Anchor and fixing

Frequently asked questions

Why is an anchor's stated load not the load it will carry?

Because the published figure belongs to a TEST, and the test named a base material the job may not have. An anchor approval states the capacity in a defined material — a particular concrete strength, a particular solid or hollow unit — at a stated embedment, edge distance and spacing. Change any of those and the capacity changes, usually downward and sometimes by a large factor: the same anchor in solid concrete, in a dense block, in a lightweight aerated block and in a perforated clay brick are four different capacities, and in the lightweight block it may be a fraction of the headline. Edge distance and spacing matter because the failure is usually the CONE of base material pulling out rather than the steel breaking, and cones that reach an edge or overlap each other are smaller. So the honest reading of a datasheet number is the capacity of an anchor-and-material pair under stated geometry, and the first question on site is what the wall is actually made of.

When is a resin anchor the right choice over a mechanical one?

In hollow or weak base materials, close to an edge, and where the fixing must not expand. A mechanical anchor works by expanding against the sides of the hole, which needs solid material to push against and generates a bursting force — so it performs badly in a hollow block, and near an edge it can split the masonry it is gripping. A resin anchor bonds along the whole depth of the hole instead, spreading load over a larger area and imposing no expansion force, which is why it is standard in perforated units with a mesh sleeve, in weak or historic masonry, and close to edges. Its cost is process discipline: the bond is only as good as the hole preparation, and a hole left full of dust can lose most of its capacity. It also has a curing time that varies sharply with temperature, and a load applied before the resin has cured is the most common way a correct specification fails on site.

What does proof loading prove, and what does it not?

It proves the installation, not the design, and keeping that straight matters. A proof test applies a defined load — typically a multiple of the working load, well below the anchor's ultimate capacity — to installed anchors, and passes those that hold it without measurable movement. What it catches is the failure the calculation cannot see: a hole drilled too shallow, a wrong drill diameter, dust left in the hole, resin cured cold, an anchor in a void behind the surface. What it cannot tell you is whether the anchor is the right one, whether the base material is what the design assumed, or whether the anchor will still hold in twenty years, because a test to a working multiple deliberately does not go near failure. This is also why it is specified as a sample across a population rather than as a test of every fixing: it is a check on workmanship, and workmanship is the variable it is sampling.