FRP-Needles as Discrete Reinforcement in ConcreteArdavan Yazdanbakhsh1; Lawrence C. Bank, F.ASCE2; Chen Chen3; and Yuan Tian4

Abstract: This paper presents a new type of discrete reinforcing element for concrete produced from either waste or new pultruded fiber-reinforced polymer (FRP) composite materials. These elements, referred to as FRP-Needles, are rigid, long, and low in aspect ratio, and havedistinct physical and mechanical differences frommacrofibers used in concrete. The FRP-Needles used in this study were produced by cuttingFRP reinforcing bar (rebar) production scrap with nominal diameter of 6 mm into rod-shape elements with length of 100 mm (aspect ratio of17). FRP-Needles were incorporated in concrete to replace 5 and 10% of coarse natural aggregate (NA) by volume. The needles did notreduce the workability or stability of concrete. The dispersion and orientation of FRP-Needles in concrete were relatively uniform. The 5 and10% replacement of NAwith FRP-Needles increased the splitting tensile strength of concrete by 22 and 33%, respectively, while reducing thecompressive strength by only 5 and 9%. The incorporation of FRP-Needles in concrete resulted in significant increases in postfailure tough-ness of concrete in both compression and tension. In a parallel study, FRP recycled aggregate (FRP-RA) was produced by cutting scrap rebarsinto cylindrical pieces with aspect ratio of 1. FRP-RA was incorporated in concrete with the aforementioned dosages to observe the effectof geometrical characteristics of FRP elements on the studied mechanical properties of concrete. The improvements achieved by usingFRP-Needles were not observed when FRP-RA was incorporated in concrete. DOI: 10.1061/(ASCE)MT.1943-5533.0002033. © 2017American Society of Civil Engineers.

Introduction

One of the impediments to using concrete in construction is its in-herent low tensile strength combined with brittleness, leading tocracking and disintegration of the material into separate pieces atrelatively low tensile stresses. This issue has been partially circum-vented at both the material and structural levels by using discreteand continuous reinforcing elements, namely, fibers and reinforc-ing bars (rebar), respectively. Concrete macrofibers (as opposed tothe smaller microfibers used for controlling shrinkage cracking)can be as long as 75 mm with aspect ratios (fiber length to nominaldiameter) ranging from 20 to 100 [ACI 544.1 (ACI 2009)]. Mostmacrofibers are approximately 50–60 mm in length and have aspectratios of 50 or higher, with nominal diameters typically lower than1 mm. Macrofibers used in concrete are either flexible (most poly-meric fibers) or semirigid (steel fibers). Steel fibers are regarded assemirigid because inclined fibers (not perpendicular to the surfaceof the crack they bridge) bend permanently when the crack growsin width. Macrofibers, particularly those made from steel and pol-ymers, have been used to reduce or replace continuous reinforce-ment (rebars) in various types of structural elements including flatmembers such as airport taxiways (Murrell 1993) and slabs-on-ground (Roesler et al. 2006).

The addition of fibers to concrete results in loss of slump[ACI 544.1 (ACI 2009)]. This is due to the significant amountof mixing water that is adsorbed on the large surface area of thefibers, and therefore is unavailable to contribute to workability.In addition, workability has a negative correlation with fiber aspectratio (Bentur and Mindess 2007) because the high-aspect-ratio fi-bers tend to entangle more easily and form agglomerations and net-works that hinder the movement of aggregates against each other.Water-reducing admixtures are used to increase the workability offiber-reinforced concrete (FRC). However, when the volumetricfiber content of concrete exceeds values as low as 0.75%, the highdemand and use of water-reducing admixtures can lead to segre-gation, i.e., the separation of aggregates from paste in concrete(Altoubat et al. 2009).

This paper investigates a new type of elongated low-surface-area discrete reinforcement for concrete, made from glass fiber–reinforced polymer (FRP) composite materials. FRP materialsconsist of fibers encased in a matrix of thermosetting resins withfiber concentrations typically in the range of 12–60% by volume(Reynolds and Pharaoh 2010). FRP composite materials haveunique physical and mechanical properties, and have important ad-vantages over many traditional construction engineering materials(Bank 2006). Those advantages typically include lower densities,higher mechanical properties in certain directions, and greaterdurability in harsh chemical and aqueous environments. For exam-ple, although the specific gravity of FRP materials is only a quarterof that of steel, they can be more than three times stronger thansteel in tension. In the past three decades, FRP composite materialshave been used in the construction industry for both externalstrengthening (e.g., epoxy-saturated carbon FRP fabrics) and inter-nal reinforcement (FRP rebars) of concrete [ACI 440R (ACI 2007);Nanni et al. 2014].

The discrete concrete-reinforcing elements investigated inthis paper are referred to as FRP-Needles. Developing a precisequantitative definition of FRP-Needles that specifies ranges fortheir geometry and property is a challenging task because theseranges can be quite large and will depend on the size of the con-crete member being reinforced with the FRP-Needles. However,

1Assistant Professor, Dept. of Civil Engineering, City College ofNew York, Steinman Hall 110, 160 Convent Ave., New York, NY 10031(corresponding author). E-mail: ayazdanbakhsh@ccny.cuny.edu

2Professor, Dept. of Civil Engineering, City College of New York,Steinman Hall 103, New York, NY 10031.

3Student Researcher, Dept. of Civil Engineering, City College ofNew York, Steinman Hall 194, New York, NY 10031.

4Graduate Research Assistant, Dept. of Civil Engineering, City Collegeof New York, Steinman Hall 194, New York, NY 10031.

Note. This manuscript was submitted on November 10, 2016; approvedon April 11, 2017; published online on July 8, 2017. Discussion periodopen until December 8, 2017; separate discussions must be submittedfor individual papers. This paper is part of the Journal of Materials in CivilEngineering, © ASCE, ISSN 0899-1561.

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FRP-Needles have distinct physical and functional differences fromconcrete macrofibers: (1) FRP-Needles are longer than macrofibers(the needles used in this study are 100 mm long); (2) FRP-Needlesare significantly larger in diameter than the thickest concrete fibers,and therefore the aspect ratios of the needles are notably lower thanthose of concrete macrofibers [the FRP-Needles used in this studyhave a diameter of 6 mm (aspect ratio of 17), making their cross-sectional area more than 50 times larger than that of most of com-mercially available macrofibers]; (3) FRP-Needles are very rigidand do not undergo plastic deformation during the failure of con-crete under load, even if they are located in the fracture zones; and(4) FRP-Needles can function as both discrete reinforcement andcoarse aggregate in concrete due to their large size and significantstiffness.

FRP-Needles can be either the target output of a FRP compositematerial production process or produced from FRP waste. Whenthe waste is production scrap and is processed into needles (a usefulproduct), the scrap and the FRP-Needles should be characterized asby-products of FRP rebar production, rather than waste. When thewaste is a FRP composite material that has reached the end of itsservice life, the FRP-Needles made from processing the waste arerecycled products. The objective of the present work is to investi-gate the effect of FRP-Needles with selected characteristics andgeometry on important properties of concrete, rather than assessinga FRP waste management strategy. However, the findings of thestudy can be used as a part of required data for such assessments.The needles used in the present study were produced by cutting thescrap from the production of FRP reinforcing bars (FRP rebars).To the best knowledge of the authors, the use of FRP-Needlesin concrete has not been studied in the past. Patnaik et al. (2013,2014) performed experimental studies on cut pieces of basalt fiber–reinforced polymer strands, which they used as discrete concretereinforcing elements. They reported results of testing concretespecimens incorporating these reinforced strands that had a lengthof 43 mm and average nominal diameter of 0.66 mm (aspect ratioof 65), dimensions that fit within the geometrical ranges of concretemacrofibers.

In a related study, scrap from production of FRP rebars withdifferent diameters were cut into short cylindrical pieces with anaspect ratio of 1, called FRP recycled aggregate (FRP-RA), andused in concrete as full and partial (40% by volume) replacementof coarse natural aggregate (NA) (Yazdanbakhsh et al. 2016). Thescrap consisted of short pieces of high-quality rebars that couldnot be sold due to their length. The results of the study showedthat concrete with structural-grade compressive strength can beproduced with FRP-RAs. However, the replacement of NA withFRP-RA resulted in a significant reduction in compressive strengthand splitting tensile strength of concrete due to the weak bondbetween FRP-RA particles (particularly the smooth saw-cut basesurfaces of the cylindrical pieces) and the mortar matrix. Develop-ing FRP discrete elements that can enhance, rather than reduce, themechanical performance of concrete is desirable and motivated thepresent study.

The main objective of the present work is to demonstratehow FRP-Needles, regardless of whether they are made by process-ing waste or as a new product, affect workability and stability offresh concrete, compressive strength, modulus of elasticity incompression, and splitting tensile strength, in addition to theload-deformation responses of concrete specimens before and afterfailure as an indication of toughness. FRP-RA incorporated con-crete batches with the same FRP contents as those of the concreteincorporating FRP-Needles were produced and tested to observethe effect of the geometry of FRP elements on the selected proper-ties of concrete. The present work is a preliminary feasibility study

and does not attempt to investigate all the concrete properties andcharacteristics required to demonstrate proof of principle for theFRP-Needles. Future work needs to be performed to study theeffect of FRP-Needles on shear strength and the mechanical per-formance of structural concrete members. In addition, the long-term performance and potential limitations of concrete withFRP-Needles will need to be investigated.

Experimental Program

Materials and Concrete Mixtures

Scrap from the production of helically wrapped and sand-coatedAslan (Seward, Nebraska) glass fiber–reinforced polymer rebarsproduced by Hughes Brothers in the United States were usedfor producing FRP-Needles and FRP-RA. Wrapping and sand-coating processes are commonly performed to enhance the bondbetween rebars and concrete. The manufacturer-reported propertiesof FRP rebars are presented in Table 1. To produce FRP-Needles,waste rebars with reported diameters of 6 mm were removed from apile of production scrap (Fig. 1) and cut with a diamond saw intocylindrical pieces with length of 100 mm (aspect ratio of 17)(Fig. 2). In the mix development phase of the study, concrete

Fig. 1. FRP reinforcing bar production scrap

Table 1. Manufacturer-Supplied Properties of the FRP Rebars (Data fromHugh Brothers, Inc.)

Characteristic Value

Tensile strength (MPa) 620–896Transverse shear strength (MPa) 150Tensile modulus of elasticity (GPa) 46Specific gravity 1.90824-h moisture absorption (%) ≤0.25Fiber content by weight (%) 70Fiber type ECR glass fibersMatrix material Vinyl Ester resin

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mixtures were produced with different contents of FRP-Needles ofdifferent sizes. FRP rebars with the lowest commercially availablediameter (6 mm) were selected to increase the aspect ratio, as wellas the number of needles per volume of FRP, thereby reducing thespacing between the needles in concrete. In addition, the length of100 mm was selected for the FRP-Needles because beyond thatlength the orientation of the needles is significantly affected bythe size of concrete molds used in the present study.

To produce FRP-RA, waste rebars with reported diametersof 6, 10, 13, and 19 mm were cut into short cylindrical pieces(Figs. 3 and 4) with nearly the same length as bar diameter (aspectratio of 1). Using a large laboratory sieve shaker, the cut bars wereproportioned to have a size gradation that fits within the limits ofthe ASTM C33 (ASTM 2013) grading size of 56 with maximumparticle size of 19 mm. The NA available in the laboratory (crushedgranite) was regraded by using the sieve shaker to achieve thesame standard gradation as that of the FRP-RA. ASTM graded

manufactured sand and Type I portland cement were used in allconcrete mixtures. The proportions of the concrete mixtures arepresented in Table 2. The control (NA) mixture was proportionedto have a target average compressive strength of 40 MPa and slumpof 75 mm, characteristics commonly specified by designers for theconcrete structures in the United States.

Two FRP-Needle-incorporated concrete mixtures, FRP-NDL-5and FRP-NDL-10, were produced using the same mix proportionsas those of the NA concrete, but with 5 and 10% of NA replacedvolumetrically with FRP-Needles, respectively. In FRP-NDL-5 andFRP-NDL-10 mixtures, the needles constitute 1.76 and 3.52% ofconcrete volume, respectively. These values were calculated fromthe mix proportions of concrete presented in Table 2 and the spe-cific gravity (SG) of FRP-Needles measured by the authors, whichis 1.96, as opposed to the reported value of 1.908 for the FRP rebarspresented in Table 1. The aforementioned needle content values arelarger than fiber content in FRC (typically less than 1.0%, beyondwhich the probability of major fiber agglomeration is high). For agiven volume of elongated elements (fibers or needles) distributedrandomly in a specified volume of concrete, increasing the diameterof the elements results in an increased spacing between them.Lower spacing between the elements is more desirable becauseit leads to a better distribution of stresses in the concrete matrix.Therefore, because FRP-Needles are significantly larger in diam-eter than fibers, higher volumetric contents were selected to avoidlarge spacing between the needles in concrete matrix. The presentstudy aims at investigating the effect of FRP-Needles on a numberof important fresh and hardened properties of concrete, as opposedto finding optimum needle content, which depends on the structural

Fig. 2. FRP-Needles cut from FRP rebars and used in the study

Fig. 3. Samples of FRP-RA produced by cutting FRP rebars with manufacturer-reported diameters of (a) 6 mm; (b) 10 mm; (c) 13 mm; (d) 19 mm

Fig. 4. FRP-RA with the ASTM C33 grading size of 56

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application of concrete. To compare the effects of FRP-Needlesand FRP-RA on the studied properties of concrete, two FRP-RA-incorporated mixtures were produced using the mix proportionsof NA concrete with 5 and 10% of NA replaced volumetricallywith FRP-RA to produce FRP-RA-5 and FRP-RA-10 mixtures,respectively.

Previous studies show that the absorption rate of cut FRP rebarsis low and full absorption requires a period of time much longerthan the setting time of concrete (Yazdanbakhsh et al. 2016). Con-sidering the low absorption rate of the FRP-RA and FRP-Needlesand their low content in concrete mixtures, no additional adjust-ment was made to the amount of mixing water to account forthe moisture absorbed by the FRP inclusions.

Specimens and Testing

For each of the five concrete mixes, one batch of concrete was pro-duced, and for each batch six cylinders with diameter of 150 mm andheight of 300 mm were cast. The orientation of the needles (similar tothat of stiff macrofibers) near mold surfaces is affected by the processof molding, leading to preferential alignments. Some standards of test-ing FRC beams [ASTM C1399 (ASTM 2015c); ASTM C1609(ASTM 2012)] require that the dimensions of the mold be at leastthree times the length of the fibers. However, ASTM C1609 allowswaiving the three times fiber length requirement when longer fibersare used to permit casting specimens with desired dimensions. Themolds selected in this study have the largest dimensions (150 by300 mm) specified by ASTM C39 (ASTM 2015b) for performingcompressive strength and splitting tensile strength tests. After the pro-duction of each batch and before casting the cylinders, a slump test[ASTM C143 (ASTM 2015d)] was performed. In addition, visual ob-servations were made for mix stability and potential segregation of theneedles in the FRP-Needle-incorporated concretes. The cylinderswere demolded 24 h after casting and placed in an environmentalchamber with relative humidity (RH) of 98% at room temperature.

All the specimens were tested on the same date 28 days aftercasting. On the day of testing, three specimens from each mix varia-tion were tested for static modulus of elasticity in compression[ASTM C469 (ASTM 2014)] and compressive strength [ASTM C39(ASTM 2015b)]. ASTM C469 permits obtaining the modulus ofelasticity and strength at the same loading provided that the gaugesare expendable, removable, or adequately protected so that it is pos-sible to comply with the requirement for continuous loading givenin ASTM C39. The strain capacity of the strain gauges used in thisstudy was 0.05, which is significantly higher than that reachedwhen concrete cylinders are loaded up to 40% of their load-carrying capacities. Unbonded neoprene caps were used for thecompression tests [ASTM C1231 (ASTM 2015a)]. The specimensfor the modulus of elasticity test were removed 1 day earlier fromthe environmental chamber, towel-dried, kept in the laboratory

environment for few hours so that their surfaces became dry,and then each instrumented with two wire strain gauges mountedcircumferentially at diametrically opposite points at the midheightof the specimen. The average reading of the two strain gauges dur-ing the compression test were used to calculate the modulus of elas-ticity according to ASTM C469. The three other cylinders of eachbatch were tested for splitting tensile strength [ASTM C496(ASTM 2011)]. The load-displacement responses were recordedduring both compressive strength and splitting tensile strengthtests. In addition, after failure deformation patterns and fracturedsurfaces of the specimens were examined to study, qualitatively,the ability of the FRP inclusions to distribute stress in concrete.

Results and Discussion

Workability and Stability of Fresh Concrete

The stability of the FRP-Needle-incorporated mixes and the pos-sibility of the separation or agglomeration of the needles was a con-cern before the initiation of the study. Concrete mixtures weremonitored visually when removed from the mixer and placed inthe molds to look for (1) separation of FRP-Needles from concrete,and (2) agglomeration of FRP-Needles. The visual examination ofthe concrete batches produced during the mix development phaseand during the production of the specimens tested for this studyshow that the FRP-Needles did not segregate during mixing or cast-ing the cylinders. The high stability of the needle-incorporatedmixes was achieved without using viscosity-modifying agents orany other chemical or mineral admixtures. One main reason for thestability of FRP-Needles is that the concrete slump was relativelylow (average value of 70 mm). Segregation of coarse aggregates(and possibly other large solid elements such as FRP-Needles)in concrete is less probable when concrete workability is low(Khayat 1999). However, because concretes with slump valuessimilar to that measured in this study are used commonly in con-struction, the finding that FRP-Needles remain stable in concreteswith this range of slump is important.

The slump test results, presented in Table 2, show that the differ-ences in the slump values of the mixtures are within the margin oferror. Therefore, FRP-Needles, as opposed to fibers, when used inas high dosages as 3.52% of concrete volume, do not affect con-crete slump, and therefore do not lead to the need for additionalwater-reducing admixtures or developing specific mix proportion-ing techniques. The specific surface area of the FRP-Needles issimilar to that of the coarse aggregates they replaced. In addition,for a given volumetric dosage, the number of FRP-Needles in con-crete is much smaller than that of fibers. These facts explain theinsignificant impact of FRP-Needles on workability as measuredby slump test in this study.

Table 2. Mix Proportions of the Concrete Mixtures in Kilograms per 1 m3 of Concrete

Component

Mixture

Control (NA) FRP-NDL-5 FRP-NDL-10 FRP-RA-5 FRP-RA-10

Cement 422 422 422 422 422Water 190 190 190 190 190Sand (SSD) 683 683 683 683 683NA (SSD) 950 903 855 903 855FRP-Needles — 34 69 — —FRP-RA — — — 34 69FRP volume content (%) — 1.76 3.52 1.76 3.52Slump (mm) 70 75 70 70 65

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The potential limitations of fresh needle-incorporated concreteshould be considered in future studies on the performance of struc-tural members, as well as in application and design. For example,if concrete with FRP-Needles is used in cast-in-place concretecolumns, because the fresh concrete needs to travel all the wayto the bottom of the mold during casting, some of the needlesmay become entangled in the cage if the spacing between the steelrebars is small.

Mechanical Properties and Failure Behavior inCompression

The results from the compressive strength and the modulus of elas-ticity in compression tests are presented in Table 3. The use of bothFRP-Needles and FRP-RA in concrete resulted in relatively smallreductions in compressive strength. Replacing 5 and 10% of NA,by volume, with FRP-Needles reduced the concrete compressivestrength from 40.2 to 38.0 and 36.7 MPa (reductions of 5 and9%), respectively. The volumetric replacement of 5 and 10% ofNA with FRP-RA reduced the concrete compressive strength to37.9 and 38.9 MPa (reductions of 6 and 3%), respectively. A pos-sible reason for these moderate reductions in compressive strength

is that the angular-shape crushed stones (NA) can develop a betterinterlock compared with the cylindrical FRP-Needles or FRP-RAthat replace them. The results in Table 3 show that FRP-Needlesand FRP-RA do not have a significant effect on the modulus ofelasticity of concrete, which is expected to be due to the low volu-metric dosages of FRP in concrete mixes (1.76 and 3.52%).

The compressive load-displacement results of NA, FRP-Needles, and FRP-RA concretes are presented in Fig. 5. The curveswere horizontally shifted to account for the initial flexibility ofthe neoprene pads used for testing [ASTM C1231 (ASTM2015a)]. The magnitudes of displacement in these curves weremeasured by the embedded LVDT of the testing device, rather thanexternal LVDTs, and therefore cannot be used reliably for quanti-fication of concrete properties such as toughness. However, thecurves can be used to qualitatively compare the failure behaviorof concrete specimens with different mix proportions. In Fig. 5the load-displacement curves are shown up to the displacement val-ues at which concrete specimens crush and the load values decreasesuddenly and significantly. The figure shows that NA concretecrushes abruptly and immediately after the compressive load reachesits peak value. However, as shown in Fig. 5(a), FRP-NDL-5 andFRP-NDL-10 concretes continue to carry load for some additionalcompressive displacement before failure. FRP-RA-incorporatedconcretes, similar to NA concrete, failed abruptly in compressionafter the applied load reached the peak value [Fig. 5(b)].

Fig. 6 shows cylinders from the five concrete mixes aftercompressive failure. The NA concrete is crushed into pieces anddeep cracks can be observed in the specimen [Fig. 6(a)]. In theFRP-NDL-5 specimen, the concrete surface has been spalled butthe cylinder’s core has maintained its integrity [Fig. 6(b)]. Thespalling of the FRP-NDL-10 specimen is notably less and the lat-eral expansion is not noticeable [Fig. 6(c)], indicating a higher post-failure toughness. Both FRP-RA-5 and FRP-RA-10 specimenshave developed large and deep cracks, resulting in the separationof large portions of concrete from the cylinders [Figs. 6(d and e)].

Mechanical Properties and Failure Behavior in Tension

The splitting tensile strength results of the concrete specimens arepresented in Table 4. The results show that the volumetric replace-ments of 5 and 10% of NA with FRP-Needles increased the split-ting tensile strength of concrete from 3.42 to 4.18 and 4.55 MPa,respectively. That is, the splitting tensile strengths of FRP-NDL-5and FRP-NDL-10 concretes are 22 and 33% higher than that of NAconcrete, respectively; these changes are much more significant

Table 3. Compressive Strength (fc) and Modulus of Elasticity inCompression (E) of the Concrete Specimens and Their Mean Valuesfcm and Em

Concretetype Specimen

fc(MPa)

fcm(MPa)

E(GPa)

Em(GPa)

NA (control) 1 41.3 40.2 29.1 29.02 39.1 24.23 40.1 33.8

FRP-NDL-5 1 39.4 38.0 31.8 30.72 36.7 29.63 38.0 16.2a

FRP-NDL-10 1 36.0 36.7 42.0a 24.02 37.1 35.5a

3 36.9 24.0FRP-RA-5 1 37.4 37.9 32.2 29.5

2 39.3 27.23 37.0 29.1

FRP-RA-10 1 38.0 38.9 39.0 33.42 40.0 32.63 38.8 28.6

aThe strain gauge reading had high noise. The modulus of elasticity (E) wasregarded as an outlier and was not used for calculating the Em.

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Fig. 5. Load-displacement results from compressive strength testing of concrete specimens with (a) FRP-Needles; (b) FRP-RA; those of NA concretespecimens are also included

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compared with the 5 and 9% reduction in compressive strengthwhen 5 and 10% of NA is replaced by volume with FRP-Needles.

The replacement of 5 and 10% of NA by volume with FRP-RAresulted in 10 and 0% reduction in splitting tensile strength. Thesefindings were unexpected because a previous study (Yazdanbakhshet al. 2016) shows that the incorporation of the higher dosages of

FRP-RA in concrete would lead to a larger decrease in splittingtensile strength. Because the dosages of FRP-RA used in thisstudy are relatively low (5 and 10%), the results are sensitive tominor errors that might have occurred during mixture productionor testing. Nevertheless, the results of both the previous and presentstudies demonstrate clearly that FRP-RA, as opposed to FRP-Needles, does not increase the splitting tensile strength of concrete.Fig. 7 shows the load-displacement curves from the splitting tensilestrength tests. Fig. 7(a) shows that the postpeak tensile toughness ofFRP-NDL-5 and FRP-NDL-10 concretes, particularly the latter, aresignificantly higher than that of NA concrete. The figure also showsthat the failure strains of FRP-NDL-5 and FRP-NDL-10 concretesare much higher than that of NA concrete. FRP-RAs did notchange the failure behavior of concrete in tension [Fig. 7(b)].

During the splitting tensile strength testing of NA concrete,shortly after the applied load reached its peak value, the cylinderswere broken (split) into two separate pieces [Fig. 8(a)]. FRP-Needle-incorporated concrete specimens developed multiplecracks, but did not break into separate pieces. Figs. 8(b and c) showthe FRP-NDL-5 and FRP-NDL-10 specimens after failure andbefore unloading in the testing machine, respectively. Each speci-men has maintained its integrity as one piece although deeplycracked and with circumferences deformed from circular into ovalshapes. The figures show that, although at the end of testing theFRP-NDL-10 specimens were more deformed laterally than FRP-NDL-5 specimens, they formed cracks with smaller openingwidths. Figs. 9(a and b) show FRP-NDL-5 and an FRP-NDL-10

Fig. 6. Concrete specimens with different compositions after compressive failure: (a) NA concrete; (b) FRP-NDL-5; (c) FRP-NDL-10; (d) FRP-RA-5;(e) FRP-RA-10

Table 4. Splitting Tensile Strength (fct) of Concrete Specimens and TheirMean Values (fctm)

Concrete type Specimenfct

(MPa)fctm(MPa)

NA (control) 4 3.46 3.425 3.866 2.93

FRP-NDL-5 4 3.99 4.185 4.646 3.90

FRP-NDL-10 4 4.67 4.555 3.666 5.32

FRP-RA-5 4 2.73 3.065 3.486 2.95

FRP-RA-10 4 3.63 3.425 3.426 3.20

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concrete cylinders after unloading, respectively. The figures showthat the FRP-NDL-5 cylinder, despite having carried a lowerload, is more damaged and has developed wider cracks when com-pared with the FRP-NDL-10 cylinder. The tested FRP-Needle-incorporated cylinders could only be broken into separate pieces[Fig. 9(c)] by a hammer, two chisels, and a significant amount ofeffort by an investigator. Fig. 9(c) demonstrates the relativelyuniform distribution and orientation of the needles in the concretecylinder, despite the fact that the diameter of the concrete cylindermolds is only 50% larger than the length of the FRP-Needles. Con-crete cylinders incorporating FRP-RA were split into two piecesafter failure similar to NA concrete specimens (Fig. 10).

The remarkable increase in the tensile strength and postfailuretoughness of concrete caused by the addition of FRP-Needlescan be in part caused by the strong bond between the needlesand concrete. Past studies have shown that the bond strength ofFRP rebars can be equal or higher (Katz 1999) or up to 40% lower

(Benmokrane et al. 1996) than that of surface-deformed steel re-bars, which maintain strong interface bond in concrete matrices.A study performed on the FRP rebars used in this work (but withthe diameter of 10 mm) shows that the bond strength of FRP rebarsis only 10% lower than that of surface-deformed steel rebars inconcrete with the compressive strength of 40 MPa (Okelo andYuan 2005).

The development length of the FRP-Needles is another charac-teristic that affects the ability of the needles to carry and distributestress within the concrete matrix before and after the peak load.The guidelines of the Fiber-Reinforced Polymer ReinforcementCommittee of the American Concrete Institute (ACI) presentsthe following equation for calculating the development length(ld) of FRP rebars [ACI 440.1R (ACI 2015)]:

ld ¼αðffr=0.083

ffiffiffiffiffif 0c

pÞ − 340

13.6þ C=db· db ðin SI unitsÞ ð1Þ

Fig. 8. Concrete specimens in the testing device after splitting tensile failure: (a) FRP-RA-5; (b) FRP-NDL-5; (c) FRP-NDL-10

Fig. 9. (a) FRP-NDL-5; (b) FRP-NDL-10 specimens after splitting tensile strength test; (c) FRP-NDL-10 specimen opened up manually after split-ting tensile strength test

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where α = constant equal to either 1.0 or 1.5 depending on whetherthe distance of the rebars from the bottom of the concrete memberis lower or higher than 300 mm, respectively; ffr = tensile stresscarried by the rebar; f 0

c = compressive strength of concrete; C =lesser of one-half of the center-on-center spacing of the bars beingdeveloped and cover to the center of the bar; and db = diameter ofFRP rebar. This equation and the results of splitting tensile strengthtests can be used to roughly estimate the development length ofFRP-Needles. By visual observation of the fractured surfaces ofFRP-NDL-10 specimens, it was determined that the average spac-ing between the needles were approximately 40 mm, and that anaverage of 18 needles passed each fractured surface. Two values offfr were estimated to calculate ld before and after peak load. Forthe former case, it was assumed that both the needles and concretecarry the tensile stresses. The moduli of elasticity of the concreteand FRP were used to calculate the modular ratio and the tensilestress, ffr, carried by the needles. For this case, the developmentlength was estimated to be only 4.6 mm before tensile failure,indicating that at least 90% of the length of each needle fully con-tributes to transferring and resisting tensile stresses from the sur-rounding concrete. Because the surface of the rebars used in thestudy is effectively treated (both helically wrapped and sand-coated), the development length may be shorter than estimated.The small development length, strong bond between the needlesand concrete matrix, and the fact that the needles are stiffer thanconcrete in tension, thereby confining the lateral expansion of con-crete, all explain the remarkable ability of FRP-Needles to increasethe tensile strength of concrete. This ability makes the needles dis-tinct from concrete macrofibers, which can typically cause only amoderate increase in tensile strength (Choi and Yuan 2005; Nanni1988; Wafa and Ashour 1992).

To estimate the development length after tensile failure, it wasassumed that all the tensile stress in the fractured surface is carriedby the needles. Therefore, to estimate the highest magnitude offfr for this case, the average splitting tensile strength of FRP-NDL-10 concrete was multiplied by the area of the split section(150 × 300 mm) to find the tensile force, and then divided by totalarea of FRP-Needles that pass the section. The estimated value ofdevelopment length after tensile failure was 256 mm, more than50 times larger than the development length before peak load.Therefore, after tensile failure only a portion of the full capacityof the needles to carry tensile stress is available. However, theprogress of fracture in the specimens after the peak load was mostlydue to the formation and propagation of new cracks rather than

needles slipping out [Figs. 9(a and b)]. This observation suggeststhat the bond between the needles and the concrete is sufficientlystrong, that despite the large development length and the low aspectratio of the needles the interface continues to transfer stressbetween concrete and the needles. The random orientation of therigid FRP-Needles is another possible reason for their resistance topullout. Because the vast majority of the needles, which are verystiff and highly resistant to bending, are not perpendicular to thefractured surface, the slippage of the needles [Fig. 9(c)], even ifthe bond is weak, would be possible when additional local fracturesoccur in the concrete zone surrounding the needles.

Conclusions

The present study introduced FRP-Needles, a new type of discreteelongated concrete reinforcement with unique physical andmechanical properties. The material characteristics that are mostimportant to designers and concrete practitioners were studiedfor FRP-Needle-incorporated concrete. The findings show thatthe FRP-Needles produced and used in this study have significantpositive effects on tensile strength and postfailure toughness of con-crete without affecting its workability and stability, and only causea small reduction in compressive strength. The results argue for theconsiderable potential of FRP-Needles as concrete reinforcementand the importance of investigating (1) other material properties,particularly shear strength and resistance to impact and blast;(2) structural performance; (3) the relationship between the prob-ability of needle segregation and workability; and (4) durability ofFRP-Needle-incorporated concrete. In addition, it is importantto perform a scientific study to determine the geometry andmechanical properties of FRP-Needles required to optimize the per-formance of concrete in different structural applications. One po-tential limitation of FRP-Needles is that the flow of fresh concreteincorporating long needles in vertical deep molds, such as those ofcolumns, may be impeded by the rebars between which the needlesmay become entangled. Experimental investigations need to be per-formed to study the flow of needle-incorporated concrete in differ-ent types of rebar cages.

Acknowledgments

The support of the New York State Energy Research and Develop-ment Authority (NYSERDA) under the grant C2CUNY1 by the

Fig. 10. Fractured surfaces of concrete specimens without FRP-Needles after splitting tensile strength test: (a) NA concrete; (b) FRP-RA-5;(c) FRP-RA-10; the surfaces were sprayed with water prior to photography so that a higher contrast between the coarse aggregates and mortarmatrix can be observed

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PowerBridgeNY Program is acknowledged. The authors wishto thank Mr. Doug Gremel from Hughes Brothers, Inc., forproviding the FRP reinforcing bar production waste and technicalinformation.

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