The report's abstract states, "The objective was to document the state of the art in rubblization technology for airfield pavements and develop guidelines covering project feasibility, thickness design/material characterization, quality assurance criteria and methods, and construction/equipment issues."
Key findings quoted directly from the report include:
- Performance studies by LA, CO, AL, IL, MI and other highway agencies all show rubblization to be an effective and efficient rehabilitation alternative.
- With an exhaustive literature review, there was no documented mention of reflective cracking ever occurring from underlying rubblized PCCP on any highway or airfield project.
- (I)t does appear Erub is somewhat related to slab thickness. This research team recommends the following design ranges on airfield projects:
 For slabs 6 to 8 inches thick: Erub from 100 to 135 ksi
For slabs 8 to 14 inches thick: Erub from 135 to 235 ksi
For slabs > 14 inches thick: Erub from 235 to 400 ksi
- When the CBR method of pavement design is used, characterizing the rubblized layer as a crushed stone base (CBR=100) is conservative. We recommend using the following CBR equivalency factors (EF) to convert from rubblized to an "equivalent" stone base:
Rubblized layer 6 to 8 inches thick: EF = 1.2
Rubblized layer 8 to 14 inches thick: EF = 1.4
Rubblized layer > 14 inches thick: EF = 1.6
- There was no noted change in subgrade moduli values before and after rubblization.
- The minimum HMA overlay thickness is 5 inches if placing directly on rubblized. This must be placed in a minimum of two lifts, with the first lift at least 3 inches thick in order to achieve density. Unbound material such as P-209, RAP, etc is often placed first on the rubblized as a leveling course for runways and taxiways to reestablish grade. If an unbound layer is placed directly over rubblized, the minimum unbound layer thickness is 4 inches, and the minimum HMA overlay thickness criteria for that particular material should apply.
- The recommended particle size acceptance criteria is:
Upper half of slab: No particles > 6 inches in any dimension, and at least 75% of material (by weight) < 3 inches in any dimension.
Bottom half of the slab or below the steel: Virtually no particles > than 2X the slab thickness, up to 24 inches, in any dimension.
- On the rare occasion of an oversized piece, leave the piece in-place versus trying to remove.
- In general, we recommend the installation of a longitudinal edge drain system for airfield rubblization projects and that it is operating at least two weeks prior to the start of rubblization. It is important that the edge drain be placed next to where the rubblized layer will be constructed and extends to the top of the rubblized layer.
- With regard to the concern that a PCCP suffering from Alkali-Silica Reaction (ASR) could potentially expand after rubblization to cause future differential swelling and subsequent roughness on the pavement surface, we found no mention of this concern in the literature or in speaking with rubblization experts. In addition, there has been a long history of highway and airfield projects utilizing fractured slab technology on ASR-infected PCCP and their long-term performance has been excellent.
This report will serve as an excellent resource for those evaluating, designing, and constructing airfield rubblization and HMA overlay projects. The success of these types of projects in the future will depend on continuing research to validate the recommendations in this report.
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