concrete slab on grade evaluation 1

[versaengine]

Trying to decide what to do. Called in to evaluate cracks in 3 month old floor. Here is the situation.
Pre-engineered metal building on concrete footings with concrete slab on grade floor. Building will house a community building with a ballroom, small kitchen, basketball court, bathrooms and offices. The floor will not have a finish floor material but is to be buffed, kind of like a wal-mart exposed concrete floor. The floor overall dimensions are 140' x 200'.
floor design was 4" minimum thick, 4,000 psi concrete, with 4" max slump and poly micro fibermesh and control joints in line with all columns each way spaced 20' x 25'. Soil subgrade is reportedly firm, non-expansive with 4"-6" thick clean crushed limestone base layer and vapor barrier above. There were 6 separate pours, the contractor claims dowels were placed between cold joints. Finish appears decent. They used a vibra-screed and power floats. I have asked for but have not received concrete tickets, to see how much water was added (hope I can trust if accurate?)
There are three long irregular cracks up to 3/16" separation with no vertical offset (this is what concerned the owner) that did not occur at cut joints. I believe these are drying shrinkage cracks and imo are not excessive,
BUT I found: in 2, 3/4" diameter hammer drills the slab was 3" and 3.5" thick. An impact-rebound hammer test suggests strength of concrete is OK. The joints were cut only 1/2" deep. The concrete sub-contractor has no money. The general has no money but may have insurance.
I am going to definitely recommend more 3.75" diameter cores to better evaluate thickness. But I am concerned about long term performance. If the thickness is only 3.25" thick on average, should I recommend a strengthening layer? or just monitor? I am concerned the owner did not get what he paid for. Would a rebate be in order?
Any ideas?? Thanks for any suggestions.

 

[beton1]

Control joints are too far apart. Slab thickness is obviously questionable. Control joints should generally be 1/4 slab thickness, so likely they are not cut deep enough. Also, when were control joints cut? If over a day (24hr) then it is possible cracks started, explaining why cracks are not at joints (and the fact that joints are not proper depth). By all means, get concrete tickets to confirm if water was added. What was aggregate size (coarse aggregate likely 3/4"). A 3/16" crack is excessive; the "6 separate pours", do you mean 6 separate days? For dimensions mentioned, and thickness, it is likely the pour could have been done in one shot??

 

[dik]

When was the slab sawcut... I normally recommend approx 8 hours after finishing... 24 hours is way too late; microcracking has already determined where the cracks will occur.

20'x25' is way too large for sawcutting panel... half that may work. Timing is critical.

1/2" is way too shallow... should be an inch or inch and a half deep.

Was there an engineer of record? Did the city/town have a building inspector? Were any inspections done?

What drawings available and any professionals involved?

Any reinforcing steel? and, if so, where and what? Fibres are not a solution.

Have you talked to the community building's lawyer... I'd be putting the contractor and sub on notice that the work is unacceptable.

Dik

 

[Ron]

Agree..these are drying shrinkage cracks. Agree also, the joint spacing is way to large for 4" slab. Should be about 12 to 15 feet. Another contributor is the lack of thickness control on the slab....all of these coupled with sawing too late will cause exactly what you noted.

My concern is a crack width of 3/16" in 3 months! That's excessive, indicating a probable high water-cement ratio in the concrete. Don't depend on a rebound hammer to assess strength...it's essentially worthless for that. Core the slab in numerous places and you'll have at least 3 pieces of critical information...(1) strength from compressive test of cores (2) thickness to compare against accepted tolerance criteria (ACI 117) and (3) petrographic evaluation to show finishing technique, water added at site (retempering), air content, void structure, and with a few additional tests, can determine water-cement ratio.

 

[DaveAtkins]

I agree with everything that has been mentioned, but I would also consider the possibility that this is a structural failure. A pre-engineered metal building frame will introduce tension across the slab on grade (because the legs of each frame want to kick out), unless the footings were designed for the thrust.

DaveAtkins

 

[dik]

It wouldn't take much sun or a bit of a breeze to dry the thin slab, too...

Dik

 

[dik]

Any pix or sketch of the cracking pattern?

Dik

 

[hokie66]

No crack control reinforcement. Less than designed thickness. I think the owner definitely got less than deserved. Agree with DaveAtkins that the cracks could be direct tension cracks, particularly due to lack of reinforcement.

 

[Ron]

versaengine...finish the evaluation before you recommend anything to the owner. You have several problematic conditions that should be addressed as all have mentioned here.

If the wide cracks are perpendicular to the bents, then look as what DaveAtkins and hokie66 have noted; however, I'm not sure in 3 months you've had an event that would mobilize much in terms of thrust; although of course that could happen any time. If the cracks are parallel to the bents, then probably just shrinkage, but the width is much larger that I would expect.

 

[dik]

Building is likely clear span... How high? be a measure of the horizontal thrust.

Dik

 

[versaengine]

To all:

thank-you all for the excellent comments.

first to some of the questions raised:

This is a rural area with no permits or inspections.

The original slab design was 5" thick with 6x6,10/10 wwm.
but the contractor and owner changed the design to 4" with fibermesh but did not adjust the joint spacing.

I don't believe this is lateral thrust from columns, the steel frame is a "rigid" frame and lateral base reactions are small, and foundation was designed to support lateral forces.

No PIX at this time.

Question to all:
would there be any benefit if we:
1) cut more control joints, to give a 10'x12.5' joint spacing
2)cut the existing joints deeper
3)epoxy inject the existing irregular cracks

thanks

 

[Ron]

Since you will likely get more cracking, you can still sawcut the slab and mitigate some of the potential future cracking. The cracks that you get now will likely be farther apart than if they had occurred earlier on. Further, you might have some very tightly closed cracks that have already occurred, but are difficult to spot. Do a very careful inspection.

For late saw cuts, you have to cut the joint deeper. In your case, about 40 to 50 percent of the slab depth. With highly variable thickness even this might not be sufficient to stop all future cracks. Keep in mind that when you do this, you will lose most of the load transfer across the joint (it can be argued that you don't get much aggregate interlock in sawcut joints anyway).

Make your cuts then inject the large cracks with epoxy (Sika Sikadur 32 or similar).

I'm sure I'll get some argument here that this will be a waste of time and effort; however, it does work for the long term. Most drying shrinkage gets alleviated in the first couple of months of a slab, but it does continue to occur.

 

[hokie66]

Ballroom, basketball court...I doubt a ground and sealed concrete finish on this floor will ever be satifactory. I would suggest reconsideration of the floor finish. Grinding is not cheap, and a flexible finish would be preferable for the activities planned.

 

[dik]

If it's a rigid frame, you will likely have large lateral thrusts... greater forces if it is not too high...

If the work was done in conjunction with the owner, you may have no recourse... the owner received financial benefit for an inferior product.

Dik

 

[dik]

I'm half asleep this AM... you may have a dangerous condition!

Dik

 

[DaveAtkins]

I looked at the original post again--this building has interior columns, so the lateral thrusts probably are not very large (as you stated, versaengine).

That being said, I think the Owner is getting a crappy slab. I might consider leaving it in place if it were going to be covered with carpet, but since it is going to be exposed, I would be tempted to recommend complete replacement.

DaveAtkins

 

[hokie66]

As the owner was complicit in the decision to cheapen the slab, he is stuck with it. Versaengine's task is to make the best of a bad situation, and as the slab can't now be improved to any appreciable degree, changing the finish would seem the best approach.

 

[miecz]

If the owner suggested thinning the slab, and eliminating the steel. then I agree that he owns it. I tend to believe, though, that these changes were recommended by the contractor. If this is the case, then it is the Contractor's fault, as he/she would have guaranteed the owner a serviceable slab with the changes to the design, and didn't deliver the goods. There is no way to fix this slab at this point. Epoxy Injection will highlight the cracks and cause cracks to appear elsewhere, requiring more hideous repairs. If the Contractor goes belly up, then the fiber manufacturer should anti up for the slab replacement.

 

[Ron]

miecz brings up a good point...salesmanship of the contractor.

Unless the owner is a developer, architect, contractor or engineer, he is generally considered an "unsophisticated" buyer. In this case, presuming he is neither, then the contractor bears the responsibility for the "redesign" of the floor slab.

Further, the contractor has the obligation to perform to acceptable codes and standards. Realizing that the standard of care of the area is probably low and ill-defined, it is difficult to assert that the contractor did anything differently than other competent contractors practicing in the same area would do. Difficult, but not impossible nor even impracticable.

As for the epoxy injection, that should not be done without sawing control joints in an appropriate pattern prior to injection. As previously stated, the sawcuts have to be deeper this time.

A topping would not help this slab. The OP mentioned that the finish was decent. Assuming reasonable flatness and levelness, there is no reason for a topping other than to supplement thickness. The slab is lightly loaded, so thickness is more of a crack control issue than a load bearing issue.

>>versaengine....a 3.75" core diameter is too large for this slab thickness. Even if you have a 4" thickness in places, the L/D ratio is marginal. Consider a smaller diameter core to get a better L/D for compressive strength testing. Find out the nominal aggregate size and use the smallest diameter core that will give you 3x on the nominal aggregate. Also, if you're considering documentation for litigation, get a petrographic examination done on a couple of cores to get an idea of the effects of water added at the site, consolidation and void structure.

 

[a2mfk]

I'm piling onto the shrinkage crack bandwagon.

I would go with 8ft-10ft oc max with control joints, and they need to be placed within about 4 hours of placement. ACI 302.1R recommends 24-36 x slab thickness, or for a 4" slab, 8-12 ft. Also avoid "L" shapes and keep your "panels" at a 1.5:1 ratio max (keep your joint pattern as square as possible)..

What about after you epoxy or otherwise seal the cracks, applying an epoxy coating finish to the floor to hide the cracks? I attached a crack repair PDF you may find useful.

I look at cracked slabs as part of forensic investigations a couple of times a week, many are residential driveway slabs on grade. The number one common factor that I have observed in slabs with few to no cracks- control joints. The more, the better... In fact, I often go to this one neighborhood where the contractor knew what he was doing and spaced them at maybe 5-6 ft o.c., and very few of them ever have any cracks.