The Effect Of A Fire On Materials Construction Essay

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The construction industry can boost the widest range of use of different materials out of any industry; this includes both manmade and natural materials. All these materials have their place and are used in different ways in construction depending on their properties, for example steel is very good in tension therefore is excellent to span loads across columns, concrete is very good in compression therefore it is perfect to be the columns the load has been transferred to.

In construction it is not uncommon to combine the properties of two or more materials to gain serious advantages in a building material such as combining concrete and steel using re-bar in concrete columns or beams will make it both good in tension and compression.

At this stage the author feels it important to talk about how high the temperature in building fires can get, and why. This is important because buildings and the materials that make them can have a massive impact in this. Fires inside buildings can reach temperatures of up to 800°C and a major factor of this is thermal feedback, this is when hot gasses from a fire transfer heat to walls and other surfaces via convection the fire grows and radiation becomes dominant and radiates heat towards the fire fiercely increasing the rate of combustion.

In conjunction to this it is also very important to understand thermal diffusively as this is the property that determines how quickly heat soaks into a material, this is determined by the thermal conductivity (k), its density (p) and it specific heat capacity (cp) the equation to work this out is as follows:

The author feels both former subjects are important to understand because it is very important to ensure the correct material is chosen for the correct components of any buildings, and in order to do so we must understand the elements they could be exposed to and how they will react when exposed to them in order to increase the time line people can escape any building before flashover might occur, this is when a room or building is capable of spontaneous combustion as all the surfaces are hot enough to ignite and temperures can rise by hundreds of degrees in a matter of seconds.( J.L. Sturges 2010 week1&7)

This paper will look at how fire can change the properties on Steel, concrete, gypsum Plaster and glass

Steel

Being a group L (load-bearing) material steel is of interest to the fire safety practitioner mainly due with regard to its mechanical properties. Steel provides the unique combination of strength and stiffness as well as excellent ductility all for a very good price it is a widely used material in construction but the vey properties that make it so desirable are called in to question when exposed to server heat cause in fires. Steel is non-combustible and has a melting point of around 1450°C which cannot be reached in building fires so you may think where is the problem, well it lies in the reduction in strength and stiffness that the structure of a building relies on. This is due to the higher temperatures causing crystal defects to multiply, increasing ductility. The hotter the steel gets the higher the reduction in yield strength this is a very important property as any reduction in yield strength is also a reduction in a load carrying ability and the greater risk of plastic collapse. Strength loss for steel is generally accepted to begin at about 300°C and increases rapidly after 400°C, by 550°C steel retains about 60% of it yield strength. This is usually considered to be the failure temperature for structural steel, load bearing steel can become subject to plastic collapse (J.L Sturges 2010; T.Z. Harmathy 1993).

It should be noted that typical hot-rolled steel and typical cold worked steel behave differently in fires. When we look at cold worked steel it is know that the mechanical properties of steel can be improved by; rolling, drawing or machining etc, this will increase both the yield and ultimate strengths, though in a fire these steel which are composed of distorted elongated grains brought about by cold work in its unsuitable state, as temperatures rise above 450 °C the grains tend to resume their exuiaxed shapes, and the material losses the excess strength gained from cold working. Also unlike hot-rolled steel upon cooling it will not regain the strengths shown before the fire. This can be a very important factor for the fire safety practitioner to consider when repairs to the damaged building are being carried out, as there may-be a certain load bearing requirement the steel has to reach, this may have been compromised because of the fire. The steel may look sound but its properties could of been effected there for should be replaced or reinforced. It should be noted that the chemical makeup of steel is a more influential factor of how different temperature effect steel than cold working (T.Z Harmathy 1993).

Steel also has a high value of thermal condutivity and thermal diffusivty this means it will soak up any heat from other surrounding materials very quickly and will heat up very quicky all heat related properties will be affected. Along with the reduction in yeild strength as perviously mentioned steel will exspand in a fire, this can cause masonary walls to be pushed over and even steel columns to buckel, all of these factors can contribute to a building collaping (J.L. Sturges 2010 week 7)

There are many ways to prevent steel being exsoped to fire and one of the most common is to use other construction materials to protect steel work, two of the most common materials used are concrete and gypsum plaster this is because they have a very low value of thermal diffusivity therefore it will take a long time for heat to soak into the material this prevents steel work heating up anywhere near the tempuratures required to affect the desiered properties of steel, this will be examined further when concrete and plaster are disscussed in more detail.

Concrete

Although concrete is a simple word, concrete is far from a simple material. It is a type L/I (load bearing/insulating) material, It's commonly understood in the building industry that concrete is the most fire-resistant building material in everyday use, the meachanical and thermal properties are of great intreset to the fire saftey practitioner.

Because of concrete's high specific heat capacity, a fire will generally not cause a rapid increase in its temperature and may not cause significant damage. You cannot set concrete on fire and it does not burn, nor will it produce smoke or toxic fumes or drip molten particles during or after a fire, unlike other materials such as some metals and plastics. For these reasons concrete is highly fire resistant and, in most applications, concrete can be described as virtually 'fireproof'. This exceptional performance derives from concrete's constituent materials (i.e. cement and aggregates) which, when chemically combined within concrete, form a material that is essentially inert and, importantly for fire safety design, has a relatively poor thermal conductivity. It is this slow rate of heat transfer (conductivity) that enables concrete to act as an effective fire shield not only between adjacent spaces, but also to protect itself from fire damage ( I.A. Fletcher 2010)

Concerete is usally looked upon as a two component material, consisting of hydrated cement pate and aggregates or alternativly, cement-sand mortor and course aggregates. The properties of concrete are determined by the properties of the componets and the interfaces between them. Therefore concrete will behave in differnet ways during a fire depending on the mixture used during themanufacturing process.

As mentioned concrete has a very low thermal diffusivy therefore performs very well in a fire and depending on the funcianal requirement of the concrete i.e the load bearing capabilities or thermal properties of the concrete we can manipulate the out come of the fire performance by using differnt aggregates, for example using lightweight aggregates such as sand will act as a excellent barrier to prevent heat passing though it, this is very usefull to protect structural steel.

If we take a concrete slab 150mm thickness and heat it from the left hand side by 1000°C the temperature 50mm from the face is 350 °C, at 150mm is 200°C and at 150mm in down to 140°C all of these dramatically improve the performance of steel during a fire. Though we must remember that when concrete is used as part of a structure, the strength is often the most important requirement of concrete, in order to achieve this the most common aggregates used are siliceous (igneous) and calcareous (limestone). Calcareous aggregates and lightweight aggregates tend to retain most of their compressive strength at 650 °C but siliceous aggregates tend to retain 55% of their compressive strength (J.L Sturges 2010; T.Z. Harmathy 1993).

Spalling

When we look at the fire resistance of concrete the presence of moisture can be an advantage though it should not be of an excessive concentration as this can contribute to the cause of spalling a phenomenon that concrete is very susceptible too.

There are many factors that can contribute to this such as, high moisture content, low permeability, high local stress due to differences in thermal expansion of the cement paste and the aggregates, stresses due to the differences of thermal expansion of the concrete and the reinforcing steel. According to Kordina and Meyer-Ottens (1981) and Dougill (1983), the principal kinds of spalling are:

Local spalling, such as surface spalling, aggregate splitting, and corner separation, caused by physical or chemical changes at elevated temperatures;

'Sloughing off`, i.e. partial separation of small layers of surface material, a process that may continue slowly though-out the fire;

Explosive spalling.

Sloughing off and local spalling are both minor worries as they have little effect on the performance of the concrete but explosive spalling is a major worry as this can lead to complete disintegration of the concrete in the very early stages of a fire. However it is generally regarded that explosive spalling will not occur unless there is unacceptable high levels of moisture content in the structure (T.Z. Harmathy 1993)

When concrete is exposed to fire as the temperature rise's the behaviour of concrete becomes more influenced by the growing number of microcracks, created by the shrinkage of the cement paste and the expansion of the aggregates. Fire events with extended life and intensity will create high temperatures in the concrete mass, causing free water inside the concrete to vaporize. Because the concrete will generally have insufficient continuous pores to relieve the vapour pressures, the tensile stresses created will result in cracks that extend to the surface (T.Z Harmathy)

There are a number of physical and chemical changes which occur in concrete subjected to heat. Some of these are reversible upon cooling, but others are non-reversible and may significantly weaken the concrete structure after a fire. Most porous concretes contain a certain amount of liquid water. This begins to vaporise if the temperature exceeds 100°C, usually causing a build-up of pressure within the concrete. In practice, the boiling temperature range tends to extend from 100 to about 140°C due to the pressure effects. Beyond the moisture plateau, when the temperature reaches about 400°C, the calcium hydroxide in the cement will begin to dehydrate, generating more water vapour and also bringing about a significant reduction in the physical strength of the material.

After a fire, changes in the structural properties of concrete do not reverse themselves, as opposed to steel structures where cooling will often restore the material effectively to its original state.

This is due to the irreversible transformations in the physical and chemical properties of the cement itself. Such changes may be used as indicators of maximum exposure temperatures, based on post-fire examination of the state of the concrete surface. It should be noted that, in some circumstances, a concrete structure may be considerably weakened after a fire, even if there is no visible damage.

Gypsum

From the view point of fire safety gypsum products belong in the I (insulating) group, they are used in a wide variety of forms in construction i.e. plasterboard and plaster to skim straight onto walls etc. The raw material used to manufacture these products is either calcium sulphate hemihydrates or calcium sulphate anhydrite, both from gypsum rock, the former will not be heated above 180°C during manufacture then fine powders will added, this will end up as plaster of Paris. The latter will be heated above 180°C and will end Keene's cement.

Gypsum products perform very well in a fire and will not contribute to a fire. In its natural state, gypsum contains the water of crystallisation bound in the form of hydrates. When exposed to heat or fire, this water is vaporised, retarding heat transfer. Therefore, a fire in one room that is separated from an adjacent room by a fire-resistance rated Gypsum board assembly will not cause this adjacent room to get any warmer than the boiling point (100°C) until the water in the gypsum is gone. This along with other forms of gypsum spay can a very effective way to protect steel in fires, as the gypsum will not allow the heat to pass though to the steel work keeping the temperature at a safe working level for steel. The Gypsum becomes calcined making the Gypsum board an ablative material because as the hydrates sublime, a crumbly dust is left behind, which, along with the paper, is sacrificial.

Generally, the more layers of Gypsum board one adds, the more one increases the fire-resistance of the assembly. It is possible to increase the fire resistance of gypsum board by using glass reinforced gypsum, this is formulated by adding glass fibres to the gypsum, to increase the resistance to fires, especially once the hydrates are spent, which leaves the gypsum in powder form.

Polymers

Polymers belong in the I/F (insulating/fuel) group in respect to the fire safety practitioner they are widely used in many applications. However, most polymers, like the majority of other organic compounds, will burn readily in air or oxygen. The flammability of polymers is a serious issue and severely limits their applications

Light-weight, high-performance polymeric materials offer many advantages in these applications over conventional metal and ceramic materials, but they greatly increase the fire risk because of their flammability and possible release of toxic by-products.

Many polymers, if subjected to some suitable ignition sources, will undergo self-sustained combustion in air or oxygen . In general, nonpolymeric materials (e.g., matches, cigarettes, torches, or electric arcs) are the main sources of ignition, but polymers are frequently responsible for the propagation of a fire. A burning polymer constitutes a highly complex combustion system. Chemical reactions may take place in three interdependent regions: within the condensed phase, at the interface between the condensed phase and gas phase, and in the gas phase.

Polymer combustion occurs as a cycle of coupled events [7]: (1) heating of the polymer, (2) decomposition, (3) ignition, and (4) combustion. The polymer is first heated to a temperature at which it starts to decompose and gives out gaseous products that are usually combustible. These products then diffuse into the flame zone above the burning polymer. If there is an ignition source, they will undergo combustion in the gas phase and liberate more heat. Under steady-state burning conditions, some of the heat is transferred back to the polymer surface, producing more volatile polymer fragments to sustain the combustion cycle.

There are three types of polymer molecules: Linear; branched and cross-linked. Depending on their molecular structure they are divided into amorphous (disordered), crystalline (ordered) and partly amorphous and crystalline. Amorphous polymers tend to be classed as thermoplastics this means they can be reheated time and time again, where as crystalline polymers tend to be classed as thermosetting plastics and tend to be more heat resistant and hard but once they are set you cannot re heat or remould them.

Polymers tend to have low strengths and densities, they will quickly soften and lose strength at low temperatures (200°C +) those that do not melt will distort and become unsuitable to use after as fire i.e. thermosetting polymers. Another factor that needs consideration when using polymers is the glass transition temperature. Below the glass transition temperature the material is rigid and glassy above that temperature it becomes rubbery and deformable. Clearly fires can reach very high temperatures therefore the properties of those polymers with a low glass transition temperature will become affected sooner than those with a high glass transition temperature.

There are ways to improve on the fire performance of polymers, mixing flame retardants into the matrix during manufacture can help, but many have their doubts on how effective these really can be and don't believe the advantages outweigh the disadvantages they bring into the equation, considering the toxic fumes they give off when they do burn these lethal compounds are hydrogen chloride (HC1) and hydrogen cyanide (HCN)

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