I have to replace part of a floor downstairs. It’s the base of the house, a poured cement slab. I’ve been using bags of concrete. It seems the texture is more coarse with the concrete. I’ve filled in about half the depth, maybe 70% of the depth of the trough. Can I switch to cement to do the top part, so that I can smooth it over just like the existing parts? If I stick with concrete, how might I go about attaining a surface smooth enough to lay down ceramic tile?
Can I even LAY down a layer of cement over a layer of concrete? I mean… I’m hoping the two will just…stick to each other fine, and there won’t be any weird settling in.
I’ve mixed concrete before, and so chose that but as I’ve been finishing up the first “layer”, I realized that the surface is coarse. Smooth baby, I need smooth.
I kind of think that cement is the stuff that holds concrete together, rather than something you’d use entirely on its own. I would assume that the “cement” you’re talking about is really just concrete made with sand as an aggregate instead of lumpy gravel.
I’m not an expert cement finisher, but I think you are confused as to terminology. Cement is the manufactured fine, powdery stuff that is mixed with an aggregate, size-graduated sand and gravel, to form concrete. In order to make the surface smooth with any concrete you need to use a large trowel or flat board and pat the surface until some of the fines (i.e. the cement and sand, are brought to the surface. After the concrete has partially set you can then further work it with a trowel or finishing machine to get the degree of smoothness you want.
Make a test form, pour in a bag of ready-mix and practice doing patting to bring the fines to the surface. It’s easy once you get the hang of it.
As to putting a top layer over already set-up concrete, I think you have to use some kind of surface primer in order to get good results.
I’m pretty sure the only way to end up with a smooth concrete surface is to chip it out down to dirt and then pour it in one fell swoop, and finish it with a concrete float, which knocks all the aggregate down below the surface and gives the smooth finish. Concrete doesn’t do so well when poured in layers with bag concrete, which is intended for fence posts.
That’s the word I couldn’t bring up, the process I called patting with the board or trowel is called “floating.” I just knew someone would be along with the right term.
Cement is the binder that holds concrete together. Concrete is cement plus sand and small rocks (“aggregates”). Cement by itself is a fine dusty powder that chemically reacts with water to glue the aggregates together to create concrete.
My experience of concrete is limited, but I believe you can get it in different grades, some with fine aggregates only for a smooth surface. I do remember that when you pour concrete, you can trowel and smooth the surface. The larger aggregates sink downwards, and the sand rises, yielding a smoother surface. I think that this is dependent on how much water there is in the mix.
You shoould be able to float / trowel the concrete’s surface smooth enough to lay tiles on. If the overall floor’s not flat, there are vinyl/concrete blends expressly made for levelling floors.
Adding a layer if topping mix may work if the existing slab is sound and free of bond breakers, laitance, or other coating. The bond may be improved by shot peening, hammer scabbling, or scarification, but all three require specialized equipment and are dirty operations. Some topping mixes direct you to apply a modified admixture to the existing slab prior to adding the new pour. Ardex and Conproco are two companies with extensive lines of product for rehabbing and repairing concrete. Ardex K-15 is a pourable self-leveling topping mix which is a suitable substrate for ceramic tile. Both companies’ products are available from redi-mix dealers-you’ll have to call around to see who carries what. To do the job right, it wouldn’t hurt to get a copy of the TCA Handbook (Tile Council of America) which covers all of the dos and don’ts for ceramic installations, including surface prep.
Very cool, thanks so much !! Yes, I’ve heard of floating. I’ve been shown how to do that with an outside project, but did not know there were devices ( besides trowels ) to use to accomplish this. Considering the narrow area - it’s a channel that is now between 2-5 inches deep, runs in a jagged “L” shape , about 12 inches wide and roughly 6 feet long total, I am wondering now if I need to use a very small trowel and float it slowly to get a smooth good surface.
Indeed, I was thinking of cement with a finer aggregate. We are talking a small volume here, but I do need that top surface to be wicked smooth. Of course any minute bumps will show, since I am going to use a standard fairly thick tile cement- and a bump would be…what is the word… amplified? Accentuated?- by the tile cement layer. Bumps, we wouldn’t want.
If I can find a bag or two of a very fine cement, I think that’d do it. Still and all, the suggestion of doing a test to find out how to handle and smooth out the top layer is an excellent one. For all it matters, I can find a fine cement and make just a few cupfuls and put it into a small box, and smooth it to and fro and figure out how to work with it. Or, is this too small a volume to show me it’s characteristics?
Danceswithcats, that’s great stuff ! To find a topping layer material that is best suited for use as a substrate beneath tile is just what I want- and worth waiting to find it. I’m not far from a store that sells nothing but tiles and tile cements, spacers, grout. What a country. " Grout, you have a call on Line 5".
I’ll look for the Ardex K-15- but, I am levelling it against the surrounding orignal cement slab. I have to hope that was poured and levelled, and so any self-levelling material will settle in and match to that surface, assuming I’m careful with pouring and trowelling it out.
Here’s a technical page on K-15. Since you’re pouring a thickness greater than 1 1/2", you’ll need to add some 1/8" or 1/4" washed gravel to the mix. Place it to just even with the existing floor and remove excess with a steel trowel. K-15 doesn’t need troweling to produce a smooth surface-when we placed it during a factory training session, floor squeeges were used to move the thick liquid around-after that it did it’s own thing.
Since you’re placing ceramic on top, you don’t want a burnished steel troweled surface-a slightly rough finish will give the tile mortar a better bonding surface. Depending on the size of tile to be placed, you may end up with a fairly thick mortar bed, which is nice for hiding surface irregularities in the existing slab.
If you have small problems like duck ponds to fix, use Ardex SD-F. It can go from featheredge up to 1/2" without aggregate and trowels easily.
If you float the pour the top will be what you are calling “fine cement.” That is, it will be cement and sand with all the larger agregate lower down in the slab. If you intend to tile over the floor you really need to use something like LevelQuick, Ardex K-15 or some other leveling mix. Any local irregularities, bumps, or the joint between the old slab and you new pour will result in rapid wear of the tile at that spot and you don’t want that.
In order to make what you are calling “fine cement” (properly called mortar although mortar can also include gypsum as in plaster or stucco) you need to buy cement and sand, both of which are available in bags. I’ve forgotten the ratio of sand to cement but I think it’s something like 7 to one. Don’t take that as true and you can probably search the internet for it. Or someone will soon post it.
Small depressions in a piece of flatwork resulting from less than stellar finishing. They may be unnoticeable when the slab is dry, but when wetted, the duck ponds are the last to evaporate, or may hold standing water, making them woefully obvious.
I’ll go out in a bit to find this fine product. And read directions so very carefully.
The trough is in a place where- were there uneven dips- one’s first step into the bathroom would likely grind down against the area. Not acceptable. Must be smooth and level in the extreme.
Which brings me to the embarassing issue of shrinkage. Let’s say I just go from novitiate to master in one swipe and pour out and smooth out a very acceptable patch, so that the level I use just skims over the joint areas. When the newest and last layer of this material dries, will it shrink- thus giving me a perfectly smooth top layer that’s 1/16 inch lower than the old floor? If shrinkage IS an issue, how does one avoid such?
