Concrete, waste tire rubber, ecological materials, mechanical properties, microwave absorption, gamma radiation.
||August 6, 2018
||The Author(s) 2018. This article is published with open access at www.chitkara.edu.in/publications.
The main raw material for the construction industry is concrete; whose fundamental components are the fine and coarse aggregates, water and cement. For the obtaining of these materials are necessary activities that generate environmental deterioration, since the aggregates are extracted from quarries or river banks and for each ton made of cement is emitted into the atmosphere a great lot of carbon dioxide. In this way, the present work is developed with the purpose of contributing to the research that can help the conservation of basic natural resources through the use of waste polymers such as waste tire rubber, in the production of concrete, hoping to reduce its harmful environmental impact. This work focuses on the one hand, in the study of the effects of the incorporation to the concrete, of different proportions of scratched rubber coming from waste tires, on its mechanical properties and on its capacity for microwaves absorption. On the other hand, it is also studied the effect of aging by applying different doses of gamma radiation on the before mentioned properties, seeking with this the possibility that it can be used in the construction industry either as structural material or as a coating. Replacements were made between 5% and 25% of rubber in order to do not significantly affecting the mechanical properties of the concrete. The results of the mechanical and microwave tests performed on the different samples with different gamma radiation doses were compared and it was found that open the possibility of research with great benefits such as the use of waste tires in the designing of concrete mixtures and the improvement of its properties. It is considered important to point out the economic benefit in the context of sustainable development, which involves solving the problem of environmental pollution caused by waste tires, to achieve the welfare of the population by improving their quality of life.
1.1 Pollution by Concrete Production and Waste Rubber Tires
Concrete is one of the most important materials within the construction industry, this is due not only to its relatively low cost but also to the properties it shows such as easy placement before setting, resistance to weathering, durability, among others. For this reason, it is widely used around the world; however it has a great impact on the environment since it requires a lot of natural resources for its manufacture (sand, gravel and water). Besides, Portland cement production generates carbon dioxide that is released into the atmosphere, it is estimated that the cement industry is the responsible for the emission of one tone of CO2 per year worldwide .
In the same way, the use of rubber tires and their subsequent disposal as waste rubber tires or recycling is a serious problem for the environment. While European countries such as Germany, France, Austria recycle up to 60% of their used tires, in Mexico there is practically no such recycling, due to low environmental awareness and a non-existent control system. In fact, it is estimated that more than 30 million tires are discarded annually and only 10% is recycled and the remaining 90% end up in vacant lots, rivers and roads. Tire trailers are not only unsightly and fire risk sites, but also represent a health risk as breeding grounds for harmful fauna such as rats, cockroaches, flies and mosquitoes, which can transmit diseases. Some of these fires can last for months and even years [1, 2], so the disposal of tires in landfills is often prohibited. As a result, the number of illegal trailers has increased with the consequent environmental and health problems, since in case of fire, the air becomes highly toxic and contaminates the subsurface freezing mantles; Likewise, these sites become breeding grounds for harmful fauna such as rodents, cockroaches, snakes, spiders, scorpions, mosquitoes, etc., carriers of diseases or that represent a danger to human beings, and contribute to the deterioration of the urban image.
Currently, the renewal industry has made significant progress in the tire recycling process, achieving the separation of the three basic components: Textile fiber, steel and elastomeric. Probably the most used method for recycling tires is the reuse of these after undercutting (elimination of steel wires) [3, 4]. For example using it in bituminous mixtures in hot powder or edge for the modification of binders in asphalt pavements [6, 7, and 8]. On the other hand, the main method to eliminate waste tires is to burn them to produce steam, electricity or heat that is, using them as an alternative fuel such as in cement kilns. This is a widespread practice in the United States and Europe  and is very beneficial for the industry since the cost as a fuel is considerably lower than the original material. However, several studies have shown that the burning of tires releases substances of maximum danger to humans, such as carbon monoxide, furans, toluene, benzene and lead oxide .
In order to reduce the ecological impact that this entails, efforts have been made to reduce the consumption of nonrenewable natural resources in the manufacture of concrete. A viable option for the solution to the problem caused by scrap tires is the reuse of the tire elastomeric in the concrete as a substitute for the fine or coarse aggregate, procuring the quality of the product and, if possible, seeking the increase in desirable properties such as strength, the elasticity module and the durability. In this sense, concrete is the mixture of fine and coarse aggregates with a cementing paste composed of Portland cement and water, which, once set, forms a monolithic mass similar to rock. The paste hardens due to the chemical reaction between water and Portland cement, generating hydration products. The fine aggregates are natural or synthetic sands with maximum sizes of up to 10 mm and the coarse aggregates are rocks or gravels with maximum sizes of up to 152 mm. Both aggregates constitute between 60 and 75% of the total volume of the mixture, so they can limit the properties of the concrete and influence its performance. However, these properties depend mainly on the quality of the paste formed by water and cement, as well as the affinity they have with the aggregates and their ability to work together.
1.2 Gamma Radiation Effects
Gamma radiation (γ) is a type of high-energy electromagnetic radiation formed by photons and generally produced by radioactive elements or subatomic processes such as the annihilation of a positron-electron pair. This type of ionizing radiation of such magnitude is capable of penetrating matter more deeply than alpha or beta radiation . When a polymeric material is subjected to ionizing radiation, physical and chemical effects are produced which are a function of the nature of the polymers, as well as the radiation dose. The advantage of working with this type of macromolecules is the great sensitivity to changes in the chemical bonds, obtaining different properties in terms of crystalline, density, coefficient of thermal expansion, modulus of elasticity, permeability, as well as in the resistance to corrosion, abrasion and solvents.
With gamma radiation it is possible to modify the chemical structure of the particles or polymeric fibers through three processes: splitting or scission of chains (cross-linking) and grafting (graphing). These mechanisms contribute, to a greater or lesser extent, to modifying the mechanical behavior of polymers. The influence of each process depends on the amount of energy absorbed per unit mass (dose), the concentration, the dispersion, the atmosphere (inert or air) and the temperature at which the irradiation is carried out, as well as the post-treatment . It is reported that physical and chemical effects can be expected when a polymeric material is subjected to ionizing radiation. The polymeric macromolecules are very sensitive to changes in its chemical bonds which can result in different properties in terms of crystalline, density, coefficient of thermal expansion, modulus of elasticity, permeability, as well as corrosion resistance, abrasion and solvents. Such effects depend on the nature of the polymers as well as the dose of the radiation. Based on the aforementioned, the objective of this research work is the study of the effects of the incorporation to the concrete, of different proportions of scratched rubber coming from waste tires as the effect of aging by applying different doses of gamma radiation on its mechanical properties and on its capacity for microwaves absorption.
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Great efforts have been made in environmental matters to reduce pollution, such as the present investigation, which contemplates the use of scrap tires as reinforcement material for concrete. In general improvements are observed in the mechanical properties of the concrete when the tire recycling is added, something desired for the concrete; however, the treatment based on Gamma rays allows observing the passage of time in the material and the properties within the mixture, with results superior to the concrete witnesses. This represents a solution to the drawbacks of using these materials within the construction industry. Finally, the combination of the low cost of the waste material and the increase in the mechanical properties of the concrete, make this an attractive product with not only economic benefits, but environmental ones.
The stress vs. strain curves of the samples at its different rubber content shows two slopes where the first one corresponds to the “elastic” behavior previous to the microcracking which is observed as a change in the slope of the line. Samples with no rubber additions exhibits a fragile behavior since once the crack appears, a rapid fragmentation and collapsing of the material is observed. Samples with rubber additions exhibits an “elastic” behavior since once the crack appears, a slow fragmentation and slow collapsing of the material is observed. This is presumably due to the long rubber scratches, acts like “anchors” maintaining together the fragmented pieces and offering an additional resistance to the collapse of the sample and still charging. It is assumed that gamma radiation dose affects the mechanical properties of the samples being such effect major in the samples without rubber and in the sample with 5 vol. % rubbers because they show the highest fracture stress values. The microwaves in the non-radiated samples behave according to Lambert’s law, where the energy absorbed by the material is a function of the penetration distance and an attenuation factor. All this as a function of the percentage of rubber. The radiated samples have an ups and downs behavior for the different percentages of rubber in the sample.
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