department of Transportation (SCDOT) in pavement design at the design-build level for its economics, structural capability and speed of construction. It is now gaining increased popularity in other levels of pavement design due to its status as an environmentally friendly alternative and its structural ability to reduce the thickness of the bituminous pavement section without comprising strength.
According to Joe O'Grady, cement stabilization specialist with the Southeast Cement Association, "Using portland cement for stabilizing soils and aggregates is the most cost-effective way to gain strength and stiffness in construction today."
With this in mind, John H. Edwards III, vice president of Site-Prep, Inc. of NC and a subcontractor on the Rt. 5 project, presented the CSABC alternative to Mike Kamis, project manager with Eagle Construction Company, Inc., the general contractor on the project.
"We were initially approached by Site-Prep to alter the pavement design on this project," says Kamis. "We initially had 8 inches of uniform graded aggregate base or macadam base, coupled with 355 pounds per square yard of asphalt aggregate base, 200 pounds per square yard of asphalt binder, and 200 pounds per square yard of surface type 1C.
"The crux of their proposal was to eliminate the macadam base and the 355 pounds of asphalt aggregate base and use instead 8 inches of (CSABC). Our initial response was somewhat skeptical."
Kamis and Eagle Construction had several concerns. The first was the structural integrity of the design and SCDOT's acceptance of the design change. After a review, the SCDOT concluded that the new pavement design was equivalent to or superior to the pavement design in place. Because the change was proposed as a value-engineering alternative, York County also supported the design change.
Production was another big concern for Eagle Construction. "This job has about 1.2 million cubic yards of earthwork, 15,000 feet of pipe and 240,000 square yards of macadam base," says Kamis. "With the current workforce we had up in this portion of the state, we were going to have some difficulty meeting the schedule. We were going to have to add a ton of equipment, manpower and additional supervision, which would have challenged our overhead structure a little bit.
"The production rate was something we were vitally interested in. When I scheduled this project we had about 1,000 square yards a day of macadam base and 1,200 tons of asphalt aggregate base per day. To date right now we are achieving 3,000 square yards a day of finished CSABC product with Site-Prep out here.
"The basic math of that is 240,000 square yards at 3,000 yards a day is 80 days. I had 220 days in my schedule initially to allot to the macadam and asphalt aggregate. So, we were looking at 140 days of time saved. On a job this length, that is substantial."
During preconstruction of a CSABC project, aggregate base and cement suppliers are first confirmed and samples are obtained. A qualified and approved geotechnical firm puts up a mix design of the blended aggregate and cement at varying cement percentages. The pills are broken at seven days to determine compressive strength and to determine a job mix design formula.
A portable pug mill or comparable blending plant is erected in close proximity to the project. The aggregate base material and portland cement are procured and delivered to the site. The plant is calibrated to provide a blended material as stated in the job mix design formula. On this project, the mix included 3-percent cement with breaks at 600–800 psi. A twin-shaft rotary pug mill blends the aggregate, and cement is metered in at the proper percentage. Water is blended in the mill to bring the material to optimum moisture. The material is then discharged in an outgoing conveyor and fed into a gob hopper that holds the finished product until it is ready for loading into trucks and delivery to the job site. Site-Prep has been processing 250 tons to 300 tons per hour on the project.
At the site, the subgrade is fine graded and shaped to the required elevations and compacted to the specified density. The CSABC is delivered to the site and dumped into a suitable mechanical spreader or high-density paver capable of spreading the material to the required depth.
The high-density paver places the material very much like asphalt. According to John Edwards III, "The benefit of the paver is that we're able to get the material densified very quickly. We're getting 92–94 percent compaction from behind the back of the screed of the paver.
"There is a time limit with this product versus normal macadam. You have cement in there so it is hydrating. We have a limited amount of time from when we add water to when we have primary compaction on the base. The paver allows us to do that."
Specifications in the past have not always required a paver. Site-Prep prefers the paver because there is a decrease in segregation of the material. It densifies quickly, which reduces the amount of rolling.
"A lot of times if we set it up on a string or on automatic machine control, we can literally place it almost to grade just like asphalt," adds Edwards. "These pavers capture the moisture and give you that 92-percent density right off the bat. This morning behind the Vogele (Super 2100-2) we ran a 91.7 percent, and two passes of a double-drum asphalt roller took us to 99 percent. It is much more uniform than other methods.
"Pavers are currently not required in the SCDOT specs, but this could change as CSABC becomes more prevalent. We do know that design-build projects that have been specified for CSABC have specified the paver, and that's a testament to the consistency and quality you get with it."
After the finished grade and required density are achieved and recorded, a bituminous seal coat is applied for curing purposes and to seal the moisture in the completed base. The finished base is normally allowed to cure for a period of five days to seven days prior to allowing traffic or subsequent lifts of bituminous or concrete pavement.
According to Edwards, "The SCDOT likes to see 600 psi on a seven-day compressive strength. We have exceeded that strength requirement."
There are numerous other advantages to using a Cement Stabilized Aggregate Base Course on a project.
- The pavement section can be reduced without compromising load-bearing capacity.
- Construction time is reduced.
- Materials cost is reduced. The amount of asphalt, a petroleum product, is decreased. In addition, CSABC is not indexed on this project, so the potential of cost overruns associated with a Hot Mix Asphalt (HMA) base course is eliminated.
- CSABC reduces dependency on foreign oil and liquid asphalt.
- There are reduced CO² emissions due to lower asphalt production requirements. Furthermore, the reduced pavement section also reduces the amount of material hauling required to the project. On the Rt. 5 project as of April 2008, 1,964 tons of cement has replaced 26,356 tons of HMA asphalt aggregate, resulting in a 92.5-percent reduction in material on this portion of the pavement section.
- CSABC reduces permeability, helps keep moisture out, and maintains a high level of strength and stiffness even when saturated, such as in high water-table areas.
- Innovations in equipment technology and product specifications provide for more enhanced quality control and quality assurance, yielding a more consistent and dependable finished product.
Although CSABC has been "in the toolbox" for many years, the dramatically increasing costs of oil and asphalt and the decreasing size of federal, state and local transportation construction budgets have made it critical to look at this process again and implement it on more projects.
"This project turned out to be a great candidate (for CSABC), and the scenario played out really well with York County, Capital Management and Engineering, and Site-Prep," says Kamis.
The Rt. 5 widening project had an original completion date of May 21, 2009, but Mike Kamis has revised this. The project is now scheduled for completion in December 2008. n