Varieties, Preparation and Functions of Fiber Reinforced Concrete

 

1.Varieties

The cement-based composite material composed of cement slurry, mortar or concrete as base material and fiber as reinforcing material is called fiber-reinforced concrete. Fiber can control the further development of cracks in matrix concrete, so as to improve the crack resistance. Due to the high tensile strength and elongation of fiber, the tensile, bending and impact strength, elongation and toughness of concrete can be improved. The main varieties of fiber concrete include asbestos cement, steel fiber concrete, glass fiber concrete, polypropylene fiber concrete, carbon fiber concrete, plant fiber concrete and high elastic modulus synthetic fiber concrete etc..


2. Preparation and Functions
2.1 Steel fiber reinforced concrete
(1) Materials. The preparation of ordinary steel fiber concrete mainly uses low carbon steel fiber. For the preparation of refractory concrete, stainless steel fiber must be used. The diameter of long straight steel fiber with circular section is generally o.25 ~ 0. 75mm; The thickness of flat steel fiber is 0 150. 4mm, width 0.25 ~ 0. 9mm, and the length is 20 ~ 60mm. In order to improve the interface adhesion, there are steel fibers with hooks at the end.

 

Generally, 425 and 525 ordinary Portland cement are used for steel fiber reinforced concrete, and 625 Portland cement or alunite cement can be used for high-strength steel fiber reinforced concrete. The maximum particle size of coarse aggregate used should not exceed 15mm. In order to improve the workability of the mixture, water reducing agent or high-efficiency water reducing agent must be used. Generally, the sand ratio of concrete shall not be less than 50%, and the amount of cement shall be about 10% higher than that of ordinary concrete without fiber.

(2) Dosage. In order to ensure that the fiber can be evenly distributed in concrete, the length diameter ratio shall not be greater than 100, generally 60 ~ 80. There is a limit value for the maximum content of each specification of fiber, which is generally 0.5% ~ 2% (volume rate).

3Mixing. Steel fiber concrete shall be mixed by forced mixer. In order to make the fiber evenly dispersed in the concrete, it shall be added through shaking screen or disperser. The feeding sequence of mixing is different from that of ordinary concrete. One method is to add coarse and fine aggregates, cement and water into the mixer, mix evenly, and then add fibers into the mixer. The other method is divided into three steps. The first step is to mix the coarse and fine aggregates evenly, the second step is to add fiber for mixing, and finally add cement and water for mixing.


(4) Tamping. Different tamping methods have a great influence on the orientation of fibers. It is pumped to the warehouse without inserting, and the fiber is in three-dimensional disorder; If the plug-in vibrating device is used for tamping, most of the fibers are in three-dimensional random direction and a few are in two-dimensional random direction; When the plane vibrator is used for vibration, most of them are in two-dimensional random direction and a few are in three-dimensional random direction; By spraying, the fiber is in two-dimensional random direction on the spraying surface; Using "centrifugal method" or "extrusion method", the fiber orientation is between one-dimensional orientation and two-dimensional random orientation; If vibrated in a magnetic field, the fibers are distributed along the direction of the magnetic line of force.


(5) mechanical function. Adding steel fiber can significantly improve the mechanical functions of concrete. When the content is within the allowable range, the tensile strength can be increased by 30% - 50%, the flexural strength can be increased by 50% ~ 100%, the toughness can be increased by 10 ~ 50 times, the impact strength can be increased by 2 ~ 9 times, and the compressive strength can be increased by 15% ~ 25%. Steel fiber reinforced concrete can also reduce the dry shrinkage by 10% ~ 30%.

Steel fiber reinforced concrete has high cost and great construction difficulty, so it must be used in the projects that should be used most. Such as important tunnels, subways, airports, elevated roadbeds, spillways and explosion-proof and shockproof projects etc..


2.2 Glass fiber reinforced concrete
The fiber used in glass fiber reinforced concrete must be alkali resistant glass fiber to resist the corrosion of Ca (OH) Z in concrete. Alkali resistant glass fiber can only slow down the erosion in ordinary Portland cement. In order to greatly improve the service life, sulphoaluminate cement should be used.


Glass fiber reinforced concrete has no special requirements for coarse and fine aggregates and mix proportion, which is basically the same as steel fiber reinforced concrete. The mechanical properties of glass fiber reinforced concrete are certainly lower than that of steel fiber reinforced concrete, and the compressive strength is slightly lower than that of non fiber reinforced concrete. However, its toughness is very high, which can be increased by 30 ~ 120 times, and has good fire resistance. It is mainly used for non load-bearing and secondary load-bearing components.


2.3 Polypropylene fiber concrete
Polypropylene membrane crack fiber is a bundle of synthetic fiber, which is opened into a network, and can also be cut into short cuts with a length of 19 ~ 64mm. In order to prevent aging, it should be packed in black packaging container before use.

 

The construction process is divided into mixing method and spraying method. The amount of fiber varies with different processes. The cutting length is 40 ~ 70mm by stirring method, and the volume mixing rate is 0.4% ~ 1%; The jet method is used to shorten the length of 20 ~ 60mm and the volume mixing rate is 2% ~ 6%. Polypropylene fiber reinforced concrete has low mechanical properties. Once the concrete cracks, the fiber concrete will crack, and the compressive strength will not be significantly improved. Only the impact strength is high, which can be increased by 2 ~ 10 times. The shrinkage can be reduced by 75%. It can be used for non load-bearing plates, parking lots, etc..

 

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