Building Materials: Properties and Selection

• Building materials are essential in modern technology and construction and contribute greatly to national economies by governing construction rate and quality.
• The climatic background and economic aspects are factors affecting material choices.
• The requirements for building properties are diverse, including strength at temperature extremes and resistance to water, acids, and alkalis.
• Materials are categorized based on their properties; mineral binding materials are split into air and hydraulic-setting types.
• Understanding material qualities leads to rational selection for specific conditions.
• Standardization ensures materials meet specific quality levels, industry innovation.
• Industry advances lead to better materials and improved production techniques.
• To achieve economic efficiency, it is helpful a comparison of similar materials under specific conditions.

Physical Properties

• Density (ρ) is the mass per unit volume of a homogeneous material, measured in g/cm³.
• Density is calculated using the formula: ρ = M / V, where M is mass in grams and V is volume in cm³.
• Example densities: Brick (2.5-2.8 g/cm³), Granite (2.6-2.9 g/cm³), Portland cement (2.9-3.1 g/cm³), Wood (1.5-1.6 g/cm³), Steel (7.8-7.9 g/cm³).
• Bulk Density (ρb) is the mass of material per unit volume in natural state including pores and voids, measured in kg/m³.
• Bulk density is calculated by: ρb = M / V, where M is mass of specimen in kg and V is volume in the natural state in m³.
• Bulk density is generally expressed in kg/m³ instead of g/cm³ for convenience.
• For most materials, bulk density is less than density, except for liquids and dense materials like glass and dense stone.
• Strength and heat conductivity are affected by bulk density.
• Density Index (ρo) is the ratio of bulk density to density, indicating how filled a material's volume is.
• Density Index formula: ρo = bulk density / density.
• For building material, the density index is less than 1.0.
• Specific Weight (γ), also known as unit weight, is the weight per unit volume of material.
• Specific weight is calculated as: γ = ρ * g, where ρ is density and g is gravity.
• The specific weight of water on Earth is 9.80 kN/m³ at 4°C.
• Specific Gravity (Gs) is the ratio of the weight/mass of a material's solids to that of an equal volume of water at 4°C.
• Specific gravity formula: Gs = γs / γw = ρs / ρw.
• At 4°C, γw = 1 g/cc or 9.8 kN/m³.
• True specific gravity (Ga) excludes permeable and impermeable voids.
• Apparent specific gravity (Gm) includes voids.
• Porosity (n) is the volume of pores interspersed within a material.
• Porosity is expressed as ratio of pore volume to total volume: n = Vv / V.
• Porosity indicates qualities like bulk density, heat conductivity, and durability.
• Void Ratio (e) is the void volume ratio to solid volume: e = Vv / Vs.
• Voids refer to the vacant spaces between aggregate particles.
• Two methods exist for measuring voids: direct and indirect
• The direct method fills voids with liquid; the indirect method calculates voids by material displacement in a calibrated tank.
• Hygroscopicity is a material's ability to absorb water vapor from air, influenced by air conditions, pore characteristics, and material nature.
• Water Absorption is the ability to absorb and retain water, expressed as a percentage: Ww = ((M1 - M) / M) * 100.
• The water absorption formula by volume: Wv = ((M1 - M) / V) * 100, where M1 is the mass of saturated material, M is the mass of dry material and V is the volume of material.
• Water absorption by volume is less than 100%, but by weight, it may exceed 100% for porous materials.
• Coefficient of softening is the ratio of saturated to dry compressive strength, indicating water resistance.
• Materials are unsuitable for wet environments if the coefficient of softening is less than 0.8.
• Weathering Resistance refers to enduring wet and dry conditions without deformation or loss of strength.
• Water Permeability is the capacity to allow water to penetrate under pressure.
• Impermeable materials: glass, steel, and bitumen.
• Frost Resistance is the ability to withstand repeated freezing and thawing cycles while saturated.
• Pore water expansion can cause stress and failure during freezing.
• Heat Conductivity is ability to conduct heat, influenced by material, structure, porosity type, temperatures.
• Thermal Capacity is the ability to absorb heat, relevant in thermal stability calculations.
• Fire Resistance is the ability to avoid deformation/strength loss at high temperatures. Fire-resistive materials char or smolder with difficulty e.g. treated wood.
• Non-combustible materials do not smolder or char.
• Refractoriness is the ability to withstand prolonged high temperatures without melting/shape loss and materials resisting 1580°C+ are refractory.
• High-melting materials withstand 1350-1580°C and low-melting materials, below 1350°C.
• Chemical Resistance is the ability to resist acids, alkalis, seawater, and gases.
• Durability is the ability to withstand combined atmospheric and other factors.

Mechanical Properties

• Mechanical properties are related to strength, compression, tensile, bending, impact, hardness, plasticity, elasticity, and abrasion resistance.
• Strength is the ability to resist failure under stress from loads.
• Compressive Strength is tested using cylinders, prisms, and cubes.
• Prisms/cylinders have lower resistance than cubes, unless height is smaller than side.
• Friction forces prevent expansion when compressing a specimen; failure occurs away from press plates.
• Tensile strength tests use round bars/strips for metals, figure-eight shapes for binding materials.
• Bending Strength is tested on bars supported at ends under concentrated loads.
• Hardness is the ability to resist penetration by a harder body.
• The Mohs scale is used to find hardness.
• Elasticity is the ability to restore initial form after unloading and deformation is proportional to stress within elastic limits.
• Modulus of elasticity is the ratio of unit stress to unit deformation.
• Plasticity is the ability to change shape under load without cracking, retaining shape after load removal.

Characteristic Behaviour Under Stress

• Common characteristics under stress: ductility, brittleness, stiffness, flexibility, toughness, malleability, and hardness.
• Ductile materials can be drawn out without necking
• Example ductile materials: copper and wrought iron.
• Brittle examples: cast iron, stone, brick, concrete
• Brittle materials have little/no plasticity, fail suddenly.
• Stiff materials have high modulus of elasticity, permit small deformation.
• Flexible materials have low modulus of elasticity, bend considerably.
• Tough materials withstand heavy shocks, dependant on strength and flexibility.
• Malleable materials can be hammered into sheets, dependent on ductility/softness.
• Copper is very malleable.
• Hard materials resist scratching, denting.
• Examples of hard materials are cast iron and chrome steel.
• Materials resistant to abrasion, like manganese, are also hard.

Examine the impacts of material selection in construction beyond individual projects. Explore how construction advancements reshape material choices and how usage influences material properties. Understand direct void measurement accuracy and material property-based grouping.