Overview

The demands on an industrial floor have become ever larger in recent years. Durability in particular is ever more in the foreground. In order to substantially plan and manufacture heavy duty industrial floors, the use of steel fibre concrete as a long-lasting building material has been proven through practice. By planning the concrete floor as a jointless system in accordance with TAB-Floor™, even more unfavourable load cases such as slab edging and slab corners will be additionally avoided and no high-maintenance dummy joints will exist.

The planning of such as system comprises of more than a simple measurement of load capacity. ArcelorMittal offers comprehensive concepts for various system solutions which cater for all demands and impacts on an industrial floor.

The planning and implementation of the innovative building material - steel fibre concrete - guarantees long durability for heavy duty industrial floors.

A practical example

An industrial floor which must be used with a coil transporter with a wheel load of 220kN must be able to withstand the heaviest loads. In the following example, it is evident that a concrete floor can be manufactured with high quality, load capacity, and durability if extensive planning is carried out and concrete engineering possibilities are optimised.

In November 2005, a new industrial floor was constructed in the coil storage area of the cold rolling mill at the steelworks in Bremen. A jointless industrial floor according to TAB-Floor™ was implemented. The jointless areas which differ in size are constantly driven on by an automatic coil transporter (FTC: Floor Transport Conveyor). Therefore, a wheel load of 220 kN (2 x 11 t per wheel) and a contact pressure of 6N/mm² were taken into account when measuring. A factory-mixed Readymix steel fibre concrete with 40 kg of ArcelorMittal HE 1/50 steel wire fibre was added to the C30/37 XC4 XF1 XA1 F4 concrete.

The industrial floor was constructed with a ready to use surface, had thicknesses of between 30 and 32 cm, and was manufactured without a hard substance layer. The surfaces have been manufactured with a high evenness and are completely free of cracks. Up to the present time there have been no abrasions at all upon permanent use.

The demands on industrial floors have become ever larger in recent years. Operators and clients of new, modern halls with varying use expect innovative and above all durable constructions based on these findings. In Germany, approximately 30-40% of all industrial floors are made of steel fibre concrete. In particular, seamless systems, such as TAB-Floor™ by ArcelorMittal, make it possible for large spaces to have high evenness, maintenance–free use, and high resilience.

Policies
In terms of its composition and its properties, factory mixed steel fibre concrete meets the requirements of DIN EN 206-1/DIN 1045-2.

Design

As stated above, industrial floor components with low hazardous potential are not subject to building regulations as a rule and can be measured with the bending tensile strength of the steel fibre concrete. As a rule, measuring takes place according to the theory of elasticity in Stage 1 (sketching stage), with the model for elastic bedded slabs.

Requirements

Because of the numerous possible uses of an industrial floor, the possible requirements are very wide-ranging. The load is decisive for planning and design. The industrial floor will be designed by its load, depending on type, impact, and size subject to subsoil. In addition to its load capacity, requirements on usability are becoming increasingly larger.

Load capacity
The bending tensile strength of the concrete must not be exceeded through bending stresses resulting from exterior loads. Verification of the load capacity is recorded in the design calculations.

Ground usability

If the industrial floor is placed on subsoil with a permanent base course and it does not have to bear any further elements or it hardens, it is therefore not an element in the context of DIN 1045-1. With regard to the other cases, an approval must be requested per individual case. However, in relation to the usability of the ground, it makes sense according to DIN 1045-2 to plan and take exposure classes into consideration.

As of 01.06.2008, steel fibres in Germany must bear the CE mark for use in concrete. The general technical approvals and Ü load deformation behaviour and cracking behaviour are decisive for durability and are improved by steel fibres.

Density/impermeability

If a concrete floor slab is required, it can be used in accordance with the WU-guideline. In this instance, the cracking width limit in particular is to be established. This can be achieved very economically through a mix made from reinforced concrete and steel fibre concrete.

If the concrete floor must be sufficiently tight in order to prevent ingress of water-hazardous substances into the ground water, concrete engineering will be planned and carried out in accordance with the DAfStB guideline when dealing with water-hazardous substances.

Due to the addition of steel fibres in the concrete, the shape and depth of the crack, as well as its spread and width are positively affected. In tests, there was already a reduction in the crack widths of over 50% due to the addition of 0.38 Vol.-% (30kg/m³) steel fibres to the reinforced concrete.

Effects and stresses

The most diverse loads have an effect on concrete floors. Not only mechanical loads and wear and tear, but also chemical attacks must be taken into account in this instance.

Punctiform loads

Punctiform loads are often crucial for verifying the loading capacity of an industrial floor. These can consist of mobile loads such as lorries, HGVs, and forklifts, as well as long-term effective loads such as shelves, supports, or containers, for example.

The three loading cases, slab centre, slab edge, and slab corner are to be taken into account when measuring. Loads impacting on the edge or the corner can therefore produce higher tensions by up to 100% than the same loads on the slab centre. In contrast with conventional industrial floors with dummy joints cuts, the effect of the load on the TAB-Floor™ is taken into account with the slab centre load case.

The bending tensions of point loads are generally determined according to Westergaard. The following are accepted for the three load cases:

Block loads and surface loads
Impacts from extensively distributed loads, e.g. bulk materials, are not influential for measuring as a rule.

Mechanical stress
According to DIN 1045-2, exposure class XM, attack on concrete through stress of wear and tear, applies here for load-bearing or hardened industrial floors. It is also advisable to consider the minimum requirements on industrial floors which are not load-bearing or hardened if no further surface treatment takes place.

Abrasion resistance in the face of sliding and buffeting stresses of steel fibre concrete is amplified. The higher the steel fibre content, the greater the abrasion resistance. In addition, the impact strength is greatly increased. Tests revealed that the addition of suitable steel fibres can increase the impact strength to 20 times that of plain control concrete.

Shrinkage

Extensive elements are always heavily exposed to shrinkage. Concrete formulae for industrial floors should have a water / cement value of ≤0.55.

The shrinkage process is further aided by the wind, high temperature, and low moisture and begins on the top of the upper side of the slab. The underside is protected through its positioning. Reduction through shrinkage is therefore greater on the upper side of the slab and in extreme cases can lead to so-called curling.

Frost

The substructure must be frost protected. If necessary, a capillary breaking layer to counteract increasing moisture is required.

If concrete areas are to be used, the possibility of entering de-icing salts must be additionally taken into account. In this instance, exposure class XF2 or XF4 is to be considered.

Temperature

Concrete floor areas in halls are protected against large variations in temperature. Warming-up by means of underfloor heating is harmless because it takes place very evenly and slowly.

Chemical attack

In some warehouses, productions, and industrial operations, substances are present which attack the concrete. If a penetration of water-hazardous substances in the subsoil can be expected, this must be taken into account.

Planning

Many parameters are to be taken into account when planning for industrial floors. For the operator and the user of the industrial floor, it is even more important that certain details and minimum requirements are defined.

Subsoil
The subsoil is the basis of the whole construction. It must always be sufficiently sustainable in order for loads to be withstood. The crucial size for the design is the EV2 deformation module which is determined by a static load platform pressure test in accordance with DIN 18134. Depending on the load and the base course, the EV2 deformation module of the subsoil should be ≥40MN/m². The quality of the sealing is evident from the ratio comparison of the second load to the first load EV2/ EV1 and must be ≤2.5. Should the required values for the deformation module in the load platform not be obtained, an enhancement of the subsoil must be carried out.

Base course
The base course is located directly underneath the concrete flooring slab, on the subsoil. It is usually manufactured from gravel or grit, can be consolidated with cement and should be at least 15 cm thick. In practice, base courses are manufactured with a thickness of approximately 20-25 cm. The EV2 deformation module of the base course should be ≥100MN/m².

Joints
A joint plan must be issued for conventional industrial floors with dummy joint cuts as well as for industrial floors without joints but with joint profiles. This is in accordance with the geometry of the hall, usage, equipment, and assembly capacity.

Dummy joints must be incised as early as possible (if the implemented concrete is safe to walk on without causing any damage). They are approximately 3-4 mm wide and have a depth of approximately 1/3 of the slab thickness. The crack existing in the remaining slab thickness is 'needled' by the steel fibres and a transfer of sheer force is possible.

No dummy joints are required for jointless industrial floors without joints according to TAB-Floor™. In this instance, large, jointless fields of up to 2500 m² are implemented. The individual fields are connected to one another with joint profiles. All joint fields should have square dimensions if possible and must not exceed an aspect ratio of 1:1.5.

Surfaces
As soon as the concrete can be walked upon, the areas of the floor can be levelled out. This takes place through positioning and troweling by machine. In this instance, there are very smooth and even surfaces which can be left as finished surfaces.

If an additional wearing surface is necessary, the hard substance material  will be evenly placed on the concrete surface and then levelled out and undergo tolerance analysis as before.

Manufacturing of steel fibre concrete
The factory mixed steel fibre concrete is manufactured in accordance with DIN EN 206-1/DIN 1045-2. Furthermore, the features of the steel fibre concrete which are not regulated are emphasized in the DBV Steel Fibre Concrete pamphlet.

If possible, the fibres should be mixed into the concrete in the concrete transport system via an automatic dosing system with fibre separation.

The concrete retracts through the addition of the fibres. In order to adjust the consistency, a liquifier from the construction site is entered into the concrete mixer. A mixing time of at least 1 minute per m³ of concrete must be observed.

Building construction and assembly
The regulations in DIN 1045-3, sections 8.2 to 8.6 are applicable.

All adjoining components are to be detached, i.e. must not make contact with the concrete floor. The industrial floor is assembled and must be observed. In this instance, the usual curing times for exposure class XM are doubled. The strength and abrasion resistance of the industrial floor surface depends on the curing to a large degree.

Author: Dipl.- Ing. Wilhelm Nell (VDB)
Senior Sales Manager Steel Fibres CEE
ArcelorMittal Wire Solutions Steel Fibres Profit Centre