Enhancing Quality Control with Percent Within Limits in Nevada DOT

1. Percent Within Limits Nevada DOT Stephen Lani, P.E. Assistant Construction Engineer Darin Tedford, P.E. Chief Materials Engineer Steve Hale, P.E. Quality Assurance Engineer WASHTO April 2019

2. Outline • What is Percent Within Limits (PWL)? • The Need for PWL specifications • Development of PWL specifications • Implementation of PWL specifications • NDOT’s current PWL specifications • PWL projects to date • PWL Updates – Dispute Resolution

3. What is PWL? • PWL uses statistical analysis to determine the consistency of material produced by a contractor • PWL encourages contractors to produce consistent material thru incentives or disincentives • PWL is based upon established specification limits for a specific material property

4. What is PWL? • PWL: Percent Conforming • Percentage of the lot falling between • Lower Specification Limit - LSL • Upper Specification Limit - USL

5. What is PWL?

6. Why the Need for PWL Specifications? • Nationwide push by FHWA to utilize PWL specifications • NDOT desired a specification ensuring the contractor provide not only a specification mix but a consistent one as well • A consistent quality mix increases long term pavement durability

7. Why the Need for PWL Specifications?

8. Why the Need for PWL Specifications? • Since 2014, the following states and territories are now using PWL specifications: • Arkansas • Illinois • Nevada • Oklahoma • Puerto Rico • West Virginia • Wisconsin • GRAND TOTAL OF 35

9. Development of PWL Specifications • Research Project • University of Nevada Reno (2010) • Three Phase Project • Phase 1 – Review existing PWL specifications • Phase 2 – Develop specifications for NDOT • Phase 3 – Implement Specifications

10. Development of PWL Specifications • Created a PWL Committee • Committee was comprised of members from NDOT, UNR, FHWA, the contracting community, & the consulting community • Met throughout the course of the research project and during the drafting of the final specifications

11. Implementation of PWL Specifications • Year 1 • Implement PWL specifications on 3 contracts (1 per District) with 25,000 tons or greater of HMA (dense-grade) • Contracts 3621, 3636, & 3628 • PWLOverall = 70 to receive 100% pay

12. Implementation of PWL Specifications • Year 2 • Implementation on all contracts with 25,000 tons or greater of HMA (dense-grade) • PWLOverall = 80 to receive 100% pay

13. Implementation of PWL Specifications • Year 3 (Current and beyond) • Implementation on all contracts with 25,000 tons or greater of HMA (dense-grade) • PWLOverall = 90 to receive 100% pay

14. Current NDOT PWL Specifications • PWL is used as project control on HMA (dense-grade) for the following properties: • Bitumen ratio • Aggregate gradation • ½” or 3/8”, No. 4, No. 10, & No. 200* • In-place density • * Selection of sieves were based upon prior UNR research project titled “Impact of Construction Variability on Pavement Performance”

15. Current NDOT PWL Specifications • PWL for bitumen ratio, aggregate gradation, and in-place densities based on a sublot of 1,000 tons or end of day, whichever comes first. • The size of a lot for bitumen ratio and aggregate gradation based on 5,000 tons or five sublots, whichever comes first and the corresponding number of in-place density tests

16. Current NDOT PWL Specifications • Gradation Percentage within Limits (PWLGradation) is based upon four sieves • Different weight factors were assigned to each sieve • ½ inch for Type 2C 10% • 3/8 inch for Type 2 10% • No. 4 35% • No. 10 35% • No. 200 20%

17. Current NDOT PWL Specifications • Weigh factors are used when calculating the Overall Percentage within Limits (PWLOverall) • The following are the weigh factors for aggregate gradation, bitumen ratio, & in-place density: • Aggregate Gradation 25% • Bitumen Ratio 33% • In-Place Density 42%

18. Current NDOT PWL Specifications • PWLOverall is determined by the following: • PWLOverall = (0.25)PWLGradation+ (0.33)PWLBitumen Ratio + (0.42)PWLIn-Place Density • Cease production if the PWL for two consecutive lots is less than 70 for any one of the measured properties • Evaluate available information and determine likely cause or causes of problem and propose change for approval

19. Current NDOT PWL Specifications • The Pay Factor (PF) for each lot of plantmix is determined by the following: • PF = 55 + (0.5 X PWLOverall) • PF cannot exceed 105% • IF a lot has a PWL < 70 for any one of the measured properties, contractor is not eligible for a PF over 100% • Contractor is required to remove material with a PF < 90% at own expense • Material may be allowed to remain in place, with NDOT approval, at the corresponding pay factor

20. Current NDOT PWL Specifications • The Progress Pay Adjustment (PPA) for each lot is determined by the following: • PPA = x L x C • Where: PF = Pay Factor • L = Tonnage amount per lot • C = Bid price per ton of plantmix • PPA is paid differently from the standard pay for plantmix • Could be up to 5% or a negative number

21. Outcome of Past/Current Projects

22. PWL Updates • Dispute Resolution • Researched Other States • Partnered with Industry to Customize Nevada Test Method • Multiple Iterations • Contractors allowed to Decline procedure • 3 Levels/Rounds of Dispute Resolution

23. PWL Updates • Dispute Resolution • Nevada T-344 • Equipment Calibrated • Personnel Certified • Test Results • Statistically accurate and precise • Implementation February 2019

24. Check Testing

25. Result Verification

26. Questions