February 5, Fatigue of Asphalt Mixtures, Endurance Limit, Polymer Modifications, Healing 1.E+02 1.E+03 1.E+04 1.E+05 1.E-07 1.E-05 1.E-03 1.E-01 1.E+011.E+031.E+05 Reduced Frequency [ rad /s ] |G*| [ MPa ]  Phase  º ]. |G*| Lab|G*| GB Phase_LabPhase GB

February 5, Fatigue of asphalt mixtures 4 p bending 2 p bending 4 p bending 2 p bending direct tension indirecttension

February 5, Fatigue resistance is a specimen, not a material property p bending strain

February 5, Area in 2 and 4 p test subjected to fatigue 2 point bending test 4 point bending test Normalized height over sample Relative strain amplitude

February 5, indirect tension load contr. 4 p bending load contr. 4 p bending displ contr. N  Fatigue resistance is a specimen, not a material property

February 5, Interpretation of fatigue tests Input: half sine or haversine displacement signal Output: residual stress develops; after some time peak load is 50% of initial peak load and load signal becomes sinusoidal

February 5, Test set-up

February 5, T = 30 o C Measured load Modelled load Displacement

February 5, T = 0 o C Measured load Modelled load Displacement

February 5, Input: half sine/haversine load Output: creep displacement; Creep will most probably be failure mechanism Interpretation of fatigue tests

February 5, Conclusion Reported nr of load repetitions to failure is depending on type of test. Reported nr of load repetitions to failure is depending on type of test. Reported nr of load repetitions to failure is depending on whether test is load or displacement controlled Reported nr of load repetitions to failure is depending on whether test is load or displacement controlled Reported applied strains are depending on how test is performed (full sine vs half sine) Reported applied strains are depending on how test is performed (full sine vs half sine) Reported endurance limits are in fact specimen properties Reported endurance limits are in fact specimen properties

February 5, What is reality? LINTRACK Linear tracking device 35 – 100 kN wheel load 20 km/h Uni and bi directional Temperature controlled Built at end/beginning 1980’s/1990’s

February 5, Transversal strain vs nr of load repetitions; residual strains are not taken into account in common design procedures

February 5, Residual strains do not seem to develop in longitudinal direction

February 5, Substantial corrections needed to match lab result with practice log  log N Field fatigue Lab fatigue Shift factor (healing, lateral wander, damage propagation, stress redistribution etc )

February 5, From lab to in situ fatigue relations the big unknown, advanced theories required n can be estimated with confidence log N log 

February 5, Theory on crack growth in visco- elastic media is of help Crack growth law: dc/dN = AK n Crack growth law: dc/dN = AK n A = f(S mix, m,  t,  ) A = f(S mix, m,  t,  ) n = f(2/m and void content) n = f(2/m and void content) Fatigue law: N = k 1 (  ) -n Fatigue law: N = k 1 (  ) -n k 1 = f(A, m, specimen geometry) it is a specimen dependent parameter ! k 1 = f(A, m, specimen geometry) it is a specimen dependent parameter ! m is slope of master curve for stiffness! m is slope of master curve for stiffness! =

February 5, Geometry dependency of lab fatigue relationship N = h (1-n/2) F c  -n / A Smix n F c = c0/h  cf/h d(c/h) / [1.99(c/h) 0.5 – 2.47(c/h) (c/h) 2.5 – 23.17(c/h) (c/h) 4.5 ] n The thicker the beam the lower the fatigue life at same strain level

February 5, Size effect on 4p beam bending tests

February 5, Endurance limits Size 0.5 Size 1 Size  m/m 44  m/m 37  m/m

February 5, Effect of beam thickness on flexural strength

February 5, SBS

February 5, Simple tests to determine effect of SBS

February 5, Simple experiment to show strength of SBS modified asphalt mixtures

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February 5, Triaxial Test, Cohesion “C” and Angle of Internal Friction “  ” Failure Envelope

February 5, General Case Failure envelope can also be generated by means of tension and compression tests. Failure envelope can also be generated by means of tension and compression tests. In generalized case,  is replaced by bulk stress I 1 and  is replaced by deviator stress parameter J 2 In generalized case,  is replaced by bulk stress I 1 and  is replaced by deviator stress parameter J 2

February 5, Parameters used in the Failure Envelope Graphs I 1 =  1 +  2 +  3 I1I1I1I1  J2 J2 J2 J2

February 5, Importance of Failure Envelope I1I1I1I1  J2 J2 J2 J2 Whenmat A and When mat A and mat B have about same stiffness, then an almost similar point in I 1 – J 2 space will be obtained but mat B will perform much better Mixtures did not differ too much in terms of stiffness

February 5, Failure Envelopes at 5 o C and Strain Rate of 0.01 %/s

February 5, Failure Envelopes at 40 o C and Strain Rate 0.01 %/s

February 5, R initial Loading cycles N f,50 Fatigue Life in terms Stress Ratio R limit N

February 5, Fatigue Test Results at 20 o C and 8 Hz

February 5, Endurance Limits at 8 Hz and 20 o C Mixture S m,initial (GPa) ε limit (10 -6 m/m)

February 5, Analyzed Pavement Structures Variable thickness; Stiffness of mixtures 40, 41 and 42;  = 0.35 E = 300 MPa; h = 300 mm;  = 0.35 E = 100 MPa;  = 0.35 F = 50 kN; r = 150 mm

February 5, Required Asphalt Thickness 42/194 41/211 40/294

February 5, Conclusion Modifying asphalt mixtures with specially designed polymers can result in a significant reduction of the asphalt layer thickness or a significant increase in pavement life when thickness is kept the same Modifying asphalt mixtures with specially designed polymers can result in a significant reduction of the asphalt layer thickness or a significant increase in pavement life when thickness is kept the same

February 5, Healing of bituminous mastic (bitumen + fine aggregate < 63  m Healing of bituminous mastic (bitumen + fine aggregate < 63  m) O hr1 hr 3 hr18 hr Source: PhD Qiu

February 5, Healing of mastics

February 5, Fatigue and healing Mixture Rest period S retest / S 1st test N retest / N 1st test pen 40/60 void content 20 o C / 10 Hz end of test: S n /S i = months at 15 o C pen 40/60 void content 5 o C / 8 Hz end of test: S n / S N = months at 15 o C – 0.9 Source: PhD Pramesti and Li

February 5, Conclusions Healing is recovery of strength and should not be confused with recovery of stiffness Healing is recovery of strength and should not be confused with recovery of stiffness Most probably, stiffness recovery is due to thixotropy Most probably, stiffness recovery is due to thixotropy Healing of asphalt mixtures is mainly a flow driven process Healing of asphalt mixtures is mainly a flow driven process Long rest periods are beneficial but only at elevated temperatures Long rest periods are beneficial but only at elevated temperatures Temperature is more important than time Temperature is more important than time Healing (strength recovery) of asphalt mixtures is limited Healing (strength recovery) of asphalt mixtures is limited

February 5, Thank you for your attention