Main properties of steel fiber

Steel fiber is a new, high-performance steel fiber variety. The mix design method of steel fiber road is generally the same as that of ordinary concrete, with the following differences: dual control standards for strength (compressive strength and flexural strength); steel fiber dosage is determined according to the flexural strength required by the design; unit water consumption and sand ratio are related to fiber dosage. For every 0.5% (volume rate) of steel fiber added, the unit water consumption increases by 6kg and the sand ratio increases by 2%.
Steel fiber concrete has the same mixing, operation and construction performance as ordinary concrete. The fibers will not ball up in the concrete and are evenly distributed. It can be produced in commercial concrete mixing plants and can be used for pumping construction. The early slump loss of milled steel fiber concrete is large, with a loss of 32% in 30 minutes and a loss of 42% in 2 hours. The actual workability of steel fiber concrete is better than that of ordinary concrete with the same slump. Steel fiber concrete has good material properties. Compared with ordinary concrete, its compressive strength is increased by 2-20%; flexural strength is increased by 20-50%; splitting tensile strength is increased by 20-40%; wear resistance is increased by about 40%. Its physical and mechanical properties can fully meet the technical indicators required by supporting components such as urban road projects and inspection well covers. The rough and clean surface of steel fiber can be firmly combined with the cement paste in concrete. This is the fundamental reason why milling steel fiber improves various properties of concrete.
In addition, high-strength steel fiber concrete has been widely used in prefabrication, cast-in-place, production and construction of railway sleepers, highway expansion joints, cement concrete pavements, etc. Its excellent performance can fully achieve good technical, economic and social environmental benefits.
1. Adhesion
Since the interface bonding between steel fiber and concrete matrix is ​​mainly physical, that is, the transmission of friction shear force is the main factor, so for steel fiber itself, its bonding properties should be improved from two aspects: fiber surface and fiber shape. There are four specific methods.
Make the surface of steel fiber rough and the cross section irregular. This goal can be achieved by using the melt extraction method. Because when the steel fiber is rapidly cooled by air, the surface shrinks unevenly and becomes rough, and the cross section also shrinks into a crescent shape, increasing the contact area with the substrate. The milled steel fiber has one smooth surface and the other rough surface, which also increases the contact area with the concrete.
Plastic processing is performed on the fiber at a certain interval along the axis of the steel fiber. For example, the "Xin Ke" steel fiber of Kobe Steel, Japan, the "XOREX" steel fiber of Raybond, USA (c in Figure 1), and the "S-2" and "S-3" steel fibers of Qing'an Steel Plant. Because the surface is pressed into a prism or a waveform, the mechanical bonding force is increased.
The two ends of the steel fiber are shaped. For example, the two ends of the ingot milling steel fiber have anchoring platforms; hooks are made at both ends; and there are large-headed steel fibers extracted by the melt extraction method. Due to the anchoring effect at both ends, the pull-out resistance is improved.
The surface of the steel fiber is coated with epoxy resin and the surface is slightly rusted. This method does not improve the interfacial bonding strength as well as the previous methods, but it also has a certain strengthening effect.
Tests by Kobayashi Ik-suke, the University of Liege in Belgium, and Zhang Wengang have proved that the reinforcing effect of steel fibers with hooks is about twice that of straight steel fibers. Kobayashi Ik-suke's test shows that the effect of ribbed steel fibers is close to that of steel fibers with hooks. These special-shaped steel fibers not only improve the strength of steel fibers, but also improve the toughness. Although corrugated steel fibers have little effect on improving the strength of steel fiber concrete, they can increase the toughness exponentially.
2. Hardness
No matter which processing method is used to manufacture steel fibers, they encounter high heat and rapid cooling during the processing, which is equivalent to the quenching state. Therefore, the surface hardness of steel fibers is relatively high. When used for concrete reinforcement and mixing, bending rarely occurs. If the steel fiber is too hard and brittle, it is easy to break during mixing, affecting the strengthening effect. When producing steel fibers by the melt-drawing method, the steel fibers centrifugally ejected from the bottom of the melt-drawing wheel are still in a high temperature state and must be dispersed and cooled by rollers or vibration conveying methods. Otherwise, the steel fibers gather and the heat is difficult to dissipate, but it has an annealing effect.
3. Corrosion resistance
As for the introduction of the corrosion resistance test of steel fiber concrete, it is known that in a humid environment, the cracked steel fiber concrete member will be carbonized at the crack, and the steel fiber in the carbonized area will be corroded. The carbonization depth and the degree of corrosion will develop with time. For steel fiber concrete, the main use is the arc after cracking and the toughness after cracking. Although the crack width is smaller than that of reinforced concrete, there are cracks after all. Therefore, anti-corrosion measures should be taken for steel fiber concrete used in a humid environment, especially at the seaside. The test face proves that under the premise of ensuring that the steel fiber concrete member has the same bearing capacity, the use of steel fiber with a larger diameter can improve the corrosion resistance. The use of steel fiber coated with epoxy resin or galvanized will improve the corrosion resistance. If the construction process permits, this kind of steel fiber can be used only in the surface layer of concrete 1-2cm, and non-inductive steel fiber can also be used when necessary.