Power Floating and Troweling Concrete
Efficient Techniques Promise Higher Production
BY CARL O. PETERSON
OPERATIVE PLASTERERS AND CEMENT MASONS
JOB CORPS TRAINING PROGRAM
For 50 years power floats and trowels have been used for finishing large areas of concrete flat- work. Using a 36-inch machine, a cement finisher can finish 700 to 1500 square feet per day depending on job and weather conditions. (In contrast, a cement finisher—with a good back—and hand tools can finish 300 to 600 square feet per day.) In order to achieve the highest production rates and greatest profits, the machine must be used efficiently. Efficiency comes only with knowing the machine and its proper uses.
The size of a machine is expressed by the diameter, in inches, of a circular area outlined by the moving blades. Machines vary from a 20-inch hand held model to an 96- inch riding model. The most common machine size is a walk-behind model, 36- to 48-inch-diameter, which costs from $1500 to $2000. While there are many different machine types, the basic parts and their function remain the same (Figure 1).
The blades of the machine finish the concrete as they are swirled around the surface. Blades are classified as float (10 inches wide), trowel (6 inches wide) or combination according to their basic use. Float blades are wider than trowel blades, with the leading edges turned up so they will not penetrate or tear the fresh concrete surface. The smaller trowel blades apply more pressure to densify the surface as the concrete hardens. Combination blades, 8 inches wide, are used for both floating and troweling operations. A single blade will cost $8 to $14.
The blades are attached to a vertical rotating shaft by a spider assembly. Commonly three but sometimes four blades, equally spaced in a radial pattern, are attached to the spider assembly. A strong, rigid spider assembly is required to maintain vertical and horizontal control of the blades during the finishing operations.
An important safety feature is the guard ring, which allows the operator to move the machine close to wallsand other obstructions without the danger of blade damage or breakage. The guard ring is connected to the engine housing.
Engines for troweling and floating machines are usu- ally gasoline-powered and range in size from 3 to 11 horsepower. Electric motors are available if noise or carbon dioxide is objectionable. The engine drives through a clutch to a vertical shaft rotating the spider assembly and blades in a clockwise motion. Newly developed hydraulic machines are available which eliminate the need for gears and brakes.
The handle, which is solidly connected to the engine, contains the clutch and speed controls and pitch adjustment. Without a safety clutch the blades would stop the instant the operator let go of the handle—but the handle would spin and create a safety hazard. The rotating action of the blades can be adjusted to match the condition of the concrete surface by using the speed control. Blades are always flat during floating, but are tilted during troweling by using the conveniently located pitch adjustment control. Tilting the blades increases the finishing pressure on the concrete surface.
General principles of floating and troweling
Floating embeds coarse aggregate, removes humps and valleys and compacts the concrete surface. The concrete surface must be sufficiently hard and free of bleed water before power floating begins. The concrete should be a bit harder before power floating than it has to be for hand floating it should take only a l/8-inch footprint indentation.
Troweling is done after floating to produce a dense, smooth, hard surface. Two or more successive troweling operations may be necessary to produce the desired floor surface. Allow time between successive trowelings to permit the concrete to increase its set.
To use the same machine for both floating and troweling requires the use of combination blades or clip-on float blades. Some contractors find it more productive to purchase two machines, one equipped with float blades and the other with trowel blades. This allows floating and subsequent troweling operations to proceed with- out delay. The extra machine can also be used as a spare in case of a breakdown.
For both floating and troweling operations the angle of the blade is important (Figure 2). For floating, the blade must be absolutely. flat to prevent tearing or gouging of the surface. During the first troweling 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 troweling pass the angle of the trowel blade is slightly increased to exert greater pressure on the concrete surface.
When a trowel blade is tilted or pitched at too great an angle, an objectionable “washboard” or “chatter” surface will result. If this occurs, reduce the tilt and continue troweling until the surface is smooth and level, after which 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.
The power trowel can be guided in any direction through the handle. First of all, it is important to adjust the height of the handle to suit the operator. The operator is usually most comfortable when the handle rests on his upper thigh.
Guiding a troweling machine is simple: to move left, apply a slight upward pressure on the handle, to move right, press down slightly; to move forward, twist handle clockwise; to move backward, twist handle counter- clockwise; to rotate in one spot, hold handle in a neutral position.
Before floating and troweling begin, it is generally desirable to plan the operations to ensure complete and efficient finishing. 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.
Where a temporary screed is removed and two different placements are made 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.
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 floated and troweled out, he should wear rubber shoes to minimize surface disturbance. Some operators wrap plastic sheeting around the soles of their shoes to prevent the shoes from sticking to the concrete.
There will be times when the operator 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. For example, concrete in the sun will set faster because of the heat and 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.
Defects in the surface may also require the operator to use a different finishing pattern. Humps and valleys left after screeding and bull-floating can be corrected during power floating. Low spots, for instance, are filled by going around them in a clockwise direction, then continuing with the regular pattern. High and low spots along the form are corrected as shown in Figure 4.
Power floating and troweling principles haven’t changed much in the last 50 years. But sometimes we don’t get the best production rates with our machines because we tend to forget the correct use and operation methods. Put this article in the job trailer next to the other reading material—so that it can serve as a reminder.