Kill or Cure for In Situ Concrete Roof Decks?

Curing is the final stage in concrete construction and surface applied curing compounds are often used to ensure moisture is retained during the process. But, is the compound about to thwart the adhesion of the ensuing hot melt waterproofing system, essentially counteracting its ability to deliver a fully bonded monolithic waterproofing layer across the entire roof deck?

Bauder hot melt product manager, Andy Harper, highlights the potential pitfalls of surface applied curing compounds and shares his advice on correct selection to support the successful hot melt waterproofing of the roof deck, accepting that this can also apply to other waterproofing types.

When cement is mixed with water, the process of hydration occurs. It is this process that causes the cement to harden, developing stability and strength. If cement dries too quickly, the concrete is not fully reacted preventing it reaching its specified strength requirement. This can have serious consequences for the concrete’s quality. 
A surface applied curing compound can be used to aid the hydration process by inhibiting water loss at the critical stage after concrete pouring and ensures the success of the finished deck.

There are numerous additives that can be added to the concrete mix and for a number of different reasons. This article concentrates on surface applied curing compounds. It is these compounds which can give mixed results for the following waterproofing installations. Surface applied curing compounds are popular as they are easy, practical, and cost effective. The challenge is that not all types are suitable when the subsequent waterproofing is a hot melt system or bitumen AVCLs. 

The decision to use a surface applied curing compound can be influenced by weather. Warm and/or wind drying conditions will see the concrete installer responding timely to maintain the hydration process, relying on these types of products which sit comfortably and often form an essential part within the equipment armoury, so the concrete cures successfully.

Importantly, British Standards recognise the correct use of curing compounds and the situations where consequences will arise. BS EN 13670:2009 Execution of Concrete Structures (E), section 8.5 Curing and Protection, has these relevant items:

  1. Curing compounds are not permitted on construction joints, on surfaces to be treated or surfaces where bonding of other materials is required, unless they are fully removed prior to the subsequent operation, or are proven to have no detrimental effects on the subsequent operations.

  2. Curing compounds shall not be used on surfaces with special requirements for the surface finish unless they are proven to have no adverse effects.

Looking at key terminology used in BS EN 13670:2009 (E), 8.5 – (10) + (11), it should be noted that a surface applied curing compound is highly likely to have ‘detrimental’ and ‘adverse’ effects, and so it is important to identify which types of products may provide a problem for the successful adhesion of the waterproofing system employed.

Detection can be difficult as compounds can come in pigmented or transparent options. Whenever possible, it is better to apply a pigmented curing compound in order to see the surface of concrete that has been effectively protected and that it is visible to all.

Most hot melt waterproofing systems are compatible with pure sodium silicate based surface applied curing compounds. The lethal blow for hot melt waterproofing system adhesion can be when the following compounds are used:

  1. Resin Based Surface Applied Curing Compounds

  2. Wax Based Surface Applied Curing Compounds

  3. Wax / Resin Based Surface Applied Curing Compounds

  4. Acrylic and Chlorinated Rubber Based Surface Applied Curing Compounds

Clearly the advice would be to avoid these four options. However, if they are being considered for use, then a manufacturer’s technical services department should be consulted for the appropriate removal technique to ensure the compound does not create a barrier between the cured concrete and the waterproofing.

It should be remembered that a feature of any hot melt waterproofing system is that it can be bonded adequately to the fully cured in-situ concrete deck. This can then produce an installed waterproofing system that is, in effect, a monolithic skin across the whole deck, and when this process is completed successfully, should allow no water to track underneath it. This is highly important when loading it with insulation and hard or soft landscaping, which effectively covers the roof for its entire life. The use of an incompatible surface applied curing compound could eliminate this fundamental element if it is not reported or identified at the correct point in the process. Waterproofing manufacturer’s technical literature should give an indication of the pitfalls that can befall hot melt waterproofing systems if the surface applied curing compounds are used without following the precautions.

The use of surface applied curing compounds is advantageous for the in situ concrete deck installer, aiming to deliver a fully cured roof deck. This use can cause the waterproofing system manufacturer and approved waterproofing system installer concern as to whether the required bond to the concrete deck will be compromised.

Once installed, the existence of a surface applied compound can be difficult for the roofing contractor to detect; one of the few tell-tales is that the concrete’s surface may be smoother than is usual.  So, if it’s use was a last-minute decision and the waterproofing installer was not made aware, there is a risk that the hot melt waterproofing will be installed over the cured concrete that has an unsuitable surface for full bonding. Therefore, communication is vital with regards to the possible use of a surface applied curing compound. Failure to do so could add unforeseen costs regarding the removal and re-instatement of the waterproofing system. It is worth noting that it is always more cost effective to remove the product post curing before the hot melt waterproofing is installed.

As a roofing system manufacturer, Bauder relies on being advised by main contractors, specifiers and our approved contractor network when surface applied curing compounds are being specified for use or used as an on-site requirement so that we can comment and give clarification on the consequence of their use.

In summary

The use of curing compounds is an important subject and should be discussed with the waterproofing manufacturer at specification stage, long before roofing works commence on site to establish any possible effects on the following operations. However, as it is often a late decision affected by changing weather, notification to the installing approved contractor/waterproofing manufacturer is essential to enable them to advise on the subsequent possible issues and consequences.

Bauder gives preference to the use of water curing, wet coverings or plastic sheets in lieu of spray applied curing compounds purely on the basis that they do not pose an issue for adhesion of the waterproofing. However, we understand that this is not always possible and that the use of a curing compound which is surface applied may be necessary.

Bauder, as a responsible flat roof waterproofing system provider can give advice on the subject and offers training to its approved contractors on this and peel bond testing techniques at its Ipswich headquarters and in locations to suit the construction community. Get in touch by contacting us.