Slab questions

[jjwillac]

Hi all,

I have a 40x42 (40x30 on slab with 40x12 open-air lean-to on continuous footing) tubular steel building that was recently completed. I've got three separate questions here about the concrete. The concrete contractor didn't exactly instill confidence, so looking for a little reassurance. All the work is already done, so hopefully no major issues here. Some slab details are below questions.

1) I've noticed the slab is getting a lot of surface cracks, some of which are chipping out to form pits in the surface. I know all concrete cracks, but how much is normal? I don't think I'm worried about any structural issues, but I think I do want to do something to prevent chipping and since water penetrates the cracks when wet. Thoughts on this is normal, whether I'm correct this isn't a major concern, and how I might "fix" it?

2) Looking back at some pictures of the pour (I was out of town), I realized the contractor put the vapor barrier UNDER the crushed stone base. I know enough to realize that isn't typical, but is it an issue or become an issue at some point?

3) The slab was saw cut. My understanding is that i should blow-out the cuts, put some backer rod in, and then caulk them with no-sag masonry joint caulk. Is this right?

Slab details: 5" thick 4,000 psi with #4 rebar 2' OC poured over 4-5" compacted 3/8 minus (on an already compacted site) with vapor barrier. Continuous 12"x18" footer (frost depth here is 8-12") with four #4 rebar. Slab was cut day after pour (two 40' cuts three 30' cuts). Poured in early August, sprayed with a sealer, and forms removed the same day as pour. Contractor specifically said not to keep it wet.

 

[Half-fast eddie]

Not impressed with the surface finish in the first picture. Did he use a broken brick for the final float?

If he used a curing sealer, keeping it wet is not needed.

Correct, stuff in a backer rod, and fill with caulk. I would use a self-leveling elastomeric like NP-1.

 

[nadogail]

I have been told by concrete professionals that slabs come with two guarantees: They are guaranteed to crack. Nobody will steal a Concrete Slab.

 

[Yankee2bbq]

I’m a concrete finisher. The above statement is true. Concrete will crack. So a vapor barrier was used, meaning this slab will be for a living area (like a basement) to help stop moisture from coming up? I have never used a vapor barrier. I live in an area where we have high humidity and homes here don’t have basements. (Not many) Also, I pour mostly shop floors and sidewalks. I have sealed concrete, but only when it was cured. I have used a water sprinkler to help cure the concrete, if weather was too hot. For the saw cut joints, I have never filled with backer and filled with a flex caulk.

 

[Half-fast eddie]

I have never sealed concrete. , I have never filled with backer and filled with a flex caulk.

Talking about 2 different sealer things. One is a long-term sealer, that cannot be applied for 30 days after the pour, when the concrete has fully cured. The product being discussed here is a curing compound, it is designed to be applied after all the bleed water has gone away. Takes the place of the need to sprinkle the concrete in hot weather. Replaces the wet burlap sacks.

If you dont use caulk, how do you close up the expansion joints?

 

[Yankee2bbq]

Talking about 2 different sealer things. One is a long-term sealer, that cannot be applied for 30 days after the pour, when the concrete has fully cured. The product being discussed here is a curing compound, it is designed to be applied after all the bleed water has gone away. Takes the place of the need to sprinkle the concrete in hot weather. Replaces the wet burlap sacks.

If you dont use caulk, how do you close up the expansion joints?

Close up expansion joints? Guess I am confused here. Saw cut joint and expansion joint two different things. Expansion joint I use Nomaflex. Which is in place when concrete is being poured.

 

[Yankee2bbq]

And slick rod is used in the expansion board.

 

[jjwillac]

Thanks all, so I'm reasonably safe assuming no major issues here? Anyone have thoughts on minimizing the cracking/chipping?

So a vapor barrier was used, meaning this slab will be for a living area (like a basement) to help stop moisture from coming up?

It is a shop, but with our very wet pacific NW winters, the consensus around here is to have a vapor barrier.

Any thoughts on the vapor barrier being below the crushed rock?

 

[Copymutt]

Don’t think it would be any different than on top. Integrity can be lost when sharp aggregate punches holes due to weight of the pour or walking on it.
In solar floor apps I’ve put the barrier below on compacted sand.

 

[billconner]

I would think that the vapor barrier below the fill is only a problem if water can get to fill. I like the slabs where the vapor barrier comes up at edges inside form as best way to prevent moisture.

I'd use it in any interior slab. Even an unheated garage so metal tools are less likely to rust.

 

[Half-fast eddie]

Close up expansion joints? Guess I am confused here. Saw cut joint and expansion joint two different things. Expansion joint I use Nomaflex. Which is in place when concrete is being poured.

Really messing with the train of thought now. Nomaflex … ok that the 1/2” wide black material that gets poured in place, like they used to do cedar boards. Thats residential style, never seen that for commercial. Some commercial finishers will cast a zip strip in the pour, rip out the top filler when the slab is firm enough to walk on. Most will sawcut an almost full depth joint the next day.

When you say expansion joint, are you talking about the crack control joint that usually gets troweled into the fresh concrete?

 

[acer66]

Any thoughts on the vapor barrier being below the crushed rock?

Interesting, I have only seen it used directly under the concrete.

 

[JWILLIE1977]

Only my opinion, but from the pictures, the I think you are seeing craze cracking of the concrete surface.

Crazing of the concrete surface is one form of shrinkage cracking. It is caused by the surface of the concrete drying out. The drying of the surface of the concrete creates tension in the surface skin as it dries and shrinks at a faster rate than the underlying concrete. The tension in the "skin" then pulls apart as the dying continues. .resulting in a cracking pattern. These cracks are usually not to deep, and shouldn't affect the structural capability of the floor. As you are seeing tho, they can affect the astetics of the floor though.
Not sure what the weather conditions were on the day of your placent. .but this usually happens when the surface during placement is been exposed to low humidity, high ambient air temp or concrete temp, hot sun exposure, or any combination of these. A high water slump or w/cm ratio mix can lead to exaggerating of the issue. The more water the mix has to evaporate out of the surface, the bigger the problem is.

There are steps the contractor could have taken to recognize, limit, or even prevent this. Application of curing compound immediately after final finishing. Wet cure, etc.

Do a Google search of concrete craze cracking. . You can find tons of info from NRMCA, ACI and various other concrete construction groups. Should give you a better understanding of what it is, and how to repair it if you wish.

Vapor barrier under the crushed stone shouldnt be an issue, unless moisture can somehow get into the stone, or there is an enormous amount of holes from placement and compaction of the stone.

Hope this helps.

 

[thebmrust]

Surface problems might be because the contractor doesn’t know or doesn’t care about concrete. The vapor barrier under the base might be from change in code, lack of knowledge, or a sub contractor that doesn’t know what they were doing. If the contractor did it, then see my first sentence.

I am not a contractor, I worked as labor for my dad who has done concrete for almost 55 years and my brother who is a flatwork specialist for almost 20 years. They always tested every truck for water content and appropriate mix. They had a specific mud order and checked the paperwork for compliance. Then added water based on their time of pouring (temperature, humidity, final finish).

I have looked at work my dad did almost 40 years ago and it’s still perfect. I’ve seen work my brother did last year... looks perfect.

I’m guessing the contractor had the wrong water content based on slab thickness and temperature (important to determine cure time, slough). The finish worker probably didn’t work the slab properly either. Timing is so important for a good final product.

They have explained to me (I’m not a concrete guy) that surface damage allows for water to infiltrate and create more long term problems.

 

[ConCretin]

1) I've noticed the slab is getting a lot of surface cracks, some of which are chipping out to form pits in the surface. I know all concrete cracks, but how much is normal? I don't think I'm worried about any structural issues, but I think I do want to do something to prevent chipping and since water penetrates the cracks when wet. Thoughts on this is normal, whether I'm correct this isn't a major concern, and how I might "fix" it?

2) Looking back at some pictures of the pour (I was out of town), I realized the contractor put the vapor barrier UNDER the crushed stone base. I know enough to realize that isn't typical, but is it an issue or become an issue at some point?

3) The slab was saw cut. My understanding is that i should blow-out the cuts, put some backer rod in, and then caulk them with no-sag masonry joint caulk. Is this right?

1) The surface cracks you are see resemble crazing, which refers to shrinkage cracks in the cement paste on the surface of the slab. Crazing is generally an aesthetic concern and doesn't usually affect the serviceability of the slab.

Based on the course appearance of the cracks, there is another form of crazing that I suspect might be in play here. If a slab is placed in hot and/or windy weather, the surface hardens before the underlying concrete. If the finisher thinks the slab is ready and gets on the slab before the concrete underneath can support his weight, you get cracking in the surface.

Depending on the severity of the cracking, it might become an issue. If you are already seeing spalling, I'd be a little concerned but there's not much to be done for it at this point. Keep an eye on it and get back to us.

As others have said, a concrete slab of any size usually experience shrinkage cracks, which is why you install control joints to hide them. Crazing however is preventable by controlling water and understanding how to deal with weather conditions.

2) A vapor barrier will work fine under the stone. It's purpose simply to prevent water vapor from the soil from passing through the slab. Putting the vapor barrier underneath the stone gives bleed water a place to go rather than the surface, which is good but that moisture will take some time to work its way out.

3) This step is not absolutely necessary but it makes for a neater job. nd dry out. I'd wait a month to let the slab shrink as much as it's going to and dry out. Assuming the floor is flat, mask off the sides and use a self leveling polyurethane caulk. You can also use a non sag product and tool the joint. Slkaflex and Sonneborn make both products that are commonly used.

 

[jjwillac]

Thanks @JWILLIE1977, @thebmrust and @LLWillysfan for the detailed replies. I definitely think it is some kind of crazing. Weather was relatively cool the day the pour occurred (80s and overcast compared to 90s-110+ and blaring sun for the rest of the summer), but still very dry (we just got first measurable moisture since May). It was back to 100 the week after pour. Contractor was on the slab working surface almost immediately after pour and told my wife we could walk on it as much as we wanted or park normal personal vehicles about 6 to 8hrs after pour (we didn't put anything on it for several weeks, but did walk on it). Forms came off about 3hrs after pour. They sprayed a curing agent on the surface, but the coverage seems really inconsistent and the crazing is occurring where there was less of the curing agent.

The contractor seemed a good guy; well meaning and responsive to my questions/concerns while specing the slab, but I got the distinct impression he was unfamiliar with a lot of the things I was asking and didn't have much experience leading a job like this. I think sidewalks and patios are more of his typical job.

Unfortunately, I can see that a couple cracks near the edges are wicking water up, but hopefully that won't be an issue once I seal the control cuts (the ends of some cuts are exposed outside the building). Seems like some kind of concrete sealer is in my future...

 

[dcg9381]

FYI - I've had the same contractor pour probably 5 different things for me. Most of those slabs look great, with the last slab getting some crazing as it dried faster than he thought it would. Not worried about that slab, but it's uglier than the other.

I've used Hd6600-MMA (Methyl Methacrylate) on outdoor slabs, largely due to UV resistance and cost. It doesn't do very well with crazing cracks like that, doesn't seem to completely seal them up - maybe because they cracked after the application (not sure). Polyurea is the other thing I've used - better sealing capability. Even with crazing, you can get a pretty cool finish with stain and the above coatings. If you want to hide it, there are solid versions of polyurea that will probably hide that stuff (once it's done cracking).

 

[jjwillac]

Thanks dcg9381. I don't mind the look of it, only the pitting where the cracks come together and any functional issues with infiltration. I actually kinda like the crazed look.

What are people's thoughts on going back to the contractor on this? Think it's reasonable and worth talking with him about and seeing if he'd cover cost of sealing/repair?