BUILDING MATERIALS: THE ANCHOR BASE

The selection of an anchoring system does not depend only on the load on the anchorage. The material in which the element is to be anchored is also important. The characteristics of the building material include a particular compressive strength, which affects the selection of a fixing. The compressive strength is the pressure that the material can withstand, given in the unit of MPa. Walls of plaster, masonry or concrete have different properties and therefore require different approaches. The thickness of the material and its characteristics are also important.

Panel elements are thin-walled building materials. These panel elements, such as plasterboard panels, fiberboard, chipboard and plywood, often have a low compressive strength. Fastening by means of plugs forms a solid basis for a strong fastening, as these plugs receive their strength from expansion in the cavity at the back of the board. This is called form locking. The anchors suitable for this type are generally known as cavity anchors.

Masonry is a composite material formed of brick and mortar. The compressive strength of brick is higher than that of mortar, so anchoring in the brick is preferable. The older the building work is, the greater the difference in compressive strength and the more important it becomes to achieve anchoring in the brick. 

Bricks with a compact structure have a high compressive strength and no cavities. Bricks with up to 15% hollow space are considered to be solid bricks. The relatively high compressive strength renders them extremely suitable for fastening methods using plugs. Perforated bricks or ventilation bricks are usually made from the same high compressive strength material as solid bricks or clinkers, but they are provided with cavities. For heavy loads, special plugs should be used in hollow bricks. These plugs are designed to bridge or fill out the cavities.

Bricks with an open structure have a low compressive strength and are extremely porous. For optimum strength, special plugs must be used, such as plugs with large expansion surfaces. If these bricks also contain hollow spaces, careful selection and fitting of the right plug is required to form a reliable anchorage. Suitable plugs may be those with a very long expansion zone or injection systems consisting of a threaded rod, anchor sleeve (net) and injection mortar. Holes in perforated bricks and building materials with an open structure should only be made by rotary drilling in order to ensure that the drill holes are not too large, which would cause the webs to break away in cavity building materials. Hammer drilling increases the likelihood that the material will crumble. Drilling is faster if hard metal bits are used for rotary drilling.

The concrete material type includes lightweight concrete, normal concrete and heavy concrete. Lightweight concrete differs from normal concrete because of the addition to the mortar of materials such as pumice, Styropor, fly ash or Clinker lsolith; cement is the binding agent in all such cases. The additive, which has a lower compressive strength than the gravel in normal concrete, leads to less favorable conditions for anchorages. The maximum load-bearing capacity of constructional fastenings to concrete is extremely dependent on the compressive strength of the concrete. The compressive strength of concrete is identified by the cube compressive strength, which refers to the pressure that can be exerted on a cube measuring 15 x 15 x 15 cm before it begins to break. This value is given in the building material characteristics using a capital letter B in combination with a figure. For example, a cube compressive strength of B25 indicates that the compressive strength of this concrete is at least 25 MPa. A similar indication of strength is related to a cylinder measuring Ø16 x 32 cm, in accordance with the European Pre-standard ENV 206, in which case the designation is C20/25. Here, the first value represents the minimum characteristic cylinder compressive strength, 20 MPa. The second value gives the minimum characteristic cube compressive strength, 25 MPa.