Can I Save My Concrete Slab?

Better to jack hammer it out and do it right.

All concrete shrinks and cracks as it cures.

You can PROPERLY reinforce it with re-bar and or wire mesh or stress it with cables tightened after cure.

Prestressed concrete - Wikipedia, the free encyclopedia

The concrete should have been deeply scored every meter to help direct/hide the cracks, or poured in small sections.

Google searches and you-tube might have helped, before the pour.

The slab has 2 welded Trays in there ST 35 on the inferior and ST 25 on the superior, It's also strengthened with 16 gauge re-bar on the top side of the inferior tray. It's not a DIY. I live in the tropics and I was'nt here when the guys poured it but I saw the photos of the steel work and it looked solid and well prepared. I think it just cured too quickly after the pour. There was a product in the cement called sika hydrofuge to plasticate the concrtere. Jack hammering it out is not an option as we are in the house now and there has been severe down pours with no water penetration which mean the slabs integrity is intact. I'm sure it can be saved.

Can't help you.

Good luck.

What are these trays? What do you mean by inferior and superior. How thick is the slab? The plasticizer is to keep the concrete workable with a low water content.

A tray is welded re-bar forming a grid that you can buy pre fabricated. They come in all different dimensions, I use European termanology, You may have a different vocabulary but the sizes that were used for my slab correspond with engineering plans that I've seen over the years....My problem today is'nt what went wrong. It's quite simply can it be saved in the situation.

We need to know what is wrong to understand the present behaviour. To simply put in a support beam could make the condition worse.

Normally, in a slab supported on four sides, I would expect reinforcing in both directions near the bottom, the bottom layer in the short direction and the perpendicular bars sitting on top of them. The cover on the bottom should be about 16mm. The concrete around the rebar is expected to crack, it is designed on that basis.

It sounds as though you have two layers of rebar which is mystifying in such a thin slab. There is only a small gap between the layers; what size aggregate did you use in the concrete? The reinforcing at the top may allow for a support near the middle, but then the top surface will crack.

It may be that you don't need to do anything, but you need to hire a structural engineer to run the numbers.

Thankyou for your input, Here's a couple of pics to help you understand what kind of slab it is. I used plans on the steel work from a structural engineering plan on a house I worked on last year. I'm going to take your advice and ask the engineer to come check it out, Maybe I'm just going overboard as I was'nt here for the pour. I can take better pics of the micro cracks with my ladder in the morning and post them.

The slab is litteraly the roof of our house

It is not leaking the cracks do not go all the way through so the water issues at this time does not exist. My underground house had many cracks on the ceiling domes that did leak until I injected a water activated epoxy into them and that was the end of the water. On top of this was one meter of earth and if the photos still show on my website www.domehome.ca then you will also see bulldozers and skid steers on the roof with no additional cracking. The roof was a dome on 24 foot square modules with a 4 foot rise from wall to the skylight wells. My engineer had no concerns over the cracking and the concrete was reinforced at about 12 inch spacing near the center of the pour. At the walls 10 inches and about 5 inches at the top where the skylight wells are located. Of course the dome shape is about 10 X stronger than a flat. I would have it inspected but I suspect as you may find that these cracks are superficial however that is up to the specialist. Thanks for the photos.

So the slab is a coffin lid?

I'd get a structural engineer to look at this to make sure its not!

What's the slab for exactly?

It sounds like you had a bad mix, or more likely, dried too fast improper curing....how thick is the slab? what was the sub-base/sub-grade, and prep method? was any curing compound added to the slab or other method used to slow drying?

The slab has 2 welded Trays in there ST 35 on the inferior and ST 25 on the superior, It's also strengthened with 16 gauge re-bar on the top side of the inferior tray. It's not a DIY. I live in the tropics and I was'nt here when the guys poured it but I saw the photos of the steel work and it looked solid and well prepared. I think it just cured too quickly after the pour. There was a product in the cement called sika hydrofuge to plasticate the concrtere. It's the roof of our house, Jack hammering it out is not an option as we are in the house now and there has been severe down pours with no water penetration which mean the slabs integrity is intact. I'm sure it can be saved.

You have an unsupported 5m 50cm x 4m concrete slab?

Houston, we have a problem!!!!!!!!!!!!!!!!!

Is this a DIY project?

Ooooo, yes that could be read as "big slab 'o' overhead concrete".

Tear it out, do it right. Last thing one needs is to try to "fix" it, it fails later with catastrophic results and the authorities start looking to send someone to prison for negligence. The question to ask is, "is this project something I can have a high level of confidence in and that it will withstand any reasonable scrutiny?" If one can't honestly say yes, it's time to stop and do it right.

You live under this slab?

Oh. Well, then, my first two questions would naturally be:

1. Do you have life insurance?

2. Are the premiums paid up?

You should have a REAL Licensed Civil Engineer examine it on site and advise you. Its been there are only two kinds of cement; cracked cement; and cement that is going to crack. This is not a large slab though it seems a bit thick, 50cm?

I have lived under a 20.5cm x 7.5m x 9.1m slab for the past 30 years; it has not yet cracked though protection from uneven or fast cures, from the elements and large temperature swings is key to keeping it crack free. It also has no strikes to help propagate cracks, which is required if the slab is not being protected from large humidity and temperature swings.

One would expect cracks on the bottom face first as the bottom is under expansion while the top is under compression; also the bottom is likely far less humid than the top. Of course it can be shorn up with steel from below; it's just a shame to go to that expense as if it is not called for.

The slab is 15cm thick. The enrobage of concrete is 3 centimeters top and underside....I was looking for someone that maybe went through this positively or negatively....All info helps. Life insurance? Havent laughed that hard since I lived in the states....Thanks for your comment all the same.....Joseph Jonathan Gould, Good advice and a sound mind. Maybe I stress too much but thats how we stay safe. I'm watching the signs and I've been in construction all my life. The slab is intact and water proof but I can see tiny cracks all over the underside. I'd just rather support it now and not have to deal with it in a year or 2 if it starts to sag and bust, Anyone else been through this?

I did not rad through all of the comments thoughly. but with the size of that, that is a lot of weight.

First off, how did you come to putting a cement 'slab'.

I have hear about people do this in their attached garages, with a basement underneath. weather this was a pur, or a preengineered piece of concrete, I don't know.

No suggests here, I'm just going to state what I experienced.

We have a pre-engineered concrete mezzanine (Spancrete) in one of our plants which I discovered cracks in it where I could shine a flashlight through it.

I called the general contractor about it that built the building 18 years prior to have it inspected. The cracks came from the building settling.???

But had said it is structural sound.

I suggest you have it inspected, there is nothing we can do here.

Some photos of the slab and how it is being used would be helpful. I built an underground house and cracking was common but sealing was accomplished and house was kept dry. Important to know if it is floor slab, roof slab what ever. This one seems massively thick for some reason. To really help need photos.. cheers.

Can appreciate the problem. My brother had a similar problem where sections of his sidewalk were collapsing due to poor soil compaction and buried vegetation when the house was built. He had a company called the "Concrete Chiropractor" come in and inject grout under the slabs through a small hole drilled in the concrete. I saw him do it and it was really good. I also wouldn't have believed it if I didn't see it done. You couldn't tell there was a problem after he was done. Didn't take long and very effective. He injected grout under the slabs. He also does the usual masonry/concrete construction/repair.

Google or look in your phone book for a local outfit that is listed as a "grouting service" or "grouting contractor".

Good Luck, Old Salt

I should have also included that as the grout was injected the slab raised up. He injected in small steps until the original slab was even with the others. In the past I never though that this could be done, but he did it!

I have talked with others that have had walls and other large units raises with this method. It is also possible to raise units such as walls that are skewed from level and need to be raised up and to an even position.

Good Luck, Old Salt

So when the slab in question is in fact a roof, the solution is to inject grout into the cavity below.

Well...

I guess that will sort out the problem of having to live underneath the slab like this.

No chance, he has to call a structural engineer.

The contractor has stated that he has worked on (repaired) concrete floors several floors above ground level. Concrete floors composed of precast, reinforced and tensioned. I have no reason not to believe him since it was the customer that told me about it. Since the building (offices) was still in operation he also said the hardest part of the project was to keep gawkers from sneaking up to see what was going on. I wouldn't have believed it unless I heard it from the customer and saw the project after it was finished. No sign of the previous problems. Some things work that we personally don't expect to work.

An example of being presumptive was Alexander Graham Bell's telephone. No one believed it was working and not a trick until it was used by a Chinese language person and Chinese came out of the other phone.

The repair method was spec'd by a structural engineer, the one who designed the building. Shame on him for the poor performance! Alas, "there is more than one way to skin a cat."

Good Luck, Old Salt

I never questioned what you said. But OPs concrete slab is a ceiling. Grout underneath would fill the cavity he is going to live in, otherwise it could not support the weight of the ceiling.

As you can see your solution might have worked for a flooring but will not for OPs problem.

Likewise, the ceiling of one floor is usually the under surface of the next floor above it. If the next level up is the roof, it is still the upper surface (floor) of the ceiling on the underside of it.

Intention was not to give the OP an exact method to cure his specific problem but to open an avenue of thought. If he hadn't heard of this type of company it was to make him aware of the type of skills and work they can do. I have been told that there is a myriad of possibilities for other than sidewalk leveling work that they can do.

Good Luck, Old Salt

The span to depth ratio is high : 550 / 15 = 36. Even for a two-way spanning continuous slab.

The span to depth ratio is to ensure that you don't get undo cracking and so 36 is just over the 35 limit set by BS 8110. If you don't respect this limit, then the engineer needs to do a crack width analysis. I'm guessing this hasn't happened. You said the slab was based on a similar design ?

The fact that you are getting small cracks beneath means that this could be a contributory factor. Along with the plasticiser and maybe not respecting the curing times for the concrete with that particular plasticiser.

If the slab is designed as continuous - deduced from the reinforcement top and bottom as you have said - then you may have micro cracks on the surface durectly over the beam which we see in the photo. Maybe not. Depends on the span directions which we can't see. On the top side you may not see the cracks very easily. Have a good look directly over the beam. But you probably have a water proofing system over the slab and so that will not allow this. Any cracks on the top surface will harm the waterproofing as they are designed for certain cracking and not more.

Given that you are in the tropics, snow isn't going to be your problem. But people on your flat roof will be. It is inevitable that, during the life of the roof, a group of people will stand on this roof. Your roof may not be strong enough. The failure will not be progressive ; it will be instantaneous.

The 15cm thick slab does sound a bit thin. Especially in the tropics where 3cm cover reduces the lever arm for the rebar. And the additional rebar you mentioned is placed on top of the mesh and so reducing its effective lever arm. With only 15cm to start with, you are down to less than 10cm as lever arm. This does not bode well for the crackwidth calc.

Other common errors are :

- not putting in the lap bars for the mesh overlaps - check the photos you have to see if they are in place

- areas where additional rebar was required (over the beam for example) was it put on the top layer or did they get it the wrong way up - this will increase the deflection in the mid span

So, all in all, quite a lot to look into. You can see why an experienced engineer is needed to look at it. Make sure you pay him/her and that you get an invoice and report that says they have looked at the slab and that they say yes/no so that it is then insured on their professional liability.

And in answer to your question, yes you can probably save the slab in putting in a couple of steel beams. Cut holes in the concrete walls and slot them in. Then dry pack. Number and position of beams depends on what your SE says.