I am working on a small diorama and wondering what i can use to make concrete / blacktop

I tried using some sand paper but when i tried glueing it down it did not stick well and bubbled up

You can 'texture' some wall spackle. Lay a thin layer down on a piece of styrene or the base. Then when it's almost dry 'stipple' it with a stiff brush. You can hit it lightly with sandpaper to take down any high spots.

Another idea is heavy card stock.

Or you can pour some plaster in a dammed area on the top of a bathroom tile that has a texture you like.

Hey Themage,Somtimes i've used 800 grit wet and dry sandpaper. Had the some problum as you. Here's what it did, i sanded the back of the piece of sandpaper i was glueing down, the backing is to smooth there's nothing to bite onto. Then i sprayed on a contact cement, can be found at a craftstore of automotive center. 3M is what i bought. It did take a few times to get it right. Found that you don't need to spray on to heavy, just lite coats. Hope this works out for you, good luck man.

I usually mix some very fine sifted beach sand with wall plaster and just apply the mix to my styrofoam base. It helps if you stick some bandage gauze with PVA on the styrofoam to help the plaster adhere better. Paint and highlight.

How rough you want the concrete to look can be adjusted by the amount of fine sand you add. On a very rough estimate, less than 10 % sand to plaster by volume should do the trick.

When I need concrete... then I make some!

No kidding. Make a mixture of 30% cement, 60% very find sand and 10% very heavy sand or small pieces of broken shells that you collected on one beach or another).

Then you make a mould with wood (Balsa, or from old cigar boxes).

Make a water - wallpaper glue mixture and use this to turn the concrete mixture into a thick wet mass.

Fill the mould (which you should make really wet beforehand) and let dry.

I got perfect concrete like this, with the typical texture resulting from the real life building systems where liquid concrete is poured into wooden forms.