Fire Resistance of Structures 5. 1 Behaviour of construction materials at high temperature

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1 Fire Resistance of Structures 5 Martin Gillie School of Engineering and Electronics The University of Edinburgh 1 Behaviour of construction materials at high temperature The kind of materials used in construction and the manner in which they respond to fire, clearly has a great bearing on how a structure will respond to fire. It is therefore important to develop an understanding of the behaviour of structural materials subjected to fire in terms of both the heat transfer characteristics of the material and its mechanical response to heating. The following text will attempt to outline the behaviour in fire of the most important materials of construction in simple terms. The manner in which a material responds to fire can be classified (somewhat arbitrarily) under the following groups according to behaviour or properties 1. Chemical: Decomposition, charring. 2. Physical: Variation in density (ρ), softening, melting, spalling. 3. Mechanical: Strength as measured by yield or peak stressed ( f y for steel and f cu for concrete), stiffness as measured by the modulus of elasticity (E), creep, thermal expansion as measured by the coefficient of thermal expansion (α). 4. Thermal: Thermal conductivity (k), specific heat (c). Chemical changes such as decomposition and charring are specific to wood. Physical effects such as spalling happen in concrete and masonry. Steels soften and creep but are unlikely to melt at the maximum temperatures experienced in normal fires.the following table shows the properties that are of interest for common structural materials: Property Concrete Steel Masonry Wood Plastics Gypsum Glass Chemical Decomposition x x x x Charring x x Physical Density x x x x x x x Softening x x Melting x x Spalling x x Mechanical Strength x x x x x x x Stiffness x x x x x x x Creep x x x x x x Thermal expansion x x x x x Thermal Thermal Conductivity x x x x x x x Specific heat x x x x x x x Thermal conductivity determines the rate of heat transfer in the materials. Specific heat determines the heat absorption capacity of a material for a given rise in temperature. Both of these properties are 1

2 combined in a dimensionless measure called thermal diffusivity ( ρc k ) which determines the temperature distributions that will exist in a material at various points in time during a fire. These properties are themselves dependent upon temperature. Structural engineers are primarily concerned with mechanical response (when forces and/or displacements are imposed). For structural materials this is governed by the mechanical properties of that material, which in turn has a great bearing on the response of whole structures to fire. The main mechanical properties of interest are, the strength of a material, which is defined as the maximum stress the material can carry. The property of strength determines the load carrying capacity of a structural material. The stiffness of a material is defined as the force required for a unit deformation, measured by the Young s Modulus of Elasticity (E). This property a structural material determines the magnitudes of displacements a structure is likely to experience. Both of these quantities are generally considered constant for most materials at normal temperatures and normal loads, however they reduce at high temperatures and loads (as the material degradation due to high temperatures and loads erode a materials capability to carry stress). Creep is the strain (i.e. deformation per unit length) that occurs in materials with time. So materials continue to undergo deformation if a load is applied over a long period of time. For most materials the creep rate increases significantly at high temperatures. Most materials expand on heating (with a few notable exceptions, such as wood), and in many cases (especially in large structures with stiff restraints to expansion) this phenomena can dominate the structural response. Thermal expansion is measured by the strain induced by unit degree rise in temperature, defined as the coefficient of thermal expansion α. All of the above properties can vary with temperature and good estimates of them must be available to allow an accurate analysis of the response of structures in fire. In the following sections the effect of high temperature upon the aforementioned properties of the most important structural materials is discussed. 1.1 Steel Steel is arguably the most important structural material in modern construction. Steel is used in construction as structural steel or as reinforcing steel for reinforced concrete. Reinforcing steel can be in the form reinforcing bars or high tensile strength steel tendons in pre-stressed concrete. Structural steel is considered considerably more vulnerable to fire than reinforcing steels which are encased in concrete which has good insulating properties and so protects reinforcing steels from significant losses in strength. Steels are very good conductors and tend to be used in thin sections. They are, therefore, liable to heat up very quickly in fires if not insulated. Due to these reasons most main structural steel members are required to be insulated in current design codes. The rate of heating depends upon the parameters of thermal conductivity, specific heat and density. The density of steel is approximately 7850 kg/m 3. The thermal conductivity of steel is approx. 54 W/mK at room temperature and reduces to about half this value at 800 C. Figure 1 shows the variation of thermal conductivity with temperature. For simple calculations the specific heat of steel can be taken as approximately 600 J/kg K. Figure 2 shows the variation of specific heat with temperature. Figure 3 shows stress/strain curves for structural steel at high temperatures. The stress/strain curve is a measure of material stiffness. The Figure shows that the strength of cold rolled structural steel increases with heating up to temperatures of 300 C (the stiffness doesn t), however thereafter reduces rather rapidly. Similar curves for cold-drawn pre-stressing steel are shown in Figure 4, which show a different pattern of behaviour, notably a considerably higher rate of strength degradation. Pre-stressing steel does regain its strength upon cooling to the same degree as hot-rolled steel. In design the yield strength of steel is required, however from the Figures above it is clear that a well defined yield strength disappears when steels are heated. The concept of proof stress is used in this case. A 1% proof stress is illustrated in Figure 5. Also for design purposes, it is convenient to represent the 2

3 Figure 1: Thermal conductivity of steel Figure 2: Specific heat of steel 3

4 Figure 3: Stress/strain curves for mild structural steel Figure 4: Stress/strain curves for cold drawn pre-stressing steel 4

5 Figure 5: 1% proof stress change of mechanical properties with temperature in terms of their magnitude at ambient temperature (say 20 C). The coefficient of thermal expansion of steel, α, is around / C and increases slightly with temperature. At about 730 C, steel undergoes a phase change (energy is absorbed without increase in temperature) and a denser structure results. The heat absorbed at phase change causes delay in further temperature rise. A simple relationship provides a useful estimate of thermal expansion variation with temperature, 1 α = T (1) where, T is the temperature change in C. The property of thermal expansion of structural steel can be both beneficial to the structure or be the cause of great damage. For instance a steel beam anchored to a masonry wall can cause it to collapse by expansion (during heating) or contraction (during cooling - this can make masonry walls very hazardous in post-fire operations). Steel reinforcement in reinforced concrete loses strength at high temperatures more rapidly than structural steel, however given adequate concrete cover, it rarely reaches high enough temperatures and in general is expected to retain its properties in a fire. 1.2 Concrete Concrete has excellent fire resistance properties and maintains its integrity and strength in very high temperatures. The thermal properties of concrete depend upon the aggregate type used, due to chemical changes (crystal structure) in aggregate compounds. Three common types are; Siliceous aggregates (gravel, granite, flint), calcareous aggregates (limestone) and lightweight aggregates made from sintered fuel ash (Lytag) and expanded clay. Siliceous aggregate concretes have a tendency to spall due to high thermal conductivity of such aggregate. Calcareous aggregate concretes are relatively more sta- 5

6 Figure 6: Relative change of mechanical properties with temperature 6

7 ble. Lightweight concrete (LWC) has the best thermal properties of all, i.e. less than half the thermal conductivity (0.8 W/mK) of normal weight concrete (NWC) and consequently loses its strength at a considerably lower rate. The thermal diffusivity of LWC is only slightly lower than NWC, so the extra fire resistance in LWC comes not so much from reduced temperatures, but from the stability of the light weight aggregates at high temperatures. The typical density of NWC is 2350 kg/m 3 and that of LWC is 1850 kg/m 3. NWC initially expands with rise of temperature but progressive loss of moisture from the cement paste causes it to shrink and helps to offset thermal expansion of aggregate. Heat is used up in drying and thus reducing further the rate of temperature rise of the concrete surface, which is low to start with due to poor conductivity. The loss of strength due to dehydration is quite often confined to the surface layers. Figure 7 shows typical thermal conductivities of various concretes with temperature. Figure 8 shows the Figure 7: Thermal conductivity of various concretes varying with temperature variation of specific heat of various concretes with temperature. The coefficient of thermal expansion of concretes is of the same order as of steel, but here again the it is considerably higher for NWC, up to / C, while that of LWC is about / C. Concrete thermal expansion depends considerably upon the stress in concrete, so for large compressive stresses the thermal expansion coefficient can be considerably lower that in unstressed concrete. This is caused 7

8 Figure 8: Specific heat of various concretes varying with temperature 8

9 by the fact that creep effects become very important in concrete at around 400 C, with strains increasing considerably for small increases in temperature beyond this point. This effect is termed as transient thermal creep and it can be so large at large compressive stresses as to completely counter the effect of thermal expansion, even leading to contraction. Concrete stiffness (i.e. the slope of the stress/strain curve) also reduces significantly at high temperatures, which also results in additional strains. These effects can cause large deflections in concrete structural members. The strength loss in concrete is slow because of the low thermal diffusivity. Figure 9 shows the stress/strain curves for a type of NWC. For design purposes, the loss in concrete strength in temperature is generally Figure 9: Relative stress strain curves for dense concrete varying with temperature required. Experimentally determined values from a number of sources with a proposed practical design curve 2 is shown in Figure 10. A simpler design curve for both normal and light weight concrete is shown in Figure 11. One of the most destructive effects of fire on concrete is spalling (loss of surface material), which ranges from superficial surface damage to explosive blowout of large chunks of material. The intensity of spalling generally depends upon the type of aggregate used, with NWC much more likely to suffer spalling damage compared to LWC. Siliceous aggregates undergo sudden increases in volume at certain temperatures (through crystal structure changes). In severe fires these volume changes may be rapid enough to cause excessive internal stresses and cause the surface layers to spall, thus exposing the reinforcement, which can lead to loss of load-bearing capacity. Spalling is characterised by lines of striation 9

10 Figure 10: Strength reduction in normal weight concrete with temperature 10

11 Figure 11: Strength reduction in normal and light weight concretes with temperature and loss of surface material resulting in chipped, cracked, broken or cratered appearance. Any kind of differential expansion can lead to spalling. So rapid cooling can also cause spalling, a situation that may occur in extinguishing fires with water. In fires spalling is generally associated with high temperature. However areas of concrete or masonry under high compressive stress may spall at relatively lower temperatures further away from the hottest regions. Spalled areas may also appear lighter in colour than adjacent areas through exposure of clean subsurface material. Colour changes in concrete can be used as rough guide to the temperature reached and the residual strength, i.e. Grey: (Under 300 C) Minimal loss of strength Pink, red or red/brown: ( C) 10 to 60 % of the strength is lost (for NWC). Grey-white: ( C) 60 to 100% loss of strength. Buff: (Over 1000 C) Sintered. 1.3 Wood Wood (or timber) is a combustible material, however, it is also one of the most widely used materials of construction. It is therefore fortunate that wood possesses certain features that allow it to provide satisfactory performance in most building fires. One of these is that it is not easily ignitable, but the most important property of wood is the formation of char after ignition, shown in Figure 12. Charred wood is likely to be found in nearly all structural fires. It is the solid residue (mainly carbon) from the decomposition of wood. It shrinks as it forms and develops cracks and blisters. The analysis of char patterns and relative depth and appearance can provide vital clues about the spread of fire. The rate of char formation varies with species of wood, mainly depending upon the bulk density and incident heat flux 3 and also depends upon the density and moisture content. In combination with a low thermal 11

12 Figure 12: Char formation in wood 12

13 conductivity, and a protective layer of char, heavy timber sections can provide excellent fire resistance and therefore continue to be used, especially in North America. The concept of sacrificial timber is used in design, i.e. using a larger section of timber than necessary for carrying the design load, with the excess sufficient to protect the member through a given duration of fire. Figure 13 shows, how this concept may be used in design. Figure 13: Design concept for large section timber structural members In terms of its mechanical behaviour, wood is an orthotropic material, with considerably different properties parallel and perpendicular to the grain, as illustrated by Figure 14. The Figure also illustrates the significantly different mechanical response of wood in tension and compression (like concrete). Strength reduction in wood begins as soon as it is heated. Compressive strength reduces at a higher rate than tensile strength. Figure 15 shows the strength and stiffness reduction with temperature relative to ambient values. The loss of strength in timber sections can also be considered proportional to the rate of char formation. Figure 16 shows the strength and stiffness based on the distance from the char layer relative to ambient values. Timber does not expand on heating like steel and concrete and therefore does not threaten adjoining masonry in the same manner. 1.4 Masonry Masonry consisting of either brick-work or of concrete block-work is inherently stable in fire. The reasons for masonry wall failures are often nothing to do with the fire resistance capability of masonry materials. The following is a partial list of masonry failures due to structural reasons: Steel/Composite beams may cause instability in a connected wall through expansion (or contraction at cooling). High walls with low slenderness ratio (i.e. thin). Lack of lateral support. 13

14 Figure 14: Typical stress-strain curves for wood Figure 15: Effect on temperature on the mechanical properties of wood 14

15 Figure 16: Variation of wood mechanical properties with distance from char layer Movement of super-structure supported on masonry. Differential heating due to a progressive pre-flashover fire. Masonry can also suffer integrity failure when fire loads are excessive. Bricks can withstand temperatures of around a 1000 C and they melt at about 1400 C. In domestic fires integrity failures are more common. Most comments about spalling discussed earlier apply to masonry as well. 1.5 Glass The main use of glass in buildings is in glazing for windows and doors. In this role, glass is has little resistance to fire and generally cracks very quickly because of the temperature difference across the exposed surfaces. Double glazing does not improve this behaviour significantly. Wire reinforcement does provide relatively greater integrity, however in general glazing should not be relied upon to remain intact in a fire. 1.6 Gypsum Dehydrated gypsum is well known as Plaster of Paris, which is a white powder which sets hard after being mixed into a paste with water. In building gypsum plaster and plaster boards are widely used in interior linings and partitions. In addition to the low thermal diffusivity with the large amount of chemically bonded water in gypsum building products allows it absorb a considerable amount of heat from fires and therefore act as a cheap and effective fire retardant material. Once the water of cystallisation has evaporated, gypsum plaster-boards have practically no strength left (other than that provided by the fibre glass reinforcement). 15

16 1.7 Plastics A large variety of plastics are used in buildings. The main disadvantage is that all plastics are combustible. Certain treatments can increase ignition temperatures and inhibit flame spread, but nothing can be added to make them non-combustible. 2 Resources The following webpage provides more information on this topic References [1] P.J. Di Nenno, editor. SFPE Handbook of Fire Protection Engineering. Society of Fire Protection Engineers, [2] A.H. Buchanan. Structural Design for Fire Safety. John Wiley and Sons Ltd, [3] D. Drysdale. An Introduction to Fire Dynamics. John Wiley and Sons, 2nd edition,