Ground Floor Slab Construction

It's the middle.

Form and Pour a Concrete Slab: The Family Handyman

How To Pour a Concrete Pad - YouTube

How to Pour a Concrete Slab - Instructables

Are you a civil engineering student?

How did it go with those dual wall glass panels?

care to list your logic for mid section reinf? Many texts suggest it at top.

No i am not a student.

don't understand your last question.

"care to list your logic for mid section reinf?" NO.

"don't understand your last question." IGU Sealant Does Not Dry!

<bye>

Oh, nice of you to remember the IGU issue where the fabricator had the last word and cut off all further discussion with a rather nasty parting shot. But he is having other problems now, of the outer glass leaf cracking up all over the hemispherical structure.

But why can't you give me your reasons for the midsection reinf in the ground floor slab??

All codes, for practical reasons, require concrete cover over the reinforcing to protect it and to carry the forces back into the concrete. One rule of thumb is one third of the way down but not less than 50mm. I know of no professional engineer who would use less cover.

Satisfy yourself, web search the subject.

You need to do some research on how to design and construct a slab on grade. I've attached a link to the US Army Corps of Engineer's Engineering Manual for slabs, as follows: http://armypubs.army.mil/eng/DR_pubs/dr_a/pdf/tm5_809_12.pdf a. Steel reinforcement is always located in the bottom of the slab because the tensile forces are the highest there. The reinforcement must be held up off the subbase material to provide full concrete encasement around the bars. b. It is best to provide slab crack control joints at least 3m each way to limit concrete shrinkage cracking as well as slab creep. These joints can either be raked when the concrete has initially set-up or later saw-cutted. These joints should be at least 1/4 of the slab depth. This is another reason to have the reinforcement at the bottom of the slab so that it doesn't run through an open joint or interfere with producing a joint. Do not do a continuous concrete placement unless you are using Shinkage-Compensating Concrete (also known as ASTM Type K Concrete). You must provide concrete construction joints every 20m maximum each way if this is a large slab. c. You can run the steel reinforcement (welded wire mesh, or rebar?) continuously, with proper lap splices where needed, under each crack control joint, but it must terminate at each side of a slab construction joint or expansion joint. All raked or saw-cut crack control joints should be filled with a concrete compatible elastomeric joint filler. Termites????? HUH????? d. You haven't said whether this slab will be experiencing vehicular traffic or forklifts, or any other moving equipment. If so, it is highly advisable to install horizontal smooth steel dowel bars along the slab construction joints and the slab expansion joints for wheel load transfer. You should only oil one end of the dowels, as well as provide a plastic dowel cap on the oiled side of the joint. Dowels must be installed at slab mid-depth, and it is highly advisable to provide a slab key here as well. If you have a wall located right along a joint, you must provide a "haunched" slab. e. See above regarding installing joint filler. Frankly, a slab on grade that is only 10cm thick is a bit on the skinny side. Fine for a house slab, but that's about the limit for it. If you have heavy loads from vehicles, forklifts, bearing walls, masonry walls, and storage shelving posts (like in a warehouse) then I suggest that you thicken the slab considerably...you should consult with a registered Structural Engineer.

My source was exactly the same publication where on pg 5-12 the reinf is shown at the TOP of the slab. What say you?

As for tensile forces. I wouldn't expect any, thanks to a well compacted hard core base and light incident loads intended (just an assembly hall approx 40x20m, not for any industrial use).

We have termites this part of the world and do anti-termite treatment of floor and footings.

Thanks for the extensive consideration of my query. The rest of your recommendations will certainly be thrown around the office for a thorough discussion. But I like the idea of doing the thing in 3x3m panels cast alternately in chess board pattern, ensuring much better quality control. Again, what say you?

That detail in the USACE Engineering Manual is for highway and airport slabs which are subjected to heavy loadings & impact, as well as being subjected to very high differential shrinkage rates between the top and bottom of the slab due to environmental exposures (wind and temperature primarily). In your case, since this is a thin indoor slab without heavy impacted live loads, you can place the reinforcement at the mid-depth of the slab. It's not going to make much difference one way or another. Personally, for slabs thicker than 100cm (4"), I prefer placement of the reinforcement in the lower portion of the slab (w/ required concrete coverage per ACI 318) to provide sufficient Resisting Moment. The primary reason for inclusion of the reinforcement in a slab is to provide a degree of control against excessive concrete shrinkage cracking. It is very important to provide adequate concrete curing methods as well to limit shrinkage cracking. Here is a very good source on how to properly design and construct a concrete slab on grade: http://www.wirereinforcementinstitute.org/pages/pubs/pdf/TF%20705-R-03.pdf Unfortunately, the forum editor is on the fritz today!!! All the above paragraphs will be lumped together into a single paragraph!

we have a nominally reinforced 10cm thick slab for ground floor sitting on the usual hard core base/vapour barrier etc. You Already have? Who defined"Nominally"?What is USUAL? WHAT loads - heavy earth movers?

My question is:

a. Is the reinf placed at mid section or top, or bottom of slab? WHAT loads?

b. is it best to cast in panels of 3x3m or continuous pour? Again WHAT Loads?

c. If in panels, is the reinf too broken or is it continuous thru panels? how best to treat the butt joint for water tightness or termite safety? WHAT loads?

d. some suggest dowels between panels, with one side oiled and free to move. OK or mad? Depends -- WHAT Loads

e. If done in panels, what happens to the top finish: is it to be broken into panels as well or can it be laid independently? Depends --WHAT loads ?

Assembly hall approx 40x20m, no industrial use intended as already mentioned.

'Nominally' meant minimum required for stresses other than structural. You tell us what is usual.

Cast ALL the area monolithically AFTER THOROUGH ROLLER COMPACTION-WETTING- DRYING-Compaction

No need of any Steel rebars .

4" thk pcc Is already a waste

Wrong, wrong, and more wrong!!!!

ACI, PCI, PCA, USACE, AIA, CRSI, nearly every Licensed Professional Engineer on the planet, and I disagree with your assessment. G'day.

WOW" ....I really can't believe you would suggest NO rebar and that 4'' is to thick for a floor of this size. What have you been Smoking..?

I DO NOT SMOKE

Thanks

I agree for pad use. We filled and compacted clay to level ground, no gravel as it allows water to get under.. 4 inches could handle medium dozers, backhoes and in my shop I had a 40 ton stamp press that ran 60 % of the time. No cracking, no flaking and no water problems. Only cracking was from shrinkage upon curing. If you feel you need metal reinforcement go ahead I used some old rusty fence wire just to get rid of it on 40% of an 1800 sq. ft. pad.

Good to see a Practice AWAY From Non-Science Codes.

Didn't you say Ground Floor Slab. Means there are Columns/Footings Massive Holding down Loads from above. All these add to ensure MONOLITH.

You can put in as much steel as you wish. Psychologically dictated : " For Safe Measure". But these may actually crack the MONOLITH eventually - Rusting Iron!

Cleverer MONOLITH PCC will be using latest SuperPlasticizers(Plastificators) and using w/c ratio of 0.28>0.30 .

You will get Double the Tensile Strength in the PCC

"mukulmahant" sounds definitely south asian, and I can understand the easy care free attitude, and I am sure your recommendations are based on long experience too. But this raw unthinking self confidence is the one that is non-science and one that has contributed to steady deterioration of so much building stock in the region. The various codes cited by others are also based on experience although basics may vary or be way above those considered standard in south asia.

Frankly, I too was for a thinner slab with some reinforcement just in case and that is exactly what we have been used to all these years, with similar specs used in similar spaces for industrial use, and no complaints yet. But we decided to check and now this: so many varieties of experiences!

But casting a 20x40m hall in one pour may not at all be a good idea. But what size panels then? And what about floor finish: do we need joints there as well? What is the science behind this?

What is the science behind this? Common Sense is the SCIENCE here-- IF you must cast 4" . Cast as much as you can >cure that sector for a week > Come back to Fill last gaps in Checkerwork fashion. LEAVE expansion Joint strips at 10mx10m

If your aim is to have A Concrete Floor for people to drive in a car --Walk-Gather.... have nomore than interlocking Precast Concrete tiles-1" thick !

I am not clear on your comments. When I use term slab I am referring to the floor of a single level building such as a farm shed or a light industrial building. In this case I usually pour a footing for the perimeter and erect the structure. I try always to pour the floor or pad, (slab) after the roof is on for shade and consistency in concrete curing. There are no columns in this type of structure as it is usually a prefab factory manufactured steel structure. I have no expertise in high rise buildings but if I was to get involved I would use the services of a qualified Civil Engineer.

OP can clarify :

He began with " We have---"

'OP' refers to me I guess.

Yes we have a single storey RCC structure for an assembly hall but with columns in the middle also. After all the peripheral walls are in we will cast the floor on a hard core base topped by a vapour barrier.

I am only trying to settle the issue of this floor slab: cast it in one piece or in separate panels; if in panels, does the floor finish on top (local stone) need to match the panel sizes or can it continue over joints.

Very sticky issue, it turns out!

if in panels, does the floor finish on top (local stone) need to match the panel sizes or can it continue over joints.

Nothing to Worry about.

Cast THE Panels with gaps like 6" between Panels .

And finally do the topping in one go -filling those gaps too.

Use SuperPlasticizers . w/c ratios ~0.28 . Cure well .

Nothing will crack except rebar Iron Rust Cracks (may be) .

The answers are as follows:

we have a nominally reinforced 10cm thick slab for ground floor sitting on the usual hard core base/vapour barrier etc. My question is:

a. Is the reinf placed at mid section or top, or bottom of slab? Opinions vary.

= Generally in the middle, but as a minimum 1" to 2" from the slabs outside =

b. is it best to cast in panels of 3x3m or continuous pour?

= The casting size will depend on several factors, how hot or cold is the environment, how many people can you get to work on the pour and finishing of the slab and how many truckloads of concrete you can get delivered at any time =

c. If in panels, is the reinf too broken or is it continuous thru panels? how best to treat the butt joint for water tightness or termite safety?

= As long as the reinforcing rebar is tied together with the required overlapping then the reinforcement can be in pieces, the water tightness will depend on how you finish the ends and if you have enough space to place a expansion joint, as to the termites, they do not like cement, but will gnaw on it to open a walkway =

d. some suggest dowels between panels, with one side oiled and free to move. OK or mad?

= Of course, and if you do not oil, them, soap them or place a lubricant, then the cement will stick to the forms and you will have a difficult time to remove them =

e. If done in panels, what happens to the top finish: is it to be broken into panels as well or can it be laid independently?

= As panels =

A 10 cm concrete slab is about 4 inches, or so, thick. The associated rebar-grid will sit on some variety of (spacers/chairs/etc.) that will typically raise the rebars to be from about one to two inches ''high'', by Code. They should not lift the rebars to be within less than one inch of the floor slab ''finished'' surface, in order to inhibit spalling, etc. Thus, the only space left in such a typically thin floor slab, for the rebar-grid, is in the middle...

Also, go visit a construction site where concrete slabs are about to be poured, and validate this for yourself...

in CALIFORNIA the height of the re-bar is spec'd to code, they even sell litlle prefabed cubes at Home depot to get the height correct., what your locale requires is up to yo to determine.. the bar is usually just above the compacted dirt, not floating below the concrete surface

Better off using vinyl-coated or galvanized rebar chairs or bolsters, then those cubes to get the required concrete cover under the bars pursuant to ACI 318....

Just saying.....