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Influence of recycled slag aggregates on the conductivity and strain sensing capacity of carbon fiber reinforced cement mortars
Construction and Building Materials 184 (2018) 311–319
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of recycled slag aggregates on the conductivity and strain sensing capacity of carbon fiber reinforced cement mortars F.J. Baeza a, O. Galao a, I.J. Vegas b, M. Cano b, P. Garcés a,⇑ a b
Department of Civil Engineering. University of Alicante, Apartado de Correos 99, 03080 Alicante, Spain Tecnalia. C/Geldo, Parque Tecnológico de Bizkaia, Edificio 700, 48160 Derio, Spain
h i g h l i g h t s Cement mortars with Electric Arc Furnace slag aggregates show higher strengths. Recycled EAF slag aggregates enhance the conductivity of CF reinforced mortars. Percolation was achieved with lower CF dosage for EAF slag aggregate mortars. CF reinforced mortars with EAF slag aggregates presented strain sensing capacity. The highest sensitivity was measured for EAF slag mortars with 1.9 vol% CF.
a r t i c l e
i n f o
Article history: Received 14 November 2017 Received in revised form 15 May 2018 Accepted 27 June 2018
Keywords: EFA slag Recycled aggregates CFRCC Percolation Carbon fibre Electrical resistivity
a b s t r a c t The electrical resistivity of carbon fiber reinforced cement composites (CFRCC) has been widely studied for the functional applications of these composites. CFRCC with enhanced electrical properties can be used as strain or damage sensor, heating or deicing material, or anode in different electrochemical techniques, like chloride extraction or cathodic protection. In this work, carbon fiber reinforced cement mortars have been prepared using a conductive aggregate produced from the valorization of Electric Arc Furnace slag (EAF slag). Cement based mortars containing EAF slag aggregates revealed lower resistivity; hence, the carbon fiber percolation threshold decreased. Additionally, the strain sensing capacity of those composites with conductive aggregates was also enhanced, indicating similar sensitivity and lower dispersion than equivalent mortars containing limestone aggregates. The best linear regressions between the electrical and mechanical measures were achieved for EAF slag cement mortar containing 1.29 vol % of oxidized carbon fibers. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The construction industry is constantly developing novel solutions for the valorization of waste materials and industrial byproducts [1–5]. Some of these applications emerge as an upgraded option to turn waste materials into composites with enhanced performance. The use of supplementary cementitious materials in concrete manufacturing transformed conventional concrete into a material with higher mechanical properties and durability: e.g., silica fume [6,7], blast furnace slag [8,9], or spent cracking catalyst residue [10,11]. Currently other waste reuses are being tested as an alternative to Portland cement use in concrete, e.g. alkali activated blast furnace slags [12–14].
⇑ Corresponding author. E-mail address: [email protected] (P. Garcés). https://doi.org/10.1016/j.conbuildmat.2018.06.218 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
Electric Arc Furnace slag (EAF slag) is a by-product from the steelmaking industry, produced from the melting and the preliminary acid refining of the liquid steel. EAF slag can be easily crushed to produce granular materials for use in both civil and building applications: EAF slag powder has been used as supplementary cementitious material [15], EAF slag aggregates have been used in concrete fabrication [16] or in road construction [17]. Concrete manufactured with partial or total replacement of conventional limestone or siliceous aggregates by EAF slag aggregates exhibited higher compressive, bending strength, durability [5,18–20], and rebar-concrete bonding strength [21]. On the other hand, conductive concrete emerges as an interesting alternative giving response to new challenges in construction (smart infrastructures or monitoring of durability) [22–24]. Conductive aggregates like iron ore or slags, might contribute to enhancing conductivity for that purpose [25]. Some metallic oxides contained in steel slags reveal conductivities in the range of pitch-based carbon fiber (CF) [26].
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Concrete and, in general, those materials evidencing a brittle tension or flexural behavior have been historically combined with other elements showing superior tensile properties [27–29]. The addition of short fibers (e.g. steel, carbon, polypropylene) to the mixture can improve the mechanical performance of the fiberreinforced concrete (FRC), i.e. higher compressive strength [30], enhanced bending behavior [31], better residual properties after high temperature exposure [32]. Furthermore, all kind of fibers (even those without structural purpose) have been traditionally added to cement composites to control creep and shrinkage [8,13]. In general, fibers have been eventually used in the construction industry because of their effect on the mechanical performance. Nonetheless, over the last decades, fibers have been used for new functional purposes; especially, conductive fibers such as steel or CF [33]. Concrete is a dielectric material. However, the addition of a conductive admixture, such as carbon materials (CF [34], CNF [35], CNT [36]), transforms that composite into a conductive material. This yields the possibility of using the new cement based composite for other applications different to structural purposes. It can be therefore considered a multifunctional material. Among diverse potential applications, the most common functions are as follows: strain-sensing [37–39], damage-sensing [35,40], EMI-shielding [41,42], resistance heating [43–45] and electrical contact for chloride extraction [46–48] or cathodic protection [49–51]. This work is focused on the application of carbon fiber reinforced cement composites (CFRCC) as strain sensors. This sensing function is defined by the ability of a structural material to detect its own deformation when subjected to some external load. If a longitudinal compressive stress is applied, the electrical resistance in that direction is reduced, and vice versa when the material is under tension. Both effects are reversible in the material’s elastic range when conductive admixture is sufficiently added [52]; therefore, the baseline electrical resistance (when load is equal to zero) returns to its initial value if compressive loads are below 30% of the material’s strength [26]. The strain sensing capacity is quantified by the gage factor (GF), as the change of the electrical resistance (or resistivity) per unit strain, Eq. (1), in which: DR refers to the variation in resistance; R0 refers to the initial resistivity; Dl refers to the longitudinal deformation; l0 refers to the initial length and e refers to the longitudinal strain.
GF ¼
DR=R0 DR=R0 ¼ Dl=l0 e
ð1Þ
In order to know the limits of the CF addition, it is important to know the value of the property called percolation threshold, i.e. the minimum CF amount that guarantees a continuous path for the electrical current [34]. For a given CF geometry, the increase in CF content leads to a decrease in resistivity [37]. The typical resistivity vs CF addition function is an S-shaped curve, meaning that for values above and below the percolation phenomena it does not exhibit significant change, but it shows several orders of magnitude of decrease for CF% close to the percolation threshold [33]. Therefore, once the percolation threshold is reached, from a conductivity point of view, the increase in CF dosage does not make any sense because resistivity does not vary significantly. Each particular application requires a different level of conductivity. Hence, there is a huge number of possible combinations of type and dosages of admixtures. As a general rule, as the fibers aspect ratio increases, percolation is achieved with lower CF content [33,37,53]. Therefore, in order to achieve the highest conductivity, possible combinations of either CF or CNT would be desirable [44,54]. The use of conductive waste materials has been also tested, e.g. carbon black [55] or steel shavings [56]. In fact, a
combination of conductive admixtures has been demonstrated to be the best solution for reach balance between material costs, waste recycling and functional properties [56]. The contribution of EAF steel slag aggregates to conductive properties of cement based materials has not been extensively addressed, in previous research works thereby constituting a pioneering approach aiming to achieve novel upcycling valorization solutions for such industrial by-products. EAF slag aggregates have been used in combination with carbon fibers to improve the electrical properties of CFRCC. In this paper, the use of EAF slag fine aggregate in CF reinforced cement mortars is addressed. EAF slag aggregates exhibit higher conductivity when comparing to conventional siliceous or limestone aggregates. The new derived cement based composites are therefore expected to present enhanced conductivity, while optimizing the CF content. Hence, the main objective of this paper is to study the effect of this EAF slag fine aggregate on the electrical properties of the material: resistivity and strain-sensing capacity. As a result, the multifunctional properties of CFRCC (enhanced by conductive aggregates) will be combined with an improvement in sustainability due to the incorporation of recycled industrial by-products. 2. Materials and Methods 2.1. Materials and preparation of specimens Portland cement mortars with carbon fiber (CF) additions were fabricated. Portland cement, type CEM II B/L 32.5R, according to UNE-EN 197-1:2011 (‘‘Cement. Part 1: Composition, specifications and conformity criteria for common cement”), was used. Unsized PAN-based CF, type PANEX 35, supplied by Zoltek (see properties in Table 1). A control sample (without CF) and four CF dosages were prepared, i.e. CF% by cement mass were 0%, 1%, 2%, 3% and 4%. According to previous research works, high contents of CF could lead to a reduction in the workability of the fresh mix, thereby producing cement based composites with higher porosity [37]. Therefore, in order to achieve good workability conditions, the water/cement ratio in this work was 0.5 for CF dosages below 3%, and was increased to 0.6 for 3% and 4% CF additions. Two different types of fine aggregates (with particles sizes ranging between 0 and 4 mm) were used. Type 1 was a regular limestone aggregate from the Basque Country (North Spain). The limestone aggregates were mainly composed of calcite (95%) and dolomite (5%) used in conventional concrete fabrication, as control dosages. Type 2 was a granular valorized by-product from the steel industry (EAF steel aggregate), whose physical characteristics are detailed in Table 2, while chemical characteristics are shown in Table 3. In both cases, a 3:1 sand/cement ratio was used. EAF slag aggregates met all the requirements to be used as aggregates in mortars, as specified in the UNE-EN 13139-1 standard (‘‘Aggregates for mortar”). It is worth highlighting that water absorption of the EAF slag aggregates exhibited higher values than those revealed by the natural aggregates, as observed by other researchers [18].
Table 1 Characterization of Carbon fibers. Type
PANEX 35
Diameter Carbon content Tensile strength Elastic modulus Resistivity Density
7.2 mm 95% 3800 MPa 242 GPa 1.52 103 Xcm 1.81 g/cm3
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F.J. Baeza et al. / Construction and Building Materials 184 (2018) 311–319 Table 2 Characterization of the EAF slag aggregates. Property
Value
Particle size Sand equivalent UNE EN 933-8 Methylene blue UNE EN 933-9 Coefficient of friability UNE 83.115 EX Water absorption UNE EN 1097-6 Particle density UNE EN 1097-6 Oven-dried density UNE EN 1097-6
0–4 mm 76.7% 0.3 g/kg 10% 2.1% 3.67 g/cm3 3.59 g/cm3
100%) by EAF slag aggregates. Portland cement, type CEM II A/L 42.5R, and mixing water from the urban supply of the city of Bilbao were used. Three prismatic specimens of each dosage were prepared with the following dimensions: 40x40x160 mm3. After having been casted and demolded, the specimens were cured at lab-controlled conditions of 20 °C and RH > 99% until testing. Both flexural and compressive strength of the blended cement mortars were determined as per EN 196-1 at 7 curing days (Fig. 1). 2.3. Electrical resistivity measures
In order to optimize fiber dispersion, CF was subjected to two types of treatment, oxidation and sonication, prior to their incorporation in the mix. All samples were treated by ultrasounds, but in order to assess the effectiveness of oxidation all CF-aggregate dosages were prepared twice, once with oxidized fibers, and the other with the fibers as received from the supplier (only with sonication). The first treatment, oxidation, was conducted by placing the fibers in an electric furnace at 400 °C with an air flow of 10 ml/min for 4 h [57]. This treatment served to (i) remove any possible sizing so as to expose the carbon in the fiber, and (ii) to form oxygen-containing functional groups on the surface of the carbon in the fiber, so improving the wettability of the fiber by water and strengthen the fiber-matrix bonding [57]. Subsequent to oxidation, a second treatment was applied on the same fibers. This second treatment involved sonication after the fibers had been stirred by hand in water. Sonication was applied using an ultrasonic device; model Hielschier UP200S, at maximum power for 10 min. After these preliminary treatments, all components were mixed according to the UNE-EN 196-1:2005 standard (‘‘Methods of testing cement. Part 1: Determination of strength”), and prismatic specimens of size 160 40 40 mm3 were prepared. After casting and demolding, specimens were kept in 20 °C and RH > 99% for 28 days.
After curing for 28 days, silver electrically conductive paint was applied around the perimeter at the four interior planes that are parallel to the end surfaces, and then copper wire was wrapped around them (Fig. 2) to form four electrical contacts that are required for the four-probe method of electrical resistance measurement. The electric current input was passed through contacts 3 and 6 (Fig. 2), using an external AC/DC current source (Keithley Model 6020), while resistance measurement was made between contacts 4 and 5, using a digital multimeter (Keithley Model 2002). 2.4. Strain sensing tests The strain sensing tests were made with 40 40 160 mm3 specimens, loaded in the longitudinal direction by an electromechanical press model Microtest 10 t/2t (Servosis, S.A., Spain), with a 20 kN loading cell. In order to monitor longitudinal strains a
2.2. Compressive strength tests The compressive strength of Portland cement mortars containing fine EAF slag aggregates was studied. The normalized siliceous fine aggregates (0–2 mm) were partially replaced (0%, 50% and
Fig. 2. Specimen dimensions and electrical contact configuration: 3 and 6 are current input contacts; 4 and 5 are voltage measurement contacts.
Table 3 Chemical characteristics of the EAF slag aggregates. Composition
Fe2O3
CaO
SiO2
MgO
MnO
Al2O3
Cr2O3
Na2O
TiO2
P2O5
Cl
Value (%)
40.67
22.54
9.06
7.56
7.35
6.20
3.00
1.70
0.425
0.35
0.32
Fig. 1. Flexural and compressive strength tests of the blended cement mortars.
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strain gage was attached to the middle section (Fig. 2), and a Vishay P3 extensometer was used to register strains at 1 Hz frequency. Each regular test comprised three consecutive loading and unloading cycles. Each sample was tested at three different maximum loads (2, 4 and 8 kN, equivalent to 1.25, 2.5 and 5 MPa) and loading rates (50, 100, 200 and 400 N/s). In previous research works, a slight influence of the current intensity during the electrical measures was also observed [55]. Hence, current intensity was fixed at 0.1, 1 or 10 mA, depending on the level of conductivity of each sample. Performing each single test, samples were kept in a desiccator at room temperature and water saturated ambient (100% RH). In order to evaluate the sensitivity of each cement based composite to its own strains, the aforementioned gage factor was used, Eq. (1). For example, for a fixed strain value, the higher gage factor is, the wider the composite’s electrical response amplitude (resistivity fractional change) is. Thus, the sensor’s sensitivity will be improved.
3. Experimental results and discussion 3.1. Mechanical properties Table 4 shows the flexural and compressive strength values at 7 curing days of those blended cement mortars containing 0%, 50% and 100% fine EAF slag aggregates. The mortars elaborated even with 100% of EAF slag as fine aggregate, revealed values of flexural and compressive strength 16% and 10% respectively higher, than that exhibited by the reference mortar (0% EAF slag) at 7 curing days. The increase in mechanical strength is attributed to a denser interfacial transition zone (ITZ) between the cementitious matrix and the slag aggregates, as it was previously reported by part of this research team [16,58]. Those previous works also revealed that
Table 4 Flexural and compressive strength values at 7 days of the mortars made with 0%, 50% and 100% of EAF slag aggregates. Mortar
Flexural strength (MPa)
Compressive strength (MPa)
0% EAF slag aggregate 50% EAF slag aggregate 100% EAF slag aggregate
7.9 8.5 9.2
38.4 42.3 42.5
the ITZ evidences lower micro porosity and closer contact between the EAF slag and the cement matrix, when comparing to limestone aggregate mortars. 3.2. Electrical resistivity First, Fig. 3 shows the relationship between mass and volume fractions of each series. Four different series have been included as a function of the aggregate type (slag or limestone) and CF treatment (oxidized or not). A linear relationship between both fractions was obtained in all cases. CF oxidation treatment did not show a significant influence on the obtained results. Actually, linear regression functions were almost the same for all composites containing the same aggregates. Nevertheless, a minor difference was observed in relation to the type of aggregate. The use of slag, as fine aggregate, within the mortar led to higher volumes of CF for the same mass ratio. Those differences can be attributed to the higher specific gravity of steel aggregate when comparing to the limestone aggregate. Table 5 summarizes the resistivity values for each mix proportioning. These values were between 2.24 MOcm and 65.3 Ocm for the control sample containing limestone aggregates and a 1.29 vol % CF mortar with steel slag aggregates, respectively. As a general
Table 5 CFRCC resistivity for each aggregate type, fiber dosage and fiber oxidation. CF% (by cement mass)
CF vol%
Oxidized CF
Aggregate
Resistivity (Ocm)
0% 1% 2% 3% 4% 1% 2% 3% 4% 0% 1% 2% 3% 4% 1% 2% 3% 4%
0% 0.29% 0.58% 0.83% 1.09% 0.29% 0.58% 0.83% 1.10% 0% 0.34% 0.68% 0.98% 1.29% 0.33% 0.65% 0.97% 1.29%
– No No No No Yes Yes Yes Yes – No No No No Yes Yes Yes Yes
Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag
2.24106 7.96105 4.66105 1.10105 6.16104 6.31105 4.81104 2.21103 327.4 8.93105 2.45105 7.21104 3.68104 2.41104 1.69104 1.87103 170.1 65.3
Fig. 3. Volumetric fraction vs mass ratio (by cement mass) of CFRCC using steel slag or limestone aggregates and as received or oxidized CF. Linear regression functions are included for each aggregate-fiber combination.
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trend, mortars with slag aggregates exhibited lower resistivity values than their limestones counterparts did. Besides, the use of oxidized CFs also led to composites with enhanced conductivity, due to the aforementioned functionalization effect of fiber’s surface [57]. Fig. 4(a) includes four different series for each combination of aggregate type and fiber surface treatment. The percolation phenomena can be observed, as resistivity decreases for higher CF fractions in the composite. However, only two series (Slag + CFOX and Limestone + CFOX) could have reached the percolation threshold, as represented in Fig. 4(b), which shows a magnification of the results. This fact could entail that oxidized fibers, as expected, were much better dispersed within the matrix, considering resistivity as a method to evaluate the dispersion [26]. Non-oxidized fibers could only reduce the resistivity of control samples by 36 times (4%FC3 + limestone) or 37 times (4%FC3 + slag), whereas oxidized fibers reduce the resistivity of control samples by 6851 times (4% FC3OX + limestone) or 13,675 times (4%FC3OX + slag). The effect of higher CF dosages could be directly observed in Fig. 4(a). However, in order to differentiate the contribution to conductivity of each variable (slag aggregates, CF dosage and oxidation treatment), different resistivity ratios have been represented in Fig. 5. First, Fig. 5(a) includes the ratios between the resistivity of pastes with oxidized fibers with respect their counterparts with the same CF%, but using non-oxidized fibers. All samples with treated fibers presented higher conductivity, and most of them at least 10 times higher (qCFox =qCF 6 0:1) only because of oxidation.
315
Besides, there was a synergy between oxidation and the use of slag aggregates, as the benefits of oxidizing CF was higher for all samples with recycled slag aggregates, instead of regular limestone aggregates. Obviously, oxidation effect was also magnified when the CF dosage was increased. On the other hand, the resistivity ratio between both types of aggregates is represented in Fig. 5 (b). In this case, the conductivity enhancement caused by the slag aggregates was smaller than the effect of fiber’s oxidation. Nonetheless, all samples’ resistivity decreased more than 60% when steel slag was used as fine aggregates. Once again, the combined used of steel slag and oxidized CF showed a synergic effect with conductivity increases of at least 80%, in the dosages tested in this research. However, higher CF dosages reduced the benefits of slag aggregates, probably because electronic conductivity prevails as the fiber content increases, while slag aggregates influence mainly the electrolytic conductivity. Resistivity in plain concrete (or in general cement composites) [4] is strictly connected with porosity, pore solution chemistry and tortuosity [59]. Sun et al. [60] proposed a constitutive model for the electrical behavior of CFRCC, which comprised four possible conductive paths. (1) water slurry inside the ion-conductive media; (2) electronically conductive fibers in the slurry and conductive holes; (3) electronically conductive paths between the fibers and the continuous conductive holes; and (4) electronically conductive fibers through the conductive network past the conductive hole. It is known from experiments that when the fiber content is low, current travels
Fig. 4. (a) Resistivity vs CF volumetric fraction of mortars with steel or limestone aggregates and different fiber treatment (oxidation and sonication or only sonication). (b) Magnification of the results for the only two series that reached the CF percolation threshold.
Fig. 5. Conductivity enhancement for different CF%, expressed as the ratio between resistivity of equivalent dosages with oxidized vs regular fibers (a), and steel slag vs limestone aggregates (b).
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mainly via paths (1) and (2). As the fiber content slightly increases, transmission occurs mainly via paths (2) and (3). When the fiber content increases further, path (4) becomes the main pathway. According to reference [61], the electrical conduction in CFRC with a fiber volume fraction below the percolation threshold involves electrons and ions. The fibers affect both the electronic and the ionic conductions. Besides, surface treatment of fibers, like ozone treatment, helped the ionic conduction [61]. Fig. 5 shows a similar effect of conductivity enhancement because of fiber’s oxidation. This treatment, in addition to helping electronic and electrolytic conductions, has been proven useful to disperse fibers in cement matrix [57]. On the other hand, slag aggregates also reduced the resistivity of all cement pastes, especially those with oxidized fibers (Fig. 5). Similar effects have been reported showing synergies between CF and other particles, like carbon black [62], which influenced ion mobility along particles surfaces. Samples with steel slag aggregates achieve a fully percolated state for CF volume fractions above 1%. While mortars made with
limestone aggregates with 1.1% of CF should be also percolated. Nonetheless, due to the limited number of dosages prepared in the present study, and considering the resistivity trends observed in similar works [34], probably the percolation threshold of a steel slag-aggregate mortar would be between 3 and 4% (by cement mass), i.e. approximately for a CF volume fraction between 0.7 and 1%. As a summary, the effect of steel slag aggregates on the electrical conductivity has been evaluated, and the conductivity enhancement due to conductive aggregates has been proven. Therefore, the effect of these aggregates can be seen on either electronic (high CF dosages) or electrolytic (low CF dosages) conductions [63]. 3.3. Strain sensing properties The first objective of this research had been fulfilled, and the effect of slag aggregates on the resistivity of the CFRCC has been assessed. Therefore, the next phase was designed to test the strain
Fig. 6. Strain sensing results, resistivity and strain time functions for different conditions (type of aggregate, CF% by cement mass and oxidize or not, and different loading levels 2–8 kN).
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sensing capacity of CFRCC with recycled slag aggregates. For this purpose, all samples were tested in water saturation state after a curing period of 28 days. Each self-sensing test comprised three consecutive loading-unloading cycles up to a maximum compressive strength of 1.25, 2.5 or 5 MPa. Three different loading rates were tested for each load level. This methodology had been successfully used in prior research regarding strain-sensing capacity of cement pastes [12,37]. Fig. 6 summarizes all the different responses (resistivity and strain curves are given) obtained under different conditions, i.e. samples with different CF addition or fiber oxidation treatment, and both types of aggregates used in this research. First, Fig. 6(a) includes the behavior of the control sample of limestone mortar, in which loading cycles could be observed in the electrical response, but the water saturation conditions (necessary for nonelectronic conduction in the absence of fiber) makes the response non-reversible, given the important role played by polarization [64]. Another problem that some CFRCC present is low sensitivity at small deformations [37]. This effect was detected for mortars with oxidized CF (3% by cement mass) when tested only up to 2 kN (1.25 MPa), which generated only 55 me. However, these composites showed a good resistivity-strain correlation when loaded up to 8 kN (5 MPa), Fig. 6(d). If Fig. 6(c) and (d) are compared, the effect of oxidation treatment of CF can be discussed. The electrical response of mortars reinforced with oxidized fibers was better than if fibers were used without this treatment. This effect can also be seen in Fig. 6(e) and (f) for mortars with slag aggregates. In order to compare the sensitivity of each dosage, GF was calculated for every set of tests. Table 6 summarizes the average gage factors and dispersion values for all dosages that showed a strainsensing response. Only mortars with at least 3%CF (by cement mass), 0.83 vol% approx., showed a proper sensing behavior in water saturation conditions, in which samples were kept. Mortars with lower amounts of CF did not present a proper resistivitystrain correlation. This bad sensing behavior can be related to their higher resistivity, which makes them more sensitive to polarization phenomena due to the prevalence of electrolytic conduction. In other studies in cement and alkali activated slag composites with CF the optimal sensing behavior was observed for intermediate humidity conditions, i.e. water saturation degree between 40 and 60% [37,65]. Therefore, in future research this issue should be properly addressed to optimize the material response. The sensitivity of all composites was in the same order of magnitude, i.e. gage factors between 4 and 13. However, samples with slag aggregates and oxidized fibers showed lower dispersion values. Fig. 7 includes the change of resistivity vs strain curve, whose slope is actually the gage factor value (linear regression equations and r2 Pearson coefficients are included for each dosage). Despite limestone aggregates showing the highest sensitivity, slag-aggregate mortars with 4% oxidized CF were more stable, i.e. the dispersion was lower as observed in the higher r2 coefficient. The GF difference between the 3%CF limestone mortars and 4%CF slag mortars
Table 6 Strain sensing results, average gage factor and standard deviation (SD) for all dosages that showed proper strain-sensing response. CF% (by cement mass)
CF vol%
Oxidized CF
Aggregate
Average gage factor ± SD
3% 3% 3% 4% 4% 4% 4%
0.83% 0.98% 0.97% 1.09% 1.10% 1.29% 1.29%
Yes No Yes No Yes No Yes
Limestone Steel Slag Steel Slag Limestone Limestone Steel Slag Steel Slag
12.90 ± 2.22 4.82 ± 0.97 5.29 ± 1.03 4.86 ± 1.29 8.46 ± 1.58 7.24 ± 1.50 4.73 ± 0.36
317
Fig. 7. Fractional change of resistance vs strain curves for three different mortars.
(both with oxidized fibers, can be justified by the difference of conductivity between both composites (2210 and 65.3 Ocm respectively). Strain sensing does not require high conductivity values, but usually composites with lower conductivity show higher sensitivities (i.e. GF up to 170 in Portland cement pastes) [37]. Nonetheless, for a practical use of this technology, it would be more effective to consider lowest dispersion (i.e. response stability in time) as well as sensitivity.
4. Conclusions PAN CF reinforced cement mortars with different aggregates were fabricated. The effectiveness and viability of using a recycled electric arc furnace (EAF) slag as fine aggregates were evaluated, and the impact of this industrial waste on the electrical conductivity of cement composites was also assessed. Finally, the possibility of using these new composites as strain sensors was also studied. Based on the results of this experimental investigation, the following conclusions may be drawn: Cement mortars containing EAF slag aggregates compared to standard cement mortars (with silica aggregates) showed an increase in the flexural and compressive strengths at 7 days. The electrical resistivity of CF reinforced cement mortars was lower if steel slag aggregates (from industrial slag recycling) were used. Percolation threshold was reached at lower levels of CF content if limestone fine aggregates (percolation threshold approx. at 1.1 vol%) were substituted by slag particles (lower than 0.97 vol%). If CF underwent an oxidation treatment, the conductivity of the resulting composites was also enhanced for the same CF addition. Resistivity decreased by several orders of magnitude, especially for higher CF dosages where electronic conduction has a prevailing role. Self-sensing capacity was observed for composites with both types of aggregates—limestone and recycled EAF slag—and CF dosages higher than 0.8 vol%. A good correlation between electrical resistivity and longitudinal strain was registered with carbon fiber cement mortars with steel slag aggregates. In fact, the best linear regressions between the electrical and mechanical measures were achieved for EAF slag cement mortar containing 1.29 vol% of oxidized carbon fibers.
Conflict of interest None.
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Acknowledgments This research was partially funded by the Basque Regional Government (Hazitek 2015 Programme number: ZL-2015/00163). The authors also want to thank SIDENOR for supplying the EAF slags and to HORMIGONES Y MORTEROS AGOTE for the slag valorization (crushing, sieving and ripening process). References [1] A. Maciá, F.J. Baeza, J.M. Saval, S. Ivorra, Mechanical properties of boards made in biocomposites reinforced with wood and Posidonia oceanica fibers, Compos. Part B Eng. 104 (2016) 1–8, https://doi.org/10.1016/j.compositesb.2016.08.018. [2] D. Foti, Innovative techniques for concrete reinforcement with polymers, Constr. Build. Mater. 112 (2016) 202–209, https://doi.org/10.1016/ j.conbuildmat.2016.02.111. [3] F. Baeza, J. Payá, O. Galao, J.M. Saval, P. Garcés, Blending of industrial waste from different sources as partial substitution of Portland cement in pastes and mortars, Constr. Build. Mater. 66 (2014) 645–653, https://doi.org/10.1016/ j.conbuildmat.2014.05.089. 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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of recycled slag aggregates on the conductivity and strain sensing capacity of carbon fiber reinforced cement mortars F.J. Baeza a, O. Galao a, I.J. Vegas b, M. Cano b, P. Garcés a,⇑ a b
Department of Civil Engineering. University of Alicante, Apartado de Correos 99, 03080 Alicante, Spain Tecnalia. C/Geldo, Parque Tecnológico de Bizkaia, Edificio 700, 48160 Derio, Spain
h i g h l i g h t s Cement mortars with Electric Arc Furnace slag aggregates show higher strengths. Recycled EAF slag aggregates enhance the conductivity of CF reinforced mortars. Percolation was achieved with lower CF dosage for EAF slag aggregate mortars. CF reinforced mortars with EAF slag aggregates presented strain sensing capacity. The highest sensitivity was measured for EAF slag mortars with 1.9 vol% CF.
a r t i c l e
i n f o
Article history: Received 14 November 2017 Received in revised form 15 May 2018 Accepted 27 June 2018
Keywords: EFA slag Recycled aggregates CFRCC Percolation Carbon fibre Electrical resistivity
a b s t r a c t The electrical resistivity of carbon fiber reinforced cement composites (CFRCC) has been widely studied for the functional applications of these composites. CFRCC with enhanced electrical properties can be used as strain or damage sensor, heating or deicing material, or anode in different electrochemical techniques, like chloride extraction or cathodic protection. In this work, carbon fiber reinforced cement mortars have been prepared using a conductive aggregate produced from the valorization of Electric Arc Furnace slag (EAF slag). Cement based mortars containing EAF slag aggregates revealed lower resistivity; hence, the carbon fiber percolation threshold decreased. Additionally, the strain sensing capacity of those composites with conductive aggregates was also enhanced, indicating similar sensitivity and lower dispersion than equivalent mortars containing limestone aggregates. The best linear regressions between the electrical and mechanical measures were achieved for EAF slag cement mortar containing 1.29 vol % of oxidized carbon fibers. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The construction industry is constantly developing novel solutions for the valorization of waste materials and industrial byproducts [1–5]. Some of these applications emerge as an upgraded option to turn waste materials into composites with enhanced performance. The use of supplementary cementitious materials in concrete manufacturing transformed conventional concrete into a material with higher mechanical properties and durability: e.g., silica fume [6,7], blast furnace slag [8,9], or spent cracking catalyst residue [10,11]. Currently other waste reuses are being tested as an alternative to Portland cement use in concrete, e.g. alkali activated blast furnace slags [12–14].
⇑ Corresponding author. E-mail address: [email protected] (P. Garcés). https://doi.org/10.1016/j.conbuildmat.2018.06.218 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
Electric Arc Furnace slag (EAF slag) is a by-product from the steelmaking industry, produced from the melting and the preliminary acid refining of the liquid steel. EAF slag can be easily crushed to produce granular materials for use in both civil and building applications: EAF slag powder has been used as supplementary cementitious material [15], EAF slag aggregates have been used in concrete fabrication [16] or in road construction [17]. Concrete manufactured with partial or total replacement of conventional limestone or siliceous aggregates by EAF slag aggregates exhibited higher compressive, bending strength, durability [5,18–20], and rebar-concrete bonding strength [21]. On the other hand, conductive concrete emerges as an interesting alternative giving response to new challenges in construction (smart infrastructures or monitoring of durability) [22–24]. Conductive aggregates like iron ore or slags, might contribute to enhancing conductivity for that purpose [25]. Some metallic oxides contained in steel slags reveal conductivities in the range of pitch-based carbon fiber (CF) [26].
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Concrete and, in general, those materials evidencing a brittle tension or flexural behavior have been historically combined with other elements showing superior tensile properties [27–29]. The addition of short fibers (e.g. steel, carbon, polypropylene) to the mixture can improve the mechanical performance of the fiberreinforced concrete (FRC), i.e. higher compressive strength [30], enhanced bending behavior [31], better residual properties after high temperature exposure [32]. Furthermore, all kind of fibers (even those without structural purpose) have been traditionally added to cement composites to control creep and shrinkage [8,13]. In general, fibers have been eventually used in the construction industry because of their effect on the mechanical performance. Nonetheless, over the last decades, fibers have been used for new functional purposes; especially, conductive fibers such as steel or CF [33]. Concrete is a dielectric material. However, the addition of a conductive admixture, such as carbon materials (CF [34], CNF [35], CNT [36]), transforms that composite into a conductive material. This yields the possibility of using the new cement based composite for other applications different to structural purposes. It can be therefore considered a multifunctional material. Among diverse potential applications, the most common functions are as follows: strain-sensing [37–39], damage-sensing [35,40], EMI-shielding [41,42], resistance heating [43–45] and electrical contact for chloride extraction [46–48] or cathodic protection [49–51]. This work is focused on the application of carbon fiber reinforced cement composites (CFRCC) as strain sensors. This sensing function is defined by the ability of a structural material to detect its own deformation when subjected to some external load. If a longitudinal compressive stress is applied, the electrical resistance in that direction is reduced, and vice versa when the material is under tension. Both effects are reversible in the material’s elastic range when conductive admixture is sufficiently added [52]; therefore, the baseline electrical resistance (when load is equal to zero) returns to its initial value if compressive loads are below 30% of the material’s strength [26]. The strain sensing capacity is quantified by the gage factor (GF), as the change of the electrical resistance (or resistivity) per unit strain, Eq. (1), in which: DR refers to the variation in resistance; R0 refers to the initial resistivity; Dl refers to the longitudinal deformation; l0 refers to the initial length and e refers to the longitudinal strain.
GF ¼
DR=R0 DR=R0 ¼ Dl=l0 e
ð1Þ
In order to know the limits of the CF addition, it is important to know the value of the property called percolation threshold, i.e. the minimum CF amount that guarantees a continuous path for the electrical current [34]. For a given CF geometry, the increase in CF content leads to a decrease in resistivity [37]. The typical resistivity vs CF addition function is an S-shaped curve, meaning that for values above and below the percolation phenomena it does not exhibit significant change, but it shows several orders of magnitude of decrease for CF% close to the percolation threshold [33]. Therefore, once the percolation threshold is reached, from a conductivity point of view, the increase in CF dosage does not make any sense because resistivity does not vary significantly. Each particular application requires a different level of conductivity. Hence, there is a huge number of possible combinations of type and dosages of admixtures. As a general rule, as the fibers aspect ratio increases, percolation is achieved with lower CF content [33,37,53]. Therefore, in order to achieve the highest conductivity, possible combinations of either CF or CNT would be desirable [44,54]. The use of conductive waste materials has been also tested, e.g. carbon black [55] or steel shavings [56]. In fact, a
combination of conductive admixtures has been demonstrated to be the best solution for reach balance between material costs, waste recycling and functional properties [56]. The contribution of EAF steel slag aggregates to conductive properties of cement based materials has not been extensively addressed, in previous research works thereby constituting a pioneering approach aiming to achieve novel upcycling valorization solutions for such industrial by-products. EAF slag aggregates have been used in combination with carbon fibers to improve the electrical properties of CFRCC. In this paper, the use of EAF slag fine aggregate in CF reinforced cement mortars is addressed. EAF slag aggregates exhibit higher conductivity when comparing to conventional siliceous or limestone aggregates. The new derived cement based composites are therefore expected to present enhanced conductivity, while optimizing the CF content. Hence, the main objective of this paper is to study the effect of this EAF slag fine aggregate on the electrical properties of the material: resistivity and strain-sensing capacity. As a result, the multifunctional properties of CFRCC (enhanced by conductive aggregates) will be combined with an improvement in sustainability due to the incorporation of recycled industrial by-products. 2. Materials and Methods 2.1. Materials and preparation of specimens Portland cement mortars with carbon fiber (CF) additions were fabricated. Portland cement, type CEM II B/L 32.5R, according to UNE-EN 197-1:2011 (‘‘Cement. Part 1: Composition, specifications and conformity criteria for common cement”), was used. Unsized PAN-based CF, type PANEX 35, supplied by Zoltek (see properties in Table 1). A control sample (without CF) and four CF dosages were prepared, i.e. CF% by cement mass were 0%, 1%, 2%, 3% and 4%. According to previous research works, high contents of CF could lead to a reduction in the workability of the fresh mix, thereby producing cement based composites with higher porosity [37]. Therefore, in order to achieve good workability conditions, the water/cement ratio in this work was 0.5 for CF dosages below 3%, and was increased to 0.6 for 3% and 4% CF additions. Two different types of fine aggregates (with particles sizes ranging between 0 and 4 mm) were used. Type 1 was a regular limestone aggregate from the Basque Country (North Spain). The limestone aggregates were mainly composed of calcite (95%) and dolomite (5%) used in conventional concrete fabrication, as control dosages. Type 2 was a granular valorized by-product from the steel industry (EAF steel aggregate), whose physical characteristics are detailed in Table 2, while chemical characteristics are shown in Table 3. In both cases, a 3:1 sand/cement ratio was used. EAF slag aggregates met all the requirements to be used as aggregates in mortars, as specified in the UNE-EN 13139-1 standard (‘‘Aggregates for mortar”). It is worth highlighting that water absorption of the EAF slag aggregates exhibited higher values than those revealed by the natural aggregates, as observed by other researchers [18].
Table 1 Characterization of Carbon fibers. Type
PANEX 35
Diameter Carbon content Tensile strength Elastic modulus Resistivity Density
7.2 mm 95% 3800 MPa 242 GPa 1.52 103 Xcm 1.81 g/cm3
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Value
Particle size Sand equivalent UNE EN 933-8 Methylene blue UNE EN 933-9 Coefficient of friability UNE 83.115 EX Water absorption UNE EN 1097-6 Particle density UNE EN 1097-6 Oven-dried density UNE EN 1097-6
0–4 mm 76.7% 0.3 g/kg 10% 2.1% 3.67 g/cm3 3.59 g/cm3
100%) by EAF slag aggregates. Portland cement, type CEM II A/L 42.5R, and mixing water from the urban supply of the city of Bilbao were used. Three prismatic specimens of each dosage were prepared with the following dimensions: 40x40x160 mm3. After having been casted and demolded, the specimens were cured at lab-controlled conditions of 20 °C and RH > 99% until testing. Both flexural and compressive strength of the blended cement mortars were determined as per EN 196-1 at 7 curing days (Fig. 1). 2.3. Electrical resistivity measures
In order to optimize fiber dispersion, CF was subjected to two types of treatment, oxidation and sonication, prior to their incorporation in the mix. All samples were treated by ultrasounds, but in order to assess the effectiveness of oxidation all CF-aggregate dosages were prepared twice, once with oxidized fibers, and the other with the fibers as received from the supplier (only with sonication). The first treatment, oxidation, was conducted by placing the fibers in an electric furnace at 400 °C with an air flow of 10 ml/min for 4 h [57]. This treatment served to (i) remove any possible sizing so as to expose the carbon in the fiber, and (ii) to form oxygen-containing functional groups on the surface of the carbon in the fiber, so improving the wettability of the fiber by water and strengthen the fiber-matrix bonding [57]. Subsequent to oxidation, a second treatment was applied on the same fibers. This second treatment involved sonication after the fibers had been stirred by hand in water. Sonication was applied using an ultrasonic device; model Hielschier UP200S, at maximum power for 10 min. After these preliminary treatments, all components were mixed according to the UNE-EN 196-1:2005 standard (‘‘Methods of testing cement. Part 1: Determination of strength”), and prismatic specimens of size 160 40 40 mm3 were prepared. After casting and demolding, specimens were kept in 20 °C and RH > 99% for 28 days.
After curing for 28 days, silver electrically conductive paint was applied around the perimeter at the four interior planes that are parallel to the end surfaces, and then copper wire was wrapped around them (Fig. 2) to form four electrical contacts that are required for the four-probe method of electrical resistance measurement. The electric current input was passed through contacts 3 and 6 (Fig. 2), using an external AC/DC current source (Keithley Model 6020), while resistance measurement was made between contacts 4 and 5, using a digital multimeter (Keithley Model 2002). 2.4. Strain sensing tests The strain sensing tests were made with 40 40 160 mm3 specimens, loaded in the longitudinal direction by an electromechanical press model Microtest 10 t/2t (Servosis, S.A., Spain), with a 20 kN loading cell. In order to monitor longitudinal strains a
2.2. Compressive strength tests The compressive strength of Portland cement mortars containing fine EAF slag aggregates was studied. The normalized siliceous fine aggregates (0–2 mm) were partially replaced (0%, 50% and
Fig. 2. Specimen dimensions and electrical contact configuration: 3 and 6 are current input contacts; 4 and 5 are voltage measurement contacts.
Table 3 Chemical characteristics of the EAF slag aggregates. Composition
Fe2O3
CaO
SiO2
MgO
MnO
Al2O3
Cr2O3
Na2O
TiO2
P2O5
Cl
Value (%)
40.67
22.54
9.06
7.56
7.35
6.20
3.00
1.70
0.425
0.35
0.32
Fig. 1. Flexural and compressive strength tests of the blended cement mortars.
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strain gage was attached to the middle section (Fig. 2), and a Vishay P3 extensometer was used to register strains at 1 Hz frequency. Each regular test comprised three consecutive loading and unloading cycles. Each sample was tested at three different maximum loads (2, 4 and 8 kN, equivalent to 1.25, 2.5 and 5 MPa) and loading rates (50, 100, 200 and 400 N/s). In previous research works, a slight influence of the current intensity during the electrical measures was also observed [55]. Hence, current intensity was fixed at 0.1, 1 or 10 mA, depending on the level of conductivity of each sample. Performing each single test, samples were kept in a desiccator at room temperature and water saturated ambient (100% RH). In order to evaluate the sensitivity of each cement based composite to its own strains, the aforementioned gage factor was used, Eq. (1). For example, for a fixed strain value, the higher gage factor is, the wider the composite’s electrical response amplitude (resistivity fractional change) is. Thus, the sensor’s sensitivity will be improved.
3. Experimental results and discussion 3.1. Mechanical properties Table 4 shows the flexural and compressive strength values at 7 curing days of those blended cement mortars containing 0%, 50% and 100% fine EAF slag aggregates. The mortars elaborated even with 100% of EAF slag as fine aggregate, revealed values of flexural and compressive strength 16% and 10% respectively higher, than that exhibited by the reference mortar (0% EAF slag) at 7 curing days. The increase in mechanical strength is attributed to a denser interfacial transition zone (ITZ) between the cementitious matrix and the slag aggregates, as it was previously reported by part of this research team [16,58]. Those previous works also revealed that
Table 4 Flexural and compressive strength values at 7 days of the mortars made with 0%, 50% and 100% of EAF slag aggregates. Mortar
Flexural strength (MPa)
Compressive strength (MPa)
0% EAF slag aggregate 50% EAF slag aggregate 100% EAF slag aggregate
7.9 8.5 9.2
38.4 42.3 42.5
the ITZ evidences lower micro porosity and closer contact between the EAF slag and the cement matrix, when comparing to limestone aggregate mortars. 3.2. Electrical resistivity First, Fig. 3 shows the relationship between mass and volume fractions of each series. Four different series have been included as a function of the aggregate type (slag or limestone) and CF treatment (oxidized or not). A linear relationship between both fractions was obtained in all cases. CF oxidation treatment did not show a significant influence on the obtained results. Actually, linear regression functions were almost the same for all composites containing the same aggregates. Nevertheless, a minor difference was observed in relation to the type of aggregate. The use of slag, as fine aggregate, within the mortar led to higher volumes of CF for the same mass ratio. Those differences can be attributed to the higher specific gravity of steel aggregate when comparing to the limestone aggregate. Table 5 summarizes the resistivity values for each mix proportioning. These values were between 2.24 MOcm and 65.3 Ocm for the control sample containing limestone aggregates and a 1.29 vol % CF mortar with steel slag aggregates, respectively. As a general
Table 5 CFRCC resistivity for each aggregate type, fiber dosage and fiber oxidation. CF% (by cement mass)
CF vol%
Oxidized CF
Aggregate
Resistivity (Ocm)
0% 1% 2% 3% 4% 1% 2% 3% 4% 0% 1% 2% 3% 4% 1% 2% 3% 4%
0% 0.29% 0.58% 0.83% 1.09% 0.29% 0.58% 0.83% 1.10% 0% 0.34% 0.68% 0.98% 1.29% 0.33% 0.65% 0.97% 1.29%
– No No No No Yes Yes Yes Yes – No No No No Yes Yes Yes Yes
Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag Steel Slag
2.24106 7.96105 4.66105 1.10105 6.16104 6.31105 4.81104 2.21103 327.4 8.93105 2.45105 7.21104 3.68104 2.41104 1.69104 1.87103 170.1 65.3
Fig. 3. Volumetric fraction vs mass ratio (by cement mass) of CFRCC using steel slag or limestone aggregates and as received or oxidized CF. Linear regression functions are included for each aggregate-fiber combination.
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trend, mortars with slag aggregates exhibited lower resistivity values than their limestones counterparts did. Besides, the use of oxidized CFs also led to composites with enhanced conductivity, due to the aforementioned functionalization effect of fiber’s surface [57]. Fig. 4(a) includes four different series for each combination of aggregate type and fiber surface treatment. The percolation phenomena can be observed, as resistivity decreases for higher CF fractions in the composite. However, only two series (Slag + CFOX and Limestone + CFOX) could have reached the percolation threshold, as represented in Fig. 4(b), which shows a magnification of the results. This fact could entail that oxidized fibers, as expected, were much better dispersed within the matrix, considering resistivity as a method to evaluate the dispersion [26]. Non-oxidized fibers could only reduce the resistivity of control samples by 36 times (4%FC3 + limestone) or 37 times (4%FC3 + slag), whereas oxidized fibers reduce the resistivity of control samples by 6851 times (4% FC3OX + limestone) or 13,675 times (4%FC3OX + slag). The effect of higher CF dosages could be directly observed in Fig. 4(a). However, in order to differentiate the contribution to conductivity of each variable (slag aggregates, CF dosage and oxidation treatment), different resistivity ratios have been represented in Fig. 5. First, Fig. 5(a) includes the ratios between the resistivity of pastes with oxidized fibers with respect their counterparts with the same CF%, but using non-oxidized fibers. All samples with treated fibers presented higher conductivity, and most of them at least 10 times higher (qCFox =qCF 6 0:1) only because of oxidation.
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Besides, there was a synergy between oxidation and the use of slag aggregates, as the benefits of oxidizing CF was higher for all samples with recycled slag aggregates, instead of regular limestone aggregates. Obviously, oxidation effect was also magnified when the CF dosage was increased. On the other hand, the resistivity ratio between both types of aggregates is represented in Fig. 5 (b). In this case, the conductivity enhancement caused by the slag aggregates was smaller than the effect of fiber’s oxidation. Nonetheless, all samples’ resistivity decreased more than 60% when steel slag was used as fine aggregates. Once again, the combined used of steel slag and oxidized CF showed a synergic effect with conductivity increases of at least 80%, in the dosages tested in this research. However, higher CF dosages reduced the benefits of slag aggregates, probably because electronic conductivity prevails as the fiber content increases, while slag aggregates influence mainly the electrolytic conductivity. Resistivity in plain concrete (or in general cement composites) [4] is strictly connected with porosity, pore solution chemistry and tortuosity [59]. Sun et al. [60] proposed a constitutive model for the electrical behavior of CFRCC, which comprised four possible conductive paths. (1) water slurry inside the ion-conductive media; (2) electronically conductive fibers in the slurry and conductive holes; (3) electronically conductive paths between the fibers and the continuous conductive holes; and (4) electronically conductive fibers through the conductive network past the conductive hole. It is known from experiments that when the fiber content is low, current travels
Fig. 4. (a) Resistivity vs CF volumetric fraction of mortars with steel or limestone aggregates and different fiber treatment (oxidation and sonication or only sonication). (b) Magnification of the results for the only two series that reached the CF percolation threshold.
Fig. 5. Conductivity enhancement for different CF%, expressed as the ratio between resistivity of equivalent dosages with oxidized vs regular fibers (a), and steel slag vs limestone aggregates (b).
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mainly via paths (1) and (2). As the fiber content slightly increases, transmission occurs mainly via paths (2) and (3). When the fiber content increases further, path (4) becomes the main pathway. According to reference [61], the electrical conduction in CFRC with a fiber volume fraction below the percolation threshold involves electrons and ions. The fibers affect both the electronic and the ionic conductions. Besides, surface treatment of fibers, like ozone treatment, helped the ionic conduction [61]. Fig. 5 shows a similar effect of conductivity enhancement because of fiber’s oxidation. This treatment, in addition to helping electronic and electrolytic conductions, has been proven useful to disperse fibers in cement matrix [57]. On the other hand, slag aggregates also reduced the resistivity of all cement pastes, especially those with oxidized fibers (Fig. 5). Similar effects have been reported showing synergies between CF and other particles, like carbon black [62], which influenced ion mobility along particles surfaces. Samples with steel slag aggregates achieve a fully percolated state for CF volume fractions above 1%. While mortars made with
limestone aggregates with 1.1% of CF should be also percolated. Nonetheless, due to the limited number of dosages prepared in the present study, and considering the resistivity trends observed in similar works [34], probably the percolation threshold of a steel slag-aggregate mortar would be between 3 and 4% (by cement mass), i.e. approximately for a CF volume fraction between 0.7 and 1%. As a summary, the effect of steel slag aggregates on the electrical conductivity has been evaluated, and the conductivity enhancement due to conductive aggregates has been proven. Therefore, the effect of these aggregates can be seen on either electronic (high CF dosages) or electrolytic (low CF dosages) conductions [63]. 3.3. Strain sensing properties The first objective of this research had been fulfilled, and the effect of slag aggregates on the resistivity of the CFRCC has been assessed. Therefore, the next phase was designed to test the strain
Fig. 6. Strain sensing results, resistivity and strain time functions for different conditions (type of aggregate, CF% by cement mass and oxidize or not, and different loading levels 2–8 kN).
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sensing capacity of CFRCC with recycled slag aggregates. For this purpose, all samples were tested in water saturation state after a curing period of 28 days. Each self-sensing test comprised three consecutive loading-unloading cycles up to a maximum compressive strength of 1.25, 2.5 or 5 MPa. Three different loading rates were tested for each load level. This methodology had been successfully used in prior research regarding strain-sensing capacity of cement pastes [12,37]. Fig. 6 summarizes all the different responses (resistivity and strain curves are given) obtained under different conditions, i.e. samples with different CF addition or fiber oxidation treatment, and both types of aggregates used in this research. First, Fig. 6(a) includes the behavior of the control sample of limestone mortar, in which loading cycles could be observed in the electrical response, but the water saturation conditions (necessary for nonelectronic conduction in the absence of fiber) makes the response non-reversible, given the important role played by polarization [64]. Another problem that some CFRCC present is low sensitivity at small deformations [37]. This effect was detected for mortars with oxidized CF (3% by cement mass) when tested only up to 2 kN (1.25 MPa), which generated only 55 me. However, these composites showed a good resistivity-strain correlation when loaded up to 8 kN (5 MPa), Fig. 6(d). If Fig. 6(c) and (d) are compared, the effect of oxidation treatment of CF can be discussed. The electrical response of mortars reinforced with oxidized fibers was better than if fibers were used without this treatment. This effect can also be seen in Fig. 6(e) and (f) for mortars with slag aggregates. In order to compare the sensitivity of each dosage, GF was calculated for every set of tests. Table 6 summarizes the average gage factors and dispersion values for all dosages that showed a strainsensing response. Only mortars with at least 3%CF (by cement mass), 0.83 vol% approx., showed a proper sensing behavior in water saturation conditions, in which samples were kept. Mortars with lower amounts of CF did not present a proper resistivitystrain correlation. This bad sensing behavior can be related to their higher resistivity, which makes them more sensitive to polarization phenomena due to the prevalence of electrolytic conduction. In other studies in cement and alkali activated slag composites with CF the optimal sensing behavior was observed for intermediate humidity conditions, i.e. water saturation degree between 40 and 60% [37,65]. Therefore, in future research this issue should be properly addressed to optimize the material response. The sensitivity of all composites was in the same order of magnitude, i.e. gage factors between 4 and 13. However, samples with slag aggregates and oxidized fibers showed lower dispersion values. Fig. 7 includes the change of resistivity vs strain curve, whose slope is actually the gage factor value (linear regression equations and r2 Pearson coefficients are included for each dosage). Despite limestone aggregates showing the highest sensitivity, slag-aggregate mortars with 4% oxidized CF were more stable, i.e. the dispersion was lower as observed in the higher r2 coefficient. The GF difference between the 3%CF limestone mortars and 4%CF slag mortars
Table 6 Strain sensing results, average gage factor and standard deviation (SD) for all dosages that showed proper strain-sensing response. CF% (by cement mass)
CF vol%
Oxidized CF
Aggregate
Average gage factor ± SD
3% 3% 3% 4% 4% 4% 4%
0.83% 0.98% 0.97% 1.09% 1.10% 1.29% 1.29%
Yes No Yes No Yes No Yes
Limestone Steel Slag Steel Slag Limestone Limestone Steel Slag Steel Slag
12.90 ± 2.22 4.82 ± 0.97 5.29 ± 1.03 4.86 ± 1.29 8.46 ± 1.58 7.24 ± 1.50 4.73 ± 0.36
317
Fig. 7. Fractional change of resistance vs strain curves for three different mortars.
(both with oxidized fibers, can be justified by the difference of conductivity between both composites (2210 and 65.3 Ocm respectively). Strain sensing does not require high conductivity values, but usually composites with lower conductivity show higher sensitivities (i.e. GF up to 170 in Portland cement pastes) [37]. Nonetheless, for a practical use of this technology, it would be more effective to consider lowest dispersion (i.e. response stability in time) as well as sensitivity.
4. Conclusions PAN CF reinforced cement mortars with different aggregates were fabricated. The effectiveness and viability of using a recycled electric arc furnace (EAF) slag as fine aggregates were evaluated, and the impact of this industrial waste on the electrical conductivity of cement composites was also assessed. Finally, the possibility of using these new composites as strain sensors was also studied. Based on the results of this experimental investigation, the following conclusions may be drawn: Cement mortars containing EAF slag aggregates compared to standard cement mortars (with silica aggregates) showed an increase in the flexural and compressive strengths at 7 days. The electrical resistivity of CF reinforced cement mortars was lower if steel slag aggregates (from industrial slag recycling) were used. Percolation threshold was reached at lower levels of CF content if limestone fine aggregates (percolation threshold approx. at 1.1 vol%) were substituted by slag particles (lower than 0.97 vol%). If CF underwent an oxidation treatment, the conductivity of the resulting composites was also enhanced for the same CF addition. Resistivity decreased by several orders of magnitude, especially for higher CF dosages where electronic conduction has a prevailing role. Self-sensing capacity was observed for composites with both types of aggregates—limestone and recycled EAF slag—and CF dosages higher than 0.8 vol%. A good correlation between electrical resistivity and longitudinal strain was registered with carbon fiber cement mortars with steel slag aggregates. In fact, the best linear regressions between the electrical and mechanical measures were achieved for EAF slag cement mortar containing 1.29 vol% of oxidized carbon fibers.
Conflict of interest None.
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Acknowledgments This research was partially funded by the Basque Regional Government (Hazitek 2015 Programme number: ZL-2015/00163). The authors also want to thank SIDENOR for supplying the EAF slags and to HORMIGONES Y MORTEROS AGOTE for the slag valorization (crushing, sieving and ripening process). References [1] A. Maciá, F.J. Baeza, J.M. Saval, S. Ivorra, Mechanical properties of boards made in biocomposites reinforced with wood and Posidonia oceanica fibers, Compos. Part B Eng. 104 (2016) 1–8, https://doi.org/10.1016/j.compositesb.2016.08.018. [2] D. Foti, Innovative techniques for concrete reinforcement with polymers, Constr. Build. Mater. 112 (2016) 202–209, https://doi.org/10.1016/ j.conbuildmat.2016.02.111. [3] F. Baeza, J. Payá, O. Galao, J.M. Saval, P. Garcés, Blending of industrial waste from different sources as partial substitution of Portland cement in pastes and mortars, Constr. Build. Mater. 66 (2014) 645–653, https://doi.org/10.1016/ j.conbuildmat.2014.05.089. 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