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Influence of nano-SiO2 and nano-Al2O3 additions on steel-to-concrete bonding
Construction and Building Materials 125 (2016) 1080–1092
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Influence of nano-SiO2 and nano-Al2O3 additions on steel-to-concrete bonding R. Ismael a, J.V. Silva a, R.N.F. Carmo a,⇑, E. Soldado a, C. Lourenço a, H. Costa a, E. Júlio b a b
CERIS, Polytechnic Institute of Coimbra, Rua Pedro Nunes – Quinta da Nora, 3030-199 Coimbra, Portugal CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
h i g h l i g h t s Adding nanoparticles leads to an increase of the bond on mixtures with higher percentage of cement. The addition of nano-Al2O3 decrease the crack width when plain rebars are used. The addition of nanoparticles does not affect the cracks’ width when ribbed rebars are adopted. No additional benefit was obtained by combining nanoparticles with steel fibers.
a r t i c l e
i n f o
Article history: Received 8 March 2016 Received in revised form 25 July 2016 Accepted 29 August 2016
Keywords: Nanoparticles Concrete Steel fibers Bond Cracking
a b s t r a c t The research on nanoparticles is quite recent and potentially relevant in different contexts, including the concrete field. Fiber reinforced concrete presents several advantages, one of which its capacity to best control cracking. In this scope, the bond between steel rebars and/or fibers and the binding paste plays a most relevant role. The study herein described was developed aiming at analyzing the influence on the latter, and as a consequence on cracking, of nano-Al2O3 and nano-SiO2 additions. In the experimental analysis, the following materials were adopted: both plain and ribbed rebars, eight different concrete mixtures, two types of nanoparticles additions, namely nano-Al2O3 and nano-SiO2, and, in some mixtures, steel fibers. Overall, thirty-two pull-out tests, plus sixteen tensile tests on reinforced concrete ties, were performed. It was concluded that adding nanoparticles to the concrete paste leads to an increase of the steel-to-concrete bond, for mixtures with higher percentage of cement. It was also concluded that Al2O3 nanoparticles induce a beneficial effect in terms of cracking, decreasing the width of these when plain rebars are used. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Concrete structures play a major role in the construction sector. Recent advances are mostly due to innovations in the material technology. Although taking its first steps on this industry, nanotechnology has an important share in this scope [1,2]. Over the last years, research regarding concrete mix design in which nanoparticles or nanotubes have been used has been carried out. Generally, the percentage of nanoparticles is calculated in relation to the volume (or mass) of the binder. Due to their reduced size, nanoparticles have a high specific area and a high reactivity during the cement’s hydration process, behaving as nucleation sites [3]. Nanoparticles can be beneficial in increasing the density of the binding matrix, by reducing concrete porosity, and in ⇑ Corresponding author. E-mail address: [email protected] (R.N.F. Carmo). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.152 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
improving the interfacial transition zone (ITZ), between aggregates and the cement paste. All these changes at the micro-scale influence the concrete performance at the macro-scale, increasing both strength and durability [1,4–8]. The nanoparticles of SiO2, currently referred to as nano-SiO2, were the first to be studied for showing the best results in reducing porosity and thus increasing the matrix density [1,4]. More recently, other types of nanoparticles have also been studied, such as nano-TiO2, nano-ZnO2, nano-Fe2O3, and nano-Al2O3, and it was observed that these also enhance concrete performance [9–11]. Studies performed by Li et al. [12], considering cementitious mixtures containing fly ash (50%) and nano-SiO2 (4%), showed an increase of 19% on the hydration temperature of cement and an increase of 81% in concrete strength, at 3 days of age. Qing et al. [13] studied the influence of nano-SiO2 addition on the properties of hardened cement paste comparatively to silica fume addition and have observed that the consistency and the setting time of
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Notations fcm,cube fctm fct,fl fRm
s smax d
mean value of concrete cube compressive strength mean value of concrete tensile strength mean value of concrete flexural tensile strength mean value of concrete residual flexural tensile strength bond stress maximum bond stress relative displacement
the fresh cement paste are different. These authors have concluded that: (i) nano-SiO2 accelerates the cement hydration process, (ii) both the compressive strength and the bond strength of the paste to aggregate interface are higher, especially at early ages, when nano-SiO2 is used; (iii) with an addition of 3% of nano-SiO2, the size of calcium hydroxide crystals are reduced thus improving the interface strength. An experimental investigation about the durability properties of concrete containing small dosages of nanosilica, varying between 0.3% and 0.9%, showed the beneficial effects of nano-silica, due to the combined contribution of the nano-filler effect and the pozzolanic reaction [14]. These results indicate, by comparison with other works [5–8] in which higher dosage of nano-SiO2 was used, that a durability enhancement can be obtained with lower dosages of nano-particles, but it is mandatory to ensure a good dispersion of the these within the cement matrix. Nazari et al. conducted an experimental work with nano-Al2O3, which also revealed an increase in compressive strength using amounts of nanoparticles not exceeding 2% [15]. The addition of nano-Al2O3 also showed to have a major influence on the Young’s modulus leading to a significant increase of the latter [16]. Mukherjee et al. [17] did not find significant differences in the compressive strength of concrete with nano-Al2O3 addition, at early-ages, but refer to a change in the microstructure of the paste, more dense and with larger crystals of portlandite within the cement matrix. Nazari and Riahi [18] proved that adding nano-ZnO2 to concrete mixtures leads to a significant decrease in workability and percentage of water absorption but can increase the strength capacity.
smin savg smax wmin wavg wmax
minimum spacing between cracks average spacing between cracks maximum spacing between cracks minimum crack width average crack width maximum crack width
Regarding nano-TiO2, it can be stated that it is one of the few types of nanoparticles that has already been used in construction (e.g., ‘‘Dives in Misericordia” Church, in Rome, Italy), due to its photocatalytic properties [3]. When exposed to ultraviolet light becomes hydrophilic and can be used for anti-fogging coating or selfcleaning. Steel-to-concrete bond is fundamental to ensure the structural behavior of reinforced concrete members for both serviceability and ultimate limit states. To design the anchorage length and the lap length of rebars, as well as to define rules for cracking control, a deep knowledge of the latter is required, involving three components: i) adhesion, understood as chemical and micro-mechanical connections; ii) mechanical interlock, originated by the rebar’s ribs; and iii) friction, mobilized by a relative displacement between the two surfaces in contact, and thus highly dependent on the roughness of the interface surface (Fig. 1) [19,20]. Bearing in mind that adding nanoparticles to the binding matrix enhances concrete properties, it is likely that steel-to-concrete bond be also improved and, consequently, the cracking of reinforced concrete members be reduced. Another study [21] conducted on with this topic, but considering a less usual nanoparticle, nano-kaolinite clay, shows that the addition of the latter can improve the bonding strength of concrete to steel rebars, reducing the stiffness of concrete specimen and delay steel corrosion. In the last decades, steel fibers have been used to reinforce cementitious materials, increasing the low tensile strength [22,23]. Regarding bond of rebars to concrete, fibers provide passive confinement and improve bond capacity in terms of bond
Fig. 1. Concrete-steel bond.
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Fig. 2. (a) SiO2 nanoparticles (150,000 zoom); (b) Al2O3 nanoparticles (1000 zoom); SiO2 nanoparticles in the concrete matrix; (c) 1000 zoom and (d) 5000 zoom.
strength and, more importantly, toughness [24]. Considering the effects described above, the bond between steel fibers and concrete matrix may also be sensitive to nanoparticles addition and it is interesting to verify if any positive interaction is originated from the combination of nanoparticles with steel fibers. 2. Research significance Studies on the addition of nanoparticles to concrete mixtures are mainly focused on analyzing the material behavior in fresh state, as well as its strength and durability in hardened state. So
far, there are not any published studies on the structural behavior of concrete members incorporating nanoparticles, particularly with nano-SiO2 and nano-Al2O3. To fill this gap was the main motivation for the experimental work herein described. Being the behavior of reinforced concrete based on steel-to-concrete bond, to understand the effect of nanoparticles addition in the latter was elected as the main goal of this research study. By adding nanoparticles to the binding matrix, an increase in the bond between the steel reinforcement and the concrete is expected to occur, as a result of a combined effect of the following factors: (i) the increase of concrete tensile strength, (ii) the
Table 1 Concrete mix proportions for 1 m3 (by weight, kg). Constituents
CEM I 52,5 R Limestone Filler Superplasticizer Water 0/1 mm Sand 0/4 mm Sand 4/8 mm Granite Gravel 6/14 mm Limestone Gravel Nano-SiO2 (2%) Nano-Al2O3 (2%) Steel Fibers (0.5%)
Dosages CA
CA_Si2%
CA_Al2%
CB
CB_Si2%
CB_Al2%
CB_F0.5
CB_F0.5Al2%
230 120 0,9 161 227 681 187 755 – – –
223 120 0,9 156 228 684 188 760 7 – –
223 120 0,9 156 229 688 188 759 – 7 –
350 – 2,1 140 239 718 190 770 – – –
343 – 2,7 140 238 715 190 769 7 – –
343 – 2,4 140 240 719 190 770 – 7 –
350 – 2,1 140 238 713 189 764 – – 39
343 – 2,1 140 238 714 189 764 – 7 39
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Fig. 3. (a) Tensile strength test; (b) Young’s modulus test; (c) Bending test conducted to assess the maximum and the residual flexural tensile strengths.
Table 2 Concrete properties in hardened state.
3.1. Concrete mixtures
Concrete
fcm,cube (MPa)
fctm (MPa)
Young’s Modulus (GPa)
CA CA_Si2% CA_Al2% CB CB_Si2% CB_Al2% CB_F0.5 CB_F0.5Al2%
40.1 43.3 40.1 74.2 74.1 72.7 79.1 78.5
4.8 4.1 4.5 5.7 7.2 6.9 5.3 5.4
38.7 41.9 49.2 51.4 52.6 52.6 53.8 49.2
increase of the binding matrix density, and (iii) the increase of the number of chemical connections between the binding matrix and steel rebars. Two types of nanoparticles (Fig. 2), nano-SiO2 (nano-silica) and nano-Al2O3 (nano-aluminum), produced by Smart Innovation company, were adopted. An extensive experimental program was settled and conducted with the aim of clarifying the effect of the nanoparticles addition in both steel-to-concrete bond and concrete cracking.
3. Experimental program To study the steel-to-concrete bond stress, thirty-two pull-out tests were performed, whereas to study the cracks width and spacing, sixteen tensile tests on reinforced concrete ties were conducted. In these tests, plain and ribbed rebars, as well as eight different types of concrete mixtures, were used. Although plain rebars are not currently used, the bond between this type of rebars and concrete was studied since the adhesion component has a prominent role in these cases. In ribbed rebars, the mechanical interlock has a very significant influence on rebar to concrete bonding and makes it much more difficult analyze the effect of both chemical and micro-mechanical connections and, thus, the expected contribution of nano-roughness provided by the addition of nanoparticles to the binding matrix. For each possible combination of rebars and concrete mixtures, two pull-out specimens plus one reinforced concrete tie were tested. This section describes the materials properties, the specimens’ production, the tests set-up, and the procedures adopted.
Eight different types of concrete mixtures were produced, divided into two series, each one with a different dosage of cement, limestone filler and nanoparticles, although always totalizing 350 kg/m3. These included three reference mixtures (without nanoparticles): CA, with cement and limestone filler; CB, with cement only; and CB_F0.5, equal to the latter but with an addition of steel fibers. Based on CA, two mixtures were produced: CA_Si2%, containing nano-SiO2, and CA_Al2%, containing nano-Al2O3. Based on CB, also two mixtures were produced: CB_Si2%, containing nano-SiO2, and CB_Al2%, containing nano-Al2O3. Lastly, based on CB_F0.5, one more mixture was produced: CB_F0.5Al%, containing nano-Al2O3. Preliminary tests [25,26] revealed that nano-Al2O3 addition combined with steel fibers leads to slightly better results compared to nano-SiO2 addition. The following binder constituents were adopted: cement CEM I 52.5 R, with a density of 3.16 kg/dm3, a limestone filler, with a density of 2.70 kg/dm3, nano-SiO2, with a density of 2.22 kg/dm3, and nano-Al2O3, with a density of 3.95 kg/dm3. The nanoparticles were provided in solid state. The dosage of nanoparticles was settled as 2% (in relation to the cement mass), based on previous studies developed by Lourenço et al. [25] and Soldado et al. [26]. Several mixtures were produced using different dosages of nanoparticles, ranging from 1 to 4%, being 2% the most favorable dosage given that higher values did not lead to significantly higher strengths. This percentage is within the range described in other studies [5–8] and does not compromise the workability of concrete. However, it must be highlighted that the fluidity decreases slightly with the increase of the addition of nanoparticles but, in general, this can be compensated with a small adjustment of the superplasticizer dosage. Two coarse aggregates, a 4/8 mm granite gravel and a 6/14 mm limestone gravel, with densities of 2.63 kg/dm3 and 2.66 kg/dm3, respectively, and two fine aggregates, a 0/1 mm fine sand and a 0/4 mm medium sand, both with a 2.63 kg/dm3 density, were adopted. Steel fibers Dramix RL 45/30 BN, with a length of 30 mm, a diameter of 0.62 mm, a shape factor (l/d) of 48 (class 45), and a simple hook-end anchorage, were adopted. The dosage of steel fibers was settled as 0.5% (in volume), based on preliminary tests. Lastly, Glenium SKY 526, a third generation superplasticizer, polycarboxylates-based, was
Table 3 Maximum and residual flexural tensile strengths. Concrete
fct,fl (MPa)
fRm,1 (MPa)
fRm,2 (MPa)
fRm,3 (MPa)
fRm,4 (MPa)
CB_F0.5 CB_F0.5Al2%
5.96 6.61
4.43 4.89
2.69 2.94
1.72 1.39
1.25 0.89
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used to reduce the amount of water demand, simultaneously maintaining a high workability of concrete in fresh state. It has to be stated that the performance of concrete with 1% of steel fibers was also assessed and showed a significant loss in workability when compared to the 0.5% dosage. All mixtures were designed according to a method first developed by Lourenço, and later improved by Costa [27], being listed in Table 1 the corresponding constituents dosages.
3.2. Concrete properties The following mechanical properties of the eight adopted concrete mixtures were assessed: Young’s modulus, compressive strength, and tensile strength (Fig. 3). The Young’s modulus was determined using one prismatic specimen (100 100 400 mm3), according to E397 [28]; the concrete compressive strength was assessed using three cubic specimens, according to EN 206-1 [29] and EN 12390 [30]; and the tensile strength was determined using four diametral compression tests, in accordance to EN 12390 [30]. For concrete mixtures with steel fibers, additional tests were performed on prismatic specimens (100 100 500 mm3), two for each mixture, to evaluate the maximum and the residual flexural tensile strength, following the procedures presented in RILEM TC 162-TDF [31] and EN 12390-5 [30]. According to MC2010 [32] the concrete strength, fRm1, was 4–5 MPa for both types of concrete, and the fRm3/fRm1 ratio was equal to 0.39 for CB_F0.5 and to 0.28 for CB_F0.5Al2%. Tables 2 and 3 summarize the assessed concrete properties.
3.3. Pull-out tests Rebars with a diameter of 10 mm, S 500 steel grade, and with plain or ribbed surface, were adopted. These were embedded in cylindrical specimens (Fig. 4a), with a diameter of 100 mm and a height of 200 mm, produced with the eight different mixtures previously referred. For each possible combination, two equal specimens were produced, totaling thirty-two. The concrete was confined with a steel formwork in order to prevent its premature failure, i.e., before reaching the maximum bond stress. It was cast with the steel rebars positioned vertically at exactly the center of the specimens. To ensure these geometric requirements, as well as the lengths with and without bond, with significant precision, a set-up was specially designed and built to support the specimens’ production (Fig. 4b). The pull-out tests were performed (Fig. 5) according to EN 10080 [33]. The tensile force was applied using a hydraulic servo-actuator. The load was applied with displacement control at a rate of 0.02 mm/s. The data acquisition was performed with a frequency of 1 Hz. Both the applied force and the corresponding displacement were measured using, respectively, the load cell and the displacement transducer (LVDT) of the hydraulic servo-actuator. To ensure an accurate and precise measurement of the steel rebar slippage relatively to the concrete bulk, three LVDTs were used. 3.4. Reinforced concrete ties Sixteen reinforced concrete ties (Fig. 6) were produced with the following dimensions: 75 75 1200 mm3. This geometry has
Fig. 4. (a) Pull-out test specimens; (b) set-up for specimens’ production.
R. Ismael et al. / Construction and Building Materials 125 (2016) 1080–1092
been adopted to allow the initiation, development and stabilization of cracks before steel rebars reached yielding. Again a single (ribbed or plain) steel rebar with a diameter of 10 mm, S 500 steel grade, was positioned at the center of the cross-section. The concrete was cast with the steel rebars positioned horizontally and ensuring its correct positioning. The tensile force was applied through metallic plates positioned at the ends of the RC ties and following the same procedure adopted in pull-out tests, namely with displacement control at 0.02 mm/s. To accurately measure the axial deformation, four LVDTs were used, one placed at each edge (Fig. 6). The cracks widths were measured using a graduated ruler, at each of the four faces of the RC ties, at different time steps. Measure the cracks width at all four faces was time-consuming but crucial because the width varied along the crack length, which would not be initially expected in a tensile test. 4. Experimental results and analysis In this section, results of both pull-out and RC ties tests are presented and discussed. For the sake of clarity, all specimens were labeled according to the same procedure, presented in Fig. 7. 4.1. Pull-out tests In all pull-out tests, failure occurred as predicted, i.e., the maximum bond stress was reached without premature concrete failure. The confinement provided by the steel formwork is similar for each type of rebars and concrete, so the results can be compared within each series and the differences obtained are relative differences, which are useful to accomplish the goals of this study. The bond stress (s) was calculated as the applied force divided by the contact area between the rebar and concrete and was
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plotted versus the corresponding relative displacement (d) of the rebar regarding concrete. The obtained stress versus displacement graphics are shown in Fig. 8, for plain rebars, and in Fig. 9, for ribbed rebars. Each value is the average of two similar tests, except for P5R_CB_Si2%, since an unexpected failure near the anchorage of the metallic plate occurred in one of the specimens tested. It was not detected significant differences between the two tests. In Fig. 10, the maximum values of all considered situations are plotted. Considering the results obtained with plain rebars, it is noticeable an increase of the maximum bond stress when nanoparticles are added to concrete. For CA specimens, maximum bond stresses of 3.8 and 3.5 MPa, respectively for nano-SiO2 and nano-Al2O3 additions, were measured, higher than the value of 3.2 MPa, measured on the reference specimen (without nanoparticles). For CB specimens, a similar trend was registered, but the strength increase was considerably higher, namely 6.8 MPa and 6.7 MPa, respectively for nano-SiO2 and nano-Al2O3 additions, whereas the value measured on the reference specimen (without nanoparticles) was just 5.1 MPa. The effect of the two types of nanoparticles was very similar, although nano-SiO2 exhibited slightly better results. Results obtained with ribbed rebars also showed an increase of the bond stress when nanoparticles were added, especially nanoSiO2, in this case an increase of 5.5 MPa was registered. Comparing the plain and ribbed steel rebars, a higher relative displacement is required to achieve the maximum bond stress in ribbed rebars. This result can be explained by the fact that its components, i.e., friction and mechanical effect provided by the ribs, demand a significant slippage between rebar and concrete, circa 1.5–3.0 mm, to mobilize its full strength. From these results, as expected, it is observed that the mechanical effect is the most important component of the bond strength. Therefore, the influence of the nanoparticles on the latter, although positive, could never be very significant. For this reason, bond between plain
A – Hydraulic actuator; B – Top LVDTs; C - Reaction / supporting plates; D – Pull-out specimen; E – Bottom LVDTs;
F - Metallic frame
Fig. 5. Set-up of the pull-out tests.
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A – Hydraulic actuator; B – Metallic plates ; C - LVDTs; D – Reinforced concrete tie; E -Metallic frame Fig. 6. Tie test set-up.
Fig. 7. Label of pull-out specimens and RC ties.
rebars and concrete should be more sensitive to the effect of nanoparticles addition since, in this case, bond is mainly ensured by adhesion and micro-mechanical connections, and nanoparticles influence mostly the adhesion component. Adding steel fibers to the concrete mixture produced some improvement on the maximum bond stress, especially in tests with
ribbed rebars, probably because ribs tend to create transversal micro-cracking, due to tensile stresses that are generated around the steel rebar, and steel fibers prevent the opening of these micro-cracks. In these tests, the nanoparticles addition did not produce any increase on bond. To further discuss results, it was decided to plot the maximum bond stress versus the concrete compressive strength (Fig. 11), since this is the main concrete property and, moreover, it is directly correlated with the tensile strength. Four distinct groups can be clearly identified, as well as the relative importance of the type of rebar (plain or ribbed) and of the type of concrete mixture in the bond between rebars and concrete. Higher bond stresses are reached when ribbed rebars are used, as well as when concrete with higher compressive strength is adopted. It is also detectable that, on specimens with plain rebars, steel fibers have not a relevant effect, probably due to absence of high values of transversal tensile stresses around the rebars. This means that, in these cases, the steel fibers are not properly stretched, which is opposite of what was found on ribbed rebars.
8
8 P1P_CA P2P_CA_Al2% P3P_CA_Si2%
6
τ (MPa)
τ (MPa)
6
P4P_CB P5P_CB_Al2% P6P_CB_Si2% P7P_CB_F0,5 P8P_CB_F0,5Al2%
4
2
4
2
0
0 0
6
12
18
24
0
6
12
δ (mm)
δ (mm)
(a)
(b)
Fig. 8. Bond stress vs. displacement for plain rebars: (a) concrete CA and (b) concrete CB.
18
24
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32
32
P1R_CA P2R_CA_Si2% P3R_CA_Al2%
24
τ (MPa)
τ (MPa)
24
16
16
8
8
0
P4R_CB P5R_CB_Si2% P6R_CB_Al2% P7R_CB_F0,5 P8R_CB_F0,5Al2%
0 0
6
12
18
24
0
6
12
δ (mm)
δ (mm)
(a)
(b)
18
24
Fig. 9. Bond stress vs. displacement for ribbed rebars: (a) concrete CA and (b) concrete CB.
32
32
Plain bars
P1_CA P3_CA_Si2% P5_CB_Al2% P7_CB_F0,5
29.1
27.9 Ribbed bars 24
CA ribbed rebars
19.8
19.2 16
15.7 13.1
τ (MPa)
τmax (MPa)
24
22.5
22.4
P2_CA_Al2% P4_CB P6_CB_Si2% P8_CB_F0,5Al2%
CB ribbed rebars
16
CB plain rebars
8
6.8 3.8
3.2
5.1
8
6.7 5.2
3.5
0
Reference Si2% Al2% CA
CA plain rebars
5.4
Reference Si2% Al2% F0.5 F0.5Al2% CB
Fig. 10. Maximum bond stress.
4.2. Tie tests Based on the measured data, a steel stress versus strain graphic was plotted, for each studied situation (Fig. 12). The overall behavior of the ties is characterized by four characteristic stages. The first stage corresponds to the uncracked state, being defined by an almost linear elastic response with high axial stiffness, and it ends when the first crack appears. Immediately after, the process of crack formation begins and is accompanied by sudden stress variations. These peaks are observed in each curve of the tie. The following stage corresponds to stabilized cracking and finishes with yielding of steel reinforcement. In this stage, it is clearly detectable the tension stiffening effect, where concrete between cracks contributes to the tensile strength and reduces the rebar strain, comparatively with the deformation observed with naked rebars. The plastic stage, where the axial deformation increases under an almost constant axial stress, is incipient for some ties, well visible for others and was not detected for plain rebars. The tests were stopped when the cracks width were large and it corresponded to different load levels. This was the main reason for the differences observed in the last stage. The experimental results also show different values of the yield strength for plain and ribbed rebars, being slightly higher for ribbed rebars. The development of the cracks width during the test was recorded for each of the four faces of each tie and, for two distinct phases, the value of the minimum, average and maximum cracks width was registered. It is visible that, during the cracking stabi-
0
25
40
55
70
85
fcm (MPa) Fig. 11. Maximum bond stress vs. concrete strength.
lization phase, the tension stiffening effect, due to the contribution of concrete between cracks, is different for plain and for ribbed rebars, being slightly higher in the former case. It is also observed that, for the same stress level, the deformation is slightly lower for ribbed rebars. This difference is explained taking into account the difference in cracking patterns, namely in the average distance between cracks. Fig. 13 shows, for four ties, all the information recorded during tests: cracks identification, position and shape, and development of the cracks width, on each side, with the applied force. On TP_CB ties, for a steel stress ranging between 479 and 500 MPa an average crack width (wavg) of 0.33 mm was measured for the reference mixture, 0.27 mm for the mixture with nanoAl2O3, and 0.40 mm for the mixture with nano-SiO2. Hence, there is a slight reduction of the cracks width when concrete incorporates nano-Al2O3. The use of steel fibers also leads to a slight decrease on average of the crack width, but it has little or no effect on the corresponding maximum value (wmax). On TR_CB ties, for high levels of steel stress, it was observed a practically identical wavg of approximately 0.25 mm was observed on ties with and without nanoparticles. However, higher values of wmax were obtained on ties with nanoparticles. These results reveal that, for ribbed rebars, the effect of ribs surpasses the beneficial effect achieved by adding aluminum nanoparticles. This means that mechanical anchorage is predominant regarding adhesion. Regarding the effect of steel fibers addition, it was not noticed any significant difference between ties with plain and ties with
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600 Wmax = 1.18 Wavg = 0.71 Wmin = 0.43
450
Wmax = 0.43 Wavg = 0.30 Wmin = 0.10
300 Wmax = 0.23 Wavg = 0.17 Wmin = 0.10
150
Wmax = 0.10 Wavg = 0.09 Wmin = 0.08
Steel stress (MPa)
Steel stress (MPa)
600
Wmax = 0.58 Wavg = 0.52 Wmin = 0.45
450
300 Wmax = 0.15 Wavg = 0.13 Wmin = 0.08
150
Wmax = 0.28 Wavg = 0.20 Wmin = 0.13
T1P_CA T1R_CA
0
1
2
3
0
4
1
Average strain (x10-3)
T2R_CA_Al2%
2
Wmax = 0.38 Wavg = 0.33 Wmin = 0.30
Steel stress (MPa)
Wmax = 0.43 Wavg = 0.30 Wmin = 0.08
300 Wmax = 0.20 Wavg = 0.13 Wmin = 0.08
150
Wmax = 0.10 Wavg = 0.09 Wmin = 0.08
Wmax = 0.43 Wavg = 0.24 Wmin = 0.08
450 Wmax = 0.13 Wavg = 0.13 Wmin = 0.13
300
Wmax = 0.08 Wavg = 0.08 Wmin = 0.08
150
T3P_CA_Si2%
T4P_CB
T3R_CA_Si2%
T4R_CB
0
0 0
1
2
3
0
4
1
2
600
4
600 Wmax = 0.33 Wavg = 0.27 Wmin = 0.15
Wmax = 0.58 Wavg = 0.40 Wmin = 0.28
450 Wmax = 0.18
Steel stress (MPa)
Wmax = 0.55 Wavg = 0.27 Wmin = 0.08
Wavg = 0.14 Wmin = 0.10
300
Wmax = 0.10 Wavg = 0.10 Wmin = 0.10
150
450 Wmax = 0.55 Wavg = 0.25 Wmin = 0.08
Wmax = 0.08 Wavg = 0.06 Wmin = 0.05
300
Wmax = 0.28 Wavg = 0.20 Wmin = 0.15
150
T5P_CB_Al2%
T6P_CB_Si2%
T5R_CB_Al2%
0
1
2
3
T6R_CB_Si2%
0
4
0
1
Average strain (x10-3)
Wmax = 0.43 Wavg = 0.16 Wmin = 0.05
450 Wmax = 0.18 Wavg = 0.14 Wmin = 0.08 300
Wmax = 0.20 Wavg = 0.13 Wmin = 0.08
Wmax = 0.38 Wavg = 0.30 Wmin = 0.18 450 Wmax = 0.15 Wavg = 0.13 Wmin = 0.08
1
4
Wmax = 0.40 Wavg = 0.22 Wmin = 0.03
300 Wmax = 0.18 Wavg = 0.13 Wmin = 0.08
150
T7P_CB_F0,5
T8P_CB_F0,5Al2%
T7R_CB_F0,5
0
3
600
Wmax = 0.43 Wavg = 0.31 Wmin = 0.20
150
2
Average strain (x10-3)
Steel stress (MPa)
600
Steel stress (MPa)
3
Average strain (x10-3)
Average strain (x10-3)
0
4
600 Wmax = 0.50 Wavg = 0.44 Wmin = 0.40
450
0
3
Average strain (x10-3)
600
Steel stress (MPa)
T2P_CA_Al2%
0
0
Steel stress (MPa)
Wmax = 0.58 Wavg = 0.29 Wmin = 0.03
2
3
T8R_CB_F0,5Al2%
4
0 0
Average strain (x10-3) Fig. 12. Steel stress vs. tie deformation.
1
2
3
Average strain (x10-3)
4
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Fig. 13. Development of cracks width with the applied force.
ribbed rebars. Although wavg was slightly lower for the latter case, wmax was almost the same in both cases (Fig. 14). No additional benefit was obtained by combining nanoparticles with steel fibers in any case. On TP_CA ties, for a steel stress ranging between 493 and 500 MPa, the wavg was 0.71 mm, for the reference mixture, 0.52 mm, for the mixture with nano-Al2O3, and 0.44 mm, for the mixture with nano-SiO2. Thus, for ties produced with the lowest strength concrete, the effect of adding nanoparticles (either nano-aluminum or nano-silica) on the reduction of the cracks width is higher than for ties produced with the highest strength
concrete. The results for TR_CA ties, i.e., for ribbed rebars, follow exactly the same trend. When the cracking pattern is analyzed, it is very important to evaluate the spacing between cracks, since this parameter has a direct correlation with the cracks width. During and after each test, the distance between cracks, at each of the four faces, was documented in detail. Higher distances between cracks were measured on TP_CA ties. On T_CA ties, with the lower strength concrete, the distances between cracks were higher than on the corresponding T_CB ties, due to a lower steel-to-concrete adhesion. And as is well known the adhesion is proportional to
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On ties with plain rebars, for both type concretes, CA and CB, the addition of nanoparticles slightly reduces the spacing between cracks. Adding steel fibers that spacing is even more reduced, which is explained by the increase of bond that derives from the increase of the tensile strength of concrete. On ties with ribbed rebars, the number of cracks is clearly higher than on ties with plain rebars. When the distance between cracks is analyzed, again it is noticeable that the mechanical strength component has a more significant effect than the increase in adhesion provided by the addition of nanoparticles. For such reason, the distance between cracks was very similar on all ties with ribbed rebars, for both types of concrete, containing or not nanoparticles. Using all the information concerning the cracking pattern, it was possible to produce the diagrams shown in Fig. 16, in which the position of the cracks and the average width along the tie is shown (last measurement performed). For a better assessment of the cracks width, a color gradient was used. Fig. 14. Crack detail on a tie reinforced with steel fibers.
concrete tensile strength. Since the main influence on the crack pattern, particularly on the distance between cracks, comes from the type of rebar surface (plain or ribbed), a clear decrease on the distance between cracks on ties with ribbed rebars compared to ties with plain rebars (Fig. 15 and Table 4) was expected.
5. Conclusions To clarify and consolidate the trend of the obtained results, conducting further testing with a larger number of specimens is mandatory. Generalizing these results to other cases must be done with caution because the type of nanoparticles and the synthesis process used may also influence the results. Based on the research study herein described, the following conclusions can be drawn:
Fig. 15. Distance between cracks on ties: (a) T1P_CA and (b) T1R_CA.
Table 4 Spacing between cracks and crack width. Tie
T1P_CA T2P_CA_Si2% T3P_CA_Al2% T4P_CB T5P_CB_Si2% T6P_CB_Al2% T7P_CB_F0,5 T8P_CB_F0,5Al2% T1R_CA T2R_CA_Si2% T3R_CA_Al2% T4R_CB T5R_CB_Si2% T6R_CB_Al2% T7R_CB_F0,5 T8R_CB_F0,5Al2%
Space between cracks (mm)
Crack width (mm)
smin
savg
smax
wmin
wavg
wmax
122 140 213 129 123 89 69 113 25 34 22 29 36 25 14 32
273 252 252 187 170 160 149 159 91 95 86 87 111 85 90 88
425 364 290 245 216 231 230 205 158 156 150 145 186 145 166 144
0.43 0.40 0.45 0.30 0.28 0.15 0.20 0.18 0.10 0.08 0.03 0.08 0.28 0.15 0.05 0.03
0.71 0.44 0.52 0.33 0.40 0.27 0.31 0.30 0.30 0.30 0.29 0.24 0.40 0.27 0.16 0.22
1.18 0.50 0.58 0.38 0.58 0.33 0.43 0.38 0.43 0.43 0.58 0.43 0.58 0.33 0.43 0.40
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Fig. 16. Cracking pattern with average crack width for ties with: (a) plain rebars; (b) ribbed rebars.
The maximum bond stress increased with the addition of nanoparticles to the concrete mixture, especially for specimens produced with the highest dosage of cement. There was an increase of approximately 25% relatively to the reference concrete, both for plain and ribbed rebars; In RC ties, the addition of nano-Al2O3 was particularly effective in reducing the cracks width and in decreasing the spacing between cracks, when plain rebars were used; In RC ties, when ribbed rebars were adopted, the addition of nanoparticles did not produce a significant effect on the cracks width; The obtained results also indicate that such small particles and in reduced percentages influence mostly the adhesion component, which is particularly relevant when plain rebars were used. When the crack pattern is analyzed, the ribs of the rebars is more effectiveness than the increase in adhesion provided by the addition of nanoparticles; No additional benefit was obtained by combining nanoparticles with steel fibers.
Acknowledgments The authors thank the support of Agência Nacional de Inovação – Portugal, namely by funding the project 38702 entitled ‘‘NanoCrete: Development of Enhanced Performance Concrete by Means of Nanoparticles Additions”. Acknowledgements are extended to Smart Inovation Lda., for developing and producing the nanoparticles used in this study, as well as to Secil, BASF, Sika, Omya and Argilis, for providing the materials adopted in the latter.
References [1] F. Sanchez, K. Sobolev, Nanotechnology in concrete – a review, Constr. Build. Mater. 24 (2010) 2060–2071. [2] W. Zhu, P. Bartos, A. Porro, Application of nanotechnology in construction – summary of a state-of-the-art report – RILEM TC 197-NCM, Mater. Struct. 37 (9) (2004) 649–658. [3] F.P. Torgal, S. Jalali, Eco-Efficient Concrete: The Future of the Ready-mix Concrete Industry (in Portuguese), Betão, no 26, Associação Portuguesa das Empresas de Betão Pronto (APEB), Portugal, 2011.
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[4] B. Bhuvaneshwari, S. Sasmal, N. Iyer, Nanoscience to Nanotechnology for Civil Engineering Proof of Concepts, Recent Researches in Geography, Geology, Energy, Environment and Biomedicine, WSEAS Press, 2011, pp. 230–235. [5] E. Ghafari, H. Costa, E. Júlio, A. Portugal, L. Durães, The effect of nanosilica addition on flowability, strength and transport properties of ultra high performance concrete, Mater. Des. 59 (2014) 1–9. [6] M. Heikal, S. Aleem, W. Morsi, Characteristics of blended cements containing nano-silica, HBRC J. 9 (3) (2013) 243–245. [7] M. Oltulu, R. Sahin, Single and combined effects of nano-SiO2, nano-Al2O3 and nano-Fe2O3 powders on compressive strength and capillary permeability of cement mortar containing silica fume, Mater. Sci. Eng., A 528 (22) (2011) 7012–7019. [8] M. Jalal, E. Mansouri, M. Sharifipour, A. Pouladkhan, Mechanical, rheological, durability and microstructural properties of high performance self-compacting concrete containing SiO2 micro and nanoparticles, Mater. Des. 34 (2012) 389– 400. [9] A.M. Rashad, Effects of ZnO2, Cu2O3, CuO, CaCO3, SF, FA, cement and geothermal silica waste nanoparticles on properties of cementitious materials. A short guide for Civil Engineer, Constr. Build. Mater. (2013) 1120–1133. [10] Hui Li, Huigang Xiao, Xinchun Guan, Zetao Wang, Lei Yu, Chloride diffusion in concrete containing nano-TiO2 under coupled effect of scouring, Compos. B 56 (2014) 698–704. [11] A. Nunes, The latest innovations in the application of structural concrete (in Portuguese), in: 1st Conference on Building Materials (JMC2011), 6 April, Porto, Portugal, 2011. [12] H. Li, H. Xiao, J. Yuan, J. Ou, Microstucture of cement mortar wiht nanoparticles, Compos. B 35 (2003) 185–189. [13] Ye Qing, Zhang Zenan, Kong Deyu, Chen Rongshen, Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume, Constr. Build. Mater. 21 (2007) 539–545. [14] Hongjian Du, Suhuan Du, Xuemei Liu, Durability performances of concrete with nano-silica, Constr. Build. Mater. 73 (2014) 705–712. [15] A. Nazari, S. Riahi, S. Riahi, S.F. Shamekhi, A. Khademno, Influence of Al2O3 nanoparticles on the compressive strength and workability of blended concrete, J. Am. Sci. 6 (2010) 6–9. [16] L. Zhenhua, W. Huafeng, H. Shan, L. Yang, W. Miao, Investigations on the preparation and mechanical properties of the nano-alumina reinforced cement composite, Mater. Lett. 60 (3) (2016) 356–359. [17] Shaswata Mukherjee, Salim Barbhuiya, Hamid Nikraz, Effects of nano-Al2O3 on early-age microstructural properties of cement paste, Constr. Build. Mater. 52 (2014) 189–193.
[18] A. Nazari, S. Riahi, The effects of ZnO2 nanoparticles on strength assessments and water permeability of concrete in different curing media, Mater. Res. 14 (2011) 178–188. [19] A.S. Louro, Bond on ribbed rebars subjected to repeated and alternate actions (in Portuguese) (Ph.D. thesis), Faculty of Science and Technology, University Nova de Lisboa, Portugal, 2014. [20] M.N.S. Hadi, Bond of high strength concrete with high strength reinforcing steel, Open Civ. Enig. J. 2 (2008) 143–147. [21] S. Zhang, Y. Fan, N. Li, Bonding behavior between steel bars and concrete modified with nano-kaolinite clay, J. Southeast Univ. (Nat. Sci. Ed.) 45 (2) (2015) 382–386. [22] A.N. Dancygier, A. Katz, U. Wexler, Bond between deformed reinforcement and normal and high-strength concrete with and without fibers, Mater. Struct. 43 (2010) 839–856. [23] Abid.A. Shah, Y. Ribakov, Recent trends in steel fibered high-strength concrete, Mater. Des. 32 (2011) 4122–4151. [24] E. Garcia-Taengua, J.R. Marti-Vargas, P. Serna, Bond of reinforcing bars to steel fiber reinforced concrete, Constr. Build. Mater. 105 (2016) 275–284. [25] C. Lourenço, E. Soldado, H. Costa, R. Carmo, E. Júlio, Influence of adding nanoparticles in the rheology and performance of self-compacting concrete (in Portuguese), in: IV Congresso Ibero-americano sobre Betão Auto-compactável, FEUP, Portugal, 2015. [26] E. Soldado, C. Lourenço, H. Costa, R. Carmo, E. Júlio, Influence of Adding Nanoparticles in the Performance of the Binding Matrix (in Portuguese), 5ª Jornadas Portuguesas de Engenharia de Estruturas, Portugal, 2015. [27] H. Costa, Structural lightweight aggregate concrete. Applications in prefabrication and strengthening structures (in Portuguese) (Ph.D. thesis), Faculty of Science and Technology, University of Coimbra, Portugal, 2013. [28] E 397 – 1993, LNEC Specification. Concrete. Determination of Young’s modulus in compression (in Portuguese), 1993. [29] EN 206-1, Concrete – Part 1: Specification, Performance, Production and Conformity, European Committee for Standardization, Brussels, Belgium, 2007. [30] EN 12390, Testing Hardened Concrete, European Committee for Standardization, Brussels, Belgium, 2009. [31] RILEM TC 162-TDF, Test and design methods for steel fiber reinforced concrete, Mater. Struct. 36 (2003) 560–567. [32] fib, Model Code for Concrete Structures 2010, Wiley-VCH Verlag GmbH & Co. KGaA, 2013. [33] EN 10080, Steel for the Reinforcement of Concrete – Weldable Reinforcing Steel – General, BSI British Standard and the European Standard, 2005.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of nano-SiO2 and nano-Al2O3 additions on steel-to-concrete bonding R. Ismael a, J.V. Silva a, R.N.F. Carmo a,⇑, E. Soldado a, C. Lourenço a, H. Costa a, E. Júlio b a b
CERIS, Polytechnic Institute of Coimbra, Rua Pedro Nunes – Quinta da Nora, 3030-199 Coimbra, Portugal CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
h i g h l i g h t s Adding nanoparticles leads to an increase of the bond on mixtures with higher percentage of cement. The addition of nano-Al2O3 decrease the crack width when plain rebars are used. The addition of nanoparticles does not affect the cracks’ width when ribbed rebars are adopted. No additional benefit was obtained by combining nanoparticles with steel fibers.
a r t i c l e
i n f o
Article history: Received 8 March 2016 Received in revised form 25 July 2016 Accepted 29 August 2016
Keywords: Nanoparticles Concrete Steel fibers Bond Cracking
a b s t r a c t The research on nanoparticles is quite recent and potentially relevant in different contexts, including the concrete field. Fiber reinforced concrete presents several advantages, one of which its capacity to best control cracking. In this scope, the bond between steel rebars and/or fibers and the binding paste plays a most relevant role. The study herein described was developed aiming at analyzing the influence on the latter, and as a consequence on cracking, of nano-Al2O3 and nano-SiO2 additions. In the experimental analysis, the following materials were adopted: both plain and ribbed rebars, eight different concrete mixtures, two types of nanoparticles additions, namely nano-Al2O3 and nano-SiO2, and, in some mixtures, steel fibers. Overall, thirty-two pull-out tests, plus sixteen tensile tests on reinforced concrete ties, were performed. It was concluded that adding nanoparticles to the concrete paste leads to an increase of the steel-to-concrete bond, for mixtures with higher percentage of cement. It was also concluded that Al2O3 nanoparticles induce a beneficial effect in terms of cracking, decreasing the width of these when plain rebars are used. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Concrete structures play a major role in the construction sector. Recent advances are mostly due to innovations in the material technology. Although taking its first steps on this industry, nanotechnology has an important share in this scope [1,2]. Over the last years, research regarding concrete mix design in which nanoparticles or nanotubes have been used has been carried out. Generally, the percentage of nanoparticles is calculated in relation to the volume (or mass) of the binder. Due to their reduced size, nanoparticles have a high specific area and a high reactivity during the cement’s hydration process, behaving as nucleation sites [3]. Nanoparticles can be beneficial in increasing the density of the binding matrix, by reducing concrete porosity, and in ⇑ Corresponding author. E-mail address: [email protected] (R.N.F. Carmo). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.152 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
improving the interfacial transition zone (ITZ), between aggregates and the cement paste. All these changes at the micro-scale influence the concrete performance at the macro-scale, increasing both strength and durability [1,4–8]. The nanoparticles of SiO2, currently referred to as nano-SiO2, were the first to be studied for showing the best results in reducing porosity and thus increasing the matrix density [1,4]. More recently, other types of nanoparticles have also been studied, such as nano-TiO2, nano-ZnO2, nano-Fe2O3, and nano-Al2O3, and it was observed that these also enhance concrete performance [9–11]. Studies performed by Li et al. [12], considering cementitious mixtures containing fly ash (50%) and nano-SiO2 (4%), showed an increase of 19% on the hydration temperature of cement and an increase of 81% in concrete strength, at 3 days of age. Qing et al. [13] studied the influence of nano-SiO2 addition on the properties of hardened cement paste comparatively to silica fume addition and have observed that the consistency and the setting time of
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Notations fcm,cube fctm fct,fl fRm
s smax d
mean value of concrete cube compressive strength mean value of concrete tensile strength mean value of concrete flexural tensile strength mean value of concrete residual flexural tensile strength bond stress maximum bond stress relative displacement
the fresh cement paste are different. These authors have concluded that: (i) nano-SiO2 accelerates the cement hydration process, (ii) both the compressive strength and the bond strength of the paste to aggregate interface are higher, especially at early ages, when nano-SiO2 is used; (iii) with an addition of 3% of nano-SiO2, the size of calcium hydroxide crystals are reduced thus improving the interface strength. An experimental investigation about the durability properties of concrete containing small dosages of nanosilica, varying between 0.3% and 0.9%, showed the beneficial effects of nano-silica, due to the combined contribution of the nano-filler effect and the pozzolanic reaction [14]. These results indicate, by comparison with other works [5–8] in which higher dosage of nano-SiO2 was used, that a durability enhancement can be obtained with lower dosages of nano-particles, but it is mandatory to ensure a good dispersion of the these within the cement matrix. Nazari et al. conducted an experimental work with nano-Al2O3, which also revealed an increase in compressive strength using amounts of nanoparticles not exceeding 2% [15]. The addition of nano-Al2O3 also showed to have a major influence on the Young’s modulus leading to a significant increase of the latter [16]. Mukherjee et al. [17] did not find significant differences in the compressive strength of concrete with nano-Al2O3 addition, at early-ages, but refer to a change in the microstructure of the paste, more dense and with larger crystals of portlandite within the cement matrix. Nazari and Riahi [18] proved that adding nano-ZnO2 to concrete mixtures leads to a significant decrease in workability and percentage of water absorption but can increase the strength capacity.
smin savg smax wmin wavg wmax
minimum spacing between cracks average spacing between cracks maximum spacing between cracks minimum crack width average crack width maximum crack width
Regarding nano-TiO2, it can be stated that it is one of the few types of nanoparticles that has already been used in construction (e.g., ‘‘Dives in Misericordia” Church, in Rome, Italy), due to its photocatalytic properties [3]. When exposed to ultraviolet light becomes hydrophilic and can be used for anti-fogging coating or selfcleaning. Steel-to-concrete bond is fundamental to ensure the structural behavior of reinforced concrete members for both serviceability and ultimate limit states. To design the anchorage length and the lap length of rebars, as well as to define rules for cracking control, a deep knowledge of the latter is required, involving three components: i) adhesion, understood as chemical and micro-mechanical connections; ii) mechanical interlock, originated by the rebar’s ribs; and iii) friction, mobilized by a relative displacement between the two surfaces in contact, and thus highly dependent on the roughness of the interface surface (Fig. 1) [19,20]. Bearing in mind that adding nanoparticles to the binding matrix enhances concrete properties, it is likely that steel-to-concrete bond be also improved and, consequently, the cracking of reinforced concrete members be reduced. Another study [21] conducted on with this topic, but considering a less usual nanoparticle, nano-kaolinite clay, shows that the addition of the latter can improve the bonding strength of concrete to steel rebars, reducing the stiffness of concrete specimen and delay steel corrosion. In the last decades, steel fibers have been used to reinforce cementitious materials, increasing the low tensile strength [22,23]. Regarding bond of rebars to concrete, fibers provide passive confinement and improve bond capacity in terms of bond
Fig. 1. Concrete-steel bond.
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Fig. 2. (a) SiO2 nanoparticles (150,000 zoom); (b) Al2O3 nanoparticles (1000 zoom); SiO2 nanoparticles in the concrete matrix; (c) 1000 zoom and (d) 5000 zoom.
strength and, more importantly, toughness [24]. Considering the effects described above, the bond between steel fibers and concrete matrix may also be sensitive to nanoparticles addition and it is interesting to verify if any positive interaction is originated from the combination of nanoparticles with steel fibers. 2. Research significance Studies on the addition of nanoparticles to concrete mixtures are mainly focused on analyzing the material behavior in fresh state, as well as its strength and durability in hardened state. So
far, there are not any published studies on the structural behavior of concrete members incorporating nanoparticles, particularly with nano-SiO2 and nano-Al2O3. To fill this gap was the main motivation for the experimental work herein described. Being the behavior of reinforced concrete based on steel-to-concrete bond, to understand the effect of nanoparticles addition in the latter was elected as the main goal of this research study. By adding nanoparticles to the binding matrix, an increase in the bond between the steel reinforcement and the concrete is expected to occur, as a result of a combined effect of the following factors: (i) the increase of concrete tensile strength, (ii) the
Table 1 Concrete mix proportions for 1 m3 (by weight, kg). Constituents
CEM I 52,5 R Limestone Filler Superplasticizer Water 0/1 mm Sand 0/4 mm Sand 4/8 mm Granite Gravel 6/14 mm Limestone Gravel Nano-SiO2 (2%) Nano-Al2O3 (2%) Steel Fibers (0.5%)
Dosages CA
CA_Si2%
CA_Al2%
CB
CB_Si2%
CB_Al2%
CB_F0.5
CB_F0.5Al2%
230 120 0,9 161 227 681 187 755 – – –
223 120 0,9 156 228 684 188 760 7 – –
223 120 0,9 156 229 688 188 759 – 7 –
350 – 2,1 140 239 718 190 770 – – –
343 – 2,7 140 238 715 190 769 7 – –
343 – 2,4 140 240 719 190 770 – 7 –
350 – 2,1 140 238 713 189 764 – – 39
343 – 2,1 140 238 714 189 764 – 7 39
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Fig. 3. (a) Tensile strength test; (b) Young’s modulus test; (c) Bending test conducted to assess the maximum and the residual flexural tensile strengths.
Table 2 Concrete properties in hardened state.
3.1. Concrete mixtures
Concrete
fcm,cube (MPa)
fctm (MPa)
Young’s Modulus (GPa)
CA CA_Si2% CA_Al2% CB CB_Si2% CB_Al2% CB_F0.5 CB_F0.5Al2%
40.1 43.3 40.1 74.2 74.1 72.7 79.1 78.5
4.8 4.1 4.5 5.7 7.2 6.9 5.3 5.4
38.7 41.9 49.2 51.4 52.6 52.6 53.8 49.2
increase of the binding matrix density, and (iii) the increase of the number of chemical connections between the binding matrix and steel rebars. Two types of nanoparticles (Fig. 2), nano-SiO2 (nano-silica) and nano-Al2O3 (nano-aluminum), produced by Smart Innovation company, were adopted. An extensive experimental program was settled and conducted with the aim of clarifying the effect of the nanoparticles addition in both steel-to-concrete bond and concrete cracking.
3. Experimental program To study the steel-to-concrete bond stress, thirty-two pull-out tests were performed, whereas to study the cracks width and spacing, sixteen tensile tests on reinforced concrete ties were conducted. In these tests, plain and ribbed rebars, as well as eight different types of concrete mixtures, were used. Although plain rebars are not currently used, the bond between this type of rebars and concrete was studied since the adhesion component has a prominent role in these cases. In ribbed rebars, the mechanical interlock has a very significant influence on rebar to concrete bonding and makes it much more difficult analyze the effect of both chemical and micro-mechanical connections and, thus, the expected contribution of nano-roughness provided by the addition of nanoparticles to the binding matrix. For each possible combination of rebars and concrete mixtures, two pull-out specimens plus one reinforced concrete tie were tested. This section describes the materials properties, the specimens’ production, the tests set-up, and the procedures adopted.
Eight different types of concrete mixtures were produced, divided into two series, each one with a different dosage of cement, limestone filler and nanoparticles, although always totalizing 350 kg/m3. These included three reference mixtures (without nanoparticles): CA, with cement and limestone filler; CB, with cement only; and CB_F0.5, equal to the latter but with an addition of steel fibers. Based on CA, two mixtures were produced: CA_Si2%, containing nano-SiO2, and CA_Al2%, containing nano-Al2O3. Based on CB, also two mixtures were produced: CB_Si2%, containing nano-SiO2, and CB_Al2%, containing nano-Al2O3. Lastly, based on CB_F0.5, one more mixture was produced: CB_F0.5Al%, containing nano-Al2O3. Preliminary tests [25,26] revealed that nano-Al2O3 addition combined with steel fibers leads to slightly better results compared to nano-SiO2 addition. The following binder constituents were adopted: cement CEM I 52.5 R, with a density of 3.16 kg/dm3, a limestone filler, with a density of 2.70 kg/dm3, nano-SiO2, with a density of 2.22 kg/dm3, and nano-Al2O3, with a density of 3.95 kg/dm3. The nanoparticles were provided in solid state. The dosage of nanoparticles was settled as 2% (in relation to the cement mass), based on previous studies developed by Lourenço et al. [25] and Soldado et al. [26]. Several mixtures were produced using different dosages of nanoparticles, ranging from 1 to 4%, being 2% the most favorable dosage given that higher values did not lead to significantly higher strengths. This percentage is within the range described in other studies [5–8] and does not compromise the workability of concrete. However, it must be highlighted that the fluidity decreases slightly with the increase of the addition of nanoparticles but, in general, this can be compensated with a small adjustment of the superplasticizer dosage. Two coarse aggregates, a 4/8 mm granite gravel and a 6/14 mm limestone gravel, with densities of 2.63 kg/dm3 and 2.66 kg/dm3, respectively, and two fine aggregates, a 0/1 mm fine sand and a 0/4 mm medium sand, both with a 2.63 kg/dm3 density, were adopted. Steel fibers Dramix RL 45/30 BN, with a length of 30 mm, a diameter of 0.62 mm, a shape factor (l/d) of 48 (class 45), and a simple hook-end anchorage, were adopted. The dosage of steel fibers was settled as 0.5% (in volume), based on preliminary tests. Lastly, Glenium SKY 526, a third generation superplasticizer, polycarboxylates-based, was
Table 3 Maximum and residual flexural tensile strengths. Concrete
fct,fl (MPa)
fRm,1 (MPa)
fRm,2 (MPa)
fRm,3 (MPa)
fRm,4 (MPa)
CB_F0.5 CB_F0.5Al2%
5.96 6.61
4.43 4.89
2.69 2.94
1.72 1.39
1.25 0.89
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used to reduce the amount of water demand, simultaneously maintaining a high workability of concrete in fresh state. It has to be stated that the performance of concrete with 1% of steel fibers was also assessed and showed a significant loss in workability when compared to the 0.5% dosage. All mixtures were designed according to a method first developed by Lourenço, and later improved by Costa [27], being listed in Table 1 the corresponding constituents dosages.
3.2. Concrete properties The following mechanical properties of the eight adopted concrete mixtures were assessed: Young’s modulus, compressive strength, and tensile strength (Fig. 3). The Young’s modulus was determined using one prismatic specimen (100 100 400 mm3), according to E397 [28]; the concrete compressive strength was assessed using three cubic specimens, according to EN 206-1 [29] and EN 12390 [30]; and the tensile strength was determined using four diametral compression tests, in accordance to EN 12390 [30]. For concrete mixtures with steel fibers, additional tests were performed on prismatic specimens (100 100 500 mm3), two for each mixture, to evaluate the maximum and the residual flexural tensile strength, following the procedures presented in RILEM TC 162-TDF [31] and EN 12390-5 [30]. According to MC2010 [32] the concrete strength, fRm1, was 4–5 MPa for both types of concrete, and the fRm3/fRm1 ratio was equal to 0.39 for CB_F0.5 and to 0.28 for CB_F0.5Al2%. Tables 2 and 3 summarize the assessed concrete properties.
3.3. Pull-out tests Rebars with a diameter of 10 mm, S 500 steel grade, and with plain or ribbed surface, were adopted. These were embedded in cylindrical specimens (Fig. 4a), with a diameter of 100 mm and a height of 200 mm, produced with the eight different mixtures previously referred. For each possible combination, two equal specimens were produced, totaling thirty-two. The concrete was confined with a steel formwork in order to prevent its premature failure, i.e., before reaching the maximum bond stress. It was cast with the steel rebars positioned vertically at exactly the center of the specimens. To ensure these geometric requirements, as well as the lengths with and without bond, with significant precision, a set-up was specially designed and built to support the specimens’ production (Fig. 4b). The pull-out tests were performed (Fig. 5) according to EN 10080 [33]. The tensile force was applied using a hydraulic servo-actuator. The load was applied with displacement control at a rate of 0.02 mm/s. The data acquisition was performed with a frequency of 1 Hz. Both the applied force and the corresponding displacement were measured using, respectively, the load cell and the displacement transducer (LVDT) of the hydraulic servo-actuator. To ensure an accurate and precise measurement of the steel rebar slippage relatively to the concrete bulk, three LVDTs were used. 3.4. Reinforced concrete ties Sixteen reinforced concrete ties (Fig. 6) were produced with the following dimensions: 75 75 1200 mm3. This geometry has
Fig. 4. (a) Pull-out test specimens; (b) set-up for specimens’ production.
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been adopted to allow the initiation, development and stabilization of cracks before steel rebars reached yielding. Again a single (ribbed or plain) steel rebar with a diameter of 10 mm, S 500 steel grade, was positioned at the center of the cross-section. The concrete was cast with the steel rebars positioned horizontally and ensuring its correct positioning. The tensile force was applied through metallic plates positioned at the ends of the RC ties and following the same procedure adopted in pull-out tests, namely with displacement control at 0.02 mm/s. To accurately measure the axial deformation, four LVDTs were used, one placed at each edge (Fig. 6). The cracks widths were measured using a graduated ruler, at each of the four faces of the RC ties, at different time steps. Measure the cracks width at all four faces was time-consuming but crucial because the width varied along the crack length, which would not be initially expected in a tensile test. 4. Experimental results and analysis In this section, results of both pull-out and RC ties tests are presented and discussed. For the sake of clarity, all specimens were labeled according to the same procedure, presented in Fig. 7. 4.1. Pull-out tests In all pull-out tests, failure occurred as predicted, i.e., the maximum bond stress was reached without premature concrete failure. The confinement provided by the steel formwork is similar for each type of rebars and concrete, so the results can be compared within each series and the differences obtained are relative differences, which are useful to accomplish the goals of this study. The bond stress (s) was calculated as the applied force divided by the contact area between the rebar and concrete and was
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plotted versus the corresponding relative displacement (d) of the rebar regarding concrete. The obtained stress versus displacement graphics are shown in Fig. 8, for plain rebars, and in Fig. 9, for ribbed rebars. Each value is the average of two similar tests, except for P5R_CB_Si2%, since an unexpected failure near the anchorage of the metallic plate occurred in one of the specimens tested. It was not detected significant differences between the two tests. In Fig. 10, the maximum values of all considered situations are plotted. Considering the results obtained with plain rebars, it is noticeable an increase of the maximum bond stress when nanoparticles are added to concrete. For CA specimens, maximum bond stresses of 3.8 and 3.5 MPa, respectively for nano-SiO2 and nano-Al2O3 additions, were measured, higher than the value of 3.2 MPa, measured on the reference specimen (without nanoparticles). For CB specimens, a similar trend was registered, but the strength increase was considerably higher, namely 6.8 MPa and 6.7 MPa, respectively for nano-SiO2 and nano-Al2O3 additions, whereas the value measured on the reference specimen (without nanoparticles) was just 5.1 MPa. The effect of the two types of nanoparticles was very similar, although nano-SiO2 exhibited slightly better results. Results obtained with ribbed rebars also showed an increase of the bond stress when nanoparticles were added, especially nanoSiO2, in this case an increase of 5.5 MPa was registered. Comparing the plain and ribbed steel rebars, a higher relative displacement is required to achieve the maximum bond stress in ribbed rebars. This result can be explained by the fact that its components, i.e., friction and mechanical effect provided by the ribs, demand a significant slippage between rebar and concrete, circa 1.5–3.0 mm, to mobilize its full strength. From these results, as expected, it is observed that the mechanical effect is the most important component of the bond strength. Therefore, the influence of the nanoparticles on the latter, although positive, could never be very significant. For this reason, bond between plain
A – Hydraulic actuator; B – Top LVDTs; C - Reaction / supporting plates; D – Pull-out specimen; E – Bottom LVDTs;
F - Metallic frame
Fig. 5. Set-up of the pull-out tests.
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A – Hydraulic actuator; B – Metallic plates ; C - LVDTs; D – Reinforced concrete tie; E -Metallic frame Fig. 6. Tie test set-up.
Fig. 7. Label of pull-out specimens and RC ties.
rebars and concrete should be more sensitive to the effect of nanoparticles addition since, in this case, bond is mainly ensured by adhesion and micro-mechanical connections, and nanoparticles influence mostly the adhesion component. Adding steel fibers to the concrete mixture produced some improvement on the maximum bond stress, especially in tests with
ribbed rebars, probably because ribs tend to create transversal micro-cracking, due to tensile stresses that are generated around the steel rebar, and steel fibers prevent the opening of these micro-cracks. In these tests, the nanoparticles addition did not produce any increase on bond. To further discuss results, it was decided to plot the maximum bond stress versus the concrete compressive strength (Fig. 11), since this is the main concrete property and, moreover, it is directly correlated with the tensile strength. Four distinct groups can be clearly identified, as well as the relative importance of the type of rebar (plain or ribbed) and of the type of concrete mixture in the bond between rebars and concrete. Higher bond stresses are reached when ribbed rebars are used, as well as when concrete with higher compressive strength is adopted. It is also detectable that, on specimens with plain rebars, steel fibers have not a relevant effect, probably due to absence of high values of transversal tensile stresses around the rebars. This means that, in these cases, the steel fibers are not properly stretched, which is opposite of what was found on ribbed rebars.
8
8 P1P_CA P2P_CA_Al2% P3P_CA_Si2%
6
τ (MPa)
τ (MPa)
6
P4P_CB P5P_CB_Al2% P6P_CB_Si2% P7P_CB_F0,5 P8P_CB_F0,5Al2%
4
2
4
2
0
0 0
6
12
18
24
0
6
12
δ (mm)
δ (mm)
(a)
(b)
Fig. 8. Bond stress vs. displacement for plain rebars: (a) concrete CA and (b) concrete CB.
18
24
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32
32
P1R_CA P2R_CA_Si2% P3R_CA_Al2%
24
τ (MPa)
τ (MPa)
24
16
16
8
8
0
P4R_CB P5R_CB_Si2% P6R_CB_Al2% P7R_CB_F0,5 P8R_CB_F0,5Al2%
0 0
6
12
18
24
0
6
12
δ (mm)
δ (mm)
(a)
(b)
18
24
Fig. 9. Bond stress vs. displacement for ribbed rebars: (a) concrete CA and (b) concrete CB.
32
32
Plain bars
P1_CA P3_CA_Si2% P5_CB_Al2% P7_CB_F0,5
29.1
27.9 Ribbed bars 24
CA ribbed rebars
19.8
19.2 16
15.7 13.1
τ (MPa)
τmax (MPa)
24
22.5
22.4
P2_CA_Al2% P4_CB P6_CB_Si2% P8_CB_F0,5Al2%
CB ribbed rebars
16
CB plain rebars
8
6.8 3.8
3.2
5.1
8
6.7 5.2
3.5
0
Reference Si2% Al2% CA
CA plain rebars
5.4
Reference Si2% Al2% F0.5 F0.5Al2% CB
Fig. 10. Maximum bond stress.
4.2. Tie tests Based on the measured data, a steel stress versus strain graphic was plotted, for each studied situation (Fig. 12). The overall behavior of the ties is characterized by four characteristic stages. The first stage corresponds to the uncracked state, being defined by an almost linear elastic response with high axial stiffness, and it ends when the first crack appears. Immediately after, the process of crack formation begins and is accompanied by sudden stress variations. These peaks are observed in each curve of the tie. The following stage corresponds to stabilized cracking and finishes with yielding of steel reinforcement. In this stage, it is clearly detectable the tension stiffening effect, where concrete between cracks contributes to the tensile strength and reduces the rebar strain, comparatively with the deformation observed with naked rebars. The plastic stage, where the axial deformation increases under an almost constant axial stress, is incipient for some ties, well visible for others and was not detected for plain rebars. The tests were stopped when the cracks width were large and it corresponded to different load levels. This was the main reason for the differences observed in the last stage. The experimental results also show different values of the yield strength for plain and ribbed rebars, being slightly higher for ribbed rebars. The development of the cracks width during the test was recorded for each of the four faces of each tie and, for two distinct phases, the value of the minimum, average and maximum cracks width was registered. It is visible that, during the cracking stabi-
0
25
40
55
70
85
fcm (MPa) Fig. 11. Maximum bond stress vs. concrete strength.
lization phase, the tension stiffening effect, due to the contribution of concrete between cracks, is different for plain and for ribbed rebars, being slightly higher in the former case. It is also observed that, for the same stress level, the deformation is slightly lower for ribbed rebars. This difference is explained taking into account the difference in cracking patterns, namely in the average distance between cracks. Fig. 13 shows, for four ties, all the information recorded during tests: cracks identification, position and shape, and development of the cracks width, on each side, with the applied force. On TP_CB ties, for a steel stress ranging between 479 and 500 MPa an average crack width (wavg) of 0.33 mm was measured for the reference mixture, 0.27 mm for the mixture with nanoAl2O3, and 0.40 mm for the mixture with nano-SiO2. Hence, there is a slight reduction of the cracks width when concrete incorporates nano-Al2O3. The use of steel fibers also leads to a slight decrease on average of the crack width, but it has little or no effect on the corresponding maximum value (wmax). On TR_CB ties, for high levels of steel stress, it was observed a practically identical wavg of approximately 0.25 mm was observed on ties with and without nanoparticles. However, higher values of wmax were obtained on ties with nanoparticles. These results reveal that, for ribbed rebars, the effect of ribs surpasses the beneficial effect achieved by adding aluminum nanoparticles. This means that mechanical anchorage is predominant regarding adhesion. Regarding the effect of steel fibers addition, it was not noticed any significant difference between ties with plain and ties with
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600 Wmax = 1.18 Wavg = 0.71 Wmin = 0.43
450
Wmax = 0.43 Wavg = 0.30 Wmin = 0.10
300 Wmax = 0.23 Wavg = 0.17 Wmin = 0.10
150
Wmax = 0.10 Wavg = 0.09 Wmin = 0.08
Steel stress (MPa)
Steel stress (MPa)
600
Wmax = 0.58 Wavg = 0.52 Wmin = 0.45
450
300 Wmax = 0.15 Wavg = 0.13 Wmin = 0.08
150
Wmax = 0.28 Wavg = 0.20 Wmin = 0.13
T1P_CA T1R_CA
0
1
2
3
0
4
1
Average strain (x10-3)
T2R_CA_Al2%
2
Wmax = 0.38 Wavg = 0.33 Wmin = 0.30
Steel stress (MPa)
Wmax = 0.43 Wavg = 0.30 Wmin = 0.08
300 Wmax = 0.20 Wavg = 0.13 Wmin = 0.08
150
Wmax = 0.10 Wavg = 0.09 Wmin = 0.08
Wmax = 0.43 Wavg = 0.24 Wmin = 0.08
450 Wmax = 0.13 Wavg = 0.13 Wmin = 0.13
300
Wmax = 0.08 Wavg = 0.08 Wmin = 0.08
150
T3P_CA_Si2%
T4P_CB
T3R_CA_Si2%
T4R_CB
0
0 0
1
2
3
0
4
1
2
600
4
600 Wmax = 0.33 Wavg = 0.27 Wmin = 0.15
Wmax = 0.58 Wavg = 0.40 Wmin = 0.28
450 Wmax = 0.18
Steel stress (MPa)
Wmax = 0.55 Wavg = 0.27 Wmin = 0.08
Wavg = 0.14 Wmin = 0.10
300
Wmax = 0.10 Wavg = 0.10 Wmin = 0.10
150
450 Wmax = 0.55 Wavg = 0.25 Wmin = 0.08
Wmax = 0.08 Wavg = 0.06 Wmin = 0.05
300
Wmax = 0.28 Wavg = 0.20 Wmin = 0.15
150
T5P_CB_Al2%
T6P_CB_Si2%
T5R_CB_Al2%
0
1
2
3
T6R_CB_Si2%
0
4
0
1
Average strain (x10-3)
Wmax = 0.43 Wavg = 0.16 Wmin = 0.05
450 Wmax = 0.18 Wavg = 0.14 Wmin = 0.08 300
Wmax = 0.20 Wavg = 0.13 Wmin = 0.08
Wmax = 0.38 Wavg = 0.30 Wmin = 0.18 450 Wmax = 0.15 Wavg = 0.13 Wmin = 0.08
1
4
Wmax = 0.40 Wavg = 0.22 Wmin = 0.03
300 Wmax = 0.18 Wavg = 0.13 Wmin = 0.08
150
T7P_CB_F0,5
T8P_CB_F0,5Al2%
T7R_CB_F0,5
0
3
600
Wmax = 0.43 Wavg = 0.31 Wmin = 0.20
150
2
Average strain (x10-3)
Steel stress (MPa)
600
Steel stress (MPa)
3
Average strain (x10-3)
Average strain (x10-3)
0
4
600 Wmax = 0.50 Wavg = 0.44 Wmin = 0.40
450
0
3
Average strain (x10-3)
600
Steel stress (MPa)
T2P_CA_Al2%
0
0
Steel stress (MPa)
Wmax = 0.58 Wavg = 0.29 Wmin = 0.03
2
3
T8R_CB_F0,5Al2%
4
0 0
Average strain (x10-3) Fig. 12. Steel stress vs. tie deformation.
1
2
3
Average strain (x10-3)
4
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Fig. 13. Development of cracks width with the applied force.
ribbed rebars. Although wavg was slightly lower for the latter case, wmax was almost the same in both cases (Fig. 14). No additional benefit was obtained by combining nanoparticles with steel fibers in any case. On TP_CA ties, for a steel stress ranging between 493 and 500 MPa, the wavg was 0.71 mm, for the reference mixture, 0.52 mm, for the mixture with nano-Al2O3, and 0.44 mm, for the mixture with nano-SiO2. Thus, for ties produced with the lowest strength concrete, the effect of adding nanoparticles (either nano-aluminum or nano-silica) on the reduction of the cracks width is higher than for ties produced with the highest strength
concrete. The results for TR_CA ties, i.e., for ribbed rebars, follow exactly the same trend. When the cracking pattern is analyzed, it is very important to evaluate the spacing between cracks, since this parameter has a direct correlation with the cracks width. During and after each test, the distance between cracks, at each of the four faces, was documented in detail. Higher distances between cracks were measured on TP_CA ties. On T_CA ties, with the lower strength concrete, the distances between cracks were higher than on the corresponding T_CB ties, due to a lower steel-to-concrete adhesion. And as is well known the adhesion is proportional to
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On ties with plain rebars, for both type concretes, CA and CB, the addition of nanoparticles slightly reduces the spacing between cracks. Adding steel fibers that spacing is even more reduced, which is explained by the increase of bond that derives from the increase of the tensile strength of concrete. On ties with ribbed rebars, the number of cracks is clearly higher than on ties with plain rebars. When the distance between cracks is analyzed, again it is noticeable that the mechanical strength component has a more significant effect than the increase in adhesion provided by the addition of nanoparticles. For such reason, the distance between cracks was very similar on all ties with ribbed rebars, for both types of concrete, containing or not nanoparticles. Using all the information concerning the cracking pattern, it was possible to produce the diagrams shown in Fig. 16, in which the position of the cracks and the average width along the tie is shown (last measurement performed). For a better assessment of the cracks width, a color gradient was used. Fig. 14. Crack detail on a tie reinforced with steel fibers.
concrete tensile strength. Since the main influence on the crack pattern, particularly on the distance between cracks, comes from the type of rebar surface (plain or ribbed), a clear decrease on the distance between cracks on ties with ribbed rebars compared to ties with plain rebars (Fig. 15 and Table 4) was expected.
5. Conclusions To clarify and consolidate the trend of the obtained results, conducting further testing with a larger number of specimens is mandatory. Generalizing these results to other cases must be done with caution because the type of nanoparticles and the synthesis process used may also influence the results. Based on the research study herein described, the following conclusions can be drawn:
Fig. 15. Distance between cracks on ties: (a) T1P_CA and (b) T1R_CA.
Table 4 Spacing between cracks and crack width. Tie
T1P_CA T2P_CA_Si2% T3P_CA_Al2% T4P_CB T5P_CB_Si2% T6P_CB_Al2% T7P_CB_F0,5 T8P_CB_F0,5Al2% T1R_CA T2R_CA_Si2% T3R_CA_Al2% T4R_CB T5R_CB_Si2% T6R_CB_Al2% T7R_CB_F0,5 T8R_CB_F0,5Al2%
Space between cracks (mm)
Crack width (mm)
smin
savg
smax
wmin
wavg
wmax
122 140 213 129 123 89 69 113 25 34 22 29 36 25 14 32
273 252 252 187 170 160 149 159 91 95 86 87 111 85 90 88
425 364 290 245 216 231 230 205 158 156 150 145 186 145 166 144
0.43 0.40 0.45 0.30 0.28 0.15 0.20 0.18 0.10 0.08 0.03 0.08 0.28 0.15 0.05 0.03
0.71 0.44 0.52 0.33 0.40 0.27 0.31 0.30 0.30 0.30 0.29 0.24 0.40 0.27 0.16 0.22
1.18 0.50 0.58 0.38 0.58 0.33 0.43 0.38 0.43 0.43 0.58 0.43 0.58 0.33 0.43 0.40
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Fig. 16. Cracking pattern with average crack width for ties with: (a) plain rebars; (b) ribbed rebars.
The maximum bond stress increased with the addition of nanoparticles to the concrete mixture, especially for specimens produced with the highest dosage of cement. There was an increase of approximately 25% relatively to the reference concrete, both for plain and ribbed rebars; In RC ties, the addition of nano-Al2O3 was particularly effective in reducing the cracks width and in decreasing the spacing between cracks, when plain rebars were used; In RC ties, when ribbed rebars were adopted, the addition of nanoparticles did not produce a significant effect on the cracks width; The obtained results also indicate that such small particles and in reduced percentages influence mostly the adhesion component, which is particularly relevant when plain rebars were used. When the crack pattern is analyzed, the ribs of the rebars is more effectiveness than the increase in adhesion provided by the addition of nanoparticles; No additional benefit was obtained by combining nanoparticles with steel fibers.
Acknowledgments The authors thank the support of Agência Nacional de Inovação – Portugal, namely by funding the project 38702 entitled ‘‘NanoCrete: Development of Enhanced Performance Concrete by Means of Nanoparticles Additions”. Acknowledgements are extended to Smart Inovation Lda., for developing and producing the nanoparticles used in this study, as well as to Secil, BASF, Sika, Omya and Argilis, for providing the materials adopted in the latter.
References [1] F. Sanchez, K. Sobolev, Nanotechnology in concrete – a review, Constr. Build. Mater. 24 (2010) 2060–2071. [2] W. Zhu, P. Bartos, A. Porro, Application of nanotechnology in construction – summary of a state-of-the-art report – RILEM TC 197-NCM, Mater. Struct. 37 (9) (2004) 649–658. [3] F.P. Torgal, S. Jalali, Eco-Efficient Concrete: The Future of the Ready-mix Concrete Industry (in Portuguese), Betão, no 26, Associação Portuguesa das Empresas de Betão Pronto (APEB), Portugal, 2011.
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R. Ismael et al. / Construction and Building Materials 125 (2016) 1080–1092
[4] B. Bhuvaneshwari, S. Sasmal, N. Iyer, Nanoscience to Nanotechnology for Civil Engineering Proof of Concepts, Recent Researches in Geography, Geology, Energy, Environment and Biomedicine, WSEAS Press, 2011, pp. 230–235. [5] E. Ghafari, H. Costa, E. Júlio, A. Portugal, L. Durães, The effect of nanosilica addition on flowability, strength and transport properties of ultra high performance concrete, Mater. Des. 59 (2014) 1–9. [6] M. Heikal, S. Aleem, W. Morsi, Characteristics of blended cements containing nano-silica, HBRC J. 9 (3) (2013) 243–245. [7] M. Oltulu, R. Sahin, Single and combined effects of nano-SiO2, nano-Al2O3 and nano-Fe2O3 powders on compressive strength and capillary permeability of cement mortar containing silica fume, Mater. Sci. Eng., A 528 (22) (2011) 7012–7019. [8] M. Jalal, E. Mansouri, M. Sharifipour, A. Pouladkhan, Mechanical, rheological, durability and microstructural properties of high performance self-compacting concrete containing SiO2 micro and nanoparticles, Mater. Des. 34 (2012) 389– 400. [9] A.M. Rashad, Effects of ZnO2, Cu2O3, CuO, CaCO3, SF, FA, cement and geothermal silica waste nanoparticles on properties of cementitious materials. A short guide for Civil Engineer, Constr. Build. Mater. (2013) 1120–1133. [10] Hui Li, Huigang Xiao, Xinchun Guan, Zetao Wang, Lei Yu, Chloride diffusion in concrete containing nano-TiO2 under coupled effect of scouring, Compos. B 56 (2014) 698–704. [11] A. Nunes, The latest innovations in the application of structural concrete (in Portuguese), in: 1st Conference on Building Materials (JMC2011), 6 April, Porto, Portugal, 2011. [12] H. Li, H. Xiao, J. Yuan, J. Ou, Microstucture of cement mortar wiht nanoparticles, Compos. B 35 (2003) 185–189. [13] Ye Qing, Zhang Zenan, Kong Deyu, Chen Rongshen, Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume, Constr. Build. Mater. 21 (2007) 539–545. [14] Hongjian Du, Suhuan Du, Xuemei Liu, Durability performances of concrete with nano-silica, Constr. Build. Mater. 73 (2014) 705–712. [15] A. Nazari, S. Riahi, S. Riahi, S.F. Shamekhi, A. Khademno, Influence of Al2O3 nanoparticles on the compressive strength and workability of blended concrete, J. Am. Sci. 6 (2010) 6–9. [16] L. Zhenhua, W. Huafeng, H. Shan, L. Yang, W. Miao, Investigations on the preparation and mechanical properties of the nano-alumina reinforced cement composite, Mater. Lett. 60 (3) (2016) 356–359. [17] Shaswata Mukherjee, Salim Barbhuiya, Hamid Nikraz, Effects of nano-Al2O3 on early-age microstructural properties of cement paste, Constr. Build. Mater. 52 (2014) 189–193.
[18] A. Nazari, S. Riahi, The effects of ZnO2 nanoparticles on strength assessments and water permeability of concrete in different curing media, Mater. Res. 14 (2011) 178–188. [19] A.S. Louro, Bond on ribbed rebars subjected to repeated and alternate actions (in Portuguese) (Ph.D. thesis), Faculty of Science and Technology, University Nova de Lisboa, Portugal, 2014. [20] M.N.S. Hadi, Bond of high strength concrete with high strength reinforcing steel, Open Civ. Enig. J. 2 (2008) 143–147. [21] S. Zhang, Y. Fan, N. Li, Bonding behavior between steel bars and concrete modified with nano-kaolinite clay, J. Southeast Univ. (Nat. Sci. Ed.) 45 (2) (2015) 382–386. [22] A.N. Dancygier, A. Katz, U. Wexler, Bond between deformed reinforcement and normal and high-strength concrete with and without fibers, Mater. Struct. 43 (2010) 839–856. [23] Abid.A. Shah, Y. Ribakov, Recent trends in steel fibered high-strength concrete, Mater. Des. 32 (2011) 4122–4151. [24] E. Garcia-Taengua, J.R. Marti-Vargas, P. Serna, Bond of reinforcing bars to steel fiber reinforced concrete, Constr. Build. Mater. 105 (2016) 275–284. [25] C. Lourenço, E. Soldado, H. Costa, R. Carmo, E. Júlio, Influence of adding nanoparticles in the rheology and performance of self-compacting concrete (in Portuguese), in: IV Congresso Ibero-americano sobre Betão Auto-compactável, FEUP, Portugal, 2015. [26] E. Soldado, C. Lourenço, H. Costa, R. Carmo, E. Júlio, Influence of Adding Nanoparticles in the Performance of the Binding Matrix (in Portuguese), 5ª Jornadas Portuguesas de Engenharia de Estruturas, Portugal, 2015. [27] H. Costa, Structural lightweight aggregate concrete. Applications in prefabrication and strengthening structures (in Portuguese) (Ph.D. thesis), Faculty of Science and Technology, University of Coimbra, Portugal, 2013. [28] E 397 – 1993, LNEC Specification. Concrete. Determination of Young’s modulus in compression (in Portuguese), 1993. [29] EN 206-1, Concrete – Part 1: Specification, Performance, Production and Conformity, European Committee for Standardization, Brussels, Belgium, 2007. [30] EN 12390, Testing Hardened Concrete, European Committee for Standardization, Brussels, Belgium, 2009. [31] RILEM TC 162-TDF, Test and design methods for steel fiber reinforced concrete, Mater. Struct. 36 (2003) 560–567. [32] fib, Model Code for Concrete Structures 2010, Wiley-VCH Verlag GmbH & Co. KGaA, 2013. [33] EN 10080, Steel for the Reinforcement of Concrete – Weldable Reinforcing Steel – General, BSI British Standard and the European Standard, 2005.