64 Construction fixings

ISSUE 64 - JULY 2010 > CONSTRUCTION FIXINGS

So called “Concrete screws” have become a well accepted fixing technique within the range of fixing solutions currently available. Like most fixings they have their strengths and weaknesses and, as with most fixing types, Mark Salmon of Independent Fixing Consultants has some clear views on what they are. He outlines their development, the range of versions available, where they may be used, where they should be avoided and issues regarding installation.

What’s in a name? Well, in the case of “Concrete screws” not quite enough really. Yes, they work well in concrete but they are also a very useful solution in some types of masonry – admittedly the harder or stronger types - so the more general “Self tapping screw” is the term I prefer to use. The term “Screwbolt” is a trade name given to it by the company that developed the larger diameter versions so it’s not a name we can use generally.

Of course self tapping screws for use in concrete have been around for ages but originally only in small diameters. It was the development of the 6mm to 20mm diameter range, that started the market for these products. Now no self respecting fixings manufacturer or supplier is without a range of these products in their portfolio.

The operating mechanism is pretty obvious. After drilling a hole in the substrate (through the fixture if needed), and removing the dust with a blowout pump, the bolt is screwed in - tapping its own thread by means of sharp threads raised on the profile. Some types have serrations cut into the leading edge of these threads to aid cutting. This operating principle has led to these anchors being categorised as “Undercut” anchors within the ETA system and there are plenty of different types which now carry that most reassuring of marks along with the associated “CE”.

One of the key features claimed for these fixings is that they create “no stress in the base material” and can therefore be placed closer to edges and each other than expansion anchors. Well, there is some truth in that but there can be some stress created on installation as the threads force their way into the material. And all fixings, no matter how stress free, need some substrate around the fixing to accept the loads from the fixture so, while minimum edge and spacing distances may be relatively small, they must still be respected.

So where are they used and for what applications?

The first part of that question is straightforward. They work well in most grades of concrete and in strong masonry i.e. strong solid brickwork and stonework, but they should only be used with extreme care in weaker bricks or stones. The problem is not that they are necessarily terribly weak in these materials, although strength is obviously reduced, but simply that they are too easily over tightened if the material is too soft. Once the anchor is screwed down against the fixture and, assuming there is no gap between the fixture and the substrate, just turning the anchor by one full turn will strip the thread in the base material and leave it with no holding power. For this reason I would not generally recommend these anchors for use in any form of lightweight or thermal blockwork. The ideal location within bricks is on centreline keeping at least 40mm from the ends. Due to the high variation in strength of mortar joints it is recommended that anchors with hole diameters less than 14mm are not used into mortar joints and especially when fixing through plastered or rendered surfaces where joints cannot be seen.

The question as to where they may be used also covers internal and external locations and here the news is even more restrictive. For the most part they are zinc plated and must therefore be limited to use in dry, internal conditions. A restricted range is made with a hot dip galvanised finish which may give somewhat greater protection but, as the coating on the threads is inevitably prone to being scraped off during installation, these versions cannot be guaranteed to offer corrosion free fixing in external applications. A very restricted range is available in stainless steel for external use.

The range of applications which may be covered is nonetheless quite large and helped by a wide range of different configurations. The hexagon headed versions are undoubtedly the most frequently used type and the washer is often integral to the bolt head. Designations usually refer to the drill hole diameter.

There is, however, one range of applications for which the lack of stainless versions is not a problem and that is for temporary use, such as scaffolding ties and anchoring for steeplejacks. As they are removable it means they leave no steel parts behind in the structure to corrode after the scaffold is struck. Of course they may well start to rust during the life of a scaffold if it is in place for an extended period so an inspection regime may be warranted in these cases. Here their capabilities in brickwork are particularly useful, especially as the Construction Fixings Association has recently withdrawn its endorsement of the use of drop-in anchors for fixing scaffold ringbolts into brickwork. Their Guidance Note on this subject is in the process of being revised and will make this clear. It will almost certainly include more examples of how self tapping screws may be used for tying scaffolds. Brackets can obviously be fixed directly using hex headed versions; eye versions are also available from some manufacturers. These may have a relatively small eye, 22mm diameter, for taking hooks, while a much larger eye, capable of taking the scaffold tube itself, has recently become available. Experience with this is limited so the jury is out regarding its effectiveness. Another technique being considered is that of using versions with male projecting threads to which an internally threaded coupler may be attached to take the traditional M16 threaded ringbolt, see the illustration below.

This would ideally use the largest screws available i.e. 14mm diameter - adequate thread engagement in the coupler
is important!

Any of these techniques using self tapping screws to support scaffold tubes should be used only when the scaffolder can be confident that there will be no lateral loads applied, e.g. from settlement, as none of these techniques has any significant capacity in bending. Scaffolding contractors assure me that settlement is not an issue and, as these fixings can be relatively strong in brickwork, they appear to offer a timely replacement for the drop-in anchor in that material. Another innovation is the use of resin systems to enable self tapping screws to create a screw thread in the resin and dramatically enhance their holding power in particularly weak or soft brickwork. This technique should only be employed if endorsed, with full installation instructions, by the manufacturer of the self tapping screw. This is one area where – to my knowledge – there is as yet no ETA endorsed product so use of this technique must also be made with extreme care.

One word used above needs a little expansion - “Removable”. Just because the screw is removable does not mean it is also “RE-USABLE”.

Opinions vary on this but the Construction Fixings Association has considered the issue carefully and decided that self tapping screws should not be considered to be re-usable. The problem is that the cutting surfaces wear, even in the first time of using, and thereafter holding power may be reduced so manufacturers’ performance claims cannot be relied upon after the first use. In brickwork, which can be very abrasive and wear the threads significantly, this problem may be severe. It may also be much harder to get the screw engaged with hard substrates. The biggest difficulty – even if the manufacturer allows re-use a certain number of times - is limiting re-use to that number.

Of course their ease of installation is one key attraction but this too needs some clarification. Turning the screw into concrete can be hard work, especially if multiple installations are to be made into stronger concrete, so impact wrenches may be used to make this easier. This is why some manufacturers quote a tightening torque. It is useful for setting the impact wrench to make sure it has enough torque to drive the screw home – in concrete. However, it is not a torque which you should strive to achieve when setting the anchor normally. Correct installation is achieved by tightening the screw until the head bears against the fixture (with no gap between that and the substrate) and continuing for up to a further quarter turn to generate the required clamping force through the fixture. Trying to achieve the recommended torque in anything weaker than concrete could result in the thread stripping and the fixing failing. Impact wrenches should therefore not be used in weaker materials including brickwork or stonework.

In summary self tapping screws are a very useful, relatively strong and versatile product with some limitations and requiring respect on installation.

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Self tapping “Concrete screws”

By Mark Salmon, Independent Fixings Consultants

READ ISSUE 64 (click here)