Testing and Specifying Electrochemical Properties of Expanded Shale, Clay and Slate Lightweight Aggregates

J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 47 Journal of Civil Engineering and Materials Application http://jcema.comJournal home page: Received: 6 January 2025 • Revised: 02 March 2025 • Accepted: 25 March 2025 doi: 10.22034/jcema.2025.539472.1162 Testing and Specifying Electrochemical Properties of Expanded Shale, Clay and Slate Lightweight Aggregates Kenneth L. Fishman 1, Fariborz M. Tehrani 2* 1 McMahon and Mann Consulting Engineering and Geology, P.C., E-mail: [email protected] 2 California State University, Fresno; E-mail: [email protected] *Correspondence should be addressed to Fariborz M. Tehrani, Department of Civil and Geomatics Engineering, California State University, Fresno, Mail Stop EE 94, 2320 E. San Ramon Ave., CA 93740-8030; E-mail: [email protected] Copyright © 2025, Fariborz M. Tehrani. This is an open access paper distributed under the Creative Commons Attribution License. Journal of Civil Engineering and Materials Applications published by (ISNet); Journal p-ISSN 2676-332X; Journal e-ISSN 2588-2880. 1. INTRODUCTION 1.1. Corrosion Potential of Geomaterials everal electrochemical parameters, such as electrical resistivity, degree of saturation, pH, and dissolved salts influence the corrosivity of earthen materials [1]. Most salts promote corrosion, except for carbonate, which forms a protective scale on metals. Chloride, sulfate, and sulfide are major promoters of corrosion in steel reinforcements. Resistivity is a significant indicator of corrosivity. Key factors affecting corrosion include temperature, oxygen concentration, resistivity, pH, carbonate scaling tendency, acids, alkalis, salts, soil particle size distribution, porosity, water content, and microbial activity [2],[3]. 1.2. Electrical Conductivity/Resistivity This is a primary indicator of corrosion potential. High electrical conductivity (low resistivity) facilitates electron transport, accelerating corrosion. S ABSTRACT Electrochemical properties of earthen materials, including lightweight aggregates can be useful to assess the corrosivity and performance of metallic materials embedded within backfills. Electrical conductivity, the reciprocal of resistivity, relates to the transport of electrons during the corrosion process. Other chemical properties such as pH and ion contents interact with physical conditions like moisture content and gradation to influence the electrical conductivity. Sulfate and chloride contents are major performance measures in laboratory testing methods. These predictors are perceived as more accessible performance measures, but not necessarily more accurate, than direct assessment of corrosion like mass loss or instantaneous measures of corrosion potentials and corrosion using electrochemical measurement techniques. However, these measures do not address the full spectrum of material characteristics influencing corrosivity. This shortcoming is apparent for lightweight aggregates that exhibit different trends with respect to electrochemical properties compared to normal weight aggregates due to their characteristic porosity and moisture dynamics. Hence, application of ESCS lightweight aggregates requires modification of testing methods and specification of alternative thresholds for electrochemical properties, including electrical resistivity, pH, sulfate and chloride ion contents. This paper addresses such modifications using the outcomes of experimental investigations. Keywords: lightweight aggregates; corrosion rate; mechanically stabilized earth; electrochemical characterization; backfills J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 48 Conversely, low conductivity (high resistivity) is correlated with low corrosion rates [4]. 1.3. pH Levels The acidity or alkalinity of the soil affects corrosion rates. Highly acidic (low pH) or highly alkaline (high pH) environments can increase corrosion rates. Neutral pH levels are generally less corrosive. The pH of ESCS ranges between approximately 8 and 9.5 [2]. 1.4. Corrosive Ions The presence of ions like chlorides and sulfates significantly impacts corrosion. Chlorides, for example, can break down protective oxide layers on metals, leading to accelerated corrosion. Sulfates can also contribute to corrosion, especially in the presence of moisture. 1.5. Moisture Content Water acts as an electrolyte, facilitating the movement of ions and electrons, which is essential for the corrosion process. The highest corrosion rates are generally associated with the degree of saturation between 65% and 90%. If the soil is saturated, the resistivity may be the lowest, but the lack of oxygen leads to lower corrosion rates. 1.6. Soil Composition and Gradation The physical characteristics of the soil, including particle size distribution and porosity, affect its moisture retention and drainage properties, which in turn influence corrosivity. 1.7. Temperature Higher temperatures can increase the rate of chemical reactions, including those involved in corrosion. Thus, warmer environments can lead to higher corrosion rates. 1.8. Organic Content Organic materials in the soil can produce acids as they decompose, which can increase the soil’s corrosiveness. The presence of organics also contributes to the respiratory processes of microorganisms that can influence corrosion. 1.9. Presence of Microorganisms Certain bacteria can influence corrosion processes, either by producing corrosive substances or by directly interacting with the metal surfaces. 1.10. Type of Aggregate Lightweight aggregates, such as ESCS (Expanded Shale, Clay, and Slate), have different electrochemical properties compared to normal weight aggregates due to their higher porosity and unique moisture dynamics. This necessitates modified testing methods and alternative thresholds for assessing corrosion potential [2]. Understanding these factors helps in selecting appropriate materials and designing effective corrosion protection strategies for infrastructure projects. The major shortcoming in current practice has roots in the standard approaches to the measurement of these factors, which is the primary contribution of this paper. 2. Description and Characteristics of ESCS LWA This report emphasizes the use of rotary-kilnmanufactured expanded shale, clay and slate (ESCS) lightweight aggregates. Figure 1 exhibits the simplified and schematic production process that is largely uniform worldwide, with differences in aggregate properties stemming from the raw materials used—shale, clay, or slate (Figure 2). Consequently, the physical and mechanical properties of these manufactured aggregates show less variation compared to those from mines, recycled sources, or industrial byproducts, which lack the quality control processes of a manufacturing plant [5]. The visual attributes of rotary-kiln-manufactured lightweight aggregates, including color, shape, and size, can vary depending on the source, as illustrated in Figure 2. Post-production screening processes allow for customization of these aggregates to meet specific application needs. Despite these visual differences, the physical and mechanical properties—such as porosity, absorption, density, and strength—remain consistent. This uniformity is due to the identical chemistry of vitrified aggregates and the porous microstructure created in the rotary kiln, ensuring similar properties across products from different manufacturers located within the North America (Figure 3) or worldwide [6]. The use of alternative materials like lightweight ESCSs in North America, Europe, and Asia has become increasingly popular due to their light weight, ease of compaction, sustainability, and durability ([7]-[9]). ESCSs improve drainage and reduce corrosion compared to on-site materials. Experimental shake table studies on lightweight J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 49 backfills have shown that low density, a high internal friction angle, and high damping are crucial for the high performance of MSE systems ([10]-[14]). Figure 1. Sample aggregates stored in sealed containers to preserve the as-received condition before testing. Figure 2. Sample Expanded Shale with Coarse, Medium, and Fine Gradations (from left to right) Figure 3. ESCSI Production Plants (Courtesy of ESCSI) 3. Sulfate and Chloride Measurements 3.1. Range of measurements for ESCS LWA Sulfate and chloride measurements for Expanded Shale, Clay, and Slate (ESCS) lightweight aggregates are typically conducted to assess the durability and stability of these materials in various applications. The testing methodology influences the results through sample preparations. ESCS aggregates typically have low chloride contents, 10 to 70 ppm, and magnesium sulfate soundness loss, less than 6%, compared to 100 ppm and 30% commonly specified limits, respectively [15]. Regardless, it is not uncommon for ESCS aggregates to exhibit high sulfate contents compared to most normal weight aggregates. However, these high values may not necessarily lead to high corrosion rates. 3.2. Specified limits for MSE wall fill Most transportation agencies assess the electrochemical properties of earthen materials using AASHTO laboratory test standards established in the early 1990s. The specified methods include AASHTO T 288 for resistivity, AASHTO T 289 for pH, AASHTO T 290 for soluble sulfate content, and AASHTO T 291 for soluble chloride content. AASHTO outlines the electrochemical criteria for fill materials suitable for MSE wall construction, as detailed in Table 1 [3]. Raw Materials Rotary-Kiln Expanding PostProcessing J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 50 Table 1. AASHTO Requirements for MSE Wall Backfills [3] Parameter Acceptable Range AASHTO Standard Minimum Resistivity (Ω.cm) 3000 T 288 pH range 5-10 T 289 Maximum Sulfate Content (ppm) 200 T 290 Maximum Chloride Content (ppm) 100 T 291 Notes: Ω.cm: ohm-centimeter; ppm: parts per million 4. Objectives of the Paper This paper evaluates test methods and specification of electrochemical properties of expanded shale, clay and slate (ESCS) lightweight aggregates. The primary objective of the paper is to analyze and compare data from laboratory and field studies to determine characteristics of ESCS related to corrosivity. 4.1. Demonstrate sulfate and chloride measurements are not properly measured for ESCS LWA AASHTO T 288, T 289, T 290, and T 291 are conducted on specimens separated using a No. 10 sieve. For coarse fills with minimal or no material passing through the No. 10 sieve, obtaining enough fines for testing may require sieving a large quantity of material. This approach is impractical and can lead to inaccurate results, especially for gravel fills with very little fine material. Crushing larger aggregates to obtain finer fractions is neither appropriate nor permitted by AASHTO standards, unless breakage is expected during placement and compaction. This is because most soluble ions are concentrated on particle surfaces, with negligible diffusion through the particles. Testing the finer portion (passing the No. 10 sieve, finer than 2.00 mm) assumes these fractions are significant sources of soluble salts, which may not be true for coarse fills with little or no fine material. In such cases, an alternative testing method should be considered [3]. 4.2. Not consistent with measurements of other electrochemical properties Sulfate and chloride ion contents facilitate electron movements during the corrosion process and hence, influence electrical conductivity. These indicators often synergize with other indicators such as pH to affect the corrosion rate of embedded metals in backfills [2]. 4.3. Not useful for characterizing corrosivity in coarse backfills Existing standards such as AASHTO T 290 and T 291 address measurements of water-soluble sulfate and chloride ion contents for fine soil particles, that is passing 2-mm (#10) sieve [16]. Thus, determining these ions in coarse backfills require modification of testing methods. In response to this need, local agencies like Texas DOT have considered revised standards to measure sulfate and chloride ion contents in coarse backfills ([17]-[21]). 4.4. Effect of Moisture Content Darbin, Jailloux, and Montuelle (1988) observed that the highest corrosion rates occur at moisture saturation levels between 60% and 85% [22]. Elias et al. (2009) studied fourteen active sites in California and found that most backfills had saturation levels above 65% [23]. Therefore, it is essential to assess the impact of moisture levels in MSE wall backfill on the corrosion of embedded steel reinforcement. Soil moisture influences the properties of the soil-metal interface, electrical conductivity, and ionic migration in soils, all of which directly affect corrosion processes [24]. Consequently, corrosion of embedded metallic reinforcement is negligible under dry conditions. Research has suggested that there may be an optimal moisture level that maximizes the corrosion rate, depending on the interaction between electrical conductivity and oxygen diffusion [25]. Thus, the funicular state of moisture in soil causes more corrosion damage to ferrous materials than the pendular state. This is significant because fine soil particles, such as clay, tend to have higher saturation levels than coarser particles like silt or sand [26]. 4.5. End point Comparison of various measurement techniques reveal that sample preparation techniques influence the results. These techniques may include sample origin, sieving, water equilibration time and dilution rate, drying time and temperature, extraction time J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 51 and method, and clarifying method [27]. The applicability of results to material selection specifications depends on how well these techniques represent the actual conditions for a given application. 5. Data from Sulfate and Resistivity Measurements 5.1. MTI Data (ESCS LWA) – Lab Tests for resistivity and sulfate content include measurements on coarse and fine samples from different sources identified as LW1, LW2, LW3, LW4, LW5, LW6, LW7, LW8 and NWCx. The gradation of the coarse samples complies with common blends for geotechnical applications produced by each manufacturer that contributed samples for the MTI study [2]. Samples LW1, LW3, LW4, LW6, LW7, LW8 and NWCx include less than 10% of the particle sizes passing the No. 10 sieve. Samples LW2 and LW5 were blended aggregates with 50% to 60% particle sizes passing the No. 10 sieve and could be separated on the #10 sieve in preparation for testing in accordance with existing AASHTO standards. Samples LW1, LW3, LW4, LW6, LW7, LW8 and NWCx were tested according to Tex-620-M using the as-is gradations. For comparison purposes , a fine gradation was obtained by crushing these coarse graded materials to facilitate testing using AASHTO standards as a basis for comparison. 5.2. Sulfate - AASHTO T 290, Tex-620-M, Leachate from resistivity test box Figure 4 presents results from sulfate content measurements on 8 sources of ESCS (LW1 through LW8) and 1 normal weight sample (NWCx). Details of the test procedures are described in MTI (2023). As-is gradations were tested in accordance with Tex620-M, and the crushed samples and blended aggregates (LW2 and LW5) were separated on the #10 sieve and tested in accordance with AASHTO T 290. Additionally, tests were performed on leachate drained from the test box used for resistivity testing after soaking the aggregates for 24 hours (24-hr saturated). Figure 4. Comparison of sulfate contents of selected samples using different standards [2]. AASHTO T 290 describes two methods/techniques that may be used for measurements of sulfate content including gravimetric and turbidimetric methods. The different measurement techniques involve different sources of interference/error. For instance, employing gravimetric measurements, as opposed to the turbidimetric measurements applied by MTI (2023), makes a difference. These differences could be due to sources of interference such as the presence of sulfite or sulfide which have an effect on gravimetric measurements. All of the sulfate contents measured from samples of ESCS were higher than those observed for the normal weight soil. In general, sulfate contents measured with the as-is gradations (Tex-620-M), are lower than those measured on the finer fraction (i.e., passing the #10 sieve) via AASHTO T 290, indicating that finer gradations have higher sulfate contents compared to coarser ESCS samples. Sulfate contents measured on the fine gradations of ESCS (AASHTO T 290) range between 200 ppm and 1750 ppm with 7 out of 8 samples higher than 200 ppm, and those measured from the as-is coarse gradations (Tex-620-M) are between 200 ppm and 1450 ppm with 5 out of the 8 samples higher than 200 ppm. Tests on samples of leachate from the resistivity test box after soaking for 24 hours are approximately equal to or lower than those measured with Tex-620M, and these sulfate measurements range between 0 500 1000 1500 2000 LW1 LW2 LW3 LW4 LW5 LW6 LW7 LW8 NWCx Sulfate, mg/kg T290 620-M J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 52 100 ppm and 600 ppm with half of the measurements ( 4 out of 8) higher than 200 ppm. The comparison between results obtained with coarse or fine gradations is relatively close from sources LW1, LW4 and LW5. LW2 and LW5 are blended aggregates with nearly 55% passing the #10 sieve, such that measurements with the as-is gradation are controlled by the presence of the finer portion, but the same cannot be said for sources LW1 and LW4. However, the gradations for LWC1 and LWC4 are close to one another, and these samples have a higher sand content compared to the other coarse samples, which are mostly gravel size (> 10% sand, except for LWC8, with a gradation close to LWC4). The sulfate contents measured with LW1 were the lowest ranging between 200 and 250 ppm, and the highest sulfate contents (approximately 1750 ppm) were observed from LW2 and LW5, both of which were bended aggregates with approximately 60% passing the No. 10 sieve. 5.3. Resistivity – AASHTO T 288, Tex-129-M Table 2 presents resistivity measurements performed on samples of coarse and fine ESCS samples. Samples with fine gradations were tested in accordance with AASHTO T 288 and coarse samples tested via Tex-129-M. None of the coarse samples have more than 5% of the particle sizes passing the No. 10 sieve such that the AASHTO T 288 test standard is not applicable (see Note 1 from AASHTO T 288). As described in NCHRP Report 958 [3], the modified Texas test standard Tex-129-M can be applied to measure the resistivity of coarse samples and is useful for characterization of corrosivity. The following observations are made from the data presented in Table 2 and resistivity measurements from samples of fine and coarse ESCS. • Fine samples have low resistivity (between 1852 Ω-cm and 3848 Ω-cm) compared to the resistivity measurements from the coarse samples/as-is gradations (between 2343 Ω-cm and 43,244 Ω-cm). • Measured resistivity for half of the fine samples (4 out of 8) are less than 2000 Ω-cm including measurements from samples LWF1, LWF2, LWF4, and LWF8. • Resistivity > 3000 Ω-cm were measured from two of the fine samples including LWF6 and LWF7. • Only one coarse sample, LWC4, had measured resistivity less than 3000 Ω-cm, and the resistivity measurement from LWC8 (3264 Ωcm) is low compared to the other coarse samples with resistivity more than 15000 Ω-cm. Table 2. Resistivity Measurements on Samples of ESCS/LWF (adapted from MTI, 2023) Fine Samples Resistivity AASHTO T 288 (Ω-cm) Blended or Coarse Samples Resistivity Tex-129-M (Ω-cm) LWF1 1983 LWC1 16,133 LWF2 1852 Blended 2346 LWF3 2771 LWC3 29,050 LWF4 1882 LWC4 2715 LWF5 2646 Blended 3259 LWF6 3848 LWC6 30,385 LWF7 3553 LWC7 43,244 LWF8 1922 LWC8 3264 NWFx 9695 NWCx 24,812 5.4. Correlate sulfate and resistivity measurements Figure 5 depicts the correlation between sulfate content and resistivity measurements from samples of fine (passing the No. 10 sieve) and coarse or blended samples. For the fine samples the sulfate content and resistivity were measured in accordance with AASHTO T 290 (sulfate) and T 288 (resistivity), and the coarse or blended samples were tested according to Tex-620-M (sulfate) and Tex129-M (resistivity). These data indicate a general trend whereby sulfate content and resistivity are negatively corelated, i.e. resistivity is lower at higher sulfate contents. A good correlation between sulfate content and resistivity (R2 ≈ 0.9) is realized for the fine samples. Data points for blended and coarse samples with higher fines contents (e.g., LW2 and LW5 ) are close to the trend line of the fine samples, and those with negligible fines show substantially higher resistivity. This explains the lower coefficient of determination J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 53 (R2 ≈ 0.2) for the trendline representing blended and coarse samples as opposed to the fine samples with particle sizes that pass the No. 10 sieve. Figure 5. Electrical resistivity as a function of sulfate contents [2] 5.5. NCHRP Data (normal weight) As described in NCHRP Report 958 [3], 24 samples of normal weight and 3 samples of lightweight (ESCS) fill were included in the test program for NCHRP Project 21-11. Samples came from various sources throughout North America including New York (5 sources), North Carolina (3), South Carolina (2), Florida (1), Louisiana (1), Arkansas (1), Texas (10), British Columbia (1), and Calgary (1). The minerology of aggregate sources includes limestone (13), granite (2), sandstone (1), natural sands/silica (6), glacial till (1) and expanded clay light weight fill (2). The samples represent a broad range of gradations and compositions ranging from fine sand to coarse, clean, and open-graded, gravel. The composition is described in terms of the percentages of gravel (% retained on ¼ inch sieve), coarse to medium sand (passing ¼ inch and retained on the No. 40 sieve), fine sand (passing the No. 40 sieve and retained on the No. 200 sieve) and fines (% passing the No. 200 sieve). We summarize the composition of the materials as follows: • Three samples were predominately (i.e., more than 50%) fine sand, • Six samples were predominately coarse to medium sands, • Two samples were mixtures of fine and coarse particles, where none of the components were equal to or greater than 50% of the total, • Sixteen samples were predominately gravel varying from sandy gravels to clean and open graded gravels with no sand content, and • None of the samples had more than 5% passing the No. 200 sieve. 5.6. Sulfate AASHTO T 290 and Tex-620-M Figure 6 presents data from testing 9 samples with different gradations including one sample of lightweight fill (ESCS) collected from a site in South Carolina. Based on its gradation, the lightweight fill has the consistency of a medium to fine grained sand with approximately 65% passing the No. 10 sieve. Compared to the samples of ESCS tested by MTI (2023) this is considered as a fine LWF, similar to LWF2 and LWF5. The data in Figure 6 are organized from left to right in terms of coarseness with the finest sample (Florida) on the left hand side and the coarsest sample (Wake Forest) on the right. Sulfate contents measured on the ESCS LWF are the highest and equal to 700 ppm (not considering Tex620-J) as measured by AASHTO T 290. Measurements via Tex-620-M are lower (425 ppm) but still above the 200 ppm threshold. Measurements of sulfate contents from the normal weight samples are less than or close to 200 ppm. J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 54 Figure 6. Sulfate Contents Measured from NCHRP Project 21-11 [3]. 5.7. Resistivity – AASHTO T 288, Tex-129-M During the course of the study for NCHRP Project 21-11, 5 different test standards or proposed test standards were considered for measurement of resistivity including AASHTO T 288, ASTM G187, Tex-129-E, Tex-129-M, and ASTM WK24621. These procedures differ in the particle sizes included in the test specimen, the preparations of the specimen before testing, the size of the test box (which depends on the maximum particle size), and the moisture content at which the minimum resistivity is recorded [3]. Figure 7 presents results from resistivity measurements performed on 9 different sources of materials including 1 source of ESCS (South Carolina LWF). Similar to the presentation of the sulfate measurements depicted in Figure 6, the data in Figure 7 are organized from left to right in terms of coarseness with the finest sample (Florida) on the left hand side and the coarsest sample (Wake Forest) on the right. For materials consisting of finer gradations (e.g. Florida; MUD NY) the resistivity measured by the different test standards are similar, with all measurements within 10% of the average. For samples with coarse gradations (e.g., Raleigh, NC; Wake Forest, NC) substantial differences are observed between measurements made with AASHTO T 288 or Tex-129-M. The measured resistivity of the ESCS is close to the 3000 Ω-cm threshold, which is lower compared to 6 out of 8 of the normal weight samples. This is expected given the high sulfate contents observed for the ESCS, but two of the sources including Rochester, NY and El Paso, Texas have lower measured resistivity. These sources had high chloride contents that affected the measured resistivity of these normal weight samples. Figure 7. Resistivity Measured from NCHRP Project 21-11 [3]. J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 55 5.8. Correlate sulfate and resistivity measurements Salts influence the electrical resistivity of an aqueous solution because they dissociate into ions when dissolved in water, creating an electrically conductive solution. As the ion concentration increases, resistivity decreases due to higher ion mobility. Therefore, resistivity measurements are inversely correlated with salt content. Figure 8 depicts the relationship between measurements of salt content (including sulfate) and resistivity. This relationship merges the data from NCHRP 21-11 related to normal weight aggregates with data from ESCS LWF collected by MTI and measurements of ESCS LWF from the project in Myrtle Beach SC and crushed ESCS from Louisiana that we also included with the testing for NCHRP 2111. Open circles are for the normal weight aggregates and the orange lines represent the typical relationship between sulfate content and resistivity for normal weight aggregates [1]. The solid squares and circles are from testing different sources of ESCS LWF. This figure shows that the higher sulfate contents reported from ESCS LWF correspond to higher resistivity compared to what would be expected for normal weight aggregates with similar sulfate contents. This is especially evident from the ESCS LWF samples with reported sulfate contents greater than 1400 ppm (LW2, LW4, LW5, and LW8). Figure 8. Trends Between Measurements of Salt Content and Resistivity from Test Procedures Selected Based on Coarseness of the Sample. The ESCS data are included within the region outlined with the closed loop (maroon dot, SC LWF; yellow, Louisiana LWF crushed; and tan, Louisiana LWF uncrushed). These data show that the resistivity of the LWF samples is high compared to the measurements made on samples of normal weight aggregates with similar salt contents. 6. Data from Resistivity and Corrosion Rate Measurements Corrosion rate measurements are available from tests performed on samples of galvanized and plain steel specimens embedded within normal weight and ESCS aggregates. MTI performed measurements of corrosion rate from ESCS samples, and both normal weight and samples of LWF were included with the data collected as part of NCHRP 21-11. MTI performed corrosion rate measurements on laboratory specimens, and corrosion rate measurements reported for NCHRP 21-11 were field/in situ measurements. Both sets of measurements correspond to aggregates in the moist condition, which is the condition during service as MSE wall fill is allowed to drain and will not become saturated during normal service conditions. 6.1. ESCS LWA – Resistivity vs CR 6.1.1. MTI – lab measurements Figure 9 shows corrosion rates measured by MTI (2023) on specimens of ESCS after one month and three cycles of wetting and drying. Results of testing aqueous contents shows less resistivity for the same corrosivity. Wet conditions result in more corrosion and less resistivity than moist conditions. Corrosion rates measurements from the moist samples ranged between 1 μm/yr and 25 μm/yr. These corrosion rates J. Civil Eng. Mater.App. 2025 (March); 9(1): 47-64 ························································································· 62 which includes the two-phase solid liquid surrounding and in direct contact with the embedded steel, and which affects the corrosivity and performance of the steel. The effect of intraparticle moisture and sulfate content on the conductivity and corrosivity of interparticle moisture and sulfate should be investigated and relevant modifications to existing test procedures implemented to mitigate the effects that the intraparticle conditions have on the test results and characterization of corrosivity. One modification may involve first testing for sulfate content after achieving a saturated surface dry condition of the aggregates, followed by testing a completely saturated sample. Moisture to be extracted from the SSD condition would need to be obtained by centrifuging the sample. In this manner the sulfate content from intraparticle moisture can be distinguished from sulfate that is contained on the surfaces of the particles (interparticle sulfate content). Alternatively, samples may be tested after soaking for 72 hours such that the samples’ intraand interparticle pore spaces are filled with water. The solution extracted for testing sulfates would be decanted without centrifuging to procure only the leachate that is easily drained from surrounding the particles and none or very little from within the particles. Possible sources of interference that may be inherent to ESCS and affect different measurement techniques such as spectrophotometry, gravimetric measurements or ion exchange chromatography need to be identified. For example, turbidity measurements obtained via spectrophotometry are affected by insoluble suspended matter and gravimetric measurements may be affected by the presence of sulfites and sulfides. Whether or not these substances are present within the makeup of samples prepared from ESCS needs to be investigated, and the effects that these species or conditions have upon the measurements needs to be mitigated. 9.2. Construction Specifications (acceptable limits) Current specifications for selection of ESCS as backfill materials should be updated. One approach is to rely more on the measurements of resistivity for characterizing corrosivity of ESCS and not rely on measurements of sulfate content. Alternatively, the allowable sulfate content could be modified to be more consistent with observations of performance and not limited to 200 ppm. It is essential to consider the low density of ESCS aggregates that result in a reduction in the mass of materials per given volume of backfills compared with normal weight aggregates. A realistic measure of ion concentration per volume can be obtained using the in-place density of ESCS aggregates. Current practice is to apply the AASHTO criteria for the electrochemical properties of fill, which is a univariate approach. If one of the measured quantities including pH, resistivity, sulfate and chloride ion contents is outside of the given range or limit, the material is not considered suitable as fill for MSE wall construction. A multivariate scheme, such as the scheme described by Bady and McMahon (1984) should be implemented that considers the synergy between these parameters and other site conditions including drainage and the presence of ions that may form a protective scale and be favorable for corrosivity. 9.3. Future Studies and Testing of ESCS The following topics would improve our understanding of the interaction of ESCS with moisture and its effects on corrosivity: 1. Evaluate the effect of modified test procedures on the measurements of sulfate content for ESCS. 2. Evaluate alternatives for expressing sulfate content for ESCS on a gravimetric as well as a volumetric basis. 3. Evaluate the relationships between characterization of corrosivity and performance of embedded metals for ESCS. 4. Conduct further studies to understand the physio-chemical properties of ESCS interacting with the moisture dynamics and its effect in electrochemical properties measurement. 5. 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