Performance Evaluation of Sustainable Composite Sandwich Slabs with Fibre-Reinforced Geopolymer Concrete

Abstract

In the conventional Portland cement production, about 1.56 billion metric tonnes of CO? is emitted each year, which is a strong incentive for the wide-spread use of low-carbon alternatives. Geopolymer Concrete (GPC) is a synthesis of fly ash and GGBFS that removes cement but has quasi-brittleness and limited post-crack ductility that restricts use in Composite Sandwich Panel (CSP) wythes. This research aims to examine the influence of the novel Flattened-End Nylon Fibre (FENF) on the mechanical, durability and structural properties of the traditional and geopolymer concrete used in the construction of CSP. The methodology employed includes the evaluation of 18 fibre mixes (2 fibre type, 3 aspect ratio and 3 dosage) for workability, compressive strength, split tensile strength, flexural strength (IS 516, IS 5816), durability (ASTM C642-21) and FESEM microstructure. Four point bending tests were carried out on 6 specimens of CSP with dimensions (1500 × 500 × 125) to evaluate the structural performance. FENF at 1.5% volume fraction and aspect ratio 55 had an improvement of 18.9% flexural strengths and 24.1% split tensile strengths compared to control concrete with an obtainable workable slump. Durability was exceptional (WA = 0.61%, VPV = 1.60%). CSPs with FENF-GPC wythes (CGFN) achieved ultimate load of 36.7 kN, ductility factor of 8.7, and 10.9% higher load capacity when compared to unreinforced GPC panels. Combined with structural performance improvements, the FENF-reinforced GPCs prove to be sustainable and lightweight, steel fibre free, corrosion-resistant, and suitable for precast sandwich panel applications in seismic areas, certifying the FENF as a good alternative to steel-fibre reinforcement for precast sandwich panel construction in seismic zones.

Introduction

The construction industry faces a major challenge: meeting growing housing demands while significantly reducing environmental impacts. Portland cement production is one of the largest sources of industrial CO? emissions, with global emissions reaching about 1.56 billion metric tonnes in 2023. In India, cement-related emissions have increased substantially, creating an urgent need for sustainable, low-carbon construction materials.

Geopolymer Concrete (GPC), produced through the alkali activation of industrial by-products such as fly ash and Ground Granulated Blast Furnace Slag (GGBFS), offers a promising alternative to Portland cement because it significantly reduces carbon emissions. However, GPC has limitations, including brittleness, low flexural strength, and poor post-cracking ductility, which restrict its direct use in structural applications.

To address these limitations, the study focuses on Composite Sandwich Panels (CSPs), which consist of two thin reinforced concrete wythes separated by a lightweight Expanded Polystyrene (EPS) core. CSPs provide several advantages, including:

• About 50% weight reduction compared to solid slabs.
• Excellent thermal insulation.
• Efficient factory-controlled construction.
• Improved structural performance through composite action.

Depending on the effectiveness of shear transfer between the wythes, CSPs may exhibit fully composite, semi-composite, or non-composite behavior, with fully composite action being the most desirable for maximum load-carrying efficiency.

The study introduces a novel reinforcement material called Flattened-End Nylon Fibre (FENF). Unlike conventional smooth synthetic fibers, FENF has thermally flattened ends that create enlarged anchor heads, improving mechanical interlocking and pull-out resistance within the geopolymer matrix. This design aims to enhance the strength, ductility, and durability of GPC while avoiding corrosion and additional weight associated with steel fibers.

Research Objectives

The study aims to:

• Evaluate the effects of FENF geometry, dosage, and aspect ratio on the workability, strength, and durability of OPC and GPC.
• Optimize fiber parameters using statistical techniques such as regression analysis, Principal Component Analysis (PCA), and Response Surface Methodology (RSM).
• Assess structural performance through four-point bending tests on CSPs.
• Compare FENF-GPC panels with conventional concrete, plain GPC, straight nylon fiber GPC, and steel-fiber GPC panels.
• Establish FENF-GPC CSPs as sustainable, high-performance precast construction elements.

Key Contributions

• First comprehensive investigation of end-profiled synthetic nylon fibers in both OPC and GPC matrices.
• Multi-scale experimental evaluation covering fresh properties, mechanical behavior, durability, microstructure, and structural performance.
• Demonstration that FENF-GPC panels achieve ductility comparable to steel-fiber panels without corrosion risks or increased weight.

Literature Review Findings

Previous studies have shown that fiber reinforcement significantly improves the performance of geopolymer concrete:

• Steel fibers enhance flexural strength and post-cracking behavior.
• Polypropylene and hybrid fibers improve tensile strength, ductility, and durability.
• Fiber-reinforced geopolymer concrete (FRGPC) improves load-deflection behavior and crack resistance in sandwich panels and slabs.

Research Gaps Identified

The study identifies several gaps:

• No prior research on Flattened-End Nylon Fibres (FENF) in GPC-based CSPs.
• Limited understanding of mechanical anchorage effects of shaped-end nylon fibers in alkaline matrices.
• Lack of multi-scale investigations linking material behavior to structural performance.
• Absence of statistical optimization studies for fiber geometry and dosage in CSP applications.

Methodology

The research follows a systematic process:

1. Material characterization.
2. Optimization of fiber-reinforced concrete.
3. Development of geopolymer concrete mixtures.
4. Structural testing of composite sandwich panels.

FENFs were produced from monofilament nylon fishing lines by thermally flattening both ends, creating anchor heads approximately 2 mm in diameter, which improve bonding and load transfer within the concrete matrix.

Conclusion

A. Summary of Work This investigation was an extensive experimental study of Flattened-End Nylon Fibre-reinforced Geopolymer Concrete in Composite Sandwich Panel application. Three FRGPC mixes and six CSP structural specimens were tested on the workability, mechanical strength, durability, microstructural, and structural performance.The 18 FRC mixes, three FRGPC mixes, and six CSP structural specimens were tested in terms of workability, mechanical strength, durability, microstructural and structural performance. B. Key Outcomes 1) The slump of all the FRC and FRGPC mixes were found to be within the range of workable slump (100-115 mm) as per slab requirement as given in IS 456:2000 and only marginally lower than slump of SNF mixes but still workable. [3] 2) The best flexural strength was obtained by FENF-1.5-55 with 4.4 MPa followed by FENF-1.0-55 with 4.1 MPa, both of which performed well with an improvement of more than 18% over CC. The best split-tensile strength was obtained by FENF-0.5-35 with 3.6 MPa, which improved by over 24% from CC, and a performance level well above FENF. [3] 3) The results of statistical optimisation showed that the optimum dosage of FENF and aspect ratio were 1.5% and 55 respectively, to optimise the strength and workability. [3] 4) FENFRC showed the highest durability with WA = 0.61% after 24 hours, and VPV = 1.60% (classified ‘Excellent’ according to ASTM C642-21) due to matrix densification provided by mechanical interlocking of FENF. [3] 5) The FESEM results indicated that fibre-matrix bonding in FENFRC was better, evidenced by better fibre-matrix bonding at the flattened ends of the fibres and low interfacial porosity compared to the SNF specimens. [3] 6) CSPs with FENF-GPC wythes (CGFN) made: ultimate load 36.7 kN (+10.9% above CG), ductility factor 8.7 (the highest of all nylon-fibre panels), mid-span deflection 27.4 mm, and 20–30% narrower crack width than unreinforced GPC (with no increase in density). [3] 7) The \"warning before failure\" gradual post-peak softening was a critical feature of CGFN panels which was important for seismic-zone construction safety. [3] C. Future Scope 1) Full scale CSP testing under combined gravity and lateral loading for code level design validation. 2) Fatigue and dynamic / seismic loading characterisation of FENF-GPC CSPs. 3) Fire and thermal resistance test at high temperatures, as per IS 3809. 4) Life cycle assessment (LCA) that identifies the amount of CO2 and economic costs for the use of OPC-steel alternative. 5) Multi-scale crack control using hybrid fibre systems (macro FENF, micro PP or basalt fibres). 6) Increase in GPC-FENF dosage – optimisation of admixture to avoid workability loss.

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Copyright

Copyright © 2026 Sonali Rohit, Amit Pandey, Ragini Mishra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.