IS 456: 2000, Reaffirmed 2005, certain points

IS 456 : 2000 Reaffirmed 2005 is the fountain head of all Indian codes related to plain & reinforced cement concrete, prescribes minimum grade of concrete to be not less than M20 in R.C.C. works. (Table: 5 and foreword note “e” of IS 456 ) IS 456:2000 lays down stress on durability of concrete and that’s why specified minimum cement content for various grades in table No.5 of IS 456:2000. The maximum cement content not including fly ash and slag is 450 Kg/m3 as per clause 8.2.4.2 of IS 456:2000.
It is most important to note that the minimum grade of concrete for R.C.C. works in severe exposure conditions is M-30. All the areas exposed to coastal environment will comes under severe exposure (Table: 3 of is 456 ). In fact as per IS 456:2000 , grades lower than M-30 cannot be designed for RCC works for severe exposure.
IS 456:2000 includes the use of Portland pozzolana cement IS:1489 (part 1)
( Fly ash based ), and Portland slag cement ( IS: 455 ) in the list of cements prescribed with the caution that the type of cement selected should be appropriate for the intended use. As per BIS the minimum fly ash content in pozzolana cement is 15% and the maximum is 35%. In slag cement the minimum slag content is 25% and the maximum is 70%. IS 456:2000 has been published during July 2000 and reaffirmed during 2005. But even after elapse of 7 years of publishing this important concrete code various users of cement not understood the advantages in using pozzolana and slag cements. Besides this , some consumers are allowing OPC 53
grade cement without knowing the implications. Use of OPC 53 grade cement is not advisable unless special arrangements made for early curing. The initial strength achievement in 53 grade is very high due to the unique composition of Tricalcium Silicate and Dicalcium Silicate. Due to this the heat of hydration is very high in OPC 53 grade. The generation of heavy heat during hydration process may leads to loss of structural integrity and monolithic action. Unless arrangements made for early curing, the water evaporation takes place and leads to micro cracking and reduction in strength and service life of the structure. At the same time use of PPC and slag cement will enhance the life of the structure by giving durable concrete. These blended cements also having better resistance against Sulphates and Chloride permeability is lesser than in OPC due to impervious concrete . However, the stripping time for forms shall be enhanced to 10/7 times than in OPC. Concrete will gain strength beyond 28 days also. This is comparatively high in slag and pozzolana cements than in OPC. However the design of concrete mixes shall be based on 28 days strength (clause 6.2.1 of IS 456 : 2000). As per BIS , like OPC there are no grades in slag or pozzolana cements. But certain cement companies are misleading the consumers by giving 53 S or 53 Plus on the bags of pozzolana cement , resembling that they are 53 Grade
The combined aggregate grading of 20mm shall conform to IS: 383 and 20mm & 10mm graded aggregate shall also conform to IS:383 individual grading and may be used in the ratio of 60:40. Otherwise honeycombing may occur. Experts says that 5% honeycombing may take away 25% compressive strength of concrete. At the same time the place which has to be occupied by 10mm graded aggregate will be occupied by sand which leads to reduction in strength and badly affect the durability of concrete. Check the aggregate shape through flakiness and elongation index. It shall be within the limits specified in SP: 23. The impact and crushing values of coarse aggregate shall not exceed 45% for concrete other than for wearing surfaces and 30% for wearing surfaces like runways, roads and pavements.
Los Angeles abrasion value of coarse aggregate shall not exceed 30% for wearing surfaces and 50 % for other concrete. Average loss of weight of fine and coarse aggregates in sodium / magnesium sulphate shall not exceed the limits laid down in clause 3.6 of IS 383. The grading of fine aggregate shall conform to the values given in Table : 4 of IS 383. The deleterious materials in aggregates shall not exceed 5% as per the limits laid down in IS 383 Table :1.
Water cement ratio is another important factor which has to be maintained strictly as per the design. Mixing of excess water than designed will leads to drastical reduction in strength. At the same time , proper curing arrangements shall be made as per clause 13.5 of IS 456 and more caution shall be taken in low water cement ratios. The water used in concrete and for curing shall be potable water and meets the requirements of clause 5.4 of IS 456. Too low slump concrete for heavy reinforced areas may leads to formation of Bug holes and honeycombing. High slump concrete may leads to segregation / laitance. The water cement ratio shall be appropriate for the intended use.
As far as possible avoid using of admixtures. Check the chloride content in plasticizers before use. All plasticizers are not compatible with all cements. Check the compatibility. Excess or less usage of plasticizer than design may leads to worst quality.
Concrete is a combination of three ingredients namely cement, aggregates and water. The fourth ingredient we must put into that is our care and conscious. Then only we can create a long lasting ‘ concrete’. Remember , Almost all items used in the building are manufactured in highly sophisticated automated plants, But Engineers can only manufacture this ‘ Wonderful Concrete “.
_ K.Srinivasa Varma, MTH INDIA LTD
e-mail : srinivasavarma@india.com