 Open Access
 Total Downloads : 129
 Authors : Mohamed Abou Elmaaty Amin, Ahmed Maher Ragb, Reham Hassan
 Paper ID : IJERTV3IS110629
 Volume & Issue : Volume 03, Issue 11 (November 2014)
 Published (First Online): 19112014
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
The Effect of LongTerm Performance on the Behavior of Strengthened Slabs
Mohamed Abou Elmaaty A. Ahmed Maher Ragb Assistant Professor of Structural Engineering Professor of Strength of Material Faculty of Engineering, Fayoum University Faculty of Engineering, Cairo University
Fayoum, Egypt Cairo, Egypt
Reham Hassan
Assistant lecture,
Institute of Aviation Engineering and Technology Giza, Egypt
AbstractReinforced concrete structures often require strengthening or repair at some point during their life time. The effect of longterm performance on repaired sections is rarely taken in consideration. The repaired sections might be affected by longterm performance so they should be considered in design. Longterm effects in the form of shrinkage and creep stresses reduce the structural capacity of the repaired sections. The main goal of this research is to investigate the longterm performance of the strengthened reinforced concrete slabs [deflection, stressstrain, shrinkage strain and creep strain] by using finite element analysis. An inhouse program was modified to perform finite element tie history analysis taking into consideration the special factors of strengthened reinforced concrete slabs. A parametric study was done on repaired reinforced concrete slabs using different types of repair at different time intervals to evaluate the efficiency of repaired slab sections for longterm performance, and find which method of repaired was better at different time. The parametric study contains four simply supported slabs by using different type of strengthening, were created four models, the first was a control slab the second is strengthened using new concrete layer without reinforced, the third was strengthened new concrete layer with reinforced and the fourth was strengthened by using carbon epoxy laminated layer attached to the tension side of slab. A comparison between the result was done using the different type of repaired at different time for different load capacity.
KeywordsTypes of repair; Longterm effects; shrinkage and creep stresses; finite elements analysis.

INTRODUCTION
The aim of this study is to study the effect of long term strains (creep and shrinkage) on strengthened slabs, under the effect of different strengthening materials, Deflection, stress, and strain will be considered through this analysis. A modified finite element program was verified with experimental works to be used in studding the effect of long term performance on strengthened reinforced concrete slabs [1,2]. The studied slabs were observed under different times, such that: At time (0) days for short term, (200), (600) and (1200) days. The four slabs were
Classified as control slab with no repair layer (Un strengthened slab) and the otherthree slabs were strengthened by three types of repaired materials as strengthened slabs with new concrete layer without reinforcement, strengthened slabs
with new concrete layer with reinforcement and strengthened slabs with carbon epoxy laminate.

DESCRIPTION OF STUDIED SLABS
All slabs are simply supported with four beams with equal span (5*5) meter and (15) cm in thickness with mesh (6 Ã˜8/m) reinforcement as shown in Fig. 1. It is subjected to uniform distributed load (10ton/m2) which is divided into 10 load step. The parametric study contains 4 types of slabs, table1 summarized the description studied slabs.
TABLE1. SUMMARIZED THE DESCRIPTION OF THE STUDIED SLABS
Specimen
Slab Dimension (cm)
Thickness (cm)
Slab RFT
Type of repaired
Control Slab (S1)
500×500
15
6 Ã˜8/m
Without repair
Concrete Strengthene d(S2)
500×500
12+
repaired thickness
6 Ã˜8/m
3cm of a new concrete layer without reinforcement
Reinforced Strengthene d(S3)
500×500
12+
repaired thickness
6 Ã˜8/m
3cm of a new concrete layer with(6 Ã˜12/m) additional reinforcement
CFRP
Strengthene d(S4)
500×500
15
6 Ã˜8/m
2 layers of carbon epoxy laminates (0.12 cm )

DESCRIPTION OF FINITE ELEMENT MODEL FOR
STUDIED SLABS
A nonlinear finite element program based on updated lagrangian formulation has been prepared [3,4,5] and employed in the present study. The program was utilized by consider the effect of creep and shrinkage strains by using ACI formula [6]. Concrete behavior under the biaxial state of stress is represented by a nonlinear constitutive relationship which incorporates tensile cracking at a limiting stress, tensile stiffening between cracks and the strainsoftening phenomenon beyond the maximum compressive strength. The steel reinforcement is represented by a bilinear, strain hardening model where Bauschinger effects are considered.
The program employs a layered approach where perfect bond is assumed to exist between the various successive layers. The constitutive relationship for concrete is a nonlinear elastic model based on isotropic formulation (hypoelastic formulation). The incremental stressstrain relationship takes the form:
each layer but layer 9 is resins material (epoxy) with (0.04) cm. The last layer is a repaired material which is deferent for three repaired slabs. Last layer in concrete strengthened slab (S2) is (3) cm of high strength concrete, reinforcement strengthened slab (S3); (3) cm of high strength concrete with (6 Ã˜12/m) reinforcement. While CFRP strengthened slab (S4) have 8 layers of concrete with 1.875cm for each layer but
d1
E1
E1E2
0 d1
layer 9 is resins material (epoxy) with (0.04) cm and the last
0
0
d 1 E E E
0 d
layer is 2 layers of Carbon epoxy laminates with (0.12)cm .
2
(1 2 ) 1 2 2
2 (1)
For reinforcement strengthened slab (S3) the reinforced steel
d12
Where,
(1 2 )G'd12
is converting to layer with smart thickness as shown in Fig. 2.Table2 shows all properties of the studied slabs.
Indices 1,2 refer to the principal stress directions. and are the normal and shear stresses.
G' is the shear modulus.
and are the normal and shear strains. is the Poissons ratio.
E1 and E2 are the modulus of elasticity in direction 1 and 2 respectively.
TABLE ( 2) PROPERTIES OF THE USED MATERIAL FOR STUDIED SLABS
Property
Model 1
a Concrete properties
Concrete compressive strength (Fcu)
20 Mpa
Modulus of elasticity of concrete ( Ec )
19799 Mpa
Allowable tension stress in concrete( Ft)
2 Mpa
Concrete Strain
0.003
B new concrete layer properties
Concrete compressive strength (Fcu)
30 Mpa
Modulus of elasticity of concrete ( Ec )
24248.7 Mpa
Allowable tension stress in concrete( Ft)
3 Mpa
Concrete Strain
.003
Thickness of 2 layers
2x.023 inches
CEpoxy Resins Properties
Compressive strength
80 Mpa
Modulus of elasticity of concrete ( E )
10.5 Mpa
D Carbon Epoxy Laminated (CFRP) Properties
Thickness of 2 layer
2*0.06 cm
Allowable tension stress in CFRP (Ft)
14 MPa
Modulus of elasticity of CFRP (E)
10 Mpa
E Steel properties
Allowable steel stresses
360 Mpa
Modulus of elasticity of concrete
201 Mpa
Maximum Strain
0.13
G' 1 E E 2 E E (2)
4(1 2 ) 1 2 1 2
In order to account for the effect of confinement of micro cracking for the case of biaxial compression, use is made of the equivalent uniaxial stress strain curve where Poissons effect is totally eliminated. The following relationship then holds,
i
EO iu
E
(3)
2
1 O 2 iu
ES ic
Where Eo, Es are initial and secant modulus of elasticity respectively.
For the purpose of the parametric study, a simply supported two way slab with equal span (5.0*5.0) meter and with thickness equal (15) cm. The slab divided into 81 nodes and 128 triangle elements which the area of each element is 1953.125cm2as shown in Fig. 2. It was subjected to uniform distributed load normal to surface per unit area. The slab section for controlled slab (S1) divided into 10 layers of concrete with (1.5) cm for each layer. It has (6 Ã˜8/m) reinforcement which is convert to layer with smart thickness as shown in Fig. 2.
However, The slab section for repaired three slabs divided into 10 layers which have 8 layers of concrete with 1.5cm for
Fig. 1. Plan of Studied Slab Before Repaired
Fig. 2. Fnite element mesh and cross sections for studied slabs

RESULTS AND DISCUSSION
Considering the studied parameters of the present study, the following notes were observed:

Effect of Time Dependent Analysis on Deflection for Different Type of Slabs
It is clearly seen from Fig. 3, at time (0) the controlled slab gives the highest deflection range between (2.4 4.5) mm while the repaired slabs using carbon epoxy laminate gives lower deflection range between (0.56 1.7) mm. Otherwise using new concrete layer without reinforcement and new concrete layer with reinforcement gives deflection range between (1.7 4.2) mm and (1.46 3.9) mm respectively. Also, we can observe from Figs. 4 to 6, that as time increase the deflection of controlled slab and all repaired slabs are
increased. By comparing the repaired slab using carbon epoxy laminate at time (200), (600) and (1200) we get the smallest deflection than the others, while the controlled slab gives the largest deflection in all figures. With increasing time and load the repaired slab using new concrete layer with reinforcement reflects lower deflection than repaired slab using new concrete layer without reinforcement.
Fig. 3. Load Deflection Curve for Different Slabs Type at Time (0) Days
Fig. 4. Load Deflection Curve for Different Slabs Type at Time (200)
Days
Fig. 5. Load Deflection Curve for Different Slabs Type at Time (600) Days
Fig . 6. Load Deflection Curve for Different Slabs Type at Time (1200) Days

Effect of Time Dependent Analysis on Stress and Strain for Different Type of Slabs
It is clearly seen from Figs. 7 to 10, by increasing strain the stress increases till it reaches the maximum stress then the stresses decreases gradually ,so the ultimate strain obtained at the maximum stress. Controlled slab(S1) reflect always the lowest stressstrain curve for different time but, stressstrain curve was increased when used new concrete layer without reinforcement(S2) and used new concrete layer with reinforcement for repaired(S3). When using carbon epoxy laminated for repaired (S4) gives the highest stressstrain curve.
At time (0): The maximum stress is (32Mpa) at ultimate strain (0.0031) for S1, while the stress of repaired slabs S2 is (35Mpa) at ultimate strain (0.0032) and S3is (37Mpa) at ultimate strain (0.0032). But, for S4, it gives (50Mpa) at ultimate strain (0.0032). At time (200): The maximum stress is (33Mpa) at ultimate strain (0.0033) for S1, while the stress of repaired slabs S2 is (34Mpa) at ultimate strain (0.0032) and S3is (36Mpa) at ultimate strain (0.0032). But, for S4, it gives (49Mpa) at ultimate strain (0.0033).At time (600): The maximum stress is (34Mpa) at ultimate strain (0.0031) for S1, while the stress of repaired slabs S2 is (36Mpa) at ultimate strain (0.0032) and S3is (37Mpa) at ultimate strain (0.0032). But, for S4, it gives (47Mpa) at ultimate strain (0.0033).At time (1200): The maximum stress is (37Mpa) at ultimate strain (0.0031) for S1, while the stress of repaired slabs S2 is (39Mpa) at ultimate strain (0.0032) and S3is (40Mpa) at ultimate strain (0.0032). But, for S4, it gives (46Mpa) at ultimate strain (0.0032).
Fig. 7. Stress Strain Curve for Different Slabs Type at Time (0) Days
Fig . 8. Stress Strain Curve for Different Slabs Type at Time (200) Days
Fig . 9. Stress Strain Curve for Different Slabs Type at Time (600) Days
Fig . 10. Stress Strain Curve for Different Slabs Type at Time (1200) Days

The Effect of Increasing Load on ShrinkageStrain for Different Type of Repaired Slab at Different Time
As can be observed from Figs. 11 to 13, the effect of increasing load on shrinkagestrain for different type of repaired slab at different time. As time increase the shrinkage strain of controlled slab and all repaired slabs are increased. The controlled slab gives the largest shrinkagestrain in all figures. With increasing time and load the repaired slab using new concrete layer with reinforcement reflects higher shrinkagestrain than repaired slab using new concrete layer without reinforcement. At time (0) No shrinkagestrain. At time (200) the controlled slab gives the largest shrinkage strain while the repaired slabs with new concrete layer with reinforcement gives lower shrinkagestrain than using new concrete layer without reinforcement. The increasing in load in case of controlled slab which varies through (1 to 10) ton/m2 reflects the increasing in shrinkagestrain which was varies between (0.0016 0.0074) while the shrinkagestrain in repaired slabs using new concrete layer without reinforcement varies between (0.00125 0.007) and new concrete layer with reinforcement varies between (0.001 – 0.0066) for the same load range. At time (600) the controlled slab gives the largest shrinkagestrain while the repaired slabs using new concrete layer with reinforcement gives lower shrinkagestrain than using new concrete layer without reinforcement. The increasing in load in case of controlled slab which varies through (1 to 10) ton/m2 reflects the increasing in shrinkagestrain. Which was varies between ( 0.0026 0.008) while the shrinkagestrain in repaired slabs using new concrete layer without reinforcement varies between (0.0014 0.0073) and new concrete layer with reinforcement varies between (0.0012 0.0069) for the same load range.
At time (1200) the controlled slab gives the largest shrinkagestrain while the repaired slabs using new concrete layer with reinforcement gives lower shrinkagestrain than using new concrete layer without reinforcement. The increasing in load in case of controlled slab which varies through (1 to 10) ton/m2 reflects the increasing in shrinkage strain. Which was varies between (0.0034 0.0082) while the shrinkagestrain in repaired slabs using new concrete layer without reinforcement varies between (0.0025 0.0074) and
new concrete laer with reinforcement varies between ( 0.0023 0.007) for the same load range.
Fig. 11. Load shrinkagestrain curve for different slabs type at time
(200) Days
Fig. 12. Load shrinkagestrain curve for different slabs type at time (600) Days
Fig. 13. Load shrinkagestrain curve for different slabs type at time (1200) Days

The Effect of Increasing Load on CreepStrain for Different Type of Repaired Slab at Different Time
We can observe from Figs. 14 to 16, the effect of increasing load on creepstrain for different type of repaired slab at different time. At time (0) No creepstrain. At time (200): the controlled slab gives the largest creep strain while the repaired slabs using new concrete layer with reinforcement gives lower creep strain than using new concrete layer without reinforcement . The increasing in load in case of controlled slab which varies from (1 to 10) ton/m2 reflects the increasing in creepstrain which was varies between (0.0019 0.0082) while the creep strain in repaired slabs using new concrete layer without reinforcement varies between (0.0017 0.0075) and new concrete layer with reinforcement varies between (0.0014 0.0069) for the same load range. At time (600) the controlled slab gives the largest creepstrain while the repaired slabs using new concrete layer with reinforcement gives lower creepstrain than using new concrete layer without reinforcement. The increasing in load in case of controlled slab which varies through (1 to 10) ton/m2 reflects the increasing in creepstrain which was varies between (0.0026 0.0079) while the shrinkagestrain in repaired slabs using new concrete layer without reinforcement varies between (0.0018 0.0073) and new concrete layer with reinforcement varies between (0.0017 0.0069) for the same load range. At time (1200) the controlled slab gives the largest creep strain while the repaired slabs using new concrete layer with reinforcement gives lower creepstrain than using new concrete layer without reinforcement. The increasing in load in case of controlled slab which varies through (1 to 10) ton/m2 reflects the increasing in creepstrain which was varies between (0.0035 0.0082) while the shrinkagestrain in repaired slabs using new concrete layer without reinforcement varies between (0.0022 0.0076) and new concrete layer with reinforcement varies between (0.002 0.0072) for the same load range.
Fig. 14. Load CreepStrain Curve for Different Slabs Type at Time (200)
Fig. 15. Load CreepStrain Curve for Different Slabs Type at Time (600)
Fig. 16. Load CreepStrain Curve for Different Slabs Type at Time (1200) Days


CONCLUSIONS

When CFRP laminates were used for repairing, the deflection was decreased in rang (6279%) at time (0) While at time (200) days it was decreased in range (3269%). However, at time (600) it was decreased in range (4062%). Also it was decreased in range (45 63.5%) at time (1200) compared to control slab.

When CFRP laminates were used for repairing, the ultimate load was increased by (166%) at time (0) and time (200), while at time (600) it increased by(200%).However, at time (1200) it increased by(400%) compared to control slab.

When CFRP laminates were used for repairing, the maximum stress was increased by (56%) at time (0) While at time (200) it was increased by (48%). However, at time (600) it was increased by (38%). Also it was increased by (24%) at time (1200) compared to control slab.

The strengthening of R.C slabs using externally bonded CFRP laminated gives lowest deflection, highest stress strain curve, lowest and highest ultimate load for longterm.

Longterm had slight effect (lowest effect) on slab strengthened with CFRP laminates (less than 10% loss initial strength).

The repaired slab using new layer of concrete without reinforcement gives the lowest shrinkage and creep strain.

The ultimate load of repaired slab using new layer of concrete with reinforcement was higher than using new layer of concrete without reinforcement.

The strengthening of R.C slab using new layer of concrete with reinforcement gives slight different behavior from using new layer of concrete without reinforcement in (deflection, stress strain, shrinkage and creep strain for longterm.

The strengthening o f R.C slabs using externally bonded CFRP laminated is gives the best performance compared to the other strengthening technique for longterm.

REFERENCES

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