Depth of Sawcut? 1

[MikeManning]

Hi all, I work for a large General Contractor, and one thing that we see a lot is cracks in slabs outside of the contraction joints. We typically get blamed for these cracks, and while the blame is rightfully ours some of the time, there's a decent portion of these issues where we followed the design exactly, sawcut as specified, and still see cracking outside of the joints.

Is this expected in the engineering world?

I tried to educate myself on the design criteria for sawcuts, and it seems like there is some room for variation from the looks of it.

ACI 302.1 says that sawcuts should be at least 1/4 the slab thickness, or 1", whichever is greater, or 1/3 depth in the case of fiber-reinforcement.
That said, I've also heard some projects and engineers specify 1/3 depth regardless of mix.

I found info in ACI 302.1 about the timing of sawcuts (within 4 hours in hot weather, 12 in cold) which is generally in line with our practices.

I found ACI 360 for some criteria on joint spacing, although this same document seems to indicate that there are a lot more factors than simply the slab depth and predicted shrinkage. This is one area where our designs don't seem to be consistent, and many drawing packages don't contain joint spacing. Although, from what I read, this should be the slab designer that specifies joint locations.

What are your experiences?

Any thoughts are appreciated!
Thank you

 

[phamENG]

There are a lot of variables:

Slab thickness
Air temperature
Water content
Timing of saw cuts
Depth of saw cuts/formed joints
Type of saw can have an impact - using the wrong saw or cutting incorrectly can result in raveling of the slab at the joint
Curing methods
Slab restraint (did you cast it on poly over sand, or directly on #57 stone?)
Presence of re-entrant corners or other geometric discontinuities
Quantity and location of reinforcing (did you attempt to "lift" the WWR or rebar mat? If so, you did it wrong.) - this one is more about limiting the size of cracks than stopping them from happening

Who designs them can be a tough question. Has the floor finish been selected? Is it tile? Will movement at the joints (which can happen) telegraph through to the finished floor and crack the tile? I design them for commercial projects, but on residential I usually set requirements and have the contractor design the joints and submit to me for approval. Residential jobs usually put off finishes for the contractor to coordinate with the owner, so I let them figure out a joint plan that will work with those finishes.

 

[phamENG]

Also, the 4 and 12 hour numbers are assuming everything else is perfect. 1-2 hours is usually a better window to use so long as the concrete is hard enough. Really, the moment a worker can walk on the slab without leaving a boot print is when you should take an early entry/soff-cut saw to it and start making test cuts. Once raveling stops, repair the test area and cut your real joints.

 

[dik]

Sawcutting is critical and 12 hours is far too much IMHO. Sawcutting should occur as soon as you can get onto the slab and not have the aggregate ravel (sp?) behind the saw blade. This usually occurs from 4hrs to 6hrs after finishing. They have specialty saws that confine the concrete surface that allow you to reduce the time of sawcutting. They are called Soff-cut. Your problems with cracking are likely a result of not sawcutting soon enough.

I usually spec reinforcing cover equal to the sawcut depth so the blade does not cut the bars. Reinforcing is near the top and not the bottom for the reason that any cracking will be visible on the top... not for stress reasons.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

This is part of a 'handout' I give to Contractors and Owners (Modified slightly and included 'Reinforcing':

Reinforcing
It is common to place reinforcing steel in the upper one-quarter of the slab thickness. This is somewhat contrary to flexural mechanics. Maximum flexural moments occur at the bottom fibre of the slab. In addition, tractions produced by the base material on the bottom of the slab, tend to increase the bottom fibre tension and put the top fibres of the slab in compression. 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.

Reinforcing is often placed in a single layer near the top of the slab. It is common to place the reinforcing to provide a concrete cover equal to the depth of the sawcut.

Cracks on the top become accentuated with time due to objects moving over the top surface.

Although some jurisdictions permit reinforcing steel to be placed at five times the slab thickness, a spacing of three times the slab thickness should be considered.

The effect of reinforcing in small amounts is somewhat nebulous and its main function is to hold the concrete sections together to help develop the aggregate interlock at the fractured surface. For improperly timed, sawcut joints, is also helps distribute cracking a little better and minimises crack widths. With properly timed sawcuts the cracking is pre-determined and not random.

It is common to provide 0.2% of the concrete area as reinforcing steel area. This proportion can be increased to 0.5% or 0.6% to largely 'eliminate' visible cracking. The cracks still occur, but they are much more frequent and have a greatly reduced crack width.

Control Joints and Sawcutting
Concrete is a brittle material and it is necessary to minimise random cracking. If the minimum dimension of the slab is greater than 5m, it may also be necessary to provide proper control joints. The use of control joints should always be considered as part of the SOG construction.

Unreinforced, or minimally reinforced, slabs usually have control joints located at 35x to 40x the slab thickness, but not greater than 5m or so. This is recommended by the ACI SOG committee.

If the structure is unheated, then the sawcuts should be at a closer spacing.

It should be noted that the time for sawcutting is critical. This is more important for a thin slab. A thin slab reacts to changes in temperature, and humidity.

The sawcuts should be made with an "early entry" saw, or 'Soff-Cut' saw, that permits sawcutting within two to four hours of the completion of the floor finishing. Sawcut timing is critical. Without an early entry saw, sawcutting should commence within 4 to 6 hours after finishing. CSA A23 stipulates that sawcutting should commence as soon as possible. Concrete should have sufficient strength to prevent the aggregate from ravelling behind the saw blade. If too much time passes, sawcutting is superfluous and the location of the microcracking has determined where the cracks will form. The following should be observed:

• The depth of sawcut should be a minimum of one-quarter of the slab thickness. I usually specify a concrete cover to reinforcing equal to 1-1/2” and use this as the depth of the sawcut.

• The sawcutting pattern should be predicated on the locations of any interior columns.

• The sawcutting pattern should not cause an angle between the adjacent panel of less than 40°.

• The sawcut should continue if it terminates at the edge of the adjacent panel.

• For irregular shapes, the sawcutting pattern should be shown on the construction documents. Alternatively, the criteria for establishing the sawcutting pattern should be described.

• In addition to sawcutting, construction joints should be located at approximately every twenty metres.

• Projections or re-entrant corners in the slab that will restrain movement should be detailed so they are isolated.

After the initial shrinkage has occurred, sawcuts should be filled with a caulk material that adheres to the concrete sawcut face and provides support for the concrete adjacent to the sawcut. This can be a polyurethane material that has a hardness to prevent the ingress of particles. For heavier loaded slabs, to support the edge of the joint, the caulk hardness should be increased.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MikeManning]

Thank you dik and other commenters!
My biggest takeaway is the importance of TIME as part of this equation. I wonder if we miss that critical element while technically still "meeting spec". So that's helpful to know!

 

[JLNJ]

Other than the factors PhamENG mentioned, cement content and aggregate size play a key role in controlling shrinkage. If you provide a high-cement mix
with less big aggregate (thinking that you are doing your client and finisher a favor), you significantly increase the shrinkage and decrease the set time.

Mitigating all cracks is almost impossible. Controlling what you can control and keeping the Owner and flatwork contractor engaged is the best policy.

 

[dauwerda]

Perhaps contrary to some of dik's notes above, it's my understanding that if you have reinforcement and it is all continuous through a saw cut and does not get cut, it is essentially making the saw cut useless. Many details I have seen call for either the rebar to be partially cut, or that only half of the reinforcement be placed continuous across the saw cut line (every other bar is cut at the joint).

 

[Retrograde]

I agree with the above that the timing of the cut is critical.

Also, the early application of aliphatic alcohol, especially in hot climates.

 

[dik]

I'll have to check my notes... the intention is that the rebar is NOT cut; the concrete cover should be the depth of the sawcut...

"It is common to place the reinforcing to provide a concrete cover equal to the depth of the sawcut." You want the reinforcing in the top of the slab, not at mid depth or near the bottom. The purpose of the sawcut is to try to locate where the shrinkage crack will occur. There are numerous papers that indicate small amounts of reinforcing do little to improve the crack resistance and load capacity of concrete SOGs.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik