A ACLAND.BIZ MAPPING CRACK SUSCEPTIBILITY OF BITUMINOUS MATERIALS WITH BINDER DURABILITY Daru Widyatmoko & Ric Elliott (Scott Wilson Pavement Engineering Ltd, UK) Martin Heslop (Acland Investment Ltd, UK)

Bitumen Hardening Loss of volatiles & oxidative hardening Heat –production Heat – delivery/transportation Heat – paving operation Age – in service

When Bitumen Hardened…. Moderate age - hardening Increased stiffness Improved load spreading Improved rut resistance Excessive age - hardening Reduced flexibility Leading to embrittlement Reduced capacity to healing

Rolling Thin Film Oven Test (RTFOT) Simulation of short-term ageing Hardening occurs during the batching process (mixing of aggregate and bitumen at high temperatures) 35 g of bitumen Test temperature : 163C Air (O2) flow rate : 4000 ml/min Test duration : 75 minutes

High Pressure Ageing Test (HiPAT) Simulation of long-term ageing Modification of Pressure Ageing Vessel procedure Residue from RTFOT ageing 50 g of bitumen Test temperature : 85C Air pressure : 2.1 MPa Test duration : 65 hours

Viscoelasticity of Bitumen Bitumen is viscoelastic material Elastic response (fast recovery) at short loading time and/or low temperature -- brittle failure Viscous response (partial/slow recovery) at long loading time and/or high temperature -- ductile failure

Dynamic Shear Rheometer Controlled Stress Rheometer Parallel plate: 8 or 25 mm diameter size

DSR Oscillatory Loading time Position of Oscillating Plate Oscillating Plate Applied Stress or Strain A C B 1 cycle Bitumen C B A Fixed Plate

DSR Oscillatory Loading t max Peak Stress g max Peak Strain d time G * = Peak Strain Peak Stress Complex Modulus Phase Angle d

Stiffer 10/20 Pen Softer

More viscous More elastic Straight run Semi blown U : Unaged bitumen

Viscoelastic response of bitumen Significant reduction in phase angle (more elastic, less viscous), in particular at low temperature, may lead to increased embrittlement (PIARC 1998) May contribute to a lower capacity for healing

Control (after RTFOT) Cracked site Non-cracked site

Straight run Semi blown Note: The suffix U, R, and H denote binders at unaged, after RTFOT aged and after HiPAT aged conditions. Straight run Semi blown

Non-cracked Cracked More elastic More viscous

VET Temperatures The Viscous-Elastic Transition (VET) temperature is the temperature at a phase angle value of 45 degrees The elastic component of the complex shear (stiffness) modulus of a bituminous material equates to the viscous component. Higher VET was well correlated with the observed surface cracking on several sites in France after 7 years in service (Migliori et al 1999)

Viscous-Elastic Transition (VET) Temperature Non-cracked site Cracked site 45 VETB8 VETD6

VETD6 > VETB8 Viscous-Elastic Transition (VET) Temperature Non-cracked site Cracked site 45 VETD6 > VETB8 VETB8 VETD6

G* at VET Temperatures Independent of the viscous or elastic response of a bitumen Progressive decrease in G*VET with increased age-hardening

Straight run Semi blown Note: The description of the temperature shift U→R→H show the shift from the left to the right axis for the three interconnected data markers for each binder type. The suffix U, R, and H denote binders at unaged, after RTFOT aged and after HiPAT aged conditions.

CASE STUDIES A unique relationship was found by “mapping” G*VET and the presence of cracks observed on some major trunk roads in the United Kingdom

Binder Recovered From SMA50 Note: The suffix U, R, and H denote binders at unaged, after RTFOT aged and after HiPAT aged conditions. Note: The description of the temperature shift U→R→H show the shift from the left to the right axis for the three interconnected data markers for each binder type. The suffix U, R, and H denote binders at unaged, after RTFOT aged and after HiPAT aged conditions.

Binder Recovered from HMB15 Straight run Note: The description of the temperature shift U→R→H only applies for binders B15, C15 and D15 data; showing the shift from the left to the right axis for the three interconnected data markers for each binder type. Semi blown Note: The description of the temperature shift U→R→H show the shift from the left to the right axis for the three interconnected data markers for each binder type. The suffix U, R, and H denote binders at unaged, after RTFOT aged and after HiPAT aged conditions.

The effect of age-hardening Progressively change the rheological properties of the penetration grade binders towards stiffer, more elastic, materials. Binders tested after a laboratory long-term ageing procedure, HiPAT, appeared to be indicative of in situ binder durability for UK conditions.

VET Temperatures As the level of distress increases (e.g. cracked areas), the VET temperature increases but the complex modulus (at the VET temperature) decreases, and vice versa. As the level of age hardening increases, the VET temperature increases but the complex modulus (at the VET temperature) decreases, and vice versa.

However…. Prediction of mixture performance based on binder behaviour is not straightforward,

Mixture design, with volumetrics and buildability in mind, are the key elements for satisfactory performance, and to minimise age-hardening, minimum binder contents may need to be controlled for example by adopting the “richness factor” concept and by understanding more about the issue of aggregate-binder interaction.

Nevertheless.. Selecting good quality, more durable, bitumen in the first place will help to ensure a sustainable and durable asphalt mixture can be produced Rheological testing is a very useful tool for preliminary assessment of bitumen quality

The Mapping Method Revealed a distinctive distribution and localisation of materials with different performance A useful preliminary screening tool for identifying the potential crack susceptibility of bituminous binders Also potentially good for “forensic” analysis

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