Concrete scaling—defined as the flaking or peeling away of the near-surface portion of hardened concrete—remains one of the most common and costly forms of surface distress in exterior flatwork. While environmental factors, such as freeze-thaw cycles and the use of de-icing salts, are often blamed, improper finishing practices during placement can also be the underlying cause.
At RJ Lee Group, we routinely investigate scaling in sidewalks, drive lanes, and exterior slabs. Through petrographic analysis and field evaluations, we’ve identified several finishing-related contributors to surface scaling that are often preventable with proper construction practices.
Premature Finishing and Bleed Water Effects
One of the most common finishing errors occurs when work begins before bleeding is complete. Starting too early can trap bleed water beneath the surface, creating a weak or soft zone just below the finished layer. This subsurface condition can cause the surface to flake or scale off.
Additionally, bleed water is sometimes worked directly into the surface during floating or troweling. This increases the local water-cementitious material (w/cm) ratio, resulting in a softer, more porous paste. Such areas are less durable and more susceptible to scaling from deicer exposure and freeze-thaw cycles.
Delayed or Late Finishing
Conversely, waiting too long to begin finishing can also lead to problems. If the surface begins to dry or crust over before troweling, consolidation may be inadequate, resulting in poor paste-aggregate bonding and a weak surface layer. These conditions often lead to tearing and shallow delamination or scaling, typically evident after the first winter season.
The risk is heightened in hot, windy, or low-humidity environments, where rapid surface drying can make timing and proper finishing even more challenging.
Overworking the Surface
Even when finishing is correctly timed, excessive or aggressive surface work can introduce problems. Overworking concentrates fine material and mortar at the top, creating a paste-rich layer with minimal coarse aggregate. This layer tends to have lower strength and is more prone to abrasion and surface scaling.
More critically in colder climates, over-finishing can reduce or eliminate entrained air near the surface. Since entrained air is essential for freeze-thaw durability, its loss significantly increases the risk of early surface distress, particularly in areas exposed to deicing salts.
What ACI and Industry Guidance Recommend
The American Concrete Institute (ACI) provides clear guidance on proper finishing practices in documents such as ACI 302.1R[1], which emphasizes that finishing should not begin while bleed water is present and that water should never be added during finishing.
ACI 305R[2] addresses additional considerations for hot weather placements, where finishing timing becomes even more critical. For exterior flatwork exposed to freeze-thaw conditions and deicing salts, ACI 330R[3] highlights the importance of proper surface construction and curing to ensure long-term durability.
Why It Matters
Scaling affects more than just appearance. It compromises surface durability, increases maintenance needs, and shortens the service life of concrete slabs. For contractors, engineers, and owners, understanding how finishing practices influence long-term performance is essential to preventing this common and costly form of deterioration.
The problems outlined above are typically not immediately obvious; however, they usually become apparent very early in the life of a given placement. Petrographic analysis is required to differentiate and accurately diagnose the cause in any particular case.
At RJ Lee Group, we routinely investigate scaling-related issues through petrographic analysis and field evaluations. These methods help identify root causes of distress and provide a technical foundation for engineers and contractors to guide repair decisions and improve construction practices.
[1] ACI 302.1R-04 Guide for Concrete Floor and Slab Construction, American Concrete Institute
[2] ACI 305R-99 Guide to Hot Weather Concreting, American Concrete Institute
[3] ACI 330R-01 Guide for the Design and Construction of Concrete Parking Lots, American Concrete Institute