Spalling in Strata Buildings
Fix it now this is as cheap as it will ever get.
A very common but not well understood problem with older concrete structures is spalling. Sometimes known as “concrete cancer” it is something that you want to have to deal with. You may have heard this tem before but not completely understand what it means, is it as bad as people who have gone through the repairs make out. Simply put, YES!
That’s nice, but what is Spalling?
Concrete spalling occurs when the steel reinforcement within a concrete slab begins to rust. The rusted steel increases to approximately five times its original size, displacing the concrete and causing it to flake. Once concrete is flaking more and more damage occurs – as more water can enter the affected area and cause further rusting, thereby speeding up the process and increasing the affected area. Concrete spalling is usually caused by corrosion of the steel reinforcement bars embedded in the concrete matrix, but can be caused by other ferrous elements either fully or partially embedded in the structure. Steel frame window systems, handrails, structural I-beams, metal pipes and conduits are among the most common of the damage causing building components. Corrosion of the reinforcing steel however, is by far the most common cause of spalling and splitting in older concrete structures.
Steel reinforcement bar (rebar) is used in concrete to provide strength to the concrete that is extremely high in compressive strength but has very limited flexural or tensile strength. One of the principles of reinforced concrete is that the high alkaline content of the concrete is meant to aide in protecting the embedded steel from corrosion, for this to happen though there needs to be a minimum amount of coverage on the steel. As a general rule of thumb one inch of cover is required to protect the steel rebar, in other words, no rebar should be nearer than one inch to the concrete surface.
What causes Spalling?
There are a number of factors that can cause concrete cancer and it often depends on the unique circumstances of each case, the most common causes are;
- steel reinforcement being located too close to the surface allowing water to react with it
- poor quality steel reinforcement used
- incompatible metals used close to each other causing a reaction
- damage to concrete allowing water to enter the slab and come into contact with the steel
How can you tell if your building has Spalling?
Owners normally only become aware of concrete spalling once symptoms of the problem are visible to the naked eye or when delaminated concrete starts falling to the ground, however visible concrete defects are normally just the indicator of larger problems. Concrete staining and discolouration is often the first visible symptom, often mistaken as being caused by weathering. Concrete flaking from the surface of an affected area combined with the discolouration of the area usually indicate there are significant problems with the structure.
The development of corrosion in concrete that happens over time is generally seen as a multi-stage process.
- In the first period, aggressive substances penetrate the concrete and ultimately reach the steel, which causes the onset of corrosion called depassivation or corrosion initiation.
- In the second stage, actual corrosion takes place (its rate depends on moisture and oxygen availability). Given that properties in the Eastern Suburbs are constantly and continually exposed to sea water the expansion due to corrosion builds up tensile stresses in the concrete cover until it cracks. This cracking produces the first visible signs of corrosion, although in some cases rust stains at the concrete surface may show before cracking.
- In the next stage expansion due to corrosion proceeds, until parts of the concrete cover completely detach and spall off constituting a potential danger for users of the structure or the general public.
- In the fourth and final stage, reinforcing bar diameter loss becomes so severe as to approach the minimum required for structural stability; eventually collapse cannot be ruled out. Catastrophic Failure is imminent.
A common question asked by owners undertaking spalling repairs is, Why can’t a contractor provide a more accurate estimate for the cost of repair? In essence they should be able to provide a qualified and expert estimate once the extent of the spalling is known. However, the full extent of the spalling spread through the building is hard to quantify until work has started with all the affected concrete removed and the amount of steel exposed is known. While each property can be completely different in the time frame between the stages and cost of repairs the over all principle remains. Spalling will not stop until it is completed addressed and repaired.
Spalling concrete puts owners of property at risk of negligence suits. Damage can occur through concrete pieces falling and injuring people or property. Concrete spalling that is not attended to in a timely manner will only lead to further repair costs as the problem grows and spreads. Simply concreting or painting over the effected areas will not address the issues. In order to affect the repairs the “spalled concrete” MUST be removed and any exposed steel MUST be either replaced or cleaned and treated. It can range from minor issue only requiring cosmetic repair but left unattended it will become a major problem that can affect the structural integrity of the building and if left untreated can cause catastrophic failure of the structural elements of the building. Taking no action only results in further damage and increased costs.
This is far too expensive, so let’s just do a temporary repair.
If the cause of the original damage to the concrete is not determined, correctly addressed and eliminated, the spalling process will repeat itself on the repair material, resulting in wasted time, money and efforts, and a spalling problem still in existence. Future owners could also seek costs being incurred from the previous (current) owners for not undertaking the appropriate works now.
In one example an owners corporation was made aware of a spalling issue in 1986, the estimated cost of repair at that stage was $15,000. The owners of the day did not wish to undertake the work as it could not be funded and was left unattended until 2010. Further engineering reports were issued and tenders called for. The spalling had dramatically increased and new estimates to repair the work were obtained. The cost in 2010 was $1,100,000.00. Still no action was taken as the owners were deliberating over the costs and whether a less expensive repair could be undertaken. After a further 24 months delay the work commenced in 2012. The spalling was moving into its final stage and the building was close to being unliveable. The final cost to the repair was $2,150,000.00.
The cost of repairing concrete spalling might be high, but the cost of doing nothing is much higher.