In the past seve ral ye a r s, an increasing number ofc o n t ractors have placed concrete containingp o l y p ro pylene fibers. Fiber manufacturers have pro-moted the material as a practical altern a t i ve to the

use of welded wire fabric for control of shrinkage andt e m p e ra t u re cracking. They cite the ease with whichfibers can be added to concrete and also state thatadding fibers reduces shri n k a g e, inhibits shri n k a g ec racking, reduces permeability and improves impactand abrasion re s i s t a n c e. There is, howe ve r, conflictingdata concerning the effects of polypro pylene fibers onthe properties of concrete.

This article reviews some of the suggested applica-tions for concrete re i n f o rced with fibers and surveys re-cent studies concerning pro p e rties of the fiber- re i n-f o rced concre t e. We limited our survey to data obtainedf rom tests on concretes containing either 1.5 or 1.6pounds of collated fibrillated polypro pylene fiber percubic yard of concre t e. These are dosage rates re c o m-mended by the two major polypro pylene fiber manufac-t u re r s. Results of the testing are fra g m e n t a ry becauset h e re have been a limited number of tests and test con-ditions investigated. Few of the studies invo l ved fieldmixing of the concrete containing fibers.

REASONS FOR USING POLYPROPYLENE FIBERS

An unre s t rained concrete member will shorten in alld i rections when it dries or cools. But because most con-c rete stru c t u ral members are at least partially re s t ra i n e d ,tensile stresses build up when the concrete dries orc o o l s. The stresses are about the same as those thatwould occur if the concrete had been allowed to contra c tf reely and had then been pulled back to its ori g i n a llength. When these stresses exceed the tensile stre n g t hof the concre t e, the member cra c k s. Me a s u res that canbe taken to control this cracking include reducing thepotential shrinkage of the concre t e, providing joints toc o n t rol crack location and adding nonstru c t u ral re i n-f o rc e m e n t .

Even if joints are used to control crack location, cra c k smay still occur between joints. And in stru c t u ral re i n-f o rced concre t e, added measures may be needed to con-

t rol shrinkage and tempera t u re cracking. Goals for theengineer and contractor are to reduce the number ofc racks and to keep ones that do form from opening uptoo wide. Adding polypro pylene fibers to the concre t ehas been suggested as one way of achieving these goals.

Other suggested applications for concrete containingp o l y p ro pylene fibers include stru c t u res such as medianb a r riers that are subjected to impact loads, placementsw h e re all materials must be nonmetallic and areas re-q u i ring materials that are resistant to alkalis and otherc h e m i c a l s.

EFFECTS OF FIBERS ON FRESH CONCRETE

Slump effects

When fibers are added to the concrete slump will de-c rease (1, 2, 3, 4).* Ac c o rding to one fiber manufactur-e r’s re p re s e n t a t i ve, the reduction in slump depends onthe length of fiber used; longer fibers cause a gre a t e rslump reduction. Data from recent labora t o ry and fieldtests indicate slump losses ranging from 1⁄2 inch to slight-ly over 3 inches, but there is little correlation betwe e nslump reduction and fiber length (Table 1).

Co n t ractors are cautioned not to add water to re s t o relost slump. Even at the lower slump, workability of fiberre i n f o rced concrete is said to be adequate for placing,compacting and finishing the concre t e. Adding water

Polypropylenefibers in concrete

What do the tests tell us?

BY W. R. MALISCH

Initial Final Fiber ReferenceSlump Slump Length (see end of

(inches) (inches) (inches) article)

31⁄2 3 2 2

51⁄4 23⁄4 2 2

63⁄4 43⁄4 11⁄2 3

5 1.9 2 4

4.9 2.1 2 4

41⁄2 21⁄2 3⁄4 6

TABLE 1. EFFECT OF POLYPROPYLENEFIBERS ON CONCRETE SLUMP

w o n’t improve workability but will reduce strength andi n c rease shri n k a g e.

In some cases, such as for slipformed median barri e r s,use of a more cohesive concrete with a lower slump mayactually be beneficial. Edges of the in-place concrete areless likely to crumble and fall away, and the barrier itselfw o n’t subside as much before setting occurs.

One re s e a rcher (5) suggests that polypro pylene fibersmight act as dampers or energy absorbers during thec o n c rete compaction pro c e s s. He recommends peri o d-ic density checks on the compacted concrete to detectpoor compaction that might cause increases in poro s i t yand poor perf o rm a n c e. Data from three studies (2, 3, 4)s h ow that the fibers had little or no effect on unit we i g h tof the concre t e. Howe ve r, unit weight wasn’t measure don the in-place concre t e.

Plastic shrinkage cracking

A test similar to one described in CONCRETE CO N-S T RUCTION (September 1985, page 775) was used toc o m p a re early cracking behavior of slabs made withplain concre t e, concrete re i n f o rced with we l d e d - w i ref a b ric and concrete containing polypro pylene fibers (6).Cement content of the concrete was 494 pounds per cu-bic yard at a water-cement ratio of 0.61 and with a 1-inchmaximum size aggre g a t e. Slabs we re 2 feet wide, 3 feetlong and 2 inches thick. Edges of the concrete we re re-s t rained and surfaces of the freshly placed concrete we reimmediately exposed under rapid drying conditions.Fibers significantly reduced cracking potential as deter-mined from a weighted ave rage value that took into ac-count crack length and width.

Using a different test method than the one descri b e da b ove, a No rwegian re s e a rcher (3) exposed fiber- re i n-f o rced and plain concrete and mortar samples to seve red rying conditions shortly after casting. Doughnut-shaped concrete specimens we re cast with a steel ri n gin the center. The steel ring re s t rained shri n k a g e, caus-ing the specimens to crack as they dried. The re s e a rc h e rconcluded that even modest additions of polypro py l e n efibers resulted in a considerable reduction of the sensi-tivity of concrete and mortar to plastic shrinkage cra c k scaused by early dry i n g .

EFFECTS OF FIBERS ON HARDENED CONCRETE

Data re p o rted here are generally for concretes con-taining the manufacture r s’ recommended fiber dosagea m o u n t s, either 1.5 or 1.6 pounds of fiber per cubic yardof concre t e. This re p resents a volume concentration ofa p p roximately 0.1 perc e n t .

Drying shrinkage

T h e re have been conflicting re p o rts concerning the ef-fect of polypro pylene fibers on shrinkage that occurs af-ter concrete has hardened. A pilot study re p o rted in 1982(7) indicated that polypro pylene fibers reduced shri n k-age of plain concrete specimens by about 75 perc e n t .Howe ve r, more recent test results don’t agree with thesef i n d i n g s. One re s e a rcher (2) concluded that the totals h rinkage for concrete containing polypro pylene fibersis approximately equal to shrinkage of concrete withoutthe fibers. Another re s e a rcher (4) found that fiber con-c rete shrinks less than plain concrete but that the differ-ence in shrinkage is small. Sh rinkage of 3000 psi con-c rete with fibers was 6.8 percent less than that of plainc o n c re t e, and shrinkage of 4500 psi concrete with fiberswas 4.9 percent less than that of plain concre t e.

Di f f e rent mix pro p o rt i o n s, specimen size s, dry i n gconditions and testing pro c e d u res we re used by the re-s e a rchers in the studies mentioned above. And conclu-sions we re based upon testing of a limited number ofs p e c i m e n s. A soon-to-be-published study (8), based ona larger number of tests, concludes that polypro py l e n efibers reduce plastic and drying shrinkage when added

Plain Fiber-reinf. Fiber ReferenceConcrete Concrete Length (see end ofStrength Strength (inches) article)

(psi) (psi)

5630 6470 2 24880 5370 2 27290 7230 11⁄2 32810 2690 2 44750 4880 2 45700 5850 3⁄4 85700 5270 1 84250 4630 21⁄4 95930 6260 2 105840* 6480* 2 10

TABLE 2. EFFECT OF POLYPROPYLENE FIBERSON CONCRETE COMPRESSIVE STRENGTH

Plain Fiber-reinf. Fiber ReferenceConcrete Concrete Length (see end ofStrength Strength (inches) article)

680 700 2 2680 725 2 2530 555 2 4815 800 2 4865 870 3⁄4 8865 890 1 8570 665 21⁄4 9750 755 2 10730* 760* 2 10

TABLE 3. EFFECT OF POLYPROPYLENE FIBERSON CONCRETE FLEXURAL STRENGTH

* These specimens cured 3 days wet and 25 days dry before testing.

* These specimens cured 3 days wet and 25 days dry before testing.

at the manufacture r s’ recommended dosage ra t e s. Co n-c retes containing fibers consistently exhibited lesss h rinkage than plain concre t e. But no conclusions we remade re g a rding actual percentage differences found as afunction of the amount or type of fiber used. The re-s e a rcher stated that because of scatter in individual testdata, shrinkage testing re q u i res a large number of testspecimens in order to interpret tre n d s.

Compressive, flexural and tensile strength

Co m p re s s i ve strength comparisons between plainc o n c rete and concrete with fibers we re made in seve ra lrecent studies (2, 3, 4, 8, 9, 10). Results are summari ze din Table 2. Some re s e a rchers have concluded that therea re no significant compre s s i ve strength differences be-t ween mixes with and without polypro pylene fibers.Others have found a modest increase in compre s s i ves t rength when fibers are added.

Results of tests made on beams tested in flexure ares u m m a ri zed in Table 3. These tests (2, 4, 8, 9, 10) showeither no effect of fibers on flexural strength or a smalli n c rease in modulus of ru p t u re. Di rect and indirect ten-sile testing of concrete has indicated little improve m e n tin tensile strength when fibers are added (2, 4). It shouldagain be noted that most of the results in Tables 2 and 3a re based upon a limited number of tests.

Impact, abrasion and fatigue resistance

In two studies (2, 7), impact tests have been conduct-ed on concrete made with polypro pylene fibers. In oneof the studies, significantly improved resistance to im-pact was observed in one test but a second test showe dlittle improvement in impact re s i s t a n c e. The re s e a rc h e ra t t ributed this va riability to nonuniform fiber distri b u-tion (2). In the other impact resistance study, va ri a b i l i t yin the test results was large and the re s e a rcher expre s s e du n c e rtainty about the individual test results as well asthe test itself (7).

In one study (11), concrete containing polypro py l e n efibers demonstrated an ability to sustain large deform a-tions without shattering. Two specimens 21 inches highwith a 6x6-inch cross-section we re tested in compre s-sion. One contained polypro pylene fibers (1.5 poundsper cubic yard) and the other was made with plain con-c re t e. Both we re loaded at a deformation rate of 0.025inch per minute and the maximum load carried by eachwas approximately the same. Howe ve r, the plain con-c rete specimen fra c t u red completely after short e n i n g0.32 inch while the fiber concrete shortened 2.00 incheswithout breaking apart completely.

T h e re isn’t much re p o rted data concerning abra s i o nresistance of polypro pylene fiber concre t e. One study (2)indicated ve ry little difference in abrasion resistance be-t ween plain concrete and concrete containingp o l y p ro pylene fibers. Details of the testing method we renot given. Another study (12) indicated improved abra-sion resistance when polypro pylene fibers we re added tothe concre t e. A rotating cutter method (U.S. Army Co r p s

of Engineers method CRD-C 52) was modified by using a20-pound load instead of a 10-pound load and an abra-sion time of 6 minutes instead of 2 minutes.

Po l y p ro pylene fibers significantly increased flexura lfatigue resistance of plain concrete in one study (2).Specimens we re subjected to re p e t i t i ve loads at about 60p e rcent of the modulus of ru p t u re. Co n c rete with fiberswithstood over twice as many cycles as plain concre t e.

SUMMARY

Conclusions here are based pri m a rily upon labora t o-ry studies of concretes containing about 0.1 percent byvolume of polypro pylene fibers. This dosage corre-sponds with the manufacture r s’ recommended dosagesof 1.5 or 1.6 pounds of fiber per cubic yard of concre t e.Data on concretes containing higher volume perc e n t-ages of fiber we re n’t re v i e we d .

Evaluations of data giving conflicting results are com-plicated by the fact that many of the testing methodsused by different investigators have n’t been standard-i zed. This and the limited number of tests conducted ins e ve ral of the studies make many of the conclusions ten-t a t i ve at best.

• Adding fibers reduces the slump of concre t e. There ap-pears to be less plastic shrinkage and less plastics h rinkage cracking when concrete contains polypro py-lene fibers.

• Drying shrinkage after hardening is reduced whenfibers are added to concrete but the amount of the re-duction is difficult to predict using current testingm e t h o d s.

• Some increase in compre s s i ve and flexural strength ispossible when fibers are added to concrete and an in-c rease in fatigue resistance has been noted by one re-s e a rc h e r.

• Results of two abrasion resistance studies are contra-d i c t o ry and there are n’t enough test results to perm i td rawing a conclusion. There is also insufficient datac o n c e rning impact re s i s t a n c e.

If the effects of polypro pylene fibers on concrete pro p-e rties are to be conclusively demonstrated by labora t o-ry tests, more data are needed. Field observations of in-place concrete are perhaps a better indicator ofp e rf o rmance at this time than are the results of labora-t o ry tests. Another article in this issue gives some exam-ples of field perf o rm a n c e.

References1. Guirguis, B. E. and Potter, R. J., “Polypropylene Fibres inConcrete,” Technical Report TR/F90, Cement and ConcreteAssociation of Australia, 1985

2. Aitcin, Pierre-Claude et al, “The Use of Fibre ReinforcedConcrete for Highway Rehabilitation,” Etude #231, IGM85-305-231, Industrial Materials Research Institute, NationalResearch Council of Canada, 1985

3. Dahl, P. A., “Plastic Shrinkage and Cracking Tendency ofMortar and Concrete Containing Fibermesh,” FCB Cementand Concrete Research Institute, Norway, 1985

4. Litvin, A., “Report to Wire Reinforcement Institute onProperties of Concrete Containing Polypropylene Fibers,”Construction Technology Laboratories, Portland Cement As-sociation, 1985

5. Hannant, D. J., “Polypropylene Fibres in Concrete, Mor-tar, and Cement,” Chapter 7 in Fibre Cements and FibreConcretes, John Wiley & Sons, 1978, page 96

6. Kraai, P. P., “Crack Control Methods: Welded Wire Fabricvs. CFP Fibers,” for Fibermesh Company, 1985

7. Zollo, R. F., “Collated Fibrillated Polypropylene Fibers inFRC,” Fiber Reinforced Concrete: International Symposium,SP 81, American Concrete Institute, 1984, page 397

8. Zollo, R. F. et al, “Plastic and Drying Shrinkage in Con-crete Containing Collated Fibrillated Polypropylene Fibers,”to be published in 1986

9. Hanna, A. N., “Preliminary Evaluation of Forta Fibre Rein-forced Concrete,” performed for Forta Corporation by Con-struction Technology Laboratories, Portland Cement Associ-ation, 1981

10. “Static Load Test of Fibermesh vs. Welded Wire Fabric,”F.E.D. Report No. 5, Fibermesh Inc. (Tests performed byWiss, Janney, Elstner Associates, Inc.), 1985

11. “Concrete Shatter Resistance Under Compressive Load-ing of Fibermesh vs. Plain,” F.E.D. Report No. 6, FibermeshInc. (Tests performed by Paul P. Kraai), 1985

12. Kraai, P. P., “Abrasion Testing of Fibermesh Concrete,”for Fibermesh Company, 1984

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