Building durable structures: Considerations for concrete, waterproofing, and longevity

by Katie Daniel | November 6, 2015 11:30 am

[1]

by Alireza Biparva, B.Sc., M.A.Sc.
The U.S. construction industry is shifting perspectives from traditional building toward a green, more sustainable design sensibility. This approach includes building concrete structures that will last an appropriate lifespan with minimal maintenance and repairs—and this necessitates careful attention paid to the waterproofing strategies chosen.

If built sustainably, maintenance and repair for concrete buildings should not be as cumbersome an issue as noted in the past. While general upkeep will always contribute to a project’s lifecycle costs, it will not be nearly to the degree experienced for projects where concrete is continually needed to be worked on, and where deterioration reaches the point where full reconstruction—a daunting, and occasionally impossible task—is considered.

While routine maintenance is expected on any structure, its difference from major repairs 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 be five times what a major repair would have cost. Avoiding major repairs and replacement issues are of the utmost importance for ensuring the financial viability of a concrete structure. Furthermore, the mammoth undertaking of a complete rebuild, which in some cases is inconceivable, needs to be avoided altogether with sustainable construction design.

Unfortunately, inspection, maintenance, and repair costs of concrete constitute a major part of the recurrent costs of U.S. infrastructure. In 2006, the International Concrete Repair Institute[2] (ICRI) said the costs associated with the material’s repair, protection, and strengthening in the United States was estimated to be $18 to $20 billion dollars a year. This means the bulk of any infrastructure repair budget tends to head straight to fixing the old instead of focusing on the practices of building new, sustainable infrastructure.

According to the American Society of Engineers’ (ASCE) latest report card[3], the country’s grade sits at a D+. The report card depicts the condition and performance of the nation’s infrastructure based on physical condition and needed investments for improvement. Further to this less-than-stellar assessment, ASCE also estimates a $3.6-trillion investment will be needed by 2020.

However, if this investment were to have a lasting impact, new structures built must be as resilient and as sustainable as possible to avoid deterioration or breakdown from use. Further, as concrete makes up a vast majority of the infrastructure, the material must be engineered for the utmost durability.

Challenges with concrete
Concrete can be molded into any shape, is accessible virtually anywhere in the country, and has high compressive strength. Though porous in nature, the material has the potential to be a durable product. However, in its basic form, concrete has limitations that can leave a structure vulnerable to early deterioration.

For example, the material has low tensile strength, a relatively low strength-to-weight ratio, and low ductility. Above and beyond all, it is permeable, allowing the ingress of deleterious materials that leave it vulnerable to acids, sulfate attack, and alkali aggregate reaction. Without a shadow of a 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 that involve ensuring a concrete mix is developed with durability in mind.

Durability defined
As laid out by the Portland Cement Association (PCA) in its book, Design and Control of Concrete Mixtures, durability can be thought of 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.” With regard to the building of a concrete structure, there are four main steps that create durable concrete. They involve proper:

• mix design based on the environment in which the structure is being built (i.e. concrete being used in a hot, dry environment is done differently than one in a cold, wet and cold region);
• production, ensuring the right processes—proper temperature, amount of water, timing, and ingredients—are used to mix concrete;
• placing, with appropriate compaction to minimize voids and defects; and
• curing to achieve desirable concrete properties and to minimize the chance of cracking.

Each one of these steps is vital to the process determining whether a concrete structure will be durable. If, for instance, the placed concrete is not given enough time to cure, it will crack and allow a pathway for water saturation. Further, if the concrete is placed without the proper amount of consolidation, weakened unconsolidated concrete structure is what remains. This would institute a mechanical failure during the concreting processes, which occurs for a small percentage of the cases, so a reliable waterproofing system is required.

Nevertheless, the top cause of concrete deterioration across the country is through the infiltration of water. Water is the reason why the durability of concrete structures is being called into question on a daily basis. Thus, in order to build a durable building, that structure must be constructed to withstand the infiltration and saturation of water.

[4]

Internal and external waterproofing
To create a durable structure, one must lower the permeability of the concrete. This requires a waterproofing solution—either applied as a layer to the outside surface or internally. The choice for concrete projects is usually between an externally applied product (e.g. a cold-applied polymer-modified-bitumen [mod-bit] sheet membrane or a brush-applied liquid membrane) and an internal crystalline admixture.

The former option continues to be used on more projects around the world than the latter because it is the traditional method. However, crystalline admixtures are growing in popularity as more successful projects are brought into the open. When added to concrete, these admixtures create a reaction that causes microscopic crystals to form, filling the pores, capillaries, and hairline cracks of the concrete mass. As long as moisture remains present, crystals continue to grow throughout the concrete, reaching lengths of many inches over time. Once the concrete has cured, the crystalline chemicals sit dormant until another dose of water (such as through a new crack, or due to humidity in the air) causes the chemical reaction to begin again.

In the recent past, structures were built on land that provided a lot of space, only went a story below-grade, and faced shorter life expectancies than the 100-plus years currently expected. Structures are now built several stories below-grade and do not allow access for an external membrane repair. These buildings are also built in high-density areas, meaning repairs are highly complex, teardown highly specialized, and rebuilding costly. With these new issues comes a call for a change from the traditional.

Application
Traditional externally applied membranes require perfect application and contours; vertical application can also be difficult due to tight space constraints for applicators. Details can be problematic and require both highly skilled and suitably trained installers. These standard products must be installed in dry weather as they will not perform over damp or uncured surfaces. While the materials will protect the positive (i.e. wet side), the negative (i.e. dry side) will be exposed to other deterioration mechanisms.

Internal crystalline admixtures can be easily added directly to the ready-mix truck or at batch plant. No surface preparation or site labor is required, and there tends to be less management hassles and safety issues because less equipment and personnel are onsite. Offering both positive and negative protection, their use means reduced potential for human error and a shortened construction schedule.

For sheet- or liquid-applied membranes, field fabrication requires intensive labor and carefully supervised installation. Integral waterproofing admixtures tend to be less expensive for materials, and additional labor costs are almost non-existent. They also allow for a larger building footprint and reduce maintenance and repairs over the long term.

[5]

However, integral crystalline waterproofing strategies should not be used in applications under constant movement. This is because during the crystallization process, crystals align in a three-dimensional array that breaks when subjected to excessive movement. This means areas that require flexibility and face recurring movement—such as plaza decks or rooftops—would be better waterproofed another way.

Performance
When using externally applied waterproofing membranes, the protective layer can be easily breached by pinholes or seams. The product can become brittle with age, resulting in cracks and openings. Further, some membranes may include toxic chemical additives.

Historically, hot-applied sheet systems—known as built-up bituminous membranes—were used for below-grade concrete waterproofing. These sheets were made from alternating layers of bitumen and felt. When heated, traditional bitumen—both coal tar pitch and asphalt—releases volatile organic compounds (VOCs) and potentially carcinogenic fumes. Since the early 1990s, the bitumen system’s popularity has fallen due to an increasing number of bans on its use by governmental and regulatory agencies. Substantial steps have been taken by product manufacturers to replace these types of membranes.

Crystalline waterproofing is not affected by surface water, and actually improves over the lifespan of the structure. This is because once the concrete has cured, the crystalline chemicals sit dormant until another dose of water (such as through a new crack) causes the chemical reaction to begin again. The ability to reactivate in the presence of water gives treated concrete the ability to ‘self-seal.’ When cracks form due to curing shrinkage, settling, or seismic activity, water entering through them causes new crystals to form and grow, blocking and filling the cracks. This feature effectively allows the concrete to become even more waterproof over time. Additionally, many crystalline admixtures are certified by NSF to NSF/ANSI 61, Drinking Water Components−Health Effects Conditions, which allows them to be safely used in potable water tanks.

[6]

Repairs
Once there are leaks, repairs (or outright replacement) for external waterproofing membranes can be both expensive and complicated. Since they cannot be repaired from the negative side, access can be impossible. Crystalline products, on the other hand, will permanently seal all new cracking. When there are larger cracks, they can be repaired from the negative or dry side. This means this route is cost-effective for future work.

It is important to note this article is specifically discussing below-grade concrete waterproofing, which, once backfilled, does not allow for breaches to be detected (even with traditional waterproofing) without embarking on major excavation around the perimeter of the building.

Sustainability
External membrane materials are landfilled at the end of their life, and they also require resources to install properly, adding to waste. Crystalline waterproofed concrete, in contrast, can be recycled and reused as aggregate on a future concrete projects.

Supplementary cementitious materials
In addition to these two waterproofing strategies, another oft-cited solution to prevent water penetration is through the use of supplementary cementitious materials (SCMs). One common example is silica fume, which is an extremely fine noncrystalline silica often used in concrete structures that need high strength or significantly reduced permeability to water.

An SCM is a material added in conjunction with concrete’s basic form of 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. Engineers and concrete suppliers will sometimes point out they can use silica fume and other SCMs to product a concrete mix that has very low permeability and can be considered ‘watertight.’

SCMs are used for a variety of reasons within a concrete mixture, which includes increasing durability. According to the National Ready Mixed Concrete Association [7](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, which, in turn, will create a durable end structure.

[8]

Silica fume is used to increase the strength and durability of concrete, densifying the mixture to block the flow of water, but it is not itself a waterproofer. Essentially, it makes concrete dense, but not watertight. SCMs can contribute to reducing concrete permeability and be a complementary component in a well-proportioned mixture, but there are drawbacks when the dosage is not optimal. For instance, if an SCM is used at a really high dosage, concrete has a tendency to crack, allowing a pathway for water infiltration.

This does not mean SCMs like silica fume are not increasingly valuable for a concrete mixture. When such materials are used optimally, they can significantly improve a concrete mixture’s lifespan. However, an alternative solution must
be used in order to create a completely watertight structure that will ensure a durable building.

Conclusion
A durable structure will lead to a sustainable building. In order to achieve concrete assembly with this enduring longevity, the permeability of the concrete mix must be as low as possible. This 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 is cracked, repair can be time-consuming, costly, and sometimes impossible. If using an internal admixture, the concrete itself becomes the waterproofing barrier.

Alireza Biparva, B.Sc., M.A.Sc., LEED GA, is a research and development manager/concrete specialist working at Kryton International Inc. He has more than a decade 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). Biparva has published several research papers in international journals and conferences on concrete permeability, waterproofing, durability, and sustainability. He can be reached by e-mail at alireza@kryton.com[9].

Source URL: https://www.constructionspecifier.com/building-durable-structures-considerations-for-concrete-waterproofing-and-longevity/

---