Whats the best way of making the gaps for a tiled concrete texture in photoshop?

I need to create a tiled concrete taxiway texture and my problem is i just cant get the gaps look right. if i just use a black line there isnt any deepness and it just looks wrong. in combination with a normal map it doesnt look too bad rendered in the sim but i have noticed that other developers seem to have a technique to draw these gaps in a more realistic way. Hope someone can help
 
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