Is 456

AN OVERVIEW OF REVISIONS IN IS 456 : 2000 ‘CODE OF PRACTICE FOR PLAIN AND REINFORCED CONCRETE’

1. INTRODUCTION 1.1 Concrete is one of the most versatile materials for construction industries. Making quality concrete requires proper technical knowledge of concrete materials, its selection, proportioning, mixing, placing, compaction and curing. This has to be supplemented by efficient design, detailing, appropriate construction method, quality control, site supervision and maintenance during service. IS 456 has been introduced in order to have an uniform standard to be maintained by all the users. 1.2 Indian Standard for Plain and Reinforced Concrete – Code of Practice (IS 456) is one of the most important and widely used code dealing with concrete materials, workmanship, inspection, testing and the design requirements for concrete structures. This standard was first published in 1953 under the title ‘code of practice for plain and reinforced concrete structures for general building construction’ and was subsequently revised in 1957. The second revision of the code was published in 1964 under title ‘code of practice for plain and reinforced concrete’; thus enlarging the scope of use to structures other than general building construction also. The ultimate load method of design was also introduced in this revision. The third revision published in 1978 introduced Limit State approach to design. 1.3 The 2000 edition is the fourth revision of the standard. This revision has brought out a number of important changes with a view to keeping abreast with the rapid developments in concrete technology, to enlarge the provisions on durability consideration and also for improvements in the light of experience gained while using the earlier version of the standard. 1.4 This report is prepared to highlight all the important changes/modifications

brought out in the 2000 edition of IS 456 with a brief commentary thereupon.


2. Revisions in Section 2 : Materials, Workmanship, Inspection and Testing

2.1 Materials 2.1.1 Cement (Cl. 5.1) :

All three grades of OPC viz. 33, 43 & 53, Low Heat Portland cement and Sulphate resisting Portland cement have been included under various types of cement. In the manufacturing of concrete, the code also permits other combination of Portland cement with mineral admixtures of quality listed in Cl. 5.2.

2.1.2 Mineral Admixtures (Cl. 5.2) :

The mineral admixtures, which may be used as part replacement of cement, listed in the code are Pozzolanas (such as fly ash, silica fume, rice husk ash and metakaoline) and Ground Granulated Blast Furnace Slag.

2.1.3 Aggregates (Cl. 5.3) :

(i) Preference has been given to the use of natural aggregates as far as possible. Most of the zonal railway specifications permit use of only crushed aggregate for RCC structures. It is considered that this should be reviewed to generate greater use of natural aggregates.

(ii) The limit of sulphate content in other types of aggregates such as slag etc. has been reduced from 1% to 0.5%.

(iii) According to the revised code, 40 mm or larger size aggregates may be permitted where there is no restriction to the flow of concrete in the section.

(iv) The code recommends considering use of 10 mm nominal maximum size aggregate for thin section, closely spaced reinforcement and smaller cover.

2.1.4 Water (Cl. 5.4)

(i) From durability considerations, permissible limits of solids in water have been reduced as under:

• Sulphate - from 500 mg/l to 400 mg/l • Chloride - from 1000 mg/l to 500 mg/l for RCC works


(ii) The 1978 edition of the code allowed use of seawater under unavoidable circumstances in such RCC structures, which are permanently under seawater. But the present revision prohibits this.

2.1.5 Admixtures (Cl. 5.5)

The 1978 edition contained a single sentence permitting use of admixture conforming to IS-9103. Considering increase in the usage of several types of admixtures available in the market, the new revision has given following detailed guidelines on their use under sub clause 5.5.1 to 5.5.6. • Admixtures should not impair durability of concrete.

• Workability, compressive strength and slump of concrete should be checked for both with and without admixtures in trial mix.

• Relative density of liquid admixtures to be checked.

• Chloride content to be independently tested.

• If two or more admixtures are used simultaneously, data should be obtained to assess their interaction to ensure their compatibility.

2.1.6 Reinforcement (Cl. 5.6)

i) Use of hot rolled deformed bars conforming to IS 1139 has been

withdrawn.

ii) Use of structural steel conforming to Grade A of IS 2062 has been introduced in place of IS 226 as the latter code is superceded by the former.

iii) Cleaning of reinforcement by sand blasting or other treatment has been recommended.

iv) Reference to specialist literature has been recommended for special precautions like coating of reinforcement in exceptional cases.

2.2 Concrete (Cl. 6) 2.2.1 Grade of Concrete (Cl. 6.1)

i) Higher grade of concrete up to M 80 has been permitted (earlier revision was permitting maximum grade M 40). This is a progressive provision in line with recent trend for using high strength concrete in India and abroad. However, the code has cautioned that for high strength concrete (compressive strength greater than M 55), the design parameters given in this standard


may not be applicable and the values may be obtained from specialist literatures and experimental results.

ii) The grades of concrete have been classified in following three

groups:

a) Ordinary Grade M 10 to M 20 b) Standard concrete M 25 to M 55 c) High strength concrete M 60 to M 80

iii) The minimum grade of concrete for plain and reinforced concrete in

various exposures conditions have been revised as under:- Minimum grade of concrete Exposure condition

Plain Cement Concrete

Reinforced Cement Concrete

Mild - M 20 Moderate M 15 M 25 Severe M 20 M 30 Very Severe M 20 M 35 Extreme M 25 M 40

2.2.2 Properties of concrete (Cl. 6.2)

(i) IS 456 : 1978 allowed increase in compressive strength of concrete up to 20% depending upon the age of concrete where it could be shown that a member would not receive its full design load/stress within a period of 28 days. The new revision though acknowledges the increase in compressive strength with age but recommends design based on 28 days strength only unless there is evidence to justify higher strength for concrete of a particular structure. This is because the increase in strength depends upon the grade and type of cement, curing, environment conditions, etc. For concrete of Grade M 30 and above, the rate of increase of compressive strength with age should be based on actual investigation.

(ii) The value of Modulus of elasticity has been reduced from 5700√fck to 5000√fck, which means that the deformation of the structure will be more.

2.3 Workability of concrete (Cl. 7)

IS 456 : 1978 specified workability in terms of compacting factor, vee-bee

time and slump. But the new revision specifies workability only in terms of slump; except in case of “very low” and “very high” degree of workability, where compaction factor and flow determination method respectively have been


specified. This is probably because of the absence any correlation between the three test methods. 2.4 Durability of concrete (Cl. 8)

Major thrust/emphasis has been given on durability aspects. The durability clause has been enlarged to a great extent to include guidance on concerning factors. Detailed clause covering various requirements for durability of concrete structures has been incorporated.

2.4.1 Shape and size of member (Cl. 8.2.1)

For the first time, importance has been given to shape and design detailing to enhance durability of exposed concrete structures. Specific mention has been made regarding good drainage arrangement, adequate curing, cover to steel, chamfering of corners, surface coating, member profiling and design detailing of member intersections to ensure easy flow of concrete.

2.4.2 Exposure conditions (Cl. 8.2.2)

2.4.2.1 General Environment (Cl. 8.2.2.1)

Table 3 on environmental exposure conditions has been modified to include “very severe” and “extreme” exposure conditions. Five environmental exposure conditions (viz. Mild, moderate, severe, very severe and extreme) have been defined in this table.

2.4.2.2 Abrasive action (Cl. 8.2.2.2)

Reference to specialist literature has been recommended for durability requirements of concrete surface exposed to abrasive action.

2.4.2.3 Freezing and thawing (Cl. 8.2.2.3)

Use of suitable air entraining admixtures has been suggested for obtaining enhanced durability in case of freezing and thawing actions under wet conditions. For concrete lower than grade M 50, the mean total air content has been specified for such cases. Since air entrainment reduces the strength of concrete, suitable adjustment in the mix design may be required.

2.4.2.4 Exposure to sulphate attack (Cl. 8.2.2.4)

Table 4 (on requirements for concrete exposed to sulphate attack) has been modified to include two more classes of sulphate attack viz. Class 4 and 5. For very high sulphate concentration in class 5, use of lining with polyethylene or polychloroprene sheet or suitable surface coating has been recommended.

2.4.3 The cover to embedded steel (Cl. 8.2.3)


The nominal cover to embedded steel required from durability consideration has been related with exposure conditions vide Table 16. Tolerances for concrete cover have also been specified. In addition, minimum cover is also specified to meet different specified period of fire resistance from 0.5 hour to 4 hours.

Nominal cover has been defined as the depth of concrete cover to all

steel reinforcement, including links.

2.4.4 Concrete mix proportions (Cl. 8.2.4)

From durability considerations appropriate values for minimum cement content and the maximum free water-cement ratio applicable to 20 mm nominal maximum size aggregate have been specified in Table 5 for different exposure conditions. Adjustment for minimum cement content for other aggregate size has been given in Table 6.

2.4.5 Maximum cement content (Cl. 8.2.4.2)

A new clause 8.2.4.2 has been added specifying that cement content not including fly ash and ground granulate blast furnace slag in excess of 450 kg/m3 should not be used unless special consideration has been given to the increased risk of cracking due to drying shrinkage in thin sections or to early thermal cracking and to increased risk of damage due to alkali silica reactions. This provision may lead to increased use of mineral admixtures as a part replacement of cement particularly for higher grade of concrete leading to likely enhancement of durability.

2.4.6 The type and quality of mix constituents (Cl. 8.2.5)

For concrete to be durable, careful selection of the mix and materials is necessary so that the presence of deleterious constituents do not exceed the prescribed limits.

2.4.6.1 Chloride in concrete (Cl. 8.2.5.2)

Chloride in concrete is harmful and there is an increased risk of embedded steel being corroded. To minimize the chances of deterioration, the maximum total acid soluble Chloride content in the concrete at the time of placing for different type/use of concrete has been limited vide Table 7. As per the new code, the maximum Chloride content is 0.6 kg/m3 for RCC works; while the earlier edition limited the chloride content to 0.15% by mass of cement. Thus the new provision


is more lenient in this respect for concrete having cement content up to 400 kg/m3.

2.4.6.2 Sulphate in concrete (Cl. 8.2.5.3)

Sulphates are present in most cement and in some aggregates. Excessive amount of water-soluble sulphate can cause expansion and disruption of concrete. To prevent this, the total water-soluble sulphate content of the concrete mix expressed as SO3 has been limited to 4% by mass of cement; which is same as was provided in IS 456 : 1978.

2.4.6.3 Alkali-aggregate reaction (Cl. 8.2.5.4)

Some aggregates containing particular varieties of silica may be susceptible to attack by alkalies (Na2O and K2O) originating from cement or other sources and may produce an expansive reaction which can cause cracking and disruption of concrete. The new code has suggested taking one or more of the following precautionary measures when the service records of the particular cement/aggregate combination is not well established: • Use of non-reactive aggregate from alternative sources. • Use of low alkali OPC having total alkali content not more than

0.6% as Na2O equivalent. • Measures to reduce the degree of saturation of concrete during

service such as use of impermeable membranes. • Limiting the cement content in the mix and thereby limiting the total

alkali content. 2.4.7 Concrete in aggressive soils and water (Cl. 8.2.6)

The code has suggested that at sites where the alkali concentration are high or may become very high, the ground water should be lowered by drainage so that it does not come into direct contact with the concrete. It has also suggested additional protection like the use of chemically resistant stone facing or a layer of plaster of Paris covered with suitable fabric, such as jute thoroughly impregnated with bituminous material. 2.4.8 Concrete in Sea-water (Cl. 8.2.8)

From durability considerations, the minimum grade of concrete in sea-water or exposed directly along sea-coast has been increased from M 15 to M 20 in the case of PCC and from M 20 to M 30 in the case of RCC works. 2.5 Concrete mix proportioning (Cl. 9)


Salient revised provisions under this clause are highlighted hereunder: i) In the list of information required in specifying a particular grade of

concrete, the following new items have been added:

• Exposure conditions as per Table 4 and 5 of the standard. • Maximum temperature of concrete at the time of placing. • Method of placing. • Degree of supervision.

ii) As the guarantor of quality of concrete used in the construction, the constructor shall carry out the mix design and the mix so designed (not the method of design) shall be approved by the employer within the limitations of parameters and other stipulations laid down in this standard.

This provision is very important as the responsibility of quality assurance and carrying out the mix design has been rightly entrusted to the constructor. Also what is more important is that the results of mix rather than the method of its design has been insisted upon as the basis for finalising the mix proportion. In this context it is worth mentioning that the earlier version of this code mentioned that the procedure given in IS : 10262-1982, Recommended Guidelines for Concrete Mix Design (which was under preparation at that time), may be followed. But the new IS : 456 does not refer to the above standard either in the body of the code or in the list of referred Indian Standards appearing in Annex A.

iii) The target mean strength of the concrete mix should be equal to

the characteristic strength plus 1.65 times the standard deviation. iv) The provision regarding necessity for revision of concrete mix has

been modified. The mix design done earlier not prior to one year has been considered adequate for later works provided there is no change in source and the quality of the materials.

v) When sufficient test results for a particular grade of concrete are

not available, the value of standard deviation given in Table 8 is to be taken for design of mix in the first instance. The values of assumed standard deviation given in the above table correspond to the site control having proper storage of cement; weigh batching of all materials; controlled addition of water; regular checking of all materials, aggregate grading and moisture content; and periodical checking of workability and strength. Where there is deviation from


the above, the values given in the table shall be increased by 1N/mm2. The revised code further stipulates that as soon as the results of samples are available, actual calculated standard deviation shall be used and the mix be designed properly.

2.6 Production of concrete (Cl. 10) 2.6.1 Quality Assurance Measures (Cl. 10.1)

Some important aspects on quality assurance measures have been added. These include:

• Quality assurance to proper design, use of adequate materials and

components to be supplied by producers, proper workmanship in the execution and timely maintenance and repair during service.

• Development and implementation of a general Quality Assurance Plan (QAP) to identify the key elements necessary to provide fitness of the structure and the means by which they are to be provided and measured. The quality assurance would involve quality audit of inputs such as materials of concrete; workmanship in all stages of batching, mixing, transportation, placing, compaction and curing; and related plant, machinery and equipment.

• The QAP shall define the task and responsibility of all persons involved, adequate control and checking procedure and maintenance of adequate documentation, which should generally include: * Test reports and manufacturer’s certificate for materials,

concrete mix design details; * Pour cards for site organization and clearance for concrete

placement; * Record of site inspection of workmanship, field tests; * Non-conformance reports, change orders; * Quality control charts; and * Statistical analysis

2.6.2 Batching (Cl. 10.2) The following important additions have been made:


• To avoid confusion and error in batching, consideration should be

given to using the smallest practical number of different concrete mixes on any site or in any one plant.

• Ready-mixed concrete supplied by ready-mixed concrete plant has

been given preference. For large and medium project sites, the concrete should be sourced from ready-mixed concrete plants or from on site or off site batching and mixing plants.

• The accuracy of measuring equipment should be within ± 2% of the

quantity of cement being measured and within ± 3% of the quantity of aggregate, admixtures and water being measured.

• Volume batching may be allowed only when weigh batching is not

practical and provided accurate bulk densities of materials to be actually used in concrete have earlier been established. The mass volume relationship should be checked as frequently as necessary.

2.6.3 Mixing (Cl. 10.3)

• The mixers are required to be fitted with water measuring

(metering) devices. • Dosages of retarders, plasticisers and superplasticisers have been

restricted to 0.5, 1.0 and 2.0 percent respectively by weight of cementitious materials.

2.7 Formwork (Cl. 11)

The minimum period before striking vertical formwork to columns, walls, beams, etc. has been reduced to 16-24 hours from 24-48 hours.

2.8 Assembly of reinforcement (Cl. 12)

• Re-bending or straightening of high strength deformed bars without prior approval has been prohibited.

• Bar bending schedule is required to be prepared for all

reinforcement work. • As per IS 456 : 1978, the reduction in concrete cover was permitted

up to one-third of specified cover or 5 mm whichever is less. But the new code specifies tolerance of cover as +10 mm and –0 mm.


Thus, no reduction in actual cover from the specified cover has been permitted.

• Types of cover-blocks acceptable (concrete of same strength or

PVC) have been mentioned. This may pave the way for use of PVC cover blocks.

2.8.1 Welded Joints or mechanical connections (Cl. 12.4)

• Only up to 12 mm for high strength deformed steel bars and up to 16 mm for mild steel bars are permitted to bend aside at construction joints which can afterwards be bent back to original position.

• Reinforcement should be placed and tied in such a way that

concrete placement be possible without segregation and allow compaction by immersion vibrator.

• Within concrete mass, different types of metal in contact should be

avoided to ensure that bimetal corrosion does not take place.

2.9 Transportation, placing, compaction and curing (Cl. 13)

• Maximum permissible free fall of concrete has been stipulated as 1.5m.

• Construction joints should comply with IS 11817.

• The earlier practice of introducing a cement slurry/mortar layer between old and new concrete has been discontinued. It has now been recommended to roughen the surface of the previously poured concrete to expose the aggregate and the prepared surface should be in a clean surface dry condition when the fresh concrete is placed against it. Fresh concrete should be thoroughly vibrated near construction joint so that mortar from fresh concrete flows between large aggregates and develops proper bond with old concrete. Provision of shear keys has been recommended where high shear resistance is required at construction joints.

• The earlier code made a general statement that the curing period should not be less than 7 days from the date of placing concrete. But the new code has specified different period of curing for concrete where mineral admixtures or blended cements are used and also for concrete exposed to dry and hot weather conditions. As per the new code, the minimum curing period of concrete made with OPC is 7 days for normal conditions and 10 days for dry and


hot weather conditions. The same for concrete where mineral admixtures or blended cements are used, are 10 days for normal conditions and 14 days for dry and hot weather conditions.

• Impermeable membrane such as polyethylene sheeting to provide effective barrier against evaporation may be used as an alternative to moist curing.

2.10 Concreting under special conditions (Cl. 14) 2.10.1 Under-Water Concreting (Cl. 14.2)

• Some fundamental conceptual changes have been made in provisions on under-water concreting. The 1978 edition stated that under-water concrete should in no case be considered as ‘Design mix concrete’ and recommended to provide at least 10% extra cement than that required for the same mix placed in dry. The new edition envisages that design mix concrete alone will be used for under-water concreting and the requirement of 10% extra cement has been deleted. However, the code has further specified that the minimum cement content shall be 350 kg/m3, which would take care of possible loss of cement during under-water concreting.

• Direct placement with pump has been added as one of the methods

to be used for depositing concrete under water. 2.11 Sampling and strength of designed concrete mix (Cl. 15)

• The heading of this clause should have been ‘Sampling and strength test of concrete’ as per the earlier code since the details of sampling and testing is related to work site concrete and not to ‘design mix concrete’.

• IS : 456 – 1978 exempted strength tests at the discretion of

engineer-in-charge for concrete of quantity less than 15 m3 in unimportant building and works.

• Both 1978 and 2000 editions have specifically mentioned that in all

cases, the 28 days compressive strength shall alone be the criteria for acceptance or rejection of concrete. But the 1978 edition permitted relaxation in the frequency in 28 days compressive strength provided a satisfactory relation between 28 days compressive strength and the modulus of rupture at 72±2 hrs or 7 days or compressive strength at 7 days is established for a


reasonably long period. This relaxation has been deleted from the 2000 edition.

• The 1978 edition had given values of the modulus of rupture at

72±2 hrs and 7 days as well as compressive strength at 7 days for various grades of concrete in order to get a relatively quicker idea of the quality of concrete and possible compressive strength at 28 days. The new edition has not given these values and has rightly stated that these values should be arrived at based on actual testing because several factors may affect it.

• The new revised standard recognizes the use of ready-mixed

concrete and states that the frequency of sampling for such concrete may be decided mutually by supplier and purchaser.

2.11.1 Test results of sample (Cl. 15.4)

Both 1978 and 2000 editions state that test result of no individual specimen should vary more than ±15% of the average strength of the sample. The 2000 edition further clarifies that the test results of such sample will be invalid.

2.12 Acceptance criteria (Cl. 16)

• The acceptance criteria has been fully modified and made simpler for implementation. As per the new code the concrete shall be deemed to comply with strength requirements when both the following conditions are met:

a) The mean strength determined from any group of four non-

overlapping consecutive test results gives concrete strength greater than or equal to:

(i) fck + 0.825 x established standard deviation (rounded off to

nearest 0.5 N/mm2) or (ii) fck + 3 N/mm2 (for M15) or fck + 4 N/mm2 (for M20 and

above) as the case may be

Whichever of (i) and (ii) is greater.


b) Any individual test result should be greater than equal to fck - 3 N/mm2 (for M15) or fck - 4 N/mm2 (for M20 and above) as the case may be.

Thus, as long as standard deviation is less than about 5 N/mm2 (which can be achieved with good quality control), the criteria is satisfied when a margin of 4 N/mm2 for the mean strength over the characteristic strength is maintained. Or in other words for concrete of grade M 20 to M 50, about 85 to 93 percent of target mean strength of design mix is to be achieved at site.

• A new clause 16.2 has been added on the requirements of flexural strength.

• Vide new additional clause 16.3, the quantity of concrete

represented by strength test results has been clarified.

2.13 Inspection and testing of structures (Cl. 17)

The thrust is now more for a quality of inspection system than the inspection of final product. The following important additions have been made: • An inspection procedure should be set up covering materials,

records and workmanship to ensure that the construction complies with the design.

• Tests should be made on reinforcement and the constituent

materials of concrete in accordance with relevant standard. • A suitable quality assurance plan should be made to see that:

* Design and detail are capable of being executed to a suitable

standard, with due allowance for dimensional tolerance; * There are clear instructions on inspection standards; * There are clear instructions on permissible deviations; * Elements critical to workmanship, structural performance,

durability and appearance are identified; and * There is a system to verify that the quality is satisfactory in

individual parts of the structure, specially the critical ones.


2.13.1 Load tests for flexural members (Cl. 17.6)

The heading of this clause has been rightly changed from “Load tests on parts of structure” (as per cl. 16.5 of IS : 456 – 1978) to “Load tests for flexural members” as it describes detailed requirements of load tests for flexural members.

2.13.2 Members other than flexural members (Cl. 17.7)

This is a new provision, which states that members other than flexural members should be preferably investigated by analysis.

2.13.3 Non destructive tests (Cl. 17.8)

The earlier provision regarding non-destructive tests has been extended and non-destructive test methods like Ultrasonic Pulse Velocity and Rebound Hammer, Probe Penetration, Pullout and Maturity has been recognized to obtain estimation of the properties of concrete in the structure. Any of these methods may be adopted, in which case the acceptance criteria may be agreed upon prior to testing. However, an important provision that the non-destructive tests should be done under expert guidance contained in the earlier code has not been mentioned here.


3 Revisions in Section 3 : General design considerations 3.1 Bases for design (Cl. 18)

• New clauses on aim of design; durability, workmanship and materials; and design process have been added to include considerations for adequate durability and resistance to the effect of fire.

• The new code has encouraged the use of Limit State Method design

by shifting Working Stress Method from the body of the code (Section 6 in 1978 edition) to Annex B. It has also stated that structure and structural elements should normally be designed by Limit State Method. Where the Limit State Method can not be conveniently adopted, Working Stress Method may be used.

3.2 Loads and forces (Cl. 19)

• Live load is now termed as imposed load and consideration of snow load in accordance with IS 875 (Part 4) has been added.

• The code has simplified that in ordinary buildings, such as low rise

dwellings whose lateral dimension do no exceed 45m, the effects due to temperature fluctuations, shrinkage and creep can be ignored in design calculations.

3.3 Stability of structures (Cl. 20)

In addition to other stability criteria like overturning, sliding, etc., lateral drift criteria is also important in high rise buildings. This aspect has been recognized by the new code and the lateral sway at top has been restricted to H/500, where H is the height of the building.

3.4 Fire resistance (Cl. 21)

• A new clause on fire resistance has been incorporated on the basis of BS 8110. To meet specified period of fire resistance, the code has now stipulated requirements of minimum dimensions for structural elements (fig. 1) and nominal covers for different structural configurations (Table 16A).

• When the required nominal cover exceeds 40 mm for beams and 35

mm for slabs, additional measures such as application of fire resistance finishes, provision of fire resistance false ceiling and


sacrificial reinforcement in tensile zone have been recommended to protect against spalling.

3.5 Analysis (Cl. 22) 3.5.1 Effective span (Cl. 22.2)

The effective length of cantilever, which was missing in the earlier code, has been introduced and is to be taken as its length to the face of the support plus half the effective depth except where it forms the end of a continuous beam where the length to the centre of support is to be taken.

3.5.2 Structural frames (Cl. 22.4)

Simplified assumptions like substitute frame has been continued except where side sway consideration becomes critical due to unsymmetry in geometry or loading, in which cases rigorous analysis is recommended.

3.5.3 Moment and shear coefficients for continuous beams (Cl. 22.5)

It is interesting to note that the bending moment coefficient at middle of intermediate span has been changed from 1/24 to 1/16. The reason is not very clear unless the moment coefficient given in the earlier code was based on redistribution done inadvertently.

3.5.4 Critical section for shear (Cl. 22.6.2)

The shear failure at sections without shear reinforcement will normally occur on plane inclined at an angle of 300 to the horizontal. Thus, if the depth of the beam is ‘d’, the critical section for shear will be at a distance of d/tan300 from the face of support. The new code has clarified that the above will be applicable for beams generally carrying uniformly distributed load and where the principal load is located further than twice the distance of critical section from the face of support. Furthermore, it is worth pointing out that the above is applicable only when load is from top and support is at bottom as has been in fig. 2(a) and 2(b). In case of fig. 2(c), whole concept is changed since the support is from top and critical section for shear is at face of support only.

3.6 Control of deflection for beams (Cl. 23.2)

The span to depth ratio given in clause 23.2.1 (a) and (b) is required to be modified for tension reinforcement as per fig. 4. Earlier, the modification factor was based on area and type of steel; but this has now been rightly based on the area and actual stress of steel for tensile reinforcement. The


minimum depth of beam now required for deflection control will be comparatively less. However, it may be relevant here to mention that the span to depth ratio given in the code is only a simplified approach when rigorous deflection calculations are not made.

3.7 Slabs continuous over supports (Cl. 24.2)

The relevant earlier clause stated that slabs continuous over supports should be designed according to provisions applicable to continuous beams. This has now been corrected and states that slabs spanning in one direction and continuous over supports should be designed according to provisions applicable to continuous beams as a separate provision is there for slabs spanning in two directions and continuous over supports.

3.8 Restrained slab spanning in two directions at right angles with unequal conditions at adjacent panels (Cl. 24.4.1)

This is a new provision for slabs spanning in two directions at right angles to each other having different span lengths or different boundary conditions at far ends. Under such cases there is significant variance in bending moment at the common support. The extant practice is to design for the greater moment, resulting in uneconomical design. The present code has made provision for distribution of the moments according to relative stiffness of adjacent spans and also curtailment of reinforcement on the basis of point of contra-flexure assuming parabolic moment diagram.

3.9 Minimum eccentricity for compression members (Cl. 25.4)

It has been clarified that in case of compression members subjected to bi-axial bending, it would be sufficient to ensure that minimum eccentricity is exceeded about one axis at a time. This would simplify the design as eccentricity need not be considered in resultant direction.

3.10 Requirements governing reinforcement and detailing (Cl. 26)

• Bundling of bars larger than 32 mm diameter has not been permitted.

• For detailing of earthquake resistant construction, reference to IS 13920 has been added.

3.10.1 Lap splices (Cl. 26.2.5.1)

Provisions have been added for increase in lap length where lap for a tension bar located at top and corner of a section and where minimum cover of twice the diameter of lapped bar is not available.


3.10.2 Strength of welds (Cl. 26.2.5.2)

The following conditions in regard to strength of welds have been added: • The strength of weld should be at least as great as that of the

parent bar. • For splices in tension, 100 percent of design strength may be

assumed if welding is strictly supervised and not more than 20% of the tensile reinforcement is welded in the section.

3.10.3 Maximum spacing of reinforcement bars in tension (Cl. 26.3.3)

Maximum spacing of main reinforcement bars in slab has been reduced from 450 mm to 300 mm.

3.10.4 Nominal cover to reinforcement (Cl. 26.4)

The effective protection of steel in concrete against corrosion depends upon adequate thickness of good quality concrete. A large-scale modification has been made in the new code while recommending requirements of concrete cover, which are as under:

• The term ‘nominal cover’ has been introduced and it is defined as

the design depth of concrete cover to all steel reinforcement including links. It is the dimension used in the design and included in the drawing that has to be strictly followed at site.

• The required nominal cover has been specified on considerations of both durability and fire resistance.

• The requirement of nominal cover on durability considerations is for various exposure conditions and not related to the type of structural members.

• The nominal cover for longitudinal reinforcement bar in column should in no case be less than 40 mm nor the diameter of such bar. This value may be reduced to 25 mm in case of columns of minimum dimension of 200 mm or under, whose reinforcement bars do not exceed 12 mm.

• For footings, the minimum cover should be 50 mm.

3.10.5 Maximum spacing of shear reinforcement in beams (Cl. 26.5.1.5)


Maximum spacing of shear reinforcement in beams is reduced from 450 mm to 300 mm.

3.10.6 Minimum shear reinforcement in beams (Cl. 26.5.1.6)

In the formula for minimum cross-sectional shear of stirrup legs of shear reinforcement for beams, the term fy (characteristic strength of shear reinforcement in N/mm2) has been replaced by 0.87fy.

3.10.7 Pitch and diameter of lateral ties in columns (Cl. 26.5.3.2c)

• The minimum distance between lateral ties in columns has been modified from forty-eight times the diameter of lateral ties to 300 mm.

• The minimum diameter of lateral ties has been increased from 5 mm to 6 mm.

3.11 Expansion Joints (Cl. 27)

For design consideration of expansion joints, reference to IS 3414 has been included.


4 Revisions in Section 4 : Special design requirements for structural members and system

4.1 Concrete corbels (Cl. 28)

A new clause on design of concrete corbels on the basis of simplified assumptions of strut-and-tie system and its reinforcement detailing has been introduced.

4.2 Minimum length of reinforcement for flat slabs (Cl. 31.7.3)

A new provision has been introduced that the length of reinforcement for slabs in frames not braced against sideways and for slabs resisting lateral loads should be determined by analysis but should not be less than those given in Fig. 16.

4.3 Walls (Cl. 32)

A new clause on design of concrete walls and its reinforcement detailing has been introduced.

4.4 Nominal reinforcement for footings (Cl. 34.5)

A new clause stipulating minimum reinforcement and its spacing for footings has been introduced.

5. Revisions in Section 5 : Structural design (Limit State Method) 5.1 Limit State of Serviceability : Cracking (Cl. 35.3.2 and 43)

Where specific attention is required to limit the design crack width, the new code has included the formulae for calculation of crack width in Annex F. The permissible crack widths has been kept same as earlier code and varies from 0.30 mm to 0.1 mm depending upon type of structure and environment. It may be noted from the formula for crack width calculation that for a given section, the crack width is a function of strain in steel, depth of concrete cover and spacing of reinforcement steel. Again, permissible crack width is less when required nominal cover is more from exposure condition. Thus, if crack width is to be limited to 0.1 mm only for a given strain and higher concrete cover, the reinforcement spacing would be too congested to cause problems in concreting. On the other hand reduction in strain would result uneconomic design. However, the code has clearly stated in clause 43.1 that compliance with the minimum spacing of reinforcement bars given in clause 26.3.2 should be sufficient to control


flexural cracking and crack width given in Annex F need be calculated only when greater spacing are required.

5.2 Slender compression member (Cl. 39.7)

Note 1 under clause 39.7.1 has been added clarifying the conditions under which a column may be considered braced in a given plane.

5.3 Design shear strength of concrete (Cl. 40.2)

In both Limit State and Working Stress Methods (Table 19 and 23), the percentage area of longitudinal tension reinforcement has been added below 0.15 and above 3.0 for design shear strength calculations. This would help the designer as often the percentage areas go beyond the limits given in the table. In both these tables the value of design shear strength for M 40 for different percentage area of tensile reinforcement as given in earlier code has been retained for M 40 and above in the new code.

5.4 Maximum shear stress in concrete with shear reinforcement (Cl. 40.2.3)

In Table 20 specifying Maximum shear stress in concrete even with shear reinforcement, the value for M 40 given in earlier code has been retained for M 40 and above in the new code.

5.5 Enhanced shear strength of sections close to supports (Cl. 40.5) For any section closer to support, substantial portion of the shear will be directly transferred to the support by strut action. Thus, the shear strength for all sections from critical section for shear (refer para 3.5.4 above) to the face of support get enhanced. This aspect has been recognized for the first time in the revised edition and a new clause in this regard has been introduced both in Limit State and Working Stress Methods. Accordingly, permissible shear stress at supports has been increased. However, it is worth mentioning that the above enhancement is applicable only when the load is on top of beam and support is at bottom. If the load acts at bottom, i.e. something is hung from bottom fiber of beam, the whole concept changes and enhancement of shear at support is no more applicable.

5.6 Limit State of Collapse : Torsion (Cl. 41)

As per earlier code, the members were required to be designed for torsion, only where the torsional resistance or stiffness of member was


taken into account in the analysis of the structure. This has been modified in the new code and it states that the members are to be designed for torsion if torsion is required to maintain equilibrium of the structure. It further clarifies that no specific calculations for torsion is required in intermediate structures when the redundant restrains are released, provided torsional stiffness is neglected in calculation of internal forces.


6. Revisions in Annex B : Structural design (Working Stress Method)

• It is interesting to note that the value of the Modulus of Elasticity of concrete has been reduced by about 12%. But there is no corresponding increase in the value of modular ratio ‘m’ which has been kept same (280/3σcbc) as earlier code. The reason is not very clear.

• Due to introduction of higher grade of concrete, values of tensile stresses for concrete of grade M 45 and M 50 have been added. Similarly, values of permissible compressive stresses and bond stresses for concrete of grade M 45 and M 50 have been added in Table 21.

• In terms of Clause B-4.3 of the new code, it is now mandatory that members subjected to combine direct load and bending be designed by Limit State Method only.

• It has already been mentioned vide para 5.3 above that in Table 23,

the percentage area of longitudinal tension reinforcement has been added below 0.15 and above 3.0 for design shear strength calculations. The value of design shear strength for M 40 for different percentage area of tensile reinforcement as given in earlier code has been retained for M 40 and above in the new code.

• In Table 24 specifying Maximum shear stress in concrete even with

shear reinforcement, the value for M 40 given in earlier code has been retained for M 40 and above in the new code.

• Provision for enhanced shear strength of sections close to supports

has been added for Working Stress Method also in a similar manner as already explained vide para 5.5 above for Limit State Method.

• Modification in design for torsion as explained vide para 5.6 above has

been included in Working Stress Method also.

7. Revisions in Annex E : Effective length of columns

• A method to determine whether a column is a no sway or a sway column by computing stability index has been added vide clause E-2.

• Reference to Fig. 26 as given in line 5 of clause E-1 should be read as

Fig. 27. This is typographical mistake.

8. CONCLUSIONS

• Every revision of IS 456 has a special milestone. 1964 revision brought out Ultimate Load Method of design, 1978 is known for introduction of


Limit State concept and in the new code i.e. 2000 revision major thrust has been on Durability aspects.

• Enhancement in durability of concrete structures depends on

numerous factors to be taken care of during planning; design and detailing; construction and service. Some of these are mentioned below:

∗ Proper assessment of environment ∗ Selection of right material and mixes ∗ Efficient design and detailing including concrete cover ∗ Proper production, placement, compaction and curing of concrete ∗ Enforcement of acceptance criteria ∗ Development of Quality Assurance Plan and inspection and testing

of constituent materials and structures. ∗ Preventive maintenance The new revision of the code has modified and enlarged the clauses comprehensively to cover all factors governing durability of concrete structures.

• The new code has taken bold steps in introducing high strength/performance concrete up to grade M 80.

• Encouragement in the use of new materials like fly ash, silica fume, Ground Granulated Blast Furnace slag, various types of cements presently being manufactured, admixtures, PVC cover blocks, etc. is considered to be a progressive approach.

• Some aspects of concrete technology though very common but are not in practice to real sense like alkali aggregate reaction, preference to natural aggregates, under water concreting, etc. have been modified/elaborated.

• Introduction of certain clauses like minimum grade of concrete, increased nominal cover, etc. from durability considerations though will involve increase in initial cost but would ultimately result in reduction of overall life cycle cost of the structure due to enhanced durability.

• The new code has encouraged the use of Limit State Method design by shifting Working Stress Method from the body of the code to Annexure.

• More scientific approaches like crack width calculations, enhanced shear strength of sections near supports, maximum cement content, etc. has been introduced.


• The code has not mentioned about permeability test of concrete, which is an important check for ensuring durability. No mention has also been made about Thermo-mechanically Treated (TMT) reinforcement bars and various types of protective coating of concrete and reinforcement bars.

In this report an attempt has been made only to highlight the important changes/modification brought out in IS 456 : 2000. Many of these changes have already appeared in Railways’ Concrete Bridge Code, which was revised in 1997; earlier to revision of IS 456. But still there remain certain important changes, which are worth considering for inclusion in IRS Concrete Bridge Code.