Modern concrete construction uses various types of formwork made from different materials: timber, plywood, steel, aluminium, and plastic, with each having its own advantages and disadvantages in terms of cost, durability, flexibility, and environmental impact ((Das et al., 2016); (Devi & Yadav, 2023)). Formwork selection can also have a major impact on the surface quality and finish of concrete being cast, as well as its strength development and durability (Nilimaa et al., 2023)(Terzioglu et al., 2021). With the need to reduce material usage and adopt circular economy principles, more emphasis has been laid recently on reusable formwork that can be removed after use, cleaned, and used again (Gappmaier et al., 2024). Thus, several studies have been carried out to assess the potential of formwork reuse and have shown that, despite its vulnerabilities, timber and wooden formworks have good potential for being recycled and reused on construction sites. These are briefly summarised below:

A study by (Mukhopadhyay et al., 2022) on the resizing and reuse of construction materials identified several advantages of the reuse of timber formwork. The benefits included ease of management and handling of the used timber formwork materials for reuse, the cost efficiency achieved through reduced material usage, and improved waste generation control.

In their review study on formwork systems, (Li et al., 2022) carried out a comparison of several materials, including wooden, metal, fabric, and 3D-printed plastic formwork. They posited that although the material cost for wooden formwork is lower than for metal formwork, they have higher labour costs and offer only a medium quality surface finish. However, the wooden formwork can be used for a range of shapes, including complex geometries, and can also be recycled and reused.

(Pronk et al., 2022) adopted a case study approach to assess the potential for reuse of wooden formwork through the use of 10 construction projects. They found that although large amounts of timber formwork are used, their reuse is still limited, with the maximum number of use cycles as formwork being determined by the roughness of the formwork and the project characteristics. The authors eventually posited that although timber formwork may not be reused for elements that are visible and need to be aesthetically pleasing, they offer vast potential for other purposes, as they still possess good mechanical properties. However, the study also pointed out that several challenges need to be overcome for mainstreaming material reuse, as although waste and cost reduction can be achieved by using reclaimed materials, the design and construction process is rendered more complex and can increase the total project budget owing to additional logistics and planning issues.

Using a mathematical modelling approach, (Mei et al., 2022) argued that reusing formwork among several construction sites led to a reduction of the total cost across the whole supply chain while improving sustainability. They further posited that the reuse of materials could also prevent the underutilisation of undesired materials through the development of detailed material planning and hence lead to a significant reduction in the construction schedule as well.

(Cheng et al., 2022) elaborated on the reuse of timber formworks for traditional in-situ concrete casting. They discussed the actual use of this type of formwork, which is usually purchased in standard sizes before being processed according to the design and construction requirements, thus generating waste. Moreover, with construction schedule requirements, several sets of timber formwork are required to prevent delays, leading to a surplus of materials on the site. The authors also posited that the reuse of timber formwork is limited to 3 – 8 times before being discarded due to large deformations and significant material accumulation on the surface.

There has only been sparse research on the impacts of the reuse of wooden formwork on concrete properties, with the few studies summarised below:

(Jiang et al., 2025) investigated the impact of the choice of three formwork materials (plastic, plywood, and steel) on the aesthetic properties of concrete surfaces and found that the number and frequency of bugholes were highest for steel formwork, while minimal for the plastic one. Moreover, in terms of concrete surface roughness, the highest values were obtained for plywood, followed by steel and lastly plastic, and this mirrored the increasing formwork roughness closely after several reuses of these materials. It was also determined that while the appearance of concrete made using the different formworks was similar initially, larger than 10 times plywood formwork reuse led to a significant concrete colour difference. The authors finally concluded that the quality of concrete decreases with increasing reuse, depending on the formwork material, with the plastic formwork being mainly subject to physical damage due to friction and paste adhesion, while the plywood and metal formworks suffered from both physical and chemical damage, mainly as a result of alkali corrosion. The study also posited that increasing the reuse of plywood formwork led to a dusting phenomenon on its surface, which increased air void defects and roughness of the concrete surface, leading to poor-quality concrete.

In another study to evaluate the impacts of formwork materials on the sorptivity of concrete cover and the durability of concrete, (Aïssoun et al., 2017) compared plywood and PVC formworks, with the former being rougher and more absorbent. They observed that while the smooth and non-absorbent PVC formwork did not have any major impact on the rheological properties of fresh concrete, the rough and absorbent plywood surface led to a reduction in the water-to-cement (w/c) ratio and fluidity of the cement paste. They also noted that the wall effect, i.e., the movement of coarse aggregates near the formwork surface, was greater for plywood due to the increased water absorption and surface roughness, leading to a decrease in movement of coarse aggregates near the surface and therefore a higher packing density. Thus, concrete cast in plywood formwork had lower sorptivity of concrete cover and ultimately higher durability.

Earlier research by (Rubaratuka, 2013) found that the use of absorbent formwork materials had a beneficial effect on concrete surface properties, with an increase in concrete strength and hence durability. The author compared 4 types of materials, namely softwood timber, hardwood timber, plywood, and steel, and concluded that concrete compressive strength was highest for the softwood and plywood formwork and lowest for the steel formwork. This was explained by the fact that softwood formwork had the greatest water absorption and thus allowed greater dissipation of water and entrapped air that migrated to the interface of concrete and formwork.

The mechanism of action and impacts of water-reducing admixtures on concrete properties are well documented in the literature. This type of admixture allows for a reduction in the water content and thus a lower w/c without decreasing the workability and the slump range required, thereby leading to concrete mixes achieving higher strength, density, and durability (Bukhari & Khanzadeh Moradllo, 2025). Moreover, using admixtures allows achieving the required concrete grade with lower cement content while also making the task of concrete placement easier in difficult site conditions. Conventional water-reducing admixtures allow for a reduction in the water content by at least 5% and thus allow for densely packed cement grains for higher durability (Lokesh et al., 2023).

In Mauritius, pine timber formwork is widely used due to its versatility and local availability, especially in residential and small-scale projects. Traditionally treated as a single-use material, timber formwork contributes significantly to waste and environmental degradation. Therefore, for this study, pine timber formwork has been used. Three categories of formwork were considered: new formwork, formwork used once (1^st use formwork), and formwork already used three times (3^rd use formwork). These formworks were all obtained from the same site to ensure they all experienced the same working conditions in terms of weather and handling. Steel moulds were used as the control for comparing the results obtained. Pictures of the formworks used are shown in Figure 1 below.

The other materials used were also adopted based on usual site practices to prepare the different mixes of concrete considered in the research: Ordinary Portland Cement (CEM II 42.5), fine aggregates (0-4 mm), coarse aggregates (6- 10 mm, 10-14 mm, and 14- 20 mm), and tap water. A liquid water-reducing concrete admixture was used in this case, namely Sika Plastiment R.

Concrete Grade 35 was used for this study, with 3 different mixes considered, each having different slumps. Mix 1 consisted of concrete with a slump of 30-60mm, while both Mix 2 and Mix 3 were designed for a 60-180 slump, with the difference that Mix 3's slump was achieved with the use of an admixture and reduced water content. The BRE method (Establishment, 1988) was used to design the concrete mixes, and a mix proportion of 1:1:1 was used for the coarse aggregate (6- 10 mm, 10-14 mm, and 14- 20 mm). In total, 144 cubes of 100x100x100mm were cast, which included samples for the steel control. The casting was carried out based on BS 1881:Part 125:1983 (Institution, 1983). Table 1 shows a summary of the composition of the different mixes for 1m³ of concrete. The slump was verified after casting using the slump test in accordance with BS 1881: Part 102:1983 (Institution, 1983) procedures.

Figure 2 illustrates the slump test and demoulding of the cubes for curing and testing. Curing was done according to normal BS test procedures, with the cubes demoulded and immersed completely in a water tank until testing.

For this study, 3 tests were considered, namely the compressive strength test, the water absorption test, and the chloride content test. All these tests were carried out as per the standards, as shown in Table 2 below. The tests were performed after allowing the samples to cure for 7 and 28 days at room temperature.

The test was performed based on BS-EN 12390-3:2019. The test was carried out on the 7th and 28th day on compressive strength. Three individual samples were used for all compressive strength tests carried out. The results are illustrated in Figure 3. A total of 144 cubes were cast: 36 cubes were cast for each mould type, with 12 for each of the 3 mixes to be used as follows: 3 for 7th day compressive testing, 3 for 28th day testing, 3 for water absorption test, and another 3 for chloride testing. Each test was hence replicated 3 times. The mean values of the concrete compressive strengths were calculated and used for each mix and formwork type. It was noted that for all mixes, standard deviation values were less than 5.0 MPa, as recommended by BS-EN 12390-3:2019, thus indicating good quality concrete.

For the 7th day, the compressive strength should have reached two-thirds of its designed strength, as stated by (Weru, 2018). From the graph above, it can be observed that only the combination of Mix 2 and the third use of formwork failed to achieve this target. Testing for the 28th day compressive strength indicated that concrete cubes cast in formwork, already used once, performed satisfactorily.

However, for the formwork already used thrice, both Mixes 1 and 2 cubes failed to achieve the targeted strength. This can be explained by the changes in the formwork surface caused by the successive pouring-hardening-demoulding cycles that lead to the adherence of cement paste to the surface and thus clogging of pores, protrusions, and raising of fibres (Jiang et al., 2025). Moreover, this also leads to greater friction during demoulding, which can create microcracks in the concrete. However, the cubes made use of the admixture to meet the design characteristic strength of 35MPa. This can be explained by the reduced water content in this mix, which helped to mitigate the negative effects due to the degradation of the formwork as a result of several uses. This constitutes an important finding as it shows the efficiency of the admixture in achieving the required target strength while also demonstrating the need to shift from the usual procedure of increasing concrete workability through the addition of water in the mixture, as this is incompatible with circular formwork reuse.

The results shown in Figure 4 indicate that the water content in the concrete design mix has an effect on the water absorption of concrete for all the types of formwork considered. Mix 1 produced concrete with lower water absorption compared to Mix 2 for all formwork categories. However, Mix 3 with admixture use and reduced water content yielded the lowest water absorption for all formwork categories. Moreover, for the third use of formwork, the water absorption was in a similar range to that of the steel moulds, while those for new and first-time formwork had lower water absorption.

Figure 5, Figure 6, and Figure 7 indicate that the chlorine content decreased with increasing depth for all mixes. New formwork was seen to consistently produce concrete with the lowest chlorine ingress content, followed by first use and then third use, due to the ability of new formwork to absorb water from the mix, allowing concrete to have a lower w/c ratio. Mix 3 and new formwork produce the concrete with less chloride ion ingress due to the combined effect of a lower w/c ratio and the reduced voids, as shown in Figure 8. Mix 2 and third use formwork (at a depth of 30-50mm) was observed to produce the highest chloride content in the concrete due to the increased voids, as shown in Figure 8. This is particularly a concerning factor as reinforcements are usually placed at this depth for structural elements. Lastly, it was noted that for Mix 3, the chloride levels were maintained within the corrosion threshold of 30 – 50mm for the various types of formwork used, whereas for Mix 2, this was not achieved for the third-use formwork.

The abstract of this paper was presented at the 5th International Conference on Climate Change and Environmental Sustainability (CCES), which was held from the 5th to the 7th of October 2025.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors/individuals.

Not applicable.

The authors declare that there is no competing interest.