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Introduction
According to 1, CDW as defined by Decree 1713 of 2002 as leftover of construction, renovation, repair, or demolition of civil engineering works or related activities, this material represents a source of pollution and environmental deterioration that negatively influences water bodies, air, or soil. Therefore, there is great interest in implementing modified designs that reduce the ecological impact through reuse to promote sustainable construction and in this way ensuring economic efficiency through evaluated technical concepts 2 and 3. According to the National Council for Economic and Social Policy (CONPES) 3874 of 2016 and the Superintendence of Public Utilities (SSPD), in Colombia will have 13.8 million tons of CDW annually for the year 2035, as Bogotá currently generates around 14,027,000 m³ of CDW per year 4 due to massive demolitions. Hence, it is necessary to reuse materials as viable technical, economic, social, and environmental alternatives 5, 6, considering research conducted on these materials where they are tested and concluded to comply with local regulations 2, 7.
For the above reasons, at a national level the management of CDW has been proposed as expressed by the UN and UNDP, emphasizing the need to incorporate policies for the management of CDW in future housing with the help of international regulations to minimize energy, water, and non-renewable resource consumption; from less machine hours for the extraction material, decreasing ecological imbalance, and mitigating ecosystem threats 8. These materials have been capable of withstanding live, dead, and seismic loads 9), (10), (11, although the structural elements for these cases of modified concrete must be checked in all their characteristics what ensure an optimal mix for the stability of structures using concrete 12, These influencing characteristics include geometry (cross-sectional area), air content, flexural strength, compressive strength, durability, and the structural system 13.
One of the major issues impacting the deficit in CDW management is specified by 14, who point out that in Colombia, the interest of public entities in managing these wastes has increased due to the lack of a consolidated methodology for reusing rubble in concrete elements 15 or massive treatment. Being a recycled material, it is influenced by hardness 16, porosity 12, and permeability (3%), which produce different w/c ratios, so this also affects workability 15), (12, since an optimal ratio avoids obstacles due to quality variations of CDW depending on its type and origin 17, thereby improving the internal and external characteristics of concrete elements 15. In additional, using clay counteracts damage due to temperature variations and increases compressive strength by up to 45% 18. Another important factor for the safety of the frame model is the steel required to regulate tension when the concrete is subjected to axial forces 19. CDW reduces material costs 20) and theoretically in housing projects for low-income families 21, as it does not require material extraction, but it does require transportation, classification, and selection (size of the coarse aggregate) 8), (22.
The research to determine the impact of using concrete manufactured with CDW on the safety and durability of two-story houses in their structural design, also includes an analysis of the physical-mechanical properties provided by clay blocks of type #4, pavers, and structural bricks, as these have specifications according to 23), (24), (25), (26), (12 respectively. All the above to analyze the behavior by verifying compliance with minimum technical and regulatory standards using the ETABS application; employed as a structural analysis software for finite elements focused on building behavior. This software allows for a structural check and design to ensure the stability, safety, and durability of a building 21. As the subject of study is a "green" house constructed with modified concrete using crushed brick for a two-story housing model, parameters were established according to the NSR-10 19.
Methodology
The research focuses on evaluating the impact of using CDW on the safety and durability of a two-story house as a measure for reintegrating crushed brick and housing optimization. Therefore, an experimental study was conducted based on a literature review of studies involving crushed brick, in which the strength indicators of importance for the deterministic analysis to estimate the advantages gained by structures with modified concrete designs, such as achieving strengths above 21 MPa.
To begin, the material specifications that make up the mix design were defined, followed by a comparative analysis of the properties enhanced by the modified concrete. Accordingly, a house was selected for the analysis of the model elements, which was modelled according to real dimensions. The comparison between conventional and modified concrete regarding the percentage of reinforcement required per element was carried out, after analyzing the technical compliance of the structural system (frames) using ETABS. This defined the safety constant and the details of structural durability for the final presentation of results and conclusions on the benefits of including CDW immersed concrete technology.
Results
According to the results of the experimental development, as well as the deterministic analysis of the model in ETABS, so the impact generated by using crushed brick was determined due to it has high hardness and permeability. The types of brick varying in functionality based on characteristics such as size, holes, type of clay, and firing, are used to determine which composition offers the greatest resistance advantages for the required structural system.
Structural bricks and pavers were selected using a sieve corresponding to a size of 19 mm, complying with the normal specification given by NSR-10, which requires a size bigger than the standard 27. Table 1 presents the w/c ratios used for concrete mixes that exceed a strength of 21 MPa 27, taking into account than for each of the following three mixes they maintained an amount of water (200 kg with a density of 1000 kg/m³), cement (432.10 kg with a density of 2940 kg/m³), and sand (776.74 kg with a density of 2701 kg/m³) 28. In parallel manner, the water/cement/sand ratio was 0.463/1000/1.798, requiring only the variation of the weight of gravel and brick.
Table 1 w/c ratio with different dosages (5, 10 and 15%)
| 5% coarse aggregate replacement | 10% coarse aggregate replacement | 15% coarse aggregate replacement | ||||||
|---|---|---|---|---|---|---|---|---|
| Aggregate | Gravel | Brick | Aggregate | Gravel | Brick | Aggregate | Gravel | Brick |
| Mass (kg) | 901.88 | 47.47 | Mass (kg) | 854.41 | 94.93 | Mass (kg) | 806.94 | 142.40 |
| Density (kg/m3) | 2597 | - | Density (kg/m3) | 2597 | - | Density (kg/m3) | 2597 | - |
| Ratio | 2.087 | - | Ratio | 1.977 | - | Ratio | 1.867 | - |
In the way the data obtained from concrete with "Paver" brick were excluded, as they exhibited lower than expected strengths and were not useful for the analysis, therefore, the verification of the model was reduced to the strengths of "Number 4" and "Structural" bricks. These failed specimens, with a curing age 29 and a diameter of 18 cm (measured after curing), provided maximum load values of interest and were subsequently used for the calculation of maximum strength (Equation 1) and modulus of elasticity (Equation 2) derived from 30:
Equation 1. Maximus resistance
Equation 2. Modulus of elasticity
For better illustration, the Figures 1 and 2 show the data calculated and corrected with the percentage deviation mentioned below with respect to 21 MPa:
Compressive strength up to 28 days with respect to different dosage rates
Figure 1 Strength and modulus of elasticity curve between conventional and brick #4 modified concrete.
Note: This graph shows the capacities of modified concrete with #4 brick at the ages of 7, 14, and 28 days compared to conventional concrete. As a secondary graph, the elasticity values acquired by each concrete are expressed.
Figure 2 Strength and modulus of elasticity curves between conventional and structural brick modified concrete.
Note: This graph shows the capacities of modified concrete with structural brick at the ages of 7, 14, and 28 days compared to conventional concrete. Like the previous graph, the secondary graph refers to the elasticity acquired by each concrete. These are averaged data from 3 specimens for each percentage of CDW replaced.
The increase in capacity, both in compression and in the modulus of elasticity, compared to conventional concretes, made it possible to select the maximum strength for the model in ETABS from the non-averaged data. (Table 2)
Table 2 Maximum resistance (f'c).
| Concrete with maximum mechanical capacities | ||||||
|---|---|---|---|---|---|---|
| Brick type | Age | Replacement | f`c | Und | E | Und |
| Estructural | 28 | 5% | 21,20 | MPa | 17956,10 | MPa |
| Numero 4 | 28 | 10% | 22,55 | MPa | 18519,67 | MPa |
Note: The Structural brick type (Structural brick type (5%) with 28 days of age) offers better strength and elasticity indices among the averaged data, while the Number 4 brick was more advantageous in terms of strength without averaging.
Simultaneously, the expected maximum strength value (21 MPa) and the maximum strength value of conventionally manufactured concrete show a 9% difference (1.88 MPa) due to influential environmental factors 31 and 20, such as temperature variation, acid rain, and microbial agents 32. Therefore, the differential percentage of conventional concrete was added to the modified concretes for the deterministic analysis. (Table 3)
Table 3 Desing strength
| Self-manufactured conventional concrete with low expected strength | ||
|---|---|---|
| Conventional | 19,12 | MPa |
| Adjusted to Good practice in respect of the reduction of conventional (Design strength) | ||
| Structural | 23,08 | MPa |
| Number 4 | 24,43 | MPa |
The impact of modified concrete on structural elements of green homes by being a bioconstruction, quantifies the positive effect of incorporating CDW at 10%, promising advantages in mechanical 33 behavior for adaptation to large-scale structures with ecological benefits. Therefore, the numerical design adapted the characteristics shown in Table 4:
Table 4 Project Description: Characteristics of a house
| Country house in the village of Vanguardia-Villavicencio | ||
|---|---|---|
| House type | Model Home | Modified housing |
| Type Use of building | Group I | |
| Materials of the main structure | Reinforced Concrete | Modified Concrete |
| Roofing materials | Lightweight plate | |
| Number of Building Levels | Two (2) | |
| Seismic resistance system | Reinforced concrete frames | CDW modified concrete portal frames |
From the breakdown, the minimum area of the columns was verified using NSR-10 section C.21.6.1.1, with a height of 30 cm and a base of 30 cm, validated as they exceed the minimum required. Column dimensions for the ETABS model are a base of 35 cm and a height of 35 cm, with a cover of 4 cm (resulting in an effective height of 31 cm). Furthermore, the adapted frames for each floor comply with NSR-10 provisions, although is important to clarify that the Shell elements were discretized. Additionally, the structural configuration (columns, beams, and lightweight slabs) was adopted for the roof mezzanine shown in Figure 3 and Figure 4.
Note: The figure above shows the structural system in plan of the mezzanine floor with uniform column distribution.
Note: The figure above shows the planar structural system of the roof with uniform column distribution.
Other specifications required for conventional concrete, modified concrete, and reinforcing steel for the numerical model (performed in ETABS software), calculating the modulus of elasticity (Ec) according to compressive strength (f’c) for the concretes, resulted in the conventional concrete having an Ec of 17,872.05 MPa and the modified concrete having an Ec of 19,276.85 MPa. Additionally, the normal capacity of steel Ec was taken as 200,000 MPa. Before examining the model, the increased loads and load combinations were assigned to the model as specified by NSR-10 in Title B, as these influence the verification of the structural support based on a uniform distribution of columns, since they are all the same size (30x30 cm). (Figure 5)
Note: the modelled structure is equal both geometrically and in its load distribution.
In addition, the labels of each element are identified as shown in Figure 6 in order to detail the elements that require more or less steel.
In comparative terms between the advantages or disadvantages produced by the modified concrete with respect to the required reinforcement and conventional concrete per element, advantages were identified based on the concept of compression, since this analysis was conducted by substituting the coarse aggregate and examining the structural system's strength. The reduction of coarse aggregate and the inclusion of CDW in the mix effectively reduced the required reinforcement percentage, such visual comparison was made possible through the longitudinal and transverse sections generated by the software, considering the axes shown in the plan with an elevation of N+3.73m in Figure 7 and the plan with an elevation of N+7.51m in Figure 8.
From the tabulated quantum comparison, the average percentage of steel required for the mezzanine and roof is presented in Table 5 and Table 6. These tables were organized by left upper end reinforcements (EIS-LU), center bottom reinforcements (CB), and right upper end reinforcements (EDS-RU). Since the elements at each axis and elevation had steel in the same positions, these values were taken and averaged with respect to all elements according to their position, height, and axis of the steel.
Axes transverse to the model
Table 5 Percentages of the amount of the model with conventional and modified (transverse axes).
| AXIS | ELEVATION | CONVENTIONAL | MODIFIED | ||||
|---|---|---|---|---|---|---|---|
| LU | CB | RU | LU | CB | RU | ||
| 1' | N+3.73 | 0,35% | 0,31% | 0,35% | 0,35% | 0,30% | 0,34% |
| N+7.51 | 0,30% | 0,30% | 0,33% | 0,30% | 0,29% | 0,33% | |
| 1 | N+3.73 | 0,71% | 0,58% | 0,91% | 0,72% | 0,57% | 0,92% |
| N+7.51 | 0,59% | 0,49% | 0,52% | 0,59% | 0,49% | 0,52% | |
| 2 | N+3.73 | 0,30% | 0,30% | 0,42% | 0,29% | 0,29% | 0,41% |
| N+7.51 | 0,30% | 0,30% | 0,40% | 0,29% | 0,29% | 0,39% | |
| 2' | N+3.73 | 0,21% | 0,67% | 0,18% | 0,00% | 0,00% | 0,00% |
| N+7.51 | - | - | - | - | - | - | |
| 3 | N+3.73 | 0,49% | 0,30% | 0,56% | 0,49% | 0,29% | 0,56% |
| N+7.51 | 0,29% | 0,30% | 0,46% | 0,29% | 0,29% | 0,45% | |
| 3' | N+3.73 | 0,18% | 0,64% | 0,18% | 0,19% | 0,63% | 0,17% |
| N+7.51 | 0,18% | 0,54% | 0,00% | 0,17% | 0,53% | 0,00% | |
| 4 | N+3.73 | 0,45% | 0,34% | 0,55% | 0,42% | 0,34% | 0,38% |
| N+7.51 | 0,33% | 0,30% | 0,32% | 0,33% | 0,29% | 0,31% | |
| 5 | N+3.73 | 0,49% | 0,32% | 0,53% | 0,39% | 0,32% | 0,54% |
| N+7.51 | 0,34% | 0,30% | 0,39% | 0,34% | 0,29% | 0,38% | |
| 6 | N+3.73 | 0,61% | 0,56% | 0,75% | 0,67% | 0,56% | 0,74% |
| N+7.51 | 0,45% | 0,32% | 0,40% | 0,44% | 0,32% | 0,40% | |
Axes longitudinal to the model
Table 6 Percentage amounts of the model with conventional and modified (longitudinal axes)
| AXIS | ELEVATION | CONVENTIONAL | MODIFIED | ||||
|---|---|---|---|---|---|---|---|
| LU | CB | RU | LU | CB | RU | ||
| A' | N+3.73 | 0,40% | 0,33% | 0,39% | 0,39% | 0,32% | 0,39% |
| N+7.51 | 0,35% | 0,32% | 0,36% | 0,34% | 0,32% | 0,36% | |
| A | N+3.73 | 0,53% | 0,30% | 0,51% | 0,53% | 0,29% | 0,51% |
| N+7.51 | 0,42% | 0,30% | 0,42% | 0,34% | 0,29% | 0,38% | |
| B | N+3.73 | 0,75% | 0,47% | 0,68% | 0,74% | 0,47% | 0,69% |
| N+7.51 | 0,60% | 0,48% | 0,37% | 0,60% | 0,47% | 0,36% | |
| C | N+3.73 | 0,70% | 0,46% | 0,71% | 0,72% | 0,46% | 0,71% |
| N+7.51 | 0,54% | 0,44% | 0,53% | 0,53% | 0,43% | 0,53% | |
| D | N+3.73 | 0,73% | 0,50% | 0,71% | 0,73% | 0,49% | 0,71% |
| N+7.51 | 0,39% | 0,37% | 0,40% | 0,39% | 0,37% | 0,40% | |
Regarding the comparisons of the required reinforcement percentage by level, a reduction of one to two percent was observed in the percentage required per element. However, it is important to emphasize that when averaging the information, a significant overall reduction per level with a positive impact is evident.
Finally, for the evaluation of the safety and durability of the structural system, data from the last three specimens (exposed to the elements) that failed were managed. These were analyzed for their strength, which increased and maintained a composition that, although it did not comply with curing according to the regulations for these last specimens, demonstrated durability against extreme factors such as the avoidance of curing processes, exposure to high temperatures, and rain over 56 days (Table 7). It was shown that the specimens did not suffer wear greater than one percent. Considering this, it is simultaneously recognized that such environmental factors can generate both internal and external pathologies in the concrete or chemically change the material conditions 32. However, with the inclusion of CDW, no significant changes regarding durability were observed.
Table 7 Specimens exposed to weathering
| EXPOSED TO THE ELEMENTS | ||||||||
|---|---|---|---|---|---|---|---|---|
| Age at failure (days) | % replacement | Diameter (m) | Cylinder area (m2) | Max. load (kN) | Max. strength (MPa) | Calculated Max. Resistance (N/mm2 ) | ||
| Concrete with structural brick | ||||||||
| Outside | ||||||||
| 28 | 5% | 0,151 | 0,151 | 0,151 | 0,018 | 395,98 | 22,18 | 22,11 |
| Curing | ||||||||
| 28 | 5% | 0,151 | 0,151 | 0,151 | 0,018 | 399,98 | 22,63 | 22,34 |
| Concrete with Brick Number 4 | ||||||||
| Outside | ||||||||
| 28 | 10% | 0,150 | 0,149 | 0,151 | 0,018 | 390,48 | 16,80 | 22,10 |
| Curing | ||||||||
| 28 | 10% | 0,150 | 0,148 | 0,149 | 0,017 | 394,42 | 17,14 | 22,64 |
| Conventional concrete | ||||||||
| Outside | ||||||||
| 28 | 0% | 0,149 | 0,159 | 0,149 | 0,018 | 345,14 | 19,34 | 19,05 |
| Curing | ||||||||
| 28 | 0% | 0,151 | 0,151 | 0,152 | 0,018 | 348,63 | 19,73 | 19,38 |
Discussion of the results
The results of this research are unique and have not been carried out before, while in several investigations they only related the characteristics of concrete that offer good resistance indicators 46) and others include modified concrete in the structural analysis 15 but none focused on the positive effects in terms of concrete-dependent variables in housing, due to the adaptation of RCD that effectively manages to overcome minimum resistance values, but what had not been taken into account is the functionality that the housing actually offers when using modified concrete and the variables involved; reduction of percentage reduction, which is a dependent variable of concrete strength, for seismic resistance capacity, durability and use for green housing (47, 48, 49), so much so that this analysis was carried out using the ETABS application and allows us to determine the efficiency of the use of crushed brick immersed in the concrete mix (50, 51).
Conclusions
The results indicate that the analyzed two-story house achieves safety and durability using crushed brick within the composition of CDW-modified concrete, contributing to its strength as it exceeds the 21 MPa resistance threshold 34. Therefore, the following statements can be made about its effectiveness. In first place, its properties gain physical-mechanical advantages in compression tests (axial forces) 35, due to the reduction of coarse aggregates and the utilization of CDW 36, of way that its maximum resistance increases, the structural system's safety also increases. In second place, it is viable to include this material in large-scale projects 37 under the concept of reducing the required quantum percentages. Lastly, the ETABS model with standardized dimensions, loads, and combinations, shows that the internal forces generated by ultimate loads are effectively supported by the modified concrete.
Furthermore, the impact of using CDW on the safety and durability of a house, especially in its structural system, as well as the function and indices of concrete with the w/c ratio of conventionally used concrete 38, is of interest to most architectural projects 39. This is because it requires a smaller amount of aggregate, which reduces costs and environmental repercussions such as the pollution of surface water sources, air, soil, and landscape 40), (41. Additionally, increased compressive strength is observed in concrete with ages greater than 28 days.
In fact, if we correct the errors in the preparation, values exceeding 21 MPa are obtained, which represents a significant difference in terms of the house's safety. Although it may not reduce the steel area, it effectively decreases the required reinforcement percentage. Therefore, it is recommended to verify reductions in the steel area only when resistance values exceed 29 MPa. It should also be noted that according to the concrete's capacity (f’c), elasticity varies, so benefits can also be seen in the elasticity of elements that exceed these safety indices.
Collecting all these observations, the mix with the best conditions is given by the CDW categorized as brick #4 at 10% at an age of 28 days, with a maximum compressive strength (f’c. Max.) of 22.55 MPa. This ensures, according to the NSR-10 and its minimum parameters, good behavior against seismic movements and the capacity to support design loads 42, as it exceeds quality standards 43. Although at earlier ages it only meets the requirements for 1- and 2-story houses, it is necessary to adapt to ages greater than 28 days to meet the minimum requirements of Colombian regulations 23.
Finally, to conclude the analysis of the structural model of a two-story house concerning the safety of the green structure and its specifications with modified concrete, it guarantees the same safety and the benefit of reducing the reinforcement percentage per concrete element 44, so that it is considered that the number of reinforcing bars can be reduced only when the f'c exceeds 8 MPa; otherwise, the relatively usable percentage will be low. The durability of the modified concrete exposed to environmental conditions and the vulnerability of the cylinders demonstrated that the consistency over time did not suffer wear or changes in strength due to its exposure to harmful environmental factors for 56 days. When exposed to the elements, they maintained optimal strength for buildings.
A final additional point: durability is a significant factor for structures, and as it is not a simple factor to calculate, it becomes a dependent variable, so that the durability analysis yielded concrete compositions with suitable coating and permeability 45, without wear and with strength above 21 MPa despite external effects. This indicates effective durability in aggressive environments due to the greater porosity offered by the CDW aggregate.
