Feasible utilization of waste limestone sludge as filler in bituminous concrete

Construction and Building Materials 239 (2020) 117781

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Feasible utilization of waste limestone sludge as filler in bituminous concrete Jayvant Choudhary, Brind Kumar, Ankit Gupta ⇑ Department of Civil Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi, India

h i g h l i g h t s
Suitability of dried limestone sludge (LS) as filler in bituminous concrete mixes was assessed.
Material characterization of LS and conventional stone dust were done.
Mixes consisting both fillers were designed at four different filler contents and tested.
LS mixes delivered superior performance against rutting, fatigue and ravelling.
Cost and energy analysis were done and predicted lower manufacturing costs and lesser greenhouse gas emissions.

a r t i c l e

i n f o

Article history: Received 4 August 2019 Received in revised form 2 November 2019 Accepted 2 December 2019

Keywords: Filler Bituminous mix Dimensional limestone Limestone sludge Industrial waste Recycling Sustainable construction

a b s t r a c t The dimensional limestone industries produce a substantial amount of waste limestone sludge during the polishing of stone slabs. The storage and disposal of this non-biodegradable sludge cause issues regarding environmental pollution, shortage of disposal land, increase in transportation cost, and several other associated problems. This study investigated the usability of dried limestone sludge (LS) instead of conventional stone dust (SD) as filler in bituminous concrete mixes. The performance of SD and LS incorporated mixes in several aspects was compared, and the amount of LS needed to ensure optimum bituminous mix behavior is determined. The physical and chemical properties of both fillers were initially explored. Then bituminous concrete mixes having both fillers added at four different proportions (4, 5.5, 7.0, and 8.5% by weight of aggregates) were prepared using the Marshall mix design method and their optimum bitumen contents (OBC) were determined. The performance of both mixes was studied. LS mixes displayed paramount rutting resistance, fatigue resistance, indirect tensile strength, ravelling, and resilient modulus than conventional mixes. It mostly attributed to the fine nature of LS, which ensured its better distribution. LS mixes also had lower OBC than SD mixes due to lower porosity and bitumen extender action of LS. LS displayed a good affinity towards bitumen due to calcite in its composition, which led to excellent adhesion and moisture resistance in its mixes. The test results suggested that the utilization of chosen LS as filler at an optimum filler percentage of 6.45% can result in formation of bituminous mixes with satisfactory engineering properties, lower bitumen content as well as lower greenhouse gas emissions. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Bituminous mixes are one of the most widely used materials in the binder and surfacing course of flexible pavement networks around the world. These mixes are conventionally made up of non-renewable resources like aggregates and carbon-based bitumen binder. In these mixes, the aggregates of various sizes provide

⇑ Corresponding author. E-mail addresses: [email protected] (J. Choudhary), [email protected] (B. Kumar), [email protected] (A. Gupta). https://doi.org/10.1016/j.conbuildmat.2019.117781 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

strength by forming a rigid structure, while the bitumen binder is responsible for holding the mix together and enhancing its durability. In 2013, construction industries around the world produced approximately 92.53 million tonnes of bituminous concrete mixes for pavement construction [44,63]. Production of such a large quantity of mixes has inherent distinctiveness for environmental damage due to the continuous exploitation of natural resources, particularly aggregates. Exhaustive mining of aggregates also causes problems such as vegetation loss, loss of water retaining strata, lowering of groundwater table, and disturbance in the existing ecosystem. In many cases, authorities impose mining

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restrictions in several regions, which reduces the availability of aggregates. It consequently leads to inflation in the cost of bituminous mixes. Also, in the construction of bituminous and concrete pavements, aggregates production alone is considered a responsible factor for half of the entire greenhouse gas emission [30]. Mineral filler can be termed as the finest part of aggregates (having the consistency of flour) and consider as a vital part of the bituminous mix. Fillers are the fine mineral grains, most of which pass through a 75 mm sieve and represent up to 12% of the aggregates by weight in the bituminous mix [41]. It primarily acts as an inert component that fills voids in between aggregates in the mix and produces denser and impermeable mix. However, filler particles finer than bitumen film thickness affects the viscosity of bitumen-filler mastic and thus influences the performance of bituminous mixes against various pavement distresses [20,29,57]. Performance of bituminous mixes is ultimately linked to the various physical, morphological and mineralogical properties of the filler, its physical-chemical interaction with bitumen and its volumetric concentration in the mix [4,19,42,52,67]. Filler type and its content in the bituminous mixes also influence the mastic behavior and affect the mixing and compaction of the bituminous mixes [39,40]. So, the selection of the appropriate filler is crucial amongst field engineers. Stone dust, cement, and hydrated lime are being conventionally utilized in bituminous mix composition as fillers since they deliver satisfactory performances in the mix [5,19,18]. However, there is an immediate need for functional substitutes of conventional fillers without compromising the quality of bituminous mixes produced. Utilization of waste materials in place of conventional pavement material is a potential solution of two crucial and delicate issues: the growing problems concerning the safe disposal of waste materials and the immediate need to find viable alternatives for conventional pavement materials. Various studies were conducted to investigate the impact of solid wastes like bauxite residue [19]; brick dust [5,6]; carbide lime [19]; construction and demolition waste [5]; copper tailing [42]; fly ash [5,18]; Oil shale [14]; phosphogypsum [37] on the properties of bituminous mixes when utilized as fillers. However, most of these studies are either limited to fewer aspects of the bituminous mixes, or they used the particular waste in a limited proportion. Arguably, more extensive research on alternative sources of filler is needed to cater to the demand of conventional fillers. The dimension stone industry is one of the largest producers of non-biodegradable solid waste. Dimension stone is a vital construction material widely utilized in flooring, paving, and cladding of the building, monuments, and railway platforms. There is a wide range of dimension stones like granite, marble, sandstone, limestone, slate, and travertine produced extensively in 28 major countries around the globe [54]. The dimension limestone is considered as one of the most popular stones in the world market because of its lower cost and superior ability to take polish similar to the marble [54]. According to a report by the United States Geological Survey, the dimension limestone constitutes 45% of the USA’s market [22]. Dimension limestone after being mined undergoes finishing operations like cutting and polishing, which produces a significant amount of non-biodegradable solid wastes. The mining operations generated coarser stones as wastes, which could be utilized as alternatives to coarse aggregates in cement concrete or bituminous mixes. However, during the cutting and polishing operations of the limestone slab, the large amount of waste is generated in the form of LS. It is produced when blades are sprayed with cold water to soaks up the fine dust (Fig. 1(a)). This sludge mostly consists of suspended dust particles and is disposed of in the landfill nearby the industrial area. The water from the sludge after being evaporated leaving behind a vast amount of fine dust which occupies valuable lands affects the region’s landscape, hinders soil permeability,

(a)

(b) Fig. 1. (a). Production of LS during the polishing of limestone sludge (b). Disposal of dried sludge in the open dumping ground.

reduces soil fertility and groundwater table (Fig. 1(b)). This fine dust, after being airborne, also cause vision, bronchial, and skin problems to the neighboring residents. Hence judicious utilization of LS as filler in bituminous mixes can not only save a significant amount of conventional fillers but also systematically resolve the problems mentioned above. 2. Scope and objectives The entire objective of the study is divided into three parts:(a) Analyzing the aptness of fillers (conventional stone dust (SD) and LS) based on detailed primary characterization (b) Design of bituminous concrete mixes containing both fillers at 4 different filler contents (4, 5.5, 7 and 8.5%) at their respective optimum binder content (OBC) and compare their mechanical and durability properties (c) Analysis of cost and environmental viability of mixes. Detailed characterization of fillers was conducted according to the various tests specified in the Indian paving guidelines [41], and with the reliable tests highlighted in numerous relevant studies. Then, bituminous concrete mixes incorporating different fillers were designed, and their respective OBC’s were assessed accordingly. At their respective OBC, the Marshall, volumetric properties of mixes, as well as their behavior against rutting, cracking, fatigue, and ravelling were analyzed using relevant test parameters (Marshall quotient, indirect tensile strength, indirect tensile fatigue test and Cantabro test (dry and wet)). The performance of mixes against moisture was assessed using a modified Lottman test,

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Collection of coarse and fine dolomite aggregates

Collection of VG 30 bitumen

Collection of fillers (SD and LS)

Physical, morphological and mineralogical characterization of all fillers

Design of bituminous concrete mixes prepared with both fillers at different filler content (4, 5.5, 7, and 8.5%) using Marshall Mix design procedure (MS-2)

Evaluation of all mixes prepared at their OBC for •

•

Marshall Stability and Flow Values (MS-2) • Volumetric Properties (MS-2) • Resistance against Permanent Deformation (MS-2) Resistance against Moisture Susceptibility (AASHTO T 283) • Resistance against Cracking (ASTM D 6931) • Active and Passive Adhesion (ASTM D3625) • Resilient Modulus (ASTM D4123) • Resistance against Fatigue (EN 12697-24) • Resistance against Ravelling (NLT 352 & NLT 362) • Cost Analysis • Environmental Analysis

Analyze the effect of fillers over the performance of bituminous mixes

Determination of optimum filler content for both mixes

Conclusions Fig. 2. Flow chart describing the methodology of the study.

active and passive adhesions analysis. The load distribution ability of all mixes was computed by the calculation of indirect tensile stiffness modulus or resilient modulus of the mixes. The cost analysis and environmental analysis of all mixes were conducted by comparing the production and greenhouse gas emission of the mixes. Finally, the optimum filler content of both types of mixes was determined, which is based on various laboratory test results. The entire methodology has been summarized in Fig. 2. 3. Material properties and experimental analysis 3.1. Material

Table 1 Properties of aggregates and bitumen. Material

Aggregates Bulk Specific gravity of coarse aggregate Bulk specific gravity of fine aggregate Water Absorption of coarse aggregate (%) Aggregate Impact Value (%) Los Angeles Abrasion Value (%) Combined Flakiness and Elongation Index Bitumen

Coarse and fine dolomite aggregates were collected from a local supplier in Varanasi city (25.31°N, 82.97°E) whose properties are stated in Table 1. The gradation used to prepare the bituminous concrete mix was chosen as per MORTH [41] specification (Fig. 3). VG 30 bitumen (similar to the 60/70 penetration grade binder) was utilized in this study, whose various properties are stated in Table 1. These are two of the most widely used materials for the preparation of the bituminous concrete mix especially in the

Property

Absolute viscosity at 60 °C, (poise) Penetration at 25 °C (0.1 mm) Softening Point (°C) Ductility at 27 °C (cm) Specific gravity

Specification Used

Results

ASTM C127

2.795

ASTM C128 ASTM C127

2.720 0.374

[33] [33] [32]

11.1% 13.4% 21.3%

IS: 73 [31]

2692 62 51.5 >100 0.999

Indian scenario. Dolomite stone dust was utilized as the conventional filler and was collected locally. Dried LS was collected from the dump yard of the dimension stone industry located in Kota city

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J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781 Table 2 Characterization of limestone sludge and stone dust. Property

Limestone Sludge

Stone Dust

Specific Gravity MBV (g/kg) German filler (g) FM D50 (mm) Hydrophilic Coefficient pH Particle shape & texture (SEM)

2.650 3.75 97 3.03 09 0.80

2.698 3.25 85 5.38 21 0.77

10.22 Small size, granulous and somewhat spherical particles with rough texture Calcite (CaCO3), Quartz (SiO2), Enstatite (Mg2Si2O6)

12.57 Angular particles with slightly rough texture

Mineralogical Composition (XRD)

Dolomite (CaMg (CO3)2), Quartz (SiO2), Ertixite (Na2Si4O9)

Fig. 3. Adopted gradations for bituminous concrete mix.

(26.91°N, 75.78°E), India. It was obtained directly from the dumping ground of the industry as the dried sludge (dust), which was produced during the polishing of limestone slabs. Both conventional and waste fillers were dried in the oven and passed through the 75 mm sieve. The material which passed through the 75 mm sieve is used as filler. 3.2. Characterization properties of fillers Detailed physical and chemical properties of each filler were determined according to relevant specifications. Specific gravities of all fillers were determined using a pycnometer as per ASTM D854-14 [9] guideline. Particle size distribution curves were plotted as per ASTM D422-63 [8] specifications and were characterized using fineness modulus and mean particle size (D50). It was determined by calculating the sum of filler percentages coarser than 75, 50, 30, 20, 10, 5, 3, and 1 mm and then dividing the sum by 100 [52,64]. Particle shape and surface texture were analyzed using scanning electron microscopy (SEM) analysis. The porosity of fillers was determined as per the German filler test prescribed by the National Asphalt Pavement Association [48]. In this test, a small quantity of filler is added in continuous dosage to the 15 g of hydraulic oil until the filler oil mix loses its cohesion. The porosity of filler is inversely proportional to its German filler value. Prevalent minerals in the filler composition were evaluated using X-Ray Diffraction (XRD) investigation, which was conducted using a Rigaku benchtop XRD device operated with Cu Ka radiation at 1.5406 Å wavelengths. The analysis of Methylene blue values of fillers was done as per the EN 933 [25] specification to enumerate the harmful clay content and organic matters in fillers. pH values of each filler were determined by analyzing the pH of filler-water suspension prepared by mixing filler and water at a ratio of 1:9 by weight. The pH value was determined after continuously stirring the prepared suspension for a minimum of 2 h. Finally, the hydrophilic coefficient test of filler is conducted as per Chinese standard JTG E42 [45], to determine their relative affinity towards bitumen, in comparison to that of water. The Hydrophilic coefficient is determined by taking the ratio of volumes after the sedimentation of equal volumes of filler in water and kerosene for 72 h in an undisturbed condition. 3.3. Designing and testing of bituminous concrete mixes 3.3.1. Marshall and volumetric properties Optimum bitumen content (OBC) of all mixes was determined using the Marshall mix design procedure according to MS-2 guidelines. For every mix, specified weight (1200 g) of aggregates having gradation (Table 2) is mixed with five different bitumen contents

(4.5–6.5%). All ingredients were mixed at the mixing temperature of 162 °C determined as per the MS-2 guideline to form a loose mix. The compaction of the loose mix was done at 152 °C determined as per MS-2 guidelines [7]. For each mix, 15 samples (3 for each binder content) were prepared, and their Marshall stability, flow, and volumetric properties (voids in mineral aggregates (VMA), voids filled with bitumen, air voids were determined [12]. OBC was considered as bitumen content in the compacted specimen having 4% air voids [7,41]. The increase in the filler proportion of mix was done by a simultaneous reduction in the fine aggregate proportion to maintain the chosen gradation. After the determination of OBC for each mix, three more samples were prepared (a total of 24 samples for eight types of mixes), and the average values of Marshall and volumetric properties were compared. At OBC, average apparent film thickness (AFT) of each mix was devised according to NCHRP Report 567 [21]. Average AFT was estimated using Eq. (1).

AFT ¼

1000VBE Gmb Ss Ps

ð1Þ

where, AFT = Apparent film thickness (microns); Gmb = bulk specific gravity of mix; Ss = Aggregate specific surface (m2/kg); Ps = Aggregate content, (% by total mix weight); VBE = Effective bitumen content (% by total mix volume) 3.3.2. Resistance against rutting The Resistance of the bituminous mix against rutting was determined using the Marshall quotient, which measures mix’s resistance to shear stress and the permanent deformation (rutting) [66]. It is defined as the ratio of Marshall stability (kN) to flow (mm) value of compacted Marshall specimen (with 4% air voids) at failure. A bituminous mix that possesses higher Marshall quotient value displays superior stiffness as well as better load distribution capability, which ultimately results in its improved resistance against creep or rutting [6]. Twenty four specimens (three per filler) were prepared, and the average Marshall quotient value of each group was determined at 60 °C. 3.3.3. Indirect tensile strength The cracking resistance of each mix was determined by comparing the average indirect tensile strength (ITS) of compacted bituminous concrete mix samples according to ASTM D 6931-12 [13] guideline. As per the guidelines, testing was performed at 25 °C, and a compressive force was imposed on compacted Marshall samples (with 4% air voids) diametrically with the help of steel strips at a constant rate of 50.8 mm/min. Twenty four specimens (three per filler) were prepared, and mean ITS values for each group were compared. The ITS values were determined using the Eq. (2):

J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781

ITS ¼

2000Pmax pDT

ð2Þ

where ITS is the indirect tensile strength (kPa), Pmax is the load at failure, (N), D is the diameter of the specimen (mm) and T is the thickness of specimen (mm). 3.3.4. Resistance against moisture The modified Lottman test (AASHTO T283) is used to evaluate the moisture susceptibility of bituminous mixes [1]. The moisture susceptibility of all mixes was determined by measuring their tensile strength ratio. Two sets of Marshall specimens (conditioned and unconditioned) were prepared at 7 ± 1% air voids. Moistureconditioning of the conditioned samples was done by conducting 55 to 80% saturation; after that, they were placed in a freezer maintained at 18 ± 3 °C for 16 h. The specimens are then removed from the freezer and were placed in a water bath maintained at 60 ± 0.5 °C for 24 h. Finally, the specimens were then removed from the water bath and kept in another water bath maintained at 25 ± 0.5 °C for 2 h. The ITS of both unconditioned and conditioned set of specimens is determined at 25 °C. The tensile strength ratio was determined as the ratio of ITS of the conditioned specimen and that of unconditioned specimens. 3.3.5. Analysis of active and passive adhesion The adhesion loss in the aggregate-binder interface is one of the primarily responsible mechanisms for high moisture susceptibility in the mixes. It could be divided into two components namely, active and passive adhesions. The bitumen’s ability to completely cover the aggregates during the mixing operation of bituminous mixes can be termed as active adhesion. The effect of various fillers on the active adhesion can be analyzed by measuring time required by the mix to get uniformly coated with the binder [20]. All components of the mixes were manually mixed at their mixing temperatures. The mixing time is the total time (in seconds) elapsed between the moment of the binder addition into the mix and the moment at which all aggregates in the mix achieve 100% coating. Bitumen’s ability to remain adhered to the aggregate surface under the influence of external agents like traffic and moisture can be defined as the passive adhesion [62]. Analysis of passive adhesion analysis was carried according to ASTM D 3625-12 [10] guidelines. As per the analysis, the bituminous mix is placed in the boiling water for 10 min. After that, the mix was taken out of the water and placed over a white towel. Bitumen coating adhered to the aggregates was determined using the visual observation conducted by the team of five experts. Mixes are having high retained binder coating display superior passive adhesion and vice versa. 3.3.6. Resilient modulus Resilient modulus (Mr) is the most important input in flexible pavement design methodology because it signifies the capability of pavement layers to dispense load among them. It measures the responses of bituminous pavement layers towards the applied stresses and their corresponding strains [3,6,66]. Resilient modulus of all mixes was determined by testing standard Marshall specimens (with 4% air voids) at indirect tensile mode as per ASTM D4123 guidelines using a universal testing machine. The machine was equipped with a temperature control chamber which maintains a constant temperature during specimen conditioning and testing. The chamber was also equipped with two linear variable differential transducers to measure the skin and core temperatures of the specimens. All specimens were conditioned in the chamber for 24 h to achieve targeted testing temperature (25 °C). A haversine load pulse was applied at a frequency of 1 Hz (0.1 s load

5

and 0.9 s rest period) vertically along the vertical diameter of specimens using curved loading strips. The stress level in this test should lie in between 10 and 50% of the indirect tensile strength [11]. Hence for each mix, the load corresponding to its 10% of ITS was used for testing. The horizontal deformation was determined using two linear variable differential transducers attached at the mid-thickness at the end of the horizontal diameter. Initially, each test specimen were conditioned by the application of 100 load pulses, and subsequently, the calculation of the modulus was done by taking the average of further five load pulses. The Poisson’s ratio of each mix was assumed to be equal to 0.35 [6,42]. For each mix, three samples were used, and every test was repeated two times, and average values are taken into consideration. The resilient modulus was calculated as per Eq. (3):

Mr ¼

Pðm þ 0:27Þ t Dd

ð3Þ

where Mr is resilient modulus (MPa), P is repeated load (N), m is the Poisson ratio, t is the thickness of the specimen(mm), and dis the recoverable horizontal deformation (mm). 3.3.7. Fatigue resistance The Indirect tensile fatigue test was performed as per EN 12697-24 specification using the universal testing machine for the prediction of the fatigue life of bituminous mixtures. The specimen, specimen conditioning, and the loading procedure in the test were similar to the resilient modulus test specified in the previous section. The test was conducted under controlled stress conditions at 25 °C with a stress level equal to 40% of the indirect tensile strength of the compacted Marshall specimen (with 4% air voids). The stress level of 40% is chosen to limit the time duration of the testing. The haversine load pulse with loading and rest periods of 0.1 s and 0.4 s respectively, were as taken for the analysis. The specimen was continuously loaded until its complete failure. Failure of specimen occurred when it collapsed, or its vertical deformation reached 9 mm, whichever happened first. The load repetition underwent by the specimen before its failure is considered as its fatigue life. 3.3.8. Ravelling resistance Cantabro durability test is a relative measure of resistance to disintegration (ravelling). Although it is traditionally used to assess the durability of open and gap-graded mixes, however recent studies found the viability of test in cases of dense-graded mixes as well [23,51]. This test measures the weight loss after the breakdown of compacted Marshall specimens in the Los Angeles abrasion testing machine after 300 rotations at a speed of 33 rpm. The percent of loss in weight (Cantabro loss) acts as an indicator of durability and is related to the adhesion and cohesion of the compacted mix. In this study, samples were compacted by giving 75 blows on each side as per guidelines specified in recent research [23]. Further, dry and wet conditioning of samples was done as per Spanish norms NLT-352/86 [49] and NLT-362/92 [50], respectively. Six Marshall specimens were prepared for each mix and were divided into a group of three samples each. Three samples were placed under dry conditions at 25 °C for 24 h. Similarly, another three samples were immersed in water for 24 h and maintained at 60 °C. Finally, all samples were placed at 25 °C for 24 h in dry conditions and then tested. 3.4. Calculation of optimum filler content The inclusion of less or more filler than the optimum can harm its performance in one or multiple aspects. Various studies [3,18,29] have observed that the mixes containing particular filler content displayed superior performance in one aspect (e.g., rutting

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J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781

resistance), but failed to perform well in another aspect (e.g., moisture resistance). Hence utilization of the optimum amount of filler in bituminous mixes is crucial to ensure its reliable performance on every aspect. Previous studies [3,55] have considered the optimum filler contents in the bituminous mixes as the filler content at which mix display best performance in a particular aspect (say Marshall stability or lower OBC) or in multiple aspects simultaneously. This was done by averaging the filler contents at which the mix delivered superior performance in different aspects [3]. However, these studies have taken only a fewer number of parameters in the analysis. In this study, the optimum filler content was calculated using eleven different parameters of bituminous mixes determined in previous sections. The properties under consideration are maximum stability, minimum optimum binder content, maximum Marshall quotient, maximum tensile strength ratio, maximum ITS, maximum bitumen coverage, minimum mixing time, maximum resilient modulus, maximum fatigue life, minimum Cantabro loss (wet) and minimum Cantabro loss (dry). The optimum filler content for both mixes was determined by taking an average of filler contents corresponding to these properties using Eq. (4) It is hypothesized that the filler content determined using this equation will give optimized performance in various aspects on the field.

Optimum Filler Content F s þ F OBC þ F MQ þ F TSR þ F ITS þ F BC þ F MT þ F RM þ F FL þ F CLD þ F CLW ¼ 11 ð4Þ where Fs is filler content corresponds to maximum stability, FOBC is filler content corresponds to minimum OBC, FMQ is filler content corresponds to maximum Marshall quotient, FTSR is filler content corresponds to maximum tensile strength ratio, FBC is filler content corresponds to maximum bitumen coverage, Fs is filler content corresponds to minimum mixing time, FRM is filler content corresponds to maximum resilient modulus, FFL is filler content corresponds to maximum fatigue life, FCLD is filler content corresponds to minimum Cantabro loss (dry) and FCLW is filler content corresponds to minimum Cantabro loss (wet). 4. Results and discussion 4.1. Characterization of fillers The properties of all fillers are shown in Table 2 and Fig. 4-6. LS (2.650) has a slightly lower specific gravity than SD (2.698). LS was the finest filler as it has lower fineness modulus (3.03) and D50

100

Cumulative Passing (%)

90 80 70 60 50 40 30 20 10 0 0.001

0.01

0.1

Sieve Size (mm) Limestone Sludge

Stone Dust

Fig. 4. Gradation curves of both fillers.

(9 mm) values. SD (17.89) was the well-graded filler due to its higher Cu value. While, LS (12.71) has a relatively lower Cu value, and considered as a more uniformly graded filler. A filler containing a high amount of active fines may expand when it comes in contact with water and act as a barrier between bitumen and aggregates, which weakens adhesion in the bituminous mix [18]. There is no allowable limit for methylene blue value specified in Indian specifications. However some European countries (like Portugal) have specified its maximum allowable limit to be 10 g/kg [24]. Both SD (3.25 g/kg) and LS (3.75 g/kg) had the methylene blue value well within this permissible limit, which signified the lower active clay content in them. SEM images of both fillers were stated in Fig. 5. SD has the relatively larger and angular particles having a smooth texture, while LS has particles with shapes ranging from sub-rounded to rounded with a relatively rough texture. XRD diffractograms both fillers are shown in Fig. 6. Both of the fillers have calcium-based water-insoluble minerals in their composition, which produce mixes with high moisture resistance. LS consist of calcite in its composition, which is water-insoluble in nature and has good bitumen adhesion, which ensures good moisture resistance [15]. Similarly, SD has dolomite in its composition, which is another calcium-based water-insoluble mineral that forms moisture-resistant bituminous mixes. Fillers having low German filler values have higher Rigden voids or porosity [36]. Also, fillers having higher porosity can form bituminous mixes with higher OBC [18,19]. LS (97 g) had a lower porosity than the SD (85 g). Fillers having a hydrophilic coefficient below 1 are considered as hydrophobic fillers. They display a greater affinity with bitumen than with the water. A previous study has suggested the ideal range of hydrophilic coefficient value to be 0.70–0.85 [27]. All fillers had hydrophilic coefficient values in the prescribed range. In general, bitumen is found to be slightly acidic in nature, hence strong filler bitumen bond is expected in the case of alkaline fillers, which ultimately may result in superior stripping resistance of their mixes. Hence, the desirable pH value of filler should be higher than seven. SD and LS had higher pH values due to the presence of calcium-based minerals (calcite and dolomite) in their composition. Based on preliminary characterization, it can be said that both materials are expected to perform well and could be utilized as fillers in bituminous mixes. 4.2. Analysis of bituminous concrete mixes 4.2.1. Marshall and volumetric properties The average Marshall and volumetric properties of all mixes at OBC were determined and stated in Table 3. All mixes displayed satisfactory Marshall and volumetric characteristics as required by the Indian specifications, and it suggested that LS could be satisfactorily adopted as an alternative filler in bituminous concrete mix [41]. Marshall stability determines the strength of the mixes against the pressure as well as the horizontal and vertical stresses caused by the traffic [3]. Marshall stability of both mixes was found to be increased with filler content (Fig. 7). Improvement in Marshall stability might be due to the toughening of bituminous mastic by an increase in filler content and a subsequent decrease in OBC. This trend was in agreement with the previous studies [3,29]. Flow value indicates the deformation of the mixes and it has a linear relationship with the internal friction. The mixes having high flow value indicates plastic nature of the mix and vice versa. All mixes had flow values within the prescribed limit, which limit their possibility to be excessively plastic or brittle (Table 3). In all cases, OBC decreases with the increase in filler content (Fig. 8(a)). In bituminous mixes, aggregates are coated with bituminous mastic rather

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J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781

(a)

(b) Fig. 5. (a). SEM images LS (b). SEM images SD.

(a)

(b) Fig. 6. (a). XRD diffractograms of LS (b). XRD diffractograms of SD.

Table 3 Average Marshall and volumetric properties of bituminous concrete mixes. Type of Filler

Filler Content (% of the OBC (% of the total Ratio of filler to effective Bulk Specific Gravity VMA weight of aggregates) weight of mix) binder content (%)

VFB (%)

Marshall Flow AFT Stability (kN) (mm) (mm)

Stone Dust

4.0 5.5 7.0 8.5

6.20 5.95 5.38 5.34

0.66 0.75 1.39 1.73

2.430 2.444 2.453 2.466

17.02 16.21 15.31 14.70

74.22 74.43 74.79 72.01

12.22 13.99 15.96 16.58

3.43 3.62 3.50 3.22

7.85 7.34 6.47 5.77

Limestone Sludge

4.0 5.5 7.0 8.5

5.96 5.53 4.98 4.89

0.70 1.06 1.54 1.94

2.427 2.456 2.469 2.469

16.83 15.33 14.27 14.07

74.79 72.54 73.84 72.13

12.65 14.42 15.60 16.34

3.37 3.15 2.95 2.9

7.78 6.55 5.92 5.49

–

–

–

14.00 (min) 65–75 9.00 (min)

2–4

–

Requirements [41] 4–10

than with binder alone. It might be because less amount of binder is needed with higher filler content to make the same amount of mastic for the lubrication of aggregates in the mix [29]. Hence, at higher filler contents, a lesser amount of bitumen is needed to compact mixes to the desirable air voids. Thus here fillers display bitumen ‘‘extender” function in the mixes. This trend is in agreement with that observed in the previous studies [3,18,29,56]. LS is much finer filler than SD due to its lower fineness modulus and D50 values. Hence it displayed a greater tendency to exhibit bitumen extender function than SD. Also, LS has lower porosity

than SD as determined from his lower German filler value. Thus, LS incorporated mixes had lower OBC than SD incorporated mixes. The ratio of filler to effective binder ratio is shown in Table 3. At all filler contents, LS mixes has higher ratio of filler to effective binder ratio than conventional mixes due to their low OBC. However, at all filler binder ratio, mixes delivered satisfactory stability and durability. VMA is the volume of inter-granular voids between aggregates of compacted specimen, which includes air void and the bitumen which is not absorbed by the aggregates. Similar to the OBC,

J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781

Stone Dust

Marshall Stability (kN)

17 16

Limestone Sludge

y = -0.114x2 + 2.247x + 5.499 R² = 0.999

15 14 y = -0.127x2 + 2.600x + 3.784 R² = 0.989

13 12 3.5

Marshall Quotient (kN/mm)

8

7 y = -0.055x2 + 1.113x + 0.199 R² = 0.999

y = 0.022x2 + 0.080x + 2.878 R² = 0.998

6 5 4 3 2

4%

5.50%

7%

8.50%

Filler Content 4

4.5

5

5.5

6

6.5

7

7.5

8

Stone Dust

8.5

Filler Content (%)

Limestone Sludge

Fig. 9. Variation of Marshall Quotient of mixes with filler contents.

Fig. 7. Variation of Marshall Stability of mixes with filler contents.

Stone Dust

6.4

Limestone Sludge

OBC (%)

6 y = 0.021x2 - 0.472x + 7.791 R² = 0.941

5.6

5.2

4.8 3.5

y = 0.037x2 - 0.722x + 8.276 R² = 0.977

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

Filler Content (%)

(a) Stone Dust

Limestone Sludge

17.5

4.2.2. Rutting resistance Higher Marshall quotient value signifies the higher rutting resistance of the bituminous mix and vice versa. Marshall quotient values of all mixes are specified in Fig. 9. There was a clear relationship exists between Marshall quotient values were found to be increased with the filler content. This trend is in agreement with the results obtained in previous studies [18,56]. In general, it is observed that mixes with lower VMA and AFT resulted in higher Marshall quotient values and vice versa [20,19,21,35]. Since both VMA and AFT of mixes decreased with filler contents, Marshall quotient values were also found to follow a similar trend. LS mixes were found to have higher Marshall quotient values than SD mixes. It is expected since LS mixes also had the lowest VMA and AFT than SD mixes. Few recent studies have also observed that the use of fine fillers results in the formation of bituminous mixes with high stiffness [19,21,35]. Higher Marshall quotient of LS mixes might also be due to its finer nature. Recent studies have observed that the fine fillers have a greater potential of distribution in bituminous mixes, which resulted in their higher stiffness [19,20,43,46].

17

VMA (%)

16.5 y = 0.022x2 - 0.801x + 19.89 R² = 0.997

16 15.5 15 14.5 14 13.5 3.5

y = 0.144x2 - 2.428x + 24.25 R² = 0.998

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

Filler Content (%)

(b)

4.2.3. Indirect tensile strength Bituminous mix having a higher value of ITS has superior resistance to cracking. A trend between ITS values of mixes with filler content is shown in Fig. 10. ITS values of all mixes were found to be proportional to the filler content. It can be explained with the principle of composite mechanics. Understandably, the filler has a higher strength than the binder. So, at higher filler content, an increase in the portion of filler and a simultaneous decrease in binder content in mastic will inevitably increase its strength and as well as ITS of the bituminous mix [29]. LS incorporated mixes had higher ITS than SD mixes at all filler percentages due to its finer nature. A few studies have observed that fine fillers uniformly

Fig. 8. (a). Variation of OBC of mixes with filler contents (b). Variation of VMA of mixes with filler contents.

Stone Dust

Limestone Sludge

1100 y = -2.444x2 + 78.42x + 549.7 R² = 0.997

ITS (kPa)

VMA of all mixes were also decreased with the increase in filler content (Fig. 8(b)). SD mixes were found to have higher VMA than LS mixes, which might be due to higher porosity of SD. The voids filled with bitumen can be simply defined as the percentage of VMA filled with bitumen. The value of voids filled with bitumen of all mixes was also found to be within specification limits. AFT is the average apparent thickness of bitumen film around the aggregate particles that affect the performance of mixes against rutting and moisture susceptibility. As expected, AFT of all mixes decreases with the increase in filler content due to a decrease in OBC and voids filled with bitumen. LS mixes were found to have lower AFT than SD mixes.

900

y = 3.444x2 + 25.01x + 491.6 R² = 0.99

700

500

4

5.5

7

8.5

Filler Content (%) Fig. 10. Variation of indirect tensile strength of mixes with filler contents.

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J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781

4.2.4. Moisture susceptibility Bituminous mixes having high values of tensile strength ratio have superior resistance against moisture. Fig. 11 showed the tensile strength ratio values of all mixes. As per Indian specification, all bituminous mixes should have a minimum tensile strength ratio of 80% [41], and all mixes satisfied this criterion. This is due to the presence of dolomite (CaMg(CO3)2) and calcite (CaCO3) in the composition of SD and LS, which enhance adhesion in between bitumen and filler and form a stronger bond even in the presence of water [52]. LS mixes have a relatively lower tensile strength ratio value due to relatively higher active clay content in the composition of LS. It may also be due to the relatively lower AFT value of LS mixes than SD mixes. Similarly, TSR values of all mixes decreased with the increase in filler content. This was attributed to the decrease in OBC and AFT of the mixes with the increase in filler content. The observed results are similar to the results observed in previous studies [18,29,56].

Tensile Strength Ratio (%)

4.2.5. Active and passive adhesion Fig. 12a and Fig. 12b displayed active and passive adhesion values of all mixes. Mixes having better active adhesion takes lower mixing times to get 100% bitumen coating during the mixing process. In both mixes, active adhesion reduced with the increase in filler content. However, LS mixes had the relatively higher mixing time than SD mixes at all filler contents. Hence, they might consume relatively higher amounts of energy during the mixing at field than SD mixes. Bituminous mixes with lower mixing time and retained bitumen coverage have better active and passive adhesion, respectively. Bituminous mixes having better passive 100 y = -0.302x2 + 1.781x + 92.10 R² = 0.987

95

y = -0.033x2 - 1.886x + 102.1 R² = 0.986

90 85 80

4%

5.50%

7%

8.50%

Filler Content Limestone Sludge

105

Bitumen Coverage (%)

distribute in the mix and forms an integrated structure, which improved the ITS of the mixes [20,19,42].

y = -0.444x2 + 3.955x + 91.38 R² = 0.993

100

y = -0.333x2 + 2.233x + 96.75 R² = 0.947

95

90

85

4%

5.50%

7%

8.50%

Filler Content Stone Dust

Limestone Sludge

(b) Fig. 12b. Variation of bitumen coverage of mixes with filler contents.

adhesion displays higher bitumen coverage. Similar to the active adhesion, lowering of passive adhesion was seen with the filler content. Trends of active and passive adhesion were similar to TSR values, where SD mixes displayed superior active than LS mixes respectively. The higher alkaline nature of SD may be responsible for their good adhesion with bitumen. Other than that the higher OBC of SD mixes might also be a responsible factor for their superior active and passive adhesion than LS mixes. 4.2.6. Resilient modulus Result of Mr analysis at 10% stress level is shown in Fig. 13. For both types of mixes, a clear inverse trend was observed between the Mr values and filler contents. It was expected since OBC in both mixes decreased with the increase in the filler content. Several studies [3,2,28] have also established that the Mr of bituminous mixes increases with the decrease in binder content. Mixes containing LS also had higher Mr values than SD mixes for each filler contents. It might be due to the lower optimum binder contents of LS mixes as well as due to the finer nature of LS particles. In general, it is believed that the use of finer fillers in the bituminous mixes resulting in their higher stiffness [4,38,42]. Since LS has finer particles than SD as observed from their lower FM and D50values, the higher Mr values of their mixes were justified by the previous studies. It can be said that LS mixes have superior load distribution capabilities than conventional mixes. Hence, flexible pavements which use LS mixes in their surface layer may support similar traffic loading that of conventional SD mixes at a relatively lower layer thickness.

Stone Dust

Fig. 11. Variation of tensile strength ratio of mixes with filler contents.

4.2.7. Fatigue life Result of fatigue live analysis at 40% stress level is shown in Fig. 14. Fatigue lives of stone dust and limestone sludge

y = 1.666x2 - 11.3x + 101.0 R² = 0.994

120 110

y = 0.592x2 - 2.163x + 83.20 R² = 0.999

100 90 80 70

4%

5.50%

7%

8.50%

Filler Content Stone Dust

Limestone Sludge

(a) Fig. 12a. Variation of mixing time of mixes with filler contents

Resilient Modulus (MPa)

Mixing TIme (Sec)

130 6000 5000

y = -28.88x2 + 735.3x + 1177. R² = 0.998 y = 34.88x2 + 21.75x + 2341. R² = 0.986

4000 3000 2000 1000 0

4

5.5

7

8.5

Filler Content (%) Stone Dust

Limestone Sludge

Fig. 13. Variation of resilient modulus of mixes with filler contents.

J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781 y = -293.33x2 + 4160.3x - 8551.8 R² = 0.9031

Fatigue Life (cycles)

7000 6000

Cantabro Loss (dry) (%)

10

y = -209.11x2 + 3410.8x - 7913.1 R² = 0.9725

5000 4000 3000 2000 1000 0

4

5.5

7

8 y = 0.3089x2 - 3.7744x + 14.991 R² = 0.9178

4 2 0

4

8.5

5.5

7

8.5

Filler Content (%)

Filler Content (%) Stone Dust

y = 0.3089x2 - 3.1184x + 10.556 R² = 0.9565

6

Stone Dust

Limestone Sludge

Limestone Sludge

(a)

Fig. 14. Variation of fatigue lives of mixes with filler contents.

incorporated mixes increased up to 7% filler content. Fatigue failure in bituminous mixes is a three-stage process which includes: crack initiation (development of microcracks), crack propagation (development of macrocracks from the microcracks), disintegration (catastrophic failure of the material due to unstable crack growth) [34,57]. The increase in fatigue lives of bituminous mixes with the increase in filler content can be explained by the ‘‘crack pinning” behavior of the filler. Crack pinning is the formal term given for the mechanism which suggested that the inclusions (in this case, filler) in a multiphase composite material (in this case, bitumen filler mastic) have an interaction which leads to slowing down of the growth of microcracks [26,57,59,60]. It can be said that the filler particles in the mastic act as barriers that deflect the crack propagation (crack pinning) and thus enhance the fatigue lives of the mixes. The superior performance of LS mixes might be attributed to its lower specific gravity and fine particle size. LS has lower specific gravity than SD, and hence it occupied larger volume in the bituminous mixes at a similar weight. LS mixes also have lower OBC than stone dust mixes, hence a higher volume of LS acted as the barrier in the lower amount of bitumen, thus deflected the larger number of cracks and prolonged the fatigue lives. Previous studies have observed that mixes with higher binder contents led to lower stiffness which, resulted in larger strains and lowers fatigue lives [57]. It might be the reason behind the lower fatigue lives of SD mixes. Finer LS particles also have a greater potential for uniform distribution and forming an integrated structure in the bituminous mix, which might have increased the fatigue life of the mixes [42]. The fatigue lives of both mixes increased up to 7.0% filler content, and then a marginal drop is observed. Previous studies [47,58] have suggested that the fatigue life of bituminous mixes increase with the increase in the stiffness of the mix, when testing is conducted in controlled stress mode. In this study, the testing was conducted in controlled stress mode and the stiffness of both mixes was found to increase with the filler content (Fig. 13). Hence, the fatigue life was found to increase with the filler content [47,58,57].

4.2.8. Ravelling resistance For all mixes, Cantabro weight losses measured after wet conditioning are higher than dry conditioning, which indicated the loss of adhesion after conditioning of mixes in water (Fig. 15a). LS mixes suffered marginally higher losses after wet conditioning than SD mixes Fig. 15b. This may be due to lower bitumen film thickness of LS mixes or due to superior filler bitumen adhesion in SD mixes caused by high dolomite composition in SD. There is no well-defined trend for the dry Cantabro loss in the mixes. However, it seemed that in a dry state, losses decreased with an increase in filler content up to a limit and then marginal

Cantabro Loss (wet) (%)

Fig. 15a. Variation of Cantabro loss (dry) of mixes with filler contents

12 y = 0.0222x2 + 0.1996x + 4.6022 R² = 0.948

y = 0.01x2 + 0.2983x + 3.8792 R² = 0.9985

8

4

0

4

5.5

7

8.5

Filler Content (%) Stone Dust

Limestone Sludge

(b) Fig. 15b. Variation of Cantabro loss (wet) of mixes with filler contents.

decline is observed at higher filler content (Fig. 15b). The decrease in Cantabro loss may be due to the stiffening of mastic with the addition of the filler in the mixes which might have increased the cohesion of mix. However, at higher filler contents, excessive stiffening and lower adhesion might have increased the losses. Interestingly, LS mixes displayed lower losses at lower filler contents (4 and 5.5%) than SD mixes. Hence, it can be inferred that both mixes displayed satisfactory resistance against ravelling in both wet and dry conditions.

4.2.9. Cost analysis The economic benefit for utilizing limestone as filler was assessed after comparing the material cost required for the production of 1 m3 of all 8 types of bituminous concrete mixes. The quantities of materials were estimated as per mix design used in the study. The current unit cost of different ingredients (coarse aggregates, fine aggregates, stone dust (SD), and bitumen) for the production of the bituminous concrete mix as per Central Public Works Department, India [17] is displayed in Table 4. Being the waste material, LS is freely available except for its transportation cost. It was assumed that the transportation cost incurred in transferring conventional SD from its quarries to the production site of the bituminous mix is equal to that of transferring LS from the disposal site to the production site. LS used in this study needed no processing since it was already found to be fine in nature, and almost all of it passed through 75 mm sieve. The processing (labour) cost of LS in the worst scenario can be taken as 0.5% of the total cost needed to produce 1 m3 of bituminous concrete. The results of the economic analysis clearly show that the mixes containing LS are economical in comparison to standard mixes at all filler contents. The lesser amount of bitumen consumption by

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J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781 Table 4 Comparison of costs of various mixes. Material

CPWD Rates

Coarse Aggregate (m3) INR 1350/m3 Fine Aggregate (m3) INR 1350/m3 Stone Dust (m3) INR 1400/m3 Limestone Sludge (m3) 0 Bitumen (kg) INR 39.57/kg Cost in (INR/m3) 0.5% for Processing (INR/m3) Total Cost (INR/m3) Percentage saving in cost with respect to SD (4%) mix (%)

Quantity in 1 m3 of bituminous concrete mix produced SD (4%)

SD (5.5%)

SD (7%)

SD (8.5%)

LS (4%)

LS (5.5%)

LS (7%)

LS (8.5%)

0.316 0.496 0.034 0 153.68 7250 0 7250 0

0.318 0.486 0.048 0 147.90 7029 0 7029 3.05

0.322 0.482 0.061 0 135.25 6547 0 6547 9.70

0.322 0.469 0.075 0 133.73 6488 0 6488 10.51

0.316 0.495 0 0.035 148.20 6991 35 7026 3.09

0.321 0.491 0 0.049 138.26 6593 33 6626 8.61

0.325 0.486 0 0.063 118.04 6083 30 6113 15.69

0.325 0.474 0 0.077 123.18 5978 30 6008 17.63

LS mixes is the major responsible parameter for the saving. Cost comparison of all mixes with respect to SD mixes having 4% filler (SD (4%)) is done, and it is observed that cost of the mixes reduced with the increase in filler content. The total cost in producing 1 m3 of SD (8.5%) and LS (8.5%) mix is about 11% and 18% lower than SD (4%) mix, respectively. It should also be noted that besides these monetary benefits, utilization of LS as a filler can save a considerable amount of precious land that was used for its dumping and also have numerous environmental benefits.

4.2.10. Environmental analysis A large amount of greenhouse gases (GHG) are emitted during the production and transport of materials for the construction, maintenance, and rehabilitation of pavements [16]. The environmental benefit for utilizing limestone as filler was assessed after comparing the GHG emission occurred during the production of ingredients required to make 1 tonne of bituminous concrete mixes. GHG emission during the transportation and construction process for all mixes was assumed to be same. The quantities of materials were estimated as per mix design conducted in the study. The greenhouse gas emission is usually measured in kilograms of CO2 equivalent. The standard amount of emission by the various ingredients (coarse aggregates, fine aggregates, stone dust (SD) and bitumen) as shown in Table 5 was taken from the previous studies on the subject matter [61,65]. LS is a waste product produced during the cutting and polishing of stone slabs, therefore the CO2 equivalent for LS production is allotted with zero value in this study. The comparison of GHG emission clearly stated that the mixes containing LS are much environment-friendly than the standard mixes at all filler contents. It can be seen bitumen emit a much higher amount of GHG in comparison to aggregates. Hence, a lesser amount of bitumen consumption by LS mixes leads to their lower GHG emissions. Comparison of GHG emission of all mixes with respect to SD mixes having 4% filler (SD (4%)) is done and it is observed that GHG emission in both mixes reduced with the increase in filler content. The total GHG emission in producing 1

tonne of SD (8.5%) and LS (8.5%) mix is about 13% and 20% lower than SD (4%) mix respectively. Leaching of heavy metals from the hazardous wastes to the groundwater level can cause serious environmental and health consequences. Some researchers have conducted the leaching analysis of the bituminous mixes containing industrial wastes fillers (coal waste, copper tailing etc.), using the procedures like toxicity characteristics leaching procedure (TCLP) to assess the chemical stability of heavy metals in the wastes [46,47]. However, it is worth noting that the LS is not classified as hazardous waste and is obtained from the natural limestone. As observed from its mineralogical analysis, it mostly consisted of calcium and silica-based minerals, so TCLP analysis was not conducted in this study. After the analysis, it can be said that the utilization of LS as fillers has great advantages like the production of low-cost mixes, saving the natural aggregates, lowering of GHG emission and the formation of strong and durable mixes. The current strategy to manage waste LS is to openly discharge it in landfills, rather than utilizing it as a by-product. This unorganized dumping of LS on fertile land or along roadside causes severe degradation of land, blockage of the natural drainage, create health issues (vision, bronchial and skin disorders) in nearby residents due to airborne LS dust and damages the aesthetics of the region’s landscape [53,54]. All these problems can automatically be resolved if LS be utilized as filler in bituminous concrete mixes.

4.3. Optimum filler content According to the calculations made as per Eq. 5, the optimum filler contents for LS and SD mixes were obtained as (6.45%) and (6.59%) respectively. Taking the assumption that the properties of both mixes vary as per the trend suggested in their trend lines, calculation of various properties of the bituminous mixes expected at optimum filler contents can be done (Table 6). At their optimum filler content, bitumen consumption of LS mixes was about 8% lower than conventional SD mixes. Since bitumen is the most expensive ingredient in the mix, the lower consumption of the

Table 5 Comparison of greenhouse gas emission of various mixes. Material

kgCO2 equivalent/kg

Coarse Aggregate (kg) 0.0026 Fine Aggregate (kg) 0.0026 Stone Dust (kg) 0.0026 Limestone Sludge (kg) 0 Bitumen (kg) 0.426 Total Emission (kgCO2 equivalent/tonne) Percentage saving in total emission with respect to SD (4%) mix (%)

Quantity in 1 tonne of bituminous concrete mix produced SD (4%)

SD (5.5%)

SD (7%)

SD (8.5%)

LS (4%)

LS (5.5%)

LS (7%)

LS (8.5%)

356.44 544.04 37.52 0 62.00 28.85 0

357.39 531.38 51.73 0 59.50 27.79 3.67

359.48 520.30 66.22 0 54.00 25.46 11.75

359.71 506.43 80.46 0 53.40 25.21 12.62

357.35 545.43 0 37.62 59.60 27.74 3.85

358.98 533.76 0 51.96 55.30 25.88 10.29

361.08 522.61 0 66.51 49.80 23.51 18.51

361.42 508.84 0 80.84 48.90 23.09 19.97

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J. Choudhary et al. / Construction and Building Materials 239 (2020) 117781

Table 6 Various properties of bituminous mixes at their optimum filler contents. Property of Mix

Optimum Filler Content (%) Optimum Binder Content (%) Marshall Stability (kN) Tensile Strength Ratio (%) Indirect Tensile Strength (kPa) Marshall Quotient (kN/mm) Mixing Time (sec) Retained Bitumen Coverage (%) Resilient Modulus (MPa) Fatigue Lives (cycles) Cantabro Loss (dry) (%) Cantabro Loss (wet) (%)

Type of Mix SD

LS

6.59 5.59 15.40 90.72 806 4.36 95 98.16 3999 5483 3.53 6.28

6.45 5.16 15.25 88.56 954 5.09 97 97.29 4718 6079 3.29 6.81

bitumen may lower the cost of the produced mix. Other than that lower consumption of the bitumen might also reduce the greenhouse gas emission and the carbon footprint of the bituminous mix. In comparison to the conventional mixes, LS mixes at their optimum filler contents are also expected to display relatively higher Marshall quotient (improvement by 16.74%), higher indirect strength (improvement by 18.36%), and marginally lower dry Cantabro loss (decline by 6.80%). The major reasons attributed to this performance might be the lower porosity and finer nature of LS. However, LS mixes are also expected to show marginally lower values of Marshall stability (decline by 0.97%), tensile strength ratio (decline by 2.38%), and higher wet Cantabro loss (increase by 8.44%) than conventional mixes prepared at optimum filler content. This behavior might be due to the relatively higher active clay content as well as relatively less hydrophobic nature of LS in comparison to SD. Finally, LS mixes are also expected to display about 18% higher resilient modulus values than conventional mixes. Higher resilient modulus signifies the better load distribution capability of pavement layers; hence pavement surface layers made with LS mixes are expected to support similar volume of traffic at relatively lower layer thickness than SD mixes. This may not only save considerable amount of non renewable resources like aggregates and bitumen, but also result in lowering of labour cost and effort for their construction. 5. Conclusions This effectiveness of recycling LS as filler in the bituminous concrete mix was explored in this study. Firstly, bituminous concrete mixes having LS and SD were designed at four different filler contents (4.0, 5.5, 7.0, and 8.5%). Subsequently, the performance of all mixes prepared at their OBC was assessed through a series of laboratory investigations. Finally, the analysis of cost and environmental viability of mixes was done. Based on the analysis the following conclusions can be drawn.
LS displayed positive traits of good filler due to its fine nature, low active clay content, and hydrophobic nature. They also ensure a good bond with bitumen in the presence of water due to the high amount of calcite in it.
LS incorporated mixes also delivered Marshall and volumetric properties equivalent/superior to the SD mixes at relatively lower OBC. Marshall stability of mixes increased with the increase in filler content. While OBC of all mixes decreased with the increase in filler content.
Cracking and rutting resistance of all bituminous mixes increased with the increase in filler content. LS mixes displayed superior cracking and rutting resistances due to the fine nature of LS, as well as due to low VMA and AFT of its mixes.

Various properties of LS mixes in comparison to SD mixes at an optimum filler content

Decrease in OBC by 7.69% Decrease in stability by 0.97% Decrease in TSR by 2.38% Increase in ITS by 18.36% Increase in MQ by 16.74% Increase in mixing time by 2.11% A decrease in bitumen coverage by 0.89% Increase in Mr by 17.98% Increase in FL by 11.23% A decrease in a loss of 6.80% Increase in the loss by 8.44%


All LS mixes displayed good adhesion and satisfactory performance against moisture resistance. Overall, TSR values of all mixes decreased with an increase in filler contents due to the gradual decrease in AFT. SD mixes displayed better adhesion and superior performance against moisture at all filler contents due to lower AFT, lower active clay content and alkaline nature of SD.
LS mixes also displayed superior load distribution characteristics, and higher fatigue lives at all filler contents than SD mixes due to the finer nature and cracking pinning behavior of LS.
Utilization of LS as filler also resulted in lowering of material cost and GHG emissions up to 18% and 20% respectively. In conclusion, it can be said that the systematical utilization of LS in place of conventional SD as filler in the bituminous mix not only could be an effective solution for the safe disposal of large quantities of produced waste and ensure sustainable construction practices but also can produce superior performing mixes in a much economical manner.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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