Mechanical Properties and Durability of Latex-Modified Fiber-Reinforced Concrete : A Tunnel Liner Application

.is study assessed the mechanical properties and durability of latex-modified fiber-reinforced segment concrete (polyolefinbased macrosynthetic fibers and hybrid fiber-macrosynthetic fiber and polypropylene fiber) for a tunnel liner application. .e tested macrosynthetic fiber-reinforced concrete has a better strength than steel fiber-reinforced concrete..e tested concrete with blast furnace slag has a higher chloride ion penetration resistance (less permeable), but its compressive and flexural strengths can be reduced with blast furnace slag content increase. Also, the hybrid fiber-reinforced concrete has higher compressive strength, flexural strength, chloride ion water permeability resistance, impact resistance, and abrasion resistance than the macrosynthetic fiber-reinforced concrete..emodified fiber improved the performance of concrete, and the hybrid fiber was found to control the formation of microandmacrocracks more effectively..erefore, overall performance of the hybrid fiber-reinforced concrete was found superior to the other fiber-reinforced concrete mixes tested for this study..e test results also indicated that macrosynthetic fiber could replace the steel fiber as a concrete reinforcement.


Introduction
Utility tunnels in urban areas are generally constructed using drill and blast and mechanized tunneling methods.Although tunnel excavation using the drill and blast method is economically feasible, environmental restrictions and ground conditions must be carefully considered for a tunnel construction method selection [1].Noise, vibration, impact on the existing infrastructures, and environmental issues are typical concerns for the tunnel construction in urban spaces [1,2].For these reasons, a mechanized tunneling (e.g., shield TBM) is more favorable for an urban tunneling (e.g., subways, electric power, communications, and water tunnels) [2,3].Shield TBM tunnels typically use steel and concrete segmental liner for tunnel supports.Steel segments were initially used exclusively [4].However, as concrete performance improved, reinforced concrete (RC) segmental liners have been increasingly used due to their enhanced corrosion resistance and cost-effectiveness compared to steel liners [4].In urban areas, tunnels are usually located within the public road right of way (ROW).ey tend to follow the road's alignment, and relatively sharp curves are also required to minimize tunnel intrusions into private properties [1].For this case, edges of a reinforced concrete (RC) segmental liner can be damaged by the TBM thrust force and load eccentricity.erefore, considerations have been given to the steel segmental liners to mitigate this type of damage.However, steel segmental liners are known to be corrosive when they are exposed to an aggressive environment (Figure 1).
Recent studies have explored alternatives to minimize the steel reinforcement of RC segmental liner using steel fiber reinforcement [4,5].Steel fiber-reinforced concrete segmental liners have a better performance than typical RC segmental liners.However, durability of the steel berreinforced concrete segmental liner was questioned due to the internal corrosion of the steel reinforcement (Figure 2) [6].According to the study done by Abbas, RC and steel ber-reinforced concrete (SFRC) segments can be corroded when they are exposed to a chloride ion environment [6].ere are two mitigation measures to the corrosion issue of steel rebar and steel ber reinforcement: (1) improve the water permeability resistance by compacting the concrete and/or (2) use noncorrosive reinforcing materials.is study evaluated the utilization of noncorrosive polyole nbased macrosynthetic ber and hybrid ber as a replacement of steel ber and steel reinforcement commonly used in concrete segmental tunnel liners.Latex and blast furnace slag (an industrial by-product) were also used to improve the water permeability resistance.Many studies have discussed the use of polyole n-based macrosynthetic ber in various applications such as marine structures, shotcrete, and concrete tunnel liners due to its corrosion resistance and higher impact/static strength than steel ber.ese properties translate to better corrosion resistance and higher rebound stability than steel ber in shotcrete [7][8][9][10].However, the uidity decreases as volume fraction increases due to poorer ber dispersibility in ber-reinforced concrete, which leads to poorer segmental concrete liner performance [11,12].
is study used latex to increase the uidity and water permeability resistance.When latex is added to the concrete mix, it improves the water permeability resistance due to the formation of a latex lm, ber dispersibility, and uidity of ber-reinforced concrete [13,14].Also, the waterproofness of concrete containing ne particles of blast furnace slag improves as the generated calcium-silicate-hydrate (C-S-H) gel improves the pore system and subsequently the sulfate and chloride ion penetration resistance [15][16][17].To evaluate the feasibility of replacing commonly used steel ber-reinforced concrete with latex-modi ed ber-reinforced concrete for a tunnel liner application, this study tested the compressive strength, exural strength, and chloride ion penetration as a function of volume fraction with ne particles of blast furnace slag, steel ber, and macrosynthetic ber.A mix suitable for a latexmodi ed segment concrete was used, to which macrosynthetic ber and a hybrid ber, consisting of a combination of macrosynthetic ber + polypropylene ber, were added.e reinforced concrete was then tested for compressive strength, exural strength, chloride ion penetration, impact strength, and abrasion resistance.Previous research  Advances in Materials Science and Engineering indicated that the hybrid fiber is effective at macro-and microinternal crack formation prevention in segmental concrete liners.

Materials.
is study used ASTM Type I cement, and its properties are given in Table 1.Fine particles of blast furnace slag were added to improve the durability and waterproofness of concrete.
e blast furnace slag composition is listed in Table 2. Crushed aggregate having a maximum size of 25 mm was used as the coarse aggregate, and Table 3 gives the physical properties of the coarse aggregate.e fine aggregate had a density of 2.62 g/mm 3 .For the reinforcing fiber, bundled end-hook-type steel fiber (30 mm (length) × 0.5 mm (diameter)) and polyolefinbased macrosynthetic fiber (30 mm (length) × 1 mm (diameter)) were used; Table 4 and Figure 3 provide details.
e polyolefin-based macrosynthetic fiber (macrosynthetic fiber) is a monofilament fiber with polypropylene and polyethylene combined (Nycon Materials Co., Seoul, Republic of Korea).In this study, polypropylene fiber was applied.e polypropylene microfibers (polypropylene fiber) consisted of 100% percent virgin homopolymer polypropylene multifilament fibers containing no reprocessed olefin materials.
e report published by the 548 Technical Committee of the American Concrete Institute (ACI) indicated that pores ranging from 10 to 1000 nm in diameter damage concrete in the long term through a capillary effect [18].Polymers with a particle size of 100 nm effectively fill these internal pores.In this study, it was difficult to ensure the pore filling during the mixing or sample fabrication due to low initial fluidity caused by the fiber reinforcement.Styrene-butadiene (SB) latex was used to improve durability and initial fluidity [18].Table 5 summarizes the SB latex properties.

Mix Proportions.
is study evaluated the performance of latex-modified fiber-reinforced concrete as a function of type and volume fraction of reinforcing fiber and the use of fine particles of blast furnace slag.e mechanical properties and durability of the concrete were measured for the mixes containing steel fiber and macrosynthetic fiber with fiber volume fractions at 0.0, 0.25, 0.5, and 1.0% and with fine particles of blast furnace slag substituted at 30%.Latex was included at 10% of the binder weight.
e design compressive strengths of the concrete mixes were 40 and 60 MPa at a material age of 28 days.Tables 6 and 7 give the mix designs.

Test Methods
e compressive strength, flexural strength, and chloride ion penetration resistance of the latex-modified fiberreinforced segment concrete were measured using standard test methods.Impact resistance and abrasion resistance tests were also carried out to measure the durability of the mixes.

Compressive Strength.
Compressive strength was determined in accordance with ASTM C 39 "Testing Methods for Compressive Strength of Concrete Standard" [19].Specimens of dimensions 100 mm (diameter) × 200 mm (length) were made.ree specimens were tested at the material age of 28 days.e specimen was located at the center of the platen in the universal testing machine and loaded at constant speed until failure.e maximum load for each specimen was divided by its cross-sectional area, and the average value was reported as the compressive strength.Specimens were cured for 1 day at 23 ± 2 °C and a relative humidity of 50%, followed by water curing in a constant temperature bath at 23 ± 2 °C.Testing was done in duplicate.

Flexural Strength.
Flexural strength was evaluated using the ASTM C 496 "Testing Methods for Flexural Strength of Concrete Standard" [20].Concrete was placed in a rectangular mold (100 mm × 100 mm × 400 mm) to form three specimens for this test at the material age of 28 days.After initial curing, the specimens were immersed in a constant temperature water bath at 23 ± 2 °C.Testing was done in duplicate.

Chloride Ion Penetration.
Permeability is the most important contributor to the strength and durability of concrete.Increased permeability lowers the strength via Advances in Materials Science and Engineering crack expansion and reduces the durability through freezing, thawing, and abrasion.In this study, a permeability test was conducted in accordance with standard ASTM C 1202-94 [21].Two cylindrical specimens (100 mm (diameter) × 200 mm (length)) were prepared for each composition and tested at the material age of 28 days.e centers of the specimens were lowered to a thickness of 50 mm and then placed in a desiccator.Vacuum was applied for 3 hours to remove the trapped air, and then water was introduced into the desiccator to saturate the specimens with water.e vacuum was reapplied for 1 hour.
e vacuum pump was then turned off, and the specimens were maintained fully immersed in water for 18 ± 1 hours.Following this process, a specimen was fixed in an applied voltage cell, where the positive pole was filled with 0.3 N NaOH solution, and the negative pole was filled with 3% NaOH solution.A potential of 60 V was applied, and the current was measured over a 6-hour period.e test was repeated twice with three specimens having the material age of 28 days.Figure 4 shows the chloride ion penetration resistance setup.Table 8 (from the ASTM standard) ranks the chloride ion permeability level as a function of the charge passed.

Impact Resistance.
e ACI Committee 544 Test Method was used to evaluate impact resistance.According to this method, a rigid body of dimensions 150 mm (diameter) × 60 mm (length) is freely dropped [22].ree specimens were produced for each material age, cured for 1 day in a curing room (23 ± 2 °C, 50% relative humidity) and then cured in a water bath at a constant temperature of 23 ± 2 °C.
e test was repeated twice for three specimens at the material age of 28 days.Figure 5 shows the device used for the impact resistance test.

Abrasion Resistance.
Four cylindrical specimens were made with dimensions 150 mm (diameter) × 300 mm (length) in accordance with the ASTM C 944 standard [23].e specimens were cured in a curing room at 23 ± 2 °C and a relative humidity of 50% for 1 day, demolded, and then cured in water for 28 days in a constant temperature bath at 23 ± 2 °C.e abrasion test was performed in duplicate.e test setup is shown in Figure 6.

Results and Discussion
4.1.Compressive Strength.Figure 7 shows the compressive strength results for the mix formulations containing various amounts of fine particles of blast furnace slag.e blast furnace slag content increase in the mixes lowered the compressive strength of concrete, and similar results were found regardless of the fiber type or volume fraction for all design strengths of 40 and 60 MPa.
e fine particles of blast furnace slag behaved as a quasi-pozzolanic material.
e design strength was achieved for all mixes.Figures 8 and 9 show the compressive strength results as a function of reinforcing fiber type.is study evaluated the  4 Advances in Materials Science and Engineering Advances in Materials Science and Engineering possible use of macrosynthetic fiber to replace the settle fiber as a mitigation measure to the steel fiber corrosion in the reinforced concrete structures.Concrete mixes containing steel fiber and macrosynthetic fiber showed similar compressive strength, regardless of volume fraction.e fiber type did not appear to affect the compressive strength, which indicated that the macrosynthetic fiber can replace the steel fiber.e compressive strength increased as fiber volume fraction increased.Generally, concrete fluidity decreases with increasing fiber volume fraction due to poorer fiber dispersibility.is leads to a decrease in compressive strength.However, latex was added to the mixes to ensure fluidity, and the fiber was tempered for 2.5 minutes after the injection to increase its dispersibility.No compressive strength decrease was observed for the latex-added concrete mixes.

Flexural Strength.
e flexural strength of concrete under a load is more critical than its compressive strength for a tunnel liner application.Figures 10-12 show the flexural strength results of the concrete with blast furnace slag.e blast furnace slag content increase in the mixes lowered the flexural strength of concrete; this was observed for the design strengths of 40 and 60 MPa, regardless of the fiber type or volume fraction.As a quasi-pozzolanic material, the fine particles delayed strength development.Consequently, flexural strength decreased.
e flexural strength increased with increasing fiber volume fraction.e effect of macrosynthetic fiber was slightly greater than that of steel fiber: the flexural strength was higher at a given volume fraction because the lower density of the macrosynthetic fiber indicated a higher number of fibers per unit volume.e flexural strength results showed that macrosynthetic fiber can replace steel fiber.e water permeability resistance increased with decreasing chloride ion penetration in the mixes containing macrosynthetic fiber, while the chloride ion penetration resistance improved with increasing fiber volume fraction.
e high strength (60 MPa) concrete mix containing the macrosynthetic fiber had the highest resistance, while 0.5    6 Advances in Materials Science and Engineering and 1.0% volume fraction of the macrosynthetic ber with ne particles of blast furnace slag resulted in a "very low" rating, according to the ASTM standard 1202.All other mixes scored "low" for the water permeability with an equal value throughout its range.ose 40 MPa mixes containing macrosynthetic ber all had "low" ratings, while the mixes with added steel ber scored the "low" rating only at the 1.0% substitution level.All the other mixes had "moderate" water permeability.Additionally, both steel ber and macrosynthetic ber mixes that also included ne particles of blast furnace slag had "low" water permeability.Macrosynthetic ber performs better than steel ber to enhance the water permeability resistance; other mixes that included ne particles of blast furnace slag also showed higher water permeability resistance.

Determination of the Optimum Mix
Proportions.e macrosynthetic ber, steel ber, and ne particles of blast furnace slag were used as variables to identify the optimum concrete mix design.
e ne particles of blast furnace slag improved the chloride ion penetration resistance but reduced the initial compressive and exural strengths.Nevertheless, all of the mixes met the design   Advances in Materials Science and Engineering strengths of 40 and 60 MPa.Hence, ne particles of blast furnace slag were added to all of the mixes.A steel ber was not used for the optimum mix design determination due to its known corrosion issues.e properties of mixes containing 0.1% volume ratio of polypropylene ber-added mix were measured, and this ber addition prevented the formation of micro-and macrocracks.e mix proportions of the various tests were summarized in Table 9.

Evaluation of the Optimum Mix.
is study compared and evaluated the mechanical properties and durability of the control mix (no reinforcing ber) with the other mixes containing macrosynthetic ber or hybrid ber.
Figure 16 shows the test results for the compressive strength of the nal optimum mix proportions.e mixes containing the hybrid ber showed the highest strength, followed sequentially by those containing the macrosynthetic ber and control ber; the trends were the same for the 40 and 60 MPa cases.Generally, mixing with reinforcing ber a ects the exural strength to a greater degree than the compressive strength.It was also noted that the compressive strength did not increase signi cantly.All of the mixes achieved the 40 and 60 MPa design strengths.
Figure 17 shows the exural strength results for the optimum mix.e mix containing the hybrid ber had the  Advances in Materials Science and Engineering highest exural strength, followed by the macrosynthetic ber and control mixes. is was because the hybrid ber mix used a blend of macrosynthetic and polypropylene bers to e ectively control micro-and macrocrack formation in the concrete.e macrosynthetic ber mix also had a higher exural strength than the control mix and also controlled crack formation more e ectively than the control mix.
Figure 18 shows the chlorine ion penetration results for the nal mix composition.e mix containing the hybrid ber had the lowest penetration, followed by that containing the macrosynthetic ber and the control.For the designed 40 MPa mixes, the control mix with its value greater than 2000 C indicated moderate permeability, while the macrosynthetic ber and hybrid ber mixes had low permeability and very low permeability, respectively.For the designed 60 MPa mixes, the control mix had a low water permeability (1500 C or higher) while the addition of macrosynthetic or hybrid bers provided very low water permeability.e permeability resistance of latex-modi ed segment concrete was increased by adding reinforcing bers.Also, mixes with hybrid ber had a better permeability resistance than the mix with the macrosynthetic ber, because the presence of polypropylene ber controls the crack formation.

Advances in Materials Science and Engineering
Figure 19 shows the impact resistance results of the optimum mix.
e improvement followed the order of control mix < macrosynthetic ber mix < hybrid ber mix, because adding a reinforcing ber helped to absorb the impact.A ber reinforcement of concrete improves exural toughness and energy absorption capacity through complex steps of pullout, debonding, bridging, and fracture and consequently, increases impact resistance.erefore, mixes with macrosynthetic ber had better impact resistance than those lacking reinforcing ber.Moreover, using the hybrid blend of macrosynthetic ber with polypropylene ber suppressed the formation of micro-and macrocracks, thereby increasing the energy absorbing capacity and impact resistance.Advances in Materials Science and Engineering design case.
e higher strength was accompanied by increased brittleness, leading to macrocracks and ultimately failure.However, stronger specimens require higher numbers of impacts until failure; the crack width and number of impacts were accordingly higher for the 60 MPa design mixes in this study.

Advances in Materials Science and Engineering
Figure 21 shows the abrasion resistance results for the optimum mix.e resistance improved in the order of control mix < macrosynthetic ber mix < hybrid mix.e hybrid mix comprising a blend of macrosynthetic ber and polypropylene ber increased the abrasion resistance through a crosslinking e ect as the ber resisted separation of concrete powder and pieces during the abrasion process.Notably, for the hybrid mix, macrocracks created on the normal concrete surface by abrasion were controlled by the macrosynthetic ber, while microcracks were mitigated by the polypropylene ber, providing an overall improvement in abrasion resistance.Figure 22 shows the specimens after the abrasion test.Surface abrasion was less for the mixes containing macrosynthetic and hybrid bers than the control mix.
As a result of evaluating the mechanical performance and durability of the optimum mix according to the type of ber, the hybrid ber application showed the best performance.Generally, hybrid ber is e ective to suppress microcrack and macrocrack simultaneously in concrete [12].A micro ber of small length and diameter inhibits the generation and growth of microcracks.Macro ber with a large length and diameter inhibits the initiation and growth of microcracks.erefore, the addition of hybrid bers to concrete improves the mechanical performance and durability of concrete by simultaneously controlling microcracks and macrocracks.In this study, polypropylene bers were used for bers of small length and diameter (micro ber), and macrosyntheticbers were applied for bers of long length and diameter    Advances in Materials Science and Engineering (macro ber).erefore, mechanical properties (compressive strength and bending strength), permeability, impact resistance, and abrasion resistance were improved compared to concrete without macrosynthetic ber and concrete without ber.at is, polypropylene ber was added to concrete to suppress the generation and growth of microcracks, and macrosynthetic ber to suppress macrocracks, thereby improving concrete performance.

Conclusions
is study evaluated the mechanical properties and durability of latex-modi ed ber-reinforced concrete as an alternative to conventional steel ber-reinforced concrete.Macrosynthetic, steel, and hybrid (macrosynthetic + polypropylene) bers were used as reinforcing bers.e results of the study are summarized as follows:  Advances in Materials Science and Engineering penetration decreased with increasing fiber volume fraction.Macrosynthetic fiber provided a better water permeability resistance than did steel fiber.(4) e compressive strength, flexural strength, and chloride ion penetration resistance results were used to obtain the optimum mix composition.e optimum mix contained fine particles of blast furnace slag and macrosynthetic fiber.(5) e mechanical durability was assessed for the optimum mix by varying the reinforcing fiber type, that is, no reinforcing fiber (control), macrosynthetic fiber, and hybrid fiber (macrosynthetic fiber + polypropylene fiber).e compressive strength, flexural strength, chloride ion water permeability test, impact resistance, and abrasion resistance are followed in the order of the control mix < macrosynthetic fiber mix < hybrid fiber mix.(6) e addition of hybrid fiber effectively controlled the formation of macro-and microcracks.(7) Macrosynthetic fiber is also a feasible replacement for steel fiber in the latex-modified concrete.

4. 3 .
Chloride Ion Penetration.show the chloride ion penetration resistance results of the concrete containing reinforcing fiber and fine particles of blast furnace slag.e penetration decreased in those mixes containing fine particles of blast furnace slag because the fine particles filled the micropores of the concrete matrix.

Table 1 :
Properties of cement.

Table 2 :
Chemical compositions of blast furnace slag.

Table 3 :
Physical properties of coarse aggregate.

Table 4 :
Properties of fibers.

Table 5 :
Properties of SB latex.

Table 8 :
Standard of permeability levels by ASTM.

Table 9 :
Mixing ratio for optimum mix proportions determination.