Masonry Magazine June 1985 Page. 24
EMBEDDED STEEL IN MASONRY
continued
Types of Embedded Steel
Reinforcing Bars. Reinforcing bars are used in bond beams, within block cores, or in grout wythes between layers of masonry. They are always completely embedded in mortar and grout in order that proper bond can be developed between the steel and masonry. The alkaline environment of the mortar and grout creates a protective film on the reinforcing steel which renders the steel "passive" and prevents corrosion.
However, if the grout cover around the steel is too thin, porous, or permeable, carbon dioxide gas from the air may enter deeply and eventually lower its alkalinity. This may impair the effectiveness of the corrosion-resistant film on the embedded steel and reduce its passivity. Also, soluble chlorides in ad-mixtures, particularly calcium chloride, can have the same kind of detrimental effect on steel, thereby increasing the potential for corrosion.
Joint Reinforcement. Joint reinforcement embedded in mortar is provided with the same protection as reinforcing steel when it is embedded in grout or mortar, and can have this protection diminished in the same way. However, unlike reinforcing rods, in concrete masonry there is a portion of joint reinforcement which is usually never embedded in mortar; namely, the cross wires in hollow, face shell-bedded masonry. Since these cross wires are always exposed and do not have the protective environment of the mortar, there is a corrosion potential whenever moisture is present. This corrosion potential is significant where cross wires contribute to the tensile capacity of truss-type joint reinforcement and where cross wires are used as cavity wall ties.
Connectors. Anchors, ties, and fasteners behave very much in the same way as joint reinforcement. They are embedded in mortar to develop their strength and usually have some part exposed to the atmosphere. However, unlike joint reinforcement in which the main elements are always protected by mortar, the main load-carrying portions of anchors, ties, and fasteners are almost always significantly exposed to the atmosphere, hence susceptible to corrosion when moisture or contaminants are present.
Embedded Frames and Lintels. Steel lintels and embedded frames are also subject to corrosion. The exterior surface of these elements can be maintained along with the other exposed surfaces of a building, but the anchors and surfaces in contact with mortar and grout must be protected.
As with reinforcement and anchors, the embedded steel will receive protection from corrosion from mortar or grout; however, as with reinforcement, the use of chloride-based or other corrosive admixtures can reduce this protection and allow corrosion to take place. In the case of lintels, there is the additional problem of moisture from cavities and hollow unit cells.
Protective Measures
The following procedures can help to reduce the possibility of having all of the causes of corrosion present at the same time.
Reinforcing Bars. Reinforcing bars should be provided with adequate grout or mortar cover to prevent moisture or oxygen from attacking the steel. The usual cover recommended for reinforcing steel is shown in Table 1. Avoid the use of calcium chlorides, thiocyanates, or admixtures containing more than 0.05 percent chlorideions which can cause corrosion in the mortar or grout mix. With the advent and availability of noncorrosive and osive and nonchloride ac celerators, mortar and grount work can proceed safely in cold weather conditions.
Joint Reinforcement and Connectors. Joint reinforcement and connectors should be provided with the same protection as reinforcing steel for that part protected by mortar (see Table 1). Use corrosion-resistant or noncor roding materials in areas where they are exposed to the weather, moist conditions, or in masonry which is in contact with the ground.
Where joint reinforcement or connectors are not embedded in mortar, the effect of potential corrosion should be carefully evaluated. In interior walls, where the exposed cross wires of joint reinforcement are not subjected to corrosive conditions, no protection is needed. Exceptions to this, of course, would be walls enclosing swimming pools, high-humidity areas, or areas where a corrosive atmosphere exists. In exterior walls corrosion protection should be provided for cross wires as well as side rods.
24 MASONRY-MAY/JUNE, 1985