1. Guidelines for Airfield Rubblization
Mark Buncher, Ph.D., P.E.
2010 FAA Worldwide Airport Technology Transfer Conference
Apr 20-22, 2010

2. 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

3. 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

4. Rubblized Joint

5. PCC Rehab Method of Choice on Highways
Over 50M Sq Meters rubblized
Over 35 states
Slab thicknesses generally between 8-12in
Lots of good industry references, studies

6. Rubblization on Airfields
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

7. MHB with Guillotene Hammer at Selfridge ANGB
Guillotene typically only needed for PCC >14in
Spacing affects max particle size at slab bottom

8. Selfridge Test Pit
agency approval before full scale rubblization

9. RB-500 at WPAFB 2000 lbf blows @ 44 cycles/second
< 1 inch amplitude
9-12 inch wide passes

10. Test pits at WPAFB: Full depth slab destruction
Nom. max particle size: 12 inches

11. 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
Now referenced in FAA AC 150/5320-6E, Airport Pavement Design and Evaluation

12. Characterizing Rubblized Material - Background
Always Assumed Rubblized Equivalent to Crushed Agg Base (CAB), P-209
Stiffness Modulus (Erub) = 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

13. 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: ksi
PCC 8-14” thick: ksi
PCC >14” thick: ksi
Larger PCC pieces and steel produce higher Erub values

14. Minimum HMA Overlay Thickness Recommendations
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

15. 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)

16. Pratt KA 6” PCC, virtually no subbase, subgrade CBR of 2-4
Spec required RPB
Edge drains installed but no water ever drained
- Rubblization started OK on edge, but problems as moved toward centerline

19. Kegelman Field, OK 5”-6.5” PCC, 0-4” sand subbase, clay subgrade
RPB required
Poor drainage and “couldn’t afford” edge drains
No punch-thrus but excessive rutting (>2”)
30% of project had full depth patches

20. Shows Start-up of MHB, normal drop-height (24”) and spacing
Tullahoma TN
- 7.25” PCC, No Subbase
- Clay Subrade (CBRs: 4-12)
Shows Start-up of MHB, normal drop-height (24”) and spacing

21. Shows MHB “Modified” Rubblization Process (low drop ht – 16”, large spacing – 10”) Acceptable Surface, But Didn’t Meet Criteria

22. Protocol and Criteria for Assessing Risk to Aid in Determining Project Feasibility
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). 20 18
Moderate
Risk
Low Risk 16 14 12
Thickness of PCC and Base 10 8
High Risk 6 4 2 1 2 3 4 5 6 7 8 9 10
Subgrade CBR

23. 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

24. Proof Rolling Very Important - especially when using MHB
MHB and rollers didn’t reveal this unstable area
- HMA haul trucks and paver did…bad news!

25. 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

26. Quality Assurance
Test Strips
Test Pits
Particle Size Criteria

27. 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

28. 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

29. 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

30. 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

31. Questions?