concrete slab-on-grade cracks (driveway) 14

[PSUengineer1]

Hello. Undesireable cracks are in a 5-inch thick NWC driveway slab. I do not have pictures yet. Contractor waiting one week to cut control joints. This seems long. Looking for reference that says how long it should be between pour and cutting control joints. ACI 302.1R-96 mentions that joints should be cut early enough, but no specific timeframe could I find. Please let me know if you know of a standard I can reference in regard to how long it should be between pour and cutting control joints. Thanks.

 

[oldestguy]

If you can find a standard, fine. I'd check state highway department standards also. However, In my experience when I could examine concrete slabs at various stages after placement, the shrinkage starts almost immediately after a degree of setting is present. That is why under ideal circumstances the cutting of a joint requires the operator to not stand on the still plastic slab, but control the machine from the side. Even waiting one day is stretching it.

 

[TheDW]

Waiting a week to cut control joints makes them useless. As soon as the concrete starts to cure, it will form 'micro-cracks' that you can't see, but will propagate into big uglies eventually.

Our notes tell them to cut joints same day as pouring, 'as soon as the slab can be cut without dislodging the coarse aggregate'.

Don't know the reference offhand, but I'll try to dig it up.

 

[spieng89]

I usually dictate the contractor to begin sawcut operations as soon as concrete is able to support weight of sawcut equipment and workers with damage to finish surface. That's usually within 48hrs depending on mix and environmental conditions. One week is too long, concrete has set and hardened by then

 

[r13]

How bad the cracks look? Can you up load a photo? Just curious.

 

[Tomfh]

A week is far too long. The slab has already formed its own joints (aka cracks). Joints need to be done ASAP. Like within 24 hours. Ideally same day.

 

[JoelTXCive]

I found a clue, but no specific timeframe.

See below snip from ACI 224. It says that the concrete should be sawed "as soon as possible." But does not provide any time frames or ways of figuring out.

 

[wannabeSE]

Portland Cement Association says typically 6-18 hrs

https://www.cement.org/learn/concre...ntraction-control-joints-in-concrete-flatwork

 

[AskTooMuch]

The standard drawing I see is 12 hrs but the contractors don't follow that because if they pour in the morning, they go home before night and will not come back until next morning. So it's usually 24 hrs.

 

[TheDW]

Yeah, its common around here at least for them to rush to make a pour on Friday afternoon, and cut the joints first thing Monday.

 

[ShawThing]

Saw cutting should be done ideally within 12 to 24 hrs usually.

 

[Settingsun]

ACI 360: 4 hours hot weather and 12 hours cold.

 

[Ron]

Timing and spacing of sawcuts is something that requires familiarity with concrete technology and specific knowledge of the particular mix design, as well as the ambient site conditions at the time of placement. The times stated in ACI and PCA documents are rules of thumb to be applied by specifying such sawcutting times in the project documents. There are numerous variables that control the timing; however, the most prominent is the knowledge of site conditions and the mix design. One should know the approximate time of set of the cement being used. This can be determined if you specify that the mix design and cement certification be submitted for approval by the design engineer and architect prior to placement. The water-cement ratio and the placement/concrete temperature also affect the initial curing (hardening) of the concrete.

In general,control joints should be cut as soon as the concrete has set enough to prevent raveling during the sawcutting operation. For normal strength concrete without significant admixtures to modify the concrete, and within reasonably acceptable placement temperatures of 60F to 80F, the concrete should gain enough strength for sawcutting within about 4 to 8 hours. There are special high speed, small diamond blade saws that can be used earlier (Sofcut), but most contractors do not have these. As warned in both ACI and PCA, sawcutting should certainly be done within 24 hours of placement, preferably within 8-10 hours of placement.

Yes, the concrete sub will complain and state that they've done this hundreds of times without problems. Not true....they just usually don't get called on the problems that occur within days or weeks.

Depending on restraint conditions, microcracks can occur in concrete within a few hours. This is why you often see a crack paralleling a sawcut joint....the joint was cut too late and that crack, visible or not, had already occurred.

 

[dik]

From my project notes (These are drawing notes that are included with each project, edited as required):

CUTTING OF CONTROL JOINTS SHALL BEGIN AS SOON AS THE CONC SURFACE HAS HARDENED SUFFICIENTLY TO RESIST RAVELLING AS THE CUT IS MADE. SAWCUTTING SHOULD COMMENCE APPROX 6 TO 8 HOURS AFTER FINISHING SLAB

With a Soff-cut saw, this timing can be reduced.

Dik

 

[dauwerda]

Take a look a section 5.3 of ACI 360R - Design of Slabs-on-Ground. Here is a snip of what it says: