Frequently Asked Questions

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 120mm (5”).

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.

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.

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.

With the exception of the Twincone™ fibre, 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.

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.

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.

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.

ArcelorMittal also produces galvanized steel fibres. Please ask your local sales office for more information about which fibre types are available in galvanized finish. Galvanized 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-galvanized 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.

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.

Yes. A dry shake topping can of course be used with SFRC. Fibres in no way reduce the bond between the concrete and the topping. Rumours stating that steel fibres cause delamination are false. Delamination is normally due to the fact that there is not enough moisture on the surface of the slab to guarantee saturation of a chosen dry shake allowing for the required reaction and bond with the fresh concrete. When using dry shake toppings users should always adhere to the manufacturer’s installation and handling instructions.

Possibly. Single fibres showing on the surface of a floor cannot be completely excluded. However following correct integration, mixing and finishing procedures will limit fibres on the surface to an absolute minimum. In order to achieve this the user should ensure that the concrete mix contains enough fines (aggregates < 0.125 mm at a minimum of 420 kg/m³ including cement) and that the sieve curve analysis of the aggregates is rather uniform. While placing concrete, the user should pay particular attention to the bull floating process and or the use of a vibratory screed. When using vibratory screeds the end user should ensure it has enough power suited to the operation. Correct bull-floating provides good compaction of the surface and durability of the upper portion of the concrete. Additionally, it will ensure that fibres will be laying down parallel to the surface and will be covered by the cement paste. The addition of a dry shake topping helps to prevent fibres showing on the surface, but is not mandatory. It should also be noted, that the higher the dosage of fibres, there is an increased risk of having fibres showing on the surface. It is a well known fact as well, that the more flexible a fibre is (higher aspect ratio), the greater the risk that fibres will show on the surface after excessive and/or early power-floating. It is critical to success that the timing for power-floating is well considered.

Concrete for floors is submitted to intense loading and so should offer durability. The concrete selected should have a more or less uniform sieve curve analysis and contain enough fines to allow for proper surface compaction and closure. The use of steel fibres requires a higher cement dosage rate. Minimum cement content should be 300 kg/m³. Generally dosage rates of 310 to 350 kg/m³ or even more are used. One of the major problems in flooring is shrinkage behaviour. Therefore the cement used should be chosen such that their shrinkage is minimal and W/C ratio should not be higher then 0.5 for jointless floors and 0.55 for floors with joints. To reach these values, it is mandatory to use high range water reducers (HWRA) or plasticizers to guarantee workability as SFRC is stiffer then normal concrete. Gravel/Sand ratio should be between 1.0 and 1.3. The maximum size of the aggregates should be adapted to the fibre type and fibre dosage rate. Often a maximum aggregate size of 16mm or 20 mm is used. For lower dosage rates and/or thicker fibres, aggregate sizes up to 32 mm are possible. Concrete grades are generally C25/30 or C30/37. Higher concrete strengths should be avoided with regard to shrinkage and crack width limitation.

Data required for getting a floor design can be split into three (3) categories: soil data, load data and data in relationship with the slab to be designed. The first two categories are mandatory. Soil can be characterized in different ways. This can be done through the Westergaard modulus of subsoil reaction, the CBR-value or EV1 and EV2 values. For loading data there are a number of load types encountered: Uniformly distributed loads UDL, wheel loads from trucks or forklift trucks, point loads from racking systems or mezzanines, line loads from partition walls or block loads from machines. The customer can also express their preferences regarding the slab thickness, the fibre dosage rate and the concrete grade they want to use. AMWS will take these requests into account whenever possible in designing. An inquiry form for industrial floor design is available as a download on this web site.

The soil is the support for slabs on grade. Therefore the soil should be as uniform as possible to avoid differential settlement and have good bearing qualities to keep overall deformations to a minimum. The soil should be well compacted. Low compaction grades lead to thicker slabs and higher fibre dosage rates and these extra costs are normally higher than those where additional compaction is carried out. Ruts that remain after vehicle traffic are a sign of poor compaction. Normally point loads are very limited in depth in the soil, so that the most important criterium is a well compacted sub-base. However, for higher uniform distributed loads or in case of heavy rack systems, attention should be paid to possible soft layers which can be located several metres under the finished level of the sub-base. Such layers should always be reported to the designers, as these can lead to significant differential settlement between loaded and unloaded areas, which can result in major longitudinal cracks in the driving aisles.

In jointless design, shrinkage and its associated stresses become the main concern. Therefore special attention should be given to the concrete mix chosen with a view to minimizing shrinkage. The major factors that determine shrinkage in a chosen concrete mix are the cement type, the cement dosage rate, the content of fines and the water-cement-ratio. It is most important to control the amount of concrete cracking in an efficient way and to avoid micro-cracks developing into macro-cracks. This is best accomplished by limiting the concrete strength (max. C30/37, better C25/30) and by ensuring uniform saturation of the concrete with steel fibres. Correct layout of the floor panels, the use of high quality construction joints and proper curing materials and procedures are three (3) important factors for constructing quality jointless

Saw-cutting should be done immediately after the final finishing process and as soon as the concrete does not tear when cutting to avoid cracks occurring in the concrete during hardening. Generally saw-cut joints are performed within a period of 6 to 24 hours after finishing the floor, depending on the weathering conditions and on the hydration speed of the concrete mix. In the summer the waiting period is normally shorter, while in winter it can take more time for the concrete to reach the correct hardening. When sawing is started too early, there is a risk that fibres will be pulled out of the concrete and that the edges will tear and not be very neat. Skilled flooring contractors know quite well, when the time is right for proceeding with saw-cutting. When in doubt, the process should be tried as soon as possible in a limited and inconspicuous area. In the event of non satisfactory results the saw-cutting should be delayed by one or several hours to allow for the concrete to harden sufficiently.

Industrial floors are, compared to other concrete structures, very slender elements, where a high percentage of the surface (compared to the volume) is exposed to weathering conditions. Particularly during summer, when a dry wind is blowing, the loss of water in fresh concrete can be considerable. This leads to major non-uniform shrinkage and crazing which can reduce the durability of a slab significantly. To avoid this, all slabs should be cured immediately after the final finishing process has been completed. Curing can be done by watering the concrete surface (wet burlap method), by the application of a suitable curing compound or by covering with a plastic sheet. The curing process helps young concrete to gain in strength before shrinkage can begin

Wet spraying is currently the most common method of shotcreting. It is mainly used in tunnelling. For wet spraying, the concrete is mixed in the normal way and then sprayed with heavy pumps. The advantages are higher outputs, less rebound, less dust and more consistent quality of the shotcrete. Disadvantages are a rather high investment in equipment and less flexibility. Dry spraying method is mainly used for repair, slope stabilization and small job-sites. Here the ingredients are mixed dry and water and accelerator are added at the nozzle exit. Therefore a lot depends on the nozzleman’s skill in controlling water-cement ratio and thus producing a uniform shotcrete.

When shooting or guniting concrete at high speed against a target, not all parts of the concrete adhere to that target, where some portion of the shotcrete falls down or rebounds. Generally the rebound of steel fibres is higher than the rebound of the other aggregates. Therefore the fibre dosage rate in the hardened concrete is generally lower then the batched dosage rate. As the customer has to pay for the totality of the fibres and materials he is of course interested in having the lowest amount of rebound possible. The percentage of rebound depends on several parameters.  The most important factor is the skill of the operator or nozzleman (concrete pump parameters, distance of the nozzle from the target, spraying angle…). For the dry method the rebound will be higher  than that for wet-spraying.  When shotcreting overhead rebound will also be higher than when shotcreting horizontally. Some fibres like the FE (flat end) lead to lower rebound then other fibre types and heavier fibres have less rebound then very fine and light fibres as more of them have energy to stick to the concrete on the wall instead of rebounding. In addition to the mentioned parameters, the addition of micro-silica to the concrete helps as to reduce the rebound of fibres and aggregates and to reduce dust.

Fibre orientation and particularly fibre distribution are unfortunately not as uniform as everybody wishes. Therefore after having determined the dosage rate at which for a specific fibre, full ductility can be achieved on average, we recommend increasing the given dosage rate by about 20% to make sure that full ductility is reached at any point of the structure. This percentage is based on the fact, that for a normal fibre distribution, the single dosage rate should not deviate more then 20% from the targeted value.

Today, even “historical” applications in SFRC like industrial floors are missing standards in many countries. Therefore it’s no surprise, that for structural applications the situation is even worse. Actually the only country that has a standard for structural SFRC is Austria (OeVBB Richtlinie Faserbeton). In Germany, the new “DAfStb Richtlinie Stahlfaserbeton” will soon be published as yellow print. Alternatively, the Eurocodes allow designs based on test data and theoretical reflections where issues are not specifically addressed in the code. Structural designs from ArcelorMittal are proprietary designs based on this option.

Theoretically all metal decks on the market can be used for TAB-Deck™. However, as TAB-Deck™ takes into consideration the composite action between the steel deck and SFRC, before doing so, it is necessary to conduct a certain number of tests to characterize this composite behaviour. Test results are available today for SMD (UK) metal deck profiles TR60™, TR80™ and R51™ when used in conjunction with HE 1/50 fibres as well as for ARVAL (ArcelorMittal) Cofraplus 60 decks when HE+ 1/60 fibres are used in the SFRC.

APC-rebars are included to avoid anti-progressive collapse of a structure. After the events of September 11, 2001, where the floors of the WTC towers collapsed within seconds one after the other, the placement of such rebars became mandatory in North America. APC rebars are additional rebars, which are not taken into account for the bearing behaviour of the normal RC or SFRC structure. Their only purpose is to prevent the so-called progressive collapse. AS ArcelorMittal conducts business worldwide the decision was made to use APC rebars in all TAB-Slab™ structures to avoid differences from country to country with a view to continuity in design and construction.

Regarding TAB-Deck™, fire tests on beam-slab systems have been conducted at Warrington Fire Testing (UK) and at Effectis (F). These are two of the major fire testing facilities in Europe, where fire ratings up to two hours could be achieved. For TAB-Slab™ it is very difficult (and expensive) to carry out full-scale tests allowing the deduction of usable design criteria. Therefore heating tests were done with 0, 50 and 100 kg/m³ of steel fibres to analyze the variation of thermal conductivity. During these tests, no such increase of thermal conductivity due to the higher fibre content could be measured and it can be assumed that fibres do not change the behaviour of concrete under fire even at high dosage rates. Therefore regarding fire engineering SFRC can be addressed in the same manner as plain or reinforced concrete.