Prestressed hybrids of AAC and HPC : The BCE (Block Composed Element) building system. A conceptual study

Sammanfattning: There is an important potential for a development of a building system, if the present AACblock plants are completed with high performance concrete and pre-stressing technique. This can be done as a continuation of a present AAC production or in a site factory. Hybrid cooperation between AAC and concrete is not a new technology. Traditionally, AAC is covered with rendering. The wetted material is first sprayed with cement slurry after which comes lime/cement-based rendering which strengthens the wall and supplies a mechanical protection. AAC blocks can be used as infill members in concrete deck plates and concrete framed walls (Hellers, 1993), in which the shrinkage of the surrounding concrete locks completely the cured blocks into a stable composite. A good cooperation between AAC and concrete is a requirement for the pre-stressed hybrid. This combination has been investigated with reference to bond and moisture content. The interface must have sufficient bond to supply shear strength to the structural member. Also, the concrete should be suitable for pre-stressing which requires a minimum strength class K40. Here, K60 is applied in order to reduce creep and avoid creep failure of the AAC. A production layout for the purpose is suggested. This research consists of three parts: 1- Hybrid concrete elements The principal formulation of hybrids, built on cooperation between two concrete materials, a weak AAC and a strong HPC poured on top, shows that this combination unites the most favourable qualities of the two concretes into a structural element with rational building technology. Load-bearing capacity is good, and the fire protection is excellent. Through pre-stressing of the structural concrete, a crack-free behaviour is guaranteed up to the service limit, and deflection from dead weight (incl. floor covering and possibly half the service load) be eliminated. The most important structural elements needed in a building system could be taken as hybrids. In drawings, different members like floor- roof- and wall members, window and door lintels are shown. 2- Production system for hybrids AAC blocks form a bed containing the pre-stressing steel in slits and on which high performance concrete is poured. Pre-stressing brings the two concretes together. This is especially important for the shear capacity of a building member, by which extra dowels can be omitted. The pre-stressing force is anchored by plates directly against the cured AAC blocks. A special pre-stressing bed is not required. The production is arranged in such a way that necessary equipment (trays, form strips, wedges, locks) are circulated within twenty-four hours. The necessary manpower for this facility is analysed. Normally 8 men are needed per shift. A detailed conceptual production layout for a hybrid production plant is included for discussion. See figure 6.2 (suggested production layout). 3- Application in building The hybrid members are united at joints and through seams. Continuity over connections is achieved by filling seams with mortar grout. Reinforcement may be included to achieve ductility. By a similar method, it is possible to make connections between horizontal and vertical building members. Common connection details are shown in the report. This building method replaces the equivalent method with concrete or AAC members, and it is in fact a coordination between these two. The maximum span of floor members is up to 9 m. It makes the system suitable for modern residential house production, but also suitable for office buildings, industrial halls and other applications. See attached drawings, part 2 and part 3.