I use foam board as a base for all my dioramas. It's easy to obtain (any DIY or craft store will stock it), it's light and easily cut to size and shape. It usually comes in large sheets of about 5mm thickness which is ideal.
First cut the board to the required size, which will depend entirely on your design and it's intended location. Also, if you want to place it on a wooden "plinth" then this may also dictate matters. I often use custom made picture frames for this purpose but I am not covering this aspect in this post.
Next you need to decide the size and shape of your concrete area. This is entirely down to you but in this example I am showing a circular hard stand area surrounded by grass with a section of it opening out to the edge to connect with what is (presumably) a larger apron. By all means sketch it out on to the board as a guide. To represent the concrete, I use regular sandpaper from any hardware store. It comes in various grits - fine grit works well for 1/72 scale but you could use medium grit for larger scales. It gives a nice "rough" texture to the concrete that is very realistic.
|Sandpaper cut into slabs|
|Creating cracks in the slabs|
These slab pieces can now be glued to the base board. Use PVA glue or similar for this purpose. It helps to leave maybe a millimetre between each piece to help represent the join between slabs. The sandpaper may tend to curl upward at the edges during this process, so take your time and be prepared to spend some time flattening it down manually whilst the glues takes hold. Also, for a few randomly selected slabs you might like to add some cracks to add some realism to the end result. To do this, simply rip the sandpaper slab before gluing. Just rip, don't try to make it follow a line as the more random it is the better. Although often cracks would appear first across the corner of a slab so make sure you do a few of those. Just glue down the ripped slab in however many pieces you have ended up with. Try to make sure the edges of the rip are aligned correctly and glued down thoroughly. They will still show up as cracks and you don't want the edges of the crack to lift as this will not look right.
|Slabs all glued down|
Once everything had dried thoroughly, spray the whole concrete area with light grey primer. I use Tamiya surface primer but to be honest any spray primer (e.g. auto primer) will do just as well, although make sure you use a light grey one. Let the primer dry thoroughly.
Next, use a white paint to spray a light random coating. Just wiggle the airbrush briefly over each slab to give a subtle mottling to the concrete. Less is more!
Now to apply the finishing touches to the concrete. The joins between slabs were normally filled with bitumen which formed an irregular black line along the joins. To do this, I use Tamiya sand texture paint mixed with some flat black acrylic for colour. Basically, you need a lumpy paint and I have heard of some people using normal acrylic or enamel paint mixed with plain flour. I have not tried this myself but in theory it should work fine. The lumpiness of the paint makes for a more realistic effect than using "normal" paint. But you may have to say goodbye to your paintbrush when you have finished so use an old one. Anyhow, this needs to be applied by hand with a thin brush to all the join lines. Don't worry about keeping it neat, it would not be in real life. Just make sure you actually cover the join itself. Any creeping over the edges is fine and the messier the better.
|Bitumen seals applied, cracks highlighted|
That is basically the concrete done, now to fill in what's left with grass. There are a number of ways to create grass effects, from textured paint (don't bother) through scatters right up to static grass dispensers. The latter gives an amazing result if you want long grass, but for these purposes we will take the easy option and use grass matting. This is available from all good model stores and normally comes in a roll. It is basically a paper roll with grass flock already glued to it. It is still quite messy though, as many of the fibres will be loose and you will find everything covered in them by the time you are finished. But they are easily brushed or blown away. You need to cut the grass mat to the required shapes to fill in the black spaces in your diorama. One way to do this easily is to lay a thin sheet of A4 paper over sections of the diorama and rub the edge of a pencil over the area you know the edge of the sandpaper to be. The edge will show through and you can then cut out the paper to use as a template for cutting the grass mat.
|Grass mat cut to size|
|Mat glued down|
Working in sections of a few centimetres at a time, paste a little PVA glue along the join between the grass and the concrete. Make the edge of the glued area a bit uneven, even bulging out onto the concrete in places. Then simply sprinkle some flock over the glue, enough to cover it well. You can then blow away any surplus and you will be left with a realistic looking edge. Continue all the way around your concrete area where appropriate.
|Edges and cracks flocked|
In my example, I wanted a slightly darker green than the grass I was using, so I sprayed the grass a darker colour taking care not to spray onto the concrete area. This means using a fairly low air pressure and getting in close around the edges to avoid unwanted "green concrete".
|Grass darkened and a few stains|
And that about wraps things up. Now you can start adding whatever other items (models, figures etc) to your diorama you had in mind. And also whilst this will look great, to really finish things off you can glue the whole thing to a nicely finished wooden plinth to set off your display perfectly.
I hope this has been of interest, I wish you the very best of luck in your endeavours. Happy modelling!