pressure washing concrete

I've had an outfit that has cleaned my driveway several times in 3 year intervals. They use this rotary thing that looks like a lawn mower when doing my driveway (about 5 car lengths). I notice that when they finish and it's dry, it has a tight grab on your shoes when you walk on it... sorta like when you wear very rubbery shoes on a new shiny wood floor. The driveway is definitely clean looking when they finish it but I wonder if over repeated cleanings, it eats away at the concrete finish? I realize without knowing the pressure, etc... this question may be hard to answer but assume this company is legit and following the correct procedures for cleaning a driveway. Am I crazy or is my suspicion correct? Either way, is there a suggested time interval to clean a driveway?
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It sounds like they are using something that also has a brush in it? Regardless, even just pressure washing is going to remove some tiny amount of concrete. But you're doing it every 3 years, not every hour. You have or should have 4" or more of concrete. Cleaining it every year or so isn't going to erode enough in it's lifetime to make a difference.
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washing concrete:

Yes it does. Next time they wash, check the run off with a magnifying glass and you will see the surface particles that have been removed.
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It will slowly take away the top lime & sand but doesn't harm the gravel. But heavy rains will do that too, just slower than pressure washing. Old outside concrete shows a lot of aggregate but it's still strong concrete.
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