The two sentences that we are given: "Se prohíbe hacer fuego directamente sobre los encofrados. Si se hacen fogatas se efectuarán en el interior de recipientes metálicos aislados de los encofrados (sobre carambucos o similar, por ejemplo)" lead me to think that "carambuco" is "concrete block," as Helena said earlier. The second sentence would translate as something like "If open fires are made, they shall be made in the interior of metal receptacles isolated from the formwork (on concrete blocks, or something similar, for example)." The concrete blocks do not conduct heat, partly because they are made of concrete, which is not a particularly good conductor, and partly because they include a lot of air space, which is even less good as a conductor of heat, in the hollow spaces in the blocks. Therefore, if you make an open fire in a metal receptacle sitting on top of some concrete blocks, the heat from the fire is effectively isolated; the formwork is insulated from the fire, and the flame and the heat of the fire can't damage the formwork. In the example sentence below, the size of the carambuco is given as 20x20x40 - in centimetres, which is equivalent to 8 inches x 8 inches x 16 inches, the standard modular concrete block size in the US.