Well, one of the great fortunate numerical coincidences is, that steel and concrete have the same coefficient of thermal expansion, which is why reinforced concrete is good stuff.(Or so it claims in a book I have on this.) I suppose it must not be *exactly* the same, but pretty close. It definitely is the case that with modern bricks and mortar, the mortar tends to crack and bricks not. With ancient bricks, which may not have had as good quality control -- thermometers not invented yet, hard to control your kiln... -- that might not have been true. An interesting modern dome-making technique is, you create a balloon out of plastic sheet, inflate with fan, place rebar mesh on it held together with wires, and spray on concrete. A few days later, you have a dome, which supposedly is very strong, long lasting, and hermetically sealed. The concrete can be sprayed either on the exterior or interior of the balloon. (Foundation, doors & windows have to be created ahead of time...) -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)