# Assessing the impact of carbonation on the CFRP-mortar bond: A combined mechanical and microstructural analysis

Dania Kabalan, Amandine Celino, Frédéric Grondin, Emmanuel Roziere

► To cite this version:

Dania Kabalan, Amandine Celino, Frédéric Grondin, Emmanuel Roziere. Assessing the impact of carbonation on the CFRP-mortar bond: A combined mechanical and microstructural analysis. Construction and Building Materials, 2025, 493, pp.143162. 10.1016/j.conbuildmat.2025.143162. hal-05523395

HAL Id: hal-05523395

https://hal.science/hal-05523395v1

Submitted on 23 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
<figure><img src="image_2.png" /></figure>

# Assessing the impact of carbonation on the CFRP-mortar bond: A combined mechanical and microstructural analysis

Dania Kabalan<sup>a</sup>, Amandine Celino<sup>b</sup>, Frédéric Grondin<sup>a,b</sup>, Emmanuel Roziere<sup>a,* b</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Fiber-reinforced polymer (FRP)

Carbonation

Concrete repair

Mortar

Pull-out test

Composite/concrete adhesion

## ABSTRACT

Research on reinforcement and repair techniques for infrastructure weakened by environmental factors has been extensive, with a focus on evaluating their mechanical behaviour. One promising method in civil engineering is the use of bonded composite reinforcements, which offer a wide range of applications. When the structure is repaired or strengthened, concrete has been exposed to environmental actions for years, thus the external layer is likely to be carbonated. This study investigates the influence of initial carbonation on the adhesive bond between concrete and carbon fiber-reinforced polymer (CFRP) systems. In this experimental study, cement mortar specimens were exposed to accelerated carbonation conditions at a CO₂ concentration of 3 %. Mechanical tests were conducted on both control and carbonated samples to assess changes in adhesive properties. Pull-out tests showed that carbonation and surface preparation methods significantly improved bond strength. Notably, the bond strength showed a significant increase of 30 % within 60 days of accelerated carbonation. Carbonation improves the compressive strength of concrete by producing calcium carbonates. This study shows that, in infrastructure rehabilitation projects especially in areas with high CO₂ exposure, taking advantage of natural carbonation before applying CFRP can improve bond performance and overall durability as well as sustainability.

## 1. Introduction

The global infrastructure is aging, and with it comes an increasing demand for maintenance and rehabilitation of reinforced concrete structures. These constructions, vital to the safety and functionality of our urban environments, are subjected to various environmental actions throughout their service life. These actions, including carbonation, chloride ingress, and freeze-thaw cycles, progressively deteriorate the structural integrity of concrete, leading to significant challenges in maintaining the durability and performance of buildings and civil engineering structures. In Europe, the cost of rehabilitating reinforced concrete structures due to reinforcement corrosion is estimated to exceed 600 million euros per year [34]. This staggering figure highlights the financial burden posed by the degradation of concrete infrastructure. Given the magnitude and urgency of this issue, it is increasingly necessary to find innovative and cost-effective solutions to repair and reinforce concrete structures. One such solution that has garnered significant attention in recent years is the use of Carbon Fiber Reinforced Polymer (CFRP) [14,16,17,20,30].

By enhancing the structural integrity of aging constructions, Fiber

Reinforced Polymer (FRP) materials play a crucial role in extending the service life of these structures, while also bolstering their resilience against future stressors. Among the various FRP reinforcement techniques, such as near-surface mounted (NSM) reinforcement and fiber-reinforced polymer wrapping, externally bonded FRP (EB-FRP) is the most widely endorsed and utilized approach [23]. This method typically involves the manual application of either wet lay-up systems or prefabricated FRP sheets, bonded to the concrete surface using a cold-curing adhesive. A key advantage of the EB-FRP method lies in the strategic alignment of the fibers, which are oriented to run parallel to the direction of the primary tensile stresses [37]. This alignment maximizes the reinforcement's effectiveness in resisting tensile forces, thereby significantly improving the structure's load-bearing capacity. Additionally, EB-FRP is favored for its relatively straightforward installation process, which is both time-efficient and less labor-intensive, making it an appealing choice for field professionals. Moreover, beyond merely adding strength, EB-FRP systems enhance the durability of concrete structures by providing a protective layer that mitigates the risks of corrosion and other types of environmental damage. The simplicity of the EB-FRP installation process also translates into cost savings, as it

<footnote><sup>a</sup> Nantes Université, Ecole Centrale Nantes, CNRS, GeM, UMR 6183, Nantes F-44000, France</footnote>

<footnote><sup>b</sup> Nantes Université, Ecole Centrale Nantes, CNRS, GeM, UMR 6183, Saint-Nazaire F-44600, France</footnote>

<footnote>* Corresponding author.
E-mail address: emmanuel.roziere@ec-nantes.fr (E. Roziere).</footnote>
reduces the need for complex and time-consuming demolition or
reconstruction efforts. As a result, EB-FRP has emerged as a preferred
method for concrete reinforcement, with its growing popularity re-
flected in the increasing body of research validating its effectiveness and
widespread application [13,28,46].

A critical aspect of using CFRP for structural rehabilitation is
ensuring a strong bond between the CFRP material and the existing
concrete, as this bond is crucial for the reinforcement's long-term
effectiveness [47]. Several techniques are used for surface preparation
before the application of CFRP on concrete structures [41]. However,
during the rehabilitation process, concrete structures often undergo
natural carbonation when exposed to atmospheric CO₂, which alters the
microstructure of the cement paste and influences properties such as
strength, shrinkage, creep, and corrosion resistance [21,29,32].
Whereas carbonation is generally considered as a detrimental process
leading to reinforcement corrosion, it may also affect the bond between
concrete and CFRP in certain conditions, particularly in regions with
high CO₂ exposure such as urban areas with elevated industrial emis-
sions [26,33]. Typically, the natural carbonation process is not
accounted for during rehabilitation, yet it may have significant impli-
cations for the bonding performance between CFRP and concrete.

It is noteworthy that the idea of using carbonation as a solution to civil engineering challenges has gained increasing attention in recent years. For instance, the treatment of recycled aggregates through accelerated carbonation enhances their mechanical properties and durability, facilitating their reuse in construction. This approach is relevant given the progressive depletion of natural aggregates and the growing emphasis on sustainable building practices, especially carbon capture and long-term storage. Recent studies, such as those by [11] and [19], have demonstrated the effectiveness of carbonation treatment in improving the quality and performance of recycled materials. These developments underscore the broader potential of carbonation not only as a degradation mechanism but also as a beneficial process in concrete technology and resource conservation.

Many studies have investigated the effect of surface treatments on the bond between CFRP and concrete [24,41], but no study has addressed the combined effect of carbonation and surface treatment. This article focuses on investigating how carbonation-induced changes in the cement paste influence the bonding performance between CFRP and concrete, with an emphasis on understanding the combined effects of surface preparation and carbonation.

The main objectives of this experimental study can be summarized as
follows:

* Examine the combined effects of carbonation and surface treatment on the bonding behavior at the CFRP-mortar interface.

* Evaluate the impact of carbonation on the mechanical properties of cement-based mortar

* Investigate the bond performance between CFRP and carbonated mortar.

* Compare two surface preparation methods—sandblasting and grinding—to determine their effectiveness in enhancing CFRP adhesion under carbonation exposure.

* Investigate the effect of carbonation on the CFRP-mortar interface by linking microstructural and pore structure changes—identified through Thermogravimetric Analysis (TGA), X-ray Diffraction (XRD), water absorption, and Mercury Intrusion Porosimetry (MIP)—to variations in bond performance and overall material durability.

* Develop a comprehensive understanding of the interplay between carbonation, surface treatment, and CFRP bonding to inform strategies for improving the durability and performance of CFRP-reinforced mortar systems.

## 2. Materials and experimental program

### 2.1. Materials and sample preparation

The choice to conduct tests on mortar stems from consideration of the wall effect in concrete structures. In such structures, the outer layer of concrete is often comparable to mortar. Therefore, by testing mortar, it is possible to simulate effectively the behavior of the mortar layer present on the surface of the actual structure. This approach allows us to examine closely the adhesion properties relevant to the surface layer, which is crucial for assessing the performance of composite-reinforced structures in real applications. A water-to-cement ratio (W/C) of 0.60 was employed, which aligns with the recommendations from standard NF EN 206+A2/CN (2022) for exposure classes prone to carbonation-induced corrosion (XC). Portland cement was used as it is considered as representative of the cements used to design most of structural concrete mixtures a few decades ago, as the study focuses on aged structures.

In this research, the materials selected given in Table 1 consist of Type CEMI 52.5 N Portland cement and natural silica sand with a particle size distribution ranging from 0 to 4 mm. This particular sand was chosen to ensure that the calcium carbonate observed during the study was exclusively derived from the carbonation process of the cement paste. To improve the workability and the homogeneity of the fresh mortar, a superplasticizer was added at a dosage of 0.3 % by mass of the cement. Additionally, xanthan gum was used as a stabilizer at a concentration of 0.6 % by mass of the water. The proportions of these components were carefully optimized through an iterative process to achieve the desired homogeneity and performance of the mortar.

The experimental program, illustrated in Fig. 1, involved two types of specimens: prismatic slabs and cylindrical samples. Four mortar slabs, each measuring 30 × 30 × 5.5 cm³, were cast. The largest faces (30 × 30 cm²) were oriented perpendicularly to the casting direction. After 24 h in molds, the samples were demolded and subjected to water curing for 28 days at 23 ± 1°C. The study aimed to assess the effect of the carbonated layer on the adhesive properties of FRP composites, so only the largest surfaces of each sample were exposed to carbonation. To ensure this, aluminum foil was applied to the other faces post-curing, allowing carbonation to occur solely on the intended surfaces.

Before carbonation testing, a preconditioning phase was implemented: samples were dried at 40 °C for 28 days to stabilize their mass, followed by a 7-day period in a controlled environment (T = 20 °C, RH = 55 %) to equalize internal moisture and reach an optimal water saturation level for carbonation [43]. The drying temperature of 40 °C was selected to remove free water efficiently while avoiding any damage of hydration products. According to Ref. [12], drying at temperatures above 60 °C can lead to the loss of chemically bound water from phases such as ettringite and C-S-H, resulting in microstructural damage. Conversely, drying below 60 °C requires longer durations, during which continued hydration, potentially altering the microstructure. This temperature is also consistent with the French standard XP P18-458 (accelerated carbonation test procedure), which recommends drying concrete specimens at 45 ± 5 °C. Therefore, maintaining drying at 40 °C until mass stabilization offers a balanced approach ensuring sufficient moisture removal while preserving the microstructure of the

<table><caption>**Table 1**
Mix design of the selected mortar composition.</caption><thead><tr><th></th><th>Mortar</th></tr></thead><tbody><tr><td>Cement</td><td>500 kg/m<sup>3</sup></td></tr><tr><td>Water</td><td>302 kg/m<sup>3</sup></td></tr><tr><td>W/C</td><td>0.60</td></tr><tr><td>Sand</td><td>1320 kg/m<sup>3</sup></td></tr><tr><td>Super plasticizer</td><td>1.5 kg/m<sup>3</sup></td></tr><tr><td>Stabilizer (xanthan gum)</td><td>1.8 kg/m<sup>3</sup></td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Experimental program.</figcaption></figure>

cementitious matrix. This preconditioning step was crucial in promoting CO₂ diffusion during subsequent carbonation.

In parallel, mortar cylinders (14 cm height × 7 cm diameter) were prepared to monitor changes in carbonation depth, porosity accessible to water, pore size distribution, and mechanical properties (compressive strength and Young's modulus) over time. These cylindrical samples were prepared with adhesive aluminum applied to the top and bottom surfaces, ensuring that CO₂ diffusion occurred only through the lateral surfaces. The same preconditioning protocol was applied to these cylindrical samples.

In order to explain some of the observed phenomena and provide a detailed analysis of the results, a microstructural characterization was carried out. These observations such as thermogravimetric analysis and X-Ray diffraction should be performed on cement paste due to practical limitations arising from the presence of aggregates in these experiments and their analysis. For this purpose, cylindrical paste samples with the same water-to-cement ratio used for the mortar samples and with a stabilizer concentration of 1.2 % of the water mass were cast in plastic molds measuring 3 cm × 5 cm. Additionally; it is worth noting that the preconditioning process applied to mortar slabs and cylinders was also applied to the paste cylinders.

The commercial product Sika CarboDur M) was selected as a Carbon Fiber Reinforced Polymer (CFRP) for this study due to its widespread use in structural rehabilitation systems. This material is known for its excellent mechanical properties and durability, making it suitable for repairing and strengthening concrete structures. Sika CarboDur M is a pultruded composite lamella with a thickness of 1.4 mm and a width of 50 mm, composed of carbon fibers encapsulated in an epoxy matrix. Sikadur-30, a high-performance epoxy resin recommended by the supplier for bonding CFRP to concrete substrates, was selected for its strong adhesion, durability, and resistance to environmental degradation, making it an ideal choice for structural repair applications. The main properties of both the CFRP and the structural adhesive Sikadur-30, extracted from the datasheets, are provided in Table 2. These materials were chosen due to their proven effectiveness in real-world applications, such as the rehabilitation of bridges like Belton Lane Bridge in the UK and Grafton Bridge in New Zealand, where they demonstrated reliable performance under harsh environmental conditions and mechanical stresses [39].

<caption>Table 2</caption>

Physical and mechanical properties of CFRP and its bonding adhesive (Extracted from datasheet).

<table><thead><tr><th></th><th>Density (g/cm³)</th><th>Volumetric percentage of fibers (%)</th><th>Rupture elongation (%)</th><th>Tensile strength (MPa)</th></tr></thead><tbody><tr><td>CFRP (Sika Carbodur M)</td><td>1.60</td><td>68</td><td>1.35</td><td>3200</td></tr><tr><td>Structural adhesive (Sikadur-30)</td><td>1.65</td><td>-</td><td>1.0</td><td>24.8</td></tr></tbody></table>

## 2.2. Exposure conditions

One of the most severe environmental attacks on reinforced concrete structures is carbonation-induced corrosion. When CO₂ diffuses into cementitious materials, it reacts with hydration products, resulting in the precipitation of calcium carbonates CaCO₃ and a decrease in the pH of the interstitial solution. This change in pH can damage the protective oxide layer on the steel surface and lead to subsequent steel corrosion [7]. Due to the inherently slow nature of this process, accelerated carbonation protocol is employed. This approach is based on the control of CO₂ concentration, relative humidity, and temperature. The applied CO₂ concentration was 3 %, with a relative humidity of 60 % ± 5 % and a temperature of 20 °C. The 3 % CO₂ concentration allows accelerating the carbonation of cement-based materials while keeping the evolution of the microstructure representative of natural carbonation [6].

The application of fiber-reinforced polymer composite materials to mortar surfaces necessitates specific surface preparation techniques. The different methods widely employed by researchers are sandblasting, grinding, brushing, scarification, steel shot blasting, and brush hammering, each offering distinct benefits and drawbacks concerning achieved roughness levels, cost-effectiveness, processing duration, and ease of application [42]. Two techniques—sandblasting and grinding—were implemented in the study to assess and compare their respective impacts. These surface treatments were performed immediately after the preconditioning process and just before initiating the carbonation tests (Fig. 1). This approach was intended to minimize any potential effects stemming from preconditioning, ensuring that the observed evolutions could be directly attributed to the carbonation testing procedures.
## 2.3. Pull-out tests

A pull-out device equipped with a digital acquisition system was employed to measure the load required to detach the composite from the substrate (Fig. 2). The tensile load at failure (kN) was measured after 0, 28, and 60 days of accelerated carbonation. At each stage, when a slab was removed from the carbonation chamber, the surfaces of the FRP plates (5 × 5 cm²) and the steel component used in the pull-out test were carefully prepared with sandpaper before being bonded together and attached to the slab's surface (Fig. 2).

Initially, a commercial two-component epoxy adhesive, ISOBOND 735, was applied between the FRP plates and the steel piece, with a hardening time of 30 ± 15 min. This adhesive was chosen to ensure that failure would occur at the mortar-composite interface before any crack between the steel and composite. Subsequently, a different adhesive, Sikadur-30 (properties described in Table 2), was applied between the FRP plates and the slab surface, followed by a 24-hour curing period. The selection of Sikadur-30 was based on recommendations from the FRP plate supplier to ensure compatibility with cementitious materials. This procedure ensured optimal adhesion between the FRP plates and the steel and mortar surfaces. For each surface treatment, three measurements were taken from both the upper and lower surfaces of the slab, and the average value was calculated.

## 2.4. Microstructural characterization

### 2.4.1. Porosity accessible to water

Total porosity (i.e. the volumetric proportion of voids) was determined under vacuum according to the French standard NF P18-459. The samples were placed in a desiccator under vacuum for 4 h (±30 min), and then the desiccator was filled with water and kept under vacuum for 44 h (±30 min). The samples were then dried at 105°C to constant weight. The following weights were recorded:

* W<sub>dry</sub>: dry weight of the sample, after oven-drying at 105 °C.

* W<sub>sat</sub>: weight in air of saturated-dry surface sample.

* W<sub>hydro</sub>: hydrostatic weight, weight in water of saturated sample.

The total porosity is then obtained from the following equation:

$$ \text{Porosity}(\%) = \frac{W_{\text{sat}} - W_{\text{dry}}}{W_{\text{sat}} - W_{\text{hydro}}} * 100 \quad (1) $$

### 2.4.2. Mercury intrusion porosity

A standard porosimeter, which allows the application of pressures up to 400 MPa, was used to assess the pore size distribution of cement mortar. This pressure corresponds to a minimum pore radius of 1.5 nm. Assuming that pores are cylindrical, the radius r, penetrated by mercury at pressure P, can be calculated by the Washburn Eq. (2):

$$ r = \frac{2 * \sigma * \cos\theta}{p} \quad (2) $$

With σ the surface tension of the mercury (σ = 0.475 N/m) and θ the contact angle between the solid surface and the mercury (θ = 141°).

The mercury intrusion porosity was conducted on mortar samples of 1.5 × 1.5 × 1.5 cm³ that were dried at 40 °C for less than 24 h prior to testing; the remaining moisture content was removed during the vacuum phase prior to mercury intrusion.

### 2.4.3. Thermogravimetric analysis

Thermogravimetric analysis (TGA) is a method of thermal analysis in which changes in physical and chemical properties of materials are being observed as a result of a controlled increase in temperature. It is most widely used for quantitative measurements. The weight variations during temperature increase can be attributed to phases decomposition, allowing to identify theses phases from their decomposition temperature and their content from mass variations. This technique has been shown to be useful to assess the influence of accelerated carbonation on the microstructure of cement-based materials.

The thermal analysis was performed using a NETZSCH microbalance. A 2 cm³ sample was crushed and sieved and the resulting powder sample was collected in an alumina crucible and placed in the thermal balance. The tests were performed under a nitrogen stream in the temperature range from 20 to 1000 °C with a heating rate of 10 °C per minute. Samples of paste were tested by TGA during their carbonation. These tests were used to correlate the TGA results with the XRD result.

The analysis of the results was based on the study conducted by [19], where the thermal decomposition of various phases in cementitious materials was carefully analyzed using thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). The decomposition temperatures for the identified phases are as follows:

* **Portlandite**, Ca(OH)₂: Decomposes between approximately 400 °C and 500 °C, where a significant mass loss is observed due to the dehydroxylation of calcium hydroxide.

* **Calcite**, CaCO₃: Decomposition occurs between 600 °C and 800 °C, associated with the decarbonation of calcium carbonate.

* **Vaterite and Aragonite**, CaCO₃: These polymorphs of calcium carbonate decompose at temperatures similar to calcite, but their distinct thermal signatures can be identified in the DTG curve.

### 2.4.4. X-ray diffraction (XRD)

XRD analyses were carried out on a Malvern Panalytical Aeris instrument. It combines two technologies from Aeris and Zetium and provides full material characterization by adding information about elemental composition determined by Zetium to the phase identification from Aeris. Scanning program consisted in rotating between 7 and 70°2θ with a step size of 0.01°2θ and a time per step of 480.165 ms. XRD

<figure><img src="image_5.png" /><figcaption>Fig. 2. Digital pull-out resistance tester.</figcaption></figure>
analysis was performed on paste instead of mortar to avoid the effect of sand particles, as many peaks associated to quartz sand would have appeared in results.

The primary objective of this analysis was to quantify the phases formed and depleted due to the carbonation of cement hydration products, providing insights into the changes in the material crystalline structures. These results were further correlated with quantification data obtained from TGA to provide a comprehensive understanding of the phase transformations, particularly focusing on the mass loss associated with specific compounds during carbonation.

### 2.4.5. Scanning electron microscopy and energy dispersive spectroscopy

The polished composite samples (mortar, adhesive, and FRP) were examined using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). A 1 × 1 cm sample was cut from each slab, embedded in low-modulus epoxy resin, and polished to prepare the cross-section. This cross-section enabled the assessment of the interface quality between the FRP and adhesive, as well as between the adhesive and mortar (both carbonated and non-carbonated). The SEM-EDS analysis was conducted in a specimen chamber under vacuum at a pressure of 30 Pa and an accelerating voltage of 15 kV, utilizing backscattered electron imaging for observations.

### 2.4.6. 3D-microscope

To assess the surface roughness of mortar slabs during the carbonation process, 3D maps of the surface (2 cm × 2 cm) were generated using a Hirox RH-2000 3D microscope. This involved merging hundreds of images evenly spaced across the section to ensure comprehensive coverage and accurate representation of the surface topography. Magnification of × 50 was chosen for this evaluation. The average surface roughness, Ra in µm, is calculated directly by the microscope. On average, 12 profiles are taken, and the average roughness (Ra) is calculated based on these profiles.

## 3. Results and discussions

### 3.1. Carbonation depth of cement mortar

Fig. 3 shows the carbonation depth of the mortar slabs and cylinders as a function of accelerated carbonation time. The carbonation depth increases proportionally to the square root of the carbonation time, in accordance with a typical diffusion model [31,45].

In addition to the obtained accelerated carbonation depth values, it is noteworthy that the natural carbonation depth over the same period, assessed on a control slab, reached 2 mm only. This relatively low carbonation depth observed in the reference slab under natural conditions underscores the significant acceleration effect of the carbonation process in a controlled environment, emphasizing the importance of

<figure><img src="image_6.png" /><figcaption>Fig. 3. Carbonation depth of mortar slabs and cylinders as a function of carbonation time.</figcaption></figure>

such tests for predicting long-term durability in real-world conditions.

The cylinder samples demonstrated a slightly higher carbonation depth compared to the slabs throughout the test duration. This difference could be attributed to differences in the geometry and the microstructure between the two sets of samples. The volume-to-surface area ratio is actually a key factor influencing diffusive phenomena, such as carbonation in this study. The cylindrical samples had a ratio of 1.75 cm, whereas the slab samples exhibited a higher ratio of 2.75 cm. The cylindrical samples having a higher volume-to-surface area ratio, they consistently showed higher carbonation depth for a given carbonation time. A lower volume-to-surface ratio may promote faster carbonation due to increased exposed surface area per unit volume. However, further investigation is required to validate this proposition, as it has not been investigated in the existing literature. In addition, microstructural differences between the two geometries, such as variations in porosity, pore distribution, and connectivity, could also influence the drying and carbonation behavior [35,36].

### 3.2. Mechanical properties of mortar and FRP-mortar bond

The graph in Fig. 4 illustrates the relationship between the proportion of the carbonated section and the mechanical properties of mortar cylinders, specifically focusing on average compressive strength and Young's modulus.

The data indicates that the increase in the proportion of the carbonated section resulted in an increase in both the compressive strength and Young's modulus of the mortar. This positive correlation between the extent of carbonation and mechanical properties is consistent with previous studies, such as those by [21,27], which have reported that carbonation enhances the compressive strength of Portland cement-based materials. The increase in both compressive strength and Young's modulus suggest that carbonation contributes to the densification of the mortar matrix, leading to improved structural integrity and stiffness. This insight underscores the importance of considering carbonation effects in the long-term durability and performance of cementitious materials.

This result can also be attributed to the formation of calcium carbonates within the porosity of cementitious materials, following the carbonation reactions (3) and (4):

$$ 
\begin{gathered}
Ca(OH)_2(s) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l) \quad (3) \\
(CaO)_x(SiO_2) \cdot (H_2O)_z + xCO_2 \rightarrow xCaCO_3 + SiO_2 \cdot (H_2O)_t + (z-t)H_2O \quad (4)
\end{gathered}
$$

These mechanical improvements have important implications for real-world applications. As demonstrated in recent studies, increased compressive strength and stiffness contribute significantly to the long-term durability of rehabilitated structures. A stronger and denser mortar matrix not only enhances load-bearing capacity but also

<figure><img src="image_7.png" /><figcaption>Fig. 4. Impact of carbonation on the mechanical properties of mortar.</figcaption></figure>
improves resistance to environmental actions such as chloride ingress and freeze-thaw cycles. Several studies have confirmed the link between increased strength and improved durability, such as in Ref. [25]. Moreover, improved durability supports climate resilience, allowing structures to better adapt to changing environmental conditions over time. As highlighted in Ref. [1], achieving sustainability in concrete infrastructure requires lower environmental impact, higher durability, and resilience to climatic conditions. The repair and strengthening of carbonated structures extends their service life and reduces the need for replacement, contributing to more sustainable construction and rehabilitation strategies.

Fig. 5 illustrates the evolution of average tensile strength obtained during pull-out test as a function of proportion of carbonated section for mortar slabs treated by sandblasting and grinding. As carbonation increases, which represents the progression of the carbonated zone into the slab, a noticeable increase in tensile strength is observed. Specifically, for sandblasting, the tensile strength increased from approximately 2.8 MPa before accelerated carbonation to 3.7 MPa at 60 days of carbonation, reflecting an increase of 32 %. For grinding, the tensile strength rose from about 2.6 MPa to around 3.4 MPa at 60 days of carbonation, showing an increase of 31 %. These findings demonstrate the beneficial effect of carbonation on the bonding performance of the mortar, with the sandblasting treatment resulting in higher tensile strength compared to grinding across all carbonation zones.

Interestingly, both techniques lead to a similar trend of increasing tensile strength, suggesting that the type of surface preparation played a secondary role in the overall bonding performance. These results open new perspectives for considering natural carbonation in the on-site rehabilitation of buildings, as it could enhance the bond between rehabilitation techniques like CFRP and the treated concrete surfaces. At the same time, these findings raise many questions about the deeper effects of carbonation on the interface between composite materials and concrete. In terms of practical implications, it could be suggested that a concrete structure with a very high degree of carbonation but no corrosion of the reinforcement [2] can be repaired or reinforced with CFRP to withstand current or future loads, with two environmental benefits: extending service life and enabling sustainable CO₂ storage (carbon sequestration potential).

Based on the results of the pull-out test, a critical question arises: did the mortar peel off only from the carbonated layers, or also from the non-carbonated ones? The depth of the peeled mortar during the pull-out test was also measured and compared it with the carbonation depth, as illustrated in Fig. 6.

It is clear from the graph that mortar peeling during the pull-out test initially occurred regardless of carbonation. However, as carbonation progressed, the peeling was limited to the carbonated mortar, as shown by the alignment of the peeled mortar depth with the carbonation depth at 30 days. Beyond this point, even though carbonation kept increasing, the peeled mortar depth remained constant, indicating that peeling was

<figure><img src="image_8.png" /><figcaption>Fig. 5. Evolution of Tensile Strength as a Function of proportion of carbonated section obtained by pull-out test.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Comparison of peeled mortar depth and carbonation depth over time.</figcaption></figure>

confined to the carbonated layer. This indicates that the outer carbonated layer defines the maximum detachment zone under the applied pull-out force. Beyond this, the substrate likely becomes too strong to fail cohesively, reflecting increased mechanical resistance in deeper carbonated mortar.

Moreover, Fig. 7 visually illustrates the CFRP-Mortar interface before and after the pull-out test. Prior to testing (Fig. 7-a), the CFRP plate is well-adhered to the mortar surface. After testing (Fig. 7-b), a portion of the mortar remains attached to the CFRP, revealing a cohesive failure within the mortar matrix. This cohesive failure mode observed consistently across all tested specimens, regardless of the type of surface treatment applied. This outcome confirms the development of a strong bond at the interface, indicating that the adhesion between the CFRP and the carbonated cement-based mortar was effective and reliable in all cases.

### 3.3. Microstructural characterization of mortar and FRP-mortar interface

Fig. 8 illustrates a decrease in water porosity as carbonation progresses, with values dropping from approximately 24–20 % after 80 days of accelerated carbonation. These results are consistent with findings from several other studies [4,9,19]. During carbonation, the formation of calcium carbonates leads to pore closure, which results in a reduction in overall porosity and a denser material. This enhanced surface integrity improves the contact area between the CFRP and mortar, which is crucial for a stronger bond.

For example, the reaction that transforms one mole of portlandite into one mole of calcium carbonate results in an expansion of the solid phase volume, reducing pore space by 2 cm³ per mole of calcite [3]. In addition to reducing the total porosity, carbonation also affects the C-S-H phases and modifies the pore size distribution as illustrated in Fig. 9,

<figure><img src="image_10.png" /><figcaption>Fig. 7. Visual comparison of CFRP-mortar interface before and after pull-out testing.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. Evolution of the porosity of the mortar during carbonation.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. Evolution of the pore size distribution during carbonation.</figcaption></figure>

which shows the evolution of the pore size distribution as carbonation progresses. For this analysis, three different exposure times were selected: 0 days (uncarbonated), 28 days, and 80 days of carbonation. The results obtained by mercury intrusion porosimetry reveal a decrease in the volume of the mesopores with an access diameter $D < 100$ nm and an increase in the volume of nanopores ($D < 20$ nm). This decrease can be attributed to the formation of calcium carbonate in the pores and the partial decalcification of C-S-H during carbonation. It is worth noting that pore sizes directly influence transport properties, such as permeability [10]. The reduction of mesopores leads to a decrease in the permeability of the cement matrix [19]. This is beneficial for the long-term durability of the CFRP-mortar system, as it reduces the rate at which harmful substances penetrate and damage the bond between the two materials.

Fig. 10 illustrates the evolution of mass percentages of portlandite,

<figure><img src="image_13.png" /><figcaption>Fig. 10. Evolution of Portlandite, Calcite, and Vaterite-Aragonite contents over carbonation time for the paste samples.</figcaption></figure>

calcite, and the combined phases of vaterite and aragonite over the carbonation period with respect to the initial mass. Initially, portlandite content exhibited a sharp decline, especially in the first 42 days, as it reacted with $CO_2$ during the carbonation process, leading to the formation of calcium carbonate in the form of calcite. This rapid consumption of portlandite indicates its active role in early carbonation. Portlandite is the first major hydrate that decomposes to calcium carbonate during carbonation, leading to a moderate volume increase (DV = 12% of Portlandite) as the molar volume of calcite (36.9 $cm^3/mol$) is greater than that of Portlandite (33.0 $cm^3/mol$) [3]. It is noteworthy that the pH remains stable at around 12.5 [40] due to the important buffer effect of Portlandite.

Calcite content quickly rose during this period, reaching a point of relative stability after 42 days, suggesting that the primary carbonation reaction has reached completion, with most of the accessible portlandite converted to calcite, which is the most stable polymorph of calcium carbonate. As carbonation advances, portlandite gradually depletes, with the remaining particles becoming coated by calcite crystals. This coating slows down the carbonation of portlandite significantly compared to other hydrates, particularly C-S-H [44]. Unlike portlandite, C-S-H lacks a buffering effect, leading to a reduction in the pH of the pore solution to below 9 during carbonation [5]. This pH drop favors the formation of other calcium carbonate polymorphs, such as vaterite and aragonite, which become more prevalent after approximately 42 days of carbonation. These two polymorphs are poorly crystalline [22] and less stable phases [38]. In addition, Fig. 9 shows that after 70 days of accelerated carbonation, the mass percentage of vaterite and aragonite becomes almost equal to that of calcite. This indicates that, although they are metastable, these phases contribute significantly to the overall carbonate content during the later stages of carbonation, especially once the calcite formation begins to stabilize. Moreover [19] showed that the formation of vaterite during the later stages of carbonation resulted in a notable increase in the elastic modulus, highlighting its beneficial effect on the mechanical performance of the carbonated matrix.

Fig. 11 illustrates the XRD patterns of cement samples before carbonation and after 28, 70, and 133 days of carbonation. The initial sample, before carbonation, shows a prominent peak around 17° 2θ, corresponding to Portlandite (P), with minor peaks indicating the initial presence of Calcite (C), Aragonite (A), and Vaterite (V).

After 28 days of carbonation, the intensity of the Portlandite peak decreases, suggesting its consumption during the carbonation process. Concurrently, the peaks corresponding to Calcite become more pronounced, indicating its formation, and peaks for Aragonite and Vaterite also become noticeable, suggesting the initial formation of these polymorphs. By 70 days of carbonation, the trend continues with a further decrease in the Portlandite peak intensity, while Calcite peaks increase in intensity, as well as Vaterite and Aragonite peaks. After 133 days of carbonation, the Portlandite peaks are significantly reduced, nearing minimal intensity, while Calcite peaks reach their highest intensity,

<figure><img src="image_14.png" /><figcaption>Fig. 11. XRD of paste during carbonation.</figcaption></figure>
reflecting a major phase transformation from Portlandite to Calcite. Additionally, Vaterite and Aragonite peaks show increased intensities, suggesting ongoing formation.

These XRD results align with the thermogravimetric analysis (TGA) data, which show a decrease in the Portlandite content and an increase in the Calcite and Vaterite-Aragonite contents over time. The XRD patterns provide crystallographic evidence of these transformations, confirming the observations from TGA. Together, these analytical techniques highlight the progressive carbonation of the cement matrix, resulting in a decrease in porosity as the solid content increases due to the formation of various calcium carbonate phases.

Fig. 12 presents the SEM analysis results of two samples: before carbonation and after 60 days of accelerated carbonation of mortar. The three distinct materials—FRP, adhesive, and mortar—are clearly visible in both images. The EDS results also allow clearly identifying the three phases. High carbon concentration actually corresponds to the FRP zone, calcium in the mortar zone, and titanium in the adhesive zone as shown in Fig. 12. EDS mapping also shows that the used adhesive is a composite material made with siliceous sand and polymer matrix. What stands out is the noticeable difference in the adhesive-mortar interface, particularly within the mortar, after 60 days of carbonation. As highlighted by the white-marked area in the images, there is a mesh-like structure of finer particles in the carbonated mortar, which is distinctly different from the non-carbonated sample. The formation of this mesh-like structure suggests a refinement of the pore network and increased matrix densification due to calcium carbonate precipitation as observed in prior findings. This denser microstructure may contribute to higher contact area and improved mechanical interlock at the interface, leading to stronger and more durable adhesive bonding.

Although mechanical interlocking is commonly associated with increased surface roughness [15], the enhancement in bond performance observed in this study may instead be attributed to the growth of calcium carbonate (CaCO₃) crystals on the mortar surface. These crystals acted as microscopic pillars, providing interlocking forces with the polymer adhesive (Sikadur-30 in our case) [18] [8].

To gain a comprehensive understanding of the slab surface and the effect of carbonation on bonding performance, 3D microscopic

measurements were conducted to assess surface roughness at 0, 28, and 60 days of carbonation. These measurements considered two types of surface treatments: sandblasting and grinding. Fig. 13 illustrates the average surface roughness across the different carbonation periods.

The graph highlights a notable decrease in surface roughness (Ra) over carbonation time for both surface-preparation methods, attributable to the progressive formation of calcium carbonates during carbonation. Initially, sandblasted surfaces exhibited significantly higher roughness values (≈ 180 µm) than ground surfaces (≈ 130 µm), confirming the more aggressive texturing effect of sandblasting. Over a period of 60 days of accelerated carbonation, roughness declined for both surfaces: the ground samples fell to about 60 µm, whereas sandblasted samples fell to about 110 µm. The decrease of around 70 µm was observed in both preparation techniques indicating that the effect of carbonation itself is similar regardless of the initial surface condition.

It is interesting to highlight the correlation between the surface roughness measurements and the pull-out test results presented in Fig. 5. The sandblasted specimens had higher and homogenous surface roughness throughout the carbonation period and, at the same time,

<figure><img src="image_15.png" /><figcaption>Fig. 13. Average surface roughness of slabs at 0, 28, and 60 days of carbonation for sandblasted and ground surfaces.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 12. SEM and EDS analysis of FRP-Adhesive-Mortar interface a) before and b) after 60 days of carbonation. The white-outlined area in (b) represents the carbonated zone.</figcaption></figure>
exhibited greater tensile strength compared to the ground surfaces.

## 4. Conclusions

Externally bonded FRP has proven to be an effective way to repair and strengthen reinforced concrete structures. An experimental study has been designed to assess the influence of the initial carbonation of concrete on the bonding with FRP. The experimental program includes mechanical testing and microstructural characterization of cement mortar and the interface with adhesive and FRP. From the experimental results and their analysis, the following conclusions can be drawn:

* Carbonation significantly improves the adhesion between the fiber-reinforced polymer (FRP), the adhesive, and the cement mortar substrate. The evolution of the carbonated material layer, while showing only modest changes in compressive strength of partially carbonated mortar samples, revealed a significant increase in tensile strength determined from pull-out tests. These adhesion strength improvements of around 32 % for sandblasted surfaces and 31 % for ground surfaces, underscores the pivotal role of the carbonated layer in strengthening the bond between FRP and the substrate.

* Surface treatments such as sandblasting and grinding had a substantial impact on pull-out strength, emphasizing the importance of substrate preparation in optimizing bonding performance. Sand-blasting lead to higher tensile strength, whatever the carbonated depth.

* The results from both SEM-EDS imaging and 3D microscopic roughness measurements confirm that carbonation not only modified the microstructure of the mortar but also decreased surface roughness due to the formation of calcium carbonates. Higher roughness could be expected to result in better bonding but the results showed no direct correlation between estimated roughness and measured tensile strength, which suggests that other phenomena were involved in the evolution of bonding between cement mortar and polymer-based composite materials. The enhancement in bond performance observed in this study may be attributed to the growth of calcium carbonate (CaCO₃) crystals on the mortar surface. These crystals acted as microscopic pillars, providing interlocking forces with the polymer adhesive, thereby contributing to improved adhesion despite the reduction in surface roughness.

* The combined effects of carbonation and surface treatment significantly enhanced the adhesion of externally bonded FRP. These insights offer valuable guidelines for optimizing infrastructure rehabilitation and construction practices by taking advantage of natural carbonation prior to CFRP application, which can improve bond performance and overall durability. In addition, this research suggests that controlled exposure to CO₂—when feasible—could be considered as a preparatory phase in repair protocols. Such an approach may reduce the need for aggressive mechanical treatments or additional surface primers, thereby streamlining repair procedures and enhancing sustainability.

* The service life of a concrete structure exhibiting advanced carbonation but no reinforcement corrosion can be extended by repairing or strengthening it with CFRP, allowing it to withstand current or future loading demands and contribute to long-term carbon storage.

This work opens the path for further investigation into the long-term effects of carbonation on FRP bonding under varying environmental conditions, such as exposure to seawater, freeze-thaw cycles, and temperature fluctuations.

### CRediT authorship contribution statement

**Dania Kabalan:** Writing – original draft, Visualization, Investigation. **Emmanuel Roziere:** Writing – review & editing, Project administration, Methodology, Conceptualization. **Frédéric Grondin:**

Supervision, Resources, Investigation. **Amandine Celino:** Writing – review & editing, Methodology, Conceptualization.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The financial support from Ecole Centrale de Nantes through the PhD thesis grant of Dania Kabalan is gratefully acknowledged. The authors would also like to thank WEAMEC for their financial support through Inter-Mer project.

### Data availability

Data will be made available on request.

### References

[1] A. Adesina, J. Zhang, Impact of concrete structures durability on its sustainability and climate resiliency, Sustainability 3 (2024) 100025, https://doi.org/10.1016/j.nxsust.2024.100025.

[2] U.M. Angst, Steel corrosion in concrete – Achilles’ heel for sustainable concrete? Cem. Concr. Res. 172 (2023) 107239 https://doi.org/10.1016/j.cemconres.2023.107239.

[3] Auroy, M., 2014. Impact de la carbonatation sur les propriétés de transport d'eau des matériaux cimentaires. Université Paris-Est Marne-La-Vallée, France, Ph.D. thesis.

[4] M. Auroy, S. Poyet, P. Le Bescop, J.-M. Torrenti, T. Charpentier, M. Moskura, X. Bourbon, Impact of carbonation on unsaturated water transport properties of cement-based materials, Cem. Concr. Res. 74 (2015) 44–58, https://doi.org/10.1016/j.cemconres.2015.04.002.

[5] M. Oral Çagatay, Ercan Batur, Influence of ph on morphology, size and polymorph of room temperature synthesized calcium carbonate particles, Powder Technol. (2018).

[6] M. Castellote, L. Fernandez, C. Andrade, C. Alonso, Chemical changes and phase analysis of OPC pastes carbonated at different CO₂ concentrations, Mater. Struct. 42 (2009) 515–525, https://doi.org/10.1617/s11527-008-9399-1.

[7] J.M. Chi, R. Huang, C.C. Yang, Effects of carbonation on mechanical properties and durability of concrete using accelerated testing method, J. Mar. Sci. Technol. 10 (2002), https://doi.org/10.51400/2709-6998.2296.

[8] Y. Dai, Y. Lan, R. Wen, C. Xu, A. Al-Mansour, Z. Zhang, L. Li, Q. Zeng, K. Li, Doubling the coating-substrate pull-off strength by growth of CaCO₃ nano-crystals, Chem. Eng. J. 474 (2023) 145763, https://doi.org/10.1016/j.cej.2023.145763.

[9] T. Danner, M.R. Geiker, Long-term influence of concrete surface and crack orientation on Self-healing and ingress in cracks – field observations, Nord. Concr. Res. 58 (2018) 1–16, https://doi.org/10.2478/ncr-2018-0001.

[10] G.S. dos Reis, B.G. Cazacliu, R. Artoni, J. Torrenti, Effect of the accelerated carbonation treatment on the recycled sand physicochemical characteristics through the rolling carbonation process, J. CO₂ Util. 39 (2020) 101181, https://doi.org/10.1016/j.jcou.2020.101181.

[11] R. Feng, X. Xian, A review for accelerated carbonation improvement of recycled concrete coarse aggregates and the meta-analysis of environmental benefit assessment and cost analysis of concrete so produced, J. Build. Eng. 106 (2025) 112649, https://doi.org/10.1016/j.jobe.2025.112649.

[12] A. Gehlot, P. Kalla, A. Sain, A comparative analysis of hydration stoppage techniques in cement using oven drying, freeze drying and solvent exchange, IOP Conf. Ser. Earth Environ. Sci. 1519 (2025) 012019, https://doi.org/10.1088/1755-1315/1519/1/012019.

[13] M.F. Green, FRP repair of concrete structures: performance in cold regions, Int. J. Mater. Prod. Technol. 28 (2007) 160, https://doi.org/10.1504/IJMPT.2007.011515.

[14] M.F. Green, L.A. Bisby, A.Z. Fam, V.K.R. Kodur, FRP confined concrete columns: behaviour under extreme conditions, Cem. Concr. Compos. 28 (2006) 928–937, https://doi.org/10.1016/j.cemconcomp.2006.07.008.

[15] P. Gujar, A. Alex, M. Santhanam, P. Ghosh, Evaluation of interfacial strength between hydrating cement paste and epoxy coating, Constr. Build. Mater. 279 (2021) 122511, https://doi.org/10.1016/j.conbuildmat.2021.122511.

[16] H. Hamilton, C. Dolan, Durability of FRP reinforcements for concrete, Prog. Struct. Eng. Mater. 2 (2000) 139–145, https://doi.org/10.1002/1528-2716(200004)06:2<139::AID-PSE17>3.0.CO;2-3.

[17] Z.-H. Hao, J.-J. Zeng, G.-M. Chen, J.-G. Dai, J.-F. Chen, Durability of FRP-to-concrete bonded joints subjected to 110 months accelerated laboratory and field exposure, Eng. Struct. 305 (2024) 117681, https://doi.org/10.1016/j.engstruct.2024.117681.
[18] M. Horngies, P. Willieme, O. Gabet, Influence of the surface properties of concrete on the adhesion of coating: characterization of the interface by peel test and FT-IR spectroscopy, Prog. Org. Coat. 72 (2011) 360-379, https://doi.org/10.1016/j.porgcoat.2011.05.009.

[19] F. Kaddah, H. Ranaivomanana, O. Amiri, E. Rozière, Accelerated carbonation of recycled concrete aggregates: investigation on the microstructure and transport properties at cement paste and mortar scales, J. CO2 Util. 57 (2022) 101885, https://doi.org/10.1016/j.jcou.2022.101885.

[20] A. Kashi, A.A. Ramezanianpour, F. Moodi, Durability evaluation of retrofitted corroded reinforced concrete columns with FRP sheets in marine environmental conditions, Constr. Build. Mater. 151 (2017) 520-533, https://doi.org/10.1016/j.conbuildmat.2017.06.137.

[21] J.-K. Kim, C.-Y. Kim, S.-T. Yi, Y. Lee, Effect of carbonation on the rebound number and compressive strength of concrete, Cem. Concr. Compos. 31 (2009) 139-144, https://doi.org/10.1016/j.cemconcomp.2008.10.001.

[22] A. Leemann, Accelerated carbonation of recycled concrete aggregates and its implications for the production of recycling concrete, J. Build. Eng. (2023).

[23] M. Li, D. Shen, Q. Yang, X. Cao, C. Liu, J. Kang, Rehabilitation of seismic-damaged reinforced concrete beam-column joints with different corrosion rates using basalt fiber-reinforced polymer sheets, Compos. Struct. 289 (2022) 115397, https://doi.org/10.1016/j.compstruct.2022.115397.

[24] Lin Ye, K. Friedrich, C. Weimer, Y.-W. Mai, Surface treatments and adhesion bonding between concrete and a CFRP composite, Adv. Compos. Mater. 7 (1998) 47-61, https://doi.org/10.1163/156855198x00048.

[25] Z. Liu, W. Meng, Fundamental understanding of carbonation curing and durability of carbonation-cured cement-based composites: a review, J. CO2 Util. 44 (2021) 101428, https://doi.org/10.1016/j.jcou.2020.101428.

[26] L. López-de Abajo, M.G. Alberti, J.C. Gálvez, Pollutant concentration prediction from traffic data analysis for concrete durability studies in Madrid calle 30 urban tunnels, Tunn. Undergr. Space Technol. 144 (2024) 105477, https://doi.org/10.1016/j.tust.2023.105477.

[27] B. Lu, S. Drissi, J. Liu, X. Hu, B. Song, C. Shi, Effect of temperature on CO2 curing, compressive strength and microstructure of cement paste, Cem. Concr. Res. 157 (2022) 106827, https://doi.org/10.1016/j.cemconres.2022.106827.

[28] H.S. Maria, P. Alcaino, C. Luders, H. Santa Maria, P. Alcaino, C. Luders, April. Experimental response of masonry walls externally reinforced with carbon fiber fabrics, Proc. 8th US Natl. Conf. Earthq. Eng. (2006).

[29] A. Morandeau, M. Thiéry, P. Pangla, Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics, microstructure changes and moisture properties, Cem. Concr. Res. 56 (2014) 153-170, https://doi.org/10.1016/j.cemconres.2013.11.015.

[30] S.J. Pantazopoulou, J.F. Bonacci, S. Sheikh, M.D.A. Thomas, N. Hearn, Repair of corrosion-damaged columns with FRP wraps, J. Compos. Constr. 5 (2001) 3-11, https://doi.org/10.1061/(ASCE)1090-0268(2001)5:1(3).

[31] V. Papadakis, Fundamental modeling and experimental investigation of concrete carbonation, Acids Mater. J. 88 (1991), https://doi.org/10.14359/1863.

[32] S.E. Pihlavaara, Some results of the effect of carbonation on the porosity and pore size distribution of cement paste, Mater. Constr. 1 (1968) 521-527, https://doi.org/10.1007/BF02473640.

[33] D.V. Ribeiro, N.S. Amorim Júnior, B.S. Santos, H.A. Santana, S.A. Pinto, Proposal for classification of concrete quality based on accelerated carbonation tests, Case Stud. Constr. Mater. 19 (2023) e02466, https://doi.org/10.1016/j.cscm.2023.e02466.

[34] S. Rizkalla, T. Hassan, N. Hassan, Design recommendations for the use of FRP for reinforcement and strengthening of concrete structures, Prog. Struct. Eng. Mater. 5 (2003) 16-28, https://doi.org/10.1002/pse.139.

[35] H. Samouh, E. Rozière, V. Wisniewski, A. Loukili, Consequences of longer sealed curing on drying shrinkage, cracking and carbonation of concrete, Cem. Concr. Res. 95 (2017) 117-131, https://doi.org/10.1016/j.cemconres.2017.02.019.

[36] H. Samouh, A. Soive, E. Rozière, A. Loukili, Experimental and numerical study of size effect on long-term drying behavior of concrete: influence of drying depth, Mater. Struct. 49 (2016) 4029-4048, https://doi.org/10.1617/s11527-015-0771-7.

[37] Setunge, S., Kumar, A., Nezamian, A., 2002. Decision support tools for concrete infrastructure rehabilitation (using FRP composites).

[38] R. Sevěřík, P. Šašek, A. Viani, Physical and nanomechanical properties of the synthetic anhydrous crystalline CaCO3 polymorphs: vaterite, aragonite and calcite, J. Mater. Sci. 53 (2018) 4022-4033, https://doi.org/10.1007/s10853-017-1884-x.

[39] SIKA, 2014. Structural Strengthening with Sika Systems.

[40] K. SIMKISS, Variations in the crystalline form of calcium carbonate precipitated from artificial sea water, Nature 201 (1964) 492-493, https://doi.org/10.1038/201492a0.

[41] S. Soares, J. Sena-Cruz, J.R. Cruz, P. Fernandes, Influence of surface preparation method on the bond behavior of externally bonded CFRP reinforcements in concrete, Materials 12 (2019) 414, https://doi.org/10.3390/ma12030414.

[42] S. Soares, J. Sena-Cruz, J.R. Cruz, P. Fernandes, Influence of surface preparation method on the bond behavior of externally bonded CFRP reinforcements in concrete, Materials 12 (2019) 414, https://doi.org/10.3390/ma12030414.

[43] Thiery, M., 2005. Modélisation de la carbonatation atmosphérique des matériaux cimentaires: Prise en compte des effets cinétiques et des modifications microstructurales et hydriques.

[44] Turcry, P., Aït-Mokhtar, A., Namouliara, K., Gendron, F., 2017. Investigation expérimentale de l'influence de l'état hydrique sur la cinétique de carbonatation accélérée de pâtes de ciment.

[45] X. Wang, Q. Yang, X. Peng, F. Qin, A review of concrete carbonation depth evaluation models, Coatings 14 (2024) 386, https://doi.org/10.3390/coatings14040386.

[46] J. Xie, R. Hu, Experimental study on rehabilitation of corrosion-damaged reinforced concrete beams with carbon fiber reinforced polymer, Constr. Build. Mater. 38 (2013) 708-716, https://doi.org/10.1016/j.conbuildmat.2012.09.023.

[47] C. Zhou, L. Wang, Y. Wang, Z. Fang, Experimental study on the flexural strengthening of one-way RC slabs with end-buckled and/or externally bonded CFRP sheets, Eng. Struct. 282 (2023) 115832, https://doi.org/10.1016/j.engstruct.2023.115832.