Hairline Cracks after 3 days

[Russellwh]

Just had a 24x36 garage pad poured 4-5" thick (garage doors were not installed). Ambient temp was around 50°F and probably got to 25-30°F at night for the first 3 days. After that the high only reached 45°F. 3 control joints were used. After 3 days I am seeing random hairline cracks. Surface was worked real smooth. The cracking is not crazing from what I can tell. The cracks have not opened up in 11 days. Concrete guy says he will do a hammer test. What else should I require to ensure I do not have troubles years down the road. Jackhammer it up?

 

[Ron]

Russellwh...
If three control joints were used, then the max. size of any section should be 12x12. That's OK for spacing, but when did he saw the joints? My guess is day after placement. If so, the cracks had already occurred based on the conditions you gave. Was it protected from freezing? If not, the cracks might ultimately be the least of your worries.

What is the pattern of the cracks? Are they parallel to the cut control joints? Please give all info and then we can perhaps guide you.

The "hammer" he suggested (Swiss Hammer or Rebound Hammer) will tell you nothing about the issue and almost nothing about the concrete. It is a waste of time, but contractors often use it incorrectly to "show" there is nothing wrong with their concrete. The only real use you can get from the Swiss hammer, if used properly, it the relative uniformity of the concrete strength, but it won't actually give you the strength, just its uniformity.

 

[Russellwh]

Thanks Ron. The contractor install the metal key type control joints running from front to back, space 12'. He came back after 3 days and then made 2 perpendiculat saw cuts. Therefore I have sections ~8'x12'. The first hairline cracks ran perpendicular to the original control joints. The slab was not covered to protect from freezing. Right now after 7 days, the surface is still quite dark, but he did trowel the surface very smooth. I plan to hose off the surface today, to remove the dust from cutting and further inspect the hairline cracks. from what I have read, the hammer test only gives relative information so I will challenge him on this. I also do not believe he put any fibers in the mix. Would this have helped? On the concrete ticket, I heard the contractor noted there was too much gravel. Also, rumor has it that the concrete company got some bad sand, but I probably could not verify that since t was just hearsay.

 

[Ron]

Russellwh....The problem is that he made his sawcuts way too late! Those cuts should have been made the same day the concrete was placed...not 3 days later. It isn't the gravel in the mix, it isn't the sand, and not even the lack of fibers. The contractor screwed up by not cutting the control joints quickly.

 

[structuresguy]

I agree with Ron. Typically, the cuts need to be made only a few hours after the concrete has poured to be effective. Basically, you should only wait long enough so that you can walk on the slab without leaving footprints, 4-8 hours after pouring. Any longer than that, and the cracks have already formed.

Just curous, how was the slab cured? If the contrator did not use some form of curing (plastic sheeting, spray on membrane), then you might have some more ammunition for demanding the contractor replace the slab, or at least repair the cracks.

The Swiss hammer, also sometimes called Schmidt Hammer, test might give some usefull info regarding the concrete strength. Since the temperatures were cold, and apparently no protection from the cold was used, the concrete strength may not be as high as it should be. The hammer test might show this, if it is done correctly. Usually however, the contrators have little idea what they are doing. And without a proper sample to calibrate against, it is not reliable for determining actual strength, as Ron said.

 

[Russellwh]

Thanks Guys, this makes a lot of sense. The cracks I am seeing run perpendicular to the initial control joints. Basically, before the saw cuts, I had 3 separate sections 12x24 so the cracks developed perpendicular to the 24' length. Also, I am noticing more cracks near the garage door entrance, which would have seen more sunlight than further back in the garage, even though the temperature was in the 40's. This sugguest that the front section cured faster and developed more cracks. Right now, these are hairline cracks such that you can not really hang a finger nail on them. Is there anything that can be applied to the surface to really bring out the cracks, for inspection purposes? I know in welding, we use a dye penetrant to find cracks.

 

[Ron]

Russellwh....water will work on concrete just as dye penetrant does on metal. As it dries from the surface, the cracks will retain more water releasing it slower.

The cracks are hairline now, but they will continue to widen. If he used a high water-cement ratio concrete mix, then your cracks can get quite wide, approaching 1/4-inch in a few years....that's part of the problem with these issues...they don't show their full extent for a long time. If he used a small coarse aggregate size (#89 stone or similar, such as a "pea rock" mix), then expect shrinkage cracks to widen significantly.

If you have a broom finish, you might also have map cracking or crazing that has not yet shown itself. It gets masked for a long time with broom finishes and only starts to show when the concrete starts wearing at the surface.

As for the Swiss Hammer, do not let that be the only method of testing used to determine concrete strength and other characteristics. It can be very misleading and will often show concrete "strength" values that are relatively high compared to core strengths. As structuresguy said, correlation is necessary.

The pattern of cracking you described is exactly as expected when the contractor doesn't sawcut the joints quickly and properly.

 

[BigH]

Ron has covered the problems with cracking quite well - we get a lot of cracking on continuous concrete kerbs (oops, curbs for you Yanks) because they cut the joints too late. As for the rebound hammer - this has been discussed in detail in a number of threads but basically it is for confirming the consistency of the concrete from batch to batch and should not be used for strength purposes - other than in a very global sense.
and enjoy the Superbowl!

 

[Russellwh]

Gotta love the internet. Thanks guys. I'll let you know how this mess turns our.

Russ

 

[fdr1371]

Russellwh:

I have a similar experience to share with you. Last year we had our pad (26 x 26) done by our contractor. He was getting ready to leave and I asked him about the joints.

What joints?

He said he doesn't typically put joints on covered garages (not enough of ambient temp change). In MN that could be 50 deg. easy (would be more - but like he said because it is covered). I said no way - it will crack in about a day if you don't - that pad is way too big. Saw it in 1/4 (13' squares) and be done with it. He did.

The pad was then covered by us (I didn't want to take the chance with him after that) and kept soaking wet for 7 days with burlap and pvc sheeting (which by the way was still warm after we took it off a week later!!!). The pad (joints sealed - 1/4" backerod first and then sealed by me) has since performed very well.

Sidebar - other than that small note, our contractor did an outstanding job for us, and we are very happy with him.

Even with T & S rebar (#4 @ 12" E.W.), you are going to get cracks regardless if you don't saw-cut or joint trowel.

It can't be said enough, as it has here, that joints and curing are a vital part of the entire process. Mesh in the concrete and typical T & S rebar will only go so far.

To me it sounds like the combination of bad curing and no joints. By your posts, it sounds like he didn't even bother covering your pad? That is the biggest no-no with pavements. It was losing heat way to fast, especially in the cold air. A good rule to use is 3 days (best to use 7 days) with soaked burlap and covered w/ pvc (6-mil) to prevent evaporation.

Again, to me it sounds like he did none of the above. You may have grounds for him at least fixing the cracks (as people have said before - they will progress) if not the whole slab. These cracks are only going to get worse. Sorry to say that.

24 x 36. That is way too much area to not have joints like that. I see lots of $$$ for somebody because of this.

I hope I helped. Please ask if you need further info.

 

[Russellwh]

Well, today I spoke with the concrete guys who were out at the job site (I was at work). Basically they said these were only surface cracks (crazing). However, I thought crazing took on more of a radom pattern, similar to the They want to meet me Wednesday and do the hammer test. I questioned them (thanks to you guys) on this being a good tool for strenght verification. They skirted the issue somewhat but I am going to go ahead a let them do it in my presence. I also got a copy of the bill, this is what they SAID they installed

3500 psi 67 stone
3% accel standard
fiber
LD Hot (not sure what this means but there was not charge associate with this line item)

The funny thing is I do not believe there is any fiber in this mix as I can not see any evidence in the corners or in the waste. I remember years ago, when I had a drive way poured, you could see fibers in the waste products. So, I'm curious how they handle that issue. Anyway, any additional advice before Wednesday is much appreciated.

Russ

 

[Ron]

Russ...What you have is a "smaller than desired" aggregate for your application. With a 3500 psi mix, you probably have about 550 to 600 lbs of cement in the mix, and a water-cement ratio of perhaps 0.55. This means that you have about 40 to 45 gallons of water per cubic yard in the mix. Only about half of that water will be required for the chemical reaction of hydration of the cement (that's what causes the concrete to get hard). That leaves the remainder of the water to create additional space in the mix that will shrink as the water evaporates from the cement paste.

The 3% Accel is a set accelerator used in cold weather concreting. It causes the concrete to set up quickly so that there is a lower potential for freeze damage.

You should see evidence of the fibers in the mix.

Crazing is characterized by VERY small surface cracking commonly called "map" cracking. You cannot typically catch your fingernail on it and it only shows up early on very smooth finished concrete floors. If your cracks are linear or nearly so, they are common drying shrinkage cracks and they will affect the full depth, not just the surface.

Please carefully watch each step they take in the use of the Swiss hammer and report it back to us. I would bet at this point that they will walk onto the slab, put the hammer down, take a rebound reading, look at the scale and tell you..."this shows the concrete strength to be 4000+ psi...there's no problem with this concrete"... then they'll repeat that for another 4 or 5 locations and declare the concrete to be just fine. THIS IS NOT THE WAY THE REBOUND HAMMER SHOULD BE USED!! There are specific preparation techniques and reading techniques to be used with the rebound hammer.

Ron

 

[Russellwh]

OK Guys here is the scoop.
Computerize mix is the following for an 8 yd pour

Material Design Qty Required Batched
57/67-V 1550 lb 12400 lb 12350 lb
78 stone 300 lb 2400 lb 2360 lb
sand 1311 lb 11222 lb 11320 lb
cement 405 lb 3240 lb 3215 lb
flyash 140 lb 1120 lb 1115 lb
water 36 gal
acc-std 3% 1569.6 oz 1570 oz
hot 144 gal 144 gal
8 bags of fiber

Hammer test results are 20-24 in the back sections, of which I would expect to be curing a little slower and I do not see any hairline cracks. Middle readings are 28-30 and front readings where most cracks are seen averages about 27. The pad is 15 days old. We then measured the pad in the attached garage that is over 28 days and supplied by the same people. Readings averaged 29-30. They did not have the chart to convert these numbers but will call me later today.
They call what I am seeing as surface plastic shrinkage which they say is only on the surface and does not penetrate through. They did say that it is not crazing.

I earlier thought there was no fibers but I now see evidence that fiber was added. What I also see is the analysis at mixing shows 78 stone but my invoice ticket shows 67 stone.

Give me your thoughts

Russ

 

[fdr1371]

Russellwh:

Let me put in my 2 cents. After re-reading your posts, it is evident they did put in joints. However, it is also evident that he did not install them properly. Two days after the pad was poured is way too late. 4-5 hrs is usually the norm. I bet you had cracks in that slab within one day.

Again, back to my statement about curing. Did he spray a membrane on the pad or take any other measures during the curing process? If he did not, you certainly do have issues with your contractor. Especially since he knew (and don't tell me he didn't) it was going to get cold or close to freezing at night.

Do you know what or if any rebar was used? 4x4 or 6x6 WWF or standard #4s at some typical (usually 12-18" each way). How did he place it? Did he use chairs (if he doesn't know what those are, then your rebar is sitting on grade right now).

Don't let that hammer or the results fool you. Sure, your concrete is strong, but that isn't the issue. You can have lots of hairline cracks that don't affect the strength, especially this early in its curing. Over time you will get moisture in your cracks and then into your rebar (if you have any). Its just a lose-lose situation all together.

24x36. Like 12 yds or so. We can all do the $ for that amount. You are going to see lots of armchair advice from all of us (me included). My suggestion is to look into hiring an engineer w/ this type of specialty. It may cost you, but at what price compared to 12 yds of poorly placed concrete? He will want to take cores and do microscopic analysis.

Serious stuff, and serious $. But look, you are going to put a decent size structure on top of this. Why should you have to pay for his mistakes or poor practices? You shouldn't. You will have to look at those cracks every single time you go in and out of the garage.

Please keep us up to date with the daily decisions.

I wish you the best of luck.

Dan

 

[Russellwh]

Thanks Dan,
No rebar was used. I've got 16 yds in the pour so I'm about 5.5" to 6" deep. I think they may be a little heavy on the amount reported but I am not too concerned on 1-2 yards as I did the compacting myself and looks like I've got in excess of 5+" when measured against the perimeter wall. Also, the computer generated mix ticket shows they sent out (2) 8 yd loads. They have offered to seal the surface to prevent anything from getting into the cracks. Does anyone have a graph that they can translate the hammer test numbers? I am waiting for a call back from the concrete complany and I just want to have my ducks in a row.

 

[fdr1371]

Russellwh:

Sure no problem.

No rebar??? That is very, very odd. I will give you ACI codes which I use. I can't tell you about your fibers. The lack of rebar or mesh (did he use that?) is very strange. I would not trust fibers to control cracking. Proper curing, jointing, and rebar all must work together.

ACI 7.12.2.1 — Area of shrinkage and temperature reinforcement shall provide at least the following ratios of reinforcement area to gross concrete area, but not less than 0.0014:

(a) Slabs where Grade 40 or 50 deformed bars are used = 0.0020.

ACI 7.12.2.2 — Shrinkage and temperature reinforcement shall be spaced not farther apart than five times the slab thickness, nor 18 in.

So basically what they are saying is for a 1-ft strip (12-in), you need 0.0020*(5-in)*(12-in) or 0.12 square inches per foot. This applies both ways (seen as E.W. or each way). A good rule is 1/8 (0.125) square in per foot.

#3 bar = 0.11 si (or 1 bar every 11 inches).
#4 bar = 0.20 si (or 1 bar every 20 inches).
Or some equivalent welded wire fabric (WWF).

The last part states the maximum spacing to be the lesser of 5 times the slab thickness or 18-in. 18-in is the governing case here.

So call it #4 @ 18" and be done with it (my preference would be #4 @ 12" - that much mild rebar is cheap in the long run). Placed on chairs @ mid-slab height. People will say no, put it @ 2/3 below top of slab. Usually not, as it will most likely be stepped on and worked down to the 2/3 depth anyway.

I can't speak for any experience regarding fibers. You should really talk to someone who deals with slabs like these.

Don't be concerned about the hammer results. I'm quite sure it will be 4000 or so psi. Strength isn't the issue. If this is for a residential garage, you are highly unlikely to produce a load that would ever come close to cracking the concrete. Temperature and shrinkage is your major concern.

Sealing is ok, but you will still see the cracks, as the void of the crack will pull the sealant into itself over time - showing you where the cracks are still, even though there may be nothing penetrating that sealant.

You have said it was cold when they did their work. It may be too cold to perfom effective sealing. I can not speak to that area - just that it does have its temp limits just as concrete does.

Sorry to make this so long. You have a very major decision to make very soon. Please don't take this lightly or let them force you into doing something you may regret.

I hope the info I have provided can help you.

Good luck.

Dan

 

[Ron]

Russ,
Rebar is not necessary. As for the conversion of the numbers...don't bother. As I said before, they don't mean anything....the rebound number is good for checking the relative consistency, not strength measurement. As you can see from the numbers you gave, there is little difference between the numbers in cracked areas and non-cracked areas.

If the cracks are longer than a few inches and somewhat continuous, they are not plastic shrinkage cracks. Plastic shrinkage cracks occur BEFORE the concrete sets up and are characterized by short, discontinuous cracks where the surface starts to dry and gain some strength before the lower concrete. Plastic shrinkage cracks occur when the surface of the concrete dries faster than the "bleed" water can collect on the surface. This is usually a result of wind blowing across the surface.

 

[sda97]

Fibers help to control random shrinkage cracks but you still need to have control joints. Reinforcing in residential construction is kind of a regional thing. Wire mesh is the most typical for residential work. Reinforcing bars are a little overkill for residential. Mesh is admittedly difficult to place properly in the slab. Reinforcing allows you to use a wider spacing of control joints than you would use without reinforcing. If reinforcing is used, it should be partialy cut (say 1/2 the number of rods or bars) at the control joints. Otherwise, cracks may develop at locations other than the control joints. Did you contract for reinforcing in the slab? It sounds like the saw cuts were made a little late to prevent random cracking. If the surface of the concrete is of good quality, the slab should serve you well. May be the contractor will agree to seal the cracks. Getting a crack free slab, although not impossible, is a real gamble. Everything must go exactly right for the contractor. That is where the contractor knowledge and experience come into play.

 

[Russellwh]

I really appreciate the help from everyone. I drilled the concrete guys and to be honest, they did not try to BS me much. It was not until the end that I let them do the Swiss hammer test. One question I had right off-the-bat was on the scale, the pointer would only return to 8 when the hold button was released after a test. Now the scale goes from 0 to 100 so maybe in the relaxed state, a reading of 8 is just the calibration set point. So, has anyone seen the same? Since I questioned them, they called the factory and now have shipped back the instrument for calibration. Funny thing is, this was a brand new machine. One other question for you guys. I see where 1 yd of this mix has 300 lb #78 stone and 1550lb of 57/67-V. I know using too small of a stone increases the chance of plastic shrinkage. I quized them on this and they say this is a normal mix for them.

I've done some reading on plastic shrinkage and it looks like the finisher has a lot to do with this occurance. Do I have 2 to blame. I think the finisher has some liability since he waited 3 days to make the saw cuts and did not cover the pad after the pour.

 

[Ron]

Russ...these are not likely plastic shrinkage cracks. Read my description in previous posts. You have drying shrinkage cracks. Whoever is responsible for sawcuts is responsible for your cracks.