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Con Tech & RCC Design

Interview Questions on RCC Structures Design

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interview

  1. Characteristic Strength of Concrete –

Characteristic strength is the compressive strength of concrete tested after 28 days of 150 mm size cube. Its represented as fck and not more than 5% of the sample should fail below this value.

  • Minimum Grade of RCC –
Environmental Condition Cement Content (kg) Maximum WC Minimum Grade of Concrete
Mild 300 0.55 M20
Moderate 300 0.50 M25
Severe 320 0.45 M30
Very Severe 340 0.45 M35
Extreme 360 0.4 M40
  1. Minimum Grade of PCC –
Environmental Condition Cement Content (kg) Maximum WC Minimum Grade of Concrete
Mild 220 0.6 M20
Moderate 240 0.6 M25
Severe 250 0.5 M30
Very Severe 260 0.45 M35
Extreme 280 0.4 M40
  • Balanced (Critical), Over-Reinforced and Under-Reinforced sections –

Balanced Section – Beam sections where the concrete and the reinforcement steel both reach their allowable stress at the same time is called a balanced section. The depth of Neutral Axis xu=xumax

Over Reinforced Section – Beams where concrete reach the failure stress earlier than steel is called an over reinforced concrete. NA falls below balanced NA xu>xumax

Under –Reinforced Section – The beams where steel reaches the allowable earlier than the concrete is called under reinforced concrete. Under reinforced beam is preferable as it gives warning before failure. The actual NA is above the balanced NA xu<xumax

  • Design constants – k,j,Q –

k is the neutral axis constant –

j is the lever arm. It’s the perpendicular distance between centres of gravity of tensile and compressive forces. It is given by j = d(1-k/3)

Q is the moment of resistance constant =  

  • Minimum and Maximum Steel in Slab Beam and Column, What are the minimum and maximum spacing of rods in slab and columns ?
Member Minimum Reinf. Maximum Reinf. Spacing of Main Spacing of Stirrups
Slab 0.12% for HYSD and 0.15% for Mild Min – Ast/bd = 0.85/fy   0.04bD ie 4% of Gross cs area. Max Size of bar = 1/8 th of Total Thickness of Member  Maximum spacing of main bar is restricted to 3 times effective depth or 300 mm whichever is less. For distribution bars the maximum spacing is specified as 5 times the effective depth or 450 mm whichever is less.
Beam Min – Ast/bd = 0.85/fy   0.04bD ie 4% of Gross cs area. Max Size of bar = 1/8 th of Total Thickness of Member Min distance = not less than (i) The dia meter of the bar if dia are equal (ii)The dia of the larger bar if dia are unequal (iii) 5 mm more than the nominal maximum size of coarse aggregate. HOWEVER when needle vibrator is used the distance may be reduced to 2/3 of the nominal maximum size of coarse aggregate The min .vertical distance between two bars should be 15 mm. Minimum of (i) Sv = 2.175fyAsv/b (ii) 300mm (iii) 0.75d –For Vertical Stirrups d –  For inclined where, d = effective depth of the section  
Column 0.8% of gross area of col. 4 Nos bar for Rectangular Column And 6 Nos Minimum Dia – 12 mm   6% when bars are not lapped 4% when bars are lapped   Max. Distance – 300 mm  Ties – ¼ Dia of main bar. 6 mm whichever is higher. Spacing – Least lateral dimension. 16xMain Bar. 300 mm.   Helical pitch Min of – 75 mm 1/6 Dc  & Should Not be less than -3 Dia of bar forming helix. 25 mm.  
  • Minimum diameter of rod in Slab, Beam and Column –

Slab – 8 mm

Beam -12 mm

Column – 12 mm

  • Maximum size of aggregates – 40 mm.
  • What is 24 SWG wire or Black Annealed wire? What is its quantity per Quintal of steel?

Wire used for binding reinforcement. Required Quantity 0.8 kg to 1.4 kg depending upon the dia of bar.

  • Values of covers in Slabs, Beams and Columns and Column Footings. –

The cover depends upon the exposure condition of concrete for Mild Condition the values are –

Slab : 20mm

Beams :25mm

Columns : 40mm

Footings : 50mm.

  1. Point to be supervised before the casting of cement concrete in RCC structures?

Weather the reinforcements are provided as per the drawing.

Proper cover is provided or not.

Alignment conform to the proposed drawing.

Sufficient support for formwork is provided or not.

Shuttering oil is applied or not.

The surface is free from any kind of soil or debris. Etc.

  • Sizes of vibrators Oscillations, Maximum time of needle vibrators, and its range of vibration at one spot.

The size of needle vibrator vary from 40 to 100 mm in diameter. The period of vibration required vary from 30 seconds to 2 minute.

The normal radius of effect of needle vibrator is 0.50 to 1.0m. It would be preferable to immerse the vibrator into concrete at intervals of not more than 600 mm or 8 to 10 times the diameter of the needle.

The frequency of vibration varies up to 15000 rpm. A range between 3000 to 6000 rpm is suggested as a desirable minimum with an acceleration of 4g to 10g.

The concrete should be placed in layers not more than 600mm high.

  • Why Binders and Stirrups are used in columns and Beams? Where are they close and wide?

In columns ties are used to tie the main bar thus preventing them from buckling and to hold the main bar in position. The tie bar reduces the unsupported length of main bar thus acting as short column.

The stirrups are used in beams and they resist diagonal shear in beams and also hold the main bar in position.

They are placed close near the support and towards the centre the spacing may increase.

  • What is the minimum diameter of binders or lateral ties? Why longitudinal rods in columns are provided along the circumference and not at the center.

The minimum dia of binder in column is– ¼ Dia of main bar. 6 mm whichever is higher.

If the bars would be placed in centre the integrity of the concrete shape could not be maintained, crack would from due to bending stress also placement of concrete would become difficult for constructing a homogeneous column. Further he column would become ineffective in carrying eccentric load.

  • Value of lapping of roads and their ideal locations in Slabs, Beams and Columns.

For flexural tension Ld or 30d whichever is greater.

For direct tension -2Ld or 30d whichever is greater.

The lap length in compression is 24d.

The top reinforcement of the beam is lapped at center of the beam and the bottom reinforcement is lapped near the support. Basically where there is minimum bending moment the lap should be preferable placed there.

  • How turbid water can be used for concreting?

The turbidity may be removed by adding coagulant like alum also the water may be kept in tanks to reduce the turbidity by sedimentation process.

  • Why stirrups are not provided in slabs? –

The dead load are carried by the beam the beam would carry most of the shear also due to in sufficient space available the stirrups could not be placed for holding the bars in position in slab.

  • One Way and Two Way slabs. – When the length of the slab is greater equals to 2 times the width then it’s called one way slab and when the ratio is less than 2 its two way slab.

Stripping time of shuttering and Centering’s. Locations from Where Centering are to be stripped off in simply supported and cantilever structures?

Type of formwork Minimum period before striking formwork
a) Vertical formwork to columns, walls, beams             16-24 h
b) Soffit formwork to slabs (Props to be re-fixed immediately after removal of formwork)             3 days
c) Soffit formwork to beams (Props to be re-fixed immediately after removal of formwork)             7 days
d) Props to slabs:   Spanning up to 4.5 m  Spanning Over 4.5 m               7 days             14 days
e) Props to beam and arches:    Spanning upto 6 m    Spanning over 6 m               14 days             21 days

In simply supported structure the formwork should be stripped from the support towards mid span.

In Cantilever structure the formwork should be stripped from end towards support.

Where Rectangular and T-Beams are recommended? Value of bf ?

The rectangular beams are recommended along the periphery of the structure. T-Beams use concrete more economically hence require less steel.  

The value of bf =  for T beam

Con Tech & RCC Design

Interview Questions on RCC Concrete Technology and RCC Structures

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CONCRETE

  1. What are Nominal Mix and Design Mix –

Nominal mix is the mix proportion of aggregate where no trial mix or mix design is prepared. It’s based on previous work experience.

  • What is SSD condition – Saturated Surface Dry is a condition of aggregate where the surface if the aggregate particles are dry.  Surface absorption of water is not possible in this state.
  • Capacity of Mixer Machine, Its RPM and Mixing Time – The Capacity of mixer machine varies manual machines has drum capacity of 260-500 lit (1/2 to 2 bag of cement), – 15-20 RPM or around 30 rotation.
  • What is grading of aggregate and why it is necessary – Particle size distribution of aggregate in concrete is termed as grading. Aggregates are graded to minimize the void to obtain higher density, adequate workability and better durability.
  • What happens if 12 mm stone chips are used in place of 20 mm size stone chips?

More water will be required to obtain the same degree of workability as the surface area will increase also more cement paste will require for the same strength of 20 mm aggregates.

  • What is W/C Ratio?

Water cement ratio is the ratio of weight of water to weight of cement in concrete.

Minimum and maximum value of WC ratio. Min – 0.4 Max – 0.6 as per IS 456:2000

 Slump Values –

Placing Conditions Degree of Workability Slump (mm)
Blinding concrete: Shallow sections: Pavement Using pavers: Very Low In the ‘very low’ category of workability where strict control is necessary, for example pavement quality concrete, measurement of workability by determination of compaction factor will be more suitable than slump (IS 1199) and value of compacting factor of 0.75 to 0.80 is suggested.
Mass concrete; Lightly reinforced section in slabs, beam, walls, columns: Floors; Hand placed pavements ; Canal lining; Strip footings; Low 25-75
Heavily reinforced sections in slabs, beams, walls columns Medium 50-100
Slipform work; Pumped concrete Medium 75-100
Trench fill; In-situ piling: High 100-150
Tremie concrete; Very High In the ‘very high’ category of workability, measurement of workability by determination of flow will be appropriate. (IS 9103)
  • What is curing? How it is done and why? – Curing is the process of prevention of escaping water from the concrete. Ponding, Wet Gunny Bag, Water Sprinkling and Use of water proofing chemicals called curing compounds are methods of curing.
  • What are segregation, Laitance and Bleeding?

Segregation is the separation of coarse aggregate from the concrete mix.

Laitance is the weak layer of concrete which is comprising thin layer of cement and sand particle, too much water causes bleeding and when the cement water set over the concrete laitance is formed. Also over working of concrete or improper finishing may lead to laitance.

Bleeding is the separation of cement water from the fresh concrete.

  1. Flakiness Index and Elongation Index.

Its index number representing how thin is the aggregate compared to its mean dimension. Its represented by the percentage by weight of particles whose least dimension (or thickness) is less than 3/5th of its mean dimension.

Elongation index is the percentage of the particle (by weight) whose largest dimension (length) exceeds the 9/5th of its mean dimension.

Building Materials

Interview Questions on Building Materials

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BRICKS

  1. Composition of brick earth –

Silica or Sand 50-60%– Prevent Shrinkage, Cracking and wrapping but too much quantity can make the brick brittle.

Alumina or Clay 20-30%– It make the brick earth plastic and provide the brick its hardness.

Lime and Oxide of Iron – Malt the sand grains and bind the clay particle when heated. Iron oxide provide the brick its red color.

Magnesia – Small quantity desirable but excess makes the brick yellow.

  • Burning Temperature – 800-10000C
  •  
  • Strength of 1st Class, 2nd Class and 3rd Class, Jhama bricks – 10.5N/mm2  7.5 N/mm2 5.5 N/mm2
  •  
  • Water absorption – First Class < 20% (Preferable below 15%) 2nd Class < 22% 3rd Class < 25%
  •  
  • Frog Size and Its position in masonry – 100x40x 10-20 mm

Frog acts as a key to the joint of brick and mortar.  

  1. Stones used in civil engineering and their strength –

Granite – Abutment and Pier of Bridge,

Stand Stone – Ashlar Masonry

Marble – Floor and Ornamental Work.

  • Why bricks are soaked before their use in brick masonry?

Dry brick absorb water and to prevent water getting absorbed from mortar and for better bonding with cement sand mortar.

  • Points to be supervised during brick masonry. –

No continuous vertical joint should be formed.

For brick masonry brick should be sufficiently wet before use.

Maximum height of construction should be limited to 1.5 m.

Verticality should be checked.

Proper curing should be done.

SAND

  1. Types- River Sand, Nallah Sand, Artificial Sand etc.

Sizes- Fine Sand, Medium Sand, Coarse Sand

Uses – Cement mortar, fine aggregate of concrete, Soil improvement, Plinth filling.

Bulking of sand The phenomenon of increase in volume of sand due to moisture in sand particle.

It’s Value – The volume may increase up to 45% corresponding to the water content of about 4.6%

What is FM of Sand – It’s an index number representing the size of sand particle.

FM -2.2-2.6 Fine Sand, 2.6-2.9 – Medium Sand, 2.9-3.2 Coarse sand.

 What does it indicate, what are their values?

It indicate the average size of the sand particles.

CEMENT

  1. Chief Ingredients – Limestone or Chalk (Calcareous materials), Shale or Clay (Argillaceous materials),
  2. Calcination temperature – 13000C – 15000C
  3. Bogue’s compounds –

Tricalcium Silicate – 3CaO.SiO2 (C3S)

Dicalcium Silicate – 2CaO.SiO2 (C2S)

Tricalcium Aluminate – 3CaO.Al2O3 (C3A)

Tetracalcium Aluminoferrrite – 3CaO. Al2O3 .Fe2O3 (C4AF)

Their functions – Tricalcium Silicate and Di Calcium Silicate are the two most important compound of cement responsible for strength development. Average C3S content is 45% and C2S is 25%. C3A flash set and to prevent flash set retarder like gypsum is added to the cement.C4AF is relatively inactive compound. C3S hardens early and develops early strength. C2S hydrates slowly and contribute to ultimate strength. C2S hydrate are much superior to C3S hydrate.

  • The Types of Cement and their uses in different locations –

Ordinary Portland cement – General purpose construction.

Portland Pozzolana Cement – Marine and hydraulic construction and mass concrete constructions.

Rapid Hardening Cement – Where formwork to be removed early, road repair works and in cold weather condition.

Portland Slag Cement –Underground construction and marine construction and at coastal regions and in mass concrete structure.

Sulphate resisting Cement – Concrete in foundation and basement where soil contains sulphates, fabrication of pipes, construction of sewage treatment plant and in marine environment.

Quick Setting Cement – Grouting operations.

Low Heat Cement – Mass concrete such as dams.

Super Sulphated Cement –Foundations where chemically aggressive condition exist marine environment and sewage pipe.

Coloured Cement –Decorative purposes.

Hydrophobic Cement – Remote locations where more storage time is required.

  • Initial Setting and Final Setting Time –

30 min 600 min for OPC.

  • What is 53 Grade Cement – Strength of the cement at 28 days is not less than 53N/mm2. (Tested as per IS 4031-1988)
  • How is cement tested at site –

Appearance of the cement should be greenish grey there should not be any lumps in the cement bags.

Smooth feeling is obtained in good cement when we take between our fingers.

If we insert our hand inside a bag of cement it should feel cool.

When a hand full of cement is thrown inside a bucket of water it should float sometime before sinking.

A cake made with stiff paste of cement when kept inside water for 24 hours should retain its shape and attain some strength.

  • Weight and volume of one Bag of cement – 50 kg weight and 0.0347 cum volume.
  1. How much temperature and heat are generated when cement is mixed with water?

After 7 days – 89-90 cal per gram.

After 28 days – 90-100 cal per gram.