Synergistic Impact of Entrained Air and Fly Ash on Chloride Ingress in Concrete Pavement: An Electrical Resistivity Model Approach

Abstract

Ensuring the durability of concrete pavements against chloride ingress is critical, yet the relationship between electrical resistivity and chloride penetration remains underexplored. This study evaluates the effectiveness of entrained air and fly ash in mitigating chloride ingress using an electrical resistivity model and surface resistivity tests. Concrete samples with varying entrained air contents (0% to 10%) and Class C or Class F fly ash underwent three-year ponding tests in temperature-controlled indoor water baths and outdoor CaCl₂-NaCl brine solutions. The results indicate that lower entrained air contents led to a more rapid increase in resistivity, with concrete mixes incorporating Class C fly ash exhibiting 1.5 times greater resistivity gains than those with Class F fly ash. Surface resistivity tests revealed that reaction factors were 67% higher in specimens with 3.5% entrained air compared to 10.0%, while decreasing by 57% and 41% in concrete mixes containing Class F and Class C fly ash, respectively, across all chloride concentrations. Using back-calculated environmental factors, corrosion initiation potential in concrete pavements was projected for exposure periods of up to 50 years. These findings provide insights for optimizing entrained air and fly ash formulations to enhance pavement performance and durability.

1. Introduction

In recent years, winter maintenance strategies in the US have undergone a significant shift from passive deicing to proactive anti-icing practices [1,2]. When applied in a timely manner, anti-icing prevents the formation of bonded snow and ice, yielding superior results compared to deicing while also reducing costs [3,4,5]. Salt brine, often combined with calcium chloride (CaCl₂) or magnesium chloride (MgCl₂), is the most favored by many state departments of transportation (DOTs), due to its effectiveness in ice control. The inclusion of CaCl₂ in NaCl brine enhances its performance at temperatures below 15°F by attracting moisture and maintaining surface adherence [6]. Similarly, MgCl₂-NaCl solutions exhibit comparable efficacy, though their use is more restricted due to their potential to create dangerously slippery conditions [7]. While non-chloride deicers have attracted attention for their reduced corrosiveness and superior performance, their limited availability in large volumes and the need for specialized storage systems render them impractical and cost prohibitive. On average, non-chloride deicers cost approximately USD 2.50 per gallon, whereas conventional salt brine solutions containing CaCl₂ or MgCl₂ range from USD 0.40 to USD 0.50 per gallon [8]. As a result, chloride-based deicers remain the preferred choice, particularly as modern anti-icing strategies continue to improve service levels, reduce overall deicer usage, and offer significant cost savings, safety enhancements, and mobility benefits [9].

Despite their benefits, chloride-based deicers pose substantial challenges to the durability of concrete pavements [10,11]. The detrimental effects of these deicers, including cracking, scaling, and reinforcement corrosion, have been reported globally and extensively studied [12,13,14]. Research has identified multiple mechanisms underlying these forms of deterioration. The crystallization of salts within concrete pores disrupts pore structures, leading to scaling and spalling [15]. Hydraulic pressure from freeze–thaw (F-T) cycles exacerbates cracking, unless it is mitigated by well-connected pores [16]. Also, prolonged exposure to salt brine dissolves calcium hydroxide in cement paste, increasing porosity and compromising F-T durability through the formation of chloroaluminate crystals [17]. Furthermore, CaCl₂ and MgCl₂ initiate chemical reactions that weaken concrete, with CaCl₂ forming expansive calcium oxychloride and MgCl₂ inducing porosity through the formation of brucite and magnesium–silicate–hydrate (M-S-H) compounds [18,19,20,21]. Chloride ions further elevate corrosion risks in steel-reinforced concrete when specific temperature and humidity conditions are met [22].

The potential for chloride penetration into concrete pavement is highly influenced by the characteristics of concrete pores, including pore size distribution, interconnectivity, and the chemical composition of the pore solution [23]. These properties are largely governed by the water-to-cement (w/c) ratio, cement type, and the inclusion of supplementary cementitious materials (SCMs) and chemical admixtures. Combined with curing conditions, these parameters influence the permeability of concrete to chloride ions. While concrete pores play a crucial role in mitigating F-T damage, their complex interactions with chloride penetration remain poorly understood. Nili et al. provided insights into the effects of entrained air on concrete resistivity, but its interactions with chloride diffusion mechanisms require further investigation [24]. SCMs—particularly fly ash—are widely recognized for their role in reducing chloride ingress. Class C fly ash, rich in calcium oxide (CaO), enhances early-stage strength development, while Class F fly ash, with higher silica (SiO₂) content, promotes long-term pore refinement through pozzolanic reactions. The concentration and type of cations present in brine solutions, such as Na⁺ and Ca²⁺, can significantly affect chloride binding capacity and transport mechanisms, introducing further complexity in predicting chloride ingress [25].

Various models have been proposed to predict chloride ingress potential, ranging from empirical diffusion models to sophisticated multi-phase transport models, in cement mortars or concrete mixes with SCMs [26,27,28,29,30,31]. A pivotal advancement in this area was Andrade’s introduction of the reaction factor in electrical resistivity-based chloride ingress models, which has been widely applied as an effective tool in characterizing microstructural variations in cementitious systems [32,33]. A recent study by Pablo-Calderón et al. showed a relationship between electrical resistivity and chloride ingress in cement-based materials [34]. Similar research works by Geng et al. and Kang et al. demonstrated the influence of fly ash on electrical resistivity in concrete, further reinforcing the role of concrete resistivity as a key durability indicator [35,36]. Raczkiewicz et al. reported that blast-furnace slag cement and an air-entraining agent provided the best protection against corrosion in reinforced concrete [37]. Chloride ingress appeared to be influenced by various mix variables, such as recycled aggregates, expanded clay, lightweight mortars, and sand content [38,39,40,41,42]. Despite these developments, the combined effects of key mix variables on chloride ingress remain underexplored, especially in chloride-rich environments. Furthermore, little research has investigated the long-term behavior of chloride ingress in the absence of F-T cycles, a critical condition for regions where chloride-based deicers are used under milder winter conditions without frequent freezing. Consequently, many winter roadway maintenance practices still rely on empirical observations, leading to the overuse of deicers and premature concrete deterioration.

To address these gaps, this study explores the synergistic impact of entrained air and fly ash on chloride ingress in concrete pavements. Three-year ponding tests were conducted on laboratory specimens in both indoor and outdoor settings. Surface resistivity measurements were utilized to refine the reaction and aging factors in Andrade’s resistivity model and to develop an alternative predictive tool for corrosion initiation in concrete pavements. The findings provide actionable insights for both state and local road agencies, enabling optimized deicer selection and application strategies to mitigate premature pavement deterioration. This research lays the foundation for developing performance-based approaches to winter roadway maintenance, offering critical infrastructure management challenges.

2. Materials and Methods

2.1. Electrical Resistivity Model

The electrical resistivity of concrete refers to its ability to withstand the transfer of ions under an electrical field. Its level largely depends on the microstructural properties of concrete, including pore size, distribution, and connectivity (i.e., tortuosity) within the matrix [43,44]. As a performance-based parameter, electrical resistivity plays a crucial role in evaluating the durability and structural integrity of concrete. Its significance is further underscored by its correlation with the chloride diffusion coefficient, as described by the Nernst–Einstein equation, thereby enhancing its versatility in chloride ingress assessments [45].

D_ef = k_CI/ρ_ef (1)

where D_ef is the effective diffusion coefficient, k_CI is the environmental factor, and ρ_ef is the effective resistivity under water-saturated conditions. As steady state quantities, D_ef and ρ_ef indicate the transport of chloride ions. Therefore, chloride diffusion is regarded as a sole mechanism of chloride ingress into concrete under the influence of environmental factors.

The electrical resistivity model that is suitable for investigating chloride ingress in concrete comprises the following three distinct types of factors: environmental factor; reaction factor; and aging factor [32]. To determine the environmental factors, Andrade et al. [46] conducted an inverse analysis on test results from real structures. This empirical approach reflected the chloride diffusions within concrete, contingent upon the external chloride concentration, and required long-term observations under specified exposure conditions. Table 1 displays the values of k_CI suggested by Andrade et al. [46] for the various exposure classes designated by the European Standard EN 206+A2 [47].

The reaction factor accounts for the degree of chloride binding with cement phases, occurring at the atomic level. It dictates the amount of free chloride ions and thus indicates the rate of chloride ingress in concrete. Equation (2) shows an expression for the reaction factor.

r_CI = D_ef/D_ap (2)

where D_ap is the apparent diffusion coefficient in a non-steady state. In practice, both D_ef and D_ap can be measured through a multi-regime test. It should be noted that if the reaction factor is determined from the ratio diffusion coefficients, a porosity (% by volume) must be considered as the unit homogenization factor [48]. When both sides of Equation (1) are divided by r_Cl, an alternative expression for D_ap can be derived as follows:

D_ef/r_CI = k_CI/(ρ_ef · r_CI) (3)

D_ap = k_CI/ρ_ap(4)

where ρ_ap is the apparent resistivity (ρ_ap = ρ_ef · r_CI) that characterizes the transport and reaction behavior of chloride ions in concrete. Andrade [32] reported that the reaction factors of Type I cement were around one, while Types II or III cement tended to have higher reaction factors, in some cases, ranging from three to five. The use of SCMs could either increase or decrease chloride binding in Portland composite cement, depending on the contribution of aluminates to the change in the phase assemblage of the cement matrix [25]. However, there have been few attempts to date to incorporate the effect of the volume of artificial pores (e.g., entrained air) and their refinement into the reaction factor, especially when exposed to diverse chloride salts.

Assuming the square root relation between the diffusivity and penetration depth of chloride ions, Andrade [32] proposed an expression of corrosion initiation period (t_i), pertaining to the moment of depassivation of the reinforcement in concrete as a threshold.

t_i = x² ρ_t r_Cl/k_CI (5)

where t_i is the corrosion initiation period in year, and x is the penetration depth of the chloride threshold concentration, equivalent to concrete cover in cm when t_i is considered. ρ_t is the effective resistivity corrected over time due to concrete aging, as seen in Equation (6).

ρ_t = ρ₀ (t/t₀)^q (6)

where t is the time considered for the application of an aging factor, t₀ represents the age of the first resistivity measurement at 28 days, and q is aging factor. The aging factor accounts for the effect of pore refinement on electrical resistivity, which generally increases with time [49]. According to the work by Andrade and D’Andrea [46], suggested values for q ranged from 0.22 (Type I cement) to 0.37 (Type IIA-P) and 0.57 (Type IIA-V). Type IIA-P is Portland cement with 6–20% pozzolanas, and Type IIA-V is made up of 12% of pozzolanas and 8% fly ash. Both cement types contain air-entraining agent (AEA) [47]. It remains uncertain whether entrained air influences pore refinement and thus the aging factor.

2.2. Experimental Program

Figure 1 presents a flowchart outlining the experimental program of this study. It begins with the preparation of concrete mixes, incorporating varying levels of entrained air and Class C or Class F fly ash. Concrete samples cast into cylindrical forms undergo moisture curing for 28 days, followed by ponding tests in indoor and outdoor pools. Also, a statewide survey is conducted to identify commonly used chloride-based brines in the state of Georgia, which provides a rationale for CaCl₂ concentrations selected for the outdoor ponds. Using surface resistivity tests, the electrical resistivity of each concrete cylinder is measured to assess the combined impact of fly ash and entrained air on chloride ingress in concrete pavement. The test results lead to a new chloride ingress model framework and model factors that account for pore refinement, chloride binding, and environmental conditions. Finally, corrosion prediction in a concrete pavement is simulated, supporting long-term durability assessments of the proposed approach.

2.3. Mix Design and Concrete Samples

The Class 1 concrete mix design approved by the Georgia Department of Transportation (GDOT) was referenced to produce concrete mixes cast into 100 mm diameter, 200 mm tall cylinders. Table 2 presents the key design variables and standard values (ranges), as outlined in Section 430 of Portland Cement Concrete Pavement of GDOT’s Standard Specifications Construction of Transportations Systems [50]. This standard permits the inclusion of entrained air and fly ash in the mix, allowing their combined effects on chloride ingress to be evaluated in concrete pavements.

Table 3 presents the chemical compositions of the Portland cement and the fly ash. Type I/II cement contains high calcium oxide (CaO) content, contributing to hydration and strength development, while lower silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and ferric oxide (Fe₂O₃) compared to both fly ash classes.

Figure 2 shows the gradation charts of #810 and #57. The fineness modulus of #810 was 2.89, and the nominal maximum size of #57 was 37.5 mm.

The volume of entrained air, referred to as design air in the table, was supposed to prevent F-T damage in the concrete pavement, but the extent to which it would be effective in mitigating chloride-induced damage under non-freezing conditions has not been proven. Fly ash, the most common SCM in Georgia for concrete pavements, is required but not specified by class, despite each class having unique chemical compositions and hydration processes [51]. Both entrained air and fly ash influence the formation of pore structure and characteristics of the pore solution [25]. However, limited research has explored their combined effects on chloride ingress beyond the specified targets and ranges. Table 4 outlines concrete batch groups and design variables used to cover this uncharted territory.

Group 1 samples were cast with Class C fly ash, while those in Groups 2 and 3 were mixed with Class F fly ash. Both Class C and Class F fly ash were acquired from Georgia Power in Georgia. Batches within Groups 1 and 2 were prepared with varying amounts of air-entraining agent (AEA) to induce a wide range of entrained air contents during batching and mixing. A liquid form of AEA (EUCON AEA–92), manufactured by the Euclid Chemical Company in North Carolina, was used for the air entrainment. No AEA was added to the batch of Group 3, so that samples without entrained air could be produced as well. The volume of entrained air in each batch was measured using the pressure air meter method specified in ASTM, C231/C231M-14 [52].

2.4. Chloride Brine Solutions

Table 5 summarizes the brine blends used in all the outdoor ponding tests. Six brine blends were formulated based on the statewide survey and inputs from GDOT engineers, each comprising rock salt (NaCl) and CaCl₂ pellets obtained from a district maintenance office of GDOT in Lafayette.

Following GDOT’s proportioning and mixing practice, the solid particles were gradually dissolved in warm water using a mechanical stirrer until a pure liquid form was achieved. The concentration of CaCl₂ ranged from 0 to 25% by weight in each five gallons of water, at intervals of 5%, while maintaining NaCl at 23.3%. With the molar masses of NaCl (58.44 g/mol) and CaCl₂ (110.98 g/mol), the resulting chloride concentrations were equivalent to 3.99 mol/L in the pure NaCl solution and reached up to 8.50 mol/L in the blend containing 23.3% NaCl and 25% CaCl₂. For reference, the chloride ion concentration of sea water is about 0.54 mol/L, underscoring the aggressive exposure levels examined in this study. Table 2 covers a typical range of CaCl₂ concentrations (5 to 15%) preferred by district officers across the state. Similar blending ratios between NaCl and CaCl₂ can be observed in other states. For example, in Illinois, salt brines are mixed with 10–15% CaCl₂ for optimal melting performance [53], while in Wisconsin, liquid CaCl₂ at 29.8% is often used to enhance the effectiveness of NaCl, particularly at cold temperatures [54].

2.5. Ponding Tests

All concrete cylinders were moisture-cured at 25 ± 0.5 °C for the first 28 days and were subsequently divided into two ponding groups, with one group submerged in brine solution in outdoor buckets, and the other continuously cured in temperature-controlled water tanks (i.e., indoor pools), as depicted in Figure 3.

Over three years, the specimens fully submerged in brine solutions underwent an accelerated process of chloride ingress at ambient temperatures, simulating the conditions that concrete pavements experience during winter road maintenance. In contrast, the specimens placed in water baths went through a pore refinement process. The electrical resistivity of concrete samples was assessed using surface resistivity tests, and key parameters of the concrete resistivity model were developed to characterize chloride ingress and forecast corrosion in concrete pavements. To prevent carbonation, the ponding tests were conducted with minimal interruptions.

2.6. Surface Resistivity Tests

Figure 4 shows the Wenner probe used in the surface electrical resistivity technique to evaluate the durability of all concrete samples, in accordance with AASTHO TP 95-11 [55,56]. The Wenner probe (Resipod Concrete Resistivity Meter, Proceq, Screening Eagle Technologies, Schwerzenbach, Switzerland) features a probe spacing of 38.1 mm (1.5 inches) with a supplied current from 10 to 50 μA, as well as a wide range of resistance measurement (1 to 1000 kΩ-cm). Each contact point in the probe array carries a small water reservoir to ensure reliable electrical contact with the specimen surface [57]. In addition, at the time of testing, a saturated surface dry condition was maintained for all concrete samples [58,59].

Table 6 summarizes the experimentally determined risk levels of chloride permeability for the cylindrical samples, which are 100 mm in diameter and 200 mm in height. It indicates that the vulnerability of concrete to chloride permeability should increase when surface resistivity becomes lower than 12.0 kΩ-cm.

The reported concrete resistivity for each sample represents the average of eight measurements taken at four circumferential locations—0°, 90°, 180°, and 270°—along the longitudinal side of the sample. At each location, two resistivity readings were recorded and averaged. The Resipod meter displays the resistivity on its LCD screen for each measurement, automatically applying the appropriate geometrical factor based on the tested cylinder’s size and dimensions.

3. Results and Discussion

3.1. Concrete Resistivity of Water-Saturated Concrete Samples

The evolution of concrete resistivity is crucial for understanding pore refinement and its impact on chloride ingress. Previous studies have indicated that SCMs, particularly fly ash, can significantly alter the pore structure of concrete, influencing its electrical resistivity and durability [35,36]. Figure 5 shows the trends of conductivity (i.e., inverse of resistivity) in water-submerged concrete samples. Each data point represents the average value of six measures from six samples per batch. The standard deviations ranged from 0.1 kΩ-cm to 10 kΩ-cm. Concrete resistivity increased over time, reflecting continuous pore refinements in the concrete. Overall, the rate of resistivity increase depended on the volume of entrained air and the fly ash type. Class F fly ash samples exhibited a moderate resistivity increase, which is consistent with the delayed pozzolanic activity that gradually refines pore structures. The maximum percentage increase in this group was approximately 1064% (9.8 kΩ-cm to 114.8 kΩ-cm) at 0% entrained air (sample ID: 0.0%-F). In the samples with Class C fly ash, lower entrained air content led to faster resistivity gain, likely due to denser hydration products and reduced pore connectivity. The maximum percentage increase in resistivity was about 1608% (11.9 kΩ-cm to 203 kΩ-cm) at 2.7% entrained air (sample ID: 2.7%-C), indicating a resistivity growth rate about 1.5 times that of the Class F fly ash samples.

A previous study by Andrade and D’Andrea proposed an aging factor (q) in the concrete resistivity model to characterize the complexity of pore refinement [46]. With a q value of 0.57 suggested for Type II/A-V cement (12% Ordinary Portland Cement + 8% fly ash), the time-corrected resistivity (ρ_t) computed at t and t₀ for each sample was compared to the measured effective resistivity in the figure. This comparison revealed that this empirical q value does not fully account for the observed trends and variability of effective conductivity (ρ_ef).

This study developed a simple procedure for defining the relationship between the aging factor and the volume of entrained air for each class of fly ash. First, individual aging factors were derived from the power terms of regression lines fitted to the data. These individual regression lines were not included in the figure for simplicity. Second, the obtained aging factors were correlated with the corresponding entrained air contents using a linear function for each class of fly ash. The resulting coefficients of determination (r-squared values) for two linear functions were 0.833 for Class C fly ash and 0.880 for Class F fly ash. With the newly proposed aging factors, the resistivity ρ_t in Equation (6) was calculated (i.e., calculated resistivity) and then compared again to the measured effective resistivity (ρ_ef). Figure 6 shows the measured and calculated conductivity (inverse of resistivity). Notably, a robust strong correlation was observed between the calculated and measured resistivity values for the samples containing Class C fly ash (r² of 0.9623) and those containing Class F fly ash (r² of 0.9832) across the entrained air contents. These findings underscore the necessity of incorporating entrained air volume into resistivity-based chloride ingress models, extending Andrade’s approach to accommodate pore refinement unique to air-entrained concrete [32,33].

3.2. Chloride Ingress in Concrete—Phenomenological Observation

The outdoor ponding test provided insights into the progression of chloride ingress in concrete. Apparent resistivity, representing the electrical resistivity of the concrete samples soaked in brine, emerged as a key indicator of chloride ingress. Figure 7 shows the average apparent resistivity that varies with the chloride concentration of the brine solution the concrete samples were submerged in. The number of sample replicates was six for each of the entrained air–fly ash groups. To account for the temperature dependency of resistivity, the resistivity readings were adjusted to a reference temperature of 25 °C [60].

During the initial exposure period (approximately 45 days), the concrete resistivity increased as the chloride concentration rose from the pure salt solution (3.99 mol/L) to a blend of CaCl₂ and NaCl. This trend suggests that chloride transport dominates chloride ingress in the early stages.

A shift in trend was observed at around 200 days, as resistivity began to decrease with the increasing chloride concentration. This phenomenon likely resulted from active chloride binding. The reduced resistivity coincided with the presence of highly conductive chloride ions in the concrete pores, potentially due to the accumulation of chlorides within the cement hydrates. Chloride binding appeared more pronounced in the NaCl-CaCl₂ blends than in the pure salt solutions, confirming higher chloride binding propensity for bivalent cation (Ca++), as reported in a previous study [25]. As chloride binding progressed, fewer free chloride ions were available for ingress, slowing the overall rate of chloride ingress.

By the end of the ponding tests (around 890 days), the apparent resistivity stabilized at levels below the high-risk chloride ingress threshold of 12 kΩ-cm (see Table 5) in the CaCl₂ and NaCl blends. This illustrates that the chloride binding had reached an equilibrium. At this stage, the risk of reinforcement corrosion and concrete erosion was at its peak due to the saturation of binding sites.

These observations highlight the utility of apparent resistivity in assessing the degree and rate of chloride ingress at different chloride concentrations. However, apparent resistivity alone may not fully capture the impact of entrained air and fly ash class on chloride ingress, nor does it account for pore refinement in concrete. Furthermore, the degree and rate of chloride ingress are affected by chloride binding with the cement matrix and the transport of unbound chloride ions through concrete pores. While chloride binding is crucial in characterizing chloride ingress, direct atomic level measurements lie beyond the scope of this study. Instead, a novel approach was proposed to evaluate its contribution to chloride ingress using a chloride reaction factor, which connects apparent and effective resistivity.

3.3. Impact of Entrained Air and Fly Ash on Chloride Ingress—Chloride Reaction Factor Approach

The reaction factors (r_Cl) were calculated by the ratio of ρ_ap to ρ_ef (r_Cl = ρ_ap/ρ_ef), where ρ_ap was measured at six to seven designated times for up to 1200 days, while ρ_ef represents the porosity of each sample at 28 days. As long as concrete is fully saturated in either brine or water, this method of obtaining r_Cl is theoretically valid and offers practical benefits, as it does not require the performance of multi-regime tests for diffusion coefficients (D_ap and D_ef), which often yield a wide data scatter [61].

Figure 8 shows the variation in the reaction factors that were temperature corrected and then averaged over the duration of the ponding tests. Like the snapshot of apparent resistivity ρ_ap observed in Section 3.2, reaction factors exhibit an exponential decay across the range of chloride concentrations. At low chloride concentrations, the reaction factors are relatively high, as chloride ions are effectively bound or immobilized within the concrete matrix through mechanisms such as physical adsorption on the C-S-H gel, chemical reactions with the cementitious phases, or incorporation into hydration products [25]. However, as the chloride concentration increases, the availability of binding sites within the concrete matrix becomes scarcer, leading to a reduction in chloride binding efficiency. Once all available binding sites are fully occupied, further increases in chloride concentration rarely impact chloride binding.

Both entrained air and fly ash play a crucial role in this process. The reaction factors decrease as entrained air increases. Specifically, at each chloride concentration, the reaction factors are up to 67% higher in the concrete samples with 3.5% entrained air compared to those with 10.0% entrained air. This suggests that lower entrained air contents enhance chloride binding efficiency in concrete. Across all chloride concentrations, the reaction factors decreased by 57% in the Class F fly ash samples and 41% in the Class C fly ash samples across all chloride concentrations. However, in the pure NaCl solutions, Class F fly ash exhibited a stronger impact on reaction factors compared to Class C fly ash. This is attributed to its higher silica content in Class F fly ash, which enhances the formation of calcium–silicate–hydrate (C-S-H) gel through extended pozzolanic reactions with calcium hydroxide (Ca(OH)₂) in the concrete matrix [62]. The C-S-H gel improves pore refinement and increases chloride binding capacity by trapping chloride ions, thereby reducing their mobility and ingress. In addition, the finer particle size and the higher pozzolanic reactivity of Class F fly ash contribute to a denser microstructure, limiting chloride permeability by reducing capillary pore connectivity, particularly in environments dominated by NaCl solutions. In contrast, Class C fly ash, with a higher calcium oxide content, promotes rapid pozzolanic reactions that lead to the early densification of the concrete matrix. However, this may result in a coarser pore structure over time, which can allow for greater chloride ingress compared to Class F fly ash.

It is worth noting that the mitigating effect of Class F fly ash on chloride ingress diminishes when exposed to NaCl-CaCl₂ blends. This reduction is likely due to increased competition for chloride binding sites within the C-S-H gel, as calcium ions (Ca²⁺) have a stronger binding affinity than sodium ions (Na⁺). The presence of Ca²⁺ can alter the chloride binding capacity of the C-S-H gel, favoring the formation of calcium-rich phases, which may reduce the gel’s ability to sequester chloride ions effectively. These findings indicate that the role of fly ash in reducing chloride ingress is influenced not only by its pozzolanic properties but also by the ionic composition of the deicing environment. Further investigation at the atomic level, using techniques such as X-ray diffraction, spectroscopy, and nuclear magnetic resonance, could provide deeper insights into the microstructural changes and chloride binding mechanisms [25].

The pronounced effects of entrained air on the reaction factors are illustrated in Figure 9. At each chloride concentration, a good correlation was achieved between the reaction factor and the inverse of entrained air, with r-squared values ranging from 0.67 to 0.83. As already noted in Figure 8, samples with Class F fly ash followed the overall trend quite well across the chloride concentrations, except for the pure salt solution.

These findings confirm that the reaction factor of concrete was affected by more than the cement type; variables like entrained air and chloride concentration also play a role. By performing a multiple regression analysis on the data presented in Figure 9, an alternative model for the reaction factor can be derived. For the tested ranges of entrained air and chloride concentration, the proposed model is as follows:

r_CI = 1.286 − 0.045012 · V_e − 0.07613 · C_c (7)

where V_e = entrained air in percent and C_c = chloride concentration in M.

In comparison, the independent data published by Andrade et al. were predicted using the model reflected in Equation (7). These reaction factors were determined based on the D_ef and D_ap (Equation (2)) obtained from a multi-regime test involving 150 mm diameter, 20 mm thick concrete samples conditioned in 1 M of NaCl, and distilled water as the anolyte [33]. Figure 10 presents the prediction results for the reaction factors. Overall, the model demonstrates reasonably good predictive performance, with the exception of a sample mixed with ASTM Type II cement and 50% slag (w/c = 0.34), which exhibited some deviations. These discrepancies may stem from differences in cement composition, pore structure, chloride concentration, temperature, aging, and other influencing factors.

The model coefficients were estimated based on experimental data under controlled conditions. Therefore, uncertainties may arise when applying the model to broader datasets due to variations in concrete mix proportions, field exposure conditions, and long-term aging effects. The model’s predictive accuracy can be improved by incorporating an extensive dataset that captures diverse environmental conditions, as well as by performing sensitivity analyses to assess the impact of individual parameters on chloride ingress.

3.4. Prediction of Corrosion Initiation in Concrete Pavement

Combining Equations (5) and (6) leads to an expression for the initiation period of reinforcement corrosion, t_i, in years. Theoretically, given the three key factors of the electrical resistivity model, t_i can be calculated, but environmental factor, k_CI, is not easily determined, as it is affected by so many experimental conditions such as humidity, temperature, species of chloride ions, and so on. Thus far, very few values and associated empirical models have been proposed for a handful of illustrative examples [32,46].

In this section, a novel approach is proposed to back-calculate k_CI that is expected to induce corrosion in concrete during the target design period. Here, k_CI, is designated as a maximum allowable environmental factor that is known to be associated with the design parameters, including design period, cover thickness, chloride concentration, and entrained air. Incorporated into the expression for k_CI are the models developed for determining the aging and reaction factors, as presented in the previous sections. Models for class F fly ash were not considered for predictions due to the lack of data.

Table 7 presents the maximum allowable environmental factors forecast for a 10-year corrosion initiation period. As noted, the effect of entrained air was conspicuous at the selected chloride concentrations. As the volume of entrained decreased, concrete was better protected from corrosion, suggesting a prolonged protection period. A similar observation was made for a range of cover thicknesses (3 cm, 4 cm, 5 cm, and 6 cm).

Figure 11 shows the prediction of k_CI levels matched with the corresponding chloride concentrations (C_c) for four corrosion initiation periods—10, 15, 20, and 50 years. The cover thickness was set at 3 cm for all the k_CI predictions that were compared with the exposure designations shown in Table 1. It demonstrates that, even at the same level of chloride concentration, k_CI could differ greatly depending on the volume of entrained air, along with other design parameters.

The model’s back-calculation approach allows for the estimation of critical environmental factors to predict corrosion initiation for long-term exposure periods. With proper calibrations, this approach can offer an alternative solution to traditional tests on diffusion coefficients for the determination of k_CI, which is notorious for being quite time-consuming and destructive, with poorly replicable outputs. Moreover, an advanced design process can be achieved for a more durable and corrosion-resistant concrete pavement, where deicing with salt brine is a common practice. However, these predictions are still based on controlled laboratory conditions and may not fully capture the complexities of long-term field exposure. Therefore, future research should focus on extending the experimental duration and incorporating variable environmental conditions to enhance the model’s reliability for long-term applications.

4. Conclusions and Recommendation

This study advances chloride ingress modeling by integrating the combined effects of entrained air and fly ash into the aging and reaction factors of the concrete resistivity framework. The proposed approach improves chloride ingress predictions by accounting for pore refinement, chloride binding, and environmental conditions. The following are the key findings drawn from this three-year long experimental work:

Entrained air emerged as pivotal in inhibiting chloride ingress. Concrete samples with lower entrained air correlated with heightened chloride binding potential and reduced likelihood of reinforcement corrosion initiation by free chloride ions.

Class F fly ash was found to be effective in reducing chloride ingress in pure salt brine, but this effect was not pronounced when exposed to CaCl₂-NaCl blends.

New aging and reaction factors of the electrical resistivity model were proposed to incorporate the combined impact of entrained air and fly ash on chloride ingress. Both factors improved the model’s performance in characterizing cross-scaling relations between chloride ingress and chloride binding.

By back-calculating the maximum allowable environmental factors, predictive models were proposed for reinforcement corrosion initiation. Simulations verified the dependency of environmental factors on entrained air even at the same chloride concentration. This approach offered an alternative solution to the traditional tests on diffusion coefficients for determining environmental factors.

The economic benefits of optimized concrete mix designs can be substantial, as increased durability reduces the need for costly repairs, resurfacing, and full-depth replacements. Industry estimates suggest that extending pavement lifespan by 10–15 years through these optimizations and preventive measures can result in significant maintenance and rehabilitation cost savings by 20 to 30%. Moreover, incorporating these improvements during the initial construction is more cost-effective than reactive repairs, minimizing traffic disruptions and lowering overall life-cycle costs.

Future research should extend the duration of chloride ingress beyond the tested timeframe, while covering a wide range of cement compositions and SCMs. Incorporate predictive modeling with long-term field data can aid in developing resistivity-based design criteria for concrete pavements. The practical implementation of these models will help state DOTs optimize concrete mix designs and winter maintenance practices for greater durability and a more resilient infrastructure.

Author Contributions

Conceptualization, Y.S.; methodology, Y.S.; validation, Y.S. and J.H.K.; formal analysis, Y.S.; investigation, Y.S. and J.H.K.; resources, Y.S. and J.H.K.; data curation, Y.S.; writing—original draft preparation, Y.S.; visualization, Y.S.; supervision, Y.S. and J.H.K.; project administration, Y.S.; funding acquisition, Y.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data will be made available on request.

Acknowledgments

Special thanks to Ryujin Pyo, who collected the data and fabricated the concrete samples, and Tien Yee for their valuable insights into the experimental efforts. Their dedication and hard work were essential for the success of this study.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1 Experimental program flowchart.

Figure 2 Gradation charts of #810 (specific gravity: 2.690; absorption: 3.1%; unit weight: 1943.1 kg/m³) and #57 (specific gravity: 2.970; absorption: 3.0%; unit weight: 1608.3 kg/m³) aggregates.

Figure 3 Ponding test setups: (a) outdoor brine buckets; (b) indoor water bath.

Figure 4 Resistivity test setup with the Wenner probe.

Figure 5 Conductivity (inverse of resistivity) in water-saturated samples mixed with (a) Class C fly ash and (b) Class F fly ash at different entrained air contents. Regression lines (power functions) were plotted with Equation (6) and an aging factor (q) of 0.57.

Figure 6 Comparison of the measured and calculated conductivity (inverse of resistivity) with new aging factors for (a) Class C fly ash and (b) Class F fly ash at different entrained air contents. The regression line (a power function) is used to illustrate a high level of correlation.

Figure 7 Temporal changes in apparent resistivity in concrete samples submerged in brine solutions. Standard deviations vary from 0.1 kΩ-cm to 10 kΩ-cm. A power function regression line is fitted to average apparent resistivity to highlight the trend.

Figure 8 Average reaction factors (r_CI) determined over the entire outdoor ponding period.

Figure 9 Effects of entrained air and fly ash on reaction factors of concrete samples saturated in the blends of CaCl₂ and NaCl: (a) 0%, (b) 5%, (c) 10%, (d) 15%, (e) 20%, and (f) 25% of CaCl₂, all with 23.3% of NaCl, equivalent to chloride concentrations of 3.99 M, 4.89 M, 5.79 M, 6.59 M, 7.59 M, and 8.50 M, respectively.

Figure 10 Prediction of reaction factor (r_CI) for multi-regime test samples mixed with various cement types and w/c ratios. Sample porosity ranged from 7.28% to 12.19%.

Figure 11 Maximum allowable environmental factors and corresponding chloride concentrations at different volumes of entrained air for four corrosion initiation periods: (a) 10 years (b) 15 years, (c) 20 years, and (d) 50 years with exposure categories in EN-206 [48].

Table 1

Environmental factors for the exposure classes of EN 206.

Exposure Class	Description	kCI, cm3 Ω/Year
X0	No risk	200
XS1a	Parts of structures in contact with marine aerosol, but without direct contact with seawater (distance > 500 m)	5000
XS1b	Parts of structures in contact with marine aerosol, but without direct contact with seawater (distance < 500 m)	10,000
XS2	Parts of marine structures subjected to permanent seawater immersion	17,000
XS3	Parts of marine structures located in tidal or splash zones	25,000

Table 2

Class 1 GDOT concrete mixture design.

Ingredient	Standard Value (Range)	Type
Cement	288 kg/m3	I/II
Fly ash	70 kg/m3	Class C or F
Sand	654 kg/m3	#810
Stone	1168 kg/m3	#57
Water	114 kg/m3	-
AEA 1	-	Liquid
Design air	4.7% (3.0–6.5%)	-
Slump	38.1 mm (0.0–63.5 mm)	-
Strength	21 MPa 2	-

¹ Air Entraining Agent. ² 28-day compressive strength.

Table 3

Chemical composition of Portland cement and fly ash.

Oxide	Portland Type I/II, %	Class C Fly Ash, %	Class F Fly Ash, %
Silicon Dioxide	18–24	35–50	40–60
Aluminum Oxide	4–8	15–30	20–30
Ferric Oxide	2–6	4–10	5–15
Calcium Oxide	60–67	20–40	1–10
Magnesium Oxide	<6	1–5	1–5
Sulfur Trioxide	<3	<5	<5
Sodium Oxide	<1	1–4	0.5–4
Potassium Oxide	<1	1–4	0.5–4
Loss on Ignition	<3	<6	<6

Table 4

Concrete batch groups and design variables.

Group	Batch	Cement, kg	Sand, kg	Stone, kg	Water, kg	FA 1, kg	Slump, mm	Air 2, %	w/c	NOS 3
1	1-1	5.0	11.3	20.2	3.5	1.2 (C)	33.0	3.5	0.69	9
	1-2	7.5	17.0	30.3	5.2	1.8 (C)	83.8	6.7	0.69	15
	1-3	7.5	17.0	30.3	4.8	1.8 (C)	35.6	7.0	0.64	15
	1-4	7.5	17.0	30.3	4.8	1.8 (C)	38.1	4.1	0.64	15
	1-5	7.5	17.0	30.3	4.8	1.8 (C)	57.2	10.0	0.64	15
	1-6	7.5	17.0	30.3	4.6	1.8 (C)	31.8	5.5	0.61	15
	1-7	6.9	15.6	27.9	4.2	1.8 (C)	15.2	5.8	0.61	12
	1-8	7.5	17.0	30.3	4.6	1.8 (C)	31.8	2.7	0.61	15
2	2-1	7.5	17.0	30.3	4.6	1.8 (F)	44.5	6.0	0.61	15
	2-2	9.5	21.6	38.5	6.0	2.3 (F)	27.9	2.3	0.63	18
	2-3	4.7	10.6	19.0	3.2	1.1 (F)	31.8	2.0	0.68	9
3	3-1	7.5	17.0	30.3	4.5	1.8 (F)	19.1	0.0	0.61	14

¹ FA: fly ash and its class in parenthesis. ² Entrained air volume, %. ³ NOS: number of cylindrical samples produced.

Table 5

Six brine blends of NaCl and CaCl₂.

Blend	Concentration, % Weight
	NaCl	CaCl2	M (mol/L)
1	23.3	0	3.99
2		5	4.89
3		10	5.79
4		15	6.69
5		20	7.59
6		25	8.50

Table 6

Chloride permeability risks associated with surface resistivity [55].

Chloride Ion Penetrability	Surface Resistivity (kΩ-cm)
	100 mm × 200 mm Cylinder, r = 38.1 1
High	<12.0
Moderate	12.0–21.0
Low	21.0–37.0
Very Low	37.0–254.0
Negligible	>254.0

¹ Spacing of the Wenner probes in mm.

Table 7

Maximum allowable environmental factor (k_CI) for reinforcement corrosion initiation after 10 years of exposure for four different cover thicknesses: 3, 4, 5, and 6 cm.

Ve, %	Cc, M	Maximum Allowable kCI, cm3 Ω/Year
		3 cm	4 cm	5 cm	6 cm
3	3.99	14,246	25,326	39,572	56,984
	4.89	13,094	23,278	36,372	52,375
	5.79	11,942	21,230	33,171	47,767
	6.69	10,790	19,181	29,971	43,158
	7.59	9637	17,133	26,771	38,550
	8.50	8472	15,062	23,535	33,890
5	3.99	9985	17,751	27,736	39,940
	4.89	9081	16,145	25,226	36,326
	5.79	8178	14,539	22,717	32,712
	6.69	7274	12,932	20,207	29,098
	7.59	6371	11,326	17,697	25,483
	8.50	5457	9702	15,159	21,829
7	3.99	6900	12,266	19,166	27,598
	4.89	6191	11,006	17,197	24,764
	5.79	5482	9747	15,229	21,930
	6.69	4774	8487	13,261	19,095
	7.59	4065	7227	11,292	16,261
	8.50	3349	5953	9302	13,395
9	3.99	4681	8321	13,002	18,723
	4.89	4125	7334	11,459	16,500
	5.79	3569	6346	9915	14,278
	6.69	3014	5358	8371	12,055
	7.59	2458	4370	6828	9832
	8.50	1896	3371	5267	7584

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