Fatigue and Deflection of Asphaltic Concrete 0. A. WHITE, Engineer of Materials and Research, Oregon State

Highway Department

ABRIDGMENT

•IN THE FALL of 1956 a comprehensive study was begun on the fatigue and deflection properties of asphaltic concrete for the purpose of evaluating pavement characteristics and design criteria. The project was arranged in three parts: (a) field tests to deter­mine deflections of existing pavements under load; (b) laboratory tests on beams cut from pavements tested for deflection under load to determine the deflection life re­maining and other physical characteristics; and (c) laboratory tests on molded speci­mens to determine deflection fatigue life and other characteristics as affected by aggre­gate gradation and asphalt content.

Phase I-Deflection Tests

Deflection tests were made using a Benkelman beam and truck loaded to 18,000 lb on a single rear axle. Static measurements were made at 2 points 50 ft apart in each wheel track for each area of test. Over 1000 measurements were made in the fall and again in the spring at the same marked points. Forty-three construction projects were measured and pavement conditions noted at one or more areas of test for each project. A final pavement condition survey was made in 1964. Additional tests were made on other projects since the spring of 1957 to determine the effect of pavement age, pavement thickness, different wheel loads, and base aggregate degradation. Data concerning the type and source of base rock were obtained from the files for each project measured.

Tables 1, 2 and 3 give various comparisons of deflections for each of five types of bases. The values of deflection are averages of all readings in the category indicated for each type of base. The number of points measured for type of base were: 20 over borrow material, 130 over volcanic cinders, 84 over crushed river gravel, 174 over crushed pit gravel, and 150 over crushed quarry rock.

TABLE 1

SPRING AND FALL MEASUREMENTS-DEFLECTION IN INCHES

Range of Deflec-Percent of Points With

Type of Base Fall Avg. Spring Avg. tion for Group Spring Deflections Higher

Spring Fall Than Fall Deflections

Borrow 0. 0398 0. 0609 0. 0316 0.0805 100 Cinder 0. 0416 0. 0427 0. 0195 0. 0760 56.3 Crushed River Gravel 0. 0244 0. 0257 0. 0120 0. 0380 76. 1 Crushed Pit Gravel 0. 0193 0. 0216 0. 0115 0.0362 65. 6 Crushed Rock 0. 0179 0. 0210 0. 0077 0. 0395 70. 6

Table 4 shows the relationship between the degree of deflection and failure. Figure 1 shows the relationship between load and deflection on two different types

of base construction. The fill area is composed of a pumice type material taken out of the cut. Sponsored by Committee on Flexible Pavement Design.

154

155

TABLE 2

OUTSIDE AND INSIDE WHEEL TRACK DEFLECTIONS

Type of Base

Fall Deflections (in.) Spring Deflections (in.) Percent Inside Deflection Higher Than Outside

Borrow Cinder Crushed Gravel Crushed Pit Gravel Crushed Rock

Avg. Avg. Avg. Avg. Outside Inside Outside Inside

0. 0402 0. 0394 0. 0756 0. 0562 0. 0402 0. 0430 0. 0438 0.0417 0. 0227 0. 0221 0. 0252 0. 0262 0. 0185 0. 0202 0. 0220 0. 0203 0. 0184 0.0175 0. 0220 0. 0197

TABLE 3

TYPE OF BASE AND FAil.,URE

Fall Spring

40. 0 0. 0 37. 5 28. 5 42. 8 38. 2 34. 4 53. 2 62. 8 25. 7

Total Avg. Defl. Percent Failed Points Type of Base Spring + Fall (in.) Fall 1956 Spring 1957 Spring 1964

Borrow Cinder Crushed River Gravel Crushed Pit Gravel Crushed Rock

0. 0503 0. 0422 0. 0240 0. 0204 0. 0201

60. 0 56. 2 4.7 0. 0 0. 0

100. 0 87. 5 19. 0

0. 0 2. 8

TABLE 4

DEGREE OF DEFLECTION AND FAILURE

Percent of Points Failed Deflection Range

Fall 1956 Spring 1957 Spring 1964a

0. 0 - 0. 0200 0. 0 3. 0 2. 3 0. 0201 - 0. 0300 2. 3 9. 3 38. 6 0. 0301 - 0. 0400 35. 3 36. 0 75. 0 0. 0401 and greater 78. 9 100. 0 100. 0

aPercentages based on spring and fal I measurements within deflection range. Thus the 2.3% indicated for the 0.0-0.020 range includes the l.5% of total spring and fall measurements plus one project failed since 1957.

100. 0 96. 8 28. 5 18.7 17. 1

Table 5 shows the effect of pavement overlays and age on deflection. The increase in deflection immediately after placing is attributed to an increase in moisture content caused by condensation under the hot layer.

Table 6 shows the effect on deflection of increased deleterious fines formed in the base rock by attrition from traffic. Values of 2. 5 in. maximum height and 25 percent maximum pass No. 20 would appear to be acceptable limits for the degradation test.

Table 7 shows the predicted decrease in deflection, using the constant 0. 0038-in. decrease in deflection per inch of pavement increase, and the actual deflection obtained on three different points. (Data and procedures used to determine the value of 0. 0038-in. decrease in deflection for each inch of asphaltic concrete added are given in the text of the complete report.)

26

24

22

(J) 20 IJ.I ::I: u 18 z ,.,, 16

I

0 - 14 -z 0 12 I-

~ 10 _J

LL B LLJ 0

6

4

2

L 1720

Effect of Varying Load Averages of 26 Measurements Each Location Deflection, 10 - 3 lnch~s

Vs Axle Load, Lbs. ,

7250

AXLE LOAD,

Figure l.

13425 15000

LBS.

18000

~ t.TI en

Project

1 2 2

157

TABLE 5

REPEATED DEFLECTION TESTS

Avg. Deflections (1/1000 in.) at Age

Oil New 1 Day 1 Day Project Top Lift, 2

Mat Grade Base Lift , Total 3 Weeks 1 Yr 2 Yr 2 In. In.

1 41. 4 55. 5 44. 6 40.7 31. 0 1 38. 6 30. 3 29. 2 29.0 25. 6 2 35. 7 30. 5 27. 3

TABLE 6

REPEATED DEFLECTIONS OVER DEGRADING MATERIAL {IN THOUSANDTHS OF AN INCH)

Project 1: Hwy 30, MP 138 to 142

Oregon Degradation Test Results on Base Material at MP 138 : Age-3 yr, Fall 1956; Height-3. 0 in.; Pass No. 20-26. 2%.

1956 1957 1960

Avg. Range {16 points) 20-26 18-28 28-46

Project 2: Hwy 15, MP 85

Degradation Point 1961 1963

Height (in. ) Pass No. 20 (%}

1 Travel Lane SB 16 24 3. ·2 18. 7 2 Travel Lane NB 18 24 3. 0 18.8 3 Passing Lane SB 16 16 2. 0 16. 1 4 Passing Lane NB 16 l6 3. 1 16.7

TABLE 7

CALCULATED AND MEASURED DEFLECTIONS-9 KIP WHEEL LOAD

Original Overlay

Deflection Thickness , Constant In.

0. 0414 3. 5 0. 0038 0.0386 3. 5 0. 0038 0. 0357 1. 5 0. 0038

Theoretical Decrease

0. 0133 0. 0133 0. 0057

Predicted Deflection

0. 0281 0. 0253 0. 0300

Actual Deflection

0. 031 2 yr. 0. 0256 2 yr. 0. 0305 1 yr. 0. 0273 2 yr.

158

Phase II-Field-Cut Beams

Three beams, 4 in. wide by 28 in. long, were cut from each of the projects measured for deflection under load in 1956 and 1957. Two of each set of beams were tested in the laboratory for fatigue of repeated flexure; one beam flexed 0. 020 in. and the other 0. 040 in. The third beam was tested to determine the coefficient of thermal expansion and beam strength at -10 F. Reclaimed asphalt tests, and tests to determine asphalt content, void content, and gradation were made on samples cut from the flexed and broken beams.

No correlation could be found between the number of deflections required for failure and pavement characteristics or ingredient characteristics. Several variables existed which could not be evaluated for their effect on fatigue life. These variables included the number and degree of deflections prior to cutting the beam, asphalt film thickness , asphalt consistency or hardness prior to cutting the beams, and improvements made on Lhe deflec lion machine Lo remove disto1·tion and extrusion of the test specimen.

Differences in coefficient of thermal expansion and modulus of rupture between specimens composed of cinder aggregate and nonvesicular aggregate are as follows:

Specimen

Cinder Aggregate Nonvesicular Aggregate

Avg. Coef. of Linear Expansion

(in./ in. / deg F)

1. 51 x 10- 5

1. 268 x 10-5

Avg. Modulus of Rupture, psi,

at -10 F

479 814

The hardening of the asphalt, as indicated by the reclaimed asphalt penetration, varied from 8 to 7 4 penetration points and averaged 42. 7 points for the 42 pavements tested. Due to the variables of aggregate characteristics, asphalt content, void con­tent, mixing temperature and length, pavement age, weather conditions, and asphalt source, the reclaimed asphalt tests of penetration, thin film, and absolute viscosities are not capable of any valid comparison.

Phase III-Laboratory-Molded Specimens

Seventy-five sets of specimens were made in the laboratory. Tests were made to determine fatigue of deflection at two constant deflection amounts , void content, linear coefficient of thermal expansion, modulus of rupture at -10 F, and tests on reclaimed asphalt. Constants in the test specimens were aggregate source, asphalt source, ma­ter ial preparation, specimen fanrication, specimennandirng, and telit pr-Ocedures. Con­trolled variables were aggregate gradation and asphalt content, consisting of 25 specific gradations at three asphalt contents.

Results of tests in terms of the number of deflections required to break the speci­men were rated from 1 to 25 for each asphalt content and are presented in chart form. The total combined ratings for all 7 5 specimens show the best and second best in ability to withstand deflection on mixtures with 26 and 27. 5 percent of aggregate passing the No. 10 sieve.

The patterns formed by void contents on charts are relatively consistent for all con­ditions but there is no evidence of connection between void content and fatigue durability.

The coefficients of linear expansion of laboratory-made specimens are comparable to those of the field-cut beams. A relationship exists between asphalt content and linear expansion, as given in the table:

Identification

Lab-Made 5. 0% Asph. Lab-Made 5. 5% Asph. Lab-Made 6. 0% Asph. Avg. Lab-Made Field Cut

Coef. of Lin. Exp. (in./in./deg F x 10- 5

)

1. 286 1. 305 1. 430 1. 340 1. 286

Range, x 10- 5

1. 07-1. 46 1. 13-1. 46 1. 27-1. 64 1. 07-1. 64 1. 00-1. 53

159

The average modulus of rupture at -10 F and deflection at failure is 7 42. 5 psi and 0. 0385 in. Figures for the field-cut beams are 814 psi and 0. 0473 in., respectively.

Tests on reclaimed asphalt show a range of penetration drop from 6 to 40 penetra­tion points. There is a slight trend for less penetration drop with increased working on the fatigue machine.