1. Introduction

1.1. ELT management: state of the art

The management of tire waste, as well as rubber-based products including rubber belts used for mining, building, and manufacturing, is one of the major global environmental challenges of our era. The increasing volume of tire waste is becoming a major concern; while classified as non-hazardous waste, tire waste poses environmental and public health riks (Dabic-Miletic, Simic, and Karagoz 2021) due to the following reasons:

- Tires are not biodegradable products composed of natural and/or petroleum-based rubber as well as, steel reinforcements, textile fibers, and various chemicals like sulfur, zinc oxide and carbon black. In addition, rubber undergoes treatemts that enhance its durability and resistant, making the product more difficult to degrade and recycle.

- Used tires consume large amounts of landfill space as they are both bulky and heavy. This trend is expected to continue as there are no alternative technologies that can reduce the accumulation of rubber waste in the near future.

- When left illicitly in the landscape, tires become an ideal mosquito incubators. These mosquitoes can transmit diseases such as zika, malaria, yellow fever and dengue fever.

- They are also difficult to extinguish in the event of fire. The treatment of tire rubbers to enhance their resistance increases their calorific value, causing them to burn for longer periods. As a result, tires require more time and effort to extinguish when ignited. In addition, tire fires release gas pollutants, including $CO$, $S{O}_2$ and $N{O}_2$.

- They accumulate hazardous gases, leachates and parasites. The heavy metals and chemicals containied in used tire may gradually decompose on contact with other liquids in the landfill and leach out into the environment. Leachates may contain harmful and carcinogenic substances and contaminates soil, groundwater and farmland.

With about 1.1 billion vehicles on the roads worldwide, approximatively 1.7 billion new tires are produced per year to satisfy the demand (Forrest 2019). Hence, millions of end-of-life tires (ELT) are discarded, representing a serious threat to the environment and health (Dong et al. 2019). According to World Business Council for Sustainable Development (WBCSD 2021), about 1 billion tires worldwide (around 17 million tons) reach the end of of their life cycle every year. This trend is expected to persist and the number of ELTs could increase up to 1.2 billion by 2030 (Azevedo et al. 2012; Thomas, Gupta, and Panicker 2016,Abbas-Abadi et al. 2022; Arulrajah et al. 2019). Moreover, it is reported that the global end-of-life tire (ELT) management market is expected to grow at a compound annual growth rate (CAGR) of 4.87% over the period 2023 to 2032(ASDReports 2023). Against this backdrop, and in keeping with the challenges of the environment, the collection and recycling of used tires is becoming a vital global issue. The success of ELT recycling programs often depends on regulatory instruments, as well as the voluntary management of the system by the tire industry.

According to information collected within the framework of the tire industry's project (TIP) covering 45 countries (Argentina, Brazil, China, India, Indonesia, Japan, Mexico, Nigeria, Russia, South Africa, South Korea, Thailand, USA and 32 European countries (EU27 plus UK, Norway, Serbia, Switzerland and Turkey)), over 29.1 million tons (metric) of ELT are generated annually and 97% of ELT are recovered (China, the United States of America, and Europe recover the most ELTs). In addition, 25.6 million tons of ELT are recycled, excluding those used in civil engineering and backfill projects. This figure includes ELT collected in China and the corresponding end use is undetermined (WBCSD 2019, 2021). This means that 88% of ELT produced is treated, and the percentage rises to 90% when considering civil engineering and backfill uses. The development of ELT management is promising, particularly in countries like Argentina, Mexico, Nigeria, South Africa, Thailand and Russia, where recovery rates are still relatively low, as reported recently (Consulting 2022; Valentini and Pegoretti 2022).

Table 1. ELT generated in various countries (in metric kilotons) and recovered for various purposes.

Table 1. ELT generated in various countries (in metric kilotons) and recovered for various purposes.

Country	Total ELT generated	Energy	Material	Civil/backfill	other	Total ELT recovered	ELT recovered (%)
China (2018)	14545	0	5650	0	8895	5650	39
United States (2017)	3700	1442	1227	326	706	2995	81
Europe +(2017)	3425.5	1180	1855.5	105.5	283.5	3141	92
India (2015)	2749.8	600	2094.8	0	55	2694.8	98
Japan (2017)	849	619.5	160.5	1	68	781	92
Russia 2017)	800	6	154	0	640	160	20
Indonesia (2017)	684.4	376.4	136.9	0	171.1	513.3	75
Brazil (2017)	587.9	206.1	379.1	0	2.7	585.2	100
Thailand (2012)	515	75.4	202.3	0	237.3	277.7	54
Mexico (2017)	467.5	67.1	27.9	0	372.5	95	20
South Korea(2017)	319.4	160	120.9	0	38.5	280.9	88
South Africa (2015)	204	9.4	41.5	0	153	50.9	25
Argentina (2018)	150	0	9.6	0	140.4	9.6	6
Nigeria (2017)	113	2.8	2.8	0	107.3	5.6	5

The main recycling methods for ELT in the 45 countries listed above are energy recovery and material recovery. These methods are followed to a lesser extent by civil/mining engineering applications, including water retention basins, tire-based aggregates for road construction, backfilling, and rehabilitation of land or mining sites. Table 1 summarizes the distribution of ELT in the countries consuming the highest number of tires based on recent data extracted from the report of the World Business Council for Sustainable Development (WBCSD 2019).

The statistics presented in Table 1 highlight the need for a concerted effort to reduce the number of ELT collected but not recovered (i.e. landfilling, stockpiling and uncertain end-use). The promotion of ELT recovery and reuse is a key concern shared by many countries worldwide.

In the Middle East, millions of tires are discarded annually, presenting a significant challenge. As a short-term solution, these non-biodegradable wastes have been stockpiled in urban facilities, illegally dumped in remote areas, or discarded in landfills (Salman Zafar 2022). In Kuwait alone, around 50 million tires have been found in landfills, and a further 3 to 3.5 million are discarded each year. Severe road conditions and intense heat require tires to be renewed every two years, further exacerbating the volume of waste (Fayez Obaid Al Mazrouei 2023).

Developing countries face challenges in managing solid waste, including ELT with limited published statistics on this subject. This issue is commonly attributed to the lack of policies, regulatory frameworks, and adequate infrastructure for waste collection, sorting, recycling and recovery, and these factors vary from one country to another. For tire wastes, another factor can be added related to the limited influence of legislation on the quality of tire imports leading to the importation of second-hand tires (tire retreads) that have a limited lifespan once mounted on vehicles. As a result, ELTs in developing countries tend to accumulate in waterways and on land, posing serious health and environmental risks. Uncontrolled open dumping and open burning are the main ELTs treatment and final disposal systems employed. This waste management challenge is particularly serious in countries with rapidly growing urban areas(Ferronato and Torretta 2019; Johannes et al. 2021; UNEP 2018). In some African countries, some attempts have been made recently by researchers to explore recycling and recovery options for used tires. In Ethiopia, ELTs are shredded and used as an energy source (Tyre-Derived Fuel "TDF") in cement kilns to replace coal in the pre-calcining process (Gebreslassie, Bahta, and Mihrete 2023). In Cameroon, ELTs are valorised as materials for the manufacture of O-rings with properties close to those found on the market(Bogno et al. 2023). However, these initiatives remain relatively marginal and at limited scale.

Similar practices are observed in the 14 Pacific Island countries and Timor-Leste (PIC), where ELT production is rapidly increasing due to insufficient quality control of imported tires. Like in Africa, used tire imports tend to have a shorter life and generate large volumes of scrap tires. It is estimated that approximately 22,000 tons of tires are illegally stored, buried or burned with an annual disposal rate of 6,706 tons, or 670,600 tires. The lack of environmental management for ELT can be explained by the absence of specific legislation concerning ELT management, with the exception of occasional references to tire burning as a measure to combat atmospheric pollution in some countries (Palau, Fiji, Kiribati and Cook Islands). Samoa is the only country in the region to have implemented an import restriction. Small-scale shredding operations are underway in Papua New Guinea (PNG), on Majuro Atoll and in Palau, and studies on shredding processes are underway in Samoa. The potential use of shredded tires is primarily as clinker in PNG's cement kilns, and as a fuel source for other applications (SEREP 2022).

In Australia, an average of 450,000 tons of waste tires -the equivalent of 56 million equivalent passenger units (EPU) tires entering the waste stream- was generated between 2015 and 2020 (Tyre Stewardship Australia (TSA) 2021). The end of Life Tires "EOLTs" often end up in landfills or stockpiles that can take centuries to decompose. It should be noted that in 2009-10, Australia generated approximately 48.5 million EPU tires that entered the waste stream compared to 41.8 million in 2007-08 (Brindley et al. 2012).

Australia's end-of-life tires are categorized according to whether they are passenger, truck or off-road (OTR). Passenger tires include those of passenger vehicles, motorcycles and caravans, as well as trailers for domestic use. Truck tires are those for buses, light and heavy commercial vehicles, motor vehicles, trailers and semi-trailers, and fire-fighting vehicles. OTR tires are those employed on machinery or equipment used in sectors like agriculture, mining, construction and demolition. While Australia has generally achieved good recovery rates for passenger car, bus and truck tires, this is not the case for off-road tires “OTR”. These have consistently low recovery rates, below 11%, throughout Australia. When OTR products reach the end of their life, they are usually stored or buried on site, sent to landfill or dumped or burned illegally. Australian EOLT tires have been recovered for domestic reuse (recycling, energy recovery, civil engineering, licensed and unknown landfill), for export (reuse and retreating, recycling and energy recovery) and some are not recovered at all. Unrecovered EOLTs have been disposed of locally in pits or landfills, stockpiled or illegally dumped into the environment.

According to some sources (Brindley et al. 2012; Consulting 2015; Shanley Authors Matt Genever Kyle, Paul Randell John Rebbechi, and - 2017; Tyre Stewardship Australia (TSA) 2021, 2022), almost passengers and truck tires are recovered while the recovery rate for OTR tires is very low even it has grown since 2009-2010 (Table 2). Australia's National Waste Policy, enacted in 2018, has set an ambitious an overall ELT recovery target of 80% by 2030. To reach this goal, OTR tire recovery would need to reach 55% to 60%. For this purpose, tires derived products “TDP” recovery activities for recycling and energy recovery should increase. The promotion of TDF markets could play a significant role as an attractive alternative fuel source and an efficient starting point for ELT management.

In France, according to some sources (Pneumatiques 2023; Venice GRAF et al. 2022), all EOL collected tires are fully recovered and/or reused. For example, in 2021, 567,762 tons of tires were marketed, 572,370 tons of EOL tires were collected and fully recovered, with 15.3% reused, 35.8% recycled materials, 46.8% energy recovery, and 2% Civil Engineering. In 2020, some 477,200 tons of tires were placed on the national market. The collection rate for used tires was around 85%, not representative of performances because of the effects of the COVID19 health crisis. The main recovery methods for these tires are energy recovery in cement works (around 44%), followed by material recovery in the form of granulation (18%) and second-hand sales / retreating (16%). The processing rate in 2021 has risen sharply compared with 2020 (+29.6 points). This can be explained by the low level of marketing in 2020, due to the health crisis, and by the fact that collection in 2021 returned to levels similar to those prior to the health crisis. This level of collection has enabled a significant volume of tires to be processed, hence the sharp rise in the processing rate. On the other hand, it should be remembered that Article L. 541-10-1 (16°) of the Environment Code, introduced by Article 62 of Law no. 2020-105 of February 10, 2020 on the prevention of waste and the circular economy, provides a broader producer responsibility (EPR) system for tires and specifies that the procedures for approving eco-organizations and individual systems are applicable from January 1, 2023. The management of tire waste has been based on the EPR principle since the early 2000s. In this context, one of the objectives of this decree is to develop new "material" recovery processes for EOL tires. Over the past few years, material recovery has accounted for around 36% of the collected EOL tires, with energy recovery accounting for the largest proportion (around 46%). It should be noted that these regulations recommend to reduce energy recovery. With this regulatory limit, and considering the reconsideration of the use of waste rubber aggregates in the manufacture of synthetic turf, it's clear that new recovery strategies for EOL tires are needed.

In this context, this research work aims to establish new methods for ELT material recovery. It should be pointed out that the use of ELT for energy recovery, which involves burning tires, should be discouraged, restricted or even prohibited to promote material recycling in accordance with the waste management hierarchy (energy recovery is at the lower end of the waste hierarchy and can be considered close to disposal).

2. Materials and Methods

3. Results and discussion

3.1. Mortar at the fresh state

A testing campaign was conducted on the fresh mortar to estimate early age paste properties including density, workability, air content, and setting time of each mixture. The experimental results of density and air content are summarized in Table 5. Figure 4-a shows that air content increases linearly with replacement ratio as follows

$$a(\%)=0.04r_v+2.64 with R^2=0.93$$

Moreover, the total setting time was measured using the penetration resistance test at ambient room temperature (20 °C). The experimental results, as shown in Figure 4-b, indicate that the setting time decreases when the replacement ratio increases. These results do not suggest a specific threshold for acceptable performance, but they provide designers with ranges of variation in air content and setting time relative to the replacement ratio that can be instrumental for practical applications. The total variation in air content is approximately200% and the principles of homogenization suggest that at this level, the volume of air alone can have great effects on porosity and, consequently, on the mechanical and thermal properties (Karrech, Strazzeri, and Elchalakani 2021; Strazzeri, Karrech, and Elchalakani 2020). The increase in air content with the increase in crumb rubber aggregates “CR” fraction is attributed to their non-polar nature and their surface roughness leading to air bubbles entrapment (Figure 2b). This phenomenon has been confirmed by several authors (K. and M. 1999; Siddique and Naik 2004).

Figure 4. Variation of air content versus replacement ratio and slump of fresh mortar.

Figure 4. Variation of air content versus replacement ratio and slump of fresh mortar.

Figure 5 shows the results of slump testing for various specimens obtained according to NF EN 12350-2 and NF EN 1015-3 standards. It indicates that workability significantly decreases when the CR content exceeds 60%. These results confirm the trend observed in the literature (Sukontasukkul and Tiamlom 2012; Turatsinze and Garros 2008). Once again, Figure 5 does not provide a specific threshold for adequate workability but gives a range of variations that assist designers in selecting the appropriate mixture depending on the sought application. It should be noted that for repair mortars, EN NF 1015-2 standard recommends a spread of 175^±10 mm (obtained using EN NF 1015-3 standard) if the density of the mortar in the fresh state exceeds 1200 kg/m³. As the mixes in Table 4 have densities higher than 1200 kg/m³, it can be concluded that mortars with $r_v\le 60\%$ comply with these recommendations.

Moreover, all the mixes maintained their slumps for about 20 minutes. After this period, a sharp drop in workability was observed especially when the CR content exceeded 60%, as indicated in Figure 6. In this figure, the relative slump is denoted as $S_{sr}=\frac{S_t}{S_i}$ where S represents the designed the slump, “t” represents the time in minutes and “i” represents the initial slump.

Therefore, from a workability point of view, mortars with $r_v\le 60\%$ can be considered suitable for repair work, aligning with the recommendations of standard EN NF 1015-3.

Figure 6. Relative slump of mortar versus time.

Figure 6. Relative slump of mortar versus time.

3.2. Characterization of mortars at hardened state

a.

Density and porosity

The density of the different hardened mortars was measured after subjecting the specimens to vacuum for 4 hours and immersing them in water for 44 hours. The experimental results are represented graphically in the left-hand side “LHS” of Figure 7 It can be seen that the mortar density decreased as the ratio of crumb rubber to sand increased; this was due to the low density of rubber as compared to the density of sand. For example, fully substituting NS by CR (MCR-100%) reduced the overall density by 50% compared to the control mortar. The results obtained in terms of density variation with respect to replacement ratio agreed with published data (Sukontasukkul and Tiamlom 2012; Turatsinze, Bonnet, and Granju 2005; Turki et al. 2009). The experimental results were generally comparable with the literature as can be seen in the right-hand side “RHS” of Figure 5 (Nadal Gisbert et al. 2014; Sukontasukkul and Tiamlom 2012; Turatsinze et al. 2005; Turki et al. 2008). While existing data were limited to 50% replacement ratio, our experimental results covered a complete range of measurements (up to 100%) for the first time and showed that the density reduced with the replacement ratio according to the following equation:

$${\rho }_{ap}\left(g/c{m}^3\right)=2.17e^{-0.01r_v} with R^2=0.88$$

Figure 6. Density and water absorption coefficient of mortars.

Figure 6. Density and water absorption coefficient of mortars.

Moreover, it should be noted that for $r_v\ge 25\%$the mortar can be considered as light because its density is less than 1900 kg/m³. The standard EN 206-1 prescribes for lightweight concrete a range of density varying from 1800-2000 kg/m3 to 800-1000 kg/m3 (Classes D1.0 to D2.0). Hence the mortar can be considered lightweight for $r_v\ge 25\%$ with Class D1.0 for MCR-25% and Class D1.2 for MCR-100%.

The results in terms of water absorption are presented in Figure 7. It can be seen that the WA coefficient increases with CR content. In particular, when NS was entirely replaced with CR, WA reached its maximum and exhibited a disproportional increase. These findings are in accordance with the literature (Angelin et al. 2015; Sukontasukkul and Tiamlom 2012; Turatsinze and Garros 2008; Uygunoğlu and Topçu 2010). The increase in WA relative to CR replacement rate is well described by the following equation:

$$WA(\%)=7e^{0.01r_v} with R^2=0.89$$

Figure 8 shows the obtained results in terms of the overall porosity accessible to water (combining the pore spaces in the cement paste and between the interfaces) versus replacement ratio. It can be observed that the mortar porosity increases by increasing the CR content. This growth is slow for $r_v\le 50\%$, but it becomes significant otherwise. It can be described by the following system of equations:

$$\{\begin{array}{c}{\left(n-n_0\right)}_{(\%)}=0.02r_v(\%)+0.13 with R^2=0.922\\ {\left(n-n_0\right)}_{(\%)}=0.07e^{0.05r_v(\%)} with R^2=0.999 \end{array}.$$

Some authors established that replacing natural sand by CR particles generates pores accessible to water proportionally to the crumb rubber content (Angelin et al. 2015; Gupta et al. 2015). It should be outlined that, in this research, water to cement ratio was kept constant at 0.55, which is not the case in the previous studies (Gupta et al. 2015; Uygunoğlu and Topçu 2010). Moreover, it is important to indicate that existing studies were limited to less than 50% replacement ratio, usually with fine and coarse rubber aggregates. As part of this work, we investigate a full range of replacement ratios (from 0 to 100%) in order to obtain more comprehensive results. In addition, we focuse on fine rubber crumb to avoid extended interfacial transition zones.

The increase of the porosity can be attributed to several factors (for a given W/C ratio). It can be partially explained by the entrapment of air by rubber particles during mixing (Uygunoğlu and Topçu 2010). Some authors have attributed the increase in porosity to poor adhesion between the rubber particles and the mortar matrix, a phenomenon more pronounced with CR spheroid particles (Angelin et al. 2019; Noor Azline et al. 2022).

In the present study, SEM observations do not show decohesion between the cement matrix and the waste crumb rubber particles (Figure 9). In addition, the EDS of the sample indicates that the interface is thin compared to the grain sizes. This can be deduced by observing the variation of elemental concentrations. For example, from the left to the right, the figure starts with a large concertation of C indicating the presence of a rubber particle until 300 micrometers. Between 300 and 350 micrometers the concentration of Ca increases while the concentration of C decreases indicating a cementitious interface of about 50 micrometers. In addition, Figure 9 shows that at no moment, the concentrations reach zero simultaneously, which means that the specimen does not include major pore spaces at least in the path that is indicated in the figure. Consequently, the increase in mortar porosity cannot be ascribed to debonding between the rubber particles and the cement matrix. However, microcracks and small voids may exist at the transition zone which be responsible od the increase of the porosity (Angelin et al. 2019; Noor Azline et al. 2022).

Hence, it can be concluded from this study that the increase in porosity is mainly attributed to the higher occluded air content associated with higher CR rates. This phenomenon is particularly pronounced for $r_v>50\%$ (Figure 10)

The figure depicting the evolution of density versus porosity (Figure 11a) confirms the expected correlation between porosity and density for rubberized mortars. More importantly, it indicates that the variation of porosity versus NS-CR replacement ratio becomes non-linear close to 100% CR. This is somewhat in alignment with the behavior in terms of water absorption. On the basis of literature works and the experimental results obtained in this study a relationship between both properties is established:

$${\rho }_{ap}\left(g/c{m}^3\right)=3e^{-0.93n(\%)} with R^2=0.82$$

Likewise, an increase in WA is noticed when the porosity accessible to water rises (Figure 11b). Based on the results of the present study and those of the literature (Angelin et al. 2019; Turgut and Yesilata 2008), the evolution is expressed as follows:

$$WA(\%)=2.19e^{0.09n(\%)} with R^2=0.93.$$

Figure 11. Evolution of the mortar density and water absorption coefficient versus its porosity.

Figure 11. Evolution of the mortar density and water absorption coefficient versus its porosity.

b.

Thermal properties

The results obtained show that the thermal conductivity of the mortars decreases by increasing the CR content, highlighting the greater capacity of the mortar to resist cold and heat for a given thickness (Figure 12(a)). Indeed, the higher the thermal resistance of the product ($R=e/\lambda )$, the better its insulation properties. It is well known that shredded tire particles are almost adiabatic with $0.193\le \lambda \left(W/mK\right)\le 0.213$ (Yang et al. 2022) or $\lambda \left(W/mK\right)=0.25$ according (Xiao et al. 2019). I contrast, natural sands have higher thermal conductivity ranging between 1.8 to 3.6 W/mK depending on factors such as chemical composition (particularly quartz), grain size, porosity and degree of saturation (Xiao et al. 2019). Some authors claim that thermal conductivity of sand consisting mainly of quartz, ranges between 3 and 8 W/mK. (Lee et al. 2015). In all cases, the thermal conductivity of tire crumb rubber particles “CR” is at least 10 times lower than that of natural sand. Consequently, the thermal conductivity of rubberized mortars decreases with increasing the ratio “CR”. This decrease is enhanced when the relative particle rises. It can be observed that this is the case (Figure 12a) since the relative reduction in thermal conductivity between the reference mortar and that containing 100% CR is 82%.

The thermal inertia of mortars, a term widely used to describe the ability of a material to store heat, is characterized by the diffusivity “a “ $\left(a=\frac{\lambda }{C}\right)$ and the effusivity “e” $\left(e=\sqrt{\lambda C}\right)$. Both properties depend on the volumetric heat capacity (Figure 12b) and thermal conductivity (Figure 12 a).

Diffusivity characterizes the rate of thermal energy exchange between the mortar and its environment. Thus, good comfort requires low diffusivity so that this exchange takes place as slowly as possible. It can be observed that the increase in the CR content strongly lowered the thermal diffusivity (Figure 12c). For example, the relative decrease in diffusivity is 61% for $r_v=60\%$. However, regarding the variation of temperature, the effusivity is the preponderant property. Therefore, the repair/rehabilitation mortar must have a high thermal effusivity to store the maximum energy and mitigate the effects of heat/cold waves inside a building. The results obtained show that the effusivity does not increase with the increase of CR content (Figure 12d). It is reduced by around 20% for 60% of CR content and 64% for 100% of CR content.

Based on these results, it can be concluded, that the use of mortars incorporating up to 60% of waste tires improves comfort and thermal resistance without affecting significantly thermal inertia.

However, it should be noted that the mortars developed do not comply with the French Environmental Regulation 2020 “RE 2020”(Ademe 2023). Depending on the climatic zone, the latter recommends minimum thermal resistances R of 2.1 to 3.2 W/² for renovated wall and floor insulation solutions. Furthermore, RE 2020 generally recommends an average insulation thickness “$e$” of 15 to 20 cm for walls, 20 to 25 cm for ceilings and 10 to 20 cm for floors. Thus, for a thickness of 20 cm, the developed mortars do not meet the requirements of RE 2020, even though a substantial increase in thermal resistance exceeding 50% has been observed (Figure 13a), which can be described by the following equation:

$$R=\frac{e\left(m\right)}{\lambda \left(W/mK\right)}=0.088e^{0.016r_v(\%)} with R^2=0.965 .$$

Several authors have attempted to correlate thermal conductivity with density (Aliabdo, Abd Elmoaty, and Abdelbaset 2015; Bala and Gupta 2021; Benazzouk et al. 2008). It is proposed in this study to establish relationship between the normalized thermal conductivity $\left(\lambda /{\lambda }_o\right)$ and the normalized density $\left(\rho /{\rho }_o\right)$ in order to avoid the effects of control mortar-related parameters affecting these two properties and to study only the impact of incorporating crumbled rubber tire particles. It can be observed an increase of the normalized thermal conductivity as the normalized density increases (Figure 13b). The relationship between these two parameters is well described by the following equation.

$$\frac{\lambda }{{\lambda }_o}=0.048e^{2.978\left(\frac{\rho }{{\rho }_o}\right)} with R^2=0.88$$

where ${\lambda }_o$ and ${\rho }_o$ are the thermal conductivity and the density of the control mortar respectively.

Figure 13. Effects of incorporating tire crumb rubber particles on the thermal properties of mortars.

Figure 13. Effects of incorporating tire crumb rubber particles on the thermal properties of mortars.

c.

Drying /Shrinkage

Given the influence of crumb rubber on porosity and water absorption, it is important to analyse its impact on drying and shrinkage. Drying and shrinkage are important causes of damage in concrete structures, especially when contraction is prevented, which induces tensile stresses. It is known that shrinkage increases with the volume fraction of paste in concrete and reduces with relative humidity (Bissonnette, Pierre and Pigeon 1999). In this study, we investigated the effect of fine crumb rubber on these phenomena when humidity, water-to-cement ratio, and water content were fixed.

The results in terms of drying shrinkage of mortar with crumb rubber compared to plain mortar are shown in Figure 14. These results reveal that crumb rubber had a significant effect on drying; while MCR-0% took about 15 hours to fully dry, MCR-100% dried within 5 hours, which indicates that rubberized mortar dries 3 times faster. This can be explained by the abundance of pore spaces and high-water absorption in mortar with high CR replacement ratios. The results also show that shrinkage increased significantly with the crumb rubber content beyond 60%. This can be explained by the weaker structural performance of rubber particles compared to sand particles; the formers being much more flexible offer low confinement for mortar which leads to higher shrinkage (Sukontasukkul and Tiamlom 2012). It is known that shrinkage occurs during cement hydration as the hardening phases continue to cure (Angelin et al. 2015). Shrinkage strongly impacts the overall strength of cement mortar as it is associated with microcracks that concentrate stresses and may lead to failure. It can be noticed that the shrinkage of all the mixes is quite similar for $r_v\le 60\%$; the obtained values at 28 days vary between -875$\mu \epsilon$ to -1000 $\mu \epsilon$.

d.

Compressive strength

The mechanical properties of mortar were tested after 7, 14, 28 and 90 days of curing. The experimental results in terms of compressive strength are presented in Figure 15. It can be seen that the compressive strength increases with time during the curing process, irrespective of the replacement ratio. For example, at 10% CR content, compressive strength increased from 28.85 MPa at 7 days to 43.68 MPa at 90 days. However, the figure shows that at a fixed age, the compressive strength decreases with the CR replacement ratio. For example, specimens at age 28 days exhibited strengths of 38.52 MPa, 32.38 MPa, 19.28 MPa, 9.7 MPa, 7.7 MPa, 4.27 MPa and 2.41 MPa when the CR content was 0%, 10%, 25%, 50%, 60%, 75%, and 100%, respectively. A significant drop in resistance can be seen beyond 25%.

The NF EN 1504-3 standard defines 4 classes of repair products according to the performance of mortars: structural repair mortars (e.g. Class R4 for $f_c\ge 45MPa$ and Class R3 for ($f_c\ge 25MPa$ ) and non-structural repair mortars (Class R2 for $f_c\ge 15MPa$ MPa and Class R1 for $f_c\ge 10MPa$). It can be observed that up to 50% of CR content, the proposed mortars are within the range prescribed by the standard but can be used solely as non-structural repair products for civil engineering buildings.

It should be noted that EN 206-1 prescribes the minimum compressive strength class at 28 days for structural applications lightweight concretes at LC8/9 minimum with density classes from D1.0 to D2.0. It can be observed that mortars incorporating up to 50% CR are conform to EN 206-1. Figure 16 compares the results of the present work with previous studies; overall, our results agree with the average of published data, despite the large discrepancy that can be seen around the average (Correia et al. 2010; Onuaguluchi 2015; Sukontasukkul and Tiamlom 2012; Turatsinze and Garros 2008; Turki et al. 2009; Uygunoğlu and Topçu 2010). However, this experimental campaign covers a wider range of CR contents.

Based on the published data and the current results, an empirical expression of compressive strength at a replacement ration CR was obtained:

$$\frac{f_c}{f_{co}}=0.97e^{-0.03r_v} wit R^2=0.85$$

where $f_{co}$ is the compressive strength of control mortar, $r_v$ is the ratio of rubber/sand replacement (ranging from 0 to 100%).

Moreover, it appears that a relationship can be established between the compressive strength and the density of mortars incorporating CR (Figure 17):

$$\frac{f_c}{f_{co}}=0.002e^{6.21\left(\frac{\rho }{{\rho }_o}\right)} with R^2=0.97$$

e.

Flexure/Tensile strength

The experimental results in terms of flexural strength are presented in Figure 18. As for compressive strength, it can be seen that it increases with time irrespective of the replacement ratio. For example, at 25% CR content, flexural strength increased from 3.8 MPa at 7 days to 4.71 MPa at 90 days. However, the figure shows that at a fixed age, flexural strength reduced with the CR replacement ratio. For example, specimens of age 90 days have strengths of 9.3 MPa, 7,45 MPa, 4.71 MPa, 3.5 MPa, 2.75 MPa, 2.15 MPa, and 1.35 MPa when the CR content is 0%, 10%, 25%, 50%, 60%, 75%, and 100%, respectively. In coherence with the compressive strength tests, the results show a significant drop in resistance, as can be seen beyond 25%.

Figure 19 compares the results of the present work with previous studies in terms of compressive strength versus flexural strength. Overall, our results are comparable with the average of published data despite the discrepancy that can be seen around the average (Angelin et al. 2015; Herrero, Mayor, and Hernández-Olivares 2013; Nadal Gisbert et al. 2014; Onuaguluchi 2015; Turatsinze et al. 2005). It should be remembered that our experimental campaign covered a wider range of CR content ($0\le r_v(\%)\le 100)$. Based on the published data and the current results, an expression between flexural and compressive strengths of the different mortars is established:

$$f_c=5.36f_f with R^2=0.77$$

To assess the effect of incorporating tires crumb rubbers particles in mortars, normalized flexural strength is plotted against normalized density, and a relationship between these two properties is established (Figure 20). It can be outlined that the decrease in the relative density due to the presence of CR particles leads to a decrease in the relative flexure strength. It can be outlined that the decrease in the relative density due to the presence of CR particles leads to a decrease in the relative flexure strength:

$$\frac{f_f}{f_{fo}}=0.015e^{4.19\left(\frac{\rho }{{\rho }_o}\right)} with R^2=0.85$$

where $f_{fo}$ is the flexure strength of the control mortar.

While the NF EN 1504-3 standard for repair mortars doesn not specify a tensile strength requirement, direct tensile tests were carried out on cylindrical specimens measuring 11 cm in diameter and 22 cm in length. The results are shown in Figure 21 with a significant decrease in tensile strength observed for CR content above 25% and an overall mechanically more ductile behavior for $r_v\ge 75\%$. However, the tensile strength can be considered as satisfactory up to 60% as it is higher than 1.5 MPa.

f.

Fracture energy

Figure 22 shows the load-CMOD curves obtained for various CR replacement ratios. Similar results were obtained in terms of force versus deflection. These results underline a more resilient behaviour and greater resistance to crack propagation as the incorporation rate of crumb rubber tires particles increases. This is particularly obvious at ratios in excess of 50%.

Figure 23 depicts the fracture energy obtained as follows (RILEM 1985):

$$G_f=\frac{W+\left(m_b+2m_l\right)g{\delta }_0}{A_{lig}}$$

where W is the area below the load-deflection curve, $m_b$ is the mass of the beam portion between the support points, $m_l$ is the mass of any support arrangement excluding the machine, $g$ is the gravitational acceleration constant, ${\delta }_0$ is the deflection upon failure and $A_{lig}$ is the fracture zone area.

It can be seen that fracture energy increased when the CR replacement ratio increased. This result is coherent with the previous results suggesting the increase of microcrack networks and aggregate-cement interfaces commensurate with the crumb rubber softness. Defects, including microcracks, tend to coalesce and propagate, leading to greater energy dissipation when the CR replacement ratio increases especially for $r_v\ge 50\%$.

Figure 23. Fracture energy at various CR contents.

Figure 23. Fracture energy at various CR contents.

g.

Elastic modulus

The determination of Young’s modulus, E, for different mortar formulations is of a great interest as it is related to the creep behaviour. As shown in the LHS of Figure 24, the resonance frequency decreases with an increase in CR content in the mortar. For example, the frequency decreases from 12,526 Hz to 3,305 Hz as CR increases from 0% to 100%. This suggests that the damping ability of the mortars increases with higher CR content.

On the RHS of Figure 24, the variation of the dynamic elastic modulus with respect to CR replacement is depicted. It can be seen that the modulus follows the trends of the resonance frequency; it decreases with an increase in CR replacement suggesting a higher creep ability.

According to NF EN 1504-3, structural repair mortars should have an elastic modulus higher than 20 GPa for class R4 and 15 GPa for class R3, while there are no requirements for non-structural repair classes. It can be observed that mortars with up to 25 % of CR replacement can be used as structural repair products for civil engineering buildings based on this property, while for higher CR replacement ratios, they are more suitable for non-structural applications.

Figure 24. The resonance frequency measurements and the dynamic modulus of elasticity (Ed) according to the standard NF EN ISO 12680-1.

Figure 24. The resonance frequency measurements and the dynamic modulus of elasticity (Ed) according to the standard NF EN ISO 12680-1.

Figure 25 shows the non-linear relationship between the dynamic modulus $E_d$ and the density ${\rho }_{ap}$ obtained based on our new experimental results and published data (Nadal Gisbert et al. 2014; Sukontasukkul and Tiamlom 2012; Turki et al. 2008, 2009) The figure also confirms the agreement between our results and the published experimental data, with the distinction that our measurements encompass a broader range of crumb rubber replacement ratios.

Moreover, it can be seen that the relationship between the elastic modulus and the incorporated crumb rubber rate follows the same trend as that established for the compressive strength:

$$\frac{E_d}{E_{do}}=0.95e^{-0.03r_v} with R^2=0.79$$

where $E_{d0}$ is the elastic modulus of plain mortar and $r_v$is the ratio of rubber/sand replacement in (%).

It is also interesting to highlight that a relationship has been established linking the normalized modulus to the normalized density, demonstrating that as the density decreases, the resistance to elastic deformation also decreases for mortars incorporating tire crumb rubber particles:

$$\frac{E_d}{E_{do}}=0.0003e^{8.29\left(\frac{\rho }{{\rho }_o}\right)} with R^2=0.82$$

Figure 25. Dynamic modulus of the different mortars.

Figure 25. Dynamic modulus of the different mortars.

3.3. Pull out

The roughness values obtained for different concrete slabs where each rubberized mortar formula was applied are shown in Figure 26. The roughness values obtained are very close, ranging between 0.44 mm and 0.37mm.

The bond strength decreases when the mortar contains more CR ratio. However, adhesion remains of good quality and no decohesion has been observed.

Figure 27. Pull out tests results.

Figure 27. Pull out tests results.

The obtained bond strengths values are lower than the tensile strength values of the rubberized mortars (Figure 27). The failure mode was adhesive for $r_v\le 50\%$, cohesive for $r_v=100\%$ and mixed for $60\%\le r_v\le 75\%$.

The NF EN 1504-3 standard prescribes bond strengths $\ge 2MPa$ for Class R4; $\ge 1.5MPa$ for Class R3, $\ge 0,8MPa$ for class R2 and noi specific requirement for Class R1. It can be observed that mortars with $25\le r_v\le 60\%$ can be used as non-structural repair products.

4. Conclusions

5. Acknowledgments

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