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Optical fibers in cementitious composites (LTCM): Analysis and discussion of their influence when randomly arranged
Construction and Building Materials 244 (2020) 118406
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Optical fibers in cementitious composites (LTCM): Analysis and discussion of their influence when randomly arranged Thiago dos S. Henriques ⇑, Denise C. Dal Molin, Ângela B. Masuero Department of Civil Engineering, Federal University of Rio Grande do Sul, Brazil
h i g h l i g h t s Polymeric optical fibers (POFs) were distributed in a random manner for the production of LTCMs. The mechanical properties of the LTCMs decreased due to increases in their fiber contents. With respect to the cost-benefit ratio, the 5% POF content samples proved to be the most advantageous.
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Article history: Received 25 June 2019 Received in revised form 3 February 2020 Accepted 10 February 2020
Keywords: Cost-benefit ratio LTCM (light transmittance cement-based material) POF in a random manner
a b s t r a c t Published studies related to LTCMs (light-transmitting cement-based materials) deserve attention. Although it is a widely used commercial material, in the technical-scientific field, there is still a demand for a deeper understanding of its performance characteristics. The data presented here refer to samples with three different fiber contents (2%, 3.5%, and 5%) that were arranged randomly and compared to the reference samples that were free of polymeric optical fiber (0% POF). This work complements a related article that was previously published by the authors [1] that addresses the influence of fibers that are arranged in an orderly manner. The performance characteristics of the compressive strength, the flexural tensile strength, the water absorption via capillarity, the light transmittance, and the cost-benefit ratio (considering the Brazilian market) were verified. From the 0% (reference) to 5% (maximum POF content adopted) fiber contents, the following behaviors were found: decreases of approximately 20% and 30% in the compressive and flexural tensile strength, respectively, and an increase of 400% in the capillary water absorption. For light transmittance, from the 2% to the 5% POF contents, there was an increase of 150% in the fibers and an increase of approximately 300% in the light transmittance. Considering the Brazilian market and the cost per lux ($ / lux) of the samples with added optical fibers (2%, 3.5% and 5% POF), the following values were obtained: $ 1.61/ lux (2% POF), $ 4.19/lux (3.5% POF), and $ 1.26/lux (5% POF). Based on the cost/light transmittance ratio, the 5% POF content was the best. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The research and development of innovative materials that are applied in civil construction always deserve discussion and dissemination, not only in congresses but also in recognized scientific publications. This article is a complement to another report [1] that was previously published by the same authors in this same journal. The use of LTCMs (light-transmitting cement materials), originally developed in Hungary in 2002 and patented under the name LiTraCon [2], has been widely and commercially applied, but there is ⇑ Corresponding author at: Av. Osvaldo Aranha 99, 3°andar, Porto Alegre, RS 90035-190 Brazil. E-mail addresses: [email protected] (T.d.S. Henriques), [email protected] (D.C. Dal Molin), [email protected] (Â.B. Masuero). https://doi.org/10.1016/j.conbuildmat.2020.118406 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
still a need for a deeper understanding of its technical knowledge before it is applied, for instance, in the structural parts of a building. In the manufacture of LTCMs, the use of optical fibers is essential regarding light transmittance properties; however, POF can affect both the mechanical and durability properties of the composites. In general, steel and glass fibers, which are the most commonly used reinforcements in cementitious matrices [3], function by increasing the ductility due to the fiber reinforcement mechanisms provided by the fibers bridging the crack surfaces [4]. LTCMs greatly enhance the lighting effects, reduce the lighting energy consumption, and promote the energy conservation of buildings [5]. Translucent panels made of concrete using natural lighting have a unique appeal in several aspects such as improved moods, acceptance, and the visual comfort of the occupants [6]. Translucent concrete as a building envelope can offset some lighting
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Fig. 1. Aesthetic appearance of samples with POFs disposed in a random manner.
energy that is consumed within a room in an office [7]. Its light transmitting properties depend on a large number of optical fibers that transmit light through the cement-based product [8]. Our previous work focused on LTCMs at four POF (polymeric optical fiber) content values (0%, 2%, 3.5%, and 5%) in which the fibers were arranged in an orderly manner. The present article assesses the same material and fiber contents, but now they are arranged randomly (Fig. 1). Therefore, often in this text, some technical data of the used materials will refer to this previous work [1]. In addition to the compressive strength, flexural tensile strength, capillary water absorption, and light transmittance data, this work adds the issue of the cost-benefit ratio using the Brazilian construction market as the reference. This cost assessment has the purpose of assisting the choice between different POF content values in LTCMs, by pointing out the one with the best cost-benefit ratio. Compared to the samples prepared with ordered fibers, the random distribution of fibers in a LTCM may have an implication for the cost since its production is simpler in relation to the design, manpower and equipment. Similarly, it may be assumed that the material performance is lower due to its own experimental variability, which may cause fibers to accumulate at some points and be lacking at others. Therefore, this study aims to determine if composites with random fibers (that have theoretically lower production costs) are statistically equal, better or worse than composites with ordered fibers in terms of their mechanical performance and durability. An important point to be clarified in this study is that the term ‘‘random” as used in the text refers to samples with fibers arranged (in a fresh mortar) in thin layers in a poorly controlled manner, that is, tending to have a random arrangement. During the vibra-
tion process, some samples had greater fiber movement than others, thus losing the visual effect of layered fibers. Therefore, there was no control over the fiber spacing, but only their quantity, to ensure the correct content values. Statistical analysis in order to provide how random the fiber distribution is also presented.
2. Materials and methods The LTCM samples were produced using Portland cement, water, a fine aggregate (sand), a polycarboxylate ether-based superplasticizer, polymeric optical fibers, and silica fume. The materials are briefly described below, and more detailed information can be found in the literature [1]. CP V ARI type cement was chosen (it is equivalent to Portland cement with a high early strength III - ASTM C150). The sand (with a quartz origin) used was from a Brazilian riverbed. A polycarboxylate ether-based superplasticizer (PCE) was added to the mixtures to act as a dispersant of the cementitious material and to provide high water reduction. The ratio of this material to the cement mass was 1.0%. A percentage of 10% silica fume, replacing part of the mass of cement, was used to improve the performance (to reduce the permeability, to increase the cohesion and to avoid segregation, among other reasons) of the material. The adopted mixture ratio, in mass, was 1:0.11:2.22:0.5 (cement : silica fume : sand : water). Following the procedures of the Brazilian standard [9], consistency index tests were performed on the reference mortars (Fig. 2). The aim was to obtain a very fluid base mortar (0% POF) that was practically self-compacting to fill all the spaces between the fibers (2%, 3.5%, and 5% POF) that were subsequently inserted.
Fig. 2. Consistency index determination test: (A) mold filling with mortar, and (B) measurement of the spreading of the mixture.
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Fig. 3. Infrared spectrum of the POF.
2.1. POF (polymeric optical fiber) As the optical fibers acquired for this research in Brazil originated from China, it was not possible to obtain any physicochemical characterization from the manufacturer. Thus, it was necessary to use infrared spectroscopy to obtain a standard power spectrum and compare it with the results of other surveys. The core raw material of POFs generally consists of PS (polystyrene), PC (polycarbonate) or PMMA (polymethyl methacrylate) with very high refractive index values, whereas the coating is made with low refractive index polymers [10]. The characterization of the POFs was made using a PerkinElmer Spectrum BX FTIR (São Paulo, SP – Brazil). Initially, the POF samples were cut to approximately 3 cm in length and rinsed with acetone to remove any grease or dirt due to handling. They were then placed in the spectrometer for their analysis. At the beginning of the assay, an infrared reading was performed without the POF sample to subtract the cell spectrum from the final result. Subsequently, 10 successive readings were performed on the same sample to obtain a spectrum with high fidelity. A typical result can be seen in Fig. 3, and the wavebands are given in Table 1. PMMA is the most common material used in the manufacture of POFs [10]. However, comparing the POF infrared spectrum used in this study with the spectrum of a PS-type POF acquired by Uribe [10] and the spectrum of a PMMA-type POF obtained by Ramesh et al. [11], it can be said that it is likely that the fiber polymer used was a PS. In the qualitative characterization, the surface analysis using an SEM (scanning electron microscope) showed that it has a smooth surface with nearly no manufacturing defects (microcracks, diameter variation within the same filament, and roughness, among other aspects), as seen in Fig. 4. The diameter had an average thickness of 0.4 mm (400 lm). The calculation procedures used for the three POF content values (2%, 3.5%, and 5%) took into account the relationship between the fiber area exposed to light and the surface area translucence of the LTCM.
Table 1 Analysis of the infrared spectrum bands of the POF. Spot
Wave number (cm
A B C D E F G H I J K L M
694.55 751.45 840.80 906.60 987.76 1146.34 1191.44 1238.35 1384.11 1448.76 1599.51 1724.68 2949.08
1
)
Fig. 4. POF surface texture obtained by SEM. Magnification: 75 times.
To perform an assessment of the mechanical properties of the POF, a tensile test according to ASTM D3822/99 was run with a displacement rate of 1 mm/min. The assay was performed on an Instron 5585H testing machine. Five specimens with free length values of 200 mm were subjected to the tensile test. Table 2 shows the POF tensile properties.
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Table 2 Individual tests of the tensile strengths of the POFs [1]. Sample 1 2 3 4 5 average standard deviation coefficient of variation
Maximum load (N) 10.93 10.95 9.93 10.37 13.70 11.18 1.32 11.79%
Resistance limit (MPa) 120.37 107.58 109.38 114.26 109.05 112.13 4.69% 4.19%
The refractive index and transmission rate of the fibers were not analyzed for the main reason that LTCM pieces for civil construction have, on average, a thickness between 0.10 m and 0.50 m, that is, a very short distance with no significant transmittance losses.
Fig. 6. Device, developed in this research, for vibration by immersion in the mortar.
3. Experimental program The characteristics of the compressive strength, tensile strength, light transmittance, and capillary water absorption were compared among the LTCM samples (2%, 3.5% and 5% fiber content values) and the reference sample using mortar that was free of POF (0%). All the assays were carried out on day 28. The same mortar mix proportions (except for the POF) were used for all of the samples. 3.1. Preparation of the LTCM sets Fig. 7. Block cutting by a saw.
During the molding process, a layer of mortar and a layer of fiber were repeatedly poured into the mold until it was filled (Fig. 5). For filling the voids within the mortar and the subsequent elimination of air bubbles, a vibrating platform was used; however, it was not efficient. Therefore, a vibrating immersion device was developed consisting of a mini rotor powered by two AA size batteries with a metallic rod tip and manual control of the vibration intensity. This device was used to eliminate as many air bubbles in the mortar as possible (Fig. 6). The duration of the vibration for all molded blocks was approximately 45 s, which was enough time for the air bubbles to stop rising to the surface and to avoid any segregation. This was a careful and necessary procedure, since even when using a suitable mortar with high fluidity, there was some difficulty in molding the samples. The main challenge of this step was to ensure the correct fiber envelopment by the mortar. The LTCM sample demolding procedure was performed twentyfour hours after the preparation of the samples and subsequent saw cuts were made (Fig. 7). From each large block, five prismatic samples (4x4x16 cm) were generated; that is, for each fiber content (including 0% POF) five specimens were produced. Each sam-
ple was subjected to all four measurement tests in the following order: 1st light transmittance (non destructive); 2nd flexural tensile strength (destructive, generating two halves of each sample); 3rd capillary water absorption (non destructive); 4th compressive strength (destructive). All of the LTCM specimens remained in an environment with controlled relative humidity (95 ± 5) % and temperature (23 ± 2) °C until the performance tests on day 28. 3.2. Test methods: Compressive strength, flexural tensile strength, capillary water absorption, and light transmittance Since the specimens have randomly positioned fibers, the compressive strength tests and the flexural tensile strength tests sought to reproduce the behavior of their use as LTCMs. Since walls may tend to receive accidental horizontal loads (impacts) or even buckle (if they are very slim), the flexural tensile assay was executed by applying a load that was parallel to the fibers. In Fig. 8, an illustration exhibiting the direction of the applied loads concerning the positioning of the fibers is presented.
Fig. 5. ‘‘Layer” molding of the samples with random fibers: (A) placement of the POF layer, and (B) placement of the mortar layer.
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Fig. 8. Diagram showing the direction of the applied loads concerning the fibers of the samples.
For the flexural tensile strength tests and the compressive strength tests, loading applications of, respectively, 50 ± 10 N/s and 500 ± 50 N/s were applied to the specimens according to the Brazilian mortar standard [12]. The capillary water absorption assays were carried out according to the Brazilian standard [13] as well. The remaining two halves of the flexural tensile test specimens on day 28 were used. In this test, the samples are placed in a water bath with a depth of approximately 5 ± 1 mm. The mass comparison is done with the data obtained before and after 90 min of the material being in contact with the water. [13]. For the light transmittance evaluation tests, the same wooden device of our previous work [1] was used to evaluate the transmittance of light that occurred with the specimens with different levels of POFs. A further explanation can be found in [1]. 3.3. Statistical analysis on the fiber distribution For the statistical analysis on the fiber distribution of the samples in the hardened state, the software Image J was used. The number of specimens used to calculate the average outcome was 6 (2 for each content value of 2%, 3.5% and 5% POF). Fiber distribution has a purpose to determine if particles in a 2D image are likely to be randomly distributed, self-avoiding or build clusters. Starting with a binary image and through the Bio Voxxel plug-in, the evaluation was based on the mean nearest neighbor distance and the confidence interval of the statistical evaluation adopted was 99.9%. The average results are shown in Table 3. Among the 3 groups of samples with fibers (2%, 3.5% and 5% POF), according to the statistical analysis performed, 2 types of fiber organization were formed: fiber randomly distributed and build clusters. For the 2% POF content, the fibers were statistically organized as build clusters. This behavior can be attributed to the method of sample execution (layer of mortar and a layer of fiber repeatedly poured into the mold until it was filled), as shown in Fig. 5. As the lowest content effectively adopted (2% POF), it is possible to observe in Fig. 1(B) that there is actually a visible trend of fiber layer formation. For the other contents (3.5% and 5% POF), due to the greater amount of fibers, they tended to mix more easily so that, statistically, they can be considered as randomly arranged. 4. Results and discussion The results of the tests that were described in the experimental program were subjected to statistical analysis, and all of these results are presented in the form of tables and graphs. On the bottom line of each type of assay can be found a summary of the significant factors that were obtained through ANOVA (analysis of
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variance) and Duncan´s method. The one-way ANOVA test evaluates the difference between a single dependent quantitative variable (randomly arranged fibers) against the average of the results of the four groups that were formed by the categories of a single independent variable (the fiber content). All hypotheses tested were evaluated at a confidence level of 95%. Although it is a handy tool, the analysis of variance only reports whether or not there is a significant difference between the groups that are analyzed but does not address the difference between each group. Thus, with a pre-established guarantee that a particular conclusion is correct; the individual results were analyzed by Duncan´s method. Duncan’s method takes into consideration the examination of individual outcomes in an incorporated arrangement of determinations with a pre-set up assurance that this result is correct. This multiple comparison procedure was used to determine which groups (0%, 2%, 3.5% and 5% POF) are significantly different from each other at the confidence limit applied. The means are considered significantly different according to the established significance level of 0.05. 4.1. Compressive strength test results For the compressive strength assays the number of specimens used to calculate the average outcome was 40 (10 for each content value of 0%, 2%, 3.5% and 5% POF). The average results of tests performed on day 28 are shown in Table 4, and the ANOVA results can be found in Table 5. In Fig. 9, a graph of the average compressive strength results for all POF contents that were adopted can be seen. The response of a concrete sample to an applied stress depends not only on the stress type but also on how a combination of various factors affects the porosity of the different structural components of the concrete, including the following: the properties and proportions of the materials that make up the concrete mixture, the degree of compaction and the curing conditions [14]. According to the ANOVA evaluation, the fiber percentage had a significant effect on the compressive strength. Since random fibers are inserted into the composites, the composites tend to have their compressive strength decreased. Using Duncan’s method, it can be seen that the compressive strength values of the samples with a content of 0% POF are significantly different from those of the samples with other fiber contents. In addition, according to the same method of analysis, only the 2% and the 3.5% POF groups have no significant difference in the results for the compressive strength. Compared to the 0% POF content sample, the samples with 2% and 3.5% POF experienced 11.4% and 14.9% decreases in compressive strength, respectively. The result of the difference compared with the 5% POF specimen was more significant with a 20% decrease. 4.2. Flexural tensile strength results For the flexural tensile strength assays, the number of specimens used to calculate the average outcome was 20 (5 for each content value of 0%, 2%, 3.5% and 5% POF). The average results of tests conducted on day 28 are presented in Table 6, and the ANOVA results are shown in Table 7. In Fig. 10, a graph of the average flexural tensile strength results for all POF contents that were adopted can be seen. There are some advantages of the use of fibers in cementitious matrices, such as the following: decreased speed of crack propagation, better responses to dynamic stress and improved postcracking behavior (which causes relative plastic deformation), among others. The addition of fibers results in improved toughness and ductility in the mixtures; that is, after cracking, the material still has a residual resistance to the forces that are applied to it
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Table 3 Statistical analysis on the samples POF distribution. POF graphical distribution on the ‘X’ axis
POF graphical distribution on the ‘Y’ axis
2D particle distribution (overall)
2% POF
Measured average nearest neighbor distance = 1.04 mm Variance (mean) = 0.3626 StdDev (mean) = 0.6022 Sample size n = 1032 d.f. = 1508.0 t = 9.749341084446682 (Welch’s t-test) critical t-value = 3.098402156 confidence interval = 99.9% - –> clustering particles significant different from random distribution with p < 0.001
3.5% POF
Measured average nearest neighbor distance = 1.05 mm Variance (mean) = 0.2596 StdDev (mean) = 0.5095 Sample size n = 1500 d.f. = 2260.0 t = 2.299784114290956 (Welch’s t-test) critical t-value = 3.09431229 confidence interval = 99.9% - –> random particle distribution no significant difference to random distribution (p > 0.001)
5% POF
Measured average nearest neighbor distance = 0.77 mm Variance (mean) = 0.1988 StdDev (mean) = 0.4459 Sample size n = 2525 d.f. = 3772.0 t = 1.276813459206366 (Welch’s t-test) critical t-value = 3.092951196 confidence interval = 99.9% - –> random particle distribution no significant difference to random distribution (p > 0.001)
Table 4 Average results of the compressive strength tests on day 28. Arrangement of fibers
Fiber content (%)
Average resistance to compression (MPa)
Standard deviation
Coefficient of variation (%)
Random
0 2 3.5 5
59.90 53.06 50.96 47.99
2.13 2.09 3.28 1.53
3.56 3.93 6.44 3.19
Table 5 ANOVA (one-way) of the compressive strength of the specimens on day 28. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
384.468 88.171
3 16 19
128.156 5.511
23.26
0.00%
S
SS, sum of squares; DF, degrees of freedom; MF, mean sum of squares; F, test statistic; Probability F value, significance level of 5% (Fischer distribution); Comment S, significant effect and NS, not a significant effect.
[15]. In the flexural tensile strength tests, as well as in the research developed by Uribe [10], there was a worsening of the mechanical properties as the amount of optical fibers increased. The flexural tensile strength test results, as expected, showed greater variability than the results of the compressive strength tests due in part to the weakness of a sample to this type of stress. Accord-
ing to the ANOVA results (Table 7), the fiber content had a significant effect on the flexural tensile strength. Using Duncan´s statistical method (Fig. 10), it is observed that the 2% and 5% fiber contents and 3.5% and 5% fiber contents are considered statistically equal. Compared to the reference specimen (0% POF), the 2%, 3.5% and 5% POF contents presented decreases of 20.6%, 31.7% and 25.4%,
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Fig. 9. Average results of the compressive strength tests on day 28 and the statistical analysis results using Duncan´s method.
Table 6 Average results of the flexural tensile strength tests on day 28. Arrangement of fibers
Fiber content (%)
Average flexural tensile strength (MPa)
Standard deviation
Coefficient of variation (%)
Random
0 2 3.5 5
1.89 1.50 1.29 1.41
0.11 0.18 0.07 0.16
6.07 11.67 5.18 11.44
Table 7 ANOVA (one-way) of the flexural tensile strength of the specimens on day 28. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
1.006 0.297
3 16 19
0.335 0.019
18.06
0.00%
S
Fig. 10. Average results of the flexural tensile strength tests on day 28 and the statistical analysis results via Duncan´s method.
respectively, for the flexural tensile strength. Therefore, it can only be said that additions of fibers from the 2% POF upward decrease the flexural tensile strength compared to the specimens with 0% POF. However, there is no way to affirm that there is a statistically significant tendency to increase or decrease this resistance with the addition of fibers between 2% and 5% POF.
4.3. Capillary water absorption results For the capillary water absorption assays, the number of specimens used to calculate the average outcome was 40 (10 for each content value of 0%, 2%, 3.5% and 5% POF). The average results of tests performed on day 28 are shown in Table 8, and the ANOVA
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Table 8 Average results of the capillary water absorption tests on day 28. Arrangement of fibers
Fiber content (%)
Average water absorption by capillarity, C (g / dm2. min. ½)
Standard deviation
Coefficient of variation (%)
Random
0 2 3.5 5
0.022 0.068 0.076 0.088
0.016 0.032 0.039 0.024
74.69 46.97 51.47 27.13
Table 9 ANOVA (one-way) of the absorption of water via the capillarity of the specimens on day 28. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
0.012 0.014
3 16 19
0.004 0.001
4.91
1.32%
S
Fig. 11. Average results of the capillary water absorption tests on day 28 and the statistical analysis results via Duncan´s method.
results are given in Table 9. In Fig. 11, a graph of the average capillary water absorption results for all POF contents that were adopted can be seen. According to Duncan´s statistical analysis (Fig. 11) that was conducted from the ANOVA results (Table 9), except for the 0% POF samples, the 2%, 3,5% and 5% contents statistically belong to the same family. Only the 0% POF content was significantly different from the others. That is, any addition of polymeric optical fiber from 2% up to 5% differs from the reference specimen (0% POF) in the capillary water absorption tests. Compared to the reference specimen (0% POF), the 2%, 3.5% and 5% POF content samples presented increases of 309%, 345% and 400%, respectively, for the capillary water absorption. There are strong indications that among the main reasons for the decrease in the mechanical strength and the increase in the water absorption with the higher POF contents in the specimens is the fiber/matrix transition zone. Due to the slippery, smooth, and practically impermeable POF surface, this interaction has been impaired. The use of a low fiber density tends to generate less concentrated stress regions in the concrete that might eventually lead to a fracture [16]. The SEM tests verified the presence of voids in the fiber-cement interface (Fig. 12). This phenomenon may have contributed to a worsening of the fiber/matrix grip, possibly having a consequence on the compressive strength according to each fiber content level adopted. Since the best concrete performance is expected when the air void volume is as low as possible, it is essential to determine the
Fig. 12. SEM image of the fiber/matrix interface showing the existing voids. Magnification: 160 times.
influence of the fiber addition on the air void volume [17]. The microstructure of the interfacial transition zone, especially the amounts of the voids and the microcracks that are present, have a significant influence on the stiffness and durability of the concrete [14]. The reason for this is that a higher level of vulnerable bonding between the fiber surface and the cement matrix in a composite may be due, in part, to the existence of voids close to the interface [18]. These aspects may act as negative influences in the transition zone within the matrix. The increased water absorption and decreased mechanical strength of the LTCM samples reflect strong evidence of this.
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Thiago dos S. Henriques et al. / Construction and Building Materials 244 (2020) 118406 Table 10 Average light transmittance of the specimens in the hardened state and the comparative costs of LTCMs. Arrangement of fibers
Fiber content (%)
Average light transmittance (lux)
Standard deviation
Coefficient of variation (%)
The approximate total cost of m3 (LTCMs)*
$/ lux
Random
0 2 3.5 5
0 7.67 11.67 23.33
– 0.58 0.58 1.15
– 7.53 4.95 4.95
$ $ $ $
– 1.61 4.19 1.26
1 12.36 20.74 29.40
* As an important parameter in the framework of the comparative cost study, the real cost of the fibers (including shipment from China and VAT) was $0.90 for each 100 m/roll of PS-type POF with diameter thickness of 0,4 mm.
Table 11 ANOVA (one-way) of the light transmittance. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
852.667 7.333
3 8 11
284.22 0.500
568.44
0.00%
S
Fig. 13. Wooden device to quantify the transmittance of light: (A) apparatus design; (B) built apparatus.
One way to improve the microstructure (particularly with respect to the fiber / matrix interface) would be to use a sulfoaluminate cement instead of Portland cement, as reported by Yue et al. [8]. Sulfoaluminate cement has many advantages, such as high early strength, high ultimate strength, and low alkalinity, compared to Portland cement [8]. In addition, its expansive properties could be beneficial in the fiber / matrix transition zones. Another way to reduce poor interactions between the fibers and the cement mortar would be to use a coupling agent surface treatment such as silane. Silane coupling agents act in the interphase region, the area between an inorganic material (such as glass, metal or mineral) and an organic substrate (such as an organic polymer, coating or adhesive), and act as bonding or bridging agents to improve the adhesion between the two dissimilar materials [19]. However, the results of Yue et al. [8] showed that even with the use of a silane coupling agent, gaps were detected in the well-marked interface between the fibers and the matrix, which means that this surface treatment alternative on POFs cannot work very well. Therefore, it is necessary to develop a modified coupling agent that is specific for this type of use. 4.4. Light transmittance results For the light transmittance assays, the number of specimens used to calculate the average outcome was 20 (5 for each content value of 0%, 2%, 3.5% and 5% POF). The averages of the test results and the comparative costs of the LTCMs are presented in Table 10,
and the ANOVA results for the light transmittance are presented in Table 11. In Fig. 13, a graph of the average light transmittance results for all POF contents that are adopted can be seen. A wooden device was developed to analyze the transmittance of light through the specimens with various content values of POFs. Fig. 13 shows that the manufactured device had three main parts: a gap at the top for a light source (a bulb with a tungsten filament having an intensity of 100 W/110 V); an intermediate part with an opening for the placement of LTCM pieces; and a lower part reserved for the light transmittance measuring instrument (luxmeter). The light transmittance data for the 0% fiber content belonged to the molded test pieces without the addition of POFs. Using a sample with this content, the sealing efficiency of the apparatus that was developed in the research was tested. The results for this content showed that 0 LUX transmittance was obtained, thus confirming the accuracy of the seal. From the 2% to the 3.5% POF content samples, there was a 75% increase in the number of fibers and a transmittance gain of 52%. From the 3.5% to the 5% content samples, there was a 42% increase in the number of fibers, and consequently, the intensity of the lux doubled, which was practically a 100% increase. In terms of the material production costs, these data are essential, since between the 3.5% and 5% amounts of POFs, an LTCM with significantly better translucency with a small increase in the fiber content proves to be the most advantageous. As seen in the graph that was made from the ANOVA results of the light transmittance (Fig. 14), each of the POF levels that were
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Fig. 14. Light transmittance of the specimens with and without random fibers.
adopted belong to significantly different families. Compared to the 2% POF samples, the light transmittance of the 3.5% and 5% POF samples increased by 152% and 304%, respectively. Concerning the costs and taking as a reference the Brazilian market, a comparative study (Table 10) was made considering the light transmittance. The costs of the molds and the labor were not considered. As the introduction of the fibers and the effort needed to remove voids and bubbles were done by hand (requiring significant time), the cost of labor was not considered. For the production of LTCM pieces on a larger scale, the use of specific and automated equipment for the arrangement and insertion of the fibers is essential to obtain good productivity. Thus, we tried to show the best alternative considering the cost-benefit ratio. The cost of 1 m3 of concrete that was free of fiber (0% POF) was set at $ 1.00 as a reference. Specimens with content values higher than 5% proved to be extremely difficult to produce, as the high fiber density eventually caused mortar filling failures during molding, thus they are considered inappropriate for technical analysis. According to the table shown, although the 5% POF samples have the highest overall costs, when considering the $ / Lux rate, this content was the one that presented the best cost-benefit ratio. Therefore, this group (5% POF) can be considered to have the optimal fiber content in terms of the highest light transmittance and the most favorable costbenefit ratio. This is a reliable indicator for those who want to produce LTCMs on a larger scale when considering the light transmittance as the leading quality index.
5. Conclusions The addition of randomly arranged optical fibers in cementitious composites has, as its main objective, to meet the determined light transmittance requirements of civil construction. According to the experimental technical delimitations, increasing amounts of random POF were used in this work up to a limit of 5%. Therefore, some conclusions were made from this work. During the molding process, intensive care was given to make a high fluidity mortar, and this was also vibrated. However, it was found that this was still not sufficient to eliminate pores near the fibers. It can be said that the voids that were found around the fibers were not the result of a simple exudation, since in that case, the voids would be just under the fibers and not all around them. This phenomenon can be explained by the origin of the polymer fibers, because they are practically smooth and
without roughness. This effect of the fiber material may explain the appearance of the voids that, consequently, are the probable cause of the decrease in the mechanical strength and the increase in the capillary water absorption of the composites. Concerning the compressive strength tests, the samples presented a statistically significant worsening of their properties. The 2% and 3.5% POF contents were considered to belong to the same group statistically and had significant reductions of 11.4% and 14.9%, respectively, in their resistance compared to the reference specimen. Samples with 5% POF showed a significantly decreased strength of approximately 20% with respect to the 0% POF sample. The 5% sample is in a significantly different group and is worse than any of the other fiber content samples that were adopted. It can be said that the higher the POF content, the greater the loss of the compressive strength of the composite. For the flexural tensile strength tests, there was a statistically significant worsening in all families of samples containing fibers. Compared to the reference specimen (0% POF), there were strength decreases of 20.6%, 31.7% and 25.4% for the 2%, 3.5% and 5% POF levels, respectively. Duncan´s method shows that the limit of the loss of flexural tensile strength occurs between the 3.5% and 5% content samples, and these values are considered to be statistically equal. Since the tests for this type of stress normally present a greater variability compared to the compressive strength tests for cementitious composites, the behavior of the results that were obtained was already expected. Due to the direction in which the composite fibers were arranged, they were not effective at tensile structural reinforcement. With respect to the capillary water absorption, the 2%, 3.5% and 5% POF contents presented increases of 309%, 345%, and 400%, respectively, compared to the reference specimen (0% POF). All samples that included fibers were considered to be statistically equal to each other and significantly different from the samples without fibers. It can be said that the addition of fibers increases the porosity in the samples (according to the SEM analysis), and this may be linked to the increased water absorption. With respect to the light transmittance, all of the POF content samples were considered significantly different from the reference samples. That is, as the fibers were added, the light transmittance always significantly increased. For increased efficiency, the 5% POF content was shown to be the most advantageous by presenting an average light transmittance of
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23.33 lx, which was 100% higher than that of the 3.5% content. For large-scale productions, this behavior is fundamental since the main purpose of the material is to transmit light. In terms of the overall costs, since optical fiber is the most expensive element of the composite, its use must be rationalized. Thus, considering the Brazilian market, the cost difference between a fiber-free concrete (0% POF) and a concrete with the maximum fiber content that was used in this research (5% POF) was practically 30 times. The LTCM’s production costs, in general, are still high. However, considering the Brazilian market and the cost per lux ($ / lux) of the samples with the added optical fiber (2%, 3.5% and 5% POF), the 5% POF content was the most advantageous with a result of $ 1.26 / Lux. In general, it can be concluded that the addition of POF that was randomly distributed worsened the mechanical properties in the composite, but it did not make the material unsuitable (up to 5% POF content) for use in construction. Among the contents that were used, and considering the cost-benefit ratio, taking the light transmittance as the leading quality indicator, the LTCM with a 5% POF content proved to be the most advantageous choice. Acknowledgments The research described in this paper was financially supported by CAPES Brazil (a Foundation within the Ministry of Education in Brazil whose central purpose is to coordinate efforts to improve the quality of Brazil’s faculty and staff in higher education through grant programs) supported the research described in this paper. References [1] T. Henriques; D. Dal Molin.; A. Masuero. Study of the influence of sorted polymeric optical fibers (POFs) in samples of a light-transmitting cementbased material (LTCM). Constr. Build. Mater. 161 (2018) 305–315. Journal Elsevier. [2] http://www.litracon.hu/en.
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[3] T. Henriques, Analysis of the influence of polymer optical fiber inserted into mortar blocks. Master Thesis. Department of Civil Engineering, Federal University of Rio Grande do Sul –, Brazil (2013). [4] M. Di Prisco, G. Plizzari, L. Vandewalle, Fibre reinforced concrete: new design perspectives, Mater. Struct. 42 (2009) 1261–1281. Rilem. [5] Yue Li, Xu. Zhiyuan, Gu. Zhongwei, Research on the light transmitting cement mortar, Adv. Mater. Res. 450–451 (2012) 397–401. [6] Simonetta L. Pagliolico, Valerio R.M. Lo Verso, Annalisa Torta, Maurizio Giraud, Fulvio Canonico, Laura Ligi, A Preliminary Study on Light Transmittance Properties of Translucent Concrete Panels with Coarse Waste Glass Inclusions, Energy Procedia 78 (2015) 1811–1816, https://doi.org/10.1016/ j.egypro.2015.11.317. [7] A. Ahuja, K. Mosalam, Evaluating energy consumption saving from translucent concrete building envelope. Energy and Buildings 153, Journal Elsevier, 2017, pp. 448–460. [8] L. Yue, L. Jiaqi,G. Hui. Preparation and study of light transmitting properties of sulfoaluminate cement-based materials. Constr. Build. Mater. 83 (2015) 185– 192. Journal Elsevier. [9] Brazilian Association of Technical Standards (ABNT). NBR 13276: Mortars applied on walls and ceilings – Determination of the consistency. Index. Rio de Janeiro – Brazil, 2016 [10] L.J.M. Uribe, Development of the evaluation of translucent mortar with polymeric optical fiber. Master Thesis, Department of Civil Engineering, Federal University of Santa Catarina –, Brazil (2010). [11] Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 66 (45) (2007) 1237–1242, https://doi.org/10.1016/j.saa.2006.06.012. [12] Brazilian Association of Technical Standards (ABNT). NBR 13279: Mortar for laying and coating of walls and ceilings – determination of compressive and flexural tensile strength. Rio de Janeiro – Brazil, 2005 [13] Brazilian Association of Technical Standards (ABNT). NBR 15259: Mortar for laying and coating walls and ceilings – Determination of capillary water absorption and capillary coefficient. Rio de Janeiro – Brazil, 2003. [14] P.K. Mehta, P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials, 4th ed., McGraw-Hill Education, New York, N.Y., 2014. [15] A.D. Figueiredo, Fiber reinforced concrete. Polytechnic School of the University of Sao Paulo. University of Sao, Paulo – USP - Brazil. Free text Lecturer. (2011). [16] Yue L., Jiaqi L., Yuhong W., Zhiyuan X., Experimental study of light transmitting cement-based material (LTCM), Constr. Build. Mater. 96 (2015) 319–325. Journal Elsevier. [17] T. Ponikiewski, J. Katzer, M. Bugdol, M. Rudzki. Determination of 3D porosity in steel fibre reinforced SCC beams using X-ray computed tomography. Constr. Build. Mater. 68 (2014) 333–340. Journal Elsevier. [18] S. Zhandarov, E. Mader, Characterization of fiber/matrix interface strength: applicability of different tests, approaches and parameters, Elsevier, Composites and Science Technology. Journal, 2005. [19] S. Surface Ebnesajjad, Treatment of Materials for Adhesive Bonding, Elsevier Book. (2014).
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Optical fibers in cementitious composites (LTCM): Analysis and discussion of their influence when randomly arranged Thiago dos S. Henriques ⇑, Denise C. Dal Molin, Ângela B. Masuero Department of Civil Engineering, Federal University of Rio Grande do Sul, Brazil
h i g h l i g h t s Polymeric optical fibers (POFs) were distributed in a random manner for the production of LTCMs. The mechanical properties of the LTCMs decreased due to increases in their fiber contents. With respect to the cost-benefit ratio, the 5% POF content samples proved to be the most advantageous.
a r t i c l e
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Article history: Received 25 June 2019 Received in revised form 3 February 2020 Accepted 10 February 2020
Keywords: Cost-benefit ratio LTCM (light transmittance cement-based material) POF in a random manner
a b s t r a c t Published studies related to LTCMs (light-transmitting cement-based materials) deserve attention. Although it is a widely used commercial material, in the technical-scientific field, there is still a demand for a deeper understanding of its performance characteristics. The data presented here refer to samples with three different fiber contents (2%, 3.5%, and 5%) that were arranged randomly and compared to the reference samples that were free of polymeric optical fiber (0% POF). This work complements a related article that was previously published by the authors [1] that addresses the influence of fibers that are arranged in an orderly manner. The performance characteristics of the compressive strength, the flexural tensile strength, the water absorption via capillarity, the light transmittance, and the cost-benefit ratio (considering the Brazilian market) were verified. From the 0% (reference) to 5% (maximum POF content adopted) fiber contents, the following behaviors were found: decreases of approximately 20% and 30% in the compressive and flexural tensile strength, respectively, and an increase of 400% in the capillary water absorption. For light transmittance, from the 2% to the 5% POF contents, there was an increase of 150% in the fibers and an increase of approximately 300% in the light transmittance. Considering the Brazilian market and the cost per lux ($ / lux) of the samples with added optical fibers (2%, 3.5% and 5% POF), the following values were obtained: $ 1.61/ lux (2% POF), $ 4.19/lux (3.5% POF), and $ 1.26/lux (5% POF). Based on the cost/light transmittance ratio, the 5% POF content was the best. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The research and development of innovative materials that are applied in civil construction always deserve discussion and dissemination, not only in congresses but also in recognized scientific publications. This article is a complement to another report [1] that was previously published by the same authors in this same journal. The use of LTCMs (light-transmitting cement materials), originally developed in Hungary in 2002 and patented under the name LiTraCon [2], has been widely and commercially applied, but there is ⇑ Corresponding author at: Av. Osvaldo Aranha 99, 3°andar, Porto Alegre, RS 90035-190 Brazil. E-mail addresses: [email protected] (T.d.S. Henriques), [email protected] (D.C. Dal Molin), [email protected] (Â.B. Masuero). https://doi.org/10.1016/j.conbuildmat.2020.118406 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
still a need for a deeper understanding of its technical knowledge before it is applied, for instance, in the structural parts of a building. In the manufacture of LTCMs, the use of optical fibers is essential regarding light transmittance properties; however, POF can affect both the mechanical and durability properties of the composites. In general, steel and glass fibers, which are the most commonly used reinforcements in cementitious matrices [3], function by increasing the ductility due to the fiber reinforcement mechanisms provided by the fibers bridging the crack surfaces [4]. LTCMs greatly enhance the lighting effects, reduce the lighting energy consumption, and promote the energy conservation of buildings [5]. Translucent panels made of concrete using natural lighting have a unique appeal in several aspects such as improved moods, acceptance, and the visual comfort of the occupants [6]. Translucent concrete as a building envelope can offset some lighting
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Fig. 1. Aesthetic appearance of samples with POFs disposed in a random manner.
energy that is consumed within a room in an office [7]. Its light transmitting properties depend on a large number of optical fibers that transmit light through the cement-based product [8]. Our previous work focused on LTCMs at four POF (polymeric optical fiber) content values (0%, 2%, 3.5%, and 5%) in which the fibers were arranged in an orderly manner. The present article assesses the same material and fiber contents, but now they are arranged randomly (Fig. 1). Therefore, often in this text, some technical data of the used materials will refer to this previous work [1]. In addition to the compressive strength, flexural tensile strength, capillary water absorption, and light transmittance data, this work adds the issue of the cost-benefit ratio using the Brazilian construction market as the reference. This cost assessment has the purpose of assisting the choice between different POF content values in LTCMs, by pointing out the one with the best cost-benefit ratio. Compared to the samples prepared with ordered fibers, the random distribution of fibers in a LTCM may have an implication for the cost since its production is simpler in relation to the design, manpower and equipment. Similarly, it may be assumed that the material performance is lower due to its own experimental variability, which may cause fibers to accumulate at some points and be lacking at others. Therefore, this study aims to determine if composites with random fibers (that have theoretically lower production costs) are statistically equal, better or worse than composites with ordered fibers in terms of their mechanical performance and durability. An important point to be clarified in this study is that the term ‘‘random” as used in the text refers to samples with fibers arranged (in a fresh mortar) in thin layers in a poorly controlled manner, that is, tending to have a random arrangement. During the vibra-
tion process, some samples had greater fiber movement than others, thus losing the visual effect of layered fibers. Therefore, there was no control over the fiber spacing, but only their quantity, to ensure the correct content values. Statistical analysis in order to provide how random the fiber distribution is also presented.
2. Materials and methods The LTCM samples were produced using Portland cement, water, a fine aggregate (sand), a polycarboxylate ether-based superplasticizer, polymeric optical fibers, and silica fume. The materials are briefly described below, and more detailed information can be found in the literature [1]. CP V ARI type cement was chosen (it is equivalent to Portland cement with a high early strength III - ASTM C150). The sand (with a quartz origin) used was from a Brazilian riverbed. A polycarboxylate ether-based superplasticizer (PCE) was added to the mixtures to act as a dispersant of the cementitious material and to provide high water reduction. The ratio of this material to the cement mass was 1.0%. A percentage of 10% silica fume, replacing part of the mass of cement, was used to improve the performance (to reduce the permeability, to increase the cohesion and to avoid segregation, among other reasons) of the material. The adopted mixture ratio, in mass, was 1:0.11:2.22:0.5 (cement : silica fume : sand : water). Following the procedures of the Brazilian standard [9], consistency index tests were performed on the reference mortars (Fig. 2). The aim was to obtain a very fluid base mortar (0% POF) that was practically self-compacting to fill all the spaces between the fibers (2%, 3.5%, and 5% POF) that were subsequently inserted.
Fig. 2. Consistency index determination test: (A) mold filling with mortar, and (B) measurement of the spreading of the mixture.
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Fig. 3. Infrared spectrum of the POF.
2.1. POF (polymeric optical fiber) As the optical fibers acquired for this research in Brazil originated from China, it was not possible to obtain any physicochemical characterization from the manufacturer. Thus, it was necessary to use infrared spectroscopy to obtain a standard power spectrum and compare it with the results of other surveys. The core raw material of POFs generally consists of PS (polystyrene), PC (polycarbonate) or PMMA (polymethyl methacrylate) with very high refractive index values, whereas the coating is made with low refractive index polymers [10]. The characterization of the POFs was made using a PerkinElmer Spectrum BX FTIR (São Paulo, SP – Brazil). Initially, the POF samples were cut to approximately 3 cm in length and rinsed with acetone to remove any grease or dirt due to handling. They were then placed in the spectrometer for their analysis. At the beginning of the assay, an infrared reading was performed without the POF sample to subtract the cell spectrum from the final result. Subsequently, 10 successive readings were performed on the same sample to obtain a spectrum with high fidelity. A typical result can be seen in Fig. 3, and the wavebands are given in Table 1. PMMA is the most common material used in the manufacture of POFs [10]. However, comparing the POF infrared spectrum used in this study with the spectrum of a PS-type POF acquired by Uribe [10] and the spectrum of a PMMA-type POF obtained by Ramesh et al. [11], it can be said that it is likely that the fiber polymer used was a PS. In the qualitative characterization, the surface analysis using an SEM (scanning electron microscope) showed that it has a smooth surface with nearly no manufacturing defects (microcracks, diameter variation within the same filament, and roughness, among other aspects), as seen in Fig. 4. The diameter had an average thickness of 0.4 mm (400 lm). The calculation procedures used for the three POF content values (2%, 3.5%, and 5%) took into account the relationship between the fiber area exposed to light and the surface area translucence of the LTCM.
Table 1 Analysis of the infrared spectrum bands of the POF. Spot
Wave number (cm
A B C D E F G H I J K L M
694.55 751.45 840.80 906.60 987.76 1146.34 1191.44 1238.35 1384.11 1448.76 1599.51 1724.68 2949.08
1
)
Fig. 4. POF surface texture obtained by SEM. Magnification: 75 times.
To perform an assessment of the mechanical properties of the POF, a tensile test according to ASTM D3822/99 was run with a displacement rate of 1 mm/min. The assay was performed on an Instron 5585H testing machine. Five specimens with free length values of 200 mm were subjected to the tensile test. Table 2 shows the POF tensile properties.
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Table 2 Individual tests of the tensile strengths of the POFs [1]. Sample 1 2 3 4 5 average standard deviation coefficient of variation
Maximum load (N) 10.93 10.95 9.93 10.37 13.70 11.18 1.32 11.79%
Resistance limit (MPa) 120.37 107.58 109.38 114.26 109.05 112.13 4.69% 4.19%
The refractive index and transmission rate of the fibers were not analyzed for the main reason that LTCM pieces for civil construction have, on average, a thickness between 0.10 m and 0.50 m, that is, a very short distance with no significant transmittance losses.
Fig. 6. Device, developed in this research, for vibration by immersion in the mortar.
3. Experimental program The characteristics of the compressive strength, tensile strength, light transmittance, and capillary water absorption were compared among the LTCM samples (2%, 3.5% and 5% fiber content values) and the reference sample using mortar that was free of POF (0%). All the assays were carried out on day 28. The same mortar mix proportions (except for the POF) were used for all of the samples. 3.1. Preparation of the LTCM sets Fig. 7. Block cutting by a saw.
During the molding process, a layer of mortar and a layer of fiber were repeatedly poured into the mold until it was filled (Fig. 5). For filling the voids within the mortar and the subsequent elimination of air bubbles, a vibrating platform was used; however, it was not efficient. Therefore, a vibrating immersion device was developed consisting of a mini rotor powered by two AA size batteries with a metallic rod tip and manual control of the vibration intensity. This device was used to eliminate as many air bubbles in the mortar as possible (Fig. 6). The duration of the vibration for all molded blocks was approximately 45 s, which was enough time for the air bubbles to stop rising to the surface and to avoid any segregation. This was a careful and necessary procedure, since even when using a suitable mortar with high fluidity, there was some difficulty in molding the samples. The main challenge of this step was to ensure the correct fiber envelopment by the mortar. The LTCM sample demolding procedure was performed twentyfour hours after the preparation of the samples and subsequent saw cuts were made (Fig. 7). From each large block, five prismatic samples (4x4x16 cm) were generated; that is, for each fiber content (including 0% POF) five specimens were produced. Each sam-
ple was subjected to all four measurement tests in the following order: 1st light transmittance (non destructive); 2nd flexural tensile strength (destructive, generating two halves of each sample); 3rd capillary water absorption (non destructive); 4th compressive strength (destructive). All of the LTCM specimens remained in an environment with controlled relative humidity (95 ± 5) % and temperature (23 ± 2) °C until the performance tests on day 28. 3.2. Test methods: Compressive strength, flexural tensile strength, capillary water absorption, and light transmittance Since the specimens have randomly positioned fibers, the compressive strength tests and the flexural tensile strength tests sought to reproduce the behavior of their use as LTCMs. Since walls may tend to receive accidental horizontal loads (impacts) or even buckle (if they are very slim), the flexural tensile assay was executed by applying a load that was parallel to the fibers. In Fig. 8, an illustration exhibiting the direction of the applied loads concerning the positioning of the fibers is presented.
Fig. 5. ‘‘Layer” molding of the samples with random fibers: (A) placement of the POF layer, and (B) placement of the mortar layer.
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Fig. 8. Diagram showing the direction of the applied loads concerning the fibers of the samples.
For the flexural tensile strength tests and the compressive strength tests, loading applications of, respectively, 50 ± 10 N/s and 500 ± 50 N/s were applied to the specimens according to the Brazilian mortar standard [12]. The capillary water absorption assays were carried out according to the Brazilian standard [13] as well. The remaining two halves of the flexural tensile test specimens on day 28 were used. In this test, the samples are placed in a water bath with a depth of approximately 5 ± 1 mm. The mass comparison is done with the data obtained before and after 90 min of the material being in contact with the water. [13]. For the light transmittance evaluation tests, the same wooden device of our previous work [1] was used to evaluate the transmittance of light that occurred with the specimens with different levels of POFs. A further explanation can be found in [1]. 3.3. Statistical analysis on the fiber distribution For the statistical analysis on the fiber distribution of the samples in the hardened state, the software Image J was used. The number of specimens used to calculate the average outcome was 6 (2 for each content value of 2%, 3.5% and 5% POF). Fiber distribution has a purpose to determine if particles in a 2D image are likely to be randomly distributed, self-avoiding or build clusters. Starting with a binary image and through the Bio Voxxel plug-in, the evaluation was based on the mean nearest neighbor distance and the confidence interval of the statistical evaluation adopted was 99.9%. The average results are shown in Table 3. Among the 3 groups of samples with fibers (2%, 3.5% and 5% POF), according to the statistical analysis performed, 2 types of fiber organization were formed: fiber randomly distributed and build clusters. For the 2% POF content, the fibers were statistically organized as build clusters. This behavior can be attributed to the method of sample execution (layer of mortar and a layer of fiber repeatedly poured into the mold until it was filled), as shown in Fig. 5. As the lowest content effectively adopted (2% POF), it is possible to observe in Fig. 1(B) that there is actually a visible trend of fiber layer formation. For the other contents (3.5% and 5% POF), due to the greater amount of fibers, they tended to mix more easily so that, statistically, they can be considered as randomly arranged. 4. Results and discussion The results of the tests that were described in the experimental program were subjected to statistical analysis, and all of these results are presented in the form of tables and graphs. On the bottom line of each type of assay can be found a summary of the significant factors that were obtained through ANOVA (analysis of
5
variance) and Duncan´s method. The one-way ANOVA test evaluates the difference between a single dependent quantitative variable (randomly arranged fibers) against the average of the results of the four groups that were formed by the categories of a single independent variable (the fiber content). All hypotheses tested were evaluated at a confidence level of 95%. Although it is a handy tool, the analysis of variance only reports whether or not there is a significant difference between the groups that are analyzed but does not address the difference between each group. Thus, with a pre-established guarantee that a particular conclusion is correct; the individual results were analyzed by Duncan´s method. Duncan’s method takes into consideration the examination of individual outcomes in an incorporated arrangement of determinations with a pre-set up assurance that this result is correct. This multiple comparison procedure was used to determine which groups (0%, 2%, 3.5% and 5% POF) are significantly different from each other at the confidence limit applied. The means are considered significantly different according to the established significance level of 0.05. 4.1. Compressive strength test results For the compressive strength assays the number of specimens used to calculate the average outcome was 40 (10 for each content value of 0%, 2%, 3.5% and 5% POF). The average results of tests performed on day 28 are shown in Table 4, and the ANOVA results can be found in Table 5. In Fig. 9, a graph of the average compressive strength results for all POF contents that were adopted can be seen. The response of a concrete sample to an applied stress depends not only on the stress type but also on how a combination of various factors affects the porosity of the different structural components of the concrete, including the following: the properties and proportions of the materials that make up the concrete mixture, the degree of compaction and the curing conditions [14]. According to the ANOVA evaluation, the fiber percentage had a significant effect on the compressive strength. Since random fibers are inserted into the composites, the composites tend to have their compressive strength decreased. Using Duncan’s method, it can be seen that the compressive strength values of the samples with a content of 0% POF are significantly different from those of the samples with other fiber contents. In addition, according to the same method of analysis, only the 2% and the 3.5% POF groups have no significant difference in the results for the compressive strength. Compared to the 0% POF content sample, the samples with 2% and 3.5% POF experienced 11.4% and 14.9% decreases in compressive strength, respectively. The result of the difference compared with the 5% POF specimen was more significant with a 20% decrease. 4.2. Flexural tensile strength results For the flexural tensile strength assays, the number of specimens used to calculate the average outcome was 20 (5 for each content value of 0%, 2%, 3.5% and 5% POF). The average results of tests conducted on day 28 are presented in Table 6, and the ANOVA results are shown in Table 7. In Fig. 10, a graph of the average flexural tensile strength results for all POF contents that were adopted can be seen. There are some advantages of the use of fibers in cementitious matrices, such as the following: decreased speed of crack propagation, better responses to dynamic stress and improved postcracking behavior (which causes relative plastic deformation), among others. The addition of fibers results in improved toughness and ductility in the mixtures; that is, after cracking, the material still has a residual resistance to the forces that are applied to it
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Table 3 Statistical analysis on the samples POF distribution. POF graphical distribution on the ‘X’ axis
POF graphical distribution on the ‘Y’ axis
2D particle distribution (overall)
2% POF
Measured average nearest neighbor distance = 1.04 mm Variance (mean) = 0.3626 StdDev (mean) = 0.6022 Sample size n = 1032 d.f. = 1508.0 t = 9.749341084446682 (Welch’s t-test) critical t-value = 3.098402156 confidence interval = 99.9% - –> clustering particles significant different from random distribution with p < 0.001
3.5% POF
Measured average nearest neighbor distance = 1.05 mm Variance (mean) = 0.2596 StdDev (mean) = 0.5095 Sample size n = 1500 d.f. = 2260.0 t = 2.299784114290956 (Welch’s t-test) critical t-value = 3.09431229 confidence interval = 99.9% - –> random particle distribution no significant difference to random distribution (p > 0.001)
5% POF
Measured average nearest neighbor distance = 0.77 mm Variance (mean) = 0.1988 StdDev (mean) = 0.4459 Sample size n = 2525 d.f. = 3772.0 t = 1.276813459206366 (Welch’s t-test) critical t-value = 3.092951196 confidence interval = 99.9% - –> random particle distribution no significant difference to random distribution (p > 0.001)
Table 4 Average results of the compressive strength tests on day 28. Arrangement of fibers
Fiber content (%)
Average resistance to compression (MPa)
Standard deviation
Coefficient of variation (%)
Random
0 2 3.5 5
59.90 53.06 50.96 47.99
2.13 2.09 3.28 1.53
3.56 3.93 6.44 3.19
Table 5 ANOVA (one-way) of the compressive strength of the specimens on day 28. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
384.468 88.171
3 16 19
128.156 5.511
23.26
0.00%
S
SS, sum of squares; DF, degrees of freedom; MF, mean sum of squares; F, test statistic; Probability F value, significance level of 5% (Fischer distribution); Comment S, significant effect and NS, not a significant effect.
[15]. In the flexural tensile strength tests, as well as in the research developed by Uribe [10], there was a worsening of the mechanical properties as the amount of optical fibers increased. The flexural tensile strength test results, as expected, showed greater variability than the results of the compressive strength tests due in part to the weakness of a sample to this type of stress. Accord-
ing to the ANOVA results (Table 7), the fiber content had a significant effect on the flexural tensile strength. Using Duncan´s statistical method (Fig. 10), it is observed that the 2% and 5% fiber contents and 3.5% and 5% fiber contents are considered statistically equal. Compared to the reference specimen (0% POF), the 2%, 3.5% and 5% POF contents presented decreases of 20.6%, 31.7% and 25.4%,
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Fig. 9. Average results of the compressive strength tests on day 28 and the statistical analysis results using Duncan´s method.
Table 6 Average results of the flexural tensile strength tests on day 28. Arrangement of fibers
Fiber content (%)
Average flexural tensile strength (MPa)
Standard deviation
Coefficient of variation (%)
Random
0 2 3.5 5
1.89 1.50 1.29 1.41
0.11 0.18 0.07 0.16
6.07 11.67 5.18 11.44
Table 7 ANOVA (one-way) of the flexural tensile strength of the specimens on day 28. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
1.006 0.297
3 16 19
0.335 0.019
18.06
0.00%
S
Fig. 10. Average results of the flexural tensile strength tests on day 28 and the statistical analysis results via Duncan´s method.
respectively, for the flexural tensile strength. Therefore, it can only be said that additions of fibers from the 2% POF upward decrease the flexural tensile strength compared to the specimens with 0% POF. However, there is no way to affirm that there is a statistically significant tendency to increase or decrease this resistance with the addition of fibers between 2% and 5% POF.
4.3. Capillary water absorption results For the capillary water absorption assays, the number of specimens used to calculate the average outcome was 40 (10 for each content value of 0%, 2%, 3.5% and 5% POF). The average results of tests performed on day 28 are shown in Table 8, and the ANOVA
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Table 8 Average results of the capillary water absorption tests on day 28. Arrangement of fibers
Fiber content (%)
Average water absorption by capillarity, C (g / dm2. min. ½)
Standard deviation
Coefficient of variation (%)
Random
0 2 3.5 5
0.022 0.068 0.076 0.088
0.016 0.032 0.039 0.024
74.69 46.97 51.47 27.13
Table 9 ANOVA (one-way) of the absorption of water via the capillarity of the specimens on day 28. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
0.012 0.014
3 16 19
0.004 0.001
4.91
1.32%
S
Fig. 11. Average results of the capillary water absorption tests on day 28 and the statistical analysis results via Duncan´s method.
results are given in Table 9. In Fig. 11, a graph of the average capillary water absorption results for all POF contents that were adopted can be seen. According to Duncan´s statistical analysis (Fig. 11) that was conducted from the ANOVA results (Table 9), except for the 0% POF samples, the 2%, 3,5% and 5% contents statistically belong to the same family. Only the 0% POF content was significantly different from the others. That is, any addition of polymeric optical fiber from 2% up to 5% differs from the reference specimen (0% POF) in the capillary water absorption tests. Compared to the reference specimen (0% POF), the 2%, 3.5% and 5% POF content samples presented increases of 309%, 345% and 400%, respectively, for the capillary water absorption. There are strong indications that among the main reasons for the decrease in the mechanical strength and the increase in the water absorption with the higher POF contents in the specimens is the fiber/matrix transition zone. Due to the slippery, smooth, and practically impermeable POF surface, this interaction has been impaired. The use of a low fiber density tends to generate less concentrated stress regions in the concrete that might eventually lead to a fracture [16]. The SEM tests verified the presence of voids in the fiber-cement interface (Fig. 12). This phenomenon may have contributed to a worsening of the fiber/matrix grip, possibly having a consequence on the compressive strength according to each fiber content level adopted. Since the best concrete performance is expected when the air void volume is as low as possible, it is essential to determine the
Fig. 12. SEM image of the fiber/matrix interface showing the existing voids. Magnification: 160 times.
influence of the fiber addition on the air void volume [17]. The microstructure of the interfacial transition zone, especially the amounts of the voids and the microcracks that are present, have a significant influence on the stiffness and durability of the concrete [14]. The reason for this is that a higher level of vulnerable bonding between the fiber surface and the cement matrix in a composite may be due, in part, to the existence of voids close to the interface [18]. These aspects may act as negative influences in the transition zone within the matrix. The increased water absorption and decreased mechanical strength of the LTCM samples reflect strong evidence of this.
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Thiago dos S. Henriques et al. / Construction and Building Materials 244 (2020) 118406 Table 10 Average light transmittance of the specimens in the hardened state and the comparative costs of LTCMs. Arrangement of fibers
Fiber content (%)
Average light transmittance (lux)
Standard deviation
Coefficient of variation (%)
The approximate total cost of m3 (LTCMs)*
$/ lux
Random
0 2 3.5 5
0 7.67 11.67 23.33
– 0.58 0.58 1.15
– 7.53 4.95 4.95
$ $ $ $
– 1.61 4.19 1.26
1 12.36 20.74 29.40
* As an important parameter in the framework of the comparative cost study, the real cost of the fibers (including shipment from China and VAT) was $0.90 for each 100 m/roll of PS-type POF with diameter thickness of 0,4 mm.
Table 11 ANOVA (one-way) of the light transmittance. SOURCE
SS
DF
MS
F
Probability
Comment
A: Fiber content Error TOTAL
852.667 7.333
3 8 11
284.22 0.500
568.44
0.00%
S
Fig. 13. Wooden device to quantify the transmittance of light: (A) apparatus design; (B) built apparatus.
One way to improve the microstructure (particularly with respect to the fiber / matrix interface) would be to use a sulfoaluminate cement instead of Portland cement, as reported by Yue et al. [8]. Sulfoaluminate cement has many advantages, such as high early strength, high ultimate strength, and low alkalinity, compared to Portland cement [8]. In addition, its expansive properties could be beneficial in the fiber / matrix transition zones. Another way to reduce poor interactions between the fibers and the cement mortar would be to use a coupling agent surface treatment such as silane. Silane coupling agents act in the interphase region, the area between an inorganic material (such as glass, metal or mineral) and an organic substrate (such as an organic polymer, coating or adhesive), and act as bonding or bridging agents to improve the adhesion between the two dissimilar materials [19]. However, the results of Yue et al. [8] showed that even with the use of a silane coupling agent, gaps were detected in the well-marked interface between the fibers and the matrix, which means that this surface treatment alternative on POFs cannot work very well. Therefore, it is necessary to develop a modified coupling agent that is specific for this type of use. 4.4. Light transmittance results For the light transmittance assays, the number of specimens used to calculate the average outcome was 20 (5 for each content value of 0%, 2%, 3.5% and 5% POF). The averages of the test results and the comparative costs of the LTCMs are presented in Table 10,
and the ANOVA results for the light transmittance are presented in Table 11. In Fig. 13, a graph of the average light transmittance results for all POF contents that are adopted can be seen. A wooden device was developed to analyze the transmittance of light through the specimens with various content values of POFs. Fig. 13 shows that the manufactured device had three main parts: a gap at the top for a light source (a bulb with a tungsten filament having an intensity of 100 W/110 V); an intermediate part with an opening for the placement of LTCM pieces; and a lower part reserved for the light transmittance measuring instrument (luxmeter). The light transmittance data for the 0% fiber content belonged to the molded test pieces without the addition of POFs. Using a sample with this content, the sealing efficiency of the apparatus that was developed in the research was tested. The results for this content showed that 0 LUX transmittance was obtained, thus confirming the accuracy of the seal. From the 2% to the 3.5% POF content samples, there was a 75% increase in the number of fibers and a transmittance gain of 52%. From the 3.5% to the 5% content samples, there was a 42% increase in the number of fibers, and consequently, the intensity of the lux doubled, which was practically a 100% increase. In terms of the material production costs, these data are essential, since between the 3.5% and 5% amounts of POFs, an LTCM with significantly better translucency with a small increase in the fiber content proves to be the most advantageous. As seen in the graph that was made from the ANOVA results of the light transmittance (Fig. 14), each of the POF levels that were
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Fig. 14. Light transmittance of the specimens with and without random fibers.
adopted belong to significantly different families. Compared to the 2% POF samples, the light transmittance of the 3.5% and 5% POF samples increased by 152% and 304%, respectively. Concerning the costs and taking as a reference the Brazilian market, a comparative study (Table 10) was made considering the light transmittance. The costs of the molds and the labor were not considered. As the introduction of the fibers and the effort needed to remove voids and bubbles were done by hand (requiring significant time), the cost of labor was not considered. For the production of LTCM pieces on a larger scale, the use of specific and automated equipment for the arrangement and insertion of the fibers is essential to obtain good productivity. Thus, we tried to show the best alternative considering the cost-benefit ratio. The cost of 1 m3 of concrete that was free of fiber (0% POF) was set at $ 1.00 as a reference. Specimens with content values higher than 5% proved to be extremely difficult to produce, as the high fiber density eventually caused mortar filling failures during molding, thus they are considered inappropriate for technical analysis. According to the table shown, although the 5% POF samples have the highest overall costs, when considering the $ / Lux rate, this content was the one that presented the best cost-benefit ratio. Therefore, this group (5% POF) can be considered to have the optimal fiber content in terms of the highest light transmittance and the most favorable costbenefit ratio. This is a reliable indicator for those who want to produce LTCMs on a larger scale when considering the light transmittance as the leading quality index.
5. Conclusions The addition of randomly arranged optical fibers in cementitious composites has, as its main objective, to meet the determined light transmittance requirements of civil construction. According to the experimental technical delimitations, increasing amounts of random POF were used in this work up to a limit of 5%. Therefore, some conclusions were made from this work. During the molding process, intensive care was given to make a high fluidity mortar, and this was also vibrated. However, it was found that this was still not sufficient to eliminate pores near the fibers. It can be said that the voids that were found around the fibers were not the result of a simple exudation, since in that case, the voids would be just under the fibers and not all around them. This phenomenon can be explained by the origin of the polymer fibers, because they are practically smooth and
without roughness. This effect of the fiber material may explain the appearance of the voids that, consequently, are the probable cause of the decrease in the mechanical strength and the increase in the capillary water absorption of the composites. Concerning the compressive strength tests, the samples presented a statistically significant worsening of their properties. The 2% and 3.5% POF contents were considered to belong to the same group statistically and had significant reductions of 11.4% and 14.9%, respectively, in their resistance compared to the reference specimen. Samples with 5% POF showed a significantly decreased strength of approximately 20% with respect to the 0% POF sample. The 5% sample is in a significantly different group and is worse than any of the other fiber content samples that were adopted. It can be said that the higher the POF content, the greater the loss of the compressive strength of the composite. For the flexural tensile strength tests, there was a statistically significant worsening in all families of samples containing fibers. Compared to the reference specimen (0% POF), there were strength decreases of 20.6%, 31.7% and 25.4% for the 2%, 3.5% and 5% POF levels, respectively. Duncan´s method shows that the limit of the loss of flexural tensile strength occurs between the 3.5% and 5% content samples, and these values are considered to be statistically equal. Since the tests for this type of stress normally present a greater variability compared to the compressive strength tests for cementitious composites, the behavior of the results that were obtained was already expected. Due to the direction in which the composite fibers were arranged, they were not effective at tensile structural reinforcement. With respect to the capillary water absorption, the 2%, 3.5% and 5% POF contents presented increases of 309%, 345%, and 400%, respectively, compared to the reference specimen (0% POF). All samples that included fibers were considered to be statistically equal to each other and significantly different from the samples without fibers. It can be said that the addition of fibers increases the porosity in the samples (according to the SEM analysis), and this may be linked to the increased water absorption. With respect to the light transmittance, all of the POF content samples were considered significantly different from the reference samples. That is, as the fibers were added, the light transmittance always significantly increased. For increased efficiency, the 5% POF content was shown to be the most advantageous by presenting an average light transmittance of
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23.33 lx, which was 100% higher than that of the 3.5% content. For large-scale productions, this behavior is fundamental since the main purpose of the material is to transmit light. In terms of the overall costs, since optical fiber is the most expensive element of the composite, its use must be rationalized. Thus, considering the Brazilian market, the cost difference between a fiber-free concrete (0% POF) and a concrete with the maximum fiber content that was used in this research (5% POF) was practically 30 times. The LTCM’s production costs, in general, are still high. However, considering the Brazilian market and the cost per lux ($ / lux) of the samples with the added optical fiber (2%, 3.5% and 5% POF), the 5% POF content was the most advantageous with a result of $ 1.26 / Lux. In general, it can be concluded that the addition of POF that was randomly distributed worsened the mechanical properties in the composite, but it did not make the material unsuitable (up to 5% POF content) for use in construction. Among the contents that were used, and considering the cost-benefit ratio, taking the light transmittance as the leading quality indicator, the LTCM with a 5% POF content proved to be the most advantageous choice. Acknowledgments The research described in this paper was financially supported by CAPES Brazil (a Foundation within the Ministry of Education in Brazil whose central purpose is to coordinate efforts to improve the quality of Brazil’s faculty and staff in higher education through grant programs) supported the research described in this paper. References [1] T. Henriques; D. Dal Molin.; A. Masuero. Study of the influence of sorted polymeric optical fibers (POFs) in samples of a light-transmitting cementbased material (LTCM). Constr. Build. Mater. 161 (2018) 305–315. Journal Elsevier. [2] http://www.litracon.hu/en.
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