Construction and Building Materials 44 (2013) 751–756

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Construction and Building Materials

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Laboratory evaluation of tensile strength and energy absorbingproperties of cement mortar reinforced with micro- and meso-sizedcarbon fibers

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.03.071

⇑ Corresponding author. Tel.: +1 865 974 2608; fax: +1 865 974 2669.E-mail address: bhuang@utk.edu (B. Huang).

Ryan K. Graham, Baoshan Huang ⇑, Xiang Shu, Edwin G. BurdetteDepartment of Civil and Environmental Engineering, The University of Tennessee, Knoxville, TN 37996, USA

h i g h l i g h t s

� Effects of micro- and meso-fiber reinforced cement mortars were considered.� Tensile strength, energy absorbing properties, and workability were tested.� Factors of fiber content, fiber-length, aspect ratio were included.� Pre- and post-peak stress energy absorption were evaluated.

a r t i c l e i n f o

Article history:Received 2 December 2012Received in revised form 12 March 2013Accepted 19 March 2013Available online 24 April 2013

Keywords:Cement mortarCarbon fiberMicro fiberTensile strengthWorkability

a b s t r a c t

In this present research the potential use of micro-fibers as concrete reinforcement was investigated byblending micro- and meso-length carbon fibers into Portland cement mortar and comparing the perfor-mance of these mortars through indirect tension tests, specific gravity analysis, and flow tests. The micro-fiber reinforced mortars were observed to have greater tensile strengths than the unreinforced mixturewhile maintaining a lower air-to-cement ratio than the meso-fiber reinforced mixtures. In addition,micro-fibers proved to be of less detriment to the workability of the mortar than the meso-fibers.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Concrete is the most commonly used construction material inthe world, which may be due in large part to the fact that its prop-erties can be altered to meet the specific needs of a wide variety ofapplications [1,2]. Varying the proportions of the basic componentsof concrete–cement, water, coarse aggregate, and fine aggregate –significantly alters the properties of the fresh and hardened con-crete. The properties of a concrete mix can be further altered byemploying chemical or mineral additives – additional componentsadded to the concrete before or during the mixing process [3]. Fi-bers have been utilized to improve the tensile and bending perfor-mance of concrete. The interaction between fibers and cementmatrix is highly complex and has been studied extensively [4–7].Simply stated, however, fibers can both impede the growth of,

and bridge, micro-cracks as they form. Thus, additional energy isneeded for a failure causing macro-crack to form.

Extensive investigation has shown that carbon fiber can providesignificant reinforcement. Carbon fibers with lengths ranging frommillimeters [8,9] to a centimeter [8–11] can improve the tensilestrength [8–12] and toughness [11], as well as the flexural strength[8–11] and toughness [9], of cementitious materials. Park et al. [8]reported that cement composites reinforced with a 5.0% volumefraction of 3-mm carbon fiber improved the tensile strength bygreater than threefold, and the flexural strength just under three-fold. Chen and Chung [9] reported that the flexural strength of acement mortar was improved by 10%, 39%, and 39% by reinforcingthe mortar with 3.0-mm, 5.1-mm, and 12.7-mm carbon fibers,respectively, at 0.5% by weight of cement. Toutanji et al. [10] foundthat fiber volume fractions of 1%, 2%, and 3% increased the tensilestrength of cement by 32%, 48%, and 56%, and its flexural strengthby 72%, 95%, and 138%. The impact that carbon fibers have on thetensile and bending properties of cementitious material is quitedefinitive.

Table 2Flow test mixture proportions.

Mixture Ratio by weight of cement Fiber volume (%)

Fine aggregate Water HRWR Micro-fiber Meso-fiber

Flow control 1.52 0.37 0 0 0Flow micro 1.52 0.37 0 0.49 0Flow meso-1 1.52 0.37 0 0 0.49

752 R.K. Graham et al. / Construction and Building Materials 44 (2013) 751–756

Even though the ability of fibers to improve the tensile and flex-ural performance of a cementitious material has been validated,the amount of fiber that can be practically incorporated into amix is limited by the occurrence of balling (tangling of the fibers),which negatively affects its workability [13]. This issue, however,can be partially alleviated by using shorter fibers [13] or fibers witha decreased aspect ratio [14] or by employing chemical admixturessuch as a HRWR. Furthermore, fibers also tend to increase the aircontent of the mix [12,15].

2. Objective and scope

The objective of this study was to investigate the potential ofmicro-length, low aspect ratio carbon fibers to improve themechanical properties of cement-based materials while minimiz-ing losses in workability and mixture density.

In order to accomplish this objective, the tensile performance,specific gravity, and flow of a Portland cement mortar reinforcedby micro-length carbon fibers (micro-fibers) were evaluated andcompared to those of mortar mixtures reinforced by meso-lengthcarbon fibers (meso-fiber). The nominal fiber volume fractions ofthe micro-fiber mixes ranged from 0.5% to 3.0% while those ofthe meso-fiber mixes ranged from 0.5% to 1.5%. Indirect tension(IDT) tests were employed for the tensile strength analysis, specificgravity values were used to evaluate air content, and flow testswere used to compare the effects that micro-fibers and meso-fibershave on a the workability of cement mortar.

This preliminary study is focused on the effects of micro- andmeso-sized carbon fibers on the properties of portland cementmortar. The feasibility of using micro-length size carbon fiberin reinforcing cement concrete will be explored in futurestudies.

3. Materials

The base mixes, which also served as the control mixes, were composed of typeI Portland cement, manufactured sand, and water. The only additional componentwas high-range water reducer (HRWR), which was utilized in several of the fiber-reinforced mixes in order to avoid or mitigate workability issues engendered bythe fibers. Table 1 presents the mix proportions of the specimens for the indirecttension (IDT) test and Table 2 for flow test.

The tensile strength and modulus of elasticity of the carbon fibers used in thisstudy were 4200–4550 MPa and 230 GPa, respectively, and a specific gravity (SG) of1.8, which is lighter than both steel (SG = 7.8) and concrete (SG = 2.4). With themixture proportions presented in Tables 1 and 2 and the specific gravities of mix-ture components, the volume fractions of different components can be calculated.The volume fractions of carbon fiber in the mixtures were also presented in Tables1 and 2.

While both the micro- and meso-fibers have a rod-like geometry and diam-eters of approximately 6–7-lm, their respective aspect ratios differ significantly(Fig. 1). Note that without magnification, the micro-fibers appear as a powder-like substance, but viewed under the magnification of a scanning electronmicroscope their true fibrous nature is evident; the nominal meso-fiber lengthis 6.31 mm.

Table 1Mixture proportions of IDT specimen.

Mixture Ratio by weight of cement Fiber volume (%)

Fine aggregate Water HRWR Micro-fiber Meso-fiber

Control 1.52 0.4 0 0 00.5% Micro 1.52 0.4 0 0.49 01.0% Micro 1.52 0.4 0 0.98 02.0% Micro 1.52 0.4 0 1.97 03.0% Micro 1.52 0.4 0.004 2.95 00.5% Meso 1.52 0.4 0.004 0 0.491.0% Meso 1.52 0.4 0.008 0 0.971.5% Meso 1.52 0.4 0.011 0 1.45

4. Sample preparation and experimental procedures

4.1. IDT test and specific gravity analysis

The indirect tension (IDT) test is similar to the typical split ten-sion test for concrete, but differ most in that the specimen thick-ness is significantly reduced. Tests set-ups found in Refs. [16–18]were considered in order to obtain accurate tensile strainmeasurements.

Four micro-fiber only mixes, with volume fractions of 0.5%,1.0%, 2.0%, and 3.0%, and three meso-fiber only mixes with volumefractions of 0.5%, 1.0%, and 1.5% were prepared. One 10.16-cmdiameter by 20.32-cm tall cylinder was cast for each mixes. Themixes were prepared in a mechanical mixer. The cylinders wererodded for compaction and cured in a lime-water bath.

At seven days the cylinders were removed from the lime-waterbath, rinsed to remove any residue, weighed, and placed in a 60 �Coven to dry for specific gravity analysis. Shortly before testing onthe following day, the specimens were removed from the oven,their dry masses were recorded, and they were then cut; the endsof each cylinder were trimmed and three 5.08-cm thick sampleswere obtained from each cylinder.

The specimens were loaded at a constant displacement rate of0.0014-mm/s with measurements recorded 25 times per second.An MTS Axial–Torsional Material Test System machine was usedfor both load application and automated data acquisition. The typ-ical IDT testing arrangement used in this study is shown in Fig. 2.Horizontal displacements from both IDT specimen faces were mea-sured using linear variable differential transformers (LVDTs). Thesehorizontal displacements were then used to calculate strains oneach face, which were averaged to determine the overall specimenstrain. Stresses were calculated according to the followingequation:

rt ¼2Pptd

where rt = stress, P = axial load, t = sample thickness, and d = sam-ple diameter.

The stress and strains at peak load, modulus of resilience, andtoughness index (TI) at the strain of 0.0009 were determined fromthe average results of triplicate tests.

4.2. Flow test

Enough mortar for two tests was prepared for each mixture (seeTable 2) using the experimental arrangement shown in Fig. 3a. Thecone was filled and compacted according to ASTM 1437. The tablewas dropped at a rate adequate to achieve 25-drops in approxi-mately 15 s. Subsequently, the diameter of the mortar was thenmeasured along all four sets of inscribed diametric lines, as shownin Fig. 3b. The flow values were calculated according to the follow-ing equation:

Flow ¼ DA � DC

DC� 100%

where DA = average post-test diameter and DC = diameter of thecone.

Fig. 1. Views of the fiber at varying magnifications; (a) and (b) contrast the aspect ratios of the micro- and meso-fibers.

Fig. 2. Typical IDT experimental arrangement; note the contour fitting bearingplatens.

R.K. Graham et al. / Construction and Building Materials 44 (2013) 751–756 753

While the first of the two tests of a specific mixture was beingperformed the remaining mixture was covered with plastic formoisture retention. Upon completion and clean-up of the first testthis remaining mixture was reconstituted by mixing for one min-ute and was then tested.

4.3. Modulus of resilience and toughness index (TI)

Two parameters, modulus of resilience and toughness index(TI), were used to evaluate the performance of the mortar mixes.

The modulus of resilience describes pre-peak energy absorption,and is calculated by integrating a stress–strain curve up to thepoint of peak stress (Fig. 4a).

The TI values were calculated by integrating the area under anormalized stress–strain curve (Fig. 4b). That is, the maximumvalue along the vertical axis was reduced to unity, and then inte-gration was performed using the strain at peak load as the lowerbound of integration. The upper bound of integration was takenas either the point where a significant stress decrease took placeor at a strain of 0.0009, whichever occurred first.

For most specimens, a significant drop in stress took place wellbefore a strain of 0.0009 was attained. In such instances it wasassumed that any residual stress detected in the post failure regionwas likely the result of one or both halves of the broken specimensbecoming wedged between the bearing platens or some other fac-tor, but not true tensile resistance. The curved shape of the platens(Fig. 2) which follow the contour of the specimens increased thelikelihood that the broken halves might become wedged.

5. Results and discussion

5.1. IDT test

The relationships among fiber volume fraction, fiber size, stressand strain at peak load as determined from the IDT tests were shownin Fig. 5. The peak stress and associated strain improved consistentlywith incremental additions of meso-fibers. The results from themicro-fiber-reinforced mortars on the other hand, displayed noconsistent pattern. In general, however, the mortars containing 2%and 3% of micro-fiber were similar and superior to those containing0.5% and 1%, which were themselves grouped closely, and all fiber-reinforced mixtures performed superiorly to the control.

The modulus of resilient results are shown in Fig. 6, whichcorroborates the results of Fig. 5. The pre-peak energy absorptionincreased steadily with increasing meso-fiber content while theresults from the micro-fiber-reinforced mortars were inconsistentwith respect to fiber content. All results from the fiber-reinforcedmortars were superior to that of the control.

Fig. 3. Flow test.

(a) Modulus of resilience

(b) Toughness index (TI)

00

Stre

ss

Data

Peak Stress

Strain

0

1

0

Nor

mal

ized

Str

ess

(Uni

tles

s)

Strain

Data

Lower Limit

Upper Limit

Fig. 4. Schematic of modulus of resilience and toughness index.

Control

0.5% Micro1.0% Micro

2.0% Micro*

3.0% Micro

0.5% Meso

1.0% Meso

1.5% Meso

4.25

4.5

4.75

5

5.25

5.5

5.75

0.33 0.35 0.38 0.40 0.43 0.45 0.48 0.50 0.53 0.55

Stre

ss (

Mpa

)

Strain (mm/mm)

Fig. 5. Average peak load stress and strain of triplicate IDT specimens.

Control

0.5% M

icro

1.0% M

icro

2.0% M

icro*

3.0% M

icro

0.5% M

eso

1.0% M

eso

1.5% M

eso

0.0

0.5

1.0

1.5

2.0

2.5

kPa

Fig. 6. Average modulus of resilience from triplicate IDT specimens (error barsindicate maximum and minimum values).

754 R.K. Graham et al. / Construction and Building Materials 44 (2013) 751–756

It is clear that the micro-fibers improved the pre-peak perfor-mance of the mortar, but with much less efficiency than did themeso-fibers. As an example, Figs. 5 and 6 show that the pre-peakperformance of the 1% meso-fiber mortar paralleled those of the2% and 3% meso-fiber mortars.

The effects of fiber-reinforcing on the post-peak tensile tough-ness of the mortar are presented in Fig. 7 in the form of TI values.These results show that the relative toughness of the mortar was afactor of both fiber volume fraction and, more importantly, theaspect ratio of the reinforcing fibers. The relative toughness, orpost-peak energy absorption, was improved much more efficiently

with the meso-fibers than with the micro-fibers. This is demon-strated by the fact that the TI of the 3% micro-fiber mortar wasinferior to that of the 0.5% meso-fiber mortar.

The failure between a crack-bridging fiber and the host matrixis likely the result of either fiber–matrix debonding and pullout

Control

0.5% M

icro

1.0% M

icro

2.0% M

icro

3.0% M

icro

0.5% M

eso

1.0% M

eso

1.5% M

eso

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Fig. 7. TI values from triplicate IDT tests (error bars indicate maximum andminimum values).

Table 4Flow test results.

Mixture Fiber (vol.%) Drops Flow (%)

Per test Average

Control 0 21 139 14120 142

Micro 0.49 23 146 14423 141

Meso 0.49 25 123 12725 131

R.K. Graham et al. / Construction and Building Materials 44 (2013) 751–756 755

or fiber rupture, which may also involve some level of debonding.The mode of failure that occurs is highly dependent upon theembedment length of the fiber. Furthermore, it can reasonably beassumed that debonding/pull-out would require substantially lessenergy than would fiber rupture due to the smooth surface andhigh tensile strength and stiffness of the fibers used in this study.Therefore, the ultimate failure of the micro-fiber-reinforced mortarwas likely due to debonding and pull-out. The meso-fibers had ahigher probability of having greater embedment. Their deeperembedment would either increase the amount of energy neededfor debonding and pull-out or may have resulted in fiber rupture.This explains the relative toughness results presented in Fig. 7.

These results of the IDT tests demonstrate that micro-length,low aspect ratio carbon fiber can be used to improve the tensileperformance of cementitious materials, but that the longer meso-fibers were much more efficient in this regard.

5.2. Specific gravity analysis and flow tests

Table 3 presents the bulk specific gravity results of the fiber-reinforced mortars. The results of the specific gravity clearly showthat the meso-fiber mixes were less dense than the micro-fibermixes. This implies that the meso-fiber mixes had a higherair-to-cement ratio than did the micro-fiber mixes. The higher aircontent of meso-fiber mixes increased their porosity and may haveadverse effects on the durability and long-term performance ofthese mixes.

The results of the flow table tests are presented Table 4, whichshows that the workability of the micro-fiber-reinforced mortarwas similar to that of the control and superior to that of themeso-fiber-reinforced mortar. The intended number of table dropswas 25, per ASTM 1437. However, as documented in Table 4, thisnumber of drops was not reached for any of the control ormicro-fiber-reinforced mortar tests due to their excessive fluidity.

Despite their inferior tensile performance, the micro-fibers-reinforced mortars were superior to the meso-fiber-reinforcedmortar with respect to unintended air-to-cement content andworkability. The superior workability of the micro-fiber-reinforced

Table 3Bulk specific gravities of cylinders prior to cutting for IDT testing.

Mixture Control Micro Meso

0.5% 1.0% 2.0% 3.0% 0.5% 1.0% 1.5%

Bulk SG 2.220 2.222 2.235 2.218 2.209 2.137 2.163 2.168

mortar is accentuated by the fact that the flow tests of this mortarwere terminated prematurely (<25-drops) and yet still had anaverage flow number more than 12% greater than that of themeso-fiber mortar. This definite degradation of workability isobserved with a fiber volume fraction of only 0.5% meso-fiber.

6. Summary and conclusions

A laboratory study has been conducted to gain an understand-ing of how micro-length, low aspect ratio carbon fibers might beutilized to reinforce cement-base materials. IDT tests, specificgravity analysis, and flow tests were employed to compare theperformance of Portland cement mortar reinforced with either mi-cro-length or meso-length carbon fiber.

This study has demonstrated that micro-length, low aspect ratiocarbon fibers had some merit for improving the peak-stress,pre-peak energy absorption, and to a lesser degree, the post-peakenergy absorption of cementitious materials, while retaining theworkability and air content of the base mix. Even though longer,higher aspect ratio carbon fibers would degrade the workabilityof the mix and increase its air content, they improved the tensileproperties of the mortar with much more efficiency than did themicro-fibers. Thus, the micro-fibers investigated in this studymay not be suited as the sole reinforcing agent for cementitiousmaterials unless relatively small tensile strength/toughnessimprovements and/or workability retention is required. However,they may be apt as a supplement of meso-length fibers. Forexample, when a cementitious mixture is primarily reinforced bymeso-length carbon fiber, and poor workability limits the furtheraddition of such fiber, it may be possible to incorporate micro-length fiber in order to increase the overall fiber content andprovide further strength gains.

Acknowledgement

The authors would like to thank Toho Tenax America, Inc. whodonated the carbon fibers in this study.

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