Steel Fiber Reinforced Concrete FAQ

When subjected to tensile stress, unreinforced concrete will crack and fail. Since the mid 1800’s steel reinforcing has been used to overcome this problem. As a composite system, the reinforcing steel is designed to carry the tensile loads. Steel fibers have been used for a similar purpose since the 1970’s. When fiber reinforcing is added to the concrete mix, it transforms the brittle concrete to a ductile composite system able to develop tensile load carrying capacity. Following are frequently asked questions by those new to steel fiber reinforced concrete.

FAQ’s

1. What is steel fiber reinforced concrete? (SFrc)

2. can steel fibers be added to the readymix plant?

3. How much mixing time is required when adding steel fibers to a ready mix truck?

4. can steel fibers can be added on site?

5. can steel fibers be added to any mix?

6. Will the steel fibers ball up in the mix?

7. How will steel fibers affect my concrete mix design?

8. do steel fibers affect the concrete slump?

9. can steel fiber reinforced concrete be pumped?

10. Will steel fibers rust?

11. Will fibers stick out of the matrix after cast-in-place forms are removed?

12. For cast-in-place SFrc when is internal vibration used and when is form vibration used?

13. Are there any problems associated with casting SFrc against a water proof membrane?

14. is the concrete electrically conductive and does this lead to a risk of electrical shock?

15. Are there any safety hazards for finishers?

16. How can i quantify the differences between various steel fibers?

17. What is meant by residual strength?

18. Will steel fiber reinforced concrete wear forms or tools more than plain concrete?

19. What is the benefit of using steel fibers over synthetic micro fibers?

20. What is the benefit of using steel fibers over synthetic macro fibers?

21. Pound for pound steel fibers cost more than rebar or mesh, why should i pay more for steel fibers?

Answers

1. What is steel fiber reinforced concrete? (SFRC)
Steel fiber reinforced concrete is an alternative to traditional reinforced concrete for certain application areas. Steel fibers are a discontinous, 3-dimensionally orientated, isotropic reinforcement, once they are mixed into the concrete. Steel fibers bridge the crack at very small crack openings, transfer stresses and develop post crack strength in the concrete.

A variety of fiber types (material, shape, size...) are available, their effect in concrete varies. therefore steel fiber reinforced concrete shall never be simplified as a “concrete with steel fibers”. Steel fibers may be divided into five groups:
  • Type I - cold-drawn wire
  • Type II - cut sheet
  • Type III - melt-extracted
  • Type IV - shaved cold drawn wire
  • Type V - milled from blocks
The vast majority belongs to group i. the common and most performing anchorage type is the “hooked end”. For the same type of fiber, length/diameter and tensile strength have the biggest influence on fiber performance. the higher the l/d ratio, the better the performance of the steel fiber reinforced concrete.


2. Can steel fibers be added at the ready mix plant?
Yes. introduce steel fibers after all other ingredients are already in the truck. Set the truck mixer on charging speed and add the fibers slowly into the mixer. Mix for about 5 minutes at charging speed.
The steel fibers can also be added to the aggregate batch belt, if there is safe access.


3. How much mixing time is required when adding steel fibers to a ready mix truck?
Bekaert recommends continuing mixing at the highest drum speed for about 4 to 5 minutes after all steel fibers are added to the truck.


4. Can steel fibers can be added on site?
Yes, also adding fibers on site to the truck mixer is feasible. Gradually dose fibers in the mix, this is typically done via a conveyor belt.


5. Can steel fibers be added to any mix?
Steel fibers can be used in concrete, mortar and grout. Generally harsh mixtures, containing very few amount of fines and/or an unsteady sieve curve at a higher fiber volume can create mixing and dispersion problems. Simply mixing steel fibers into any concrete will most probably not utilize all the positive effects fibers can provide to concrete. depending on the type and amount of fibers adjustments to the concrete mix may need to be made.
For example:
  • increasing the content of fines
  • adjusting the grading curve
  • adjusting the amount of plasiticizer
For concrete strength up to an actual strength of around 8000 psi typical fibers with normal strength wire are sufficient (majority of applications). For higher concrete strength´s than middle strength or high strength fibers can be required to avoid a brittle behaviour. if special cements, aggregates or admixtures are used (seldom the case), a preliminary mix/pump test is recommended.


6. Will the steel fibers ball up in the mix?
A properly designed concrete mix is essential for avoiding fiber balling. in order to avoid the potential for fiber balling related to fibers with a high l/d (aspect) ratio (meaning high performing fibers), glued fiber technology has been developed. Glued fiber bundles will spread the glued bundles evenly on “macro level” and during mixing the bundles separate into individual fibers. in essence the glued bundle temporarily lowers the aspect (l/d) ratio of the fibers for easy mixing. that´s how balling can effectively be avoided and a homogenously mix of high performing steel fiber reinforced concrete can be achieved.


7. How will steel fibers affect my concrete mix design?
Steel fiber mix designs are similar to those commonly used for plain concrete mixes. recommended aggregate gradations and mix proportions are provided in local standards. Using the largest practical top size aggregate and a well-graded combined aggregate blend as opposed to a gap-graded blend can minimize shrinkage. Steel fibers may cause a reduction in slump due to their stiffness. this does not necessarily equal a reduction in workability. depending on ambient temperatures and placement method, mid-range water reducers are commonly used to enhance workability for mixes with more than 30 to 40 pounds per cubic yards of steel fibers.


8. Do steel fibers affect the concrete slump?
Yes, the addition of steel fibers at typical dosage rates of 25 to 65 lb/yd3 will reduce the apparent slump by 1” to 3”. However, this does not necessarily equal a reduction in workability. Use of vibratory consolidation, restores the workability to the SFrc.


9. Can steel fiber reinforced concrete be pumped?
Yes, but expect a 0.4” to 1.2” slump loss through the hose depending on the steel fiber dose rate, ambient temperatures and hose length. A midrange water reducing agent (MrWr) is commonly used to enhance workability and ease of flow through pump lines. High-range water reducers (HrWr) may be required in some cases. typically, a 4” diameter hose is required.


10. Will steel fibers rust?
For indoor applications such as tunnels and warehouses, no. For outdoor applications such as pavements some minor rusting may occur. experience in highways and industrial pavements indicate that while individual fibers corrode at the surface, staining of the concrete surface does not occur. overall aesthetics and serviceability are maintained even with the presence of individual fiber corrosion. indoor Applications- Surface fibers in typical indoor tunnels or manufacturing floor applications remain bright and shiny under normal environmental conditions.

Outdoor Applications Without cracks- experience has shown that concrete specified with a 28-day compressive strength over 3000 psi, mixed with standard water/cement ratios, and installed with methods that provide good compaction, limit the corrosion of fibers to the surface skin of the concrete. When surface fibers corrode, there is no propagation of the corrosion more than 0.008” beneath the surface. Since the fibers are short, discontinuous, and rarely touch each other, there is no continuous path for stray or induced currents between different areas of the concrete. outdoor Applications With cracks-laboratory and field-testing of cracked SFrc in environments containing chlorides has indicated that the cracks in concrete can lead to corrosion of the fibers passing across the crack. However, small cracks (crack widths < 0.008”) do not allow corrosion of steel fibers passing across the crack. if the cracks wider than 0.008” and are limited in depth, the consequences of this localized corrosion are not structurally significant.


11. Will fibers stick out of the joints after cast-in-place forms are removed?
Fibers can only protrude from forms where there is a joint. they can not protrude in the middle of a form. this can be minimized if the joints are caulked before concrete placement. However, it is not always possible to calk every joint. the number of protruding fibers is a function of the precision of the joints and the fiber dosage.

Wider joints will catch more fibers than tighter joints. After the formwork is removed, the fibers can be quickly knocked down with a hand sanding block or a small angle grinder.


12. For cast-in-place SFRC when is internal vibration used and when is form vibration used?
For cast insitu, internal vibration is the most used option to consolidate the concrete. Form vibration is generally used in the precast industry.

When steel fiber concrete is cast into form work a small amount of vibration of the forms helps keep the fibers from touching the forms and thereby from being visible when the forms are removed. For example, during casting of steel fiber reinforced precast structures, the forms are vibrated to consolidate the concrete. this action results in an almost fiber free surface of the structures. So allowing a short period of form vibration in the all cast-in-place structures, in addition to internal vibration where possible, will provide the best finished surface.


13. Are there any problems associated with casting SFRC against a water proof membrane?
No failures of the plastic liner due to fiber punctures have ever been identified. the abrasion from sharp aggregates during placement of the concrete poses just as big of a threat to the liner as do the steel fibers. After placement the fibers tend to move around and re-orient themselves during vibration which relieves any pressure of an individual fiber on the liner created during placement. Many projects using SFrc are constructed with cast-in-place and sprayed shotcrete directly in contact with water proof membranes.


14. Is the concrete electrically conductive and does this lead to a risk of electrical shock?
typical steel fiber reinforced concrete contains less than 0.5% vol. steel fibers and hardly more than 0.75% vol. those fibers are discontinuous and not connected to each other. tests only show a slight decrease in electrical resistivity due to the addition of steel fibers. However, the resistance to current flow is still substantial. effects from moisture content and aggregate composition are much more dominant than the addition of steel fibers.


15. Are there any safety hazards for finishers?
No, with good jobsite safety practices in place steel fibers shall not impose any safety concerns. Please refer to our safety data sheets for further information.


16. How can I quantify the differences between various steel fibers?
the post crack strength of steel fiber concrete is a material property which is commonly used to differentiate fiber performance. this will typically be determined with a bending test and is often referred to as the residual flexural strength (see below). For the same concrete composition, steel fiber performance is a function of fiber length, diameter, aspect ratio, anchorage and tensile strength. A fiber dosage alone has no performance related value at all. AStM international has two flexural testing procedures for fiber reinforced concrete. the two testing procedures are AStM c1399 Standard test Method for obtaining Average residual-Strength of Fiber-reinforced concrete and AStM c1609 Standard test Method for Flexural Performance of Fiber-reinforced concrete. bekaert propose the use of AStM c1609. AStM c1399 can lead to inflated post crack flexural strengths due to favorable fiber orientation and the use of a steel plate to control the first crack’s energy release.


17. What is meant by residual strength?
the residual flexural strength is equal to the post crack flexural strength of steel fiber concrete corresponding to a certain deflection in a beam bending test. it is a value from testing which has been introduced for the design of steel fiber concrete.


18. Will steel fiber reinforced concrete wear forms or tools more than plain concrete?
not more than concrete.


19. What is the benefit of using steel fibers over synthetic micro fibers?
Steel fibers are not a replacement of synthetic micro fibers and vice versa. both fiber types provide very different properties to concrete so that the applications fields do not overlap. rather than a substitute, both fiber types may be used complementary. While steel fibers offer post crack strength and thus act as reinforcement, synthetic micro fibers reduce cracking due to plastic shrinkage and improve the fire resistance of concrete. they do not provide any reinforcing effect.


20. What is the benefit of using steel fibers over synthetic macro fibers?
Mechanically anchored steel fibers have been proven as reinforcement, even for structural application. Steel fibers are made from a material with well known engineering properties; e modulus, Poisson’s ratio, tensile strength and creep. the e-modulus of steel is greater than that of concrete. thus, the steel fibers pick up the stresses quickly and affect the cracking process immediately. the long term load carrying capacity of the steel fiber reinforced concrete is significant. Steel fibers have a material specification of AStM A820. Macro synthetic fibers come in a large variety and have very different material properties. Macro synthetic fibers do not have a material specification in AStM. All macro synthetic fibers do have an e-modulus lower then that of concrete and relatively low tensile strengths. thus, macro synthetic fibers need a certain crack width to occur prior to being able to engage in the concrete and then only moderate post crack strength values can be achieved. Macro synthetic fibers are also subject to creep which makes the long term loading capacity of the fiber lower or non existent. the rate of creep can be increased with increased ambient temperatures.

There are at least four factors to review when considering reinforcement; Modulus of elasticity, Poisson’s ratio, tensile strength and creep.


21. Pound for pound steel fibers cost more than rebar or mesh, why should I pay more for steel fibers?
Steel fiber concrete compresses the construction schedule, allows for alternative construction methods or design solutions and increases durability. When a project is delivered quicker with fewer efforts and labor, the higher costs of the steel fibers are overcompensated by the savings.

In certain applications the volume weight of steelfibers is lower as the rebar for a similar reinforcing effect. For those applications the cost/reinforcement is lower for steelfibers.

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