Steel Fibre-Reinforced Structural Lightweight Concrete with Waste-Derived Green Aggregates: A Review of Mechanical Performance and Crack Behaviour

Structural lightweight aggregate concrete (SLWAC) is a specialised class of concrete developed to reduce its unit weight while retaining adequate load-bearing capacity for structural applications. According to the American Concrete Institute (ACI) 213R-14 [1], concrete is classified as structural lightweight when its oven-dry density lies between 1120 and 1920 kg/m³ and its 28-day compressive strength is not less than 17 MPa. These limits should be viewed as engineering criteria rather than absolute physical boundaries. However, the classification provides a practical basis for distinguishing SLWAC from other lightweight concrete systems in terms of density and strength.

The reduced density of SLWAC is achieved primarily by replacing normal-weight coarse aggregate, wholly or partly, with lightweight aggregate (LWA), whose highly porous internal structure lowers concrete density while still providing sufficient strength for structural applications. This porous structure is fundamental to the behaviour of SLWAC, as it enables substantial weight reduction while simultaneously influencing its mechanical and durability performance. The application of SLWAC dates back more than a century, emerging in shipbuilding during the First World War before spreading to high-rise construction in the United States and Europe [2]. Today, it is used in long-span bridges, offshore structures, precast elements and tall buildings, where reduced self-weight can provide structural and economic advantages [3]. Its long history indicates that SLWAC has established applications in structural construction, while current research continues to focus on improving its performance and sustainability.

Interest in SLWAC has been renewed by the growing demand for sustainable construction. Concrete is the second most consumed material after water, and its production accounts for roughly 8-9% of global anthropogenic greenhouse-gas emissions, mainly through cement manufacture [4]. By reducing structural dead load, SLWAC can enable smaller foundations, slimmer columns and lighter beams, potentially reducing the consumption of concrete and reinforcing steel and their associated embodied energy and carbon. Its lower mass can also reduce earthquake-induced inertial forces and decrease the demand on lateral-load-resisting systems [5]. These environmental benefits may be further enhanced when LWAs are produced from recycled or industrial waste, as this can reduce the consumption of virgin resources and divert waste from landfill [6].

Despite these advantages, the porous nature of LWAs remains a major challenge for the wider application of SLWAC. It can reduce stiffness and tensile performance, resulting in a lower modulus of elasticity, higher water absorption and a more brittle failure mode than normal-weight concrete [7, 8]. Since these limitations are closely associated with the characteristics of the aggregate phase, improving SLWAC performance requires approaches that enhance stress transfer and control crack propagation. Among the available approaches, steel fibre reinforcement has been widely investigated as an effective method for mitigating the brittle behaviour of SLWAC. Steel fibres bridge developing cracks, transfer tensile stresses across cracked sections, and can improve toughness, post-cracking capacity and energy absorption [9].

Accordingly, this review focuses on steel-fibre-reinforced SLWAC, with particular emphasis on green and recycled lightweight aggregates, including engineered plastic-glass aggregates. Although steel fibres and lightweight aggregates have each been extensively investigated, studies examining their combined behaviour remain comparatively limited, particularly for engineered recycled plastic-glass aggregates. This review therefore critically evaluates the current literature on lightweight aggregates, fibre-matrix interaction, the influence of steel fibres on the mechanical performance of SLWAC, and the resulting crack behaviour and failure mechanisms, while identifying key research gaps that require further investigation.

The scope of this review is limited to steel-fibre reinforcement because steel fibres have been among the most extensively investigated fibre types for SLWAC. Although other fibre types, including polypropylene, basalt, glass and carbon fibres, have also been investigated, they are beyond the scope of the present review.

Lightweight aggregates are the primary constituent responsible for reducing concrete density while maintaining acceptable structural performance. Their porous internal structure lowers unit weight and thermal conductivity but also influences mechanical properties, water absorption, permeability, durability, and the quality of the ITZ. Lightweight aggregate should therefore not be treated as one variable among several in SLWAC mix design, because many of the benefits and limitations of the concrete originate directly from its pore structure and physical characteristics [33].

Table 2. Comparison of common conventional and green lightweight aggregates used in structural lightweight aggregate concrete (SLWAC)

Lightweight aggregates are commonly classified as natural or artificial. Natural LWAs are formed through geological processes, whereas artificial LWAs are produced through thermal expansion or industrial processing. Although this classification is convenient, aggregate origin alone is not a reliable predictor of concrete performance. Aggregates from the same general category may differ considerably in pore connectivity, particle strength, surface texture, absorption, and outer-shell structure. These characteristics are more directly related to SLWAC behaviour than the source of the aggregate itself (Table 2).

A recent development within the artificial LWA category is the emergence of waste-derived green aggregates. LiGrA® is a manufactured lightweight green aggregate produced by Gramoler Pte Ltd (Singapore) through the valorisation of waste materials, including recycled glass, recycled plastics, fly ash, and other industrial by-products, into a stable and safe aggregate form suitable for structural and non-structural applications. This zero-waste manufacturing approach aims to reduce material costs, improve thermal insulation performance, and support sustainable construction practices by diverting waste from landfill.

The porous structure of LWA provides reduced dead load, improved thermal insulation, and enhanced fire resistance [41]. However, the same structure may cause high water absorption, low stiffness, reduced workability, and poor crack resistance when the mixture is not properly optimised [41, 42]. This dual effect is central to understanding lightweight aggregates: every reduction in density may be accompanied by a mechanical or durability penalty. Consequently, reported benefits should not be evaluated separately from the limitations associated with achieving them.

Table 2 summarises the origin, density, water absorption, mechanical performance, advantages, limitations, and potential applications of the four aggregate types discussed in this section, providing a basis for the comparative discussion that follows.

3.1 Conventional lightweight aggregates

3.1.1 Expanded clay

Expanded clay is one of the most widely used artificial LWAs. It is manufactured by heating selected clays in rotary kilns above approximately 1100 ℃, causing gas expansion and producing a cellular internal structure, typically consisting of a hard outer shell surrounding a porous core [43, 44]. Expanded-clay concretes can achieve compressive strengths above 30 MPa at densities considerably below those of normal-weight concrete [34, 45]. Its rough surface can also improve mechanical interlock and the aggregate-paste interface.

Nevertheless, compressive-strength values reported for expanded-clay concrete should be interpreted cautiously because they depend not only on the aggregate but also on the mixture proportions and matrix characteristics adopted in each investigation. Therefore, reporting structural strength without considering the conditions under which it was achieved limits meaningful comparison between studies. Expanded clay also exhibits relatively high water absorption and lower stiffness, which can reduce elastic modulus and increase drying shrinkage [27]. It should thus be regarded as a well-established LWA rather than a material free from the typical limitations of lightweight aggregates.

3.1.2 Pumice

Pumice is a natural volcanic aggregate characterised by a highly vesicular structure that provides very low density and good thermal insulation [36]. Compared with expanded clay and shale, pumice generally has lower crushing strength and higher absorption because of its interconnected pores. These characteristics can increase permeability and weaken the aggregate-paste interface unless the mixture and aggregate moisture condition are carefully controlled [37].

Water stored within pumice pores can contribute to internal curing [46], but this benefit should not be treated as automatic. It depends on the capacity of the aggregate to absorb and later release water, as well as on appropriate pre-soaking. Without moisture control, pumice may instead absorb mixing water and disturb the intended mixture proportions. Its low baseline strength should also be considered when evaluating reported percentage improvements, because a large relative increase does not necessarily indicate superior absolute performance.

3.1.3 Expanded shale

Expanded shale is produced by thermally expanding shale rock to form a porous but mechanically stable aggregate [13]. Compared with pumice, it generally provides higher compressive strength and lower permeability because of its denser outer shell and finer pore distribution. Its angular morphology may further improve aggregate-paste bonding and stress transfer [31].

Shale-based lightweight concretes commonly exhibit densities of approximately 1700-1900 kg/m³, making expanded shale suitable where a balance between strength, durability, and density is required [22]. However, comparisons suggesting that shale performs better than clay or pumice should be treated as indicative rather than conclusive. Such conclusions are often drawn from separate studies that used different concrete matrices, curing conditions, and testing procedures rather than from direct comparisons under identical conditions.

Figure 1 compares the reported bulk density and water absorption of the principal lightweight aggregate types. Two observations are relevant here. First, absorption does not decrease systematically with increasing bulk density; aggregates of comparable density differ by more than an order of magnitude in water uptake, indicating that pore connectivity and outer-shell continuity govern absorption more strongly than total porosity alone. Second, the scatter within a single aggregate type is comparable to, or larger than, the scatter between types: reported absorption for pumice alone spans approximately 3-20%. This supports the position adopted in this review that aggregate origin is a weak predictor of engineering behaviour. The comparison should nonetheless be interpreted cautiously, because absorption is strongly dependent on the immersion period adopted and the source studies do not follow a common test duration (see Table 3 for the individual data sources).

1.png

Figure 1. Bulk density and water absorption of representative lightweight aggregates

Table 3. Sources for the lightweight aggregate density and water absorption data shown in Figure 1

Overall, conventional lightweight aggregates have a relatively mature and predictable evidence base. However, their performance remains governed by the same fundamental trade-off: increased porosity reduces density but may compromise stiffness, absorption resistance, and crack behaviour. This limitation has encouraged the development of engineered aggregates produced from recycled and waste-derived materials.

3.2 Green and recycled lightweight aggregates

Environmental concerns and carbon-reduction targets have increased interest in engineered green lightweight aggregates produced from recycled and waste-derived materials. These aggregates aim to reduce the consumption of virgin resources and divert waste from landfill while retaining acceptable structural and durability performance [13, 63]. Recycled glass and post-consumer plastics are among the most frequently investigated feedstocks. Glass-based aggregates are attractive because of their low density and silica-rich composition, which may assist particle packing and matrix densification [64], whereas plastic-based aggregates can reduce concrete density while providing a route for plastic-waste utilisation [40].

Plastic-based aggregates have been reported to reduce unit weight by approximately 21-29% compared with conventional coarse aggregates while maintaining acceptable structural performance in the reported studies. Their hydrophobic surfaces may also result in relatively low water absorption values of approximately 1.2-1.6% [39]. However, although the environmental rationale for these aggregates is well established, the engineering evidence supporting their long-term structural application remains comparatively limited. Available studies are fewer and less consistent than those concerning conventional LWAs, particularly with respect to durability, interfacial behaviour, and performance under prolonged loading or environmental exposure.

The main engineering challenge is the variability of recycled feedstocks. Differences in their physical and chemical characteristics can affect particle stability, absorption, interfacial bonding, and long-term behaviour. In particular, the hydrophobic and relatively low-energy surfaces of waste plastics can reduce adhesion to cement paste and weaken the ITZ [40, 65, 66]. Hydrophobicity therefore presents a potential trade-off: it may reduce water absorption and alter moisture transport, but it may also reduce the aggregate–paste bond responsible for stress transfer and influence crack initiation.

This behaviour distinguishes plastic-based aggregates from conventional porous LWAs. Conclusions developed from rough and absorptive ceramic aggregates cannot necessarily be transferred directly to hydrophobic recycled aggregates because the governing interfacial and moisture-related mechanisms differ. In particular, the internal-curing and moisture-exchange effects associated with conventional porous aggregates may be less pronounced when the recycled aggregate exhibits low water absorption.

Pelletisation, surface treatment, and thermal treatment have been investigated to improve particle stability, bonding, and pore characteristics [67]. However, these approaches are mainly supported by individual experimental investigations rather than widely established design procedures. Their effectiveness may also depend strongly on the characteristics of the original waste material and the processing conditions, making direct comparison between different engineered aggregates difficult.

Among engineered green LWAs, aggregates combining recycled plastic and recycled glass, such as LiGrA, have been investigated at high replacement levels, of the order of 80%, with substantial density reduction reported in the corresponding studies. Their low absorption may address one of the principal limitations of conventional porous aggregates; however, the hydrophobic plastic component may also... contribute to weaker interfacial bonding and influence crack initiation at the aggregate-paste boundary. These competing characteristics make crack control and stress transfer particularly relevant when assessing concrete containing such aggregates.

Comprehensive evidence on the combined behaviour of recycled plastic-glass lightweight aggregates and steel fibres remains limited. Existing studies often examine density reduction, mechanical performance, or durability separately, while fewer studies evaluate how fibre reinforcement interacts with the distinctive interfacial characteristics of hydrophobic recycled aggregates. The research gap is therefore not simply the limited use of recycled LWAs, but the insufficient understanding of how steel fibres interact with, and potentially mitigate the consequences of, their weaker aggregate-paste bond and associated crack development.

Figure 2 compares the typical density ranges reported for structural lightweight concrete produced with expanded clay, pumice, expanded shale, and recycled plastic-based green lightweight aggregates. The ranges were compiled from representative experimental studies reported by Chung et al., Ahmad et al., Zhao et al. and Alqahtani [7, 27, 38, 39]. Rather than establishing a fixed ranking between aggregate types, the figure illustrates the variation in reported concrete density and the broader range associated with engineered recycled aggregates. The density values are presented in kg/m³ for consistency; no unit conversions were required, as all four cited sources report density directly in kg/m³. Such comparisons should be interpreted cautiously because the reported densities were obtained using different mixture proportions, replacement levels, and moisture conditions. The figure should therefore be regarded as a literature-based contextual comparison rather than a direct experimental comparison between aggregate types. Table 4 summarises the origin, density, strength, and key characteristics of the four aggregate types discussed in this section, to support direct comparison alongside the density data presented in Figure 2.

2.png

Figure 2. Typical density ranges of structural lightweight concrete produced with expanded clay, pumice, expanded shale, and green recycled lightweight aggregates

Table 4. Comparison of common conventional and green lightweight aggregates used in structural lightweight aggregate concrete (SLWAC)

The mechanical performance of SLWAC includes compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, toughness, and post-cracking behaviour. These properties are governed by the combined effects of aggregate strength and stiffness, matrix quality, ITZ characteristics, and fibre reinforcement [5, 83]. However, these factors do not contribute equally to every property. The lightweight aggregate largely controls compressive strength and stiffness, whereas steel fibres are most effective in properties governed by cracking and post-cracking stress transfer. This distinction is essential because reporting all mechanical improvements together can overstate the overall influence of fibres. The principal quantitative trends are consolidated in Figure 4.

4.png

Figure 4. Effect of steel fibres on structural lightweight aggregate concrete (SLWAC)

5.1 Compressive strength

Compressive strength is the main property used in structural design. In SLWAC, it is strongly influenced by the lightweight aggregate, which is generally weaker and more porous than normal-weight aggregate. Whereas failure in normal-weight concrete often develops around the aggregate-paste interface, fracture in SLWAC frequently passes through the lightweight aggregate because of its lower crushing strength [22, 34].

This behaviour creates a strength ceiling. Once the matrix becomes stronger than the lightweight aggregate, further improvement in matrix quality does not produce a proportional increase in concrete strength because the aggregate becomes the controlling phase [84-86]. This concept is important when interpreting high-strength claims in lightweight concrete. A stronger matrix cannot increase compressive strength indefinitely, and the achievable performance remains bounded by the strength of the aggregate.

The strength-ceiling effect becomes particularly relevant when recycled green aggregates such as LiGrA are used at high replacement levels. Under these conditions, the properties of the recycled aggregate increasingly govern the failure mode and final load capacity [39]. Accordingly, compressive strength should not be interpreted only as a reflection of matrix quality, especially in mixtures containing large volumes of low-density aggregate.

Lower density does not correspond to a fixed loss in strength. Concrete mixtures of similar density may achieve different strengths because aggregate crushing resistance, pore structure, bonding, and matrix characteristics also influence performance. The density-strength relationship should therefore be regarded as a general trade-off rather than a fixed or universal correlation between the two properties.

Steel fibres improve compressive strength mainly through lateral confinement rather than through direct strengthening of the matrix. As concrete expands laterally under compression, fibres crossing internal cracks restrain deformation, delay crack development, and improve stress redistribution. Reported gains are commonly around 10-21% at fibre contents of 0.5-0.75% by volume, while increases of up to approximately 76% have been reported in pumice-based systems at higher dosages [5, 34, 87].

The wide difference between these reported gains indicates that there is no single fibre effect applicable to all lightweight concretes. Large percentage increases may partly reflect a weak reference mixture rather than an exceptionally strong fibre contribution. Therefore, improvements should be interpreted together with baseline strength, aggregate type, fibre dosage, and absolute strength values. Comparing percentage gains alone may give a misleading impression of fibre effectiveness.

Fibres at 0.75% have been reported to increase load capacity by approximately 20% and reduce brittle crushing through improved internal crack resistance and stress redistribution [88, 89]. Nevertheless, the compressive contribution of fibres remains limited by aggregate crushing. This explains why compressive-strength gains are generally smaller than improvements in tensile, flexural, and post-cracking properties.

5.2 Splitting tensile strength

Splitting tensile strength is associated with crack initiation, shear resistance, and structural integrity under service loading. Plain SLWAC generally has lower tensile strength than normal-weight concrete of similar compressive strength because the porous aggregate has limited tensile capacity. This weakness may be more pronounced when hydrophobic recycled aggregates are used at high replacement levels, as reduced aggregate-paste adhesion can encourage crack initiation near the interface [32, 34, 39].

Porous conventional lightweight aggregates may partly improve the ITZ through the micro-pump effect. During mixing, they absorb water from the surrounding paste, locally reducing the water-to-binder ratio and contributing to a denser interface [22, 32]. However, this mechanism depends on the aggregate's ability to absorb water. It should not be assumed to occur to the same extent in hydrophobic recycled aggregates with low water affinity.

Steel fibres are especially effective in splitting tension because they directly resist the mechanism governing failure. Fibres crossing developing cracks transfer tensile stress, delay crack coalescence, and reduce the rapid loss of capacity after crack initiation [90, 91]. This direct correspondence between fibre action and tensile failure explains why improvements in splitting tensile strength are commonly greater than those in compressive strength.

A fibre volume fraction of 0.75% has been reported to increase splitting tensile strength by approximately 68-104% relative to non-fibrous lightweight concrete [92, 93], while Mohamed et al. [34] reported improved tensile resistance and reduced brittle behaviour across several fibre dosages. These large percentage gains should nonetheless be interpreted cautiously because they depend strongly on the initial tensile strength of the control mixture. A large relative improvement does not necessarily indicate a high absolute tensile capacity.

The range of 0.25-0.75% therefore provides a practical band for evaluating tensile enhancement in lightweight concrete containing recycled green aggregates [94]. The value of this range lies not in guaranteeing a universal optimum, but in allowing the relationship between dosage, crack control, and workability to be assessed without automatically assuming that the highest fibre content will provide the best overall performance.

5.3 Flexural strength

Flexural strength is governed by tensile resistance in the tension zone and by the ability of the material to sustain load after cracking. Plain SLWAC generally exhibits lower flexural strength than normal-weight concrete because of the weaker tensile capacity of the lightweight aggregate and its tendency to fracture under bending. The response becomes more sensitive to aggregate-paste bonding when hydrophobic recycled aggregates are used at high replacement levels [34, 39].

Steel fibres improve flexural behaviour by bridging cracks in the tension zone and maintaining stress transfer after matrix cracking. Fibre-reinforced lightweight beams therefore show higher residual flexural strength and a more gradual post-cracking response than plain beams [95]. Improvements of approximately 17-31% have been reported at fibre contents up to 0.75%, with larger gains generally associated with increased crack-bridging capacity [34, 96].

The flexural evidence supports the tensile-strength findings because both properties are controlled by crack formation and propagation. This agreement provides stronger mechanistic support for fibre reinforcement than compressive-strength results alone. Nevertheless, peak flexural strength should not be used as the only indicator of improvement. Residual strength, crack distribution, and the ability to maintain load after the first crack are equally important when evaluating fibre performance.

These improvements are particularly relevant for members subjected to repeated loading, vibration, and dynamic actions, where the retention of capacity after cracking contributes to safety and serviceability [97]. The main benefit of fibres in bending is therefore not merely a higher peak strength, but a less abrupt transition from uncracked to cracked behaviour.

5.4 Modulus of elasticity

The modulus of elasticity of SLWAC is lower than that of normal-weight concrete of comparable strength because composite stiffness is strongly governed by the relatively low stiffness of the lightweight aggregate. Typical values range from approximately 10 to 25 GPa, compared with 25-35 GPa for normal-weight concrete [22]. At high replacement levels of recycled lightweight aggregate such as LiGrA, particle stiffness becomes an increasingly dominant influence on the modulus of the composite [39].

Steel fibres have only a limited influence on elastic modulus because the modulus is measured within the pre-cracking linear-elastic range, whereas fibres contribute mainly after cracking begins. Reported increases are generally below 10% at fibre dosages of 0.25-0.75% [34, 68, 98].

This limited effect is analytically important because it defines what fibre reinforcement cannot achieve. Fibres may improve crack resistance, residual strength, and toughness, but they cannot substantially correct the low stiffness caused by lightweight aggregates. Consequently, reductions in modulus and the resulting serviceability implications should be treated as inherent characteristics of high-replacement SLWAC rather than as weaknesses that fibre reinforcement can fully remove.

The comparatively low modulus may increase deformation under service loads, although it can also contribute to energy dissipation under dynamic and seismic loading [99]. This characteristic should therefore be evaluated according to the structural application rather than described only as a disadvantage.

5.5 Toughness and post-cracking behaviour

Toughness represents the energy absorbed up to and beyond the peak load and is the property in which steel fibres most clearly transform lightweight concrete behaviour. Plain SLWAC typically fails with limited deformation and loses load capacity rapidly after cracking because of its low tensile resistance and brittle aggregate fracture. Steel fibres maintain stress transfer across cracked regions, enlarge the area under the load-deformation curve, and produce a more gradual failure response [56].

The principal contribution of fibres is therefore not simply an increase in peak load. Their greater value lies in preserving residual capacity after the matrix has cracked. Through crack bridging, fibres improve toughness index, residual flexural strength, ductility, and energy absorption while maintaining structural continuity after first cracking [71, 100, 101].

Figure 5 illustrates this distinction. Plain lightweight concrete loses capacity abruptly after reaching its peak load, whereas fibre-reinforced concrete sustains a longer descending branch. The additional area under this branch represents the greater energy absorbed during post-cracking deformation. The figure should therefore be interpreted as demonstrating a change in failure mode rather than only an increase in strength.

5.png

Schematic load-deflection response of plain versus steel-fibre-reinforced SLWAC, prepared by the authors. Plain concrete fails abruptly at peak load (brittle), whereas steel fibres provide residual post-cracking capacity; the shaded area denotes the additional toughness (energy absorption).

Increasing fibre dosage has been associated with improved ductility and delayed brittle failure [9]. Material-scale toughness and member-scale ductility should, however, not be treated as interchangeable. A larger area under a specimen load-deflection curve demonstrates improved material energy absorption, whereas a change in beam failure mode depends additionally on member geometry, reinforcement arrangement, and structural loading. Both forms of evidence support fibre use, but they describe different levels of performance.

Overall, the literature shows a consistent division in the role of steel fibres. Their influence on compressive strength and elastic modulus is comparatively limited because these properties remain controlled mainly by aggregate crushing strength and stiffness. Their greatest contribution occurs in splitting tension, flexure, toughness, and residual behaviour, where crack bridging directly changes the failure process. Table 5 and Table 6 summarise the reported quantitative and qualitative effects of steel fibres across the studies cited in this section, grouped by compressive/splitting-tensile strength (Table 5) and flexural strength/other performance aspects (Table 6).

Table 5. Reported effects of steel fibres on the compressive and splitting tensile strength of Structural lightweight aggregate concrete (SLWAC)

Table 6. Reported effects of steel fibres on flexural strength and other mechanical performance aspects of structural lightweight aggregate concrete (SLWAC)

The magnitude of these improvements is not governed by fibre content alone but results from the combined influence of lightweight aggregate characteristics, fibre geometry, fibre volume fraction, matrix composition, and the quality of the fibre–matrix interface. Consequently, direct comparison of reported mechanical improvements across different studies should be undertaken with caution, since variations in mixture proportions, curing conditions, specimen geometry, and testing procedures can significantly influence the measured performance. This highlights the importance of interpreting published results within their specific experimental context rather than relying solely on percentage improvements. Such critical interpretation provides a more reliable basis for evaluating the effectiveness of steel fibres in SLWAC and for identifying realistic directions for future research.

This review critically examined the mechanical performance and crack behaviour of SLWAC reinforced with steel fibres. The main conclusions are as follows.

SLWAC reduces self-weight and offers structural and environmental advantages, but its porous and relatively low-stiffness aggregates reduce tensile capacity, elastic modulus, and crack resistance while promoting brittle fracture through the aggregate.

Lightweight aggregate selection is the main factor governing performance. Conventional aggregates such as expanded clay, pumice, and shale have a comparatively mature evidence base, whereas engineered green aggregates, including recycled plastic-glass aggregates such as LiGrA, remain insufficiently characterised in fibre-reinforced structural applications.

The environmental benefit of recycled lightweight aggregates does not guarantee equivalent engineering performance. Hydrophobicity may lower absorption but may also reduce aggregate-paste adhesion, promote segregation, and alter crack initiation. These competing effects must be evaluated together rather than individually.

Steel fibres act primarily through crack bridging and post-cracking stress transfer. They reduce crack localisation, limit crack width, redistribute tensile stresses, and sustain residual load after matrix cracking. Their main contribution is therefore the modification of crack development and failure mode rather than uniform improvement of all mechanical properties.

Fibre benefits are greatest in tensile-governed properties. Reported increases in splitting tensile strength and flexural strength are generally larger than those in compressive strength because fibres directly bridge tensile cracks, whereas compressive strength remains limited by aggregate crushing.

The effect of fibres on the modulus of elasticity is minor because modulus is governed mainly by aggregate stiffness and is measured before significant cracking. Steel fibres should therefore not be expected to correct the serviceability limitations caused by the low stiffness of lightweight aggregates.

Reported percentage gains should be interpreted together with the baseline performance of the control mixture. Very large increases may reflect a weak reference concrete rather than a universally superior fibre response, and percentage improvements alone are insufficient for comparing different lightweight systems.

Hooked-end fibres generally provide better pull-out resistance and longer-lasting stress transfer than straight fibres. However, their effectiveness still depends on matrix quality, fibre dispersion, and the condition of the fibre-matrix interface.

Fibre contents around 0.5-0.75% frequently provide a practical balance between mechanical improvement and workability. This range should not be treated as a universal optimum, because the most suitable dosage depends on fibre geometry, aggregate characteristics, bond quality, workability, and mixing conditions.

The decisive contribution of steel fibres lies in toughness, residual strength, and failure-mode control. Fibres transform the response from sudden and localised cracking towards a more gradual and energy-absorbing process, although material-level toughness should not be assumed to guarantee structural-level ductility without member testing.

Existing literature remains fragmented. Mechanical properties, crack patterns, failure modes, and microstructural characteristics are commonly examined separately, preventing a complete understanding of how fibre reinforcement operates in recycled lightweight systems.

Realising the full potential of steel-fibre-reinforced SLWAC requires integrated studies that connect peak strength with crack width, crack distribution, residual capacity, fibre pull-out, aggregate fracture, ITZ condition, and structural failure mode. This need is especially important for hydrophobic recycled lightweight aggregates, where conventional mechanisms cannot be assumed to remain valid.

Overall, the available evidence confirms that steel fibres are an important enabling component for extending the structural application of sustainable lightweight concrete. Their value lies not in eliminating the inherent limitations of lightweight aggregates, but in controlling the brittle cracking and rapid loss of capacity that would otherwise restrict their structural use.