Concrete is inorganic and non-combustible mass which makes them excellent to perform against fire or exposure in fire and heat. But, being non-combustible doesn’t mean it is a poor conductor of heat or it is heat resistant.
In fact, it has very high specific heat capacity and good thermal conductivity thus can transfer and store heat for a long time influencing the surrounding material and environment along with self-degradation in strength.
So, Heat and fire resistant concrete simply mean the concrete which is able to maintain its stability and strength along with the thermal integrity or thermal insulation for a certain rated time, withstanding fire and heat at standard fire rating test perform.
The concrete which is needed to be constructed in fire zones such as Chimney, refractory lining, nuclear plants is specially designed as Fire resistant concrete.
And, the normal structure containing concrete as structural components are designed with fire-resistant as safety methods doe probable fire accident that may cause in building and its period od extension.
The major effect of heat and fire exposure over concrete is a loss in the strength of concrete and its degradation by spalling and shelling.
How does the strength of concrete is affected by rising heat?
The strength loss in concrete with the rising up of heat takes place stage by stage. On initial heating of concrete, it will lose only “Absorbed” water i.e. the water concrete has absorbed from external moisture present in the environment that is present on the surface and pores of it.
After the complete evaporation of such evaporable water, the concrete now starts to lose “Adsorbed” water i.e. water bonded by electrochemical force inside the concrete with cement paste products. The removal of such water leads to micro-cracks and some loss in strength about 10%.
Above 150°C, heat causes the starting of degradation loss of bounded water from Silicate hydrates and Calcium hydroxide. And, above 300°C the loss is prominent and strength loss occurs.
In a range of 350°C to 400°C, the calcium hydroxide dehydrates to give CaO(Calcium Oxide). The formed CaO can even cause post-fire damage, i.e. even if the concrete temperature is cooled down the formed CaO still can cause the crack by swelling upon reaction with moisture or external water (like from a fire extinguisher).
In the temperature range 500°C to 650°C, the loss in the strength of concrete is about 50% – 75% of the original strength. The degradation process and loss in strength continue till 850°C – 900°C. At that time the concrete strength becomes 70% – 80% of the original strength.
Technique to make Heat and fire resistant concrete
1) Heat refractory cement
Concrete with high Calcium Aluminate cement (heat refractory cement) is excellent for refractory lining, industrial floor topping in foundries. But this cement gives low strength to concrete. So generally prefer foe lining rather than structural design.
2) Aggregates selection
Siliceous aggregate is a poor selection for heat resistant concrete as they have a large and differential coefficient of thermal expansion. Moreover, at higher temperatures, their volume changes because of their phase transformation (temperature >570°C)
Limestone aggregate is a good selection for fire-resistant concrete as they have a very closer coefficient of thermal expansion than that of cement.
Moreover, they have surplus benefits of using doe fire-resistant. On heating, at660°CCaCO3break downs and at 740°C MgCO3 break downs releasing Carbon dioxide which is an endothermic reaction and thus absorb the heat due to fire.
Moreover, the CO2 even gives blanket protection against heat transfer. The residual remnants also further have a low thermal conductivity which again further prevent heat transfer through concrete.
3) Steel reinforcement
During the design of the heat resistant reinforced concrete, the designated static load on elements if the structure is kept as 55% of yield strength of steel.
This is done because at 550°C the strength of steel is 55% of yield strength at room temperature. So even at high heat (creating a temperature of nearly about 550°C), the steel can still support the static load without failure.
And note that the melting point of steel is 1450°C, so the design for such type of heat resistant concrete is focused on slowing or preventing of the collapse of a structure due to load at higher temperature rather than melting of steel as the main concern is fire protection.
So if the heat is expected to be extremely high then other special steels is needed to be selected.
4) Fly ash
Fly ash and ground granulated blast furnace slag are very less reactive and inert in nature. Moreover, they contain a low quantity of free CaOH2in hydrated form so they are very useful to create fire and heat resistant concrete.
They enhance the mechanical properties of concrete even at high temperatures. And also, they decrease spalling of concrete due to high temperatures.
Design of Heat and fire resistant concrete
The minimum section size is required for heat resistant concrete in order to prevent the fire spread through conduction. Moreover, concrete is needed to be designed with special consideration and material in order to make them fire/heat resistant.
In such design, the matrix strength loss and heat transfer are targeted to compensate with the provision of minimum concrete section size along with the degree of fire-resistant required with help of fire ratings.
The codes and practices use for designing fire and heat resistant concrete are, BS 8110(BSI1985) in which part 1 guides for simple thumb rules for section size and cover to reinforcement; and part 2 guides for special cases along with special material to be used.
Other mostly used or referral codes are IS 1642:1989(Fire safety design of a building ), IS3809:1979 (Fire resistant test) and ACI code –216.1 – 14 (19) (Determination of fire resistance od concrete).
Some other methods to improvise fire resistance and heat of concrete are;
- Use of sand cement render and gypsum plaster
- Fire protection boards covering concrete externally
- Use of fine polypropylene fiber within concrete
- Use of light weight aggregate (mostly air entrained aggregate like pumice)
I hope this article on “Heat and fire resistant concrete” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha