This is a case study to show how Concrete Preservation Technologies (CPT) new generation of cathodic protection products can significantly increase the lifetime of a reinforced concrete corrosion affected structure. It also demonstrates how the testing undertaking by CPT helped to formulate a cost effective targeted corrosion prevention solution for the Interchange Bridges.
Background
Hertfordshire County Council, for 20 years, has been a leading authority in Highways Asset Management. Hertfordshire have recently been rolling out asset management principles for the management of their bridge stock.
ISO 55000 defines Asset Management as the "coordinated activity of an organization to realize value from assets" in practice for bridges this means strategic targeted interventions to tackle deterioration and protect the future life of the bridge stock. Hertfordshire have been working with Geoffrey Osborne to deliver a 4 year specialist framework dedicated to bridge asset management.
Rush Green is a busy interchange for the meeting of the A414 and A10 at Ware in Hertfordshire and as such is a critical structure on the county highways network. The bridges are in an environment where corrosive chlorides form salt spray from the road is inevitable. Hertfordshire were looking to extend the life of the two interchange bridges that had been significantly affected by corrosion to the steel reinforcing. The four, cast in situ abutment walls supporting the elevated roundabout had been identified as exhibiting signs of chloride induced corrosion with staining and spalling of the concrete.
Mechanics of Corrosion
When steel corrodes it expands and exerts a tensile force resulting in delamination and spalling of the concrete cover, in effect an early warning that should be heeded. One of the most common causes of steel reinforcement corrosion on bridges is from chlorides contained in de-icing salts, which typically run onto the substructure due to blocked drainage systems or leaking deck joints. The chlorides are deposited on to the surface of the structure and find their way through the pores and/or defects (such as cracks) in the concrete cover to the steel reinforcing.
The acidic agents destroy the naturally occurring protective oxide passive layer on the reinforcing steel bars (known as 'rebar') generated by the high alkalinity of the surrounding concrete material. Steel corrosion is an electrochemical reaction. Once the protective passive layer is compromised a corrosion cell forms on the steel surface (allowing ions to flow from anode and cathode). Once this happens, the electrochemistry takes over and corrosion propagates. If left unchecked, complete loss of steel section will result.
Carbon dioxide (carbonation) is another acidic corrosion agent. Found naturally in the atmosphere it reacts with water to form carbonic acid which neutralises the protective alkaline compounds in the concrete. Coupled with chloride contamination it can exacerbate the corrosion attack by freeing up bound chlorides in the concrete - acting as sort of a lubricant in the process.
CPT Testing Services – Pinpointing the problem
After initial meetings with Hertfordshire, CPT were commissioned in conjunction with Geoffrey Osbourne Ltd to undertake a full corrosion survey of the structure involving visual assessment, hammer tap, half-cell potential mapping, chloride profiling and carbonation depth analysis. An iterative approach is adopted by CPT in the testing process to accurately establish the location extent and source of the corrosion damage. This allows meaningful interpretation of the test data for the client along with recommendations for remediation options.
Osborne, as Principal Contractor, planned, organised and coordinated the access facilities including traffic management to facilitate the smooth operation of the testing process. An initial visual survey of the four abutment walls was carried out both from ground level and from a Mobile Elevated Work Platform (MEWP). The objective was to identify visual defects such as spalling, cracks, rust/corrosion stains etc. and to record the positions on the elevation drawings
Measuring the steel corrosion potential gives an indication of the corrosion environment around the steel. The half-cell potential testing technique measures the potential of the steel by replicating two halves of a battery (see diagram above). One half is the measuring electrode cell and the other is the steel – the concrete material acting as the electrolyte. The electrode is moved along the surface and measured at given intervals - a more negative reading (using conventional procedure) would be indicative of more corrosive environment. The measured steel potentials are plotted to produce a potential map of areas that are either actively corroding, at risk from corrosion or not corroding.
To corroborate the findings of the half-cell testing Chloride Profiling was undertaken. Samples at three incremental depths were taken at 5-25mm 25 – 50mm and 50 to 75mm and sent to the laboratory for testing. It is important to know how far the chlorides have ingressed into the structure as the chlorides ions are an aggressive corrosion agent. To make matters worse chloride induced corrosion is a catalytic reaction, so the chlorides are not consumed in the corrosion process and remain at the steel - unfortunately in this scenario it is the steel rebar that is being consumed!.
A cover depth survey was carried out in accordance with BS 1881 part 204:1986. The electromagnetic cover-meter measures the depth of the steel rebar and allows areas of low cover to be identified which are more susceptible to ingress of corrosive agents.
Survey results
The testing uncovered significant variability in the degree and extent of corrosion over the four structures tested. All the abutments were wetted in some degree by chloride contaminated water (de-icing salts) leaking from deck joints and running off the bearing shelves. It was also clear that water had been splashed onto the walls by passing vehicles and had wicked up from the ground at the wall base. Wetting and drying had led to the concentration of chlorides at the concrete surface leading to diffusion through to concrete cover to the steel.
From the half-cell potential results a potential map was produced which showed that the most of the half-cell values more negative than -300mv were in the top and bottom areas of the abutments. This meant that a targeted corrosion protection approach would be a cost effective option for prolonging the life of the structures.
Solution – low cost long term protection
Hertfordshire wanted to achieve a 25 year minimum lifetime of protection for Rush Green. Due to the presence of chloride salts and the damp/wet conditions in the areas where concrete repairs would be carried out (bearing shelf and wall base) it is likely that the repairs would suffer from incipient anode corrosion whereby reinforcement adjacent to the repair begins to corrode (Broomfield – Corrosion of Steel in Concrete - 1997). This is likely to lead to further cracking delamination and spalling and the requirements for further repair.
As mentioned previously the testing results indicated that protection would only be required for the top and bottom sections of the four abutment walls. After consulting with CPT a targeted cathodic protection design was undertaken by the Hertfordshire's 'in-house' engineering design partner Opus Arup. The design was to ISO standard 12696: 2012. The design incorporated CPT DuoGuard Hybrid anodes which are ideal for use as part of a targeted corrosion prevention system to halt ongoing corrosion and prevent future damage to the structure.
DuoGuard (Hybrid Anode)
The hybrid (two stage) anodes were installed into the contaminated areas of concrete and connected in four zonal areas to a power supply. In the first phase an initial charge was delivered using a temporary power source, which draws the chloride ions to the anode and produces passivating alkaline hydroxides at the steel. In this environment the protective passive layer is reinstated on the steel and corrosion activity is halted.
Installation of the DuoGuard anode – the whole system is discrete.
The hybrid anode was then switched to the sacrificial anode mode where a galvanic current is passed to the steel to maintain the environment created during phase 1. In this phase the steel can be protected for up to 50 years with no power supply required. The anodes act like individual mini batteries, which are self-contained and only produce current when required thus conserving the anode material. Being totally responsive to the local environment around the steel, no supply power rectifiers are required to operate and regulate the system.
This allows savings of up to typically £10k per year in maintenance, software updates and consulting fees compared to traditional impressed current systems. Over 25 years this will save Hertfordshire potentially £250k in whole life care costs alone. Also the suitability of DuoGuard for the targeted cathodic protection approach adopted meant that installation costs at Rush Green would be significantly less than for a traditional impressed current scheme.
All elements of the installation were coordinated and organised by Geoffrey Osbourne Ltd working closely with CPT to ensure the programme met the allotted timeframes. As part of the works Osborne also replaced the worn expansion joints and renewed the surfacing over the bridges. Because Duoguard is targeted intervention utilised specifically on deteriorated areas, it is a fast technique to deploy. This allowed Osborne to programme the works to minimise disruption to the travelling public.
Monitoring - for peace of mind
CPT MN15 (Manganese Oxide) Reference Electrodes and data logging equipment have been installed to the four abutments to facilitate long term assessment of corrosion susceptible areas. This allows Hertfordshire Council to observe any changes in the condition of the steel reinforcement with real time access to corrosion data from day one. After the first impressed current phase (externally powered) the steel was measured as passive which indicated that enough alkaline hydroxides had been produced electrochemically to reinstate the protective passive layer back onto the steel. Now the steel was under control it meant the system could be switched to the galvanic mode for the rest of the 25 years plus treatment period.
All CPT's products are manufactured in the UK at our facility at Nottingham.
The Directors of CPT have produced over 100 technical publications in the field of corrosion and concrete protection. This includes paper 800024 published Nov 2008, in the "Proceedings of the Institution of Civil Engineers – Hybrid Corrosion protection of Chloride-contaminated Concrete".
For more information please contact; Martin Davison, CPT (Concrete Preservation Technologies Ltd) on 0115 9724238, email [email protected]
Unit 1, Palmer Business Court
Manor House Road
Long Eaton
Nottingham
NG1LR
United Kingdom
Geoffrey Osbourne Ltd
[email protected]
www.osborne.co.uk
