HOW DO WE DEFINE PERMEABILITY? Inter-connected air voids which allow air, water, and moisture vapor to infiltrate a pavement structure from the surface and / or base.
WHY IS PERMEABILITY A PROBLEM? Stripping – Asphalt binder is separated from the aggregate by the continued presence of water and moisture vapor.
TEST PROCEDURE OVERVIEW OHD L-44 MEASUREMENT OF WATER PERMEABILITY OF COMPACTED PAVING MIXTURES
Mold specimen to 7% air voids, 75mm height. Saturate specimen, apply thin film of Vaseline to sides
Place specimen on permeameter and position sealing tube, which is covered by a rubber membrane
Place upper cap assembly on specimen and clamp down. Inflate membrane to 14 psi.
Ensure that water is flowing through the specimen at a constant rate and begin timing when water level reaches 65mm.
Measure either: 1) Time for water level to drop to zero 2) Water level after 30 minutes
The coefficient of permeability, k, is determined using the corrected water temperature. The initial specification was a maximum of 125 x 10 -5 cm/s
IF WE HAD A PERMEABILITY TEST AND SPECIFICATION, WHY WERE THE PAVEMENTS STILL PERMEABLE? Could we change the permeability test to better identify problem mixes? Could we change the Superpave specification in some way to make them inherently less permeable?
1 st Step – Check the Permeability Test Equation k = a L A t ln ( h1 h2 ) C where:k = coefficient of permeability, cm/s a = inside x-sectional area of cylinder, cm 2 L = avg. thickness of the test specimen, cm A = x-sectional area of test specimen, cm 2 t = elapsed time between h1 and h2, s h1 = initial head across test specimen, cm h2 = final head across test specimen, cm C = temp. correction for viscosity of water
1 st Step – Check the Permeability Test Equation The 65 cm tall tube of water could totally drain through the specimen in 5-1/2 minutes and still meet k = 125 x 10 -5 cm/s Dropping 45 cm in 30 minutes results in k = 12.5 x 10 -5 cm/s
A review of existing mix designs at that time demonstrated the relationship between nominal maximum aggregate size and permeability MIX NMS 1 3/4 1/2 3/8 MEETS 12.5 x 10 -5 67.6%77.9%80.9%100%
INITIAL IN-HOUSE RESEARCH HIGHLIGHTED THE FOLLOWING: Confirmed that permeability increases as mix coarsens Confirmed that permeability increases as the density at N ini gyrations decreases The method used to determine 93% density for permeability testing is critical (CoreLok vs. Air Wt, Imm. Wt, SSD Wt) The difference between the methods increases as the mix gets coarser
CONTINUED RESEARCH Based on the data from the initial research, a test matrix with 1 NMS mixes at evenly spaced combined gradations was proposed. The mixes ranged from very coarse to very fine. Each mix was molded in increments from 91% to 98% density
The difference between the methods of determining density increases with mix coarseness and lower density
Absorption also increases with coarser mixes and lower density
There is a clear relationship between permeability and absorption
The three coarsest mixes stood out very clearly. Attaining 94% roadway density is critical.
RESULTING CHANGES TO ODOTS PERMEABILITY TEST & SPECIFICATION Specify OHD L-14 as the method by which to obtain the density of the permeability specimens Lower the maximum permeability from 125 x 10 -5 cm/s to 12.5 x 10 -5 cm/s Add minimum AC contents for each Superpave mix
RESULTING CHANGES TO ODOTS SUPERPAVE SPECIFICATION Specify a minimum density of 85.5% @ N ini gyrations Fine up Superpave gradation to make the allowable broad band more similar to to our previous A, B, C, and D blends Remove 1-1/2 NMS mixes from the specification
New specifications were enacted in June 2004. All ODOT Field Divisions are reporting a marked decrease in permeability issues.
The new specifications alone cannot guarantee that the roadway will not be permeable. There is one more important factor: ROADWAY DENSITY
DATA FROM OUR FIRST FIELD INVESTIGATION OF A PERMEABLE PAVEMENT: (AT LONGITUDINAL JOINT) LIFT% DENSITY 6 LEFT6 RIGHT S482.686.1 S383.387.2 S2 (2 nd Lift)88.893.4 S2 Base90.690.2
DATA FROM OUR NEXT FIELD INVESTIGATION OF A PERMEABLE PAVEMENT: (AT LONGITUDINAL JOINT) LIFT% DENSITY 6 LEFT6 RIGHT S491.289.3 S3 (2 nd Lift)84.191.3 S3 Base83.391.1
ODOT intends to adopt a longitudinal joint specification similar to TxDOTs in the next few months. Based on their experience, we believe that permeability issues will be largely resolved.