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Maximum plan dimensions for slab on grade
2

Maximum plan dimensions for slab on grade

Maximum plan dimensions for slab on grade

(OP)
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.

RE: Maximum plan dimensions for slab on grade

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.

RE: Maximum plan dimensions for slab on grade

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

RE: Maximum plan dimensions for slab on grade

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.

RE: Maximum plan dimensions for slab on grade

36x4" is 12 ft.

RE: Maximum plan dimensions for slab on grade

...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.

RE: Maximum plan dimensions for slab on grade

(OP)
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?

RE: Maximum plan dimensions for slab on grade

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.

RE: Maximum plan dimensions for slab on grade

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

Thanx a bunch!
Thats what I wanted to hear smile

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.

RE: Maximum plan dimensions for slab on grade

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.

RE: Maximum plan dimensions for slab on grade

(OP)
"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.

RE: Maximum plan dimensions for slab on grade

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.

RE: Maximum plan dimensions for slab on grade

(OP)
"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).

RE: Maximum plan dimensions for slab on grade

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

Dik

RE: Maximum plan dimensions for slab on grade

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

RE: Maximum plan dimensions for slab on grade

(OP)
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 ? smile

RE: Maximum plan dimensions for slab on grade

(OP)
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.

RE: Maximum plan dimensions for slab on grade

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.

RE: Maximum plan dimensions for slab on grade

(OP)
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/pu...
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.

RE: Maximum plan dimensions for slab on grade

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.

RE: Maximum plan dimensions for slab on grade

(OP)
I agree partialy (there is contact of slab and the soil where there is no heaving) with you on that BUT show me just one egsample calculated using this procedure that reinforces slab in 2 layers!
Few egsample I found only reinforced slab in top layer, no bottom reinforcement for postive moments!
Beams are reinforced in top and bottom layer.
Slab reinforcement that was calculated and provided only in the top layer was usualy less then 0,5% wich in turn mean that there will be shrinkage cracks---can anyone correct this last statement couse I cannot understand this value of 0,5% that controls shrinkage when suspended floorslabs (1st, 2nd,3rd floor) have less then 0,5% reinforcement for bending and there where never any shrinkage cracks!

RE: Maximum plan dimensions for slab on grade

(OP)
I found this great article for stiffened slabs on grade:
http://www.wirereinforcementinstitute.org/pages/pu...
On the last pages theres an solved egsample.
You can see clearly that the slab is reinforced ONLY in the middle of the slab!?
SInce this is in the middle I can only assume that this is for shrinking purposes.
If it would be for flexure it had to be positioned in the upper or bottom part to adress tennsion forces in section.
Can someone plase tell me whats the amount of slab reinforcement provided in the egsample ( in percentage )
min. reinforcement needed for beams and slabs in bending is 0,15% for EUROCODE.

Thanx and Merry Christmas everyone! smile

RE: Maximum plan dimensions for slab on grade

(OP)
Hi guys!
I found this instruction manual for software called SlabWOrks (made by mr. Eric Green) wich is used for designing stiffened slabs on grade on expansive clays.
Manual is located here:
http://www.scribd.com/doc/161912886/SlabWorks-Manu...
I will quote some sections here and then hopefully discuss with you.bigears

page 22/95
"Overall structural performance of stiffened slabs is generally independent of the
performance of the thin slab in the areas between beams. This portion of the
foundation slab is generally intended only to acts as a separator between the
building and the soil below. However, if thermal or shrinkage cracking is noted
in these areas, many owners will perceive the foundation is in a failed state. This
is particularly important if the owner anticipates the use of tile or stone finishes.
Therefore, performance expectations with respect to slab cracking should be
discussed with the owner and architect prior to design.
In the past, many engineers have relied on the minimum temperature and
shrinkage steel requirements from ACI-318 (0.18% steel).
These guidelines are
intended for elevated structural slab and are not applicable to slabs-on-grade.
This is discussed in the commentary to the latest version of ACI-318.
The engineer should instead refer to ACI 224 “Control of Cracking” for guidance
on controlling cracking of slabs-on-grade. Generally, cracking in stiffened slabs
is controlled with bonded reinforcement, and control joints are not used.
According to ACI 224, 0.50%-0.60% steel is required to control cracking with
steel alone. Control joints can be installed, with the control joints located midway
between the stiffening beams. Control joints near beams will not be effective because the
beams restrain the concrete from movement."


I found an interesting thread here on the forum on MINIMUM reinforcement requirements for members in bending.
It seems that ACI codes made this a bit confusing but generaly it was agreed that a ratio of 0,18% is the minimum reinforcement that should be provided anywhere in the member where tension occurs.
If you have moment appearing in the top and in the bottom of a slab you should provide 0,18% ratio in the top portion of the slab and also 0,18% in the bottom of the slab.
This wll ensure ductile behaviour of a member because of a slightly higher moment capacity o the cracked section vs. uncracked section.

The sentense "These guidelines are intended for elevated structural slab and are not applicable to slabs-on-grade" is where I get confused and if you read further in the manual, page 89/95, Quote:
"As discussed in the commentary, ACI 318-05 minimum steel requirements
(0.18%) for elevated slabs are not applicable to slabs-on-grade. Instead, refer to
ACI 224 “Control of Cracking” for directions on controlling cracking of slabs-ongrade. Generally, cracking is
controlled with a combination of steel and control joints. According to ACI 224,0.50%-0.60% steel is required
to control cracking with steel alone."


0,5-0,6% ratio is VERY HIGH reinforcement amount and I cannot understand with this.
So why doesnt rule apply also to elevated slabs wich are not resting on the ground and wich usualy have higher bending moments?
Would this rule also then apply to raft structures since they are also constructed as jointless, let say for residential house a 12inch raft, plan dim. 50ft x 35ft would need a 0,5% reinforcement ratio if poured all at once (no joints that is).
Please correct me couse I dont wanna belive that this is rational design and Ive designed raft with 0,18% both sides many times.
Was I wrong?


RE: Maximum plan dimensions for slab on grade

The reason for placing the steel in the top is to minimise cracking on the top surface which is subject to 'wear and tear'. Cracking of the top surface is not aesthetically pleasing because it is visible. Cracking of the underside is not so noticeable <G>.

Dik

RE: Maximum plan dimensions for slab on grade

If this is becoming an intractable problem, have you considered using a "California" - style post-tensioned slab?

5-1/2" to 7-1/2" thick

12" deep perimeter "beam"

RE: Maximum plan dimensions for slab on grade

As a result of this thread, I've put together some items for Slab-on-Grade design and have attached these. If anyone has any comments, please add them (*.pdf files can be annotated). I'll combine the result into something a little more coherent and post it in E-T Journal.

Dik

RE: Maximum plan dimensions for slab on grade

(OP)
Hi guys.
I found here an lecture on stiffened SOG on swell soils.
https://ceprofs.tamu.edu/briaud/Stiffened%20Slabs%...
It explains design procedure using Abaqus model wich simulates edge up and drop lift.
I was thinking of maybe using winkler soil model wich would be applied depending on the scenario.
edge drop - no surface support (k modulus) on perimetar of the slab
edge lift - no surface support (k modulus) in the center part of the slab
Design members and required reinforcement but ensuring that maximum permissable deflections for downstand beams are within code limits wich will ensure that there are no cracking in walls and partitions above.
What do you think?

One thing I noticed in the lecture above, is that they only reinforce 4inch slab with ONLY ONE layer of reinforcement.
I cannot understand this.
Is this for flexure or shrinkage ?

RE: Maximum plan dimensions for slab on grade

Sounds a bit bogus. If the rebar is in the center, then you get equal, but very small, capacities in both bending moments, positive and negative.

RE: Maximum plan dimensions for slab on grade

(OP)
But if your bending moments in slab are very small it could work....?

RE: Maximum plan dimensions for slab on grade

you cannot possibly put two layers of reinforcement in a 4 inch thick slab.

and why would you design a slab that thin anyway?

RE: Maximum plan dimensions for slab on grade

(OP)
Seems that guys have done it succsesfuly, so I dont see any problems?!
Slab is thiner becasue of the downstand beams wich greatly enlarge the ovearll stiffnes of the structure as a whole.

RE: Maximum plan dimensions for slab on grade

But after all the expense of providing the beams, why not use a "California" - style post tension slab on grade? If the soil is classified non-active, it can be only 5-1/2" thick, and there you have more reliable precedence.

RE: Maximum plan dimensions for slab on grade

(OP)
Post tensioning is something that hasnt been much used in small residential project in my country and it only used in highway bridge design.
So Im not going to be considering it at all.

RE: Maximum plan dimensions for slab on grade

(OP)
People again, I need you advice.
Plase look at the picture attach.
For edge heave situation there will be lost of contact in the midle portion of the entire slab wich will couse a load transfer from structure to parts that remain in contact with the ground. Since perimetar beams are usualy place under load bearing slabs they will produce the bigest pressure on the ground. When edges are lifted up and due to lost of contact the loads will be transfered to edge beams that in contact with ground but NOW the pressure will be increased substentialy!
I havent seen this be controled in any of the design methods PTI, WRI, BRAB...
How safe is to ignore this effect?

For "edge heave state" you can clearly see that due to lost of contact between slab and ground you will get positive bending moments in the slab from self-weight and area loads on the ground floor, yet in previous egsamples and case studeis no bottome reinforcement was ever used in the slab.
Why?

RE: Maximum plan dimensions for slab on grade



Quote (mar2805 (Structural))

Seems that guys have done it succsesfuly, so I dont see any problems?!

concrete cover on the bottom of slab on grade should be at least 1 1/2 inches and top could be as thin as 1 inch of cover. Assuming two layers of #4 bars in a 4 inch thick slab, then you have just a half an inch of space between the two layers of steel. That is just too tight for adequate concrete placement, can't be done.

If you want to design a structural slab on grade with double mats of steel bar reinforcement, than you will just have to buck up and make it a bit thicker than 4 inches.

RE: Maximum plan dimensions for slab on grade

(OP)
Agree with you on that, but if you look at my previous post I dont undersatnd why they never provide bottom rinforcement in slab for edge heave situation?

RE: Maximum plan dimensions for slab on grade

The system illustrated, with a 4" slab and edge beams only, is not appropriate for sites subject to high shrink and swell conditions. A "raft slab" is one system used for such sites, and a raft incorporates a grid of stiffening ribs.

RE: Maximum plan dimensions for slab on grade

(OP)
Sorry the sketch is wrong.
I was thinking of stiffened SOG with a system of perimetar and internal beams (waffle raft).
The question on reinforcing 4inch slab in waffle raft with ONLY ONE layer of top reinforcement, is still not clear.

RE: Maximum plan dimensions for slab on grade

(OP)
Im I BLIND??!!
Flanged beam effect, effective width of the beam for positive moments!!!!!

RE: Maximum plan dimensions for slab on grade

(OP)
Guys, for your answer on joints, provide joints "36 times the slab thickness and not greater then 15 feet".
My question is, with how much reinforcement is this statement vaild?
I found an article wich describes 5 methods for providing reinforcement in slabs on grade. (attach)
"Subgrade drag procedure" gave always the minimum percentage of reinforcement (aboth 8 times less)
Why such a drastic difference, if the reinforcement is considered as non-structural !?!

RE: Maximum plan dimensions for slab on grade

Maybe you are conflating "shrinkage and temperature steel" and "structural steel". More shrinkage and temperature steel is required in SOG applications because shrinkage and temperature has a more dominant affect than in an exposed structural slab where there is less restraint to movements caused by shrinkage and temperature.

RE: Maximum plan dimensions for slab on grade

(OP)
Im sorry, but I dont understand you...

RE: Maximum plan dimensions for slab on grade

Slabs-on-ground may require more shrinkage and temperature steel than a overhead structural slab due to the slab-on-ground being more restrained from movements due to shrinkage and temp.(i.e. curling).

RE: Maximum plan dimensions for slab on grade

(OP)
I dont think youre right.
SOG are restrained only on the bottom side since this part is in contact with soil and friction develops.
Suspended slabs are cast in formwork wich on the bottom sides also couses friction + slab is connected sideways with reinforcing bars with other elements such as walls, beams...

RE: Maximum plan dimensions for slab on grade

ACI 350 requires two to three times the amount of steel for members subjected to environmental exposure conditions (depending on the distance between movement joints) as compared to ACI 318 minimum temperature and shrinkage steel for elevated slabs (0.18%).

RE: Maximum plan dimensions for slab on grade

just for a fun fact of the day...not in response to your last post

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