To be honest, anything smaller than 1/32nd scale doesn't really need the 'texture' of grit papers or similar, as this can look too coarse against the model.
Painted card, shaded and 'dirtied' will look equally as good, as long as the expansion joints aren't overdone.
I usually mark out the 'concrete panels', then lightly score the marked lines to leave an indentation, which can then be toned, lightly, if required, to represent tar sealer, or just left 'as is'.
It's best to glue this to a solid surface, such a MDF, to prevent the base board from warping at some point in the future. The use of such materials as 'styro-foam' or cork are not really stable enough, and will warp eventually.
Here's an example in 1/48th scale, where the plain card was airbrushed with a 'concrete' shade, then had darker colours misted over the top, and stains etc added by the use of both thinned paint and eyebrow pencil, smudged onto the surface.