International Concrete Abstracts Portal

This study proposes a new design formula for the development of positive moment reinforcement in tension. A review of current design code provisions for the end anchorage at simply supported beams shows unsatisfactory requirement for flexural bond strength and that an additional length beyond simple supports is needed. The code provisions neglect a tensile force increase due to shear force and hence, the formulas assume zero tensile force at simple support locations. This paper shows that the treatment for both the flexural bond strength and anchorage requirements is necessary for the safe detailing of reinforcement at beam end regions. Investigation of bond-related failures in these regions shows that it is necessary to differentiate between the anchorage force and flexural bond strength along the bar. Comparison between bond strengths required by current design concept and the proposed formula shows a need for modification of current code provisions for end anchorage.

Production of high performance concrete (HPC) depends on several factors including the use of low water-to-cementitious material ratio, proper dosage of high-range water-reducer (superplasticizer), and a careful selection and dosage of a mineral pozzolanic admixture such as silica fume. Most of the improvement in the strength and durability characteristics of the hardened concrete is attributed to the filler effect and pozzolanic action of the fine size silica fume. Results of tests conducted at the American University of Beirut (AUB) on tension lap splices in full-scale beam specimens and on reinforcing bars anchored in eccentric pullout specimens, indicate that the replacement of part of the cement by an equal weight of silica fume resulted in reduction in bond strength. The reduction was independent of specimen type, bar size, super-plasticizer dosage, and casting position. The mode of failure of the high strength concrete beam specimens was a very brittle bond splitting failure. Tests were conducted to check the effect of transverse reinforcement in the splice region on the bond strength and mode of failure. This paper will provide an overview of the research performed at AUB.

Inhomogeneous stress situations are prevailing in some engineering problems, such as around corroding steel bars or around aggregate particles expanding due to alkali-silica reaction. Micro-mechanics and macromechanics will significantly differ in those cases. The present article focuses on experiments simulating the effect of steel bar corrosion on four cementitious composites. For that purpose, prismatic specimens containing an excentrically located slightly tapered cylindrical hole, where subjected to controlled push-through of a metal cone of similar shape. Behaviour in the pre-peak range was reflected by strain gauges, and in the post-peak region by clip gauges. Simulateneously, acoustic emission measurements were performed. Various aspects of the tests have been highlighted before in publication to which is referred when relevant. This paper merely presents illustrative data, evidencing typical micro- and macro-mechanical processes taking place under the given conditions. Successively, on macro-level, the elastic range terminates at Discontinuity Point (DP), at ultimate (BOP) a stage of quasi-plastic yielding is obtained, and a mechanism is formed at Crack Opening Point (COP), after which energy due to bar expansion is stored in opening up of only the leading crack. On micro-level, dispersed crack initiation and coalescence in radial direction starts at the interface. this process gradually concentrates in one of the two ‘weakest’ sections of the covercrete. This process slows down, whereupon cracks are initiated at the exterior of the second weakest section. Here they propagate to coalesce with the interor microcracks to form the major crack.

The objective of this research was’ to investigate the strength and serviceability, viz cracking behaviour and flexural stiffnesses, of structurally damaged reinforced concrete beams strengthened in flexure by incorporating thin ferrocement laminates reinforced with additional longitudinal bars onto the soffits (tension face) tested under quasi-static and cyclic loadings. Results from seven test beams are discussed. The beams were fabricated and precracked under quasi-static mid-span loading to three levels of damage: 1) flexural failure with crushing of concrete and yielding of tension reinforcements viz 115%, 2) 100% and 3) 90% of their respective theoretical flexural capacities. They were then repaired and strengthened after unloading. Four of the strengthened beams were tested to failure under quasi-static loading while the remaining three were subjected to cyclic sinusoidal loads up to 150,000 cycles. It was found that, under quasi-static loading, the structurally damaged and then repaired and strengthened beams exhibited improved performance compared to their behaviour during the precracking stage. Their performance was comparable to a control beam strengthened in its virgin state showing only marginally larger reopened crack widths. Under cyclic loading however, the presence of a distinct ferrocement/concrete interface at the soffit of the beams resulted in cracking at the interface under repeated loading of high load levels.

Our laboratory fitted a highway bridge near Geneva (Switzerland) with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting of two parallel pre-stressed concrete beams supporting a 30 cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the behaviour between the old and the new concrete, low-coherence fiber optic sensors were chosen in order to measure the displacements of the fresh concrete during the setting phase and to monitor its long term deformations. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The curvature of the bridge is measured locally at multiple locations along the bridge span by installing sensors at different distances from the neutral axis. By taking the double integral of the curvature and respecting the boundary conditions, it is then possible to retrieve the deformation of the bridge. The choice of the optimal emplacement of the sensors and the mathematical model required to interpreter the results are also presented.