Why did cracks form over the rebar grid in my concrete footings?

Question

I am building a house and we just poured concrete 3 days ago and now it has some cracks,

This is the footer with reinforcement :

This is footer after pouring concrete :

These are some cracks that we have now, some of them follow the shape of bars some of them are just random , these are the worst cracks overall :

Notes: Temperature condition now is between 5 to 15 °C in (Baku , Azerbaijan), We used 16mm iron bars for footer, We ussed M30 concrete, We have never put water in concrete after pouring they said its not necessary because its cold. They didn't use vibrator when they poured concrete. The ground is rocky and solid.

Questions: What did we do wrong ? Are these Cracks Dangerous ? What should we do to improve it now ?

Answer 1

This is known as "plastic settlement cracking", a type of shrinkage crack. It's the result of the rebar being too close to the surface. As water settles out of the concrete, it tends to shrink downward. If the rebar is close to the surface the concrete tends to pull around it and gaps appear.

Plastic settlement cracks can form in young concrete, within the first few hours after placing. As water moves upward through the mixture, the denser constituents move downward. This downward movement may be obstructed by the top layer of reinforcement or by the shuttering.

The plastic concrete may arch over the top of individual reinforcing bars, bringing the surface into tension. Cracks may develop at regular spacing and usually follow the line of the uppermost bars, giving a series of parallel cracks; there may also be shorter cracks at right angles over the bars running in the opposite direction.

https://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=19

I'm not an engineer, but I don't think this is a cause for concern. The cracks are shallow and won't affect the performance of your footings. It's obviously not an aesthetic issue. Just bear it in mind for your next pour, be it foundation walls or slabs. As others have mentioned, vibration can enhance settling while the concrete is still fluid and prevent this problem.

If you want to protect against premature corrosion of the rebar, apply foundation waterproofer to the surface after you build your walls. I don't advise waterproofing before you build the walls as this could affect lateral stability.

Answer 2

Potentially there are three issues. The cracks could be caused by any single issue or any mix of the issues.

1. Not enough clear cover over the rebar mat.

2. The concrete mix contained too much water (high slump), which usually is the result of adding water during concrete placement to make it more workable (self- floatable). High slump concrete mix is likely to suffer from plastic shrinkage, which is characterized by excessive shrink/reduction in volume due to rapid replacement of water by the solids through gravity, and crack forms on where the resistance is greatest - the rebars.

3. Surface wasn't protected from the extreme cold/dry weather which causes rapid moisture loss and early setting/drying of the surface layer that led to early drying shrinkage, unlike the mature shrinkage crack, the early drying shrinkage crack starts near the surface layer and at where the physical condition is the weakest - again, over the rebar.

A note for issue 2, if the concrete was properly vibrated, you might have caught the problem early - as the concrete would have been covered by excessive bleeding water, with noticeable settlement right before finishing/ending the job.

I don't claim I am an expert on concreting, but if you pay attention to the issues above, I think the result will be better.

Answer 3

You have a problem. The cracks * are * a risk.

The problem is, that if water penetrates - and it will, quickly or slowly, over months or decades, it will corrode the steel. Corroding steel expands with great force, forcing the concrete open, and exposing the steel to more water flow,and deeper.

Over time, you end up with this kind of problem if you are unlucky....... an entire building collapse because some part of the concrete structural elements don't have the needed strength, and steel and rust are degrading it constantly.

What to do

The problem you have is, stopping water ever getting into those cracks. But not many things guarantee that.

For example you can cover with waterproof cement/concrete, but the place they join will let water in. You can coat with tanking product or rubber/bitumen paint, but it is not likely to protect the work enough over time, and as the ground shifts over the years.

(Normally, water can penetrate concrete, but it doesn't matter. It has to travel through about 60-75mm of concrete to reach the steel. The cement in concrete is very alkaline, and when the water has to get through that thickness of concrete first, the alkali level and access to free water and oxygen effectively stops the steel+water+oxygen process (that causes corrosion), from ever happening. Cracks, however small, are like an opening in the defences, and can allow corrosion to happen instead.)

I had this problem once. The solution we reached was to sandblast or grind the surface of the concrete to get past any surface deposits and powder. Then use a building industry epoxy coating, which bonds to concrete "for the life of the building". We used Sika products. Sikadur 31, 32 and Combiflex (all "Normal") are all suitable Sika products. These are products specifically designed for concrete repair and protection, where there are cracks, degradation, exposed steel, or a risk of penetration+corrosion. We found the 31 a bit stiff,the 32 a bit runny, and the combiflex just right. Its sold as part of a combiflex system, but is also a valid epoxy in its own right, and has a good spreadable texture. Spread it on, and important, not just the crack but a good 60 or 70mm each side of every crack as well. Work it into cracks and into the surface where you can. When it sets, the next day, water won't be able to penetrate and reach the crack without traveling through 70mm+ of concrete, which is the protection the steel should have had. Once set, you can build on it exactly as you would build on the original concrete.

Not cheap, but a lot cheaper than digging and redoing, specialist concrete repair/waterproofing contractors, or finding out the hard way later.