infrastructures

Article New Trends for Reinforced Structures: Some Results of Exploratory Studies

Ricardo N. F. Carmo 1,* ID and Eduardo Júlio 2 1 CERIS and Polytechnic Institute of Coimbra, Rua Pedro Nunes—Quinta da Nora, 3030-199 Coimbra, Portugal 2 CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] * Correspondence: [email protected]; Tel.: +351-239-790-200

Academic Editors: Jónatas Valença, Susana Lagüela López and Pedro Arias-Sánchez Received: 21 September 2017; Accepted: 24 October 2017; Published: 27 October 2017

Abstract: Today, the concrete sector is being pushed to innovate in order to better address current challenges with higher competitiveness and more sustainable solutions. Different research studies have been conducted all over the world in which novel approaches and paths were proposed. It is important to spread information to define new strategies for the future of this industry. The enhancement of concrete properties and the impact of these changes in structural design are some of the topics analysed in those studies. This paper presents four experimental studies conducted by the authors where different and structural members were tested. The common goal of these studies was to develop innovative solutions with high performance and low environmental impact. The scope of the first study was the structural behaviour of members produced with lightweight aggregate concrete (LWAC). Results of several beams, ties, and slabs are herein presented and analysed. The advantage of using glass fibre–reinforced polymer (GFRP) was addressed in a second study, and main results obtained with this type of are also herein presented. Recent advances in nanotechnology led to the development of incorporating nanoparticles into the binder matrix. Typically, these nanoparticles have a diameter of 10–300 nanometers and are added to the mixture to reduce the porosity and increase the density of the binder matrix, improving the mechanical properties and durability. To analyse their influence on steel-to-concrete bonding and on the shear and flexural behaviour of the beams was the main goal of the third study herein described. Finally, a new concept to produce members with high durability using a special , which was the goal of the fourth study, is also herein presented.

Keywords: concrete; innovation; sustainable; LWAC; GFRP; nanoparticles; durability

1. Introduction The construction sector is constantly in development, although the visible effects on society take more time to be detected compared to other industries, like the aeronautical and the automotive industries. This is mainly due to the fact that construction is more conservative and because products must be cost effective. Today, the pressure on construction is to have innovative solutions with higher performance, higher eco-efficiency, and lower costs, but innovation in concrete structures can only take place through new materials or new building techniques. Research studies [1–6] conducted by the authors in the last decade aimed at contributing to the knowledge on concrete as building material, and new types of concrete with enhanced properties were developed: lightweight aggregate concrete (LWAC) with very reduced density and high

Infrastructures 2017, 2, 17; doi:10.3390/infrastructures2040017 www.mdpi.com/journal/infrastructures Infrastructures 2017, 2, 17 2 of 12

strength,Infrastructures ultra-high 2017, 2, 17 performance concrete (UHPC) with ultra-high strength and ultra-high2 of durability, 12 concrete incorporating different types of nanoparticles to enhance its mechanical properties, and low binderstrength, concrete ultra (LBC)-high withperformance reduced concrete dosage(UHPC) while with exhibitingultra-high adequatestrength strength.and ultra-high durability, concrete incorporating different types of nanoparticles to enhance its mechanical Other studies [7,8] also conducted by the authors addressed concrete’s reinforcing material, properties, and low binder concrete (LBC) with reduced cement dosage while exhibiting adequate sincestrength significant. advances were also experienced in this scope. Namely, several types of fibres (steel, glass, carbon,Other studies and polypropylene) [7,8] also conducted are currently by the authors adopted addressed in the mixture concrete to’s increase reinforcing the material, tensile strength of (fibresince significant reinforced) advances concrete, were and also reinforcing experienced bars in (rebars)this scope in. N glassamely fibre, several reinforced types of polymer fibres (steel, (GFRP) are usedglass, in severecarbon, environmental and polypropylene) locations. are currently adopted in the mixture to increase the tensile strengthA brief of overview (fibre reinforced) on the concrete, studies referredand reinforcing and major bars conclusions(rebars) in glass drawn fibre arereinforced herein polymer presented. (GFRP) are used in severe environmental locations. 2. LWACA brief Structures overview on the studies referred and major conclusions drawn are herein presented.

2. ThisLWAC section Structures presents a summary of three experimental works developed to analyse the structural behaviour of members produced with lightweight aggregates concrete (LWAC). The density of the This section presents a summary of three experimental works developed to analyse the developed LWACs varies between 1400 and 2000 kg/m3 and its compressive strength can reach 90 MPa. structural behaviour of members produced with lightweight aggregates concrete (LWAC). The Thisdensity type of the concrete developed is fundamental LWACs varies to between develop 1400 more and competitive 2000 kg/m3 structuraland its compressive solutions strength in addition to thecan evident reach benefits90 MPa. obtained This type with of concrete the reduction is fundamental in the structure’s to develop dead more weight, competitive especially structural in horizontal members.solutions Particularly in addition into seismicthe evident regions, benefits this isobtained quite relevant with the because reduction the in mass the andstructure stiffness’s dead reduction of theweight, structure especially also decreasesin horizontal the members. effect of theParticularly seismic actionin seismic [9–13 regions,]. this is quite relevant becauseThe widespreadthe mass and usestiffness ofLWAC reduction in of construction the structure hasalso createddecreasesthe the needeffect of for the studies seismic on the characterizationaction [9–13]. of its mechanical properties and on the structural behaviour of members produced with thisThe type widespread of concrete. use LWACof LWAC presents in construction characteristics has created quite different the need from for concretestudies on with the normal density:characterization the Young’s of its modulusmechanical is properties lower, the and stress-strain on the structural relation behaviour is different, of members presenting produced a more with this type of concrete. LWAC presents characteristics quite different from concrete with normal pronounced linear elastic region and a more fragile failure, and the binder matrix usually exhibits density: the Young’s modulus is lower, the stress-strain relation is different, presenting a more higherpronounced strength linear which elastic compensates region and the a more intrinsic fragile lower failure strength, and the of binder lightweight matrix aggregatesusually exhibits [14, 15]. higher strength which compensates the intrinsic lower strength of lightweight aggregates [14,15]. 2.1. Strength, Stiffness, and Ductility of LWAC Beams 2.1.The Strength, principal Stiffness aim, and of Ductility this study of LWAC was theBeams analysis of the load capacity, stiffness, and ductility of LWACThe beams principal submitted aim of this to study pure was bending the analysis [16]. Forof the this load purpose, capacity, three stiffness, sets and of testsductility were of carried out,LWAC one for beams each submitted considered to pure parameter: bending tensile[16]. For reinforcement this purpose, three ratio, sets transversal of tests were reinforcement carried out, ratio, andone concrete’s for each compressiveconsidered parameter: strength. tensile Thirteen reinforcement beams with ratio, 3000 transversal mm length, reinforcement 120 mm width, ratio, and and 270 mm heightconcrete were’s compressive tested. The strength. load was Thirteen applied beams through with 3000 a hydraulic mm length, servo-actuator 120 mm width, using and 270 displacement mm controlheight and were a constanttested. The speed load ofwas 0.01 applied mm/s thr (Figureough a1 ).hydraulic During servo the tests,-actuator measurements using displacement were made to control and a constant speed of 0.01 mm/s (Figure 1). During the tests, measurements were made to characterise the beams’ behaviour. More specifically, support reactions and displacements at critical characterise the beams’ behaviour. More specifically, support reactions and displacements at critical pointspoint weres were recorded. recorded.

(b)

(a) (c)

Figure 1. (a) Test setup and instrumentation; (b) Brittle failure; (c) Ductile failure. Figure 1. (a) Test setup and instrumentation; (b) Brittle failure; (c) Ductile failure. Infrastructures 2017, 2, 17 3 of 12 Infrastructures 2017, 2, 17 3 of 12

A summary of the main conclusions is as follows: (i) When concrete strength remains constant, the deformation capacity of LWAC beamsbeams decreasesdecreases significantlysignificantly with the increase of the tensile reinforcement ratio;ratio; (ii)(ii) The The vertical vertical displacements displacements and and curvatures curvatures in in critical critical sections sections increase increase with with the increasethe increase in concrete in concrete strength, strength, and this and effect this is moreeffect evident is more in beamsevident with in lowerbeams tensile with reinforcementlower tensile ratios;reinforcement (iii) The ratios; ductility (iii) index, The ductility in this case index, the in curvature this case ductilitythe curvature index, ductility defined index, as the defined ratio between as the theratio curvature between correspondingthe curvature corresponding to the ultimate to moment the ultimate and the moment curvature and corresponding the curvature corresponding to the yielding moment,to the yielding showed moment, that the showed plastic deformations that the plastic are de mainlyformations influenced are mainly by the tensileinfluenced reinforcement by the tensile ratio (Figurereinforcement2); (iv) Beamsratio (Figure without 2); transversal (iv) Beams confinementwithout transversal in the central confinement region in of the beamcentral presented region of athe more beam fragile presented failure a comparativelymore fragile failure to beams comparatively with transversal to beams confinement; with transversal (v) Higher confinement; transversal (v) reinforcementHigher transversal ratios reinforcement also led to an ratios increase also of led the to load an increase capacity. of the load capacity.

90 8.0

ρ = 2.96% 72 6.0 ρ = 2.27%

54 ρ = 1.62% 4.0 1/r μ M (kN.m) ρ = 1.12% 36

ρ = 0.54% 2.0 18

0 0.0 0 24487296120 0.00 0.75 1.50 2.25 3.00 1/r (×10-3 m-1) ρ (%) (a) (b)

Figure 2. (a) Moment curvature relation, M-1/r; (b) Ductility index vs. tensile reinforcement ratio. Figure 2. (a) Moment curvature relation, M-1/r; (b) Ductility index vs. tensile reinforcement ratio.

2.2. Punching Capacity of Thin LWAC Slabs 2.2. Punching Capacity of Thin LWAC Slabs Combining reduced weight and good mechanical performance, LWAC is an efficient solution Combining reduced weight and good mechanical performance, LWAC is an efficient solution for flat slabs. Therefore, another experimental study was carried out to analyse the influence of the for flat slabs. Therefore, another experimental study was carried out to analyse the influence of the punching capacity in thin LWAC slabs [17]. Six slabs of 1000 × 1000 × 100 mm3 were produced punching capacity in thin LWAC slabs [17]. Six slabs of 1000 × 1000 × 100 mm3 were produced without shear reinforcement using three different types of LWAC with the same density, 1900 kg/m3, without shear reinforcement using three different types of LWAC with the same density, 1900 kg/m3, and with equal longitudinal reinforcement ratio, namely Ø12 rebars spaced 100 mm in both and with equal longitudinal reinforcement ratio, namely Ø12 rebars spaced 100 mm in both directions. directions. The average compressive strength was 29, 42, and 54 MPa. Two equal slabs for each type The average compressive strength was 29, 42, and 54 MPa. Two equal slabs for each type of concrete of concrete were produced. The steel amount was defined in order to avoid a premature bending were produced. The steel amount was defined in order to avoid a premature bending failure. failure. Hot-rolled ribbed rebars S500NR-SD were adopted, presenting an average yield strength of Hot-rolled ribbed rebars S500NR-SD were adopted, presenting an average yield strength of 545 MPa. 545 MPa. The slabs were supported at the four corners on semi-spherical supports, allowing the The slabs were supported at the four corners on semi-spherical supports, allowing the slabs free slabs free rotation at these points, and a concentrated force was applied at the centre of the slabs rotation at these points, and a concentrated force was applied at the centre of the slabs (Figure3a). (Figure 3a). The loaded area was 120 × 120 mm2, and the force was applied using a hydraulic The loaded area was 120 × 120 mm2, and the force was applied using a hydraulic servo-actuator with servo-actuator with displacement control at a rate of 0.02 mm/s. During the tests, the load and the displacement control at a rate of 0.02 mm/s. During the tests, the load and the support reactions were support reactions were measured using load cells, and the deformation at critical points was measured using load cells, and the deformation at critical points was measured with linear variable measured with linear variable differential transformers (LVDTs) with a frequency of one record per differential transformers (LVDTs) with a frequency of one record per second. The tests stopped when second. The tests stopped when shear-punching failure occurred, identified by a significant drop in shear-punching failure occurred, identified by a significant drop in the strength capacity. the strength capacity.

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180

P (kN)

135

kII

90

45 kI S1-LC30 S2-LC30 S1-LC45 S2-LC45 S1-LC60 S2-LC60 0 0 4 8 12 16 δ (mm) (a) (b)

Figure 3. (a) General view of test set-up; (b) Curve load—displacement at the centre of the slab (P-δ). Figure 3. (a) General view of test set-up; (b) Curve load—displacement at the centre of the slab (P-δ).

Based on the experimental results, cracking and maximum loads, displacements, rotations, stiffness,Based failure on the modes, experimental and cracking results, patterns cracking were and analysed. maximum The experimental loads, displacements, results also rotations, allowed stiffness,assessing failure the applicability modes, and of cracking EC 2, fib patterns MC 2010, were and analysed. ACI 318 Theto thin experimental LWAC slabs results [18–20]. also The allowed main assessingconclusions the of applicability this study are: of EC (i) 2,thefib sizeMC and 2010, type and of ACIaggregate 318 to are thin essential LWAC slabsto predict [18–20 the]. Thepunching main conclusionscapacity of ofLWAC this study slabs are: without (i) the sizespecific and reinforc type of aggregateement; (ii) are the essential concrete to strength predict the has punching a direct capacity of LWAC slabs without specific reinforcement; (ii) the concrete strength has a direct influence influence on the shear punching strength, increasing from 127.6 to 175.7 kN, when flcm increases from on29 theto 54 shear MPa punching (Figure 3b); strength, (iii) the increasingpredictions from of punching 127.6 to 175.7strength kN, using when both flcm ACIincreases 318 and from MC 29 2010 to 54for MPa LWAC (Figure thin3 b);slabs (iii) are the safe; predictions and (iv) ofthe punching measured strength angles of using the failure both ACI punching 318 and ‘cone’ MC 2010 are very for LWACsimilar thin to the slabs value are recommended safe; and (iv) the by measured EC 2. angles of the failure punching ‘cone’ are very similar to the value recommended by EC 2. 2.3. Exterior Beam-To-Column Joints of LWAC 2.3. Exterior Beam-To-Column Joints of LWAC This study aimed at evaluating the influence of rebar detailing, LWAC strength, and This study aimed at evaluating the influence of rebar detailing, LWAC strength, and reinforcement reinforcement ratio on the joints’ behaviour. The beam-to-column joints play a key role in the ratio on the joints’ behaviour. The beam-to-column joints play a key role in the behaviour of reinforced behaviour of reinforced concrete frames under monotonic and dynamic actions, and if not properly concrete frames under monotonic and dynamic actions, and if not properly computed and detailed, computed and detailed, their strength capacity can be lower than expected. To properly execute the their strength capacity can be lower than expected. To properly execute the joints can also be joints can also be complex, especially when small cross sections with a significant number of rebars complex, especially when small cross sections with a significant number of rebars are adopted because are adopted because this hinders concrete casting. The outer beam-to-column joints are more this hinders concrete casting. The outer beam-to-column joints are more vulnerable than the inner vulnerable than the inner ones due to uneven geometry and unbalanced structural behaviour ones due to uneven geometry and unbalanced structural behaviour accompanied by less effective accompanied by less effective confinement conditions [21–27]. confinement conditions [21–27]. Considering the lack of studies considering these conditions, an experimental study [28] Considering the lack of studies considering these conditions, an experimental study [28] focused focused on the behaviour of exterior beam-to-column joints made with LWAC and under monotonic on the behaviour of exterior beam-to-column joints made with LWAC and under monotonic loads was loads was conducted. Five specimens were produced combining two different reinforcement ratios conducted. Five specimens were produced combining two different reinforcement ratios and LWACs and LWACs with two different strengths. The concrete adopted had a compressive strength of 35 with two different strengths. The concrete adopted had a compressive strength of 35 and 55 MPa, and 55 MPa, and the tensile reinforcement ratio was 1.12 and 2.96%. Four specimens had the same and the tensile reinforcement ratio was 1.12 and 2.96%. Four specimens had the same cross-section, cross-section, 120 × 270 mm2, equal for columns and beams, and in the remaining, it was changed to 120 × 270 mm2, equal for columns and beams, and in the remaining, it was changed to 220 × 270 mm2 220 × 270 mm2 to allow a better anchorage of the longitudinal rebars of the beam. The lengths of to allow a better anchorage of the longitudinal rebars of the beam. The lengths of column and beam column and beam were 1500 and 1200 mm, respectively (Figure 4a). were 1500 and 1200 mm, respectively (Figure4a). The experimental results demonstrated that adequate detailing of the reinforcement in the The experimental results demonstrated that adequate detailing of the reinforcement in the beam-to-column joint considerably reduces cracking and increases the load capacity. More beam-to-column joint considerably reduces cracking and increases the load capacity. More specifically, specifically, the longitudinal reinforcement of the beam should be anchored at the outer face of the the longitudinal reinforcement of the beam should be anchored at the outer face of the column with column with a hook, and a high volume of concrete in compression is required when high a hook, and a high volume of concrete in compression is required when high reinforcement ratios reinforcement ratios are used. These specifications are essential for the internal balance of forces are used. These specifications are essential for the internal balance of forces between the beam and between the beam and the column. This study also showed that the column’s transversal the column. This study also showed that the column’s transversal reinforcement within the node is reinforcement within the node is fundamental to transfer horizontal forces from the outer to the fundamental to transfer horizontal forces from the outer to the inner face, avoiding concrete spalling in inner face, avoiding concrete spalling in the outer region, and ensuring a correct behaviour of the the outer region, and ensuring a correct behaviour of the joint. The increase of LWAC strength did not joint. The increase of LWAC strength did not have a significant impact in the joints’ behaviour for low reinforcement ratios, whereas for specimens with higher reinforcement ratios, the maximum Infrastructures 2017, 2, 17 5 of 12 have aInfrastructures significant 2017 impact, 2, 17 in the joints’ behaviour for low reinforcement ratios, whereas for specimens5 of 12 with higher reinforcement ratios, the maximum load increased, the tensile reinforcement was better Infrastructuresload increased, 2017, 2 , 17the tensile reinforcement was better mobilized, and more ductile failure5 ofwas 12 mobilized,observed and (Figure more ductile4b). failure was observed (Figure4b). load increased, the tensile reinforcement was better mobilized, and more ductile failure was observed (Figure 4b). 80

BC joint 5 8060

BC joint 5 BC joint 4 6040 F (kN)

BC joint 4 BC joint 2 4020 F (kN) BC joint 3 BC joint 1 BC joint 2 20 0 BC joint 3 0BC 16324864 joint 1 δ (mm) 0 (a) 0 16324864(b) δ (mm) Figure 4. (a) View of test set-up; (b) Load—displacement at the edge of the beam. Figure 4. (a) View(a) of test set-up; (b) Load—displacement at the(b edge) of the beam.

3. GFRP ReinforcingFigure 4. ( aBars) View of test set-up; (b) Load—displacement at the edge of the beam. 3. GFRP Reinforcing Bars Reinforced concrete structures present the risk of steel rebars’ corrosion. The required 3. GFRP Reinforcing Bars Reinforcedprotection depends concrete on structuresthe environmental present exposure the risk and of on steel the design rebars’ service corrosion. life of the Thestructure. required protectionTypically,Reinforced depends the qualityconcrete on the and environmentalstructures the thickness present exposureof thethe concreterisk and of on coversteel the rebars’ designare specified corrosion. service based life The on of therequiredthe structure.latter Typically,protectionparameters. the quality depends Thereand are,on thehowever, the environmental thickness other solutions of exposure the concreteto el animinate,d on cover the or design at areleast specifiedservice to minimize, life basedof thethe problemstructure. on the of latter parameters.Typically,steel corrosion, There the quality are, such however, asand the the adoption otherthickness solutions of stainlessof the to concrete eliminate,steel or cover even or are atnon-metallicleast specified to minimize, reinforcement,based onthe the problem latteras for of parameters. There are, however, other solutions to eliminate, or at least to minimize, the problem of steel corrosion,instance glass such fibre–reinforced as the adoption ofpolymer stainless (GFRP) steel orrebars. even non-metallicThese are suitable reinforcement, for very asaggressive for instance steelenvironments corrosion, and/orsuch as when the adoptionhigh durability of stainless is required. steel or even non-metallic reinforcement, as for glass fibre–reinforced polymer (GFRP) rebars. These are suitable for very aggressive environments instanceThe glass GFRP fibre–reinforced rebars also have polymerthe advantage (GFRP) of presenrebars.ting These high aretensile suitable strength for and, very in addition,aggressive of and/or when high durability is required. environmentsbeing easy to and/orcut and when carry. high Their durability disadvantages is required. are th at they cannot be bent at the construction site TheandThe GFRPthey GFRP exhibit rebars rebars low also strengthalso have have to the the shear advantageadvantage and compressiv of of presen presentingeting forces. high highRibs tensile of tensile GFRP strength strengthrebars and, are in and,also addition, different in addition, of of beingbeingfrom easy easythose to to cut ofcut steel and and carry.rebars,carry. TheirTheir and disadvantages the Young’s modulus are are th thatat theyis theysignificantly cannot cannot be bent belower, bent at thewhich at construction the constructionundoubtedly site site and theyandaffects they exhibit the exhibit structure’s low low strength strength behaviour to to shear shear [29,30] and and compressive(Figurecompressiv 5a). eTo forces. forces. analyse Ribs Ribs these of ofGFRP differences, GFRP rebars rebars are an also areexperimental alsodifferent different fromfrom thosestudy those ofwas steel carriedof steel rebars, out rebars, andusing theand both Young’sthe steel Young’s and modulus GFRP modulus re isbars significantly is [31]: significantly (i) 36 pull-out lower, lower, whichtests which were undoubtedly undoubtedly produced, affects18 the structure’saffectswith steelthe structure’s behaviourrebars and behaviour [1829 ,with30] (Figure [29,30]GFRP 5rebars,(Figurea). To co5a). analysembining To analyse these different these differences, rebar differences, diameters an experimentalan experimentaland concrete study was carriedstudystrengths was out carried(Figure using out5b); both using (ii) steel 12 both 1200 and steel × GFRP100 and × 100 GFRP rebars mm re3[ bars31ties]: were[31]: (i) 36 (i)produced, pull-out 36 pull-out six tests testswith were weresteel produced, rebarsproduced, and 18 18six with with steel rebars and 18 with GFRP rebars, combining different rebar diameters and concrete steel rebarswith GFRP and rebars. 18 with The GFRP ties were rebars, reinforced combining with on differente rebar applied rebar diametersat the centre and of the concrete cross section strengths strengths(Figure 5c). (Figure 5b); (ii) 12 1200 × 100 × 100 mm3 ties were produced, six with steel rebars and six (Figure5b); (ii) 12 1200 × 100 × 100 mm3 ties were produced, six with steel rebars and six with GFRP with GFRP rebars. The ties were reinforced with one rebar applied at the centre of the cross section rebars.(Figure The1000 ties5c). were reinforced with one rebar applied at the centre of the cross section (Figure5c). σ (MPa) 1000 E = 60 GPa 750σ (MPa) E = 60 GPa 750 E = 200 GPa 500

E = 200 GPa 500 250 GFRP bar Steel bar 250 0 GFRP bar 0 6 12 18 24 ε (x10-3) Steel bar 0 (a) (b)(c) 0 6 12 18 24 ε (x10-3) Figure 5. (a) Stress-strain relation of steel and glass fibre–reinforced polymer (GFRP) rebars; (b) (a) (b)(c) Pull-out tests; (c) Tensile tests of reinforced ties. Figure 5. (a) Stress-strain relation of steel and glass fibre–reinforced polymer (GFRP) rebars; (b) Figure 5. (a) Stress-strain relation of steel and glass fibre–reinforced polymer (GFRP) rebars; Pull-out tests; (c) Tensile tests of reinforced ties. (b) Pull-out tests; (c) Tensile tests of reinforced ties. Infrastructures 2017, 2, 17 6 of 12 Infrastructures 2017, 2, 17 6 of 12

Considering the results of pull pull-out-out tests, the two types of rebars (steel and GFRP) have a similar bond strength strength,, but the anchorage length of GFRP rebarsrebars must be double, considering its maximum strength. The The ties reinforced with GFRP present deformations higher than those recorded in identic ties reinforced withwith steelsteel rebars rebars during during state state II, II, i.e., i.e., between between the the development development of theof the first first crack crack and and the theinstant instant steel steel rebars rebars start start yielding. yielding. The The stiffness stiffness KII isKII greatly is greatly affected affected by theby the Young’s Young modulus’s modulus of the of therebars’ rebars material’ material and itand is slightlyit is slightly affected affected by the by variation the variation of the of concrete’s the concrete strength’s strength (Figure 6(Figurea). For 6a). the Forsame the level same of stress,level of the stress, cracks’ the width cracks is’ inverselywidth is inversely proportional proportional to the Young’s to the modulus Young’s ofmodulus the rebars’ of thematerial. rebars In’ material. ties with In GFRP ties with rebars, GFRP cracks’ rebars, width cracks is, on’ width average, is, on 3.3 average, times higher 3.3 times than higher that of than ties withthat ofsteel ties rebars with steel (Figure rebars6b). (Figure 6b).

80 500 y = 454.91x + 185.08

60 400

y = 139.41x + 139.53 40 300 F (kN) (MPa) σ

20 200 Steel bar GFRP tie GFRP bar Steel tie Linear (Steel bar) Linear (GFRP bar) 0 100 0 2.5 5 7.5 10 0 0.5 1 1.5 2 δ (mm) w (mm)

(a) (b)

Figure 6. (a) Comparison between of ties reinforced with steel and GFRP rebars; (b) Correlation Figure 6. (a) Comparison between of ties reinforced with steel and GFRP rebars; (b) Correlation between the reinforcement stress and the cracks width. between the reinforcement stress and the cracks width.

4. Structures Made with Nanoconcrete 4. Structures Made with Nanoconcrete Over the last few years, several studies on concrete mix design, considering different additions Over the last few years, several studies on concrete mix design, considering different additions of of nanoparticles, have been carried out intensively. The rational use of nanoparticles, i.e., particles nanoparticles, have been carried out intensively. The rational use of nanoparticles, i.e., particles with with diameters varying between 0.1 and 100 nm, with a high specific surface-volume relation [32,33], diameters varying between 0.1 and 100 nm, with a high specific surface-volume relation [32,33], and with high reactivity during the cement’s hydration process, is mainly used to make the and with high reactivity during the cement’s hydration process, is mainly used to make the cementitious matrix more compact, thus improving the mechanical properties and durability of cementitious matrix more compact, thus improving the mechanical properties and durability of concrete [34,35]. The nanoparticles of silica (SiO2), usually named nano-SiO2, are one of the most concrete [34,35]. The nanoparticles of silica (SiO ), usually named nano-SiO , are one of the most used used in this field because they have shown good2 results in increasing the matrix2 density. Other types in this field because they have shown good results in increasing the matrix density. Other types of of nanoparticles that have also been studied as additions to concrete mixes are nano-TiO2, nanoparticles that have also been studied as additions to concrete mixes are nano-TiO2, nano-ZnO2, nano-ZnO2, nano-Fe2O3 and nano-Al2O3 [36,37]. nano-Fe O and nano-Al O [36,37]. Studies2 3 on the addition2 3 of nanoparticles to concrete mixtures are focused mainly on the material Studies on the addition of nanoparticles to concrete mixtures are focused mainly on the behaviour and secondarily on the structural behaviour of concrete members incorporating material behaviour and secondarily on the structural behaviour of concrete members incorporating nanoparticles. Most recent experimental studies conducted by the authors aimed at understanding nanoparticles. Most recent experimental studies conducted by the authors aimed at understanding the effect of the addition of nanoparticles on the steel-to-concrete bond [38,39] and on both the shear the effect of the addition of nanoparticles on the steel-to-concrete bond [38,39] and on both the shear strength and the bending strength of RC beams [40]. To characterize the steel-to-concrete bond strength and the bending strength of RC beams [40]. To characterize the steel-to-concrete bond stress, 32 pull-out tests (Figure 7a) and 16 tensile tests were performed. The latter were conducted on stress, 32 pull-out tests (Figure7a) and 16 tensile tests were performed. The latter were conducted reinforced concrete ties, and the goal was to analyse the cracks’ width (Figure 7b,c). In these tests, on reinforced concrete ties, and the goal was to analyse the cracks’ width (Figure7b,c). In these tests, plain and ribbed rebars, as well as eight different types of concrete mixtures, were used [38]. The plain and ribbed rebars, as well as eight different types of concrete mixtures, were used [38]. The effect effect of adding nanoparticles on steel fibres-to-paste bond was also analysed using a modified of adding nanoparticles on steel fibres-to-paste bond was also analysed using a modified pull-out test pull-out test to monitor the fibre’s slip versus the applied load (Figure 7d) [39]. The strength capacity to monitor the fibre’s slip versus the applied load (Figure7d) [ 39]. The strength capacity of beams with of beams with nanoparticles was evaluated by testing 16 reinforced concrete beams (Figure 7e,f) [40]. nanoparticles was evaluated by testing 16 reinforced concrete beams (Figure7e,f) [40].

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(a) (b) (c)

(d) (e) (f)

Figure 7. (a) Pull-out tests in specimens with nanoparticles addition; (b) Tensile tests in RC ties with Figure 7. (nanoparticlesa) Pull-out addition; tests in specimens(c) Cracks’ width; with (d nanoparticles) Pull-out test to addition;study the fibre (b)’s Tensile slip; (e) testsBeams in’ test RC ties with nanoparticlesset-up; addition; (f) Beams’ failure. (c) Cracks’ width; (d) Pull-out test to study the fibre’s slip; (e) Beams’ test set-up; (f) Beams’ failure. Based in these studies, the following conclusions were drawn: (i) Adding nanoparticles to the concrete mixture influences mostly the adhesion component, which is particularly relevant when Basedplain in theserebars studies,are used. ( theii) Regar followingding the conclusionscracks’ width, the were effect drawn: of the rebars (i) Adding’ ribs is predominant nanoparticles to the concrete mixturecompared influencesto the addition mostly of nanoparticles, the adhesion thus leading component, to a higher which increase is in particularly bonding (Figure relevant 8a). when (iii) The mechanical component of bond provided by the hook at the end of the fibres is, as expected, plain rebars are used. (ii) Regarding the cracks’ width, the effect of the rebars’ ribs is predominant much higher than the adhesion and friction components, and it exceeds considerably the comparedimprovement to the addition reached of with nanoparticles, the nanoparticles thus addition leading. (iv) to In a spite higher of the increase latter, the in nanoparticles bonding (Figure 8a). (iii) The mechanicaladdition leads component to an increase of bondof the binding provided paste by strength the hook and at thus the to end an increase of the fibresof the is,bond as expected, much higherstrength than (Figure the adhesion 8b). (v) The and nano friction-ZnO addition components, has a negative and iteffect exceeds on the considerablybond strength, probably the improvement due to a chemical reaction with the fibres’ coating (Figure 8c). (vi) Finally, the effect of nanoparticles reached with the nanoparticles addition. (iv) In spite of the latter, the nanoparticles addition leads in the beams’ behaviour is influenced by the dosage of the cement adopted in the mixture. For beams to an increaseproduced of the with binding higher dosages paste strength of cement, and the thusaddition to anof increasenano-SiO2 ofor thenano bond-Al2O3 strengthleads to an (Figure 8b). (v) The nano-ZnOincrease inaddition the shear strength has a negative. However, effect they have on thealmost bond no influe strength,nce in probablythe bending due strength to a chemical reaction with(Figure the 8d). fibres’ coating (Figure8c). (vi) Finally, the effect of nanoparticles in the beams’ behaviour is influenced by the dosage of the cement adopted in the mixture. For beams produced with higher dosages of cement, the addition of nano-SiO2 or nano-Al2O3 leads to an increase in the shear strength. However, they have almost no influence in the bending strength (Figure8d). Infrastructures 2017, 2, 17 8 of 12 Infrastructures 2017, 2, 17 8 of 12

32 16 Plain bars 29.1 Ribbed bars 27.9 Fibers with hook-end 24 12 22.4 22.5

19.8 (MPa)

19.2 τ 16 15.7 8 (MPa) 13.1 max τ

Bond stress, stress, Bond 4 8 Fibers without hook-end 6.8 6.7 5.1 5.2 5.4 3.2 3.8 3.5 0 0 0481216 0123456789Different mixtures Slip (mm) (a) (b)

2.4 120

Beams without nanoparticles 1.8 90 (MPa) τ 1.2 60 F (kN)

Bond stress, stress, Bond 0.6 30 Mixture with nano-ZnO

0 0 048121602468 Slip (mm) δ (mm) (c) (d)

Figure 8. (a) Maximum bond stress on fibres; (b) Bond-slip curves of the fibres with and without the Figure 8. (a) Maximum bond stress on fibres; (b) Bond-slip curves of the fibres with and without the hook-end; (c) Bond-slip curves of the fibres without the hook-end anchorage and with the hook-end; (c) Bond-slip curves of the fibres without the hook-end anchorage and with the nanoparticles nanoparticles added to the binding paste; (d) Load-displacement curves of beams. added to the binding paste; (d) Load-displacement curves of beams.

5. Structures with Ultra-High Durability Concrete Cover 5. Structures with Ultra-High Durability Concrete Cover After Lafarge patented Ductal and released it to the market [41], several researchers all over the After Lafarge patented Ductal and released it to the market [41], several researchers all over the world developed different mixtures for ultra-high-performance concrete (UHPC), which is also world developed different mixtures for ultra-high-performance concrete (UHPC), which is also called called ultra high–performance fibre-reinforced concrete (UHPFRC). Most of these studies were ultra high–performance fibre-reinforced concrete (UHPFRC). Most of these studies were focused on focused on the material properties, and only few addressed the behaviour of structural UHPC the material properties, and only few addressed the behaviour of structural UHPC members. In this members. In this latter group of studies is included the work by Bruwhiler et al., developed at EPFL, latter group of studies is included the work by Bruwhiler et al., developed at EPFL, which uses UHPC which uses UHPC to improve the performance of both existing and new concrete structures [42,43]. to improve the performance of both existing and new concrete structures [42,43]. The research developed at CERIS, although also focused in structural UHPC, has a somehow The research developed at CERIS, although also focused in structural UHPC, has a somehow different goal and focus. The goal is to enhance durability, which is why the authors often refer to different goal and focus. The goal is to enhance durability, which is why the authors often refer to ultra-high durability concrete (UHDC) instead of UHPC, and to reduce the carbon footprint of the ultra-high durability concrete (UHDC) instead of UHPC, and to reduce the carbon footprint of the composite UHPC-concrete member. The main characteristic of this UHPC is an ultra-compact matrix composite UHPC-concrete member. The main characteristic of this UHPC is an ultra-compact matrix with reduced porosity, giving rise to an excellent performance in terms of durability and mechanical with reduced porosity, giving rise to an excellent performance in terms of durability and mechanical properties. The compressive strength can be higher than 150 MPa, and tests performed to assess the properties. The compressive strength can be higher than 150 MPa, and tests performed to assess the absorption, corrosion rate of steel, and the porosity proved that UHPC is particularly water absorption, corrosion rate of steel, and the porosity proved that UHPC is particularly adequate adequate for reinforced concrete structures located in very aggressive environments [4,44]. The for reinforced concrete structures located in very aggressive environments [4,44]. The composition of composition of these UHPC usually contains fibres, nanoparticles, , and a large amount of these UHPC usually contains fibres, nanoparticles, fly ash, and a large amount of cement, between cement, between 900 and 1100 kg/m3, which is substantial higher than the cement dosage in normal 900 and 1100 kg/m3, which is substantial higher than the cement dosage in normal concrete [45]. concrete [45]. Thus, aiming at reducing the environmental impact associated with the production of Thus, aiming at reducing the environmental impact associated with the production of these UHPC, these UHPC, it was developed at CERIS the ‘Eco-UHPC’ concept [3,45]. it was developed at CERIS the ‘Eco-UHPC’ concept [3,45]. Although UHPC has been adopted in different types of reinforced concrete structures, authors Although UHPC has been adopted in different types of reinforced concrete structures, have focused on a specific application: the production of a ‘super skin’ that enhances the durability authors have focused on a specific application: the production of a ‘super skin’ that enhances the of current concrete members. This concept was developed in the scope of a research project called durability of current concrete members. This concept was developed in the scope of a research ‘Intelligent Super Skin—Enhanced Durability for Concrete Members’ funded in 2004 by the Portuguese Science and Technology Foundation. In this project, several UHDC mixtures were developed and characterized. Also, for the first time, the concern with eco-efficiency was included in Infrastructures 2017, 2, 17 9 of 12 project called ‘Intelligent Super Skin—Enhanced Durability for Concrete Members’ funded in 2004 by the Portuguese Science and Technology Foundation. In this project, several UHDC mixtures were Infrastructures 2017, 2, 17 9 of 12 developed and characterized. Also, for the first time, the concern with eco-efficiency was included inthe the UHDC UHDC mixture mixture design design [45]. [45 ].Presently, Presently, a a vast vast ex experimentalperimental campaign campaign is is in progress, aimingaiming atat analysinganalysing thethe best best way way to applyto apply this innovativethis innovative ‘eco-super ‘eco-super skin’, consideringskin’, considering different different types of concretetypes of forconcrete the bulk for (LWAC the bulk and (LWAC LBC, besides and LBC, current besides concrete) current and concrete) different and types diff oferent surface types conditions of surface for theconditions interface for between the interface the cover between and the the bulk cover (Figure and9 the). The bulk characterization (Figure 9). The of characterization the bond between of the the UHDCbond coverbetween and the concreteUHDC bulk,cover considering and the all concrete the variables bulk, above-mentioned, considering all is alsothe a goalvariables that differentiatesabove-mentioned, the authors’ is also a research goal that from differentiates other studies. the authors’ research from other studies.

(a) (b)

(c) (d)

Figure 9. Different surface conditions for the interface between the cover and the bulk of RC Figure 9. Different surface conditions for the interface between the cover and the bulk of RC members: members: (a) Smooth; (b) Indented; (c) Smooth with micro connectors; (d) treated with water jet. (a) Smooth; (b) Indented; (c) Smooth with micro connectors; (d) treated with water jet.

The tests were performed in beams and slabs to analyse the contribution of the ultra-high durabilityThe tests concrete were cover performed in the structural in beams behaviour, and slabs tonamely analyse in terms the contribution of strength, stiffness, of the ultra-high cracking durabilitypattern, deformation, concrete cover and in ductility the structural (Figure behaviour, 10). Some namelytests have in termsalready of been strength, performed, stiffness, but cracking results pattern,are not yet deformation, analysed. and ductility (Figure 10). Some tests have already been performed, but results are not yet analysed.

(a) (b)

Figure 10. (a) Beam with indented surface; (b) Beam’s test. Infrastructures 2017, 2, 17 9 of 12

the UHDC mixture design [45]. Presently, a vast experimental campaign is in progress, aiming at analysing the best way to apply this innovative ‘eco-super skin’, considering different types of concrete for the bulk (LWAC and LBC, besides current concrete) and different types of surface conditions for the interface between the cover and the bulk (Figure 9). The characterization of the bond between the UHDC cover and the concrete bulk, considering all the variables above-mentioned, is also a goal that differentiates the authors’ research from other studies.

(a) (b)

(c) (d)

Figure 9. Different surface conditions for the interface between the cover and the bulk of RC members: (a) Smooth; (b) Indented; (c) Smooth with micro connectors; (d) treated with water jet.

The tests were performed in beams and slabs to analyse the contribution of the ultra-high durability concrete cover in the structural behaviour, namely in terms of strength, stiffness, cracking Infrastructurespattern,2017 deformation,, 2, 17 and ductility (Figure 10). Some tests have already been performed, but results10 of 12 are not yet analysed.

(a) (b)

Figure 10. (a) Beam with indented surface; (b) Beam’s test. Figure 10. (a) Beam with indented surface; (b) Beam’s test.

This solution is interesting for enhancing the durability without being required high amounts of cement, since UHDC is only considered in the cover, thus resulting in a more economic and sustainable solution, and it has the additional advantage of being applicable to both new and existing structures, i.e., it can also be adopted in rehabilitation and strengthening interventions.

6. Final Considerations This overview of some recent studies conducted by the authors shows that there is a huge potential for innovation in the structural concrete area. Currently, the paradigm is to improve the performance considering, simultaneously, strength, durability, eco-efficiency, and cost, from a life-cycle perspective. The experimental studies conducted by the authors in recent years, herein presented, are contributions in this domain and the following main conclusions drawn are synthesized next. Regarding LWAC, it can be stated that it is possible to design LWAC structures with the prescribed strength and ductility, requiring special care with both transversal and longitudinal reinforcement ratios. LWAC structures can be a very competitive solution with the advantage of significantly lowering the structures’ dead weight. In aggressive environments where the risk of steel corrosion is high, non-metallic rebars, like GFRP rebars, are an interesting alternative to stainless steel. Nevertheless, these rebars require double anchorage length compared to current steel rebars, and the stiffness of the structure in state II is highly affected by the low Young’s modulus of this reinforcing material. In addition, when GFRP rebars are adopted, the expected cracks’ width is, on average, 3.3 times higher compared with the reference situation of using steel rebars. Results showed that the cracks’ width is inversely proportional to the Young’s modulus of the adopted reinforcing material. The experimental studies performed to analyse the effect of using nanoparticles additions in concrete mixtures showed that the cementitious matrix is more compact when nanoparticles are added, thus also improving both the mechanical properties and the durability. It was also demonstrated that, regarding steel fibres reinforced concrete, the hook at the end of the fibres has a much higher effect than the improvement reached with the nanoparticles addition. Moreover, it was found that nano-ZnO affects negatively the fibre-to-matrix bond strength, probably due to a chemical reaction with the fibres’ coating. Regarding the effect of nanoparticles addition on beams behaviour, it was concluded that the shear strength increases with the nanoparticles addition and the strength capacity increases slightly, whereas the effect on the bending strength is not significant. Finally, the new concept, named by the authors as ‘super skin’, proved to be a promising approach to enhance the durability of concrete structures. Using a normal concrete in the core and an UHDC only in the cover, the result is a high-performance solution, combining all the current concerns previously identified. In fact, it proved to be cost effective, to improve durability, to increase strength, and to be eco-efficient according to a life-cycle perspective. Studies on this subject are still in progress and will be published soon.

Conflicts of Interest: The authors declare no conflict of interest. Infrastructures 2017, 2, 17 11 of 12

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