Keywords

1 Introduction

Today, energy and waste management are two of the main concerns of humanity [1]. In Europe, around 5 million tonnes of tyres are produced per year [2] and buildings account for 40% of the total energy consumption [3]. According to the same source, roughly 75% of the EU building stock is energy inefficient. Moreover, thermal bridges may be responsible for up to 30% of heat losses in buildings [4]. To achieve the ambitious goal of carbon-neutrality by 2050, improving energy efficiency in buildings is very relevant.

Over the last few years, alternatives to the traditional constructive method have emerged and proliferated worldwide, such as the lightweight steel-framed (LSF) system, which offers some advantages in fields such as production, transportation, durability, adaptability and construction economy [5]. However, if not correctly designed and built, the thermal bridges created by the high thermal conductivity of steel can penalize the thermal performance of the exterior envelope of buildings and, consequently, their overall energy efficiency [5].

Regarding end-of-life tyre (ELT) waste management, the Directive 1999/31/EC on waste landfill has prohibited the landfilling of used whole tyres since 2003 and shredded tyres since 2006. Therefore, with decreasing disposal options and increasing production, the volume of used tyres is becoming a major waste management issue.

In March 2020 the European Commission adopted the new updated European Circular Economy Action Plan (ECEAP), which will be implemented not only to waste materials (such as ELT), but also to the construction industry and built environment. Regarding this issue, a more recent and advanced construction system, such as the LSF system, may play a relevant role in this transition to a circular economy, due to some of their advantageous features, such as, high suitability for modular construction, easier reuse and adaptability [5].

The research project Tyre4BuildIns [6] was conceived to develop a new cost-effective eco-friendly thermal insulation material, made from a mixture of recycled tyre rubber (waste) and an advanced high-performance insulation material (aerogel) [7], which will be used mainly, but not exclusively, as a thermal break for LSF building structures [8].

In this study, modular construction and the LSF system are assessed to evaluate their suitability for implementing circular strategies in buildings. Furthermore, the Tyre4BuildIns research project is briefly described, with a focus on the main achievements in the management of waste from ELT (recycled tyre rubber and textile fibres), as well as in favour of improved thermal performance and improved energy efficiency of LSF building elements.

2 LSF Modular Construction

2.1 Modular Construction

Modular constructions are prefabricated buildings that consist of repeated components called modules [9]. One can identify two-dimensional (2D) panels, with or without openings, but also three-dimensional (3D) volumetric units, with or without full fixtures, much more complex than the two-dimensional ones. Timber, concrete, or steel can be used separately or in hybrid systems in different forms.

Modularity involves constructing units away from the building site and then delivering them to the site. The installation of the prefabricated unit is completed on site using cranes. The modules can be placed in various ways, i.e. side-by-side, end-to-end, or stacked, allowing for a variety of configurations and styles. Once the module is placed in position, they are joined using specific connections. These connections tie the individual modules together to form the overall structure of the building [10]. Modular buildings refer to the application of a variety of structural systems and building materials, rather than a single type of structure.

According to Rogan et al. [11], the attributes of modular construction are:

  • reduced construction costs, especially when combined with economy-of-scale production (10%);

  • shorter construction time on site (50–60%);

  • increased profitability of the industry due to economy of manufacturing scale;

  • increased site productivity (up to 50%) and reduced labour force on-site;

  • greater certainty of completion on time and to budget;

  • much reduced wastage in manufacture and on site—improved sustainability;

  • greater reliability and quality.

Some other benefits can be identified, such as:

  • lower weight, which provides lower foundation loads and expense, including transportation, and the ability for roof-top extensions of existing buildings;

  • less disruption at construction sites due to multiple truck movements compared to conventional on-site construction;

  • design for deconstruction, to encourage future reuse and recycling of products and materials;

  • design for flexibility to extend building lifetimes and, where possible, further extend the life of buildings by renovation and refurbishment.

These benefits may be quantified in a holistic assessment of the costs and value of modular construction in relation to more traditional alternatives.

Lawson et al. [12] identified two different types of building modules according to their load transfer mechanisms. The first type is the corner supported module, where loads are transferred from edge beams to the supporting corner columns to the ground or a floor. The second type is the load bearing modules, where the loads are transferred from the side walls of the module to the ground.

Lusby-Taylor et al. [15] identified prefabricated construction in 5 categories according to the way it is manufactured and assembled on site. They can be (1) modular (volumetric) construction: production of three dimensional units in controlled factory conditions prior to transportation to site; (2) panelised construction, where flat panel units are produced in a factory and assembled on site to produce a three-dimensional structure; (3) hybrid (semi-volumetric) construction: combines both panelised and modular approaches; (4) off-site manufacturing (OSM) subassemblies and components: covers approaches that fall short of being classified as systemic OSM, but which utilise several factory fabricated innovative subassemblies or components in an otherwise traditionally built structural fabric, e.g. roof cassettes, pre-cast concrete foundation assemblies, but excluding window, door sets, roof trusses; and (5) non-OSM, where this category is intended to encompass schemes utilising innovative housing building techniques and structural systems that fall outside the OSM categories.

2.2 Lightweight Steel-Framed System

Light steel framing (LSF) is an integral part of modular construction as it is strong, lightweight, durable, accurate, free from long-term movement and has been well-tested in a wide range of applications.

LSF comprises galvanised cold-formed steel lipped channel sections of 70–150 mm depth in the wall panels, and 150–300 mm deep C-sections or lattice joists in the floors. The usual spans are of 3.6 m, but up to 6 m can be achieved, which can eliminate internal load-bearing walls and, therefore, leads to flexibility in internal space planning. The prefabricated wall panels are typically of the storey height and the length depending on transportation and lifting.

The following types of modules can be used Lawson et al. [12] in the design of buildings using either fully modular construction or mixed forms of steel construction: (1) 4-sided modules; (2) partially open-sided modules; (3) open-sided (corner-supported) modules; (4) modules supported by a primary structural frame; (5) non-load bearing modules; (6) mixed modules and planar floor cassettes, and; (7) special staircase or lift modules.

These systems may also have some drawbacks that could penalise their thermal behaviour and energy efficiency. Thermal bridges, originated from steel studs and reduced thermal inertia, are two major examples of these possible drawbacks [5]. Steel may be fire-resistant, but it also tends to attract extreme heat, due to its thermal conductivity, and home interiors can be affected during hot summers, given their usual reduced thermal inertia. Fortunately, this problem can be solved by adopting some strategies to increase their thermal mass.

Compared to timber, steel-framed homes need additional attention to insulate because steel framing could require a thermal break to be included as part of the insulation process. If a thermal break is not included, which can be an insulating strip along the steel flanges [13, 14] or an adequate External Thermal Insulation Composite System (ETICS), then the cold steel can cause condensation within the walls of the home, and, over time, the moisture can compromise the integrity and durability of the wall.

The light steel frame is enhanced with the use of other materials such as cladding, insulation, and internal fittings, making it a truly sustainable option that is also energy efficient, when adequately designed. In addition, LSF supports creative and innovative design with a wide range of combinations with other materials.

Moreover, structural steel in LSF constructions is inherently recyclable but is also reusable. These and many other attributes, such as design for deconstruction and design for flexibility, are becoming increasingly significant in the context of the circular economy. The circular economy keeps resources in use for as long as possible, extracting the maximum value from them while in use, and then recovers and regenerates products and materials at the end of each service life through recycling and reuse.

3 The Tyre4BuildIns Research Project

3.1 Main Objectives

The Tyre4BuildIns research project [6] started on 26 July 2018 and had a total duration of four years. The main goals were as follows:

  1. 1.

    Develop a new eco-friendly and cost-effective insulation composite material based on recycled tyre rubber and other insulation materials;

  2. 2.

    Evaluate and optimize the performance of this new composite insulation material by characterizing its properties (hygrothermal, acoustic, fire reaction, mechanical resistance and durability);

  3. 3.

    Optimize the use of the new insulation material in building elements (e.g. walls) in order take maximum advantage of it regarding thermal and acoustic performance, and;

  4. 4.

    Assess the environmental impacts and cost of this new insulation material from a life-cycle perspective.

3.2 Main Tasks and Sub-Tasks

To achieve these objectives, the team make use of a research plan containing six main tasks and several sub-tasks, as illustrated in Fig. 8.1.

Fig. 8.1
An illustration depicts six main tasks and their sub-tasks in Tyre 4 BuildIns. The main tasks include state-of-the-art. 2. development of new thermal insulation composites, 3. performance evaluation of new composites, 4. development and performance evaluation of L S F elements, 5. L C A, and 6. dissemination.

Tasks and sub-tasks of the research project Tyre4BuildIns

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Task 1 (State-of-the-art) aimed to keep the team up to date on the research topics addressed in this project, given the frequent related new scientific advances. Task 2 (Development of new thermal insulation composites) aimed to develop a new eco-friendly and cost-effective insulation composite material based on recycled tyre rubber and aerogel materials, this being the most important task of this research project. Task 3 (Performance evaluation of new composites) purposed to characterise the previously developed composite materials to evaluate their performance. Task 4 (Development and performance evaluation of LSF elements) aimed to evaluate the thermal and acoustic performance and optimize the use of the material in LSF walls in order to take maximum advantage of it. Task 5 (Life cycle assessment) focused on a life cycle assessment of the new insulation material in terms of environmental impacts and costs. Task 6 (Dissemination) involves the dissemination of the results of the research project using a website [6] and the organisation of a technical workshop [16].

3.3 Research Team and Main Outputs

The research team included the participation of the Chemical and Civil Engineering Departments of the University of Coimbra. While the first department focused on the development of the innovative composite insulation material, the second aimed at its evaluation and optimization at the building element level (e.g., as thermal break strips (TBS) in LSF walls). The main outputs achieved until now [6], include: 21 scientific publications (e.g. books and book chapters, articles in international and national scientific journals); 14 communications in international and national scientific meetings; 9 PhD and Master thesis concluded; 1 computational application, and; 2 provisory patents applications (INPI and WIPO [17]).

3.4 Manufacturing Process of the New Rubber-Silica Aerogel Insulation

Among these outputs, it should be highlighted the new rubber-silica aerogel composite reinforced with fibres, where the rubber and the fibres can be obtained from recycled tyres [18], and for which the manufacturing process is now protected by an international patent application to WIPO—World Intellectual Property Organisation [17]. Figure 8.2 illustrates the main steps to reclaim the end-of-life tyres, from the process of recycling their components to the conversion into superinsulating thermal break strips (TBS) for LSF walls [19].

Fig. 8.2
Six photographs of the main steps to reclaim the end-of-life tyres, including 1. end-of-life tyres, 2 a. recycled rubber, 2 b, recycled textile fibers, 3. devulcanized rubber sol with silica sol, 4. rubber-silica gel, 5. rubber-silica gel, rubber-silica aerogel, and 6. superinsulation of L S F walls.

From end-of-life tyres to rubber-silica aerogel and their application as super-insulating thermal break strips in LSF walls

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The manufacturing process to produce the superinsulation composite made of silica aerogel and recycled tyre rubber reinforced with fibres was described in Reference [18]. First, the recycled tyre rubber was dissolved using a Peracetic acid alcoholic solution, to obtain a sol of rubber which can be promptly mixed with the silica sol. The latter is obtained from hydrolysis and condensation of silica precursors (tetraethyl orthosilicate and organosilanes). Notice that a “sol” is a colloidal suspension made from tiny solid particles in a continuous liquid medium. Next, a gelification process occurs to obtain a rubber-silica gel. Just before gelification, this solution was poured over reinforcement fibres inside containers. Several reinforcement fibres were tested, including recycled tyre textiles, polyester fibres, silica fibres and glass wool.

3.5 Thermal Performance of the New Insulation Composites

The thermal conductivity of these new composite insulation materials was measured using two different methods: (1) Transient Plane Source (TPS) method using a Thermal Constants Analyzer TPS 2500 S (Hot Disk, Göteborg, Sweden), with two similar samples maintained at 20 °C, and; (2) Heat Flow Meter (HFM) method using an HFM 436/3/1 Lambda (EN 1946-1:1999), from NETZSCH (Selb, Germany), at 23 °C.

The measured thermal conductivities for each measurement technique and for the different reinforcement fibres used, are listed in Table 8.1. The conductivities measured using the HFM method exhibit smaller values than those of the other method (TPS). Notice that the HFM method is more accurate for insulating materials and the test samples are larger: 21.5 × 21.5 × 1.6 cm3 [18]. When the thermal performance of the several new composite insulation materials are compared, it can be concluded that the best performance was achieved when using the polyester fibres (16.4 ± 1.0 mW/m/K), while the worst was obtained by the glass wool reinforcement fibres (28.7 ± 1.7 mW/m/K).

Table 8.1 Measured thermal conductivities of the new fibre-reinforced rubber-silica aerogels (Adapted from Lamy-Mendes et al. [18])
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3.6 Enhanced Thermal Performance of LSF Walls

These new superinsulation composite materials may have many applications in buildings, not only in the thermal domain, but also in acoustics given their good vibration attenuation, e.g., floating floors [20] or LSF walls [21]. In the Tyre4BuildIns research project [6] this new superinsulation composite material was mainly evaluated as thermal break strips (TBS) in LSF partition [14] and facade walls [13].

Moreover, some numerical simulations were performed, as well as measurements of thermal resistance under controlled lab conditions. Table 8.2 lists the measured conductive \(R\)-values for partition and facade load-bearing walls without TBS (“Reference”), as well as the thermal resistances when using a single aerogel TBS on the outer steel stud flange (“1 TBS”), and two aerogel TBS (one in the outer flange and another on the inner flange). The thermal resistance improvement was for the LSF partition walls, ranging from +42% (1 TBS) up to +77% (2 TBS), relative to the \(R\)-value of the reference wall (1.558 m2·K/W). The thermal performance improvement in the LSF facade walls was smaller, ranging from +15% up to +28%, relative to the corresponding reference \(R\)-value, i.e., 3.200 m2·K/W. This reduced \(R\)-value increment was expected due to the existence of an external thermal insulation composite system (ETICS), which reduces the thermal bridge effect originated by the steel frame.

Table 8.2 Measured thermal resistances of load-bearing LSF walls
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4 Final Remarks

In this paper, the lightweight steel-framed (LSF) system and the research project Tyre4BuildIns were presented within the scope of buildings, modular constructions, circular strategies, waste management and energy efficiency for a more sustainable built environment. First, a short review about modular construction and the main advantages of the LSF system was presented. After, the Tyre4BuildIns research project was described, including the main objectives, tasks and subtasks, the research team and the results, the manufacturing process of the new rubber-silica aerogel insulation material, thermal conductivities achieved and the enhanced thermal performance of LSF walls using thermal break strips (TBS).

It was concluded that, compared to traditional construction (e.g., reinforced concrete and masonry), LSF modular construction system can contribute significantly to a more sustainable built environment, being more suitable for the implementation of circular strategies (e.g., design for deconstruction and flexibility), due to its inherent advantages. Moreover, the Tyre4BuildIns research project allowed to develop a high-performance thermal insulation composite material based on recycled tyre rubber and aerogel. This new eco-friendly thermal insulation has a thermal conductivity similar to the state-of-the-art commercial aerogels available in the market. When used as TBS along the steel stud flanges in LSF building elements, allowed to significantly enhance their thermal performance. This increase in thermal resistance reached +42% and +77% compared to a reference partition LSF wall, when using one and two TBSs, respectively. For LSF facade walls, the increase in \(R\)-value was more modest (+15% and +28%, respectively), due to the influence of ETICS, which mitigated the importance of the steel frame thermal bridge effect.

Regarding the eventual reuse of the new TBS, it is facilitated by the way these TBS are fixed to the steel stud flanges, using screwed mechanical connections. Thus, the TBS could be easily removed after unscrewing the bolts and removing the adjacent sheathing panels (e.g., gypsum plasterboard or OSB).

5 Funding and Acknowledgements

This research was funded by FEDER funds through the Competitivity Factors Operational Programme–COMPETE, and by national funds through FCT, Foundation for Science and Technology, within the scope of the project POCI-01-0145-FEDER-032061. The authors also want to thank the following companies, partners of the Tyre4BuildIns project: Pertecno, Gyptec Ibéria, Volcalis, Sotinco, Kronospan, Hulkseflux, Hilti, and Metabo.