HMA Perspective

1
HMA Perspective

• Joe P. Mahoney
• 20th Annual Joint Technical Conference
• February 2008

2
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMApossibilities for future
  changes?
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

3
WSDOT SpecificationsThen and Now
4
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

5
Quote from Manual of Instructions, Construction,
California Division of Highways, 1938

• Mixing
• The temperature of the mixture when leaving the
  plant shall be within the limits as specified.
  Every effort should be made to maintain a uniform
  temperature, as loads of asphalt mixture of
  widely different temperatures will result in a
  rough pavement.The street assistant shall keep a
  record of the temperature of the mixture and
  notify the plant assistant of any variation.

Bottom line It was at least partially understood
70 years ago that uniform HMA mix temperatures
were a critical factor in quality paving.
6
WSDOT Density ControlExcerpts from 1935 WSDOT
Std Spec

• asphaltic concrete shall be compacted by
  rolling with approved gasoline rollers.
• One roller shall be provided for each 1,200
  square yards of surface to be rolled per eight
  hours.
• At least one roller shall be of the
  three-wheeled type, weighing not less than six
  tons.

Bottom line Production of about 900 ft per lane
per roller per 8 hours.
7
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

8
WSDOT Density ControlExcerpts from 1957 WSDOT
Std Spec

• The Contractor will be required to place two
  smooth-wheeled rollers and one self propelled
  pneumatic-tired roller on each project
• The initial, or breakdown rolling shall be
  accomplished withsmooth-wheeled rollers.
• The pneumatic-tired roller shall be used to
  knead and compact the pavement mixture following
  the initial rolling and preceding the final
  rolling.

9
A Few Quotes from the 1963 WSDOT Standard
Specifications

• Compaction
• The Contractor will be required to place two
  smooth-wheeled power rollers and one
  self-propelled pneumatic-tired roller on each
  project to roll and compact the pavement
  mixture.
• All rolling shall proceed as directed by the
  Engineer.

And that was all there was, in essence, for a
compaction specification.
10
Another Quote from the 1963 WSDOT Standard
Specifications

• Hauling
• The asphalt concrete mixture shall leave the
  mixing plant at a temperature between 260 and
  350?F, and when deposited on the road it shall
  have a temperature not less than 250?F.

The 250?F mat requirement originated in Los
Angeles in 1898 (65 years earlier).
11
A Few Quotes from the 1969 WSDOT Standard
Specifications

• Compaction
• Asphalt concrete mixture shall not be deposited
  on the road if the compaction cannot be completed
  before dark.
• Airflow readings for compaction control during
  paving construction shall be taken at the rate
  and locations determined by the
  Engineer.Additional compaction may be required
  on any section where the airflow index is less
  than the established minimum.
• Except on the wearing course.breakdown
  compaction shall be with the pneumatic tired
  roller. On the wearing course.a single coverage
  with the steel wheel roller shall precede
  pneumatic tired rolling.

12
WSDOT Density ControlExcerpts from 1969 WSDOT
Std Spec

• The airflow meter will be used to evaluate the
  degree of compaction and may be confirmed by
  density measurements at the option of the
  Engineer.
• Asphalt Concrete mixture shall not be deposited
  on the road if the compaction cannot be completed
  before dark.

Job requirements push specification changes.
13
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

14
1972 WSDOT Standard Specifications Binder Grades
and Mix Gradations

• Mix Gradations
• Class B
• Class D
• Class E
• Class F
• Class G

• Binders
• 40-50
• 60-70
• 85-100
• 120-150
• 200-300

15
1991 Standard Specifications

• Adapted statistical based HMA specification
• Early non-stat vs statistical trials done in
  1989.
• TMD minimum of 91 set as the basis for computing
  the Pay Factorsome confusion remains about this
  today. Why?
• QL (Xm LSL)/S
• where
• QL lower quality index
• Xm mean of test results
• LSL lower specification limit which for
  compaction 91
• S standard deviation of test results

16
Major HMA performance problems during the 25 year
period

• Rutting
• Stripping

17
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

18
A ten year spanmore or less

• 1996 first Superpave section placed by WSDOT
• 1997 2 of WSDOT HMA Superpave.
• 2002 50 of WSDOT HMA Superpave. Superpave 19
  and 12.5 mm mixes added to Classes A, B, D, E, F,
  and G.
• 2004 Term Superpave dropped from the Standard
  Specifications. Four mix types listed
• HMA Class 1 inch
• HMA Class 0.75 inch
• HMA Class 0.5 inch
• HMA Class 0.375 inch

19
Other changes/additions

• Maximum layer thicknesses have ranged from about
  (max thickness)/(NMAS) 3 to 4 (1963 SS) to 3 to
  5 for more recent Standard Specs. However, what
  should a minimum t/NMAS be? More on this shortly.
• Full adaptation of the PG binder system and use
  of volumetrics in pay factors began in 2002.
• Superpave research began nationally in 1987. From
  start of research to implementation 15 years.

20
Other changes/additions

• Temperature differentials
• 1995 relationship between temperature
  differentials and low HMA densities resolved.
• 2006 Standard Specifications Section added that
  specifies if 4 or more low cyclic density areas
  (lt89 of TMD) measured in a lot, the unit price
  reduced 15.
• From research to specification 11 years

21
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

22
WSDOT Binder Specifications
1935 1948 1957 1974 1996 2004
Pen Pen Pen AR AR and PBA PG
40-50 40-50 AR-2000 AR-2000W PG 58-22
50-60 51-60 AR-4000 AR-4000W PG 58-28
60-70 61-70 60-70 PBA-6 PG 58-34
70-80 PBA-6GR PG 64-22
80-90 86-100 85-100 PG 64-28
90-110 PG 64-34
110-130 121-150 120-150 PG 70-22
150-180 151-200 PG 70-28
180-210 201-300 200-300 PG 70-34
210-250 PG 76-22
PG 76-28
W designation added earlier than 1996.
23
WSDOT Aggregate Specifications
Test 1935 1948 1957 1972 2004
Wear (Deval) 5
Degradation Factor 30/Surf 20/Base 30/Surf 20/Base
Wear (LA Abrasion) 30 30 30 30
Sodium Sulphate 10
Fractured Faces () 100/1 75/2 75 75 75 B,D,G 50 E,F 90/2 ( 10 mil) 90/1 ( 10 mil)
Sand Equivalent 45 45 45
Liquid Limit 25
Plastic Index 1
24
WSDOT Classes of Asphalt Mixesw/ Maximum
Aggregate Size (inches)
Class 1935 1948 1957 1972 2002 2004
A 1.25 1.0 1.25 0.75
B 1.0 0.875 0.625 0.625 0.75
C 0.5 0.625 0.5
D 0.5 0.5 0.5 0.5
E 2.0 1.25 1.25 1.25
F 0.75 0.75 0.75
L 0.625
G No. 10 0.25 0.5 0.5
1.0 NMAS 1.5
0.75 NMAS 1.0
0.5 NMAS 0.75
0.375 NMAS 0.5
25
(No Transcript)
26
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

27
NCHRP Report 531 (NCAT Study)

• A number of State DOTs reported higher than
  desirable air voids in Superpave designed mixes.
• Mixes with 19 mm nominal maximum aggregate size
  were particularly a problem (many states switched
  to 12.5 mm NMAS mixes).
• The National Cooperative Research Program awarded
  a contract to the National Center for Asphalt
  Technology to address the issue (NCHRP Project
  9-27).

NCHRP 531 published in 2004.
28
NCHRP Report 531 (NCAT Study)

• Studies going back over 40 years1 have shown that
  for dense graded mixes, air voids of about 8 and
  above tend to be permeable.
• SMA mixes with air voids above 6 tend to be
  permeable.
• Dense graded mixes that are coarse graded
  (below the 0.45 power curve) are particularly
  susceptible to this problem.

Zube (1962), Bulletin 358, Highway Research Board.
29
NMAS vs Permeability
Source Cooley, et al (2002) and WSDOT Pavement
Guide
30
Permeability vs In-Place Air Voids
Source NCHRP Report 531
31
Measurement of HMA Air VoidsAASHTO T166 vs
Vacuum Seal (Corelok)
Fine-Grained
On Max Density Curve
Coarse-Grained
SMA
32
Superpave Mixes t/NMAS vs Air Voids
33
SMA Mixes t/NMAS vs Air Voids
34
Major Findings from NCHRP 531

• Rapid cooling of lift a primary reason for low
  density in thinner layers (lower t/NMAS)
• Higher HMA permeabilities largely due to
• Higher air voids following field compaction
• Lower VMA
• For improved compactability
• t/NMAS 3 for fine-grained mixes
• t/NMAS 4 for coarse-grained mixes
• t/NMAS 4 for SMAs
• NCAT recommends in-place air voids between 6 to
  7 or less.

For a 19 mm mix, the minimum lift thickness 76
mm or 3
35
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

36
Superpave Compaction Gyrations
Design ESALS (E80s) _at_ 20 years (millions) Typical Roadway
lt0.3 Light traffic, local roads, city streets with no significant bus or truck traffic.
0.3 to lt3.0 Collector streets and local access roads, majority of county roads.
3.0 to lt30.0 Heavy traffic city streets, most U.S. highways, some rural Interstates.
?30.0 Majority of urban and rural Interstates, truck scales, high climbing lanes.
37
Superpave Mix Design Requirements

• Required Density Based on the number of
  gyrations made in the gyratory compactor.
• Initial Ninitial is a measure of mix
  compactability. Mixes that compact too quickly
  tend to be tender. Target is about 11 air
  voids (depends on design ESAL level).
• Design Ndesign is the number of gyrations needed
  to produce a density in the mix equivalent to the
  expected HMA density in the field after the
  indicated number of ESALs (target is 4 air
  voids).
• Maximum Nmax is the number of gyrations required
  to produce a density in the lab that should never
  be exceeded in the field. Air voids must be at
  least 2.

38
Changes due to NCHRP Project 9-9
Design ESALS (E80s) _at_ 20 years (millions) Compaction Parameters Compaction Parameters Compaction Parameters
Design ESALS (E80s) _at_ 20 years (millions) Ninitial Ndesign Nmax
lt0.3 6 50 75
0.3 to lt3.0 7 75 115
3.0 to lt30.0 8 100 160
?30.0 9 125 205
Current table (ASTM D6925)
Design ESALS (E80s) _at_ 20 years (millions) PG Binder (76-XX) Compaction Parameters Compaction Parameters Compaction Parameters
Design ESALS (E80s) _at_ 20 years (millions) PG Binder (76-XX) Ninitial Ndesign Nmax
lt0.3 lt76 50
lt0.3
0.3 to lt3.0 lt76 65
0.3 to lt3.0 76 50
3.0 to lt30.0 lt76 80
3.0 to lt30.0 76 65
?30.0 lt76 100
?30.0 76 80
Future table?
Marginal Value
Marginal Value
Note Assumption is PG76-xx is a modified binder
(experiment used SBS).
39
Why the changes for lab compaction?

• Mixes too dry, i.e., not enough binder.
• Raveling a commonly reported problem.
• Permeability of mixes too high.

40
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

41
Performance Tests for HMA Mix Design

• NCHRP Report 465 (2002) Simple Performance Test
  for Superpave Mix Design.
• NCHRP Report 513 (2003) Simple Performance
  Tester for Superpave Mix Design First-Article
  Development and Evaluation.
• NCHRP Report 547 (2005)
• Simple Performance TestsSummary of
  Recommended Methods and Database.

42
Performance Tests for HMA Mix Design

• Some State DOTs will adopt the SPT. How many?
• Many State DOTs are adopting wheel testers such
  as
• Hamburg Wheel Tester
• Accelerated Pavement Analyzer
• Others
• The Hamburg Wheel Tester is the most popular at
  this time.

43
Topics

• Time periods for WSDOT HMA Standard Specs
• 1930 to 1950
• 1950 to 1970
• 1970 to 1995
• 1995 to 2006
• Summary
• Recent research on HMA
• Lift thickness
• Recent Superpave mix design changes
• Performance Tests
• NCAT Test Track II results

44
Summary of NCAT Test Track II Results

• NCAT report released during December 2006.
• Results from the 2nd round of test sections
  constructed and trafficked at the National Center
  for Asphalt Technology, Auburn University.

45
TopicsNCAT Test Track II Results

• NCAT Test Track Phase I
• NCAT Test Track Phase II
• Overview
• Mixture performance studies
• Structural performance

46
NCAT Test Track I
Photo sources NCAT
47
NCAT Test Track I46 test sections
48
NCAT Test Track ILoading configuration
Trailer single axles 9,100 kg (20,000 lb)
49
NCAT Test Track I

• Built in 2000.
• 2 years of truck loadings, 10 million E80s
  applied to 46 test sections.
• All sections experienced the same environmental
  effects.
• Results
• Superpave and SMA designed mixes performed well.
  Very little rutting with a variety of HMA mixes.
• Quality of constructed HMA excellent.

50
NCAT Test Track II

• 23 sections remained to receive another 10
  million E80s.
• 22 sections reconstructed.

51
NCAT Test Track Results

• Examine the following
• Modified versus non-modified binders
• Fine-graded versus coarse-graded HMA mixtures
• Air voids on rutting
• Prediction of rutting from performance tests.

52
NCAT Test Track II ResultsModified vs Unmodified
Binders
10 mm rut depth is max for WSDOT
Test Track I Modified binders 60 less
rutting. Test Track II Modified binders 55 less
rutting.
53
NCAT Test Track Results

• Examine the following
• Modified versus non-modified binders
• Fine-graded versus coarse-graded HMA mixtures
• Air voids on rutting
• Prediction of rutting from performance tests.

54
NCAT Test Track II ResultsFine vs Coarse Graded
Dense Mixes
Gradation Compaction (number of passes) Compaction (number of passes) Temp at start of compaction Density following compaction ( Gmm) Binder Content () Rutting following 10 million E80s
Gradation Vibratory Static Temp at start of compaction Density following compaction ( Gmm) Binder Content () Rutting following 10 million E80s
Coarse 5 1 115C (240F) 94.4 6.8 4.2 mm
Fine 2 1 112C (233F) 94.3 7.4 5.6 mm
Note Binders were same for each coarse and fine
paired sections although binders types varied
between paired sections. Some modified, some not.
55
NCAT Test Track II ResultsFine vs Coarse Graded
Dense Mixes--Conclusions

• Fine graded mixes easier to compact. NCAT
  experience agrees with State DOT reports.
• Binder contents, on average, about 0.6 higher
  for fine graded.
• Rutting differences were small between the coarse
  and fine gradations.

56
NCAT Test Track Results

• Examine the following
• Modified versus non-modified binders
• Fine-graded versus coarse-graded HMA mixtures
• Air voids on rutting
• Prediction of rutting from performance tests.

57
NCAT Test Track ResultsLab Air Voids vs Rutting
Test Track I
Rut depth (mm) after 10 million E80s
Lab air voids ()
Test Track II
Rut depth (mm) after 9 million E80s
Lab air voids ()
58
NCAT Test Track ResultsLab Air Voids vs
RuttingConclusions

• Lab air voids (Va) are not a good predictor of
  HMA rutting (within the range evaluated).
• Need performance test to estimate rutting.

• Study by Willoughby and Mahoney (2007) for WSDOT
  concluded that no significant difference exists
  between projects with pay factors based on
  non-volumetric or volumetric bases. Recent study
  for Caltrans examined mix characteristics that
  most affect HMA performance and those are
• Rutting
• Asphalt content
• Degree of compaction
• Aggregate gradation
• Fatigue cracking
• Degree of compaction
• Pavement thickness
• Asphalt content

59
Conclusions

• Performance needs (higher traffic, nighttime
  paving, noise reduction, etc) push specification
  changes.
• Specifications have evolved from how to
  language toward statistically or performance
  based. However, that does not mean specifications
  get simpler or shorter.
• Binder changes reflect the importance of that mix
  ingredient. Modified binders now far more
  commonand for good reason.
• Recent research will continue to push
  specification changes. Typical time from research
  to implementation 10 to 15 years.

60
The End
and thanks!