**1. Introduction**

The intention of this article is to study construction in order to propose a sustainable method in which it can be carried out. One possible way to achieve this is through the application of lean manufacturing in construction processes.

The lean concept [1] was born in the 1930s through the observation of processes in Toyota factories [2] and, since then, it has typically been applied in productive or industrial processes through the use of different techniques and tools, such as 5s, SMEN, QFD, TPM, Kamban and Pocayoke, etc., achieving adjusted production. With sustained applications over time, the continuous improvement and optimisation of different productive processes are achieved.

For this process, the lean methodology seeks the reduction of "waste," commonly in seven categories: Overproduction, waiting time, transport, excess procedures, inventory, movements, defects and the non-contribution of the operator in obtaining ideas that can improve processes.

As can be seen, this waste produced in the manufacturing industry is easily extrapolated to the building sector, as the construction process is considered to be an industrial or productive process [3].

Combining the use of the lean methodology in construction [4] and BIM (Building Information Modelling) [5] for integrated building managemen<sup>t</sup> would be the best way to improve sustainability in construction, thereby achieving certain SDGs goals, such as 9, 11 and 12 [6].

The production process in construction should be understood as an integrated process as are other industrial activities. However, at present, it is very fragmented, categorised as independent fields such as design, focused on the speciality of architecture; the execution

**Citation:** Awad, T.; Guardiola, J.; Fraíz, D. Sustainable Construction: Improving Productivity through Lean Construction. *Sustainability* **2021**, *13*, 13877. https://doi.org/10.3390/ su132413877

Academic Editors: Roberto Alonso González Lezcano and Marc A. Rosen

Received: 26 October 2021 Accepted: 8 December 2021 Published: 15 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of the project, split between architects and engineers; and construction, carried out by independent companies separate from the fields of action of those previous, where architects and engineers are also present.

When considering any other process and development in the industrial field, the activities, tasks, and work are coordinated, managed, and directed in a very different way to building processes.

Construction, in order to be included in an industrial dynamic, should combine the following stages: Design, devising the space, the project, the transfer to production, and finally, the construction of the work, as well as the post-sale follow-up.

An analysis of today's buildings has many characteristic aspects that are different from typical industrial production, where all production activities are usually controlled by the same entity or company and fall under the same methodology, standards, regulations, and common performance objectives.

Lean techniques can be applied to all steps of the construction process, which encompass lean project definition, lean design, lean supply, lean assembly, and others [7]. The focus is on how a lean design and lean assembly can be combined to contribute to a sustainable production process.

The production of buildings is carried out in three phases: Design, engineering, and assembly [8].

In development, there are the following three stages:


The results of this development are oriented towards obvious economic advantages, such as a cost reduction, which is socially necessary. The deadline, which is an aspect directly linked to the cost, will also be addressed more than the economic aspect.

Other advantages of the reorientation of building to an industrial environment include quality assurance, sustainability [9] and digitalisation.

Unlike manufacturing, construction is a project-based production process. Lean construction [10] is concerned with the integral pursuit of simultaneous and continuous improvements in all dimensions of the built and natural environment: Design, construction, commissioning, maintenance, recovery, and recycling.

#### **2. Materials and Methods**

The methodology is based on an analysis of the elements involved in the design and construction process. Through the use of lean tools, productivity is improved, which results in construction being sustainable. The following five aspects are developed below: An analysis of the current situation, the discourse of the method, an analysis of UNE-EN 15643-3-2012, the definition of the programme and the meaning of construction as engineering through a price decomposition analysis.

Firstly, an analysis is conducted of the current state of the art [2], the traditional building processes and the challenges of new buildings related to the industrialisation available. We also analysed the advances achieved and the possible solutions for the application in the construction industry.

#### *2.1. Analysis of the Current Situation: Industrial versus Traditional Construction*

Since the end of the 1980s, the lean philosophy has been adopted in the construction sector with a focus on efficiency, mainly motivated by economic competition [11].

In terms of industrialisation, there are differences between products and technology. Products refer to construction components available on the market, and technologies to the methodologies for on-site assembly.

2.1.1. Products and Technologies

Nowadays, sufficient materials, products and systems are available to develop a more advanced building technology [12] than traditional building systems of the past. Products:


Figure 1 is a study of the combination possibilities between the origin of materials and the technology used.

**Figure 1.** Products and technology (own source).

Incoherent construction happens when technology and product categories that do not correspond are mixed. Outside the "cloud" areas, advanced industrial design technological products are combined with traditional, archaic construction.

#### 2.1.2. Technology and Design

This is an analysis of building systems and technologies and their connection to the elements of programme design and space distribution.

Sometimes, architecture that is intended to use advanced construction systems is paradoxically executed with handcrafted construction. This results in incoherence between design and construction.

Two construction systems can be distinguished as shown in Figure 2.

**Figure 2.** Design and technology (own source).

System based on ancient techniques and materials: from archaic eras, based on earthy materials and hydraulic joining methods, which combine water with binders that set over time.

Modern systems based on external technologies: Aeronautical, naval [13] and others, which provide lighter materials, composites of elements with combined properties, according to more optimized requirements, as well as more rationalised execution methods.

There are four possible outcomes distinguished by the design of the habitat and the construction technology provided.


The first two possibilities are coherent and parallel between the design criteria and the technology chosen for their execution; however, the latter two produce an incoherent result.

#### *2.2. Stages of the Construction Project*

Once the current situation has been analysed, reviewing the building project [6] can lead to an improvement in the construction process, resulting in a better level of quality, lower cost, and shorter execution time.

The complete cycle is reviewed—design, project, and assembly—in order to obtain a better performance in terms of cost and construction time. This process review, which is carried out within the lean construction methodology [15], seeks the optimisation of each process of components, resources and materials.

According to Koskela, there are two main processes in a construction project:


Processes can be characterised by their cost, duration, and value to the customer [16]. A lean analysis in construction must be based on the order of stages established in construction projects, as set out in the UNE-EN 15643-3-2012 standard: Terminology of Engineering Services in Buildings, Infrastructures, and Industrial Installations.

To analyse the complete construction process, from the design to the material execution, the description of the UNE-EN 15643-3-2012 standard is used.

UNE-EN 15643-3-2012 is a reference to sustainability in construction.

From the following chart, Figure 3, we have chosen the following stages of development of a construction project. Those that will best fit the above-mentioned arguments of "design" and "material execution" are the following:

0. Initiative:


1. Start:


2. Design:



Among the four design stages listed in the UNE-EN 15643-3-2012, the conceptual design and the preliminary design belong to the architecture project group, while technical design and detail design would be in the engineering execution group.

On an architectural level, in the project development phase, we will analyse the architectural design manners of the habitat in order to find lines of improvement in the organisation of the programme.

For the second group—engineering—oriented to the material execution of the project, the economic configuration is studied, with price base and deadlines.

The main variables to be considered in costs are:


Materials are the products that enter the construction site for assembly or joining. Manpower is the human resources necessary to carry out the work. The auxiliary resources are the means to facilitate such work, tools, and machines.

By analysing the architectural elements, the way to optimise cost efficiency is studied, from the revised design aimed at improving the productivity of execution to the cost components and construction times to be reduced, depending on the construction process itself.

The next subsection is an analysis of each of these two groups, framed in the "architecture" and "engineering" of the construction processes, including the stages set out for each of them, the particular developments of the project, seeking options for cuts and elimination of superfluous elements, typical of lean methodology in construction.

#### *2.3. Design: Definition of the Programme*

The industrial engineer [17], who designs cars, railways, aeroplanes, and ships, surpasses the construction engineer by using mechanically processed materials and methods for rational production. The problem has to be approached from three interdependent factors: economy, technology and method.


**Figure 3.** Based on UNE-EN 15643-3-2012: sustainability in construction: sustainability assessment in buildings.

> The economic factor consists of analysing cost reduction, the technological one of providing the materials, products, and components, all based on mass production means, and the method, which deals with the rational composition of the living space and its design, on the other hand.

> From the historical analysis of habitat needs, two common strategies can be observed: the revision of the way of living, its needs or corresponding consumption of space and the search for technologies that facilitate its execution, with the aim of economising costs and economic effort.

> Thus, from new solutions and revised approaches to the habitat scheme, the aim is to achieve a new rational industrialised building.

> Before moving on to industrial production in construction, the requirements and use of the spaces have to be clear.

> The design of space is looking for housing with rational use. As a result of this, there are habits that have to be revised, rationalising the organisation of living space.

> A building system that combines design and execution will facilitate the typical construction processes, based on industrial construction engineering, transferring methodologies that improve the productivity of construction on-site, and that will optimize lean construction.

> This section begins to focus the analysis on the design of the "product" itself, which is the habitat.

#### 2.3.1. Functional Analysis

Facing a new form of construction affects both the specific execution process and the design of the space.

It is necessary to delve into the design from the beginning, analysing the habitat or way of living, its relations and internal functions and its needs and uses; in short, its functional programme.

The in-depth study of this section is enormously complex as it is an extensive field of analysis, in all its cultural, sociological, and psychological components; many authors, analysts and architecture professionals have been working for some time on the exclusive subject of housing, including its political implications since the last century, with enormously committed positions.

In this sense, the CIAM congresses, Le Corbusier's Athens Charter [18] and all that follow are both in favour and broad critical positions: Team X and Hadraken's diagrammatic methodology [19], Japanese Metabolists [20], and Archigram [21]. Up to the present day, this is a debate that is still unfinished.

The issue of housing has long been a problem of difficult satisfaction for society.

At the beginning of the 20th century, the social and economic panorama began to change, with the studies of the Industrial City by Tony Garnier [22], in which new approaches to an integrated habitat were presented.

From the early years of Russian constructivism, through the successive trends of modern architecture, solutions of housing units have been collected (Le Corbusier), in the approaches of minimal housing, on arguments of hygiene and health or in the revision of a new way of life, open to nature and today's society, for example, Los Angeles architecture with case study houses.

Likewise, even designs have been revised to facilitate a better and more rational construction: the five points of Le Corbusier, Bauhaus school [23], Prouvé, even in Spain, the contribution of Rafael de la Hoz [24] stands out, looking for solutions in architectural elements.

On the other hand, pioneering engineers, using new materials such as steel, concrete, plastics, and laminated wood, in line with the industrialisation of building, such as E.L. Ransome [25,26] in the use of prefabricated concrete parts in the USA or Freyssinet in France, with the evolution of pre-stressed concrete [27], provide applications in civil and industrial fields.

Governments, city councils and schools of design and architecture universities have always been gradually getting involved, looking for the solution to the population's need for housing. This began with the pressing problems that arose after the Second World War, where Europe lost an enormous number of buildings and housing and looked for quick and emergency solutions, from which the first applications of mass prefabrication [28] emerged but did not turn out to be the desired solution.

#### 2.3.2. Rational Design: The Architectural Project

This section analyses the minimum elements necessary for the habitat.

There is no justification for the fact that every house in a suburban neighbourhood has a different typology: different floor plans, facades and building materials. Throughout Europe, the old farmhouse, and the townhouse of the average citizen in the 18th century show a similar layout.

Except for the typologies revised during the modern movement, in search of the minimum house solution, the way of living is still traditional, falsely conditioned by the construction of load-bearing walls, where rooms are enclosed between walls and elongated corridors.

Housing should be designed according to the requirements of the family unit but tending to standardise the common parts, eliminating unnecessary uses and spaces and generalising related spaces.

The optimisation of the habitat focuses on the strategy of reducing architectural components through function and use, using architectural elements that configure spaces according to their use.

According to A. Moles, the necessary minimum elements of a space configuration can be determined as [29]:


In this strategy of minimums can be referenced the design modes for a revised habitat result, with the saving of architectural resources, which ultimately saves costs, preserving user value.

As an example, we can propose solutions, taken from the courses at the Madrid School of Architecture, taught by A. Fernández Alba in the 1970s [30].

In these solutions, the concepts of minimal conceptual strategies applied to the proposed spaces are tested (Figure 4).

**Figure 4.** Interior perspective: rehearsal of the organisation of living space: J. Guardiola.

#### Elements of Design

A more controlled design of the habitat conditions, programme, space and uses favours the rationalisation of the execution on the elements of the construction: Walls, installations or façades.

The basic functions can be pointed out, which will help rethink less traditional housing solutions and involve more current approaches among the needs of the habitat.
