Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment
Reexamination Certificate
2001-02-28
2003-02-25
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
C205S704000, C205S734000
Reexamination Certificate
active
06524465
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method of electrochemical treatment of prestressed concrete structures reinforced by rebars or PC steel tendons. In particular, it relates to a method of electrochemical treatment for regeneration of carbonated concrete structures having low concrete alkalinities, concrete structures containing chloride ion and concrete structures containing aggregate which can undergo alkali-aggregate reaction in the concrete.
2. Background Art
The combination of concrete having high compressive strength and steel having high tensile strength imparts a good dynamic balance between compressive strength and tensile strength to concrete structures, as in prestressed concrete structures, and therefore is widely used in various important structures, especially in bridges and many huge and long structures such as roads, railroads and warehouses.
Besides, concrete is generally excellent in resistance to the environment such as water, fire and sunlight. Since the high alkalinity of concrete at a pH value of from 11 to 13 allows the steel in it to form a passive film on the surface which protects it from corrosion, concrete structures such as prestressed concrete have been considered as durable and permanent structures.
However, the longevity of concrete structures is now questioned as concrete structures deteriorate in durability due to various factors, though they used to be considered as permanent structures.
Phenomena called carbonation of concrete and chloride attack account for deterioration of concrete structures.
Carbonation is a phenomenon in which calcium hydroxide resulting from hydration of cement reacts with atmospheric carbon dioxide to yield calcium carbonate and accompanied by decrease in the concrete alkalinity from the normal pH value 11-13 due to the carbonate. When the pH value decreases to about 10 at which the steel starts to corrode due to rupture of the passive film on the steel, concrete structures lose the balance of strength and deteriorate in durability drastically.
Such deterioration of concrete structures develops into rusting of the steel in the concrete and breakage of the steel and is a serious problem from both structural and visual aspects.
Meanwhile, at the seaside and the like, sea water splashes onto the concrete surfaces of concrete structures. When the salt in sea water penetrates to the steel inside the concrete through the concrete pores, the chloride ion ruptures the passive film on the steel and hence induces corrosion.
Further, if insufficiently desalinated sea sand is used as aggregate in concrete, the concrete contains a large amount of chlorides from the beginning, and as a result, steel can not form a passive film sufficient to prevent corrosion.
Concrete structures deteriorate in durability with development of cracks on the concrete and corrosion of the steel as mentioned above.
Such deteriorated concrete structures have been repaired mainly by so-called sectional restoration, which comprises “scraping” concrete from rust on the steel or “scraping” cracks or defects on the concrete and then filling new concrete or mortar.
Sectional restoration only repairs visible deteriorations such as rust on steel and cracking and scaling of concrete and is not applicable at all to concrete with unidentifiable deterioration, namely concrete endangered by visibly unrecognizable deterioration which is going on latently.
To remove the primary causes of carbonation of reinforced concrete structures and chloride attack, application of electrochemical repair methods is is closed (JP-A-1-176287 and JP-A-2-302384).
One of these methods comprises supplying direct current continuously between the steel surrounded by carbonated concrete and an electrode which is placed on the concrete surface or surrounded by alkaline concrete with an alkalinity at a pH value of 11 or above and re-alkalinizes detrimentally carbonated concrete to a pH level of 10 or above by migrating alkaline substances such as sodium or potassium hydroxide from the alkaline surroundings.
The other method is useful for chloride-contaminated concrete and removes chloride ions in the concrete from the surface by supplying direct current continuously between the steel embedded in the concrete and an electrode on the surface of the concrete.
However, because these methods use voltage higher than hydrogen evolution, they have the side effect that the water in the concrete voids is electrolyzed into hydrogen gas continuously on the surface of the steel as the cathode.
PC steel tendons, which are usually high tensile steel and kept under high tension in concrete, embrittle metallographically as they absorb and store hydrogen gas in their structure and break due to hydrogen embrittlement. As a result, prestressed concrete structures fail dynamically and eventually collapse. In fact, some collapses of prestressed concreted structures in the past are considered to be attributed to hydrogen embrittlement of PC steel tendons.
Therefore, although electrochemical repair is the most appropriate to repair concrete structures, care has been needed to apply electrochemical repair to prestressed concrete structures so far due to the side effect called hydrogen embrittlement.
In order to prevent hydrogen gas evolution on the cathode, an attempt to supply direct current at a voltage of about 1.0 V or below has been made. However, this repair method can not reform the concrete and only can prevent corrosion of the steel, and this means that it is necessary to supply direct current permanently. Therefore, there is a problem with maintenance and durability of the electrification equipment and with practicability.
DISCLOSURE OF THE INVENTION
Under these circumstances, the present inventors conducted extensive research to solve the above-mentioned problems and have found out that the hydrogen stored in PC steel tendons during electrochemical treatment diffuses quickly on discontinuation of current supply, and therefore that electrochemical repair can be applied to prestressed concrete structures without the worst result, breakage of the PC steel tendons. On the basis of the discovery, the present inventors have accomplished the present invention.
Namely, the present invention provides (1) a method of electrochemical treatment of prestressed concrete which comprises supplying a direct current between a steel tendon embedded in the prestressed concrete as a cathode and an anode on the surface or inside of the concrete at a voltage higher than the hydrogen evolution potential, wherein an effective tensile force acting on the PC steel tendon embedded in the concrete is not greater than 80% of the tensile strength of the PC steel tendon, (2) the method of electrochemical treatment of prestressed concrete according to (1) wherein the effective tensile force is reduced to 80% or below by placing tendons outside of the cross-sections of the concrete to shift part or all of the stress on the PC steel tendon inside the cross-sections of the concrete to the tendons, (3) the method of electrochemical treatment of prestressed concrete according to (1) wherein the effective tensile force is reduced to 80% or below by providing more support points to the prestressed concrete, (4) a method of electrochemical treatment of prestressed concrete which comprises supplying a direct current between a steel tendon embedded in the prestressed concrete as a cathode and an anode on the surface or inside of the concrete at a voltage higher than the hydrogen evolution potential, wherein the voltage is adjusted to less than the hydrogen evolution potential at least once during electrochemical treatment, and then electrochemical treatment is resumed at a voltage not lower than the hydrogen evolution potential, (5) the method of electrochemical treatment of prestressed concrete according to (4) wherein a duration of adjusting the voltage to less than the hydrogen evolution potential during electrification is at least one day, and (6) the method of electrochemical treatment of prestress
Ashida Masanobu
Ishibashi Kouichi
Denki Kagaku Kogyo Kabushiki Kaisha
Phasge Arun S,.
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