SPRAYED-UP FRP STRENGTHENING
FOR CONCRETE STRUCTURES
Tomoki Furuta1, Toshiyuki Kanakubo2, Takeshi Nemoto3,Keisuke Takahashi4 and Hiroshi Fukuyama5
1

Bando Chemical Industries, LTD.
Tsuchiyama, Hiraoka-cho Kakogawa, 675-0104 Japan
2
Institute of Engineering Mechanics and Systems, University of Tsukuba
Tennohdai, Tsukuba-city, 305-8573, Japan
3
Showa Highpolymer Co., LTD., Tokyo Laboratory
1019-1, Tomizuka-cho, Isesaki-city, 372-0833, Japan
4
Mitsubishi Chemical Functional Products, Inc.
8-2, Marunouchi 1, Chiyoda-ku, Tokyo, 100-0005, Japan
5
Building Research InstituteTachihara, Tsukuba-city, 305-0802, JapanABSTRACT
A new repair and strengthening method by spraying carbon or glass chopped fiber with vinyl ester
resin upon concrete structures is introduced. This paper reported the outline of this new strengthening
method and the results of experimental programs for RC columns and bond / anchor test between FRP
and concrete. This method is engineered systemically for the purpose of repair and strengthening ofconcrete structures using sprayed-up FRP composites. Carbon or glass fiber chopped with 1.5 or 2
inch-length is sprayed with vinyl ester resin using the air-compressed spray machine on the surface ofconcrete structures directly. The anti-symmetrical loading test of rectangular columns was carried out
for the purpose of confirming fundamental behaviors of RC structures strengthened by sprayed-up
FRP system. As the result, it can be recognized that the similar shear reinforcing effect is obtained ineither way spraying or fiber sheet. Anchor method by the advantage of sprayed FRP’s flexibility isinvestigated. Anchor bolts or slits are set on the concrete surface, FRP sprayed in and on them.
Compared with fiber sheet, the bond strength by sprayed FRP has over the equivalent of one by sheet,when the thickness is set having equal rigidity. By filling the FRP to the slit, the mechanical bearing
can be expected.

KEYWORDS
FRP, spray up, carbon fiber, glass fiber, vinyl ester, column, bond, anchor, slit

INTRODUCTION
Nowadays, strengthening by post casting concrete, steel plate jacketing, fiber reinforcements such ascarbon, aramid, and glass are utilized as seismic strengthening methods for concrete structures.
Recently, a seismic strengthening method by wrapping continuous fiber sheets has often been used,since the constructibility and durability is superior. However, materials using continuous fibers areexpensive. On the spread of seismic strengthening for buildings and infrastructures in future, simple
methods of strengthening with low cost should not only be suggested, but also seismic behaviors
should be cleared.
In this study, a new, inexpensive, and simple strengthening method for concrete structures is discussed
and suggested in order to improve future seismic strengthening. This method using short fibers withvinyl ester is a new combination of materials as seismic strengthening. Chopped short fibers of carbon
and glass with vinyl ester resin are sprayed in place on the concrete structures. It is called“Sprayed-Up FRP (Fiber Reinforced Polymer).” Benefits of using vinyl ester resin in thisstrengthening method are that it takes shorter time to harden the resin than epoxy resin. In addition,
the mechanical properties of vinyl ester resin are the same as the one of epoxy resin.
In this paper, the outlines of this method and the results of column test under the anti-symmetricalloading are reported. In addition, the bond behavior between FRP and concrete, and anchoring
behavior using slit (groove) are reported.

OUTLINE OF SPRAYED-UP FRP STRENGTHENING METHOD
Figure 1 illustrates the idea of the sprayed-up FRP strengthening method for reinforced concretebuildings. Photo 1 and 2 show the construction site of sprayed column specimens. In this method,resin is carried through a narrow hose by an air compressor. The resin is mixed with short fibers suchas carbon or glass at a tip of the narrow hose. The mixed materials are sprayed directly on a surface tobe reinforced. After that, the surface is made flat by a roller. The resin will be hardened and the whole
sprayed structure will be reinforced with FRP. This method makes seismic strengthening possible thatall structure members, which are columns, beams, walls, and slabs, are monolithic since it is possible
to reinforce an entire interior structures in building structures.

Figure 1: Sprayed-up FRP strengthening

Photo 1: Sprayed fiber

Photo 2: Surface curing

The installing procedure of the sprayed-up strengthening is as follows;
Step 1. Base arrangement; Surface of concrete is polished by a disc-sander and cleaned by air.

Step 2. Primer resin coating; Primer resin is applied to the surface in order to make highlyadhesive between concrete and putty/resin.
Step 3. Putty arrangement; Dent areas and steps on concrete surface are filled with putty andmake the surface flat in order to prevent from partial stresses of FRP and air voids on concrete.
After putty dried, the surface is sanded.
Step 4. Resin coat; In order to make fibers more adhesive, resin is coated first by a spray gun.
Step 5. Spraying (Photo 1); Resin and short fiber are sprayed on concrete at a same time by aspray gun. The lengths of the carbon fiber and glass fiber are 2.0 inches and 1.5 inches,respectively.
Step 6. Impregnation (Photo 2); Entrapped air is rolled out.
In this study, in order to compare structural behaviors of sprayed-up FRP to the ones of continuous
fiber sheet strengthening, preliminary arrangements as Step 1 through 3 are done. However, it is a goal
to obtain sufficient seismic behaviors by taking only after Step 4.

MECHANICAL PROPERTIES OF SPRAYED-UP FRP
A total of sixty coupon specimens of each sprayed-up FRP were prepared as Type A (JIS K7054) testpieces. Thirty specimens were made with carbon fiber (CF-SU), and the other thirty specimens forglass fiber (GF-SU). Based on a tensile test method for plastics reinforced by glass fiber (JIS K7054),
a tensile test was carried out for these Type A test pieces. In order to compare the differences ofcharacteristics between sprayed-up FRP and fiber sheets, another sixty Type B (JIS K7054) test pieces
were also prepared and tested in the same tensile test. Thirty of them were made by carbon fiber sheet(300g/m2) with one layer (CF-HU). The other thirty were made by glass fiber sheet (444g/m2) with 4layers (GF-HU). The specified thickness of sprayed-up FRP for CF-SU and GF-SU is 3.0mm and4.5mm, respectively.
Table 1 and Figure 2 show the tensile test results and the typical stress-strain diagram (σ-ε),
respectively. The measured sectional areas including
resin are applied, and elastic modulus is a secant
σ(MPa)
stiffness of the 10%-50% tensile strength. The tensile
strength of CF-SU is about one tenth of the one of400
CF-HU. GF-SU is about one fifth of GF-HU. The
tensile strength of CF-SU is approximately the same300
as the one of GF-SU. Regarding elastic modulus,
CF-HU
GF-HU
CF-SU is about one forth of CF-HU. GF-SU is also
CF-SU
200
about one forth of GF-HU. The elastic modulus ofGF-SU
CF-SU is approximately twice as large as the one of100
GF-SU. From these results, the thickness of sprayedup CFRP needs to be about 6.5mm in order to expect
00.5
1
1.5
the similar strength of specimens with one layer of2()
ε％carbon sheet (300g/m ). As shown in Figure 2, σ-εFigure 2: Stress – strain relationship for FRP
relationships for sprayed-up FRP is not perfect linear.
test pieces
Table 1: Tensile test for test pieces of FRP
Thickness (mm)Width (mm)Maximum load (kN)
Tensile strength (MPa)Elastic modulus (GPa)Ultimate strain (%)

CF‐HU
Average STDV
0.654
0.096
24.787 0.076
18.23
1.52
1124
77
64.20
2.27
1.76
0.11

GF‐HU
CF‐SUGF‐SU
Average STDV Average STDV Average STDV
2.949 0.143 3.372 0.596 4.614
0.617
24.785 0.057 24.917 0.095 24.794 0.049
38.43
3.66
9.43
3.02
12.92
2.33
527.0
57.0
117.1
41.8
112.9
12.2
28.16
1.65
15.24
4.88
7.669
0.569
1.87
0.14
0.78
0.19
1.48
0.18

ANTISYMMETRICAL LOADING TEST FOR COLUMNSSpecimens
Table 2 shows the properties of specimens. Four specimens were prepared. The dimension of these all
four specimens was 300mm wide and 300mm deep and the shear span ratio is 1.5. The parameters
were FRP reinforcements, which were sprayed-up CFRP (CF-SU), sprayed-up GFRP (GF-SU), carbon
fiber sheet (300g/m2) with one layer (CF-HU), and glass fiber sheets (444g/m2) with four layers(GF-HU). Resin of vinyl ester was utilized for all specimens. The amount of FRP reinforcements weredesigned so as for the FRP stiffness (elastic modulus times thickness) to be equal. Deformed rebarsD13 (yield strength = 341MPa) and D6 (yield strength = 413MPa) were used as main bars and hoops,respectively. The designed normal weight concrete strength was 30MPa. The maximum diameter of
the aggregates was 20mm. The measured compressive strength at the loading age is 34.2MPa. Table 3
shows the material properties of FRP. Regarding FRP, the material property was identical to Table 1.
The properties of sheets are calculated by considering the cross-section as the designed thickness of
only the fiber.
Table 2: Column specimens
Specimen

Section
(mm)

CF-HU
GF-HU
CF-SU
GF-SU

300×300

Hoops
Main bar

12-D13

Arrangement

D6@160FRP(Specified values)pw (%)

Elastic modulus (GPa)Thickness

0.13

240
54
1280.167mm×1
0.163mm×43.0mm
4.5mm

Table 3: Mechanical properties of FRP
Tensile strength
Elastic modulus(MPa)
(GPa)
CF-HU
4451*
254.1*
GF-HU
2378*
127.1*
CF-SU
117.1
15.24
GF-SU
112.9
7.669
* For sectional area of only fiber (not included resin)IDloading

Loading System and Instrumentation
Each specimen was subjected to antisymmetrical bending moment in a cyclicmanner. The drift angles were from 1/400 rad to1/15rad. The axial force was kept constant at63kN as the axial stress was 0.2 of concrete
strength. Measuring items were horizontal andvertical displacements between the top andbottom stubs, and strains of main bars, hoops
and FRP as shown in Figure 3. Tri-axial strain
gages were used for measuring strains ofsprayed-up FRP.

Ultimate strain
(%)
1.76
1.87
0.78
1.48150

HU Series: Uni-axial gages
SU Series: Tri-axial gages

150150150
150
150

Loading
direction

Perpendiculardirection

Figure 3: Location of strain gages

Ultimate Strength and Shear Force versus Drift Angle Curves
Table 4 and Figure 4 show the test results and shear force versus drift angle curves, respectively. Allfour specimens were yielded in flexure. After that, FRP for CF-SU and GF-SU was ruptured at thedrift angle, -1/20rad and +1/20rad, respectively. Both fiber sheet reinforcements and sprayed-up FRPreinforcements give the similar results on the effectiveness of shear reinforcements. As shown inTable 4, eQy for all four specimens were approximately the same as calculated bending capacity of258.8kN. Figure 4 shows that GF-HU has the best performance on the deformation capability until thedrift angle 1/20rad. The deformation capabilities declined in the order, CF-SU, GF-SU, and CF-HU,accordingly. The failures for sprayed-up specimens are shown in Photo 3 and 4.
Table 4: Results of column testSpecimen
Load at yielding eQy (kN)posMaximum loadnegeQmax (kN)average
Drift angle at yielding Ry (×10−2rad)
Drift angle at ultimate
posR u (×10−2rad)neg

CF-HU267.1274.2
277.2
275.7
0.91
2.47
3.00

GF-HU
264.6
296.6
321.0
308.8
0.77
>6.67
>5.00

CF-SU
257.4
300.0
303.2
302.0
0.60
5.00
3.32

GF-SU
268.4
285.6
302.0
293.8
0.83
3.27
2.01

Q (kN)
300

P-δEffect-505R (x10-2rad.)
CF-HU
GF-HU
CF-SU
GF-SU

-300

Figure 4: Q-R curves

Photo 3: CF-SU

Photo 4: GF-SUFRP Strain Distributions
Figure 5 illustrates strain distributions of specimens CF-HU and CF-SU at peak steps of each drift
angle. The strains for sprayed-up FRP specimens at loading direction surface were small. Regarding
strains at the corners of specimens, sheet reinforced specimens had few negative strains. However,
R=1/400
R=1/200
R=1/200(1）R=1/100(2)Perpendicular
direction
Up

R=1/67(1)R=1/67(2)R=1/50(1)R=1/50(2)Corner

R=1/33(1)R=1/33(2)R=1/20
R=1/15

Loading
direction
Max Principal Strain
Min Principal Strain
Perpendicular direction

CF-HU

Tensile-2000 -1000
CompCorner01000

2000 (μ）

Loading direction

Down0 0.2 0.4 0.6 0.8

0 0.2 0.4 0.6 0.8

0 0.2 0.4 0.6 0.8

0 0.2 0.4 0.6 0.8-0.4-0.2 0 0.2 0.4

0 0.2 0.4 0.6 0.8

UpCF-SUDown
ε(%)

Figure 5: Strain distributions of FRP

-2250

ε (μ）

2250 -2250

ε（μ）

2250

-2250

ε （μ）

2250

Figure 6: Principal strains of sprayed-up FRPsprayed-up FRP specimens had many negative strains. It shows that the flexural stiffness ofsprayed-up FRP specimens is larger than the one of sheet reinforced specimens and that compression
strains at the surface of corner portions occurs.
The principal strains and those directions of FRP of specimen CF-SU at the drift angle, 1/50rad areshown in Figure 6. At the principal strain directions for sprayed-up FRP specimens, horizontal tension
and vertical compression at perpendicular face to the loading direction occurred. The strains at thecorner portions show tension and compression at the same face to the loading direction. However,strains at the ends of specimens were influenced by the stubs in some degrees. The strain directions
were distorted because of this influence. In the loading direction, both the maximum and minimum
principal strains were tension around the top of the column at the positive drift angle and around thebottom of the column at the negative drift angle.

BOND AND ANCHOR TEST BETWEEN FRP AND CONCRETE
Sprayed-up FRP has flexibility for the concrete surface at the construction site. FRP spray-on methods
were studied by providing slits or anchor bolts to bond and anchor FRP on the concrete. Two different
series of experiments were carried out, which were bond test by double shearing and anchor test formeeting corners between beams and slabs or columns and walls.
Specimens for Bond Test
600

100Specimens for bonding between FRP and50
100 Slit or Bolt
concrete were prepared as shown in Figure 7. Steel barD22Knotch
The specimen consists of a concrete prism
FRP(100 x 100 x 600mm) cracked at the center,l20using a hammer on the notch, after the 10θ
90° dreinforcing with FRP. The two steel bars also
have no connection, which means that the twoNo.5
No.6〜9No.10
prisms are connected only through the FRP.
Figure 7: Bond test specimen
Specimens No.1 through No.4 and No.11 had
no slits. In order to investigate each FRP bond behavior, FRP was sprayed on specimens under theexact same conditions as column specimens for specimens No.1 through No.6. The FRP of specimens
with slits was expected mechanical bearing to concrete. The parameters of specimens were slit types,
the number of slits, and diameters of anchor bolts. The list of specimens is shown in Table 6 with test
results. Concrete for No.1 through No.6 and No.7 through No.11 was normal weight concrete withcompressive strength of 34.2 MPa and 25.7 MPa, respectively. The specified thickness o f sprayed-up
FRP was 8.0mm for No.7 through No.11. Static tensile load was applied with displacement controlled
2MN loading machine. Load and crack width of notch at center of specimens were measured. FPR
strains were measured by strain gages with the interval of 15mm.
Results and Analysis of Bond Test
Table 5 and Figure 8 show a list of resultsand FRP strain distribution at maximum
displacement, respectively. In specimens
with no slit, No.1 through No.4, themaximum loads of sprayed-up FRPspecimens were larger than the ones ofspecimens with FRP sheets. Since FRP
stiffness is equal to or larger than FRP

0.5
ε(%)

No.10-2
No.9-3No.5-4No.6-3

0-0.5
0100
200
Position from free end (mm)Figure 8: Strain distributions of FRP300

sheets, the bond strength of FRP in the sprayed-up method is equal to or larger than the ones in sheet
reinforcements. Because specimens No.5, which have a slit without filling FRP, showed bond failures,
there is no indication that a slit works as FRP anchor. From the strain distribution (Figure 8) forspecimen No.5, strains were negative values around the slit. It indicates that flexural force occurred toFRP and that FRP was peeled along the slit and lost bond. According to specimens No.6, which havean angled slit as a parameter (Figure 7), fixative behavior was sufficient. The strain distribution
indicates that there was no compression strain. The stiffness between the slit and FRP were improvedby filled with FRP in the slit. The mechanical bearing between a slit and FRP was capable.
The fixative behaviors for specimens No.7 through No.9 were sufficient, though the bonding strengths
for these specimens No.7 through No.9 were unknown by concrete failure. Since specimens with slit
angle 90 degree did not failed in bond, filling FRP in slits or increasing the stiffness of FRP in the slit
makes the bond behavior more capable, comparing to specimens No.5 and No.6. Slit depth 10mm or
deeper can give the sufficient FRP bonding, because specimen No.9-1 failed in bond. In series No.10,
which has anchor bolts, FRP fixed by M16 bolts was ruptured at a location where the anchor bolt wasfastened. The anchor bolt diameter was too large to the FRP width. For FRP fixed by M6 bolts, theanchor bolts themselves were ruptured. M6 bolts were too small to the FRP width. It is certain that
there should be an appropriate anchor bolt size to FRP width.
Table 5: Results of bond test
No.

Width l(mm)

1-1~3
2-1~3
3-1~3
4-1~3
5-1
5-2
5-3
5-4
6-1
6-2
6-3
7-1
7-2
7-3
8-1
8-2
8-3
9-1
9-2
9-3Slit, Bolt
Depth d(mm) Angleθ Number

CF-HU
GF-HU
CF-SU
GF-SU-

20

2040

40

20

10-1
10-2
10-3

101051020
51020
51020

90 deg121275 deg
60 deg45 deg

CF-SU
GF-SU
GF-SU

75 deg
190 deg

75 degGF-SU
Thickness
8.0mm

Bolt M6
Bolt M12
Bolt M16

11* BF: Bond failure

Maximumload (kN)ID

FR: FRP Rupture CF: Concrete failure

16.58
14.40
26.27
15.91
18.43
16.55

Failurepattern*

BF

BF→FR

14.82
15.09
33.75
19.78
18.25
33.06
36.67
37.39
29.04
37.8433.70

BF
BF→FR

33.55
38.59
36.36

BF

FR

CF

CF

BF→AR
CF
FR
28.08
BF
AR: Bolt failure
30.89
44.72
45.45

Specimens for Anchor Test at Meeting Corner
Figure 9 shows a typical specimen. The loading was a tensile test fixing one end. The parameter of thespecimens were slit sizes, anchor bolt diameters, and the number of anchor bolts. The concrete
compressive strength was 25.7MPa of normal weight concrete. The same type of FRP as No.7 throughNo.11 of the bond test mentioned before was used.
LoadLoaddTabθ

FRPWWFRPdθ

No.1〜3
NobondSlitdW

BondedregionConcrete

4040

No.4〜5

No.6〜8Figure 9: Anchor test specimen

Photo 5: Specimen

Photo 6: After loading

Results and Analysis of Anchor Test
The test results are shown in Table 6. The results of the anchor test indicate that specimens with a slit
obtained sufficient anchoring because these specimens failed in FRP ruptured or concrete failure.
Specimens No.1 and No.2 had FRP rupture. This indicates that a slit angle 90 degrees gives sufficientanchoring behavior of FRP. Specimens with anchor bolt reinforcement had 2 to 2.5 times as large
maximum load as specimens with no anchor No.9, because the anchor bolt fixed the FRP at thebottom. However, specimens with slit had extremely larger maximum load than the ones with anchor
bolt reinforcement. It indicates that slit reinforcement is better method to anchor FRP.
Table 6: Results of anchor test
No.
123456789Width w (mm)4020
4040Slit, Bolt
Depth d10Angle θ90 deg

ID

Maximum load(kN)

GF-SUThickness
8.0mm

30.7232.67
37.05
28.73
31.83
12.86
18.87
25.22
6.70

75 deg1075 deg
1075 deg
Bolt M12
Bolt M16
Bolt M6×8-M12M16Failure Patter
FRP Rupture
FRP Rupture
FRP Rupture
FRP Rupture
Concrete failure
FRP rupture
Bolt yield
Debonding

CONCLUSIONSThis strengthening method by sprayed-up carbon and glass fibers with vinyl ester resin is possible toapply to concrete structures. When sprayed-up CFRP that was the same strength as 300g/m2 one layerof carbon sheet was expected, approximately 6.5mm of FRP thickness needed to be sprayed. There
was no large difference on deformations, ultimate strengths, or FRP strains between columns
reinforced by FRP sheet and sprayed-up FRP. The bond and anchor strength was improved by filling
FRP into a concrete slit or by angling a concrete slit.
This study verified that sprayed-up FRP strengthening was possible to expect the same structural
behaviors as the fiber sheet reinforcement. The sprayed-up FRP strengthening method is inexpensive
and simple to construct. This method will facilitate the future strengthening for concrete structures.