Concrete structures are inevitably subjected to fire and high temperatures throughout their lifespan. These conditions can lead to detrimental changes in the properties of concrete and even result in structural failure. Therefore, it is crucial to comprehend the factors that influence the performance of concrete during fires. This article aims to provide insights into the key factors that dictate how concrete behaves under fire conditions.
Inevitably, concrete structures are exposed to fire and high temperature during its life service which creates changes in concrete properties detrimentally and also sometimes causing failure. Therefore, it is considerably significant to understand factors that control concrete performance during fires. This article would shed light on the factors that govern the performance of concrete during fire.
Fig.1: Concrete Element Exposed to Fire
Factors Affecting Performance of Concrete during Fire
The performance of concrete during a fire is influenced by several factors. One crucial factor is the water-to-binding material ratio, which affects the overall density and strength of the concrete. A lower water-to-binding material ratio typically results in a denser and stronger concrete, which can better withstand the effects of fire.
The moisture content of the concrete also plays a significant role. Excessive moisture can lead to steam generation and potential explosive spalling of the concrete when exposed to high temperatures. On the other hand, insufficient moisture content can cause rapid dehydration and cracking of the concrete during a fire.
The type of aggregate used in the concrete mixture is another factor that affects its fire performance. Aggregates with low thermal conductivity, such as lightweight aggregates, tend to provide better fire resistance compared to dense and highly conductive aggregates. The selection of the appropriate aggregate type is crucial for enhancing the fire resistance of the concrete.
The inclusion of supplementary cementitious materials (SCMs) in the concrete mix can also improve its fire performance. SCMs, such as fly ash, silica fume, or slag, can enhance the overall fire resistance and reduce the potential for spalling. These materials react with calcium hydroxide during the hydration process, resulting in the formation of additional hydration products that enhance the concrete’s fire resistance.
Lastly, the addition of fibers to the concrete mix can enhance its resistance to fire. Fibers, such as polypropylene or steel fibers, reinforce the concrete matrix and help to control cracking and spalling under high temperatures. The presence of fibers improves the overall durability and fire resistance of the concrete.
In summary, the performance of concrete during a fire is influenced by factors such as the water-to-binding material ratio, moisture content, type of aggregate used, supplementary cementitious materials, and the inclusion of fibers. These factors, when appropriately considered and optimized, can contribute to the improved fire resistance of concrete structures.
Water to Binding Material Ratio
The water to binding material ratio is a significant factor affecting concrete performance in fire situations. A lower ratio (W/B) generally leads to better concrete performance when exposed to fire. Studies have demonstrated that high W/B ratios (around 0.6) cause a more significant reduction in compressive strength and modulus of elasticity compared to low W/B ratios (ranging from 0.28 to 0.35). This trend holds true for both lightweight concrete and concrete made with alternative materials like fly ash and slag. It is important to note that concrete with a low W/B ratio tends to experience spalling, which is the breaking off of surface layers, at lower temperatures than concrete with a high W/B ratio.
Moisture Content of Concrete
Moisture content greatly impacts concrete performance in fire situations. Higher moisture levels in concrete increase the risk of spalling, which occurs when high vapor pressure within the pores causes the concrete to break apart. Moisture content depends on the relative humidity and the type of coarse aggregate used. Concrete exposed to fire will likely experience spalling if the relative humidity exceeds eighty percent.
Type of Aggregate Used to Produce Concrete
Aggregate plays a significant role in the performance of concrete when exposed to fire, as it typically constitutes around 60-70% of the concrete volume. The properties of the aggregate have a considerable influence on how well the concrete withstands fire conditions. Three commonly used types of aggregates in concrete production are carbonate aggregates (e.g., limestone), siliceous aggregates (e.g., granite and sandstone), and lightweight aggregates (e.g., expanded clay and ceramsite sand). These different types of aggregates bring their own characteristics and behaviors to the concrete mix, impacting its fire resistance and overall performance.
Fig.2: Carbonate Aggregate for Concrete
Fig.3: Sandstone Aggregate
Fig.4: Lightweight Aggregate
Supplementary Cementitious Materials
Generally, blending cementitious materials such as blast furnace slag and fly ash improve the performance of concrete subjected to fire. But the influence of cementitious materials is not the same and type of aggregate is also affect concrete performance. It is demonstrated based on tests that, compressive strength of conventional concrete would be entirely lost at temperature of 1050C whereas concrete produced using cement plus 80% slag replacement would lose around 82% of its compressive strength. Additionally, supplementary materials would increase the resistance of concrete to spalling during fire. Unlike other supplementary materials, silica fume would outperform by normal concrete when they are exposed to the same degree of fire.
Fig.5: Fly Ash for Fire Resistance of Concrete
The performance of concrete varies depending on the type of aggregate used. Concrete made from carbonate aggregate exhibits greater fire and spalling resistance compared to concrete made from siliceous aggregate. This is attributed to the larger specific heat of carbonate aggregate. A higher specific heat in the aggregate improves the concrete’s resistance to spalling under fire conditions. Additionally, carbonate aggregate offers enhanced fire resistance due to factors such as durability and ductility. Lightweight aggregate, on the other hand, is expected to perform well in fire situations due to its low thermal conductivity and high resistance to heat. Tests have shown that concrete made from lightweight or carbonate aggregate maintains its compressive strength when exposed to temperatures of 648.80°C, while concrete made from siliceous aggregate loses half of its compressive strength under the same temperature conditions.
Fig.6: Silica Fume for Fire Resistance of Concrete
Fibers in Concrete
The addition of fiber to concrete improves its performance when exposed to fire. Specifically, the inclusion of polypropylene fiber has been shown to enhance the concrete’s resistance to spalling, which is the breaking or cracking of the surface layer. However, the addition of polypropylene fiber does not significantly enhance the mechanical properties of the concrete. To improve spalling resistance, it is recommended to mix 0.1 to 0.5 percent of polypropylene fiber into the concrete mixture. If high temperatures are expected, it is advisable to use long fibers. Another type of fiber that can be added to concrete is steel fiber. While the spalling resistance of concrete with steel fiber is not as effective as that of concrete with polypropylene fiber, it generally improves the mechanical properties of the concrete. Therefore, the addition of fibers enhances the performance of concrete when exposed to fire.
Fig.7: Polypropylene Fiber to Increase Fire Resistance of Concrete
References
Several studies have explored the mechanical properties of concrete at high temperatures. In a 2015 review published in Construction and Building Materials, Ma et al. examined the subject in detail. They discussed the effects of elevated temperatures on concrete and provided insights into its mechanical behavior under such conditions.
Another overview of structures exposed to extreme high temperatures was conducted by Yehia and Kashwani in 2013. Their study, published in Civil Engineering, highlighted the performance of structures when subjected to extreme heat. They discussed various aspects related to the behavior and response of structures under these conditions.
Examining the fire performance of high-strength concrete structural members, Kodura conducted a study in 1999, which was published in Construction Technology Update. Kodura investigated the behavior of high-strength concrete structures when exposed to fire, evaluating their fire resistance and performance.
In 2007, Kodura and Phanb published a paper in the Fire Safety Journal, focusing on the critical factors that influence the fire performance of high-strength concrete systems. They discussed the key elements that determine the behavior and response of these systems under fire conditions, shedding light on important considerations for their design and performance.
These studies collectively contribute to our understanding of the mechanical properties of concrete at high temperatures and the performance of structures exposed to extreme heat. They provide valuable insights into the behavior of high-strength concrete under fire conditions, informing the design and assessment of structures in such scenarios.