By Kayla Hanson, P.E.
Reclining in your favorite patio chair on a hot summer day, you kick back as the blazing sun beats down on your sweat-beaded arms. You sink your hand into the frigid, ice-cube-filled water in the cooler beside you and pull out a cold can of your favorite summer drink. You pop the tab and it sprays a cool, much-welcomed mist as it cracks open, releasing the pressure in the can. The carbonated liquid fizzes as you take your first refreshing sip. Few things are as welcomed as this on a hot summer afternoon. Carbonation isn’t always so welcomed, though. When it comes to concrete, carbonation poses a threat to durability, strength, and ultimately, safety of products in service.
How does carbonation in concrete happen?
Carbon dioxide is all around us. It’s in the air we breathe and in our water. In addition to giving our fountain drinks that welcomed bubbly sparkle or making dough rise, it’s also capable of penetrating a concrete surface and dissolving in pore water, which forms carbonic acid.1 The carbonic acid that forms in the concrete’s pores reacts with calcium hydroxide, a desirable product of cement hydration because it inherently strengthens the concrete matrix, and produces calcium carbonate.2
The main reactions involved in carbonation consume calcium hydroxide and produce calcium carbonate. This process basically replaces the relatively large calcium hydroxide molecules with relatively small calcium carbonate molecules. This replacement reduces concrete’s pH and increases its porosity. Although the concrete’s absolute volume may remain the same, the increase in porosity reduces the product’s relative volume and decreases its strength.
Factors affecting the rate of carbonation
Carbonation begins at the concrete surface and slowly penetrates deeper, typically at a rate of about .039 inches per year in high quality concrete with a low water-cementitious material ratio. For carbonation to progress in severity to bi-carbonation, where carbonation reactions continue to penetrate the concrete at greater depths, additional carbon dioxide is needed at those levels. Additional carbon dioxide can travel further into concrete via cracks, which serve as a transport mechanism for gases, water and other contaminants. A high w/cm also increases the likelihood of bi-carbonation, which ultimately results in porous, friable and weak concrete.
Besides forming within the concrete matrix, calcium carbonate can also form on the surface of the concrete and develop a weak, chalky exterior coating if fossil fuel-burning heaters are used incorrectly during cold weather concreting. The carbon dioxide emitted by the heaters reacts with calcium hydroxide on the concrete’s surface and ultimately produces the chalky calcium carbonate coating. The coating creates a layer around cement particles in the plastic or semi-plastic concrete and prevents the particles from hydrating further. To prevent the weak exterior layer of calcium carbonate from forming and detrimentally affecting the precast product, the NPCA Quality Control Manual prohibits use of such heaters for accelerated curing prior to initial set.
Water is essential for carbonation, and a relative humidity ranging from about 50% to 70% creates an ideal environment for the reactions to occur. Concrete with a relative humidity lower than 40% is less susceptible to carbonation because there is insufficient water to dissolve carbon dioxide. At a relative humidity in excess of 90%, when pores are filled with water, carbonic acid is unable to penetrate the saturated pores and diffuse throughout the concrete, again preventing carbonation.
Population density and geographic location are other factors that can influence carbonation.
“It seems to be worse in big cities where there are more fossil fuels being burned,” said Jesse Osborne, precast and underground segment manager with Euclid Chemical. “You see a bit more in the North than you do in the South. Two of the biggest factors are industrialism and humidity.
“Also the fact that the North is usually colder means you’re running a lot more furnaces and machines and the slower setting concrete is exposed to the elements longer.”
Understanding carbonation-induced pH reduction
Carbonation’s most significant effect on concrete is reduced durability, a direct result of pH reduction. For clarification of how this pH reduction actually happens, it’s important to understand that pH, a term deriving from the German words for “power of hydrogen,” is a measure of the hydrogen ion concentration in a water solution. The pH scale ranges from 0, representing very acidic substances such as car battery acid, to 14, representing very alkaline substances like sodium hydroxide, also known as lye. Pure water is considered neutral at a pH of 7.
Calcium hydroxide contains two hydrogen ions and is a main source of strength in a concrete matrix. Calcium carbonate contains zero hydrogen ions. After some of the calcium hydroxide is consumed during the carbonation process and is partially replaced by calcium carbonate, hydrogen ions still exist in the matrix but at a lower concentration. The weaker hydrogen ion concentration reduces the concrete’s pH.
A pH reduction in steel-reinforced concrete is more troublesome than in non-steel-reinforced concrete. The highly alkaline environment of concrete, which usually has a pH in excess of 12, creates a protective, passivating oxide layer around steel, protecting the reinforcement from corrosion. Carbonation is capable of reducing a concrete’s pH to a value less than 9, which significantly weakens, or could even eliminate, the steel’s protective layer.
It’s important to note that the pH scale is logarithmic, meaning that each step on the scale changes the pH by a factor of 10. This means a five point drop on the pH scale, as experienced by a concrete sample with an initial pH of 13 that drops to 8 after bi-carbonation, is a 100,000-fold reduction in the concrete’s pH.
Testing for carbonation
Tests for carbonation typically require a core to be extracted or a sample to be cut from the concrete. In most cases, a solution of phenolphthalein is applied to the freshly fractured face of the concrete sample to determine the presence and depth of carbonation.
A concrete surface wetted with the phenolphthalein solution will either remain its original color or turn a shade of fuchsia. The telltale fuchsia color appears only when the concrete’s pH is greater than 9.5, indicating little-to-no carbonation. A vibrant shade of fuchsia that appears immediately upon application indicates a pH much higher than 9.5, while a softer shade of fuchsia that appears slowly indicates a pH closer to 9.5. In cases where the pH has been reduced to less than 9.5 – implying excessive carbonation has occurred – there will be no change in the concrete’s color.
The depth of carbonation can be determined after the phenolphthalein solution has been applied to the fractured concrete surface. A typical cross section of a carbonated specimen will exhibit no color change in the outermost portion, indicating carbonation, and will display a shade of fuchsia toward the center of the piece. At times, the carbonation threshold will be faint fuchsia, while the innermost concrete will be a bright shade. The carbonation depth can be determined by measuring the distance from the outer edge of the concrete piece toward the center of the specimen, ending the measurement where the concrete has turned fuchsia.
When factors other than atmospheric carbon dioxide, such as improper or inadequate curing are suspected of causing the carbonation, other tests can be employed.
“In those cases, petrographic examination of can include various acidic reaction coloration testing, microscopic examination and potentially X-ray diffraction,” Osborne said.
Resisting carbonation and its effects
To avoid carbonation, remember that it’s more prevalent in concrete with a high w/cm, high water content, low cement content, or where a short curing period or inappropriate curing methods are used.
Concrete containing large quantities of supplementary cementitious materials, namely fly ash or slag, may also experience an increased rate of carbonation when compared to concrete with the same w/cm made without SCMs. Typically, concrete containing SCMs gains strength at a lower rate than conventional portland cement concrete, therefore additional curing time may be required for concrete containing SCMs to ensure sufficient carbonation resistance.
Lastly, highly permeable concrete, often the result of cracks or excessive porosity, has an increased likelihood for carbonation. When it does occur, the rate of carbonation in poor-quality concrete such as this is greater than in high-quality, properly cured pieces.
Steel-reinforced concrete is exceedingly more susceptible to corrosion if the concrete carbonates. Increasing concrete cover over embedded steel can help resist corrosion, as the length of time required for the carbonation to reach the depth of the steel will be greater.
Following quality practices when manufacturing precast concrete is important for a number of reasons, including avoiding carbonation. Using the NPCA Quality Control Manual and ensuring employees are properly trained makes a big difference. For questions on carbonation or quality control, get in touch with the NPCA technical services team.
Kayla Hanson, P.E. is a technical services engineer with NPCA.
1: carbon dioxide (CO2) dissolves in pore water (H2O) to produce carbonic acid (H2CO3): CO2 + H2O ↔ H2CO3
2: carbonic acid (H2CO3) reacts with calcium hydroxide (Ca(OH)2) to produce calcium carbonate (CaCO3) and water (H2O): H2CO3 + Ca(OH)2 → CaCO3 + 2H2O
Portland Cement Association, Design and Control of Concrete Mixes, 15th Edition
International Concrete Repair Institute, icri.org/publications/2014/PDFs/marapr14/CRBMarApr14_Kakade.pdf
Concrete Structures Part- II, Zahid Ahmad Siddiqi