February 6, 2020
by Kris Till
Waterproofing structures has always been an important and often costly consideration for designers and builders. Canada’s weather can be extreme, and climate change poses additional challenges for those involved in protecting the structure from water damage.
Whether one is trying to keep water in or out of the structure or protect the concrete itself from the deteriorating effects of water, the decisions made during construction have major cost implications over the long term. This creates financial stress for building owners, facility mangers, and developers.
Concrete is one of the world’s most widely used materials. It is the choice of many building professionals because of its durability, versatility, unique engineering properties, and availability of constituent materials.
However, as a porous and permeable material, concrete is vulnerable to water penetration. Water passes through pores, voids, and capillaries, especially under hydrostatic conditions. It also cracks, thereby allowing water to pass freely into the structure and flood, or through to the reinforcing steel to start the corrosion process.
The damage water does to concrete used in infrastructure projects and in below-grade foundations is staggering. The Construction Waterproofing Handbook says, “Water continues to damage or completely destroy more buildings and structures than war or natural disasters” (Read the Construction Waterproofing Handbook by Michael T. Kubal).
Through long-term deterioration like freeze-thaw cycles and corrosion of the reinforcing steel, water infiltration can weaken the structural integrity and shorten a structure’s lifespan. Rising sea levels and other long-term effects of climate change, as well as the expectation concrete structures will last longer, make waterproofing concrete more essential than ever. Designs made during construction will affect future spending.
Types of waterproofing
There are two industry approaches for waterproofing: external and internal. Membranes usually last one to 10 years, and manufacturers would warrant the product will be free from defects. However, since the material is installed, it is never warranted against leaks. Having said that, some manufacturers work with installers to offer two- to three-year extended warranties.
Crystalline manufacturers offer 10- to 25-year warranties on their products. Furthermore, some manufactures offer 10-year labour and materials warranty to undertake repairs.
The external approach uses membranes applied to the surface of the concrete. Membranes are divided into two categories—fluid-applied and sheet-applied. Fluid-applied products are typically used as dampproofing, while sheet membranes are required when there is an increased risk from hydrostatic pressure and other environmental properties. The concept of waterproofing is easy—the membrane forms a barrier, thereby preventing water from touching the concrete.
However, there are drawbacks to this method. Traditional waterproofing membranes are usually installed after the concrete is placed. In vertical applications, extra space is required around the perimeter. This can significantly reduce the building’s footprint and limit its design. For example, taking 1 m (3 ft) of space away from the property line can result in a significant loss in building footprint. This loss is amplified as the structure gets higher, and can equate to millions of dollars in lost revenue. The acceptance of structural shotcrete has allowed membranes to be used in blindside applications and gets around this limitation, but puts a significant amount of risk in the installation of the membrane, which is why multiple waterproofing systems are typically used in areas where hydrostatic pressure exists.
Additionally, highly skilled labour is required to apply the membranes in dry weather after the concrete is cured.
Surfaces have to be properly prepared and the application needs to be precise. Fluid-applied systems are dependent on the installer spraying consistently. Although quality control steps, such as measuring mil thickness and manufacturer training programs, can be undertaken, the waterproofing comes down to the capability of a single person installing in perfect conditions. With sheet-applied systems, every seam needs to be installed 100 per cent correctly or the area will leak. The larger the project, the more detail is required and higher the chance of error or damage during installation.
Even if installed perfectly, traditional surface-applied waterproofing is susceptible to damage and tends to deteriorate over time. Early punctures can take place from backfilling or other work during construction. There is no way to inspect the membranes after backfilling has been completed. It is challenging to inspect the membranes after installation and impossible after backfilling. Excavating to repair is also a costly process.
External waterproofing is an installed product, and construction schedules suggest it is time-consuming. It can add days, weeks, or on large complex projects, months to the schedule. While these are not completely hidden costs, there is very little understanding on the true expenses that are associated with installing and repairing membranes over the life of a concrete structure. A general contractor will understand the cost to him for the physical installation, and typically build a conservative schedule to accommodate. However, the costs that are associated with building time into the general construction schedule are usually unknown. If waterproofing was not needed, it would be easier to schedule and fast track construction and better understand the actual cost of waterproofing.
Also known as integral waterproofing, internal waterproofing is the method of adding the waterproofing component to the concrete at batching stage. These include densifiers, hydrophobic admixtures, and crystalline admixtures. However, not all of these components are suitable for waterproofing against water under pressure.
The American Concrete Institute (ACI) classifies admixtures for concrete as permeability reducing admixtures (PRAs) and then groups them as either PRA for non-hydrostatic conditions (PRAN) or PRA for hydrostatic conditions (PRAH). The former is suited for dampproofing and the latter for waterproofing. PRANs are classified as unreactive while PRAHs are reactive.
Crystalline admixtures under the PRAH category are recommended for long-term waterproofing in hydrostatic conditions and intended to replace the need for conventional labour-intensive membrane systems. With crystalline admixtures, the insoluble crystals are produced in the presence of water and reduce the permeability of the interior concrete matrix over time. The crystals grow throughout the concrete resulting in permanent waterproofing impervious to damage and deterioration. The key benefit is the admixture becomes a part of the concrete. This not only removes the need for a membrane, but also eliminates the risks of punctures, installation, and deterioration that can occur in external membranes used for waterproofing.
According to ACI, a PRAH waterproofing admixture must have the ability to lessen water under pressure and reduce concrete’s permeability by 50 to 90 per cent. Concrete permeability and water penetration tests like DIN 1048-5, Testing concrete; testing of hardened concrete (specimens prepared in mould), and EN BS 12390, Testing hardened concrete. Depth of penetration of water under pressure, take crystalline-treated samples and compare them against control samples by applying 0.5 MPa (72.5 psi) of water pressure for three days. Both samples are cracked open and water penetration is measured. The treated sample is then calculated as a reduction in permeability over the control. As mix designs differ, results will too—this is why ACI provides such a broad range.
Another ACI guideline is PRAH must be able to self-seal the micro-cracks that may form in the concrete over its life. Since a crack can become a gateway for water, and small cracks could turn into large ones, self-sealing is a critical property. The more reliable the long-term reaction of the crystalline admixture, the lower the life-cycle costs.
Differences in crystalline admixtures
It is important to remember not all crystalline admixtures are the same. Performance and reliability is based on the chemical reaction taking place, and the amount of reactive material used.
The original crystalline admixture was invented in 1981 and commercialized in 1983. It was developed based on a chemical reaction between unhydrated cement and water, which is reliable due to the abundance of unhydrated cement left over in the concrete after it has cured. Any time water enters through a new crack, a reaction takes place between the crystalline technology, unhydrated cement, and water. This creates a reliable reaction (i.e. self-sealing will always occur when using materials that react with unhydrated cement) for self-sealing small cracks over the life of the structure, thus reducing costly repairs and blocking water-borne chlorides from attacking the reinforcing steel.
In the process of concrete creation, cement and water react to produce calcium silicate hydrate (CSH) gel—the glue binding concrete together—and CH. The reaction that takes place creates crystal growth.
Since the late 1990s, many crystalline admixtures have come on the market with a technology that reacts with the by-products of cement hydration, otherwise known as calcium hydroxide (CH) or free lime.
However, there is a major disadvantage with free lime. It can leach out of concrete as time passes, resulting in depletion of the key component for crystalline admixtures to react in the way it does. Additionally, some concrete mix designs are using more supplementary cementitious materials (SCMs) like fly ash. These SCMs also react with CH and compete in the acquisition of free lime. This too affects the admixture’s self-sealing performance.
As concrete ages, micro-cracks are inevitable. If the crystalline-treated concrete is unable to self-seal cracks, water will find its way into the structure and not only create leaks, but also initiate corrosion.
Chapter 15 of ACI 212.3R-10, Report on Chemical Admixtures for Concrete, says that crystalline admixtures reacting with unhydrated cement particles achieve benchmark performance levels. This is due to the shape and size of the crystal. Even though most crystalline admixtures produce needle-shaped crystals, one can see stark differences between the materials. Products reacting with unhydrated cement will typically have needle-shaped crystals that are obvious at 30x magnification, while those reacting with the by-products of cement hydration require a magnification of 2000 to 5000x to see anything resembling a needle shape. This means the crystals designed to react with unhydrated cement are able penetrate deeper and interlock more effectively, resulting in better performance in the mix design, effective reduction in permeability, and improved ability to self-seal wider cracks with higher flow rates.
Specifiers should be aware not all crystalline admixtures use the same chemistry. Unless the product is reacting with unhydrated cementing materials, there is a risk of lower performance and concerns about reaction over the life of the structure.
SCMs: Not the answer for waterproofing
A general misconception surrounds the use of SCMs for concrete waterproofing. SCMs are materials like fly ash, slag, and silica fume that are added to the concrete at the time of batching to replace a quantity of Portland cement as a cost-saving measure. Dependent on the material used, SCMs can offer a number of performance benefits to the concrete, such as improved compressive strength, increased workability, and added durability. Specifiers also use SCMs to reduce permeability, which is one of the reasons for the added durability. The densification of the concrete, along with a proper concrete cover of the reinforcing steel, makes it easy to understand how an SCM slows water from passing through concrete. However, specifiers need to understand SCMs do not enhance the ability of the concrete to self-seal or stop the water when cracking occurs. In fact, working with SCMs can be tricky and cause more cracks to develop in the concrete. This allows water and water-borne chlorides to pass to the reinforcing steel. For this reason alone, it will require extensive preventative and corrective repairs throughout the building’s life cycle.
There are various factors to consider when waterproofing concrete. Those involved in specifying concrete must carefully consider waterproofing options. Whether one is waterproofing concrete from the harsh environmental conditions in Canada, or to contain water under hydrostatic pressure, there is risk everywhere. The wrong decision can have major cost implications over the long term.
Kris Till is a product manager at Kryton International. He has 20 years of experience in the construction industry and is an expert in Kryton’s proprietary technologies and how they can add value building professionals. Till can be reached at firstname.lastname@example.org.
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