http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

Concrete reinforced with irradiated nylon fibers

Gonzalo Martínez-Barreraa)

Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA), Facultad de Química,Universidad Autónoma del Estado de México, Km.12 de la carretera Toluca-Atlacomulco,San Cayetano 50200, Mexico; and Laboratory of Advanced Polymers & Optimized Materials(LAPOM), Department of Materials Science and Engineering, University of North Texas,Denton, Texas 76203-5310

Carmina Menchaca-CamposCentro de Investigación en Ingeniería y Ciencias Aplicadas (CIICAp), Universidad Autónoma delEstado de Morelos, Cuernavaca, Morelos 62210, Mexico; and Center for the Study of Matter atExtreme Conditions (CeSMEC), Florida International University, Miami, Florida 33199

Susana Hernández-López and Enrique Vigueras-SantiagoLaboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA), Facultad de Química,Universidad Autónoma del Estado de México, Km.12 de la carretera Toluca-Atlacomulco,San Cayetano 50200, Mexico

Witold BrostowLaboratory of Advanced Polymers & Optimized Materials (LAPOM), Department of MaterialsScience and Engineering, University of North Texas, Denton, Texas 76203-5310

(Received 25 May 2005; accepted 3 November 2005)

Polymeric fibers have been used since the 1980s for improvement of the concrete.However, high mechanical performance has been obtained at high cost and usingcomplex technologies. At least two parameters are important here: dimensions andsurface characteristics of the fibers. We have modified nylon 6,12 fiber surfaces by5, 10, 50, and 100 kGy gamma irradiation dosages. Tensile strength of the irradiatedfibers was determined and then the fibers mixed at 1.5%, 2.0%, and 2.5% in volumewith Portland cement, gravel, sand, and water. The compressive strength of the fiberreinforced concrete (FRC) was evaluated and the results were compared with resultsfor similar materials reported before. The highest values of the compressive strengthof FRC are seen for fibers at 50 kGy and 2.0% in volume of fiber; the strength is122.2 MPa, as compared to 35 MPa for simple concrete without fibers. We advance amechanism by which the fiber structure can be affected by gamma irradiation resultingin the compressive strength improvement of the concrete.

I. INTRODUCTION

Micro- or macro-synthetic nylon fibers have been inuse since the early 1980s for secondary temperature-shrinkage reinforcement in shotcrete and concrete. In themicro case, monofilaments and fibrillated shapes areused; the monofilaments are available in various lengthsfrom 13 to 38 mm, with the 20-mm size used most often.Since these fibers are very thin, their number per weight(fiber count) is in the range of millions per 1 kg ofconcrete, for example, 77 million/kg when using fiberswith 19 mm length.1

Several improvements can occur when micro-synthetic nylon fibers are added to the concrete, typicallyat dosage rates of 0.6 to 0.9 kg/m3 of mixture.

The first potential improvement is reduction in crazecracking due to compressive stress, plastic shrinkagecracking, and plastic settlement. The reduction in crack-ing and settlement prior to setting produces concrete withimproved long-term durability.

The second improvement relates to increasing highimpact resistance by lowering the extent of stretch-ing and pull-out of the fibers, which occur for largestrains, resulting in failure of the matrix at relativelylow loads. It would be advantageous if the fiber re-inforced concrete (FRC) could be designed to sup-port an increasing load after the cracking of the matrix.This may be achieved by improving the stress trans-fer from the matrix to the fibers after matrix failure.The transfer depends on the aspect ratio of the fibersand on interfacial-shear strength.1 Alternative mecha-nisms of craze formation are discussed in detail byDonald.2

a)Address all correspondence to this author.e-mail: gonmar@uaemex.mx

DOI: 10.1557/JMR.2006.0058

J. Mater. Res., Vol. 21, No. 2, Feb 2006 © 2006 Materials Research Society484

http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

The third improvement involves creation of a three-dimensional (3D) network of reinforcement with supe-rior fiber/mix bonding. The integration of fibers evenlydisperses stress throughout the reinforcement network,modifies the micro-macro cracking mechanism, and en-hances durability. Fibers are proactive crack-fighters thatincrease the usable life of shotcrete and concrete.

The final potential improvement is reduction of bleed-ing and thus the number of bleed channels, resulting inless water migration to the concrete surface. This actionhelps to control the water/cement ratio and produces aconcrete with less permeability, greater strength, and im-proved toughness.

For dosages of 1.8 kg/m3 or more, the synthetic fibersyield a superior secondary reinforcement for precast con-crete, shotcrete, security and seismic applications, indus-trial slabs, and ultrathin white topping. The increasedfiber volume and decreased fiber spacing improve con-crete’s residual strength, impact, and penetration resis-tance, fatigue strength, and ultimately reduce the poten-tial of catastrophic failure.

We used nylon fibers to reinforce laminated compos-ites based on cement matrices; the fiber volume anddiameter were varied, and changes in three distincttoughness criteria, namely impact energy W, fracture en-ergy G, and toughness index I100 were pursued. It isknown that W increases with increasing fiber volumefraction and fiber diameter. As advocated by Muhua andcoworkers,3 I100 is a good criterion of toughness of fibermat laminated cement composites.

The nylon-FRC can sustain a higher stress level thancement when loads are applied in the flexural mode.Kurtz and Balaguru report for a low fiber volume fraction(0.1%) sustainable stress much lower than the postcrackstrength.4 It is important to test different ranges of stresslevels (creep-time behavior) when nylon-FRC beamshave to be used: load sustained indefinitely (low stress),creep failure (intermediate stress), and rapid failure (highstress). Apparently, adding nylon fibers to concrete hasvery little effect on the tensile and bending strength.

Balaguru used nylon single-filaments with lengthsvarying from 19 to 50 mm and found shrinkage crackreduction of cement composites during the initial andfinal setting periods.5 The 19-mm-long fibers are moreeffective in lean mortars and concrete than they are inrich cement mortar.

It is also important to consider the directions the fibersassume when introducing them into the mixing system toachieve uniform distribution; a wet-mix allows a moreuniform distribution of the fibers in the mixture than adry-mix. The former also prevents the fly-away phenom-enon at the point of discharge from the nozzle.

Macro-synthetic fibers are distinguished by their typi-cally longer length (with 38 mm considered a minimum),greater fibril cross-section, and dosages rates from 3 to

9 kg/m3 of mixture, higher than those of micro-fibers.The macro-synthetic fibers provide shrinkage benefitsand post-first crack advantages. Furthermore, the addedoption of using fibers at elevated dosages levels maylimit the damaging effects of a seismic event or concus-sive energy. In such cases the fibers hold the elements ofthe fractured concrete together, reducing the potential forcatastrophic failure as well as the danger of fragmentsflying away at a variety of directions from the face of theconcrete.

One can add elevated dosages of macro-synthetic fi-bers exceeding 35 kg/m3. In such a case, the fibers con-tribute to the post-first-crack properties of shotcrete andconcrete, among them toughness and average residualstrength. There exists a reduction in plastic and dryingshrinkage cracking, an increase of the impact and surfaceabrasion resistance and fatigue strength, as well as a re-duction in permeability.

Balaguru and coworkers have added 4.75 kg/m3 ofnylon-6 fibers to concrete thus improving the postcrackresistance; the fibers are not affected by the alkaline en-vironment in concrete, demonstrating their long-termdurability in the composite.6 This was established fol-lowing an accelerated aging process of concrete in lime-saturated water at 50 °C and measuring the flexuraltoughness.

There are three main routes toward improvement ofmechanical properties of FRC, namely by: (i) varyingthe diameter/length ratio and the concentration of fibers,7

(ii) a modification of the matrix, and (iii) an alteration ofsurface characteristics. We have decided to explore theuse of gamma radiation, particularly to modify the sur-faces of the fibers. It well known that the gamma radia-tion causes modifications of the polymer structure viathree main processes: scission, crosslinking and graftingof chains.8–13 Other procedures used, such as chemicalattack or thermal treatment, are costly and time consum-ing. We expected that the ionizing energy can improvecompatibility between irradiated fibers and brittle ce-ramic matrix. It is also known that in certain cases thecrystallization can be achieved through breaking chainsin the amorphous zones of the fibers.14,15

In the case of nylon monofilaments subject to gammairradiation, for low doses (up to 50 kGy), there is anincrease in the temperature of fusion due to the partialand reparable damage that the fibers suffer; that is, thereexists a self-healing process. This has been reported byone of us and coworkers16 and the effect assigned tobreaking the C–N bonds. However, the effect is insuffi-cient to counteract an ongoing scission process; appar-ently an oligomer re-polymerization is taking place.16,17

Consequently, a permanent damage occurs; the oligo-mers so formed are responsible for a decrease in thetemperature of fusion. Thus, a chain scission duringirradiation caused by the Compton effect occurs, mainly

G. Martínez-Barrera et al.: Concrete reinforced with irradiated nylon fibers

J. Mater. Res., Vol. 21, No. 2, Feb 2006 485

http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

in the amorphous zone of the fibers, as described previ-ously.9

The crystallization process can be followed by Ramanspectroscopy. The band intensities become higher andincrease when the radiation doses increases.11,16 Varia-tions in the length of methylene sequences are seen, par-ticularly at 2880 and 2846 cm−1. We recall that methy-lene bending bands are used as intensity self standardsfor qualitative analysis since these vibrations are in-dependent of intermolecular interactions. Moreover, CHbending at 1440 cm−1 and CH stretching vibrations at2900 cm−1 vary with the irradiation dosages.18

An increase in the crystallinity due to the re-polymerization of the fibers is accompanied by chainreorientation. Thus, crystallization control in nylon fibersresults in some degree of control of the resulting me-chanical properties—compressive strength and impactresistance in particular.

In the present work, we studied gamma radiation ef-fects on nylon fibers and their use for mechanical im-provement of Portland concrete. On this basis, we pro-pose a novel technology for manufacturing polymerreinforced concrete.

II. EXPERIMENTAL

A. Specimen preparation

Before preparing the concrete specimens, one set ofcrystalline Nylon 6,12 fibers (Dupont, Wilmington, DE)whose diameters varies from 30 to 40 �m were cut to theapproximate length of 5 mm. The fibers so obtained wereirradiated and mixed into the concrete. The effect of thefiber length was not investigated.

For preparing the concrete specimens, natural silica assand (according to ASTM C-778) and gravel from a localcompany were used, as well as Portland cement (CruzAzul, Monterrey, NL, Mexico), together with crystallineNylon 6,12 fibers. Five different lots identified by A, B,C, D, and E were prepared, each one a different day.

The proportions of components in the concrete were1/2.75 for cement/aggregates. The water/cement ratio of0.485 was used according to ASTM C-305. The fibercontents were 1.5%, 2.0%, and 2.5% in volume. Aftermixing, the concrete cylindrical specimens (2 in. diam-eter and 4 in. long) were placed in a controlled tempera-ture room at 23.0 ± 3.0 °C for 20–72 h, with the surfaceexposed to moisture in air and no less than 50% humidityaccording to ASTM C-511.

B. Mechanical tests

The tensile tests on the fibers were carried out in aZwick dynamometer (Zwick, Ulm, Germany) accordingto the ASTM D638 standard with the test speed of10 mm/min.

The compressive tests of the concrete cylindricalspecimens were carried out in an Instron Universal Test-ing machine Model 1125 (Norwood, MA) according tothe ASTM C-109M standard. The testing allowed toler-ance for the specimens was 28 days ± 12 h and the chargespeed was between 91 and 184 kg/s, holding the chargeuntil reaching the maximum value to assure the reliabilityof the test.

C. Morphological characterization

Taking account the length and diameter of the fibersand the expected gamma irradiation effects, the follow-ing contrasting process was used. First the fibers weresubmerged in OsO4 for 48 h and cooled in liquid nitrogenfor 0.5 h. Then the fibers were vacuum-coated with car-bon (coating thickness between 3 and 10 nm) in avacuum pump (E.F. Fullam) at 50 mTorrs. Finally, thefiber surfaces were analyzed by scanning electronmicroscopy (SEM) in a JEOL model JSM-5200 machine,in the secondary-electron mode.

D. Irradiation procedure

Crystalline Nylon 6,12 fibers were exposed to varyinggamma radiation doses using a 60Co source. The experi-ments were performed in air at the room temperature;the dosages were 5, 10, 50 and 100 kGy at the dose rateof 6.10 kGy/h; the fibers were placed in packets of 50into a capillarity tube. The irradiation was provided by a651 PT Gammabeam Irradiator manufactured by theAtomic Energy of Canada Ltd. (AECL, Chalk River,Ontario, Canada), and located at the Institute of NuclearSciences of the National Autonomous University ofMexico.

III. RESULTS

In Fig. 1 we show average values of the tensile stressat yield points of the fibers as a function of the irradiation

FIG. 1. Tensile stress at yield point of the nylon fibers.

G. Martínez-Barrera et al.: Concrete reinforced with irradiated nylon fibers

J. Mater. Res., Vol. 21, No. 2, Feb 2006486

http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

dose. We observe that at the 50 kGy dose the tensilestress at yield amounts to 701.6 MPa, that is, 143.9%above the value for non-irradiated fibers. The irradiationenergy causes structural modifications of the fibers.Above 50 kGy, the tensile stress at yield point valuesdecrease to 387.2 MPa for 100 kGy, approximately40.8% more with respect to raw fibers. Similar tensilestrength values have been reported by Olivares and co-workers19 for crystalline nylon fibers subjected togamma irradiation: 510 MPa for raw nylon and decreas-ing to 400 MPa when reaching 100 kGy. The irradiationgenerates the formation of low molecular weight oligo-mers created by scissions of the chains. The oligomersact as plasticizing agents, enabling re-crystallization.

Another important mechanical parameter characteriz-ing effects of addition of fibers into concrete is the tensilestrain; its behavior is shown in Fig. 2. For raw fibers,application of the tensile stress results in the averagevalue of the strain � 17.4%. When the radiation dose isincreased, the strain increases up to 22.0% for 50 kGy,and then decreases down to 10.1% for 100 kGy. Similarvalues have been reported before, with the percentage ofelongation at break being a maximum of 16% and aminimum of 6% when the gamma irradiation doses in-crease in comparable intervals.19 Moreover, it has beenreported that the degree of crystallinity changes whengamma irradiation is applied, increasing from 15.5% to19.5% when 15 kGy radiation is applied, and then de-creasing to 17.5% for 100 kGy.16 In crystalline nylon,the effects of gamma radiation can be quantified follow-ing either the (002) crystallographic plane or the reflec-tion at a low angle (5°), which reflects the spatial ar-rangements of the crystallites.19

Two further important features can be observed: (i) amaximum value of the strain is obtained for 50 kGy(22%) and (ii) a minimal strain value at 100 kGy is lowerthan the strain for raw fibers by 40.4%. Thus, values of

both stress and strain at yield point decrease at least 40%when 100 kGy of radiation is applied; we have reportedsimilar results for irradiated polypropylene fibers mixedwith concrete.20

Average data for stress and strain are reported inFigs. 1 and 2. Figure 3 displays two consecutive stagesof behavior caused by irradiation. In the first stage, thetensile stress and the tensile strain are increasing (fromunirradiated to 50 kGy); in the second stage both theseparameters are decreasing (from 50 to 100 kGy). More-over, a large elastic zone and thus high elastic modulaeare obtained, accompanied by high strain values (22% onthe average). Thus, we have created a reinforced concretewith high elastic modulus and high deformability, a fea-ture not typical for hydraulic concrete.

Clearly, such mechanical behavior is a consequence ofmorphological changes, which we have analyzed byscanning electron microscopy (SEM). For raw fibers, ho-mogeneous surfaces are observed (Fig. 4). Applicationof the ionizing energy at 5 kGy results in formation ofsmall crazes (Fig. 5) that correspond to the increment of65.7% for tensile stress. When the irradiation energy is

FIG. 3. Tensile stress versus strain of nylon fibers.

FIG. 4. SEM micrograph of nylon fibers.FIG. 2. Tensile strain at yield point of the nylon fibers.

G. Martínez-Barrera et al.: Concrete reinforced with irradiated nylon fibers

J. Mater. Res., Vol. 21, No. 2, Feb 2006 487

http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

increased to 10 kGy, the morphology begins to showscraps (Fig. 6) while the tensile stress and tensile strainincrease; thus, chain scission is apparently taking place.In general, regions with less crystallinity and with largerinterfaces per unit volume are damaged more easily. At50 kGy, minor irregularities can be observed on the fibersurfaces (Fig. 7), apparently a result of crosslinking ofthe polymer caused by the ionizing energy. At this stage,the maximum values of both tensile stress and the tensilestrain are reached. Finally, when mechanical propertiesdeteriorate at high energies (100 kGy), visible smallspherical objects are formed on surfaces of the fibers;see Fig. 8.

In crystalline regions, two phenomena take place:melting temperature decrease and the heat capacity in-crease along with increasing gamma radiation. As con-cluded by Xenopoulos and Wunderlich, changes in therelative number of methylene and amide groups as wellas hydrogen bond scissions in the chain can explain theobserved behavior in the melting temperature as a func-tion of the dose.21 Indeed, the damage in semicrystallinepolymers induced by gamma radiation spreads from the

surface inward, and at least three stages can be seen. Inthe first stage, the radiation-induced chemical reactionsoccur solely in the noncrystalline regions. A second stageinvolves the decrease of microcrystal size with radiationdose, with activation energy smaller than that of firststage. Finally, in the third stage, given the supplied en-ergy, the chemical bonds within the crystallites are af-fected, including C–N bonds. Moreover, the meltingpoint depression is due to the addition of low molecularweight nylon oligomers.

The compressive strength results for all concretespecimens are shown in Table I; the averages are dis-played in Fig. 9. It is important to observe in Table Ivariations of values according to the lots (A to E). Thedifferences are small, indicating adequate mixing.

In Fig. 9, values vary from 31.1 to 37.5 MPa for con-crete with nonirradiated fibers; we have obtained similarvalues for concrete containing polypropylene fibers (35.9to 41.7 MPa).20 These values are comparable to 35 MPafor simple concrete (SC). Moreover, following the analy-sis for concrete without irradiated fibers (Fig. 9), thehighest values are obtained when adding 2.0% fiber. The

FIG. 5. SEM micrograph of nylon fibers irradiated at 5 kGy.

FIG. 6. SEM micrograph of nylon fibers irradiated at 10 kGy.

FIG. 7. SEM micrograph of nylon fibers irradiated at 50 kGy.

FIG. 8. SEM micrograph of nylon fibers irradiated at 100 kGy.

G. Martínez-Barrera et al.: Concrete reinforced with irradiated nylon fibers

J. Mater. Res., Vol. 21, No. 2, Feb 2006488

http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

initial bond between the fiber and the concrete can beattributed to physical adhesion and static friction causedby the surface finish of the fiber. There is a noticeabletendency of the fiber to slip and to show compressiveresistance.

In general terms, the compressive strength values ofthe concrete are increasing when increasing the radiationdose in the fibers, but detrimental behavior is observedfor higher dosages (100 kGy).

When the irradiation dose is increased, the compres-sive strength of the concrete, the tensile stress, and thetensile strain of the irradiated fibers also increase. Werecall that, in the case of nylon filaments, the extrusionprocess by which the fibers are obtained is known toinduce a preferential orientation of the polymer chainalong the main axis of the fiber, allowing support of highcompressive strengths. When the elongation value dimin-ishes (above 50 kGy), each fiber becomes harder, result-ing in the formation of small spheres on the fiber surfacesvisible in Fig. 8. It is possible to infer that a mechanismof load transfer between the concrete and fiber when

external load is applied is such that the highest compres-sive values for concrete are obtained for the highest strainvalues of the fiber.

The ionizing energy generates more contact surfaceeither by producing wall-brick-formation or small-sphere-formation on the fiber surfaces. In either case, aconsequence is a larger contact area between the fibersand the hydrated cement phase. Eventually, the concretewill split parallel to the fibers, and the resulting crackwill propagate out to the surface. Nevertheless, an in-creasing fiber contact surface with the concrete resists theload by traction forces. In other words, the structuresformed on the fiber surfaces (wall-brick or small sphereformation) act also at non-zero angles with respect to thelongitudinal axis of the fibers; they perform a similar roleas metal rods used as curl-shaped reinforcements in con-crete buildings. One presumes that each fiber supportsthe load not only in the direction of its longitudinal axisbut also at other angles. As a result, the release energy isdiminished, and the opening of the splitting crack isavoided. In general, the splitting cracks follow the re-inforcing fibers, and the bond transfer drops rapidly un-less reinforcement is provided to restrain the opening ofthe splitting crack.

The highest compressive values for all concretespecimens are achieved when adding 2.0 vol% fibersirradiated at 50 kGy. Thus, it is necessary to add moreenergy and a higher content of nylon-irradiated fibersthan to concrete manufactured with polypropylene-irradiated fibers. In the latter case, the maximumvalues are obtained when adding 1.5 vol% irradiated fi-ber at 10 kGy.20

There is a compressive strength improvement of292.3% with respect to SC, a higher percentage than forconcrete containing polypropylene-irradiated fibers

TABLE I. Compressive strength for concrete reinforced with gamma-irradiated nylon fibers.

Specimenno.

Radiationdose (kGy)

Vol%nylon fiber

Compressive strength (MPa)Average(MPa)A B C D E

A 0 0 29.87 25.45 29.35 24.34 23.54 26.511 0 1.5 32.50 35.56 38.24 30.84 34.26 34.282 5 1.5 78.80 99.56 76.73 95.56 73.20 84.773 10 1.5 92.56 107.40 121.80 114.27 110.87 109.384 50 1.5 74.15 108.80 78.40 107.94 108.46 95.555 100 1.5 113.20 85.54 96.09 84.92 117.40 99.436 0 2.0 41.50 34.33 33.37 40.32 38.28 37.567 5 2.0 95.30 80.32 106.23 86.70 100.50 93.818 10 2.0 129.30 122.83 104.73 133.31 98.26 117.689 50 2.0 109.00 118.32 139.20 133.39 111.36 122.25

10 100 2.0 119.10 129.45 98.33 108.26 111.32 113.2911 0 2.5 39.56 33.62 26.50 25.27 30.85 31.1612 5 2.5 76.98 83.26 61.20 73.52 66.84 72.3613 10 2.5 76.11 71.68 88.50 91.20 78.76 81.2514 50 2.5 101.60 79.21 84.29 106.48 82.26 90.7615 100 2.5 84.17 88.60 84.17 78.52 82.39 83.57

FIG. 9. Compressive strength of the reinforced concretes with irradiated-nylon fibers.

G. Martínez-Barrera et al.: Concrete reinforced with irradiated nylon fibers

J. Mater. Res., Vol. 21, No. 2, Feb 2006 489

http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

(189.4%).20 Above 50 kGy, the values go down; never-theless, an improvement of 168.1% remains.

For concrete containing 1.5 vol% irradiated fiber, thehighest compressive values are obtained for 10 kGy,different than 50 kGy necessary for 2.0 and 2.5 vol%irradiated fibers. Both results point out that the compres-sive behavior depends on two parameters: fiber contentand radiation dose. We conclude that, when adding morefibers, it is necessary to apply much more ionizing en-ergy, going from 10 to 50 kGy. At the microstructurelevel, the gamma radiation causes modification of thefiber surfaces, seen as either wall-brick or small spherestructures. A combination of both these structures pro-vides the highest compressive strength at 50 kGy. A fur-ther increase in the irradiation does (to 100 kGy) resultsin domination of the small sphere formation. A com-parison with metal rods used for reinforced concretebuildings, where the curls to some degree stop the open-ing of the splitting crack, is worthwhile here as well.Even at high irradiation doses, there is an improve-ment caused by the nylon fibers addition as comparedwith the SC.

IV. CONCLUDING REMARKS

In general, the improvement in the compressivestrength of the concrete caused by the irradiated fibersdepends on: (i) the dose applied to the fibers and (ii) thefiber content in the concrete. There is an optimum levelin the irradiation dose applied, and for that dose also anoptimum fiber concentration. At that optimum, namely50 kGy and 2.0% fiber concentration, the results aredistinctly better than at lower and higher nylon fiberconcentrations and/or other doses. We note that the pres-ent results constitute a part of a larger project on effectsof irradiation on properties of polymers and compos-ites.9,10,20,22–24

ACKNOWLEDGMENTS

Financial Support of the Autonomous University ofthe State of Mexico (UAEM) in Toluca (Grant No. 1982/UAEM) and the help of Mr. Epifanio Cruz García andMr. Francisco García Flores in the sample irradiationperformed at the Nuclear Sciences Institute of the Na-tional Autonomous University of Mexico are acknowl-edged. Ms. Aydeé Rojas Uribe, holder of an undergradu-ate fellowship in the College of Chemistry of UAEM hasparticipated in the experiments. We thank Traian Zaha-rescu, Institute for Electrical Engineering, Bucharest, forhelpful discussions and one of the reviewers for con-structive comments that resulted in improved perspicuityof this paper.

REFERENCES

1. Z. Zheng and D. Feldman: Synthetic fibre-reinforced concrete.Prog. Polym. Sci. 20, 185 (1995).

2. A.M. Donald: Reinforcement of Plastics, in Performance of Plas-tics, edited by W. Brostow (Hanser, Munich and Cincinnati,2000), Chap. 13.

3. T. Muhua, L. Jinping, and W. Keru: The toughness of nylon fibermats laminated MDF cement composites. Cement Concrete Res.24, 1185 (1994).

4. S. Kurtz and P. Balaguru: Postcrack creep of polymeric fiber-reinforced concrete in flexure. Cement Concrete Res. 30, 183(2000).

5. P.N. Balaguru: Contribution of fibers to crack reduction ofcement composites during the initial and final setting period. ACIMater. J. 91, 280 (1994).

6. A. Khajuria, K. Bohra, and P. Balaguru: Long-term durability ofsynthetic fibers in concrete, in Durability of Concrete, edited byV.M. Malhotra (ACI 2, Detroit, MI, 1991), p. 851.

7. J.J. Beaudoin: Handbook of Fiber-Reinforced Concrete: Prin-ciples, Properties, Developments and Applications (Noyes Publi-cations, Park Ridge, NJ, 1990), p. 214.

8. G. Burillo, R.L. Clough, T. Czvikovszky, O. Guven, A. Le Moel,W. Liu, A. Singh, J. Yang, and T. Zaharescu: Polymer recycling:Potential application of radiation technology. Radiat. Phys. Chem.64, 41 (2002).

9. C. Menchaca, A. Alvarez-Castillo, G. Martínez-Barrera,H. López-Valdivia, H. Carrasco, and V.M. Castaño: Mechanismsfor the modification of nylon 6,12 by gamma irradiation. Int. J.Mater. Prod. Technol. 19, 521 (2003).

10. G. Martínez-Barrera, V.M. Castaño, and R. Rodríguez: Studies onthe rubber phase stability in gamma irradiated polystyrene-SBRblends by using FTIR and Raman spectroscopy. Radiat. Phys.Chem. 69, 155 (2004).

11. C. Menchaca, A. Alvarez-Castillo, H. López-Valdivia,H. Carrasco, H. Lara, P. Bosch, and V.M. Castaño: Structuralanalysis of crystalline nylon 6,12 exposed to gamma radiation. Int.J. Polym. Mater. 51, 769 (2002).

12. D.M. Timus, C. Cincu, D.A. Bradley, G. Craciun, and E. Mateescu:Modification of some properties of polyamide-6 by electron beaminduced grafting. Appl. Rad. Isotopes 53, 937 (2000).

13. X.C. Zhang, M.F. Butler, and R.E. Cameron: The ductile–brittletransition of irradiated isotactic polypropylene studied using si-multaneous small angle x-ray scattering and tensile deformation.Polym. 41, 3797 (2000).

14. B.J. Holland and J.N. Hay: Thermal degradation of nylon poly-mers. Polym. Int. 49, 943 (2000).

15. P.N. Thanki, C. Ramesh, and R.P. Singh: Photo-irradiation in-duced morphological changes in nylon 66. Polym. 42, 535 (2001).

16. C. Menchaca, L. Rejón, A. Alvarez-Castillo, M. Apátiga, andV.M. Castaño: Structural analysis of crystalline nylon 6,12 ex-posed to gamma radiation. Int. J. Polym. Mater. 48, 135 (2000).

17. M.A. Malek, A. Renreng, and Ch.S. Chong: Mechanistic modelfor bond scission in a polymeric system by radiation. Radiat.Phys. Chem. 60, 603 (2001).

18. M. Olivares, H. López-Valdivia, G. Vázquez-Polo, M.A. Mondragón,R. Lima, E. Martínez, and V.M. Castaño: FT-Raman analysis ofthe effects of gamma radiation on nylon 6-12 filaments. Polym.Bull. 37, 221 (1996).

19. M. Olivares, H. López-Valdivia, G. Vázquez-Polo, H. Carrasco,A. Alvarez-Castillo, E. Oliva, and V.M. Castaño: Studies on theeffects of �-radiation on the mechanical properties of Nylon 6-12fibers. Polym. Bull. 36, 629 (1996).

G. Martínez-Barrera et al.: Concrete reinforced with irradiated nylon fibers

J. Mater. Res., Vol. 21, No. 2, Feb 2006490

http://journals.cambridge.org Downloaded: 19 Sep 2013 IP address: 148.218.100.182

20. G. Martínez-Barrera, E. Vigueras-Santiago, S. Hernández-López,C. Menchaca-Campos, and W. Brostow: Mechanical improve-ment of concrete by irradiated polypropylene fibers. Polymer Eng.Sci. 45, 1426 (2005).

21. A. Xenopoulos and B. Wunderlich: Thermodynamic properties ofliquid and semicrystalline linear aliphatic polyamides J. Polym.Sci. Phys. 28, 2271 (1990).

22. G. Martínez-Barrera, W. Brostow, V.M. Castaño, and J. Horta:

Gamma irradiation effects on impact strength and thermal prop-erties of SBR-toughened polystyrene. Polimery 49, 9 (2004).

23. G. Martínez-Barrera, C. Menchaca, D. Pietkiewicz, and W. Brostow:Polystyrene + styrene-butadiene blends: Mechanical and morpho-logical properties. Mater. Sci. Medziagotyra 10, 166 (2004).

24. W. Brostow, V.M. Castaño, and G. Martínez-Barrera: Gammairradiation effect on polystyrene + SBR blends: Morphology andmicrohardness. Polimery 50, 27 (2005).

G. Martínez-Barrera et al.: Concrete reinforced with irradiated nylon fibers

J. Mater. Res., Vol. 21, No. 2, Feb 2006 491