The efficiency of using CFRP as a strengthening technique for reinforced concrete beams subjected to blast loading
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Abstract
Some structures may be subjected to blast loading while in service. This may cause damage or failure to the structural elements. This paper examines the performance of reinforced concrete beams using carbon fiber reinforced polymer (CFRP) when subjected to blast loading. The experimental data including damage and deflection were collected from a previous investigation and numerical analysis was then performed using ABAQUS software. Furthermore, the single degree of freedom (SDOF) model was used to complement the findings from numerical analysis. Following the good correlation between the experimental and numerical data, further analysis was performed on reinforced concrete beams strengthened with carbon fiberreinforced polymer (CFRP). Using CFRP was found to enhance the load capacity and energy absorption and to reduce the central deflection. In addition, IsoDamage curves were produced for each beam, thus allowing the assessment of damage to be predicted.
Keywords
Impact Dynamic Blast Isodamage CFRP EnergyIntroduction
Equations 4 to 10 were used as input parameters in the numerical analysis
There are many methods to analyze this type of loads. Some of them are related to finite element modeling programs (Hibbitt et al. 2011), and the other related to dynamic analysis theory such as single degree of freedom analysis (SDOF) and multidegree of freedom analysis (MDOF) (Temsah et al. 2017a, b). This paper will present both finite element and dynamic analysis procedure to verify a blast experiment, and therefore to derive what is called the Isodamage curve.
Based on the degree of damage many strengthening methods may be used. One of these methods is using CFRP (Carbon Fiber Reinforced Polymers) which has a significant effect on both shear and flexural strengthening of beams and slabs (Jahami et al. 2018). Erki and Meier (1999) and Boyd et al. (2008) examined the use of either CFRP or FRP on damaged reinforced concrete beams. They found that the beams have regained about 95% of their original flexural strength. Moreover, Pham (Pham and Hao 2017) studied the effect of CFRP sheets on RC beams subjected to impact loading. It was found that the strengthening of RC beams with FRP can considerably increase the impact resistance in both flexure and shear. However, the main failure occurred between the RC face and CFRP sheet through delamination.
As can be noticed, there has been extensive research on the use of CFRP in reinforced concrete beams for strengthening against blast loading (Urgessa et al. 2005); (Mosalam and Mosallam 2001); (De Lorenzis and La Tegola 2005). However, there is a little work for determining the required explosive mass to cause a specific degree of damage for RC beams strengthened with CFRP.
Aim and objectives
The aim of this research is to derive numerically, using the ABAQUS software, an Isodamage curve for a reinforced concrete beam subjected to blast loading with and without CFRP sheets. In addition, the exact mass of TNT to cause a specific degree of damage will be determined for a known standoff distance.
Data collection
Number and dimensions of beams
Beam  Dimensions (mm)  TNT (Kg)  Standoff distance (m) 

B21  100 × 100 × 1100  0.36  0.4 
B22  0.45  0.4  
B23  0.51  0.4  
B24  0.75  0.4 
Numerical modeling
The factors “b_{c}” and “b_{t}” relate the inelastic and plastic strains. They can be determined using the results of curve fitting of cyclic tests, and have approximate default values of 0.7.
Mechanical properties of concrete
Parameter  Static condition  Dynamic condition  

Elastic modulus (MPa)  E  29,725  46,341 
Poisson’s ratio  υ  0.2  0.2 
Density (Kg/m^{3})  ρ  2400  2400 
Compressive strength (MPa)  f’_{c}  40  56.8 
Peak compressive strain (mm/m)  \(\grave{\varepsilon}_{\text{c}}\)  2.3  3.2 
Tensile strength (MPa)  f _{t}  3.86  5.48 
Dilation angle (°)  ψ  36  36 
Eccentricity  ɛ  0.1  0.1 
Biaxial to uniaxial strength ratio  f_{b0}/f_{t0}  1.16  1.16 
Second stress invariant ratio  K  0.67  0.67 
Viscosity parameter  μ  0  0 
Mechanical Properties of Steel reinforcement
Parameter  Static condition  Dynamic condition  

Elastic modulus (MPa)  E  200,000  200,000 
Poisson’s ratio  υ  0.3  0.3 
Density (Kg/m^{3})  ρ  7850  7850 
Yield strength (MPa)  f _{y}  395  590 
Ultimate strength (MPa)  f _{u}  501  620 
Model calibration
Midspan deflection for experimental, SDOF, and finite element modeling analysis
Sample  TNT (Kg)  Standoff distance (m)  Midspan deflection (mm)  Error (%)  

Exp  SDOF  ABAQUS  SDOF  ABAQUS  
B21  0.36  0.4  9  7.9  8.8  12.2  2.22 
B22  0.45  0.4  25  22.1  23.5  11.6  6 
B23  0.51  0.4  35  31.3  32.1  11.1  8.29 
B24  0.75  0.4  40  34  36.3  15  9.25 
Top and bottom damage
Beam sample  Compression Fracture zone width (cm)  Tensile fracture zone Width (cm)  

Experiment  ABAQUS  Error %  Experiment  ABAQUS  Error %  
B21  0  0  0  0  0  0 
B22  8  6  25  7  5.9  15.8 
B23  10  9  10  12  10.8  10.4 
B24  12  10.3  14.6  15  13.8  8.3 
Strengthening beams using CFRP
Referring to Fig. 25, it can be noted that using CFRP increased the required explosive mass needed to cause the same damage to the unstrengthened sample (B22). The percentage increase varied between 10 and 50%.
Conclusion

The prediction of the response of blast loaded reinforced concrete beams using a finite element program such as ABAQUS is possible. The builtin CONWEP model that simulates the blast load pressure is an accurate tool that can be used in similar simulations to find the blast response for structural elements with deferent boundary conditions.

The builtin concrete damage plasticity model (CDP) was simulated successfully the nonlinear behavior of concrete under impact loading. It requires taking into consideration the strain rate effect on the concrete damage plasticity parameters to get an acceptable response with minimal error.

It is possible to form the IsoDamage curve for any structural elements using Finite Element Program such as ABAQUS. This can be accomplished using several combinations of TNT mass and standoff distance to obtain the equivalent pressure and impulse values.

Using CFRP reduced the extent of damage of the reinforced concrete beams when subjected to blast loads. As the number of layers of CFRP increased as demonstrated by the midspan deflection. The midspan deflection for beams with 4 layers of CFRP is 30% lower than that of a beam without CFRP. However, using more than 2 layers does not cause a further decrease in deflection.

A reinforced concrete beam is able to resist higher blast load in the presences of CFRP. Strengthening the beam with four layers with CFRP can increase the TNT mass by 50% to cause the same damage compared with a beam without CFRP.

Using CFRP sheets in reinforced concrete beams increased the absorbed energy of the beam. Higher absorption energy is associated with a reduced number and propagation of cracks.
Notes
References
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