Pressure washing driveway

I have a concrete drive lined with large oak trees. They stain the driveway and about 40% of it is black. In the past I've used a pressure washer which works great. The driveway is bright and new looking. It is ready to be blasted again, but I am wondering if maybe I am doing more harm than good. I know better than to pressure wash an asphault shingle roof, but what about my concrete driveway? If I use a garden hose, it only gets it slightly clean. It will go from black to a dark grey.
I don't want to use harsh chemicals, since it may kill the grass along the edges. And the drive is very big so it would take lots of chemicals - it takes all day to pressure wash.
Any suggestions?
Thanks
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In alt.home.repair

While I'm sure this will work, what is the purpose of mixing weak bleach aka Clorox with strong bleach aka pool shock? Just mix the shock in water. A 1 lb bag of shock is enough to give you 10ppm in 10-15,000 gallons so it will make a hell of a strong brew in a 5 gallon bucket.
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You won't be flushing the solution all over the place - just use a push broom (or something similar) to coat the driveway. You won't need to scrub it and more than likely you won't even need to rinse it off. The waiting period is for it to oxidize and lose its strength. I've never had any grass or surrounding plants damaged.
Jim

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