PRESERVATION OF STEEL IN CONCRETE 123. Tests have been made to find the value of Portland cement concrete as a protection of steel or iron from corrosion. Nearly all of these tests have been of short dilration (from a. few weeks to several months); but they have clearly shown, when the steel or iron is properly imbedded in concrete, that on being removed therefrom it is clean and bright. Steel removed from concrete containing cracks or voids usually shows rust at the points where the voids or cracks occur; but if the steel has been completely covered with concrete, there is no corrosion. Tests have shown that if corroded steel is imbedded in concrete, the concrete will remove the rust. To secure the best results, the concrete should be mixed quite wet, and care should be taken to have the steel thoroughly imbedded in the con crete.
124. Cinder vs. Stone Concrete. A compact cinder concrete has proven about as effective a protection for steel as stone concrete. The corrosion found in cinder concrete is mainly due to iron oxide or rust in the cinders, and not to the sulphur. The amount of sul phur in cinders is extremely small, and there seems to be little danger from that source. A stcei-frame building erected in New York in 189S-had all its framework, except the columns, imbedded in cinder concrete; when the building was demolished in 1903, the frame showed practically no rust which could be considered as having developed after the material was imbedded.
125. Practical Illustrations. Cement washes, paints, and plasters have been used for a long time, in both the United States and Europe, for the purpose of protecting iron and steel from rust. The eagineers of the Boston Subway, after making careful tests and investigations, adopted Portland cement paint for the protection of the steel work in that structure. The railroad companies of France use cement paint extensively to protect their metal bridges from corrosion. Two coats of the cement paint and sand are applied with leather brushes.
A concrete-steel water main on the Monier system, 12 inches in diameter, inches thick, containing a steel framework of and steel rods, was taken up after 15 years' use in wet ground, at Grenoble, France. The adhesion was found perfect, and the metal absolutely free from rust.
William Sooy Smith, M. Am. Soc. C. E., states that in removing a bed of concrete at a lighthouse in the Straits of Mackinac, twenty years after it was laid, and ten feet below water surface, imbedded iron drift-bolts were found free from rust.
A very good example of the preservation of steel imbedded in concrete is given by Mr. H. C. Turner (Engineering News, Jan. 16,
190S). Mr. Turner's company has recently torn down a one-story reinforced-concrete building erected by his company in 1902, at New Brighton, Staten Island. The building had a pile foundation, the piles being cut off at mean tide level. The footings, side walls, columns, and roof were all constructed of reinforced concrete. The portion removed was 30 by 60 feet, and was razed to make room for a five-story building. In concluding his account, Mr. Turner says: "All steel reinforcement was found in perfect preservation, excepting in a few cases where the hoops were allowed to come closer than 1 inch to the surface. Some evidence of corrosion was found in such cases, thus demon strating the necessity of keeping the steel reinforcement at least inch from the surface. The footings were covered by the tide twice daily. The concrete Avas extremely hard, and showed no weakness whatever from the action of the salt water. The steel bars in the footings were perfectly preserved, even in cases where the concrete protection was only inch thick." 126. Tests by Professor Norton. Prof. Chas. L. Norton made several experiments with concrete bricks, 3 by 3 by S-inch, in which steel rods, sheet metal, and expanded metal were imbedded. The specimens were enclosed in tin boxes with unprotected steel, and were exposed for three weeks. One portion was exposed to steam, air, and carbon dioxide; another to air and steam; another to air and carbon dioxide; and another was left in the testing room. In these tests, Portland cement was used. The bricks were made of neat cement of 1 part cement and 3 parts sand; of 1 part cement and 5 parts stone; and of 1 part cement and 7 parts cinders. After the steel had been imbedded in these blocks three weeks, they were opened and the steel examined and compared with specimens which had been unprotected in corresponding boxes in the open air. The unprotected specimens consisted of rather more rust than steel; the specimens imbedded in neat cement were found to be perfectly protected; the rest of the specimens showed more or less corrosion. Professor Norton's conclusions were as follows: 1. Neat Portland cement is a very effective preventive against rusting.
2. Concrete, to be effective in preventing rust, should be dense and without voids or cracks. It should be mixed wet when applied to steel.
3. The corrosion found in cinder concrete is mainly due to iron oxide in the cinders, and not to sulphur.
4. Cinder concrete, if free from voids and well rammed when wet, is about as effective as stone concrete.
5. It is very important that the steel be ;dean when imbedded in concrete.