concrete pour damaged by heavy rain 3

[doger]

we were placing 4in. class c concrete sidewalk that had 6% air. It started raining hard before we could get it covered with plastic and the covered surface was soaked. Also ,walk that had set for an hour was not covered and the tool marks were obleterated buy the rain.We were able to refinish all of the walk to a decent appearence although some of the surface was sandy.
What can be expected as far as problems and are there any test that would tell us if this would be reliable sidewalk?

 

[Ron]

The most like affect will be surface deterioration in the form of crazing, scaling, or erosion. You mentioned a "sandy" texture at the surface. This indicates part of the cement paste was likely lost, so these areas will be susceptible to erosion/abrasion loss. In areas where the paste was diluted with rainwater but otherwise remained intact, you will see crazing or map cracking due to a high water-cement ratio at the surface and the higher shrinkage associated with it. I will assume you have put a texture finish such as with a light brooming, so this will decrease the appearance of these map cracks, but it won't decrease their occurrence. They'll still be there, just harder to see!

Having said all that, this is a sidewalk so the traffic is light and the consequence of durability issues is also small. Unless someone complains about the aesthetics, it will probably perform OK. Did they get what they paid for? Probably not, but again, the consequences of failure are low.

 

Interesting to know. I am planning a pour for a driveway and sidewalk, and the forecast is light rain. What effect will "light rain" have on the concrete if it rains while finishing, and how long typically do I have to wait before I don't have to worry? It will have a thickened edge of 10 inches, the balance is 4 inches.

 

[Ron]

MagTral...if the rain is light enough so as not to "pockmark" the surface, it will actually be beneficial. Do not let it collect on the surface, however. Once you have done your broom finish, cover the surface with dry sand and let it rain. This will provide critical moisture during the initial curing stage.

 

[bwb]

any comment on what the long term effect is if the surface *was* heavily pockmarked, and then the contractor re-broomed the surface and re-applied curing compound ?

 

[Ron]

To BWB....the typical result would be scaling and/or dusting. You would also see significant map cracking in the long term, hidden only by the broom finish for the short term.

 

This is a timely discussion. My builder recently poured the driveway on my new home and I checked it out today. It is lightly pockmarked from rain with a 'dark spot' or mark approximately every sq inch. Is this serious enough to require repair or should I simply try to get some monetary compensation for the loss in aesthetics?

 

[Ron]

The dark spots you mention could be bleeding from the aggregate, usually a result of concrete that it too wet.

To have the contractor replace the concrete would likely require a more serious breach than just the rain markings on the surface. You could; however, press the point and perhaps get some compensation...not only for loss of aesthetics, but loss of long term surface durability.

 

[kme]

I recently had the same problem but on a thin (3&quot structural slab (Hambro floor system) where it started raining on a 4000 s.f. pour. It continued to rain for most of the day so the finishers had a really tough time. The result is an extremely poor quality surface that has absolutely no durability. Our only recourse on this project is to grind the slab and apply a thin set epoxy topping. If this were an exterior exposure (i.e. a driveway) and this happened to me, I would DEFINATELY remove the entire slab as this is going to cause you nothing but grief.

This had raised an interesting professional issue for me as the project structural engineer though -- who holds responsiblity for the stupid decision to go ahead with the pour (my rebar inspection was 2 days prior to the pour date). What can one do to verify that the structural integrity of the slab is still there (the hambro system typically uses steel joists spaced at 4 feet on center with a 3" concrete topping reinforced with WWM - it is a composite system).

 

[scoopgh]

We had our footings for our new home poured seconds before a heavy rain. The surface has some dark spots and some of the gravel is at surface. After the rain I saw about half in or more water sitting on top of the concrete that remained until it evaporated.
What affect could this have on the concrete?

 

My builder poured footers today for foundation. It rained shortly after. I checked their progress after work and found the footers to be underwater. What long term effect will this have on the structure?

 

[Ron]

twib...assuming your concrete was not actually diluted by the rainwater (that would occur if substantial water was present when the concrete was placed), the water after initial set could actually have a positive effect, since it moderates the curing and provides better conditions to mitigate initial shrinkage.

Make sure the subgrade was not disturbed by the additional water and make sure the top surface of the concrete is relatively "hard". If both of these are OK, it is probably not a big issue (assuming you have good concrete to start).

 

[Jim6758]

Ron is correct. It is not the water that causes the damage. Water is often used as a curing agent. It is the water moving across the surface and eroding it that creates the problem. I've seen fairly moderate rains do extensive damage on concrete surfaces that were on an appreciable slope. The mechanical effort applied to mask the damage can create additional damage.

 

[axle]

i just had my patio done. The surface was nice and smooth, then as the contractor started doing the stamping, it started raining. they tried to cover it but couldn't in time, and the result is bad aethetics and spots. The contractor is unhappy with his work and suggested pouring an additional inch on top and doing it over- are there any problems i could have with that?

 

[shpoindexter]

We just had cement poured for a patio on which brick pavers and blue stone will be placed. Less than 12 hours later 4 to 5 inches of rain fell - the concrete was not covered. Will this ruin the concrete. How strong will it be? What should we do?

 

[doger]

UPDATE: I went back to the concrete side walk that was the subject of my August 2000 post. The whole area the was affected by the heavy rain shows no visible difference from side walk not poured in the rain!