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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