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
A Rivet is around bar of steel or wrought iron provided with a head on one side & tail on the other side.
There are two types of riveted joints
1.Lap Riveted Joints
In Lap joints one member is placed above the other & both members are connected by rivet just like in the below image
2.Butt Riveted Joints
In butt joints 2 Members are kept aligned and another third member is used to join 2 members(Refer Below Picture)
IMPORTANT TERMS USED IN RIVETING
- Nominal Diameter- It is the diameter of the shank of rivet before riveting.
- Effective Diameter (Gross Diameter)- It is the diameter of the hole it fills after riveting.
- Pitch- It is the centre to centre distance of two adjacent rivets measured parallel to the direction of force.
- Diagonal Pitch- It is the diagonal distance between centres of 2 rivets.
- Gauge Line or Rivet Line- It is the line along which the rivets are placed. The perpendicular distance between the adjacent gauge lines is known as guage
- Margin- It is the distance between the centre of rivet hole to the nearest edge of plate.
- Tacking Rivets- are those rivets which are used to connect long lengths of members in the members of roof trusses. These are provided at suitable distance so that the members may act as one unit
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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.
Reinforced concrete column is a compression member and transfers the loads from structure to the ground through foundations. There are three types of concrete columns based on its height and lateral dimension. Long columns are those whose ratio of height to least lateral dimension is more than 12. When the height to least lateral dimension is less than 3, it is called a pedestal and if it is between 3 and 12, it is called as a short column.
The load carrying capacity and modes of failure of a reinforced concrete column is based on the slenderness ratio. Slenderness ratio is the ratio of the effective length Le and least lateral dimension of the column as per Indian and British Standards. But as per American Concrete Institute Code of Practice, the slenderness ratio is defined as the ratio of effective length of column to its radius of gyration, which is same as used for structural steel design as per IS Code. Effective length of a column depends on its support conditions at ends.
Based on the slenderness ratio of the column, there are three reasons of failure of reinforced concrete columns. The columns are assumed to be centrally loaded (no eccentric loads).
- Column Failure due to Pure Compression:
When reinforced concrete columns are axially loaded, the reinforcement steel and concrete experiences stresses. When the loads are high compared to cross-sectional area of the column, the steel and concrete reach the yield stress and column fails without undergoing any lateral deformation. The concrete column is crushed and collapse of the column is due to the material failure. To overcome this, the concrete column should have sufficient cross-sectional area, so that the stress is under the specified limit. This type of failure is generally seen in case of pedestals whose height to least lateral dimension is less than 3 and does not experience bending due to axial loads.
- Column Failure due to Combined Compression and Failure:
Short columns are commonly subjected to axial loads, lateral loads and moments. Short columns under the action of lateral loads and moments undergo lateral deflection and bending. Long columns undergo lateral deflection and bending even when they are only axially loaded. Under such circumstances when the stresses in steel and concrete reach their yield stress, material failure happens and RCC column fails. This type of failure is called combined compression and bending failure.
- Column Failure due to Elastic Instability:
Long columns are very slender, i.e. its effective length to least lateral dimension is more than 12. Under such condition, the load carrying capacity of reinforced concrete columns reduces drastically for given cross-sectional area and percentage of reinforcement steel. When such type of concrete columns are subjected to even small loads, they tend to become unstable and buckle to any side. So, the reinforcement steel and concrete in such cases reach their yield stress even for small loads and fail due to lateral elastic buckling. This type of failure is unacceptable in practical concrete constructions. Code prevents usage of such long columns for slenderness ratio greater than 30 (for unbraced columns) for the use in concrete structures.