V. Mandapaka, I. Basheer, K. Sahasi & P. Vacura CalTrans, Sacramento, CA B.W. Tsai, C. L. Monismith, J. Harvey & P. Ullidtz UCPRC, UC-Davis, CA

Transcription

1 Application of four-point bending beam fatigue test for the design and construction of a long-life asphalt concrete rehabilitation project in Northern California V. Mandapaka, I. Basheer, K. Sahasi & P. Vacura CalTrans, Sacramento, CA B.W. Tsai, C. L. Monismith, J. Harvey & P. Ullidtz UCPRC, UC-Davis, CA

2 Introduction Caltrans adopted Mechanistic Empirical design method (CalME) to design long-life flexible pavements. Rutting and Fatigue are the main modes of failure. (Fatigue being the scope of this paper) 4 Point Bending Beam Fatigue test (LLP AC1) based on AASHTO T321 has been adopted by Caltrans to determine fatigue properties of HMA materials.

3 Introduction contd. A pavement section on I-5 in Tehama County has been used to illustrate the use of fatigue data to develop fatigue performance specifications Project location: I-5 Tehama County, PM 37.5/41.5 Structural section and material details. OGFC 0.1 ft PG PM 0.3 ft PG (25% RAP) ft PG Rich Bottom 0.2 ft Old CTB 0.5 ft Aggregate Subbase+ Subgrade

4 Objective The objective of this paper is to present the methodology for utilizing the four-point bending (4PB) beam fatigue test (AASHTO 321) to: Obtain the stiffness master curves required for determining the fatigue damage model parameters necessary for CalME, and Determine the fatigue performance specifications for the three HMA materials proposed for use on the project.

5 4PB testing equipment Testing can be performed under different: Frequencies (To simulate traffic speed) Temperatures (to consider climatic effect) Strain levels 4 PB beam testing setup

6 HMA stiffness master curves HMA stiffness master curve is developed using 4PB beam fatigue frequency sweep test. Several tests were performed at 11 frequencies: 15, 10, 5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01 Hz Three temperatures: 10C, 20C and 30C Two strain levels : 200 and 400 micro strain Note: The test was performed for each HMA material that was used on this project.

7 HMA master curve equation log ( ) E i = δ exp α ( β + γ1log( tr) ) 1 2 E i = the intact modulus; α 2, β 1, γ 1, δ 1 αt=model parameters,; tr = lt visc ref visc at tr = reduced time; viscref = reference viscosity; log10 log10 ( visc) = A + VTS log10 T A and VTS =constants, T is temperature.

8 Fatigue Model In CalME, it is assumed that fatigue damage causes the HMA modulus to decrease. HMA fatigue curve is developed using 4PB beam fatigue test. The data obtained from the Frequency Sweep test was used to determine fatigue model parameters. 1 temperature (20C); 2 strains (200 and 400 microstrain); 4 frequencies data was considered for the analysis.

9 Fatigue model equations log ( E) = δ + 1+ α exp ( 1 ω) ( β + γ log( tr) ) ω = damage ω MN = MN p α T α = exp α 0 + α1 o 1 C µε MN p = A µε ref β E E ref γ Ei E MN= No. load repetitions; MN p = Permissible # load repetitions; T= HMA average temperature; A, α 0, α 1, β, γ, δ = model parameters (β= 2*γ ), µε= tensile strain at the bottom of HMA; E= current damaged modulus; E i = intact modulus; µε ref and E ref = reference strain and modulus. ref δ

10 Comparison between measured E/Ei and calculated E/Ei (PG64-28PM)

11 Comparison between measured E/Ei and calculated E/Ei (PG64-10RAP)

12 Comparison between measured E/Ei and calculated E/Ei (PG64-10RB)

13 Fatigue damage model parameters Materi al type A α 0 α 1 ε ref β E ref γ δ RMS PG64-28PM PG64-10RAP PG64-10RB Nonlinear model optimization was used to calculate model parameters. All parameters were uploaded into the CalME software with other material parameters. Pavement structure was analyzed using the Incremental-Recursive (I-R) method.

14 Performance Specifications Performance Specifications were developed to: provide the contractor a quantitative measure of the quality of materials that can be used in each HMA layer. have a quality assurance of HMA materials Confidence Band Concept was used to statistically determine the lower bound of fatigue life at specified strain levels.

15 F 1 α Confidence Band Concept Ln ( Nf ) = a + bln( strain) Lower bound: yˆ0 2 F 1 α S Yˆ 0 Upper bound: yˆ0 + 2 F 1 α S Yˆ 0 F 1-α =(1-α)-percentile of F-distribution with 2 & n-2 degrees of freedom S 2 Yˆ = S 2 Y x 1 + n 2 ( x ) 0 x 2 ( x ) 0 x Variance of Ln(Nf) i S 2 Y x ( y yˆ ) = n 2 i i square of residual standard error of the regression equation 2 x 0 = calculated Ln(strain) ŷ 0 = calculated LnN f

16 Lower bounds for 95% confidence band for mixes at 20 0 C Strain Ln(Strain) PG64-10RB w/lime (LB) Ln(Nf) PG64-10RAP w/lime (LB) Ln(Nf) PG64-28PM w/lime Lower Bound(LB) Ln(Nf)

17 95% confidence band for PG64-28PM HMA (with 1.2% lime added, AC = 5.2%, AV = 6.0%) tested at 20 0 C % Confidence Band: PG64-28PM w/lime microstrain (mean = 4.64E+10) 400 microstrain (mean = 1.55E+08) Upper Bound Ln(Nf) (Nf = 72,826,467) (Nf = 404,570) Lower Bound Ln(strain)

18 95% confidence band for PG64-10RAP HMA (with 1.2% lime added, AC= 5.38%, AV= 6.0%) tested at 20 0 C Ln(Nf) % Confidence Band: PG64-10RAP w/lime 200 microstrain (mean = 3,169,823) 400 microstrain (mean = 103,672) Upper Bound (Nf = 935,232) (Nf = 24,933) Lower Bound Ln(strain)

19 95% confidence band for PG64-10RB HMA (with 1.2% lime added, AC = 5.5%, AV = 3%) tested at 20 0 C Ln(Nf) % Confidence Band: PG64-10RB w/lime 200 microstrain (mean = 29,246,517) 400 microstrain (mean = 629,332) Upper Bound (Nf = 2,841,407) (Nf = 65,133) Lower Bound Ln(strain)

20 Lower bound fatigue life at 400 and 200 microstrain levels HMA type Fatigue life at 400 microstrain Fatigue life at 200 microstrain PG PM 15% RAP 404,570 72,826,467 PG % RAP 24, ,232 PG Rich Bottom 65,133 2,841,407

21 Conclusions The 4PB tests were used to determine the fatigue model parameters and master curves as they are necessary inputs to the California M-E design software, CalME. The 4PB fatigue test has enabled the integration of construction quality requirements (performance specifications) with the ME design of flexible pavement The minimum fatigue life at a given strain level for each material for a 95% confidence level was specified as the performance criteria for each material.

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