What Is Spalling?
Spalling of concrete at fires is the breaking-off of layers of a concrete surface in response to applied heat.
Spalling may be either localized and widespread depending upon the fire and concrete condition, particularly moisture content, and the susceptibility to the break-up of heat-unstable aggregate particles.
Causes of Spalling in Concrete
On prolonged heating areas of concrete cover may also fall away, a process that is sometimes called ‘sloughing.’
The processes causing sloughing aren’t generally reported, although it is noted that it occurs from corners of beams and slabs and seems to spread along a plane of weakness parallel to the outer surface.
Because ‘sloughing‘ occurs late in a fire exposure, it is considered by some as being of less concern than explosive spalling that occurs earlier upon exposure to fire.
Understanding explosive spalling is important because of the potential for loss in the section of the concrete element, the depth of fire-affected concrete, and the reduced protection to embedded steel.
Spalling is a frequently observed phenomenon in a fire, more prominently on soffits of slabs and on beams because of the greater exposure to heat and possibly heat ‘entrapment.’
It is not certain that this frequent observation is fully anticipated by design codes, and this is discussed in more detail below in the section on design codes.
The prediction of risk of spalling occurrence has not proven easy despite considerable research. The propensity to spall is influenced by the moisture content of the concrete.
The permeability of the concrete, the rate of heating, the nature of the aggregate, and the load applied to the concrete.
Although these separate contributing mechanisms have been identified, their relative contribution and their interaction are less well understood.
There are, however, general trends that can be established. Concretes in a moist or saturated condition will spall faster and, more extensively, the drier the concrete.
Some guidance (Malhotra, 1984; Concrete Society, 1998) suggests that moisture contents greater than 3% by weight will lead to spalling, although this ‘limiting’ value may be affected by the permeability of the concrete; the lower the permeability, the higher the risk of spalling, a higher temperature also increases the risk.
Some research also shows that elements under higher structural load during the fire have increased risk of spalling. These issues are discussed in greater detail in the following sections.
Measures to Prevent Concrete Spalling
In a concrete floor slab, Early-entry dry-cut helps prevent spalling.
Apply good water to the finished surface to prevent water from entering into a concrete.
Prevent excess water at concrete mixture since it leads to a concrete spalling.
Proper working management of concrete during pouring to ensure a provision of adequate cover and placement of joints at correct positions.
Repairing Concrete Spalding
There is a number of solutions that can be offered depending on the severity of a condition.
The solution to this may require that less labor-intensive repairs be done. Here are some of the ideas you could use to Repairing a spalled concrete.
- Once you notice a damaged concrete, if it’s less than 1/3 of its thickness, the repair could be done. But when it’s deeper, you might install a steel bar and might require a full restoration.
- When the rebar is exposed or corroded, it should be cleaned before it’s restored. Use a wire brush to clean the steel bar surface very well before you apply a protective coating. It could help to stop or prevent this.
- Once the problem is on the horizontal surface or within the driveway, plaza, or like sidewalks, you might need to apply a cement overlay. Once the overlay set is completed, the waterproofing membrane could be applied to stop also prevent concrete spalling.
- To carry out the successful repair, ensure that you remove a concrete near about 1.5 inches (35mm) deep. You should then clean and remove all debris before the repair commences.
- You can do that patching, so they extend 4″ beyond the area affected by concrete spalling.
- If you want to get better results, cut a rectangular part of the concrete so that the patching may be done also get the concrete contained within the specified area.
- Remove and clean all debris and ensure that all particles are removed. It might be cleaned with water, but the excess water should be removed with a sponge. The surface must dry up before the compound is applied.
- The old surface could be rough so as to allow for better bonding inside a new concrete.
- The best patching materials include epoxy also Portland cement.
- The patch material should be mixed very well, but you should consider that they may react or mix with what you want to use within 15 minutes.
- Ensure that the air temp. is above 40 degrees Fahrenheit if you want the patching to work well.
- Ensure you only use materials that have a similar expansion coefficient with that of the existing concrete.
- Before the joint is repaired, ensure it is restored properly so as to give some allowance for the expansion of the slab.