Heat resistant concrete or refractory concrete have properties of handling extreme temperatures. Installation and applications of heat resistant refractory concrete is discussed.
The placement curing and applications of heat resistance and refractory concrete will be explored in the following sections.
Fig.1: Heat Resistant Concrete or Refractory Concrete
- Heat Resistant Concrete or Refractory Concrete -Installation and Applications
- Placement and Compaction of Heat Resisting Concrete or Refractory Concrete
- Curing of Heat Resisting Concrete or Refractory Concrete
- Drying and Firing of Heat Resistant Concrete or Refractory Concrete
- Reinforcement in Heat Resistant Concrete
- Shrinkage and Thermal Expansion of Heat Resistant Refractory Concrete
- Strength of Heat Resistant Refractory Concrete After Firing
- Applications of Heat Resistant Concrete or Refractory Concrete
Heat Resistant Concrete or Refractory Concrete -Installation and Applications
Following details about heat resistant refractory concrete is discussed:
- Heat resistant concrete or refractory concrete placement and compaction
- Curing of heat resistant concrete concrete
- Drying and firing of heat resistant refractory concrete
- Reinforcement in heat resisting concrete or refractory concrete
- Shrinkage and thermal expansion of heat resistant concrete
- Strength after firing
- Applications of heat resisting refractory concrete
Placement and Compaction of Heat Resisting Concrete or Refractory Concrete
The placement and compaction of heat resistant concrete and refractory concrete is substantially significant. Like ordinary concrete, the heat resisting and refractory concrete is placed and cured, and neither specific tool nor special skills are required.
Regarding formworks, standard materials are utilized and when precast elements are used, dimensions must be considered carefully. If the location is not accessed easily and cannot be casted normally, then the application by gunning is considered and it is conducted by especially skilled contractors.
Curing of Heat Resisting Concrete or Refractory Concrete
The main aim of concrete curing is to keep concrete moisture and continue hydration reaction for concrete to obtain sufficient strength. Inadequate curing will lead to not only dusty and friable concrete surface but also it causes the concrete to fail under service loads. So, curing of calcium aluminum cement (CAC) based concrete is considerably crucial.
Curing of heat resistant concrete and refractory concrete is similar to conventional concrete but the calcium aluminum cement concrete curing must begin within 3-4 hour after the placement because of quick hardening and great heat evolution.
Drying and Firing of Heat Resistant Concrete or Refractory Concrete
After the concrete curing is finished, there will be considerable amount of free water in concrete. Unless this free water is expelled, concrete spalling cannot be avoided when concrete is exposed to fire.
Before concrete is subjected to fire, it is advised to remove as much free water as possible by driven drying out at 100oC or natural drying and if the degree of heating exceeds 100oC up to 350oC, then the hydration cement water is eliminated.
It is considerably important to apply heating carefully and the plane of heat application is based on number of factor such as thickness, concrete type, and the purpose for which the project is constructed.
A typical concrete heating plane involves heating concrete for six hours at minimum temperature of 50oC up to 500oC, then it will be increased to reach the service temperature.
There are cases in which concrete drying-out is not easy and cannot be conducted properly, for example, when the concrete thickness is larger than 500mm. So, it is recommended to create proper passage for water vapor to release. This can be achieved by increasing concrete porosity through the addition of organic fibers or porous aggregate.
It is not permitted to apply heating unless the concrete is allowed to be fully wetted in specific cases, for example, external storage over winter period.
Reinforcement in Heat Resistant Concrete
If steel bars are embedded in heat resisting refractory concrete that is exposed to large heating degree, then paying careful attention to the reinforcement application is a must.
Not only does the high temperature lead to decrease in steel-concrete bond and possibly melting at severe temperature, but it can also cause concrete spalling and influencing steel properties.
It is noted that, the bond between concrete and steel decline at 300oC temperature and if it is increased, the concrete starts to spall and develop crack. At higher temperature, steel reinforcement can lose its function and the presence of steel in concrete will no longer be beneficial.
Recommendation regarding the stated problem includes the steel placement away from heated surface of concrete and the steel reinforcement should not be heated at greater than 300oC.
It is possible to utilize special reinforcement, for instance mild steel and steel fiber in number of cases like heavily industrial areas. The latter possess the ability to withstand larger temperature degree compared with the former.
Shrinkage and Thermal Expansion of Heat Resistant Refractory Concrete
Cracks are commonly developed when heat resisting refractory concrete is exposed to fire because of shrinkage caused by losing water.
Not only does these cracks likely to close at service life but they also cannot create issues if waste materials are not permitted to enter cracks, otherwise the width of the cracks will increase when the concrete is heated again.
Strength of Heat Resistant Refractory Concrete After Firing
Prior to firing, ordinary castables, which contain about 15-25% cement by weight, begin to harden after 3-4 hours of concrete placement and it achieves most of its strength after one day.
When concrete is subjected to heating, its strength development is associated with combined and free water and when the degree of temperature is further increased the changes in strength will be related to the reaction between calcium aluminum cement and aggregate.
When castables concrete is heated to around 500C, hydraulic bond is decreased and this leads to decrease in concrete strength. When heating degree exceeds 500oC, the ceramic bond, which is based on cement and aggregate type, between aggregate and cement is formed at this stage. The concrete show increased strength as it tested cooling but exhibit decreased strength at it tested prior to cooling.
Low cement castables concrete exhibits increased strength both at hot and cooling condition. This type of cement performs substantially well when exposed to high degree of temperature.
Applications of Heat Resistant Concrete or Refractory Concrete
Application of heat resistant concrete or refractory concrete includes fire training areas which may include wide flat surface areas, full-scale rooms or two storey buildings, fire, stair cases used during fire training, foundry floors, domestic flues, fireplaces, and chimneys.
Regarding fire training area, in addition to subjecting concrete to fire, forming chemical as a consequent of burning materials which is used to create fire, is highly possible and this material attack the concrete in the area.
Fig.2: Fire Extinguisher Training Area Using Heat Resisting Refractory Concrete
As far as foundry floors are concern, it is a type of structure that can be exposed continuous heating and thermal shocks in addition to abrasion and impacts. So, concrete that can withstand not only high temperature but also impacts and abrasion must be employed. For example, calcium aluminum cement concrete combine with a synthetic calcium aluminate aggregate.
Fig.3: Foundry Floors using Heat Resisting Refractory Concrete
Chimneys are usually subjected to heating and possible chemical aggression due to the occurrence of acid in flues.
Fig.4: Chimney in Buildings using Heat Resisting Refractory Concrete