Quality
Waterjet Newsletter – 08/29/2006
Waterjetting Effects on Concrete Moisture
Level This was a subject of controversy
in the cleaning and surface preparation industry. Last year Todd A. Shawver
conducted a series of tests and published a paper* to provide a scientific
answer that clears up the previous speculation. Here are the main points of
his paper. The effectiveness of water jetting
for surface preparation has been proven. Its percentage of surface
interference failure is only 7%, compared to 38% for abrasive blasting, and
31% for jack hammering. Water jetting is capable of removing invisible
contaminants, one of the main causes of coatings failures. It also eliminates
dust and does not disrupt other nearby work. Among current practices for concrete
coating removal, one of the most common methods is shot blasting with
recyclable steel shots. Some of these steel shots can escape the vacuum
system and become a potential source of coating failures. Failure to
completely remove invisible contaminants and creating dust are also the
disadvantages of shot blasting. However, with water jetting, there
has been a concern that it may increase the moisture level in the concrete
and thus cause blistering, delaminating and other forms of coating failures.
To provide a scientific answer to this question, a series of tests were done
in a controlled condition. The tests were done on smooth and clean concrete
surfaces. The target environmental condition was a sunny day with 75°F and 50% relative humidity. The
actual condition was 77°F and 55% relative
humidity as well as a light to variable wind at 3 miles per hour. The
moisture level of the concrete surface was measured with a specialized tool,
which measures the electric impedance of the concrete. The subject concrete
surface was divided into 4 different zones. Zone 1 was base concrete, used as
a reference. Its moisture was measured and the value was 3.4% throughout the
test duration. Zone 2 was prepared with a “SpinJet”
at 40 kpsi and 6 gpm
within a time frame of 7 seconds. A vacuum recovery system was employed to
remove the waste water. The moisture level was increased from 3.4% to 4.5%
immediately after cleaning, but then drop to 2.9% after just 2 minutes. The
reason why afterwards moisture value was lower (2.9% compared to 3.4%) was
because the reading prior to cleaning was affected by some “Iron” deposits.
The same test was repeated on the cleaned surface and the result confirmed
this (same 2.9% prior to and 2 minutes after cleaning). Zone 3 was similar to
Zone 2 but without vacuum recovery. The moisture level dropped back down to
3.3% after 25 minutes. Zone 4 was soaked with running water for a 3 hour
period to simulate saturation from precipitation. The moisture level returned
to 3.4% after 35 minutes. The table below shows the moisture variation in the
tested period.
The conclusion was that water
jetting at 6 gpm and 40 kpsi
with a vacuum recovery system does not cause any measurable increase in
moisture level of concrete surfaces. * Shawyer, Todd. A. (2005) The effects of UHP surface
preparation on concrete moisture levels using UHP water jets, in Mohamed
Hashish (Ed.), Proceedings of the
2005 WJTA American Waterjet Conference, Houston, Texas, USA, August 21-23, Paper
5B-4. |
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