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abrogard 07-11-2010 06:24 PM

Work A Slab 15 Hours After Pouring?
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My wife and I poured a slab yesterday, about 4 square metres x 100mm.

We're trying to learn. We don't really know how to finish a slab.

The concrete seemed very wet. As best I can understand from reading forums and other web stuff you don't move onto the next stage of finishing until the concrete is dry again: having got wet on the last working when moisture and cement rise to the surface.

But it never seemed to get dry.

The moisture got thicker, more pastier, like a slurry, but it didn't get dry as much as four hours after the pouring, when it was dark and bedtime and we had to give up on it.

I could get a smooth finish but always got a line, like a ridge, left by the corner of the trowel.

We were trying to get that smooth, dark, shiny, slippery surface that you see on professional jobs. Not because we need it or even particularly want it, but so's we know what's what, how to get it, how to not get it, how to get the finish we do want.

But the trowel never got to slipping smoothly over the surface, it was always scratching, making a grating noise, you could feel the particles of sand, even though it was pushing over a thick slurry.

O.K. That sets the scene?

So now, this morning, 15 hours after the pour, I find I can make a fingermark in the concrete. Not 'IN' the concrete itself, but in the layer of slurry on the surface.

So I thought I'd ask. Is that malleable surface slurry there this morning the stuff that we should work to get the 'polished' finish?

Is the whole trouble that the concrete was too wet?

Or is it too cold?

Or did I work it too much whilst trying to do the job?

Or what?

I'll attach a couple of pics. One showing the general finish we've got. Another showing the 'trowel ridges' and another showing the scuffed appearance I created this morning when I gave a tentative swipe with the trowel across this still soft top layer. Which kinda demonstrates how workable (?) it still is 15 hours later.


ab :)

Daniel Holzman 07-11-2010 07:55 PM

First off, you are pretty brave to place your own concrete without help from an experienced hand. That said, the concrete looks like it was too wet to begin with. Did you mix it yourself, or buy transit mix?

Second, it looks like you overworked the concrete. When you first place concrete, it should be as dry as possible, to get maximum strength. Just enough water to make it workable. If you buy transit mix, they know exactly the right proportions of sand, cement, aggregate and water, however sometimes they will supply excessively wet concrete in order to keep it in the mixer longer.

When you first place the concrete, the objective is to get it level with the forms by doing the minimum amount of work to the surface. This usually means screeding with a board. After it has set for a few hours, the surface should be hard enough to walk on if you use a small sheet of plywood under your feet. At that point, you are going to begin your finishing, using either a mechanical finisher or hand finishing technique if you did not rent an electrical finisher. This is where you can really use the help of someone who is experienced, finishing concrete is hard work, and even harder to get that mirror look it sounds like you are looking for.

Under no circumstances do you want to bring water and slurry to the surface, it will cure to a weak, easily broken surface. The best finishers work their concrete very dry, but of course they are experienced and are capable of handling dry, difficult to move concrete. As I said, it is harder than it looks. Specialized finishes require other techiques, if you are interested I suggest you purchase a book on concrete technique, there are numerous tricks to getting a wide range of surface textures.

Yoyizit 07-11-2010 08:07 PM

I'd go looking for an accelerant or some chemical that can fix your curing problems.

Gary in WA 07-11-2010 08:46 PM

You may find the slab totally rough under the slurry:

Be safe, Gary

abrogard 07-11-2010 09:27 PM

Daniel, you can see all that from my pics? Glad I posted them. I wish I could read a surface that way. Can it be explained to me? It'd be useful for checking my own progress.

It's all bad news but it'll be good to get on the right track.

Thanks, GBR and yoyizit. I'll look for accelerants and I'm always looking for information.

Looks like it was too wet. We mixed it ourselves in our old mixers. I measured carefully and there shouldn't have been too much water but I guess the premix was pretty wet and I didn't allow for it.

For all my reading it has been very hard to get concrete (excuse the pun) information. This Forum has given me more in this one question than I've been able to get anywhere else anytime.

I think I've got the guts of it now: DO NOT work until bleeding stops. That seems to be the MAIN thing. That NRMCA thing is the best thing I've seen.

I hope we've still got a useable slab. It is only one corner of a shed slab and will never have heavy traffic on it - though it will have much light traffic in the doorway - it is right in front of the doorway.

I can fix the mixing easily enough. I was slack letting it get like that. First time we've ever worked together, wife and I, on concreting, and first time we've tried working two mixers at once and I just blew my cool, I guess.. got overconfident and didn't think straight.

Thought I had the water measured right and had it waiting in a bucket beside the mixer at the start of each load and thought it was good that I couldn't possibly get too much in there because there was only that measured amount available. Famous last words. Actually, it turns out, the measured amount was too much, probably because of the wet premix.

But I can fix all that. I can make good concrete I think. A demonstration of how dry it can be would be good - readymix I watch delivered onsite here and there is always very runny...

But the finishing - previously I haven't finished it at all. Just screeded, quick wooden float, cover with plastic and leave it.

Told by a man in the business that the strength comes from the finishing and I should finish it. See the dusty top of the stuff I've done before. See the mirror finishes (which we don't want, but it is a goalpost, a marker of the far end of the ability) - and decided to learn about it.

I welcome, of course, any further comments and/or help but I think maybe it has all been said, now - the big, big thing is: 'Don't work until bleeding finishes'.

Hope our slab will be good enough for its job.


ab :)

jogr 07-11-2010 10:16 PM

When you say your premix was "pretty wet" do you mean the bags of dry concrete mix? Those need to be dry until used. If your bags got wet before use then you might have a problem as the cement (the "glue" component of concrete) activates as soon as it gets wet and won't reactivate later (I'm probably not using the right word).

Gary in WA 07-11-2010 10:17 PM

This should keep you busy........

Be safe, Gary

abrogard 07-12-2010 02:29 AM

No, the 'premix' that I'm talking about is a mixture of aggregate and sand. You add 1 part of cement to 5 of it and finish with a 1:2:3 mix. It was wet because it has been raining.

Thanks for the Google search link, it is the best list of hits (just looking at the first page) I've seen on the subject.

I'll read my way through most of it. I'm soaking up info on the subject. I'll maybe publish my findings - or what I learn - on the web later, at the same place I've put up my little 'concrete calculator' -


joed 07-12-2010 08:50 AM

You can't use formula for the water when mixing your own. You have to look at the batch and see if it needs more water. The amount of water used depends on how wet your components are to start with. The mix should not be like soup but more like a very stiff pudding.
It looks to me like the surface was trowelled too much too soon. The top finish will most likely all chip off.

creamaster 07-12-2010 02:49 PM

Not to derail the OP's thread but along the same lines... How long is a good time to wait before removing the forms and trowel the sides if need be? Also what is a good material to use on the forms so the concrete wont stick to the forms, I was told to use motor oil wiped on by someone?

kwikfishron 07-12-2010 02:59 PM

199 Attachment(s)

Originally Posted by creamaster (Post 468564)
Not to derail the OP's thread but along the same lines... How long is a good time to wait before removing the forms and trowel the sides if need be? Also what is a good material to use on the forms so the concrete wont stick to the forms, I was told to use motor oil wiped on by someone?

You spray the forms with diesel.

abrogard 07-12-2010 06:52 PM

I'm feeling really sad about my slab. Do you think I'll need to dig it out now or will it be okay for a year or two?

What's going to happen assuming, what seems pretty certain, it was drastically too wet? The top will be crumbly, dusty? It will crack?

But how quick and how savage is this going to be?

About the formwork:

I've found I don't need any oiling and when I try it the success rate is variable and it's a messy business. I don't need oiled formwork because I use plastic sheet underneath and I just bring it up at the sides, inside the formwork.

That suits my way of working because I'm just doing small jobs at home. I can reuse that formwork wood, then, for other jobs, where I couldn't if it were oiled and spoiled.

Of course, that wouldn't apply to folks doing different work where they've got, for instance, purpose made steel formwork or such.

I just throw it in for what use it may be, wherever, whenever.


jomama45 07-12-2010 11:25 PM

abro -

3 major factors I can see that are major contributors to your concrete issues:

- You're mixing your own concrete, which doesn't have air-entrainment to help capture "bleed" water.
- You're pouring on plastic, which means all the water from the concrete has to rise to the surface, as none is allowed to exit into the sub-base.
- You're covering with plastic before the concrete is finished????? I think that you may have misunderstood something you read somewhere, you don't need/want to cover the concrete until the finishign is complete, and the slab will not be damaged from placing the covering on it.

There really are many, many variables that go into the "finishability" time of concrete, but 15 hours in summer seems FAR too long, even if you're on the North Pole.

To put things into perspective, we placed a bunch of sidewalk today at 1:30 in the afternoon. Finished and broomed by 4:00 in partly cloudy skies & 90% humidity. Forms stripped & back in the trailer by 4:30. Modrate rain by 7:00 with no issues to the new concrete. Remember though, this is reay-mix concrete, so it sets (especially the top surface) substantially faster than site-mixed concrete.

If you're stuck mixing on site, your best bet is to change at least 2 of the 3 issues listed above, or at least try to aquire some bulk "air" from a local ready-mix plant.

stadry 07-13-2010 04:44 AM

if conc were left to the finishers, it'd all be self-consolidating AND self-leveling :laughing: no longer would they need trowels, kneeboards, mason's brushes, or helicopters ( troweling machines ).

' brave ' combined w/ignorance can be deadly especially w/conc,,, unless you can develop the good ' eye ' & ' hand ' of a concretist :whistling2: find some experienced help, hire 'em, watch, & LISTEN :yes:

for the most part, all our work has a broom finish ( driveways & sidewalk ) even after the edging's done,,, far's the wet ' pre-mix ', plants have the same issues & adjust water accordingly,,, to my knowledge, there isn't a densifyier made that'll turn weak paste ( cement/sand/water } into an acceptable hard dust-free wearing surface :no:

abrogard 07-13-2010 05:29 AM

I'm in the winter here. South Australia. Daytime temp about 15 degrees Celcius.

The slab is under cover, in a shed.

I don't put the plastic on top until after finishing. Normally that's very, very little finishing that I do. I've just tried to be extra clever on this one and run into the wettest mix I've ever done, too.

Tried to hire experienced help but the guys on the concreting gangs are working so well for such good money they won't accept hourly work - they'll only give quotes for doing the whole job for you.

They quoted for a 6 metre x 9 metre x 100mm slab $AU 1000 - that's just for the pouring and finishing, I do everything else, formwork, plastic, reo, get the readymix.

How's that compare with the scene in the States I wonder?

That air entrainment thing's interesting. It is designed to catch water? To keep the slab damp? To improve curing?

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