K category: Concrete works

Measures to reduce the adhesion of concrete to formwork

The adhesion force of concrete to formwork is influenced by adhesion (sticking) and shrinkage of concrete, roughness and porosity of the surface. With a high adhesion force between concrete and formwork, the work of stripping becomes more complicated, the labor intensity of the work increases, the quality of concrete surfaces deteriorates, and the formwork panels wear out prematurely.

Concrete sticks to wood and steel formwork surfaces much more strongly than to 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 weakly wetted materials (for example, textolite, getinax, polypropylene) the adhesion of concrete is several times lower.

Therefore, to obtain surfaces High Quality You should use cladding made of textolite, getinax, polypropylene, or use waterproof plywood treated with special compounds. When adhesion is low, the concrete surface is not disturbed and the formwork comes off easily. As adhesion increases, the concrete layer adjacent to the formwork is destroyed. This does not affect the strength characteristics of the structure, but the quality of the surfaces is significantly reduced. Adhesion can be reduced by applying aqueous suspensions, water-repellent lubricants, combined lubricants, and concrete retarding lubricants to the surface of the formwork. The principle of operation of aqueous suspensions and water-repellent lubricants is based on the fact that on the surface of the formwork a protective film, which reduces the adhesion of concrete to formwork.

Combined lubricants are a mixture of concrete set retarders and water-repellent emulsions. When making lubricants, sulfite-yeast stillage (SYD) and soap naft are added to them. Such lubricants plasticize the concrete of the adjacent area, and it does not collapse.

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

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

- Measures to reduce the adhesion of concrete to formwork

The amount of adhesion between concrete and formwork reaches several kgf/cm2. 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 to demonstrate adhesion. 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. Kc values ​​for different types formworks are equal to: small-panel - 0.15, wooden - 0.35, steel - 0.40, large-panel (panels made of small panels) - 0.25, large-panel - 0.30, volumetric-adjustable - 0.45, for block - forms - 0.55.
Wood, plywood, untreated steel and fiberglass are well wetted and the adhesion of concrete to them is quite large; concrete has little adhesion to weakly wettable (hydrophobic) getinax and textolite.
The contact angle of ground steel is greater than that of untreated steel. However, the adhesion of concrete to polished steel is reduced slightly. This is explained by the fact that at the interface between concrete and well-treated surfaces the contact continuity is higher.
When an oil film is applied to the surface, it becomes hydrophobized, which sharply reduces adhesion.
The roughness of the formwork surface increases its adhesion to concrete. This occurs because a rough surface has a larger actual contact area compared to a smooth surface.
The highly porous formwork material also increases adhesion, since the cement mortar, penetrating into the pores, forms reliable connection points during vibration compaction. 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 is greater 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 separation, the formwork is easily removed, its surface remains clean, and the concrete surface has 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.
Aqueous suspensions of powdery substances, inert to concrete, are simple and cheap, but not always effective means to eliminate the adhesion of concrete to the formwork. The principle of operation is based on the fact that as a result of the evaporation of water from suspensions before concreting, a thin protective film is formed on the forming surface of the formwork, which prevents the adhesion of concrete.
Most often, a lime-gypsum suspension is used to lubricate formwork, which is prepared from semi-aqueous gypsum (0.6-0.9 parts by weight), lime paste (0.4-0.6 parts by weight), sulfite-alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).
Suspension lubricants are erased by the concrete mixture during vibration compaction and contaminate concrete surfaces, as a result of which they are rarely used.
The most common water-repellent lubricants are based on mineral oils, EKS emulsol or salts fatty acids(soap). After their application to the surface of the formwork, a hydrophobic film is formed from a number of oriented molecules, which impairs the adhesion of the formwork material to concrete. The disadvantages of such lubricants are contamination of the concrete surface, high cost and fire hazard.
The third group of lubricants uses the properties of concrete to set slowly in thin butt layers. To slow down setting, molasses, tannin, etc. are added to the lubricants. The disadvantage of such lubricants is the difficulty of regulating the thickness of the concrete layer.
The most effective are combined lubricants that use the properties of forming surfaces in combination with retarding the setting of concrete in thin butt layers. Such lubricants are prepared in the form of so-called reverse emulsions. In some of them, in addition to water repellents and set retarders, plasticizing additives are introduced: sulfite-yeast stillage (SYD), soap naft or TsNIPS additive. During vibration compaction, these substances plasticize the concrete in the butt layers and reduce its surface porosity.
ESO-GISI lubricants are prepared in ultrasonic hydrodynamic mixers, in which mechanical mixing of the components is combined with ultrasonic mixing. To do this, pour the components into the mixer tank and turn on the mixer.
The ultrasonic mixing unit consists of circulation pump, suction and pressure pipelines, distribution box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The liquid supplied by the pump under overpressure 3.5-5 kgf/cm2, flows out of the vibrator nozzle at high speed and hits the wedge-shaped plate. In this case, the plate begins to vibrate at a frequency of 25-30 kHz. As a result, zones of intense ultrasonic mixing are formed in the liquid with the simultaneous division of components into tiny droplets. Mixing duration is 3-5 minutes.
Emulsion lubricants are stable; they do not separate within 7-10 days. Their use completely eliminates the adhesion of concrete to the formwork; they adhere well to the forming surface and do not contaminate the concrete.
These lubricants can be applied to the formwork using brushes, rollers and spray rods. At large quantities shields, a special device should be used to lubricate them.
The use of effective lubricants reduces harmful effects on the formwork of some factors. 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.
Anti-adhesive agents have a good effect 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. This coating completely eliminates the adhesion of concrete to the formwork and can withstand up to 35 cycles without repair.
Despite their rather high cost, anti-adhesive protective coatings are more profitable than lubricants due to their multiple turnover.
It is advisable to use panels 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

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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 formwork when laying concrete and insufficiently dense formwork;
- violation of the design dimensions of structures;
- bad seal concrete mixture when laying it in 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 shape reinforced concrete elements. In this case, the elements take on the appearance of strongly bent structures, vertical surfaces acquire bulges. Deformation of the formwork can lead to displacement and deformation reinforcement cages and grids and changes bearing capacity 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 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. 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 has melted in summer time and the base slab of the building turned out to be 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 temperature conditions and 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 received widespread monolithic reinforced concrete structures, 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 formwork.
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, floor concrete should be insulated when heated by laying it on top polyethylene film layer of 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.

The amount of adhesion between concrete and formwork reaches several kgf/cm2. 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. Anti-adhesive protective coatings based on polymers have a good effect. 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 are not pre-treated, 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 of 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 presupposes 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;
the composition should not contain volatile substances that are hazardous to human health;
the ability to stay on a surface (vertical and horizontal) for 24 hours at an air temperature of +30 degrees Celsius.