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Pressure Washing 3000 psi Concrete at 2000 psi. Will It Erode?

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racookpe1978

racookpe1978

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Not a on-the-job question, but it relates to concrete wear-and-tear.

Assume a 30-year-old "nominal concrete" driveway, no rebar.
If I am pressure washing it to remove mildew and years of accumulated tree sap and gunk with a "Big Box" rented pressure washer rated at 2000 psi, will the washing force erode the concrete?
Question was asked by she-who-must-obeyed, and a neighbor. My answer was "no."

My reasoning was two fold:
A test area with the nozzle held 1/2 inch from the concrete for 20 seconds showed no erosion of the concrete nor any breaking out of the aggregate from the matrix. Aggregate was already visible at the top of the concrete: it was not "smooth" before testing. Normal nozzle position was 2-3 inches from the concrete being swept side-to-side with a constant but overlapping motion, so in one place for 20 seconds was far in excess of what was normally done. Figure an average total water impact area of 2-1/2 inches x 1/4 inch "fan" when it hit the concrete.

second was my assumption that a 2000 psi washer is putting out 2000 psi at the PD pump outlet. Down a 50 ft, 1/4 internal diameter hose, through hose and washer fittings, and then out the nozzle reduces the pressure even more.
Going out the nozzle itself spreads out the force over a bigger area, and so reduces the final pressure even more. But the water at the nozzle hole is much faster than than the slower water in the hose -> so is the impulse force more important than pressure?
 
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No science to back it up but I would think not. The pressure washer isn't going to be enough to break apart the bond strength of the concrete elements and unless you are power washing it for years on end I don't think it will wear/erode away the surface at all.

My guess...the concrete is fine until you hit a spot that might have already started to crack away or break look (near joints, edges, etc). You might lose some concrete here, but not simply erode away the surface.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi
 
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No. The "pressure" noted on pressue washers is the pressure just behind the opening of the nozzle. The sudden expansion as the water crosses the nozzle causes a tremendous head loss. No, you wouldn't want to hold it on your skin, but erosion of good concrete won't occur unless you direct the stream from the nozzle in the same location for extended periods of time (many minutes!).

Further, the distance between the nozzle and the surface to be cleaned makes a big difference. The farther away the nozzle is from the surface, the lower the contact impingement pressure will be. Once the distance gets over about 12 to 15 inches, it's affect is similar to a garden water hose with a concentrator nozzle.
 
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if you have an old mortar-set brick paver walkway..... kiss those joints goodbye.

off-topic. but something my dad brings up everytime we grill at his house. wasn't me.
 
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If your not careful you can alter the surface finish and end up with a striped driveway. I've written my name in concrete with a pressure washer. With a 0º tip you can put a hole in asphalt.

Terry
KI6FCI
 
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When I was young I was a mechanic at a rental equipment company. We rented pressure washers and would warn people that the 4000 psi models "could", and the 6000 psi models "would" gouge concrete surfaces, especially using the 0° tips. Of course that's all relative to your concrete quality and how much degradation its seen through the years. We also had a 20,000 psi trailer-mounted unit that people would actually use for concrete hydrodemolition.
 
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We power wash our patio every year, using a pulsating (swirl) jet on a 2500 psi machine. There is always some fine "sand" in the mix of effluent, which I presume is from the top layer of the concrete eroding away, either sand in the concrete or cement particles. So far, in 10+ years, we haven't worn down to see the larger aggregate.
 
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This was done by a former employee with a 1500 psi pressure washer using a 25º tip. The property owner was less than thrilled. Luckily the parking lot is going to be seal coated after they finish painting the complex. Concrete is much harder than asphalt, but can still suffer the same type of damage.
Asphalt.jpg


Terry
KI6FCI
 
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@Ron,

At what length hose is the pressure behind the nozzle rated? If the hose is longer, then the pressure is reduced due to the additional head loss in the hose. Also, the head loss at the nozzle is basically the pressure behind the nozzle as once the water leave the tip, it is at atmospheric pressure. All of the pressure has been converted to water velocity (minus heat and sound)
 
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Well, it was a 50 fifty foot hose, 3/8 inch OD, probably <1/4 inch ID.

Flow rate low, less than 2 gpm so I did not expect much pressure loss due to flow alone, which is why I'd expect - as noted above - that almost all of the potential (pressure) energy is converted in kinetic energy (sound, velocity) at the nozzle exit. Then, as the distance from the nozzle increases and the spray "footprint" increases (see photo of damage from "handwriting" with a pressure washer above) less and less energy is available to impact the concrete.

Many interesting and inciteful comments, my thanks to all who contributed their experiences. Robert
 
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Yes, you can erode concrete with a pressure washer. The action of the water is "micro", while concrete compressive strength is "macro", and the force put on the surface of the concrete (that is, the particles at the surface) isn't compressive when sprayed, it creates a tensile action on individual particles. Also, the strength of the hydrated cement paste only holds when it is resisting, in aggregate, against compression in confined conditions. This is why the size of a concrete compression test sample is important. If it was not, then samples could be the size of mortar cubes. However, the effect will be insignificant as long the tip is away from the surface.

As to the pressures involved, the damage that can happen to skin and underlying tissues due to pressure washers (and paint sprayers) is tremendous. Imagine a pint of water or driveway cleaner forced under the skin and into the tissue because you accidentally hit your leg with the tip.
I once saw a guy at a self-serve car wash inflate the brand new paint on the roof of his car like a balloon.
 
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The damage in my above picture was totally unintentional. The employee was removing paint splatter. The wet pavement (asphalt or concrete) masks the damage to the surface finish until it dries. It is a very minor, but very visible, form of damage.

A friend of mine likes to tell the story of his friend (professional painter) who was cleaning his airless sprayer of oil based paint. He reached into the bucket of solvent to see if the gun was spraying. After a week in the hospital he was able to keep his arm. The nozzle broke the skin at his wrist and injected paint thinner up his forearm.

Respect your tools and what they are capable of doing.

Terry
KI6FCI
 
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It's a matter of degree, isn't it? Water, even at lower pressures, erodes rocks over time. Solid granite stair steps erode from thousands of people stepping on them; there's no discernible incremental erosion for any given step, but the cumulative effect is there. Cotton thread can cut stainless steel, over time.

So, the technical answer is there IS erosion, but for the limited exposure you are applying, the erosion is not discernible.

TTFN
faq731-376
7ofakss

Need help writing a question or understanding a reply? forum1529
 
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Entirely too many "professional" painters persist in treating airless spray guns like conventional guns, to the extent of removing the tip guards. All too frequently it results in an incident like the one described above, or worse.
 
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