The Performance of Steel Fiber Reinforced Concrete
Steel fiber reinforced concrete is a fast-developing composite material with short steel fibers distributed in random directions. As shown in the table below, a 2% steel fiber dosage is enough to improve various properties of concrete. Steel fibers not only improve the tensile, flexural, and shear strength of concrete, but also improve the crack resistance, ductility and toughness of concrete.
|Performance||Compared with ordinary concrete|
|Early age cracking resistance||1.5-2.0 times|
|Tensile strength, flexural strength||1.5-1.8 times|
|Elongation||approximately 2.0 times|
|Compressive strength||1.0-1.3 times|
|Shear strength||1.5-2.0 times|
|Impact resistance||5-10 times|
|High temperature resistance||significantly improved|
|Freeze-thaw resistance||significantly improved|
The mechanical strength of steel fiber reinforced concrete
Though the compressive strength of steel fiber reinforced concrete increases with the increase of fiber content, the effect of steel fibers is not significant in improving this indicator. This is because, in the vibration procedure, most of the fibers are aligned in a direction that is parallel to that of the vibrating table. The size of aggregate particles is the main factor that influences compressive strength.
The improved shear strength is due to the presence of steel fibers, which increases the compression deformation of the specimen and improves its toughness under compression. When shear failure occurs, unlike ordinary concrete which is divided into three sections, steel fiber reinforced concrete can still maintain its integrity. So steel fiber reinforced concrete is suitable for the parts that bear the greatest shear force.
Steel fiber reinforced concrete also has higher flexural strength than ordinary concrete. Under flexural loading, the steel fiber reinforced concrete tension zone cracks and the neutral axis moves upward. There are still some fibers and the bond strength with the base material in the tension zone to bear the tension, which increases the toughness the flexural strength. Whereas, ordinary concrete fractures quickly, resulting in brittle failure.
Toughness and crack resistance of steel fiber reinforced concrete
Toughness is the property of absorbing energy before the material is broken under pressure. Crack resistance refers to the property of steel fibers to reduce cracks and prevent the further development of cracks in the brittle concrete matrix. The toughness of steel fiber reinforced concrete increases greatly with the increase of the number of steel fibers, and is also related to the bond strength between the fibers and the base material. If the strength of the base material is improved, the toughness of the fiber concrete is correspondingly improved.
The impact resistance of steel fiber reinforced concrete
Steel fiber concrete has excellent impact resistance. Under the action of dynamic load, when the cracks expand in the concrete, steel fibers being pulled out by overcoming the bond strength with the base material, or reaching the yield strength and being broken, both require a lot of energy. A drop weight impact test of 0.4mm x 0.5mm x 25mm cut sheet steel fibers with different content used in a 30cm span, CF55 grade (China JG Standards), 10cm x 10cm x 40cm test specimen shows that the numbers of impact resistance in blows of concrete with 0.5%, 1%, 1.5% fiber content are 3-4 times, 11-12 times, and 21-22 times that of ordinary concrete, respectively. It can be seen that the impact resistance of steel fiber reinforced concrete is greatly improved with the increase of fiber content, which is of great importance to improve the safe and reliable operation of projects.
Fatigue resistance of steel fiber reinforced concrete
Steel fibers improve the bending fatigue performance of concrete. Compared with ordinary concrete, the fatigue limits at two million cycles of CF80 steel reinforced fiber concrete with 1%, 1.5% fiber content are increased by 10%, 15% respectively. When the fatigue stress ratio is 0.7, the fatigue life of concrete with 1%, 1.5% fiber content can be extended by 1.2 times, 6.5 times respectively.
The durability of steel fiber reinforced concrete refers to the corrosion resistance, freeze-thaw resistance and the ability to withstand the damage caused by the corrosion of steel fiber.
(1) If the concrete has good compactness and corrosion resistance, the fibers will not cause much harm to the concrete. Materials like silica fume are added to improve the compactness and reduce the permeability of concrete to chloride ions.
(2) Steel fibers can greatly improve the freeze-thaw resistance of concrete, especially when concrete has a high air content (within 5%).
(3) The size of the steel fiber cross section is very small for the entire concrete. Due to the high alkalinity of cement in concrete (pH value is 12-13), steel fibers produce a passive film in the alkaline medium, which can prevent fibers from rusting.