Wood wool slab: a deleterious material
What is wood wool, and why is it associated with structural damage?
What is wood wool slab?
Wood wool slab is, in fact, not made of wool. It is instead formed by mixing shredded timber with a cement paste. It was used in the ’60s and ’70s when building commercial buildings using in-situ concrete; wood wool slab was used as the permanent formwork structure that the concrete was laid on.
Because wood wool is also a good insulator, it is particularly common to find it used at roof level.
However, wood wool slab used as formwork is now designated as a ‘deleterious material’, because it can damage a building’s structure.
Why is wood wool slab a deleterious material?
A deleterious material is one that can cause damage or harm to the individual, the environment or to the buildings and infrastructure; wood wool slab falls into the category of materials that can harm the structural integrity of a building. But why does its presence indicate a risk of structural failure?
It’s because wood wool slab compresses easily. When concrete is laid over it, unless great care is taken, the slab can compress unevenly, resulting in variable compaction of the concrete.
This in turn can lead to gaps forming in the concrete (known as ‘honeycombing’), resulting in steel reinforcements within the concrete becoming exposed and vulnerable to moisture.
Another known issue is the fine aggregate in the wet concrete bleeding into the woodwool slab, leaving behind only course, no-fines concrete. This concrete is susceptible to moisture ingress.
Moisture contact with the steel reinforcements can lead to corrosion of steel reinforcements, and ultimately, their failure.
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Finding wood wool slab on a building survey
Wood wool slabs form part of the permanent structure of many of the buildings it was used in, and it is not unusual to discover it when surveying a 20th century commercial building. However, depending on how the structure was built, it can be difficult or impossible to view the concrete above to visually assess its condition.
Sometimes, the structure was designed with concrete ribs; in those cases, a limited area of concrete can be seen and inspected. It is important to remember, however, that even with this design, there will still be a large proportion of the concrete that is not visible.
For this reason, intrusive investigations are often recommended if woodwool slab is found. This is particularly important if there are any signs that the property has not been well maintained, or that water ingress might have occurred. Your surveyor will look for signs of current or past water ingress and damp, and will carefully inspect the condition of the walls, masonry and rainwater goods. This will allow them to to assess the level of risk, and will inform their recommendations.
If intrusive tests are required, they will be used to check for concrete cover of the steel reinforcements, along with any deterioration or defects to the reinforcement.
If severe damage is identified, the whole structure may need to be replaced. In cases where the concrete may be compromised, but the reinforcements remain sound, the wood wool can be stripped out and replaced with another material.
In all cases where wood wool slab is found, regular inspection and careful maintenance are critical to ensuring the structure remains safe.
What to do if you are concerned about wood wool slab
The first step is normally to instruct an inspection by a chartered surveyor or structural engineer. If there are other issues you would like investigated at the same time, or if you are taking on a new property and need to get a thorough insight into all items of concern, then a full structural survey would be recommended. Alternatively, a specific structural inspection might be the most appropriate option if the presence and condition of wood wool slab is your only concern.
The survey report will cover the condition of the building and the visual assessment of the slab and any visible areas of concrete and steel. As discussed above, the report will also advise on whether further investigations are required to accurately determine the condition of the structure and the level of risk of structural failure.
Another key action point for anyone concerned about wood wool slab is to ensure that they are properly maintaining their building. Failure related to wood wool is a result of water ingress corroding steel reinforcements; therefore, prevention of water ingress and damp is criticial. Careful maintenance and quick repair of roofs, flashings, skylights, windows, mortar joints, masonry and render will reduce the risk significantly. For large properties, a planned preventative maintenance schedule can help with budget forecasting and programming works.
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