by Catherine Howlett | January 6, 2013 10:14 am
by Nickie Ramm, EIT
Observations of numerous building envelope failures in stucco (i.e. portland cement plaster) indicate it may lack durability when correct installation practices are not performed. One of the most common problems is cracked stucco—and with the increasing trend of using the material as a cladding, the situation will likely get worse.
Historically, stucco has been a durable material, so cracking is not inherent; rather, the observed distress results from design and construction errors such as poor consolidation, inadequate control joints, or non-uniform thickness.
Hydration is essential to achieving durable stucco. It is the principal mechanism of converting portland cement into a solid binder. Improper hydration leads to decreased strength development, layers’ delamination, and cracking. Hydration is attained by incorporating proper curing methods during each step of the application process. American Concrete Institute’s (ACI’s) Guide to Portland Cement-based Plaster suggests curing plaster methods ensuring continued hydration should be used.
Curing of stucco achieves two things: long-term strength development and improved substrate conditions for application of exterior finishes. ASTM C926, Standard Specification for Application of Portland Cement-based Plaster, defines stucco curing as “the act or processes of producing a moisture environment favorable to cement hydration, resulting in the setting or hardening of plaster.” Continual wetting of the material encourages increased hydration of the cement particles. As the percentage of cement particles undergoing hydration increases, the pores containing air and water are replaced with hydration products, resulting in increased strength. Hydration can only be achieved in the presence of water, which is why curing should be required for all products containing portland cement.
Today, curing is often left to the installer’s discretion, despite requirements in project specifications, codes, and standards. As a result, the curing method is guided by popular misconceptions and misinformation. For example, some contractors may believe humidity in the air is sufficient for proper curing, but, this is not supported by national standards.
Stucco requires fully developed tensile strength to resist cracking due to frequent stresses within the system. The plaster must be strong enough to transfer the forces induced by self-weight, wind loads, frame shrinkage, and soil movements. Each of these forces acts independently and can be magnified when they occur simultaneously. Strength is developed over time if the following conditions are met:
The relative humidity is considered the controlling condition; structure is determined by the mix design and rate of hydration. The plaster’s RH should be controlled by implementing proper curing methods.
Many people understand concrete (the most common portland cement system) requires moist-curing to develop full strength, but stucco is more likely to lose moisture than concrete due to the application’s thickness; the thin cross-section is more susceptible to drying out making moist-curing critical to performance.
Providing an adequate substrate
The performance of any stucco application is controlled by the base coat. This consists of either a two-layer application of a scratch and brown coat or a single-layer base coat that performs both functions (called a ‘one-coat’ system, even though an additional finish coat is subsequently applied). The base coat is required to act as the primary cladding layer and the substrate for an architectural finish. Poor curing of the base coat can result in the system’s failure to perform these intended functions.
In a traditional three-coat stucco system, the finish coat must be cured using the same practices as base coats. Moist-curing can be terminated after 48 to 72 hours, but the finish coat should remain uncoated for at least seven days to prevent coating failure or poor bonding. It is not generally recommended to moist-cure proprietary finish coats with integral color, as this can cause discoloration. For proprietary finishing coats, follow the manufacturer’s recommendations.
Experience has shown a lack of curing can result in failures that develop quickly after construction (or before substantial completion). Other long-term consequences of not properly curing stucco—including shortened service life, corrosion of metal lath and accessories, and water penetration—are being researched.
Blistering of the finish coating applied before full curing of plaster can occur, as shown in Figure 1. Stucco, when fresh, has a high pH level (i.e. more than 11), similar to concrete. As it cures, the pH level falls and the surface can accept a finish coating. If applied to uncured stucco, the finish coating can lose adhesion. This finish coat was applied before the base coat was fully cured, resulting in severe blistering (i.e. saponification) an acid-base reaction. Applying the finish coating early can also inhibit curing of stucco by limiting water absorption, resulting in a permanently soft stucco surface.
Widespread cracking associated with shrinkage
Shrinkage cracks occur when the plaster dries out quickly due to moisture evaporation within the mix. Shown in Figure 2, shrinkage is often accommodated with control joints at regular intervals, but in severe drying conditions, control joints are not enough to prevent cracking within the panels.
Delamination (Figure 3) occurs in composite materials when two adjacent surfaces do not achieve an adequate mechanical or chemical bond to resist the stresses imposed. In stucco, this adhesion failure can occur at the plaster-to-substrate interface or between two layers of plaster. Bonding, both to the substrate and subsequent layers, is controlled by proper substrate preparation, application techniques, and the plaster’s sufficient strength gain. In this application, the plaster finish coat was applied before the base coat developed adequate strength, which is dependent on hydration.
To enhance stucco durability, the best choice a designer can make is to specify, and enforce, proper curing in the contract documents. Simply referencing standards is not enough, because they conflict and are not typically stringent. Standards can also be vague and do not refer to the active process of adding water to the surface to prevent it from drying during the specified period. Relying on the weather conditions is not an acceptable curing method.
For example, Northwest Wall and Ceiling Bureau’s (NWCB’s) Stucco Resource Guide only suggests curing the first (i.e. scratch) coat for a minimum of 48 hours. Conversely, Texas Bureau of Lathing and Plastering’s (TBLP’s) Systems Manual does not require moist-curing when the humidity in the air is over 75 percent. Stucco manufacturers typically call for moist-curing stucco a minimum of 48 hours.
There are two widely accepted methods for curing stucco: surface-misting (or fogging) and the double-back method.
The best curing method is surface-misting with water. This includes lightly misting the surface of each layer of plaster two to four times daily for 48 to 72 hours after application. The method can be easily modified—but not eliminated—to accommodate the weather conditions as needed. Portland Cement Association’s (PCA’s) Portland Cement Plaster/Stucco Manual suggests it is generally enough to fog the surface with water at the workday’s beginning and end. The water should be applied as a mist or fog, avoiding over-saturating the plaster; a heavy stream of water could damage the plaster if enough strength has not been developed. Cold water applied to hot stucco can also cause distress from thermal shock.
The misting application should be increased in weather conditions with high drying potential; this includes high temperatures or wind speeds following plaster installation. It is best to shield the plaster from direct sunlight and wind exposure using wind breaks or plastic sheeting. The shield should be used in addition to surface misting, not in lieu of it.
The aim should be to maintain the plaster’s constant dark brown color for 48 hours following installation. If the plaster becomes light gray in appearance, the plaster has experienced excessive drying and will be susceptible to various failures. It is also recommended to pre-wet the surface of the first or second coat before installing the brown or finish coat; this will improve bonding between the layers and improve the scratch coat’s long-term strength gain.
The double-back method means the application of the second (i.e. brown) coat over the scratch coat as soon as the plaster can accept the application without damage. It is the only alternate method permitted by the International Building Code (IBC) to the curing recommendations shown in Figure 4.
There are several benefits to utilizing the double-back method. The brown coat’s application results in the excess mix water being absorbed by the scratch coat. This improves the two layers’ bonding and encourages further hydration of the cement particles in the scratch coat, resulting in improved strength. Additionally, the second layer of plaster increases the plaster’s overall thickness, which decreases the drying potential and provides more pores to retain moisture for curing. (See Thomas C. Geary’s article “Double-back Application of Plaster,” at stuccoguru.com.)
Caution should be used when deciding to specify the double-back method, as there are conditions requiring the scratch coat’s full curing before subsequent layers are applied. Low temperatures during application slow strength development; they require longer than normal for the scratch coat to achieve enough strength to receive the brown coat. Additionally, the double-back method should not be used if the substrate consists of flexible backing, such as metal lath applied over open framing.
Double-backing is only a curing method for the scratch coat and does not relieve the applicator from properly curing the brown coat. Following the double-back method, the brown coat should be protected from direct sun and wind exposure and the surface should be misted as previously detailed.
Enforcing project specifications is difficult unless the moist-curing language is strong. It is recommended to include the following requirements, in addition to thorough drawings and specifications for proper stucco and accessory product selection and installation.
If the contract documents include control joints drawn at regular intervals, the provisions laid out in ASTM C926 and C1063, Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-based Plaster, are strictly adhered to during construction and the stucco is properly moist cured, it is reasonable to believe the stucco application will not crack and with proper maintenance will last more than 100 years. (See the article, “How Long Does Stucco Last?” posted on PCA’s website, www.cement.org.)
Nickie Ramm, EIT, has bachelor’s and master’s degrees in architectural engineering from the University of Texas at Austin. She is a project engineer at Building Diagnostics Inc., where she specializes in failure investigation, remedial design, architectural peer review, new construction monitoring, and diagnostic testing for building envelope systems. Ramm also participates in the company’s research group, The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. She can be reached via e-mail at email@example.com.
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