FAQs

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 categories:

• Soil data
• Load data
• 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. ArcelorMittal will take these requests into account whenever possible in designing.

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 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.

Compared to other concrete structures, industrial floors are 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.