Fibre reinforced lightweight aggregate concrete: Fid-Bau Portal

Fibre reinforced lightweight aggregate concrete

König, Gert / Holschemacher, Klaus / Weiße, Dirk / Schneider, Holger

Lightweight Aggregate Concrete (LWAC) plays more and more a significant role in the design and construction of innovative and economic concrete structures in Europe after years of stagnation. Small bridges like the Karl-Heine pedestrian bridge in Leipzig, Germany, large span bridges like the Stolma bridge, Norway, the Gravity Base Tank for the BP Harding field, Great Britain, and facade structures like the Kai-Center in Düsseldorf, Germany, demonstrate the economic and technical advantages of LWAC. In this paper the experimental programme and its results about LWAC reinforced with fibres is described. The aim was to reduce the brittle material behaviour of LWAC by adding steel and/or polypropylene fibres. Within two test series 16 different fibres or fibre mixes were used with two different fibre contents. The main task was to find an optimal fibre proportion for the used LWAC mix and to compare the performance of the fibres with results stated in the literature. One of the findings is that steel fibres increase the ductility of LWAC, but some fibres showed a complete different behaviour than known from normal weight concrete. The flexural tensile strength-deflection-curves approve that long wire fibres with hooked ends show a high efficiency. Interesting is the almost same performance of both glued wire fibres, but with different aspect ratios, with a higher fibre content. Furthermore it could be shown that glued fibres are more effective than singled ones with almost the same geometry. With the proportion of polypropylene fibres increased to 1.5 kg/m³ the effectiveness of the steel fibres is substantially higher. Mill cut fibres behaved disappointingly; there was almost no change due to the higher fibre content. It could be shown that steel fibres can enhance the ductility, but the selection of the appropriate fibres is more crucial than for normal weight concrete. The force transfer via adhesion between the concrete matrix and the fibres is considerably lower for LWAC, therefore straight fibres perform quite poorly. The forces can be preferably transferred via hooked ends. The load carrying hooked ends of the fibres must be anchored in the concrete with a certain distance from the crack plain, otherwise cone break-out failure would occur. For LWAC this spacing must be increased by the ratio between the tensile strength of lightweight and normal weight concrete. On the other hand a large fibre length harms the workability of the concrete. Overall wire fibres performed better than mill cut or cut sheet fibres, especially glued wire fibres with hooked ends and an aspect ratio of lid = 65 showed very good results in both fresh and hardened concrete properties. Fibre cocktails out of steel and polypropylene fibres also showed an adequate behaviour. Vital is only a minimum content of the polypropylene fibre. Another important application area of fibre cocktails is concrete exposed to fire. Here both fibres harmonize perfectly.