Questions with Solution

Transcription

1 Questions with Solution Q 1: For making fresh concrete, the quantity of water is expressed in the ratio of (a) Coarse aggregates (b) Fine aggregates (c) Cement (d) None of these Explanation: In a mix proportion, the quantity of all ingredients are expressed in terms of cement, even the quantity of water. Answer: (c)

2 Q 2: The dimension of cube cast for finding the strength of concrete is (a) 100 mm (b) 150 mm (c) 200 mm (d) 250 mm Explanation: The dimension of the cubes cast for finding the strength of concrete is 150mm in dimension. Answer: (b)

3 Q 3: The number given for Indian Standard code for plain and reinforced concrete is (a) 123 (b) 456 (c) 465 (d) 564 Explanation: IS 456: Indian Standard for Plain and Reinforced Concrete- Code of Practice. Answer: (b)

4 Q 4: The number given for Indian standard code of practice for design loads (other than earthquake) for buildings and structures is (a) 456 (b) 800 (c) 875 (d) 876 Explanation: IS 875- Indian Standard Code for Practice for Design Loads (other than Earthquake) for building and structures Part I- Dead Loads- unit weights of building materials and stored materials Part II- Imposed Load Part III- Wind Loads Part IV- Snow Loads Part V- Special Loads and Combinations Answer: (c)

5 Q 5: In the concrete grade M20, the number 20 means (a) 20 N/mm 2 (b) 20 N/m 2 (c) 20 kn/m 2 (d) Testing after 20 days Explanation: M20 indicates that the mix has a characteristic strength of 20 MPa or 20 N/mm 2 at 28 days. Answer: (a)

6 Q 6: The workability of concrete is measured by (a) Core test (b) Permeability test (c) Fineness test (d) Slump test Explanation: The workability is measured by Slump cone having a top diameter of 100mm, base diameter of 200mm and a height of 300mm. Answer: (d)

7 Q 7: The time dependent deformation at constant loading is known as (a) Tension (b) Fracture (c) Shrinkage (d) Creep Explanation: If we assume a load is applied to the concrete, we find that it has an effect on the long-time deformations of the concrete sample. There is an instantaneous strain which is called the "elastic" strain since it can be removed by unloading the sample. It is observed that there is a gradual increase in strain for days after a stress has been applied to concrete and kept constant. This is called the "creep" strain. Creep can thus be defined as the increase in strain under a sustained stress; and since this increase can be several times as large as the strain on loading, creep is of considerable importance in structural mechanics. (A more detailed discussion is provided in discussion forum) Answer: (d)

8 Q 8: The presence of voids in concrete will (a) (b) (c) (d) reduce its strength increase its strength retard setting increase its density Explanation: Presence of voids provides the weaker zone in the matrix. There will be always micro-void in between the hydrated products. But presence of excess void will provide pathways for cracks, to branch out faster. Hence strength capacity will reduce. More formation of cracks will result in interconnected void systems which is again harmful for durability in the long run. Interconnected voids will have more ingress of moisture and air from the outer environment which may result in carbonation, sulphate attack on concrete or the chloride attack on the reinforcements. New products are formed which will give pressure on the surrounding mortar. Also the air within the voids may expand giving pressure on the surrounding concrete. If the generated stress is more than the tensile capacity of the concrete more cracks will generate. So this will be chained propagation of cracks which will ultimately result in failure. Answer: (a)

9 Q 9: The cubes and cylinders are stored under water or placed in constant temper- ature at 100% relative humidity before testing (a) (b) (c) (d) to provide proper curing to enhance workability to increase water cement ratio to decrease water cement ratio Explanation: Concrete that is allowed to dry out quickly undergoes considerable early age shrinkage which can cause shrinkage cracks. Besides curing also ensures the cement hydration reaction to progress steadily producing calcium silicate hydrate gel making the concrete denser thereby decreases the porosity and enhances the physical and mechanical properties of concrete. Answer: (a)

10 Q 10: The lateral ties are provided in columns (a) (b) (c) (d) to decrease shear strength to resist axial force to resist lateral buckling none of these Explanation: The lateral ties are provided to resist lateral buckling. Answer: (c)

11 Q 11: The modular ratio used in working stress method means (a) (b) (c) (d) the ratio of moduli of elasticity of steel and coarse aggregates the ratio of moduli of elasticity of concrete and aggregates the ratio of moduli of elasticity of concrete and steel the ratio of moduli of elasticity of steel and concrete Explanation: Consider a composite beam of steel (1) and concrete (2). E1 = Young s Modulus of Steel σ1 = Stress in Steel ε1 = Strain in Steel E2 = Young s Modulus of Concrete σ2 = Stress in Concrete ε2 = Strain in Concrete Strain in steel = ε1 = Strain in concrete = ε2 = But strain of whole system should be same i.e. or or or ε1 = ε2 = = = where m= Modular ratio = Ratio of Young s moduli of steel to concrete. Answer: (d)

12 Q 12: The characteristic strength of a material means that the value of the strength of the material (a) (b) (c) (d) below which not more than 2 percent of the test results are expected to fall below which not more than 5 percent of the test results are expected to fall below which fifty of test results are expected to fall below which no test results are expected to fall Explanation: Assuming that the strength follows this normal distribution 5% of cubes are allowed to fall below the characteristic strength. (A more detailed explanation is provided in the discussion forum) Answer: (b)

13 Q 13: In ultimate limit state, the partial safety factor for dead load is (a) (b) (c) (d) 1.0 for dead and live load combination 1.2 for dead and live load combination 1.5 for dead and live load combination 1.5 for dead, live and wind load combination Explanation: Refer Table 18, page- 68, IS 456: 2000 Answer: (c)

14 Q 14: In limit state of serviceability, the partial safety factor for wind load is taken as (a) 1.0 (b) 1.2 (c) 1.5 (d) 0.9 Explanation: Refer Table 18, page- 68, IS 456: 2000 Answer: (a)

15 Q 15: The concrete is (a) (b) (c) (d) strong in both compression and tension weak in compression and weak in tension weak in compression and strong in tension strong in compression and weak in tension Explanation: Concrete is a heterogeneous material with the aggregates being connected by the mortar system. There is a very narrow interfacial transition zone between the two filled with the hydrated products. This is the weakest link connected through Van der Waals force. When the system undergo compression the load is transferred from aggregate to aggregate. Now under tension the aggregates are torn away from each other and the transition zone which was hold together through weak Van Der Waals force gives away to much less amount of stress. Thus concrete is strong in compression and weak in tension. Answer: (d)