Sun Xuewei, Gao Peiwei, Zhang Wanlei
Nanjing University of Aeronautics and Astronautics Zhonglu Jiaotong Technology Co., Ltd
Abstract: The accumulation of discarded tires has become an environmental hazard. This paper reviews the research status of the working properties (slump, air content, unit weight), mechanical properties (compressive strength, tensile strength, fracture toughness, fatigue performance) and durability (freeze-thaw resistance, chloride ion penetration resistance) of existing rubber concrete, summarizes the influence of rubber morphology and dosage of rubber in rubber concrete on concrete, and analyzes the application of rubber concrete in pavement engineering. The results show that the waste tire is crushed and processed into rubber particles with a certain shape, size and gradation and mixed into concrete as aggregate, and the good elasticity and deformation ability of waste tire rubber and its alkali corrosion resistance can not only open up a new way for waste rubber treatment, but also improve some properties of concrete, including slump, deformation ability, impact toughness and durability. At the same time, some suggestions are put forward for the related research and popularization of rubber concrete.
Keywords: waste tires; rubber concrete; working performance; mechanical properties; engineering applications;
About author:SUN Xuewei (1981-), male, from Dongtai, Jiangsu, Ph.D. candidate, senior engineer, research direction: new materials, new technologies, structures, traffic safety research of highway and water transportation engineering.
1 Introduction
With the rapid development of the automobile industry, the accumulation of a large amount of rubber waste has gradually evolved into an environmental problem that cannot be ignored. According to the World Health Organization, more than 50% of the 1.5 billion car tires produced worldwide each year are discarded or buried untreated [1]. The storage of waste tires not only occupies land resources, but also the large-scale and long-term accumulation of tires is easy to cause fires. When tires burn, they release volatile gases, heavy metals, and other harmful compounds. In addition, the stock provides a breeding ground for rats, mosquitoes, and other pests.
Many people have proposed solutions to the problem of waste tire inventory, such as full tires can be used as tire packs for road embankments and retaining walls, and granular rubber can be added to asphalt binders for asphalt pavement. Due to the better slip resistance, fatigue cracking resistance, and durability of rubber asphalt, the school replaced grit with processed waste tires on the playground. Tire chips can be used for insulation and as a substitute for soil/aggregate materials in civil engineering applications.
In the 90s of the 20th century, the use of recycled waste tires expanded to a relatively new product called rubber concrete [2]. Rubber concrete uses Portland cement as the binder. Studies have shown that rubber concrete has a very positive application prospect, such as paving pavement. Kardos A J [3] investigated the feasibility of using commercially processed rubber powder to replace some fine aggregates in P-grade pavement concrete mixtures in the state of Rollado, USA, with substitution volumes ranging from 10% to 50%, and tested the working and mechanical properties of the concrete. According to the test results, the mixture with 20% and 30% replacement ratios can fully meet the requirements of Class P pavement, and the recovered waste tire pellets do not show any abnormal increase in strength under different replacement amounts. At the same time, the environmental performance of the rubber-concrete mixture was tested by leaching test, and the results showed that the material did not pose a threat to human health.
Concrete grade requirements are currently given in the road and bridge construction code guidelines. Rubber concrete is usually designed at a low slump and is used in slipform pavers or curbs and gutters. The maximum aggregate size varies from 20mm~40mm, depending on the pouring form and whether the transverse reinforcement is configured. The fracture modulus (flexural strength) of 28d pavement concrete measured in the field is 4.5MPa, and the value measured in the laboratory is 4.8MPa. The in-situ compressive strength at 28 days of age is 31MPa. The minimum amount of cementitious material is 236kg, and the maximum water-glue ratio is 0.44. Concrete requires 4%~8% air to ensure good durability and resistance to freeze-thaw cycles. The rubber-concrete mixture should have the same range of air content to have good freeze-thaw resistance.
Rubber concrete has a wide range of application prospects in pavement engineering, and it has very practical engineering significance to comprehensively analyze its working performance, basic mechanical properties and comprehensive mechanical performance under dynamic load. This paper summarizes the research status of the working properties and mechanical properties of rubber concrete, provides a reasonable direction for follow-up research, and plays an important role in the promotion and application of rubber concrete in road engineering.
2 Basic material properties of tire rubber
Tires are made from natural and synthetic rubber elastomers derived from a mixture of oil, gases and metals. The addition of other ingredients, such as carbon black, polymers, steel, and additives, can improve the performance of the tire [4]. Table 1 summarizes the basic properties of tires and compares them with those of mineral aggregates.
Table 1 Comparison of basic properties of tire rubber and mineral aggregate Download the original image
The specific gravity of tire rubber is estimated according to the standard test method of density, relative density (specific gravity) and coarse/fine aggregate absorption in the "Test Regulations for Highway Engineering Aggregates" (JTG E42-2005) [5]. The gravity characteristics of tire fragments are significantly different from those of sand and gravel coarse and fine aggregates, the specific gravity of tire fragments is greater than 1, and they will not float when immersed in water, but the rubber particle fragments will float on the water surface and will not discharge water, which will have a certain impact on the mixing mechanism of rubber concrete. The specific gravity of tire rubber is less than half of the mineral aggregate, and during transportation, the volume of rubber coarse aggregate transported each time will be 2~2.5 times the volume of sand and gravel coarse aggregate. The dead weight of rubber concrete is also correspondingly reduced compared to ordinary concrete.
The modulus of elasticity is the ratio between the applied stress and the measured strain, reflecting the ability of a material to resist deformation. The modulus of elasticity of sand and gravel ranges between 42MPa~84MPa, while gravel is much larger. The elastic modulus of tire rubber is much lower as compared to grit. In concrete, tire rubber behaves as weak inclusions. Researchers have established some theoretical models to explain the compressive failure mode of rubber concrete cylinders [6], and thus established a mathematical expression of the relationship between the strength of rubber concrete and the amount of rubber [7]. The Poisson's ratio of tires is 0.5, which is close to the performance of mineral aggregates, and there are no reports on the relationship between Poisson's ratio and the properties of rubber concrete.
In summary, making waste rubber tires into rubber particles or powder through mechanical shearing is an important form of rubber waste reuse into cement-based materials. The addition of rubber particles can make concrete obtain many excellent properties, such as reducing the weight of concrete, improving concrete slump, increasing concrete fluidity, improving toughness, sound reduction and heat insulation, fatigue resistance, wear resistance and crack resistance, impermeability, freeze-thaw resistance and other characteristics.
3 Working properties of rubber concrete
Slump, air content, and unit weight are common indicators to evaluate the working performance of concrete. Raghvan D et al. [8] found that a mortar containing rubber particles obtained similar or better workability compared to a control mortar without rubber particles, while other researchers found that slump decreased as the rubber content increased. Khatib Z K et al. [9] noted that when rubber accounts for 40% of the total aggregate volume, the slump of the mixture is almost zero. The working performance of fine-grained rubber concrete is better than that of coarse-grained rubber concrete. Tire particles will produce a rough surface during processing, and during the concrete mixing process, the rough surface is easier to absorb air, so the air content in rubber concrete is higher than that of rubberless ordinary concrete. At the same time, rubber is a hydrophobic material, and the surface is not easy to absorb moisture, so that more air adheres to the rubber particles. As the percentage of rubber content in the total aggregate volume increases, the specific gravity of rubber particles is much lower than that of mineral aggregates, resulting in a decrease in the unit weight of concrete. The increased air content due to the increase in rubber further reduces the unit weight of the concrete.
Therefore, the effects of rubber particles on the working properties of concrete are summarized as follows: the slump and unit weight of concrete mixture decrease with the increase of rubber content; The air content in the mixture increases with the increase of rubber content.
4. Mechanical properties of rubber concrete
4.1 Compressive and tensile mechanical properties
Size, surface texture, and rubber content affect the compressive and tensile strength of rubberized concrete. Eldin N et al. [6] pointed out that when the coarse aggregate is replaced by 100% tire fragments, the compressive strength is reduced by about 85% and the splitting tensile strength is reduced by 50%. Rubberized concrete exhibits ductile failure under compressive and tensile loads and is able to absorb more energy [10].
The compressive strength and tensile strength of rubber concrete decrease with the increase of tire particle content, as shown in Table 2. The main cause of strength loss is the poor adhesion of the cementitious product to the surface of the rubber particles. Chemical treatment of rubber particles can improve the bonding properties of the interfacial transition zone (ITZ) between the rubber aggregate and the cementitious material. These methods include pretreatment of polyacrylamide, pretreatment of pressure-aging vessels, pretreatment of silanes, sodium hydroxide immersion, magnesium chloride cement, etc. [4]. The compressive strength of concrete pretreated with rubber particles is 16%~57% higher than that of concrete containing untreated rubber aggregates.
Table 2 Relationship between the strength of rubber concrete and the content of rubber particles Download the original figure
4.2 Fracture toughness and fatigue properties
Toughness is the energy absorption capacity of the specimen and is defined as the area under the load-deflection curve of the bending specimen. Rubber concrete is able to withstand additional loads after the ultimate load and has higher toughness than concrete without rubber particles. With the increase of rubber content, the rubber concrete samples showed a trend of gradual failure rather than brittle failure.
Many scholars have done relevant research on the fracture properties of rubber concrete. Liu F et al. [17] used rubber particles with diameters of 25 mm, 50 mm, and 75 mm, and the incorporation ratios were 5%, 10%, and 15%, respectively, to study the bending test of rubber concrete, and the results showed that when the rubber content was 5% and the rubber particle size was 50mm, the fracture energy of the material was the largest, but it was only about 5% higher than that of ordinary self-compacting concrete. Luo Surong et al. [18] measured the load-displacement curve and crack opening displacement of the three-point flexural tensile specimen, and applied the double-K model of fracture mechanics to calculate the fracture energy and fracture toughness.
The improvement of the toughness of rubber concrete is also beneficial to improve its load-bearing capacity in the fatigue state. Liu Miaoyan et al. [19] used acoustic emission technology to carry out three-point bending beam fracture tests on rubber concrete with different rubber content under fatigue and static load loads, and studied the relationship between the damage degree and acoustic emission characteristic parameters of rubber concrete under fatigue and static load loads. The degree of damage of rubber concrete can be judged by parameters such as the amplitude, energy and accumulated energy of acoustic emission. The results show that [20] shows that the damage process of concrete can be divided into three stages for the analysis of the amplitude change under fatigue load, and the damage process of concrete can be divided into two stages for the analysis of the cumulative energy change. Under the action of static load, the damage process of concrete can be divided into two stages according to the analysis of amplitude parameters, and the damage process of concrete can be divided into three stages according to the analysis of energy parameters. The acoustic emission signal increases under fatigue load due to the increase of rubber content, while under static load it decreases due to the increase of rubber content. Incorporating rubber into concrete not only improves the concrete's ability to withstand damage, but also significantly increases the crack-blocking ability of concrete.
In summary, the fracture test of rubber concrete shows that the crack resistance and fatigue performance of the specimen can be significantly improved by adding rubber to the concrete. The existing studies mainly analyze the fracture parameters at the macroscopic level, while the interaction and mechanism between rubber particles and other parts of concrete need to be systematically studied.
4.3 Rubber concrete durability
There is less literature on the durability of rubber aggregate concrete. Savas B Z et al. [21] studied the rapid freeze-thaw durability of rubber concrete, in which the specific gravity of rubber mixtures was 10%, 15%, 20%, and 30%. After 300 freeze-thaw cycles, the durability coefficients of the mixture with rubber particle content of 10% and 15% were greater than 60, while the durability coefficient of the mixture with rubber particle content of 20% and 30% were less than 60. The weight loss rate of each mixture increases with the number of freeze-thaw cycles. Studies conducted by Paine A et al. [22] suggest that rubber particles may be used as antifreeze/thawing agents in concrete.
Concrete samples with good chloride permeation resistance will pass the 1000~2000 coulombs (low permeability) test method of ASTM C1202 Electro-Indication Test Method for Chloride Ion Permeation Resistance of Concrete. Gesoglu M et al. [23] evaluated the effect of chloride ion penetration in silica fume-containing rubber concrete, and the results showed that rubber can significantly inhibit the penetration of chloride ions, and the use of silica fume can significantly reduce the permeability of chloride ions, especially for rubber concrete.
5 Application of rubber concrete
Due to the stable chemical properties of rubber and its stable existence under strong alkali conditions, the application of waste rubber in concrete structures has a good prospect, and rubber concrete has been widely used in various types of construction, and the results are remarkable.
Rubber concrete has good impact resistance and fatigue properties, and is very suitable for pavement engineering. The application of rubber in road engineering can not only improve the fatigue life of concrete pavement, but also play a role in sound absorption and noise reduction. At present, a typical practical engineering case for the construction of rubber concrete pavement is a load-bearing pavement paved by rubber concrete in Gudino, Spain in 2000 [24]. In order to solve the problems of expansion and contraction of ordinary concrete pavement, the Florida Department of Transportation (FDOT) replaced fine and coarse aggregates with granular rubber particles, which made the concrete pavement elastic, solved the expansion and contraction cracks, and prolonged the service life of the pavement [25].
Due to the good toughness and seismic performance of rubber concrete, the use of this material as the basic material of railway sleepers can greatly improve its service life. At present, in 2003, Qingdao Luye Rubber Co., Ltd. of China and Maple Leaf Holding Group Co., Ltd. of Canada jointly carried out a sleeper project using rubber concrete as the laying material [26].
In addition, rubber concrete can also be used as an environmentally friendly and energy-saving material with good sound absorption and heat insulation in the construction of civil buildings. At the same time, airport runways, impact barriers, sports fields, buildings in the nuclear industry and important military buildings can be paved or constructed with rubber concrete. It is of great significance to recycle waste rubber and rationally use it in engineering construction to alleviate the environmental problems caused by the rapid social and economic development of the mainland.
6 Conclusion
The number of waste tires is increasing year by year, and the accumulation and storage of tires will consume more and more land resources, and they are vulnerable to fire, which is a potential threat to the environment and human health. This paper investigates the potential for reuse of tire fragments as pavement concrete aggregates. Rubber concrete does not significantly reduce the cost of concrete, nor does it reduce the environmental impact of concrete itself, but it helps to eliminate the inventory of scrap tires and reduce the potential threat to the environment from inventory. In this paper, the typical working properties and hardening properties of rubber-concrete mixtures in existing studies are summarized, including slump, air content, and unit weight, and hardening properties include compression, bending, splitting tensile strength, fracture toughness, permeability, and freeze-thaw resistance. The properties of rubber concrete are summarized as follows:
(1) With the decrease of rubber content, the slump increases. When replacing 10% coarse aggregate with tire pellets, tire pellets have no effect on the workability of concrete. The working performance of a mixture mixed with tire particles or low cement content is significantly reduced.
(2) With the increase of tire particle content, the air content generally showed an increasing trend. Regardless of the cement content, the unit weight decreases linearly with the increase of rubber aggregate content.
(3) When the replacement amount of coarse aggregate is 10%, the compressive strength decreases by 32%, and the larger the replacement amount, the greater the decrease in compressive strength. The content of cement and rubber particles had an effect on the compressive strength of rubber concrete. Concrete with low cement content has low compressive strength. As the particle content of the tire increases, the compressive strength decreases.
(4) With the decrease of tire particle content, the bending strength increases. Concrete with a low amount of cement can withstand additional bending loads after cracking. When the amount of coarse aggregate is replaced, the splitting tensile strength decreases by at least 18%, and with the increase of rubber particle content, the splitting tensile strength is further reduced.
(5) Under the action of compression, bending and splitting load, the deformation of concrete mixed with rubber particles is greater than that of ordinary concrete, and the bending toughness is also improved.
(6) The chloride ion permeability of rubber concrete at 28 days of age is higher than that of ordinary concrete, and the influence of air mixture on the permeability of rubber concrete is greater than that of rubber particles. Rubber concrete has good freeze-thaw resistance.
The existing research results show that the ITZ adhesion between rubber particles and concrete matrix is an important factor affecting the mechanical properties of rubber concrete, and the existing treatment methods are not ideal for improving the bonding performance, and the toughness of rubber particles has not been fully utilized, and more economical and reliable interface improvement methods need to be further studied. At the same time, the existing studies mainly focus on the influence of rubber particle content and particle size on the related properties of concrete, but there are few studies on the relationship between the characteristics of rubber particles and the properties of concrete, which also leads to the practical engineering application of rubber concrete.
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