SECTION VIII. FINISHING
5-56. The finishing process provides the desired final concrete surface. There are many ways to finish concrete surfaces depending on the effect required. Sometimes there is only a need to correct surface defects, fill bolt holes, or clean the surface. Unformed surfaces may require only screeding to proper contour and elevation, or a broomed, floated, or troweled finish may be specified.
5-57. The top surface of a floor slab, sidewalk, or pavement is rarely placed at the exact specified elevation. Screeding brings the surface to the correct elevation by striking off the excess concrete. Using a tool called a screed, which is a template having a straight lower edge to produce a flat surface or a curved lower edge to produce a curved surface, move it back and forth across the concrete using a sawing motion as shown in Figure 5-17. With each sawing motion, move the screed forward a short distance along the forms. (The screed rides on either wood or metal strips established as guides.) This forces the excess 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, which will stop the tearing action in most cases. Hand-screed surfaces up to 30 feet wide, but expect diminished efficiency on surfaces more than 10 feet wide. Three workers (excluding a vibrator operator) can screed about 200 square feet of concrete per hour. Two of the workers operate the screed while the third pulls excess concrete from the front of the screed. Screed the surface a second time to remove the surge of excess concrete caused by the first screeding. This surge is when the concrete seems to grow out of the form into a convex shape.
Figure 5-17. Screeding operation
5-58. If a surface smoother than that obtained by screeding is required, work the surface sparingly using either a wood or aluminum-magnesium float, or a finishing machine. The wood float is shown in use in view 2 of Figure 5-18. Begin floating immediately after screeding while the concrete is still plastic and workable. Floating has three purposes: to embed aggregate particles just beneath the surface; to remove slight imperfections, high spots, and low spots; and to compact the concrete at the surface in preparation for other finishing operations.
Figure 5-18. Wood floats and floating operations
Do not overwork the concrete while it is still plastic or an excess of water and paste will rise to the surface. This fine material will form a thin weak layer that will scale or wear off under use. To produce a coarse texture as the final finish, float the surface a second time after it partially hardens. Use a long-handled wood float for slab construction as shown in view 3 of Figure 5-18. Use an aluminum float the same way as the wood float, to give the finished concrete a much smoother surface. To avoid cracking and dusting of the finished concrete, begin aluminum floating when the water sheen disappears from the freshly placed concrete surface. Do not use either cement or water as an aid in finishing the surface.
5-59. For a dense, smoother finish, follow floating with steel troweling (see Figure 5-19). Begin this procedure when the moisture film or water sheen disappears from the floated surface and the concrete has hardened enough to prevent fine material and water from working to the surface. Delay this operation as long as possible. Too much troweling too soon tends to produce crazing and reduces durability. However, too long a delay in troweling makes the surface hard to finish properly. Troweling should leave the surface, smooth, even, and free from marks and ripples.
Figure 5-19. Steel finishing tools and troweling operation
Avoid all wet spots if possible. When they do occur, do not resume finishing operations until the water has been absorbed, evaporated, or mopped up. When a wear resistant and durable surface is required, it is poor practice to spread dry cement on the wet surface to absorb excess water. Obtain a surface that is fine-textured, but not slippery, by a second light troweling over the surface with a circular motion immediately following the first regular troweling, keeping the trowel flat against the surface. When a hard steel-troweled finish is specified, follow the first regular troweling with a second troweling only after the concrete is hard enough that no paste adheres to the trowel, and passing the trowel over the surface produces a ringing sound. During this final troweling, tilt the trowel slightly and exert heavy pressure to compact the surface thoroughly. Hair cracks usually result from a concentration of water and fines at the surface due to overworking the concrete during finishing operations. Rapid drying or cooling aggravates such cracking. Close cracks that develop before troweling by pounding the concrete with a hand float.
5-60. Produce a nonskid surface by following the floating operation. After waiting 10-15 minutes, broom the concrete before it hardens thoroughly with a straw broom of fibers at least 4 1/2 inches long. The grooves cut by the broom should not be more than 3/16 inch deep. When severe scoring is not desirable, such as in some floors and sidewalks, produce the broomed finish using a hairbrush after troweling the surface once to a smooth finish. However, when rough scoring is specified, use a stiff broom made from either steel wire or coarse fiber. The direction of scoring when brooming should be at right angles to the direction of any traffic.
5-61. The most uniform and attractive surface requires a rubbed finish. A surface finish having a satisfactory appearance can be produced simply by using plywood or lined forms. As soon as the concrete hardens, rub the surface first with coarse carborundum stones so that the aggregate does not pull out. Allow the concrete to cure before the final rubbing with finer carborundum stones. Keep the concrete damp while rubbing. To properly cure any mortar used as an aid in this process and left on the surface, keep it damp for 1 to 2 days after it sets. Restrict the mortar layer to a minimum, because it is likely to scale off and mar the surface appearance.
5-62. A machine finish is needed when the concrete takes its initial set, but still remains in workable condition. Technical Manual (TM) 5-331D describes the operation and maintenance of machine finishers.
5-63. Set both the screeds and vibrators on the machine finisher to produce the specified surface elevation as well as a dense concrete. Generally, a sufficiently thick layer of concrete should build up ahead of the screed to fill all low spots completely. The sequence of the operation is: screed, vibrate, then screed again. If forms are in good alignment and firmly supported, and if the concrete has the correct workability, only two passes of the machine finisher are needed to produce a satisfactory surface. Adjust the second screed to carry enough concrete ahead of it so that the screed continually contacts the pavement.
5-64. Hand floating often does more harm than good; overworking can cause the wearing surface to deteriorate. Scaling is a good example. However, sometimes there may be a need to use a longitudinal float to decrease variations running lengthwise in the surface. Such a float is made from wood, 6 by 10 inches wide and 12 by 18 feet long, fitted with a handle at each end. It is operated on form-riding bridges by two workers. The float oscillates longitudinally as it moves transversely (crosswise). A 10-foot straightedge pulled from the center of the pavement to the form removes any minor surface irregularities as well as laitance. The surface should have no coating of weak mortar or scum that will later scale off. Unless the workers use considerable care as the straightedge approaches the form, it will ride up on the concrete causing a hump in the surface, especially at construction and expansion joints. When the water sheen disappears, obtain the final surface finish by dragging a clean piece of burlap along the pavement strip longitudinally. This operation, called belting, requires two workers, one on each side of the form. Be sure to round all pavement corners with an edging tool, clean out expansion joints, and prepare them for filling.
5-65. As soon as possible after removing the forms, knock off all small projections, fill tie-rod holes, and repair any honeycombed areas. Section XII in this chapter covers concrete repair in detail.
CLEANING THE SURFACE
5-66. Concrete surfaces are not always uniform in color when forms are removed. If appearance is important, clean the surface using one of the methods described below.
CLEANING WITH MORTAR
5-67. Methods of cleaning with mortar includes--
Using a solution. Clean the surface with a cement-sand mortar consisting of 1 part portland cement and 1 1/2 to 2 parts fine sand. Use white portland cement for a light-colored surface. Apply the mortar with a brush after repairing all defects. Then immediately scour the surface vigorously using a wood or cork float. Remove excess mortar with a trowel after 1 or 2 hours, allowing the mortar to harden enough so that the trowel will not remove it from the small holes. After the surface dries, rub it with dry burlap to remove any loose material. Mortar left on the surface overnight is very difficult to remove. Complete one section without stopping. There should be no visible mortar film remaining after the rubbing.
Rubbing with burlap. An alternate method is to simply rub the mortar over the surface using clean burlap. The mortar should have the consistency of thick cream and the surface should be almost dry. Remove the excess mortar by rubbing the surface with a second piece of clean burlap. Delay this step long enough to prevent smearing, but complete it before the mortar hardens. Allow the mortar to set several hours and cure for 2 days. After curing, permit the surface to dry and then sand it vigorously with number 2 sandpaper. This removes all excess mortar remaining after the second burlap rubbing and produces a surface having a uniform appearance. For best results, clean concrete surfaces in the shade on a cool, damp day.
5-68. Completely remove surface stains, particularly rust, by lightly sandblasting the surface. This method is more effective than washing with acid.
CLEANING WITH ACID
5-69. Use this method when surface staining is not severe. Precede acid washing by a 2-week period of moist curing. First, wet the surface and while it is still damp scrub it thoroughly using a stiff bristle brush with a 5 to 10 percent solution of muriatic acid. Remove the acid immediately and thoroughly by flushing with clean water. If possible, follow the acid washing with four more days of moist curing. When handling muriatic acid, be sure to wear goggles to protect your eyes and take precautions to prevent acid from contacting hands, arms, and clothing.
|David L. Heiserman, Editor||
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Revised: June 06, 2015