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:
- unhydrated cement particles are present;
- plaster’s relative humidity (RH) remains above 80 percent; and
- there is space within the pore structure for new hydration products to form. (For more, see the Portland Cement Association’s [PCA’s] 2002 Design and Control of Concrete Mixtures, by Steven H. Kosmatka, et al.)
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.