Strengthening of Concrete Structures with Textile Reinforced Mortars: State-of-the-Art Review
Publication: Journal of Composites for Construction
Volume 23, Issue 1
Abstract
Textile reinforced mortars (TRM), also known in the international literature as textile reinforced concrete (TRC) or fabric reinforced cementitious matrix (FRCM) materials, have been widely studied during the last two decades as they constitute a promising alternative to the fiber reinforced polymer (FRP) retrofitting solution for strengthening of reinforced concrete members. This paper presents a state-of-the-art review on the strengthening of concrete structures with TRM. First, the tensile and bond behavior of TRM is described. Next, an overview of studies on the use of TRM for flexural, shear, confinement, and seismic retrofitting of concrete or RC members is included, and the key parameters are investigated.
Introduction
Over the last decades, the issue of upgrading existing structures has been of great importance because of their deterioration, ageing, environmental induced degradation, a lack of maintenance, or the need to meet current design requirements. Fiber reinforced polymers (FRP) have been widely used as an externally applied reinforcement of existing structurally deficient structures over the last three decades due to their favorable properties (i.e., high strength to weight ratio, corrosion resistance, ease and speed of application, and minimal change of geometry). However, the FRP strengthening technique has a few drawbacks, which are mainly associated with the use of epoxy resins—namely, high cost, poor performance in high temperatures, inability to apply on wet surfaces, and incompatibility with substrate materials (concrete or masonry). In an attempt to alleviate the problems arising from the use of epoxies, researchers have suggested the replacement of organic (epoxy resins) with inorganic (mortar) matrix. However, the penetration and impregnation of fiber sheets, in this case, has been very difficult due to the size of the granules in the mortar; even a fine mortar cannot impregnate fiber bundles as resins do. Improved bond conditions between fibers and matrix in mortar-based composite materials were achieved when continuous fiber sheets were replaced by textiles. At first, the new textile-based composite materials were given (in Europe) the name textile reinforced concrete (TRC) or textile reinforced mortar (TRM); strictly speaking, the inorganic matrix is not classified as concrete, because of the very small size of aggregates. In the US the materials were given the name fabric reinforced cementitious matrix (FRCM) systems; a possible problem in this term is that the matrix may not be cementitious (e.g., hydraulic lime).
Textile-based composite materials have been studied extensively during the last two decades, since they can be used for the construction of new prefabricated structural elements (e.g., Curbach and Jesse 1999; Brameshuber et al. 2001) or for the strengthening of existing structures (e.g., Triantafillou et al. 2006; Triantafillou and Papanicolaou 2006; Bournas et al. 2007). The use of textiles in prefabrication as well as in the retrofitting of existing concrete or masonry structures was summarized in Triantafillou (2016). Selected case studies of strengthening applications of concrete and masonry structures were presented by Bournas (2016).
TRM combines high-strength fibers in the form of textiles (with open-mesh configuration) with inorganic matrices, such as cement- or hydraulic-lime-based mortars. TRM is low-cost, friendly for manual workers, fire resistant, compatible with concrete and masonry substrate materials, and can be applied on wet surfaces or at low temperatures. For all these reasons, the use of TRM is progressively becoming more attractive for the strengthening of existing structures, in parallel to the widely used FRP. Although the first applications of TRM systems were in concrete elements, strengthening of typical or historical masonry structures with TRM seems to be very promising, considering the limitations of FRP systems (i.e., Cascardi et al. 2017; Leone et al. 2017; Kariou et al. 2018). The continuously increasing interest of the research community in using TRM for the structural retrofitting of concrete or masonry structures is depicted in Fig. 1, where the number of publications are plotted against the publication year [Scopus search with keywords (1) textile-reinforced mortar, or (2) FRCM, or (3) textile-reinforced concrete, and (4) strengthening; last accessed on April 2, 2018]. Almost 60% of the publications are research articles in peer-reviewed journals; the rest are mostly conference papers.
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Textile mesh materials used as reinforcement of TRM composite materials consist of fiber rovings arranged in two or more directions. The fiber rovings are spaced apart to allow for the formation of a mesh. Perforations between the fiber rovings enable some sort of mechanical interlock between the reinforcement and the matrix. The coating of nonmetallic textiles with polymers improves the stability of the textile material and the mechanical interlock between the textile and the matrix. However, coated textiles are stiffer, and, like steel fabrics, they cannot be easily applied to complex geometries (e.g., U-shaped or fully wrapped jackets). Fig. 2 shows textiles that have been used as reinforcement in TRM systems. The mesh size of commercially available nonmetallic textiles that are used for strengthening applications (i.e., carbon, glass, basalt, or polyphenylene bezobisoxazole fiber textiles) typically varies between 8 and 30 mm, whereas their weight is usually between 150 and , depending on the fiber material. Steel fabrics consist of unidirectional steel cords, each one comprising a number of twisted steel filaments [Fig. 2(e)]; their density typically varies between 1 and .
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The composition of mortar used as matrix in TRM systems significantly affects its response as a composite material, because the impregnation of fibers with mortar is quite important for achieving a good bond between the fibers and the matrix. The mortar has to include fine granules and should have a plastic consistency, good workability, low viscosity (for easy application to vertical or steep surfaces) and sufficient shear strength (to prevent the debonding of the composite material from the substrate); hence cement-based mortars are widely used as matrix of TRM. The mechanical properties of mortar, namely the flexural strength and the bond between the matrix and the fiber rovings, can be significantly improved by adding polymers.
The procedure of strengthening with TRM jacketing includes the following steps: (1) surface preparation; (2) the application of a first layer of mortar at the dampened concrete surface; (3) impregnation of the textile fibers with mortar (this operation is repeated until all textile layers have been applied and covered by mortar); and (4) application of a final layer of mortar on the top of the final textile layer.
The development of guidelines for designing concrete and masonry members strengthened with TRM is required for enabling their wide use in real applications. ACI 549.4R (ACI 2013) constitutes the first attempt at guidelines for designing and constructing externally bonded textile-based systems for the repair and strengthening of concrete and masonry buildings.
This paper provides a comprehensive state-of-the-art review on the use of TRM systems for strengthening with a focus on concrete members or structures; it covers a broader range of applications compared to the recent study of Awani et al. (2017). It provides a critical review of more than 100 past studies and enables readers to identify gaps in the existing literature that could be investigated in future work. This introductory section is followed by the description of the tensile behavior and bond aspects of TRM. Next, an overview of the flexural, shear, confinement, and seismic retrofitting of concrete members with TRM is presented in a systematic way, including method description, failure modes, and the effect of key parameters investigated. The main conclusions are summarized at the end of this paper.
Mechanical Behavior of TRM Composites
Tensile Behavior of TRM
Several researchers have investigated the tensile behavior of TRM as a composite material (Colombo et al. 2011; Contamine et al. 2011; Hartig et al. 2012; Zhu et al. 2011; Ascione et al. 2015; Malena and De Felice 2014; Arboleda et al. 2016; De Santis et al. 2017; D’Antino and Papanicolaou 2018). The tensile properties of TRM, namely the ultimate stress and strain and the modulus of elasticity, can be defined through tensile tests in which the fibers rupture (ideally at the central region of TRM coupons). In most studies, monotonic tensile tests have been carried out on TRM specimens; however, Zhu et al. (2011) investigated the effect of strain rate on the mechanical properties of TRM coupons, conducting dynamic tensile tests.
A variety of TRM coupon geometries have been tested and different gripping methods have been used. Plate-type TRM coupons have been widely used, but bone-shaped or dumbbell [Figs. 3(a–c)] specimens (Colombo et al. 2011; Hartig et al. 2012; Ascione et al. 2015) have also been tested to enable failure in the central region of the coupon [Fig. 3(c)]. According to De Santis et al. (2017), the gripping method strongly affects the failure mode of TRM coupons. Specifically, slippage of the fibers through the mortar before rupture was usually observed in case of the clevis grip method (Arboleda et al. 2016; D’Antino and Papanicolaou 2017; De Santis et al. 2017), but this failure was prevented when a clamping grip (providing sufficient gripping pressure) was applied (Arboleda et al. 2016; De Santis et al. 2017). In-depth discussion on comparisons between different test setups available in the literature can be found in the recent study of D’Antino and Papanicolaou (2018), which confirmed the strong influence of the test setup on tensile test results.
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The typical tensile response of a TRM coupon comprises three distinct branches, as shown in Fig. 3(d): (1) the specimen remains uncracked; (2) the development of multiple cracks after the first cracking occurs; and (3) the cracking pattern has fully developed and the increase in resistance is due to the textile itself until the fibers rupture.
The spacing and width of cracking depend on the quantity and type of the textile reinforcement as well as on the bond between the textile fibers and the mortar (De Felice et al. 2014). Based on the results presented in D’Antino and Papanicolaou (2017), the use of coating enabled uniform load distribution, hence the mechanical characteristics of the coated textiles were significantly improved.
Colombo et al. (2011) studied the behavior of TRM at high temperatures, conducting tensile tests on TRM coupons. The specimens were first exposed to high temperatures, and after a two-hour stabilization phase they were cooled down before testing. The researchers concluded that after exposure to 200°C TRM coupons kept their strength, whereas the stress and strain significantly decreased after the exposure of specimens to 400°C and 600°C due to the deterioration of the textile mesh coating.
Bond Aspects of TRM
The study of the bond between TRM and concrete substrate is of crucial importance, as it determines the effectiveness of TRM jacketing as a strengthening material. The condition (dry or coated) and geometry of fiber rovings, the degree of impregnation of fibers with mortar, and the quality of concrete substrate surface preparation are the key parameters that affect the bond between TRM and concrete substrate. The bond of TRM to concrete has mainly been investigated through single-lap [Fig. 4(a)] and double-lap [Fig. 4(b)] shear tests. In single-lap shear tests, a TRM strip is externally bonded to one side of a concrete block, and then a tensile load pulls out the TRM strip while the concrete block is fixed (Sneed et al. 2014; Carloni et al. 2015; D’Antino et al. 2015; Sneed et al. 2015; D’Antino et al. 2016a, c; Carloni et al. 2017; Sabau et al. 2017). In double-lap shear tests, TRM strips are bonded to both sides of two concrete blocks that are only connected with TRM strips, and the concrete blocks are subjected to tensile loading up to the failure of the TRM (Ortlepp et al. 2006; D’Ambrisi et al. 2012; Awani et al. 2015; Raoof et al. 2016; Raoof and Bournas 2017c). Sneed et al. (2015) compared the results obtained from single-lap and double-lap direct shear tests. Based on this study, the displacement versus load curves as well as the failure modes obtained from both tests were identical. However, single-lap shear tests had some drawbacks compared to double-lap shear tests; the results obtained from the double-lap shear tests were less scattered compared to those obtained from single-lap shear tests, whereas in some single-lap shear tests rupture of the TRM was observed outside of the bonded length.
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The majority of studies investigated the bond between PBO TRM and concrete substrate. The bond between carbon TRM and concrete has also been studied by a few researchers, but studies on the bond between glass or steel TRM and concrete are quite limited. The failure modes observed in bond tests are (1) slippage of fibers through the mortar; (2) debonding of TRM with part of concrete; (3) debonding of TRM in the concrete–mortar interface; and (4) rupture of TRM. More details regarding the characteristics of each failure mode are presented in subsequent sections. In most studies, the specimens failed due to slippage of the fibers through the mortar. Debonding of TRM strips with part of concrete was also observed in some specimens, whereas rupture of TRM strips was mainly observed in the case of glass TRM jackets. A description of the main parameters investigated is given subsequently.
The bonded length was a parameter investigated in most studies (D’Ambrisi et al. 2012; D’Antino et al. 2013, 2014; Sneed et al. 2014; Tran et al. 2014; Awani et al. 2015; D’Antino et al. 2015; Ombres 2015a; Sneed et al. 2015; D’Antino et al. 2016a; Raoof et al. 2016; Sabau et al. 2017). It was concluded that ultimate load and bond capacity increase nonlinearly with the bonded length. Based on the results presented in the aforementioned studies, the effective bond length for PBO TRM with mesh size and nominal thickness of and 0.045 mm, respectively, was between 250 and 350 mm, while the corresponding value for carbon TRM with mesh size and nominal thickness of and 0.095 mm, respectively, was between 200 and 300 mm.
The effect of the width of PBO TRM strips has also been investigated by several researchers (D’Antino et al. 2013, 2014; Sneed et al. 2014; D’Antino et al. 2015; Sneed et al. 2015), who reached the conclusion that there is no width effect. D’Ambrisi et al. (2012), Ombres (2015a), and Raoof et al. (2016) investigated the effect of the number of layers, concluding that an increase in the number of TRM layers resulted in nonproportional bond capacity increase. Moreover, a change in the failure mode from slippage of fibers through the mortar to debonding of TRM layers with part of the concrete was also observed when the number of TRM layers increased. This shift in the failure mode occurred when the number of PBO layers increased from one to two (D’Ambrisi et al. 2012; Ombres 2015a) and occurred with carbon TRM strips when three layers were applied instead of two.
Concrete surface preparation has also been a parameter under investigation. D’Antino et al. (2015) compared untreated concrete surfaces with surfaces that were prepared through sandblasting, concluding that the surface preparation had a limited role in the effectiveness of TRM, since the corresponding specimens in this study failed due to slippage of the fibers through the mortar. Raoof et al. (2016) compared two different surface preparations, grinding with a grid of grooves and sandblasting, and they reported that both methods gave the same results. Furthermore, Raoof et al. (2016) investigated the effect of concrete strength and concluded that decreasing the concrete strength by 50% results in a slight reduction (of 7.5%) of the bond capacity; the failure mode was not changed. In the same study, the effect of coating (application of epoxy resin to the dry textile material) was also investigated. Coating the textile material resulted in a bond capacity increase and a change in the failure mode from slippage of fibers through the mortar to TRM debonding at the textile–mortar interface.
Carloni et al. (2017) studied the effect of loading rate (which was varied from 0.00042 to ) and method of displacement control [linear variable differential transformer (LVDT) or machine stroke]. Based on their results, the loading rate affected the ultimate load of the tests, whereas the type of displacement control influenced the initial stiffness and the peak load for certain rates. Ortlepp et al. (2006), D’Ambrisi et al. (2012), and Focacci et al. (2017) used the experimental results obtained from bond tests to calibrate the local bond–slip relation, which is important in the modeling of the structural behavior of strengthened RC elements. The proposed model provided reliable results; however, it should be mentioned that the obtained results are only valid for the specific types of mortar and fibers tested.
Finally, Raoof and Bournas (2017c) investigated the bond between TRM or FRP and concrete substrate at high temperatures by conducting double-lap direct shear tests. Both FRP and TRM specimens were tested at 20°C, 50°C, 75°C, 100°C, and 150°C, and TRM specimens were additionally tested at 200°C, 300°C, 400°C, and 500°C. Based on the results, the bond of TRM is quite strong at high temperature. The bond capacity of FRP dramatically dropped after exposure to high temperatures, but the bond capacity of TRM was only slightly affected by the high temperatures. Regarding failure modes, in FRP specimens cohesive failure was observed up to 50°C, but adhesive failure at the concrete–resin interface was observed at 75°C. In the case of TRM specimens, cohesive failure was observed over all tested temperatures (20°C–400°C).
Flexural Strengthening of RC Beams or Slabs
Method Description
Strengthening of beams or slabs in flexure is achieved by bonding TRM layers to the tensile face, which is typically the soffit of a beam [Fig. 5(a)] or the bottom face of a slab. Textile reinforcement is provided at the regions where additional moment capacity is needed. In addition, a sufficient anchorage length should be provided. Additional measures to improve anchorage conditions could be taken by applying methods similar to the ones used for FRP (i.e., U-strips or spike anchors; Koutas et al. 2014; Bournas et al. 2015).
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In the flexural strengthening of beams or one-way slabs, not all the fibers of a textile are utilized in carrying tensile forces. In the typical case of a bidirectional textile (with fiber rovings in two orthogonal directions) only the fibers parallel to the member axis are stressed in tension; the rest simply contribute to the development of the mechanical interlock mechanism—hence, the cost-effectiveness of this strengthening technique is questionable. However, in the case of two-way slabs, fibers in both directions are effective.
General Behavior
The effectiveness of the flexural strengthening of RC beams and slabs with TRM has been experimentally investigated in the last decade in numerous publications (Bruckner et al. 2006; Papanicolaou and Triantafillou 2006; Triantafillou 2007; Jesse et al. 2008; Bösche et al. 2008; Papanicolaou et al. 2009; D’Ambrisi and Focacci 2011; Ombres 2011, 2012; Schladitz et al. 2012; Elsanadedy et al. 2013; Babaeidarabad et al. 2014; Loreto et al. 2013; Verbruggen et al. 2016; Aljazaeri and Myers 2017a; Ebead et al. 2017; Escrig et al. 2017; Koutas and Bournas 2017; Pino et al. 2017a, b; Raoof et al. 2017; Raoof and Bournas 2017d). Targeting them to assess their flexural capacity, beam or slab elements have been subjected to monotonic three- or four-point bending. In the majority of the studies the tested elements comprise either medium-scale beams (effective span around 2.0 m) or one-way slabs strengthened with textiles made of carbon, glass, basalt, or PBO fibers. Steel fabrics made of high-strength steel cords have also been used in combination with cement-based mortars for the flexural strengthening of RC beams (e.g., Napoli and Realfonzo 2015; Sneed et al. 2016; Escrig et al. 2017). What differentiates beams from one-way slabs is the section geometry and the absence of steel stirrups in the case of slabs. Strengthening of two-way slabs was first reported by Papanicolaou et al. (2009) and later by Koutas and Bournas (2017).
Fig. 5(b) shows simplified load versus deflection curves that illustrate the effect of strengthening as reported in the majority of the studies. Three linear branches up to the maximum load describe the flexural behavior in three stages: (1) the uncracked stage up to the point of first concrete cracking, (2) the cracked stage up to the point of steel yielding, and (3) the stage of plastic hinging in the case of unstrengthened elements or the stage of full textile activation up to the ultimate load in the case of strengthened elements.
The presence of the strengthening layers in some cases increases the initial stiffness at the uncracked stage (Bösche et al. 2008; Jesse et al. 2008; Papanicolaou et al. 2009; Schladitz et al. 2012; Escrig et al. 2017; Koutas and Bournas 2017; Pino et al. 2017a; Raoof et al. 2017), but in other cases this increase is negligible. From the literature survey it seems that the lower the initial stiffness of the unstrengthened element, the higher the effect of strengthening (this is usually the case with slab elements). An increase in the concrete cracking load has also been reported by Jesse et al. (2008), Escrig et al. (2017), Koutas and Bournas (2017), and Raoof et al. (2017), and can be noticed in the load-deflection curves reported by Papanicolaou et al. (2009) and Schladitz et al. (2012). This increase indicates some activation of the fibers in tension prior to concrete cracking.
At the second stage, multiple cracking of concrete results in activation of the strengthening layers and a stiffer behavior with respect to an unstrengthened element is observed along with an increase in the yield load (Triantafillou 2007; D’ Ambrisi and Focacci 2011; Ombres 2011; Elsanadedy et al. 2013; Babaeidarabad et al. 2014; Loreto et al. 2013; Sneed et al. 2016; Escrig et al. 2017; Pino et al. 2017a; Raoof et al. 2017). From the instant of steel yielding and beyond, the contribution of TRM to the flexural resistance becomes significant. Any additional load after that point is carried almost solely by the TRM layers until failure occurs and the ultimate load is reached. With few exceptions (i.e., shear failure, concrete crushing), failure is attributed to the loss of the strengthening action, which can be either progressive or abrupt (D’Ambrisi and Focacci 2011). After a significant loss of strength, the residual flexural capacity of the strengthened element approaches the plastic moment capacity of the unstrengthened element.
Failure Modes
Several failure modes have been reported in the literature, highlighting the complexity of the mechanical behavior of the TRM strengthening system. Apart from failure modes similar to those for FRP strengthening systems, additional failure modes have been observed in most of the studies. Fig. 6 illustrates schematically all the reported failure modes that will be described subsequently. In general, an RC element strengthened in flexure with TRM may fail due to loss of the strengthening action [Figs. 6(a–f)] or due to concrete failure [Figs. 6(g and h)]. The failure modes can be categorized as follows:
1.
Slippage of the fibers within the matrix [Figs. 6(a) and 7(a)]; this has been reported by D’Ambrisi and Focacci (2011), Ombres (2011), Babaeidarabad et al. (2014), Loreto et al. (2013), Sneed et al. (2016), Aljazaeri and Myers (2017a), Ebead et al. (2017), Koutas and Bournas (2017), Pino et al. (2017a), and Raoof et al. (2017). This failure mode is related to low impregnation of the fibers with the mortar and to the poor chemical bond at the fiber–matrix interface. Partial rupture of the outer fibers of the rovings may occur due to their better impregnation compared to the core fibers. Slippage occurs at the region of maximum moments, whereas load drop due to loss of strengthening action in this case is gradual and smooth. The use of U-wraps at the end of beams as a means of providing anchorage to the flexural strengthening layers may cause slippage of the fibers through the matrix away from the region of maximum moments (Sneed et al. 2016; Raoof et al. 2017).
2.
Debonding at the concrete–matrix interface [Figs. 6(b and c)]; this has been reported by D’Ambrisi and Focacci (2011), Ombres (2011, 2012), Elsanadedy et al. (2013), Babaeidarabad et al. (2014), Aljazaeri and Myers (2017a), and Pino et al. (2017a). The loss of the bond between the concrete and the matrix is the reason for this mode of failure. The detachment of the TRM layer can either start from the region of maximum bending moments due to the development of flexural cracks or can initiate from the ends. The first case is usually described as intermediate crack debonding; in this case, debonding propagates toward the support [Fig. 6(b)]. The second case is usually described as end debonding; as shown in Fig. 6(c), the detachment of the TRM propagates toward the midspan. Providing a short anchorage length for the TRM can result in end debonding (Ombres 2012). The load drop in both cases is sudden, indicating the brittle nature of debonding.
3.
Debonding at the matrix–textile interface, or interlaminar shearing [Figs. 6(d) and 7(b)]; this has been reported by Triantafillou (2007), D’Ambrisi and Focacci (2011), Napoli and Realfonzo (2015), Sneed et al. (2016), and Raoof et al. (2017). This failure mode comprises a fracture surface within the TRM thickness, which is at the interface between a textile layer and the mortar. In this case, the bond at the concrete–matrix interface is stronger than the shear bond at the interface that fails. Thus, part of the strengthening material remains attached to the soffit of the concrete element. The use of relatively low shear strength mortars and/or the use of high tensile strength textiles that have small grid size can cause shearing between layers of mortar and textile. This failure mode can also be observed when coated textiles are used (Raoof et al. 2017).
4.
Debonding from the concrete surface accompanied with peeling off of the concrete cover [Figs. 6(e) and 7(c)]; this has been reported by Loreto et al. (2013), Ebead et al. (2017), and Raoof et al. (2017). In this case, debonding initiates from an intermediate flexural or shear-flexural crack and propagates toward the end of the TRM reinforcement. Part of the concrete cover remains attached to the composite material, indicating a strong bond between the mortar and the concrete. This failure mode typically leads to higher flexural capacities compared to the aforementioned Failure modes 1, 2, and 3, and usually occurs when high-strength mortars are combined with more than one layer of high-strength textiles.
5.
Fiber rupture [Figs. 6(f) and 7(d)]; this has been reported by Bösche et al. (2008), Jesse et al. (2008), Schladitz et al. (2012), Elsanadedy et al. (2013), and Raoof et al. (2017). When the textile fibers at the region of maximum moment are subjected to high tensile stresses, they rupture in a single section. This mechanism is brittle, leading to a sudden load drop.
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The five aforementioned failure modes are associated with the loss of the strengthening action. Nevertheless, there is always the case where an element might be strengthened in such a way that failure is eventually associated with concrete damage. In particular, concrete crushing [Fig. 6(g)] has been reported by Ombres (2011) in beams with high steel reinforcement ratio and low TRM reinforcement ratio. Concrete crushing in that case prevented the propagation of debonding, which had just been initiated at the location of an intermediate crack. In the case of excessive flexural strengthening, shear failure of the element may precede flexural failure [Fig. 6(h)]. Related to this, Papanicolaou et al. (2009) and Koutas and Bournas (2017) have observed punching shear failures in two-way slabs, and D’Ambrisi and Focacci (2011) have reported shear failure of the concrete web in short beams with low internal shear reinforcement ratio. It is crucial, therefore, for design engineers to account for this possibility and avoid excessive flexural strengthening that may result in shear or punching shear failure of retrofitted elements.
Strengthening effectiveness strongly depends on the failure mode. If strength increase is the target, then premature failure modes (i.e., fiber slippage or early debonding) are linked with less effective performance compared to failure that exploits the properties of the strengthening material (i.e., fiber rupture or debonding at high strains). The role of different parameters on the effectiveness of the flexural strengthening system is described in the following section.
Effect of Parameters on the Effectiveness of Flexural Strengthening
Depending on various parameters, such as number of layers, material properties, and the RC member’s reinforcement details, the effectiveness of the strengthening system can vary significantly. Next, we summarize how different parameters affect the performance of beams and slabs, according to information collected from the literature survey.
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By increasing the amount of externally applied reinforcement (expressed by the number of layers), flexural capacity increases. However, this correlation is not consistent among different studies. In some cases (Bösche et al. 2008; Papanicolaou et al. 2009; Schladitz et al. 2012; Elsanadedy et al. 2013; Koutas and Bournas 2017) the increase was nearly proportional to the number of layers, but in others (D’Ambrisi and Focacci 2011; Ombres 2011, 2012; Babaeidarabad et al. 2014; Raoof et al. 2017) this was not verified. This inconsistency is linked to the possible development of different failure modes when the number of layers increases.
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Increasing the number of layers can alter the failure mode. In particular, in certain cases reported by D’Ambrisi and Focacci (2011), Ombres (2011), Loreto et al. (2013), Babaeidarabad et al. (2014), Ebead et al. (2017), Pino et al. (2017a), and Raoof et al. (2017), the use of textiles with more than one layer may suppress slippage of the fibers. The failure mode was altered to debonding at the matrix–concrete interface, interlaminar shearing, or debonding with peeling off of the concrete cover.
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The effect of the internal steel reinforcement ratio was investigated by Bösche et al. (2008), who concluded that for lower ratios, the strengthening effect is significantly higher. As an indication, for the same retrofitting scheme the flexural capacity of a slab was increased by approximately 10% and 30% for ratios of 0.2% and 0.5%, respectively.
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Jesse et al. (2008) reported that coating textiles with a polymer adhesive can improve the strengthening effectiveness by 67%. Similar effects from coating have also been reported by Raoof et al. (2017); in that study, a 55% enhancement in the strengthening effectiveness was recorded when one layer of carbon fiber textile was coated with epoxy adhesive two days before the strengthening application.
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The influence of mortar type was investigated by D’Ambrisi and Focacci (2011) and Elsanadedy et al. (2013). The former highlighted the importance of the chemical bond between the fibers and the mortar by using a special type of mortar which resulted in higher (by approximately 28%) load-carrying capacity and debonding strain when compared to conventional mortars. Elsanadedy et al. (2013) compared the performance of conventional versus polymer-modified cementitious mortars and concluded that the latter are better, because they result in better bond action at the concrete–mortar interface.
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The effect of textile geometry was briefly investigated by D’Ambrisi and Focacci (2011), with no clear results. A change in the TEX (defined as mass in grams per 1,000 m) of the fiber rovings in the transverse direction resulted in slightly better performance, but this increase could be considered in the range of experimental scatter.
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D’Ambrisi and Focacci (2011), Sneed et al. (2016), Escrig et al. (2017), and Raoof et al. (2017) tried to improve the effectiveness of the strengthening layers by providing anchorage with the use of additional TRM U-strips at the full length or only at the two ends of the strengthening layers. According to their results, there was no strong evidence that the extra measures helped to increase load-carrying capacity. In the cases in which U-strips were applied only at the two ends of beams (Sneed et al. 2016; Escrig et al. 2017; Raoof et al. 2017), debonding of the TRM strip was prevented and failure was due to slippage of the fibers through the matrix. As Sneed et al. (2016) concluded, the latter indicates that traditional anchorage solutions for FRP composites might not be effective in the case of TRM composites.
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Aljazaeri and Myers (2017a) and Pino et al. (2017a) examined the performance of PBO-FRCM-strengthened RC beams subjected to fatigue loading. Both studies concluded that the strengthening of beams improved their fatigue performance, with the level of improvement being influenced by the amount of external reinforcement. In addition, in both studies the residual flexural capacity of the retrofitted beams that survived 2 million cycles was very close to the capacity of the strengthened beams not subjected to fatigue.
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Raoof and Bournas (2017d) investigated the effect of high temperatures (150°C) on RC beams strengthened in flexure and concluded that TRM reached an average of 55% of its ambient temperature effectiveness. This behavior was advantageous over FRP-strengthened beams; FRP completely lost its effectiveness when subjected to the same heating conditions.
TRM versus FRP in Flexural Strengthening
Triantafillou (2007), D’Ambrisi and Focacci (2011), and Raoof et al. (2017) compared the effectiveness of TRM versus FRP systems in increasing the flexural capacity of RC beams. With a limited number of results, Triantafillou (2007) concluded that carbon TRM is less effective (by about 30%) than equivalent carbon-FRP (CFRP) systems in terms of flexural capacity enhancement. D’Ambrisi and Foccacci (2011) obtained a similar effectiveness between PBO TRM and CFRP systems, but they found that—in agreement with Triantafillou (2007)—carbon TRM systems are less effective (by 30%–70%) than CFRP because of the development of different failure modes (fiber slippage for TRM versus debonding for FRP). Raoof et al. (2017) also concluded that the effectiveness of TRM systems was less than that of FRP systems and that the TRM versus FRP effectiveness ratio was sensitive to the number of textile layers; an increase of layers from one to three resulted in an increase of the effectiveness ratio from 0.47 to 0.80.
Design Aspects
For design purposes, ACI 549.4R (ACI 2013) proposes a maximum value of tensile strain in the TRM composite equal to 0.012, which is multiplied by the tensile modulus of elasticity of the cracked composite to yield the design tensile strength of the external flexural reinforcement. Ombres (2011) suggested that available analytical models for FRP-strengthened RC beams can estimate with good accuracy the ultimate flexural capacity of the beams, given that premature failure modes (e.g., slippage of fibers, debonding from the concrete substrate) are avoided and rupture of the fibers occurs. Based on limited experimental data, Raoof et al. (2017) concluded that if failure is due to debonding of TRM from the concrete substrate (including part of the concrete cover), the debonding stress in TRM can be estimated using the provisions of fib (2010) for the case of FRP-strengthened beams. Finally, Koutas and Bournas (2017) provided simple design equations to estimate the flexural moment of resistance of two-way RC slabs strengthened with carbon fiber textile composites.
Shear Strengthening of RC Members
Method Description
One of the most common needs when assessing the strength of RC structures under current code requirements is the shear strengthening of RC beams or bridge girders. This is due to insufficient amounts of shear reinforcement, corrosion of existing shear reinforcement, low concrete strength, and/or increase in the applied load. Moreover, shear strengthening is also required to ensure a ductile flexure-type failure mode.
TRM is applied at critical shear spans as side-bonding (not recommended), U-wrapping, or full wrapping. It should be mentioned that fully wrapped jackets can only be used for the shear strengthening of concrete columns; their application is not feasible in beams or bridge girders because of the presence of concrete slabs or decks, respectively. Anchorage systems, including mechanical devices or spike or textile-based anchors, have also been used to improve the anchorage conditions of side-bonded and U-shaped jackets, similar to FRP jackets. Fig. 8(a) shows a beam strengthened with U-shaped TRM jackets and tested under symmetric four-point monotonic loading; Fig. 8(b) shows a T-beam strengthened with U-shaped TRM jackets combined with textile-based anchors.
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General Behavior
A number of studies have investigated the use of TRM for the shear strengthening of RC beams over the last decade (Triantafillou and Papanicolaou 2006; Bruckner et al. 2008; Al-Salloum et al. 2012; Blanksvard et al. 2009; Si Larbi et al. 2010; Al-Salloum et al. 2012; Contamine et al. 2013; Azam and Soudki 2014; Baggio et al. 2014; Escrig et al. 2015; Jung et al. 2015; Loreto et al. 2015; Ombres 2015b; Tetta et al. 2015; Trapko et al. 2015; Awani et al. 2016; Tetta et al. 2016; Tzoura and Triantafillou 2016; Aljazaeri and Myers 2017b; Tetta et al. 2018a), the vast majority of which were on small- or medium-scale beams with rectangular sections. The beams in the aforementioned studies were tested under (symmetric or nonsymmetric) monotonic three-point or four-point bending, except for the ones in Tzoura and Triantafillou (2016), which were tested as cantilevers under cyclic loading, simulating shear strengthening near supports.
Fig. 9 depicts the simplified load versus deflection (measured at the load application position) curves and illustrates the effect of strengthening as reported in the majority of the studies. Two linear branches up to the maximum load describe the shear behavior: (1) the uncracked stage up to the point of first concrete cracking, and (2) the cracked stage up to the failure of the specimen. The descending branch can be quite smooth with relatively soft load degradation (pseudoductile failure mode) or quite rough with sudden drop of load (brittle failure mode) depending on the failure mode observed in the TRM.
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Failure Modes
The failure modes observed in RC beams strengthened in shear with TRM jackets are summarized as (1) debonding of the TRM jackets with peeling off of the concrete cover, in the case of side-bonded or U-wrapped TRM jackets [Fig. 10(a)]; (2) fracture of TRM jacketing (mainly observed in fully wrapped jackets or glass and basalt side-bonded or U-wrapped jackets) [Fig. 10(b)]; and (3) local damage of the jacket, including slippage of the fibers through the mortar [Fig. 10(c)]. The first two failure modes were also widely observed in RC beams strengthened in shear with FRP jackets; slippage of the fibers through the epoxy resin was prevented in FRP jackets (tested at ambient temperature) because of the high strength of the epoxy resins usually used in FRP applications. It should be mentioned that the failure mode related to slippage of the fibers through the mortar was observed in TRM specimens strengthened with dry/uncoated textile materials (e.g., Loreto et al. 2015; Tetta et al. 2015, 2016; Tzoura and Triantafillou 2016) but was suppressed when coated textile materials were applied (e.g., Awani et al. 2016).
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Reinforced concrete members strengthened in shear may also fail in shear compression, that is, due to concrete compression prior to failure of the TRM jacket, as reported in the studies of Al-Salloum et al. (2012), Azam and Soudki (2014), and Loreto et al. (2015). Finally, flexural failure was also reported in the studies of Triantafillou and Papanicolaou (2006), Ombres (2015b), and Tetta et al. (2015); this failure mechanism should always be the target in any project involving the shear strengthening of structures in service. Note that activation of the last two failure mechanisms (concrete compression or flexural failure) does not allow calculation of the TRM contribution to shear capacity. Hence, it is convenient to avoid such failures in research projects.
The failure mode significantly affects the effectiveness of TRM jacketing. In particular, TRM is nearly as effective as FRP jacketing in increasing the shear capacity of RC beams when failure is associated with debonding of the jacket (Awani et al. 2016; Tetta et al. 2016), but it becomes less effective when failure is attributed to damage (e.g., fracture) of the jacket instead of debonding (Triantafillou and Papanicolaou 2006; Tetta et al. 2015; Tzoura and Triantafillou 2016). Full exploitation of the tensile capacity of the textiles is achieved when failure of specimens is associated with rupture of the fibers.
Effect of Parameters on the Effectiveness of Shear Strengthening
The shear capacity of beams was significantly increased, up to 150% or 190%, by applying nonanchored or anchored U-shaped TRM jackets, respectively. The key parameters investigated in these studies and the conclusions thereof are summarized subsequently.
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The effect of increasing the amount of external reinforcement on the shear capacity of (nonanchored) TRM-strengthened beams has been studied by most researchers by means of (1) increasing the number of layers (Triantafillou and Papanicolaou 2006; Bruckner et al. 2008; Al-Salloum et al. 2012; Jung et al. 2015; Loreto et al. 2015; Tetta et al. 2015; Awani et al. 2016; Tetta et al. 2016; Tzoura and Triantafillou 2016; Tetta et al. 2018a); (2) increasing the ratio of width over spacing () of discontinuous TRM strips (Contamine et al. 2013; Ombres 2015b); and (3) applying textile materials of different weights (Azam and Soudki 2014; Tetta et al. 2016; Tzoura and Triantafillou 2016; Tetta et al. 2018a). The key conclusion was that increases in the amount of external shear reinforcement resulted in significant but nonproportional increases in the shear resistance. When the number of layers was increased, a change in the failure mode was witnessed in the case of dry/uncoated textile materials. In particular, the failure of specimens that received one (Loreto et al. 2015; Tetta et al. 2015) or two (Tetta et al. 2016) textile layers was associated with slippage of the fibers through the mortar. In contrast, the failure mode of specimens that received more than two textile layers (Loreto et al. 2015; Tetta et al. 2015, 2016) was attributed to debonding of the TRM jacket combined with peeling off of the concrete cover. Thus, increasing the number of layers suppressed damage of the jacket (e.g., slippage of the fibers through the mortar) and shifted the damage to the concrete substrate.
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The strengthening configuration, namely side-bonded, U-shaped or fully wrapped jacketing, has been investigated in a few studies. Azam and Soudki (2014) concluded that side-bonded and U-shaped jackets exhibited similar performance in terms of strength, contrary to Jung et al. (2015) and Tetta et al. (2015), who reported that U-shaped jackets are much more effective than side-bonded jackets in increasing the shear capacity of beams. As expected, fully wrapped jacketing is the most effective strengthening configuration (compared to side-bonding or U-wrapping), based on the results presented in Tetta et al. (2015). However, it should be mentioned that the use of closed jackets is not feasible in beams of typical RC buildings or bridge girders because of the presence of concrete slabs or decks, respectively.
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Debonding of side-bonded or U-shaped TRM jackets constitutes a common failure mode, similar to the case of FRP jackets. In this context, the use of end anchorage of TRM open jackets, so as to delay or even prevent their early debonding from the concrete substrate, has been examined. Bruckner et al. (2008) and Tzoura and Triantafillou (2016) used a mechanical end-anchorage system (comprising steel sections anchored into the flange by bolts) in T-beams strengthened in shear with glass or carbon TRM U-jackets. Despite the fact that the effectiveness of the TRM jackets was dramatically improved [up to 500% based on the results presented in Tzoura and Triantafillou (2016)], an anchorage system with metallic components may be susceptible to corrosion and its use may result in tearing failure of the textiles due to stress concentrations. A possible solution to this problem could rely on the use of nonmetallic materials. Baggio et al. (2014) and Tetta et al. (2016) investigated the performance of spike anchors (Fig. 11), which are versatile, noncorrosive, lightweight, and compatible with the materials used for TRM jackets. However, Baggio et al. (2014) were not able to assess the performance of carbon-based spike anchors due to premature rupture of the glass TRM jacket, both in the nonanchored and in the anchored jacket. In contrast, Tetta et al. (2016) applied textile-based anchors as an end-anchorage system for carbon or glass U-shaped TRM jackets and found that textile-based anchors significantly improved (up to 150%) the effectiveness of the carbon TRM jackets; however, glass-anchored TRM jackets failed due to rupture before failure of the anchorage system.
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Blanksvard et al. (2009), Awani et al. (2016), and Aljazaeri and Myers (2017b) investigated the effect of the amount of internal shear reinforcement ratio on the shear capacity of TRM-strengthened beams. Based on the results presented in Awani et al. (2016), in which all the strengthened beams failed due to diagonal shear, the contribution of the TRM jacket to the total shear resistance is not practically affected by the amount of stirrups.
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Recently, Tetta et al. (2018a) investigated the effect of shear span-to-depth ratio in RC beams strengthened in shear with U-shaped TRM jackets; they concluded that the shear span-to-depth ratio has no effect on the failure mode nor on the contribution of the jacket to the total shear resistance of the beams.
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Regarding the geometry of the textile materials used in TRM, Tetta et al. (2016, 2018a) concluded that different (dry/uncoated) carbon fiber textile geometries having the same reinforcement ratio resulted in different load increases and failure modes. This effect of the geometry of the textile fiber material is drastically mitigated by increasing the reinforcement ratio.
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The use of polymers in the TRM matrix was studied by Al-Salloum et al. (2012), who reported that polymer-modified cementitious mortars are more effective than unmodified ones.
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Regarding fiber orientation, Al-Salloum et al. (2012) concluded that textiles with fibers at were slightly more effective than textiles with orientations of , contrary to the results presented in Trapko et al. (2015), who concluded that the highest shear capacity gain was obtained from TRM strips aligned at 90o to the longitudinal axis of the member.
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Loreto et al. (2015) reported that the increase in shear capacity was slightly lower in specimens with low concrete compressive strength; hence, they concluded that the strength of concrete marginally affects the response of strengthened beams.
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Torsional strengthening of RC with TRM was investigated by Schladitz and Curbach (2012) and Alabdulhaby et al. (2017). Schladitz and Curbach (2012) conducted tests on beams with round (300-mm-diameter) or rectangular () cross sections strengthened with 4 or 6 layers of fully wrapped (coated) glass-based textiles. The results showed that the torsion load-carrying capacity and serviceability (based on crack width and crack spacing) of concrete members significantly improved (up to 240% and 272% in the case of round and rectangular cross sections, respectively) by using TRM jacketing. Moreover, as expected, the torsional resistance increased with an increase in the number of textile reinforcement layers. Based on the results presented in Alabdulhady et al. (2017), fully wrapped PBO TRM jacketing significantly increased the torsional capacity of rectangular RC beams, contrary to U-wrapped PBO jackets.
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TRM versus FRP in Shear Strengthening
The comparison of TRM with FRP jackets has been investigated by several researchers who tested beams strengthened in shear with the same textile materials combined with different adhesive materials, namely mortar for TRM jacketing and epoxy resin for FRP jacketing (Triantafillou and Papanicolaou 2006; Tetta et al. 2015; Awani et al. 2016; Tetta et al. 2016; Tzoura and Triantafillou 2016). Triantafillou and Papanicolaou (2006), and Tzoura and Triantafillou (2016) reported that closed or anchored TRM jackets are less effective than their FRP counterparts because of slippage of the fibers through the mortar. Based on tests on both medium-scale and full-scale T-beams, Tetta et al. (2015), Awani et al. (2016), and Tetta et al. (2016) reported that TRM jackets can be practically as effective as equivalent FRP U-shaped jackets when failure of both systems is attributed to debonding of the jacket with concrete cover separation. Tetta and Bournas (2016) compared TRM with FRP jackets in the shear strengthening of concrete beams subjected to high temperatures; they concluded that side-bonded and U-shaped TRM jackets are much more effective than their FRP counterparts when specimens are exposed to temperatures in the range 100°C to 150°C.
Design Aspects
Triantafillou and Papanicolaou (2006) first presented a design methodology for shear-strengthened RC members with TRM, that is, adopting the so-called effective strain of the external reinforcement, which is also used in FRP-strengthening design procedures. ACI 549.4R (ACI 2013) proposes a maximum value of effective strain for design purposes equal to 0.004, which is multiplied by the tensile modulus of elasticity of the cracked composite to yield the design tensile strength of the external shear reinforcement. To estimate the effective strain, Ombres (2015b) used the equations proposed by Monti and Liotta (2007) for FRP-strengthened RC beams, and applied an effectiveness coefficient equal to 0.5, reflecting the reduced effectiveness of TRM compared to FRP systems. Escrig et al. (2015) developed empirical equations from limited experimental data to calculate the effective strain of the TRM for fully wrapped, U-shaped or side-bonded jackets, given that debonding failure is avoided. Very recently, Tetta et al. (2018b) conducted a comprehensive study, and by collecting all the available data (which were grouped based on the observed failure modes), proposed a simple design approach for calculating the contribution of the TRM jacket to the total shear resistance, including new expressions for all failure modes described previously.
Concrete Confinement for Increased Load and Deformation Capacity
Method Description
The wrapping of concrete elements with TRM layers [Fig. 12(a)] aims to provide passive confinement stresses when the elements are subjected to axial compression and, hence, to increase their compressive strength and deformation capacity. The need of extra confinement is typical for columns with low axial capacity and poor detailing. Confinement with TRM can also be useful for seismic retrofitting of columns, which is discussed in another section of this paper.
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The typical method of application is wet lay-up on properly prepared surfaces. An overlap of the last layer is usually provided in the hoop direction to avoid early debonding [Fig. 12(b)]. Bidirectional textiles (with fiber rovings in two orthogonal directions) are the most commonly used so far in the literature. However, when they are applied in a configuration, only the fibers in the hoop direction contribute to the development of confining stresses. This suggests that optimum textile geometries with different materials in each direction are still to be developed.
Similar to FRP confinement, wrapping with textiles can easily be applied in circular or rectangular sections, provided that the fibers are uncoated and the corners of the rectangular section have been properly rounded. Based on the authors’ experience, corner radius should be at least equal to , with a minimum value of 25 mm (where is the smaller of the two column sides). This assumes that the available concrete cover is adequate. Textiles with coated fibers can be used for wrapping under certain conditions—the degree of coating impregnation must be low, and a relatively large mesh size will further make the application easier.
General Behavior
The application of open-mesh textiles in combination with inorganic matrices as a means of increasing the axial capacity of concrete through confinement has been reported in the studies of Triantafillou et al. (2006), Bournas et al. (2007), Peled (2007), Ortlepp et al. (2009), Di Ludovico et al. (2010), Garcia et al. (2010), Trapko (2013), Colajanni et al. (2014), Ombres (2014), Trapko (2014), Ombres and Verre (2015), Thermou et al. (2015), Cerniauskas et al. (2016), Ombres (2017), and Yin et al. (2016). With the aim of assessing axial load and deformation capacity, concrete prisms (with the axial dimension at least two times larger than the other two) were subjected to monotonic concentric compression in the axial direction; in a few cases the loading was eccentric (Trapko 2014; Ombres and Verre 2015). In the majority of the studies, the tested elements comprised unreinforced concrete cylinders with a 150-mm diameter and 300-mm height. However, rectangular-section column-type elements have also been studied, with sections ranging from to and heights between 300 and 1,500 mm. Carbon, glass, basalt, aramid, and PBO fiber textiles were used in the aforementioned studies, with carbon and PBO textiles being the most commonly studied. The use of steel fabrics made of high-strength steel cords was also reported in the study of Thermou et al. (2015).
Fig. 12(c) illustrates typical simplified axial stress–axial strain curves for unconfined and TRM-confined concrete. Although the presence of the jacket may increase the initial elastic stiffness of concrete, the contribution of strengthening mainly affects the postelastic behavior of concrete. It is only after cracking that lateral deformations develop and the jacket is activated in tension due to the lateral expansion of the concrete. As a result, the effect of strengthening is twofold: (1) an axial load-carrying capacity increase, and (2) an axial deformation capacity enhancement. The slope of the postelastic ascending branch of the confined concrete strongly depends on the external reinforcement ratio, the axial stiffness of the jacket, and the shape of the cross section [a low reinforcement ratio may result in zero or even negative slope (Colajanni et al. 2014), particularly when combined with a rectangular cross section (Triantafillou et al. 2006)]. At large axial and particularly lateral deformations, failure of the TRM jacket occurs, resulting in load drop, which can vary from rather sudden to quite smooth and progressive. The failure modes are discussed in the following section.
Failure Modes
Failure in TRM-confined concrete is always induced because of the loss of the strengthening action. Two different failure modes have been reported in the literature and are described as follows:
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Debonding from the end of the lap [Fig. 13(a)]. In this case debonding initiates at the end of the lap and is indicated by a vertical crack that appears at the location at which the textile terminates. It occurs at the interface between the mortar and the last layer of textile (interlaminar shearing), and its development depends on various parameters. A low tensile strength mortar (e.g., Triantafillou et al. 2006), a short overlap length (e.g., Thermou et al. 2015), a very dense mesh that does not allow a good impregnation with mortar (e.g., Thermou et al. 2015), or a combination of any of the above, may promote the debonding failure mechanism.
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Fracture of the jacket due to hoop stresses [Fig. 13(b)]. If sufficient overlap length is provided for the applied amount of TRM reinforcement, failure of the jacket initiates due to fracture of fibers in the hoop direction when they reach their tensile strength. In this case, failure of the TRM jacket is usually gradual (Triantafillou et al. 2006; Bournas et al. 2007; Peled 2007; Di Ludovico et al. 2010; Garcia et al. 2010; Colajanni et al. 2014; Ombres 2014; Thermou et al. 2015) because of the initiation of fracture in a limited number of fiber bundles (or steel cords in the case of steel fabrics), which propagates rather slowly in the neighboring bundles until concrete crushing is extensive and the load drops at very low levels. In elements with rectangular section, fracture initiates at one of the corners due to high concentration of stresses or due to buckling of the longitudinal steel reinforcement (Bournas et al. 2007). The speed of fracture propagation may vary depending on the amount of reinforcement; a higher number of layers may lead to a more abrupt failure in the corners of rectangular sections compared to a more gradual failure when fewer layers are used (Triantafillou et al. 2006).
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Effect of Parameters on the Confinement Effectiveness
The effect of the various parameters examined in the literature on the strengthening effectiveness and the behavior of TRM-confined concrete elements is discussed subsequently.
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By increasing the reinforcement ratio (mainly by increasing the number of the applied TRM layers), both the compressive strength and the ultimate strain capacity of the confined concrete increase. This has been confirmed by all studies in the literature. The increase is usually nonproportional to the amount of reinforcement, with the effectiveness decreasing as the number of layers increases. A clear trend is not easy to define because of the relatively large scatter of experimental results, with varying types of materials and possibly different quality control. However, Ombres and Mazzuca (2016) have developed semiempirical prediction models based on the majority of available data in the literature.
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The section geometry greatly affects the strengthening effectiveness. Similar to FRP jackets, TRM jackets are more efficient when they confine elements with cylindrical sections. This has been confirmed by Triantafillou et al. (2006), Ortlepp et al. (2009), and Colajanni et al. (2014). The reason for this is the uneven confining action in the case of a rectangular section (which is higher at the four corners) compared to the uniform confinement in the case of a circular section.
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It has been confirmed by Triantafillou et al. (2006), Ombres (2014), Thermou et al. (2015), and Ombres and Mazzuca (2016) that the unconfined concrete strength significantly affects the effectiveness of TRM jackets in increasing the axial load-carrying capacity of confined concrete; the effectiveness of the jackets is higher for lower values of unconfined concrete strength.
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The effect of mortar strength has been investigated by Triantafillou et al. (2006) and Garcia et al. (2010). Triantafillou et al. (2006) compared two cement mortars and concluded that two layers of carbon TRM jacketing with the higher (by 24%) flexural strength mortar was 58% more effective than the lower-strength mortar in terms of increasing the strength of concrete through confinement; the failure mode in the former case involved fracture of the jacket and in the latter case debonding. The corresponding increase was marginal (4%) when four layers of the same textile were used, mainly because the failure mode was the same in all tested specimens with four layers. Garcia et al. (2010) reported that increasing the flexural strength of mortar by 13% resulted in increased effectiveness of the TRM jackets with one or two layers of basalt fiber textiles by 8% or 6.5%, respectively.
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Increasing overlap length can increase the compressive strength and axial deformation capacity of TRM-confined concrete elements only if debonding at the end of the lap is suppressed. For instance, Thermou et al. (2015) managed to supress debonding and further enhance the strength of 150-mm-diameter cylinders confined with steel fabrics by increasing the overlap length from 120 to 360 mm. In contrast, Yin et al. (2016), who tested square columns with internal steel reinforcement, did not record any difference in the ultimate load when they increased the overlap length by four times, mainly because the initial length was adequate to prevent debonding and failure in all cases was due to fracture of the jacket at the corners.
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Ombres (2017) studied the effect of fiber orientation on the compressive behavior of PBO fiber TRM-confined cylinders, with the angle of the fibers varying from 90° to 30° with respect to the cylinder axis. He concluded that the optimal orientation of the fibers is 90°, with the other orientations having a reduced effectiveness even after being normalized to the reinforcement ratio.
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Trapko (2014) and Ombres and Verre (2015) investigated the effect of eccentricity in vertically loaded square columns confined with PBO fiber TRM jackets. The common conclusion of these studies was that, although the jackets enhanced the load capacity of the columns, the gain in strength was inversely proportional to the ratio , where is the eccentricity and is the height of the section.
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The effect of exposing TRM-confined concrete to elevated temperatures has been investigated by Trapko (2013), Cerniauskas et al. (2016), and Ombres (2017). However, the testing method and the temperature range varied significantly among these studies. Trapko (2013) exposed carbon fiber TRM-confined cylinders to 40°C, 60°C, and 80°C for 24 h, and then performed compression tests immediately after the end of the heating period. Cerniauskas et al. (2016) tested carbon fiber TRM-confined cylinders under a steady-state thermal regime, varying from 20°C to 400°C for different specimens. Finally, Ombres (2017) exposed PBO fiber TRM-confined cylinders to cycles of thermal conditioning with temperatures ranging from 50°C to 250°C, and then performed compression tests at ambient temperature. In all studies, a limited decrease in the peak load compared to specimens not exposed to high temperature was reported (5%–20%) for temperatures up to 250°C; this was attributed to a decrease in the strength of the concrete. However, at 400°C, Cerniauskas et al. (2016) recorded strength enhancement, which needs further investigation.
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The effect of exposing TRM-confined square concrete columns to chloride wet-dry cycles was investigated in the study of Yin et al. (2016), who found that after 90 wet-dry cycles the axial load carrying capacity of columns was reduced by approximately 5%; when combined with a sustained load at a stress ratio of 0.4, the decrease in strength reached almost 6%. Yin et al. (2016) reported generation of corrosion products at the interface between the TRM jacket and the concrete, which affected the bonding properties.
TRM versus FRP in Concrete Confinement
Finally, Triantafillou et al. (2006) and Bournas et al. (2007) compared the effectiveness of TRM versus equivalent FRP jackets. Based on tests on concrete cylinders, Triantafillou et al. (2006) concluded that TRM jackets are approximately 80% and 50% less effective (compared to FRP) in increasing the strength and ultimate strain of confined concrete, respectively, whereas Bournas et al. (2007), based on column-type specimens with square section, concluded that TRM jackets were only 10% less effective in increasing both strength and ultimate strain capacity. The difference in the two studies is mostly related to the different section of the specimens. Nevertheless, the effectiveness of TRM jackets is believed by the authors of the aforementioned studies to strongly depend on mortar mechanical properties.
Design Aspects
ACI 549.4R (ACI 2013) provides equations for an idealized bilinear constitutive law of TRM-confined concrete, with the effective tensile strain in the composite material being limited to a value of 0.012.
Seismic Retrofitting of RC Members
Seismic Retrofitting of Columns
TRM jacketing is applied as a means of confining the plastic hinge [Figs. 14(a and b)] of old-type RC columns designed with poorly detailed reinforcement. Under seismic loading, the ultimate deformation capacity of substandard columns is low because of premature failure at the ends of the columns (especially those of the ground and first floors), typically due to bar bucking or bond failure in the case of short lap splices. Full-height jacketing is provided when shear strengthening of columns is seismic areas is required [Fig. 14(c)].
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Bar Buckling
The work of Bournas et al. (2007) demonstrated that TRM jacketing is quite effective (and equal to its FRP counterpart) as a means of increasing the cyclic deformation capacity and energy dissipation of old-type RC columns with poor detailing by delaying bar buckling. In that study, three full-scale RC columns with smooth longitudinal bars were constructed (unstrengthened and FRP- or TRM-jacketed in the plastic hinge region) and subjected to reversed cyclic loading with constant axial load. The geometry of the columns, the reinforcement details, and the general setup of the test are shown in Fig. 15. Details are provided in Bournas et al. (2007).
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The load versus drift ratio curves for these specimens are given in Fig. 16. The control specimen failed due to bar buckling at the column base, which limited its deformation capacity [Fig. 16(a)]. The behavior of the two retrofitted columns was practically identical [Figs. 16(b and c)] for Columns R2 and M4, respectively, and far better than their unretrofitted counterpart, with an increase in deformation capacity by a factor of more than two.
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In a subsequent study, Bournas et al. (2009) demonstrated that the effectiveness of TRM jackets in columns with limited deformation capacity due to bar buckling was also quite high (for the case of columns with deformed longitudinal bars) and clearly higher than the effectiveness of identical FRP jackets. The superior performance of TRM over FRP jackets in the case of bar buckling was attributed to the capacity of TRM jackets to resist local stresses. This occurs because TRM jackets are able to deform outward without early fiber rupture, owing to the relatively low composite action between fibers and mortar that allows for higher local deformations (e.g., slip of fibers within rovings). For the same reason, TRM jackets have been proven quite effective in providing local confinement in RC columns strengthened in flexure with near surface mounted (NSM) reinforcement (Bournas and Triantafillou 2009, 2013). The local confinement with TRM jackets was quite effective in controlling buckling of the NSM reinforcement, thus enabling this reinforcement to reach higher strains at failure. An in-depth analytical investigation of the problem of bar buckling is presented in Bournas and Triantafillou (2011a).
Bond Failure at Regions of Lap-Spliced Bars
The effectiveness of TRM jackets in confining RC columns with limited capacity due to bond failure at lap-splice regions was investigated by Bournas et al. (2009). Based on simulated seismic tests [Fig. 15(a)] conducted on six full-scale columns [Fig. 15(b)], it was shown that TRM jackets are quite effective as a means of increasing the cyclic deformation capacity and the energy dissipation of old-type RC columns with poor detailing by preventing splitting bond failures in columns with inadequate lap splices. It may be concluded that TRM confining jackets provide substantial gains in the lateral strength and deformation capacity of cyclically loaded RC columns with short lap splices at the base of the columns. Compared with equal stiffness and strength FRP jackets, TRM jackets are characterized by a slightly reduced effectiveness in terms of deformation capacity for columns with short lap splices and with the same effectiveness for columns with longer lap lengths. An in-depth experimental investigation, as well as an analytical model for columns with lap splices, is given in Bournas and Triantafillou (2011b).
Finally, the high effectiveness of TRM jacketing as a measure of improving plastic hinge behavior was also confirmed through nearly full-scale testing of a two-story RC building, as reported in Bousias et al. (2007).
Seismic Retrofitting of Beam–Column Joints
Fig. 17 presents a typical strengthening configuration applied to a beam–column joint. The effectiveness of TRM jackets in the seismic retrofitting of deficient beam–column joints was evaluated by Al-Salloum et al. (2011). In that study, TRM and FRP jackets were used for enhancing the shear strength and ductility of seismically deficient exterior beam–column joints. Five specimens, representing nonseismically detailed joints with inadequate joint shear strength due to the lack of shear reinforcement, were constructed. Two of them were tested without retrofitting as reference specimens, whereas the remaining three were strengthened using TRM, CFRP, and glass-FRP (GFRP) jackets, respectively. The test results demonstrated that TRM can effectively improve both the shear strength and deformation capacity of seismically deficient beam–column joints to an extent which is comparable to the strength and ductility achieved by well-established CFRP and GFRP strengthening of joints. It was found that identical load (shear strength) increases were achieved by using TRM and FRP jackets, but due to the higher ductility of TRM, the energy dissipation capacity of TRM-strengthened beam–column joints was substantially higher than the corresponding FRP-strengthened ones.
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Seismic Retrofitting of Masonry-Infilled RC Frames
The contribution of masonry infills to the in-plane seismic resistance of existing RC buildings is quite high, especially before the separation of the infill from the surrounding frame occurs but also during large cycles of imposed deformations near collapse. Koutas et al. (2015a, d) proposed TRM jacketing as an alternative retrofitting technique [Fig. 18(a)] to convert masonry infilling to a more reliable source of resistance by guaranteeing its contribution over the whole spectrum of structural response. They used TRM jacketing on nearly full-scale, as-built, and retrofitted three-story masonry-infilled frames [Fig. 18(b)], which were subjected to cyclic loading.
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Based on the test results presented in Koutas et al. (2015a), the TRM retrofitting scheme resulted in an enhanced global response of the infilled frame, both in terms of lateral strength and deformation capacity [Figs. 19(a and b)]. An increase of more than 50% in the lateral strength was observed, accompanied by a more than 50% higher deformation capacity at the top of the structure at the ultimate strength state. Moreover, the TRM-retrofitted RC frame dissipated about 25% more energy than the unretrofitted one for the same loading history. The effect of retrofitting on the lateral stiffness of the first story was an almost twofold increase for low drift levels (up to 0.5%); this became less pronounced at higher drift levels. It was suggested by the authors that the application of TRM over the entire surface of infills should be supplemented with an adequate infill–frame connection if a reliable resisting system is to be obtained. This was achieved in that study by using custom-fabricated, textile-based anchors, thoroughly investigated by Koutas et al. (2014). Finally, it is also worth mentioning that TRM jacketing proved to be effective in withstanding large shear deformations through the development of a multicrack pattern and by introducing an efficient load transferring mechanism at the local level. The experimental results obtained by Koutas et al. (2015a) were found to be in good agreement with a strut-and-tie analytical model described in Koutas et al. (2015d).
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Akhoundi et al. (2018) studied the performance of TRM-strengthened masonry-infilled RC frames on two nearly half-scale specimens subjected to in-plane cyclic loading. The technique they used was similar to that used by Koutas et al. (2015a). Based on their results, strengthening the masonry infills and connecting them to the RC frame by simply extending the retrofitting layers to the faces of the columns and the beam yielded an increase in lateral stiffness and ultimate strength of about 40%.
Moreover, very recently Koutas and Bournas (2018) conducted an experimental study on the use of TRM jackets as a means of improving the out-of-plane performance of masonry-infill walls in RC frames. The effect of different connection configurations between the infill wall and the RC frame members, as well as the effect of the wall’s thickness, were investigated. It was concluded that the TRM strengthening technique was highly effective in increasing the out-of-plane load capacity of the infill walls. The strengthening effectiveness factor varied between 4 and 5 for single-wythe wall specimens and was equal to about 1.5 in the case of double-wythe wall specimens. Moreover, the energy absorption of the retrofitted walls was enhanced from 140% to 260%, compared to the control ones, with the connection configuration playing an important role. Finally, the authors concluded that connecting single-wythe walls to the front or the back side of the frame resulted in 30% and 40% increases, respectively, compared to walls without connection to the frame.
Finally, the use of TRM jacketing for strengthening of masonry infills has been combined with thermal insulation materials for providing concurrent seismic and energy retrofitting. The concept of combined seismic and energy retrofitting with advanced materials was proposed for the first time and investigated experimentally for the case of masonry subjected to out-of-plane (Triantafillou et al. 2017) and in-plane (Triantafillou et al. 2018) loading. These studies introduced for the first time the combination of TRM with standard or even highly fire-resistant thermal insulation materials. A similar system for the concurrent seismic and energy retrofitting for the case of RC building envelopes was proposed by Bournas (2018). The combined retrofitting (TRM + insulation material) was found to be economically efficient, as the payback period of the retrofitting intervention can be significantly reduced for seismic zones when energy is applied concurrently with seismic retrofitting because of large savings (of about 30%) related to labor costs.
Conclusions
The application of textile reinforced mortars, also known in the international literature as textile reinforced concrete or fabric reinforced cementitious matrix materials, in the field of strengthening and seismic retrofitting of concrete structures is critically reviewed in this paper. The review covers the tensile and bond behavior of TRM, as well as strengthening for flexure, shear, and through confinement; special attention is also given to seismic retrofitting. The review is critical, with a view toward describing the key parameters under investigation. Comparisons with the widely used FRPs are also discussed.
Overall, for all the studies considered in this paper, the strengthening of concrete members with TRM is concluded to be an efficient technique to increase the ultimate flexural or shear capacity of RC members with typical geometries. TRM increases their stiffness and, hence, their performance under serviceability loads. In addition, cracking is better controlled. Concrete confinement results mainly in large axial deformation capacity and, to a lesser degree, in enhanced compressive strength. Seismic retrofitting of RC columns or joints results in ductility increases and energy dissipation enhancements. Finally, masonry-infilled RC frames are stiffer and stronger when strengthened with TRM, with improved seismic performance in their plane direction. Although the literature includes one document with design guidelines and a few studies with design recommendations, compact and reliable design equations are still missing to date. This is mainly attributed to the quite-limited experimental data, as well as to the fact that failure modes in TRM systems are quite complex, not easy to predict, and strongly affect their performance.
The authors believe that the strengthening and seismic retrofitting of concrete structures with textile-based composites is a highly promising technique, which attracts increasing attention of the international scientific community. Future work in this field should be directed at optimizing the textile reinforcement, understanding the durability of the strengthening system (including high temperatures), and establishing design guidelines in the context of current design formulations.
Acknowledgments
The second and the last authors acknowledge funding through the Marie Curie ENDURE program (European Network for Durable Reinforcement and Rehabilitation Solutions, 607851).
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Volume 23 • Issue 1 • February 2019
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