Swedish companies have been directing their R&D efforts into the repair and maintenance of dams and other concrete structures
ADEREGULATED energy market and an increased number of actors have increased the pressure on energy prices and lowered economic margins in Sweden. Political and environmental factors have also limited prospects for further expansion of hydro power projects.
The average age of the country’s existing hydro power infrastructure is 45 years, and replacing these would cost in the region of US$14.6B. A great part of this is capital connected to buildings and concrete structures and the cost-effective use of existing structures is a very important issue. Sufficient service life and minimised running costs must be aimed for.
With this in mind the repair and maintenance of concrete structures should be carried out using the correct method; at the right point in time, to the correct quality and with optimal
In order to carry out effective repairs and maintain the condition of a concrete structure it is necessary to define the actual status of the structure.
Knowledge of ongoing degradation and degradation processes are essential. Therefore, it is necessary to devise appropriate methods to collect this information.
Swedish utility Vattenfall Utveckling has devoted a substantial amount of research and development into concrete repair methods and ways in which to optimise the maintenance management of dams and hydro plants.
The research programme has mainly been financed by Elforsk (an R&D company sponsored by all the major hydro power companies in Sweden) as well as other organisations who have experienced similar problems with concrete structures.
One of the key-projects has been the development of a manual for maintenance management which can be used as a model for all ongoing research projects. The manual has specifically focused on concrete dams and highlights research needs. It is divided into the six following processes as shown in the figure above.
• Requirements specifies requirements from authorities and other demands from power companies. Both specific technical demands and functional requirements for concrete dams have been collected.
• Inspection and condition control means basic maintenance or extended analysis. A continuous follow up of a dam is made easier when routine inspections are made with methods that give information of the functional status. A system with status-indices is developed to indicate the type and the seriousness of occurring damages.
• Optimisation of measures deals with methods like life-cycle-cost analysis and cost optimisation with probabilistic methods. The results from this process will be recommendations of what measures to take.
• Planning of measures presents a number of different repair methods and repair materials. It could also be measures to minimise the load or slowing down the process that is causing damages. The results from this process will be tender documents or action plans.
• Accomplishment is most commonly performed by a contractor that is trying to fulfil the demands from the fourth process.
• Feedback contains advice on how a measure or repair should be followed up.
To collect information about the results obtained from different type of repairs a database has been created. It can be used to follow up repairs and to find examples of suitable measures to be taken for different types of damage. From the database it can be seen that the most frequently occurring damage on Swedish concrete dams is similar to that experienced in the US.
An important tool in preliminary investigations is the collection of concrete samples by drilling out cores. The importance of how the cores are drilled and handled after drilling is essential to draw the right conclusions about the status of the dam. Erroneous drilling can induce cracks and lead to incorrect conclusions about cracks inside the dam. Cavities in a dam are indicated by a ‘loss of core’. The systematisation of how to mark the cores and any loss is therefore very important. Vattenfall Utveckling has developed guidelines and a software programme to minimise the number of mistakes.
An important question to answer when damages occur on dams is how severe the loads and stresses are to the dam and how they will affect the loadbearing capacity now and in the future.
A manual has been developed in an EC-project called Concrete Technology Vetting (CONTECVET). The guideline is aiming to define methods for how degradation will affect loadbearing capacity. In the manual, degradation due to alkali-silica reactions, reinforcement corrosion and frost action is taken into account.
Methods that can be used to define the status on existing structures without drilling cores will be more and more important in the future. A number of commercially available methods have been tested on 11 different types of damage found on concrete dams to evaluate their adaptiveness.
Ultra sonic pulse velocity (UPV) is one of the methods that upon evaluation seems to be very useful in the future.
Research focus over the past few decades has been in the construction of new infrastructure. The development of cost-effective repair methods for large structures like dams has been neglected. Repair and renewal of large concrete structures in the hydro power area give rise to specific requirements. Compatibility and full interaction between existing parts of the structures and the repair material is essential. Water-tightness and durability are also central parameters.
Generally Swedish dams are in a good state of repair but the most common type of damage found includes: leaching; frost damages due to one-sided water pressure; abrasion, erosion and cavitation due to flowing water; chemical degradation due to incorrect choice of materials; cracking due to restraints or static and dynamic loads; and scaling and delamination of concrete in the spillways.
The technique to pump grout in to cracks and cavities is suitable for repair of concrete structures, eg for leached concrete. Vattenfall Utveckling has previously investigated the rheology of cement-based grouts. This is of major importance to achieve maximum penetrability in cracks or leached areas in concrete.
Shotcrete is commonly used for strengthening or upgrading damaged concrete structures. Experience has shown a generally good quality of shotcrete repairs. Degradation due to steel fibre corrosion in cracks has also been investigated. Two and a half years of field exposures with cracked shotcrete containing steel fibres show very small amounts of corrosion in the river environment. The influence of the crack width and anode to the cathode ratio seems important. Other research has been focusing on the influence of rheological properties of the fresh shotcrete and understanding of bond mechanisms. All the work is being compiled in a Swedish guideline for shotcrete repairs.
Patch and surface repair mortars
A common measure to repair surface damages is to use different types of mortars. To make it easier to choose between the different types (cement, epoxy, polymer modified mortars etc.) a test programme has been developed. The focus has been test-methods for common properties needed on concrete dams. The guidelines also give suggestions on suitable requirements for different parts of a dam structure. A demonstration of the programme has also been performed consisting of parameters like compressive strength, bond strength, water-tightness, frost resistance in bond zone etc.
The technique can be used to repair dams on the upstream surface without using caissons. New developments on anti-washout admixtures have largely improved the results on underwater concreting. One critical parameter is the influence on the durability against frost action. Conventional scaling test show a durable behaviour but tests on internal degradation show extensive damages. Current work at Vattenfall Utveckling is aiming to develop concepts for achieving durable underwater concrete.
The reaction between cement and water give rise to elevated temperatures in the concrete. In structures with large dimensions or in structures with large restraints from surrounding concrete this can give rise to thermal cracking. Today it is possible to simulate the heat of hydration and the following heat development and also the most mechanical properties in the concrete.
In co-operation with Lund University of Technology, Vattenfall has developed the commercially available FEM-program HACON. With HACON it is possible to estimate the risk for cracks and to evaluate different measures to be taken to prevent thermal cracking.
Utilisation of residuals in concrete repairs
In the development of new repair methods one concept is to combine this with utilisation of residuals. One of the projects has been to use residuals from the steel industry. After rolling of steel billets at Ovako Steel, Sweden, (a member of the SKF Group) a so-called hot scarfer is used to remove surface defects from the steel billets. The residue from the hot scarfer is called SHV, mainly consisting of iron oxide and has a spherical shape (grain size 0-2 mm).
The rounded shape of the particles in SHV should bring a better flowability (rheology) to the concrete. Earlier tests which have added SHV to conventional concrete have shown that SHV has a positive effect on workability. Similar tests with SHV in self-compacting concrete have also shown promising results.
Another possible application for utilisation of SHV is underwater concrete where no vibration or extra energy should be needed during pouring. Tests have been performed to find out the possibility to combine the positive properties of SHV with a mortar with aluminate cement and coarse synthetic aggregates.
The tests show that the use of SHV improve the workability significantly. Another result is that there is a decrease in compressive strength. If higher compressive strength is demanded the mix composition has to be compensated by increased cement content or lowered w/c-ratio. The freeze-thaw durability is acceptable for an exposed ‘mould’ surface. If the strength is increased it will also result in lowered permeability and therefore probably increased freeze-thaw durability. It can also be seen that the resistance to wear from running water at high pressures seems very good.