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Application of coal waste powder as filler in hot mix asphalt
Construction and Building Materials 66 (2014) 476–483
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Application of coal waste powder as filler in hot mix asphalt Amir Modarres ⇑, Morteza Rahmanzadeh Department of Civil Engineering, Babol Noshirvani University of Technology, Iran
h i g h l i g h t s This study focussed on application of coal waste powder (CWP) in hot mix asphalt. The use of CWP had no detrimental effects on stability and stiffness of HMA. The moisture resistance indices in CWP mixes was even higher than the reference mix. The CWP mix had higher toughness index or flexibility than the reference mix. Combination of CWP and limestone in equal proportion attained a mix with higher water resistance.
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Article history: Received 28 April 2014 Received in revised form 1 June 2014 Accepted 3 June 2014
The main objective of this research was to investigate the effect of coal waste powder as filler material in hot mix asphalt. After sampling the coal waste material from a coal washing plant, it was processed to achieve natural coal waste filler. Furthermore, after incinerating the natural coal waste powder at 750 °C, the coal waste ash was produced. The results of an X-ray diffraction test revealed pozzolanic compounds which encouraged the application of these materials as active filler in hot mix asphalt. The main laboratory program consisted of Marshall stability, indirect tensile strength and resilient modulus tests conducted in dry and saturated conditions. Based on the obtained results in comparison to the reference mix (i.e. a mix containing limestone powder) the coal waste and its ash resulted in higher stability and resilient modulus. Furthermore, the combination of coal waste and limestone powders in equal proportion resulted in a desirable mix with high water resistance. Moreover, the water sensitivity of mixes was also improved by coal waste powder and especially its ash. Considering the stress–strain curve obtained from indirect tensile strength the toughness index parameter was determined which is an indicator of energy absorbency or material flexibility. Results indicated that the hot mix asphalt containing coal waste powder exhibited more flexible behavior than the reference mix. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Coal waste Filler Hot mix asphalt Limestone Water resistance
1. Introduction The continuing rapid growth in traffic demands along with the increase in allowable axle loads, necessitates higher quality for highway paving materials. The main highway authorities’ obligation is to provide safe, economical, durable and smooth pavements that are capable of carrying anticipated loads. To achieve this goal, many experts, engineers, and researchers are devoted to selecting paving materials that can reduce the severity and density of distress and improve the overall performance of asphalt pavements [1]. Because of high construction costs, research studies should concentrate on correct design and choosing appropriate materials which can increase the construction efficiency and extend the ⇑ Corresponding author. Tel.: +98 9111163215. E-mail addresses: (A. Modarres).
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http://dx.doi.org/10.1016/j.conbuildmat.2014.06.002 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
pavement service life [2]. Research studies showed that the strength of hot mix asphalt (HMA) depends on different factors such as filler and aggregate type, and bitumen grade. Among these, filler material plays a major role in various properties of HMA, especially those related to mixture compatibility and aggregatebitumen adhesion [3]. Furthermore, it also affects several HMA properties such as workability, moisture sensitivity, stiffness, durability, fatigue behavior and long term characteristics of HMA. [4]. Fillers vary in physical and chemical properties, shape and texture, size, and gradation. Therefore, selection of suitable filler is very vital for ideal performance of HMA [5]. Nowadays due to environmental and economic concerns the use of recycled waste materials in road pavements has considerably extended [6]. In this regard, several research studies have been performed by environment and transportation organizations which relate to using recycled waste materials as filler in pavement applications [7–9]. In recent researches, materials such as recycled waste lime, phosphate
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waste filler, baghouse fines, municipal solid waste incineration ash and waste ceramic materials were satisfactorily used as filler in different asphalt mixes [10–13]. Sargin et al. evaluated rice husk ash (RHA) as filler in HMA. Based on their report the combination of 50% RHA and 50% limestone powder as filler had the best result and caused improvement in Marshall Stability and reduced Marshall flow in comparison to reference specimens [14]. Chen et al. studied the potential use of recycled fine aggregate powder as filler in asphalt mixture. The recycled aggregates used in the abovementioned study were by-products of the concrete pavement recycling process. The research outcomes indicated that recycled fine aggregates can improve the water sensitivity and fatigue resistance properties of the mixes studied. However, it was reported that the use of recycled fine aggregates may have contrary effects on the low-temperature characteristics of HMA [15]. Sung et al. investigated the potential use of waste lime as filler material in HMA. Finally they inferred that the use of waste lime as mineral filler can improve the permanent deformation, stiffness and fatigue properties of HMA [10]. During a laboratory investigation, Thanaya et al. examined the effect of coal ash on the technical properties of hot and cold mix asphalts. They obtained results showing that the coal ash was a suitable material for use as filler in both cold and hot mix asphalts. According to their recommendation, both cold and hot mixes containing coal ash were suitable to be used in low to medium trafficked areas, pedestrian ways and foot paths [16]. According to the literature waste coal has already been used in concrete blocks for paving, soil stabilization, building materials, production of cement and blended cement. Dos Santos et al. used coal waste as fine aggregate in concrete blocks for paving. Considering the mechanical properties and environmental safety they determined the proper coal waste content that could be used as a part of fine aggregates [17]. Fríasa et al. used coal waste as a pozzolanic additive in blended cement. Results revealed that using up to 20% coal waste by the weight of cement in cement paste increased the 7-day compressive strength of cement mortar [18]. In another study coal waste was used to stabilize base and subbase materials. It was concluded that incorporating coal waste increased the 7, 28 and 90 days compressive strength of stabilized materials [19].
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Coal is one of the most abundant resources used to produce energy. Coal production across the world is about 5.5 billiard tons per year and the volume produced in Iran reaches about 310 million tons per year. The Alborz Markazi coal washing plant which is located in the north of Iran, is one of oldest mines of Iran with 557 million tons of probable reserves. At present, more than 2 million tons of coal waste dumps are available which leads to several environmental problems [20]. Generally the issue of pollution in coal waste is due to pyrite oxidation. When pyrite and materials containing iron are exposed to open air or water or both, they undergo rapid oxidation which results in acidic water. Pollution originating from acid mine drainage (AMD) is considered the most important water pollutant formed around coal mines and coal washing factories. It contains ferric sulfate and other materials that might contaminate water resources [21]. AMD contains high quantities of iron, SO2 and various quantities of toxic metals [22,23]. One of the solutions for the environmental issue is the use of these coal wastes in various industries such as highway construction. Coal waste powder (CWP) contains pozzolanic compounds including silica and alumina and can have similar properties to type F pozzolans. The goal of the present study is to investigate the applicability of CWP as filler material in HMA.
2. Coal waste production process Coal preparation involves physical processes like regulation, gradation and reducing of mineral substances such as ash and sulfur which improve the coal quality. The most important of these operations include screening, cleaning, crushing and separation. Common methods used for coal preparation are gravity concentration (Jig) for coarse and intermediate size coal and the floatation process which is applied for fine size coal [24]. The coal waste used in this study was prepared from the Alborz Markazi coal washing plant located in the northern part of Iran. Raw coals obtained from different mines located in this area are gathered to undergo the preparation process in this coal washing plant. The schematic of the coal washing process is shown in Fig. 1. Raw coals are mixed together and after crushing by special crushers they are screened and divided into two parts. The first part includes coal pieces larger than 80 mm which are returned
Fig. 1. The schematic of a coal washing plant.
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Fig. 2. The coal waste deposit of Alborz Markazi coal washing plant.
by conveyors to be processed again. The second part consists of coal particles with a maximum size of 80 mm which are conveyed to the next screening system. Coal particles in the size range of 0.5–80 mm discharged on to the conveyor belt and are sent to the jig machine. During the gravity preparation process a part is converted into coal concentrate and the remainder part is sent to waste dumps. The fine coal grains smaller than 0.5 mm are sent to the flotation process section in which a portion of coal is converted into coal concentrate and the remaining part is sent to waste dumps. The wastes obtained from the gravity preparation method (Jig machine) were used in this research. About 40–45% of the coal plant entry is waste. From 1989 to 2011, the coal washing plant has produced and dumped huge volumes of coal waste around the plant area which is depicted in Fig. 2. The waste dump with ten meters height and approximate weight of 1.5 million tons, covers an area of about 2 hectares [25].
3. Materials 3.1. Aggregates In this research crushed and sharp-edged aggregates were used to prepare the asphalt mixes. The aggregate gradation was a continuous gradation with a maximum size of 25 mm. The gradation test was performed according to ASTM: D3515 [26]. Fig. 3 shows the aggregate gradation curve and related gradation specification limits. Also Table 1, presents the technical properties of the studied aggregate.
Fig. 3. The aggregate gradation used to prepare the HMA mixes.
Table 1 Technical properties of studied aggregates. Property (unit)
Standard
Result
Bulk specific gravity (gr/cm3) Apparent specific gravity (gr/cm3) Water absorption (%) Crushing value (%) Los angeles abrasion (%) Soundness (%)
ASTM: ASTM: ASTM: ASTM: ASTM: ASTM:
2.46 2.63 1.01 2.3 15.6 1.5
C127 C127 C127 D5821 D131 C88
3.2. Bitumen The bitumen which was used to prepare the HMA mixes was a 85/100 penetration grade bitumen produced in Tehran oil refinery. The basic physical characteristics of this bitumen have been given in Table 2.
3.3. Fillers The filler materials used in this study consisted of limestone powder (LS), coal waste powder (CWP) and coal waste ash (CWA). The amount of these fillers was selected as equal to 7% by total mass of aggregates. The chemical compositions and other physical properties of these fillers have been given in Table 3. The chemical compositions were determined by X-ray diffraction testing according to ASTM: E1621-05 [27].
3.3.1. Limestone (LS) Limestone is a sedimentary stone which is mostly composed of calcium carbonate. It has been commonly used as filler material in HMA [28]. As presented in Table 3 the main LS components were CaO, SiO2 and loss on ignition (LOI). CaO is an alkali composition which contributes to increasing the bitumenaggregate adhesion and improves the HMA resistance against detrimental effects of water which ultimately reduces the potential of aggregate stripping [28].
Table 2 Basic physical characteristics of studied bitumen. Property (unit)
Standard
Result
Softening point (°C) Penetration (25 °C) Flashing point (°C) Specific gravity (gr/cm3) Ductility (cm)
ASTM: ASTM: ASTM: ASTM: ASTM:
48 91.72 310 0.99 >100 cm
D36 D5 D92 D70 D113
SiO2 is a pozzolanic compound. Pozzolans consist of silica or silica – aluminate compositions that have slight cohesion, but in a soft powder form and with the presence of water and lime can produce cemented compounds [29]. However, there are also pozzolans which have self-cementing properties [30]. The loss on ignition (LOI) is reported as part of an elemental or oxide analysis of a mineral. The volatile materials lost during ignition usually consist of combined water (i.e. hydrates and labile hydroxy-compounds) and carbon dioxide from car-
A. Modarres, M. Rahmanzadeh / Construction and Building Materials 66 (2014) 476–483 Table 3 Chemical compositions, gradation and specific gravity of CWP, CWA and LS. Component
CWP (%)
CWA (%)
LS (%)
A – Chemical compositions Sio2 34.8 AL2O3 14.53 Fe2O3 3.89 MgO 0.868 CaO 0.0513 0.27 P2O5–P2O3 Na2O 0.27 K2O 2.39 MnO 0.02 TiO2 0.983 L.O.I 40.96 Total 100 Percent finer
55.63 23.25 8.09 1.54 2.28 0.27 0.59 3.96 0.05 1.63 2.01 100
17.95 0.46 0.052 3.64 46.9 0.04 0.08 0.10 0.14 0.03 29.95 100
Size (mm)
CWP (%)
CWA (%)
LS (%)
B – Gradation 0.0750 0.0376 0.0182 0.0086 0.0014 Filler
100.00 76.19 63.49 34.92 6.25 CWP (%)
100.00 85.94 75.00 45.31 12.70 CWA (%)
100.00 57.81 44.20 25.50 4.25 LS (%)
C – Specific gravity Gs
2.324
2.123
2.412
bonates. Although there is not a consensus about the harmful effects of these components, there is no doubt that they only act as filler materials without any cementitious properties [29].
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The chemical compounds analysis of coal waste ash is presented in Table 3. As presented the value of LOI after incineration at 750 °C was reduced by 39%. The chemical composition of CWA demonstrated in Table 3 is similar to type N or F fly ash, since the sum of Fe2O3, Al2O3 and SiO2 is more than 70% and the LOI value is 2.01% [29,30]. Fig. 4, shows an image of CWP and CWA powders. As seen in Fig. 4A in the natural state, CWP is a black powder whereas after the incineration process the powder color altered to brown (Fig. 4B).
4. Experimental design 4.1. Mix design In this study, the Marshall method was used to determine the optimum bitumen content and prepare HMA according to the ASTM: D1559 standard method [31]. For the reference mix (i.e. the mix that contained limestone powder as filler) 18 HMA mixes were prepared at six different bitumen contents between 3.5% and 6% by total weight of mixture at 0.5% increments. After preparing specimens Marshall stability and flow, bulk and maximum specific gravities, air void content, VMA and VFA parameters were determined. Based on these parameters, optimum bitumen content was determined as equal to 5.2%. Different mix compositions that were evaluated in this study are presented in Table 4. Based on laboratory analysis the change in optimum bitumen content for various compositions presented in this table was less than 0.2%. Therefore in order to better investigate the effect of filler type equal bitumen content was selected for all studied mixes. 4.2. Marshall stability tests
3.3.2. Coal waste powder (CWP) In this research, the coal wastes available in the Alborz Markazi coal washing plant were used which is located in Mazandaran province in the north of Iran. This factory uses two processing methods (Floatation and Jig) for coal washing. In this study the coal waste which was obtained via the Jig machine preparation process was used. After sampling from the Jig deposit, coal samples were dried in an oven at 60 °C until achieving a constant weight. Then dried samples were crushed and screened on the No. 200 sieve and the passing part was used in this study. According to results of the Atterberg limits test, the CWP studied was a nonplastic filler. The chemical composition of CWP and its gradation are presented in Table 3. In comparison to LS powder, CWP contained higher pozzolanic and LOI content and negligible CaO. Furthermore, compared to other fillers it has intermediate gradation and specific gravity. 3.3.3. Coal waste ash (CWA) In order to reduce the existing coal content, coal waste was incinerated at 450, 750, 950 and 1200 °C. Afterwards, the LOI test was conducted on the ashes produced, needless to say that the value of LOI decreased with temperature increase. Since the rate of LOI reduction at higher than 750 °C was very slight, this temperature was selected as the incineration point. It should be noted that lower incineration temperatures reduce energy consumption and help environmental conservation.
Marshall stability and flow were based on the ASTM: D1559 method, conducted at 60 °C and a loading rate of 50 mm/min for each mix type [31]. Marshall stability shows the ability of asphalt concrete to resist against shoving and rutting. Flow shows the ability of asphalt concrete to resist gradual settlement and deformation without cracking [6]. 4.3. Indirect Tensile Strength (ITS) During the ITS test, a cylindrical sample is subjected to compressive loads between two loading strips which create tensile stress along the vertical diametric plane causing a splitting failure [32,33]. The tensile strength of the specimen is calculated according to Eq. (1).
ITS ¼
2000Pmax ptd
Fig. 4. Image of CWP (A) and CWA (B) used in this research.
ð1Þ
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Table 4 Studied mix compositions. Mix no.
Filler combination
Mix type
1 2 3 4 5 6
100% limestone 75% coal waste powder, 25% limestone 50% coal waste powder, 50% limestone 25% coal waste powder, 75% limestone 100% coal waste powder 100% coal waste ash
LS LCP1 LCP2 LCP3 CP CA
where ITS: indirect tensile strength, kPa, P: peak load, N, d: diameter of the specimen, mm, t: thickness of the specimen, mm. 4.4. Indirect tensile toughness index (TIit) The TIit parameter was determined using a normalized stress– strain curve obtained in the ITS test. This parameter is an indicator of material flexibility. In fact toughness is the ability of a material to adsorb energy and deform without fracturing [34]. Fig. 5, depicts a typical normalized ITS curve. In order to obtain the normalized curve, recorded stress values were divided by the tensile strength of related specimen (i.e. the maximum stress value). Therefore, a dimensionless stress–strain curve was obtained with a maximum value of 1 on the vertical axis versus the real strain values on the horizontal axis. Dimensionless indirect tensile toughness index (TIit) is defined as presented by Eq. (2).
TIit ¼
Ae Ap e ep
ð2Þ
In this equation the Ae parameter is the total area under the normalized stress–strain curve up to the point of complete break (e). As shown in Fig. 5, this parameter was obtained by summation of partial areas under the normalized ITS curve between each two recorded strains up to the (e) point. Furthermore, as shown in Fig. 5, Ap is equal to the total area under the normalized curve up to the peak strain point (ep) which similarly obtained by summation of partial areas up to the peak stress value on the normalized stress–strain curve. For an ideal flexible material TIit parameter is equal to 1; for an ideal brittle material with no post-peak load carrying capacity, the value of the TI parameter is equal to zero [34]. 4.5. Resilient modulus (Mr) The resilient modulus of bituminous mixes, which is determined according to the ASTM D4123-04 method, is one of the stress–strain measurements used to evaluate the elastic properties of these mixes [35,36]. It is well known that most paving materials are not elastic but experience some permanent deformation after each load application. However, if the load is small compared to
the strength of the material and is repeated for a large number of times, the deformation under each load repetition is nearly completely recoverable and proportional to the load and can be considered as elastic. In Mr testing, the total deformation is the sum of instantaneous and recoverable deformations which represent the elastic and visco-elastic behavior of the material, respectively [37]. For an applied dynamic load of P in which the resulting horizontal dynamic deformations have been measured, the total Mr value is calculated from Eq. (3) [35]:
Mr ¼
Pðl þ 0:27Þ tdh
ð3Þ
where P is the maximum dynamic load, N; l the Poisson’s ratio (assumed 0.35); t the specimen length, mm; dh is the total horizontal recoverable deformation, mm. In the Mr test the loading frequency was set as equal to 1 Hz, including 0.1 s loading and 0.9 s recovery time. Both ITS and Mr tests were performed at 20 °C. Furthermore, the stress level in the Mr test was selected as equal to 20% of ITS. 4.6. Water sensitivity Moisture conditioning is used to evaluate the effects of water saturation on compacted asphalt mixture in the laboratory. The main role of filler is to improve the durability or mixture resistance of HMA. Since there is little experience about using coal waste material as filler in HMA, the moisture susceptibility of studied mixes was precisely evaluated by three moisture resistance indices using the results of Marshall stability, ITS and Mr tests for unconditioned and conditioned specimens. To evaluate the water sensitivity by the Marshall method, the Marshall stability ratio (MSR) was calculated according to Eq. (4).
MSR ¼
Ms 100 Md
ð4Þ
where Ms: the average stability of three specimens which were placed into the water bath at 60 °C for 24 h; Md: the average stability of three specimens which were placed into the water bath at 60 °C for 30 min. In the second method the moisture resistance of HMA mixes was examined by the ITS method. The moisture conditioning was used to evaluate the effects of water saturation on compacted asphalt mixtures in the laboratory. The mix specimens conditioning was performed according to AASHTO T283 by immersing the specimens in water and exposing them to a vacuum to achieve saturation levels up to 80%. For conducting this test, samples were equally divided into conditioned and unconditioned groups, then tensile strength ratio (TSR) was calculated according to Eq. (5).
TSR ¼
Stcon 100 St unco
ð5Þ
where Stcon is the average tensile strength of conditioned specimens, kPa and Stunco the average tensile strength of unconditioned specimens, kPa. The TSR index is used to predict the potential of asphalt concrete stripping. A TSR of 80% or more has been standardized as a minimum approved value for HMA. In the third method the water sensitivity of specimens was evaluated by the resilient modulus test which was measured for conditioned and unconditioned specimens. Finally the stiffness modulus ratio (SMR) was calculated based on Eq. (6):
SMR ¼ Fig. 5. Normalized stress–strain curve of ITS test.
Mr con 100 Mr unco
ð6Þ
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where Mrcon is the average resilient modulus of conditioned HMA specimens, MPa and Mrunco the average resilient modulus of unconditioned HMA specimens, MPa. Similar to the ITS test the minimum criterion for mix approval was selected as equal to 80%. 5. Results and discussion In this section at first, the results of tests are compared for mixes Nos. 1–5 as defined in Table 4. Then mixes containing each individual filler material, No. 1, No. 5 & No. 6, are compared together. 5.1. Marshall stability
Fig. 7. Results of ITS test for different HMA mixes.
Results of the Marshall stability test are depicted in Fig. 6. As can be seen HMA stability increased by incorporating coal waste powder. The minimum and maximum stability were obtained for LS (i.e. the reference specimen) and CP samples, respectively. For specimens containing both filler materials the mix stability continuously increased by increasing the CWP content. However the Marshall stability of all specimens was more than 800 kg which is the minimum required quantity for heavy loading conditions. 5.2. ITS Fig. 7 illustrates the results of the ITS test for various studied mixes. According to this figure the changes in ITS for the studied mixes were less than for Marshall stability. Unlike the Marshall test the ITS of the LS specimen was slightly higher than the CP mix. Furthermore, the combination of limestone and CWP fillers resulted in higher ITS quantities. The LCP2 specimen was the optimum combination of these filler materials which revealed maximum ITS among all tested HMA mixes.
Fig. 8. Comparison between the stress–strain curves of HMA mixes.
5.3. Indirect tensile toughness index (TIit) In order to evaluate the flexibility of studied HMA mixes the normalized stress–strain curves were drawn and the parameters as presented in Eq. (2) were calculated for each curve. Fig. 8, shows the stress–strain curves of LS, LCP1 and LCP3 mixes. As can be seen all curves had to some extent a similar peak strain point (ep), but the point of complete break (e) was different for each mix. Based on this figure the LCP1 & LCP3 mixes had the maximum and minimum break strains (e), respectively. Fig. 9, compares between the TIit of the studied mixes. The reference mix containing LS showed weaker energy absorbency than other mixes. Application of coal waste powder alone increased the material flexibility. However, equal combination of limestone and coal waste powders resulted in the maximum flexibility in HMA. Higher toughness index indicates higher crack resistance of
Fig. 9. Comparison between the indirect tensile toughness index of studied HMA.
material [34]. Therefore, it could be concluded that incorporating the coal waste powder not only improved the HMA stability and indirect tensile strength but also had beneficial effects on material flexibility. 5.4. Resilient modulus (Mr)
Fig. 6. Results of Marshall stability for different studied HMA mixes.
Results of the Mr test have been presented in Fig. 10. According to this figure, the stiffness of mixes increased by increasing the coal waste powder content. The maximum Mr value was obtained for the LCP3 specimen. Comparison between the results of Figs. 6, 7 and 10 showed that a combination of coal waste and limestone powders resulted in optimized mechanical properties. As presented in Table 3 CWP contained pozzolanic compounds (i.e. Sio2 & Al2O3) which could improve the bitumen-aggregate surface adhesion. Moreover, this very fine material could even increase the internal cohesion of the bitumen phase and finally increase
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Fig. 10. Results of Mr test for various HMA mixes.
the mix strength. Likewise, limestone powder contained about 47% CaO which contributes to aggregate-bitumen adhesion and could itself increase the mix strength. Therefore, the combination of LS and CWP resulted in higher ITS and Mr than other mixes. Not only did this combination increase the strength and stiffness of HMA mixes but also it improved the material flexibility which is desired for asphalt mix, especially at lower temperatures and repeated heavy loading conditions. Likewise, as well as the ITS results, which were almost the same for LS and CP mixes, using coal waste powder resulted in higher Marshall stability and Mr than the limestone powder. The latter result could be related to gradation and specific gravity of these materials. As presented in Table 4B, the coal waste powder has finer gradation and slightly lower specific gravity than limestone. The specific surface area (SSA) is increased as the particle size becomes finer. It means that filler material interacts with bitumen and aggregate surface at a higher contact area which then improves the bitumen-aggregate interaction and bonding quality between the aggregates and bitumen [38].
5.5. Moisture sensitivity Effects of the studied fillers on moisture damage indices have been shown in Fig. 11. As mentioned earlier a retained strength or stability of 80% is usually accepted for HMA mixes after the conditioning period. According to Fig. 11 all mixes satisfied this criterion. However, the maximum values were obtained for mixes containing both coal waste and limestone powder. The maximum MSR, TSR & SMR quantities were obtained for LCP3, LCP1 & LCP2 mixes, respectively. Likewise, the CP mix revealed similar moisture resistance to the LS mix. Results confirmed that coal waste powder behaves as an active filler which could improve the aggregate-bitumen adhesion. In mixes containing both coal waste and limestone powders the existence of lime and pozzolanic components could create cementitious compositions in the presence of water. The latter phenomenon might be the reason for the ratios being equal or even higher than 100% for mixes containing both limestone and coal waste powders.
Fig. 11. Effects of studied fillers on moisture damage indices.
Property (Unit)
LS
CP
CA
Marshall stability (kg) Marshall flow (mm) ITS (kPa) TIit Mr (MPa) MSR (%) TSR (%) SMR (%)
875 3.13 739 0.185 1697 98 88 84
1020 3.00 734 0.228 1980 96 91 86
1260 2.50 812 0.206 2933 98 95 92
5.6. Comparison to coal waste ash (CA) Table 5 compares between the various studied properties of LS, CP and CA, mixes. Based on this table, using CWA in HMA resulted in higher Marshall stability, ITS and stiffness than the other studied fillers. This is logical because after the incineration process at a constant additive weight the pozzolanic compositions proportion increases which could have beneficial effects on the bitumenaggregate bonding. However, incorporating CWA to some extent reduced the Marshall flow and toughness index of HMA. In order to better recognize the effect of these fillers on rheological properties of bitumen and finally the deformability of HMA, the kinematic viscosity of bitumen-filler blend was determined based on ASTM: D2170 for all filler materials at different temperatures [39]. The filler-bitumen blends were prepared by mixing the filler and bitumen with an equal combination. Fig. 12, shows the viscositytemperature diagram which was obtained for each blend. As shown at all temperatures the CWA-bitumen blend had the maximum viscosity. This figure indicates that incorporating CWA as filler material in HMA increases the viscosity of bitumen phase which will then increase the mix stiffness and reduce the deformability of material. According to Table 5, the moisture damage indices of all mixes were more than 80%. Therefore all examined mixes showed the desired moisture resistance. However, the maximum quantity was obtained for CWA. This could be related to higher viscosity of the CWA-bitumen blend which resulted in thicker bitumen film around the aggregate particles (i.e. higher effective bitumen content). Apart from the technical benefits of CWA, preparing the CWA for use in HMA needs incineration equipment which increases the total costs of HMA production. Furthermore, the incineration process increases the total consumed energy and environmental pollution. Therefore, since the use of natural coal waste powder (CWP) had no detrimental effects on the HMA mix properties especially those related to moisture damage, application of this additive is more favorable than CWA.
Fig. 12. The temperature-viscosity diagram of studied filler-bitumen blends.
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6. Conclusions Based on the laboratory analysis and obtained results of this research the following conclusions could be drawn: (1) In comparison to the reference mix containing limestone powder as filler material, incorporating coal waste powder and its ash increased the Marshall stability, indirect tensile strength and resilient modulus of HMA mixes. (2) The HMA mix containing coal waste powder exhibited higher toughness or energy absorbency than the reference mix. Therefore, this material had no detrimental effects on HMA flexibility which is an important property at lower temperatures and heavy repeated loading conditions. (3) The equal proportion blend of limestone and natural coal waste powders revealed optimized properties in most of the examined characteristics. Therefore this blend is also recommended to be used as filler material in HMA mixes. (4) Because of high LOI content and existence of water absorbent compounds in natural coal waste powder the moisture resistance of HMA mix was precisely examined by three indices consisting of MSR, TSR & SMR. The minimum ratio of 80% was selected as an acceptance criterion for all these parameters. Based on the obtained results all mixes satisfied this criterion. (5) For all studied properties, using coal waste ash resulted in higher stability and stiffness than the two other additives. However, preparing the CWA for use in HMA needs incineration equipment which increases the total costs of HMA production. Furthermore, the incineration process increases the total consumed energy and environmental pollution.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Application of coal waste powder as filler in hot mix asphalt Amir Modarres ⇑, Morteza Rahmanzadeh Department of Civil Engineering, Babol Noshirvani University of Technology, Iran
h i g h l i g h t s This study focussed on application of coal waste powder (CWP) in hot mix asphalt. The use of CWP had no detrimental effects on stability and stiffness of HMA. The moisture resistance indices in CWP mixes was even higher than the reference mix. The CWP mix had higher toughness index or flexibility than the reference mix. Combination of CWP and limestone in equal proportion attained a mix with higher water resistance.
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 28 April 2014 Received in revised form 1 June 2014 Accepted 3 June 2014
The main objective of this research was to investigate the effect of coal waste powder as filler material in hot mix asphalt. After sampling the coal waste material from a coal washing plant, it was processed to achieve natural coal waste filler. Furthermore, after incinerating the natural coal waste powder at 750 °C, the coal waste ash was produced. The results of an X-ray diffraction test revealed pozzolanic compounds which encouraged the application of these materials as active filler in hot mix asphalt. The main laboratory program consisted of Marshall stability, indirect tensile strength and resilient modulus tests conducted in dry and saturated conditions. Based on the obtained results in comparison to the reference mix (i.e. a mix containing limestone powder) the coal waste and its ash resulted in higher stability and resilient modulus. Furthermore, the combination of coal waste and limestone powders in equal proportion resulted in a desirable mix with high water resistance. Moreover, the water sensitivity of mixes was also improved by coal waste powder and especially its ash. Considering the stress–strain curve obtained from indirect tensile strength the toughness index parameter was determined which is an indicator of energy absorbency or material flexibility. Results indicated that the hot mix asphalt containing coal waste powder exhibited more flexible behavior than the reference mix. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Coal waste Filler Hot mix asphalt Limestone Water resistance
1. Introduction The continuing rapid growth in traffic demands along with the increase in allowable axle loads, necessitates higher quality for highway paving materials. The main highway authorities’ obligation is to provide safe, economical, durable and smooth pavements that are capable of carrying anticipated loads. To achieve this goal, many experts, engineers, and researchers are devoted to selecting paving materials that can reduce the severity and density of distress and improve the overall performance of asphalt pavements [1]. Because of high construction costs, research studies should concentrate on correct design and choosing appropriate materials which can increase the construction efficiency and extend the ⇑ Corresponding author. Tel.: +98 9111163215. E-mail addresses: (A. Modarres).
[email protected],
[email protected]
http://dx.doi.org/10.1016/j.conbuildmat.2014.06.002 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
pavement service life [2]. Research studies showed that the strength of hot mix asphalt (HMA) depends on different factors such as filler and aggregate type, and bitumen grade. Among these, filler material plays a major role in various properties of HMA, especially those related to mixture compatibility and aggregatebitumen adhesion [3]. Furthermore, it also affects several HMA properties such as workability, moisture sensitivity, stiffness, durability, fatigue behavior and long term characteristics of HMA. [4]. Fillers vary in physical and chemical properties, shape and texture, size, and gradation. Therefore, selection of suitable filler is very vital for ideal performance of HMA [5]. Nowadays due to environmental and economic concerns the use of recycled waste materials in road pavements has considerably extended [6]. In this regard, several research studies have been performed by environment and transportation organizations which relate to using recycled waste materials as filler in pavement applications [7–9]. In recent researches, materials such as recycled waste lime, phosphate
A. Modarres, M. Rahmanzadeh / Construction and Building Materials 66 (2014) 476–483
waste filler, baghouse fines, municipal solid waste incineration ash and waste ceramic materials were satisfactorily used as filler in different asphalt mixes [10–13]. Sargin et al. evaluated rice husk ash (RHA) as filler in HMA. Based on their report the combination of 50% RHA and 50% limestone powder as filler had the best result and caused improvement in Marshall Stability and reduced Marshall flow in comparison to reference specimens [14]. Chen et al. studied the potential use of recycled fine aggregate powder as filler in asphalt mixture. The recycled aggregates used in the abovementioned study were by-products of the concrete pavement recycling process. The research outcomes indicated that recycled fine aggregates can improve the water sensitivity and fatigue resistance properties of the mixes studied. However, it was reported that the use of recycled fine aggregates may have contrary effects on the low-temperature characteristics of HMA [15]. Sung et al. investigated the potential use of waste lime as filler material in HMA. Finally they inferred that the use of waste lime as mineral filler can improve the permanent deformation, stiffness and fatigue properties of HMA [10]. During a laboratory investigation, Thanaya et al. examined the effect of coal ash on the technical properties of hot and cold mix asphalts. They obtained results showing that the coal ash was a suitable material for use as filler in both cold and hot mix asphalts. According to their recommendation, both cold and hot mixes containing coal ash were suitable to be used in low to medium trafficked areas, pedestrian ways and foot paths [16]. According to the literature waste coal has already been used in concrete blocks for paving, soil stabilization, building materials, production of cement and blended cement. Dos Santos et al. used coal waste as fine aggregate in concrete blocks for paving. Considering the mechanical properties and environmental safety they determined the proper coal waste content that could be used as a part of fine aggregates [17]. Fríasa et al. used coal waste as a pozzolanic additive in blended cement. Results revealed that using up to 20% coal waste by the weight of cement in cement paste increased the 7-day compressive strength of cement mortar [18]. In another study coal waste was used to stabilize base and subbase materials. It was concluded that incorporating coal waste increased the 7, 28 and 90 days compressive strength of stabilized materials [19].
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Coal is one of the most abundant resources used to produce energy. Coal production across the world is about 5.5 billiard tons per year and the volume produced in Iran reaches about 310 million tons per year. The Alborz Markazi coal washing plant which is located in the north of Iran, is one of oldest mines of Iran with 557 million tons of probable reserves. At present, more than 2 million tons of coal waste dumps are available which leads to several environmental problems [20]. Generally the issue of pollution in coal waste is due to pyrite oxidation. When pyrite and materials containing iron are exposed to open air or water or both, they undergo rapid oxidation which results in acidic water. Pollution originating from acid mine drainage (AMD) is considered the most important water pollutant formed around coal mines and coal washing factories. It contains ferric sulfate and other materials that might contaminate water resources [21]. AMD contains high quantities of iron, SO2 and various quantities of toxic metals [22,23]. One of the solutions for the environmental issue is the use of these coal wastes in various industries such as highway construction. Coal waste powder (CWP) contains pozzolanic compounds including silica and alumina and can have similar properties to type F pozzolans. The goal of the present study is to investigate the applicability of CWP as filler material in HMA.
2. Coal waste production process Coal preparation involves physical processes like regulation, gradation and reducing of mineral substances such as ash and sulfur which improve the coal quality. The most important of these operations include screening, cleaning, crushing and separation. Common methods used for coal preparation are gravity concentration (Jig) for coarse and intermediate size coal and the floatation process which is applied for fine size coal [24]. The coal waste used in this study was prepared from the Alborz Markazi coal washing plant located in the northern part of Iran. Raw coals obtained from different mines located in this area are gathered to undergo the preparation process in this coal washing plant. The schematic of the coal washing process is shown in Fig. 1. Raw coals are mixed together and after crushing by special crushers they are screened and divided into two parts. The first part includes coal pieces larger than 80 mm which are returned
Fig. 1. The schematic of a coal washing plant.
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Fig. 2. The coal waste deposit of Alborz Markazi coal washing plant.
by conveyors to be processed again. The second part consists of coal particles with a maximum size of 80 mm which are conveyed to the next screening system. Coal particles in the size range of 0.5–80 mm discharged on to the conveyor belt and are sent to the jig machine. During the gravity preparation process a part is converted into coal concentrate and the remainder part is sent to waste dumps. The fine coal grains smaller than 0.5 mm are sent to the flotation process section in which a portion of coal is converted into coal concentrate and the remaining part is sent to waste dumps. The wastes obtained from the gravity preparation method (Jig machine) were used in this research. About 40–45% of the coal plant entry is waste. From 1989 to 2011, the coal washing plant has produced and dumped huge volumes of coal waste around the plant area which is depicted in Fig. 2. The waste dump with ten meters height and approximate weight of 1.5 million tons, covers an area of about 2 hectares [25].
3. Materials 3.1. Aggregates In this research crushed and sharp-edged aggregates were used to prepare the asphalt mixes. The aggregate gradation was a continuous gradation with a maximum size of 25 mm. The gradation test was performed according to ASTM: D3515 [26]. Fig. 3 shows the aggregate gradation curve and related gradation specification limits. Also Table 1, presents the technical properties of the studied aggregate.
Fig. 3. The aggregate gradation used to prepare the HMA mixes.
Table 1 Technical properties of studied aggregates. Property (unit)
Standard
Result
Bulk specific gravity (gr/cm3) Apparent specific gravity (gr/cm3) Water absorption (%) Crushing value (%) Los angeles abrasion (%) Soundness (%)
ASTM: ASTM: ASTM: ASTM: ASTM: ASTM:
2.46 2.63 1.01 2.3 15.6 1.5
C127 C127 C127 D5821 D131 C88
3.2. Bitumen The bitumen which was used to prepare the HMA mixes was a 85/100 penetration grade bitumen produced in Tehran oil refinery. The basic physical characteristics of this bitumen have been given in Table 2.
3.3. Fillers The filler materials used in this study consisted of limestone powder (LS), coal waste powder (CWP) and coal waste ash (CWA). The amount of these fillers was selected as equal to 7% by total mass of aggregates. The chemical compositions and other physical properties of these fillers have been given in Table 3. The chemical compositions were determined by X-ray diffraction testing according to ASTM: E1621-05 [27].
3.3.1. Limestone (LS) Limestone is a sedimentary stone which is mostly composed of calcium carbonate. It has been commonly used as filler material in HMA [28]. As presented in Table 3 the main LS components were CaO, SiO2 and loss on ignition (LOI). CaO is an alkali composition which contributes to increasing the bitumenaggregate adhesion and improves the HMA resistance against detrimental effects of water which ultimately reduces the potential of aggregate stripping [28].
Table 2 Basic physical characteristics of studied bitumen. Property (unit)
Standard
Result
Softening point (°C) Penetration (25 °C) Flashing point (°C) Specific gravity (gr/cm3) Ductility (cm)
ASTM: ASTM: ASTM: ASTM: ASTM:
48 91.72 310 0.99 >100 cm
D36 D5 D92 D70 D113
SiO2 is a pozzolanic compound. Pozzolans consist of silica or silica – aluminate compositions that have slight cohesion, but in a soft powder form and with the presence of water and lime can produce cemented compounds [29]. However, there are also pozzolans which have self-cementing properties [30]. The loss on ignition (LOI) is reported as part of an elemental or oxide analysis of a mineral. The volatile materials lost during ignition usually consist of combined water (i.e. hydrates and labile hydroxy-compounds) and carbon dioxide from car-
A. Modarres, M. Rahmanzadeh / Construction and Building Materials 66 (2014) 476–483 Table 3 Chemical compositions, gradation and specific gravity of CWP, CWA and LS. Component
CWP (%)
CWA (%)
LS (%)
A – Chemical compositions Sio2 34.8 AL2O3 14.53 Fe2O3 3.89 MgO 0.868 CaO 0.0513 0.27 P2O5–P2O3 Na2O 0.27 K2O 2.39 MnO 0.02 TiO2 0.983 L.O.I 40.96 Total 100 Percent finer
55.63 23.25 8.09 1.54 2.28 0.27 0.59 3.96 0.05 1.63 2.01 100
17.95 0.46 0.052 3.64 46.9 0.04 0.08 0.10 0.14 0.03 29.95 100
Size (mm)
CWP (%)
CWA (%)
LS (%)
B – Gradation 0.0750 0.0376 0.0182 0.0086 0.0014 Filler
100.00 76.19 63.49 34.92 6.25 CWP (%)
100.00 85.94 75.00 45.31 12.70 CWA (%)
100.00 57.81 44.20 25.50 4.25 LS (%)
C – Specific gravity Gs
2.324
2.123
2.412
bonates. Although there is not a consensus about the harmful effects of these components, there is no doubt that they only act as filler materials without any cementitious properties [29].
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The chemical compounds analysis of coal waste ash is presented in Table 3. As presented the value of LOI after incineration at 750 °C was reduced by 39%. The chemical composition of CWA demonstrated in Table 3 is similar to type N or F fly ash, since the sum of Fe2O3, Al2O3 and SiO2 is more than 70% and the LOI value is 2.01% [29,30]. Fig. 4, shows an image of CWP and CWA powders. As seen in Fig. 4A in the natural state, CWP is a black powder whereas after the incineration process the powder color altered to brown (Fig. 4B).
4. Experimental design 4.1. Mix design In this study, the Marshall method was used to determine the optimum bitumen content and prepare HMA according to the ASTM: D1559 standard method [31]. For the reference mix (i.e. the mix that contained limestone powder as filler) 18 HMA mixes were prepared at six different bitumen contents between 3.5% and 6% by total weight of mixture at 0.5% increments. After preparing specimens Marshall stability and flow, bulk and maximum specific gravities, air void content, VMA and VFA parameters were determined. Based on these parameters, optimum bitumen content was determined as equal to 5.2%. Different mix compositions that were evaluated in this study are presented in Table 4. Based on laboratory analysis the change in optimum bitumen content for various compositions presented in this table was less than 0.2%. Therefore in order to better investigate the effect of filler type equal bitumen content was selected for all studied mixes. 4.2. Marshall stability tests
3.3.2. Coal waste powder (CWP) In this research, the coal wastes available in the Alborz Markazi coal washing plant were used which is located in Mazandaran province in the north of Iran. This factory uses two processing methods (Floatation and Jig) for coal washing. In this study the coal waste which was obtained via the Jig machine preparation process was used. After sampling from the Jig deposit, coal samples were dried in an oven at 60 °C until achieving a constant weight. Then dried samples were crushed and screened on the No. 200 sieve and the passing part was used in this study. According to results of the Atterberg limits test, the CWP studied was a nonplastic filler. The chemical composition of CWP and its gradation are presented in Table 3. In comparison to LS powder, CWP contained higher pozzolanic and LOI content and negligible CaO. Furthermore, compared to other fillers it has intermediate gradation and specific gravity. 3.3.3. Coal waste ash (CWA) In order to reduce the existing coal content, coal waste was incinerated at 450, 750, 950 and 1200 °C. Afterwards, the LOI test was conducted on the ashes produced, needless to say that the value of LOI decreased with temperature increase. Since the rate of LOI reduction at higher than 750 °C was very slight, this temperature was selected as the incineration point. It should be noted that lower incineration temperatures reduce energy consumption and help environmental conservation.
Marshall stability and flow were based on the ASTM: D1559 method, conducted at 60 °C and a loading rate of 50 mm/min for each mix type [31]. Marshall stability shows the ability of asphalt concrete to resist against shoving and rutting. Flow shows the ability of asphalt concrete to resist gradual settlement and deformation without cracking [6]. 4.3. Indirect Tensile Strength (ITS) During the ITS test, a cylindrical sample is subjected to compressive loads between two loading strips which create tensile stress along the vertical diametric plane causing a splitting failure [32,33]. The tensile strength of the specimen is calculated according to Eq. (1).
ITS ¼
2000Pmax ptd
Fig. 4. Image of CWP (A) and CWA (B) used in this research.
ð1Þ
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Table 4 Studied mix compositions. Mix no.
Filler combination
Mix type
1 2 3 4 5 6
100% limestone 75% coal waste powder, 25% limestone 50% coal waste powder, 50% limestone 25% coal waste powder, 75% limestone 100% coal waste powder 100% coal waste ash
LS LCP1 LCP2 LCP3 CP CA
where ITS: indirect tensile strength, kPa, P: peak load, N, d: diameter of the specimen, mm, t: thickness of the specimen, mm. 4.4. Indirect tensile toughness index (TIit) The TIit parameter was determined using a normalized stress– strain curve obtained in the ITS test. This parameter is an indicator of material flexibility. In fact toughness is the ability of a material to adsorb energy and deform without fracturing [34]. Fig. 5, depicts a typical normalized ITS curve. In order to obtain the normalized curve, recorded stress values were divided by the tensile strength of related specimen (i.e. the maximum stress value). Therefore, a dimensionless stress–strain curve was obtained with a maximum value of 1 on the vertical axis versus the real strain values on the horizontal axis. Dimensionless indirect tensile toughness index (TIit) is defined as presented by Eq. (2).
TIit ¼
Ae Ap e ep
ð2Þ
In this equation the Ae parameter is the total area under the normalized stress–strain curve up to the point of complete break (e). As shown in Fig. 5, this parameter was obtained by summation of partial areas under the normalized ITS curve between each two recorded strains up to the (e) point. Furthermore, as shown in Fig. 5, Ap is equal to the total area under the normalized curve up to the peak strain point (ep) which similarly obtained by summation of partial areas up to the peak stress value on the normalized stress–strain curve. For an ideal flexible material TIit parameter is equal to 1; for an ideal brittle material with no post-peak load carrying capacity, the value of the TI parameter is equal to zero [34]. 4.5. Resilient modulus (Mr) The resilient modulus of bituminous mixes, which is determined according to the ASTM D4123-04 method, is one of the stress–strain measurements used to evaluate the elastic properties of these mixes [35,36]. It is well known that most paving materials are not elastic but experience some permanent deformation after each load application. However, if the load is small compared to
the strength of the material and is repeated for a large number of times, the deformation under each load repetition is nearly completely recoverable and proportional to the load and can be considered as elastic. In Mr testing, the total deformation is the sum of instantaneous and recoverable deformations which represent the elastic and visco-elastic behavior of the material, respectively [37]. For an applied dynamic load of P in which the resulting horizontal dynamic deformations have been measured, the total Mr value is calculated from Eq. (3) [35]:
Mr ¼
Pðl þ 0:27Þ tdh
ð3Þ
where P is the maximum dynamic load, N; l the Poisson’s ratio (assumed 0.35); t the specimen length, mm; dh is the total horizontal recoverable deformation, mm. In the Mr test the loading frequency was set as equal to 1 Hz, including 0.1 s loading and 0.9 s recovery time. Both ITS and Mr tests were performed at 20 °C. Furthermore, the stress level in the Mr test was selected as equal to 20% of ITS. 4.6. Water sensitivity Moisture conditioning is used to evaluate the effects of water saturation on compacted asphalt mixture in the laboratory. The main role of filler is to improve the durability or mixture resistance of HMA. Since there is little experience about using coal waste material as filler in HMA, the moisture susceptibility of studied mixes was precisely evaluated by three moisture resistance indices using the results of Marshall stability, ITS and Mr tests for unconditioned and conditioned specimens. To evaluate the water sensitivity by the Marshall method, the Marshall stability ratio (MSR) was calculated according to Eq. (4).
MSR ¼
Ms 100 Md
ð4Þ
where Ms: the average stability of three specimens which were placed into the water bath at 60 °C for 24 h; Md: the average stability of three specimens which were placed into the water bath at 60 °C for 30 min. In the second method the moisture resistance of HMA mixes was examined by the ITS method. The moisture conditioning was used to evaluate the effects of water saturation on compacted asphalt mixtures in the laboratory. The mix specimens conditioning was performed according to AASHTO T283 by immersing the specimens in water and exposing them to a vacuum to achieve saturation levels up to 80%. For conducting this test, samples were equally divided into conditioned and unconditioned groups, then tensile strength ratio (TSR) was calculated according to Eq. (5).
TSR ¼
Stcon 100 St unco
ð5Þ
where Stcon is the average tensile strength of conditioned specimens, kPa and Stunco the average tensile strength of unconditioned specimens, kPa. The TSR index is used to predict the potential of asphalt concrete stripping. A TSR of 80% or more has been standardized as a minimum approved value for HMA. In the third method the water sensitivity of specimens was evaluated by the resilient modulus test which was measured for conditioned and unconditioned specimens. Finally the stiffness modulus ratio (SMR) was calculated based on Eq. (6):
SMR ¼ Fig. 5. Normalized stress–strain curve of ITS test.
Mr con 100 Mr unco
ð6Þ
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481
where Mrcon is the average resilient modulus of conditioned HMA specimens, MPa and Mrunco the average resilient modulus of unconditioned HMA specimens, MPa. Similar to the ITS test the minimum criterion for mix approval was selected as equal to 80%. 5. Results and discussion In this section at first, the results of tests are compared for mixes Nos. 1–5 as defined in Table 4. Then mixes containing each individual filler material, No. 1, No. 5 & No. 6, are compared together. 5.1. Marshall stability
Fig. 7. Results of ITS test for different HMA mixes.
Results of the Marshall stability test are depicted in Fig. 6. As can be seen HMA stability increased by incorporating coal waste powder. The minimum and maximum stability were obtained for LS (i.e. the reference specimen) and CP samples, respectively. For specimens containing both filler materials the mix stability continuously increased by increasing the CWP content. However the Marshall stability of all specimens was more than 800 kg which is the minimum required quantity for heavy loading conditions. 5.2. ITS Fig. 7 illustrates the results of the ITS test for various studied mixes. According to this figure the changes in ITS for the studied mixes were less than for Marshall stability. Unlike the Marshall test the ITS of the LS specimen was slightly higher than the CP mix. Furthermore, the combination of limestone and CWP fillers resulted in higher ITS quantities. The LCP2 specimen was the optimum combination of these filler materials which revealed maximum ITS among all tested HMA mixes.
Fig. 8. Comparison between the stress–strain curves of HMA mixes.
5.3. Indirect tensile toughness index (TIit) In order to evaluate the flexibility of studied HMA mixes the normalized stress–strain curves were drawn and the parameters as presented in Eq. (2) were calculated for each curve. Fig. 8, shows the stress–strain curves of LS, LCP1 and LCP3 mixes. As can be seen all curves had to some extent a similar peak strain point (ep), but the point of complete break (e) was different for each mix. Based on this figure the LCP1 & LCP3 mixes had the maximum and minimum break strains (e), respectively. Fig. 9, compares between the TIit of the studied mixes. The reference mix containing LS showed weaker energy absorbency than other mixes. Application of coal waste powder alone increased the material flexibility. However, equal combination of limestone and coal waste powders resulted in the maximum flexibility in HMA. Higher toughness index indicates higher crack resistance of
Fig. 9. Comparison between the indirect tensile toughness index of studied HMA.
material [34]. Therefore, it could be concluded that incorporating the coal waste powder not only improved the HMA stability and indirect tensile strength but also had beneficial effects on material flexibility. 5.4. Resilient modulus (Mr)
Fig. 6. Results of Marshall stability for different studied HMA mixes.
Results of the Mr test have been presented in Fig. 10. According to this figure, the stiffness of mixes increased by increasing the coal waste powder content. The maximum Mr value was obtained for the LCP3 specimen. Comparison between the results of Figs. 6, 7 and 10 showed that a combination of coal waste and limestone powders resulted in optimized mechanical properties. As presented in Table 3 CWP contained pozzolanic compounds (i.e. Sio2 & Al2O3) which could improve the bitumen-aggregate surface adhesion. Moreover, this very fine material could even increase the internal cohesion of the bitumen phase and finally increase
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A. Modarres, M. Rahmanzadeh / Construction and Building Materials 66 (2014) 476–483 Table 5 Comparison between the properties of LS, CP and CA, mixes.
Fig. 10. Results of Mr test for various HMA mixes.
the mix strength. Likewise, limestone powder contained about 47% CaO which contributes to aggregate-bitumen adhesion and could itself increase the mix strength. Therefore, the combination of LS and CWP resulted in higher ITS and Mr than other mixes. Not only did this combination increase the strength and stiffness of HMA mixes but also it improved the material flexibility which is desired for asphalt mix, especially at lower temperatures and repeated heavy loading conditions. Likewise, as well as the ITS results, which were almost the same for LS and CP mixes, using coal waste powder resulted in higher Marshall stability and Mr than the limestone powder. The latter result could be related to gradation and specific gravity of these materials. As presented in Table 4B, the coal waste powder has finer gradation and slightly lower specific gravity than limestone. The specific surface area (SSA) is increased as the particle size becomes finer. It means that filler material interacts with bitumen and aggregate surface at a higher contact area which then improves the bitumen-aggregate interaction and bonding quality between the aggregates and bitumen [38].
5.5. Moisture sensitivity Effects of the studied fillers on moisture damage indices have been shown in Fig. 11. As mentioned earlier a retained strength or stability of 80% is usually accepted for HMA mixes after the conditioning period. According to Fig. 11 all mixes satisfied this criterion. However, the maximum values were obtained for mixes containing both coal waste and limestone powder. The maximum MSR, TSR & SMR quantities were obtained for LCP3, LCP1 & LCP2 mixes, respectively. Likewise, the CP mix revealed similar moisture resistance to the LS mix. Results confirmed that coal waste powder behaves as an active filler which could improve the aggregate-bitumen adhesion. In mixes containing both coal waste and limestone powders the existence of lime and pozzolanic components could create cementitious compositions in the presence of water. The latter phenomenon might be the reason for the ratios being equal or even higher than 100% for mixes containing both limestone and coal waste powders.
Fig. 11. Effects of studied fillers on moisture damage indices.
Property (Unit)
LS
CP
CA
Marshall stability (kg) Marshall flow (mm) ITS (kPa) TIit Mr (MPa) MSR (%) TSR (%) SMR (%)
875 3.13 739 0.185 1697 98 88 84
1020 3.00 734 0.228 1980 96 91 86
1260 2.50 812 0.206 2933 98 95 92
5.6. Comparison to coal waste ash (CA) Table 5 compares between the various studied properties of LS, CP and CA, mixes. Based on this table, using CWA in HMA resulted in higher Marshall stability, ITS and stiffness than the other studied fillers. This is logical because after the incineration process at a constant additive weight the pozzolanic compositions proportion increases which could have beneficial effects on the bitumenaggregate bonding. However, incorporating CWA to some extent reduced the Marshall flow and toughness index of HMA. In order to better recognize the effect of these fillers on rheological properties of bitumen and finally the deformability of HMA, the kinematic viscosity of bitumen-filler blend was determined based on ASTM: D2170 for all filler materials at different temperatures [39]. The filler-bitumen blends were prepared by mixing the filler and bitumen with an equal combination. Fig. 12, shows the viscositytemperature diagram which was obtained for each blend. As shown at all temperatures the CWA-bitumen blend had the maximum viscosity. This figure indicates that incorporating CWA as filler material in HMA increases the viscosity of bitumen phase which will then increase the mix stiffness and reduce the deformability of material. According to Table 5, the moisture damage indices of all mixes were more than 80%. Therefore all examined mixes showed the desired moisture resistance. However, the maximum quantity was obtained for CWA. This could be related to higher viscosity of the CWA-bitumen blend which resulted in thicker bitumen film around the aggregate particles (i.e. higher effective bitumen content). Apart from the technical benefits of CWA, preparing the CWA for use in HMA needs incineration equipment which increases the total costs of HMA production. Furthermore, the incineration process increases the total consumed energy and environmental pollution. Therefore, since the use of natural coal waste powder (CWP) had no detrimental effects on the HMA mix properties especially those related to moisture damage, application of this additive is more favorable than CWA.
Fig. 12. The temperature-viscosity diagram of studied filler-bitumen blends.
A. Modarres, M. Rahmanzadeh / Construction and Building Materials 66 (2014) 476–483
6. Conclusions Based on the laboratory analysis and obtained results of this research the following conclusions could be drawn: (1) In comparison to the reference mix containing limestone powder as filler material, incorporating coal waste powder and its ash increased the Marshall stability, indirect tensile strength and resilient modulus of HMA mixes. (2) The HMA mix containing coal waste powder exhibited higher toughness or energy absorbency than the reference mix. Therefore, this material had no detrimental effects on HMA flexibility which is an important property at lower temperatures and heavy repeated loading conditions. (3) The equal proportion blend of limestone and natural coal waste powders revealed optimized properties in most of the examined characteristics. Therefore this blend is also recommended to be used as filler material in HMA mixes. (4) Because of high LOI content and existence of water absorbent compounds in natural coal waste powder the moisture resistance of HMA mix was precisely examined by three indices consisting of MSR, TSR & SMR. The minimum ratio of 80% was selected as an acceptance criterion for all these parameters. Based on the obtained results all mixes satisfied this criterion. (5) For all studied properties, using coal waste ash resulted in higher stability and stiffness than the two other additives. However, preparing the CWA for use in HMA needs incineration equipment which increases the total costs of HMA production. Furthermore, the incineration process increases the total consumed energy and environmental pollution.
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