International Burch university Course : Building Construction Technology IArchitecture department Date : xx / xx / xxxxSarajevo LECTURE NO.6INTRODUCTION TO BUILDING MATERIALS Building Construction Technology I Professor : Prof.dr.Nerman Rustempasic Assistant : M.Sc. Ahmed El Sayed
INTRODUCTION Any material which is used in construction of residential or commercial buildings is dubbed as building material. The choice of building material depends on : the size and nature of building, its design, intended purposes, availability of resources location. Usually building materials are classified as natural synthetic materials
BRIEF INTRODUCTION TO BUILDING MATERIALS Rock : Easily, one of the most solid and durable material used in constructions, Rock is a very dense material so it gives a lot of protection too. Dry-stone walls have been built for as long as humans have put one stone on top of another. Mostly Stone buildings can be seen in most major cities, some civilizations built entirely with stone. Question : name two draw-backs for stone.
Mud and Clay : Mud and clay are the most commonly used materials in residential buildings. Buildings made primarily of mud and clay can easily endure many years. Using mud and clay in buildings is a very good option for warm places, Question : Why using mud and clay in bulidings is a very good option for warm places ? Soil and especially clay is good thermal mass; it is very good at keeping temperatures at a constant level. Homes built with earth tend to be naturally cool in the summer heat and warm in cold weather.
Wood : A natural material for building dwellings for thousands of years, Wood was also used to make Churches in the past. Question : What is the main problems with wood structures ? Wood is an aesthetically pleasing material that never goes out of trend completely, Wood obtained from certain plants is quite durable, however low quality wood is open to many extremities. These days wood is mostly used for making cabinets, furniture or wardrobes.
Metals / Steel : Metal is used as structural framework for larger buildings such as Skyscrapers, or as an external surface covering. Steel is a metal alloy whose major component is iron, and is the usual choice for metal structural building materials. It is strong, flexible, and if refined well and/or treated lasts a long time. Question : What is metal’s prime enemy ? The lower density and better corrosion resistance of aluminium alloys and tin sometimes overcome their greater cost.
Glass : Glassmaking is considered an art form as well as an industrial process or material. Clear windows have been used since the invention of glass to cover small openings in a building.Question : What makes glass different from other building materials ? Glass is generally made from mixtures of sand and silicates, in a very hot fire stove called a klin and is very brittle. Very often additives are added to the mixture when making to produce glass with shades of colors or various characteristics. The use of glass in architectural buildings has become very popular in the modern culture.
Plastic : The term plastics covers a range of synthetic or semi- synthetic organic condesition or polymerzation products that can be molded or extruded into objects or films or fibers. Plastics vary immensely in heat tolerance, hardness, and resiliency. Combined with this adaptability, Plastic is a light, flexible substance, used mostly for piping in buildings. Their name is derived from the fact that in their semi-liquid state they are malleable, or have the property of plasticity.
Concrete : Concrete is made by mixing cement, sand, gravel and water, while the structures are made using steel bars. The most common form of concrete is Portland cement concrete, which consists of mineral aggregate (generally gravel and sand), portland cement and water. Concrete is another material known for its durability It is more convenient to use as far as portability and molding is concerned. For a concrete construction of any size, as concrete has a rather low tensile strenght, it is generally strengthened using steel rods or bars (known as rebars).
FUNDAMENTAL PROPERTIES OF BUILDING MATERIALS Parametars of state / structural characteristics Physical properties Mechanical properties
Density : Ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the substance. The bulk density of soil depends greatly on the mineral make up of soil and the degree of compaction. Bulk density = mass of soil/core volume The bulk density of soil is inversely related to the porosity of the same soil: the more pore space in a soil the lower the value for bulk. Specific density ( specific mass ) is the mass of apsulutly dense material. ( 100% solid material ).
Porosity : Porosity or void fraction is a measure of the void (i.e., "empty") spaces in a material, and is a fraction of the volume of voids over the total volume. Effective porosity (also called open porosity) : Refers to the fraction of the total volume in which fluid flow is effectively taking place. Ineffective porosity (also called closed porosity) : Refers to the fraction of the total volume in fluids or gases are present but in which fluid flow can not effectively take place and includes the closed pores.
Hydro-physical properties : Hygroscopicity : is the capacity of a product (e.g. cargo, packaging material) to react to the moisture content of the air by absorbing or releasing water vapor. Water absorption : The amount of water absorbed by a composite material when immersed in water for a stipulated period of time. Moisture : Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, fruit, or wood Water permeability : The rate of water flow in gallons per day through a cross section of 1 square foot under a unit hydraulic gradient, at the prevailing temperature. Shrinking and swelling : Swelling soils are soils or soft bedrock that increase in volume as they get wet and shrink as they dry out
Thermo-technical properties : Thermal conductivity : is the property of a materials ability to conduct heat Question : Explain ? Fire resistance : A fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. Thermal diffusivity : The thermal diffusivity is a measure of the transient heat flow through a material. Specific heat : The specific heat is a measure of the amount of energy required to change the temperature of a given mass of material.
Melting point : The melting point is the temperature at which a material goes from the solid to the liquid state at one atmosphere. Thermal expansion coefficient : The thermal expansion coefficient is the amount a material will change in dimension with a change in temperature. Thermal shock resistance : Thermal shock resistance is a measure of how large a change in temperature a material can withstand without damage. Thermal shock resistance is very important to most high temperature designs.
Viscosity : Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Viscosity describes a fluids internal resistance to flow and may be thought of as a measure of fluid friction. Question : Why is viscosity important if we are talking about solid materials ?
Frost resistance : The ability of building materials in a wet condition to withstand many cycles of freezing and thawing without disintegrating. The basic cause of the disintegration of materials acted upon by low temperatures is that the water filling the pores of the material expands when it freezes. Frost resistance depends primarily on the structure of the material: the larger the pores that water can penetrate, the lower frost resistance will be. The frost resistance value is the number of cycles of freezing and thawing the material can undergo before losing 25 percent of its initial strength or 5 percent of its weight. Question : How to improve frost resistance ?
Acoustic properties : The study of sound and sound phenomena led to a scientific discipline called Acoustics. Acoustic absorption is that property of any material that changes the acoustic energy of sound waves into another form, often heat, which it to some extent retains, as opposed to that sound energy that material reflects or conducts. The absorptivity of a given material is frequency-dependent and is affected by size, shape, location and the mounting method used. Porous insulative materials such as mineral wool or glass wool are effective sound absorbers. Question : What about metals ?
The mechanical properties of a material describe how it will react to physical forces. The deformation that takes place is called the STRAIN, while the force causing the deformation is known as the STRESS. The strain may be a change in size (length, area or volume), while the stress may be : forces of tension (that tend to increase length), compression (that tend to reduce length), or, shear (where parallel planes of a body tend to slide over each other.
Stress is measured in units of force per unit area of cross- section (N.m-2), and is commonly given the symbol σ (greek "sigma"). Since the dimensions of stress are the same as those for pressure, stress is frequently measured in pascals. Strain is a pure number, and is given the symbol ε (greek "epsilon"). A material is said to be ELASTIC if, when deformed by an applied force, it returns to its original shape when the force is removed. Permanent deformation may occur if the stress is too large. However, many structural materials also present the property of RIGIDITY, in that they will offer resistance to stress.
The relationship between stress and strain in elastic materials was investigated by Robert Hooke, and led to what is known as Hookes law."For an elastic material, the strain is directly proportional to the stress.“
Definition : The point up to which the stress and strain are linearly related is called the proportional limit. The largest stress in the stress strain curve is called the ultimate stress. The stress at the point of rupture is called the fracture or rupture stress. The region of the stress-strain curve in which the material returns to the undeformed state when applied forces are removed is called the elastic region. The region in which the material deforms permanently is called the plastic region. The point demarcating the elastic from the plastic region is called the yield point. The stress at yield point is called the yield stress.
The permanent strain when stresses are zero is called the plastic strain. The off-set yield stress is a stress that would produce a plastic strain corresponding to the specified off-set strain. A material that can undergo large plastic deformation before fracture is called a ductile material. A material that exhibits little or no plastic deformation at failure is called a brittle material. Hardness is the resistance to indentation. The raising of the yield point with increasing strain is called strain hardening. The sudden decrease in the area of cross-section after ultimate stress is called necking.
Strenght : Strength has several definitions depending on the material type and application. Before choosing a material based on its published or measured strength it is important to understand the manner in which strength is defined and how it is measured. When designing for strength, material class and mode of loading are important considerations. For metals the most common measure of strength is the yield strength. For most polymers it is more convenient to measure the failure strength, the stress at the point where the stress strain curve becomes obviously non-linear. Strength, for ceramics however, is more difficult to define. Failure in ceramics is highly dependent on the mode of loading.
Elastic limit : The elastic limit is the highest stress at which all deformation strains are fully recoverable. For most materials and applications this can be considered the practical limit to the maximum stress a component can withstand and still function as designed. Beyond the elastic limit permanent strains are likely to deform the material to the point where its function is impaired.
Proportional limit : The proportional limit is the highest stress at which stress is linearly proportional to strain. This is the same as the elastic limit for most materials. Some materials may show a slight deviation from proportionality while still under recoverable strain. In these cases the proportional limit is preferred as a maximum stress level because deformation becomes less predictable above it.
Yield Strength : The yield strength is the minimum stress which produces permanent plastic deformation. This is perhaps the most common material property reported for structural materials because of the ease and relative accuracy of its measurement. The yield strength is usually defined at a specific amount of plastic strain, or offset, which may vary by material and or specification. The offset is the amount that the stress-strain curve deviates from the linear elastic line. The most common offset for structural metals is 0.2%.
Ultimate Tensile Strength : The ultimate tensile strength is an engineering value calculated by dividing the maximum load on a material experienced during a tensile test by the initial cross section of the test sample. The ultimate tensile strength helps to provide a good indication of a materials toughness but is not by itself a useful design limit. Conversely this can be construed as the minimum stress that is necessary to ensure the failure of a material.
True Fracture Strength : The true fracture strength is the load at fracture divided by the cross sectional area of the sample. Like the ultimate tensile strength the true fracture strength can help an engineer to predict the behavior of the material but is not itself a practical strength limit.
Ductility : Ductility is a measure of how much deformation or strain a material can withstand before breaking. The most common measure of ductility is the percentage of change in length of a tensile sample after breaking. This is generally reported as % El or percent elongation.
Toughness : Toughness describes a materials resistance to fracture. It is often expressed in terms of the amount of energy a material can absorb before fracture. Tough materials can absorb a considerable amount of energy before fracture while brittle materials absorb very little. Neither strong materials such as glass or very ductile materials such as taffy can absorb large amounts of energy before failure. Toughness is not a single property but rather a combination of strength and ductility. Materials with high yield strength and high ductility have high toughness.
Fatigue ratio : The dimensionless fatigue ratio f is the ratio of the stress required to cause failure after a specific number of cycles to the yield stress of a material. Fatigue tests are generally run through 107 or 108 cycles. A high fatigue ratio indicates materials which are more susceptible to crack growth during cyclic loading.
Loss coefficient : The loss coefficient is another important material parameter in cyclic loading. It is the fraction of mechanical energy lost in a stress strain cycle. The loss coefficient for each material is a function of the frequency of the cycle. A high loss coefficient can be desirable for damping vibrations while a low loss coefficient transmits energy more efficiently. The loss coefficient is also an important factor in resisting fatigue failure. If the loss coefficient is too high, cyclic loading will dissipate energy into the material leading to fatigue failure.