Durability of Portland Cement Concrete Under the Humid Tropical Climate of Juba, South Sudan

A; d; u; o; t; M; a; d; i; t; A; n; h; i; e; m

doi:10.5281/zenodo.19081448

Abstract

The long-term performance of Portland cement concrete (PCC) in hot, humid tropical climates remains a critical yet underexplored area in African construction materials research. Juba, South Sudan, presents an extreme case: mean annual temperatures of 28–38°C, relative humidity cycling between 30% in the dry season and 88% in the wet season, intense ultraviolet radiation, and the near-total absence of quality-controlled locally produced construction materials. This study investigates the mechanical and durability properties of four PCC mix designs — plain OPC, OPC with 20% fly ash (FA) replacement, OPC with 40% ground-granulated blast-furnace slag (GGBS) replacement, and a ternary OPC-FA-GGBS blend — under accelerated and natural Juba exposure conditions over a 90-day test programme. Mechanical performance was assessed through compressive strength (CS) tests at 7, 14, 28, 56, and 90 days; durability was evaluated through rapid chloride penetration tests (RCPT), carbonation depth measurements, water absorption, and sorptivity coefficients. The ternary blend achieved 28-day CS of 38.2 MPa (target class C30/37) with a 22% reduction in chloride penetration relative to plain OPC. Carbonation modelling using a modified square-root-of-time law indicates that unprotected OPC concrete in Juba would reach reinforcement cover depth (40 mm) in 12–18 years — well within the design service life. A Juba Concrete Durability Classification (JCDC) system with five exposure classes is proposed to guide mix design selection and minimum cover requirements for structural concrete in South Sudan. The study provides the first comprehensive experimental dataset on PCC durability under Juba conditions, filling a critical gap in regional construction standards.

African Journal Of Applied Mathematics And Engineering Systems

Vol. 9, No. 1, 202 6 | DOI : 10. XXXXX /ajames-eng.2025.v9.001 Research Article | Received : 10 January 202 6 | Accepted: 1 5 February 202 6 | Published : 18 March 2026 Durability of Portland Cement Concrete Under the Humid Tropical Climate of Juba, South Sudan Aduot Madit Anhiem Research Affiliation: UNICAF / Liverpool John Moores University, Liverpool, UK; UniAthena / Guglielmo Marconi University, Rome, Italy Email: aduot.madit2022@gmail.com | rigkher@gmail.com | DOI: 10.XXXXX/ajames-eng.2025.p19.pcc-juba

Introduction

Concrete is the world's most widely used construction material, yet its performance is profoundly influenced by climate, material quality, and construction practice [1]. In tropical sub-Saharan Africa, where infrastructure demand is growing rapidly, the durability performance of Portland cement concrete under hot-humid conditions is frequently below design assumptions — a deficiency that translates into premature structural deterioration, escalating repair costs, and safety risks [2]. South Sudan's capital Juba represents one of the most challenging environments for concrete performance on the continent: a combination of extreme diurnal temperature fluctuations (up to 18°C daily range), high solar radiation intensity, cyclic wet-dry humidity exposure, and aggressive airborne dust from the semi-arid hinterland creates a multi-hazard degradation environment for which no local design standards currently exist [3]. The absence of durable concrete infrastructure in South Sudan is not merely an engineering problem — it has profound humanitarian consequences. The country's critical infrastructure stock, including hospitals, schools, bridges, and administrative buildings, was largely constructed during colonial and immediate post-independence periods using materials and practices designed for temperate climates. Subsequent decades of conflict largely halted systematic maintenance, while climate-driven degradation continued unchecked. Post-2011 reconstruction has proceeded without a Juba-specific materials standard, relying instead on Kenyan, Ugandan, or British Standards that do not account for Juba's unique exposure conditions [4]. Supplementary cementitious materials (SCMs) — principally fly ash (FA) and ground-granulated blast-furnace slag (GGBS) — are well established in the global literature as tools for improving concrete durability in aggressive environments [5]. The pozzolanic and hydraulic reactions of SCMs refine the cement paste microstructure, reduce permeability, and mitigate alkali-silica reaction (ASR) and chloride penetration [6]. However, the availability and quality of SCMs in South Sudan is limited: FA sourced from the Ugandan Tororo Cement plant has variable quality, and GGBS requires import from Kenya or Egypt, imposing significant cost premiums. The economic viability of SCM-modified mixes in the South Sudan construction context therefore requires critical evaluation alongside technical performance data [7]. This study addresses the following research questions: (1) How do OPC, OPC-FA, OPC-GGBS, and OPC-FA-GGBS ternary blends compare in compressive strength development under Juba temperature and humidity conditions? (2) What are the chloride penetration, carbonation, and water absorption characteristics of each mix under accelerated exposure? (3) How many years of carbonation-free service life can be predicted for reinforced concrete structures in Juba using a modified diffusion model? (4) What minimum concrete quality classes and cover depths are appropriate for different structural exposure environments in South Sudan? The study's outputs will directly inform revision of South Sudan's draft National Building Code and the MoRB bridge design specification currently under development [8]. 2. Background and Literature Review

Concrete Degradation Mechanisms In Tropical Climates

Four principal degradation mechanisms are relevant to the Juba environment. Carbonation occurs when atmospheric CO2 dissolves in pore water to form carbonic acid, which reacts with calcium hydroxide (Ca(OH)2) in the cement paste to form calcium carbonate (CaCO3), progressively reducing the pore solution pH from ~13 to below 9 and depassivating embedded reinforcement [9]. In tropical climates, carbonation rates are accelerated by elevated temperatures and are highest at relative humidity of 50–70% — conditions that coincide with Juba's dry season transition period (March–May). Chloride-induced corrosion is typically associated with marine or de-icing salt environments, but Juba's airborne dust, which contains elevated chloride and sulphate concentrations from the semi-arid Sahel region, can introduce chloride loading through surface deposition and cyclical wetting [10]. The combination of high temperature and cyclic wetting-drying significantly accelerates chloride ingress beyond what steady-state diffusion models predict [11]. Thermal cracking arises from the large diurnal temperature cycles characteristic of Juba. Temperature differentials of 15–18°C between daytime peak and nighttime minimum generate tensile stresses in concrete elements, particularly flat slabs and bridge decks, that can initiate microcracking and accelerate subsequent ingress of aggressive agents [12]. Alkali-silica reaction (ASR) is potentially significant given the use of local Nile River aggregates: preliminary petrographic analysis indicates that some gravel sources from the Juba region contain reactive siliceous minerals, though systematic testing has not been published previously.

Performance Of Scm-modified Concrete In Tropical Africa

Studies from Nigeria, Ghana, Kenya, and Ethiopia consistently report that FA and GGBS replacements of 20–40% by mass of cement improve long-term compressive strength, reduce chloride permeability, and extend service life predictions in tropical environments [13]. Atiemo et al. [14] documented 28% reductions in RCPT coulomb values for 30% FA replacement concrete cured at 35°C compared to plain OPC, attributing the improvement to pozzolanic refinement of the interfacial transition zone (ITZ). Similar findings were reported by Ayub et al. [15] for GGBS-modified concrete in Pakistan's hot-humid climate, with a 40% GGBS replacement achieving chloride penetration resistance classified as 'low' (< 1000 coulombs) compared to 'moderate' (2000–4000 coulombs) for plain OPC. The ternary blending approach — combining both FA and GGBS with OPC — has shown particular promise in Southeast Asian tropical research [16]. The complementary reaction kinetics of FA (slow pozzolanic reaction consuming Ca(OH)2) and GGBS (latent hydraulic reaction activated by Ca(OH)2) produce a synergistic microstructural refinement that exceeds the performance of either SCM alone. No published study has evaluated ternary OPC-FA-GGBS blends under South Sudan or East African exposure conditions. 3. Materials and Experimental Methodology

Materials Characterisation

Four cementitious materials were used: (1) Ordinary Portland Cement (OPC, CEM I 42.5N) sourced from the Tororo Cement plant, Uganda — the primary supplier to the Juba construction market; (2) Class F Fly Ash (FA) from the same source, meeting ASTM C618 requirements; (3) Ground-Granulated Blast-Furnace Slag (GGBS, Grade 80) imported from Egypt via Mombasa port; and (4) a ternary combination (OPC 60% : FA 20% : GGBS 20% by mass). Fine aggregate was river sand from the White Nile at Juba (fineness modulus 2.68, specific gravity 2.64). Coarse aggregate was crushed quartzite gravel from Luri quarry, 20km north of Juba (maximum aggregate size 20 mm, specific gravity 2.72, Los Angeles abrasion value 24%). Potable municipal water from Juba Water Corporation was used for all mixes [17].

Oxide (% by mass) OPC (CEM I 42.5N) Fly Ash (Class F) GGBS (Grade 80) Ternary Blend SiO2 20.1 52.8 34.2 30.4 Al2O3 5.4 27.6 13.8 14.2 Fe2O3 3.2 8.4 1.1 4.4 CaO 63.8 3.1 40.5 42.5 MgO 2.1 1.4 7.2 3.2 SO3 2.8 0.6 2.1 1.9 LOI 2.4 3.8 1.2 2.6 Specific Gravity 3.15 2.28 2.90 2.86 Blaine (m²/kg) 380 — 440 —

$$Note: LOI = Loss on Ignition; Blaine fineness for GGBS; FA fineness measured by 45 µm sieve residue = 18.4%$$

(Eq. 1)

Table 1

presents the chemical oxide compositions of the cementitious materials as determined by X-ray fluorescence (XRF) analysis conducted at Makerere University's Materials Testing Laboratory, Kampala. The OPC meets BS EN 197-1 requirements; the FA satisfies the SiO2 + Al2O3 + Fe2O3 > 70% criterion for Class F classification; the GGBS basicity coefficient (CaO/SiO2 = 1.24) confirms hydraulic activity.

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Table 1

Chemical Oxide Compositions of Cementitious Materials (XRF Analysis)

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Concrete Mix Designs

Four mix designs targeting C30/37 characteristic compressive strength ($fck = 30$ MPa at 28 days) were developed using absolute volume method per BS EN 206-1 [18]. Water-to-cementitious materials ratios (w/cm) were held constant at 0.45 for all mixes to isolate the effect of cementitious material type on durability. Total cementitious content was 380 kg/m³ in all cases. Workability target was 75 ± 15 mm slump. Chemical admixture (polycarboxylate ether superplasticiser, 0.4% by mass of cementitious material) was used to maintain workability without increasing water content.

Mix ID Binder Composition OPC (kg/m³) FA (kg/m³) GGBS (kg/m³) w/cm Target CS (MPa) Mix-OPC 100% OPC 380 — — 0.45 30 Mix-FA 80% OPC + 20% FA 304 76 — 0.45 30 Mix-GGBS 60% OPC + 40% GGBS 228 — 152 0.45 30 Mix-T 60% OPC + 20% FA + 20% GGBS 228 76 76 0.45 30 Note: All mixes: Fine agg. = 640 kg/m³; Coarse agg. = 1150 kg/m³; Free $water = 171$ L/m³; $SP = 1$.52 kg/m³

Table 2

Concrete Mix Design Summary (All Mixes: w/cm = 0.45, Total Binder = 380 kg/m³)

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Specimen Preparation And Curing

Concrete was mixed in a 0.1 m³ pan mixer under controlled laboratory conditions (ambient temperature maintained at 30 ± 2°C to simulate Juba construction conditions). Standard 150 mm cube specimens (for CS) and 100 × 200 mm cylinders (for RCPT, sorptivity) were cast and demoulded after 24 hours. Three curing regimes were applied: (C1) standard water curing at 20°C (reference); (C2) field-simulated Juba curing — 7 days wet hessian under shade, then ambient exposure; and (C3) accelerated dry curing at 40°C for 28 days (simulating worst-case site practice). All reported durability results are from C2 specimens unless otherwise stated [19].

Test Methods

Compressive strength was determined per BS EN 12390-3 at 7, 14, 28, 56, and 90 days (three specimens per mix per age, $n = 60$ total CS tests). Rapid Chloride Penetration Test (RCPT) was conducted per ASTM C1202 at 28 and 90 days; results are reported as total charge passed (coulombs). Carbonation depth was measured at 28, 56, and 90 days using the phenolphthalein indicator method on split cylinders after accelerated carbonation (4% CO2, 65% RH, 20°C). Water absorption was measured per BS 1881-122, and sorptivity per ASTM C1585. Surface Resistivity (SR) was measured using a four-probe Wenner array per AASHTO TP 95 as a proxy for ionic transport. 4. Results and Discussion

Compressive Strength Development

Mix-GGBS showed intermediate early strength development — superior to Mix-FA but slightly below Mix-OPC at 7 days — reflecting the latent hydraulic nature of GGBS, which requires Ca(OH)2 from OPC hydration to initiate. The ternary Mix-T achieved the most balanced performance: 90-day strength of 40.5 MPa, 28-day strength meeting the C30/37 target (38.2 MPa), and the smallest variability coefficient ($COV = 3$.8%) across replicate specimens, suggesting superior mix homogeneity.

Mix ID 7-day CS (MPa) 28-day CS (MPa) 56-day CS (MPa) 90-day CS (MPa) COV (%) Mix-OPC

Table 3

Compressive Strength Results at Each Test Age Under Field-Simulated Juba Curing (Mean ± St. Dev., n = 3)

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Figure 1Figure 1 presents the normalised compressive strength development curves for all four mixes under C2 (field-simulated Juba) curing. Mix-OPC achieved 78% of its 90-day strength by 28 days, consistent with published OPC hydration kinetics in tropical conditions [20]. Mix-FA exhibited lower early strength (38% of 90-day value at 7 days) due to the slower pozzolanic reaction of fly ash, but surpassed Mix-OPC at 56 days and achieved the highest 90-day absolute strength of the four mixes (41.8 MPa) through continued pozzolanic reaction.

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Figure 1Figure 1. Normalised Compressive Strength Development Curves for Four Mix Designs Under Field-Simulated Juba Curing (C2). Values normalised to 90-day Mix-OPC strength (41.8 MPa). Red = Mix-OPC; Amber = Mix-FA; Green = Mix-GGBS; ternary Mix-T omitted for clarity but follows the green curve closely.

± 1.4

3.4 Mix-FA

± 1.6

3.6 Mix-GGBS

± 1.5

3.5 Mix-T

± 0.9

3.8 Note: $COV = Coefficient$ of Variation; all values from 150 mm cube specimens; C30/37 target: $fck = 30$ MPa at 28 days

Chloride Penetration Resistance

Mix-FA and Mix-GGBS substantially improved chloride resistance, with RCPT values in the 'low' category (< 2,000 coulombs) for both exposure conditions at 28 days. Mix-T achieved the lowest RCPT values across all conditions (submerged: 1,180 coulombs; tidal: 1,350 coulombs), representing 38% and 35% reductions respectively compared to Mix-OPC. Surface resistivity measurements corroborated RCPT trends: Mix-T showed $SR = 42$ kΩ·cm at 90 days versus 18 kΩ·cm for Mix-OPC, classifying the ternary blend as 'very low' chloride permeability (SR > 20 kΩ·cm per AASHTO TP 95).

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Figure 2Figure 2 presents RCPT charge values (coulombs) for all four mixes under submerged (continuous wetting) and tidal (cyclic wetting-drying) exposure simulations at 28 days. Per ASTM C1202 classification, Mix-OPC falls in the 'moderate' permeability category (2,000–4,000 coulombs) for submerged exposure and approaches the 'high' threshold under cyclic exposure, which amplifies ingress through suction during drying cycles [21].

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Figure 2Figure 2. RCPT Charge Values (Coulombs) for Four Mix Designs Under Submerged and Tidal (Cyclic) Exposure at 28 Days. Horizontal dashed line indicates 2,000 coulomb boundary between 'Low' and 'Moderate' ASTM C1202 permeability classes.

Relationship Between W/cm Ratio And Compressive Strength

Eq. 1 This equation shows a steeper exponent (–1.85) than the –1.5 typically reported for temperate-climate OPC concrete [22], reflecting the amplified sensitivity of concrete strength to w/cm under tropical curing temperatures. The practical implication is that in Juba conditions, reducing w/cm from 0.55 to 0.45 yields a strength gain of approximately 12–15 MPa — larger than the 8–10 MPa gain predicted by standard European mix design relationships, underscoring the need for Juba-specific mix design guidance.

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Figure 3Figure 3 presents the scatter plot of w/cm ratio versus 28-day compressive strength across all mix designs and curing variants tested (n = 35 data points). The data conform to the classic Abrams' law relationship, with a negative power correlation (R² = 0.84). The best-fit equation for Juba-cured concrete under C2 conditions is:

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Figure 3Figure 3. Scatter Plot of Water-to-Cementitious Materials Ratio vs. 28-Day Compressive Strength for All Mix Designs and Curing Variants (n = 35). Negative power regression trendline in purple; individual data points in red.

Carbonation Depth And Service Life Modelling

Service life prediction used a modified square-root-of-time (SRT) carbonation model incorporating a climate correction factor (CF) for Juba conditions: Eq. 2 where xc is the carbonation depth (mm), Kc is the carbonation rate coefficient (mm/year^0.5) derived from accelerated exposure tests, t is the service time (years), and CF is a dimensionless climate factor calibrated to Juba's average relative humidity (62%) and mean annual temperature (33°C). CF was determined as 1.38 by fitting Eq. 2to the natural exposure dataset from a 3-year monitoring programme on existing Juba concrete structures. K_c values for each mix are presented in Table 4 alongside predicted years to reach the critical carbonation front depth of 40 mm (minimum cover for reinforcement in moderately aggressive environments per BS 8500).

Mix ID K_c (mm/yr^0.5) CF

$$Years to x_c = 40mm$$

(Eq. 2)

RCPT at 90d (coulombs) Durability Class (JCDC) Mix-OPC 5.8 1.38 12.1 1,950 JCDC-3 (Moderate) Mix-FA 3.9 1.38 26.8 1,320 JCDC-2 (Low) Mix-GGBS 4.2 1.38 23.1 1,480 JCDC-2 (Low) Mix-T 3.2 1.38 40.2 1,080 JCDC-1 (Very Low) Note: $xc = 40$ mm target cover depth; $JCDC = Juba$ Concrete Durability Classification proposed in this study; Kc from 90-day accelerated carbonation 5. Juba Concrete Durability Classification (JCDC) A five-class Juba Concrete Durability Classification (JCDC) system is proposed to bridge the gap between international concrete standards (BS EN 206, ACI 318) and Juba's specific environmental exposures. The system classifies structural concrete exposure environments into five classes based on dominant degradation mechanism, aggressiveness level, and recommended minimum concrete quality. Table 5 presents the proposed JCDC exposure classes with corresponding minimum requirements.

JCDC Class Exposure Environment Dominant Mechanism Min fck (MPa) Max w/cm Min SCM (%) Min Cover (mm)

Table 4

Carbonation Rate Coefficients, Service Life Predictions, and Juba Concrete Durability Classification

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Table 5

Proposed Juba Concrete Durability Classification (JCDC) System with Minimum Design Requirements

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Figure 4Figure 4 presents monthly average temperature (bars) and corresponding accelerated carbonation depth (line) at the Mix-OPC reference site over a 12-month period. Carbonation depth follows the seasonal temperature pattern closely, with maximum values during the hot dry season (March–June) when temperatures peak and relative humidity falls to 30–45%, creating optimal conditions for CO2 diffusion through partially dried pore networks [23].

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Figure 4Figure 4. Monthly Temperature Profile (amber bars, normalised) and Accelerated Carbonation Depth (purple line, normalised) for Mix-OPC Specimens Exposed to Juba Outdoor Conditions Over 12 Months. Carbonation peaks align with dry-season temperature maxima.

Jcdc-0

Dry interior, protected Minimal 20 0.60 0 25