1 To determine the modulus of rupture, the size of test specimen used is-150 x150 x700 mm.
2 the water in the mix tends to rise to the surface while placing and compacting, is called-bleeding.
3 Lean mixes bleed more as compared to rich ones.
4 Bleeding can be minimized by adding pozzuolana finer aggregate.
5 Bleeding can be increased by addition of calcium chloride.
6 the ingredients to separate from each other while placing the concrete
7 Workability of concrete is directly proportional to-grading of the aggregate.
8 Workability of concrete is inversely proportional to-time of transit.
9 Approximate value of shrinkage strain in concrete, is-0.0003.
10 Air entrapment in the concrete increases-workability.
11 The approximate value of the ratio bet ween direct tensile strength and flexural
strength is- 0.5.
12 Strength of concrete increases with-increase in fineness of cement.
13 The compressive strength of 100 mm cube as compared to 150 mm cube is always-more
14 Increase in the moisture content in concrete-reduces the strength.
15 As compared to ordinary portland cement, use of pozzuolanic cement-increases shrinkage.
16 Admixtures which cause early setting, and hardening of concrete are called-accelerators.
17 The most commonly used admixture which prolongs the setting and hardening time is-gypsum.
18 The percentage of voids in cement is approximately-40%.
19 The strength of concrete after one year as compared to 28 days strength is about-20 to 25% more
20 Modulus of rupture of concrete is a measure of-flexural tensile strength.
21 In order to obtain the best workability of concrete, the preferred shape of aggregate is-rounded
22 The effect of adding calcium chloride in concrete is-to increase shrinkage,to decrease setting time.
23 Bulking of sand is maximum if moisture content is about-4%.
24 Finer grinding of cement-affects only the early development of strength.
25 Poisson's ratio for concrete-increases with richer mixes.
26 Concrete is the most widely used man-made construction material in the world It is obtained by mixing cementing materials, water and aggregates, and sometimes admixtures, in required proportions. The mixture when placed in forms and allowed to cure, hardens into a rock-like mass known as concrete.
27 The hardening is caused by chemical reaction between water and
cement and it continues for a long time, and consequently the concrete grows stronger with age.
28 The hardened concrete may also be considered as an artificial stone in which the voids of larger particles (coarse aggregate) are filled by the smaller particles (fine aggregate) and the voids of fine aggregates are filled with cement.
29 Cement Portland cement, the most widely used cementing ingredient in present day concrete comprises phases that consist of compounds of calcium, silicon, aluminum, iron and oxygen.
30 Aggregate These are primarily naturally occurring, inert granular materials such as sand, gravel, or crushed stone. However, technology is broadening to include the use of recycled materials and synthetic products.
31 Chemical admixtures These are the ingredients in concrete other than Portland cement, water, and aggregates that are added to the mixture immediately before or during mixing to reduce the water requirement, accelerate/retard setting or improve specific durability characteristics.
32 Supplementary cementing materials Supplementary cementing materials, also called mineral additives, contribute to the properties of hardened concrete through hydraulic or pozzolanic activity.
33 The concrete without reinforcement is termed as plain cement concrete or simply as concrete. The process of making concrete is called concreting.
34 Segregation is the separation of coarse aggregate and bleeding is the separation of cement paste from the main mass. The segregation and bleeding result in a poor quality concrete.
35 The concrete of grades lower than M15 is not suitable for
reinforced concrete works and grades of concrete lower than M30 are not to be used in the prestressed concrete works.
36 Concrete is economical in the long run as compared to other engineering materials. Except cement, it can be made from locally available coarse and fine aggregates.
37 Concrete possesses a high compressive strength, and the corrosive and weathering effects are minimal. When properly prepared its strength is equal to that of a hard natural stone.
38 The green or newly mixed concrete can be easily handled and molded or formed into virtually any shape or size according to specifications. The formwork can be reused a number of times for similar jobs resulting in economy.
39 It is strong in compression and has unlimited structural applications in combination with steel reinforcement. Concrete and steel have approximately equal coefficients of thermal expansion.
40 Concrete can even be sprayed on and filled into fine cracks for repairs by the guniting process.
41 Concrete can be pumped and hence it can be laid in difficult positions also.
42 It is durable, fire resistant and requires very little maintenance.
43 Concrete has low tensile strength and hence cracks easily. Therefore, concrete is to be reinforced with steel bars or meshes or fibers.
44 Fresh concrete shrinks on drying and hardened concrete expands on wetting. Provision for construction joints has to be made to avoid the development of cracks due to drying shrinkage and moisture movement.
45 Concrete expands and contracts with the changes in temperature. Hence, expansion joints have to be provided to avoid the formation of cracks due to thermal movement.
46 The setting time decreases with rise in temperature up to 30°C and vice versa.
47 For an ordinary Portland cement, the initial setting time should not be less than 30 minutes and final setting time should not be more than 600