Page 64 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

ENVIRONMENTAL ISSUES 47

Ultrahigh-Pressure Water Jetting with Abrasive Injection. The ultrahigh-pressure
water jet method can be enhanced by the addition of disposable abrasives to the jet stream.
The result is rated highly effective, with advantages and disadvantages similar to those of
the previously described water jetting methods.
Hand-Tool Cleaning. Manually operated impact tools and scrapers can be used to
remove paint and mill scale. This method is relatively inexpensive, but is relatively ineffec-
tive compared to other methods. Since only small amounts of localized dust and debris
are created, workers may have a false sense of security about exposure, thus making it
difficult to enforce personal protective equipment requirements.

Power-Tool Cleaning. Power tools such as chippers, needle guns, descalers, wire brushes,
sanding disks, and grinding wheels can be used to remove paint, rust, and scale from the
bridge surface. This is a labor-intensive method. The resulting quality of preparation of the
surface may be inadequate, depending on the condition of the coating being removed.
Airborne dust is generated, and workers must be properly protected.
Power-Tool Cleaning with Vacuum Attachment. In another version of the previous
method, a vacuum attachment is added around power tools and debris. This has the disad-
vantage that accessibility in tight areas is reduced because of the shroud and vacuum attach-
ment. On irregular surfaces, a seal may be difficult to maintain, and airborne leaded dust
may be present. Because a seal typically minimizes dust, workers may not be aware when
it has slipped and they thus require additional respiratory protection.
Power-Tool Cleaning to Bare Metal. Power tools can also be used to clean to the bare
metal. This method adds such tools as scarifiers (rotary peening tools) to the power-tool
set and can achieve a generally higher level of surface preparation. More dust is created,
and higher levels of worker protection and training are required. Productivity is low, and
a high quality of surface preparation may not be achieved in inaccessible or heavily pitted
areas.
Power-Tool Cleaning to Bare Metal with Vacuum Attachment. A modification of the
previous method contains dust and debris using a shroud and a vacuum attachment around
the scarifying power tools, creating a seal with the bridge surface. This has the same dis-
advantages as the method of power-tool cleaning with vacuum attachment, but with addi-
tional training required on the equipment and greater cost to achieve bare-metal standards.

Chemical Stripping. Chemical stripping agents can be applied to the surface, left in place
for several hours, and then scraped off along with paint, rust, and scale. The surface must
then be flushed with water and the chemical agent neutralized. The rinse material must be
contained and disposed of properly. This method virtually eliminates airborne debris.
Personal protective clothing must be worn during the removal process to prevent dermal
contact with leaded debris. However, not all chemicals are effective on all paints, and few
will remove all the rust and scale.

Sponge Jetting. In the sponge jetting method, compressed air is used to propel polyurethane
particles (sponge) that may be seeded with abrasives against the bridge surface. The debris
and sponges are collected and sorted. The sponges can then be reused. The quality of surface
preparation is similar to that from other blast cleaning methods, but the productivity is lower.
The amount of debris is significantly reduced because of the recycling of the sponges. Visible
dust is reduced, although containment and personal protection gear must be maintained as in
other blasting methods. Costs of the equipment and abrasives are high.