Investigating the physical and mechanical properties of concrete containing rubber crumb

Introduction

In today's era, the existence of waste materials resulting from various physical and chemical processes is one of the important problems of industrialized and developing countries. So that extensive research is being carried out for methods of recycling or disposing of them to minimize the damage to the environment. In this regard, construction researchers, like other manufacturing and recycling industries, have made progress in extracting waste materials. Among them, we can mention the use of sawdust in the production of light bricks, the use of industrial slags and slags from metallurgical operations, and many other materials. One of the things that enters the environment as non-recyclable materials is used car tires. In the current research, the possibility of using used tires in concrete is investigated.

Research shows that used tires are made of materials that cause environmental pollution due to their non-degradation under normal conditions. One of the ways to decompose them is to burn.

But the gas from burning causes a lot of pollution. Based on the research, one of the ways to remove these materials is to use them in concrete. From a macroeconomic point of view, when using these materials in concrete, the following issues should be considered:

Collection, preparation and transportation costs

Environmental pollution.

Saving stone materials used in concrete by replacing them with stone materials in the construction industry,

Concrete is considered as one of the most important and most consumed construction materials. The concrete used in any of the structural and non-structural materials, such as sewer pipes, building foundations, tables adjacent to aqueducts, beams and columns, overflows, nuclear power plants, sidewalks and many other cases, is necessary to have specific characteristics and characteristics appropriate to their application.

Therefore, the role of the type of material added or replaced in concrete should be determined in terms of changing the physical, mechanical and durability characteristics. The expansion of the automobile industry in the world has led to the parallel increase in tire production in the world. This has made the existence of used tires and their disposal methods one of the most important problems. Apart from the environmental problems caused due to lack of decomposition, it provides a suitable environment for the growth of mosquitoes. Since the early 1990s, a lot of research has been done to recycle these waste materials. Using it in asphalt and concrete is considered as a good option.

In this research, the possibility of using used tires in concrete is investigated. In order to understand the performance of concrete, it is necessary to briefly discuss some of the most important physical, mechanical and durability characteristics of concrete. Then, the results of the research, which includes the following tests, are presented to check the characteristics of concrete containing rubber:

compressive strength, tensile strength, bending strength

1-2- research objectives

making concrete with rubber crumb

examining the compressive, tensile and bending strength of concrete

investigating the possibility of using concrete with rubber crumb in Special structures such as industrial foundations

1-3- Properties of concrete

1-3-1- Compressive, tensile and bending resistance

Compressive resistance is one of the most important and useful properties of concrete. In most construction applications, concrete is used due to its high compressive strength. In general applications, the amount of compressive strength is usually used as a criterion for measuring other properties of concrete, such as tensile strength, bending strength, etc. However, no exact relationship between compressive strength and tensile strength, modulus of elasticity, wear resistance or permeability has been stated.

But a series of statistical relationships stated in certain cases can be used in engineering applications to some extent. Despite this, in cases where other properties of concrete are of particular importance, relevant tests should be performed and one should not rely solely on the compressive strength results.

For example, as the sample size decreases, its compressive strength increases, while the modulus of elasticity decreases. Therefore, in engineering applications, the modulus of elasticity should be obtained based on the test method standard.

One of the ways to check the strength of materials is to use fracture mechanics. Fracture mechanics investigates the stress and strain behavior of brittle and homogeneous materials.Concrete can be considered as a brittle material. Although it shows a little plastic behavior. On the other hand, it is difficult to consider concrete as a similar material. Because its properties are different and somewhat isotropic in different applications. But despite this failure mechanics helps a lot in understanding the behavior of concrete.

Griffiths theory is used to justify the failure in brittle cases. According to this theory, the applied stress is concentrated at the tips of cracks and other defects and the maximum stress accumulates at the crack tip. The larger the crack and the smaller the tip radius, the greater the stress concentration [1]: (1-1) where c is half of the central crack (inside the part) and is equal to the length of the surface crack, r is the crack tip radius, the applied stress and the maximum stress at the tip. It is Turkish. When the stress reaches the amount of stress required to break the piece, the piece breaks: (1-2) Fracture resistance of brittle objects, w is the work required to break and E is the modulus of elasticity. The concentration of tension is in three directions. But the weakest state of material resistance is when the crack direction is perpendicular to the stress direction. Therefore, other factors, such as the orientation size and the shape of the sample are also effective.

For a larger sample, there is a higher probability of failure and cracking. In fully brittle materials, the energy released during fracture is sufficient to continue the fracture. Concrete cannot behave as a brittle material due to the presence of voids caused by unhydrated cement or other elastic materials in concrete. In this case (tensile stress), crack growth is at the interface of large rocks.

When loading, although the applied forces are compressive, but these forces produce tensile stress at the tips of internal cracks and other defects, which causes the crack to spread. In the uniaxial tensile state, failure occurs in the plane perpendicular to the direction of stress. In uniaxial compressive state, the fracture occurs in two parallel planes to apply stress. This is due to the creation of tensile stress due to the application of compressive stress applied in the direction perpendicular to it at the crack tip. In this case, cracks with the same angle as the applied stress are created, and due to the increase in the number of shear planes, failure occurs in them. This is the failure pattern when the applied stress is direct. During the test, due to the friction created between the concrete surface and the jaw (steel plates), in addition to the axial stress, the horizontal stress component is also created. To solve this defect and reduce the effect of friction, a sample with a length/width ratio greater than 2 is used.

1-4- Effective factors in concrete strength

Although porosity is the most important factor in concrete strength, but due to the fact that porosity can be measured or calculated in solid engineering (because the degree of hydration is not determined correctly), the most important factors in concrete strength are the ratio of water to cement, the degree of quality density. Aggregate (granulation, resistance and its surface) is aggregate size, age and temperature. Below is the effect of each of these factors:

1-4-1-water-cement ratio

The strength of concrete depends on the strength of cement paste. The strength of cement increases with an increase in the amount of cement and decreases with an increase in the amount of water and air. In 1918, Mr. Abrams expressed the following classic formula for concrete strength: (1-3) where x is the volume ratio of water to cement and the results for 28 days express the values ??of A and B as 7 and 14000 psi, respectively. This formula is a special case of the original feret formula, which expresses the strength of concrete in terms of its components: (1-4) K is constant, vc is the volume of cement, vw is the volume of water, and a is the volume of air. In this formula, the degree of compression is included because it indirectly refers to the volume of air filling the holes. In the case where vibration is done, a lower water-cement ratio can be used and concrete with higher strength can be made. For the case where the vibration is done manually, we need more water-to-cement ratio.

1-4-2- Age

With the increase of concrete age, hydration reactions increase, and as a result, the adhesion of concrete components to each other increases.