ELECTROCHEMICAL TREATMENT OF REINFORCED CONCRETE

Abstract: A system for electrochemical treatment of reinforced concrete comprises a plurality of power supply units (1a, 1b, 1c, 1d, 1e) for delivering electrical power to the concrete. Each power supply unit is connected, through a multiplexer (2), to a computer (3) programmed to control the setting up of the system and its operation. Each power supply unit has a microprocessor which includes non-volatile memory in which is stored the history of output current and anode/cathode potential and which is accessed at intervals by the computer (3).

Description: Title: Electrochemical Treatment of Reinforced Concrete Field of the Invention This invention relates to the electrochemical treatment of reinforced concrete by real alisation or desalination. The invention is not concerned with cathodic protection. Background to the Invention The most common cause of deterioration of reinforced concrete is corrosion of the reinforcing steel bars. During corrosion iron oxides are formed which occupy a greater volume than the constituent steel. The pressure caused by the expansion leads to spalling (cracking) of the concrete surrounding the corroded reinforcing bar. The reinforcing steel bars are normally passivated, chemically inert, when embedded in concrete. This is due to the alkalinity of the pore water within the concrete which allows a passivated layer of iron oxide to exist on the steel surface. This protects the steel from chemical attack. There are two main mechanisms by which this passivity can be destroyed, carbonation and chloride attack. In carbonation, carbon dioxide from the atmosphere is absorbed and dissolved in the concretes pore water to form carbonic acid. This acid reacts with the alkaline calcium hydroxide present in the concrete, producing calcium carbonate and water. As the calcium hydroxide is consumed the alkalinity of the concrete drops. This in turn causes the passivating layer of iron oxide to break down and the now unprotected steel corrodes due to the presence of oxygen and water within the concrete pores. Realkalisation is highly effective in the treatment of reinforced concrete which has been damaged by the natural process of carbonation. Typically, an anode grid is applied to the external surface of the concrete which is then encapsulated with a cellulose fibre hydrated with a sodium carbonate electrolyte. A series of electrical connections is made to the steel reinforcement, which acts as a cathode and in this way positively charged sodium ions are attracted towards the steel reinforcement, and pH level is restored. The presence of chlorides in the concrete is the other main cause of corrosion. The chloride ions attack the passivating layer of iron oxide, despite the alkaline environment and help break it down. This occurs when the chloride concentration exceeds 0.6 of the hydroxyl concentration. Furthermore, the chloride ions are not consumed but only act as catalysts in speeding up corrosion. Desalination is used to restore concrete where excessive choloride salts have contaminated the concrete either at the construction stage or by later ingress. The process closely resembles realkalisation in reverse but with water as the electrolyte. A potential is applied across the electrical connection, attracting the negatively charged chloride ions to migrate towards the external anode. One popular form of implementing electrochemical treatment is to use a temporary anode made of titanium mesh for chloride removal in desalination or steel mesh for realkalisation. The anode system consists of papier mache applied as a layer before and after securing the anode mesh to the concrete surface. This is kept wet (with water) for the duration of the treatment. A d.c. power supply is used to create the necessary electrical conditions. Its positive terminal is connected to the temporary anode mesh, whilst its ground terminal is connected to the reinforcing steel bars. Anode and cathode connections are made at regular intervals across the concrete surface to ensure even current densities and care must be taken in ensuring electrical continuity between the anode and cathode. In particular, short and open circuits must be avoided and each anode/cathode connection must be adjusted for similar resistance. The absolute end point of electrochemical treatment is determined by chemical analysis of the concrete. However, a good indication can be obtained by monitoring the electrical parameters of the concrete. For example, in realkalisation the resistance of the concrete falls as the process progresses. A constant resistance condition is reached which is characteristic of realkalisation completion. In desalination, the degree of chloride removal is related to the amount of electrical charge passed through the concrete, among other things. As presently implemented, electrochemical treatment requires skilled operators to set up the correct electrical conditions and de-bug the system if necessary. Furthermore, manual monitoring of both the electrical parameters of the process and its physical set-up is necessary to ensure smooth progress of the process. The aim of the present invention is two-fold. Firstly, to de-skill the setting up of the electrical conditions necessary for electrochemical treatment, and secondly, to automate the monitoring and measurement of the salient electrical parameters during treatment. Disclosure of the Invention According to the invention there is provided a system for electrochemical treatment of concrete, comprising at least one power supply unit for delivering electrical power to the concrete, the or each power supply unit having a microprocessor controlling the electrical power delivered, and control means linked to the or each power supply unit for controlling the corresponding microprocessor and thereby controlling the application of electrical power from the or each power supply unit to the concrete. Preferably, each microprocessor stores information concerning the power delivered by the corresponding power supply unit, and the control means accesses the stored information to obtain a record of the power delivered as a function of time. The control means may be a computer programmed to control the power supply units, and the computer preferably communicates with each power supply unit by means of a hard wired link, although radio links may be possible. In a preferred embodiment, the computer is connected to a communications multiplexer which in turn is connected to the power supply units in a daisy chain or star configuration. Each power supply unit has an output connected by a plurality of resistors in parallel to respective anode connections, the voltage drop across each resistor being measured to enable the current flowing to each anode connection to be derived. A system according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the system, Figure 2 is a circuit diagram of one of the power supply units illustrated in Figure 1, Figure 3 shows parallel resistors connected to the output of each power supply unit, and Figure 4 shows diagrammatically how a microprocessor of each power supply unit performs its functions. Referring to Figure 1, the system effects treatment by realkalisation and comprises a plurality (in the example illustrated, five) d.c. power control units la to 1e connected in a star configuration (as 1a, 1b, 1c) or in a daisy chain configuration (as 1c, 1d, 1e) to a communications multiplexer 2. The multiplexer 2 is in turn connected to a computer 3, either directly or via a modem for long distance remote control. Each power control unit has anode and cathode outlet connections, the anode connections being connected to an anode net 4 placed against the concrete to be treated, and the cathode connections being made to the steel reinforcing bar in the concrete. An irrigation unit 5 is supplied with electrolyte, in this case sodium carbonate, which irrigates the anode net 4 and is supplied thereto by a pump 6. Calcium carbonate can be used as the electrolyte. The pump 6 is fed from a reservoir 7 of electrolyte and is controlled from the computer 3 through the multiplexer 2. The reservoir may incorporate means for detecting chloride level, in order to monitor treatment. Referring to Figure 2, each power supply unit is powered from a single phase 240v, 50Hz mains supply 8. The supply is connected to the primary of a transformer 9 having a centre-tapped secondary feeding two thyristors 10 which modulate and rectify the input signal. Smoothing filters 13 remove the unwanted a.c. component from the output. The output current is maintained by a Hall effect sensor 14 which, through a feedback loop 15, controls the firing angle of the thyristors 10 to maintain the output current (from output 16) at a preset level, preferably 1 ampere per square metre of anode. This level can be set manually, by a current set knob 17, and the output voltage may similarly be set manually by a voltage set knob 18. However, output current and voltage are preferably controlled automatically, the power supply unit having a microprocessor which is controlled by the computer 3. A subsidiary circuit 19, powered from the transformer 9, has voltage outputs which are used to ensure that the datum voltage of the power supply unit is at the same potential as the concrete being treated. To achieve the purpose of this invention the following extra circuit features are incorporated. a) The output from each power supply unit is split into parallel paths each of which incorporates a series resistor 20 as shown in Figure 3. Each path forms a separate anode connection. The voltage drop across each resistor 20 can be measured and hence the current flowing in each anode connection determined. This enables short or open circuit conditions to be identified. b) The current set circuitry which is manually adjusted by knob 17 is supplemented or replaced by a digital to analogue converter which allows for digital programming . c) The microprocessor in each power supply unit sets the D to A converter, determines the anode currents and anode/cathode potential and stores this data in non volatile memory at regular intervals. The microprocessor accepts instructions from the computer 3 on the set-up conditions, via the communications multiplexer 2, and transfers measurement data to the computer 3 upon request. Lastly, it controls the power supply display and keyboard (Figure 4) should manual operation be desired. The links between the microprocessor and the above functions are shown in Figure 4. The communications multiplexer 2 is a unit which links the serial port of the computer 3 to one or more power supply units 1a to 1e. It is possible to provide for communication over a bus such as GPIB. This however limits both the number of power supply units in a system and also the distance between the computer and the power supply units to either 1 power supply unit at a distance of 15 metres or 15 power supply units separated by 1 meter cables . Normally, a computer can only interface with one unit over its serial port. The port connections include a transmit connection, receive connection, ground connection and control connections. To overcome the problem of single unit interface, the communications multiplexer has one port compatible with the computers serial link. Its other port connects to the multiplicity of power supply units in a star or daisy chain fashion. The power supply units only have two connections, namely a ground connection and a transmit/receive connection. When the computer is transmitting information all power supply units receive it but only the one addressed acts upon it. When the computer requests information, the addressed power supply unit provides it. All communication is serial in time, the computer never transmits and receives simultaneously. This communication system is low cost and does not limit either the number of power supply units or the distance between the power supply units and the computer. The computer 3 is programmed with expert software. The expert software can reside on any computer with a serial port which can be connected directly or via a MODEM to the communications multiplexer. It performs the following functions. a) Guides the operator through the set-up process and from the electrical parameter measurements directs the debugging process. b) Sets up the power supply units and adjusts their settings as necessary. c) Monitors the electrical parameter changes during the process and instructs the operator if unexpected behaviour occurs. d) Controls automatic electrolyte application system if present or instructs operator to apply electrolyte when needed. e) Instructs operator of process completion and generates process reports. As realkalisation proceeds, sodium ions dissociate and are attracted towards the cathode. Soon after the system is powered up, there is a drop in resistance between the anode and the cathode. Simultaneously, hydroxyl ions are formed at the cathode which further increases the alkalinity in the vicinity of the steel reinforcement. Because the pH has been raised to a level which prevents the formation of corrosion produced, the further decay is arrested. Long term durability is achieved due to the alkalinity of the electrolyte in the cover concrete. A system according to the invention may effect treatment by desalination. For desalination, the system would be similar to that of Figure 1, but the electrolyte would be water instead of sodium (or calcium) carbonate. For example, to desalinate a concrete mullion (with internal steel reinforcement) of a concrete building by the invention, polycarbonate panels are mounted in spaced relationship to the exposed mullion surfaces to define a tank (or shuttered enclosure) around the exposed surfaces of the mullion. An anode mesh is suspended within the tank and the tank is filled with water to form the electrolyte. Units identical to units 1a to 1e, but now serving as desalination units instead of realkalisation units, are connected to the anode. The steel reinforcement becomes the cathode. The units are linked to a multiplexer and remote computer in a manner similar to Figure 1. The system ensures that a substantially uniform current density of 1 ampere per" square metre is applied to the anode. As treatment proceeds, the chloride ions are attracted towards the positively charged anode and the resistance between the anode and the cathode drops. The chloride ions pass into the water which is charged on a regular basis. Because the chloride ion concentrations at the steel reinforcement in the concrete is reduced (by in excess of 95%) the alkalinity increases around the steel reinforcement and a durable repair is achieved. The invention enables the setting up and operation of electrochemical treatment of reinforced concrete to be accurately controlled and monitored. Individual currents and voltages applied to the concrete are not only accurately controlled, in accordance with a predetermined program in the computer, but the applied currents, voltages and Ampere/hours are also stored, so that the history of the treatment is preserved and can be used to modify ongoing treatment, as necessary, and to terminate treatment automatically. Isolated locations can be treated reliably and simultaneously. After realkalisation or desalination by the invention, the components of the system are removed, leaving a durable repair. It is also a feature of an expert management system according to the invention that it allows the current supply to the anode to be phased, in that selection is possible for periods of rest and predetermined intervals during both realkalisation and desalination treatment time. This facility allows the migration of produced hydroxyl ions away from the reinforcement and, therefore, reduces the very high pH established during prolonged application of the system. Furthermore, another facility is available to calculate the amount of hydroxyl ions generated during the treatment period based on the physical properties of the concrete, the nature capability to predict the diffusion of generated ions away from the steel reinforcement after treatment based on Fick's 2nd law of diffusion. Because of the nature of the expert system, it is possible to have a virtually infinite selection of application of power and rest period during any particular treatment.

Claims: Claims 1. A system for electrochemical treatment of reinforced concrete, comprising at least one power supply unit for delivering electrical power to the concrete, the or each power supply unit having a microprocessor controlling the electrical power delivered, and control means linked to the or each power supply unit for controlling the corresponding microprocessor and thereby controlling the application of electrical power from the or each power supply unit to the concrete. 2. A system according to claim 1, wherein each microprocessor stores information concerning the power delivered by the corresponding power supply unit, and the control means accesses the stored information to obtain a record of the power delivered as a function of time. 3. A system according to claim 1 or 2, wherein the control means is a computer programmed to control the power supply units. 4. A system according to claim 3, wherein the computer communicates with each power supply unit by means of hard wiring. 5. A system according to claim 3 or 4, wherein the computer is connected to a communications multiplexer which in turn is connected to the power supply units in a daisy chain or star configuration. 6. A system according to any of the preceding claims. wherein each power supply unit has an output connected by a plurality of resistors in parallel to respective anode connections, the voltage drop across each resistor being measured to enable the current passing to each anode connection to be derived. 7. A system according to claim 6, wherein the output current of each power supply unit is settable electronically by means of the microprocessor under the control of the control means. 8. A system according to claims 2 and 7, wherein the microprocessor includes non-volatile memory in which is stored at time intervals the output current and the anode/cathode potential. 9. A system according to any of the preceding claims, wherein each power supply unit includes a Hall effect sensor for determining the output current of the power supply unit. 10. A system according to claim 9, wherein the output of the Hall effect sensor is applied to a feedback loop which acts to maintain the output current of the corresponding power supply unit at a preset level, or within preset limits. 11. A system according to claim 10, wherein the output current is varied by thyristors the firing angle of which is controlled in the feedback loop by the output of the Hall effect sensor. 12. A system according to any of the preceding claims, wherein each power supply unit is powered by single phase mains supply, namely 240 volts a.c. at 50Hz in the UK. 13. A system according to any of the preceding claims and including a pump for pumping electrolyte over an anode net to which the outputs of the power supply units are connected. 14. A system according to claim 13, wherein the pump is controlled by the control means, so that supply of electrolyte is coordinated with control of the power supply units.

Inventors: HAMMOND ANDREW DUNCAN, HAMMOND, ANDREW, DUNCAN
Applicants: HAMMOND ANDREW DUNCAN, MAKERS IND LIMITED, MAKERS INDUSTRIAL LIMITED
Application year: 1993
Main classification: C04B41
IPC codes: C04B41, C23F13, E04G23