According to a recent study, in addition to the moisture of the concrete substrate, its alkalinity must also be taken into account when installing plastic mats. The plastic coating should not be installed directly on top of the concrete, as the alkaline moisture of the concrete surface can damage the floor structure glued from the plastic coating and thus cause indoor air problems. Rakennustoollisuus RT recommends that a low-alkaline screed layer of at least five millimeters thickness is always used as a base during installation.
Plastic carpets have gained a bad reputation when the prevailing installation method has in some locations led to the emission of volatile organic compounds and indoor air pollution. Inadequate drying of the concrete before installation of the plastic mat has often been considered the main cause of these types of indoor air problems.
The maximum values of the relative humidity of concrete are determined in the general quality requirements for construction works (RYL). In many cases, however, problems have occurred, even though the humidity of the substrate has been within the limit values.
In the just-completed first phase of the Tampere University research project, it has been confirmed that there is another, at least equally important factor behind the problems: the alkaline moisture of the concrete, i.e. the high pH value of the surface to be coated. It is caused by the by-product formed during the hardening reaction of concrete.
According to the study, in addition to humidity requirements, it would be at least as important to set requirements for the alkalinity of the coating substrate as well.
"The indoor air problems associated with floor coverings have not been completely eliminated, although special attention has been paid to moisture management in construction in recent years. Symptoms have occurred, for example, in quite new school buildings. That's why we launched a research project in 2017, the information obtained from which can now effectively prevent harmful emissions", expert from Rakennusteollisuus RT Jani Kemppainen says.
Damage to plastic-covered floors has been studied as part of the University of Tampere's research project on the moisture-technical and chemical properties of materials. The research has been financed by Rakennusteollisuus from RT's industry associations Building Construction Industry, Concrete Industry and the Floor and Wall Covering Association, as well as several material suppliers, manufacturing companies and contracting companies.
The study found out what effect humidity has on starting the chemical reaction between the glue and plastic carpet softeners. In the first phase, different moisture concentrations, different leveling materials, adhesives and plastic mat materials were investigated.
In their preliminary conclusions, researchers from the University of Tampere state that alkaline moisture is the main factor in damage to a plastic-coated glued floor structure, and that installing plastic coating directly on top of concrete cannot be recommended because of that.
As a corrective measure, it is proposed that a low-alkaline (max. pH 11,5) leveling layer be used as the base of the plastic-coated glued floor covering. When this is done, the paving work can otherwise be done with the current relative humidity requirements of concrete used in RYLs.
In the second phase of the research project, the plan is to continue the tests done in the laboratory with test pieces and possibly find out the effect of the water vapor permeability of the plastic mat on the drying speed.
The contractor can find instructions to ensure successful paving in the Concrete Floor Moisture Management and Paving publication from 2007 and SisäRYL 2013. The Floor and Wall Covering Association has also published a concise guide to plastic coverings - How to cover correctly.
- Instructions: This is how you cover correctly
More information
- Jani Kemppainen, agent, phone 040 541 8090
- Merja Vuoripuro, director of communications, tel. 040 587 2642
(first name.surname@rakensuusteollisuus.fi)