Corrosion of embedded steel in reinforced concrete (RC) is a world-wide problem, that reduces structural performance and lifespan. Chloride attack may be a result of seawater, de-icing salts or contaminated admixtures, brought on by ingress of chlorides into the concrete.
Electrochemical Chloride Extraction (ECE) is a non-destructive treatment for contaminated RC structures, that due to uncertainty of treatment times and applied current densities, is only 50% effective. It is often diesel powered has an environmental impact and often very costly due to the long treatment times.
To improve the efficiency of ECE the influences of concrete resistance, cement type and duration of treatment have been investigated in an experimental programme.
The use of Photovoltaic (PV) panels to improve the efficiency of ECE is presented which replace fossil fuels as a power source enabling a more environmentally sustainable treatment. These findings will increase the life span of vital infrastructure and reduce expensive ongoing repairs with decreased traffic congestion and inconveniences associated with bridge repairs.
How Electrochemical Chloride Extraction Works:
A titanium mesh, submerged in an electrolyte, is used to create an anode at the concrete surface and connected to the positive terminal of the DC supply. The embedded steel reinforcement is then made cathodic by connecting it to the negative terminal of the DC supply. This arrangement is shown in Figure 2.
Chlorides exist in the form of free, chemically bound and physically absorbed ions. The free chloride ions exist in the pore water solution of the concrete from de-icing salts or from sea-water. Under the effect of an electric field, adsorbed chlorides are released, which leads to an increase in free chloride concentration in the pore solution.
Due to ECE treatment, the free chlorides are removed quickly. When the current is switched off, the dissolution of chemically bound chlorides leads to re-establishing of the equilibrium between chemically bound and free chlorides.
The photovoltaic effect was first noted in 1839, when Becquerel observed that “electrical currents arose from certain light induced chemical reactions”. Later on in 1905, Einstein described the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel prize in physics.
In order to investigate the feasibility of using a PV panel to power ECE, the system must be able to provide a steady voltage and include battery storage for night-time use.
By measuring the surface and internal resistivity of concrete, one may gain an insight into the electrical requirements of ECE and optimised for a more efficient process. The resistivity of concrete increases rapidly during the first 20 days of moist storage, but after 30 days it becomes almost constant. Since conduction can be regarded as electrolytic in nature, the initial increase in resistivity is probably due to the continued hydration of the concrete.
The experimental work focused on attaining a resistivity value of concrete used in previous works. The resistivity was of particular interest due to its variability in different cement types. In order to measure the resistance of the concrete, a concrete slab (245mm wide x 245mm deep x 100mm thick) was cast along with 6 cubes for compressive strength testing at 7 and 28 days.
The mix was designed for compressive strength of 35MPa using CEMI with a w/c of 0.5. The moisture content of the aggregate and sand was measured prior to casting in order to achieve the desired w/c ratio.
ECE Treatment in Ottawa:
Work carried out by Bennett and Fong explored the trial application of ECE on a bridge deck in Ottawa. The deck had a surface area of 300m 2, of which half was treated and half left as a control. The start up current for both North and South spans was 48A with a total treatment area of 68m2.
ECE Treatment in Virginia:
A field trial carried out by Clemeña and Jackson on a highway overpass in Virginia used high currents for ECE treatment. With an area of 174m 2 , the start-up current for ECE
was around 160A, equating to around 4000A-hr.
Cost of Diesel Generator:
The cost of a diesel generator for an 8-wek period ranges from around €10,000 to €15,000. For the field trial carried out by Bennett and Fong, the fuel consumption was 6000 litres of diesel equating to €6000.
APPLICATION OF PV TO AN EXISTING STRUCTURE
Photovoltaic technology is always being improved with new efficient panels being brought to the market constantly. During the period of writing this paper, the most efficient commercially-available panel was made by Panasonic with an output at low (200W/m2) and normal (800W/m2) irradiance levels .
The following conclusions have been drawn:
- ECE treatment on large areas of RC structures requires a very high current. This prevents PV from being employed to power ECE in large areas.
- Localising the treatment reduces the area by focusing on key structural elements enabling ECE to be powered solely using photovoltaics.
- Measuring the resistivity of concrete tailors the PV system for specific requirements.
- RETScreen 4 together with simplified design methodologies are effective tools for preliminary sizing of PV arrays.
Source: Dublin Institute of Technology
Authors: Sean Bond | Lynette O’Callaghan | Niall Holmes | Brian Norton