This study evaluated the performance of latex-modified fiber-reinforced concrete (RC) segments as a function of the substitution level of microsilica and type of reinforced fiber, to address the problem of corrosion of steel segments and steel-reinforced fiber segments, which are commonly used to shield tunnel-boring machine (TBM) tunnels in urban spaces. Our study compared macro synthetic, steel, and hybrid (macro synthetic fiber + polypropylene fiber) reinforcing fibers. The substitution levels of microsilica used were 0, 2, 4, and 6%. The target strengths were set at 40 and 60 MPa to test compressive strength, flexural strength, chloride ion penetration resistance, and impact resistance. Testing of latex-modified and fiber-reinforced segment concrete showed that the compressive strength, flexural strength, and chloride ion penetration resistance increased with an increasing substitution level of microsilica. These improvements were attributed to the densification of the concrete due to filling micropores with microsilica. Micro synthetic fiber was more effective in terms of improved compressive strength, flexural strength, and chloride ion penetration resistance than steel fiber. These results were due to the higher number of micro synthetic fibers per unit volume compared with steel fiber, which reduced the void volume and suppressed the development of internal cracks. The optimal microsilica content and fiber volume fraction of micro synthetic fiber were 6% and 1%, respectively. To evaluate the effects of the selected mixtures and hybrid fibers simultaneously, other mixing variables were fixed and a hybrid fiber mixture (combination of macro synthetic fibers and polypropylene fibers) was used. The hybrid fiber mixture produced better compressive strength, flexural strength, chloride ion penetration resistance, and impact resistance than the micro synthetic fibers.
As underground tunnel construction in urban spaces usually causes severe environmental problems, complaints because of noise and vibration, and infrastructural effects, the shielded tunnel-boring machine (TBM) construction method is generally used for urban tunnel cutting [
However, segments constructed using steel fiber may have poor durability due to internal corrosion of the steel fibers, in addition to a decreased life span resulting from corrosion of the steel fibers and steel bars in steel-reinforced fiber segments [
There are also studies on the application of high-strength concrete in segment concrete. Using high-strength concrete can enable a reduction in the thickness of the segment and improves water permeability resistance and other properties [
This study evaluated the performance of microsilica, which can serve as a highly strengthening, noncorrosive polyolefin macro synthetic fiber substitute for the steel fibers and steel-reinforcing bars traditionally used in segment concrete. Compared with steel fiber, polyolefin macro synthetic fibers have better impact/static strength ratio and cannot corrode. These properties have encouraged its use in various structures such as marine structures, shotcrete, and concrete linings [
Previous studies have focused on the application of latex-modified reinforced segment concrete with blends of polyolefin and polypropylene fibers [
The cement used in this study was ASTM Type 1; its properties are listed in Table
Properties of cement [
Type of cement | Fineness (cm2/g) | Specific gravity | Stability (%) | Setting time | Compressive strength (MPa) | |||
---|---|---|---|---|---|---|---|---|
Initial (min) | Final (min) | 3 days | 7 days | 28 days | ||||
Portland | 3,200 | 3.15 | 0.02 | 220 | 400 | 20.3 | 30.2 | 38.7 |
Properties of microsilica.
Chemical composition (%) | ||||
---|---|---|---|---|
SiO2 | Al2O3 | Fe2O3 | CaO | Others |
90–98 | 0.4–0.9 | 1-2 | 0.2–0.7 | 2-3 |
Physical properties of coarse aggregate [
Type of aggregate | Specific gravity | Absorption (%) | FM | ||
---|---|---|---|---|---|
Bulk | Bulk (SSD) | Apparent | |||
Crushed coarse aggregate | 2.80 | 2.65 | 2.83 | 0.35 | 6.92 |
Properties of fibers [
Type of fiber | Elastic modulus (GPa) | Density (g/mm3) | Length (mm) | Diameter (mm) | Tensile strength (MPa) | Aspect ratio (L/D) | |
---|---|---|---|---|---|---|---|
Steel | Bundrex type | 200 | 7.8 | 30 | 0.5 | 1100 | 60 |
Macro synthetic | Crimped type | 10 | 0.91 | 30 | 1 | 550 | 30 |
Geometry of the reinforcing fibers [
A technical report published by the Technical Committee 548 of the American Concrete Institute (ACI) indicated that pores ranging from 10 to 1,000 nm in diameter damage concrete over the long term through a capillary tube effect. Polymers with a particle size of 100 nm can effectively fill these internal concrete pores. In this study, it was difficult to achieve sufficient mixing and compaction during mixing and segment fabrication because of low initial fluidity, in turn due to the fiber reinforcement method applied. This study used styrene-butadiene (SB) latex to improve durability and the initial fluidity. Table
Properties of SB latex [
Solids content (%) | Styrene content (%) | Butadiene content (%) | pH | Density (g/mm3) | Surface tension (dyne/cm) | Particle size (Å) | Viscosity (cps) |
---|---|---|---|---|---|---|---|
49 | 34 ± 1.5 | 66 ± 1.5 | 11.0 | 1.02 | 30.57 | 1,700 | 42 |
Table
The study evaluated the performance of latex-modified fiber-reinforced segment concrete as a function of microsilica addition. Macro synthetic fibers and steel fibers were mixed at the volume ratio of 1.0%, and 0.0, 2.0, 4.0, and 6.0% of the cement weight was substituted by microsilica to evaluate changes in the mechanical properties and chloride ion penetration resistance of the segment. The performance evaluation results are listed in Table
Mix proportions.
Design strength (MPa) | Unit weight (kg/m3) | |||||||
---|---|---|---|---|---|---|---|---|
W | C | S | G | Microsilica | Latex | Steel fiber | Macro synthetic fiber | |
40 | 150 | 500 | 1184 | 466 | 0 | 50 | 80 | — |
490 | 10.0 | |||||||
480 | 20.0 | |||||||
470 | 30.0 | |||||||
150 | 500 | 1184 | 466 | 0.0 | 50 | — | 9 | |
490 | 10.0 | |||||||
480 | 20.0 | |||||||
470 | 30.0 | |||||||
|
||||||||
60 | 143 | 625 | 1087 | 467 | 0.0 | 62 | 80 | — |
612.5 | 12.5 | |||||||
600 | 25.0 | |||||||
587.5 | 37.5 | |||||||
143 | 625 | 1087 | 467 | 0.0 | 62 | — | 9 | |
612.5 | 12.5 | |||||||
600 | 25.0 | |||||||
587.5 | 37.5 |
This study evaluated the compressive strength, flexural strength, and chloride ion penetration resistance of latex-modified fiber-reinforced segment concrete as a function of the substitution level of microsilica and identified the mixture having the optimal performance. Polypropylene fiber was added to the mixtures at the unit volume of 0.1% to test for impact resistance, compressive strength, flexural strength, and chloride ion penetration resistance.
Compressive strength was determined using the ASTM C 39 standard “Testing Methods for Compressive Strength of Concrete.” Specimens of dimensions 100 mm (diameter) × 200 mm (length) were prepared, and the test was repeated twice for three specimens, which were tested at a material age of 28 days. Each specimen was cured for 1 day in a curing room at 23 ± 2°C and a relative humidity of 50%, followed by water curing at a constant temperature of 23 ± 2°C [
Flexural strength was determined using the ASTM C 496 standard “Testing Methods for Flexural Strength of Concrete.” Concrete was placed directly in a rectangular mold (100 × 100 × 400 mm) to fabricate three specimens, which were tested at a 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 [
Water permeability is the most important property affecting the strength and durability of a segment concrete [
Standard of permeability levels by ASTM.
Charge passed (coulombs) | Chloride permeability |
---|---|
>4,000 | High |
2,000∼4,000 | Moderate |
1,000∼2,000 | Low |
100∼1,000 | Very low |
<100 | Negligible |
Mixing ratio for optimal mix proportions determination.
Design strength (MPa) | Type of mix | Unit weight (kg/m3) | |||||||
---|---|---|---|---|---|---|---|---|---|
W | C | Microsilica | G | S | Latex | Macro synthetic fiber | Polypropylene fiber | ||
40 | Control | 150 | 470 | 30 | 1184 | 466 | 50 | — | — |
Macro synthetic fiber | 9 | — | |||||||
Hybrid fiber | 9 | 0.9 | |||||||
|
|||||||||
60 | Control | 143 | 587.5 | 37.5 | 1087 | 467 | 62 | — | — |
Macro synthetic fiber | 9 | — | |||||||
Hybrid fiber | 9 | 0.9 |
Impact resistance was determined according to the test method of the ACI Committee 544, whereby a rigid body of dimensions 150 mm (diameter) × 60 mm (length) is freely dropped [
Figure
Compressive strength by microsilica content. Target strength of (a) 40 MPa and (b) 60 MPa.
Figure
Flexural strength by microsilica content. Target strength of (a) 40 MPa and (b) 60 MPa.
Generally, concrete is brittle, and has low tensile strength, energy absorption, and crack resistance [
This compared the results of previous studies using steel fiber with the current studies using crimped macro synthetic fiber [
Figure
Chloride ion penetration by microsilica content: (a) 40 MPa and (b) 60 MPa.
This study used macro synthetic fibers, steel fibers, and microsilica as variables in our study and evaluated their applicability to segment concrete. In particular, we evaluated the applicability of macro synthetic fiber as a substitute for steel fiber in steel fiber-reinforced segment concrete. This study fixed the fiber volume fraction to 1 vol (%) and determined the appropriate replacement ratio of microsilica. The control specimens were steel fiber-reinforced segment concrete. Additionally, to study the application in tunnels, 1 vol% (9 kg/m3) was used in shotcrete containing macro synthetic fiber. The appropriate mixing ratio was also determined.
Recent studies with steel fiber-reinforced concrete determined the optimal mixing ratio based on postcracking behavior [
Microsilica performed optimally at the 6% substitution level. The effects of blends of fibers (hybrid fiber reinforcement) were evaluated with respect to possible improvement of the performance of the latex-modified fiber-reinforced segment concrete. Blends of fibers of different lengths, diameters, and types are also known to improve the performance of concrete because they can effectively control the micro- and macrocracks that form in the concrete [
Figure
Compressive strength of the optimum mixture.
The mixtures made with a hybrid fiber mixture had higher flexural strength than those made with a macro synthetic fiber. This is because the addition of polypropylene fibers increased the number of fibers per unit volume, subsequently increasing the internal tensile strength (Figure
Flexural strength of the optimum mixture.
Figure
Chloride ion penetration of the optimum mixture.
Figure
Impact resistance of the optimum mixture.
This study evaluated the mechanical properties and durability of fiber-reinforced, latex-modified segment concrete as a function of fiber type and microsilica substitution level. Macro synthetic, steel, and hybrid fibers (macro synthetic fibers + polypropylene fibers) were used as reinforcing fibers. The substitution levels of microsilica were 0, 2, 4, and 6%. Ultimately, this study showed the utility of latex-modified segment concrete containing macro synthetic fibers and hybrid fibers as a replacement for corrodible steel fiber in TBM tunnels. The test results are summarized as follows: The compressive strength, flexural strength, and chloride ion penetration resistance of latex-modified fiber-reinforced segment concrete increased with increasing substitution level of microsilica. This was because the fine microsilica particles filled the internal concrete micropores to create a more compact concrete. The compressive strength of latex-modified fiber-reinforced segment concrete did not significantly increase when the macro synthetic fiber was used as the reinforcement. The macro synthetic fibers had better flexural strength and water permeability resistance than steel fibers. This is because the macro synthetic fibers had a higher number of fibers per unit volume, which effectively suppressed internal crack formation and formed a more compact concrete. The compressive strength, flexural strength, and chloride ion penetration resistance results demonstrated that the 6% substitution level of microsilica and the 1% volume fraction of macro synthetic fiber were the optimal mixing ratio. The effects of the selected mixture and hybrid fibers were compared by holding other variables constant. The mechanical properties and durability of a macro synthetic fiber mixture and a hybrid fiber mixture were compared. The hybrid fiber mixture had excellent compressive strength, flexural strength, permeability, and impact resistance. The addition of reinforcing fibers improved the performance of latex-modified concrete, and the hybrid fiber mixture effectively controlled the formation of wide and narrow cracks in segment concrete, thereby improving the performance. This study evaluated the use of macro synthetic fiber as a reinforcing fiber in segment concrete. Additional theoretical and experimental studies are needed to develop a predictive model of the flexural behavior and establish the minimum and optimal fiber volume fractions for practical use.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This research was supported by the basic science research program through the National Research Foundation of Korea (NRF) funded by Ministry of Education (NRF-2016R1D1A3A03918587 and NRF-2013R1A1A4A01011776).