[salamt]

I need your help to advise me how to repair severe surface cracks while building my house.

The cracks appeared due to a mistake during the casting of a fresh concrete on site to build above grade slab for the first floor (ground floor ceiling). the cracks appeared on the surface withing 24Hrs due to high water content. I have asked for help and opinions from many structural engineers in the workplace and business offices but only one advise from a colleague was convincing. The advise is to use Low Viscosity Epoxy resin for bonding (ex. Sikadur 52). The tricky part is that floor casting (12 Inches Thick) was done in two stages because it was hard to work out this thick layer of concrete. First layer is OK, tested with Schmidt hammer reads 32 that means more than 3600 psi, but the one on top (with the apparent cracks) was giving a very low strength result, Schmidt hammer reads 19 and that means little below 2000 psi. The thickness of this layer is ranging from 2-4 inches. The cracks width is < 5mm.

I hope pictures below help

Thank you

 

[salamt]

Looks like I need to have one more post to be able to add pictures. Sorry for that.

Before Casting Pictures:

Casting on site day (one hour later when I arrived):

 

[salamt]

 

[Daniel Holzman]

Not sure where you are located. You have a 12 inch thick reinforced concrete slab of substantial size, looks more like a commercial building than a house. I would be discussing the cracking with the designer and the contractor, this looks like expensive construction with a poor result, and you certainly want to offer the contractor the opportunity to correct this unfortunate problem.

 

[Canarywood1]

I agree with Daniel, what doe's the contractor say, and was the entire slab poured in 2 stages??

 

[HDS]

Would core samples need to be taken to test the concrete?

 

[Msradell]

No matter how you look at it that's completely unacceptable concrete work for new construction! It's up to the contractor to correct it with appropriate means specified by a structural engineer. The best course of action would be to completely remove it and replace it but I'm sure the contractor is going to fight you all the way on doing that.

In the 1st place it should have all been poured at one time instead of 2 separate pours. At the very least there is a cold joint between the 2 layers which could result in separation over the long-term and that top layer is obviously very poorly done and probably doesn't meet the specifications that were in the design. There is definitely no excuse for poor workmanship like that.

 

[salamt]

Not sure where you are located. You have a 12 inch thick reinforced concrete slab of substantial size, looks more like a commercial building than a house. I would be discussing the cracking with the designer and the contractor, this looks like expensive construction with a poor result, and you certainly want to offer the contractor the opportunity to correct this unfortunate problem.

I agree with Daniel, what doe's the contractor say, and was the entire slab poured in 2 stages??

I have discussed it with Structural Engineers and unfortunately it is very common in the middle east to do this bad behavior of casting slabs in 2 stages. This makes it easier on the contractor to work out the concrete using the vibrator first stage and leveling in the second. The commercial buildings are done properly though. The 12 inches is standard it is a combination of enforced flat slab, beams and hollow blocks beneath the slab.

Would core samples need to be taken to test the concrete?

The concrete test company recommended Schmidt Hammer test where concrete from the first stage passed with 32 N/mm2 and second stage failed with average reading of 19.2 N/mm2

No matter how you look at it that's completely unacceptable concrete work for new construction! It's up to the contractor to correct it with appropriate means specified by a structural engineer. The best course of action would be to completely remove it and replace it but I'm sure the contractor is going to fight you all the way on doing that.

In the 1st place it should have all been poured at one time instead of 2 separate pours. At the very least there is a cold joint between the 2 layers which could result in separation over the long-term and that top layer is obviously very poorly done and probably doesn't meet the specifications that were in the design. There is definitely no excuse for poor workmanship like that.

I could push for that but I don't want to affect the house with vibrations specially these crack are not very structural as the 90% of the enforcing bars were covered by the good concrete.

I have another contractor to fix the issue and we agreed to do the following:
- Sound clean the surface of the concrete.
- Using a small grinder, grind a v-notch no more than 1/2 inch because the cracks are less than 0.2 inch.
- Using Air blower, blow the concrete dust from inside the cracks.
- Gravity feed low viscosity epoxy resin (most probably it will be Sikadur 52)

The Engineers claim this will provide both bonding and strengthening for the concrete slab. I am still waiting for the quotation but not so optimistic and expecting a commercial level figures.

Blaming is not currently working so well because the contractor blames the redimix company and vise versa. I will try to hire 3rd party inspecting company to see if that will work.

 

[WhatRnsdownhill]

the top concrete is garbage and should be taken off and new concrete to spec poured in its place..if a structural engineer that you trust will sign off on the repairs..then good luck..

 

[Canarywood1]

The Schmidt hammer test is not very reliable, a core sample would be much better, done by a testing labratory.

 

[Msradell]

The Engineers claim this will provide both bonding and strengthening for the concrete slab. I am still waiting for the quotation but not so optimistic and expecting a commercial level figures.

Blaming is not currently working so well because the contractor blames the redimix company and vise versa. I will try to hire 3rd party inspecting company to see if that will work.

You shouldn't be the one hiring the 3rd party inspector, the contractor should be the one doing that since he has to provide an acceptable solution to the problem. He is responsible for the problem even if it was a result of the Redimix company. If the problem originated with too much water in the mix as you stated in your 1st post he should've been able to see that by checking the slump of the concrete before placing it!

Since you are the customer is the contractor he has to develop and proves that the solution uses to fix the problem is that acceptable solution by engineering standards and to you!