May 17, 2016
By Jackson So, B.A.Sc., EIT
Major infrastructure that affects the lives of tens of thousands on a daily basis must last for its expected life (at the very least)—otherwise, a ripple effect is caused to government budgets, negatively affecting each citizen in some capacity. When infrastructure fails prematurely, action must be taken to discover why it happened, enabling the engineering community and infrastructure owners/managers to amend the practices going forward to preserve durable growth throughout city networks.
Failing structures may fall into a spectrum of different issues, but the underlying cause of prematurely problematic concrete is often the same—water penetration. Rainwater and humidity can find their way into the porous networks within the concrete through the smallest entry point; the result is slow, but often catastrophic, damage to the structure. Given enough time, water will support deterioration, corrosion, rusting, and/or change to all building materials.
In particular, any concrete structure in close proximity with water faces myriad life-shortening processes. Reinforced concrete infrastructure, found in marine environments, commonly face reduced life spans due to exposure to extreme environmental conditions, which allow water and waterborne chlorides to penetrate through the concrete to the reinforcing steel. This contact results in corrosion and expansive cracking, leading to premature deterioration.
Bridges, tunnels, and retaining walls are all types of infrastructure that are affected by the highly destructive nature of marine environments. Other types of infrastructure may be less obvious, such as sewer pipes or water tanks, but often must deal with high hydrostatic pressure and the corrosive elements that are brought in by seawater nonetheless.
How water attacks concrete structures
Since nature cannot be ‘stopped,’ design/construction professionals must instead direct careful attention to how nature operates and plan accordingly when designing, constructing, and maintaining concrete structures. Not all damage is the result of spectacular floods, storms, or newsworthy global-warming events—some are slow and unseen, but the outcome can be just as damaging.
All concrete infrastructure found in or near marine environments will face reduced life spans unless steps are taken to protect them. They are at a higher risk of corrosion due to the constant wet-dry-wet cycle that forces water through capillary tracts and micro-pores found in the concrete. When adding in the extreme freeze/thaw weather conditions experienced throughout Canada, the protection of these structures must be of utmost importance.
Water permeability ultimately determines the rate of deterioration, with some of the deterioration mechanisms threatening marine concrete infrastructure being:
Corrosion of steel reinforcement
There are three essential components necessary for corrosion to take place in reinforced concrete:
Eliminating one of these components will mitigate the damages due to corrosion. This is both why there is no corrosion in dry concrete, and why it is important to have low-permeability concrete to prevent the movement of water and the harmful chemicals in solution from reaching the steel reinforcements.
Overall, concrete serves as a great host for the rebar. Due to the former material’s high alkalinity, the latter develops a passive layer that provides a protective barrier to the steel. In this state, concrete normally provides excellent corrosion protection, but the passive layer can be broken down over time due to atmospheric carbon dioxide. This causes carbonation, which lowers the pH of the concrete and destabilizes the passive layer. However, this is a slow process and the overall rate depends on the density of concrete and humidity of the exposed environment. Durable concrete with low permeability can reduce the rate of carbonation, in addition to slowing the rate of water penetration necessary for corrosion to occur.
Poor-quality concrete has more connected pores and larger capillaries; this increases the potential for the ingress of detrimental substances into the concrete. Substances such as chlorides can enter into the concrete through the pore network, leading to the breakdown of the passive protection layer around the rebar. Without the passive iron oxide film protecting the steel, corrosion is able to commence at a much higher rate.
The most common type of sulfate attack is through external means, whereby water containing dissolved sulfate penetrates the concrete. This is usually the result of high-sulfate soils and groundwater, but can also be caused by atmospheric or industrial water pollution, bacteria in sewers, or even just regular seawater. (For more information, see the article, “Sulfate Attack in Concrete and Mortar,” posted at www.understanding-cement.com/sulfate.html).
A sulfate attack will typically change the composition and microstructure of the concrete and lead to extensive cracking, expansion, and loss of bond between the cement paste and the aggregate.
Occasionally, certain aggregates can react with the alkali hydroxides in concrete, causing slow deterioration of the concrete through expansion and cracking. The hairline cracks that develop are an invitation for water to cause corrosion of the rebar even in above-grade structures.
There are two forms of alkali-aggregate reaction:
The first is the more concerning type of reaction, as it is more common to find aggregates containing reactive silica materials. (ACR is relatively rare.)
With ASR, the silica in these aggregates reacts with alkali hydroxide in concrete, forming a gel that swells by absorbing the water in the surrounding cement paste, or any water that finds its way into the concrete. As the gel absorbs more moisture, the swelling effect can cause long-term damage to the concrete by inducing expansive pressure. Cracking is often an indicator ASR is present, with the cracking often located in areas with a frequent supply of water or moisture.
Freeze/thaw actions will likely cause deterioration to non-air entrained concrete. When water freezes to ice, it occupies nine per cent more volume. With no available space for this increase in volume, freezing can distress concrete, leading to hairline cracks. Thawing will then allow water to penetrate through the cracks and with each freeze/thaw cycle increase the number and size of hairlines, resulting in greater damage to the concrete.
Some noticeable signs of freeze/thaw damage are spalling and scaling of the concrete surface, surface parallel cracking, or exposed aggregate.
Durability critical for protecting coastal environment
In keeping the water out of concrete, damage to the structure as a whole—from corrosion, freezing, and other water-caused effects—can be eliminated. Taking the proposed liquefied natural gas (LNG) project along coastal British Columbia into account, an immense amount of concrete is being used for all of these planned projects, some of which have already invested hundreds of millions of dollars in preparation.(For more, see Kitimat.ca’s “Major Projects” page on economic development. Visit www.kitimat.ca/EN/main/business/invest-in-kitimat/major-projects.html) The region constantly battles some of the weather challenges most damaging to concrete.
Part of the Canada Starts Here: B.C. Jobs Plan, which was launched in 2011 by the B.C. Liberal Party, focused on strengthening local economies by getting B.C. products to new markets. The main goal surrounding this objective was to distribute liquified natural gas (LNG) to overseas investors through three LNG facilities located along B.C.’s Northern Coastline.
Kitimat and Prince Rupert are two of the proposed locations for LNG terminals. Being port cities, concrete structures in these locales face a regular barrage of sea spray and salt. Prince Rupert in particular is known as one of Canada’s wettest cities, experiencing more than 2550 mm (100 in.) of rain, annually. The temperature drops just below freezing on a regular basis during winter, resulting in constant freeze/thaw cycles throughout that time of year.
Diego Orozco, a civil engineer specializing in LNG tank construction, project, and risk management, worked on an LNG terminal project in Mexico. He attests to the massive amount of concrete required by these types of facilities.
“The Mexico project used over 80,000 m3 [105,000 cy] of concrete for the outer shells of the storage tanks holding LNG, as well over 35,000 m3 [177,000 cy] of additional concrete to construct the jetty and other buildings within the processing area,” he explained.
One of the most likely LNG projects to go ahead in Kitimat, B.C., would itself be using more than 150,000 m3 (196,000 cy) of concrete in that project alone. Orozco further goes on to state the durability of the concrete is critical to the long-term success of this LNG infrastructure.
If these projects move ahead as expected, the quality and durability of this vast amount of concrete may be the only barrier between damaging materials and sensitive marine environments, where delicate ecosystems depend on the status-quo for survival, meaning the construction cannot fail or damage the region.
Thus, proper waterproofing of the concrete is vital to ensure the durability of the infrastructure is set and maintained at the highest level possible. To create a durable structure, the permeability of the concrete must be lowered. This requires a waterproofing solution that reduces permeability, while enhancing the durability of the concrete structure.
Internal crystalline admixtures
Internal crystalline admixtures have been growing in popularity over the past decade as an increasing number of high risk, successful projects have been brought to the forefront. This type of waterproofing is simple to apply, as the admixture is added directly to the concrete mixture itself.
Once the crystalline concrete mixture is poured, the waterproofing capabilities of the admixture actually improve their effectiveness over the lifespan of the structure because of the technology’s ability to reactivate and ‘self-seal’ cracks. Further, crystalline products permanently seal new hairline cracks, and if larger cracks occur, they can be repaired from the negative or dry side. This means future repairs or maintenance are cost-effective and simple to achieve. Crystalline technology not only provides waterproof concrete, but also essentially increases the durability of the structure as a whole by lowering permeability.
The fact is, building durable infrastructure leads to sustainable construction. In order to avoid major repair bills that are less than effective, or being forced to fork out sometimes hundreds of millions in premature replacement costs, water must be kept out of the concrete structure with a long-term solution. In keeping the water out, damage to the structure as a whole can be substantially reduced.
Jackson So, B.A.Sc., EIT, is a concrete permeability specialist with Kryton International Inc. He has been involved with the research surrounding concrete waterproofing even before graduating from the University of British Columbia in 2006 with a degree in materials engineering. So may be reached by e-mail at email@example.com.
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