Technical note ·
What causes concrete spalling, and when does a patch last?
Spalling is a symptom, not a diagnosis. The cause decides whether a patch will last.
What spalling is
Spalling is the loss of pieces of concrete from the surface or edges of a slab or wall. It leaves pits, broken edges and shallow craters. On commercial properties it shows up most at pavement joints, dock edges, door thresholds and in wheel paths.
The common causes on Texas commercial sites
Spalls look alike, but they come from different mechanisms. Identifying the right one decides the repair.
- Joint-edge spalling. Sand and stones get into unsealed joints. When the slab expands in summer heat it bears on that debris and breaks off the joint edges. This is the most common cause on parking lots and truck courts.
- Reinforcement corrosion. Steel close to the surface corrodes when water and chlorides reach it. Rust takes up more volume than the steel it replaces and pushes the concrete cover off. Exposed, rusted bar in the bottom of a spall is the giveaway.
- Weak surface layer. Concrete that was overworked, finished with bleed water on the surface or poorly cured can have a weak top layer that scales or delaminates under traffic and weather.
- Impact and point loads. Steel-wheeled carts, forklift tines and trailer landing gear chip edges that have lost support or were never armored.
Why patches fail
Most patch failures come from preparation, not material. Industry repair guidance from ACI and ICRI agrees on the essentials: remove all unsound concrete, saw-cut the perimeter so the repair has defined edges instead of feathered rims, clean and roughen the surface so the repair can bond, and treat any exposed reinforcement. A good repair material placed over dust, loose concrete or a feathered edge will debond.
The second most common failure is using the wrong material for the depth. Repair mortars have a minimum and maximum placement depth on their data sheets. A product rated for half an inch won't hold in a three-inch hole, and a deep-repair concrete won't bond as a skim coat.
Patch or replace?
Partial-depth patching is right when the damage is confined to the upper part of a slab that is otherwise sound and well supported. If a panel is cracked through, rocks when trucks cross it, or sits over base that has washed out, a patch won't fix it, and that section should be removed and replaced.
Preventing the next spall
Most joint-edge spalling is preventable. Keeping pavement joints sealed keeps debris and water out, and that one maintenance step protects both the joint edges and the base under the slab. Re-inspect joints yearly and reseal where sealant has hardened, split or pulled away.
References
- American Concrete Institute, PRC-546-14: Guide to Concrete Repair. www.concrete.org
- International Concrete Repair Institute, Guideline 310.1R: Surface Preparation for the Repair of Deteriorated Concrete Resulting from Reinforcing Steel Corrosion. www.icri.org
- American Concrete Institute, PRC-201.2: Guide to Durable Concrete. www.concrete.org