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What is Steel Fibre Reinforced Concrete (SFRC)?

SFRC is concrete to which steel fibres are added to replace completely or partially the traditional reinforcement. Typical fibre dosage rates are between 15 and 50 kg/m³. For structural applications, the dosage rate can be increased up to 100 kg/m³.

Steel fibres reduce and control the shrinkage of concrete. They bridge cracks that appear in concrete thereby guaranteeing a certain level of post-cracking ductility and help to prevent micro-cracks, which are always present in concrete, from developing into macro-cracks. For typical dosage rates, this post-cracking behaviour is generally partially ductile. For higher dosage rates and premium fibres, full ductility including strain hardening can be achieved.

How do steel fibres perform in concrete?

Steel fibres are normally deformed at their ends or over their entire length. These deformations allow for better anchorage of the fibre within the concrete matrix than that obtained by the simple friction between steel and concrete.

Steel fibres are more or less uniformly distributed throughout the concrete. Their orientation is spatial. Typical spacing between two fibres is between 10 and 25 mm. Thus, steel fibres can block micro-cracks at their inception and subsequently prevent them from developing into macro-cracks. In the event a macro-crack does develop, the steel fibres act as tiny rebars, where the main anchorage is obtained through the deformation devices of the fibres.

How can I test and determine the performance of SFRC?

The performance of SFRC can be measured in different ways by standardized load-bearing tests. Today the most common way is to use simply supported SFRC beams with a cross section of 150 x 150 mm².

Possible standards for doing this are Japanese JSCE/SF4, Dutch CUR 35, German DBV-Merkblatt or RILEM guidelines. Another way of testing SFRC is to use circular or square SFRC panels with central point loads or a line load. This type of tests generally leads to results with less deviation than those obtained through beam tests. Common documents used in conjunction with plate testing are the EFNARC procedure, Swiss SIA 162/6 or French BEFIM rules.

In all cases, the load-deformation behaviour during testing is registered in order to characterize the post-cracking behaviour of SFRC and to derive design values.

What is the difference between steel fibres and micro-synthetic fibres?

Micro-synthetic fibres are very thin short fibres primarily made of polypropylene (PP).

Typical dosage rates are from 600 to 900 g/m³ of concrete. Unlike steel fibres, micro-synthetic fibres do not offer concrete post-cracking bearing capacities. Concrete with micro-synthetic fibres remains as brittle as plain concrete. Micro-synthetic fibres only have a positive influence on the fresh concrete. By reducing the amount of bleed water from fresh concrete, they reduce the plastic shrinkage of said concrete. After the concrete has hardened and is drying, they partially restore this water to the concrete, thus creating a curing effect. In other words, they slow the rate at which moisture evaporates from the surface of the concrete. The same effect can be obtained with steel fibres, by adding only the correct amount of allowable water for a given concrete mix, using adequate super-plasticizers as well as concrete mixes that are stable yet workable and by curing of the concrete based on state of the art technology as soon as is possible after the final finishing of the surface.

What are structural applications?

Generally engineers make a distinction between structural and non-structural applications. Applications are considered structural where a global or partial failure of the given element would lead to a subsequent failure of other elements accompanied by loss of human life or heavy material damages. 

As an example, industrial floors on ground are non-structural applications as cracking of the slab will not be followed by a collapse of the whole building. On the other hand a raft would be considered to be a structural element as such a subsequent collapse could happen.

What is concrete shrinkage?

In order to guarantee the workability of concrete, you need to add more water then required for the chemical reaction also called hydration. This excess water remains in the concrete. A portion of that water leaves or evaporates during the drying process of the concrete. This leads to shrinkage of the concrete. If the concrete element cannot move freely, the concrete will then be submitted to internal tensile and flexural stresses which can cause cracking of the element. Shrinkage is critical in the early stages of the concrete. Drying shrinkage slows down over time until finally coming to a complete stop after 2 or 3 years. Shrinkage is an inherent property of concrete. It can only be delayed by applying proper curing procedures or reduced by choosing an appropriate concrete mix. The main parameters influencing the shrinkage of concrete are the added water content, the design W/C ratio, the content of fines (cement, fly ash, slag, fillers, micro-silica,…) and the type of cement. When concrete is submerged in water, the opposite behaviour takes place; the concrete expands. When the concrete then begins to dry, shrinkage begins again.

How can I guarantee that fibres will be distributed uniformly in the concrete?

The main parameter for the fibre distribution is the mixing time.
Normally one (1) minute mixing time per m3 of concrete is required to ensure uniform fibre distribution. When the fibres are added directly into the truck mixer containing ready mixed concrete, the same rule applies. However the drum should rotate at full speed and the minimum mixing time should not be less then five (5) minutes.

A second parameter that influences the uniform distribution is the aggregate grading. The sieve curve of the aggregates should be continuous and the maximum aggregate size should be adapted to accommodate the volume of fibres added. It should be noted that fibre distribution is considered uniform when all single dosage rate measurements based on a volume of ten (10) litres of fresh concrete do not deviate more then 20% from the targeted dosage rate.

Can I use steel fibres in self compacting concrete (SCC)?

Yes, steel fibres can be used in SCC.

The user should be aware that due to the high slump of SCC, the fibres will be largely oriented in the direction of the concrete flow. In the event this direction differs from the direction of the main tensile stresses, the user should refrain from using steel fibres as the only reinforcement in the element.

Does the addition of steel fibres influence the workability of concrete?

Yes, the addition of steel fibres reduces the slump thus increasing the stiffness of concrete.

It can be assumed that with typical fibre dosage rates of 20 to 40 kg/m³, the consistency is modified by one class (slump reduction of 25 mm to 45 mm) To compensate for the reduced workability SFRC should always incorporate the use of high-range water reducing admixtures like super-plasticizers.

Can SFRC be pumped without problems?

Yes, SFRC can be pumped even at high dosage rates (100 kg/m³). 

As with other pumpable concretes, the concrete mix must contain enough fines to provide stability thus preventing segregation of the components. 

When pumping SFRC the minimum hose diameter should not be less than 120 mm (5”).

What is fibre balling and how can I avoid this?

A fibre ball is a bunch of fibres sticking together during fibre integration in the concrete mix. Fibre balls can block the concrete pump and will cause pockets or areas of unconsolidated concrete, voids in hardened concrete, which can lead to crack formation.

There are two types of fibre balls, wet balls and dry balls. The dry fibre ball is the most common type, which forms during fibre integration, when the fibres are initially added. The fibres are not integrated correctly, so that a bunch of fibres is covered by mortar, which through the rotating movements in the mixing drum forms a ball, usually of 100 mm to 150 mm in diameter. This can be avoided by choosing an adequate fibre integration method. When breaking open a dry fibre ball, you will only find fibres and fine sand inside. It should also be noted that some fibre shapes ball more easily (at the same aspect ratio) than other shapes and that for a given shape, the risk of balling increases as the aspect ratio (length over diameter) increases. The shape and aspect ratio should be two (2) of the considerations when choosing a particular fibre. Wet fibre balls are formed during mixing. This can happen if the mixing time is too long, if the sieve curve analysis of the aggregates is not continuous and if the maximum aggregate size is too large for a given application thus inhibiting the homogenous mixing process to disperse the fibres uniformly; or if the concrete is segregating. As wet fibre balls are formed after the fibres have been distributed in the concrete mass, these balls will contain other aggregates including fine sand. Glued fibres can also form wet fibre balls. When fibre balls occur it is very important to determine which type of fibre ball has formed as this is the key in troubleshooting both types.

How can I determine the fibre dosage rate on fresh or on hardened concrete?

The best way to check fibre dosage rates is to do so with fresh concrete as this allows for corrective actions to take place prior to concrete placement. Normally the concrete volume to be analyzed should be at least ten (10) litres to avoid large deviations in the obtained results. The fines can be washed out from the fresh concrete and the fibres extracted by a magnet. This procedure is rather simple but time consuming. Therefore the use of the ArcelorMittal dosometer is recommended for this purpose. This tool allows for easy and safe collection of the SFRC sample while acting as a simple and safe mechanism for separating the steel fibres from the concrete constituents. Here the fibres are extracted directly from the fresh concrete magnetically. The extracted fibres must then be dried and weighed to calculate the corresponding dosage rate. Determining the fibre dosage rate on hardened concrete is not very accurate. This is mainly due to the fact, that it can only be done on drilled concrete cores and that the volume of such cores does not normally exceed 2.5 litres. The concrete must be crushed and the fibres then extracted from the crushed material. As the risk of missing or damaging some of the fibres is rather high, the measured dosage rate is normally lower than the actual value. Today, more sophisticated methods have made their appearance on the market by measuring the dosage rate using electromagnetic induction on cubes of fresh or hardened concrete. The disadvantage of this method is that it is a rather expensive tool requiring a calibration of the equipment with the used fibres before being able to use it for measurements on site.

Which parameters influence workability of SFRC?

With regard to workability, the main parameters are the fibre shape, the aspect ratio, the fibre length and the volume of fibres per m³ of concrete. Generally the same parameters which decrease the workability conversely increase the performance. Therefore it is important to find a compromise between workability and performance.

What are the benefits of high strength premium fibres?

All steel fibres available on the market are pulled out of the concrete matrix in ultimate limit state. As the majority of the fibres have deformed shapes, the material must yield in these deformation points in order to allow pull-out. Therefore the higher the yield or the tensile strength of the base material, the higher will be the pull-out resistance and the better the anchorage. High strength premium fibres show higher performance than normal grade fibres. Therefore quite often it makes sense to use these fibres in higher strength concrete.

What is ductility and post-cracking behaviour?

Concrete is a brittle material. When submitted to tensile stresses it shows an elastic behaviour up to the cracking of the material. Once the crack arises, the resistance is zero, which means that the collapse is very sudden and brutal.
To avoid such an unpredictable behaviour, the brittle material concrete is reinforced by traditional rebar or by steel fibres. These methods of reinforcement guarantee that the bearing capacity loss after cracking is not total. This is referred to as post-cracking behaviour. Post-cracking behaviour can be partially or fully ductile. Full ductility means that the cracked section has a higher resistance than the un-cracked section and shows a so called strain-hardening. When, during the post-cracking behaviour, the bearing capacity is reduced yet not falling to zero, we refer to this as partial ductility or strain-softening. With traditional fibre dosage rates, SFRC typically shows just such a partial ductile behaviour, where the level of ductility depends on the performance of the chosen fibre and dosage rate of the particular fibre. With higher dosage rates and premium fibres, it is possible to achieve full ductility with SFRC.

Can steel fibres replace traditional reinforcement in any application and if not, why not?

When SFRC demonstrates strain-softening behaviour, it can only be used for non-structural or temporarily applications when completely replacing traditional reinforcement. For use in structural applications, full ductility is required in order to avoid collapse after cracking. However even in these circumstances the substitution is limited to elements with low and medium bending moments and a multi-directional bearing behaviour, like slabs for instance. For elements with high flexural requirements in one direction, like beams, the complete substitution with steel fibres is not possible. This is mainly due to the limited length of a single fibre and the absence of possible moment redistribution. Nevertheless, in these cases fibres can always be combined with other reinforcement methods.

Is the fibre distribution in the concrete absolutely uniform?

If SFRC is mixed correctly the fibre distribution will be rather uniform. Fibre distribution can be considered as uniform, when each single measurement does not deviate more then 20% from the specified dosage rate value and when the mean value does not deviate more then 10% from that same value. Measurements should be done with minimum concrete volumes of ten (10) litres through a series of at least three (3) measurements. The SFRC samples should be taken after discharge has begun at the following intervals: about one-third of the mixer’s volume, during the middle and toward the end of the load. Samples should not be taken at the very end of the load.

Does ArcelorMittal produce galvanized steel fibres and what are the benefits of such coated fibres?

ArcelorMittal also produces galvanised steel fibres. Please contact your local sales office for more information about which fibre types are available in galvanised finish.

Galvanised fibres are generally about 50% more expensive then the same fibres without zinc coating. Zinc coating is normally not required for steel fibres. In fact, even after carbonisation, concrete cover of about 2 mm is sufficient to protect fibres from corroding. In heavy chloride environments the required cover can increase up to 7 mm. Non-galvanised fibres which are exposed on the surface will of course corrode, leaving some rust stains. However this is purely aesthetic and has no influence on the bearing capacity of SFRC. For this reason plain steel fibres should not be used for architectural or exposed to view concrete. Furthermore, due to their reduced diameter, rusting steel fibres do not have the power to spalling off the concrete surface as rebars typically do when corroding.

What is the difference between loose and collated fibres?

The main reason for collating fibres together with water soluble glue is to improve the fibre integration into the concrete without forming balls.

The same effect can be achieved by using the right integration tools, but of course at additional costs. Some glues may contain a certain amount of air entraining agents which can increase the total air content in concrete.

The customer should therefore pay attention when using collated fibres in combination with extra air entrainer as this might result in excessive air content, which can significantly reduce the concrete strength, make concrete pumping more difficult and disturb the surface aspect of industrial floors through upcoming air bubbles.