Beltrami and Hubbard counties planned pavement rehabilitation on a shared road, presenting a unique opportunity to directly compare two stabilizers: engineered emulsion and Base One, a proprietary mix. While there were some variations in performance and cost, both met design values for pavement strength.
Local transportation agencies rely on stabilized full-depth reclamation (SFDR) as a pavement rehabilitation strategy. The process creates a supporting recycled road base layer by pulverizing the existing pavement, adding a stabilizer, compacting the layer and overlaying with a surface layer of asphalt.
Previous research by the Local Road Research Board (LRRB) found that SFDR produced a stronger base than regular aggregate of the same thickness. While LRRB has sponsored additional research on full-depth reclamation, different stabilizers have not been directly compared.
“These road projects presented a unique opportunity to compare stabilizing products side by side. While both products met design goals, it will be helpful for county engineers to understand the differences in costs and long-term performance,” said Michael Marti, director, SRF Consulting Group, Inc.
Beltrami and Hubbard counties had each planned a rehabilitation project in 2020 for a segment of road that bordered both counties. Each county used a different stabilizer: Hubbard County used Base One and Beltrami County used engineered emulsion. LRRB saw a unique opportunity to help county engineers directly compare two commonly used products.
What Was Our Goal?
The goal of this project was to compare the performance and costs of two stabilizing additives used in the full-depth reclamation process.
What Did We Do?
Conditions of the two-lane roadway that crossed both counties were evaluated before, during and after the SFDR project. Beltrami County’s approximately 3.5-mile-long segment experienced a higher traffic load than Hubbard County’s 1.3-mile-long segment.
MnDOT’s PathWeb digital road data collection tool reviewed photos of the roadway’s existing condition, showing damaged pavement with breaking edges, wide cracks and frequent patches. The tool also presented a graphical representation of pavement condition indices, including the international roughness index and measurements of rutting, faulting and cracking. Based on this data, both segments were considered in fair condition.
Pavement design values for both road segments were the same for speed (55 mph) and load (10 tons). Although the Hubbard County segment had a stronger subgrade, the Beltrami County segment was expected to carry a higher average daily traffic load over the 20-year design period. The design granular equivalency (GE) values also varied, with Beltrami County assigning a 1.5 GE value to the engineered emulsion while the Base One used by Hubbard County’s SFDR was assigned a GE of 1.25.
Investigators also reviewed the application rates, total quantities and costs per mile of the stabilization processes and total construction costs. Three sets of strength and stiffness tests conducted before placing the final overlays indicated the initial performance of the two products. Dynamic cone penetrometer (DCP) tests were performed before injection of the stabilizing agent and then after compaction of the SFDR layer. Core sampling of the SFDR layer on both segments was followed by falling weight deflectometer (FWD) tests.
Additionally a visual survey, coring and FWD testing were performed on the road segments after one year of service, including a winter season.
What Did We Learn?
In general, both stabilizing products resulted in pavements that exceeded their 10-ton design value. The DCP measure before compaction was comparable between the two segments. After compaction, the Base One segment’s value unexpectedly decreased, which may have been due to lingering pavement moisture.
The core test on the engineered emulsion indicated the dry indirect tensile strength was very close to the design values. The Base One SFDR layer did not produce a solid core because it is an unbound base, which has implications for thermal cracking protection. The FWD analysis revealed both roadway segments exceeded design capacity.
“This comparison reinforced our practice of choosing a stabilizer based on the design values, though now our decisions will be better informed,” said Bruce Hasbargen, public works director, Beltrami County.
After one year of service, both segments were in visibly good condition despite some low-severity transverse cracking. The segment treated with Base One had over five times as many cracks as the segment with engineered emulsion, which was attributed to the bound layer providing higher strength at low temperatures.
As during construction, cores could not be obtained from the Hubbard County Base One SFDR layer because it was not bound. The Beltrami County core segments indicated an increased strength, potentially due to additional curing. The FWD tests revealed a 6% increase in Beltrami County’s segment and a 2% decrease in the Hubbard County segment, however it was still above the 10-ton design value.
Comparing the product costs, the engineered emulsion at $88,391 per mile was eight times the cost of Base One at $10,625 per mile. Total construction costs for the Beltrami County segment, which required an extra mix design step and different equipment, was about 39% higher than the Hubbard County segment.
Road conditions over the next several years will provide additional data for Beltrami and Hubbard county engineers to consider when choosing stabilizers for the SFDR process. In the meantime, they will continue to evaluate all available products and choose those that best suit the specific needs of the roadway and are attainable given available resources.