tensile properties of synthetic fiber reinforced mortar

Cement & Concrete Composites 12 (1990) 29-40

Tensile Properties of Synthetic Fiber Reinforced Mortar Youjiang Wang

School of Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, LISA

Victor C. Li a, & Stanley Backer b

"Department of Civil Engineering, t'Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

(Received 5 July 1989; accepted 27 October 1989)

Abstract

This paper reports on an experimental study of synthetic fiber reinforced mortar. The fibers used included aramid, high-strength high-modulus polyethylene, and polypropylene, and they were ran- domly mixed in the matrix at volume fractions below 3%. Tensile prope~.~.es of the composites were measured by the direct ter~ile test under both monotonic and cyclic loading. Workability and the drying shrinkage of the comps, sites are also reported.

Keywords: Fiber cement composites, synthetic fibers, tension tests, toughness, shrinkage, tensile strength, cyclic loading, workability, composite materials, fracture properties, cracking (fra(.'tur- ing).

INTRODUCTION

Concrete, a heavily used construction material, has low tensile strength and low toughness. Recent research has demonstrated that fiber reinforcement at low fiber volume fractions can significantly improve the tensile behavior of concrete. It has been suggested that the tensile stress-crack separation (o-6) curve can be used to characterize the tensile behavior of fiber reinforced cement and concrete (FRC). !,2 In principle, the response of an FRC structure can

*To whom correspondence should be addressed at: Depart- ment of Civil Engineering, University of Michigan, Ann Arbor, M! 48109-2125, USA.

29

be predicted from such a 0-6 curve which describes the material constitutive relation along the matrix cracks in the structure, and from the material stress-strain relationship which applies elsewhere in the structure. In addition, the widely recognized R-curve behavior associated with the formation and growth of the fiber bridging zone in the tip region of a macroscopic crack can be predicted once the 0 -6 relationship is known. Further, the o -6 curve directly reflects the composite internal structure and deformation mechanisms, therefore providing the fundamental information for the purpose of material engineering. For these reasons, considerable interest has been generated in experimental evaluation of the direct tensile behavior of FRC, 3-i° although most of the work has been on the use of steel fibers as the reinforcing elements.

This paper reports on an experimental study of the tensile behavior of synthetic fiber reinforced cement composites. Fibers used in the FRC test specimens included both high-performance fibers (aramids and high-modulus high-strength polyethylene) and low-cost fibers (polypropylene). The properties of these fibers are presented. The tensile properties of FRC were studied by the direct tensile test under monotonic and cyclic loading. Stress versus crack separation curves and fracture energies of FRC are reported. Observa- tions of fiber reinforcement effects on the workability of freshly mixed FRC and on the drying- shrinkage of FRC are also described. The tensile failure mechanisms of these FRC composites have been studied and are reported elsewhere.m ~" ~ 2

Cement & Concrete Composites 0958-9465/90/$3.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

30 Youjiang Wang, Victor C Li, Stanley Backer

MATERIALS

The following materials were used throughout to form the mortar matrix of FRC.

Cement: POrtland cement Type HI, a high early strength cement (sieved before use) Sand: mortar sand passing through a # 8 sieve (size = 2-46 ram) Water: local tap water at room temperature Superplasticizer: Daracem-100* containing sodium and potassium salts of polynaphthalene sulfonic acid, sodium lignosulfonate and formaldehyde, classified as ASTM C-494 Type A, F, or G high-range water-reducing ad- mixture.

Table 1 lists the fibers tested, their generic types, deniers, and the manufacturers. Denier is a measure of fiber linear density expressed as the weight in grams of 9000 m of fiber. It is usually the only quantity provided by synthetic fiber producers regarding the fiber cross-sectional area. Assuming a circular fiber cross-section, the fiber diameter, dr, in mm, can be calculated from

~ 9 4D d~ = 00"0"=7' (1)

where D is the fiber denier and y is the fiber density in g/cm 3. The values of fiber strength and elastic modulus provided by the fiber manufacturers are often in grams per denier, or gpd, which can be converted into the engineering stress unit of GPa from

Stress in GPa = 0'088267' × (stress in gpd) (2)

Staple (discontinuous) fibers of Kevlar 49, Technora, Spectra 900 and Herculon PP were used in FRC. The staple lengths of these fibers were specified by the manufacturers. For each fiber, an identical continuous filament yarn was

*Registered trademark of W& R Grace Co.

also supplied by the manufacturer on which testing of fiber properties was conducted. All these fibers were uncrimped and used in the tests in the as-received state. No attempt was made to remove or modify the fiber surface finishes applied by the fiber manufacturers. The surface finishes of Spectra 900 and Herculon PP, according to their manufacturers, make the fibers readily dispersible in aqueous media. The finishes on the aramid fibers used in FRC did not seem to possess such a property.

FIBER PROPERTIES

Single-fiber tensile tests were conducted to measure the tensile strength and elastic modulus of fibers listed in Table 1. The fiber test specimen was prepared by attaching a fiber separated from a filament yarn onto a pair of cardboard tabs using adhesive tapes and fast-curing epoxy glue, as shown in Fig. 1. The fiber gage length was 25 ram. Multiple tests were performed on an /nstron machine equipped with pneumatic grip- ping jaws, at a crosshead speed of 50.8 mm/min for nylon and Herculon PP fibers and 12.7 mm/min for other fibers. A curve of load versus crosshead displacement was recorded for each test and the fiber tensile properties were calculated. The results are presented in Table 2 along with values from the manufacturers' literature. Typical stress-strain curves for some of the fibers are shown in Fig. 2. All the high-performance fibers,

~ 4 0 mm .-~ 25 m m - ~

fast curing epoxy cardboard

Fig. 1. Specimen configurations for the fiber tensile test.

Table I. Synthetic fibers tested

Fiber Generic type D e n i e r Manufacturer

Kevlar 49 Aramid 1.5 Kevlar 149 Aramid 1"5 Technora Aramid 1.5 Spectra 900 Polyethylene 10.2 Herculon PP Polypropylene 3 Nylon Nylon 66 6

E.I. du Pont de Nemours & Co. E.I. du Pont de Nemours & Co. Teijin Ltd, Japan Allied Corporation Hercules Incorporated E.l. du Pont de Nemours & Co.

Tensile properties of synthetic fiber reinforced mortar

Table 2. Tensile properties of synthetic fibers tested

31

Fiber Test results

Strength Modulus N a

(god) (GPa) (gpd) (GPa)

Literature data

Strength Modulus e'~ b Density (%) (g/cm')

(gpd) (GPa) (gpd) (GPa)

Kevlar 49 25.4 3-31 534-1 69-8 18 Kevlar 149 20.8 2-72 767.6 100.3 16 Teclmora 30.2 3.94 458-8 59.9 20 Spectra 900 . . . . . Herculon PP 1-8 0-14 - - - - 18 Nylon 9.5 0.96 - - - - 20

23 34) 900 117 2-5 1-44 18 2.4 1110 145 1-47 1-47 25 34) 570 70 4-4 1-39 30 2-6 1400 120 3-5 0-97 . . . . . " 0.91 . . . . . 1-14

u N = N u m b e r of tests. he~' = Fiber elongation at rupture. 'e~ = 364.4% from fiber tests. Li terature data not available.

40

35

30

25

20

lb

10

5

0

2.0

Spectrs 900

Kevlar 4 9 ~ ~

St ra in (Yo)

(a)

Technora

1.5

1.0

0.5

0.0

Herculon PP

o 100 2d0 360 400 Strain (Y~)

(b) Fig. 2. Tensile test curves for fibers: (a) aramid and Spectra 900 fibers; (b) Herculon PP fiber. T h e stress-strain curve for Spectra 900 fiber is f rom manufacturer ' s literature)3

Kevlar 49, Kevlar 149, Technora, and Spectra 900, exhibited an almost linear stress-strain relationship up to failure. The Herculon PP fibers tested manifested an elastic-plastic tensile behavior, with a failure strain of 364"4%. Such behavior indicated that the fiber was an undrawn filament and the plastic deformation cor-

responded to the drawing process taking place in the fiber. Where applicable, the measured fiber strength was in good agreement with the literature value. However, the measured fiber modulus was always lower than that given in the literature, because the fiber gage length used was relatively short and, presumably, extension and slippage occurred under the adhesive tapes, thus interfer- ing with the displacement measurement. A much longer specimen gage length is required for accurate measurement of the fiber modulus.

FRC MIXES F O R T H E E X P E R I M E N T A L STUDY

The properties of FRC were studied by the direct tensile test. The effect of some fiber reinforce- ments on the drying shrinkage was also studied with shrinkage-rest-aining specimens. The reinforcing fibers and the mix designations for these tests are listed hl T~ble 3. Except for mix SH, normal-strength mortar was used throughout as the specimen matrix for which the cement/sand/ water ratio by weight was 1/1/0.5. Liquid superplasticizer was also added to improve the workability, at a dosage of 17-6 ml/kg of cement, or 1"5% of matrix volume. High-strength concrete was used as the matrix for mix SH, which con- tained 38.5% by total volume of crushed granite aggregates (maximum size= 10 mm), 1% by total volume of superplasticizer, and silica fume. The cement/water/silica fume ratio by weight for mix SH was 1/0.25/0.133.

Details on fiber reinforced high-strength concrete (including mix SH) can be found elsewhere. '4 The procedures of preparation of fiber reinforced normal-strength mortar (all mixes except SH) are described as follows.

32 Youjiang Wang, Victor C. Li, Stanley Backer

Table 3. FRC mixes for the experimental study

Mir Fiber Vf Lf df designation (%) (ram) (/urn)

K1 Kevlar 49 2 6.35 12 T1 Technora 1 6.35 12 T2 Teclmora 2 6.35 12 T3 Technora 3 6.35 12 T4 Technora 1 12.7 12 T5 Technora 2 12.7 12 S 1 Spectra 900 1 12.7 38 $2 Spectra 900 2 12.7 38 SH" Spectra 900 0.6 6.35 38 Pl Herculon PP 2 5 22 P2 Herculon PP 2 10 22

Technora 1 12.7 12 HI and

Spectra 900 1 12.7 38

"High-strength concrete matrix.

A trial batch was mixed containing 11.35 kg of cement, 11.35 kg of sand, 5.675 kg of water, and 200 ml of superplasticizer. The total volume yielded was 13-595 liters. This volume yielding was assumed to apply to all the FRC matrices prepared for this study and the fiber weight needed for each specimen was determined from this yield information and the fiber density.

Mixing was done in an OMNI Mixert, Model OM-30AV. Instead of using moving blades, which often cause fiber clumping, to achieve the mixing action, this mixer uses a flexible rubber drum wobbled externally to cause random movement of particles to be mixed. It is also possible to control the vacuum inside the mixer during mixing.

The following procedures were followed to prepare the FRC samples. Cement and sand were first dry-mixed for 1 rain; fibers were then added to the mix, and dry mixing was continued for another minute before water containing the superplasticizer was added to the mixer. The mixer rid was tightly closed and the vacuum pump was turned on for the vacuum in t~e mixer to reach 650-700mmHg, then the valve between the mixer and pump was closed; the mixer was turned on at high speed for 1 rain, then at low speed for another minute, and finally at high speed for 1 rain, with a total mixing time of 3 rain.

After mixing, the fresh mix was carried in a bucket and poured into the specimen molds which had been sprayed with fluorocarbon mold- release dry lubricant. A vibrating table was used to achieve compaction. The specimens were

tRegistered trademark of the Chiyoda Technical and In- dustrial Co. Ltd.

covered with plastic sheets for setting before they were removed from the molds 15-24 h later. The shrinkage specimens were left in the air after- wards. The tensile specimens were stored in water at 22°(2 for curing until they were tested at an age of 14 days. The tensile specimens were removed from the water tank for surface grinding and notch cutting, and then put back into the water tank sometime during the curing process. The specimen dimensions will be given below along with the test methods and procedures.

WORKABILITY OF FRESH FRC

In this study, Daracem-100 superplasticizer was used to improve the workability of the fresh FRC mixes. Quantitative measurements of the workability were not carried out. Instead, the workability was judged by the deformability of the fresh mixes during molding. Good workability was observed in all the mixes except mix T3, and specimen compaction was easily achieved by use of a vibrating table. The workability of mix T3 (containing 3% of Teclmora) was extremely poor and the mix had no flowability, as shown in Fig. 3 along with mix P1 which had normal workability. For mix T3, the specimen molds had to be hand-filled and compacted.

DRYING SHRINKAGE OF FRC

Concrete shrinks during setting and initial curing, which may result in wide cracks in concrete structures when the concrete is restrained from free movement. The effect of fiber reinforcement on drying shrinkage was studied with the shrinkage- restraining specimens, illustrated in Fig. 4, which were similar to those used by Swamy & Stavrides? 5 The test specimen was cast around a steel tube which by its high stiffness resisted the tendency for the FRC/mortar specimen to shrink radially. Comparisons were made between the shrinkage crack patterns of FRC specimens and those of plain mortar.

Figure 5 shows two typical mortar specimens after air-drying in the laboratory for 7 days. Major shrinkage cracks can be seen from these pictures and the amount of circumferential shrinkage can be estimated at 0.5%. In contrast, only fine cracks, not recognizable in photographs with equal magnification, developed in various FRC specimens under the same conditions. The crack pat-

Tensile properties of synthetic fiber reinforced mortar 33

Fig. 3. Mix T3 (left) containing 3% of Technora fibers and mix P 1 (fight) containing 2% Herculon PP fibers after mixing.

terns for different FRC and mortar specimens were traced onto transparent films and are illustrated in Fig. 6. Unless the approximate crack widths are explicitly indicated, the cracks in Fig. 6 are all judged as fine cracks w;.th a width of 0.05 mm or less. It can be inferred from these results that fiber reinforcement can reduce or prevent macrocracks in concrete structures due to drying shrinkage. In addition, the amount of drying shrinkage under restrained conditions appeared to be reduced by fiber reinforcement. Further, as is experimentally evident in the FRC containing high-modulus aramid fibers, many of the shrinkage cracks emanating from the steel ring have been arrested.

r ~ A l \ l "

~ . f V V ~ ~'~. A A J "

~ A A ~

~.J V V ~ f ~ . A A i ~

~J V V ~., ~ A A ~

~ . f V V ~

! i ° I

I i ! i

Cross s e c t i o n a l v iew

Fig. 4. Illustration of the specimen for test of FRC drying shrinkage. Dimensions in ram.

TENSILE BEHAVIOR OF FRC

Because of the brittle nature of concrete, valid direct tensile testing of concrete and FRC is always difficult to carry out, for the accuracy of the direct tensile test is strongly dependent on the experimental conditions. To ensure a stable uniaxial tensile test, the testing equipment should satisfy the following basic requirements: (1) the loading fixture should be stiff so as to avoid unstable unloading; (2) no initial undefined stress field on the specimen should be introduced by specimen misalignment; and (3) the fixture should have high rotational stiffness to prevent specimen


Fig. 5. Shrinkage cracks in unreinforced mortar specimens.

ends from rotating. Intricate modifications to the testing machines are often needed to meet such requirements.

In this study, an improved method was developed and used to measure the tensile behavior of FRC. The loading fixture consisted of a pair of loading plates directly tightened to a testing frame and the specimen was attached to the plates with epoxy adhesives. This method is relatively simple and gives satisfactory results for fiber reinforced normal-strength mortar.

Specimen preparation and test procedures Specimens for the direct tensile tests were cast in Plexiglas molds of inner dimensions 76.2 mm × 76.2 mm x 279.4 mm. Sometime 5-10 days from casting, the specimens were removed from the curing water tank for surface grinding and notch cutting. Each specimen was cut into two parts, a longer one with a length of about 155 mm for direct tensile test, and a shorter one with a length of about 120 mm for splitting tensile test. The free surface of the specimen was ground in a water- cooled grinding machine to form a smooth surface to ensure subsequent precision notch cutting. The specimen cast end, opposite to the cut end, was also ground to remove the top layer of 3 mm thickness. This removal of the weak surface layer was necessary for the tensile specimen to be firmly glued to the loading plates. Notches were cut across the midsections of all four sides of the specimen using a diamond saw of 1.6 mm thickness. The notch depth was 12.7 rnm. After grind-

ing and notch cutting, the specimens were returned to the curing tank. All the tests were performed at a specimen age of 14 + 1 days. Finally the specimens were removed from the water tank the day before testing, and allowed to air-dry in the laboratory.

Nominal dimensions of the tensile specimens are shown in Fig. 7. Actual areas of the net specimen cross-sections were measured after the tests and used for calculations of stresses.

The test fixture used in the direct tensile test, shown in Fig. 8, consists of a pair of steel loading plates tightly connected to a servo-hydraulic Instron testing machine. The FRC specimen was attached to the plates using fast-curing epoxy adhesive. By elimination of 'soft' connections between the specimen and the machine, this set- up takes furl advantage of the stiffness of the machine frame to minimize the release of strain energy in the testing fixture and to restrain the end rotations of the specimen. In addition, the in-situ curing of the adhesive excludes specimen misalignment and thus undesirable bending strains in the specimen.

Two LVDTs (linear variable differential trans- formers) were used to monitor the crack opening displacement, both with a displacement range of 5.08 ram. The LVDTs were mounted on two opposite sides of the specimen with aluminum holders glued to the specimen surface. The nominal measuring gage length spanning the notches was 12.7 mm. Signals corresponding to load, crack openings of the two sides, and the

Tensile properties of synthetic fiber reinforced mortar 3 5

(a)

(b)

(c)

(d) Fig. 6. Crack patterns developed in restrained shrinkage test. (a) Plain mortar, Vf=0; (b) mix KI: Kevlar 49 fibers, Vf=2%, Lf-- 6.4 ram; (c) mix PI: Herculon PP fibers, Vf= 2%, Lf= 5 nun; (d) mix P2: Herculon PP fibers, Vf= 2%, Lf = 10 man.

36 Youjiang Wang, Victor C Li, Stanley Backer

machine piston displacement were recorded by a microcomputer.

The direct tensile test was carried out by con- trolling the movement of the Instron machine actuator piston. The loading rate was 0.005 mm/s for the first 1 mm of displacement (200 s), and about 0.025 mm/min for the rest of the test. To avoid premature specimen failures during adhesive curing, proper test procedures should be followed. Details of the test method and the test procedures can be found elsewhere.~ ~. ~6

Splitt ing tensile test Splitting tensile tests were also performed to measure the cracking strength of FRC. The

w

t

. . q

p

Fig. 7.

| i

n o t c h e d c r o s s s e c t i o n

Dimensions (mm) of direct tensile test specimens.

specimens, with dimensions of 76-2 m m x 76.2 m m x 120 mm, were prepared at the same time as the direct tensile specimens. The test set-up is shown in Fig. 9. The specimen was loaded with a pair of hexagonally cross-sectioned rods, the side width of which was 5 mm. The test speed was 0.005 mm/s. The load at matrix cracking, P, identified from the load-displacement plot, was

Fig. 9. Set-up for the splitting tensile test.

~ load cel l ( f ixed to f r a m e )

4 s t ee l c o n n e c t o r d i a m e t e r 76 h e i g h t 73

/

~ , ~q- s t e e l load ing p la te 100 by 100 by 25.4

r-- ~' ~ LVDT

-" ......... ~ e p o x y a d h e s i v e

a c t u a t o r p i s t on

(a) Fig. 8.

(b) Loading f~ture for the direct tensile test. (a) Schematic illustration; (b) photograph. Dimensions in mm.


used to calculate the splitting tensile stress, o.,, which is given approximately by the same equa- tion for the splitting tensile test of a cylinder:

2P ,~,= (3)

:rA

where A is the area of the specimen cross-section between the loading rods.

Test results and discussion FRC mixes listed in Table 3 were tested in direct tension and splitting tension. Six specimens were prepared and tested for each mix (except mix SH) and at least five tests were completed without unexpected specimen failure. Three specimens were tested for mix SH. For each specimen tested, the maximum stress was calculated from the maximum load divided by the net area of the specimen cross-section, and the fracture energy per unit area was computed from the total area under the stress-crack opening width (0-6) curve. These results, together with the splitting tens~e strength, are given in Table 4. Average 0-6 curves for these FRC mixes, shown in Fig. 10, were obtained from both the 0-6 curves for monotonic loading tests and the envelope a -6 curves for cyclic loading tests.

From Table 4 it is seen that the tensile strength measured by the direct tensile tests is generally consistent with that by the splitting tensile test. With the reinforcement of 1-2% of low-modulus Herculon PP fibers (mixes P 1 and P2), the specimen strength is not expected to be influenced strongly by the fibers. Because of their low yielding stress, the Herculon PP fibers could not

provide significant resistance to crack opening, and the FRC post-cracking behavior as indicated by the 0-6 curves for mixes Pl and P2 (Fig. 10) is unsatisfactory. Although the total energy absorp- tion of mix P2 was impressive, the low post-cracking stresses of the Herculon PP FRC are undesirable, particularly in consideration of the serviceability requirement for concrete structures.

Depending on the fiber length and the volume fraction, the strengths of aramid FRC (mixes K1 and T l-T5) are about 40-90% higher than that for FRC with Herculon PP fibers. Fractographic observation revealed that most fibers in aramid FRC formed local bundles and that the matrix crack was deflected around or along these fiber bundles, thus resulting in such strength increase. As indicated in Fig. 10, however, the stresses of aramid FRC decrease rapidly with increase in crack opening, due to the relatively low resistance of fiber bundles to splitting.

For Spectra FRC (mixes S1 and $2), the tensile strength is not significantly different from that for FRC with Herculon PP fibers. However, the post- peak stress of Spectra FRC decreases with crack opening less rapidly than does that of aramid FRC. Significant amounts of Spectra fibers in these composites were observed to be uniformly distributed in the matrix. Here the major mech- anism contributing to the post-cracking resistance was fiber pull-out. As a result, the fracture energy of Spectra FRC is higher than that of most of the aramid FRC mixes.

When Spectra 900 and Technora fibers are blended together as reinforcement, the performance of such hybrid FRC (mix HI) is improved over the FRC using only one species of

Table 4. Tensile strength and fracture energy of FRC

Fiber Mix Vf Lf Direct tensile no. ('X,) (ram)

(Mea) (CV%)

Splitting tensile Fracture energy

(MPa) (CV%) (k J/m:) (CV%)

Kevlar 49 K 1 2 6.35 3.96 11.9 Technora T 1 1 6.35 3.31 5.0

T2 2 6.35 3.11 6.3 T3 3 6-35 3.65 12.2 T4 1 12.7 3.49 10.9 T5 2 12.7 4.17 4.1

Spectra 900 S 1 1 12.7 2.39 8-2 $2 2 12.7 2-70 1.9

SH 0.6 6.35 4.21 5.4 Herculon PP P 1 2 5 2.21 9-5

P2 2 10 2.14 13.3 Technora { [ 1 12.7 and H 1 3-40 2.7 Spectra 900 1 12.7

3"65 9"3 1-31 13-4 3"26 7.7 1.42 17"7 3-58 8"0 1.28 13"4 4"15 12-1 1.87 18-2 3"66 4.1 2.13 30"9 3"96 7.7 4-36 11"0 2.84 7.2 5.98 10"3 2"82 9"i 5"62 16"1

0"39 10"1 2-58 9"6 1"44 8.8 2"02 7'8 4.58 12.4

3.60 5"1 3-80 9.0

38 You/iang Wang, Victor C. Li, Stanley Backer

5 . 0

4 . 8

3 . 0

• 2 " 0 " I +D gO

1 . 0

T4

, T3

.Sl

,5H

Mix Fibre V! L! d! designation % mm IAm

KI Kevlar 49 2 6.35 12 TI Teclmora 1 6.35 12 T2 Technora 2 6.35 12 T3 Technora 3 6.35 12 T4 Technora I 12.T 12 T5 Technora 2 12.7 12 SI Spectra 900 I 12.7 38 $2 Spectra 900 2 12.T 38 SH Spectra 900 0.6 6.35 38 PI Hercuion PP 2 5 22 P2 Herculon PP 2 10 22 HI Tech. & Spec. l&l 12.7&12.7 12~38

0 . 0

Fig. I0.

2 . 0 4 . 0 G.O Ci-ack w i d t h (ms )

Stress versus crack separation curves for FRC.

s.e~r- - - r - -~ i 'r--'-~--'-r--'-~ F ,r~,~'~'~r ]. ~ - - - ~

4 , 6

A

3 . e

1' B

B, 2 . 8 4 . 8 f i . 8

C r a c k w i d t h (me) (a)

S . o ~ . . . . I . . . . I . . . . I . . . .

4 . 0 - -

"~ 3 . e "

v

2 . 6

1 . 8

e . e , , i , , , , 2 . 0 4 . 0 6 . 8

Ca, a©k w i d t h ( a m )

(b)

Fig. ! I. Stress versus crack separation curves for cyclic loading tests. (a) Mixes TI, T2 and T4; (b) mix SI. The zero-stress zero crack opening width origins for specimens T2 and T4 have been shifted horizontally to avoid overlapping of the curves.


fibers (mix S1 or T4), with both relatively high strength and high fracture energy. High cracking tensile strength is observed in mix SH of high- strength concrete matrix. However, as expected from the low volume fraction and short fiber length used in this mix, the post-cracking stress and fracture energy of mix SH are very low.

It is noted that even for FRC mixes exhibiting relatively low fracture energies, these energy absorptions are still much higher than that of plain mortar, typically 0.01 kJ/m 2, and that of concrete containing coarse aggregates, typically 0.1 kJ/m 2.

The formation of fiber bundles in the composite matrix effectively reduces the number of fibers interacting with the cement matrix. This seems to be the reason for the unexpected weak dependence of FRC tensile behavior on the fiber volume fraction observed in most cases. Further improvement in FRC behavior arid cost effective- ness can be expected if increased uniformity of fiber distribution in the matrix is achieved and the fiber/matrix bond characteristics are properly controlled. ~7 Detailed discussions on the tensile failure mechanisms observed in these tests can be found in a separate paper. 12

The o -6 curves for specimens tested with cyclic loadings are shown in Fig. 11. Comparison between the envelope curves in this figure and the o -6 curves for the monotonic loading tests (Fig. 10) indicates no noticeable difference, suggesting that under cyclic loading the primary effect of fiber reinforcement is on the envelope curves.

CONCLUSIONS

The direct tensile test is one of the most important measurements to characterize the properties of FRC. Such tests were conducted on FRC containing 1-3% synthetic fibers, including aramid (Kevlar 49, Teclmora), polyethylene (Spectra 900), and polypropylene (Herculon PP). Stress versus crack opening width (0-6) curves were obtained and fracture energies from the areas under the 0 -6 curves were measured. These tensile properties were dependent on the uniformity of fiber distribution and the failure mechanisms. Toughness improvements of FRC over plain mortar were found to be at least 2-3 orders of magnitude. With the reinforcement of aramid fibers, the tensile strength of the composites was also found to be improved by as much as 90%.

Limited cyclic loading tests were also conducted. Test results indicated that the envelope

load versus crack separation curves for cyclic loading tests were essentially the same as those for monotonic loading tests of corresponding specimens. Kevlar 49 and Herculon PP fibers were mixed in the mortar matrix to form FRC specimens for the shrinkage ~-sts. Under restrained conditions, the test results indicated a reduction in macrocracks due to fiber reinforcement.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the sup- port of the Shimizu Construction Co. Ltd and the Program of System Engineering for Large Struc- tures at the National Science Foundation. Helpful comments of Professors T. Gutowski and P. Schwartz are acknowledged.

R E F E R E N C E S

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10. Kormeling, H. A. & Reinhardt, H. W., Strain rate effects on steel fiber concrete in uniaxial tension. Int. J. Cement Composites and Lightweight Concrete, 9 (4) (November 1987) 197-204.

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Engineering, Massachusetts Institute of Technology, 1989.

12. Wang, Y., Li, V. C. & Backer, S., Tensile failure mechanisms in synthetic fiber reinforced mortar. J. Mat. Sci. (in press).

13. allied Corporation, Spectra high performance fibers for utility applications. Manufacturer’s literature, Allied Signal Fibers Division, Petersburg, VA, USA, 1985.

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tensile properties of synthetic fiber reinforced mortar

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