Masonry Magazine June 1985 Page. 23
PROTECTION OF
EMBEDDED STEEL
IN MASONRY
by MARIO J. CATANI
Corrosion problems with reinforcement in concrete highways, bridge decks, and steel stud brick veneer systems have called attention to the protection given to conventional reinforcement in masonry walls. When steel is used to reinforce masonry, to tie masonry wythes together, or to anchor it to other materials, it has generally been accepted that there will be adequate protection against corrosion of the steel so that corrosion does not become the cause of a failure. The same could be said about steel lintels, window frames, and door bucks. For a number of reasons, it has recently become more important to reconsider this assumption.
The amount of corrosive and potentially corrosive pollutants in the air has increased in many areas: the use of thinner walls and single-wythe walls increases the potential for moisture penetration into masonry, the use of insulation in cavities has resulted in the relocation of the point at which the dew point of building vapor is reached: and the increased use of calcium chloride in mortar as an accelerator are only some of the changes that are taking place that affect the behavior of steel embedded in or attached to masonry.
These developments have the effect of increasing the hostility of the environment in which frames, lintels, reinforcing steel, joint reinforcement, anchors, and ties must exit, making it important to provide adequate corrosion protection.
How Does Steel Corrode?
The corrosion of steel results from an electrochemical process, one in which both chemical processes and the flow of electricity are involved. The flow of electricity can originate from stray currents in the building, but the frequency of this phenomenon is rather low in modern buildings. Most likely, corrosion takes place electrochemically in galvanic or electric cells.
About the Author
Mario Catani is president of Dur-O-Wal, Inc., Northbrook, Ill., a principal manufacturer of masonry wall reinforcement systems. He is vice chairman of the Masonry Research Foundation Advisory Council and chairman of the joint ASCE/ACI Committee 530 on Masonry Structures. Catani earned a master's degree in civil engineering from Newark (N.J.) College of Engineering. Reprinted with permission of The Construction Specifications Institute from The Construction Specifier, January, 1985.
Figure 1
Electrochemical Corrosion Cells
Anodic
Electrolyte
Site
Conductor
(steel bar)
Cathodic
Site
Organization, Inc, May 1981
Electrolyte
In order for these cells to be practically operative, they require the presence of an electrolyte, that is, water containing dissolved substances whose ions can conduct electricity and two metallic connected surfaces which generate different electric potentials with respect to the electrolyte in contact with each of them.
This electric potential can result from two dissimilar metals (such as steel and copper) coupled in the same electrolyte, or two similar metals coupled in different electrolytes. The different electrolytes in the latter case can come about as a result of different moisture concentrations, dissolved oxygen levels, or concentrations of other dissolved substances at different places along the same piece of metal.
The two metallic surfaces having different electrical potentials are called cell electrodes: one, the anode; the other, the cathode. Electricity flows from the anode to the cathode through the electrical connection, then back to the annode through the electrolytes (see Figure 1). It is important to note that the electrodes in a corrosion cell need not be separated by a recognizable distance. They can be so close to each other as to be nearly in the same place.
For corrosion to take place, two additional conditions must exist. First, there must be some oxygen present and, in the case of steel embedded in grout or mortar, the normally protective film encasing steel must be destroyed. The total length of time to which steel is exposed to relative humidities of about 75 percent has been shown to be one of the most significant factors affecting corrosion, whether humidity results from direct water penetration, atmospheric humidity, or condensation.
Other factors affecting the rate of corrosion are air pollutants (particularly sulfur dioxide), contaminants in mortar (particularly calcium chloride), and temperature. Corrosion protection strategies should be directed primarily toward the elimination of these conditions and then to protection from their effect where the conditions cannot be completely eliminated.