1. Introduction

Concrete is the most extensively used construction material worldwide, consisting of Portland cement, sand, crushed stone, and water. With an annual consumption of nearly five billion tons, its usage significantly surpasses that of steel in many countries, generating concerns about the availability of its components, particularly natural aggregates. Large-scale extraction of these aggregates is degrading ecosystems and causing regional shortages, with global consumption of sand and gravel projected to reach 30–50 billion tons per year [1] [2].

In Metropolitan Lima, aggregate production has reached critical levels. In 2022, 4,365,653 cubic tons of concrete, 915,413 tons of sand, and 573,741 tons of construction stone were produced, with significant annual growth linked to environmental impacts and illegal exploitation of natural resources [3].

This study investigates the use of ground glass as a partial substitute for coarse sand in concrete to improve compressive and flexural strength, testing proportions of 15%, 20%, and 25% in plain concrete applications. Performance is evaluated at 7, 14, and 28 days of curing, promoting a sustainable alternative that reduces dependence on natural resources and incorporates recycled materials.

2. Related Work

Previous research supports the viability of using ground glass in concrete. Dadouch et al. (2024) observed that replacing up to 10% of cement with ground glass maintained compressive strengths of 35–40 MPa at 28 days, alongside an 8–10% reduction in porosity, improving durability [4]. León and Rázuri (2020) demonstrated that replacing 10–20% of fine aggregate with finely ground recycled glass improved the compressive strength of concrete, with the 15% replacement showing the highest strength gains [5]. Similarly, Huapaya and Valdivia (2019) found that 15% replacement of fine aggregate with recycled glass increased compressive strength by 56% after 14 days and 19% after 28 days, compared to standard concrete [6]. Gebremichael et al. (2023) identified an optimal mix substituting 10% cement, 15% sand, and 20% gravel with ground glass, achieving a compressive strength of 29 MPa at 28 days, with improved sulfate resistance and water absorption below 5% [7].

In addition to compressive behavior, some studies have also addressed the flexural performance of glass-modified concretes. For instance, Amin et al. (2023) comprehensively evaluated the physico-mechanical and durability properties of sustainable ultra-high-performance concrete (UHPC) incorporating different proportions of cement replacement with recycled glass. Their findings showed that the optimal substitution of 20% Portland cement with recycled glass resulted in a compressive strength of 176.3 MPa, a tensile strength of 18 MPa, a flexural strength of 25.7 MPa, and an elastic modulus of 57.82 GPa at 28 days of curing. This level of replacement also significantly reduced water permeability, achieving values of 1.28 × 10^−11 cm/s [8].

While these studies validate the potential of recycled glass in concrete mixtures, most focus on narrow aggregate replacement ranges or cement substitution, and often without considering broader structural properties. In contrast, our study offers a more comprehensive assessment by testing multiple proportions of coarse sand replacement with crushed glass and evaluating both compressive and flexural performance. By identifying optimal proportions that improve mechanical behavior, this research contributes to a more robust and sustainable approach to concrete production.

3. Materials

In this study, strategically selected materials were used to evaluate the performance of a concrete modified with ground glass as a partial substitute for sand. Each material complies with specific regulations to ensure its quality and compatibility with the project objectives, especially in terms of strength and sustainability.

1. Cement

This study used Cement Andino Ultra, a type of Portland cement that meets NTP-334.082 and ASTM C-1157 standards, making it suitable for structural concrete. It demonstrates higher compressive strength than conventional cement, achieving 27.8 MPa at 3 days, 36.3 MPa at 7 days, and 46.6 MPa at 28 days (see Table 1).

Additionally, Andino Ultra Cement has an initial setting time of 146 minutes, within the regulatory range, and offers durability features such as low alkali-reactive expansion and resistance to sulfate attack, ensuring concrete stability and longevity in demanding environments [9].

Table 1: Physical and chemical properties of Andino Ultra HS type Portland cement.

3.2. Sand

For fine aggregate, sand from the Pampa Azul quarry with controlled granulometry was used. This sand complies with ASTM C33 standards for fine aggregates and underwent granulometric analysis following ASTM C136 and NTP 400.012 regulations, which define test methods for particle size distribution in aggregates (see Figure 1). The sand has a loose unit weight of 1572.05 kg/m³, a compacted unit weight of 1791.19 kg/m³, a density of 2610 kg/m³, an absorption percentage of 1.19%, a moisture content of 1.33%, and a fineness modulus of 3.10, ensuring a homogeneous mix and adequate workability (see Table 2).

Table 2: Properties of aggregates.

3. 3. Crushed stone

Crushed stone with a nominal size of 0.0127 m, sourced from the Pampa Azul quarry, was used as the coarse aggregate. This material meets the ASTM C33 standard for coarse aggregates and was evaluated through g            ranulometric analysis following ASTM C136 and NTP 400.012 regulations, which define test methods for determining the particle size distribution in aggregates (see Figure 2). The crushed stone has a loose unit weight of 1582.74 kg/m³, a compacted unit weight of 1717.41 kg/m³, a density of 2640 kg/m³, an absorption percentage of 0.75%, and a moisture content of 0.16%, contributing to the strength of the concrete mixture (see Table 2).

3. 4. Ground glass

For this study, recycled ground glass from basic float window glass was used, selected as a partial substitute for sand in proportions of 15%, 20% and 25%. This glass was crushed until reaching a particle size like that of sand, allowing for uniform distribution in the mixture and optimizing its interaction with the cement. The particle size curve of the ground glass is shown in Figure 3, where it can be observed that the glass complies with the particle size limits established for fine aggregates.

3. 5. Additive

This study utilized the SikaCem plasticizing admixture, compliant with ASTM C494 (types A and D) standards for chemical admixtures in concrete. SikaCem is a chloride-free additive that improves the placement and compaction of concrete, enabling up to a 15% reduction in water content, which enhances workability, cohesion, and density.

The recommended dosage ranges from 0.25 to 0.5 dm³ per 42.5 kg of cement, equivalent to 0.7%–1.4% of the cement's weight. With a density of 1,200 ± 20 kg/m³, SikaCem ensures accurate dosing and uniform integration into the mix [10].

4. Methodology

This study follows an experimental methodology, focusing on the laboratory testing and evaluation of an innovative approach that incorporates recycled ground glass as a partial substitute for natural sand. Compressive and flexural strength tests were carried out at the Concrete Laboratory of the Peruvian University of Applied Sciences (UPC), ensuring reliable and standardized procedures for assessing the mechanical performance of the mixtures.

4. 1. Concrete mix design

The mix design followed the American Concrete Institute (ACI 211) guidelines to establish proportions partially replacing coarse sand with ground glass at 15%, 20%, and 25%. A controlled water-cement ratio ensured consistency and reliable comparisons with conventional concrete.

A plasticizing additive was included to maintain workability, optimizing settlement and facilitating placement and compaction. Table 3 specifies the material dosages, detailing the quantities of cement, aggregates, glass, additive, and water in kg/m³ for each mix variant.

Table 3: Concrete design dosage in kg/m³.

4. 2. Curing process

The samples were demoulded after 24 hours and cured in water at 296.15 ± 275.15 K for 7, 14 and 28 days, following the specifications of ASTM C511 and Peruvian regulations NTP 339.183, which govern the preparation and curing of concrete specimens in the laboratory.

4. 3. Mechanical tests

The mechanical performance of concrete with ground glass was assessed through tests conducted according to Peruvian and international standards. Compressive strength tests were performed on cylinders (0.1 m diameter, 0.2 m height) following NTP 339.185 and ASTM C39 standards, using a 2,000,000 N capacity hydraulic press. Samples were tested at 7, 14, and 28 days to determine compressive strength.

Flexural strength tests were carried out on prismatic specimens (0.10 m × 0.10 m × 0.40 m) in accordance with NTP 339.086 and ASTM C78 standards, using a third point loading configuration. Tests were performed at 7, 14, and 28 days, allowing for the evaluation of the flexural behavior of mixtures with different proportions of ground glass.

5. Results

In this study, the results obtained from the experimental program are presented, focusing on the behavior of concrete in the fresh state when natural fine aggregates are partially replaced with different percentages of ground glass. The analysis includes slump, temperature, and air content tests to evaluate the influence of glass on the concrete’s properties and overall performance.

5. 1. Fresh State Tests

In the fresh concrete, three tests were performed to investigate how the glass affects the behaviour of the concrete during this stage. The test results are shown in Table 4.

Table 4: Fresh Concrete Test Results.

An increase in slump is observed as the substitution percentage of fine aggregate with glass increases, indicating that the replacement with glass improves the workability of the concrete, making it more fluid. This increase in workability is because glass does not absorb water, leaving more water available for the concrete mixture [11]. Despite this increase in workability, the slump values of the samples remain within the variation range allowed by the Peruvian Technical Standard, which is ±1". This ensures that, although more workable, the concrete is still suitable for use.

The concrete temperature increases as the percentage of glass replacing the fine aggregate in the mixture decreases. This phenomenon suggests a clear relationship between the amount of glass and the heat generated. Concrete with 15% glass shows higher temperature than with 0% because, with the introduction of ground glass, a pozzolanic reaction occurs between the glass and cement, generating additional heat [11]. This reaction does not occur in concrete without glass, hence the lower temperature. However, as the glass percentage increases to 20% or 25%, the amount of glass surpasses the optimal level for this reaction, and the glass begins to behave as an inert material, reducing the heat released and allowing faster thermal dissipation compared to the 15%.

The V25 design, with 25% glass, achieves the highest air content (2.50%), indicating that this glass proportion significantly increases the trapped air in fresh concrete. The V15 and V20 variants have air contents of 2.10% and 2.30%, respectively. Although both values are higher than the control, the increase is less pronounced than in V25, indicating that using 15% and 20% ground glass also increases porosity, but more moderately. This progressive increase in air content reflects that a higher percentage of ground glass results in more air trapped in the concrete.

5. 2. Hardened State Tests

To analyze the behavior of the concrete in the hardened state, a compression strength test was conducted. The results of these tests are shown in Table 5.

Table 5: Compressive and Flexural Strength Test Result.

There is an optimal increase in compression strength with 15% glass, surpassing the control concrete by 4.91%. With 20% glass, the strength increases by 3.11% compared to the control, but with 25%, the strength decreases by 14.04%. This is because a higher glass percentage (20% and 25%) makes the mixture more porous and less dense. This happens because the glass has a smooth surface that prevents particles from packing as efficiently as sand, creating more voids and less cohesion in the mixture [11]. This reduction in density decreases the compression strength.

There is an optimal increase in flexural strength with 15% recycled ground glass, surpassing the control concrete by 18.36%. With a 20% glass replacement, the flexural strength increases even further, achieving an improvement of 20.52% compared to the control mix, indicating that this proportion can be considered the most effective for applications requiring higher flexural performance. However, with a 25% glass replacement, the flexural strength decreases by 5.46% compared to the control, demonstrating that an excessive glass content negatively affects the mechanical behavior. This occurs because higher glass percentages (such as 25%) produce a more porous and less dense matrix, as the smooth surface of the glass hinders proper particle bonding, creating internal voids that reduce the mixture's capacity to resist flexural stresses [12].

The flexural performance reached a different percentage than the compressive strength because both parameters responded to different mechanisms. At 15% replacement, the glass particles promoted greater compaction and a more effective pozzolanic reaction, which densified the matrix and optimized compressive strength. In contrast, at 20% replacement, although the compactness was not the most favorable for compressive loads, the higher presence of fine particles acted as a microstructural reinforcement, delaying crack propagation and improving flexural strength.

5. 3. Economic Analysis

As the fine aggregate is increasingly replaced with glass, the cost of concrete preparation per cubic meter decreases. The 15%, 20%, and 25% designs reduce the cost per cubic meter by 1.94%, 2.59%, and 3.23%, respectively, compared to the control design. This is because recycled glass, being cheaper than natural sand, reduces the costs associated with material acquisition and transportation. This approach also favors sustainability by recycling glass waste and reducing the need to extract natural resources.

6. Discussion

In this study, the results obtained from incorporating ground glass as a partial replacement for fine aggregate are analyzed and compared with findings from previous research. This comparison allows for the validation of our experimental results and helps identify trends regarding the optimal glass replacement percentage and its impact on the compressive strength of concrete.

6.1. Results of Selected Studies

6.2. Comparative Analysis

The results of our study regarding compressive strength closely align with those from León Reyes, D. J. C., & Rázuri Cueva, D. A. (2020) and Paredes Bendezú, A. (2019). Both studies demonstrate that the optimal glass replacement level is 15%, as beyond this percentage, the compressive strength of the concrete tends to decrease [5][13]. Additionally, the study by Huapaya Tenazoa, D. A. & Valdivia Farromeque, J. I. (2019) shows that adding glass between 6% and 9% increases compressive strength [6]. When combined with the first two studies, these findings indicate that concrete with glass enhances compressive strength up to 15%, while between 15% and 20%, strength tends to decrease. Although our study focuses on concrete with a different compressive strength than these studies, their information validates the results obtained in our research on concrete with F’c=315 kg/cm².

6.3. Durability of Concrete with Ground Glass as Fine Aggregate Replacement

This study focused on evaluating the short-term mechanical performance of concrete at 7, 14, and 28 days, without including specific durability tests. However, recent research suggests potential effects of recycled glass on long-term durability.

An experimental study reported that the water penetration depth decreased from 0.0377 m in the control to 0.0367 m with 5% glass, but progressively increased to 0.0453 m (10%), 0.0463 m (15%), 0.0473 m (20%), and 0.0517 m (25%), indicating that low replacement levels reduce permeability, while higher contents increase porosity [14]. Another study found that replacing up to 30% of fine aggregate with glass optimized chloride resistance, achieving chloride diffusion coefficients close to 7 × 10⁻⁹ cm²/s, whereas higher glass contents reduced this performance [15]. Furthermore, under freeze–thaw cycles, mixtures with 15% glass retained over 85% of their dynamic modulus after 25 cycles, compared to less than 60% retention for higher replacement levels [15]. Additionally, a systematic review on fine glass aggregate demonstrated that replacement levels between 10% and 30% generally improve durability-related properties, including resistance to ionic transport, chemical attack, and thermal cycling, without significantly increasing the risks associated with alkali–silica reactivity (ASR) [16].

Overall, the literature indicates that replacing sand with 15% to 20% recycled glass powder can enhance concrete durability, particularly against chloride ingress, thermal cycles, and aggressive environmental conditions. Future studies are recommended to evaluate the durability of mixtures V15, V20, and V25 using standardized protocols such as NT-TC 85 (chloride penetration), ASTM C1012 (sulfate attack), and ASTM C1260 (ASR expansion) to validate their long-term reliability.

7. Conclusion

In conclusion, incorporating recycled ground glass as a partial substitute for sand in concrete offers a sustainable and cost-effective solution for the construction industry. The study demonstrates that replacing sand with 15% ground glass optimizes compressive strength, achieving a 4.91% increase, while a 20% replacement maximizes flexural strength with a 20.52% improvement compared to the control mix. However, higher replacement levels, such as 25%, lead to reductions in both compressive and flexural strengths, indicating a threshold beyond which mechanical performance is compromised. Additionally, this approach reduces production costs by up to 1.94%, promotes waste reuse, minimizes natural resource extraction, and mitigates environmental impacts, aligning with sustainability goals. These findings highlight the potential of recycled ground glass as an innovative and eco-friendly material for structural concrete applications.

References