Fråga Experten: Förstörs betongytan av saltet?

Som restaurangägare saltar man terrass och trappa. Kan man lita på frystestad betong? Klarar sig betongytan så att inte ytan blir förstörd av saltet.
/Jan-Erik

Betongkonstruktioner utomhus i
Sverige utsetts ofta för cyklisk frysning i kombination med fuktig miljö. För att klara detta blandar man i ett luftporbildare tillsatsmedel (typ tensid) så betongen får en lufthalt på minst 4,5% (om 16 mm sten). En ej lufttillsatt betong riskerar annars att få skador typ avfalgning/avskalning på ytan. Inre frysning är inte så vanligt, men kan ske om vatten-cement-talet (vct) är högt i kombination med hög fuktlast. Förutom en minsta lufthalt behöver betongen ha ett någorlunda lågt vct för att klassas som frostresistent/frostbeständig (vct max 0,55) i miljöer med sötvatten. Är det däremot saltvatten (tösalter eller havsvatten) förvärras risken för frostaflagning betydligt, framför allt när man har en salthalt på ca 3%. Saltet innehåller som regel klorider (NaCl, natriumklorid), vilket dessutom ökar risken för armeringskorrosion.

För att klara frosten i salt utomhusmiljö brukar man sänka vct ytterligare (max 0,45) och använda ett anläggningscement, och dessutom frystesta betongen enligt Svensk standard SS 137244. För att armeringen inte skall korrodera inom dimensionerad livslängd brukar tösaltade konstruktioner ha vct på max 0,40 (i kombination med tillräckliga täckskikt). Så är din trappa och terrass gjuten med en frystestad betong (med saltvatten, metod II A) klarar den att tösaltas utan att riskera froststkador, och har den dessutom ett vct på max 0,40 samt tillräckligt täckskikt (ex min 35 mm för 50 års livslängd) är armeringen skyddad. I övrigt skadas inte betongen av salt.

Oskar Esping, tekn. dr. Thomas Betong och Thomas Concrete Group

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