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

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Low RiskModerate

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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?