Effect of manufacturing methods on tensile properties of fibre concrete

This Paper presents the first results of the experiments of fibre reinforced concrete tested using a newly developed uniaxial tensile test. The aim was to develop a tensile test set-up in which the specimen supports have no restraind freedom of movements. This goal has been achieved by means of an arrangement based on ‘pendulum-bars’. Different filling/casting methods were used for optimizing the fibre distribution and orientation aimed at an increase of the tensile strength will increase. Results of a test series, where the filling method, the type and the amount of fibres are varied is presented in this paper. 2 FILLING METHODS AND MATERIAL In order to optimize the filling process and the resulting fibre orientation and distribution three different filling methods were performed. The ‘conventional’ method (C), where the concrete was filled from the top and the material did not flow, but falls in the mould (Fig 1); the ‘filling’ method (F), where the concrete was filled in the mould in such a way that the material flowed a little (Fig. 2) and the ‘climbing’ method (CL), where the material was filled/pumped from the bottom to the top of the mould (Fig 3). Figure1. Sketch (a to d) and images (e to f) of the filling sequence of a tensile specimen with the ‘conventional’ method. The arrows show the direction of the flow of the material during the filling process. Figure 2. Sketch (a to d) and images (e to f) of the filling sequence of a tensile specimen with the ‘filling’ method. The arrows show the direction of the flow of the material during the filling process. Due to the available equipment in the laboratory (no concrete pump was available) the ‘climbing’ method was performed in such a way, that the material first was filled in the ‘filling’ method specimen and there, the material flowed trough a connecting part and climbed up into the ‘climbing’ method specimen (Fig. 4). The difference between the conventional and the filling method is in the flow of the material. Figure 1b shows that as soon as the material starts to fill the mould, the material starts to flow upwards and concrete swirls develop. This also happens when the mould is almost filled (Fig. 1c). Figure 2b and c show that the material starts to flow downwards the more the mould is filled. No swirls develop and the fibres can properly align in the direction of the material flow. Figure 3. Sketch (a to d) and images (e to f) of the filling sequence of a tensile specimen with the ‘climbing’ method. The arrows show the direction of the flow of the material during the filling process. Figure 4. Sketch and image of the ‘U-Shaped’ mould for the tensile test specimens To ensure that the fibres did not segregate and the material was capable to self-level the diameter of the small slump flow of the fresh material had to be inbetween 20 and 26 cm. The concrete matrices only differed in the amount of super-plasticizer because of the aim to have equal rheological properties for all the mixtures. In most of the mixtures additional super-plasticizer was added to increase the flow ability of the material. Three mixtures with only one type of steel fibre and two hybrid fibre concretes with three different types of steel fibres were investigated. An overview of all the mixtures and their respective rheological properties is given in Table 1. The matrix was composed as follows: 1000 kg/m CEM I 52.5R, 200 kg/m fly-ash, 100 kg/m Microsilica and 800 kg/m sand with a maximum diameter of 1 mm. This composition of the materials guaranteed a compact material structure where the fibres were quite perfectly embedded. The water-binderratio (w/b-ratio) was taken as low as possible to improve the strength, i.e. the w/b-ratio was 0.18. To keep the mixture self-compacting Glenium® superplasticizer was between 2.3% and 2.6% of the cement weight. The used fibres were Stratec steel fibres with different shapes and geometries. Straight steel wire fibres with a length of 6 and 12 mm and a diameter of 0.15 and 0.2 mm respectively and undulated steel fibres with a length of 30 mm and a diameter of 0.6 mm were used. The steel fibres had a tensile strength of 2400 MPa. Fresh concrete properties were tested using slump flow tests Stähli (2004) and the air contend was measured using the test according to EN 12350-7. Table 1. Overview of the concrete mixtures 3 0 0 0 3 0 0 0 3 1 1 1 3 1 1 Super-plasticizer [%] 2.0 2.0 2.6 2.0 2.3 Additional superplasticizer [%] 0.3 0.3 0.4 0.3 0.15 x 0.6 mm fibre [Vol.-%] 3 1 3 0.20 x 12 mm fibre [Vol.-%] 3 1 1 0.60 x 27 mm fibre [Vol.-%] 3 1 1 Small slump flow [mm] 20.5 23 23 26 27 Large slump flow [mm] 62.5 67 78 76 84 Fresh concrete density [kg/m] 2428 237