Constraints in the masonry

In addition to sufficient stability of a building, sufficient safety against the occurrence of cracks is also important. Prerequisite for a proper assessment of crack safety of components and buildings made of masonry is the sufficiently precise knowledge of the deformation and stability characteristics to be considered, essentially: Shrinkage, crawling, tensile strength, modulus of elasticity and shear stability.

Not every “little crack” represents a defect to be negotiated or that justifies a claim. On a case by case basis it must be checked whether a crack is only a blemish, which can be easily eliminated, or whether the crack can severely impact the usability and thus renovation measures are required. The goal is therefore to avoid cracks in the planning and implementation already. This requires that the key influences on the occurrence of cracks are known and the certainty of cracks can be determined sufficiently precisely.

Cracks appear if the load (stress) on the component becomes greater than the strength. For short-term demand the short-term strength, for long lasting demand the permanent strength is decisive. For long lasting demand it must be considered on the one hand that the strength is about 20 % less than results in short-term testing indicate, on the other hand long lasting stresses are relieved by relaxation.

Stresses occur through the impact of loads (e.g. loads on the walls from storey ceilings), but also via obstructed form change, which can appear with or without load impact. Included in the first mentioned type are the elastic form changes from short-term load impact and creep from long-term load impact, while moisture and heat expansion form changes belong to the second type. The shrinking, which occurs due to the drying (release of moisture) and leads to a reduction in volume or shortening, has particular significance. Swelling in ceramic building materials, such as masonry bricks and tiles, and also in porous concrete is partially reversible.

If the shortening of a component due to shrinkage and possibly cooling is hindered by the storage conditions (connection with other, differently deforming components), stresses arise from the hindered part of the deformation. Due to the comparably small strength the risk of cracks is greater for pull, shear and push stresses than for compressive stress. The size of the deformation difference and the deformation-impaired part, i.e. the stress-generating part between adjacent components, is always essential. It depends on the load, the component dimensions, the climatic conditions (temperature, relative humidity), the placement conditions (“degree” of component connection: obstruction degree), as well as the building material characteristics (deformation properties, tensile strength). Knowing them is therefore a significant prerequisite for an estimate of the crack safety.

Basically shrinkage occurs at constant drying conditions faster in the beginning, later increasingly less with the drying duration. Key influences on the shrinkage progression are

Such changes, i.e. repeated shrinkage after getting wet, can occur if the masonry blocks absorb precipitation moisture at the storage location, at the construction site and in unprotected masonry and release it again in between. The first shrinkage usually begins in the production-moist state, however, always without previous drying possibility.

With stones, the shrinkage is only finished after two months to one year. For masonry a shrinkage duration of three to five years is to be expected, whereby 90 % of the total shrinkage is already reached after about one year. Significant differences in the course of shrinkage of the blocks in the direction of block height and length or in the masonry vertically and parallel to the horizontal joints do not occur.
As a final value for moisture expansion (shrinkage, chemical swelling) for lime-sand blocks, concrete blocks and granulated slag blocks, DIN 1053-1 lists a calculated value of -0.2 mm/m and a value range of -0.1 to -0.3 mm/m, for porous concrete blocks a calculated value of -0.2 mm/m and a value range of +0.1 to -0.3 mm/m, for light-weight concrete blocks using pumice a value of -0.4 mm/m and a value range of -0.2 to -0.5 mm/m. The calculated value for masonry bricks is listed at 0 mm/m, however, with a value range of +0.3 to -0.2 mm/m. The value range thereby lists the maximum range that is possible based on current insights. Examinations have also shown that the storage times of the blocks after production have a significant influence on the shrinkage. For light-weight concrete building materials in long-term tests on blocks and walls final shrinkages of about -0.2 mm/m were determined.

The consideration exclusively of the shrinkage of the wall components often represents an inadmissible simplification. A building structure experiences deformations dependent and independent of load. Additional deformations can be: