Construction and Building Materials 47 (2013) 111–117

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Construction and Building Materials

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Enhancement of the durability characteristics of concretenanocomposite pipes with modified graphite nanoplatelets

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.05.002

⇑ Corresponding author.E-mail addresses: peyvandi@msu.edu, amirpasha.peyvandi@gmail.com (A. Pey-

vandi).

Amirpasha Peyvandi a,⇑, Parviz Soroushian a, Anagi M. Balachandra b, Konstantin Sobolev c

a Dept. of Civil and Environmental Engineering, Michigan State University, 3546 Engineering Building, E. Lansing, MI 48824-1226, USAb Metna Co., 1926 Turner St., E. Lansing, MI 48906, USAc Dept. of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, WI 53211, USA

h i g h l i g h t s

� Durability performances of dry-cast concrete mix with graphite nanoplatelet were evaluated.� Graphite nanoplatelets were surface modified to improve their dispersion in mixing water.� Improvements in durability characteristics of concrete pipes were realized by adding of nanomaterial.

a r t i c l e i n f o

Article history:Received 26 February 2013Received in revised form 2 May 2013Accepted 3 May 2013Available online 1 June 2013

Keywords:Graphite nanoplateletSurface modificationDispersionConcrete pipesSorptivityDurabilityAcid resistanceSanitary sewerFiber

a b s t r a c t

Enhancement of cement-based materials with graphite nanomaterials builds upon and complements theestablished practice of reinforcing cement-based materials with micro-scale fibers. Among graphitenanomaterials which have reached industrial-scale production, graphite nanoplatelets offer favorableeconomics for use in concrete. The planar geometry of graphite nanoplatelets is another distinguishingfeature of this nanomaterial, which favors its role towards enhancement of the transport and durabilitycharacteristics of concrete. An experimental investigation was conducted in order to determine the con-tributions of graphite nanoplatelets to the durability of dry-cast concrete pipes in aggressive sanitarysewer environment. Laboratory studies were followed with industrial-scale production of concrete pipesincorporating graphite nanoplatelet and/or micro-scale polyvinyl alcohol (PVA) fibers. Special attentionwas given to dispersion and interfacial interactions of graphite nanoplatelets in concrete. For this pur-pose, nanoplatelet surfaces were modified by a polyelectrolyte, and the modified nanoplatelets were firstdispersed in a fraction of the mixing water prior to addition to concrete. Significant improvements in thetransport and durability characteristics of concrete pipes were realized by the addition of modifiedgraphite nanoplatelets. These improvements could be attributed to the close spacing and high specificsurface area of graphite nanoplatelets, which effectively hinder sorption of moisture and aggressivechemicals into concrete, and control growth of microcracks under aggressive exposures.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Concrete pipes, as integral elements of infrastructure systems[1], are utilized in sanitary and storm water sewer systems [2,3].Maintenance and repair of concrete pipes represent growing eco-nomic burdens, especially in the aggressive sanitary sewer envi-ronment [1,4].

Concrete pipes are susceptible to chemical and biogenic attacks[5–9]. Microorganisms existing in the sanitary sewer environmentproduce sulfuric acid, which is responsible for a severe degradationmechanism of concrete pipes which is generally termed ‘‘Microbial

Induced Concrete Corrosion’’ [3,6,9,10]. Gypsum (CaSO4�2H2O)forms as a result of the reaction of sulfuric acid and calciumhydroxide (CH); it further reacts with calcium monosulfate hydrateto form the needle-shaped ettringite (3CaO�Al2O3�3CaSO4�32H2O).The resulting gypsum and etrringite products occupy more spacethan the initial reaction compounds, causing expansion and crack-ing of concrete. The products of degradation are removed by thepipe internal flow expediting the corrosion process [5,8,11].

The solutions investigated for improving the resistance of con-crete pipes against such aggressive conditions include refinementof concrete mix design [12–15], use of sulfate-free aggregates[16], use of pozzolanic materials [17], fiber reinforcement of con-crete [3], use of different chemical admixtures [18], surface treat-ment [19], and proper mixing and thorough curing methods [20].Progress in nanotechnology has opened new prospects for

112 A. Peyvandi et al. / Construction and Building Materials 47 (2013) 111–117

enhancement of the barrier and durability characteristics of con-crete [21–25]. The present investigation studies the effects ofgraphite nanoplatelet (GP) on the barrier and durability character-istics of concrete in sanitary sewer pipes.

Concrete materials provide acceptable levels of engineeringproperties (compressive strength, moisture resistance and durabil-ity) at a relatively low cost [21–24]. Concrete, however, offers lim-ited toughness and impact resistance. Micro-fibers are used forimproving the toughness and impact resistance of concrete. Themicro-scale diameters and relatively low aspect (length-to-diame-ter) ratios of these fibers, and their viable volume fractions in con-crete, however, yield relatively large fiber-to-fiber spacing andrelatively small specific surface area of fibers. These features limitthe effectiveness of micro-scale fibers in controlling the flaw(microcrack) size and propagation, and in improving the barrierand durability characteristics of concrete. Micro-scale fibers, onthe other hand, are relatively long, and their extended (frictional)pullout can dissipate substantial energy. One can expect a desiredsynergy between micro-scale fibers and nanomaterials becausethey function at different scales, and also because nanomaterialscould benefit the pullout behavior of micro-scale fibers. Nanomate-rials with distinct geometric and engineering properties could beused towards overcoming these drawbacks, and for enhancingthe transport and durability characteristics as well as the tensile/flexural strength of concrete. Relatively low-cost graphite nanom-aterials offer the promise for rendering unique reinforcement qual-ities in concrete which bring about balanced gains in mechanicaland durability characteristics [25]. Tortuous diffusion paths im-posed by the presence of GPs can significantly benefit the moisturebarrier and durability characteristics of concrete.

Initial research on enhancement of cementitious materialsthrough introduction of nanomaterials has largely focused on theuse of carbon nanotubes [3,8,9,16,19,26] and the use of ‘‘nanonets’’was proposed [25]. These exploratory investigations have mostlyused high-performance cementitious paste and sometimes mortaras the matrix incorporating nanomaterials [27,28].

The study reported herein evaluated the effects of nano- and/ormicro-scale reinforcement on the durability characteristics of (dry-cast) concrete pipes. The focus of this investigation was on themoisture sorptivity and acid resistance of concrete pipes [28],which determine their service life in sanitary sewer applications.Eight full-scale concrete pipes were produced at a manufacturingplant using plain concrete and concrete with nano- and/or micro-scale reinforcement. Samples were produced using the concreteprepared at industrial scale; samples were also cored from con-crete pipes. Polyvinyl alcohol (PVA) fiber which complements rel-atively high mechanical properties with desired acid resistance[29,30] was selected as the micro-scale reinforcement. GPs wereselected for use in the project because of their distinctly low cost(among nanomaterials), industrial-scale availability, desired geo-metric attributes (including the planar geometry which favorstheir contributions to barrier attributes of concrete), and highmechanical qualities.

Fig. 1. TEM micrograph of graphite nanoplatelet (GP) with planar geometry.

2. Experimental program

2.1. Materials

The raw materials used for production of concrete pipes were natural coarseaggregate with 19 mm maximum particle size, natural sand (2NS) with maximumparticle size of 4.75 mm, Type I Portland cement, Class F fly ash, ground granulatedblast furnace slag, water-reducer (Catexol 1000 NP) and set-retarder (Catexol 1000R). The cement: fly ash: slag: sand: coarse aggregate: water weight ratios in thisdry-cast concrete mix were 0.70:0.14:0.16:2.6:2.1:0.36. The water-reducer andset-retarder dosages were 28 and 60 g per 4.5 kg of cementitious materials, respec-tively. The 28-day compressive strength of plain concrete was 27 MPa. Graphitenanoplatelets which are produced commercially were purchased from XG Sciences.Modified GPs were dispersed in 10% of the concrete mixing water. The graphite

nanomaterial considered in this investigation was GP with 6–8 nm thickness and25 lm average planar dimension. The specific gravity of GP was 2 g/cm3 (Fig. 1).Polyvinyl alcohol (PVA) fibers were used as micro-scale reinforcement; these PVAfibers had a length of 13 mm, and a diameter of 100 lm. The specific gravity ofPVA fibers was 1.3 g/cm3.

Benefits of modified GPs, used alone or in combination with (micro-scale) PVAfibers, towards enhancing the durability characteristics of concrete pipes were eval-uated through industrial-scale production and experimental evaluation of concretepipes with diameter of 53 cm and length of 2.44 m. A total of eight pipes were man-ufactured (Fig. 2) according to the procedure described in the following: (i) twowith plain concrete; (ii) two with GP at 0.2 vol.% of cementitious materials(0.05 vol.% of concrete); (iii) two with PVA fibers at 3.2 vol.% of cementitious mate-rials (0.8 vol.% of concrete); and (iv) two with GP at 0.2 vol.% and PVA fibers at3.2 vol.% of cementitious materials. The nanoplatelet volume fraction was selectedbased on laboratory-scale optimization studies which targeted achievement of bal-anced gains in the mechanical and sorption characteristics of concrete. The dosageof PVA fibers was selected based on the previous experience with normal-strengthconcrete.

2.2. Modification of graphite nanoplatelets

Optimal bonding of GPs to the matrix enables mobilization of the exceptionallyhigh tensile strength and the frictional resistance against the tremendous surfacearea of GPs toward achievement of high strength and toughness levels. The optimalbonding requirements can be met by careful surface modification of graphitenanomaterials. Modified GPs can play multi-faceted roles toward enhancing themechanical, physical and functional attributes of cementitious materials.

The surface modification method used in this study involved grafting of Polyac-rylic Acid (PAA) onto GPs for introducing high concentrations of hydroxyl and car-boxyl (OH and COOH) groups on GP surfaces (Fig. 3). A detailed description of thismodification technique has been presented by authors elsewhere [31]. The hydro-xyl and carboxyl groups introduced on modified GP benefit dispersion of nanoplat-elets in water by making their surfaces more hydrophilic and (in the presence ofpolymers) via electrosteric stabilization. The hydroxyl and carboxyl groups also en-able bonding to cement hydrates via coordinate bond formation with Ca2+ ions incement hydrates (Fig. 3), strong cationic and anionic interactions, and other second-ary bonding.

Laboratory studies indicated that modification of GP with PAA at 1.0:0.1 GP-to-PAA weight ratio enhances the contributions of GP to the flexural strength ofcementitious matrices. The required amount of PAA was added to GP prior to dis-persion in water.

2.3. Dispersion of graphite nanoplatelets in water

GPs were dispersed in 10% of the concrete mixing water using the procedureoutlined below (the dispersed nanoplatelets was then added to the mixer usedfor preparation of the concrete):

(1) The required amount of GP was added to water, and the mix was stirredovernight (�12 h); and

(2) The mix was sonicated for 10 min at 30, 45, 65 and 75% of maximal power(400 W) with 1-min intervals between the intensity increments; and

(3) Repeated step (2).

Fig. 2. Eight concrete pipes fabricated with four different combinations of graphitenanoplatelet (GP) and PVA fiber dosages.

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2.4. Pipe production

The pipes were produced using the dry-cast method which employs a zero-slump normal strength concrete that is consolidated using a combination of vibra-tion and pressure. By ordinary standards, dry-cast concrete mixes with relativelylow water content would be considered unworkable. However, they become use-able through the use of specialized consolidation techniques, such as intense vibra-tion and packing. Dry-cast concrete products enable efficient use of a limitednumber of molds in high quantity production. Conditions because freshly consoli-dated products can be demolded without slumping or cracking under their ownweight. Given the relatively low workability of dry-cast concrete mixtures, pan-type mixers are commonly used to ensure their through blending. The approachdeveloped in this project for production of dry-cast concrete nanocomposites wassimilar to that commonly used with plains dry-cast concrete, except that: (i)nanomaterials were pre-dispersed in 10% of the concrete mixing water; and (ii) fi-bers were added directly to the mixer. All the pipes fabricated and tested in the pro-ject had the internal diameter of 53 cm, wall thickness of 6.9 cm, and length of8 feet. The pipes were reinforced with a 3 � 6 – W2 �W2.5 (W2 circumferentialand W2.5 longitudinal) wire mesh. After concrete placement and consolidation un-der pressure, concrete pipes were subjected to steam curing at 60 �C over 10 h, andwere then cured at ambient conditions. Pipes were subjected to structural tests(Three-Edge-Bearing test) 28 days after production (Fig. 4). Samples were thencored from the less stressed (and uncracked) regions of pipes for performance ofdurability tests. These cored samples represent concrete subjected to the indus-trial-scale production and service loading conditions experienced by pipes.

2.5. Concrete specimens

Concrete specimens were produced in the pipe production plant using the con-crete materials prepared at industrial scale, consolidated using a combination ofvibration and pressure similar to those applied to concrete pipes, steam cured fol-lowing the procedures used in the plant, and then exposed to ambient temperatureand 50% relative humidity prior to testing at 28 days of age. Three cylindrical spec-imens with 10 cm diameter and 5 cm height were produced from each concrete mix

Fig. 3. Interactions of hydroxyl/carboxyl groups used to modify the surface of graphine n

for sorption tests. Three prismatic 10 cm � 10 cm � 40 cm specimens were pro-duced for acid attack tests. Two sorption specimens were also cored from each pipe(four for each concrete mix), Fig. 5, the cores divided by half to produced rectangu-lar (10 � 6.9 cm) cross section for sorption test. These specimens were cored afterperformance of three-edge bearing tests from less stressed (uncracked) regions ofpipes. These specimens were cut into two halves which provide rectangular outersections for exposure to water in sorption tests.

2.6. Test procedures

2.6.1. SorptivitySorptivity tests (ASTM C 1585) were conducted on specimens which were ex-

posed on one surface to water (with all other surfaces sealed). The specimens wereweighed after 1, 5, 10, 20, and 30 min, 1, 2, 3, 4, 5 and 6 h, and 1, 2, 3, 4, 5, 6, 7, 8, 9and 10 days. For this purpose, the specimen’s wet surface (exposed to water) wasquickly dried with a tissue, and the specimen weight was measured prior to re-exposure of the surface to water.

2.6.2. Acid resistanceAcid resistance tests were performed on prismatic specimens using a modified

version of ASTM C 267. Concrete specimens were exposed to a 3% sulfuric acid solu-tion at room temperature over a period of 90 days. The acid solution was replacedevery 15 days. The 3% sulfuric acid solution was selected to be consistent with otherlaboratory investigations of concrete acid resistance in sanitary sewer environment[14]. After 90 days of exposure to acid attack, specimens were washed to removeany loose particles, and then tested in flexure; weight and thickness were alsomeasured.

2.6.3. FlexureFour-point flexure tests (ASTM C 78) were performed (in unaged condition and

after aging) on prismatic concrete specimens with 100 mm2 cross sections over aspan of 30 cm.

3. Experimental results and discussion

3.1. Structure of concrete nanocomposite

The planar geometry of GPs and their close spacing within con-crete (Fig. 6) make them an effective additive against sorption ofmoisture and aggressive solutions into concrete, and also againstmicrocrack inception and growth. The closely spaced nanoplateletsforce tortuous diffusion paths into concrete, which significantlybenefit the barrier qualities of concrete. This, together with controlof microcracks by such closely spaced nanoplatelets of high spe-cific surface area, benefits the durability characteristics of concretenanocomposites in aggressive environments.

Scanning electron microscope observations of the fractured sur-faces of (dry-cast) concrete pipes (Fig. 7a) pointed at non-uniformdispersion of graphite nanoplatelets within cementitious matrix.Areas of high nanoplatelet concentration could be detected(Fig. 7b) where bonding of cementitious paste to nanoplateletsseemed imperfect. The non-uniform dispersion of graphite

anoplatelet with cement hydrates (calcium silicate hydrate and calcium hydroxide).

Fig. 4. (a) Three-Edge Bearing Test Set-Up; (b) front view of three-edge bearing test.

Fig. 5. Sorption test specimens cored from pipes.

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nanoplatelets within concrete could be attributed to the relativelylow cementitious paste content and the dry nature of the dry-castconcrete mixtures used in this investigation.

Fig. 6. SEM images concrete incorporating graphite nanoplatelets (GPs) at fracturedsurface.

3.2. Sorptivity test results

Figs. 8 and 9 show the result of sorptivity test which were per-formed on specimens that were either cored from concrete pipes(Figs. 8a and 9a) or produced from the concrete mixtures used inconcrete pipes (Figs. 8b and 9b). The specimens cored form loadedpipes had higher moisture sorption rates, which could be attrib-uted to microcracking of concrete pipes under structural loading,and probably reduced level of concrete consolidation in large pipeswhen composed with small laboratory specimens. Major and sta-tistically significant (at 0.05 significance level) gains in resistanceto moisture sorption of concrete cores and specimens were real-ized with introduction of GPs (alone or in combination with PVAfibers). Concrete materials with GP alone or in combination with

PVA fibers produced statistically comparable sorptivity test results.PVA fibers produced a slight rise in concrete sorptivity, which mayhave resulted from the minor loss of fresh mix workability withintroduction of PVA fiber.

3.3. Resistance against acid attack

After 90 days of acid attack, the visual appearance (Fig. 10) aswell as the measured values of weight loss (Fig. 11a) and thicknessloss (Fig. 11b) of concrete specimens with different reinforcementsystems after 90 days of acid attack pointed at some improvementsin acid resistance with GP, synthetic microfiber (PVA) and espe-cially hybrid (GP/PVA) reinforcement. Flexural strength test results(Fig. 12), on the other hand, highlighted the significant gains in acidresistance of concrete realized with introduction of GPs (and, to alesser extent, PVA fibers). This finding supports the hypothesis thatthe contributions of GPs towards moisture barrier qualities andmicrocrack resistance yield important benefits to the acid resis-tance of concrete. Reasoning of the observation that GP and GP/PVA reinforcement significantly benefited the reinforcement ofconcrete flexural strength under acid attack (when compared withplain concrete and that PVA fiber reinforcement) in spite of the factthat the corresponding benefits towards control of weight and

Fig. 7. SEM images of graphite nanoplatelets (GPs) in dry-cast concrete; (a) low magnification, and (b) high magnification at fractured surfaces.

Fig. 8. Rate of moisture sorption of concrete with different reinforcement conditions; (a) specimens cored from concrete pipes, and (b) specimens obtained from concretemixtures.

Fig. 9. Moisture sorption test results (means and standard errors) over 10 days; (a) specimens cored from concrete pipes, and (b) specimens obtained from concrete mixtures.

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thickness loss were not as significant requires furtherinvestigations.

4. Conclusions

Concrete pipes were produced at industrial scale in order to as-sess the effects of nano-scale (graphite nanoplatelet) and/or micro-

scale (PVA fiber) discrete reinforcement on the moisture transportand durability characteristics of concrete. The dry-cast method ofconcrete pipe production, due to low binder content and freshmix workability, affected thorough dispersion of nanoplatelets inconcrete. Graphite nanoplatelets were modified via physisorptionof polyacrylic acid (PAA) polyelectrolyte in order to improve theirdispersion and interfacial interactions in concrete. Tests wereperformed on specimens cored from concrete pipes, and also on

Fig. 10. Visual appearance of concrete specimens with different reinforcement systems after 90 days of acid attack (a), typical plain concrete prismatic specimens prior to (b)and after (c) 90 days of exposure to acid attack.

Fig. 11. Measured values of weight and thickness loss (means and standard errors) after 90 days of acid attack for concrete specimens with different reinforcement systems;(a) weight loss ratio of original weight, and (b) thickness loss ratio of original thickness.

Fig. 12. Reduction of flexural strength test results (means and standard errors) ofconcrete materials with different reinforcement systems after 90 days of acidattack.

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specimens prepared from concrete materials produced at indus-trial scale. Graphite nanoplatelets at low dosages (0.05 vol.% ofconcrete) were found to significantly improve the moisture trans-port performance and acid resistance of concrete without or withPVA fiber reinforcement. These improvements could be attributedto the tortuous diffusion paths forced by the closely spaced nano-platelets of planar geometry, and also to the microcrack arrestqualities of the closely spaced nanoplatelets with high specific sur-face area and desired interfacial bonding to cement hydrates. Not-ing that microbial-induced acid attack on concrete pipes is a major

concern in sanitary sewer environments, the implementation ofproposed graphite nanoplatelet and PVA fiber reinforcement inPortland cement based systems designed for severe exposures isthe most promising.

Acknowledgments

The authors wish to acknowledge the support of National Sci-ence Foundation (NSF) (Award No. IIP-1142455) for the project.The support of Northern Concrete Pipe Inc. (Charlotte, MI) towardsproduction and experimental evaluation of pipes is alsoacknowledged.

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