1

Innovative technique of textile reinforced mortar (TRM) for flexural strengthening of

reinforced concrete (RC) beams

A. Baiee, I. Rafiq and A. Lampropoulos, University of Brighton, UK

ABSTRACT

Performance degradation of existing RC structures due to aging and environmental effects made

strengthening of RC structures a global issue. Bond between the substrate member and new

strengthening layer is considered a threshold for any successful strengthening technique. This study

explores the efficiency of using cementitious high strength connectors in preventing the debonding of

TRM strengthening layer from strengthened RC beams. The experimental program includes two parts. In

first part, the effect of strength, ratio, diameter and distribution of connectors, for smooth and rough

surfaces, on the tensile bond strength are examined. The applicability of these connectors is investigated

in second part by means of RC beams strengthened with TRM comprising four and eight textile basalt

fibre layers. The results demonstrate that the inclusion of cementitious connectors changed the failure

mode from debonding failure to desired flexural failure. The proposed improvement exhibited increasing

to cracking and ultimate load up to about 140% and 93%, respectively. However, a significant reduction

in ductility of all strengthened beams was observed in comparing with the control specimen.

NOMENCLATURE

C cement

G coarse gravel

S sharp sand

S.S silica sand

S.P superplasticizer

W water

fc cubic compressive strength

ft splitting tensile strength

P applied compressive load

fb splitting bond strength

ρc ratio of cementitious connectors

Aco area of cementitious connectors

Ac area of interface

n number of connectors.

d diameter of connector (mm).

D and L the short and long dimensions of half

cylinder (mm), respectively

μδ ductility index

δu deflection at ultimate load

δy deflection at steel yielding

1. INTRODUCTION

Many of the existing RC structures were

constructed based on old design codes that do

not supply the safety requirements

recommended by present design codes

especially for seismic and fire aspects.

Fibre Reinforced Polymer (FRP) composites

has been widely used for strengthening RC

structures. This technique has favourable

properties such as high strength to weight ratio

and resistivity to corrosion but also has

drawbacks mainly attributed to the organic

epoxy resins used to bind the fibres [1]. In

addition, epoxy has many weak points such as

poor fire resistance; high cost; and

inapplicability on wet surfaces or at low

temperatures; hazards for the manual worker;

poor thermal compatibility with the base

concrete [2,3,4,5].

Therefore, inorganic binder (cementitious

matrix) has been investigated to be used instead

of epoxy. Recently, textile fibres cementitious

technique has been investigated for

strengthening masonry and RC structures. This

technique is based on using textile polymer

fibres bonded with the substrate concrete by

using cementitious matrix [3]. There are

different forms of textile cementitious

composite; Textile Reinforced Mortar (TRM),

Textile Reinforced Concrete (TRC), Fibre

Reinforced Cementitious Mortar (FRCM) and

Mineral Based Composites (MBC). All these

forms have similar components with a slight

difference in property of the cementitious

2

matrix. This study adopted TRM form as a

textile strengthening technique.

Most of studies that investigated using TRM

in strengthening masonry structure explained a

debonding failure mode of the strengthening

layer [6,7,8]. Therefore, anchors to improve the

bond strength have been investigated.

Garmendia et al. [8] and Ismail and Ingham [9]

found using anchors can changed the failure

mode from de-bond to basalt breakage of

strengthening layer. However, Bernat et al. [10]

observed a reduction in the enhancement of the

capacity of masonry brick walls strengthened

with carbon textile reinforced mortar in

presence of carbon spike anchors. That

reduction may be attributed to the insufficient

penetration of anchors within the mortars which

in turns affect the adhesive bond between the

textile fibres and the wall.

In field of RC structures, similar debonding

observation of the TRM strengthening layer

was reported by many researchers

[3,11,12,13,14]. Some researchers studied

improve the bond by improving the adhesive

properties of the matrix [4,14,15]. The

enhancement of the strengthened beams was

attributed to the increasing the adhesive bond of

the matrix because of increasing the calcium

silicate hydrate (C–S–H) with reducing the

amounts of ettringite in form of Al2O3-Fe2O3-tri

(AFt) or calcium hydroxide (Ca(OH)2).

However, the improvement in bond strength

was insufficient to achieve the desired

enhancement of the strengthened members.

Therefore, some researchers investigated using

anchors to improve the bond strength [11, 16,

17]. Inspite of the achived enhancement but its

still unsufficient to provide the required bond

between the substrate concrete and

strengthening layer. That may be due to lack of

interlocking between these anchors and the

mortar.

In general, bond between old concrete and

new cementitious layer depends mainly on the

adhesive bond (chemical bond) and physical

bond of the interfacial zone (interface) between

the two layers. Chemical bond is resulted from

a chemical forces during the hydration of

cement and forming calcium silicate hydrate

(C–S–H) [18]. As the (C-S-H) increases at the

interface, the chemical bond strength increases.

Many researchers found that using cementitious

matrix with high compressive strength can

exhibit high bond strength with old concrete

[19,20,21,22].

The physical bond depends mainly on the

mechanical interlocking between the old and

new concrete. The interlocking is highly

effected by the surface preparation of the

substrate concrete. Many researchers found

that higher roughness can lead to high bond

strength [23,24,25,26,27,28,29,30].

It can be stated that bond between the

substrate member and new strengthening layer

is considered a threshold for any successful

strengthening technique. Therfore, the present

work investigate improving the bond strength

of strengthened RC beams with TRM by

enhancing the chemical and physical bond of

the interface. The chemical enhancement

includes using high strength mortar as a matrix

for TRM. While, high strength cementitious

connectors is investigated to enhance the

physical bond.

2. EXPERIMENTAL PROGRAME

2.1 METHODOLOGY

The experimental work consisted two parts;

the first part deals with investigation the effect

of cementitious connectors on the splitting

bond strength of hybrid cylinders. The

objective of this part was to determine the

connector ratio which can explain bond

strength approximately equals to the splitting

tensile strength of the substrate concrete.

Hybrid cylinders are made up of half cylinder

concrete which casted and cured for 28 days

and half mortar casted after preparation the

surface of the cured half concrete cylinder.

The second part included casting five RC

beams and after 28 days water curing, the TRM

strengthening layer was casted and cured in

water for 28 days. All RC beams were tested

3

under three points flexural loading. The

investigated parameters included; strength of

the mortar of TRM (normal and high strength

mortar) and amount of textile basalt fibre (four

and eight layer). In addition, the mechanical

properties of substrate concrete and TRM

strengthening layer were determined.

2.2 MATERIALS PROPERTIES

Ordinary Portland cement type CEMI 52.5N

that satisfies BS EN 197-1:2011 standards for

Portland cement were used for casting concrete

of beams, half cylinders and mortars of TRM.

Crushed gravel with10mm maximum aggregate

size analogous to BS EN 12620:2002+A: 2008

was used. Sharp sand with maximum size

aggregate 4mm satisfying BS EN

12620:2002+A:2008 was used for casting

concrete. For mortar, fine silica sand that

satisfies the requirements of BS EN

12620:2002+A:2008 was used. Fosroc

Auracast200 high performance concrete

superplasticizer based on polycarboxylate

polymers was used to improve the workability

of mortar. Textile basalt fibres with opening

meshes size 5mm were used for composing the

TRM strengthening layer, table 1 presents the

main properties of these fibres.

Table 1: Properties of textile basalt fibres Thickness (mm) 0.6 – 0.7

Melting point (C˚) 1350

Type of coating Styrene-acrylic latex

Maximum load (N/5cm) 1873.86 (warp)

4416.29 (weft)

Elongation at beak (%) 4.26 (warp)

10.26 (weft)

2.3 MIX PROPORTIONS

Mix proportions of cylinders samples and RC

beams were designed to provide concrete grade

C40/50. Cubic specimens (100×100×100) mm

that satisfies BS EN 12390-3:2009 were used to

assess the compressive strength. Splitting

tensile test of cylinders (100 mm diameter and

200mm length) was carried out to determine the

tensile strength of the concrete at 28 days

according to BS EN 12390-6:2009. Standard

slump test was carried out according to BS EN

12350-2:2009 to assess the workability of

concrete. The obtained slump values of the

mixes ranged between 75 to 100 mm.

Two different mortar grades were used in this

study; M50 with compressive strength about 50

MPa for NSM and M100 for HSM. To provide

sufficient penetration of mortar inside the holes

of the substrate concrete and textile basalt fibre,

high workability should be provided. Flow

table test that satisfy the BS-EN 1015-3:1999

was conducted to assess the workability. The

results of the mortar flow ranged between 225

and 250 mm. The mix proportions and

mechanical properties of concrete and mortars

are listed in table 2.

2.4. BOND BETWEEN CONCRETE

AND MORTAR

Hybrid cylinders were tested under splitting

compressive load to assess the bond strength

between old concrete and new mortar. Hybrid

cylinders are made up of half cylinder normal

strength concrete (NSC) which casted and

curing for 28 days and half mortar casted after

preparation the surface of the cured half

cylinder. The hybrid specimens cured again for

28 days before testing. The process of casting

and testing is shown in Fig. 1. In this work, the

ratio of connectors is the total holes area

divided by the contact area (interface area)

between the old concrete and new mortar.

Different ratios; 0.85, 1.7, 2.5, 5 and 10 of the

connectors were investigated.

…………. (1)

………….. (2)

Splitting bond strength is evaluated according

to the following equation:

………………. (3)

4

Fig. 1. Process of casting and testing of hybrid cylinder

Two surface preparations were adopted; as

casted (AC) and rough (R). The diameter of the

cementitious connector ranged between 6mm

and 12mm, with an average depth equal to

10mm. These dimensions were selected after

many trials to avoid destroying the substrate

half cylinder concrete, see Fig.2. Rough surface

was achieved by roughening the half concrete

cylinders using needle scaler hammer for

removing 1-2 mm from the surface. Also,

normal strength mortar investigated to assess

the effect of mortar properties on the bond

strength. Control specimen represents the

results of splitting tensile strength of normal

strength concrete. The configurations of the

tested hybrid cylinders are reported in table 3.

As casted Rough

As casted with connectors Rough with connectors

Fig. 2. Prepared surface texture of the substrate concrete

Table 2: Mix proportions and mechanical properties of the adopted mixes

Mix C

(kg/m3)

G

(kg/m3)

S

(kg/m3)

S.S

(kg/m3)

S.P.

(kg/m3)

W

(kg/m3)

Fc -

7days

(MPa)

Fc -

28days

(MPa)

Ft

(MPa)

NSC 450 800 785 -- -- 225 44.1 54.4 3.78

NSM 540 -- -- 1350 15 270 40.3 51.3 3.1

HSM 1050 -- -- 1050 45 210 85.3 104.8 5.7

Table 3: Configuration of hybrid cylinders

Specimen Surface Connectors’ ratio

(%)

CONT -- --

R0 R --

AC0 AC --

RONSM R --

ACM0.85-6C AC 0.85

ACM0.85-6E AC 0.85

ACM1.75 AC 1.75

ACM2.5 AC 2.5

ACM5.0 AC 5

ACM10 AC 10

RM1.75 R 1.75

RM0.85-6C R 0.85

RM0.85-6E R 0.85

NSC

HSM

5

3ɸ 8mm 2ɸ 6mm

P

345mm 345mm

750mm 150mm

150

mm

2.5 RC BEAMS STRENGTHENED WITH

BASALT TRM

Normal strength concrete (NSC) beams with

dimensions (150x150x750mm) reinforced with

three bars of diameter 8mm in longitudinal

direction and 6mm diameter for shear

reinforcement were adopted. The yield stress of

the bars was 380 MPa. The reinforcement

details are illustrated in Fig.3

After preparing the required reinforcement of

the RC beams, concrete was casted in steel

moulds.

Fig. 3. steel reinforcement details of RC beams

After water curing period (28 days), needle

scaler hammer was used to removing 1-2 mm

from the surface of RC beams before casting

the TRM layer. After that, hammer drill

machine was used to make holes inside

substrate concrete beams with diameter equal to

7mm. The applied ratio of connectors was 8.75,

see Fig.4.

Roughnening of surface

Drilling the holes

Fig. 4. Process of preparing RC beams

Five RC beams were tested under three-point

flexural loading. One RC beams without

strengthening (CON) to serve as a control

beam. One beam strengthening with normal

strength TRM and without connectors (B1).

One beam was strengthened with four layers of

high strength mortar TRM and with connectors

(B2). Like this beam, two beams (B3) and (B4)

with eight strengthening layers was tested to

investigate the effect of number of layers on the

performance of the strengthened beams. For all

strengthened beams, prefixed of basalt fibre and

then casting mortar was used. Only B3 a hand-

lay-up method was applied for casting the TRM

strengthening layer, see Fig. 5. Table 4 presents

the details of the investigated beams. Zwick

device with ultimate load 200kN was used for

testing with rate of loading 0.25 mm/min, see

Fig. 6.

Table 4: Detailed of the tested beams Beam Type of

mortar

Ratio

connectors

No. of

layers

CON --- 0 0 B1 NSM 0 4

B2 HSM 0.0875 4

B3 HSM 0.0875 8

B4 HSM 0.0875 8

a. pre-fixed of fibres b. hand-lay-up

Fig. 5. Details of casting TRM on RC beams

6

Fig. 6. Test set up of RC specimens

3. TEST RESULTS AND DISCUSSION

3.1 BOND TEST

Three specimens for each case were adopted.

HSM specimens exhibited 20% increasing in

bond strength in comparison with NSM

counterparts. This attributed to the presence of

high cement content in the HSM mixture,

which leads to increase the C-S-H components.

HSM explained bond with the particles of

substrate concrete higher than the bond

between components of substrate concrete as

shown in Fig. 8. The obtained results are listed

in table 5.

The results showed that the lowest bond

strength was observed for AC0 specimens

because of untreated surface (as cast) is usually

weak. However, the inclusion of connectors

improves the bond strength because of

increasing the contact area at the interface and

reaching stronger layer than the surface.

Adhesive bond of smooth surface without

connectors depends only on the chemical

interaction between old concrete and new

mortar. While in presence of connectors, the

adhesion bond will depend on the tensile

strength of connectors in addition to the

chemical interaction. That means as the tensile

strength of the connectors increased, the

adhesive bond increased.

In addition, it was found a negligible effect of

the distribution of the connectors on the bond

strength. All connectors exhibited perfect bond

inside the substrate concrete, see Fig.8. Despite

the achieved enhancement of bond because of

application the cementitious connectors, but

they still provide bond strength lower that

rough surface without connectors. Specimen

ACM1.75 explained about 58% of the rough

surface counterpart (RM1.75). That explains

the importance of removing the outer thin layer

of the substrate member.

Table 5: Summary of bond test results

Specimen fb (MPa) C.O.V (%)

CONT 3.78 4

R0 2.85 2

AC0 1.0 18

RONSM 2.37 4

ACM0.85-6C 1.57 4

ACM0.85-6E 1.6 13

ACM1.75 1.71 5

ACM2.5 1.74 12

ACM5 2.03 9

ACM10 2.29 2

RM1.75 2.91 13

RM0.85-6C 3.04 6

RM0.85-6E 3.51 6

However, for rough surface, the results

showed that the distribution of connectors can

affect the bond strength. All specimens with

connectors explained higher bond strength than

counterparts without connectors.

The highest achieved bond strength was with

using rough surface and the connectors

distributed at the edges of the interface. Despite

the high coefficient of variation (c.o.v.) of most

results, it can be stated rough surface with

inclusion cementitious connectors can provide

bond strength equal to the bond strength

between the components of substrate concrete.

3.2 RC STRENGTHENED BEAMS

Flexural failure was observed for all tested

beams except B1, where it exhibited de-

bonding of the strengthening layer at the mid

span before reaching ultimate load. The de-

bonding of strengthening layer started at load

equal to 50.2kN. While, B2 explained perfect

bond between the substrate RC and TRM

strengthening layer until reaching the ultimate

tensile capacity of the basalt fibre. Fig. 9 shows

the crack pattern at failure load of the tested

beams.

LVDT-1

LVDT-2

Actuator

Support

Support

345mm 345mm

7

ROHSM AC0 RONSM ACM0.0085-6C ACM0.0085-6E ACM0.0175

ACM0.025 ACM0.05 ACM0.1 RM0.0175 RM0.0085-6C RM0.0085-6E

Fig. 8: Failure mode of bond specimens

The strengthening layer (TRM) of beam B2

explained a flexural failure and the failure was

due to fail of basalt fibre mesh. While a de-

bonding failure was observed at the mid span of

beam B1, see Fig9. This de-bonding reduced

the enhancement of the strengthening layer to

the ultimate strength from 30% to 21%, see

Fig.10. In addition, the debonding limited the

enhancement up to 21%. Where, there is no

expected enhancement if the amount of basalt

fibres increased.

The highest increasing in ultimate capacity

about 93% was observed for beam B3.

However, beam B4 exhibited lesser increasing

in ultimate capacity about 79% than beam B3.

Both beams were strengthened with the same

properties of connectors and textile fibres. That

was due to the difference in method of

application the textile layers, which affect the

location of fibres within mortar. The textile

fibres in beam B3 were closer each to other,

which lead to loading them at the same time

and presenting higher tensile strength than

beam B4. This explains the effect of fibre

position within the matrix on the tensile

strength of the TRM layer. All results are

reported in table 6.

Moreover, beams strengthening with eight

layers of textile basalt fibres exhibited cracking

load (load at first crack) higher than the

ultimate load of the control beam. That means it

can be convert a RC beams from cracked

members under ultimate loading to un-cracked

member under the same amount of loading

which enhanced the durability of the

strengthened members.

All strengthened beams explained higher initial

stiffness than control beam, see Fig. 11. The

ductility of the tested beams is calculated based

on deflection ductility index based on the

following equations:

μδ = δu / δy ……….. (4)

Table 6: Results of tested beams

Specimen Pc

a

(kN)

∆cb

(mm)

Puc

(kN)

∆ud

(mm) Pc/ Pco

e ∆c /∆co

f Pu/ Puo

g ∆u /∆uo

h Failure mode

CON 22.1 1.2 45.5 12.1 -- -- -- -- Flexural

B1 29.6 1.6 54.9 4.7 1.34 1.33 1.21 0.39 Debond-flexural

B2 31 1.5 59 4.1 1.4 1.25 1.3 0.34 Flexural

B3 53.2 2.5 87.6 8.3 2.41 2.08 1.93 0.69 Flexural

B4 51.7 2.3 81.6 5 2.34 1.92 1.79 0.41 Flexural a. Load at first crack (kN). b. Deflection at first crack (mm). c. Ultimate load (kN). d. Deflection at ultimate load

(kN). e. Load at first crack of control beam (kN). f. Deflection at first crack of control beam (mm). g. Ultimate

load of control beam (kN). h. Deflection at ultimate load of control beam (mm).

8

Fig. 9. Failure mode of strengthening RC beams

Fig. 10. Enhancement of strengthening

All strengthened beams explained lower

ductility than control beam due to the increase

the cross section of the strengthened beams as

shown in table 7. Beam B1 explained higher

ductility than beam B2 because of the

debonding which leads to reduce the cross

section of the strengthened beams. In addition,

the stiffness of TRM with HSM is higher than

NSM counterpart. Beam B3 and B4 exhibited

approximately same ductility because both

presented high bond strength until ultimate

load.

Table 7: Ductility of the tested beams

Specimen δu (mm) δy (mm) μδ

CON 11.56 1.2 9.6

B1 2.8 0.93 3

B2 2.2 1.1 2

B3 5.6 1.85 3

B4 3.1 1.15 2.75

Fig. 11. Load deflection curves of the tested

0

50

100

150

CON B1 B2 B3 B4

Incre

asin

g in

lo

ad

(%

)

Beams

ultimate load

cracking load

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15

Load

[kN

]

Deflection [mm]

CON B1

B2 B3

B4

CON

B1 B2

B3 B4

9

4. CONCLUSIONS

The main conclusions that can be drawn are

as following:

- The application of cementitious

connectors improves the bond strength

of smooth surfaces. However, this

improvement is less effective than

rough surface. It is effective with rough

surface to achieve bond strength higher

than substrate concrete.

- High strength mortar exhibits higher

bond strength up to about 20% than

normal strength mortar counterparts.

- Application of high strength

cementitious connectors improves the

bond strength between the substrate

concrete and TRM strengthening layer.

This improvement depends on the

properties and amount of textile fibres.

- Strengthening RC beams with eight

layers of textile basalt fibre increase the

ultimate load up to 93% and the

cracking load up to 140%.

- All strengthened beams explained lower

ductility than control beams because of

the increasing in cross section of the

strengthened beam. The reduction in

ductility affected by properties of the

bond and TRM strengthening layer.

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