Pressure Washing Concrete Driveway

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sleddriver

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First, I did a search on this term here to see if it had already been covered. No joy.

Second, I'm using a neighbors PW'er with a 5HP Honda engine. Pump is a 2500 psi unit rated at 2.5 gpm. I started out with a white wide-fan tip (25°?), then switched to a rotating jet nozzle, rated to 3000psi. Unable to find anything about how large the orifice is.

Driveway is 33yrs old, 50' long. Over time, some sort of green algae/moss/fungi/??? will "lock in" to the upper porous surface and resist all but the most direct pressure and closest contact. When this stuff is wetted, it will swell and become very slippery and deep green. Over time it'll turn the concrete deep gray. When wet, the driveway will look nearly black.

Anyone know what this is?

I poured common household ammonia on a 5' x 5' patch, then brushed it thoroughly. The bristles did turn a deep green. Since NH3 is effective against mold/mildew and is quite alkaline, I was hoping it would kill this first. It did appear to have a small effect.

Next I tried common household bleach, straight from the bottle (~ 9% soln). It began to produce white foam, break up some of the algae, after brushing. So it was more effective than NH3, but not so much that it killed it off nor loosened it's tenacious grip.

Next step would be to try a fungicide or something with copper in it or even a stronger soln of bleach.

Or go with sheer force: Larger PW'er and/or a circular concrete cleaning disk with rotating arms underneath.

The rotating single jet nozzle was a HUGE improvement over the wide-fan nozzle I was using: Much more force applied. Not sure if this unit is properly "matched" to the 2500psi/2.5gpm pump though.

I did finish up last night and this morning, the whole driveway is a very light tan color. Looks completely different. No longer slippery when wet, nor black/green.

Anyone have any ideas? Recipes? Alkaline or acidic? Fungicide? Algicide?
 
1 c. TSP + 1c. Bleach + 1 gal. water.

I live in the PNW and fighting that is an annual joy. What you see accumulate in 20 years, we get in one or two. This solution works great and replicates the higher-dollar stuff in home stores. TSP is cheap and at your local HD or Lowes. Spray it on with a garden sprayer and let it work. 15 minutes is usually good. Best to do it NOT in direct summer sunlight or it evaporates too quickly. It will make the follow-up pressure work much easier and the TSP will help lift and remove the mess so it doesn't cling to the surface.
 
We have black mold here. Grows on tree sap drippings. I use 50/50 bleach/water. Apply it with a garden sprayer, let it sit for a while then pressure wash it. I hadn't considered putting some detergent in it. Good idea.
 
Originally Posted By: Oro_O
1 c. TSP + 1c. Bleach + 1 gal. water.

I live in the PNW and fighting that is an annual joy. What you see accumulate in 20 years, we get in one or two. This solution works great and replicates the higher-dollar stuff in home stores. TSP is cheap and at your local HD or Lowes. Spray it on with a garden sprayer and let it work. 15 minutes is usually good. Best to do it NOT in direct summer sunlight or it evaporates too quickly. It will make the follow-up pressure work much easier and the TSP will help lift and remove the mess so it doesn't cling to the surface.


+1
 
Copper or zinc is going to be the long term solution. Mix up some copper sulphate and spray it on the worst areas. It doesn't take much, maybe 1lb in a 5 gal bucket? It's cheap in places like Home Depot.
 
Originally Posted By: hatt
A real pressure washer does wonders.



+1 You need something strong to kill the mold, look into Jomax solution. Then a good strong power washer to to the grunt work.
 
Originally Posted By: demarpaint
Originally Posted By: hatt
A real pressure washer does wonders.



+1 You need something strong to kill the mold, look into Jomax solution. Then a good strong power washer to to the grunt work.


I would bet if you use the boat bottom cleaner you won't need much of a pressure washer to get it off.
 
Originally Posted By: DriveHard
Originally Posted By: demarpaint
Originally Posted By: hatt
A real pressure washer does wonders.



+1 You need something strong to kill the mold, look into Jomax solution. Then a good strong power washer to to the grunt work.


I would bet if you use the boat bottom cleaner you won't need much of a pressure washer to get it off.
Probably not. But with my washer I don't need to fool with $27/gal jugs of acid that eat concrete.
 
Originally Posted By: Oro_O
1 c. TSP + 1c. Bleach + 1 gal. water.

I live in the PNW and fighting that is an annual joy. What you see accumulate in 20 years, we get in one or two. This solution works great and replicates the higher-dollar stuff in home stores. TSP is cheap and at your local HD or Lowes. Spray it on with a garden sprayer and let it work. 15 minutes is usually good. Best to do it NOT in direct summer sunlight or it evaporates too quickly. It will make the follow-up pressure work much easier and the TSP will help lift and remove the mess so it doesn't cling to the surface.
I'm going to try this on the front walk & porch. May up the bleach % a bit. I already have TSP.

If I don't do this every couple of years, it gets quite bad & slippery. Thanks!
 
I tried dilute muriatic (HCL) acid years ago. It bubbled up and etched the concrete, but didn't remove the black/green fungi/algae, surprisingly enough.
 
Originally Posted By: hatt
Originally Posted By: DriveHard
Originally Posted By: demarpaint
Originally Posted By: hatt
A real pressure washer does wonders.



+1 You need something strong to kill the mold, look into Jomax solution. Then a good strong power washer to to the grunt work.


I would bet if you use the boat bottom cleaner you won't need much of a pressure washer to get it off.
Probably not. But with my washer I don't need to fool with $27/gal jugs of acid that eat concrete.


Exactly. Throw enough chemicals at it and you could probably scrub it clean with the garden hose and a stiff brush if you have the time, energy, and enthusiasm.
 
Don't use an acid on concrete. It will roughen up the surface and make things stick even worse next time.

I've used it to prepare a garage floor for epoxy coating. The concrete feels rough after the acid bath... like fine sandpaper
 
Last edited:
Originally Posted By: xfactor9
Don't use an acid on concrete. It will roughen up the surface and make things stick even worse next time.

I've used it to prepare a garage floor for epoxy coating. The concrete feels rough after the acid bath... like fine sandpaper
No worries. No acid used on concrete in years...decades even...can't remember.

Spent a few hrs. researching on various sites yesterday. Always enjoyed chemistry. Evidently bleach works more effectively in an alkaline soln, thus the use of TSP & other alkalines. Surfacants help the mix stick to vertical walls without running off. Found an interesting site dedicated to fungicide use: Where, when, how, what type.

My driveway's so clean, I have to wear shades!
 
Originally Posted By: sleddriver
My driveway's so clean, I have to wear shades!

No one in my neighbor ever washed their driveways, but we sweep our driveways once in a while.
 
Originally Posted By: sleddriver


My driveway's so clean, I have to wear shades!


You must have studied Nuclear Science.
wink.gif
 
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