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Use of Carbon Fibres for Strengthening Use of Carbon Fibres for Strengthening of Concrete and Masonry Structuresof Concrete and Masonry Structures

The work presents some aspects regarding design, parametric studies using S&P FRP design software and experimental testing made in INCERC.

The experimental testing was made for finding the mechanical properties of carbon fibre and the behaviour of elements strengthened with laminates CFC.

For the design and parametric studies were used (according the design software for S&P FRP system) partial safety factors for EC2, BS 8110 and ACI 318.

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Technical and economical advantages in connection with strengthening works execution are:

lower execution duration

possibility of performing the strengthening execution without activity interruption

small quantities of materials to handle

lower noses (noise, dust)

lower cost (less than classical proceedings/methods).

Main advantages regarding structure behaviour under seismic action are:lower weight : construction weight is not increased so inertial forces due to seismically action do not become greater

increase of shear strength: allows the leading of fracture to a ductile failure mechanism;

increase of ultimate deformation: hysteretic dissipated energy become greater.

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DATA REGARDING THE CHARACTERISTICS OF COMPOSITE MATERIALS

Composite materials (polymers reinforced with fibres) are manufactured from high strength and rigidity fibres (carbon, aramide, glass or high strength polyethylene) included/embedded in an epoxy resin.

Tensile elasticity modulus: Elasticity modulus of glass fibre reinforcement is approximately 25% from steel modulus. Carbon fibre modulus is bigger than glass fibres modulus.

Compression strength: FRP bars have a less favourable behaviour on compression than on tensile. Compression strength depends on the fibre type (plain or ribbed).

Compression modulus: Unlike tensile modulus, compression modulus depends on bars diameter, type, fabrication control and length/diameter ratio of the sample. Compression elasticity modulus is lower than tensile modulus.

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Shear strength: FRP shear strength is very low, so that they can be cut with saw transversal on their longitudinal axis.

Fatigue strength: CFRP bars have a better fatigue strength than steel.

Factors which affect the mechanical properties - FRP type composite properties depend on factors like: loads duration and history, temperature and moisture. These factors are correlated so that it is difficult to determine each element’s influence.

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REINFORCED CONCRETE ELEMENTS AND STRUCTURES STRENGTHENING

Fibre strengthening effects - Beam can be strengthened for increasing their flexural bearing capacity. The most effective way is to apply carbon fibres on beam bottom face. Also, a main purpose is increasing of shear capacity.

Experimental research regarding beam strengtheningExperimental research was made in INCERC for studying the behaviour of beam type elements strengthened with carbon fibres; In table 2.1 test results are presented experimental researches were made for determination of bearing capacity for a beam 2.20m long, having a 12.5 x 25cm section;Beam reinforcement was considered damaged in proportion of 100% (broken reinforcement) (Table 2.2 beam, type C).Also, results obtained on strengthened beam were compared with results obtained on tests of two beams, having the same section (one beam un- strengthened - type B - and the other strengthened - type A -with carbon strips on bottom). Table 2.2 includes the load levels and deflection values for all 3 beams. Figure 1 shows load – deflection relation.

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Table 2.1- Technical characteristics of carbon fibre strips (SIKA CARBODUR S512)

Flexural strength(N/mm2)

Tensile strength(N/mm2)

Bond strength(N/mm2)

Shear strength between carbon

band and resin

(N/mm2)

Shear strength between carbon band,

resin and concrete(N/mm2)

2199 2796 7.66 3.35 5.44

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Table 2.2- Load levels and deflection values for all 3 beams

Beam type Load stepP (tf)

Deflection

F1 F2 F3

(A)strengthened

(reinforcement 2 φ 8mm)

1.15 0.09 0.09 0.08

1.64 0.14 0.14 0.12

2.26 0.26 0.28 0.25

3.45 0.66 0.71 0.63

4.60 1.05 1.10 1.04

failure

(B)unstrengthened(reinforcement

2 φ 8mm)

0.5 0.03 0.03 0.03

1.0 0.08 0.08 0.08

1.5 0.12 0.12 0.12

2.25 0.22 0.23 0.22

2.85 1.01 1.20 1.01

3.0 2.34 2.87 2.73

failure

(C)strengthened

(fractured reinforcement)

0.5 0.02 0.02 0.02

1 0.09 0.10 0.08

1.25 0.11 0.13 0.10

1.65 0.18 0.25 0.17

1.75 0.30 0.51 0.28

failure

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Fig.1. Load - Deflection unstrengthened state (beam type B), strengthened state (beam type A) and strengthened state (beam type C)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.5 1 1.5 2 2.5 3

Deflection (cm)

Loa

d (

tf) beam type A

beam type B

beam type C

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OBSERVATIONS REGARDING CARBON FIBRE STRENGTHENED BEAM

Bearing capacity of the strengthened beam with fractured reinforcement type C was similar to a reinforced concrete beam having a 0.5% reinforcement ratio. Brittle failure occurred but it was announced by a big crack width.

For the beams type A and B:

On load levels up to service loads, beam behaviour (strengthened and un-strengthened) do not substantially differ (regarding the crack and deformation aspects). Although it can be observed smaller values for deflection on bigger load levels for strengthened beam.

On load levels greater than service level, beam behaviour differs significantly. Unstrengthened beam has a typical behaviour for reinforced concrete elements subjected to flexural loads, after reinforcement yields the element have important/big deflection to constant loads. Failure mode is a ductile one, in concrete in compression (after reinforcement yield). Behaviour of strengthened beam with carbon fibres is different from flexural reinforcement elements. On bigger load levels, the element does not have important deflections but cracks width become greater. Generally, the failure occurs due to strip separation.

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Parametrical studies

Parametrical studies enclosed in this paper were done with S&P Clever Reinforcement Company from Germany.

Studies have followed the influence of different parameters in connection with concrete section, concrete class, and reinforcement type and needed moment for strengthened section on CFRP reinforcement.

Calculation had in view the Eurocode 2, BS 8110 and ACI 318 provisions loads coefficients and reinforced concrete section.

In calculation a single parameter variation was considered (the other remained constant). So, in figs.2.1, 2.2, 2.3 and 2.4 results are presented for the following cases:

- section height variation (Table 2.3, fig.2.1)- concrete compression strength variation (Table 2.4, fig.2.2)- reinforcement strength variation influence (Table 2.5, fig.2.3)- needed moment variation (Table 2.6, fig.2.4)

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From graphics slope/gradient result that CFRP reinforcement

depending on different parameters enframes in “classical behaviour” of

reinforced concrete elements. Reinforcement strength has the main

influence (fig.2.3). Also, necessary needed exterior moment influence the

reinforcement (CFRP type) quantity (fig.2.4).

Section height (fig.2.1) also influences (by level arm of internal forces)

the needed CFRP quantity. Compressive strength has the smallest

contribution. But it must be mentioned the importance of the tensile

strength and the quality of the concrete cover for the bond strength

between CFRP and concrete.

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Steel bars

Steels cable

Glass fibres bars

Glass fibres cables

Carbon fibres cables

Aramidefibres cables

Tensile strength

(MPa)

483-690

1379-1862

517-12071379-1724

1650-2410

1200-2068

Yield limit(MPa)

276-414

1034-1396

not the case

not the case

not the case

not the case

Tensile elasticity modulus

(GPa)

200 186-200 41-55 48-62 152-165 50-74

Ultimate elongation

(mm/mm)

>0.10 >0.040.035-0.05

0.03-0.045

0.01-0.015

0.02-0.026

Compression strength

(MPa)

276-414

not the case

310-482not the

casenot the

casenot the

case

Thermal dilatation coefficient

(10-6/C)

11.7 11.7 9.9 9.9 0 -1.0

Specific weight(g/cm3)

7.9 7.9 1.5-2.0 2.4 1.5-1.6 1.25

Table 1 – Comparison between mechanical proprietiesof fibres and steel

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00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

33.23.4

40 45 50 55 60 65

h (cm)

Af

FR

P(c

m2)

Fig.2.1.Influence of the height of beam

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00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

33.23.4

15 25 35 45 55 65

Strength class of concrete (N/mm2)

Af F

RP

(cm

2)

Fig.2.2. Influence of the strength class of concrete

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00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

33.2

200 300 400 500 600

Strength of reinforcement (N/mm2)

Af F

RP

(cm

2 )

Fig.2.3. Influence of the reinforcing steel

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Fig.2.4.Influence of the bending moment

00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

33.23.4

15 20 25 30 35

M (kNm)

Af

FR

P (c

m2)

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Fig. 2.5.Reinforced concrete column strengthened with

FRP sheets

Fig.2.6.Testing ensemble for columns

Load - deflection

-6

-4

-2

0

2

4

6

8

-20 -15 -10 -5 0 5 10 15 20

mm

*10

00

Kg

f

Fig. 2.7.Load – deflection diagram forcolumn strengthened with FRP sheets

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Fig.2.8. Masonry wall strengthened with FRP plates

Load-deflection

-50-40-30-20-10

01020304050

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5

(cm)

(KN

)

Load-deflection

-80

-60

-40

-20

0

20

40

60

80

-2 -1.5 -1 -0.5 0 0.5 1

(cm)

(KN

)

Fig.2.9.Testing ensemble for masonry elements

Fig.2.10. Load–deflection diagram for masonry wall

Fig.2.11. Load – deflection diagram for masonry wall strengthened with FRP plates

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Fig.2.12.Strengthened beam

Fig.2.13. Strengthened slab

Fig.2.14. Strengthened column

Fig.2.15. Strengthened masonry

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