Scaling, as defined by the American Concrete Institute (ACI) Committee 116, is the “local flaking or peeling away of the near-surface portion of hardened concrete or mortar.” Sometimes called mortar flaking when it occurs just over the aggregates near the surface, it is primarily a physical action created by hydraulic pressures from repeated freeze-thaw cycles within the concrete. The expansive forces caused by the formation of ice are exacerbated with deicing chemicals, which increase both the saturation of the concrete and the number of freeze-thaw cycles. The distress mechanisms of scaling are complex on both a microscopic and macroscopic level.
Michigan is in a severe exposure climate where exterior concrete is subjected to continuous moisture, cycles of freezing and thawing, and use of deicing chemicals. Therefore, exterior concrete must be proportioned with durable ingredients designed for the climate with an entrained air-void system using proper placement, finishing, curing and protection to resist hydraulic pressures (stresses) that can promote scaling.
The most common causes of scaling are related to one or a combination of the following factors:
- Lack of, or inadequate, curing and protection. Do not let the concrete surface dry out after placement. Cement reacts with the water in the mix. Once the water is gone, the chemical reaction stops and the concrete stops gaining strength. A surface that dries out before it reaches strength will be weak. Curing compounds and/or secured plastic sheeting can be used to keep the surface from drying during the first few days, allowing it to gain full strength. Curing is always critical, but it is more critical on comfortable feeling low humidity days than on hot humid days.
- Improper finishing operations that work or trap water at the surface, resulting in a high water-cementitious ratio and low strength, creating a low-durability surface layer. Delayed or extended finishing will also allow early drying of the exposed concrete surface, prior to curing application, which can result in a weak surface.
- The use of non-air-entrained concrete or too little entrained air, resulting in a non-durable concrete mix. A poor air-void system may also be created at the surface via over-manipulation of the plastic concrete during finishing operations.
- Using concrete with low strength or excessively high water-cementitious ratio will allow for deeper penetration of water and deicing chemicals.
- Exposure of new concrete to freeze-thaw cycles before it has been adequately cured, and not allowed to air dry. Intentional application or indirect exposure (from vehicle traffic) to deicing chemicals at this early age greatly increases the likelihood of scaling.
- Exposure to aggressive/corrosive salts and fertilizers. Never use calcium or magnesium-based deicers on concrete. Sodium chloride (table salt, rock salt, or safe salt) may be used in moderation to melt the ice.
- Misunderstanding the use of supplemental cementitious materials (SCMs) may lead to scaling. Properly designed, finished, and cured mixes containing SCMs have the same resistance to scaling as 100% portland mixes. SCMs generally improve strength, durability, and water tightness. However, they also extend the concrete set time. Understanding set time and when to perform the varying finishing operations is crucial to the overall durability of a mix containing SCMs.
To read more about preventing and minimizing scaling and repair or treatment of scaled concrete surfaces download our full bulletin below.
If you have any questions please contact Steve Waalkes at 800.678.9622 or by email at [email protected].