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Welcome to the Precast Flooring Federation

FULL SCALE FIRE TESTS SHOW BENEFIT OF USING PRECAST HOLLOWCORE FLOORING

Concrete as a material has excellent and proven fire resistant properties; it does not burn, does not add to the fire load and thus does not aid the spread of fire.

Concrete combined with prestressing strands, or prestressing wires, to form precast prestressed hollowcore concrete floor units has been widely used for over 50 years in many types of building construction including residential, industrial and leisure applications. The benefits of using hollowcore floor units are many including high load carrying capacity, long spans, durability, speed of erection, provision of an immediate working platform, good thermal and sound insulation, as well as providing a floor with excellent fire resistant properties without the need for the application of secondary fire protection systems.

Structural design codes have enabled precast hollowcore floors to be designed with up to 4 hours fire resistance by using tabulated data that gives minimum dimensions for the depth of concrete cover to the prestressing strand or wires as well as overall depth of the floor slab to be used. Where actual fires have occurred in buildings with hollowcore floors the performance of the floor has generally exceeded the design specification, safely maintaining structural integrity.

Fire tests carried out in test furnaces on relatively small sections of hollowcore units in Europe indicated that there was a risk of premature shear failure occurring in hollowcore flooring during the heating cycle of a fire. These results were disputed by some experts as the test samples were not considered to be acting in a structural manner representative of that which occurs in a building where different restraints due to a number of factors can be imposed on the floor slab.

It was decided by the Precast Flooring Federation in conjunction with the International Prestressed Hollowcore Association and other member companies of the British Precast Concrete Federation to undertake two full scale fire tests, as part of an ongoing product performance development programme, to see if premature shear failure was truly a potential problem, and by the use of significant instrumentation to better understand how precast hollowcore flooring behaves during the heating and cooling phases of a realistic fire.

British Precast were responsible for co-ordinating the test programme and appointed Professor Colin Bailey, a recognised fire expert, of Manchester University to undertake the design of the test regime to equate to a one hour standard fire test, including the fire load, and be responsible for producing an independent analysis of the results. A separate order was placed with the Building Research Establishment to undertake the construction and testing, including full instrumentation. BRE having great expertise in undertaking this type of full scale testing following the recent full scale fire tests of concrete and steel structures at Cardington. The two tests were carried out under the direction of the BRE’s Mr Tom Lennon at their new large scale fire test facility at Teeside in March 2007.

Both test structures were of single storey construction using 200mm deep hollowcore floor units forming an area of 18m x 7.5m supported 3.6m above ground level on structural steelwork with infill blockwork panels. Plasterboard fire protection was applied to the steelwork and blockwork but not the soffit of the floor units. Openings to the front of the test structures represented window openings and provided ventilation to the fire as would occur in a real structure. Structural loading was applied to the hollowcore floor area by the use of 1 tonne sandbags positioned to give a Live Load of 4.0+1.0Kn/m2, significantly more than the minimum used for office loading of 2.5+1.0Kn/m2. The difference between the two tests was that one had peripheral tie details to give the floor different edge restraint characteristics to the other.

The fire load was provided by forty timber cribs designed with the intent that the fire intensity followed the standard parametric fire curve for a one hour duration fire. Due to the size of the ventilation openings the resultant fires were in fact more severe than the fire design parametric curve.

During the tests average atmospheric temperatures in excess of 10500C, were recorded by thermocouples within the test structures, compared to 9000C required by the standard parametric fire test curve, however the degree of spalling of the concrete soffits was negligible.

Both tests were extremely successful with the hollowcore floor areas maintaining their structural integrity for the full heating and cooling cycles of the tests. There was no shear failure that the small standard furnace tests had indicated might occur, and these tests demonstrate that such unrepresentative small scale testing should be treated with extreme caution.

An immense amount of recorded data including temperatures, temperature distribution and deflections showing how the flooring behaved during the tests was obtained for analyses to the benefit of the hollowcore flooring industry. A technical paper will be published towards the end of 2007, written by Professor Bailey and Mr Lennon, which will provide more in-depth information on these successful full scale tests.

The tests demonstrate that for a fire resistant floor the use of prestressed hollowcore units should be the first choice solution, enabling rapid construction without the programme and cost effects of providing secondary fireproofing systems.

Norman E Brown
Head of Engineering Services – British Precast
Secretary – Precast Flooring Federation

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