Reinforced concrete is regarded as one of the ideal structural materials which comprises concrete with high compressive strength and reinforcing bars with high tensile strength. However, concrete has been pointed out that it consumes a large volume of energy and emits a lot of carbon dioxide during its manufacturing. In order to lower such environmental burdens of concrete structures, a number of studies and approaches have been carried out. The voided slab is also suggested as a new method to reduce the environmental burden since voided section of the slab would use less concrete compared with the normal reinforced concrete slab. However, no studies have evaluated the CO2 emissions and environmental performance of voided slabs. The purpose of this study was to evaluate the structural performance of voided slabs and empirically corroborate their environmental influence. The flexural performance test was carried out based on the variables of the depth of slab, types of the void former materials, and the hollowness ratio. In addition, comparison of the emission of CO2 was also performed by considering the hollowness ratio and types of void former materials over the normal reinforced concrete slab. The structural performance of the voided slab was similar or slightly higher than the normal reinforced concrete slab. The yield strength of specimens was increased approximately 10∼30% over the anticipated yield strength. Based on this result, it is considered that the voided slab would be sufficient to structural performance and beneficial to plane planning in buildings. In general, it is considered that the voided slab would be beneficial to both structural and environmental aspects. However, the test results in this research showed that the voided slab would emit more carbon dioxide emissions compared to the normal reinforced concrete slab. The main source of more CO2 emissions in the voided slab was the anchoring materials. In this research, wires were used to fix the void former materials to the reinforcing bars. In order for the voided slab to become a more eco-friendly and sustainable material, new anchoring methods such as use of recycled materials, new void former materials without anchoring, or other eco-friendly materials should be applied to reduce the emission of CO2.
Reinforced concrete is regarded as one of the ideal structural materials and is commonly used in the architectural, engineering, and construction (AEC) industry [
However, studies have shown that concrete consumes a large volume of energy and emits large quantities of carbon dioxide [
The AEC industry is regarded as one of the main actors emitting large volumes of carbon dioxide and consuming a significant amount of energy [
A number of academics have proposed that an effective approach to reduce CO2 emissions in the AEC industry is to replace conventional materials with low-carbon ones [
Many researchers have emphasised the effectiveness of reducing CO2 emissions through application of high-strength concrete. Tae et al. [
Additionally, Park et al. [
Along with the implementation of high-strength concrete, some researchers have proposed ensuring the optimal design of concrete and material selection in the design stage. González and Navarro [
There are also other approaches to reduce CO2 such as recycling and developing optimal design programmes. Lee et al. [
While CO2 reduction strategies such as selection of alternative materials and optimizing design might be considered a micro-perspective, there is also a “macro-approach,” which considers the entire building system rather than its individual parts or materials. Nadoushani and Akbarnezhad [
Cole [
In this study, the depth of slab, type and presence of void former materials, and hollowness ratio were used as the main variables to test the flexural strength of concrete slab specimens. The details of the dimensions and properties of the specimens are summarised in Table
Properties of specimens.
Specimens | Type of slab | Depth of slab (mm) | Specification of void former materials |
---|---|---|---|
S1 | Normal reinforced concrete slab | 210 | — |
S2 | Voided slab | 210 | Sphere shape |
Diameter 100 mm | |||
S3 | Voided slab | 210 | Oblate shape |
Diameter 170 mm | |||
Height 110 mm | |||
S4 | Normal reinforced concrete slab | 169 | — |
Details of the specimens. (a) S1. (b) S2. (c) S3. (d) S4.
To evaluate the flexural performance of the voided slab specimens, the test specimens were simply supported as shown in Figure
Test setting of the voided slab. (a) Flexural test setup diagram (unit: mm). (b) Test setting and loading configuration.
All the specimens were reinforced by 10 mm and 13 mm deformed bars, and
Details of the void former materials and anchoring. (a) Details of the void former materials and anchoring. (b) Photo of the void former materials
The detailed characteristics of the materials used in the voided slab are shown in Table
Design of concrete mixture.
Designed compressive strength (MPa) | w/c (%) | s/a (%) | Unite content (kg/m3) | Air content (%) | ||||
---|---|---|---|---|---|---|---|---|
Water | Cement | Fine aggregate | Coarse aggregate | Admixture | ||||
27 | 45.7 | 44.3 | 108 | 333 | 821 | 1086 | 3.33 | 2.0 |
Note: w/c is the water/cement ratio, and s/a is the sand/aggregate ratio.
The reinforcing bars used in the specimens were named D10 and D13 based on the diameter; their nominal yield strength was
Specifications and test results of reinforcing bars.
Types | Diameter (mm) | Yield strength (MPa) | Ultimate strength (mm) |
---|---|---|---|
D10 | 10 | 519 | 630 |
D13 | 13 | 531 | 651 |
It is necessary to calculate the life cycle of voided slabs to evaluate their CO2 emissions. Based on ISO 14044 [
The transportation stage refers to CO2 emissions during transporting the ingredients of voided slabs to the manufacturing site. These occur during the manufacturing of the voided slabs from the electricity, gas, oil, etc., used in the manufacturing plant. The system boundary of life cycle CO2 emissions of voided slabs is depicted in Figure
The system boundary of the voided slabs.
Carbon dioxide emissions from the raw materials of voided slabs were calculated as the sum of the quantity of individual components, which were concrete, reinforcing bars, and void formers. The CO2 emissions of concrete were evaluated through the sum of multiplication of the quantity of each ingredient utilised for producing 1 m3 of concrete and the CO2 emission factor for producing concrete. The following equation was used for computation of the CO2 emissions of a unit of concrete:
The CO2 emission factors of cement, aggregates, and water given in the Korea Life Cycle Database (LCI DB) [
Reference of life cycle inventory database.
Material | Unit | Source |
---|---|---|
Ordinary Portland cement | kg | The Korea LCI DB (South Korea) |
Coarse aggregate | kg | The Korea LCI DB (South Korea) |
Fine aggregate | kg | The Korea LCI DB (South Korea) |
Chemical admixture | kg | Overseas LCI DB (ecoinvent) |
Water | kg | The Korea LCI DB (South Korea) |
HDPE | kg | The Korea LCI DB (South Korea) |
Iron wire | Kg | The Korea LCI DB (South Korea) |
The CO2 emissions of transportation occur as individual components of the voided slabs are delivered to the manufacturing site. The number of vehicles, distance from the origin to the manufacturing plant, and fuel efficiency of each vehicle were considered when assessing CO2 emissions. The speed of the vehicles and the traffic situation were not considered. Equation (
Here, the quantity of CO2 emitted from transporting a unit of manufactured voided slab is CO2
The CO2 emission of voided slabs from the manufacturing process is the sum of consumed energy and unloading of raw materials and manufacturing of reinforcing bars and HDPE. The energy consumption of the manufacturing process was estimated based on the process of manufacturing which was divided into loading, storage, transportation, and mixing. The types of energy sources used in the voided slabs manufacturing process were electricity, diesel, liquefied natural gas (LNG), and water. The calculation of CO2 emissions during the manufacturing is shown in the following equation:
The flexural test results of the voided slab specimens are summarised in Table
Test results of flexural performance.
Specimens | Initial cracking | Yield load | Ultimate load | |||
---|---|---|---|---|---|---|
Load (kN) | Displacement (mm) | Load (kN) | Displacement (mm) | Load (kN) | Displacement (mm) | |
S1 | 14.4 | 1.5 | 53.9 | 24.3 | 73.3 | 216.4 |
S2 | 9.2 | 1.0 | 60.7 | 34.0 | 70.3 | 141.8 |
S3 | 5.6 | 0.7 | 46.0 | 29.2 | 54.5 | 175.1 |
S4 | 5.4 | 0.7 | 38.1 | 40.4 | 45.6 | 213.2 |
Load-displacement curves of specimens. (a) S1 specimen. (b) S2 specimen. (c) S3 specimen. (d) S4 specimen.
The comparison between the expected value of cracking strength and yield strength, calculated in accordance with the Structural Concrete Design Code and Commentary in South Korea [
Comparison between anticipated and actual values.
Specimen | A | B | C | C/A | C/B |
---|---|---|---|---|---|
Anticipated strength (kN) | Anticipated strength with anchoring (kN) | Ultimate strength (kN) | |||
S1 | 50.8 | — | 73.3 | 1.443 | — |
S2 | 52.5 | 59.6 | 70.3 | 1.339 | 1.180 |
S3 | 53.1 | 57.5 | 54.5 | 1.026 | 0.948 |
S4 | 38.5 | — | 45.6 | 1.184 | — |
The CO2 emissions of each component of the voided slab were 334.0749 kg-CO2/m3 for concrete, 2.5266 kg-CO2/kg for D10 reinforcing bars, 2.4858 kg-CO2/kg for D13 reinforcing bars, 2.034 kg-CO2/kg for wires, and 2.06 8kg-CO2/kg for HDPE (Tables
CO2 emission of concrete in product stage.
Items | Raw materials | Transportation | |||||
---|---|---|---|---|---|---|---|
A | B | C = A |
D | E | F | G | |
kg/unit | kg-CO2/kg | kg-CO2/unit | Location | km | kg-CO2/kg-km | kg-CO2/kg | |
|
|||||||
Ordinary Portland cement | 333.000 | 0.948000 | 315.6840 | Damyang | 277.00 | 6.06 |
5.590 |
Fine aggregate | 821.000 | 0.000152 | 0.124792 | Hadong | 322.00 | 1.16 |
3.067 |
Coarse aggregate | 1086.00 | 0.007740 | 8.405640 | Gyeonggi, Gwangju | 37.60 | 1.16 |
0.474 |
Chemical admixture | 3.330 | 0.002050 | 0.006826 | Gyeonggi, Anseong | 66.000 | 1.16 |
0.003 |
Sub sum | 324.232274 | 9.133 | |||||
Manufacturing | 1 m3 | 0.71 kg-CO2/FU | 0.710000 | ||||
Total | 334.078975 |
CO2 emission of voided slabs components in product stage.
Items | CO2 emission | Transportation | |||
---|---|---|---|---|---|
A | B | C | D = A |
||
Location | Distance | Factor | |||
HDPE | 1.875 | Shihwa | 70.40 | 2.735 |
0.193 |
CO2 emission + D | 2.068 | ||||
Wire | 1.732 | Dangjin | 110.54 | 2.735 |
0.302 |
CO2 emission + D | 2.034 | ||||
Rebar | 2.5266 | Dangjin | 110.54 | 2.735 |
0.302 |
D10 | CO2 emission + D | 2.8286 | |||
Rebar | 2.4858 | Dangjin | 110.54 | 2.735 |
0.302 |
D13 | CO2 emission + D | 2.788 |
The CO2 emissions of specimens (unit: kg-CO2/FU).
Ingredients of the voided slab | S1 | S2 | S3 | S4 |
---|---|---|---|---|
Concrete | ||||
Volume (m3) | 0.21 | 0.17 | 0.16 | 0.17 |
CO2 emissions (kg-CO2) | 69.59 | 57.66 | 53.36 | 55.89 |
|
||||
Rebars | ||||
D10 | ||||
Weight (kg) | 8.38 | 6.71 | 8.38 | 8.38 |
CO2 emissions (kg-CO2) | 23.72 | 18.97 | 23.72 | 23.72 |
D13 | ||||
Weight (kg) | 4.97 | 3.97 | 4.97 | 4.97 |
CO2 emissions (kg-CO2) | 13.85 | 11.08 | 13.85 | 13.85 |
|
||||
Wires | ||||
Weight (kg) | — | 19.86 | 7.76 | — |
CO2 emissions (kg-CO2) | — | 34.42 | 13.44 | — |
HDPE | ||||
Weight (kg) | — | 3.90 | 3.98 | — |
CO2 emissions (kg-CO2) | — | 7.31 | 7.45 | — |
|
||||
Total emissions (kg-CO2/FU) | 107.15 | 129.44 | 111.82 | 93.45 |
Total CO2 emissions with anchoring.
The specimen S2 was made of voided slab of approximately 16% hollowness ratio. In this specimen, 64 spherical void formers of 100 mm diameter were used to fill the hollow sections of the slab. The volume of concrete used was 0.17 m3, approximately 0.04 m3 less than that used in the reference model S1. Reinforcing bars applied in the S2 specimen were 6.71 kg of D10 and 3.97 kg of D13. In addition, 19.86 kg of wires (
The specimen S3 was voided slab model with about a 22% hollowness ratio, including 25 oblate shape void formers. The oblate shape void formers were 170 mm in length and 110 mm in height, and each component had a volume of 1,903,594 mm3. The concrete applied in the S3 specimen was 0.16 m3, and the quantity of CO2 emitted from this concrete was 53.36 kg-CO2. The rebars used in the S3 specimen were 8.38 kg for D10 and 4.97 kg for D13. The CO2 emissions of each type of reinforcing bars in specimen S3 were 23.72 kg-CO2 for D10 and 13.85 kg-CO2 for D13. The total CO2 emissions from deformed bars were 37.57 kg-CO2. Furthermore, 7.76 kg of wires (
The S4 specimen was made of 0.17 m3 of concrete which was comparable with the amount of concrete used in the S2 and S3 specimens. It was designed to compare the structural performance with the voided slabs. The mass of the reinforcing bars applied to the S4 specimen was 8.38 kg for D10 and 4.97 kg for D13. The carbon dioxide emitted by the S4 specimen was 93.45 kg-CO2/FU.
CO2 emission variation based on hollowness ratio.
The first specimen, S1, was an ordinary slab which is normally applied to reinforced concrete buildings in South Korea. The depth of this slab was 210 mm, which is the minimum depth of slab for prevention of noise complaints between floors of apartment housing in South Korea. The emitted carbon dioxide from the S1 specimen was 107.15 kg-CO2/FU. Concrete was the highest carbon dioxide emitter among all of the components of this reinforced concrete slab. Reinforcing bars were the second largest source of CO2 emissions in this specimen.
The S2 and S3 models were voided slab specimens that showed 129.44 kg-CO2/FU and 111.82 kg kg-CO2/FU, respectively. The S2 specimen had a 15.96% ratio of hollowness to the total volume of concrete. The hollow section of the slab was filled with void formers made from HDPE. The void formers were spherical, with 100 mm diameter, and 64 balls were inserted in the specimen. Twenty-five oblate shape void formers were buried in the S3 slab to create a ratio of 22.66% hollowness to concrete. The size of each component was 170 mm in length and 110 mm in height. All the void formers in both specimens were anchored by wires to prevent buoyancy or separation between reinforcing bars during the placing and curing of concrete. The amount of concrete used in the specimens was 0.17 m3 for S2 0.16 m3 for S3, respectively. Based on the reduction of concrete use in the slab, approximately 12 and 16 kg-CO2/FU less CO2 were emitted from the S2 and S3 specimens, respectively. As the CO2 emissions from concrete decreased as the concrete use was reduced, the emissions from concrete also decreased in response to this tendency. However, the total CO2 emissions from specimen S2 and S3 were higher than those in the S1 specimen. The S2 specimen showed about 21% increase of carbon dioxide emissions compared to the S1 specimen.
In addition, the S3 voided slab indicated approximately a 4% rise of CO2 emissions over the S1 specimen. The reason for the increase of CO2 emissions in the voided slabs might be the application of wires for anchoring the void formers. It is unavoidable for voided slabs to utilise anchoring materials such as wires, deck-plate, and wire mesh; for example, in this research, wires were utilised for the anchoring component of the voided slab. The amount of wires applied in both voided slab specimens was quite large: 19.86 kg for S2 and 7.76 kg for S2. Based on these data, the emitted carbon dioxide from the S2 and S3 specimens was 34.42 kg-CO2 and 13.44 kg-CO2, respectively. These results show that about 27 and 12% more carbon dioxide was emitted from anchoring wires, and anchoring wires accounted for a significant proportion of CO2 emissions in the voided slab (Figure
CO2 emissions reduction ratio by the shape of void former materials.
Furthermore, the CO2 emissions of the voided slab exhibited different features depending upon the shape of the void formers. Two different shapes of materials were applied to fill the hollow section of the concrete slab in this research. The S2 specimen, into which spherical void formers were inserted, showed higher CO2 emissions than the S3 specimen, into which oblate shape ones were inserted. Oblate shape materials emitted approximately 18 kg-CO2/FU less carbon dioxide than spherical ones. The reason for higher emissions of CO2 from spherical materials might be that the individual spherical void formers were smaller than the oblate shape ones. As the size of each former was smaller, more anchoring wires may have been required to harness the formers in the voided slabs overall. As shown in Table
Total CO2 emissions without anchoring.
The CO2 emissions of S2, S3, and S4 specimens were 95.0241, 98.3753, and 93.4536 kg-CO2/FU, respectively. The reduced amount of CO2 compared to the reference model S1 was 12.1265 kg-CO2/FU for S2, 8.7754 kg-CO2/FU for S3, and 13.6971 kg-CO2/FU for S4 (Figure
CO2 variation by void ratio without anchoring.
In summary, the CO2 emissions of higher void ratios were lower in this research. The reason for this tendency is the application of more anchoring materials, since the size of the individual components in the higher hollowness ratio was smaller than in the lower hollowness ratio. Thus, more utilisation of materials led to the occurrence of more carbon dioxide emissions in this research.
It is generally known that voided slabs are environmentally friendly and beneficial for a sustainable environment. In addition, utilisation of less concrete would lead to lower energy consumption and carbon dioxide emissions from voided slabs. However, the carbon dioxide emissions of voided slabs showed higher CO2 emissions compared to normal reinforced concrete slabs in this research.
The structural performance of the voided slab proved to be comparable to a normal reinforced concrete slab. In this study, the flexural performance of the voided slab was similar or slightly higher than the normal reinforced concrete slab. From a structural perspective, the voided slab would be appropriate to apply to long span structures due to its light weight, as well as to prevent noise complaints between floors in apartment housing in South Korea. Moreover, the application of the voided slab would make it possible to remove beams in reinforced concrete structure. The voided slab is thus an alternative to lightweight materials.
When comparing the carbon dioxide emissions of normal reinforced concrete slab and voided slab, the CO2 emissions of the voided slab were higher than those of the normal reinforced concrete slab, considering all the materials used in the voided slab such as concrete, void former materials, reinforcing bars, and wires for anchoring. In particular, the amount of anchoring wires to manage buoyancy occupied a considerable proportion of the total amount of carbon dioxide emissions, approximately 25% in each specimen. Based on these results, replacing or reducing current anchoring materials to more environmentally friendly ones should be considered to lower the amount of CO2 emissions in the voided slab.
Additionally, the variation of carbon dioxide emissions by the application of different types of void formers (i.e., spherical and oblate shape) was studied. In this study, oblate shape void formers emitted 4.35% in S2 and 20.80% in S3 additional carbon dioxide. Since the overall size of the spherical type was smaller than the oblate one, more wires were used to anchor the void formers to the reinforcing bars. The anchoring wires were one of the main sources of CO2 emissions in the voided slab.
It is normally considered that plastics or other petrochemical products might be less effective than other materials for environmental performance. The CO2 emission per unit weight of petrochemical material (i.e., HDPE in this study, which was used for void materials) was higher than other components of voided slab. However, the amount of HDPE applied in the specimens was smaller than other materials such as concrete, reinforcing bars, and wires. Thus, the influence of carbon dioxide emissions of HDPE in the voided slab was not significant compared to other materials.
In order for the voided slab to meet both environmental and structural performance requirements, current anchoring methods using wires, deck-plate, cable ties, and so forth should be replaced with more eco-friendly or recycled materials. Additionally, other methods to connect void former materials directly to reinforcing bars should be developed, in order to reduce the application of anchoring materials in the voided slab. Based on such research approaches, further studies should consider the optimal design of the voided slab with using optimal materials which would meet both structural and environmental performance.
This research was limited to the slab component of a building, but further studies should be carried out to confirm the reduction of construction materials and variation of carbon dioxide emissions in the entire building through application of the voided slab. Although this study only focused on the case of South Korean construction industry, this study would also provide a useful reference for assessing the emission of CO2 from the voided slab to other countries. It is considered that further studies should be also carried out to validate the cases of voided slab in other countries.
Moreover, this study only dealt with the emission of carbon dioxide for the voided slab during the manufacturing phase. However, it is recently considered that the significant impact of operational carbon of a building’s life cycle. As for this reason, further research should be conducted to assess the influence of operational carbon for the voided slab system compared to the normal reinforced concrete structure. Along with the consideration of operational carbon for the voided slab system, other factors such as geographical aspect which would impact the transportation distance and energy consumptions.
The purpose of this research was to confirm the flexural performance as well as the variation of carbon dioxide emissions from voided slabs. The flexural performance was assessed based on the variables of the depth of slab, types of the void former materials, and hollowness ratio. In addition, CO2 emission comparisons were also conducted considering the hollowness ratio and types of void formers compared to the normal reinforced concrete slab. The system boundary of the voided slab was limited to the production stage, known as cradle to gate, in accordance with ISO 14044 [
The structural performance of the voided slab was similar or slightly better than the normal reinforced concrete slab. The yield strength of specimens increased approximately 10–30% over the anticipated yield strength. Based on this result, it is considered that the voided slab has sufficiently good structural performance and would be beneficial to planning in buildings.
The results of assessment of the CO2 emissions showed that the voided slabs emitted more carbon dioxide compared to the normal reinforced concrete slab, regardless of the hollowness ratio and the types of void formers. In this research, the slab with a higher hollowness ratio emitted less carbon dioxide emissions than that with a lower hollowness ratio. Additionally, the voided slab would require additional materials to anchor the void formers to the reinforcing bars in order to prevent buoyancy of the formers during the placing and curing of concrete.
In general, a number of studies have shown that voided slabs are beneficial both structurally and environmentally. However, the results of this study showed that voided slabs emitted more carbon dioxide compared to normal reinforced concrete slabs. The main source of this additional CO2 from the voided slab might be the materials anchoring the void formers to the reinforcing bars. In this research, wires were used to fix the void formers, and they were a reasonably large source of CO2 emissions. In order for the voided slab to become a more eco-friendly and sustainable material in buildings, new anchoring methods such as use of recycled materials, new void formers without anchoring, or other eco-friendly materials should be developed to reduce the emission of CO2.
The data used to support the findings of this study are included within the article. In addition, some of the data used in this study are supported by the references mentioned in the article. If you have any questions regarding the data, the data to support of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03032279).