QUESTION: We have been asked to treat an interior concrete slab's surface inside a store. The concrete is about three years old, but the store is only now ready to open. The unheated shell was exposed to winter weather for two seasons.
The design originally called for a stained concrete floor. This option is being discussed due to a localized surface problem with the concrete. The floor near the front bay windows and the entry door is blemished with crusty, flat chunks of broken concrete, as if the upper crust of the slab has been lifted from the rest of the mass. The pieces are almost entirely composed of hardened paste.
A structural engineer believes the building is sound. The owner wants to follow the store's original design and avoid the expense of a floor treatment such as stone or tile. He has asked us to advise him of the feasibility of repairing the damaged surface, followed by installing a stained/polished floor surface.
What could have caused the problem and how should we repair it? And what types of concrete floor treatments are suitable if we solve the surface defect issue?
ANSWER: The surface distress is probably scaling. The American Concrete Institute (ACI) defines scaling as the “local flaking or peeling away of the near-surface portion of hardened concrete or mortar.” It looks like the inside of the slab has pushed up to peel away the smooth concrete surface area.
In the “Guide for Concrete Floor and Slab Construction,” ACI Committee 302 explains how scaling occurs. “Scaling is primarily a physical action caused by hydraulic pressure from water freezing within the concrete; it is not usually caused by chemical corrosive action. When pressure exceeds the tensile strength of concrete, scaling can result if entrained air voids are not present in the surface concrete to act as internal pressure relief valves.”
If possible, try to track down the delivery receipts from the concrete pour. They may help explain the current problem.
Prominent causes of scaling include: Poor quality concrete, meaning that the water/cement ratio may be greater than 0.50; a localized area in which the contractor poured at an excessive slump; overworking of wet concrete and/or premature finishing operations by the contractor; or inadequate curing.
But because the floor slab is sound, there is a probably another contributing factor. From your description, it's possible that when the concrete was originally ordered, poured, and placed, all parties expected a fast-track completion. So the contractor may have ordered concrete with no air entrainment. As the floor sat exposed, the concrete slab near the window opening may have frozen and become distressed.
ACI 302 warns of concrete placed in winter. The committee suggests that new concrete exposed to freezing and thawing before it has been adequately cured achieved a compressive strength of 4000 psi (28 MPa) and allowed to air-dry may scale. The likelihood of scaling is even greater when someone applies deicing chemicals at this early age. Another factor is inadequate slope to drain water away from the slab. Moisture-saturated concrete is more susceptible to damage from freezing and thawing than drier concrete.