Fiber reinforcement for refractory concrete

Whenever an industrial process requires large quantities of heat for the manufacture of new products or semi-finished goods, refractory materials must be used to protect the structure. These materials are applied as packing on the structures they are designed to protect. There are many refractory materials and attempts are still being made in the field of Applied Research to find new materials or techniques for protecting against heat. The most important types of refractory packing are refractory masonry (using pre-formed elements such as blocks, bricks, etc.) and on-site castings. This latter approach makes use of concrete prepared with ground refractory material that is riddled to control the particle size and binders capable of ensuring good hydraulic setting at room temperature and good ceramic setting at working temperatures. Concrete may be applied to the structures requiring protection in a variety of manners: cast on site with the aid of formwork, plastered on or applied like gunite, i.e. sprayed with special pumps. Refractory concrete has advantages in that it is plastic, sets quickly and ensures high performance, but it also has some limits deriving from its fragility and shrinkage. During drying and firing, refractory concrete undergoes a negative deformation (i.e. contraction) which, together with the stresses induced by abrupt temperature variations, may cause cracks in the material which trigger off a rather rapid deterioration due to heat-induced fractures. However, as early as the 1970s it was realized that this problem could be avoided by adding Steel Fibers to the concrete mix design. In fact, the presence of Steel Fibers makes the mix tougher and more compact and therefore less fragile; they also act as reinforcement against the spread of cracks induced by shrinkage and thermal stresses.
When designing a fiber-reinforced refractory packing, one must carefully consider the following aspects:
  • refractory material;
  • fiber size;
  • aspect ratio and quantity of fibers;
  • type of metal making up the fibers;
  • aggressiveness of the furnace atmosphere.
 
 
Different kinds of fibers are available in the market; among them, sheet fibers have an advantage in that they are free of lubricants, unlike needle fibers obtained from drawn wire. Moreover, their irregular surface and higher Surface/Volume ratio ensure a more effective bond with the mix. The size of the fibers - their length in particular - is a very important parameter and must be fixed according to the method in which the concrete is to be applied. More specifically, for sprayed concrete (gunite) it is advisable to use fibers no longer than 25 mm, even though with current spray-gun technology it is possible to reach 35-40 mm.
From this standpoint needle-shaped fibers have technological drawbacks in that while they are travelling along the rubber tubes and in the tip of the nozzle, they may cause obstructions to flow. Determining the quantity of fibers to use may be the most delicate problem in the design of fiber-reinforced refractory concrete: in fact, this parameter will necessarily depend on several characteristics of the fibers themselves, in particular the Aspect Ratio (sometimes also referred to as slenderness), defined as the ratio L/D, L being the fiber length and D the actual or equivalent fiber diameter. Every Aspect Ratio is associated with a critical quantity below which the fiber reinforcement will fail to perform its function. The type of steel will be chosen according to the operating temperature to which the refractory concrete will be exposed, the mix design and characteristics of the furnace atmosphere. The choice must be made taking into account the fact that the effectiveness of the fibers is conditioned by the steel's resistance to heat corrosion and its residual tensile strength at high temperatures. Therefore, when choosing which steel alloy to use, it is important to remember that chromium and nickel, respectively, are the elements capable of ensuring said features.
Using steel fibers in the preparation of refractory concrete allows the following advantages to be obtained from the composite:
  • higher compression, flexural and shear strength of the material against coactive heat-induced stresses;
  • greater hardness of the surface layer and thus high resistance to mechanical shocks and abrasive agents;
  • greater ductility of the material and thus a higher Ultimate Strength of the packing structure with a consequent increase in fatigue strength (vibrations, etc.);
  • higher resistance to cracking both in the setting and firing phases;
  • higher resistance to abrupt temperature changes;
  • capacity to withstand loads even after cracking.
The FIBROCEV technical service is at the disposal of designers, whom it may aid, backed by its own experience, in identifying the optimal fiber reinforcement according to the engineering problems to be solved.