Sandwich structure is a high-risk structure
“Double concrete slab structure” refers to a structural solution where a second concrete casting is made on top of thermal insulation installed on a foundation slab. This structure, also called a sandwich structure, is classified as a high-risk structure in the KH 90-00394 guideline for condition inspections in connection with housing transactions (Kuntotarkastus asuntokaupan yhteydessä; suoritusohje; 2007) in cases where mineral wool or wood wool insulation has been used as the thermal insulation, or where there are water and heating pipes in between the concrete slabs.
The sandwich structure has been in use from the 1960s to the present day. It is a functional and safe structural solution, provided that its condition is regularly monitored and inspected.
Why is the sandwich structure prone to damage?
Damage to a structure is usually caused by moisture from the soil entering the structure. Specifically, moisture transferring through the lower concrete slab to the thermal insulation or water seeping through the foundation to the thermal insulation.
Moisture ingress into structures is typically caused by inadequate or missing underdrainage or by too fine-grained artificial fill under the bottom concrete slab. Base floor structures with a crawlspace below the load-bearing concrete slab are also challenging, as moisture can be transferred from the air of the crawlspace to the structure or moisture can condense from the cooler space underneath into the structure.
Water, sewer and heating pipes often run in the insulation space between the slabs, which means that pipe leaks or water coming from the outside very often cause problems in the structure. Moisture condensing on the surface of the pipes can also pose challenges.
The risk of structural damage can usually be reduced by placing thermal insulation underneath the bottom slab and installing water pipes inside a conduit.
What harm can the high-risk structure cause?
A sandwich structure can cause extensive damage to the structures of a building. A pipe leak in the insulation space can soak a large part of the structure, and the damage can go undetected for several months.
A prolonged leak can cause mould damage, rot damage to timber walls and microbial growth in the thermal insulation. Microbial growth can be accelerated by nutrients in the water and sawdust left in the insulation space during renovations.
Condition of the structure can be determined by opening it
Verifying the condition of a sandwich structure on your own is challenging, as the structure cannot be reached. A musty smell like in a cellar can be an indication of a damaged high-risk structure, but the structure in question can only be located by looking at the building documentation and opening the structures.
To determine the condition of the structure, it is always necessary to open the structure and carry out additional surveys to a sufficient extent. The surface of a sandwich structure is concrete, which means that part of the top slab must be removed or other similar difficult opening of structures must take place.
How can the structure be repaired?
The correct repair method can be determined by examining the structure. In many cases, incorrect repairs will only cause more problems. Both the repairs and the inspection of the structure should be left to an expert.
An example of the high-risk structure:
The images are for illustrative purposes only and do not reflect the exact structure.
What is a high-risk structure?
A high-risk structure refers to a type of structure which has been found in practice and based on structural surveys to be highly susceptible to damage. The structure usually complied with the building regulations and guidelines valid at the time when it was built, and the susceptibility to risk was not recognised until at a later point in time. As a result, the structure is no longer used.
Typically, damage to high-risk structures is caused by moisture entering the structure, either through the soil from the outside or from the indoor air in the form of water vapour. An example of a commonly observed high-risk structure is a false plinth.
The condition of the high-risk structure, i.e. whether the risk has been realised, should always be investigated. This is usually possible only by opening the structure.