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STUCCO

LEARNING OBJECTIVE:

Upon completing this section, you should be able to identify the composition of stucco, and state the procedures for mixing, applying, and curing.

"Stucco" is the term applied to plaster whenever it is applied to the exterior of a building or structure. Stucco can be applied over wood frames or masonry structures. A stucco finish lends warmth and interest to projects.

COMPOSITION

Stucco is a combination of cement or masonry cement, sand and water, and frequently a plasticizing material. Color pigments are often used in the finish coat, which is usually a factory-prepared mix. The end product has all the desirable properties of concrete. Stucco is hard strong, fire resistant, weather resistant, does not deteriorate after repeated wetting and drying, resists rot and fungus, and retains colors.

The material used in a stucco mix should be free of contaminants and unsound particles. Type I normal Portland cement is generally used for stucco, although type II, type III, and air-entraining may be used. The plasticizing material added to the mix is hydrated lime. Mixing water must be potable. The aggregate used in cement stucco can greatly affect the quality and performance of the finished product. It should be well graded, clean, and free from loam, clay, or vegetable matter, which can prevent the cement paste from properly binding the aggregate particles together. Follow the project specifications as to the type of cement, lime, and aggregate to be used.

APPLICATION

Metal reinforcement should be used whenever stucco is applied on wood frame, steel frame, flashing, masonry, or any surface not providing a good bond Stucco may be applied directly on masonry.

The rough-floated base coat is approximately 3/8 inch thick. The finish coat is approximately 1/4 inch thick. Both are shown in figure 7-18 applied to a masonry surface. On open-frame construction (fig. 7-19), nails are driven one-half their length into the wood. Spacing should be 5 to 6 inches OC from the bottom. Nails should be placed at all corners and openings throughout the entire structure on the exterior.

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Figure 7-18.—Masonry (two-coat work directly applied).

 

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Figure 7-19.-Open-frame construction.

The next step is to place wire on the nails. This is called installing the line wire. Next, a layer of water-proof paper is applied over the line wire. Laps should be 3 to 4 inches and nailed with roofing nails. Install wire mesh (stucco netting), which is used as the reinforcement for the stucco.

Furring nails (fig. 7-20) are used to hold the wire away from the paper to a thickness of three- eighths of an inch. Stucco or sheathed frame construction is the same as open frame except no line wire is required. The open and sheathed frame construction requires three coats of 3/8-inch scratch coat horizontally scored or scratched, a 3/8-inch brown coat, and a 1/8-inch finish coat.

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Figure 7-20.-Several types of furring nails.

Stucco is usually applied in three coats. The first coat is the scratch coat; the second the brown coat; and the final coat the finish coat. On masonry where no reinforcement is used, two coats maybe sufficient. Start at the top and work down the wall. This prevents mortar from falling on the completed work. The first, or scratch, coat should be pushed through the mesh to ensure the metal reinforcement is completely embedded for mechanical bond. The second, or brown, coat should be applied as soon as the scratch coat has setup enough to carry the weight of both coats (usually 4 or 5 hours). The brown coat should be moist-cured for about 48 hours and then allowed to dry for about 5 days. Just before the application of the finish coat, the brown coat should be uniformly dampened. The third, or finish, coat is frequently pigmented to obtain decorative colors. Although the colors may be job-mixed, a factory-prepared mix is recommended. The finish coat maybe applied by hand or machine. Stucco finishes are available in a variety of textures, patterns, and colors.

Surface Preparation

Before the various coats of stucco can be applied, the surfaces have to be prepared. Roughen the surfaces of masonry units enough to provide good mechanical key, and clean off paint, oil, dust, soot, or any other material that may prevent a tight bond. Joints may be struck off flush or slightly raked. Old walls softened and disintegrated by weather action, surfaces that cannot be cleaned thoroughly, such as painted brickwork, and all masonry chimneys should be covered with galvanized metal reinforcement before applying the stucco. When masonry surfaces are not rough enough to provide good mechanical key, one or more of the following actions may be taken:

Old cast-in-place concrete or other masonry may be roughened with bush hammers or other suitable hand tools. Roughen at least 70 percent of the surface with the hammer marks uniformly distributed. Wash the roughened surface free of chips and dust. Let the wall dry thoroughly.

Concrete surfaces may be roughened with an acid wash. Use a solution of 1 part muriatic acid to 6 parts water. Note: Add muriatic acid to the water; never add water to the acid. First, wet the wall so the acid will act on the surface only. More than one application may be necessary. After the acid treatment, wash the wall thoroughly to remove all acid. Allow the washed wall to dry thoroughly.

CAUTION

When your crew members are using muriatic acid, make sure they wear goggles, rubber gloves, and other protective clothing and equipment.

You can quickly rough masonry surfaces using a power-driven roughing machine (such as that shown in figure 7-21) equipped with a cylindrical cage fitted with a series of hardened steel cutters. The cutters should be mounted to provide a flailing action that results in a scored pattern. After roughing, wash the wall clean of all chips and dust and let it dry.

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Figure 7-21.—Power-driven roughing machine.

Suction is absolutely necessary to attain a proper bond of stucco on concrete and masonry surfaces. It is also necessary in first and second coats so the following coats bond properly. Uniform suction also helps obtain a uniform color. If one part of the wall draws more moisture from the stucco than another, the finish coat may be spotty. Obtain uniform suction by dampening the wall evenly, but not soaking, before applying the stucco. The same applies to the scratch and brown coats. If the surface becomes dry in spots, dampen those areas again to restore suction. Use a fog spray for dampening.

When the masonry surface is not rough enough to ensure an adequate bond for a trowel-applied scratch coat, use the dash method. Acid-treated surfaces usually require a dashed scratch coat. Dashing on the scratch coat aids in getting a good bond by excluding air that might get trapped behind a trowel-applied coat. Apply the dash coat with a fiber brush or whisk broom, using a strong whipping motion at right angles to the wall. A cement gun or other machine that can apply the dash coat with considerable force also produces a suitable bond. Keep the dash coat damp for at least 2 days immediately following its application and then allow it to dry.

Protect the finish coat against exposure to sun and wind for at least 6 days after application. During this time, keep the stucco moist by frequent fog-spraying.

Mixing

Mixing procedures for stucco are similar to those for plaster. Three things you need to consider before mixing begins are the type of material you are going to use, the backing to which the material will be applied, and the method used to mix the material (hand or machine). As with plaster, addition of too much of one raw ingredient or the deletion of a raw material gives you a bad mix. Prevent this by allowing only the required amount of ingredients in the specified mix.

Applying

Stucco can be applied by hand or machine. Machine application allows application of material over a large area without joinings (joinings are a problem for hand-applied finishes). To apply stucco, begin at the top of the wall and work down. Make sure the crew has sufficient personnel to finish the total wall surface without joinings (laps or interruptions).

Curing

The curing of stucco depends on the surface to which it is applied, the thickness if the material, and the weather. Admixtures can be used to increase workability, prevent freezing, and to waterproof the mortar. Using high-early cement reduces the curing time required for the cement to reach its initial strength (3 days instead of 7). Air-entraining cement is used to resist freezing action.

COMMON FAULTS

There are times when the finish you get is not what you expected. Some of the most common reasons for discoloration and stains are listed below:

  • Failure to have uniform suction in either of the base coats;
  • Improper mixing of the finish coat materials;
  • Changes in materials or proportions during the work;
  • Variations in the amount of mixing water;
  • Use of additional water to retemper mortar; and
  • Corrosion and rust from flashing or other metal attachments and failure to provide drips and washes on sills and projecting trim.

 

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