Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Textile Reinforced Concrete (TRC) for precast Stay-in-Place formworkelements

I.C. PapantoniouCivil Engineering Department, University of Patras, Patras, Greece

C.G. PapanicolaouStructural Materials Laboratory, Civil Engineering Department, University of Patras, Patras, Greece

ABSTRACT: The main goal of the present study is to experimentally investigate the response of structuralelements cast against thin-walled stay-in-place formwork elements made of Textile Reinforced Concrete (TRC).TRC comprises an innovative composite material consisting of fabric meshes made of long woven, knitted or evenunwoven fibre yarns (e.g. carbon, glass or aramid) in at least two (typically orthogonal) directions embedded ina cementitious matrix (mortar or fine-grained concrete). The experimental investigation described in this studywas carried out on two types of reinforced concrete specimens: the first one included 22 beam-type specimensincorporating flatTRC stay-in-place formworks and the second one included 11 prismatic column-type specimenscast into permanent precast TRC shafts.

1 INTRODUCTION

Construction management is always focused on theminimization of two inter-related factors: the dura-tion of construction and the total (life-cycle) cost ofa structure. The use of Stay-in-Place (SiP) – or per-manent – formwork elements addresses this goal andoffers additional benefits such as reduced maintenancecosts and improved safety (by reducing hazards duringconstruction). SiP formwork is a structural elementthat is used to contain the placed concrete, mould itto the required dimensions and remain in place forthe life of the structure (Wrigley, 2001). Permanentformwork elements are distinguished into participat-ing and non-participating ones; the former contributeto the strength of the structure through compositeaction with the cast-in-place parts of it, while thelatter make no strength contributions. Several dif-ferent SiP formwork systems have been developedincorporating a variety of materials, such as steel,timber and fiber reinforced polymers (FRP). Highcorrosion susceptibility, poor durability and low fireresistance are, respectively, some marked drawbacksof the aforementioned systems. The alleviation ofthese drawbacks may be realized by using cementitiouscomposite materials consisting of inorganic matrices(cement-based mortars or micro-concrete) reinforcedby non-corrosive fiber yarns arranged in a grid struc-ture. The generic term for these materials is TextileReinforced Concrete (TRC) [RILEM, 2006]. In thiscase, composite behavior is achieved mainly through

mechanical interlock between the matrix and the gridopenings; furthermore, the fibers’ properties may befully exploited since the quantity and the orientationof the yarns can be selected according to the designrequirements (contrary to the fiber reinforced con-crete, in which fibers are randomly distributed andoriented). The flexibility of the textile meshes allowsfor the design of thin SiP formwork elements withcomplex geometry. This study presents the results ofan experimental investigation conducted on concreteelements cast against thin-walled TRC SiP formwork.

2 EXPERIMENTAL PROGRAM

2.1 Beam-type specimens

Beam-type specimens measuring 1500 mm in length,150 mm in width and 100 mm in total height, were sub-jected to four-point bending.The following parameterswere examined in order to investigate their influenceon the flexural behavior of the composite elements:

• Fiber reinforcement ratio (corresponding to 1 and 2layers);

• Fiber rovings’ coating (polymeric and none);• Treatment of theTRC/cast-in-situ concrete interface

(smooth and rough);• Spacing of the textile reinforcement layers (1 mm

and 6 mm);• Filaments’ material (e-glass and carbon).

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Figure 1. Beam-type specimen geometry.

The construction of the specimens was dividedin two stages. Initially, a thin-walled flat TRC ele-ment (measuring 1500 mm × 150 mm × 12 mm, as inlength × width × height) was produced by applicationof two 6 mm thick mortar layers, within which the tex-tile reinforcement (either in 1 or 2 layers) was placed.In the case of double-layer textile reinforcement, a thinmortar layer (1 mm in thickness) was used to separatethe textiles and to ensure that an adequate quantity ofmatrix material would protrude through the grid open-ings in order to provide better bond conditions. In mostspecimens, the exposed surface of the TRC elementwas roughened while in a fresh state by the formationof longitudinal grooves using a hand tool. In orderto investigate the influence of the formwork’s surfacetreatment two TRC elements received smoothening.Following a 24-hour curing period the specimen’s mainpart (88 mm in thickness) was cast on top of the TRCand the completed beam was stored in a curing cham-ber (20◦C, 98%RH) for 28 days (until testing). For 15specimens the main part consisted of plain concrete,while for the remaining specimens reinforced concretewas used. Steel reinforcement comprised two longitu-dinal bars (with a diameter of 8 mm), positioned at aneffective depth of 75 mm (resulting in a reinforcementratio equal to approx. 9‰). No transverse reinforce-ment was used. The specimen geometry is shown inFigure 1.

For the TRC matrix material a commercial inor-ganic binder was used consisting of a blend of cemen-titious powders and a low fraction of polymers. Thewater to cementitious materials ratio was equal to 0.23;the compressive and flexural strength (measured at 28days) were equal to 24 MPa and 6.3 MPa, respectively.The mean 28-day compressive strength of the concretewas equal to 21.2 MPa. The average yield stress of thesteel reinforcement was equal to 571.6 MPa.

Two basic types of commercial textiles with equalquantity of fiber rovings in two orthogonal direc-tions (0◦/90◦) were used. The first type comprisedhigh-strength carbon fiber rovings and the second onee-glass fiber rovings. In either textile the width of

Figure 2. Textile structure.

Figure 3. Specimen construction stages.

each roving was equal to 3 mm and the clear spacingbetween rovings was 7 mm. The weight of fibers in thetextiles was 348 g/m2 and 480 g/m2 for carbon fibersand e-glass fibers, respectively. The nominal thicknessof each layer (based on the equivalent smeared dis-tribution of fibers) was 0.095 mm and 0.092 mm forthe carbon fiber textile and the e-glass fiber textile,respectively. The resulting volumetric ratio of fibers inthe TRC element (fiber volume per TRC unit volume)was approximately equal for both carbon (7.9‰) ande-glass fibers (7.6‰). The geometric fiber reinforce-ment ratio for either type of fibers was approximatelyequal to 1‰ per textile layer. The guaranteed tensilestrength of the fibers (as well as of the textile, when thenominal thickness is used) in each direction was takenfrom data sheets of the producer equal to 3350 MPa(carbon) and 1600 MPa (e-glass). The elastic modu-lus of the fibers was 225 GPa (carbon) and 70 GPa(e-glass). The fiber rovings in most of the textileswere uncoated, but in a small number of textiles theywere polymer-coated. The textile structure is shown inFigure 2. Figure 3 illustrates stages of the specimens’construction.

A total of 22 specimens were constructed: 12specimens comprised plain concrete main parts,7 specimens comprised reinforced concrete mainparts and 3 were used as control specimens. Thespecimens designated as “Con” and “Conm” consistedof plain concrete and were cast against a retriev-able and a SiP TRC formwork, respectively. Specimen“ConS” was also cast against a retrievable formwork

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Figure 4. Test set-up for the beam-type specimens.

and consisted of reinforced concrete. Specimens aregiven the general notation XF, where X denotes thenumber of textile layers of the TRC element (1 or2) and F denotes the type of fibers of the textiles(C for carbon and G for e-glass). Indices “c” and“s”, where applicable, denote polymer-coated textilesand smooth TRC/concrete interface, respectively. Thefollowing specimens with plain concrete main partswere constructed: 1C, 2C, 1Cc, 2Cc, 1Ccs, 1G, 2G,1Gc, 2Gc and 1Gcs. The notation of specimens withreinforced concrete main parts is complemented withthe letter “S”.These specimens are the following: 1CS,2CS, 1CcS, 1GS, 2GS and 1GcS.

Three additional specimens with plain concretemain parts were constructed in order to investigate theeffect of the textile inter-layer spacing along with thecombined use of carbon and e-glass fiber textiles andthe combined use of coated and uncoated e-glass fibertextiles. More specifically, specimens CGl and CGpwere both cast on top of TRC elements each con-taining one layer of carbon and one layer of e-glassfiber textiles, the only differences being the spacingof the two layers (6 mm for CGl and 1 mm for CGp)and the resulting TRC thickness (18 mm for CGl and12 mm for CGp). Specimen GcG was cast on top ofa TRC element reinforced with two layers of e-glassfiber textile, one coated (closest to the bottom of thespecimen) and one uncoated.

All specimens were tested under monotonic four-point bending at a total span of 1400 mm using astiff steel frame. The displacement-controlled load-ing protocol was applied using a vertically positioned500 kN MTS actuator. The load application pointswere 50 mm distant from the mid-span. The displace-ment application rate was equal to 0.016 mm/sec.Displacements were measured at mid-span using anexternal rectilinear displacement transducer (of 50 mmstroke capacity) mounted on one side of the specimen(Fig. 4).

Figure 5. Column-type specimens.

2.2 Column-type specimens

Prismatic column-type specimens with square crosssection (224 × 224 mm) and 500 mm height weretested under concentric monotonic compressive load-ing. The construction procedure of these specimensresembled the one of the beam-type specimens. First, athin-walled TRC SiP shaft was constructed measuring500 mm in height and 12 mm in thickness. Following acuring period of 28 days the main part of the specimenwas cast in the TRC formwork consisting of rein-forced concrete. The reinforcement was of the samegrade as the one used for the beam-type specimensand comprised a 460 mm high steel cage formed by4 longitudinal bars (12 mm diameter) and square ties(8 mm diameter) with a spacing of 100 mm (Fig. 5).

The parameters under investigation were:

• Filaments’ material (e-glass and carbon);• Fiber rovings’ coating (polymeric and none);• Matrix material;• Fiber rovings’ orientation.

The TRC column-type formwork elements werecast in a custom-made modular metallic mould mea-suring 500 mm in height. Two sets (each of four steelplates) formed a 12 mm gap in which the cement basedmortar was hand-pumped through a plug at the bottomof the mould. The plates’ edges were chamfered at acurvature radius of 15 mm. Prior to fixing the platestogether a single-layer textile was put in place (in themiddle of the space between the inner and the outerparts of the mould) allowing for a 200 mm overlap.

Four different textiles were used in the TRC shafts.The first two were identical to the ones used for theproduction of the flat TRC SiP formwork elements,while the other two comprised polymer-coated fiberrovings (carbon for one of the textiles and e-glass forthe other) oriented at +45◦/−45◦. Apart from rovingorientation the textiles differed in no other aspect.

Two types of inorganic mortars were used: the firstone (mortar I) was identical to the mortar used for theflat SiP formwork elements (measured 28-day com-pressive and flexural strength were equal to 23.1 MPaand 6.88 MPa, respectively); the second mortar (mor-tar B) was of higher compressive strength (51.7 MPa)

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Figure 6. Load versus mid-span deflection curves.

and of lower flexural strength (2.5 MPa) than mortar I.Both mortars included a moderate quantity of super-plasticizer so that high fluidity and pumpability couldbe achieved.

A total of 11 specimens were constructed. One spec-imen (designated as “Con”) was cast in a retrievableformwork and was used as the control specimen. Theremaining specimens were cast in the TRC moulds.The mean 28-days compressive strength of the con-crete used in this specimen series was equal to 21 MPa.Mortar I was used for the production of half of theTRC shafts, whereas mortar B was used for the remain-ing ones. The specimens receive the general notationMFO, where M denotes the mortar type (I or B), Fdenotes the type of fibers in the textile (C for carbonand G for e-glass) and O denotes the roving orientation(90 for 0/90◦ and 45 for +45◦/−45◦). Index “c” is usedto distinguish the polymer-coated textiles. Accordingto the The following hybrid (composite) specimenswere produced: BC90, BCc45, IC90, ICc45, BG90,BGc90, BGc45, IG90, IGc90, IGc45. All specimenswere stored in a curing chamber (20◦C, 98% RH)for a time span of 28 days (i.e. until testing). Axialconcentric monotonic compression was exerted onall specimens in a displacement control mode ata rate of 0.035 mm/sec using a 4000 kN capacity

loading machine. Axial deformations were measuredby an external lvdt (with a gauge length of 250 mm)positioned at mid-height.

3 RESULTS AND DISCUSSION

3.1 Beam-type specimens

The load versus mid-span deflection curves for allspecimens are given in Figure 6. Higher load anddeflection at failure was recorded for all compositespecimens in comparison to the control ones. Speci-mens Con and Conm responded in an almost identicalmanner (failing at the same load and deflection) dueto a single crack that formed in the mid-span. Failureof plain concrete specimens cast on top of uncoatedcarbon TRCs was attributed to progressive pull-outof fiber rovings from the inorganic matrix. Failureof their e-glass counterparts was the result of thesleeve filaments’ damage (i.e. of the fracture of indi-vidual fibers found on the perimeter of the rovings)caused by stress concentrations in the cracks’ edges.For the same type of specimens (i.e. comprising plainconcrete main parts) the use of polymer-coated tex-tiles and the increase of the textile reinforcementratio led to a denser cracking pattern with smaller

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Figure 7. Crack pattern for beams cast on top of carbonTRCs.

crack widths (Fig. 7). This fact is indicative of homo-geneous stress distribution in the TRC element andof high stress transfer capacity between the mortarand the textile. Shear failure of specimen 2Cc con-firms this observation. Coating of the rovings seemsto improve textile/mortar bond conditions and fiberalignment by stabilizing the fiber bundles at the textilejoints and by preventing their relative slippage in theseareas. Furthermore, the coating procedure results intopartial impregnation of the fiber bundles, providingstrain compatibility between the inner and the outerfilaments.

According to the experimental results the effect ofthe TRC/concrete interface roughness is rather smallbut deserves more experimental attention since it isstrongly dependent on: the roughness scheme (i.e. themagnitude of surface undulations); the mechanicalproperties and the maximum aggregate size of cast-in-place concrete; and the elapsed time between TRCand concrete casting (i.e. the effect of chemical bond).Regardless of the surface treatment that the SiP form-works received in this investigation all TRC/concreteinterfaces remained intact during testing.

By comparison of the response of specimens CGpand CGl it is concluded that the increase of the matrixthickness between the textile layers (and the con-sequent increase of the TRC thickness) has smallinfluence in regard to ultimate load-bearing capac-ity and deformability. The observed differences areclearly attributed to the change of effective depth.

Crashing of concrete in the compression zone forspecimens with steel reinforced main parts was thecause for load-bearing capacity reduction. The steelreinforcement (acting as crack arrestor) led to densecrack patterns with small crack widths. The effectof polymer coating of the textiles in this case wasalso beneficial but less pronounced. The quantifiedinfluence of the rovings’ polymer coating and the tex-tile reinforcement ratio is given in Tables 1 and 2,respectively.

Table 1. Influence of polymer-coated textiles (coated vsuncoated).

% increase atmaximum

Specimens Load Deflection

Main parts made of plain concreteSingle layer carbon TRC 1C↔1Cc 112.9 89.9Double layer carbon TRC 2C↔2Cc 26.0∗ 0.8Single layer e-glass TRC 1G↔1Gc 13.1 672.1Double layer e-glass TRC 2G↔2Gc 147.5 469.7

Main parts made of reinforced concreteSingle layer carbon TRC 1CS↔1CcS 10.7 0.0Single layer e-glass TRC 1GS↔1GcS 1.5 40.9

* Shear failure

Table 2. Fiber reinforcement ratio effect (higher vs lowerreinforcement ratio).

% increase atmaximum

Specimens Load Deflection

Main parts made of plain concreteUncoated carbon TRCs 1C↔2C 129.9 49.1Coated carbon TRCs 1Cc↔2Cc 36.0 −20.9*Uncoated e-glass TRCs 1G↔2G 42.9 354.9Coated e-glass TRCs 2Gc↔2Gc 212.6 235.6

Main parts made of reinforced concreteUncoated carbon TRCs 1CS↔2CS 22.7 14.3Uncoated e-glass TRCs 1GS↔2GS 8.3 16.2

* Shear failure

The theoretical value of flexural capacity was cal-culated by a cross section analysis based on idealizedmaterial constitutive laws for concrete in compressionand carbon or e-glass fiber textile (and steel whereapplicable) in tension and on the classical assumptionsof plane cross sections and negligible contribution ofthe concrete below the neutral axis. Uniform stressdistribution across each roving’s cross section and per-fect TRC to concrete bond was assumed. Possiblefailure modes included textile fracture (prior or aftersteel yielding, if applicable) and concrete compressivecrushing.

For all specimens except those comprising plainconcrete main parts cast on top uncoated TRC form-work elements the calculated values of flexural capac-ity were in good agreement to the experimental ones.Specimens incorporating uncoated TRCs exhibitedlower experimental moment capacity in comparisonto the theoretical value. Lack of full exploitation ofthe textile properties in these specimens is attributedto the inability of the matrix material to penetrate and

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Figure 8. Coefficient of effectiveness for specimens 2G,2Gc, GcG.

impregnate the fiber yarns. This fact leads to the inho-mogeneous stress distribution between the outer andthe inner filaments and thus the former carry largertensile stresses in comparison to the latter [this has alsobeen reported by Voss and Hegger (2006)]. As a result,the well-bonded e-glass outer filaments fail in tensionand rapid crack width opening occurs until the drycore filaments are fully stressed. For the case of car-bon rovings, differential stress distribution results intelescopic sliding of the filaments. The ratio of exper-imental to theoretical value of the flexural momentcapacity can be considered as a coefficient of effec-tiveness (cef ) of the textile reinforcement. Based onthe above discussion this ratio was found to be closeto unit for specimens comprising reinforced concretemain parts and for plain concrete specimens withpolymer-coated TRCs. For uncoated TRCs supportingplain concrete toppings the coefficient of effectivenessdepends on the fiber volume ratio and on the type offibers (e.g. cef equals to 0.32 and 0.56 for specimens1C and 2C, respectively). The influence of polymer-coated textiles becomes clearer in Figure 8, where cefis plotted against the polymer-coated textile contentfor specimens 2G, 2Gc and GcG.

3.2 Column-type specimens

All specimens exhibited similar response under con-centric monotonic compressive loading. The normal-ized axial stress versus axial strain curves are shownin Figure 9. Normalized stress is defined as the ratio ofthe compressive stress fcc over the specimen’s concretecompressive strength fc (specimens were cast withconcrete of slightly different compressive strength,with a mean value equal to 21 MPa). Mortar B TRCshafts were cracked prior to testing due to shrinkage.These specimens failed under lower maximum load(by 10–18%) compared to the control specimen. Betterresponse was observed for specimens cast in mor-tar I TRC shafts. Yet, these specimens (but one) alsofailed under lower maximum load (by 6% on overage)compared to the control specimen. This was due toearly local buckling of the TRC element, as explainedhereafter.

Figure 9. Normalized axial stress versus axial strain curves.

From the early stages of loading vertical crack-ing and flaking of the matrix material was evident onthe TRC elements, resulting in the low contributionof the thin-walled TRC elements to the load-bearingcapacity of the specimens. Until the peak load wasreached, growing of the vertical cracks width wasobserved (especially at the specimens’ corners wherestress concentration was present) and local buckling ofthe formwork occurred at the vicinity of the load appli-cation areas (top and bottom parts of the columns);this was accompanied by mortar spalling. The con-fining action of the TRC formwork was activated aftermaximum load was reached, resulting into the increaseof post-peak deformability. Buckling of the verticalsteel bars was observed in a later stage of the test (at50% load carrying capacity reduction) causing textilerupture total failure of the specimens.

High compressive strength/low shrinkage matri-ces (e.g. fiber reinforced mortars) are recommendedin order to avoid the premature failure of TRC ele-ment. Additionally, the installation or the formationof shear connectors on the inner side of the TRCelements is expected to improve composite actionbetween the SiP formwork and the core concrete. Itis pointed out that the main purpose of use of theseinnovative composite elements is not augmenting theload carrying capacity of axially loaded structural

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members. The main structural advantage of these sys-tems, if used as participating formwork, lies in theincrease of deformability. In this study (using lowtextile reinforcement ratios), the experimental resultsindicate an increase in deformability by 25% to 75%(corresponding to the axial strain at a post-peak axialstress equal to 80% of the specimen’s compressivestrength) compared to the monolithic specimen.

4 CONCLUSIONS

The experimental investigation presented in this studyprovides useful information on the behavior of com-posite TRC/concrete structural elements which can beused for the conduction of large scale tests. Further

research is needed in order to establish a designmethodology for TRC SiP formwork elements that isbased on realistic structural behaviors.

REFERENCES

RILEM Technical Committee 201, “Textile Reinforced Con-crete”, State of the Art Report (2006), RILEM Publica-tions Press.

Voss, S., Hegger, J., “Dimensioning of textile reinforcedconcrete structures”, in Proceedings of the 1st Interna-tional RILEM Symposium Textile Reinforced Concrete(Aachen, Germany, Sep 9–14, 2006), RILEM PublicationsPress, 151–160.

Wrigley, “Permanent formwork in construction”, Construc-tion Industry Research and Information Association(CIRIA), London 2001.

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