Brunton’s Blog – Concrete decks and direct bonding
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Concrete decks and direct bonding
It goes without saying that any roofing system that is attached to any substrate must have adequate resistance to forces of nature or loading otherwise imposed to ensure the thermal and waterproofing performance of the system.
At a recent industry technical meeting there was some quite detailed discussion on the matter, specifically concrete decks and when they are acceptable in a fully adhered warm roof situation. This prompted me to write this blog as a way of reminding all of the importance of the issue and as always, pleased to accept comment from members and others on the subject to continue to build and share knowledge within the SPRA community.
In-situ concrete decks
Concrete decks as we know come in all shapes, sizes and finishes. Some are delivered pre-cast to be installed on site, some are effectively manufactured on site (in-situ) by delivering and pouring a specified concrete mix. In simple terms the two different approaches, pre-cast or in-situ poured, require careful consideration regarding the suitability or readiness when accepting the first bonded layer, the air and vapour control layer (AVCL). In this blog I discuss only the suitability of in-situ concrete decks for the adhered option but point out that there are very capable alternative methods of attachment such as mechanical fixing and ballasted options again which have their own important criteria concerning deck suitability. I hope to discuss these alternative options as well as pre-cast decks in future blogs.
It is critical that a good bond is established between the AVCL and the substrate, but the question is often raised regarding the concrete substrate and it’s ‘readiness’ including dryness to accept an adhered membrane. Screeds for levelling or inducing falls can also have significance by surface drying quicker however also soaking up water depending on their porosity.
Formwork can have a decisive impact on the drying out process of a concrete deck. Permanent formwork often used in the construction of in-situ concrete decks for rapid construction and subsequently overlaid with a waterproof membrane as AVCL can potentially trap in the deck excess construction water. This can prevent the surface from drying properly putting at risk the attachment of the first bonded layer, applied by conventional hot pour or torch methods. Correct adhesion will not be possible unless the concrete deck substrate is properly dry. A means of mitigating this effect would be the use of perforated formwork allowing a more even drying process to upper and lower faces of the concrete surface or better still protection of the slab by temporary roof facilities during drying out as referred to in BS6229. Highlighting these issues to contractors and designers can assist in providing better, more secure conditions for adhering membranes.
Manufacturers have their own requirements concerning assessment of whether the concrete deck is dry enough and can often recommend a method for establishing this by testing and therefore these instructions should always be followed. Whatever test method is used records should be kept of the results and be representative of the area to be covered as a whole therefore this may mean testing several areas depending on roof size.
Another crucial aspect for achieving adequate bonding of the AVCL in a warm roof is the smoothness of the finished concrete surface. Our SPRA Design Guide states that the concrete deck should be finished with a smooth, nib-free float finish. This is important again to ensure adequate bonding and no potential damage to the membrane in direct contact with the concrete surface. There are many potential surface finishes possible with concrete today and the product is frequently used as a finished surface for walls, soffits and internal building areas. So, getting the right finish, while critical for the specialist roofing contractor, should not be an onerous task for the main or specialist concrete contractor. Important however to make it very clear what finish is required for the best bonding potential. If in any doubt about surface finish PD CEN/TR 15739:2008 contains simple methodology for assessing surface characteristics of concrete and could be used in some cases even though it’s main concern is for assessing precast concrete structures. Ensuring the correct falls for roof drainage is a separate issue from surface smoothness finish and will also be discussed in a separate future blog.
Bonding to in-situ concrete containing admixtures
Admixtures (chemicals added to the concrete mix during production) are increasingly used in concrete for a variety of reasons; an example being, to accelerate the curing process. It is important that any admixtures used are compatible with the membrane and bonding agent going directly above to ensure adequate attachment of subsequent layers and no detrimental chemical impact.
The SPRA Design Guide (2018) gives guidance on many of these issues in Section 5 Workmanship p41 and should be consulted in addition, as always, to the membrane manufacturer’s recommendations
Dr Ronan Brunton B.Sc MBA GMICE, Technical Manager
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