Question: During the summer of 2012, we placed several concrete sidewalks and ADA curb ramps for a commercial property. The flatwork was integrally colored with both pattern-stamped and broomed finishes. The owner was pleased with our work, but now the owner wants large areas replaced because of surface deterioration. Last spring, the surface of the concrete started to flake and scale off. In addition to normal pedestrian foot traffic, the concrete was exposed to lots of freezing rain, sleet, snow, and deicing salts during the past winter.
Because the owner is a good customer, we’ve agreed to replace the damaged concrete. But what caused the scaling? Did the deicing salts cause or contribute to the scaling? How do we prevent this type of concrete deterioration from occurring on future architectural flatwork projects?
Answer: Scaling is the local flaking or peeling away of the near-surface portion of hardened concrete, and it is frequently rated according to its severity. Light scaling does not expose the coarse aggregate particles. Medium scaling involves the loss of mortar to depths from 3/16 to 3/8 inch with exposure of the coarse aggregate particles. Severe scaling includes the loss of mortar surrounding the coarse aggregate particles from 3/8 to 3/4 inch in depth. If the depths of mortar loss exceed 3/4 inch and coarse aggregate particles have been lost, the scaling is rated as very severe.
Regardless of the rating, scaling is unsightly and may lead to poor drainage and trip hazards.
Scaling of concrete flatwork is basically a physical phenomenon related to the expansive pressures created by freezing water. Hydraulic pressures are created by the 9% expansion that occurs in the near-surface zone of the concrete when the growing ice crystals displace the unfrozen water in the capillaries and pores of the cement paste and aggregates.
If entrained air bubbles are not present in the near-surface concrete to act as internal pressure-relief valves, hydraulic pressures can easily exceed the tensile strength of the surrounding cement paste. When this occurs, subhorizontal microcracks start to form and the top surface begins to separate from the body of the concrete. Successive freeze-thaw cycles eventually cause the top surface to flake and scale off.
The primary causes of scaling include:
1. Permeable and poor quality concrete due to:
- High water to cementitious materials ratio (w/cm)
- Using aggregates that aren’t resistant to freezing and thawing
- Overworking of wet concrete
- Premature finishing operations
- Inadequate curing
- Low compressive strength at the surface.
2. Little or no entrained air due to:
- Failure to use an air-entrained concrete
- Working out or destroying the entrained air bubbles along the surface by overworking overly wet concrete in premature finishing operations
- Total air content that’s too low.
3. Total air content that’s too low to resist the effect of deicing chemicals used for snow and ice removal.
4. Prematurely sealing the surface, which creates a thin, weak layer directly beneath the surface.
5. Inadequate thermal protection, allowing freezing of the surface at an early age.
6. Exposing new concrete to freezing and thawing before it has been adequately cured, achieved a minimum compressive strength of 4000 psi, and allowed to air dry. Also, exposing concrete to deicing chemicals at an early age greatly increases the risk of scaling.
7. Inadequate slopes to properly drain surface water. Saturated concrete is more susceptible to damage from freezing and thawing than drier concrete.
For your project, a combination of factors including winter exposure conditions (moisture and deicing chemicals) most likely caused the scaling. To determine the exact cause or causes of the scaling, perform a condition survey and a petrographic examination, including the microscopical determination of the air-void system parameters of the hardened concrete using drilled core samples.
Surface damage by freezing and thawing is exacerbated when concrete is exposed to deicing chemicals, especially by heavy or repeated applications. Deicing chemicals increase the normal hydraulic pressures produced when water in concrete freezes.
How to prevent scaling
For scale-resistant concrete, use a maximum w/cm ratio of 0.45 and a minimum 28-day concrete strength of 4500 psi. Placing concrete with a low w/cm ratio, adequate strength, and proper curing will reduce the permeability along the top surface. Limiting the amount of water and deicing chemicals that can penetrate into the concrete also increases the scale resistance of the concrete.
Use air-entrained concrete that meets the requirements of the table shown on page CS-56. As shown, the total air content requirements increase as the maximum size of the coarse aggregate decreases. Exercise caution and do not damage or destroy the entrained air bubbles along the near-surface zone of the concrete by overworking overly wet concrete. Also, do not prematurely seal the surface. This can trap rising bleedwater and entrapped air directly beneath the surface, creating a weak layer that is prone to scale.
Be sure to install sufficient slopes to properly drain the surface to minimize the external source of water. Also, consider using an exterior concrete sealer to reduce the penetration of water and deicing chemicals into the concrete. Careful attention to mix design, finishing, and curing should produce scale-resistant concrete, even when exposed to reasonable amounts of deicing chemicals.
Kim Basham, PhD, PE, FACI, is president of KB Engineering. He specializes in concrete construction, troubleshooting, nondestructive testing, forensics, and repair. E-mail email@example.com; visit www.kbengllc.com.