Identification of Time Wastage Towards Lean Production for IBS: A Case Study of Precast Hollow Core Slab Production

Industrialized Building System (IBS) is defined as the invention of Modular Coordination (MC) of which begins from planning to utilization in construction projects by applying standard manufacturing designs. The standard progression of a project would involve the component’s designs, sourcing, manufacturing, fabrication and installation on that particular project [1, 2].

1.1 Lean production

Researchers reported on the trend of Lean methodologies having said that Lean production was also founded on the idea of Kaizen; which directly translates as continuous improvement [3]. A concept or philosophy which has been employed far and wide to gather information, constantly assessing all processes and in time apply continuous improvement [4, 5]. In addition, Lean emphasized on giving value to the customers through three types; value added, unnecessary non-value and finally necessary non-value activities [6]. Dogan and Yagli [7] mentioned to assist in the identification of activities that do or do not create value to the customer, one method stood out from the rest which is the Value Stream Mapping (VSM) method [7-9]. VSM employs graphical representation through value stream mapping icons constructing the overall progress of a product through the production line from the supplier to the customer [3, 10]. Proceeding the VSM establishment which also identifies as Current Value Stream Mapping (CVSM), an improved version reformed from CVSM is produced which is known as Future Value Stream Mapping (FVSM). Improvement results are quantifiable through CVSM and FVSM comparison [11, 12]. Ciano et al. [13] provided evidence that manufacturing companies with revenues from a range of 9.5 to 100 million Euros practiced Lean Production allowing them to employ continuous improvements in their businesses [14, 15]. These businesses ranged from manufacturing metal products, mechanical, electronic components to healthcare sealant. Through this assessment of processes and activities, it is evident that Lean can be a great asset in helping companies make the right choices and benefit greatly [9, 16, 17]. These studies highlight the importance of Lean Production in reducing waste and improving efficiency in manufacturing processes. However, there is limited research on the application of Lean tools in the context of IBS, particularly in the production of precast hollow core slabs. This study builds on these findings by applying Lean tools such as Value Stream Mapping and Cause-and-Effect Matrix to identify and address time wastage in IBS production [13, 18-21].

Badir et al. [22] reported production of houses grew ten times as much once new building systems and factory produced building components are employed in the construction industry. Despite the benefits, there is a sizable amount of waste produced during the manufacturing of any IBS components. This waste accumulation would lead to financial, quality and time losses if left unchecked. Henceforth, deploying Lean Production in any IBS construction is regarded as an excellent strategy in mitigating losses. Lead Production in simpler terms are the principle or techniques of which the aim is to lessen or eradicate waste [23, 24].

While application of Lean Production has been thorough in the manufacturing industry such as employing numerous tools and techniques to eliminate waste, that has not been the case for most of the IBS construction manufacturing in Malaysia [9, 15, 25]. Though countermeasures are in place to tackle the problem of solid waste produced from the construction industry, such waste that is hidden from sight such as manpower loss, non-strategic methods, improper material handling or ineffective machinery utilization still plague the industry [26, 27]. Researchers agreed that with the reduction of wastage from the construction industry not only will the manufacturers save capital and time but also contributing to saving the environment, to be in the correct path towards supporting Sustainable Development Goals (SDG) [2, 28-30].

Manufacturing of an IBS components involved few phases of which the most essential one is the production phase of the components [31, 32]. This phase consumed the most time, involved the most processes, and had the most activities occurred during the production. By concentrating on time wastage at any section of the production line, there is a strong likelihood that reducing time wastage will boost productivity during the manufacturing of IBS component [33, 34]. Therefore, applying Lean Production principle in this case, it is possible to reduce lead times, increase productivity, and eliminate wasteful resources while producing the best possible outcomes.

This paper is aimed to identify time wastage known as non-value adding activities (NVAAs) in the production line of a precast hollow core slab (HCS), as a first step towards Lean Production development.

Based on the preliminary inquiry with the selected IBS manufacturing company, the process flow was established prior to the site visit for data collection. The flow of the production processes for the precast HCS are shown in Figure 2.

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Figure 2. Process flow of precast HCS production

As shown by the process flow chart in Figure 2, it can be seen that the process started with bed cleaning and oiling. The process continued with the rebar arrangements and prestressing of the prestressed steel before concreting and casting commences. After the QA or QC inspection passed with an approval stamp, the precast HCS was cured until targeted strength of the concrete was achieved and later finalized with the segmentation and storage.

Subsequently, site visit and physical verifications of each process were documented. Each process in Figure 2 was compiled physically as shown in Figure 3.

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Figure 3. Physical observation of the process flow for precast HCS production

The process of manufacturing precast HCS started with bed cleaning by two to three general workers using machinery. This is then followed by manual hand pump spray for the oiling as shown in Figure 3(i). After the bed is cleaned and set, prestressed rebars were installed according to the required design specifications using the extruder machine. The rebars were stressed along the bed, clamped and tightened using hydraulic jack at the end point of the manufacturing bed as demonstrated in Figure 3(ii).

Once the rebar specification was in order, shown in Figure 3(iii) is the commencement of the concreting phase. Specification approved concrete mix from the batching plant was poured into the extruder machine using a system of conveyor belt. The concrete casting phase ensued immediately after with up to eight workers; with each one worker handling the extruder machine and conveyor belt, respectively, four to five workers employed to apply the finishing surface, while a supervisor was observing the whole process to maintain quality. General workers were seen using hand tools such as spades and brooms to assist in the finishing surface as demonstrated in Figure 3(iv).

Once the concrete casting work reached a certain range, a QA or QC inspector inspected the constructed precast HCS to notify the workers of any additional amendments that needs rectification. Figure 3(v) exhibits an example of a QA or QC inspector checking the strength of the newly casted precast HCS by projecting his body weight onto the slab. After the quality of the precast HCS was checked thoroughly, a blanket of plastic canvas was wrapped upon the slab for curing purposes as shown in Figure 3(vi).

The following day after the precast HCS completed its curing period and achieved the design concrete strength, sections of the slab were cut using a cutting machine operated by an operator as shown in Figure 3(vii). Finally, an overhead crane was used to transport the segmented precast HCS for storage. The precast HCSs were then stacked vertically up to six to seven pieces as shown in Figure 3(viii), awaiting to be transferred to the site. The production details of this particular case study of the precast HCS and its operation processes for the mentioned project are listed in Table 1 and Table 2, respectively.

Table 1 lists the details of the operation processes of the precast HCS including the dimensions, manpower consumption and machineries involved in the production. Meanwhile, Table 2 provides the details of each operation process of the assembly production line. This data points will then be structured to develop the Work Breakdown Structure (WBS) and Cycle Time (CT) summary as shown in Table 3 and Table 4, respectively.

Table 1. Precast HCS production details

Table 2. Precast HCS component processing operation

Table 3. WBS for precast HCS production

Table 4. Summary of average CT

WBS divides all tasks into main tasks that will organize each individual work delivery in a way that helps to define and coordinate the project's full work scope. To identify time wastage for any type of production work will need an identification of the major processes impacted by inadequate or incompetent work delivery. Devi and Reddy [37] explained the importance of WBS in estimating cost, risk and time of a project. This can be accomplished by referring to an organized and adequately defined scope of the total project that was structured by an appropriate WBS interpretation. By observation, the CT results from more than 40 sets of observation for each process on the manufacturing of the precast HCS, are summarised in Table 4. Using the parameters and data gathered prior to the observations, Current State Value Stream Mapping (CSVSM) was then executed as portrayed in Figure 4.

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Figure 4. CSVSM findings of precast HCS production

As shown in Figure 4, the mapping for the overall manufacturing process of the precast HCS was developed. The chronology of each process involved was clearly defined with emphasised on time wastage marked by the red triangles between the processes. Each average process cycle and waiting time were finely depicted by summing up the total lead time and value-added time. The total lead time for this particular component production is 177.50 minutes, while the total processing time is 1327.13 minutes.

Putting together the total lead time and total CT enabled the research to proceed in calculating Value Added (VA) ratio for the manufacturing of the precast HCS. VA ratio is calculated to map out whether there was time wastage in the process. The calculation for VA ratio is shown in the following equation:

VA Ratio $=\frac{ { Total\ Processing\ Time }}{ { Total\ Lead\ Time }}=\frac{1327.12 {\ mins}}{177.50 {\ mins}}=7.5$

The VA ratio of 7.5 indicates that the total lead time is shorter than the total processing time. This shows that there is a considerable amount of time wastage in the process. It is also worth to note the VA ratio from the CSVSM findings can also be used as a comparison to calculate the hypothetical improvement percentage from the Future Value Stream Mapping (FVSM), of which was not pursued in this study.

From the CSVSM findings, it can be identified that the highest CT of the overall process is the curing time. This is because the curing process of the precast HCS will require approximately one day for the hydration cycle and reaching its intended targeted strength. As seen in Figure 3(vi), the manufacturing company had taken an initiative to wrap the precast HCS using plastic canvas in order to provide an ample condition for curing as the process can be affected by weather and temperature changes from its surroundings. From the observations and interviewing the personnel involved in the manufacturing company, the study reached an understanding that the CT of each process was finely tuned and unwarranted for improvement. Therefore, this study shifted its focus to examine the waiting time between each process. The summary of the average CT for waiting time is listed in Table 5.

Table 5. Summary of average CT for waiting time

When providing numerical values followed by measurement units, a regular space or non-breaking space should be left between each value and its measurement unit. This rule also applies to the unit for litre, which is recommended to be capitalized as 'L'. However, no space is used for percentages and degrees Celsius (e.g., 35%, 234°C).

Following the examination of the waiting time, it was apparent that the largest was before the bed setting and also between the bed setting and rebar setting processes with an average CT of 75 minutes and 40 minutes, respectively.

Upon close inspection, the largest waiting time before the bed setting was found to be closely connected with the process of rebar setting as the workers involved with the bed cleaning and oiling will only start their work when the rebar setting workers were ready to start. This idle time before the bed setting was then considered redundant as this process could not stand on its own without the direct commencement of the rebar setting. Therefore, the quality of bed setting will be detrimental with the longer gap in between both processes.

Following that, the second highest waiting time was the idle time between the beds setting and rebar setting with an average CT of 40 minutes. Upon close observation, multiple factors were contributing to this high lead time including mismanagement of manpower changeover, overlapping crane practice and poor management of materials placement layout.

The research proceeds with the identification of time wastage by employing an approach from Productivity Development Team (2002), by creating ranking of waste magnitude as shown in Table 6. This was later used to develop checklists of major wastage findings for each process as listed in Table 7.

Based on Table 7, the ‘seven deadly wastes’ of each process was identified and given weightage from 3 (define as lots of waste) to 0 (define as no waste). It was identified that the process in between bed cleaning and rebar setup was deemed to have the highest total waste magnitude and therefore needs of improvements. This was then followed by the process between concrete preparation and concrete casting. The least of the major waste in terms of ranking for the area of improvement was the time before the process of bed cleaning.

Table 6. Checklist for major waste findings (Productivity development team, 2002)

Table 7. Levels of waste magnitude (Productivity Development Team, 2002)

The study was then proceeds to further focus on the wastage in between the process of bed cleaning and rebar setup. Each of the ‘seven deadly wastes’ attributed to the total waste magnitude for the process; therefore, a separate checklist to further identify the area of improvements are documented in Table 8, Table 9 and Table 10 for each conveyance, processing and idle time, respectively.

Table 8. Conveyance waste-finding checklist

Table 9. Processing waste-finding checklist

Table 10. Idle time waste-finding checklist

Table 8 to Table 10 lists the waste magnitude together with the proposed improvement plans for each waste that was considered as a general contributor to the total time wastage of the production. Amongst the description of waste that needs improvement includes the flow of production, modes of process, manpower management and machinery setup. In addition, idle time waste-findings checklist tops the waste magnitude with total of 9, slightly higher that conveyance at 8 and the lowest was the processing waste at 3.

The research proceeds to explore into the listed improvement plan by getting feedbacks from the executives of the company at the management level and the workers at the processing level. Identification on the area of improvements from the feedbacks given by the workers will allow to figure out of the sources for each wastage. Figure 5, shows the Ishikawa Diagram or the fishbone diagram as a visual aid in showing the sources for the high queue time of the focused improvement area. Varma and Lal [35] also mentioned about the use of cause-and-effect matrix, fishbone diagram, why-why analysis and failure mode effect analysis to identify the root cause of wastage complications in the industry.

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Figure 5. Ishikawa diagram

The fishbone diagram depicted in the Figure 5 was split into four main categories which are the equipment, process, manpower and materials. Each category was due for an improvement and therefore elimination of these sources for wastage will pave the way to incorporate Lean Manufacturing into the process.

As a means to eliminate time wastage identified by the fishbone diagram, this study opted to adapt the approach of Cause-and-Effect Matrix (CEM). In addition, Varma and Lal [35] also incorporates the idea of using CEM to investigate the root cause of wastage in the industry. CEM represents the relationship between elements that contributed to an issue (the cause) and the desired outcome or action (the effect). Four major contributions were identified in this study as the causes which include equipment, process, manpower and materials.

The effects from these causes include timely product delivery, less downtime and quality of product. The operation Engineer and supervisor in this project with at least five years of experience helped and rectified the inputs throughout the study of this CEM. Scores are given accordingly depending on the correlation of the process input to the respective process outputs. Table 11 shows the matrix that was developed, while Table 12 shows the proposed magnitude of correlations to be assess based on the effect.

Table 11. Cause-and-effect matrix (CEM) findings

Table 12. Magnitude of correlation

Table 13. Process input details on wastage frequency and percentage

Table 14. Root causes of time wastage in precast HCS manufacturing and improvement measures

Expert opinions from employees who are in direct responsibility to the manufacturing of the precast HCS were shared upon the research and used to evaluate the root cause of wastage based on a rating scale of 0 (no correlation) to 9 (very high correlation). The root cause weightage is multiplied with the customer prioritization magnitude to calculate the total percentage of each and every process wastage. Table 12 shows that the causes that contributed to the most time wastage in the production are the machinery functions not fully utilized and the conventional method of bed cleaning with a score of 111 and 106, respectively.

The analysis continued with the development of the Pareto chart based on the CEM analysis as a visual aid in highlighting the root cause for time wastage in the production. Table 13 summarises the time wastage sources of the precast HCS production categorised from the highest total frequency to the lowest, while Figure 6 illustrates the Pareto chart of the time wastage sources.

Various figures can be accepted. Several examples cited from papers published in previous IIETA journal issues are as follows. Please pay special attention to how much line spacing is allowed in different cases.

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Figure 6. Pareto chart of time wastage sources from precast HCS production

Figure 6 shows the Pareto chart developed from Table 13, highlighting the root causes of time wastage in the production that is needed to be the prioritization for the improvement’s efforts. The line from the graph represents the cumulative percentage of wastage from the processes while the bar represents the total frequency of the given process input. It is evident that inefficient machinery utilization and conventional method of bed cleaning contributed to the highest percentage of time wastage in the precast HCS production. Suggested improvements for the production are given in Table 14. The identified time wastage, particularly in machinery utilization and bed cleaning, has significant practical implications for production efficiency and cost. Inefficient machinery use leads to prolonged cycle times, while the traditional bed cleaning process contributes to delays between production stages. Addressing these issues through improved machinery utilization and automated bed cleaning systems could reduce lead times and enhance overall productivity.

This study contributes to the field by applying Lean production tools, such as Value Stream Mapping and Cause-and-Effect Matrix, to identify and address time wastage in the production of precast hollow core slabs within the IBS framework. The findings provide a practical reference for manufacturers aiming to optimize production processes, reduce lead times, and enhance overall efficiency in IBS component production.