In the design of R.C.C. structures, what should be the tensile strength of concrete?
If you are referring to the contribution of cement in the tensile strength of composite RCC system, then -to be on the safe side- it should be probably be set to zero. i.e. all tensile loads are carried by the reinforced elements (e.g. steel bars etc).
Cement does not have particularly good tensile properties and it is known to behave in a brittle manner. It is also exhibits much larger variance in tensile strength compared to compressive strength. As such, one should not rely on its tensile strength.