Effect of fiber synergy on strength & durability of high-performance concrete

The distinct fiber addition to concrete can improve weak performance in tension. Due to its higher performance, fiber hybridization is becoming more and more competitive. In this study, the hardened and durability characteristics of performance-based concrete reinforced with polypropylene, basalt and hooked end steel fibers are investigated. Total 12 mixes are prepared to evaluate the different characteristics of performance-based concrete. The influence of basalt fibers (0.2, 0.25, 0.5, 0.75 & 1%), polypropylene fibers ( 0.025, 0.03, 0.035, 0.05 & 1%), hooked end steel fibers (0.225, 0.475, 0.65, 0.75, 0.80 & 1%) to explore the impact of fiber synergy, various hybrid fibers with maximum volume fraction of 1% were used out of total concrete volume with and without copper slag (20% & 40%) replacement with fine aggregate are investigated on the hardened characteristics including; compressive strength, flexural strength, splitting tensile strength. Moreover, durability properties including; water absorption, electrical Resistivity, for performance-based concrete are also conducted. Out of the 12 mixes considered for this study, PP0.05B0.2S0.75CS40 performs better and exhibit excellent performance in terms of strength and durability.

highest values. Among all the combinations, (Forta ferro-medium density) FF-MD fiber displayed the highest splitting tensile strength, measuring 5 MPa. The fact that residual flexural strength improved dramatically from 36% to 79% is undoubtedly evidence of the contribution of fibers to concrete's post-cracking behavior. The mix design with the lowest Forta Ferro and RheoShore fiber dosages appears to be the most appropriate for the intended application of the four.
In their research, Emad A H Alwesabi et al (2019) shown that hybrid fibers (0.1% PP + 0.9% MS) had greater elastic modulus, CS, STS, and other mechanical properties. The FS of concrete was greatly increased by the mixes containing 1.0% MS. The mechanical characteristics of concrete were negatively impacted by the 1% PP mixes.
According to research by Bilal Masood et al (2019), the workability of recycled aggregate concrete (RAC) is higher than that of the natural aggregate concrete (NAC) because RAC contains more free water. Comparatively speaking, RAC mixes have more air content than NAC mixes did. The fresh density of concrete mixes was decreased by substituting NCA for RCA. Comparing RAC to its NAC mix competitors, the compressive strength of RAC dropped.
PP and BF both attain the best mechanical performance at a content of 0.1%, according to research by Xinyu Hu and Yihong Guo (2019). The workability of concrete is not significantly affected by PP and BF hybridization when compared to single fibers at the same concentration [32][33][34][35][36].
Dehong Wang et al (2019) discovered that the highest synergy effect of hybrid fibres occurred when the volume percentage of basalt fibre was 0.15% and the volume fraction of polypropylene fibre was 0.033%.
Iman Sadrinejad et al (2018) investigated how hybridizing poly-olefin (PO) with PP fibers might increase mixtures' compressive and splitting tensile strengths by up to 7.5% and 23%, respectively, in comparison to control mixtures. These fibers' hybridization did not have a beneficial impact on the post-cracking behavior during the flexural test. large volume fiber content, the detrimental effects of PO and PP fiber hybridization were evident. For actual use, it is advised to use PO and PP fibers at volume fractions of 0.9% and 0.1%, respectively.
In their study, Doo-Yeol Yoo et al (2017) discovered that the flexural performance suffered when short fibres length of fiber to diameter of fiber (l f /d f of 13/0.2) were used in place of long fibres (l f /d f of 30/0.3). Long fibers were used, which significantly enhanced complimentary energy.

Problem statement
Concrete faults become more noticeable as the service life lengthens, which causes numerous issues in engineering applications. As concrete ages, local failure mechanisms that regulate its ability to bridge are reduced bonding capacity, fibre pull-out, and fibre sliding. The linear elastic fracture mechanics form the foundation of the fibre spacing theory. It is believed that the concrete has flaws and microscopic fissures. The crack enlarges as a result of the stress concentration created under the influence of the external force. By using fibre, the crack can be successfully stopped from spreading. This study represents the utilization of hybrid fibres which greatly reduces the effect due to shrinkage in construction practices.

Materials used in mix design
As per ACI Committee 363R -92, the main raw ingredients used to develop the concrete mix design for HSC were cement, fly ash, silica fume, coarse aggregates, fine aggregates, Polypropylene fibers, basalt fibers, hooked end steel fibers, polycarboxylate ether (PCE) based superplasticizer, and water.

Cement
OPC of 53 grade cement has a minimum compressive strength of 53 MPa. The cement met the requirements of IS 8112-2013. Table 2 displays the chemical make-up of the 53-grade cement used in OPC. As shown in table 1, all required cement tests have been completed before casting and compared to OPC 53 grade standards.

Admixtures
A pozzolan substance called fly ash admixture was acquired from Trans Delhi Signature City, Ghaziabad. Fly ash utilized in the mix design was Grade 1 with requirements that complied with IS 3812 codes. According to IS 456-2000, FA is typically used as a partial substitution for cement. According to table 2, this grade of fly ash has been identified as having the following chemical makeup. The widely used mineral additive silica fume, according to IS 5388: 2003, was employed in the mix design to create high-strength concrete. According to the specifications supplied by ASTM C-1240, AASHTO M307, and IS Code, the desired results were obtained for the silica fume admixture as indicated in table 3. An excellent water content reduction, slump retention admixture, and poly carboxylate ether (PCE) chemical admixture were utilized. According to Marsh cone test, the ideal concentration of the superplasticizer (SP) was 1.12%. The parameters were in accordance with IS 9103: 1999, and table 4 lists the superplasticizer's physical characteristics.

Aggregates
A neighboring site in New Delhi, India provided Badarpur sand (fine dust) of zone II, which was used as fine aggregates in accordance with IS 383: 1970. The tests' fine dust was produced to fit through a sieve with a 4.75 mm opening. Based on the results of the sieve analysis test, the fineness modulus of sand utilized in the experiment was determined to be 2.80. The values obtained using the pycnometer approach, which took into account water absorption and the specific gravity of the fine dust, were 1.2% and 2.60, respectively. The fine aggregate (FA) was subjected to sieve analysis to produce the particle size distribution curve depicted in figure 1. For the design mix, coarse aggregates (CA) with a nominal size of 10 mm were employed, and their fineness modulus was once more assessed using a sieve analysis test. It was determined to be 7.2 The results obtained using the wire basket method were 0.2% and 2.8, respectively, for water absorption and the specific gravity of the coarse aggregates.

PP, basalt and hooked end steel fibers
Polypropylene fiber, Basalt Fiber and Hooked End Steel fibers were taken from Dilshad Composites, Ghaziabad and is utilized throughout the research work. Hooked end steel fiber is having 60 mm length and 0.75 mm diameter, polypropylene fiber is having 12 mm length and 0.022 mm diameter, basalt fiber is having 12 mm length and 12 micrometer diameter.

Mix design proportion of concrete
To comprehend the mechanical behavior of high-strength concrete and the viability of hybrid fibers, twelve mix proportion compositions were established. Table 6 displays the numerous mixed design compositions that were created. Basalt fiber, polypropylene fiber, and hooked end steel fiber were used as hybrid fibers. Table 2 focusses on various mix proportion. PP0B0C0CS0 shows the control mix. Mix PP0B0S0CS20 and PP0B0S0CS40 represents 0% Polypropylene, 0% Basalt, 0% Hooked End Steel fibers and Copper Slag of 20% and 40% respectively. Similarly, PP0.025B0.5S0.475CS40 represents 0.025% Polypropylene, 0.5% Basalt, 0.475% Hooked End Steel fibers and Copper Slag of 40%. In a similar manner, all the remaining mix designations were nomenclature.
Initially, CA and FA were introduced to the concrete mixer for batching, and the mixer received automated revolutions for roughly a minute. At this point, mineral additive silica fume, fly ash, and cement were also added. The determined amount of water and SP was added, and the mixture was properly homogenized before being slowly added to the mixer. Finally, the appropriate dose of hybrid fibers was then equally distributed throughout the dry mixer, and the mixer once more received automatic revolutions for another minute. For around three minutes, all the mix components received enough revolutions to produce a satisfactory blend.
The mixing process was now halted, and the prepared concrete mixture was poured into molds with the dimensions 150 * 150 * 150 mm and 500 * 100 * 100 mm, respectively, for the manufacture of cubic and prismatic beam samples for subsequently conducting compressive and flexure tests. Using a shaking table to create a dense mixture for generating a high strength concrete mix, the prepared concrete mix was poured in three layers to achieve enough compaction. The set samples were also demolded after 24 h, and they were then cured in water for 28 days. The samples were ready for testing for ultrasonic pulse velocity tests after 28 days of water curing, prior to flexure and compressive strength tests in accordance with IS 516-1959 and IS 456-2000, these tests were conducted.   strength incorporated hybrid fibers in the specimen. A load of 0.035 to 0.1 mm per minute was applied to the specimen. Under displacement control, a 0.6 mm min −1 deflection rate was performed. This test method and residual loads that take into consideration the L/600 and L/150 deflection were used to calculate the peak load (P) because aggregate can only be 20 mm in size, the size of the beam specimen that was taken is  Where 'L' stands for the beam's length, 'd' for its depth, 'P' for its failure load, and 'f' for its flexural strength.

Split tensile strength
Three cylinders for each type of mix were subjected to STS testing in accordance with ASTM C39 specifications. Every minute, 12 MPa of loading occurs. For testing, 36 cylinders of hybrid fibers in varied proportions were put to each sample. The specimen is 150 mm × 300 mm in size. From 0.7 to 1.4 MPa min −1 of load was applied to the ST test until failure. STS Determined using the formula provided in the Indian standard, in accordance with ASTM C496 standards.
In this formula, L denotes the cylinder's length, D its diameter, P its failure load, and fst its split tensile strength.

Specific electrical resistivity and water absorption
With an AC-Impedance spectroscopic technique, the particular electrical resistivity was determined using a 1.0 kHz frequency and 1.0 Ω-m ultimate capacity. According to BS 1881-72, cubic specimens were subjected to water absorption tests. In water, samples were cured for 27 days at 22 degrees centigrade. After that, the samples were dried for at least 14 days in an oven that was set at 45 degrees Celsius. If the values obtained from two successive measures of mass differed by more than 0.5% of the smaller value, the specimens were put back in the oven for an additional 24 h drying period. The weights of the samples were then measured over a longer period of time until there was a 0.5% variation between any two successive measurements. The cured specimens were then wiped with a dry paper towel, submerged in tank filled with water for half an hour and 168 h, and weighed on a 0.01 g balance. The early and final water absorptions are referred to in the current study as occurring at 0.5 h and 7 days, respectively. The remaining tests were conducted at 7, 28, and 91 days.

Effect of hybrid fibers on compressive strength of concrete
As, it is very clear from figure 3 and table 3 that compressive strength of hybrid fibers of 12 mixes have different strength at 7 days and 28 days of curing. The findings after minute monitoring of tested cubes in CTM reveals that control mix PP0B0S0CS0 has compressive strength of 76.5 MPa. In addition to this on increasing the dosage of hybrid fibers the behavior of tested samples changed and maximum compressive strength is reported for mix PP0.05B0.2S0.75CS40 i.e., 78 MPa which is 1.96% greater than control mix PP0B0S0CS0. After that, for all the mixes strength starts decreasing with a similar trend. The mix PP0.035B0.215S0.75CS40 shows lowest compressive strength out of 12 mixes prepared. Because the failure is in the compressive axis and not due to fracture, the concentration of fibers in compression does not significantly increase. Although compressive strength in terms of capacity to carry loads does not satisfy the criteria, the increased fiber dosage in cubes indicates that the fracture width has decreased. Observed comparable results on varied fiber percentages and stated that steel, polypropylene fiber and basalt fiber influence on compressive strength is quite minimal. This might be caused by an increase in the amount of fiber in the mixture, which creates voids and lowers the Intermediate transition zone (ITZ) between the fibers and other elements. Table 3 also demonstrates that, depending on the replacement amount of steel, polypropylene fiber and basalt fiber PP0.05B0.2S0.75CS40 mix shows maximum strength. The mix of 0.05% PP, 0.2% Basalt fiber and 0.75% steel fibers produced the maximum compressive strength out of all the steel, PP fiber and basalt fiber combinations taken into consideration in this investigation.   supplementary cementitious materials including silica fume and is due to the stronger connection between the wet cement matrix and the aggregate. Due to the conversion of calcium hydroxide, which typically occurs on the surface of aggregate particles, into CSH in the presence of reactive silica, the connection is reinforced. The reason of increased flexural strength incorporating hybrid fibers is the densest inter-particle packing in the concrete matrix and strong bond. Table 4 also demonstrates that, depending on the replacement amount of steel, polypropylene fiber and basalt fiber PP0.05B0.2S0.75CS40 mix shows maximum strength. The mix of 0.05% PP, 0.2% Basalt fiber and 0.75% steel fibers produced the maximum flexural strength at 28 days of curing out of all the steel, PP fiber and basalt fiber combinations taken into consideration in this investigation. As a result, hybrid fiber-reinforced concretes lost some of their flexural strength when PP fibers were 0.15%, basalt fiber was 0.2% and steel fiber was 0.65% and starts decreasing after all the mixes.

Effect of hybrid fibers on Split tensile strength of concrete
As, it is very clear from figure 5 and table 5 that split tensile strength of hybrid fibers of 12 mixes has different strength at 28 days of curing. The findings after minute monitoring of tested beams in CTM reveals that control  i.e., 7 days and significantly reduce concrete's capacity to absorb water than controlled concrete mix PP0B0C0CS0 which is having 1.53% (ultimate) water absorption. As a result, various mixes examined in this research displayed water absorption value low, which suggested 'excellent' concrete quality, in accordance with these classifications. Those with poor water absorption results are a result of the concrete mixtures' low porosity and restricted pore connectivity.

Effect of hybrid fibers on specific electrical resistivity
Given that corrosion in reinforced concrete is significantly influenced by electrical resistivity, it is one of the most crucial characteristics of concrete durability. It has been demonstrated in the past that corrosion of concrete reinforcement is not likely to occur above an electrical resistance of 120 Ω-m, which is the limit for corrosion propagation in internal steel reinforcing bars. The microstructure of concrete becomes denser due to the presence of supplementary cementitious materials and hybrid fibers, and secondary calcium silicate hydrate is created as a result of its pozzolanic reaction (C-S-H). The C-S-H gel, which is a well-known source of concrete strength, increases the volume of solid phases and inhibits the development of capillary pore systems. Concrete resistivity, one of the endurance characteristics of concrete, is improved as a result of these events. Electrolytes in the pores of composite materials and the presence of conductive materials like steel fibers in fiber-reinforced specimens both have a significant impact on the electrical resistance of concrete. The findings show that the electrical resistance of concrete was greatly decreased by addition of hybrid fibers. From  concrete have different effects on how electrically resistive the concrete is. Polypropylene fiber and basalt fiber addition makes mixes more porous and slightly lowers the electrical resistance of concrete. The electrical resistivity of concrete is greatly reduced when all hybrid fibers are added because of the fibers' high conductivity.

Conclusion
The research's findings and analysis can be used to draw the following conclusions: (1) All of the mechanical qualities of concrete are enhanced by the addition of hybrid fibers. The copper slag & silica fume particles serve as a microfiller, densifying the transition zone and strengthening the link between the matrix and the aggregate. Additionally, it has been discovered that silica fume works better in the later phases of curing to increase concrete strength at increasing fiber dosage.
(2) The mechanical properties of all fibers including basalt, PP and hooked end steel fiber are improved as the fiber content is raised. This is because fibers can prevent cracks from spreading, lessen the amount of stress concentrated at the tips of fissures, and slow down the rate at which cracks expand.
(3) The two key elements that contribute to the enhanced performance of PP0.05B0.2S0.75CS40 are the higher tensile strength and elastic modulus of steel fibers with compatible bonding with other two fibers.
(4) The addition of hybrid fibers with supplementry cementitious materials increases the features of the transition zone and has a major impact on the durability properties of concrete. The early and final water absorption were reduced by 25% and 36%, respectively, when fiber hybridization takes place.   (6) Concrete's ability to absorb water is significantly reduced when copper slag at 40% and hybrid fibers are combined. The mixture containing 0.05% PP, 0.2% Basalt fibers and 0.75% steel fibers has been discovered to have the lowest water absorption of all fiber-reinforced concretes.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).