By Claude Goguen, P.E., LEED AP
Durability. Out of precast concrete’s many attributes, this is the one that makes it among the most-used building and infrastructure materials on the planet.
Sure, products must meet precise dimensions, required strength and varying levels of appearance, but nothing beats the significance of long service life, which affects other important attributes such as sustainability and resiliency.
Threats to concrete durability can originate from many different sources, but there is one primary strategy that will protect against the majority of those risks: reducing the permeability of the concrete and densifying it to prevent intrusion of harmful materials.
Low permeability concrete can be achieved through the use of quality materials and proper manufacturing practices. Permeability reducing admixtures also can be a valuable asset in the quest for enhancing durability. There are many options for permeability reducing admixtures on the market, so it’s important to know which to choose for each application.
Transport Mechanisms of Liquids and Gases into Concrete
In order for a liquid or gas to enter or travel through concrete, it needs help from specific types of conduits. These conduits can take the form of:
Pores are air- or water-filled voids in concrete that can vary in size and number depending on multiple factors. Water added to concrete serves two purposes: to hydrate cement and create paste and to make the concrete workable so it can be placed. Excess water left behind after the cement hydration may remain in the concrete after it hardens and create water-filled pores or dry out and leave air-filled pores.
Entrapped air also is a type of pore. Pores can be separate distinct voids or connected by capillaries. The number and volume of pores are measured as porosity, which is the ratio of the volume of voids in a material to the total volume of the material and is expressed as a percentage.
Capillaries are microscopic channels left behind in hardened concrete from when water travels through fresh concrete. For example, when concrete is placed and heavier materials in the mix – such as coarse aggregate – ever so slightly sink, extra mix water can get pushed toward the concrete surface. The water that appears at the surface as a result of this action is known as bleed water. The channels left behind from that movement are calle capillaries – sometimes referred to as capillary pores.
Finally, cracks may form after internal fracturing of the paste. Most cracks are too small to see with the naked eye. These “microcracks” are not large enough to cause any structural issues, but they could facilitate the passage of liquids or gases.
Microcracks may heal through a process known as “autogenous healing.” This can occur without the addition of any special admixtures as long as the crack is relatively stable and there is water and cementitious compounds that can react and produce material to fill the crack. The water’s velocity flowing through the crack will affect the healing rate. 1
Good concreting practices aim to minimize these pores, capillaries and cracks and keep ingress and passage of liquids and gases through concrete to a minimum. That ingress is measured as permeability. While porosity and permeability are not directly proportional, a higher porosity contributes to a higher permeability. Excessive bleeding could lead to more capillaries, which also could facilitate the entry of liquids.
Water can enter concrete using one of two primary mechanisms: absorption or ingress under pressure.
Absorption is a mechanism that can occur in the absence of hydrostatic pressure. Sometimes, this mechanism is referred to as “wicking.” Liquids can be drawn into the concrete because of capillary absorption. It’s the same force that draws water up into a piece of paper towel or blood from the tip of a finger into a small tube. If you place a dry concrete cylinder in a shallow dish full of water, moisture will absorb and wick upward counter to gravitational pull.
Ingress under pressure means there is a hydrostatic pressure or hydraulic head pushing water into the concrete. In other words, the structure is submerged and water is forced into the concrete by that pressure, which increases with depth. Concrete surfaces buried below the water table are subjected to hydrostatic pressure. The surface above the water table does not experience that pressure but still could allow ingress through absorption.
Of the two mechanisms, absorption tends to be the fastest means of water ingress.
While permeability technically applies to water under pressure, it generally is used to describe the passage of liquids through concrete under both mechanisms.
It is practically impossible to make concrete completely impermeable, but permeability can be reduced to a point where it no longer poses a threat to a structure’s durability. This can be achieved through optimizing mix proportions as well as good manufacturing and curing processes. It also can be achieved by using certain supplementary cementitious materials (SCMs) or admixtures that help fill voids in the concrete. These products are sometimes called densifiers, waterproofing admixtures or permeability reducing admixtures (PRA).
While there are many PRAs on the market, they can be broken down into two main categories, as defined in ACI 212.3R, “Report on Chemical Admixtures for Concrete,” Chapter 15.2:
- PRAN. Permeability reducing admixtures – non-hydrostatic. These types of admixtures, also referred to as damp-proofers, are designed to prevent the absorption of liquids that are not under hydraulic pressure. Most PRANs consist of a hydrophobic or water-repellant material. Others contain finely divided solids that increase density and help resist passage of liquids and gases by forming a precipitate and partially blocking voids.
- PRAH. Permeability reducing admixtures – hydrostatic. These types of admixtures often are called water-proofers and are designed for hydrostatic pressure conditions. Many PRAHs are crystalline based while some rely on other technologies to densify the concrete. Paul Laskey, national manager for concrete innovation with Sika, said, “In the presence of water, the ingredients of these crystalline products react to form non-soluble crystals that fill and plug the pores and microcracks in concrete.” The reaction may occur with calcium hydroxide (CH) or cement particles to create additional calcium silicate hydrate (CSH) gel and other precipitates. It is a similar mechanism to that of using a pozzolan that reacts with CH to form more CSH, which can fill additional voids in the concrete and reduce permeability. Paul Derby, technical consultant with Xypex, said, “Crystalline development is initiated with the excess mix water. Additionally, water curing of the concrete helps improve crystalline development and reduce permeability, and in the case of liquid holding structures, the sooner they are put into immersion the better as it also aids in crystalline development.”
Some PRAHs are hydrophobic or may contain a blend of hydrophilic and hydrophobic elements. Some products use polymer-based agents. These admixtures contain monomers, which react with calcium and moisture to polymerize and form hard globules similar to rubber to block voids.
PRA technology has come a long way in recent years and continues to evolve.
Sam Lines, engineering manager with Concrete Sealants, said, “As science is allowing us to know more about the cement hydration process, researchers are learning new and innovative approaches to creating nano-scale products which reduce the transport properties of liquids and gases through concrete. In the next decade, the industry will see even more options available to the precast industry to reduce permeability, enhance durability and provide environmentally green solutions.”
Other potential benefits of PRAs
Some PRAs can bring about additional benefits to plastic and hardened concrete.
- Strength enhancement. Some PRAs can result in increased concrete strength. Pores can’t withstand loads, so it’s logical that fewer pores equates to higher strength. CSH primarily is responsible for strength development in concrete. Therefore, when PRAs react with CH to create more CSH, this helps enhance strength. The interfacial transition zone (ITZ) denotes the very thin region of cement paste around the aggregate particles. In some cases, CH or pores can form in the ITZ, thus negatively affecting strength. PRAs that reduce CH, fill pores and enhance ITZ quality also increase strength. It’s important not to consider the potential PRA-induced strength increase in the final design as this increase can vary significantly.
- Set retardation or acceleration. PRAs can influence setting times. Those that do mostly extend set times, which can end up enhancing hydration and overall concrete quality. If set retardation is desired, then that can be a benefit of certain PRAs. Conversely, some accelerate the hydration process, which also can be beneficial in the right conditions. Accelerated hydration can lower bleeding, which reduces capillaries and allows finishers to start sooner. It’s important to determine the set time impacts through trial batches and testing so any necessary adjustments can be made.ration
- Workability. Some PRAs enhance workability by acting a bit like a water reducer. This could be because of the specific proprietary chemical formulation or the actual addition of a high-range water reducer (HRWR). This could result in slightly higher spread or slump test results.
- Efflorescence control. Efflorescence is a phenomenon where a deposit forms on the surface of a concrete or masonry product caused by evaporation of a salt-containing solution. The deposit usually is white or slightly off-white. This process is promoted by water ingress where the solution moves back out to the surface to evaporate. Repetitive cycles of wetting and drying are a cause, but the primary contributor is high concrete permeability. Many times, PRANs are sufficient to slow permeability enough to prevent moving liquids and thus preventing efflorescence.
- Self-healing. Microcracks may self-heal when the admixture reactivates in the presence of moisture, a reaction occurs and they are filled with crystalline deposit. It is common that concrete self-seals hairline cracks up to 0.5 mm (0.02 in.).
Choosing a PRA
By virtue of the name, PRAs are designed to reduce permeability, but this is not usually the end goal when using these admixtures.
The end goal usually includes one or a combination of the following: controlling efflorescence, maintaining an architectural appearance or extending durability in a mild or aggressive environment.
Standards and Tests
Many PRAs are compliant with ASTM C494, “Standard Specification for Chemical Admixtures for Concrete” under the classification Type S – specific performance admixture3. Some PRAs also may fall under other classifications if they contain additional admixtures or satisfy specific criteria. This standard helps ensure quality and uniformity but is not a performance test.
The most common types of permeability tests for PRAs include:
- Army Corps of Engineers test CRD C48-92, “Standard Test Method For Water Permeability Of Concrete.”4 This test method consists of subjecting a concrete sample to water under 200 psi (1.38 MPa) of pressure. After several days, the water loss reservoir is converted to a permeability value.
- German standard DIN 1048 (Part 5) Testing of Hardened Concrete, Water Permeability.5 Measures water penetration into concrete samples subjected to 72.5 psi (0.5 MPa) of hydrostatic pressure over a period of three days.
- ASTM C1585, “Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic-Cement Concretes.”6 This test method measures absorption of water into a concrete sample. This is achieved by drying the sample to a known moisture content, weighing it, exposing the bottom to water for a period of time and weighing it again.
- ASTM C1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.”7 This test method often is called the rapid chloride permeability (RCP) test. It was primarily designed to measure concrete’s ability to resist chloride ion penetration. The test records a total electrical charge passed through a specimen in coulombs, and that is converted to an estimated permeability rate. This test is popular because of its speed but is not always the most accurate test to determine permeability depending on mix design constituents.
Many PRAs come in powder form while some come in liquid form. Most powdered products come in pulpable, shreddable or water-soluble bags, which allows the user to simply throw the entire bag into the mixer. Whether the bag is added to the mixer, or the packaging is opened and the admixture is added manually, considerations must be given to the ease of doing so with every batch, and mitigation of any associated safety risks.
“One of our precast producers uses the product in bulk and has installed a dedicated hopper and auger system to dispense the admixture into the mixer,” Derby said.
The PRA dosing and sequencing must be determined by the product supplier. They will need details of your mix design and mixer and may want to run some tests prior to suggesting a dosage rate and batching sequence. Most powdered admixtures have dosage rates around 1% to 2% by weight of cementitious materials, while dosage rates for liquid admixtures range from just a few ounces to 40 ounces per 100 pounds of cementitious material.
Dosages also will depend on use of concentrated and pigmented formulas.
Sequencing of powdered PRAs tend to be at the very beginning of the mix cycle or with aggregates.
Mark Bury, product manager – admixture systems with Master Builders, said, “Our product should be added up front in the empty mixer, followed by the water addition and mixing for a few seconds. Immediately after that, add the remaining ingredients and continue mixing. The product can also be added with the aggregates or placed on the aggregate belt.”
Some liquid admixture suppliers recommend adding their product with mix water or to a wetted mix and warn against late addition to avoid delayed or even accelerated set times.
Potential Impacts on Plastic Concrete Properties and Interaction with Other Materials
Those considering using PRAs will want to know how they affect plastic concrete properties such as set times and slump or spread. They also will want to know potential influences on short- and long-term concrete strength.
Most PRAs have minimal negative interactions with other admixtures and cementitious materials, but suppliers may suggest slight modifications such as air entrainment dosage and HRWR type and/or dosage.
Many PRAs offer a pigmented option that will turn the concrete a specific shade.
“Some precast producers utilize a tinted version of the admixture for identification purposes to prove that the admixture is in the mixture, and to identify the treated structures in their inventory,” Bury said.
Great Option to Boost Durability
Permeability reducing admixtures can be a reliable means of enhancing the inherent durability of a high-quality precast concrete structure, but remember that they will not fix poor quality concrete. The work has to be put in up front to optimize raw materials and mix proportions, control moisture levels, keep the water-to-cementitious materials ratio low and properly place and cure the product. Once that system is in place and working well, PRAs can offer additional reduction in permeability as well as the associated boost to one of precast concrete’s most important attributes: durability. PI
Claude Goguen is the director of outreach and technical education at NPCA.
- Testing Self-Sealing Properties of Concrete – Gupta, Biparva, Azarsa, ACI, Concrete International https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&id=51714489
- ACI 212.3R, “Report on Chemical Admixtures for Concrete,” ACI, https://www.concrete.org/publications/internationalconcreteabstractsportal/m/details/id/51688972
- ASTM C494, “Standard Specification for Chemical Admixtures for Concrete,” https://www.astm.org/Standards/C494
- ACE CRD C48-92, “Standard Test Method For Water Permeability Of Concrete,” Army Corps of Engineers, https://www.wbdg.org/ffc/army-coe/standards/crd-c48
- DIN 1048:5 Water Permeability Test. Deutsches Institut für Normung, https://www.din.de/en
- ASTM C1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration,” https://www.astm.org/Standards/C1202.htm
- ASTM C1585, “Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic-Cement Concretes,” https://www.astm.org/Standards/C1585.htm