Three-fibre-type hybrid fibre concrete

Rheological and fracture investigations on hybrid fibre concrete were performed to develop a material with improved tensile strength. The hybrid fibre concrete that was used in this investigation contained fibre mixtures of three different types of fibres. In order to optimise such a material, a four point bending test with continuously justified external loads ('pendulum-bars four-pointbending test') was developed. This paper presents results of four-point bending test on cut and moulded specimens and three-point bending tests on moulded specimens. Also, comparisons of results of different bending tests from different authors, mixtures, test arrangement and boundary conditions are presented. test and a large one with the inverted Abrams cone, were performed. In some series both the large and the small test were carried out. The aim was to determine the correlation between the results obtained from these tests. The small cone is originally a mortar flow test. The geometries of the two cones are quite different (Fig. 1). The small cone was used to find self-compacting mixtures which were used for the mechanical tests. The test with the small cone was performed the same way as the large slump flow test with the inverted Abrams cone but upright. Two perpendicular diameters were measured after 10 to 20 seconds, when the concrete stopped flowing and the average value of these measurements were calculated. A mixture is considered self-compacting when the values of the large slump flow test are larger than 650mm (Markovic et al. (2003) and Ramsburg (2003). The latter one also reports that the difference between the inverted and upright Abrams cone SCC test is less than 1%. The criterion for self compactness with the small cone was an average diameter of a minimum of 230mm . To develop self-compacting HFCs the small slump test was carried out because of its easier handling. As soon as the mixtures which were used for further investigations were found with the small cone, rheological test with both the inverted Abrams cone and the small cone were performed. Figure 1. Geometry of the slump flow cones 2.1 Rheology and testing fresh HFC It is necessary to assure that the mixing and testing procedure is optimised and kept uniform for all the samples. A recommended procedure for mixing and rheological testing of HFC yielded good results is the following: Step 1: All the tools needed should be prepared in advance before the start of the mixing. It is important to wet all the equipment, i.e. slump flow cones and table, trowels and mixer. Step 2: The microsilica slurry (1:1) is prepared. Step 3: Aggregates are weighted in a different vessel than binder (i.e. as fly ash and cement). Step 4: Water and plasticizer are weighed in a separate vessel. Step 5: Binders (including microsilica slurry) are weighed in one vessel. Step 6: Water-plasticizer mixture is poured into the cement-binder mixture and is mixed for four minutes. Step 7: It is ensured that the material is well mixed and no residue sticks to a wall of the mixing vessel or to the mixing blades. Step 8: All the aggregates are added into the mixture and mixed for 11⁄2 min. Step 9: The mixture consistency is checked by a trowel. Should there be any material which is not properly mixed, the mixing is continued for another minute. Step 10: All the fibres are added and mixing proceeds for another two minutes. At the end the consistency of the mixture is again controlled by a trowel. The criterion to decide whether to mix for another minute or not are: (i) no fibres stick together (hedgehog) or they are not well distributed and (ii) no fibres stick to the mixer blades or the wall of the mixing vessel. Step 11: Fresh concrete properties are tested using the slump flow tests. It is important that all the tools needed for these tests are damp but not wet, and that the test is made within one minute after the end of the mixing process. It is recommended to dispose all the concrete that was needed for the slump flow test. Step 12: The oiled formworks for specimens for mechanical testing are filled immediately after the end of the fresh properties testing procedure.