Influence of carbon black on the mechanical and electrical properties of asphalt mixtures

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 14, October 2007, pp Influence of carbon black on the mechanical and electrical properties of asphalt mixtures Perviz Ahmedzade* & Tacettin Geckil Department of Civil Engineering, Firat University, Elazig, Turkey Received 16 October 2006; accepted 10 September 2007 The effect of carbon black used as a filler on the mechanical and electrically conductive properties of two types of asphalt mixtures was investigated. The asphalt mixtures in which carbon black and limestone fillers having the same aggregate gradation were prepared and optimum bitumen content was determined by Marshall stability test. Mechanical characteristics of the carbon black and control mixtures were evaluated by Marshall stability, indirect tensile stiffness modulus, creep stiffness, and indirect tensile strength tests. Based on these tests values it is clear that the best results were obtained from mixtures with carbon black filler. Obtained volume resistivity values of the all mixtures indicate that mixtures with carbon black filler have higher electrical conductivity than that of the control mixtures. The results of investigation showed that carbon black improves both mechanical and electrically conductive properties of the asphalt mixtures. Over the years, road structures have deteriorated more rapidly than expected due to increase in traffic density, axle loading and tyre pressure, and an insufficient degree of maintenance. This has resulted in the need to enhance the properties of existing asphalt material. Asphalt is a complex multi-phase material comprising mineral aggregate, filler, bitumen and air. The filler material is considered to be one of the main components of asphalt mixes. The characteristics of mineral filler material have been investigated to determine their potential in upgrading performance of asphaltic concrete, especially by increasing hot mix stability and durability to alleviate problems with rutting and shoving. The use of certain mineral fillers offers enhanced opportunity for increased stiffness and strength in asphalt concrete 1. Filler material is generally produced by crushing and screening of aggregates, limestone, Portland cement, carbon black, and fly ash. It is reported that the use of these substances, especially carbon black, is beneficial to the durability, the wear resistance, and the temperature susceptibility of the mix 2,3. The idea of using carbon black as a reinforcing agent for asphalt was initiated by Martin 4. Martin reported that the dispersing of 3% carbon black by the weight of asphalt appears to have no significant effect on the susceptibility of asphalt binder to temperature. * For correspondence ( pahmedzade@firat.edu.tr) One of the major subsequent research works in the use of carbon black was conducted by Rostler et al. 5. They commented that the poor performance of carbon black in Martin s test results was caused by poor dispersion of carbon black into the asphalt, low carbon black concentration, and the addition of oil fluxing to obtain workable viscosity. They also performed photomicrographs of carbon black dispersion in asphalts and reported that the carbon black particles should have a mean particle diameter of less than microns in order to be integrated into the internal phase of the asphalt cement. Terrel et al. 6 reported on their research at the University of Washington concerning the properties of lignin and the use of carbon black as a reinforcing agent. The carbon black used in that research was Microfil 8. In general, they found that the addition of small amounts of carbon black to mixtures made of lignin and asphalt significantly improved the mixture s stability, increased the stiffness modulus of the mixture, and improved its tensile strength. Their work indicated that treating asphalt cement with carbon black could improve the binder s temperature susceptibility and increase the moisture resistance of the mixes. Yao and Monismith 7 found that the addition of 15-20% by weight of Microfil 8 improved the fatigue life, resilent modulus, and resistance to rutting of asphalt mixes. Button et al. 8 also reported that the addition of 15% Microfil 8 in AC-5 asphalt significantly increased the

2 AHMEDZADE & GECKIL: MECHANICAL AND ELECTRICAL PROPERTIES OF ASPHALT MIXTURES 359 resistance to permanent deformation as compared to that of straight AC-20. Recently, various types of carbon-based materials have been used as additives to produce conductive asphalt concrete mixtures 9,10. Carbon-based material such as is synthetic graphite was used in Snowfree (electrically conductive asphalt pavement system) formulated by Superior Graphite Co., with cooperation of the FAA and Flood Testing Laboratories. This system is based on an electrically conductive asphalt pavement that uses synthetic graphite, asphalt and electricity to heat the airport runway surface to melt snow and ice 11. This paper presents the result of a study that investigated the use of carbon black as a filler in asphalt mixtures. The main objective of this study is to determine the influences of carbon black on the mechanical and electrically conductive properties of asphalt concrete mixtures. Experimental Procedure Materials AC-10 and AC-5 asphalt cement were used in this study. Both asphalts were obtained from Turkish Petroleum Refineries Corporation (TUPRAS). Table 1 gives a summary of the results of some tests performed on the asphalt cements. In this study, crushed limestone was used as a mineral aggregate and it was obtained from quarries around Elazig and which is mainly used for highway construction. Table 2 gives some properties of coarse and fine aggregates. Two types of fillers were used in this study. First type of a filler is limestone dust obtained from sieving the aggregate from No.200 sieve. The limestone filler s specific gravity is g/cm 3. Second type of a filler is carbon black (HAF N-326) directly taken from TUPRAS. Carbon black properties are given in Table 3. The combined gradation of aggregate is shown in Table 4. Preparation of samples Two different aggregate mixtures were used for preparing Marshall specimens. In the first aggregate mixture, limestone was used as a coarse and a fine aggregate, and a filler. In the second aggregate mixture, type and amount of coarse and fine aggregates remained the same, but carbon black was used instead of limestone as a filler. Since Marshall mix design is a volumetric method, therefore the amount of carbon black as filler in aggregate mixture was taken to be equal to the limestone in terms of its volume. Two types of aggregate mixtures and bitumens (AC-10; AC-5) were heated in an oven at a temperature of at least 165 C. AC-10 and AC-5 bitumens were added to the first and the second types of aggregate mixtures separately. Then the bitumenaggregate mixtures were mixed mechanically for s, and Marshall control (AC-10/LS; AC-5/LS) and carbon black (AC-10/CB; AC-5/CB) specimens were prepared. Asphaltic concrete mixture specimens were prepared in triplicate for each aggregate mixture formulation. Table 1 Physical properties of the asphalt cements Properties Standard Asphalt cement types AC-10 AC-5 Specific gravity (g/cm 3 ) at 25 C Ductility (cm) at 25 C Penetration, (0,1mm), 100g, 5s Softening point ( C) Fraass breaking point ( C) Kinematic viscosity, 135 C, (um 2 /s) Penetration index (PI) Penetration viscosity number (PVN) ASTM D70 ASTM D113 ASTM D5 ASTM D36 IP 80 ASTM D Table 2 Properties of limestone mineral aggregate Properties Standard Aggregate Coarse Fine Abrasion loss (%) ( Los Angeles) Frost action (%) (with Na 2 SO 4 ) Specific gravity (g/cm 3 ) Specific gravity (g/cm 3 ) ASTM DC-131 ASTM C-88 ASTM C-127 ASTM C ,

3 360 INDIAN J. ENG. MATER. SCI., OCTOBER 2007 Table 3 Carbon black properties Properties Standard Carbon black (HAF N-326) PH Specific gravity (g/cm 3 ) Sulfur content (%) Ash content (%) Dibutyl phthalate (DBP) absorption (ml/100g) Specific surface area (CTAB) (m 2 /g) Pellet hardness 20 (g) ASTM D-1512 ASTM D-1513 ASTM D-1619 ASTM D-1506 ASTM D-3493 ASTM D-3765 ASTM D-3313 Table 4 Combined aggregate gradation Testing Program Thermogravimetric analysis (TGA) Thermal analysis was performed in nitrogen atmosphere by using a TGA-50 thermobalance at a heating rate of 10 o C min -1. Marshall stability, flow and Marshall quotient tests Marshall stability and flow tests were carried out on compacted specimens at various binder contents according to ASTM D1559. The Marshall test is an empirical test in which cylindrical compacted specimens, 100 mm diameter by approximately 63.5 mm high are immersed in water at 60 C for min and then loaded to failure using curved steel loading plates along a diameter at a constant rate of compression of 51 mm/min. The Marshall stability value (in kn) is the maximum force recorded during compression whilst the flow (in mm) is the deformation recorded at maximum force. The binder contents at maximum bulk specific gravity, maximum stability, 4% air voids in the total mixture, and 80% voids in the aggregate mass filled with binder are used in order to determine the optimum binder content 12. The Marshall quotient (MQ) (kn/mm), calculated as the ratio of stability (kn) to flow (mm) and thereby Sieve size Total cumulative passing (%) 19 mm (3/4 ) 12.5 mm (1/2 ) 9.5 mm (3/8 ) 4.75 mm(#4) 2.00 mm(#8) 1.18 mm (#16) mm (#30) mm (#50) mm (#100) mm (#200) representing an approximation of the ratio of load to deformation under the particular conditions of the test, can be used as a measure of the material s resistance to permanent deformation in service 13. Indirect tensile stiffness modulus (ITSM) test Stiffness modulus is considered to be a very important performance characteristic of the road-base and base-course layers. ITSM test defined by BS DD 213 is a nondestructive test and has been identified as a potential means of measuring this property. The test was performed by Universal Testing Machine (UTM- 5P) 14. The ITSM S m in MPa is defined as: S m L( ν + 0,27) =... (1) Dt where L is the peak value of the applied vertical load (N), D is the mean amplitude of the horizontal deformation obtained from two or more applications of the load pulse (mm), t is the mean thickness of the test specimen (mm), and v is the Poisson s ratio (a value of 0.35 is normally used). During testing, the rise time, which is measured from when the load pulse commences and is the time taken for the applied load to increase from zero to a maximum value is set at 124 ± 4 ms. The load pulse, defined as the period from the start of the load application until the start of the next load application is equated to 3.0 ± 0.05 s. The peak load value is adjusted to achieve a peak transient horizontal deformation of 0.005% of the specimen diameter. The test is normally performed at 20 C but for this investigation, additional tests were performed at 0 C and 40 C. Creep stiffness test One method for the assessment of resistance to permanent deformation is the creep test. Typical conditions under which the unconfined static uni-axial creep test is carried out are: (i) standard test temperature 40 C, for very hot climates 60 C; (ii) preloading for 2 min at 0.01 MPa, as a conditioning stress; (iii) constant loading stress during the test equal to 0.1 MPa; (iv) duration of test: 1 h loading and 1 h unloading 15. During the test, axial deformation is measured as a function of time. Thus, knowing the initial height of the specimen, the axial strain, ε, and therefore the stiffness modulus S mix, at any loading time can be determined:

4 AHMEDZADE & GECKIL: MECHANICAL AND ELECTRICAL PROPERTIES OF ASPHALT MIXTURES 361 S mix = applied stress (σ) cumulative irrecoverable axial strain (ε) (2) Indirect tensile strength (ITS) test The indirect tensile strength test (ITS) is performed at loading rate of 51mm/min by using the Marshall apparatus. The ITS test involves loading a cylindrical specimen with compressive loads that act parallel and loading the vertical diametrical plane. The ITS test is carried out to define the tensile characteristics of the asphalt concrete which can be further related to the cracking properties of the pavement. To compute the ITS, according to the maximum load carried by a specimen at failure, the following equation is used: 2P max ITS = (3) πtd where P max is the maximum applied load (kn), t is thickness of specimen (mm), d is diameter of specimen (mm). Resistance to moisture damage Moisture susceptibility of asphalt mixtures is defined as the vulnerability of the mixture to be damaged by water. As moisture is collected within the asphalt mixture, it can damage the bond between the asphalt binder and the aggregates resulting in stripping. The moisture susceptibility of asphalt mixtures was evaluated using the AASHTO T283 test. The specimens sorted into two subsets were approximately equal. One subset was conditioned by vacuum saturating with distilled water to 55-80% of the air void volume. These specimens were then placed in a freezer at -18 C for a minimum of 16 h. After removal from the freezer, they were placed in a 60 C water bath for 24 h. The specimens are then placed in a 25 C bath for two hours. Also at this time the unconditioned specimens were placed in the 25 C bath. After these two hours of temperature stabilization, the indirect tensile strength was determined on all the specimens. The ratio of conditioned indirect tensile strength to dry indirect tensile strength is calculated from following equation: (kpa), ITS dry is average indirect tensile strength of dry subset (kpa). Mixture with tensile strength ratios less than 0.7 are moisture susceptible and mixtures with ratios greater than 0.7 are relatively resistant to moisture damage. Electrical resistivity The electrical resistivity was measured on Marshall specimens by electric resistivity testing apparatus. This test apparatus was built in accordance with ASTM D The apparatus consists of a Keithley 6517A high resistance electrometer, a ring electrode, a guarded electrode, and a top electrode in the form of plate (Fig. 1). The sample was placed on the bottom electrode, the top electrode was bolted down on top of the sample. A voltage of 500V was then applied between the top and bottom electrodes and the current was measured by the Keithley 6517A. The volume resistivity is calculated from the following equation: S ρ V = R (5) t where ρ V is volume resistivity (Ω cm), S is the effective area of the guarded electrode (cm 2 ), t is thickness of the sample (cm), R is measured resistance (Ω). Results and Discussion Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) was used to characterize the thermal behaviour of the AC-10, AC- 5 and carbon black (Fig. 2a c.) The initial decomposition temperatures of these materials, which were determined by TGA, were as following: 227 C (AC-10), 225 C (AC-5) and 550 C (carbon black). ITSR ITS ITS cond = (4) dry where ITSR is indirect tensile strength ratio, ITS cond is average indirect tensile strength of conditioned subset Fig. 1 Electric resistivity test apparatus diagram

5 362 INDIAN J. ENG. MATER. SCI., OCTOBER 2007 have better results of Marshall stability and flow values than control mixtures. Because AC-10/CB and AC-5/CB mixtures have high stability and low flow values, that is an important criteria for Marshall tests. The values of MQ of the asphalt concrete mixtures containing carbon black filler are higher than that of the control mixtures. It is well recognized that the MQ is a measure of the materials resistance to shear stresses, permanent deformation and hence rutting. According to all Marshall test results it should be noted that AC-10/CB asphalt concrete mixture has the best results compared to all other mixtures. Fig. 2 Thermogravimetric analysis curves of (a) AC-10, (b) AC-5 and (c) carbon black Fig. 2 shows that the initial decomposition temperatures of the asphalt cements and carbon black are higher than the temperature of mixing and preparing of the asphalt concrete samples. Marshall stability and flow AC-10/LS, AC-5/LS, AC-10/CB and AC-5/CB asphalt concrete mixtures with 4.0, 4.5, 5.0, 5.5, and 6.0% bitumen content by mass of aggregate were prepared in order to determine optimal bitumen content (o.b.c). The o.b.c for the AC-10/LS, AC-5/LS, AC-10/CB, and AC-5/CB mixtures was as following: 5.0%, 4.85%, 4.95%, and 5.1%, respectively. Marshall design results were obtained from compacted specimens at the o.b.c of each aggregate mixture formulation and each result based on an average of three test specimens is given in Table 5. The Marshall stability of asphalt concrete reflects its ability to resist shoving and rutting under traffic. The Marshall flow is the ability of asphalt concrete to adjust to gradual settlements and movements in the sub-grade without cracking. The flow can be considered as an opposite property to the stability. As it can be seen from Table 5 carbon black mixtures Indirect tensile stiffness modulus (ITSM) and creep stiffness The ITSM results for control and carbon black mixtures at 0 C, 20 C and 40 C are given in Table 6. The stiffness modulus of AC-10/CB and AC-5/CB asphalt mixtures at 40 C increased by 1.87 and 2.51 times as compared to the AC-10/LS and AC-5/LS asphalt mixtures respectively. However, the stiffness modulus of the carbon black mixtures at 0 C and 20 C was only slightly higher than that the control mixtures. Moreover, ITSM values of AC-10/CB mixture are higher than AC-5/CB mixture values. The values obtained from creep stiffness test are shown in Table 6. There is a substantial improvement in the resistance to permanent deformation of the carbon black mixtures as indicated by higher creep modulus at standard test temperature 40 C and for very hot climates 60 C in this test as compared to the control asphalt mixtures. In addition to this, creep stiffness value of AC-10/CB mixture has good result in comparison with AC-5/CB mixture. The best results from both ITSM and creep stiffness tests were obtained by AC-10/CB asphalt concrete mixture. Indirect tensile strength Table 7 presents the indirect tensile strength test results. The addition of carbon black as a filler to AC- Table 5 Marshall design results Property AC-10/LS AC-5/LS AC-10/CB AC-5/CB Optimum binder content (%) Aggregate bulk specific gravity (g/cm 3 ) Mix bulk specific gravity (g/cm 3 ) Air void (%) Voids in mineral aggregate (%) Marshall stability (kn) Flow (mm) MQ (kn/mm) ,

6 AHMEDZADE & GECKIL: MECHANICAL AND ELECTRICAL PROPERTIES OF ASPHALT MIXTURES asphalt mixture increased both ITS dry (in 1.2 times) and ITS cond (in 1.3 times) values compared to AC- 10/LS. The addition of carbon black as a filler to AC- 5 asphalt mixture increased both ITS dry (in 1.4 times) and ITS cond (in 1.6 times) values compared to AC- 5/LS. The ITSR values were found to be higher for the AC-10/CB and AC-5/CB mixtures than for the AC-10/LS and AC-5/LS mixtures, respectively. Therefore, it can be said that the addition of carbon black as a filler improves bonding properties between the bitumen and aggregate and results in a higher water and temperature resistance. Electrical resistivity The electrical volume resistivity values for control and carbon black mixtures are given in Table 8. It is seen from this table that the volume resistivity of AC- 10/CB and AC-5/CB asphalt mixtures decreased to and , respectively, as compared to the resistivity of AC-10/LS ( ) and AC- 5/LS ( ) asphalt mixtures. And hence carbon black mixtures have higher electrical conductivity than control mixtures. Conclusions The following conclusions can be drawn from this study: Table 6 Indirect tensile stiffness modulus and creep stiffness test results Property Indirect tensile stiffness modulus at (MPa) (a) 0 C (b) 20 C (c) 40 C Creep stiffness at 1 h loading at (MPa) (a) 40 C (b) 60 C AC- 10/LS AC- 5/LS AC- 10/CB AC-5/CB (i) According to the results obtained from thermogravimetric analysis, it should be noted that the initial decomposition temperatures of the AC-10, AC-5 and carbon black filler are higher than the temperature of mixing and preparing of the asphalt concrete samples. (ii) Marshall stability and flow tests results show that the mixtures with carbon black filler have better results than mixtures with limestone filler. It means that carbon black used as a filler in AC-10/CB and AC-5/CB asphalt concrete mixtures increased stability and decreased flow values, and, hence mixtures with carbon black filler have higher MQ. That is an indicator of high stiffness and resistance to permanent deformation. (iii) Based on the ITSM test results, it can be seen that the stiffness modulus values of carbon black mixtures especially at 40 o C are higher than the control mixtures but at lower temperatures (0 o C; 20 o C) those values tend to converge. The creep stiffness of carbon black mixtures is significantly higher than that of the control mixtures. The higher creep stiffness of the mixtures with carbon black filler indicates better rutting resistance. (iv) The tensile strength values of the both conditioned and unconditioned specimens with carbon black filler are found to be greater than the specimens with limestone filler. Relying on these results it can be concluded that the addition of carbon black in asphalt mixtures leads to the enhancement of resistance against moisture damage as well as moisture induced rutting damage. Moreover, carbon black helps AC-10/LS AC-5/LS AC-10/CB AC-5/CB Table 7 Indirect tensile strength test results Table 8 Volume electroresistivity data Volume resistivity (Ω cm) Property AC-10/LS AC-5/LS AC-10/CB AC-5/CB Indirect tensile strength of dry subset, ITS dry (kpa) Indirect tensile strength of cond. subset, ITS cond (kpa) Indirect tensile strength ratio, ITS cond / ITS dry ,07 0, ,41 452,87 0,

7 364 INDIAN J. ENG. MATER. SCI., OCTOBER 2007 by improving the adhesive bond between aggregate and asphalt binder, reduces the affinity of asphalt for water, and as a filler, helps in increasing the stiffness of the mixture. (v) The electrical conductivity of mixtures with addition of carbon black filler is higher compared to control mixtures. The carbon black mixtures with improved electrical conductivity can be used as a conductive asphalt mixture in thermo-electrical asphalt pavement that allows to melt snow and ice by heating. In this study, AC-10/CB asphalt concrete mixture containing carbon black filler showed the best results for all the tests. Finally, it can be said that AC-10 asphalt cement should be preferred to AC-5 asphalt cement used with carbon black filler to prepare asphalt concrete mixture. References 1 Benson F C & Martinez D F, Evaluation of several mineral fillers for use in item 340 hot-mix asphaltic concrete, Report FHWA/TX/85/1, Texas Transportation Institute, USA, Roberts F L, Kandhal P S, Brown E R, Lee D Y & Kennedy T W, Hot mix asphalt materials, mixture design, and construction, NAPA Education Foundation, Maryland, USA, Rogue R, Birgisson B, Tia M, Kim B & Cui Z, Guidelines for use of modifiers in Superpave mixtures: evaluation of Ground-Tire-Rubber (GTR), Report 4910/4504/718/12, University of Florida, USA, Martin K G, Preliminary microviscometer studies of carbon black bitumen dispersion, Proc Australian Road Research Board, 1 (1962). Rostler F S, White R M & Dannebege E M, Proc Assoc Asphalt Paving Technol, 4 (1977) 376). Terrel R L & Rimstrington S, Evaluation of wood lignin as a substitute or extender for asphalt, Report FHWA/RD- 80/125, FHA, Washington DC, Yao Z & Monismith C L, Proc Assoc Asphalt Paving Technol, 32 (1987). Button J W, Little J L, Kim Y & Ahmed J, Proc Assoc Asphalt Paving Technol, 56 (1987). Fitzgerald R L. Novel applications of carbon fiber for hot mix asphalt reinforcement and carbon-carbon pre-forms, Master s thesis, Michigan Technological University, USA, Shaopeng W, Liatong M, Zhonghe S & Zheng C, Carbon, 43(7) (2005) ). Derwin D, Both P, Zaleski P, Marsey W & Flood Jr W, Snowfree heated pavement system to eliminate icy runways, SAE Int Paper No , Resistance to plastic flow of bituminous mixtures using the Marshall apparatus, ASTM D Zoorob S E & Suparma L B, Cem Concr Compos, 22 (2000) ). British Standards Institution, Method for the determination of the indirect tensile stiffness modulus of bituminous mixtures, Draft for development DD-213, British Standards Institution, Method for the determination of creep stiffness of bituminous aggregate mixtures subject to unconfined uniaxial loading, Draft for development DD-185, Standart test method for direct current resistance or conductance of insulating materials. ASTM D257-91, 1998.