New Applications for Steel Fibre-Reinforced Concrete and Combined Reinforcement
After years of research and development not only recommendations but also codes for the design and construction of steel fibre reinforced concrete are available. The German Guideline for example is an addition to the structural concrete code of Germany and is taken up by the building authority in the list of official building materials. The Model Code 2010, which is supposed to be state of the science and serves as a template for an upcoming Eurocode version, has dedicated a whole chapter to the design and construction of steel fibre reinforced concrete with or without traditional reinforcement (combined reinforcement). Further guidelines are worked out in different countries.
Aforementioned standards cover a new range of applications, for example foundation slabs of multi-storey buildings, clad rack buildings, structural floors, fluid tight floors and many more slab types subject to high structural and serviceability requirements.
The application area has been expanded thanks to extensive committee work but also trendsetting progress has been achieved for steel fibre products as such. After years of research new types of steel fibres are introduced in the market which lift performance to another level.
The question about the "need" of new fibre types for SFRC finds its answer in considering all recent possibilities of applications which have evolved over years of experience with this construction material and which are after years of committee work supported by guidelines, standards and codes. In particular the German Guideline SFRC, which is taken up in the list of official building materials is a guideline with code character and enables plenty of applications for SFRC and for combined reinforcement.
The Model Code 2010 is widely regarded as state of the science and might be one of the documents that will serve as a template for a future Eurocode version in which SFRC will likely be engaged. Within the scope of Model Code 2010 SFRC is taken up and subject to design and construction for structural applications.
A brief review of steel fibre properties
The following review outlines three of the essential parameters of steel fibre properties. The performance of SFRC is mainly based on these steel fibre material properties. The following prescription holds for typical concrete strength classes between C20/25 and C35/45.
The anchorage of steel fibres is designed to allow a controlled pull out behavior under advanced deformation. Herein the resistance against pulling out of the concrete matrix plays a crucial role. The proven system is the typical end hooked type of anchorage. It offers sufficient resistance against pulling out and yet assures the mechanism of controlled fibre pull-out.
The tensile strength of a steel fibre needs to be aligned to the type of anchorage. That is the way how the tensile strength capacity can be utilized level best whilst ductility remains. For a wire of higher tensile strength without an adaption of the anchorage strength the tensile capacity will - as a matter of fact- not be approximately utilized. In case of too strong anchorage without adaption of tensile strength a brittle material behavior can be observed, the result of which is the fibre snapping. Normal strength fibres with a typical single end hook have proven to be best for steel fibres customary in the market.
Ductility might be the term and the material property that is mostly linked to SFRC. Concrete reacts in a brittle manner, and typically so do fibres. For all typical steel fibres known in the market ductility is established by the aforementioned pull out procedure. It is not a ductility of the wire type!
The new steel fibre generation
Bekaert has created fibre families in order to clearly distinguish between fibres of different performance levels. All well-known fibres with typical hooked end anchorage and a normal tensile strength of around 1100 N/mm² are clustered into the Dramix® 3D fibre family. All typical applications apply to these fibre types and the principle mechanism of fibre pull out remains.
A logical evolvement was to increase the tensile strength to a high tensile strength wire of 1500 N/mm² and to improve accordingly the type of anchorage (double hook) in order to utilize the wire strength effectively. This development lifts performance to a higher level but further sustains the pull out mechanism as previously described. Bekaert has clustered these fibres into the Dramix® 4D fibre family. The Dramix® 4D series is designed with optimal serviceability in mind.
A totally novel development for steel fibres is offered by the Dramix® 5D series. The uniqueness of this fibre type lays in the mechanism of action of the SFRC. The anchorage is designed as a perfectly shaped hook that is fully restrained into the concrete and does not follow the typical pull out procedure. Ductility is hence not ensured by fibre pull out. On the contrary for a steel fibres specific material, a distinctive treated wire type, serves as source material: An ultrahigh tensile strength wire that itself disposes of huge strain capacity (figure 1).
At full anchorage of the 5D fibre type snapping will be avoided due to the strain capacity of its material. This is a totally new principle for SFRC, an approach akin to structural concrete. This mode of action enables to utilize the tensile strength capacity at its maximum, and give logical reasoning to use an ultrahigh tensile strength wire. Figure 2 illustrates the utilization of tensile strength of a Dramix® 3D, 4D and 5D fibre in a reference concrete: The optimized utilization of wire tensile strength for the 5D fibre type is clearly visible. This investigation is based on a pull out test of a single fibre.
As a performance measure for SFRC statically determined beam tests are well established. Multiple cracking can be observed for 5D fibres used in these beam tests which indicates a performance beyond the flexural strength of concrete. 5D fibres obtain strain hardening material behavior at decent/usual dosage rate in these statically determined beam tests; behavior that to date was not associated with steel fibres. The 5D fibre type is designed for the most demanding conditions particularly for verifications in the ultimate limit state design of concrete structures. Figure 3 illustrates typical results in a beam test according to EN 14651 for 3D, 4D and 5D fibres.
In order to check the performance of the new Dramix® 5D fibre type large scale testing on elastic bedded slabs has been conducted at the University of Kaiserslautern. Comparison has been drawn to typical 3D fibres which have also been tested in the same manner. The large scale tests have proven the high performing level of the new 5D fibre types. In comparison with conventional fibre types the 5D fibres have shown an impressive performance with both, higher load bearing capacity and a much more pronounced multiple cracking effect with considerably smaller crack openings.
To verify material properties but also serving as a basis for a back calculation beams have been cast and tested according to EN 14651 and the determinations of the German Guideline. The beam tests with the 5D fibres have clearly outlined strain hardening in these single spanned beams, a material behavior that was never associated with typical steel fibres and rather reflects the behavior of structural concrete.
The market possibilities
Well known application areas
The use of SFRC is mainly associated with industrial floors, underground works, precast and minor residential applications. With regards to flooring typically non-structural floors (the building is erected on foundations, then the floor is laid later and does not interfere with the integrity of the building) are a core business. Different construction methods like saw cut or jointless floors for internal or external areas for various kinds of use are established and will remain the core application areas for SFRC. Many more wellestablished applications are illustrated in figure 6. Reluctance existed towards structural applications, the key reason being a lack of standards and limited performance of typical steel fibres.
New application areas for SFRC with focus on SLS and/or ULS
Large surfaces, intensive use, no joints:
Seamless industrial floors are increasingly replacing jointless floors. Whereas jointless floors still have contraction joints every 40 meters or less, seamless floors have no joints whatsoever, no matter how large the surface of the floor is. The optimized crack control and high impact resistance of the Dramix®4D series in combination with only a top mesh provides a system with intensive usage, and reduced maintenance and repair costs.
Fluid tight floors, water tight structures and coated slabs
The Dramix® 4D series has been specifically designed to affect cracks between 0.1 and 0.3mm, enabling you to create durable fluid and/or water tight structures with the most stringent serviceability requirements. Combined reinforcement can also be used as the substrate for hard thin toppings such as epoxy layers and other coatings. Along with only one top mesh a crack width limitation designed for the specific SLS requirement can be applied.
Figure 7 shows a typical illustration of a fluid tight floor. A coating was applied in order to assure tightness. The slab is subject to a very stringent crack width limitation in order to allow the coating to remain undamaged. A combined solution with only a top mesh + high performing SFRC is a most practical, economical and time saving way of construction.
Structural floors and seismic floors
Industrial floors are usually ground-supported and not interfering with the integrity of the actual building. However, there are structural floors on which the entire building is erected. Those floors additionally act as foundation slab that is bracing and carrying the entire building load.
Figure 8 shows the raft foundation of a 30m high production facility. The whole building is erected on the slab with cantilever columns exerting loads of more than 5 MN and 2 MNm into the slab. An additional requirement was for a seamless construction with crack width limitation to 0.2 mm. Design and execution was in combined reinforcement saving around 60% of the traditional reinforcement that would otherwise have been required. The tremendous effect of this solution was the time saving outcome and the practicability (e.g. use of Bamtech meshes enabled).
In seismic areas, floors function as a tie beam for structural elements, such as columns and pad foundations. Significant uplift forces, and in plane forces during a seismic event, have to be dealt with. A combined solution offers a practical, economical and time saving solution. By using higher performing fibres like the Dramix® 5D fibre a considerable amount of traditional reinforcement can be replaced.
Floor on piles
SFRC floors on piles have already been carried out from time to time though with stricter limitations in terms of pile distances, slab thickness and additional amount of reinforcement. All as to date executed SFRC piled floors were usually solutions with additional reinforcement along the pile grid or with a piece of mesh above the piles. Because of its exceptional load bearing capacities, Dramix® 5D steel fibres enable the construction of floors on piles without traditional reinforcement. This does not only save time during construction, but also creates new possibilities for floors on piles.
Clad Rack foundations
Clad Rack warehouses are any type of storage system in which the shelving facility is part of the building structure, thereby avoiding the need for the civil works of a conventional building. For this type of warehouse, the shelving facility not only supports the load of the stored goods, but also the load of the building envelope, as well as external actions, such as wind, snow and seismic actions. Most clad rack buildings are automatic systems (AS/RS) using robotic equipment for handling loads.
Accordingly the foundation of this racking system is a real raft foundation that additionally has to fulfil the requirements of a floor. The raft is executed before the rack system is erected; meaning temperature needs to be considered for a monolithic slab type. A typical solution with SFRC can be in combination with or even without the use of mesh or any other traditional reinforcement methods. Because of its unique capabilities, the 5D series provides utmost strength and durability to preserve the integrity of the clad rack structure from downward forks, uplift from wind loads and seismic forces. The elimination of traditional reinforcement can achieve significant savings
Steel fibre reinforced concrete has been used for years in foundation slabs of residential buildings. The legal possibility to design this kind of load bearing structure was supported by local general approvals. However foundation slabs have been limited to certain loads and size measurements. Due to recent codes (e.g. German Guideline) there is no limitation, neither of applicable loads nor to the size measurement. As such raft foundations of multi storey buildings of any kind can be executed with SFRC, or combined reinforcement respectively. Since these are in most cases heavy loaded rafts, big in size and subject to a stringent crack width limitation combined reinforcement is mostly applied. As a rule of thumb about 50% of the traditional reinforcement gets replaced. This is clearly depending on the SFRC performance wherefore the use of Dramix® 4D or 5D fibres are particularly favorable and will generate bigger savings.
The raft foundation of the building illustrated in figure 10 was carried out in combined reinforcement. One main reason for the decision towards SFRC was to minimize shear studs and shear reinforcement. With the solution using combined reinforcement both has been achieved. Bending reinforcement and reinforcement for crack width requirement has mainly been reduced. On top most foreseen shear studs and shear reinforcement have been completely skipped by the combined solution. As such cost saving, time saving and constructability have been the main reason for this solution. The key applications of the extended application area for SFRC are illustrated in figure 6. Different elements other than those mentioned herein are certainly also possible.
6. Conclusions and perspective
The application area for SFRC and especially for combined reinforcement is more plentiful than ever. Thanks to widely gained experience, proven economic feasibility and validation in terms of codes and standards the way into new applications has been cleared. With the development of new steel fibre products an essential element has been complemented. All puzzle parts are assembled in order to cover the extended possibilities for steel fibre reinforced concrete in the well-known manner of an economic, durable and time saving construction art.
-  Deutscher Ausschuß für Stahlbeton (DAfStb): Richtlinie Stahlfaserbeton (German Committee for Structural Concrete DAfStb guideline steel fibre concrete), Edition 2010
-  Fib Model Code 2010, Final draft, Volume 1 and 2
-  Deutscher Ausschuß für Stahlbeton 1996: Richtlinie für Betonbau beim Umgang mit wassergefährdenden Stoffen, September 1996, Beuth-Verlag GmbH, Berlin
-  European Committee for Standardization: EC2-1, Design of Concrete Structures – Part 1: General rules and rules for building
-  Deutscher Ausschuß für Stahlbeton: Winterberg R., Einfluss von Stahlfasern auf die Durchlässigkeit von Beton, Heft 483, Beuth-Verlag GmbH, Berlin
-  Brite-Euram: BRPR-CT98-0813, Test and design methods for steel fibre reinforced concrete
-  EN 14651: 2005 Test method for metallic fibered concrete - Measuring the flexural tensile strength (limit of proportionality (LOP), residual)
-  Deutscher Beton- und Bautechnik-Verein e.V., DBV-Merkblatt Stahlfaserbeton, Fassung Oktober 2001, Eigenverlag, Berlin
-  RILEM TC 162 TDF, Design of Steel fibre reinforced concrete – Method, recommendations, Material and Structures, March 2001
-  U. Gossla, Bodenplatten aus selbst verdichtendem Stahlfaserbeton
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