Mechanical properties of slurry-infiltrated fiber concrete (SIFCON) as sustainable material with variable fiber content

For any member, sustainability is very important, which refers to the capacity to endure. Normal concrete has low tensile strength and could not exhibit significant tensile strain margins. Accordingly, it is essential to implement strategies that can enhance the behavior of conventional concrete before using it as structural concrete. Therefore, using Slurry-Infiltrated Fiber Concrete (SIFCON) composites is a current innovation in the field of civil engineering that has a considerable influence. This study presented an experimental study to improve the mechanical efficiency of a SIFCON mix with varying fiber content. To satisfy this objective, several SIFCON mixes, incorporating hook-end steel fiber with three different volume fractions (6, 7, and 8.5%) were prepared. The prepared mixtures performed compressive strength tests at 7 and 28 days, splitting tensile strength tests, and flexural strength tests both in the fresh and hardened states. The results pointed out that the fresh state mortar had proper filling and flow ability properties, satisfying the spread diameter requirement (between 240 and 260 mm) for SIFCON mortar, which is equal to 249 mm. For mechanical performance, the maximum compressive strength increased by 53% after 28 days for the mix’s SF of 8.5%. Furthermore, when compared to the reference mix, splitting, and flexural enhancement increased by 44% and 91%, respectively. On the other hand, the failure shape of SIFCON for all tested mixes has an impact on strengthening, that’s assigned to the high vast area of SF.


Introduction
Concrete is the most famous material used widely in the modern construction of buildings it is cheap, strong, durable, and suitable.At an airport, the SIFCON (Slurry-Infiltrated Fiber Concrete) slab was placed into an excavated area.During the one-year service period, the precast slabs used in airport taxiways to support the loads for the wheeled aircraft showed no cracking due to the excellent impact resistance of SIFCON.As a result, the concrete field has effectively made immense progress with the creation of Fiber Reinforced Concrete (FRC).Fibers were used as building materials for many centuries.Fiber types available were fibers made of steel, glass, synthetic materials, and sample organic and inorganic fibers, whereas steel fibers were the most commonly used in construction projects [1].Steel fibers having straight and hooked ends have been widely employed in structural applications.The incorporation of steel fibers is known to improve a variety of properties of concrete, including flexural strength, tensile splitting strength, durability, stiffness, fatigue resistance, impact resistance, and reduced creep and shrinkage [2]- [4].Steel fibers' (SF) benefits depend on a number of variables, including the type, form, length, cross-section, fiber content, matrix strength, mix design, and concrete mixing [5].The fiber content in FRC ranges from 1-3% by volume, whereas high SF content produces a highperformance material known as a concrete slurry (SIFCON).SIFCON is a reducing building material with high strength and huge ductility.SIFCON was originally founded in 1979, by Prof. Lankard in his Research in his laboratory in Columbus, Ohio, USA, He discovered that the content of steel fibers in a 1232 (2023) 012025 IOP Publishing doi:10.1088/1755-1315/1232/1/012025 2 cement matrix could be Augment substantially [6], [7].The increased fiber content of SIFCON obviously results in a different manufacturing method.SIFCON begins by putting fiber into the forms.Following a rich, flowable mixture, the forms are filled with a slurry made of cement and fine aggregate.SIFCON matrix has a high cementitious concentration but no coarse aggregates [8].
SIFCON has important potential for construction purposes where essential (or unusual) loads are experienced while still in service.Moreover, it exhibits the "Fiber lock" behavioral phenomena, which is considered to be the cause of its exceptional stress-strain properties.Nevertheless, many research studies have been officiated to quantify the mechanical behavior of SIFCON members, including compressive, splitting, flexural strength, and stress-strain tests.For instance, Sashidhar [9] the strength when the steel fibers using low strain strength of 390 MPa as metakaolin and silica fume in the cement mortar.The results discovered that the compressive strength of SIFCON was over 88% of normal concrete.Additionally, it has been proposed that the matrix uses metakaolin and silica fume increased SIFCON's compressive strength by (11 to 14%).Also, the experimental tests Yazıcı et al. [10] the study presented the effect of the difference of steel fiber volume fraction on the mechanical of SIFCON characteristics.The results of the test declared that the increase in the volume of the fiber will increase flexural strength and durability.This exercise had been better when the fiber volume was 10%.Further, Kim and Choi [11] have shown that increasing the workability of cement slurry by adding silica fume to 10% by weight of cement and a superplasticizer to 0.5%.Steel fibers with hooked ends and volume friction varying from 4% to 10% were used.Test results have also found that the compressive strength of SIFCON increased from around 1.59 to 2.68 times that of cement paste, while the improvement in tensile strength was about 2.51 to 8.77 times too high.However, Augment the volume fraction of steel fiber improved the ductility and rigidness of SIFCON.In 2015, Giridhar et al. [12] Achieved the effect of many kinds of steel fiber volume fractions (4, 6, and 8%) on SIFCON mechanical characteristics.Hooked-end steel fibers with two different aspect ratios were used.One of the resulting conclusions was that longer fibers had lower compressive strength than smaller fibers.Ali [8] showed experimental research to study the effect of several influences on SIFCON behavior.These variables include volume fraction of fiber (6, 8.5, and 11%), type of SIFCON mortar (utilizing fly ash and/or silica fume as a cement replacement), and fiber kind (utilizing micro steel fiber, hooked end fiber, and hybrid fiber that vary in aspect ratio and geometry).The results revealed that using microfiber or hybrid fiber showed better mechanical properties compared with hooked specimens.
It can be seen from the abovementioned research studies that very few experimental studies have investigated the mechanical behavior of SICON mixes.Therefore, further experimental research is required in this field.The purpose of this research is to determine the possibility of improving the mechanical performance of SIFCON mixes.For this purpose, several mixes were made using three volume parts (6%, 7%, and 8.5%) of steel fiber and tested in fresh and hardened states.Hooked end aspect ratio steel fiber 60 was used in this study.Slump flow tests were utilized initially, while compressive, splitting tensile and flexural strength were used for the test.

2.1.
Materials Locally usual ordinary Portland cement I which is normally notedness (Krasta) met the requirements of the Iraqi Specification (IQS) [13] and was used in this study.Normal sand with a volume of 0.6 mm, limited gravity of 2.59, sulfate content of 0.13%, and absorption of 2% was utilized to guarantee combined infiltration using steel fiber, which specifies IQS [14] see Table 1.In this study, all mixing and curing of SIFCON specimens were done using ordinary tap water.Master GLENIUM® 54 satisfied ASTM C-494-05 [15] specification and was used as a superplasticizer (SP), the properties of the used SP are shown in Table 2. Hooked-end steel fibers provided by Sika Company were used in this study (see figure 1).The tensile strength, specific gravity, diameter, length, and aspect ratio of the used fibers were 1200 MPa, 7.8, 0.51 mm, 30 mm, and 60, respectively.

2.2.
Mix Proportioning A series of trial slurry mixes were prepared to obtain an appropriate mix with the desired fresh qualities by measuring the viscosity, fluidity, and filling capacity in the fiber network without segregation or pore pockets according to the requirement for SIFCON slurry.Following this, determine the volumetric ratio of steel fibers utilized.One of the cast mixes with normal concrete is used as a reference specimen.Additionally, three SIFCON concrete mixes with identical mix ratios (1:1) (cement: fine aggregate) in terms of weight were also prepared in this experimental study.The water-to-cement ratio was kept constant at 0.29 for all prepared mixes, and the cement content and fine aggregate were 885 kg/m 3 .The addition of steel fiber was done volumetrically at three percent, which is 6, 7, and 8.5 percent.Details of prepared SIFCON mixes are shown inTable 3.

Concrete Casting and Testing
This experimental work focused on fresh and hardened SIFCON mixtures.Slump flow tests in accordance with EFNARC [16] were performed.For each mix of concrete, six 100*200 mm cylinders, and three100*200 mm cylinders were cast to study the material's compressive strength and splitting tensile strengths, respectively, and three prisms 100*100*500 mm for flexural strength.A unique mixing method was employed to produce a homogeneous SIFCON matrix.First, binders, sand, and other components were combined in a big bowl.The remaining dry ingredients have then been combined with 50% of an SP.The final product was blended using high-speed spinning for 10 minutes.The steel fibers were first placed in molds, and then the slurry was added while vibrating (see Figure 2).After casting, the samples were taken out from the molds after 24 hr.and left to cure for 28 days in the laboratory's water curing tank.The compressive strength and division tensile strength were tested based on BS EN 12390-3 [17] and BS EN 12390-6 [18] consecutively, while the flexural strength examinations of the samples were holed out according to ASTM C78 [19].

3.1.
Fresh Properties To satisfy the flow criteria through the thick fiber bed, SIFCON mortar must be sufficiently liquid and fine.Therefore, one of the most essential tests for the creation of SIFCON mixes should be considered the fresh state test.The mini slump flow test can be considered the quickest and simplest test for the fresh state of SIFCON mortar.Additionally, slump flow test according to EFNARC [16], can estimate the filling ability and horizontal flow for SIFCON mortar, besides show uniformity of the slurry and its segregation resistance through the visual observation during the test.This test is conducted using apparatus shown in figure 3. The used small slump flow testing method has dimensions of 100 mm for the base, 70 mm for the top, and 60 mm for the height, correspondingly.A baseplate was used on level, stable ground, and then the slump cone was centered on the base plate.Mortar was poured into the cone, which was then removed to allow the mortar to flow freely.Last but not least, as shown in figure 3, the slump flow in mm (which is the average of diameters for two perpendicular directions of the mortar) was measured.The range of the SIFCON mortar's ideal spread diameter (240-260 mm).In this study, the slump flow for the mortar mixture was 249 mm.  4 and Table 5 illustrate the effect of the main consideration parameters (size fraction of steel fibers) on the compressive strength of the tested mixes at 7 and 28 days.According to the result, the compressive strength of the mixes with fibers (mixes M2, M3, and M4) was in the range of 63 to 94 MPa at 7 days and ranged from 74 to 114 MPa at 28 days, while it was from 18 and 32 MPa for a mix without steel fibers (mix M1) at 7 and 28 days, respectively.When SF is used at 7%, compressive force increases by about 22% at 7 days and 21% at 28 days compared with that of the reference mix (mix with 6% SF).However, when 8.5% SF was used, compressive strength enhanced only 49% and 53% at 7 and 28 days, respectively, in comparison with the control mix, which means there was an enhancement (22% and 27% at 7 and 28 days) in the compressive strength when compared with 7% SF content was used.This improvement in compressive strength is attributed to a stronger bond between the fiber and matrix interfaces created by rising the fiber size fraction up to 8.5%, as well as the influence of steel fiber in bridging the growth of microcracks, resulting in higher composite strength.All composites have a significantly higher compressive strength.when random fiber arrangement is used.Due to the fact that the uniaxial compressive test was conducted with oriented fiber placement and two independent loading typesloading parallel to the fibers and loading perpendicular to the fibers-were employed.
On the other hand, table 5 illustrates the failure mechanisms of tested cylinders for all mixes.It is obvious a large superficial area of SF fills up the gaps in the cement grains, which causes it to have an impact on increasing the SIFCON mixes' compressive strength.According to this phenomenon, there is no limited effect of the opposite side's long direction whereas fibers constrain the lateral deformation in one direction.The mixed kind of failure (separation and shear) was noticed when random fibers were used, and as a result, the random fibers enhanced the compressive strength of SIFCON specimens.4. For the mixes with fibers (M2, M3, and M4), splitting strength ranged from 12 to 17 MPa.Figure 4 illustrates the impact of the volumetric ratio of steel fibers on the tensile strength of SIFCON.The Practical values proved that SIFCON has superior tensile properties compared with normal concrete.Compared with the reference mix (mix M2), the splitting strength of the mix incorporating 7% SF increased by about 21%, while mix 8.5% SF increased by 44%.However, splitting strength dropped only 74% for normal concrete (mix M1) in comparison with the control mix.This enhancement could be attributed to the fact that binding fiber is naturally available in SIFCON where the mechanism of bridging of fiber led to control of the micro-cracks.In another hand, the using of a hooked end type of steel fiber produces increasing in the bond between mortar and fiber, hence an enhancement in SIFCON mechanical properties [8].As mentioned before, this remarkable increase in splitting strength can be associated with an increased aspect ratio of hooked SF which leads to an increase in tensile strength.
In this study, the other conclusion adopted in Table 5 found the ability of the added SF can be controlled to micro-cracks by preventing and bridging the fiber's mechanism.Additionally, employing Hook end steel fiber develops the bond between the fiber and matrix, which enhances the mechanical characteristics of SIFCON.

Flexural strength.
The flexural test results of tested mixes were presented in Figure 5.The differences in the mix densities are associated with the differences in steel fiber volume fractions.It is evident that mixes with fibers (M3 and M4) had greater flexural values than reference mixes (mix M2), increasing by 74% and 91% respectively.Among all the investigated fiber mixes, the improvement in flexural strength is due to increasing the percentage of steel fibers.This behavior is consistent with results from previous research [20] which observed that for a similar aspect ratio, higher fiber percentages resulted in higher ultimate flexural strength.The mortar matrix serves as their struts, and the fibers work as connections, creating a weblike structural structure through the matrix that resembles a space truss.The internal tensile forces in the mortar matrix are distributed and transferred to the fibers embedded in the mortar; however, the mortar matrix carries compression in addition to carrying larger internal tensile forces when there are more fibers implanted in it.When the fiber content exceeds a certain limit, these phenomena become more violent, reducing the ability for slurry to penetrate the web-like structure.Consequently, an air void will be created, and the bond between the matrix and the fibers could deteriorate.On the other hand, table 5 indicates that the failure mechanisms effect by the volume fraction of SF.Therefore, this may be explained by the sturdier interface zone between the binder and the fibers, improving the bond's strength and preventing the development of microcracks that can cause flexural failure.Generally, SF is a highly reactive pozzolana material that reacts with calcium hydroxide to form calcium silicate hydrate, a binder material that is identical to what is produced when Portland cement is hydrated.And as a result, the matrix phase improves, which also increases the binding strength between the fibers and the matrix and flexural strength [21].

Conclusions
This study presented a research instrument on fresh and mechanical properties of SIFCON mixes prepared with variable fiber content.The test results pointed out that: • The produced SIFCON slurry was found to have proper filling and flow ability properties, satisfying the spread diameter requirement (between 240 and 260 mm) for SIFCON mortar, which is equal to 249 mm.This consistency allows for effective slurry penetration into the dense fiber network without blocking, honeycombing, or bleeding.• Hardened properties demonstrated that the compressive force of SIFCON mixes incorporating 7% and 8.5% fiber increased by about 21% and 53% at 28 days compared with the control mix (mix with 6% fiber).• Hardened performance of concrete mixes made with 7% and 8.5% fiber showed an enhancement of about 21% and 44% in splitting strength, compared with that of reference mix.• It was also revealed from this work that the differences in the mix flexural strength are dominated by fiber content.Among all the prepared mixes, they increased by 74% and 91% for 7% and 8.5% mixes compared with the reference mix.• The failure shape of SIFCON for all steel fiber ratios have an impact on strengthening of tested SIFCON mixtures, which attributed to SF's huge surface area fills in the spaces between the cement grains.

Figure 4 .
Figure 4. Splitting strength of the tested concrete mixes.

Figure 5 .
Figure 5. Flexural strength of the tested mixes.

Table 1 .
Sieve analysis result of the fine aggregate.

Table 3 .
Constituent materials of the prepared concrete mixes, kg/m 3 .

Table 5 .
Modes failure of SIFCON tested mixes.