Experimental Investigation on Fly Ash Based Concrete Mixed with Glass Fiber

Abstract

This study explores the behavior of fly-ash based glass fiber reinforced concrete developed using different combinations of fiber lengths in M30 and M35 grades. In this study, 20% of cement was replaced with Class-F fly ash to improve sustainability and long-term performance, while alkali-resistant (AR) glass fibers were added at a constant dosage of 1% by weight of cementitious material. Three fiber lengths (6 mm, 12 mm, and 24 mm) were used in single-length and graded combinations of two as short graded (6 mm + 12mm length fiber), long graded(12 mm + 24 mm length fiber), and combined graded(Short Graded + Long Graded)to understand how fiber size and distribution influence concrete performance. The brittle nature of conventional concrete and its susceptibility to early-age cracking remain major concerns in structural applications, particularly under flexural and tensile loading conditions. This study presents a comprehensive experimental investigation on fly ash-based Graded Glass Fiber Reinforced Concrete (GGFRC) developed for M30 and M35 grades. Fresh properties were evaluated using slump tests, and hardened properties were assessed through compressive, flexural, and split tensile strength tests at 7 and 28 days. Results indicate that workability decreased by approximately 8–18% with increasing fiber length due to increased internal friction and fiber interlocking. However, graded fiber systems demonstrated improved dispersion efficiency compared to mono 24 mm fiber mixes. Compressive strength showed marginal improvement at 28 days (3-7%) due to continued pozzolanic reaction of fly ash and enhanced microstructural densification. More significant improvements were observed in flexural strength (12-22%) and split tensile strength (10-18%), confirming enhanced crack-bridging efficiency and stress redistribution mechanisms. Graded fiber systems exhibited superior post-cracking behavior, reduced crack width, and improved ductility compared to mono-fiber systems.A cost analysis revealed that 20% fly ash replacement reduced cement consumption and partially offset fiber cost. While fiber addition increased material cost by approximately 3.5% per cubic meter compared to conventional concrete, the strength enhancement resulted in an improvement of 12% in strength-to-weight efficiency. When normalized against flexural performance, the cost per unit strength decreased, indicating improved structural economy. The study concludes that optimized fiber grading significantly enhances mechanical performance, structural efficiency, and crack resistance while maintaining acceptable workability. From a sustainability perspective, 20% cement replacement reduces embodied carbon without compromising strength. The results recommend graded fiber reinforcement as a viable approach for structural concrete requiring improved tensile performance and durability

Introduction

Concrete, though widely used, has low tensile strength and develops microcracks that reduce durability and structural performance. To overcome this, Fiber Reinforced Concrete (FRC) is used, where fibers improve crack resistance and energy absorption. This study focuses on Graded Glass Fiber Reinforced Concrete (GGFRC), which combines different fiber lengths (short and long) to control both micro and macro cracks, enhancing overall performance.

Experimental work was conducted on M30 and M35 concrete with 20% fly ash replacement and 1% glass fibers of varying lengths (6 mm, 12 mm, 24 mm), including mono and graded combinations (short graded, long graded, and combined graded). Results show that graded fiber mixes outperform mono-fiber mixes in strength, deformation capacity, and energy absorption, while also maintaining better workability balance.

Key findings include:

• Workability: Short fibers improve slump, long fibers reduce it, while combined graded fibers provide balanced workability.

• Compressive Strength: Slight improvement, especially at later ages, with better performance in hybrid (graded) mixes.

• Flexural Strength: Significantly improved due to effective crack-bridging, especially in balanced fiber combinations.

• Split Tensile Strength: Markedly increased, with optimal performance in well-balanced graded fiber mixes.

• Cost: Only a small increase (~3–4%) despite improved mechanical properties and durability.

Conclusion

Based on the experimental investigation on graded glass fiber reinforced concrete (GGFRC) with fly ash in M30 and M35 grades, the following conclusions are drawn: A. Fresh Properties 1) In both M30 and M35 concrete, workability decreases progressively with increase in effective fiber length due to higher interlocking and resistance to flow caused by longer fibers. 2) Mixes containing shorter fibers exhibit better slump values because of improved dispersion and reduced fiber entanglement, whereas mixes dominated by longer fibers show significant loss of workability. 3) In SG hybrid mixes, higher proportion of shorter fibers improves slump performance; conversely, LG mixes demonstrate reduced workability due to greater presence of longer fibers affecting rheological behaviour. 4) CG mixes maintain moderate and comparatively stable slump values in both grades, indicating balanced fiber distribution and controlled flow characteristics. 5) M35 concrete consistently shows higher slump than M30 for similar fiber combinations, attributed to its richer paste content, improved lubrication effect, and better tolerance to fiber incorporation. 6) Overall, workability in both grades is primarily governed by effective fiber length and its percentage distribution, along with matrix richness and paste quality. B. Compressive Strength Behaviour 1) In both M30 and M35 concrete, early-age compressive strength shows only small changes after adding glass fibers. This indicates that fibers mainly influence later-age performance rather than initial strength development. 2) At 28 days, compressive strength increases gradually with increase in effective fiber length. Longer fibers help in controlling cracks and improving load transfer inside the concrete. 3) Mono-length fiber mixes show steady improvement in strength as fiber length increases, due to better crack-bridging action. 4) In SG mixes, SG-III shows better compressive strength. Short fibers help in controlling micro-cracks and improve bonding at the paste–aggregate interface. 5) In LG mixes, LG-III gives higher compressive strength because longer fibers are more effective in restricting crack growth and improving internal resistance under load. 6) In CG mixes, CG-III shows the best and most consistent strength development. The combination of different fiber lengths controls both micro and macro cracks, which improves overall load-carrying capacity. 7) M35 concrete shows overall higher compressive strength than M30 due to its denser matrix, higher cement content, reduced porosity, and better fiber–paste bonding, which also leads to improved stress distribution and stronger fiber anchorage. 8) Hybrid fiber mixes perform better than mono-length mixes, especially in M35 concrete, because the richer paste content improves fiber–matrix interaction, provides better tolerance to fiber inclusion, and results in more uniform and consistent strength gain compared to M30. C. Flexural Strength Performance 1) Flexural strength increases with fiber length in both grades, with M35 consistently showing higher values due to its denser and stronger matrix. 2) Mono-length mixes show steady improvement, while graded combinations enhance stress transfer and crack control efficiency. 3) In SG mixes, higher proportion of 12 mm fibers improves performance (SG-I most effective); in LG mixes, balanced proportion (LG-I) provides more uniform behaviour than dominance of longer fibers alone. 4) CG-I exhibits superior flexural strength in both grades, confirming effective tensile stress redistribution and crack-bridging ability. 5) M35 utilizes graded fiber proportions more efficiently under bending due to improved fiber–paste interaction. D. Split Tensile Strength Performance 1) Split tensile strength increases from reference to mono-length fiber mixes in both grades, with progressive improvement as fiber length increases due to enhanced crack-bridging action. 2) In SG mixes, moderate proportion of longer fibers (SG-I) yields better tensile strength, while excessive short fibers reduce effectiveness. 3) In LG mixes, LG-II provides optimum tensile performance; dominance of a single longer fiber length leads to marginal reduction because of fiber interaction effects. 4) CG-I achieves superior tensile strength in both grades, confirming that balanced multi-length fiber distribution improves crack control and tensile stress redistribution. 5) M35 consistently exhibits higher split tensile strength than M30 due to denser matrix structure and stronger fiber–paste bonding, demonstrating better utilization of graded fiber synergy. E. Cost Analysis 1) For M30 grade concrete, the total cost increased from ?7052/m³ (Reference Mix) to ?7278/m³ (Modified Mix), resulting in an increment of ?226.5/m³, which corresponds to a 3.21% increase. 2) For M35 grade concrete, the cost increased from ?7312/m³ (Reference Mix) to ?7560/m³ (Modified Mix), giving an increment of ?247.5/m³, equivalent to approximately 3.4% increase. 3) Although 20% cement replacement with Class-F fly ash reduces cement consumption from 408.29 kg for M30 to 326.29 kg for M30 and 440 kg to 352 kg for M35, the inclusion of 1% AR glass fiber (by weight of cementitious powder) increases the overall binder cost due to its high unit rate (?180/kg). 4) The marginal cost increase ?226 per m³ for M30 and ?248 per m³for M35 is technically justified considering the enhanced tensile strength, flexural strength, crack resistance, and durability performance achieved in fiber-reinforced concrete.

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Copyright

Copyright © 2026 Rajwardhan Kumar, Dr. Hemant Sood. 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.