Introduction
The use of distributed fibres within a concrete mix as a replacement for steel reinforcing bars or fabric has long been established for the casting of mass concrete structures such as ground bearing floor slabs. For suspended RC floors it has remained common practice to specify steel bars and fabric. Composite floor slabs consist of a thin gauge profiled steel sheet that acts as both a shuttering system and as the main tensile reinforcement. In these slabs there is a reduction in the amount of reinforcement that is required to be embedded in the concrete but there has still been a requirement for fabric or bars to supplement the composite decking to satisfy all structural reinforcement requirements
In the late 1980’s the convenience to be gained by replacing the fabric in composite floor slabs with fibre reinforcement was recognised, with the fibres distributed evenly throughout the concrete mix improving the resistance to crack development. The principal fibre types available at that time were polypropylene micro-fibres and steel fibres. It was not until more recently that synthetic structural fibres have become available.
The main purpose of adding any of these fibre types is not to prevent cracking in concrete but to control it. Uncertainty over the ability of fibres to replace all or part of the reinforcement in a suspended composite floor slab construction identified the need for an extensive research programme.
The traditional roles of supplementary steel reinforcement
The HOLORIB and RIBDECK profiles available from Richard Lees Steel Decking are all designed to act as shuttering during the slab construction stage, and then as tensile reinforcement to the slab once the concrete has cured. By combining these two functions all of the reinforcement is by necessity near to the soffit of the slab. Additional reinforcement may then be required to resist tensile stresses in the concrete from any of the following:
- Drying shrinkage cracking during concrete curing.
- Thermal cracking through repeated heating and cooling cycles.
- Settlement cracking.
- Negative bending over supports.
- Cantilevers.
- Supplementary fire reinforcement steel, when the exposed decking reduces in strength.
- Lateral distribution of loads (steel decking is a one-way spanning element).
- Framing reinforcement around openings.
- Lateral reinforcement of composite beam flanges.
Research
In 1988 Richard Lees Steel Decking commissioned a full-scale slab fire test at Warrington Fire Research Centre, using HOLORIB as the composite decking profile. The slab was reinforced with a standard dosage of polypropylene micro-fibres and successfully passed all the criteria necessary to achieve a fire rating on the construction tested. Despite this success there was concern over the size and extent of cracking that occurred in the zone of negative bending over the intermediate support beam. At that time research into the use of synthetic micro-fibres was suspended.
In 2003 a renewal in interest in the use of synthetic fibres to replace steel bars and fabric led to the development of a cooperative research programme between Richard Lees Steel Decking and Grace Construction Products. The fibre proposed by Grace Construction Products was STRUX® 90/40, one of a new generation of structural synthetic fibres. STRUX® 90/40 is a flat fibre manufactured from a co-extrusion of two polymer types. Physically the fibres are 40mm long with a cross-sectional aspect ratio of 90.
Fire test slabs were cast using both HOLORIB and RIBDECK E60 profiles, with STRUX® 90/40 fibres added to the normal weight concrete mix. Once cast the slabs were allowed to dry for a period in excess of 6 months to allow the free moisture content level to reduce to that representative of a real building. The slabs measured approximately 4.5m x 3.0m and after delivery to the Warrington Fire Research Centre were built in to the roof of the furnace with the metal decking soffit directly exposed to the furnace flames. This effectively modelled the most onerous end bay condition within a large floor slab construction. The top surface was subjected to a uniformly distributed load (typically 6.7 kN/m2) for the duration of the test, which followed the procedures set out in BS EN 1365-2:2000. Each test was successful when judged on the failure criteria for insulation, integrity, and load bearing capacity outlined in the test code.
In a separate research initiative a series of small scale push tests were completed at Cambridge University to demonstrate the continued ductility of welded shear studs when embedded in fibre reinforced concrete. These tests followed the recommendations of the forthcoming Eurocode 4 and demonstrated a slight improvement in both ductility and shear resistance against conventional concrete push tests.
A third research programme has been used to compare the transverse shear strength of fibre reinforced concrete against that of conventional reinforcement. Bath University was commissioned to complete a series of Hofbeck tests, where concrete samples were failed in pure shear. The results of these tests again demonstrated a slight improvement in performance when structural synthetic fibres replaced conventional reinforcement.
Application of results
Different types of fibre can be used to control some of the common causes of cracking in concrete but of the synthetic options available only structural fibres can satisfy all of the criteria investigated in the current research programme. It is similarly true that different types of composite floor decking can be used but it is important to realise that each profile is unique in its combination of shape, yield strength, and ability to bond to the concrete. These results therefore only serve to demonstrate the successful combination of HOLORIB and RIBDECK as provided by Richard Lees Steel Decking, with STRUX® 90/40 as provided by Grace Construction Products.
Why synthetic fibres?
Certain types of steel fibres can be used as a replacement for traditional steel fabric in a similar way to that investigated here for synthetic structural fibres, but they do introduce practical problems that synthetic fibres avoid. One characteristic of suspended composite floor slabs is that the concrete is most frequently delivered to the work area by pump. The addition of steel fibres to the mix can adversely affect the ease with which the concrete flows through the pipes, whereas the addition of STRUX® 90/40 to the mix does not. It is also quite common for the surface of composite floor slabs to be power floated and the presence of steel fibres at the slab surface will be detrimental to the quality of this type of finish.
Operations on site
STRUX® 90/40 fibres are supplied in 2.3kg bags that can be added directly to the concrete mix. The paper bags dissolve in the wet concrete and release the fibres, which quickly disperse throughout the mix. By the time that the concrete has been placed the STRUX® 90/40 fibres are distributed throughout the concrete section and are available to control cracking at the point of initiation.
In most instances the concrete will be pumped up to the steel decking and then spread, compacted and levelled to provide the finished floor. A pumped concrete mix will have been designed with an even distribution of aggregate sizes, ranging upwards from fine sand particles to coarse aggregates with a diameter typically in the region of 20mm. The geometry of the STRUX® 90/40 fibres allows the fibres to fit easily into this mix matrix. Adding any fibres to the mix will result in a slight decrease in the workability, increasing the work necessary to place and compact the concrete. To counter this stiffening of the mix by the introduction of STRUX® 90/40 at a dosage of 5.3 kg/m3, a superplasticiser is added. The target is usually to achieve a medium to high level of workability (i.e. 120 – 150mm slump) to allow efficient pumping.
A well-designed mix containing the correct dosage of STRUX® 90/40 and superplasticiser will ensure that there is no excessive wear of the pump. This is a direct function of the flexibility of the synthetic fibres, especially when compared with rigid steel fibres, as is demonstrated by there being no increase in pumping pressures when compared to normal concrete mixes. Once discharged on to the HOLORIB or RIBDECK sheets the fibre reinforced concrete is easily spread, compacted, levelled and finished to the standard normally expected.
Conclusions
Removal of the traditional reinforcement from the construction of composite floor slabs will yield considerable benefits to the overall construction programme. This is now possible for a range of design situations using HOLORIB or RIBDECK composite floor decking with STRUX® 90/40 synthetic structural fibres. The nett result can be both project economy and efficiency.
When considering the substitution of synthetic structural fibres for steel fabric and bars the cost to benefit ratio should be assessed after consideration of the following:
- Improved safety – reduces the number of heavy items to be lifted to level, the chances of overloading the steel decking when used as a working platform, the need for breaching of perimeter safety rails, the number of areas left undecked to allow crane access, a reduction in potential trip hazards, and a reduction in manual handling tasks.
- Logistics – reduction in vehicles delivering materials to site, storage space requirements on site, and crane hook time.
- Programme – reduction in construction time associated with installation of fabric and loose rebar.
- Design – improved crack control and flexural stiffness, a high strength to weight ratio with up to 2 hours fire resistance, and an assurance that the slab is reinforced where required as the fibres are evenly distributed throughout the slab.
