Full Depth Reclamation (FDR) is an increasingly popular alternative for roadway rehabilitation. The process is simple; a roadway reclaimer or mixer is used to physically mix an existing roadway in place. The existing asphalt and a certain portion of the base is blended together, often with an additive to help stabilize the new composite material. This new composite base is then compacted, graded and topped with a new asphalt surface course. This process is not recommended in all instances but can be a good option in case where the base material is inconsistent or in remote areas where the cost of hauling in new material, or hauling out the millings, becomes prohibitive.
One of the key cost factors is the additive used to assist in the stabilization. The two common additives are asphalt emulsion or Portland cement. There are pros and cons to both. An asphaltic binder remains flexible and resistant to cracking, however it can be expensive. Portland cement is considerably cheaper and produces a very strong composite base but you often have issues with cracking, which will eventually reflect up through the asphalt. Many times a combination of both materials can be used but it requires extensive analysis and the end product is only as consistent as the existing material you started with.
So is it critical to include a stabilizing additive at all? Not necessarily. This has been done on private roadways for some time, the contractor typically referring to the process simply as “reclaiming”. However on public roadways it becomes more important to achieve a particular strength and reliability. On the Fort Denaud Road project in Hendry County, Johnson Engineering worked with the Hendry County Engineering and Road and Bridge Departments to develop a more formalized process to reclaim the road without the expensive additives. As part of the geotechnical analysis, a series of tests were conducted to estimate the structural capacity of the composite base section. During construction a test strip was performed utilizing a roadway reclaimer traveling at various, forward speeds. The mixed material was not stabilized. It was collected and tested for Limerock Bearing Ratio (LBR) and gradation to determine the optimum forward speed of the reclaimer. Special construction specifications were developed and the road was built, without any additives…other than the water needed to help with compaction. The end result was a new roadway, fully reclaimed in place with a homogenous composite base, a smooth ride and a significantly lower price tag then other stabilized options.
For more information contact Ryan Bell, P.E. at [email protected].