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What is Carbonation depth testing? 

Carbonation depth testing measures the average distance from the surface of a concrete element, structure or sample to the depth carbonate ions have penetrated the concrete and reduced the alkalinity of the cement paste. 

Carbonation depth shown by phenolphthalein solution Figure 1. Carbonation depth shown by phenolphthalein solution

What is the principle behind Carbonation depth testing? 

Concrete is extremely alkaline (pH = 12+) but carbonation reduces this alkalinity to below 9.5. This causes decomposition of hydration products, de-passivation of reinforcement and cracking of the concrete cover; all of which make the concrete extremely vulnerable to corrosion. 

Carbonation depth schematic and image of tested slab Figure 2. Carbonation depth schematic and image of tested slab

Phenolphthalein solutions turn from clear to pink below a pH around 9.5. As such when this compound is sprayed on fresh fractured concrete its stains the surface and the carbonated area is easily identifiable. 

What is Carbonation depth testing used for? 

 

Deterioration process 

Defects 

Control of repairs 

Carbonation 

Reinforcement corrosion 

Spalling 

Corrosion cracks 

Concrete replacement 

Re-alkalization 

 

How do I carry out Carbonation depth testing? 

Carbonation depth testing is an extremely simple test to carry out requiring only basic equipment. This test is extremely popular worldwide with a wide range of standards documents available dictating similar best practice procedures such as  BS EN 14630:2006 and ISO 1920-12:2015. A simple step by step procedure to carry out carbonation depth testing is presented below: 

  • A coring schedule must be developed based on the test objectives aND completed based on correct sampling procedures. This could be a systematic random sampling of an entire structure or a targeted investigation of specific areas at risk of carbonation and corrosion. 

  • Once cores have been removed, they must be split down their centre. A core splitter is designed for this purpose however a test rig for tensile splitting tests would also be suitable.  

Core splitter and Tensile splitting test rig Figure 3. Core splitter and Tensile splitting test rig

  • If required prepare the phenolphthalein solution by mixing 1g of phenolphthalein with 50g of ethylic alcohol and 50g of water and apply evenly to the fractured concrete surface using a spray bottle. 

  • Wait for the solution to react and observe the colour change. 

  • Measure Carbonation Depth from the surface to the colour change boundary using vernier callipers. 

What equipment and expertise are required for Carbonation depth testing? 

This test is extremely simple an only requires a core of concrete from the element under investigation, and a Phenolphthalein indicator solution (1g phenolphthalein to 50 ml of water and 50 ml of ethylic alcohol) and vernier callipers to measure the carbonation depth. 

Phenolphthalein indicator solution and vernier callipers Figure 4. Phenolphthalein indicator solution and vernier callipers
What are the advantages of Carbonation depth testing? 

  • Well established, familiar technique.

  • Cheap, easy and quick to carry out.

  • Gives definitive measure of carbonation depth. 

  • Assesses concrete mix quality and effectiveness of curing. 

What are the disadvantages of Carbonation depth testing? 

  • May underestimate carbonation depth in concrete with pH 9.5-11.  

  • Only provides information about the carbonation depth and not about other forms of degradation or the overall condition of the concrete. 

  • Collecting samples to perform this test requires a portion of the concrete member be damaged and the tested area to be destroyed. Therefore, several samples can’t be taken from a single location to validate results. 

How accurate is Carbonation depth testing? 

Carbonation depth is normally recorded with an accuracy of 0.5 mm in intervals of 10 mm. 

What are the limits of Carbonation depth testing? 

The carbonation zone is of finite width and corrosion can be initiated when pH is lower than 11. Corrosion may therefore initiate a few millimetres ahead of the carbonation front as measured by the phenolphthalein which only begins to change colour below pH 9.5.  

The test method only indicates that corrosion is possible to initiate and gives no indication of the rate of deterioration due to carbonation.  

For high alumina cement concretes this test is unreliable as the presence of calcium aluminate phases can lead to a different mechanism of carbonation than the calcium silicate hydrate (C-S-H) phases predominant in Portland cement concrete. Additionally, high alumina concretes are often more porous, which could lead to faster carbonation penetration and an uneven carbonation front, complicating the measurement and interpretation of carbonation depth. 

Carbonation based corrosion of reinforced concrete Figure 5. Carbonation based corrosion of reinforced concrete
Ancillary information 

Maturity of test: > 10 years 

Qualification & interpretation : Specialised lab 

Service disruption: No 

Preliminary works: Yes 

Time consumption     Low (< one hour) 

Cost                        Low 

Access to element 1 face 

References and further information 

BS EN 14630:2006 - Products and systems for the protection and repair of concrete structures. Test methods. Determination of carbonation depth in hardened concrete by the phenolphthalein method. 

ISO 1920-12:2015 - Testing of concrete - Part 12: Determination of carbonation depth in hardened concrete by the phenolphthalein method. 

BS EN 12504-1:2019 Testing concrete in structures - Cored specimens. Taking, examining and testing in compression 

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