The pros generally outnumber the cons of using a concrete bathtub. Pro arguments include the material's versatility, allowing a wide gambit of bathtub shapes and sizes, as well as durability and the smooth yet easily gripped surface of concrete. Cons of concrete are that it has a high cost of labor and can crack when exposed to changes in temperature.
Versatility is the primary argument for commissioning or creating a concrete bathtub. Concrete is a very moldable material, so it can be made into just about any shape or size. A concrete tub can be molded in a factory and delivered to where it will be installed, or it can be molded at the site, allowing for larger designs that might not otherwise fit through doors to get to the installation site. Not only is size infinitely adjustable, so is the shape of the tub. Seats, ridges, and any other addition can be included in the design of the tub.
Another argument for using a concrete bathtub is the durability of the material. It's not very likely that a bathtub will be exposed to very much weather, but in the case that it is, concrete is incredibly weatherproof. Concrete is also fireproof, so it is a good material to use when building appliances or furniture. Fire and weather, though rarely, can destroy property, so it's a good idea to use durable materials within the home to minimize devastation when natural or unnatural disasters strike.
An argument that also speaks to the durability of concrete is that it holds up very strongly against compression. This is a good feature to have when there are higher risks of things collapsing on top of the concrete bathtub. Obviously, no one hopes that her roof or a large tree will collapse onto her bathtub, but concrete is one of the few materials that can withstand such high levels of compression. It takes a lot of force to crumble concrete.
Concrete is great for making bathtubs because of the smooth surface it creates when it dries properly. This is somewhat limited to the skill of the person molding and shaping the bathtub, but generally concrete dries to an ideal surface for a bathtub. It is smooth enough to be comfortable when the bather sits inside, but it's also just rough enough to allow the feet to grip the floor and avoid dangerous shower spills.
The main con of using a concrete bathtub is the material's high labor cost. The material itself is relatively inexpensive, which is a pro argument if someone is planning to mold the bathtub on his or her own. Most people, however, prefer to leave that to a professional, and expert concrete craftsmanship does not usually come cheaply. Mainly because of this reason, there are not very many bathtubs made from concrete.
Another argument against using a concrete bathtub is that the material tends to crack from temperature changes. Cracking is caused only by extreme temperature changes, so it won't be affected by normal use. In certain extreme weather climates, this material is not recommended for use because it cannot withstand sudden drops and rises in temperature. Creating the tub can be problematic because concrete has the tendency to crack when cooling and hardening.