PSI - Issue 64

9

Francesco Bencardino et al. / Procedia Structural Integrity 64 (2024) 932–943 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

940

(a)

(b)

Figure 6. Strengthened RC frames: (a) front view and cross-section (dimensions in mm) and (b) image of the intervention realized.

The rehabilitation intervention required a total length of 73 m of C-FRP plates, 100 mm wide and 1.2 mm thick. Each plate was 9800 mm long. The lightweight and flexibility of C-FRP allowed for easy installation without the need for heavy scaffolding. Furthermore, the quick application enabled the conclusion of the work within the scheduled timeframe (one week). The entire structure was repaired using 15 kg of C-FRP plates. Today, 30 years after the intervention, the structure is in excellent condition, showing no signs of degradation. This serves as a clear and evident example on the effectiveness of FRP as a strengthening system and demonstrates their promising durability properties. As reported in section 2.3, today, about three decades after the intervention was carried out, there are many guidelines available for designing strengthening interventions using C-FRP systems. For instance, in accordance with CNR DT 2013/R1, the maximum allowable design strain of the composite can be evaluated using the following equation:

   

   



fk

min

;

=



a 



(2)

fd

fdd



f

where η a is a conversion factor for internal exposure (taken as 0.95) ; γ f is a partial factor equal to 1.10; ε fk is the ultimate characteristic strain of the chosen laminate; ε fdd is the maximum strain of the FRP plate before debonding. The latter coefficient must be evaluated by dividing the design strength to delamination of the FRP reinforcement according to mode 2 or intermediate debonding ( f fdd,2 ) by the elastic modulus of the FRP plate ( E f ). The maximum stress f fdd,2 at which the FRP plate can work without intermediate debonding occurring can be determined using the following equation:

k 2 E 





q

f

Fd,2

f

=



(3)

fdd,2

t



f ,d

f

Where k q is a coefficient that considers the load distributions and is equal to 1.25 for distributed loads (1.00 in all other cases); γ f,d is a partial factor regarding the FRP’s debonding from the support ranging from 1.20 to 1.50 and it is assumed 1.20; Γ fd,2 is the design value of the specific fracture energy for intermediate debonding, determined by the following formula:

k k

b G CF 

,2

(4)

f

f

 =





Fd

cm ctm

,2

in which f cm is the mean value of cylindrical compressive strength of concrete; f ctm is the mean value of tensile strength of concrete (assumed 1.57 N/mm 2 ); k G,2 is a corrective factor equal to 0.10 mm regardless of the type of reinforcement; CF is the confidence factor taken as 1.00 and k b is the geometrical corrective factor defined by the following equation: