CONSTRUCTION FIXINGS Is it just load
Figure X Surface roughness from hammer drilling is the “Key” to resin anchor strength.
The second feature plays an important role and that is its
meandering shape. When we drill into concrete the drill bit is deflected first to one side and then the other by the aggregate the drill tip meets on its journey. And in materials without aggregates, like brick or stone, it’s us that causes the drill bit to wander simply because we can’t hold the drill straight. So we have a rough surface in a meandering hole. When we
load the fixing we increase the stress in the bonds between the resin and the hole sides and between the resin and the anchor. If we keep increasing that load eventually the bond between the resin and the hole sides will break and, if we’re testing a fixing when this happens, we will only see a slight reduction in the load.
The meandering hole – even after the bond is broken pulling an anchor out of this shape takes a high force.
This failure of the resin bond happens at the level of the
surface roughness of the base material but once the bond is broken then pulling the awkwardly shaped plug of hard resin from the awkwardly shaped hole can take virtually as much force as the initial failure load as gradually more of the resin and base material are sheared off. This can be regarded as a safe mode of failure, as the anchor does not just pop out of the hole, but a slightly misleading one. The time when this can be confused with load relaxation
is when we are proof testing anchors to check the quality of installation and when there happens to be a problem with the installation such that the anchor fails at a load close to the proof load. In the years that I spent site testing over thirteen thousand anchors of all sorts - many of them resin anchors - I actually came across this more times than you might imagine, in both concrete and masonry. From the tester’s point of view what may happen is this. Let’s
say we’re testing an M12 anchor with a recommended load of 8kN to a proof load of 1.5 times that i.e. 12kN. We load the anchor carefully to 12kN and leave it to settle. After a minute or so we notice a slight loss of load to say 11.9kN.
82 Fastener + Fixing Magazine • Issue 69 May 2011
relaxation? Or has the anchor failed? We increase the load back to 12kN. If the anchor is well founded the initial loss of load was just relaxation and further relaxation will be hardly noticeable. If, on the other hand, the bond has broken then when we reapply the 12kN load again it will reduce again and we will eventually be aware that the anchor is pulling from the substrate. In fact the bond probably started to fail at a load less than 12kN we just didn’t notice it. The trouble is that each time we reapply the load we need to give it some time, say a minute or so, to see if it is relaxing or not. We just have to be patient. But of course there would need to be something wrong for the anchor to fail at a proof load. Maybe the installer had not cleaned the hole as thoroughly as he should have. Maybe he had not pumped any resin to waste before pumping into the hole. Maybe the anchor had been tightened before the resin had cured properly and the bond had inadvertently been broken in the process. Sadly this happens far more frequently than it should as many installers who I meet openly tell me: “You don’t need to allow the full curing time, the resin cures in ten minutes.” when the manufacturer’s instructions talk about three or four times that. And the problem with tightening before the anchor is cured is that we can induce a pretty high load in the anchor when we tighten it (too high if we don’t use a torque wrench) and the overload we then cause only breaks the resin bond locally but we don’t realise it as the anchor doesn’t just pop out of the hole. Even testing an anchor at the manufacturer’
relaxation, due to creep of the resin, than if left much longer So there are a few conclusions we can draw from this.
1. A reduction in load during a test may not mean failure – it may just be load relaxation.
2. Re-applying the proof load with no movement of the anchor shows it was just load relaxation.
3. When proof testing resin anchors we need to take time to differentiate between relaxation and failure.
4.
If we’re selling resin anchors to installers we should take the opportunity of reminding them about correct installation especially thorough hole cleaning (sell them a brush), pumping the first resin to waste, allowing the full curing time and not over tightening.
s recommended curing time will show more .
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