Indian Standard Plain & Reinforced Concrete

A STUDY OF NEW IS 456:2000 VISVIS-À-VIS IS 456:1978 BY: SURENDRA PRASAD SURVEYOR OF WORKS -II O/ 0 PCE BSNL, LUCKNOW

INDIAN STANDARD PLAIN & REINFORCED CONCRETE CODE OF PRACTICE (Fourth Revision) PREFACE:IS: 456 (fourth revision) was adopted by BIS, after the draft finalized by the Cement and concrete sectional committee had been approved by the Civil Engineering Division Council. The code was first published in 1953 and has undergone revisions in 1957, 1964, 1978 and 2000. This revision was taken up with a view to keeping abreast with the rapid development in the field of concrete technology and to bring in further modifications/improvement in the light of experience gained while using the earlier version. In recent years, durability of concrete structures has become cause of concern to all concrete technologists. Keeping the trend world over, this

revision codifies the durability requirements in order to introduce in-built protection from factors affecting a structure. A durable concrete is one, which performs, satisfactorily in the working environment during its anticipated exposure conditions during service. Main characteristics influencing the durability are its permeability to the ingress of water, oxygen, carbon dioxide, chloride, sulphate and other deleterious substances. The factor influencing durability is environment, cover to steel, type and quality of materials, the cement content and water cement ratio, workmanship and the shape and size of the member.

This revision has made specific provisions in respect of each of these factors. The inclusion of requirements for durability in this code is the most significant change vis-à-vis IS 456-1978. Other changes have been listed under the headings of new and modified clauses of this review.

REVIEW OF NEW CLAUSES OF IS 456: 2000 DETAILS OF NEWLY ADDED CLAUSES : 1. CLAUSE 1.1.1 For the purpose of this standard, plain concrete structures are those where reinforcement, if provided is ignored for determination of strength of structure. 2. CLAUSE 5.2 MINERAL ADMIXTURES New IS code permits use of fly ash, silica fume, rice husk ash and metakaoline in the category of pozzuolanic admixtures and ground granulated blast furnace slag in the category of non pozzola nic admixtures. In all these cases IS code specifies that uniform blending with cement shall be ensured. 3. CLAUSE 8.2 REQUIREMENTS OF DURABILITY New code defines requirements for durability under following heads: 1. Shape and size of member – The shape and size of members should be such as to promote good drainage of water and to avoid standing pools and rundown of water. 2. Exposure Conditions - The general environment to which concrete will be exposed has been categorised into mild, moderate, severe, very severe and extreme.

3. Abrasives – Special literatures may be referred to, for durability of concrete surfaces exposed to abrasive action in case of machinery and metal tyres. 4. Freezing and Thawing action - Percentage of air entrainment has been defined to get enhanced durability for grades of concrete lower than M50 and for 20mm and 40mm nominal size of aggregates. 5. Exposure to sulphate attack – New code gives recommendations for the type of cement, maxm. Free water/cement ratio and minm. Cement content, which are required at different sulphate concentrations in near- neutral ground water having pH of 6 to 9. 6. Nominal cover has been specified for different exposure conditions. For mild, moderate, severe, very seve re and extreme conditions the cover shall not be less than 20,30,45,50 and 75 mm respectively. 7. Concrete mix proportions - Approximate values for minimum cement content and the maximum free water cement ratio are given for different exposure conditions. 8. Mix constituents – For concrete to be durable, careful selection of the mix and materials is necessary, so that deleterious constituents such as chlorides, sulphates and alkali-aggregate reaction do not exceed the limits. 9. Concrete in aggressive soils and water – Where structures are partially submerged or are in contact with aggressive soils or water on one side only, the ground water shall be lowered by drainage so that it will not come into direct contact with the concrete. 10. Compaction, finishing and curing – Adequate compaction without segregation should be ensured by providing suitable workability. Overworking the surface and the addition of water/cement to aid in finishing shall be avoided. 11. Concrete in seawater - Concrete in seawater or exposed directly along the seacoast shall be at least M20 Grade in the case of plain concrete and M30 Grade in case of Reinforced concrete. Care shall be taken to protect the reinforcement from exposure to saline atmosphere during storage, fabrication and use. 4. CLAUSE 21.0 FIRE RESISTANCE. Minm. Requirement of concrete cover, member dimensions for concrete members, to have required fire resistance has been given. 5. CLAUSE 22.4.2.1 SUBSTITUTE FRAME (OLD CLAUSE 21.4.2)

Where side sway consideration becomes critical due to unsymmetrical geometry or loading, rigorous analysis may be required. 6. CLAUSE 28.1 CONCRETE CORBELS Special design requirements of concrete corbels are dealt in this clause. 7. CLAUSE 34.5 NOMINAL REINFORCEMENT IN FOOTINGS The new code specifies that minm. Reinforcement and spacings shall be as per requirements of the solid slab. The nominal reinforcement for concrete sections of thickness greater than 1m shall be 360 mm2 per metre length in each direction on each face. This provision does not supersede the requirement of minm. Tensile reinforcement based on the depth of the section. 8. CLAUSE 40.5 ENHANCED SHEAR STRENGTH OF SECTIONS CLOSE TO THE SUPPORT The enhancement of shear strength may be taken into account in the design of sections near a support by increasing design shear strength of the concrete to 2 d cc / av provided that design shear stress at the support remains less than ccmax.In this case ‘d’ is the effective depth, ‘cc’ is the shear strength of the concrete and ‘av ’ is the distance of the failure plane from the edge of the support. If shear reinforcement is required a formula has been given to calculate the same. 9. ANNEXURE F CALCULATION OF CRACK WIDTH Formulas have been given for calculation of the crack width.

REVIEW OF MODIFIED CLAUSES OF IS 456: 2000 DETAILS OF CLAUSES WHICH HAVE BEEN MODIFIED IN NEW CODE : 1. CLAUSE 5.1 CEMENT (OLD CLAUSE 4.1 CEMENT) In the new code ten types of cements are specified for use. All the three grades of Portland cement, namely 33 grade, 43 grade and 53 grade have been included. Both flyash based and calcined clay based Portland Pozzuolana cement has been covered. In

addition low heat Portland cement and sulphate resisting Portland cement have also been included. 2. CLAUSE 5.3.3 SIZE OF AGGREGATE (OLD CLAUSE 4.2.4) Earlier code specified that for RCC work, aggregates having nominal size of 20mm are considered satisfactory. As per new code, for most of the work 20 mm aggregate is suitable. Where there is no restriction to the flow of concrete into sections, 40 mm or larger size may be permitted. 3. CLAUSE 5.4 WATER (OLD CLAUSE 4.3) Permissible limits for sulphates and chlorides for RCC work have been reduced from 500 mg/l and 1000 mg/l respectively in the old code to 400 mg/l and 500 mg/l respectively in the new code. 4. CLAUSE 5.6 REINFORCEMENT (OLD CLAUSE 4.6) Hot rolled deformed bars conforming to IS: 1139-1966 existing in old code has been removed along with Rolled steel made from structural steel conforming to IS: 2261975.In their place Structural steel conforming to Grade A of IS 2062 has been substituted. 5. CLAUSE 6.1 GRADES OF CONCRETE (OLD CLAUSE 5.1) Earlier code specified grades from M10 to M40, whereas the new code specifies grades from M10 to M80. 6. CLAUSE 6.2.3.1 MODULUS OF ELASTICITY OF CONCRETE (OLD CLAUSE) The modulus of Elasticity can now be assumed as Ec= 5000( fck)1/2 in the earlier code it was 5700 instead of 5000. 7. CLAUSE 7.1 WORKABILITY (OLD CLAUSE 6.1) In the absence of proper correlation between compacting factor, vee bee time and slump, workability has now been specified in terms of slump only. 8. CLAUSE 9.2 DESIGN MIX CONCRETE (OLD CLAUSE 8.2) As per new code the contractor shall carry out the mix design and the mix so designed, shall be approved by the Employer. The target mean strength of the concrete shall be equal to the characteristic strength plus 1.65 times the standard deviation. The values of standard deviation if not calculated shall be assumed as per values given in the concrete

for different grades of the concrete. Minm. grade of concrete recommended for RCC shall be M20. 9. CLAUSE 10.2 BATCHING (OLD CLAUSE 9.2) The accuracy of the measuring equipment shall be within + - 2% of the quantity of cement being measured and within + - 3% of the quantity of aggregate, admixtures and water being measured. There was no such limit in the old code. 10. CLAUSE 11.3 STRIPPING TIME (OLD CLAUSE 10.3 ) Vertical formwork to columns, walls and beams can now be removed after 16 to 24 hours. The period for the same work was 24 to 48 hours in the old code. 11. CLAUSE 13.2 PLACING OF CONCRETE (OLD CLAUSE 12.2) The maximum permissible free fall of concrete has been defined as 1.5 m. No such value was specified in the old code. 12. CLAUSE 13.4 CONSTRUCTION JOINTS (OLD CLAUSE) This clause has been generalised and the numerical limits placed on the thickness of fresh mortar and concrete on old concrete, which were there in the old code have not been removed. 13. CLAUSE 16.0 ACCEPTANCE CRITERIA (OLD CLAUSE 15.0) Acceptance criteria of the concrete have been changed. Minm. Acceptable mean strength and individual strength has been defined for acceptability both in respect of compressive strength and flexural strength. 14. CLAUSE 17.0 INSPECTION (OLD CLAUSE 16.0) The clause on inspection has been modified to give more emphasis on quality assurance. 15. CLAUSE 22.2 EFFECTIVE SPAN (OLD CLAUSE 21.2) Effective length of cantilever has been added. It shall be taken as its length to the face of the support plus half the effective depth except where it forms the end of the continuos beam, where length to the centre of the support shall be taken. 16. CLAUSE 23.2.1 MODIFICATION FACTOR FOR TENSION REINFORCEMENT Graph has been modified. Earlier it was given in terms of mild steel, Fe 415 and Fe500 but now it has been given in terms of Fs, where

Fs= 0.58 fy x Area of cross- section of steel required. Area of cross-section of steel provided. 17. CLAUSE 24.4.1 RESTRAINED SLABS WITH UNEQUAL CONDITIONS AT ADJACENT PANEL. The values of moments obtained from coefficients given for two-way slabs have to be modified in accordance to the provisions of this clause, for restrained slabs with unequal conditions at adjacent panels. 18. CLAUSE 26.2.5.1 LAP SPLICES (OLD CLAUSE 25.2.5.1) Where lap occurs for a tension bar located at the top of a section as cast and minm. cover is less than twice the dia. of the lapped bar ,the lap length shall be increased by a factor 1.4 . Where lap occurs for a tension bar located at the corner of a section and the minimum cover to either face is less than twice the diameter of the lapped bar or where the clear distance between the adjacent laps is less than 75mm or 6 times the diameter of the lapped bar, whichever is greater, the lap length should be increased by a factor of 1.4. These provisions were not there in the old code. 19. CLAUSE 26.2.5.2 STRENGTH OF WELDS (OLD CLAUSE 25.2.5.2) The design strength of mechanical connection can now be assumed as 100% of the strength of joined bars. Earlier this value was 80%. 20. CLAUSE 26.3.3 MAXIMUM DISTANCE BETWEEN BARS IN TENSION (OLD CLAUSE 25.3.2) In slabs the horizontal distance between parallel reinforcement bars shall not be more than three times the effective depth or 300 mm whichever is smaller. Earlier it was 450mm instead of 300 mm. 21. CLAUSE 26.4.1 NOMINAL COVER TO REINFORCEMENT ( OLD CLAUSE 25.4) Nominal cover has now been linked to durability and hence to degree of exposure. The values for mild, moderate, severe, very severe and extreme are 20,30,45,50 and 75mm respectively. For footings the minimum cover is recommended to be 50mm. 22. CLAUSE 26.5.1.5 MAXM. SPACING OF SHEAR REINFORCEM ENT (OLD CLAUSE 25.5.1.5)

The maxm. Spacing of shear stirrup shall not exceed 0.75d or 300mm whichever is less. Earlier it was limited to 450mm instead of 300 mm. 23. CLAUSE 26.5.1.6 MINM. SHEAR REINFORCEMENT (OLD CLAUSE 25.5.1.6) Minm. Shear reinforcement shall be such that : ASV >= 0.4 B sv 0.87 fy Where Asv= Area of stirrup B= breadth Sv =spacing of stirrup Fy = characteristic strength of steel Earlier 0.87 was not there in the denominator. 24. CLAUSE 26.5.3.2 PITCH OF LATERAL TIES (OLD CLAUSE 25.5.3.2) Pitch shall not be more than the least lateral dimension nor 16 times the smallest dia. of the longitudina l bar nor 300 mm. In the earlier code the third condition was 48 times the dia. of transverse reinforcement instead of 300 mm. 25. CLAUSE 32.0 WALLS (OLD CLAUSE 31.0) Walls have been dealt in greater detail in the new code. The code now gives empirical design method for walls subjected to in plane vertical loads. Formulas have now been given for effective height, axial strength of the wall, for design of walls for horizontal shear and design of shear reinforcement. Earlier clause that walls can be designed as columns and then their strength increased depending upon the slenderness ratio , has now been removed. 26. CLAUSE 35.3.2 CRACKING (OLD CLAUSE 34.3.2) New code gives a formula for calculation of crack width, which was not there in the old code. It limits the crack width to 0.3 mm in general cases, to 0.2 mm in cases where cracking in tensile zone is harmful and to 0.1 mm in particularly aggressive environments. Old code specified that in particularly aggressive environments crack width should not exceed 0.004 times the nominal cover to the main reinforcement. 27. ANNEXURE B WORKING STRENGTH METHOD (OLD CODE SECTION 6) Everything is same except that values of different parameters are now given up to M50. Earlier values were given for grades up to M40.

28. ANNEXURE ‘E’ EFFECTIVE LENGTH OF COLUMNS (OLD CODE APPENDIX ‘D’) To determine whether a column is a no sway or a sway column, a formula has been given to calculate stability index. If stability index is less than or equal to 0.04 then the column in the frame may be taken as no sway column.