How to Prepare Steel Fiber Concrete

Steel fiber concrete should meet the structural design requirements for strength grades (including compressive strength, tensile strength, flexural toughness ratio, etc.).

The volume fraction of steel fibers in steel fiber concrete should be determined according to design requirements; if there are no design requirements, it should not be less than 0.35%; for deformed steel fibers with high strength (tensile strength not less than 1000 MPa), it should not be less than 0.25%; the amount of steel fibers in prestressed steel fiber concrete should be about 80 kg/m.

The water-cement ratio of fiber reinforced concrete should not be greater than 0.50, and for steel fiber concrete with durability as the main requirement, it should not be greater than 0.45. The cement content (or total amount of cementitious materials) per cubic meter of concrete should not be less than 360 kg.

The consistency of steel fiber concrete can be determined with reference to the consistency required for similar projects with ordinary concrete. Its slump value can be 20 mm smaller than the required value for corresponding ordinary concrete, and its Vebe consistency value is the same as the required value for corresponding ordinary concrete.

• The type, specification, and quality of steel fibers should comply with design requirements. For ordinary steel fiber concrete, low-carbon steel fibers can be used. For refractory concrete, stainless steel fibers must be used. The diameter of long, straight, round-section steel fibers is generally 0.25~0.75 mm; the thickness of flat steel fibers is 0.15~0.4 mm, the width is 0.25~0.9 mm, and the length is 20~60 mm. To improve interface bonding, steel fibers with hooked ends can be chosen.

  
  

• Cement. Steel fiber concrete generally uses 425, 525 ordinary Portland cement, while high-strength steel fiber concrete can use 625 Portland cement or aluminate cement.

  
  

• For the preparation of high-strength steel fiber concrete, it is advisable to use hard-textured, well-graded river sand, with a fineness modulus not less than 2.4.

  
  

• Coarse aggregates should be selected from hard-textured, well-graded crushed limestone, granite, diabase, etc., with a particle size not larger than 20 mm and 2/3 of the steel fiber length. To improve the workability of the mixture, a water-reducing agent or a high-efficiency water-reducing agent should be used. The sand ratio of the concrete should generally not be less than 50%, and the cement content should be about 10% higher than ordinary concrete without fibers.

To ensure the fibers are evenly distributed in the concrete, the aspect ratio should not be greater than 100, typically between 30-80. Each fiber specification has a maximum allowable dosage limit, generally ranging from 0.5% to 2% (by volume).

• It is preferable to use a specialized fiber concrete mixer with a distributing device to mix steel fiber concrete.

  
  

• The mixing process should ensure that the steel fibers are evenly dispersed in the mixture without clumping. It is recommended to use the method of dry mixing the steel fibers, cement, and coarse and fine aggregates first, then adding water for wet mixing.

  
  

• The sequence of material feeding, mixing method, and mixing time for steel fiber concrete should be determined through on-site homogeneity tests. The mixing time should be 1-2 minutes longer than that for ordinary concrete.

Compaction

Different compaction methods have a significant impact on the orientation of the fibers.

• If pumped into the mold without internal vibration, the fibers will be randomly oriented in three dimensions;

  
  

• If using an immersion vibrator, most fibers will be randomly oriented in three dimensions, with a few being randomly oriented in two dimensions;

  
  

• If using a surface vibrator, most fibers will be randomly oriented in two dimensions, with a few being randomly oriented in three dimensions; if using a spraying method, the fibers will be randomly oriented in two dimensions on the sprayed surface;

  
  

• If using the "centrifugal method" or "extrusion method", the fiber orientation will be between one-dimensional alignment and two-dimensional random orientation;

  
  

• If vibrating in a magnetic field, the fibers will align along the magnetic field lines.