Construction Building Mat 2009 2

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Rehabilitation of reinforced concrete axially loaded elements

with polymer-modified cementicious mortar

Carlo Pellegrino *, Francesca da Porto, Claudio Modena

Department of Structural and Transportation Engineering, University of Padova, Via Marzolo 9, 35131 Padova, Italy

a r t i c l e i n f o

Article history:

Received 21 November 2008Received in revised form 16 May 2009

Accepted 18 June 2009

Keywords:

Reinforced concrete

Repair mortars

Interface

Cracking

Columns

a b s t r a c t

The aim of the paper is to investigate the compatibility and the efficiency of the rehabilitation interven-

tion on reinforced concrete columns with polymer-modified cementicious mortar. This paper presents

the results of experimental tests on axial behaviour of reinforced concrete columns, with square cross-

section, repaired by polymer-modified cementicious mortar. Tests were repeated varying repair thick-

ness, which included or did not include the steel reinforcement on one face of the square column. Despite

this type of intervention is quite common in practice, the effect of repair thickness on the intervention

efficiency, in relation to the existing steel reinforcement configuration, had not been previously studied

in detail for axially loaded elements.

Results were discussed and compared with those from control columns, which were tested in non-

damaged, non-repaired conditions. The main findings of this work can be summarized as follows. The

repair cannot restore the load-bearing capacity of non-damaged control columns, although they give

acceptable results. Repairs that include the longitudinal reinforcement show good properties, with stable

behaviour, sharing of loads, and plasticization of the material before failure, whereas thin repairs that do

not include the reinforcement do not have adequate performance due to premature debonding. Non-lin-

ear numerical models also confirmed the different behaviour of the two types of repair.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

The field of rehabilitation and strengthening of reinforced con-

crete structural elements shows an increasing interest for existing

constructions and various projects have been carried out around

the world over the past two decades. Structural strengthening

and repairing is aimed at increasing or restoring the load-bearing

capacity of the element, due to changes in conditions of use (e.g.

increased loading) or deterioration and damage of the concrete

structure (for example due to environmental conditions or seismic

events). Historically, steel has been the primary material used to

strengthen concrete structures. Bonded steel plates or stirrups

have been applied externally to successfully repair reinforced con-

crete elements. However, using steel as a strengthening element

adds additional dead load to the structure and normally requires

corrosion protection. Externally bonded fiber-reinforced polymers

(FRP) sheets/plates exhibit several attractive properties, such as

low weight-to-strength ratios, non-corrosiveness, and ease of

application. A number of experimental programs and analytical

studies have been developed in the last few years at the University

of Padova on flexural [19,28], shear [16,17,18] and bond behaviour

[21,20] of FRP strengthened elements. In this context, adding or

applying mortar, spraying concrete or mortar with the aim of reha-

bilitating and/or strengthening of existing reinforced concrete

structures is also a possible way of intervention with a more tradi-

tional and common material [5].

Emberson and Mays [1] carried out one of the first extensive

experimental studies on the influence of mechanical and physical

properties of repair mortars, applied on axially loaded (in tension)

reinforced concrete elements. They numerically modelled the axial

load transfer through repair and substrate in the linear elastic

range. They also worked on flexural elements, and studied the ef-

fect of repairs applied either in the compression or tension regions

of reinforced concrete beams [2]. Following, most research focused

on flexural elements. For example, Hassan et al. [9] tested the com-

patibility of cementicious, polymer, and polymer-modified mortar

repairs to concrete. Ro et al. [24] tested beams designed to fail in

flexure, after localized artificial corrosion at midspan and localized

patch-repair with three types of mortar (cement based, epoxy resin

binder, and polymer-modified mortar). Park and Yang [15] tested

eight beams repaired in the tension region with ordinary Portland

and polymer-modified cement mortar. They varied reinforcement

ratio and repair length. Shannag and Al-Ateek [26] tested 30

under-reinforced concrete beams, repaired in the tension region

with five materials: ordinary Portland cement and four types of

fiber-reinforced cementicious materials. Once repaired, the beams

were tested as they were or after accelerated corrosion. Nounu and

0950-0618/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2009.06.025

* Corresponding author. Tel./fax: +39 049 8275618.

E-mail address: [email protected] (C. Pellegrino).

Construction and Building Materials 23 (2009) 31293137

Contents lists available at ScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t
http://dx.doi.org/10.1016/j.conbuildmat.2009.06.025mailto:[email protected]://www.sciencedirect.com/science/journal/09500618http://www.elsevier.com/locate/conbuildmathttp://www.elsevier.com/locate/conbuildmathttp://www.sciencedirect.com/science/journal/09500618mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2009.06.025


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Chaudhary [14] compared ordinary Portland cement with free

flowing micro-concretes, obtaining better results with the latter.

Kim et al. [12] applied fiber-reinforced cementicious materials at

the intrados of reinforced concrete beams with and without stir-

rups. Recently, Jumaat et al. [11] made a review of various repair

materials and techniques for reinforced concrete beams.

Experimental review of ten different repair methods for axially

loaded columns, which took into account not only the structuralperformance and failure modes, but also the applicability and

cost-efficiency of the repairs, was carried out by Ramirez [23],

who found good results for methods based on application of

cementicious materials. Among the wide literature on repair of

reinforced concrete columns oriented to the seismic field (a com-

prehensive literature review can be found in [6], few authors con-

sidered the case of jacketing with mortar or concrete without

adding additional reinforcement. Fukuyama et al. [7] tested eight

damaged columns, repaired with different techniques, and found

that replacement of cover with concrete can restore the original

column shear strength and deflection capacity, without any in-

crease of cross-section or steel percentage. The use of shrinkage

compensating mortar can even improve strength, although faster

strength degradation may occur. In general, the main aim of

cementicious repair of concrete columns is to assist the repaired

columns to carry the axial load, particularly when a significant

amount of material in compression is lost due to the action of cor-

rosion. Rahman et al. [22] numerically studied the problem of dry-

ing shrinkage and creep in cementicious repair of reinforced

concrete columns. Shambira and Nounu [25] experimentally

studied the long-term behaviour of this type of intervention. They

applied two types of localized repairs, polymer and polymer-mod-

ified mortars, on one side of axially loaded columns. Despite the

short-term behaviour was acceptable, the long-term behaviour

worsened due to high shrinkage, and shrinkage forces induced par-

asite bending in the columns. Mangat and OFlaherty [13] applied

seven different ordinary and polymer-modified cementicious

materials, on the unpropped compression members of two existing

bridges. The case studies showed that repairs displayed structuralinteraction with the structure, and those made with stiff materials

were more efficient than others, which is in disagreement with

other results. For example, Sharif et al. [27] assessed the effective-

ness of localized repairs on two sides of axially loaded columns.

They applied two cementicious materials with low and high elastic

modulus, under loaded and unloaded conditions. They demon-

strated that repairs are structurally effective only if applied on un-

loaded columns, or become effective only if further loads are added

to the columns. The load distribution between the repair layer and

the concrete is even, only if the elastic modules of the two materi-

als are similar.

Some issues related to the rehabilitation of reinforced concrete

columns, such as the choice of the repair material properties and

its geometric configuration with the aim of improving the compat-ibility of the intervention, are thus still objects of research. In this

framework, the effect of thickness of repair material on the effi-

ciency of rehabilitation interventions, in relation to the existing

steel reinforcement configuration, was not studied in detail for axi-

ally loaded elements.

The objective of this study is to give some new insights on val-

idating the effectiveness of repair intervention with polymer-mod-

ified cementicious mortar repairs applied to square columns, under

axial loads, to recover the original properties of columns. In partic-

ular the aim is to verify the effect of repair thickness (including

steel reinforcement or not), on cracking pattern and, in general,

structural behaviour of axially loaded element. For this reason,

experimental and numerical study on square columns, subjected

to axial loads, repaired with polymer-modified cementicious mor-tar is carried out. The repair material had similar mechanical prop-

erties, slightly higher tensile strength than the concrete substrate

and two different thicknesses (including steel reinforcement or

not) and was applied over the entire length of one face of the col-

umns. Experimental results were compared in terms of cracking

pattern, ultimate capacity, axial and transversal strains with those

from control columns, which were tested in non-damaged, non-re-

paired conditions.

Simplified three-dimensional numerical models, implementingnon-linear constitutive laws for materials, were developed to sim-

ulate the behaviour of control and repaired columns.

2. Experimental program

The main objective of the experimental program was to assess the static behav-

iour under axial loading of six reinforced concrete columns repaired with polymer-

modified cementicious material. The following subsections describe the specimens

used for experimental testing, the materials adopted for their construction and re-

pair, and the testing procedure.

2.1. Design and preparation of specimens

Six columns were made with square section of 300 300 mm area, 20 mm con-

crete cover, 0.8 m total height. Longitudinal reinforcement was constituted by four

12 mm diameter reinforcing bars. Stirrups having 8 mm diameter were placed at

140 mm spacing. The six columns were divided into three test series. Two speci-

mens were used as control columns (P00_a;b) and were tested in non-damaged/

non-repaired conditions. Other four columns were repaired on one face. They were

cast leaving the reinforcement non-covered with a curing process of 28 days. After

this period, the non-covered surface was prepared. The preparation of the surface

included roughening, cleaning of dust, powders and any impurities to improve

the adhesion between concrete core and mortar, and wetting. The polymer-modi-

fied cementicious mortar for repair was applied after eventual evaporation of water

in excess.

Repair mortar was 50 mm thick and included the longitudinal reinforcement on

two columns (P50_a;b). 15 mm of repair mortar were sufficient to obtain the origi-

nal 300 300 mm section on other two columns (P15_a;b). In the latter case, repair

was intended to restore only the concrete cover. Fig. 1 shows the details and Table 1

lists the data of the tested columns.

2.2. Materials

The main mechanical properties of concrete were experimentally evaluatedafter 28 days curing. Average compressive strength on 150 150 150 mm cubic

specimens, determined from results of four samples casted during the column

800 800

P0_a;b

P

300

800

2 12

300

2 12

300

P15_a;b

P

300

2 12

300

2 12

300

P50_a;b

P

300

2 12

300

2 12

300

15 50

Stirrup spacing 140mm

Fig. 1. Dimensions, rebars and repairs arrangement of columns.

3130 C. Pellegrino et al. / Construction and Building Materials 23 (2009) 31293137


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construction was 34.8 N/mm2. Mean tensile strength, measured by splitting tests on

three cylindrical samples having diameter 150 mm and height 300 mm, was 3.19 N/

mm2. Elastic modulus was not measured, but according to the measured cubic com-

pressive strength and Eurocode 2 formulations [3], it was assumed to be around

32,500 N/mm2.

Ribbed bars used for longitudinal reinforcement and for transversal reinforce-

ment were both tested in tension. Mechanical properties were similar for the two

types of bars; mean yield stress was 532 N/mm2 and mean tensile strength was

628 N/mm2. Strain at failure was 25%.

Finally, the cementicious material, used for repairing all columns, was pre-

mixed, tixotropic, polymer-modified mortar with high-strength hydraulic binders

and aggregates having maximum thickness of 4 mm. This product has high bond

properties, low CO2 and vapour permeability, limited shrinkage. It is generally used

for cover repair in reinforced concrete structures. Mechanical properties of the re-

pair mortar were measured on samples having dimensions of 40 40 160 mm,

cast during the repair interventions on columns. These samples were tested after

28 days curing. Density of hardened mortar was 2170 kg/m3. Mean tensile strength

deducted from six flexural tests was equal to 3.48 N/mm2. Mean cubic compressive

strength (six samples) was 39.6 N/mm2, mean elastic modulus (three samples) was

26,200 N/mm2. Table 2 compares the mechanical properties of the concrete support

and the repair material. It can be seen that the measured compressive strengths dif-

fer for 4.8 MPa, the measured tensile strengths differ for about 0.3 MPa and con-

crete and mortar elastic modules differ for less than 10 kN/mm 2.

2.3. Testing procedures

Axial tests on columns were carried out monotonically, under a 10,000 kN load-

ing machine, with loads increased between 0.5 and 2.5 kN/s. Pressure transducer

mounted on theloadingmachine wasusedto measurethe applied loads.The control

columns were instrumented with six strain transducers (DD1; 100 mm measuring

base), placed at mid-height along the columns. Four DD1 were placed on two adja-

cent orthogonal faces, two in the horizontal and two in the vertical direction, to

measure transverse and axial strains. On the other two faces, two horizontal DD1

were placed close to the column corners, in order to gather information on possible

instability of the reinforcement. Two linear variable differential transducers (LVDT)

with 600 mm measuring base were also placed vertically on two faces, to measure

overall axial strains of the columns. For the repaired columns, other two strain

transducers (DD1) were placed across the repair layer-concrete column interface,

to gather information on the behaviour of the interface. The other instruments were

placed on two adjacent orthogonal faces, one of which was repaired and the other

left in the original conditions, in order to gather information on the behaviour of

the repair material. Fig. 2 shows thetest setupand instrumentationon thefour sides

of a repaired columnand some details of displacement andstrain transducers on the

repair layer and across the interface between concrete support and mortar layer.

3. Test results

In thefollowing themain resultsof theexperimental programare

shown in terms of failure modes, cracking patterns, stress vs. (axial

and transversal) strain curves of undamaged and repaired columns.

3.1. Failure modes and ultimate loads

All columns showed compressive failure with crushing of con-

crete. Vertical and sub-vertical cracks generally developed close

to one column end, where damage was concentrated. Cracks also

connected each other close to the column corners, with spalling

of the reinforcement cover at corners. Differences in failure modes

were determined by presence and thickness of the repair. Fig. 3

shows the crack patterns of the tested columns.

For the control columns P00_a and P00_b, failure occurred by

crushing of concrete, respectively, at the lower and at the upper

end of the column. Cracks had vertical or sub-vertical patterns

and were distributed on the four column sides. Larger cracks were

located close to the column corners and were connected to those

on the adjacent column face. Ultimate loads reached by the control

columns, 2929 and 2869 kN, corresponded to average stresses on

the cross-sections of 31.0 and 31.9 N/mm2 (see Table 3).

Columns P15_a and P15_b were repaired on one side only, with

15 mm thick mortar layer. At failure, cracks had vertical or sub-

vertical patterns and were distributed on the three non-repaired

column sides. They were localized at the upper end of column

(P15_a) and on the overall specimen height (P15_b). Larger cracks

were located close to the column corners and were connected to

those on the adjacent column faces. However, in both specimens,

the repair did not crack, but clearly debonded from the concrete

substrate (Fig. 4) at 1901 and 1575 kN, corresponding to average

stresses on the cross-sections of 21.1 and 17.5 N/mm 2 (76% and

58% of ultimate load). Ultimate loads were 2507 and 2709 kN,

respectively, corresponding to average stresses on the cross-sec-

tions of 27.9 and 30.1 N/mm2, on average 92% of the ultimate

capacity reached by the control columns (see Table 3).

Columns P50_a and P50_b were repaired on one side only with50 mm thick mortar layer, and showed different behaviour. Cracks

had vertical or sub-vertical patterns at failure and were distributed

on the four column sides, including the repaired one. The repair

only partially debonded from the concrete substrate at the column

corners (Fig. 5), at 2430 and 2160 kN, corresponding to average

stresses on the cross-sections of 27.0 and 24.0 N/mm2 (93% and

86% of ultimate load). Ultimate loads were 2606 and 2501 kN,

respectively, and corresponded to average stresses on the cross-

sections of 29.0 and 27.8 N/mm2, on average 90% of the ultimate

capacity reached by the control columns (see Table 3).

3.2. Stresses and strains

Fig. 6 shows the stressstrain curves of three columns: withoutrepair (P00_a), with 15 mm thick repair (P15_a) and with 50 mm

thick repair (P50_a). In these diagrams, axial strains (compression)

are plotted positive and transversal strains (tension) negative.

Fig. 7 compares all stressaxial strain curves, with axial strains

measured on the non-repaired sides of the columns (above) and

on the repairs (below; in this diagram, axial strains of control col-

umns are left as reference values). Fig. 8 compares all stresstrans-

versal strain curves, with transversal strains measured on the

non-repaired sides of columns (above) and on repairs (below; in

this diagram, transversal strains of control columns are left as

reference values). In the diagrams of Fig. 7 and 8, axial and trans-

versal strains are both plotted positive.

In control columns, the strains measured on orthogonal faces

had the same trend and the stressaxial strain relationship was al-most parabolic (see P00_a in Fig. 6). The ratio of transversal to axial

Table 1

Details of specimens.

Type of element/test Section (mm2) Longitudinal reinforcement ql (%) Transversal reinforcement qw (%) Condition Designation

Tension Compression

Column 300 300 4U12 0.50 1U8/140 mm 0.24 Control column P00_a; P00_b

Axial Repair 15 mm P15_a; P15_b

Repair 50 mm P50_a; P50_b

Table 2

Mechanical properties of concrete and repair mortar.

Property Concrete Mortar

Density of hardened material (kg/m3) 2380 2168

Mean cubic compressive strength (N/mm2) 34.8 39.6

Mean elastic modulus (kN/mm2) 32.5a 26.2

Mean tensile strength (N/mm2) 3.19 3.48

a Evaluated on the basis of EN 1992-1-1.

C. Pellegrino et al. / Construction and Building Materials 23 (2009) 31293137 3131


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strains in the elastic phase, on average, was about 0.2. Axial

strains at ultimate load, on average, were 2.45 103, i.e. failure

occurred after concrete plasticization (see Table 4).

The columns repaired with thick mortar layer (see P50_a in

Fig. 6) showed similar behaviour. The strains measured on orthog-

onal faces had the same trend but, in this case, they were gauged

on the repaired and the non-repaired sides of the column, which

were thus working together, although the repair always gave high-

er values of strain. The ratio of transversal to axial strains mea-sured on the non-repaired sides of the columns in the elastic

phase, on average, was around 0.22. Despite the higher strains

measured, also the ratio of transversal to axial strains measured

on the repairs was, on average, 0.20 (see Table 4). The repairs par-

tially debonded at the column corners at average stress level of 90%

of the ultimate capacity. When this circumstance occurred, repair

mortar had already started plasticizing on both columns (average

axial strains on repair of 2.33 103; Table 4). At that point,

the stiffness of the repaired columns, which initially was higher

than that of the control columns, decreased and became lower(Fig. 7). Debonding, however, was not complete since the non-re-

STRAIN TRASDUCER

LVDT TRASDUCER

STRAIN

TRASDUCERS

LVDT TRASDUCER

Fig. 2. Test setup and instrumentation for axial tests on columns.



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paired and repaired sides of the columns kept working together

(see P50_a in Fig. 6) until reaching the ultimate load. Average axial

strains on the non-repaired and repaired sides of P50_a and P50_b

at ultimate load were 2.25 103 and 3.01 103, i.e. failure

occurred with plasticization of both concrete on the non-repaired

column sides, and mortar repairs (see Table 4).

The columns repaired with thin mortar layer (see P15_a in

Fig. 6) presented different behaviour. The strains measured on

orthogonal faces, i.e. on the repaired and the non-repaired sides

of the columns, had the same trend only during the first elastic

phase. During this phase, the ratio of transversal to axial strains

measured on the non-repaired sides of the columns, on average,was again around 0.21, and it was slightly lower on the repaired

sides, where the axial strains were higher (in average 0.17; Table

4). At stress level of 76% and 58% of the ultimate capacity (in P15_a

and P15_b, respectively), the repairs debonded from the concrete.

When this circumstance occurred, the repairs could not carry load

any more, as demonstrated by the strain release (see P15_a in

Fig. 6; axial strains of repair for P15_a and P15_b in Fig. 7 and

transversal strain across the interface between concrete support

and mortar layer for P15_a and P15_b in Fig. 8). In this case, plas-

ticization had not yet started (average axial strain on repaired and

non-repaired sides: 0.76 103 and 1.19 103; Table 4). At

that point, the stiffness of the repaired columns, which was ini-

tially higher than that of the control columns, decreased and be-

came lower (Fig. 7). At ultimate load, average axial strains on thenon-repaired sides of P15_a and P15_b were 2.18 103, while

the values on the repairs were not significant (Table 4). Failure thus

occurred immediately after plasticization of concrete on the origi-

nal, non-repaired portion of the columns, without any contribution

of the mortar repairs.

The stressstrain curves of undamaged (P00_b), and repaired

(P15_b, P50_b) columns were not shown in Fig. 6 as they are sim-

ilar to the corresponding (P00_a), and (P15_a, P50_a) columns.

Stressstrain curves for P00_b, P15_b and P50_b columns are com-

pared with the others in Figs. 7 and 8.

4. Finite element modeling of tested columns

Three-dimensional non-linear finite element models were usedto simulate the experimentally observed behaviour. The Straus7

code (G+D Computing [8]) was used for the numerical analyses.

Eight-node solid elements were used to model concrete substrate,

while beam elements were used for the steel reinforcement. In re-

paired columns type P15 and P50, the mortar repair was also mod-

elled with eight-node solid elements, having properties different

from those adopted for concrete. The interface between concrete

substrate and mortar repair was modelled by means of link ele-

ments with tension cut-off. Translation of nodes at the upper and

lower bases was restrained in the two orthogonal horizontal direc-

tions to reproduce friction between upper and lower faces of the

columns and the loading machine plate. Simplified parabolic

stressstrain relationships derived from Hognestad [10] wereadopted for the concrete substrate and the mortar repair, while

elasto-plastic bilinear relationship with hardening was used for

steel. The properties of materials were derived from the experi-

mental tests and are listed in Table 5. The Poisson ratio of the con-

crete and steel were assumed according to Eurocode 2 [3] and

Eurocode 3 [4], respectively. The Poisson ratio of the mortar was

assumed equal to that of the concrete. The described models were

used to carry out non-linear static analyses, under constantly

increasing loads.

Fig. 9 compares experimental and numerical stressstrain

curves. For the non-damaged, non-repaired columns (type P00),

the model reproduces very well the mean stressstrain curves

of the two tested specimens until peak value. Initial elastic stiff-

ness and value of ultimate load are well reproduced, as can beseen by the stresstransversal strain curve and stressaxial

strain curve. Only the latter is slightly stiffer in the model. In

the case of columns type P15, repaired with 15 mm thick mortar

layer the model reproduces quite well the values of initial elastic

stiffness and ultimate load, as can be seen by the stresstrans-

versal strain curve and stressaxial strain curve obtained on

the non-repaired side of the column. The axial strains on the re-

pair had trend similar to that measured on the non-repaired side

of the column only during the first elastic phase, and the model

still gives good results. During experimental tests, at stress level

between 76% and 58% of the ultimate capacity, the repairs deb-

onded from the concrete. Although the model cannot reproduce

the subsequent strain release, the numerical curve presents a

sudden discontinuity at the upper bound of this range of stres-ses. The model is thus able to show the debonding of repair. This

Fig. 3. Crack pattern of columns at failure.



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phenomenon is also described by the model displacement in

horizontal direction (see Fig. 10). In the case of columns type

P50, repaired with 50 mm thick mortar layer, the model is not

able to get the difference in the initial elastic stiffness of the ori-

ginal column and the repair, but can average the two trends and

does not present any discontinuity (Fig. 9), nor evidence marked

debonding, according to experimental evidence.

5. Discussion

The experimental results obtained on axially loaded columns

showed that repairs on one side of the columns could not re-estab-

lish completely the load-bearing capacity of the non-damaged con-

trol columns. Ultimate load of the repaired columns was on

Table 3

Results of axial tests.

Column D ebonding loa d ( kN ) ( a) U ltima te loa d (k N) (b) (a )/ (b)

P00_a 2929

P00_b 2869

P15_a 1901 2507 0.76

P15_b 1575 2709 0.58

P50_a 2430 (partial) 2606 0.93

P50_b 2160 (partial) 2501 0.86

Fig. 4. Failure with debonding of mortar layer (P15_b).

Fig. 5. Cracking of mortar and along the interface (P50_a).



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average 91% that of the control columns. Thin repairs (15 mm),

substituting only the reinforcement cover, showed premature deb-

onding from the concrete substrate, on average at 67% of the ulti-

mate load. After debonding, the repairs were not effective, as

revealed by strain analysis, and the entire load was transferred to

the original, non-repaired portion of the columns. When the repair

layer included the reinforcement (50 mm thick), the global behav-

iour of the repaired columns was improved. Debonding of repair

from the concrete substrate was limited and occurred only at the

column corners, on average at 90% of ultimate load. After partial

debonding, the repairs kept on collaborating with the concrete

support, as revealed by strain analysis. Plasticization of both con-

crete on the non-repaired column sides and mortar repairs re-vealed that both portions of the repaired columns were

contributing to the column capacity, even in the non-linear phaseuntil failure.

Simplified three-dimensional numerical models, implementing

non-linear constitutive laws for materials, could simulate fairly

well not only the behaviour of control columns, but also that of re-

paired columns. In the case of thin repairs, the models evidenced

the premature debonding of the repairs. Similar models can be

thus effectively used for assessment and design of interventions.

6. Conclusions

In this work an experimental investigation to control the effec-

tiveness of polymer-modified cementicious mortar repairs applied

to square columns under axial loads is carried out. The repair

material had similar mechanical properties and slightly higher ten-sile strength than the concrete substrate and was applied over the

entire length of one face of columns. The aim is to give some new

insights on validating the effectiveness of such materials in recov-

ering the properties of non-damaged, non-repaired columns, veri-

fying the effect of repair thickness (including the steel

reinforcement or not), on cracking pattern, strength and deforma-

bility of axially loaded element.

The main conclusions arising from the experimental tests show

that polymer-modified cementicious mortars, with limited shrink-

age and mechanical properties similar to those of the concrete sub-

strate, can be effective for the repair of reinforced concrete

columns. The effectiveness of the intervention depends also on po-

sition and thickness of the repair layer.

For columns repaired on one side only, the repairs cannot re-store the load-bearing capacity of non-damaged control columns,

Average stress-strain on P00_a

0

5

10

15

20

25

30

35

-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Strain [10-3]

Stres

s[N/mm

2]

transv. strain column


axial strain column

axial strain column


0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/m

m2]


transv. strain interface

transv. strain repair

axial strain column

axial strain repair


0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/mm

2]

transv. s train column

transv. strain interface

transv. strain repair

axial strain column

axial strain repair

Fig. 6. Stressstrain curves of undamaged (P00_a), and repaired (P15_a, P50_a)

columns.

Average stress-axial strain on column

0

5

10

15

20

25

30

35

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Strain [10-3

]

Stress

[N/mm

2]

P00_a (column)P00_b (column)P15_a (column)P15_b (column)P50_a (column)P50_b (column)

Average stress-axial strain on repair

0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/mm

2]

P00_a (column)P00_b (column)P15_a (repair)P15_b (repair)P50_a (repair)P50_b (repair)

Fig. 7. Stressaxial strain curves, on original column and on repair (all specimens).



8/9

although they give acceptable results (91% of the ultimate capacity

of the control columns). In any case, repairs that include the longi-

tudinal reinforcement show good properties, with stable behav-

iour, sharing of loads, and plasticization of the material before

failure, whereas thin repairs that do not include the reinforcement

do not have adequate performance due to premature debonding.

Hence, from the practical point of view, a rehabilitation interven-tion on axially loaded elements that include the longitudinal steel

reinforcement may be generally recommended since it is more

effective than that involving only the concrete cover without

including steel bars. In fact, the action of the transverse steel al-

lows sharing of loads between the mortar and the concrete core

until failure, avoiding premature debonding of the mortar layer,

when the steel bars are included in the repair layer.

Average stress-transv. strain on column

0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/mm

2]

P00_a (column)P00_b (column)P15_a (column)P15_b (column)P50_a (column)P50_b (column)

Average stress-transv. strain on repair

0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/mm

2]

P00_a (column)P00_b (column)P15_a (repair)P15_b (repair)P50_a (repair)P50_b (repair)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fig. 8. Stresstransversal strain curves, on original column and on repair (allspecimens).

Average stress-strain on P00

0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/m

m2]

P00_a transv. columnP00_b transv. column

P00 transv. col. model

P00_a axial columnP00_b axial column

P00 axial col. model


0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/mm

2]

P15_a transv. columnP15_b transv. columnP15 transv. col. modelP15_a axial columnP15_b axial columnP15 axial col. modelP15_a axial repairP15_b axial repairP15 axial rep. model


0

5

10

15

20

25

30

35

Strain [10-3

]

Stress[N/mm

2]

P50_a transv. columnP50_b transv. columnP50 transv. col. modelP50_a axial columnP50_b axial columnP50 axial col. modelP50_a axial repairP50_b axial repairP50 axial rep. model

-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

-2 0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Fig. 9. Experimental and numerical stressstrain curves of undamaged (P00), andrepaired (P15; P50) columns.

Table 4

Stresses and strains of axial tests.

Column Stress (N/mm2) 1/3 Ultimate load (a)/(b) Stress (N/mm2) Debonding Ultimate Load

Transv. strain (a)

(103)

Axial strain (b)

(103)

Transv. strain

(103)

Axial strain Stress (N/mm2) Axial Strain

(103)

P00_a 10.3 0.08 0.41 0.19 31.0 2.41

P00_b 10.6 0.08 0.39 0.20 31.9 2.52

P15_a Column 9.3 0.06 0.31 0.21 21.1 0.20 0.97 27.9 2.25

Repair 0.09 0.51 0.18 0.23 1.30 0.79

P15_b Column 10.0 0.06 0.30 0.21 17.5 0.13 0.55 30.1 2.11

Repair 0.06 0.38 0.15 0.06 1.08 0.54

P50_a Column 9.7 0.05 0.23 0.20 27.0 0.25 1.48 29.0 2.28

Repair 0.12 0.74 0.16 0.43 2.25 3.30

P50_b Column 9.3 0.05 0.24 0.23 24.0 0.29 1.06 27.8 2.21

Repair 0.24 1.00 0.24 0.54 2.40 2.71



9/9

The behaviour of axially loaded columns repaired with thin or

thick repairs can be also reproduced by simplified three-dimen-

sional non-linear finite element models.

Acknowledgements

The authors gratefully acknowledge Tassullo S.p.A. that pro-vided materials and specimens for experimental testing, and Eng.

Diego Testolin for his contribution to the experimental investiga-

tion developed during his MSc thesis. The experimental tests were

carried out at the Laboratory of Structural Materials Testing of the

University of Padova, Italy.

References

[1] Emberson NK, Mays GC. Significance of property mismatch in the patch repairof structural concrete. Part 2: Axially loaded reinforced concrete members.Mag Concrete Res 1990;42(152):16170.

[2] Emberson NK, Mays GC. Significance of property mismatch in the patch repairof structural concrete. Part 3: Reinforced concrete members in flexure. MagConcrete Res 1996;48(174):4557.

[3] European Committee for Standardization. Eurocode 2 Design of concretestructures. Part 1-1: General rules and rules for buildings. EN 1992-1-1,Brussels, Belgium; 2004.

[4] European Committee for Standardization. Eurocode 3 Design of steelstructures. Part 1-1: General rules and rules for buildings. EN 1993-1-1,Brussels, Belgium; 2005.

[5] European Committee for Standardization. Products and systems for theprotection and repair of concrete structures Definitions, requirements,quality control and evaluation of conformity. Part 3: Structural and non-structural repair. EN 1504-3, Brussels, Belgium; 2005.

[6] Fib. Seismic assessment and retrofit of reinforced concrete buildings. State ofArt Report, Bulletin 24. Federation International du Beton: Lausanne(Switzerland); 2003.

[7] Fukuyama K, Higashibata Y, Miyauchi Y. Studies on repair and strengtheningmethods of damaged reinforced concrete columns. Cement Concrete Compos2000;22:818.

[8] G+D Computing. Straus7 users manual. Sydney; 2005.[9] Hassan KE, Brooks JJ, Al-Alawi L. Compatibility of repair mortars with concrete

in a hot-dry environment. Cement Concrete Compos 2001;23:93101.[10] Hognestad E. A study of combined bending and axial load in reinforced

concrete members, Bulletin No. 399. University of Illinois, Urbana ILUSA: Engineering Experimental Station; 1951.

[11] Jumaat MZ, Kabir MH, Obaydullah M. A review of the repair of reinforcedconcrete beams. J Appl Sci Res 2006;2(6):31726.

[12] Kim JHJ, Lim YM, Won JP, Park HG, Lee KM. Shear capacity and failurebehaviour of DFRCC repaired RC beams at tensile region. Eng Struct2007;29:12131. doi:10.1016/j.engstruct.2006.04.023.

[13] Mangat PS, OFlaherty FJ. Influence of elastic modulus on stress redistributionand cracking in repair patches. Cement Concrete Res 2000;30:12536.

[14] Nounu G, Chaudhary Z-UL-H. Reinforced concrete repair in beams. ConstrBuild Mater 1999;13:195212.

[15] Park SK, Yang DS. Flexural behaviour of reinforced concrete beams withcementitious repair materials. Mater Struct 2005;38:32934. doi:10.1617/14051.

[16] Pellegrino C, Modena C. FRP shear strengthening of RC beams with transversesteel reinforcement. J Compos Constr 2002;6(2):10411.

[17] Pellegrino C, Modena C. FRP shear strengthening of RC beams: experimentalstudy and analytical modelling. ACI Struct J 2006;103(5):7208.

[18] Pellegrino C, Modena C. An experimentally based analytical model for shearcapacity of FRP strengthened reinforced concrete beams. Mech Compos Mater2008;44(3):23144.

[19] Pellegrino C, Modena C. Flexural strengthening of real-scale RC and PRC beams

with end-anchored pre-tensioned FRP laminates. ACI Struct J2009;106(3):31928.[20] Pellegrino C, Modena C. Influence of FRP axial rigidity on FRP-concrete bond

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[25] Shambira MV, Nounu G. On the effect of time-dependent deformations on thebehaviour of patch-repaired reinforced concrete short columns. Constr BuildMater 2000;14:42532.

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Table 5

Model material properties.

Material fc (N/mm2) ecu (%) fct (N/mm

2) ectu (%) fy (N/mm2) ey (%) ft (N/mm

2) et (%) E (N/mm2) m ()

Concrete 28.90 0.18 3.19 0.01 32,500 0.20

Mortar 32.87 0.25 3.48 0.01 26,200 0.20

Steel 532 0.27 628 25 200,000 0.30

Fig. 10. Model displacement in horizontal direction for P15 at debonding (left) and

P50 at same vertical load level (right).

http://dx.doi.org/10.1016/j.engstruct.2006.04.023http://dx.doi.org/10.1016/j.engstruct.2006.04.023http://dx.doi.org/10.1617/14051http://dx.doi.org/10.1617/14051http://dx.doi.org/10.1061/(ASCE)0899-1561(2005)17:2http://dx.doi.org/10.1061/(ASCE)0899-1561(2005)17:2http://dx.doi.org/10.1016/j.cemconcomp.2006.05.013http://dx.doi.org/10.1016/j.cemconcomp.2006.05.013http://dx.doi.org/10.1061/(ASCE)0899-1561(2005)17:2http://dx.doi.org/10.1617/14051http://dx.doi.org/10.1617/14051http://dx.doi.org/10.1016/j.engstruct.2006.04.023

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