The text of the report presented at the conference by the head of the Laboratory for Testing Building Materials and Structures Dmitry Nikolaevich Abramov “The main causes of defects in concrete structures”

In my report, I would like to talk about the main violations of the technology for the production of reinforced concrete work encountered by our laboratory employees at construction sites in Moscow.

- early formwork removal.

Due to the high cost of formwork in order to increase the number of cycles of its turnover, builders often do not adhere to concrete curing conditions in the formwork and carry out formwork removal at an earlier stage than is required by the design cards and SNiP 3-03-01-87. When dismantling the formwork, the adhesion of concrete to the formwork is important when: large adhesion makes it difficult to dismantle. Deterioration in the quality of concrete surfaces leads to defects.

- manufacturing is not rigid enough, deforming when laying concrete and not sufficiently dense formwork.

Such formwork receives deformations during the period of laying the concrete mixture, which leads to a change in the shape of reinforced concrete elements. Deformation of the formwork can lead to displacement and deformation of reinforcing cages and walls, a change in the bearing capacity of structural elements, and the formation of protrusions and sagging. Violation of the design dimensions of structures leads to:

If reduced

To decrease bearing capacity

In case of increase to increase of their own weight.

This type of violation of observation technology in the manufacture of formwork in building conditions without proper engineering control.

- insufficient thickness or lack of a protective layer.

It is observed with incorrect installation or displacement of the formwork or reinforcement cage, the absence of gaskets.

Serious defects in monolithic reinforced concrete structures can be caused by poor quality control of reinforcing structures. The most common are violations:

- non-compliance with the design of reinforcing structures;

- poor-quality welding of structural components and joints of reinforcement;

- the use of highly corroded fittings.

- poor compaction of the concrete mix during installation  into the formwork leads to the formation of shells and caverns, can cause a significant decrease in the bearing capacity of elements, increases the permeability of structures, contributes to corrosion of reinforcement located in the zone of defects;

- laying of the stratified concrete mix  does not allow to obtain uniform strength and density of concrete throughout the entire volume of the structure;

- use of too hard concrete mix  leads to the formation of shells and caverns around the reinforcing bars, which reduces the adhesion of the reinforcement to concrete and causes the risk of corrosion of the reinforcement.

There are cases of adhesion of concrete mixture to reinforcement and formwork, which causes the formation of cavities in the body of concrete structures.

- poor maintenance of concrete in the process of hardening.

During the maintenance of concrete, temperature-moist conditions should be created that would ensure that the water required for the hydration of the cement is retained in the concrete. If the hardening process proceeds at a relatively constant temperature and humidity, stresses arising in concrete due to volume changes and caused by shrinkage and thermal deformation will be insignificant. Typically, concrete is coated with plastic wrap or other protective coating. In order to prevent it from drying out. Overdried concrete has significantly lower strength and frost resistance than normally hardened concrete; a lot of shrinkage cracks appear in it.

When concreting in winter conditions with insufficient insulation or heat treatment, early freezing of concrete may occur. After thawing such concrete, he will not be able to gain the necessary strength.

Damage to reinforced concrete structures is divided according to the nature of the effect on the bearing capacity into three groups.

Group I - damage that practically does not reduce the strength and durability of the structure (surface shells, voids; cracks, including shrinkage, with openings not exceeding 0.2 mm, and also, under the influence of a temporary load and temperature, the opening increases by no more than 0 , 1mm; concrete chips without exposure of reinforcement, etc.);

Group II - damages that reduce the durability of the structure (corrosive cracks with an opening of more than 0.2 mm and cracks with an opening of more than 0.1 mm, in the area of ​​working reinforcement of prestressed spans, including along sections under constant load; cracks with an opening of more than 0.3 mm under temporary load; voids of the shell and chips with exposed reinforcement; surface and deep corrosion of concrete, etc.);

Group III - damage that reduces the structural bearing capacity of the structure (cracks not specified by calculation either in strength or endurance; inclined cracks in the walls of beams; horizontal cracks in the joints of the slab and spans; large shells and voids in the concrete of the compressed zone, etc. .).

Damage to group I does not require urgent measures, they can be eliminated by coating at the current content for preventive purposes. The main purpose of coatings for damage of group I is to stop the development of existing small cracks, prevent the formation of new ones, improve the protective properties of concrete and protect structures from atmospheric and chemical corrosion.

In case of damage of group II, repair provides an increase in the durability of the structure. Therefore, the materials used must have sufficient durability. Cracks in the area of ​​the arrangement of bundles of prestressed reinforcement, cracks along the reinforcement are subject to obligatory sealing.

In case of damage of group III, the bearing capacity of the structure is restored according to a specific symptom. The materials and technologies used should provide strength characteristics and durability of the structure.

To eliminate the damage of group III, as a rule, individual projects should be developed.

The constant growth of volumes of monolithic construction is one of the main trends that characterize the modern period of Russian construction. However, at present, the massive transition to the construction of reinforced concrete can have negative consequences associated with a fairly low level of quality of individual objects. Among the main reasons for the low quality of constructed monolithic buildings, the following should be highlighted.

Firstly, the majority of regulatory documents currently in force in Russia were created during the era of priority development of prefabricated reinforced concrete construction, therefore their focus on factory technologies and insufficient study of the issues of construction from monolithic reinforced concrete are quite natural.

Secondly, most construction organizations lack sufficient experience and the necessary technological culture of monolithic construction, as well as poor-quality technical equipment.

Thirdly, an effective quality management system for monolithic construction has not been created, including a system of reliable technological quality control of work.

The quality of concrete is, first of all, the correspondence of its characteristics to the parameters in regulatory documents. Rosstandart approved and are operating new standards: GOST 7473 “Concrete mixtures. Specifications ", GOST 18195" Concrete. Rules of control and strength assessment. " GOST 31914 “High-strength heavy and fine-grained concrete for monolithic structures” should come into force, the standard for reinforcing and embedded products should become effective.

The new standards, unfortunately, do not contain issues related to the specifics of legal relations between construction customers and general contractors, manufacturers of building materials and builders, although the quality of concrete work depends on each stage of the technical chain: preparation of raw materials for production, concrete design, production and transportation of the mixture, laying and maintenance of concrete in the structure.

Ensuring the quality of concrete in the production process is achieved through a variety of conditions: here, modern technological equipment, the presence of accredited testing laboratories, qualified personnel, unconditional compliance with regulatory requirements, and the implementation of quality management processes.

Candidates of tech. Sciences Ya. P. BONDAR (TsNIIEP homes) Yu. S. Ostrinsky (NIIES)

To find methods of concreting in the sliding formwork of walls with a thickness of less than 12-15 ohms, the interaction forces of the formwork and concrete mixtures prepared on solid aggregates, expanded clay and slag pumice were studied. With the existing technology of concreting in sliding formwork, this is the minimum allowable wall thickness. For stucco concrete, expanded clay gravel of the Beskudnikovsky plant with crushed sand from the same expanded clay and slag pumice made from melts of the Novo-Lipetsk Metallurgical Plant with fishing line obtained by crushing slag lemza.

Expanded clay of grade 100 had vibration compaction, measured on a N. Ya. Spivak instrument, 12-15 s; structural factor 0.45; bulk density of 1170 kg / m3. Slag-grade slag-concrete of grade 200 had a vibro-compaction of 15–20 s, a structural factor of 0.5, and a bulk density of 2170 kg / m3. Heavy grade 200 concrete with a bulk density of 2400 kg / m3 was characterized by a draft of a standard cone of 7 cm.

The forces of interaction of the sliding formwork with concrete mixtures were measured on a test setup, which is a modification of the Kaza-randa instrument for measuring the forces of a single-plane shear. Installation is made in the form of a horizontal tray filled with concrete mix. Across the tray, test rails were laid from wooden blocks, sheathed on the surface of contact with the concrete mixture with strips of roofing steel. Thus, the test rails simulated a steel sliding formwork. The slats were kept on concrete mix under loads of various sizes, simulating the pressure of concrete on the formwork, after which the forces causing horizontal movement of the slats on concrete were recorded. A general view of the installation is given in Fig. one.

Based on the results of the tests, the dependence of the interaction forces of the steel sliding formwork and the concrete mix t on the concrete pressure on the formwork a (Fig. 2), which is linear, is obtained. The angle of inclination of the graph line with respect to the abscissa axis characterizes the friction angle of the formwork on concrete, which allows you to calculate the friction forces. The value cut off by the line of the graph on the ordinate axis characterizes the adhesion forces of the concrete mix and formwork t, which are independent of pressure. The friction angle of the formwork on concrete does not change with an increase in the duration of fixed contact from 15 to 60 minutes, the magnitude of adhesion increases in this case by 1.5-2 times. The main increment of adhesion forces occurs during the first 30-40 minutes with a rapid decrease in increment over the next 50-60 minutes.

The adhesion force of heavy concrete and steel formwork 15 minutes after compaction of the mixture does not exceed 2.5 g / ohm2, or 25 kg / m2 of contact surface. This amounts to 15-20% of the generally accepted value of the total interaction force of heavy concrete and steel formwork (120-150 kg / m2). The bulk of the effort falls on the friction forces.

The slower increase in adhesion forces during the first 1.5 hours after concrete compaction is explained by an insignificant number of neoplasms in the process of setting concrete mix. According to studies, in the period from the beginning to the end of setting of the concrete mix, redistribution of mixing water in it between the binder and aggregates occurs. Neoplasms develop mainly after the end of setting. The rapid increase in adhesion of the sliding formwork to the concrete mix begins 2-2.5 hours after compaction of the concrete mix.

The specific gravity of the adhesion forces in the total value of the interaction forces of heavy concrete and steel sliding formwork is about 35%. The main part of the effort falls on the friction forces, determined by the pressure of the mixture, which varies with time under conditions of concreting. To verify this assumption, the shrinkage or swelling of freshly formed concrete samples was measured immediately after vibration compaction. During the molding of concrete cubes with a rib size of 150 mm, a textolite plate was placed on one of its vertical faces, the smooth surface of which was in the same plane with the vertical face. After the concrete was compacted and the sample was removed from the vibrating table, the vertical faces of the cube were freed from the side walls of the mold and the distances between the opposite vertical faces were measured with a mass for 60–70 min. The measurement results showed that freshly formed concrete immediately after compaction shrinks, the magnitude of which is the higher, the greater the mobility of the mixture. The total value of bilateral precipitation reaches 0.6 mm, i.e., 0.4% of the sample thickness. In the initial period after molding, the swelling of the freshly laid concrete does not occur. This is explained by contraction in the initial stage of concrete grabbing in the process of redistribution of water, accompanied by the formation of hydrated films that create large surface tension forces.

The principle of operation of this device is similar to the principle of the conical plastometer. However, the wedge-shaped form of the indenter allows you to use the design scheme of a viscous bulk array. The results of experiments with a wedge-shaped indenter showed that To varies from 37 to 120 g / cm2 depending on the type of concrete.

Analytical calculations of the pressure of the concrete mix layer with a thickness of 25 ohms in the sliding formwork showed that the mixtures of the accepted compositions, after their compaction by vibration, do not exert active pressure on the formwork sheathing. The pressure in the “sliding formwork - concrete mixture” system is due to the elastic deformations of the shields under the influence of the hydrostatic pressure of the mixture during its compaction by vibration.

The interaction of the sliding formwork panels and compacted concrete in the stage of their joint work is reasonably well modeled by passive repulsion of the viscoplastic body under the influence of pressure from the side of the vertical retaining wall. The calculations showed that with a unilateral action of the shuttering board on concrete masses) to displace a part of the massif but on the main glide planes, a pressure increase is required that significantly exceeds the pressure that occurs under the most unfavorable combination of laying and compaction conditions. When the shuttering boards are pressed bilaterally on a vertical - layer of concrete of limited thickness, the pressing forces necessary to displace compacted concrete ps to the main slip planes acquire the opposite sign and significantly exceed the pressure required to change the compression characteristics of the mixture. Reverse loosening of the compacted mixture under the action of two-sided compression requires such a high pressure, which is unattainable when concreting in sliding formwork.

Thus, the concrete mix, laid according to the rules of concreting in sliding formwork with layers 25-30 cm thick, does not exert pressure on the formwork panels and is able to perceive the elastic pressure arising from them during vibration compaction.

To determine the interaction forces arising in the process of concreting, measurements were carried out on a full-size sliding formwork model. A sensor with a membrane of high-strength phosphor bronze was installed in the molding cavity. The pressures and efforts on the lifting rods in the static position of the installation were measured by an automatic pressure meter (AID-6M) during vibration and lifting of the formwork using an N-700 photo-oscilloscope with an 8-ANF amplifier. The actual characteristics of the interaction of steel sliding formwork with various types of concrete are given in the table.

In the period between the end of the vibration and the first rise of the formwork, a spontaneous decrease in pressure occurred. which remained unchanged until the formwork began to move up. This is due to the intense shrinkage of the freshly formed mixture.

To reduce the interaction forces between the sliding formwork and the concrete mixture, it is necessary to reduce or completely eliminate the pressure between the formwork panels and compacted concrete. This problem is solved by the proposed concreting technology using intermediate removable shields (“liners”) from thin (up to 2 mm) sheet material. The height of the liners is greater than the height of the molding cavity (30-35 ohms). The liners are installed in the molding cavity close to the shields of the sliding formwork (Fig. 5) and immediately after laying and compaction. The concrete is alternately removed from it.

The gap (2 mm) remaining between the concrete and the formwork, after removing the shields, protects the formwork shield, which straightens after elastic deflection (usually not exceeding 1-1.5 mm) from contact with the vertical surface of the concrete. Therefore, the vertical faces of the walls, freed from the liners, retain their shape. This allows concreting thin walls in the sliding formwork.

The fundamental possibility of forming thin walls with the help of liners was tested during the erection of full-size fragments of walls 7 cm thick made of expanded clay concrete, slag concrete and heavy concrete. The results of test moldings showed that light-concrete mixtures better correspond to the features of the proposed technology than mixtures with dense aggregates. This is due to the high sorption properties of porous aggregates, as well as the cohesive structure of lightweight concrete and the presence of a hydraulically active dispersed component in light sand.

Heavy concrete (albeit to a lesser extent) also shows the ability to maintain the verticality of freshly formed surfaces with its mobility of not more than 8 cm. When concreting civil buildings with thin intra-apartment walls and partitions according to the proposed technology, two to four pairs of liners from 1.2 to 1.6 m, providing concreting of walls with a length of 150-200 m. This will significantly reduce concrete consumption compared to buildings constructed according to the adopted technology, and increase economic efficiency their construction.

The adhesion of concrete to the formwork is affected by adhesion (adhesion) and shrinkage of concrete, surface roughness and porosity. With a large force of adhesion of concrete to the formwork, the formwork is complicated, the complexity of the work increases, the quality of the concrete surfaces deteriorates, the formwork panels prematurely wear out.

Concrete adheres to wood and steel formwork surfaces much more strongly than plastic ones. This is due to the properties of the material. Wood, plywood, steel and fiberglass are well wetted, therefore, the adhesion of concrete to them is quite high, with poorly wettable materials (for example, textolite, getinaks, polypropylene), the adhesion of concrete is several times lower.

Therefore, to obtain high-quality surfaces, it is necessary to use claddings from textolite, hetinax, polypropylene or use waterproof plywood treated with special compounds. When the adhesion is small, the concrete surface is not broken and the formwork easily leaves. With an increase in adhesion, the concrete layer adjacent to the formwork is destroyed. This does not affect the strength characteristics of the structure, but the surface quality is significantly reduced. Adhesion can be reduced by applying aqueous suspensions, hydrophobizing lubricants, combined lubricants, lubricants - concrete retarders to the formwork surface. The principle of action of aqueous suspensions and hydrophobic lubricants is based on the fact that a protective film forms on the surface of the formwork, which reduces the adhesion of concrete to the formwork.

Combined lubricants are a mixture of concrete setting retarders and water repellent emulsions. In the manufacture of lubricants, they add sulfite-yeast vinasse (SDB), soap-oil. Such lubricants plasticize the concrete of the adjacent area, and it does not collapse.

Lubricants - concrete setting retarders - are used to obtain a good surface texture. By the time of dismantling, the strength of these layers is slightly lower than the bulk of concrete. Immediately after stripping, the concrete structure is exposed by washing it with a stream of water. After such washing, a beautiful surface with uniform exposure of coarse aggregate is obtained. Lubricants are applied to the formwork panels prior to installation in the design position by pneumatic spraying. This method of application provides uniformity and a constant thickness of the applied layer, and also reduces lubricant consumption.

For pneumatic application, spray guns or fishing rods are used. More viscous lubricants are applied with rollers or brushes.

Hello dear readers! All our and your questions are answered today by master Vadim Aleksandrovich. Today we will talk about the features of pouring concrete into the formwork.

Hello Vadim Alexandrovich!

Hello! First of all, I want to say that this work is quite complicated and very responsible, and it is better to entrust the professionals to fill the floors and load-bearing walls than to try to do it yourself. Let's get to your questions.

1. Do I need to somehow prepare the formwork and reinforcement?

The formwork is lubricated with a special aqueous emulsion lubricant (Emulsol) in order to separate the formwork from the hardened concrete. Although there were cases at a construction site when they were poured into an ungreased formwork and then it was torn off. Also, the formwork is pulled together with special screeds that are inserted into the tubes between the shields.

2. Is the method of filling horizontal forms different from vertical?

Virtually no different. Verticals are a little harder to tamp.

3. Please tell us how to pour concrete.

The method of pouring is determined by the project (TCH). It is desirable to fill the entire formwork immediately, pouring layers is undesirable, otherwise you will have to make notches with a perforator for better adhesion of the layers. Vertical forms must be filled in whole.

4. How to connect layers if nevertheless we fill with layers? Well, we didn’t have enough concrete for pouring the whole thing.

As I said, we make notches with a puncher for hardened concrete.

5. What are the secrets to evenly filling?

There are no secrets, there are general rules: We fill it in different places and not in one, scatter it with shovels in all its shape, then - ram it with a vibrator to a smooth glossy surface in order to remove all voids and concrete uniformly filled the formwork. However, if the concrete is of poor quality, but it is very necessary to fill it, then you cannot use a vibrator - all the water will flow out and the concrete will not seize. In this case, you just need to knock on the formwork. But try to avoid such cases - build for yourself.

6. How does the density of the solution affect the fill?

A thick solution is difficult to evenly distribute and compact. Before pouring, add water to the mixer. Too liquid - and again bad, when tamping all the water will flow out and the concrete will not seize. If we do it ourselves, then we add cement and sand, if we are brought ready, we are sent to the factory due to non-compliance.

7. I heard that concrete heats up when solidified. Is this a problem and is it necessary to deal with it?

Yes, this is a problem and it needs to be fought. In the heat, it is necessary to pour the formwork with cold water, otherwise the concrete will crack. And in the cold, on the contrary, we warm up.

8. If we do not keep track and the concrete is cracked, how to fix it?

Small cracks are permissible, the maximum crack size is indicated in the design documentation, if the size is exceeded, then we take a jackhammer and beat off. Otherwise, it will fall apart after a while. After all, cracks significantly reduce the strength of the structure.

Thank you very much for the consultation Vadim Alexandrovich. We and our readers are very grateful.