PSI - Issue 11

R. Capozucca et al. / Procedia Structural Integrity 11 (2018) 402–409 R. Capozucca, E. Magagnini, M.V. Vecchietti / Structural Integrity Procedia 00 (2018) 000–000

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concrete element leads to changes in its dynamic response so that in many cases, experimental vibration monitoring of strengthening through dynamic tests is a convenient, non-destructive method [Capozucca, 2009; Salawu, 1997]. In the last decades, repairing reinforced concrete (RC) structures using composite materials has become a significantly common technique in field practice using different strengthening techniques. The method investigated in this paper is the Near-Surface Mounted (NSM) technique which uses Fiber Reinforced Polymer (FRP) rods inserted into grooves on concrete covers [De Lorentis and Nanni, 2001]. Examples of NSM using steel rods for RC structures go back to the early 1950s. The advantages of NSM FRP rods compared to steel as strengthening is that they are easier and quicker to assemble due to the lightness of the strengthening materials, the slimness of the grooves attributable to the higher traction resistance, and FRPs’ better resistance to corrosion. The NSM technique appears more advisable of externally bonded FRP reinforcements to damage deriving from collision, high temperature and fire [De Lorentis and Teng, 2007]. The availability of strengthening with NSM FRP rods depends on maintaining the bond between rods and concrete although many factors affect bond mechanism: bond length, the diameter of the rods used, the type of FRP material employed, rods’ surface configuration and groove size [Sharaky et al., 2013;De Lorentis and Nanni, 2001; De Lorentis and Nanni, 2002; Hassan and Rizkalla, 2004]. NSM FRP rods are prone to show greater slips than steel reinforcement due to potentially lower bond shear stress of FRP materials [Capozucca, 2013], to the presence of surrounding adhesive layers and local cracking in the cover concrete [De Lorentis et al., 2002; Perera et al., 2009; Focacci et al., 2000]. Investigations and theoretical studies addressing the bond behaviour of FRP rods in RC elements [Capozucca, 2009; Capozucca, 2013] have been developed. On the other hand, a non-destructive method that appears convenient to apply for the analysis of response of beams strengthened with FRP rod [Capozucca, 2013] is the free vibration analysis. The basic concept behind vibration monitoring is that dynamic characteristics are functions of structures’ physical properties, therefore any change caused by damage results in change in dynamic response [Salawu, 1997]. In the NSM of strengthening bond-slip may be influenced by the cracking of concrete and loss of adhesion of rods which can modify frequency values and beams’ modes of vibration [Capozucca, 2013]. Over the years, many studies based on frequency change measures have been developed to detect damage in uniform beams and significant researches have been carried out in the analysis of damage to RC beams. Dynamic tests with frequency change measures have demonstrated that the method is useful to detect damage in the case of RC/PRC elements [Capozucca, 2013]. This paper analyses the effects of damages due to bending cracking of concrete and loss of bond of NSM glass FRP (GFRP) rectangular rods on the static and dynamic responses of strengthened RC beams. An investigation was developed to evaluate the experimental vibration response of two RC beams; one beam was built and examined experimentally by static tests increasing bending moment and by tests of free vibration after strengthening by NSM GFRP rectangular rod. The non-strengthened beam was tested to evaluate frequency value changes due to also a series of notches on concrete cover. The experimental results include both the static bending tests and the dynamic tests measuring the beams’ natural vibration modes and the frequency values. Experimental vibration tests have been carried out on RC beams without and with strengthening by GFRP rod assuming free-free ends and hinged ends. In the paper the envelope of Frequency Response functions (FRFs) obtained by the dynamic experimental tests are shown and the changes of natural frequency values are correlated to the damage degree of beam elements. A comparison of experimental and theoretical results is shown and discussed.

Nomenclature f c , f y

strength of concrete and steel E s ,E c ,E f Young’s modulus of steel, concrete and GFRP λ eigenvalue ω, f i , r circular frequency, frequency value and mode of vibration D i damage degree for notches D* i damage degree for cracking of concrete ε c , ε s strain at compressive concrete and on steel bar Δf i /f D*i difference between frequencies