STRENGTHENING OF NORMAL AND HIGH STRENGTH CONCRETE CORBELS WITH HORIZONTAL AND INCLINED STRIPES OF CARBON FIBER

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 7, Issue 3, May–June 2016, pp. 271–282, Article ID: IJCIET_07_03_027

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STRENGTHENING OF NORMAL AND HIGH

STRENGTH CONCRETE CORBELS WITH

HORIZONTAL AND INCLINED STRIPES OF

CARBON FIBER

Asst. Prof. Aamer Najim Abbas and Eng. Wahig Abrahim Abd Al-kareem

Al-Mustansiriya University, Iraq

ABSTRACT

In this study, there were two modes of applying the carbon fiber strips on

reinforced concrete corbels: the first one is application three horizontal strips

and its width is (50 mm) and the other is applying three inclined strips with

angle about (45°), the both modes applied on the two faces of concrete corbel

specimens. Two types of concrete were used in this study; normal strength (28

MPa) and high strength concrete (57 MPa). Each types strengthening with

two modes of carbon fiber stripes.

The ultimate and cracking capacity of tested specimens were improved as

a result of strengthening with carbon fiber strips, in addition to development

of energy absorption and stiffness characteristics.

Key words: Strengthening, Carbon Fiber, High Strength Concrete, Carrying

Capacity, Cracking Capacity, Energy Absorption, Deflection.

Cite this Article: Aamer Najim Abbas and Wahig Abrahim Abd Al-Kareem,

Strengthening of Normal and High Strength Concrete Corbels with Horizontal

and Inclined Stripes of Carbon Fiber. International Journal of Civil

Engineering and Technology, 7(3), 2016, pp.271–282.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3

INTRODUCTION

Sometimes cracks develop on the concrete surface in a structure because of weakness

designing, incorrect placement of bars of reinforcement, temperature variations,

change in use of building or due to any other unforeseen reason. In order to improve

the strengthening of structures, one of the most common strengthening methods is

wrapping by carbon fiber sheets. (1)

Carbon fibers could be described as high-performance materials. They could be

defined as fibers containing at least 92 % weight of carbon gained by the controlled

pyrolysis of convenient fibers. The carbon fibers have been intensively used because

Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem


they mostly have efficient tensile features, excellent creep resistance, good thermal

and electrical conductivities, high thermal and chemical stabilities, low densities and

the absence of oxidizing agents.(2)

Carbon fibers are used in composites with a lightweight matrix. Where strength,

stiffness, lower weight, and outstanding fatigue characteristics are critical

requirements, carbon fiber composites are perfectly appropriated to applications. Also

carbon fiber composites can be used in the occasion because of the importance of high

temperature, high damping and chemical inertness.(3)

The main target of using carbon fibers as reinforcement is their resistance to

corrosion and rusting. The advantages of using carbon fiber in reinforced concrete

depends on an important factors such as length, shape, cross section, bond

characteristics of carbon fibers and fiber content.(4)

Carbon fiber has been produced with higher flexural strength; higher shear

strength and higher modulus of elasticity, hence enhance the deflection of structural

members. (4)

The main benefits of (CFRP) could be described as increase the durability of

strengthening members against alkalis, aggressive materials and corrosion, achieve

very high strength with tensile strength greater than (2400MPa) and modulus of

elasticity greater than (165 GPa), enhance the fire resistance of structures, saving the

cost of maintains, decrease the construction period, it's light weight as a result its

easily handling and transportation in the form of rolls, CFRP meets the requirements

of using in all wanted lengths , decrease the mechanical fixing and excellent fatigue

resistance. But the disadvantages of CFRP could be described as it's susceptible to

mechanical impacts and the high cost. (5)

CORBELS DIMENSIONS AND REINFORCEMENT DETAILS

It had been casted and tested six concrete corbels, three of them is normal strength

concrete (NSC), and the other corbels is high strength concrete (HSC). All specimens

designed according to shear until failure, see Table (1).

The dimensions of column supporting the both opposite sides of corbels were (150

mm depth x 200 mm width x 650 mm height) , and the concerned column reinforced

at corners with (4ɸ10 mm) deformed longitudinal bars and (8 mm) diameter of closed

ties with spacing (150mm) center to center.

The dimensions of corbels are shown in Figure (1) below, Corbels were reinforced

with (3Ø12mm) of deformed steel bars as a main reinforcement.

Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined

Stripes of Carbon Fiber


Figure 1 Description of Tested Corbels

Table 1 Details of Concrete Corbels

Specimen

Number Type of Concrete

Mode of Carbon Fiber

Arrangement

C1 NSC ---

C1T NSC Horizontal Strips

C1S NSC Inclined Strips

C4 HSC ---

C4T HSC Horizontal Strips

C4S HSC Inclined Strips

Concrete Compressive Strength

According to (BS1881:Part116)(6)

, the compressive strength test of concrete

( ) was performed using standard 150 mm3 concrete cubes and 150 mm ×

300 mm concrete cylinders respectively. Two types of concrete mixes are used (NSC)

of (28) MPa and (HSC) of (57) MPa compressive strength.

Splitting Tensile Strength (ft)

According to (BS1881: Part 117)(7)

the splitting tensile test had been carried out in

the laboratory by using (150mm*300mm) concrete cylinder, see Table (2).



Table 2 Tensile Strength of Concrete

Type of Concrete Tensile Strength (MPa)

Normal Concrete 1.41

High Strength Concrete 2.92

Flexural Strength (fr)

The flexural strength can be expressed by the modulus of rupture. It is carried out by

using (100*100*500) mm prism, loaded with two point loads hydraulic machine of

(50 kN ) capacity according to (ASTM C78-02)(8)

, see Table (3).

Table 3 Modulus of Rupture of Concrete

Type of Concrete Modulus of Rupture

(MPa)

Normal Concrete 5.16

High Strength Concrete 7.88

Static Modulus of Elasticity (Ec)

The modulus of elasticity is calculated by plotting the stress to strain diagram of

loaded axially cylinder. As recommended by (ASTM C469)(9)

the chord-modulus

method has been used, see Table (4).

Table 4 Static Modulus of Elasticity of Concrete

Type of Concrete Static Modulus of Elasticity (MPa)

Normal Concrete 33205

High Strength Concrete 129159

Testing of Steel Bars

According to ASTM A615 (10)

, the tensile strength test of steel bars was

performed, see Table (5).

Table 5 Properties of Steel Bars

Nominal Diameter

(mm) Bar Type

fy

(MPa)

fu

(MPa)

Es**

(GPa)

Elongation

%

8 Deformed 412 591 200 10.8

10 Deformed 404 566 200 10.3

12 Deformed 401 548 200 11.1

Cement

Tables (6) and (7) illustrate the physical and chemical properties of cement used in

this research. The test was performed according to American Specifications ASTM-

C150 (11)

.




Table 6 Physical Properties of Cement

Physical Properties Test Results

Fineness using Blain air permeability apparatus

(m2/kg)

392

Initial setting time using Vicat's instruments (hr: min.) 1:48

Final setting time using Vicat's instruments (hr: min.) 4:51

Safety (soundness) using autoclave method (%) 0.01

Compressive strength for cement paste cube

(70.7mm) at : (3days) in (N/mm2) or (MPa)

22.49

Compressive strength for cement paste cube

(70.7mm) at : (7days) in (N/mm2) or (MPa)

26.6

Table 7 Chemical Composition of Cement

Compound Name Compound Chemical

Composition % (weight)

Silica SiO2 19.20

Alumina Al2O3 5.31

Iron Oxide Fe2O3 3.68

Lime CaO 63.77

Magnesia MgO 1.93

Sulfate SO3 2.21

Insoluble Residue I.R. 1.11

Loss On Ignition L.O.I 3.68

Tricalcium aluminates C3A 8.29 (From X.Ray

diffraction)

Lime Saturation Factor L.S.F 0.83

Fine Aggregate

Natural sand with fineness modulus of (2.69) is used. Fine aggregate characterized by

rounded particle shape and smooth textures. Only the passing sand from the sieve

(4.75mm) is requires achieving the requirement of mixing. The grading is shown in

Table (8).

Table 8 The Grading of Fine Aggregate (Sand)

No. Sieve size

(mm)

Present work of fine

aggregate (% passing)

BS882:1992 limit (%

passing)(12)

1 10 100 100

2 5 92.48 89-100

3 2.36 81.83 65-100

4 1.18 53.46 45-100

5 0.6 62.43 25-80

6 0.3 41.03 5-48

7 0.15 8.22 0-15



Coarse Aggregate

The maximum size of crushed gravel is (14mm). The grading is shown in Table (9),

which confirms to the BS882:1992 Specification(12)

.

Table 9 The Grading of Coarse Aggregate (Gravel)

No. Sieve size

(mm)

Present work of

coarse aggregate

(% passing)

BS882:1992 limit (%

passing)(12)

1 20 100 100

2 14 93.72 90-100

3 10 73.6 50-85

4 5 4.2 0-10

5 2.36 0 0

Admixtures

In order to produce high strength concrete mixes, super-plasticizer based on poly

carboxylic ether must be used. Also, it can be called (high range water reducing agent

HRWRA). Glenium51 is one of the new generation of polymer which mainly used in

designed super-plasticizer; the normal dosage for Glenium51 is ((0.5-0.8) L/100kg) of

cement. Table (10) illustrates the typical properties of super-plasticizer.

Table 10 Typical properties of Glenium 51

No. Main action Concrete super plasticizer

1 Color Light brown

2 pH. Value 6.6

3 Form Viscous liquid

4 Chlorides Free of chlorides

5 Relative density 1.08 – 1.15 gm/cm3 @ 25C

6 Viscosity 128 30 cps @ 20C

7 Transport Not classified as dangerous

8 Labeling No hazard label required

Carbon Fiber

The technical data information of CFRP sheets and epoxy that used in this work can

be clearly seen from Tables (11) and (12) respectively.

Table 11 SikaWrap Hex-230C (Carbon Fiber Fabric) Technical Data *

Property Results

Fiber type High strength carbon fibers

Fiber orientation The fabric equipped with special weft fibers which

prevent Loosening of the roving (heat set process).

Areal weight 225 g/m2

Fabric design thickness 0.13 mm (based on total area of Carbon fibers).

Tensile strength of fibers 3500 MPa

Tensile E – modulus of fibers 230 GPa

Elongation at break 1.5 %

Fabric length/roll ≥ 45.7 m

Fabric width 305/610 mm




Table 12 Sikadur-330 (Impregnating Resin) Technical Data*

Property Results

Appearance Comp. A: white

Comp. B: grey

Part A+B mixed: light grey

Density 1.31 kg/l (mixed)

Mixing ratio A : B = 4 : 1 by weight **

Open time 30 min (at + 35C◦)

Viscosity Pasty, not flowable

Application temperature + 15C◦ to + 35◦C (ambient and substrate)

Tensile strength 30 MPa (cured 7 days at +23◦C)

Flexural E-modulus 3800 MPa (cured 7 days at +23◦C)

Concrete Mix Proportions

According to ACI committee 211.1-91(13)

, two concrete mixtures were designed.

Anyway, to ensure that the required strengths (28 and 57 MPa) were achieved, many

trial mixes were made. The proportions of the suitable mix are as given in Table (13).

Table 13 Concrete Mixes

DISCUSSION AND RESULTS

General Behavior and Failure Patterns

At the early stages of load application, the specimens appear high stiffness and show

high resistance to loads until appearance of the first crack, the vertical displacement is

small and no cracks appear.

After appearance of the first crack, the stiffness begins to decrease and the vertical

displacement begins to increase. At this stage, the flexural cracks begin to appear at

the tension face of corbels near the column; the cracks are narrow and increase with

load increment.

At the advanced stages of loading, diagonal shear cracks start to develop near the

supports and propagate quickly towards the column face with an angle about 60°.

These cracks are wider than the flexural cracks. The failure is sudden and

uncontrolled except the strengthened specimens with carbon fiber strips and steel

fibers which the failure is more ductile than the un-strengthened specimens. The

plates (1) to plate (6) show the failure mode of tested specimens.

(MPa)

Cement

(Kg/m3)

Sand

(Kg/m3)

Gravel

(Kg/m3)

W/C Superplastizer

Liter/m3

ـــــــ 0.46 1200 542 418 28

57 510 590 1000 0.32 4



Load – deflection Behavior

By studying the load- deflection curve in Figure (2) and Figure (3), it has been found

that all the specimens exhibit three stages as indicated below

1. The first linear stage

This stage started from the beginning of load application with relatively straight line

until the appearance of first crack, this stage represent the elastic behavior of concrete

and steel just because the stresses in concrete and steel virtually small as compared

with later stages, in other words the specimens returned to the original manner after

the load releasing because there is no local slipping between concrete and steel. At

this stage of load – deflection curves, the specimens have the highest value of flexural

rigidity. 2. The second linear stage

In this stage there was a change in the slope of load versus deflection curves in

comparison with pre cracking phase. This stage started from the appearance of first

crack up to the yield of tensile steel bars, this stage characterized by plastic behavior.

In other words, specimens did not return to the original manner after the load

releasing. At this stage, the specimens have less value of stiffness because extension

of the flexural cracks and the loose of bond characteristics between the concrete

Plate (1) Failure Mode of

Corbel (C1) Plate (2) Failure Mode of

Corbel (C1T) Plate (3) Failure Mode of

Corbel (C1S)


Corbel (C2)


Corbel (C2T)


Corbel (C2S)




matrix and steel bars. Thus, the less toughness of structural member means large

deformations. 3. The non-linear stage

This stage starts at the yield of tensile steel bars until the failure of specimens. At this

stage, the specimens have stiffness less than the previous stages because of the

increasing the number of cracks, width of cracks and loose of bond between steel and

concrete.

Figure 2 Load-deflection Curve of Normal Strength Concrete Corbels

Figure 3 Load-deflection Curve of High Strength Concrete Corbels

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5

Corbel - C1-

Corbel -C1S-

Corbel -C1T-

0

100

200

300

400

500

600

700

0 1 2 3 4 5

Loa

d-k

N

Deflection-mm

Corbel -C2-

Corbel -C2S-

Corbel -C2T-



STRENGTHENING EFFICIENCY OF CARBON FIBER STRIPS

Effect of Carbon Fiber Strips on carrying capacity

The using of carbon fiber strips in strengthening the concrete corbels effectively

enhances the ultimate strength of the specimens. The improvement of strengthening

corbels with horizontal strips less than the other specimens with inclined strips

because the later are perpendicular on cracks propagation and this configuration play

a vital rule in delaying the extension of cracks such as the specimen C1S which have

an increasing ratio about 60% over the reference specimen C1, while the

improvement of specimen C1T reached to 16.445 in comparison with same reference

specimen. Accordingly, the increasing of the specimen strength of C2T is about

67.167% while the specimen C2S is about 69.667% in comparison with reference

specimen C2, see Table (14).

Table 14 Effect of Carbon Fiber Strips on Carrying Capacity

Specimen No. Ultimate Loading

Capacity

Percentage of

Improvement (%)

C1 225 ----

C1T 262 16.445

C1S 360 60

C2 300 ----

C2T 501.5 67.167

C2S 509 69.667

Effect of Carbon Fiber Strips on Cracks Appearance

The strengthening mode does not effect on the appearance of first crack in normal

concrete corbels. The amount of improvement is about (63.636%) for specimens C1T

and C1S in comparison with reference specimen C1. But, the improvement of high

strength concrete corbels is clearly seen in specimens (C2T and C2S) which have the

ratio of improvement about 116% and 60% respectively in comparison with reference

specimen C4, see Table (15).

Table 15 Effect of Carbon Fiber Strips on Cracking Capacity


Capacity

Percentage of

Improvement (%)

C1 110 ----

C1T 180 63.636

C1S 180 63.636

C2 125 ----

C2T 270 116

C2S 200 60

Effect of Carbon Fiber Strips on Energy Absorption

The energy absorption of tested specimens can be calculated from the area under the

load-deflection curve.

The using of carbon fiber strips as a strengthening method improved the energy

absorption of tested corbels. The strengthened specimens of normal strength concrete

C1T and C1S increased about 68% and 237.21% respectively as compared with




reference specimen C1. Also, the high strength concrete corbels C2T and C2S have an

improvement about 13.886% and 36.795% respectively over reference specimen C2,

see Table (16).

The inclined strengthening configuration achieved good energy absorption in

comparison with horizontal configuration.

The amount of energy absorption gives an indication about its ductility; the

greater amount means high ductility. Therefore, the strengthened specimens have a

ductility more than the non-strengthened specimens, and the inclined configuration of

carbon fiber gives ductility greater than the horizontal one.

Table 16 Effect of Carbon Fiber Strips on Energy Absorption


Capacity

Percentage of

Improvement (%)

C1 934.585 ----

C1T 1570.116 68

C1S 3151.518 237.21

C2 3980.419 ----

C2T 4533.148 13.886

C2S 5445.033 36.795

Conclusions

1. The ultimate carrying capacity of tested corbels is affected positively by using carbon

fiber strips as a strengthening method.

2. The cracking capacity of normal strength concrete corbels does not affected by using

carbon fiber strips, while the high strength concrete corbels achieved good

improvement.

3. The amount of energy absorption increased as a result of using carbon fiber strips.

4. The using of carbon fiber strips does not exchange the failure type of tested corbels.

5. The strengthened corbels by carbon fiber strips have a good stiffness in comparison

with un-strengthened corbels.

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