3/27/2016. Scope. NCHRP 9 53 Properties of Foamed Asphalt for Warm Mix Asphalt Applications

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1 NCHRP 9 53 Properties of Foamed Asphalt for Warm Mix Asphalt Applications Texas Transportation Institute Texas A&M University Center for Transportation Research University of Texas Foaming Will water affect strength and durability? How long will the effect of the foam last? How do different asphalts foam? How much temperature reduction can be achieved? Do foaming techniques produce the same quality and quantity of foam? What is the effect of anti foaming agents? How will polymer modified binders behave? How will other additives interact with foaming? Will mix design need to be modified? Scope Characterize foamed and the effects of foam on binders and mixtures Make testing as practical as possible by: Minimizing complexity Making tests applicable to lab and field Simplifying equipment and making it as rugged as possible while detecting sensible differences in measurements Minimizing cost of equipment and testing 1

2 Objectives Determine what properties of foamed asphalt binders relate to asphalt mixture performance and Develop laboratory foaming and mixing protocols that may be used to design asphalt mixtures Formation of foam Binder Foaming 1. Gas (steam) diffuses and mixes with the fluid under pressure in a mixing chamber 2. Gas bubbles nucleate and expands until pressure of gas is balanced by external pressure (surface tension + atmosphere + overburden) 3. Bubbles move upwards in the fluid (or fluid drains from the walls of the bubbles) and collapses Types of foam 1. Polyhedral foam volume of gas >> volume of the fluid fluid very thin films separating the gas 2. Spheroidal foam volume of gas < volume of fluid fluid a relatively thick film separating the gas Binder Foaming Polyhedral with thin film Spheroidal with thick film 2

3 Asphalt Foaming Most Popular Method for Producing WMA Improves Particle Coating by: Increasing Volume Reducing Mass Viscosity Methods Zeolite Wet Sand Plant Foaming Systems Binder Foaming Typical foam characteristics Foam rapidly collapses during the first few seconds Stable bubbles, with gradually reducing size, rise to the top and collapse Factors Affecting Asphalt Foaming Source of Asphalt Grade of Asphalt Presence of Anti Foaming Chemicals Water Content Presence of Polymers Presence of Anti Strip Additives Other Factors Water Pressure? Atmospheric Conditions? Water Chemistry? 3

4 Field Mechanical Mixing Venturi Mixing Expansion Chamber Shear/Colloid Mixer Air Atomized Water Atomized Water Lab Expansion Chamber Air Atomized Water Foaming Methods Field Foaming A Few Available Devices Field Foaming HP 4

5 Laboratory Foamers Wirtgen Instrotek PTI Foaming Measurements Expansion Ratio ER = Vol max /Vol final Half Life HL = t vmax t vhalf Rate of Decay Bubble Size Distribution Uses of Tests Evaluate foamability of binders Optimize moisture content Lab Field Mix design for performance testing Relate to coatability Relate to workability 5

6 Expansion Ratio and Decay After 30 sec. 3% Moisture 1% Moisture 6

7 DEVELOP LABORATORY MIX METHODS Coatability Coarse aggregate > 3/8 in Mix at 275 F, 60s, STOA 2h at 240 F Submerge mix and aggregates in water for 60 min Measure water absorption Calculate coatability Index (CI) = relative diff i i/ Coatability 21 7

8 Effect of Polymers and Asphalt Source Workability Set foamer to desired water content Add foam to aggregate at design asphalt content Mix for 90 sec Condition for 2 hrs at 275 (HMA) or 240 (WMA) Compact in SGC capable of monitoring shear stress Compact to maximum shear stress and record Workability 8

9 Workability Effect of Polymers and Binder Source INITIAL FIELD TRIAL Plant in Austin, Texas with a Maxam foaming unit Validate the applicability of the laboratory binder foaming setup in a field environment Assess the difference in container size (1 gallon vs. 5 gallon) 9

10 EFFECT OF CONTAINER SIZE 5 gallon laboratory vs. field had comparable values Higher ER for 1 gallon vs. 5 gallon in the laboratory by about a factor of 2 28 EFFECT OF EXTENSION PIPE Extension pipe likely cooling the binder and clogging the line ER max with the extension pipe ~ 65% of the ER max without the extension pipe 29 COMPARISON OF PLANT FOAMING UNITS Two plants in Ohio Gencor Foamer Terex Foamer Sampling Issues Low overhead clearance Valve heating Limited binder flow Distance between the sampling port and equipment setup 30 10

11 COMPARISON OF FOAMING UNITS ER max 2.2% > ER max Gencor 1.5% Binder not sensitive to increasing water content 31 DEVELOP FOAM MIX DESIGN PROTOCOL Materials Selection Laser/Camera Measurement on Laboratory Foaming at 160 C & 1% water Meets foaming criteria? Yes No SUPERPAVE MIX DESIGN Aashto r35 Modify asphalt binder or add/modify foaming additive W opt% Lowest τ max CI > CI HMA Workability Evaluation on Foamed Mixture at 1, 2, and 3% water Coatability Evaluation on Foamed Mixture at 1, 2, and 3% water OPTIMUM WATER CONTENT (W opt%) DETERMINATION Optimum Binder Content Indirect Tensile (IDT) Strength per AASHTO T 283 Pass AASHTO/DOT Specification Performance Evaluation Specimen Fabrication for HMA and Foamed WMA at W opt% Resilient Modulus (M R) per ASTM D 7369 or Dynamic Modulus (E*) per AASHTO 342/TP79 Equivalent or better performance than HMA Hamburg Wheel Tracking Test (HWTT) per AASHTO T 324 or Flow Number per TP79 Fail AASHTO/DOT Specification MIX DESIGN PROTOCOL VALIDATION Asphalt plant in Huntsville, Texas Accushear foaming unit Validation Steps 1. Visit the plant to perform binder foaming tests and sample materials (water content during operation 5.5%) 2. Apply mix design protocol to obtain W opt % 3. Revisit the plant, adjust the water content to W opt %, perform binder foaming test, sample materials 33 11

12 MIX DESIGN PROTOCOL VALIDATION MIX DESIGN PROTOCOL VALIDATION W opt% Lowest τ max CI > CI HMA Workability Evaluation on Foamed Mixture at 1, 2, and 3% water Coatability Evaluation on Foamed Mixture at 1, 2, and 3% water OPTIMUM WATER CONTENT (W opt%) DETERMINATION Optimum Binder Content Min 1.5% water content 1.5% water content > CI HMA MIX DESIGN PROTOCOL VALIDATION Improved max at W opt = 1.5% vs. 5.5% & HMA 36 12

13 MIX DESIGN PROTOCOL VALIDATION HMA = Foam 37 Tensile Strength Ratio HMA = Foam 38 Hamburg Results 13

14 MIXTURE EVALUATION SUMMARY Developed and verified workability and coatability laboratory test methods Developed and validated mix design protocol with plant data (field trial) W opt usually between 1 and 2% Additional foaming water does not translate into better workability or coatability Findings Increasing moisture contents did not improve workability and coatability The best workability generally occurred between one and two percent moisture Workability and coatability were better for neat asphalt for same mixing and molding temperature Workability improves with higher mixing temperatures for foamed asphalt The mix design procedure for optimum moisture content was validated through a field trial 14