Water Permeability Test DIN 1048

[GRCIVIL]

I am working in a testing laboratory which conducts concrete tests. We conduct water permeability test for hardened concrete as per BS EN 12390 as well as DIN 1048 test methods. In both the standards, for the specimen preparation; it has indicated that immediately after demoulding the specimens; the surface (intent to apply the water pressure) shall be subjected to wire brushing. Both the standards does not specify the reason for wire brushing nor the extent of brushing required. We are accredited for this particular test as per ISO/IEC 17025 standards. Recently, we have been questioned by one of the consultant regarding the extent of wire brushing required. Can anyone suggest the need for brushing the surface and any reference which identifies the extent of wire brushing required.