Here we'll address the problems and solutions when drilling into this often stubborn material-- how to avoid hitting rebar, the procedure if you do, which drill bits are suitable, etc.. Please also note our general drilling guidelines for mounting wire trellises.
Concrete is a mixture of three components: 1) an aggregate-- pebbles/gravel/sand, 2) water, and 3) cement-- which can be manually spread or poured into a mould where it then sets. In order to increase the strength of this material, it is often reinforced with a latticed structure of steel rods or ‘rebar,’ which concrete can be poured over and set around. Concrete with a distinctly high inclusion of rebar is known as ferroconcrete. This rebar, in turn, is often divided into stronger sections of reinforcement with thin wires that secure the rods during concrete casting to maintain firm positioning. The overlay of reinforced concrete used externally is 3.5 - 4 cm, so holes drilled up to and below 3.5 cm are generally unproblematic. Concrete surfaces can be left bare and untreated. However, for aesthetic reasons they can also be painted, plastered or tiled.
First, determine the wall thickness by drilling a test hole and ensure that it is not a 'thin concrete wall (< 8cm thick).' It is best to keep the drilling depth 1-2 cm shallower than the wall's thickness to prevent spalling (splintering or chipping) on the inner wall. Due to the density of ferro-concrete, a hammer drill is usually needed - for which we provide high quality drill bits. A hammer drill is sufficient for small holes, but it is worth noting that special drills with concrete cutting geometry such as SB 77777 are more suitable than universal masonry drills like SB 44444. Pre-determine your drill points and drill initially without the impact (percussive) motion of the hammer drill, to prevent spalling in the surface layer, especially with exposed concrete. Please refer to our general guidelines on drilling if unsure.
If you hit metal or your drilling stops on its own, you have probably encountered rebar. Clean the drill hole and shine a spotlight into it. If you see a 'thin' (up to 5 cm) iron rod, it can be cut through with our hammer drill bits by simply continuing to drill. Be persistent! Severing any thinner auxiliary wires in the concrete (see above) will often go unnoticed. In the case of greening commercial industrial structures, any severance of inner reinforcement should be clarified with the site management. Strong steel bars should not be severed--multiple perforations of the reinforcing steel can affect the structural stability. For the special case "prestressed concrete" please see below. Read on for how to troubleshoot botched drilling...
If you think you hit rebar, confirm whether you have hit a vertical or horizontal rod by pulling out of the hole and checking out the inside with a flashlight; then, seal the hole thoroughly with weatherproof white filler to guard against water damage and consequent rusting of the inner reinforcement. Once the filler has set, a new hole can be drilled and the mount(s) fastened as planned.
If necessary, you can also drill a bit skewed/angled such that you bypass the rebar. Mounts of the easy, light, or heavy kits can be bent in a vice with a hammer to fit exactly, the remaining space filled with composite mortar. Alternatively, with medium eco and classic kits, if you realise after screwing them into place that they are crooked, you can gently hammer them into the desired level position.
This method is valid for all our mounts that use metric thread screws; it limits the drilling depth to that of the external wall such that you do not drill at all into the rebar zone. This means drilling will be much easier; however, because of the shallow drill depth, it also means that special attention must be paid to the angle of the drill bit. If you do not drill straight, the circumference of the hole will be enlarged and lose tension, compromising the stability of the mount. Once a successful hole has been drilled, insert the threaded shaft, followed by a suitable brass plug/anchor, which can then be shortened accordingly using a buzz saw. For medium kits, the WM 10081 holder is suitable. However, if the reinforced wall has only been plastered over, it will not be strong enough to support the weight of a brass wall anchor; once the hole is drilled it must be secured in the wall with composite mortar. The same procedure is used for wood threaded screws.
In order to increase stability in exceptionally large structures, like industrial ceilings, highway bridges, etc., ‘prestressed’ concrete has been developed. This is when the metal rods (tendons) are subjected to immense lateral strain pulling in opposite directions or ‘stressed.’ Prestressed concrete has two forms: pre and post-tensioned. In pre-tensioned concrete, the tendons are stressed before being cast in the concrete, whereas in post-tensioned concrete, the tendons are stressed within the concrete using hydraulic jacks. Drilling for the mounting of trellis systems must be done with the utmost care as damage to/severing of the tendons can completely compromise the structural integrity!