Vector Corrosion Blog
Post-Tension Tank InvestigationMarch 7th, 2020
- Project Name: Yankee Gas Phase 3
- Structure: Liquid Natural Gas Tank
- Location: Waterbury, CT
- Client: Kleinfelder
- Technologies: 4 mm Diameter 360 Degree Articulating Borescope
- Project Issues: Identification of Voids and Strand Condition in Post-Tensioned Ducts
- Project Date: June, 2014
- Contractor: N/A
Vector Corrosion Services (VCS) performed an investigation of a liquid natural gas (LNG) tank in Waterbury, CT to identify the extent and condition of voids located in the tank’s vertical post-tensioning ducts.
The vertical post-tension ducts extend 150 ft up the LNG tank wall. During the grouting of the tendons the 7-wire stand created a capillary effect, which drew water up its length. This allowed for water to rise to the top of the duct and the grout to settle creating a void in the top of the duct. The tank was 7 years old and the owner was concerned about the presence of soft grout, strand corrosion, and strand section-loss. Access into the side of the tank was not an option due to the significant height of the tank and also the extremely large amount of reinforcing steel blocking access to the tendons from the exterior surface of the tank.
VCS implemented a borescope, 9 m long and 4 mm diameter, with a 360 degree articulating tip to visually inspect the interior of the ducts. The borescope was able to entry the duct through the vent port located in the trumplate. The inspection objectives were to identify the presence of a void, evaluate the condition of the exposed strands, identify if soft grout was present, and sample any water contained in the duct for chloride and sulfate content, and pH. The borescope allowed for the visual inspection of the duct while minimizing the impact to the structure. The borescope collected video and images of the duct interior, strand condition, anchor plate condition, and the grout/void interface.
The inspection resulted in VCS identifying that the strands were in good condition, despite the presence of the void. There was no soft grout and the trapped water maintained an alkaline environment. As a result VCS worked with the project’s structural engineer, grout supplier, post-tension installer, and general contractor to recommend possible repair solutions and material selection.