Frost apron

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A frost apron is used for shallow foundations of structures without a basement, the lower edge of which does not reach the frost-free depth . In the case of foundations on frost-sensitive soils ( cohesive soils made of silt , loam as well as sand and gravel with a significant proportion of fine-grained particles), a strip is then added all around the base plate at the frost-exposed edge, which extends down to the frost line to prevent surface water from running under the foundation and freezes there in winter . A geotechnical assessment can be used to determine whether a soil is sensitive to frost.

In Germany, a frost apron should generally be placed at least 80 cm below the final upper edge of the site and is usually poured at the same time as the concrete floor slab .

Frost aprons can influence the load-bearing behavior of floor slabs and must therefore be taken into account in the statics with regard to the settlement behavior and the associated distribution of the bedding.

Instead of the concreted frost apron, the foundation can also be lined with gravel or crushed stone (so-called frost protection material) up to the frost line. In the case of low-energy and passive houses, the floor slab can then be more easily surrounded with a layer of thermal insulation panels. Alternatively, foam glass gravel can be used, which also serves as thermal insulation and drainage . In the case of cohesive soils (clay, loam, etc.), efficient drainage (e.g. ring drainage) should be provided to prevent rainwater and meltwater from the surrounding area from seeping into the gravel pack and freezing in the event of frost.

Alternatively, a strip of heat-insulating material can be placed around the foundation to prevent the ground under the foundation from freezing.

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