Maximum plan dimensions for slab on grade 2

[mar2805]

Hi folks!
What are maximum plan dimensions of a 4" thick slab on grade that will not crack due to shrinking problem?

I can use a "drag formula" for 1-layer reinforced SOG, but what aboth unreinforced slab?
Is it safe to put an very very small value of reinforcement area in the drag formula and calculate joint spacing?
Thank you.

 

[haynewp]

Unreinforced (or minimally reinforced) slabs are usually designed where I am for control joints at about 36x slab thickness in inches. This is backed up by an ACI slab on grade document. I don't use the reinforced slab on grade methods to control cracking because there is too much pulling up the reinforcing going on when pouring. If you use you mats of rebar instead of WWR then the drag formula method may be fine but I haven't used it before.

 

[cvg]

12 - 15 feet is typical. 36x might be excessive

 

[ajk1]

Assuming this slab is within a heated building at about 70 deg. F or so, the usual recommendation is to provide sawcut joints at 36 times the slab thickness, but not greater than 15 feet, in each direction. For a 4" slab, this would mean 12 foot centres each direction. The columns will probably end up determning the sawcut spacing, since there should be a sawcut at each column centreline. So if the columns are at 20 foot centres, the sawcust should be at 10 foot centres.

The sawcuts should be made with an "early entry" saw that permits sawcutting within 2 to 4 hours of the completion of the floor finishing operation. If they wait to the following day to make the sawcuts, then you have lost some of the benefit of them. For a 4" slab, the sawcuts should be 1" deep if made using the early entry saw.

The maximum length between construction joints should also be specified (perhaps 60 feet?).

You can never guarantee that there will be no cracking, but you can minimize cracking by taking a number of steps, including providing the contractor with a layout of the sawcut joints so that there are no "re-entrant corners" that will initiate a crack, and no projections through the slab that will restrain the slab movement; such projections should be isolated from the surrounding slab.

The maximum shrinkage of the concrete mix can be spacified under CSA A23.1 (eg. you can specify 0.04% maximum shrinkage at 28 days when tested as per CSA A23.1 Clause __). However there is extra cost for the concrete.

I generally don't like a 4" slab because you cannot use the 1.5" coarse aggregate in it, since slab thickness must be at least 3 times the nominal coarse aggregate size. A 4.5" or 5" slab allows use of 1.5" coarse aggregate, which allows design of a concrete mix with less shrinkage than a mix with 3/4" aggregate.

The base under the slab should also be considered and carfully specified.

Curing is best done by covering with polyethylene sheet kept in place at least 4 days, preferably 7 days. If this is not possible, a good curing agent can be used (specify one with a high solids cantent) if the concrete is not CSA C-XL classiifcation (I assume it is not in your case).

There are a number of good publications on slab on grade. Unless the slab is not important (eg. if it will be covered by carpet) I suggest that you get someone who knows about slab on grade to write the specification.

Inspection of the work as it is carried out is also a good idea.

If the slab is exposed to deicing chemicals (such as a parking garage exposed to deicing chamicals tracked in, exterior landscape areas, etc.), then there are important additional requirements.

If the building is unheated, then the sawcuts should be closer.

Slab on grade is best placed after the roofof the building is in place.

Slab on grade is one of the most trouble prone of all concrete elements, so be careful.

Hope this helps a bit.

 

[haynewp]

36x4" is 12 ft.

 

[ajk1]

...one more thing: if there is to be an adhered architectural finish, it is best not to use a curing compound unless you are prepared to shotblast it off; cure with polyethylene sheet, or much better is to cure with a "curing blanket" Do not use burlap.

 

[mar2805]

Ok thanx.
Did some seraching and recommendation for joints seems to be "every 2-3 feet for every inch thicknes of the slab"
What do you think?

If the SOG is provided ,instead of the joints, with downstand beams (eg.30" in height), wich increase stiffnes of the slab, can joints be discharged and everything be poured at once?

"Slab on grade is best placed after the roofof the building is in place"
Why is this?

 

[hokie66]

mar2805,
You first need to understand the purpose of control joints. They are provided in slabs on grade to control direct tension cracking due to drying shrinkage which is restrained. The restraint for a single thickness slab is imposed by friction with the supporting soil. If you introduce downturned ribs in the slab, that provides a greater degree of restraint, so for the same thickness of slab with the same amount of crack control reinforcement, you would expect more and/or wider cracks.

It is preferable to place a slab on grade in an enclosed rather than exposed environment because the external factors affecting curing are reduced. However, this is usually only possible when the slab is non-structural, i.e. the structure above does not rely on the slab for vertical support.

 

[mar2805]

"If you introduce downturned ribs in the slab, that provides a greater degree of restraint"

Thanx a bunch!
Thats what I wanted to hear

Hokie66, another question thats been buging me.
SOG are considered as pavements that must be designed not to crack when loaded.
This is controled with a rule that tension strenght of concrete will be never reached, when SOG is loaded.
IF reinforcement is provided it is provided in upper 1/4 of the slab wich will control shringing cracks.
Shrinking cracks can howerevr also be controled with propper joint spacings and slab thickness.

This placement of reinforcement is a bit confusing to me.

SOG usualy have big "+ bending moments" (bottom of the slab) and smaller "- moments" (top of the slab).
IF there are some accidential overloads wich will couse breaking wouldent be wiser to put that constructive reinforcement in the bottom of the slab (big + moments)
But then again that amount of reinforcement should be equal to minimum reinforcement needed for a flexural member wich is much greater in area then for shrinkage control.
Your thoughts.

 

[hokie66]

Slabs on grade are not normally designed as flexural elements. In the case of the 4" slab which you talked about in a building, this is really not a pavement as such but rather just a working surface. In many cases, cracking of these thin internal slabs can be tolerated, provided it does not render the slab unserviceable.

Your quandary about placement of reinforcement is a topic which is confusing to many of us. The force which causes the slab to crack is due to shrinkage being resisted by friction with the soil, so the force is at the bottom of the slab, and that is where the slab cracks tend to initiate. I agree with you that the optimum position for reinforcement is at the bottom, but not for the reason you gave.

 

[mar2805]

"The force which causes the slab to crack is due to shrinkage being resisted by friction with the soil, so the force is at the bottom of the slab, and that is where the slab cracks tend to initiate"

I must say that placement of the reinforcement in the top for shrinkage seems logical for me.
Imagine your SOG as a pice of paper that you put on a table.
Now put your all of your five fingers on the paper in a circular shape and star pulling fingers close.
Since the slab is restarined at the bottom, when it starts shrinking it will curl upwards, "lost of contact" with the soil at the perimetar, wich will result in a bending moment in the top middle of the slab.
When M > Mcr youll get a crack.

 

[Ron]

If your SOG will have traffic on it, it should be designed for flexure and repetitive loading. That is typical for rigid pavement design. In general, cracking is a totally different issue.

If you design the section as a pavement with no reinforcement and limit the flexural stress at the bottom of the slab to 50% or less of the flexural strength of the concrete, the number of repetitions of load is irrelevant. However, a 4" SOG will not handle high wheel loads. Further, the thickness control on a 4" SOG has to be very tight to prevent exactly what hokie66 noted..."random" drying shrinkage cracks. I placed quotes around "random" because I am of the opinion they are not random, just uncontrolled.

As for placing shrinkage reinforcement, there will likely always be multiple schools of thought on that. Shrinkage cracks are wider at the top than the bottom, so in one sense it is better to reinforce near the top. As hokie66 noted, the restraint stress starts at the bottom so why not mitigate the stress and reinforce at the bottom. In the end it makes little difference and is better to have a good mix design, good thickness control, quality placement and above all, proper location and spacing of control joints placed at the proper time. Spacing should be 24 to 36 times the thickness in inches. Joints should be cut the same day as placement and cut to a depth of 25% of the thickness.

 

[mar2805]

"If your SOG will have traffic on it, it should be designed for flexure and repetitive loading"
Im not shure I understand you.
You mean section should be designed as for limit state design of RC structures (cracked, steel yiled, and concrete fully expoited in compression, rectangular stess diagram for concrete).

 

[dik]

It should be noted that the time for sawcutting is critical... moreso, for a thin slab...

Dik

 

[Ron]

mar2805...no...design the slab as a pavement section.

 

[mar2805]

1....."If your SOG will have traffic on it, it should be designed for flexure and repetitive loading"

2....."If you design the section as a pavement with no reinforcement and limit the flexural stress at the bottom of the slab to 50% or less of the flexural strength of the concrete, the number of repetitions of load is irrelevant"

"...design the slab as a pavement section"

?!

So theres no difference in designing 1 and 2 ?

 

[mar2805]

I forgot to ask, how much reinforcement should be provided to slab to fully ignore the use of joints?
I think I read 0.5% but I might be wrong.

 

[Ron]

Yes. 1 and 2 are for pavement section design.

In my opinion, leaving out the joints is a mistake. You will get cracks even with plenty of reinforcement...you just don't always know where they'll show up.

 

[mar2805]

Ron thank you very much.
If you dont mind Ive would like to ask another question expecialy regarding joints for RESIDENTIAL stiffened slabs on grade.
This is something that I recently discovered when searching for a foundation system for a lightweight, wooden frame, 2 story residential structure on plastic soils.
Its something like a waffle system made from slab on grade with a system of perimetar ground beam but also with a system of internaly provided downstand beams.
I found WRI article here:

http://www.wirereinforcementinstitute.org/pages/pubs/pdf/TF 700-R-07_Slab_on_Ground.pdf

wich explains this system and they also made an egsample compared to BRAB report.
For stiffened residential slabs on grade it seems that providing joints is NOT usual practice!
How to deal with this problem (skrinkage cracks) then or is this situation somewhat different because the slab is now stiffened due to downstand beam system?
Thank you.

 

[structSU10]

In that situation, the slab is designed as a structural slab, spanning between the down turned beams. You consider that the soil under the slab will not always in in contact with the slab, so it must span from beam to beam. In this case, you need to design and reinforce the slab like a normal elevated slab.