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:
- alkali-silica reaction (ASR); and
- alkali-carbonate reaction (ACR).
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.