Short Term Creep Tests of Low Strength Rectangular Concrete Members

Jacketed with Carbon FRP Sheets

Cem Demir

PhD Cand., Struct. And Earthquake Eng. Lab., Dept. of Civil Eng., Istanbul Tech. Univ., Istanbul, Turkey

Aygul Aydogmus

Researcher, Struct. And Earthquake Eng. Lab., Dept. of Civil Eng., Istanbul Tech. Univ., Istanbul, Turkey

Alper Ilki

Prof., Struct. And Earthquake Eng. Lab., Dept. of Civil Eng., Istanbul Tech. Univ., Istanbul, Turkey

ABSTRACT

Usage of low strength concrete is common in many developing countries all around the

World. For example, in Turkey, the approximate average concrete compressive strength of

existing reinforced concrete buildings constructed before 1990s is around 10 MPa. Since

the concrete strength is foreseen higher during structural design, the sustained axial stresses of columns of such existing buildings may be remarkably high (70~90% of axial capacity).

These high axial stresses sometimes cause collapse of columns or overall structure due to

effects of creep of concrete. The service life of such structures can be enhanced through

external jacketing of columns with FRP sheets. The efficiency of such a rehabilitation is

higher in case of low strength concrete. In this study, the short term creep behavior of

carbon FRP sheet jacketed low strength (~7 MPa) rectangular concrete prisms is

investigated under varying levels of sustained axial stresses. The safe lifetime of the specimens are extrapolated considering the variations in axial and transverse strains

measured during short term creep tests (48~96 hours). In addition, the residual capacities of

carbon FRP jacketed rectangular prism specimens are investigated after being subjected to short term creep tests.

KEYWORDS

carbon fiber, confinement, low strength concrete, rectangular cross-section, retrofit, creep, sustained loading

S1A04

1. INTRODUCTION

Many structures built with low strength concrete

exist all around the world, particularly in the developing countries. In the case of Turkey, the

average concrete compressive strength can be

assumed to be around 10 MPa for buildings

constructed before 1990s [1 and 2].

Although, majority of these structures lack engineered design procedures, even the engineered

ones frequently suffer from low concrete quality.

Consequently, when the actual concrete strength used during the construction is lower than that of

foreseen at the structural design phase, the

sustained axial stresses of columns may be remarkably high (70~90% of column axial

capacity). These high axial stresses sometimes

cause collapse of columns or overall structure due

to creep effects on concrete. Along with the low

concrete quality, reinforcement detailing problems

and inadequate utilization of transverse

reinforcement negatively affect the load bearing

and ductility performance of these structures. The

service life of such structures can be enhanced through external jacketing of columns with FRP

sheets.

Although significant number of experimental

studies exists for investigation of the behavior of

fiber reinforced polymer (FRP) confined concrete

members under short-term loading, limited

number of studies has targeted the behavior under

sustained axial loads [3-6].

In this paper, results of short-term creep tests and

residual capacity tests performed on Carbon FRP

(CFRP) confined specimens with square cross-sections are presented. In order to

investigate the case of extremely low concrete

quality, that can be encountered for an average

compressive strength of 10 MPa, eleven prisms

with the unconfined standard cylinder concrete

strength (f′co) of 5.3 MPa at 28 days are included

in the study. Eight of the specimens were

externally confined with three layers of CFRP

sheets and three unconfined specimens are tested as unconfined reference specimens. Main

parameter of the study is the ratio of sustained

axial stress to the axial capacity of the specimen

2. EXPERIMENTAL PROGRAM

2.1 Specimen Preparation Square cross-sectioned prism specimens were cast

using low strength concrete. Cement, water, sand, gravel, crushed stone weights used in the concrete

mix were 197, 231, 563, 852, 272 (kg/m3),

respectively. Portland cement with a 28 days compressive strength of 32.5 MPa was used in the

mixture and maximum aggregate size in the

mixture was 15 mm. Concrete prisms with

dimensions of 150x150x300 mm were removed

from the steel molds one week after casting and

were cured for seven days. Corners of all specimens were rounded to 30 mm radius. Eight of

the specimens were wrapped with three plies of

CFRP sheets in transverse direction. CFRP sheets were bonded with epoxy adhesive and 150

mm overlap was formed at the end of the wrap.

During the wrapping phase; a spacing of 10 mm was left at the top and bottom ends of the

specimens to avoid direct loading of the CFRP

sheets. Finally, all prisms were capped with sulfur

and graphite. Views from the preparation phases of

the specimens can be seen in Fig. 1.

Fig.1 Specimen preparation

Tensile strength and Young’s modulus of the

CFRP sheets used were 3800 MPa and 240000 MPa, respectively. Ultimate elongation of the

CFRP sheets was 1.55%. The design thickness

provided by the manufacturer was 0.117 mm.

The test matrix of the study is presented in Table 1.

Accordingly, two unconfined and two CFRP

confined prisms are subjected to monotonic

loading as reference specimens. Short-term creep

tests consist of three distinct sustained axial stress

levels: 3.37, 3.13 and 2.76 times the unconfined

compressive strength of concrete (f′cun) that was obtained as 6.8 MPa from S0A and S0B reference

prisms. These stress levels respectively correspond

to 90, 83 and 73% of the confined concrete strength (f′cc) obtained from S3A and S3B CFRP

confined reference specimens. The single

unconfined SRI specimen, which was subjected to

a much lower sustained stress level than the

confined specimens, aims to establish a

comparison with respect to the retrofitted ones. It should be noted that, the short-term creep tests

were carried out at a time span of 48 hours, except

the SRE and SRF prisms whose loading duration was extended to 96 hours.

The short-term creep tests of specimens were ceased only when failure occurred, or upon

reaching the targeted loading duration. After

completion of the sustained loading duration, the

specimens were subjected to monotonic loading

until failure was achieved.

Table 1 Tested square cross-sectioned prisms

SpecimensRetrofit

(plies) Sustained stress

Sustained load

duration

(hours)

Failure

during

sustained

loading

S0A,S0B - - - - S3A, S3B 3 - - -

SRA,SRB 3 3.37 f′cun, 0.90 f′cc 48 No, Yes

SRC,SRD 3 3.13 f′cun, 0.83 f′cc 48 No, Yes

SRE,SRF 3 2.76 f′cun, 0.73 f′cc 96 No, No

SRI - 0.85 f′cun 48 Yes

2.2 Test Setup An Instron Satec 1000 RD universal testing

machine (with a compression capacity of 5000 kN) available at Istanbul Technical University,

Civil Engineering Faculty, Construction Materials

Laboratory was utilized for monotonic and

sustained compressive loading of the specimens.

Five kN of pre-load was applied to the specimens

prior to testing. Tests of the monotonically loaded reference specimens were done under

displacement control. In the case of short term

creep tests, loading to the target sustained stress

level was done under force control and the load

was kept constant during the designated time

period. The residual capacities after the sustained

loading were obtained by monotonically loading

the specimen up to failure which was done under

displacement control.

Four linear variable displacement transducers

(LVDTs) of TML CDP-25 type in the gage length

of 150 mm were attached to the specimen through a compressometer for measurement of average

axial strains. The compressometer compatible with

the square cross-section of the prisms was designed and manufactured particularly for this

study. Lateral strains at mid-height were also

measured by four surface strain gages on each side.

The strain gages had a gage length of 60 mm. For

measurement of loads, TML CLP-100CMP type

load cell with a capacity of 1000 kN was used in parallel to built-in load cell of Instron loading

system. A specimen with the above mentioned

instrumentation can be seen in Fig. 2.

Fig.2 Instrumented specimen

Data acquisition was maintained via a 50 channel

TML ASW-50C switch box and a TML TDS-303

data logger. During the short-term creep tests, the displacement, strain and load data was collected at

every one second automatically until target stress

level was reached, then continued at 20 seconds

intervals until the end of sustained loading test.

3. RESULTS AND OBSERVATIONS

3.1 Monotonic Tests Two unconfined square cross-sectioned prisms (S0A and S0B) were tested under 0.002 strain/min

loading rate yielding to an average compressive

strength of 6.8 MPa. Average axial deformation corresponding to peak stress was 0.0023 mm/mm.

In addition to these two unconfined specimens,

two identical prisms wrapped with 3 plies of CFRP sheets (S3A and S3B) were also tested

under the same loading rate.

Axial stress-strain diagrams obtained by these

reference tests are presented in Fig. 3. The strain

values in this figure are obtained from the LVDTs since most of the vertical strain gauges were not

operational until the end of the tests. As also

observed in [7], the enhancement in strength and deformability for FRP confined specimens was

significantly high for the low strength concrete.

Comparison of CFRP confined specimens tested under monotonic loading with the unconfined ones

reveals that the average confined concrete strength

and the strain corresponding to strength increases to 25.5 MPa and 0.0527 mm/mm, respectively .

The lateral deformation data presented in Fig. 4

for lateral strains of CFRP confined specimens

were collected by using the strain gauges at

mid-heights of the specimens. Although the maximum lateral strains could reach 0.0132 and

0.0129 for S3A and S3B specimens, respectively,

the average value was approximately 0.0112. This corresponds to 72% of the ultimate tensile strain

given by the manufacturer.

0

10

20

30

0 20000 40000 60000

Str

ess

(MP

a)

Strain (microstrain)

S0AS0BS3AS3B

Fig.3 Axial stress-strain curves of confined and

unconfined reference specimens

0

10

20

30

-12000 -8000 -4000 0

Str

ess

(MP

a)

Lateral strain (microstrain)

S3A

S3B

Fig.4 Axial stress-lateral strain curves of confined

reference specimens

3.2 Short-Term Creep Tests Two confined specimens for each of the three

different sustained axial stress levels were

subjected to continuous axial load for targeted time durations. Sustained axial stress levels chosen

for the CFRP confined prisms become equal to 73,

83 and 90% of the confined concrete strength

(f′cc=25.5 MPa). Lowest sustained stress level (0.73 f′cc) corresponds to 2.76 f′cun, which is much

higher than the axial load capacity of an

unconfined concrete member. Apparently, the 0.83 and 0.90 f′cc stress levels were even higher, leading

to axial stresses of 3.13 and 3.37 f′cun, respectively.

The single unconfined concrete prism (SRI) was

tested under short-term creep load that

corresponded to 0.85 f′cun.

During sustained loading tests, as expected, the

behavior up to the target sustained stress level was

identical to monotonic loading tests. However, one specimen from each of the specimen couples (SRB

and SRD) that targeted 0.83 and 0.90 f′cc (3.13 and

3.37 f′cun) sustained stress levels, unexpectedly failed in the vicinity of the target stress levels

(Table 1). Similarly, the unconfined SRI specimen

failed three minutes after reaching the 0.85 f′cun

stress. The premature failures of the CFRP

confined specimens are shown in the lateral

strain-loading duration diagram (Fig. 5). The

premature failures of the specimens SRB, SRD

and SRI are attributed to the possible variation in

concrete compressive strength and workmanship errors, which might have led to non-uniform

stresses and strains of external FRP jacket

-12500

-10000

-7500

-5000

-2500

0

0 500 1000 1500 2000Time (s)

Lat

eral

str

ain

(m

icro

stra

in)

SRB

SRD

Fig.5 Premature failures of SRB and SRD prisms

In case of specimens that could reach the target

stress level, the lateral strains initially exhibited a

tendency to increase (Fig. 6). This increase was more remarkable for the SRA and SRC specimens

that were subjected to higher stress levels than the

SRE specimen. However, later on the lateral strains became stabilized and none of these three

specimens reached failure during the short-term

creep loading phase.

In order to predict the life-time of the specimens

subjected to sustained loading, a power type

regression analysis was performed on the curves

presented in Fig. 6. Similar power type relationships for creep-time behavior were also

reported by [8-10] for plain concrete and [3, 4, 6

and 11] for FRP confined concrete. Analysis on lateral strain-time curves of SRE and SRF with

lowest sustained stress level (0.73 f′cc or 2.76 f′cun)

showed that the specimens were unlikely to fail in

practical duration. However, specimens with 0.83

f′cc (SRA) and 0.90 f′cc (SRC) addressed service

life durations that can be easily exceeded in actual conditions. These results are also supported with

the premature failure of two other specimens (SRB

and SRD) loaded to these stresses. During the analysis it has been assumed that the specimen

fails when the trend line, generated by using

lateral strain-time data shown in Fig. 6, reaches the FRP effective ultimate lateral strain (rupture

strain). The FRP rupture strain was taken as 11000

microstrain, which was determined to be the

average rupture strain during the monotonic

compression tests. As an exception, since the

specimen SRA exceeded 11000 microstrain during

the sustained loading and identical SRB specimen

prematurely failed at a lateral strain of 10400

microstrain, its service life can be considered as practically short.

-12500

-10000

-7500

-5000

-2500

0

0 100000 200000 300000 400000

Time (s)

Lat

eral

str

ain

(m

icro

stra

in)

SRE

SRC

SRA

Fig.6 Lateral strain variation with time

It should be noted that the applied sustained stress

levels were extremely high and quite unlikely for

actual existing structures. However, extremely poor concrete and/or extreme loading conditions

may cause unexpectedly high sustained axial

stresses. In such cases, FRP confinement arises as an effective retrofitting technique for prevention of

potential damage due to creep of existing concrete

members.

3.3 Residual Capacities Although some of the CFRP confined concrete

prisms failed at higher creep loads, the remaining

ones were virtually undamaged after creep tests. In order to investigate the residual load capacities of

remaining specimens, sustained loads were

removed and the specimens were left unloaded for thirty minutes. In the next step, the specimens

were reloaded monotonically under 0.002

strain/min loading rate, until failure. During

unloading and reloading phases, data acquisition

was continued. View of the SRA and SRF

specimens after the residual capacity test is presented in Fig. 7.

Fig.7 SRA and SRF specimens after testing

Failure modes of the monotonic compression tests (S3A and S3B) and residual capacity tests were

similar. The sudden rupture of the external CFRP

jacket generally initiated near the center of a side and extended along the 1/3 of the specimen height.

In few cases, as also observed in SRA specimen,

FRP rupture was close to one of the corners.

The axial stress-strain curves obtained from

residual strength tests are shown in Fig 8. In this

figure, it is also possible to compare these curves

with the monotonically loaded confined and

unconfined prisms.

Despite the utilized extremely high sustained

stresses, it can be observed that sustained load did not have negative effects on confined concrete

strength and deformability characteristics. All

short-term creep test specimens, which were reloaded to failure, achieved greater axial and

lateral strain values than specimens tested under

short-term monotonic loading (Figs. 8 and 9).

Surprisingly, the increase in strength and

deformation capacities was even more pronounced

for higher sustained stresses (Table 2). This

observation may be attributed to the creep characteristics of the CFRP sheets, which resisted

higher tensile strains with respect to CFRP sheets,

wrapped around members, which are directly tested under monotonic loads.

0

10

20

30

0 22000 44000 66000

Str

ess

(MP

a)

Strain (microstrain) Fig.8 Residual capacity test axial stress-strain curves

Table 2 Lateral strains at failure

SpecimensSustained stress

Lateral

strain at

failure

(microstrain)

Failure

during

sustained

loading

S3A - 11646 -

S3B - 10767 -

SRA 3.37 f′cun, 0.90 f′cc 13890 No

SRB 3.37 f′cun, 0.90 f′cc 10400 Yes

SRC 3.13 f′cun, 0.83 f′cc 12565 No

SRD 3.13 f′cun, 0.83 f′cc 9230 Yes

SRE 2.76 f′cun, 0.73 f′cc 11750 No

SRF 2.76 f′cun, 0.73 f′cc 11508 No

0

10

20

30

-15000 -10000 -5000 0

Str

ess

(MP

a)

Lateral strain (microstrain) Fig.9 Residual capacity test lateral stress-strain

curves

It is very important to note that while it was

observed that FRP confinement played an important role for the creep performance of the

tested members, the efficiency of confinement was

remarkably less than reported by [3] for the

circular specimens. It is thought that, this

difference mainly stemmed from the distribution

of the stresses in the cross-sections. In the case of circular cross-sections, the distribution of the

stresses is uniform, whereas in the case of square

cross-sections, the distribution of stresses is quite

irregular due to concentrations at the corners. So,

it is very important to approach the results of creep

tests of FRP confined circular specimens carefully

before generalizing the obtained results for

noncircular specimens.

4. CONCLUSIONS

(1) External FRP confinement of low strength

rectangular members is efficient to increase

the service life under extremely high axial

stresses with respect to their axial strength.

(2) The enhancement in the creep performance is

relatively less for rectangular specimens with

respect to circular ones.

(3) The specimens tested under sustained loading exhibited a slightly better performance during

residual strength tests in terms of strength and

deformability with respect to their counterparts tested directly under monotonic

compression.

ACKNOWLEDGEMENT

The authors wish to thank ISTON Company for

supplying the concrete of the specimens.

Contributions of BASF Turkey Company is

greatly acknowledged for financially supporting the second author and supplying the FRP and

epoxy materials. Tests were carried out at the

Construction Materials Laboratory of Istanbul

S3A

S3B

SRA

SRC

SRE

S0A

S3A

S3B

SRA SRC

SRE

S0B

Technical University. The authors would like to thank Prof.Dr. Mehmet Ali Tasdemir for making

usage of Instron machine at this laboratory

possible.

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