BALANCED MIX DESIGN APPROACH. Robert Lee, P.E. Flexible Pavements Director

Transcription

1 BALANCED MIX DESIGN APPROACH Robert Lee, P.E. Flexible Pavements Director TxAPA Annual Meeting

2 Initial Thoughts Dry mixes are prevalent in our industry. Lowering N-design and raising lab molded densities will only help so much. Restrictive specifications and general notes are not long term solutions. Continuing to increase binder replacement without addressing mix performance is not sustainable. We need to increase our understanding of the factors which drive mix performance to help us better optimize our mixes

3 It s a Nationwide Problem Several tests sections on NCAT test track are dedicated to investigating long term pavement performance. Researchers across the country are looking at better ways to design longer lasting pavements. NAPA recently created the Pavement Performance Task Group in response to concerns over states issues with pavement performance. 3

4 Some Questions We Need to Ask Ourselves Is going to SP the answer? Is getting rid of RAP and RAS the answer? Is disallowing modifiers like REOB and PPA the answer? IS DOING THINGS THE WAY WE VE ALWAYS DONE THEM THE ANSWER? What about traffic and climate? Are universal volumetrics (e.g., VMA and air voids) controlling designs without regard to traffic and climate? 4

5 A Better Approach LET S STOP USING THE RECIPE TO DETERMINE IF THE CAKE IS GOOD We need to define our expectations and open up the recipe to meet the end result What defines a good cake? It Tastes Good What defines a good mix? Lasting Performance Optimized Mix Design Approach Foundational Points Use what works Eliminate what doesn t Be simple and practical Good doesn t have to be complicated and complicated isn t always good.

6 What We Need And What You Want Design For Performance, Not Economics CONTRACTORS WANT A LEVEL PLAYING FIELD

7 What is a Balanced Mix Design? Balanced Mix Design o Optimize the mix to provide needed performance and balance between stability and durability.

8 Optimized Mix Design Approach Framework Mix Performance Evaluation Stability Durability BALANCE

9 Optimized Mix Design Approach Asphalt Mixes Mix Performance Evaluation Stability Durability

10 Balanced Mix Design Approach - PG Number of Passes (12.5 mm Rut Depth) OT Number of Cycles HWT OT y = x R 2 = y = 2E-07x R 2 = Asphalt Content (%)

11 Optimized Mix Design Approach LRA Mix Performance Evaluation Stability Cantabro Durability

12 Cantabro Loss, % Balanced Mix Design Approach Limestone Rock Asphalt Type I D Fine Hveem Stability 12

13 Optimized Mix Design Approach Framework Material Selection Optimize JMF Mix Performance Evaluation Workability Aggregates Gradation Stability Mixing Binder, Modifiers & Additives Binder Content Durability Compaction Recycled Materials Volumetrics Adjustments Segregation

14 Optimized Mix Design Approach Framework Material Selection Aggregates SAC Frictional Properties Binder, Modifiers & Additives ΔTc MSCR COMPATIBILITY

15 DTc using BBR log (Stiffness or S) m-value (ability to relax) m = 0.3 S = 300MPa Temperature Temperature S based continuous grade Differen ce = DTc m-value based continuous grade Binders with DTc > 6 o C

16 Optimized Mix Design Approach Framework Optimize JMF Gradation Bailey Method Volumetrics VMA

17 Optimized Mix Design Approach - Evaluation Mix Performance Evaluation Stability Durability

18 Hamburg Analysis 18

19 Load, lbs Maximum Load, lbs OT Data Analysis Area used for Fracture Energy Maximum Load First Crack Retardation Rate Power equation representing load reduction curve y = ax b, a=1-400 G f = W A Displacement, in Number of Cycles The fracture energy and crack retardation rate are plotted together on a design interaction plot

20 TOUGHNESS Critical Fracture Energy, lbs-in./in.2 Overall View of All Mixtures Crack Progression Rate FLEXIBILITY CAM TOM-F TOM-C SMA-C SMA-D SMA-F SMAR-C SMAR-F SP-C SP-D SP-B Type D Type C Type B 20

21 Critical Fracture Energy, lbs-in./in.2 Example Lab Standard Mix SuperPave Type D PG Limestone Aggregate SAC B Stripping Inflection Point Crack Progression Rate 21

22 Final Thoughts Some form of a balanced mix design framework is probably in our future There is a lot of work at the national level already in progress towards utilizing this type of approach Skid IFI HWT VMA OT Constraints: Large ships turn slow A change in our thought process MSCR ΔTc 22

23 QUESTIONS?