Carbon steel is widely used a construction material of choice for buildings due to its inherent strength. Steel columns, trusses, girders and beams are the critical building components, failure of which usually leads to progressive collapse of the local and global structures. However, steel has high thermal conductivity which can cause it to lose its structural strength and stiffness as temperature increases. Expansion can also occur.
A non-uniform temperature distribution leads to thermal curvature. Any resistance to free movement of axial thermal expansion will induce internal stresses within the steel member making it frangible. The steel critical temperature for fire protection is the temperature that can cause structural collapse in a fire, and is often taken as 538 °C, as per ASTM E119, which corresponds to the point where it loses 50% of its load bearing capacity.
It is important to understand its mechanical and physical properties to understand how temperature affects steel. The melting point of steel is 1540 °C. Critical temperature is 720 °C, at 538 – 566 °C, carbon steel only retains about 50% of its yield strength. Steel can regain 100% strength if temperature does not exceed 720 °C.
Typical grades of structural steel are A36, A572, A588 and A514. They have yield strengths of 33000-35000 psi, and tensile strength 360000-70000 psi.
Rapid cooling of the steel elements heated between 704 and 843 °C during firefighting operations can also lead to some of the steel microstructure transform into martensite which increases hardness, but introduces brittleness. Below this temperature this transformation does not occur.
The purpose of PFP is to slow down propagation of heat to the structure bearing materials, e.g. steel beams and columns, to give time to evacuate the building before any catastrophic collapse of structural components could occur. Typically, the PFP materials are designed to withstand up to 2 hours of fire exposure per BS 476: Part 21, AS1530 Part 4, and up to 3 hours per ASTM E119 for intumescent products and up to 4 hours for cementitious products.
Most of the PFP materials used can be broken down into the three main categories:
• Cementitious products
• Mineral and fibre board systems
• Intumescent coatings
Cementitious products include cement and concrete, as well as lightweight materials such as mineral fibre, vermiculite or perlite. These products have relatively low thermal conductivity slowing the transmission of heat to the underlying structural steel. Lightweight mesh reinforcement may be required to increase durability for the structures subject to vibration or where mechanical damage is likely.
Fire protection boards are mineral boards (i.e. made of calcium silicate or calcium sulphate) reinforced with fibres and fillers, and can be further optimised for durability, e.g. resistance to humidity and freeze-thaw. The boards are manufactured in the controlled environment at a factory with strict tolerances and measurements. They require extensive supporting constructions, e.g. noggings, and require glue, staples, nails, etc. to be held together. It is typically easy to inspect their quality of installation.
Intumescent (which means “swell up”) coatings are composed of inorganic components contained in a polymer matrix: a combination of an acid source (ammonium phosphate, APP), a carbon source (pentaerythritol, PER) and a blowing agent (melamine). At the early stages of a fire, a large amount of thermal energy is absorbed by the coating, whose temperature increases rapidly. When the temperature of the coating reaches a critical temperature, the polymer matrix melts and degrades to form a viscous fluid. The inorganic acid source in the coating undergoes thermal decomposition normally at temperature of 100-250 °C. At temperature 280 – 350 °C, the blowing agent within the coating decomposes to release large amounts of gas of which some fraction is trapped within the molten matrix. The molten fluid hardens and releases residual volatile to form char. Convective currents are suppressed, and the thermal radiation does not have a “direct” path through the char to the substrate. This behaviour is complex and until now no agreeable model is available to simulate it.
Intumescent coatings are lightweight and can be applied in relatively thin coats. Other advantages include attractive appearance, and small volume occupation. They can easily follow the geometry of structural steel and are generally of relatively low cost. Intumescent coatings can be applied by airless spray, roller or brush, and cure and dry rapidly. Standard QA/QC requirements apply for surface preparation, application and inspection. Workshop application of these coatings has the advantage of better controlled application conditions. However, any structural materials with pre-applied intumescent coatings require careful transportation to minimize damage in transit.
Fireproofing coatings are tested for durability using time-temperature curves in accordance with UL 1709 for hydrocarbon fire exposures. ASTM E-119 is no longer used, as it does not adequately represent the fire exposure experienced in oil and chemical facilities. ASTM E-119 time-temperature curve however can still be used for non-hydrocarbon fires.
Dmitry Sidorin
