Trade Journals for the Concrete Industry

The sector of interest of small-medium elements to be used on road infrastructures, hydraulic works, urban furniture and subdivisions, is moving more and more worldwide towards prefabrication. The reasons of this trend arise from important critical issues encountered during on-site construction, including the difficulty of finding workers able to make these elements on site, in the higher cost of on-site construction, in the longer time required for the curing of the concrete and in the difficulty of complying with specifications identifying precise geometric shapes and high mechanical performance requirements. The vibration ensured by external motorvibrators installed on the frame on the mold is considerable and adjustable thanks to the combination of the vibrating wave generated by multiple motorvibrators, with variation of centrifugal masses, variations of rotation directions and rotation speed. Each motorvibrators create a centrifugal force equal to 20 kN. The compaction of the concrete has the function of guaranteeing to the fresh conglomerate the greater compactness, homogeneity and density possible. The final resistance of the concrete is heavily influenced by the level of compaction achieved during the set-up. The resistance is also directly proportional to the ambient temperature, to the water-cement ratio of the mixture used in this technology, equal to around 0,38-0,40. The ideal concrete for the immediate demoulding with external vibration is of semidry consistency, with slump equal to S1. The increase of the portion of concrete for the same water, for the well-known rules of the concrete technology, allows you to increase the mechanical strength to material compression, reaching values up to 60 Mpa. Since the first instants after the demoulding, the material reaches the minimum capacity necessary to bear its weight without deformation. This characteristic is achievable pro- vided that the vibration is appropriate, the recipe optimized with correct water/cement ratio and right quantity of large aggregates, preferably crushed rock rather than natural rounded, and that it is eventually provided the use of accelerating additives in the mixture. The uniqueness of the mold destined for the production of a certain product, allows you to obtain the perfect equality between the various elements, ensuring perfect alignments and coupling, quality particularly appreciated in case of replacement of elements in infrastructural lines. VibroCast machine, available in both stationary and mobile version, both manual and semi-automatic, provides the operator a protected position with a wide view. The machines is also equipped with safety interlocks in compliance with the current European machine regulation. This allows you to avoid air handling of elements and molds, both in rotation and in translation, and to reduce considerably the use of forklifts reducing the risk of accidents. Generally speaking, the immediate demoulding technology, compared with the production by wet casting in static moulds, could be subject to come limitations in particular cases, i.e. the need to have a minimum flaring of the elements, the impossibility di make recesses of big dimensions, the possibility that the element could lower some millimetres after the demoulding, the difficulty to create shapes with high and thin walls, a less smooth finishing of the visible surfaces. Most of the particular issues of these limitations can be overcome, it is precisely on this focus that the experimental and observational activity of Expomat, the company owner of the brand VibroCast, is concentrated. Tank in vibrated concrete 190 cm height The flaring, also called conicity, is the dimensional ratio between the base and the top of the element, and it is necessary to allow the release of the concrete element from the mold. This conicity, to be applied to both external and internal walls, is based on different factors and is usually equal to a few mil- limetres. In some cases, the conicity is not acceptable for the final customer and therefore different solutions can be found to allow the forced ejection of the element from the mols. A solution between all is the application of internal extractable cores inside the mold. Another constraint is the capability of creating notches or holes of big dimensions on the walls of the elements, be- cause there is a risk of collapse of the wall of the product just demoulded in correspondence of the recesses. Experimental activity has recently been carried out on this to observe the behaviour of pipes and box culverts in presence of holes for canalizations, actually finding that with the use of the proposed solutions, optimal capacity of resistance to collapse can be achieved. The alternative that can be always adopted is the insertion of elements filling the holes to be left in the cast during the first hour after the demoulding, just the time of the first grip of the concrete. The aggregates can be non-returnable, recoverable or permanent. The reinforcement inside of the element helps the shapes during the first minutes but attention has to be paid to the recipe because a cracking may occur on the top of the element in case of slight deformation of the concrete part compared to the reinforcing cage which is instead non-deformable. Either the reinforcement collapse in the same way or it is necessary to work on water, large aggregates, vibration time and chemicals.