Building Materials

S.K. Duggal

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781351462976
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería civil

This text on building materials includes discussion of structural clay products, rocks and stones, wood, materials for making concrete, ferrous and non-ferrous metals, and miscellaneous materials.

Chapter
1
PRINCIPAL PROPERTIES OF BUILDING MATERIALS

1.1	Introduction
1.2	Physical Properties
1.3	Mechanical Properties
1.4	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.

The importance of standardisation cannot be overemphasised. It requires the quality of materials and manufactured items to be not below a specific standard level. However, the importance of standardisation is not limited to this factor alone, since each revised standard places higher requirements upon the products than the preceding one, with the effect that the industry concerned has to keep up with the standards and improved production techniques. Thus, the industry of building materials gains both in quantity and quality, so that new, more efficient products are manufactured and the output of conventional materials is increased.

To develop products of greater economic efficiency, it is important to compare the performance of similar kinds of materials under specific service conditions. Expenditures for running an installation can be minimised by improving the quality of building materials and products. Building industry economists are thus required to have a good working knowledge, first, of the building materials, second, of their optimum applications on the basis of their principal properties, and, third, of their manufacturing techniques, in order that the buildings and installations may have optimum engineering, economic performance and efficiency. Having acquired adequate knowledge, an economist specialising in construction becomes an active participant in the development of the building industry and the manufacture of building materials.

1.2 Physical Properties

Density

Defined as the mass of a unit volume of homogeneous material denoted by

ρ=MVg/mm3

where

M = mass (g)

V = volume (mm³)

Density of some building materials is as follows (g/mm³):

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 is the mass of a unit volume of material in its natural state calculated as

ρb=MVg/mm3

where

M = mass of specimen (g)

V = volume of specimen in its natural state (mm³)

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 building materials are as follows (g/mm³):

Brick	1.60-1.80
Granite	2.50-2.70
Sand	1.45-1.65
Pine Wood	0.50-0.60
Steel	7.85

Density Index is the ratio

d0=bulk densitydensity=ρbρ

It indicates the degree to which the volume of a material is filled with solid matter. For almost all building materials do is less than 1.0 because there are no absolutely dense bodies in nature.

Porosity is defined as the degree to which volume of the material is interspersed with pores. It is expressed as a ratio of the volume of pores to that of the specimen. 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 the other hand, walls of buildings are commonly built of materials, featuring considerable porosity.

Hygroscopicity is the property of a material to absorb water yapour 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 to absorb and denotes the ability of the material retain water. It is expressed as percentage in weight or of the volume of dry material:

Ww=M1-MM­100Wv=M1-MV­100

where

M₁= mass of saturated material (g)

M = mass of dry material (g)

V = volume of material including the pores (mm³)

Water absorption by volume is always less than 100 per cent, whereas that by weight of porous material may exceed 100 per cent.

The properties of building materials are greatly influenced when saturated. The ratio of compressive strength of material saturated with water to that in dry state is known as coefficient of softening. For materials like clay which soak readily it is zero, whereas for materials like glass it is one. Materials with coefficient of softening less than 0.8 should not be recommended in the situations permanently exposed to the action of moisture.

Weathering Resistance is the ability to endure alternate wet and dry conditions for a long period without considerable deformation and loss of mechanical strength.

Water Permeability is the capacity 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 o...