Section 5: Full Depth Reclamation/Recycling (FDR)
This rehabilitation procedure entails pulverizing the old pavement structure, blending in a stabilizing agent, compacting, adding additional material, and resurfacing. This procedure is meant to address deep structural problems ranging to depths as great as 12 in. in the existing structure. This is the practical limit of recycler cut-in and subsequent compaction of the recycled layer. Deeper problems must be addressed by temporary stockpiling, such as windrowing removed material in the right of way (ROW) or otherwise moving existing material off site. Efficiency is improved by using a specialized train to reprocess the existing structure. New base and surfacing is then applied to provide appropriate additional structure, ride quality, skid, weatherproofing, etc. It is imperative that representative samples be taken from the layers to be recycled, preferably in similar gradations as would be produced by the recycling process, in order to evaluate stabilizer type and content in the lab. This may require the use of a small milling machine or a larger continuous flight auger working in one or more representative locations. Additional aggregates may be necessary to improve the gradation if the resulting blend is poor and/or if the depth of desired stabilization is greater than the depth of existing materials above the subgrade. Alternatively, additional passes of the pulverizer can be considered to achieve the desired gradation. Cutting into the existing subgrade should be avoided as this may introduce excessive fines into the blend that may be detrimental to the moisture susceptibility of the recycled layer.
“ The WirtgenTM Cold Recycling Manual” offers a sequence of steps for developing a mix design suitable for the recycled layer and is summarized here:
- Anchor: #HJRNPUKS
- Select stabilizer based on cost, availability (sufficient quantity, quality), suitability with respect to type and quality of material to be recycled, and desired (design) properties of the recycled mix. Stabilizer suitability must address the plasticity and quantity of the fines in addition to the overall gradation. The environment also must be considered; preliminary research has shown that fly ash and emulsified asphalt do not work well in high moisture environments. These sensitivities to moisture may be mitigated when used in combination with other additives (lime-fly ash or cement with emulsified asphalt). Also, where freezing conditions are expected, chemical stabilized materials should be in place well before (one month) the onset of freezing weather. Anchor: #EUXOTBAY
- Prepare samples (test methods Tex-120, Tex-121, Tex-126, and Tex-127), using varying percentages of stabilizer; bring samples to optimum fluid content (water plus any additional liquid such as emulsified AC) for compaction. Anchor: #XFTAMRHX
- Compact sample using standardized compaction procedures. Anchor: #PASCYQWI
- Cure samples. Anchor: #GMRKXXQK
- Subject samples to unconfined compressive strength (UCS) and moisture susceptibility testing. Optimum stabilizer content for chemical stabilizers should target the sample UCS at 300 psi. Moisture susceptibility evaluation should limit dielectric to < 10 after a 10-day capillary rise and an 85% retained UCS.
Additional guidelines can be found in TTI report 4182-1, “Field Performance and Design Recommendations for Full Depth Recycling in Texas.”