A Guide to Concrete Carbonation and Accelerated Carbonation Testing

Man began using concrete 5,000 years ago to build monuments that could stand the test of time.

The ancient Egyptians used a primordial form of concrete to build their pyramids

The Romans used it to build their massive aqueducts, which enabled the rapid growth of the empire.

In 1824, Joseph Aspdin invented Portland cement, which would go on to create the modern wonders we still enjoy today.

However, many of the ancient buildings have collapsed or are at risk due to the carbonation of the concrete.

But what is carbonation, and what makes concrete so special? Read on to learn more and find out how manufacturers conduct accelerated carbonation testing.

What is concrete?

Concrete is a composite material consisting of a matrix and a binder. The matrix is ​​an aggregate of rock material, usually limestone, granite, or gravel.

The binder is usually Portland cement and holds the matrix together.

According to the Bogue equations, Portland cement contains between 60-67% CaO, 17-25% SiO2, 3-8% Al2O3, 0.5-6% Fe2O3, 0.1-4% MgO, 1.3-3% SO3, and 0.4-1.3% alkali.

Its chemical composition makes it resistant, strong and gives it the ability to lose plasticity and gain compactness with the subsequent addition of water.

Water is added to a dry powder and aggregate, resulting in a soft substance that the worker can manipulate into any shape.

Structures made with Portland cement tend to use steel bar reinforcement, creating a strong structure that can support a large weight without bending or breaking.

However, the greatest threat to concrete structures is the process of carbonation.

What is concrete carbonation?

Carbonation is the chemical reaction between carbon dioxide CO2 and cement hydration compounds in concrete structures.

The carbonation rate depends on many physical characteristics, such as:

• On-site preparation
• Production and protection
• The design of the structure

In addition to the above, other factors also play an important role, such as the location of the structure and its exposure to contaminants.

Finally, environmental factors, such as heat, rain, snow, and humidity, also play a role in carbonation.

What are the effects of carbonation? Why is it harmful?

Carbonation works its way deeper into the concrete over time, generally at a rate proportional to the square root of time.

It takes only a few hours for the concrete surface to react with CO2 in the air. The reaction can reach a depth of 1 mm for thick concrete and 5 mm for porous and permeable concrete.

This process causes a domino effect: first of all, the pH is reduced, that is, the natural alkalinity of the concrete is lowered from pH13 to pH8.

The lower pH in turn causes the passivation layer around the steel to break down, exposing the reinforcing steel to water and air, thus causing it to rust.

The rusted steel then expands, pushing against the surrounding concrete.

The concrete eventually cracks, weakening the structure and putting it at serious risk.

Carbonation is one suspected cause, among many, of building collapses in Lagos, Nigeria—an example of the damage untested concrete can cause.

There is some good news,.

Most people associate the carbonation process with the weakening of reinforcements and the shrinkage of steel, but not all effects are negative.

Carbonation also increases the compressive and tensile strength of concrete. The key is to find the right balance. To do this, you need accelerated carbonation tests.

What is the Accelerated Carbonation Test?

In simple terms, this test is nothing more than anin-depth investigation to accelerate carbonation in a controlled environment.

It's a way for concrete professionals to test their product and determine its ability to last under certain conditions. This test is performed by testing its corrosion resistance. It goes without saying, therefore, that a correct diagnosis of any problems can be crucial from a safety and prevention perspective.

Each test uses different parameters based on the type of conditions the concrete may be exposed to.

Some of these factors include:

• • The concentration of CO2 gas – This is a particularly important factor for concrete used in the most polluted cities
  • Humidity – Humidity is best at 50%-70%. There's less water when there's humidity. Too much water inhibits the diffusion of CO2.
  • Temperature – In warmer environments, there is a higher rate of carbonation
  • Water to cement ratio: Determines the pore system of the concrete and the degree of hydration
  • Porosity and permeability

The problem is that it is difficult, if not impossible, to control all these variables without the proper equipment.

Any small change in these variables can lead to faulty test results and the creation of inadequate or unsafe constructions.

Indeed, even when researchers control for these factors, there can still be a difference between what is observed and what actually happens in a real natural environment.

It is impossible to control every variable, but it is possible to increase the chances of success of the investigation if you use the right test chamber.

To perform these tests in a controlled laboratory, FDM provides a test chamber_CO2 , capable of simulating enriched atmospheres and maintaining stable temperature and humidity parameters required by the EN 12390-12 standards.

Standards and Procedures for Testing Concrete for Carbonation

The recommended profiles for carbonation tests are:

• CO2 levels at 4%
• Relative humidity levels at 65%
• Setting the temperature to 20°C
• Active CO2 control
• An efficient circulation fan
• A reference mix
• Calibration and proper recording of results

It is recommended to condition the samples in laboratory air for one month at 20-23°C.

If you are testing concrete with a protective coating, you will need to sandblast the samples before applying the protector, let them “cure” for three days, and then bring them to the laboratory at least three months after selecting them.

The ideal thickness for the test is between 40 and 50 mm. This will provide enough material to evaluate the results.

fig.1: Experimental test of accelerated carbonation

Remedies for carbonation of concrete

As you can easily imagine, there are no remedies or palliatives that can easily resolve a problem related to concrete that has undergone a carbonation process.

Very often, the only solution is to carry out major renovations where no more serious problems such as collapses or irreparable damage have already occurred.

The only viable option is to work to prevent the problem by carrying out all the necessary tests and analyses before the concrete is produced and used en masse for construction work. 

How to Find the Right Test Chamber

Here are some suggestions for understanding the main assessments to be made during the investigation phase with a concrete test chamber.

• Measure different variables for each type of concrete being tested.
• Apply different values ​​to these variables based on where the concrete will be used.

  For example, you'll need to increase the moisture content for testing concrete used in Florida compared to testing in Kansas. You'll also need to account for Florida's year-round heat compared to Kansas's cold winters.

• Use a test chamber that allows for the same variations as the experiments.
• Prefer a test chamber that is durable over time, especially since testing for accelerated carbonation is an ongoing process.

We can help you find your ideal chamber

We offer a wide range of test chambers to meet any testing need.

You can choose between various sizes and two different controllers, depending on whether you are running stability or dynamic tests.

Contact us and we will find the ideal chamber for your concrete carbonation tests

Discover the FDM solution for concrete testing: our concrete test chamber complies with EN 12390-9 and UNI EN 1367-1 standards, with an automatic immersion system and programmable freeze-thaw cycles.

The Reference Standards for Carbonation Tests

Accelerated carbonation tests are regulated by several international standards that define operating parameters, specimen preparation, and evaluation criteria:

• EN 13295 — Determination of carbonation resistance of hardened concrete according to the European standard procedure (1% CO₂, 20°C, 60% RH).
• EN 14630 — Method for determining carbonation depth by the phenolphthalein test.
• UNI EN 12390-12 — Italian specification for accelerated carbonation testing on hardened concrete, under defined environmental conditions (3% CO₂, 20°C, 57% RH).
• ISO 1920-12 — International standard for testing of concrete for resistance to carbonation.
• RILEM TC 56-MHM — Technical recommendations of the RILEM committee for the measurement of carbonation depth.
• ASTM C1202 — Complementary practice for evaluating chloride ion permeability, often used in conjunction with carbonation tests.

Frequently Asked Questions about Concrete Carbonation

How long does an accelerated carbonation test take?

A standard test according to EN 13295 or UNI EN 12390-12 typically lasts 28 to 70 days, with periodic measurements of the carbonation depth. Long-term tests for the validation of special cementitious formulations can extend up to 180 days. The advantage of the accelerated test (with 3-4% CO₂ compared to 0.04% atmospheric CO₂) is a 50-100-fold reduction in testing times compared to natural exposure.

What are the optimal environmental conditions for testing?

The standards converge on a narrow range: 20 ± 2°C temperature, 57-65% relative humidity , and 1-4% CO₂ concentration. Humidity is the most critical parameter: below 50%, carbonation slows because there is no water for the reaction; above 75%, the concrete pores become saturated and CO₂ cannot penetrate.

How is carbonation depth measured?

The standard method according to EN 14630 is the phenolphthalein test: the specimen is broken and a 1% alcoholic phenolphthalein solution is sprayed onto the fresh surface. The non-carbonated area (pH > 9) turns pink-violet, while the carbonated area (pH < 9) remains colorless. The depth is measured from the outer edge to the colored front, in millimeters.

Do you need a dedicated climate chamber or is an air-conditioned chamber sufficient?

For standard-compliant testing, a dedicated carbonation chamber with simultaneous CO₂, temperature, and humidity control, plus data logging, is required. A climate-controlled chamber cannot control the CO₂ concentration or maintain it stable over time. FDM carbonation test chambers integrate a CO₂ dosing system with feedback and concentration maintenance within ±0.2%.

Authoritative Sources

• ISO 1920-12 — Testing of concrete — Determination of carbonation resistance.
• RILEM — International Union of Laboratories and Experts in Construction Materials.
• fib — International Federation for Structural Concrete.
• UNI — Italian Standardization Organization.