![]() The stucco mix is characteristic of the aggregates and materials available in each location. A more conservative design is to install a control joint over the juncture. Where concrete and block abut, local experience has shown that placing strip lath over the joint and securing it to the concrete and the block, will minimize cracking. This will pre‐align and hide the crack in a nice looking joint. As a precaution, a control joint is recommended at the juncture of block and frame walls. In walls where block construction abuts another type of construction, the dissimilarity of the bases will cause a crack in the stucco. Proper spacing of control joints should follow the guidelines established in ASTM C 1063, “Installation of Lathing and Furring for Portland Cement Based Plaster.” If metal plaster base is used over a block wall, the same recommendations must be followed as with frame construction. This is one of the greatest benefits of using block for a base. Control Jointsīecause stucco and concrete block are similar materials, properly applied and adequately bonded stucco need only be jointed at the control joints in the base wall. Where this method is chosen, control and expansion joints and there necessary plaster accessories must be included in the assembly and installed in accordance with ASTM C 1063. The purpose for the paper is to make sure that there is a uniform un-bonded condition and that the stucco isn’t adhering to the block in some spots and not in others, a condition which promotes cracking. It is extremely important that the right bonding agent be used and that it be applied in strict accordance with the manufacturer’s recommendations.ģ) Metal Plaster Base Assembly – A paper backed, self‐furring metal lath is recommended for this application and must be adequately anchored to the wall. Some are surface applied to the block prior to the application of the scratch coat, while others are integrally mixed into the stucco mortar. Consider the other options available:ġ) Dash‐Bond Coat – A thick slurry of Portland cement and fine sand plus sufficient water that is dashed by hand or machine onto concrete, masonry, or older plaster surfaces to provide a mechanical bond for succeeding plaster.Ģ) Bonding Agents – There are several types and variations of these materials. If a bond problem is suspected, don’t proceed. The Bondīond adequacy must be determined before the scratch coat is applied. Cracks or voids in the mortar joints should be addressed prior to application of stucco. The surface should be clean and free of form oil, loose block, efflorescence, mortar drippings, roof tar, etc. ![]() The wall should be inspected for suitability before plastering. The wall should be properly aligned to eliminate large variations in stucco thickness and plane tolerances as define in ACI 530 Chapter 3. In a block wall that is to receive stucco, the joints are not tooled or struck instead they are cut flush with the block.įloating or sponging of mortar joints is prohibited as this practice reduces the mechanical keying required for good bond. 3) and waiting for the water to be absorbed prior to the application of stucco, the absorption rate can be controlled and the success of the suction bond can be ensured. There is, however, a practical solution to this problem. This condition also can cause additional shrinkage and cracking. Since cement requires water for hydration, this “dry out” removes the necessary ingredient for proper strength gain in the stucco/block interface and good suction bond. A block with a high absorption rate is capable of removing water too rapidly from the stucco mix. ![]() If this condition occurs, the options are to use a dash bond coat, a bonding agent, or if absolutely necessary, a metal plaster base assembly.Īnother consideration for suction bond is the absorption rate of the block. However, if the water beads up as if on wax paper, suction bond will be negligible. If the water is absorbed into the block, suction bond is possible. It is a relatively simple procedure to determine if a block wall will provide suction bond. This bond creates a knitting effect at the interface of the stucco and block. ![]() Suction bond is the absorption of water and Portland cement from the mortar into the surface of the block. This type of block provides little mechanical bond for the stucco, but may still be considered an excellent base if suction bond can be developed. In some parts of Florida the standard concrete block has a smooth texture. This enables the stucco to interlock with the surface of the block providing a mechanical key bond. Properly done, it’s a great combination.Ĭoncrete block to receive stucco should have an open or coarse texture. The two materials are made from the same basic ingredients and are extremely compatible. Concrete block makes an exceptional base for Portland cement-based plaster (stucco).
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