Bituminous PWT
(Percent Within Tolerance)
Steven L. Koser, P.E.
Pennsylvania Department of Transportation
Gary L. Hoffman, P.E.
PA Asphalt Pavement Association
www.dot.state.pa.us
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Spec Limits and Goal Posts are Similar
www.dot.state.pa.us
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Quality Measure:
Percent Within Tolerance (PWT)
• Estimates the percentage of material within
specification limits
– Assumes normal distribution
– Area equals 1.0 or 100%
1.0
or 100%
www.dot.state.pa.us
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Standard Deviation
• A measure of the variability (i.e. spread) of data
Spread
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Standard Deviation: Equation
• Sample standard deviation (s)
∑(Xi - X)2
s=
s = - 1∑(X - X)
i
2
n-1
www.dot.state.pa.us
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PWT
• Efficiently captures mean and standard deviation
in one quality measure
X - mean
s - standard deviation
-3s
-2s
-1s
X
1s
2s
3s
www.dot.state.pa.us
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Percent Within Tolerance
• Lookup estimated PWL from Q for a specified
number of samples (n)
LSL
QL → PWLL
PWLL
-3s
-2s
-1s
X
1s
2s
www.dot.state.pa.us
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3s
Percent Within Tolerance
• Lookup estimated PWL from Q for a specified
number of samples (n)
QU → PWLU
USL
PWLU
-3s
-2s
-1s
X
1s
2s
www.dot.state.pa.us
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3s
Percent Within Tolerance
LSL
PWLL
+
USL
- 100 =
PWLU
LSL
PWL
PWLU + PWLL - 100 = PWL
USL
X
www.dot.state.pa.us
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Payment Plan with Incentive
AQL=90
80 90 100
Estimated PWL
Pay
Disincentive
70
Pay
Incentive
95 100 105
Pay factors
www.dot.state.pa.us
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Number of Agencies
Number of States using PWT for
Hot Mix Asphalt
30
25
Source
FHWA QA
Assessment
20
15
10
5
0
2014
2010
2008
www.dot.state.pa.us
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What Does PWT Drive?
• Tighter adherence to
producing job mix
formula
• Tighter adherence to
field density spec.
requirements
www.dot.state.pa.us
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Advantages of PWT
• Well suited for low bids to
achieve quality
• Contractors = bonuses for
tighter adherence to targets
• Contractors = reduced
payments for loose adherence to
targets
• Moves focus to targets (NOT
minimums)
www.dot.state.pa.us
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What’s different with PWT spec?
• Adds bonus structure (maximum 4%)
• Adds mix gradation (PCS) as part of
payment
• Modifies current “goal posts” approach for
100% payment (good or no good)
• Results in many fewer 100% payments and
spreads these out (bonus and penalty)
www.dot.state.pa.us
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Payment Equation Changes
• Current specification (50% mix, 50%
density)
• 25% asphalt content
• 25% #200 sieve
• 50% field density
• PWT specification (50% mix, 50% density)
• 30% asphalt content
• 10% #200 sieve
• 10% primary control sieve (new)
• 50% field density
www.dot.state.pa.us
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Common to All PWT Specs
• Defective lots can be left in place at
70% pay by DE (previously 50% pay)
• Allows contractor to terminate lot
 Allows contractor to limit risk when
early QC results indicate an issue
 Must stop paving
 90% maximum pay
 Must R&R if defective by test results
www.dot.state.pa.us
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Current Status
• Three (3) methods thru CT process:
1. PWT-LTS (Laboratory Testing Section)
• Fully approved (includes FHWA)
• Acceptance at LTS
• PWT-LTS Use Guidelines/District Memo
2. PWT-LA (Local Acceptance)
• Producer/Contractor Results, Department
Verification, F-test and t-test (Not Practical)
3. PWT-HOLA (Hands On Local Acceptance)
• Department Acceptance, Producer/Contractor Lab
www.dot.state.pa.us
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PWT-LTS
2016 Construction Season
• Projects advertised after October 13,
2015 (considered)
• Districts decide if PWT-LTS is viable
• Gain experience with PWT
• PWT-LTS Incentive/Disincentive
Spreadsheet
www.dot.state.pa.us
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PWT-LTS
• Very similar to current process
• Allows contractor option to expedite
sample delivery to Harrisburg





Inspection staff secures samples
Secure samples given to contractor
Contractor delivers to LTS at his cost
LTS verifies security prior to testing
Priority given to expedited samples
 Test Turnaround Goal (Days)
www.dot.state.pa.us
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What is Local Acceptance?
• HMA/WMA Material
samples collected as
usual
• Acceptance testing
performed at:
• Proficient producers
lab
• or another mutually
acceptable lab
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• No LTS acceptance
testing (except for dispute
resolution situation)
www.dot.state.pa.us
Mixture Sample Collection
Contractor
Pulls Sample
Representative
Secures
Sample
Contractor
Transports
Sample
Representative
Tests Sample
Locally
Representative
Splits Sample
LTS Tests IA
Sample
www.dot.state.pa.us
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Gmm Verification Testing
• Federal Aid Projects & Projects on the NHS
• Local Testing
www.dot.state.pa.us
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Staffing LA Testing
• Districts allocate certified
plant inspectors to perform
testing
• Shortage in qualified staff
• Use Consultants?
• May need to coordinate
testing for out of District
sources
www.dot.state.pa.us
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Contractor’s Lab Assessment
• Local acceptance lab
needs AMRL on-site
proficiency assessment
• Every 2-years
• Assessment of
equipment used for
acceptance testing
www.dot.state.pa.us
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Independent Assurance
• Required by 23 CFR 637
• First Lot & randomly once every
10 Lots thereafter
• Performed at LTS on split
samples
• Ensures testing equipment
used is:
• properly calibrated
• operating correctly
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• Ensures sampling & testing is
performed correctly
www.dot.state.pa.us
Local Acceptance QC Plan
• Separate LA QC Plan
required to detail specifics
• 3 weeks before paving
• Must identify lab and
equipment for testing
www.dot.state.pa.us
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Local Acceptance Equipment Requirements
• Dedicated equip. for Dept. use
• Dept. Representative:
• Reviews equipment
• Disapproves equip. not
meeting requirements
• Provides Final Lab
Acceptance
• Dept. Rep. accessibility to local
lab
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Sample Transportation and Security
• Tamper proof coolers
with:
• Clasps
• unique, unopened
padlock
• Capable of relocking for
storage
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Sample Transportation and Security
• Alternative: Coolers
w/uniquely numbered
security tags
• Only 1 lot per cooler
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Sample Security
• Sealed containers:
• Inspected for security
• @ opening, samples & paperwork examined
• Evidence of compromised security  Dept.
may:
• Reject for testing
• Suspend paving operations
• and require additional samples to be taken
www.dot.state.pa.us
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Sample Paperwork
• Paperwork (TR-447) is the same
• Worksheets keep records of test results
• Paperwork to be retained for 3-years
www.dot.state.pa.us
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Lot Testing
• Testing begins when lot is
complete
• LA testing takes
“priority” over other
testing (i.e., QC)
• Contractor’s tech. may
witness  not required
• Contractor may saw cut
density cores for testing
www.dot.state.pa.us
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Department Concerns/Issues
Equipment:
• Enough equipment
• Inspectors familiar
with equipment
• Space for QC testing
AND acceptance
testing @ same time
www.dot.state.pa.us
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Department Concerns/Issues
Manpower:
• Need more trained & certified inspectors
• Challenge  dedicated LA testing staff, AND
meet all other Materials Unit duties (i.e.,
aggregate, concrete, SOS, records audits, etc.)
Training:
• Many inspectors are certified, but have little
experience doing testing
www.dot.state.pa.us
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Cons
PWT-LTS
PWT-HOLA
PWT-LA
• Oven
Correction
Factor
• Test
Turnaround
Time
•
•
•
•
•
•
•
•
•
Manpower
Equipment
IA
AMRL
Manpower
Equipment
IA
AMRL
Large Projects
Only
• F-test/t-test
www.dot.state.pa.us
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Pros
PWT-LTS
PWT-HOLA
PWT-LA
• Manpower
• Equipment
• All Projects
• Oven
Correction
Factors
• Test
Turnaround
• Gmm
Verification
• Staff
Knowledge
• AMRL
• All Projects
• Oven
Correction
Factors
• Test
Turnaround
• Gmm
Verification
• Staff
Knowledge
• AMRL
www.dot.state.pa.us
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Questions?
www.dot.state.pa.us
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