Comparative Life Cycle Assessment of Concrete Elements: Cast In Situ, Semi-Precast and Precast

Abstract

The construction industry is moving forward with the aim to maintain process sustainability. In this aspect, the role of life cycle assessment is essential in determining the appropriate material or method. Hence, this study aims to compare the environmental impacts of a concrete frame office structure, built with different construction techniques, including cast in situ, precast, and semi-precast methods of construction. The analysis adopts the CML 2001 characterisation method to present and compare the environmental impacts from cradle to gate and, therefore, the extraction and production of raw materials, manufacturing, and construction phases of these technologies. Results show that the total energy consumption for the three methods of construction is similar, with a variation of less than 5%. The highest energy consumption phase is associated with the extraction and production of cement. In fact, the precast and semi-precast methods were found to have almost 30% higher global warming potential (CO₂ equation) than the cast in situ method per functional unit due to the utilisation of higher cement content in the mix. Hence, the environmental impacts associated with each phase will help the concrete construction industry to develop and improve its efficiency while adopting more sustainable measures.

1. Introduction

The construction industry is reforming the globe by fulfilling society’s demands [1]. On the other hand, the major environmental challenge is also triggered by this industry due to the construction materials production and the operation of buildings [2]. The carbon emissions (CO₂) from buildings are much greater than energy and contribute up to 39% in total [3]. As anticipated, if conventional construction methods are pursued for the long term, it will enormously shatter the natural environment [4]. At present, one of the primary materials used for the construction of modern structures is concrete. The main constituents of concrete are Portland cement, limestone or granite aggregate, sand, and water [5]. Only the cement itself is responsible for 5–7% of total carbon emissions [6]. Other cementitious materials, such as pulverised fuel ash and other admixtures, are usually added to achieve the required properties of the mix [7]. With time, modernisation occurred even in concrete formation, and it is no longer just a mixture of basic materials [8].

There are different ways in which concrete buildings can be constructed. The more conventional way is known as cast in situ construction, where fresh concrete is brought to the site, poured into forms, compacted, and cured in place [9]. Nonetheless, this method leads to a high amount of waste with higher carbon emissions [10]. On-site construction is a source of carbon emissions, which are contributed by the heavy machinery and materials transportation. Also, waste treatment management and embodied carbon in temporary materials lead to carbon emissions, making the construction industry terrible for ruining the natural environment [11]. An alternative type of construction is precast or modular construction. It involves the pre-manufacturing of different concrete elements in a plant [12], under controlled conditions, following the same standards, codes, and building laws as orthodox built projects; however, it is less time-consuming in comparison to the conventional construction method. In today’s time, the facilities are not only simplified but also multifaceted and can be built with modular construction. This process is not only a fast way of constructing, but also more cost-efficient than site construction. In addition, there are no weather restrictions to modular construction and, with a proper design of the structural system, it can uphold the seismic load as well [13]. The process of precast construction includes casting individual members into moulds, compacted, cured, and stored until sufficient strength for lifting is obtained. These are then transported to the building site, lifted, and placed into their final position [14]. Precast concrete construction is gaining popularity, especially in medium to large projects, where construction time has to be minimised as much as possible to improve efficiency whilst reducing costs [15]. In smaller residential projects, the cast in situ construction method is mostly used. However, there is also a common tendency to combine both cast in situ and precast elements, such as precast prestressed slabs supported on cast in situ beams, columns, or concrete masonry unit walls [9].

These two construction techniques result in different environmental impacts due to several variations. Such variables are the quality and quantity of concrete used in each element [16,17]. Precast elements use a higher grade of concrete, to which a higher amount of cement is used to improve the compressive strength, resulting in higher carbon emissions per cubic metre [18]. However, when a reinforced cast in situ element and a precast prestressed element of the same design strength are compared, a lower volume of concrete is needed [19]. Concrete with a higher grade has better durability than one with a lower grade; hence, less maintenance is needed throughout the service life of the building [20], although higher strength classes may require more reinforcement and, therefore, embodied carbon. Another significant variable is the quantity and quality of steel. When compared to a cast in situ reinforced concrete element with the same design strength, a precast prestressed element requires a lower quantity of steel to produce. Although there is a significant amount of material savings, when considering the perspective of a life cycle assessment (LCA), one has to consider the amount of energy needed to produce the required force of prestressing and the resulting CO₂ emissions [21,22,23]. Other variables include the mould, formwork, and falsework preparations. Precast element moulds are usually made of steel, while the traditional formwork used for cast in situ concrete is usually made of timber such as plywood. Steel has a much higher embodied carbon dioxide (ECO₂) than timber. On the other hand, steel moulds can be reused more than 400 times, while timber formwork must be replaced after being reused between five and seven times. Due to these distinctions, the environmental impact of both scenarios must go through a thorough analysis of the different stages involved in both construction techniques in order to be able to observe the different outcomes [24,25,26].

LCA concerning the comparison of steel and concrete building frame structures has been performed by Oladazimi, Mansour and Hosseinijou [27], Seyedabadi, Karrabi [28], and Balasbaneh and Ramli [29]. The LCA technique began developing due to growing public environmental awareness and increasing concern about the exploitation and degradation of natural resources [30]. Due to this fact, industries and businesses started taking into consideration and investigating how their activities are affecting the environment and determining possible solutions to minimise their impact. This was tackled by implementing pollution prevention measures and by adopting environmental management systems through the LCA approach [31]. The International Organisation for Standardisation (ISO) states that LCA is the “compilation and evaluation of the inputs, outputs and the potential environmental impacts of a product system throughout its lifetime” (ISO 14040, 2006). In LCA, sustainability considerations are not only based on a finished product, but on the impact of all the stages throughout its entire lifetime [2,32].

Table 1. Comparison of three construction methods.

Criteria/Requirement	Cast In Situ	Semi-Precast	Precast
Concrete Grade	Low–Medium	Medium–High	High
Cement Content	Low–Medium	Medium–High	High
Steel Reinforcement Usage	High	Medium	Low
Formwork Demand	High	Medium	Low
Labour Intensity	High	Medium	Low
On-site Construction Time	Long	Medium	Short
Durability	Medium	Medium–High	High
Transportation	Low	Medium	High

shows the key differences between cast in situ, semi-precast, and precast construction methods.

While recent studies focus on ReCiPe-based works related to modular concrete and low-carbon cement, the current study relies on conventional construction practices. For this reason, CML 2001 was employed to provide a regional and comparable baseline. Although an LCA approach enables decision-makers to identify which construction processes or materials have lower environmental impacts, the existing literature offers limited cradle-to-gate comparisons of cast in situ, precast, and semi-precast concrete systems using conventional mixes. To address this gap, the current study develops an LCA methodology for a reinforced concrete structure and compares the environmental impacts of these three construction techniques. The analysis highlights which of the technologies exhibits higher environmental burdens under typical construction environments. Given this context, the study investigates the environmental implications of conventional cast in situ, precast, and semi-precast concrete construction methods.

2. Methodology

2.1. Site Selection

An office building was designed to be used as a basis for comparison. The building consists of a five-storey concrete frame structure designed in compliance with Eurocode 2 (BS EN 1992). A considerable part of the study consisted of calculating the quantities of different materials, which are required for the construction, and an extensive bill of quantities (BOQ) was generated for the building location in Malta. The reference building is a multi-storey 5-floor frame structure. Three concrete construction techniques were compared in this assessment. These are the cast in situ, semi-precast, and precast construction methods. In the case of the cast in situ structure and semi-precast structure, the building consists of 4 × 4 bays, that is, 5 multi-storey frames with a primary beam span of 6.4 m centres and spacing of 4.2 m centres between frames.

On the other hand, with the introduction of prestressed hollow concrete slabs, a higher span could be achieved for the same load requirements, thus reducing the number of beams and columns. The layout for the precast prestressed slabs building consists of 4 multi-storey frames with a primary beam span of 8.5 m centres and a spacing of 8.4 m between frames. Materials used for construction are almost all imported from Europe with the exception of aggregate, which is still extracted from local quarries. However, this local resource is being depleted at a fast pace [33]. Due to this fact, it is assumed that all materials used in this study are imported, with the exception of water, which is stored in reservoirs from rainwater. The three-dimensional views of both cast in situ/semi-precast and precast systems are shown in Figure 1.

The Life Cycle Inventory (LCI) that deals with the production of concrete is compiled in the United States. However, it is implemented in this study because it is one of the most recent and most reliable inventories to date of this study. The inventories of all the other materials and processes are based on European data, including emission data from local electricity generation. The functional unit adopted in this LCA is a 1-metre-square (1 m²) floor area of a concrete structure with a service life of 50 years. This unit is the basis for comparison between the distinct construction methods. A direct comparison between the quantities of materials was required for each construction method to satisfy the goal of this study with the chosen functional unit. For the purpose of this analysis, stairwells, lift cores, and ramps were not taken into consideration as the study is solely based on the primary members comprising slabs, beams, columns, and foundations.

2.2. Data Collection

The goal of the LCA presented in this study was to provide information on different concrete construction methods. LCA study requires primary data on the consumption of energy and materials, along with their emissions resulting from the processes involved in the system. Data requirements for this study include the types and quantity of material used for the production of a concrete building, the production processes of concrete elements, the lifting and modes of transport to the desired site, and the lifting requirements on site. Ideally, all data concerning all the inputs and outputs is compiled; however, in practice, only the most significant data is compiled, which is subject to availability, the time frame for data collection, and financial reasons. Further details on data collection are provided in

Table 2. Collected Data Type and Source.

S. No	Data Type	Source
1	Raw materials and production of concrete	LCI of Portland Cement Concrete [34]
2	Raw materials and production of steel and timber	Ecoinvent v2.0 [35], IDEMAT 2001 [36]
3	Production of precast elements	Through personal communication and direct contact with the manufacturers of the equipment used in the process
4	Transportation, lifting and construction	Existing Literature [37]
5	Electricity generation and emissions	Enemalta Annual Report 2009 and Financial Statements 2008 [38], and from the National Greenhouse Gas Emissions Inventory Report for Malta 1990–2008 [39]

.

2.3. LCA Method and System Boundary

SimaPro 7.1 was the LCA software tool used for the modelling and calculation of LCI data and environmental impacts. The LCI was compiled from published LCI reports, industry manufacturers and extensive databases which are readily available in SimaPro 7.1, including Ecoinvent v.2 and IDEMAT 2001. The chosen method for carrying out the impact assessment is CML 2 Baseline 2001, as it provides nine impact categories relevant to this study, including Abiotic depletion; acidification; eutrophication; global warming (GWP 100); human toxicity; freshwater aquatic ecotoxicity; marine aquatic ecotoxicity; terrestrial ecotoxicity; and photochemical oxidation. The characterisation factor was the primary measure for the impact assessment comparison in this study. The impact assessment was carried out based on CML, which ensures methodological consistency; however, the uncertainties remain due to the dataset’s generic nature. The EN 15804-based impact assessment was not considered as it only focuses on the core impact categories, while CML 2001 provide a broader environmental profile.

The LCA was conducted in accordance with the specified guidelines by the International Organisation for Standardisation in ISO 14040, Environmental Management—Life Cycle Assessment—Principles and Framework. The system boundary, illustrated in Figure 2, encompasses all processes from cradle to gate, including concrete production, plant operations, site activities, and transportation between the plant and the construction site. The LCA concentrates on the processing and construction phases of the whole cycle, whilst the upstream profiles, namely, the stages prior to the plant operations, which include the acquisition of raw materials and production of cement, aggregate, water, steel reinforcement, moulds, formwork, falsework, and electricity, are imported directly into the system boundary.

3. Results

3.1. Material Quantification

Each individual element was designed in accordance with BS EN 1992. The total volume of concrete was calculated by multiplying the cross-sectional area by the length of each member. This was then multiplied by the total number of identical members to acquire the total volume of concrete. The BOQ used for the inventory phase is presented in

Table 3. Bill of Quantities.

Element	Cast In Situ Structure	Semi-Precast Structure	Precast Prestressed Slab Structure
	(Ton)	(Ton)	(Ton)
Slabs
Concrete	916	Cast in situ: 531.8 Precast: 277.9	875.9
Steel	15.1	13.1	15.3
Polystyrene Blocks	N/A	0.86	N/A
Beams
Concrete	433.9	416.5	318
Steel	13.8	9.57	7.7
Columns
Concrete	214.2	205.6	109.8
Steel	7.6	5.8	5.4
Foundations
Concrete	34.3	32.9	21.9
Steel	1.4	1.4	0.274
Total
Concrete	1598.4	1467.1	1325
Steel	37.9	29.9	28.6

. For this purpose, the element of prestressing was only considered in precast prestressed slabs. It was also estimated that beams and columns require the same quantities of concrete for both cast in situ and semi-precast buildings due to the fact that the loads and spans are identical—the only difference being their mix.

Figure 3 and Figure 4 represent the quantities of concrete and steel subdivided into the structural elements. It can be observed that slabs are the most concrete-consuming structural elements in all three construction methods, followed by beams, columns, and to a lesser extent, foundations. A similar interpretation can be made with respect to steel reinforcement quantification, where a significantly smaller use of material is needed for beams, columns, and foundations for the semi-precast and precast structures.

3.2. Life Cycle Inventory

3.2.1. Material Inputs

Material inputs for the construction of three distinct office buildings were obtained from the BOQ. Other materials needed during the production and construction processes, such as moulds, were either calculated from the drawings or input directly with the right proportion from other data sources. Each material input into the LCI had its own upstream profile, which is imported directly into the system process of SimaPro. Most of these profiles were readily available in the databases of SimaPro, where such a tool is used both as a data source and as a modelling tool for the environmental impacts. Where some materials were not available, such as prestressing cables, a new process was created to form the material according to its chemical composition. Other materials, such as releasing agents, which constituted a relatively small fraction of the total mass, due to unavailability, were replaced by similar materials. A list of all input materials is presented in

Table 4. Material Description.

Material	Cast In Situ	Semi-Precast	Precast
	(Ton)	(Ton)	(Ton)
Concrete
Ready-Mix Concrete	1598.4	564.7	0
Precast Concrete	0	900	1325
Metals
Reinforcing Steel	37.9	37.8	13.1
Prestressing cables	0	0	15.3
Falsework *	0.0223	0.00107	0.00057
Forms *	0.184	1.24	0.368
Polystyrene
High-Density Polystyrene Blocks	0	0.86	0
Timber
Formwork *	38	0	0
Oils
Lubricating Oil	0.084	0.060	0.061

* Values are divided by the number of repeated use; i.e., 400 for steel and 5 for timber.

.

3.2.2. Electricity Inputs

Electricity inputs for plant and construction processes are presented in

Table 5. Electricity Description.

Plant Operations	Cast In Situ				Semi-Precast				Precast
kWh/ton kg Product	S	B	C	F	S	B	C	F	S	B	C	F
Universal Extruder	-	-	-	-	-	-	-	-	1.3	-	-	-
Crosscut Saw	-	-	-	-	-	-	-	-	0.24	-	-	-
Dispatcher	-	-	-	-	-	-	-	-	0.7	-	-	-
Stressing Pump	-	-	-	-	-	-	-	-	0.16	-	-	-
Feeding Gantry	-	-	-	-	-	-	-	-	0.43	-	-	-
Overhead Cranes	-	-	-	-	8.61	1.74	3.62	-	3.16	1.74	3.62	-
Vibrating Tables	-	-	-	-	0.53	1.53	1.53	-	-	1.53	1.53	-
Site Operations
Tower Crane Lifting	3.2	3.2	3.2	3.2	3.04	2.88	2.88	3.2	2.88	2.88	2.88	3.2
Onsite Vibration	1.6	1.55	1.55	1.55	1.01	-	-	1.55	-	-	-	1.55
Total: (kWH/ton) *	4.8	4.75	4.75	4.75	13.2	6.15	8.03	4.75	8.87	6.15	8.03	4.75

Where S: Slabs, B: Beams, C: Columns, F: Foundations. * Actual values may differ slightly due to rounding.

. Values were input into the model as KWH/ton of product. These were then multiplied by their respective total mass to determine the actual number of kilowatt-hours needed for the entire process, as shown in Figure 5. Electricity input for the batching plant mixer is not included in the table below, as it is already included in the upstream profile of concrete.

3.2.3. Transportation Inputs

An arbitrary distance of 5 km was chosen as a starting point for transportation comparison. Although this situation did not give the actual definite values since the buildings were not constructed, it still gave a consistent end result from the estimations that were undertaken. Estimates with reference to the amount of material that can be carried per voyage have been performed for each individual element. Transportation values were input into the model as km/ton. These were then multiplied by their respective total mass to determine the actual number of kilometres needed for the entire process, as shown in

Table 6. Transportation Description.

Element km	Cast In Situ	Semi-Precast	Precast
Slabs	304	569.8	316
Beams	250.36	149.8	133.6
Columns	212.8	102.96	54.9
Foundations	21.36	21.36	13.7
Tower Crane	10	10	10
Concrete Vibrator	150	50	-
Total	948.52	903.92	528.2

Note: Values shown above are imported directly from the model and are hence exclusive of any possible differences attributable to rounding.

.

3.3. Life Cycle Impact Assessment

The data provided for the impact categories corresponds to a functional unit of 1 m² of floor area. According to ISO 14044, the primary focus during the LCIA phase is the process of characterisation and the factors are established to assess the relative contribution of LCI results to the outcomes within each impact category.

3.3.1. Global Warming Potential

The global warming potential (GWP) of the construction methods is shown in Figure 6. GWP is characterised by the extraction and production of concrete in the three scenarios, followed by that of reinforcing steel. Total CO₂ emissions range from 99.9 kg/m² for cast in situ, 140 kg/m² for semi-precast, to 144.5 kg/m² for precast construction. Impacts for semi-precast and precast construction are similar, with an additional contribution of almost 45% when compared to cast in situ construction.

3.3.2. Abiotic Depletion

Abiotic depletion, as shown in Figure 7, was characterised by the use of steel and concrete in the three scenarios. Reinforcing steel was the main contributor (61.9%) to the cast in situ construction, followed by concrete (26.7%) and the process of plywood (10.7%), whilst remaining processes added up to 1.6%. Semi-precast was balanced between reinforcing steel (50.8%) and concrete (40.4%), followed by polystyrene (4.7%), whilst the remaining processes added up to 4.1%. Precast construction impacts were shared between concrete (49.8%), prestressing steel cables (23.5%) and reinforcing steel (21.4%), whilst the remaining processes add up to 9.7%. Cast in situ construction contributes to abiotic depletion of a total of 0.251 kg Sb equation/m², followed by semi-precast, with 0.191 kg Sb equation/m², and by precast construction with 0.141 kg Sb equation/m².

3.3.3. Acidification

Acidification was characterised by a number of processes, with concrete being the major contributor, followed by steel and electricity. As seen in Figure 8, the highest acidification emissions were related to precast construction, having a value of 0.486 kg SO₂ equation/m², followed by semi-precast, emitting 0.46 kg SO₂ equation/m², whilst cast in situ construction releases 0.364 kg SO₂ equation/m².

3.3.4. Eutrophication

Semi-precast and precast constructions contributed to similar impact levels of eutrophication as shown in Figure 9, releasing a total of 0.0546 kg PO₄ equation/m² and 0.0542 kg PO₄ equation/m², respectively. In both cases, concrete, being the highest contributor, shared 68.9% and 75%, respectively, followed by steel (reinforcing steel and/or prestressing steel) with a total share of 24.9% and 19%, respectively. Cast in situ construction released 0.0495 kg PO₄ equation/m², which was about 10% less than the other two methods. Concrete shared 53.8%, followed by reinforcing steel at 35.3% and plywood at 7.5%. In all construction methods, electricity and the remaining processes added up to less than 4% of the total releases.

3.3.5. Human Toxicity

1,4-dichlorobenzene (DB) is an aromatic compound and, in LCA, it is used as a reference substance due to its well-studied toxic effects on the environment and human well-being. The human toxicity impact category, Figure 10, showed that once more, semi-precast and precast methods had similar values, with a total contribution of 43.2 kg 1, 4 DB equation and 42.7 kg 1, 4 DB equation/m². In semi-precast construction, concrete (ready-mix and precast mix) shared 77.4% of the total impact, followed by reinforcing steel at 19.4%. For precast construction, concrete shared 84.5%, followed by reinforcing and prestressing steel, adding up to 11.2%. On the other hand, cast in situ construction released 36.9 kg PO₄ equation/m², which was about 15% less than the other methods. Concrete shared 63.3%, followed by reinforcing steel, 29.3%, and plywood, 6.02%.

3.3.6. Freshwater Aquatic Ecotoxicity

This impact category was dominated by steel products in three scenarios, as it is illustrated in Figure 11. The cast in situ and semi-precast methods were dominated by reinforcing steel, which shared more than 95% of the total impact. Cast in situ construction released 11.1 kg 1, 4 DB equation/m^2, while semi-precast construction releases almost 8.5 kg 1, 4 DB equation/m². On the other hand, precast released less than half (43%) of the emissions of cast in situ, totalling a value of 4.78 kg 1, 4 DB equation/m². Regarding emissions, 62.7% were generated from reinforcing steel, whilst 32.5% were associated with the process of wire drawing of prestressing steel. The reinforcing steel dominates freshwater aquatic ecotoxicity due to the metal emissions and other toxic substances. This is why the impacts of this vary with the mode of construction.

3.3.7. Marine Aquatic Ecotoxicity

Figure 12 illustrates marine ecotoxicity impacts and, proportionally, it is identical to that of freshwater aquatic ecotoxicity for the three scenarios. Cast in situ construction releases 10,700 kg 1, 4 DB equation/m², semi-precast releases 7880 kg 1, 4 DB equation/m², whilst precast releases 4240 kg 1, 4 DB equation/m². The extraction of raw materials used in steel emits metals and chemical pollutants, which have an adverse impact on marine life. That is why the cast in situ, semi-precast, and precast methods have the highest, moderate, and lowest marine ecotoxicity.

3.3.8. Terrestrial Ecotoxicity

As indicated in Figure 13, the highest terrestrial ecotoxicity impact for cast in situ and semi-precast construction was related to reinforcing steel with a share of 69.3% and 63.4%, respectively, whilst that of precast was almost shared between concrete and steel (reinforcing steel and prestressing steel) with a percentage of 56.4% and 43%. The total terrestrial ecotoxicity of cast in situ added up to 0.662 kg 1, 4 DB equation/m², followed by semi-precast, releasing 0.562 kg 1, 4 DB equation/m², whilst precast construction’s impact was almost 2/3 that of cast in situ, totalling a value of 0.386 kg 1, 4 DB equation/m².

3.3.9. Photochemical Oxidation

Photochemical oxidation impact values were quite similar for the three scenarios, as the variation between the highest and lowest impact is less than 8% as shown in Figure 14. Reinforcing steel was the highest contributor (56.9%) to cast in situ construction, followed by concrete (32.1%), and plywood (6.65%), whilst electricity and the remaining processes added up to 4.3%. The highest impacts for semi-precast construction were shared by concrete (ready-mix and precast mix) and reinforcing steel, with percentages of 45.8% and 42.9%, respectively, followed by electricity (6.29%), whilst remaining processes added up to 2.5%. Precast construction impacts related to concrete (47.4%), followed by prestressing steel (31%), reinforcing steel (15.1%), and electricity (4.1%), whereas the remaining processes added up to 2.4%. Precast construction contributed a total of 0.0258 kg C₂H₄ equation/m² to photochemical oxidation, followed by semi-precast, with 0.0246 kg C₂H₄ equation/m², and by cast in situ construction with 0.0239 kg C₂H₄ equation/m².

3.4. Energy Use

The total energy consumption required for cradle-to-gate for cast in situ, semi-precast, and precast constructions was almost identical, considering the uncertainties input into the model. A meagre percentage difference of almost 5% exists between the most and least consuming. The total embodied energy of each structure was found directly from the total energy consumption. The embodied energy of the cast in situ structure is 2.29 TJ; that of semi-precast is 2.34 TJ, whilst that of precast is 2.4 TJ. Figure 15 shows energy consumption per functional unit, i.e., per square metre floor area. The total energy presented below included energy requirements for all extraction, electricity, transportation, production, and construction processes involved prior to the use phase.

The primary energy source for all three construction methods was generated from non-renewable fossil fuels. In fact, these account for around 80% for cast in situ construction and approximately 95% for semi-precast and precast structures. A difference of 15%, resulting from renewable biomass energy, can be observed in the cast in situ method of construction due to the higher consumption of wood used to prepare formwork. Other energy sources include non-renewable nuclear energy, renewable water, and wind, solar, and geothermal energy. However, these result in less than 5% of the total energy sources, as shown in Figure 16.

4. Discussion

It is clearly visible that, with the introduction of precast elements, a considerable amount of material savings can be realised. The semi-precast building exhibits a reduction in concrete of 8.2% when compared to the cast in situ building. Furthermore, with the introduction of prestressed elements, additional material savings achieved as a reduction of 17.1% in concrete is exhibited. On the other hand, it is shown that, more or less, the same amount of steel is required for both cast in situ and semi-precast, while a reduction of almost 25% is exhibited in the totally precast structure. The current outcome provides a localised baseline instead of a direct comparison with international studies due to the fact that it is region-specific. The geographical factors influence the environmental impacts; however, the results are consistent with the existing body of literature related to LCA, where concrete and steel are the key contributors to environmental impacts.

Similar results have been quoted in various international LCA studies of concrete structures. For instance, Habert and Roussel [40] and Chen, Habert [41] mentioned in their studies that the cement and reinforcing steel utilisation boost the global warming potential and other impact categories, without any influence of the region. However, it is evident that the alternative construction systems save the materials utilisation when shifting from cast in situ to prefabricated construction, as supported by the literature [42,43]. These findings indicate that, though the current study analysis is regional-based, the outcome is similar to the other regional findings.

4.1. Life Cycle Interpretation

4.1.1. Global Aspects

Figure 17 illustrates the impact assessment of all categories using the characterisation indicator. Graph comparisons confirm that cast in situ accounts for higher impacts in abiotic depletion, freshwater aquatic ecotoxicity, marine aquatic ecotoxicity and terrestrial ecotoxicity impact categories. Semi-precast accounts for higher impacts in eutrophication and human toxicity, whereas precast construction results in higher impacts in acidification, global warming potential, and photochemical oxidation impact categories.

From the impact assessment, it is apparent that impact categories with higher values associated with the cast in situ method of construction relate to the quantity of reinforcing steel used in the building and, to a lesser extent, the higher use of timber formwork. This was because cast in situ construction uses a larger quantity of steel and timber when compared to the other methods. It was also noted that in these specific impact categories, the percentage difference is considerably high, approximately between 16 and 26% when compared to semi-precast and between 42 and 57% when compared to precast construction. The findings align with the other structural LCA studies in which cast in situ systems often show a greater impact in the categories which are linked to site-intensive operations [44,45].

It was also noted that in the remaining five impact categories, semi-precast and precast systems exhibited relatively similar values, with a maximum difference of 4%. Cast in situ construction exhibited a remarkable 15%, 25%, and 29% decrease in human toxicity, acidification and global warming potential impacts, respectively; the main factor being the use of precast concrete mix in both semi-precast and precast systems, whereas similar values were accounted for in the use of steel. Precast mix still exhibited higher impacts when compared to ready-mix concrete, even though a lesser amount of concrete was required in the other constructions. The study conducted by Passer, Lasvaux [46] has also reported the similarities between semi-precast and precast impact categories, where it was concluded that the hybrid systems inherit the advantages and drawbacks of both prefabricated and in situ processes.

Data was more evenly distributed for eutrophication and photochemical oxidation as the disparity between cast in situ and the other methods decreases. This was due to the fact of higher utilisation of timber formwork in cast in situ construction, which had to be replaced after a small number of reuses, resulting in an increase of 6.7% and 7.5% for photochemical oxidation and eutrophication impacts, respectively.

From a global point of view, it can be concluded that energy consumption was comparable for the three construction techniques, having precast construction consumption slightly higher than semi-precast (1.9%) and cast in situ (5%) methods. Precast and semi-precast resulted in a significantly higher impact on global warming potential (29%) when compared to cast in situ. Cast in situ had a significantly higher impact on abiotic resource depletion, water, and marine and land ecotoxicity, whereas semi-precast and precast methods have higher impacts in the remaining impact categories. These comparable conclusions are also supported by other LCA studies where precast systems exhibit lower on-site energy consumption but higher factory-level energy requirements [42,47].

4.1.2. Local Aspects

When taking only into account the local aspect, it is clear that semi-precast construction was the most energy-consuming method of construction (34.6 MJ/m²), while cast in situ and precast consume relatively similar amounts of energy (22.5 MJ/m²) as illustrated in Figure 18. This was also reflected in the global warming potential impact assessment, as it was proportional to the amount of energy consumed. The primary cause for semi-precast construction having a significantly higher impact was because it contains a combination of both techniques. Plant operations were still higher than those of precast as more lifting was required due to larger amounts of elements. On the other hand, this construction method still required intensive site operations as, apart from lifting and placing precast elements into place, ready-mix concrete is brought to the site, lifted, vibrated, and cured. This strong influence of plant operations and transportation is also highlighted in several studies, where limited local material availability and short internal distances shift burdens between production and transport phases [46,48].

Cast in situ includes the lowest plant operations energy consumption since most of the processes were performed on site. The only plant operations it required were the batching plant mixing and reinforcing steel cutting and bending, which consumed almost 12% of the total energy consumption. However, cast in situ construction involved the highest energy demand in transportation and site construction. Most of the energy consumption in precast construction was performed in the plant, totalling a percentage of 61.9% of the entire energy required. Transportation and construction energy demands added up to less than half of that required for the other methods, making it the most efficient method in the transportation and construction phase.

When considering the global warming potential impact, a similar outcome of energy consumption was exhibited, as shown in Figure 19. The highest emissions resulted from semi-precast construction (6.7 kg CO₂ equation/m²), followed by precast (4.9 kg CO₂ equation/m²) and cast in situ (4.3 kg CO₂ equation/m²). It was noted that although energy consumption was similar for both cast in situ and precast, slightly higher CO₂ emissions were exhibited by precast. This was because, when compared to the energy consumption, where transportation influences were relatively high, the influence of transportation for GWP was lower, resulting in fewer CO₂ outcomes for cast in situ construction [41,42,49].

5. Conclusions

This study intends to perform a comparative LCA, comparing cast in situ, semi-precast, and precast concrete methods of construction. In this manner, LCA from cradle to gate of a five-storey concrete frame office building, having a service life of 50 years, has been performed, in accordance with the specified guidelines by the International Organisation for Standardisation in ISO 14040. SimaPro 7.1 was the LCA software tool used for modelling and calculation of life cycle inventory data and environmental impacts. The system boundary set from cradle to gate includes the inputs and outputs for production of raw materials, outputs from electricity generation, transportation for raw material production and construction, plant operations, and construction processes. The system boundary excludes local electricity inputs, capital goods and human labour, finishes and services installation, use maintenance and repairs, demolition, and waste and recycling. Based on the LCA analysis, the following outputs were drawn:

• The single most important factor affecting GWP impact was directly related to the cement content in the mix. Total CO₂ emissions resulting from C25 ready-mix concrete is 262 kg/m^3, whereas for C50 precast concrete mix, it is 490 kg/m³. It was observed that several impact assessment categories are directly attributed to the choice of concrete mix that was obtained from the predefined concrete datasets within the SimaPro software.
• The overall environmental impact of polystyrene in semi-precast construction was minimal; hence, in all other impact categories, it was considered under “Remaining Processes”.
• Semi-precast construction resulted in the highest electricity consumption since it utilised both cast in situ and precast methods together. Electricity consumption was 15,601 kWh, followed by precast, using 10,809 kWh whereas the least electricity-consuming method is cast in situ, with 7965 kWh. These values were attributed to plant operations and construction only.
• Cast in situ construction resulted in the highest transportation demand, having a total distance of 949 km, followed by semi-precast, with 904 km, and to a lesser extent, precast, with a total transportation distance of 528 km.
• The total energy consumption required for cradle-to-gate for all construction methods is similar, with an average value of 1047 MJ/m² floor area. On a local aspect, the most energy-demanding method is attributed to semi-precast, having 32.6 MJ/m², whilst cast in situ and precast constructions have similar demands, using around 22.5 MJ/m².
• Precast and semi-precast methods resulted in similar quantities, with values almost 30% higher than that of the cast in situ method of construction. This significant difference was directly related to the cement content in the precast mix. On a local aspect, semi-precast construction resulted in the highest CO₂ emissions, having between 28 and 36% higher impacts than precast and semi-precast, respectively. The impact difference between precast and cast in situ construction is 12%. However, when compared to cast in situ, precast emits less than half the amount of CO₂ equivalents during transportation and construction, making it the most efficient method on site.
• Cast in situ construction exhibits significantly higher impacts in abiotic depletion, freshwater, marine and terrestrial ecotoxicity. These results were related to the higher use of steel in this construction method.
• Semi-precast and precast had similar higher impacts in acidification, eutrophication, human toxicity, and photochemical oxidation. These impacts were related to the use of concrete.
• Overall, it can be concluded that, from a global aspect, the energy consumption is comparable for the three construction techniques, having precast construction consuming slightly higher than semi-precast (1.9%) and cast in situ (5%) methods. From the local perspective, the semi-precast construction has significantly higher energy consumption, as it requires both extensive plant and site operations due to mixed construction. It is also evident that cast in situ and precast construction have comparable energy consumption. Cast in situ has the least amount of plant operations but the highest impact on transportation and site operations. On the other hand, precast has the highest plant operations in a controlled environment and the least extensive transportation and site operations. This implies that although they have similar energy consumptions, precast has an advantage that site operations are minimised, resulting in fewer labour operations and traffic impacts overall.

6. Limitations and Future Direction

Based on the scope of the study, the following limitations and future directions have been drawn:

• The impact results’ validity relies on LCI data and, in this study, it was on secondary databases, providing a well-established characterisation. The outcome should be interpreted as indicative, instead of considering it as an absolute value.
• The focus of the current study was limited to LCA only; however, in future, life cycle cost (LCC) can be integrated with it to observe the practical relevance.
• Although the adoption of low-carbon cements or renewable electricity could influence the impact on the environment, this study was based on current material mixes and electricity profiles. Hence, low-carbon cements and renewable electricity aspects could be considered in future research.