Cyclic bond behaviour of plain bars. Part I: Experimental investigation Gerardo M. Verderame, Paolo Ricci * , Giovanni De Carlo, Gaetano Manfredi Department of Structural Engineering, University of Naples ‘‘Federico II”, Via Claudio 21, 80125 Naples, Italy article info Article history: Received 29 October 2008 Received in revised form 13 July 2009 Accepted 21 July 2009 Available online 4 September 2009 Keywords: Old type RC element Seismic assessment Plain round bar Bond Pull-out test Cyclic load abstract In recent years there has been wide use of non-linear dynamic analysis for the evaluation of existing structures in areas of high seismic risk. In the simulation of the cyclic behaviour of elements in reinforced concrete (RC) its bonding performance cannot be ignored. There are numerous numerical models devel- oped in recent decades which can explicitly account for the slip between reinforcing bars and the sur- rounding concrete through the definition of the local hysteretic bond–slip relationship. As for plain bars, commonly used in reinforced concrete buildings before the 1970s, there are almost no references to hysteretic bond mechanisms. This paper describes the results of a series of monotonic and cyclic pull-out tests aimed at the assess- ment of the bond performances of plain round bars. Monotonic behaviour is characterized by a first ascending branch, up to the maximum bond strength; in this phase, as the slip increases, chemical–phys- ical adhesion and micro-interlocking between the cement paste and the indentations of the bar surface progressively activate. During post-peak phase the only frictional contribute is present, gradually degrad- ing towards a minimum value as the slip increases. The two parameters, maximum bond strength and minimum frictional bond, in spite of the high variability shown, seem to reflect well the literature indi- cations about bond performances of plain bars. In cyclic field, where the only frictional mechanism is present, experimental results show a significant degradation of bond capacities. Hysteretic cycles show a singular shape, characterized by a reloading phase showing a slight reduction for slip values approach- ing zero and a subsequent increase in bond stress towards the maximum imposed slip. Bond stresses in cyclic field show a high variability, but it is generally possible to recognize a decreasing trend both with the number of cycles and with the maximum imposed slip. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Many of the old type reinforced concrete (RC) constructions in areas characterized by high seismic risk were designed only for gravitational loads or with respect to obsolete seismic codes. Ab- sence of construction details and of the design principles typical of modern seismic codes generally lead to the absence of a hierar- chy of resistances and therefore to conditions of low ductility. The present Italian technical standards [1], on a level with the most modern of international codes [2,3], allow us to determine the seis- mic capacity of such constructions with recourse to non-linear analysis methodologies. The use of such methods of analysis, how- ever, requires knowledge of the real post-elastic rotational capaci- ties of each element of the construction (beams, columns) both in monotonic field, for non-linear static analysis, and in cyclical field, for non-linear dynamic analysis. It seems obvious that assessment of the real deformation mechanisms (flexural, shear and fixed end rotation) of RC elements have to take into account the interaction mechanisms of the interface between reinforcing bars and the sur- rounding concrete, mainly in cyclic and post-elastic fields. As far as deformed bars are concerned, the scientific literature reports a large number of research programs aimed at the defini- tion of local bond–slip relationship [4–9]. The considerable amount of tests has allowed, over time, the investigation of the principle parameters which influence the bond performance: the geometri- cal and mechanical characteristics of the reinforcement, the mechanical characteristics of the concrete, the cast direction, the confinement exercised by the concrete cover and the transversal reinforcement, and finally the cyclic load conditions. The existing literature on bond mechanisms of plain bars, which dates largely from the first half of the last century, is much less rich and detailed especially with regard to the aspects of cyclic and post-elastic nature. In fact, most of the tests carried out on plain reinforcing bars are basically aimed at the assessment of maximum bond strength. The first mention of the deformability characteris- tics of bond–slip relationship are to be found in the work of Abrams [10], which reports the results of about 1500 pull-out tests carried out in displacement control. According to the author, there are two different mechanisms of load transfer between the bar and 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.07.002 * Corresponding author. Tel.: +39 0817683672; fax: +39 0817683491. E-mail address: paolo.ricci@unina.it (P. Ricci). Construction and Building Materials 23 (2009) 3499–3511 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat