Concrete Slab Pour Issue

[jeffdan17]

Maybe this is the right place to ask this question, maybe it’s not. But I’m going to ask for some opinions anyway.

I just had my concrete slab poured for our garage a couple of weeks ago (5/23). I have family who work for a general contractor and they are doing the work for me on the side. I also happen to work for a materials company and was able to get concrete at a nice discount. Win-win, right?

We ordered 17 CY, and got 2 trucks (8.5/each). The first truck went great. And we thought the second truck did too. But after some time, whenever the concrete was hardening and ready to trowel, we noticed that the concrete from the second truck was still much softer than the first. To cut to the chase, it hardening way slower than the concrete from the first truck, and right where the second truck started pouring, there are 3 areas that have raised up noticeably, and I now have what looks like “bubbled” concrete there. The only difference we noticed between the trucks (after the fact), was that the second truck driver did not mix his concrete before dispensing it. Otherwise, the concrete guys say it was the same mix.

So, my guys that did the concrete say there was definitely something different about the second truck’s mix. And after talking with the salesman at work and sending him some pics, he said it is 100% “concrete shrinkage cracking”, and never did physically come out to look at it (on the day of the pour, or when I sent him pics last week). Our call was cut short, and I have yet to hear back from him.

Here are some pics. What do y’all think? It’s been over 2 weeks now so I’m not really sure what solutions there are to fix it, even if I decide to try to pursue having that done. But there’s very noticeably some really uneven concrete there. Thanks for any input!

 

[PCustoms]

The only difference we noticed between the trucks (after the fact), was that the second truck driver did not mix his concrete before dispensing it.

Huh?

 

[jeffdan17]

Huh?

My terminology probably isn’t right. You know the big barrel on a concrete mixer truck? The first truck idled his truck up and spun the barrel, mixing the concrete inside before pouring it. The second truck did not. We didn’t realize it until after the fact.

 

[Mattilac]

Oh boy, those cracks are really disturbing looking.

Paging @ConCretin

 

[ConCretin]

It's obviously challenging to diagnose a problem from pictures but I think it's safe to say it's not normal “concrete shrinkage cracking”. If I had to venture a guess, I'd say the second load wasn't adequately mixed in the truck. The cracked areas look like there's not enough cement, which would also explain the slow set. The fact that the problem is so localized suggests it wasn't a lack of cement in the whole load, which would be unlikely in any case.

It's not necessary to spin the drum before discharging concrete if you didn't need to add water and as long as the concrete had time to mix properly on the way from the plant. Do you recall if the drum was turning as the truck arrived? How far is it to the plant?

If the areas are small and just on the surface, it might not affect the overall serviceability of the slab if you can live with the appearance. One solution is to remove and replace the defective areas, which wouldn't be pretty or possibly replace the whole slab if it's structurally compromised. The other option would be to overlay the slab with a new one. In any case, it appears to be defective concrete and the responsibility of the supplier. Good luck.

 

[jeffdan17]

Thanks for the information, @ConCretin! To answer your questions, I do not recall if the drum was turning when the truck arrived. He pulled in while we were finishing up with the first truck. The plant is approximately 14 minutes away.

With where the areas are located, I can probably live with it as long as there isn’t any concern with structural integrity. I plan on parking my subcompact tractor in that space. But it is certainly noticeable.

 

[C-S-H]

Definitely something wrong with the second batch, maybe beyond insufficient mixing. You should always do a slump test, or at least handle the concrete, before you commit to pouring the batch in the form. Also, read the batch tickets. The second truck could have been carrying a retempered reject batch from another job. Between the plastic tearing in the lean areas, and the segregation of the air void system in other areas, we would already be chipping this slab up.

 

[Kurt4440]

Definitely something wrong with the second batch, maybe beyond insufficient mixing. You should always do a slump test, or at least handle the concrete, before you commit to pouring the batch in the form. Also, read the batch tickets. The second truck could have been carrying a retempered reject batch from another job. Between the plastic tearing in the lean areas, and the segregation of the air void system in other areas, we would already be chipping this slab up.

I agree, check the batch ticket first then the drum counter. Slump and temperature next.
If they added material to a rejected load, I would want them to pay for removal and replacement.
Those pictures indicate more than shrinkage cracks.

 

[jeffdan17]

Update: I was finally able to speak to the salesman yesterday and he visited the slab this morning. He informed me that the batch tickets from both trucks are the same, and that the raised areas are structurally sound. Therefore, there’s nothing he can do at this point. If it were his slab, he would just let it be. In so many words, the raised areas are not bad enough to consider paying to have them grinded down or removed and replaced. So, it is what it is at this point I suppose.

 

[mm08822]

What was the weather conditions during the pours? Was any water added to the surface? Was there any cold joints created between truck 1 & 2?

Does the slab elevation matter? Could you add another 4" pour on top before framing starts?

Provisions for tieing the 2 slabs together and a watertight perimeter seal would be needed.

 

[Jim greengo]

My terminology probably isn’t right. You know the big barrel on a concrete mixer truck? The first truck idled his truck up and spun the barrel, mixing the concrete inside before pouring it. The second truck did not. We didn’t realize it until after the fact.

It should've been spinning from the time it was dumped in the truck.

 

[jeffdan17]

What was the weather conditions during the pours? Was any water added to the surface? Was there any cold joints created between truck 1 & 2?

Does the slab elevation matter? Could you add another 4" pour on top before framing starts?

Provisions for tieing the 2 slabs together and a watertight perimeter seal would be needed.

Weather was around 70 degrees, sunny, and almost no wind. Max wind gust that day was 3mph. We did not add water once it was off the truck, no. No cold joints between trucks.

Slab elevation does matter, unfortunately. So I’m not sure that would be an option.

 

[jeffdan17]

It should've been spinning from the time it was dumped in the truck.

That’s my understanding as well, but I’m being told that isn’t an issue.

 

[djjsr]

I had some rough high spots in my driveway after it was poured. Smoothed them out with my little 3x21 belt sander and a few coarse belts. I think they were Gator brand, 36 grit that I got from Menards. It was easier than I thought.

 

[Kurt4440]

That’s my understanding as well, but I’m being told that isn’t an issue.

Someone is not being truthful.
The drum needs to turn from the time that it is loading until it is time to discharge.
Do you have copies of the batch tickets?

 

[ConCretin]

Just to clarify the drum issue before we go down a rabbit trail. The drum spins slowly from the plant to the job site, which I assume occurred with both trucks. I believe that the OP stated that the first truck spun the drum up to a higher rpm for a few minutes (presumably because water was added onsite). The second truck did not. There is nothing wrong with this although if the load wasn't on the road long enough to thoroughly mix the concrete, a little extra mix time might be the difference.

 

[ConCretin]

Update: I was finally able to speak to the salesman yesterday and he visited the slab this morning. He informed me that the batch tickets from both trucks are the same, and that the raised areas are structurally sound. Therefore, there’s nothing he can do at this point. If it were his slab, he would just let it be. In so many words, the raised areas are not bad enough to consider paying to have them grinded down or removed and replaced. So, it is what it is at this point I suppose.

Why am I not surprised by such a mealy mouthed response. There was clearly something wrong with the concrete that at a minimum, resulted in surface defects. I doubt the slab has any major structural issues and it would be understandable if you chose not to pursue any further remedy but that was a pathetic response to legitimate concerns.

 

[mm08822]

Update: I was finally able to speak to the salesman yesterday and he visited the slab this morning. He informed me that the batch tickets from both trucks are the same, and that the raised areas are structurally sound. Therefore, there’s nothing he can do at this point. If it were his slab, he would just let it be. In so many words, the raised areas are not bad enough to consider paying to have them grinded down or removed and replaced. So, it is what it is at this point I suppose.

Yeah, that's a self-serving response to keep his company out of trouble with you. I doubt if he were in your shoes would he have the same opinion.

If you didn't accept that response then he probably would have started down the path of poor placement and finishing techniques all of which are undocumented/ unverifiable and puts the burden on you to prove otherwise. Even if you had a sample tube poured somewhere during the pour, it would not have necessarily been from and under-mixed agglomeration of materials in the drum.

 

[Stelzer]

I'll leave the concrete advice to the concrete professionals. The only thing I'll add is I'd request actually seeing the batch tickets and not taking the salesman's word for it.

 

[geneg]

I'll leave the concrete advice to the concrete professionals. The only thing I'll add is I'd request actually seeing the batch tickets and not taking the salesman's word for it.

Always ask for the ticket from every load. The driver should indicate if he added water to the load. Another thing to prevent things like this from being swept under the rug is to get some test cylinders from a lab & have a couple filled from every truck. Set them aside unless something like this comes up. Actually paying the lab to break a few cylinders gives you peace of mind knowing that you got what you paid for.

Every time you walk past it- you'll regret letting it go.

But then, hind sight is 20-20. Go up the ladder from the salesman at the concrete supplier. The management/owners of the company are usually concerned about more than sales commissions.

Good luck.

 

[jeffdan17]

Always ask for the ticket from every load. The driver should indicate if he added water to the load. Another thing to prevent things like this from being swept under the rug is to get some test cylinders from a lab & have a couple filled from every truck. Set them aside unless something like this comes up. Actually paying the lab to break a few cylinders gives you peace of mind knowing that you got what you paid for.

Every time you walk past it- you'll regret letting it go.

But then, hind sight is 20-20. Go up the ladder from the salesman at the concrete supplier. The management/owners of the company are usually concerned about more than sales commissions.

Good luck.

Yes sir. I did get the tickets from the drivers, but they don’t lost all of the batch details. I will see if I can get them from the salesman.

The tricky part of this is that I work for this company. It’s a large building materials company, and I work in the office in another division. I’m not afraid to speak up, but it’s more of a matter of being sure that this is a legitimate concern, and I’m not just crying wolf.

 

[jeffdan17]

Why am I not surprised by such a mealy mouthed response. There was clearly something wrong with the concrete that at a minimum, resulted in surface defects. I doubt the slab has any major structural issues and it would be understandable if you chose not to pursue any further remedy but that was a pathetic response to legitimate concerns.

Yes, that is where I’m at on it. Even if the issue doesn’t get addressed, I’m sorely disappointed that there isn’t any acknowledgement of error on the supplier side. I’m not a concrete expert, but I have no idea how you can say that there was not a problem of some sort with the mix.

 

[Mattilac]

That concrete ain't right, period. I'm no expert but I don't see how poor finishing techniques could cause it to crack like that, and only where the first batch meets the second batch.

I would be very unhappy but I'm fairly picky.

 

[jkeyser14]

Yes sir. I did get the tickets from the drivers, but they don’t lost all of the batch details. I will see if I can get them from the salesman.

The tricky part of this is that I work for this company. It’s a large building materials company, and I work in the office in another division. I’m not afraid to speak up, but it’s more of a matter of being sure that this is a legitimate concern, and I’m not just crying wolf.

Your company should be luckily it happened to an employee and not an outside customer. It doesn't look right and I would want it fixed.

 

[ConCretin]

To be honest, I had to look this up. I questioned whether a 15 minute transit time was adequate to mix the concrete and it appears it's marginal at best. According the the article below, the drum on a concrete truck turns at 2 to 4 rpm during transit. At the top of that range, a 15 minute transit time would result in 60 revolutions. The recommended number of revolutions is 70-100.

This might explain why the concrete from the truck that spun up its drum for an additional period on site was fine while the concrete from the truck that didn't had issues.

What factors affect the mixing time?

A. The importance of thorough mixing for the development of strength and uniformity throughout the batch has long been recognized. With a proper loading se

www.concrete.org

 

[Notgrownup]

Can you just chip away the flakes and pour some surfacer?

 

[Rusted Nut]

This is what we call a “plug”, because they will plug up a concrete pump. It's a ball of sand and cement that didn’t get mixed with water. Most of our pours are structural with an on-site special inspector. If the inspector sees a plug larger than 3” coming down the chute, they will reject the entire load from that truck.

On a slab pour, I would think the finishers would have seen this either when placing or finishing and taken a shovel to it. If this were me, I’d saw cut the area out and patch it back, being a non structural slab, rest of the load is probably fine.