Procedures for finishing slabs-on-ground Chapter 7

Chapter Topics Striking off (screeding) the concrete Bull floating or darbying Highway-type straightedge Waiting period Edging and jointing Brooming Floating Troweling

Chapter Topics Machine float and trowel direction and patterns Floating-to-troweling sequence Finishing when setting time varies from load to load Floating edges Walk-behind basics Hand floating and troweling Blade tilt

Overview of Finishing Steps

Finishing to Obtain F-numbers

Striking Off (Screeding) Screeding is the act of striking off the surface of the concrete to a predetermined grade. Immediately done after spreading and consolidating. Types of screeds – Hand held – Vibrating – Laser-guided

Rigid vs. Wet Screeds Support for the screeding process Rigid screeds – Temporary pipe or rail, edge forms, bulkheads, adjacent hardened concrete Wet screed – A ribbon or pad of fresh concrete used as guides in screeding the rest of the fresh concrete Use wet screeds for F F 20/F L 20 or lower and use rigid screeds for higher F-number

Bull Floating or Darbying Smooths the surface removing high and low spots. Done immediately after screeding before bleed water appears. Bullfloat used on large areas. Darby used for congested areas. Use only one and not both (bull float or darby).

Highway Straightedging Use to improve flatness and levelness. Use immediately after screeding before bleed water appears. Use in cutting and filing operation. Use multiple times in different directions.

Waiting Period Further finishing can’t begin until bleeding has stopped and concrete is firm enough to start floating – No visible bleed water on surface – Footprint indentation of ¼ inch or less Finishing can’t be done with bleed water; so remove it if necessary. – Drag heavy rubber hose across surface – Use squeegee to push water off surface

Edging Commonly done on sidewalks, driveways and steps. Leaves a neater, slightly rounded edge that is less likely to chip under traffic or when forms are removed. Before using edges, good practice to use a pointed trowel (vertical sawing motion) to loosen the concrete from the side form. Place edger flat and then tilt up slightly in the direction the tool is moved.

Jointing Jointer or groovers are used to make contraction joints in exterior flatwork; driveways and sidewalks. Mark the location of the joint with a string line or chalk line on opposite side forms or on the concrete surface. To start the joint, push the groover into the concrete and move it forward while applying pressure to the back of the tool. After the joint is cut, turn the groover around and pull it back over the grooved joint to produce a smooth finish.

Brooming Most common final treatment for exterior flatwork; provides slip resistant surface. The rough texture is made in the surface by running the broom crosswise (perpendicular) to the direction of travel. Several textures can be produced by using various types of brooms and different amounts of pressure while the concrete is stiffening. Rough brooming typically follows the bull floating pass. If a fine broom finish is required, the concrete is often floated and lightly troweled.

Brooming Gently place the broom on the opposite edge of the surface and pull the broom toward you. When the broom is close to the near edge, lift it up and return it to the far edge. Let the next pass slightly overlap the previous pass. Always pull the broom, never push it. This allows more of the broom face to contact the concrete surface and gives better control. Pushing the broom may cause surface paste to build up on the broom that can damage the surface during the push. The broom should be dampened but not dripping with water.

Floating After the waiting period, floating is done to: – Embed the large aggregate beneath the surface mortar layer; – Remove slight humps or other imperfections and produce a plane, level surface; and – Compact and further consolidate the surface in preparation for other finishing. Float when bleed water sheen is gone and footprint indentation of ¼ inch or less.

Troweling Immediately after floating. No troweling should ever be done unless the surface has first been floated. Troweling produces a hard, smooth surface that will be easy to clean and maintain. For first troweling, a blade at least 4-3/4 in. wide is preferred, and it should be kept as flat against the surface as possible. For additional trowelings, smaller trowels are used, and these are tilted slightly up in the direction of travel to increase the pressure on the trowel.

Machine Direction and Patterns Use power float and trowels in controlled patterns. Start the first float pass perpendicular to the direction of last finishing procedure. Typically that last procedure is bull floating, so the first float pass is perpendicular to that. Subsequent float passes are perpendicular to previous float passes. Each pass in the alternating direction refines the flatness of the floor until the concrete stiffens to a point where it can no longer be moved. Floating or troweling in only one direction creates a wave, and repeated floating or troweling in only one direction accentuates the wave. These waves lower the floor flatness and levelness and are difficult to remove during restraightening if that procedure is used.

Machine Direction and Patterns For double-riders, the weight of the machine is centered over each rotor that is attached to the blades. This concentrates the weight at the center of the blades, with less weight (and pressure) at the outer edge of the blade. This creates a trough at the center of each rotor. Overlapping each pass by ½ minimizes the depth of the trough left by the previous pass. After the first pass, center the rotor of the machine over the mortar ridge. Each pass erases the mortar ridge left by the previous pass.

Floating Edges If done properly, machine floating the slab edges at least once before the hand finisher begins edge work can save a lot of hand labor. If the edges are low and more concrete must be brought to them, use a ride-on or walk-behind machine on a forward pass along the edge, with the float blades or pan on or over the edge form. If the edges don’t need more concrete, or if you aren’t sure, make a backward pass with the ride-on or walk- behind machine.

Walk-Behind Basics Use the handle to guide the machine. To move left, apply a slight upward pressure on the handle, to move right, press down slightly. To move forward, twist the handle clockwise. To move backward, twist the handle counterclockwise. To rotate in one spot, hold the handle in a neutral position. Do not stay in one spot for too long. Move the machine across the slab in a sweeping motion. Wipe out footprints during each pass.

Blade Tilt For both floating and troweling operations, the angle of the blade is important. For floating, the blade must be absolutely flat to prevent tearing or gouging of the surface. 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 in. above the concrete surface.