Cement-treated base technology has been around for a while, but the last time it was used in Alabama was in the 1980s. Low aggregate prices and low trucking costs made other approaches such as soil aggregate base more feasible, and the result was that cement-treated base fell out of the spotlight in favor of other approaches.
But that was before changing economics repainted the picture.
"In recent years, aggregate costs have gone up significantly," notes Robert Taylor, P.E., director of the Alabama office of the ACPA's Southeast Chapter. "Trucking costs have risen dramatically as well. The result is that the use of cement-treated base has become much more cost effective, and contractors and owners are looking at it once again."
Cement-treated base is a mixture of aggregate material and/or granular soils with measured amounts of Portland cement and water. The mix hardens after compaction and curing to form an excellent base material, which is then overlaid with a bituminous or concrete wearing course to complete the pavement structure. Cement-treated base is used as a pavement base for projects such as highways, roads and streets, parking areas, airports, and in other applications. Cement-treated base can be mixed in place or in a pug mill.
To showcase how cement-treated base can be used on current projects, the American Concrete Pavement Association's Southeast Chapter recently hosted an open house in Covington County, Ala., where Bullard Excavating and subcontractor A.G. Peltz are using cement-treated base on two adjoining projects on U.S. 84.
In letting the projects, the Alabama Department of Transportation offered four base alternates to bidders. These included the following options:
- Soil aggregate base course, Type C, road mixed to 6-inch compacted thickness (two 6-inch lifts);
- Cement-treated base, 8-inch compacted thickness;
- Crushed aggregate base course, Type B, plant mixed, 8-inch compacted thickness; or
- Granular soil base, Type C, road mixed, 6-inch compacted thickness (two 6-inch lifts).
On each of the U.S. 84 contracts, the decision was made to build the new roadway cement-treated base. One of the projects involves about 58,000 square yards of cement-treated base; the second includes about 210,000 square yards of cement-treated base.
Cement treated base can be mixed in place or mixed off-site in a pugmill. The U.S. 84 projects are using material mixed off-site, utilizing a pug mill which is in operation at a borrow site near the jobs.
To produce the cement-treated base, the construction team utilized cement from CEMEX. After being mixed in the pug mill, the cement-treated base was transported to the work site in dump trucks. There, A.G. Peltz used an ABG Titan High Density paving machine, distributed in the United States by Ingersoll Rand, to place the material in a single lift. The Titan machine, manufactured in Germany, finds other applications in placing not only HMA but also zero-slump materials such as roller compacted concrete (RCC). It was thus very well suited for placement of zero-slump cement-treated base.
After placement, the cement-treated base was compacted by rollers. After curing, it will be overlaid with a suitable wearing course to complete the new lanes.
Approximately 75 people attended the open house and paving demonstration, which drew representatives from many parts of the industry. The list of attendees included Alabama Department of Transportation engineers, Federal Highway Administration engineers, county engineers, and city engineers from across the state.
Following a tour of the site, guests were treated to a fish fry sponsored by CEMEX and enjoyed a fried catfish dinner with all the trimmings.
But the most important part of the open house, Taylor notes, was that it gave engineers and other industry representatives a new perspective on an old construction technique.
"Across the southeast, in light of increases in aggregate and trucking costs, we feel that cement-treated base will gain even wider acceptance in the future," he says. "Alabama DOT feels that it is equal to other base materials, and in the future we expect that use of CTB will only continue to grow."