Reasons for concrete sticking to formwork. Grozdov V.T. Defects in building structures and their consequences - Defects in monolithic reinforced concrete structures caused by violations of the technology of their construction. e. Ironing of concrete surface
Text of the report presented at the conference by the head of the Testing Laboratory building materials and structures by Dmitry Nikolaevich Abramov “The main causes of defects in concrete structures”
In my report I would like to talk about the main violations of iron production technology concrete works faced by our laboratory staff at construction sites city of Moscow.
- early demoulding of structures.
Due to the high cost of formwork, in order to increase the number of cycles of its turnover, builders often do not comply with the conditions for curing concrete in the formwork and remove formwork at an earlier stage than the requirements of the project technological maps and SNiP 3-03-01-87. When dismantling formwork, the amount of adhesion between concrete and formwork is important: high adhesion makes formwork removal difficult. Deterioration in the quality of concrete surfaces leads to the occurrence of defects.
- production of insufficiently rigid formwork that deforms when laying concrete and is not dense enough.
Such formwork undergoes deformation during the laying of the concrete mixture, which leads to a change in shape reinforced concrete elements. Deformation of the formwork can lead to displacement and deformation of reinforcement frames and walls, changes bearing capacity structural elements, the formation of protrusions and sagging. Violation of the design dimensions of structures leads to:
If they decrease
To reduce load-bearing capacity
In case of increase, their own weight increases.
This type of violation of observation technology during the manufacture of formwork under construction conditions without proper engineering control.
- insufficient thickness or absence of a protective layer.
Observed when incorrect installation or displacement of the formwork or reinforced frame, lack of gaskets.
To serious defects of monolithic reinforced concrete structures may result from poor control over the quality of reinforcement of structures. The most common violations are:
- non-compliance with the structural reinforcement design;
- poor quality welding structural units and reinforcement joints;
- use of heavily corroded reinforcement.
- poor compaction of the concrete mixture during laying into the formwork leads to the formation of cavities and cavities, can cause a significant decrease in the load-bearing capacity of elements, increases the permeability of structures, and promotes corrosion of reinforcement located in the defect zone;
-laying laminated concrete mixture does not allow obtaining uniform strength and density of concrete throughout the entire volume of the structure;
- use of too hard concrete mixture leads to the formation of cavities and cavities around the reinforcing bars, which reduces the adhesion of the reinforcement to the concrete and causes the risk of corrosion of the reinforcement.
There are cases of concrete mixture sticking to reinforcement and formwork, which causes the formation of cavities in the body of concrete structures.
- poor care of concrete during its hardening process.
When caring for concrete, it is necessary to create such temperature-humidity conditions that would ensure that the water necessary for hydration of cement is retained in the concrete. If the hardening process takes place at a relatively constant temperature and humidity, the stresses arising in the concrete due to changes in volume and caused by shrinkage and temperature deformations will be insignificant. Usually concrete is covered with plastic film or other protective coating. In order to prevent it from drying out. Overdried concrete has significantly less strength and frost resistance than normally hardened concrete; many shrinkage cracks appear in it.
When concreting in winter conditions If insulation or heat treatment is insufficient, early freezing of the concrete may occur. After thawing, such concrete will not be able to gain the necessary strength.
Damage to reinforced concrete structures is divided into three groups according to the nature of the impact on the load-bearing capacity.
Group I - damage that practically does not reduce the strength and durability of the structure (surface cavities, voids; cracks, including shrinkage ones, with an opening of no more than 0.2 mm, and also in which, under the influence of temporary load and temperature, the opening increases by no more than 0 ,1mm; concrete chips without exposing reinforcement, etc.);
Group II - damage that reduces the durability of the structure (corrosion-dangerous 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 the working reinforcement of prestressed spans, including along areas under constant load; cracks with an opening of more than 0.3 mm under temporary load load; shell voids and chips with exposed reinforcement; surface and deep corrosion of concrete, etc.);
Group III - damage that reduces the load-bearing capacity of the structure (cracks not included in the calculations either in terms of strength or endurance; inclined cracks in the walls of beams; horizontal cracks in the interfaces of the slab and spans; large cavities and voids in the concrete of the compressed zone, etc. .).
Damage of group I does not require urgent measures; they can be eliminated by applying coatings during routine maintenance for preventive purposes. The main purpose of coatings for group I damage is to stop the development of existing small cracks, prevent the formation of new ones, and improve protective properties concrete and protect structures from atmospheric and chemical corrosion.
In case of damage of group II, repair ensures an increase in the durability of the structure. Therefore, the materials used must have sufficient durability. Cracks in the area where bundles of prestressed reinforcement are located and cracks along the reinforcement are subject to mandatory sealing.
In case of damage of group III, the load-bearing capacity of the structure is restored according to a specific feature. The materials and technologies used must ensure strength characteristics and durability of the structure.
To eliminate group III damage, as a rule, individual projects must be developed.
Constant growth in volumes monolithic construction is one of the main trends characterizing the modern period of Russian construction. However, at present, a massive transition to construction from monolithic reinforced concrete may 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, most of the regulatory documents currently in force in Russia were created in the era of priority development of construction from precast reinforced concrete, so their focus on factory technologies and insufficient elaboration of the issues of construction from monolithic reinforced concrete are completely natural.
Secondly, most construction organizations do not have sufficient experience and the necessary technological culture of monolithic construction, as well as poor-quality technical equipment.
Thirdly, not created efficient system quality management of monolithic construction, including a system of reliable technological quality control of work.
The quality of concrete is, first of all, the compliance of its characteristics with the parameters in regulatory documents. Rosstandart has approved and is in force new standards: GOST 7473 “Concrete mixtures. Specifications", GOST 18195 "Concrete. Rules for monitoring and assessing strength." GOST 31914 “High-strength heavy and fine-grained concrete for monolithic structures", should become a valid standard for reinforcement and embedded products.
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, design of concrete, production and transportation of the mixture, laying and maintaining concrete in structures.
Ensuring the quality of concrete during the production process is achieved thanks to a complex various conditions: here and modern technological equipment, and the presence of accredited testing laboratories, and qualified personnel, and unconditional implementation regulatory requirements, and implementation of quality management processes.
The amount of adhesion between concrete and formwork reaches several kgf/cm 2 . This complicates stripping work, deteriorates the quality of concrete surfaces and leads to premature wear of formwork panels.
The adhesion of concrete to formwork is influenced by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the formwork's forming surface.
Adhesion (sticking) is understood as a bond caused by molecular forces between the surfaces of two dissimilar or liquid bodies in contact. During the period of contact between concrete and formwork, favorable conditions are created for adhesion to occur. The adhesive (adhesive), which in this case is concrete, is in a plastic state during the laying period. In addition, in the process of vibration compaction of concrete, its plasticity increases even more, as a result of which the concrete moves closer to the surface of the formwork and the continuity of contact between them increases.
Concrete sticks to wood and steel formwork surfaces more strongly than to plastic ones due to the latter's poor wettability.
When removing formwork, there can be three tearing options. In the first option, adhesion is very small, and cohesion is quite high. In this case, the formwork is torn off exactly along the contact plane. The second option is adhesion more than cohesion. In this case, the formwork is torn off along the adhesive material (concrete). The third option is that adhesion and cohesion are approximately the same in magnitude. The formwork comes off partially along the plane of contact between the concrete and the formwork, and partly along the concrete itself (mixed or combined tearing). With adhesive tearing, the formwork is easily removed, its surface remains clean, and the concrete surface is of good quality.
As a result, it is necessary to strive to ensure adhesive separation. To do this, the forming surfaces of the formwork are made of smooth, poorly wetted materials or lubricants and special anti-adhesive coatings are applied to them.
Formwork lubricants, depending on their composition, operating principle and operational properties, can be divided into four groups: aqueous suspensions; hydrophobic lubricants; lubricants - concrete set retarders; combined lubricants.
The use of effective lubricants reduces the harmful effects of certain factors on the formwork. In some cases, lubricants cannot be used. Thus, when concreting in sliding or climbing formwork, the use of such lubricants is prohibited due to their penetration into the concrete and a decrease in its quality. Good effect provide anti-adhesive protective coatings based on polymers. They are applied to the forming surfaces of shields during their manufacture, and they withstand 20-35 cycles without re-application and repair. A phenol-formaldehyde-based coating has been developed for plank and plywood formwork. It is pressed onto the surface of the boards at a pressure of up to 3 kgf/cm2 and a temperature of + 80° C.
It is advisable to use boards whose decks are made of getinax, smooth fiberglass or textolite, and the frame is made of metal corners. This formwork is wear-resistant, easy to remove and provides good quality concrete surfaces.
When working with monolithic structures made of reinforced concrete, it is worth paying attention to the characteristics of the adhesion of concrete to formwork, where the value reaches several kg per square centimeter. Due to the adhesion, stripping the reinforced concrete structure will be more difficult, in addition, this process may worsen itself concrete surface, namely, its quality. And the formwork panels may even collapse before the specified time. To prevent this from happening, ubts.kiev.ua is now available, which solves all these problems.
Due to the factors described below, concrete adheres to the formwork:
concrete undergoes adhesion and cohesion;
shrinkage of concrete occurs;
formwork adjacent to a reinforced concrete structure may have a rough or porous surface.
At the moment when concrete is laid, its state is plastic, so it is considered an adhesive substance, due to which a process called adhesion occurs (when concrete sticks to the formwork). When the material is compacted, the plasticity index of the concrete may increase, causing it to adhere to the surface of the formwork.
The adhesion process can be different, depending on the material that was used to produce the formwork surface: concrete will stick more strongly to wood and steel. Plastic products, due to their less wettability, adhere least to concrete.
If plywood, steel, wood or fiberglass materials do not pre-treat, they will be easily wetted, which will ensure high-quality adhesion to concrete. Less significant coefficient of adhesion with getinax and textolite, since they belong to the category hydrophobic materials.
Wetting can be reduced by surface treatment, which is the application of an oil film to it, as a result of which the adhesion process will be significantly reduced. Due to shrinkage, not only adhesion, but also adhesion can decrease: due to high shrinkage, there is a high probability that shrinkage cracks will appear in the contact zone, which affects the weakening of adhesion.
If stripping of a monolithic concrete structure is required, then three methods are now available, thanks to which tearing is carried out removable formwork:
high cohesion index and low adhesion index. In this situation, it is necessary to tear off the formwork along the contact plane;
the level of adhesion exceeds cohesion. The formwork will be torn off using a material that is adhesive (concrete);
approximate equivalence between adhesion and cohesion. This situation involves a separation of a mixed (combined) type.
The first option is the most optimal, since it allows you to easily remove the formwork, keeping its surface clean, and also preserve the quality of the concrete itself. In this regard, adhesive separation should be ensured more often than others. It is available in the following situations:
when the forming surface is made of a smooth material that is poorly wetted;
the forming surface was treated with a special lubricant or special anti-adhesive coatings.
Form release agent must meet the following requirements:
after its use, no oil stains should be left on the concrete surface;
the contact layer of concrete should not become less durable;
high level fire safety;
should not be included in the composition volatiles, which are dangerous to human health;
the ability to stay on a surface (vertical and horizontal) for 24 hours at an air temperature of +30 degrees Celsius.
Hello dear readers! Master Vadim Alexandrovich answers all our and your questions today. Today we will talk about the features of pouring concrete into formwork.
Hello Vadim Alexandrovich!
Hello! First of all, I want to say that this work is quite complex and very responsible, both pouring the floors and load-bearing walls It’s better to leave it to professionals than to try to do it yourself. Let's get started with your questions.
1. Is it necessary to prepare the formwork and reinforcement in any way?
The formwork is lubricated with a special water-based lubricant (Emulsol) in order to separate the formwork from the hardened concrete. Although at a construction site there were cases when they poured it into ungreased formwork and then tore it off. The formwork is also tightened with special ties, which are inserted into the tubes between the panels.
2. Is the method of filling horizontal forms different from vertical ones?
Almost no different. Vertical ones are a little more difficult to compact.
3. Please tell us how to pour concrete.
The method of pouring is determined by the project (TKP). It is advisable to pour the entire formwork at once; pouring in layers is undesirable, otherwise you will have to make notches with a hammer drill for better adhesion of the layers. Vertical forms must be filled entirely.
4. How to connect layers if we still fill them with layers? Well, we didn’t have enough concrete to fill it entirely.
As I already said, we make notches with a hammer drill on the hardened concrete.
5. What are the secrets of uniform filling?
There are no secrets, there are general rules: We pour it in different places and not in one place, spread it with shovels over the entire form, then compact it with a vibrator until it has a smooth, glossy surface in order to remove all voids and the concrete evenly fills the formwork. However, if the concrete is of poor quality, but it really needs to be poured, then you cannot use a vibrator - all the water will leak out and the concrete will not set. In this case, you just need to knock on the formwork. But try to avoid such cases - build for yourself.
6. How does the thickness of the solution affect the pouring?
A thick solution is difficult to evenly distribute and compact. Before pouring, you need to add water to the mixer. Too liquid - and again it’s bad; when compacting, all the water will flow out and the concrete will not set. If we do it ourselves, then we add cement and sand; if they bring it to us ready-made, then we send it to the factory due to non-compliance.
7. I heard that concrete heats up when it hardens. Is this a problem and do we need to deal with it?
Yes, this is a problem and needs to be dealt with. In hot weather it is necessary to water the formwork cold water, otherwise the concrete will crack. And in cold weather, on the contrary, we warm it up.
8. If we didn’t pay attention and the concrete cracked, how to fix it?
Small cracks are acceptable maximum size cracks indicated in project documentation, if the size is exceeded, then take a jackhammer and beat it off. Otherwise it will fall apart on its own 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 to you.
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10. DEFECTS IN MONOLITHIC REINFORCED CONCRETE STRUCTURES CAUSED BY VIOLATION OF THEIR CONSTRUCTION TECHNOLOGY
The main violations of the work production technology leading to the formation of defects in monolithic reinforced concrete structures include the following:
- production of insufficiently rigid, highly deformed when laying concrete and insufficiently dense formwork;
- violation of the design dimensions of structures;
- poor compaction of the concrete mixture when laying it in the formwork;
- laying stratified concrete mixture;
- use of too hard concrete mixture with thick reinforcement;
- poor care of concrete during its hardening process;
- use of concrete with a strength lower than the design strength;
- non-compliance with the structural reinforcement design;
- poor-quality welding of reinforcement joints;
- use of heavily corroded reinforcement;
- early demoulding of the structure;
- violation of the required sequence of stripping vaulted structures.
The production of insufficiently rigid formwork, when it receives significant deformations during the laying of the concrete mixture, leads to large changes in the shape of reinforced concrete elements. In this case, the elements take on the appearance of strongly bent structures, and vertical surfaces acquire convexities. Deformation of the formwork can lead to displacement and deformation of reinforcement frames and meshes and a change in the bearing capacity of the elements. It should be borne in mind that the dead weight of the structure increases.
Loose formwork promotes leakage cement mortar and, in connection with this, the appearance of shells and cavities in concrete. Sinks and cavities also arise due to insufficient compaction of the concrete mixture when it is laid in the formwork. The appearance of sinkholes and cavities causes a more or less significant decrease in the load-bearing capacity of elements, an increase in the permeability of structures, promotes corrosion of reinforcement located in the zone of sinkholes and cavities, and can also cause the reinforcement to pull through in concrete.
Reducing the design dimensions of the cross-section of elements leads to a decrease in their load-bearing capacity, while an increase leads to an increase in the dead weight of the structures.
The use of stratified concrete mixture does not allow obtaining uniform strength and density of concrete throughout the entire volume of the structure and reduces the strength of concrete.
The use of too hard a concrete mixture with dense reinforcement leads to the formation of cavities and cavities around the reinforcing bars, which reduces the adhesion of the reinforcement to the concrete and creates the risk of corrosion of the reinforcement.
When caring for concrete, temperature and humidity conditions should be created that would ensure that the water necessary for cement hydration is retained in the concrete. If the hardening process takes place at a relatively constant temperature and humidity, the stresses arising in the concrete due to changes in volume and caused by shrinkage and temperature deformations will be insignificant. Typically, concrete is covered with plastic film or other protective coating. It is also possible to use film-forming materials. Concrete maintenance is usually carried out within three weeks, and when using concrete heating - after its completion.
Poor maintenance of concrete leads to overdrying of the surface of reinforced concrete elements or their entire thickness. Overdried concrete has significantly less strength and frost resistance than normally hardened concrete; many shrinkage cracks appear in it.
When concreting in winter conditions with insufficient insulation or heat treatment, early freezing of the concrete may occur. After thawing, such concrete will not be able to gain the necessary strength. The final compressive strength of concrete subjected to early freezing can reach 2-3 MPa or less.
The minimum (critical) strength of concrete that provides the necessary resistance to ice pressure and subsequent preservation at positive temperatures of the ability to harden without significant deterioration in the properties of concrete is given in Table. 10.1.
Table 10.1. The minimum (critical) strength of concrete that concrete must acquire by the time of freezing (available only when downloading full version books in Word doc format)
If all the ice and snow were not removed from the formwork before concreting, then sinkholes and cavities appear in the concrete. An example is the construction of a boiler house in permafrost conditions.
The boiler room was based on a monolithic reinforced concrete slab, into which the heads of piles immersed in the ground were embedded. A ventilated space was provided between the slab and the soil to insulate the soil from heat penetrating through the boiler room floor. Reinforcement outlets were made from the top of the piles, around which ice formed, which was not removed before concreting. This ice melted in the summer and the base slab of the building was supported only by the outlets of the reinforcement from the piles (Fig. 10.1). The reinforcement outlets from the piles were deformed under the influence of the weight of the entire building and the base slab received large uneven settlements.
Rice. 10.1. Diagram of the states of the monolithic base slab of the boiler room (a - during concreting; b - after the ice remaining in the formwork has melted): 1 - monolithic slab; 2 - ice left in the formwork; 3 - pile reinforcement; 4 - pile (available only when downloading the full version of the book in Word doc format)
Inconsistency with the design of the strength of concrete and reinforcement of structures, as well as poor-quality welding of reinforcement outlets and intersections of rods, affects the strength, crack resistance, and rigidity of monolithic structures, as well as similar defects in precast reinforced concrete elements.
Minor corrosion of the reinforcement does not affect the adhesion of the reinforcement to the concrete, and, consequently, the operation of the entire structure. If the reinforcement is corroded in such a way that the corrosion layer peels off from the reinforcement upon impact, then the adhesion of such reinforcement to concrete deteriorates. At the same time, along with a decrease in the bearing capacity of elements due to a decrease in the reinforcement cross-section due to corrosion, an increase in the deformability of the elements and a decrease in crack resistance are observed.
Early stripping of structures can lead to complete unsuitability of the structure and even its collapse during the stripping process due to the fact that the concrete has not gained sufficient strength. The stripping time is determined mainly by temperature conditions and the type of formwork. For example, the formwork of the side surfaces of walls and beams can be removed much earlier than the formwork of the lower surfaces of bending elements and the side surfaces of columns. The last formwork can be removed only when the strength of the structures is ensured against the influence of its own weight and temporary load acting during the period construction work. According to N.N. Luknitsky, removal of the formwork of slabs with a span of up to 2.5 m can be carried out no earlier than the concrete reaches 50% of the design strength, slabs with a span of more than 2.5 m and beams - 70%, long-span structures - 100%.
When stripping vaulted structures, the circles at the lock must first be released, and then at the heels of the structure. First release the manger at the heels, then the vault will rest on the circles in its locking part, and the vault is not designed for such work.
Currently, monolithic reinforced concrete structures have become widespread, especially in multi-storey housing construction.
Construction organizations, as a rule, do not have the appropriate formwork and rent it. Renting formwork is expensive, so builders reduce its turnover period as much as possible. Usually stripping is done two days after laying the concrete. At this rate of construction of monolithic structures, particularly careful study of all stages of work is required: transporting the concrete mixture, laying concrete in the formwork, preserving moisture in the concrete, heating the concrete, insulating the concrete, monitoring the heating temperature and the strength gain of the concrete.
To reduce the negative impact of concrete temperature differences, you should choose a minimum permissible temperature heating concrete during stripping.
For vertical structures(walls) concrete heating temperature can be recommended 20°C, and for horizontal (floors) - 30°C. In the conditions of St. Petersburg, for two days the average air temperature does not reach 20°C and, especially, 30°C. Therefore, concrete should be heated at any time of the year. Even in April and October, the author was not able to see the heating of concrete at construction sites.
IN winter time Floor concrete should be insulated when heated by laying it on top polyethylene film layer effective insulation. And in many cases this is not done. Therefore, floor slabs concreted in winter have a concrete strength on top that is 3-4 times less than on the bottom.
When stripping the formwork, a temporary support is left in the middle of the section of the floor slab in the form of a stand or section of formwork. Also, temporary supports should be installed before stripping strictly vertically across floors, which is also often not observed.
Since the strength of the concrete walls during stripping does not reach the design value, it is necessary to make an intermediate calculation to determine the number of floors that can be erected in winter.
There is a large shortage of instructional literature on monolithic reinforced concrete, which affects its quality.
