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Tuesday 27 February 2018

METHODS OF CONCRETE CURING
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CONCRETE CURING:
The process of protecting the moisture of concrete surface and enhancing the hydration of cement is known as curing. The physical properties of concrete totally depend on the hydration of cement. If curing is not done properly concrete will be failed to acquire its full strength. Improper concrete curing may also lead the concrete to crack.

METHODS OF CONCRETE CURING:
1. SHADING:
By this method, the evaporation of water is locked in the concrete surface. It also protects the surface from heat, wind etc. In cold climates, it prevents the concrete from freezing by preserving heat of hydration of cement.
2. COVERING THE SURFACE WITH WATERPROOF PAPER:
In this method, the concrete surface is covered with wet gunny bags or waterproof papers to avoid water loss and to protect the concrete from further damage. This method gives satisfactory results for curing of concrete slabs and pavements.
3. SPRINKLING WATER:
In this method, water is sprayed on the concrete with the help of nozzles at proper intervals. This method is not so effective due to the difficulty of keeping the concrete surface be moist all the time.
4. PONDING:
Ponding is the most common method that is adopted for curing of floors, slabs, pavements etc. In this method, concrete surface is first covered with moist wrapper for 24 hours. After that, the covers are then removed and small amount of clay puddles are constructed around all the area. Then water is filled for final curing.
5. MEMBRANE CURING:
In this method concrete surface is covered by waterproof membranes or sealing compounds such as bitumen emulsion, wax, rubber latex emulsion, water repellant, plastic films etc. The membrane protects the water loss from concrete. It is seen membrane curing for 28 days give equivalent strength to two weeks moist curing.
6. STEAM CURING:
Steam curing is done by increasing the temperature of concrete in wet condition. This method allows the concrete to achieve its full strength within a short time, thus curing is also finished within short time. Steam curing is mostly adopted for the production of precast members.

Friday 23 February 2018

#One way slab & Two way slab

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One Way Slab

One way slabs are such slabs in which the loads are shared only by the shorter direction of the slab. In above figure (a) represents one way slab. Here ratio of longer span of slab to the shorter span of slab is greater than 2. One way slab being supported on two opposite sides only leads to the structural action in single direction.

Analysis and Design of One-way Slab:

Design steps:

Step 1: Preliminary Design:

Find depth of slab using deflection control criteria.

Take Span/d = 25 for simply supported slab.

= 30 for continuous slab.

D ≥ Dmin = 100mm for normal design.

= 125mm for earthquake resistant design.

Step 2: Analysis of slab:

Analyze the slab considering the slab as beam of 1m width and find respective loads, maximum design bending moment and maximum design shear force.

Step 3: Design of Slab:

Verify depth of slab:

Determine ‘Ast’ and ‘Φ’ and number of bars in shorter direction of slab.
Slab section is normally designed as Singly Reinforced Under Reinforced Rectangular Section.

Ast = Mu/(0.87fy(d-0.416xu)) which should not be less than Astmin.

Check slab for shear.
Step 4: Check slab at limit state of serviceability in deflection.

l/d ≤ αβγδγ

Step 5: Detailing of reinforcement.

Curtail or bent surplus bars.
Check development length at support.
Ld ≤ 1.3Ml / Vu + ld

Provide min. Ast as distribution bars in longer direction of slab.
Two Way Slab

Two way slabs are such slabs in which the loads are shared by both the shorter and longer direction of the slab. In above figure (b) represents one way slab. Here the ratio of longer span of slab to the shorter span of slab is less or equals to 2. Two way slab being supported on four sides the load will be carried in both the directions.

Analysis and Design of Two Way Slab:

Design Steps:

Step 1: Preliminary Design.

Find depth of slab using deflection control criteria.

Take span/d = 28 for simply supported slab and 32 for continuous slab.

D ≥ Dmin = 100mm for normal design and 125mm for earthquake resistant design.

Step 2: Analysis of Slab.

Slab is analyzed by IS code method. (Ref. Annex D-1 and Annex D-2).

Step 3: Design of Slab.

Verify depth of slab:

Determine ‘Ast’ and ‘Φ’ and number of bars in shorter direction of slab.
Slab section is normally designed as Singly Reinforced Under Reinforced Rectangular Section.

Ast = Mu/(0.87fy(d-0.416xu)) which should not be less than Astmin.

Check slab for shear in shorter direction.
Step 4: Check slab at limit state of serviceability in deflection in shorter direction.

l/d ≤ αβγδγ

Step 5: Detailing of reinforcement:

Curtail/bend the surplus bars as per detailing rules of code. (Ref. Annex)
Check development length at support.
Ld ≤ 1.3Ml / Vu + ld

Provide torsion reinforcement at the discontinuous edge of slab as per codal provision.

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