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Hindawi Publishing CorporationAdvances in Civil EngineeringVolume 2010, Article ID 985843, 19 pagesdoi:10.1155/2010/985843

Review Article

From Material Level to Structural Use of Mineral-BasedComposites—An Overview

Katalin Orosz,1, 2 Thomas Blanksvard,3 Bjorn Taljsten,1, 3 and Gregor Fischer1

1 Department of Civil Engineering, Technical University of Denmark, Brovej, Building 18, 2800 Kgs. Lyngby, Denmark2 Group Material Technology, Norut Narvik Ltd, P.O. Box 250, 8504 Narvik, Norway3 Division of Structural Engineering, Lulea University of Technology, 97187 Lulea, Sweden

Correspondence should be addressed to Katalin Orosz, [email protected]

Received 22 April 2009; Revised 17 January 2010; Accepted 4 March 2010

Academic Editor: Tarun Kant

Copyright © 2010 Katalin Orosz et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper surveys different material combinations and applications in the field of mineral-based strengthening of concretestructures. Focus is placed on mechanical behaviour on material and component levels in different cementitious composites;with the intention of systematically maping the applicable materials and material combinations for mineral-based strengthening.A comprehensive description of a particular strengthening system developed in Sweden and Denmark, denominated as Mineral-based Composites (MBCs), together with tests from composite material properties to structural elements is given. From tests andsurvey it can be concluded that the use of mineral-based strengthening system can be effectively used to increase the load bearingcapacity of the strengthened structure. The paper concludes with suggestions on further development in the field of mineral-basedstrengthening.

1. Introduction

The existing civil engineering infrastructure is a very impo-rtant element of the economical potential of a majority ofthe countries worldwide. A large number of today’s bui-ldings, transportation systems, and utility facilities are builtwith reinforced concrete and many of these systems arecurrently reaching the end of their expected service life.Additionally, increased loads and traffic flows, reuse, andongoing deterioration even affect the durability of structuresthat are less than 20 years old.

There exist several repair and strengthening methods thatcan be applied to existing concrete structures for this pur-pose, such as cross section enlargement of critical elements,span shortening with additional supports, external/internalpost tensioning, and steel plate bonding or strengtheningwith fibre-reinforced polymer (FRP) composites. Since theend of the 1980s, the use of FRP has been researched andapplied increasingly for the rehabilitation of existing concretestructures. Externally epoxy-bonded FRP systems have beenproven to be an effective strengthening method in repairingor strengthening structures and a large amount of literature

is published on this topic; see, for example, [1–7]. FRPreinforcements can be used in numerous ways to strengthena structure, For example, by bonding plates or sheets with ahigh-quality epoxy to the surface of concrete, timber, or evensteel structures. There are also methods to wrap columnsfor enhanced ductility and strengthening systems where FRProds are embedded in the concrete surface. Systems replacingthe epoxy with polymer-modified mortars have been recentlydeveloped. For example, continuous fibre sheets can beembedded in a layer of mortar to provide, for example,confinement to a column; however, lately these sheets havebeen replaced by textiles due to bond issues related todifficulties with penetration as described in [8]. Biaxial ormultidimensional FRP textiles are used in Textile-Reinforcedconcrete (TRC) [9, 10] systems or Textile-Reinforced mortar(TRM) jacketing, see; for example, [11].

In this paper a state-of-the-art report using mineral-based FRP strengthening systems is presented togetherwith information about recent research at Lulea Universityof Technology (LTU), Sweden, and Technical Universityof Denmark (DTU). The literature survey is selective topublished applications in which a fibre component has been

2 Advances in Civil Engineering

Material

Component

Pull out Dog bone

Fibres

Mortar ECC

Strain, deformation

Ten

sile

stre

ss

WST

Confinement

Shear

StructuralFlexure

Figure 1: From material to structural level.

used together with a cementitious bonding agent, referredto as “mineral-based strengthening” in general. The authorsalso involved research and results from the field of ductilecementitious mortars which could be used together with anFRP component resulting in a high-performance strength-ening material. The research significance within this paperis the mapping of possible design of different mineral-basedstrengthening systems. In addition, a systematic mappingof the novel strengthening system named mineral-basedcomposites (MBCs), developed at LTU and DTU, going frommaterial science to composite behaviour and some outlinesfor applications to existing structures are presented.

2. Definition and Development ofMineral-Based Strengthening Systems

“Mineral-based strengthening” in a broader perspectivemay be referred to any kind of a strengthening system inwhich a fibre component is embedded into a mineral-basedbinder to repair or strengthen existing concrete structures.Mineral-based strengthening in this context would includeapplications as surveyed in Section 3 from textile-reinforcedmortar/concrete through fibre-reinforced cement to FRPgrid applications.

Mineral-based strengthening systems are originallyderived from other externally bonded FRP systems. Themost commonly used adhesive to bond the FRP element tothe surface of the structure (mainly concrete) is the epoxyadhesive. The use of epoxy has proven to give excellentforce transfer. It bonds to the majority of surfaces (concrete,steel, timber, etc.) and shows to be durable and resistant tomany different environments. However, some drawbacks canbe identified. Firstly, epoxy as a bonding agent may createproblem in the working environment, secondly, epoxy isrecommended to have a minimum application temperature,often above 10◦C, and thirdly, epoxy creates diffusion-closed(sealed) surfaces which may imply moisture and freeze/thawproblems for concrete structures. There might also be bondproblems applying epoxy to wet or humid surfaces. To avoidsome of these problems alternative strengthening systemshave been researched and are currently being developed.

In mineral-based strengthening systems, the traditionalepoxy bonding agent to adhere the FRP to the concretesurface is being replaced by cementitious matrices to bondthe fibre material to the concrete surface. Mineral-basedstrengthening systems are made by replacing part of thecement hydrate binder of conventional mortar with poly-mers which, with the addition of fibre composites, become ahigh-performance external strengthening system for existingconcrete structures.

3. Concept of Mineral-Based Strengthening

Mineral-based strengthening systems are dealt within threedifferent levels. At the material level, the raw materialsused in a mineral-based composite system such as binders(different quasibrittle or strain hardening mortars), FRPreinforcement (dry fibres, textiles, grids) and the mostimportant properties of those are defined (Figure 1). At thecomponent level, larger strengthened elements are discussed,for example, beams strengthened in flexure and shear. Atthe level of structural behaviour, field applications can bementioned.

The intersection between material and component levelwould contain the different interactions between con-stituents and the effects of those on the structural behaviour(bond transfer mechanisms, fibre bridging and strain hard-ening).

3.1. Materials for Mineral-Based Strengthening. A list ofrequirements is proposed in [11] which should be met fora successful and efficient mineral-based strengthening. Themortar phase should have very low shrinkage deformations,high workability (application should be possible using atrowel, or shotcreted), high viscosity (application shouldnot be problematic on vertical or overhead surfaces), lowrate of workability loss (application of each mortar layershould be possible while the previous one is still in afresh state), and sufficient shear (hence tensile) strength,in order to avoid premature debonding. In case E-glassfibre textiles are used, the mortar-based matrix should beof low alkalinity. Li [12], also adds a few requirements on

Advances in Civil Engineering 3

Concrete structure

Interaction

Mineral based strengtheningsystem

Binders Interaction Fibre composites

Qu

asi-

brit

tle

mor

tars

Stra

inh

arde

nin

gm

orta

rs

Fibr

es

Mat

rix

Figure 2: Overview of the constituents and possible interaction inmineral-based strengthening systems.

the “future concrete” which the authors feel relevant for aconcrete-like repair and strengthening material as well: the“concrete” should be highly ductile with the ability of “yield”like a metal when overloaded to prevent unpredictable andsudden failure, highly durable and sustainable. These justifyinvolving ductile mortars into the MBC system.

Mapping of the materials that have been or are promisingto be used in mineral-based strengthening systems is shownin Figure 2. The integrated materials in the mineral-basedstrengthening systems can be divided in two main groups,binders and fibre composites. The components used inpractical applications are detailed in the following.

3.1.1. Binders. Binders in practical applications are eitherquasibrittle, conventional polymer-modified or more duc-tile, strain hardening mortars.

Quasibrittle Mortars. Polymer-modified mortar (PMM) isthe most widely used [13], suitable mortar in a mineral-based strengthening system. Polymeric admixture, or cementmodifier, is defined as an admixture which consists of apolymeric compound that acts as a main ingredient whenmodifying or improving the properties such as strength,deformation, bond strength, or durability of mortars andconcretes. The polymer-modified mortars or concretestherefore contain two types of binder, the hydraulic cementand the polymeric admixture. Polymeric admixtures can belatexes, powdered emulsions, water soluble polymers andliquid resins. Adding polymeric compounds to the fine-grained mortar phase is also common to enhance mechanicalproperties of Textile-Reinforced concrete (TRC) or mortar.In [14], improved interfacial bond is achieved by a secondarypolymeric cohesive matrix within the mortar phase. Ingeneral, the properties of a polymer-modified mortar (orconcrete) depend significantly on the polymer content orpolymer-cement (P/C) ratio rather than the water-cementratio compared with ordinary cement mortar [15].

Increasing the P/C ratio to about 10%–15% by weighthas shown to increase the flexural strength. A P/C ratio

higher than 15% by weight decreases the mechanical strength[15, 16]. Another source, [17] states that an addition ofpolymeric dispersion up to even 20% by weight results in ahigher tensile strength of the (Textile-Reinforced) concrete.

To further enhance the properties of a PMM, forexample, workability, flowability, mechanical properties, andso forth, of the mortars, superplasticizers, silica fume, fly ashand reinforcing fibres can be used.

Fibres in the matrix can be chopped or milled fibres.The fibres must be easily dispersed in the mixture, musthave suitable mechanical properties, and must be durablein the highly alkaline cement matrix. They are randomly(in a good mix, uniformly) distributed throughout themortar/concrete. Continuous fibres are more effective inincreasing strength in a certain direction compared torandomly distributed short fibres, but they are not easilymixed into the cement matrix and their high cost does notallow them to be widely used. Different types of fibres arecurrently used such as steel, glass, carbon, polyvinyl-alcohol(PVA), polypropylene, nylon and natural fibres dependingon the structural needs. Using a small percentage of carbonfibre addition (∼0.5%), a considerable increase in flexuralstrength is achieved relative to unreinforced mortar [18].Other researchers have investigated new type of fibres, suchas ceramic fibres [19]. The results show that the flexuralstrength of mortar can be increased and also the durabilityof this ceramic fibre-reinforced mortar is much better thanthat of alkali-resistant (AR) glass fibre. The use of steelfibres is the most common solution to enhance toughnessof a steel fibre-reinforced concrete (conventional FRC). Forenhanced ductility, first cracking strength or ultimate tensilestrength, other types of fibres may be more suitable. Theaddition of small volume fractions of synthetic fibres (upto 2%) to the mortar can improve the toughness of themortar [20]. Among these fibres, the polypropylene fibresare very popular in concrete and the nylon fibres are recentlybecoming more widely used [21]. The propylene fibresreinforce the concrete performance under flexure, tension,impact blows and plastic shrinkage cracking. On the otherhand, the nylon fibres improve the performance after thepresence of cracks and sustained high stresses. In [22], bothtypes of fibres were compared and the results showed a betterimprovement of the properties when using the nylon fibresthan the polypropylene fibres. Other researches [23, 24] arefocusing on the use of recycled PET (polyethylene tereph-thalate). PET fibres made from beverage bottles were usedsuccessfully up to 3% in (normal) concrete [23]. Anotherstudy on the durability in aggressive environments of a PET-reinforced concrete [24] emphasizes the sustainability andenvironmentally friendliness of such concretes since the PETfibre has a long decomposition time (over 100 years tocompletely degrade).

Some supplementary materials should be used for coun-teracting the insufficiencies brought about by the addition offibres, such as increase in porosity or decrease in compressivestrength. For example, substituting a part of the cement(20%) by silica fume increases the compressive strength ofthe resulting mortar and provides a reduction of porosity,which leads to an increase of the flexural strength. Adding


silica fume in concretes also increase the interfacial bondstrength and interfacial fracture energy by about 100% dueto its smaller particle size and thereby the ability to increasethe density of the microstructure of a mix [25, 26].

In concrete prepared with ordinary Portland cement,the interfaces between the hydrated cement matrix and theaggregates are the weakest link [27]. The incorporationof industrial by-products such as fly ash in concretes cansignificantly enhance basic properties in both the fresh andhardened states [28, 29]. It is well known that blendingcement with fly ash or other supplementary cementing mate-rials improves the rheological properties of the fresh concreteand the engineering properties of hardened concrete [30–32]. Fly ash in Textile-Reinforced concrete is widely usedto densify the grain structure [33] resulting in an improvedbond between textile and concrete. Superplasticizers can alsobe used to improve consistency and workability.

Strain Hardening Mortars. Engineered Cementitious Com-posites (ECCs) are another type of binder which can beused together with an FRP component [34]. Both in thecase of repair and strengthening, the failure mechanismsthat lead to the need for repair/retrofit of a structure ofteninvolve fracture. This can be overcome by materials withimproved toughness, ultimate tensile strength and ductility,which justifies involving ECC into the FRP strengthening.This micromechanically designed material invented in theearly 1990s represents a particular class of HPFRCC (High-Performance Fibre-reinforced Cementitious Composites),exhibiting strain hardening behaviour and multiple crack-ing during the inelastic deformation process [35]. Sinceits introduction, ECC has undergone major evolution inboth material development and the range of applications.Recently, it is often referred to as SHCC (strain hardeningcementitious composites) due to its tensile (pseudo) strainhardening effect of (steel reinforced) ECC, see more in detaillater, which has been previously documented by Fischer andLi [36].

Besides common ingredients of cementitious compositessuch as cement, sand, fly ash, water and additives, ECCutilizes short, randomly oriented polymeric fibres (e.g.,polyethylene, polyvinyl alcohol) at moderate volume frac-tions (1.5%–2%). In contrast to some other types of fibres,there is a strong chemical bond between PVA and mortarwhich needs to be reduced [37]. This is done by a chemicalcoating applied on the fibre surface and additional fly ash inthe mix in order to prevent premature PVA fibre rupture andthus achieve a more ductile failure mode, characterized bypull-out from the cement matrix.

The tensile strain capacity of ECC is several hundredtimes that of normal concrete [12] and the fracture tough-ness of ECC is similar to that of aluminium alloys [38].Furthermore, the material remains ductile even when subjectto high shear stresses [39]. The compressive strength of ECCranges from 40 to 80 MPa depending on mix composition,the high end similar to that of high strength concrete. ECChas typically an ultimate tensile strength of 5–8 MPa and astrain capacity ranging from 3% to 5%.

There are a number of characteristics of ECC that makeit attractive as a repair material. According to Li [40],the unique feature of ECC is its ultra high ductility. Thisimplies that structural failure by fracture is less likely incomparison to normal concrete or steel fibre-reinforcedconcrete (FRCs). As a consequence, ECC has been usedin a wide range of applications where ductility and/orenergy absorption performance or damage tolerance of thematerial is an important criterion (seismic and nonseismicstructural applications, see, e.g., [41–45]. In contrast toquasibrittle repair materials, ECC can eliminate prematuredelamination of the strengthening layer or surface spallingin an ECC/concrete repaired system [46]. Spall resistanceof ECC in the surroundings of a corroded rebar in slabshas been investigated by Kanda et al. [47], where ECCaccommodated the expansion by a “plastic yielding” processthrough radial microcracking [12]. Interface defects can beabsorbed into the ECC layer and arrested without formingspalls, thus extending the service life [38]. Suthiwarapiraket al. [48] showed that ECC has significantly higher fatigueresistance than that of commonly used repair materialssuch as polymer mortars. It also has a good freeze-thawresistance and restrained shrinkage crack control [41].ECC-reinforced shear beams behaved in a ductile mannereven without additional (steel) shear reinforcement andremained ductile even for short span shear elements whichtypically fail in a brittle manner with normal concrete [49].Under shear, ECC develops multiple cracking with cracksaligned normal to the principal tensile direction. Becausethe tensile behaviour of ECC is ductile, the shear responseis correspondingly ductile. ECC as a strengthening materialalso offers excellent crack control. When an ECC structuralelement is loaded in flexure or shear beyond the elastic range,the inelastic deformation is associated with microcrackingwith continued load carrying capacity across these cracks[40]. The microcrack width is dependent on the type offibre and interface properties (generally less than 100 micronwhen PVA fibre is used). The tight crack width in ECChas advantageous implications on structural durability [12].When used as strengthening or repair material, fine cracksalso prevent penetration of substances [50]. The spacingbetween multiple cracks in a typical ECC is on the order ofseveral mm, while the crack widths are limited to the order of200 μm. In standalone applications, maximum crack widthin ECC is a material property independent of the embeddedreinforcement, unlike in RC or conventional FRC in whichcracks widths are influenced by the steel reinforcement [12].However, crack distribution of ECC as repair material ismore concentrated adjacent to an existing crack in the basesubstrate [12].

“Flowability” or “self-compactability” of ECC refers tothe ability of the material that it can flow under its ownweight and fill the formwork properly. Despite the presenceof (short, chopped, randomly oriented, typically PVA orPE) fibres, a self-compacting ECC is able to fill in eachcorner of the formwork without external vibration required[51]. This would also ensure a good bond to the embeddedreinforcement (such as steel or FRP).


Uniaxial fibresrods/sheets

Textile fabrics2- or 3D

Grids2- or 3D

C

L

Figure 3: Different types of fibre composites.

3.1.2. Fibre Composites. Commonly used FRP reinforce-ments are shown in Figure 3. FRP rods are typically usedas internal reinforcement or for near-surface mountedreinforcement (NSMR). Other pultruded profiles used arethin composite plates with a typical cross section of 1.4 ×80 mm; however none of these systems will be detailedwithin this paper. Uniaxial fibres or fibre sheets, textiles andgrids used together with mineral-based bonding agents areother types of FRP reinforcement systems, these systems aredescribed more in detail below.

Fabrics and Textiles (Knitted, Woven, or Unwoven). Differ-ent kinds of fibres used as FRP reinforcement and theirmechanical properties are detailed, for example, in [52, 53].There exist a few continuous long fibre applications, wherenonimpregnated unidirectional fibres may be embedded in amortar phase [21], but the impregnation of continuous fibresheets with mortars is very difficult resulting in a poor bondbetween fibre and mortar [8]. Fibres used in most mineral-based strengthening systems usually come in the form of atextile or fabric (woven or knitted) or grid (with rigid joints),where the latter is impregnated and held together by an epoxyresin which also shapes the composite.

Fabrics are unidirectional or multidirectional FRP com-posites. They are widely used in Textile-Reinforced concretesystems. The most commonly used textiles are the bidirec-tional woven fabrics, in which the yarns are woven togethercrossing them orthogonally, having a width of 300–600 mm,in large spools cut at the workplace. The density of theyarns directly affects the penetrability of the adhesive intothe fabric, higher density leads to lower penetrability asthe mechanical interlock acts through the fabric openings[11]. Woven fabrics are easy to adapt to any surface whichmakes them very suitable for wrapping (U-warp or fullwrap) or confinement. The flexible shape also representssome disadvantages. The reinforcing efficiency is lowerthan that in straight yarns due to the crimped geometryof the woven fibres; however, this crimped geometry isadvantageous in providing mechanical anchoring betweenthe woven fabric and the cement matrix in a polymer-modified mortar. Fabrics having relatively complicated yarn

shapes may enhance the bonding and improve the compositeperformance [11]. However, very complex textile geometriesmay cause premature failure of some of the yarns resulting inlower strength than the nominal strength of the fibrous phasewould be, as reported in [54].

Bi- and Triaxial Grids with Rigid Joints. An FRP grid isa multidirectional prefabricated composite. For externallybonded reinforcement, grids are made of continuous fibresalternating in two directions and impregnated with a resinto form a 2D-cross laminate grid structure with rigid joints.The resin is usually epoxy and its role is to hold the fibrestogether and shaping the composite. The fibre amount andgrid spacing often vary in the two perpendicular directions.FRP grids are typically produced in large rolls and thencut into the required dimensions. Compared to wovenfabrics, FRP grids have improved mechanical propertiesand in general, have more rigid connection points. Theimproved performance is due to the aligned fibres in a certain(predefined) direction and that there are no unpenetratedfibres or fibre bundles with would prematurely fail (furtherdiscussed in Section 4.2 under TRC). Thinner grids areflexible and can be adapted to curved surfaces.

4. Existing Mineral-BasedStrengthening Systems

4.1. Continuous Dry Fibres. The use of continuous dry fibresis published in [55] where a pretreatment with silica fumeand high amounts of polymers improved the bond betweencarbon fibres and cementitious matrices. It was found thata pretreatment with silica fume and relatively high amountsof polymers improved the bond behaviour of carbon fibreto the cement. Another report [56] has been published onlarge-scale tests of ordinary concrete beams strengthenedwith a cementitious fibre composite where the strengtheningcomposite consisted of a unidirectional sheet of continuousdry carbon fibres and a polymer-modified mortar, applied byhand lay-up method. Both flexural and shear strengtheningwere investigated. From the tests it was concluded that themethod works and that considerable strengthening effectscan be achieved. However, in comparison with epoxy-bonded carbon fibre sheets using the same amount ofcarbon fibre, the strengthening effect for the mineral-basedcomposite strengthening system was approximately half. Thereason for this is most likely due to the reduced interfacialbond strength between the mortar and the carbon fibre-reinforcement. This is also emphasized by [57], where it wasfound that the loading capacity of the cementitious carbonfibre composite is influenced by the amount of fibres in thetow. If the cementitious matrix penetrated into the interior ofthe carbon fibre tow, a higher number of filaments would beactive during loading, leading to the increase in load carryingcapacity.

Another way of using continuous dry fibres, usually inthe form of sheets is the fibre-reinforced composites (FRCs)strengthening system (which may also use dry fabrics), asdescribed by, for example, [58]. Depending on the geometryof the fibres and the strengthening purpose, the composite


plates can be made as thin as 2 mm. The sheet or fabric is cutinto chosen dimensions and the fibre geometry is submergedinto cement slurry (matrix) for a better penetration. Theimpregnated sheet or fabric is then removed from theslurry and immediately bonded to the concrete surface. Animproved penetration of the matrix into the fibres can beachieved by using multiple layers of continuous carbon fibreswhere all single layers were impregnated in cementitiousslurry before bonded to the concrete surface. This procedurewas also suggested in [51]; however not executed.

4.2. Textile-Reinforced Mortar (TRM) or Textile-ReinforcedConcrete (TRC). Textile-Reinforced Concrete (TRC) is madefrom a cementitious matrix and layers of oriented, contin-uous fibres (technical textiles) as for reinforcement. HereTRC is only discussed as a strengthening material andnot as a material also suitable for stand-alone applicationssuch as thin shells. The most commonly used bondingagent for applying the textile on the structure is the finegrained concrete or mineral-based mortar which has a grainsize of less than 1 mm and provides high strength andflowable consistencies. Less than 2 mm of concrete or mortarthickness is needed between textile layers due to the smallmaximum aggregate size of the concrete mix [9]. Alkali-resistant glass (AR-glass) or carbon is used most often as thefibrous material. The fibres can be in the form of a woventextile or be unidirectional and held together by a yarn inthe perpendicular direction in order to make the materialeasier to work with. The fabrics can be manufactured with amaximum number of four reinforcing directions dependingon the load direction [9] and can therefore be tailored (thefilaments are intentionally aligned in the direction of thetensile stresses leading to an increase in their effectiveness).There are several designs of textile fabrics depending on theload case and the positioning of the fabrics. Most fabrics arebi- or multiaxial warp knits and woven fabrics since theyoffer a great flexibility of properties and are suited for manycases.

In a TRC strengthening system, the transfer of the bondforces from one layer to another through the bonding jointmust be guaranteed to ensure a full composite action. Thebond behaviour of TRC is a complex property that dependson both the textile and the matrix. Due to the reinforcementgeometry, only the outer filaments are directly in contactwith the concrete, hence the load bearing capacity of a TRCdepends mainly on the proportion of the outer to the innerfilaments, and just a certain part of the tensile strength ofthe concrete can be activated due to the limited contactsurface [14, 59]. Significant improvement in load capacitycan be achieved if the textile is previously penetrated withliquid polymers [17, 59] or epoxy- resins [17]. However,there is a risk that in an impregnated textile, a higher numberof activated filaments would lead to abrupt failure of allfilaments when the tensile strength of those is exceeded [17].The same study also highlights that the tensile strength of aTRC (dogbone) specimen can be increased most effectivelyby both impregnating the textiles with polymers and addingpolymer modifiers to the concrete.

TRC systems exhibiting mechanisms of distributedcracking and strain hardening behaviour (see Section 5) havebeen used for stand-alone structural applications such asextremely thin and slender structures [60], or for repairand strengthening of existing structural members [9, 10].Confinement strengthening of short columns both withepoxy and cement-based mortars has been investigated by[8]. Effectiveness of the TRM jacketing compared to theepoxy-impregnated counterparts resulted in a reduced effec-tiveness (80% for strength and 50% for ultimate strain). Thestrengthening effect increased with the number of confininglayers and was highly dependent on the tensile strength ofthe applied cement mortar. Failure was more ductile in caseof the TRM jacketing than in the epoxy-bonded jackets dueto the slowly progressing fracture of individual fibre bundles.Shear strengthening of concrete beams has been examinedin [61] on six identical RC beams with different bondingagents (epoxy and cement matrix), one or two layers oftextile in different arrangements (conventional wrapping andspirally applied textiles). Two layers of mortar-impregnatedTextile-Reinforcement in the form of either conventionaljackets or spirally applied strips were sufficient to increasethe shear capacity of the beams tested more than 100% overthe unstrengthened beam, preventing sudden shear failuresand allowing activation of flexural yielding (as was the casewith the resin-based jacket). The TRM system; however,was found to be about 50% less effective than the FRP-strengthened beam. Other experiments [9, 10, 62] have beencarried out on T-beams which were designed with minimumshear reinforcement but a great amount of longitudinalflexural reinforcement in order to prevent flexural failure.The strengthening fabric was a multiaxial textile appliedas a U-wrap in an angle of 45◦ to the load direction,aligned with the principal stresses in the web of the T-beamto be strengthened. Varying parameters were the numberof textile layers (2–6) placed with or without mechanicalanchorage in the compression zone. The adhesive tensileload carrying capacity is crucial in the performance of thesystem—this determines how many layers can be anchored tothe web without any additional mechanical anchorage. Testsrevealed the importance of mechanically anchoring of thereinforcement. A load increase could be achieved with a fewlayers of textile even without anchorage to the compressivezone; however, when the adhesive tensile bond between theweb and the reinforcement was exceeded, the TRC layerspeeled away and therefore the reinforcement fails.

4.3. Mineral-Based Composites (MBCs). MBCs (mineral-based composites) are defined as a system in which aFRP grid is applied onto the surface of the structure tobe strengthened with a cement-based mortar as bondingmatrix. MBC “inherits” the properties and behaviour of itsconstituents, for example, brittleness or tension softeningbehaviour when the linear elastic fibre material is usedtogether with quasibrittle cementitious binders. However,its behaviour can significantly be changed/altered and thematerial can be enforced to exhibit more ductility, multiplecracking or strain hardening in tension in a modified


MBC where the mortar is a PVA-reinforced engineeredcementitious composite (ECC) [63, 64]. In the recent devel-opment of MBC, two strengthening approaches meet andresult in a high-performance hybrid strengthening material.One is finding the most suitable combinations of a FRPreinforcement and an existing mortar (which, dependingon the application may be ordinary or polymer-modifiedmortar), and the other one is to strengthen/develop themortar itself with different short fibres (in practice, mostlyPVA) or involve existing, ductile mortars in order to improvethe interaction between grid and mortar, tensile properties ofthe composite, crack spacing and crack widths, ductility andalso if possible, fracture energy.

The FRP in MBC applications is normally a two-dimensional carbon fibre grid. As matrix, polymer-modifiedquasibrittle mortars (PMMs) are typically used. Between thetwo components little or no chemical bond exists. Thereforethe bond strongly relies on mechanical bond, contraryto epoxy-based FRP systems. The precut CFRP grid withvarying grid spacing and thickness is embedded between tworelatively thin layers of mortar.

To achieve a good bond between the base concrete andthe mortar, the surface of the base concrete needs to beroughened, for example, by sandblasting or water jetting, inorder to remove the cement laitance. The surface preparationmethod normally is sandblasting. The strengthening layerscan be applied by hand (hand lay-up) or shotcretingwhen strengthening large surfaces. The hand lay-up methodincludes prewetting the base concrete with water for 1–3days depending on the conditions of the base concrete andthe surrounding climate. The moisture conditions in thetransition zone between the base concrete and mortar arefurther discussed in [65], where it is found that the best bondis obtained when the base concrete has just dried back froma saturated surface. Prior to mounting the MBC system thebase concrete surface has to be primed using a silt-up product(primer) to prevent moisture transport from the wet mortarto the base concrete. A first layer of mortar is immediatelyapplied to the primed surface. Next, the CFRP grid is placedon the first layer of mortar followed by an additional layer ofmortar being applied on top of the grid. The thickness of themortar depends on the maximum grain size in the mortar.

Reliability of the system highly depends on the bondin two interface layers, between the concrete substrate andthe first layer of mortar and in the between the grid in themortar; see [66].

5. Mineral-Based Strengthening atMaterial Level

On the interface between the base concrete and the strength-ening layer interface, such as between the fibre compositeand the mortar, bond is crucial for the performance ofthe strengthening system. In a TRC or MBC system, bondbetween FRP and mortar relies mostly on mechanicalinterlocking (physical bond). If the mortar contains PVAfibres, a chemical bond is also being built up within themortar phase. Good (but not overly strong) bond togetherwith the deformation compatibility of the mortar and

PVA fibres, results in fibre bridging and as a consequence,multiple cracking and overall strain hardening, resulting in a“stronger” and more ductile mortar. These mechanisms aredetailed below.

5.1. Bond between Base Concrete and Strengthening Layer.When an existing RC element is to be strengthened, thetensile force developed in the strengthening layer must beintroduced into the original concrete of the member by bondforces. Failure can initiate from an interfacial defect causingdelamination of the whole strengthening layer in the case ofa “weak” bond and spalling in the case of an overly “strong”bond [35].

Bond strength in a mineral-based system generallydepends on the adhesion in the interface, friction, aggregateinterlocking and possibly, time-dependent factors [52].

The sensitivity of a structure to bond strength dependshighly on what kind of load it is subjected to. The forcetransmission into the original concrete when strengtheningfor flexure occurs over a relatively large length (surface). Incontrast, the force transmission via shear strengthening takesplace only in the range behind the last shear crack [9]. Asa result, significantly shorter anchorage lengths and “betterbond”, and in some cases, additional anchoring is requiredfor shear strengthening. An example is a TRC-reinforced T-beam strengthened for shear [9] where if the U-wrap aroundits web is not anchored into the slab via an L-shaped steelsection the TRC reinforcement has a tendency to peel awayfrom the web.

Incompatibility between the base concrete and the repairmaterial in repair and/or strengthening systems can lead tothe same bond problem and render the system ineffectiveresulting in debonding or spalling of the strengthening layerespecially if the load is applied parallel to the bond line of thecombined system. If there is an incompatibility in the elasticmodulus of the base concrete and the mortar, it will result inthe incompatibility of the deformations on the interface asthe “weaker” material will have larger deformations [67]. Ifthe load bearing capacity of the material or the bond at thetransition zone is exceeded by the transferred load, failurewill occur. Recommendations for the modulus of elasticityof the repair or strengthening material suggest a modulusat least 30% larger than the modulus of elasticity for thebase concrete [68]. When the bonding agent tensile loadbearing capacity is exceeded in the interface between thebase concrete and the strengthening layer, the strengtheningwould peel off the structure. To prevent debonding, it isusual to prescribe a minimum adhesive tensile strengthof the original concrete substrate which, for example, inGerman codes is 1.5 N/mm2 [9]. If the surface is roughenedadequately, by means of removing the deteriorated partsand sandblasting, a good bond can be guaranteed. Anotherparameter that influences the compatibility of the combinedrepair or strengthening system is the drying shrinkage. Asthe fresh repair or strengthening material has a tendencyto shrink, the more or less older base concrete acts asa rigid foundation that restrains these movements. Thesedifferential movements cause tensile stresses in the repair orstrengthening material and compressive stresses in the base


Strain hardening,multiple cracking Localisation

HPFRCC

FRC

Concrete

Strain, deformation

Ten

sile

stre

ss

Short fibre reinforced mortar/concrete [70]

(a)

Crackingformation Stabilised cracking

TRC

Textile

Failure

Strain, deformation

Ten

sile

stre

ss

Textile reinforced mortar/concrete [73]

(b)

Figure 4: (a) Strain hardening and strain softening mortars. (b) Tensile behaviour of TRC.

concrete. Creep in this context can be an advantage sinceit releases parts of these stresses. Shrinkage incompatibilityis more associated with cement-based mortars while thepolymer-modified mortars (PMC) show better shrinkagecompatibility and epoxy mortars proved to have the bestshrinkage compatibility [68].

5.2. Bond of FRP in a Mortar. The mineral-based strength-ening systems are strongly dependent on the bond betweenfibre composite and mineral-based binder. Using a non-impregnated textile fabric with a high density (high fibrefilament amount) will make it hard for the mineral-basedbinder to penetrate. This type of inferior bond or penetrationof textile fabrics has been the focus for researchers developingthe TRC system [8, 11, 14]. Using an epoxy-impregnatedCFRP grid, as in the MBC system, relies on physical bondbetween CFRP and mortar. This physical bond can originatefrom direct mechanical anchorage (joints in the grid,transversal tows of the grid), frictional forces on the interface,aggregate or fibre interlock (if the mortar contains longerfibres) but there is no chemical bond between the FRP andthe mortar as in an epoxy-bonded system there would be.

When using strain hardening mortars, there is a chemicalbond between the mortar fine-grain aggregates and thePVA fibres which affects bonding of the embedded FRPreinforcement in the mortar. PVA has a high chemicalbond and reaches its peak load at relatively small pulloutlength. During the extraction process, it has a tendency tobreak instead of pulling out from the mortar. In order toincrease the opening of an individual crack and enhance thecomposite stress-strain behaviour of PVA-ECC, the chemicaland frictional bond of the PVA fibre is to be decreased, forexample, by means of particular surface treatment (coating)[38] or by modification of the fibre/matrix interface transi-tion zone. Bond can be lessened, for example, by adding flyash to the mix.

Bond characteristics influence the mechanism of loadtransfer between reinforcement and concrete, and thereforecontrol the concrete (or mortar) crack spacing, crack width,required concrete cover to the reinforcement, and the rein-forcement development length. The behaviour of strength-ened concrete structures thus depends on the integrity ofthe bond. Most literature to date has been published aboutbond of FRP bars in concrete or other cementitious matrices.Very little literature is found on the bond of FRP grids to acementitious material which is more complex because of thegeometry of the grid, including the formation of the joints.

5.3. Multiple Cracking—Crack Control through Strain harden-ing Materials. The deformation behaviour of cementitiouscomposites (concrete, fibre-reinforced concrete) and highperformance fibre-reinforced cement composites (HPFR-CCs) is typically distinguished according to their tensilestress-strain characteristics and postcracking response [69].In [70], it is shown that brittle matrices, such as plain mortarand concrete, lose their tensile load-carrying capacity almostimmediately after formation of the first crack as illustrated inFigure 4.

The addition of fibres in conventional fibre-reinforcedconcrete (FRCs) can increase the toughness of cementitiousmatrices significantly; however, their tensile strength andespecially strain capacity and ductility beyond first crackingare not enhanced [71]. FRC is therefore considered to be aquasibrittle material with tension softening behaviour, thatis, a decaying load and immediate localization of compositedeformation at first cracking in the FRC matrix. In contrast,strain hardening fibre-reinforced cementitious composites(SHCCs), which is a particular class of HPFRCC (highperformance fibre-reinforced cementitious composites), aredefined by an ultimate strength higher than their firstcracking strength and the formation of multiple cracking


during the inelastic deformation process. Under tension,it initially exhibits multiple cracking with associated overallhardening, which later changes to softening as fracture local-izes [72]. The crack continues to open, and the embeddedfibres are pulled out of the matrix (or break, depending onwhat matrix/fibre composition is used), which still generatessome resistance against crack opening within the tensilesoftening regime. When all fibres are completely pulled out(or broken), no resistance is left, that is, the tensile stress isequal to zero.

The contribution of a cementitious matrix to the load-deformation response of reinforced concrete or ECC is theso-called “strain hardening” or “plastic hardening”. Tensilesoftening versus tensile hardening behaviour is illustratedin Figure 4(a). Textile-Reinforced concrete exhibits a similarhardening behaviour [73, 74]; however the cracking processis clearly not as uniform as in an ECC (Figure 4(b)). Inthe precracking regime (Stage I), the stresses are distributeduntil the first cracking strength of the concrete is reached.First cracking occurs at the point where the ultimatetensile strength of the concrete is reached and the FRPreinforcement will carry the entire applied load in thevicinity of the transverse crack location. The load carriedby the concrete in the uncracked segments is transferred tothe FRP reinforcement at the crack location via interfacialbond. After first crack formation (Stage IIa, “crackingformation”), the load-deformation curve shows a smallincrease in loading capacity as compared to that in StageI due to the formation of additional transverse cracking.The member is softened by the formation of additionalcrack(s) and the load increase per deformation increment isdecreasing with each crack until Stage IIb when a stabilizedcrack pattern is reached (postcracking regime). When thefinal crack pattern is being formed, the slope of the stressstrain relationship becomes steeper again, which mainlyresults from the Young’s modulus of the fibre-reinforcement.Crack widths are also governed by the FRP reinforcementand the bond characteristics to the surrounding concretematrix. At this stage of a stabilized crack pattern (Stage III)no further cracking occurs. Here, mainly the FRPs determinethe stiffness of the member. Stage III is characterized eitherby slippage in the fibre tows instead of yielding of steelreinforcement, or by linear elastic deformations until theFRP ruptures in a brittle manner upon reaching its tensilefailure strain, as detailed in [75]. In practice, final failurenormally is a combination of fibre slippage and rupture [73].

Due to the more homogeneous nature (as a consequenceof randomly distributed, well mixed short fibres) of ECC,there is a gradually increasing part of the curve smootherthan for TRC because of the fibre bridging characteristicsof the PVA fibres compared to those of a textile mesh. InECC, there is no definite distinction between stages IIa andIIb (crack saturation) [36]. The (pseudo)strain-hardeningbehaviour in ECC is achieved by sequential development ofmatrix multiple cracking [76]. “Multiple cracking” meansthat under tension, cracks are consecutively formed after firstcracking, and become evenly and closely spaced. Deforma-tion then is often expressed in terms of strain instead ofcrack opening displacement. Multiple cracking results in the

improvement in “ductility, toughness, fracture energy, strainhardening, strain capacity and deformation capacity undertension, compression or bending” [77].

The strain hardening ensures that a structural elementmade of such a composite material will maintain its stabilityalso after cracking. This is a significant difference betweenall conventional FRC and SHCC materials as shown inFigure 4. Deformation of ECC is uniform on a macroscaleand considered as pseudostrain, which is a material property[36], in contrast to other reinforced concrete-like materialssuch as conventional FRC. As a consequence of the strainhardening, the tensile strain capacity and ultimate strengthin the cracked state of ECC are much higher than that ofconventional (steel) fibre-reinforced concrete.

5.4. Fibre Bridging. Since the overall hardening behaviourhighly relies on fibre bridging, it is important to knowhow high bridging stresses arise in the fibres. This wouldalso limit and determine which materials are usable asreinforcing fibres. For very flexible fibres which have a highstrain capacity (polymeric fibres), a snubbing effect (fibredowel action arising when a fibre is not loaded in thedirection of the initial straight fibre) may occur reducingthe fibre critical length leading to premature failure. Forelastoplastic fibres (steel), local yielding can happen, as forelastic-brittle fibres (carbon, glass) fibre failure may happendue to bending of fibres [39]. The fibre bridging analysisis about relating the single fibre bridging stress to thecrack opening which is a function of fibre and interfaceproperties and mode of failure [78]. Fibre properties mayinclude the fibre modulus, ultimate tensile strength, fibrelength, diameter, and the interface properties are the cohesivestrength, fracture properties and frictional properties. Themicromechanics of fibre bridging is described more in detailby [79]. In a composite, the embedded fibres in the mortarwould often bridge a crack at an angle. Inclined fibre pulloutwould depend on a greater number of parameters, involvingthe inclination angle of the fibre to the matrix crack plane,local fibre/matrix interaction properties and possible agingeffects (interfacial properties and fibre/matrix interactionchanging with time) [79].

Fibre bridging after first cracking of the cement matrix iswell detectable in PVA-reinforced mortars. For MBC systems,the mechanism is important because with adequately chosenmortars, a MBC system can be altered/improved resultingin a more efficient strengthening material under tension orshear.

6. Testing MBC Systems

As previously shown in Figure 1, the three (material, com-ponent and structural) levels of mineral-based strengtheningmust be linked together. It is not within the scope ofthis paper to generalize about all existing mineral-basedstrengthening systems; however linking can be done and ispresented for mineral-based composites through experimen-tal investigations carried out at LTU and DTU.

Mineral-based composites are, as defined earlier, aparticular instance of mineral-based strengthening in which


137.8 400

980

69.5 20

160

160α

(a) Uniaxial tension

500 100 500 20 100

500

428

α

(b) Wedge splitting test

P PSGφ12 s100 φ10 s50

2φ16 Ks50012φ16 Ks500

250 1250 750 750 s s

4000

180

500

(d) Shear strengthening

3980

1000

10 φ 8 Ks500

Reference slab

Additional steelreinforcementslab no. 2

14 φ 8 Ks500

MBC strengtheningslab no. 3, 4 and 6

Epoxy bonded sheetsslab no. 5

1333 1334P P

386010 φ 8 Ks500

100

(c) Flexural strengthening

Figure 5: Experimental tests for tensile and fracture behaviour: (a) uniaxial tensile test on dogbones. (b) wedge-splitting test, (c) flexuralbeams, and (d) shear beams.

FRP grids are adhered to the concrete substrate by meansof (quasibrittle or strain hardening) mortars. By usingECC as the mortar phase in MBC, different force transfermechanisms come into play which did not exist in the initialphase of MBC research where the mortar was a quasibrittlePMM. In addition to that, MBC research has mostly focusedon beams strengthened in flexure and shear, and much lessattention has been paid to repairing structural members inwhich significant tensile forces may emerge. Before applyingthe MBC as a strengthening material in conditions where thestructural member is subject to tensional or splitting forces(or bending combined with axial forces), it is necessary toproperly characterize the tensile behaviour. Knowledge ofrelated parameters such as the tensile strain-crack openingcurve makes possible estimations, for example, about brittle-ness in compression and tension or shear capacity. Suitabletest methods are being searched, which quantify the tensileproperties of MBC directly (dogbone tests) or indirectly(WST, giving the splitting tensile strength).

This section will give insight in the MBC behaviourfor tension, flexure and shear. Tests have been performedon the tensile behaviour of the MBC system, dogbone andwedge splitting tests (WST), along with flexural and shearstrengthening, see Figure 5 where all tests are shown.

6.1. Experimental Set-Up. A summary of tests conductedat LTU and DTU is shown in Figure 5. Uniaxial tensileand wedge-splitting tests aim to characterize the tensileand fracture properties of the strengthening material. Shear

and flexural strengthening (Figure 8) involving MBC leadtowards the structural level and applications as outlined inFigure 1. Short descriptions of each test are summarizedbelow, and further references are given to a more detaileddescription of each.

6.2. Material Properties. All the material properties for thematerials used in the four different tests are summarized inTable 1 for the CFRP grids and Table 2 for the binders.

6.3. Linking Material and Component Levels

6.3.1. Uniaxial Tensile Tests. New test method and test spec-imens have been developed for testing MBC under uniaxialtension in [64]. The bare strengthening material was tested,without considering the interaction with a real structureto be strengthened. Quasibrittle polymer-modified mortarsand ECC have been investigated together with an embeddedCFRP grid in the mid-plane of the dogbone-shaped speci-mens, focusing on crack development and the influence ofthe applied mortar’s ductility on the behaviour in tension.

Dimensions of the dogbone-shaped test specimens were160 × 160 × 980 mm, with a representative test sectionof 400 × 160 × 20 mm. Two different grids with differentgrid spacings and three chosen grid orientations (0◦—longitudinally placed grid, 90◦—transversally placed grid,15◦—rotated grid) were tested in different combinations,more in detail presented in [64]. The number of the CFRPtows in the representative test section, tensile direction, were


Table 1: Mechanical properties for the fibres in each tow of the CFRP grids used in experimental tests [53].

Material DirectionTow spacing Tensile strength E modulus Failure strain

[mm] [MPa] [GPa] [‰]

Grid S Transverse 25 5214 366 14.0

Grid S Longitudinal 24 3546 278 12.5

Grid M Transverse 43 4620 389 11.9

Grid M Longitudinal 42 4219 404 10.5

Grid L Transverse 72 3772 423 8.6

Grid L Longitudinal 70 3877 320 12.8

Table 2: Material properties for binders used in experimental tests.

Material DescriptionTensile strength Compressive strength

[MPa] [MPa]

PMM Polymer-modified mortar 2.4 53.2

ECC Engineered cementitious composite 3.0 60.0

7 (small grid) and 4 (medium grid). The specimens werefixed in self-centering, custom-made clamps.

In Figure 6(a), quasibrittle and ductile mortars arecompared and related to the linear elastic behaviour of thepure CFRP grid. Behaviour of all specimens was nearlyidentical up to first cracking irrespective of which mortar isused. After first cracking, ECC specimens show a significantstrain hardening behaviour with both small (S) and medium(M) grids which is represented as the contribution of themortar compared to the shifted lines of the pure grid. TheECC specimens, after the first crack, show a further loadincrease until the peak load. These curves are smooth dueto the fibre-bridging characteristics of the cracks in ECC.Curves of quasibrittle mortars are more jagged and have asignificant drop in load carrying capacity right after the firstcrack (and after further crack formation). This fact showsthat the increased ductility of the ECC mortar has a positiveeffect on the interaction with the CFRP through the fibrebridging effect. It seems to prevent premature failure of theCFRP grid which was observed in the specimens with brittlemortars, with significantly reduced axial tensile strength.

Figure 6(b) illustrates the effect of the grid orientation.The apparent better performance of the grid when rotatedtransversally may be due to an epoxy surplus on thetransversal tows resulting in improved mechanical anchorageor, more likely, due to the joint shape (between longitudinaland transversal tows of the grid) and associated deformationcapacity of the grid joints when loaded in a certain direction.

6.3.2. Wedge Splitting Tests. Brittleness of concrete-like mate-rials is usually evaluated by means of the post failurebehaviour in tension governed by the stress versus crackwidth relation (σ-w), the so-called softening behaviourwhich is a basic property of a (conventional) concretedescribed by the tensile strength, the maximum crack widthand the fracture energy, which corresponds to the area underthe stress versus crack width curve. WST (wedge splittingtest), originally introduced by [80] and further improved

by [81] is a suitable method, traditionally used for brittlematerials, for obtaining the splitting tensile strength alongwith the postpeak behaviour, and an estimation on thefracture energy.

WST is an extension of the uniaxial test because theMBC-strengthening is applied on a concrete surface. In arecently published study [63], behaviour of a CFRP gridreinforcement in two different directions (0◦ and 45◦, resp.)together with quasibrittle and ductile mortars has beeninvestigated and evaluated through recording and comparingcrack widths, crack patterns, splitting load versus crackopening displacement curves and fracture energy valuesfor the MBC-strengthened concrete specimens. Externaldimensions of the plain concrete test specimen were 500 ×500 × 100 mm, after strengthening with a representative testsection of 400 × 420 × 140 mm where 140 mm is the totalthickness after strengthening on both sides. The length ofthe starter notch together with the groove is 50 mm. Thebottom groove served to ease proper vertical positioning andacted as a line support for the heavy specimens. The CFRPgrid was applied on both sides (ten tows bridging the crackon both sides, in the nonrotated position), placed betweentwo 10 mm thick layers of mortar, applied on the concretesurface roughened with sandblasting and treated with aprimer to enhance bond on the concrete-mortar interface.Tests were run deformation-controlled, that is, through thecrack opening displacement kept constant at 0.3 mm/min,by a clip gauge mounted at the tip of the notch in themidplane of the specimen. During the test, vertical load andcrack opening displacement (COD) are recorded, hence thesplitting load can be calculated.

The resulting splitting force versus crack opening dis-placement plots show that the splitting tensile strengthof concrete can be significantly improved with MBC-strengthening, resulting in an increase of 30–110% in peaksplitting load compared to the unstrengthened referencespecimen (Figure 7). ECC has given significantly higherfailure loads and similarly to the uniaxial tests, it preventedpronounced drops in load capacity. Ductility is significantly


0 1 2 3

Displacement (mm)

0

4

8

12

16

Load

(kN

)

ECC: Grid MECC: Grid SPMM: Grid M

PMM: Grid SGrid MGrid S

(a)

0 1 2 3

Displacement (mm)

0

4

8

12

16

Load

(kN

)

PMM: Grid M: α = 90◦



(b)

Figure 6: (a) All dogbone specimens with grid S and M placed in longitudinal (0◦) direction. (b) Effect of grid orientation (transverse (90◦),longitudinal (0◦) and rotated in 15◦).

0 5 10 15 20 25

Crack opening displacement (mm)

0

20

40

60

Split

tin

glo

ad(k

N)

ECC, α = 0◦

ECC, α = 45◦

Reference

(a)

0 5 10 15 20 25

Crack opening displacement (mm)

0

20

40

60

Split

tin

glo

ad(k

N)

PMM: α = 0◦

PMM: α = 45◦

Reference

(b)

Figure 7: Splitting force versus COD curves for specimens with highest peak load per series, against reference specimen (plain concrete).

enhanced by all grid-mortar combinations resulting inan extreme increase of fracture energy which is directlyproportional to the area under the curves.

Bond provided by both mortars was excellent leading toCFRP rupture. By applying PVA-reinforced ductile ECC asbonding agent, improved performance, significantly higherfracture energy, multiple cracking and enhanced ductilitywere observed, caused by improved bond between grid andmortar due to the refined grain structure, the bridging effectof the embedded fibres working against crack opening andvia direct mechanical interlock with the grid.

Compared to a uniaxial tensile test, WST also hasprovided information on the softening part of the curve afterpeak load, revealing tendencies in how fast or how graduallyload capacity decreases until all crack bridging fibre tows

and embedded fibres in the mortar are broken. The slopesof the Fs versus COD curves reveal that the tensile ductility(recorded peak load and its tendency to decrease to zero)depends more on the grid orientation than on the mortarquality, unlike the increase in the energy absorption/fractureenergy which seems to be “mortar-dependent” and issignificantly higher for the ECC specimens.

6.4. Linking Component and Structural Levels

6.4.1. Slabs Strengthened in Flexure. The test set-up andgeometry for the slab specimens is shown in Figure 5 anda more detailed overview and results are also published in[82, 83]. Three of the slabs were strengthened using the MBCsystem, one specimen with one CFRP grid, one specimens


0 50 100 150 200

Mid-span displacement, d (mm)

0

10

20

30

40

50

60

Tota

lloa

d,P

(kN

)

No.1. ReferenceNo.2. Extra steelNo.3. NSMG 1 layer, sandedNo.4. NSMG 1 layer, no sandNo.5. CompositeNo.6. NSMG 2 layers, no sand

No.1

No.2

No.3

No.4

No.5

No.6

Figure 8: Load versus mid-span displacement of flexural strength-ened specimens.

with a sanded CFRP for better mechanical anchorage andone specimen with double CFRP grid. In addition onespecimen was strengthened using epoxy-bonded carbonfibre sheets and yet one specimen containing four extraflexural steel reinforcement bars, the extra reinforcement wascalculated to correspond to the strengthening. Thus, all ofthe specimens were designed to have similar failure loads,except the specimen with dual CFRP grid. The thicknessof the strengthening layer was approximately 10 mm anda quasibrittle mortar was used as binder. In all MBC-strengthened specimens the CFRP grid had a fibre content of159 g/m2 (Grid M). For the epoxy-based strengthening, thecarbon fibre sheets had a fibre content of 200 g/m2. The totalcarbon fibre amount in the tensile direction of the specimenscorresponded to 20 mm2/m (CFRP grid) and 62 mm2/m(carbon fibre sheet) respectively.

All of the strengthened slabs increased the ultimatebearing capacity in comparison to the nonstrengthenedreference beam. Strengthened specimens using one layer ofCFRP grid failed at a total load of 35 kN and specimenwith epoxy-bonded sheet failed at 41 kN while showinga stiffer behaviour compared to the latter. The specimencontaining four extra flexural bars failed at a load of 38 kN,similar loads to the specimens with only one grid. However,the specimen with extra steel reinforcement showed stifferresponse compared to both the specimens strengthened withsheets and one CFRP grid. It is also noted that sanding thesurface of the grid (specimen No. 3) led to premature ruptureof the CFRP grid. Strengthening the specimen with duallayer of CFRP grid will have a positive effect on the loadbearing capacity. Thus this specimen (No. 6) had the highestultimate load capacity of 51 kN. All of the specimens with the

MBC system failed by fibre rupture while the epoxy-basedstrengthening system failed by a mix mode of debondingand fibre rupture. Comparative load-displacement curves areplotted in Figure 8.

6.4.2. Beams Strengthened in Shear. Beams strengthened inshear had a rectangular cross section and were 4.5 m long andhad a height of 0.5 m. Again, these results are described inmore detail in [53, 66]. Note, in Figure 8, that the beams wereheavily shear reinforced in one shear span and had inferior tono shear reinforcement in the other shear span. Thus, onlythe shear span containing no or little shear reinforcementwas strengthened. In these experiments, the influence ofdifferent grids and internal steel shear reinforcement ratioswere studied. In all test the quasibrittle PMM was used asa binder of the strengthening system. The PMM was alsoused in the dogbone and WST tensile test. For testing theinfluence of different CFRP grids, the carbon fibre amountwas varied from 66, 98, to 159 g/m2 (Grid S, L, and M). Allof these specimens had no internal shear reinforcement inthe strengthened shear span. For specimens with differentinternal shear reinforcement the different distances was s =350 mm and s = 250 mm, see also Figure 5. The internalshear reinforcement was monitored by six strain gaugesmounted on both two stirrups closest to the load, seeFigure 5. In addition, six strain gauges were mounted on thevertical CFRP tows in the grid at the same locations as thestrain gauges mounted on the stirrups.

All of the specimens with no internal shear reinforcementand strengthened with the MBC system failed by fibrerupture of the vertical tows. For strengthened specimenswith different internal reinforcement amounts the failuremode changed from a brittle shear failure to a more ductilecompressive/crushing failure. The ultimate loads for allspecimens are summed in Table 3 together with compressive-and tensile strength of the concrete, and failure modes(S-shear failure and C-compression/crushing). From theresults of the specimens strengthened with different gridsit was clear that the specimens with highest carbon fibreamount had the highest load bearing capacity. Since thespecimens containing internal shear reinforcement wereheavily monitored by strain gauges, the shear behaviourand strain development from initial load stages could bemeasured. Here, the strain development in the stirrups wasreduced for beams strengthened with the MBC system. Thestrains monitored in the CFRP grid indicated that high strainconcentrations were apparent in the vicinity of cracks.

Typical strain readings from internal shear reinforcementand CFRP grid are shown in Figure 9 for specimens withand without strengthening corresponding to specimens withconcrete compressive strength of 47–52 MPa and internalstirrup distance of 350 mm (Specimens C35s3 and C35s3-Grid M in Table 3). Figures 9(a) and 9(b) shows that strainsin the (steel) stirrups are reduced in all MBC-strengthenedbeams even at relatively low load levels (100–200 kN) whencompared to the nonstrengthened ones. As the shear loadincreases, the favourable strain reduction of the MBC systemis more pronounced for a strengthened specimen comparedto a non strengthened specimen.


Table 3: Ultimate shear loads and failure modes, along with description of tested shear beams.

Beam Binder CFRPStirrup distance Failure load

Failure modeCompressive strength Tensile strength

[mm] [kN] [MPa] [MPa]

C40s0 — — — 123.5 S 44.8 2.9

C40s0∗ — — — 126.7 S 36.3 2.5

C40s0-M PMM — — 141.9 S 53.6 2.8

C40s0-Grid M PMM Grid M — 244.9 S 53.6 2.8

C40s0-Grid M PMM Grid M — 241.9 S 53.6 2.8

C40s0-Grid S∗ PMM Grid S — 208.1 S 32.5 2.7

C40s0-Grid M∗ PMM Grid M — 251.9 S 35.1 3.0

C40s0-Grid L∗ PMM Grid L — 206.4 S 44.8 2.5

C35s0 — — — 130.6 S 47.0 2.7

C35s3 — — 350 346.0 C+S 47.6 3.1

C35s3-Grid M PMM Grid M 350 336.9 C 52.4 3.0∗

Tests were run not deformation-controlled but load-controlled at 10 kN/sec.

In the vertical CFRP tows, a significantly uneven straindistribution is visible, with locally high strains in the vicinityof forming shear cracks. These strains are increasing rapidlyas the shear crack is opening.

6.5. Field Implementations. The MBC-strengthening systemshould be able to be implemented in various field strength-ening applications such as, harbour structures (less sensi-tive to moisture compared to epoxy-bonded strengtheningsystems), large structures that requires higher compatibilityof the strengthening system to the base concrete structure,low temperature applications, applications that demand dif-fusion openness and for concrete applications that excludesthe use of epoxies. In addition, the MBC system can be usedin tunnelling or mining applications instead of using steelfibre-reinforced shotcrete. In the latter, the combination ofusing ECC as a binder should further improve the ductilityand crack bridging ability of the MBC system. Pilot studiesfor in-situ production techniques of the MBC system usingquasibrittle mortars as binders have been investigated in[82]. The results from this study clearly show that sprayingthe MBC system can be made in an easy and efficient manner.The application technique is done by mounting the CFRPgrid on preattached studs and then the binder is beingsprayed. The thickness of the MBC system is controlled bythe chosen length of the studs.

6.6. Discussion and Conclusions. Uniaxial tests have revealedthat the most commonly used grid (medium-sized, M-grid)performs better when placed in transversal direction. Thisleads to the need of studying the effect of grid joint forma-tions (i.e., the grid is “woven” out of two perpendicular towsof CFRP and the grid joint looks differently depending onthe direction). The uneven distribution of the residual epoxyon the grid surface in the two perpendicular directions couldalso contribute to the direction-dependent performance.

Flexural specimens strengthened with MBC had higherultimate load compared to the epoxy-bonded sheets of com-paring the fibre amount in the tensile direction. However, the

failure mode for the MBC was fibre rupture while the epoxy-bonded carbon fibres failed by a mix mode of debonding andfibre rupture. However, trying to improve the bond betweenthe mineral-based binder and the CFRP grid by the useof sand-coated grids has negative effects on ultimate loadbearing capacity. It appears that sanding the surface createsdiscrete increases in strain and leading to premature failure,this was however not monitored. But as shown in the shearstrengthening in Figure 9(b), by the use of monitoring strainsin the grid, discrete high strains occur in the vicinity ofcracks.

All MBC-strengthened shear beams, which had nointernal shear reinforcement, failed by rupture of the verticaltows in the grid. From this series of specimens it wasconcluded that increasing the fibre amount will also increasethe ultimate failure load, to what extent is not within thescope of this paper. But increasing the fibre to a certainlevel should imply failure in the intermediate transition zonebetween binder and fibre composite. For beams withinternalreinforcement the failure mode changed from a brittleshear failure to a more ductile compressive failure. Forthe specimens withinternal reinforcement it could be seenfrom strain gauge monitoring that the MBC-strengtheningsystem reduced strains in the steel reinforcement even for lowshear loads. Thus, this indicates that the MBC-strengtheningsystem can be used for crack width reduction in the servicelimit state.

MBC tests have shown that strain hardening mortarscan be successfully used together with embedded FRPreinforcement and the fibre bridging behaviour of ECC“compensates” for the brittleness of the grid and preventspremature failure of it by retaining fast and rigid deforma-tions in the grid joints. This may enable a better utilizationof the grid.

To the authors’ knowledge, ECC has been implementedinto the MBC system for the first time. Due to its fibrebridging mechanisms and strain hardening behaviour, ECCshall further be tested as a bonding agent for mineral-basedstrengthening systems. Preliminary results from pullout testscarried out by the authors have shown that if the application


0 2000 4000 6000

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Figure 9: Strain development at different shear loads. (a) in stirrups without MBC and (b) in stirrups with MBC and (c) strains in verticalCFRP tows.

method is correct and the mix is properly compacted aroundthe CFRP grid, there is a nearly perfect bond on the interfaceeven at short embedment lengths (45 mm) using bothpolymer-modified quasibrittle mortars and ECC. Thereforein a real structural element, the failure mode is expected tobe fibre rupture.

Traditionally, ECC has been used up to its maximumstrain or deformation capacity (especially where energydissipation is the major concern). In an ECC-based MBC, theFRP component is fully used up to its maximum potential;however the ECC is not because the FRP reinforcementfails prior to that the ECC would reach its maximum straincapacity. In a structural member made from pure ECC, themaximum strain capacity would be characterized by thehighest yet sustainable crack density with evenly distributedfine cracks.

Despite the fact that when combined with FRP, themaximum strain capacity of the ECC is not fully utilized,

ECC improves the MBC system significantly. Its mainadvantages are that it accommodates larger elongations thanFRP therefore it will not be the weak link in strengthening,it enforces the failure mode to be more ductile [49]and prevents or reduces sudden brittle deformations. Itsdocumented spalling resistance [46, 47] is also very attractivewhen designing externally bonded strengthening systems.

6.7. Future Research for MBC

6.7.1. Materials. Strain hardening mortars together withFRP grids should be further tested especially in shear beamsor other shear sensitive structures where it should yield amore predictable and ductile failure mode.

Tri- or other multiaxial grids could also be effectivelyused, in particular for shear strengthening. More flexiblegrids, in which the joints are not rigid using a moreelastic matrix for impregnating the fibres, would most likely


accommodate larger deformability of the grids for the use of,for example, wrapping. Also investigations of incorporatinghigher fibre amount in the CFRP grid should be made forfurther increasing the load bearing capacity.

When using ECC as binder, further investigation isneeded on whether (and to what extent) the large improve-ment in load capacity is due to a direct tensile contributionfrom the ECC or the fact that it is capable to retain grid jointsfrom fast and brittle deformations breaking the grid or toslow down this process, hence preventing premature failurein the grid.

6.7.2. Structural Behaviour and Future Applications. Influ-ences on the bond in the transition zone between fibre com-posite and binder should be investigated when increasing thefibre amount in the FRP grids. The investigations done sofar on the MBC system has shown no problems concerningthe bond to the base concrete structure, future applicationsinvolving enhancing the load bearing capacity should befollowed by an experimental program involving the bond tothe base concrete.

For field applications the influence of shrinkage shouldalso be investigated, especially when incorporating ECC as abinder. However, traditional precautions can be taken whencommercially available PMM is used as binders.

Regarding future applications, stay-in-place formworkcould be one new approach, especially for the combinationof using concrete with low-quality as a core material andthen using high-quality binders together with a FRP grid as aprotective outer layer of remaining formwork.

To incorporate the MBC system for the use in civil engi-neering structures design guidelines should be developed forestimating the load bearing capacity.

7. Discussion and General Conclusions inMineral-Based Strengthening

7.1. Guidelines. One of the most important aspects forimplementing these mineral-based systems for the use inreal structures is that there exist engineering design methodsfor calculating the load bearing capacity. To the authors’knowledge, there exists no general and commonly accepteddesign for the mineral-based system. These guidelines shouldbe made on a sound physical basis dealing with both thefavourable aspects at the service limit state (crack bridgingand crack reducing effects) together with the load bearingincrease in the ultimate limit state.

7.2. Production Methods and Procedures. For strengtheninglarger surfaces, sprayed ECC together with FRP grid rein-forcement may be a solution. Although this paper has notdealt with sprayable ECC there has been publications aboutthe topic, see, for example, [84]. As the tensile strength ofthe pure ECC is not significantly different from that of aconcrete, combining it with FRP would be useful when thetensile stresses significantly exceed the upper load bearingcapacity of an additional pure ECC strengthening layer.

7.3. Suitable Materials. Perhaps it is an advantage tostrengthen a structure using a textile for increasing the

ductility at ultimate failure load due to the relatively poorbond between binder and fibre composite. However, forservice limit state a full utilization of interfacial bondbetween FRP and mortar is desirable to maintain economy.This justifies the applicability of grid-type reinforcement ifthe shape of the cross-section allows a grid to be used. It isimportant to emphasize that epoxy coating, compared to thetextiles used in TRC, significantly improves the interactionbetween mortar and FRP because of the difficulties of themortar to penetrate dry fibre bundles.

An alternative to TRC when ductility is an issue is strain-hardening mortars together with grids as published in [63,64]. Strain-hardening mortars also offer the advantage of amore ductile and predictable failure mode which would bebeneficial for strengthening shear sensitive structures.

It is assumed that ECC, when combined with grids,would be able to ensure a more even stress distributionamong the grid tows along a crack line because of its fibrebridging effect. Then the peak strains in the adjacent gridtows (as shown, e.g., in Figure 9(b)) would be reduced andredistributed to the neighbouring tows hence making thefailure more predictable.

New matrices could also be introduced for impregnatingthe FRP component. A common method for strengtheningcolumns or beams is wrapping with textiles or continuoussheets/dry fibres. However, wrapping is not possible whenusing an epoxy-impregnated carbon fibre grid due tothe rigidness and brittleness of the matrix. Using textilesmakes wrapping around corners much easier [83]. Usinga semielastic matrix, for example, latex, which still ensuresrigid connection points but allows wrapping around corners,could be a beneficial solution for ensuring rigidity, anchorageand effectiveness of the fibres.

7.4. Interactions. Further research should be directed to theinfluence of bonding, both between base concrete and binderalong with the transition zone and between binder andfibre composite. In MBC, having perfect bond between fibrecomposite and quasibrittle binder could lead to prematurefailure in the FRP. In TRC, bond between textile and mortaris significantly weaker and there is a limitation of how manylayers of textile can be used effectively without anchorageproblems (if there is no additional mechanical anchorage)and debonding. This also limits the maximum fibre amountapplicable in a certain direction and therefore the maximumstrengthening effect achievable by a TRC system.

Penetration of the FRP component has been a problem-atic issue with several existing systems (e.g., dry fibres/sheets,TRC). Using nonimpregnated sheets, grids or textiles willgenerate larger slips and inferior effective strain over theroving cross section with possibly overloaded yarns. Usingimpregnated fibres (fibres imbedded in an epoxy matrix) willcreate a more effective strain distribution in the FRP tow suchas in case of FRP grids.

7.5. Other Issues. Further research should also be directed tothe durability and fatigue aspects of mineral-based strength-ening. In seismic regions, ductility and energy dissipationcapacity is of importance, in this regard ECC-based systems


which use additional fibre composites as reinforcement couldbe further developed and investigated.

Acknowledgments

The research work presented in this paper was performed atthe Technical University of Denmark and Lulea Universityof Technology, financed by the Norwegian Research Councilthrough the strategic institute program RECON at NorutNarvik Ltd, the Swedish road administration, and thedevelopment fund of the Swedish construction industry.Sto Scandinavia should also be acknowledged for supplyingmaterials in the experimental studies.

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FromMaterialLeveltoStructuralUseofMineral-Based Composites ...downloads.hindawi.com/journals/ace/2010/985843.pdf · based Composites (MBCs), together with tests ... the ﬁbre material