Lucie Vandewalle

Shear Capacity of Concrete Beams Containing Longitudinal Reinforcement and Steel Fibers

This paper presents results of a large testing program that has been carried out to investigate the shear capacity of concrete beams containing longitudinal reinforcement and steel fibers. Two models were used for the calculations: RILEM TC 162-TDF and the model Imam. The comparison results show that the RILEM model is a conservative approach, especially underestimating the contribution of the stirrups and the influence of the shear span to depth ratio. The Imam calculates the contribution of the stirrups in the same way as the RILEM. The Imam, however, is fairly good for SFRC beams containing longitudinal reinforcement but without stirrups.

Are current concerete strength tests suitable for high strength concrete

Some important factors are investigated that may have an effect on compression test results of high strength concrete, including the size and shape of the test specimen as well as the mould material. Compression tests were performed on 6 different specimen types with a total of 360 specimens cast from 18 different high strength concrete mixtures (f′c=82–117 MPa). The compressive strengths of the different specimen types were compared and conversion factors between them were established. The comparative results indicate that for each 50 mm increase in cube size there is a ∼5% drop in the compressive strength. It was also found that the concrete strength of 150 mm×300 mm cylinders was, on average, 94.5% of the 150 mm cube strength. Moreover, plastic moulds yielded concrete with a strength lower than that of the concrete cast from iron moulds, and the reduction in compressive strength to the plastic moulds increased with increasing mould size.ResumeUn programme expérimental a été mené pour vérifier lapplicabilité et la validité des méthodes dessai actuelles dans le domaine du béton de haute performance. Cet article décrit des essais visant à déterminer linfluence éventuelle de quelques paramètres importants sur la résistance à la compression du béton de haute performance. On a fait varier entre autres la taille et la forme de léprouvette, ainsi que les moules. Des essais de compression ont été exécutés sur 6 types différents déprouvettes et de moules: des cylindres de 150 mm×300 mm en acier, des cubes de 100 mm en acier et en plastique, des cubes de 150 mm en acier et en plastique, et des cubes de 200 mm en acier. Au total 360 éprouvettes, résultant de 18 mélanges différents de béton à haute résistance, ont été examinées, et on a obtenu des valeurs de résistance à la compression (mesurée sur cylindre) variant de 82 à 117 MPa.On a alors effectué une comparaison de ces valeurs pour les différents types déprouvettes et établi des coefficients de conversion. Cette comparaison a montré que la résistance à la compression diminuait denviron 5% à chaque augmentation de 50 mm de la dimension du cube. On a aussi constaté que la résistance du béton des cylindres de 150 mm×300 mm atteignait en moyenne 94.5% de celle des cubes de 150 mm. De plus, lemploi de moules en matière plastique entraînait une résistance moins élevée que pour le béton coulé dans des moules en acier. Avec ces mêmes moules en plastique, la baisse de la résistance à la compression était denviron 10% pour les cubes de 150 mm, alors que cette différence se limitait à 7% pour les cubes de 100 mm. On peut en conclure que lemploid de petites éprouvettes (cubes de 100 mm ou cylindres de 100 mm×200 mm) convient, et est même recommandé pour essayer le béton de haute performance.

Chloride penetration and carbonation in self-compacting concrete

In this research program, both the steady-state and the non-steady-state migration test are used to determine the chloride diffusion coefficient D (m2/s) of 8 different self-compacting concrete mixes and 1 reference, traditionally vibrated, concrete mix. Simultaneously, the carbonation behaviour of those mixes was investigated. Here fore, the carbonation depth was measured at regular points in time according to NT BUILD 357 (1989) and a carbonation constant A (mm/√year) was deduced. Concerning the chloride diffusion coefficient, test results revealed that the determination of the steady-state migration coefficient according to NT BUILD 355 (1997) is far from easy and question marks could be placed beside the corresponding diffusion coefficient but an explanation for the observed anomalies could not be found yet. The non-steady-state diffusion coefficient according to NT BUILD 492 (1999) was used in order to rank the different concrete mixtures. The carbonation constant could best be measured using an inflated CO2-concentration, resulting in a more linear behaviour of the carbonation depth in function of time. Besides, using this carbonation constant, results reveal that the concrete mixtures could be ranked in the same way as they were by the non-steady-state chloride diffusion coefficient.

The cracking behaviour of SFRC beams containing longitudinal reinforcement

ABSTRACT Durability of concrete is in a great extent determined by the crack widths that are formed in the structure and that allow water to infiltrate in the concrete so that corrosion of the reinforcement bars can occur. The analysis of the cracking behaviour in concrete has always been a topic of interest for investigation by many scientists. In general it is assumed that the durability of the structure is assured when the crack widths are limited to 0.3 mm. One of the often-used approaches to limit the crack widths is the use of steel fibre reinforced concrete (SFRC). Steel fibre concrete is generally known to reduce crack widths because of its post-cracking tensile strength. At the Department of Civil Engineering of the Catholic University of Leuven, Belgium, a test program has been executed on 19 full-scale SFRC beams containing longitudinal reinforcement. All beams have been tested in four-point bending. The tests were performed in different load steps until failure of the beam. The load steps were chosen so that the beams failed after 10 to 15 steps. At each load step the crack widths and spacing were measured. The results of the test program illustrate the strong beneficial effect of steel fibres on the crack widths as well as on the crack spacing. The addition of fibres to the concrete can lead to a reduction of the crack width of up to 40%. The crack widths of the beams of the test program have been calculated by means of the new Rilem guideline as well as with a newly developed physical cracking model for reinforced SFRC beams. The physical model takes into account the bond between the reinforcement bars and the SFRC matrix as well as the influence of the steel fibres on the stress in the reinforcement bars. A comparison of the calculated results and the experimental results shows that there is a relatively good correlation between the two. The input parameters for the calculation model are the concrete compressive strength, the dimensions of the beam, the position and diameter of the reinforcement bars, the tensile strength and the post-cracking tensile strength of the SFRC material and the bond stress-slip relation. The new calculation model provides a very good understanding of the crack formation process. It also creates the possibility to determine for example the necessary post-cracking strength of the SFRC, given that the crack width of the beam must be lower than a certain value. Furthermore, also the influence of the bond stress-slip relation can be taken into account. This creates the possibility to use the crack model for other types of reinforcement than steel rebars (e.g. GFRP rebars).