May be this question would be answered by micro mechanics, as I know there are two kind of adhesion between fiber and matrix, the chemical bond and the frictional bond. thanks to micro mechanic you can find the pull out load in terms of frictional and chemical bond. it is worthy to be mentioned that finding specifications of fiber, matrix and fiber/matrix needs accurate experiments. in this regard you can see the articles of professor Victor Li . the main purpose of his researches is to reach strain hardening and multiple cracking instead of local cracking and softening response of the cement based composites.