Low Cost Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) with Flash Metakaolin

Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) presents remarkable mechanical and durability properties. Concretes can be considered as UHPFRC when compressive strength and direct tensile strength reach respectively reach 150 MPa and at 7 MPa at 28 days [1]. The UHPFRC matrix is generally composed by a high amount of cement and silica fume as binder, steel fibers, very low Water/Binder (W/B) ratio and high rang water reducer. Special heat treatment is often required in order to achieve compressive strengths over 150-200 MPa. This work focuses on the development of low cost UHPFRC with less energy in production process in order to widespread its use. To reach this objective, some mix criteria have been set up: use of local materials, substitution of silica fume by less expensive and more available flash metakaolin, reduced cement content, without heat treatment. Four UHPFRC mixtures have been fixed. The reference one incorporates silica fume. In the second one, metakaolin substitutes weight by weight silica fume. Two other mixtures have been designed using a wet packing method [2] and a metakaolin water absorption test [3], in order to optimize the metakaolin/cement mass ratio and to reduce cement content. Workability and mechanical tests have been carried out. Results show that the weight by weight substitution of silica fume by flash metakaolin leads to a slight reduction (about 5%) of strengths but a more important reduction of cost (about 10% of UHPFRC material cost per m3). The optimization of metakaolin/cement ratio allows reaching equivalent performances of silica fume reference mix and reducing cement content without significant strengths decrease.