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 machine

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 thFigure 1e 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 trowelingFigure 2 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.

 Machine movement

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 Figure 3pattern 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 Figure 4can 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.

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