Journal of the Civil Engineering Forum.
January 2026, 12.
:40-52 DOI 10.
22146/jcef.
Available Online at https://jurnal.
id/v3/jcef/issue/archive Sustainable Concrete Using Ground Granulated Blast Furnace Slag and Polypropylene Fibers: Flexural Behavior of RC Beams Mardiana Oesman* .
Muhamad Irfan Nurdin.
Muhammad Miftahul Riza.
Wahib Hasan Prasetyo Department of Civil Engineering.
Politeknik Negeri Bandung.
Bandung.
INDONESIA *Corresponding author: mardianaoesman@polban.
SUBMITTED 16 July 2025 REVISED 06 September 2025 ACCEPTED 23 September 2025 ABSTRACT As sustainability becomes a central focus in the construction industry, the combined use of supplementary cementitious materials and discrete fiber reinforcement offers an innovative pathway to enhance both environmental and structural performance.
This study investigates the mechano-microstructural interaction between a dense Ground Granulated Blast Furnace Slag (GGBFS)Aebased matrix and polypropylene (PP) fibers in reinforced concrete (RC) beams, emphasizing the performance trade-offs among key mechanical The experimental program comprised two phases.
First.
GGBFS replacement levels of 30% and 45% .
y binder mas.
were evaluated for compressive strength to identify the optimal matrix.
Second.
PP fibers were incorporated at 0, 3, 5, and 7 kg/mA into the selected matrix.
Tests under standardized curing conditions measured compressive strength, flexural load capacity, ductility, toughness, and stiffness.
Microstructural analysis assessed fiberAematrix bonding quality and crack-bridging mechanisms.
The 30% GGBFS mixture achieved the highest compressive strength in the optimization phase.
Fiber inclusion produced distinct performance trade-offs: 3 kg/mA delivered the best combination of strength and toughness, 5 kg/mA maximized ductility, and 7 kg/mA yielded the highest initial stiffness but slightly reduced post-peak energy absorption.
These findings demonstrate that no single fiber dosage is universally optimal.
the choice should be based on prioritizing specific performance criteria.
Microstructural observations revealed dense interfacial transition zones and effective fiber anchorage in GGBFS-rich matrices, enhancing crack control and delaying propagation.
This studyAos primary contribution lies in establishing a clear link between microstructural features and quantified mechanical trade-offs, providing a framework for performance-based mix design.
The identified trade-offs also offer direct guidance for performance-based design, enabling engineers to tailor mix compositions to targeted applications such as seismic resilience, deflection-sensitive spans, or impact-resistant members.
KEYWORDS GGBFS.
Polypropylene fiber.
Reinforced concrete beams.
Flexural behavior.
Mechano-microstructural interaction.
Performance-based design.
A The Author.
This article is distributed under a Creative Commons Attribution-ShareAlike 4.
0 International license.
1 INTRODUCTION
The mechanical performance of reinforced concrete (RC) beams is fundamentally governed by the synergy between the cementitious matrix and the reinforcement system (Ebead et al.
, 2017.
Qin et al.
, 2020.
Turker and Torun, 2020.
Khan et al.
, 2022.
Michalik et al.
, 2022.
Shao et al.
, 2022.
Syll and Kanakubo, 2022.
El Mennaouy et al.
, 2.
When the concrete matrix is densified through supplementary cementitious materials such as Ground Granulated Blast Furnace Slag (GGBFS) fly ash, silica fume, metakaolin, rice husk ash, and when micro- to macro-scale reinforcement is introduced via fibers such as polypropylene (PP), kevlar, carbon, basalt, or steel fibers, an interactive mechanism occurs at both macro and micro levels, (Hawileh et al.
, 2017.
Qin et al.
, 2019.
Hussain et al.
, 2020.
Liu et al.
, 2021.
Prasanna et al.
, 2021.
Ahmad et al.
, 2022.
Lakshmi et al.
, 2022.
Hossain et al.
, 2022.
Xiong et al.
Helmi and Alraimi, 2024.
Hendra et al.
, 2.
The GGBFS enhances matrix compactness through its pozzolanic reaction and latent hydraulic activity, re- fining pore structure and reducing microvoids (Hawileh et al.
, 2017.
Hussain et al.
, 2020.
Prasanna et al.
Simultaneously.
PP fibers act as discrete crack arresters, bridging microcracks and redistributing tensile stresses across the matrix (Qin et al.
, 2019.
Ahmad et al.
, 2022.
Lakshmi et al.
, 2.
While previous studies have separately examined the benefits of GGBFS on compressive strength and durability .
nzbay et al.
, 2016.
Li.
Zhang.
Song.
Liu and Zhang, 2018.
Majhi and Nayak, 2019.
Imran et al.
, as well as the crack-bridging and toughnessenhancing role of PP fibers (Li et al.
, 2016.
Li.
Chi.
Xu.
Shi and Li, 2018.
Gali and Subramaniam, 2019.
Hatami Jorbat et al.
, 2020.
Ryos et al.
, 2020.
Bhogone and Subramaniam, 2021.
Guo et al.
, 2021.
Liang et al.
Lakshmi et al.
, 2022.
Yan et al.
, 2.
, little is known about their combined mechano-microstructural interaction within a structural element.
Specifically, the question remains: how does the microstructural Vol.
12 No.
1 (January 2.
refinement from GGBFS-based dense matrices interact with the fiberAematrix bonding mechanisms of PP fibers to influence stiffness, ductility, and toughness in RC This study addresses this gap by designing a two-stage experimental program.
In the first stage, different GGBFS replacement levels .
% and 45% by binder mas.
were evaluated to identify the mixture with the highest compressive strength.
In the second stage.
PP fibers were introduced at varying dosages .
, 5, and 7 kg/mA) into the selected GGBFS matrix to investigate their effect on flexural performance.
The analysis focuses on quantifying the trade-offs between stiffness, strength, ductility, and toughness, with the aim of developing performance-based design guidance for sustainable, fiber-reinforced concrete in structural applications such as seismic zones, deflection-sensitive spans, and impact-resistant members.
2 METHODS
1 Materials Characterization The cementitious binder consisted of Ordinary Portland Cement (OPC) conforming to ASTM C150 Type I specifications and Ground Granulated Blast Furnace Slag (GGBFS) sourced from a local steel plant.
Fine aggregate was natural river sand, and coarse aggregate consisted of crushed stone with a maximum size of 20 PP fibers used in the study had a nominal length of 54 mm, a diameter of approximately 72 m, a tensile strength of 550 MPa, and a specific gravity of 0.
Aggregate properties were determined according to Indonesian National Standards (SNI).
Cement and GGBFS were evaluated for specific gravity, while reinforcing steel was examined through tensile strength testing to determine yield and ultimate strengths.
All material characterization results are presented in Table 1.
2 Mix Design and Preparation of GGBFS Concrete The experimental program consisted of two stages.
Stage 1-GGBFS matrix optimization: Two concrete mixtures were prepared with GGBFS replacing Portland cement at 30% and 45% by mass of binder.
The total binder content was fixed at 540 kg/mA, and the water-to-binder ratio .
was maintained at 0.
40 for all mixes.
A constant superplasticizer dosage of 0.
by binder mass was used to achieve the target slump.
Stage 2- Fiber-reinforced concrete development: The mixture with the higher 28-day compressive strength from Stage 1 was selected as the control matrix for fiber addition.
PP fibers were introduced at dosages of 0, 3, 5, and 7 kg/mA.
All mixes maintained the same binder composition, aggregate proportions, and admixture dosage as in Stage 1.
Journal of the Civil Engineering Forum 3 Specimen Preparation and Curing Concrete was mixed in a pan mixer following ASTM C192.
Fibers were gradually added after the aggregates and binder were homogenized, ensuring even distribution and avoiding balling.
Cylindrical specimens .
mm y 200 m.
were cast for compressive strength testing, while RC beams .
mm y 150 mm y 1000 m.
with steel reinforcement .
yo10 mm in tension, 2yo10 mm in compression, stirrups yo8 mm @ 100 m.
were prepared for flexural testing.
All specimens were demolded after 24 hours and water-cured at 23 A 2 AC until the designated testing ages.
4 Testing Procedures Workability was assessed using the flow table test for Stage 1 mixtures, and the slump test for Stage 2 mixtures.
Compressive strength tests were conducted at 7, 28, and 56 days following SNI 03-1974-2011, with three specimens tested for each age and mix, and the mean values reported.
Flexural performance was evaluated at 56 days using a four-point bending test in accordance with ASTM C78.
LoadAedeflection behavior was recorded with a linear variable displacement transducer (LVDT) positioned at mid-span.
The parameters measured included peak load (Pmax ), ultimate load (Pult , defined as 80% of Pmax on the descending branc.
, initial stiffness .
lope of the initial linear portion of the loadAedeflection curv.
, ductility index .
atio of ultimate deflection to yield deflection, as suggested by Ling et al.
), and toughness .
rea under the loadAedeflection curve until failur.
Crack patterns were photographed at both Pmax and Pult , crack width and distribution were qualitatively analyzed to relate cracking behavior to mechanical performance.
3 RESULTS
1 Compressive Strength of Concrete with GGBFS and PP Fibers Compressive strength tests were conducted at curing ages of 7, 28, and 56 days to examine the influence of GGBFS replacement on strength development.
The results are presented in Table 2 and Figure 1.
The mixture containing 30% GGBFS achieved the highest compressive strength at 28 days .
42 MP.
and continued to gain strength up to 56 days .
64 MP.
comparison, the 45% GGBFS mixture exhibited lower strength at both ages, while the control mix without GGBFS showed the highest early strength but a less pronounced gain after 28 days.
At the early age of 7 days, the control mixture 31 MPa, significantly higher than both slag-containing mixes, which registered 14.
17 MPa Journal of the Civil Engineering Forum Vol.
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Table 1.
Results of material characterization tests Material / Test Type Test Standard Result Cement Specific gravity SNI 2531:2015 93 g/cm3 Ground Granulated Blast Furnace Slag (GGBFS) Specific gravity SNI 2531:2015 84 g/cm3 Coarse Aggregate Compacted unit weight Loose unit weight Saturated surface-dry (SSD) specific gravity Bulk specific gravity (SSD) Apparent specific gravity Water absorption Passing sieve No.
SNI 03-4804:1998
SNI 03-4804:1998
SNI 1970:2016
SNI 1970:2016
SNI 1970:2016
SNI 1970:2016
SNI 03-4142:1996
50 g/cm3 41 g/cm3 Fine Aggregate Compacted unit weight Loose unit weight Saturated surface-dry (SSD) specific gravity Bulk specific gravity (SSD) Apparent specific gravity Water absorption Passing sieve No.
Organic matter content
SNI 03-4804:1998
SNI 03-4804:1998
SNI 1970:2016
SNI 1970:2016
SNI 1970:2016
SNI 1970:2016
SNI 03-4142:1996
SNI 2816:2014
77 g/cm3 72 g/cm3 Number 1 Reinforcing Steel Yield strength .
y ) Ultimate tensile strength .
u ) SNI 2052:2017 SNI 2052:2017 95 MPa 34 MPa .
% GGBFS) and 15.
27 MPa .
% GGBFS).
This result reflects the well-established phenomenon that GGBFS replacement reduces early-age strength due to the slower kinetics of the pozzolanic and latent hydraulic reactions compared to the rapid hydration of Portland cement clinker minerals.
While this early reduction in strength may be a concern for applications requiring rapid formwork removal or early loading, it is important to note that such limitations can be mitigated through adjustments in the curing regime or supplementary acceleration methods.
Between 7 and 28 days, a notable strength gain was observed in the 30% GGBFS mixture, surpassing the control mixture at 28 days.
The continuous availability of calcium hydroxide from cement hydration promotes ongoing secondary reactions with GGBFS, leading to the formation of additional calcium silicate hydrate (CAe SAeH) gel.
This additional CAeSAeH fills microvoids within the paste, thereby refining the pore structure and improving the density of the hardened matrix.
The slower yet prolonged hydration of GGBFS is advantageous for long-term structural performance, as it contributes to higher later-age strengths, reduced permeability, and improved durability.
Table 2.
Compressive strength development over time for varying GGBFS replacement levels By 56 days, all mixtures exhibited further strength development, though the relative gains varied.
The control mixture improved modestly to 26.
37 MPa, while the 30% GGBFS mix reached 30.
64 MPa, maintaining its superior performance.
The 45% GGBFS mixture also improved to 27.
45 MPa but remained lower than the 30% mix.
This confirms that excessive GGBFS content
can lead to dilution of clinker phases and insufficient calcium hydroxide for complete activation of the slag
GGBFS
7 day (MP.
28 day (MP.
56 day (MP.
Control
30% GGBFS
45% GGBFS
Vol.
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Journal of the Civil Engineering Forum Figure 1 Compressive strength development over time for concrete mixtures with varying GGBFS replacement levels .
%, 30%, 45%) at 7, 28, and 56 days.
particles, resulting in a plateau or reduction in laterage strength.
This finding is consistent with Oner and Akyuz .
, who reported optimal GGBFS replacement levels between 30% and 40% for achieving the best balance between early and later-age strength.
From a structural engineering perspective, the implications of these results are significant.
For applications where long-term load-carrying capacity, low permeability, and high durability are prioritizedAisuch as bridge decks, marine structures, and high-rise building columnsAiusing 30% GGBFS provides an optimal balance between strength development and sustainability Additionally, the reduced early-age strength of GGBFS concrete can be advantageous in massive pours, as the slower heat evolution lowers the risk of thermal cracking.
Comparing these findings with the review by Muralidharan et al.
on supplementary cementitious materials, the strength development pattern observed here aligns with the general behavior of slag-blended systems, which typically underperform at early ages but outperform plain cement mixes at later ages, especially beyond 56 or 90 days.
This suggests that even higher long-term strength gains could be expected if testing were extended to 90 days or one year.
In summary, the 30% GGBFS mixture demonstrated the most favorable strength development profile, balancing initial strength reduction with substantial later-age This performance, combined with its environmental benefits through reduced cement content, supports its suitability for sustainable structural concrete 2 Effect of PP Fibers on Compressive Strength The influence of polypropylene (PP) fiber content on the compressive strength of concrete with a 30% GGBFS matrix was evaluated at 28 and 56 days.
The results, summarized in Table 3 and Figure 2, indicate that fiber addition had a measurable effect on both early and later-age compressive strength.
At 28 days, the control mix without fibers (PP.
13 MPa, whereas all fiber-reinforced mixtures showed significantly higher strengths.
The PP3 mixture .
kg/mA fibe.
reached the highest value at 73 MPa, representing a 43.
9% improvement over the The PP5 and PP7 mixtures recorded slightly lower strengths of 34.
40 MPa and 34.
28 MPa, respectively.
This trend suggests that moderate fiber content enhances compressive strength, but additional fibers beyond an optimal dosage yield diminishing returns.
At 56 days, a similar pattern was observed.
The control mix improved to 37.
66 MPa, narrowing the gap with the fiber-reinforced mixes.
Nevertheless.
PP3 maintained the highest strength .
49 MP.
, followed closely by Table 3.
Compressive strength of concrete specimens incorporating varying PP fiber contents PP fiber variations 28 day (MP.
56 day (MP.
0 kg/m3 3 kg/m 5 kg/m 7 kg/m Journal of the Civil Engineering Forum Vol.
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Figure 2 Compressive strength at 28 and 56 days for concrete mixtures with varying PP fiber contents .
, 3, 5, 7 kg/mA) in the selected 30% GGBFS matrix.
PP5 .
22 MP.
and PP7 .
06 MP.
The sustained advantage of PP3 at later ages suggests that an optimal fiber dosage not only bridges microcracks but also integrates effectively within the dense GGBFS matrix, maintaining good fiberAematrix bonding and reducing stress concentrations under load.
The improvement in compressive strength at moderate fiber content can be explained by several mechanisms.
First, the presence of PP fibers helps to arrest microcrack formation during early shrinkage and under service loads, thereby preserving the integrity of the cement paste before significant damage develops.
Second, in a dense GGBFS matrix, the interfacial transition zone (ITZ) between fibers and paste is enhanced due to the refined pore structure, leading to better stress Third, fibers may contribute to load sharing within the composite during compression, particularly when cracks initiate, by restraining crack opening and redistributing stresses.
However, at higher dosages .
Ae7 kg/mA), the slight reduction in strength is likely due to fiber balling and reduced workability, which can introduce entrapped air and localized weak zones.
This effect has been reported in prior studies, such as (Banthia and Gupta, 2006.
Bertelsen et al.
, 2.
, which noted that excessive fiber volume can disrupt paste continuity and lead to compaction difficulties, particularly in mixes with a lower water-to-binder ratio.
In the present study, this phenomenon was more pronounced given the already low w/b ratio and dense matrix provided by GGBFS, which leaves little tolerance for workability loss without mechanical vibration adjustments.
From a practical standpoint, these results suggest that incorporating 3 kg/mA of PP fibers into a 30% GG44 BFS concrete matrix provides an optimal balance between compressive strength enhancement and workability.
For structural applications where compressive strength is a primary performance requirementAisuch as columns, prestressed beams, or heavily loaded slab elementsAithis dosage offers tangible benefits without introducing mixing or placement challenges.
It is also important to note that while compressive strength is a critical property, it is not the sole determinant of structural performance, particularly in fiber-reinforced systems.
The full benefit of PP fibers emerges in post-cracking behavior, ductility, and energy absorption, which will be discussed in the subsequent flexural performance sections.
Nevertheless, establishing that fiber inclusion at optimal levels does not compromise, and can even enhance, compressive strength is essential for validating the viability of such composites in practical structural applications.
In summary, the experimental findings confirm that moderate PP fiber content .
round 3 kg/mA) enhances compressive strength in a dense GGBFS matrix, while higher contents may slightly reduce strength due to workability issues.
These observations align with existing literature and reinforce the need for careful optimization of fiber dosage in performance-based concrete mix design.
3 Flexural Behavior of RC Beams The flexural performance of RC beams incorporating 30% GGBFS with varying PP fiber dosages .
, 3, 5, and 7 kg/mA) was evaluated through four-point bending tests at 56 days.
The loadAedeflection curves (Figure .
demonstrated that all beams exhibited an initial Vol.
12 No.
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Journal of the Civil Engineering Forum Figure 3 LoadAedeflection curves of RC beams with 30% GGBFS matrix and varying PP fiber contents .
, 3, 5, 7 kg/mA), showing Pmax and Pult points.
linear elastic response followed by nonlinear behavior until failure.
The inclusion of PP fibers enhanced both peak load (Pmax ) and post-cracking performance compared to the control beam (PP.
As shown in Table 4, the highest maximum load was achieved with PP3 .
0 kN), representing a 21.
95% increase over the control beamAos 20.
5 kN.
This improvement indicates that a moderate fiber dosage effectively contributes to crack-bridging and stress redistribution, allowing better utilization of the tensile reinforcement before peak load is reached.
The PP5 and PP7 beams recorded slightly lower Pmax values .
5 and 23.
0 kN, respectivel.
compared to PP3, suggesting that excessive fiber content may reduce workability and lead to uneven fiber distribution (AuballingA.
, which in turn impairs matrix homogeneity and load transfer efficiency.
capacity of beams beyond peak load, enabling them to sustain greater deflections before failure.
The ductility index, defined as the ratio of ult to yield , reached its maximum in PP5 .
, which was 43.
higher than the control.
The superior ductility at this dosage may be attributed to an optimal combination of fiber dispersion and matrixAefiber bond strength, which provides effective crack control without inducing the stiffness-related limitations observed in PP7.
Higher ductility is particularly beneficial in seismic design, as it allows the structure to absorb and dissipate energy over a longer deformation range.
Deflection at maximum load also increased with fiber inclusion, most notably for PP3 .
94 m.
compared to PP0 .
71 m.
This result reflects enhanced strain capacity before peak loading, indicating that fibers help delay crack localization.
However, in the PP7 beam, despite a higher fiber content, the deflection at Pmax was lower .
88 m.
than that of PP3 and PP5, suggesting that excessive stiffness at high fiber dosages can limit elastic deformation capacity.
Initial stiffness, calculated from the slope of the initial linear portion of the loadAedeflection curve, increased consistently with fiber content: from 14.
27 kN/mm in PP0 to 24.
10 kN/mm in PP7.
This stiffening effect can be attributed to the improved integrity of the matrix, where GGBFS-induced densification reduces microvoids, and fibers restrain incipient microcracks, thus delaying the onset of nonlinear deformation.
However, while high stiffness improves serviceability by reducing deflections under working loads, overly stiff beams .
uch as PP.
may experience reduced post-cracking deformability, which could be undesirable in applications requiring high ductility.
When considering post-peak behavior, all fiberreinforced beams displayed a more gradual load reduction compared to the brittle drop observed in the control beam.
The ultimate deflection .
lt ) was highest in PP7 .
74 m.
, followed closely by PP3 .
00 m.
and PP5 .
41 m.
, while the control beam reached only 34.
80 mm.
This demonstrates that fiber inclusion significantly enhances the deformation Energy absorption .
rea under the loadAedeflection curve up to Pmax ) was highest for PP3 .
66 kNAm.
, an increase of 68% over the control.
This indicates that PP3 achieves the most efficient fiberAematrix interaction in the elastic and early post-cracking phases.
Energy dissipation .
rea beyond Pmax until failur.
continued to increase with fiber content, peaking in PP7 .
11 kNAm.
The higher dissipation in PP7 sug45 Journal of the Civil Engineering Forum Vol.
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Table 4.
Recapitulation of flexural performance of FRC beams Behavior
PP0
PP3
PP5
PP7
Max Load (Pmax , kN) Deflection Pmax .
Ultimate Load .
N) Deflection Pult .
Initial Stiffness .
N/m.
Ductility Index Yield Load .
N) Deflection Yield .
Energy Absorption .
NAm.
Energy Dissipation .
NAm.
Toughness .
NAm.
gests that although peak load capacity is slightly lower, the beam can release stored elastic energy more gradually, reducing the risk of sudden brittle failure.
The overall toughness .
um of energy absorption and dissipatio.
was highest for PP3 .
31 kNAm.
, closely followed by PP5 and PP7.
This reinforces the conclusion that a moderate fiber content achieves the best overall flexural performance by balancing strength, ductility, and toughness.
These findings are in agreement with previous research on synthetic fiber-reinforced concrete.
For example.
Shi et al.
reported that PP fibers in optimized dosages enhance both strength and post-cracking performance, while excessive volumes may reduce workability and uniformity.
Similarly.
Ding et al.
highlighted that the fracture toughness of fiber-reinforced concrete is maximized at intermediate fiber dosages due to optimal crack-bridging efficiency.
From an engineering perspective, the results suggest that for applications prioritizing strength and toughnessAisuch as bridge deck slabs or industrial floorsAiPP3 provides the most balanced performance.
For seismic applications where ductility is critical.
PP5 may be more advantageous.
Conversely, where serviceability deflection control is the main concern, as in long-span beams.
PP7Aos higher initial stiffness could be beneficial, provided the potential reduction in ductility is acceptable.
In summary, the flexural behavior results highlight the importance of dosage optimization in fiber-reinforced GGBFS concrete.
Moderate fiber contents .
Ae5 kg/mA) offer substantial improvements in load capacity, deformation ability, and toughness, whereas excessive fiber volumes may lead to marginal gains or even trade-offs in certain performance aspects due to workability challenges and stiffnessAeductility interactions.
4 Crack Pattern Observation The crack patterns observed at maximum load (Pmax ) and at ultimate load (Pult ) for all beam specimens are shown in Figure 4-7.
Across all beams, the first visible cracks consistently appeared in the constant moment region between the two loading points, which is consistent with flexural behavior under four-point bending.
However, the progression, distribution, and width of cracks varied significantly depending on the PP fiber Figure 4 Crack pattern at peak load (Pmax ) for control beam (PP.
without fibers, showing single dominant midspan crack indicating brittle failure.
For the control beam (PP.
shown in Figure 4, the first crack appeared at a relatively low load and propagated rapidly through the depth of the beam.
Subsequent cracks developed sparsely and concentrated near midspan, with limited branching.
At Pmax , one dominant crack became the primary failure path, widening significantly in the post-peak stage.
This crack localization is characteristic of plain concrete beams, in which once the tensile capacity of the matrix is exceeded, stress transfer to reinforcement is immediate and concentrated, leading to rapid softening and brittle behavior.
In contrast, the fiber-reinforced beams exhibited more distributed and finer cracks.
The PP3 beam developed multiple microcracks at lower load levels, but their growth rate slowed due to fiber bridging.
At Pmax .
Vol.
12 No.
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Journal of the Civil Engineering Forum potentially limiting energy dissipation through microcracking.
Figure 5 Crack pattern at peak load (Pmax ) for beam with 3 kg/mA PP fibers (PP.
, showing multiple fine cracks and reduced crack width due to effective crack bridging.
cracks remained narrower, and additional secondary cracks formed away from the main flexural zone, indicating improved stress redistribution.
In the postpeak stage, the cracks continued to propagate gradually, with widths remaining within a moderate range due to the restraint provided by the fibers.
Figure 6 Crack pattern at ultimate failure load (Pult ) for beam with 5 kg/mA PP fibers (PP.
, exhibiting widely distributed narrow cracks consistent with highest ductility index.
The PP5 beam displayed an even denser network of cracks, extending over a wider portion of the span.
The high crack density suggests that fibers effectively delayed the coalescence of microcracks into dominant cracks, which is beneficial for ductility.
This finding aligns with the high ductility index recorded for PP5.
The distribution of cracks also suggests a more uniform strain distribution along the tensile face of the beam, which is advantageous for seismic resistance because multiple yielding zones are preferred over a single plastic hinge formation.
Figure 7 Crack at ultimate failure load (Pult ) for beam with 7 kg/mA PP fibers (PP.
, showing extensive distribution of fine cracks and gradual failure mode with high energy dissipation.
For the PP7 beam, although cracks were initially narrow, their density was lower than that in PP5.
The increased stiffness of the high-fiber matrix limited deformation in the early stages, but once cracking initiated, several cracks propagated more abruptly.
This behavior reflects a balance shift: while high fiber content restrains crack width effectively, it can also reduce the number of cracks by increasing the elastic stiffness.
A key observation across all fiber-reinforced beams is that crack widths at Pult remained smaller than those in the control beam, even for the most heavily loaded This improvement in crack control is directly related to the fiber-bridging mechanism, whereby PP fibers spanning across crack faces carry tensile forces after matrix cracking, reducing opening displacement.
This mechanism is supported by microstructural studies (Ding et al.
, 2020.
Shi et al.
, 2.
, which show that PP fibers anchor into the surrounding paste, mobilizing pull-out resistance and friction along the fiberAematrix From a durability standpoint, improved crack control has significant implications.
Narrower cracks limit the ingress of chlorides, sulfates, and other aggressive agents, enhancing the service life of structural elements in harsh environments, such as coastal infrastructure and bridge decks.
In this context, the combination of GGBFSAiknown for its pore refinement and chloride binding capacityAiwith PP fibers offers a dual mechanism for durability enhancement: microstructural densification and mechanical crack restriction.
In summary, the crack pattern observations confirm the quantitative flexural performance results: moderate fiber dosages .
Ae5 kg/mA) promote distributed microcracking, higher ductility, and better energy dissipation, while excessive dosages .
kg/mA) improve stiffness and crack width control but may reduce crack density and ductility.
This nuanced understanding of crack behavior is essential for performance-based design, where serviceability, durability, and ultimate strength must be balanced for specific applications.
4 DISCUSSION
The experimental results provide a comprehensive understanding of the mechano-microstructural interaction between a dense GGBFS-based cementitious matrix and discrete PP fiber reinforcement in RC beams.
The discussion in this section integrates compressive strength, flexural behavior, and crack pattern observations, offering insights into the mechanisms governing performance and their implications for structural concrete design.
1 Role of GGBFS in Matrix Densification and Strength Development The compressive strength results confirmed that partial replacement of Portland cement with 30% GGBFS produced a dense and refined matrix that surpassed the strength of plain cement concrete beyond 28 days.
Journal of the Civil Engineering Forum The microstructural densification was attributed to the pozzolanic and latent hydraulic reactions of GGBFS, which consume calcium hydroxide (Ca(OH)CC) and generate secondary CAeSAeH gel, reducing the porosity of the paste and improving the quality of the interfacial transition zone (ITZ).
Studies on high-performance concrete have reported strength enhancement at 28 and 49 days with 30Ae50% GGBFS replacement (Yun et al.
, 2.
Similarly, microstructural investigations of blended systems with GGBFS and silica fume revealed reduced porosity, increased formation of CAeSAeH and CAeAAeSAeH gels, and improved durability and compressive strength (Chen et al.
, 2.
The replacement range of 30Ae40% GGBFS is frequently considered optimal, providing a favorable balance between later-age strength gain and early-age strength Recent studies emphasize that mixtures within this range achieve the best trade-off between mechanical and durability performance (Ahmed, 2024.
Dandaboina et al.
, 2.
However, higher replacement levels, such as >40% GGBFS, may lead to lower long-term strength due to clinker dilution and insufficient Ca(OH)CC for complete slag activation.
This limitation has been confirmed in recent investigations, which observed increased porosity and reduced strength in mixes with very high SCM contents (Sadok et al.
, 2021.
Dandaboina et al.
, 2.
From a practical standpoint, the strength gain beyond 28 days is particularly advantageous for infrastructure subjected to sustained or gradually increasing service In massive structural elements, the slower hydration of GGBFS can also mitigate thermal cracking by reducing peak hydration temperatures.
2 PP Fiber Contribution to Compressive Strength The incorporation of PP fibers at 3 kg/mA into the optimized 30% GGBFS matrix enhanced compressive strength by approximately 44% at 28 days and about 5% at 56 days compared to the fiber-free control.
This improvement at a moderate fiber dosage can be explained by enhanced crack resistance under compression, where fibers bridge internal flaws and restrain microcrack propagation during loading.
However, increasing fiber dosage beyond 3 kg/mA did not yield additional strength gains, and in some cases slightly reduced compressive strength.
This reduction can be attributed to reduced workability and increased risk of fiber balling, leading to air void entrapment and localized paste discontinuities.
These results align with (Mohamed and Zuaiter, 2.
, who emphasized the importance of optimal fiber content to avoid negative effects on matrix homogeneity.
Vol.
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3 Flexural Behavior:
Toughness Interactions StrengthAeDuctilityAe The flexural test results reveal that PP fibers significantly modify the loadAedeflection response of RC The 3 kg/mA dosage (PP.
yielded the highest peak load (Pmax ), demonstrating improved cracking resistance and stress transfer prior to reinforcement The 5 kg/mA dosage (PP.
provided the highest ductility index, indicating superior deformation capacity before failure, while the 7 kg/mA dosage (PP.
maximized initial stiffness and post-peak energy dissipation.
The differences in performance can be linked to fiberAe matrix interaction mechanisms.
At moderate dosages, fibers are well-dispersed, enhancing crack-bridging efficiency and maintaining good workability.
At higher dosages, despite increased stiffness and post-cracking resistance, the risk of non-uniform distribution and matrix discontinuities becomes more significant, reducing the efficiency of fiber reinforcement in the elastic range.
This finding is consistent with Jang et al.
, who reported that optimal fiber dosages enhance both flexural strength and ductility, whereas excessive fiber content can reduce these benefits due to workability challenges.
4 Crack Control and Durability Implications Crack pattern observations demonstrated that PP fibers effectively transformed the fracture mode from brittle localization .
n control beam.
to distributed microcracking .
n fiber-reinforced beam.
The narrower crack widths in fiber-reinforced beams not only improved structural aesthetics and serviceability but also provided substantial durability benefits by reducing pathways for aggressive agents.
When combined with GGBFSAos chloride-binding capability and reduced permeability, the fiber-induced crack control could significantly extend service life, particularly in marine and salt-exposed environments.
These synergistic benefits are highly relevant to performancebased durability design, which is increasingly emphasized in modern infrastructure projects.
5 Trade-Offs Guidance Performance-Based Design The results highlight important trade-offs between stiffness, strength, ductility, and toughness.
For example, while PP7 exhibited the highest stiffness, it also had the lowest ductility among fiber-reinforced beams.
Conversely.
PP5 achieved the highest ductility but a slightly lower peak load than PP3.
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12 No.
1 (January 2.
This finding underscores the importance of selecting the appropriate fiber dosage based on the specific performance priorities of the structure.
For strengthdominated applications, such as heavily loaded beams or short-span girders.
PP3 provides the most favorable balance between load capacity and structural efficiency.
In cases where ductility is the primary requirement, as in seismic frame beams or plastic hinge zones.
PP5 offers superior deformation capacity before failure.
Meanwhile, for serviceability-critical structures, including long-span floor beams or deflection-sensitive members.
PP7 delivers the highest stiffness and effective control of crack widths, thereby enhancing overall service performance.
Such selections align with the concept of performance-based design, where material composition is optimized to meet project-specific serviceability, safety, and durability targets rather than pursuing a single AumaximumAy property value.
6 Limitations and Future Research Directions While the study provides valuable insights, several limitations should be acknowledged.
First, the testing program was limited to 56 days.
extended testing to 90 days or beyond would provide a more complete understanding of long-term strength and durability performance, particularly given the continued hydration potential of GGBFS.
Second, microstructural analysis .
SEM.
EDS) was not conducted within this experimental phase but would offer direct evidence of fiberAe matrix bonding and pore refinement mechanisms.
Finally, field-scale validation is necessary to confirm laboratory results under real placement, compaction, and curing conditions.
Future research should also explore hybrid fiber systems, where PP fibers are combined with steel or basalt fibers, potentially enhancing both tensile and compressive properties while improving post-cracking behavior.
Additionally, the influence of fiber geometry .
ength, aspect ratio, and surface textur.
on GGBFSbased concrete performance warrants further investigation.
Journal of the Civil Engineering Forum consistent with the widely reported optimal GGBFS replacement range of 30Ae45%.
The incorporation of PP fibers further contributed to strength and flexural performance improvements.
moderate dosage of 3 kg/mA provided the greatest gain in compressive strength, with increases of up to 44% at 28 days, whereas higher dosages .
Ae7 kg/mA) yielded no additional benefits due to reduced workability and fiber Flexural tests demonstrated superior performance across all fiber-reinforced beams compared to the control.
Specifically, the PP3 mixture achieved the highest peak load capacity.
PP5 provided the greatest ductility index, and PP7 offered superior initial stiffness and post-peak energy dissipation.
Beyond mechanical performance.
PP fibers effectively transformed brittle crack localization into distributed microcracking, significantly limiting crack widths at ultimate load.
Coupled with the reduced permeability and chloride-binding capacity imparted by GGBFS, this synergy enhances durability and service life in aggressive environments.
These findings highlight the importance of tailoring fiber dosage to design priorities: PP3 for strength-critical applications.
PP5 for ductility-driven performance, and PP7 for serviceability and crack control.
Overall, the integration of 30% GGBFS with optimally dosed PP fibers demonstrates a viable pathway toward sustainable and high-performance structural concrete.
The observed synergy improves mechanical behavior, crack control, and long-term durability in ways that are aligned with modern performance-based design frameworks.
Future work should include extended curing studies, detailed microstructural characterization, and full-scale field trials to validate these results and refine practical design guidelines.
DISCLAIMER
The authors declare no conflict of interest regarding the funding, authorship, or publication of this research.
5 CONCLUSION
ACKNOWLEDGMENTS
This study investigated the combined effects of GGBFS and PP fibers on the mechanical behavior of reinforced concrete beams, with particular emphasis on compressive strength, flexural response, and crack development.
Experimental findings revealed that replacing 30% of Portland cement with GGBFS produced the highest later-age compressive strength, surpassing the control mix at both 28 and 56 days.
This enhancement is attributed to microstructural densification through secondary CAeSAeH formation, which is The authors would like to express their gratitude to PT Krakatau Semen Indonesia for providing the GGBFS material used in this research.
The authors also acknowledge PT Global Pro Asia, the country distributor of Kratos in Indonesia, for supplying the polypropylene Journal of the Civil Engineering Forum
REFERENCES