For perfect execution and durability of works in reinforced concrete, it is essential that the reinforcements be positioned correctly and protected within the concrete casting. For this purpose, spacers are used to keep the reinforcements in position during the casting and throughout the setting phase.
The spacers are commonly made of plastic or cement mortar, so as not to initiate any corrosion of the reinforcement bars, and have a shape and geometry so as to minimize the surface area in contact with the formwork for technical and aesthetic reasons. They must also be able to support the load transmitted from the reinforcement bars until the hardening of the concrete.
During the execution of works in concrete, particular attention must be given to the lack of continuity in the casting which gives rise to “cold joints”, real lines of separation between consecutive castings carried out at different times (in which the new concrete comes into direct contact with the part cast previously that may even be hardened) and which penalizes the work from the structural and hydraulic seal point of view. A good scheduling of the arrival of cement mixers by the construction firm should enable the best organization of the daily castings so as to minimize the cold joints in the casting. Often, however, cold joints are unavoidable considering the large dimensions of the works. In these cases, the cold joints of the casting should be arranged suitably by the site engineer while taking particular precautions to avoid increases in the lack of continuity of the casting: for example, the surface of interruption can be placed in a dovetailed formwork or formwork of another shape suited to improve the attachment of the subsequent casting.
Products which are commonly used to guarantee the hydraulic seal for the casting’s cold joints as well as the expansion joints (interruptions in the continuity of the work arranged intentionally) are the WATERSTOPs. These are PVC sections with a particular conformation, with or without bulb, so as to guarantee perfect anchoring to the concrete. The material, which is resistant to the aggression of atmospheric and chemical agents found in air and water, maintains suitable flexibility even at low temperatures and is able to resist the moderate longitudinal and transverse stress that occurs during the settling of the structure.
THICKNESS OF LEVELING FOR THE PREFABRICATION
The installation of prefabricated structures is characterized by the need to perform a perfect “assembly” of the various elements (constructed in the plant or on site) in a context, the building site, which usually has irregular support surfaces. To resolve this problem and obtain the correct alignment of the prefabricated elements, the so-called levelling shims are used. Shims with various characteristics, dimensions and shapes are available depending on the different needs. The positioning of the infill panels on the perimeter surface and that of the pillars in the so-called sleeves are typical interventions in which the levelling shims are essential.
REBAR CONNECTION SYSTEM
When the cold joints in the casting (real lines of separation between consecutive castings carried out at different times in which the new concrete comes into direct contact with the part cast previously) are unavoidable, then it is necessary to adopt particular techniques and use metal reinforcements crossing the cold joint surfaces to provide a more efficient joining and guarantee structural continuity.
Robin is a prefabricated case in zinc-coated steel inside which there are reinforcement rods with increased adherence, in order to guarantee a better anchoring of the concrete. The cold joint rods of the casting originally remain housed in the case so as not to hinder the operation of the first casting. Only after the formwork is removed will they be straightened.