Page 63 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

46 CHAPTER ONE

Open Abrasive Blast Cleaning with Expendable Abrasives. In this method, compressed
air propels blasting grit against the coated surface. The spent blasting grit is then collected
for disposal. The major advantages of this method are that contractors are familiar with
this long-practiced method, it is very effective in creating a superior surface preparation,
it reaches areas otherwise difficult to access, and it is relatively quick (seperate con-
tainment considerations). The major disadvantage of this method is that it creates a high
level of leaded dust and large quantities of debris that typically must be disposed of as haz-
ardous waste. The additional containment requirements, hygiene training, and personal
protection equipment requirements increase the cost of removal.

Open Abrasive Blast Cleaning with Recyclable Abrasives. In this method, metallic abra-
sives are used to remove the paint. The abrasives can be separated from the debris (paint, rust,
mill scale) and reused. The volume of dust and debris is reduced as compared to open abra-
sive blast cleaning with expendable abrasives, but the effectiveness and the ability to reach
inaccessible areas are the same. Additional disadvantages are contractors’ unfamiliarity with
the method and the special care that must be taken to keep the blasting grit moisture-free to
avoid rusting and clumping. Should the abrasive dust escape containment, it may cause rust
spots on the surfaces where it settles. Because the grit is recycled, higher concentrations of
airborne lead dust within the containment area will have to be considered for worker safety.
Closed Abrasive Blast Cleaning with Vacuum. A third method is to apply a compressed-air
propellant from a nozzle fitted with a localized containment assembly that employs a vacuum.
The recycled metallic grit, dust, and debris are vacuumed as the surface is blasted. This method
is rated as highly effective, both in surface preparation and in containment of dust and debris,
but the rate of cleaning is slow. The greatest limitation of this method is that the containment
mask must be held tightly to the surface of the structure, reducing the method’s effectiveness
on irregular and inaccessible surfaces. The containment method confines the blast spray pat-
tern so that only small surface areas are being blasted at any one time. This requirement, along
with the need to maintain a tight seal, is arduous and leads to operator fatigue.

Wet Abrasive Blast Cleaning. In the wet abrasive method, water is injected into a stream
of slag abrasive propelled by compressed air. This method is effective both in dust control
and in the quality of surface preparation; however, the amount of waste produced is sub-
stantial and difficult to clean up. Inhalation hazard is greatly reduced with this method, but
the potential for ingestion still exists.
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High-Pressure Water Jetting. High pressure water (20,000 lb/in or 138 MPa) propelled
against the surface is effective without the use of grit. This method reduces dust to negli-
gible levels; however, the potential for ingestion still exists. The water is voluminous and
difficult to capture in containment. The method is not effective in removing paint from rel-
atively inaccessible areas or in removing mill scale. A rust inhibitor is usually used as part
of this method, which may affect the applied coating.
High-Pressure Water Jetting with Abrasive Injection. Combining the previous method
with abrasive injection results in all the advantages and disadvantages of the previous
methods but with the additional complication of having grit in the disposal water. It is con-
sidered highly effective in removing mill scale and paint from inaccessible areas.

Ultrahigh-Pressure Water Jetting. Even more highly pressurized water (up to 40,000 lb/in 2
or 276 MPa) can be propelled against the surface without the use of grit. This method is
more efficient in removing paint than the high-pressure water jetting method; however, the
main advantages and disadvantages of the high-pressure water jetting method still apply.