concrete surface peeling while power troweling 2

[garyinnepa]

While in the final stages of power troweling a floor, where your beginning to get the dark smooth finish, every once in a while the top skin or 1/8 inch of the top surface of the floor will detach and peel off the underneath. At this point the floor is to hard to go back over it with a trowel to try and work it back in. I need to wet the area and the loosened cement in order to fix it. When this happens it takes a considerably long time to fix because besides th visually obvious detachment, you have to check the immediate surrounding area with your trowel where you find that much of that has detached as well. Does anyone have any idea what causes this? Are my blades tipped to much, not enough? There usually is alot of drag on the trowel at this point. I've had it happen under differant pouring conditions. Inside, outside, sun, no sun, jigged and bullfloated, or just screated with no jigging. I wait till the concrete is done bleeding before executing the final finish. I sometimes keep the top from drying to fast by wetting down with water, or covering with plastic, untill its ready for me to start floating. I usually dont get on the floor until I can walk on it without sinking in with my feet.

Thanks gary

 

[dik]

This is usually caused by starting to trowel at the wrong time. Are you using a ride on trowel and with pan floats?

You should not be 'tempering' the surface with water! This can lead to dusting and future spalling.

Dik

 

[msucog]

i have seen this before from two different stand points so i'll pass along my opinions for what it's worth. one is the way that dik mentions and my opinion is that you could be hopping on it too soon. the other was where i saw a mat slab was being poured (3' thick with underlying slab and moisture barrier so no moisture leaving the bottom side) and mechanical power screed was used. the problems did not become completely evident until a few days later when the upper 1/8 inch flaked off. i was present during one of the pours and could only narrow down a few things that i saw as being a problem. one was that the concrete was not being consolidated enough for the rate of placement. secondly, some loads had higher slumps. thirdly, the power screed was causing the water to bleed out in puddles (but it wasn't very obvious) possible because it was over vibrating the surface. once these puddles set, they ended up breaking out later. those were my observations anyway.
maybe try covering with plastic (or don't--seems to generally work fine either way but depnds on weather) and let cure a little longer before beginning to slick it off. as i'm sure you are aware, be careful not to let the slab take off and get away from you though. see if the material testing firm or supplier can send out their concrete expert to provide some input based on what they see. hope this helps.

 

[garyinnepa]

Thanks Dik, I use a walk behind power trowel. First float blades, than finishing blades. The last floor this happened on, It was out in the open, and at the time I started to float it the sun was hitting it pretty hard. If I would have waited any longer before getting on it, I'm sure I would not even have been able to float all the imperfections out of it, it simply would have been to hard. The peeling up does not occur days later. But rather as I'm doing the final finishing. I've had occurences where I finish troweled to soon, while the concrete was still bleeding water, and you culd see the bubbles in the floor, from the water coming up getting trapped under the top skin. If you poke the bubble with a trowel, water comes out just like popping a blister on your skin. This could certainly lead to peeling later on as the water has seperated the skin from the base. The peeling I'm talking about is happening during the very last faze of the finishing. One way you could duplicate what I'm talking about is if you stopped your power trowel, turned it off, and went to lift it off the floor. And instead of turning it a little as you lifted it(moving the blades), you simply lifted stright up, pulled the top layer right off (by suction) You could probably do this with a regular hand trowel as well. This does not happen all the time, but happens often enough to be a concern. And like I've said before, its happening under so many varous conditions, that I'm having trouble pinning down the cause.

 

[dirtsqueezer]

If the sun was hitting it pretty hard when you troweled it, there is a possibility I see. Could be that the top skin of the slab was cured faster in the hot sun than the concrete below it, hardening up first, then breaking and cracking loose into your 1/8" flakes. That seems pretty plausible to me. I don't think there's anything you could really have done about it, if that's any consolation. Maybe if you could move a cloud in overhead...

 

[shin25]

Sometimes, a situation like this may happen for the mix containing silicafume. Because these mixes are very dense and sticky and does not bleed that well. The recommendate solution for this kind of situation is, while finsihing water mist (vapour) needs to be continually sprayed.