SP 188-23

Study on Structural Performance of Reinforced
Concrete Columns with Waist-High Walls
Strengthened by Carbon Fiber Reinforced
Plastic Sheets
by H. Nagai, T. Kanakubo, Y. Jinno, Y. Matsuzaki, and S. Morita

Synopsis:
This research aims on understanding the influence of the strengthening
method using carbon fiber reinforced plastic (CFRP) sheets on the performanceof 13 reinforced concrete columns with waist-high walls. Main parameters of thespecimens include the thickness and height of the attached walls, axial load, andthe amount of CFRP sheets. The strengthening method is done by wrapping theupper portion of the column with sheets in a closed form. Also, the waist-highwalls are equipped with holes where the carbon fiber strands (CF-anchor) are
inserted and anchored to the ends of the sheets attached only on the surface of thelower portion of the column. Test result show that there is a great improvement in
the ductility of the shear-failure type specimens when strengthened with CFRP.
Also with an increase in the amount of the sheets, the ductility is furtherenhanced. The specimens strengthened using CFRP with CF -anchor also show
higher ductility than the specimen where only the upper column is strengthened.
The bending and shear strengths of the columns with waist-high walls can be calculated by using effective column height he considering the presence of the wallportions.

Keywords: carbon fiber; column; plastic; reinforced concrete255

256 Nagai et al.
Hitoshi Nagai: Graduate student, Masters Program, University of Tsukuba,Japan
Toshiyuki Kanakubo: Assistant Professor, Dr.E., Institute of Engineering
Mechanics, University ofTsukuba, JapanYasuo Jinno: Research Engineer, Institute of Technology, Shimizu Corporation,Japan
Yasuhiro Matsuzaki: Dr.E., Professor, Dept. of Architecture, Science Universityof Tokyo, Japan
Shiro Morita: Dr.E., General Building Research Corporation, JapanINTRODUCTION

The South Hyogo Earthquake, which occurred at January 17, 1995 in Japan,
damaged a quite large number of structures, such as buildings, wooden houses,bridges, offshore constructions, and so on. The epicenter was positioned near tothe city of Kobe, which is one of the oldest cities in Japan. A maximum acceleration of over 800 gal was observed at JMA Kobe. A huge number of old buildingsare still existing throughout Japan. If an earthquake with the same magnitude as
the last one occur in another city, a lot of buildings will be destroyed. It is essential to strengthen or upgrade old buildings.
Two methods for strengthening or upgrading buildings are popular in Japan.
One is by increasing the sectional area by post-casting concrete, and the other isby confining members by steel plates. These methods, however, need many processes, much time, good techniques and heavy machines. Recently, a strengthening method using continuous fiber, such as carbon, glass and aramid has been themain focus because of its simplicity in construction and economy. The simplest
way to use the fiber for strengthening is to wrap the members by sheets. Many
investigations and studies concerning fiber sheet strengthening are now going on
for both building and civil engineering structures in Japan. However, the effect ofstrengthening and the structural performance of the sheet reinforcement are still
unclear at present. Also, previous studies dealt mostly on the square column andonly a very few on columns with side walls which are used in actual building
construction.
TEST PROGRAMS
Specimens

Thirteen reinforced concrete columns with waist-high walls are tested. The
list of specimens and their dimensions are shown in Table 1 and Fig. 1, respectively. The following are the common specifications of the specimens. The column cross section is 300x300 mm and the specified concrete compressive
strength is 24 MPa. The column bars used are 4-D13 (only No.6-2 used 4-<j>l3)
and D6, for the main and lateral reinforcement, respectively, while the verticaland horizontal bars in the walls are 2-D4@80. The ratio of lateral reinforcement,pW> is 0.13% and the ratio of horizontal wall bars, Psh' is 0.17%. Also the clear

Fiber Reinforced Polymer Reinforcements 257
span length is 900 mm, and the shear span ratio is 1.5.
Two types of carbon fiber sheets are used for strengthening the specimens,
the 150g!m2 and 300g!m2 types, in terms of weight per unit area. Strength andelastic modulus is 4500MPa, 4170 MPa and 273GPa, 257 GPa, respectively. The
strengthening method is done by wrapping the upper portion of the column withsheets in a closed form. Also, the waist-high walls are equipped with holes where
the carbon fiber strands (CF-anchor) are inserted and anchored to the ends of thesheets attached only on the surface ofthe lower portion of the column. Method of
reinforcing is shown in Fig. 2. Main parameters of the specimens include thethickness and height of the attached walls, axial load, amount of CFRP sheets,
wall position (center side or eccentric side of column) and main bar type(deformed or round bar).
Mechanical properties of concrete, the reinforcement, carbon fiber sheet, anda photo of the CF-anchor are shown in Table 2, 3, 4 and Photo 3, respectively.
Cyclic loading is provided for all specimens using the antisymmetrical loading system shown in Fig. 3. The loading is carried out by controlling the drift
angle (R). The loading history applied to all specimens is R = ±l/400, ±1/200 radians once, R = ±1/100, ±1/67, ±1150, ±1133 radians twice and R = ±1/20, +1115
radians once. The shear force, relative displacement between the upper and thelower stubs and the strains of the reinforcements, CFRP sheet and CF-anchor aremeasured.
TEST RESULTS

Failure Pro~:ress
The progress of the experiment until the loading cycle to R = 1/400 radians
was almost the same for all strengthening .;;pecimens. First, shear cracks commenced on the waist-high walls on both sides. Next, bending cracks occurred at
the both ends of columns and extended as the displacement increased.
For specimens No.l-0, No.2-0, No.3-0, and No.5-0 shear cracks wereobserved on the upper columns extending toward both compression edges. At theloading cycle to R = 11395 radians, the maximum load was attained and a drop in
the shear force capacity was observed for the succeeding cycles. These specimens
showed brittle behavior and that the yielding of main bars were not observed.
This failure type is defined as diagonal tension failure (DT).
For other specimens, yielding of main bars (F) occurred at the loading cycleof R = 1/100 radians. For these specimens strengthened by CFRP sheets with CFanchor (No.l-1, 1-2, 1-3, 2-2, 3-2, 4-2 and 5-2}, a rupture of sheet (SR) after theyielding of main bars was observed coupled with a remarkable decrement in theshear force capacity. For specimens No.l-C2, where only the upper column isstrengthened, shear failure (S) at the lower column portion adjacent to the waisthigh walls was observed after yielding of the main bars. For specimens No.6-2,
whose main bars are round steel bars, bond failure (BO) was observed. Photographs of representative specimens after loading are shown in Photo 4.

258 Nagai et al.
Comparison of Shear Forc~Drift Angle Skeleton Curves
Shear force versus drift angle skeleton curves of specimens are shown in Fig.
5. For specimens not strengthened by CFRP sheets (No.l-0, No.2-0, No.3-0,No.5-0), the toughness of the column decrease as the thickness height and eccentric position are increased. For specimens strengthened with CFRP sheets (No.l1, No.l-2, No.l-3), maximum loads attained are greater compared with specimen
No.l-0 by about 25%. And the ductility is greatly improved, as the amount ofCFRP sheet is increased. The following described the influence of the size of thewall, strengthened range, axial force and shape of main bars, on the ductility ofthe column basing from the results of specimen No.l-2. For the specimens with agreater wall thickness (No.2-2) and greater wall height (No.3-2), it was observedthat the maximum loads attained are almost identical. However, there was adecreased in the ductility of the column. On the other hand, for the specimen having a wall at an eccentric position (No. 5-2), the ductility of the column increased
compared to specimen No.l-2. This reason for such is that there is an increase in
the length ofthe shear span the wall is positioned eccentrically compared when it
is positioned at the center. For spe<:imen where in only the upper column isstrengthened (No.l-C2), both the maximum load and the ductility decrease ascompared with specimen No.l-2. Moreover for specimen loaded with a higher
axial force (No.4-2), an increase in the maximum load attained and a decrease in
the ductility are obsel'Ved. Lastly, for the specimen using round main bars (No.62), there was a decrease in the maximum load but an increase in the deformation
capacity compared with specimen No.l-2. This is mainly because the bondstrength of the round steel bars with the concrete is weak compared to deformedbars.

DISCUSSION
Strengths Evaluation
Following formulas are used to calculate bending and shear strengths.
eQmu = 2Me /H
Me = 0.5ag ·cry· g 1 • D + 0.5N ·v(l- b·D·crN)
8g1= d 1 ID

eQmu: calculated bending strength

Me:bending moment of the independent columnag: total sectional area of the main bars

cry: yield strength of the main bars

d1:distance between centers of tensile and compressive main bars

N: axial force(I)Fiber Reinforced Polymer Reinforcements 259
D: column depthb: column width:column length to evaluate the effect of waist-high wallsHhe, he, h is defined as follows
he:column length of independent column

he: effective height (l)1 + qs)he= l - k(1 - ~)hh ~b 1 + qw

{j_(qs=pshXCJslfFe, qw=pwxcr w/Fc'k=1.69 (shear-failure type),=1.65 (flexural-failure type): thickness of the attached walls: total length column:concrete compressive strength

kthFe

but whencot<J>= min {2.0,j 1 I (Dtan8),

Jvc ·cr I L,<Pw · crw) -1.0}
8260 Nagai et al.
O"wy: yield strength of lateral reinforcement

O"wf:ultimate strength of the CFRP sheet (=2350MPa)ash:yield strength of waist-high wall bars

Pw

: shear reinforcement ratio of the lateral reinforcement

Pwf: shear reinforcement ratio of the CFRP sheet

Psh:shear reinforcement ratio within the wall

Vc:effective concrete compressive strength (=0.7-a8 /2000)

vcf: effective ultimate strength of CFRP sheet (=0.6)</>: angle of the concrete compressive strut in the truss mechanismjt: 7/8dCalculated shear strength cQsu versus observed maximum load eQsu relationship are shown in Fig. 8. Y and X axis values are standardized by calculated
bending strength cQmu. The adaptability of the use of column height (he, he, h)in the calculation of the bending and shear strength is investigated. Three figurescorrespond to the cases of h, he and he for calculate strengths. It can be observedthat there is a good correspondence between the calculated and experimental values when he is used for column height. However, for specimens having a highaxial force (No.4-2), further investigation on the use of he is necessary.
Energy Absorption Capacity
It is known that the absorbed energy represents the seismic capacity of thespecimens directly. Transitions of absorbed energy (Eac) for each specimen areshown in Fig. 6. The specimens, which are strengthened using CF-anchor (No.ll,No.l-2, No.l-3), show an increase in the energy absorption capacity as theamount of CFRP is increased. Taking specimens No.l-1 as the bases, it isobserved that the capacity of specimens No.l-2 is increased to about 1.3 times,
and that of No.l-3 to about 1.5 times. On the other hand, with regard to specimens No.l-2, No.2-2, No.3-2 and No.5-2, because of the similarity in the Eaccurves, it can be stated that the height and the position of the attached walls doselittle influence the energy absorption capacity of the columns. However, for thespecimen having a great wall thickness (No.2-2), the Eac is greatly reduced at R=1120 radians. The specimen where only the column is strengthened (No.l-C2)
and the specimen whose axial force is higher (No.4-2) show higher Eac than thatof specimen No.l-2 before reaching the loading angle at failure. Also the capacity of specimen No.4-2 is increased to about 1.4 times ofNo.l-2 before failure.
The specimen whose main bar is round steel bars (No.6-2) shows a lower capacity than specimen No.l-2 reduced to about 80%. Therefore, the ductility of thecolumn can be improved greatly by the strengthening with CF-anchor.
Strain Distribution

Fig. 7 shows the strain distributions for the sheet and CF-anchors at the peakof each loading cycle. The left graphs show the distribution for sheets and CFanchors at the perpendicular side to the loading direction, and the right ones show

Fiber Reinforced Polymer Reinforcements 261
those of the parallel side. The dotted lines in the figures indicate the yield strainof hoops.
In the most of the specimens, the average strains in the sheets are larger thanyield strain of hoops. This shows that the strengthening method using CFRP
sheets is effective. Also, the strains in the sheets are observed to decrease as theamount of CFRP sheets is increased. The observed strains in the CF-anchors are
smaller than 2000jl.
CONCLUSIONS

1. There is a great improvement in the ductility of the shear-failure typespecimens when strengthened CFRP sheet. Also with an increase in theamount of the sheets, the ductility is further enhanced.
2. The specimens strengthened using CF-anchor also show higher ductility
than the specimen where only the column is strengthened.
3. The bending strength and shear strength of reinforced concrete columns
with waist-high walls can be calculated using the effective column heighthe. But for cases of a high axial force, the good correlation between
calculated and experimented values is not observed.
4. The specimens not using CFRP sheets, it was observed that the toughnessof the columns is decreased as the wall thickness, height and eccentricity of
the wall is increased. Moreover, for specimen strengthened by CFRP sheets,a decrease in the toughness of the column is also observed as the thickness
and height of the wall is increased. However, there is an increase in thetoughness of the column as the eccentricity of the wall is increased.
5. The observed strains in the CF-anchor are lower than 2000jl which showsthat the CF-anchor has enough factor of safety comparing it to the ultimate
strength of the CFRP sheets.
ACKNOWLEDGMENT

This research is conducted under the research project organized by the SR-CF
Research Associate. The authors acknowledge Mr. Kouhei Maruyama for hissupport.
REFERENCES(I) T. Kyutoku: Study on Structural Factors influencing Ultimate Strength of Reinforced Concrete Column with Spandrel Walls, Summaries of technical papers of annual meeting Architectural Institute of Japan, pp.395 - 396, 1986. 8 (in Japanese)262 Nagai et al.
TABLE 1-LIST OF SPECIMENS.
V\xial Effective Height ~ickness Sheet weight
Mainforce height of walls of walls(g/m2)Xbars(kN) h. (em)' hw (em) tw (em)lay_erSpecimen Section
No.1-0No.1-1
No.1-2-64.3No.1-3424

No.1-C2 l4~w
4-D13

No.2-0No.2-2
No.3-0No.3-2
No.4-2
No.S-0No.5-2~~

No.6-2~4-c/)13

*~

7.5
30

60.5

1051.445

64.3

30-300X1

300X1
300X1951
7.5
424

150X1
300X1
150X1
300X1
300X1
only column

300X1
300X1=1-k(l- ~ )~(::;:) qs=Psh X as/ Fe , qw=Pw X a wy/Fc ,k= 1.69(Shear-failure type), k= 1.65(Flexural-failure type)

TABLE 2-MECHANICAL PROPERTIES OF CONCRETE.

Type

NormalNormalCompressive
strength

Splittingstrengthas(J SP_(MPI!}_(MPa)Elastic
modulus
Ec(GPa)

25.2

2.49

23.0

27.4

2.49

25.4

25.2

2.02

22.2

22.3

1.96

21.1

26.8

2.25

24.6

25.8

2.06

24.0

Specimen
No.1-0No.1-C2
No.l-1 '"'"'1-3
No.2-0No.3-0No.S-0No.4-2
No.2-2
No.3-2
No.5-2
No.6-2Fiber Reinforced Polymer Reinforcements 263

TABLE 3-MECHANICAL PROPERTIES OF REINFORCEMENT.
Identification(Nominaldiameter)

Ultimate
strengtha,, (MPa)

Yieldstrength(Jsv (MPa)Elastic
modulusEs (GPa)Specimen

D13 (13mm)

494341

185

No.1-0""5-2

tP 13 (13mm)

457

325

203

No.6-2

D6(6mm)

532

413

193

No.1-0""6-2

D4(4mm)

385

250

211

No.1-0""6-2

TABLE 4-MECHANICAL PROPERTIES OF CARBON FIBER SHEETS.
Weightper unitarea(glm2)
150
306

Thickness
(mm)
0.835
0.167

Specific
gravity(glcm 3)1.80
1.80

Ultimate
strength

Elastic
modulus
E!
(GPa)
273
257

(Jfu

(MPa)
4500
4170

0~

g.Y'l_.P-~,
1200

Fig. 1-Dimensions of specimens (unit: mm).

;-· ,.~t .'- *===='- -[" ~ 3_Q_Q300 300 j- 900-

264 Nagai et al.

CF-anchor

1'1'1:11111111~11'1'111
Fig. 2-Method of reinforcing.

ReactionwallFig. 3-Asymmetricalloading system.

No.l-0No.l-1

Fig. 4-Specimens after loading.

Photo 3 CF-anchor

Fiber Reinforced Polymer Reinforcements 265-No.1-0--- No.2-0-·-·- No.3-0-No.5-0Q(I<N)

P-o Effect-1

Fig. 5-Shear force-drift angle skeleton curves.

',,~I',".

0 1/1001/67 1/50 1133
Dnft angle R(rad.)0 1/1001/67 1/50 1/33
Dlift angle R(rad.)18

ir===0--T"Ii:: 1=:=~~:Lf II~

j--o-No.6-2'"12r 1~

IIIW'';;/''

1: ~~.

:::0 1/1001/67 1/50 1/33
Dnft angle R(rad.)Fig. 6-Transition of absorbed energy.

:1/20

1/20

266 Nagai et al.

No.1-1

No.1-1-I-

No.1-1
No.1-3--

No.1-3-0-:R=l/200 -.6.-:R=l/100 -D-:R=l/50 -0-:R=l/33
white : positive loading

black : negative loading

Fig. 7-Strain distribution in sheets and CF anchors.

Fiber Reinforced Polymer Reinforcements 267"<!;.

2~8D~01!1~1.5~

"'0.5

0.5

0.5

1.5

21.50.5

cOsu/cOmu (h)@I DT

0No.1-0F---+SR No.1-1e F---+SR No.1-2

t::. F
No.1-3
A F---+SNo1-C2
IQJ DTNo.2-00 F---+SR No.2-2iA DT
No.3-0• F---+SR No.3-2\1 F---+SR No.4-2V DT
No.5-0T F---+SR No.5-2() F---+BO No.6-2

• 0"

0.5

0.5

1.5

2cOsu/cQmu (he)2cOsu/cQmu (he)Fig. 8-Calculated shear force casu - maximum load eOsu.