Leaf "shadows" on a new commercial sidewalk, with a sealcoated parking lot right up against the curb.
Every fall we get this call. A new, light-colored sidewalk out front of a store, and leaves sat on it for about a week. Now it's covered in dark leaf "shadows," and there's an asphalt parking lot right up against the curb.
This isn't dirty concrete. It's stained concrete. That changes how you handle it. Here's how I do it.
The Process
- Clear the leaves first.
Get every leaf off with a blower or a broom before you do anything else. You can't treat a stain that's still got a leaf sitting on it.
- Know what you're standing on.
Yes, this concrete is new. But it's commercial concrete, and that matters. Residential sidewalks and patios are commonly poured somewhere around 2,500–3,000 PSI (often higher where there's freeze-thaw). Commercial sidewalks and parking areas are commonly spec'd around 4,000–4,500 PSI, and heavy-traffic and industrial areas go 5,000 PSI and up. New commercial concrete is a lot stronger than new residential concrete.
That said, any brand-new pour is still curing — concrete is generally considered to reach its design strength around 28 days. If you don't know how old the pour is, ask the property manager or GC before you start, and always test a spot.
- You don't need a surface cleaner.
This concrete isn't soiled. It's stained from leaves sitting on it for about a week in the fall. Leave the surface cleaner on the trailer.
- Rinse with plain water first.
Before any chemical goes down, rinse the whole area with plain water.
- Apply Groundskeeper at 30:1.
Now apply Groundskeeper at 30:1. Groundskeeper needs no dwell time — you can rinse it almost immediately.
Rinse it off really well with the pressure washer. That's the only job the pressure washer has on this one: rinsing.
- Apply about 3% sodium hypochlorite (bleach).
Next, put down a solution of about 3% sodium hypochlorite — bleach. You can downstream it or use a dedicated pump.
If you downstream, you'll probably need multiple applications, because most downstreamers aren't getting you to 3%. There are some new ones on the market claiming they will.
It's not a big area, so a simple pump-up sprayer works fine too.
Let it dwell about 5 minutes, then rinse.
- Test a spot. If you need it, go stronger — with NO surfactant.
Test a spot. If those two processes didn't get everything, this is your third process: once the concrete has been through the first two, spray down a stronger solution with no surfactant. That's important — make sure there is no surfactant in it.
Walk away for about 15–20 minutes. It will probably have eradicated every leaf stain.
Why Not Just Nuke It?
Some people will look at this and say it's too complicated. They'll tell you to just nuke it.
Look at the picture again. There's asphalt right next to that sidewalk. That parking lot is basically a painted surface — a seal coat. You don't want to damage it.
Protecting the Asphalt
If you want to take extra precautions, spray a non-caustic, high-alkaline degreaser on the asphalt before you apply the other chemicals. It acts as a barrier, protecting the surface and neutralizing anything that lands on it.
When you're done, clean and rinse the asphalt too. Don't take care of one mess and create another. Once the concrete is finished, clean up the asphalt and rinse it down really well so no chemical or runoff is left sitting on that seal coat.
Bottom Line
Clear it, rinse it, Groundskeeper, bleach, test, and only go stronger if you have to — with no surfactant — while you protect the asphalt next to it. Then clean and rinse that asphalt before you leave.
This is the difference between a professional and Billy Bob.
Pressure Washing Institute — PressureWashingInstitute.org
Power Wash Network — PowerWashNetwork.com
