Pavement Overlay Design Michael Gawn Principal (Newcastle) Why Overlay? Cheaper than reconstruction Suitable for shorter life span (asset prioritisation) Faster than reconstruction/stabilisation
Types Of Overlay Several types: Granular Ranging from 50 mm to 200 mm or so, pavement material (can be stabilised) Asphalt Ranging from 40 mm to 100 mm or so, asphalt (can be several layers). Can be thin or deep lift Concrete Mainly over existing rigid pavements (stitched in)
Types Of Overlay Limits: Granular overlays up to 108 ESA Asphalt overlays up to 107 ESA Basis for Overlay Design Based on pavement deflections (and curvature) Need to ensure sufficient existing structural thickness Improving road structure to allow it to satisfy
increased traffic load (i.e. life) Remember drainage is an important consideration Data Collection Deflection testing of road Benkelman beam Falling weight deflectometer (40 kN) Deflectometer
Data Collection Curvature = D200 D0 Beaming Take readings regularly (say 10 m to 20 m in both
wheel paths) Alternating wheel paths is usually undertaken Record rebound deflections at: 200 mm 600 mm 900 mm 2700 mm 9000 mm Very important to take pavement temperature Data Collection
Pavement Condition Inspect pavement for defects:
Rutting (can be indicator of subgrade failure) Shoving (subgrade failure or poor gravel) Cracking (cemented material?) Edge break (thinning on edge?) Austroads Guide to Visual Assessment of Pavement Condition Draw maps Extent of damage Type of damage Drainage conditions!!! In-ground service trenches
Overlay Design - Types Heavy Patching In-situ Stabilisation (incorporating existing)
Improve conditions where existing gravels have high plasticity Allows thinner increase in pavement thickness Re-uses some existing material
Granular Overlay Can be used to correct short sections where full reconstruction is not economical Where finish level constrained (i.e. kerb and gutter) Can be cheaper than AC
Use where levels not constrained Asphalt Overlay Thinner than granular Greater structural improvement Overlay Design Procedure Segment Pavement into areas of similar
One outlier result service trench? deflection performance Based on coefficient of variation (CV) Design Deflection what deflection needs to CV = Standard Deviation / Mean (of deflection be to cope with projected traffic
results) Segments should ideally by greater than 100 m Remove result (heavy patch) and remaining road meets deflection criteria Overlay Design Procedure Characteristic Deflection
Indicator of overall performance of pavement segment: = mean of deflection readings f = factor for road function (how busy the road is) Where <30 readings, max deflection can be better Overlay Design Procedure
Characteristic Curvature Mean of curvature results in segment Curvature = D200 D0 Overlay Design Procedure Determine design deflection from design traffic Example: For 106 ESA design deflection is 1.18 mm
Overlay Design Procedure Adjust deflection data for Method of collection Seasonal variation Pavement temperature
Deflection Standardisation Factor - Method Deflection Standardisation Factor - Method Deflection Standardisation Factor - Temperature WMAPT = mean weighted average pavement temperature See Austroads Tmeas = temperature at time of testing As AC gets thinner correction gets smaller
NOTE: Different graphs for Benkelman beam v FWD No correction needed for <25 mm AC Also correction curve for curvature Deflection Standardisation Factor - Season Correction for time / season of testing Applied to deflection and curvature Note for silt subgrades could be much greater Design Procedure Example
We have determined parameters Characteristic deflection = 1.2 mm Done in November with Benkelman Beam AC thickness 40 mm Pavement Temperature = 36 degrees Going to mill off 25 mm of AC Adjust for temperature WMAPT/Tmeas = 1.3
Design Procedure Example 1.05 Adjust for temperature WMAPT/Tmeas = 1.3 and therefore adjustment factor of 1.05 Seasonal factor (temperate climate in Nov) therefore 1.0 Asphalt thickness
Design Procedure Example Seasonal factor (temperate climate in Nov) therefore 1.0 Most cases Design Procedure Example We have determined parameters
Characteristic deflection = 1.2 mm Done in November with Benkelman Beam AC thickness 40 mm Pavement Temperature = 35 degrees Possibly going to mill off 25 mm of AC Kerb and Guttering Corrected CD = 1.58 mm
Corrected CC = 0.33 mm For milling depths of up to 50 mm, increase CD and CC by 15% to 25% for each 25 mm of asphalt milled Design Procedure Asphalt Overlay Determine thickness required to avoid asphalt fatigue Two thicknesses can work very thin <40 mm or above curved line Example: For 105 ESA design
deflection is 1.6 mm Either AC >88 mm (say 90 mm) or less than 50 mm (say 40 mm) Design Procedure Asphalt Overlay Can adjust traffic and hence overlay thickness for nonstandard asphalts (provided resultant overlay is less than 50 mm) Design Procedure Asphalt Overlay
Asphalt overlay thickness required from characteristic deflection Based on WMAPT of 25 and C320 binder Design Procedure Asphalt Overlay Where WMAPT > 25 or lower stiffness binder used adjust overlay Design Procedure Granular
Use following chart for granular overlay thickness required Need 140 mm Overlay Design Example For our example:
Asphalt overlay 40 mm ok for curvature and deflection (or 90 mm AC) If mill 20 mm existing: need 65 mm for deflection (but this would be an issue for asphalt fatigue and would need 95mm AC for curvature) Granular Overlay Need 140 mm (but existing kerb and gutter)
Pavement Overlay Design THANKS
