You can use the value for normal concrete, since there is no  investigation on Poisson's ratio for FRC to my knowledge. 

In dispersed systems like composites with short fibers the properties are completely dominated by the matrix phase. That is you case, I suppose.

Deviations from what said above are evident only at very high fiber volume content (say 40-50%) and very high aspect ratio, l/d (length to diameter ratio) of the fibers, namely , l/d= 100-10000. .

Please see the attached paper. It contains Poisson's ratio measured for various strengths of fibre concrete.

Article Manufacturing ultra-high performance concrete utilising conv...

Thank you everyone for your explanation and help.