How do cracks arise
Settling cracks - causes, prevention, treatment
If cracks appear in the structure, this is annoying but not always avoidable. They are caused by tensions that are exceeded in the materials of the wall, ceiling or floor. On the one hand, the cause can be planning errors if the built-in materials were not brought in properly or appropriately. However, tensions can also develop through subsidence of the soil.
Settlement always results from the slow subsidence of a building due to a gradual, even compaction of the subsoil.
This type of settlement can be calculated and included in the planning after assessing the subsoil before the start of construction. New buildings still settle in the first seven years.
Causes of the development of settlement cracks
1. Uneven subsidence of the subsoil
1.1. Condition of the subsoil
Different soils with different settlement behavior
1.2. Stress superimpositions
Uneven foundation depths of neighboring buildings can lead to superimposition of loads and uneven settlement
1.3. Overlay of pressure from neighboring structures
1.4. Different construction loads
The building is too long and has insufficient structural rigidity
2. Construction defects
Incorrectly executed foundations can also cause settlement cracks.
This can e.g. B. be about different loads from a building or its geological conditions, which are not taken into account when planning the foundations. But even a foundation that is not frost-free can lead to settlement cracks.
3. Changes in the environment
- due to floods and undercutting of the ground
- Earthquake tremors
- due to a changed groundwater level
- Soil subsidence due to extensive soil erosion in mining areas
Prevention of settlement cracks
1. Assessment of the building site prior to the start of construction
- Settlement behavior
A detailed soil examination of the property before the start of construction protects against a faulty foundation on an inhomogeneous subsoil and uneven layering of the soil. In the case of a subsoil survey, however, it must be ensured that the sampling depth of the boreholes is well below the foundation depth of the planned foundations. To be on the safe side, at least two to three probes are recommended. Drilling should also be carried out, in which so-called pile-driving cores are drawn in order to be able to assess the layer structure of the subsoil below the foundation components. These results provide information about the load-bearing capacity of the subsoil and, as a result, the optimal planning of foundations and the basement of a building.
- Water conditions
Drill soundings also provide information about the location of the groundwater in the subsoil. The groundwater level and its fluctuation ranges are decisive for the safe planning of a structure, whether z. B. a masonry cellar can be built at all, or it is better to opt for a waterproof concrete cellar.
Example of a drilling profile of an exploration well
2. Ground improvement through soil replacement
If the subsoil has insufficient load-bearing capacity, it is possible to eliminate the problem by replacing the soil. Cohesive and unsuitable soils such as soft silt, clay and organic soil are replaced by non-cohesive soils such as sand and gravel. The replacement material is introduced and compacted in layers of 30 to 40 cm.
3. Compaction of the subsoil
If the soil is basically stable and only the density is insufficient, it can also be compacted. Surface or depth vibrators are used for non-cohesive soils. Cohesive soils can be improved by tamping with the addition of gravel or crushed stone.
4. Deep foundations
Deep foundations also offer an alternative for non-cohesive soils in the upper layers of the earth. In this case, the building loads are not transferred directly below the building into the subsoil, but into deeper, load-bearing soil layers. This can be solved either over a large area with a foundation plate or as a strip foundation. If the main load of a building is carried by columns, then these require a point or single foundation. This is carried out through so-called pile foundations. Wood, steel or concrete piles are used as material. Deep foundations can only be planned in detail after a comprehensive soil survey.
5. Injection of solids
In the case of non-cohesive soils or fissured rock, the bearing capacity can be increased by injecting cement suspensions or gelling solutions. High-pressure floor mortaring is used primarily for underpinning existing properties. The existing non-cohesive soil is loosened with high pressure water flushing and bound with cement suspension.
Treatment of settlement cracks
Before a sensible treatment and long-term elimination of any settlement cracks that have occurred, a precise research into the cause should be carried out. For this purpose, the type of cracks must be examined and assessed more closely.
Hairline cracks are z. B. not more than 0.2mm wide and usually harmless. It becomes problematic when it is wider. In such cases it makes sense to observe and document the development of the crack. To do this, you can take regular photos, mark the beginning and end of the crack with a pen and date, or attach a plaster mark that will tear open if the crack continues to widen. In this way you can check whether movements can be expected on the crack flanks in the future. In addition to the technical assessment of the crack, this assessment has a major impact on treatment, repairing or just concealing the crack.
If the crack grows or other circumstances indicate a possible construction defect, a building expert should definitely be called in.
Important details for crack diagnosis:
- Crack width
- Depth of crack
- Distribution and course of the cracks
- Crack offset parallel and perpendicular to the component surface
- Crack age
- future movements to be expected on the crack flanks
Treatment options are:
1. Fill cracks and plaster the wall
Small, harmless cracks, the walls of which can be proven to no longer “work”, can be repaired and permanently resiliently closed.
- The crack should first be scratched out and spread with a screwdriver
- Then spray out the crack with acrylic and smooth the surface with a spatula.
- After the acrylic has dried, the filled area should be covered with self-adhesive anti-tear tape made of fiberglass.
- Now the place can be plastered, wallpapered or painted.
- The crack should be created in a V-shape before further treatment so that the edges gain more adhesive surface. It can be pretreated with a power cutter or a hammer and chisel.
- Then paint the cleaned areas with deep primer.
- Before applying the plaster, lay a strip of reinforcement fabric in the crack area to solidify it.
- Now the crack can be plastered in two layers with the base and top plaster.
2. Use of spiral anchors and anchor mortar
With the use of spiral anchors and anchor mortar in masonry, cracks can be removed in the long term with relatively little effort. The anchor mortar is placed in the bed joints that are milled out at certain intervals. Then the spiral anchors are embedded perpendicular to the crack and grouted with a second layer of anchor mortar. Similar to the reinforcement in concrete, the anchors take on the tensile forces in the masonry. The building is held together and the crack can no longer expand.
3. Crack grouting
Crack grouting is an injection process in which so-called drill packers are inserted into the masonry. The correct spacing of the positioning is important here so that they cross the crack. The crack is then dammed up and mortar, cement suspension or injection resin is injected.
Treatment of the causes of long-term instability
1. Foundation reinforcement
One way to stabilize the building is to reinforce the existing foundations. For this purpose, the subsoil below the foundation is stabilized by injecting a special resin. The procedure can also be used to raise the foundation back to its original level. This prevents instability and further crack expansion.
During the post-foundation, the structural loads are guided to greater depths with the help of piles. For this purpose, pre-pressed segment piles are brought under the foundations from a head pit in a very small space. Segments are pressed into the earth until the required pre-pressing load is reached.
This flexible method enables the pile to work optimally so that the building can be supported without further cracking. If the existing inclinations of the structure or the component are too great, the structure can also be lifted.
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