PSI - Issue 37

Jamal A. Abdalla et al. / Procedia Structural Integrity 37 (2022) 660–667 Abdalla et al./ Structural Integrity Procedia 00 (2019) 000 – 000

661

2

and their ductility ( Abokwiek et al. 2021, Zhang et al. 2017, Al-Hacha et al. 2004; ElMaaddawy et al. 2009; Hadi et al 2012). Prior to the advent of CFRP, steel plates were used as externally strengthening materials, while steel bars were used as near surface mounted (NSM) strengthening material. The use of steel plates and bars as NSM is not encourage due its susceptibility to corrosion. CFRP bars have been used as NSM with and without wraps due to their high tensile strength and their high resistance to corrosion (Obaidat et al 2021; Gajdosova et al. 2013; Chellapandian et al. 2019; Triantafyllou et al. 2015). The NSM-FRP reinforcement technique requires minimal surface preparation and increases the flexural capacity of the structural element significantly, yet the bonding might control the failure. As a result, CFRP confinement is used in order to provide higher bonding, ductility and energy absorption to the structural element. Multiple parameters were tested with the aim of examining the effects of NSM-CFRP wrap strengthening in short columns. Recently CFRP strips have been used as NSM due to their high strength and large surface area that assists in increasing the bonding surface with concrete and therefore assist in the transfer of larger loads (Abokwiek et al. 2021; Khorramian et al 2019; Barros et al. 2008 ) . Abokwiek et al. (2021) carried out an extensive experimental investigation to study the efficiency of the use of NSM-CFRP strips confined with CFRP wraps to strengthen RC columns. The tested columns were strengthened with different number of CFRP strips that were wrapped with different number of layers of CFRP sheets. The columns were tested under axial and uniaxial bending to study the effect of NSM-CFRP strips, number of CFRP layers, and the eccentricity ratio on the capacity of the strengthened specimens. They observed that, the load-carrying capacity of the confined specimens strengthened with strips in axial and uniaxial bending increased significantly and reached up to 95% over the control specimen. This paper presents several empirical models that were developed using the experimental results obtained in this study. The models cover the unstrengthened and three strengthened schemes for four different eccentricity ratios. The axial load capacities predicted by the nonlinear empirical models showed a nearly perfect fit to the experimental results. Therefore, the developed empirical models can be used for predicting the capacities of RC columns strengthened with CFRP strips and wraps when subjected to axial load and uniaxial bending Tests of 15 specimens consisting of the same RC columns but with various NSM-CFRP strip numbers, CFRP confinement layers, and loading eccentricity ratios were performed. A ready mixed concrete with 22 MPa cylinder strength at 28 days was used in preparation of all specimens. All columns were made of a tied short column, 1000 mm high, 200 x 200 mm cross-section, 4Ø12 longitudinal reinforcement ratio (1.13% of gross area), and Ø6@200mm c/c closing ties with two extra closed ties, of the same dimensions, at either end of the column to prevent the column from failing prematurely between the corbels and the columns. The steel reinforcement used has a yield and tensile strength of 500 and 540 MPa, respectively. To ensure sufficient eccentricity, a corbel with a depth of 300 mm was connected to both ends of the specimen. All specimens have the same size and detailing. The CFRP strips, wraps and their compatible epoxies were used from the same manufacturer. Their mechanical properties were obtained from the manufacturer technical data sheets and they are summarized in Table 1 (Sika 2003; Sika 2006; Sika 2009; Sika 2014). Table 1 shows the material properties of CFRP strips and wraps. The load eccentricity ratio used were zero, 0.25, 0.5 and 0.75. The number of CFRP strips used were 0, 4 and 8 strips corresponding to 0S, 4S and 8S in specimen designation, respectively. Finally, the number of CFRP wraps were 0, 2 and 4 wraps correspond to 0W, 2W and 4W in the specimen designation, respectively. Figure 1 shows a sketch of two typical specimens that were used in this study. with different eccentricity ratios. 2. Materials and Test Specimen

Table 1. CFRP strips, wraps and their corresponding epoxies mechanical properties. Material Type Thickness t f (mm) Modulus of Elasticity E f (GPa)

Ultimate Tensile Strength f fu (MPa)

Strain at Rupture ε fr (%)

CarboDur®

Plate

1.5

165

3100

>1.7

Sikadur®-30 LP

Epoxy

-

10

>25

-

SikaWrap®300C Composite 0.17

230

3900

1.5 0.9

Sikadur®-330

Epoxy

-

4.5

30