Improving the mechanical properties of concrete

The purpose is to improve the mechanical properties of concrete

The Spanish Instruction on Structural Concrete (EHE-08 – RD 1247/2008) is the name given to the Spanish regulations on the calculation and safety of concrete structures. It is mandatory for all structures where concrete is used in Spain

For the purposes of this Annex, fiber-reinforced concretes (HRF) are defined as those concretes that include short fibers randomly distributed in their mass in their composition. The approach is general for all types of fibers, although it must be borne in mind that the fundamental basis of the knowledge available is for steel fibers

The use of fibers in concrete has a structural purpose when its contribution is used in calculations related to one of the ultimate or service limit states and its use may involve the partial or total replacement of reinforcement in some applications

Fibers will be considered to have no structural function when fibers are included for other purposes such as improving fire resistance or cracking control

The incorporation of fibers modifies the non-linear behavior of structural concrete, especially in traction, preventing the opening and propagation of fissures

Therefore, the application of nonlinear analysis may be especially recommended in cases where the fibers constitute an important part of the concrete reinforcement

Likewise, given the ductility introduced by the presence of fibers, the principles for the application of the linear analysis method with limited redistribution and the plastic calculation methods are considered valid

The combination of conventional reinforcement and fibers can be an alternative to reduce the amount of conventional reinforcement in regions where there is a high density of reinforcement that hinders the correct concreting of the element

The geometry of the fiber has an important impact on the adherent characteristics of the fiber with the concrete

Steel fibers must be in accordance with UNE 83500-1 and, according to the manufacturing process, are classified into:

  1. Drawn (Type I)
  2. Cut into sheets (Type II)
  3. Hot scraped (steel shavings) (Type III)
  4. Other(molten steel fibers,…)(Type IV)

Fiber length is recommended to be at least 2 times the size of the largest aggregate

The use of lengths of 2.5 to 3 times the maximum size of aggregate is usual. Furthermore, the diameter of the pumping pipe (in the case of sprayed concretes) requires that the length of the fiber is less than 2/3 of the diameter of the pipe. However, the length of the fiber must be long enough to give the matrix sufficient adhesion and avoid pulling out too easily

At the same length, fibers of reduced diameter increase the number of them per unit of weight and make the fiber framework or network denser. The spacing between fibers is reduced when the fiber is thinner, being more efficient and allowing a better redistribution of load or stress

Slenderness is a parameter that relates length to diameter of the fiber cross section

High values of fiber slenderness increase the performance of the concrete, but in turn, the possibility of forming hedgehogs in the fiber concrete mix increases.

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