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.
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.
4. Apply Repair Material Mixture
- There are a variety of products to use depending on the size of the repair, its deep.
- If the spalling depth less than 1/3 of the concrete thickness, the concrete usually can receive a surface repair.
- However, for damages greater than 1/3 of the concrete depth, steel bars may need to be installed and a full depth restoration may be required.
- Roughen affected surface and apply suitable adhesion agent to improve bonding.
- After that, patch the area using suitable admixture, as can be seen in Fig.4.
- The best recommended patching material is Portland-cement-based or epoxy.
- It is required to select patching material with thermal expansion close to that of existing concrete.
- Consider ambient temperature during patching since it affects the patching material.
- The ambient temperature shall be above 4.5C.
- Damage on driveways, walkways, and other horizontal surfaces may be repaired with a cementitious overlay.
- If affected area is adjacent to a joint, ensure that it is restored properly so as to give some allowance for expansion of the member.
Using a margin trowel, scrape off the excess product and apply the finishing touches to the surface to level it, as shown in Fig.4.
Cure the material adequately to make sure that it gain required strength.
7. Apply Pain or Coat on Repaired Surface
After the material is set completely, then apply paint or waterproofing membrane to avoid re-occurrence of spalling.