Flexural Fatigue Strength, Endurance Limit, And Impact .
17TRANSPORTATION RESEARCH RECORD 1226Flexural Fatigue Strength, EnduranceLimit, and Impact Strength of FiberReinforced ConcretesV.RAMAKRISHNAN, GEORGEY. Wu,AND G. HosALLIIn many applications, particularly in pavements, bridge deck overlays, and offshore structures, the flexural fatigue strength andendurance limit are important design parameters because thesestructures are designed on the basis of fatigue load cycles. Thispaper presents the results of an extensive experimental investigation to determine the behavior and performance characteristicsof the most commonly used fiber reinforced concretes (FRC) subjected to fatigue loading. A comparative evaluation of fatigue properties is presented for concretes with and without four types offibers (hooked-end steel, straight steel, corrugated steel, and polypropylene) at two different quantities (0.5 and 1.0 percent byvolume), using the same basic mix proportions for all concretes.The test program involved the determination of fresh concreteproperties, including slump, vebe time, inverted cone time, aircontent, unit weight, and concrete temperature; and the determination of hardened concrete properties, including flexural fatiguestrength, endurance limit, and impact strength. The addition ofthe four types of fibers caused a considerable increase in the flexural fatigue strength and the endurance limit for 4 million cycles,with the hooked-end steel fiber providing the highest improvement(143 percent) and the straight steel and polypropylene fibers providing the least. The impact strength was increased substantiallyby the addition of all four types of fibers, with straight steel fiberproducing the lowest increase.The recent interest in reinforcing portland cement basedmaterials with randomly distributed fibers was spurred bypioneering research on fiber reinforced concrete (FRC) conducted in the United States in the 1960s. Earlier work (1-19)has established that the addition of steel fibers improves thestatic flexural strength, flexural fatigue strength, impactstrength, shock resistance, ductility, and failure toughness inconcrete.In many applications, particularly in pavements and bridgedeck overlays, the flexural fatigue strength and endurancelimit are important design parameters because these structures are designed on the basis of fatigue load cycles. Thegreatest advantage of adding fibers to concrete is the improvement in fatigue resistance. Plain concrete has a fatigue endurance limit of 50 to 55 percent of its static flexural strength(15-17). A properly designed FRC can achieve a 90 to 95percent endurance limit. Theoretically, with a higher endurance limit, the concrete cross sections could be reduced.Alternatively, using the same cross section could result in alonger life span or higher load carrying capacity or both.However, the research cited above involved small-scale,V. Ramakrishnan and G. Hosalli, South Dakota School of Minesand Technology, Rapid City, S. Dak. 57701. G. Y. Wu, Naval CivilEngineering Laboratory, Port Hueneme, Calif. 93043-5000.independent pilot projects for various types of fibers. A needremained for an extensive scientific investigation to determinethe fatigue performance characteristics of the most commonlyused types of fibers and mix proportions. There was a furtherneed to evaluate the comparative fatigue behavior of varioustypes and quantities of fibers. Particularly, little informationis available about the flexural fatigue behavior of concreteswith different types and quantities of fibers.The primary objective of this research was to determinethe behavior and performance characteristics of FRC subjected to fatigue loading. The other major objectives were To determine the fresh concrete properties includingworkability, balling characteristics, and finishability of concretes reinforced with four types of fibers (hooked-end steel,straight steel, corrugated steel, and polypropylene) and tocompare their properties with those of corresponding plainconcrete; To study the effect on the fresh and hardened concreteproperties due to the addition of the four types of fibers at0.5 and 1.0 percent by volume of fibers to a plain concretemix; and To conduct a detailed investigation of the flexural fatiguestrength including the endurance limit for concretes with andwithout the four types of fibers in two different quantities,using the same basic proportions for all concretes.MATERIALS, MIXES, AND TEST SPECIMENSMaterialsFibersThe following four types of fibers were used in this investigation:1. Type A. The 2-in.-long hooked-end fibers used wereglued together side by side into bundles with a water-solubleadhesive. During the mixing process, the glue dissolved inwater and the fibers separated into individual fibers, creatingan aspect ratio of 100.2. Type B. The straight fibers used were made from lowcarbon steel with a rectangular cross section of 0.009 in. x0.030 in. and a length of 0.75 in. It has an aspect ratio ofapproximately 40.3. Type C. The 2-in.-long corrugated fibers used were pro-
TRANSPORTATION RESEARCH RECORD 122618duced from a mild carbon steel. The diameter of the fiber (orequivalent diameter) was 0.03 to 0.05 in. with an aspect ratioof about 40 to 65.4. Type D. The %-in.-long polypropylene fibers used werecollated, fibrillated fibers .CementASTM Type I/II (dual purpose) portland cement was used.Coarse AggregateThe aggregates used were blended in two sizes: (a) in a mixture of 60 percent aggregate with a 1-in. maximum size, and(b) 40 percent aggregate with a %-in. maximum size . Themixture satisfied ASTM C33.Fine AggregateThe fine aggregate used was natural sand. It had a waterabsorption coefficient of 1.64 percent and a fineness modulusof 3.02.AdmixturesA superplasticizer satisfying the requirements of ASTM C494for chemical admixtures and an air-entraining agent satisfyingthe requirements of ASTM C260 were used .MixesThe same proportions were used for the plain (control) andFRC mixes for the entire investigation. The water-to-cementratio was maintained at 0.4 for all the concretes . For flexuralfatigue testing, two mixes each for plain and Type A, B, C,and D fibers were made with 0.5 percent and 1.0 percent byvolume of fibers. The control mix design was as follows:CementCoarse aggregateFine aggregateAir co ntent658 lb/yd'1,560 lb/yd'1,560 lb/yd'5 1.5 percentTESTS FOR HARDENED CONCRETEFlexural Fatigue TestThird point loading was used in the flexural fatigue strengthtest. The test beams had a span of 18 in. and were subjectedto a nonreversed fluctuating load.The lower load limit was set at 10 percent of the averagemaximum load obtained from the static flexure test. The upperload limit was set at 90 percent of the average maximumflexural load for the first beam in each mix , and the fatiguetest was run between these limits. If the beam failed beforecompleting 2 million cycles, the upper limit was reduced forthe next specimen. If the beam survived, another beam wastested at the same upper load as a replicate. Three specimenswere tested at each maximum load level.The frequency of loading used was 20 cycles/sec (Hz) forall tests. The control and monitor system for all tests consistedof a MTS 436 control unit, a Hewlett-Packard oscilloscope,and a digital multimeter working with a MTS load cell .Impact TestThe impact specimens were tested at 28 days by the dropweight test method (7) . Equipment for this test consisted of A standard, manually operated, 10 lb (4.54 kg) weighthammer with an 18 in. (457 mm) drop (ASTM D1557); A 2.5 in. (63.5 mm) diameter hardened steel ball; and A flat steel base plate with a positioning bracket and fourpositioning lugs.The specimen was placed on the base plate within the positioning lugs with its rough surface upward . The hardened steelball was placed on top of the specimen within the positioningbracket, and the compactor was placed with its base on thesteel ball. The test was performed on a smooth, rigid floor tominimize the energy losses. The hammer was dropped consecutively, and the number of blows required to cause thefirst visible crack on the top of the specimen was recorded.The impact resistance of a specimen to ultimate failure wasalso measured by recording the number of blows required toopen the cracks enough that the pieces of the specimen touchedthree positioning lugs on the base plate.Test SpecimensTEST RESULTS AND DISCUSSIONFor the fatigue test, 18 beams of 6 in . x 6 in . x 21 in . (152mm x 152 mm x 533 mm) were cast in each of plain , 1.0percent fiber, and 0.5 percent fiber concretes. Cylinders6 in. x 2.5 in. (152 mm x 64 mm) were made for the impacttest.Fresh Concrete PropertiesRoom temperature, humidity, and concrete temperature wererecorded to ensure that all the mixes were tested under similarconditions. The room temperature and humidity varied in therange of 18 to 27 C and 33 to 58 percent, respectively . Theconcrete temperature range was 20.4 to 27.2 C.TESTS FOR FRESH CONCRETEThe freshly mixed concrete was tested for slump (ASTM C143),air content (ASTM C231), fresh concrete unit weight (ASTMC138), temperature, time of flow through an inverted cone(ASTM C995) , and vebe time.WorkabilityThree test were done to determine the workability of themixes: slump, inverted cone time, and vebe time. The test
Ramakrishnan et al.19results indicated that, in general, satisfactory workability canbe maintained even with a relatively high fiber content. Thiswas achieved by adjusting the amount of superplasticizer used;the water-to-cement ratio remained constant (0.4) for all mixes.Balling tendency for the straight steel fiber mixes wasobserved at 1.5 percent fiber volumes. To avoid balling, thefibers had to be carefully sprinkled by hand. The concretehad poor workability and more bleeding and segregation withhigher quantities of polypropylene fibers. In all other mixeswith an appropriate quantity of fibers, there was no balling,bleeding, or segregation. Even though slump values decreasedwith increasing amounts of fibers, no difficulty was encountered in placing and consolidating the concrete in thelaboratory.It seems that the relationship between vebe time and slumpfor each fiber type is not affected by fiber contents for therange tested in this investigation. However, the relationshipis different for other types of fibers, and markedly differentfor hooked-end fibers. The rheological properties of freshconcrete with hooked-end steel fibers are different than thosefor other fibers. This may be due to the higher frictionalresistance for movement in hooked-end fibers.The relationship between vebe time and slump is independent of the air content. Fibrous concrete has less slumptha n plain concrete. In general, FRC seems to be more workable under vibration lhan is indicated by the 'stump. Nevertheless, the energy needed to compact the cohcrete appearsto be proportional to the fiber content in the concrete.The inverted cone test was specially developed (12) tomeasure the workability of FRC in the field. Since both theinverted cone test and the vebe test are based on the energyrequirements for flowability and compaction, there is a linearcorrelation between the two tests. This facilitates the transferof laboratory test results to field practice more accurately.Hardened Concrete PropertiesFlexural Fatigue BehaviorThe fatigue properties of FRC were evaluated thoroughly inthis study. Beams made with plain concrete and concretesreinforced with 0.5 percent and 1.0 percent by volume offibers were tested for flexural fatigue. Three specimens weretested at each strength level. Figures 1 through 11 present thevarious relationships between the number of cycles (N), logN, fatigue strengths, and endurance limits. Based on the datapresented in these figures, the following three main propertiesare discussed: Fatigue strength, Endurance limit expressed as a percentage of modulusof rupture of plain concrete, and Endurance limit expressed as a percentage of its modulusof rupture.Fatigue StrengthFatigue strength iftmax) is defined as themaximum flexural fatigue stress at which the beam can withstand 2 million cycles of nonreversed fatigue loading.The fatigue strength was increased substantially with theaddition of fibers to the concrete, as shown in Table 1 andFigure 1. The fatigue strength was 508 psi for plain concreteand 549 psi and 676 psi, respectively, for concrete mixes reinforced with 0.5 percent and 1.0 percent corrugated steel fiber.The increase in fatigue strength was 8 percent and 33 percent,respectively.Graphs of flexural fatigue stress versus the number of cyclesare shown in Figures 2 and 3. The relationship is curvilinearuntil the fatigue strength of that particular concrete is reached,then the line becomes parallel to the X-axis; the same behaviorcan be observed for all concretes. Figures 4 and 5 presentfatigue flexural stress versus the logarithm of the number ofcycles for all the concretes. These figures reveal a linearrelationship between fatigue stress and log N. The fatiguestrengths of concretes with and without fibers are comparedFinishabilityExcellent finishability was achieved with the appropriate dosage of superplasticizer.TABLE 1 FATIGUE PROPERTIES OF CONCRETES WITH DIFFERENT TYPES OFFIBERSFiber TypeAFiber Content(%) 0.5ffmax (in psi)749cBPlainD1. 0 0.51. 0 0.51. 0 0.51. 0 Cone.1242 559594 549676 478508508EL1(%)95158 7176 7186 616565EL2(%)7685 6759 7055 706565ffmax - flexural strength.Endurance limit expressed as a percentage of modulus ofrupture of plain concrete.Endurance limit expressed as a percentage of its modulus ofEL2 rupture.EL1 -
20TRANSPORTATION RESEARCH RECORD 1226150c:::::J PLAIN!Em TYPE A110.-TYPE B120100§;;!TYPE Cc:::::J TYPE D----- .900go800."'"- .JI . ---. . .:- 700- en;.ea 60 -.;I -.;---- --60PLAI::500.30CONTROL0TYPED- -20rl-'------- - - - - - - - - - -- - - - - - - - '1.0 %0 .5 %TYPE C - - - -l!---TYPER----- . ---- IE 0PERCENTAGE CF FI BERFIGURE 1 Fatigue strength.IE IIE 2LOG OFIE 3IE 4NUMBER CF CYCLESIE 5IE 6FIGURE 4 Fatigue stress versus log N for 0.5 percent fiberfatigue beams.130.120aTYPE B 110.PLAIUTYPE RTYPE C.TYPE D100NO fflll.UC\[.:t:::90--- A - -- - - - - -- -- ---- --- -- - ·-. .--* t:-- - -EPLAI::-- '-. - .-. i:f- - -0- 0.D - - -'i ""'""- -.--:-:-.:. i-'-4; --.:::500 - .:::::::::a-- -----------l "'---1-- -- - ---- ·TYPE R - - -TYPE B - - TYPE C ---TYPE 0 - -dO O -----,.---.,.-- ---.---.---.---.---r-- 0500100015002000t,'UMSSR OF CYCLES25003000l ln thouut'ldl3 00 00FIGURE 2 Number of cycles versus fatigue stress for 0.5percent fiber fatigue beams.200El"T'"!.90180- -----TYPE A----1700160150 TYPE B - - TYPE C - . TYPED - --- . --- - - --- - -- - - - --- -- ---400JOO200 ---.-----.---.------.---r---.---.-- 000I 00I OD2 DO25003000NUH.8ER OF CYCLES tt"' t'hot1o11.1u111 dl3 004000FIGURE 3 Number of cycles versus fatigue stress for 1.0percent fiber fatigue beams.IE 2IE 3IE 4LOG OF NUMBER CF CYCLESIE 5IE 6FIGURE 5 Fatigue stress versus log N for 1.0 percent fiberfatigue beams.- - - - -- - -- -- - - -- - - . . .PLAINIE Iin Figure 1. As can be seen, the fatigue strength increaseswith the fiber content for all fiber types. However, there is alarger increase in the fatigue strength with hooked-end fibers(47 percent and 144 percent, respectively, for 0.5 percent and1.0 percent fiber contents) than with other fibers. The smallestincrease in fatigue strength was found with polypropylene andstraight steel fibers (see Table 1).Endurance Limit Expressed as a Percentage of Modulus ofRupture of Plain ConcreteThe endurance limit (EL 1 ) isdefined as the maximum flexural fatigue stress at which thebeam could withstand 2 million cycles of nonreversed fatigueloading, expressed as a percentage of modulus of rupture ofplain concrete.Figure 6 compares the endurance limit values for all fiberconcretes and plain concrete. Beams with 0.5 percent and 1.0percent corrugated steel fiber contents show an appreciable
21Ramakrishnan et al.USING h IPLAIN CONCRETE)I. TYPE A-TYPE BI.c:::::J TYPE Cea TYPE0nrrID PLAIN CONCRETE .4.2CONTROL0.5 %PERCENTAGE OF FI BER1.0 %FIGURE 6 Comparison of FRC and plain concrete forendurance limit EL 1 nnm PLArn TYPE A-TYPE Bea TYPECc:::::J TYPE 0CONTROL0.5 %PERCENTAGE OF FI BER1.0 %FIGURE 7 Comparison of FRC and plain concrete forendurance limit EL2 increase in endurance limit expressed as a percentage of modulusof rupture of plain concrete. The endurance limit was 71percent for the mix with 0.5 percent fiber content and 86percent for the mix with 1.0 percent fiber content, whereasthe endurance limit for plain concrete was 65 percent. Thus,the endurance limit was increased by 9 percent and 32 percent,respectively, when 0.5 percent and 1.0 percent of fiber contentsby volume were added to the concrete. The highest increasewas experienced with hooked-end fiber (46 percent and 143percent for 0.5 percent and 1.0 percent fiber contents,respectively) and the least increase with straight andpolypropylene fibers (see Table 1).is misleading and shows some fibers unfavorably. For example, corrugated steel fiber concrete with 1.0 percent fibercontent by volume had a high fatigue strength compared withplain concrete, although it has a lower endurance limit. Thisalso indicates that the increased benefit due to the increasedfiber content is not proportional at higher quantities of fibers.For Type C fibers, the endurance limit was 70 percent forthe mix with 0.5 percent fiber content and 55 percent for themix with 1.0 percent fiber content (Table 1). The limit forthe 1.0 percent mix is low because its modulus of rupture washigh compared with that of plain concrete. Hence, theimprovement in endurance limit is evident only when theendurance limit is expressed as a percentage of plain concretemodulus of rupture.With an increase in fiber content, the apparent decrease inendurance limit expressed as a percentage of its modulus ofrupture was also true with straight steel fiber and polypropylenefiber. The endurance limits for the straight steel fiber concreteswere 67 percent and 60 percent, respectively, for 0.5 percentand 1.0 percent fiber contents. They were 70 percent and 67percent, respectively, for the concretes with 0.5 percent and1.0 percent of polypropylene fiber contents (see Table 1).However, the endurance limits for the hooked-end steel fiberconcretes were 76 percent and 82 percent, respectively, for0.5 percent and 1.0 percent fiber contents, which shows anincreasing trend with the increase in fiber content. Thisphenomenon may also be a function of the aspect ratio of thefiber. Further research is necessary to study this aspect morethoroughly.It was also observed that the variability in fatigue strengthof the concrete with 1.0 percent fiber content is high comparedwith the concrete with 0.5 percent fiber content. Some of thebeams that had much lower values than the mean were studiedclosely; when a fiber count in the fracture zone was performed,it was found that they had a subnormal number. Theinconsistency in the distribution of the fibers, particularly inthe tension zone, is inherent in fiber concretes with randomlyoriented fibers. This is probably the main reason for the highvariability in fatigue and static flexural strengths.Graphs of the ratio of flexural fatigue stress to modulus ofrupture lfrma)f,) versus the number of cycles are presented inFigures 8 and 9, respectively, for 0.5 percent and 1.0 percentPLAIN.9.;"' .8"' j.7 ;.,. .--TYPE A - - - - -DTYPE B - - - oTYPE C - - - --NO f"AILUP.CTYPE 0 -,. -"'.:;· . --- -o-.---- -- --- - - -- ----- - - -::- -.[l"- --. . -- -y- I : ::oO :::!::: . -- .:. - -- . --::r:-.6Endurance Limit Expressed as a Percentage of Its Modulus ofRuptureThe endurance limit of concrete (EL 2 ) can alsobe defined as the flexural fatigue stress at which the beamcould withstand 2 million cycles of nonreversed fatigue loadin
pioneering research on fiber reinforced concrete (FRC) con ducted in the United States in the 1960s. Earlier work (1-19) has established that the addition of steel fibers improves the static flexural strength, flexural fatigue strength, impact strength, shock resistance, ductility, and failure toughness in concrete.
materials with randomly distributed fibers was spurred by pioneering research on fiber reinforced concrete (FRC) con ducted in the United States in the 1960s. Earlier work (1-19) has established that the addition of steel fibers improves the static flexural strength, flexural fatigue strength, impact
Table 2-10 Available flexural strength, kip-ft Rectangular HSS (Brake Pressed) Table 2-11 Available flexural strength, kip-ft Square HSS (Roll Formed) Table 2-12 Available flexural strength, kip-ft Square HSS (Brake Pressed) Table 2-13 Available flexural strength, kip-ft Round HSS Table 2-14 Available flexural st
fatigue (endurance) limit, ferrous and titanium alloys. This value is the maximum stress which can be applied over an infinite number of cycles. The fatigue limit for steel is typically 35 to 60% of the tensile strength of the material. Fatigue strength. is a term applied for nonferrous metals and alloys (Al, Cu, Mg) which do not have a fatigue .
concrete strength, effect of fiber distribution on flexural strength of ultra-high strength concrete has been investigated. The ultimate flexural strength and time to first crack have been affected [1]. The placing methods control fiber distribution and mechanical performance of short-fiber reinforced concrete [2, 3]. The flexural behavior
flexural tensile strength of concrete given as, f r ¼ 0:827 h0;1 f c 2/3, where f c is compressive strength and h is depth of beam in mm. NCHRP (2004) has proposed an equation to deter-mine the flexural tensile strength (MOR) at different age, if 28-day flexural tensile strength of concrete is given, as MORðtÞ 1þlog 10 t 0:0767 0 .
3. Flexural Analysis/Design of Beam3. Flexural Analysis/Design of Beam REINFORCED CONCRETE BEAM BEHAVIORREINFORCED CONCRETE BEAM BEHAVIOR Flexural Strength This values apply to compression zone with other cross sectional shapes (circular, triangular, etc) However, the analysis of those shapes becomes complex.
Chapter 3 3.1 The Importance of Strength 3.2 Strength Level Required KINDS OF STRENGTH 3.3 Compressive Strength 3.4 Flexural Strength 3.5 Tensile Strength 3.6 Shear, Torsion and Combined Stresses 3.7 Relationship of Test Strength to the Structure MEASUREMENT OF STRENGTH 3.8 Job-Molded Specimens 3.9 Testing of Hardened Concrete FACTORS AFFECTING STRENGTH 3.10 General Comments
Tank Gauge) API 2350 categorizes storage tanks by the extent to which personnel are in attendance during receiving operations. The overfill prevention methodology is based upon the tank catagory. Category 1 Fully Attended Personnel must always be on site during the receipt of product, must monitor the receipt continuously during the first and last hours, and must verify receipt each hour .
