Experimenting with "Blown Out" and Cracked concrete.

Looks great.

How about some details on how you did this. 

It looks great.

Very nicely done!  I second ZAC's request

Interesting effect - yes, the "how-to" info would be good.

It was surprisingly easy and fun. I'm really happy with the way it turned out.

I had some free time and was working on a tunnel portal. I decided to go with a poured concrete portal and gave it a try as a change up to the carved rock portals.

First I carved out and lightly sanded foam insulation board.

Then mixed and applied acrylic colors with a brush to simulate concrete. Tan, gray and white. Weathered the bottoms of the cracks and holes with black and immediately used a water soaked brush in light downward strokes. 

Used a dry brush green, yellow, drop of black, to simulate mossy mold. Then water soaked brush immediately with downward strokes.

The rebar is 22 AWG solid wire. I used needle nose pliers and pushed it into the foam to form the grid. Then used orange, raw umber and black to simulate rust.

Gave it a couple weather washes of diluted black. This really made the imperfections and gravel effect pop out.