Assessment of the Post-Thermal Performance of Concrete Modified with Treated and Untreated Crumb Rubber

Abstract

Crumb rubber, obtained from discarded tires, presents a sustainable alternative in the construction industry, particularly in rubberized concrete. Treated crumb rubber offers improved mechanical performance; however, limited reports are available on its behavior at elevated temperatures. This study investigates the performance of rubberized concrete containing treated and untreated crumb rubber when exposed to elevated temperatures. The treatments employed are chemical (sodium hydroxide (NaOH)) and physical (cement coating) methods. M30-grade concrete was used as a control mix, and crumb rubber (CR) was added by replacing a portion of the fine aggregate. In order to mitigate the strength reduction, silica fume and polypropylene fibers were added. An optimal mix was determined using Taguchi’s L9 orthogonal array, by varying proportions of crumb rubber, silica fume (SF), and polypropylene (PP) fiber. The ideal mix contained 10% CR, 5% SF, and 0.2% PP fiber based on compressive strength. Specimens were cured for 28 days and exposed to temperatures of 200 °C, 400 °C, 600 °C, and 800 °C for 1 h. Mechanical properties such as compressive strength, split tensile strength, and modulus of elasticity were evaluated, along with an ultrasonic pulse velocity test. The results indicate that treated crumb rubber enhances bonding, improving the mechanical and thermal performance of rubberized concrete under high temperature.

1. Introduction

A considerable amount of natural resources are required for the preparation of concrete, the most widely used construction material. The ongoing extraction and use of natural aggregates affect the environment and deplete natural resources. The use of waste materials, from industrial and agricultural sectors in place of conventionally used aggregates, has gained popularity in recent years. Several agricultural and industrial waste products are used as supplementary cementitious materials for the preparation of concrete. The viability of using these waste products was investigated in a number of studies, and showed encouraging results for improving concrete qualities [1,2,3,4]. Apart from these materials, plastic waste, discarded glass, wood shavings, sawdust, rubber, marble dust, and debris from building and demolition are also utilized in construction [5,6,7]. Several researchers have investigated the use of recycled aggregates in concrete, such as low-quality recycled coarse aggregate [8], waste concrete fines (WCFs) [9], recycled aggregate (RA), and recycled powder (RP) [10], all of which support sustainable construction practices. The application of these waste materials is intended to recycle them and lessen their environmental impact, thereby promoting sustainability in the construction industry.

Crumb rubber (CR) is a very promising eco-friendly alternative to concrete that successfully addresses the environmental issues related to tire disposal and provides innovative ways to maximize the properties of concrete. With advancements in technology, the incorporation of CR into construction materials holds the potential to revolutionize infrastructure development by providing a more durable, sustainable, and environmentally friendly alternative.

Numerous studies have documented the incorporation of CR into concrete mixtures, which has the twin benefits of waste reduction and improvement in properties. Concrete mixtures that contain CR perform better in terms of fatigue, flexibility, and impact resistance [11,12]. The use of rubber in concrete possesses advantages such as low density, higher impact strength and toughness, better ductility, and better sound insulation property [13]. The CR particles act as a partial replacement of conventional aggregates, providing flexibility, reduced cracking, and enhanced structural integrity. It is reported that as the rubber content continues to increase, the loss in strength also increases [14,15,16].

Previous studies have reported that treated CR exhibits superior performance compared to untreated CR [17]. Researchers used chemical surface treatment methods like NaOH, silane coupling agent, coating with normal cement, blended cement with silica fume, and blended cement along with sodium silicate to improve rubber–cement bonding. Based on a study, it is reported that sodium hydroxide (NaOH) treatment improved the mechanical performance of rubber concrete and enhanced its long-term durability [18]. Comparing the treated CR concrete specimens with the untreated ones, treated CR concrete demonstrated a notable improvement in electrical insulation and chloride impermeability [15]. Different concentrated NaOH solutions (0.1 mol, 0.5 mol, and 1 M) were applied to CR for varying periods (2 and 24 h), where the use of NaOH treatment with 24 h soaking exhibited impeccable recovery in strength [19]. Rubberized concrete degrades at a very slow rate compared to regular concrete, and the specimen’s components are difficult to separate [20]. It is observed that the rubber after heating to 200 °C will develop a hard outer shell that encourages strong bonding with the concrete matrix, thus increasing the strength recovery compared to untreated rubber [21]. At an elevated temperature, the split tensile strengths showed much more variation than the compressive strengths [14]. Empirical equations for the mechanical properties of rubberized concrete were developed as a function of temperature [16].

Numerous researchers have examined the viability of using CR as a replacement for fine aggregate in concrete technology. While few studies have focused on the treatment of CR to improve the properties of rubberized concrete relative to conventional concrete, very limited research has focused on the performance of both treated and untreated CR concrete when exposed to elevated temperatures. Therefore, the present research explores the viability of treated CR concrete under varying elevated temperature conditions, ranging from 200 °C to 800 °C, and evaluates its strength parameters in comparison with conventional concrete. In addition, earlier studies have shown that incorporating CR into concrete generally results in a reduction in strength. To address this limitation, the present study explores the combined use of CR with silica fume and PP fibers, aiming to mitigate the strength loss and improve the overall performance of concrete. This integrated approach defines the novelty of current research.

2. Materials and Methods

2.1. Materials

The materials used in the current study include cement, fine aggregate, coarse aggregate, silica fume, PP fiber, crumb rubber, water, and sodium hydroxide (NaOH) solution.

The type of cement used is PPC (Portland Pozzolana Cement), manufactured by Malabar Cements, Walayar, India. The specific gravity, fineness, and normal consistency of cement were 2.97, 5.5%, and 34%, respectively. The fine aggregate used in the study is Manufactured Sand (M sand) of zone 2 conforming to IS 2386 (Part 3):1963 (2002). The coarse aggregate used had a maximum nominal size of 20 mm. The specific gravity and water absorption of fine and coarse aggregate were 2.61, 1% and 2.74, 0.5%, respectively. The water used for this study is potable water with a pH of approximately 7.

The CR particles used in the study are shredded tire waste material and are obtained from Krishna rubber product, Coimbatore, India. The particle size distribution for CR and fine aggregate is shown in Figure 1. The specific gravity of CR is obtained as 1.16.

Energy-dispersive X-ray spectroscopy (EDS) analysis is carried out to determine the chemical elements that are present in the CR (Figure 2). The analysis shows carbon (C) as the major element along with oxygen (O) and traces of zinc (Zn), silica (Si), sulphur (S), and iron (Fe).

Silica fume with a specific gravity of 2.2 is used in rubberized concrete to enhance the matrix density, strength, and thermal resistance, which can possibly occur at elevated temperatures.

PP with a density of 900 kg/m³ is used to enhance the crack resistance and reduce the splaying of concrete when exposed to elevated temperatures. It possesses a melting point of ~160 °C, which can create microchannels to release the vapor pressure and thus can prevent the explosion of concrete at higher temperatures. Sodium hydroxide, which is used for the chemical treatment of CR, has a specific gravity of 1.5, bulk density of 1040 kg/m³, and pH of 14.

2.2. Treatment of Crumb Rubber

In the untreated CR concrete, the CR particles as received are properly cleaned using water washing and dried in an oven at 40 °C (Figure 3a). The water washing is intended to remove any impurities and foreign matter present in the CR.

The treatment of CR is carried out using both chemical and physical approaches. In the chemical route, NaOH treatment is carried out to improve rubber–cement bonding by increasing the surface roughness and hydrophilicity of CR [17]. This process can reduce the interfacial voids, which usually weaken the rubberized concrete under heat. The required quantity of rubber particles was soaked in 1 M NaOH solution for 24 hrs. The rubber particles were then washed to remove the alkali solution. Rinsing with water was continued until the water’s pH was maintained within a 7 ± 0.1 range. Finally, treated rubber particles (Figure 3b) are subjected to air drying for a period of 24 h.

In the physical treatment, a coating is provided over the surface of the CR using cement slurry. The coating provides a thin cementitious layer on CR to improve bonding with the cement matrix [17]. It also plays the role of a thermal buffer to delay rubber decomposition under elevated temperatures. A water-to-cement ratio of 0.35 and a rubber-to-cement ratio of 0.6 were used for the coating, determined through a trial-and-error approach. Fresh cement paste was prepared and subsequently mixed with rubber particles to ensure uniform coating. The treated rubber is then dried for 24 h and sieved to conform with the particle size gradation curve of fine aggregate (Figure 3c).

Scanning electron microscopy (SEM) was employed to decipher the microstructure of untreated and treated CR. From the SEM images presented in Figure 3a, it is shown that the untreated crumb rubber displays a smooth surface, which can reduce the bonding with cement paste.

Figure 3b shows an SEM image of NaOH-treated rubber. It is observed that NaOH treatment enhanced the surface roughness, and can thus improve the interfacial bonding by mechanical interlocking of the rubber and cement matrix. Figure 3c shows an SEM image of cement-coated rubber, which presents a dense and granular surface structure, hence indicating the uniform adherence of cement particles.

Since this study is focused on the behavior of rubberized concrete under elevated temperature, the mass changes of untreated and treated CR as a function of temperature were also evaluated using thermogravimetric analysis (TGA) with a Derivative Thermogravimetry (DTG) overlay as presented in Figure 4. The sample was heated from room temperature up to 700 °C. The green curve represents the weight loss of the sample with increasing temperature, while the blue curve illustrates the derivative thermogram (DTG), indicating the rate of decomposition at various temperatures. From the curve, it is observed that, in all cases, major decomposition starts around 300 °C, with a maximum rate of decomposition at 378.01 °C for untreated, 382.71 °C for NaOH-treated, and 381.30 °C for cement-coated CR. Subsequently, the rate of decomposition slows down and nearly tapers off by 442.02 °C for untreated, 458.96 °C for NaOH-treated, and 455.67 °C for cement-coated samples. The sharp DTG peak at 378.01 °C, 382.71 °C, and 381.30 °C reveals a major decomposition event, which can be assigned to a phase transition, volatilization, or the degradation of a major component like a polymer or organic substance. In case of untreated CR (Figure 4a), the weight plateaus off at approximately 29%, indicating a total loss of approximately 70.98% from the initial state. The remaining 29% could be inorganic residue, char, or filler. In the case of NaOH-treated rubber (Figure 4b), the weight plateaus at 33.61%, indicating a total loss of 66.39% of the original sample, whereas the cement-coated sample (Figure 4c) shows lower weight loss of 54.99%, which is attributed to the effectiveness of the coating provided over the surface. By serving as a thermal barrier, the cement coating reduces heat transfer to the rubber crumbs, effectively delaying their degradation at elevated temperatures. Based on the test results, it can be observed that the CR undergoes substantial thermal degradation in the range of 300 to 475 °C. The application of NaOH treatment and cement coating resulted in reductions in weight losses by 6.45% and 22.53%, respectively, in comparison with untreated CR.

The differences in temperature decomposition and weight loss among untreated CR, NaOH-treated CR, and cement-coated CR can be attributed to variations in surface bonding and thermal stability. The untreated CR, with its smooth and hydrophobic surface, exhibits weak interaction with the cement matrix, leading to accelerated degradation and higher weight loss. In contrast, NaOH-treated CR develops a rougher and more reactive surface, which enhances adhesion with the cement paste, thereby delaying thermal degradation and reducing weight loss. Cement-coated CR provides the highest stability, as the cement layer forms a protective barrier around the rubber particles, improving interfacial bonding and shielding them from direct heat exposure. Consequently, cement-coated CR demonstrates the least weight loss and the greatest thermal resistance.

Thus, these findings highlight the positive influence of surface treatments on the thermal resistance of CR under elevated temperature conditions. A lower weight loss in the TGA curve is indicative of greater thermal stability. Among the treatments, the cement coating exhibited the highest thermal stability, followed by NaOH treatment, while untreated rubber showed the greatest degradation.

2.3. Mix Proportion and Methods

Concrete of M30 grade was adopted in the investigation, and the mix design presented in

Table 1. Mix proportion for control mix.

	Cement (kg/m3)	Water (kg/m3)	Coarse Aggregate (kg/m3)	Fine Aggregate (kg/m3)	Crumb Rubber	W/C Ratio
CM	447	204	1119	610	0	0.45

was formulated following IS 10262:2019. To mitigate the reduction in strength caused by the replacement of fine aggregate with CR, silica fume was incorporated as a partial replacement for cement, and PP fibers were added to improve shrinkage resistance and tensile strength.

In line with this, the study evaluated three key parameters—namely, the weight percentages of CR, PP fiber, and silica fume. The levels and values for the parameters are shown in

Table 2. Parameters and their levels.

Variable Parameters	Level 1	Level 2	Level 3
Silica fume (%)	0	5	10
PP fiber (%)	0	0.2	0.4
Crumb rubber (%)	0	10	20

, as obtained from previous literature [22]. A three-level orthogonal array design (L9) was utilized in the study to determine the most effective combination out of nine possible mixes. Based on the Taguchi method, nine experimental runs were designed, as detailed in

Table 3. Mix proportion and nomenclature.

Sample Name	Silica Fume (%)	PP Fiber (%)	Crumb Rubber (%)
CM	0	0	0
S0P0.2C10	0	0.2	10
S0P0.4C20	0	0.4	20
S5P0C10	5	0	10
S5P0.2C20	5	0.2	20
S5P0.4C0	5	0.4	0
S10P0C20	10	0	20
S10P0.2C0	10	0.2	0
S10P0.4C10	10	0.4	10

, which also outlines the naming convention. ‘CM’ denotes the control mix (conventional concrete), while ‘SxPyCz’ refers to mixes incorporating admixtures and CR —where ‘x’ represents the percentage of silica fume, ‘y’ indicates PP fiber content, and ‘z’ signifies the crumb rubber replacement level. The signal-to-noise ratio (S/N) was used to quantify the performance and assess how noise affected it.

The samples prepared based on the nine mix combinations (Table 3) were cast and kept for water curing for a period of 28 days. After the curing was completed, the compressive strength of the nine mixes was tested. With the help of ANOVA and the S/N ratio, the optimal mix for the rubberized concrete was determined. Further studies were carried out on this optimal combination with treated CR, using NaOH treatment and cement coating, as a replacement for fine aggregate. These samples were exposed to elevated temperatures ranging from 200 °C to 800 °C, with an interval of 200 °C for a period of 1 h. Various mechanical properties of the rubberized concrete, such as compressive strength, split tensile strength, modulus of elasticity, and UPV, along with microstructural characteristics, were assessed before and after exposure to elevated temperatures and compared with those of conventional concrete.

After 28 days of water curing, the concrete specimens were exposed to temperatures ranging from 200 °C to 800 °C by placing them in an electric furnace for a duration of one hour. After exposure to the desired temperature, the specimens were allowed to cool inside the furnace until they reached ambient temperature, and testing was carried out immediately thereafter.

Different tests like the compressive strength test, split tensile test, ultrasonic pulse velocity test, and modulus of elasticity test were carried out to study the mechanical characteristics of the specimens before and after temperature exposure. Compressive strength was assessed using cubical specimens of size 150 mm using a compression testing machine of 2000 kN capacity, in accordance with IS 516 (Part 1/section 1): 2021 [23]. The split tensile test, an indirect method for assessing concrete’s tensile strength, is estimated using concrete cylinders measuring 100 mm in diameter and 200 mm in height, conforming to IS 516 (Part 1/section 1): 2021 [23]. The concrete’s stiffness and ability to deform under applied loads are determined by obtaining the modulus of elasticity, using a cylindrical specimen of size 150 mm in diameter and 300 mm in height. An extensometer is used along the longitudinal axis of the specimen to measure deformation and the test is carried out conforming to IS 456:2000 (Reaffirmed 2005) [24]. Apart from the mechanical property assessments, morphological studies such as scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were also carried out to understand the surface characteristics of the specimens after exposure to elevated temperatures.

3. Results and Discussion

Optimum Mix Combination

With the help of the Taguchi method, nine combinations were arrived at, as reported in Table 3. The specimens for these combinations were prepared and water-cured for 28 days. After that, the compressive strength test was performed and the test results are presented in Figure 5. Three samples were tested in each category and the average values are reported.

The control mix provided a compressive strength of 36.69 MPa and the strength reduced with the incorporation of CR. When 10% CR was added in the mix as a replacement for fine aggregate along with 0.2% PP fiber, the strength reduced to 27.54 MPa. With 20% CR along with 0.4% PP fiber, the compressive strength further reduced to 20.2 MPa. The observed reduction in strength is attributed to the inadequate bonding between rubber particles and the cement paste, as CR possesses a smooth, hydrophobic surface that restricts the adhesion. This also increases the voids in concrete and reduces density, which results in a weaker microstructure and increased porosity. In addition, CR has a lower stiffness and strength compared to natural aggregates, which reduces the overall load-bearing capacity of the concrete.

To increase the strength and improve the shrinkage resistance, silica fume was used as a partial replacement to cement along with PP fiber. Furthermore, PP fibers can act as vents during elevated temperature exposure by developing pores, as they start melting at approximately at 160 °C. With 10% silica fume, the strength decreased slightly compared to the 5% addition, which may be due to the agglomeration of silica fume at higher loading levels. Also, the pozzolanic reaction between silica and calcium hydroxide (CH) from cement hydration contributes to strength. Nevertheless, after a certain limit, available CH may not be sufficient for further reaction, limiting the strength.

Based on the test results reported in Figure 5, the main effect plot for the S/N ratio is presented in Figure 6. The S/N ratio is used in the Taguchi method to improve the robustness of a product or process by minimizing the effects of uncontrollable factors (noise).

ANOVA is performed to understand the contribution of each parameter, and the results are presented in

Table 4. Contribution of each parameter to compressive strength.

Source	Degree of Freedom	Sequential Sum of Squares	Contribution	Adjusted Sum of Squares	Adjusted Mean Square	F-Value	p-Value
Silica fume	2	29.042	7.27%	29.042	14.521	12.14	0.076
PP fiber	2	3.062	0.77%	3.062	1.531	1.28	0.439
Crumb rubber	2	365.158	91.37%	365.158	182.579	152.62	0.007
Error	2	2.393	0.60%	2.393	1.196
Total	8	399.654	100.00%

. Table 4 shows that CR plays a major role in compressive strength with a contribution of 91.37%, followed by silica fume with 7.27% and PP fiber with 0.77%. Accordingly, by analyzing the S/N ratio and ANOVA table, the optimal mix design is selected and presented in

Table 5. Optimal mix proportion.

Parameter	Silica Fume	Crumb Rubber	PP Fiber
Compressive strength	5%	10%	0.2%

.

Further studies on elevated temperature were carried out on this optimum mix combination, consisting of treated and untreated CR. The test results are also compared with the conventional M30 concrete without CR and admixtures, exposed to elevated temperature.

4. Experimental Studies–Optimum Mix Combination

4.1. Compressive Strength

In the current study, the compressive strength test was carried out conforming to IS 516 (Part 1/section 1): 2021 [23]. After water curing for 28 days, rubberized concrete with treated as well as untreated rubber was tested for compressive strength and compared with normal concrete at ambient as well as elevated temperature, and the test results are presented in Figure 7.

The compressive strength decreased with an increase in temperature. At ambient temperature, the compressive strength of rubberized concrete without treatment reduced by 12.78%, whereas with NaOH treatment the reduction reduced to just 1.88%; the cement-coated mix showed an improvement of 3.57% compared to the control. The reduction in strength may be due to the less rigid surface and hydrophobic nature of CR compared to the M sand as presented in the SEM image shown in Figure 3a. Apart from this, the smooth surface reduces the cohesion between the cement paste and the crumb rubber particles, which provides a weak interfacial transition zone (ITZ) in the concrete, which results in more voids in the concrete and hence can result into lower load carrying capacity [16]. For NaOH-treated rubber, the increase in strength might be due to better bonding between treated rubber with cement mortar compared to untreated ones. This is due to the improved surface roughness provided by the NaOH treatment, as evident from the SEM image presented in Figure 3b. For cement-coated rubber, applying a layer of cement paste to rubber before mixing it with concrete creates a solid, appropriate interface with the surrounding cement matrix. As a transitional layer, the cement coating improves adhesion and reduces the weak ITZ, which is typically caused by the hydrophobic, smooth surface of the rubber particles. This improves the efficiency of the cement matrix and rubber particles’ stress transmission. The silica fume in the rubberized concrete densifies the concrete and increases the strength by reacting with calcium hydroxide to form more calcium silicate hydrate (CSH) gel.

With an increase in temperature, the compressive strength is reduced in all the concrete combinations. At 200 °C, there is a reduction of 7.4% for the control specimen, whereas the rubberized concrete shows a reduction of 15.12% for the untreated samples and 2.83% and 6.84% for NaOH- and cement-coated samples, respectively, compared to corresponding ambient temperature strength (Figure 7). This reduction may be due to the evaporation of free water and surface water from the concrete, which results in increased internal stresses to create micro-cracks on the surface, causing degradation in concrete [25]. The addition of CR, which has a lower specific gravity than M sand, causes strength loss in rubberized concrete. Also, PP fiber in the rubberized concrete starts melting, which leaves space for internal vapor pressure, which is developed as part of elevated temperature, to escape. This prevents the concrete from undergoing sudden spalling. The slightly better performance of the treated CR-added concrete is attributed to the better bonding of CR with concrete compared to non-treated one. After NaOH treatment, CR becomes rough and more compatible with cement paste, improving the bonding between the rubber and cement paste and hence the strength. Also, the improvement in the cement-coated rubber mix might be due to the coating of rubber with cement, which creates a strong outer surface. Not only does it creates better bonding with the cement matrix, but the overall compactness and integrity of concrete are also improved. Even though the PP fiber completely melted at this stage, the dense silica fume matrix contributed to strength.

At 400 °C, the control specimen showed a reduction of 25.59%, which is due to the evaporation of water from the hydrated concrete (in CSH), which is referred to as dehydration [26]. More decomposition of the CSH gel begins at this stage. The rubberized concrete with untreated, NaOH-treated, and cement-coated mix showed a reduction of 28%, 22.22%, and 19.73%, respectively, which is also attributed to the dehydration of concrete due to the evaporation of water from the hydrated concrete, as well as the decomposition of CR as explained in the TGA curve (Figure 4a). The slightly improved performance in the NaOH-treated and cement-coated concrete mix is due to the surface modification of rubber and better bonding between rubber and the cement matrix after treatment.

The strength reduction is 33.16% for the control, corresponding to a temperature exposure at 600 °C, whereas the rubberized concrete shows an equal reduction of 34% for the untreated one, and an almost equal reduction of 29% for both NaOH- and cement-treated CR, compared to the ambient temperature. The reduction in strength is due to the significant dehydration of the CSH gel and also the full decomposition of calcium hydroxide, at around 500–550 °C [16]. Due to the dense matrix in the rubberized concrete with the addition of silica fume, micro-cracks become more significant and result in a reduction in strength.

As the temperature reaches 800 °C, a drastic reduction in strength is observed for the control specimen, of 65.84% at ambient temperature. The strength performance of untreated CR shows a reduction of 70.06%, whereas NaOH- and cement-coated CR show 63.88% and 61.05% reduction, respectively, compared to that at ambient temperature. At this temperature, the CSH decomposition is drastic, which shows a high strength loss and the concrete becoming brittle and porous.

From 600 °C to 800 °C, major strength loss is noted for rubberized concrete due to the complete degradation of CR and the changes in the properties of the hydrated products in the concrete at the elevated temperature. However, the treatment provides better protection to the CR as shown by the improved compressive strength compared to conventional concrete.

Thus, it can be observed that from ambient temperature to 800 °C, the strength loss is 7.5–65.84% for the control mix whereas for the rubberized concrete it was in the range of 15.12 to 70%, 2.83 to 63.88%, and 6.84 to 61.05% for untreated, NaOH-treated, and cement-coated CR concrete, respectively, compared to respective strength values at ambient temperature. This shows the efficacy of the treatment on CR to enhance the strength of the concrete compared to untreated CR.

The failure of specimens exposed to elevated temperature and subjected to compressive loading is shown in Figure 8 for both normal as well as rubberized concrete. A detailed explanation and comparative insights of behavior of the different mixes at elevated temperatures after being subjected to compressive load are presented in

Table 6. Behavior of conventional and rubberized concrete exposed to elevated temperature after compressive strength test.

Temperature	Conventional Concrete	Untreated CR Concrete	NaOH-Treated CR Concrete	Cement-Coated CR Concrete
Ambient	Cube is intact with cracks on both edges.	Intact with minor cracks on edges, due to the influence of silica fume and PP fibre.	Intact with minimum surface cracks.	Intact and clean surface with minor cracks.
200 °C	Spalling of concrete starts from the surface with visibly smaller cracks on faces. Chemically bound water starts losing at ~105 °C.	Surface was relatively intact, with lesser cracks than the normal concrete; may be due to the influence of silica fume and PP fibre present. Rubber softens but remains, internal flexibility can reduce cracking. PP fibre melts and provide voids, for vapor to escape.	Minor surface cracking is observed. Rubber begins softening; PP fibres may start melting. Spalling present at edges.	Minor cracks, cube remains intact. Rubber softens but coating provides thermal buffer. Spalling present at edges. Surface appears slightly lighter, maybe due to moisture loss.
400 °C	Visible deterioration on the surface and cracks observed with degradation on the edges.	Spalling of surface concrete starts along with cracking. Rubber starts decomposing, produces voids.	Increased surface cracking with reduced spalling, due to NaOH treatment, compared to rubberized concrete; some edge degradation. Rubber partially decomposes, internal void formation.	Slight surface cracking, still cohesive. Cement coating delays rubber decomposition; matrix remains stronger. Surface appear slightly lighter, maybe due to moisture loss.
600 °C	Severe cracking with broken surface. Decomposition of calcium hydroxide (~400–500 °C). Becomes brittle, and structural integrity deteriorates quickly above 400 °C.	Rubber burns off, leaving large internal voids, reducing strength. Similar deprivation to normal concrete with more discoloration and flakiness.	Surface chipping, moderate cracking, and visible spalling. Degraded rubber and melted fibres leave voids; strength deteriorates. Better performance than untreated cube at same temperature.	Moderate cracking, but cube shape well retained. Surface appear slightly lighter, maybe due to moisture loss.
800 °C	Almost complete disintegration with a major loss of material. Complete disintegration of the cement matrix.	Severe crumbling due to absence of rubber and collapse of weakened matrix. Wide surface damage along with charring, as the crumb rubber and PP fibre have completely burned out and left voids.	Severe damage, edge collapse. Crumb rubber fully degraded; thermal stress and internal voids caused collapse. Some cohesion compared to untreated crumb rubber concrete. Coating protects rubber longer; helps maintain some post-fire compressive strength.	Significant damage, but some cohesion in structure remains. Coating protects rubber longer; helps to maintain some post-fire compressive strength.

.

Based on the failure pattern, less severe surface cracking is observed in treated CR concrete than untreated CR concrete. It is observed that NaOH treatment improves bonding, but high temperatures still lead to structural deterioration due to the organic rubber content and fibre melting. Some cubes show edge spalling, but core integrity is relatively preserved, as a result of better heat resilience due to improved rubber bonding.

Cement-coated CR clearly provides better resistance to elevated temperature exposure, likely due to delayed rubber decomposition, better ITZ, and less abrupt void formation compared to untreated or NaOH-treated rubber. Better overall cube shape retention, even at higher temperatures. Some cracking visible but more cohesive and less fragmented than NaOH-treated samples, which implies better thermal performance, likely from the additional cementitious coating.

4.2. Split Tensile Strength

The split tensile test was carried out conforming to IS 516 (Part 1/section 1): 2021 [23], using cylindrical specimens of size 100 mm by 200 mm. After curing for 28 days, rubberized concrete with treated as well as untreated rubber was tested for split tensile strength and compared with the normal concrete at ambient as well as elevated temperature (Figure 9). As anticipated, the split tensile strength reduces in rubberized concrete, as the addition of CR causes voids in the rubberized concrete. CR, which has a hydrophobic nature, develops weak bonding between the cement mortar and CR aggregate, resulting in crack initiation and further propagation under tensile stresses [27,28,29]. But the comparative lesser reduction is attributed to the addition of silica fume and PP fiber. As in the case of compressive strength, the silica fume in the rubberized concrete densifies the concrete by filling the voids created by the addition of CR. PP fiber enhances the early-age crack control by preventing plastic shrinkage cracks and also improves the fire resistance of the concrete [16].

At ambient temperature, the tensile strength of control concrete is 3.4 MPa, whereas the rubberized concrete without treatment shows 2.9 MPa, 14.7% lesser than that of conventional concrete. In treated rubberized concrete, the cement-coated rubber mix shows a strength of 3.65 MPa, which is 7.35% higher compared to the control mix, and NaOH treatment shows a strength of 3.2 MPa, which is 5.88% lesser compared to the control. For cement coating and NaOH-treated rubber, the increase in strength is attributed to the surface modification and better bonding between treated rubber with cement paste compared to untreated ones.

The split tensile strength is decreased with an increase in temperature, which may be due to the evaporation of moisture inside the concrete as well as due to the internal cracks induced by the temperature variation [16]. At 200 °C, a loss of 17.65% for the control, 31.03% for untreated CR, 12.81% for NaOH-treated, and 19.45% for cement-coated rubber mix compared to the ambient temperature of corresponding mixes are observed, maybe due to the moisture loss and formation of micro-cracks in the concrete.

At 400 °C, a strength reduction of 35.29%, 41.72%, 28.12%, and 31.5% is noticed for the control, untreated, NaOH-treated, and cement-coated mix, respectively, compared to corresponding ambient temperature results. The higher reduction in rubberized concrete is attributed to the complete melting of the rubber particles at this temperature, as evident from the TGA graphs (Figure 4). At this temperature the CR lost its bonding with the cement paste and completely melted, which create voids in the concrete, resulting in the formation of major internal cracks and hence a reduction in tensile strength. Also, at this stage the degradation of calcium hydroxide begins, which results in a degradation in strength.

When the temperature is increased to 600 °C, the control experiences a reduction of 49.41%, whereas the concrete with untreated CR shows a reduction of 52.75%, and NaOH- and cement-coated concrete showed values of 41.25% and 46.3%, respectively, from the corresponding ambient temperature values. This is due to the complete melting of CR in the range of 300 to 475 °C, as shown in the TGA graph, after which both mixes behave in the same manner. The aggregate cement bonding reduces at this stage, which results in severe cracking which is extended to the surface. The hydrated product, CH, fully decomposes at this stage and C-S-H gel decomposition starts [16], which results in strength reduction.

At 800 °C, severe degradation occurs, and the concrete is structurally compromised. A reduction of 76.47% observed for the control, whereas the rubberized concrete shows a reduction of 79.31% for untreated rubber, and 73.43% and 75.34% corresponding to NaOH- and cement-coated CR, respectively, compared to that at ambient temperature.

Hence, for temperatures from 200 °C to 800 °C, the strength value was decreased in the range of 17.64 to 76.47% for conventional concrete, whereas for the rubberized concrete it was in the range 31.03 to 79.31% for untreated, 12.81 to 73.43% for NaOH-treated, and 19.45 to 75.34% for cement-coated CR-added concrete, respectively.

Figure 10 shows the internal view of normal concrete and rubberized concrete specimens at elevated temperature after the split tensile test. Figure 10a shows the conventional concrete. Here, as the temperature increases due to loss of moisture, micro-cracks are visible. At higher temperatures, widespread cracks are visible due to the weakening of the bond between cement paste and aggregate. Figure 10b shows the internal view of the rubberized concrete after the split tensile strength test. From the figure it is visible that at higher temperatures the rubber starts to degrade, and the black dots due to the degradation of rubber, as well as the cracks, are visible. The aggregate color also starts to change at high temperature.

4.3. Modulus of Elasticity

Modulus of elasticity is a critical parameter to measure concrete’s ability to undergo elastic deformation under applied loads. The results presented in Figure 11 show that the rubberized concrete has a lower modulus of elasticity compared to normal concrete due to the flexible nature of CR compared to the natural fine aggregates. The other reason for the decrease in elasticity value might be due to the weak adhesion between cement paste and the rubber particles.

At 200 °C, the normal concrete has a reduction of about 26.82%, whereas the rubberized concrete had a reduction of about 21–25% with and without treatment. However, the elasticity value of the treated CR concrete is marginally higher than that of the untreated one. This might be because the treatment improved the bonding between the cement paste and the CR.

At 400 °C, the reduction in the modulus for control concrete was up to 57.14%, whereas the CR concrete exhibited a percentage reduction of 62% for untreated and 51–53% for treated concrete compared to the ambient temperature. This drastic reduction is attributed to the dehydration of calcium hydroxide crystals in the range of 400 to 600 °C. Also, in CR concrete, the melting of the CR takes place in the range of 300 to 450 °C, as provided in the TGA graph (Figure 4). Apart from this, above 600 °C, the CSH gel experiences a drastic reduction. The dehydration of hydrated products and the development of more voids in the CR concrete due to the melting of CR weakens the ITZ in the concrete, and reduces the stiffness and modulus of elasticity of concrete specimens [25,30,31,32].

From 400 °C to 800 °C, the decrease in the modulus of elasticity was high and in the range of 57 to 95% for normal concrete and 62 to 95.59% for untreated rubberized concrete, whereas it is in the range of 51 to 95% for treated rubber concrete compared to ambient temperature. At high temperature, the decomposition of C-S-H gel occurs, which reduces the stiffness and decreases the modulus of elasticity value. For rubberized concrete at high temperature, the thermal degradation of rubber occurs, i.e., rubber particles start to melt, which leads to the formation of voids.

4.4. Ultra Sonic Pulse Velocity (UPV) Test

The UPV test is a non-destructive test to assess the internal compactness and homogeneity of the concrete. In the current study, the concrete cube after exposure to elevated temperature is subjected to the UPV test to assess the internal composition after exposure (Figure 12). Normal concrete possesses a velocity of 4.5 km/s at ambient temperature, while rubberized concrete with and without treatment has a value in the range of 4.2 to 4.4 km/s. The reduction in UPV values after exposure from 200 °C to 600 °C is due to improper bonding between rubber and cement paste and void formation. At 600 °C and 800 °C, complete failure of concrete starts, which is also justified by the UPV values.

As per IS 516 (Part 5)-2021 [33], the velocity criterion for concrete of above M25 grade is greater than 4.5 km/s represents excellent quality, between 3.75 to 4.5 km/s is good, and below 3.75 km/s is doubtful. Accordingly, from Figure 12, it is observed that the quality of concrete is not good after 400 °C, as shown by the UPV values.

4.5. Microstructural Analysis

The microstructure characterization for conventional as well as rubberized concrete was performed at normal temperature, 200 °C, and 400 °C, using scanning electron microscopy (SEM), and the results are presented in Figure 13, Figure 14, Figure 15, Figure 16 and Figure 17.

4.5.1. Control Mix Concrete

Figure 13 presents the SEM images of control mix concrete, where needle-like ettringite crystals are well developed along with dense C-S-H gel clusters with a fibrous architecture. The existence of a dense microstructure indicates strong inter-particle bonding, with very little porosity or cracking, which indicates proper hydration of the cementitious compounds. At 200 °C, thermal effects tend to start deteriorating the microstructure of the concrete. As ettringite breaks down, C-S-H starts to dry out, and seems to be more amorphous and fragmented. A less compact matrix and an increase in micro-cracking indicate a decline in mechanical strength. At 400 °C, significant thermal damage occurs in the concrete, as shown in Figure 13c, which shows a fragmented and degraded appearance. At this stage, C-S-H dehydration accelerates with the breakdown of calcium hydroxide. This leads to increased brittleness, decreased strength, and a notable loss of cohesion. The structure deteriorates and becomes permeable.

4.5.2. Untreated Rubber Mix Concrete

A heterogeneous matrix with slight micro-cracking and poor interfacial bonding with the cement paste is depicted in the image (Figure 14a) at ambient conditions. Large, smooth, and hydrophobic rubber particles are probably a contributing factor to the weak ITZs. This could result in early-age micro-cracking and decreased mechanical strength. Though silica fume and PP fiber slightly improve the texture of the matrix, it is insufficient to strengthen the weak rubber interaction. Figure 15 presents the EDS of untreated CR at a normal temperature, which shows the presence of silica fume.

From Figure 14b it is observed that, at 200 °C, more pronounced micro-cracks and surface roughness are evident, as rubber begins to disintegrate and lose its bond with the matrix. ITZ deteriorates due to differential thermal expansion, exhibiting instability and poor adhesion. Although silica fume aids in maintaining matrix cohesion, the response is dominated by fiber and rubber degradation. At 400 °C (Figure 14c), the matrix develops significant microstructural degradation with wide voids, most likely as a result of PP fiber vaporization and rubber disintegration. The concrete’s strength and durability are much reduced and the ITZ totally disintegrates, compromising the structural integrity of the material. At this point of thermal degradation, silica fume’s densification effect is no longer effective.

4.5.3. NaOH-Treated Rubber Mix Concrete

Figure 16a, which presents the SEM of NaOH-treated rubber mix concrete at ambient temperature, shows a dense, compact surface with minimal micro-voids. The NaOH treatment improved the surface roughness and compatibility of CR, which strengthens its bond with the cement paste. When compared to untreated rubber concrete, this results in a more cohesive microstructure with reduced weak zones, indicating an enhancement in the ITZ. The addition of silica fume promotes packing and reduces porosity. Thus, at ambient temperature, the concrete structure shows excellent interfacial bonding, reduced porosity, and a homogeneous microstructure due to the combined impacts of silica fume, PP fiber integration, and NaOH treatment.

At 200 °C, concrete with NaOH-treated rubber exhibits moderate thermal deterioration, with microfractures and considerable surface degradation, as shown in Figure 16b. PP fiber deterioration begins at approximately 200 °C, compromising crack-bridging capacity, but the presence of silica fume likely helps retain some structural integrity. Debonding at the rubber interface is noticeable, but not as severe as in untreated specimens.

The surface is significantly fractured after exposure at 400 °C, with large angular cracks and visible surface deformation, as the majority of organic constituents in rubber undergo degradation (Figure 16c). Most of the PP fibers are likely vaporized and NaOH-treated rubber no longer maintains a strong bond, though it may have slowed the degradation rate initially. NaOH treatment and silica fume may help retain microstructural remnants, but significant deterioration is evident.

4.5.4. Cement-Coated Rubber Mix Concrete

At ambient temperature, the microstructure is compact and dense, suggesting minimal micro-cracking and robust mechanical interlocking. With a coating layer that lessens the weak ITZ, CR seems to be firmly bound to the matrix. The micro-cracks appear to be bridged and held in place by the embedded PP fibers. Needle-like ettringite crystals are also visible in the image. As a key hydration product, ettringite regulates the setting of cement by preventing flash setting.

As can be seen from Figure 17b, considerable thermal degradation has begun at 200 °C, exhibiting the first indications of micro-cracking and increasing porosity close to the rubber–cement contact. The rubber–cement contact exhibits a slight detachment, which suggests early-stage thermal stress degradation. Even though the silica fume still contributes to matrix densification, the thermal stresses start to surpass its benefits.

The SEM image at 400 °C (Figure 17c) displays substantial thermal degradation, a significantly fractured surface with numerous micro-cracks, and a rough morphology. The presence of visible gaps and fissures indicate a lack of cohesive structure. The rubber particles have deteriorated at this temperature, as evident from TGA test results, and PP fibers have completely melted or evaporated, leaving behind pores that have increased porosity. Thus, the strength has been significantly reduced due to a weaker, porous, and broken structure caused by the melting of PP fibers and the thermal disintegration of rubber.

5. Conclusions

The current study explores the behavior of rubberized concrete incorporated with admixtures, silica fume, and PP fiber, under elevated temperature exposure. By recovering used tires and incorporating crumb rubber into concrete with silica fume and polypropylene (PP) fibers, the project seeks to improve the sustainability of construction materials. The silica fume and PP fibers compensate for the strength reduction due to the addition of CR.

Based on the ANOVA results, 5% silica fume by weight of cement, 10% CR by weight of fine aggregate, and 0.2% PP fiber by volume of concrete are obtained as the optimum combinations, and further tests on treated and untreated CR are carried out on this combination. Specimens prepared with conventional concrete, untreated, and treated CR concrete were heated to temperatures between 200 °C and 800 °C, and their modulus of elasticity, compressive strength, tensile strength, and ultrasonic pulse velocity (UPV) were measured before and after exposure. CR was treated with cement coating and NaOH to enhance its bonding qualities. Characterization studies such as SEM, EDS, and TGA are also carried out to support the experimental results.

The results obtained from conducting these tests led to the following conclusions:

• Loss in strength occurs as crumb rubber percentage increases.
• SEM analysis of treated CR indicated better surface morphology and bonding compared to untreated rubber particles. TGA confirmed major degradation of CR between 300 and 475 °C and shows the better thermal stability of treated CR concrete.
• From 200 °C to 800 °C, the compressive strength loss is 7.5–65.84% for the control mix, whereas for the rubberized concrete it was in the range of 15.12 to 70.06%, 2.83 to 63.88%, and 6.84 to 61.05% for untreated, NaOH-treated, and cement-coated crumb rubber concrete, which is due to the dehydration of concrete due to the evaporation of water from the hydrated concrete, as well as the decomposition of CR.
• Split tensile strength dropped more rapidly than compressive strength under heat, especially for rubberized concrete. From temperatures of 200 °C to 800 °C, the split tensile strength value was decreased in the range of 17.64 to 76.47% for the control mix, whereas for the rubberized concrete it was in the range of 31.03 to 79.31%. But the treated CR concrete shows a comparatively better performance in the range of 7 to 71.66% for NaOH-treated and 19.45 to 75.34% for cement-coated concrete, respectively.
• The modulus of elasticity was lower in rubberized mixes due to the flexible nature of CR. But the treatment improved the performance compared to untreated CR concrete. UPV values also decreased at higher temperatures due to the degradation of PP fiber and CR, which reduces the compactness of the specimen.
• The microstructural characterization studies confirm the better performance of the treated CR compared to untreated CR at elevated temperatures.
• Based on the test results, it is observed that cement-coated CR offers the best thermal and structural performance under elevated temperatures compared to all other combinations. Cement coating outperforms NaOH treatment at high temperatures, likely due to enhanced thermal shielding and stronger interface with the cement paste. NaOH treatment provides improvement over untreated rubber but is slightly less effective than cement coating at high temperatures. Also, the combined use of silica fume and PP fibers enhances early-stage crack resistance and vapor pressure relief, contributing to better fire performance. Thus, it can be inferred that rubberized concrete with treated CR incorporated with admixtures is promising for structural applications and moderate-load structural applications, especially when treated and optimized.

The incorporation of CR as a replacement for aggregates is an eco-friendly approach in construction that minimizes landfill waste, preserves natural resources, and promotes a circular economy. The current study explored the possibility of treated CR-modified concrete as a viable option for structures exposed to elevated temperatures, supporting several UN Sustainable Development Goals (SDGs). This initiative supports SDG 11 (Sustainable Cities and Communities) by improving infrastructure resilience, SDG 12 (Responsible Consumption and Production) by facilitating tire recycling and minimizing the consumption of natural aggregates, and SDG 13 (Climate Action) by decreasing the carbon footprint through sustainable resource utilization. Furthermore, it contributes to SDG 9 (Industry, Innovation, and Infrastructure) by promoting innovative methodologies in construction materials for the enhanced durability and sustainability of structures.