Text 2 . MATERIALS SCIENCE: PLASTICS

Text 1. BUILDING MATERIALS IN CONSTRUCTION

Introduction

Materials that are used for structural purposes should meet several requirements. In most cases it is important that they should be hard, durable and easily fastened together.

Main Body

1. The most commonly used materials are wood, stone, brick, concrete, steel, glass, plastics, etc. They all differ in hardness, durability, strength, weight, fire- and decay-resistance and, naturally, cost.

Wood is the most ancient structural material. In compar­ison with steel wood is lighter, cheaper, easier to work with and its mechanical properties are good. On the other hand, wood has certain disadvantages. First, it burns and is there­fore unsuitable for fire-proof buildings. Second, it de­cays.

Stone belongs to one of the oldest building materials used by man. Stone is characteristic of many properties. They are mechanical strength, compactness, porosity, sound and heat insulation, and fire-resistance. Stone is widely used for foundations, walls and steps of buildings, for the supports of piers, and bridges, and for finishing and decorating all sorts of structures.

Bricks were known many thousand years ago. Bricks are hard and easily fastened together with the help of mortar. A brick building is strong, durable and weather resistant.

Concrete is referred to as one of the most important materi­als. Mass concrete was employed by the Egyptians and the Romans but the use of steel reinforcement did not begin until the nineteenth century. Concrete is a mixture of cement, sand and crushed stone, made into a paste with water. It forms a hard, durable mass and is used largely for the foundations and walls of houses and for structures under water.

Steel has come into general use with the development of industry. Its manufacture requires special equipment and skilled labour.

Glass and plastics are also widely used nowadays in the construction of different kinds of buildings. The raw mate­rials employed in the manufacture of glass are limestone, sand, soda ash, sodium sulfate, cullet (broken glass), and a small amount of aluminia. Glass is unaffected by gasses and most acids.

Plastics is a name for various organic deriva­tives of resin, cellulose, and protein.

2. All building materials are divided into three main groups:

1). Primary (main) building materials such as rocks and artificial stones, timber and metals are used for bearing structures.

2). Secondary (auxiliary) materials are used for the interior parts of the buildings, for the interior finish of structures.

3). Cementing or binding materials such as lime, gypsum and cement are the three materials most widely used for the purpose of bind­ing together masonry units, such as stone, brick and as constituents of wall plaster.

Building materials can be further categorized into two sources, natural and synthetic. Natural building materials are unprocessed or minimally processed by industry, such as timber, sand, lime or stone. Whereas synthetic materials are made in industrial settings after some human manipulations, such as plastics and petroleum based paints. Cement, clay products and concrete are also examples of artificial buildings materials.

Conclusion

The designer must be able to select and adapt such materials of construction that will give the most effective result by the most economical means. In this choice of mate­rials for any work of construction the civil engineer must consider many factors. These factors include availability, cost, physical properties of materials and others.

Text 2 . MATERIALS SCIENCE: PLASTICS

Materials science or materials engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering. This science investigates the relationship between the structure of materials and their properties. It includes elements of applied physics and chemistry, as well as chemical, mechanical, civil and electrical engineering. With significant media attention to nanoscience and nanotechnology in the recent years, materials science has been propelled to the forefront at many universities.

Metals, polymers and ceramics constitute an important part of materials science. Polymers are the raw materials (the resins) used to make what we commonly call plastics. Plastics consisting of one polymer are referred to as simple. Thus, organic glass (plexiglass) consists of one syn­thetic resin. But in the building field we usually deal with complex plastics, e.g. plastics consisting of a polymer and other components.

As is known, the term plastics covers a range of synthetic or semi-synthetic organic condensation or polymerization products that can be molded or extruded into objects or films or fibers. Their name implies that in their semi-liquid state they are malleable, or have the property of plasticity. Plastics vary immensely in heat tolerance, hardness, and resiliency. Combined with this adaptability, the general uniformity of composition and lightness of plastics ensure their use in almost all industrial applications today.

Polymers which are in current widespread use include polyethylene, polypropylene, polyvinyl-chloride, polystyrene, nylons, polyesters, acrylics, polyurethane, and polycarbonates. Plastics are generally classified as “commodity”, “specialty” and “engineering” plastics.

PVC (polyvinyl-chloride) is a commodity plastic; it is widely used, inexpensive, and annual production quantities are huge. This plastic has an incredible array of applications, and its fabrication and processing are simple and well-established. To modify its material properties various chemicals and compounds are added to the polymer base.

Polycarbonate would be normally considered as engineering plastic. Engineering plastics are valued for their superior strengths and other special material properties. They are usually not used for disposable applications, unlike commodity plastics.

It should be noted here that the dividing line between the various types of plastics is not based on material but rather on their properties and applications. For instance, polyethylene (PE) is a cheap polymer commonly used to make disposable shopping bags and trash bags, and is considered a commodity plastic, whereas Medium-Density Polyethylene (MDPE) is used for underground gas and water pipe.

Plastics combine all the fine characteristics of a building material owing to their inherent valuable and diverse properties. The architects and engineers have also turned to them to add beauty to modern homes and of­fices. Being a comparatively recent invention plastics have found today a wide application not only in construction, but many other industrial fields (machine-build­ing, aviation, textile industry, etc.).

Date: 2015-12-11; view: 2446