Reinforcement Alternatives for Timber–Concrete-Composite Floor Slabs

Abstract

Timber–Concrete Composite (TCC) floor systems offer a sustainable and structurally efficient solution by combining the tension capacity of timber with the compressive strength of concrete. Despite their advantages, the environmental impact of concrete and conventional steel reinforcement remains a concern. This thesis explores the use of alternative reinforcement methods and materials, focusing on loose basalt fibres, to evaluate their mechanical and environmental performance in TCC slabs.

The central research question is: “What are the mechanical and environmental implications of using a suitable alternative reinforcement method and material in timber-concrete-composite (TCC) floor systems, assessed against a case study?”

A standard floor element from the DPG Media building was selected as the case study. Eight reinforcement alternatives were evaluated through a multi-criteria analysis (MCA), considering parameters such as strength, ductility, sustainability, and buildability. The most promising option, loose basalt fibre reinforcement, was selected for further comparison against the original steel mesh-reinforced design.

In the MCA, each reinforcement alternative was scored from 0.0 to 100.0 per criterion, with scores linearly interpolated between the best and worst performers. To reflect the priorities of this study, environmental impact was weighted twice as heavily as performance capability, which itself was weighted four times more heavy than buildability and cost. The final weights assigned were: sustainability (0.5), performance capability (1.0), and buildability and cost (each 0.125). Performance capability included two equally weighted sub-criteria, ensuring it did not disproportionately influence the overall outcome. The total score for each alternative was calculated by multiplying the criterion weights with the respective scores and summing the results.

To test the robustness of the MCA outcome, a sensitivity analysis was performed on both the weighting scheme and scoring method. This confirmed that the selection of basalt fibre reinforcement remained consistently high across variations, reinforcing confidence in the methodology and its conclusions.

Numerical modelling was conducted to assess crack formation due to shrinkage (using LS-DYNA) and structural capacity under horizontal wind loading (using GSA Oasys) for the selected reinforcement alternative. LS-DYNA models were developed for three scenarios: non-reinforced, steel mesh-reinforced, and basalt fibre-reinforced slabs. A smeared cracking approach was used to estimate crack widths under expected shrinkage. The slab was supported with pinned edges and discrete spring elements representing the stiffness of notched connections with dowels. The steel mesh model was validated against the Eurocode analytical method, yielding a crack width of 0.19 mm, which complies with the Eurocode’s Serviceability Limit State (SLS) requirements of 0.40 mm. These limits are primarily based
on corrosion prevention. For basalt fibre, which is corrosion-resistant, a maximum crack width of 0.70 mm was adopted based on aesthetic considerations found in literature. The model results exceeded both analytical predictions and crack width limits:
• Non-reinforced slab: 0.95 mm
• Basalt fibre-reinforced slab: 0.96 mm
• Steel mesh-reinforced slab: 1.35 mm

A separate GSA model was developed to assess stress distribution in the top concrete layer of the TCC slab under horizontal loading, comparing standard steel mesh and basalt fibre reinforcement. For extra validation, the values are also compared to the values from the SCIA-model from the documentation of the original design. The unity check for steel mesh was 0.55 in the GSA model and 1.0 in the SCIA model. For basalt fibre, an additional safety factor was applied due to its brittle nature, resulting in unity checks of 0.31 (GSA) and 0.43 (SCIA). These results demonstrate the superior mechanical performance of basalt fibre, supporting the MCA-based material selection.

Environmental impact was assessed using a cradle-to-gate Global Warming Potential (GWP) analysis for life-cycle-stages A1-A3, based on available Environmental Product Declarations (EPDs). Fibre-based reinforcements showed significant reductions in carbon footprint when considering only the reinforcement material. However, for a fair comparison, both concrete and reinforcement must be considered. Since fibre-reinforced concrete typically requires a higher cement content per 𝑚3 of concrete than steel-reinforced concrete.
GWP values per square meter of TCC floor (reinforcement only):
• Steel mesh: 4.05 𝑘 𝑔 𝐶𝑂2 𝑒𝑞/𝑚2
• Basalt fibre: 0.45 𝑘 𝑔 𝐶𝑂2 𝑒𝑞/𝑚2
GWP values for combined concrete and reinforcement:
• Steel mesh: 16.93 𝑘 𝑔 𝐶𝑂2 𝑒𝑞/𝑚2
• Basalt fibre: 17.40 𝑘 𝑔 𝐶𝑂2 𝑒𝑞/𝑚2
Comparison of results for the GWP of only reinforcement and combination of reinforcement and concrete matrix emphasizes the importance of evaluating the entire concrete-reinforcement system. To refocus on the reinforcement material, a concrete mix using eco2cem, a lower GWP cement alternative, was studied. The adjusted GWP values are:
• Steel mesh with eco2cem: 11.26 𝑘 𝑔 𝐶𝑂2 𝑒𝑞/𝑚2
• Basalt fibre with eco2cem: 11.43 𝑘 𝑔 𝐶𝑂2 𝑒𝑞/𝑚2

The analysis revealed that fibre reinforcement, when applied with the same cross-sectional height as steel mesh, results in higher GWP values. However, using low-GWP concrete and considering potential design optimizations, such as reduced cross-sectional height due to the elimination of corrosion-sensitive steel and the associated need for concrete cover, could make basalt fibre a more attractive alternative.
An additional finding was the high GWP contribution of dowels, measured at 12.97 𝑘 𝑔 𝐶𝑂2 𝑒𝑞/𝑚2 . The high GWP value for the dowels is most likely due to the high-level of detail and intervention during the manufacturing, leading to a more energy intensive process.

The findings indicate that these two performance aspects are strongly interconnected, primarily through the concrete mixture rather than the reinforcement alone. Basalt fibre reinforcement relies on its bond with concrete for structural efficiency, while the environmental impact in terms of GWP is largely determined by cement content. Using conventional fibre quantities from literature led to an overdesigned structure with a GWP exceeding that of the reference DPG TCC floor. This demonstrates that optimizing the concrete mixture is essential for achieving both structural adequacy and sustainability, even when
reinforcement selection is the primary focus.

This research demonstrates that basalt fibres can meet structural performance requirements and improve the sustainability of TCC floor systems, particularly when focusing on the reinforcement material. It also underscores the necessity of evaluating all components of the system together. The developed MCA offers a framework for assessing novel reinforcement strategies in terms of both mechanical behaviour and environmental impact.

Recommendations for future research include:
• Expanding data on bio-based fibre-reinforced concrete.
• Experimental validation of fibre-reinforced concrete behaviour.
• Development of design codes for fibre reinforcement
• More comprehensive EPDs to support life cycle assessments of emerging materials