Cure for crumbling concrete
By Einar Myrenget
Photo: Gunnfrid Holan
Senior lecturer Øystein Vennesland of the Department of Structural Engineering at NTH has developed a new method to stop corrosion in the steel reinforcement in concrete.
The Body of Evidence - NTH Central Building I
Vennesland´s method of realkalization of buildings was tested four years ago on Central Building I on the Gløshaugen campus at NTH. At that time, the steel reinforcement in the building had more or less rusted away since it was built in the 1960s. The concrete samples that were recently taken showed that the experiment four years ago was a success. The concrete has regained its original PH value strength and the corrosive attack has stopped.
The method is easy to use, it is inexpensive and can save NOK millions in maintenance costs. The method which Vennesland and John Miller in the consulting firm Noteby (Norway) have patented will now be implemented abroad.
Corrosion in steel reinforcement is a problem for innumerable housing co-operatives and other home owners in Norway. After twenty-thirty years, depending on the quality of the concrete, rust spots appear on the surface and soon after pieces of concrete may break off. In Norway, the buildings most vulnerable are those built at the end of the 1960s and the beginning of the 1970s. At that time the concrete often was of bad quality. An additive containing salt was used to make the concrete harden. As a result, the steel reinforcement rusts more quickly.
Vennesland and Miller have developed methods for fighting corrosion. One is known as realkalization and the other electrochemical chloride removal. Realkalization is a method that can be used on buildings.
"After the iron reinforcement is built up like a skeleton, the concrete is poured in. The concrete is alkaline and has a pH of 12-13. (A pH of 7 is neutral, less than 7 is acidic and over 7 is alkaline.) As the years go by, the carbon dioxide from the air reduces the pH and at a certain pH-level it naturally begins to rust. After that, the steel rusts away in peace and one day rust spots appear on the surface of the concrete and, at worst, blocks of concrete begin to fall off," Vennesland tells.
The new method consists of simply running electricity through the concrete and to the steel reinforcement. A negative terminal is attached to the steel, and the wall is sprayed with a cellulose material containing crystal soda which is then connected to a positive terminal. The current is then switched on and allowed to flow for three to four days. During this time the concrete regains its original pH value and becomes as good as new with respect to corrosion protection. Thus the concrete is realkalized and it will stay alkaline.
CHLORIDE REMOVAL
The other method is called electrochemical chloride removal. The problem occurs when salt attacks the concrete. This problem is most common on quay structures, docks and bridges. Salt slowly but surely seeps into the concrete, and when it reaches the reinforcement, the steel begins to rust. As with realkalization, a negative electrode is attached to the iron reinforcement while a cellulose coating on the surface of the concrete is connected to a positive. After the current has been flowing for three to four weeks, the negative terminal attached to the reinforcement has pressed the salt out of the concrete to the cellulose coating. The coating can be subsequently removed and sprayed clean. The concrete has regained its strength and the chlorides have been drawn out with the help of electricity.The method which Vennesland and Miller have patented, seems fascinatingly simple. It was in the middle of the 1980s, when Vennesland was working at Noteby, that he and Miller first considered using the simple laws of physics to solve problems in this field. The two colleagues then patented the method, but Vennesland admits it wasn´t an overnight success.
"We fumbled around a bit in the beginning, and it took years to completely develop the method. We experimented with one of Norway´s largest housing co-operatives, Risvollan in Trondheim, without much success. That was several years ago, and at that time we hadn´t discovered that we could spray wood fiber - cellulose - on the walls as a coating. The tests we´ve done now are giving much better results," says Øystein Vennesland.