Steel Fiber

Introduction

What is Steel Fiber?

Steel fibers mixed into the concrete can provide an alternative to the provision of conventional steel bars or welded fabric in some applications.

Today, industrial floors and pavements are major applications for steel-fiber-reinforced concrete. In the United Kingdom, several million m2 of steel-fiber-reinforced slabs have been installed over the past ten years, both for ground-supported and pile-supported floors. Other major applications for fiber-reinforced concrete include external paved areas, sprayed concrete, composite slabs on steel decking and precast elements.

Fibers are often used to replace the nominal conventional steel fabric in ground bearing slabs. Steel fibers are increasingly being used in suspended ground floor slabs on piles to replace much, and in many cases all, of the reinforcement. Savings in the cost of supplying and fixing the conventional welded fabric reinforcement that is replaced can offset the extra cost of adding fibers to the concrete. There may also be health and safety benefits resulting from the reduced handling of reinforcement. Also, problems caused by the misplacement of conventional steel in the depth of the slab are avoided.

During recent years, steel fiber reinforced concrete has gradually advanced from a new, rather unproven material to one which has now attained acknowledgment in numerous engineering applications. Lately, it has become more frequent to substitute steel reinforcement with steel fiber reinforced concrete. The applications of steel fiber reinforced concrete have been varied and widespread, due to which it is difficult to categorize. The most common applications are tunnel linings, slabs, and airport pavements.

Many types of steel fibers are used for concrete reinforcement. Round fibers are the most common type and their diameter ranges from 0.25 to 1.05 mm. Deformed fibers in the form of a bundle are also used. The main advantage of deformed fibers is their ability to distribute uniformly within the matrix.

Over many years, much research works performed to render concrete with flexural and tensile strength with its natural high compressive strength. The result is reinforced concrete having both longitudinal steel to provide bending and torsional strength and transverse steel (stirrups) to provide shear and torsional strength in flexural members. In case of compression member with longitudinal and transverse steel (tie or spiral) are used to provide compression plus bending strength. Nowadays short length steel wire or thin steel sheets are used to increase flexural strength, crack resistance and explosion resistance.

This special reinforcement is called Steel fiber and the resulting concrete is steel fiber reinforced concrete. This type of concrete is widely used in abrasion suspected wearing surface.

Steel fiber concrete flooring can provide superior resistance to minimize cracks in hardened concrete, as well as maximum resistance to withstand heavy loads, either dynamic or static. If you decide to use steel fiber concrete flooring, you can select to use a ‘joint-less floor’. Joint-less floors are floors that have minimal joints, providing spaces without joints as large as 40 or 50-meter span wide.

Steel fiber dosage will vary greatly upon the projects intended use, and the types of mesh being replaced. Common dosages are in the range between 20-30kg/m3 to 40-50kg/m3 for joint-less floors. Trowelling concrete will help to embed steel fibers into the concrete surface producing a better finish product. Steel fibers will enhance to crack resistance of the concrete, and they can also be used to replace or supplement structural reinforcement. It only can be done through a structural engineer and with proper guidance.

Effect of Fibres Utilized with Concrete

Fiber reinforced concrete is a composite material comprised of Portland cement, aggregate, and fibers. Normal unreinforced concrete is brittle with low tensile strength and strain capacity. The function of the irregular fiber distributed randomly is to fill the cracks in the composite. Fibers are generally utilized in concrete to manage the plastic shrinkage cracking and drying shrinkage cracking. They also lessen the permeability of concrete and therefore reduce the flow of water. Some types of fibers create greater impact, abrasion and shatter resistance in the concrete. Usually, fibers do not raise the flexural concrete strength. The quantity of fibers required for a concrete mix is normally determined as a percentage of the total volume of the composite materials. The fibers are bonded to the material, and allow the fiber reinforced concrete to withstand considerable stresses during the post-cracking stage. The actual effort of the fibers is to increase concrete toughness.

How to use Steel Fibers in Concrete?

Typically the fibers are added at the batch plant, just after all concrete aggregates are being mixed. Some people would request to have the fiber added at the job site, but then the QA/QC should have more control on how much fiber is added. The steel fiber manufacturer can provide guidance on how to mix and the amount needed to obtain the desired results.

Be aware that if you add too much fiber, it might show up at the surface when finishing the concrete so be cautious about the amounts being mixed.

The importance of the total system

Just adding steel fibers to a load of concrete doesn’t ensure success. Steel fiber in concrete represents only one part of the system. There are other important elements to consider, including subgrade preparation, concrete mix design, and the total water in a mix.

The condition of the subbase is critical. The subgrade under a slab must have adequate drainage, be properly compacted, and have a flat, smooth surface. The installation of a good vapor barrier system also is recommended. Concrete placed over mud and water puddles shouldn’t be allowed. These areas should be removed, replaced with suitable material, and compacted before concrete is placed. The goal is to create a smooth surface to the underside of a concrete slab to freely move on when shrinking—slabs that get caught by irregularly shaped subgrade can become stressed enough to crack.

It’s wise to work out good aggregate distributions for a mix. Well-graded mixes require less cement, yielding stronger concrete. They also require less total water, so there is less shrinkage. The compressive, flexural, and tensile strength of concrete is largely due to the concrete mix design, not the addition of steel fiber. High flexural strength is especially necessary for quality concrete, steel fiber installation.

Deciding on the dosage of steel fiber to include in an application is important. For instance, to increase joint spacing on a project while still providing crack control might require 40 pounds per cubic yard of steel fibers added to a good low-shrinkage mix. Increasing joint spacing can be achieved by adding the right amount of fibers (and the right type) to a good concrete mix, adding the right amount of water, and placing it on a well-prepared subbase.

Uses and application of Steel fiber
Tunnel linings, Manholes, Risers, Burial Vaults, Septic Tanks, Curbs, Pipes, Covers, Sleepers ,Roller compacted concrete with steel fibers

Application Fields

Advantages

Several advantages can be obtained from HardTape Steel fiber , for example:

Features

The uses of Steel fiber in concrete can improve its many properties. The benefits of using Steel fibers in concrete are as follows:

1. Steel fibers are generally distributed throughout a given cross-section, whereas reinforcing bars or wires are placed only where required.
2. Steel fibers are relatively short and closely spaced as compared with continuous reinforcing bars of wires.
3. It is generally not possible to achieve the same area of reinforcement to an area of concrete using steel fibers as compared to using a network of reinforcing bars of wires.
4. Steel fibers are typically added to concrete in low volume dosages (often less than 1%), and be effective in reducing plastic shrinkage cracking.
5. Steel fibers typically do not significantly alter free shrinkage of concrete, however, at high enough dosages they can increase the resistance to cracking and decrease crack width.

Reinforcement

MIX DESIGN OF SFRC（steel fiber reinforcement concrete）

As with any other type of concrete, the mix proportions for SFRC(steel fiber reinforced concrete) depend upon the requirements for a particular job, in terms of strength, workability, and so on. Several procedures for proportioning SFRC mixes are available, which emphasize the workability of the resulting mix. However, there are some considerations that  are particular to SFRC. In general, SFRC mixes contain higher cement contents and higher ratios of fine to coarse aggregate than do ordinary concretes, and so the mix design procedures the apply to conventional concrete may not be entirely applicable to SFRC. Commonly, to reduce the quantity of cement, up to 35% of the cement may be replaced with fly ash. In addition, to improve the workability of higher Fiber volume mixes, water reducing admixtures and, in particular, superplasticizers are often used, in conjunction with air entrainment. The range of proportions for normal weight SFRC.

Steel fiber reinforced concrete is a composite material having fibers as the additional ingredients, dispersed uniformly at random in small percentages, i.e. between 0.3% and 2.5% by volume in plain concrete.

SFRC products are manufactured by adding steel fibers to the ingredients of concrete in the mixer and by transferring the green concrete into moulds. The product is then compacted and cured by the conventional methods.

Segregation or balling is one of the problems encountered during mixing and compacting SFRC. This should be avoided for uniform distribution of fibers. The energy required for mixing, conveying, placing and finishing of SFRC is slightly higher.

Use of pan mixer and fiber dispenser to assist in better mixing and to reduce the formation of fiber balls is essential. Additional fines and limiting the maximum size of aggregates to 20mm occasionally, cement contents of 350 kg to 550 kg per cubic meter are normally needed.

Steel fibers are added to concrete to improve the structural properties, particularly tensile and flexural strength. The extent of improvement in the mechanical properties achieved with SFRC over those of plain concrete depends on several factors, such as shape, size, volume, percentage and distribution of fibers.

Plain, straight and round fibers were found to develop a very weak bond and hence low flexural strength. For a given shape of fibers, flexural strength of SFRC was found to increase with aspect ratio (ratio of length to equivalent diameter).

Even though higher ratios of fibers gave increased flexural strength, workability of green SFRC was found to be adversely affected with increasing aspect ratios. Hence the aspect ratio is generally limited to an optimum value to achieve good workability and strength.

Grey suggested that the aspect ratio of less than 60 are best from the point of handling and mixing of fibers, but an aspect ratio of about 100 is desirable from a strength point of view. Schwarz, however suggested aspect ratio between 50 and 70 is more practicable value for ready mix concrete.

In most of the field applications tried out of date, the size of the fibers varies between 0.25 mm and 1.00mm in diameter and from 12mm to 60mm in length, and the fiber content ranged from 0.3 to 2.5 percent by volume. Higher contests of fiber up to 10% have also been experimented. Addition of steel fibers up to 5% by volume increased the flexural strength to about 2.5 times that of plain concrete.

As explained above, mixing steel fibers considerably improves the structural properties of concrete, particularly tensile and flexural strength. Ductility and post cracking strength, resistance to fatigue, spalling and wear and tear of SFRC are higher than in the case of conventional reinforced concrete.

How Workability is affected?

If you opt-in to add fiber to your concrete mix, be aware that there will be some changes in the way you manage this concrete. First of all, the slump will be affected, and it is recommended to add a super plasticizer to enhance the slump and make the concrete a little more fluid. Not all steel fiber can be used as a substitute for steel reinforcement, so make sure that your structural engineer has reviewed and analyzed the loads before proceeding.

Specifications

Hooked End Bright Glued Steel Fiber

HardTape Glued Steel Fiber: Cold-drawn hooked ends steel fiber is manufactured by a quality base low carbon steel bar, which has excellent mechanical properties including high tensile strength. Hence, the average tensile strength of the reinforced fiber surpasses 1100MPa. Owing to high strength and uniform distribution of fibers, stresses can be fully dispersed and cracking propagation being effectively controlled.

Refers to loose end hooked fiber glued together in a raw way. The big difference between loose and glued fibers is that the former is easy to block together and the latter is easy to disperse uniformly.

Specifications

• Diameter   :       0.75 mm
• Length     :        60mm
• Aspect Ratio :       80
• Tensile Strength:       ≥1100Mpa
• Material      :      Low carbon steel bar – Coating: Non, Bright.
• Packaging    :      25kg per paper bag.

Hooked End Bright Loose Steel Fiber

HardTape Loose Steel Fiber:Cold-drawn hooked ends steel fiber is manufactured by a quality base low carbon steel bar, which has excellent mechanical properties including high tensile strength. Hence, the average tensile strength of the reinforced fiber surpasses 1100MPa. Owing to high strength and uniform distribution of fibers, stresses can be fully dispersed and cracking propagation being effectively controlled.

Diameters of HardTape loose steel fiber shape: