Search

Since the beginning of the construction of structures with reinforced concrete, it has been known that concrete presents a variability that should be taken into account. In modern codes, this variability implies the use of a characteristic strength corresponding to a 5% fractile of the distribution of strength. The actual relation in Eurocode 2 between the characteristic and the mean strength fck = fcm ? 8 MPa was introduced several years ago by Rusch and is integrated in CEB or fib model codes since 1978. In this article, it is presented how the relation was obtained and it is discussed if this relation is still valid considering the fact that the range of concrete strengths is now larger. Considering the scatter of the SD, the relation proposed by Rusch could still be used but engineers should keep in mind that the SD on site could be very different from the one predicted by means of the relation between fck and fcm.

Concrete shells have been widely used in the past as economical and suitable solutions for a number of structures such as roofs, silos, cooling towers or offshore platforms. Taking advantage of their single or double curvature, bending moments and shear forces are limited and the structure develops mostly membrane (in-plane) forces, allowing to span large distances with limited thicknesses (typically of only some centimetres). In the last decades, the advances on numerical modelling, formwork erection and concrete technology open a new set of possibilities for use of concrete shells. In this paper, the design and construction of a shell with the form of an ellipsoid (93×52×22 m) and with variable thickness between 100 and 120 millimetres is described. The shell was built using sprayed concrete and also ordinary concrete in some regions. A number of tailored solutions were also adopted such as post-tensioning, addition of fibres and shear studs to ensure satisfactory performance both at serviceability and ultimate limit states.

The flexural capacity of reinforced concrete (RC) beams can be increased by applying carbon fibre reinforced polymers (CFRP) in the form of fabric or laminates. Concrete elements strengthened with bonded FRP systems are often showing a brittle failure mode with premature delamination of the FRP ply at the concrete-FRP interface. This can be quite catastrophic and is undesirable since it does not allow exploiting the FRP mechanical properties to be fully utilized. The failure is often brittle. It has been was demonstrated that the use of anchoring systems (i.e., U-wrap or anchor spikes) are effective in order to raise the ultimate load; failure of the beam remains essentially brittle with a substantial drop of the load after the peak. This paper basically focuses on flexural strengthening with pre-cured FRP laminates applied on RC beams with different anchoring devices chemically and mechanically applied. One beam was maintained as un-strengthened control sample. Two beams were strengthened using CFRP pre-cured laminates: for one of them, glass fibre anchor spikes were applied, the other was kept without anchoring. Two other beams were strengthened using hybrid pre-cured laminates having high longitudinal bearing strength: the laminate was bonded on one beam (steel anchors were also used to prevent/delay peeling) while was mechanically fastened (MF system) to the other one. The results showed that the ultimate load of the strengthened beams was 35-80% higher than the control beam, depending of the type of strengthening system applied. The lower value corresponds to the beam strengthened using CFRP without anchor spikes, whose use increased the ultimate load of about 14%. The best performance of the strengthening was obtained for the beams strengthened with the hybrid laminates: steel anchors reduced the load drop after the peak while, in the case of hybrid system and mechanically fastened CFRP, the use of the MF system resulted in a pseudo-ductile mode of failure. The experimental results were compared with analytical results obtained from the application of recently developed guidelines, that were published by the Italian Research Council (CNR). It is shown that design equations, even if material safety coefficients are disregarded, resulted conservative in both cases of unanchored and anchored FRP plate.