In our opinion, any investigation of attacked, eroded or weathered surfaces, where the principal aims are to formulate a solution involving a durable repair and subsequent protection, must involve:
- A detailed study of the environment of exposure, e.g. water, effluent and treatment process chemistry, with reference study of unaffected, protected or sheltered surfaces, so that the processes of degradation can be fully understood and factored into the design of the solution.
- A qualitative investigation of the affected surfaces, to assess the extents of penetration of any aggressive agents, beyond the obviously degraded surface zones.
- A quantitative investigation of the affected structures and surfaces, to assess the practicalities of undertaking the preparatory, repair and protective coatings procedures, together with an evaluation of the quantities involved (surface areas and depths).
The combination of an increasing population, climate change and associated water shortages, together with modern requirements for drinking water quality, require some Water Authorities to use more aggressive chemistry, to adjust the composition of at least some raw waters. Raw water abstracted from some rivers, reservoirs and boreholes may be contaminated with pollutants /dissolved solids and the chemical treatments or dosing both facilitate efficient processing, the quality of the final product and the supply of sufficient drinking water to satisfy demand.
However, in some cases, the water passing through treatment plants has been found to be aggressive towards both the concrete forming the plant, protective coatings applied to the concrete surfaces and the various metal sluices, pipes and other mechanical services.
In our experience some affected plants had experienced upwards of 25mm surface losses in less than 5years, with protective coatings, applied to protect against degradation of the concrete substrate softening in less than 1year and completely dissolving within 2years, despite being “resistant to low pH and chemical attack”.
In our opinion, affected works, including the water passing through them, from raw inlet to final supply, and every dosing stage in between and the affected concrete surfaces should be subjected to a detailed investigation. The concrete surfaces should be assessed, in detail, to quantify the surface areas affected, the depths of degradation and penetration of any aggressive agents into the ‘sound’ concrete behind.
The water passing through the plant, at every dosing stage, should be analysed for general composition, but focussing on constituents potentially deleterious to concrete and other plant infrastructure and assessed for ‘aggressiveness’.
From this repair methodologies, such as methods of cutting-back, and accurate quantities for both cutting-back and reinstatement can be evaluated. The extents of any reinforcement corrosion can also be evaluated, together with quantities of replacement reinforcement required.
Once the chemistry of the ‘system’ is understood and the aggressiveness of the water established suitable protective coating products and systems can be considered, although considering the cost implications of shutdowns and subsequent failures, trials of various coatings could be undertaken in-situ.
Essentially, the chemical / bacteriological degradation of effluents produces hydrogen sulphide and other gases, which would normally vent to atmosphere. However, when covered, for health, safety and odour / environmental control the gases condense on concrete surfaces above the effluents, with the condensate comprising various acids and other potentially deleterious agents. The cement matrix, binding the concrete and any vulnerable aggregate constituents, when exposed to acids, will degrade and dissolve, leaving the insoluble concrete residue nominally in-situ, unless there is a mechanism to ‘wash’ or ‘abrade’ the residue away. Once degradation or dissolution of the surfaces reaches the reinforcement it will also variously corrode and dissolve the steel. This can lead to significant section loss through walls and roofs, with the potential for structural failure, particularly when degradation occurs hidden within enclosed chambers and channels.
Although experience suggests that the rate of surface degradation outstrips the rates of penetration of potentially aggressive agents, into the ‘sound’ concrete behind, in our opinion, this should additionally be checked. Aggressive agents such as sulphate and chloride should be included, but a review of the composition of chemical wastes should be undertaken to ensure all possibilities are resolved.
An investigation, to assess, in detail, to quantify the surface areas affected, the depths of degradation and penetration of aggressive agents into the ‘sound’ concrete behind should be undertaken. From this repair methodologies, such as methods of cutting-back, and accurate quantities for both cutting-back and reinstatement can be evaluated. The extents of any reinforcement corrosion can also be evaluated, together with quantities of replacement reinforcement required.
Suitable protective coating products and systems can then be considered, although considering the cost implications of shutdowns and subsequent failures, trials of various coatings could be undertaken in-situ.
The degradation of concrete surfaces and protective coatings exposed to aggressive waters and atmospheres is a relatively recent phenomena, in terms of the understanding of the detailed processes involved and, more importantly, the formulation, application and durability of protective coatings with sufficient resistance to attack, within what must be considered as an extremely aggressive environment. We would therefore suggest that the Customer and all other parties involved need to understand that due to the aggressive nature of the environment of exposure, both physically and chemically, applied coatings will need regular monitoring and timely maintenance, to ensure durability of the system.
There have been coatings ‘failures’ (failures and perceived failures) in these environments and we would suggest that these must be taken into consideration as a part of the design process for remediation.
Repairing and protecting concrete surfaces that will be subjected to high levels of H2S and possibly also exposure to effluents with high / low pH / variable aggressiveness indices, is extremely difficult to achieve. Inlet works are particularly difficult because of the potential for high rates of flow and the abrasive nature of effluents.
The channels and chambers in inlet works are also particular tight which means that the usual methods for cutting back, washing down, surface preparation and the application of coatings, e.g. applying coatings by hot spray, are not practicable, with other methods potentially time consuming to ensure the quality required for success.
In order to cope with fluctuations in pH and chemical aggressiveness, we would normally recommend a Polyurea coating. However these have low adhesion to concrete (typically 0.5 MPa) which makes them unsuitable for high flow, abrasive, situations such as inlet works. On the other hand two pack epoxies have excellent bond /adhesion to concrete, although they may not be able to cope with high /low pH, or chemical aggressiveness. The latter are also rigid and any cracking in the substrate will be mirrored in the coating leading to a risk of failure.
Whatever coating is proposed we would expect to see at least some blistering, due to osmosis, which should not necessarily be considered as a defect. Such coatings can still work as effective barriers.
Whilst we are happy to offer advice on coatings based upon our experience, we are not prepared to accept design responsibility which we see as lying with a specialist Consultant. It is also worth pointing out that we can only apply coatings that are readily available in the market and whilst we will select the best available coatings, the Customer must accept that there is a risk of failure even when it has been installed 100% correctly, with extensive QA/QC records to prove it.