Requirements for Steel Fiber in Steel Fiber Concrete

The steel fiber used for preparing steel fiber concrete are generally low-carbon steel. Stainless steel can also be used for projects with special requirements. The requirements for steel fiber in steel fiber concrete mainly include aspects such as the strength, size, shape, aspect ratio, and technical performance of the steel fiber.

Strength of Steel Fiber

Engineering practice and tests have shown that when steel fiber concrete fails, it is often because the steel fiber is pulled apart. Therefore, it is essential to enhance their toughness, but it is unnecessary to excessively increase their tensile strength. If the material achieves high tensile strength through quenching or other intense hardening methods, its texture becomes hard and brittle. Hard and brittle steel fiber is easily broken during mixing, reducing the reinforcement effect. Therefore, as long as the steel is not easily brittle, steel fiber with relatively high strength can generally meet the requirements.

Size of Steel Fiber

The size of steel fiber used in preparing steel fiber concrete is mainly determined by the reinforcement characteristics and ease of construction. If the steel fiber is too thick and short, the reinforcement characteristics of the steel fiber concrete will be poor. If the steel fiber is too long and thin, they are prone to clustering during mixing.

Tests have shown that the appropriate size of steel fiber is: for cylindrical cross-section straight steel fiber, the diameter is generally within the range of 0.25 to 0.75mm; for flat steel fiber, the thickness is 0.15 to 0.40mm, and the width is 0.25 to 0.90mm. These two types of steel fibers generally have a length within the range of 20 to 60mm. Hooked bundled steel fiber is made by bonding 20 to 30 fibers together with water-soluble glue, with the diameter of single steel fiber being 0.30 to 0.50mm and the length being 40 to 60mm. After bonding, the aspect ratio of the fiber bundles is 20 to 30. These bundled fibers, when encountering water during mixing, can dissociate into single fibers and distribute evenly in the concrete.

Test data shows that incorporating 2% of 0.5mm×0.5mm×30mm steel fiber into 1m³ of concrete results in a total surface area of 1600m². This is approximately 320 times the total surface area of eighteen 16mm diameter, 5.5m long steel bars of the same mass. Appropriately increasing the total surface area of steel fiber can enhance the bond strength between the steel fiber and the concrete.

Shape of Steel Fiber

Tests have fully demonstrated that in order to increase the bond strength between the steel fiber and concrete, the surface area is often increased, or the surface of the steel fiber is processed into a concave-convex shape, such as wavy, dumbbell-shaped, hooked ends, or flat shapes. However, engineering practice has also shown that if the surface of the steel fiber is concave-convex, it is only significantly effective in improving bond strength with concrete when oriented in the same direction. When uniformly dispersed, it may not be effective. Additionally, steel fiber should not be processed too thin or too fine, as they are prone to break during mixing and increase costs.

Aspect Ratio of Steel Fiber

To evenly distribute steel fiber in concrete, the steel fiber incorporated into the concrete should have an appropriate aspect ratio, generally not exceeding the critical aspect ratio of the fiber. When using single steel fiber, the aspect ratio should not be greater than 100, generally controlled within 60 to 100. The geometric parameters of steel fiber suitable for various concrete structures are listed in Table 1.

Steel Fiber Price

In steel fiber concrete, the amount of steel fiber added is about 2% of the concrete volume. Although the amount added is not too much, the price is about four times that of thick steel plates. Therefore, steel fiber price is relatively high, making it a significant factor hindering their widespread use. This must be considered when designing steel fiber concrete and selecting steel fiber.