Flexural Behavior And Toughness Of Fiber Reinforced Concretes
TRANSPORTATION RESEARCH RECORD 122669Flexural Behavior and Toughness ofFiber Reinforced ConcretesV.RAMAKRISHNAN, GEORGEY. Wu,AND GrRISH HosALLIThis paper presents the results of an extensive investigation todetermine the behavior and performance characteristics of themost commonly used fiber reinforced concretes (FRC) for potentialairfield pavements and overlay applications. A comparative evaluation of static flexural strength is presented for concretes withand without four different types of fibers: hooked-end steel, straightsteel, corrugated steel, and polypropylene. These fibers were testedin four different quantities (0.5, 1.0, 1.5, and 2.0 percent by volume), and the same basic mix proportions were used for all concretes. The test program included (a) fresh concrete properties,including slump, vebe time, inverted cone time, air content, unitweight and concrete temperature, and hardened concrete properties; (b) static flexural strength, including load-deflection curves,first-crack strength and toughness, toughness indexes, and postcrack load drop; and (c) pulse velocity. In general, placing andfinishing concretes with less than I percent by volume for all fibersusing laboratory-prepared test specimens was not difficult. However, the maximum quantity of hooked-end fibers that could beadded without causing balling was limited to 1 percent by volume.Corrugated steel fibers (Type C) performed the best in fresh concrete; even at higher fiber contents (2 percent by volume), therewas no balling, bleeding, or segregation. Higher quantities (2 percent by volume) of straight steel fibers caused balling, and higherquantities of polypropylene fibers (2 percent by volume) entrappeda considerable amount of air. Compared with plain concrete, theaddition of fibers increased the first-crack strength (15 percent to90 percent), static flexural strength (15 percent to 129 percent),toughness index, post-crack load-carrying capacity, and energyabsorption capacity. Compared with an equal 1 percent by volumebasis, the hooked-end steel fiber contributed to the highest increase,and the straight steel fiber provided the least (but appreciable)increase in the above-mentioned properties.Previous research by Ramakrishnan (1-5 ,9-13) and others(6-8) has established that the addition of fibers to concreteconsiderably improves static flexural strength, impact strength,shear and torsional strength, direct tensile strength, fatiguestrength, shock resistance, ductility, and failure toughness.The degree of these improvements, however, depends on thetype, size, shape , and aspect ratio of the fibers.The research cited above involved small-scale, independentpilot projects for various types of fibers. Yet an extensivescientific investigation was still needed to determine the performance characteristics of the fibers and mix proportionsmost commonly used in field practice. Evaluation of the comparative behavior and properties of various fiber types atdifferent fiber contents was also necessary. Furthermore, lackof sufficient information on the toughness and static flexuralbehavior of concretes with different types and quantities ofV. 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.fibers underscores the need for more research. This information is essential for fiber reinforced concrete (FRC) inpotential airfield overlay applications.OBJECTIVESThe objectives of this investigation are as follows: 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 fresh and hardened concrete properties due to the addition of the four types of fibers at 0.5,1.0, 1.5, and 2.0 percent by volume for steel fibers and 0.1,0.5, 1.0, and 2.0 percent by volume for polypropylene fibersto a plain concrete mix; and To conduct a detailed investigation of the static flexuralbehavior, including first-crack strength, modulus of rupture,load-deflection curve, post-crack deformation characteristics,post-crack load drop, and toughness indexes .MATERIALS, MIXES, AND TEST SPECIMENSMaterialsFibersThe following four types of fibers were used in this investigation:1. Type A. The 2-in .-long hooked-end steel fibers usedwere glued together side by side into bundles with a watersoluble adhesive. During the mixing process, the glue dissolved in water and the fibers separated into individual fibers,creating an aspect ratio of 100.2. Type B. The straight steel fibers used were madefrom low carbon steel with a rectangular cross section of0.009 in. x 0.030 in. and a length of 0.75 in . Their aspectratio was approximately 40.3. Type C. The 2-in.-long corrugated steel fibers used wereproduced from a mild carbon steel with an aspect ratio of 40to 65 .4. Type D. The polypropylene fibers used were collated,fibrillated, and % in. long .
70TRANSPORTATION RESEA RCH RECORD 1226CementCl38), temperature, time of flow through an inverted cone(ASTM C995), and vebe time.ASTM Type I/II (dual purpose) portland cement was used .TESTS FOR HARDENED CONCRETECoarse AggregateThe aggregates used were maximum size 3ls in. and maximumsize 1 in. They were blended in a mixture of 60 percent aggregate with a 1-in. maximum size and 40 percent aggregate witha %-in. maximum size. The mixture satisfied ASTM C33.Fine AggregateThe fine aggregate used was natural river sand with 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 .Cylinders were tested for compressive strength (ASTM C39)and static modulus (ASTM C469) at 28 days of age.STATIC FLEXURE TESTBeams were tested at 28 days for static flexural strength (ASTMC1018) and pulse velocity (ASTM C597) . Some of the beamswere also tested at 7 days . Toughness indexes were calculatedusing the load-deflection data. According to ASTM Cl018,third point loading was applied to the beams in the staticflexural test. The span length was 18 in. (457 mm). Deflectionwas measured at mid span using a dial gauge accurate to 0.001in. (0.0254 mm). This test was a deflection-controlled test,with the rate of deflection kept in the 0.002 to 0.004 in./minrange as per ASTM Cl018. The loads were recorded at every0.002-in. increment in deflection until the first crack appeared.Thereafter, the loads were recorded at different intervals.TEST RESULTS AND DISCUSSIONMixesAspect RatioThe same proportions were used for the plain (control) andFRC mixes. The mix design is as follows:CementCoarse aggregateFine aggregateAir content658 lb/yd 31,560 lb/yd'1,560 lb/yd'5 1.5 percentThe water-to-cement ratio was maintained at 0.4 for allconcretes.For the static flexure strength test, two mixes without fibersand four mixes each for Type B and C fibers were made with0.5, 1.0, 1.5, and 2.0 percent by volume (66, 132, 198, and264 lb/yd', respectively). In the case of Type A fibers (hookedend), the maximum quantity of fibers that could be addedwithout creating balling was 132 lb/yd 3 (1 percent by volume) .Therefore, only three mixes with 0.5, 0.75, and 1.0 percentby volume (66, 99, and 132 lb/yd 3) of hooked-end fibers wereused. For Type D fibers (polypropylene), four mixes with 0.1,0.5, 1.0, and 2.0 percent by volume (1.5, 7.5, 15, and 30lb/yd 3 ) were made.It is well known that the aspect ratio of straight fibers has aconsiderable effect on the performance of fresh and hardenedconcrete. Yet it is not practical to assess the relative value ofthe aspect ratio with regard to deformed or modified fibers(corrugated, hooked, collated, or fibrillated) . Hence, the aspectratio was not selected as a parameter for study. In this investigation, four commercially available and commonly used fiberswere selected. Each has substantially different apparent aspectratios (Type A fiber has twice as much as that of Types Band C). The cost difference between the fibers is also substantial (one fiber costs twice as much as another fiber).Fresh Concrete PropertiesRoom temperature, humidity, and concrete temperature wererecorded to ensure that all the mixes were combined undersimilar conditions. The room temperature and humidity varied in the range of l8 -27 C and 33 percent-58 percent,respectively. The concrete temperature range was 20.4 -27.2 C.Test SpecimensFor static flexural testing, beams of size 6 in . x 6 in. x 21in. (152 mm x 152 mm x 533 mm) were cast. Cylinders 6in. x 12 in. (152 mm x 305 mm) were cast for compressionand modulus of elasticity tests. Specimens were made with amechanical table vibrator.TESTS FOR FRESH CONCRETEThe freshly mixed concrete was tested for slump (ASTM C143),air content (ASTM C231), fresh concrete unit weight (ASTMWorkabilityThree tests were performed to determine the workability ofthe mixes: slump, inverted cone time, and vebe time. Testresults indicated that, in general, satisfactory workability canbe maintained even with a relatively high fiber content forcorrugated steel fiber concretes. This was achieved by adjusting the amount of superplasticizer used; the water-to-cementratio remained constant (0.40) for all mixes. For the plainconcrete, about 675 cc of superplasticizer was needed. Themix with 2 percent fibers by volume required 1735 cc of super-
71Ramakrishnan et al.plasticizer, representing an increase of 157 percent. Thesuperplasticizer dosage varied from 860 cc to 1735 cc for thefiber concrete.For the straight steel fiber mixes, balling tendency wasobserved with higher fiber quantities. In the case of hookedend steel fibers, the maximum amount of fibers that could beadded without inducing balling and segregation was 1.0 percent by volume.With higher quantities of polypropylene fibers, the concretehad poor workability and more bleeding and segregation. Formix D4 with 2.0 percent by volume of polypropylene fibers,the water-to-cement ratio was increased to 0.49, and a higherquantity of superplasticizer was added to obtain better workability. In spite of this, the concrete was difficult to place andfinish, resulting in bleeding and segregation. Concretes withhigher quantities of polypropylene fibers also had higherquantities of entrapped air.Based on test results, the relationship between vebe timeand slump for each type of fiber is not affected by fiber contents for the range tested in this investigation. The relationshipis different, however, for other types of fibers.The inverted cone test was specifically developed to measure the workability of FRC in the field. Since both the invertedcone test and the vebe test are based on the energy requirements for flowability and compaction, a linear correlationexists between the two tests.FinishabilityGood finishability was achieved with an appropriate dosageof superplasticizer.Hardened Concrete PropertiesTABLE 2 HARDENED CONCRETE PROPERTIESFOR 1.0 PERCENT FIBER CONTENT(psi)Ee(10 6 OTES: compressive strength at 28 daysEe static modulus at 28 daysf, modulus of rupture at 28 daysf;TABLE 3 HARDENED CONCRETE PROPERTIESFOR 1.5 PERCENT FIBER CONTENTFiberType1;(psi)Ee(10 6 65NOTES: compressive strength at 28 daysEe static modulus at 28 daysf, modulus of rupture at 28 daysf;TABLE 4 HARDENED CONCRETE PROPERTIESFOR 2.0 PERCENT FIBER CONTENTCompressive Strength and Static ModulusThe average values of compressive strength (f ) and staticmodulus of elasticity (EJ for different concretes with fourtypes of fibers and different volumes are shown in Tables 1through 4. Each value in the tables represents the average offour tests.The compressive strength was 7 ,040 psi for plain concrete.For the fibrous concrete with 0.5 percent and 1.0 percent fiber(psi)Ee(106 13,8821,120D1,4701.4412,120440NOTES;TABLE 1 HARDENED CONCRETE PROPERTIESFOR 0.5 PERCENT FIBER CONTENT(psi)(10 6 TES: compressive strength at 28 daysEe static modulus at 28 daysf, modulus of rupture at 28 daysE, static modulus at 28 daysf, modulus of rupture at 28 days1,D1;J; compressive strength at 28 dayscontents, the strength decreased slightly except for Type Afiber. For higher quantities of fibers (1.5 percent and 2.0percent by volume), there was appreciable reduction in compressive strength with Type B and Type C fibers and a tremendous decrease with Type D fibers. The low compressivestrength is due to the entrapped air content (13.9 percent)and low unit weight. The lower strength in fibrous concretesmay be attributed to the difficulty in controlling the air content. The decrease in strength is also due to the increase inyield with a consequent reduction in cement factor. In thisinvestigation, a basic mix proportion was maintained, andfibers in different quantities were added without considering
TRANSPORTATION RESEARCH R ECO RD 122672the fiber factor. If optimum mix proportions were obtainedby trail mixes and used for different fiber contents, then thesame compressive strengths could have been maintained.Pulse VelocityThe results for pulse velocity are also given in Tables 1 through4. The ;wernge pulse velocity it 28 (fays w i s 14,184 fps witha maximum of 14,639 fps (3.21 percent) and a minimum of13,764 fps (2.96 percent), indicating a significant degree ofconsistency and quality control. The results also demonstratethat fiber content has little or no effect on pulse velocity .Finally, it indicates that the addition of steel fibers does notaffect the elastic wave transmitting property of concrete.Static Flexural Strength (Modulus of Rupture)Results of static flexural strength (f,) are tabulated in Tables5 and 6. The values given in the tables represent the averageof four test results. Within-test standard deviation and coefficient of variation values calculated for all the mixes werefound to be very low. However, the variations between thebeams for a given type of mix are likely to be larger for fiberreinforced specimens than those without fibers. This is dueto the difficulty of achieving the same uniform distribution ofthe random oriented fibers . For approximately the same firstcrack deflection, there is an increase in first-crack load forthe concretes with increasing fiber content. It can also beobserved that the flexural strength increases by 25 percent,32 percent, 72 percent, and 81 percent for the concretes withcorrugated steel fiber contents 0.5 percent, 1.0 percent, 1.5percent, and 2.0 percent compared with that of plain concrete.For 2.0 percent by volume polypropylene FRC, the compressive strength was very low, and hence the flexural strengthwas also significantly low. Similarly, for 1.5 percent and 2.0percent fiber volumes of Type Band C fibers, the compressivestrengths were low, and hence the flexural strengths were less.As a result, the direct flexural strength comparison may bemisleading. Figure 1 illustrates the true effect of adding different types of fibers and quantities to a basic plain concretemix. The maximum increase in flexural strength occurred whenType A fibers (hooked end) were added (Table 1). The increasewas higher when higher quantities of fibers were added forall four fiber types . The smallest increase occurred in the caseof polypropylene and straight steel FRC. Appreciable increasein flexural strength occurred when corrugated steel fibers wereadded . Higher quantities caused higher increases up to 2.0percent by volume of fibers.The values of fr/ for different types of fibers with various fiber contents are shown in Figure 2. A linear relationshipbetween the fiber quantity expressed as a volume percentage(p1) and the normalized flexural strength (f,/ ) is indicatedfor each type of fiber. However, the relationship varies fordifferent types of fibers . Therefore , four separate linear equations were obtained for four types of fibers and are usedTABLE 5 MODULUS OF RUPTURE (FLEXURAL STRENGTH)Fiber TypeAcBDFiber f,J,!Vf'cf,f,!Vf'cN OTES:f, modulus of rupturer; co mpressive stre ngthTABLE 6 FLEXURAL STRENGTH AT FIRST CRACKFiber TypeAFiber 3511.35NOTES:he/Vf'cDhefrc flexural strength at first crackr; compressive slrcuglhhe
73Ramakrishn an et al.Load-DeOection BehavioraIID PLAIN TY PER- TYPE B T Y PEc:Jo. 1 xCONTROL0.5xo. 75 %1. 0xCTYPE Dt.5x2. 0xPERCENTAGE OF FI BERFIGURE 1 Fiber content vs. f,l'\/R.2019181716151413lVPE R0TYPE BA significant performance difference of concretes with andwithout fibers is observed in the load-deflection curves, firstcrack strengths, and toughness indexes.Load-deflection curves are a standardized method of quantifying the energy a beam absorbs during its load-inducedflexural deflection. The area under the curve represents theenergy absorbed by the beam .Load-deflection curves were drawn using the data from thestatic flexure test. Typical load-deflection comparison curvesare given for the four fiber contents used in this investigation:0.5 percent , 1.0 percent , 1.5 percent , and 2.0 percent by volume for Types A, B, C, and D , respectively . These curvesare shown in Figures 3 through 6. Unlike plain concrete, FRCdoes not fail in a brittle, catastrophic manner at the formationof the first crack under a clearly identifiable maximum load.Well before signs of significant material distress are visible,the load-deflection curve becomes nonlin ear; microscopicexamination of the specimen revealed fin e cracks. An increasein fiber content caused an increase in first-crack strength forall fiber types as shown in Figures 3 through 6. As explainedTYPE C TYPED 2423222112 .,.,Itto920 676ai,50 . ,08 (I0.5FIGURE 2I1.5PER CENTA GE OF FI BEH2 10.68) Vf'cfrb (2.613p1 7.773)Vf'cttto932t0Percentage of fiber vs. f,l'\/R.( 4.82p11312 2. 5below. These equations are valid for fiber contents varyingfrom 0.5 to 2.0 percent by volume except for Type A fiber(the experimental range used in this investigation). Equation 1 is only valid for fiber contents from 0.5 to 1.0 percentby volume . This range covers almost all the fiber contentscurrently used in the field .frac)(32191817t6t50, 05.!. t5Vf'c.3.35.4( I ne h lFIGURE 3 Comparison of load-deOection curves for hookedend steel fiber.(1)o2 .0/. FI BER BERMOl. 5% FJBER BEAM 0 . 51. FIBER BERMl. D% FIBER BEAM(2)ai,f rc (4. 667pf 6.667).25.2DEFLECT JON(3)0 )(c (frd (l.1 84pf 9. 032)Vf'c(4)where f,., f,b , f m and f,d are the modulus of rupture of concretes with Type A, B, C, and D fib ers , respectively, and p1is the fiber content expressed as a volume percentage. Theequations are valid only for the aspect ratios used in thisinvestigation.0 ,100200300DEF LEC TI ON 10.00 1 iol00FIGURE 4 Comparison of load-deflection curves for straightsteel fiber.
74TRANSPORTATION RESEARCH RECORD 1226242322ai,0)(c 0,0-SERIES0.1% ; 0.5%:21201918171615141312ti109B76The load-deflection curves for different types of fibers arecompared for O.S percent, 1.0 percent, 1.S percent, and 2.0percent by volume fiber contents
Flexural Behavior and Toughness of Fiber Reinforced Concretes V. RAMAKRISHNAN, GEORGE Y. Wu, AND GrRISH HosALLI This paper presents the results of an extensive investigation to determine the behavior and performance characteristics of the most commonly used fiber reinforced concretes (FRC) for potential
A.2 ASTM fracture toughness values 76 A.3 HDPE fracture toughness results by razor cut depth 77 A.4 PC fracture toughness results by razor cut depth 78 A.5 Fracture toughness values, with 4-point bend fixture and toughness tool. . 79 A.6 Fracture toughness values by fracture surface, .020" RC 80 A.7 Fracture toughness values by fracture surface .
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.
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
ASTM C 1604 Flexural Strength FLEXURAL STRENGTH KING MS-D3 UG 28 Days 6.8 MPa (985 psi) ASTM C 78 KING MS-D3 UG MF and KING MS-D3 UG ST 28 Days 8.0 MPa (1160 psi) ASTM C 78 Flexural toughness ASTM C 1550 STEEL FIBERS KING MS-D3 UG STA Peak applied load Toughness as a function of flexure 5 mm 10 mm 20 mm 30 mm 40 mm 8992 lbf (40 Kn)
Steel fiber reinforced concrete (S.F.R.C.) is distinguished from plain concrete by its ability to absorb large amount ofenergy and to withstand large deformations prior to failure. The preceeding characteris tics are referred to as toughness. Flexural toughness can be measured by taking the useful area underthe load-deflectioncurve in flexure.
Toughness indexes were evaluatedaccording to ASTM C1018, and Japan Society of Civil Engineers (JSCE) standard test methods for measuring flexural toughness of fiber-reinforced . values may differ from the result obtained by the load-controlled procedure of ASTM C78 [4]. Some criticisms of the ASTM C1018 were that is was necessary to measure .
D. sassafras and C. aperoluni in order of increasing toughness. Leaf toughness increased with age up to class 3 (except in D. e.rcrlsa) and then decreased slightly between maturity (class 3 and 4) and sen- escence (class 5). Leaf toughness did not increase during age class 3; rather, the toughness values
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