O F RELEVANT STANDARDS

All the codes on material that have been referred to in this chapter listed as follows:

It has come to the notice of the authors that same educational institutions conducting commercial tests have been adapting the method even for high strength reinforcing steel specimens. As a result, yield strengths greater than the real values have been erroneously reported. Hence, this note of caution.

MATERIAL PROPERTIES 71 Standards on Cement

IS 269 : '1989 - Spkcificaiion for 33 Grade ordinary Portland cement (fourth revision);

IS 8112 : 1989 Specification for 43 Grade ordinary Portland cement (first revision);

IS 12269 : 1987 -Specification for 53 Grade Portland cement;

IS 8041 : 1990 -Specification lor rapid hardening Portland cement (second revision);

IS 455 : 1989 -Specification for Portland slag cement (fourth revision);

IS 1489 : 1991 -Specification for Portland pozzolana cement I based (third revision);

P a r t : Calcined clay based (third revision);

IS 8043 : 1991

-

Spccificntion for hydrophobic Portland cement (second revision);

IS 12600 : 1989 for low heat Portlandccment;

IS 12330 ^: 1988 Specification for sulphate resisting Portland cement;

IS 8042 : 1978 -Specification for Portland white cement (first revision);

IS 8043 : 1991 Specilication Tor hydrophobic Portland white (second revision);

IS 6452 : 1989 Spccificntian for high cement for structural use (first revision);

IS 6909 : 1990 for supersulphated cement (first revision);

IS 4031 : 1988 of physical tests for hydraulic cement;

Standards on Aggregate, Water and Admixtures

IS 383 : 1970 - Specification for coarse and fine aggregates from natural sources for concrete (second revision);

IS 9142 : 1979 Specification for artificial lightweight aggregates for concrete masonry units;

IS 2386 (Parts 1-8) of tests for aggregate for concrete;

IS 3025 (Parts 17-32) of sampling and test (physical and chemical) for wntcr and waste water;

IS 9103 : 1999 -Specification for admixtures for concrete (first revision);

IS 3812 : 1981 - Specification lor for use as pozzolana and admixture (first revision);

IS 1344 : 1981 -Specification for calcined clay pozzolana (second revision);

Standards on Concrete

IS 10262 : 1982 -Recommended guidelines for concrete nux design;

72 REINFORCED CONCRETE DESIGN

IS 1343 : 1980 - Code of Practice for Prestressed Concrete (first revision);

S t a n d a r d s o n R e i n f o r c i n g S t e e l

(Part 1) : 1982 - Specification for mild and medium tensile steel bars for concrete (third revision);

IS 1786 : 1985 - Specification for high strength deformed steel bars for concrete reinforcement (third revision);

IS 1566 : 1982 Specification for steel wire fabric for concrete reinforcement (second revision);

IS 2062 : 1999 - Steel for general structural purposes- Specification revision);

IS 1608 : 1995 -Mechanical testing of Metals Tensile testing (second revision).

2.1 What are the types of cement that are for (a) mass resistance to suiphate attack?

2.2 How can the development of strength and heat of hydration be controlled in cement manufacture?

2.3 Can the use of excessive cement in concrete be

2.4 What do the terms setting and mean, with reference to cement paste?

2.5 What is the basis for deciding the maximum size of coarse aggregate in concrete work?

2.6 What is meant by segregation of concrete? Under what circumstances does it take place?

2.7 What is by of concrete, and how is it 2.8 Discuss the role of water in producing 'good' concrete.

2.9 Mention the types of 'admixtures' and their applications.

MATERIAL PROPERTIES

2.10 (a) Define characteristic strength. Determine the 'mean target strength' required for the mix design of M25 concrete, assuming moderate quality control.

2.1 Enumerate the steps involved in the Indian Standard method o f design.

2.12 Why is the strength different from the cylinder for the same grade of concrete?

2.13 Can concrete be assumed to he a linear elastic material? Discuss.

2.14 Distinguish between static modulus and dynamic modulus of elasticity of concrete.

2.15 Discuss the variations of lateral and volumetric strains that are observable in a typical uniaxial compression test on a concrete prism.

2.16 Why the Code limit the compressive strength of concrete in structural design to 0.67 , and not

2.17 Is the modulus of rupture of concrete equal to its direct tensile strength?

Discuss.

2.18 The standard flexure test use of a 'third-point loading'. Is this necessary?

Can a single point load at be used as alternative?

2.19 Why is it not possible to determine the shear strength of concrete subjecting it to a state of pure shear?

2.20 What is the advantage of confinement of concrete? Give suitable examples to illustrate your point.

2.21 What does 'creep of concrete' Is creep harmful or beneficial?

2.22 How is it that deflection of a simply supported reinforced concrete beam increases due to shrinkage of concrete?

2.23 Consider a simple portal frame (with fixed base) made of reinforced concrete.

Sketch the approximate shape of the deflection curve caused (a) a uniform shrinkage strain, (h) a uniform temperature rise.

2.24 Consider the temperature gradient across the shell thickness of a reinforced concrete chimney (with tubular cross-section). Where would you provide reinforcing steel to resist tensile stresses due to the effect of temperature alone (caused the emission of hot gases): close to the outer circumference or close to the circumference? Justify your answer.

2.25 How would you define 'durable concrete'? Discuss the ways of ensuring durability.

2.26 Cite two each for the five categories of 'environmental exposure' described in the Code.

2.27 Describe the main factors that affect the permeability of concrete.

2.28 Discuss briefly the factors that lead to corrosion of reinforcing steel.

REINFORCED WNCRETE DESIGN

2.29 What steps can a designer adopt at the design stage to ensure the durability of reinforced concrete offshore structure?

2.30 What is meant by strain hardening of steel? How is it related to the grade of reinforcing Steel?

2.31 What is meant by cold-working of mild steel? How does it affect the structural properties of the steel?

2.32 What is Bauschinger Where is it relevant?

Neville. A.M.. of Concrete, Second edition, Publishing Co., London, 1973.

Mehta, P.K. and P.J.M., Concrete Microstructure, Properties Materials, Indian edition, Indian Concrete Institute, 1997.

Neville, A.M. and J.J., Concrete Technology, ELBS edition, London, 1990.

Design of Concrete Mixes, Special Publication Bureau of Indian

standards, 1982.

P.S. and P.K.. Concrete Mix Design Practice -Need for a Fresh Approach, Indian Journal, May 1990, pp 234-237

Price, W.H., Factors Influencing Concrete Strength, Journal ACI, 47, Feb. 1951, pp

-Guide for Use of Admixtures in Concrete, ACI Committee Report 212.2 81, Conc. Detroit, Michigan, USA, 1981.

Cement and Concrere Terminology, ACI 116) Special Publication SP-19, Conc, Detroit, Michigan, USA, 1967.

Ellingwood, B. and T.V. and G. and C.A.,

Development of a Probability Based Load Criterion for American National Standard Special Publication National Bureau of Standards,

1980.

2.10 - Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete. ACI Standard 211.1-81, Am. Conc. Detroit, Michigan, USA, 1981.

2.11 ^- Standard Practice for Selecting Proportions for Structural Lightweight Concrete, ACI Standard 211.2-81, Am. Conc. Detroit, Michigan, USA, 1981.

2.12 ^- Standard Practice for Selecting Proportions for No-Slump Concrete, ACI Standard 211.3-75 (revised), Am. Conc. Detroit, Michigan, USA, 1980.

2.13 Teychenne, D.C., Franklin, R.E., H.C., Design of Normal Concrete Mixes, Dept, of Environment, Her Majesty's Stationary Office, London, 1975.

2.14 Hardened Concrete 'Tests and Properties of

ASTM Publication No. 169-A, Am. for Testing and

Materials, 1966, pp

2.15 Hsu, T.T.C. et Microcracking of Plain Concrete and the Shape of the Stress-Strain Curve, Journal ACI, 60, Feb. 1963,

I

MATERIAL PROPERTIES 75

2.16 Kupfer, H., H.K. and H., Behaviour of Concrete Under Stresses, ACI, 66, Aug. 1969, pp

2.17 Hognestad, E., N.W. and D., Concrete Stress Distribution in Ultimate Strength Design, Journal ACI, 52, 1955,

2.18 K.H. and L.G., Stress-Stmix f o r Concrete, Journal Vol. 57, July 1960, 1-28.

2.21 -Building Code Requirements for Reinforced Concrete, ACI Standard 89, Am. Conc. Dctroit, Michigan, USA, 1989.

2.22 P.S. and D., Strength of Tubular R C Tower Sections Under Indian Concrete Journal, Feb. 1995,

2.23 Wright, P.J.F., an Indirect Tensile on Cylinders, Magazine of Concrete Research, No. 20, 1955, p. 87.

2.24 Tasuji, M.E., Slate, F.O. A.H., Stress-Strain Response Fracture of Concrete Loading, Journal ACI, Vol. 75, July 1978, pp

2.25 B. and Pister, K.S., Strength of Concrere Under Combined Stresses, ACI, 55, Sept. 1958,

2.26 F.E., A. and Brown, R.L., A Study of the Failure of

Concrete of Illinois Engineering

Experimental Station, Bulletin No. 1928.

2.27 et Strength of Under Multi-axial States, ACI

Publication SP-55, Am. Conc. Michigan, USA, 1978, pp 131.

2.28 Park, R. and T., Concrete Structures, John Sons, New York, 1975.

2.29 ACI Committee 209, Prediction of Creep, Shrinkage and Temperature Effects in Concrete SP-27, Am. Conc. Detroit, Michigan, USA, 1971, 51-93.

2.30 CEB-FIP, International for the Construction of

Concrere du de

la 1970.

2.31 Gouthaman, A. and D., Increased Cover in IS 456 (2000) Crack-width in RC Slabs, Indian Concrete Journal, Sept.

2001,

2.32 ACI Committee 201, to Durable Concrete, Journal ACI, Vol. 74, 1977, pp

Explanatory on Indian Standard Code of for

(IS Special Publication Bureau of Indian Standards, New 1983.

76 REINFORCED CONCRETE DESIGN

2.34 Purushothaman, P., Concrete Elements Analysis and Design. Tata Publication Co. Ltd., New 1984.

3.1 INTRODUCTION

Having gained a general overview of reinforced concrete structures (Chapter and an understanding of the basic material properties (Chapter 2), it is time to get into the actual details of the design process. This chapter introduces the basic concepts relating to criteria in reinforced concrete design.

3.1.1 Design

The aim of structural design is to design a structure so that it fulfils its intended purpose during its intended lifetime with adequate safety (in terms of strength,

. stability and structural integrity), adequate serviceability (in terms of stiffness, durability, and economy.

Safety that the likelihood-of (partial or total) collapse of the structure is acceptably low not only under the normal expected loads (service loads), but also under abnormal but probable overloads (such as due to earthquake or wind).

Collapse may occur due to various possibilities such as exceeding the

capacity, overturning, sliding, buckling, fatigue fracture, etc. Another related aspect of safety is structural integrity (see Section 15.1.3). The objective here is to

the likelihood of progressive collapse.

Serviceability implies satisfactory performance of the structure under service loads, without discomfort to the user due to excessive deflection, cracking,

Other considerations that come under the purview of serviceability are durability, impermeability, acoustic and thermal etc. A design that adequately the 'safety' requirement need not necessarily satisfy the 'serviceability' reqnirement. example, a thin reinforced concrete slab can be made safe against collapse (by suitable reinforcement); but if it is too thin, it is likely to result in deflections, crack-widths and permeability (leakage), and the steel becomes vulnerable to corrosion (thereby affecting durability).

REINFORCED CONCRETE

the design . safety can enhance this increases the cost of the structure. In considering ove cost associated with increased should be w

and serviceability; but economy, the increased eighed against the potential

...

. .

losses that could result from any damage.

3.1.2 , Design Philosophies

Over the years, various design philosophies have evolved in different parts of the world, with regard to reinforced concrete design. A 'design philosophy' is built up on a few fundamental (assumptions), and is reflective of a of thinking.

The earliest codified design philosophy is the working stress method of design (WSM). Close to a hundred years old, this traditional method of design, based on linear elastic. is still surviving in some countries (including although it i s now sidelined by states design philosophy. In the recent (2000) revision of the Code (IS the provisions relating to the WSM design procedure have been relegated from the main text of the Code to an Annexure (Annex "so as to give emphasis to limit state design" (as stated in the 'Foreword').

Historically, design procedure to follow the WSM was the ultimate load method of design (ULM), which was developed in the Based on the (ultimate) strength of reinforced concrete at ultimate loads, it evolved and gradually gained acceptance. This method was introduced as an alternative to WSM in the ACI code in 1956 and the British Code in 1957, and subsequently in the Indian Code (IS 456) in 1964.

Probabilistic concepts of design developed over the and received a major impetus from the mid-1960s onwards. The philosophy was based on the theory that the various uncertainties in design could he handled more rationally in

framework of probability theory. The risk involved in the design was quantified in of offailure. Such probabilistic methods came to be known as methods. However, there was little acceptance for this theory in professional practice, mainly because the theory appeared to be complicated and intractable (mathematically and numerically).

In order to gain code acceptance, the probabilistic approach had to be simplified and reduced to a deterministic format involving multiple (partial) safety factors (rather than probability of failure). The European Committee for Concrete and the International Federation for Prestressing were among the earliest to introduce the philosophy of stales (ISM) of design, which

is in concept [Ref. Based on the recommendations.

LSM was introduced in the British Code CP 110 (1973) [now BS 8110 and the Indian Code IS 456 (1978). In the United LSM was intmduced in a slightly different format design and design) in the ACI 318-71 (now

Thus, the past several decades have witnessed an evolution in design philosophy - from the traditional 'working stress method', through the 'ultimate load method',

to the modern 'limit states method' of design.

history, consult Ref

DESIGN CONCEPTS 3.2 WORKING STRESS METHOD (WSM)

This was the traditional method of design not only for reinforced concrete, but also for structural steel and timber design. The conceptual basis of WSM is simple. T h e method basically assumes that the structural material behaves in a linear elastic manner, and that adequate safety can be ensured by suitably resmcting in the induced the expected (service loads) on the structure.

As the specified ('allowable') are kept the material

strength in the initial phase of the stress-strain curve), the assumption of linear elastic behaviour is considered justifiable. The ratio of the strength of the material to the permissible stress is often referred to as the factor of safety.

The stresses under the applied loads are analysed by applying the methods of 'strength of materials' as simple bending theory. In order to apply such methods to a composite material like reinforced concrete, (due to bond) is assumed, whereby the strain in the reinforcing steel is assumed to he equal to that in the adjoining concmte to it is bonded. Furthermore, as the in concrete and steel are assumed to be linearly related to their strains, it follows that the stress in is linearly related to that in the adjoining concrete by a constant factor (called the ratio), defined as the ratio of elasticity of to that of concrete.

However, the main assumption of linear elastic behaviour the tacit assumption that stresses under working loads can be kept within the 'permissible stresses' are not found to be realistic. Many factors are responsible for this

-

such as the long- term effects of creep and shrinkage, the effects of stress concentrations, and other secondary effects. All such effects result in significant local increases in and redistribution of the calculated stresses^t. Moreover, WSM does not provide a realistic measure of the actual factor of safety underlying a design WSM also fails to discriminate between types of loads that act simultaneously, but have different of uncertainty. This can, at times, result in very

designs, particularly when two different loads (say, dead loads and wind loads) have counteracting effects [Ref.

in defence against these other shortcomings levelled against WSM, it may be stated that most structures designed in accordance with WSM have generally performing satisfactorily for many years. The design usually results in relatively large sections of structural members (compared to ULM and thereby resulting in better serviceability performance (less deflections, crack-widths, under the usual working loads. The method is also notable for its essential simplicity - in concept, as well as application.

It may also he noted that although WSM has been superseded by the limit states method in design code for general RC structures (IS it continues to

F o r example, in case of reinforced concrete columns subjected sustained service it is found that of takes with to such an extent that the 'permissible' stress in reinforcing will not only exceeded, but the stress is even likely to reach yield stress thereby upsetting the assumptions and of WSM based on a constant modular ratio [Ref.

be the accepted method of design in India for special structures such as RC bridges water tanks (IS 3370) and chimneys (IS 4998).

In document Reinforced Concrete Design-PILLAI & MENON-FULL (Page 46-51)