Materials Engineering PDF

# Materials Engineering ## 1.2.8 ​​Civil Materials Engineering The choice of material for civil engineering systems construction is influenced by a variety of factors, including the initial and life-cycle maintenance cost, mechanical properties, durability, ease of construction, and aesthetics (Ho...

# Materials Engineering ## 1.2.8 ​​Civil Materials Engineering The choice of material for civil engineering systems construction is influenced by a variety of factors, including the initial and life-cycle maintenance cost, mechanical properties, durability, ease of construction, and aesthetics (Ho, 2003). To make informed decisions on material choices, the materials engineer typically solicits mechanistic or empirical data on the performance of the material in response to environmental factors that include usage and climate/weather. Thus, civil materials engineering involves the investigation of the properties of construction materials such as the raw ingredients for construction (e.g., cement, water, steel, aggregates, subgrade, subbase/base courses, etc.) and mixed products (e.g., asphaltic and Portland cement concrete, etc.) to ascertain their suitability or to recommend ways to enhance their properties for that purpose. The field of civil materials engineering is an interdisciplinary one that investigates the relationships between the composition and structure of materials and their properties. With the current explosion in nanoscience and nanotechnology, materials engineering is playing a more visible role in many institutions. Materials engineering also includes forensic engineering and thus covers the study of the failure of civil engineering systems. The nature and behavior of any material are governed by its constituent elements and the manner in which it was synthesized. Materials engineers seek to understand the fundamental structure and behavior of existing materials with a view to expanding their uses and the development of new or enhanced materials with specific desired properties. They relate the atomic structure of that material to the properties and performance of the material when it is used in a given application. ## The Future of Materials Science and Engineering (MSE) In the near future, the study and application of materials in civil engineering is expected to include geosynthetics (geotextiles, geomembranes, and geogrids). Enhanced versions of these products will be used in embankments on soft foundations or to protect erosion-prone slopes (Holtz, 1991). Due to the adoption of intelligent materials and intelligent designs, there is expected to be an increasing number of energy-efficient buildings and other civil structures (Apelian, 2007). As case in point, Germany's Institute of Solar Energy Systems has developed a technique that uses a thin layer of material containing microencapsulated paraffin to carry out temperature equalization; when temperature inside the building rises above 24°C, the enclosed paraffin in the wall melts, leading to heat reduction in the room. Then at times of low temperature, the paraffin solidifies, releasing the stored heat, leading to energy savings and pollutant reduction. The future seems to be promising for discoveries in intelligent, green, and energy-efficient materials. Another example of future trends in this area is exemplified by roofing system applications such as the Teflon-coated fiberglass membrane roof that was used for the Riyadh International Stadium in Saudi Arabia and self-healing bioconcrete. Future world needs are projected to include recyclable or biodegradable materials. Environmental quality will be enhanced by the use of new biodegradable natural plastics for packaging of goods. Other similar materials including fiber-reinforced polymers (Figure 1.17) will see increased use due to their desirable engineering properties, low life-cycle cost, and contribution to sustainable development. As designers of structural systems demand less weight with greater strength, the focus will be on lightweight structural materials, specifically in the areas of alloys that can be stiffened to the extent needed). The properties of strength, ductility, weight, # Building materials ## Principal Properties of Building Materials - Introduction - Physical Properties - Mechanical Properties - Characteristic Behaviour Under Stress - Exercises ## 1.1 Introduction Building materials have an important role to play in this modern age of technology. Although their most important use is in construction activities, no field of engineering is conceivable without their use. Also, the building materials industry is an important contributor in our national economy as its output governs both the rate and the quality of construction work. There are certain general factors which affect the choice of materials for a particular scheme. Perhaps the most important of these is the climatic background. Obviously, different materials and forms of construction have developed in different parts of the world as a result of climatic differences. Another factor is the economic aspect of the choice of materials. The rapid advance of constructional methods, the increasing introduction of mechanical tools and plants, and changes in the organisation of the building industry may appreciably influence the choice of materials. Due to the great diversity in the usage of buildings and installations and the various processes of production, a great variety of requirements are placed upon building materials calling for a very wide range of their properties: strength at low and high temperatures, resistance to ordinary water and sea water, acids and alkalis etc. Also, materials for interior decoration of residential and public buildings, gardens and parks, etc. should be, by their very purpose, pleasant to the eye, durable and strong. Specific properties of building materials serve as a basis for subdividing them into separate groups. For example, mineral binding materials are subdivided into air and hydraulic-setting varieties. The principal properties of building materials predetermine their applications. Only a comprehensive knowledge of the properties of materials allows a rational choice of materials for specific service conditions. ## 1.2 Physical Properties **Density (ρ)** is the mass of a unit volume of homogeneous material denoted by $p = \frac {M}{V}$ g/cm³ where: - M= mass (g) - V= volume (cm³) Density of some building materials is as follows: | Material | Density (g/cm²) | |---|---| | Brick | 2.5-2.8 | | Granite | 2.6-2.9 | | Portland cement | 2.9-3.1 | | Wood | 1.5-1.6 | | Steel | 7.8-7.9 | **Bulk Density (ρ_b)** is the mass of a unit volume of material in its natural state (with pores and voids) calculated as $ρ_b = \frac {M}{V}$ kg/m³ where: - M= mass of specimen (kg) - V= volume of specimen in its natural state (m³) Note: Bulk density may be expressed in g/cm³ but this presents some inconveniences, and this is why it is generally expressed in kg/m³. For example, the bulk density of reinforced cement concrete is preferably expressed as 2500 kg/m³ rather than 2.5 g/cm³. For most materials, bulk density is less than density but for liquids and materials like glass and dense stone materials, these parameters are practically the same. Properties like strength and heat conductivity are greatly affected by their bulk density. Bulk densities of some of the building materials are as follows: **Density Index (ρ_i)** is the ratio, $ρ_i = \frac {ρ_b}{ρ}$ It indicates the degree to which the volume of a material is filled with solid matter. For almost all building materials ρ_i is less than 1.0 because there are no absolutely dense bodies in nature. **Specific Weight (γ)** also known as the unit weight) is the weight per unit volume of material, $γ = ρ ⋅ g$ Where: - γ = specific weight (kN/m³) - ρ = density of the material (kg/m) - g = gravity (m/s²) Specific weight can be used in civil engineering to determine the weight of a structure designed to carry certain loads while remaining intact and remaining within limits regarding deformation. It is also used in fluid dynamics as a property of the fluid (e.g., the specific weight of water on Earth is 9.80 kN/m³ at 4°C). The terms specific gravity, and less often specific weight, are also used for relative density. **Specific Gravity (G)** of solid particles of a material is the ratio of weight/mass of a given volume of solids to the weight/mass of an equal volume of water at 4°C. $G = \frac {γ_s}{γ_w} = \frac {ρ_s}{ρ_w}$ At 4° C γ_w = 1 g/cc or 9.8 kN/m³ **Porosity (n)** is the degree to which volume of the material of the material is interspersed with pores. It is expressed as a ratio of the volume of pores to that of the specimen. $n = \frac {V_v}{V}$ Porosity is indicative of other major properties of material, such as bulk density, heat conductivity, durability, etc. Dense materials, which have low porosity, are used for constructions requiring high mechanical strength on other hand, walls of buildings are commonly built of materials, featuring considerable porosity. Following inter relationship exists between void ratio and the porosity. $n = \frac {e}{1+e}$ **Void Ratio (e)** is defined as the ratio of volume of voids (V_v) to the volume of solids (V_s). $e = \frac {V_v}{V_s}$ **Hygroscopicity** is the property of a material to absorb water vapour from air. It is influenced by air-temperature and relative humidity; pores-their types, number and size, and by the nature of substance involved. **Water Absorption** denotes the ability of the material to absorb and retain water. It is expressed as percentage in weight or of the volume of dry material: $W_w = \frac {M₁-M}{M}$ × 100 $W_w = \frac {M₁-M}{V}$ × 100 where: - M₁ = mass of saturated material (g) **Chemical Resistance** is the ability of a material to withstand the action of acids, alkalis, sea water and gases. Natural stone materials, e.g. limestone, marble and dolomite are eroded even by weak acids, wood has low resistance to acids and alkalis, bitumen disintegrates under the action of alkali liquors. **Durability** is the ability of a material to resist the combined effects of atmospheric and other factors. **Weathering Resistance** is the ability of a material to endure alternate wet and dry conditions for a long period without considerable deformation and loss of mechanical strength. **Water Permeability** is the capacity of a material to allow water to penetrate under pressure. Materials like glass, steel and bitumen are impervious. **Frost Resistance** denotes the ability of a water-saturated material to endure repeated freezing and thawing with considerable decrease of mechanical strength. Under such conditions the water contained by the pores increases in volume even up to 9 per cent on freezing. Thus the walls of the pores experience considerable stresses and may even fail. **Heat Conductivity** is the ability of a material to conduct heat. It is influenced by nature of material, its structure, porosity, character of pores and mean temperature at which heat exchange takes place. Materials with large size pores have high heat conductivity because the air inside the pores enhances heat transfer. Moist materials have a higher heat conductivity than drier ones. This property is of major concern for materials used in the walls of heated buildings since it will affect dwelling houses. **Thermal Capacity** is the property of a material to absorb heat described by its specific heat. Thermal capacity is of concern in the calculation of thermal stability of walls of heated buildings and heating of a material, e.g. for concrete laying in winter. **Fire Resistance** is the ability of a material to resist the action of high temperature without any appreciable deformation and substantial loss of strength. Fire resistive materials are those which char, smoulder, and ignite with difficulty when subjected to fire or high temperatures for long period but continue to burn or smoulder only in the presence of flame, e.g. wood impregnated with fire proofing chemicals. Non-combustible materials neither smoulder nor char under the action of temperature. Some of the materials neither crack nor lose shape such as clay bricks, whereas some others like steel suffer considerable deformation under the action of high temperature. ## 1.3 Mechanical Properties The important mechanical properties considered for building materials are: strength, compressive, tensile, bending, impact, hardness, plasticity, elasticity and abrasion resistance. **Strength** is the ability of the material to resist failure under the action of stresses caused by loads, the most common being compression, tension, bending and impact. The importance of studying the various strengths will be highlighted from the fact that materials such as stones and concrete have high compressive strength but a low (1/5 to 1/50) tensile, bending and impact strengths. **Compressive Strength** is found from tests on standard cylinders, prisms and cubes-smaller for homogeneous materials and larger for less homogeneous ones. Prisms and cylinders have lower resistance than cubes of the same cross-sectional area, on the other hand prisms with heights smaller than their sides have greater strength than cubes. This is due to the fact that when a specimen is compressed the plattens of the compression testing machine within which the specimen is placed, press tight the bases of the specimen and the resultant friction forces prevent the expansion of the adjoining faces, while the central lateral parts of the specimen undergoes transversal expansion. The only force to counteract this expansion is the adhesive force between the particles of the material. That is why a section away from the press plates fails early. The test specimens of metals for tensile strength are round bars or strips and that of binding materials are of the shape of figure eight. **Bending Strength** tests are performed on small bars (beams) supported at their ends and subjected to one or two concentrated loads which are gradually increased until failure takes place. **Hardness** is the ability of a material to resist penetration by a harder body. Mohs scale is used to find the hardness of materials. It is a list of ten minerals arranged in the order of increasing hardness (Section 3.2). Hardness of metals and plastics is found by indentation of a steel ball. **Elasticity** is the ability of a material to restore its initial form and dimensions after the load is removed. Within the limits of elasticity of solid bodies, the deformation is proportional to the stress. Ratio of unit stress to unit deformation is termed as modulus of elasticity. A large value of it represents a material with very small deformation. **Plasticity** is the ability of a material to change its shape under load without cracking and to retain this shape after the load is removed. Some of the examples of plastic materials are steel, copper and hot bitumen. ## 1.4 Characteristic Behaviour under Stress The common characteristics of building materials under stress are ductility, brittleness, stiffness, flexibility, toughness, malleability and hardness. The ductile materials can be drawn out without necking down, the examples being copper and wrought iron. Brittle materials have little or no plasticity. They fail suddenly without warning. Cast iron, stone, brick and concrete are comparatively brittle materials having a considerable amount of plasticity. Stiff materials have a high modulus of elasticity permitting small deformation for a given load. Flexible materials on the other hand have low modulus of elasticity and bend considerably without breakdown. Tough materials withstand heavy shocks. Toughness depends upon strength and flexibility. Malleable materials can be hammered into sheets without rupture. It depends upon ductility and softness of material. Copper is the most malleable material. Hard materials resist scratching and denting, for example cast iron and chrome steel. Materials resistant to abrasion such as manganese are also known as hard materials.