Core Filling for Fire Protection

30thMarch 2021
Core Filling for Fire Protection

Steel is widely used in the construction of multi-storey buildings all over the world.  It has proven to be cost-effective, sustainable, efficient, and also offers many advantages when it comes to health and safety. Technological advancements have also further developed steelworks and their fire protection which are critical in achieving building structures with a high fire protection rating.
Performance of steelwork when subjected to fire is determined through a series of fire tests using an analytical fire safety engineering approach.  This approach is aimed at producing the best results when it comes to controlling or preventing the spread of a fire.

Core Filling - Thin Film Intumescent Coatings - Permax

The structural fire performance of steelworks is determined by its heated perimeter to cross sectional area ratio or Hp/A.  Tests have shown that structural steel with hollow sections produce very poor performance when subjected to heat or fire.  Due to the limitations of fire rating products, in many circumstances, the FRL is only achieved once a minimum Hp/A level is met, yet because these steel sections are hollow, they need to be core-filled in order to achieve the required fire protection ratings.

When to Consider Core Filling?

The use of core fill blocks should be considered for hollow sections of steelworks. By filling the hollow sections with concrete, the load-bearing capacity of steel is greatly increased. Concrete filled in hollow steel not only adds to its structural integrity but also acts as a heat sink.

The Core Filling Process

A typical core-filling procedure follows three steps:

  1. The steel column to be filled in is placed in an upright position.
  2. When filling concrete block cores, a specified concrete mixture is poured or pumped into the hollow steel column.
  3. A hole is drilled at the top of the steel column to act as vent, which is then covered after the whole concrete-filling process is completed.

With the above process, a composite section is then produced.  This will significantly strengthen the section and increase its load carrying capacity.   In case of fire, the core-filled structure will absorb heat.   On the other hand, when incorporated into your building design with an external fire retardant such as an intumescent coating (sometimes called fire paint), the exterior of the core-filled steel will reduce the rise in steel temperature during a fire scenario.

Core Filling – What you should know

In construction there are circumstances where what has been deisgned is not always what is best for the fire rating aspects of the build. At times, core-filling serves to improve durability under fire conditions, however, core filling does impact other aspects of the build:

  • Core filling adds weight to the building structure which can mean increased use of structural steel
  • Delayed timelines
  • Increased construction costs

There is no doubt that concrete core-filling improves hollow steel sections’ structural-fire performance and durability under actual fire conditions. However, determining when to use it, or understanding what other options exist could save you significant sums of money in the project cost, which makes it imperative to determine and use the right calculation and mixture for core filling by using the standards outlined in the fire and building safety requirements.
If you are unsure how to interpret these guidelines, speak to a Permax fire rating expert to discuss your fire protection needs and let us help you design the right solution.


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Permax constantly update the documentations based on the new fire testing outcomes and change of standards and regulations. To ensure the documents you read are up-to-date, please contact the Permax technical team.