Spalling

Spalling is the detachment of surface fragments from concrete, masonry, brick, or stone. In structural engineering, it most commonly refers to concrete spalling — the breaking away of cover concrete to expose or nearly expose the reinforcing steel beneath. It is one of the most visible indicators of structural deterioration and one of the defects most frequently documented in condition surveys.

The most common cause of concrete spalling is corrosion of embedded steel reinforcement. When chloride ions or carbonation reach the reinforcement, the steel begins to corrode. The corrosion products (iron oxides) have a greater volume than the original steel — sometimes up to four times greater. This expansion exerts outward pressure on the surrounding concrete, eventually cracking and detaching the cover layer. The resulting spalled surface exposes the reinforcement to further atmospheric attack, accelerating the cycle of deterioration.

Other causes of spalling include freeze-thaw action (water absorbed into porous concrete expands when frozen, fracturing the surface over repeated cycles), alkali-silica reaction (a chemical reaction between certain aggregates and cement alkalis that produces an expansive gel), fire damage, impact loading, and poor original construction (insufficient cover, high water-cement ratio, or inadequate compaction).

In survey reports, spalling is typically described by location, areal extent, and depth. Shallow spalling (up to 25mm) may affect only the cover without exposing reinforcement. Deep spalling exposes or undercuts the reinforcement and represents a more immediate structural concern, particularly in thin elements such as suspended slabs or balconies where cover is limited.

Documenting spalling accurately requires both context photographs — showing the affected panel or element in relation to adjacent structure — and detail photographs that capture the depth, texture, and extent of the breakaway. 360° panoramic photography is particularly useful for spalling surveys because it allows the full extent of affected areas on a façade or soffit to be captured in a single frame, with the spatial relationship to grid references preserved.

Remediation of spalling concrete typically involves breaking out all loose and delaminated material, treating or replacing corroded reinforcement, and reinstating the cover with a compatible repair mortar. The extent of the repair programme depends on the surveyor's assessment of total affected area, a figure that a rigorous photographic condition survey makes considerably easier to quantify and communicate.