Building codes in Sweden forbid all placing of concrete at temperatures below 40 degrees F, unless appropriate precautions are taken to prevent freezing. Neither must the temperature of the concrete fall below 40 degrees F during the first 3 days after placing. Most major construction work proceeds unhindered throughout the winter, however, using heated ready mixed concrete containing a calcium chloride accelerator. Forms are cleaned of snow and ice by means of steam sprayed through a pipe with a flattened end. Insulation is provided chiefly by plastic mattresses filled with rock wool; these are claimed to be lighter and less expensive than any other material.