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Basic Civil Engineering Fourth Edition  About the Author M S Palanichamy is currently serving as the Vice Chairman of the Tamil Nadu State Council for Technical Education. He holds a bachelor’s degree in Civil Engineering from PSG College of Technology and MTech and...

Basic Civil Engineering Fourth Edition  About the Author M S Palanichamy is currently serving as the Vice Chairman of the Tamil Nadu State Council for Technical Education. He holds a bachelor’s degree in Civil Engineering from PSG College of Technology and MTech and PhD degrees from the Indian Institute of Technology, Chennai. His teaching experience spans a period of over 35 years. Dr Palanichamy was a faculty of Anna University in the Structural Engineering Department for a decade. He served as Professor and Head of the Civil Engineering Department and then as the Principal of Mepco Schlenk Engineering College, Sivakasi. Later, he served as the Vice Chancellor of Tamil Nadu Open University, Chennai, for two terms. He has published four books and 60 papers in his professional career. Also, four candidates have obtained their PhD degrees under his guidance. He is a member of various professional bodies like American Concrete Institute (AUI), Indian Roads Congress (IRC), Indian Buildings Congress (IBC), Institution of Engineers (India), Indian Concrete Institute (ICI), Institution of Values, Indian Geotechnical Society, Institution of Public Health Engineers (India), Indian Institute of Public Administration, etc. He has also served as a member of National Executive Council of the Indian Society of Technical Education, All India Council for Technical Education and the Engineering Accreditation Committee, NBA, New Delhi. He was the Chairman, Board of Studies (Civil Engineering) of Madurai Kamaraj University, Anna University, Coimbatore Institute of Technology, Coimbatore, and Thiagarajar College of Engineering, Madurai, under the autonomous system. He has also served in the governing councils of many technical institutions and private universities in Tamil Nadu. Basic Civil Engineering Fourth Edition M S Palanichamy Vice Chairman Tamil Nadu State Council for Technical Education Chennai, Tamil Nadu Tata McGraw Hill Education Private Limited NEW DELHI McGraw-Hill Offices New Delhi New York St Louis San Francisco Auckland Bogotá Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal San Juan Santiago Singapore Sydney Tokyo Toronto  Tata McGraw-Hill Published by Tata McGraw Hill Education Private Limited, 7 West Patel Nagar, New Delhi 110 008. Basic Civil Engineering, 4/e Copyright © 2011 by Tata McGraw Hill Education Private Limited No part of this publication may be reproduced or distributed in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers. The program listings (if any) may be entered, stored and executed in a computer system, but they may not be reproduced for publication. This edition can be exported from India only by the publishers, Tata McGraw Hill Education Private Limited. ISBN (13): 978-0-07-070796-2 ISBN (10): 0-07-070796-0 Vice President and Managing Director—McGraw-Hill Education, Asia-Pacific Region: Ajay Shukla Head—Higher Education Publishing and Marketing: Vibha Mahajan Deputy Manager—Acquisition (Science, Engineering & Mathematics): H R Nagaraja Asst Development Editor: Gopakumar Executive—Editorial Services: Sohini Mukherjee Jr. Production Manager: Anjali Razdan Deputy Marketing—Manager (SEM & Tech Ed.): Biju Ganesan General Manager—Production: Rajender P Ghansela Asst. General Manager—Production: B L Dogra Information contained in this work has been obtained by Tata McGraw-Hill, from sources believed to be reliable. However, neither Tata McGraw-Hill nor its authors guarantee the accuracy or completeness of any information published herein, and neither Tata McGraw-Hill nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that Tata McGraw-Hill and its authors are supplying information but are not attempting to render engineering or other professional services. If such services are required, the assistance of an appropriate professional should be sought. Typeset at Bharati Composers, D-6/159, Sector-VI, Rohini, Delhi 110 085, and printed at Print Shop Pvt Ltd., No. 4/310, Gandhi Street, Kottivakkam, Old Mahabalipuram Road, Chennai 600 096. Cover: Print Shop Pvt Ltd. RYLQCRBZRCALR The McGraw-Hill Companies  Contents Preface xi 1. Civil Engineering and Materials 1.1-1.54 1.1 Introduction 1.1 1.2 Civil Engineering 1.2 1.3 Construction Materials—Bricks 1.5 1.4 Stones 1.10 1.5 Cement 1.15 1.6 Cement Concrete 1.23 1.7 Steel Sections 1.34 1.8 Wood 1.42 1.9 Plastics 1.47 1.10 Properties of Building Materials 1.48 Short-Answer Questions 1.52 Exercises 1.54 2. Simple Stresses and Strains 2.1-2.40 2.1 Definition of Mechanics 2.1 2.2 Units 2.1 2.3 External Forces 2.3 2.4 Internal Forces 2.3 2.5 Compound Member 2.11 2.6 Composite Member 2.12 2.7 Torsion 2.13 Illustrative Examples 2.15 Short-Answer Questions 2.37 Exercises 2.38 vi Contents 3. Geometric Properties of Sections 3.1-3.25 3.1 Centre of Gravity 3.1 3.2 Moment of Inertia of Plane Areas 3.2 3.3 Perpendicular Axis Theorem 3.2 3.4 Parallel Axis Theorem 3.3 3.5 Radius of Gyration of Plane Areas 3.3 Illustrative Examples 3.4 Short-Answer Questions 3.22 Exercises 3.22 4. Buildings 4.1-4.41 4.1 Introduction 4.1 4.2 Classification of Buildings 4.1 4.3 Components of Buildings 4.2 4.4 Building by Laws 4.3 4.5 Orientation of Buildings 4.6 4.6 Ventilation 4.8 4.7 Acoustic Requirements 4.9 4.8 Selection of Site 4.10 4.9 Substructure 4.12 4.10 Objectives of a Foundation 4.12 4.11 Site Inspection 4.12 4.12 Soils 4.12 4.13 Loads on Foundations 4.16 4.14 Essential Requirements of a Good Foundation 4.17 4.15 Types of Foundation 4.17 4.16 Caisson Foundation or Well Foundation 4.26 4.17 Failure of Foundations and Remedial Measures 4.27 4.18 Foundations for Machinery 4.28 4.19 Foundations for Special Structures 4.31 Short-Answer Questions 4.40 Exercises 4.41 5. Superstructure 5.1-5.69 5.1 Introduction 5.1 5.2 Brick Masonry 5.1 5.3 Stone Masonry 5.9 5.4 RCC Structural Members 5.18 5.5 Columns 5.23  Contents vii 5.6 Lintels 5.25 5.7 Roofing 5.28 5.8 Flooring 5.40 5.9 Damp-Proofing 5.51 5.10 Plastering 5.53 Illustrative Examples 5.61 Short-Answer Questions 5.66 Exercises 5.68 6. Dams 6.1-6.14 6.1 Introduction 6.1 6.2 Purpose of Dams 6.2 6.3 Components of a Reservoir 6.2 6.4 Selection of Site 6.2 6.5 Classification of Dams 6.3 6.6 Geological Effects 6.12 Short-Answer Questions 6.13 Exercises 6.13 7. Bridges 7.1-7.18 7.1 Introduction 7.1 7.2 Necessity of Bridges 7.1 7.3 Site Investigation 7.1 7.4 Preliminary Data to be Collected 7.2 7.5 Components of a Bridge 7.3 7.6 Technical Terms 7.5 7.7 Classification of Bridges 7.6 7.8 Culverts 7.14 7.9 Causeways 7.16 Short-Answer Questions 7.17 8. Tall Structures 8.1-8.7 8.1 Introduction 8.1 8.2 Need for Tall Structures 8.1 8.3 Material Used in Construction of Tall Structures 8.2 8.4 Practical Applications 8.2 8.5 Basic Design Features 8.4 Short-Answer Questions 8.7 Exercises 8.7 viii Contents 9. Surveying 9.1-9.36 9.1 Introduction 9.1 9.2 Importance of Surveying 9.1 9.3 Objectives of Surveying 9.1 9.4 Types of Surveying 9.1 9.5 Classification of Surveys 9.2 9.6 Principles of Surveying 9.4 9.7 Measurement of Distances 9.5 9.8 Measurement of Angles 9.9 9.9 Levelling 9.17 9.10 Determination of Areas 9.23 9.11 Contouring 9.27 Illustrative Examples 9.30 Short-Answer Questions 9.34 Exercises 9.35 10. Environmental Engineering 10.1-10.32 10.1 Introduction 10.1 10.2 Sources of Water 10.1 10.3 Elements of Protected Water Supply 10.5 10.4 Sewage Treatment (Sanitary Engineering) 10.11 10.5 Septic Tank 10.14 10.6 Oxidation Ponds 10.15 10.7 Plumbing 10.18 10.8 Solid Waste Management 10.27 Short-Answer Questions 10.31 Exercises 10.31 11. Roads 11.1-11.19 11.1 Introduction 11.1 11.2 Road Transport Characteristics 11.1 11.3 Benefits of a Good System of Roads 11.2 11.4 Classification of Roads 11.3 11.5 Methods of Construction of Roads 11.9 11.6 Traffic Signs 11.13 Short-Answer Questions 11.19 Exercises 11.19 12. Railways 12.1-12.15 12.1 Introduction 12.1  Contents ix 12.2 Comparison of Railways and Roadways 12.1 12.3 Advantages of Railways 12.2 12.4 Permanent Way 12.2 12.5 Components of a Railway Track 12.3 12.6 Basics of Points and Crossings 12.10 Short-Answer Questions 12.15 Exercises 12.15 13. Airports 13.1-13.20 13.1 Introduction 13.1 13.2 Functions of Air Transport 13.1 13.3 Airports 13.1 13.4 Airport Planning 13.2 13.5 Selection of Site 13.2 13.6 Classification of Aerodromes 13.3 13.7 Components of an Aircraft 13.4 13.8 Characteristics of an Aircraft 13.4 13.9 Components of an Airport 13.6 13.10 Lighting Arrangement at Runways, Taxiways and Apron 13.10 13.11 Windrose Diagram 13.12 13.12 Airport Zoning 13.12 13.13 Airport Capacity 13.13 13.14 Imaginary Surfaces 13.13 13.15 Fillets 13.14 13.16 Blast Pad 13.15 13.17 Helipad 13.15 13.18 Landing Aids 13.16 13.19 Standards 13.18 Short-Answer Questions 13.20 Exercises 13.20 14. Docks and Harbours 14.1-14.17 14.1 Waterways 14.1 14.2 Functions of Water Transport 14.1 14.3 Coastal Structures 14.2 14.4 Classification of Ships 14.2 14.5 Seaports 14.2 14.6 Port Structures 14.6 14.7 Berthing Structure 14.6 x Contents 14.8 Docks 14.8 14.9 Transit Sheds 14.10 14.10 Warehouses 14.11 14.11 Dolphins 14.12 14.12 Fenders 14.12 14.13 Mooring Accessories 14.14 14.14 Buoy 14.15 14.15 Navigational Aids 14.16 Short-Answer Questions 14.17 Exercises 14.17 15. Interior Design 15.1-15.15 Part A—Interior Design 15.1 15.1 Interior Design 15.1 15.2 Functional Requirement of an Interior Designer 15.1 15.3 Basic Elements of Interior Design 15.2 15.4 Design Principles 15.4 15.5 Interior Design for Spacious Rooms 15.6 15.6 Interior Design for Comfortable Rooms 15.6 15.7 Interior Design for Theme Rooms 15.7 15.8 Interior Design of Corners in Buildings 15.7 15.9 Interior Design of Living Areas 15.8 15.10 Interior Design of Cooking Areas 15.8 15.11 Interior Design of Dining Areas 15.8 15.12 Interior Design for Home Offices 15.9 15.13 Interior Design for Sleeping Areas 15.9 15.14 Interior Design for Bathrooms 15.9 15.15 Interior Design for Public/Private Buildings 15.9 15.16 Storages 15.11 Part B—Landscaping 15.11 15.17 Landscaping 15.11 15.18 Elements of Landscape Architecture 15.11 15.19 Specialization in Landscape 15.12 15.20 Landscape Products 15.12 15.21 Landscape Materials 15.12 15.22 Water-Efficient Landscaping 15.13 15.23 Design Guidelines for Interior Landscaping 15.13  Preface Basic Civil Engineering, taught at the first-year level enables the students to get a good glimpse of all the major concepts of civil engineering. These topics would be dealt with in greater detail in the higher semesters. This book has been revised to meet the requirements of a first-year course on Basic Civil Engineering offered by the major technical universities and institutions across the country. It provides a concise coverage of all the major concepts in one single volume in a lucid style. This book will be useful not only to the first-year BE students, but also to the Diploma and AMIE students. It will also serve as a good reference material for those preparing for competitive examinations. Key Features New chapter on Interior Design and Landscaping New sections on Building Types and Components; Water Sources; Water Purification: Chlorination; Water Demand Estimation; Sanitary Engineering: Definition and Terms; Solid Waste Management; Methods of Constructing a Concrete Road; Caisson Foundation; Sand and Testing of Sand; Brick Testing Adequate coverage of all the key fundamental topics Lucid writing style Rich pedagogy: 219 Illustrations 212 Short Answer Questions 220 Exercise Questions 65 Solved examples Chapter Organisation The book is organized in 15 well-written chapters. Civil Engineering and Materials are introduced in Chapter 1. Chapters 2 to 6 are devoted to Buildings, Superstructures, Dams, Bridges and Tall Structures respectively. Simple Stresses and Strains are explained in Chapter 7. The principles and techniques of Surveying are covered in Chapter 8. Environmental protection and upkeep is a major concern of the twenty-first century. xii Preface Keeping this in view, Water and Environmental Engineering are discussed in Chapter 9. Chapters 11 to 14 can be treated as a group and are dedicated to Roads, Railways, Airports, and Docks and Harbours. Finally, Interior Designing and Landscaping are taken up in Chapter 15. The book is presented in a simple, but comprehensive manner. Solved problems and illustrative diagrams have been included to explain the various concepts. Exercises are appended at the end of each chapter to provide adequate practice to the students and to help them comprehend the subject. Acknowledgements I am deeply indebted to Tata McGraw Hill Education, particularly to Mr H R Nagaraja, for the speedy publication and excellent quality of the book. I hope that this book will be well received by students and teachers alike. Any suggestions from the readers for the improvement of the book are welcome. M S Palanichamy Feedback Constructive suggestions and criticism always go a long way in enhancing any endeavour. We request all readers to email us their valuable comments/views/feedback for the betterment of the book at [email protected], mentioning the title and author name in the subject line. Also, please feel free to report any piracy of the book spotted by you.  Chapter 1 CIVIL ENGINEERING AND MATERIALS 1.1 INTRODUCTION Engineers have probably contributed more to the shaping of civilisation than any other professional group. In every society, the role of engineers is to develop technological applications to meet practical needs. For example, the application of an electrical system is to provide power to a city, a water wheel is to run a mill, an artificial heart is to prolong life, etc. The systems that supply our food, water, fuel, power, transportation network, communication and other conveniences are the products of engineering skill. Despite the essential part engineers play in the above progress and in the well-being of humanity, their exact role is imperfectly understood. Engineering is the art of converting knowledge into useful practical applications. An engineer is a person, who plays the key role in this process of conversion. Since engineering is the profession which serves people, their environment is an important consideration. Often, there have been difficulties in distinguishing engineers from scientists. It is difficult to determine where the work of the scientist ends and that of the engineer begins. The basic distinction between the linked professions of science and engineering lies in their goals. Scientists aim to invent while engineers strive to use the inventions effectively to cater to the needs of mankind. For example, the German physicist Heinrich Hertz discovered radio waves while Guglielmo Marconi developed wireless telegraphy using radio waves, a feat of engineering. And after the scientific principles of nuclear fission were established, the hard work of creating atomic weapons and useful power plants was accomplished by electrical, chemical and mechanical engineers. 1.1.1 History of Civil Engineering In earlier times, the work of an engineer was confined to public works (surveying, construction, roads, water supply, irrigation, transport, etc.) simple mechanics (using only the force of water or human and animal power) and to military needs. This continued until the late 19th century. However, the modern developments in technology led to the grouping of engineering into several branches, each requiring specific knowledge related to the particular branch. Mining and metallurgy was the first to be recognised as a separate branch in 1871. Mechanical engineering in 1880, electrical engineering in 1884 1.2 Basic Civil Engineering and chemical engineering in 1908 were the next to be recognised as separate branches. Since then, numerous other branches have come into being. The history of development of housing facilities reveals that humans have been moulding their environment throughout the ages for more comfortable living. Egyptians constructed huge pyramids. Romans developed arches for vaults and domes. During the Gothic period of architecture (1100–1500 AD), churches with pointed arches and ribs supporting masonry vaults were constructed. The arched ribs were supported by stone pillars. These structures led to the idea of development of framed structure. 1.2 CIVIL ENGINEERING Civil engineering is that branch of engineering which aims to provide a comfortable and safe living for the people. Shelter, one of the primary needs of mankind, is provided by civil engineers. The efficient planning of water supply and irrigation systems increases the food production in a country. Shelters, apart from just being shelters, have been constructed by civil engineers to provide a peaceful and comfortable life. The engineering marvels of the world, starting from the pyramids to today’s thin shell structures, are the results of the development in civil engineering. Communication lines like roads, railways, bridges, etc., without which development is impossible, are fruits of civil engineers’ work. 1.2.1 Scope of Civil Engineering Any discipline of engineering is a vast field with various specialisations. The major specialisations of civil engineering are listed below: 1. Structural engineering 2. Geotechnical engineering 3. Fluid mechanics, hydraulics and hydraulic machines 4. Transportation engineering 5. Water supply, sanitary and environmental engineering 6. Irrigation engineering 7. Surveying, levelling and remote sensing Structural Engineering Structural engineering is the most important specialisation in civil engineering. The construction of a structure needs efficient planning, design and method of construction to serve the purpose fully. Generally there are five major steps in any construction project. These include the following: 1. Positioning and arranging the various parts of the structure into a definite form to achieve best utilisation. 2. Finding out the magnitude, direction and nature of various forces acting on the structure. 3. Analysing the structure to know the behaviour of the various parts of the structure subjected to the above forces.  Civil Engineering and Materials 1.3 4. Designing the structure such that its stability under the action of various loads is ensured. 5. Executing the work with selected construction materials and skilled workers. Geotechnical Engineering For the efficient functioning of any structure built on earth, the behaviour of soil must be known. Geotechnical engineering gives the basic idea about the soil. This branch also deals with the following aspects: 1. The properties and behaviour of soil as a material under “soil mechanics”. 2. The various types of foundations for a structure, for a machine, etc. and their suitability. Geotechnical engineering also deals with the analysis, design and construction of foundation. Fluid Mechanics, Hydraulics and Hydraulic Machines Fluid mechanics deals with the properties and behaviour of fluids at rest or in motion. The principles of fluid mechanics can be applied to daily life as in the case of the flight of planes, the movement of fish in water, and the circulation of blood in the veins. The design of hydraulic structures, such as dams and regulators, require the force exerted by water and the behaviour of water under pressure. Machines which utilise the hydraulic energy are called hydraulic machines. For example, turbines use potential energy of water to generate power. Pumps are devices which utilise mechanical energy to lift water. The efficient working of the above machines depends upon the fluid behaviour which is dealt with in this discipline. Transportation Engineering The development of a nation mainly depends on the communication facilities available. A nation’s wealth is measured in terms of the road and railway facilities available. There are three modes of transportation, viz. land, water and air. This specialisation deals with the design, construction and execution of the communication routes. The different branches of transportation engineering include the following: highway engineering deals with the planning and designing of roads, railway engineering deals with the railway tracks, harbour engineering deals with the harbours and airport engineering deals with the airports. Water Supply, Sanitary and Environmental Engineering Without food man can survive for days but not without water. The responsibility of providing potable (drinking) water to the public and disposing the waste water safely is that of a civil engineer. The sources of water are precipitation and underground water. Water supply engineering deals with the location, collection of water, its treatment methods, tests for standard limits and efficient supply of water. Used water, solid wastes, toxic wastes, etc., cannot be disposed directly since these affect the environment. Hence these have to be treated and tested for the standard limits and then disposed. Sanitary engineering deals with the collection of used water, their treatment methods and effective disposal which safeguards the whole world. The natural 1.4 Basic Civil Engineering and artificial wastes generated and released into the atmosphere have upset the natural equilibrium. Anthropogenic or human-induced pollutants have overloaded the system. The role of an environmental engineer is to build a bridge between biology and technology by applying all the techniques to the job of cleaning the debris. Environmental engineering deals with the methods of protecting the environment from the deleterious effects of human activity which would result in the improvement of environmental quality for the well-being of mankind. Irrigation Engineering Irrigation may be defined as the process of supplying water by man-made methods for the purpose of land cultivation. Irrigation engineering includes the study and design of works related to the control of river water and the drainage of waterlogged areas. Thus, irrigation engineering deals with the controlling and harnessing of various resources of water, by constructing dams, reservoirs, canals, head works and distribution channels to the cultivable land. Surveying, Levelling and Remote Sensing Before starting any important civil engineering project, such as the construction of railways, highways, dams and buildings, it becomes necessary to have a detailed survey map showing accurate boundary of the project area. Surveying is defined as an art of collecting data for mapping the relative positions of points on the surface of the earth. Levelling is the process of determining the relative heights of the points on the surface of earth in a vertical plane. The main purpose of the survey work is to prepare the plan of the object to be surveyed. Various instruments are used to measure and collect the necessary information to draw the plan. Remote sensing uses the technique of obtaining the data about an area by taking aerial photographs. The intelligent interpretation gives a clear picture of the terrain. 1.2.2 Functions of a Civil Engineer Civil engineering incorporates activities such as construction of structures like buildings, dams, bridges, roads, railways, hydraulic structures, water supply and sanitary engineering. Various functions of a civil engineer are listed below. 1. Investigation The first function of a civil engineer is to collect the necessary data that is required before planning a project. 2. Surveying The objectives of surveying is to prepare maps and plans to locate the various structures of a project on the surface of earth. 3. Planning Depending on the results obtained from investigation and surveying, a civil engineer should prepare the necessary drawing for the project with respect to capacity, size and location of its various components. On the basis of this drawing, a preliminary estimate should be worked out. 4. Design After planning, the safe dimension of the components required are worked out. With this dimension a detailed drawing is prepared for various components and also for the whole structure and a detailed estimate is also calculated.  Civil Engineering and Materials 1.5 5. Execution This function deals with the preparation of schedules for construction activities, floating of tenders, finalisation of contracts, supervision of construction work, preparation of bills and maintenance. 6. Research and Development In addition to the above-mentioned works, a civil engineer has to engage himself in research and development to achieve economy and to improve the efficiency to meet the present and future needs. 1.3 CONSTRUCTION MATERIALS—BRICKS 1.3.1 Introduction As an engineer, one must know about the materials used in the construction site. All structures are constructed of materials known as engineering materials or building materials. It is necessary for an engineer to be conversant with the properties of such materials. The service conditions of buildings demand a wide range of materials with specific properties. Hence the properties of the materials are to be studied properly to select suitable building materials. In this section and in the subsequent sections, the properties and uses of some building materials, such as bricks, stones, cement, concrete and steel are discussed. The common brick is one of the oldest building materials and it is extensively used at present because of its durability, strength, reliability, low cost, etc. Bricks are obtained by moulding clay in rectangular blocks of uniform size, then by drying and burning these blocks in brick kilns. 1.3.2 Qualities of Good Bricks 1. Bricks should have perfect edges, well-burnt in kilns, copper coloured, free from cracks with proper rectangular shape and of standard size (19 × 9 × 9 cm). 2. Bricks should give a clear ringing sound when struck with each other. 3. Bricks must be homogeneous and free from voids. 4. The percentage absorption of water by weight should not be greater than 20 per cent for first-class bricks and 22 per cent for second-class bricks when soaked in cold water for 24 hours. 5. Bricks should be sufficiently hard, i.e., no nail impression must be present when scratched. The average weight of bricks should be 3–3.5 kg. 6. Bricks should not break when dropped from a height of 1 m. 7. Bricks should have low thermal conductivity and should be soundproof. 8. Bricks should not show deposits of salts when immersed in water and dried. 9. The minimum crushing strength of bricks must be 3.5 N/mm2. 1.6 Basic Civil Engineering 1.3.3 Classification of Bricks Bricks are classified based on the manufacturing process adopted. The classification is given as follows: 1. First-class bricks are table-moulded and of standard shape. These comply with all good qualities of bricks and are used for superior and permanent works. 2. Second-class bricks are ground-moulded and burnt in kilns. The surfaces of such bricks are rough and are slightly irregular in shape. Such bricks are used with a coat of plaster. 3. Third-class bricks are ground-moulded and are burnt in clamps. These bricks are not hard but rough with irregular and distorted edges. These give a dull sound when struck with each other. They are used for unimportant and temporary structures and at places where there is less rainfall. 4. Overburnt bricks with irregular shape and dark colour are classified as the fourth class bricks. These are used as aggregates for concrete in foundations, floors, roads, etc. 1.3.4 Uses of Bricks 1. Bricks are mainly used for the construction of walls. 2. Bricks when moulded in the shape of a gutter can be used as drains. 3. Bricks with cavities known as hollow bricks can be used for insulation purposes and because of their light weight they are more useful in speedy constructions. 4. Paving bricks prepared from clay containing higher percentage of iron can be used for pavements, since they resist abrasion in a better way. 5. Bricks with holes are used in multi-storied framed structures. 6. Fire bricks made of fire clay can be used as a refractory material. 7. Sand-lime bricks are used for ornamental work. 8. Bricks are used in the construction of compound walls, columns, etc. Broken pieces of bricks are used as aggregates in concrete. 9. Bricks of superior quality can be used in the facing of a wall. 10. Bricks are used in the construction of chimneys and other special works. 1.3.5 Constituents of a Brick 1. Alumina It is the chief constituent of clay. A good brick should have 20–30 per cent of alumina. This imparts plasticity to the earth. 2. Silica It exists in clay in a free or combined form. A good brick earth should contain about 50–60 per cent of silica. The presence of silica prevents cracking, shrinking and warping of raw bricks. It imparts uniform shape to bricks. The durability depends on proper proportion of silica.  Civil Engineering and Materials 1.7 3. Lime Up to 5 per cent of lime is desirable in good brick earth. It prevents shrinkage in raw bricks. Sand alone is infusible, but it fuses at kiln temperature due to the presence of lime. Bricks may melt and lose their shape due to excess of lime content. 4. Oxide of iron This gives the red colour to bricks. A small quantity of iron oxide up to 5 or 6 per cent is desirable. 5. Magnesia This imparts yellow tints to bricks and it reduces shrinkage. Advantages of using bricks The following are the advantages of bricks over other construction materials, like stone, concrete, etc. (a) Bricks are cheaper and easy to handle. (b) They are of standard size and hence easy to have proper bonding. (c) Consumes less mortar when compared to stone masonry. (d) Labour required for brick masonry is less. (e) Brick walls can be raised to a larger height, when compared to stone masonry. (f) Because of regular size the surface of wall will be plane and given a neat appearance. (g) Brick masonry consumes, less mortar for plastering. (h) Easy to drill holes for fixing service connection line. (i) Bricks have low thermal conductivity and high sound insulation properties. (j) They possess very high resistance to fire. (k) They are non-combustible and non-inflammable. Disadvantages of using bricks (a) The compressive strength of brick is less compared to stone and concrete. (b) Water absorption is more than that of stone or concrete. (c) Only a selected variety of clay can be used for manufacture of bricks. (d) Kilns are required to be constructed for manufacturing bricks. (e) It has got a very low tensile strength compared to other building materials. 1.3.6 Tests on Bricks The following are the field tests by judgment for assessing the quality of bricks. Field tests 1. The bricks should be truly rectangular in shape with sharp edges and plane faces and of the same size. 2. They should be hard and well burnt and should give a metallic ringing sound when struck with a steel rod. 1.8 Basic Civil Engineering 3. They should be of uniform red colour and of fine texture. 4. When the bricks are dropped on the ground from one metre height, they should not crack or break. 5. They should be free from cracks, fissures, pebbles or nodules of free lime. Lab tests 1. Test for water absorptions (a) 3 samples of clean well dried bricks are taken and their dry weight is found out individually. (b) The bricks are then immersed in water for 24 hours. (c) After 24 hours, the bricks are taken out, surface dried and weighed in a balance and wet weight found out. (d) If the wet weight of each bricks is W2, the percentage water absorption of each brick W2 – W1 = ________ × 100 W1 (e) The average percentage of water absorption of three samples is the water absorption of the bricks. Required standard The average absorption should not be greater than 20%. Too much of water absorption indicates under-burnt condition and poor strength. 2. Test for efflorescence (For the presence of salt) Salts like sulphates of calcium, magnesium, sodium and potassium present in the brick will cause efflorescence on the brick surface, when they get dissolved in water. Bricks containing too much of salt are less resistant to weathering and will have poor strength. 1. Three samples of bricks are immersed in good water for 24 hours. 2. After 24 hours, the bricks are taken out and examined for white patches of salt on the surfaces. 3. If the white patches of salt present are heavy, the bricks are poor and are to be rejected. 4. If the white patches present are small to medium, the bricks can be accepted. 3. Test for compressive strength The load carrying capacity of bricks is increased, as the compressive strength increases. (a) Three samples of bricks are taken and immersed in good water for 24 hours. (b) After 24 hours of immersion, the bricks are taken out and surface dried. (c) Each brick is placed on the compression testing machine and the load on the brick is gradually increased until the brick fails. The failure load of each brick is found out. (d) Average failure load of the 3 bricks is the compressive strength of the bricks.  Civil Engineering and Materials 1.9 Requirement standards 1. Country Bricks Æ 35 to 50 kg/cm2 2. II Class bricks Æ 50 to 75 kg/cm2 3. I Class bricks Æ 75 to 125 kg/cm2 1.3.7 Manufacture of Bricks The following are the four processes involved in the manufacture of bricks. 1. Preparation of brick earth 2. Moulding of bricks 3. Drying of bricks 4. Burning of bricks 1. Preparation of Brick Earth Preparation of brick earth involves the following operations. (i) Removal of loose soil (ii) Digging, Spreading and Cleaning (iii) Weathering (iv) Blending (v) Tempering (i) Removal of loose soil The top layer of the loose soil about 20 cm depth contains lot of impurities and hence it should be taken out and thrown away. (ii) Digging, spreading and cleaning The earth is then dug out from the ground. This earth is spread into heaps about 60 cm to 120 cm height. All the undesirable matters like stones, vegetable matter, etc. are removed. Lumps of clay should be converted into powder form. (iii) Weathering The earth is then exposed to atmosphere for softening. The period of exposure varies from weeks to full season. (iv) Blending The clay is then mixed with suitable ingredients. It is carried out by taking a small portion of clay every time and by turning it up and down in vertical direction. (v) Tempering This is done to make the whole mass of clay homogeneous and plastic. Required water is added to clay and the whole mass is kneaded under the feet of men or cattle. When bricks are manufactured on a large scale, tempering is usually done in a pug mill. A pug mill consists of a conical iron tub with cover at its top. A vertical shaft with horizontal arms is provided at the centre of iron tub. Several cutting blades are attached to this horizontal arm. The clay with water is put inside the mill and the vertical shaft is rotated by bullocks or stream, diesel or electric power. Due to the action of the horizontal arms the clay is thoroughly mixed and tempered. 2. Moulding of Bricks The tempered clay is then sent for the next operation of moulding. There are two methods of moulding. (i) Hand moulding (ii) Machine moulding 1.10 Basic Civil Engineering 1. Hand Moulding This is done by a mould which is a rectangular box with open at top and bottom. It may be of wood or steel. Following are the ways of hand moulding: (a) Ground moulding (b) Table moulding (a) Ground moulding First a small portion of ground is cleaned and levelled. Fine sand is sprinkled over it. Moulding is started from one end of the ground. Mould is dipped in water and kept on the ground and clay is pressed by hand nicely so that all is again dipped in water and it is placed just near the previous brick to prepare another brick. Process is repeated till the ground is covered with bricks. A mark of depth about 10 mm to 20 mm is placed on raw brick by a pallet during moulding. This mark is called as frog. After the bricks become sufficiently dry, they are sent for the next process of drying. (b) Table moulding This should be done by an experienced supervisor. The moulder stands near a table of size about 2 m × 1 m. Clay, mould water pots, stock board, strikes and pallet boards are placed on this table. Bricks are moulded on the table and sent for the next process of drying. 2. Machine moulding When bricks are manufactured in huge quantity at the same spot then moulding is done by machines. These machines contain a rectangular opening of size equal to the length and width of the brick. The Tampered clay is placed in the machine and as it comes out through the opening under pressure it is cut into strips by wire fixed in frames. Arrangement is made in such a way that strips of thickness equal to that of the brick are obtained. The machine moulded bricks have sharp edges and corners, smooth external surface and uniform texture. 3. Drying of Bricks After the bricks are moulded they are dried. This is done on specially prepared drying yards. Bricks are stacked in the yard with 8 to 10 bricks in each row. Bricks are dried for a period of 5 to 12 days. During drying it must be protected from wind, rain and direct sun. Sometimes, bricks, are dried artificially by hot gases from kiln. But there is change of warping of bricks in case of artificial drying. After drying, the bricks are sent for the next operation of burning. 4. Burning of Bricks Burning imparts hardness and strength to bricks and makes them dense and durable. It must be done carefully and properly because underburnt bricks remain soft and hence cannot carry loads and overburnt bricks become brittle and hence, break easily. Burning of bricks is done either in clamp or in kilns. 1.4 STONES Building stones are obtained from rocks. It is essential to have some knowledge about rocks in order to study the properties of stones. Rocks are mainly classified into igneous  Civil Engineering and Materials 1.11 rocks, sedimentary rocks and metamorphic rocks. Igneous rocks are formed by the cooling of the molten material from beneath the earth’s surface. Stones from these rocks are said to be harder. Granite which is widely used in building constructions is a good example. Sedimentary rocks are formed by the deposition of weathering products on existing rocks. Deposits are in layers and when load is applied along the layers these rocks easily split. Metamorphic rocks are formed by the change in character of the pre-existing rocks. These will be hard if the basic rock is an igneous rock. 1.4.1 Qualities of Good Stone 1. The crushing strength of stone should be greater than 100 N/mm2. All igneous rocks have a strength around 100 N/mm2 and some of the metamorphic rocks also satisfy this requirement. Sedimentary rocks have a lower strength. 2. Stones must be decent in appearance and be of uniform colour. Light coloured stones resist weathering action in a better way and hence preferred. 3. Stones must be durable. For the stones to be durable, their natural bed must be perpendicular to the direction of pressure. 4. Stones should be such that these can be easily carved and dressed. This property is opposed to strength and hardness but this depends upon the situation in which the stone is used. 5. For a good building stone its fracture should be sharp and clear. 6. If the stone is to be used in road work, it should be hard enough to resist wear and tear. 7. A good building stone must have a wear less than 3 per cent. If it is equal to 3 per cent, it is just tolerable while if it is more than 3 per cent it is not satisfactory. 8. Stones must be fire resistant, i.e. these must retain their shape when a fire occurs. Limestone resists fire up to about 800°C. Sandstones can resist fire in a better way. Argillaceous stones are poor in strength, but resist fire to some extent. 9. A good stone should not contain quarry sap which is nothing but moisture present in the stones. 10. A good building stone must have a specific gravity greater than 2.7. 11. A good stone must have a compact, fine, crystalline structure, strong and durable. 12. A good stone should not absorb water more than 0.6 per cent by weight. It must be capable of withstanding effects of atmosphere. 13. A good building stone must be acid resistant and free from any soluble matter. Stones with exposed faces are acted upon by various atmospheric agencies such as wind, frost, living organisms, alternate wetness and drying, movement of chemicals and rain water. Stones can be prevented from the effects of these agencies if preserved properly. Coal tar, linseed oil, paint, paraffin, a solution of alum and soap, and a solution of baryta are some of the commonly used preservatives. 1.12 Basic Civil Engineering 1.4.2 Uses of Stones Stones are used 1. In the construction of buildings from the very ancient times. 2. For foundations, walls, columns, lintels, arches, roofs, floors, damp-proof courses, etc. 3. For facing works in brick masonry to give a massive appearance. 4. Since stones are hard, these can be used for pavements. 5. As a basic material for concrete, moorum of roads, calcareous cements, etc. 6. As ballast in railways, flux in blast furnaces, blocks in construction of bridges, piers, abutments, retaining walls, lighthouses, dams, etc. 1.4.3 Quarrying of Stones Quarrying is the process of extracting stone blocks from existing rocks. It is done at some depth below the top surface of rock where the effects of weathering are not found. Quarrying of soft and hard rocks is done by the following methods: 1. Digging, heating or wedging In soft rocks like limestone and marble, stones are obtained by digging, heating or wedging using hand tools, namely, pick-axes, hammers, chisels, etc. 2. Blasting In hard and dense rocks, stones are obtained by blasting using explosives. 1.4.4 Dressing of Stones Stones obtained after quarrying have rough surfaces and are irregular in shape and size. Dressing is the process of cutting the stones to a regular shape and size and the required surface finish. The purposes of dressing are: 1. To prepare the stones for a suitable size for any handling and transport. 2. To prepare the stones into a regular shape and pleasing appearance, with neat horizontal and vertical mortar joints between the adjacent stones. 3. To make hammer-dressed surface, tooled surface, polished surface, rubbed surface or cut-stone surface to suit a particular stone masonry. 4. To secure proper bedding in stone masonry. 1.4.5 Testing of Building Stones To determine the suitability of stones for construction work, the following tests are conducted: 1. Hardness test Hardness of a stone is tested by a pen knife which will not be able to produce a scratch on a hard stone. Hardness number is determined using Mohr’s scale of hardness.  Civil Engineering and Materials 1.13 2. Impact test Impact test is carried out on an Impact Testing Machine to determine the toughness of a stone. In this test, a cylinder of 25 mm diameter and 25 mm height is taken out from the sample of the stone. A steel hammer of 2 kg weight is allowed to fall axially on the cylinder from 1 cm height for the first blow, 2 cm height for the second blow, 3 cm height for the third blow, and so on. The blow at which the specimen breaks is noted. If it is the nth blow, n represents the Toughness Index of the stone. 3. Test for crushing strength In this test, a cube of sample stone of size 40 mm × 40 mm × 40 mm is tested in a compression Testing Machine. The rate of axial loading on the cube is 13.7 N/mm2/ minute. The maximum load at which the stone crushes is noted. Crushing strength of the stone per unit area is the maximum load at which its sample crushes or fails divided by the area of the bearing face of the specimen. Maximum load at failure That is, Crushing Strength of stone = _______________________ Area of bearing face 4. Fire resistance test The stone, which is free from calcium carbonate, can resist fire. The presence of calcium carbonate in the stone can be detected by dropping a few drops of dilute sulphuric Acid which will produce bubbles. 5. Electrical resistance/ Water absorption test As the electrical resistance of a wet stone is less, the stone should be non-absorbent. In this test, a stone of known weight is immersed in water for 24 hours. Then, it is weighed again. Percentage absorption of water by weight after 24 hours = (Increase in weight/Original weight of the stone). 6. Attrition test/Abrasion test Attrition test is carried out to determine the percentage of wear of stones used for the construction of road. It is carried out in Deval’s Attrition Testing Machine. In this test, some known weight of stone pieces are taken and put in the Deval’s Attrition Test Cylinder. The cylinder is rotated about its horizontal axis at the rate of 30 rpm for 5 hours. Then, the contents of the cylinder are sieved. The quantity of material retained on the sieve is weighted. Percentage Wear = (Loss in Weight/ Initial Weight) × 100. 7. Acid Test In this test, a specimen stone is kept for 1 week in the solution of sulphuric acid and hydrochloric acid. The corners of stones with high alkaline content turn roundish and loose particles will get deposited on its surface. Such types of stones are unsuitable for smoky atmosphere. 8. Smith’s test This test indicates the presence of earthly matter in the stone. In this test, the sample of the stone is broken into small pieces and put into a test-tube containing clear water. The test-tube is then shaken vigorously. The direct colour will directly show the presence of argillaceous matter. 9. Crystallization test This test determines the durability or weathering quality of a stone. In this test, a sample of stone is immersed in the solution of sodium sulphate 1.14 Basic Civil Engineering and dried in hot air. The process of wetting and drying is carried out for 2 hours. The difference in weight, if any, is recorded. Little difference in weight indicates durability and good weathering quality of the stone. 10. Microscopic test This is a geologist’s test. In this test, the sample of stone is subjected to microscopic examination to study the following properties: Mineral constitution Texture of stone Average grain size Nature of cementing material Presence of pores, fissures and veins. 11. Freezing and thawing test In this test, the specimen stone is kept in water for 34 hours. It is then placed in a freezing mixture at –12° C for 24 hours. It is then thawed (warmed) to atmospheric temperature. The procedure is repeated several times and the behaviour of the stone is studied. 1.4.6 Types of Building Stones and their Uses 1. Granite It is obtained from igneous rocks. It is hard, durable and available in different colours. It is highly resistant to weathering and has good crushing strength. It can take mirror-like polish. Uses: Granite is used for the construction of walls, columns and bridge piers. It is used for steps, sills and facing works. Also, it is used as ballast for road metal, rail metal, rail track and coarse aggregate for concrete. It is unsuitable for carving. 2. Basalt and Trap Basalt and Trap are also quarried from igneous rocks. These are hard, tough and durable and available in different colours. Uses: Basalt and Trap are used for constructing masonry floors, ornamental or decorative works and as road metal. 3. Chalk Chalk belongs to sedimentary variety. It is pure white stone, soft and easy to form powder. Uses: Chalk is used in preparing glazer’s putty and also as colouring material in the manufacture of Portland cement. 4. Limestone It is derived from sedimentary rocks. It is easy to work. It consists of a high percentage of calcium carbonate. Uses: Limestone is used for the manufacture of cement. It is also used for floors, steps, walls and as road metal. 5. Sandstone It belongs to sedimentary variety. Its structure shows sandy grains. It is easy to work and dress. It is available in different colours. Its strength is low. Uses: It is used for different building works like facing works, carving, steps, walls, columns and as road metal.  Civil Engineering and Materials 1.15 6. Laterite It is derived from metamorphic rocks. It is sandy clay stone. It is porous and soft. It can easily be quarried in blocks. It contains high percentage of iron oxide. Uses: It is used for wall construction, rough stone masonry work and as road metal. 7. Gneiss Gneiss is metamorphic in nature. It is easy to work and splits into thin slabs. Uses: It is used as thin slabs for flooring, street paving, rough stone masonry work. etc. 8. Marble Marble is metamorphic. It can take good polish. It can be easily cut with saw and carved. It is available in different colours. Uses: Marble is used for flooring in the form of slabs, wall lining, facing work, steps, columns, etc. It is used for interior decoration and such ornamental works. Taj Mahal is built fully of white marbles. 9. Gravel It is available in river beds in the form of pebbles of any kind of stone. Uses: It is used for surfacing road. It is also used in concrete. 10. Slate Slate is metamorphic. It is black in colour and can be split easily. Uses: It is used as roofing tiles, paving works and as damp-proof course in buildings. 11. Quartzite It is metamorphic. It is hard, durable, brittle and crystalline. It is difficult to work. Uses: It is used in rubble masonry, concrete aggregate, retaining walls and as road metal. 1.5 CEMENT Cement is obtained by burning at a very high temperature a mixture of calcareous and argillaceous materials. The calcined product is known as clinker. A small quantity of gypsum is added to the clinker and is pulverised into very fine powder known as cement. On setting, cement resembles a variety of sandstone found in Portland in England and is, therefore, called Portland cement. 1.5.1 Good Qualities of Cement 1. The colour should be uniform. 2. Cement should be uniform when touched. Cement should be cool when felt with hand. If a small quantity of cement is thrown into a bucket of water, it should sink. 3. Cement should be free from lumps. 4. Cement mortar at the age of three days should have a compressive strength of 11.5 N/mm2 and tensile strength of 2 N/mm2. Also, at the age of seven days, compressive 1.16 Basic Civil Engineering strength should not be less than 17.5 N/mm2 and tensile strength should not be less than 2.5 N/mm2. 5. In cement, the ratio of percentage of alumina to that of iron oxide should not be less than 0.66. 6. When ignited, cement should not lose more than 4 per cent of its weight. 7. The total sulphur content of cement should not be greater than 2.75 per cent. 8. The weight of insoluble residue in cement should not be greater than 1.5 per cent. 9. Weight of magnesia in cement should not exceed 5 per cent. 10. The specific surface of cement as found from the fineness test should not be less than 2250 mm2/gm. 11. The initial setting time of cement should not be less than 30 minutes and the final setting time shall be around 10 hours. 12. The expansion of cement should not be greater than 10 mm when soundness test is conducted. 1.5.2 Uses of Cement 1. Cement mortar, a mixture of cement and sand, is used for masonry work, plastering, pointing and in joints of pipes, drains, etc. 2. Cement is the binding material in concrete used for laying floors, roofs and constructing lintels, beams, weather sheds, stairs, pillars, etc. 3. Construction of important engineering structures, such as bridges, culverts, dams, tunnels, storage reservoirs, lighthouses and docks needs cement. 4. The manufacture of precast piles, pipes, garden seats, artistically designed urns, flower pots, dust bins, fencing post, etc., requires cement. 5. For underwater construction, quick setting cement is used. Rapid hardening cement is used for structures requiring early strength. 6. White and coloured cements are used for imparting coloured finishes to the floors, panels and exterior surfaces of buildings. 7. Expansive cements, which expands while setting, can be used in repair works of cracks. 1.5.3 Types of Cement By changing the chemical composition and by using different raw materials and additives, many types of cements can be manufactured to cater to the need of the construction industry for specific purposes. Different types of cements are classified as Portland and Non-Portland cement. 1. Rapid-hardening Cement This cement is similar to the ordinary Portland cement. As the name suggests, it develops strength rapidly. The rapid rate of strength development is attributed to the higher fineness of grinding. This cement is used where high strength is  Civil Engineering and Materials 1.17 required instantly in initial stages. For example, repair works, early removal of formwork, etc. 2. Sulphate-resisting Cement Ordinary Portland cement has less resistance to the attacks of sulphates. This type of cement with higher silicate content is effective in fighting back the attacks of sulphates. This is used for the construction of sewage treatment works, marine structures and foundations in soils having large sulphate content. 3. Low-heat Cement This cement hardens slowly but produces less heat than the other cements while reacting with water. This can be used in mass concreting works like construction of dams, etc. 4. Quick-setting Cement This cement sets very quickly. This is due to the reduction of gypsum content in the normal Portland cement. It is used for underwater construction and also for grouting operation. 5. Portland pozzolana Cement Pozzolana is a siliceous material. Portland pozzolana cement is produced by grinding Portland cement clinker and pozzolana with gypsum. It produces less heat of hydration and offers greater resistance to the attack of aggressive water. 6. High-alumina Cement This cement generates high heat while reacting with water and causes high early strength development. So this cement can be used for generating high early strength in cold climates. 7. Air-entraining Cement This cement is produced by mixing a small amount of an air- entraining agent with ordinary Portland cement. By adding this, the properties of concrete can be changed and it also increases the frost resistance of hardened concrete. 8. Masonry Cement This cement has great plasticity, workability and water retentivity as compared with ordinary Portland cement. This is used for masonry constructions in making mortars and plasters. 9. Expansive Cement This cement produces an expansion in concrete during curing. As a result of expansion, cracks due to shrinkage of concrete are avoided. So, this can be used for filling the cracks by grouting and also to overcome cracks formation in reinforced cement concrete structures. 10. Hydrophobic Cement This is a water-repellent cement and is of great utility when the cement has to be stored for longer duration in wet climatic conditions. This cement also improves the workability of concrete. 11. Coloured Cement Coloured cement consists of ordinary Portland cement with 5 to 10 per cent of pigment for colouring. This is used for aesthetic purposes. 12. White Cement The colour of this cement is white and it has the same properties of ordinary Portland cement. This can be used for architectural purposes and for manufacturing coloured concrete, flooring tiles, etc. 13. High-strength Cement Certain special works require high strength concrete. To improve the strength a higher content of C3S and higher fineness are incorporated in ordinary Portland cement. This cement can be used for railway sleepers, prestressed concrete, precast concrete and air-field works. 1.18 Basic Civil Engineering 1.5.4 Mortar 1. Definition The term mortar is used to indicate a paste prepared by adding required quantity of water to a mixture of binding material (cement or lime) and fine aggregate (sand). The above two components of mortar, namely, the binding material and fine aggregate are sometimes referred to as the matrix and adulterant respectively. The matrix binds the particles of the adulterant. The durability, quality and strength of mortar will mainly depend on the quantity and quality of the matrix. The combined effect of the two components of mortar is that the mass is able to bind the bricks or stones firmly. 2. Types of Mortars The mortars are classified on the basis of the following: (a) Bulk density (b) Type of binding material (c) Nature of application (d) Special mortars (a) Bulk density According to the bulk density of mortar in dry state, there are the following two types of mortars. (i) Heavy mortars The mortars having bulk density of 15 kN/m3 or more are known as heavy mortars and they are prepared from heavy quartzes or other sands. (ii) Lightweight mortars The mortars having bulk density less than 15 kN/m3 are known as lightweight mortars and they are prepared from light porous sands, pumice and other fine aggregates. (b) Type of binding material The type of binding material used for a mortar is according to several factors such as expected working conditions, hardening temperature, moisture conditions, etc. According to the type of binding material, the mortars are classified into the following five categories. (i) Lime mortar In this type of mortar, lime is used as binding material. The lime may be fat lime or hydraulic lime. The fat lime shrinks to a great extent and hence, it requires sand to the extent of about 2 to 3 times its own volume. The lime should be slaked before use. This mortar is unsuitable for water-logged areas or in damp situations. The lime mortar has a high plasticity and it can be placed easily. It possesses good cohesiveness with other surfaces and shrinks very little. It is sufficiently durable, but it hardens slowly. It is generally used for lightly loaded above-ground parts of buildings. (ii) Surkhi mortar This type of mortar is prepared by using only surkhi instead of sand or by replacing half of sand in case of fat lime mortar. The powder of surkhi should be fine enough to pass BIS No. 9 sieve and the residue should not be more than 10 per cent by weight. Surkhi mortar is used for ordinary masonry work of all kinds in foundation and superstructure. But it cannot be used for plastering or pointing since surkhi is likely to disintegrate after some time. (iii) Cement mortar In this type of mortar, cement is used as binding material. Depending upon the strength required and importance of work, the proportion of cement to sand by  Civil Engineering and Materials 1.19 volume varies from 1:2 to 1:6 or more. It should be noted that surkhi and cinder are not chemically inert substances and hence, they cannot be used as adulterants with matrix as cement. Thus, only sand can be used to form cement mortar. The proportion of cement with respect to sand should be determined with due regards to the specified durability and working conditions. The cement mortar is used where a mortar of high strength and water-resisting properties is required such as underground constructions, water-saturated soils, etc. (iv) Gauged mortar To improve the quality of lime mortar and to achieve early strength, cement is sometimes added to it. This process is known as gauging. It makes lime mortar economical, strong and dense. The usual proportion of cement to lime by volume is about 1:6 to 1:8. It is also known as a composite mortar or lime-cement mortar and it can also be formed by the combination of cement and clay. This mortar may be used for bedding and for thick brick walls. (v) Gypsum mortar These mortars are prepared from gypsum binding materials such as building gypsum and anhydrite binding materials. (c) Nature of application According to the nature of application, the mortars are classified into the following two categories. (i) Bricklaying mortars The mortars for bricklaying are intended to be used for brickwork and walls. Depending upon the working conditions and type of construction, the composition of masonry mortars with respect to the kind of binding material is decided. (ii) Finishing mortars These mortars include common plastering work and mortars for developing architectural or ornamental effects. The cement or lime is generally used as binding material for ordinary plastering mortar. For decorative finishing, the mortars are composed of suitable materials with due consideration of mobility, water retention, resistance to atmospheric actions, etc. (d) Special mortars Following are the various types of special mortars which are used for certain conditions. (i) Fire-resistant mortar This mortar is prepared by adding aluminous cement to the finely crushed powder of fire-bricks. The usual proportion is one part of aluminous cement to two parts of powder of fire-bricks. This mortar is fire-resistant and is, therefore, used with fire-bricks, for lining furnaces, fire places, ovens, etc. (ii) Lightweight mortar This mortar is prepared by adding materials such as saw dust, wood powder, etc. to the lime mortar or cement mortar. Other materials which may be added are asbestos fibres, jute fibres, coir, etc. This mortar is used for sound-proof and heat- proof construction. (iii) Packing mortar To pack oil wells, special mortars possessing the properties of high homogeneity, water resistance, predetermined setting time, ability to form solid waterproof plugs in cracks and voids of rocks, resistance to subsoil water pressure, etc. have to be formed. The varieties of packing mortars include cement-sand, cement-loam and cement- sand-loam. The composition of packing mortar is decided by taking into consideration the hydrogeologic conditions, packing methods and type of timbering. 1.20 Basic Civil Engineering (iv) Sound-absorbing mortar To reduce the noise level, the sound-absorbing plaster is formed with the help of sound-absorbing mortar. The bulk density of such a mortar varies from 6 to 12 kN/m3 and the binding materials employed in its composition may be Portland cement, lime, gypsum, slag, etc. The aggregates are selected from lightweight porous materials such as pumice, cinders, etc. (v) X-ray shielding mortar This type of mortar is used for providing the plastering coat to walls and ceiling of X-ray cabinets. It is a heavy type of mortar with bulk density over 22 kN/m3. The aggregates are obtained from heavy rocks and suitable Admixtures are added to enhance the protective property of such a mortar. 3. Properties of Mortar Following are the properties of a good mortar: 1. It should be capable of developing good adhesion with the building units such as bricks, stones, etc. 2. It should be capable of developing the designed stresses. 3. It should be capable of resisting penetration of rain water. 4. It should be cheap. 5. It should be durable. 6. It should be easily workable. 7. It should not affect the durability of materials with which it comes into contact. 8. It should set quickly for speedy construction. 9. The joints formed by mortar should not develop cracks and they should be able to maintain their appearance for a sufficiently long period. 4. Uses of Mortar Following are the uses of mortar: 1. To bind the building units such as bricks, stones, etc. into a solid mass. 2. To carry out pointing and plaster work on exposed surfaces of masonry. 3. To form an even and soft bedding layer for building units. 4. To form joints of pipes. 5. To improve the general appearance of a structure. 6. To prepare moulds for coping, corbels, cornice, etc. 7. To serve as a matrix or cavity to hold coarse aggregates, etc. 8. To distribute uniformly the super incumbent weight from the upper layer to the lower layer of bricks or stones. 9. To hide the open joints of brickwork and stonework. 10. To fill up the cracks detected in the structure during maintenance process, etc. 5. Selection of Mortar Depending upon the nature of civil engineering work, suitable type of mortar should be selected or recommended. Table 1.1 shows the types of mortars to be used for various civil engineering constructions.  Civil Engineering and Materials 1.21 Table 1.1 Selection of mortars S.No. Nature of work Type of mortar 1. Construction work in waterlogged areas and Cement or lime mortar in the proportion 1:3, lime exposed positions being eminently hydraulic lime 2. Damp-proof courses and cement concrete roads Cement mortar in the proportion 1:2 3. General RCC work such as lintels, pillars, slabs, Cement mortar in the proportion 1:3, the concrete stairs, etc. mix being 1:2:4 4. Internal walls and surfaces of less importance Lime cinder mortar proportion, being 1:3. Sand is replaced by ashes or cinder 5. Mortar for laying fire-bricks Fire-resisting mortar consisting of 1 part of aluminous cement to 2 parts of finely crushed powder of fire-bricks. 6. Partition walls and parapet walls Cement mortar in the proportion 1:3 or lime mortar proportion 1:1. Lime should be moderately hydraulic lime 7. Plaster work Cement mortar in the proportion 1:3 to 1:4 or lime mortar proportion 1:2 8. Pointing work Cement mortar in the proportion 1:1 to 1:2 9. Reinforced brickwork Cement mortar in the proportion 1:3 10. Stone masonry with best varieties of stones Lime mortar in the proportion 1:2, lime being eminently hydraulic lime 11. Stone masonry with ordinary stones, Lime mortar in the proportion 1:2 or cement mortar brickwork, foundations, etc. proportion 1:6. Lime should be eminently hydraulic lime or moderately hydraulic lime 12. Thin joints in brickwork Lime mortar in the proportion 1:3, lime being fat lime 1.5.5 Sand 1. Classification of sand According to the nature of source, sand is classified into two groups: (a) Natural Sand (b) Artificial Sand (a) Natural sand Is the one which is carried by the river water and is quarried from the river bed, when the river becomes dry. (b) Artificial sand Is the one which is the outcome of crushing and breaking stones into different sizes of stone aggregates in a stone crushing plant (or) crushed gravel sand. 2. Qualities of Good Sand (a) Sand should be clean, hard and durable and preferably dry. (b) It should be free from mica, chemical salts, organic and inorganic impurities and outer foreign matters. 1.22 Basic Civil Engineering (c) It should preferably be free from clay, silt and fine dust. In case if the presence of them is unavoidable, they should not be present by more than 5% by weight (or 7% by volume). (d) Sand particles should be well graded and shall have sizes ranging from (150 micron) 0.15 mm to 4.75 mm. (e) The fineness modulus of sand shall be from 1.6 to 3.5. 3. Uses of Sand (a) It is used for making mortar and concrete (b) It is used for filling in the basement of buildings to receive the flooring concrete. (c) It is used as a binding material on the top of bituminous road. (d) It imparts mechanical strength to the mortar and prevents shrinkage and cracking of mortar while setting. (e) It forms major portion of mortar and reduces the cost of mortar. (f) It is mixed with expensive clay soils to stabilise them and prevent cracking of clay soils due to seasonal moisture changes. 4. Tests on Sand The following tests are conducted to find out the suitability of sand. (a) Sieve analysis and fineness modulus test (b) Test for bulkage of sand (c) Test for silt content (a) Sieve analysis and fineness modulus test The sand is sieved through 1. S. Sieves 4.75 mm 2.36 mm, 1.18 mm, 600 micron, 300 micron and 150 micron sieves and percentage retained in each sieve is found out. Fineness modulus of sand = sum of the percentages retained in each sieve divide by 100. Requirement A fineness modulus of 1.6 to 2.0 for sand for plastering mortar and a fineness modulus of 2.5 to 3.5 for sand for concrete and a fineness modulus of 2.0 to 3.0 for sand for masonry mortar may be sufficient. (b) Test for bulkage of sand: bulking of sand The volume of dry sand will increase due to the presence of water in the sand up to about 25% of water content and thereafter it will decrease and become equal to its dry volume, when it is saturated with water. This increase in volume of sand is known as bulking of sand. River sand will generally be wet and its volume will be more than the dry volume. Hence, it is necessary to know the bulking of sand to allow for its increase in volume in the volume batching of concrete and mortar. The increase in volume of sand is found out from the test for bulkage of sand. Test for bulkage A small quantity of wet sand is poured into a glass measuring jar and rammed by a small rod of dia 6 mm. and level of sand is noted (say H1). Now water is poured into the cylinder until the sand is submerged and the glass jar is well shaken and now the level of sand is noted. (say H2). H2 will be less than H1 and sand is saturated when it is submerged.  Civil Engineering and Materials 1.23 (H1 – H2) Percentage bulkage of sand = _________ × 100 H2 (c) Test of silt content A small quantity of sand is poured into a glass measuring jar. Now water is poured until sand is well submerged in water. The glass jar is now shaken several times so that the silt and dust layer floats at the top of sand layer. The level of sand layer (excluding silt layer) is noted (say H2). The top level of silt layer above sand is noted. (say H1). (H1 – H2) The percentage of silt by volume = _________ × 100 H2 1.6 CEMENT CONCRETE Cement concrete is a mixture of cement, sand, crushed rock and water which when placed in the skeleton of forms and allowed to cure, becomes hard such as stone. Concrete has attained the status of a major building material in all branches of modern construction and hence it is necessary to know the properties and uses of concrete. 1.6.1 Properties of Concrete 1. It has a high compressive strength and its strength depends on the proportion in which cement, sand, stones and water are mixed. 2. It is free from corrosion and there is no appreciable effect of atmospheric agents on it. 3. It hardens with age and the process of hardening continues for a long time after the concrete has attained sufficient strength. 4. As it is weak in tension, steel reinforcement is placed in it to take up the tensile stresses. This is termed as ‘Reinforced Cement Concrete’. 5. It shrinks in the initial stage due to loss of water through forms. The shrinkage of cement concrete occurs as it hardens. 6. It has a tendency to be porous. This is due to the presence of voids which are formed during and after its placing. 7. It forms a hard surface, capable of resisting abrasion. 1.6.2 Uses of Concrete 1. Concrete can be made impermeable by using hydrophobic cement. This is used for the construction of RCC flat-roof slabs. 2. Coloured concrete is used for ornamental finishes in buildings, park lanes, separating lines of road surfaces, underground pedestrian crossings, etc. 3. Light weight concrete is used in multi-storeyed constructions. 4. No-fines concrete is one in which sand is eliminated. This can be used for cast-in- situ external load bearing walls of single and multi-storey houses, retaining walls, damp-proofing material, etc. 1.24 Basic Civil Engineering 5. Concrete is mainly used in floors, roof slabs, columns, beams, lintels, foundations and in precast constructions. 6. It is used in massive structures, such as dams and bridges. 7. Concrete is used in the construction of roads, runways, playgrounds, water tanks and chimneys. 8. It is used in the construction of sleepers in railways. 9. Prestressed concrete is a relatively new type of concrete which is used in many constructions particularly in the construction of bridges. 10. Concrete trusses are also used in factory constructions. 11. Concrete is used in the construction of bunkers, silos, etc. 12. It finds a place in the construction of nuclear reactors because of its high shielding capacity for the radioactivity. 13. Thin economical shell construction are possible with the use of concrete. 1.6.3 Reinforced Concrete Plain concrete is very weak in tension and cannot be used in the construction of lintels, roof slabs, beams, etc. in which the bottom fibres of them are subjected to tensile stresses. Figure 1.1 explains how a loaded beam or a slab is subjected to a flexural action when it is laid over an opening known as span. The top portion is compressed while the bottom portion is stretched. As concrete withstands compression but not tension, steel rods are embedded in the bottom portion to withstand the tension. A combination of concrete and steel is known as reinforced cement concrete and is widely used in various situations. Reinforcing bars are available from 6–32 mm diameter and of 22 feet length. They may be of mild steel or Tor steel and may be plain or twisted. Load Load Span Span (a) A plain concrete member (b) A reinforced concrete member subjected to flexure subjected to flexure Fig. 1.1 Flexure action  Civil Engineering and Materials 1.25 1.6.4 Advantages of Reinforced Concrete 1. Reinforced concrete is a versatile building material and can be used for casting members of any shape. 2. It has good resistance to fire, temperature and weathering actions. 3. RCC construction is easy and fast. 4. The component materials used for preparing RCC are easily available. 5. Monolithic construction is possible with the use of RCC. This increases the stability and rigidity of the structure. 6. RCC is tough and durable. 7. Maintenance of RCC construction is very cheap. 8. With proper cover, RCC can be made free from rusting and corrosion. 1.6.5 Types of Concrete 1. Light-weight concrete One of the disadvantages of normal concrete is the high self- weight which has a density of 2200 to 2600 kg/m3. This heavy self-weight causes heavy load and increases the haulage and handling costs. In order to make an economical concrete, attempts were made in the past to reduce the self weight of concrete. As a result the light weight concrete was developed whose density varies from 300–1850 kg/m3. Advantages of light-weight concrete (a) It has low density. (b) It has low thermal conductivity. (c) It lowers haulage and handling costs. Types of light-weight concrete (a) Light-weight aggregate concrete (b) Aerated concrete (c) No-fine concrete (a) Light-weight aggregate concrete By replacing the usual mineral aggregate by cellular porous or light weight aggregate, light-weight aggregate concrete can be produced. Light- weight aggregate can be classified into two categories namely natural and artificial light- weight aggregate. Natural light-weight aggregates are (i) Pumice (ii) Diatomite (iii) Scoria (iv) Volcanic cinders (v) Saw dust (vi) Rice husk 1.26 Basic Civil Engineering Artificial light-weight aggregates are (i) Artificial cinders (ii) Foamed slag (iii) Bloated clay (iv) Sintered fly ash (b) Aerated concrete By introducing gas or air bubbles in mortar, aerated concrete can be produced. This concrete is a mixture of water, cement and finely crushed sand with air or gas introducing agents. There are several ways in which aerated concrete can be manufactured. One important way is by the formation of gas or air bubbles using finely powdered metal (usually aluminium powder). Chemical reaction takes place in the concrete and finally large quantity of hydrogen gas is liberated which gives the cellular structure. (c) No-fine concrete By omitting sand fraction from the aggregate, no-fine concrete can be produced. This concrete is made up of only single-sized aggregate of size passing of 20 mm and retained on 10 mm coarse aggregate, cement and water. The single sized aggregate makes a good no-fine concrete, which in addition gives large voids and hence is light in weight. It also offers an architecturally attractive look. Out of the three main groups of light-weight concrete, the light-weight aggregate concrete and aerated concrete are more often used than the no-fine concrete. 2. High-density concrete The concrete whose unit weight ranges from about 3360–3840 kg/m3 and which is about 50 per cent higher than the unit weight of normal concrete is known as high-density concrete. The high-density concrete is mainly used in the construction of radioactive shields. High-density concrete is made by using such a heavy-weight aggregate whose specific gravity is more than 3.5. The aggregates used in this type of concrete should be clean, strong, inert and relatively free from deleterious material. Normally barite, magnetite, lemonite are used to make high-density concrete. To produce high density and high strength concrete, it is necessary to control the water – cement ratio, correct admixture and vibrators for good compaction. 3. Polymer Concrete Air voids and water voids are present in the conventional concrete due to improper compaction, high water-cement ratio and some other causes. Due to compaction, these voids are formed and the strength of the concrete is naturally reduced. There are number of methods available to reduce the air voids but none of these methods could really help to reduce the water voids. The impregnation of monomer and subsequent polymerisation is the latest technique adopted to reduce the inherent porosity of the concrete, to improve the strength and other properties of concrete. This type of concrete is known as polymer concrete. Types of polymer concrete 1. Polymer Impregnated Concrete (PIC)  Civil Engineering and Materials 1.27 2. Polymer Cement Concrete (PCC) 3. Polymer Concrete 4. Partially impregnated and surface coated polymer concrete The following are the monomers normally used in polymer concrete. 1. Methyl melthacrylate (MMA) 2. Styrene 3. Acrylonitrite 4. t-butyl styrene Impregnation of these monomers improves the compressive strength, tensile strength, flexural strength of concrete and gives the concrete a high freeze thaw resistance and also high resistance to sulphate and acid attack. Applications of polymer impregnated concrete 1. Prefabricated structural elements 2. Prestressed concrete 3. Marine works 4. Desalination plants 5. Nuclear power plants 6. Sewage works—pipes and disposal works 7. For waterproofing of structures 8. Industrial applications 4. Fibre-reinforced concrete Plain concrete possesses a very low tensile strength, limited ductility and little resistance to cracking. Due to its poor tensile strength, internal micro cracks are present in concrete which leads to brittle fracture. To improve the tensile strength of concrete one of the method used is that of the conventional reinforced steel bars and the other way is by introducing fibres in the concrete and thereby increasing the inherent tensile strength of concrete. In order to reduce the microcracks, addition of small, closely spaced and uniformly dispersed fibres are used. These fibres act as crack arrester and substantially improve its static and dynamic properties. This type of concrete is known as Fibre Reinforced Concrete (FRC). Some of the fibres used are steel fibres. polypropylene, nylons, asbestos, coir, glass and carbon. The property of concrete may vary depending upon the type, diameter, length and volume of fibres. Steel fibre is one of the most commonly used fibres. Most of the times round fibres are used. The diameter of such fibres may vary from 0.25–0.75 mm. The use of steel fibres may improve the flexural, impact and fatigue strength of concrete. Applications of fibre-reinforced concrete Normally, FRC are used in air field, road pavements, industrial floorings, bridge decks, canal lining, explosive resistant structures, refractory linings, etc. It can also be used in pre-cast products like pipes, boats, beams, staircase steps, wall panels, roof panels, manhole covers, etc. 1.28 Basic Civil Engineering 1.6.6 Testing of Fresh and Hardened Concrete 1. Testing of fresh concrete Fresh concrete or plastic concrete is a freshly mixed material which can be moulded into any shape. The most important property of fresh concrete is its workability. Workability The term workability is used to describe the ease or difficulty with which the concrete is handled, transported and placed between the forms with minimum loss of homogeneity. However, this gives a very loose description of this vital property of concrete which also depends on the means of compaction available. For instance, the workability suitable for mass concrete is not necessarily sufficient for thin, inaccessible or heavily reinforced sections. The compaction is achieved either by ramming or vibrating. The workability, as a physical property of concrete alone irrespective of a particular type of construction, can be defined as the amount of useful internal work necessary to produce full compaction. If the concrete mixture is too wet, the coarse aggregates settle at the bottom of the concrete mass and the resulting concrete has a non-uniform composition. On the other hand, if the concrete mixture is too dry, it will be difficult to handle and place it in position. To correlate these two conflicting conditions proportions of various components of concrete mixture should be carefully decided. The important facts in connection with workability are as follows: 1. If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability. 2. If the strength of concrete is not to be affected then, the degree of workability can be obtained in following ways: (i) by slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too wet. (ii) by adding a small quantity of water cement paste in the proportion of original mix, in case the concrete mixture is too dry. 3. A concrete mixture for one work may prove to be too stiff or too wet for another work. For instance, stiff concrete mixture will be required in case of vibrated concrete work while wet concrete mixture will be required for thin sections containing reinforcing bars. 4. The workability of concrete is affected mainly by water content, water – cement ratio and aggregate–cement ratio. 5. The workability of concrete is also affected by the grading, shape, texture and maximum size of the coarse aggregates used in the mixture. In order to measure the workability of concrete mixture, various tests are developed. Tests such as flow test and compaction test are used mostly in laboratory. The slump test, which is commonly used in the field, is briefly described below. It should, however, be  Civil Engineering and Materials 1.29 remembered that numerous attempts have been made to correlate workability with some easily determinable physical measurement. Although they may provide useful information within a range of variation in workability but none of these tests is fully satisfactory. At the same time, the slump test does not measure the workability of concrete. It is simply useful in detecting variations in the uniformity of a mix of given nominal proportions. Slump test The standard slump cone, as shown in Fig. 1.2 is placed on the ground. The operator holds the cone firmly by standing on the foot pieces. The cone is filled with about one- fourth portion and then rammed with a rod which is provided with bullet nose at the lower end. The diameter of the rod is 16 mm and its length is 60 mm. The strokes to be given for ramming vary from 20 to 30. The remaining portion of the cone is filled in with similar layers and then the top of concrete surface is struck off such that the cone is full of concrete. The cone is then gradually raised vertically and removed. The concrete is allowed to subside and then the height of concrete is measured. The slump of concrete is obtained by deducting the height of concrete after subsidence from 30 cm. Table 1.2 shows the recommended slumps of concrete for various types of concrete and Table 1.3 shows the classification of concrete mixes on the basis of slump. Top 10 cm Cone Outline of cone Slump Handle Concrete mix 30 cm 20 cm Base Base plate 20 cm (a) The cone filled with concrete paste (b) The settled paste after the cone is removed Fig. 1.2 Slump test 2. Testing of Hardened Concrete 1. Compressive Strength It may be defined as the maximum compressive load that can be taken by concrete per unit area. It has been shown that with special care and control, concrete can be made to bear loads as high as 80 N/mm2 or even more. In practice, however, concrete with compressive strength between 10–50 N/mm2 can be easily made on the site for common type of construction. 1.30 Basic Civil Engineering Table 1.2 Recommended slumps of concrete S.No. Type of concrete Slump 1. Concrete for road construction 20 to 40 mm 2. Concrete for tops of curbs, parapets, piers, slabs and walls that are horizontal 40 to 50 mm 3. Concrete for canal linings 70 to 80 mm 4. Concrete for arch and side walls of tunnels 90 to 100 mm 5. Normal RCC work 80 to 150 mm 6. Mass concrete 25 to 50 mm 7. Concrete to be vibrated 10 to 25 mm Table 1.3 Classification of concrete mixes Slump Nature of concrete mix No slump Stiff and extra stiff mix From 10 to 30 mm Poorly mobile mix From 40 to 150 mm Mobile mix Over 150 mm Cast mix The compressive strength, also called the crushing strength of concrete is determined by loading axially cube shaped (or cylindrical shaped) specimens made out of the concrete. The tests are carried out 3 days, 7 days and 28 days after the casting of the samples. It is the 28 days compressive strength which is taken as a standard value for concrete of a particular batch. It has been observed that the compressive (crushing) strength of concrete is influenced by a very large number of factors. The most important of these factors are the following: (i) Types of cement The composition, quality and age of the cement used in making concrete influences its strength. Thus, cement that has been stored for considerable time make concrete of lower strength despite all the other factors being the same. Cement with higher proportions of tri-calcium silicates produce concrete that show higher strengths, at least in earlier stages. Similarly, finer the particle size of the cement, higher is the ultimate compressive strength. Sand and coarse aggregates are the other two essential components (ii) Nature of aggregates of concrete. A good bond between cement and the aggregate is possible only when the latter have sharp edges, clean surfaces and rough texture. Smooth and rounded aggregates result in comparatively poor bonds. Similarly, the aggregates used in concrete making should have in themselves, good compressive strength. For example, if chalk (very soft limestone) is used in making concrete instead of massive limestone, the resulting concrete will be weak in compressive strength because of the poor strength of the aggregate. (iii) Water – cement ratio The compressive strength decreases, in general, with increasing water–cement ratio (other things being the same). Hence, when minimum water just to  Civil Engineering and Materials 1.31 ensure complete hydration of the cement is used, the resulting concrete will give maximum compressive strength on proper compaction. (iv) Curing conditionsGreat importance is attached to proper curing of concrete after its laying for obtaining maximum compressive strength. Incomplete curing and intermittent drying of concrete during the curing period may cause a loss in the compressive strength to the extent of 40 per cent or even more. (v) Weather conditions The same concrete placed in different weather conditions like extremely cold, dry and hot, may develop different strength values. The cause is related to incomplete hydration of the cement in the concrete. (vi) AdmixturesCertain admixtures are added to the concrete at the mixing stage for some specific purposes. It has been observed that certain admixtures especially calcium chloride, increase the compressive strength. Some other admixtures (e.g. air entraining agents), however, affect the compressive strength adversely if proper controls are not maintained on water–cement ratio. Improper mixing of the concrete and careless transport and (vii) Methods of preparation storing may result in poor strength despite best cement and aggregates used in it. It is the workmanship that determines the quality of the concrete work in ultimate analysis. A skilled worker can produce best concrete works despite some other deficiencies. An incompetent worker, however, may spoil the entire work despite being given the best designed concrete mix. The voids left in the concrete on compaction and curing have a profound influence on the strength of the concrete. 2. Tensile strength Plain concrete (without steel reinforcement) is quite weak in tensile strength which may vary from 1/8 to 1/20 of the ultimate compressive strength. It is primarily for this reason that steel bars (reinforcement) are introduced into the concrete at the laying stage so as to get a concrete which is very strong in compression as well as in tension. In plain concrete, tensile strength depends to a great extent on the same factors as the compressive strength does. Tensile strength of concrete becomes an important property when it is to be used in road making and runways. It is determined by using indirect methods. In one of such methods, it is derived from the flexural strength tests. In these tests, a beam of concrete is cast in standard dimensions depending upon the nominal size of the aggregate. The beam is properly cured and tested after 28 days. It is simply supported from below and equally loaded at