Distinguish between retarders and accelerators. Under what circumstances are they used? Name some products and their dosage for use as accelerating and retarding admixtures.
RETARDERS:- It is an admixture that slows down the chemical process of hydration so that the concrete remains plastic and workable for long time.
Circumstances in which they are used:- They overcome the accelerating effect of high temp. on setting properties of concrete in hot weather concreting. They are also used in oil-well construction [ where depth – 6000m and temp. - 200°c ]. They are commonly used in RMC.
ACCELERATORS;- Accelerating admixture are added to concrete to increase the rate of early strength development in concrete. Accelerators improve the rate of chemical process of hydration so that the concrete gains strength early.
Circumstances in which they are used:-
RETARDER:- Calcium sulphate, starch, sugar, acid or salts of acids ( like lingo sulphonic acid).
ACCELERATOR:- Soluble carbonates like silicates fluosilicates, organic compound like triethenolamine.
Introduce Flow table test. How can sulphate attack be controlled?
Explain Kelly Ball to measure the workability of concrete. Mention the limitation of the test.
What are flaky and elongated aggregates? How can the shape of aggregate influence strength of concrete? Following is the result of a sieve analysis of 500 gm aggregate:
|IS sieve size||10 mm||4.75 mm||2·36 mm||1·18 mm||600 micron||300 micron||150 micron||lower than 150 micron|
|Weight retained in gm||0||10||50||50||95||175||85||35|
Find out the 'Fineness Modulus' of the sample