Reinforcing of concrete

Reinforcing methods. The use in reinforced concrete of reinforcement like bars, meshes, frameworks and other elements (Fig. 14.1) is intended mainly for resisting tensile stresses (Fig. 14.2). Apart from the principal reinforcement, embedded fittings for connection of structures during assembling, lifting loops, distributive reinforcement are also applied in elements.

There is ordinary and prestressed reinforcing. Although ordinary reinforcing, increases the load-carrying ability of structures, it has some limited possibilities, predefined by insignificant stretchability of concrete – 0.1-0.15 mm/m. Stresses in the tensioned reinforcement are small at such strains and make up approximately 20-25% of its calculated strength. Increase in concrete strength increases its stretchability insignificantly. That is why cracks exist in the tension regions of concrete structures at comparatively low loadings. On formation of cracks, deflections are increased, moisture and gases get to the cracks and corrosion of the steel reinforcement becomes inevitable.

This shortcoming may be overcome through the use of prestressed structures, first carried out in practice in 1928 by the French engineer E. Freyciet.

The essence of prestressing is demonstrated in the concrete compression with the tensioned reinforcement shown in (Fig. 14.3). In order to change the sign of tension, which is acting in the concrete of prestressed structure, it is required foremost to neutralize the compression of concrete. At the same time it is required to mean that possible deformation of concrete at a compression exceeds limit tension in 20-25 times.

The most important consequences of prestressing is increasing of crack resistance, reinforcement saving and decline of structures mass or their enlargement. Reinforcement saving is caused by possibility of the use of high-strength steel which can not be rationally used for ordinary reinforcing. In the later case, tension of high-strength steel reinforcement increases with the increase of working stress in comparison with ordinary steel, which results in crack appearance in the tension region of reinforced concrete element and loss of load-carrying ability by it.

Due to prestressing it is possible to produce structures (slabs, beams, trusses) for covering of large spans (more than 9 m), thin-walled spatial structures (doubly curved shells, panels-shells span-sized 12, 18 and 24 m) for structures of differing purposes, etc.

Production of large diameter pipes for penstocks, supports of high-voltage lines of electricity transmissions and a series of other structures are constructed using prestressed concrete.

The application of prestressed concrete is considerably extensive and includes precast elements used for construction of dams, locks, structures of hydroplants, etc.

During the production of precast reinforced concrete elements prestressing can be conducted either before the concrete hardens or after it has acquired some strength. The first method (pre-tension) is more widespread. The essence of it is that the reinforcement is placed in a form, fixed and then tightened. The form is filled with concrete mixture and the reinforcement freed of tension after the concrete has hardened. For the second method (post-tension), reinforcement is placed in special channels left in concrete and tightened after the concrete hardens. The required adhesion of the tensioned reinforcement with concrete is achieved by injection of cement mortar in the channels. In both cases reinforcement now released of tension, tends to return back to its initial position, shortens and wrings out the reinforced concrete elements.

It is also possible to cause tensioning of reinforcement, due to expansion of cement after the concrete has acquired a strength of 10-20 MPa. The stretched reinforcement has sufficient adhesion with concrete, wrings it out and provides the effect of self-stressing.

A reliable adhesion with concrete is obtained with corrugated bars, twisted reinforcement, and also reinforcement which have additional anchor devices set on their ends.

Stretching of reinforcement is realized by mechanical, electro- thermal, electro- thermo-mechanical and chemical methods. The mechanical stretching of reinforcement is carried out by hydraulic jacks and by other devices. Electro-thermal stretching is based on using the linear expansion of reinforcement when heated by an electric current.

The ends of the extended reinforcement fasten in grip vice (at stretching on stressing abutments) or by anchors (at stretching on a concrete), as a result there is tension in it.

Reinforcement becomes taut by mechanical device and simultaneously heated by an electric current during the electro-thermal stretching.

Chemical tensioning occurs through use of expanding cements which have high energy of expansion.

Types of reinforcement. Reinforcing of structural elements and structures are carried out using steel bars and wire mesh. For ordinary reinforcing, steel bars and corrugated wire profile are widely used as reinforcement. Hot-rolled, thermal fixed bar reinforcement and also high-strength wire and reinforcement ropes are mainly used for tension reinforcement.

The basic types of reinforcement elements for reinforced concrete structures are reinforcing fabrics, flat and spatial cages. Fabrics are made from a wire and used as an assembling reinforcement. Flat cages (mats) are made of work and distributive bars, using them as bearings elements. Spatial cages can have a rectangular, t-shaped and round cross-section. They are used for reinforcing of columns, beams, pipes and others like that.

Prestressed precast reinforced concrete structures are reinforced by separate wires and bars, by wire strands and bundled burs, and also by wire packages with the different number of strings. A choice of reinforcement, which becomes taut, depends on the type of products and equipment for tendon jacking.

Production of reinforcement includes: preparatory operations; welding of bars from separate short small twigs, marking and cut of bars on the elements of required length, bend and grant to them the set project configuration, drafting and welding of reinforcement products.

The most effective process of producing reinforcement are welding operations. The contact butt welding is applied for connection of reinforcement bars at their purveyance for providing of subsequent non-waste cutting (Fig. 14.4). The contact point welding is widespread for producing reinforcement products (Fig. 14.5). The essence of contact welding is for connection of metallic elements when warming-up by an electric current and application of mechanical effort. The most heat release takes place at the bar joints as a result of passing current through weldable elements.

Mechanized and automated lines, equipped by highly productive equipment are applied for the production of reinforcement elements and structures.

Corrosion of reinforcement. The alkaline character of the internal environment of concrete is a favorable factor for the protection of the reinforcement and causes surface "passivation" of steel. However for highly porous concrete, passivation of reinforcement steel is violated as a result of penetration of carbon dioxide and other gases from the environment. Corrosion of reinforcement at humidity greater than 80% is the most intensively observed process. Steel reinforcement is affected by highly aggressive environment such as those which exist in the wet and hot climate of marine environments with the air saturated by salts. Corrosion of reinforcement can also be initiated by chloride salts added to the concrete to accelerate the hardening process. They are allowed to be applied only for reinforced concrete structures from non-stress working reinforcement with more than 5 mm diameter, intended for exploitation in non-aggressive gas and water environments.

An effective method of prevention of reinforcement’s corrosion in concrete is by use of high density concrete and sufficient thickness of the protective cover layer, which depends on humidity and the aggressiveness of the environment.

Date: 2015-12-18; view: 814