If you want to make a tarmacadam surface, simply take a sheet of wet-and-dry sandpaper (usually black or very dark grey in colour) with the correct 'scale' size grit on it and stick it to the diorama base! You can then add the tar lines by cutting them into the paper and painting them black (or use a black marker pen and a ruler). Watch, however, as the paper will mark very easily and fingers slid on the surface will leave a grey line of sanded skin!!
To make the concrete hardstanding, look carefully at the inside of a breakfast cereal box! Many of them are a grey/sand coloured card which can be dusted with pastels and have the tar lines put on with a ruler and a marker pen.
Always look for the easiest solution! Hope these comments help.