PRESSURE ZONE AND TENSION ZONE
All types of loads that act on building components, such as inherent weight, traffic and wind shear, result in forces and tensions, which can lead to deformations and even cracks. The load is an important cause, but cracks can also be produced by the contraction of the concrete as a result of imposed forces and deformations or by external influences such as earthquakes.
As an example, a sag in a bridge produces a compressive force in the upper section of the bridge, the pressure zone. Tensile forces arise in the lower half of the bridge, which is the tension zone. Concrete has a very limited capacity for absorbing tensile forces. Where tensile forces can be expected, concrete is often constructed with steel reinforcement, which is better able to absorb these forces. As concrete cannot withstand the same tension as the steel reinforcement, the concrete eventually develops small cracks at the place where the steel reinforcement has taken on the tensile forces. The two effects result in countless fissures, which are barely visible to the naked eye, and it referred to as cracked concrete. As a result of changed loads or temperature differences, cracks can occur even in structures that have existed for a long time.
All anchors that are suitable for use in concrete can be used in the pressure zone, while anchors that continue to expand and those that are fully enclosed in the drill hole by means of form locking are suitable for use in the tension zone. Examples of anchors allowable in the tension zone are undercut anchors, post-expansion segment anchors, sleeve anchors and some chemical anchors. Undercut anchors take on the form of a cylindrical or conical drill hole. The oversize of the conical part prevents the anchor from pulling out of the hole, even when a crack occurs; cracks increase the diameter of drill holes. Post-expansion segment anchors even out the cracks, as the taper is pulled deeper into the expansion area. Both of these types of anchoring are suitable for use both in the tension zone and for absorbing shock loads. However, not every anchor approved for the tension zone is suitable for transferring loads in a concrete construction that has been cracked by earthquake.
HOLLOW-CORE SLABS
Hollow-core slabs are a special group within the prefab concrete elements. Because the reinforcement does not allow drilling into walls, the constructional fastenings are, in fact, always made into the upper or lower shell of the core. The thickness of the upper and lower shell usually varies between 25 and 40 mm. To maximize the available shell thickness, fastenings systems that use form locking are recommended.
FAILURE MECHANISMS
Overloading of fastening points, incorrect mounting and insufficient load-bearing capacity of the base material can lead to the failure of an anchorage. The various failure mechanisms can be divided into four groups.
ANCHOR BASE FRACTURING
The anchor base can fracture due to an excessive load, inadequate load-bearing capacity of the base or if the setting depth is too low.
SPLITTING
The building material can split as a result of several conditions: if the required edge distances and axial spacing are not observed, the dimensions of the building component are too small, or the expansion pressure of the anchorage is too high.
ANCHOR IS PULLED OUT
An excessively high load or incorrect mounting can lead to an anchor being pulled from the building material.
STEEL FRACTURING
If the diameter of the anchor or screw is too small in relation to the forces that arise, the steel can fracture. Steel fracturing can also occur if lower-quality steel was used. The steel breaks at the point with the smallest diameter.
The load on the anchorage and the characteristics of the base material are important for the selection of the anchor. The permissible load indicated by manufacturers should be treated with caution. To interpret this information correctly, it is also important to know what this value is based on: anchor base fracture, splitting, extraction of anchor or steel fracture. Besides this qualification, it is naturally important to know the building material on which the indicated permissible load is based. To prevent the failure of an anchorage, demands are also placed on the mounting of the anchor or plug. Knowledge of the drill hole depth, edge and axial spacing, fitting depth and drill-hole cleaning are extremely important for creating a reliable anchorage.
