Standby Service Water Basins – Roof Slab Cracking

Project Type
Projects Icon - Technical Services

Project Description

Underwater Coating Application to Seal Cracks (Nuclear Power Project – applicable to any industry with concrete infrastructure)

The concrete roof slabs of SSW Basins A and B demonstrated full thickness cracking of the concrete slab covering a large portion of each basin.  The roof slab extends below the water line in each basin.  A previous report recommended sealing the cracks on both the atmospheric and immersion faces of the slab to prevent exposure of the rebar to excessive moisture intrusion.  The atmospheric side was sealed by the application of a coating system over the entire roof slab.

A flexibilized underwater epoxy supplied by TFT of Houston was selected for repair of the identified cracks. Extensive testing was performed to verify that the material could be applied underwater on an overhead surface.  The selected material demonstrated the ability to cling to an overhead surface even under the turbulent conditions produced by the divers exhausted air bubbles.  It also demonstrated sufficient post cure adhesion to the concrete surface.

High pressure water at 4,500 to 4,800 psi was used to remove loose material and clean deposits from the surface of cracks and adjacent concrete.
Plural component caulk guns were used to mix and dispense coating material.  This eliminated pot-life concerns and allowed efficient underwater transport and dispensing of coating.  Storage and dispensing were carefully tracked to permit control of all material dispensed within the Basins.  Material was applied at a wet film thickness of 10 to 20 mils.  Wet film thickness was checked periodically during application with a wet film thickness gauge.

The application method forced the coating as deeply as possible into the crack.  Coating extended out approximately 2” on either side of the crack but did not overlap any unprepared surface.  After the initial pass to apply the coating, several subsequent passes were necessary to adjust wet film thickness and ensure the coating did not pull away from the substrate.

A total of 3,583 feet of crack equaling approximately 1,800 square feet was coated.  Inspection of the applied coating was performed by ANSI N45.2.6 certified Level II and Level III Coating Inspectors.  The inspection was conducted under the supervision of a Level III Coating Inspector.

Visual inspection of the applied coating shows that the coating was applied per the approved procedure.  Post cure adhesion tests were performed and adhesion values were fund to be acceptable.  A follow-up inspection was performed in August 2004.  All coating appears to remain tightly bonded to the surface with no evidence of cracking or disbonding.