Is 456

AN OVERVIEW OF REVISIONS IN IS 456 : 2000 ‘CODE OF PRACTICE FOR PLAIN AND RE INFORCED 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 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 • Chloride -

from 500 mg/l to 400 mg/l 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)

2.2

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.

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

Is 456

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