McLeod County uses cement-stabilized FDR in pilot project
Gravel roads provide reliable, all-year access to landowners throughout Minnesota. Although a gravel road is not the “dirt” road citizens often complain about, it can be bumpy, dusty, and subject to soft areas during the spring thaw. Gravel roads are also costly for the county to maintain, especially for higher traffic volumes, requiring replacement gravel and regular blading.
McLeod County Road 54 was a gravel road serving growing rural residential and farm-to-market needs. As a county road, even though the 180-vehicle-per-day traffic count met standards for paving, no state or federal funds were available for a conventional reconstruction and paving project.
John Brunkhorst, P.E., county engineer, considered a number of options for improving the road, including calcium chloride application, bituminous stabilization, cement stabilization, and hot-mix paving. John determined that a cement stabilized with full-depth reclamation—CSFDR—had the best potential for improving the road within cost constraints and public acceptance.
The county retained American Engineering and Testing to perform a mix design to determine the depth of reclamation, percent cement, and optimum moisture content based on the aggregate and soil conditions. Because of the potential for expanding CSFDR to other county gravel roads, several test sections were constructed with 8-inch and 10-inch depths and 5%, 6%, 7%, and 8% cement added to determine the optimum design. The goal was to develop a durable and economical mix design with a compressive strength of about 250 psi, enough strength to support traffic but flexible enough to avoid acting as monolithic crack-prone slabs.
The year before the project, drainage was improved and the road bladed to a uniform cross section. An average of 4 to 6 inches of aggregate was in place, so full-depth reclamation would include about 4 inches of earth from the underlying road embankment.
The general construction process was as follows:
- Cement was spread on the grade by a GPS-calibrated semi-trailer spreader.
- A reclamation machine, basically a GPS-enhanced rototiller with the capability to add precise amounts of moisture, tilled the section to the design depth and injected the appropriate amount of water. Sections of 200 to 300 feet were constructed to ensure shaping and compacting would take place before the cement-stabilized mixture set.
- A pad foot roller compacted the material.
- A motor grader bladed the section to a 4% crown. Offset staking provided horizontal alignment control.
- A smooth drum roller performed final compaction.
- A bituminous tack coat, double seal coat, and fog seal were applied to seal the surface and act as a final wearing course.
American Engineering and Testing assisted during construction with field tests to ensure the specifications for moisture, cement content, and density were achieved.
The county has developed a testing and monitoring program to measure performance of the various test sections, including MnDOT ride quality measurements, FWD strength, and perhaps ground penetrating radar evaluation. Local OPERA funds from the LRRB will be sought to assist with monitoring.
Preliminary conclusion: strength is adequate. Establishing realistic public expectations for ride is important. It is a stabilized gravel road, not a high-level pavement. The county is also evaluating the desirability of centerline striping.
McLeod County has developed a carefully engineered pilot project throughout the planning, designing, and construction process. It is planning a testing and monitoring program so sound conclusions can be drawn. We look forward to future reports.
—Al Forsberg, retired Blue Earth County Engineer