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[quote user="MidlandMike"]Cement curing is an exothermic/heat producing process,[/quote] As a civil engineer in structural design I know that and usually the curing stops below 14°F till it gets warmer again. [quote user="MidlandMike"]so it seems that spraying cement on to frozen ground would be an incompatible combination.[/quote] At the first glance that seems to be right but on the other hand there are lots of tunnel all over Europe that have been built that way. Fahrlachtunnel Mannheim, Germany: <a href="http://www.daub-ita.de/fileadmin/images/daub/TunnelDB/UBD1995134-02.jpg">http://www.daub-ita.de/fileadmin/images/daub/TunnelDB/UBD1995134-02.jpg</a> Frostkoerper = Frozen soil; Spritzbetonschale = Sprayer concrete shell; Bauzustand: Intermediate state; Endzustand: Final state; I have never been in tunneling so I don't know the concrete recipe. The temperatures of the frozen surface are said to be between 23°F and 11°F. <hr /> Regards, Volker Edit: I have browsed for information about sprayed concrete on frozen soil. What I found is as follows. At first the thickness is determined according structural requirement. Than are 2'' added which act as insulation and are neglected in calculations. At last is checked if the hydration heat is high enough to ensure hydration/curing of the concrete. A Portland cement is chosen as it delivers the highest hydration heat of all cement types. Additionally an accelerator is added. Here is a link to a German tunnel project in Berlin written in English: dc.engconfintl.org/cgi/viewcontent.cgi?article=1008&context=shotcrete
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