Bangladesh Army
Engineering Core (Civil)

57 BMA SPECIAL COURSE
Exam Date: 13-08-2021
Total marks: 100

Time : 1hour

01. A rectangular column of 15” x 15” section has reinforcement of 8 no. 9 bars. If the value of fc’ is 4000 psi and fy is 60000 psi, find the ultimate design axial load of the column

Solution:

𝐴𝑔 = 15 x 15 = 225 sq.inch
𝐴𝑠𝑡 = 8 x 1 ≈ 8 sq.inch

𝑃𝑢 = 𝜑 𝛼 [0.85 𝑓𝑐′ (𝐴𝑔 – 𝐴𝑠𝑡) + 𝐴𝑠𝑡 𝑓𝑦]
So, 𝑃𝑢 = 0.8 x 0.65 [0.85 x 4 x (225 – 8) + 8 x 60] = 633.256 K

Ultimate design axial load of the column = 633 k

02.What is Punching Shear , Give example .

Solution:

Punching shear:

The punching shear is a failure mechanism in structural members by shear under the action of concentrated loads. The action of concentrated loads is on a smaller area in the structural members. Punching shear is a type of failure of reinforced concrete slabs subjected to high localized forces.This type of failure is catastrophic because no visible signs are shown prior to failure. Punching shear failure disasters have occurred several times in this past decade.

Example:

Some examples of the occurrence of concentrated loads on a slab are a column, particularly on a pad foundation, and wheel loads. This same type of failure could also happen in another way. Turning the structure upside down we get a flat slab supported by a column, where there is a high concentration of shear force around the column head.

Punching shear checks are carried out for the following elements generally to avoid punching shear failures.

  1. Normal Slab
  2. Flat Slab
  3. Pile Caps
  4. Footings
  5. Raft foundations

03. Short Note
i) Coagulation
ii) Portland Cement
iii) Marshal Mix Design
iv) Steady Uniform Flow

Solution:

i) Coagulation
The coagulation process involves adding iron or aluminum salts, such as aluminum sulphate, ferric sulphate, ferric chloride or polymers, to the water. These chemicals are called coagulants, and have a positive charge. The positive charge of the coagulant neutralizes the negative charge of dissolved and suspended particles in the water. When this reaction occurs, the particles bind together, or coagulate (this process is sometimes also called flocculation). The larger particles, or floc, are heavy and quickly settle to the bottom of the water supply. This settling process is called sedimentation. The following diagram illustrates the basic reactions and processes that occur during coagulation.

Process of Coagulation, Flocculation and Sedimentation

In a water treatment facility, the coagulant is added to the water and it is rapidly mixed, so that the coagulant is circulated throughout the water. The coagulated water can either be filtered directly through a medium filter (such as sand and gravel), a microfiltration or ultrafiltration membrane, or it can be moved to a settling tank. In a settling tank, or clarifier, the heavy particles settle to the bottom and are removed, and the water moves on to the filtration step of the treatment process.

ii) Portland Cement

Portland cement is the product obtained by pulverizing clinker, consisting of hydraulic calcium silicates to which some calcium sulfate has usually been provided as an interground addition. It was termed portland cement because its hydration product resembled a building stone from the Isle of Portland off the British coast. The first patent for portland cement was obtained in 1824 by Joseph Aspdin, an English mason. The specific gravity of portland cement particles is about 3.15. There are four primary phases in portland cement: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). The strength and other properties of concrete are mainly derived from the hydration of tricalcium and dicalcium silicates. The composition of any of these phases in a particular clinker will not be precisely in the composition indicated.

iii) Marshall Mix Design

The Marshall Mix Design method was originally developed by Bruce Marshall of the Mississippi Highway Department in 1939. 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.

The Marshall Mix Design method consists mainly of the following steps:

(i) Determination of physical properties, size and gradation of aggregates.

(ii) Selection of types of asphalt binder.

(iii) Prepare initial samples, each with different asphalt binder content.
For example, three samples are made each at 4.5, 5.0, 5.5, 6.0 and 6.5 percent asphalt by dry weight for a total of 15 samples. There should be at least two samples above and two below the estimated optimum asphalt content.

(iv) Plot the following graphs:

(a) Asphalt binder content vs. density
(b) Asphalt binder content vs. Marshall stability
(c) Asphalt binder content vs. flow
(d) Asphalt binder content vs. air voids
(e) Asphalt binder content vs. voids in mineral aggregates
(f) Asphalt binder content vs voids filled with asphalt

(v) Determine the asphalt binder content which corresponds to the air void content of 4 percent

(vi) Determine properties at this optimum asphalt binder content by reference with the graphs. Compare each of these values against design requirements and if all comply with design requirements, then the selected optimum asphalt binder content is acceptable. Otherwise, the mixture should be redesigned.

marshall-mix-design

iv) Steady Uniform Flow

In fluid mechanics, uniform flow means the flow in which the flow parameters like velocity, pressure, temperature does not vary with positions. Whereas, in steady flow the flow properties does not vary with time.i.e. the velocity at a certain point is same as it was 5 seconds ago. So, steady uniform flow means the flow in which the flow properties does not vary with position as well as time.

1: Schematic view of a river with a steady, uniform flow with sloping... |  Download Scientific Diagram

04.What is CBR test ? How it is conducted ?

California Bearing Ratio of Soil:

The California Bearing Ratio or CBR test is performed in construction materials laboratories to evaluate the strength of soil subgrades and base course materials. Those who design and engineer highways, airport runways and taxiways, parking lots, and other pavements rely on CBR test values when selecting pavement and base thicknesses.

CBR Test:

The California Bearing Ratio (CBR) Test is used to estimate the bearing value and the mechanical strength of highway subbases and subgrades. It requires a reaction load, usually a Land Rover or similar, to provide the force to the plunger and CBR press.

Procedure For Penetration Test:

  1. Place the mould assembly with test specimen on the lower plate of penetration testing machine. To prevent upheaval of soil into the hole of the surcharge weights, 2.5 kg annular weight shall be placed on the soil surface prior to seating the penetration plunger after which the remainder of the surcharge weights shall be placed.
  2. Seat the penetration piston at the center of the specimen with the smallest possible load, but in no case in excess of 4 kg so that full contact of the piston on the sample is established.
  3. Set the load and deformation gauges to read zero. Apply the load on the piston so that the penetration rate is about 1.25 mm/min.
  4. Record the load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10 and 12.5 mm.
  5. Raise the plunger and detach the mould from the loading equipment. Take about 20 to 50 g of soil from the top 30 mm layer and determine the moisture content.

05.What is Permeability and What are the factors affecting it ?

Soil permeability:
Soil permeability is the property of the soil to transmit water.The permeability of a soil is a measure of the ability of soil to allow water to pass through it. It is typically represented by the letter ‘k’ 

Factors Effecting Permeability of Soils:

Following are factors effecting permeability of soils.

  1. Size of soil particle
  2. Specific Surface Area of Soil Particle
  3. Shape of soil particle
  4. Void ratio
  5. Soil structure
  6. Degree of saturation
  7. Water properties
  8. Temperature
  9. Adsorbed water
  10. Organic Matter

06. What is initial and final setting time?

Initial Setting Time of Cement:

Initial setting time of cement is defined as the time elapsed between the moments when water is added to the cement to the time when the cement paste starts losing its plasticity. It can also be defined as the time elapsed between the moments when water is added to the cement to the time when the Vicat square needle penetrates a depth of 33-35 mm from the top (5 to 7 mm from the bottom) of the mould of the Vicat Apparatus.

Final Setting Time of Cement:

Final setting time of cement can be defined as the time elapsed between the moments when water is added to the cement to the time when the cement paste has completely lost its plasticity. Final setting time of cement can also be defined as the time elapsed between the moments when water is added to the cement to the time when square needle makes impression on the paste, while the annular collar fails to do so.

07.Draw SFD BMD


Solution:

SFD:


BMD:

08. What is the Role of Site Engineer during Slab Casting?

Solution:

Roles and Responsibilities of Civil Site Engineer during slab casting: It is the responsibility of the civil engineer to check the steel installed in it before any concrete is done. 

Check list for slab casting.

  • Check the dimension and arrangement of reinforcement bars as per drawing.
  • Check the spacing of main steel and distribution steel.
  • The electrical lines should be placed as per plan.
  • Check the joint and supporting member of formwork. And it should be greased or oiled.
  • IN-IN dimension of formwork.
  • Horizontal level of formwork.
  • Ensure the location of the sunken slab and verify the location with the help of a typical plan.
  • The prepared mould should be free of any dirt.
  • Concrete should be poured evenly.
  • Use of vibrator is necessary for even compaction of concrete to avoid honeycombing.
  • The casting should be completed in a single operation to avoid joint formation

09. What are the Advantage and Disadvantage of Pre-Cast Pile

Solution:

Precast concrete piles are cast, cured and stored in a yard before they are installed in the field mostly by driving.

ADVANTAGES OF PRECAST CONCRETE PILES:

  1. Reinforcement used in the pile is not liable to change its place or get disturbed
  2. The defects in pile can be easily identified after the removal of forms, and these defects (such as presence of cavity or hole) can be repaired before driving the pile.
  3. The cost of manufacturing will be less, as a large number of piles are manufactured at a time.
  4. Precast concrete piles can be driven under water. If the subsoil water contains more sulphates, the concrete of cast in situ piles would not set. Thus precast concrete piles have added advantage in such a circumstance.
  5. Precast concrete piles are highly resistant to biological and chemical actions of the sub soil.
  6. Better quality control can be implemented as compared to bored cast in situ piles.
  7. These piles can be constructed in various cross-sectional shapes such as circular, octagonal or square.

DISADVANTAGES OF PRECAST CONCRETE PILES:

  1. These piles are usually very heavy. So special equipments are required for handling and transportation.
  2. Sufficient care must be taken at the time of transportation, otherwise piles may break.
  3. For embedding these piles in field heavy pile driving equipment is required.
  4. These piles are costly as extra reinforcement is required to bear handling and driving stresses.
  5. The length of the pile is restricted since it depends upon the transport facility.
  6. Once constructed, it is not possible to increase the length of the pile (as per the site demand)
  7. If the pile is found to be too long, during driving, it is difficult and uneconomical to cut. Also cutting of extra length results in the wastage of material.
  8. Driving these piles created a lot of noise pollution.

REFERENCE: Building Construction by Dr. B.C. Punima

10.Amplification of the following

i)ASTM
ii)AASHTO
iii)EIA
iv)FAR
v)PPA


Solution:

i)ASTM
American Society for Testing and Materials

ii)AASHTO
American Association of State Highway Officials

iii)EIA
Environmental Impact Assessment


iv)FAR
Floor Area Ratio

v)PPA
Public Procurement Act


AUTHOR: 
Engr.Towhidul Islam
Upazila Assistant Engineer,LGED

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