Repair of reinforced concrete structures: when is it possible and how to do it correctly

Recommendation points

• Causes and types of damage to be repaired
• Basic repair methods
• Crack treatment
• Elimination of large defects
• When exposing the reinforcement cage
• Rules to be followed

Which is cheaper: dismantle the reinforced concrete structure and reassemble it, or carry out repairs, extending the service life by tens of years? The portal offers a selection of technologies for the repair of foundations, columns, walls, floors and other structures found in private construction.

Causes and types of damage to be repaired

Almost all concrete products are deformable and can change shape when the existing loads change. The concrete mix itself has extremely low tensile strength, which is compensated by the reinforcement of the structure. However, this does not completely solve the problem: cracks still form in concrete, but of a microscopic size and much more numerous.

Thanks to reinforcing ties, concrete acquires durability: small cracks do not affect the strength characteristics in any way, and the permissible crack opening limit itself is determined by the technical standards for the product. However, over time, even small cracks expand due to friction and erosion, moreover, it is possible that the operating conditions of reinforced concrete will change to less favorable, which will cause accelerated wear of the structure.

Outwardly, such phenomena manifest themselves as the opening of wide and deep cracks, as well as in the form of delamination of concrete parts, up to the exposure of the reinforcing cage. The main reason is water, which saturates the pores and, expanding when freezing, destroys the concrete shell from the inside. If the water gets to the reinforcement, the latter corrodes intensively, while the swelling rust prevents the adhesion of concrete with metal and tears the structure from the inside.

In total, there are 5 degrees of damage:

1. Dirt, cavities and shrinkage cracks.
2. Chips, peeling and deterioration of the surface.
3. Continuous gouges, rust spots, inactive cracks up to 0.2 mm.
4. Cracks more than 0.2 mm, formation of stalactites.
5. Chipping off large pieces with bare reinforcement, the formation of voids and delamination.

The types of damage are distinguished:

• not affecting the properties of the structure;
• shortening the service life (thinning of the protective layer);
• reducing the bearing capacity (exclusion of part of the reinforcement from work due to exposure).

In general, we can say that if, due to the exposure of the protective layers of concrete, no deformation of the reinforcement cage has occurred, the product can be restored relatively easily. The exception is cases of deep corrosion – when, when trying to get to the intact rust of the reinforcement, it is found that the entire frame is affected by it. Then the product is recognized as unserviceable and must be replaced..

Basic repair methods

There are two types of repairs – with and without restoration of the bearing capacity. In the latter case, the actions are cosmetic in nature – cracks, chips and shells are filled, the surface is leveled. Such types of repairs can be performed both during the construction process to eliminate casting defects, and during operation..

Repair with restoration of bearing capacity is performed in the following cases:

• With an increase in the load on the structure above the design.
• When operating conditions deteriorate.
• With accelerated wear of the structure and damage of 3 types 4-5 degrees.

The essence of any repair is to restore the mass of concrete and fill with fresh mortar those areas where delamination and shedding occurred. If reinforcement is visible, the main task during the repair is to prevent the spread of corrosion and to achieve excellent adhesion of the mixture to the metal. Often for these purposes, repair compounds with polymer components are used..

There are three main repair methods:

1. Treatment of cracks by filling them.
2. Restoration of the shape and planes by applying a layer of concrete to the surface.
3. Restoration of the connection of concrete with reinforcement and sealing of areas of bare frame.

Reinforcement of reinforced concrete structures is distinguished separately. By increasing the weight and dimensions of the structure, an increase in the bearing capacity is achieved. In such cases, after the simplest preparation, they simply perform tides to the product, which, after hardening, take on part of the load..

Crack treatment

When repairing cracks, you need to understand whether they are active. In many cases, the appearance of thin cracks indicates that the structure changes shape due to temperature fluctuations, seismic activity, or due to changes in soil properties. Such deformations, as a rule, are reversible, that is, cracks are closed after restoring their original shape. If, at the moment of opening, they are filled with an incompressible material, after the reverse deformation of the structure, an even wider crack will appear on the back of the product..

It is customary to eliminate active cracks with compounds that retain the ability to stretch and compress at least 50% of their original dimensions. It is good to treat cracks up to 0.3 mm by surface sealing using epoxy resins ELD-283, ELD-552. Wider cracks are filled to the full depth by injection: holes are drilled along the entire length of the split with a step of 10-15 cm, through which epoxy is injected with a syringe. This repair method helps to significantly reduce the water absorption of concrete, eliminate the penetration of moisture and atmospheric oxygen to the reinforcement.

Concrete injection

To distinguish active cracks from inactive ones, beacons made of densely mixed stucco are attached to them. On an inactive crack, the beacon will not crack or fall off. If the lighthouse has burst, and the crack on it is even, without chips, and there is a visible gap, this indicates that the fault is currently expanding. If on a burst lighthouse the edges of the fault are torn and crumbled, or one part of the lighthouse has fallen off, the crack is currently shrinking.

If a survey by lighthouses during the year showed that the crack is inactive, it can be repaired with an incompressible material, such as fine sand concrete of the same grade from which the structure was cast. First, it is necessary to open the crack – remove chips and crumbling particles at the edges, blow with compressed air. Depending on the width, an inactive crack can also be treated with either a surface seal with a spatula or a grout injection.

Elimination of large defects

If the repair of cracks was not carried out in a timely manner, destructive phenomena affect the protective layer of concrete, which leads to chipping and crumbling of large fragments. At the same time, the surface of the concrete is dotted with wide, but shallow cracks, pits, when tapping with a hammer, a booming sound is heard – voids have formed under the surface.

In such cases, the protective concrete layer is restored, after removing all the remains that have undergone destruction. Quite often, it is necessary to clean off thick layers of loose and crumbling concrete from the surface of gouges and holes. It cannot be left, because the strength of the damaged concrete is much lower than the design.

The cleaned surface is subjected to invoicing – with the help of embossing, many small serifs are left, contributing to a better adhesion of the new layer to the old one. The edges of the area to be repaired are cut with a chisel, forming an even edge with a depth of at least 10 mm.

The application of the concrete mass is carried out either with a trowel in layers, or by spraying under pressure – this method is the most technological and effective. After the restoration of the protective layer, the concrete requires maintenance for 7-10 days, which consists in periodic moistening of the surface and tracking the boundaries of the embedded areas.

When exposing the reinforcement cage

If the destruction of concrete occurred to the depth of the reinforcement, repairs must be carried out urgently. Further weakening of the mass can lead to the fact that the reinforcing rods bend and the structure will irreversibly lose its properties.

First of all, it is necessary to cut the concrete into the depth of the reinforcement and make sure that corrosion has not touched the metal at a depth of more than 50–60 mm. If the damage is deeper, the frame should be exposed until areas unaffected by corrosion appear. If, due to rust, the diameter of the reinforcement has decreased by more than 10%, the frame must be reinforced. To do this, a new one is tied to the existing bar, the length of the overlap with intact reinforcement must be at least 40 diameters.

Also, when eliminating defects with bare reinforcement, it is practiced to bind to the frame of the reinforcing mesh. It avoids slipping of a thick layer of fresh concrete and at the same time prevents the opening of new cracks in the problem area.

Steel elements must be thoroughly cleaned of rust, efflorescence and other contamination. The fittings should be washed well with an acid-free rust converter and then covered with thick cement laitance using a brush. Without waiting for it to dry, the damaged area is filled with fresh concrete. The procedure for preparing the edges and surfaces of large gouges has been described above..

Rules to be followed

There are three main requirements for repairing reinforced concrete products. The main one is that the repair composition must be identical, or at least compatible with the one that was used in the construction of a particular part of the concrete structure. There are no particular problems in terms of compatibility with polymeric materials, however, mixtures based on a cement binder should be selected carefully, taking into account the type and grade of cement, the type and fraction of the filler, the water-cement ratio and the presence of additives. If the repair area turns out to be non-uniform, due to the difference in physical properties, the fresh composition will quickly break off the surface..

The second rule is to observe the temperature regime. Repair work is prohibited if the temperature in the local area exceeds 10 ° C or below +5 ° C. In addition, there are requirements for structures experiencing an increased effect of thermal expansion – building frames, supports, floor beams, reinforced concrete products operated in direct sunlight. Their average temperature at the time of repair should not exceed 20 ° С.

Rule three: at the time of repair, the concrete should not be saturated with water. It is only allowed to wet the surface and flush the cracks before adding the repair compound, but you should wait until the moisture is completely absorbed. If it is not possible to dry the concrete structure, a modifier should be added to the repair mix to ensure adhesion to wet concrete..