What is the main cause of concrete spalling?
In reinforced concrete, the most common cause is rebar corrosion. As the embedded steel corrodes it expands and pushes the cover concrete off from the inside. Corrosion usually starts when carbonation or chlorides from de-icing salt or marine exposure reach the steel. Freeze-thaw cycling, fire, insufficient cover and alkali-silica reaction are the other main drivers. Spalling is a post-cure durability mechanism that develops over months or years, not a fault present when the forms come off.
What is the difference between concrete spalling and scaling?
Both involve surface loss, but they differ in depth and cause. Scaling is shallow loss of the surface mortar, leaving a pitted, peeling top layer, and is driven mainly by freeze-thaw and de-icing salts on a weak or over-finished surface. Spalling is deeper: it breaks into the cover concrete and is often driven by something underneath pushing out, such as corroding rebar. If the damage exposes aggregate or steel and goes into the cover, it is spalling rather than scaling.
Is concrete spalling serious?
It depends on depth, location and cause. A flaking patch on a residential driveway is mostly cosmetic and a trip hazard. Spalling on a parking deck, balcony, bridge or facade is more serious, because it usually signals active rebar corrosion that reduces the steel section, and falling fragments are a safety hazard. Spalling that exposes corroding rebar on a suspended slab, balcony or column should be assessed by a qualified structural engineer rather than judged from the ground.
How do you repair spalled concrete?
Sound the area to find all the delaminated concrete, then break back to a solid substrate, cutting behind any corroding rebar. Clean the exposed steel to bright metal, remove the corrosion product and apply a protective coating where specified. Apply a bonding agent, place a repair mortar or micro-concrete matched to the depth and exposure, and cure it properly. Skipping the steel treatment is why many cosmetic patches fail within a couple of years. Deep or structural spalling needs an engineered repair and a structural assessment.
Does the formwork plywood cause or prevent concrete spalling?
No. Spalling is a durability outcome decided by concrete cover, mix quality, exposure and time, long after the forms are gone. The forming stage has a narrow, front-end influence only: the form and rebar geometry set the concrete cover that protects the steel, and clean, on-time striking avoids chipping green edges and corners. A sound form panel helps present a dense surface and strip cleanly, but no panel prevents spalling on its own.
