ICE - Institution of Civil Engineers - Virtual Library

In this work, eight types of aggregates that are representative of French production are tested for thermal stability. Afterwards, the influence of this stability on the mechanical characteristic of concrete under fire is studied using strength tests at high temperatures. Two kinds of aggregates, one representative of the siliceous type and the other of the calcareous type, are then selected to perform an experimental programme for studying the evolution of mechanical proprieties of concrete when varying other parameters, principally linked to admixtures (i.e. vibrated or self-compacting concretes). The results of this study are compared to the Eurocode 2-1-2 and the French standard NF P 92-701(known as DTU–FB). A good correlation with the predictions of the standards is found, and the consideration of a single curve for the strength variation with temperature, without distinction between the aggregates, is suggested.

This paper mainly reports an experimental investigation on the residual mechanical and fracture properties of normal and high-strength concrete (HSC) with and without fly ash after exposure to high temperatures ranging from 200 to 600°C. The fracture behaviour of concrete was investigated using the three-point bending notched beam test, according to RILEM recommendations. The effects of concrete strength, temperature level and the presence of fly ash on the fracture and mechanical properties of concrete were investigated. Some studies on the aggregate–cement paste transition zone have been made by means of a scanning electron microscope (SEM). The mechanical properties of concrete specimens significantly decreased with increase in temperature level, the extent of which depended on the presence of fly ash in the mix. In general, fly ash concrete performed better in residual mechanical and fracture properties especially at relatively high temperature levels than concretes without fly ash. Also, fewer and finer microcracks were identified in fly ash concrete under the SEM study.

A numerical model that can simulate the cracking behaviour of a reinforced concrete (RC) structure subjected to monotonic loading is introduced. A criterion to take into account the biaxial behaviour of concrete is designed on the basis of Kupfer's experimental results, and the equivalent uniaxial stress–strain relationship with a tension-stiffening effect is used for an effective simulation of the inelastic behaviour induced from the cracking of concrete. Moreover, an improved numerical model to implement the bond-slip effect in RC structures, in which the consideration of the bond-slip effect is virtually impossible in the numerical analysis as the reinforcing steels are uniformly distributed through the entire domain, is developed in this paper. Instead of using the bond-link elements that connect the concrete and steel nodes with the spring elements, a relationship between the steel stress and the end-slip obtained from an idealised pull-out specimen is implemented at the end face of RC structures. The use of these boundary spring elements makes it possible to simulate precisely the cracking behaviour of the RC structures affected most by the bond-slip. Finally, correlation studies between the numerical results and the experimental results are conducted in order to verify the soundness of the introduced boundary spring element model.

A theoretical ductility study based on full-range flexural behaviour is carried out for symmetrically reinforced concrete columns cast of normal- and high-strength concrete, with the axial load covering both tension and compression. The analysis employs the actual stress–strain curves and takes into account the stress-path dependence of concrete and steel reinforcement. From the complete moment–curvature curve obtained, the moment capacity, flexural ductility, balanced failure load, variations of neutral axis depth and steel stress are studied. In particular, the effects of axial load and confinement are evaluated. Generally speaking, the higher the axial load, the steeper is the descending part of the moment–curvature curve and the lower is the flexural ductility. Confinement is observed to help maintain the resisting moment up to a much larger curvature at the post-peak stage and therefore it benefits the flexural ductility. Its benefit to the moment capacity is, however, minor. The effect of concrete tensile strength is only observed for columns under relatively low axial load at service stage and has virtually no effect on moment capacity and flexural ductility.

The equivalent frame method (EFM) was developed for two-way slabs as a simple approximate method. Current design codes (ACI 318-05, Eurocode 2 and BS 8110) permit the EFM for the analysis of two-way slab systems under gravity loads, as well as lateral loads such as seismic loads. The EFM, however, was developed based on a semi-empirical formula, derived from the behaviour of two-way slab systems under gravity loads. This study shows that the EFM is not appropriate in accurately predicting the response of two-way slab systems under lateral loads. To improve the EFM under lateral loads, this study proposes a modified equivalent frame method (MEFM). To verify the MEFM, an idealised two-storey flat plate frame is considered and analysed by both the MEFM and elastic finite-element (FE) analysis for estimating slab moments and lateral drifts. Analysis results obtained from the conventional EFM and effective beam width method (EBWM) are also compared. The lateral stiffness, based on the proposed MEFM, was also compared with experimental data.

The interfacial zone (ITZ) around aggregate in concrete is a crucial element in mechanical and in transport systems. Computer simulation of cement is used to approach this phenomenon. For particle-packing in the fresh state, the Software Package for the Assessment of Compositional Evolution (SPACE) system relies on a dynamic generation algorithm, reflecting production conditions. The evident effects of spatial position of hydrating particles and hydration products on structure formation of cement call for use of stereological methods. Morphological and structural evolution of the ITZ from the fresh into the hardened state is quantified by composite as well as configuration parameters. The results reveal that in the fresh state of particle packing, the ‘wall effect' reflects the decline in cement particle density in the vicinity of the aggregates, leading to a more porous ITZ, the extent of which depends on technological parameters (water-to-cement (w/c) ratio, cement fineness), but also on the type of parameters characterising material structure. As hydration proceeds, only moderately higher porosities are observed within the ITZ than in the bulk paste. The disproportional high physical bond strength in the ITZ at low w/c ratio remains apparent throughout the hydration process.

A study to develop the parameters of the models for predicting the deformation characteristics of normal-strength concrete incorporating slag-blended cements at early age is presented in this paper. By comparing with the previous experimental results, it was found that the basic shrinkage and creep factor decreased significantly owing to the expansion of slag concrete during the first week of moist curing. When utilising the developed models for estimating the crack width, it was found that the initial crack width of concrete subjected to restraint drying shrinkage is reduced if the slag content level increased up to 50 and 65% in the concrete mixture. Further, slag concrete tends to have less crack width and lower number of cracks in the long term compared with that with no slag.