Temporary Corrosion Protection and Bond of Prestressing Steel

Four types of tests were carried out, using unprotected specimens as references:

1. Condensing humidity tests with steel plates12: steel plates were subjected to an alternating climate for 3 days (8 hours at 50 °C [122 °F] and 100% RH followed by 16 hours at room temperature and less than 100% RH);

2. Condensing humidity tests with wires and strands13: wires and strands were subjected to an alternating climate for 28 days (8 hours at 40 °C [104 °F] and 100% RH followed by 16 hours at room temperature and less than 100% RH);

3. Semi-immersion tests14 in deionized water: wires and strands were semi-immersed into deionized water with access to air (21 days at 23 °C [73.4 °F], 50% RH); and

4. Semi-immersion tests14 in a saturated calcium-hydroxide solution: wires and strands were semi-immersed into a saturated calcium-hydroxide solution with access to air (21 days at 23 °C [73.4 °F], 50% RH).

After the corrosion tests, the specimens were visually inspected using an optical microscope.

The condensing humidity tests with steel plates showed that Rust-Ban 310 protected the steel from corrosion. Shellcool M3 and Aseol Milem-23-31 provided a moderate corrosion protection. After the tests, many rust spots were visible on the metal surfaces. The unprotected reference specimens were covered with countless rust spots.

The condensing humidity tests with wires and strands demonstrated that the Rust-Ban 310 concentration must be at least 25%. The 5% aqueous emulsion did not protect the steel from corrosion.

This finding was confirmed by the semi-immersion tests in deionized water (refer to Fig. 1(a)). Only Rust-Ban 310 used as a concentrate and as a 25% aqueous emulsion protected the steel from corrosion. The two other agents exhibited a relatively poor or nonexistent corrosion protection, even as concentrates (100%).

The semi-immersion tests in a saturated calcium-hydroxide solution showed no signs of corrosion for Rust-Ban 310 and Aseol Milem-23-31 used as concentrates and little rust formation in the three-phase zone for the 25% aqueous emulsion of Rust-Ban 310. Shellcool M3 and the 25% aqueous emulsion of Aseol Milem-23-31 had no corrosion inhibition effect for the chosen test conditions (refer to Fig. 1(b)).

Because of their high viscosity, concentrated oils are not suitable for practical applications as temporary corrosion-protective agents for post-tensioning tendons. Therefore, based on the results of the laboratory tests that indicated consistently good corrosion protection behavior of Rust-Ban 310 used as a 25% aqueous emulsion, this product was selected for the further bond investigations.

Field tests

Practical exposure tests7 were carried out by the Laboratory for Corrosion and Materials Integrity at Empa from November 2000 to April 2001, using eight transverse deck post-tensioning tendons (each) from the Limmat River bridge at Siggenthal and one of the twin highway bridges at Corcelles in Switzerland (refer to Fig. 2). The tendons were equipped with removable anchor heads at both ends. The tendons were prestressed, left ungrouted for 6 months, and then detensioned, removed, and replaced by new tendons.