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November 12 - November 18, 2012

Is it cost-effective to pressure wash a bridge or other structure before abrasive blasting? What pressures, flow rates, and procedures are recommended?



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Selected Answers

From Martin Neumann of Neumann Co. on April 5, 2013:
     Thank you, Shane Hirvi. I think the answer is, it just depends.

From Atanas Cholakov of ACT on January 21, 2013:
     Yes, it is cost-effective to pressure wash/jet prior to abrasive blast cleaning. If the existing coating system is beyond repair, high pressure water jetting can be conducted at 70 MPa – 210 MPa (10 000psi – 30 000psi). At this pressure range, corrosion oxides, paint and other unwanted contaminants are blown away. Chemical contamination will be removed, but at various degree of effectiveness. Therefore, a chemical agent for salt removal is recommended. In this scenario you have two essentials: chemical reaction and mechanical force working on the surface. The emerging substrate will quickly get hue (flash rust) as it dries.
      The aim of the abrasive blast will be to remove the flash rust and tightly adherent paint if any is left and to re-create surface profile. The cleaned surface is tested for soluble salts content. Info for salt content level is obtained from coating supplier. Reference standards: NACE No.5 / SSPC – PC12 NACE No. 2/SSPC-SP 10 for abrasive blast.

From James Johnson of Chlor*Rid International, Inc. on November 20, 2012:
     Is it economically feasible to pressure wash before abrasive blasting? Perhaps the question should be reversed - Since all bridges have road film, general dirt and grime,and often salt contamination, it is a known fact some of those deposits will be blasted onto an abrasive-blasted surface. It is also a fact each coating manufacturer's data sheet says to apply it to a clean, uncontaminated surface. Is it economically feasible to NOT pressure wash before blasting? Shall we, as an industry, concentrate on how thin of ice we can skate on and chance premature failure, or should we concentrate on how we can achieve the maximum lifespan of an expensive new protective coating? Shall we gauge cost by the out of pocket amount or calculate cost on an annual basis for the life of the coating system? Either way, in most states pressure washing is very economical, while other states appear to be driven by environmental concerns, which does drive the cost up considerably. The compliance cost will affect overall cost and/or lifespan.

From Michael Beitzel of Modjeski & Masters inc on November 15, 2012:

     Water may be cheap but pressure washing is not always. Before this question can be answered, several conditions need to be determined and questions answered. Are there currently any existing surface contaminants in the form of chlorides, nitrates or sulphates present on the structure and at what level?

     A test pressure washing should be conducted and the effluent collected, sampled, and tested to determine if any toxic materials are contained in the total suspended solids or other properties of the waste water that would require total containment of the wash water.

     If soluble salts are found on the steel, was pressure washing effective in removing them to acceptable levels? The test pressure washing would also be a good time to determine the pressures to be used and the size nozzel and stand-off distance as they may impact the characterization of the waste water generated. If salt removal additives are necessary, will their use require total containment and collection, treatment,and disposal of the waste water. If the structure is over a body of water, it may be necessary to pre-clean to remove gross deposits of dirt and debris prior to pressure washing. A test abrasive blast cleaning without pressure washing may also be helpful to determine what surface contaminant conditions are actually generated.

From Rich Todd of Thomas ind coatings on November 14, 2012:
     Yes, water is cheap and you don't want to impregnate the steel with contaminants, causing failure before the warranty is up. 2000 psi is the minimum pressure I would use. If for some reason there is salt present, I would wash with Chlorid.

From shane hirvi of superior consulting & Inspection services, Inc on January 24, 2013:
     Is it cost-effective to pressure wash a bridge? Well, there are a few things that would factor into any answer whether it is yes or no. 1) Has testing confirmed the presence of soluble salts in sufficient quantities to be detrimental to the long-term viability of the proposed coating system? If you don't have a contamination issue, then no, it is a waste of taxpayer money. 2) Does the bridge have lead paint? 2a) Are you in a state that typically operates under NPDES permit? 2b) Are you planning on containing/filtering the water? 3) Are you using a fungicide prior to the pressure washing? 4) If you are in a state that salts the roads, would you wash the whole bridge or just the areas, and those underneath, that have a level of contamination, in sufficient quantities to be detrimental to the coating system? 5) Are you in a cold-weather state executing this work in the winter season?
     Washing for the mere sake of washing seems to be a costly endeavor. There are rules and regulations that must be observed, and it is the responsibility of the owner to ensure the safety of the traveling public, ensure that the lakes and rivers are free from dangerous substances, to create an environment where the safety of the industrial worker is provided for and they are beholden to the tax payers who fund these projects. This industry is stuffed full with cut and paste specifications requiring all manner of surface prep pulled out from specs from another part of the country who experience vastly different environmental conditions.    
      Is it cost-effective to power wash a bridge in west Texas? Probably not. Is it cost- effective to power wash over a pristine trout stream dumping lead-laden water into that stream? Probably not. Is it cost- effective to power-wash the splash zone and the area underneath on a bridge in northern Michigan? Probably. Is it cost-effective to wash the top of that bridge when testing has determined no contamination? No, probably not.
     We need more common sense approaches in this industry--this is not a one size fits all business. Test first and then figure out a game plan. Don't start just doing things because it might be the right thing to do in a certain part of the country. As a contractor, you have rights and responsibilities as a part of the contract you signed. Be very aware of them.

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Tagged categories: Abrasive blasting; Power washing; Surface preparation


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