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The Properties Of Concrete In Plastic Stage are:
Plastic stage of concrete is that stage where concrete can be transported easily, moulded or poured into any place
- Good Workability:
Workability is the most important property of concrete. It may be defined as ‘ The ease with which the concrete can be mixed, transported, placed and compacted. ‘ A workable concrete mix doesn’t results in bleeding or segregation.
- Freedom from segregation :
- Freedom from bleeding :
A good concrete should be free from bleeding. The appearance of Water, along with some particles of cement and very fine sand on the surface of freshly placed concrete is known as bleeding.
- Prevention from harshness :
The concrete mix which doesn’t give smooth surface with a certain amount of trowelling is known as harsh mix.
USEFUL TIPS FOR CIVIL ENGINEERING INTERVIEW
GRADE OF CONCRETE:
M5 – 1 : 4 : 8
M10 – 1 : 3 : 6
M15 – 1 : 2 : 4
M20 – 1 : 1.5 : 3
M25 – 1 : 1 : 2
CLEAR COVER TO MAIN REINFORCEMENT:
Footings : 50 mm
Raft Foundation Top : 50 mm
Raft Foundation Bottom : 75 mm
Raft Foundation Side : 75 mm
Beam : 25 mm
Strap Beam : 50 mm
Column :40 mm
Slab : 15 mm
flat Slab : 20 mm
Staircase : 15 mm
Retaining Wall : 20 – 25 mm
Water Retaining Structures : 2 0- 30 mm
Maximum water absorption by bricks – 15%
Compressive strength of bricks – 3.5 N/mm2
Density Of Bricks- 1600-1920 Kg/m3
Minimum thickness of slab – 125 mm
Dimension tolerance for cubes – +2
Maximum free fall of concrete – 1.50 m
Lapping should not be used for the bars having larger dia than 36 mm.
Binding wire required for steel reinforcement – 8 kg per MT
3 samples should be taken for every 100 m2 in core cutting test.
Maximum chair spacing – 1 m.
Minimum dia should be used in dowels rod – 12 mm.
Hook for strriups (one side) – 9D
No. of strriups = (clear span/spanning) + 1
Length of main steel in cantilever anchorage – 69D.
Minimum no. of bars in square column – 4
Minimum no. of bars in circular column – 6
Minimum dia of main bars and distributors in the slab – 8 mm.
Maximum dia of main bars and distributors in the slab – 1/8 of slab thickness.
All reinforcement should be free from mill scales, loose rust, and coats of paints , oil or any other substances.
Initial setting time should not be less than 30 minutes.
Final setting time should not be greater than 10 hours.
REQUIRED CURING DAYS:
Super sulfate cement – 7 days
Ordinary portland cement – 10 days
Cement with minerals and admixtures – 14 days.
SLUMP VALUE (IS456):
Lightly reinforced concrete : 25 – 75 mm.
Heavily reinforced concrete : 75 – 100 mm.
Trench fill : 100 – 150 mm ( for in-situ & tremie).
1 – 5 m3 : 1 No.
6 – 15 m3 : 2 No..
16 – 30 m3 :3 No.
31 -50 m3 : 4 No.
Above 50 m3 : 4 + 1 no. of addition for each 50 m3.
There are 2 Types of cover provided to reinforcement in Civil Engineering
Clear Cover is the length between outer end of reinforcement to outer edge of concrete.
2. Nominal Cover
Nominal cover is the length from center of reinforcement to outer edge of concrete.
Have you ever been asked for density of Cement, Sand, Aggregates & Water along with density of concrete in an job interview ?
If not you were lucky because here are the densities of all
Density of Concrete – 2400 Kg/m3
Density of Cement – 1440 kg/m3
Density of Water – 1000 Kg/m3
Density of Dry Sand – 1600 Kg/m3
Density of Wet Sand – 1850 Kg/m3
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Density of 20mm Aggregate – 1600 Kg/m3
Density of 10 mm Aggregate – 1700 Kg/m3
Density of Fly Ash – 750 Kg/m3
What is Workability ?
Workability of concrete is the ease with which it can be mixed, transported & placed in position in homogeneous state.
Workability of concrete can be found out by performing following tests
- Slump Test
- K-Slump Tester
- Compacting Factor Test
- Flow Test
- Kelly ball test
- Vee Bee Consistometer Test
Links will be provided soon for the above tests **Stay tuned ©Civilmafia.in
Following are the points to be checked before pouring concrete for any structural member.
1. Rebar details must be correctly followed as per drawings.
2. Rings/stirrups provide strength to columns and beams and prevent the concrete from spreading out and cracking. Hence rings should not be missed out as it could cause structural failure . Its a good idea to get rings pre-fabricated from contractor who specializes in this kind of a job
3.Ring spacing along the column or beam rebar must be correctly followed.
4.Cover blocks must be placed all around the rebar to prevent the rebar from touching the shuttering
5. Ensure that the shuttering alignment is correct and shuttering is vertical.
6. Make sure to take atleast 2 samples of concrete before casting for 7 day and 28 day testing. Fill the moulds with concrete and cure them before sending them to the quality lab.
7. Do an onsite slump test of concrete by using a slump cone
8. Make sure that the inner sides of shuttering material are smooth and smeared with shuttering oil
VIBRATION OF CONCRETE WHY IMPORTANAT?
Vibrating concrete is a must so that concrete can achieve its design properties
Vibrating concrete makes it more fluid and hence it flows into the corners of the formwork
Air-gaps in concrete are significantly reduced and results in a more consistent density of concrete both at top and bottom
However over-vibrating the concrete will result in segregation of aggregates.
One must be careful that vibrator does not disturb the rebar arrangements especially in areas where rebars are very dense.
For optimum compaction the frequency of the vibrator should be between 10,000 to 12,000 vibrations per minute.
Following are the clear cover normally provided to Reinforced Cement Concrete structures, Please note that this may vary as per the Structural DesignCLEAR COVER TO MAIN REINFORCEMENT:
FOOTINGS : 50 mm
RAFT FOUNDATION.TOP : 50 mm
RAFT FOUNDATION.BOTTOM/SIDES : 75 mm
STRAP BEAM : 50 mm
GRADE SLAB : 20 mm
COLUMN : 40 mm
SHEAR WALL : 25 mm
BEAMS : 25 mm
SLABS : 15 mm
FLAT SLAB : 20 mm
STAIRCASE : 15 mm
RET. WALL : 20/ 25 mm on earth
WATER RETAINING STRUCTURES : 20/30 mm
PLINTH BEAM:30 mm
PILING:50 to 75 mm
Below is only a general guideline for removal of formwork and not to be followed without the approval of your structural consultant.
a) Removal of formwork from walls, columns and vertical surfaces – 24 hours
b) Removal of formwork from slabs while keeping the props – 4 days
c) Removal of formwork from beams while keeping the props – 7 days
d) Removal of props under slabs
1) Slab span upto 4.5m – 7days
2) Slab span above 4.5m – 14 days
e)Removal of props under beams
1) For beams upto 4.5 m – 14 days
2) For beams above 4.5m – 21 days
Please remember after removal of formwork, proper curing of concrete is really important.