What is Spalling Concrete? Their Causes and Repair
Madeh Izat Hamakareem
Reading time: 1 minute
Spalling is break away of concrete surface which often extends to the top layers of reinforcing steel. Spalls may be 150 mm or more in diameter and 25mm or more in depth, and the occurrence of smaller spalls also possible as per ACI 302.1R-4. Spalling of concrete affects a broad variety of structures including framed buildings, multi-storey car parks, bridges, jetties, tanks and bunds.
Spalling leaves the concrete surface patchy and pitted while exposing aggregate underneath. As soon as spalling spotted, it should be properly repaired otherwise it compromises concrete surface integrity and jeopardizes ultimate capacity of the member. Added to that, spalling could distort aesthetic appearance of the structure.
Causes of Concrete Spalling
Corrosion of embedded reinforcing steel, because the corrosion products (rust) occupy more volume than the original steel, and the resultant pressure spalls the concrete.
Fire exposure due to which free water in concrete change to stem and create internal expansive pressure in concrete, consequently spall off concrete.
Freeze and thaw cycling
Expansive effects of Alkali Silica Reaction
Inadequate depth of cover over reinforcement
Low quality concrete cover over reinforcing steel.
Joint spalls are often caused by improperly constructed joints lead to joint spalling
Bond failure in two-course construction due to Differences in shrinkage between topping and base courses, drying of the bonding grout before the topping concrete is placed etc.
Fig. 1: Mechanism of Spalling in Reinforced Concrete Slab Due to Steel Corrosion
Measures to Prevent Concrete Spalling
In concrete floor slab, Early-entry dry-cut helps prevent spalling.
Apply a good water sealant to the finished surface to prevent water from entering into the concrete.
Proper management of concrete during pouring to ensure the provision of adequate cover and placement of joints at correct positions.
Prevent excess water in concrete mixture since it leads to concrete spalling.
Repairing Process of Spalling of Concrete
Repair of concrete spalling vary based on severity of the damage, type and location of concrete structure. It should be performed based on recommendations of applicable codes such as ACI Code and BS EN 1504.
1. Clean Spalled Concrete Area
Clean the area to be repaired from any dirt to eliminate bond breaker agents (Fig. 3). If heavy stains and dirt build up, then a pressure washer may be needed to properly clean the surface.
2. Remove Loose Concrete
Remove loose and broken concrete from the area using hammer and chisel.
Sometimes, shot blasting or grinding may be needed for proper surface preparation.
It is recommended to remove about 3.81cm deep of concrete to conduct a successful repair.
It is advised to strongly tap the surrounding area with hammer to check for possible deterioration.
Hollow sound is a sign of deteriorated concrete and should eliminated. This prevent the need for future repairing of concrete.
It is advised to cut the rectangular part of the concrete (as shown in Fig. 3) so that the patching could be done and get the concrete contained within the specified area. This offers better results.
Finally, eliminate dust and debris using vacuum.
3. Clean and Coating Corroded Steel Bars
Clean the rebar from corrosion or other dirt using wire brush (Fig.2) and then, apply a protective coating to avoid rusting as illustrated in Fig. 3.