If you are pouring your own concrete the following cement slab finishing techniques should be helpful. The idea is to produce a progressively smoother, more even surface with each progression. You will use a different tool for each step in the process.
Unfortunately it is not common to make a concrete slab to use just for practice. So start small, maybe with a trash can slab. Over time as you master the techniques through experience you will be able to produce professional results.
Concrete is a mixture of Portland cement, sand, ash and aggregate. Aggregate is gravel and is of various sizes. Depending on what mixture you have in your cement, weather conditions will affect the pouring, finishing and curing of concrete. Check with the manufacturer for specific details.
The screeding step should be started almost immediately after pouring the concrete. Screeding levels the concrete with the form and starts forcing larger pieces of aggregate below the surface.
A screeding tool, known as a screed, or strike-off board, is any two by four that is perfectly straight and extends at least six inches beyond the edge of the form for the concrete.
Slowly and firmly run the screed on end over the concrete, filling in low spots until the concrete is level with the top of the form. This is often a two man job, depending on the width of the form, one at each end of the screed.
The next step requires use of a darby. A darby is made by screwing a handle of some type onto a straight piece of one by four. For a longer darby you may want two handles. A darby should be slightly longer than half way across the slab. This is done to fill in the holes and even out marks left over from screeding.
This step will continue the process of forcing the larger aggregate down and leaving the fine cement and sand closer to the surface. Sweep the darby in wide arcs over the surface of the concrete. You should not pass over the slab more than a few times because this can result in all the fine particles and sand forming a weak surface at the top of the concrete slab.
After smoothing the concrete with the first two steps, water will begin to pool on the surface of the cement. This is normal and temporary. It will reabsorb into the concrete.
It is very important to wait until all the water has been reabsorbed before moving on to the next step. Failure to wait will result in a weak surface when the concrete dries.
Edging and Grooving
When all the water is gone the concrete will begin to harden. When a firmly pressed, gloved thumb leaves only a one fourth inch deep impression, it is ready for the next step. Run an edger around the perimeter of the slab.
This will continue to press large aggregate deeper under the surface as well as smooth the edge. Use the edger to round the outside edge of the concrete into a smooth, neat edge.
Slabs need grooves about every four feet. Driveways and garages need them about every ten feet. These grooves help prevent cracking. Use a straight edge to make the grooves.
If the soil underneath moves and causes cracks, these grooves will absorb the cracks in a controlled pattern. To be effective these grooves need to be about one fourth the depth of the slab.
Floating and Troweling
The final steps of floating and troweling remove the marks left by edging and produce the finished surface of the concrete. Floating produces a rougher texture and can be left to harden this way.
If you prefer a smoother surface follow floating with a steel trowel. A household broom or a special concrete broom can be brushed across the surface to give it a texture.
A concrete finishing trowel is made of steel. A wooden float serves the same purpose depending whether or not a smooth surface is desired. If the concrete slab is indoors, a steel trowel will provide a smoother surface.
For outdoor concrete slabs, a wooden trowel will produce a less smooth surface. This may be helpful and desired to prevent slippery conditions on the concrete slab in times of rain, snow and ice.
The concrete will need to set or cure for several days. You can mist it with water throughout this period. Concrete set in direct sun should be covered with roofing felt or some other material for the curing period.
Prolonged exposure to fresh concrete can result in burns to your skin. Wear safety goggles, gloves, long sleeves and rubber boots when working with wet concrete.