Here we provide advice, address the problems and solutions to the drilling of this often stubborn material: How to avoid hitting rebar, procedure if you do, what drills are suitable, etc. Please also note the general instructions for drilling work for the mounting of wire trellises.
Concrete consists of three components, 1. The aggregate, i.e. 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 3.5-4cm, so holes drilled up to and bellow 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 walls thickness by drilling a test hole and ensure it is not a 'thin concrete wall’. It is advised to keep the drilling depth 1-2cm lower than the walls 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, however, 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-determined your drill points and drill initially without the impact motion of the hammer drill, to prevent spalling in the surface layer, especially with exposed concrete. Please refer to the general remarks about drilling if unsure.
If, during concrete drilling, you hit metal you have probably encountered rebar. Clean the hole shine a focussed light source into it. If you see an iron rod approximately 5mm diameter it can be severed when persistently drilled with our hammer drill bits. Severing any thinner auxiliary wires in the concrete (see above) will often go unnoticed. In the case of reintroducing plants to commercial industrial structures, any severance of inner reinforcement should be clarified with a site management. Strong steel bars should not be severed, especially if multiple perforations of the reinforcing steel effect the stability of the structure. For the special case of prestressed concrete please see below. Solutions for botched drilling are as follows:
First, confirm whether you have hit a vertical or horizontal rod but pulling out of the hole and illuminating the inside, 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 holder(s) fastened as planned.
Sometimes, when drilling, the direction of the hole can become skewed or interrupted by a rod of rebar. If you have drilled and notice the shaft of an Easy, Light and Heavy kit are not lying straight you can drill the hole wider to allow adjust the position of your anchor then fill the remaining space with composite mortar. Alternatively, with '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 suitable for all applications that use metric screws and limits the drilling depth to that of the external wall, meaning you do not drill into the reinforced area. This makes drilling much easier because the hole will be particularly shallow, however, 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, thus compromising the security of the implanted screw. Once a successful hole has been drilled, insert the threaded shaft, followed by a suitable brass anchor, which can then be shortened accordingly using a buzz saw. For medium sized 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. This procedure is also relevant to anything with a wooden thread.
In order to increase stability in exceptionally large structure such as 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 growth aids must be done with the utmost care as damage to/severing of the tendons can completely compromise the structural integrity!