Introduction

In India, with over 1210 million inhabitants, about 143,449 tonnes of household waste are generated daily, of which about 111,000 tonnes are collected and about 35,602 tonnes are disposed of [1]. The Central Environmental Control Board (CPCB) of India claims that waste production per person has increased exponentially (0.26 kg/day to 0.85 kg/day). It is estimated that between 80 and 90 percent of municipal solid waste is disposed of in landfills without the use of proper management techniques, polluting the air, water, and soil [2, 3]. If municipal solid waste is not adequately managed, by the end of 2047, more than 1400 km2 of land would be needed for solid waste management. Our homes, schools, hospitals, and businesses all produce municipal solid waste (MSW), which includes leftover food, used clothes, newspapers, bottles, household appliances, furniture, paint, batteries, etc., are included. About 40–60 percent of municipal solid waste in India is biodegradable, 30–50 percent is inert, and 10–30 percent is recyclable. However, research has shown that the physical makeup of Indian MSW has changed over time. Using biodegradable (biological treatment) or waste-to-energy (thermal treatment) technologies is the best way to utilise combustible waste and compostable waste. In biological treatment, waste components are broken down by microorganisms in a favourable environment. This process results in the breakdown of the biodegradable organic waste into gaseous products and water molecules, leaving carbon-rich by-products (compost). However, in thermal processes such as incineration, waste parts are burned in a controlled oxygenated environment to recover the maximum amount of thermal energy possible without polluting the air. Incinerating municipal waste reduces it by 90–95 percent of its original volume and creates incinerated residue. Most of the waste is incinerated during the incineration process and broken down into gases such as carbon dioxide (CO2), water vapor, and hazardous gases, which are cleaned by a sophisticated flue gas cleaning system. Bottom ash and fly ash are two general categories for the inorganic ferrous and non-ferrous metal wastes generated from waste incineration. Bottom ash is non-combustible and is removed from the incinerator as a slag-like solid residue. Depending on the composition of the waste, 20–25 percent by weight of the incinerated waste is BA. Most of the slag is disposed of in landfills, only a tiny part is recyclable. The disposal of MSWI-BA bottom ash is expensive. The search for better, cheaper, and more environmentally friendly ways to recycle garbage has become urgent due to concerns about landfills drying up and the soaring cost of waste disposal. The limited availability and high cost of traditional paving materials have also prompted research into using waste products such as bottom ash as a supplement or partial replacement for natural paving materials. Bottom ash consists mainly of oxides of Si, Fe, Ca, Al, Na and K, making it compositionally comparable to traditional road material, and its effect on road surface properties can be demonstrated to be technically, economically, and ecologically justifiable [4]. A comprehensive investigation characterized the chemical waste from MSWIBA, revealing its potential to release leachable components and influence groundwater, emphasizing proper landfill disposal [5]. Incorporating MSWIBA in bituminous mixes improved stability with increasing ash concentration, conforming to leaching regulations. However, the Los Angeles abrasion value of the bottom ash was found subpar [6]. Partial replacement of fine aggregates of bituminous concrete mixes with coal ash and bottom ash reduced Indirect Tensile Strength (ITS) values impacting the Tensile Strength Ratio (TSR) values. The TSR values increased substantially when hydrated lime was added [7]. The use of MSWIBA as a partial aggregate substitute showcased cost-saving benefits and waste diversion, though with a lower dynamic modulus [8]. Coal slag as filler improved Hot Mix Asphalt (HMA) strength, stiffness, and flow with optimized bitumen and coal slag ratios [9]. The use of coal MSWIBA in pavement construction was found successful, showing that it can perform similarly to natural aggregates when the particle size distribution is adjusted appropriately [10,11,12]. Substituting virgin aggregates with MSWIBA met Superpave requirements but caused Voids in Mineral Aggregates (VMA) reduction and higher bitumen content [13]. Chemical, physical, and mechanical analysis of MSWIBA revealed its potential use as a road base material with the addition of lime treatment [14]. MSWI-BA may be dangerous due to high levels of water-leachable components. The most typical procedures of stabilisation utilising bituminous binders are used to handle such hazardous materials [15].

The aim of this research is therefore to see how well bottom ash can be used as a partial replacement for fine aggregates in bituminous mixtures. Bituminous Concrete Grade II (BC-II), according to MoRT&H specifications, was used in this study [16].

Materials used

The viscosity-graded binder VG-40 was used in this study. Table 1 summarises the basic properties of the binder used. It was discovered that the Nominal Maximum Aggregates Sizes (NMAS) of coarse aggregates (10 mm NMAS and 20 mm NMAS), and fine aggregates (stone dust), acquired from a local quarry, met the requirements of the MoRT&H (Ministry of Road Transport and Highways) [16]. The basic properties of aggregates are tabulated in Table 2.

Table 1 Basic properties of VG 40 Binder
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Table 2 Basic properties of Aggregates
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It was observed that MSWI-BA contained some sheet metal, broken glasses, and ceramic fragments. The MSWI-BA consists of irregularly shaped particles with significant surface roughness and a porous microstructure, supporting previous density and absorbance results. The MSWI BA was subjected to a combined analysis using Scanning Electron Microscopy (SEM) and X-ray fluorescence (XRF) techniques to obtain comprehensive insights into the morphological and chemical characteristics of the ash. SEM analysis revealed that the MSWI BA consisted of agglomerated and irregularly shaped particles with a rough surface texture with open pores. Conversely, the bottom ash displayed a coarser and irregular morphology with large-sized particles. The SEM examination further identified the predominance of various glassy spheres, spheroids, and aggregates with irregular forms in the bottom ash. The results of SEM analysis are presented in Fig. 1. On the other hand, XRF analysis provided valuable insights into the crystalline structure and elemental composition of the bottom ash. The major elements identified through XRF analysis conducted on the same source samples included SiO2 (54.3%), CaO (12.8%), Al2O3 (9.7%), Fe2O3 (5.4%), and MgO (3%), with minor quantities of K2O, Na2O, P2O5, TiO2, SO3 and Cl. The elemental composition results from XRF analysis are tabulated in Table 3 [26].

Fig. 1
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SEM images of MSWI-BA at different magnifications (a) MSWI-BA × 1.0 K; (b) MSWI-BA × 5.0 K; (c) MSWI-BA × 10.0 K; and (d) MSWI-BA × 20.0 K

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Table 3 Elemental composition of MSWI-BA
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When used in the wearing course of the pavement, the rough texture of the material should increase its skid resistance, although irregularly shaped particles can still affect the compatibility of the material. Table 4 summarises the basic properties of MSWI-BA. The comprehensive particle diameter range of MSWI-BA extended from 0 to 9.5 mm, offering a holistic representation of the particle size distribution. However, to gain deeper insights into the grain size distribution of MSWI-BA particles suitable for replacing the bituminous mix, a wet sieve analysis that specifically focused on particles passing through a 4.75 mm sieve was performed according to IS 2420-4 (1985) [32]. As the research focused specifically on the replacement of Bituminous mix with Bottom ash particles within the 0 to 4.75 mm sieve range, the resulting grain size distribution data for particles passing through the 4.75 mm sieve has been diligently reported in Table 5 and visually represented in Fig. 2. Because the gradation requirement of particles passing a 4.75 mm sieve matched the particle size distribution of MSWI-BA particles, the finer grade for wearing coarse BC-II was used in this study. MoRT&H has laid out the requirements for these gradations [16]. The final obtained gradation is illustrated in Fig. 3. The BA was procured from Timarpur-Okhla Waste Management Co. Pvt. Ltd. (TOWMCL), Delhi.

Table 4 Basic properties of MSWI-BA
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Table 5 Gradation of MSWI-BA
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Fig. 2
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Grain Size Distribution of MSWI-BA

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Fig. 3
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Gradation for BC-II

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Experiments

Bituminous mix design

Six different percentages were employed to replace the fine aggregates (passing a 4.75 mm IS sieve) in the asphalt mixes to study the effects of MSWI-BA on those combinations: 0% (control mixture), 10%, 20%, 30%, 40%, and 50%. The BC-II mix that complies with MoRT&H specification is considered for the study [16]. The asphalt institute MS-2 was referred for the Marshall mix design and determination of optimum binder content (OBC) [33]. Hot bitumen was added to the mixture to prepare the MSWI-BA asphalt mix. Compaction was applied to Marshall Specimens at 75 blows on each side at 155 °Celsius mixing temperature. The Marshal mix design approach was used for the mix design (ASTM D 6927–15) [34]. Three specimens were prepared for each ash replacement and asphalt cement amount. Thus, a total of 90 specimens were produced.

The increased porosity of the MSWI-BA and higher asphalt absorption within the aggregate blend led to incomplete coating of aggregate particles, resulting in subsequent disintegration upon compaction. As a result, until an acceptable coating was noticed, tests were conducted for each percentage of ash concentration in increments of 0.5 percent asphalt content. Using trial and error, the initial proportion of asphalt material was raised as the ash content increased. As a consequence, the asphalt concentration for the various combinations ranged from 4.25 to 8 percent. The specimens were then cooled to room temperature.

The bulk specific gravity (Gmb) of the prepared samples were determined as per ASTM D 2726-19 [35]. A separate, loose mix with a binder content near the expected OBC was prepared for the determination of the theoretical maximum specific gravity (Gmm) as per ASTM D 2041-03a [36]. Specimens were then transferred to a preheated water bath having a temperature of 60 °C for 30 to 40 min. Marshall stability and flow tests were performed on these samples following the specification laid out in ASTM D6927-15 [34]. As indicated by the test technique, the Marshall sample is set underneath the Marshall testing head. Compressive loading is applied at a consistent pace of 51 mm/minute until the failure of the sample. Marshall stability is the maximum load at which the specimen fails, whereas the flow value is the measure of the deformation of the sample, which is measured using a flow meter.

Similarly, the entire process is repeated at different binder contents, and a series of Marshall stability, flow, Gmm, Gmb, percent air voids (Va), and density values were obtained. Separate graphical plots of each parameter were made against different bitumen contents. In this study, the bitumen content with respect to 4 percent air void was considered the optimum, and the other obtained parameters like VMA, stability, unit weight, and flow values were checked to be under the specified limits as per the Ministry of Road Transport and Highways (MoRT&H) Specifications [16].

Bituminous mix performance tests

The most widely used test methods for determining the moisture sensitivity of an asphalt mixture are the AASHTO T-283 indirect tensile test protocols, also known as modified Lottman tests [37]. The indirect tensile test involves loading a cylindrical specimen along its vertical diametral plane [38]. According to AASHTO T 283, a total of six Marshall samples were prepared at 7 ± 0.5% air voids (by volume of mix); out of these, three were tested as unconditioned, and the other three were tested as conditioned specimens. The tensile strength ratio (TSR) is the ratio of the average indirect tensile strength of the conditioned specimens to the average indirect tensile strength of unconditioned specimens. It is expressed as a percentage. Conditioning of the specimen was done by keeping the specimens in a water bath maintained at 60 °C for 24 h and then curing them at 25 °C for 2 h before commencing the test. The unconditioned test was conducted at 25 °C. MoRT&H suggests a minimum of 80% TSR to make the mix resistant to moisture damage [16].

The resilient modulus was calculated as per ASTM D 4123-82 [39]. The test was run using the IPC Global Universal Testing Machine. For each replacement percentage, three Marshall samples were tested. A Haversine loading waveform, 0.1 s of loading followed by 0.9 s of rest and an assumed Poisson's ratio of 0.35 were used to complete 100 conditioning cycles. A load equivalent to 10% of the indirect tensile strength of the mix was applied during the MR test. The MR test was performed at two different temperatures of 25 °C and 35 °C. This was done to assess how well the blend performed in terms of stiffness as the temperature increased.

The dynamic creep test was carried out according to European standards at 40 °C (EN 12697-25) [40]. As per Indian conditions, a recommended tyre pressure of 560 kPa was applied in the uniaxial creep test at 40 °C to simulate stress in an asphalt concrete surface layer [41]. A seating load of 100 kPa was applied to ensure that the actuator and sample make firm contact. One thousand eight hundred cycles are used as a termination condition. The samples must first be preconditioned for at least 2 hours at 40 °C. The conditioning time should be increased if there is a discrepancy between skin and core temperatures.

Environmental evalation

The samples of MSW-BA and asphalt mixtures were analysed for the presence of lead (Pb) using the toxicity characteristic leaching procedure (TCLP) adopted by both the Central Environmental Control Board (CPCB) of India and the U.S. Environmental Protection Agency (EPA). The extraction of the leachable elements is done using USEPA Test Method 1311, and the testing procedure will be based on the Toxicity Characteristic Leaching Procedure (TCLP) [42]. TCLP is an analytical method that simulates sanitary landfill contaminant leaching in waste samples.

This test was performed to simulate the leaching that may occur if MSWI-BA is used in bituminous mixes. The solid waste samples can be classified as hazardous or non-hazardous based on the concentrations of the TCLP constituents and the guidelines set forth by the CPCB, India TCLP regulatory standard as per Schedule II of Hazardous and Other Wastes, Rules, 2016 [43].

Results and discussion

Marshall mix design

Five Marshall indices (Marshall Stability, Flow, % Air voids and Voids in Mineral Aggregates and Marshall Quotient) are exhibited in Figs. 4, 5, 6, 7 and 8. Table 6 indicates the OBC values for different mixes. It is to be noted that the results illustrated are the average value of the results of three samples. The binder demand increases with an increase in the MSWI-BA replacement. The aggregate blend in the MSWI-BA asphalt mixture had higher asphalt absorption due to the higher porosity of the BA particles. The bitumen fills the pores of the aggregates, which are present in the aggregates; thus, there is a higher absorption of bitumen over the surface of aggregates. Hence, there is a significant rise in the optimum binder content of the mix as the BA replacement increases. Mixes prepared with the least percentage replacement, that is 10% has the least OBC for the BC-II mix. The mix with 50% replacement being the highest replacement dosage considered in the study yields a higher optimum binder content. The bitumen required for the 10% replacement of MSWI-BA was 4.0% more than that required for the control mix. This accounted for a 3.84% increase in the optimum binder content compared to the control mix.

Fig. 4
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Marshall Stability v/s Binder Content

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Fig. 5
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Marshall Flow v/s Binder Content

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Fig. 6
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% Air Voids v/s Binder Content

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Fig. 7
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VMA v/s Binder Content

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Fig. 8
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Marshall Quotient for different mixes

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Table 6 Optimum Binder Content for different mixes
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Marshall stability value was highest for the mix with 10% MSWI-BA replacement with an increase of 4.16%. Whereas higher MSWI-BA replacement corresponding to 50% resulted in 20.10% lesser stability compared to the Marshall stability obtained for the control mix. The higher asphalt binder content leads to lesser bonding due to the increased film thickness and the rough surface of aggregates becoming smooth. Other than for the 40% replacement mix, a substantial decreasing trend in the Marshall stability can be observed with further addition in the MSWI-BA content. For an effective asphalt pavement, the flow value should be in the range of (2–5) mm. The mixes with 40 and 50% replacement of MSWI-BA showed a flow value more than 5 mm which is beyond the permissible limit. The results obtained from other mix dosages are well within the tolerance limits specified by MoRT&H specifications [16].

The Marshall Quotient is a sort of pseudo-stiffness that measures the material's resistance to permanent deformation. From Fig. 8, it can be observed that the mix containing 20% of MSWI-BA as fine aggregate replacement is the stiffest among other mixes, followed by the 10% replaced mix and the Control Mix. The results show that the mixes containing MSWI-BA with lower replacement percentages are stiffer than the conventional BC-II mix. They tend to have more resistance to permanent deformation when compared with the control mix. By analysing these Marshall parameters, the mix with 20% replacement yields better results compared to other mixes due to its better stability, low flow value, and compatible optimum binder content required. In addition, other volumetric properties are well within the limits set as per MoRT&H [16].

From the ITS test, it can be observed that bituminous mixes with 10% MSWI-BA replacement as fine aggregate resulted in the lowest tensile strength and crack resistance of any other mix. It has been observed that the indirect tensile strength of the mix increases with an increase in ash content. At higher ash content (= 50%) ITS value was the highest. This is due to the fact that with an increase in ash content, the optimum binder content also increased subsequently, which increased the tensile strength. The tensile strength ratio (TSR) values are also plotted in Fig. 9. MoRT&H suggests a minimum of 80% TSR, so those mixes offer good resistance to moisture damage [16]. The TSR values are higher for mixes containing MSWI-BA, which shows that these mixes offer more resistance to moisture-induced damage. This may be due to the presence of calcareous materials in the MSWI-BA, which naturally act as filler. As a result of the stronger bond between the bitumen and the aggregates, removing the bitumen from the coated aggregates becomes difficult. All six mixes satisfy the specifications given by MoRT&H [16].

Fig. 9
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ITS & TSR Values for different mixes

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Modulus of resilience

From Fig. 10, it has been observed that the modified mix with 50% MSWI-BA replacement is the stiffest among all other mixes. The mixes containing BA are stiffer when compared with the control mix. Thus, they offer more resistance to permanent deformation when compared to control mixes. The MR value decreases when the temperature increases to 35 °C. Essentially, the MR value of the mix must increase as the OBC content increases for each % increase in BSWI-BA replacements. The modulus of resilience, on the other hand, significantly decreases as the temperature rises. This effect is predictable given that it is well-known that the stiffness of a material reduces as its temperature rises.

Fig. 10
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MR values at 25 °C and 35 °C

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The material expands and the free volume rises as a result of the movement of molecules and collision at higher temperatures. Temperature causes both the space between the molecules that make up a substance and atomic vibration to increase.

Rutting evaluation using dynamic creep test

The dynamic creep test for both the control mix and the bituminous mix whose fine aggregates are replaced has been done at 40 °C. From Fig. 11, the increment in stiffness for the mixes having bottom ash percentages of 20%, 30%, 40%, and 50% as fine aggregate replacement is less than the control mix. Due to the decrease in stiffness, the resistance to permanent deformation increases. It has been observed that as BA replacement dosages increase by 20% the resistance to permanent deformation decreases. Thus, Rutting resistance shows a negative correlation with the % of MSWI-BA replacement. In CM, 40% and 50% replacement mixes, the secondary deformation is linear, which shows continuous and ductile failure throughout the 1800 cycles. However, in the case of 10%, 20%, and 30% replacement mixes, it shows the slightest deformation; the deformation increases suddenly and then stabilizes, which leads to an increase in the slope of the deformation curve. This might happen due to the brittle failure of the mix, whereas other mixes show progressive failure throughout the test cycles.

Fig. 11
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Permanent Actuator deformation for different mixes

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Environmental evaluation

Table 7 summarises the heavy metal concentrations (Lead as Pb) of leachates from TCLP tests that were conducted on the MSWI-BA. The results showed that Lead is present, but the TCLP results of MSWI-BA showed that all Lead concentrations in leachates were below the CPCB, India TCLP regulatory standard as per Schedule II of Hazardous and Other Wastes Rules, 2016 [43]. As a result, the MSWI-BA used in this study could be classified as non-hazardous waste. Also, after being mixed with bitumen, the heavy metal concentration (Lead as Pb) of leachates was undetectable (less than 0.1 mg/L). This means that the trace element and heavy metal concentrations in MSWI-BA could be controlled after mixing it with bituminous binder, as the binder is highly hydrophobic and prevents any pollutant from leaching out. The test results indicate MSWI-BA would be well encapsulated by a bituminous binder and could be used as an aggregate substitute for a bituminous mix. The results obtained are in good agreement with those of other researchers [44].

Table 7 Metal concentrations (Lead as Pb) of leachates from TCLP tests
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Conclusions

By replacing the fine aggregate of wearing course BC-II with MSWI-BA the following results were observed.

The stability of the mix increased for a 10 percent BA replacement and then started reducing substantially for a further increase in replacement dosages. With the addition of MSWI-BA, the optimum binder requirements of the mixes increased, increasing bitumen costs, while aggregate costs also decreased since BA is free of cost. According to the results, blends containing MSWI-BA at lower replacement percentages are stiffer than the traditional BC-II blend. When compared to the control mix, they often exhibit greater resistance to permanent deformation. The mixes with 50% MSWI-BA as a substitute for fines are found to be the stiffest among all mixes considered. Bituminous mix with 10% MSWI-BA replacement as fine aggregate produced the lowest ITS and TSR values of any mix tested. It has been observed that the indirect tensile strength and tensile strength ratio of the mix increase with the increase in MSWI-BA replacement. At higher ash content (= 50%) ITS value was the highest. With the addition of MSWI-BA, the affinity of coated aggregates towards bitumen is increased, thus offering more resistance to moisture damage. Bituminous mix with 50% BA as fine aggregate replacement provided higher resistance to moisture-induced damage due to the presence of higher bitumen content, and BA being a calcium-rich material, itself acts as an anti-stripping agent. All of the mix varieties were found to be capable of resisting moisture-induced damage as they have a TSR value > 80%. The cost evaluation provided an economic insight into different mix varieties. The mix with 30% MSWI-BA replacement resulted in an economic mix with 0.96% cost reduction compared to the control mix.

The heavy metal concentrations (Lead as Pb) were within legal limits, according to the TCLP data for the MSWI-BA. It was also shown that using MSWI-BA instead of natural aggregate in bituminous mixes would pose minimal environmental problems since the bituminous binder is good at lowering metal concentrations in leachates. The test findings of the current study showed that, in terms of both physical qualities and environmental safety, the use of MSWI-BA in hot-mix asphalt mixes is viable.

As a result of this work, the researcher recommends replacing up to 20% of the fine aggregates with a physically strong, better performing, and more economically efficient bituminous mix to be used as a wearing course.