Behaviour of Construction Materials at High Temperature

Concrete is a composite material consisting of cement paste and aggregates, each responding differently to thermal exposure. This dual nature creates complex behavior patterns when subjected to high temperatures.

Physical and Chemical Changes

When concrete is heated above 100C, free water begins to evaporate. Between 100C and 300C, physically bound water is released, causing internal pressure building that can lead to spalling (explosive surface failures). Around 200C, the calcium silicate hydrate (C-S-H) gelthe primary binding agent in hydrated cementbegins to decompose.

Strength Degradation

From 300C to 800C, progressive dehydration of calcium hydroxide occurs, significantly reducing compressive strength. At approximately 400C, silica in the aggregates undergoes a polymorphic transformation from -quartz to -quartz, accompanied by a volumetric expansion that generates internal stress. Above 800C, complete disintegration of the cement paste structure occurs, leading to severe loss of mechanical properties.

Factors Affecting Performance

• Aggregate type (siliceous aggregates are more susceptible to damage than carbonate aggregates)
• Moisture content (higher moisture increases risk of spalling)
• Concrete density and porosity
• Presence of reinforcing steel

Protective Measures

Concrete's fire resistance can be enhanced through the addition of polypropylene fibers (which melt and create pathways for steam escape), the use of lightweight aggregates, and the incorporation of supplementary cementitious materials that improve high-temperature performance.

Steel's mechanical properties are highly temperature-dependent, making its behavior during fires a critical consideration in structural design.

Thermal Response

Steel begins to lose its strength at relatively low temperatures compared to other construction materials. At approximately 200C, yield strength starts to decrease. By 400C, steel retains only about 60% of its ambient temperature strength, and at 600C, this drops to roughly 50%. At temperatures above 700C, structural steel approaches what engineers consider its "collapse point."

Thermal Expansion

Steel's coefficient of thermal expansion (approximately 1210 per C) causes significant elongation when heated. This expansion can induce additional stresses in constrained members, potentially leading to deformation or connection failures before strength loss becomes critical.

Creep and Stress Relaxation

At elevated temperatures, steel exhibits increased creep (continued deformation under constant load). This time-dependent behavior accelerates structural deformations during prolonged fire exposure, potentially leading to premature failure even if immediate strength retention appears adequate.

Wood's response to high temperatures involves complex chemical and physical transformations that lead to charring, combustion, and eventual structural degradation.

Thermal Decomposition

When wood is exposed to heat, it undergoes pyrolysisthe thermal decomposition of organic material in the absence of oxygen. This process begins around 150C and accelerates significantly at 250-300C, producing combustible gases, water vapor, and leaving behind a carbonaceous char residue.

Charring Rate

One of wood's beneficial characteristics is its predictable charring rate, which typically ranges between 0.4-0.7 mm per minute for most structural timbers when exposed to fire. The char layer acts as insulation, protecting the unburned wood beneath and maintaining the structural integrity of the remaining section.

Properties After Fire Exposure

Even after fire exposure, the uncharred portion of wood retains most of its mechanical properties. This predictability allows engineers to design timber structures based on residual cross-section calculationsa design approach that leverages wood's natural fire resistance.

Masonry materials, including brick, concrete blocks, and stone, generally demonstrate good fire resistance, though their behavior depends on composition and moisture content.

Brick Behavior

Fired clay bricks typically withstand temperatures up to 1200C without significant loss of strength. Their expansion coefficient is relatively low, reducing thermal stress. However, mortar joints between bricks are more vulnerable and often fail before the brick units themselves, leading to wall instability.

Concrete Masonry

Concrete blocks generally perform well in fires, with resistance dependent on the aggregate type and density. Lightweight concrete masonry units often perform better than normal weight units due to their lower thermal conductivity and reduced moisture content.

Moisture Effects

One of the main failure mechanisms for masonry during fires is explosive spalling caused by entrapped moisture converting to steam and creating internal pressure. This particularly affects high-strength concrete masonry units with low permeability.

Fiber-reinforced polymers have gained popularity in construction for strengthening and rehabilitation applications, but their performance at high temperatures requires careful consideration.

Matrix Degradation

The polymer matrix in FRP composites begins to soften and degrade when exposed to temperatures approaching its glass transition temperature (typically 60-150C for thermosetting resins). This degradation occurs at much lower temperatures than for traditional construction materials.

Fiber Response

While the fibers themselves (carbon, glass, or aramid) have much higher temperature resistance, the matrix degradation leads to loss of composite action and drastic reduction in overall strength and stiffness.

Protective Solutions

Improving the fire resistance of FRP systems typically involves applying thermal barriers, using inherently fire-resistant resins, or incorporating intumescent coatings that expand when exposed to heat, creating an insulating char layer.

Gypsum products, including plasterboard and gypsum plaster, provide effective fire protection through their unique thermal properties.

Dehydration Process

When exposed to fire, gypsum undergoes endothermic dehydration, releasing water vapor in a process that absorbs substantial heat energy. Approximately 20% of gypsum's mass is chemically bound water, which is gradually released between 80C and 200C.

Insulating Effect

This endothermic reaction slows temperature rise on the unexposed surface, making gypsum products effective fire barriers. Specialized fire-resistant gypsum boards contain additives such as vermiculite or glass fibers to further enhance their fire performance.

Applications

Gypsum-based materials are extensively used in passive fire protection systems, including wall assemblies, shaft walls, and enclosures for structural elements, providing crucial fire resistance ratings for buildings.

Standardized fire testing provides the data necessary to evaluate material performance and compliance with building codes.

Fire Resistance Tests

Fire resistance tests expose structural elements to a standard temperature-time curve (such as the ASTM E119 or ISO 834 curve) while subjecting them to design loads. The tests measure performance in three categories:

1. Structural Integrity: Ability to support design loads
2. Thermal Insulation: Limiting temperature rise on unexposed side
3. Fire Containment: Preventing passage of flame and hot gases

Standardized Tests

• ASTM E119: Standard Test Methods for Fire Tests of Building Construction and Materials
• ISO 834: Fire Resistance Tests Elements of Building Construction
• EN 1365: Fire Resistance Tests for Loadbearing Elements
• BS 476: Fire Tests on Building Materials and Structures

Material-Specific Testing

Specialized tests evaluate specific material behaviors, such as:

• TGA (Thermogravimetric Analysis) measuring mass changes with temperature
• DSC (Differential Scanning Calorimetry) measuring heat flow and phase changes
• High-temperature mechanical testing evaluating strength, modulus, and creep at elevated temperatures