Post: Captain ( Engineering Core)
1. What is the quality of good bricks? Write three field test.
Characteristics of 1st class bricks are given below,
- A good brick should be of proper shape and standard specified size.
- The color of a good brick should be copper red color.
- When a brick is struck by a hammer or against another brick, it should emit a clear metallic ringing sound, it should not be dull.
- The structure should be homogeneous, compact and free from any defects.
- If a brick is dropped from about a height of 1m on a hard ground or on another brick, it should not break.
- When a brick is scratched with finger nail it should not leave any impression on the brick.
- A good brick (1st Class) should not absorb water by not more than 20% of its own Dry weight when immersed in water for a period of 24 Hours
2. What is pre-stressed concrete? Write the fundamentals.
Pre-stressed concrete is a concrete block which is pre-tensioned during its construction.
Here the concrete is pre-stressed in tension so when the service load is applied the entire section remains in compression. Stress distribution diagram will always show compressive strength in the pre-cast pre-stressed concrete block.
Whereas reinforced cement uses reinforcing bars to resist the tension stresses which develops below the neutral axis during sagging moment. The stress distribution diagram for the same would show compression above the neutral axis and tensile stress below the neutral axis hence the tensile stresses developed are counteract by reinforcin
3. Write down short notes on
a. Working stress
b. Bearing stress
c. Modulus of rigidity
d. Modulus of elasticity
e. Bulk modulus of elasticity
f. Elastic curve
Working stress: Safe working stress is known as the maximum allowable stress that a material or object will be subjected to when in service.
Bearing stress: Bearing stress is Contact pressure between two separate bodies. It can be defined as Compression force divided by the characteristic Area perpendicular to it.
Modulus of rigidity: Modulus of rigidity or shear modulus is the rate of change of unit shear stress with respect to unit shear strain for the condition of pure shear within the proportional limit. Modulus of Rigidity can be experimentally determined from the slope of a stress-strain curve created during tensile tests conducted on a sample of the material.
Modulus of elasticity: Modulus of elasticity (MOE) is the rate of change of unit stress with respect to unit strain under uniaxial loading within the proportional (or elastic) limits of the material.
Modulus of elasticity: Bulk modulus is termed as the proportion of volumetric stress related to the volumetric strain of a specified material, while the material deformation is within elastic limit.
Elastic curve: The curve assumed by the longitudinal axis of an originally straight elastic strip or bar bent within its elastic limits by any system of forces
4. Write down the short notes on
a. Bearing capacity
d. Shear strength
Bearing capacity: It is defined as the capacity of the soil to bear the loads coming from the foundation. The pressure against which the soil can easily withstand against load is called allowable bearing pressure.
Compaction: Soil compaction is defined as the method of mechanically increasing the density of soil. Air during compaction of soil is expelled from the void space in the soil mass and therefore the mass density is increased. Compaction increases the strength characteristics of soils, which increase the bearing capacity of foundations constructed over them.
Permeability: Permeability in fluid mechanics and the earth sciences is a measure of the ability of a porous material to allow fluids to pass through it.
Shear strength: Shear strength is the maximum shear stress which a material can withstand without rupture.
5. What is the difference between tie bars and dowel bars?
Tie bars: Tie bars are deformed rebar or connectors used for holding faces of rigid slabs in contact to maintain aggregate interlock. Tie bars are not load transferring device. For instance, tie bars are used in longitudinal joints in concrete pavement.
Dowel bars: Dowel bars are smooth round bars which mainly serve as load transfer device across concrete joints. They are placed across transverse joints of concrete pavement to allow movement to take place. Where movement is purposely designed for longitudinal joints, dowel bars can be adopted.
6. Draw the diagram of flexible pavement.
7. Describe marshal mix design
The Marshall stability and flow test provides the performance prediction measure for the Marshall mix design method. The stability portion of the test measures the maximum load supported by the test specimen at a loading rate of 50.8 mm/minute. Load is applied to the specimen till failure, and the maximum load is designated as stability. During the loading, an attached dial gauge measures the specimen’s plastic flow (deformation) due to the loading. The flow value is recorded in 0.25 mm (0.01 inch) increments at the same time when the maximum load is recorded. The main idea of the Marshall Mix Design method involves the selection of the asphalt binder content with a suitable density which satisfies minimum stability and range of flow values.
8. What is the difference between disturbed and undisturbed soil sample?
a) Disturbed soil: When the natural conditions of a sample such as its structure, texture, density, natural water contents or the stress conditions are disturbed then the sample is called as disturbed soil. By using shovel from auger cutting these samples can be recovered.
b) Undisturbed soil: Without disturbing the natural conditions of a soil sample such as its structure, texture, density, natural water contents or the stress condition the sample obtained is called undisturbed soil. This type of soil sample cannot be recovered and it retains the original properties of the soil mass as much as possible.
9. Draw SFD and BMD for the following beam.
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Engr. Abu Jobair
Assistant Engineer (Civil)
Bangladesh Water Development Board