Concrete solutions to plant ageing26 September 2018
With a lifetime of 60 years or more, nuclear power plant concrete structures are subject to ageing and degradation. Mark Lemon outlines some typical refurbishment solutions.
THERE ARE A HUGE NUMBER of reinforced concrete structures in nuclear power stations, from containment structures to cooling towers and water intake structures, as well as concrete pipes, masonry and underground structures. Effective inspection, repair and protection of these structures is required to ensure they are fit for service throughout plant service life and during decommissioning.
In untreated concrete, carbonation is nearly impossible to avoid when there is exposure to the elements. In this process, carbon dioxide slowly penetrates the surface and reacts with the moisture and calcium hydroxide within the concrete. It forms calcium carbonate, which initially hardens the concrete raising the compressive strength, but over time in the alkaline environment it starts to break down. Depending on the permeability, concrete carbonation may advance at a rate of 1–5mm per year. As carbon dioxide travels deeper into the concrete it reaches the steel reinforcement. As the concrete around the steel is broken down by carbonation the corrosive effects of air and water rust the steel, causing it to expand, spall and crack. This causes failures in the surrounding area.
Some deterioration of concrete structures can be attributed to original design errors, inadequate structural design and lack of attention to detail. Construction defects such as voids due to inadequate compaction or the inaccurate positioning of joints or rebar can also degrade concrete structures.
In power stations there are numerous other factors which affect the ageing, carbonation or degradation of concrete. They are often located near the coast, to make use of seawater for cooling, so chloride ion-induced corrosion is a common threat, as is the threat of freeze-thaw cycles, exposure to moisture and thermal exposure. There are specific threats from the power station processes, in particular chemical attack and irradiation.
Inspection and testing
The distress and degradation of concrete can manifest itself in numerous ways including spalling, cracking, voids, delaminations and exposure of rebar, rust and exposure of aggregate. Often, evaluating concrete structures and materials requires a combination of test methods, since no single technique will detect all potential degradation factors. The most common methods include visual inspections; removal, testing and analysis of material; or a combination these.
Following condition assessment, a refurbishment solution can be specified. Typically cementitious or epoxy mortars are used to rebuild the concrete surface.
A range of different coating products are available, depending upon substrate compatibility, life span and the film thickness required to provide the necessary cover.
In 2017, CSC Services completed a concrete spalling remediation programme at EDF Energy’s Hinkley Point B nuclear power station in Somerset, UK. Over 10t of specialist repair mortars and structural strengthening products were used to rectify the degradation of reinforced concrete at various areas across the station, including the flask bay and turbine hall.
This included use of a waterborne, cementitious modified polymer coating. A 2mm coating is equivalent to 100mm of good quality concrete cover, and provides a complete barrier to water under 10 bar pressure. Being cement- based, it became integral to the substrate as a result of a chemical reaction and it will have a design life equivalent to that of the concrete to which it is applied. This will protect the structures from moisture intrusion and further corrosion, reducing the need for ongoing maintenance work for the often extended design life of the station.
A similar project at Dungeness B used a cementitious coating system. A surface water drainage chamber required refurbishment. Although intact, there were voids in the mortar and cracking around pipe penetrations. The defects had to be repaired to ensure environmental compliance was maintained. A protective lining was required that would tolerate the heat from the water being discharged.
CSC Services repaired the chamber and lined it with Cemprotec E942 from Flexcrete. This water-based cementitious system gives a hard, durable coating with excellent resistance to water, chloride ions, oxygen and aggressive chemicals.
Cathodic protection systems can be installed to help prevent or mitigate corrosion of concrete rebar, other concrete reinforcement or concrete pipe. Systems can use sacrificial anode cathodic protection, impressed current cathodic protection, or both.
CSC Services installed a cathodic protection system for EDF Energy at Hartlepool power station in July 2016 to protect the corroded drumscreen chamber 3. A further system was installed at drumscreen chamber 1 in January 2018 for EDF Energy’s contract partner Cape.
Carbonation and chloride ingress of the reinforced concrete structure had been caused by sea salt passing through the drumscreen as part of the cooling process. A cathodic protection system was specified, to address corrosion across the entire area treated. This could not have been achieved with conventional repair methods without removing all the concrete where salt or carbon dioxide had penetrated.
Following the hydro-demolition of spalled concrete, a site inspection quantified the structural reinforcement replacement bar and anode requirements, 580 sacrificial anodes were installed and several hundred metres of structural reinforcement replacement bar in a range of dimensions fitted. Over 22,000kg of concrete was spray- applied to return the structure’s surface to profile. Access ladders, platforms and framework were then replaced.
This work was undertaken during the statutory outage at the station in under 3000 person hours, which was a significant achievement. CSC Services was able to manage and successfully complete all stages of the work.
Rebar protection and strengthening
Protective coatings can also be applied to steel reinforcement before the application of relevant concrete repair mortars. CSC Services recently reinstated a structural beam within the reactor building at Hartlepool, installing a steel reinforcement protection system to the rebar.
Flexcrete’s Steel Reinforcement Protector 841 is a corrosion preventative, flexible coating. It forms a highly alkaline coating, with a degree of elasticity which not only protects the steel from aggressive acidic gases, moisture and chlorides, but prevents further rust formation with a passive chemical coating on the steel surface.
A high strength structural repair mortar (Flexcrete’s Monomix HD) was then used to return the concrete back to initial profile. Flexcrete’s Monolevel FC fairing coat was used to re-profile the concrete, filling minor blow holes and defects prior to overcoating.
To provide structural strength, carbon fibre plates were installed to the underside of the concrete beam. Improving the durability of the structure, Sika CarboDur Ecarbon fibre plates will improve fatigue resistance and reduce stress in the steel reinforcement, preventing further cracking.
To complete the reinforcing system, SikaWrap – 301C carbon fibre wrap was used. Wrapped around the beam in a series of overlapping layers, the wrap will further strengthen the reinforced concrete. The impervious wrap material will also protect structures from moisture intrusion and further corrosion, reducing the need for future maintenance work.
Author information: Mark Lemon, Managing Director, CSC Services