Presentation on theme: "SuperPave Considerations Roy D. McQueen, P.E. Roy D. McQueen & Associates, Ltd. www.rdmcqueen.com 703 709-2540 For presentation at 2012 FAA Hershey Conference."— Presentation transcript:
SuperPave Considerations Roy D. McQueen, P.E. Roy D. McQueen & Associates, Ltd For presentation at 2012 FAA Hershey Conference
Overview Review EB 59A Background on Issues Research Results AAPTP Gyratory FAA Gyratory ERDC SRA FAA HTPT Requirements to Complete Specification
References in EB 59A TAI Superpave Mix Design, Superpave Series No. 2 (SP ‑ 2) TAI Performance Graded Asphalt, Binder Specification and Testing, Superpave Series No. 1 (SP-1) Interim Item P ‑ 401 Plant Mix Bituminous Pavements (SUPERPAVE) EB-59A
Policy: Modification to Standards Gross aircraft weights <100,000 pounds: approval at Regional Office Gross aircraft weights > 100,000 pounds: approval by AAS-100
What’s are the Big Differences Between FAA’s SuperPave & Marshall Specs? The Compactor and sample size! Volumetrics measured the same Compaction (bulk sp.g.) measured the same Mix design & acceptance criteria are slightly different It’s still aggregate, sand, binder and air!
4” Diameter Mold 6” Diameter Mold
Binder Grade Selection and Grade Bumping Based on Gross Aircraft Weight* Determine binder requirements from the LTTP Bind software using 98 percent reliability with no traffic or speed adjustments. Increase the high temperature grade by the number of grade equivalents indicated (1 grade is equivalent to 6 degrees C) below. Use the low temperature grade as determined from LTTP Bind. (see NOTES) Aircraft Gross WeightHigh Temperature Adjustment to Binder Grade All Pavement Types weight < 12, < weight < 100,000 1 weight > 100,000 2 NOTE: PG grades above a –22 on the low end (e.g. 64–16) are not recommended. Limited experience has shown an increase in block cracking with -16 or -10 grade asphalts. *Same requirement for Marshall Mix
AAPTP Study Binder Selection The base high-temperature PG grade should be determined using LTPPBind 3.1, for a surface layer (depth of layer surface = 0 mm), using a reliability of 98 %. The EHEs for both taxiways and runways are calculated using: EHEs = 10.4 (design tire pressure in lb/in 2 / 120) 2 annual departures. The high-temperature PG grade is then determined using LTPPBind 3.1, using the calculated value for EHEs as the design traffic level.
AAPTP Study Binder Selection For runways: LTPPBind 3.1 (“fast” traffic condition). For taxiways without stacking, speed adjustment for “slow” traffic For taxiways with some stacking, grade bumping: the high-temperature PG grade should be increased by 6C; for taxiways with frequent stacking, the grade should be increased by 12C. The high-temperature PG grade may be reduced one level (6C) for lifts which are entirely 75 mm or more below the pavement surface.
PG+ Criteria Polymer Modified Asphalts Rule of “90” “Gray” area for sum ~90, e.g., PG Elastic Recovery (60% to 70%) typical for this region to ensure polymerization at proper % Criteria varies by state
Primary EB59A SuperPave Mix Design Criteria > 60,000 lbs. 85 Gyrations 4% VTM VMA: 13% - 14% VFA: 65% to 78% Dust to asphalt ratio Coarse & Fine FAA > 45 < 60,000 lbs. 60 Gyrations 4% VTM VMA: 13% - 14% VFA: 65% to 78% Dust to asphalt ratio Coarse & Fine FAA > 42 A coarse gradation is defined as a gradation passing below the restricted zone. The restricted zone is defined in the Asphalt Insitute’s Manual Superpave, SP-2.
Gradation Requirements Runways – same as current P-401 Taxiways Control Points Restricted Zone ?
Off Maximum Density Line – Higher VMA
EB 59A SuperPave Acceptance Criteria > 60,000 lbs. 2.5% < VTM < 85 gyrations Compaction L = 92.5% G mm < 60,000 lbs. 2.5% < VTM < 60 gyrations Compaction L = 92.5% G mm
There are problems with EB59A mix design and acceptance criteria that need to be resolved.
BACKGROUND ON ISSUES
FAA Standards for production and placement of hot mix asphalt (HMA) pavements have been in place for more than 50 years. So, why change? Because we have to. No one is supporting Marshall. Modifications to both Federal and State Highway standard requirements have led to the SuperPave Design process and the use of the Gyratory Compactor
Major Issues Associated With Adopting SuperPave Required number of gyrations for mix design Volumetrics – appropriate level of VMA and VTM Gradation Requirements Field Compaction Standard
Establishing Design Gyrations Need to establish N design for the gyratory compactor Performance equivalent to well performing Marshall mixes Validation testing on a variety of mixes
Stated Differently: Make sure the new stuff works as good as the old! Quality Issues Legal Defensibility
Average PCI at Civil Airports HMA Pavements Source: Report DOT/FAA/AR
Overview of FAA P-401 75 blow Marshall for heavy duty Design VTM: 2.8% - 4.2%, 3.5% typical VMA typically 1% higher than EB 59A TSR for moisture susceptibility (75% - 80% min) Compaction function of lab Marshall density PWL acceptance: Density: 90% above 96.3% 98% average VTM: 90% between 2% and 5% 3.5% average Limits based on actual construction data
Density Limit Derivation 10 PD 90 PWL L 98% L = 98% (1.3%) = 96.3% Zs
Air Voids Limits Derivation L=2%U=5% 3.5% D L = 2% + (1.28x0.65%) = 2.8%D U = 5% - (1.28x0.65%) = 4.2% Zs 2.8%4.2% 0.7%
Primary Differences Between P-401 Marshall and P-401 Superpave P-401 Marshall* Impact Compactor 90% > 96.3% Marshall Avg.~ 98% lab density 50 or 75 blows 2.8% - 4.2% design VTM 2% to 5% acceptance 1% higher VMA Volumetric + Strength test P-401 Superpave** Gyratory Compactor 90% > 92.5% MTD Avg.~ 94.5% MTD 60 or 85 Ndes 4% design VTM 2.5% to 5.5% acceptance 1% lower VTM Strictly volumetric * Limits are based on construction data ** Limits not based on construction data
Major Issue: N design AAPTP Study FAA Studies ERDC SRA Advanced Asphalt Technologies Soiltek Nomenclature: N equivalent = equivalent N corresponding to 75 blow Marshall N design = design N for development of standard
SUMMARY AAPTP STUDY
AAPTP Study Approach for Ndes: Compare In-place Density to Orig Ndes Compare with Marshall for Equivalent Performance AMPT Performance Tests Mixes: Included southwest, west Coast Mixes Not all well-performing – some poor Several military mixes Did not use P-401 volumetrics
N equiv Results 75-blow Comparisons Range: 32 to 59 Avg. = 49, STD = 10 50-blow Comparisons Range: 25 to 40 Avg. = 36, STD = 11 Volumetric criteria different from P-401: VMA 1% lower & VTM 1/2% higher. This may be reflected in low Nequiv to meet EB59A volumetrics at same %AC as P-401
N design Values Based Upon Research Tire Pressure, psi Ndesign Less than to More than Recommended N design Values for Designing Airfield Mixes Tire Pressure, psi Ndesign Less than to More than Indicates that EB 59A N-des may be problematic. No variability analysis. AAPTP N design
SUMMARY ERDC STUDY
ERDC Study N des from comparative Marshalls Mixes developed from P-401 Specification requirements, i.e., well performing mixes Not considered 75-blow Marshall, only P-401 volumetrics, i.e. VMA & 3.5% VTM
Variables Mineralogy: Limestone, Granite, Gravel Aggregate Size: ½, ¾, 1 inch Max Gradation: Coarse & Fine Sides of P-401 Band Sand: 10% Nat’l & 100% Crushed Binder: PG & PG Nequiv Range: 25 to 125
Analyses of Variability (1) Sand: N=75 (all crushed) vs. N=59 (10% natural) p<0.001, significantly different Aggregate Type: Gravel: N=50 Granite: N=84 Limestone: N=69 p<0.001, significantly different
Analysis of Variability (2) Aggregate Size: ½ inch: N=72 ¾ inch: N=66 1 inch: N=80 p=0.051, not significantly different Gradation: Fine: N=80 not significantly different Coarse: N=69 p=0.047, not significantly different Polymer vs. neat binders not significantly different
Conclusions Variability too cumbersome to warrant multiple compaction levels N design based on arithmetic average of 69 with a recommended value of 70 EB 59A N equiv criterion may be problematic Validation study scheduled for
SUMMARY FAA GYRATORY STUDY
Objectives Establish guidance for N-design Establish specifications for designing HMA using SGC that provides performance equivalent to Marshall mixes Verify on a range of well performing mixes More comprehensive than other studies
Critical Issues N-design consistent with 75 Marshall blows Effect of switch to SGC on performance SGC could also result in subtle changes in aggregate gradation to meet volumetrics Volumetric and compaction Issues: VTM & VMA limits % MTD vs. % laboratory
Program to Establish N design Phase 1: Determine N equiv equivalent to 75- blow Marshall air voids (G mb ) Suggest N design based on volumetrics Phase 2: Validate N design based on comparative performance tests at N design and N equiv
Mix Variables (1) All well-performing mixes Various mineralogy Gneiss Dolomite Granite Gravel Basalt Argillite Diabase
Mix Variables (2) Nominal Maximum Aggregate Size 12.5 mm 19.0 mm 25.0 mm Varying natural sand content (0%, 7.5%, 15%) Binders Neat asphalt Polymers: Elastomeric (SBS) and Plastomeric (Novophalt)
N-equivalent Results Average: 62 Minimum: 34 Maximum: 99 Standard deviation: 16 Like other studies – range is large
Phase 2: Performance Evaluation What is affect of any asphalt content and/or gradation changes needed to meet volumetric N design on rut and fatigue resistance and durability?
Phase 2 Experiment Design Test at N equiv and N des Rut resistance AMPT E and flow number APA rut depth Fatigue resistance Durability from ASTM D 4867 (modified Lottman)
AMPT Flow # APA Rut Depth Rut Resistance
Conclusions Superpave gyratory compaction level of 70 gyrations will provide similar volumetrics on average to 75 blows of a Marshall hammer. Converting an existing Marshall design to a gyratory design can be done by a slight adjustments in asphalt binder content and in some cases aggregate gradation. Mixes designed with Ndesign = 70 achieved slightly better rut resistance than 75-blow Marshall. Mixes designed using gyratory compaction with Ndesign = 70 and 75-blow Marshall compaction exhibited similar fatigue resistance.
FAA High Tire Pressure Study
Background Aircraft wheel loads and tire pressures are increasing: L ~ 65,000 lbs. p > 240 psi Reported pavement failures in hot climates overseas
Study Objectives Evaluate the rutting, durability and fatigue performance of asphalt mixes at the extreme boundary of operation with respect to tire pressure, wheel load, temperature and (low) speed.
HTPT Study Elements Full scale tests at NAPTF heated pavements Laboratory tests with different binders and different temperatures: Binder Sensitivity: DSR, MSCR. HMA Performance: Indirect Tensile (IDT) APA rut resistance AMPT flow number Fatigue Test Matrix: Limestone, Dolomite, Granite aggregates PG 64, , and TLA blend binders Combine with Gyratory results for updating EB59A and SuperPAVE Specification Development
Conclusions All Studies
Conclusions EB 59A N-equiv appears to be problematic EB 59A AAPTP ERDC FAA – 70 Volumetric and performance comparisons support N des = 70 Other Considerations Volumetrics Compaction Standard
Other Considerations - Volumetrics Effect of 1% lower VMA and ½% higher air voids with EB 59A compared to P- 401: 1.5% lower % AC by volume (~ 0.7% by wt.) Effect of potentially lower %AC on durability, e.g., film thickness, stripping
Why is VMA Important? V a & VMA Related and V a is a pay item! Low VMA mixes are sensitive to minor variations in asphalt content Low VMA mixes can become tender Low VMA mixes may not allow for sufficient film thickness to ensure durability
Other Considerations - Compaction Effect of using %MTD in lieu of % lab for compaction control: Example 1: 6% in-place air voids 4% laboratory air voids, V a 98% field compaction Example 2: 6% in-place air voids 2% laboratory air voids, V a 96% field compaction
What is the combined net effect of: Raising VTM? Lowering VMA? Inconsistent (lower?) compaction
Ultimate Objective of All Studies New P-401 SuperPAVE specification !