Tag Archives: slab

Bridges : Deck slab & Solid slab ??

What is the difference between a deck slab and a solid slab? by Santosh Kulkarni https://www.quora.com/What-is-the-difference-between-a-deck-slab-and-a-solid-slab/answer/Santosh-Kulkarni-37?share=df35b072&srid=zM5g

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Gross cost of a water tank

Dear Friends ,

In the previous article, we planned a water tank .

We arrived at the dimensions of the water tank.

[ Read the post here ]

Now let us calculate the total cost for this construction as follows :

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Multi-span concrete slabs …Expert speaks .

Answer by Patrick Barry:

There’s a ton of ways of doing this that vary between contractors, countries, local codes (eg earthquake considerations) and speed vs expense of construction.

1) The simplest:
Cast the beams and slab of the entire floor at once:

+ Good for checking, last minute alterations and easy to do a concrete boom pump.
– A lot of labour required and not exactly quick, if the formwork’s not sealed properly you might have cracks or honeycombing. It’s not impossible to cast level changes (eg if you want the bathroom floor a step lower) but it can be a pain. If the spacers get displaced during concreting the steel might move out of position vertically (not good, but something that applies to nearly all methods)

2)
Since the formwork is on the bottom (tension for simple slabs), some designs use permeant formwork. The concrete is cast on top and the two make a composite slab together. The trick is to make sure there’s enough shear connectors (often steel studs) to stop them separating under strain.
Some variants use steel shuttering:

Some use precast panels that lock together (often with some small extra rebar used on top to bind them strongly, and tie them into the beams.

Design of these panels can get very complex, with polystytrene blocks included to reduce weight with minimal loss of strength.

3) Beyond that, fully precast slabs can be used, although they typically are finished with a very thin layer of slurry on the top to ensure smoothness.

I’m unaware of any of these slabs types that are truly continuous over beams or supports, so we’re entering the realm of cheating the question. Since the concrete is already at full strength, construction can proceed as fast as the cranes can lift:

And then beyond that, at the current top limits of pre-manufactured slabs, you have finished (tiled!) floors as used here. They’re not concrete though, using steel truss for the reduced weight.

How are multispan concrete slabs constructed?

Yeah..the Slab is finally cast

Overview:
In this series  we are looking at the various stages in the construction of a building.
So far we have covered from land acquisition up to mobilization and from mass excavation up to columns of the first floor.
This article brings you the intricacies and steps in casting the roof slab.

For the purpose of clarity, the tasks have been grouped under:

A. Pre-concreting care:
B. During concreting care:
C. Post -concreting care:

All the Activities ,however, have been sequenced in a continuous serial nos from 1 to 47.

The activities sequence:

A.Pre-concreting care:

Columns:
1.Columns up to beam bottom
2.Curing of columns
3.marking of beam bottom on columns
4.fixing of runner blocks on columns

Beam bottoms & sides:
5.Fixing of beam bottom
6.Supports below beam bottom
7.Inside face of beam: side fixing

Slab centering:
8.Slab centering in plates
9.Gaps fixing with planks & closing of gaps qith iron strips or masking tape.
10.Fixing of Accrow spans
11.Levelling of centering plates
12.Keeping camber in the slab

Reinforcement laying:
13.laying of beam reinf.
14.Laying of slab reinf.
15.placing chairs below bent up bars

Electrical conduits/ fixtures:
16.Laying electrical conduits/ fan hooks/ down pipes from beams

Sunk portion:
17.Keeping pipe sleeve in the sunk portion for drainage pipe

Securing:
18.Placing cover blocks for beams & slabs
19. Fixing clamps for beams
20.Fixing tie bars in sleeves for deep beams

Joint inspection:
21.Steel measurement and checking
( jointly with clients engineer)
22.Laps counting
23.Extra bars counting

Arrangement for casting:
24.Installing the concrete pump
25.Fixing the pipes for concrete
26.scheduling the transit mixer ( TM)
27.Final checking by consultant
28.pour card approval

B. Care during concreting :

29.Placing concrete
30.Compaction by needle vibrator
31.levelling
32.slab thickness verification
33.finishing
34.Removing excess debris from slab

C.Post concreting care:

35.casting of specimen cubes
36.providing mortar bunds for curing
37.ponding with water
38.striking off beam sides
39. striking off slab centering
40. striking off beam bottom
41.Hacking the concrete surface
42.Sunk portions cleaning
43.waterproofing
44.ponding with water
45.Testing of cubes on 7th & 28th days
46.Reconciliation of materials
47.Bills

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Bending backwards  for bent up bars ?

Overview :
A particular shape of reinforcement called “Bent up bar” is provided in the flexural members such as beam & slab.This article brings the practical aspect of these bars to you in a very simple language.
Purpose:
If I tell you that bent up bars are provided to take care for the diagonal shear would it suffice ?
Probably no.because then  you would like to first know what is diagonal shear .
So, instead of going further ,let us say that in a beam or slab , there are two types of internal stresses ;
1.Bending moment : Main bars in a beam or slab cater to this stress.
2.Shear stress : this can be further re-grouped into:
2.1 Vertical shear stress: Stirrups are provided to provide resistance against this stress.
2.2 Diagonal shear or Diagonal tension : Bent up bars develop resistance to counter this stress.
In short , if you see a beam you would observe micro cracks near supports .These are due to diagonal tension.
BENT UP BARS IN A BEAM
BENT UP BARS IN A BEAM
Bent up bars for beams:
The bent up bars run at bottom in the middle half span of the beam.The bending up normally is done at a distance of 1/4 of the span from the supports.
Difficulties:
Usually the reinforcement cage of the beam is tied at the location of the beam only. There  is restricted space in between the two beam sides and hence inserting the bent up bars in between the top & bottom bars is cumbersome for the fitters.

Can it be avoided ?

The designers have developed alternate method for bent up bars in a beam as follows ;

1. Provide curtail bars for middle half ( L/2 ).

2. Provide EOS bars ( Extra over support ) on the Top side for a distance of L/4 from the supports.

3. Reduce the stirrup spacing to half in the L/4 span near supports on either sides.

 Bent up bars for slabs:

Difficulties:
Bent up bars in the slab are very susceptible to the movements of men & machinery on the slab before & during casting .Comparatively, the bent up bars in the beam are quite sturdy after getting positioned.
1.If the concreting is done with pump then the laying of the pipe across the slab results in getting the bent up bars disturbed  due to continued jerks in the pipeline.
2.If the concrete is lifted to the slab by hoist /lift and carries to specific spots by trolley or carts,then the tire of the carts jump again & again on the elevated bent up bars and eventually bends them down.

 Can it be avoided ?

Yes.It is possible to protect the bent up bars getting disturbed by providing ample number of strong chairs below them
It must be made compulsory to have a regular path made for the empty carts.
A pathway can also be made for the personnel walking on the slab.
For the concrete pipe stands are available on which the pipes are mounted and thus bent up bars are protected.