Durability of concrete and steel structures in marine environments and Desalination Plants:

1.0 CONCRETE/STEEL BEHAVIOUR TOWARDS MARINE ENVIRONMENTS:Marine environments have a substantial aggressive response to concrete and steel in its proximity. The behaviour of concrete structures has seen its main problem arising from steel reinforcement becoming corroded.

As it is understood, concrete is a complex composite material. Concrete under tension is substantially weak and in practice it is best to reinforce such structures with steel in order to improve its tensile properties. Concrete within marine environments are no different. In these environments, deterioration of concrete and its reinforcement are accelerated by chemical reactions that attack the concretes aggregate which results in corrosion damage to the reinforcing steel. With the aid of chloride, corrosion of steel reinforcement causes spalling (figure 1 and 2) within concrete itself is a unique problem seen regularly.

To understand why concrete corrodes, it is important to firstly note that there exists a significant amount of calcium hydroxide and small amounts of alkali rudiments (i.e. sodium or potassium) within concrete. These elements are responsible for giving concrete such a high alkalinity status. Alkalinity works in the favour of steel, as it helps the steel obtain a surface film. This can keep the steel passive and hence protect it from corrosion.

As mentioned by Reis (2011), concrete structures in marine environments are often exposed to salty spray or continual submersion in saltwater (figure 3). The chloride ions will slowly but steadily penetrate the concrete structure to where the protective layer in-houses the steel reinforcement. This causes steels protective alkali rudiments to become destroyed and in turn allow steel to corrode.

2.0 INITIAL PROCEDURES BEFORE CONSTRUCTIONS:Fortunately there are numerous ways in order to prevent the effects of corrosion. To implement such a system, a thorough understanding is needed before money is invested on such systems. With respect to this desalination project, it would be best to firstly:

Conduct a comprehensive on-site inspection to allow for the structure to be analysed in detail. This can be obtained from CAD drawings or any past inspection report.

Get an understanding for the environmental conditions which would include having current and tidal data on hand.

Conduct analysis on the water so that salinity, impurity and temperature readings can be obtained.

This inspected data will allow for a future corrosion control system to be picked. It is critical to not allow corrosion to be prolonged without any means of protection. This can have a costly result on the desalination project along with compromising safety.

Figure 1: Slab Spalling.

Figure 2: Typical column Spalling.

Figure 3: Corrosion due to atmosphere and chloride

attack.

DURABILITY SYSTEMS TO PREVENT STEEL CORROSION:Cathodic Protection/Impressed DC Voltage:Cathodic protection (figure 4) is a method which involves connecting the structure at question with a metallic material (i.e magnesium, zinc or aluminium). In very basic chemistry notation, this is known to be a galvanic cell where the more active material (an anode) provides a flow of electrons towards the structure (cathode). At some stage a low DC voltage can be implemented between the anode and cathode.

This idea is implemented into full scale units because it has the ability to protect the structure at the demise of the anode that is being continually sacrificed. Done correctly, this can become a very simple and cost effective method applied to any steel exposed in this environment. It must be kept in mind that these systems are more complex and are beneficial at greater voltages. These greater voltages allow for remote anodes locations, which has the capability of improving the efficiency of current along the protected cathode. Larger voltages are just as ideal in concrete structures as well.

Sacrificial Anode System:Sacrificial anode systems (figure 5) are simpler to implement as appose to impressed voltages although the anodes need to be placed at precise locations. All it takes is attaching an anodic material onto the protected steel member in the environment at question. These systems have inadequate pitching power in order to protect, hence the spacing and the abundance of anodes is fairly important.

The advantages of such a product are that they are relatively easy to install throughout the structure, cathodic interference is unlikely, easy to maintain and replace and there is a very low risk of overprotecting. Although the set back with this system is that the anodes won’t work very well in high resistivity environments and that the anodes themselves can increase the structural weight of the entire unit.

Coating and Encapsulation: As stated by Reis (2011), a great point was established that in addition to cathodic protection, marine member coatings and encapsulations in marine environments are crucial to the well being of engineered solutions. As discussed above, cathodic protection establishes itself with the help of an electrolyte, electric circuit, anode and cathode. This would not work with structures that are exposed to airy environments as air is a very poor electrolyte and has the ability to prevent current flowing from anodes to cathode materials. Additional protection to members is therefore necessary and that is why coating and encapsulations are needed along with cathodic systems in many marine constructions.

Encapsulation (figure 6) has very beneficial results as it can be used on either steel or concrete members in marine water. Although as stipulated by Reis, he noted that “encapsulations, due to their volume, composition, and the thermal changes in the marine environment, exhibit a tendency

Figure 4: Aluminium anodes on steel structure.

Figure 5: Sacrificial Anode example.

Figure 6: Encapsulation of steel members.

to crack over time”. It is fairly important to note that if using these encapsulation designs, they will exert very large loading on the members coated and when cracks start to appear, these breaches can cause rapid failure. Breached members (which are difficult to notice/monitor) can have little to no protection on its structural members, as water intrusion will result in the ultimate failure of the encapsulation.

DURABILITY SYSTEMS/DESIGNS TO PREVENT CONCRETE CORROSION FAILURE:Concrete exposure conditions:Concrete and steel design in and within a desalination plant is incredibly stressed to premature failure. In order to justify such designs, a quick reference to the Australian Standards is preferred. In order to get the best prolonged life of concrete structures, it is best to design for different atmospheric conditions within the Desalination Plant. Such atmospheric conditions are known to be:

Wet Blanket Turbulent Flow Variable Water Level Constant High Water Level

As established by Papworth (2009), AS3600 dictates that 50MPa concrete with 45mm (min) cover, should be capable of dealing with the differentiating conditions. It is important to note that concrete in the presence of splashed zones (near leaking pipes etc) where no run off is seen, has the possibility of being severely exposed to corrosion, no matter what concrete grade or cover measurements may have been applied.

Instead of draining off (like other zones on the plant) the seawater hangs in these sections as ponds on the surface of the work area. This seawater is then drawn into the concrete floors because of the capillary action (this is a relatively fast process). Furthermore, additional cementitious materials (like slag , silica fume or fly ash) may need to be considered in areas of “splash zones” not so much spray zones as these compounds will aid in minimising corrosion.

Variable Water level Constant High Level

Wet Blanket Very Turbulent Water Level

Figure 7: Different atmospheric exposures in counted within a desalination plant.

Figure 8: “splash” areas causing problems to both concrete and steel.

In conditions like variable water level and constant high level (as discussed above), these need special consideration when being constructed. Papworth (2009) discussed that in these zones, chloride (ions that are a catalyst to corrosion) builds up substantial easy on the surface of the concrete. This is so because water splashes (figure 9) and hangs around in the area until it is sucked in and dried out by the concrete. The ingression of these chloride ions and the depth of their penetration are dependent on a term that Papworth (2009) uses frequently, concrete “sorptivity”. A easy term to understand the phenomenon.

The moment in which chlorides seeps in to become in contact with the reinforcement bars is known to be called the critical chloride level. This is where corrosion begins and the structures ability to withstand loading is compromised further with time.

It is expected that the time involved for corrosion to initiate for a 50MPa Concrete structure with respect to its cover is:

The time calculated above has been done so with respect to the surface chloride levels. It can be noted that concrete with different cemetitious materials can perform outstandingly well in harsh conditions. Thereby it is recommended for higher end life expediency, which results in cement with slag material in it would perform really well, while it is cheap and well abundant within Victoria).

FINAL RECCOMMENDATIONS:AS3537 AND AS3600 RECOMMENDATIONS:Australia is recently uncovering the severity of exposure given to concrete and steel structures within constructed or intended desalination plants. This fast severity of corrosion/deterioration has still not become fully encompassed by an Australian Standard, although enough evaluation and testing have been done in order to provide this nation with the proper safety to uphold the need for durable structures.

In many of these desalination plants, galvanic, atmospheric and fretting corrosion are the most common types of corrosion. Given that the initial procedures before construction are conducted as discussed prior, then the following recommendations can be given to prevent such types of corrosion on the structure.

Figure 9: Time involved for corrosion to begin on a 50MPa

To (Time in Years to Corrode)

C (Concrete Cover mm)

Sc (Measured Chloride Level)

GP (General Purpose Cement)

CSF (Condensed Silica Fume)

Figure 9: Concrete in the presences of water

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Furthermore, the institute of concrete Australia has worked in liaison with Mr. Papworth (a reputable civil engineer) in order to revise and begin a recommended practice for all concrete water retaining structures (known in AS3735). The following points will give a brief run down towards what would be expected of such designs seen in underwater D Class Exposure settings listed in AS3537 and AS3600:

Immersed Inactive Water Condition design Recommendation (D1): This condition stipulates that the concrete is permanently immersed on all faces of the structure. The standards has seemed to of requested any concrete under such conditions, to have a strength of 40MPa concrete along with a 30mm cover. This would most likely become adequate of a 100 year lifetime as the corrosion rate becomes restricted by oxygen starvation. Immersed Agitated Flowing Water Condition design Recommendation (D2):

Very similar to the above, although the oxygen concentrations in the seawater would be much higher. The standards here request much higher protection therefore 40MPa concrete with 45mm cover are adequate for the 100 year life expectancy. Immersed but with Air on the other side Condition design Recommendation (D3):

This is mostly seen when seawater penetrates a concrete wall and establishes a pressure head along with water evaporations, whilst exposing the materials to salty spray. This can potentially lead to huge salt build up in the concrete wall. Here, simple wall thicknesses and concrete grades can ultimately be guessed by the aid of pressure head calculations.

INTERNAL PLANT SLAB RECOMMENDATION:The plants slab is another aspect of interest. Seawater, brine and de-mineralised water leaks all over the surface of such structures, are quickly absorbed into the slab as seen in figure 8. The location of the leaks or where the leaks are establish from, are never known for sure. This is partially why these plants floors should always be coated with a sealant. These coated floors need to be regularly maintained and there construction are ultimately simple. The slab can rest on the ground with a single reinforcement layer which would enable for a simple cathodic protection system to be applied within it.

BRINE PIPING RECOMMENDATION:Any reinforced concrete piping that discharges brine (saturated salt water) can substantially deteriorate the piping especially when it is not flowing full. Conventional concrete piping is not sufficient to provide adequate resistance. Ground nodes would become intelligently implemented as requested by Papworth (2009). Such a pipe work can see itself being encompassed by a ribbon of anodes running beside and underneath the pipe trench. This of course is used only if spacing is not a big deal in design (as ribbon anodes take up a lot of space).

Seawater intake and outtake pipes that run completely full can become considered as permanently immersed on all faces which means that conventional design of having 40MPa with a 30mm cover as discussed above is adequate.

The design of normal concrete piping can reduce the effect of corrosion significantly, although piping made out of steel fibre precast units with a dense plastic like paste in and around the piping can substantially be economical in constructing it and be a very durable consideration

Figure 10: Underground half flowing brine pipe.

towards corrosion. Unlike concrete piping, these units do not crack easily to become susceptible to corrosion attacks. Any cracking of 0.1mm on these pipes would still establish a long life cycle.

IMPRESSED DC VOLTAGE OR CATHODIC PROTECTION:Basic Recommendation:Cathodic Protection for Steel Structures Impressed DC high voltage for Concrete Structures

Choosing between the two methods is substantially important to get right. The reason as to why it was choose was because it has been understood from conducting such tests in high school that, sacrificial anode systems are generally easier than impressed current systems. The only tedious issue about it is that, it needs to have a whole array of sacrificial anodes placed throughout the structure at precise locations of one another, although this does a great job in protecting steel structures.

Sacrificial anodes are substantially superior to that of the impressed DC systems. DC distributed patterns are more catered towards concrete structures (low-conductivity units) where larger voltages can be used to protect such constructions effectively where the reinforced iron bars are within the concrete.

COATING AND ENCAPSULATION (WITH RESPECT TO CONCRETE):Basic Recommendation:Cathodic Protection for Steel Structures Impressed DC high voltage for Concrete Structures

The above was chosen to be a good idea to have on structures that are excessively exposed. For steel it would be a good last resort to have it coated/galvanised to comply with Australian Standards. As for any concrete extremely exposed to the marine environment should have itself encapsulated (epoxy linned with fiber-reinforced polymers). Coating and Encapsulation should only be applied in extreme cases as a last resort due to their costs and extra loading it gives to the entire structure.

IMPORTANT CONSTRUCTION METHODS:Not only are the systems and materials important to have a plant and its components complying with the 100 year duration, the construction method influence the durability of steel and concrete too. The main points that need to be taken into consideration are: Implement underground seawater and brine tanks (to which a cathodic system can help it

from corroding). This would reduce the amount of concrete and rebar reinforcement costs needed to support such structures when placed underground.

Make sure that whatever joints between tanks and piping, they have at least 3 levels of waterproofing to establish a safe guard against any leakage that would discourage steel and concrete durability.

Insure that rack probes are installed in and around the plant to allow for the rebars to be monitored progressively for any corrosion.

REFERENCES:

Robert, L 2010, Durability Performance of Australian Commercial Concrete Modified with Permeability Reducing Admixture, UNSW School of Civil & Environmental Engineering, New South Whales, viewed 10 May 2011, < http://www.xypex.com.au/assets/test_reports/chloride_resistance/Paper_Presented_by_Bob_Munn_Concrete05_Melbourne.pdf>

Reis, T 2011, Substructure Inc, Portsmouth NH viewed 10 May 2011, <http://substructure.com/>

http://www.bcrc.com.au/Papers/Concrete-Structures/Structures-Papers/2009-07-Water-Journal-Desalination-Plants.aspx