Wood is resistant to common organic acids at room temperature and at slightly higher temperatures. Inorganic acids, except very dilute ones, and alkaline solutions attack wood, particularly at higher concentrations and elevated temperatures. All oxidizing solutions of salts, acids, and alkalies rapidly attack the wood substance. Many woods are not permeable in spite of their highly porous structure.
This low permeability of many woods is due to the depositions of extractives and tylose formation during the conversion of sapwood to heartwood. Wood like other organic materials decomposes on heating, giving off water vapors, flammable and noncombustible gases, and smoke. The ignition temperature of wood is about C F but even before reaching this temperature the wood becomes brown when exposed for longer periods to C F. This darkening is accompanied by loss of weight, shrinkage, and reduction in strength. Factors affecting the ignition temperature of wood are heat conductivity, moisture content, density, and heat capacity.
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To make the wood more fire resistant, fire retardants are incorporated into the wood structure. Wood is also subject to decay by microorganisms such as fungi, which grow and nourish themselves on the wood substance in the presence of air of suitable humidity and temperature. When completely submerged in water, wood does not decay because of the Lack of air. An exception is the so-called dry rot, which is produced by a special kind of microorganism capable of absorbing the necessary moisture from sources outside the wood itself.
To prevent decay, the wood is subject to a variety of chemical treatments which have the purpose of protecting wood from degradation as the result of action of fungi, wood-destroying insects termites , marine borers, discoloration, fire, and weathering. Various methods are used to improve the dimensional stability or strength of wood when exposed to the atmosphere while in service. These methods have resulted in the development of such commercially important forms of modified wood as impreg, compreg, and plywood.
Impreg is made by impregnating thin sheets of wood veneer with a phenolic resin so that the cell cavities are filled and bonded together with resin, which is then cured at a temperature of to C to F to produce crosslinks.
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Treated veneers are usually assembled together and glued with the same resin. Compreg is also made by impregnating veneer wood with resin, but the curing takes place under pressure sufficient to compress most species of wood to a specific gravity of 1. Both treatments greatly increase the chemical resistance, decay resistance, electrical resistance, and dimensional stability of the wood. Plywood is made by bonding together an odd number of thin sheets of wood 0.
The main purpose is to overcome the directional properties of wood and to obtain a material more uniform in all directions. The dimensional stability is only slightly improved. A wood plastic composite is produced by impregnating a natural untreated wood with a liquid monomer such as methyl methacrylate, acrylonitrile, or styrene, followed by polymerization induced by ionizing radiation. The resultant material is much harder than the original wood and has much greater compressive strength and abrasion resistance, showing greatly improved dimensional stability because of low moisture absorption.
It also exhibits improved shear and static bending strength but retains natural woodgrain and color.
CONCRETE Concrete is a building and structural material obtained by mixing cement, a mineral aggregate, and water in suitable proportions so that the result is a plastic and workable mass that can be molded into any desired shape. Concrete is one of the most important materials of construction, and it ranks second only to steel in its many different industrial applications.
The quality of concrete depends on the properties of the materials used, the methods of batching and mixing, and the methods of construction.
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The basic materials of concrete are cement, mineral aggregate, and water. Older methods are based on the rule of thumb instead of on scientific principles. Mixes with cement sand gravel volume proportions of Experience indicates, however, that except for cement-rich mixtures, a still larger proportion of sand is necessary to obtain the desired workability. Hence most of the mixtures now used show cement sand gravel proportions of The amount of water added is estimated by eye alone to produce the required workability for any particular application.
Design methods for concrete mixes are based on two basic principles: the water cement ratio and the absolute-volume principle. The absolute-volume principle states that a unit volume of concrete is the sum of the absolute volumes of its components. Additional factors, such as the aggregate grading, the size and shape of the particles, the surface characteristics of the aggregate. The water-cement ratio represents the water necessary for the hydration of the cement compounds see section , for lubrication of the particles of the mix, and for making up evaporation and other losses during mixing and placing of the concrete.
Illustrative problem shows a typical calculation used in the design of a concrete mix. If the concrete contains entrained air, in the calculation of the proportions of concrete ingredients. Apart from its many advantages as a material of construction, concrete has certain limitations and disadvantages.
These are its low tensile strength, which is 10 or 15 times smaller than the compressive strength, shrinkage and expansion, thermal volume changes, and permeability. These deficiencies can be prevented or considerably reduced by using reinforced concrete, mechan ical prestressing, or special additives during mixing fresh concrete. Recently various concrete polymer combinations have been used to produce the materials with greatly improved strength and durability that can be used for a variety of important industrial applications such as nuclear reactors, highways, pipes, storage tanks, and many others ASPHALT CONCRETE Asphalt concrete is competitive with cement concrete for use in construction of highways, roadbeds, and other surfaces.
Asphalt concrete is made by mixing a suitably graded hot aggregate produced from crushed rock with asphalt cement. The terms asphalt, asphaltic bitumen, and bitumen are generally synonymous. Asphalt is a black to dark brown solid or semisolid material consisting predominantly of mixtures of hydrocarbons that are completely soluble in carbon disulfide. Asphalt is a thermoplastic material and it softens on heating and hardens on cooling.
The viscosity temperature relationship of asphalt is a very important factor determining its properties and suitability for many applications. Because of its chemical nature, asphalt is resistant to most nonoxidizing acids and corrosive salts, but it is attacked by concentrated sulfuric acid and is soluble in many organic solvents.
Asphalts are processed to asphalt cements, liquid asphalt products known as cutbacks, and asphalt emulsions. Asphalt cements are solid or semisolid products and require heating to convert them into a fluid state before application. Cutbacks are solutions of asphalt cement in organic solvents such as kerosene, naphtha, gasoline, and others. Depending on the amounts of solvents and their boiling pomts they are divided in rapid curing, medium curing, and slow curing products. Emulsified asphalts find numerous applications in small repair jobs on pavements especially when the road surface is wet.
Asphalt cements include five grades different in their viscosity level or softening point. Asphalt concrete is used as the hot mix of asphalt cement with suitably graded mineral aggregate spread and compacted on rolling at temperatures between and 80 C to F.
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It cures immediately on cooling and the material can be recycled. A schematic cross section of a slab of an asphalt concrete is shown in Fig An important factor affecting the durability of asphalt concrete is good adhesion between the stone surface and the asphalt cement serving as a binder. The surface hardens after a certain amount of solvents evaporates.
Products can also be spread hot over well-prepared road surfaces and subsequently covered by a layer of crushed rocks of suitable grading and then compacted. Such asphalt pavements perform well only under light traffic and are mainly used for small jobs or for temporary road surfacing. Emulsified asphalt is also sometimes used for small repair jobs, mainly patching, especially when the road surface is wet.
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An asphalt emulsion is mixed on the spot with suitable graded aggregate and placed in holes and compacted. After the emulsion breaks down, the resultant patch is pretty strong and weather resistant. Concrete is a heterogeneous material characterized by a high compressive strength average quality concrete 28 MPa psi 1 but a low tensile strength that is 10 or 15 times smaller than the compressive strength. When a concrete member is bent, failure occurs on the tension side of the member, resulting in cracks in the concrete mass.
To overcome this weakness, reinforced concrete has been designed in which steel in the form of rods, wires, bars, or fabric is embedded in the fresh concrete. This minimizes the development of tensile stresses in concrete and produces material of much greater strength in compression. In a reinforced concrete, steel bars or rods carry the tensile stresses that the concrete so poorly resists.
Here the terms E s and E C denote the Young moduli for steel and concrete, respectively. It can be easily shown that if the tensile stress in the reinforcement steel rods exceeds the value of the compressive strength of concrete e. For example, assume the applied tensile stress on the slab of reinforced concrete is 70 MPa 10, psi ; then the tensile strain developed in the steel rod reinforcement will be If the tensile strength of concrete is assumed to be one-tenth of its compressive strength, i.tuokapenmontclas.ga
Lecture Notes on Composite Materials
Thus, if the concrete is not to crack in tension, the stress in the reinforcement steel should be below 28 MPa psi. For many applications cracking in the tensile zone is not detrimental to the general behavior of the member, since the bond between steel and concrete is sufficiently high to prevent the width of the cracks from becoming significant.
However, if the width of the cracks is excessive, then concrete may become too permeable to moisture and gases, and corrosion of the steel reinforcement may occur.
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