Doç. Dr. / Assoc. Prof. Dr. Hasan YILDIRIM
İ.T.Ü Faculty of Civil Engineering
1. Introduction
A concrete, which comprises calcium aluminum cement (CAC) as a binding agent and is heat-resistant and suitable for higher temperatures, is a refractor concrete. The heat resistant or refractor aggregates and filler materials are used for this type of concrete. The limit value between the heat-resistant and the refractory concrete is approximately 1000˚C but some sources say that it is after 1500˚C. However, the spectrum range for higher temperature resistance can start from 300–400˚C and go up to 2000˚C and over. High-class calcium aluminum cement is utilized to reach these values.
2. Calcium Aluminum Cement (CAC ) and Portland Cement (PC)
It has rapidly become apparent that CAC has some heat resistance and refractor properties although it is not particularly produced to be exposed to heat impact. It was first used in 1930s and its use was significantly increased in 1950s. The use of Ordinary Portland Cement (OPC) at high temperatures is limited for some reasons. With having completed the hydration, Ca(OH) that makes up the large part of Portland Cement becomes dehydrated at 2 500˚C to be converted into CaO.
3. Refractor Concrete
A concrete that can be used up to at 1000˚C is considered heat resistance. This is a reliable point although there is no exact limit. Most refractor concretes can be used at a temperature much lower than 1000˚C and sometimes higher than that. What matters is to select a suitable concrete for the temperature to which it will be exposed.
In the cases where refractor concrete is replaced with heat-resistant concrete, the operations are similar as with the ordinary concrete. However, the concrete manufactured with CAC is required to be prepared on the construction site and then placed as it is usually unable to be supplied by ready-mixed concrete plants. But bear in mind that the points to take into consideration when preparing and placing the ordinary concrete should also be considered when dealing with the concrete with CAC. The relationship between the shrinkage, durability, wearing and the water/cement ratio, compactness and cement content is important. There are also other parameters to be considered for a concrete to display a good performance.
The concrete manufactured with grey CAC (39% of Alumina) normally shows an adequate resistance when it is exposed to 1000˚C. Actually for some type of aggregates (e.g. vermizulit, perlite, basalt, etc), the characteristic of aggregate, rather than the characteristic of cement, can limit the performance of concrete against higher temperature. In some cases it is vice versa and increased resistance of aggregate improves resistance of concrete made with CAC against heat effect.
4. Refractor Insulating Concrete
Thermal insulation properties of concrete depend on the density and the aggregate is directly affected by the density of concrete. The density of light aggregate varies between 100 and 110 kg/m3 (an example of light aggregate: perlite) and 600 and 800 kg/m3 (an example of medium light: sintered PFA). Thermal conductivity of concrete made with light aggregate is between the range of 0.15 to 0.5 w/m˚K (Figure 3).
Such concretes are used in heat insulators together with the refractor concretes. The refractor concrete exposed to heat is enveloped with the light concretes to provide thermal insulation so that the heat loss can be reduced. CAC is placed into mould in single piece as it will be used as a binding agent for both of the concretes.
Figure 3: The relationship between the thermal conductivity and the density after incineration of heat-resistant insulating concretes
High temperature insulating concretes are made of specific aggregates. An example of these aggregates is alumina or micro porous calcium hexa-aluminate (CaO-6Al2O3). In this case approximately 70-80% of Al2O3 CAC will be required as the heat would be greater than the limit of grey CAC.
5. Wear and Heat Resistant Concretes
Heat resistant concretes should also be resistant to wear to a certain extent because of the wear caused by physical contact or gas particles. Most high temperature concretes have a reasonable wear resistance.
However, there are some cases requiring concrete to be resistant against heat, thermal shock and wearing (areas of fire drill). Suitable synthetic aggregates are available for such conditions. These aggregates are generated by breaking up and grading calcium alumina clinkers. The chemical structure of this aggregate is similar to grey CAC.
Such aggregates are hydraulically bound to CAC matrix in the concrete, which improves the strength and wear resistance of concrete. Increased harness of aggregate reduces wear. Such concretes are used in many locations exposed to heat, wearing, thermal shock, and chemical effects.
6. Refractory Concrete Resistant to Higher Temperature
Refractory concretes are used in thermal insulators, and metallurgy, ceramic and cement industry. They are prefabricated to be used on construction sites.
Currently there are many types of refractory concrete. The type of refractory concretes depends on the cement content and installation. They include Conventional Castables (CC), Low Cement Castable (LCC), Ultra Low Cement Castables(ULCC), Self Flow Castables (SFC) and spray casting.
The cement ratio in CC concretes is 15-25% by weight as in ordinary concretes. The service temperature of concrete depends on the combination of CAC used and refractory aggregate.
The performance of conventional casting concretes under a higher temperature depends on their chemical structure and particularly on the amount of lime (CaO) contained. A conventional casting composed of 15% of high class CAC (70-80% of Al2O3) and alumina aggregate contains 2.5-4.5% of CaO. Using micron size silica fume in concrete and developments in LCC and ULCC provide decrease in cement ratio thus in lime ratio.
7. Concretes Containing Low and Ultra Low Cement
The lime in calcium alumina cement (CAC) is the most important factor affecting the behavior of refractory concrete under high temperatures. 80% of alumina CAC approximately contains 18% of CaO. Although CaO was reduced in cement with higher alumina content (e.g. 90%), the required commercial achievement was not acquired. Therefore, low lime content in refractory concrete is ensured by reducing cement content.
Production of low cement concrete was improved during 1970s. This study used particle-packing theory. However, distribution of theoretical grain size could not applied due to absence of aggregates in 0,1 to 1 μ grain size in practice but use of silica fume solved the problem.
Such material allowed continuity of theoretical curve of grain size distribution of concrete when it was less than 0,1μ. So, cement content was reduced by 5-8%. This concrete is known as Low- Cement Castables (LCC). Developments in the technology and common use of chemical additives have reduced the cement ratio by 2% more. This new cement ratio is called Ultra-Low-Cement Castables (ULCC).
The greatest advantage of LCC and ULCC is that they show a better performance against higher performance in terms of mechanical properties compared to the conventional production. Due to such property, refractory concrete has replaced the refractory tiles used up to the present.
8. Self-Flow Castables Concretes
Self-flow castables (SFC) is refractory conjugate of self-compacting concrete. It was really hard to place low and ultra low cement concrete into especially large and complex geometric moulds. Ongoing improvements in chemical additives and attention on grain size distribution allowed development of selfflow castables concrete. This facilitated the use of SFC yet did not generalize the production of LC and ULC.
9. Constitution of Heat Resistant and Refractory Concrete
9.1 Placement and Compression
Placement of high resistant and refractory concrete is quite important as conventional concrete. No equipment or skill is required other than placement of conventional concrete. The moulds used are the same. But the size of moulds should be paid attention to. Mixture ratio of concrete and mortar should be observed.
9.2 Cure
A strict attention must be paid to provide appropriate cure conditions for the concretes used CAC cement. Poor cure might result in dusty and weak concrete surface and collapse failure in use. The purpose of cure phase is to maintain the damp in concrete and to ensure hydration has been completed. Instant strength gain and higher hydration temperature require CAC concretes to start curing 3-4 hors after placement and to maintain for 24 hours..
9.3 Drying and Firing
There is still considerable amount of water in the concrete after curing. The concrete must be preheated to remove water in the concrete in order to prevent breaking up and spattering parts in case of a fire. The drying period of concrete is crucial prior to putting into service. Natural drying is recommended prior to exposing to higher temperature, or drying the concrete by exposing to 100°C is required to remove free water as much as possible.
After drying, hydration water is removed from concrete when it is exposed to 100–350°C. This is the bound water in the hydrated concrete and is not removed from the concrete in drying stage.
Heating schedule is of capital importance during the initial firing. This schedule varies by thickness and type of concrete and place of application. The concrete must be heated at 50°C per hour and when the temperature reaches to 500°C it must be kept at this temperature for 6 hours. Then the concrete is continued to be heated until it reaches its serviceable temperature. It is recommended to keep the sections greater than100 mm in thickness at different temperatures.
If it is not possible to provide appropriate drying and firing conditions or section thickness is too large (>500mm), the required conditions to remove water in the concrete must be ensured. Such conditions are ensured by using porous aggregate and organic fibers.
No other specific firing is needed after initial burning at a temperature that concrete will be exposed to when using. However, the concrete should not be exposed to water and saturated again after this stage.
9.4 Reinforcement
The state of steel reinforcement in concrete exposed to higher temperature needs to be considered. The difference in thermal expansion between the concrete and the steel at temperatures (300°C) to which heat resistant or refractory concrete is exposed results in reduced adherence. Cracks and breaking up in the concrete can occur at higher temperatures especially where thick steel profiles are used. Sudden decreases in tensile strength of normal steel occur when the temperature exceeds 400°C. In this case, the presence of steel in concrete is insignificant.
Therefore additional reinforcement may be required. The reinforcement in floor concretes must be placed as distant as possible from the surfaced exposed to heat. The temperature exposed by reinforcement should not exceed 300°C. Light steel mesh can be used. Steel mesh should not be placed higher than the center point of mould height.
If required, steel fibers can be used in heavy industrial zones. Stainless steel fibers show better performance than standard steel at higher temperatures.
9.5 Shrinkage and Thermal Expansion
It is usual to have cracks in the concrete after initial firing. This results from the ceramic reaction between the dehydration shrinkage and aggregate and the cement. These cracks get closed up due to reheating when the concrete is put into use. Thermal expansion after initial firing is returned following cooling and heating cycle (Figure 4). If filling of cracks is avoided, it will not cause any problems. Heat effect generated afterwards does not extend the width of cracks.
Figure 4: Heating and cooling cycle of concrete
9.6 Resistance after Firing
The development of resistance prior to firing for ordinary concretes and CAC concretes is similar. Hardening starts with setting of concrete (3-4 hours) and 90 % of the final resistance is achieved after 24 hours.
During the initial drying and firing cycle, changes in the resistance depends on the loss of free and chemically bound water and the reaction between the CAC and the aggregate used. Typical heat resistance relationship of ordinary and LCC concretes is shown in figure 5.
Figure 5. Fraction module of ordinary and LCC concrete
The resistance is reduced in ordinary concrete at 500°C due to impaired hydraulic bond. The decrease in resistance up to the temperature where the ceramic bond was started between the aggregate and the cement when the temperature was increased continued at a low level. The ceramic bond between the cement and aggregate varies depending on the type of aggregate and cement used at 900 to 1200°C. This bond is actually formed when softening point of concrete is reached.
When the concrete resistance is determined under the heat effect without allowing concrete to cool, it is gradually decreased as the temperature is increased. The ceramic bonds are formed and the resistance is increased when the test is performed after the concrete has been cooled up. Since formation of liquid phase is less in LCC concretes, the resistance of both testing systems is higher than the ordinary concrete. LSS concretes show better performance at higher temperatures as they are commonly used in iron and steel industry.
10. Application Areas
The followings are the areas where refractory concrete is applied. The area of use for this concrete does not include iron and steel industry where the temperature is 1800°C or over.
10.1 Residential Flue Ducts, Furnaces and Flues
CAC and heat resistant precast concrete made with aggregate fired in the furnace are often used for residential flue ducts. Although the temperature reaches up to several hundreds of centigrade in flue ducts, this type of concrete can resist to 1000–1100°C. However, the temperature can go up to 900°C in case of flue fires. Fire bricks or chamotte is used in locations such us fire place and furnace where the temperature reaches to 1200°C. CAC based concrete is used for industrial kiln shafts. This concrete is resistant to acids and hazardous effects caused by flue gases, as well as to fire.
10.2 Foundry Floors
Molten liquid metal is likely to spill on the floors of industrial buildings. Thus, the concrete must be resistant to thermal shock and higher heat effect caused by liquid metal. The concrete must also be resistant to wear and collision. Some basalts and granites are resistant to such cases. However, some exceptional CAC and synthetic calcium alumina aggregate composites have been proven to be partially suitable for such applications.
10.3 Fire Drill Areas
It depends on the drill area for each concrete. Fire drill areas are exposed to periodic cycles of combustion and extinguishing. Burning material can be different types of materials such as hydrocarbon, wood, paper, furniture, rubber, etc. Burning materials can chemically damage the concrete during fire in addition to heating effect.
Fire tests can be performed on large floors, in fire rooms and even in a two-storey building. If such buildings are made of CC, they can be out of service in short period of time. On the other hand, specific performance levels expected from concrete can be provided by using CAC and synthetic calcium alumina aggregate.
11. The Effects of Higher Temperature over High Resistant Concrete
To show better performance in areas of application, concretes have been produced which have been developed in the past twenty years and of which compressive strength is 80 Mpa or over using either chemical or mineral additives (Kalifa et al, 2000; Neville 2000).
High resistant and durable concrete is being studied in many aspects. Although high performance concrete provides advantages in many ways when used for ferroconcrete buildings, the brittle structure is the weakest point (Poon et al, 2004). High performance concrete loses its characteristic when exposed to higher temperature and can be seriously damaged, e.g. breaking up and disintegration, compared to the ordinary concrete. The reason for breaking up and spattering at higher temperature is the density of high performance concrete. The tightness of inner structure reduces resistance to fire and makes high performance concrete more exposed to higher temperature effect compared to the ordinary concrete. (Kalifa et al, 2000; Chan et al, 2000).
The use of mineral additives such as silica fume, fly ash and cinder is most effective way of making high performance concrete (Poon et al, 2004). The resistance of concrete containing silica fume to a higher temperature depends on the amount of additives and resistance level. The strength of high resistant concrete with silica fume greater than 20% is lower than ordinary concretes. When the temperature exceeds 300ºC’I, gel absorbed water is released; the pressure in capillary void of high performance concrete is increased because the size of capillary voids is small causing large tensions in the concrete. Resulting pressure effect causes cracks and breaking up (Yeğinobalı, 2002; Baradan et al, 2002; Yüzer et al, 2004). Poon et al (2001) investigated the effects of higher temperature over high resistant concrete in his study and presented experimental assays involving silica fume concrete samples, and one of the assays showed that the compressive strength of concrete samples with 14-20% of silica fume and 170 MPa was increased at any temperatures lower than 350°C while there was a sudden drop at higher temperatures, and reported that damages such as cracks, spattering parts and breaking up at 650 °C. Another example of the same study showed that silica fume in samples containing silica fume of 10% did not provide any benefits for the concrete at a higher temperature.
