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Concrete

Miscellaneous useful points on Concrete (Part-II)

Contd. from Part-I….

* Concrete that is strong may not be durable too. While strength is a very important characteristic, it does not necessarily ensure durability in concrete. For example, concrete mix having excess chloride in it may show good 7 days & 28 days strength and yet may have durability issues. That’s because the excess chloride could easily lead to corrosion of reinforcing steel in the concrete and thus creating serious durability issues in the course of time.

* Durability of concrete depends largely on: maximum water to cement (w/c) ratio, minimum cement content, thickness of cover provided to re-bars, type of cement, extent of chlorides & sulphates present in concrete etc.

* IS codes limits chlorides to 0.15% by mass of cement for ordinary concrete & 0.06% for prestressed concrete. Besides, sulphates from all sources shuld not exceed 4% by mass of cement used in concrete.

* IS codes classify concrete as ordinary concrete (M20 to M30); standard concrete (M25 to M55) & high strength concrete (M60 to M80). SP-23 suggests that concrete of grades M5 & M7.5 need not be designed and nominal mixes can be adopted in stead.

* As a thumb rule, the greater the 7 day compressive strength of concrete, the greater would be the 28 day compressive strength. Hence, 7 day concrete cube compressive strengths are good early indicators of future strength.

* Water curing is not necessary for concrete poured in freezing or near freezing temperature. Some codes prescribe that below – 1¤C ambient temperature, the temperature of the mix should be maintained at around 15¤C & when poured at around 10¤C.

* While pouring concrete in extreme cold weather, air entraining agents may be induced in order to achieve workability at low w/c ratio as well as to provide resistance to freezing & thawing. Sometimes, CaCl2 is used with care to accelerate setting time of concrete in such weather. Well insulated formwork may be used to prevent loss of moisture from concrete poured in such weather.

* While setting time of cement is a well known subject & is commonly tested in most project sites, there also is something called setting time of concrete tests on which are conducted in laboratories for various purposes even though the same are not quite common in construction project site laboratories. Initial setting time of concrete is defined by IS code as the elapsed time after initial contact of cement & water, required for mortar sieved from freshly made concrete to reach a penetration resistance of 3.43 N/mm2. Similarly, final setting time of concrete is defined as the elapsed time after initial contact of cement & water, required for mortar sieved from freshly made concrete to reach a penetration resistance of 26.97 N/mm2.

* Setting time of concrete may be determined by Penetration Resistance apparatus (spring reaction type, IS-8142).

Contd. in Part-III….

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About clazcons

B. Tech. (Civil Engineering)

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