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Old 08-19-2010, 09:57 AM   #1
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Concrete finishing questions

I would be interested in hearing from concrete experts on this. I have been pouring some concrete slab as a base for flagstone. Since the flagstone will be mortared on top of the slab the finish is not important. But I have been trying to finish and edge the slab just to get some experience at doing this. I have been encountering the following issues.

While finshing with steel trowel (after floating) sometimes even thought the surface looks smooth a piece of aggregate pops out leaving a hole. Is this common durng the finishing process. Do I just fill the hole with some fresh concrete or are these filled after the concrete has cured. Should this even be happening.

Similarly with edging - when I run the edging tool it quite often hits the larger aggregates and rips it out leaving a torn edge. I've looked at some online videos on finishing/edging and I never see that happeneing while they are doing it. btw I'm using general purpose pre-mixed concrete (4000 PSI) with 3/8" aggregate.


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Old 08-19-2010, 11:03 AM   #2
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do a quick 'edging' of the slab to push down the rocks, and then follow it with the steel trowel to flatten, followed by the edger once more.
the first edging will push down the rocks and provide some 'cream' to the finish. when you edge the second time, the rock problem will/should not happen since the cream is there.


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Old 08-19-2010, 11:39 AM   #3
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Do not leave holes to be filled later. Trowel over until the hole fills back in.
You should be doing a very basic finish and edging when pouring and then about an hour later when the concrete is partially setup you do your finish trowel job.
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Old 08-19-2010, 12:02 PM   #4
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Thanks for the replies guys. I will try the edging method suggested. I guess I should also poke a trowel inbetween the form and the concrete (for the side) after the first edging to push the aggregate down.

Yes, I have been floating the top first and then finishing with the steel trowel once it starts setting up. For the most part that works well with a good finish except when the aggregate pops up. Maybe I need to float with heavier pressure. I'm also doing a second and third troweling somethings since I still gets some edge marks with the first troweling. Sometimes on those passes the aggregate pops up on what looks like a smooth surface.

I'm using a wood float which leaves tears in the surface but accoding to the video and advice on the concrete network this is a good thing as it allows the bleed water to evaporate. I then finish up with the steel trowel and then a broom finish (to help the mortar stick for the next phase).

One thing I'm a bit confused is that in some online articles and even books they show the finishing process as one step whereas in others it is shown as a two step process (floating + troweling).
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