Gently place the concrete into the bottom of the mold. Do not push the concrete into position as this cause the aggregate to scratch the bottom of the mold and could move the rebar, wire mesh out of position.
Vibrate the concrete into position throughout the base of the mold. You can gently tap on the sides of the mold with a mallet or hammer. Tap on the underside of the workbench top and/or use a concrete vibrator. We recommend the use of a concrete vibrator, as previously described, as this will ensure even distribution of the concreter throughout the bottom of the mold. Small air bubbles should form on the top of the concrete as it is being vibrated. The air bubbles are the air that was trapped in the concrete rising to the surface as it is being vibrated. Continue to vibrate the concrete until very few air bubbles are being formed.
Vibrating the concrete is critical in order to have the concrete flow around the rebar, wire mesh, fill all the corners and remove any air pockets.
Do not over vibrate the concrete as this will cause all of the larger pieces of aggregate to sink to the bottom of the mold, which will affect the appearance of the concrete countertop and as well if the aggregate is not evenly distributed throughout the concrete it will not lose its optimum strength.
Note: When vibrating the concrete keep an eye on the wire ties that are holding the rebar and wire mesh off the bottom of the mold. If those wire ties slip, the rebar and wire mesh reinforcement will fall to the bottom of the mold.
Take extra effort to vibrate around any mushroom type spacers that you may have installed. The spacer is a spot that is prone to hold an air pocket and not allow the concrete to fully surround it.
Once you have vibrated the concrete add additional concrete to any low spots that may have been created by vibrating. Double check the edges of the mold as this is a common spot for the concrete to be below the top of the mold.
Note: Do not over fill the mold with concrete as this makes for extra effort when using the screed over the surface of the concrete.
Once you are sure that you do not have to vibrate the concrete any more, cut the wires that are holding the rebar and wire mesh in position. Use a pair of snips or side cutters to cut the wire below the surface of the concrete. Move the pieces of wire over the outside edge of the mold so that they are not in the way and will allow you to use the top of the edges of the mold as a rest as you screed the surface.
Using a straight, clean 2 x 4 or piece of aluminum, screed the surface of the concrete in a sawing motion, moving forward along the length of the mold. The screed should push a small amount of concrete in front of it.
Note: Finishing the concrete is where patience is definitely a virtue. One of the most common mistakes made by the novice is to rush the finishing process. The concrete, in order to be finished properly must be allowed to sit for periods of time between each of the finishing processes. If you do not wait until the concrete is ready for each stage of the finishing process all of your previous work will have been wasted.
Once the surface of the concrete has been completely screeded from one end to the other, you can take a small break. The concrete, after about a half an hour should become dull. At this point you should use a magnesium or wood hand concrete float, as shown in Figure E, to smooth the surface of the concrete, ensuring that none of the aggregate is visible.
Note: If, while using the float, water appears on the surface of the concrete, you should stop the float process and wait until the surface becomes dull again.
Figure E - Magnesium hand concrete float
Once you have completed floating the concrete you can take another break and wait for the concrete to be at a point where it can be troweled. Depending on the temperature and humidity in the room where the concrete is resting this may take from a half an hour to an hour and a half. The concrete is ready to be troweled when it gives only slightly when you press your finger against it. You should check the surface every 15 minutes.
A properly troweled surface will have a sheen without any water. If you get water on the surface of the concrete as you are troweling you are too premature with your troweling. Stop the troweling process and wait a little longer.
Initially the steel concrete trowel, as shown in Figure F, should be basically held flat against the concrete. Wait a few minutes and then trowel the surface again holding the trowel at a slight angle.
Figure F - Steel concrete trowel
When troweling, ensure that the edges of the concrete countertop that will be visible when it is installed are perfectly smooth. Any deformities in a visible edge will standout when the concrete countertop is installed.
How to Make Concrete Countertops - Index