Guidelines for Airfield Rubblization Mark Buncher, Ph.D., P.E. 2010 FAA Worldwide Airport Technology Transfer Conference Apr 20-22, 2010
Jan 18, 2016
Guidelines for Airfield Rubblization
Mark Buncher, Ph.D., P.E.2010 FAA Worldwide Airport Technology Transfer Conference
Apr 20-22, 2010
Presentation Outline
• Background on Rubblization
• Key Findings from AAPTP Project 04-01– Thickness Design Considerations– Assessing Feasibility on Weak PCC Sections – Recommendations for “Marginal” Candidates– Recommendations for Quality Assurance
Note: Many more details in conference paper and Final Report
What is Rubblization?
• Fracturing techniques that:– Rubblizes PCC into high quality agg. base – Eliminates slab action and inherent distresses– Destroys bond between concrete and any steel
• Converts failed rigid system into new flexible one– Utilizing all existing pavement layers
• Very Green Design Alternative
Rubblized PCC @ Joint
• Over 50M Sq Meters rubblized 1994-2004– Over 35 states
• Slab thicknesses generally between 8-12in
• Lots of good industry references, studies
PCC Rehab Method of Choice on Highways
• 30 airfield projects in US through 2006– Shared between MHB and RPB– PCC thicknesses from 6 to 26 inches
• Early questions about very thick slabs on airfields. Resolved in 2002…– Resonant Pavement Breaker (RPB) at Wright-
Patterson AFB, OH• Up to 26 inches
– Multi-Head Breaker (MHB) at Selfridge ANGB, MI• Up to 21 inches
Rubblization on Airfields
MHB with Guillotene Hammer at Selfridge ANGB
• Guillotene typically only needed for PCC >14in• Spacing affects max particle size at slab bottom
Selfridge Test Pit agency approval before full scale rubblization
RB-500 at WPAFBRB-500 at WPAFB
• 2000 lbf blows @ 44 cycles/second
• < 1 inch amplitude
• 9-12 inch wide passes
Test pits at WPAFB:Test pits at WPAFB:– Full depth slab destructionFull depth slab destruction– Nom. max particle size: 12 inchesNom. max particle size: 12 inches
AAPTP Project 04 – 01
• Objective– Document state-of-the-art rubblization– Develop guidance regarding project
feasibility, structural design, construction, quality control, etc
• Final Report completed 2008 and posted on www.aaptp.us – Now referenced in FAA AC 150/5320-6E,
Airport Pavement Design and Evaluation
Characterizing Rubblized Material - Background
• Always Assumed Rubblized Equivalent to Crushed Agg Base (CAB), P-209 – Stiffness Modulus (Erub) = 50 - 60 ksi
• Literature Suggests This is Conservative
• O4-01 Approach– Reviewed Literature for Back-calculations of
Rubblized – Performed New Back-calculations on Several
Projects
– Examine Data to Predict Erub
Conclusions on Material Characterization
• Data range of in-service Erub: 100 to 430 ksi
– Avg of 205 ksi
• Erub closer to HMA base than CAB
– For Layer-Elastic designs• PCC 6-8” thick: 100-135ksi• PCC 8-14” thick: 135-235ksi• PCC >14” thick: 235-400ksi
• Larger PCC pieces and steel produce higher Erub values
Minimum HMA Overlay ThicknessRecommendations
• If HMA Placed On Rubblized Material– 5 inches Minimum HMA
• Minimum 2 lifts, but 3 preferred (for smoothness)• 1st lift: minimum thickness of 3 inches (for density)
• If Unbound Material Placed On Rubblized– Use Existing Criteria (3 - 4 inches Min. HMA)
• Structural Design May Require Greater HMA Thickness
Assessing Suitability of Project for Rubblization
• Not All Sections May Be Strong Candidates
• Possibly Marginal Candidate If Slabs Are Thin (<8”) With Poor Underlying Support– Thin to No Subbase– Weak Subgrade (often saturated)– Typical of WWII Built (Now GA) Airfields– 13 of the 30 Known Airfield Rubblization Projects
were <8” PCC.– Was an Issue on 3 of these 13
• Pratt KA, Kegelman OK, Tullahoma TN (all RW Projects)
Pratt KAPratt KA-6” PCC, virtually no subbase, subgrade CBR of 2-46” PCC, virtually no subbase, subgrade CBR of 2-4
-Spec required RPBSpec required RPB
-Edge drains installed but no water ever drainedEdge drains installed but no water ever drained
- Rubblization started OK on edge, but problems as moved - Rubblization started OK on edge, but problems as moved toward centerlinetoward centerline
Kegelman Field, OKKegelman Field, OK- 5”-6.5” PCC, 0-4” sand subbase, clay subgrade5”-6.5” PCC, 0-4” sand subbase, clay subgrade
- RPB required RPB required
- Poor drainage and “couldn’t afford” edge drainsPoor drainage and “couldn’t afford” edge drains
- No punch-thrus but excessive rutting (>2”)No punch-thrus but excessive rutting (>2”)
- 30% of project had full depth patches30% of project had full depth patches
Shows Start-up of MHB, normal drop-height (24”) and spacing
Tullahoma TNTullahoma TN - 7.25” PCC, No Subbase- 7.25” PCC, No Subbase - Clay Subrade (CBRs: 4-12)- Clay Subrade (CBRs: 4-12)
Shows MHB “Modified” Rubblization Process (low drop ht – 16”, large spacing – 10”) Acceptable Surface, But Didn’t Meet Criteria
Assessing risk of having inadequate structural support for effective rubblization (resulting in inconsistent breakage, large and shifting PCC particles, punch-thus or rutting from construction equipment).
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Subgrade CBR
Th
ickn
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High Risk
Low RiskModerate
Risk
Protocol and Criteria for Assessing Risk to Aid in Determining Project Feasibility
Recommendations on Avoiding Problems for Marginal Candidates
• Conduct assessment protocol before starting– Determine relative risk over entire project– Identify areas of high, medium and low risk
• Avoid wet season for rubblizing
• Install edge drain system before rubblization– Eases rubblization and improves long-term
performance– Exceptions:
• if one exists and is working• if subgrade is self-draining
MHB and rollers didn’t reveal this unstable areaMHB and rollers didn’t reveal this unstable area
- HMA haul trucks and paver did…bad news!- HMA haul trucks and paver did…bad news!
Proof Rolling Very Important - especially when using MHB
Other Designer Recommendations for Marginal Candidates Only
• Consider provision for “Modified” Rubblization– Waive particle criteria
• Consider other design options– Conventional Crack and Seat
• Separate bid item for full depth patching – Provides competitive price
• Keeping Perspective– Out of 30 airfield projects, 13 had PCC <8”– 10 of those 13 did not reveal instability issues
Quality Assurance
• Test Strips
• Test Pits
• Particle Size Criteria
Test Strips
• For contractor to demonstrate effective rubblization and rolling practices
• Provides area for test pit
• Minimum: 300 ft long by one slab width
• New test strip for each unique feature
Test Pits
• Excavate from test strip
• Include transverse and longitudinal joint
• Determine if spec criteria is met– Full depth fracture
• Particle size criteria (next slide)
– Steel “substantially” debonded• Dowels can be sawed
• At free edges, allow larger PCC pieces– Due to lack of support
Particle Size Acceptance Criteria
• Upper half of slab– All particles < 6”– 75% of material (by weight) < 3”
• Bottom half of slab or below steel– All particles < 2x slab thickness
Significant Non-finding from AAPTP 04-01
• No documented findings of any reflective cracking on any rubblization project – Hundreds of Highway Projects– Over 30 Airfield Projects– Dating back into the early 1990s
Questions?