Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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PAPER REF: 5652

TEXTILE REINFORCED MORTAR SYSTEM AS A MEANS FOR

CONFINEMENT OF MASONRY STRUCTURES EXPOSED TO

ELEVATED TEMPERATURES

Krevaikas D. Theofanis(*)

Xi’an Jiatong-Liverpool University, Department of Civil Engineering

111 Ren Ai Road, Suzhou, Jiangsu, 215123, China (*)Email: theofanis.krevaikas@xjtlu.edu.cn

ABSTRACT

The present paper demonstrates the initial results of an ongoing experimental and analytical

research programme on masonry structures confined with Textile Reinforced Mortars

(TRMs). The application of externally bonded TRM as a means of masonry confinement, a

subject never addressed before, is investigated in this experimental programme. In the present

experimental stage three series of uniaxial compression tests, were conducted on small scale

masonry columns having as the only variable the ambient temperature. The purpose of this

experimental investigation was to examine the performance of TRM jackets exposed to

elevated temperatures. All specimens had rectangular 240*240 mm2 cross-sections with a

corner radius of 20 mm and were strengthened with one layer of Carbon fiber textile having a

weight of 220 g/m2 and a tensile strength of 8.40 MPa. The carbon fiber textiles were

externally bonded to masonry using mortar-as a binder-that contained polymeric additives. A

total of 15 specimens were divided into three groups comprising three, three and six

specimens, respectively. The first two groups each comprising three specimens were tested

after 8 hours of exposure to 100 oC and 200 oC degrees respectively. The six specimens in

the third group were tested under room temperature conditions. In addition three unreinforced

control specimens were prepared (one for each group) and tested under the same

environmental conditions. From the results obtained, the performance of the TRM jacket

proved to be unaffected by the exposure to elevated temperatures and this gives an initial

advantage as compared with the FRP strengthening technique. Further experimental and

analytical investigations will be made in the future in order to examine the behavior of TRM

confined masonry axially loaded under elevated temperatures and/or real fire conditions.

Keywords: mortar, masonry, textile reinforcement

INTRODUCTION

Unreinforced masonry structures, usually termed URM, are numerous worldwide. Since that

type of construction is one of the oldest men used from the beginning of civilization most of

masonry structures are old and hence prone to failure. Old URM structures have been proven

to be susceptible to lateral loading such as intense winds or moderate to large earthquakes. A

research on the causes of earthquake fatalities has proved that approximately 60% of life

losses, in the second half of 20th century were attributed to URM failures [Coburn AW,

2002]. To the aforementioned poor structural behavior should be added the natural decay of

the material components due to ageing. On the other hand in most of the cases, especially for

old heritage URM structures, the design and construction was not adequate. The main reason

for that was the fact that the construction was based mostly on everyday life practice, low

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level of quality control and old, if any, seismic codes rather than consistent and scientific

knowledge. All those factors led to an increased interest worldwide towards intervention

methods especially in seismic prone areas.

Through the past decades numerous techniques have been developed and applied in order to

increase the structural efficiency of URM in terms of both strength and ductility. The static

interventions including shotcrete jackets and overlays, pre-stressing using external or internal

steel ties, as well as externally bonded fiber reinforced polymers (both woven fabrics and

prefabricated strips) and near-surface mounted (NSM) FRP reinforcement. The FRP-based

method was well received by the civil engineering community mainly because of its flexible

and yet efficient solutions to problems that the conventional methods could not address. FRPs

with their high strength and stiffness to weight ratio, minimal effect on the mass and stiffness

of the structure, very good behavior under severe environmental conditions and ease and

speed of application with minimum disturbance to the occupants, offered a very promising

alternative in strengthening and rehabilitation of old URM structures.

It was those advantages that gave a boost to numerous studies and experimental programs

worldwide aiming to examine different aspects of masonry’s behavior strengthened with FRP

materials. Within that context studies have been made in order to verify the structural

behavior of URM structural elements strengthened with FRPs using pre-stressing

[Triantafillou & Fardis, 1997]; confinement [Valuzzi MR., 2003; Krevaikas & Triantafillou,

2005]; externally bonded strips or overlays [Schwegler, 1994;Laursen, 1995;Ehsani MR,

1997; Triantafillou TC., 1998; Badoux, 2002; Fam, 2002; Moon FL., 2002; Marcari G.,

2003;Stratford T., 2004; Krevaikas & Triantafillou, 2005; Ehsani MR., 1999; Velazquez-

Dimas & Ehsani, 2000; Albert ML., 2001; Hamilton & Dolan, 2001; Hamoush SA., 2001;

Kuzik MD., 2003; Ghobarah & El Mandooh Galal,2004;Tumialan G.,2001; Li T., 2005];

near-surface mounted reinforcement [Valuzzi MR., 2001; Foraboschi P., 2004].

Although the FRP-based technique emerged as very promising and innovative there are still

some drawbacks that need to be solved. Mainly those setbacks are related to the nature of the

organic resins used as polymeric matrices to bind or impregnate the fibers. Some of those can

be attributed to the poor behavior of epoxy resins at temperatures above the glass transition

threshold (Tg); the relatively high cost of epoxy matrices; the weather conditions (in very

humid environments or in very low temperatures the application is not allowed); the

conditional compatibility with some of the substrates (for clay masonry units there is an

inherent incompatibility) and the difficulties of assessment in the aftermath of earthquakes.

A promising alternative to the above problems is the use of inorganic matrices known as

cementitious composite systems. Again some weaknesses of those systems having to do with

the non-well established impregnation of the fiber sheets attributed to mortar’s granularity

have to be properly handled. Because of that inherent weakness of cement-based matrices, the

fundamental property of binders, which is the ability to wet and impregnate the individual

fibers is lost [ACI 549.2R-04, 2004]. In order to improve the bond conditions and the fiber-

matrix interaction woven fabrics can be replaced by textiles. The outcome of that combination

is a composite system alternatively known as Textile Reinforced Mortar (TRM). These

systems constituted by two components, the open weave fiber fabrics and the inorganic

mortars. Usually those open weave fabrics are made of either Carbon fibers or Basalt fibers. It

is the chemical consistency of those fibers (polybenzoxozole; PBO) that allows them to bond

directly to cementitious mortar matrices and therefore improve the interaction between those

two components [Peled & Bentur, 2002].

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Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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Although research on the field of textile reinforced cementitious materials started since late

70’s it was during the 90’s when the first promising results started to appear. Since the

beginning of 2000 the number of researchers getting involved with the use of Fiber

Reinforced Mortars has increased. Initially TRMs were used as externally bonded

reinforcement of concrete in order to increase the flexural [M. A. Taylor, 1975; Curbach &

Brueckner, 2003] and shear strength [Curbach & Ortlepp, 2003; Triantafillou & Papanicolaou,

2005, 2006; Brueckner, 2005] and as a means of confinement [Triantafillou TC, 2006].

In the case of strengthening URM structures using textiles combined with mortars dated back

to late 90’s but it was until recently that a bit more attention has received on the subject.

Previous research gave results on diagonal compression tests on tuff masonry wallettes

[Faella C., 2004], confined masonry columns [Krevaikas T., 2005; Nurchi & Valdes, 2005]

and masonry walls under cyclic loading conditions [Catherine G. Papanicolaou, 2006, 2007].

In the present study masonry column specimens constructed from solid clay masonry units

and general purpose mortar confined with a layer of carbon fiber cement matrix system (one

layer of bi-directional carbon fiber fabric in polymer-modified cement binder) were tested

under axial loading after they had been exposed to elevated temperatures. The specimens

demonstrated a considerable increase in strength and deformability despite their exposure to

high temperatures. The main objective of this preliminary study is to examine the overall

performance of the TRM jackets as a means of confinement at temperatures beyond the glass

transition threshold (Tg).

EXPERIMENTS

The main objective of the present study is to investigate the features that characterize the

performance of TRMs as a means of confining masonry under elevated temperatures. In total,

three groups of specimens were prepared and subjected to axial load after being exposed to

different ambient temperatures. All the specimens were constructed using solid clay masonry

units with dimensions 230mm (length), 100 mm (width) and 45 mm (height), bonded together

with a mortar containing cement and lime as binder, at a water : cement : lime : sand ratio

equal to 0.9:1:3:7.5 by weight(the same proportion as in Krevaikas & Triantafillou, 2005).

The cross-sectional area of all specimens was 240x240 mm (aspect ratio 1:1). Each specimen

comprised of bricks placed in five rows with four bed joints in between as shown in Fig.1.

The thickness of mortar was approximately 10 mm except for the head joints where it was

increased in order to maintain the desired cross-section aspect ratio. The corners of all

specimens were rounded using a grinding machine at a radius of 20 mm. Within each group,

specimens were wrapped with one layer of bi-directional Carbon fiber textile, applied through

the use of polymer modified mortar.

Details about the specimens in each group are given in table 1. The notation that has been

given to all specimen types was FN_A_RX_T, where F=fiber type (C for carbon), N= number

of layers (1 layer of Carbon fiber textiles), A = aspect ratio (1:1 in the present study), X=

radius at the corners (20 mm in the present study) and T= ambient temperature (20 oC, 100

oC,

and 200 oC). For example, C1_1_R20_200 denotes specimens wrapped with one layer of

carbon fiber textile with square cross-section (aspect ratio 1) rounded at 20mm, and tested

after being exposed to 200 oC.

The configuration described above was designed in order to be similar (with some minor

differences) to the one used in previous experiments (Krevaikas & Triantafillou, 2005) so that

the results obtained from the current experimental program can be easily correlated. A total of

15 specimens were fabricated and divided in three groups of three, three and six specimens

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respectively. For each group an unreinforced control specimen was prepared and tested under

the same conditions. All 12 specimens strengthened using one layer of the proprietary Tyfo

EP-C Carbon Fabric system combined with Tyfo C-matrix polymer-modified mortar.

After rounding the corners, two holes were drilled 11 cm apart along the height, on one of the

sides of each specimen and two threaded rods were fixed using epoxy resin glue in order for

the external LVDT to be attached. Before wrapping the Carbon Fabric all masonry surfaces

were polished using an electric grinder.

Fig. 1 - Configuration of masonry specimens

After removing the loose particles and dust form the surface of the specimens, using wet

sponge, an initial layer of polymer modified mortar was applied to the masonry, in order to

achieve the desired surface evenness. After that stage, wrapping of the textiles was applied

keeping the fibers aligned to the specimen’s circumferential direction. The mortar was applied

simultaneously during the wrapping of the Carbon fiber textile. In order to evenly distribute

the mortar and the impregnation of fibers to be homogeneous a trowel was used. The above

procedure is illustrated in Fig.2. The finishing end of the textile overlapped the starting end by

approximately 100 mm.

The strength of the mortar was determined according to EN 1015-11 (1993), using a

servohydraulic universal testing machine. Flexural testing was carried out on 40X40X160 mm

hardened mortar prisms, at an age of 28 days. The mould used for the preparation of mortar

prisms, was identical with the one specified in EN 1015-11 (1993). For each batch of mortar,

three prisms were obtained and cured under the same laboratory environmental conditions as

the masonry specimens. All prisms were subjected to three point bending test, at a span of 140

mm, with a constant loading rate equal to 5 N/s. Each of the two fractured parts subsequently

subjected to compression until failure using two 40x40 mm bearing steel platens placed on the

top and bottom of each specimen part. The mean compressive strength of mortar was 5.27

MPa.

20 mm

240 mm

240 mm

≈300 mm

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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(a) ( b) ( c) (d)

(e) ( f) ( g)

Fig. 2 - Preparation of specimens: a) rounding the corners and fixing the threaded rods; b) Polishing the

surfaces; c) Initial layer of mortar; d-g) wrapping of the textile

The masonry units were subjected to compression test following the procedure proposed in

BS EN 772-1: 2011, using a universal compression testing machine. Six masonry units were

tested and prepared according to BS EN 772-1: 2011. Additional steel platens were used in

order to fill the distance between the bricks and of the machine’s load imposition device. The

normalized mean compressive strength of masonry units was 31.70 MPa. Finally, the

following properties (average values) were provided by the supplier of the fiber textiles and

the cementicious mortar: Elastic modulus and tensile strength of the Textile reinforced mortar

(design values) = 0.43 GPa and 7.14 MPa respectively.

The main objective of the test procedure was to examine the effect that the ambient elevated

temperatures have on the behavior of TRM confining jackets. A secondary objective is to

examine the effect of confinement using TRMs on all of the masonry specimens subjected to

axial loading applied monotonically under a force control mode in a compression testing

machine of 3,000 kN. The specimens of group B and group C were exposed to 100 oC and

200 oC prior to testing by placing them in heating chambers for eight hours. At the time of

testing all specimens, even those exposed to elevated temperatures were under room

conditions. Loads were measured using a load cell, and displacements were obtained using

external linear variable differential transducers (LVDTs) mounted on the walls, at a gauge

length of 110 mm in the middle part of each specimen.

RESULTS AND DISCUSSION

The experimental results are given separately for each group of specimens, categorized

according to the ambient temperature in which they were exposed. In table 1, the compressive

strength and ultimate strain for each group are summarized.

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Table 1 Specimen notation and summary of test results

Specimen type

Compressive

strength

(MPa)

Average

compressive

strength

(MPa)

Ultimate

strain

(εΜuc)

Group A

Co_20 1.19 - 0.0048

C1_1_R20_20 1.52

1.78

0.0476

C1_1_R20_20 1.97 0.0196

C1_1_R20_20 1.24 0.0196

C1_1_R20_20 1.69 -

C1_1_R20_20 2.85 0.018

C1_1_R20_20 1.40 -

Group B

Co_100 2.43 0.016

C1_1_R20_100 1.17

1.56

0.0008

C1_1_R20_100 1.54 0.0243

C1_1_R20_100 1.99 0.0116

Group C

Co_200 2.56 0.0044

C1_1_R20_200 1.89

2.52

0.035

C1_1_R20_200 3.37 0.033

C1_1_R20_200 2.32 0.030

(-): External LVDT failure

In fig.3 and fig. 4 the stress versus strain relationship is given both for the control and the

reinforced specimens of all three groups.

Fig. 3 Stress vs. Strain relationship for the control specimens of three groups.

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Fig. 4 - Stress vs. Strain graphs for the reinforced specimens of three categories.

(1) Specimens under room temperature conditions (20 oC)

The control specimen failed following a brittle pattern in which vertical cracks were formed

throughout the entire height before the ultimate stress had been reached (fig.5). The failure

mode of the reinforced specimens was similar to the one that the FRP-wrapped specimens

exhibit. In the early stages of the loading and before the ultimate strength of masonry core had

been reached the external jacket was not contributing to the overall behavior. Close to

ultimate strength, small vertical cracks started to show on the TRM jacket, a sign that the

masonry core was starting to expand significantly. After the compressive strength of the

masonry core was exhausted the external TRM jacket was fully activated. That was apparent

from the extensive swelling of the external reinforcement and the wide vertical cracks that

were uniformly distributed throughout the height of the specimens. At the final stage, failure

occur at the side of the specimen where the overlap has been made by initiation of debonding

from the end of the lap (Fig. 6 a)-d)). At that stage, that behavior was attributed to the low

compressive strength of masonry and therefore once the specimen’s core was completely

deteriorated the external jacket became very flexible and failed. However, the strengthened

specimens exhibit a considerable increase of the ultimate load, by about 38% and of the

ultimate strain by a factor of 7. Although failure started at the end of lap it was not abrupt but

rather smooth.

(2) Specimens exposed to 100 oC

All the specimens of group B were exposed for eight hours to 100 o

C ambient temperature

prior to testing. The control specimen behaved in the same way as the one of group A and

failed following a brittle pattern in which vertical cracks were formed throughout the entire

height before the ultimate stress had been reached. The major difference was the increase in

compressive strength which was almost doubled and that was indicative of the influence that

temperature has on masonry. The failure mode of the TRM-wrapped specimens was identical

to the one that group A specimens exhibited. In the early stages of loading micro-cracking of

the external jacket occurred followed by swelling and extensive wide cracks after masonry

reached its compressive strength, until failure. At the final stage failure occur at the side of the

specimen where the overlap has been made again by initiation of debonding (Fig. 6 a)-d)).

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That behavior contradicts the initial assumption that the failure of the jacket at the

overlapping side of the specimen occur due to the masonry’s low compressive strength. Since

the control specimen exhibited an increase in the compressive strength the aforementioned

assumption seems to be non-valid. The TRM-wrapped specimens displayed a slight decrease

in ultimate strength by 5.7 % and in ultimate strain by a factor of 3.4 when compared with the

results obtained from group A. Again the observed failure was not abrupt.

(3) Specimens exposed to 200 oC

The specimens of group C were exposed for eight hours to 200 oC ambient temperature prior

to testing. The behavior of the control specimen was identical with that of group B and

displayed a considerable increase in compressive strength.

Fig. 5 - Vertical cracking in control specimens

Although the TRM-wrapped specimens exhibited the same behavior until the ultimate

strength of masonry core has been reached at the final stage failure occurred when the lateral

expansion reached the capacity of the TRM jacket, which failed by fracture at the corners

(Fig. 7). Although the average ultimate strength was the same for the unreinforced and

reinforced specimens, an individual TRM-wrapped specimen displayed an increase in

strength by 31%, while the other two specimens displayed a decrease of 35% and 10%

respectively. However the reinforced specimens displayed a considerable increase in ultimate

strain by a factor of 2. The failure in this case was rather smooth initiated from a small

number of fibers and propagated slowly in the neighboring bundles.

From the graphs in fig.8 it is obvious that the elevated ambient temperature primarily affects

the behavior of the masonry core. Comparing the results for the TRM-wrapped specimens in

different temperatures it can be seen that those of group A (ambient temperature 20 oC) and

group B (ambient temperature 100 oC) are similar. Also specimens of group A and B

displayed similar behavior at failure. The results for group C specimens are different from

group A and B showing an increase in ultimate strength and a different mode of failure. The

influence of elevated ambient temperature affects mostly the deformability of the TRM-

wrapped specimens, showing a decrease of 25% and 34% for group B and C respectively

compared with the results obtained for group A and the mode of failure. The notable

difference between group C specimens mode of failure (failed when the TRM reached its

tensile strength) and group A and Group B specimens (debonding from the end of the lap)

indicates that above certain ambient temperature the interlaminar shear strength of the TRM

jacket has been affected. Further investigation needed in order to clearly define the

temperatures above which there is a change in the bond conditions of the composite system.

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

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a) b) c)

d)

Fig. 6 - Failure mode of group A and group B specimens a) micro-cracking of the TRM jacket; b)

vertical wide cracks distributed throughout the height; c) onset of failure at the point of overlap; f)

failure of TRM jacket at the point of overlap.

Fig. 7 - Fracture of the TRM-jacket at the corners.

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0 20 40 60 80 100 120 140 160 180 200 220 240

0.0

0.6

1.2

1.8

2.4

Ultimate strength(MPa)

Temperature (oC)

Influence of temperature on the ultimate strength of unreinforced specimens

0 40 80 120 160 200 240

0

1

2

3

4

TRM-wrapped specimens in room temperature

TRM-wrapped specimens exposed to 100 oC

TRM-wrapped specimens exposed to 200 oC

Stress (MPa)

Temperature (oC)

(a) (b)

Fig. 8 - Influence of ambient elevated temperature on a) URM specimens and b) TRM-wrapped specimens

CONCLUSIONS

Based on the results obtained from the test procedure presented in this paper the following

conclusions can be drawn:

The ultimate strength of URM specimens was increased after being exposed to elevated

temperatures affecting the overall behavior of the TRM-wrapped specimens.

For the TRM-wrapped specimens tested under ambient temperatures of 20 oC and 100

oC

the basic failure mechanism was the same depending on the interlaminar shear strength of

the composite jacket. Specimens of group A and B failed after debonding started at the

end of the lap. That explains why both group of confined specimens reached the same

ultimate compressive strength.

The TRM-wrapped specimens tested after being exposed to an ambient temperature of

200 oC increased their ultimate strength benefited from the fact that failure occurred away

from the anchorage area. In this case the composite jacket gradually failed after rupture of

fiber bundles started at the corners.

The exposure in elevated temperatures had a major effect in the deformability of the

TRM-wrapped masonry columns decreasing the ultimate strain of the reinforced

specimens exposed to 100 oC and 200

oC by a factor of 0.25 and 0.34 respectively when

compared with those of group A (ambient temperature of 20 oC).

For all groups of specimens the failure occurred in a rather smooth fashion giving ductile

characteristics to the overall behavior.

In general the TRM confining system used in this experimental program appears to be

quite effective after exposure to elevated ambient temperatures and substantially increased

the ultimate axial strain of the specimens. Nevertheless further investigation needs to be

carried out in order to determine the actual effect that ambient temperature has not only in

the interlaminar shear strength of the composite jacket but in the bonding conditions

between the TRM and the masonry substrate as well.

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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ACKNOWLEDGMENTS

The author wish to thank the final year student Wu Jicheng who provided assistance in the

experimental program. The work presented herein was funded by Xi’an-Liverpool University

within the framework of the program SURF.

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