sustainability

Article Evaluation of Mechanical Properties of Concrete Reinforced with Eucalyptus globulus Bark Fibres

Claudia Mansilla 1, Mauricio Pradena 1,2,* , Cecilia Fuentealba 2 and Andrés César 1 1 Departamento de Ingeniería Civil, Universidad de Concepción, Edmundo Larenas 219, Casilla 160-C Correo3, Concepción 4030000, Chile; [email protected] (C.M.); [email protected] (A.C.) 2 Unidad de Desarrollo Tecnológico, UDT, Universidad de Concepción, Casilla 4051, Concepción 4030000, Chile; [email protected] * Correspondence: [email protected]



Received: 14 September 2020; Accepted: 12 November 2020; Published: 1 December 2020


Abstract: Concrete is a material with high compressive strength, but predisposed to shrinkage cracking, rapid cracks propagation, and brittle failures. The incorporation of fibre is an acceptable solution to reduce these limitations. However, high cost and energy consumption related to man-made fibres have placed natural fibres as an attractive sustainable alternative, especially considering that different natural fibres are industrial waste (as the Eucalyptus globulus bark fibre). Still, natural fibres can produce an important reduction of concrete strength. Hence, the objective of this study is to evaluate the effects of Eucalyptus globulus bark fibre in traditional concrete mechanical properties as compressive and flexural strength. For this, an experimental program was developed in such a way that reduces the results uncertainties and increases the power of decision regarding the percentage and fibre conditions of the samples. The results indicate that, unlike other natural fibres, the traditional mechanical properties have a slight reduction and acceptable workability. This fact is more evident in the samples with 0.50% fibre with respect to the weight of cement. Therefore, reinforcing mortars and concrete with Eucalyptus globulus bark fibres emerges as an eco-friendly building alternative to reuse this industrial waste.

Keywords: compressive strength; Eucalyptus globulus; flexural strength; mechanical properties; natural fibre; reinforced concrete; sustainability

1. Introduction Globally, concrete is one of the most used materials in the construction industry, with an estimating production of 30 billion tons per year [1]. Despite being a material known for its high compressive strength [2], the concrete material is predisposed to shrinkage cracking, rapid cracks propagation, and brittle failures [3,4]. In this context, the addition of fibres in the concrete mixture has proven to be an acceptable solution to minimize these limitations [5–7]. In effect, fibres improve the post-cracking behaviour and the residual strength of concrete, which is important for the overall performance of concrete members under monotonic and cyclic loading [8–10]. Fibres are a promising non-conventional reinforcement in concrete elements because they alters the brittle failures to ductile (pseudo-ductile) ones, improving the deformation capability, reducing cracking and presenting higher energy dissipation regarding to the non-fibrous elements [11–14]. Fibres can also contribute to minimize the corrosion problems in concrete structures, because fibres limit the formation and cracking increment, and they can replace part of the steel reinforcement [10,15]. The fibres consist of thin and short filaments that are randomly distributed throughout the structure [16,17]. According to their composition they are classified as steel, glass, synthetic or natural

Sustainability 2020, 12, 10026; doi:10.3390/su122310026 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 10026 2 of 19 fibres [18]. However, the vulnerability of steel to corrosion and the high cost and energy in the manufacture of synthetic fibres have placed natural fibres as an attractive reinforcement alternative [19]. In addition, man-made fibres are non-biodegradable, and when they are disposed of in landfills, they can cause pollution by releasing heavy metals and other pollutants into the ground water as well as in the soil [20,21]. Natural fibres as reinforcement material have been used in construction since the early 19th century [22]. Between the applications of concrete with natural fibres reinforcement is possible to find the construction of pipes, silos, slabs on grade, airports and pavements, tunnelling, rock stability, shotcrete, and others [23–25]. Compared to man-made fibres (steel, glass and synthetic), natural fibres are less expensive, locally abundant (sometimes as a waste), renewable, lightweight and biodegradable [26–30]. Moreover, natural fibres have very low embodied energy [31], i.e., sum total of the energy used at each step of the process needed to create a particular finished product. The carbon footprints as well as the embodied energy of natural fibres are very low as compared to artificial ones [32]. Furthermore, the health and safety concerns during their handling, processing, and mixing are less [33]. Therefore, from a sustainable perspective, natural fibres are postulated as a promising reinforcement material for concrete using natural assets and with a non-hazardous impact on the environment [33–35]. This is especially relevant in the construction industry, which is responsible for a large amount of non-renewable resources and (only in Europe) of 30% of the emission of carbon dioxide gases [15]. In addition, studies conducted by Soto, Ramalho and Izquierdo [36], and Okeola, Abuodha and Mwero [37] report that sisal fibres limited the cracks propagation in the reinforced concrete. In fact, fibre addition is an accepted technique for controlling shrinkage cracking, which is a cause of premature deterioration [38]. Llerena [39] identified that the increase in the ductility of the compound is due to the vegetable fibre. Sen and Reddy [40] compared the effectiveness of green retrofitting schemes made of sisal Fibre Reinforced Polymer (FRP) with schemes of carbon FRP and glass FRP in reinforced concrete beams. The failure mode performance of sisal FRP was similar to the ones of carbon FRP and glass FRP. Moreover, sisal FRP promoted ductile failure of beams with sufficient warnings unlike carbon FRP and Glass FRP, which underwent sudden debonding. Ribeiro et al. [41] identified the high water retention and reduction of surface temperature as a countermeasure for urban heat islands when natural bagasse fibres are incorporated in concrete blocks for pavements. However, the incorporation of natural fibre can produce a significant reduction in the concrete compressive strength. There is more reduction as the amount of fibres increase [42]. For example, Yalley and Kwan [43] studied the mechanical performance of coconut fibre reinforced concrete with 0.25%, 0.50% and 0.75% with respect to the weight of the cement. Compared to the control samples (without fibre), the results show a decrease in compressive strength between 11% and 33%. Similarly, Zakaria et al. [35] report that incorporating 0.75%, 1.75%, and 3.52% of jute fibre, with respect to the weight of cement, reduces the compression strength between approximately 6% and 35%. Moreover, Okeola et al. [37] mentioned that adding sisal fibre in 0.5%, 1.0%, 1.5%, and 2.0% with respect to the weight of cement, causes a compressive strength reduction by 4.22%, 11.54%, 18.18%, and 25.30%, respectively. Finally, the study performed by Yursa et al. [44] shows that the addition of concrete with bamboo fibres in 0.5%, 1.0%, and 1.5% of the weight of cement decrease the compressive strength in 1.67%, 5.44%, and 12.00%, respectively. As concrete strength, especially compressive strength, is a fundamental characteristic related with the massive use of this material, the incorporation of waste into the concrete, requires a strength evaluation of the concrete-waste composite. In fact, that incorporation can generate different technical benefits, but if the strength of the concrete-waste composite is significantly less than the reference concrete, the practical possibilities to be massively used in different concrete applications can be very limited. However, from a sustainable point of view, it is important to produce a composite Sustainability 2020, 12, 10026 3 of 19 concrete-waste that effectively has practical possibilities to be applied. For that reason, the strength evaluation of that composite is fundamental. This is even more evident when the waste is natural fibres because different authors report a reduction of the compressive strength as the amount of fibres increases [35,37,42–44]. Then, being the amount of fibres to incorporate in the concrete limited to a small quantity, a significant reuse of waste fibres will be related with the massive use of concrete, which will depend of the strength of the concrete-fibre composite. That is an important reason why several studies have focused on evaluating the performance of mixes with different natural fibre dosages. The final goal of that process is to determine optimal proportions to be incorporated into the concrete [45–48]. Those proportions will be related, between other factors, with the particular fibre under study, the treatment applied during its incorporation and the uniformity of volume distribution of the fibre in the mix [29,49]. The present article focuses in the application of natural fibres produced with bark of Eucalyptus globulus (E. globulus). In Chile, E. globulus has a relevant role as one of the most used tree species in the forestry industry [50,51]. In particular, at the Biobio region, which concentrates forest plantations, 4.2 billion of wood are consumed per year [51]. This generates bark corresponding to 9.2% of the total volume of the tree [52] and represents approximately 386 thousand m3 of annual bark. This is not an isolated case. In fact, Eucalyptus plantations cover large areas around the world (>20 million hectares) in countries, such as Australia, Spain, Portugal, Kenya, Brazil, Uruguay, and Chile [53]. Therefore, the disposal and reuse of Eucalyptus bark is a problem in different countries and regions. In effect, the bark is the external protection of the tree and it needs to be extracted before the wood can be commercially used. In this way, the bark is an industrial waste. In particular, at the Biobio region in Chile, part of the E. globulus bark waste generated is used to produce energy. However, compared to other biomass products, the process is inefficient due to its fibrous structure, low heating value and higher ash content [54]. Therefore, it is necessary to develop alternatives that contribute to the reuse of waste. Although there have been several studies related to concrete reinforcement with natural fibres of different origins [36,37,45–48,55], to the best of the authors’ knowledge, there is a lack of research considering fibres from the bark of the species E. globulus, and its impact on concrete performance. This is an important fact, because bark is distinctive of a tree, and even they are different between Eucalyptus species. Actually, bark is an important element of tree identification [56]. Furthermore, as bark protects the tree from the environment where it is located, the bark characteristics are geo-dependent as well. Geo-dependency is fundamental when dealing with waste materials and concrete, especially because the alternatives of solutions must be practical and possible to apply. This, from a sustainable perspective, will allow to effectively reuse the industrial waste. Actually, geo-dependency is one of the characteristics of the concrete material that make it so popular, as it is possible to use local raw materials. The work presented in this article is part of a bigger investigation about the effect of E. globulus bark fibres as concrete reinforcement. However, for the reasons explained before, it is fundamental to know if the bark fibre of E. globulus significantly reduces the strength of the composite concrete-waste. Hence, the objective of the present article is to evaluate traditional mechanical properties of concrete reinforced with E. globulus bark fibres. This is especially relevant considering that natural fibres are known for producing a significant reduction of the concrete strength. In this way, for the particular case of E. globulus bark fibres, the article includes the evaluation of the influence of different fibre proportions and fibre treatments on the compressive and flexural strength of the composite concrete-waste. Additionally, a preliminary evaluation of the durability of the fibres in the concrete is made, considering that the durability of the mixes could eventually be affected by volumetric changes of the fibres due to their high-water absorption capacity and their potential weakness against the alkalinity of the cementitious matrix [57–59]. Sustainability 2020, 12, 10026 4 of 19

2. Materials and Methods

2.1. Materials

2.1.1. Cement Cement from two different suppliers typically used by the construction industry in Chile were considered in the research. This choice was based on the importance of developing practical sustainable alternatives towards reusing industrial waste. Based on the Chilean standard NCh 148 [60], both are classified as standard grade Portland pozzolanic cement (max. 30% pozzolanic). The properties of the cements used in the study are presented in Table1.

Table 1. Cement characterization.

Properties Cement 1 (C1) Cement 2 (C2) Requirements Autoclave expansion (%) 0.1 0.1 1.0 max. Initial setting (h:m) 02:40 01:30 01:00 min. Final setting (h:m) 03:40 05:50 12:00 max. Compressive strength (kg/cm2) 3 days 270 7 days 320 187 180 min 28 days 410 275 250 min

2.1.2. Aggregates Table2 presents the properties of the fine and coarse aggregate (20 mm maximum size) used in the samples. Both aggregates satisfy the minimum requirements for the production of mortars and concrete [61]. Table 2. Physical properties of fine and coarse aggregates.

Physical Properties Fine Aggregate Coarse Aggregate Loose bulk density (kg/m3) 1.57 1.49 Bulk density consolidated (kg/m3) 1.66 1.63 Relative density dry (kg/m3) 2.66 2.67 Absorption (%) 2.05 1.05 Percentage of compacted Voids (%) 40.98 44.0 Materials finer than the No. 200 sieve (%) 0.2 0.6 Fineness Modulus 3.12 6.67 Organic Impurities No -

2.1.3. Eucalyptus globulus Bark Fibre The E. globulus bark fibres were provided by the Unidad de Desarrollo Tecnológico, UDT (Technological Development Unit) of the Universidad de Concepción, Chile (Figure1). The fibre is characterized by having a light brown colour (Figure1), absorption of 350%, length between 10 mm and 20 mm, and 2 mm average thickness. The chemical composition is presented in Table3. Previously, the fibres were screened in order to use only material retained in 3/8” sieve. Sustainability 2020, 12, x FOR PEER REVIEW 5 of 19

Sustainability 2020, 12, 10026 5 of 19 Sustainability 2020, 12, x FOR PEER REVIEW 5 of 19

Figure 1. Eucalyptus globulus bark fibre.

Table 3. Chemical composition of E. globulus bark fibre.

Component % Dry Solid Base Ethanol/Water Extractives 7.43 ± 0.03 FigureCellulose 1. Eucalyptus globulus bark 49.91 fibre. fibre. ± 2.56 Table 3. ChemicalHemicellulose composition of E. globulus 18.12 ± 4.16bark fibre. Table 3. ChemicalKlason Lignin composition of E. globulus 17.60 ± 0.49bark fibre. ComponentAsh % Dry 7.62 Solid ± 0.32 Base Component % Dry Solid Base Ethanol/AbsorptionWater Extractives (%) 7.43 3500.03 Ethanol/Water Extractives 7.43± ± 0.03 Cellulose 49.91 2.56 Cellulose 49.91± ± 2.56 2.2. Experimental Program Hemicellulose 18.12 4.16 ± KlasonHemicellulose Lignin 17.60 18.12 0.49± 4.16 ± In order to increase the informationKlasonAsh Lignin possible to obtain 7.62 17.60 0.32from± 0.49 the samples behaviour, the ± experimental program was dividedAbsorption intoAsh three (%) consecutive stages 7.62 350 (Figure± 0.32 2). This strategy was chosen to narrow the range of study basedAbsorption on the results (%) obtained in the 350 previous phase. In this way, stage 2.2.1 Experimentalconsiders the Program evaluation with two cements and high percentages of bark fibre (2% and 5%). According to the results obtained in stage 1, the cement with the best performance is chosen for the 2.2. Experimental Program nextIn orderphases. to In increase a similar the way, information decisions possible are taken to obtainregarding from to the the samples percentage behaviour, of bark thefibre experimental based on programtheIn results order was of dividedto the increase previous into threethe phases. information consecutive Then, at stage stagespossible 3, (Figure only to theobtain2). percentage This from strategy ofthe fi wasbresamples with chosen the behaviour, to best narrow results thethe rangeexperimentalat stage of study 2 is programevaluated. based on was the divided results obtainedinto three in consecut the previousive stages phase. (Figure In this 2). way, Thisstage strategy 1 considers was chosen the evaluationto narrowTo studythe with range twothe potential cementsof studyand influencebased high on percentages ofthe the results high obtaabsorption of barkined fibre in of the (2%the previous andfibre 5%). in thephase. According cement In this mortar to way, the resultsand stage obtained1 considersconcrete in behaviour, stagethe evaluation 1, the the cement fibres with are with twoincluded the cements best in performancedry and and highsaturated ispercentages chosen state. Moreover, for of the bark next to fibre phases.evaluate (2% Inpotential and a similar 5%). way,Accordingdurability decisions to effects, the are results taken at stage regardingobtained 2 the in tofibres stage the percentageare 1, treathe tedcement ofwith bark with a paraffinic fibre the basedbest emulsionperfor on themance results and isthe ofchosen thepH previousof for the the phases.nextcementitious phases. Then, In at matrixa stagesimilar is 3, measured. onlyway, thedecisions percentage are taken of fibre regarding with the to best the resultspercentage at stage of bark 2 is evaluated. fibre based on the results of the previous phases. Then, at stage 3, only the percentage of fibre with the best results at stage 2 is evaluated. To study the potential influence of the high absorption of the fibre in the cement mortar and concrete behaviour, the fibres are included in dry and saturated state. Moreover, to evaluate potential durability effects, at stage 2 the fibres are treated with a paraffinic emulsion and the pH of the cementitious matrix is measured. FigureFigure 2. 2.Experimental Experimental programprogram with three three consecutive consecutive stages. stages.

ToFigure study 3 the presents potential the details influence of the of thefirst highstage. absorption This phaseof consists the fibre of a in pre-factorial the cement analysis mortar of and concretecement behaviour, mortars. The the fibrestests were are included performed in dryon sp andecimens saturated with state. dimensions Moreover, of 40 to evaluate× 40 × 160 potential mm durabilityaccording e fftoects, the atRéunion stage 2Internationale the fibres are des treated Labora withtoires a et para Expertsffinic des emulsion Matériaux, and thesystèmes pH of de the cementitiousconstruction matrix et ouvrages is measured. (RILEM) (Figure 3) [62]. The samples were made with both type of cements,

estimatingFigure3 apresents compressive the detailsstrength of of the 41 MPa first stage.and slump This between phase consists3 cm and of8 cm. a pre-factorial These target analysisvalues ofwere cement chosen mortars. based TheonFigure the tests importan 2. wereExperimental performedce of developing program on specimens with practical three alternatives consecutive with dimensions towardsstages. of effectively 40 40 reusing160 mm × × accordingindustrial to waste. the R Theéunion amount Internationale of added fibre des was Laboratoires 0% (control etsample), Experts 2%, des and Mat 5%é withriaux, respect systè tomes the de constructionweightFigure of 3cement. et presents ouvrages In addition,the (RILEM) details the of (Figure fibres the first were3)[ 62stage. incorporated]. The Th samplesis phase in a were dryconsists and made saturatedof witha pre-factorial both state. type ofanalysis cements, of estimatingcement mortars. a compressive The tests strength were performed of 41 MPa andon sp slumpecimens between with 3dimensions cm and 8 cm. of These40 × 40 target × 160 values mm wereaccording chosen to basedthe Réunion on the importance Internationale of developing des Labora practicaltoires et alternatives Experts des towards Matériaux, effectively systèmes reusing de industrialconstruction waste. et ouvrages The amount (RILEM) of added (Figure fibre 3) was[62]. 0%The (control samples sample), were made 2%, with and 5%both with type respect of cements, to the weightestimating of cement. a compressive In addition, strength the fibresof 41 MPa were and incorporated slump between in a dry 3 cm and and saturated 8 cm. These state. target values were chosen based on the importance of developing practical alternatives towards effectively reusing industrial waste. The amount of added fibre was 0% (control sample), 2%, and 5% with respect to the weight of cement. In addition, the fibres were incorporated in a dry and saturated state. Sustainability 2020, 12, x FOR PEER REVIEW 6 of 19

Sustainability 2020, 12, 10026 6 of 19 Sustainability 2020, 12, x FOR PEER REVIEW 6 of 19 Figure 3. Work plan description of stage 1.

In the preparation, the consistency of all samples was evaluated by means of the reduced Abrams cone slump method, according to NCh 2257/3 [63]. The compressive and flexural strength at 28 days was evaluated in accordance with the American Society for Testing and Materials standard ASTM C 270 [64], using multi-test equipment SERCOMP 7 controls. Figure 4 presents the details of the second stage. This phase consists in a factorial analysis of cement mortars RILEM specimens made with cement 1. The compressive strength and slump projected in the design are the same as in stage 1. The fibre was added in proportions of 0% (control sample), 0.5%, and 1.0% in relation to the weight of cement. In addition, the fibre was added in a dry,

saturated state, and treated with a paraffinic emulsion composed of 10% paraffin and 90% water. This last case was included to evaluateFigure 3.theWork potential plan description effects on of stage the 1.fibres durability. In effect, the modification of the fibre surfacesFigure with 3. physical Work plan or description chemical ofagents stage 1.is a mitigation strategy to protect In the preparation, the consistency of all samples was evaluated by means of the reduced Abrams them against the potential degradation caused by the alkaline environment of the cementitious matrix. cone slump method, according to NCh 2257/3 [63]. The compressive and flexural strength at 28 days In Infresh the state,preparation, the consistency the consistency of the cement of all samplesmortars wassamples evaluated was evaluated by means according of the reduced to NCh wasAbrams evaluated cone in slump accordance method, with according the American to NCh 2257/3 Society [63]. for The Testing compressive and Materials and flexural standard strength ASTM at C 2257/3 [63] and the pH values recorded with a Hanna pH-meter model pH21. This allows monitoring 27028 [64 days], using was multi-testevaluated in equipment accordance SERCOMP with the American 7 controls. Society for Testing and Materials standard the alkalinity levels of the cementitious matrix while evaluating the performance of the fibre ASTMFigure C 4270 presents [64], using the multi-test details of equipment the second SERCOMP stage. This 7 controls. phase consists in a factorial analysis of reinforced mortars. cementFigure mortars 4 RILEMpresents specimens the details made of the with second cement stage. 1. TheThis compressive phase consists strength in a factorial and slump analysis projected of Similar to stage 1, the compressive and flexural strength of the samples was evaluated, but in in thecement design mortars are the RILEM same as specimens in stage 1. Themade fibre with was ce addedment 1. in proportionsThe compressive of 0% strength (control sample),and slump 0.5%, this phase not only at 28 days, but also at 7, 14, and 50 days according to ASTM C 270 [64]. andprojected 1.0% in relationin the design to the are weight the same of cement. as in stage In addition,1. The fibre the was fibre added was addedin proportions in a dry, of saturated 0% (control state, For the evaluation of durability, an accelerated aging process was carried out once the samples andsample), treated 0.5%, with and a para 1.0%ffi nicin relation emulsion to the composed weight of of cement. 10% para In addition,ffin and 90%the fibre water. was This added last in case a dry, was had completed 28 days of curing. The test is based on the application of 20 cycles of 24 h. Each cycle includedsaturated to evaluatestate, and the treated potential with eaff paraffinicects on the emulsi fibreson durability. composed Inof e10%ffect, paraffin the modification and 90% water. of the This fibre consists of 18 h of oven heating at 105 ± 5 °C followed by 6 h of immersion in water at 20 °C. At the end surfaceslast case with was physical included or chemical to evaluate agents the is potential a mitigation effects strategy on the to protectfibres durability. them against In theeffect, potential the of the 20 cycles, flexural and compression strength tests were performed [65,66]. degradationmodification caused of the by fibre the surfaces alkaline with environment physical or of chemical the cementitious agents is matrix.a mitigation strategy to protect them against the potential degradation caused by the alkaline environment of the cementitious matrix. In fresh state, the consistency of the cement mortars samples was evaluated according to NCh 2257/3 [63] and the pH values recorded with a Hanna pH-meter model pH21. This allows monitoring the alkalinity levels of the cementitious matrix while evaluating the performance of the fibre reinforced mortars. Similar to stage 1, the compressive and flexural strength of the samples was evaluated, but in this phase not only at 28 days, but also at 7, 14, and 50 days according to ASTM C 270 [64]. For the evaluation of durability, an accelerated aging process was carried out once the samples had completed 28 days of curing. The test is based on the application of 20 cycles of 24 h. Each cycle consists of 18 h of oven heating at 105 ± 5 °C followed by 6 h of immersion in water at 20 °C. At the end of the 20 cycles, flexural and compression strength tests were performed [65,66].

Figure 4. Work plan description of stage 2. Figure 4. Work plan description of stage 2. In fresh state, the consistency of the cement mortars samples was evaluated according to NCh 2257/3 [63] and the pH values recorded with a Hanna pH-meter model pH21. This allows monitoring the alkalinity levels of the cementitious matrix while evaluating the performance of the fibre reinforced mortars. Similar to stage 1, the compressive and flexural strength of the samples was evaluated, but in this phase not only at 28 days, but also at 7, 14, and 50 days according to ASTM C 270 [64]. For the evaluation of durability, an accelerated aging process was carried out once the samples had completed 28 days of curing. The test is based on the application of 20 cycles of 24 h. Each cycle consists of 18 h of oven heating at 105 5 C followed by 6 h of immersion in water at 20 C. At the ± ◦ ◦ end of the 20 cycles, flexural andFigure compression 4. Work plan strength description tests of were stage performed 2. [65,66]. Sustainability 2020, 12, x FOR PEER REVIEW 7 of 19

Figure 5 presents the details of the third stage. This phase consists of the factorial analysis of Sustainabilityconcrete cubic2020, samples12, 10026 (200 mm) and beam samples (150 × 150 × 530 mm). Cement 1 was used in7 of the 19 preparation, estimating a compressive strength of 43.5 MPa and slump between 5 and 7.5 cm. The fibresFigure were5 incorporated presents the in details a dry ofand the saturated third stage. state. This The phasefibre dosages consists considered of the factorial were analysis0% (control of concretesample) and cubic 0.5% samples in relation (200 mm)to cement and beam weight. samples (150 150 530 mm). Cement 1 was used in × × the preparation,In the preparation, estimating the consistency a compressive of all strength samples of was 43.5 evaluated MPa and by slump means between of the 5Abrams and 7.5 cone cm. Theslump fibres method were according incorporated to ASTM in a dryC143/C143M and saturated [67]. At state. this stage, The fibre docility, dosages compressive considered and were flexural 0% (controlstrength sample) were evaluated and 0.5% at in ages relation 7, 21, to and cement 28 days, weight. according to ASTM C94/C94M [68].

Figure 5. Work plan description stage 3. Figure 5. Work plan description stage 3. In the preparation, the consistency of all samples was evaluated by means of the Abrams cone 2.3. Samples Mix Dosage slump method according to ASTM C143/C143M [67]. At this stage, docility, compressive and flexural strengthTable were 4 presents evaluated the at dosage ages 7, of 21, the and cement 28 days, mortar according samples to ASTMevaluated C94 in/C94M the factorial [68]. phase. The cement mortar mixes were designed following the design principles presented by the Chilean 2.3.Highway Samples Agency Mix Dosage [61] and Egaña [69]. Table4 presents the dosage of the cement mortar samples evaluated in the factorial phase. The cement mortar mixes were designedTable 4. Cement following mortar the mix design dosage. principles presented by the Chilean Highway Agency [61 ] and Egaña [69]. Cement 1 Cement 2 % Fibre 0 0.5 1 2 5 0 2 5 Table 4. Cement mortar mix dosage. Cement (kg) 559.6 559.6 559.6 559.6 559.6 506.9 506.9 506.9 Water (L) 223.8 223.8Cement 223.8 1 223.8 223.8 263.6 Cement 263.6 2 263.6 Fine Aggregate% Fibre (kg) 1488.6 0 1485.8 0.5 1455 1 1449.4 2 1460.6 5 1409.3 0 1388.2 2 1384 5 Fibre (kg) 0 2.8 5.6 11.2 28 0 10.1 25.3 Cement (kg) 559.6 559.6 559.6 559.6 559.6 506.9 506.9 506.9 Water (L) 223.8 223.8 223.8 223.8 223.8 263.6 263.6 263.6 ConcreteFine Aggregatemixtures (kg)were designed1488.6 1485.8 based on1455 the American1449.4 1460.6 Concrete 1409.3 Institute 1388.2 (ACI)1384 method [70]. Table 5 presentsFibre the (kg) mix designs 0 for the 2.8 fibre conditions 5.6 11.2 evaluated 28 (0% and 0 0.5%). 10.1 25.3

Table 5. Concrete mix dosage. Concrete mixtures were designed based on the American Concrete Institute (ACI) method [70]. Table5 presents the mix designs for the% fibre Fibre conditions evaluated 0 (0% 0.5 and 0.5%). Cement (kg) 436.8 436.8 TableWater 5. Concrete(L) mix dosage. 190 190 Fine aggregate% Fibre (kg) 0 793.9 0.5 746.6 Coarse aggregate (kg) 945.4 945.4 Cement (kg) 436.8 436.8 FibreWater (kg) (L) 190 0 190 2.2 Fine aggregate (kg) 793.9 746.6 Coarse aggregate (kg) 945.4 945.4 Fibre (kg) 0 2.2 SustainabilitySustainability2020 2020, 12, 12, 10026, x FOR PEER REVIEW 8 of 819 of 19

3. Results 3. Results 3.1. Stage 1: Cement Mortar Pre-Factorial 3.1. Stage 1: Cement Mortar Pre-Factorial 3.1.1. Fresh State Behaviour 3.1.1. Fresh State Behaviour Table 6 presents the consistency results of the fresh mixes contained by the reduced Abrams coneTable slump6 presents method the [63]. consistency results of the fresh mixes contained by the reduced Abrams cone slump method [63]. Table 6. Consistency results of the pre-factorial cement mortar mixes.

Table 6. ConsistencyMortar results Mix of the pre-factorial Slump (cm) cement mortar mixes. MortarMC-C1 Mix Slump4.5 (cm) MC-C2 4.1 MC-C1 4.5 MD-2-C1 3.4 MC-C2 4.1 MD-2-C2MD-2-C1 2.9 3.4 MD-5-C1MD-2-C2 1.5 2.9 MD-5-C2MD-5-C1 1.0 1.5 MS-2-C1MD-5-C2 4.0 1.0 MS-2-C2MS-2-C1 3.0 4.0 MS-2-C2 3.0 MS-5-C1 2.5 MS-5-C1 2.5 MS-5-C2 2.0 MS-5-C2 2.0

The results of Table 6 show that as the amount of fibre increases the workability of the mixes decreases.The results In fact, of regardless Table6 show of the that type as of the cement amount or the of fibrecondition increases of the the fibre workability (dry or saturated), of the mixesthe decreases.mixes with In 5% fact, fibre regardless were the of lowest the type slump of cement values. orIn thecomparison, condition the of themixes fibre with (dry 2% or bark saturated), fibre thepresents mixes withhigher 5% slump fibre values, were the but lowest always slump lower values.than the In control comparison, samples. the Other mixes investigations with 2% bark have fibre presentsfound similar higher results, slump values,i.e., as the but amount always of lower fibre than increases, the control a reduction samples. of workability Other investigations is produced have found[35,37]. similar results, i.e., as the amount of fibre increases, a reduction of workability is produced [35,37]. InIn addition, addition, it it is is observed observed thatthat mixesmixes with satu saturatedrated fibres fibres have have slightly slightly higher higher slump slump values values thanthan the the ones ones with with dry dry fibres. fibres. ThisThis cancan be explained explained by by the the fact fact that, that, in in saturated saturated condition, condition, the the fibres fibres areareunable unable to to absorb absorb water. water. OnOn the contrary, as as dry dry fibres fibres absorb absorb water, water, there there is isless less water water in the inthe mix, mix, whichwhich affects affects the the workability. workability.

3.1.2.3.1.2. Compressive Compressive Strength Strength FigureFigure6 presents6 presents the the results results ofof thethe samplessamples compressivecompressive strength strength with with their their standard standard error. error. ControlControl samples samples show show the the best best results.results.

Figure 6. Pre-factorial cement mortar results, compressive strength: (a) Cement 1; (b) Cement 2. Figure 6. Pre-factorial cement mortar results, compressive strength: (a) Cement 1; (b) Cement 2. Compared to samples with 5% fibre, the results indicate that the addition of 2% fibre in dry Compared to samples with 5% fibre, the results indicate that the addition of 2% fibre in dry and and saturated state causes less compressive strength loss. However, in both cases, the strength saturated state causes less compressive strength loss. However, in both cases, the strength reduction reductionis significant. is significant. This behaviour This behaviour agrees with agrees the withresults the found results by found other by researchers other researchers working working with withincorporation incorporation of natural of natural fibres fibres into into thethe concrete concrete [35,37,42,43]. [35,37,42,43 The]. The strength strength reduction reduction can can be be associatedassociated to to the the workability workability reduction, reduction, understanding understanding that that compaction compaction is moreis more di ffidifficultcult and and the the voids presence increases [37,43]. This situation can be clearer in the case of 5% fibre, considering the amount of workability reduction presented in Section 3.1.1. This high amount of fibres introduced in the cement Sustainability 2020, 12, x FOR PEER REVIEW 9 of 19

voids presence increases [37,43]. This situation can be clearer in the case of 5% fibre, considering the amount of workability reduction presented in Section 3.1.1. This high amount of fibres introduced in the cement mortar, and the associated workability reduction, can also produce fibres concentration due to the lack of dispersion (Figure 7). The result is a heterogeneous composite material with weak spots [35].

Sustainability 2020, 12, 10026 9 of 19 mortar, and the associated workability reduction, can also produce fibres concentration due to the lack of dispersion (Figure7). The result is a heterogeneous composite material with weak spots [35].

Figure 7. Cement mortar samples with 5% E. globulus bark fibres: (a) top view; (b) side view. Figure 7. Cement mortar samples with 5% E. globulus bark fibres: (a) top view; (b) side view. When comparing the effect of the type of cement, it is observed that the greatest compressive strengthWhen is comparing obtained with the Cement effect of 1. the type of cement, it is observed that the greatest compressive strength is obtained with Cement 1. 3.1.3. Flexural Strength 3.1.3. FlexuralFigure 8 Strengthshows that in all cases the flexural strength decreases as the fibre content increases. In fact,Figure for both8 shows types that of cements, in all cases the the best flexural result amon strengthg fibre decreases samples as is the for fibre the case content with increases. 2% dry fibre. In fact, In this case, the flexural strength achieved is 89% and 98% of the control samples strength. However, for both types of cements, the best result among fibre samples is for the case with 2% dry fibre. In this when comparing the effect of the cement used, it is observed that the strength values are higher for case, the flexural strength achieved is 89% and 98% of the control samples strength. However, when Cement 1. comparing the effect of the cement used, it is observed that the strength values are higher for Cement 1.

Figure 8. Pre-factorial cement mortar results, flexural strength: (a) Cement 1; (b) Cement 2. Figure 8. Pre-factorial cement mortar results, flexural strength: (a) Cement 1; (b) Cement 2. The results showed similar trends to those reported by other researchers, in the sense that the The results showed similar trends to those reported by other researchers, in the sense that the mechanical strength decreases as the fibre content increases [35,37,43]. The possible explanations mechanical strength decreases as the fibre content increases [35,37,43]. The possible explanations behind this phenomenon are basically the ones presented in Section 3.1.2, with the addition of the behind this phenomenon are basically the ones presented in Section 3.1.2, with the addition of the bonding between fibre and the cementitious matrix. In effect, bonding between these two materials bonding between fibre and the cementitious matrix. In effect, bonding between these two materials can be important for the nature of the flexural mechanism. Momoh and Osofero [71] attributes as well can be important for the nature of the flexural mechanism. Momoh and Osofero [71] attributes as well thisthis factor factor with with the the concrete concrete flexuralflexural strength re reductionduction due due to to the the incorporation incorporation of ofoil oilpalm palm broom broom fibres.fibres. Since Since all all samples samples mademade with Cement 2 2 show show results results below below the the target target strength, strength, it was it was decided decided notnot use use this this cement cement in in the the followingfollowing phases. Additionally, Additionally, considering considering that that the the incorporation incorporation of 5% of 5% fibresfibres produces produces significant significant strengthstrength reduction and and the the best best performance performance were were obtained obtained by byadding adding 2% 2% ofof fibre, fibre, it it was was decided decided to to evaluate evaluate atat thethe second phase phase lower lower fibre fibre contents contents than than 2%. 2%.

3.2. Stage 2: Cement Mortar Factorial

3.2.1. Fresh State Behaviour Table7 presents the slump, by means of the reduced Abrams cone [63], and the pH values. Sustainability 2020, 12, x FOR PEER REVIEW 10 of 19

3.2. Stage 2: Cement Mortar Factorial

3.2.1. Fresh State Behaviour Sustainability 2020, 12, 10026 10 of 19 Table 7 presents the slump, by means of the reduced Abrams cone [63], and the pH values.

TableTable 7. 7.Cement Cement mortalmortal factorial, consistency, consistency, and and pH pH of of the the mixes. mixes.

Mortar MixMix Slump (cm)(cm) pH pH Values Values MCMC 4.1 12.9 12.9 MD-0.5MD-0.5 3.0 12.8 12.8 MD-1.0MD-1.0 3.0 12.9 12.9 MS-0.5MS-0.5 3.0 12.7 12.7 MS-1.0MS-1.0 3.0 12.8 12.8 MEm-0.5MEm-0.5 3.5 12.4 12.4 MEm-1.0 3.0 12.0 MEm-1.0 3.0 12.0

TheThe results results indicate indicate thatthat forfor allall cases the inclusion inclusion of of fibre fibre slightly slightly reduces reduces the the mix mix docility. docility. However,However, the the 1 cm1 cm slump slump reduction reduction isis relativelyrelatively constant, slight, slight, and and cannot cannot be be necessarily necessarily attributed attributed toto an an increment increment or or state state of of the the added added fibrefibre [[72].72]. WhenWhen comparing comparing the the pH pH measurementsmeasurements with the the co controlntrol sample, sample, it itis isobserved observed that that in inall allcases cases thethe pH pH decreases. decreases. However, However, these these reductions reductions are are small small beingbeing thethe standardstandard deviationdeviation 0.67. Hence, Hence, it it is possibleis possible to conclude to conclude that that no no significant significant changes changesin in thethe concreteconcrete alkalinity alkalinity are are produced produced between between samples with and without fibres. samples with and without fibres.

3.2.2.3.2.2. Compressive Compressive Strength Strength Figure 9 shows the average compressive strength of the tested samples at the factorial phase. Figure9 shows the average compressive strength of the tested samples at the factorial phase.

Figure 9. Cement mortar factorial results, compressive strength. Figure 9. Cement mortar factorial results, compressive strength. Figure9 shows that for all ages, the greatest strength was obtained with the control sample. Figure 9 shows that for all ages, the greatest strength was obtained with the control sample. Regarding fibre samples, the best results at 28 days are obtained for the MS-0.5 sample, obtaining 88% Regarding fibre samples, the best results at 28 days are obtained for the MS-0.5 sample, obtaining of88% the controlof the control sample sample strength. strength. This result This result is followed is followed by the by MD-0.5 the MD-0.5 sample sample with with 86%. 86%. The lowestThe resultlowest isthe result MEm-1 is the sampleMEm-1 withsample 77%. with 77%. AtAt 50 50 days, days, the the best best performance performance isis presentedpresented by by the the MD-0.5 MD-0.5 sample, sample, which which reaches reaches 85% 85% of ofthe the controlcontrol sample sample strength. strength. The The MS-1 MS-1 andand MEm-1MEm-1 samples show show the the lowest lowest results results with with 72%. 72%. ComparingComparing the the results results of of samples samples withwith incorporated fibre, fibre, the the addition addition of of 0.5% 0.5% fibre fibre gives gives better better compressioncompression strength strength results results than than 1%. 1%. ThisThis factfact is maintained regardless regardless of of the the fibre fibre treatment. treatment. The better results obtained in this phase in comparison to the pre-factorial stage can be attributed to the fact that less amount of fibres reduces the possibility of fibre concentration (Figure 10) and production of weak spots [35]. Sustainability 2020, 12, x FOR PEER REVIEW 11 of 19

The better results obtained in this phase in comparison to the pre-factorial stage can be attributed to the fact that less amount of fibres reduces the possibility of fibre concentration (Figure 10) and production of weak spots [35]. Sustainability 2020, 12, x FOR PEER REVIEW 11 of 19

The better results obtained in this phase in comparison to the pre-factorial stage can be attributed

Sustainabilityto the fact 2020that, 12less, 10026 amount of fibres reduces the possibility of fibre concentration (Figure 10)11 ofand 19 production of weak spots [35].

Figure 10. Cement mortar samples with 0.5% E. globulus bark fibres.

3.2.3. Flexural Strength Figure 11 shows the average flexural strength at ages 7, 14, 28, and 50 days. At 28 days, the fibre samples with the best performance are MD-05 and MS-1. They reached 90% of the control sample strength. The lowestFigure results 10. Cementwere obtained mortar samples by MEm-1, with with 0.5% 73% E. globulus of the control bark fibres.fibres. sample strength. At 50 days, the fibre sample with the best performance was the MS-1 with 94% of the control 3.2.3. Flexural Strength 3.2.3.sample Flexural strength. Strength This result is slightly higher than the ones obtained by samples MD-0.5 and MD-1 withFigure 93% 11and shows MS-0.5 the with average 92% of flexural the control strength sample at agesstrength 7, 14, respectively. 28, and 50 days.Again, At the 28 samples days, the with fibre Figure 11 shows the average flexural strength at ages 7, 14, 28, and 50 days. At 28 days, the fibre samplesfibres treated with the with best paraffin performance emulsion are presented MD-05 andthe lowest MS-1. strength They reached regarding 90% to ofthe the control control samples, sample samples with the best performance are MD-05 and MS-1. They reached 90% of the control sample strength.with 73% The and lowest 80% for results MEm-1 were and obtained MEm-0.5, by respectively. MEm-1, with 73% of the control sample strength. strength. The lowest results were obtained by MEm-1, with 73% of the control sample strength. At 50 days, the fibre sample with the best performance was the MS-1 with 94% of the control sample strength. This result is slightly higher than the ones obtained by samples MD-0.5 and MD-1 with 93% and MS-0.5 with 92% of the control sample strength respectively. Again, the samples with fibres treated with paraffin emulsion presented the lowest strength regarding to the control samples, with 73% and 80% for MEm-1 and MEm-0.5, respectively.

FigureFigure 11. 11.Cement Cement mortarmortar factorial results, flexural flexural strength. strength.

AtThe 50 days,flexural the strength fibre sample reached with by samples the best with performance 0.5% and 1.0% was is thevery MS-1 close withto the 94% control of thesamples, control sampleespecially strength. for the This cases result highlighted is slightly before. higher Furthermor than thee, ones the results obtained at 50 by days samples indicate MD-0.5 that the and fibre MD-1 withsamples 93% and keep MS-0.5 winning with strength 92% of redu thecing control the gap sample with strength the control respectively. samples. This Again, fact suggest the samples that the with fibressamples treated with with bark para fibreffin emulsion can have presented a later thestre lowestngth development strength regarding that is to worthy the control to further samples, withinvestigations. 73% and 80% for MEm-1 and MEm-0.5, respectively. The flexural strengthFigure reached 11. Cement by samples mortar with factorial 0.5% results, and 1.0% flexural is very strength. close to the control samples, especially for the cases highlighted before. Furthermore, the results at 50 days indicate that the fibre samplesThe keepflexural winning strength strength reached reducing by samples the gap with with 0.5% the and control 1.0% is samples. very close This to factthe control suggest samples, that the samplesespecially with for barkthe cases fibre highlighted can have a later before. strength Furthermor developmente, the results that is at worthy 50 days to indicate further investigations. that the fibre samplesSimilar keep to winning what was strength expressed reducing in Section the gap 3.2.2 with, the the bettercontrol results samples. obtained This fact in thissuggest phase that with the respectsamples to with stage bark 1 can befibre attributed can have to thea lesslater amount strength of fibres,development which reduces that is the worthy possibility to offurther fibre concentrationinvestigations. (Figure 10) and production of weak spots [35]. There are no significant differences in adding the fibres in dry or saturated state. Therefore, it is not essential to dry the fibre before adding it to the mixture. With respect to the fibres treated with paraffin emulsion, similar to the compressive behaviour, there are no strength advantages related with their application. Sustainability 2020, 12, x FOR PEER REVIEW 12 of 19

SustainabilitySimilar 2020 to, what12, x FOR was PEER expressed REVIEW in Section 3.2.2, the better results obtained in this phase 12 with of 19 respect to stage 1 can be attributed to the less amount of fibres, which reduces the possibility of fibre concentrationSimilar to (Figure what was10) and expressed production in Section of weak 3.2.2, spots the [35]. better results obtained in this phase with respectThere to stage are no 1 significantcan be attributed differences to the in less adding amou thent offibres fibres, in drywhich or saturatedreduces the state. possibility Therefore, of fibre it is notconcentration essential to (Figure dry the 10) fibre and before production adding of it weak to th spotse mixture. [35]. With respect to the fibres treated with paraffinThere emulsion, are no significant similar to differencesthe compressive in adding beha theviour, fibres there in dry are or no saturated strength state.advantages Therefore, related it is withnot essential their application. to dry the fibre before adding it to the mixture. With respect to the fibres treated with paraffin emulsion, similar to the compressive behaviour, there are no strength advantages related Sustainability 2020, 12, 10026 12 of 19 3.2.4.with theirDurability application.

3.2.4.As Durability mentioned before, a preliminary evaluation of the fibres durability in the cementitious matrix 3.2.4.is made Durability though accelerated aging process. Figure 12 presents the compressive strength results of the As mentioned before, a preliminary evaluation of the fibres durability in the cementitious matrix samplesAs mentioned after that before,process. a As preliminary only slight evaluation variations are of the produced fibres durability in all cases, in it thecan cementitiousbe concluded matrixthat is made though accelerated aging process. Figure 12 presents the compressive strength results of the is madethe aging though process accelerated does not agingaffect process.the compressive Figure 12strength. presents the compressive strength results of the samples after that process. As only slight variations are produced in all cases, it can be concluded that samples after that process. As only slight variations are produced in all cases, it can be concluded that the aging process does not affect the compressive strength. the aging process does not affect the compressive strength.

Figure 12. Cement mortar factorial results, compressive strength of aged samples. Figure 12. Cement mortar factorial results, compressive strength of aged samples. In a similarFigure way, 12. CementFigure mortar13 presents factorial the results, flexur compressiveal strength strength results of at aged 28 dayssamples. after the aging process. In aIn similar a similar way, way, Figure Figure 13 presents 13 presents the flexuralthe flexur strengthal strength results results at 28 at days 28 afterdays theafter aging the process.aging process.

Figure 13. Cement mortar factorial results, flexural strength in aged samples. Figure 13. Cement mortar factorial results, flexural strength in aged samples. Figure 13 shows that, in all cases, less flexural strength is obtained after applying the cycles. Figure 13 showsFigure that, 13. Cement in all cases, mortar less factorial flexural results, strength flexural is obtained strength inafter aged applying samples. the cycles. MD- MD-0.5 is the most affected case with a 34% reduction compared to the 28-day resistance. On the other 0.5 is the most affected case with a 34% reduction compared to the 28-day resistance. On the other hand, MEm-1Figure 13 is theshows samples that, in with all cases, the lowest less flexural strength strength reduction is obtained (12%). after However, applying it isthe not cycles. possible MD- to associate0.5 is the the most flexural affected strength case with reduction a 34% withreduction the fibre compared addition. to the This 28-day is because resistance. the controlOn the sampleother shows as well a considerable reduction (23%), after the application of the cycles. Furthermore, it is not possible to associate durability benefits to the paraffinic emulsion treatment. Hence, considering the results presented in Figure 13 and the compressive strength ones showed by Figure 12, it is possible to conclude that adding fibres not necessarily affects the mixes durability. A possible explanation is the composition of the typical cements used in Chile, which are the ones incorporated in the present investigation. In effect, the typical Chilean cements are pozzolanic, being the pozzolana, a well know mitigating factor of potential degradation of natural fibres in the cementitious matrix. Sustainability 2020, 12, x FOR PEER REVIEW 13 of 19

hand, MEm-1 is the samples with the lowest strength reduction (12%). However, it is not possible to associate the flexural strength reduction with the fibre addition. This is because the control sample shows as well a considerable reduction (23%), after the application of the cycles. Furthermore, it is not possible to associate durability benefits to the paraffinic emulsion treatment. Hence, considering the results presented in Figure 14 and the compressive strength ones showed by Figure 13, it is possible to conclude that adding fibres not necessarily affects the mixes durability. A possible explanation is the composition of the typical cements used in Chile, which are the ones Sustainability 2020, 12, 10026 13 of 19 incorporated in the present investigation. In effect, the typical Chilean cements are pozzolanic, being the pozzolana, a well know mitigating factor of potential degradation of natural fibres in the Stagecementitious 2 results matrix. indicate as well that treating fibres with paraffinic emulsion does not produce performanceStageadvantages. 2 results indicate This as canwell bethat related treating with fibres the with fact paraffinic that para emulsionffinic emulsiondoes not produce prevents a performance advantages. This can be related with the fact that paraffinic emulsion prevents a good good fibre-cementitious matrix interaction. Therefore, application of the paraffinic emulsion is not fibre-cementitious matrix interaction. Therefore, application of the paraffinic emulsion is not considered in stage 3. Finally, the incorporation of 0.5% fibre presented the most favourable results considered in stage 3. Finally, the incorporation of 0.5% fibre presented the most favourable results and,and, therefore, therefore, it is it the is the percentage percentage of of fibre fibre considered considered in stage stage 3. 3.

3.3. Stage3.3. Stage 3: Concrete 3: Concrete Factorial Factorial

3.3.1.3.3.1. Fresh Fresh State State Behaviour Behaviour TableTable8 shows 8 shows the slumpthe slump recorded recorded using using the the Abrams Abrams cone method method [67]. [67 ].

TableTable 8. 8.Concrete Concrete factorial results, results, docility. docility.

ConcreteConcrete Mix Mix Slump Slump (cm) (cm) CC 7.0 CC 7.0 CD-0,5 7.0 CD-0,5 7.0 CS-0,5 8.0 CS-0,5 8.0

The CS-0.5 mix shows a slight increase in its docility. However, this value is within the range of Thepermissible CS-0.5 variation mix shows [68,73], a slight and increasethus it is not in itsnecessarily docility. associated However, with this the value added is withinfibre. the range of permissibleIn contrast, variation the [68 incorporation,73], and thus of itdry is notfibre necessarily did not show associated variation with with the respect added to fibre.the control Insample. contrast, the incorporation of dry fibre did not show variation with respect to the control sample.

3.3.2.3.3.2. Compressive Compressive Strength Strength The concreteThe concrete compressive compressive strength strength results results areare presented in in Figure Figure 14. 14 .

FigureFigure 14. 14.Concrete Concrete factorialfactorial results, results, compressive compressive strength. strength. At all ages, the greatest strength is obtained by the control sample. However, the CD-0.5 sample has slightly lower values. In fact, after 28 days its strength only differs 0.98 MPa of the control sample, being 0.82 MPa the standard error. Hence, the difference is very close to the error. The CS-0.5 sample reached a 28-day difference of 1.8 MPa respect to the control sample strength. The good results obtained can be explained in a similar way than the results of stage 2 described in Section 3.2.2. Other researchers have also found their best compressive strength results with lower amount of natural fibres [35,37,42,43]. In general, the compressive strength decreases more, for similar percentage of fibre, than the results presented in this section. Actually, the compressive strength reductions of those investigations varies between 4.22% and 11.00% for 0.50% fibre reinforcement with respect to the weight of cement [35,37,43]. In the present investigation the best result differs only 2.00% of the control sample, being the difference very close to the standard error with a coefficient of variation 1.73%. Sustainability 2020, 12, x FOR PEER REVIEW 14 of 19

At all ages, the greatest strength is obtained by the control sample. However, the CD-0.5 sample has slightly lower values. In fact, after 28 days its strength only differs 0.98 MPa of the control sample, being 0.82 MPa the standard error. Hence, the difference is very close to the error. The CS-0.5 sample reached a 28-day difference of 1.8 MPa respect to the control sample strength. The good results obtained can be explained in a similar way than the results of stage 2 described in Section 3.2.2. Other researchers have also found their best compressive strength results with lower amount of natural fibres [35,37,42,43]. In general, the compressive strength decreases more, for similar percentage of fibre, than the results presented in this section. Actually, the compressive strength reductions of those investigations varies between 4.22% and 11.00% for 0.50% fibre reinforcement Sustainabilitywith respect2020 to, 12 the, 10026 weight of cement [35,37,43]. In the present investigation the best result differs only14 of 19 2.00% of the control sample, being the difference very close to the standard error with a coefficient of variation 1.73%. This is a very promising result towards developing practical alternatives for Thiseffectively is a very reusing promising the resultbark fibres towards of Eucalyptus developing globulus practical species, alternatives considering for eff thatectively concrete reusing strength, the bark fibresand ofparticularlyEucalyptus compressive globulus species, strength, considering is related that with concrete the massive strength, use and of particularlyconcrete and compressive different strength,numerous is related of concrete with applications. the massive use of concrete and different numerous of concrete applications.

3.3.3.3.3.3. Flexural Flexural Strength Strength FigureFigure 15 15 presents presents the the results results ofof thethe concreteconcrete flexural flexural strength. strength.

FigureFigure 15.15. Concrete factorial results, results, flexural flexural strength. strength.

AtAt 28 28 days, days, the the best best results results areare obtainedobtained by the control sample. sample. It Itis isobserved observed that that the the CS-0.5 CS-0.5 samplesample has has a a 0.20 0.20 MPa MPa reduction, reduction, beingbeing 0.22 MPa MPa th thee standard standard error, error, i.e., i.e., the the difference difference cannot cannot be be consideredconsidered significant. significant. The The CD-0.5CD-0.5 samplesample shows a a reduction reduction of of 0.7 0.7 MPa MPa (13%). (13%). TheThe good good results results obtained obtained cancan bebe explainedexplained in a a similar similar way way than than the the results results of ofstage stage 2 described 2 described inin Section Section 3.2.3 3.2.3..

4.4. Discussion Discussion TheThe present present article article focuses focuses onon thethe mechanicalmechanical pr propertiesoperties traditionally traditionally evaluated evaluated in inmortars mortars and and concrete,concrete, i.e., i.e., compressive compressive and and flexural flexural strength.strength. TheThe results results presented presented indicateindicate that that mortars mortars and and concretes concretes with with 0.5% 0.5% and and 1.0% 1.0% of bark of barkfibre fibreof E. of E.globulus globulus developdevelop an an acceptable acceptable behaviour, behaviour, although although lower lower when when compared compared with with samples samples without without fibre.fibre. Nevertheless, Nevertheless, when when comparing comparing thethe results with with the the ones ones obtained obtained with with other other types types of ofvegetable vegetable fibres [35,37,43], it is observed that the strength reduction is low, particularly in the case of the reinforced concrete. Although reports indicate that the incorporation of natural fibres can produce a significant reduction in concrete strength [42], which is not the case in this study. The explanation behind this phenomenon is related with the technical factors discussed in Section3 and the fact that both concrete and fibre are geo-dependent materials, i.e., their characteristics and properties depend on a series of environmental and geographical factors influencing the materials properties. For example, availability of plant fibres more resistant to aggressive environments, such as those of high alkalinity. In this aspect, E. globulus bark fibres may be more resistant to these environments. In addition, Chilean most common cements are pozzolanic, being the pozzolana a well know mitigating factor of potential degradation of natural fibres in the cementitious matrix. Geo-dependency is fundamental when dealing with waste materials and concrete, especially since the alternatives of solutions must be practical and possible to apply in order to effectively reuse Sustainability 2020, 12, 10026 15 of 19 the waste. Hence, they must be feasible not only from a technical point of view but also from an economic perspective including, for instance, transportation costs. Actually, geo-dependency is one of the characteristics of the concrete material that make it so popular, as it is possible to use local raw materials. The promising results obtained in the phase of the investigation presented in this paper can contribute to reduce the waste material, for instance, at the Chilean forest industry,especially considering the massive use of concrete in Chile. In fact, it is estimated that over 70% of the Chilean constructions are built with this material [74]. Furthermore, concrete is the material of greater volume used by man and it is very difficult to replace in numerous infrastructure applications [75]. The present phase of the research was focused on the evaluation of traditional mechanical properties as compressive and flexural strength. However, considering the different potential benefits of incorporating fibres in the concrete material, it is planned to specifically evaluate other engineering properties in the coming research phases. Those engineering properties include the shrinkage cracking progression, tensional post-cracking behaviour, residual strength, energy dissipation, and ductility of the concrete reinforced with E. globulus bark fibres. In the specific case of ductility, although was not experimentally evaluated in this phase of the research, it was possible to visually observe that samples with fibres were clearly less brittle than the control ones, i.e., without bark fibre reinforcement. Finally, it is important to highlight the importance of the present phase of the research. In effect, although fibres can improve different engineering properties of the concrete material (as the ones mentioned in the previous paragraph), if the strength of the composite concrete-fibre is significantly reduced regarding to the reference concrete, the possibilities to effectively reuse the waste are very limited. This is due to the fact that concrete strength is related with the massive use of this material in numerous different applications. Then, as the percentage of natural fibres possible to incorporate in the concrete material is low, the massiveness factor is fundamental to reuse significant amounts of waste natural fibres. In this respect, the present phase of the investigation has demonstrated that the incorporation of E. globulus bark fibre does not reduce significantly the compressive and flexural strength of the concrete material.

5. Conclusions The work presented in this article is part of an ongoing project. However, at the present state, very valuable conclusions are possible to obtain regarding to the mechanical properties traditionally evaluated in cement mortars and concrete, i.e., compressive and flexural strength. Considering that the incorporation of natural fibres can produce a significant concrete strength reduction, the most important findings of this study were the results obtained for the concrete compressive and flexural strength. In effect, there is only a slight strength reduction when 0.5% of E. globulus bark fibre (with respect to the cement weight) is included. Furthermore, one of the results indicate that the strength difference cannot be considered significant as the reduction is less than the standard error. Those are very important results because concrete strength is a fundamental characteristic related with the massive use of this material. Actually, compressive strength is a typical property tested in real structures when they are being built. For certain structures, as pavements, the flexural strength is tested instead. In addition to the favourable results of strength, the consistency results of the optimal bark fibre proportions are also appropriate, which is important as well, because in real-world structures, workability is also tested. Moreover, workability is directly related with the process of concrete mixing, placing and compacting. Moreover, adequate mix consistency also means that the composite concrete-bark fibre can be applied with traditional construction equipment. Even more, natural fibres as bark fibres of E. globulus are not abrasive to the processing equipment as some man-made fibres. Hence, the article presents optimal results and proportions to promote the practical use of bark fibre of E. globulus in numerous areas of concrete applications as, roadways, residential, agricultural, commercial, tunnelling, precast concrete products, airports, warehouses, waterways, elevated decks, Sustainability 2020, 12, 10026 16 of 19 structural reinforcement, and bridges, between others. Even more, the results exhibit that the inclusion of bark fibre of E. globulus not only does not reduce the concrete strength significantly, but even it is possible to obtain a compressive strength over 42 MPa, which would open the possibility to high strength concrete. The inclusion of E. globulus bark fibre in the manufacture of mortars and concrete is a good alternative to counteract the problems associated with the use and disposal of this waste. Moreover, it contributes to minimize the environmental impact caused in the construction, for instance, by the production of man-made fibres. In effect, man-made fibres are non-bio-degradable, and when they are disposed of in landfills, they can cause pollution by releasing heavy metals and other pollutants into the ground water as well as in the soil. On the contrary, natural fibres are renewable, biodegradable, less expensive and locally abundant, many times as a waste, as it is the case of E. globulus bark fibre. Furthermore, the carbon footprints as well as the embodied energy of natural fibres are very low as compared to artificial ones. Therefore, from a sustainable perspective, the use of natural fibres in concrete are an important alternative to artificial fibres, using natural assets and with a non-hazardous impact on the environment. This is especially relevant in the construction industry which is responsible for a large amount of non-renewable resources and emission of carbon dioxide gases. Considering the favourable results obtained, the use of E. globulus bark fibre in concrete can be a relevant contribution to promote sustainable construction materials. The results presented in this article are an interesting antecedent to consider the reinforcement of concrete with Eucalyptus bark fibre, especially in countries with important areas of Eucalyptus, such as Australia, Spain, Portugal, Kenya, Brazil, or Uruguay. However, to evaluate the mechanical properties of those barks into the concrete, specific research is necessary. In fact, the work presented in this article, attempts to contribute to the research related with understanding the properties of concrete reinforced with Eucalyptus bark fibres and its possibilities as an eco-friendly construction alternative to the concrete reinforced with man-made fibres. In effect, although there are numerous investigations about natural fibres, there is a lack of research related with the use of bark fibres of the E. globulus species in the concrete material. This is relevant because bark can be different between trees and even among Eucalyptus species.

Author Contributions: Conceptualization, M.P., C.M., and C.F.; methodology, M.P., C.F., and C.M.; formal analysis, C.M., M.P., and C.F.; writing, C.M., A.C.; writing—review and editing, M.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by ANID PIA/Apoyo CCTE AFB170007. Conflicts of Interest: The authors declare no conflict of interest.

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