The word “screed” comes from the Middle English word shred, as in a long strip or piece of wood. In England, a screed is used to measure the thickness of plaster or finishing layer of mortar over concrete.
In North America, screed has evolved into what ACI 302 defines as “a tool used to strike off the surface of concrete to a specific grade,” frequently set by edge forms.
Screeds can come in a number of varieties, each with its own purpose and niche, as well as its own benefits and drawbacks.
The bar screed is exactly what it sounds like—a narrow, flat piece of wood or metal (often magnesium) that can be up to 20 feet in length. Magnesium screeds often are preferred by contractors, because wood tends to warp or bend over time. A bar screed typically is used by kneeling down and passing the tool over the concrete.
To eliminate some of the backbreaking work involved in concrete placement, manufacturers make mechanized versions of the bar screed. These machines—the most common type of screed—have upright handles and mounted gas-powered motors that provide vibration. The screed floats on top of the concrete in the middle of the slab away from the side forms. Because it floats in this manner, the operator's skill determines the flatness of the surface. An inexperienced operator can introduce regularly spaced ridges and troughs parallel to the bar.
A truss screed is made with a framework and interfaces with the concrete in the same manner as a bar screed. The framework maintains straightness to allow longer widths. This type rides on the side forms to maintain the grade level. Truss screeds tend to be used most commonly with concrete pavements and can span the whole pavement surface at once, saving time.
The friction screed, also known as a striker or roller screed, functions differently than the previously mentioned screeds. It moves and compacts as it rolls back and forth over the concrete, helping to fill in any low spots. The machine consists of a metal tube that spins in the opposite direction of the screed's movement. “It doesn't pull up cream,” says Dave Mitchell, president, Bunyan Industries, making it suitable for pervious concrete.
One more innovation in screed technology is an articulating roller screed made by Lura Enterprises. It features a universal joint in the middle, allowing the contractor to make crowned or inverted surfaces.
A staple in screed technology has been the laser screed, patented by Somero Enterprises in the 1980s. The laser screed relies on a laser to determine the level of the concrete surface. To make this work, the look of the screed has changed dramatically. In this case, a laser screed is a wheeled machine with an extendable arm that reaches out over the concrete. The screeding occurs as the arm is drawing inward over the concrete. The laser screed considerably improves floor flatness. Because the laser maintains a constant grade across the entire floor, it is not subject to the inconsistencies of eyeballing.
Laser screeds have carved a niche for themselves in large acreage pours such as warehouses. They are fast and accurate, but don't fill low spots well, so the smart contractor learns how to over fill to get around this situation.
Somero has further refined the laser screed model with several types for both larger and smaller slabs. The 17,000-pound, ride-on SXP model can screed 240 square feet per pass, and the smaller walk-behind Copperhead can be used on softer subgrade or over rebar.
Regardless of what type of screed a contractor uses, screeding is only the first step in providing a good concrete flatwork surface. But doing this properly and efficiently makes the rest of the process go smoothly.