Experimental Analysis of the Post-Fire Structural Performance of Corroded Geopolymer Concrete Columns
- Open Access
- 01.01.2026
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Abstract
1 Introduction
Traditional concrete made with cement is one of the most widely used construction materials worldwide due to its versatility, availability, cost-effectiveness and performance. However, cement production is a significant contributor to global carbon emissions, accounting for nearly 8% of the total emissions each year [1]. To mitigate its environmental impact, alternative materials with lower carbon footprints are a key requirement towards achieving a more sustainable future. Geopolymer concrete (GPC), introduced by French chemist Prof. Joseph Davidovits, is a promising alternative that requires no cement in its production [2]. Despite its potential benefits, GPC adoption has been limited due largely to a lack of awareness as well as scientific and practical performance information. Unlike conventional cement concrete, GPC does not require water for curing; instead, it gains strength through polymerization, with air-dried samples achieving higher strength than those cured in water [3]. Additionally, GPC promotes sustainability by incorporating industrial by-products as source materials, reducing both production costs and carbon emissions [4, 5]. GPC has also demonstrated the ability to attain compressive strengths of up to 175 MPa while maintaining excellent workability, classifying it as an ultra-high-performance concrete [6].
GPC shows sustainability advantages and, in many cases, the mechanical and durability performances were superior then cement concrete. But GPC possess drawbacks such as the type of source material used for the production of concrete [7]. Moreover, the oxides present in the source material influence the hardened properties of the concrete. In addition to the oxides of the source materials, heat curing is required for the fly ash-based GPC to attain adequate mechanical properties [8], this is because of the slow reactivity of the fly ash particles. In general, GPC attains its strength through the alkali activation process, which requires aggressive alkaline solutions. The concentration and the ratio of the alkaline solution influence the physical, mechanical, and durability performance of the concrete. Lowering the concentration of the sodium hydroxide results in lowering the strength of the concrete [9].
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Marine infrastructure, including offshore structures, relies heavily on concrete, which is prone to deterioration from seawater and atmospheric exposure [10]. Repairing these structures is costly and challenging, making corrosion resistance a critical factor for long-term durability [11]. Studies have shown that increasing the sodium hydroxide concentration in GPC mixtures reduces chloride penetration and corrosion in steel reinforcement, with lower-grade GPC being more susceptible to degradation [12]. Fly ash and slag-based GPC composites exhibit superior corrosion resistance compared to cement-based concrete, with higher slag content further reducing corrosion rates [13]. Under accelerated testing conditions, GPC specimens demonstrated corrosion rates between 10 and 20 μm/year, significantly lower than the 40 to 60 μm/year observed in cement concrete [14]. Shaikh [15] further confirmed that GPC exhibits greater resistance to corrosion than conventional concrete.
The fire and post-fire performance of GPC has also been studied. While exposure to temperatures between 250 °C and 750 °C reduces its impact resistance, GPC shows lower susceptibility to spalling than cement concrete at 700 °C [16, 17]. Lightweight GPC developed with pumice and expanded clay demonstrated optimal performance when composed of a 50:50 fly ash-to-slag ratio, even after exposure to temperatures up to 800 °C [18]. In contrast, cement concrete completely disintegrates at 1100 °C, whereas GPC retains residual strengths of 5.5 MPa to 15 MPa, indicating superior fire behavior [19]. The performance of metakaolin-based GPC remains stable up to 600 °C, with residual strength ranging from 28% to 63% of the original values, while cement concrete experiences significant cracking at 400 °C, leading to premature strength loss [20, 21]. Furthermore, fibre-reinforced polymer-based GPC slabs exhibit higher flexural capacity than cement concrete counterparts, even after exposure to 550–600 °C [22]. Mathew and Joseph [23] reported that increasing the fire temperature decreases the ultimate load capacity, ductility, and stiffness of GPC beams, with 64% of ductility lost at 800 °C.
Both corrosion and fire exposure negatively impact structural performance, but high-temperature exposure (500–800 °C) has a more severe effect than corrosion (10%–20% degradation) [24]. Fire alters the physico-chemical properties of the concrete matrix, causing softening, whereas corrosion primarily weakens the reinforcement bars [25]. Concrete strength initially increases with rising temperature but declines beyond a critical threshold, while corrosion progressively reduces both compressive and flexural strength. At 600–750 °C, the flexural capacity of reinforced concrete beams is significantly affected [25]. Further, yield and ultimate load capacities decrease notably in specimens with 4% and 8% corrosion when exposed to 400 °C and 600 °C, respectively, and the combined effects of corrosion and fire lead to severe structural deterioration [26]. Corroded beams exposed to fire exhibit similar failure profiles to uncorroded beams, but corrosion-induced cracking accelerates heat penetration, making reinforcement bars more brittle and leading to sudden failure [27]. Corroded specimens also experience greater deformations during fire exposure, with more rapid failure under lower temperature differentials [28].
In this context, the current paper examines the fire performance of GPC columns following accelerated corrosion. The specimens were exposed to fire temperatures of 925 °C and 986 °C, with heating rates in accordance with the ISO 834 [29] standard fire. The paper proceeds with a description of the test programme, as well as a detailed analysis of the results. The residual performance of corrosion-damaged, fire-exposed GPC columns is compared with both reference and corroded-only specimens.
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2 Test Methodology
This section details the experimental programme adopted in the study, encompassing specimen preparation, test methods, and the desirable outcomes. The programme aimed to investigate the fire performance of geopolymer concrete (GPC) columns after exposure to corrosion. A total of six column specimens were cast and cured under controlled conditions before being subjected to an accelerated corrosion process to simulate deterioration. Following this, the specimens were exposed to elevated temperatures in an electric furnace, after which their physical and mechanical properties were assessed. The methodologies employed for each phase of the experiment are described in the following subsections.
2.1 Specimen Preparation
Locally available Class F Fly Ash and slag was used as the binder material for the production of the concrete used to cast the specimens. A sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) solution, with a ratio of 1:1.5 and an NaOH concentration of 12 M, was employed as the alkaline activator. In this study, grade M30 GPC was considered, following the guidelines outlined in IS 17,452 [30]. The mix design used in this study is presented in Table 1.
Table 1
Concrete mix design
Binder (kg/m3) | Aggregates (kg/m3) | Activator (kg/m3) | Activator to binder ratio | |||
|---|---|---|---|---|---|---|
Fly Ash | Slag | Coarse | Fine | NaOH | Na2SiO3 | |
200 | 200 | 1086 | 950 | 89 | 134 | 0.56 |
The GPC mix, as given in Table 1, was prepared in the laboratory available mixer unit by first dry-mixing the binder and the aggregates for 2 min to achieve homogeneity. The alkaline activator solution (NaOH and Na₂SiO₃) was prepared 24 h before the casting process to allow for temperature stabilisation and complete dissolution of NaOH pellets. The activator was gradually added to the dry mix, followed by an additional 3–4 min of mixing until a uniform, workable consistency was achieved. The mix was then poured into column moulds measuring 800 mm in length with a square cross-section of 150 mm × 150 mm. Additionally, cube moulds of 150 mm × 150 mm × 150 mm were prepared to assess the residual strength of the concrete after heating. The columns were reinforced with 10 mm diameter bars as the main reinforcement and 8 mm diameter ties, spaced at 100 mm center-to-center. The reinforcement used in the column was Fe415 grade with a reinforcement ratio of 1.4%. The yield and tensile strength of a 10 mm diameter bar were 383.86 MPa and 551 MPa. Similarly, the yield and tensile strength of an 8 mm diameter bar were 360 MPa and 527 MPa. After casting, the specimens were left to set under ambient conditions for 24 h before being demoulded and cured in laboratory conditions for 28 days. Following the curing period, the specimens were subjected to the accelerated corrosion process, as outlined hereafter, and subsequent heating and testing.
2.2 Accelerated Corrosion Procedure
After curing, a set of specimens underwent exposure to accelerated corrosion conditions to simulate the corrosion process in a controlled laboratory environment. The accelerated corrosion method used in this study was based on the procedure described by Jayasree et al. [31]. To induce corrosion in the reinforced concrete (RC) columns, one of the main reinforcement bars was extended 30 cm beyond the concrete surface at one end of the member, as shown in Fig. 1. This exposed bar functioned as the anode in the electrochemical corrosion process. The RC column specimen was then submerged in an electrolyte solution containing 3.5% sodium chloride (NaCl), as shown in Fig. 1. A stainless-steel section placed inside the tank served as the cathode for the electrochemical process [32, 33]. Figure 2 shows the experimental test setup for the accelerated corrosion process. To accelerate corrosion, a 6 V DC power supply was applied to the anode (i.e. the exposed reinforcement bar). The duration of current flow required to complete the electrochemical process was estimated using Faraday’s law, expressed in Eq. 1:
$$\:\text{m}=\:\frac{\text{M}}{\text{F}}\:\times\:\frac{\text{I}}{\text{z}}\:\times\:\text{t}$$
(1)
where m is the mass of the steel reinforcement, M is the atomic weight of iron (AMS), I is the current (Amp), t is the duration (s), F represents the Faraday’s constant and z is the ionic charge (96485 A/s).
Fig. 1
Accelerated corrosion test setup
Fig. 2
Experimental accelerated corrosion test setup
The half-cell potential method was employed to determine the corrosion rate in reinforced concrete elements. After the corrosion period, the specimens were removed from the immersion tank containing the salt solution. A voltage probe was connected to the exposed reinforcement bar, while a sensing probe filled with a copper sulphate solution was placed on the surface of the concrete column. A small voltage was applied to the reinforcing bars through the voltage probe, and the sensing probe measured the voltage drop on the concrete surface. The recorded potential values, typically in millivolts (mV), were used to estimate the rate of corrosion.
2.3 Heating Procedure
Before initiating the heating phase, all specimens were stored under laboratory ambient conditions (temperature: 29 ± 2 °C; relative humidity: 60 ± 5%) for 7 days following the corrosion process to ensure consistent moisture levels. Following the corrosion process, the column specimens were subjected to high-temperature exposure in an electric furnace with a maximum temperature capacity of 1200 °C. All of the specimens were heated in accordance with the standard fire curve given in ISO 834 [26]. Two heating durations were employed: (i) 60 min, and (ii) 90 min. Table 2 gives an overview of the specimens examined. Figure 3 shows the heating of concrete specimens. Figure 4 shows the time-temperature curve of the column specimens (Ref-H2 and CR-H2).
Table 2
Specimen descriptions
Specimen Name | Specimen Description |
|---|---|
Ref | Reference (benchmark) specimen |
Ref-H1 | Reference specimen subjected to 60 min of heating |
Ref-H2 | Reference specimen subjected to 90 min of heating |
CR | Corroded specimen (after accelerated corrosion) |
CR-H1 | Corroded specimen subjected to 60 min of heating |
CR-H2 | Corroded specimen subjected to 90 min of heating |
Fig. 3
Heating of concrete column in the electric furnace
Fig. 4
Time-temperature response in different locations
2.4 Measurements Taken During and After Testing
2.4.1 Crack Width and Mass Loss
In addition to the column specimens, the cube specimens were exposed to elevated temperatures for the same heating durations. Following exposure, physical changes such as colour alterations and crack formation were examined. The crack width was measured using a concrete crack microscope. Furthermore, the mass loss of the concrete was determined by comparing the specimen’s weight before and after exposure to elevated temperatures, with the variation in mass representing the extent of material loss.
2.4.2 Compressive Strength
The compressive strength of the concrete cube specimens was assessed to evaluate the residual strength of the concrete after high-temperature exposure. The specimens were tested using a Compression Testing Machine (CTM), with the loading rate determined according to ASTM C39 [34] guidelines. The compressive strength was calculated as the ratio of the ultimate load to the cross-sectional area of the specimen.
2.4.3 Axial Resistance
After heating, the reinforced concrete column specimens were tested under axial compression using a Universal Testing Machine (UTM) to evaluate their load-bearing capacity and deformation characteristics. The column deformation during loading was recorded using a linearly varying differential transducer (LVDT). Figure 5 illustrates the axial compression testing setup for the reinforced concrete columns in the UTM.
Fig. 5
Axial compression test on columns
3 Results and Discussion
3.1 Concrete Quality and Workability
A slump cone test was conducted to evaluate the workability of the freshly prepared geopolymer concrete (GPC) in accordance with ASTM C143 [35]. The concrete was placed in a cone mould with a top diameter of 10 cm, a bottom diameter of 20 cm, and a height of 30 cm. After filling the mould, it was lifted, and the slump was measured as the difference in height between the concrete and the mould. The recorded slump values were 80 mm, 82 mm, and 81 mm for three successive trials, indicating good and repeatable workability.
3.2 Half-Cell Potential
To assess corrosion levels in the reinforced concrete specimens, half-cell potential measurements were performed using a CuSO4 electrode following the point measurement method. Electrical connections were established with the embedded steel bars, and potential readings were recorded at 20 different locations across the column surfaces. The observed potential values ranged from − 508 mV to −550 mV. According to IS 516 Part 5, Sect. 2 [36], values below − 500 mV indicate severe corrosion, with a probability of corrosion exceeding 90%. The variation in potential values suggests localized differences in corrosion activity, likely influenced by moisture distribution, cracks, and internal structural defects. Figure 6 presents the half-cell potential values, while Fig. 7 illustrates the corrosion damage observed on the reinforced concrete column specimens.
Fig. 6
Half cell potential values read during testing
Fig. 7
Corrosion damage on the reinforced concrete column specimens
3.3 Physical Observations and Mass Losses
Following exposure to elevated temperatures, the concrete specimens were visually examined for colour changes, surface cracks, and potential spalling. Figure 8 illustrates the colour transformation before and after heating. From Fig. 8, it can be seen that light brown colour stains were visible before corrosion and dark brown colour stains after heating. The reference specimens that were not exposed to accelerated corrosion procedures (Ref, Ref-H1, Ref-H2) developed surface cracks due to thermal incompatibility between the core and surface temperatures, leading to internal pressure release through cracking. However, no spalling was observed, as the concrete edges remained intact. On the other hand, the corroded specimens (CR) exhibited light brown stains before heating, which darkened after exposure, but spalling was still absent regardless of heating duration and temperature.
Crack widths were measured using an Elcometer, as shown in Fig. 9. The Ref-H1 specimen exhibited a crack width of 0.4 mm. In comparison, Ref-H2 had a slightly higher width of 0.5 mm, as shown in Fig. 9, indicating greater thermal stress-induced damage with longer heating durations. Similarly, corroded specimens CR-H1 and CR-H2 showed crack widths of 0.4 mm and 0.5 mm, respectively, confirming a trend of increasing crack width with prolonged exposure due to thermal stresses, moisture evaporation, and material expansion.
Mass loss was evaluated by comparing these values before and after exposure to heating and/or corrosion. After 60 min of heating, the concrete lost 4.4% of its mass, as illustrated in Table 3, primarily due to the evaporation of free water from the pore structure. Following 90 min of heating, the mass loss increased to 4.7%, representing a 24% higher reduction compared to the 1-hour exposure. The mass loss was found to be consistent with the findings of other researchers (e.g [37, 38]. This additional loss is attributed to the dehydration of chemically bound water in the gel phases, which weakens the internal structure and accelerates the degradation [39, 40]. The results indicate a nonlinear trend in mass loss with increased heating duration. Corrosion alone does not cause significant loss in mass but reduces the strength and durability by weakening the bond between the concrete and steel reinforcement [41]. Corrosion exacerbates fire damage by promoting initial cracking and increasing permeability, leading to more severe mass loss [42].
Table 3
Results from the post-fire tests
Specimen | Mass loss (%) | First crack load (kN) | Loss in load at first cracking (%) | Ultimate load (kN) | Loss in ultimate load (%) |
|---|---|---|---|---|---|
Ref | - | 250 | 0 | 710 | 0 |
Ref-H1 | 4.4 | 140 | 44 | 490 | 31 |
Ref-H2 | 4.7 | 78 | 69 | 340 | 52 |
CR | 1.1 | 90 | 64 | 440 | 38 |
CR-H1 | 5.4 | 30 | 88 | 210 | 70 |
CR-H2 | 5.6 | 15 | 94 | 150 | 79 |
Fig. 8
Colour change before and after heating CR specimens
Fig. 9
Images showing the crack widths in the concrete specimens
3.4 Residual Compressive Strength
The initial compressive strength of the GPC was measured as 33.2 MPa, which was determined as the average value from three samples. Specimens subjected to elevated temperatures exhibited significant strength reductions. After 60 min of heating, the residual compressive strength decreased to 20 MPa, reflecting a 41% loss due to thermal cracking and degradation of binding phases. After 90 min, the strength further dropped to 15.3 MPa, corresponding to a 54% reduction compared with the original value. The additional loss suggests progressive thermal damage, with the 1.5-hour heated specimens experiencing 24% greater strength loss than those exposed for 60 min. This indicates that the thermal resistance of GPC declines non-linearly with prolonged exposure, likely due to internal morphological degradation and increased crack propagation. Shorter heating durations result in moderate strength reductions, while prolonged exposure leads to more severe and accelerated degradation [43, 44]. Kodur [45] and Phan and Carino [46] reported a loss of 25–50% of the concrete strength when exposed to a temperature of 500–600 ℃, with further reductions developing after exposure to higher temperatures. Other researchers [47, 48] found that corrosion reduces the load capacity by anywhere between 30 and 70% due to the reduction in both steel cross-sectional area as well as bond strength. The combined effects of fire and corrosion accelerates strength loss, often leading to more than 80% reduction in load capacity [41, 42].
3.5 Residual Column Behaviour
3.5.1 Load Versus Axial Deformation
After exposure to elevated temperatures followed by subsequent cooling (air), the column specimens were tested under axial compression using a universal testing machine (UTM) to evaluate their structural performance. The applied load was recorded via a data acquisition system integrated with the UTM, while the corresponding deformation was measured using a series of linear variable differential transformers (LVDT). Figure 10 illustrates the load-deformation behaviour of the tested columns. The reference (Ref) specimen exhibited the highest load-bearing capacity, demonstrating superior structural performance compared with those exposed to corrosion and/or elevated temperatures. The Ref-H1 specimen showed a slightly lower response, followed by the corroded (CR) specimen. The remaining specimens exhibited progressively lower load-deformation profiles, reflecting the detrimental effects of corrosion and elevated temperatures on mechanical performance. These results highlight the influence of external factors on the deformation capacity and overall structural integrity of the reinforced concrete columns. Figure 11 presents images of the concrete columns after testing, showing damage caused by corrosion and heating. The brown colour in the specimens represents the corrosion stains in the CR specimens, which were confirmed through Fig. 11. Corrosion leads to loss of reinforcement area, reducing its load-carrying capacity. It was shown elsewhere [49, 50] that a 10–20% loss in reinforcement diameter can lead to a strength loss of 50%. Moreover, the formation of rust results in the generation of stresses inside the concrete matrix leading to the development of microcracks and the weakening of bond [51]. Pre-existing corrosion-induced cracks may provide a pathway for temperature penetration and further exacerbate the bond degradation process. Corrosion-induced cracks serve as stress concentrators, making them susceptible to thermal degradation.
Fig. 10
Load versus axial deformation responses
Fig. 11
Damage to concrete columns after testing showing the influence of corrosion and heating
3.5.2 Load at First Cracking
The load at which the first visible crack appeared was recorded for each specimen, and the results are given in Table 3, as well as the reduction (as a %) in this value for each condition, relative to the corresponding value for the unheated and uncorroded specimen, Ref. It is evident that the Ref specimen exhibited the highest first crack load at 250 kN, indicating greater resistance to cracking under axial compression. In contrast, the CR specimen recorded a significantly lower first crack load of 90 kN. The Ref-H1 specimen showed a 44% reduction in first crack load compared with Ref, while the Ref-H2 specimen experienced a 69% reduction. Among the corroded specimens, the CR specimen exhibited a 64% reduction compared to the Ref specimen. The CR-H1 and CR-H2 specimens displayed even greater losses, with reductions of 88% and 94%, respectively. The results given in Table 3 highlight the percentage loss in the first crack load for all tested columns, clearly showing that corroded specimens subjected to heating experienced the most severe reductions, exceeding 85% compared to the Ref specimen.
3.5.3 Ultimate Load
Table 3 also presents the ultimate load of the column specimens, illustrating the effects of corrosion and elevated temperature on their load-carrying capacities. The Ref specimen exhibited the highest ultimate load at 710 kN. In contrast, the CR specimen recorded a significantly lower value of 440 kN, representing a 38% reduction compared to Ref. Among the heated specimens, Ref-H1 showed a 31% reduction in load-carrying capacity owing to the 60 min of elevated temperature exposure, while Ref-H2 experienced a greater reduction of 52%. Corrosion-induced heated specimens exhibited even more pronounced reductions, with the CR-H1 and CR-H2 specimens showing ultimate load losses of 71% and 79%, respectively, compared to the Ref specimen. The residual strength of the specimens after corrosion and heating ranged from 22% to 30% of their initial strength, highlighting the significant impact of environmental and accidental factors on structural performance. These findings reinforce the detrimental effect of combined corrosion and high-temperature exposure on the structural integrity of reinforced concrete columns.
3.5.4 Residual Stiffness Properties
Stiffness represents the rigidity of column specimens and is a key indicator of structural integrity. Stiffness is measured as the ratio between the ultimate load to its corresponding deformation. In this study, the stiffness of concrete specimens was evaluated before and after exposure to corrosion and/or elevated temperature to assess the impact of these conditions. Table 4 presents the stiffness of the tested columns under different scenarios, as well as the percentage loss (-), or gain, shown as +, relative to the unheated and uncorroded Ref column. The Ref specimen exhibited a stiffness of 101 kN/mm. Interestingly, after heating for one hour (Ref-H1), the stiffness increased to 122.5 kN/mm, showing a 20.8% improvement. This increase is attributed to the activation of geopolymeric gel phases, where unreacted binder particles enhance rigidity. The CR specimen exhibited a stiffness of 98 kN/mm, only slightly lower than the Ref specimen, suggesting that corrosion alone had a limited impact on stiffness. However, when corrosion and heating were combined, a significant reduction in stiffness was observed. The CR-H1 specimen exhibited a 49 kN/mm stiffness, while the CR-H2 specimen showed a further decline to 36.5 kN/mm, compared with the Ref specimen. Compared to the heated reference specimens, the CR-H1 specimen exhibited a 60% reduction in stiffness relative to Ref-H1, while CR-H2 showed a 58% reduction compared to Ref-H2. These results highlight the severe deterioration in stiffness caused by the combined effects of corrosion and elevated temperature. While heating alone (Ref-H1) is shown to maintain or even enhance stiffness, the presence of corrosion in conjunction with heating (CR-H1 and CR-H2) led to substantial stiffness degradation. This finding highlights the need to account for these factors when assessing the long-term structural performance of reinforced concrete elements. It is noteworthy that other researchers [52] found that the stiffness of reinforced concrete columns subjected to corrosion declines by 10–40% depending on corrosion level. It is noteworthy that it was previously shown [37] that member stiffness can reduce by up to 70% under the combined effects of corrosion and post-fire exposure conditions.
Table 4
Residual structural properties (stiffness, ductility and EAC) following exposure to elevated temperature and/or corrosion
Specimen | Stiffness (kN/mm) | Loss/gain in stiffness (%) | Ductility index | Loss/gain in ductility index (%) | EAC (kNmm × 103) | Loss/gain in EAC (%) |
|---|---|---|---|---|---|---|
Ref | 101.4 | 100.0 | 3.5 | 0.0 | 147.3 | 0.0 |
Ref-H1 | 122.5 | + 20.8 | 2.5 | −28.6 | 18.2 | −87.6 |
Ref-H2 | 87.1 | −14.1 | 2.6 | −25.7 | 11.1 | −92.5 |
CR | 97.7 | −3.6 | 2.6 | −25.7 | 20.1 | −86.4 |
CR-H1 | 48.8 | −51.9 | 5.3 | + 51.4 | 9.05 | −93.9 |
CR-H2 | 36.5 | −64.0 | 5.1 | + 45.7 | 3.7 | −97.5 |
3.5.5 Ductility
Ductility refers to an elements or material’s ability to undergo significant deformation before fracture, a critical factor governing the structural resilience of reinforced concrete columns. The ductility index is the ratio of the deformations at the ultimate load and at first cracking, respectively. In the current study, the ductility of all columns was evaluated to assess the effects of corrosion and heating and the results are given in Table 4. The Ref specimen exhibited a ductility index of 3.5. For the heated specimens, Ref-H1 and Ref-H2, the ductility indices were 2.5 and 2.6, respectively, indicating a reduction in ductility due to elevated temperature exposure. Similarly, the CR specimen had a ductility index of 2.6, representing a 25.7% reduction compared to the Ref specimen, which highlights the negative impact of corrosion on the material’s deformation capacity.
In contrast, the combined effect of corrosion and heating significantly increased ductility. The ductility indices of CR-H1 and CR-H2 specimens were 5.3 and 5.1, respectively, representing increases of 53% and 49% compared to their uncorroded heated counterparts. This trend suggests that while heating or corrosion alone reduces ductility, their combined effect enhances it. The improved ductility in corrosion-induced heated specimens is attributed to stress redistribution within the material. These findings highlight the complex interaction between corrosion and heating, demonstrating that while each factor individually weakens ductility, their combined effect enhances the deformation capacity of reinforced concrete columns. Fire affected corroded specimens shows plastic deformation before failure, through the development of microcracks [41].
3.5.6 Energy Absorption Capacity
The energy absorption capacity (EAC) of the concrete specimens was assessed based on the area under the load-deformation curves, which reflects their ability to dissipate energy under applied loads. Table 4 presents the EAC values of the tested columns under different conditions. The EAC was calculated using Simpson’s Rule. This numerical integration method provides higher accuracy than the trapezoidal rule by approximating the curve between three consecutive points with a second-order polynomial. For a set of n + 1 evenly spaced deformation points Δ0, Δ1, …, Δn with corresponding loads P0, P1, …, Pn, the Simpson’s Rule formula is illustrated in Eq. 2:
$$\:\text{E}\text{A}\text{C}=\:\frac{\text{h}}{3}[{\text{P}}_{0}+{\text{P}}_{\text{n}}+4\sum\:_{\text{o}\text{d}\text{d}\:\text{i}=1}^{\text{n}-1}{\text{P}}_{\text{i}}+2\sum\:_{\text{e}\text{v}\text{e}\text{n}\:\text{i}=2}^{\text{n}-2}{\text{P}}_{\text{i}}]$$
(2)
where h is the uniform deformation interval (mm), and Pi are the loads (kN) at each point. The result is expressed in kNmm.
The reference (Ref) specimen exhibited the highest energy absorption capacity at 147 × 10³ kNmm. In contrast, the heated specimens showed significant reductions, with Ref-H1 and Ref-H2 specimens absorbing 18.2 × 10³ kNmm and 11.1 × 10³ kNmm, representing reductions of 87.6% and 92.5%, respectively, compared to the Ref specimen. Corrosion also led to a substantial loss in energy absorption capacity, as the CR specimen exhibited 20.1 × 10³ kNmm, an 86.4% reduction from the Ref specimen. The combined effect of corrosion and heating resulted in the most severe losses, with CR-H1 and CR-H2 specimens absorbing only 9 × 10³ kNmm and 3.7 × 10³ kNmm, corresponding to reductions of 93.9% and 97.5%, respectively, compared to Ref. When comparing the heated specimens to their corroded counterparts, CR-H1 exhibited 50% lower energy absorption than Ref-H1, while CR-H2 showed a 66% reduction compared to Ref-H2. These results demonstrate that both corrosion and heating significantly affect the energy dissipation capability of reinforced concrete columns, with their combined effect leading to the most severe losses. While heating alone significantly reduces energy absorption, its combination with corrosion exacerbates the degradation, highlighting the detrimental impact of these factors on structural performance.
3.5.7 Toughness Factor
The toughness factor, which quantifies a material’s ability to absorb energy and deform plastically before fracture, was evaluated under various exposure conditions to assess the impact of corrosion and heating on the structural performance of reinforced concrete columns. The toughness factor was determined based on the expression given in Eq. 3, and the results are given in Table 5:
$$\:\stackrel{-}{{{\upsigma\:}}_{\text{b}}}=\:\frac{\text{T}\text{b}}{{{\updelta\:}}_{\text{t}\text{b}}}\:\times\:\:\frac{\text{l}}{{\text{b}\text{h}}^{2}}\:$$
(3)
where, \(\:\stackrel{-}{{{\upsigma\:}}_{\text{b}}}\) is the toughness factor (N/mm2), Tb is the axial toughness (Nmm), \(\:{{\updelta\:}}_{\text{t}\text{b}}\) is a deformation value corresponding to mid height (mm), and l, b and h are the member length, breadth and height, in mm, respectively. The Ref specimen exhibited the highest toughness factor at 8.2 MPa. Heating alone led to a substantial reduction, with Ref-H1 and Ref-H2 specimens showing toughness factors of 1 MPa and 0.6 MPa, representing reductions of 87.8% and 92.7%, respectively. Similarly, the CR specimen displayed a toughness factor of 1.1 MPa, an 86.6% reduction compared to the Ref specimen. The combined effects of corrosion and heating resulted in the most severe degradation. The CR-H1 and CR-H2 specimens exhibited toughness factors of 0.5 MPa and 0.2 MPa, corresponding to reductions of 93.9% and 97.6%, respectively, compared to the Ref specimen. When comparing heated specimens to their corroded counterparts, CR-H1 exhibited 50% lower toughness than Ref-H1, while CR-H2 showed a 66.6% reduction compared to Ref-H2. These results highlight the detrimental effects of heating and corrosion on the toughness of reinforced concrete columns. While heating alone caused a substantial reduction, the presence of corrosion further exacerbated the degradation.
Table 5
Residual structural properties (Toughness factor, strength and deformation index) following exposure to elevated temperature and/or corrosion
Specimen | Toughness factor (MPa) | Loss/gain in toughness factor (%) | Strength index | Loss/gain in strength index (%) | Deforma-tion index | Loss/gain in deforma-tion index (%) |
|---|---|---|---|---|---|---|
Ref | 8.2 | 0.0 | 3.6 | 0.0 | 3.9 | 0.0 |
Ref-H1 | 1.0 | −87.8 | 4.7 | + 31.3 | 3.1 | −20.9 |
Ref-H2 | 0.6 | −92.7 | 5.7 | + 59.4 | 3.3 | −16.2 |
CR | 1.1 | −86.6 | 5.6 | + 58.9 | 3.5 | −10.8 |
CR-H1 | 0.5 | −93.9 | 8.4 | + 136.6 | 8.6 | + 121.6 |
CR-H2 | 0.2 | −97.6 | 15 | + 322.5 | 6.8 | + 76.0 |
3.5.8 Strength Index
The strength index of the reinforced concrete columns was evaluated under different exposure conditions to assess their structural performance. The strength index was determined as the ratio between the ultimate load and the load at 0.001 strain, and the results are given in Table 5. It is shown that Ref-H1 exhibited a 31.3% increase, reaching 4.67, while Ref-H2 showed a further increase to 5.7. In the case of the corroded specimen, the strength index was 5.6, representing a 58.9% increase compared to Ref, possibly due to localized hardening effects induced by corrosion. The combined effects of corrosion and heating resulted in a further increase in strength index. The CR-H1 specimen achieved 8.4, a 136.6% increase over Ref, while CR-H2 reached 15, marking a 322.5% increase. Comparing corrosion-induced heated specimens to their heated-only counterparts, CR-H1 exhibited a 44% higher strength index than Ref-H1, while CR-H2 showed a 62% increase over Ref-H2. These results indicate that the combination of corrosion and heating significantly enhanced the strength index, particularly for specimens subjected to prolonged heating (CR-H2). While heating alone contributed to increased strength, the presence of corrosion further amplified this effect, suggesting that certain exposure conditions may alter stress distribution within the material. Therefore, these effects must be carefully considered in the context of overall structural performance and long-term durability.
3.5.9 Deformation Index
The deformation index, which quantifies a concrete column’s ability to sustain deformation under applied loads, was evaluated for specimens subjected to varying exposure conditions, including corrosion and heating. The deformation index was determined as the ratio between the deformation at the ultimate load and the deformation at 0.001 strain in the concrete, and the results are given in Table 5. It is shown that the Ref specimen had a deformation index of 3.9. Of the heat-exposed specimens, Ref-H1 exhibited a 21% drop to 3.0, while Ref-H2 showed a 16% drop to 3.2, indicating that heating contributed to a marginal improvement in deformation capacity, particularly with prolonged exposure. For the corroded specimen (CR), the deformation index was 3.4, representing a 11% low compared to the Ref specimen. This suggests that corrosion may introduce localized changes that enhance the material’s ability to deform before failure. The combined effects of corrosion and heating resulted in the most significant increases. The CR-H1 specimen had a deformation index of 8.6, marking an 54% increase over the Ref specimen, while CR-H2 exhibited 6.8, reflecting an 55% increase. When compared to their heated-only counterparts, CR-H1 demonstrated an 64% higher deformation index than Ref-H1, and CR-H2 showed an 52% increase over Ref-H2. These results highlight the substantial influence of corrosion and heating on the deformation behaviour of concrete columns. While heating or corrosion alone had a moderate impact, their combined effect significantly enhanced deformation capacity. However, this increased deformation is likely to be associated with reduced stiffness, potentially compromising overall structural integrity under service conditions.
4 Analysis of the Structural Properties Following Exposure to Heating and/or Corrosion
This section presents a detailed analysis of the relationships between various structural performance parameters following exposure to corrosion, elevated temperatures, and their combined effects. The results are analysed with a view to understanding how these degradation mechanisms influence the behaviour of reinforced GPC columns and to draw useful insights relevant to structural design in harsh and/or fire-prone environments.
4.1 Stiffness and Load Capacity
Table 4 clearly show that stiffness and ultimate load capacity are positively correlated across all specimens. The unexposed reference specimen (Ref) exhibited the highest stiffness (101.4 kN/mm) and the greatest load-bearing capacity (710 kN). Conversely, the corroded and heated specimens demonstrated significant degradation in both properties. Notably, the CR-H2 specimen, which was subjected to both corrosion and 90 min of heating, recorded the lowest stiffness (36.5 kN/mm) and ultimate load (150 kN), corresponding to reductions of 64% and 78.9%, respectively.
This relationship is expected, as stiffness is fundamentally linked to the material’s ability to resist deformation under load. Degradation of the concrete matrix due to thermal cracking, combined with loss of reinforcement cross-section and bond strength from corrosion, leads to a reduction in both stiffness and strength. Interestingly, the stiffness of Ref-H1 (122.5 kN/mm) was higher than the unheated Ref specimen, likely due to thermally activated geopolymerisation processes that temporarily improved rigidity. However, this effect was not sustained following longer heating durations or in the presence of corrosion.
4.2 Balance Between Ductility and Strength
The ductility index, representing the ratio of deformation at ultimate load to that at first crack, displayed a complex relationship with strength degradation. While heating or corrosion alone reduced ductility (e.g., Ref-H1 and CR both showing ~ 26% loss), the combined exposure in CR-H1 and CR-H2 resulted in increased ductility indices of 5.3 and 5.1, respectively—more than 45% higher than Ref. This increase in ductility is combined with severe strength losses. CR-H1 and CR-H2 experienced reductions in ultimate load of 70.4% and 78.9%, respectively, compared with Ref. This suggests that, although the material became more deformable under load, this came at the expense of strength and stiffness. Such behaviour can be attributed to the development of distributed microcracks due to corrosion and heating, which permit greater strain accommodation prior to failure. However, from a design perspective, this increased ductility may not be beneficial if accompanied by substantial strength loss.
4.3 Toughness and Energy Dissipation
The toughness factor and energy absorption capacity (EAC) are critical indicators of a column’s ability to withstand dynamic or accidental loads, such as those experienced during a fire. Both properties showed dramatic reductions following exposure to elevated temperatures and/or corrosion. The reference specimen recorded the highest EAC (147 × 10³ kNmm) and toughness factor (8.2 MPa). Following 90 min of heating alone (Ref-H2), these values fell by more than 92%. For the CR-H2 specimen, exposed to both corrosion and prolonged heating, the EAC dropped to 3.7 × 10³ kNmm (97.5% loss) and the toughness factor reduced to 0.2 MPa (97.6% loss). These results illustrate the compounded effects of corrosion and fire in significantly reducing the energy dissipation capacity of the structure. Such degradation is consistent with the combined breakdown of the binder matrix and reinforcement-concrete bond, which impairs the ability to absorb energy through plastic deformation. The presence of pre-existing cracks and corrosion-induced porosity likely intensifies the rate at which thermal damage propagates during fire exposure.
4.4 Combined Effects of Temperature Exposure and Corrosion
The results presented from the test programme indicate that the combined impact of corrosion and heating is not simply additive but can have even more detrimental effects. For instance, while corrosion alone reduced the ultimate load by 38% and 90-minute heating alone led to a 52% reduction, the CR-H2 specimen experienced a 79% loss—greater than either of the individual effects. Similarly, while corrosion and heating alone had negligible or even slightly positive effects on ductility and strength index, their combination led to dramatic shifts. CR-H2 showed a strength index increase of 322% and a deformation index increase of 76% over Ref, yet these gains are misleading in the absence of meaningful load-bearing capacity. These findings highlight the complex interaction between damage mechanisms. Corrosion appears to exacerbate the susceptibility of the concrete to thermal degradation, both by introducing internal stress risers and by increasing permeability, facilitating deeper heat penetration.
4.5 Implications for Structural Design
The findings from this study have important implications for both structural design and fire safety assessment, particularly in environments where corrosion is a credible risk. Current fire design provisions, such as those in Eurocode 2 and ACI 216, largely consider fire exposure in isolation and do not explicitly account for pre-existing deterioration due to corrosion. However, this study has shown that the combined effects of corrosion and elevated temperatures result in significantly more severe degradation than either factor alone, with losses in stiffness, ultimate strength, and toughness exceeding 70% in some cases. A key insight is that structural capacity cannot be reliably inferred from fire performance alone when prior corrosion exists. Corrosion not only reduces cross-sectional area and bond strength, but also increases permeability and internal cracking, which accelerates heat penetration and exacerbates thermally induced degradation. These mechanisms can occur simultaneously, and their compounded effects are non-linear, meaning that design assumptions based on independent damage models may underestimate the actual risk.
Importantly, although some specimens displayed increased ductility and strength index following combined exposure, these enhancements occurred alongside critical reductions in load capacity and stiffness. Therefore, these increases should not be interpreted as improved performance, but rather as indicative of more compliant and degraded failure modes. From a design perspective, the following considerations are important:
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Fire resistance ratings should be carefully considered to reflect prior corrosion exposure, particularly in marine, industrial, or aging infrastructure where chloride-induced corrosion is prevalent. Incorporating corrosion-adjusted material properties in thermal analysis and structural models may yield more realistic fire design outcomes.
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Increased cover depth or enhanced corrosion protection may be necessary in fire-critical regions of structures exposed to aggressive environments. This may include using corrosion-resistant reinforcement, such as stainless steel or FRP bars, especially in perimeter columns or locations with high thermal gradients.
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Geopolymer concretes, while showing promising fire resilience compared to OPC-based mixes, are still vulnerable to combined corrosion and fire exposure. Further development of GPC formulations with improved bond durability and thermal stability may be beneficial, particularly for use in critical infrastructure.
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Structural assessment protocols following fire exposure should be modified to account for the condition of reinforcement prior to the fire event. Traditional residual strength evaluations may overestimate safety margins in elements with underlying corrosion damage.
5 Conclusions
This study investigated the post-fire structural performance of geopolymer concrete (GPC) columns subjected to accelerated corrosion and elevated temperatures, simulating combined environmental and accidental damage. Corrosion alone caused over 60% reductions in both first crack and ultimate loads, while heating led to progressive, non-linear degradation in compressive strength, mass, and stiffness. The combined effects were most severe, with up to 79% loss in ultimate load, 64% reduction in stiffness, and over 90% decline in toughness and energy absorption. Although increases in ductility and strength index were observed in corroded–heated specimens, these reflected more compliant failure modes rather than improved performance. Stiffness showed strong correlation with ultimate load, making it a reliable indicator of residual capacity.
However, it is also important to recognise the limitations of the study. The investigation was limited by the use of half-cell potential measurements without direct quantification of reinforcement mass loss, and by the absence of tensile strength testing, which is critical for understanding cracking and bond behaviour. The specimen geometry also prevented buckling failure, and load-controlled testing restricted insights into post-peak deformation. These limitations suggest that future research should incorporate combined electrochemical and physical corrosion assessments, tensile strength testing, varied slenderness ratios, and deformation-controlled loading to better capture the full degradation mechanisms and improve predictive models for fire-exposed, corroded GPC structures. The findings underscore the need to revise current design codes, which do not account for the compounded effects of corrosion and fire, potentially leading to unsafe overestimations of structural capacity in deteriorated concrete elements.
Acknowledgements
Authors are thankful to Karunya Institute of Technology and Sciences for providing “Structural Fire Testing Laboratory” to carry the work.
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