Comparison of Conventional and Precast Half Slab Work Methods (Case Study: Japfa Office Building Daan – Mogot)

In the construction of vertical buildings, the implementation method used affects the productivity of construction. The development of construction system innovation in Indonesia which continues to grow has led to a new system, one of which is the precast system. Precast systems have been proven to be able to increase productivity in terms of construction time and costs, as well as the value of waste costs, which results in little from conventional methods. The main objective of this study was to compare the efficiency of the method of implementing floor slabs in the field by comparing the variables of field implementation, costs, time, quality control, and material waste costs of the conventional and precast half - slab methods. The analysis was carried out using the unit price is based on the Jakarta Worker’s Wage Unit Price 40 year XXVII - 2021 edition. The index coefficient (conventional) refers to the Minister for Public Works and Human Settlements Regulation No. 1 of 2022, and the index coefficient (precast half slab) refers to SNI 7832:2012. Scheduling was performed using Microsoft Projects. The results show that the precast half - slab method is more efficient than the conventional method.


Introduction
In the process of constructing a construction project, it is mandatory to consider the work methods because each method used affects the productivity of the construction.Each project generally uses conventional construction methods.Conventional methods in projects generally uses formwork from plywood, which has an impact on costs and waste material in the environment.In addition, weather affects the quality and time of work (conventional), which is often not in accordance with plans or specifications [1].The Japfa Office Building Daan-Mogot Project, located on Jalan Daan-Mogot Raya KM 12 No. 9, Cengkareng, West Jakarta consists of eight floors, 1 one roof floor, and a helipad floor.The construction of this building was estimated to take one year.The Japfa Office Building Daan-Mogot project has an area of 24,621 m 2 and a floor area of 16,848 m 2 .The construction of projects with large volumes, particularly floor slab structures, is highly dependent on the selection of implementation methods in the field.Therefore, with the existence of this precast half slab system or method, it is hoped that it can be useful in increasing productivity values in construction in terms of time and cost without compromising good quality [2].
The half-slab is a precast plate that is printed half of the planned plate thickness, which is then added to the casting in the area above the precast plate (overtopping) in a conventional way [3].There are considerations in using the precast half-slab method; namely, the environment around the project will be cleaner because it can reduce the use of formwork made of wood, reduce construction costs, and cut time faster with good quality.There is an important aspect that becomes a determining factor in 1324 (2024) 012012 IOP Publishing doi:10.1088/1755-1315/1324/1/012012 2 increasing the productivity of construction projects: efficiency.A project is said to be successful when seen based on short work completion time, minimal cost, good quality and quality control, and little waste material produced [4].
Efficiency is a comparison between planning and implementation, where, in practice, savings are made in terms of greater time and costs, so that the efficiency that occurs is higher without reducing the quality of work.Time and cost efficiencies are important in construction projects.Time efficiency can speed up project work so that it can be completed on time or faster than the specified target.Cost efficiency can have an impact on contractor profits, because the material used can be maximized without reducing the quality of work.Cost efficiency can also be useful for avoiding the material waste generated during construction.Quality control is also an important component in determining the implementation method in the field because it has a big influence on the sustainability of construction projects where in terms of time and cost efficiency, there is also the quality of work that must be achieved [5].

Analysis of Implementation Precast Half Slab
The precast half slab is a plate, half of which has been produced at the factory and will then be surface cast (overtopping) directly in the field.Time, cost, and quality are the three most important aspects of the project work process.The precast half-slab method has several advantages over conventional methods as follows) [6].
x Relatively fast execution time x Simplification of the implementation of the construction of the number of workers used less x Its production is not affected by the weather x Can reduce construction costs x Guaranteed concrete quality However, apart from the advantages of precast element structures, they also have some limitations and drawbacks, as follows: x It is uneconomical for the production of a small number of element types x Damage may occur during the shipping process which causes the element structure to be imperfect x Requires people with special expertise due to the need for detailed planning on the connection In the process of carrying out the work the precast half slab method is simpler and faster than the conventional method.According to SNI-2847-2013, Article 13.6.1.2, in modeling floor panels it is preferable to have a square shape, with a ratio of length and width of more than 2 or less than ½.In modeling, precast half-slab plate components are divided into two or three parts to facilitate the fabrication and installation process of the project [7].
Budget cost plans play an important role in project sustainability.The construction cost structure was divided into two components: direct and indirect.Direct costs are directly related to the implementation of construction projects in the field.For example, material costs, labor wage costs, and the costs of using heavy equipment and other equipment.Indirect costs are those that are not directly related to project construction in the field.Examples include the cost of building a field office and mess for employees and workers, employee salaries, licensing fees, and insurance costs [8].
Scheduling determines when a project can be completed along with the sequence of work in a timely manner and at an economical cost.The scheduling method is divided into several types, one of which is the Precedence Diagram Method (PDM).There are four types of constraints or relationships between activities with one another in a project, namely [9].
x Finish to start (FS): an activity can start if the previous activity has been completed.
x Start to start (SS): an activity starts if the previous activity start.In other words, the activity cannot be started if the activity has not yet started.x Finish to finish (FF): an activity is completed if the previous activity is completed.
x Start to finish (SF): an activity is finished if the previous activity start.Productivity is the ratio between input (total resources used) and output (results obtained).from the results of these comparisons to evaluate what has happened in the project and what should happen [10].
Quality control is important to ensure that the work is in accordance with what was previously planned.Conventional quality control methods are strongly influenced by the weather and require considerable handling and monitoring.The quality control of the precast half-slab method for making precast plates is carried out at the factory; handling and supervision are only carried out during installation in the field, and the weather has little effect on overtopping iron work and overtopping casting [11].
Waste material is the remaining material used or the excess material generated during the construction process.Waste is classified into two types during construction: residual material caused by the demolition or destruction of old buildings (demolition waste), and residual material caused by construction processes that are no longer used (construction waste).Construction waste is classified into two categories based on its type: Direct Waste is material leftover that arises from construction projects because it is damaged and cannot be reused.Indirect Waste is the remaining material that occurs in construction projects due to errors in material calculations, resulting in an excess of either the volume of material used or too much material purchased.Therefore, it is necessary to apply Construction Waste Management to manage construction waste in three ways [12]: x Reuse is the act of reusing leftover material that occurs in the field to support other work.
x Recycling is the process of recycling leftover materials or waste at the project site to a processing plant to be made into new products that can be useful and have sale value.x Salvage is the act of moving goods (remaining construction materials and waste) from the project site to be disposed of in the landfills, resold, and donated to third parties.

Methodology
The data processing in this study consisted of primary and secondary data.Primary data are obtained from direct observations in the field, such as the results of observations and interviews with operations and engineers in the field.The primary data obtained from the results of observations in the Japfa Office Building Daan-Mogot project are conventional floor slab implementation methods; interviews with project managers and field operations divisions regarding resources, materials, equipment, and implementation risks; and project material ordering data.Meanwhile, secondary data are obtained from the Japfa Office Building Daan-Mogot project, which is usually used as reference data for the cost of conventional method plates, consisting of shop drawings, implementation schedules, and the Budget Cost Plan [13].
The purpose of this research was to compare the efficiency in terms of implementation method, cost, time, and waste cost, as well as quality control of slab work using the conventional and precast half-slab methods.Waste cost data analysis is waste material produced in the field outside the residual value of the material that can be resold to generate profit [14].Material ordering data were obtained from interviews with the purchasing department.

Efficiency Analysis of Precast Half Slab
The data obtained in this study came from observations and interviews with operations and engineers in the field, as well as other supporting data, such as conventional method plate costs, shop drawings, implementation schedules, budget plans, and implementation methods.The outline preparation of work stages and, implementation methods are used to determine the sequence of implementation, what heavy equipment and materials are needed, and the number of workers needed in each work activity.In this study, the design of a precast half-slab was planned.
Cost analysis can be performed by multiplying the volume of each job by the value of the work unit price analysis.The stages of the cost analysis were.
x Calculating the volume of work based on shop drawings x The work unit price analysis is calculated by multiplying the unit work index by the unit price.This analysis refers to the Minister for Public Works and Human Settlements Regulation No. 1 of 2022 for the conventional method, and SNI 7832:2012 for the precast half-slab method.The unit price, will be taken from the 40 year XXVII-2021 Jakarta Provincial Worker's Wage Unit Price List.
x The volume of each work item with work is multiplied with a work-unit price analysis to obtain the budget cost plan of each method.There is a relationship between the cost and work volume in conventional concrete systems and half-slab precast concrete, where the greater the volume of precast concrete work, the smaller the costs required.Meanwhile, if the volume of precast concrete work is smaller, the cost of the work elements is higher.
The analysis time can be calculated by dividing the volume of each job by the productivity of the workers or the tools used.In calculating the total duration of each construction method, scheduling was used using the Precedence Diagram Method (PDM) using the Microsoft Project application.The following is the sequence of time analysis stages.
x Calculating the volume of work based on shop drawings based on the division of work zones on the project: x Calculating work productivity using the formula: x Calculate the duration of the work using the formula:

Duration of work =
Volume of work Productivity of work x Make sequencing and scheduling with the PDM method.By using the Microsoft Project application The waste analysis from the results of direct observation and interviews with the site manager of the Japfa Office Building Daan-Mogot project, can be calculated using the following formula [15]: x To get the waste value:

Results and Discussions
In this study, the efficiency of the implementation of the conventional method and precast half slabs can be observed from several variables, namely field implementation, cost, time, quality control, and wastage of material costs.A comparison of the implementations in the field is presented in Table 1.This shows that the half-slab precast method is used at the factory, which is then sent to the project site.The half-slab precast method is simpler but requires skilled workers in the installation process.The total cost in the field of the precast half-slab method shown in Table 3.This shows that the cost of using the precast half-slab method floor slab work approximately IDR 13,971,075,986.98A comparison of the costs in the field is shown in Table 4.This shows that the use of the cheaper and more efficient half slab precast method can save costs of approximately IDR 3,427,361,685.14.The processing times in the field of the conventional method shown in Figure 1.This shows that the times of using the conventional method floor slab work approximately 71 days.

Figure 1. Scheduling Conventional Method
The processing times in the field of the precast half-slab method shown in Figure 2.This shows that the times of using the precast half-slab method floor slab work approximately 45 days.

Figure 2. Scheduling Precast Half Slab Method
A comparison of the processing times in the field is presented in Table 5.This shows that using the precast half-slab method is faster and more efficient, and can save approximately 26 days.A comparison of the waste costs of materials for work in the field is presented in Figure 3.This shows that using the precast half-slab method is more efficient, and can save material approximately IDR 408,524,075.

Conventional Method
Precast Half Slab Method Waste Cost Comparison (Rp289.699.699).This value is outside the value of the total profit material that can be resold, so it is only calculated based on the waste material produced in the field.The value and factors that cause material waste produced in floor plate work, it is necessary to create waste management that can be implemented in order to minimize the waste material in the field is presented in Table 7.
Tabel 7. Construction Waste Management Material Reuse Recyle Salvage Formwork 9 The remaining formwork produced, if the condition is still proper, can be used to build or repair buildings and workers' facilities or by seeking several parties who wish to buy the formwork waste. - In general, formwork that cannot be used anymore will then be disposed of directly by the project to the Final Disposal Site (TPA).Rebar D10 9 The remaining pieces of iron that are still possible to use can be used in the next job.9 The remaining pieces produced can be used to support other work -9 If it cannot be used again, then it will be sold to scrap iron collectors so that the remaining iron material is not wasted.
Wiremesh Overtopping The remaining pieces of iron that are still possible to use can be used in the next job. - If it cannot be used again, then it will be sold to scrap iron collectors so that the remaining iron material is not wasted.Concrete K-35 The remaining concrete produced is used to construct and repair temporary access roads on the project.9 The remaining concrete is used to make decking concrete.9 The remaining concrete is used to construct or repair buildings and workers' facilities.
-9 If it cannot be used again, then it will be thrown away without being able to be used again The work of the precast half-slab method for floor slabs in quality control is classified as easier than that of conventional method is shown in Table 8.Because precast fabrication is performed in a factory, quality control can be guaranteed through several tests.The most prominent advantage of the precast half-slab method is that it is not significantly affected by rainy weather.Rainfall is a major problem in the conventional method because casting cannot be performed during rainy conditions.

Concrete Topping Work
The slump test was carried out on the ready-mix, and compressive strength test after 28 days.

Weather Effects Anticipate the weather during the casting work
Based on the analysis that has been carried out, it can be seen that in the work using the conventional method, there are quite long stages, such as preparation for installation of formwork, work on ironing, casting, and dismantling of formwork, maintenance of concrete, and its implementation waiting for each other, and the process of forming and casting must coincide with the installation of reinforcement beams that are linked to the top of the previous floor column so that they are bound to each other.Although the half-slab precast method is faster because it has simpler stages, the implementation is not mutually exclusive because the precast concrete has already been printed at the factory, so all that remains is the installation process.Subsequently, overtopping and casting ware carried out.
A comparison of the work time for floor slabs with the precast half-slab method requires a shorter processing time than the conventional method, with a difference of 26 days or 37% faster.In addition, the use of the half-slab precast method is proven to save costs of Rp3.427.361.685.14, which is 20% more efficient than conventional plate systems.In terms of the waste cost generated by the two methods when using the precast half-slab method, the resulting waste material is less than that of conventional methods.The most waste material generated by the conventional method is waste from formwork wood by 14.84% while the most waste material is generated by the precast half-slab method, namely K-35 Overtopping concrete waste by 22.63%.

Conclusions
From the results of the analysis that was carried out on the implementation of the conventional plate method and the precast half-slab method, the following conclusions were obtained.
x The results of the conventional method analysis require a fee of Rp17.398.437.672 with an implementation time of 71 days and are affected by the weather.x The results of the analysis of floor slab work using the precast half slab method required a cost of Rp 13.971.075.986 with an implementation time of 45 days and was not significantly affected by the weather.x The results of the analysis of floor slab work using the precast half-slab method produce a waste cost of 2% or Rp289,699,699 of the total cost of the precast half-slab method with the largest percentage of waste material, namely, K-35 Overtopping concrete of 22.63%.The waste cost generated by the conventional method is 4% or Rp698,223,774 of the total cost of the conventional method, with the largest percentage of waste material, namely formwork wood, by 14.84%.This value is outside the value of the total profit material that can be resold, so it is only calculated based on the waste material produced in the field.
x The difference in plate work between the conventional method and the half-slab precast method in terms of cost is IDR 3,427,361,685.14 (20% more efficient), while the time is 26 days (37% faster), and the waste cost generated by the half-slab precast method is smaller than that of conventional methods.The precast half-slab construction method is faster than conventional construction methods but can be more expensive because of the small volume of plate construction and is not typical.This will be considered in future studies.The use of the half-slab precast method can be more expensive in terms of costs when used in non-typical building structures, but it is proven to be faster in terms of processing time, and the cost of the resulting waste material is minimal.Based on the Sustainable Development Goals (SDG) 2030, this research supported the 9 th goal which is the Industry, Innovation and Infrastructure goal.This paper pointed out the advantages and disadvantages of construction methods that can be applied in the construction industry.
ܹ=Σ‫−ܤ‬Σ‫ܧ‬ Where: ܹ = Waste Σ‫ܤ‬ = Purchased quantity Σ‫ܧ‬ = Estimated quantity required x To get the percentage value of waste: %ܹ=(ܹ÷Σ‫)ܤ‬ × 100% Where: %ܹ = Waste Percentage W = Waste ΣB = Purchased quantity x To get the waste cost value: Waste Cost = % Work Weight x %W x Total contract value Where: Waste cost = Waste cost value W = Waste %Work weight = %Work weight of structure items Total contract value = Total contract value of work items

Table 1 .
Implementation of Each MethodsThe total cost in the field of the conventional method shown in Table2.This shows that the cost of using the conventional method floor slab work approximately IDR 17,398,437,672

Table 2 .
Cost of Conventional Method

Table 3 .
Cost of Precast Half Slab Method

Table 4 .
Cost Efficiency

Table 6 .
Waste Cost Efficiency