Analysis of Mechanical Properties of Fiber Reinforced Concrete Using RCC and PCC

Abstract

The addition of macro fibers to concrete pavements has been used to improve the cracking of concrete pavement, reduce slab thickness and contribute to increasing the joint spacing. A laboratory test was carried out in the study to analyze the impact of fiber reinforced concrete (FRC) on plain cement concrete (PCC) and roller compacted concrete (RCC), determining the flexural strength by performing ASTM-1609 tests and compressive strength by ASTM C-39 tests. Two synthetic fiber types selected with different geometries and different dosages (0.25% and 0.5% by volume) were tested for both RCC and PCC. To examine the effect of fiber contents and property, statistical testing was done using strength-test data. The test result showed that flexural strength was not affected by fibers. As fiber content increased, both residual strength (F₆₀₀ and F₁₅₀) and specimen toughness (T₁₅₀) increased for each fiber type. To the contrary, the compressive strength of specimens with higher fiber contents was lower in every case. Fiber properties including length and shape affected the residual strength of RCC more, than PCC. It is notable that the residual strength of RCC and PCC with the same fiber condition is very similar, even though the mix design and compressive and flexural strengths are different. In this paper, the strength-test data results are discussed, and the factors affecting the test results and the limitations of the testing methods are suggested.

1. Introduction

Over the years, roller compacted concrete (RCC) has been used for pavements in storage areas, municipal and industrial roads [1] with the availability of design procedures for industrial pavements [2] and roadways [3]. Roller compacted concrete has become a popular alternative pavement type because of the expedient construction benefits (earlier opening to traffic), reduced material and construction costs, increased sustainability rating, and structural capacity [4]. RCC has reduced the amount of cement due to the increase in the percentage of aggregate content, continuous gradation, and compaction energy [5]. In the case of fiber reinforcement in RCC with the addition of fibers, the flexural toughness and residual strength increases. Properly designed macro fibers in RCC lead to thickness reduction and crack width reduction which gives enhanced aggregate interlock joints and a reduction in the rate of crack deterioration [6,7]. Plain cement concrete (PCC) has low flexural strength and crack-holding capacity. The addition of fibers tremendously increases its structural characteristics and allows a reduction in concrete pavement thickness [8,9]. Therefore, the addition of fibers to concrete has become a common approach to improve its crack-holding capacity. A series of research projects has been carried out to investigate the physical and mechanical properties of RCC or PCC with the addition of different types of fibers.

Zhao et al. (2024) [10] investigated the influence of recycled-tire steel fiber length on the mechanical properties of steel fiber-reinforced concrete, claiming that fibers with a length of 21 mm are the optimum for the mixture design. Otieno and Mushunje (2021) [11] investigated the influence of replacement of fine mineral aggregates with crumb tire rubber of different particle sizes on the creep and shrinkage of PCC, claiming that it has similar creep and shrinkage deformation characteristics to that of mineral aggregate concrete. Isa et al. (2020) [12] contended that adding the hybrids of recycled-tire steel cords and recycled-tire steel fibers can achieve the desired flexural strength of the mixture. Akhtar et al. (2022) [13] concluded that 0.25%–0.5% recycled-tire steel fiber can enhance the ductility and imperviousness of concrete. Alwesabi et al. (2021) and Neves et al. (2024) [14,15] proved that adding recycled-waste tires to concrete is beneficial to its fracture characteristics.

Besides adding tire fiber, basalt, PVA, waste glass, and sisal fibers were also utilized in concrete mixtures. Yang et al. (2023) [16] performed three-point bending testing and concluded that AP interleaving is confirmed as an effective method in improving the interlayer structure and flexural performance of basalt fiber-reinforced polymer composites. Similarly, Meyyappan and Carmichael (2021) [17] confirmed that basalt fiber-reinforced concrete can enhance the strength properties of concrete. Alomayri (2021) [18] investigated the mechanical properties of basalt fiber-reinforced fly ash-based geopolymer pastes with various contents of additions, concluding that 3% nano CaCO₃ in basalt fiber-reinforced geopolymer paste presented the highest values of compressive strength. Yildizel et al. (2022) [19] proposed the optimal mixture design for basalt fiber-reinforced RCC. Wang et al. (2023) [20] investigated the physical and mechanical properties of basalt and sisal fiber-mixed concrete, concluding that the optimal hybrid fibers volume fraction was calibrated at 0.3%. Mitani et al. (2022), Yao et al. (2024), Min et al. (2024), Rao et al. (2022) and Wan et al. (2020) [21,22,23,24,25] confirmed that adding PVA fibers can contribute to the enhancement of the mechanical properties of the mixture. Gencel et al. (2022), Ibrahim (2021), Zeybek (2022), Ali et al. (2021) and Gholampour et al. (2022) [26,27,28,29,30] reported that the addition of waste glass improves the physico-mechanical and durability properties of the concrete. Jamshaid et al. (2022) [31] concluded that the addition of sisal improves the mechanical properties of the concrete when the content of additions is up to 2%. Sreekumaran et al. (2024) [32] confirmed that sisal hybrid fiber-reinforced concrete has a similar mechanical property compared with the usage of steel fibers in concrete when its addition is within 25%. Kerche et al. (2022) [33] investigated the physical and mechanical properties of hybrids of PVA, waste glass, and sisal fibers, reporting that adding glass fiber can achieve a higher flexural stiffness and modulus.

To thoroughly investigate the flexural strength and compressive strength of RCC and PCC with reinforcement of synthetic macro-fibers, a series of tests was conducted in this study. ASTM C1399 and C1609 [34,35,36,37,38] are the most common tests for evaluating FRC materials for bridge deck, deck overlays and pavement overlays (ACI Committee 544 2018). ASTM-1609 provides several performance measures of the combined interaction between concrete and macro fibers. The residual strength (f₁₅₀) and the equivalent flexural strength ratio (RT,150) are calculated from the monotonic-load deflection curve of a flexural beam specimen till 1/8 in deflection. There are shortcomings to the test as well. Banthia and Islam [39] discussed the limitations of the ASTM-1609 test and raised numerous concerns specifically concerning the loading rate [40].

Adding fibers to RCC and PCC and conducting comparative experiments helps to comprehensively evaluate the performance of fibers in these two types of concrete, providing a basis for engineering applications and gaining a deeper understanding of the material characteristics of the two types of concrete. This is of great significance for the design and construction of concrete structures. Since there are limited studies carried out previously on studying the effects of macro fibers (specifically synthetic fibers) in RCC, the objective of the paper is to compare the mechanical properties obtained for RCC and PCC mixed with fibers and discuss the influence of different types and volumes of fibers in the flexural strength of concrete. The study will also address the limitations of the ASTM-1609 test and address solutions for the same.

2. Testing Materials

2.1. Mix Design Properties

The performance of RCC and PCC highly depends on the mixture design. An appropriate water content and water-to-cement ratio intrinsically influences the workability and strength of the concrete. The strength of concrete has an inverse relationship with the water-to-cement ratio. However, slightly increasing the water content is beneficial to the workability. Therefore, balancing between them is critical for concrete mixture design. In addition, aggregate gradation has a significant influence on the performance of concrete. In this study, oven-dried coarse dolomite, intermediate dolomite, and natural sand are chosen based on the 0.45 power maximum density curve, which showed a satisfactory result in the previous study [6]. In terms of types of macrofibres, two different synthetic fibers with different surface textures are used in this study. As for the fibers’ dosage, 0.2% to 0.5% by volume is a typical range of the fiber content. Additionally, Min et al. (2024) [23] claimed that a significant improvement in the fatigue life of the concrete was observed when adding PVA fiber at a volume fraction of 0.2%. Similarly, Akhtar et al. (2022) [13] concluded that 0.25%–0.5% by volume fibers can enhance the ductility and imperviousness of concrete. Therefore, the amounts of 0.25% and 0.5% by volume fibers are used for each type of synthetic fiber. The mixture proportions of each component for RCC are shown in

Table 1. Mixture proportions of RCC (kg/m³).

Batch (Kg)	0.25% Volume Fiber	0.5% Volume Fiber
Coarse Dolomite (Oven dry)	534.89	534.29
Intermediate Dolomite (Oven dry)	962.78	961.74
Natural Sand (Oven dry)	641.85	641.16
Cement	288.67	288.67
Water	157.27	157.27
Fibers	2.33	4.67
Total	2587.81	2587.81

. The mixture proportions of each component for PCC are shown in

Table 2. Mixture proportions of PCC (kg/m³).

Batch (lb)	0.25% Volume Fiber	0.5% Volume Fiber
Coarse Dolomite (Oven dry)	1048.53	1044.71
Natural Sand (Oven dry)	656.46	657.36
Cement	337.99	338.46
Water	192.28	192.55
Fibers	2.10	4.29
Total	2237.37	2237.37

.

2.2. Information about Fibers

Two different synthetic fibers are used in this study. The detailed information is shown in

Table 3. Information about fibers.

Characteristic	A	B
Fiber Name	BarChip MQ58	Fibermesh 4 Roads
Material	Bi-Component Polymer	blend of two homopolymer polypropylene fibers: - Macro-monofilament fibers - Micro synthetic grade fibers
Length	58 mm	65 mm
Tensile Strength	640 Mpa	613 Mpa (Macro-monofilament fibers)
Texture	Continuous Embossing	Continuous Embossing (Macro-monofilament fibers) Flat (Micro synthetic fibers)

.

3. Concrete Beam and Cylinder Fabrication

3.1. Mixing Procedure

The mixing procedure was conducted based on ASTM C192 (2018), described as follows. Weight the materials including oven dried coarse dolomite, intermediate dolomite, natural sand, cement, water, and fibers separately. Mix the coarse dolomite, intermediate dolomite, natural sand, and half amount of the water in the mixer for one minute. Add cement and the other half amount of water into the mixture mixing for three minutes. Add fibers evenly into the mixture mixing for two minutes. Check whether fibers are balling or clumping, and evenly distribute fibers in the mixture by hand, and mix for one more minute (Figure 1).

3.2. Casting Procedure

Based on the ASTM D1557 and ASTM C1435, electric vibrating hammer is used to mold RCC beams and cylinders. Three replicate beams and three cylinders are fabricated for each fiber type and fiber dosage for RCC as well as PCC. The beam is 18 in. in length and has 6 in. by 6 in. cross section. The cylinder is 4 in. in diameter and 8 in. in height. The beams are subjected to third-point loading testing for flexural performance of fiber-reinforced concrete at 15 to 18 days, and the cylinders are subjected to compressive strength testing at 28 days (Figure 2).

4. Testing Procedure

4.1. ASTM 1609 Third-Point Loading Testing for Flexural Performance of FRC

ASTM C1609-12 is used as the test method to analyze concrete beam performance with reinforced fibers. The testing is based on a simple supported beam under third-point loading (Figure 3). Instead of regular load-controlled testing, it is controlled by the mid-span vertical deflection. The testing terminates when the mid-span vertical deflection reaches L/150. Typically, when L is equal to 18 in., L/150 is equal to 0.12 in.

As for the dimension of the beam specimen in this study, 6 in. square cross section and 18 in span is adopted.

Several parameters are of great interest in this testing method, including peak strength, residual strength at net deflection of L/600, residual strength at net deflection of L/150, specimen toughness, and equivalent flexural strength ratio. The first-peak strength, usually the peak strength, is the indication of beginning of cracking. The residual strength indicates the capability of holding the cracks by fibers after cracking. The specimen toughness indicates the capacity of energy absorption of the specimen.

The following formulas explain the way to calculate such parameters. Figures of the specimens after testing are attached in Appendix D.

$$f_1={\displaystyle \frac{P_{1}L}{b{d}^2}}$$

(1)

$$f_{150}^D={\displaystyle \frac{P_{150}^{D}L}{b{d}^2}},$$

(2)

$$f_{600}^D={\displaystyle \frac{P_{600}^{D}L}{b{d}^2}},$$

(3)

$$R_{T,150}^D={\displaystyle \frac{150T_{150}^D}{f_{1}b{d}^2}}\times 100(\%),$$

(4)

where

• $P_1$ is the peak load as well as the first-peak load;
• L is the span length of beams;
• b is the width of the cross section, and d is the depth of the cross section;
• $f_1$ is the peak strength as well as the first-peak strength;
• $P_{150}^D$ is the load obtained from load-deflection curve when the mid-span vertical deflection reaches $L/150$;
• $f_{150}^D$ is the residual strength of FRC at the net deflection of L/150;
• $P_{600}^D$ is the load obtained from load-deflection curve when the mid-span vertical deflection reaches $L/600$;
• $f_{600}^D$ is the residual strength of FRC at the net deflection of L/600;
• $T_{150}^D$ is the area under the load-deflection curve from 0 to L/150;
• $R_{T,150}^D$ is the equivalent flexural strength ratio.

4.2. ASTM C39 Compressive Strength Testing

ASTM C39 is used to analyze the concrete cylinder compressive strength. By applying a compressive axial loading to the cylinder specimen at a loading rate until the cylinder is failed, calculate the strength of the concrete cylinder by loading at the failure divided by the cross section of the cylinder. The loading rate should be in the range from 12 MPa/min to 18 MPa/min. The cylinder specimen alignment should be verified since unbounded caps are used in this study. Figures of the specimens after testing are attached in Appendix E.

$$f_{cm}={\displaystyle \frac{4P_{max}}{\pi D^2}}$$

(5)

where

• $P_{max}$ is the maximum load when cylinder fails;
• D is the average measured diameter;
• $f_{cm}$ is the compressive strength.

5. Testing Results

5.1. Compressive Strength

Figure 4 and

Table 4. Compressive strength testing results.

	Compressive Strength [MPa]
Mixture ID	Average	COV (%)
PCC A 0.25	42.001	0.764
PCC A 0.5	40.997	3.612
PCC B 0.25	45.044	1.304
PCC B 0.5	42.150	3.357
RCC A 0.25	64.357	1.415
RCC A 0.5	59.858	3.827
RCC B 0.25	51.476	4.063
RCC B 0.5	45.158	1.219

show the compressive strength properties. As expected, the compressive strength of the RCC specimens is higher than that of the PCC specimens. For every PCC and RCC specimen, specimens which have 0.5% volume fiber have a smaller compressive strength than specimens which have 0.25%.

5.2. Flexural Strength

5.2.1. Strength Properties

Figure 5 and

Table 5. Flexural performance testing results.

	MOR [MPa]		F600 [MPa]		F150 [MPa]		T150 [Nm]		R150 [%]
Mixture ID	Average	COV (%)	Average	COV (%)	Average	COV (%)	Average	COV (%)	Average	COV (%)
PCC A 0.25	3.942	4.80	0.430	34.85	0.446	31.55	14.627	17.51	14.754	12.84
PCC A 0.5	4.321	6.78	0.816	26.90	0.934	39.07	22.297	38.87	21.230	31.24
PCC B 0.25	4.370	6.08	0.601	54.46	0.484	67.88	21.315	-	21.535	-
PCC B 0.5	4.408	4.04	1.301	1.81	1.170	7.02	31.685	1.65	28.525	3.90
RCC A 0.25	5.893	7.91	-	-	-	-	-	-	-	-
RCC A 0.5	6.111	5.50	0.835	-	0.921	13.54	-	-	-	-
RCC B 0.25	6.031	4.33	0.433	-	0.441	32.46	-	-	-	-
RCC B 0.5	5.654	14.27	1.137	25.62	1.251	15.21	-	-	-	-

show the flexural performance testing results. For all strength parameters, the coefficient of variation (COV) is calculated to account for the data point dispersion. Peak flexural strengths (MOR) are calculated for all cases including PCC and RCC, but the residual strength (F₆₀₀ and F₁₅₀), specimen toughness (T₁₅₀) and equivalent flexural strength ratio (RT,150) of some specimens could not be calculated because of the testing failure after peak strength (Appendix C). The detailed reason for the testing failure will be discussed in the following Discussion Section. The load-deflection curves of RCC and PCC are plotted and compared in Figure 6. RCC samples were subjected to reloading as this failed at the initial attempt. Detailed discussion of the initial failure is found in Section 6.7.

5.2.2. Statistical Analysis

Table 6. Statistical Analyses of Flexural strength characteristics.

Test No.	Mixture ID	MOR [MPa]	std.dev	t-Test	Two Sided p-Value	Statistically Significant?
1	PCC A 0.25	3.942	0.189
	PCC A 0.5	4.321	0.293	−1.88224	0.156349	No
2	PCC B 0.25	4.370	0.266
	PCC B 0.5	4.408	0.178	−0.20758	0.848849	No
3	PCC A 0.5	4.321	0.293
	PCC B 0.5	4.408	0.178	−0.44173	0.688585	No
4	PCC A 0.25	3.942	0.189
	PCC B 0.25	4.370	0.266	−2.27181	0.085556	No
5	RCC A 0.25	5.893	0.466
	RCC A 0.5	6.111	0.336	−0.65492	0.548271	No
6	RCC B 0.25	6.031	0.261
	RCC B 0.5	5.654	0.807	0.769449	0.522077	No
7	RCC A 0.25	5.893	0.466
	RCC B 0.25	6.031	0.261	−0.4465	0.685498	No
8	RCC A 0.5	6.111	0.336
	RCC B 0.5	5.654	0.807	0.904236	0.432524	No

,

Table 7. Statistical Analyses of Residual strength characteristics, F₆₀₀.

Mixture ID	F600 [MPa]	std.dev	t-Test	One Sided p-Value	Statistically Significant?
PCC A 0.25	0.430	0.150
PCC A 0.5	0.816	0.219	−2.51038	0.033015	Yes
PCC B 0.25	0.601	0.327
PCC B 0.5	1.301	0.024	−3.02126	0.101744	No
PCC A 0.5	0.816	0.219
PCC B 0.5	1.301	0.024	−3.80898	0.031265	Yes
PCC A 0.25	0.430	0.150
PCC B 0.25	0.601	0.327	−0.68931	0.307895	No

and

Table 8. Statistical Analyses of Flexural strength characteristics, F₁₅₀.

Mixture ID	F150	std.dev	t-Test	One Sided p-Value	Statistically Significant?
PCC A 0.25	0.446	0.141
PCC A 0.5	0.934	0.365	−2.16242	0.059651631	No
PCC B 0.25	0.484	0.328
PCC B 0.5	1.170	0.082	−3.51385	0.036157444	Yes
PCC A 0.5	0.934	0.365
PCC B 0.5	1.170	0.082	−1.09404	0.194060157	No
PCC A 0.25	0.446	0.141
PCC B 0.25	0.484	0.328	−0.18385	0.432928007	No
RCC B 0.25	0.441	0.143
RCC B 0.5	1.251	0.190	−5.89291	0.00207353	Yes
RCC A 0.5	0.921	0.125
RCC B 0.5	1.251	0.190	−2.50748	0.043565324	Yes

shows the statistical analysis results for the flexural strength (MOR) and residual strength (F₆₀₀ and F₁₅₀), respectively. For MOR values, a t-test with a 95% confidence interval was conducted to compare the effect of fiber contents and fiber properties. Since our hypothesis based on the previous research is that the flexural strength is not affected by fibers [6,9], a two-tailed t-test was adopted. For residual strengths, it is assumed that the fiber content and fiber properties affect values, thus a one-tailed t-test was conducted. Based on the reference value 0.05, statistical significance was shown in the tables.

6. Discussion

6.1. Effect of Fiber Contents

To figure out the effect of fiber contents for flexural strength and residual strength, a t-test for each specimen with different fiber contents was conducted. Table 6 shows the t-test results for flexural strength of the specimens. Test numbers 1 and 2 are the testing results for PCC with different fiber contents, and test numbers 5 and 6 are the test results for RCC with different fiber contents. As expected, all case results are not statistically significant, which means that the flexural strength is not affected by fiber content. This is because the role of fibers is holding the crack after peak stress, so fibers have no influence on flexural strength before cracking.

Table 7 and Table 8 shows the statistical test results for residual strengths F₆₀₀ and F₁₅₀. Due to the flexural testing failure, all the F₆₀₀ values and some F₁₅₀ values of RCC could not be calculated. For all cases including PCC and RCC, the residual strength of specimens with 0.5% fibers is greater than that of specimens with 0.25% fibers, from 89% to 184%. This is also clearly shown in Figure 4.

From Table 7, the t-test result for PCC with fiber A shows that more fiber increases the F₆₀₀ residual strength statistically. Although the t-test result from PCC with fiber B is not statistically different with fiber content, the p-value is almost at threshold (0.05). In the case of F₁₅₀ residual strength, both PCC and RCC with fiber B are statistically affected by fiber content. Again, the F₁₅₀ residual strength of PCC with fiber A is not statistically different with fiber content, but the p-value is almost at threshold (0.05). Considering that the threshold p-value is frequently used up to 0.1, it is clearly shown that the higher fiber content increases the residual strength of both RCC and PCC.

Although the specimen toughness and equivalent flexural strength ratio were only calculated for PCC specimens, it also shows the effect of fibers. In Table 5, for both fibers, specimen toughness was increased from 32% to 52% by using more fiber content. According to ASTM 1609, specimen toughness is a measure of the energy absorption capacity of the test specimen.

As the fiber contents increased, the values of both residual strength (F₆₀₀ and F₁₅₀) and specimen toughness (T₁₅₀) increased for each fiber type. This result is in accordance with previous research on FRC [6,9].

In Table 4, the compressive strength of specimens with higher fiber content is lower in every case. Increased fiber content may prevent aggregate contact and create a weaker zone in the specimen.

6.2. Effect of Fiber Properties

To examine the effect of different fiber properties, the test results of PCC and RCC with the same fiber content and different fiber materials were compared and statistically tested. Table 6 shows that the flexural strength of specimens with different fibers is not statistically different. As discussed in previous paragraphs, this is because fibers have no influence on flexural peak strength.

Table 8 shows the residual strength (F₁₅₀) of each specimen and the statistical testing results. For PCC specimens, although the F₁₅₀ values of specimens with Fiber B were greater than those of specimens with Fiber A, a comparison between specimens with different fibers shows no statistical difference. That means the fiber material difference did not affect the residual strength of the PCC beams. However, the residual strength of RCC specimens with fiber B is 36% greater than the residual strength of RCC specimens with fiber A, and statistical testing results also shows the significant difference. That means that fiber B is better than fiber A, in terms of the residual strength of RCC.

Unlike PCC specimens, the RCC mixing procedure and gradation makes fiber bent and tortuous. This may result in the fiber length at the failure plane not being enough to resist the tensional stress. Considering Fiber B is 12% longer than fiber A, a longer fiber length could be a reason for the greater residual strength. Another possible reason is the texture of fibers. Half of the fiber B specimens were very flexible and flat, whereas all the fiber A specimens were continuously embossing and stiffer. The flexible and flat component of fiber B may allow the fiber to have a tortuous and bendy shape along the aggregates in RCC.

From Table 4, the compressive strength of the RCC specimens with fiber B is significantly lower than that of the RCC specimens with fiber A. Having longer and flexible fibers brings better residual strength but decreases compressive strength because it prevents particle contacts in RCC.

6.3. Residual Strength of RCC and PCC

Table 9. Residual strength of RCC and PCC.

Fiber, Content (%)	F600 [MPa]			F150 [MPa]
	RCC	PCC	Difference (%)	RCC	PCC	Difference (%)
A, 0.25	-	0.430		-	0.446
A, 0.5	0.835	0.816	2.3	0.921	0.934	1.4
B, 0.25	0.433	0.601	11.9	0.441	0.484	8.9
B, 0.5	1.137	1.301	12.6	1.251	1.170	6.5

shows the comparison of residual strength between RCC and PCC specimens. Even though the RCC and PCC specimens have a different mixing design, and compressive and flexural strength, the residual strength of specimens with the same fibers and same fiber contents shows an almost identical trend of residual strength. This result could be evidence that the residual strength is governed by the fiber property and fiber contents, not the concrete mixing properties.

6.4. Comparison of Testing Results with Other Studies in the Literature

The comparison of mechanical properties of FRC between this study and others in the literature is displayed in

Table 10. FRC mechanical properties comparison between this study and others in the literature.

	Flexural Strength (MPa)	Compressive Strength (MPa)
PCC PVA 1.50, literature [41]	4.34	46.86
PCC PVA 0.20, literature [23]	4.50	-
PCC Basalt 0.50, literature [17]	4.88	34.08
PCC Glass 10.0, literature [28]	5.93	18.10
RCC without fibers, literature [19]	4.35	35.15
RCC without fibers, literature [6]	4.65	46.70
PCC A 0.25, this study	3.942	42.001
PCC A 0.50, this study	4.321	40.997
PCC B 0.25, this study	4.370	45.044
PCC B 0.50, this study	4.408	42.150
RCC A 0.25, this study	5.893	64.357
RCC A 0.50, this study	6.111	59.858
RCC B 0.25, this study	6.031	51.476
RCC B 0.50, this study	5.654	45.158

. The comparison results indicate that the fibers A and B used in this study have a higher performance at the same dosage compared to fibers added in other studies.

6.5. Effects Analysis of Fiber-Reinforced Concrete

6.5.1. Cost Analysis

The economy of FRC is mainly reflected in the following aspects. Firstly, although the addition of fibers increases the cost per unit volume of concrete, from the perspective of total lifecycle cost, FRC often has a lower total cost than ordinary concrete due to its reduced frequency of maintenance and replacement. Furthermore, FRC has excellent crack resistance, which helps to reduce leakage problems caused by cracks and lower additional costs due to water damage.

6.5.2. Environmental Impact Assessment

FRC has significant environmental benefits in the production process. FRC reduces cement consumption and correspondingly lowers carbon emissions during production by adding different types of fibers, which helps to slow down the trend of global climate change. Secondly, FRC has higher crack resistance, impermeability, and durability, which means that FRC is less prone to cracking and leakage during use, thereby reducing the frequency of repairs and replacements and lowering the maintenance costs. Meanwhile, due to the better durability and longer service life, it further reduces the generation of construction waste and lowers the pressure on the environment. In addition, FRC also has good renewability. With the continuous development of the construction industry, the problem of construction waste is becoming increasingly serious. The fiber materials in FRC can be recycled and reused to reduce the amount of construction waste.

6.5.3. Concrete Workability

After adding fibers to concrete, the slump of concrete tends to decrease, indicating that the addition of fibers reduces the fluidity of concrete. The mechanism of the influence of fibers on the workability of freshly mixed concrete is that the fibers are randomly distributed in the mixture, providing support for the various components of the mixture, causing an increase in internal friction and a decrease in the fluidity and workability of the mixture. Meanwhile, the addition of fibers increases the surface area and occupies a large amount of water, thereby reducing the slump of concrete.

6.6. Factors’ Effects on Testing Results

6.6.1. Curing Time

Curing time is a key determinant to the strength of concrete specimens for both PCC and RCC. The strength of concrete specimens would have been increasing during the 28 days after casting. However, not all of the specimens were well cured by the moisture due to the damage of the moisture curing room, which had a significant influence on the strength of the specimens. To be specific, six RCC beams mixed using fiber type A were cured in the moisture room for 2 days and were dry cured for 18 days. Six PCC beams mixed using fiber type A were dry cured for 17 days. Six RCC beams mixed using fiber type B were dry cured for 17 days. Six PCC beams mixed using fiber type B were dry cured for 15 days. All the twenty-four cylinders were dry cured for 28 days.

6.6.2. Eccentricity

Certain samples while testing using the ASTM-1609 test had an eccentricity in the third-point loading. According to ASTM 1609 specifications, third-point loading should be applied without any eccentricity to samples. The results of the samples that had eccentricity caused the low peak as well as residual strength. Samples for RCC 0.25% fiber A sample 3, 0.50% fiber A sample 2, RCC 0.50% fiber B samples 1 and 3, 0.25% fiber B sample 3, and for PCC 0.5% fiber A sample 1 and 0.5% fiber B sample 2 had eccentricities while loading. Some lower values from the samples that had eccentricity were excluded from data analysis. Load-deflection curves for the samples are attached in Appendix A.

6.6.3. Compaction

As for the compaction process, the RCC beam and cylinder specimens were cast according to ASTM D1557-12 and ASTM 1435 standard. A vibrating hammer was used to cast the specimens. However, the hammer rod was broken when molding the RCC mixture with fiber type A. Since the casting process could not be stopped otherwise new mixtures should be made again and there were only two beams and three cylinders left, we chose to put the head of the vibrating hammer on the mixture and put the rod in contact with it to continue compaction. We expected that the strength of these beams and cylinders might be lower than others because of the nonuniform distributed load of compaction. However, the results show that the strength of these beams and cylinders is at the same level as the others.

6.7. Limitation of Testing Methods

ASTM 1609 is one of the most common tests to analyze the performance of FRC. However, ASTM 1609 still has several limitations that would make it difficult to obtain a stable load-deflection curve.

The first factor is related to ‘closed-loop control’ machines. According to Banthia and Islam [39], there is a huge probability that a closed-loop machine cannot control stability. Many factors rather than the testing material itself would influence controlling stability. In this study, the stiffness of the loading frame is a key determinant of obtaining a stable load-deflection curve because the loading frame itself is not sufficiently stiff. The machine would wrongly recognize that the testing beam has been failed, especially for RCC which has a higher flexural strength. Since the sudden stop of the testing machine, the load-deflection curve could not be obtained.

The second factor is the loading rate in the ASTM 1609 specifications. According to Banthia and Islam [39], the specified loading rate in ASTM 1609 is too high especially for low-dosage fiber-content concrete. When they conducted ASTM 1609 testing for 1.0 kg/m3 fiber content FRC following a specified loading rate, the stable load-deflection curve could not be obtained. However, if they reduced the loading rate to some extent, a stable load-deflection curve could be acquired. Therefore, they decided to reduce the loading rate to 0.001 to 0.015 mm/min when the net deflection was below L/900 and increase the loading rate to 0.02 to 0.15 mm/min after the net deflection reached L/900.

7. Conclusions

(1)

The flexural strength of the concrete is not affected by the type and volume of fibers.

(2)

The volume and type of fibers significantly affected the residual strength of both RCC and PCC. Both RCC and PCC showed a marked increase in the residual strength with the increase in the fiber content.

(3)

The performance of fiber B was found to be much better than fiber A in terms of the residual strength for RCC. The longer fiber length and flexible components of fiber B could be a reason for this difference in residual strength. Fiber B is more suitable for use in RCC.

(4)

There is a decrease in the compressive strength of FRC for each RCC and PCC.

(5)

The residual strength of RCC and PCC with the same fiber condition is very similar, although the mix design and compressive and flexural strength are different. This could be because the residual strength is governed by fiber property and fiber content, not the concrete mixing properties.