14.5. Fiber concrete
Composite materials, which include short segments of different fibers in the cement matrix, are classified as fiber reinforced concretes or dispersed-reinforced concretes. Different types of fibers from steel, glass, synthetic materials, asbestos, carbon and others can be used as fiber.
The disperse reinforcement of cement stone by fibers substantially increase its specific strength in particular with respect to the tension, bending, crack-resistance, impact resistance, vibration influences, abrasive resistance, etc. Application of fiber reinforced concrete allows:
- the realization of effective constructive solutions, for example, thin-walled structures without the bar reinforcement or reticulated distribution rods;
- a decrease in labor expenses for the reinforcing works and to increase the degree of mechanization and automation of the production of the reinforced structures.
I
t
also offers the possibility of applying new, more productive methods
of molding the reinforced structures, for example, pneumatic
sputtering, roller molding, etc.
Fibers increase the bearing capacity of the concrete matrix. As a result of the difference between the values of the elasticity coefficient of the reinforcing fibers and the matrix, the transfer of load in the contact zone through the matrix to the fiber is accomplished.
Typical stress-strain diagram of fiber in fiber-reinforced concrete includes three zones (Fig.14.11):
І - zone of the elastic strain energy of both the matrix and of the fiber;
ІІ - zone of the alternate displacement of stress on the matrix and the fibers and the formation of microscopic cracks;
ІІІ - zone of the perception of load by the fibers.
In the ІІІ zone, the complete destruction of material is stopped by the adhesion strength in the contact zone or by the strength of some fibers only.
If the length of the fibers (l) is less or equal to the critical length (lcr), the breaking energy is determined by the energy required for the extraction of fibers. At l lcr the material is destroyed with the breaking of fibers and the breaking energy is inversely proportional to the length of the fiber. The breaking energy is proportional also to the diameter of the fiber. The increase of the tensile strength of fiber reinforced concrete can be achieved by the application of fiber cables and therefore by increasing their working diameter.
Portland cement based fiber concrete, reinforced by steel fibers (steel-fiber concrete) are the most widespread. Steel fibers are usually represented by the sections of wire. Fibers can have different cross sections - round, oval and other sizes from 0.2 to 1.6 mm and length from 10 to 160 mm. The surface of fibers can be shaped; processed by etching and can be smooth. The quantity of fibers introduced into the concrete in the majority of the cases varies from 0.5 to 2% by volume. Introduction of steel fibers into the concrete in quantity of 1-1.5% in volume increases its tensile strength to 100%, bending strength by 150-200%, and compressive strength rises to 10-25%.
Due to the higher crack-resistance of steel-fiber concrete it possesses an increased(1.5-2 times) frost, heat and fire resistance, as well as water impermeability. The valuable qualities of steel-fiber concrete are the increased abrasion resistance, impact and dynamic strength. Thus, the wear-resistance of steel-fibro-concrete is increased by about 50% and the impact strength by about 10-12 times.
Application of steel-fiber concrete is promising both in the precast and in monolithic structures. The application areas include: road and aerodrome paving, sprayed layers for the revetment of the mines of tunnels and fire-retardant stack linings, thin-walled and ribbed floor slabs, the elements of shells, piles, ties and others.
The effectiveness of the application of the steel-fibro-concrete structures is made evident by the observable decrease of labor expenses for the reinforcing works, the reduction of steel and concrete contents and the structures thickness, combination of the technological operations of the preparation of concrete mixture and reinforcement, improvements in the durability of structures and reduction in the expenses for routine repair.
It is expedient to combine fiber and bar reinforcement i.e. the application of the combined reinforcement to resist significant loads in the structure.
The introduction of steel fibers into the concrete mix is characterized by specific technological difficulties connected with the clumping, the formation of “urchins”, and the complexity of proper consolidation of material. The fiber is introduced into a preliminarily mixture of cement, water and aggregate or the aggregates and fibers are premixed at first, and then the cement and water are added. The preparation of fibers concrete involves the use of mixers with pulsing action on the mixture and other special mixers.
There is a positive experience of the application of glass-fiber concrete in construction, which makes it possible to reduce significantly the mass of structures additionally. The positive experience is obtained by including cement paste or mortar of alkali-durable fibers (up to 5%) throughout the mass. Tensile and bending strength of concrete reinforced by glass exceeds the strength of the ordinary concrete by about 2-3 times even after 10 years of air storage. The maximum deformation due to the maximum stretching effort in the glass-reinforced concrete,s is 10 times higher than for the non-reinforced concrete.