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Fatigue Behavior of Air-Entrained Concrete Dah-yinn Lee, F. Wayne Klaiber, and Jeff W. Coleman, Department of Civil

Engineering, Engineering Research Institute, Iowa State University

Almost all rigid-pavement design procedures used today include consid· eration of the fatigue strength of concrete, but the fatigue curves used do not take into account the effects of entrained air. This paper pre· sents the results of a study of the flexural fatigue strength of concrete in which the only variable was the percentage of entrained air. All other variables (e.g., type of cement, water-cement ratio , and type of aggre· gate) were held constant. Five sets of concrete beams that had air con· tents that varied from 2.8 percent (natural air, i.e., no air-entraining agent added) to 11.3 percent but a constant water-centment ratio of 0.41 were tested under one-third-point loadinQ. The stresses were varied from zero to a predetermined maximum that corresponded to a percentage of the modulus-of-rupture strength of the given beam. Within each air· content group, a minimum of five beams were tested at each of four stress levels: 60, 70, BO, and 90 percent of modulus of rupture. A total of 112 specimens of 15.2 x 15.2-cm (6 x 6-in) cross section were tested. The results are presented as fatigue curves that relate the ratio flexural stress: modulus of rupture to the number of allowable load repetitions and show a decrease in fatigue strength as air content increases.

Concrete is subject to fatigue failure, and because high­way concrete slabs are subjected to many repetitions of traffic loads during their service lives, the importance of fatigue in concrete pavement performance and design is self-evident. The flexural stresses in concrete pave­ment siabs are criticai; thus, fatigue due Lo flexural stress should be used for concrete pavement design. Fatigue tests in compression, although useful for many design applications, do not provide information useful to the designer of pavements. Loading schemes that subject concrete specimens to flexural loading more realistically duplicate conditions encountered in the field.

The first concrete fatigue tests that used flexural loading were carried out by Feret in 1906 (1). How­ever; the most important investigations relative to concrete pavement design were those carried out by Hatt (2) at Purdue University (1922-24) and by Clemmer (3) and Older (4) at the Illinois Department of Highways -(1921-23), which served as a basis for the develop­ment of the Portland Cement Assoc iation (PCA) design curve for the fatigue strength of concrete pavement (5).

Clemmer devised a unique machine in which 15.2--x 15.2- x 91.4-cm (6- x 6- x 36-in) concrete beams were cantilevered out from a central hub. Load was applied by rotating a pail· of rubber -tired wheels about the hub. Test beams were subjected to 40 load applications/ min. Clemmer found that the endurance limit for concrete was between 51 and 54 percent of the modulus of rupture, as determined by a static test, for up to 2 million cycles of load application. More recent investigations have not shown the existence of a fatigue limit in concrete, at least up to 10 million cycles of load application (6).

Hatt's flexural tests were significantly different from Clemmer's, but led to similar conclusions. Specimens 10.2 x 10.2 x 76.2 cm (4 x 4 x 30 in) were tested as cantilever beams . Each specimen was sub­jected to sti·ess reversal at a r ate of 10 cycles/ min. Hatt; felt t hat the 40 load applications/ min used by Ciemmer was too ia1:1t iu compan: with actual :road con­ditions and discontinued testing overnight to allow for rest periods that would better duplicate field con­ditions.

Of special significance is the fact that the fatigue response of concrete in the laboratory was also re-

fleeted in actual pavement performance and the results of road tests in terms of the decrease of the service­ability index and the development of cracks wltb in­creasing number of load applications (1, !!., ~ 10, .!_!, 12) .

The most widely used fatigue curves for plain con­crete pavement design are those developed by PCA (6, 13). The current PCA curve can be expressed as foi= lows:

SR= 0. 972 - 0.08281og N

where SR = stress ratio = ratio of flexural stress to modulus of rupture and N = number of allowable load repetitions.

(! )

Almos t all modern rigid-pavement design methods recognize the importance of the fatigue life of concrete and consider not only the anticipated masses but also the number of heavy-axle loads that will be applied during the design pavement life. The AASHTO Interim Design Procedure, for example, uses the total number of equivalent 80-kN [18 000 lbf/in2 (18-kip)] single­axle loads applied during the design life for the design of thickness. The PCA design procedure evaluates the accun1ulated fatigue effects of all hoavy-~~le lcn.d ap­plications during the pavement life.

The fatigue strength of concrete is affected not only by the magnitude of load and the number of load applica­tions but also by other factors such as type and quality of aggregate, moisture condition, rate of loading, cur­ing conditions, and age of concrete at testing. How­ever, one important variable that has not been in­vestigated is the effect of entrained air: The pavement design curves used at present do not directly take en­trained air into account.

AIR-ENTRAINED CONCRETE IN PAVEMENT

The use of admixtures that cause the entrainment of air in concrete is considered by many to be one of the most important developments in concrete technology in recent decades. The principal use of this technique has been in pavement concrete. Entrained air improves concrete mainly in two ways : (a) it improves the resistance of the concrete to freezing and thawing and deicing chem­icals and (b) it improves the workability and decreases segregation of freshly mixed concrete.

Today, air-entrained concrete is recommended for all structures under conditions of severe exposure and for all pavements regardless of climatic conditions (5, 13, 14 , 15). The entrained-air content recommended -depends on the maximum size of t he aggregate , and usually x·anges f'rom about 5 percent for a 5.1-cm (2-in) aggr egate t o about 8 percent for a 1.3-cm (0.5-in) aggregate (6) . For pavement concrete, the speci­fied air content is usually 6 ± 1 percent.

In recent years, because of the greatly increased use of deicing chen1icals, it has been found necessary to incorporate higher levels of air in certain concrete structures, such as highway bridge decks. Further­more, there is reason to suspect that, because of con­cern on the part of the contractor to achieve desired workability and to meet minimum air-content require-

ments, the actual air content in field-placed concrete could sometimes be higher than that recommended.

FATIGUE BEHAVIOR OF AIR-ENTRAINED CONCRETE

The effects of air content on most concrete properties (such as compressive strength, workability, durability , and creep) are relatively well understood. However, in­formation about the effect of air content on the flexural fatigue strength of plain concrete is nearly nonexistent.

A search of the literature of the past 70 years found only two studies of fatigue strength of air-entrained concrete. The first of these was carried out by Gregg (16) and involved flexural fatigue testing of beam speci­mens in which one of the variables was air entrainment. The results of this project are of limited interest be­cause different cements and different loadings were used in the various beams. Also, the air-content variable was not singled out for comparison. The s econd study was carried out by Antrim and McLaughlin (17), who perfor med axial compression fatigue tests ontwo types of concrete, one containing entrained air and the other containing only natural air. One of the conclusions of this study was that air-entrained con­crete has a longer fatigue life at low stress levels but a shorter fatigue life at high stress levels. Thus, to our knowledge, no work has been done to determine the effect of entrained air on the flexural fatigue strength of concrete.

PURPOSE AND SCOPE

The purpose of this study was to evaluate the effects of air entrainment on the fatigue strength of plain con­crete and to establish preliminary fatigue curves for air-entrained concrete to be used in concrete pavement design.

The scope of the work included flexural fatigue tests of concrete at various levels of air entrainment, pre­pared with one aggregate and grading, one type of cement, and at one water-cement ratio. All other vari­ables, such as age of concrete when tested, tempera­ture, and curing conditions, were held constant.

The flexural fatigue tests were supplemented by five other types of measurement:

1. The variation of the modulus of rupture with air content,

2. The variation of the compressive strength with air content,

3. The variation of the modulus of elasticity with air content,

4. The variation of the air content when measured by different techniques, and

5. The variation of the concrete microstructure with air content (by scanning electron microscopy and mercury penetration porosimetry) .

This paper will summarize only a portion of the test­ing program. A more detailed analysis of the results and details of the supplementary testing is given else­where (18).

TEST PROGRAM

Five laboratory mixes in which air content was the only variable were studied. Because uniformity of mix was of the utmost importance, all specimens that had a given air content were prepared from one batch of con­crete. Beams 15.2 x 15.2 x 91.4 cm were cast for modulus-<?f-rupture testing and flexural fatigue testing,

21

and 15.2-cm diameter x 30.5-cm (12-in) long cylinders were prepared for control of compressive strength. At the time of casting, plastic air content and slump were measured . After an initial curing period of 24 to 48 h, the specimens were stored submerged in water until testing, at which time they were sealed in polyethylene bags to maintain their saturated moisture condition. The effect of age was minimized by testing all batches at an age of 28 to 56 d. The 91.4-cm-long flexural­test specimens permitted testing the modulus of rupture on one end of the beam and fatigue on the remaining portion. The fatigue tests were run at percentages (60, 70, 80, or 90) of the modulus-of-rupture strength determined on the other end of the beam. The dynamic cycler used was modified so that the beams were sub­jected to the same one-third-point loading as was used in the modulus-of-rupture tests . The applied load produced stresses in the bottom fiber of the specimen that varied from zero to a tensile stress equivalent to a given percentage of the modulus-of-rupture stress. All specimens were loaded at a rate of 5 to 7.5 Hz.

Materials

Type 1 portland cement was used and care was taken to ensure that all the cement came from one manufactur­ing batch. A crushed limestone was used as the coar.se aggregate, and a concrete sand was used as the fine aggregate. The gradations of the coarse and fine aggre­gate used meet state specifications. A commercially available vinsol resin was used as the air-entraining agent for all batches. The amounts used in each batch were varied to obtain different air contents. The water­cement ratio was 0.41 for all batches.

Results

The physical properties of the five batches of concrete tested are shown in Table 1 in order of increasing air content. Batch designations (i.e., A, B, and so on) represent order of pour (i.e., A was first, B second, and so on). No air-entraining agent was added to batch C (2.8 percent air); thus, this percentage can be con­sidered as entrapped or natural air. The variations with air content of the various properties are in general agreement with the findings of other studies . One hundred twelve beams were subjected to flexural fatigue testing.

Within each air-content group, beams were tested at four different stress levels. With one exception, a minimum of five beams within each of the groups was tested at each stress level. All tests were run a mini­mum of 2 million cycles; a few specimens loaded to 60 percent of their modulus-of-rupture strengths did not fail before 2 million cycles, thus testing was terminated.

Visual examination of the failure faces of the modulus-of-rupture specimens and the fatigue speci­mens at different air contents showed that there is no difference in appearance between the modulus-of-

Table 1. Physical properties of concrete studied.

28-<i Modulus Modulus Air Unit Compressive of of Content Slump Mass strength Elasticity Rupture

Batch (~) (cm) (kg/ m') (MPa ) (GPa) (MPa)

c 2.8 2 .5 2498 37.0 34. 7 5,94 A 3.5 3.2 2347 34.5 29 .2 5.64 E 6.4 11.4 2242 25 .1 22 .3 4.02 D 10.2 12 . 7 2118 18.0 18.3 3.33 B 11.3 9.5 2130 18.8 16.1 3 .50

Note: 1cm=0.39 in, 1kg/m'=0.0625 lb/ft3 , and 1MPa=145 lbf/in2•

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rupture failure surface and the fatigue-failure surface for beams that have the same air content. However, there are differences among the failure surfaces of beams that have different air contents. The failure surfaces of the low-air-content specimens showed failure predominantly through the coarse aggregate; those of the high-air-content specimens showed some failure of the aggregate, but their main failure is be­tween the cement-paste matrix and the aggregate. Linear regression analyses between the logarithm of SR and the logarithm of N were performed. Specimens that did not fail before 2 million cycles of load were assigned a fatigue life of 10 million cycles. Composite S versus N curves for the various air-content spec­imens are given as a semilog plot in Figure 1. By com­paring the other curves with that for the specimen

Figure 1. Composite S versus N curves for five concretes.

UJ

"' :::> ..... Cl. :::>

"' u.. 0

V) :::> -' :::> 0

90

80

70

that has 2.8 percent air, i.e., the natural air-content specimen, one can see the decrease in fatigue strength as the air content increases. Figure 2 shows the 95 percent confidence limits for the 11.3 and 2 .8 percent air-content specimens. These confidence limits over­lap only slightly at the ends of the ranges, indicating that, although the precise locations of the middle-range curves may not be statistically significant, the trend of the data is unmistakable. Thus, it can be concluded that air content has an undeniable effect on the fatigue strength of plain concrete in flexure. Because of the semilog plot, it is not obvious from the curves, but the effect of air is more significant at lower ranges of stress than at higher ones. For example, at the 70 per­cent stress level, the anticipated fatigue lives of con­cretes containing 11.3 and 2.8 percent air are 30 000 and

i ---- 2. 8% AIR !;: ------ · 3. 5% AIR

-·-·- b. 4:; AIR UJ u 0:: UJ Cl.

Figure 2. Confidence limits for plain concrete containing 2.8 and 1 1.3 percent entrained air.

UJ

"' :::>

60

50

90

::;:: 80 :::>

"" u.. 0

V)

~ 70 0

i

50

........ ___ 10. 2% AIR --.. - ... 11. 3% AIR

? 10 10~

LOG NUMBER OF CYCLES TO FAILURE

10 102 103 104 LOG NUMBER OF CYCLES TO FAILURE

.. ....

200 000 cycles respectively, and at the 65 percent stress level, these values are 200 000 and 1 700 000 cycles respectively. The difference in the fatigue lives of the two concretes at 70 percent of the modulus of rupture is 130 000 cycles, and at 65 percent, it is 1 500 000 cycles. The lower stress ranges are crucial with re­spect to pavement design, which makes this divergence of critical importance.

CONCLUSIONS

1. The fatigue behavior of plain concrete in flexure is affected by its air content. Fatigue strength de­creases as air content increases.

2. The modulus of rupture, the compressive strength, the modulus of elasticity, and the unit weight of con­crete all decrease as the air content of the concrete in­creases.

3. As the air content increases, the failure of con­crete subjected to fatigue occurs increasingly at the interface of the aggregate and the cement paste. On a macroscopic level, the fatigue failure surface is similar to the modulus-of-rupture (static) failure surface .

ACKNOWLEDGMENTS

This report summarizes a portion of a research program designed to study the effects of air entrainment in plain concrete on the fatigue strength. The program was con­ducted by the Engineering Research Institute of Iowa State University and funded by the Highway Division of the Iowa Department of Transportation. The opinions, findings, and conclusions expressed are ours and not necessarily those of the Iowa Department of Transporta­tion.

REFERENCES

1. G. M. Nordby. Fatigue of Concrete: A Review of Research. Journal of the American Concrete In­stitute (ACI, Proc. Vol. 55), Vol. 30, No. 2 Aug. 1958, pp. 191-219.

2. W. K. Hatt. Fatigue of Concrete. HRB, Proc. Vol. 4, 1924, pp. 47-60.

3. H. F. Clemmer. Fatigue of Concrete. ASTM, Proc. Vol. 22, Pt. 2, 1922, pp. 409-419.

4. C. Older. Highway Research in Illinois. Trans., ASCE, Vol. 87, 1924, pp. 1180-1222.

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5. Thickness Design for Concrete Pavements. PCA, Skokie, IL, 1966.

6. G. E. Troxell, H. F. Davis, and J. W. Kelly. Composition and Properties of Concrete. McGraw­Hill, New York, 1968.

7. W. N. Carey and P. E. Irick. Relationships of AASHO Road Test Pavement Performance to De­sign and Load Factors. HRB, Special Rept. 73, 1962, p. 259.

8. W. R. Hudson and B. F. McCullough. An Exten­sion of Rigid Pavement Design Methods. HRB, Highway Research Record 60, 1964, pp. 1-14.

9. G. K. Ray, E. G. Robbins, and R. G. Packard. Concrete Pavement Design Today. Proc., ASCE Conference on Pavement Design for Practicing Engineers, Atlanta, GA, 1975, pp. 1-2.

10. G. K. Ray. History and Development of Concrete Pavement Design. Journal of the Highway Divi­sion, Proc., ASCE, Vol. 90, No. HWl, Jan. 1964, pp. 79-101.

11. A. S. Vesic and S. K. Saxena. Analysis of Struc­tural Behavior of AASHO Road Test Rigid Pave­ments. NCHRP, Rept. No. 97, 1970.

12. E. J. Yoder and M. W. Witczak. Principles of Pavement Design. Wiley, New York, 1975.

13. W. Lerch. Basic Principles of Air-Entrained Concrete. PCA, Skokie, IL, 1953, p. 36.

14. G. W. Hollen and M. E. Prior. Factors Influenc­ing Proportioning of Air-Entrained Concrete. ACI, Publ. SP-46, 1974.

15. C. E. Wuerpel. Purposeful Entrainment of Air in Concrete. Hercules Powder Company, Wilming­ton, DE, 1970.

16. L. E. Gregg. Experiments With Air-Entrainment in Cement Concrete. Kentucky Department of Highways, Bull., No. 5, Sept. 1947.

17. J. DeC. Antrim and J. F. McLaughlin. Fatigue Study of Air-Entrained Concrete. Journal of the American Concrete Institute (ACI, Proc. Vol. 55), Vol. 30, No. 11, May 1959, pp. 1173-1182.

18. D. Y. Lee, F. W. Klaiber, and J. W. Coleman. Fatigue Behavior of Air-Entrained Concrete. Engineering Research Institute, Iowa State Univ., Ames; Iowa Department of Transportation, Final Rept., July 1977.

Publication of this paper sponsored by Committee on Mechanical Prop­erties of Concrete.

Pavement-Layer Modular Ratios From Dynaflect Deflections R. A. Jimenez, Arizona Transportation and Traffic Institute, College of

Engineering, University of Arizona

Pavement systems and individual layers were evaluated by using the de­flections obtained with the Dynaflect device. The pavements were con· sidered to be three-layer systems, and the deflection data were used to estimate ratios of the elastic moduli of the adjacent layers (Kl and K2) . These ratios reduce the system from three values of elastic modulus to

two values of modular ratio. Certain assumptions were made related to pavement factors that affect deflections and also the elastic modulus of the asphalt concrete layer (El). A graphical trial-and-error procedure was used to match the surface deflections calculated by using the Chevron program for the Dynaflect load with the actual deflections measured on