How to Finish Concrete
by Rick Menefee
RDM Concrete Placement, Inc.
If you are a do-it-yourself type of person and determined to try to do your own concrete work, I strongly recommend you forget it and let professionals do it for you. If that doesn't discourage you, maybe you do have the tenacity to actually get it done.
I say that to try to convince you that concrete is not a trade that can be learned entirely from a book but must be practiced over a lifetime. I've been in the trade for over 25 years and to this day can still say that almost every time I finish concrete I learn something new.
It would be a good idea to start small with a 4 x 4 slab at 4 inches thick, or something like that, so you can get the feel of what it's like to work with concrete. This can be done with about 10 bags of pre-mix concrete.
Establish the Grade
The first thing you must consider is the finished grade or elevation. This is the height of the concrete surface once it's completed. Where exactly do you want it to be? If this slab is to be connected to the door of a house, it must be a minimum of 3 inches lower than, and a maximum of 7.5 inches below, the floor you're stepping off of. These are codes established in most major cities. Check your local building codes.
Consider the Slope
It is usually recommended to slant any outdoor surface away from the house so that water will run off properly when it rains. This is called the slope. The standard slope should be a minimum of 1/8- inch per foot and 1/4-inch per foot maximum.
Setting String Lines
The outline of the slab should then be established by setting string lines to the exact shape of the 4x4 slab. If you already have 3.5 inches beneath the string line when you string it, no digging will be required. If you do not have this minimum clearance below the string line, then it's time to dig out for the forms. Sometimes the ground is so high that you must first dig out the outline of the project just to get down far enough to get the strings into place, then dig out for the forms.
Once a minimum of 3.5 inches is established below the string line, all the way around the outside of the string, the forms can be installed using straight 2x4s turned on their side.
Setting the Forms
The way to do this is to stake the board at the two ends first by placing vertical stakes on the outside of the board at both ends and nailing the board up to the height of the string.
Once this is done, place two "kickers" at 45-degree angles to hold the ends from moving horizontally, and then nail them to the form.
Perfecting the Sub-grade
Once you have all of your forms in place, it is time to perfect the sub grade. I cannot emphasize enough how important this process is. If you don't dig out enough it will add to the potential cracking problem, yet if you dig out too much you may not have enough concrete to complete the job.
The sub grade is simply the area under which the concrete will be placed. For four-inch concrete, it is necessary to have a consistent 3.5 inches dug out in each direction, because if you grade everything to a minimum of 4 inches you will likely run short of concrete. The reason for this is that since you are placing the concrete over an imperfect surface there will be places that are more than 4 inches deep. The closer you get the sub grade to a consistent 3.5 inches, the more accurate your concrete volume will be.
The best way to accomplish this is to use a 2x4 turned on its side as a gauge. This will help to make sure you have a consistent 3.5 inches below your forms in each direction. (A 2x4 is actually 1.5 x 3.5 inches.)
If you did not have to dig out for the forms because the sub grade was already low enough for you to place your strings and forms without digging, you will probably need to add some fill.
A 4x4 pad is something that can be placed using pre-mix concrete bags that can be purchased at any of the home-improvement stores. An eighty-pound bag of pre-mix will cover about 8 square feet, 1 inch thick.
Here are the tools you will need:
- A flat shovel.
- A straight 2X4 long enough to reach from one form to the other.
- A wheelbarrow large enough to mix at least 1 bag of pre-mix.
- A wooden or magnesium float tool.
- An edge tool.
- A finish trowel.
- A light bristle or horse hair broom.
- Access to water.
The best way to do this is to first dampen the sub grade so that when the concrete hits the ground the dry dirt or rock does not take all of the water out of the mix like a sponge. If you are in an area where there is a lot of humidity, and temperatures are below 60 degrees Fahrenheit, many of the tips in this text will be over-kill. Some textbooks speak vehemently against watering the sub grade. I think this may be because they were written back east where things are done a little differently, and the standard of acceptable concrete finishing is generally not as exacting as here in the warm climates.
In the desert we know that we have to dampen the sub grade. Once this is done go ahead and mix the concrete to the specifications recommended on the bag. Of course, if it is too dry to work with, don't be afraid to add a little more water. Keep in mind, though, that the more water you put in the more the mix will shrink as it sets up, and the longer it may take for the mix to set up.
When you have your first wheelbarrow of "mud" ready, go ahead and dump it in the forms, but place it consistently around the bottom of the entire square. Do not try to start on one side and perfect the shape right away as you continue to place subsequent wheelbarrows. The way to use this mix is to keep dumping each wheelbarrow full on top of the previous one so that the final load is placed on the top.
Finishing the Slab
Once you have the forms filled up, drag a straight board across the top of the pad keeping the two ends tight from form to form. If there is a crown to the board, put the crown up so that you will not end up with a dip in the pad that holds water. Keep a little concrete handy in case the surface sinks as the water starts to come off.
Now that the forms are full and your pad is the shape you want it, take a magnesium or wood float and tamp down the entire top of the surface to get the rocks down into the mix. This will allow the surface to be finished smoothly later on. If the tamping process causes the surface to become misshapen, go over it again with the board you used to shape it the first time.
Now you should be looking at a consistently shaped, relatively flat surface. If that is the case, go ahead and use a magnesium or wooden float over the entire surface. To use a float properly, keep it flat as you move it across the surface. Do not try to tilt it very much, as you might with a trowel later, but tilt it just enough to keep the leading edge from digging in the direction you're moving. Do not worry about little lines of water and cream that might form on the outside edge of the tool, as you move it, as long as they are not reshaping the surface. They can be smoothed out in a later phase.
Now leave it alone for a while. You should be looking at a flat surface with only the insignificant water-line marks from your initial float process. There should also be a shiny, watery sheen over the entire surface of the pad. This is where patience comes in.
The Hydration Process
When the watery sheen has evaporated off the surface, the hydration process has begun. This is critical. If you continue to try to shape and perfect the pad before the hydration process gets going, you may not be able to get a good finish on it later. After the water comes off, use an edge tool to get the edge-role you are looking for. Lift up the leading edge of this tool only enough to keep it from digging in the direction you are moving. Use the same edge tool each time.
Using the Edge Tool
Notice that the edge tool, which is made of blue steel, affects the surface of the concrete a little differently than the floats do. This is because the floats are made of magnesium, or wood which is very porous, and helps to pull the water on out and get that evaporation (or hydration) process started. Once you have edged the pad, use the float tool on it again.
Using the Mag Float
This time, keep it as flat as you can just as you did the first time, only now be more attentive to the flatness of the work. If you keep the float completely flat as you move it across the surface you will be able to tell if there are high or low spots. This is how we continue to get the work as flat as possible. If you notice that there is a hole under the float as you move it, do not try to push the float down to make up for it, or you may be digging a bigger hole. If the hole is quite drastic, you might need to add a little more concrete that you have left in the wheelbarrow. If the hole is very slight, check to see if there are high spots that can be pushed with the float over to the hole. This is accomplished by keeping the float flat while allowing any excess material to roll up in front of the leading edge as you move it.
You can also roll some surface cream into the edges in order to cover any rocky areas that may have showed up after the initial edging. Be careful not to push rock into the edge, it may be cause for frustration later.
"Reading" the Concrete
Ideally, each phase of finishing should be done twice. The second time a surface tool is used it should be used from a different direction than it was the first time. This is also a part of the flattening out process. You might notice an imperfection by crossing up your patterns this way that you might otherwise miss.
At this point you should have a pretty good idea of how this project is beginning to shape up. You should also be waiting for the water to evaporate off once again before continuing to the next phase.
Once the water has completely disappeared from the surface, again use the same edge tool on it that you used before and edge it again. Leave little or no marks from the edge tool on the outside of the tool. If you are leaving a deep line on the pad side of the edge tool it will be difficult or impossible to get it out after going much further. A little bit of a line is OK at this time.
Another common mistake often made with this tool is called "rolling the edges." You can spot a rookie a mile away if the edge tool is being pushed down so hard in the early stages that the edge is becoming much lower than the form. This will cause the over-all pad to have a slight bevel, or roll, to it for the entire width of the edge tool instead of being completely flat.
Now the concrete should be starting to change to a slightly lighter greenish color. This indicates that the next phase of the setting up process is beginning to occur. It would be appropriate to try out your trowel on the pad now. The correct trowel to use might be a 4-inch wide rectangular shaped trowel maybe 16-inches long. Yes, you can use a rounded plaster trowel; however, for keeping the surface flat a rectangular shaped trowel is better.
Move the finishing trowel across the surface slowly and continuously while lifting up slightly on the leading edge. If you can move it without lifting it up you may notice that it leaves the surface with a sort of orange peal texture. This is called, "fuzzing". This will help to let the hydration process continue. You can also wipe out your edge tool marks now with the trowel.
Depending on the ambient temperature you might not want to go too far away at this point. The nature of pre-mix concrete makes it difficult to get a really fine broom finish on it. It just usually has a lot more sand in it than you typically see from a quality ready-mix company. It also tends to have a little higher cement content than factory mud. It's often rated at 3500 pounds per-square-inch, or better. Sometimes a mix called pos-mix is available and is a little closer to an actual factory ready-mix.
If you can press down on the surface of the concrete without leaving finger holes, it may be ready for a final trowel attempt. Another indication of what stage it's in is, again, the color. It may be starting to turn a little more grayish than green.
If you do not have a nice consistent edge from the edge tool, this may be the last chance to fix that. If it is necessary to try to fix a bad edge now, try holding the rectangular trowel on it's end radically, and sweep it across the surface of the pad. This can produce a little bit of soft cream that you can put into the bad edge, and then use the edge tool to properly shape it. Be careful not to push down too hard, but be firm. Whatever scratch you cause by doing this must be fixed right away. Try using the trowel with the leading edge picked up a little higher now than before. If you have done a good job of getting the pad completely flat through the repetition of tool usage and the crossing up of your patterns, you should not be seeing many trowel marks from the outside edge of your rectangular trowel. A narrower finishing trowel is usually preferred at this point, maybe a 3-inch by 12-inch trowel.
If your edge is properly shaped there is no need to edge it again. If your are happy with the surface shape - no holes, humps, or trowel marks - there is nothing left to do but wait a while to let the final bit of hydration occur. Let it air a bit.
If you think you are ready to complete the project, take a soft broom, (like a horse hair broom), wet it and gently drag it across the surface in strips until the entire surface has been covered. If the surface gets ripped up too much, trowel it back down right away and wait a while until you can get that fine broom finish on it.
A broom finish is the most common finish for outdoor concrete. City sidewalks tend to be a little rougher than typical residential patios. This is because the cities want to make sure that no one slips on it when it's wet. Cold regions also will broom the concrete much rougher than desert regions simply because of the icy condition that occurs in the winter. Either way it is always preferred and in fact governed by construction codes that a texture must be placed on the surface of all outdoor concrete. This is because concrete with a smooth trowel finish becomes as slick as glass when it is wet. You can literally skate on it with tennis shoes. If someone goes down on your property because of the slippery surface, you could be liable for their injuries.
Once you have completed the pad, you can walk away if the temperature is under 85 degrees or so. If the ambient temperature is any hotter than that, a curing compound should be used to cover the pad so that it will not continue to hydrate too fast. It may also be covered with hay or dark plastic, but these tend to leave hay lines or plastic crease marks.