CS270933B1 - Method of concrete mixtures consolidation during concrete elements production - Google Patents

Abstract

The method of compacting the concrete mix in concrete production is by a vacuum vibration, in which the fresh mix is first subject to the vacuum treatment and then under the last vacuum, it is subject to dynamic compacting, and within further compacting, it is subject to an area load (thrust) by increasing the pressure in a low-pressure chamber. This method allows production of a concrete with surprisingly great strength.<IMAGE>

Description

(57) A method of compacting concrete mixtures in the manufacture of concrete parts by vacuum-lining, in which the fresh mixture is first subjected to a vacuum, then subjected to a dynamic compaction during continuous vacuum and subjected to a flat pressure during subsequent compaction by increasing the pressure in the vacuum chamber. The method makes it possible to produce surprisingly high-strength batons.

(11), (13) Bl (51) Int. Cl. ⁵ В 28 В 1/08

The invention relates to a method of compacting concrete mixtures in the manufacture of concrete members in a forming apparatus using vacuum, vibration and atmospheric down pressure processes. In the technology of production of concrete structures and later in the production of concrete components, effective methods of processing concrete mixtures were sought, which would ensure the effective formation of the structure of concrete. It was therefore generally sought to achieve the highest possible density of the concrete mixture components during the molding process, thus ensuring maximum conditions for utilizing the bond strength of the concrete binders used. Higher concrete density thus allows either the improvement of the quality of the products or, on the contrary, the use of economical doses of binders, eg cement in the composition of the concrete mixture, to achieve the desired grade of concrete. Therefore, a variety of processes have been applied in the production of concrete products by applying dynamic effects or even pressure on the concrete mixture, i.e. different modes of vibration, impact shaking or compression of the concrete mixture into molds or combinations thereof, e.g. high levels of compaction and density of the concrete as well as its good shaping and settling to the walls of the mold. This should also achieve high product strength, durability and relatively good smoothness of the parts after demolding, while reducing the number of surface air bubbles and eliminating the overall structure of the product.

such procedures are specified, for example, in patents;

- ČSSR Author's Certificate No. 199419 specifying the method of compacting concrete mixtures with lower vibrations using vertical harmonics 12 to 35 Hz;

- USSR Author's Certificate No. 1 207 772 A presenting an efficient method of forming large parts of rigid concrete mixtures, applying a 50 Hz lower vibration, 0.5 mm amplitude and subsequent double pressure pulses to a vibratory press for 400 and 500 mm respectively. When dosing 580 kg of cement grade. The M 500 per concrete achieves a strength of up to 78 MPa with a concrete density of up to 99%.

However, the above-mentioned methods of compacting concrete by dynamic effects or pressure do not, however, make it possible, in particular for concrete mixtures, to achieve a stiffer consistency, which would be optimal in terms of cement utilization and in terms of concrete properties. The concrete mixture contains, in addition to binders and fillers, water and a large amount of air, which can reach up to 35% by volume of the concrete mixture with a solid consistency. The air in the structure of the concrete causes a significant reduction in quality and, as an undesirable component, it must be removed during compaction of the concrete by compaction. However, especially for tougher concrete mixtures, the existing systems and procedures for vibrating, shaking or crushing the concrete fail to ensure that the components of the concrete mixture are efficiently liquefied or displaced efficiently so that all residual air bubbles, eg wedged between aggregate grains, , thus achieving a full concrete structure without these failures.

Control calculations and laboratory concrete fins show that fresh concretes made from mixtures with lower water content, worse workability, using all the existing Intensive compaction processes, contain 2 to 5 or more volumetric air prooents contained in the volume of concrete, preferably 1% (e.g., said Soviet invention). Although higher compaction values can be obtained for concrete mixtures of soft to liquid consistency, they obtain better workability due to the increased water content of the concrete mixture. At the same time, water causes higher cementitious porosity, unevenness of the concrete structure and lower quality in terms of strength. These deficiencies can be compensated in part only by higher amounts of cement, which is economically disadvantageous.

Another way to achieve better workability of concrete and those plasticizers that reduce the surface tension of the water and its components of the concrete mix. While this enables higher and good product shaping, these processes also require chemical additives, where at the same time in the field of energy-saving mixtures of plastic or rigid con- crete concrete structures, the application of internal coefficient of cement concentration is increased. does not apply.

In an effort to overcome the difficulties in compaction and forming of concrete, the existing force and dynamic compaction procedures were supplemented by procedures of concrete vacuuming, which in principle brought new possibilities. Generally known are procedures of so-called concrete vibrovacuation. These allow good molding of complex products using well-workable concrete shifts with excess water, whereby a sudden vacuum of the concrete through the contact filtration and suction system reduces the water content of the concrete by suction and some compaction of the structure is achieved.

The disadvantage of vibrovacuation is in particular the demands on filtration systems of technological equipment, long vacuum time corresponding to low speed of water evacuation from concrete depth, ie approx. 5 to 10 mm / min. Despite the increase in the strength of the concrete treated by this process by appearance to the original composition, ie the water / cement ratio (v / c) of the concrete mix used, the obtained concrete structure remains largely porous with open capillarity. The density of concrete in terms of the newly obtained ratio v / c of concrete due to vacuum is very low due to the residual air content of the concrete. On the other hand, compaction of the structure resulting from depressions inside the concrete capillaries also causes the shrinkage of the concrete mass during vacuuming, and thus the possible formation of cracks, eg around mold inserts, reinforcement, etc., which is undesirable. Therefore, revibrations have been introduced which partially eliminate this phenomenon, but enforce efficient vibration systems capable of reshaping fresh concrete with already stiffer consistencies achieved by previous vacuum of concrete. As a result, the original advantage, i.e. to work only with easily workable concrete, which, after forming the element under vacuum action, changes the properties corresponding to a higher v / c, is lost.

Another method of forming and compacting concrete by means of vacuum is a process known as implosive concrete forming. In this method, the concrete mixture of soft to liquid consistency is thrust by atmospheric pressure into the evacuated mold space. The concrete mixture is compacted by impact and at the same time fills the mold tightly. There are other steps, such as water extraction, but the excess water is harmful and the relative cement consumption is too high.

A method of vibroforming in a vacuum chamber is also known, wherein the concrete mixture in the mold is subjected to simultaneous vacuuming, vibration and external pressure inside the chamber by a pressing plate which is pressed by atmospheric pressure.

However, the sealing plate of the movable plate causes considerable technical difficulties. Furthermore, with a high downforce, the mobility of the inner grains of the Fresh Concrete is greatly reduced and the downforce compaction is prematurely worsened. Furthermore, the outer element is often disturbed during demolding.

A common drawback of, inter alia, the methods of compacting concrete mixtures under reduced pressure conditions is that the deaeration of the concrete mixture is not as perfect as it would correspond to the relative pressure reduction at which compaction is carried out. The air in the concrete mixture is thinner, eg 10 times than at atmospheric pressure, but physically and theologically this air behaves the same in compaction, and therefore the same percentage of failures in fresh concrete will remain as in the case of concrete compaction under normal atmospheric pressure conditions. If the concretes thus treated are nevertheless of higher quality than concretes produced by conventional processes, the improvement is due to the partial densification of the structure and the reduction of the porosity of the concrete caused by internal depression of the residual percentage of air enclosed in the concrete. active compaction process, the principle of partial compaction due to capillary shrinkage. In terms of compaction, especially in the case of economical mixtures, the consistency of the concretes produced is of 0.95 to 0.905, which is not very satisfactory due to the compaction limits.

CS 270933 Bl

SUMMARY OF THE INVENTION The method of compacting concrete mixtures is carried out by a combination of vacuum, vibration and pressure increase processes such that in the first stage the concrete mixture is vacuumed to a vacuum value of at least 30 kPa for a period of 1 s to 10 min; it is vibrationally compacted, and during the vibrational compacting, the pressure of the surrounding air acting on the compacted concrete is increased during the third phase.

In comparison with the currently unsatisfactory methods of concrete compaction, which are essentially the basic principles of concrete elements production, the present process of concrete mixtures can achieve an exceptional value of density, concentration and utilization of cement putty and thus excellent properties of concrete or economics of concrete elements production. The method of compacting concrete mixtures according to the general definition of clause 1 of the invention relates to the processing of concrete mixtures in the manufacture of prefabricated concrete parts, allowing the processing of economical, worse-workable mixtures of tougher consistency, ie.

The aim of the process is to reduce as much as possible the proportion of residual air in the concrete after compaction, whose harmfulness to the concrete properties is obvious for concretes of common applications, and at the same time to achieve maximum composition densities of other concrete components with limited cement consumption.

Schematically, the basic concrete compaction process is shown in the drawing. Compaction of the concrete mixtures according to the invention is the vacuuming of the concrete mixture for a period of time 1, then subjecting the mixture to vibrations, shaking and the like in time periods 2, 6, during the final phase 3 of these dynamic effects of concrete compaction. pressure reduced by previous vacuum.

The whole process of concrete processing is carried out in an enclosed space sealed against the external atmospheric pressure p1, pressure controlled through the shutters leading to a pressure reducing device, i.e. a vacuum pump, or to a pressure increase, which can be easily ensured, for example.

The first phase χ of the concrete mixture evacuation process causes the gas component to be released from the mixture, especially air and later also vaporized water vapors of low partial pressure if the concrete mixture is evacuated with higher vacuum values relative to the external atmospheric air pressure. concrete exists.

The first stage 1 of the process is a prerequisite for limiting the amount of residual air in the concrete to be treated, and a higher air dilution in the concrete mixture is beneficial in this respect.

The field of the mentioned concrete mixtures of stiffer consistency belongs to the area of concrete mixtures in a high degree of voids, in general up to 35 or more prooent for mixtures of workability around 30 in VeBe. The voids allow rapid exhaustion of air mass from the entire volume of the concrete mixture without significant unevenness of the rate of aspiration from the cross-section of the mixture and without the transport phenomena of increased water evacuation from the mixture, which is not the aim of the procedure.

Vacuum mixtures can therefore be advantageously carried out in a non-contact way under the vacuum bell, etc. Due to the requirement for easy release of the gas phase from the mixture, it is intentional to pre-compact the concrete mixture before vacuuming in principle undesirable. Pre-compacting caused by the transport of mixtures or the like before the concrete elements are formed can only be accepted to the extent that the vacuum of the volume of the concrete mixture is even and does not cause changes in the water distribution in the mixture, ie undesirable differences in workability of the mixture.

The next step 2 of the concrete compaction process according to the invention is the densification of the components of the concrete mixtures under the influence of dynamic influences, eg vibrations, shaking, etc.

The choice of these parameters is essentially analogous to conventional dynamic compaction techniques under normal conditions under the influence of existing atmospheric pressure. The effect of these dynamic influences on their own and in conjunction with the evacuation performed4 still has a decisive effect on a higher concrete density than normal, ie if the compaction process were terminated at this stage of the process.

The desired final effect of the obtained high concrete density is achieved by incorporating the third step 3 of the so-called compaction pressure pulse process. In the final phase 3 of the dynamic concrete compaction processes used, which allow the concrete to flow and allow gravity to initially densify the concrete components, an increase in the pressure on the concrete is introduced during these processes by increasing the ambient atmospheric pressure reduced by previous vacuum. Due to the simultaneous liquefaction of the concrete, this pressure is transferred to the whole volume of fresh concrete and causes both the condensation of water vapor in the concrete and the permanent compression of the residual air bubbles inside the concrete.

The results of technological tests of these procedures showed an extraordinary density of concrete, around even over 2,500 kg / m ³ of concrete weight, while at the same time control calculations confirmed zero air content in the volume of fresh concrete. The absence of air bubbles was confirmed in comparison with the usually treated concretes on both the cuts and the surface of the concrete test elements.

The level of positive increments of concrete compaction, as compared to the conventional concrete compaction level, achieved according to the known compaction methods, can generally be different but positive due to the possible variability of the intrinsic parameters of the compaction process steps according to clause 1 of the invention. However, in order to achieve higher or maximum concrete compaction effects, it has been found appropriate to select partial parameters of the technological steps by compaction procedures within the following limits. ___________________ _________ ________

When the initial vacuum of the concrete mixture is carried out as a first step of said compaction processes generally to a higher final vacuum value, the achievable value of the concrete compaction rate is also generally higher. Vacuum of concrete mixtures carried out to an absolute pressure of at least 30 kPa is already a clear technological limit, ensuring that the increments of density, reinforcement and other required properties of concrete will be significantly influenced by the concrete processing by the given process. When concrete mixtures are used, which are already at normal or slightly elevated temperatures, even around 25 ° C, moisture at the concrete mixture gradually evaporates at an absolute pressure of 30 kPa. Evaporation occurs when the pressure is reduced on a portion of the energy-unbalanced water molecules before the boiling point of the water is reached, and at the same time is supported by the movement of air within the mixture, which is still in a gap-like state. At the level of lower vacuum values, absolutely around 30 kPa, besides diluting the air in the gaps of the concrete mixture, it is partially replaced with water vapor, which partially or locally displaces air from the concrete mixture. During further gradual compaction of the trapped bubbles in the concrete structure, when they are formed by steam, they have the possibility of back condensation when the pressure is increased in the final stage of the concrete structure by vibrations or the like. This is manifested by the fact that at a given location, when the pressure increases, the bubble not only shrinks as the concrete vibrates, but is eliminated completely. This phenomenon is positively reflected in the integrity of the concrete structure.

This principle is consistently developed in such a way that the initial vacuum of the concrete mixture is recommended to be carried out up to a vacuum value that is less than or equal to the water vapor pressure corresponding to the initial temperature of the concrete mixture. When this condition is met boiling water and practically complete displacement of air from the concrete mixture and its replacement by saturated steam of low partial pressure. It should be noted that the necessary amount of water converted by evaporation to completely fill the gaps of the concrete mixture with saturated water vapor is very low. Under vacuum reduced pressure conditions in the concrete mixture, at an absolute pressure value of eg 10 kPa (- approx. only about 15 g / m ³ of concrete weight. Although the actual values will be different, but not of the order of magnitude, this value is only less than about 1/100% of the usual amount of mixing water contained in 1 weight of concrete. Using the principle of total displacement of air from the mixture by the generation of steam within the volume of the mixture, it is necessary to take into account the laws of temperature and partial pressures of saturated water vapor.

At a temperature of, for example, 45 ° C, the steam pressure of steam is 9.6 kPa, at 35 ° C the partial pressure is 5.6 kPa, at 25 ° C the partial pressure is 3.2 kPa.

The application of these parameters ensures that the effective further compaction process of the concrete mixture in the sense of the above-mentioned methods will reliably achieve the full structure of fresh concrete without any bubbles and remains of enclosed air in its structure.

Desired vacuum values are recommended to be achieved within the concrete mixture within 120 s. Apart from shortening the working cycle of the concrete processing, the importance of a higher concrete vacuum rate is that in the case of a lower void volume of the mixture, resp. Its higher tackiness or larger layers of concrete, where the permeability of the concrete mixture cavities to the vacuum is partially limited, the rapid start-up of the vacuum causes differential pressures inside and outside the concrete mixture which can overcome the cohesion of the putty. Conversely, with slow vacuuming of the concrete mixture, air extraction takes place by means of a minimum of permeability of channels and capillaries in the concrete mixture, which may cause undesirable local changes in the workability of the concrete. This change may occur, for example, due to the capillary transport of a portion of the water in the closed air ejected mixture, or due to local evaporation of moisture from the collecting duct walls within the mixture where high flow rates and long-term surface drying occur locally. However, there is no risk of local variations in the workability of the concrete to be treated when there is sufficient surface area of the concrete mixture to be suctioned and a rapid vacuum depressurization process. The process thus contributes to achieving greater uniformity of structure, strength and other properties of the concrete produced. It should be noted that this type of failure, or causes of possible structural unevenness, is in itself considerably less severe than, for example, unevenness in the processing of concrete mixtures of soft to liquid consistency, which arises over the height of concrete elements, but generally accepted and they are also respected in the design standards of structural reinforcing concrete elements.

The amount of increased density is obtained by the concrete treated according to the present invention primarily during the compaction pulse, i.e. the last step of the compacting process. In this step, during the prolongation of the dynamic method of compacting the concrete by vibrations or the like. vacuum.

Parametrically, this process can be controlled by a number of technological variables, the value of the down pressure or the degree (%) of compensation of the generated negative pressure with respect to the existing value of the external atmospheric pressure, time, method and speed of pressure equalization. The optimization of these parameters, in view of the best compaction results, depends at the same time on the condition and use of the previous concrete processing parameters (temperature, pressure, etc.).

As to the effect of the pressure equalization speed during the compaction step 3, the thrust exerts an apparent interaction ee by the introduced vibration and the speed of introduction of this thrust is also decisive for the quality of the compaction achieved. According to the tests carried out, too slow the application of the downforce does not produce optimal results for the possible degree of concrete reinforcement (see example 2),

In this sense, it is recommended and reduced from above the pressure equalization time during the application of pressure during concrete compaction to less than 120 s. In terms of pressure value, the applied pressure during vibration can reach values close to 100 kPa without special measures. vacuuming the concrete mix of the processed concrete. It can be expected that a higher downforce ensures better compression of the residual bubbles of gases contained in the concrete than a lower downforce, and therefore ensures a higher degree of compaction of the concrete. However, these relationships are not linear and have not been confirmed by testing the need for a maximum degree of pressure equalization during compaction to achieve the maximum density, or the degree of concrete reinforcement, as evidenced by the results of the experiments. These facts are probably related to the ratio of the proportion of air residues and the proportion of vacuum, the water vapor produced inside the concrete mixture, which condenses back during the final compaction phase. The elimination of water vapor bubbles no longer requires an additional pressure increase for compression after they have exceeded the threshold value corresponding to the vapor condensation point, since they do not exist. On the contrary, the question of the size of the applied down pressure remains essential to limit the size of the residual air bubbles. Overall, however, the downforce is generally positive and applies dynamically together in the final stage of concrete compaction in vibration to overcome the friction values of the concrete components. Therefore, it is not appropriate to limit the pressure increase values in the final stage of concrete compaction excessively. Therefore, it is appropriate in the final compaction phase to introduce a concrete pressure by increasing the atmospheric pressure reduced by a previous vacuum of at least 40% of the existing pressure difference to the external atmospheric pressure. According to the tests carried out, the utilization of a thrust pressure of 60% of the given pressure difference yielded practically identical results of concrete quality as concretes produced by the application of a thrust pressure of 100%,

On the other hand, when the downforce was completely omitted, the vibration method, etc. was identical, the concrete strength was reduced by about 25% by about 4% by its bulk density. These concretes showed even a slight deterioration in quality compared to reference concrete samples compacted by a vibration-free process.

Example 1

Compaction of concrete mixtures of workability class 30 with VEBE with variable cement PC 400 ^{9 and} common technological parameters

- cement PC 400, real bond after 28 days pressure 42,8 MPa; thrust 7.0 MPa

- gravel 0/8 relative ratio of abs, volumes 42; 58

- drags 8/16 ·

- temperature of the concrete mixture; Deň: 22 ° C

compaction according to the invention;

Step 1; vacuum concrete mixture to abs, pressure 10 kPa

Step 2; concrete vibration is forced lower circular, time 150 s; frequency 50 Hz, effective acceleration value 2 g

Step 3; (compaction impulse)

- vibration ditto, time 60 s

- pressure in the range of 100% of the pressure difference, with an equalization time of 60 s

Table 1

Variables and control parameters of technology and concrete quality results

Marking of the test В 25 В зо В 35 В 45 cement dosage kgm “^ h.b. 250 зоо 350 450 v / c ratio О, 5бО 0.493 0.417 0.350 actual workability concrete, mixes in VeBe 32 28 32 33 concrete strength after 28 days MPa 50.4 54.5 65.9 69.1 hmotnoet volume of fresh BET ° ^nu _kgm -3 2 528 2 533 2 529 2 528

Example 2

Compaction of test concrete mixes of the vyrobovatelnoati class 30 with VeBe at dosages of 250 and 350 kg / π bh, b, cement PC 400 in variable time of compaction pressure application

Table 2

Test designation E 21 E 31 F 21 F 31 PC 400 cement dosing kgm ^ h, b. 350 250 cement bond - pressure MPa - thrust MPa 42.8 7.0 48.9 7.9 ratio of fractions of aggregates 0/8 and Θ / 16% 42 * 58 41 : 59 ratio v / c - 0.493 1 0.590 workability of concrete mix 8 VeBe 31 29 30 30 temperature of the mixture ° C 25 25 27 Mar: 27 Mar: compaction Step 1 - Vacuum kPa (absolute vacuum) 10 10 5.0 5.0 Step 2 - vibrate with (see note) 150 90 135 90 Step 3 - (impulse of concrete compaction) - vibration time в 60 120 75 120 - downforce% 100 ALIGN! 100 ALIGN! 80 100 ALIGN! - downforce insertion time s 60 120 75 120 concrete strength after 28 days MPa 63 ,! 58.1 54.3 51.5 density of fresh ^concrete kg / _m 3 hb 2 519 2 530 2 503 2 511

Note: used the lower circular forced vibration frequency 50 Hz, effect, acceleration values

- 2 g

The effects of compaction of concrete mixtures according to the present invention have been experimentally tested and compared with the theoretical assumptions of the current state of the art in concrete processing technology 8 with very good results. Using selected technological parameters of this concrete processing system, the compressive strength of 60 MPa was also experimentally achieved using concrete mixture containing only 250 kg of PC 400 cement per 1 weight of concrete.

Claims (4)

SUBJECT OF THE INVENTION 1. A method of compacting concrete mixtures containing cement binder, aggregate and water intended for the production of concrete members by a combination of vacuum, vibration and pressure increase processes, characterized in that in the first stage, the concrete mixture is vacuumed to a vacuum value of at least 30 kPa for a period of from 1 s to 10 min, in the second phase it is vibrationally compacted and during the vibrational compacting the pressure of the surrounding air acting on the compacted concrete is increased during the third phase. 2. The method according to claim 1, wherein the vacuum achieved by the concrete underpressure value is less than or equal to the water vapor partial pressure corresponding to the initial temperature of the concrete mixture. 3. A process according to claim 1, wherein the pressure equalization time during compaction is less than 90 e. 4. A process according to any one of claims 1 to 3, characterized in that the increase in ambient atmospheric pressure reduced by the previous vacuum is carried out by at least 40% of the pressure difference produced.