• Bernard Erlin

    Bernard Erlin

Some metals usually get along fine in portland cement mortar and concrete. Some do not, for some it depends. Iron and steel are among the first kind. Some metals are fine as long as the concrete or mortar doesn't contain certain halides (e.g., chloride, bromide, and iodide), or become carbonated. So, we would like to discuss some of these and go beyond the most popular subject of corrosion, iron (Fe) and steel, and include aluminum (Al), zinc (Zn), and concrete.

We know that corrosion of steel in reinforced concrete causes severe distress in structures. Corrosion can occur when concrete loses its high alkalinity (about a pH of 12.5 to 15). Reinforced concrete must be of high quality—primarily low water-cement (w/c) ratio—to maintain its high alkalinity, which it does by minimizing depth of carbonation. We examined concrete from a 90-year-old surge tank exposed outside to atmospheric carbonation—depth of carbonation was 3/32 inch—the result of a low 0.42 w/c ratio.

  • William Hime

    William Hime

All concrete structures exposed to air to some degree gradually carbonate. We've investigated many cases of corrosion distress to reinforced concrete due to “age,” or shallow concrete cover over steel. The age/carbonation factor can be overcome by low w/c ratios—shallow cover cannot. So, minimum depths of cover over steel, such as required by ACI, need to be followed.

A denizen of concrete, sometimes lurking in the background, is chloride (Cl-). It can cause attack to and corrode steel, and regardless of pH can cause corrosion to some grades of stainless steel, galvanized steel (by definition, the galvanize coating is zinc), and aluminum. Millions of dollars were spent repairing masonry structures and concrete elements made using an admixture that chemically broke down and very slowly liberated chlorides into mortar and concrete. Embedded prestressing strand and other steel reinforcements, wires, lifting anchors, attachments, shelf angles, and galvanized reinforcing bars corroded sufficiently to crack their concrete and masonry encasements.

Calcium chloride is a common admixture. It expedites cement hydration that accelerates strength development and generates heat that can even keep concrete from initially freezing. Using the right amount of calcium chloride is OK for nonreinforced concrete. If you need to keep hydration going on cold days, additions of up to 2% calcium chloride is thought to be fine but can be disastrous in steel-reinforced mortar and concrete having minimum internal relative humidity of 85%. It also increases drying shrinkage—dramatically at higher w/c ratios.

Complicating this is the reaction of metals with the highly alkaline portland cement paste. Some metals such as aluminum or zinc react with highly alkaline solutions. Fresh concrete can have a pH from 12.5 (textbook) to 15, or more. Fortunately, the practical range is about -1 to +15. It all depends upon how many hydrogen ions or hydroxyl ions you can stuff into the water!

Hydrogen gas can be liberated in fresh concrete containing galvanized steel (where the zinc coating reacts). Thomas A. Edison patented aluminum powder portland cement mixtures because ensuing reactions with portland cement alkalies release hydrogen gas creating foamed concrete useful for insulation purposes. Hydrogen gas also has been generated when aluminum scraped from the aluminum beds of dump trucks used for hauling concrete, or aluminum pumping hoses, reacted with portland cement alkalies and inadvertently caused high air entrainment and attendant low concrete strengths.

“Corrosion” of concrete itself also can occur. Virtually any acid will deteriorate or corrode concrete. The corrosion depends upon concrete quality (e.g., w/c ratio), the acid concentration and, the kind of acid, in which the extent of corrosion varies with solubility of the corrosion product. Concrete corrosion in rare cases occurs when it is exposed to extremely strong bases. The corrosion depends upon aggregate type. This type of corrosion usually is more theoretical than practical.

Concrete is a very forgiving. But corrosion has hurt structures, and damages and needed repairs have hurt the pocketbook. As far as we know, for nongovernment involvement, it can cost someone up to the tune of nearly $50 million.