Expanded Polystyrene-Modified Lightweight Concrete for Thermally Improved Rural Housing in High-Andean Regions

The high Andean regions of Peru face severe climatic conditions, especially during frosty seasons, when temperatures can drop below 0 ℃. This situation, intensified by global climate change and the increase in atmospheric CO₂, has led to numerous cases of acute respiratory illnesses and premature deaths, primarily affecting children and the elderly [1]. In July 2010, the El Niño phenomenon caused a wave of extreme cold in South America, resulting in at least 45 deaths in the Peruvian highlands due to respiratory infections. In areas such as the Peruvian–Bolivian highlands, these events are recurrent and are aggravated by the thermal deficiencies of homes traditionally built with adobe or rammed earth [2-4]. Although these materials are accessible and inexpensive, their limited thermal insulation capacity poses a risk to the health and quality of life of residents. In this context, concrete construction represents a viable alternative because of its availability and strength. However, conventional concrete has high thermal conductivity, making it inefficient in cold climates unless technically modified with insulating materials.

To address this problem, recent studies have explored the incorporation of lightweight and thermally efficient materials, such as expanded polystyrene (EPS) beads, in concrete mixtures. EPS beads are thermoplastic polymers with a closed-cell structure containing up to 98% air, which significantly reduces heat transfer and decreases density while maintaining adequate strength [5, 6]. In Russia, EPS concretes have achieved compressive strengths between 0.28 and 4.22 MPa and thermal conductivities as low as 0.073 W/(m·℃), demonstrating strong potential for insulation applications [7-10]. In Latin America, studies in Colombia have combined Building Information Modeling (BIM) with phase-change materials (PCMs) to improve thermal comfort, while in Mexico, mortars combining limestone and recycled waste such as rubber and PET have been developed to balance thermal and mechanical performance [11, 12].

In Peru, national research has also produced encouraging results. In Macusani, for example, a classroom equipped with bioclimatic technology reached an indoor temperature of 13.36 ℃ compared to 6.60 ℃ in a conventional classroom, significantly improving comfort levels [13]. In Abancay, an insulating mortar incorporating wheat straw and cork raised the interior temperature by up to 4.5 ℃ without compromising mechanical strength (2.56–9.07 MPa) [14–16]. Other experiences in Chachapoyas proposed Styrofoam and plant-based mattresses to enhance adobe insulation, while in Tacna, adding EPS and rubber powder improved thermal efficiency without affecting structural integrity up to 10% inclusion [17-20].

Despite these advances, gaps remain in the literature. There is limited research on EPS-modified concretes simultaneously evaluated under real thermal conditions and controlled mechanical testing. Few studies have built full-scale housing prototypes to correlate indoor thermal performance with compressive and flexural strength, and replicable methodologies for Andean regions are scarce.

Accordingly, this study proposes a comprehensive experimental evaluation of concrete incorporating EPS beads at 4%, 6%, and 8% replacement levels. The analysis focuses on compressive and flexural behavior, fire resistance under exposure to 1000 ℃, and the thermal comfort of a prototype dwelling constructed in the high-altitude district of Calquis, Cajamarca. Laboratory tests were performed following ASTM and Peruvian Technical Standards, while on-site monitoring compared the thermal behavior of an EPS-based dwelling with a conventional one. This integrated approach addresses the identified research gaps by: (i) combining structural and thermal analysis within a unified experimental design, (ii) constructing and monitoring a real-scale module in high Andean conditions, and (iii) documenting replicable construction processes for similar contexts.

The outcomes of this research directly contribute to the Sustainable Development Goals (SDGs): SDG 3 (Good Health and Well-Being), by mitigating cold-related illnesses; SDG 9 (Industry, Innovation, and Infrastructure), by promoting the use of innovative, low-cost materials; and SDG 11 (Sustainable Cities and Communities), by advancing resilient, affordable housing for vulnerable Andean populations. Its replicability is reinforced through standardized mix designs, compliance with national regulations, and field validation, providing technical evidence for future guidelines, local codes, and sustainable social housing programs in cold-climate regions of Peru.

3.1 Physical tests

3.1.1 Particle size analysis

Summary tables of the physical and mechanical properties of the aggregates are presented, together with the results of axial compressive strength of cylindrical specimens, flexural strength of beam specimens, and compressive strength of cylindrical specimens after exposure to 1000 ℃.

Table 2 indicates a continuous particle-size distribution for the fine aggregate, with the largest fractions retained between sieves No. 30 and No. 8, and only ≈ 1.77% passing the No. 200—which meets ASTM C33 limits. The cumulative percentages remain within the lower/upper limits prescribed by the standard across the sieve range.

Table 2. Fine aggregate (FA) particle size test

Figure 3 graphically indicates the same result: the granulometric curve of the fine aggregate lies within the ASTM C33 envelope, showing a slight tendency toward a coarser grading.

The particle size distribution test of fine aggregates was performed in the university laboratory using sieves No. 4, 8, 16, 30, 50, 100, and 200, and the corresponding pans. The results obtained were as follows: the material retained most was obtained with sieve No. 30, the average particle size was 3/8 in., the fineness modulus was 2.82, and the particle size distribution curve is within the range allowed by the particle size distribution spindle. In conclusion, the tested material is suitable for further investigation, as shown in Table 2.

Figure 4 shows the particle size distribution results obtained for the coarse aggregate in the concrete laboratory of Universidad Privada del Norte. The granulometric analysis was performed using the following sieves: 1 1/2 in., 1 in., 3/4 in., 1/2 in., 3/8 in., No. 4, and pan. The highest retained percentage was observed in the 1/2 in. sieve. The maximum aggregate size was 1 1/2 in., the nominal maximum size was 3/4 in., and the fineness modulus was 2.09. The particle size distribution curve remained within the permissible grading limits, indicating that the coarse aggregate was suitable for the subsequent concrete mixture design and experimental testing.

Table 3. Moisture content of fine and coarse aggregates

Table 3 shows the procedure and results of the test performed on the fine aggregate to obtain the moisture content of the fine and coarse aggregates. For this test, 3 samples of each of the aggregates were used to average the results and obtain more accurate data. The average moisture content of the fine aggregate was 5.04%, and the average moisture content of the coarse aggregate was 0.57%, results that will be used for the design of concrete in the different dosages.

3.1.2 Loose and compacted unit weight of fine aggregate

Table 4 presents the results and representative images from the tests conducted on fine aggregate to determine its loose and compacted unit weights. Three samples were used in this test, and the average values obtained are 1486.68 kg/m³ for the loose unit weight and 1593.27 kg/m³ for the compacted unit weight. These values are subsequently used in the design of concrete mixtures with different dosages.

Table 5 presents the results and representative images from the tests conducted on coarse aggregate to determine loose and compacted unit weights based on three samples. The average values obtained are 1456.39 kg/m³ for the loose unit weight and 1488.31 kg/m³ for the compacted unit weight. These values are subsequently used in the design of concrete mixtures with different dosages.

Table 4. Loose and compacted unit weight test of fine aggregate (FA)

Table 5. Loose and compacted unit weight test of coarse aggregate (CA)

3.1.3 Absorption and specific gravity

The results of the specific gravity and water absorption tests for fine aggregates, which are essential for mix design development, are presented in Table 6. The fine aggregate exhibits a specific gravity of 2.68 g/cm³ and a water absorption of 2.92%.

Table 6. Absorption and specific gravity test of fine aggregates (FAs)

Table 7. Absorption and specific gravity test of coarse aggregates (CAs)

Similarly, the results of the specific gravity and water absorption tests for coarse aggregates are shown in Table 7. The coarse aggregate has a specific gravity of 2.59 g/cm³ and a water absorption of 1.15%.

3.1.4 Workability

The slump test measures the workability of the concrete mix, which impacts how easily it can be molded into the formwork. Therefore, the slump in inches was determined for the four concrete mix designs.

Table 8 and Figure 5 show the influence that the different dosages of polystyrene pearls have on the composition of the concrete, where it is observed that the polystyrene pearls increase the slump value, but still maintain a plastic consistency in the mixture, which guarantees adequate workability.

Table 8. Workability test with different mix designs

3.2 Mechanical tests

3.2.1 Compressive strength

Table 9, Figure 6, and Figure 7 show that the use of polystyrene beads in the composition of the concrete has a slight negative effect at all curing ages, moderately decreasing the f'_c value as the EPS content increases. The standard mix reached an average compressive strength of 301.56 kg/cm² (≈ 29.58 MPa) after 28 days of curing. The mixes with 4%, 6%, and 8% polystyrene beads reached average strengths of 296.45 kg/cm², 296.39 kg/cm², and 295.63 kg/cm², respectively, demonstrating only a minimal decrease in axial load capacity as the EPS dosage increased.

Although the target design strength was 21 MPa, the higher strength obtained in the control mix is fully consistent with the overdesign margin recommended by the ACI 211.1-91 method, which suggests an excess of approximately 25–35% to ensure that the specified strength is achieved under field conditions. Since the specimens were produced and cured under controlled laboratory conditions with high-quality local aggregates and optimized water–cement ratio, it is technically reasonable that the resulting compressive strength exceeded the nominal design value. Therefore, this difference does not indicate an error in mix design or material control, but rather reflects the controlled environment and intrinsic quality of the materials used.

Table 9. Summary of f'_c test of cylindrical samples of the standard concrete and with polystyrene beads at 4%, 6%, 8%

Considering both the mechanical behavior and the fresh properties, the 4% EPS dosage was selected as the most suitable proportion for the construction of the prototype housing module in the district of Calquis.

The relatively high standard deviation (SD) observed in the 4% EPS mix (SD = 8.97 MPa) is attributed to the heterogeneous distribution of EPS beads within the cementitious matrix, which produces localized porosity and variable density. This variability is inherent to lightweight concretes containing polymeric inclusions and remains within the range reported by Siddique et al. [13]. Despite this dispersion, the ANOVA test indicated no statistically significant difference in compressive strength (p > 0.6).

3.2.2 Compressive strength of specimens exposed to 1000 ℃

Table 10 and Figures 8–10 present the results and their comparisons, showing that the incorporation of polystyrene beads into concrete exposed to 1000 ℃ led to a reduction in compressive strength compared with the reference concrete. After 28 days of curing, the reference sample at 0 ℃ reached an average compressive strength of 29.58 MPa, while the reference sample exposed to 1000 ℃ reached 28.22 MPa. The concrete mixtures containing 4%, 6%, and 8% polystyrene beads and exposed to 1000 ℃ achieved average compressive strengths of 27.93 MPa, 26.60 MPa, and 26.32 MPa, respectively, indicating a gradual reduction in resistance to axial loading. Exposure to 1000 ℃ produced a measurable reduction in compressive strength, although the reduction was not statistically significant for the selected comparisons. Compared with the reference concrete at 0 ℃, the strength decreased by 4.52% for the reference concrete exposed to 1000 ℃, by 5.58% for the mixture containing 4% polystyrene beads, by 10.07% for the mixture containing 6% polystyrene beads, and by 11.02% for the mixture containing 8% polystyrene beads. The maximum reduction was therefore observed in the mixture with 8% polystyrene beads; however, all mixtures remained within the acceptable strength range. Based on these results, the selected percentages of polystyrene beads were considered suitable for the proposed concrete mixtures, although lower replacement levels showed better agreement with the reference design.

Table 10. Summary of f'_c test of cylindrical specimens for the 4 designs subjected to 1000 ℃

3.2.3 Flexural strength of beams

Table 11 and Figure 11 show the use of polystyrene beads in the composition of the concrete for the production of beams subjected to flexural testing. The standard sample reached an average value of 4.87 MPa after 28 days of curing. The mixtures contain the inclusion of 4% polystyrene beads, an average value of 4.50 MPa; with the equivalent addition of 6% polystyrene beads, a value of 4.07 MPa; and with the addition of 8% polystyrene beads, a value of 3.96 MPa, demonstrating a decrease in resistance to flexural loading loads. The design is considered acceptable at all addition percentages and remains within the required range; considering this, it was decided to add 4% polystyrene beads as it best fits the standard design.

Table 11. Summary of modulus of rupture (M_r) test of concrete with polystyrene beads at 4%, 6%, 8%

3.3 Thermal comfort tests

Tables 12-13 and Figures 12-16 show the performance of the housing module constructed with 4% EPS beads in the district of Calquis, where thermal comfort was evaluated and compared with both a conventional module in the area and the outdoor ambient temperature at the different times of day. Measurements were taken during two distinct climatic conditions:

During the summer season (08/11/2024), 17hourly samples were recorded between 05:00 and 20:00 h. The average outdoor temperature was 8.93 ℃, the conventional module reached 12.01 ℃, and the EPS-based module achieved 13.82 ℃. The relevant and consistent comparison—against the conventional module—indicates a +1.81 ℃ (≈ 15%) improvement in indoor temperature.

During the rainy season (10/01/2025), 17 samples were taken under similar conditions. The average outdoor temperature was 7.56 ℃, the conventional dwelling reached 9.89 ℃, and the EPS-based module achieved 12.41 ℃, showing a +2.52 ℃ (≈ 25%) increase compared with the conventional module.

These values were recalculated to ensure a consistent reference baseline, taking the conventional module as the control rather than the outdoor environment. The results indicate that the use of EPS significantly improves indoor thermal comfort under both seasonal conditions, with statistically significant gains (p < 0.001). The statistical results presented in Table 14 indicate that the inclusion of EPS as a fine aggregate replacement up to 8% did not produce statistically significant variations in the compressive strength of concrete at 28 days (p > 0.6). A one-way ANOVA test, rather than a Pearson correlation, was applied to compare the mean compressive strengths among the four EPS replacement levels (0%, 4%, 6%, 8%), because EPS content was treated as a categorical factor in this analysis. Where appropriate, Welch’s t-test was further used for pairwise comparisons to account for possible unequal variances among groups. The ANOVA and Welch’s t-test results show that the observed strength reductions, which were below 2%, fall within the expected variability of cement-based materials and can be explained by the relatively weak mechanical bonding at the EPS–cement interface.

However, the flexural strength showed a marginal reduction with borderline (p ≈ 0.05), suggesting that the lower stiffness and smooth surface of EPS particles may create interfacial discontinuities that restrict tensile stress transfer and slightly weaken the bending capacity. These results indicate that, although EPS has a limited effect on tensile-related properties, it does not compromise compressive performance, which is the primary load-bearing criterion for non-structural concrete elements.

Table 12. Outdoor temperature samples taken from a conventional module and the module made with expanded polystyrene (EPS) concrete blocks on a clear day in Calquis

Table 13. Outdoor temperature samples taken in a conventional module and a module made with expanded polystyrene (EPS) concrete blocks during the rainy season in Calquis

Under high-temperature exposure at 1000 ℃, the ANOVA results indicated no statistically significant reduction in compressive strength (p > 0.6), suggesting that the material maintained acceptable mechanical stability when EPS was incorporated at moderate dosages (≤ 4%). This behavior may be associated with the thermal response of EPS and the resulting pore structure after high-temperature exposure; however, the mechanism should be interpreted cautiously, as EPS can soften, melt, and decompose under elevated temperatures. The paired t-tests for the thermal performance data showed highly significant differences (p < 0.001), indicating that the observed indoor temperature improvements of +1.3 ℃ in summer and +2.4 ℃ during the rainy season were unlikely to result from random variations but represented measurable improvements in indoor thermal conditions. Thus, the data suggest a clear mechanical–thermal balance: EPS can improve thermal performance without statistically compromising compressive performance, provided that its dosage remains within the approximately 4% range.

Table 14. Statistical analysis of the mechanical and thermal behavior of expanded polystyrene-modified concrete

Table 15. Summary of statistical and technical effects

In Table 15, the synthesis of results reinforces the dual behavior of EPS in concrete—mechanically neutral but thermally advantageous. The mechanical effects were statistically non-significant (p > 0.6), indicating that moderate EPS incorporation maintains the functional strength of the concrete matrix and complies with NTP 339.034 requirements. The flexural effect was marginally significant (p ≈ 0.05), which aligns with theoretical predictions of reduced stress transmission through polymeric inclusions. On the other hand, the thermal effect exhibited strong statistical significance (p < 0.001), highlighting the capacity of EPS to act as a heat barrier by reducing conductive and convective heat flow through the concrete mass. Overall, the trade-off analysis indicates that the 4% EPS dosage achieves the optimal balance between structural reliability and thermal efficiency, representing a practical and statistically validated solution for high-Andean construction contexts.

This combined interpretation supports the selection of EPS concrete as a sustainable, low-cost material capable of mitigating heat loss in cold climates while maintaining acceptable mechanical behavior for non-structural or lightly loaded applications. The statistical validation strengthens the reliability of the findings, ensuring that the improvements in thermal performance are not anecdotal but quantitatively significant and replicable under similar environmental and experimental conditions.

Despite the positive outcomes, this study has several limitations. The experiment was conducted in Calquis, Cajamarca, which restricts the generalization of the results to regions with different climatic and construction conditions. The housing module tested was small-scale; therefore, the structural and thermal behavior in larger or multi-story buildings remains to be validated.

Additionally, only one type of EPS was assessed, without considering variations in density or particle size, which may influence mechanical and thermal performance. The thermal evaluation was limited to the summer and rainy seasons and did not cover a full annual cycle, thereby constraining long-term performance assessment.

Furthermore, mechanical tests were conducted under controlled laboratory conditions rather than dynamic or seismic loading scenarios. Another limitation was the lack of a comparable housing structure with equivalent dimensions and materials in the study area, which prevented direct performance comparison.

Future research should focus on large-scale validation, long-term thermal monitoring, and the integration of digital modeling tools (e.g., BIM-based simulations) to improve the applicability and scalability of the findings.