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Carbonation Resistance in Concrete: Uncovering Long-Term Durability

Introduction

Carbonation serves as an important indicator of deterioration in reinforced concrete structures, particularly in marine and industrial environments where exposure to atmospheric $CO_2$ accelerates degradation. This process reduces the alkalinity of concrete, compromising the passive protection of embedded steel reinforcement and increasing the risk of corrosion.

This study evaluates the carbonation resistance of Xypex-treated concrete panels constructed in 1995. These reinforced concrete panels were placed in the splash zone at Lascelles Wharf, Port of Geelong, Australia. They have been subjected to aggressive exposure conditions, including the marine environment and bulk chemicals and fertilizers from off-loading operations. Comparative assessments conducted in 2014 (CHEM-142) and 2021 (CHEM-155) examined whether carbonation ingress had progressed over time.

Mechanism of Carbonation and its Impact on Concrete Durability

Carbonation occurs when $CO_2$ penetrates the porous network of concrete and reacts with calcium hydroxide $(Ca(OH)_2)$, forming calcium carbonate $(CaCO_3)$. This reaction lowers the pH of the concrete matrix from its naturally high alkaline state (≈12.5-13.5) to below a critical threshold (≈9), leading to depassivation of embedded steel reinforcement.

The carbonation reaction is:

$$CO_2+Ca(OH)_2⟶CaCO_3+H_2O$$

The rate of carbonation is influenced by several factors, including relative humidity, permeability, water-to-cement ratio, and exposure conditions. The pH reduction typically follows this approximation:

$$ pH=pH_0-kx_c $$

Where:

$pH_0$: is the initial pH of the concrete.
$k$: is the constant that reflects how rapidly the pH decreases with increasing carbonation depth (in pH per unit of depth) and is typically determined through experimental data or field measurements.
$x_c$: is the carbonation depth at a given time.

Once the pH of the concrete surrounding the reinforcement decreases below the critical pH threshold, it destabilizes the passive oxide layer on the steel—primarily composed of iron oxide $(Fe_2 O_3$ or $Fe_3 O_4)$ —increasing the susceptibility to corrosion, which leads to cracking, spalling, and structural weakening. To understand at what depth carbonation will cause the pH to fall below the critical value, initiating the corrosion process in the steel reinforcement, the depassivation condition equation is used:

$$x_{c, c r i t}=\frac{p H_0-9}{k}$$

Where:

$pH_0$: is the initial pH of the concrete.
$k$: is the constant that reflects how rapidly the pH decreases with increasing carbonation depth (in pH per unit of depth).
$x_{c, c r i t}$: is the carbonation depth at a given time.
$9$: is the critical pH threshold for depassivation.

Thus, when the carbonation depth reaches x_(c,crit), the pH around the reinforcement drops below 9, leading to the depassivation of the steel.

Experimental Procedure

A worker drills and collects concrete cores from the surface of the completed Lascelles Wharf, Australia, to perform independent testing. — A worker drills concrete cores for independent testing at Lascelles Wharf, Australia.

The carbonation resistance of Xypex-treated concrete was assessed through the following testing procedures:

Core Extraction: Five core samples were extracted from the Xypex-treated slabs, built in 1995.
Phenolphthalein Indicator Test: A 0.1% phenolphthalein solution was applied to freshly fractured core surfaces to determine the carbonation depth. A color change to red/purple indicates uncarbonated concrete, while a colorless reaction indicates carbonation.
Measurement and Analysis: The depth of carbonation was recorded and compared to untreated control samples from other structures.

Results

All five core samples exhibited a staggering 0 mm carbonation depth in 2014 and in 2021, confirming that carbonation ingress was non-existent even after 26 years of exposure. These results suggest that Xypex crystalline technology effectively helps to maintain the concrete’s alkalinity, protecting embedded reinforcement from carbonation-induced deterioration.

Conclusion