The crystal structure of the catalytic core of murine terminal deoxynucleotidyltransferase (TdT) at 2.35 A Ê resolution reveals a typical DNA polymerase b-like fold locked in a closed form. In addition, the structures of two different binary complexes, one with an oligonucleotide primer and the other with an incoming ddATP-Co 2+ complex, show that the substrates and the two divalent ions in the catalytic site are positioned in TdT in a manner similar to that described for the human DNA polymerase b ternary complex, suggesting a common two metal ions mechanism of nucleotidyl transfer in these two proteins. The inability of TdT to accommodate a template strand can be explained by steric hindrance at the catalytic site caused by a long lariat-like loop, which is absent in DNA polymerase b. However, displacement of this discriminating loop would be suf®cient to unmask a number of evolutionarily conserved residues, which could then interact with a template DNA strand. The present structure can be used to model the recently discovered human polymerase m, with which it shares 43% sequence identity.

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The application of textile reinforced mortar (TRM) as a means of increasing the shear resistance of reinforced concrete members is investigated in this study. TRM may be considered as an alternative to fiber reinforced polymers (FRP), providing solutions to many of the problems associated with application of the latter without compromising much the performance of strengthened members. Based on the experimental response of reinforced concrete members strengthened in shear it is concluded that textile-mortar jacketing provides substantial gain in shear resistance; this gain is higher as the number of layers increases and, depending on the number of layers, is sufficient to transform shear-type failure to flexural failure. TRM jackets were provided in this study either by conventional wrapping of fabrics or by helically applied strips. Both systems resulted in excellent results in terms of increasing the shear resistance. However, compared with their resinimpregnated counterparts, mortar-impregnated textiles may result in reduced effectiveness. Modelling of reinforced concrete members strengthened in shear with TRM jackets instead of FRP ones is presented by the authors as a rather straightforward procedure by the proper introduction of experimentally derived jacket effectiveness coefficients. From the limited results obtained in this study it is believed that TRM jacketing is an extremely promising solution for increasing the shear resistance of reinforced concrete members.Résumé Cet article décrit une recherche expéri-mentale sur l'application du mortier renforcé par textile (MRT) comme des moyens d'augmenter la résistance au cisaillement des membres en béton armé. MRT peutêtre considéré comme une solution de rechange aux polymères renforcés de fibres (PRF), fournissant des solutionsà plusieurs des problèmes liés a l'application du dernier, sans compromettre beaucoup la conduit des membres renforcés. La conclusion dérivé de l'évaluation de la réaction expérimentale des membres renforcés au cisaillement est que la chemise au textile-mortier fournit un augmentation substantiel dans la résistance au cisaillement; cet augmentation est plus grandà mesure que le nombre de couches augmente et, selon le nombre de couches, est suffisant pour transformer l'échec au cisaillementà l'échec au fléchissement. Dans l'étude présentée ci-dessous les chemises MRT ontété fournies par emballage conventionnel des textiles ou par des bandes appliquées dans une manière hélicoïdale. Les deux systèmes ont eu des résultats excellents en termes d'augmentation de la résistance au cisaillement. Cependant, comparésà leurs contreparties imprégnées de résine, les textiles imprégnés du mortier peuvent résulterà une efficacité réduite. La modélisation des membres en béton armé renforcés au cisaillement avec des vestes de MRT au lieu de PRF est présenté par les auteurs dans une manière directe par l'introduction 94 Materials and Structures (2006) 39:93-103 appropriée des coefficients expérimentalement dérivés de l'efficacité de la chemise. Par le...

Mortar-based composite materials appear particularly promising for use as externally bonded reinforcement (EBR) systems for masonry structures. Nevertheless, their mechanical performance, which may significantly differ from that of Fibre Reinforced Polymers, is still far from being fully investigated. Furthermore, standardized and reliable testing procedures have not been defined yet. The present paper provides an insight on experimental-related issues arising from campaigns on mortar-based EBRs carried out by laboratories in Italy, Portugal and Spain. The performance of three reinforcement systems made out of steel, carbon and basalt textiles embedded in inorganic matrices has been investigated by means of uniaxial tensile coupon testing and bond tests on brick and stone substrates. The experimental results contribute to the existing knowledge regarding the structural behaviour of mortar-based EBRs against tension and shear bond stress, and to the development of reliable test procedures aiming at their homogenization/standardization