Concrete finishing is done after floating to produce a dense, smooth, hard surface. Two or more successive passes may be necessary to produce the desired floor surface. Take the necessary time between successive passes to permit the concrete to increase its set.
Read our previous post if you missed it: A Beginner's Guide to Troweling Concrete - Part 1
Some contractors find it more productive to use two machines:
- The first one equipped with float blades
- and the other with finish blades.
This allows floating and subsequent finishing operations to proceed without delay. The extra machine can also be used as a spare in case of a breakdown.
For both floating and finishing operations, the angle of the blade is important (Figure 1).
Here we can see the angle of the blade is flat. Begin first pass with blade raised slightly (unless an old blade) and with each successive troweling, increase the angle of the blade (maximum should not exceed more than 1" [2.5cm]).
For floating, the blade must be absolutely flat to prevent tearing or gouging of the surface. During the first pass, an old blade may be used flat because of its slight curvature due to wear. If a new blade is used on the first troweling pass, it should be slightly tilted. On each successive pass the angle is slightly increased to exert greater pressure on the hardened surface.
The increased blade pressure provides a dense, durable concrete surface.
When a trowel blade is tilted or pitched at too great an angle, you may encounter chattering. If this occurs, reduce the tilt and continue troweling until the surface is smooth and level. The tilt may again be slightly increased. The tilt is excessive if the leading edge of the blade is more than 1 inch above the concrete surface.
Before floating and finishing begin, it is good to plan the operations to ensure an efficient process. Figure 3 shows a pattern for floating and troweling. Note that the power floating is started in the direction perpendicular to the direction of bull floating.
If a second floating pass is made it should be at a right angle to the first pass. Also, successive troweling operations are perpendicular to initial troweling. Each successive finishing pass should overlap the previous pass by half the width of the machine. This type of finishing pattern will ensure complete slab coverage and minimize surface imperfections.
Multiple Slabs Poured Adjacently
When two different slabs are poured adjacent to each other, the concrete may be softer on one side of the screed location than it is on the other side. In this case the machine should run parallel to the seam where the two concretes come together. The blades should overlap the seam by just a few inches. If a machine is run across the seam, a bump will result.
There will be times when you must deviate from the planned finishing patterns. Some areas will set faster than others and these must be finished in the order they are setting. When outside, concrete in the sun will set faster because of the heat. When inside, concrete near door openings may set faster because of being exposed to the wind. Usually, though, concrete placed first should be floated and troweled first.
When the concrete is ready, float and finish the surface in a regular pattern. Make sure that each successive pass overlaps the previous pass by half the machine width as shown in Figure 2.
Bull floating is done side-to-side while the power floating is done around edge (as shown). Trowel in a north-south pattern on first pass and an east-west pattern on second.
Remember that the machine should not be operated in one spot for very long. The machine is moved across the slab in a sweeping motion—much as in hand finishing.
While the operator’s footprints will be troweled out, he should wear rubber boots to minimize surface disturbance.
Correcting Excess and Valleys
Defects in the surface can be fixed by using a different finishing pattern. Humps and valleys left after floating can be corrected during the finishing process. Low spots are filled by going around them in a clockwise direction, then continuing with a normal pattern. High spots along the form can also be corrected as shown in Figure 3.
Note the direction of machine travel relative to the defect.
When there is a valley, operate the machine in this manner in order to fill it (left) in order to pull concrete into it. When there is excess concrete (right), trowel in a direction that moves the concrete away from the form.
Completing all of these steps and adhering to these guidelines will help ensure that you create the perfect troweled slab.