Seeing a crack in your building but not knowing what to do? Well, read this first...

Following the Opal Tower and the Mascot Tower incidents recently revealed in Sydney, there appeared to be a noticeable increase in number of inquiries regarding cracking observed in their buildings at construction stage, defect liability stage and serviceability stage of the building life cycle from Residents, Owners corporations, Builders, Developers made to Structural Engineers. Whilst we all want to find out if our buildings are structurally safe to live in, all stakeholders are urged to get answers/facts about the observed cracks, however, with slightly different agenda from one stakeholder to another. We will get to those agenda later in this article.

See the 'wiggly' line pattern in the photo above? If you are looking for answers of the following three frequently asked questions, you should keep reading:

1. What is it?
2. Why did it happen?
3. How to 'deal' with it?

The answers to the What, Why and How questions above would enable you to further understand how professional engineers may identify, consult and resolve a reported cracking-related issue. This article does NOT cover cases where crack happened during construction stage of a building project. Follow me on LinkedIn for future articles that cover structural related construction stage matters.

What is it?

It appears to be a crack on the surface of a concrete floor slab. BUT what is that supposed to mean? Well, let's start with reading some guidance from some official sources such as the Table below which provides guidelines of how to classify a crack in concrete slab in a residential building.

The above table does not necessarily cover all cases. Long story short, if the crack width is measured narrower than 2mm and the change in level is smaller than 15mm, it would be classified as Damage Category '2' which means it is most likely structurally safe (but not always). It may also mean that there would be no threat to your safety immediately or long-term in most cases (i.e. Non-Structural Crack). A quick way to check the width of the crack would be try to insert 2 back-to-back credit cards (generally a credit card is about 1mm thick) into the crack.

If the cracks were observed to be in Damage Category 3 or 4, it would most likely need structural attention in terms of remediation work for restoring its short-term and long-term structural performance. i.e. a Structural Crack

Why did it happen?

Trust me, you would be eager to find out the cause(s), only if the crack belonged to Damage Category '3' or '4'. Here is why:

Are you ready? The common causes of cracks in buildings are: Thermal movement, corrosion of reinforcement, chemical reaction, moisture changes, creep, foundation movement, soil settlement, shrinkage, elastic deformation, overloading, environmental stress, high-risk design concept, bad quality materials, poor construction practices, wrong concrete poured, weather effects during concrete poor, unreasonable wear and tear, poor design detailing, poor specification, poor maintenance, poor workmanship in concrete curing and etc; or any combinations of the above causes. Where do you even start?

These are the answers that you may get from a structural engineer assessing the case. I have personally encountered all above causes in my engineering experience. In general, we, as engineers would first use the Method of Exclusion to narrow down to the possible causes as efficiently as possible by undertaking the following actions:

• Conduct a site inspection to measure and record crack width, pattern and deformation of slab

• Perform a localised desktop analysis and calculation to understand the existing structure based on theoretical inputs
• Check construction stage photos, construction method used by Builder, load history, material record documents to compare theoretical model with actual as-built condition
• Perform sensitivity analysis to simulate cases that could cause the cracks and deformations of slab found on site
• Request more testing to confirm as-built material properties if needed.

The above assessment process may take anywhere between 1 to 3 weeks or even more. Due to the fact that there could be lack of factual information and many variable in the above actions, the conclusion following the assessment could be quite opinionated and sometimes vague if it was believed NOT caused by 'design fault'. Fair enough in my opinion! There could be no right or wrong answer here. Thus let's leave the 'Why Did It Happen' questions to the insurers and solicitors for now.

For the time being, why don't we focus on the possible resolutions instead?

How to 'deal' with it?

Before knowing the 'How', we have to know our Goal.

From a general point of view, our goal would be for the building to look and perform as per it was originally design/anticipated for its purpose.

However, from a structural point of view, we simply want to make sure the building would continue performing as per it was originally design/anticipated for its purpose.

The resolution could be very different pending the goals and different stakeholders need. Thus let's understand further about the stakeholders first.

The Residents typically just want the cracks to 'disappear' by having them filled, repaired, rendered or re-mediated at no cost if the cracks were found within the defect liability or Builder's warranty period. A typical resident passes his/her the finding to the Owners corporation or Strata committee for action.

The Owners corporation and the Strata committee usually has two choices: - 1) Contact the Builder or Developer for their action; or when issues could not be resolved via 1), then 2) Engage a third-party consultant to assess the significance of the crack. These group of stakeholders make decisions following expert's advice.

The last thing that the Builder and Developer want to hear is that there was a crack found in their building. They may quickly engage a structural engineer they know (who may have designed the original structure) to investigate the issue. Sometimes, the insurance company wants a certificate confirming the structural integrity of the building before insuring the property as part of their due-diligence activities.

Not all type/category of crack requires same level of structural attention. For Structural Cracks, the engineer must assess the situation to design the most effective remedial methodology to restore the design performance of the structure. For Non-Structural Cracks, it could be up-to the Builder and the property owners to decide and negotiate on a mutually agreed commercial outcome to aesthetically 'cover' the cracks.

Repair Methodology

The repair of cracks can be achieved with the following techniques:

Partial demolition and rebuild, epoxy-injection grouting, routing and sealing, flexible sealing, stitching using steel or carbon fiber strips, providing additional external reinforcement, drilling and plugging, providing additional vertical support or steel beam, grouting, dry packing, rendering, surface coating and etc.

The selection of the method would depend on the purpose of the repair, cost, timing, structural risk, commercial risk, reputation risk and cause of crack.

Prevention of Cracks

Some researches have been carried out that though it is impossible to guarantee against cracking yet attempts can be made to minimise development of crack. Some prevention could be taken care of during the construction process of a slab element.

• Engage an experienced and qualified engineering entity
• Allow for adequate number of pour joints, slip joints and movement joints
• Allow for extra mesh steel sheets
• Minimise cast-in conduits or pipes (or at least minimise cross-overs and grouping when laying conduits)
• Strictly monitor concrete supply quality
• Ensure vertical supports are properly connected to slab as per design (some permanent formwork system such as Dincel or Rediwall may adversely affect the effectiveness of connections between slab and wall/column if wall formwork continues above the deck for a particular pour.
• For Post-tensioned (PT) slabs, the construction program should allow the designed timing of stressing cables
• Avoid pouring under extreme weather or temperature
• Allow labour and budget for proper curing for 7 to 10 days
• Control and seek engineer's approval on any proposed loading on the slab due to heavy machinery, equipment, scaffolding, construction vehicle and etc.

Any lack of attentiveness can lead to a cause for damage in the building in its future, which can also lead to failure of the structure. The occurrence of cracks cannot be stopped but particular measures can be taken to restrict them to reduce the level and degree of consequences. The potential causes of crack can be controlled if proper consideration is given to structural design concept, construction material, construction stage load level and construction technique used.

P.S. If you learnt something useful from this article, please like, comment and follow.

If your residential or commercial property is due for a structural audit, please feel free to contact me on Linkedin for further advice.

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