The results of laboratory and field tests to investigate the performance of different temporary corrosion protection methods for prestressing steel are presented. It is demonstrated that a particular emulsifiable oil product applied to the prestressing steel showed by far the best corrosion protection behavior. Using this product, a pullout test with a long embedment length was performed on a post-tensioned seven-strand tendon with a plastic duct and compared with a reference test using untreated strands. Compared with the untreated strands, a bond shear stress reduction by a factor of approximately 2.5 was observed. It is shown that, generally, this reduction does not significantly influence the load-deformation response of post-tensioned concrete members and that the emulsifiable oil does not need to be removed before grouting of the tendons.Keywords: bond; corrosion; duct; durability; grout; post-tensioning; prestressed concrete; prestressing steel; test.
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INTRODUCTION
One of the most important means to ensure the durability of post-tensioned concrete structures is an effective multilayer protection of the prestressing steel, provided by the surrounding concrete, duct, and grout.1 Because, depending on the construction process, the tendons remain ungrouted for a certain period of time, temporary corrosion protection measures may have to be implemented. Typically, temporary corrosion protection measures are required if tendons manufactured on the construction site remain ungrouted for more than 6 weeks or if they are left ungrouted after being tensioned for more than 2 weeks.2 Whereas there is little guidance on suitable temporary corrosion protection methods, it is generally required that they are not harmful to the prestressing steel, the duct, or the bond between the prestressing steel and the grout.3
Temporary corrosion protection methods include (1) the application of emulsifiable oils to the prestressing steel; (2) the continuous pumping of dry air through the duct to reduce the relative humidity to values below 40 to 50%; and (3) the creation and maintenance of an inert gas atmosphere in the duct using nitrogen.
Emulsifiable oils can easily be applied, are inexpensive, and affect neither the steel nor the alkaline protection provided by the grout.4 Not all emulsifiable oils are suitable for corrosion protection, though, and the protective oil film on the steel can be damaged during transport and tendon installation. Furthermore, emulsifiable oils change the tribology of the steel and thus reduce the bond action between steel and grout; this was confirmed by a number of pullout tests using single strands and wires.5-7
Both the dry air and the inert gas method are rather sophisticated and costly. Installation at the construction site is complex, the system may leak, and there is a danger of condensation. The limited available test data indicate that the methods can be used for temporary corrosion protection up to 1 year.8-10 Recent investigations, however, showed that the same quality of corrosion protection over the same time period can be obtained by applying emulsifiable oil.11
This paper reports on laboratory and field corrosion tests to investigate the performance of different temporary corrosion protection methods. It is demonstrated that a particular emulsifiable oil product, used as a 25% aqueous emulsion, showed a consistently good corrosion protection behavior under all test conditions. Using this product, a bond test on a post-tensioned seven-strand tendon with a plastic duct embedded in a long reinforced concrete prism was performed and compared with a reference test using untreated strands. It is shown that compared with the untreated strands, the average bond shear stresses were reduced by a factor of approximately 2.5. The significance of this finding is discussed.
RESEARCH SIGNIFICANCE
Four types of laboratory corrosion tests with three emulsifiable oil products in three concentrations resulted in the selection of one superior product with a particular concentration. Field tests during a winter semester on 16 transverse deck post-tensioning tendons of two road bridges demonstrated the practical suitability of the selected product and its superiority compared with other corrosion protection methods for parallel-wire and multi-strand tendons, both with steel and plastic ducts. Two large-scale pullout tests on post-tensioned seven-strand tendons with long embedment lengths provided reliable data to quantify the bond reduction of treated versus untreated strands for the selected product.
CORROSION TESTS
Laboratory tests
Laboratory tests under controlled conditions were performed to select an appropriate corrosion-protective agent for the subsequent field and bond tests. Steel plates measuring 100 x 75 x 2 mm (3.94 x 2.95 x 0.08 in.), prestressing wires 7 mm (0.28 in.) in diameter, and prestressing strands 15.7 mm (0.62 in.) in diameter were used as specimens for the laboratory tests7 at the Laboratory for Corrosion and Materials Integrity of the Swiss Federal Laboratories for Materials Testing and Research (Empa). The specimens were dipped into the corrosion-protective agents for 2 minutes, and then they were given time to air dry under standard conditions (23 °C [73.4 °F], 50% relative humidity [RH]) for at least 16 hours. The corrosion-protective agents included Rust-Ban 310, Shellcool M3, and Aseol Milem-23-31, used as 5% and 25% aqueous emulsions and as concentrates (100%).
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