Strength properties and load-bearing behaviour

The property values of masonry can vary greatly due to the numerous stone-mortar combinations that occur in practice and sometimes deviate significantly from those of other building materials.
Masonry is a building material that is primarily suitable for components subject to compressive stress. The ability to withstand tensile, bending and shear loads is much lower than that of compressive loads. Masonry is therefore primarily used for the transfer of vertical loads.
However, wind, earth pressure and earthquakes also cause horizontal loads which, depending on the arrangement of the walls in the structure, can result in a load in the plane of the wall or at right angles to it. For example, external walls are subjected to bending tensile and plate shear as a result of earth pressure or wind, while bracing walls are usually subjected to shear in the direction of the slab.

For the design of structural components, the relevant strength properties or the load-bearing and deformation behaviour of the masonry must be known. In Eurocode 6 and the associated national annexes, the relevant strength properties of masonry are specified depending on the material combinations and the design equations to be applied are given.
These are based on models that were derived in the past on the basis of highly simplified assumptions and theoretical and experimental investigations. Therefore, they cannot always adequately represent the load-bearing and deformation behaviour of today's new developments in masonry construction.

The research work at ibac aims to fundamentally and systematically investigate the load-bearing and deformation behaviour of masonry - also taking into account previously neglected influencing variables.
The investigations are carried out both experimentally on the building material, on composite and wall test specimens and with the help of numerical calculation methods. From this, the material laws of the masonry units, the masonry mortar and the composite are determined for the respective load case.
With the development of correlations, existing models are to be adapted or new models developed that describe the compressive, flexural and shear load-bearing behaviour of masonry more accurately and enable better utilisation of the materials and material combinations.

Deformation properties and crack resistance

As with other comparable building materials, masonry is occasionally damaged by cracking.
For decades, ibac has been intensively involved in researching the property values that influence crack resistance.

These are

  • Deformations (shrinkage, swelling, creep),
  • the stiffness and
  • the relaxation


of the masonry.

In addition to assessment procedures for crack safety with recommendations for the avoidance of crack damage, options for the repair of cracked structural components are also developed.

Masonry reinforcement

The tensile strength of masonry is much lower compared to the compressive strength.
When using very light, thermally insulating masonry blocks with low bulk density or filigree hole structure, lower tensile strengths can occur. This also results in lower bending tensile and shear strengths, which are often insufficient when assuming increased horizontal loads.

Analogous to reinforced concrete, the flexural and shear load-bearing capacity of masonry components can be increased by means of reinforcing steels. These are embedded in mortar in the bearing joint or encased in mortar in vertical masonry block recesses.
In Germany, the design of such components is currently very uneconomical. Due to gaps in knowledge about the load-bearing capacity of this construction method, the safety factor on the material side was set at a value of 10 for the time being.

The ibac is working intensively on the load-bearing and composite behaviour of conventionally reinforced masonry. The aim is to incorporate these findings into the design of reinforced masonry in the National Annex of Eurocode 6 so that this construction method can be used in practice.

Another modern way of reinforcing building components is the use of textile fabrics, as shown by the latest building material developments in the concrete sector. As a new research focus, textiles embedded in mortar are being investigated to reinforce existing masonry and to increase the flexural and shear load-bearing capacity of new masonry structures.
The experiences from the special research area of textile-reinforced concrete and the exchange with European partners in this field, who have been dealing with reinforced masonry under earthquake loads for a very long time, are directly incorporated.

Fatigue behaviour and durability

Light mineral building materials for load-bearing wall components are of great importance for meeting the thermal insulation requirements of residential and office buildings.
Lightweight concrete and aerated concrete blocks can be produced today with gross densities of approx. 250 kg/m³ at a compressive strength class of 1.6 N/mm².

In order to investigate the fatigue behaviour of small masonry piers made of lightweight concrete blocks with a low strength class, fatigue load stands were designed and further developed at ibac. These enable the application of a permanent load defined by a constant, previously calibrated pressure. The creep strength of aerated concrete blocks can be tested on small cylindrical test specimens.

In addition to the creep behaviour, the durability of masonry building materials is also of relevance to safety. To ensure durable aerated concrete, the responsible standardisation committee has limited the maximum shrinkage to 0.4 mm/m.
The relationship between the deformation behaviour and the strength properties of aerated concrete is also being investigated at the ibac in collaboration with the "Mineralogy and Inorganic Chemistry" working group at the microstructural level.