Can steel fibres be added to the readymix plant?
Yes. Introduce steel fibres after all other ingredients are already in the truck. Set the truck mixer on charging speed and add the fibres slowly into the mixer. Mix for about 5 minutes at charging speed.
The steel fibres can also be added to the aggregate batch belt, if there is safe access.
What is steel fibre reinforced concrete (SFRC)?
Steel fibre reinforced concrete is an alternative to traditional reinforced concrete for certain application areas. Steel fibres are a discontinous, 3-dimensionally orientated, isotropic reinforcement, once they are mixed into the concrete. Steel fibres bridge the crack at very small crack openings, transfer stresses and develop post crack strength in the concrete.
A variety of fibre types (material, shape, size...) are available, their effect in concrete varies.
Therefore steel fibre reinforced concrete shall never be simplified as a “concrete with steel
fibres”. Steel fibres 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 fibre, length/diameter and tensile strength
have the biggest influence on fibre performance. The higher the l/d ratio, the better the
performance of the steel fibre reinforced concrete.
How much mixing time is required when adding steel fibres to a ready mix truck?
We recommend continuing mixing at the highest drum speed for about 4 to 5 minutes after all steel fibres are added to the truck.
Can steel fibres be added on site?
Yes, also adding fibres on site to the truck mixer is feasible. Gradually dose fibres in the mix, this is typically done via a conveyor belt.
Will the steel fibres ball up in the mix?
A properly designed concrete mix is essential for avoiding fibre balling. In order to avoid the potential for fibre balling related to fibres with a high L/D (aspect) ratio (meaning high performing fibres), glued fibre technology has been developed. Glued fibre bundles will spread the glued bundles evenly on “macro level” and during mixing the bundles separate into individual fibres. In essence the glued bundle temporarily lowers the aspect (l/d) ratio of the fibres for easy mixing. That´s how balling can effectively be avoided and a homogenously mix of high performing steel fibre reinforced concrete (SFRC) can be achieved.
Can steel fibres be added to any mix?
Steel fibres can be used in concrete, mortar and grout. Generally, harsh mixtures, containing very few amounts of fines and/or an unsteady sieve curve at a higher fibre volume can create mixing and dispersion problems. Simply mixing steel fibres into any concrete will most probably not utilize all the positive effects fibres can provide to concrete. Depending on the type and amount of fibres 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 plasticizer
For concrete strength up to actual strength of around 8000 psi typical fibres with normal strength wire are sufficient (majority of applications). For higher concrete strength´s than middle strength or high strength fibres 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.
How will steel fibres 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 fibres 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 fibres.
Do steel fibres affect the concrete slump?
Yes, the addition of steel fibres 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.
Can steel fibre reinforced concrete (SFRC) be pumped?
Yes, but expect a 0.4” to 1.2” slump loss through the hose depending on the steel fibre 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.
Will steel fibres rust?
For indoor applications such as tunnels, no.
For outdoor applications such as pavements some minor rusting may occur. Experience in highways and industrial pavements indicate that while individual fibres corrode at the surface, staining of the concrete surface does not occur. Overall aesthetics and serviceability are maintained even with the presence of individual fibre corrosion. Indoor applications surface fibres in typical indoor tunnels 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 fibres to the surface skin of the concrete. When surface fibres corrode, there is no propagation of the corrosion more than 0.008” beneath the surface.
Since the fibres 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 fibres passing across the crack. However, small cracks (crack widths < 0.008”) do not allow corrosion of steel fibres 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.