This photo gives a better view of the concrete driveway leading to each side of the weighbridge “plate”. The slabs of plaster were cast in styrene form work. They are about 2mm thick and were stained in a very dilute mix of rubbing alcohol and india ink, much thinner than the mix I use to stain wood. The cracks and chips you may be able to see weren’t really deliberate: I managed to break bits of the slabs when extracting them from the molds.
Morpeth In O-Scale 
Looks great Trevor, I think you will find that the accidental breagages will ultimately enhance the realism of old concrete.
John