Construction and Building Materials 25 (2011) 195–200

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Construction and Building Materials

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Effect of deicing solutions on the tensile strength of micro- or nano-modifiedasphalt mixture

Shu Wei Goh a, Michelle Akin b, Zhanping You a,*, Xianming Shi b,c,**

a Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931-1295, United Statesb Western Transportation Institute, Montana State University, PO Box 174250, Bozeman, MT 59717-4250, United Statesc Department of Civil Engineering, Montana State University, 210 Cobleigh Hall, Bozeman, MT 59715, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 March 2010Received in revised form 19 May 2010Accepted 19 June 2010Available online 24 July 2010

Keywords:DeicerTensile strengthMoisture susceptibilityNano-modified asphalt mixtureModified asphalt mixture

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.06.038

* Corresponding author. Tel.: +1 906 487 1059.** Corresponding author. Tel.: +1 406 994 6486.

E-mail addresses: sgoh@mtu.edu (S.W. Goh), mi(M. Akin), zyou@mtu.edu (Z. You), xianming_s@coe.m

This paper reports on the potential benefits of micro- or nano-sized materials for asphalt mixtures usedon pavements, specifically when they are exposed to water or deicing solutions. Asphalt mixtures wereprepared with various amount of nanoclay and/or carbon microfiber, and compacted using the Super-pave™ gyratory compactor. Moisture susceptibility and deicer impacts were assessed by exposing thesamples to water or deicing chemicals (NaCl, MgCl2 and CaCl2), and seven freeze–thaw cycles, in a mod-ified AASHTO T283 test. Comparisons of micro- or nano-modified asphalt mixtures exposed to deicers aremade based on results of indirect tensile strength tests, which preliminarily demonstrate the great poten-tial of using microfibers and nanoclays in asphalt mixture for improved performance. Based on theresults, it was found that the addition of nanoclay and carbon microfiber would improve a mixture’smoisture susceptibility performance or decrease the moisture damage potential in most cases. Thedetailed effects of deicing solutions on the tensile strength of micro- or nano- modified asphalt mixtureare discussed in this paper.

� 2010 Elsevier Ltd. All rights reserved.

1. Background

Deicers are frequently used in cold climates for snow and icecontrol to maintain high levels of service on winter roadways,which translate to safety, mobility and productivity benefits. How-ever, the growing use of these materials has raised concerns overtheir effects on pavements, bridge decks, motor vehicles, and theenvironment. The deleterious effects of deicers on Portland cementconcrete (PCC) and their corrosion to metals in transportationinfrastructure are well known and have been recently summarized[23,24].

Compared with PCC pavement, asphalt pavement features ahigher chemical resistance and is generally less affected by chlo-ride-based deicers. Hence until now, there have been no specifi-cations developed in the United States assessing and addressingthe negative impacts of deicers on asphalt pavements. Further-more, only limited research has been conducted to study theinteractions between asphalt and deicers [8,23]. Severalconsequences of extensive use of deicers have been documentedin Nordic countries and Canada, including the loss of skid resis-

ll rights reserved.

chelle.akin@coe.montana.eduontana.edu (X. Shi).

tance [7] and damage within the asphalt pavement structure[8,9]. Some studies have documented that damage to the asphaltmixture in the presence of deicers is greater than compari-sons performed with only water. Researchers reported thateffects attributable to deicers were observed especially whenpavement underwent freeze–thaw cycles [6,23]. The main reasonwas that the strength and elasticity of asphalt mixtures werereduced which led to more severe freeze–thaw damage [9,14].A recent study [16] also confirmed that the use of acetate-baseddeicers could accelerate degradation of asphalt mixture viaemulsification of the binder during the modified boiling watertest [3]. Due to the widespread use of deicers on asphaltpavements, a study of potential mitigation measures iswarranted.

Numerous studies have been devoted to enhancing the perfor-mance and/or durability of asphalt binder or mixture throughphysical or chemical modifications, all of which may offer insightsin mitigating the deleterious effects of deicers. One method wasto modify asphalt mixtures using an additive to improve thedurability and ductility. Polymer modifiers such as styrene–butadiene rubber (SBR), styrene–butadiene–styrene (SBS), ethylenevinyl acetate (EVA), and polyethylene had been one of the mostpopular methods to improve the ductility and elasticity of asphaltpavement [27,32]. In China, researchers have studied perfor-mance and rheological properties of montmorillonite (MMT)

196 S.W. Goh et al. / Construction and Building Materials 25 (2011) 195–200

modified asphalt [29–31]. MMT is the clay that forms microscopicplaty micaceous crystal and it has been widely used as a modifierto improve thermal, mechanical, and barrier properties of poly-mers [2,21,22,33]. Researchers indicated that the addition ofMMT was able to increase the softening point and viscosity ofthe asphalt at high temperature [29]. Organo-montmorillonite(OMMT) as an additive was also demonstrated to increase ductil-ity retention rates of asphalt and to improve its thermo-oxidativeaging resistance [28].

Even though engineers are interested in the material propertiesat the macro- and meso- scales, the phenomena at nano- and mi-cro- scales provide fundamental insight for the underlying interac-tions defining the physico-chemical behavior of materials.Previously, additives such as nanoparticles (e.g., nanoclay) andmicrofiber (e.g., carbon microfiber) were found to have the poten-tial to improve material strength and fatigue characteristics whileenhancing ductility and other durability properties of engineeringmaterials [12,13,17,20,34]. Extensive research on using microfiberto enhance concrete performance has been conducted and thesestudies have proven that the addition of carbon microfiber im-proves concrete durability [4,17,18,20]. However, there are only alimited number of studies on the use of carbon microfiber to rein-force asphalt pavement. Currently there is also the need to explorethe combined use of and the potential synergism between micro-and nano-sized modifiers in improving the properties of asphaltmaterials, especially when they are exposed to deicer solutions.

2. Objectives

The objectives of this research are to modify an asphalt mixturewith two materials – nanoclay and carbon microfiber; and to eval-uate its performance in terms of tensile strength after exposure towater or various deicer solutions.

3. Experimental design

A traditional hot-mix asphalt (HMA) mixture was designedaccording to the Superpave™ method and utilized a PG 58-28binder. The optimum asphalt content was 5.2% by weight, basedon the air void requirements at 9 and 150 gyrations in theSuperpave™ gyratory compactor. The aggregate was supplied bythe Knife River Corporation in Belgrade, Montana. The HMA pucksprepared for indirect tensile strength testing utilized the optimummix design, but were compacted under 200 gyrations and had anestimated air viod content of approximately 1%. The samples werefabricated to the size of 150 mm (±5 mm) in diameter and 95 mm(±5 mm) in height to meet the test specification.

For most pucks, the traditional mix was modified with nanoclayand/or carbon microfiber in various combinations, from zero to2.0% by mass of bitumen. The properties of the nanoclay (polysi-loxane-modified montmorillonite, with bulk density of 0.251 g/cm3 and aspect ratio of 200–400) and carbon microfiber (with ten-sile intensity of 670 MPa and tensile elastic modulus of 30 GPa)have been reported elsewhere respectively [10,25]. Prior to com-bining and mixing the aggregate and bitumen for the HMA pucks,the heated bitumen was mechanically mixed with the nanoclayand/or carbon microfiber with a high shear force to ensure uniformdistribution. The required mass of bitumen for the pucks was sub-sequently measured from the modified mixture.

A total of 141 pucks were prepared with 17 different mixes uti-lizing various combinations of the nanoclay and carbon microfiber.Triplicate specimens were prepared for each scenario – dry ambi-ent exposure or wet freeze–thaw exposure. Thus, 51 pucks wereprepared for the control (dry) specimens. The remaining 90 puckswere exposed to water or liquid deicing chemicals in freeze–thaw

conditions in a modified version of the AASHTO T283 testprocedure [1]. Three different deicing chemicals commonly usedon roads were used: NaCl, MgCl2, and CaCl2. The deicers were usedeither as liquids at their eutectic concentration or diluted with tapwater at dilution rates of 1:4, 1:10, or 1:15 (deicer:water).

The pucks exposed to water or deicers underwent freeze–thawcycling with intermittent soaking. To begin, the pucks were soakedin the deicer or water for 12 h at room temperature. Each puck wasthen covered with plastic wrap, sealed in a plastic (Ziploc) bag, andplaced in an environmental chamber for 12 h at �17.8 �C. The con-ditioned pucks were exposed to seven 24-h cycles. The experimen-tal design for the modified asphalt bitumen and HMA exposure issummarized in Table 1.

4. Results and discussion

4.1. Tensile strength testing

The purpose of tensile strength testing is to evaluate asphaltmixture’s fatigue potential and moisture susceptibility. Previousresearch has indicated the tensile strength of hot-mix asphalt is re-lated to fatigue cracking [5]. A higher tensile strength means as-phalt pavement can tolerate higher strains before failing (i.e.cracking). Additionally, the moisture susceptibility of the asphaltmixture can be evaluated by comparing the tensile strength of as-phalt mixtures exposed to wet and dry conditions. In this study,the tensile strength of all 141 samples were tested based on AASH-TO T283 [1]. Fig. 1 shows the tensile strength testing setup, andFig. 2 shows a typical result from the indirect tensile strength test.

4.2. Comparison of tensile strength for control mixtures and mixtureswith nanoclay and carbon microfiber

A summary result of the tensile strength ratio (TSR) for samplesconditioned with water is shown in Fig. 3. The value of TSR is cal-culated as:

Tensile strength ratio ¼ Tensile strength of conditioned sampleTensile strength of dry sample

In Fig. 3, it is observed that the modified samples have similar orhigher TSR values compared to the unmodified control mixture.Typically, the final result for TSR testing would have a value of lessthan 1.00 because it is expected that the conditioned sampleswould suffer moisture damage and exhibit lower tensile strength;this phenomenon was observed in the control sample. However, itwas found that more than half of the samples modified with nano-clay and/or carbon microfiber exhibited TSR values greater than1.00. This indicated that the sample after conditioning has highertensile strength. The best mixture in this case was the mixturemodified with 1.5% nanoclay and 1.5% carbon microfiber. Overall,it was concluded that the addition of nanoclay and carbon micro-fiber would reduce a mixture’s moisture susceptibility in mostcases.

4.3. Effect of nanoclay in the asphalt mixture

The effects of nanoclay were evaluated using the mixes withvariable amounts of nanoclay, no carbon microfiber, and exposedto water (not deicers) or dry conditions. The effect of nanoclay isshown in Fig. 4 (tensile strength of dry and conditioned mixtures)and Fig. 5 (tensile strength ratio). Based on Fig. 4, it is observed thatthe tensile strength of dry specimens decrease with increasingamounts of nanoclay; however, if conditioned with water, thespecimens with nanoclay demonstrate higher tensile strength. Asexpected, TSR values increase with increasing amounts of nanoclay

Table 1Experimental design.

Nanoclay (%) Carbon microfiber (%) Exposurea Nanoclay (%) Carbon microfiber (%) Exposurea

0 0 MgCl2 (1:15) 1.5 0 NaCl0 0 Water 1.5 0 Dry0 0 Dry 1.5 0.75 NaCl (1:15)0 0.75 Water 1.5 0.75 CaCl2 (1:10)0 0.75 NaCl (1:4) 1.5 0.75 Dry0 0.75 Dry 1.5 1.5 Water0 1.5 CaCl2 (1:10) 1.5 1.5 Dry0 1.5 Dry 1.5 2 CaCl2 (1:15)0 2 NaCl 1.5 2 MgCl2 (1:4)0 2 CaCl2 (1:4) 1.5 2 Dry0 2 Water 2 0 CaCl2

0 2 Dry 2 0 Water0.5 1.25 NaCl 2 0 NaCl (1:15)0.5 1.25 MgCl2 2 0 Dry0.5 1.25 CaCl2 2 0.75 MgCl2 (1:4)0.5 1.25 Dry 2 0.75 Dry1 0 Water 2 1.5 MgCl2 (1:15)1 0 MgCl2 (1:10) 2 1.5 Water1 0 Dry 2 1.5 Dry1 0.75 CaCl2 2 2 NaCl (1:10)1 0.75 Dry 2 2 Dry1 1.5 NaCl (1:10)1 1.5 MgCl2

1 1.5 Dry1 2 Water1 2 Dry

a Dry, water, or deicer type and dilution ratio (NaCl, MgCl2, or CaCl2).

Fig. 1. Indirect tensile strength testing setup.

Fig. 2. Typical result for indirect tensile strength.

S.W. Goh et al. / Construction and Building Materials 25 (2011) 195–200 197

(Fig. 5). This finding is consistent with a recent study conducted byYeganeh et al. [26] in which nanoclay dramatically improved theadhesion strength and compression modulus. It was hypothesizedthat during the conditioning, the well-dispersed nanoclay particlesenhanced adhesion of the fine aggregates to bitumen [11,15], thusleading to improved strength properties of the asphalt mixture. Inour study, the nanoclay used was polysiloxane-modified montmo-rillonite, which features a hydrophobic end with the siloxane

groups (thus compatible with the bitumen phase) and a hydro-philic end with the montmorillonite i.e. hydrated sodium calciumaluminium magnesium silicate hydroxide (thus compatible withthe aggregate phase).

4.4. Effect of carbon microfiber in the asphalt mixture

The effect of carbon microfiber was evaluated using the mixeswith variable amounts of carbon microfiber, no nanoclay, andexposed to water (not deicers) or dry conditions. The effect ofcarbon microfiber on the tensile strength of dry and conditionedspecimens is shown in Fig. 6 (tensile strength) and Fig. 7 (tensilestrength ratio). Dry specimens with carbon microfiber exhibitedlower tensile strength than the unmodified control specimens.However, for the conditioned samples, it was observed that thetensile strength and TSR was highest for specimens modifiedwith 0.75% carbon microfiber. A previous study has shown thatcarbon microfiber tends to reduce the rate of deterioration dueto freezing and thawing cycles, and scaling due to freezing inthe presence of deicers [19]. A similar phenomenon could beresponsible for these results. Although not shown in the figures,samples with both nanoclay and carbon microfiber at 1.5% andconditioned with water (no deicers) exhibited the greatest ten-sile strength.

Fig. 3. Comparison of tensile strength ratio for control mixture, and mixtures modified with nanoclay and carbon microfiber.

Fig. 4. Comparison of tensile strength for different amounts of nanoclay.

Fig. 5. Comparison of tensile strength ratio for different amounts of nanoclay.

Fig. 6. Comparison of tensile strength for different amounts of carbon microfiber.

Fig. 7. Comparison of tensile strength ratio for different amounts of carbonmicrofiber.

198 S.W. Goh et al. / Construction and Building Materials 25 (2011) 195–200

4.5. Effect of deicers on the tensile strength of asphalt mixtures

Due to the complex experimental design of specimens usingvarious amounts of nanoclay, carbon microfiber, and deicer typesand dilution ratios, trends exhibited by deicers and modifiers aresubtle. Furthermore, the impacts from deicers on asphalt pave-ments have been relatively mild until acetate- and formate-baseddeicers were used recently. The mechanism of deterioration wassuspected to be a combination of chemical reactions, emulsifica-tions, distillation, and the generation of additional stresses inthe asphalt concrete [23]. As mentioned earlier, the deicers usedin this study were MgCl2, NaCl and CaCl2. The tensile strengthof specimens exposed to MgCl2 is shown in Table 2. In general,

higher tensile strengths of asphalt mixture have been found inweaker deicer solutions [23]. For this work, the overall tensilestrengths in the weakest deicer solutions were greater than whenthe specimens were conditioned with water, regardless ofwhether the asphalt mix was modified with micro- or nano-materials.

The tensile strength of specimens exposed to NaCl is shown inTable 3. Unlike with MgCl2, a trend of the effect of the concentra-tion of NaCl solutions on the tensile strength of asphalt mixeswas not clearly demonstrated (tensile strengths are high and low

Table 2Effect of MgCl2 on tensile strength.

Nanoclay (%) Carbon microfiber (%) Conditioned with MgCl2 Conditioned with water

Deicer/water dilution ratio Tensile strength (kPa) Tensile strength (kPa)

1.0 1.5 MgCl2 (1:0) 708.0 – –0.5 1.25 MgCl2 (1:0) 685.3 – –2.0 0.75 MgCl2 (1:4) 790.6 – –1.5 2.0 MgCl2 (1:4) 763.3 – –1.0 0.0 MgCl2 (1:10) 665.9 Water 715.02.0 1.5 MgCl2 (1:15) 953.8 Water 918.30.0 0.0 MgCl2 (1:15) 875.8 Water 670.3

Table 3Effect of NaCl on tensile strength.

Nanoclay (%) Carbon microfiber (%) Conditioned with NaCl Conditioned with water

Deicer/water dilution ratio Tensile strength (kPa) Tensile strength (kPa)

1.5 0.0 NaCl (1:0) 971.1 – –0.0 2.0 NaCl (1:0) 844.6 Water 655.40.5 1.25 NaCl (1:0) 724.2 – –0.0 0.75 NaCl (1:4) 841.8 Water 884.72.0 2.0 NaCl (1:10) 777.0 – –1.0 1.5 NaCl (1:10) 755.4 – –1.5 0.75 NaCl (1:15) 931.3 – –2.0 0.0 NaCl (1:15) 680.3 Water 754.5

Table 4Effect of CaCl2 on tensile strength.

Nanoclay (%) Carbon microfiber (%) Conditioned with CaCl2 Conditioned with water

Deicer/water dilution ratio Tensile strength (kPa) Tensile strength (kPa)

2.0 0 CaCl2 (1:0) 806.4 Water 754.51.0 0.75 CaCl2 (1:0) 728.7 – –0.5 1.25 CaCl2 (1:0) 719.4 – –0.0 2.00 CaCl2 (1:4) 710.3 Water 655.40.0 1.5 CaCl2 (1:10) 903.1 – –1.5 0.75 CaCl2 (1:10) 853.6 – –1.5 2.0 CaCl2 (1:15) 876.7 – –

Table 5Comparison of samples’ tensile strength using different deicer type.

Nanoclay (%) Carbonmicrofiber (%)

Conditioning: deicerand dilution ratio(deicer:water) or water

Tensilestrength (kPa)

0.0 2.0 Water 655.4NaCl (1:0) 844.6CaCl2 (1:4) 710.3

0.5 1.25 NaCl (1:0) 724.2CaCl2 (1:0) 719.4MgCl2 (1:0) 685.3

1.5 0.75 NaCl (1:15) 931.3CaCl2 (1:10) 853.6

1.5 2.0 CaCl2 (1:15) 876.7MgCl2 (1:4) 763.3

2.0 0.0 Water 754.5NaCl (1:15) 680.3CaCl2 (1:0) 806.4

S.W. Goh et al. / Construction and Building Materials 25 (2011) 195–200 199

for both strong and weak dilutions). However, the highest tensilestrength was associated with asphalt mixtures modified with1.5% nanoclay, regardless of deicer concentration and presence ofcarbon microfiber. Compared to specimens conditioned withwater, full-strength NaCl actually showed increased tensilestrength, whereas diluted NaCl was associated with reduced ten-sile strength. One possible explanation for this behavior is thatspecimens soaked in full-strength deicing solutions would be lesssusceptible to freezing temperatures because full-strength deicersdon’t freeze at �17.8 �C, whereas diluted deicers and water willfreeze. The deicer compounds could contribute to additionalexpansive pressures during freezing and result in lower tensilestrengths compared to water. However, the effects of weak MgCl2

solutions are not consistent with the effects of weak NaCl solu-tions, thus chemical factors likely play a larger role than physicalfactors.

The tensile strength of specimens exposed to CaCl2 is shown inTable 4. In general, higher tensile strengths are associated withspecimens conditioned with weaker CaCl2 solutions; this is consis-tent with the observations of MgCl2. Conditioning with CaCl2 in-stead of water also appears to contribute to increased tensilestrengths, whereas this was only observed for the weakest MgCl2

solutions and strongest NaCl solutions.Comparisons between the different deicers can be drawn from

the tensile strengths shown in Table 5. If the asphalt was modified

with carbon microfiber (regardless of whether nanoclay is alsopresent) and conditioned with NaCl instead of water or other deic-ers, the overall tensile strength was higher. However, the highestthree tensile strengths (>850 kPa) were associated with asphaltmodified with 1.5% nanoclay and conditioned with weak deicers.

200 S.W. Goh et al. / Construction and Building Materials 25 (2011) 195–200

Asphalt mixtures conditioned with MgCl2 exhibited weaker tensilestrengths (<800 kPa).

5. Conclusions

Indirect tensile strength tests were performed on asphalt mix-tures modified with nanoclay and/or carbon microfiber in a modi-fied AASHTO T283 test. Hot-mixed asphalt samples wereconditioned in either water or deicing solutions of various concen-trations and were then exposed to seven freeze–thaw cycles. Whilemore testing and analysis are necessary to further understand andexplore the use of nano- and micro-sized materials in asphalt, thefollowing conclusions can be drawn from the current laboratorytesting results:

1. The addition of nanoclay and carbon microfiber would improvea mixture’s moisture susceptibility performance or decrease themoisture damage potential in most cases.

2. As the nanoclay dosage increased, the tensile strength of dry(not conditioned) asphalt mix samples decreased, but increasedfor samples conditioned in water.

3. The addition of carbon microfiber decreased the tensile strengthof dry samples, but specimens with moderate amounts of car-bon microfiber (0.75%) and conditioned in water exhibited thegreatest tensile strengths.

4. In general, asphalt mixtures modified with 1.5% nanoclayappear to offer high tensile strengths and less susceptibility towater or deicers.

5. In general, weaker solutions of NaCl and stronger solutions ofMgCl2 and CaCl2 are associated with higher tensile strengthsof asphalt mixture.

Note that the conclusions were made based on overall observa-tions of the trends in the data as well as the average values in var-ious scenarios. While more in-depth analysis may help furtherelucidate the complex cause-and-effect relations between asphaltmix design/deicer exposure and resulting mixture performance,these trends are not expected to change.

Acknowledgments

The asphalt samples were prepared and conditioned at theWestern Transportation Institute at Montana State University withfunding from the US Department of Transportation Research andInnovative Technology Administration. The authors would like toacknowledge the contributions by Seth Stevens to this researchthrough his assistance with preparing and conditioning the asphaltsamples. The mixture testing work was completed in the Transpor-tation Materials Research Center at Michigan Technological Uni-versity, which maintains the AASHTO Materials ReferenceLaboratory (AMRL) accreditation on asphalt binders and mixtures.

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