ABSTRACT

This paper presents the results obtained from the evaluation of the pre-stressed concrete piles of the General Rafael Urdaneta Bridge on Lake Maracaibo (PGRU) and the field tests carried out for the cathodic protection of its reinforcements with sacrificial anodes.

This paper also presents the results of the seven years experience with A1-Zn-in in-test piles that has led to their use for protecting the bridge?s piles.

The evaluation was carried out by visual inspection and by using physical-chemical and electrochemical analysis. The latter which were able to detect severe corrosion problems in the reinforcement.

The results show that the corrosion in the reinforcement is due to the procedure that was used to connect them to the cap. This has led to exposure of the reinforcement to the aggressive Lake Maracaibo environment, and their subsequent corrosion.

INTRODUCTION

It is well known that reinforced concrete structures are the ideal solution that structural engineers give to any demanding application, from the mechanical as well as the corrosion point of view. The last twenty years have seen a significant surge in the use of this type of structure, so special emphasis has been laid on identifying the causes of problems associated with these structures, brought about by the surrounding environment. Of these, corrosion of the reinforcement is one of the problems to which many studies have been addressed. The General Rafael Urdaneta Bridge on Lake Maracaibo (PGRU, Figure 1), is one such structure. It is a cable-stayed bridge located in northwest Venezuela, facing the Caribbean. One of Venezuela?s most important socio-economic structures, PGRU is the only transportation link between the economic centers in and around the City of Maracaibo with the oil-rich eastern lake shore and the rest of Venezuela. Designed and built to replace ferries plying from Maracaibo to and from the eastern lake shores, it now spans shipping lanes cut through to the sea to accommodate deep-draught super tankers. When it was built, it was one of the largest and longest bridges in the world. Spanning a relatively fresh-water but increasingly salt-water lake with a length of 8,678.6 m, it boasts a four-lane, 17.4 m deck with 134 openings, supported on 2184 piles of different types: hollow pre-stressed and post-tensioned or pre-fabricated reinforced concrete.

The encroaching salt water, high temperatures and humidity have exposed PGRU to an unanticipated corrosive environment. Recent studies carried out by Petroleos de Venezuela, S.A. (PDVSA) have revealed that Lake Maracaibo?s corrosivity has increased from 16 mpy/1 962 to 180 mpy/1 993, i.e., a chloride increase from 400 to 5000 ppm. It is important to note that, under normal exposure conditions, the corrosion of the steel is negligible, because it is passivated by the formation of a protective iron oxide film. However, as chlorides penetrate, they destroy the passive film, generating severe localized corrosion. Corrosion problems in the structure of PGRU have been reported ever since 1964.

These have attacked both the reinforcement and the cables, which were all replaced in 1980. In 1985, Rincon et all inspected the first 19 supporting piles. They found corrosion damage. This led to studies and research contemplating the application of cathodic protection to these piles, based on the following considerations: a) This part of the structure cannot be replaced or repaired; b) Cathodic protection is the only system that can be applied to slow down corrosion at this stage. Sacrificial anodes were selected instead of impressed current for several reasons: installation is easier, maintenance less costly and the possibility of

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