ABSTRACT High levels of chloride were found in cross head beams on a motorway bridge in the northwest of England. The design made access for testing repair and protection difficult as steel buried up to 800mm deep within the cross head was exposed to high levels of chloride. Tests and trials were carried out to see to what extent the inaccessible steel could be cathodically protected from a surface applied anode.
After successful trials, a full impressed current cathodic protection system was installed using a mixed metal oxide titanium mesh anode with a shotcrete overlay. Detailed calculations of steel surface areas and current requirements were carried out. After nine months of monitoring, it is apparent that the system could have been designed assuming a maximum current density requirement of less than 10mA/m2 of steel surface area rather than the 20mA/m2 used in the design.
Embedded reference electrodes 700mm deep in concrete measured polarization of steel 800mm from the anode on the surface. This showed that embedded probe anodes were not required to protect the highly congested steel deep inside the cross head.
INTRODUCTION Impressed current cathodic protection is one of the few ways of being certain of controlling chloride induced corrosion of reinforced concrete structures. There is now a wide range of anodes ranging from conductive metallic and carbon pigmented organic coatings, mixed metal oxide coated titanium mesh with a cementitious overlay, conductive mortar, and ribbons and probe anodes embedded in the concrete.
Each anode type has its strengths and weaknesses. These are in terms of cost and ease of application, durability, wear resistance, and changes to the appearance, profile and loading of the structure. The selection of the optimum anode for a particular application may be dictated by the element being protected or a range of anodes may be suitable for contractors to choose their own design and offer their best price in a competitive bid.
However, for unusual situations it can be worthwhile carrying out trials to select the best anode system. In this case, inaccessible steel needed to be protected. Previous experience had shown that probe anodes could be placed in drilled holes near the steel. However, there is a high risk of hitting steel in congested areas when installing the anodes, making them expensive and difficult to install. It was therefore decided to investigate whether a surface applied anode could distribute current adequately to the steel near the surface and deep within the element. If so, there would be cost savings in terms of installation risk and time. Reduced lane closure times would reduce inconvenience and costs to motorway users.
CATHODIC PROTECTION OF HALF JOINTS
Half joints are frequently used on British motorway bridges. They are essentially bearing shelves built into either side of a cross head beam. Deicing salt leakage down the joint leads to reinforcement corrosion. Because of lack of accessibility, they are difficult to repair and protect.
Figure 1 shows the soffit of a motorway bridge in Northwest England, undergoing testing. It can be seen that the prestressed concrete longitudinal beams are flush with the soffit of the cross head beam. Figure 2 is a cross section of a cross head beam supporting the longitudinal prestressed beams and the bridge deck. A previous trial on a similar cross head beam on an elevated motorway section in west London was reported elsewhere(1). In the earlier case, there was reasonable access to most of the corroding surfaces as the cross head beam projected below the longitudinal beams. Also there was no evidence of high chloride levels at the steel close to th