Rebar connections

Rebar connections, electrical continuity of reinforcement and stray currents... 

The anode system may be a single component, such as flame sprayed zinc, or multiple component such as a titanium mesh with a cementitious overlay. It may consist of a single continuous anode such as mesh or coatings, or multiple anodes connected together such as ribbon anodes or the discrete rod anodes. All anodes require electrical connections to the power supply. As for the rebar connections these should be duplicated for redundancy. 

Coated reinforcement MacroceU formation may be important in the case of epoxy-coated rebars (Section 15.4) in chloride-contaminated concrete if there are defects in the coating and the coated bars are electricaUy connected with uncoated passive steel bars in deeper parts of the structure. Small anodic areas are created at the defect points of coated rebars in contact with chloride-contaminated concrete, while the uncoated passive rebars provide a cathodic surface of much greater size. In these situations the macroceU can result in considerable anodic current densities and can significantly accelerate the attack on corroding sites. This is why coated rebars should be electrically isolated from uncoated bars. 

Presence of different metals. Rebars of carbon steel in certain cases can be connected to rebars or facilities made of stainless steel or copper. This type of coupling, which in other electrolytes would provoke a considerable degree of corrosion in carbon steel by galvanic attack, does not cause problems in the case of concrete any different from those provoked by coupling with normal passive steel. In fact, the corrosion potential of passive carbon steel in concrete is not much different... 

Rebars not entirely embedded in concrete. Macrocell corrosion can occur when there are macroscopic defects in the concrete (cracks with large width, honeycombs, delaminations, etc.) or when there are metallic parts connected to the rebars that are only partially embedded in the concrete. This case is important for structures immersed in seawater or in aggressive soil. Besides being subjected to direct attack, those parts in direct contact with water or soil may also undergo more severe attack caused by the galvanic coupling with steel embedded in concrete. 

To describe consequences of macrocell corrosion, let us assume that inside the concrete there are only a passive rebar and an active one. If these are not electrically connected, the corrosion potential and the corrosion rate of the passive rebar are p cor and Ip <.or d those of the active rebar are , oor and I, cor (Figure 8.3). A driving voltage for the macroceU is then available and is equal to ... 

Often the use of stainless-steel reinforcement is hmited to the outer part of the structure (skin reinforcement) or to its most critical parts for economical reasons. Furthermore, when stainless-steel bars are used in the rehabihtation of corroding structures, they are usually connected to the original carbon-steel rebars. Concern has been expressed with regard to the risk of galvanic corrosion of carbon steel induced by coupHng with stainless-steel bars. Actually, the galvanic corrosion that can arise when stainless steel is used in partial substitution of carbon steel has to be compared with that which takes place in the absence of stainless steels [30]. 

In order to ensure the confinement of transverse reinforcement and longitudinal rebar for wall type piers shown in Fig. 1(b), cross ties were arranged to connect the longitudinal rebar in the columns. The spacing of the cross ties should be less than the minimum side of rectangular section or 12 times the diameter of longitudinal rebar, whichever smaller. 

Use a cover meter to locate the steel and determine rebar spacing. Make a connection to the steel either by exposing it or using already exposed steel. The connection must be metal to metal and secure. Self-tapping screws are frequently used. 

The negative sign is by convention and will depend upon how the leads are connected to the reference electrode and the rebar from the millivoltmeter. 

Gowers et al. (1992) have used the linear polarization technique with embedded probes (reference electrode and a simple counter electrode) to monitor the corrosion of marine concrete structures. This technique was described previously without reference to isolating the section of bar to be measured (Langford and Broomfield, 1987). By repeating the measurement in the same location on an isolated section of steel of known surface area the corrosion rate of the actual rebar can be inferred. The main problem is the long-term durability of electrical connections in marine conditions. Also the probes should ideally be built into the structure during construction. A desirable but rare occurrence. Recent developments in corrosion monitoring are described in Chapter 5. 

An important aspect when considering cathodic protection or the other electrochemical techniques is zero cover or tie wire touching the surface. If there is a direct metallic connection between the anode and the rebar it could short out the electrochemical protection if a surface applied anode is under consideration. Excessive shorts have caused CP systems to be... 

Epoxy coated rebars present particular problems to determining the corrosion condition of the Steel. In the first place the bars are electrically isolated from the concrete except at areas of damage. The size and locations of the areas of damage are obviously unknown. Attempts to carry out reference electrode potential surveys and linear polarization measurements have therefore been unable to come up with definitive criteria for corroding and non-corroding areas. The other problem is that the bars are isolated from each other, therefore a connection must be made to each bar measured to be sure that there is electrical contact. 

In order to install cathodic protection, continuity can be established by welding in extra rebars. However, at Florida DOT one approach has been to expose bars in damaged areas, grit blast them clean and apply arc sprayed zinc directly onto the steel and then across the steel surface. This provides galvanizing directly on the steel and SACP to the steel embedded in the concrete. Multiple continuity connections are established by the sprayed zinc. 

In experimental systems in.stalled on bridges in the USA, sections of rebar have been isolated from the network and connected via an ammeter to the rest of the steel. This allows current flows into a particular area to be monitored. Similarly, an area of anode has been isolated and current flow measured to determine how much current the anode is delivering. The.se control and monitoring systems have been used in experimental system.s. In routine installations, none of this will be necessary. The amount of anode consumed can be assessed by occasional coring and the anode replaced when necessary. 

J ore vater may not reach the steel, especially in areas of undercutting. It is also very difficult to interpret the measurements. For corrosion rate measurements with the linear polarization technique it is impossible to calculate the area of corrosion. For rehabilitation it is necessary to make electrical connections between all the rebars for cathodic protection or chloride removal. 

There are problems with applying cathodic protection, chloride removal and re alkalization to FBECR because the rebars are electrically isolated, Special care must be taken to connect each rebar together, This problem does not arise with the welded cages where each cage is continuous. 

Zinc spraying of plastics or phenolic-impregnated asbestos is used to provide reflecting surfaces, and the large open-air dishes used in electronic applications are so coated. In impressed current protection of rebar in concrete, the exterior of the concrete is sometimes zinc sprayed (Morrow, 1991) to give a uniform current distribution (see subsection on impressed current systems in Section II. E) in such cases, it is often policy to avoid direct electrical connection between zinc and rebar, since the consequent use of zinc as a sacrificial anode could be counterproductive if only a limited amount of zinc is present to protect large areas of steel. 

In the field one must locate the bar and drill or core to make an electrical connection. If there is good continuity between the bars, one connection can allow for testing at multiple sites. However, in the case of poor continuily, e.g., epoxy-coated rebars, multiple connections will be needed. 

The instrumentation needed for LPR measurements is more complicated than for half cell measurements in addition to the reference electrode a counter electrode (CE) has to be placed on the concrete surface. The current from the CE to the rebar network (WE) is regulated by a special device, a po-tentiostat. After having selected the measuring position (preferably based on half cell potential field), proper connections to the rebar and between CE and concrete surface are made, the corrosion potential is measured and then a potential of Ecorr 5 or 10 mV is imposed. The resulting current I is recorded in order to obtain the AE/AI ratio. 

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