Strengthening of short splices in RC beams using Post-Tensioned Metal Straps

This paper investigates the effectiveness of a novel and cost-effective strengthening technique using Post-Tensioned Metal Straps (PTMS) at enhancing the bond behaviour of short lap spliced steel bars in reinforced concrete (RC) beams. Twelve RC beams with a short lap splice length of 10d b (d b = bar diameter) at the midspan zone were tested in flexure to examine the bond splitting failure. The effect of confinement (no confinement, internal steel stirrups or external PTMS), bar diameter and concrete cover were examined. The results show that, whilst unconfined control beams failed prematurely due to cover splitting, the use of PTMS confinement enhanced the bond strength of the spliced bars by up to 58 % and resulted in a less brittle behaviour. Based on the test results, a new analytical model is proposed to predict the additional bond strength provided by PTMS confinement. The model should prove useful in the strengthening design of substandard lap spliced RC elements

Residual behavior of steel rebars and R/C sections after a fire

The mechanical properties of various steel bars exposed to high temperature ("residual" properties) are experimentally investigated up to 850 °C, with reference to a number of steel and bar types (carbon and stainless steel; quenched and self-tempered bars; hot-rolled and cold-worked bars; smooth and deformed bars). The aim is to clarify to what extent the thermal sensitivity of the different bars affects the ultimate capacity of a typical R/C section subjected to an eccentric axial force, past a fire ("residual" capacity). As usual in the design of R/C sections under combined bending and axial loading, the ultimate behavior is represented through the "M-N envelopes", where the materials strength decay due to high temperature is taken into account. The results show that quenched and self-tempered bars (QST), very popular in Europe, are more temperature-sensitive above 600 °C than the carbon-steel bars extensively used in the States and nowadays rarely used in Europe. Furthermore, the best response is exhibited by the stainless-steel bars, provided that they are hot rolled, as it is generally the case for medium- and large-diameter bars. Similar conclusions can be drawn for the sections reinforced with the different bar types. 2009 Elsevier Ltd. All rights reserved

Mechanisms governing the compressive strength of unconfined and confined rubble stone masonry

In the safety assessment of masonry constructions the estimate of the compressive strength of masonry is often required, especially for those structures subjected to high stresses or stress concentration. Focusing on rubble stone and conglomerate-like masonries, a typology which is quite common in historical constructions, a theoretical formulation identifying the phenomena governing the collapse mechanism was recently proposed for the evaluation of the compressive strength of unconfined and confined masonries. The theoretical formulation also provides useful insights on the design of the necessary confinement stress level, anytime improvement of the compressive strength and ductility of the masonry walls is needed. In this paper the reliability of the theoretical predictions is substantiated by an experimental study. Specific and challenging tests on masonry samples replicating historical rubble stone masonry elements and the inner core of three leaf walls are conceived to offer experimental evidences of the phenomena governing collapse, such as the cohesion, the aggregate interlocking and the residual tensile strength after cracking, as well as the possible confinement stress level. The experimental investigation also provides preliminary values of the parameters affecting the masonry compressive strength

Bond slip model in cylindrical reinforced concrete elements confined with stirrups

An analytical model able to evaluate the bond-slip law of confined reinforced concrete elements is developed and presented in this paper. The model is based on the studies developed by Tepfers and by den Uijl and Bigaj on the thick-walled cylinder model and extended to the case of the presence of transverse reinforcement. The bond strength and the considered failure modes (splitting or pull-out failure) are expressed as a function of the geometrical (concrete cover and transverse reinforcement) and mechanical (concrete strength) parameters of the element. The application of the proposed methodology allows to forecast the failure mode, and equations for the bond-slip law are finally proposed for a range of steel strain lower than the yielding one