What is the best way to hand float a small slab

[MoreCowbell]

I am ready to start placing my basement slab doing one small (8'x8') section at a time (thank for the idea BridgeMan) . I am trying to educate myself on the finishing process before i start mixing.

Everything i read says this:
1) Screed
2) Bull float immediately after screeding to push the aggregate down
3) Hand float after bleed water dries
4) Steel trowel to a smooth finish

I'm wondering how to adapt the bull float step to a small slab. Can I just use a wood hand float to do that?

Also, I would certainly welcome any other tips related to what I'll be doing. Thanks.

 

[nealtw]

while you waite for the pros, this is interesting. http://www.agweb.com/article/Tips_to_Better_Concrete_205045/

 

[MoreCowbell]

Thanks nealtw. That was a useful link. My test slab section will be tomorrow evening. My fingers are crossed.

 

[oldognewtrick]

Don't forget pics of the pour.

 

[MoreCowbell]

I'll need to wait another couple of days on that photo. I forgot to grab a shot between troweling and keeping it wet for curing. I pulled the plastic back tonight but the moisture just ruins the shot.

 

[MoreCowbell]

Here are some photos. They don't really show much detail. One slab section is done curing and the new one is just about to get some plastic laid on top.



I have one question... On the second slab section I was not able to trowel the surface as smoothly as I did with the first one. I think I did everything the same. It seems like the aggregate in the concrete was right at the surface and was preventing a smooth trowelling. Instead I got a lot of pinholes.

I'm assuming that I did not float the concrete properly and the aggregate was not pushed down enough. I hit it with the wood float right after screeding, then with the aluminum float after the bleed water dried. Are there any tips or tricks to doing this?

Thanks.

 

[BridgeMan]

Small voids showed up because you didn't bring up enough fines (cement and sand) to fill them. Usually accomplished during the steel troweling step, where you need to really push down hard as you make sweeping passes with the trowel. The concrete surface should "sing" to your trowel, with somewhat of a hissing sound.

 

[MoreCowbell]

Thanks BridgeMan. I just laid my 3rd slab section but I think I've gone from bad to worse (unrelated to my earlier problem). I think I know what I did wrong but I'm hoping to get some expert advice here before doing the next section.

I laid and wood floated the first two sections early in the day so that I was able to wait the 8 hours or so for the bleed water to dry so I can mag float it and then the finish troweling. Those sections came out fine aside from the slight gravely feel during the final troweling I spoke of earlier.

However, I laid and wood floated this 3rd section at night. I forgot about the time it takes for the bleed water to dry. So I stayed up as late as I could and mag floated and trowelled it only a few hours later while the surface was still rather soupy. The problem is that large portions of the surface are peeling off in paper thin flakes. Here is a picture (don't mind the wet appearance - I'm keeping it wet to cure it)...


I suspect that I trowelled too early and worked all that remaining moisture into the surface and weakened it. Is that right? Thanks.

 

[BridgeMan]

The scaling that you experienced on the most recent placement is the result of a water-cement ratio that's way too large, in the upper surface of your pour. And you're right, it was probably caused by working it too soon, instead of allowing the bleed water to first evaporate off. Unfortunately, there's not much that can be done to correct it, other than removal and replacement.

It's also possible that you are simply pouring too wet in the first place, instead of going with a stiffer mix that would result in less bleed water. A 3" slump is close to ideal, and is indicated by gobs of concrete "standing up" on a flat shovel instead of flowing off and flattening. You know you're pouring too wet if, when you spade the mix to consolidate it, it immediately closes up around the spade marks instead of them staying open.

For interior placements like yours, it's often a good practice to set up a fan blowing air across the finished surface, and turn up the heat, all in an effort to hasten evaporation of bleed water. Another option, although I've only done it a few times, is to broadcast dry Portland cement uniformly across the wet surface and then float it in with a hand (resin or mag) float. Doing so enables steel troweling to be done sooner, but there's a possible drawback of over-working the finish and thus also weakening it.

 

[MoreCowbell]

Thanks BridgeMan. I did mix it a little too wet, but where I really went off the rails based on what you are saying is that I trowelled way too early. It was like a slurry I was pushing around.

My next slab section will be this weekend. I'll make sure to not use so much water this time and also wait until the bleed water is gone before trowelling.

I just want to make sure I have the proper sequence. Here is what my understanding is:
1) Screed
2) Lightly float with wood float to flatten
3) After bleed water dries, float with mag float
4) Wait until surface firms up to where finger cannot leave print, then trowel

Do I have that right? Thanks again.