Spalling Concrete Yard

[engstructural]

Hi,

I have a residential project where a concrete yard area was poured. Approx. one and a half months later a cold snap came (temperatures 0 to -10 Deg Celceus). The concrete has spalled significantly on the surface and along the edges.

As the concrete was placed for this length of time I would have expected the frost not to have significanlty effected it.

The yard was salted in the frost, I would have expected the salt to have caused more significant spalling but again only whwn the concrete was curing. Along the walls of the house where the concrete has some shelter the concrete is OK.

Anyone any experience of this? Could the main reason for this be due to the quality of concrete and was the forst then the trigger for the damage?

 

[Ron]

What is the depth of spalling? Was the concrete air entrained? How was it finished?

Yes, the salt can affect the surface, but as noted, usually during the early stages, not later.

Does the spalling resemble small flakiness at the surface, exposing the coarse aggregate, and occurring in sporadic patches of perhaps several square feet (0.5 m^2) each?

 

[Zambo]

Ron, nice to see you converting to metric units, square feet etc always give me a problem.

It sounds like there was already something wrong with the surface allowing water to get in (perhaps areas of lamination), but then the freezing has caused spalling.

 

[Ron]

Zambo...yeah, that's likely the culprit. Has to do with both the mix design and finishing process. Screwed either way!

As for the metric, the US toys with the idea but hasn't adopted it (even though there was a congressional edict in the 1870's that declared it to be the US standard!). I taught a course in metric units to contractors for several years. The best thing I could tell them was to get a measuring tape in metric units and match the units on the tape with the units on the drawing!

Brain damage avoided!

 

[beton1]

Generally you do not want to salt in the first season. However, there are questions to be answered first as Ron has initiated. Mix design, supplier, finishing, conditions at pour etc.

 

[engstructural]

The concrete has a brushed finish. Adjacent to the walls of the dwelling etc. it seems to be in perfect condition.

The spalling has occured all over the slab where there was no additional shelter from the walls - it is small flakes at the edges but on the surface of the slab the spalling is made up of small to large(2" dia) patched all over the depth varies but is approx. 5-6mm in the worst cases. Also the corners have broken away as you might expect.

The contractor is an experienced concrete man as stated he has only seen this once before approx. 5 yrs ago in the same area but the concrete was from a diffenent supplier. I have also inspected his work before and it has been fine.

 

[hokie66]

I would call what you describe scaling rather than spalling, but perhaps that is just me.

Causes of scaling can be overtroweling or troweling while bleed water is present, too much water in the mix, too little air entraining, inadequate curing, and using deicing salts too soon in the life of the concrete. Unfortunately, all these possible causes occur frequently.

 

[jike]

How was it cured?

How much air entrainment was provided?

It is not a good idea to put salt down the first year.

 

[engstructural]

The concrete had a brushed finish - after being tamped it was simply brushed with a rough brush. No trowelling.

Tne concrete was likely to be 20N/mm2 compressive strength or possibly 30n/mm2. Air entrained concrete would not have been used. Usually the temperatues are not to cold when we are pouring concrete so this is not required.

 

[concretemasonry]

Air entrainment use is not related to the temperature while pouring and curing. - It is to improve the freeze/thaw resistance during later exposure.

Since you had 0C to -10C, it is obvious that air entrainment should have been used since these temperature swings could be common later in the life of the concrete. Here, even for controlled professional designs, suppliers may require a liability waiver to signed by the purchaser. If it is just a residential driveway, it can be difficult to even buy anything less than 4000 psi air entrained concrete at any price.

Deicers and cycling add up to problems even for air entrained concrete.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Zambo]

If I was supervising a brushed finish I would expect first tamping or screeding to get to the correct level, then trowel to close the surface, then brush for the finish.

Without trowelling I think there may have been surface cracks in the tamped concrete which would then have been hidden by the brushing. Perhaps near the walls the concrete was shaded from the sun at the time of casting which reduced any cracks at that stage.

 

[Ron]

Zambo...just had that argument with an opposing attorney in a court case. You're right. Just floating then brooming doesn't protect the concrete. Too much surface porosity that can come back to haunt. You're procedure is exactly the way it should be....not if only the concrete finishers would listen!