The results from the techniques discussed above and a discussion of the results are presented in the subsequent sub-sections.
4.1. Visual Inspection
During the visual inspection phase, it was observed that certain areas exhibited adverse effects of corrosion, primarily evident through cracking and spalling. Notably, since the structure remained distant from any potential chloride ingress source, it was deduced that chloride infiltration couldn't be attributed as the cause for the corrosion and that carbonation was its likely cause.
Furthermore, the visibly damaged locations were mainly confined to columns adjoining bathrooms or columns housing electrical wiring between floors. These instances of corrosion-induced damage were predominantly located on the externally exposed beams and columns that were subject to recurrent wetting and drying cycles. Importantly, no main structural members displayed indications of swelling or excessive deflection. Distinct stains of rust were observed on parapet walls, and clear spalling, and delamination were also observed. It was observed that while the exposed outer surface of the columns had a clear cover between 15 mm and 35 mm, the inner faces had a clear cover of 25 mm to 65 mm. Corrosion-induced cracks were observed only on the outer surfaces and not on the inner surfaces.
Subsequent to the preliminary inspection, six locations were designated for core-strength assessment, encompassing three beams and three columns. Additionally, six core samples were retrieved for the assessment of porosity and sorptivity. Furthermore, eight columns, equally divided between east (front) and west (back) orientations, were earmarked. Selection was based on visible distress indicators – three columns exhibited significant cracks (GF-CF-4, GF-CB-2, & GF-CB-4) notably column GF-CF-4 had a clear depth of 15mm, two columns presented minor crack indications (GF-CF-3, & GF-CB-1), and three columns displayed no evident cracking or rust marks (GF-CF-1, GF-CF-6, & GF-CB-5); notably, one of the latter, GF-CF-6, was positioned on the inner side of a column featuring a cover depth of 64mm. (GF - ground floor, CF - Column in front, CB - Column in back; numeric digit signifies column number).
4.2 Phase Composition
The X-ray diffraction (XRD) analysis was initially conducted on samples obtained from the non-carbonated zone (identified using a pH indicator), the carbonated zone, and a non-carbonated sample subjected to accelerated carbonation at 3% carbon dioxide concentration. Figure 3 (a) illustrates the diffractogram of these samples. The characterized samples primarily consisted of quartz (Q), with minor portions of chlorite, muscovite, and carbonate phases. Notably, the sample from the non-carbonated zone exhibited peaks of portlandite, however peaks of calcite were still observable, indicating the initiation of carbonation even at these locations. This was probably because although carbonation starts to reduce the pH of concrete below 12, phenolphthalein, which was used as an indicator for non-carbonated concrete, shows colour change at a pH of around 9. Upon comparing the naturally carbonated and accelerated carbonated diffractograms, it becomes evident that stable calcite and metastable polymorphs of calcium carbonate, namely aragonite and vaterite, are formed under both conditions. In the accelerated condition, the intensity of calcite is higher, with minor quantities of aragonite and vaterite peaks. Conversely, under natural conditions, a significant quantity of vaterite is present as peaks can be distinctively observed, while the peak intensity of calcite is reduced [20, 21, 24, 31]. The latter observation indicates the hindered or slowed down polymorphic transformation of vaterite to calcite which is thermodynamically more stable form of calcium carbonate. Low temperature and limited water availability in pores during natural carbonation have been characterized as one of the reasons for this slowed down transformation [32–34].
Additionally, Fig. 3 (b) shows the thermogravimetric loss curves of samples for the non-carbonated, naturally carbonated, and accelerated carbonated environments. The presence of portlandite is characterized by a decomposition peak between 400–500℃, which is evident in the non-carbonated sample but absent in both naturally and accelerated carbonated samples. The presence of calcium carbonate in the non-carbonated sample is confirmed by thermogravimetric analysis (TGA) as well, indicating the initiation of carbonation. The first derivative (DTG) curves for both natural and accelerated carbonation are similar, revealing the presence of poorly crystalline calcium carbonate polymorphs. Due to the presence of amorphous and poorly crystalline calcium carbonate polymorphs, the decomposition of calcium carbonate initiates from 400℃ onwards. However, the absence of clear, sharp, and distinct decomposition peaks makes it challenging to calculate the mass loss from any specific phase in which calcium carbonate would be present [10, 35–37]. It is essential to note that the samples for TGA analysis underwent no specific preparation and were tested as such after extraction and grinding to a particle size below 45µm. Consequently, the decomposition peak in the range of 30–300℃ could be influenced by the presence of moisture, as observed in the accelerated carbonated sample. However, the presence of moisture has no effect on the portlandite and calcium carbonate decomposition peak and quantification [38].
4.3 Pore solution
The results of chemical analysis conducted on the representative cold-water extracted pore solution samples from non-carbonated, naturally carbonated, and accelerated carbonated specimens are presented in Table 1. Notably, concrete carbonation leads to a decrease in the concentration of alkali metal ions [39, 40], with a more significant reduction observed in the concentration of potassium ions in accelerated carbonation. This greater decrease in potassium ions concentration in accelerated carbonation may be attributed to the intensified decalcification of calcium silicate hydrate (C-S-H) [24]. This decalcification results in elongated silica chain lengths and increased absorption of alkali metal ions [21, 41–43]. Surprisingly, sodium ions peak was not detected in the chromatograph for any of the samples and, therefore, are not mentioned. Furthermore, the concentration of anionic species such as chloride and sulphate were observed to increase upon carbonation. The increase in sulfate concentration can be attributed to the carbonation of ettringite, while the rise in chloride ions concentration may result from the release of physically absorbed and chemically bound chloride during the carbonation of C-S-H and Friedel’s salt, respectively [21, 40, 44, 45].
Additionally, carbonation led to a decrease in the ionic conductivity and pH of the extracted pore solution. Although in systems like cement paste pore solution, electrical conductivity is not linearly related to ionic concentration [46], a decrease in ionic conductivity indicates a decrease in the concentration of charge-carrying species in the solution upon carbonation, primarily sodium, potassium, and hydroxide ions. The lower concentration of potassium ions and pH in accelerated carbonation conditions explains the lower electrical conductivity compared to naturally carbonated samples [47].
Table 1
Ionic concentration, and chemical parameters of concrete pore solution
Sample | \(C{l}^{-}\) conc. mmol/kg | \(S{O}_{4}^{2-}\) conc. mmol/kg | \(C{a}^{2+}\) conc. mmol/kg | \({K}^{+}\) conc. mmol/kg | pH | Conductivity mS/cm |
Non-carbonated | 0.91 | 3.69 | 5.58 | 3.68 | 12.35 ± 0.07 | 3.55 ± 0.15 |
Naturally carbonated | 1.69 | 7.93 | 7.62 | 2.17 | 9.70 ± 0.10 | 1.30 ± 0.07 |
Accelerated carbonated | 1.74 | 6.97 | 4.90 | 0.04 | 9.20 ± 0.12 | 0.89 + 0.01 |
4.4 Porosity and Sorptivity
Sorptivity and porosity measurements were performed on specimens derived from cores extracted from the structure, and the outcomes are depicted in Fig. 4. A discernible trend is evident – the porosity values undergo reduction upon concrete carbonation, irrespective of whether natural or accelerated. This signifies a reduction in the available paths for the movement of gases and moisture through the pore structure post-carbonation. Further substantiating this observation, a decline in the sorption values is apparent in Fig. 4 following carbonation. This reduction lends additional support to the argument that the passage of water is impeded subsequent to carbonation. Nevertheless, according to the classification by Alexander et al. (1999) [48], the sorptivity values for all cases are rated as very poor.
Remarkably, both accelerated and natural carbonation yield comparable reduction in porosity and sorption values when compared to non-carbonated samples. This implies that both methods induce similar changes in the pore-structure. Consequently, it can be deduced that the accelerated carbonation technique at 3% carbon dioxide is suitable for investigating transport properties and corrosion in carbonated conditions.
4.5 Mechanical Properties
Location for extracting the cores was identified with help of the rebar locator. Extracted cores were prepared such that diameter to length ratio remained 1:2. The load was converted to equivalent cube strength using a correction factor recommended in IS516 2014. The measured core crushing strength values are presented in Table 2. Using the approximate conversion factors listed in IS516, it was found that the concrete in the columns met requirements for M20 grade and that in beams met the requirements for M15 grade.
Table 2
Equivalent cube strength and carbonation depth of the cores extracted.
Location of Core | Height/Diameter | Corrected Load (kN) | Equivalent Cube Strength (MPa) | Average Cube Strength (MPa) |
GF-CF-5 | 2.19 | 48.86 | 16.55 | 17.20 |
GF-CB-4 | 2.18 | 56.94 | 19.11 |
FF-CF-6 | 2.18 | 47.53 | 15.93 |
GF-BF-4 | 2.18 | 44.68 | 14.98 | 15.02 |
GF-BB-5 | 2.17 | 41.86 | 14.00 |
FF-BF-7 | 2.18 | 47.82 | 16.08 |
4.6 Corrosion Parameters
4.6.1 Carbonation Depth
The carbonation depth of samples taken from various locations in beams and columns, as measured by spraying the phenolphthalein indicator, is shown in Fig. 5. It is worth noting that samples from the slab and facade were found to be fully carbonated and, therefore, have not been included in the plot. In all the structural members, the carbonation depth had penetrated beyond the rebar level. This indicates an active state of corrosion, provided that moisture and oxygen are present. Members with potential exposure to alternate wetting and drying, such as those affected by rain splashes or being in proximity to washrooms, exhibited a lower degree of carbonation (e.g., GF-CB-1, GF-CB-2, GF-CB-4, & GF-BF-5). Conversely, members situated indoors, particularly those in the kitchen area (e.g., GF-CF-3, FF-BF-5, & GF-BF-4), displayed a higher rate of carbonation. Notably, no chimneys were present in the kitchen; however, some apartment units had exhaust fans installed. The Column FF-CF-6 exhibited a carbonation depth of approximately 144mm, despite having a member thickness of 230 mm only. This anomaly can be attributed to the immediate proximity of another column adjacent to it. The variance in moisture content is a driving factor behind the disparate carbonation rates witnessed across these locations.
4.6.2 Half-cell Potential
The figure displayed in Fig. 6 illustrates the distribution of half-cell potential values, collected through measurements against the copper-copper sulphate electrode. These measurements were taken across a 3 x 6 grid, maintaining a spacing of 15cm. An interesting correlation was observed – columns that exhibited visible crack marks manifested notably negative half-cell potential values. Employing the criteria outlined in ASTM C 876 [30], the analysis indicated that out of the total eight columns investigated, four columns displayed a likelihood of corrosion exceeding 90%. Conversely, one column recorded a half-cell potential value below − 200mV, suggesting a low probability of corrosion occurrence. The status of corrosion occurrence in the remaining columns appeared uncertain, neither conclusively indicating its presence nor absence. Additionally, it was noted that the half-cell potential values decreased when the concrete surface was splashed and saturated before measurement (refer to Fig. 6), albeit this increase was small. The observed reduction in the half-cell potential can be attributed to the decrease in concrete resistance caused by the saturation of pores in the concrete [49, 50]. The relatively small increase in the half-cell potential values upon saturation indicates that the test method can also be reliably used in unsaturated members.
4.6.3 Surface Resistivity
Surface resistivity measurements were conducted with careful consideration to ensure complete saturation of the top surface. The surface resistivity values collected in both orthogonal directions for various columns are graphically presented in Fig. 7. A noteworthy finding was the similarity in values recorded for both directions, indicating accurate measurement positioning – away from the directly underlying rebar. Despite this, certain columns displayed notable scatter in their data points, a phenomenon that could be attributed to the presence of conductive and insulated cracks, as outlined in [51].
Applying Langford and Broomfield's classification [52], which is grounded in the Wenner four-probe technique, it was deduced that out of the eight columns under investigation, three columns are assessed to be at a high risk of corrosion, three columns are categorized as being at low risk, and the remaining two columns fall within the spectrum of low to moderate risk of corrosion.
4.6.3 Corrosion rate
The RapiCor device was employed for the in-situ measurement of corrosion rates. Alongside the primary corrosion rate data, the machine also furnishes supplementary parameters such as resistivity and half-cell potential. The corrosion rates, resistivity values, and half-cell potentials obtained from the RapiCor measurements are shown in Fig. 8.
Applying the interpretation criteria from the RapiCor results as detailed in [53], it was found that five out of the eight columns (GF-CF-4, GF-CF-3, GF-CB-2, GF-CB-5, and GF-CB-4) exhibit a high corrosion rate. Two columns (GF-CF-1 and GF-CB-1) are associated with a moderate risk of corrosion, while one column (GF-CF-6) showcases a low corrosion rate. Notably, column GF-CB-4 manifests an exceedingly high corrosion rate of 98.87µm/year, signifying a substantial loss in cross-sectional area that demands immediate attention.
An interesting observation pertains to column GF-CF-6, which, was shielded from external exposure and having a cover depth of 64mm, displays an unusually low corrosion rate. This could potentially be attributed to insufficient moisture, a key driving factor in the corrosion process.
4.6.4 Corrosion Parameters Analysis
The results of carbonation depth assessments revealed that the carbonation front had penetrated beyond the rebar level for all the structural members, indicating a de-passivated state of the reinforcement. However, not all members exhibited indications of actively corroding reinforcement in measurements such as half-cell potential, surface resistivity, and corrosion rate. A correlation analysis between surface resistivity and half-cell potential values collected from the structure revealed a general trend of decreasing half-cell potential with a reduction in resistivity values (refer to Fig. 9). Moreover, classification based on corrosion rate values obtained from the RapiCor machine was performed, distinguishing between negligible corrosion (corrosion rate ≤ 5 µm/year), moderate corrosion (corrosion rate between 5–10 µm/year), and high corrosion (> 10 µm/year).
It was noted that when surface resistivity exceeded 40kΩ‧cm and half-cell potential values were less than − 200mV, no visible signs of corrosion were evident. Conversely, in instances where half-cell potential values dropped below − 350mV and surface resistivity fell below 12.5kΩ‧cm, observable cracking signs were present in the investigated members. For values falling between these extremes, determining the extent of corrosion in the member proved challenging, although it was confirmed to be in a state of corrosion, emphasizing the necessity of employing various non-destructive techniques concurrently and conducting a visual inspection of the reinforcement. This comprehensive approach becomes crucial when corrosion is the suspected cause of deterioration in the structure.