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Pre-wetting of concrete substrates: why?

03 May 2021 by MAPEI

Manufacturers recommend pre-wetting of cementitious substrates (concrete, screeds, plasters) before applying cementitious mortar over-lays eg grouts, waterproofing slurries, etc. There is a good reason for this and it is explained here.

Cementitious substrates, to varying degrees, are absorbent due to the presence of capillary pores. This can be demonstrated by pouring drops of water on a bare concrete surface. The droplets will shortly disappear, sometimes in a matter of seconds.

Overlay mortars are prepared for use by mixing cement product formulations with water. The resultant hydration reaction creates the binder which (a) keeps the mass together and gives it body and mechanical strength, and (b) develops adhesion bond to the substrate.

However, porous surfaces are “thirsty” and absorb water from the freshly-mixed mortar at the interface, possibly leading to incomplete hydration of the cement where the bonding is expected to occur, and causing weak adhesion. This problem is prevented by pre-wetting the substrate to “saturated, surface-dry” (SSD) condition, which means all the capillary pores are filled and will no longer draw water from the applied material but the surface is free from standing water.

Rightly, SSD condition can only be assured by soaking the substrate and mopping up or sweeping away all excess water afterwards. If soaking is not practical, at the very least, hosing-down liberally is recommended. You can perform a simple test to check that pre-wetting is properly done by placing your palm on the substrate; it should feel damp without wetting your hand. Apply the overlay mortar immediately.

(Reference must be made to the manufacturer’s technical data sheet for instructions on use of particular products.)

Founded in Milan in 1937, Mapei is today the world leader in the production of adhesives and chemical products for the building industry. We implemented our internationalisation strategy in the 1960’s and this has enabled us to be closer to the needs of regional markets and reduce transport costs to a minimum. The Group now has 89 subsidiaries with 83 production facilities in 36 different countries. We have always placed a lot of emphasis on research. In fact, we channel 5% of annual turnover into Research & Development and 70% of our research work is aimed at the development of eco-sustainable and eco-compatible products.An unrivalled offerOur products offer clients the most specific and most advanced solutions in the world of building. Users of Mapei products can see for themselves, day after day, how this choice leads to optimising work, maximum application yield and perfect results, under all conditions. We are not just a supplier; we are also a partner for all those who operate in this sector with a comprehensive range without rivals. Our name is synonymous with innovation and every year we create thousands of new formulations to meet all your needs. We supply more than 5,000 technologically advanced products worldwide that take into consideration the widest variety of requirements imaginable and, in so doing, we help improve the characteristics of buildings, from anti-seismic upgrading to the healthiness of our surroundings.Expertise in the construction sector born from on-site experienceThrough a constant exchange of ideas and experiences with clients and builders’ merchants, we use their input as a starting point to shape innovation and reformulate their suggestions in the form of new products to expand the range of lines and references. From the smallest to the largest sales points for ceramics and building materials, our quality is not limited to the product itself, but extends to the competence of the sales team, the passion of our retailersand the value of their advice. Our clients can also take advantage of our Technical Services Department and their constant, expert support, from the design phase right up to the execution phase, to guarantee an end result that will always live up to their expectations.


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