Screeding

The top surface of a floor slab, sidewalk, or pavement is rarely placed at the exact specified elevation. Screeding brings the surface to the required elevation by striking off the excess concrete. Two types of screeds are used in concrete finishing operations: the hand screed and the mechanical screed.

Hand Screed

Hand screeding requires a tool called a straightedge. This is actually a templet, usually a 2 by 4; having a straight lower edge to produce a flat surface or a curved lower edge to produce a curved surface. Move the screed back and forth across the concrete using a sawing motion, as shown in Figure 65. With each sawing motion, move the screed forward an inch or so along the forms. This forces the concrete built up against the screed face into the low spots. If the screed tends to tear the surface, as it may on air-entrained concrete due to its sticky nature, either reduce the rate of forward movement or cover the lower edge of the screed with metal. This stops tearing the tearing action in most cases.

You can hand screed surfaces up to 30 feet wide, but the efficiency of this method diminishes on surfaces more than 10 feet wide. You must screed the surface a second time to remove the surge of excess concrete caused by the first screeding.

A mechanical screed as shown in Figure 66 usually consists of a beam (or beams) with a gasoline engine or electric motor and a vibrating mechanism mounted in the center of the beam. Most mechanical screeds are quite heavy and usually equipped with wheels to help move them around. You may occasionally encounter lightweight screeds not equipped with wheels. These are easily lifted by two crew members and set back for the second pass if required.

The speed at which to pull the screed is directly related to the slump of the concrete; the less the slump, the slower the speed; the more the slump, the faster the speed. On the finishing pass of the screed, there should be no transverse (crosswise) movement of the beam; the screed is merely drawn directly forward riding on the forms or rails. For a mechanical screed, a method is provided to quickly start or stop the vibration. This is important to prevent over vibration when the screed might be standing still.

Mechanical Screed

The mechanical screed is being used more and more in construction for striking off concrete slabs on highways, bridge decks, and deck slabs. This screed incorporates the use of vibration and permits the use of stronger, and more economical, low-slump concrete. It can strike off this relatively dry material smoothly and quickly. The advantages of using a vibrating screed are greater density and stronger concrete. Vibrating screeds give a better finish, reduce maintenance, and save considerable time due to the speed at which they operate. Vibrating screeds are also much less fatiguing to operate than hand screeds.

Concrete is usually placed 15 to 20 feet ahead of the screed and shoveled as close as possible to its final resting place. The screed is then put into operation and pulled along by two crewmembers, one at each end of the screed. It is important to keep sufficient concrete in front of the screed. Should the concrete be below the level of the screed beam, voids or bare spots will appear on the concrete surface as the screed passes over the slab. Should this occur, place a shovelful or so of concrete on the bare spot, and lift up the screed and ease it back past this spot for a second pass. In rare cases, the screed crew will work out the void or bare spot with a hand-operated bull float, rather than make a second pass with the screed.

The vibration speed will need to be adjusted for particular mixes and different beam lengths. Generally, the stiffer the mix and the longer the beam, the greater the vibration speed required. The speed at which the screed is moved also affects the resulting finish of the slab. After a few minutes of operation, a satisfactory vibration pulling speed can be established. After the vibrating screed has passed over the slab, the surface is then ready for broom or burlap finishing.

Where possible, it is advisable to lay out or engineer the concrete slab specifically for use of a vibrating screed. Lay out forms in lanes of equal widths, so that the same length screed can be used on all lanes or slabs. If possible, any vertical columns should be next to the forms, so that the screed can easily be lifted or maneuvered around the column.

There are three important advantages of using a vibrating finishing screed. First, it allows the use of low-slump concrete, resulting in stronger slabs. Second, it increases the density of the concrete, resulting in a superior wearing surface. And third, in the case of floor slabs, troweling can begin sooner since you can use drier mixes, which set up more quickly.