Screeding Concrete

The information on this page is drawn from material produced by NIOSH. The original PDFs are available at the bottom of the page. 6/1/09

Problems with manually screeding concrete

In its simplest form, concrete is a mixture of paste and aggregate (sand and rock). The paste, composed of cement and water, coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a rock-like mass known as concrete. This mixture and the ratios are critical to the workability, integrity and quality of the end product. The increased forces needed to screed/rod and rake can take their toll on the worker. Screeding/rodding is the first step in the process of leveling and smoothing justpoured concrete. In one way or another, screeding/rodding involves pulling a device over the concrete in order to even out and fill in the surface prior to the completion of finer finishing processes such as floating or troweling. The awkward torso bending, high wrist/hand forces could lead to an overuse injury to the back, arm, wrist or hand.

• Lots of forward bending causes awkward torso posture 79% of the time
• Holding the "rod" (2" x 4" board) requires a pinch grip with extreme wrist flexion
• Repetitive hand/arm activity (exceeds the ACGIH TLV for HAL)
• High hand forces are required to pull the "rod" to smooth the concrete
• Frequent arm reaching (greater than 90 degrees) is required

Different tools to screed concrete

Motorized Screed

The motorized screed consists of a blade or plow that floats on the concrete, one or two fuel powered motors that vibrate the blade, and the metal support tubing and handles to hold when pulling/guiding it backwards.

• The design allows for an upright posture 92% of the time
• Pinch grips are eliminated
• Hand arm repetition is reduced to within moderate risk (ACGIH TLV for HAL)
• Carrying the motorized unit is required and forces for lifting are increased
• Requires less effort than hand screeding ²

This type of screed eliminates both screeding in a stooped position and the need for repeated arm and shoulder movements. ²

How It Works

The motorized screed consists of a blade or plow that floats on the concrete, one or two gasoline motors that vibrate the blade, metal support tubing, and handles to hold when you operate it.

It works best for small to medium-sized jobs. ²

Ergonomic Features

Rotating, height adjustable handles are available to accommodate differing height operators. Additionally, an anti-vibration system and a carrying handle are options.

Adjustable handle angles allow for different sized operators to maintain a more neutral wrist posture.

Benefits for the Worker and Employer

A motorized screed should reduce a worker's chance of developing muscle and joint injuries. This equipment greatly reduces the physical effort needed for hand screeding, and eliminates the frequent and prolonged stooping. Little effort is needed to move the plow over the concrete surface.

Screeding with motorized equipment can be faster than hand screeding. Many contractors report improvements in productivity. The vibration of the blade improves the consolidation of the concrete and reduces time spent "bull floating" the surface.

There are some drawbacks. Although you can work around electrical or plumbing stubs, some hand screeding may still be necessary. Also, it can be difficult to move the screed to and from the work location. A single-engine screed weighs around 50 lbs., and can be awkward to lift and carry. Some screeds have a quick-release system to remove the plow from the frame, which makes carrying easier.

Vibration can also be a problem. It is important to protect workers from hand-arm vibration syndrome (HAVS), a nerve disorder that can become disabling. NIOSH measured vibration levels on three types of motorized screeds. Two had the gasoline engine placed at the bottom of the frame and above the plow. One screed had the engine placed on a single shaft, and the operator held the shaft below the engine. Vibration levels for the two types with the engine at the bottom were below the current recommended guidelines to prevent HAVS. The third screed, which was also older and poorly maintained, gave off much higher vibration that could exceed current HAVS guidelines. Higher vibration levels are expected when the engine is connected to the frame or shaft that the operator must grip. When buying a motorized screed, ask about vibration levels and test drive the screed. ²

Cost

• A single-engine motorized screed costs around $1,500. ²
• A twin engine motorized screed is approximately $3800 - $4000 and requires 2 operators.

Best suited for

The single or twin engine motorized screed can be used in medium to large jobs that may have some electrical or plumbing stubbing. If rebar and/or Styrofoam insulation under the slab is called for, the motorized screed is ideal.

For More Information

• Products related to this solution are described at CPWR's Supplement to NIOSH's Simple Solutions.
• Local contractor tool and equipment suppliers or rental companies may be another source of information on products.
• For general information on this solution, check CPWR's Construction Solutions Database and eLCOSH - Electronic Library of Occupational Safety and Health.

Roller Screed

The roller screed is a seamless pipe (6' to 43' long) that spins in the opposite direction it is pulled. It is powered by a motor and when pulled over a surface between guide rails containing the concrete, the roller screeds off the surplus concrete and evens the surface. The continuous rotation compacts the concrete yet brings mortar up to the surface to facilitate the finishing operation. The lead person operates the powerend while a second worker pulls and guides the other end in unison with the lead. The surface usually requires 3-5 passes before ready for finishing. The roller screed can be used on inclines and with a winch to pull it up hill.

• The design allows for an upright posture 17% of the time but refers to slightly leaning back - not bending forward
• No neck bending
• Pinch grips and wrist flexion are eliminated.
• Hand arm repetition is reduced to within moderate risk (ACGIH TLV for HAL)
• Minimal lifting is required
• Heavier models can require up to 170 lbs of pull force to pull the roller back on the first or second swipe inst.
• Lifting the roller from section to section may or may not be required (Heavier roller can be a 2- 3 person lift and upwards of 70-80 lbs.)

Ergonomic Features

Most units have an optional winch system that enables the roller screed to be pulled up inclines. The lighter unit ("Spin Screed") weighs less than 100 pounds.

Cost

The cost of a "roller screed" is approximately $7,000 - $9,000 and requires two operators. The "Spin Screed" costs approximately $1,100.

Best suited for

The roller screed can be used on large, wide surfaces with temporary pipe rails to guide it. It can also be used for tilt-up wall panels and long, narrow and open surfaces. Another advantage is that it can be used on up to 45° inclines (with winch)

Walk-Behind Laser Screed

The "CopperHead" screed is a 660 lb., selfpropelled, laser-guided, two-wheeled machine with a 10' wide vibrating plow. The operator guides the machine into place, puts it into reverse at the chosen speed, guides it over the desired section of concrete, raises the plow off of the concrete (by pushing down on the handles), and guides it to the next section to be screeded. While it is selfpropelled, it can be tiring on the arms to maneuver because of the pushing down on the handles to lever it up off the concrete and just getting it in place for the next section. The laser system maintains the plow at the desired height regardless of the direction the "CopperHead" is traveling. The walk-behind laser screed can be used only where accessible (usually ground floor jobs) and on jobs that have relatively few obstructions and are too small for the larger laser screed.

Benefits

• The design allows for an upright posture 95% of the time
• Pinch grips and wrist flexion are eliminated.
• Hand arm repetition is reduced to within moderate risk (ACGIH TLV for HAL)
• Minimal lifting is required
• Very low push/pull force required

After an average follow-up period of 225 days, one construction company that implemented a laser screed achieved the following results ¹ :

• The CTD incidence, lost days and restricted days rates all remained at zero;
• The ROI for a laser screed is undeterminable at this time due to lack of data to accurately measure;
• The turnover rate (per 200,000 hours worked) changed from 202 to 138 — 32-percent improvement;
• The risk factor score for the affected task was 45 before the laser screed was used, and was 1 afterward — a 98-percent improvement;
• The company found that it could use the laser screed on 63 percent of the square footage it poured. Productivity, measuredin square footage per man hour, increased 40 percent to 45 percent as a result of using the laser screed.

Ergonomic Features

The unit is self-propelled so there are minimal pushing and pulling forces. Easy to move forward and backward. Additionally, it has multidirectional capability which may reduce the material handling demands on the operator.

Cost

The cost of a Copperhead screed is approximately $36,000 and requires only one operator

Best Suited for

The walk behind laser screed can be used in medium to large jobs that may have some electrical or plumbing stubbing. If rebar and/or Styrofoam insulation under the slab is called for, the laser screed is ideal.

Resources

¹ The information in this section is drawn from "Ergonomics Best Practices for the Construction Industry" from Ohio Ergo Best Practices.

² Information in this section is drawn from "Simple Solutions: Ergonomics for Construction Workers" published by NIOSH.

Laser_Screed_tipsheet.pdf
Roller_Screeder_tip_sheet.pdf
Vibratory_Screeding_tipsheet.pdf