July 10, 2015
By Alireza Biparva, B.Sc., M.A.Sc.
When it comes to buildings, routine maintenance is expected on any structure as wear and tear is inevitable. However, the difference between small repairs and a major retrofit or rehabilitation is significant. Referring to De Sitter’s Law of Fives, a major repair can be expected to cost roughly five times what routine maintenance would have cost. An all-out replacement will then cost five times what a major repair would entail. So, avoiding large-scale work of this nature is of the utmost importance for the financial viability of a concrete structure.
With much of Canada’s infrastructure built in the 1950s and ’60s, deterioration has been occurring for more than half a century with much of the budget heading toward repair, maintenance, and inspection, instead of building for the population growth. This handicaps the future insofar much of the country’s infrastructure—including roads, highways, and bridges—are in dire need of repair. Governments shoulder this expensive task of inspection and repair. In Ontario alone, an Auditor General’s report in 2009 reported there were 14, 800 bridges with an average age of 30 years and 3500 determined to be in either fair or poor condition. Further, the Federation of Canadian Municipalities (FCM) estimates the country will incur a $123-billion infrastructure deficit, growing by two billion every year. This does not say anything about building infrastructure for a growing population—this is only repairing that with which is already underserving.
Repairs also have hidden costs that affect the bottom line of projects. These include traffic delays incurred due to inspection, maintenance programs and needed repairs. There is also the time needed, fuel costs associated, and the health impact from the pollution set-off in implementing procedures and projects. For instance, the Port Mann Bridge in Coquitlam, B.C., caused disruption to vehicles crossing—the substantial delays meant more pollution given off by idling vehicles, and further, less people using the bridge and finding longer routes to make it across. The final issue was the $3.3-billion investment (approximately) into building a new bridge altogether.
The Canadian government is investing heavily into infrastructure building, but will not be able to run the deficit to a point that continual maintenance and repair is needed. Moreover, the infrastructure built must be done so that it is made as resilient and as sustainable as possible to thwart deterioration from external factors, refuse to breakdown from use, and handle the constant impact of natural weathering hazards. To do so, concrete must be built for the utmost durability.
Concrete challenges, concrete solutions
Concrete comes with many advantages—it can be moulded into virtually any shape, is accessible anywhere on the planet, and has high compressive strength. Though porous in nature, concrete has the potential to be a durable product. However, in its basic form, the material has limitations that can leave a structure vulnerable to early deterioration.
Concrete has low ductility, tensile strength, strength-to-weight ratio and is susceptible to cracking. Above and beyond all, however, concrete is permeable. It allows the ingress of deleterious materials leaving it vulnerable to the attack of chemicals such as acids, sulfate attack, and alkali aggregate reaction. Without doubt, the ingress of moisture, which brings with it different chemicals, is the biggest concern for concrete structures. Thankfully, there are solutions to these limitations, all pointing to ensuring a concrete mix is developed with durability in mind.
This is not to say durable concrete will not require maintenance and even repair from time to time—both remain givens to ensure the safety and service life standards of the concrete structure are met. Moreover, a durable structure will lessen the burden of the costly maintenance and repair needed. This means the lifecycle costs (i.e. total cost of ownership over the life of an asset, including those costs associated with maintenance and repair) will be significantly lower over a longer lifespan with a durable structure.
Building durable structures, regardless of the material used, has definitely become mainstream knowledge. This is especially true with regard to concrete structures, given that durability directly results in a sustainable structure.
The Portland Cement Association defines durability as:
The ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties with minimal loss of mass in an aggressive environment.
Regarding the building of a structure, there are four main steps to create a durable concrete structure:
Each one of these steps is critical to the process that determines a concrete structure’s durability. For instance, if the placed concrete is not given enough time to cure, it will crack and allow a pathway for water penetration. Further, if the concrete is placed without the proper amount of consolidation, it leads to low strength, and increased permeability.
A durable concrete structure can be created by following the previous steps; however, ignoring these steps will cause a cracked and highly permeable material. As a porous material, concrete can allow water to migrate through, corroding steel reinforcement, bringing in harmful chemicals, or causing other short- or long-term issues requiring serious attention. Thus, in order to build a durable structure, a reliable waterproofing system must be included to help the concrete withstand the infiltration and saturation of water.
Internal and external waterproofing
The predominant cause of concrete deterioration is the infiltration of water. To create a durable structure, one must lower the permeability of the concrete. In order to lower the permeability of the concrete, one must apply a waterproofing solution. This means either waterproofing the concrete with a layer on the outside of the structure or waterproofing from within.
More often than not, the waterproofing measure for concrete projects across Canada is decided between surfaces, or externally applied, membrane and an internal crystalline admixture. The former option continues to be used on more projects because it is still known as the traditional method used for concrete waterproofing. However, crystalline admixtures are becoming more popular as more successful projects are brought into the open. The industry seems to be changing, shifting perspectives from the traditional to the innovative and sustainable.
In the recent past, structures were built on land that provided a lot of space, only went a storey below-grade, and the life expectancy of the structure was shorter. Fast-forward to the present—buildings have sustainability measures they must live up to, including a long service life. Most of the traditional membranes deteriorate over time and have a life much less than service life of today’s structures. Buildings are now built three and four storeys below-grade—a strategy that does not allow access for an external membrane repair. These buildings are also built in high-density areas, meaning repairs are highly complex; in other words, it takes a special skill set for tear-down and then can be quite costly with respect to rebuild. With these new issues comes a call for a change from the traditional.
The table in Figure 1 provides a general comparison, using application, performance, repair, and sustainability as the areas of concern.
Advantages with supplementary cementitious materials
Another solution to prevent water penetration is through the use of supplementary cementitious materials (SCMs) such as silica fume. These components are materials added in conjunction with concrete’s basic form—water, portland cement, and fine and coarse aggregate. These SCMs make up a portion of the cementitious material within a concrete mixture, which means the proportion of cement in a given mixture will lessen as SCMs are added. Some engineers and concrete suppliers point out they can use silica fume and other materials to produce a concrete mix that has very low permeability and can be considered watertight.
SCMs are used for various reasons within a concrete mixture, which includes a means to increase durability. According to the National Ready Mix Concrete Association (NRMCA):
These materials modify the microstructure of concrete and reduce its permeability thereby reducing the penetration of water and water-borne salts into concrete.
Thus, the addition of SCMs can aid in creating a watertight structure that, in turn, will create a durable end structure.
However, silica fume, and other SCMs used to increase concrete strength and durability by densifying the mixture to block the flow of water, is not in and of itself a waterproofing material. SCMs can contribute to reducing concrete permeability and be complementary components in a well-proportioned mixture, but they do not make the material watertight. Further, they must be used at an optimal dosage or drawbacks will be seen. For instance, the use of silica fume in concrete shows a tendency for cracking, resulting in water penetration and corrosion of reinforcing steel. This increased densification of the concrete often creates an illusion of waterproofing capability. In actuality, the use of silica fume will accelerate the deterioration process when cracking occurs.
This means optimally specified SCMs can significantly improve a concrete mixture’s lifespan, but an alternative solution must be used to create a completely watertight structure that will ensure a durable building.
In order to achieve a durable concrete structure, the permeability of the concrete mix must be as low as possible, which can be achieved with the proper waterproofing method. An externally applied membrane wraps itself around the concrete, protecting it from water ingress with a layer of material. However, if the shell fails, repair can be time-consuming, costly, and sometimes impossible. If using an internal membrane, the concrete matrix will be improved and will become the waterproofing barrier. By having a proper internal waterproofing system, your structure will be protected during its service life and not just during the short warranty period.
Alireza Biparva, B.Sc., M.A.Sc., LEED GA, is research and development manager/concrete specialist at Kryton International Inc. He has more than 20 years of experience in the field of concrete permeability. Biparva oversees several research projects focusing primarily on concrete permeability studies and the development of innovative products and testing methods for the concrete waterproofing and construction industries. He is an active member with the American Concrete Institute (ACI), and has given more than 300 presentations on concrete permeability, waterproofing, durability, and sustainability around the world. Biparva can be reached by e-mail at firstname.lastname@example.org.
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