Timber-Reinforced Concrete and Its Application in Portugal

Abstract

Traditional construction is an important skill. This paper intends to contribute to the literature on the topic of timber-reinforced concrete. First, some traditional Portuguese timber solutions are introduced. Second, some traditional Portuguese timber-reinforced concrete solutions are presented. Third, a sustainable concrete that uses waste from industrial timber aggregate is proposed. Some experimental work is also presented, along with some material properties. Fourth, an expedited experimental study of slender timber-reinforced concrete columns is introduced. Subsequently, some traditional Portuguese solutions for increasing the connection between timber and concrete are presented. The benefits of combining traditional techniques with modern materials for achieving greener and more resilient building solutions are emphasized.

1. Introduction

Traditional construction provides a way to learn about sustainable building solutions. In the North of Portugal, this concept is particularly relevant. This region is notably rich in traditional construction practices, supported by the abundant availability of local raw structural building materials. These include stone, timber, suitable soil, and water. As a result, the principal traditional building techniques used in this region are stone masonry and Tabique. Given the age of these structures, it is evident that they possess considerable resilience. Furthermore, they are well integrated into the surrounding landscape. The most commonly used stones are granite and schist, while the primary timber species employed are pine (Pinus pinaster) and Portuguese oak (Quercus Faginea). Several researchers have investigated traditional stone masonry and Tabique structures, as documented in references [1,2,3,4], respectively. The traditional timber-framed Pombalino construction system was developed right after the 1755 earthquake of Lisbon and was the first technical regulation addressing seismic resistance. Nowadays, it is considered a cornerstone in the Portuguese Local Seismic Culture and influences vernacular architecture in Southern Portugal [5,6]. This innovative system, required during the reconstruction of Lisbon, included a three-dimensional braced timber frame structure called the “Gaiola Pombalina”, which allowed flexibility and resistance to horizontal forces, ensuring the structural integrity of buildings during seismic events [7,8]. In addition, Tabique, a traditional earth and timber construction method from Portugal, carries a lot of cultural values in the Trás-os-Montes e Alto Douro region; however, many of these Tabique structures currently exist in highly deteriorated states, making the conservation of such rich architectural heritage very relevant [9,10]. The use of timber structural elements, especially Glulam, in Portugal has become a fast-moving area due to the competitive properties of other construction materials. It reflects the trends towards using more environmentally friendly and greener options in general within Portugal when it comes to construction. The demand for timber is largely based on two factors, i.e., its early structural capability and being a renewable resource, so it attracts green-conscious developers and builders [11]. There is also a growing interest in exploring the load-bearing potential of Portuguese hardwoods, such as poplar, which aligns with the broader objectives of the construction sector to enhance sustainability, particularly through the use of renewable resources. This trend reflects Portugal’s efforts to promote the use of local resources while contributing to a more sustainable built environment through the application of native hardwoods to structural uses. These new developments in timber construction demonstrate Portugal’s commitment to balancing modern construction demands with principles of environmental stewardship [12]. Traditional timber solutions are based on sustainability and conservation, as they are naturally sustainable since the construction is based on local and natural materials. These require minimum energy use and produce low emissions, making them environmentally friendly. Additionally, the rehabilitation and conservation of traditional timber structures have greatly emphasized compatible materials and techniques. This approach preserves the historic value of these structures while guaranteeing their continued environmental value in the name of sustainability and a conservationist future [13,14].

In fact, rehabilitation of heritage is one of the most promising aspects of the Portuguese building industry [15]. It is important to note that traditional reinforced concrete and steel construction methods are already established within the Portuguese context. Furthermore, the combination of timber and concrete is an existing building technique [16]. In fact, cement can agglutinate and protect timber or timber-based elements such as cement-bonded particleboard [17]. Combining timber and concrete presents an interesting building solution that may contribute to reducing the environmental impact of reinforced concrete (RC) structures. It can also lead to more durable constructions and more efficient structural behaviour, particularly by benefiting from the lighter weight of the structure. This latter advantage is especially relevant for prefabricated and modular reinforced concrete construction techniques. The main objective of this document is to provide information about traditional Portuguese timber-reinforced concrete structural building elements, while also exploring the potential of using this approach in slender timber–reinforced concrete columns. In this context, a detailed experimental study was conducted and is presented in Section 4 of this paper. Following this introduction, the paper is structured as follows: first, some traditional Portuguese timber-reinforced concrete solutions are presented, including Tabique walls, timber floor reinforcement, and chimney structures. Subsequently, a study on slender timber-reinforced concrete columns is introduced. In addition, traditional Portuguese techniques designed to enhance the connection between timber and concrete are also discussed. Finally, concluding remarks are provided. It is concluded that combining timber and concrete can serve as an environmentally friendly construction solution, a fact already demonstrated by traditional Portuguese building practices.

2. Examples of Traditional Portuguese Structural Timber Solutions

Historically, timber has been one of the most important materials in traditional Portuguese construction, showing versatility and sustainability [1,2,18,19]. At the structural level, timber has been widely used in various building components due to its availability, mechanical properties, and adaptability. The left image in Figure 1 shows timber pavement with the traditional construction practice using wooden beams to support flooring systems. These beams provide structural integrity while offering flexibility under loads. The layout often consists of a series of longitudinal beams carried on either walls or secondary timber elements. This geometry reflects the efficient use of materials and ease of assembly, typical of vernacular Portuguese architecture. The right image in Figure 1 presents a particular timber roof structure representative of the traditional construction type. The truss-like structure in the primary load-carrying frame of the roof is made from timber rafters. The sloping configuration provides good drainage for rainwater, while the application of the terracotta tiles acts to insulate and provide weather protection. This is indeed a very exemplary system with a successful combination of timber and local materials that contributes to both functionality and aesthetic appeal. The visible geometry of the roof indicates careful consideration of load distribution and resistance to wind forces, showing the ingenuity of traditional design solutions.

Thus, we can conclude that timber is a traditional structural building material.

At the same time, we also realise that timber works well with stone (Figure 1, Left), ceramic (Figure 1, Right), and earth (Figure 2, Left).

The last dwelling shown in Figure 3 is a good example of a resilient traditional Portuguese building. In fact, it is over one hundred years old. Since it has been abandoned and neglected for a long time, it has been fading slowly instead of facing a premature collapse. This example of a dwelling also has an additional particularity, which is that the timber structural elements of the exterior walls are covered with earth. Earth protects the timber by increasing its durability and fire resistance.

It is important to emphasize that until the early 20th century, cement was not yet proliferating in the Portuguese building industry.

3. Examples of Traditional Portuguese Timber Reinforced Concrete Solutions

After the above-mentioned period, cement became more available in Portugal. Therefore, cement-based mortar and concrete started to be used more often with timber. For instance, the partition wall in the left image of Figure 4 exemplifies this building technology solution. The vertical timber boards and the cement-based mortar applied on both sides of the wall are visible. Considering the compressive strength of the cement-based mortar, we are dealing with a mixture of structural building elements. As seen in the right image of Figure 4, a timber pavement was reinforced with a layer of cement-based concrete. This reinforcement was local and aimed to increase the pavement’s resistance in this area because it is related to a kitchen floor (the case study in Figure 4 [right]). An enormous granite stone had to be placed to work as a fireplace. Additionally, the cement-based concrete layer also aimed to increase fire resistance and improve the waterproof behaviour of the pavement. It is important to note that the cement-based concrete layer had been reinforced with timber elements, as shown in the right image of Figure 4. Additionally, an example of a traditional Portuguese timber-reinforced concrete chimney is shown in Figure 5 (left and right). In both cases, it is noted that the construction materials (e.g., timber and concrete) are very well preserved. No pathologies can be observed because the concrete preserves timber elements. The timber preservation is needed to improve the durability of wood and avoid damage from environmental/biological attacks for better sustainability in different applications. A major reason for preserving timber is to prevent biological deterioration caused by fungi, insects such as termites and wood borers, bacteria, and marine borers, all of which can weaken or degrade timber structures. Timber preservation also increases the longevity of the timber, reducing replacement cycles and extending the service life of structures like bridges, poles, and buildings. Additionally, timber is vulnerable to moisture and weathering, which can cause warping, cracking, and decay; applying preservatives helps to maintain dimensional stability and prevent these issues. Some treatments also enhance timber’s fire resistance, which makes timber safer to use in construction work.

From a cost perspective, conservation is cost-saving in terms of maintenance and replacement, and therefore it is a cost-saving strategy in the long run. Furthermore, prolonging the life of timber reduces the demand for new wood, resulting in forest conservation and maximising the utilisation of natural resources in a sustainable manner. Some treatments also improve the mechanical performance of timber, which can make it stronger and more resistant to mechanical stress, particularly for load-carrying structures such as railway sleepers, utility poles, and marine pilings.

4. Study of Slender Timber-Reinforced Concrete Columns

4.1. Slender Concrete Columns

The traditional building solutions presented above show that timber and concrete can work together. Therefore, they can be adopted in new construction for this purpose. At the same time, this technical advantage can be considered for other structural concrete elements. In this context, slender timber-reinforced concrete column represents one such application.

Usually, a slender concrete column is an architectural requirement. This type of column tends to have a small cross section such as the slender reinforced concrete columns (Figure 6, left), a slender MICADO reinforced concrete column that includes a PVC pipe as a mould, or slender steel columns (Figure 6, right).

In addition, we can also come across this possibility with modular reinforced concrete solutions. The MICADO reinforced concrete modular solution [20] is one example (Figure 7, left).

Alternatively, for a reinforced concrete or steel slender column, we may use a timber-reinforced concrete slender column. As a first step, a simple model (Figure 7, right) was created to understand the behaviour of this system.

4.2. Timber Reinforced Concrete Slender Columns

In recent years, much attention has been paid to the use of timber as a reinforcing material in concrete, mainly as a means to reduce the environmental impact of traditional reinforced concrete structures. This section presents a novel approach to the design of a slender column of timber-reinforced concrete, where both the concrete and reinforcement are based on timber. In other words, the experimental work concerning the study of a new timber-based concrete is described (Section 4.2.1). The preliminary experimental work concerning the proposal of a timber-based reinforcement column is also presented (Section 4.2.2). The results obtained are promising.

4.2.1. Proposal of a Timber-Based Concrete

This study aims to evaluate the possibility of using a wooden material as a replacement for the aggregate in ordinary concrete. The aims of this study include the following.

(a)

The production of concrete that is expected to be used in construction activities where very little compressive strength is needed. These activities include but are not limited to non-load-bearing elements or areas with low suspected seismic activity.

(b)

More environmentally sustainable concrete made with a new natural and renewable material as a substitute to minimise the carbon emissions associated with ordinary concrete use and production.

For this purpose, we set out to use carpentry wood waste for this concrete, and as depicted, this waste has a very large and varied composition of different solid wooden and wood waste with different lengths and volumes (Figure 8, left).

As can be seen in the left image of Figure 9, this type of waste has dimensions much larger than those of concrete aggregates; as such, it was necessary to adapt it. For this purpose, an existing cutting machine for carpentry was used (Figure 8, right). However, after this procedure, the pieces of wood were still larger than normal gravel (Figure 9, left). To correct this, a jackhammer was used, and the wood waste aggregate began to present a more appropriate particle size, as shown in the right image of Figure 8.

In addition to this wood waste aggregate (M), Portland cement (C) class C32, river sand (A), and water (W) were also used in the manufacture of the concrete under study. At this stage, a W/C/A ratio of 1:1.5:1.5 (in volume) and a W/C ratio of 0.5 were considered. The traditional aggregate was also completely replaced with wood waste aggregate. When preparing the first mixtures, it was found that it was convenient to wet the wood waste aggregate first (Figure 9, right). After four days of drying under laboratory thermo-hygrometric conditions, the specimens were removed (see Figure 10).

Using timber as an alternative to current aggregate (e.g., granite gravel, for instance) may be a user-friendly option when strength is not the key mechanical property. In fact, timber may be local, renewable, organic, and does not require heavy industry, among other sustainable attributes.

The weights of the specimens were measured regularly at 7, 14, and 28 days of age during the drying process, which was carried out under controlled laboratory conditions. Such a systematic approach allowed for a thorough understanding of the moisture loss over time and provided useful information about the drying behaviour of concrete. The gradual nature of the drying process is graphically represented in Figure 11, which illustrates the decrease in weight as a function of time.

Manufactured specimens exhibited an average mean density of 1474 kg/m³. Considering this density, compared with conventional normal-vibrated-density concrete with density values of about 2300–2400 kg/m³ at ordinary strength in different water conditions, concrete is lightweight with a lower density below 1800 kg/m³ as cited by Portuguese Standard NP EN 206 [21]. Seeing that the average density of the test specimens is lower than this, the concrete under study can be considered lightweight due to its low specific mass. Since it has been established that the concrete is lightweight, the range of applications that the concrete may have made possible could also come with several advantages: reduced dead load, improved thermal insulation, and increased durability. However, the trade-offs involved in using lightweight concrete, such as possible reduction in compressive strength and increase in susceptibility to shrinkage, need to be explored against its benefits. Further investigation of the actual drying and what happens to various properties during drying to fully explain the nature of lightweight concrete might consider the concrete under different curing conditions or composed of different kinds of aggregates, as well as with variations in active admixtures.

Figure 11 shows the drying behaviours of three specimens, P1, P2, and P3, as their weight decreases with time, indicating water evaporation through the curing process, a common trend for concrete material as water dries out with hydration and exposure to the atmosphere. Such drying has a considerable influence on the mechanical behaviour of the concrete, especially in terms of compressive strength, as excessive water evaporation may result in internal stress and possible cracking. At 28 days, the specimens were tested for compressive strength, with the results detailed in

Table 1. Compressive strength results and some mechanical properties.

Parameter	P1	P2	P3
Mixing Date	06/17/2022	06/17/2022	06/17/2022
Test Date	07/15/2022	07/15/2022	07/15/2022
Age of Specimen (days)	28	28	28
Storage Conditions	Room temperature	Room temperature	Room temperature
Dimensions of Specimen (cm)	15 × 15 × 15	15 × 15 × 15	15 × 15 × 15
Mode of Failure	Normal	Normal	Normal
Weight of Specimen (kg)	4.823	5.032	5.093
Failure Load (kN)	77.7	98.6	94.3
Failure Stress (MPa)	3.5	4.4	4.2

, revealing an average compressive strength of 4 MPa. This is below the minimum threshold of 8 MPa for lightweight concrete as specified by NP EN 206-1:2007. This underperformance therefore raises concerns about the suitability of the current concrete mix design, especially with the incorporation of wood waste aggregates, which usually present higher water absorption and lower stiffness than conventional aggregates, leading to a reduction in compressive strength. The results for individual specimens show variability in mechanical performance: P1 exhibited the lowest failure stress at 3.5 MPa, P2 the highest at 4.4 MPa, and P3 slightly lower at 4.2 MPa. These differences may stem from variations in specimen density or non-uniformity in mixing and curing. Despite these differences, all specimens failed in a “normal” and predictable way, as predicted for concrete under compressive loading, which is further confirmed by Figure 12, which shows the failure patterns. Hence, this lower compressive strength would mean the inclusion of finer or treated wood wastes along with pozzolanic additives, such as silica fume, that will increase the bond strength through the optimisation of the water-to-binder ratio. Also, this workability, coupled with porosity, can be further enhanced through the use of chemical admixtures like superplasticisers. This extreme drying may be counteracted by different curing alternatives, including water curing and sealed curing that allow for better hydration, which may then aid in achieving a higher compressive strength. The obtained compressive strength of 4 MPa is lower than the minimum requirements set for lightweight concrete in NP EN 206-1:2007, with a minimum value of 8 MPa. This result needs further investigation regarding other mix designs or further modification of the present mix, particularly with regard to wood waste aggregates, to attain better mechanical properties of the concrete. Long-term mechanical and durability tests, such as tensile strength and freeze-thaw resistance, along with microstructural examination by SEM, are recommended to assess the feasibility of this material for practical applications.

In the preliminary phase of the current study, the aim was also to understand the ability of concrete based on wood waste to adhere to metallic and wooden elements. To this end, a circular tubular steel profile (Figure 13, left) and a circular wooden element (Figure 13, right) were used. The two structural elements did not undergo any type of treatment. After concreting and drying, it was observed that in the case of the metal tube, there was good adhesion between the two materials (Figure 13, left). In contrast, in the case of the wooden element, there does not seem to have been good adhesion between the materials. In fact, a joint emerged between the wood waste-based concrete and the wood element (Figure 13, right). In the latter case, perhaps pre-wetting the wooden element or ensuring some surface roughness will allow adhesion between materials. With these quick tests, it was also realised that it is possible to create a covering layer with concrete based on wood waste.

The research was also supposed to investigate the fire-resistant capacity of the studied concrete and its efficiency in case of fire for the protection of the structural elements. For that purpose, a quick-fire resistance test was conducted on the specimen concrete, as depicted in Figure 14 (right). Heating one side of the specimen with a controlled heat source—a torch—for 10 min, as shown in Figure 14 (left), enables the execution of the test. For the entire duration of the test, no evidence of combustion was recorded on the wood-waste-based concrete specimen. This was evident enough that the material did not show any visible damage, and there was no evidence of smoke formation or the release of toxic gases. These results show that the concrete, even with the addition of wood waste granulate, has a high resistance to fire. The layer of concrete with wood residue also appeared to provide good thermal insulation for the steel pipe embedded in it. The temperature readings followed the trend below. Before the test, the steel pipe had an initial temperature of 26 °C. After 3 min of exposure, the temperature remained constant. A gradual rise in temperature was realised after only 6 min of testing. At the end of the 10-min test, the inner surface of the steel pipe reached a temperature of 42 °C. However, the temperature on the surface of the specimen facing the torch flame reached as high as 153 °C, as reflected in Figure 14 (right). These results indicate that the concrete made from wood waste is not only fire-resistant intrinsically, but also acts as an effective insulation barrier to minimise heat transfer through it to the underlying structural elements like steel. This makes it a prospective material for fire-resistant construction.

At this stage, only the steel tube cube was tested under fire. In fact, the main goal was to verify whether the new proposed concrete was fire resistant itself and also provided fire protection, which was validated by this expedited test.

Fire protection can be further improved by combining concrete with other insulating materials, such as glass wool. For instance, a glass-wool-concrete cover has demonstrated better performance in maintaining lower steel temperatures during fire exposure [22].

Other combined solutions include using rockwool products with different densities to improve the fire resistance of steel structures [23]. Due to its internal moisture content and because it forms a char layer while burning, wood is essentially thermally insulating. A char layer acts like an insulation barrier that slows the rate of heat transfer to steel members. Because the wood is moist, this would delay temperature rise and perhaps allow the structural steel members to survive longer in fire conditions [24]. Based on the above observations, one can reasonably infer that the concrete cover mixed with wood waste provides some shielding to the steel members during a fire. A similar assumption can be made for the case of wood members, but this assumption needs more empirical verification through extensive testing to establish its authenticity. Additionally, the concrete cover based on wood waste could have the potential to delay the oxidation of the steel elements; however, it is just speculation at this stage and requires further experimental research to be confirmed. Concrete structures are prone to spalling under high temperatures, which can expose the steel members to direct fire; however, the addition of fibres, including organic fibres like those from wood waste, can help prevent fire-induced spalling by creating pathways through which steam can escape, thereby reducing internal pressure build-up. The thermal mass effect of concrete due to the partial replacement of wood waste will also increase the fire endurance time due to the higher internal moisture concentration that delays the temperature rise within the concrete. Besides this, the wood-waste-incorporated concrete will increase sustainability by recycling waste materials, reducing construction costs, thus proving to be a cost-effective and environmentally friendly solution in terms of fire resistance [24,25,26]. Also, in this context, some of the tested concrete specimens had been immersed in water in a container for one week. The specimens, while being removed, did not exhibit any signs of material disintegration, hence having a certain resistance to water-induced degradation and a higher durability.

4.2.2. Proposal of Timber-Based Reinforcement

Alternatively, instead of reinforced concrete or steel slender columns, we may use a timber-reinforced concrete slender column. As a first step, a simple model (Figure 15) was used to understand the behaviour of this system.

It was noticed that this technical solution may be promising. At this stage, the adhesion between timber and concrete raises more concerns. However, according to traditional Portuguese building techniques, we can find some guidance, described in the following.

5. Examples of Traditional Solutions to Increase the Connection Between Timber and Concrete

Traditional solutions to enhance the connection between timber and concrete have been largely documented in the historical and cultural context, mainly in countries like Portugal with a rich architectural heritage. Using the irregular shape of timber elements, moisturising the surface of the timber elements, and making the surface of the timber elements rugose (Figure 16, left) are some possible options.

These techniques provide not only improvements in structural safety but also manifest the ingenuity of traditional craftsmanship in solving problems related to the compatibility and durability of materials [27]. One of the approaches includes using irregularly cut timber pieces that provide a mechanical interlock with concrete, increasing the area of contact and thus improving the bond between the materials. Smearing moisturiser on timber surfaces before embedding them in concrete improves adhesion by allowing concrete to penetrate through the porous structure of wood [28]. Other methods that have been very effective are those where the rugosity of timber elements (Figure 16, left) provides a textured interface to give a good bond to the concrete. The addition of a steel net to the covering layer of concrete, as shown in the right image of Figure 16, reinforces this system considerably. With this approach, stress sharing becomes highly uniform, localised failure is reduced, and seismic advantages due to this technique may prevent the collapse of the structural members during earthquakes [7]. This technique, when combined with vertical timber elements nailed to the timber pavement, enhances anti-seismic behaviour due to the addition of flexibility and energy dissipation capabilities [29]. These traditional solutions underpin the need for material compatibility and innovative design in creating strong and resilient structures, thus serving as a useful inspiration for modern engineering practices, especially in seismic areas. By learning and combining these ancient technologies, today’s architects and engineers can develop hybrid timber–concrete systems that integrate traditional knowledge with modern technology for stronger and more sustainable construction.

6. Conclusions

Traditional Portuguese construction can indeed yield a wealth of information in terms of sustainable and resilient buildings. The use of local, natural, and reusable materials like timber, stone, and earth in vernacular architecture represents an environmentally friendly approach that could be used to inform modern green development. These traditional methods not only bring sustainability benefits but also offer inherent seismic resistance in many cases. For example, traditional Portuguese timber frames—including the remarkable “gaiola” system, a timber frame structure—were widely applied to roofs, floors, and reinforced masonry walls, showing excellent seismic resistance. Timber was also used in light-framed partition walls (Tabique) and timber-reinforced masonry walls (frontal walls). Similarly, in northern Portugal, stone masonry, mainly with granite and schist, was widely used, while the Alentejo region showed traditional architecture with stone, adobe, and fired brick. The ingenuity of traditional stone construction is further manifested by unique examples, such as the round stone houses of the village of Piódão, constructed without mortar. Earth-based techniques, such as adobe, rammed earth (taipa), and wattle and daub (Tabique), were used significantly in regions like Estremadura, Alentejo, and Algarve, by taking advantage of local materials and adapting well to climatic conditions.

In contrast, modern concrete construction creates severe environmental challenges. The production of cement, especially, accounts for some 8% of man-made CO₂ emissions, has a high demand for water and sand, and contributes to urban heat islands and surface runoff, although light-coloured concrete can help to reduce some of the effects. With these considerations in mind, sustainable approaches in modern construction can learn from traditional Portuguese techniques while maintaining the integrity of structures. Hybrid systems, which are a mix of traditional timber elements and concrete, can give increased durability and help with fire resistance while reducing the environmental impact. Improved concrete mixes made using recycled materials and industrial by-products, such as fly ash and silica fume, as cement substitutes can massively reduce the carbon footprint of concrete. This also saves cultural heritage through the adaptive reuse of traditional structures, rather than the environmental cost of new construction. Biomimicry, inspired by traditional techniques that are adapted to local environments, can also be used to develop more sustainable modern designs.

Merging traditional Portuguese construction wisdom with that of modern technologies and materials results in buildings that are sensitive to the environment, have cultural relevance, and are resistant to seismic activity. This integrated approach can improve building life cycles and contribute to a more sustainable built environment in general.