Mechanical Performance of Steel-Fiber-Incorporated Rubberized Concrete for Rigid Pavement Applications

Sustainable solutions are required to mitigate waste tire accumulation. Using rubberized concrete (RuC) in rigid pavements is a viable and relatively environmentally friendly strategy for partially replacing fine/coarse aggregates with crumb rubber. However, crumb rubber affects the mechanical properties of concrete. Although RuC is more flexible (ideal for road pavements) than conventional concrete, its strength is lower than that of the standard concrete used in rigid pavements. Consequently, the crumb rubber generated from discarded tires cannot be used sustainably. The primary objective of this study was to characterize RuC mixed with manufactured steel fibers to improve the strength of the mixture. Fine aggregates (sand) were partially substituted with various proportions (10%, 20%, and 40%) of crumb rubber (size:1.70–2.36 mm) while incorporating 0.5% steel fibers (by volume). The compressive strength, splitting tensile strength, and flexural strength of the concrete deteriorated with increasing rubber content, consistent with the previously reported results. However, steel fiber incorporation improved the mechanical behavior of RuC, thus enhancing the compressive strength, splitting tensile strength, and flexural strength of the pavement structure. The developed steel-fiber-incorporated RuC design enhances the usability and economic value of RuC as well as minimizes the adverse environmental impact of concrete pavement technology.


Introduction
In Thailand, road transportation is considered the primary mode of transportation owing to the extensive road network that covers the entire country and facilitates efficient connectivity between various regions.Regardless of the agencies responsible, the total road length was substantial, reaching 702,567 kilometers [1].Roads are generally categorized into two main structural types: flexible pavements, which consist of asphalt or bitumen surfaces, and rigid pavements, which are made of concrete.The selection of a pavement structure is determined by factors such as traffic conditions and usage.Concrete pavements are increasingly being preferred owing to their high strength, chemical resistance, and durability.However, the properties of concrete, including its susceptibility to shrinkage and expansion owing to changes in moisture and weather conditions, can lead to cracking.To mitigate this problem, concrete roads often incorporate joints and reinforcing steel to control cracking, making their construction more time consuming than that of flexible pavements.Moreover, the driving experience of these two types of pavements differs significantly, primarily influenced by the high strength and presence of joints in concrete pavements.Therefore, enhancing the flexibility, reducing the steel reinforcement, and increasing the joint spacing in concrete pavements are areas of research and development aimed at creating sustainable construction materials.
Concrete, a widely used construction material, consists of cement, water, and aggregates (both coarse and fine).Concrete road surfaces typically include subgrades, sub-bases, and concrete surface layers.Reinforcement, usually steel bars or wire mesh, is added to improve tensile strength and resist cracking.However, owing to environmental concerns related to the production and usage of concrete, efforts have been made to develop alternative materials, including recycled materials or the incorporation of additional materials to reduce cement consumption or replace scarce local materials.One alternative material is Rubber has unique properties, such as flexibility, elasticity, impact absorption, and vibration dampening.These properties reduce the impact and vibrations transferred from the road surface to vehicles, thereby contributing to driver comfort and safety.With the increasing production of rubber products, managing rubber waste has become a challenge, necessitating environmentally friendly solutions [2].The recycling of rubber into construction materials, specifically road surfaces, has emerged as a sustainable approach.In 2014, researchers led by Gesoglu et al. [3] found that adding rubber particles to concrete significantly reduced the modulus of elasticity and compressive strength owing to the reduced load-bearing capacity of rubberized concrete (RuC).Thus, the development of concrete road surfaces with rubber aggregates aims to provide an environmentally friendly and efficient solution for managing rubber waste, reducing environmental pollution, and enhancing road performance.This research project aims to develop a prototype of RuC for rigid road construction (JPCP) by grinding rubber waste materials to add economic value and reduce environmental impact.It is crucial to balance the use of rubber aggregates in concrete to ensure optimal performance, because excessive rubber replacement can negatively affect the modulus of elasticity and load-bearing capacity of concrete.
Concrete is a commonly used construction material comprising cement, water, and aggregates (both coarse and fine).The concrete used in road surface structures typically consists of subgrade, subbase, and concrete surface layers.In the construction of concrete surfaces, temperature or dowel bars are often used to reinforce the concrete, and the use of cement and steel in large quantities can have environmental implications for production and usage.For instance, in the production and use of cement, 1 kilogram can emit up to 0.9 kilograms of carbon dioxide (CO2) is emitted [4], contributing up to 7% of the total industry emissions.Concrete can be further developed by incorporating other materials, such as various types of fibers, including steel, synthetic, and natural fibers, to enhance its strength and resistance to cracking while reducing the need for traditional steel reinforcement, typically mixed at a rate of 0.4% to 2% by volume of concrete.Current research in this field has often focused on the use of alternative materials and additional mixtures to reduce the use of cement or substitute materials that may be locally scarce.Materials discarded from industries or recycled from various sectors are being studied and developed for practical applications.One example is the incorporation of rubber powder derived from waste tires into concrete.This approach provides an environmentally friendly solution for rubber waste, making it reusable.The structure of RuC includes rubber particles, a concrete matrix, and steel reinforcement, and offers a sustainable solution to the problem of rubber waste disposal.Research has shown that changing the modulus of elasticity and compressive strength is significantly related to the addition of rubber granules to concrete, reducing these properties by up to 17% -25% for coarse rubber particles and 18%-36% for fine rubber particles.Therefore, to maximize the efficiency of RuC, the quantity of rubber particles added should be appropriate, as excessive amounts can significantly reduce these properties [5].
Consequently, this study aims to develop a prototype concrete material for road construction that does not require traditional steel reinforcements (temperature steel and dowel bars).This reduces the use of steel reinforcements and minimizes the formation of construction joints in the concrete.The use of crumb rubber and fibers will be explored to improve the material properties while increasing the utilization of recycled materials, adding economic value, and promoting sustainable environmental practices.The additive used in the testing was a type of Superplasticizer called Naphthalene Sulfonate (Type F), according to ASTM C494 [6], which helps increase the flowability of concrete without the need to add water, making it easier for pouring and surface finishing.Additionally, it reduces the risk of segregation, which is the separation of aggregates from the cement in concrete.
2.1.5Fibers.The end-hooked Dramix 4D 65/60BG steel fibers are shown in figure 3. The parameters of the circular cross-sectioned steel fiber parameters were: aspect ratio (length/diameter) of 65, length of 60 mm, diameter of 0.9 mm, ultimate tensile strength of 1,255 MPa, Young's modulus of 200 GPa, and density of 7.80 g/ 3 .

Mixture proportion
The design of the concrete mix proportions according to the (American Concrete Institute) for concrete with a minimum compressive strength of 350 kg/cm 2 (cylinder) is listed in table 1.The design used crumb rubber to replace fine aggregate in proportions of 10%, 20%, and 40% by volume.Another series was considered in this study by combining crumb rubber with steel fiber 0.5% by volume of concrete.Mixing was performed using a machine at a constant speed of 30 rpm.After mixing, samples of size 200 mm × 100 mm (height × diameter) samples were cast to test their compressive strength and splitting tensile strength.Additionally, samples of 150 mm × 150 mm × 500 mm (width × length × height) were prepared to test the tensile strength.All the mix proportions were cured by leaving the concrete wet for 24 h before curing in water for 3, 7, or 28 days.

Preparation of rubber aggregate
When incorporating crumb rubber into a concrete mixture, the compatibility between crumb rubber and concrete must be considered.This is owing to the differences between the volumetric properties of rubber and rock aggregates, which exhibit varying characteristics.Therefore, crumb rubber must be soaked in water for at least 24 hours to achieve saturated surface-dry conditions before being mixed with concrete [7].
2.4.1 Properties of fresh concrete.The following tests were conducted to determine fresh RuC's properties and fluidity.The slump test was performed according to ASTM C143 [8].It involves measuring the slump of the concrete, which indicates its workability.The result is obtained by measuring the drop in the center of the concrete specimen after the mold is removed.The flow test was conducted using a standard flow table as ASTM C124 [9].The test measures the concrete's flowability by observing its spread when lifted and dropped onto the table from a height of 12.7 mm.This test was performed 15 times within 15 s to assess how the concrete spreads.

Mechanical properties.
The following tests were conducted to test the mechanical properties of hardened concrete.The compressive strength test was performed on three cylindrical concrete specimens with dimensions of 200 mm × 100 mm (height × diameter) at ages 3, 7, and 28 days.This test was carried out by ASTM C39 [10].The tensile strength test was conducted on three cylindrical concrete specimens with dimensions of 200 mm × 100 mm (height × diameter) at ages 3, 7, and 28 days.This test was performed following ASTM C496 [11].The flexural strength performance test was conducted on three beam specimens with dimensions of 150 mm × 150 mm × 500 mm (width × length × height), which were tested after 28 days of concrete curing following ASTM C79 [12].

Results and discussion
3.1 Fresh properties 3.1.1Workability.Table 3 shows the slump of all the concrete mixes.The steel fiber content was 0.5% (Mix SF0.5), causing a 30% lower slump than that of the control mix.The addition of steel fibers to concrete reduces the amount of cement paste available as a lubricant between aggregate particles by increasing the internal surface area.Steel fibers increase the internal friction between concrete components, which is the cause of slump reduction.All the mixes achieved the targeted slump, and no segregation, bleeding, or excessive balling was observed in any mixture.4 shows the unit weight results for the concrete mixes.The figure indicates that adding 0.5% steel fibers by volume of concrete to mix SF0.5, did not result in a significant difference in unit weight compared to the control mixture.In contrast, when crumb rubber was included in the mix, the unit weight of the concrete decreased noticeably compared with that of the control mix.Specifically, the unit weight decreased by 55, 70, and 106 kg/m 3 when 10%, 20%, and 40% fine aggregates were used, respectively.The unit weight decreased owing to the low specific gravity of crumb rubber.They found that RuC with 0.5% steel fibers exhibited a similar decrease.When 10%, 20%, and 40% of the fine aggregate was replaced with crumb rubber in a mixture containing 0.5% steel fibers and crumb rubber, the unit weight decreased by 20, 40, and 75 kg/m 3 respectively.The unit weight reduction owing

Test procedure
to the addition of rubber was comparable in concrete with and without steel fibers.This reduction in unit weight may be advantageous for various structural and nonstructural applications involving concrete with rubber content.

Hardened properties
3.2.1 Compressive strength.From the compressive strength tests conducted on cylindrical samples at ages of 3, 7, and 28 days, the results shown in figure 5 were analyzed.The compressive strength test results of the concrete at 28 days for the average compressive strength of normal concrete were found to be 354.1 ksc.The mix containing steel fiber 0.5% by volume of concrete (Mix SF0.5) was found to have a considerable improvement in compressive strength, with the highest increase compared to the control mixture, with a 23% increase at 3 days, 3% at 7 days, and 4.7% at 28 days.The decline from including crumb rubber offsets the increase in compressive strength owing to this enhancement.For RuC with steel fibers, the compressive strength decreased by 4.2%, 22.0%, and 49.0% when the replacement ratios were 10%, 20%, and 40%, respectively.In contrast, the reduction rates for rubberized concrete mixtures without steel fibers were 6.0%, 28.0%, and 38.0%, respectively.A comparison of the reduction rates between the two groups revealed that crumb rubber tended to decrease the compressive strength of both regular and steel-fiber-reinforced concretes at a similar rate.This reduction in compressive strength occurred because of the interfacial transition zone (ITZ) that formed between the rubber particles and cement paste [13].The ITZ is a region of weakness that occurs around the perimeter of rubberized concrete.7, the tensile strength test results indicate a reduction in the tensile strength of concrete when rubber particles were used to replace fine aggregate in all proportions.The decreases in the tensile strength of the concrete, as presented in table 4, were 3.6%, 7.2%, and 20% compared with the tensile strength of the control concrete.The data indicated a positive correlation between rubber aggregates and steel fibers.When fibers are added to concrete, their tensile strength increases.As shown in table 4, the strength increased by 73%, 62%, and 58% when comparing the RuC-incorporated steel fiber mixes SF0.5R10, SF0.5R20, and SF0.5R40 to RuC mixes without fibers R-10, R-20, and R-40.However, the effect is even greater when fibers are added to rubberized concrete.This is likely because rubberized concrete has a low strength, making it easier for steel fibers to improve their strength when a lower stress is present.This is particularly true when concrete contains rubber aggregates.3.2.3Flexural strength.Bending loads cause critical stresses in pavement structures.Flexural strength (also known as the fracture modulus) is used as a decisive assessment factor for pavement strength [14].
The flexural strengths are compared in figure 7, indicating a reduction in the flexural strength of the concrete when rubber particles were used to replace the fine aggregates in all proportions.The decreases in the flexural strength of the rubberized concrete, as presented in table 4, were 3.6%, 7.2%, and 20%, respectively, when compared to the flexural strength of the control mix.The inclusion of steel fibers in RuC led to a significant increase in flexural strength.Thus, it can partially reduce the negative impacts of using recycled rubber particles to replace natural aggregates.When comparing the flexural strength of rubberized concrete incorporated with steel fiber mixes SF0.5R10, SF0.5R20, and SF0.5R40 with the flexural strength of the RuC mixes without fibers R-10, R-20, and R-40, the flexural strength increased by 32%, 62%, and 132%, respectively.Flexural strength (ksc)

Mix ID
The results of the study of the properties of rubberized concrete incorporated with steel fibers, in which waste tire rubber is replaced as a fine aggregate and blended with steel fiber, by testing its compressive, tensile, and flexural strengths can be summarized as follows: • The average unit weight decreased by 2.9%, 3.4%, and 4.3% of the control mixture when 10%, 20%, and 40% crumb rubber was incorporated into the concrete mixtures, respectively.A similar trend was observed for mixtures with the same replacement ratios when steel fibers were added to rubberized concrete.• The inclusion of rubber aggregates led to a reduction in the compressive strength, with higher reductions at greater rubber replacement percentages.Slight improvements in the compressive strength were obtained after using steel fibers (0.5% by volume) improves compressive strength, with notable increases of 23% at 3 days, 3% at 7 days, and 4.7% at 28 days.• The splitting tensile strength was reduced by increasing the crumb rubber content.The tensile strength decreased by 3.5%, 7.0%, and 20% for the rubber particle contents of 10%, 20%, and 40%, respectively.However, using steel fiber 0.5% in the rubberized concrete mix enhanced the indirect tensile strength by 73.6%, 61.8%, and 58.7%, respectively.• Their study revealed that when the rubber particles were increased by 10%, 20%, and 40%, the flexural strength decreased by 5.30%, 24.40%, and 49.90%, respectively.However, the incorporation of 0.5% steel fibers resulted in significant improvements in the flexural strength, with enhancements of 32.06%, 62.41%, and 132.07%, respectively.This study highlights the importance of incorporating of steel fibers into RuC to enhance its mechanical properties for rigid pavement applications.By blending waste tire rubber as a fine aggregate substitute with steel fibers, improvements in the compressive, tensile, and flexural strengths were observed.This combination not only elevated the structural integrity of the pavement but also presented a sustainable solution for managing rubber waste.The addition of steel fibers offsets the typical reduction in strength associated with rubber inclusion, demonstrating a viable method for improving both the environmental impact and economic value of concrete pavements.This approach represents a promising advancement in concrete technology, combining sustainability and performance in construction materials.

3 2. 1
Materials 2.1.1Cement.The material used in this study was general-purpose hydraulic cement (GU) composed of cement clinker, gypsum, calcium components, and additives that provide high compressive strength, making it suitable for construction projects that require strength and durability.Furthermore, they are environment-friendly. 2.1.2Natural aggregates.The coarse aggregate (CA) material with a maximum size of 19.05 mm (3/4") has a specific gravity of 2.60.The fine aggregate (FA) material had a fineness modulus (FM) of 2.8, and the specific gravity under dry surface conditions was 2.59.The fine and coarse aggregates exhibited the distribution characteristics shown in figure 1.

Figure 4 .
Figure 4. Unit weight of all concrete mix.

Figure 6 .
Figure 6.Splitting tensile strength of the tested concrete mixes.

Figure 7 .
Figure 7. Flexural strength of the tested concrete mixes.

Table 1 .
Chemical properties of crumb rubber.

Table 3 .
Concrete mix proportions

Table 4 .
Mechanical properties of all concrete mixes.