June 9, 2014
This is the second of three articles on the design and construction of concrete industrial floors. Tony Hulett of Face Consultants looks at the modern and possible future developments in the structural design of ground-supported industrial floors found primarily in warehouses and distribution centres, as described in Chapter 7 of Technical Report 34 Fourth Edition(1)
In practical terms, the changes in the 4th Edition of TR34 have had relatively little impact on design thickness of slabs in comparison with the 3rd Edition(2). This judgement is based largely on the commonly used fabric reinforced slab designs incorporating sawn joints.It is possible that there may be more impacts in respect of fibre reinforced slabs as more guidance has been provided on minimum ductility requirements and the effects of fibres at joints. New guidance is given on steel stay- in- place construction joints and on the use of saw cut joints in fibre reinforced floors.Guidance is now provided for taking account of ground support in punching calculations.
The minimum ductility requirement has been increased to 50%. This applies to both steel fabric and to fibre reinforcement. For fibres, this is unlikely to increase the design thickness of most jointless slabs. It will however restrict the use of very low steel fibre contents such as the 10 kgs per cubic metre reported as being used in slabs with sawn induced joints. Although this appears to be more of a mainland European phenomenon, it also applies to synthetic fibres used in sawn slabs seen in the UK. TR34 gives clear advice on the use of such designs as discussed later.One aspect of design which has not been considered is that of minimum fibre quantities for jointless slabs. Historically, 35 – 40 kgs of a 50 mm or 60 mm fibre was used. While this appears to still be the case in the UK, recently, designs in mainland Europe have been seen with as little as 18 kgs of fibre per cubic metre. The case is made that finer fibres are being used and that, therefore, the total number of fibres has increased resulting in better crack control. This is hard to accept at face value because the claimed ductility value of the lower fibre content is reduced to approximately 55% compared with the previously typical 70 to 80%. Although there may not be a precise correlation between flexural tensile strength and axial tensile strength, one must be an indicator of the other. This is an area for future research.Enhancement of punching capacity by the addition of steel fibres has been reduced by 50 %. This action was taken because it was felt that there was insufficient data to substantiate the 3rd Edition position. This has little effect on ground supported slabs where the effect has largely been balanced by the enhancement of the ground support that can be taken. Punching capacity is of much greater importance in pile supported designs and it was felt that a more conservative position was appropriate. This will be reported in the next of these articles.The enhancement effect of steel fibres on punching at dowels was shown to be unjustified and is not permitted in the 4th Edition.
Positive advice is now given about the poor performance of continuous plate dowels. Commonly known as Omega joints, these have been shown to be inadequate with common failure caused by shear of the concrete above or below the continuous plates. These continuous plate dowels have been superseded for some considerable time by discrete plate dowels in most floors. They are however still prevalent on mainland Europe and have been used by some flooring contractors until recently in the UK.The use of sawn joints in fibre reinforced slabs has proved to be highly contentious. These were found in the UK on a number of new floors in the late 1990’s. Joints were left with no load transfer capacity and a number failed under the actions of fork lift trucks.The UK floor constructors learnt the hard lesson and soon abandoned the practice. As the UK market was more disposed to using steel mesh fabric in sawn floors, the number of projects affected was limited.Meanwhile in Europe, floor designs using fibres have generally been jointless. However, fibre reinforcement with sawn joints has become noticeably more prevalent in recent years. Some of these are already causing difficulties with considerable damage at some large distribution centres such as can be seen in Figures 1 and 2. These are presenting major headaches for developers, property owners and tenants alike. The same situation has been seen in the UK with the use of synthetic fibres and sawn joints.The trend towards fibre sawn slabs is also accompanied by a trend towards very low levels of fibre reinforcement. Designs have been seen with just 10 kgs of steel fibre. The floor is therefore effectively unreinforced as it is difficult to envisage that a cracked concrete section with such a small amount of fibre can experience any redistribution of moment. It is also certain that such small amounts of fibre cannot restrain surface cracks. TR34 now recommends that sawn joints should be avoided in fibre reinforced floors unless additional load transfer measures are used. In practical terms, this means that sawn joints should not be cut in floors reinforced with fibres only.
It is well recognised that joints are the weakest points in a floor and it has been proved to be the case that the installation of armoured joints has not been a panacea for the problem of damage by truck wheels and in particular by pallet trucks. Typical and in this case early damage at a joint can be seen in Figure 3.
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