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.
Give the physical characteristics of 53 grade OPC.
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
What is an air entraining agents? Give some examples. What is efflorescence in concrete structure?
Design concrete mixes of M30 to suit the following data per IS:10262-1982 :
Characteristic cube strength = M-30, type of cement - ordinary Portland, fine aggregate natural river sand conforming to grading zone II of table-4 of IS:383-1970 coarse aggregate-crushed (angular) coarse aggregate of 20 mm maximum size conforming to IS:383 code requirements, specific gravities of cement, sand and coarse aggregate are 3·14, 2·63 and 2·61 respectively. Type of exposure - mild, Degree of quality control - very good. Degree of workability = 0·08. use IS:10262-1982.
Discuss the role of Gel-space ratio. How can it be determined? Calculate the gel-space ratio and theoretical strength of a sample of concrete made with 500gm of cement with 0.5 w/c ratio on full hydration and 40% partial hydration. Discuss the factors which influence creep in concrete?