Experimental investigation of reinforced cement concrete and nylon fiber reinforced concrete beamABSTRACT
Concrete is brittle substance whose tensile strength and flexural strength are low and may easily crack and its lifespan is limited. The proposed research aims at assessing how the addition of nylon fiber affects the mechanical and workability of M30 grade reinforced cement concrete (RCC). The originality of this study is that the nylon fibers used are used at optimal dosage (0.25% and 0.5% by volume) in order to improve both strength and durability without impairing the workability. Six series of specimens were cast and experimented on compressive, split tensile and flexural strength, and slump cone testing of fresh concrete workability. The findings showed that the compressive, split tensile and flexural strengths were increased by 11, 14 and 17 percent respectively by the addition of 0.5 percent nylon fiber over plain M30 concrete. The enhancement in mechanical performance is attributed to improved fiber matrix interfacial bonding and crack-bridging mechanisms provided by the dispersed nylon fibers. The fibers effectively delay crack propagation and redistribute tensile stresses within the concrete matrix. Engineering wise, the nylon fiber-reinforced concrete is a cost-effective and structural-friendly substitute of structural application in which enhanced tensile and flexural strengths are needed.
Keywords:
Nylon fiber; Reinforced cement concrete; Compressive strength; Tensile strength; Flexural strength; Fiber reinforced concrete.
1. INTRODUCTION
There is an increasing pressure on the construction industry of the world to use construction materials and technology that minimizes the embodied carbon yet does not compromise its stability and functionality [1]. The other most feasible method of achieving this balance is by inclusion of fibers in cementitious composites to increase tensile, flexural and cracking behavior [2]. FRC includes more post-cracking ductility and energy absorbing characteristics as compared to conventional concrete [3]. It is revealed that the use of optimized fiber systems can be useful to the environment due to the extended service life and reduced maintenance schedule [4]. The natural fibers such as jute, coir, flax and sisal are attractive as it can be renewed and low cost of energy is used in the production [5]. They have also been related to low shrinkage cracking and excellent toughness at moderate exposure levels due to their incorporation [6]. However, natural fibers are destroyed in the alkaline environment, and this lowers their application in concrete structure in the long-term [7]. The concept of natural and synthetic yarns has therefore been proposed on what it can achieve to retain the durability as well as sustainability [8]. Artificial polymeric fibers polypropylene, polyester, synthetics out of basalt and nylon are typical in order to boost impact and fatigue strength [9]. These fibers are dependent on factors such as aspect ratio, bonding property, quality of dispersion in the matrix as well as efficiency [10]. The influence of fiber volume fraction on fresh and hardened properties is very intense and surplus of it can result in a low workability and fiber balling [11]. The best fiber contents as set by past studies to be below 1.0 percent by volume have been determined to be used to ensure that the macro-synthetic systems are functioning properly and acquiring the right mechanical benefits [12]. There is also the need to ensure consistency through proper mix proportioning and addition of chemical admixtures in order to achieve consistency when fibers are being introduced [13]. Similar testing regimes and standard specimen preparation are necessary as well to guarantee a large volume of data comparison [14]. The first studies have established that length and distribution of the fiber has a significant contribution to the load-bearing and deformation behaviors of the concrete [15]. The fact that the dosage and composition of the matrix can alter the resilient modulus and volumetric stability due to the addition of fibers is also mentioned in other experiments [16]. The reviews also indicate that testing and dosage control stability are essential to be maintained in order to interpret the fiber effects correctly [17]. Interesting to note in recent is the nylon fiber in that it has been found to have high tensile strength, elasticity and alkali attack resistance [18]. It has been found that nylon fibers can increase splitting tensile and flexural strength primarily due to bridging cracks and reduced crack propagation [19]. Despite all the advantages, nylon fibers possess the capacity of reducing slump and flowability upon bad dispensation or even when lots of them are used [20]. The available literature reveals that the two percentages that can be the best dosage are 0.2–0.8 percent nylon by volume due to their capacities to increase the strength and workable consistency [21]. Hybrid composite nylon polypropylene or natural fiber research discovered synergistic enhancement of the toughness and post crack strength [22]. Experiments on durability confirm that synthetic fibers can be applied to limit crack width and to slow down the diffusion of chlorides to improve long service life [23]. Nevertheless, the strength of the increment varies with concrete grade, the kind of aggregate and curing regime meaning that the reaction is mixture-dependent [24]. Though it is not a secret that much effort has been put into basalt and polypropylene fibers on high-performance and pavement concrete, few studies have systematically investigated the low dosage of nylon on ordinary M30 reinforced cement concrete (RCC) [25]. The researchers recommended these three articles that are combined to highlight the significance of fiber geometry, dosage sensitivity and standardized testing to measure these benefits [26]. As a result of one of these studies, it has been established that compressive and flexural strengths are very sensitive to the fiber length and percentage in pavement-grade mixtures [27]. The other one demonstrated that fiber addition has an effect on the resilient modulus and expansion, and that the dosage of the fiber has to be calibrated in all situations [28]. The third one indicated the necessity of standard of tests and modification in order to be able to transfer laboratory results to the field performance [29]. Unlike such investigations, the present study relies on the classical M30 RCC using two dosages of tiny nylon fibers (0.25% and 0.5% by volume) that is used to maintain the workability constant and increase strength [30]. Slump, compressive, split-tensile and flexural strength are measured by the paper, which is further analyzed to visually analyze the crack development via photography [31]. Finally, the behaviour is explained on engineering principles, such as fiber pull-out energy, bridging efficiency, and post-peak ductility to explain why certain rules can be feasible when it comes to nylon fibre dosage to be used in RCC [32].
1.1. Novelty
Even though past research has conducted an inquiry on nylon fiber reinforcement in concrete, minimal research has been conducted jointly to analyze mechanical strength, durability properties, and fire resistance behavior of M30 grade concrete at the Indian climatic conditions. The current study presents the overall analysis of the strength improvement, the ductility performance, the water absorption and the high temperature performance; thus, it presents the synthesized analysis of structural applicability.
2. MATERIALS AND METHODS
2.1. Materials
2.1.1. Cement
The experimental investigation used ordinary Portland Cement (OPC) of 53 grade that meets the requirements of the IS 12269:2013. To ensure that the cement had the desired physical and chemical properties, the cement was examined as per the IS 4031 (Part 115):1996. The characteristics are critical in determining the quality, consistency, and strength development behaviour of the concrete.
2.1.2. Physical properties of cement
The physical parameters shown in Table 1 including specific gravity, fineness and setting times were determined to be suitable to be used in the reinforced and fiber-reinforced concrete. The findings indicate that the cement is up to the standard requirement of OPC 53 grade.
The values are a pointer to the fact that the cement is well grounded, moderate setting time, and forms the necessary compressive strength, so it serves both the plain cement and fiber reinforced cement.
2.1.3. Chemical composition of cement
The oxides, mainly calcium oxide (CaO), silicon dioxide (SiO2), aluminum oxide (Al2O3) and ferric oxide (Fe2O3), are part of the constituents that form different phases of clinkers that determine the strength and durability properties of cement, and include: C3S, C2S, C3A, and C4AF are shown in Table 2.
The chemical analysis was as per IS 4032:1985, which indicates the methods to be used to analyze the content of oxides in cement.
2.2. Fine and coarse aggregates
2.2.1. Physical properties of aggregates
The fine and coarse aggregates are also important to influence the strength, workability, as well as the durability of concrete. River sand and smashed granite aggregates were used in the study locally.
The fine aggregate meets Zone II limits of grading, which gives RCC mixes an appropriate workability. The two aggregates have acceptable water absorption and strength characteristics, and they contribute to the sustenance of the target workability and durability of the concrete. The properties are shown in Table 3.
2.2.2. Chemical properties of aggregates
A chemical examination was done to make sure that deleterious material like chlorides, sulphates and organic matter that can adversely impact cement hydration or corrosion of reinforcement is absent.
Results of the tests indicate that both fine and coarse aggregates are chemically stable and can be used in reinforced construction. The Properties are shown in Table 4.
2.3. Nylon fibers
2.3.1. Description and source
In this study, nylon fibers shown in Figure 1 were chosen as the reinforcing additive due to their high tensile strength, elasticity and crack resistance. The fibers meet the standards of IS 16481:2022 (Synthetic Fibers for Concrete — Specification) and ASTM C1116/C1116M: 2010 on fiber-reinforced concrete. The fibers of nylon are monofilament in nature, and they are evenly mixed into the compound so that toughness after cracks and shrinkage of plastics are minimized. They were acquired with the certified manufacturer, so their dimensions and mechanical behavior were of the same order.
2.3.2. Physical and mechanical properties of nylon fibers
The selected nylon fibers are of good tensile capacity, flexible and have a high melting point, thus can be used in M30 grade of RCC without interfering with the fresh properties of concrete. Table 5 contains the properties of nylon fibers.
2.3.3. Engineering basis for fiber dosage
Nylon fibers were added in two amounts; 0.25 percent and 0.5 percent of concrete. The values have been chosen according to the optimization results given in the past studies which have shown that nylon fiber content in this range gives significant enhancement in tensile and flexural strength without having negative effects on workability. The trial mixes ensured that these percentages showed equal dispersion without balling or segregation of the fibers.
2.4. Microstructural characterization
2.4.1. X-ray diffraction (XRD) analysis
The X-ray diffraction (XRD) was conducted in order to determine the crystalline phases found in the control and nylon fiber-reinforced concrete samples. A Bruker D8 Advance diffractometer with Cu-Ka radiation (1.5406 A) was used to perform the test. The voltage and the current used in this instrument were 40 kV and 30 mA respectively. The diffraction data were measured in a scanning range 2 = 10.0o -80.0o degree with step size = 0.02 and a step time = 48s. Testing in a coupled 2θ -theta scanning mode was applied to the samples. The diffraction patterns obtained were measured in order to determine the major functional group of hydration products and mineral phases that were found in the concrete matrix. Figure 2 shows sample for XRD test.
2.4.2. Fourier transform infrared spectroscopy (FTIR) analysis
The chemical bonding and functional groups of the samples of concrete were investigated by using Fourier Transform Infrared Spectroscopy (FTIR) analysis. A standard FTIR spectrometer was used to analyse in the wavenumber range of 400–4000 cm3 -1 with a TGS detector. To enhance accuracy of the signal, the samples were scanned at a resolution of 4 cm-1 and 16 accumulations. Incident radiation angle was set at 45 and cosine apodization and automatic gain settings were used to get the spectra. Peak-picking analysis was used to identify the key functional groups that were related to the cement hydration products and matrix interaction. Figure 3 shows sample for FTIR test.
2.5. Mix design and sample preparation
The concrete mix design was designed to have a compressive strength of M30 (30 MPa) in 28 days of the concrete mix which complies with IS 10262:2019 – Concrete Mix Proportioning (Guidelines) and IS 456:2000 Plain and Reinforced Concrete (Code of Practice). The design was to balance between the strength and workability and at the same time allow the incorporation of the nylon fibers at two dosages of 0.25 and 0.5 percent by volume of concrete.
2.5.1. Mix proportioning
The control and fiber-reinforced concrete proportions were made out of a mixture of several trial mixes conducted to reach a satisfactory workability level and desired strength. Water to cement ratio (w/c) was kept at 0.45 as this has offered an acceptable balance between workability and strength. The flow characteristics were increased by the addition of a superplasticizer (Conplast SP 30) at a weight of cement of 0.8 percent to guarantee the consistent dispersion of the fibers. Figure 4 and Table 6 shows mixing of M30 grade concrete.
To ensure that excess water variation was not created, the aggregates were all in saturated surface dry (SSD) condition before the batch was mixed. The mixture was combined in a tilting drum mixer and then cast.
2.5.2. Fiber addition and mixing procedure
The nylon fibers were slowly incorporated to the dry mixture of cement, sand, coarse aggregates to allow a uniform distribution and not to allow clumping of the fibres. Upon the attainment of uniform dry blending, water containing superplasticizer was added. In order to attain reproducibility, every batch was mixed between 3 and 4 minutes once fibers were added to allow the mix to remain wet and the fibers to be evenly dispersed throughout the entire mix.
2.5.3. Casting of specimens
Compressive, split-tensile and flexural strength tests were to be done on concrete specimens cast. The IS 516 (Part 1):2021 steel moulds of standard size were utilized. The next dimensions were adopted as specimen.
Following casting, each of the specimens was enwrapped in wet burlap during 24 hours to reduce the loss of moisture. After being demoulded, they were curved in a curing tank containing clean water at 27 ± 2 o C at 7, 14 and 28 days. Figure 5 and Table 7 shows the casted specimens details.
2.5.4. Testing procedures
2.5.4.1. Workability test
The slump cone test (IS 1199:1959) was used in measuring workability. The experiment was carried out as soon as the mixture was mixed to determine the effect of adding a nylon fiber to fresh concrete.
2.5.4.2. Compressive strength test
A compression testing machine (CTM) of 2000 kN was used to conduct the compressive strength test according to IS 516 (Part 1):2021. Cube specimen was tested on three mixes and three ages of curing and the average value was taken.
2.5.4.3. Split tensile strength test
The split-cylinder test method was used to test cylindrical specimens based on IS 5816:1999. The load was put as a uniform force along the vertical diameter of the cylinder until failure and tensile strength calculated.
2.5.4.4. Flexural strength test
The two-point loading method by IS 516 (Part 2): 2021 was used to long-term flexural test the concrete prism. Modulus of rupture was determined in relation to the highest force applied and the size of the specimen.
2.5.4.5. Failure observation and crack pattern analysis
After testing, the specimens were all visually inspected and photos on failure patterns and crack propagation were taken to be comparatively assessed between the control mix and fiber reinforced concrete mixes. The orientation and crack width and distribution were monitored so as to learn about the efficiency of the nylon fibers in crack bridging.
2.5.5. Curing and durability assessment
Potable water was used to cure all the specimens kept at 27 ± 2 C temperature until the date of testing. Other samples were withheld in order to carry out durability tests to determine water absorption, surface deterioration and shrinkage cracking resistance to IS 516 (Part 5):2021.
2.5.6. Fiber ratio selection engineering basis
An 0.25 and 0.5 percent of nylon fiber content have been selected depending on previous optimization research works, which had reported an optimum enhancement in mechanical and durability behavior of low fiber volume fractions. Research 20182024 has shown that higher fiber doses (>0.75%) can lower the workability, and result in clustering, but lower doses offer a better balance between workability and strength properties. In the given study, these two ratios were thus considered to approve their influence over compressive, tensile, and flexural strength performance of traditional M30 grade RCC.
3. METHODS
3.1. Slump test
The Slump test is a simple and fast method which is applied to determine the ease at which fresh concrete needs to be handled prior to being poured. It assists in defining whether the concrete mix is of correct mixture that is neither too dry nor too wet. It does settle down somewhat when it hardens, the amount of settling being noted. This value is a test or slump value that is used to determine the workability of the concrete. The slump is defined as the mixture becoming easier to work and free flowing but low slump is more rigid, less working mix. The test is used in the industry to determine that all concrete batches are consistent and easy to lay and finish. Table 8 shows the slump value.
The value of slump of the M30 grade of concrete were determined on varying mix proportions in order to test their workability. The normal concrete (CC) mix had a slump of 89 mm compared to natural fiber-reinforced (NF) concrete mixtures, which had slump of one 84 mm and one 81 mm. The findings shown in Figure 6 tells that the addition of natural fibers to the mixture of M30 concrete to the M30 concrete will in some way reduce its slump in that it will have a moderate decrease in the workability to that of the normal mix.
3.2. Mechanical tests
3.2.1. Compressive strength test
Compressive strength is another extremely significant characteristic of concrete that expresses the capability of concrete to withstand forces, which push it or press it against each other. In other words, it tells us the amount of load that the concrete is able to cope with before crushing. Curing period is pending at 7 or 28 days. Once the specimen is cured, it is then put under a compression test machine whereby a steadily increasing weight is applied to it until the breakage of the concrete has happened. The maximum load that it can take is divided by the area of compressive strength in N/mm2 or Mpa. It is employed to identify the conditions of the concrete being strong and reliable enough to be used safely in construction.
The compressive strength behavior of M30 grade concrete under the conventional concrete (CC) and that of natural fiber-reinforced (NF) was evaluated over various timeframes of curing. The CC mix in Figure 7 and Table 9 showed lower strengths of 14.10 N/mm2, compared to the NF mixes which showed an enhancement of approximately 16 and 27 percent, respectively. At 14 days, the CC mix gained 21.90 N/mm2, and the NF mixes gained 24.60 N/mm2 and 26.10 N/mm2 which represent approximately 12 and 19 percent. The trend persisted at 28 days, where the CC mix was 30.10 N/mm2 in comparison to 33.00 N/mm2 and 34.10 N/mm2 in the NF mixes, which are nearly 10 and 13 percentages, respectively. These comparisons evidently reveal the success of natural fiber incorporation to improve compressive strength of M30 concrete with time.
3.2.2. Split tensile strength test
Split tensile strength is an experiment on the ability of concrete to oppose forces of pulling or cracking. This test informs us about what concrete does when it is about to break because it is mostly compressive-resistant and not tension-resistant. The test does not pull at the concrete, instead, a round horizontal concrete cylinder is applied in a machine. A load is applied along its length, and makes it apart. It is referred to as the split tensile test or Brazilian test. It is related to the tensile strength by an indirect measure. It is generally conducted in standard sizes and in accordance with tests such as IS 5816 or ASTM C496. Tension strength attained by this test tends to be 8 to 15 percent of compressive power of the concrete. It is a productive and fast method of testing the concrete resistance to cracking in the real buildings.
Normal concrete (NC) nano fiber (NF) filled blends of M30 grade concrete were found to have split tensile strength at 7, 14 and 28 curing days. Tensile strength of the CC mix in Figure 8 and Table 10 shows that, at 7 days was 2.30 N/mm2, whereas NF mixes recorded higher values of 2.76 N/mm2 and 2.88 N/mm2 with an increment of about 20 and 25 percent respectively relative to control mix. With 14 days, strength of CC mix was 2.95 N/mm2 and strength of NF mix was 3.22 N/mm2 and 3.40 N/mm2 increasing approximately by 9 per cent and 15 per cent respectively. At 28 days, tensile strength was 4.00 N/mm2 which was of the CC mix, and NF mixes were in the extremely high values of 4.65 N/mm2 and 4.80 N/mm2 respectively, which are 16 percent and 20 percent more respectively. This implies that the ability to incorporate nano fibers in concrete can contribute greatly to the split tensile strength of concrete at any curing stage.
3.2.3. Flexural strength test on beam
Flexural strength is a measure of the strength of a beam to withstand bending loads. It indicates the maximum stress a concrete beam can bear before it breaks under a load. In structural elements like beams and slabs, bending is a common type of stress due to applied loads during service. As the load increases, the beam starts to bend the top side is compressed while the bottom side is stretched, which may lead to cracking at the bottom. The test continues until the beam fails, and the highest stress it endures before breaking is considered its flexural strength. Figure 9 shows the loading frame for flexural testing on the beam.
The relationship between load and deflection indicates a gradual increase in deflection with rising load. At lower loads, such as 2 KN and 4 KN, the deflection was relatively small at 0.86 mm and 1.12 mm, respectively. As the load increased to 10 KN and 20 KN, the deflection rose to 1.91 mm and 3.38 mm, showing a steady progression. Beyond 30 KN, the deflection continued to grow, reaching 5.28 mm at 30 KN and 7.42 mm at 40 KN. Interestingly, between 42 KN and 44 KN, shown in Table 11 deflection slightly fluctuated around 7.35–7.36 mm, indicating a possible stiffness variation or onset of material nonlinearity. As the load increased further to 54 KN and 56 KN, the deflection jumped to 9.85 mm and 11.27 mm, suggesting a steeper rise. At the peak load of 62 KN, the deflection reached 11.76 mm, indicating a significant deformation as the structure approached its load-bearing limit. Overall, Figure 10 shows that the increasing deflection with higher loads, with notable acceleration in deflection at higher load levels.
The flexural strength test for concrete with 0.25% nylon fibers in Figure 11 and Table 12 reveals how the material bends and stretches as the load increases. At the beginning, with just 2 kN applied, the deflection is quite small at 0.72 mm, but it steadily grows, reaching 2.41 mm by 8 kN. There’s a slight dip at 10 kN where the deflection drops a bit to 2.24 mm, but the overall upward trend quickly continues. By 20 kN, the deflection is 3.73 mm, and it keeps rising to 5.80 mm at 30 kN. Around 32 kN, the deflection jumps more noticeably to over 7 mm, with a small dip at 34 kN, but then it climbs again. From there, the deflection steadily increases, nearing 10 mm at 48 kN and pushing past 12.7 mm at 60 kN. At the highest load tested, 64 kN, the deflection reaches just over 13 mm. These results show that while the nylon fibers help the concrete resist bending, the material still gradually deforms more as the load gets heavier, with some small variations likely due to the way the fibers and concrete interact under stress.
The flexural strength test for concrete with a 0.5% nylon fiber content in Figure 12 and Table 13 highlights how the material responds to increasing loads by bending more noticeably. At the very beginning, with just 2 KN applied, the deflection measures 1.26 mm, which is clearly greater than the deflection seen in the mix with 0.25% nylon at the same load. As the load continues to rise, the deflection grows steadily reaching 3.32 mm at 14 KN and 6.33 mm at 30 KN both values consistently higher than those for the lower fiber content. When the load increases further, the deflection climbs to nearly 11 mm at 48 KN and around 14 mm at 60 KN, showing that the higher nylon content allows the concrete to flex more under stress. At the maximum load of 66 KN, deflection reaches 15.23 mm, exceeding the peak deflection observed in the 0.25% mix. This pattern suggests that boosting the nylon fiber ratio enhances the concrete’s ability to bend and absorb energy before failure, making it more flexible and tougher.
3.3. Durability tests
3.3.1. Water absorption test
Water absorption is the measure of how much water hardened concrete can draw into its pores, expressed as a percentage of its dry mass. It is mainly affected by the concrete’s pore size, compaction level, curing quality, and the presence of cracks. A lower absorption value usually means the concrete is denser, less permeable, and more durable against weathering and chemical attack.
The Nylon fiber reinforcement effect on the compressive strength of the composite is shown in and Table 14 Figure 13. The unreinforced sample which is the control sample has compressive strength of about 29.78 Mpa. The compressive strength is increased by 10 percent in the addition of 0.25 percent Nylon fibers (NF1) to 32.77 Mpa. Adding more and more fiber content to 0.5% (NF2) also enhances the strength to 33.92 Mpa, which is 14 percent stronger than the control. This progressive increase implies that Nylon fibers are useful to carry loading on the composite as reinforcement. The fibers would work by sealing microcracks and evenly redistributing applied loads throughout the matrix preventing localized failure and improving the stiffness and toughness of the composite. Nylon fibers enhance crushing properties as well as deformation of the composite under mechanical load hence it is more applicable in structural use where strength is the main consideration. Simultaneously, the graph suggests that there is a sharp decline in the water absorption as the nylon content increases. The absorptivity of the control composite water is 4.1 and it reduces to 3.2 and to 2.9% in NF1 and NF2 respectively. This reduction shows that nylon fibers help in decreasing the porosity and permeability of the composite. Nylon fibers help to seal the spaces and cracks in the matrix to form a smaller and tight structure which helps to limit the channels of water entry. Also, the hydrophobicity of nylon will not promote the affinity of moisture, which further reduces its ability to absorb water. Such a trait is very important to use in an environment where the wet exposure or moisture is anticipated since the minimization of water absorption is of great importance, ensure that the composite strength is not diminished easily like swelling or freeze-thaw damages, as well as increase the material service life.
3.3.2. Fire resistance test
Fire resistance describes how long a concrete element can endure high temperatures while still performing its intended structural role. The main influencing factors include section thickness, concrete density, type of coarse aggregate, moisture level, reinforcement cover, and any special additives. Fibers, in particular, can improve fire resistance by reducing the risk of explosive spalling, allowing the concrete to keep its shape and strength for longer during a fire.
Fire resistance evaluation was conducted by exposing specimens to elevated temperatures in a controlled electric furnace. The heating regime followed a gradual temperature rise up to 800°C over a duration of three hours. Temperature was monitored using embedded thermocouples.
The fire performance was assessed based on:
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Visible cracking
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Surface spalling
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Residual compressive strength after cooling
The improved fire resistance of nylon fiber-reinforced specimens is attributed to the formation of micro-channels due to fiber melting, which reduces internal vapor pressure and minimizes explosive spalling.
The test was conducted against two other important properties, which include the 28-day compressive strength of the composite and its fire resistance. The compressive strength with a 28 days period is an essential indicator of long-term mechanical strength, which is the process in which the material solidifies and acquires strength after it has been cured. Table 15 and Figure 14 shows the test score correspondingly reproduces the original compressive strength trend, as the NF1 and NF2 samples would score higher scores compared to the control, which in turn supports the fact that nylon fiber reinforcement does not only ensure instantaneous strength benefits but also continues to provide and further provides the benefits of the same strength over time. Another important property of safety-critical applications would be the fire resistance. The findings show the control composite to resist fire up to about 2 hours before structural collapse. As the content of nylon fiber is raised to 0.25, 2.5 hours will be added to fire resistance; a 0.5 content will add 3 hours. The inflammatory enhancement is an indication that nylon fibers enhance the thermal stability of the composite. The fibers would serve as thermal barrier which will lower the rate of heat transfer through the composite, maintain the cohesion of the matrix, and slow the rate of crack formation and collapse at high temperatures. Better fire resistance is helpful in the formation of the safety profile of the composite, still, it is even more fitting to be used in construction, infrastructure, and industrial use, where fire is also a threat. In summary, these results suggest that nylon reinforcers are multifunctional to composite materials. Addition of nylon, besides enhancing mechanical properties, limits absorption of water and decreases resistance to fire which is important in addressing key performance parameters on structural integrity, durability, and safety. The enhancements are quantitatively proportional to the degree of nylon addition with high concentrations of the same realizing higher enhancements. This renders the nylon fibers an extremely efficient and efficient addition to the manufacturing of the advanced composites that can address the challenging demands of the high stakes engineering tasks. Nylon reinforcement is a promising field of breakthrough in the material science and construction technology in the extension of composite lifespan and performance in different environments.
3.4. Microstructural analysis
3.4.1. XRD analysis
X-ray diffraction (XRD) pattern of the sample indicates a number of diffraction peaks in the 2th range of 10o–80o which means that the sample has crystalline phases in its composite matrix. There is a noticeable and acute peak at a certain 2θ range that is within 26o–28o, indicating that there must be quartz (SiO2) that is frequently used in cementitious makeup and fine aggregates. Further peaks of diffraction in the 30o–36o range could be attributed to calcium silicate phases and calcite (CaCO3) generated during the hydration of cement. Figure 15 shows that the fact that minor peaks are observed at higher diffraction angles, around 40o–50o and higher around 70o also serve to suggest that there are secondary crystalline compounds present in the hydrated cement matrix. The high and sharp edges prove the crystalline nature of the mineral phases used in the material. The introduction of nylon fibers does not cause any significant change to the basic crystalline structure of the cement matrix but is used to reinforce it mechanically without eliminating the mineralogical phases that existed previously.
3.4.2. FTIR analysis
Fourier Transform Infrared (FTIR) spectrum of the sample used in the area between 4000 and 500 cm-1 indicates a number of typical absorption bands which represent functional groups available in cement-polymer composite. A continuous absorption band at approximately 3400 cm-1 can be attributed to OH stretching vibrations which corresponds to the availability of water molecules and hydration product like calcium hydroxide in the cement matrix. Figure 16 shows the presence of a weak band around 2900 cm-1 signifies the presence of C-H stretching vibrations, which proves the existence of polymeric chains of the deposited nylon fibers. Amide group vibrations and H-O-H bending are the reason of the absorption band in the range of 1600–1650 cm-1, and this is yet another indication of the interaction between the nylon fibers and hydration products. Moreover, high absorption peaks observed in 1000–1100 cm, -OH indicate that the silicate phases of the cementitious material are found in the form of Si-O-Si and Si-O repeats. The sharp peaks at 600–500 cm-1 are related to metal-oxygen bonding and vibrations in the structure of mineral compounds in the hydrated cement structure. Such spectral characteristics prove the presence of polymeric nylon fibers and cement hydration products, which are good to be combined under the composite system.
4. RESULTS AND DISCUSSION
The current experimental study focuses on the mechanical and durability characteristics of M30 grade concrete that is reinforced with nylon fibers at the dosage of 0, 0.25 and 0.5 percent of the weight of cement. Compressive, split tensile, and flexural strengths, water absorption and fire resistance under concrete were tested on concrete specimens, in order to identify how the addition of fibers impacted the general performance.
4.1. Compressive strength
The compressive strength results indicate a consistent improvement with increasing nylon fiber dosage. The control mix (CC) achieved 28-day compressive strength of 30.10 MPa, while the 0.25% fiber mix (NF1) reached 33.00 MPa, representing an increase of approximately 9.6%. The 0.50% fiber mix (NF2) exhibited the highest strength of 34.10 MPa, corresponding to an improvement of about 13.3% compared to the control mix. Similar increasing trends were observed at 7 and 14 days, indicating that fiber incorporation enhances both early-age and later-age strength development. The improvement can be attributed to the crack-bridging effect of nylon fibers, which restricts microcrack propagation and improves stress transfer within the cement matrix.
4.2. Split tensile strength
A significant enhancement was observed in split tensile strength with fiber addition. At 28 days, the control mix recorded 4.00 MPa, whereas NF1 and NF2 achieved 4.65 MPa and 4.80 MPa, respectively. This corresponds to increases of approximately 16.3% and 20% over the control mix. The improvement was more pronounced in tensile behaviour than in compressive strength, highlighting the effectiveness of nylon fibers in resisting crack initiation and propagation. The fiber–matrix interaction enhances post-cracking resistance and reduces brittle failure.
4.3. Flexural load-deflection behaviour
The load-deflection response further confirms the beneficial effect of fiber reinforcement. The control beam reached an ultimate load of 62 kN with a maximum deflection of 11.76 mm. The 0.25% fiber beam exhibited a slightly lower ultimate load of 54 kN with 11.35 mm deflection. In contrast, the 0.50% fiber beam achieved the highest ultimate load of 66 kN and a maximum deflection of 15.23 mm, representing a 6.45% increase in load capacity and approximately 29.5% improvement in deformation capacity compared to the control beam.
The enhanced deflection capacity indicates improved ductility and energy absorption due to the fiber bridging mechanism, which delays crack widening and enables sustained load transfer beyond initial cracking.
The findings that were summarized gave a conclusion that the experimental study to be conducted will measure the effects of varying levels of recycled nylon fibers on the strength and durability of the M30 grade concrete. The control mix (M0) was characterized by the compressive strength of 33.4, split tensile strength of 2.81, flexural strength of 4.15, 4.10 per cent water absorption, and 2 hours of fire resistance. The compressive strength rose to 37.8 MPa, the split tensile strength rose to 3.42 MPa and flexural strength improved to 4.72 Mpa when 0.25% nylon fiber (M1), was added. The absorption of water was reduced to 3.20%. Fire resistance was also raised to 2.5 hours, which means that incorporation of fibers provided better strength and durability. The concrete which possessed a higher fiber content of 0.50% (M2) performed even better with compressive strength of 39.5 Mpa, split tensile strength of 3.58 Mpa and flexural strength of 4.94 Mpa. Meanwhile, water uptake decreased further to 2.90% and fire resistance went up to 3 hours. The factual result of this study is that the addition of nylon fibers strengthens the concrete matrix, increases crack resistance and ultimately advances the overall durability through the reduction of water penetration and heat resistance. Compressive strength was found to be higher in the 0.5% fiber mix (39.5 MPa) and 33.4 Mpa in the control mix with the highest change of approximately 27 percent and 13 percent at 7 days and 28 days respectively. This growth could be explained by the fiber bridging effect in which the nylon fibers inhibit the spread of the microcracks during hydration, improve the stress distribution, and postpone the coalescence of the cracks hence increasing the load-bearing capacity. The same was stated confirmed that early-age compressive strength is enhanced with synthetic fibers by means of internal crack confinement [33]. Split tensile strength also had a significant increase with an improvement of 20–25 per cent in the early stages of curing. This was mostly improved by the high bond between the fibers and the matrix that allowed the fibers to bridge the microcracks and take tensile forces that would otherwise result to brittle failures. This process enhanced ductility and post-cracking strength that is needed in structural applications where indirect tensile loads prevail. Glass fractured specimens when under a microscope showed thinner and more branched cracks in fiber-reinforced mixes which proved the role of fiber in crack control. The study results correlate that nylon fiber composites are more ductile and have lower brittleness [34]. The positive effect of the fiber reinforcement was also confirmed by flexural strength tests, as the beam with 0.5% nylon fibers had the best deflection capacity and energy absorption before breaking down. The nonlinear load deflection behavior indicated that fiber-reinforced beams could maintain higher deformation until failure, which provided a more ductile load-deflection behavior in comparison to the brittle behavior of the control specimens. The mechanism of energy dissipation in which fibers cross bridging cracks, and take loads by a series of bridging processes has been attributed to the higher flexural toughness. The use of the nylon fibers also demonstrated similar trends in terms of increasing flexural strength, as well as decreasing the crack widths. Durability-wise, the fiber addition decreased the water absorption of 4.1 in the control mix to 2.9 in the 0.5% fiber mix, which showed that the concrete mix had created a denser and less permeable concrete matrix [35]. This effect can be attributed to the pore-blocking effect of fibers that interferes with the continuity of the capillaries and reduces the entry of water and harmful agents. Thus, the fiber-reinforced concrete will be more resistant to permeability, chloride attack, and freeze-thaw damages, and will have a long life span. Fire resistance was also enhanced significantly with the control mix sustaining a structural integrity of approximately 2 hours with fiber-reinforced mixes standing the test of 2.5 to 3 hours. This is mainly attributed to better thermal stability because when the nylon fibers melt, they create microchannels through which the vapor is escorted out avoiding the explosive spalling that otherwise would have damaged the bond between the cement paste and the aggregates. The addition that fibers have the benefit of increasing thermal durability [36]. The present study in comparison to the previous ones on polypropylene and glass fiber reinforced concretes reveals that nylon fibers have high tensile and flexural values at lower dosages. The dosage of 0.5% fiber was found as the best percentage, which attained an equal measure of strength, ductility, and durability. The originality of the study is the successful use of the recycled nylon fibers that indicated that upon appropriate dispersion, recycled nylon fibers can greatly contribute to the mechanical and durability characteristics at low volumes and low costs. This renders nylon fiber-reinforced concrete a potential, sustainable, structural use in demanding mechanical and environmental performance.
4.4. Microstructural analysis
The addition of nylon fibers to concrete will result in a considerable enhancement in its overall functionality, which is well demonstrated by X-ray diffraction (XRD) and Fourier Transformer Infrared (FTIR) analysis. The XRD patterns also identify the existence of significant crystalline phases like calcium silicate hydrate (C–S–H), calcite (CaCO3), and quartz (SiO2), which are the key hydration products that enhance the development of strength in cementitious systems. These phases are determined by the clear and, consequently, enhanced peaks pointing to a more complete hydration process and better densification of the matrix. The fact that there are no other crystalline phases is another confirmation that the nylon fibers do not chemically react with cement matrix but are a reinforcing component in the cement which is stable and inert. In spite of this, their existence leads to a more sophisticated and miniature microstructure by limiting the growth and propagation of microcracks. These findings are also supported by the FTIR analysis which shows that the sample contains major functional groups of OH stretching vibrations, which are the hydration products, SiO the Si bonds, which are the silicate structures, and CH stretching vibrations, which are the polymeric nature of the nylon fibers. Amide group vibrations are observed, which proves the presence of interaction of nylon fibers and products of cement hydration, which means that interfacial bonding is strengthened in the composite structure. Further, the fine changes in the peak positions and alterations in intensity are signs of enhanced structural homogeneity and low connectivity of pores. This enhanced fiber-matrix interaction helps to achieve better distribution of the stress and transfer of loads effectively under the applied loads. After such microstructural adjustments, the nylon fiber reinforced concrete has a higher mechanical strength, such as high tensile strength, increased flexural strength and toughness. The fibers are successful in bridging cracks and slowing down their development, therefore, diminishing brittle failure and enhancing ductility. Also, the decreased porosity and better compactness of the matrix add to the increased durability properties, including reduced water uptake, increased environmental degradation resistance, and greater fire resistance. On the whole, the joint use of XRD and FTIR tests indicates clearly that the incorporation of nylon fibers contributes to the creation of a more dense, cohesive, and structurally sound concrete matrix, and, as a result, enhances the performance of a concrete to a substantial degree both in terms of strength and strength.
5. CONCLUSION
When nylon fibers are added to the reinforced cement concrete, it enhanced the mechanical and durability properties of reinforced concrete much better than plain concrete. Compressive, split tensile and flexural strengths were also improved at 0.25 percent and 0.5 percent fiber contents, with the 0.5 percent mix recording the most improvement. It proves the beneficial role of fibers in tension resistance and crack retardation. This has mostly been enhanced because of the fiber bridging mechanism where nylon fibers regulate microcracks, equally distribute stress and bind together cement paste and aggregates resulting in higher load bearing capacity and toughness of the concrete. Nylon fiber was used in terms of durability as it increased the resistance of the concrete to environmental and thermal stresses. The fiber-reinforced mixes had reduced water absorption and enhanced fire resistance as compared to the control mix, which demonstrates that they have a denser microstructure that can block moisture ingress and chemical assault. The enhanced thermal resistance also tells about the stability of the material in high-temperature conditions that guarantee the long lifespan and higher safety performance of the material. Of the proportion tested, the 0.5 percent dosage of nylon fiber was observed to be the most effective in terms of balance in strength, ductility and durability. Even though workability was slightly reduced, this dosage was still within acceptable construction limits and thus, this dosage was the best in terms of practical use. Engineering wise, nylon fiber reinforced concrete is more flexible and energy absorbing to loading and thus reduces the possibility of abrupt brittle failure. Such augmented mechanical behaviors are especially useful in the impact affected structures or vibration/cyclic loaded structures where the post-cracking load bearing capability of the material contributes to safety and serviceability. Although the promising results are obtained, more studies are required to assess long-term performance in the form of shrinkage, creep, and fatigue behavior. A precise cost-benefit analysis would also prove useful to identify whether the use of the large scale is economically feasible. Also, comparative analysis of other types of synthetic and natural fibers such as polypropylene, basalt, steel, coir, or jute might be useful in future to determine the best type of reinforcement material in different structural and environmental environments.
6. DATA AVAILABILITY
The entire dataset supporting the results of this study was published in the article itself.
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