Constraints in the masonry
Explanations for occurring tensions 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
- the structure (pore volume, pore size distribution, binding agent and type of aggregate),
- the component size,
- the drying conditions (relative humidity, air flow),
- the number of extreme temperature changes,
- the number of shrinkage-swelling changes.
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:
They occur immediately after placing the load and are also complete at that time. The load stems from the building structure’s own weight (walls, ceilings, floors, plaster) and the so-called traffic load (use load from walking and furnishing). The elastic deformations are therefore not complete until commissioning of the building
With constant stress and after occurrence of the elastic deformation the building structure continues to deform over a certain period of time. In building practice creeping deformations are complete after two to five years. The creeping deformations are greater than the elastic deformations by about a factor or 2. Also because of the creep deformations the plaster should be applied as late as possible.
Amount and time progression for setting depend on the building soil and the groundwater conditions.
The building materials (light-weight concrete, masonry mortar, plaster, concrete components) manufactured with binding agents (lime and/or cement) reduce their dimensions with increasing drying (shrinkage). The process is mostly reversible (swelling). In building practice shrinkage is complete after two to five years. So this is only one reason for selecting the time for plastering as late as possible.
Depending on the change in temperature the dimensions of the components change. This results in a “working” of the cracks. Frequently occurring cracks are (independent of building material used) parapet cracks, usually in the vertical course of the reveal, but also those running vertically in the parapet area or from the bottom corners of the windows diagonally outward. This is often caused by horizontal tensile stresses at the upper edge of the parapets due to the spreading of the pressure trajectories, eccentrically introduced camber support forces and increased shrinkage stresses due to the reduction in cross-section through the openings. In the area of window parapets people frequently do not notice that a butt joint in the extension of the reveal represents a pre-fabricated breaking point for the parapet break to be expected.
To avoid tears in the parapet area a constructive reinforcement should be planned, e.g. inserting at least 2 reinforcement rods-Ø 6 mm in the last, upper horizontal joint. The rods should reach 50 cm into the lateral masonry on both sides. Alternatively, even if not as a full replacement, reinforcement of the plaster base in the parapet area can be implemented.
Due to the deformation differences only diagonally inserted fabric (under 45° to the horizontal joint direction) is effective in the area of the extension of the reveal for accepting pulling forces. Reinforcement fabric in flush-mounted plaster with low-strength light plasters also require a larger overlap width/anchoring length than the usual 10 cm. At least 50 cm are recommended.
Depending on the base plaster, should it be firmer than the base (light-weight blocks), tearing can also occur suddenly due to different deformation criteria. As a rule the material strength should decrease from the inside towards the outside. Therefore mineral light-weight plasters or mineral fibre-light-weight plasters are the ideal addition as plaster systems for thermal insulating masonry.
The renovation of tears can often be carried out via targeted overcoating. Renovation should always occur after abatement of the deformation of the overall construction, which should be expected at the earliest three years after completion of the shell construction. Cracks with a width of more than 0.2 / 0.3 mm are to be widened and then force-locked with mortar.
According to the latest case law, cracks in interior walls with crack widths ≤ 0.3 mm and in exterior walls with crack widths ≤ 0.2 mm are assumed not to impair the serviceability of the masonry under any circumstances and are described as blemishes.