Technical Papers
 

Some useful information on various building materials


Grading of aggregates

IS 456: 2000, certain points

Coarse aggregates

Fine aggregates

Limits of deleterious materials

Cements

Oxide composition of ordinary portland cement

Basic properties of cement

Ordinary portland cement 33 grade, 43 grade, 53 grade

Portland slag cement - IS : 455-1989

Portland puzzolana cement (IS : 1489 part-1)

High strength deformed bars

Chemical composition –
the ladle analysis of steel when made as per relevant parts of IS : 228*

Cross sectional area and mass (clause 5.2 of IS 1786)

Tolerances on nominal mass of HSD bars

Is Crushed Stone Sand is an ideal substitute to replace Natural Sand entirely in Concrete ?

Crushed stone sand



IS 456: 2000, 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 10 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. 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 “.




Note-1: For crushed stone sands, the permissible limit on 150 micron IS Sieve is increased to 20 percent. This does not affect the 5 percent allowance permitted in 4.3 applying to other sieve sizes.

Note-2: Fine aggregate complying with the requirements of any grading zone in this table is suitable for concrete but the quality of concrete produced will depend upon a number of factors including proportions.

Note-3: Where concrete of high strength and good durability is required, fine aggregate conforming to any one of the four grading zones may be used, but the concrete mix should be properly designed. As the fine aggregate grading becomes progressively finer, that is, from Grading Zones I to IV, the ratio of fine aggregate to coarse aggregate should be progressively reduced. The most suitable fine to coarse ratio to be used for any particular mix will, however, depend upon the actual grading, particle shape and surface texture of both fine and coarse aggregates.

Note-4: It is recommended that fine aggregate conforming to Grading Zone IV should not be used in reinforced concrete unless tests have been made to ascertain the suitability of proposed mix proportion.


Aggregates Impact Value - The aggregate impact value may be determined in accordance with the method specified in IS: 2386 (Part IV) – 1963. The aggregate impact value shall not exceed 45 percent by weight for aggregates used for concrete other than for wearing surfaces and 30 percent by weight for concrete for wearing surfaces, such as runways, roads and pavements.

Aggregate Abrasion Value – Unless otherwise agreed to between the purchaser and the supplier, the abrasion value of aggregates, when tested in accordance with the method specified in IS: 2386 (Part IV) – 1963 using Los Angeles machine, shall not exceed the following values:

a)For aggregates to be used in concrete for wearing surfaces.     -30 percent

b)For aggregates to be used in other concrete                           - 50 percent

Deleterious Materials – Aggregates shall not contain any harmful material, such as pyrites, coal, lignite, mica, shale or similar laminated material, clay, alkali, soft fragments, sea shells and organic impurities in such quantity as to affect the strength or durability of the concrete. Aggregates to be used for reinforced concrete shall not contain any material liable to attack the steel reinforcement. Aggregates which are chemically reactive with alkalis of cement are harmful as cracking of concrete may take place.


NOTE-1 – The presence of mica in the fine aggregate has been found to reduce considerably the durability and compressive strength of concrete and further investigations are underway to determine the extent of the deleterious effect of mica. It is advisable, therefore, to investigate the mica content of fine aggregate and make suitable allowances for the possible reduction in the strength of concrete or mortar.

NOTE-2 – The aggregate shall not contain harmful organic impurities (tested in accordance with IS: 2386 (Part-II) 1963] in sufficient quantities to affect adversely the strength or durability of concrete. A fine aggregate which fails in the test for organic impurities may be used, provided that, when tested for the effect of organic impurities on the strength of mortar, the relative strength at 7 and 28 days, reported in accordance with 7 of IS: 2386 (Part VI) – 1963 is not less than 95 percent.


Basic Properties of Cement Compounds

The two silicates, namely C3S and C2S which together constitute about 70 to 80 percent of the cement , control the most of the strength giving properties. Upon hydration both C3S and C2S give the same product called calcium silicate hydrate (C3S2H3) and calcium hydroxide. Tricalcium silicate (C3S) having a faster rate of reaction accompanied by greater heat evolution develops early strength. On the other hand, dicalcium silicate (C2S) hydrates and hardens slowly and provides much of the ultimate strength. It is likely that both C3S and C2S phases contribute equally to the eventual strength of the cement. C3S and C2S need approximately 24 and 21 percent water by weight, respectively, for chemical reaction but C3S liberates nearly three times as much calcium hydroxide on hydration as C2S. However, C2S provides more resistance to chemical attack.


Types of Cement:

Five types of cements are in general use. They are:

1) Ordinary Portland cement (OPC)           33 grade
2) Ordinary Portland cement (OPC)           43 grade
3) Ordinary Portland cement (OPC)           53 grade
4) Portland slag cement (PSC)
5) Portland Pozzolana Cement (PPC)


OPC 33 grade means the minimum 28 days compressive strength of cement is 33 MPa. Similarly 43 & 53 grades. All the three grades of ordinary Portland cement are produced from the same materials. But the higher strengths are achieved by increasing the tricalcium silicate (C3S) content and also by fine grinding of the clinker.
The BIS limits for the various types of cements are mentioned here in below:





















STRIPPING TIME

Forms shall not be released until the concrete has achieved a strength of at least twice the stress to which the concrete may be subjected at the time of removal of form work. The strength referred to shall be that of concrete using the same cement and aggregates and admixture, if any, with the same proportions and cured under conditions of temperature and moisture similar to those existing on the work.

While the above criteria of strength shall be the guiding factor for removal of form work, in normal circumstances where ambient temperature does not fall below 15?C and where ordinary Portland cement is used and adequate curing is done, following striking period may deem to satisfy the guideline given in 11.3 of IS-456:

For other cements and lower temperature, the stripping time recommended above may be suitably modified.
The number of props left under, their sizes and disposition shall be such as to be able to safely carry the full dead load of the slab, beam or arch as the case may be together with any live load likely to occur during curing or further construction.

Where the shape of the element is such that the form work has re-entrant angles, the form work shall be removed as soon as possible after the concrete has set, to avoid shrinkage cracking occurring due to the restraint imposed.

Frequency for concrete sampling
The minimum frequency of sampling of concrete of each grade shall be in accordance with the following:



DETERMINATION OF STANDARD DEVIATION: The standard deviation refers to minor changes in the mix which might be made at site either to correct consistently higher results or vice-versa, where as the materials remain the same, and particularly the water-cement ratio. But note that at least 30 samples should be used for this ‘purpose, according to the test results on specimens are obtained from the field as the concreting proceeds

.