1 Introduction

Scrap tyres, also known as waste tyres or end-of-life tyres (ELT), refer to vehicle tyres that are unfit to be used for the intended purpose. Their generation and disposal have nowadays turned into a pressing concern around the globe due to the associated environmental consequences and health concerns. According to statistics, 17 million MT of scrap tyres are reported to be generated annually in the world [1, 2]. Figure 1a illustrates the distribution of scrap tyre generation per capita in various countries. Most of the developed countries generate a significant amount of scrap tyres in comparison to the developing nations. Out of the generated scrap tyres, only 1.53 million MT, i.e. 4.3% of the total scrap tyre generation is being recycled [3]. Figure 1b shows the methods by which the scrap tyres are recycled and recovered globally. Non-recovered scrap tyres are typically dumped into a landfill, which is the least preferable disposal choice as it causes ecological imbalance and is subjected to health hazards [4,5,6]. Moreover, openly dumped tyres are potential sources of uncontrolled fire due to their high energy content [5, 7]. Yet, these management strategies owe their pros and cons including, impediments associated with processing, limitations in usage, carbon footprint and costly process [4, 5]. As a result, research has been geared towards finding alternative avenues to discard scrap tyres sustainably. One of the promising methods proposed in the past was using scrap tyres as raw material in civil engineering construction [8]. This will yield two prominent benefits: a cost-effective strategy to replace conventional construction and an environmentally friendly method of waste disposal. Evaluating the potential of deploying scrap tyres in civil engineering construction has been extensively explored under various contexts.

Fig. 1
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Scrap tyre generation in different countries and their recovery methods [9,10,11,12,13,14,15,16]

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Scrap tyres of whole or derived products are typically deployed in civil engineering applications such as embankment construction, retaining wall backfill and subgrade improvement as a standalone layer or blend [17,18,19,20,21,22]. Using tyre crumb in the aforementioned civil engineering elements brings down the vertical stresses due to the lightweight property of the tyre and hence reduces lateral stresses exerted on supporting structures [23]. In addition, the tyre-derived products (TDP) can be blended with other materials including aggregates, bitumen and cement [8]. They effectively reduce the contact between aggregates to improve their resistance to impact, static and abrasive forces [24]. Figure 2 illustrates the typical arrangement of particles when aggregate is blended with tyre crumb. In this context, the tyre crumb particles act as a cushion to negate typical forces exerted on aggregates during road construction and use. However, the cushion act performed by tyre crumb adversely affects the compaction characteristics of the aggregates and tyre crumb mix as the applied compaction energy is dissipated by the resilient property of tyres [24]. Reduction in compaction and hence the density of the mix may lead to unfavourable strength characteristics. This contrasting phenomenon about engineering characteristics emphasises the need of evaluating the presence of tyre crumbs in aggregate.

Fig. 2
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Schematic diagram of typical particle arrangement of crushed rock and tyre crumb

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Discarding scrap tyres or their derivatives using them in road construction seems an attractive option as it will allow large quantities of scrap tyres to be disposed of with minimum effort [8, 25]. However, it should be explored in depth to ensure that the presence of them on roadways does not compromise the characteristics of the road infrastructure. Researchers have explored the utilisation prospects of a variety of tyre crumbs to be deployed in road elements such as base/subbase and subgrade. The evaluations revealed the fact that the size and type of aggregate and tyre crumb had a profound influence on the final properties of the mix [26]. As illustrated in Fig. 2b, the size of the rubber determines the placement of it in the aggregate skeleton for crushed rock with fixed particle size distribution. The positioning of the tyre crumb affected the degree of contact between aggregates and hence dictates the engineering properties of the final mix.

TDPs are generated by reducing the size of scrap tyres by mechanical (ambient) or chemical (cryogenic) means [27, 28]. ASTM D6270 [29] defines the sizes of ranges of TDPs as follows: < 425 μm, < 425 μm–12 mm, < 425 μm–2 mm, 12–50 mm, 50–305 mm and 50 × 50 × 50 mm–762 × 50 × 100 mm. According to published literature, extensive works have been conducted to assess the impact of tyre crumbs on various engineering properties for the application of pavement layers. Table 1 summarises the studies which have been performed under a variety of contexts to evaluate the impact of tyre crumb size on engineering properties. Natural aggregates and recycled aggregates have been customarily tested for their property enhancement with the presence of scrap rubber of different sizes ranging from 0.4 to 20 mm. The assessments revealed that compaction-related properties such as dry unit weight and strength parameters such as California bearing ratio (CBR) and resilient modulus (MR) decreased with tyre size and replacement level. However, hydraulic conductivity, impact and abrasive resistance, rutting and fatigue resistance increased favourably.

Table 1 Details of tyre crumb sizes used in previous research
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Tasalloti et al. [9] found that compressibility of the scrap tyre and rounded gravel mix was affected by the scrap tyre composition and size. Moreover, the authors claimed that large-sized tyre crumbs were preferred in roadbases as they could minimise leaching of harmful metals. A similar study mixed coarse aggregates with different sizes of scrap tyres, and found that density, CBR and MR of the mixes decreased with the increase in tyre crumb composition [8]. Further, the study recommended to keep the replacement level of scrap tyres below 5% for satisfactory performance. Resilient moduli characteristics of TDPs and waste rocks were examined by Arulrajah et al. [30]. The findings from the study showed that TDPs by weight up to 2% was found to be suitable for the roadbase/subbase applications. Saberian et al. [31] examined the permanent deformation behaviour of recycled concrete aggregate (RCA)/crushed rock (CR) with two tyre crumb sizes. The authors discouraged the use of tyre crumb with RCA/CR in roads with heavy loadings. The experimental study conducted by Xiao et al. [26] discovered that using tyre crumbs in asphalt mixes increased voids in mineral aggregates and improved rutting resistance regardless of tyre crumb sizes. Various forms of scrap tyres have been already deployed in the construction of geotechnical elements such as embankments, retaining wall backfills, subgrades and subbases [17,18,19, 21, 22]. The introduction of scrap tyres contributed to reduce settlement and lateral forces on the structures due to lower self-weight of the fill material and increase vibration resistance. The field applications have generally scoped down to embankments and backfills. Mackenzie and Saarenketo [21] deployed tyre bales to strengthen peat layer of the subgrade and reported a considerable improvement in the subgrade. From an environmental viewpoint, the literature reported that the tyre crumbs laid well above the water levels released no harmful substances such as ferrous ions and manganese [32]. Further, to evaluate the contamination level when scrap tyres are used for road construction, the leachate can be analysed and checked against the maximum thresholds recommended for different trace metals in standards [33]. Overall, the review reveals the potential of employing the mixture of scrap tyres and unbound coarse aggregates in road construction and the pertaining limitations. The studies have been mostly conducted in the North American region or in developed countries, which have access to advanced technology. In the recent past, disposal of scrap tyres using landfills in developing nations have caused detrimental environmental issues [34, 35]. Instead of dumping scrap tyres in landfills, deploying scrap tyres in road construction could be a viable option to reduce the environmental problems in developing countries. Designs in developing countries, especially the ones in tropical regions, follow the standards stipulated in the Overseas Road Note 31 (ORN31) [36]. Furthermore, contemporary research in this context lack cost-related computations and analyses, which is vital for countries with a limited budget for road infrastructure development.

Developing nations are now facing an infrastructure boom, which has always been impeded by the scarcity of quality construction materials. It is also noted that a significantly large fraction of roads often experience low traffic volume with a negligible amount of high occupancy vehicles or heavy vehicles. These roads can be laid with cost-effective alternative materials other than the conventional ones, which are often cost-prohibitive. However, the lack of intention towards innovative construction and the absence of extensive studies in the local context have hindered the application of cost-effective and innovative materials.

As such, this study evaluates the effect of tyre crumb size on the mechanical characteristics of crushed rock substituted with tyre crumb for the application of road pavement in developing countries. The proposed evaluation method is inexpensive and complex-free, which can be conducted without intense supervision or advanced technology. In addition, such cost-effective method of construction greatly benefits developing nations to overcome budget-associated constraints related to road construction. In addition, the proposed strategy provides alternative means to dispose scrape tyre in a safer way. The findings of this study might contribute to devise standard protocols pertaining to the usage of scrap tyres in road construction. The availability of standards and formulation of national policies could persuade the industries in developing countries to adopt sustainable practices in road construction.

2 Materials and Methods

2.1 Characterisation of Constituent Materials

This study used a dense graded crushed rock as base-course aggregate, which was subjected to the different replacement levels of tyre crumbs of two different size ranges. Crushed rock is of nominal maximum size of 20 mm (S20) and tyre crumbs are of two size ranges; 0.6–6.3 mm (T0.6–6.3) and 6.3–12.5 mm (T6.3–12.5). As there are no stringent guidelines available about the gradation of tyre crumbs, this study selected the lower and upper limits of tyre crumb size as 0.6 and 12.5 mm, respectively, as widely reported in the literature. This range was bisected to have two size ranges, 0.6–6.3 mm and 6.3–12.5 mm. Figure 3 shows the particle size distribution of crushed rock and tyre crumbs used in this study. The selected composition of crushed rock was as per the standard specifications for road bases as specified in Overseas Road Note 31 (ORN31) [36]. According to ASTM D6270, tyre crumbs T0.6–6.3 and T6.3–12.5 are categorised as granulated rubber.

Fig. 3
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Particle size distributions of tyre crumbs and crushed rock

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During the preparation of crushed rock sample for testing, first, the particles of different proportions were sieved and blended to obtain the required gradation as shown in Fig. 3. From this stockpile, representative samples for each testing were reduced using quartering and riffling techniques described in ASTM standard [40].

This study used scrapped motorcycle tyre as it is commonly available in Sri Lanka in abundant quantity. The waste tyres were collected free of charge from a local mechanic shop located in Kilinochchi, Sri Lanka. It was thoroughly washed with water to remove the impurities and left in the air for a few hours until it became completely dry. Thereafter, steel reinforcement placed around the inner perimeter of the tyre was removed by cutting it manually using an angle grinder. The net weight of the tyre after the removal of the reinforcement was 2.4 kg. The whole tyre was cut into two or three parts and fed into the shredder machine as shown in Fig. 4. The shredder machine was operated for 20 and 40 s to generate tyre crumbs of size ranges 0.6–6.3 mm and 6.3–12.5 mm, respectively. The duration of the shredding operation required to obtain the respective size was decided through a trial-and-error process by conducting multiple runs. The shredded tyre crumbs were subjected to sieve analysis according to ASTM C136/C136M [41] to obtain the required particle size distribution.

Fig. 4
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Details of the shredder machine

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Before commencing the experiments on the mix, crushed rock and tyre crumbs were separately characterised through standard laboratory experiments [42,43,44]. The results obtained from the characterisation tests are summarised in Table 2. The properties of both crushed rock and tyre crumb were found to be following the corresponding standard requirements stipulated for road base materials [36, 45]. This ensured that the constituents were defect-free and did not have any influence on the mix properties.

Table 2 Results of material characterisation
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2.2 Mix Proportions

The constituent materials were added in predefined proportions to make crushed rock-tyre crumb mixes with alternative compositions. Published literature suggest various ranges for the inclusion of tyre crumb in crushed rock starting from 0% up to 25% for different aggregate types for a range of applications such as retaining wall backfill, embankments, road bases and subbases [8, 13, 24, 47]. In addition, the studies recommended being vigilant about the drastic change in compaction-related-strength characteristics when adding an excessive amount of tyre crumb [24, 48, 49]. Bearing the associated repercussions pointed out in the literature and to strike a balance between compaction characteristics and impact/abrasion/static resistance properties, this work adopted the tyre crumb replacement in the range of 0–8% by dry weight of the crushed rock.

Table 3 details the mix proportions used in this study. Crushed rock was substituted with 2, 5 and 8% of tyre crumbs of two size ranges 0.6–6.3 mm and 6.3–12.5 mm. Each mix was assigned a mix ID for distinct identification. The mix ID T20.6–6.3 refers to crushed rock substituted with 2% tyre crumb of size range 0.6–6.3 mm. T0 is the control mix with pure crushed rock.

Table 3 Mix proportions used in the study
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2.3 Characterisation of Crushed Rock and Tyre Crumb Mix

The road base/subbase materials experience various types of forces during the construction and operation. Design standards have recommended a series of tests to ensure the proper functionality of pavement. Accordingly, each mix was tested for various engineering properties as per the standards given in Table 4. Each test was repeated on six identical samples to reduce random error due to unknown reasons. The experimental conditions were all kept the same between all tests to minimise the impact of extraneous variables. In total, 210 mix samples were tested to determine the mechanical characteristics.

Table 4 Tests conducted on mixes and the respective standards
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Initially, a modified Proctor compaction test was conducted on a pure crushed rock sample (T0) as per BS standard [53] to determine the maximum dry density (MDD) and optimum moisture content (OMC). The crushed rock sample was thoroughly mixed with alternative quantities of water and compacted in a standard mould (Fig. 5a). The measured dry densities and the respective moisture contents are plotted in Fig. 5b. From the density–moisture relationship curve, the MDD and OMC were determined to be 2.436 Mg/m3 and 6.2%, respectively. The remaining mixes with tyre crumb substitution were prepared with the measured OMC.

Fig. 5
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Compaction test arrangement and results

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2.3.1 Compacted Bulk Density

First, the required weight of aggregate and the tyre crumb for each mix composition were measured separately. The measured constituents were mixed for 2 min using a laboratory planetary mixer until a homogenous mix was achieved. Thereafter, water was added at optimum moisture content and continued mixing thoroughly for another minute. The mix was then placed in a standard compaction mould in five layers and each layer was given 27 compaction blows with a 4.5 kg rammer. The blows were given uniformly across the specimen as recommended in the modified Proctor compaction test [53]. Having cast the specimen, the wet density and respective moisture content were measured from which the dry density of each mix was computed.

2.3.2 Aggregate Impact Value Test

AIV tests were conducted on samples with particle sizes ranging from 10 to 14 mm according to the BS standard [51]. The required amount of sample for each test was measured and washed thoroughly and dried in an oven at a temperature of 105 °C for 24 h. The dried samples were then placed in the AIV mould with alternative tyre crump compositions and were subjected to impact loading as stipulated in the standard. The amount of fines produced by the impact load was weighed and expressed as the percentage of the dry weight of the original sample, as AIV.

2.3.3 Ten Per Cent Fines Value Test

TFV tests followed almost similar procedures to the AIV test during sample preparation. The dried samples were subjected to static loading using a compression testing machine at a uniform loading rate of 2 mm/min for 10 min, as recommended in BS standard [52]. The static load required to produce ten per cent fines of the dry weight of the original sample was determined as TFV.

2.3.4 Los Angeles Abrasion Value Test

LAAV tests were conducted on a batch of samples with particle sizes ranging between 9.5 and 19 mm as per ASTM standards [50]. A clean, oven-dried sample of the required weight was fed into the drum of the LAAV testing machine along with the recommended amount of abrasive chargers. The drum was subjected to 500 revolutions at a rate of 33 revolutions per minute as recommended by ASTM standards [50]. The LAAV testing machine was stopped after completing the revolutions and the material was discharged into a tray. The amount of fines generated was expressed as the percentage of the dry weight of the original sample, as LAAV.

2.3.5 California Bearing Ratio Test

The sample preparation for the CBR test resembled that of the modified Proctor compaction test. The prepared cylindrical samples of alternative mix compositions were subjected to penetration using a standard plunger as per BS standard [53] by applying a load at a nominal rate of 1 mm per minute. The load and corresponding penetration were recorded at 0.25 mm intervals and were plotted in a load-penetration curve. The load required to make a penetration of 2.5 and 5.0 mm were recorded, from which the CBR values were estimated.

2.4 Cost Estimation

To conduct a financial analysis regarding the proposed mix, costs incurred during the production and construction of road pavement layers were determined. For the cost estimation, a granular road base pavement type defined in ORN31 to carry a traffic load belonging to the T3 traffic class that was laying on a subgrade of strength class S5 was considered. According to the design standards defined in ORN31, this type of pavement was recommended to have three layers including a subbase of 100 mm thickness, a road base of 175 mm thickness and an appropriate surface dressing. The width of the road section was assumed to be 5.5 m throughout. The average cost incurred in procuring aggregate was 8544.00 LKR (23.07 USD) per cubic metre that included transport and labour cost associated with loading/unloading and spreading of aggregates as per the highway schedule rates (HSR), Sri Lanka [54].

The cost of scrap tyres was assumed to be zero as it could be obtained free of charge from the local markets. A tyre shredding machine with a 15 kW power rating that operated with three phase power supply was used to shred the scrap tyre into tyre crumbs of two size ranges. The transport cost per km that includes loading and unloading, and electricity tariff per unit were obtained as 20.00 LKR (0.05 USD) and 11.00 LKR (0.03 USD) from highway schedule rates (HSR), Sri Lanka [54] and local industrial rates [55], respectively. Due to the absence of precise information, the depreciation cost of the shredding machine and labour cost associated with the production of one cubic metre of tyre crumb that includes initial preparation, shredding, and mixing was assumed to be 1.00 LKR (0.003 USD) and 5.00 LKR (0.014 USD), respectively.

With all the costs discussed above, the cost estimations were performed for a road length of 1 km. The costs to construct a road base for the abovementioned road section with alternative mix compositions of crushed rock and tyre crumb were estimated separately for comparison. To further correlate the cost and performance, incurred cost-to-mix property ratios were computed including cost/compacted density, cost/TFV, cost/CBR, cost/AIV and cost/LAAV.

3 Results and Discussion

The results from each test were carefully inspected and outliers were removed manually. The remaining values of each parameter were averaged out for alternative mix compositions. These average values were compared across mix compositions to determine the impact of the inclusion of tyre crumbs of various sizes in crushed rock. The 0% tyre crumb represents pure crushed rock without any tyre crumb substitution. Therefore, all the measured parameters for 0% replacement level are the same irrespective of the tyre crumb size.

3.1 Density

Figure 6 displays the variation of average compacted density with the replacement level of the tyre crumb. As expected, the compacted density reduced with the increase in tyre crumb composition in the mix. This could be attributed to the resilient property of the tyre crumb, which absorbed the applied compaction energy that prevented the mix from being compacted to the fullest degree of what it could have been. Similar declining nature in compacted density with the addition of tyre crumb has been widely reported in the literature [9, 24, 48]. When the size of the tyre crumb increased, for the same replacement level, the compacted density improved compared to that of the mix with a smaller tyre crumb (T0.6–6.3). This phenomenon contradicted the results reported by Tasalloti et al. [9]. This might be due to the adopted size range to represent the larger tyre crumb. The adopted size range might have contributed to filling the voids in between aggregates rather than negating the compaction by positioning themselves between aggregates. This nature rather facilitated a slight increase in the compacted density. The reduction rates of compacted densities were calculated concerning the control specimen (0% tyre crumb) and are illustrated in Fig. 6b. Reduction in densities of the mixes for both size ranges was found to be linearly proportional to the tyre crumb replacement levels up to 8%. The reduction rates were higher in mixes with smaller-size tyre crumbs (T0.6–6.3) than that of the larger (T6.3–12.5). For the same tyre crumb replacement level, the reduction of the larger tyre crumb (T6.3–12.5) was slightly less than that of the smaller-size tyre crumb (T0.6–6.3). This finding emphasises the need to investigate other size-related parameters such as the aspect ratio of tyre crumbs and the alignment of tyre crumbs in an aggregate framework. Overall, crushed rock substituted with tyre crumb of size 6.3–12.5 mm yielded better compacted densities than that of 0.6–6.3 mm. Furthermore, all the mixes satisfied the condition for the minimum requirement for compacted bulk density of subbase and embankment [45].

Fig. 6
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Variation of compacted density with tyre crumb replacement level and the reduction trend

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3.2 Mechanical Characterisation

The change in AIV when the replacement level of the tyre crumb increased is illustrated in Fig. 7. As expected, the AIV values steadily reduced for mixes with both tyre crumb sizes. 2% tyre crumb replacement was not that significant in reducing AIV, as a significant fraction of the tyre crumbs might have been trapped into the voids of the aggregate rather than positioning themselves at the aggregate interfacial zones to negate the impact forces. Due to this, the effect of tyre crumb size on AIV values at 2% replacement was not that prominent. However, a significant reduction was observable after a 5% replacement level. The reduction in AIV for the mixes with smaller tyre crumbs tended to attain a saturation level beyond 5%, i.e. the rate of reduction in AIV values reduced. This indicates that the smaller sized particles have limited ability to cut down the interaction between aggregates, which caused a relatively low impact on AIV. On the contrary, large tyre crumbs reduced the contact between aggregates greatly and hence contributed to reducing AIV. This increased the rate of reduction in AIV relatively higher when the substitution levels of large tyre crumbs increased in the mix. For higher replacement levels (8%), the large-size tyre crumb (T6.3–12.5) contributed greatly to the reduction of AIV compared to the smaller ones. Importantly, all the mixes met the maximum requirement for AIV for road bases stipulated in standards [45].

Fig. 7
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Variation of aggregate impact value (AIV) with tyre crumb replacement level and the reduction trend

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Figure 8 shows the effect of tyre crumb size and replacement level in TFV. The TFV values increased with the tyre crumb replacement levels. Tyre crumbs absorbed static load applied on crushed rock and tyre crumb mix and prevented the aggregates from being pulverised. This reduced the amount of fines produced and hence the TFV increased. Even the mixes with 2% large tyre crumbs exhibited notable improvement in TFV. This indicates that large-sized tyre crumbs aligned themselves between aggregates effectively to resist interactive forces between aggregates due to static loading. On the contrary, smaller sized tyre crumbs got trapped between the aggregate skeleton and provided relatively low resistance. For 5% replacement, the improvement in TFV was almost the same for mixes with both size ranges. This again raises concern about the influence of the aspect ratio of tyre crumbs on the positioning between aggregates, which requires a detailed investigation. For the 8% replacement level, the size effect contributed immensely to reducing the impact from static loading and hence improved TFV. However, all the reported TFVs were above the minimum requirement set for road bases in ORN 31 [36]. A similar trend in TFV was reported in the literature [24].

Fig. 8
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Variation of ten per cent fines value (TFV) with tyre crumb replacement level and the reduction trend

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LAAV were observed to reduce with the increase in tyre crumb content in the mixture. The reduction in LAAV values concerning the control values, shown in Fig. 9b, evidently shows that the reduction in LAAV is significantly large in large-size tyre crumbs (T6.3–12.5). When the tyre crumb size increased, they contributed to cutting down the interaction between aggregates and hence reduced the chances of aggregates being subjected to abrasive forces. This in turn reduced the fines generated by abrasive forces and hence decreased the LAAV. The replacement level of 2% for both size ranges was trivial as the available tyre crumbs were insufficient to reduce the contact between aggregates and abrasion between them. When the replacement level of large-size tyre crumbs (T6.3–12.5) increased from 2 to 5%, the additional large tyre crumbs contributed more to reducing the contact between aggregates that reduced the abrasion between aggregates and hence produced a notable reduction in LAAV. Further increase in large-size tyre crumbs significantly contributed to reducing LAAV by cutting down the contact between aggregates. The reduction in LAAV reported for 8% tyre crumb replacement level was 2 and 31% for smaller tyre crumbs (T0.6–6.3) and larger tyre crumbs (T6.3–12.5), respectively. Li et al. [37] reported similar LAAV results when recycled crushed concrete or crushed rock were used with tyre crumbs of size ranges 0.4–0.6 mm or 10–15 mm.

Fig. 9
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Variation of Los Angeles abrasion value (LAAV) with tyre crumb replacement level and the reduction trend

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CBR values displayed in Fig. 10 were found to reduce steadily with the increase in tyre crumb replacement for both tyre crumb sizes. A sharp drop in CBR values was observed when the content was increased to 2%. As per Fig. 10b, the reduction rate of CBR was relatively high in smaller tyre crumb sizes (T0.6–6.3) than that of larger (T6.3–12.5). This could be attributed to the reduction in the density of the mix, which made the penetration of the plunger easier when the tyre crumb replacement level increased. For higher levels of tyre crumb replacements, the mixes with large-size tyres exhibited relatively higher CBR values as they had higher densities. A drop in CBR values was recorded by many researchers [8, 56, 57]. The mixes with 2% tyre crumb replacements for both sizes marginally satisfied the requirements for subbases recommended in standards [36].

Fig. 10
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Variation of California bearing ratio (CBR) with tyre crumb replacement level and the reduction trend

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3.3 Cost Analysis

The ratios between costs and various mix properties were estimated for alternative crushed rock and tyre crumb mixes and are displayed in Fig. 11. The higher compacted density or CBR or TFV is preferred at the lowest possible cost. For these parameters, the mix with the lowest ratio would be the best both in terms of performance and cost. On the other hand, lower AIV or LAAV at low cost is favoured in selection. Since the reduction in both cost and AIV or LAAV is preferred, their respective ratios cannot reveal tangible information.

Fig. 11
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Variation of the cost to compacted density, CBR, TFV, AIV and LAAV ratios of alternative crushed rock and tyre crumb mixes

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The cost/compacted density ratio depicted in Fig. 11a shows a steady increase in tyre crumb content for the mixes with both smaller and larger tyre crumbs. The relationship between cost/compacted density ratio and tyre crumb content followed a linear variation (positive). This could be due to the variation in the rate of change in compacted densities with tyre crumb replacement levels. In addition, the reduction in cost with the increase in tyre crumb content contributed to this cause. The lowest cost/compacted density ratio was obtained for the mix with smaller tyre crumbs (T0.6–6.3) at a replacement level of 2% as the mix composition had the lowest reduction in compacted density compared to its competitor. The relationship between the cost/CBR ratio and tyre crumb content shown in Fig. 11b followed an exponential variation. The drastic drop in CBR values with the addition of tyre crumb might be the underlying reason behind this. The difference between cost/CBR ratios was observed to increase drastically beyond 5% tyre crumb substitution. On the contrary, the cost/TFV ratio shown in Fig. 11c steadily decreased as TFV continued to improve with the increase in tyre crumb substitution. It exhibited a linear relationship (negative). The lowest cost/TFV ratio was recorded for larger tyre crumbs (T6.3–12.5) at an 8% substitution level. Figure 11d shows the variation of cost and AIV with the tyre crumb replacement level. For both tyre crumb sizes, cost and AIV steadily decreased. A drastic drop in cost was observed when the tyre crumb content was increased from 0 to 2%, and the cost continued to decline at a lower rate thereafter. This implies that the tyre crumb processing cost increased significantly and compromised the reduction in aggregate cost after a 2% tyre crumb replacement. The lowest cost and AIV values were recorded at the replacement level of 8% for the mix with a smaller tyre crumb (T0.6–6.3). LAAV values and respective costs of the mixes shown in Fig. 11e exhibit a similar trend to that of the AIV. The same reasons given for AIV can be put forward to this scenario as well. The lowest cost and LAAV were obtained for the mixes with 8% substitution of smaller tyre crumb (T0.6–6.3).

4 Conclusions

As the developing nations greatly struggle with the absence of quality material for construction and on the other hand combat with containing non-biodegradable wastes, this study aimed at facilitating such countries with a complex-free, laboratory experimental-based method to design appropriate mixes for road base/subbase construction using conventional construction materials and waste materials. As such, this study investigated the application perspectives of crushed rock substituted with scrap tyres for the construction of a road base/subbase of road pavement, specifically the effect of tyre crumb sizes on mechanical properties. The conclusions drawn from the study are summarised below:

  • Compacted densities steadily reduced with the increase in tyre crumb replacement levels for the mixes with both smaller (T0.6–6.3) and larger tyre crumbs (T6.3–12.5).

  • AIV reduced with the increase in tyre crumb content for mixes with both tyre crumb sizes. The larger tyre crumb contributed to reducing AIV by 27% of the control at the replacement level of 8%.

  • Larger size tyre crumbs contributed to reducing LAAV significantly. For the mix with a larger tyre crumb, a 32% reduction in LAAV of the control mix was obtained at an 8% replacement level.

  • TFV values improved steadily for both tyre crumb sizes and yielded an improvement of 17–21% of the control mix at a replacement level of 8%.

  • CBR values dropped drastically for a 2% replacement level for both tyre crumb sizes and continued to reduce at a slow rate for further addition of tyre crumb. Mixes with both tyre crumb sizes marginally satisfied the minimum requirement for subbases.

  • Cost analysis revealed that mixes with smaller tyre crumbs are slightly better in terms of cost to compacted density or CBR or TFV. For AIV and LAAV, the lowest cost and better performance were obtained in mixes with smaller tyre crumbs.

This research, therefore, recommends deploying scrap tyres with crushed rock for subbase construction for roads with low traffic volume. Further improvements in CBR values can extend the proposed method to roads with large traffic volumes. For developing countries with budget restrictions and facing issues with piling scrap tyres, the proposed strategy can benefit both financially and environmentally. It is also recommended to devise standard design protocols regarding the usage of scrap tyres in road construction, which will not only regulate the application but also persuade the authorities and professionals to switch to innovative construction practices.

The proposed study can be further extended to explore the response of dynamic loading on road pavement. This could be done by performing a cyclic triaxial test to determine the resilient modulus and permanent deflection of mixes. This would enhance the application potential of the proposed strategy to various pavement types.