Saw Joints In Structual Slabs

[wayniac3]

I am designing a 9 inch thick, elevated, one-way reinforced (continuous top and bottom), structural concrete slab. The slab and associated support members (grade beams) will be formed and poured seperately. The slab has a cover of 1-1/2" typical top and bottom. The longitudinal reinforcing is #9 @ 11" oc top and bottom. The transverse reinforcing is #5 @ 16" oc top and bottom. I have details concerning construction joints on the drawings. The question has now come up concerning 1" deep sawn control joints for surface crack control. While I understand the architectural concerns (cracks telegraphing into finishes) and structural concerns (limiting cracking to a more desireable location), I think there may be other options and am not sold on the idea of sawing joints in the slab. Is there an ACI or industry standard on sawing joints in structural slabs?

 

[kieran1]

Why would you sawcut a reinforced slab. Can you not design the reinforcement to limit the cracks.

Kieran

 

[wayniac3]

I agree - I have applied the appropriate ACI equations for temperature steel. However, I am getting pressure from others on the project to cut the slab based on their past experiences.

 

[SteelPE]

Hahaha,

I had a similar experience a 6 months ago. I refused to let the GC cut the slab. Haven't heard anything but I don't know if that is good or bad.

Now placement of construction joints is a different matter. You may have to give some thought if the GC need to place some construction joints. I placed mine over the tops of the grade beams (seemed logical to me).

 

[ron9876]

How long is the slab.

 

[dik]

As ron9876 noted, it's important to know how large the slab is, it's intended purpose, and the exposure to the elements (1-1/2" cover?). With structured slabs, they are not normally sawcut, but they should not be long enough to pose problems. With a continuous 9" slab, I would think that your supporting beams are approx 18' or so on centre or less depending on loading. What are your supporting elements? beams, walls, ???. How is the slab in the opposite direction supported? Do you have issues at the corners of your supports? A little more info might be useful.

Dik

 

[JAE]

With structured slabs, they are not normally sawcut

I agree with this - we never sawcut structural slabs.

 

[wayniac3]

ron9876 & dik,

The slab is a one-way reinforced slab with spans ranging from 16ft to 18ft in the longitudinal direction. The slab is doweled into supporting grade beams bearing on drilled piers at the spacing noted above. The are no supporting members in the transverse direction except at the perimeter of the slab. The construction joints are to placed at the contractor's discretion and locations are specified parallel to the longitudinal steel spaced at approximately 16ft to 18ft oc (over the pier centerlines). The saw joints run in both directions at the spacings indicated above. The slab area is approimately 28000sf. The slab dimensions are approximately 150ft (longitudinal) x 200ft (transverse).

Another thought that has crossed my mind is cambering the slab. I have followed ACI Table 9.5 in determining the thickness of the slab. Is this thickness adequate for long term deflection or should I consider adding camber?

 

[TXStructural]

What kind of cracking are they expecting? You will always be better off improving performance rather than mitigating problems.

Crack width will always be smaller where more steel crosses the crack. If you want tight, closely-spaced cracks, provide closely-spaced reinforcement (welded wire reinforcement or small diameter reinforcing bars) near the top of the slab. Where the cracks are sawcut, you lose part of the aggregate interlock which keeps the two sides aligned.

Provide diagonal deformed bars across reentrant corners and at column corners. Place additional transverse reinforcement across stiff supports running parallel to the main reinforcement.

Pay close attention to the concrete mix requirements. Low water-cement ratio, well-graded aggregates with largest practical large aggregate size, and use of a high range water-reducing admix and a viscosity modifier will minimize shrinkage and allow good placement rates at little or no cost premium. Also, specify the strength of concrete necessary, but stronger is not better since the amount of shrinkage increases with higher cement content, and the modulus increases with strength, both of which may result in more or wider cracks.

 

[ron9876]

Pile supported ground floor slabs can have cracking problems due to restraint from the piles. I wouldn't go 200 ft. without some type of release. I prefer expansion/contraction joints. I would probably put joints at about mid way both directions instead of sawcuts. Of course you could use shrinkage compensating concrete instead of joints.

A 1" deep sawcut in a 9" reinforced slab is a waste of money in my opinion. Besides it won't do much since your slab will still be bound to the grade beams.

This discussion also depends on the use of the slab and the owners tolerance for cracks.

 

[dik]

Intended purpose and exposure?

It appears that the slab is a structured main floor slab. Is this correct? Is it on void form? Can you use a flat plate/slab, without beams... might be some savings... I'll see if I can dig up a typical dwg that shows reinf placing.

Dik

 

[hokie66]

Another vote for no sawcutting. Either provide planned movement joints or reinforce heavily enough to control (not prevent) cracking. Sounds like the longitudinal reinforcement is enough, but you may consider more in the other direction. Shrinkage cracking due to restraint by the previously cast beams is the issue.

 

[wayniac3]

dik,

Intended purpose is a cafeteria with a kitchen. The slab is the main floor slab. There is a 12" void below the concrete grade beams (shrink/swell soils). The entire slab and grade beam system will be formed no void boxes - customer requirement.

hokie66,

How would I quantify additional transverse steel?

 

[JAE]

You simply do not need to sawcut a slab like this. Structural slabs, especially over a void do not require them.

 

[dik]

and the floor surface is tile? or sheet goods?

Dik

 

[wayniac3]

dik,

The floor surfaces consist of tile. I believe quarry and porcelin.

 

[hokie66]

wayniac,
For a strong degree of crack control, as in water retaining structures, about 0.6%Ag reinforcement is required. You have about 2.0% in the primary direction, but only 0.42% in the secondary direction. That may be enough for your purposes, but providing #5@12" T&B would be preferable, in my opinion that is.

How do you remove the formwork from such a shallow undercroft?

Preventing cracking in tiled floors has more to do with the tiling than with the slab. The tiles require control joints, and they should be laid using flexible adhesive, and perhaps on a fibre cement underlay. As structural engineer, don't accept responsibility for the architect's specification.

























































































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