Choosing the Right Blast Method for Architectural Restoration
April 12, 2007
Contractors in the architectural restoration business ask one question all the time: how should we blast it? Media blasting – pneumatic abrasive cleaning – has proven effective in many architectural restoration applications, including mold remediation and smoke and fire cleanup.
We’re typically talking about larger jobs, where the contractor is restoring large portions of the structure of a building – joists, trusses, roof decking, studs, foundations and crawlspaces, to name a few. These applications usually require the removal of surface contaminants like soot and charring, or microbial growth, as well as a thin layer of damaged wood. Some applications may even require more aggressive cleaning like the removal of burnt, or possibly mold-infested, paint. Abrasive blasting can clean more square footage – usually at a lower cost, and almost always to a higher standard – than manual alternatives like sanding, brushing or scrubbing (Photo 1, 2 and 3).
Media blasting in general is a proven, effective process. Not everyone uses it, but its use and recognition increases daily. Nationally accepted estimating software has pricing set up for media blasting applications. Select restoration training schools are teaching the application of the blasting process for architectural restoration. Restoration contractors and their customers are now able to expect a higher level of cleaning, with quicker job completion and, ultimately, lower costs to both.
Back to the question: how should we blast it? Obviously, you want to use compressed air, usually in large volumes, mixed at the right ratio with an abrasive, hopefully in small volumes, to “blast” the contaminated surfaces clean. The not-so-obvious part has to do with the choice of the abrasive blasting method to be employed.
The abrasive in this case, at least in the last several years in the architectural restoration industry, has typically proven to be either dry ice or baking soda (Photo 4), with contractors having to decide which method might be best for their real-world application. While the media blasting process is now somewhat mainstream, both baking soda and dry ice remain exotic blast media with flow and handling characteristics that require specialized blast equipment to be used productively. Both abrasives have their own characteristic effect on the blasted surface (Photo 5 and 6). So while the results are often close enough for direct comparison, the processes are different, requiring significantly different equipment.
When purchasing equipment, a great many restoration contractors choose one process or the other based on financial considerations; not everyone can purchase every tool available. If you must choose one process or the other, choose the process that offers the most effective cleaning on the highest percentage of the jobs you will blast. Even if you have access to both tools, you will need to decide, at some point, which process to use on each unique job site.
When considering media blasting, consider not only the cleaning requirement, but also the job setting, and the clean-up requirements among other concerns. To highlight this, let’s consider a few specific examples of actual restoration applications ideal for media blasting:
- Job No. 1: Smoke and fire restoration in a 3,000-square-foot home, including both structural and finished woodwork.
- Job No. 2: Mold remediation in a 1,500-square-foot residential crawl space.
- Job No. 3: Smoke and fire restoration in a large automotive parts-in-process warehouse.
Job No. 1 involves smoke and fire restoration in a large custom home, including both structural cleaning, and restoration of finished woodwork and masonry. In this particular case, the structural wood could have been cleaned with either process. Both dry ice and baking soda will remove soot and light charring from wood. Baking soda tends to strip faster than dry ice, due to the angular nature of the soda crystals; however, either process would probably do the job.
One of the somewhat surprising differences between these two processes in this application is the deodorizing effect of the baking soda on the soot. Baking soda acts as a buffering agent, literally neutralizing a great deal of the odor-causing material (soot) in the structure. The real clincher here, though, is the finished woodwork and masonry. Baking soda is able to clean soot from finished wood, brick and mortar, and many other building materials with less damage, and with a better finish, than dry ice could (Photo 7 and 8). This job was completed with baking soda blasting. The contractor, the insurance adjuster and the homeowner were all thrilled with the results. Job No. 2 involves mold remediation in a 1,500-square-foot residential crawl space. With 24 to 36 inches of headroom in the crawlspace, there was room enough to access the subfloor and joists for blasting, but the poor prospect for adequate ventilation caused some real concerns. Dry ice, or solid carbon dioxide, changes from a solid to a gas – a process known as sublimation – when blasted onto a hard surface. This minimizes the amount of debris to clean up when the blasting is completed; however, it poses its own particular set of problems. Too much CO2 in an enclosed space like this one can drive out the oxygen and pose a risk of oxygen deprivation. OSHA and the EPA require that operators not be exposed to CO2 in concentrations higher than 25,000 parts per million, and no more than 5,000 parts per million on a time average weighted basis. A recent Canadian study suggests that CO2 concentrations in the area of the operator in a typical job site exceeded 23,000 ppm even with the addition of ventilation.
In other words, a supplied air system for the operators, and oxygen level monitoring is the bare minimum personal protection, were dry ice to be employed here. While this is commonly done, supplied air systems can be cumbersome (you are connected to a hose while crawling around), expensive (each operator in the space has the same requirement), and lacking in protection (depending on the interpretation, OSHA might suggest that possible oxygen deficient environs would even require Self Contained Breathing Apparatus).
Baking soda was chosen as the blast medium for this application. Vane axial fans ducted to an external dust sock were used to create negative air, and maintain good visibility. Clean up was minimized by laying down a layer of plastic before blasting commenced. When the blasting was completed the plastic layer was removed, along with most of the spent blast media and debris. The blasted surfaces were HEPA vacuumed, and a new vapor barrier was installed. The baking soda did not pose any substantial hazard in the application, while achieving the intended results quickly and efficiently (Photo 9 and 10). Job No.3 involves cleaning light smoke residue in a large warehouse. The damage was mostly contained to the galvanized metal ceiling/roof and the metal roof trusses. The warehouse was critical to the operation of the adjoining automotive assembly plant. The proposed plan called for dividing the warehouse into thirds, with restoration operations taking place in one third, while the inventory was shifted and somewhat normal business operations took place in the other two thirds. The work would proceed from one third of the warehouse, into the others as each third was completed.
The immense size of the warehouse, with ceiling heights of over 30 feet, as well as occasional forklift traffic, meant that tight containment of the shifting work areas would have been extremely difficult to set up and maintain. Due to the dust level associated with the baking soda process, tight containment and proper air movement would have been required to reduce cross contamination into the working areas of the warehouse. This would have been prohibitively cumbersome. The large workspace, coupled with loose containment and mechanical air movement minimized CO2-level concerns. The lower dust and debris level of the dry ice process caused a minimal amount of cross contamination into the working areas of the warehouse, and no disruption of the adjoining plant operations. In addition, the after-blast cleaning was limited to a simple sweep of the general area and repainting of the ceiling and trusses, with little concern for the lower area of the structure. Baking soda would surely have required a more extensive cleaning of the entire work area. (Photo 11)
This is not an exhaustive study of the pros and cons of these two unique processes. Every restoration job that comes your way presents unique challenges. In a perfect world, you’d always have the perfect tool, close at hand, to solve your problem. This world isn’t perfect, however, and sometimes you have to choose. When it comes to media blasting for architectural restoration, think it through. Then, choose the process that fits the job best, and relax; your job just got a lot easier.