Introduction

The construction industry is one of the largest and most important industrial sectors in the world, receiving around 30 to 40% of global investment resources (Chel; Kaushik, 2018). According to the Brazilian Institute of Geography and Statistics (IBGE, 2023), the increase in Gross Domestic Product (GDP) by 2.9% in 2022 was only achieved due to the 6.9% increase in GDP in civil construction in the same period. However, the sector is responsible for high levels of pollution, a consequence of the energy consumed during the extraction, processing, operation, manufacturing and transport of raw materials. The carbon embodied annually in construction materials grew from 346.2 million tons in 2000 to 1,757.5 million tons in 2018, an increase of 407.7%, being considered one of the largest emitters of greenhouse gases in the world. This value represents 40% of the total emission of gases into the atmosphere. If that wasn't enough, Brazil produces 224,000 tons of urban solid waste per day, of which 59% is construction and demolition waste (CDW), with only 16% being recycled, around 25 million cubic meters (ABRELPE, 2022; ABRECON, 2021).

In face of the current climate crisis and global warming, carbon dioxide emissions from the construction industry must be drastically and immediately reduced. This way, with a view on the most economically and energetically efficient life cycle possible and, an understanding that the materials used in construction are responsible for 15 to 20% of the embodied energy of a building (Chel; Kaushik, 2018), professionals returned to using local resources for their buildings, in particular, soil. According to Hamard, et al. (2016), the interest in reducing the environmental and social impacts of civil construction has led to a renewed interest in construction with soil.

The use of soil can contribute to reducing the emission of pollutant gases into the atmosphere (Brinkmann; Wiehle, 2023). When there is no implementation of industrial additives, the soil can be reused without losing its mechanical characteristics, or even left in place without any risk of pollution (Préneron; Magniont; Aubert, 2018). According to Morel et al. (2001), when using local materials, the energy incorporated in construction reduces by up to 215% and the impact of transport drops by up to 453%. In fact, soil has numerous environmental benefits that prove the reduction in energy consumption and CO₂ emissions resulting from construction (Shukla; Tiwari; Sodha, 2009; Torgal; Jalali, 2012; Sameh, 2014).

However, the great variability and heterogeneity of the soil, the lack of standardization and quality control in the manufacturing and construction process, generate changes in the mechanical behavior of elements made with soil and, consequently, make it difficult to evaluate the performance of these buildings and, thus, are highlighted as some of the disadvantages of using this material (Silva et al., 2015; Torgal, Jalali, 2012; Barnaure; Bonnetb; Poullain, 2021; Fages et al., 2022). According to Sameh (2014), there are still many doubts about the use of soil as a construction material when compared to conventional materials that dominate the market, such as concrete and steel.

However, over the years, construction techniques for construction with soil have been improved (Gomaa et al., 2021), such as masonry made with compacted soil bricks (TSC), a technique introduced in the 1950s, with the development of the manual press known as CINVA-RAM. The technology of pressed soil-cement bricks aims to minimize construction costs and the degradation of nature, since soil is an abundant construction material, easy to obtain and low cost. It can be reused on the construction site, reducing transportation costs. It has small energy consumption in the extraction of raw materials and does not require the burning process (Segantini; Wada, 2011; Uchimura, 2006; Rosa; Santos, 2013).

Currently, discussions about soil stabilization with cement have intensified. Damme and Houben (2018) argue that the gain in resistance caused by stabilization does not justify the consequent emission of CO₂ generated. According to the authors, the gains are not significant, and the material loses its sustainable status. However, according to Dahmen, Kim and Plamondon (2018), soil blocks stabilized with 4% cement have 46% less embodied carbon when compared to conventional concrete blocks. In their research, Chel and Kaushik (2018), compared five types of building blocks: stone blocks, fired clay, soil-cement, hollow concrete, and steam-cured mud. The authors concluded that soil block stabilized with 6% cement were the most energy efficient, corresponding to 23,5% of the energy of fired clay bricks, considered the most polluting. When 7-8% of cement is used, this value rises to 30% of the embodied energy of fired clay bricks (Reddy; Jagadish, 2003). According to Minke (2006), the on-site preparation, transport and handling of soil-cement bricks requires 1% of the energy required for the production, transport and handling of ceramic and concrete blocks, producing practically no environmental pollution.

Considering that waste is still minimally recycled in civil construction, the construction industry has incorporated new concepts and technical solutions aimed at the sustainability of its activities in all phases of the construction method (Oliveira, 2011; Lima et al., 2015). Currently, several studies have been carried out with the aim of prove the potential of incorporating this waste into soil-cement blocks and bricks with the view to improve their mechanical behavior and durability (Kongkajun et al., 2020; Seco, 2018; Abhilash et al., 2020; Malkanthi; Wickramasingle; Pereira, 2021). The use of Construction Waste (CDW), whether recycling or reusing, appears as a sustainable alternative, contributing to the promotion of the practice, and even giving it status as a noble material. According to ABRECON (2021), CDW recycling is not only sustainable development and environmental preservation, but also a job and business opportunity, boosting the country's economy.

Currently, Brazilian regulations (ABNT, 2012a) classify these bricks only as sealing masonry, but in practice they are widely used in buildings that do not include pillars and beams, whose purpose is to receive and transfer the efforts of the building to the foundation and subsequently to the ground. Therefore, the walls are called structural due to the structural load demands of the building.

For structural analysis purposes, due to the expensiveness of testing full-size walls, the dimensions of the machinery, transport of samples, time and cost, tests are commonly carried out using smaller specimens, called prisms and small walls (Sarhat; Sherwood, 2014; Nalon et al., 2020; Thaickavil; Thomas, 2018; Chang; Messali; Esposito, 2020; Joyklad et al., 2022; Huamani et al., 2022). The structural masonry design regulations (ABNT, 2020; BSI, 2005; ASTM, 2023; BIS 1987; CSA, 2014; AS, 2017) establish two methods for determining their mechanical properties, they are: mathematical models based on the mechanical characteristics of the components, unit, mortar and grout, individually; and experimental tests on the masonry assembly (Nalon et al., 2020).

Currently, several studies have been carried out with the objective of proving prove the potential of incorporating waste into soil-cement blocks and bricks in order to improve their mechanical behavior, durability (Nascimento et al., 2021; Nadia; Fatma; Nasser, 2023; Dhanjode; Nag, 2022; Barros et al., 2020; Vilela et al., 2020) and even its thermal behavior (Hany et al., 2021; Muñoz et al., 2020).

Studies have demonstrated the feasibility of replacing part of the soil in these bricks with construction and demolition waste (CDW) (Kongkajun et al., 2020; Seco et al., 2018, Abhilash et al., 2020; Malkanthi; Wickramasingle; Pereira, 2021), with the aim of reducing the accumulation of waste generated, the vast majority of which are discarded inappropriately, causing environmental problems. However, these studies analyzed bricks and prisms (with bricks only overlapping - without mortar in the laying joint) of young age subjected to compression stresses, and detailed analyses of their structural behavior with more advanced age were not reported.

Within this context, considering the scope of sustainability, and the possible consequences when it is not considered in different areas, this work aims to collaborate with the civil construction sector, through the study of a practice that aims to contribute to the reuse of CDW aiming for its incorporation in the development of soil-cement brick masonry. For this purpose, the compressive strength, deformability, and failure mode of the structural masonry, made with modular soil-cement bricks with partial reuse of CDW, were analyzed through instrumented tests of prisms with laying mortar, aged 530 days, using an extensometer.

Materials and methods

Materials

The soil used is native to the city of Caarapó-MS, has a characteristic profile of a Dystrophic Red Oxisol, of medium texture, clay content less than 30%, classified as Clay Sand by NBR 6502 (ABNT, 2022a). The waste, class A (CONAMA, 2002), collected at a Construction Waste Processing Plant located in the Municipality of Dourados-MS, came from the crushing and sieving process on a 4.8 mm mesh, composed of remains of ceramic materials, mortars and concrete, reddish in color, classified by NBR 6502 (ABNT, 2022a) as Silty Sand with Gravel. The cement used was CP II Z-32, according to NBR 16697 (ABNT, 2018).

The characterization of the soil and the Soil-CDW mixture used in the manufacture of the bricks was carried out in accordance with the procedures and standards: compaction, characterization and determination of moisture content tests by NBR 6457 (ABNT, 2024); specific mass by NBR 6508 (ABNT, 1984); particle size analysis (sieving and sedimentation) by NBR 7181 (ABNT, 2016c); liquidity limit by NBR 6459 (ABNT, 2016a); plasticity limit by NBR 7180 (ABNT, 2016b); compression by NBR 7182 (ABNT, 2016d); compaction test by NBR 12023 (ABNT, 2012c). The characteristics and morphology of the soil and residues are presented in Table 1 and Figures 1 and 2.

The mortar mixture used to make the prisms was 1: 0.656: 7.234 (cement; lime; sand), in mass, with a resistance of 2.9 MPa (ABNT, 2016f), and in accordance with that advised by Parsekian, Hamid and Drysdale (2013), Mohamad et al. (2017) and NBR 16868-1 (ABNT, 2020). The lime used was CH III (ABNT, 2003). The sand used, coming from the Porto São José deposit - PR, has a fineness modulus of 1.61, maximum characteristic dimension of 0.6 mm, real specific mass of 2.65 kg/dm³, unit mass of 1.46 kg/dm³ and compacted unit mass of 1.60 kg/dm³ (ABNT, 2022b; ABNT, 2021). To prepare the mortar, the sand and hydrated lime were homogenized in a concrete mixer, 16 hours in advance, in accordance with NBR 13276 (ABNT, 2016e). The mortar consistency index obtained was 260±10 mm, measured in accordance with NBR 13276 (ABNT, 2016e). The determination of compressive strength followed standards NBR 13279 (ABNT, 2016f), being characterized by NBR 13281 (ABNT, 2023).

Mix design and molding of bricks

To choose the content of each material in the mixture to be used in making the bricks, dosages composed of soil, cement, with additions of 0% (reference), 20%, 40% and 60% of CDW in relation to soil dry mass. A cement content of 10% was used in relation to the dry mass of the soil and CDW mixture.

The mixtures were tested for their optimal humidity and maximum apparent dry specific mass through the compaction test in accordance with NBR 12023 (ABNT, 2012c), method A, using a small cylinder, with 3 layers and applying 26 blows per layer with a small socket (Normal Energy 600 kJ/m³).

The materials were weighed separately. The sand was added in a rotating concrete mixer, right after the waste and cement. The materials were mixed until homogenized, and then water was added, little by little, using an adapter with a sprinkler system, taking the optimum humidity, determined in the compaction test, as a reference. The mixture was kept in rotation for approximately three minutes and, after turning off the concrete mixer, it was conveyed to the press by a conveyor belt system.

The bricks were made using a hydraulic press from the Eco Máquinas brand (Figure 1b), producing one block at a time, measuring 12.5 cm x 25 cm x 6.5 cm (width x length x height) and fittings in the internal holes of 6 cm in diameter, allowing direct binding between the bricks when executing the masonry. The brick molding process can be seen in Figure 3.

During molding, it was found that to obtain good quality bricks, a moisture content below the optimum moisture content found in the Proctor Normal compaction test was necessary. Attempts to mold the bricks with optimum humidity resulted in pieces with cracks and subsequent wear. This was also observed in research by Grande (2003) and Souza (2006). This behavior can be explained considering that the amount of energy applied and the compaction processes used in the compaction test and in the press are different. Therefore, it was decided to add less water to the soil-cement-CDW mixture to avoid breaking the bricks.

The bricks were removed from the press, placed on pallets, protected from wind and direct sunlight. The curing process lasted 7 days, where they were sprayed with water for 3 hours during the day and night, so that they remained moist during the curing process. Difficulties were encountered in manufacturing bricks in mixtures with 60% CDW in the brick production process. The difficulty in molding the bricks resulted in many losses, so it was decided to reduce the amount of CDW to 50% for manufacturing the bricks.

Characterization of bricks and prisms

In total, 50 bricks (providing time off, in case of brick breakage or the need to redo a test) were made for each chosen feature, which were tested in accordance with standards NBR 8492 (ABNT, 2012b) for their compressive strength (7 bricks), dimensional analysis (10 bricks) and water absorption (3 bricks).

By analyzing the results obtained, the sample with 50% CDW was chosen for the manufacture of bricks and prisms. The chosen bricks were broken at ages of 14, 28, 56 and 530 days to analyze their compressive strength as a function of time, in days.

Due to the lack of a standard regarding structural masonry made with soil-cement bricks, the execution and testing of the prisms followed the recommendations of standards NBR 15961-2 (ABNT, 2011b) and NBR 15812-2 (ABNT, 2010b) regarding structural masonry with concrete and ceramic blocks, respectively. Six prisms were made, formed by overlapping two bricks, wetted beforehand and placed in a 5 mm layer of cement mortar and capped for the compression resistance test, with a 2 mm layer of sulphur, to regularize the surface in contact with the press and avoid possible tension accumulation. The prisms were identified and maintained in ambient curing conditions, protected from the elements, until they were 530 days old, when they were ruptured.

To measure the displacements, four linear inductive variable displacement transducers (LVDT) with a sensitivity of 0.001 mm were used, schematized as illustrated in Figure 4a. After installing the LVDTs, the prisms were placed in a universal testing press model MVE–100. To apply the load, a manual steel hydraulic jack model P80, with a capacity of 20 tons, was used. Two steel plates were added, positioned one at the top of the prism and the other at the bottom of it, 80 mm thick, as suggested by Mohamad (2017). Loads were measured using a load cell with a capacity of 10 tf (Figure 4b). In order to preserve the integrity of the LVDTs, they were removed before breaking the test piece.

The tests were conducted so that each load increment corresponded to 10% of the probable failure load, maintaining a residence time of 3 min for each load. This was done until reaching a load close to 50% of the expected breaking load, in 5 increments, as suggested by NBR 15961-2: Annex A (ABNT, 2011b). It was decided not to unload the load.

The modulus of elasticity (EP) was determined in the range corresponding to the secant curve between 5% and 30% of the prism's rupture stress (ABNT 2011b). A first prism was tested by applying a gradual and uninterrupted load until rupture to know the probable strength.

Various standards and authors (BSI, 2005; ABNT, 2011a, 2010a, 2020; CSA, 2014) correlated the modulus of elasticity (E_p) of the masonry with its compressive strength (f_m), according to Equation 1

In the literature, there is a wide variety in the values of the coefficients “a” and “b”. Therefore, with the results for the modulus of elasticity and compressive strength, the correlation between them will be calculated using Equation 1.

Results and discussions

Dosage of soil-cement-CDW mixtures

Figure 5 shows the optimum compaction moisture (w_ót) and maximum apparent dry mass (ρ_dmáx), found in the soil compaction test and soil-CDW mixtures in different proportions, carried out according to NBR 7182 (ABNT, 2016d), without soil reuse at Normal energy (600 kJ/m).

The increase in the percentage of construction and demolition waste in the mixture causes the compaction curve to shift, increasing the value of the optimum humidity and decreasing the maximum apparent dry mass. It was also observed that adding 20% CDW to the soil-cement mixture reduced its maximum apparent dry mass by approximately 4% and increased the optimum humidity by 7%. The addition of 60% of CDW caused a drop of approximately 10% in the maximum apparent dry mass and increased the optimum moisture content of the mixture by 25%. The reduction in the dry apparent specific mass may be related to the particle size of the CDW, which has larger grains than the soil grains, increasing the void ratios in the sample (worst particle packing). The increase in moisture content may have been influenced by the characteristics of the waste, which comes from cementitious and ceramic materials (greater porosity, therefore greater water absorption), generating the need to increase the amount of water to ensure good compaction (Silva Neto et al., 2021).

For a preliminary assessment of the compressive strength of the mixtures, broken samples were evaluated after 7 days in a humid chamber. The results can be seen in Table 2 and Figure 6.

It can be seen in Table 3 that the addition of waste increased the strength of the soil-cement mixture for all situations. Replacing the soil with 20%, 40% and 60% CDW in the soil-cement mixture resulted in an increase in preliminary compressive strength of approximately 9.5%, 16.0%, 24.8%, respectively. This occurred due to the increase in larger particles with greater roughness (Figure 1) that can improve anchorage with cement and thus increase compression forces. Another point to highlight refers to the fact that CDW is very porous and tends to absorb more water for the same condition suitable for optimal compaction. Therefore, the effective water/cement ratio tends to be lower and this can lead to greater brick strength (Silva Neto et al., 2022). This additional water for absorption tends to have less impact on the porosity and thus the density of the brick (Silva Neto et al., 2021).

After preliminary analyses, based on maximum use of waste, the mixture with 60% CDW was chosen for making the bricks and subsequently making the prisms.

Soil-Cement-CDW Bricks

The dimensional analysis, carried out in accordance with NBR 8491 (ABNT, 2012a), did not present results exceeding the established limit and are shown in Table 3. Therefore, they are suitable for use in relation to these parameters.

After being manufactured, the bricks had a dry mass of 2,660.12 g, apparent dry specific mass of 1.600 g/cm³, maximum dry specific mass of 1.790 g/cm³ and a degree of compaction of 89%. The water absorption found was 17.4% at 7 days, and 16.9% at 14 days, meeting the specifications of NBR 8491 (ABNT, 2012a), noting that absorption decreased with the increase in the age of the specimens. The compressive strength of the bricks, as a function of age in days, can be seen in Figure 7.

The bricks made with the addition of 60% CDW reached the minimum compressive strength required by NBR 8491 (ABNT, 2012a) at 14 days. It was observed that the brick gained strength with age, reaching an average strength of 2.7 MPa after 56 days. However, even reaching the necessary strength, in the manufacturing process of bricks with 60% CDW there were significant losses. This behavior can be explained because CDW is composed of material that is more granular than soil, reducing its workability and initial cohesion, making molding difficult. For this reason, it was decided to reduce the amount of CDW to 50% and manufacture new bricks, which were analyzed for their mechanical (Figure 7) and physical characteristics (Figure 8) and used to make the prisms.

The bricks made with the addition of 50% CDW (Figure 7) reached the minimum compressive strength required by NBR 8491 (ABNT, 2012a) at 28 days. The action of time increased the strength of the bricks by approximately 15% when the age increased from 28 to 56 days and 22% from the age of 56 days to 530 days. In other words, a significant gain in strength was observed depending on the age of the bricks. This behavior is a consequence of the internal reactions that continued to occur in the bricks, more specifically to dicalcium silicate (C₂S), a compound present in cement and responsible for the gain in resistance at advanced ages. There may also have been pozzolanic reactions between the cement and the soil and ceramic particles from the CDW. As seen in Figure 8, 50% CDW met the physical parameters of NBR 8491 (ABNT, 2012a) with a whistle being used.

This same behavior, of increasing compressive strength with the age of the bricks, was found in research by: Souza (2006), for bricks made with 12 different mixes composed of soil, cement and concrete residue tested at ages of 7, 28 , 56, 120, 240 days; Sturm, Ramos and Lourenço (2015), regarding the resistance of hollow soil-cement blocks tested at 7, 14, 28 and 56 days and Gutierrez et al. (2015), for soil-cement blocks with cement contents of 7. 10, 13 and 16% for ages 7, 14, 28, 60 days and also for older ages (19 years).

Assessment of prisms

The average compressive strength of the prisms at 530 days was 1.56 ± 0.06 MPa. According to the results of the compressive strength versus deformation curves of the prisms presented in Figure 9.

The efficiency factor (average f_{p /}f_brick) was 56.6%, values are close to those found by Reddy and Gupta (2008) and Jabri et al. (2022). The soil-cement-CDW behaves as a brittle material, that is, it presents little deformation before breaking. It was found that the loss of stiffness can be generated by: propagation of internal cracks, crushing located in the horizontal and vertical joints and/or opening of the vertical mortar joint at the interface with the brick.

As for the failure mode (Figure 10), all prisms showed similar behaviors, propagation of vertical cracks at the corners and vertices in the prism units, propagating, in some cases, to the joint, generating horizontal cracks in the mortar. The first cracks were observed at a strength/rupture stress ratio between 0.70 and 0.85. Zanatta (2015) reported the same failure mode for the same prisms, aged 50 days, and first cracks at a strength/rupture stress ratio of 0.80.

A value like that found through the equation proposed by EN 1052-1 (BSI, 1999) and Eurocode 6 (BSI, 2005), and by Kaushik et al. (2007) (Figure 11). It was noticed that the Brazilian standards NBR 15961-1 (ABNT, 2011a), NBR 15812-1 (ABNT, 2010a) and NBR 16868-1 (ABNT, 2020) underestimated the ultimate rupture stress and authors Lumantarna, Biggs and Ingham (2014) and Nalon et al. (2020) overestimated it. It should be noted that the standards relate to masonry with ceramic and concrete blocks. Therefore, it should be noted that the behavior of soil-cement-CDW bricks tends to present values slightly different from conventional ones.

The elastic modulus of the prisms reached an average of 1.017 ± 0.127 GPa. By relating the average modulus of elasticity to the characteristic strength of the prisms, it was possible to find the equation that described, in a more satisfactory way, the value of the modulus of elasticity of the soil-cement-CDW masonry as a function of the characteristic resistance of the prisms (f_pk) (Equation 2).

Where:

E_m = modulus of elasticity of soil-cement-CDW masonry; and

f_pk = characteristic of simple compressive strength of the prism.

In other words, coefficients “a” equal to 753 and “b” equal to 1. With the results obtained for the characteristic compressive strength of the prisms, the modulus of elasticity of the masonry was calculated, using the equations proposed by some regulations (Figure 12).

The closest value was of the equation proposed by NBR 15961-1 (ABNT 2011a), for structural masonry of concrete blocks, and NBR 16868-1 (ABNT, 2020). Similar to what was found by Morel et al. (2013), who states that cement provides rigidity to the soil and the behavior of brittle materials, similar to concrete and stone.

Conclusions

The study proved the feasibility, in different proportions, of partial use of CDW in the soil-cement mixture to bricks’ produce. The insertion of CDW contributed to increasing the compressive strength of the material, however, it is possible to verify a limit to its use, since its particle size does not provide the initial cohesion necessary for the brick manufacturing process. As the percentage of CDW increases, the clay fraction in the mixture decreases, making it difficult to demould and handle the bricks after pressing. The results of this research suggest that this percentage of clay is around 9%.

The mixtures with a higher percentage of CDW obtained more granular particle size and consequently larger voids, which resulted in a decrease in their apparent dry mass. In addition to influencing the weight of the manufactured bricks, the increase in the size and quantity of voids increased their water absorption.

During the brick manufacturing process, it was found that optimal humidity did not guarantee the production of quality bricks, causing cracks and breakage of components, making it necessary to reduce the moisture content by three percentage points (3%). This behavior was observed by other authors and can be justified by the difference between the compaction methods used on the sample for the compaction test and in the manufacture of bricks by pressing, mainly in terms of the type of energy applied and its magnitude.

The average compressive strength found for bricks with 50% CDW was 2.81 MPa. An increase of 22% was observed when comparing the average compressive strength of bricks aged 56 days and 530 days, respectively. In other words, the value of compressive strength continued to increase under the influence of time.

The value found for the average compressive strength of the prisms, after 530 days, was 1.59 MPa. The efficiency factor of 56.59% is in accordance with that found by other authors for prisms made with compacted soil bricks. The longitudinal axial deformations measured in the prisms were linear and were around 40 to 45% of their individual ultimate compressive strength. The first visible cracks were observed in the prisms at a stress/strain ratio between 0.70 and 0.85. They presented brittle rupture in the vertical direction, indicating tensile failure in the bricks, a behavior observed by other authors in prisms made with mortar that is more resistant than bricks, with propagation of vertical cracks occurring in the corners and vertices of the prisms. The equation that best described the relationship between the modulus of elasticity and the characteristic compressive strength of prisms was E_m=753.f_pk. When compared with the equations proposed for structural masonry made of ceramic and concrete blocks, it was noticed that the masonry made with hollow soil-cement-CDW bricks behaved more rigidly than the masonry made with ceramic blocks (E_m=600.f_pk.) and less rigid than those made with concrete blocks (E_m=800.f_pk.). The equations proposed by Brazilian standardization are those that are closest to the values found in this research.

Acknowledgements

The authors acknowledge National Council for Scientific and Technological Development (CNPq - grants 304596/2022-1 and 409493/2023-6), Coordination for the Improvement of Higher Education Personnel financial support.

References

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