How to Finish a Concrete Slab on Grade
Concrete is not my friend, it ranks right there with my ability to do plumbing. My Dad was awesome, he could do either of these with ease. Neither of these tasks, done by me personally, have given me results I am pleased with. So I hire professionals. For those who are braver than I, here are words of wisdom from those experts.
If you have a small concrete slab or floor and you want to achieve a smooth finish, using a steel trowel is going to be your best bet.
A hand trowel will give you a very smooth concrete finish. Know the proper procedure on how and when to start using it.
Basic steps for finishing concrete are:
After pouring concrete let it firm up enough so you can only press your fingers into surface about 1/4 of an inch.
On your first finishing pass, use a mag float to smooth surface. With moderate pressure, move mag float in a back and forth motion. This motion will change surface texture surface, working up some cream and cement paste.
Once you’ve gone over entire surface, let it dry a little before using your hand trowel.
Start troweling using same back and forth motion as you did with mag float. Slightly tip trowel edge up a bit in the direction you’re moving it. Go over the same area 2 – 3 times and it will start to look smoother.
Don’t worry if you’re leaving a few small trowel lines, you will remove them on next pass.
After troweling entire surface, let concrete dry a bit more.
Trowel surface again same as first time you troweled it. Surface should be drier and harder now. Smooth out your trowel marks from previous pass, surface should be looking smoother.
Continue this process as many times as necessary to get desired smooth finish you want.
If you have a large concrete floor or slab, the best way to get a smooth finish is to use a power trowel.
Learning how to use a power trowel will take some practice.
If you’re renting one, make sure rental company shows you how to start, hold on to, and move machine right and left. This is very important for your own safety.
Most important part of getting your concrete floor smooth using a power trowel is knowing when to start.
If you start troweling too early, you could potentially create humps, dips, or a wavy surface you won’t be happy with.
If you start too late, it’ll be hard to get concrete as smooth as you like.
There’s a couple ways to test concrete to see if it’s ready to start power troweling.
Press your fingers into concrete. If you can only press into concrete about 1/8 to 1/4 inch, this tells you concrete is close to or ready to start power troweling.
If you can only press your fingers into surface 1/8 inch, try walking on the surface. If you only sink into the surface 1/16 to 1/8 inch with your feet, concrete is ready to power trowel.
IMPORTANT: If you start power troweling and you start “sinking” in more than this or power trowel starts digging into surface and creating waves or humps, just stop and wait a little longer then try again.
If concrete seems ok, then run trowel in an east to west motion across the surface, covering the entire floor. Start furthest away from where you want to stop and work your way backwards.
When you’ve troweled entire surface you may have to stop and let concrete “dry out” awhile before going over it again.
Time it takes it to “dry out” will be greatly dictated by outside temperature and if it’s in direct sun.
It should take multiple passes with power trowel to get a very smooth finish. Maybe three to five depending on how fast concrete is curing.
Cross your pattern each time you power trowel surface. If you started with an east/west pattern, go north/south next time and so on.
If you want a non-slip surface on your concrete, then a broom finish is what you’re looking for.
In my opinion, learning how to do a broom finish is a little easier than doing a smooth steel trowel finish.
If you’re broom finishing basics are the same.
Knowing when to start is key to success. Much like above, you start finishing process when:
Start to mag float surface when you can only press your fingers into surface about 1/4 inch.
After you mag float surface, drag broom across surface in a backwards motion pulling it towards you.
Continue this process working your way from one end to the other until you’ve broomed the entire surface.
If you can’t reach the entire area from outside edges, you will need a set of concrete skids to get onto the concrete.
With patience and courage, you can successfully finish a concrete slab!
I have been finishing concrete off and on since I was 18 years old. It is without doubt the most diverse application of elements and differentials that can affect what one would term the perfect finish on a slab. The most important lesson is that set time is relative to so many variables such as temperature, ground moisture, vapor barrier, slump, and sack of mix. There are three other items which affect working time and these are air , excellerants, and super p. The last three can help or hinder when used together. Super P can help decrease the amount water used while increasing workability. However, when used with air Super P can create too much air in the mix . The other term called bleeding is important as it affects when finishing can proceed without endangering top separation of slab. Allowing the bleeding process keeps the water from being trapped when a steel trowel finish traps it beneath the surface.
Thank you for adding your insights!