The word Spall in engineering refers to the fragments of a material that is broken off from a large object. Spalling is also called spallation. It is the surface failure in which the materials like concrete, brick, limestone, etc break off from the object or structure due to the presence of moisture, corrosion, weathering, etc. Similarly, Spalling of concrete is the surface failure in which concrete breaks off from the concrete object or structure. Concrete Spalling is also called Concrete Cancer.
Spalling of concrete affects the broad variety of structures like jetties, bridges, framed structures, bunds, tanks, etc. As soon as spalling occurs it must be cured because the size of spall goes on extending over time which may lead to difficulties in repair/ maintenance, exposure of rebars to the atmosphere and sometime it may lead to failure of the structure or structural member. It also distorts the aesthetic view/appearance of the structure. Generally, spall of concrete maybe 150mm or more in diameter and 25 mm or more in-depth. There is also the possibility of occurrence of smaller spall as per ACI 302.1R-4.
How Spalling of Concrete Occurs?
The main reasons for spalling in the concrete are firing and corrosion.
Spalling mechanism due to Firing
When concrete is exposed to fire then free water gets evaporated and creates internal pressure which makes the materials bond weaker. This leads to spalling.
Spalling mechanism due to Corrosion
Steel bars are used in concrete to provide strength (i.e. Steel provides high compressive strength but low tensile strength). High alkaline content of the concrete protects the embedded steel from corrosion. So there is a rule that no bars should be nearer than 1 inch from the concrete surface. This also protects the rebar from external moisture. Poor consolidation or compaction of concrete also leads to rusting of steel as it allows the ingress of oxygen and moisture inside due to the presence of a large number of voids. So precautions should be taken during compaction.
Despite precautions, corrosion in steel may occur. Rust / Corrosion has more volume than the original material which creates pressure. This resultant pressure causes the spalling.
Causes of Spalling of Concrete
1. Corrosion of embedded reinforcement steel
The volume of rust is more than original material that creates the resultant pressure which leads to spalling of concrete in Reinforced Concrete.
2. Exposure of concrete to fire
When concrete is exposed to fire; free water in the concrete evaporates resulting in the weak bond between concrete materials. This leads to spalling.
3. Freeze and thaw cycling
If water freezes, its volume can expand by about 9%. As the water in humid concrete freezes, it yields pressure in the capillaries and pores of the concrete. If the pressure surpasses the tensile power of the concrete, the cavity will dilate and break which results in spalling.
4. Insufficient depth of concrete over rebars
There should be the provision of a minimum of 1 inch of concrete depth over rebars. Lower depth mostly causes the exposure of rebars to moisture absorbed by the concrete. This results in corrosion of rebars.
5. Improperly constructed joints that causes joint spalling
Improperly constructed joints leads to the weak bond between materials at the joint that causes the spalling of concrete in joints.
6. Poor compaction of concrete
When compaction of concrete is poor; air voids present inside the concrete causes the loose bond of materials that leads to spalling. Poor compaction also causes the rusting of rebars by allowing the ingress of moisture through voids.
As we know; concrete expands in warm weather and contracts in cold weather. Seasonal expansion and contraction of concrete causes the breaking and spalling of concrete.
8. Poor concrete mix
If materials used and their ratio is not suitable then it leads to improper bonding causing to spalling.
9. Old age of the concrete structure
It is the natural process. When concrete gets very old; Its efficiency starts dropping that causes the weakness in the bond of materials resulting in the spalling.
10. Chemical Attacks
Chemical attack also deteriorate the concrete when it is exposed to different environmental conditions like acid rain have series effects on the concrete. Generally, sulphate attack is common in concrete.
11. Exposure of reinforced concrete to chloride ions
The chance of spalling increases as the number of chlorine increases. When the moisture is present at the threshold limit of chlorine, it causes series spalling-deterioration.
Measures to prevent spalling
Some measures to prevent the spalling of concrete are:
1. Proper mix design of materials.
2. Proper compaction of concrete during the construction of the structure.
3. Provision of good water sealant to concrete to stop the ingress of moisture.
4. Provision of adequate depth to concrete cover over rebars. (minimum 1 inch)
5. In the concrete floor slab, Early-entry dry-cut helps prevent spalling.
6. Avoid excess water use in the concrete mixture.
Repair of Spalling of Concrete
Repair work of spalling of concrete varies based on the location of concrete structure and severity of the damage. Steps of repairing spalling are:
1. Clean the spalled area
Clean the spalled area to eliminate the bond breaker agents by hand or pressure washer.
2. Remove Loose Concrete
a. Use hammer and chisel to remove loose and broken concrete.
b.The removal of 3.81cm deep of concrete is recommended for better repair.
c. Tapping the surrounding area of spalled area with the hammer is advised to check the future deterioration.
d. Hollow sound represents deteriorated concrete. Such concrete should be eliminated to prevent the need for future repair work.
e. It is suggested to cut the spalled area in regular shape (i.e. either in square or in a rectangular shape). This makes patching easy and effective.
f. Finally, remove dust and debris using a vacuum.
3. Cleaning and Coating rusted steel bars
Clean the surface of the steel bar from dust and rust using the wire brush and apply the anti-rusting coating or paint.
4. Apply repair material mixture
a. There are various materials for spalling repair of concrete. Select the materials according to the size of the repair required.
b. If the depth of spalling is less than 1/3 of the concrete thickness; surface repair is done.
c. If the depth of spalling is more than 1/3 of the concrete thickness; Installation of steel bars and full depth restoration may be required.
d. Apply adhesion agent on the affected surface to increase the strength of bonding.
e. After that, patch the area using suitable admixture.
f. Epoxy or Portland cement-based materials are recommended for patching.
g. Selected material for patching should have close thermal expansion value to that of existing concrete.
h. Ambient temperature during patching should be considered as it affects the patching material.
i. The ambient temperature should not be less than 4.5 degrees C.
j. Repair of driveways, walkways and other horizontal surfaces can be done using cementitious.
Finishing of the surface is done by providing levelled surface using a trowel or other means.
Curing is done to provide maximum strength and durability to the applied concrete. Minimum 7 days of curing is recommended if surface repair is done and 28 days for full depth restoration.
7. Coating or Application of Paint
After material is set; application of paint or application of waterproofing membrane is done to avoid future spalling.