Construction of working seams for concreting - why is this done?
It is known that the concrete should be laid on the entire volume of installed formwork fences. But their laying can be done in layers, and it is quite possible to fill the next layer until the previous one is seized. In this case, the strength of the concrete monolith does not deteriorate.
But if laying of layers was conducted with such interruptions, that the previous layer managed to harden, then the next casting is carried out with a number of requirements. Their goal is to reduce the negative impact on the strength of the construction of the working seam between solid old and liquid new concrete.
Only at first glance it seems as if nothing special is in these works. In fact, they represent a rather complex process, requiring knowledge of technology and compliance with stringent requirements for its implementation. The formation of cold seams, also called working, building or stitching, presents a real headache. And this is understandable, given their negative impact on strength.
The working joint of concreting can be formed as a result of organizational, technological, and constructive reasons. Obviously, you can move the working time, eliminate downtime, duplicate the required equipment and increase the carrying capacity of the forests. But it is very difficult to completely eliminate the formation of concreting concretions, at least from the point of view of the accepted practice of creating complex structures by separate coupled blocks.
Seam work concreting - the rules for obtaining a quality weld
The above mentioned rules establish: in case of work with interruptions for more than two hours, they are renewed after reaching the previous layer of one-and-a-half strength( 1.5 MPa) in order to preserve its structure. At the same time, it is necessary to clean the hardened surfaces from cement films mechanically( metal brush, milling, sandblasting), with obligatory washing with a water jet and drying with an air jet of the place of such processing.
It is necessary to take into account the opinion of practitioners who say that the construction of working joints during concreting, even after five hours, will give a stronger joint than the same complex of surface treatment of fully cured concrete. As a rule, concrete mixes are filled horizontally and layer by layer.
In the case of a large mass, the technique of stepped concreting is used, when the laying time of a new layer is no more than the setting of the previous one. The general rule of the admissibility of the place of the working seam is its device in the cross section, which is not critical for the strength of the structure, but the inadmissibility of seams is relevant in places of the greatest shearing force.
For complex vaults, bunker, bridge and arch structures, other similar structures, the places of seam construction are completely specified in the working projects. For the elements, joints are arranged in agreement with the designers. The rules establish, for example, for columns, the possibility of a seam on the top of the support foundation, of the lower or upper boundary of beams or runs between columns, etc.
For walls and columns, the seams are arranged horizontally, taking into account the appearance of the building. It is not allowed to lay the layer at an angle: for vertical seams, it is mandatory to use limiters at the minimum length. For the exact identification of seam areas, the project documentation and recommendations of norms and rules of load-bearing and enclosing structures are used( SNiP 3.03.01-87).
Cold seam concreting - how to increase the reliability of construction with seams
If necessary, to increase the setting time of the layers, special hardening moderators called plasticizers are used. The slowing down of grasping by the use of sulfite-yeast mating is permissible only in the summer season and is not recommended in other seasons.
There are a lot of methods and methods for practical increase in the strength of the bond at the seam between the layers, in addition to the mandatory machining described above. For example, the use of a horizontal or graded seam. This method is recommended when pouring small elements of tape structures.
Primer compositions are used for better adhesion to hardened concrete. A similar result is achieved by glue or bitumen compositions.
Obvious task is one - to increase the strength of the joint. For concrete works of high-rise housing construction, in agreement with the technical supervision bodies, a technological seam of concrete casting can be used, including the use of special reinforcement techniques using one or more grids( wireframes of reinforcement, fine-mesh, etc.), double-sided keys of their galvanized metal and t. P.
In addition to the seams considered, concrete types of structures may be envisaged for the design of concrete structures. With temperature fluctuations, linear dimensions change in order to avoid critical stresses in concrete structures, temperature expansion joints( compression, expansion, warping) are arranged.
Such seams are located across( expansion and compression), along( warpage) of the elements and cut into hardened concrete with a diamond disc. Seams, after preliminary preparation, are filled with special elastic compensators on different basis( mastics, rubber cords, polymers), capable of easily deforming.