E. Pozzo

The influence of axial load and rate of loading on experimental post-elastic behaviour and ductility of reinforced concrete members

This research investigates the post-elastic behaviour of reinforced concrete members under combined bending and axial loads, considering different concrete strengths and types, reinforcing percentages (always symmetric), axial loads and loading rates.Sensible differences between experimental results, for usual and high test loading rates, and those obtained by ‘classical’ analysis are pointed out. One of the most interesting results shows that disagreement was reduced (to low quotas for not high axial loads) if we took into account certain phenomena neglected by theory (such as tensile strength of concrete, tension stiffening and the effect in reinforcement strength of transverse constraint caused by the presence of concrete) while structural ductility values appeared fairly probative, at least with the conventional collapse assumed.The strong decreases that ductility can undergo with increasing loading rate are also pointed out, indicating that it is therefore hazardous to utilize results obtained with usual test procedures in the seismic field.

Rheological model of concrete in the dynamic field

SommarioViene qui tentata la definizione del modello reologico del calcestruzzo, o cemento armato non parzializzato, in campo di sforzi dinamici sufficientemente lontani dalla rottura; fine al quale avevamo di recente già indirizzato una serie di indagini, volte a chiarire i diversi aspetti del comportamento sperimentale.Pur con lacune di dettaglio, il modello trovato sembra correttamente interpretare ilmeccanismo dissipativo del nostro materiale, ed è — contrariamente a correnti schematizzazioni viscose — a caratteristica prevalentemente elastoplastica, con smorzatori di tiposolid-friction.Il lavoro si conclude trattando della determinazione sperimentale dei parametri del modello e di particolari soluzioni approssimate del moto.SummaryThe definition of the rheological model of concrete, or uncracked reinforced concrete, in the field of dynamic stresses sufficiently far from breaking, is here attempted; an aim towards which we had already recently directed a series of investigations to clarify the different aspects of experimental behaviour.Even with gaps in detail, the model found seems to correctely interpret the dissipativemechanism of our material, and it is — contrary to current viscous schematizations — of a prevalently plasto-elastic characteristic, with dampers of asolid-friction type.This work concludes dealing with the experimental determination of the parameters of the model and with particular approximate solutions of motion.

Reliability of testing procedures for reinforced concrete structures experimentally checked on bending-stressed beams

The investigation, carried out to check the reliability of testing procedures for reinforced concrete structures reported in Eurocode 2 (1988), has revealed situations in which bending-stressed beams, even when seriously defective, could be judged correctly built because residual deflections after the first or second short-term loading may remain within the limit set by EC2 for passing the test. Even an eventual check on elastic deformability, at least as usually carried out, does not substantially improve the reliability, which definitely improves only if the theoretical deflections are calculated with particular care (among other things, by experimentally measuring the concrete elastic modulus and by introducing the real value for the homogenization coefficient for steel areas instead of the conventional one); but, though it appears capable of revealing anomalies in the stress level in concrete, it fails to reveal a lack of concrete strength. It can be concluded that these procedures need improvement.ResumeLes études menées pour vérifier la fiabilité des modes opératoires d’essai des structures de béton armé, décrites par l’Eurocode 2 (1988), ont mis en évidence des cas où des poutres contraintes en flexion, même lorsqu’elles étaient très défectueuses, ont pu être jugées correctes, du fait que, après le premier ou le deuxième chargement de courte durée, des courbures résiduelles peuvent subsister dans la limite admissible de l’Eurocode 2.Même une vérification éventuelle de la déformabilité, du moins telle qu’elle est effectuée en général, n’améliore pas notablement la fiabilité, qui ne progresse nettement qui si les flèches théoriques sont calculées avec un soin particulier, notamment en mesurant expérimentalement le module élastique du béton et en introduisant la valeur réelle du coefficient d’homogénéisation pour les zones d’armature au lieu du coefficient conventionnel; cependant, si on peut ainsi détecter des anomalies dans le niveau de contrainte du béton, on ne peut déceler un défaut de résistance du béton. En conclusion, ces méthodes demandent à être améliorées.