Slab-on-grade crack 3

[haynewp]

See attached slab layout, photo, and mix. The crack is approximately 75 ft long.

1. A 1.5 to 1.0 slab panel ratio per ACI is approximately met for the control joint layout.
2. 3 times the slab thickness in feet was used (this is a 5" slab) per ACI so the max spacing is 15 ft for control joints.
3. 6X6-W5.5xw5.5 WWR at 1.5" below the top of the slab was called for but not counted on for crack control. The WWR most likely pulled up during pouring.
4. Vapor retarder is directly under the slab.

There is an identical control joint layout North of the area shown that did not crack and the crack stopped as shown on the right side of the CMU wall as the same control joint pattern continued on the left side of the wall. I have a couple of ideas but I wanted to get other opinions on the crack formation.

 

[hokie66]

Why is it a problem?

 

[haynewp]

It will remain an exposed area (for laundry carts and foot traffic) and the owner pointed it out. Mainly the owner asked is the issue since it is not even in a public building.

 

[mike20793]

When did they saw cut the control joints? They may have waited too long and the crack was already forming by the time they cut them in.

 

[haynewp]

That is what I suspect. It has been a long time since the slab was poured as the building has been under construction, then delayed, now finally finished. I am not sure yet when they actually cut the joints relative to the pour and they may not even remember at this point.

 

[dik]

Some questions... what was the strength, what was the slump, when was it sawcut and how was it cured?

Dik

 

[haynewp]

ASTM C 309 curing compound. I am hunting for the inspection report at this area. It's been over 1.5 years since it was poured so I doubt they will accurately remember when the joints were cut relative to the pour. I think that is the most likely cause of the crack, late saw cut at this area.

 

[dik]

concur... that and too high a slump... and curing if windy and hot.

Dik

 

[jayrod12]

Too high of a slump could be a red herring, Super P in mixes is super common now. We stopped specifying slump a long time ago, except for piles/caissons where we want to ensure self consolidation at the bottom of the hole we specify a minimum 5" slump. Otherwise it's up to the contractor to specify the slump, but their specified slump must be on the ticket.

 

[dik]

Too high a slump, without super-P...

Dik

 

[OldBldgGuy]

If you want one rule to take to the bank: concrete cracks. Where, why, & when are all questions that can be answered with more or less certainty depending on the circumstances, but typically owners think cracks are signs of problems and contractors think suck it up, and you're supposed to be the magician that reconciles it.

 

[bowlingdanish]

and you're supposed to be the magician that reconciles it.

You forgot to add- with minimal wage and complete liability!

Oh we are a silly bunch.

 

[dik]

OBG: That's why it has to be a proper mix, and sawcut at the right size and time, and use a proper cure... you can, generally, influence the shrinkage crack locations so they are not so ugly, straight, and where you want them.

Dik

 

[hokie66]

Cracks don't necessarily affect serviceability of a slab, unless they are very wide. The width of cracks can be controlled by adequate reinforcement, but the fabric in haynewp's slab is not enough to help. By memory, I think ASTM C309 allows acrylic "curing" compounds, and they don't do any good.

 

[SrVaro]

To expand on the statement that "concrete cracks" and the comments about proper mix, placement, etc the general intent is to make the concrete crack where we want it to, and to control that crack size. Thus the term control joint being used by many. One question that has not been asked, on the left side there are a couple small walls with grade beams below, were re-entrant control bars added in this area? It appears two cracks started at these locations, and moved to form one between the laid out joints. This is more than likely especially if the joints were cut "late", but even if cut at the proper time this still could have occurred.

 

[dik]

SrVaro:
Not a through joint... just a slab 'weakening' to cause the cracking to occur. Aggregate interlock generally provides for shear between concrete panels. It has to be done at the right time.

Dik

 

[haynewp]

It's not acrylic. Curing blankets were also submitted but I doubt they were used.

 

[epoxybot]

Is there a paint or water-based coating on the slab? What was the specified finish, hard steel trowel? The concrete looks mottled, like the finish lacks uniformity. Would that be an indicator of too high a slump, as in difficultly bringing cement paste to the surface? You could always check the saw cut depths with the rod end of a caliper. They should be 25% or 1-1/4". I assume this was part of a larger slab pour. The crack doesn't appear to be out of plain. As long as the laundry carts are not steel wheels, I don't see a service issue. If laundry supplies are intended for this area and durability is a concern, I'd be more concerned about the finish. You could lightly sound the notch in the "Y" for soundness. I think that is the only point were a spall might be waiting to happen. Finish-wise how does this area compare to the rest of the pour or previous pours?

 

[haynewp]

Water based stain, trowel finish, other areas look a little better in the photos I have but not much close up shots.

 

[XR250]

If you want one rule to take to the bank: concrete cracks.

You could make any slab job not have visible cracks. It just ain't practical time and dollarwise for most projects.