7. FINISHING CONCRETE Learning Objective: Upon completing this section, you should be able to describe the finishing process for the final concrete surface. The finishing process provides the final concrete surface. There are many ways to finish concrete surfaces, depending on the effect required. Sometimes you only 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 trowelled finish may be specified. 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 screed. 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 7-44. 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 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. Three workers (excluding a vibrator operator) can screed approximately 200 square feet of concrete per hour. Two of the workers work the screed while the third pulls excess concrete from the front of the screed, You must screed the surface a second time to remove the surge of excess concrete caused by the first screeding. Figure 7-44.-Screeding operation. 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 abetter 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. A mechanical screed (figure 7-45) usually consists of a beam (or beams) and a gasoline engine, or an 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 crewmembers and set back for the second pass if required. Figure 7-45.-Mechanical screed. The speed at which the screed is pulled 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. 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 that sufficient concrete is kept 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, a shovelful or so of concrete is thrown on the bare spot, and the screed is lifted up and earned 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. Forms should be laid out in lanes of equal widths, so that the same- length screed can be used on all lanes or slabs. It should also be planned, if possible, that any vertical columns will be next to the forms, so that the screed can easily be lifted or maneuvered around the column. There are four important advantages of using a vibrating finishing screed. First, it allows the use of low-slump concrete, resulting in stronger slabs. Second, it reduces and sometimes eliminates the necessity of hand tamping and bull floating. Third, it increases the density of the concrete, resulting in a superior wearing surface. And fourth, in the case of floor slabs, troweling can begin sooner since drier mixes can be used, which set up more quickly. HAND TAMPING Hand tamping, or jitterbugging (figure 7-46), is done after the concrete has been screeded. Hand tamping is used to compact the concrete into a dense mass and to force the larger particles of coarse aggregate slightly below the surface. This enables you to put the desired finish on the surface. The tamping tool should be used only with a low-slump concrete, and bring only just enough mortar to the surface for proper finish. After using the jitterbug, you can go directly to using the bull float. Figure 7-46.-Hand tamp (Jitterbug).FLOATING If a smoother surface is required than the one obtained by screeding, the surface should be worked sparingly with a wood or aluminum magnesium float (figure 7-47, view A) or with a finishing machine. In view B, the wood float is shown in use. A long-handled wood float is used for slab construction (view C). The aluminum float, which is used the same way as the wood float, gives 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 cement or water as an aid in finishing the surface. Figure 7-47.-Wood floats and floating operations. Floating has three purposes: (1) to embed aggregate particles just beneath the surface; (2) to remove slight imperfections (high and low spots); and, (3) to compact the concrete at the surface in preparation for other finishing operations. Begin floating immediately after screeding while the concrete is still plastic and workable. However, do not overwork the concrete while it is still plastic because you may bring an excess of water and paste to the surface. This fine material forms a thin, weak layer that will scale or quickly wear off under use. To remove a coarse texture as the final finish, you usually have to float the surface a second time after it partially hardens. EDGING As the sheen of water begins to leave the surface, edging should begin. All edges of a slab that do not abut another structure should be finished with an edger (figure 7-48). An edger dresses corners and rounds or bevels the concrete edges. Edging the slab helps prevent chipping at the corners and helps give the slab a finished appearance. Figure 7-48.-Edger. TROWELING If a dense, smooth finish is desired, floating must be followed by steel troweling (figure 7-49). Troweling should begin after the moisture film or sheen disappears from the floated surface and when the concrete has hardened enough to prevent fine material and water from being worked to the surface. This step should be delayed as long as possible. Troweling too early tends to produce crazing and lack of durability. However, too long a delay in troweling results in a surface too hard to finish properly. The usual tendency is to start to trowel too soon. Troweling should leave the surface smooth, even, and free of marks and ripples. Spreading dry cement on a wet surface to take up excess water is not a good practice where a wear-resistant and durable surface is required. Wet spots must be avoided if possible. When they do occur, however, finishing operations should not be resumed until the water has been absorbed, has evaporated, or has been mopped up. Figure 7-49.-Steel finishing tools and troweling operations. Steel Trowel An unslippery, fine-textured surface can be obtained by troweling lightly over the surface with a circular motion immediately after the first regular troweling. In this process, the trowel is kept flat on the surface of the concrete. Where a hard steel-troweled finish is required, follow the first regular troweling by a second troweling. The second troweling should begin after the concrete has become hard enough so that no mortar adheres to the trowel, and a ringing sound is produced as the trowel passes over the surface. During this final troweling, the trowel should be tilted slightly and heavy pressure exerted to thoroughly compact the surface. Hairline cracks are usually due to a concentration of water and extremely fine aggregates at the surface. his results from overworking the concrete during finishing operations. Such cracking is aggravated by drying and cooling too rapidly. Checks that develop before troweling can usually be closed by pounding the concrete with a hand float. Mechanical Troweling Machine The mechanical troweling machine (figure 7-50) is used to good advantage on flat slabs with a stiff consistency. Mechanical trowels come with a set of float blades that slip over the steel blades. With these blades, you can float a slab with the mechanical trowels. The concrete must be set enough to support the weight of the machine and the operator. Machine finishing is faster than hand finishing. However, it cannot be used with all types of construction. Refer to the manufacturer’s manual for operation and maintenance of the machine you are using. Figure 7-50.-Mechanical troweling machine. BROOMING A nonskid surface can be produced by brooming the concrete before it has thoroughly hardened. Brooming is carried out after the floating operation. For some floors and sidewalks where scoring is not desirable, a similar finish can be produced with a hairbrush after the surface has been troweled once. Where rough scoring is required, a stiff broom made of steel wire or coarse fiber should be used. Brooming should be done so that the direction of the scoring is at right angles to the direction of the traffic. GRINDING When grinding of a concrete floor is specified, it should be started after the surface has hardened sufficiently to prevent dislodgement of aggregate particles and should be continued until the coarse aggregate is exposed. The machines used should be of an approved type with stones that cut freely and rapidly. The floor is kept wet during the grinding process, and the cuttings are removed by squeegeeing and flushing with water. After the surface is ground, air holes, pits, and other blemishes are filled with a thin grout composed of one part No. 80-grain carborundum grit and one part portland cement. This grout is spread over the floor and worked into the pits with a straightedge. Next, the grout is rubbed into the floor with the grinding machine. When the filings have hardened for 17 days, the floor receives a final grinding to remove the film and to give the finish a polish. All surplus material is then removed by washing thoroughly. When properly constructed of good-quality materials, ground floors are dustless, dense, easily cleaned, and attractive in appearance. SACK-RUBBED FINISH A sack-rubbed finish is sometimes necessary when the appearance of formed concrete falls considerably below expectations. This treatment is performed after all required patching and correction of major imperfections have been completed. The surfaces are thoroughly wetted, and sack rubbing is commenced immediately. The mortar used consists of one part cement; two parts, by volume, of sand passing a No. 16 screen; and enough water so that the consistency of the mortar will be that of thick cream. It may be necessary to blend the cement with white cement to obtain a color matching that of the surrounding concrete surface. The mortar is rubbed thoroughly over the area with clean burlap or a sponge rubber float, so that it fills all pits. While the mortar in the pits is still plastic, the surface should be rubbed over with a dry mix of the same material. This removes all excess plastic material and places enough dry material in the pits to stiffen and solidify the mortar. The filings will then be flush with the surface. No material should remain on the surface above the pits. Curing of the surface is then continued. RUBBED FINISH A rubbed finish is required when a uniform and attractive surface must be obtained. A surface of satisfactory appearance can be obtained without rubbing if plywood or lined forms are used. The first rubbing should be done with coarse carborundum stones as soon as the concrete has hardened so that the aggregate is not pulled out. The concrete should then be cured until final rubbing. Finer carborundum stones are used for the final rubbing. The concrete should be kept damp while being rubbed. Any mortar used in this process and left on the surface should be kept damp for 1 to 2 days after it sets to cure properly. The mortar layer should be kept to a minimum thickness as it is likely to scale off and mar the appearance of the surface. EXPOSED AGGREGATE FINISH An exposed aggregate finish provides a nonskid surface. To obtain this, you must allow the concrete to harden sufficiently to support the finisher. The aggregate is exposed by applying a retarder over the surface and then brushing and flushing the concrete surface with water. Since timing is important, test panels should be used to determine the correct time to expose the aggregate. CURING CONCRETE Adding water to Portland cement to form the water-cement paste that holds concrete together starts a chemical reaction that makes the paste into a bonding agent. This reaction, called hydration, produces a stone-like substance—the hardened cement paste. Both the rate and degree of hydration, and the resulting strength of the final concrete, depend on the curing process that follows placing and consolidating the plastic concrete. Hydration continues indefinitely at a decreasing rate as long as the mixture contains water and the temperature conditions are favorable. Once the water is removed, hydration ceases and cannot be restarted. Curing is the period of time from consolidation to the point where the concrete reaches its design strength. During this period, you must take certain steps to keep the concrete moist and as near 73°F as practical. The properties of concrete, such as freeze and thaw resistance, strength, watertightness, wear resistance, and volume stability, cure or improve with age as long as you maintain the moisture and temperature conditions favorable to continued hydration. The length of time that you must protect concrete against moisture loss depends on the type of cement used, mix proportions, required strength, size and shape of the concrete mass, weather, and future exposure conditions. The period can vary from a few days to a month or longer. For most structural use, the curing period for cast-in-place concrete is usually 3 days to 2 weeks. This period depends on such conditions as temperature, cement type, mix proportions, and so forth. Bridge decks and other slabs exposed to weather and chemical attack usually require longer curing periods. Figure 7-51 shows how moist curing affects the compressive strength of concrete. Figure 7-51.-Moist curing effect on compressive strength of concrete. Curing Methods Several curing methods will keep concrete moist and, in some cases, at a favorable hydration temperature. They fall into two categories: those that supply additional moisture and those that prevent moisture loss. Table 7-2 lists several of these methods and their advantages and disadvantages. METHODS THAT SUPPLY ADDITIONAL MOISTURE.— Methods that supply additional moisture include sprinkling and wet covers. Both these methods add moisture to the concrete surface during the early hardening or curing period. They also provide some cooling through evaporation. This is especially important in hot weather. Sprinkling continually with water is an excellent way to cure concrete. However, if you sprinkle at intervals, do not allow the concrete to dry out between applications. The disadvantages of this method are the expense involved and volume of water required. Wet covers, such as straw, earth, burlap, cotton mats, and other moisture-retaining fabrics, are used extensively in curing concrete. Figure 7-52 shows a typical application of wet burlap. Lay the wet coverings as soon as the concrete hardens enough to prevent surface damage. Leave them in place and keep them moist during the entire curing period. If practical, horizontal placements can be flooded by creating an earthen dam around the edges and submerging the entire concrete structure in water. Table 7-2.-Curing Methods | METHOD | ADVANTAGE | DISADVANTAGES | | Sprinkling with Water or Covering with Burlap | Excellent results if kept constantly wet | Likelihood of drying between sprinklings; difficult on vertical walls | | Straw | Insulator in winter | Can dry out, blow away, or burn | | Moist Earth | Cheap but messy | Stains concrete; can dry out; removal problem | | Pending on Flat Surfaces | Excellent results, maintains uni-
form temperature | Requires considerable labor; undesirable in freezing weather | | Curing Compounds | Easy to apply and inexpensive | Sprayer needed; inadequate coverage allows drying out; film can be broken or tracked off before curing is completed; unless pigmented, can allow concrete to get too hot | | Waterproof Paper | Excellent protection, prevents drying | Heavy cost can be excessive; must be kept in rolls; storage and handling problem | | Plastic Film | Absolutely watertight, excellent protection. Light and easy to handle | Should be pigmented for heat protection; requires reasonable care and tears must be patched; must be weighed down to prevent blowing away | Figure 7-52.-Curing a wall with wet burlap sacks. METHODS THAT PREVENT MOISTURE LOSS.— Methods that prevent moisture loss include laying waterproof paper, plastic film, or liquidmembrane-forming compounds, and simply leaving forms in place. All prevent moisture loss by sealing the surface. Waterproof paper (figure 7-53) can be used to cure horizontal surfaces and structural concrete having relatively simple shapes. The paper should be large enough to cover both the surfaces and the edges of the concrete. Wet the surface with a fine water spray before covering. Lap adjacent sheets 12 inches or more and weigh their edges down to form a continuous cover with closed joints. Leave the coverings in place during the entire curing period. Figure 7-53.-Waterproof paper used for curing. Plastic film materials are sometimes used to cure concrete. They provide lightweight, effective moisture barriers that are easy to apply to either simple or complex shapes. However, some thin plastic sheets may discolor hardened concrete, especially if the surface was steel-troweled to a hard finish. The coverage, overlap, weighing down of edges, and surface wetting requirements of plastic film are similar to those of waterproof paper. Curing compounds are suitable not only for curing fresh concrete, but to further cure concrete following form removal or initial moist curing. You can apply them with spray equipment, such as hand-operated pressure sprayers, to odd slab widths or shapes of fresh concrete, and to exposed concrete surfaces following form removal. If there is heavy rain within 3 hours of application, you must respray the surface. You can use brushes to apply curing compound to formed surfaces, but do not use brushes on unformed concrete because of the risk of marring the surface, opening the surface to too much compound penetration, and breaking the surface film continuity. These compounds permit curing to continue for long periods while the concrete is in use. Because curing compounds can prevent a bond from forming between hardened and fresh concrete, do not use them if a bond is necessary. Forms provide adequate protection against moisture loss if you keep the exposed concrete surfaces wet. Keep wood forms moist by sprinkling, especially during hot, dry weather. FORM REMOVAL Forms should, whenever possible, be left in place for the entire curing period. Since earl y form removal is desirable for their reuse, a reliable basis for determining the earliest possible stripping time is necessary. Some of the early signs to look for during stripping are no excessive deflection or distortion and no evidence of cracking or other damage to the concrete due to the removal of the forms or the form supports. In any event, forms must not be stripped until the concrete has hardened enough to hold its own weight and any other weight it may be carrying. The surface must be hard enough to remain undamaged and unmarked when reasonable care is used in stripping the forms. Curing Period Haunch boards (side forms on girders and beams) and wall forms can usually be removed after 1 day. Column forms usually require 3 days before the forms can be removed. Removal of forms for soffits on girders and beams can usually be done after 7 days. Floor slab forms (over 20-foot clear span between supports) usually require 10 days before removing the forms. Inspections After removing the forms, the concrete should be inspected for surface defects. These defects may be rock pockets, inferior quality ridges at form joints, bulges, bolt holes, and form-stripping damage. Experience has proved that no steps can be omitted or carelessly performed without harming the serviceability of the work. If not properly performed, the repaired area may later become loose, crack at the edges, and not be watertight. Repairs are not always necessary, but when they are, they should be done immediately after stripping the forms (within 24 hours). Defects can be repaired in various ways. Therefore, let’s look at some common defects you may encounter when inspecting new concrete and how repairs can be made. RIDGES AND BULGES.— Ridges and bulges can be repaired by careful chipping followed by rubbing with a grinding stone. HONEYCOMB.— Defective areas, such as honeycomb, must be chipped out of the solid concrete. The edges must be cut as straight as possible at right angles to the surface or slightly undercut to provide a key at the edge of the patch. If a shallow layer of mortar is placed on top of the honeycomb concrete, moisture will form in the voids and subsequent weathering will cause the mortar to span off. Shallow patches can be filled with mortar placed in layers not more than 1/2-inch thick. Each layer is given a scratch finish to match the surrounding concrete by floating, rubbing, or tooling or on formed surfaces by pressing the form material against the patch while the mortar is still in place. Large or deep patches can be filled with concrete held in place by forms. These patches should be reinforced and doweled to the hardened concrete (figure 7-54). Patches usually appear darker than the surrounding concrete. Some white cement should be used in the mortar or concrete used for patching if appearance is important. A trial mix should be tried to determine the proportion of white and gray cements to use. Before mortar or concrete is placed in patches, the surrounding concrete should be kept wet for several hours. A grout of cement and water mixed to the consistency of paint should then be brushed into the surfaces to which the new material is to be bonded. Curing should be started as soon as possible to avoid early drying. Damp burlap, tarpaulins, and membrane-curing compounds are useful for this purpose. Figure 7-54.-Repair of large volumes of concrete. BOLT HOLES.— Bolt holes should be filled with small amounts of grout carefully packed into place. The grout should be mixed as dry as possible, with just enough water so it compacts tightly when forced into place. Tie-rod holes extending through the concrete can be filled with grout with a pressure gun similar to an automatic grease gun. ROCK POCKETS.— Rock pockets should be completely chipped out. The chipped out hole should have sharp edges and be so shaped that the grout patch will be keyed in place (figure 7-55). The surface of all holes that are to be patched should be kept moist for several hours before applying the grout. Grout should be placed in these holes in layers not over 1/4 inch thick and be well compacted. The grout should be allowed to set as long as possible before being used to reduce the amount of shrinkage and to make a better patch. Each layer should be scratched rough to improve the bond with the succeeding layer and the last layer smoothed to match the adjacent surface. Figure 7-55.-Repairing concrete with dry-packed mortar. Where absorptive form lining has been used, the patch can be made to match the rest of the surface by pressing a piece of form lining against the fresh patch. View A of figure 7-56 shows an incorrectly installed patch. Feathered edges around a patch lack sufficient strength and will eventually break down. View B of the figure shows a correctly installed patch. The chipped area should be at least 1-inch deep with the edges at right angles to the surface. The correct method of screeding a patch is shown in view C. The new concrete should project slightly above the surface of the old concrete. It should be allowed to stiffen and then troweled and finished to match the adjoining surfaces. Figure 7-56.-Patching concrete. |
