CN119841569A - Artificial defect aggregate and preparation method and application thereof - Google Patents

Abstract

The invention relates to the technical field of cement-based composite materials, in particular to an artificial defect aggregate, a preparation method and application thereof. The invention provides an artificial defect aggregate which is formed by drying a sizing agent covered on the surface of a basic aggregate, wherein the basic aggregate is at least one of slag, river sand, quartz sand, ceramsite and waste glass, and the sizing agent is prepared by mixing acrylic acid sizing agent and carbon nano tube powder. The concrete of the invention can reduce the remarkable reduction of compressive strength while introducing artificial defects to improve the stability of tensile property.

Description

Artificial defect aggregate and preparation method and application thereof

Technical Field

The invention relates to the technical field of cement-based composite materials, in particular to an artificial defect aggregate, a preparation method and application thereof.

Background

Concrete is one of the most commonly used engineering building materials at present, which has excellent compressive strength. However, because of the interface transition area between the aggregate and the matrix in the concrete, the tensile capacity of the concrete is weaker, and larger cracks are easy to occur, which further accelerates the corrosion of the reinforcing steel bars in the concrete and the failure of the whole structure. With the development of the construction of the infrastructure in China, in the practical application process, the durability of the concrete material and the service life under severe environments are more and more emphasized. In order to solve the problem, the fiber concrete is produced by taking clean slurry or mortar as a matrix and taking fibers as a reinforcing phase, the prepared concrete can well overcome the defects of low tensile strength, poor ductility and whole brittleness of the traditional concrete, and the width of cracks can be effectively reduced through bridging of the fibers, so that the durability of the concrete is further improved, and even the self-healing capacity of the concrete is endowed.

Currently, engineering cement-based composite materials (ECC), a special concrete material, is gaining increasing attention. The ECC has the characteristics of multi-joint cracking and tensile strain hardening, has ultrahigh ductility (more than 200 times of common concrete), and has great self-healing potential, and the crack width can be controlled below 100 micrometers in the stretching process. The concrete is very suitable for working conditions with complex stress conditions, high requirements on durability and severe service environments. However, the tensile properties of ECC, i.e. the multi-slit cracking phenomenon and ductility, have a large variability, mainly because in the ECC matrix, there is a difference in the dispersion of the fibers and the distribution of the original defects in the matrix, resulting in a large difference in the difficulty of cracking and bridging ability of the fibers at various parts of the matrix. When the ECC is broken under the action of external force, a cracking mode with larger difference appears on the matrix, so that the stability of ECC performance is seriously influenced, and the expression of high-ductility characteristics of the ECC in practical application is hindered. In order to solve the problem, the method commonly used at present is to add artificial defects in the matrix, so that the distribution of the defects in the matrix is more uniform, the cracking difficulty of each part of the matrix is consistent, which is beneficial to ECC to realize multi-slit cracking and even saturated multi-slit cracking, and more microcracks mean larger tensile elongation, namely tensile ductility. However, the reported artificial defects such as rubber particles, expanded shale, plastic pellets and the like all have obvious defects, namely, the compressive strength of the concrete is greatly weakened.

Therefore, there is a need to find an efficient artificial defect aggregate that weakens the compressive strength as little as possible while ensuring high ductility of the ECC.

Disclosure of Invention

First, the technical problem to be solved

In view of the above-mentioned shortcomings and disadvantages of the prior art, the present invention provides an artificial defect aggregate, a preparation method and an application thereof, which solve the technical problem that compressive strength is significantly reduced when tensile property stability of concrete is improved by introducing the artificial defect aggregate.

(II) technical scheme

In order to achieve the above purpose, the main technical scheme adopted by the invention comprises the following steps:

in a first aspect, the embodiment of the invention provides an artificial defect aggregate, which is formed by drying a basic aggregate and slurry covered on the surface of the basic aggregate, wherein the basic aggregate is at least one of slag, river sand, quartz sand, ceramsite and waste glass, and the slurry is prepared by mixing acrylic acid slurry and carbon nano tube powder.

Optionally, the mass ratio of the carbon nanotubes to the acrylic slurry is (0-3): 7, for example, 0:7, 1:7, 2:7 or 3:7.

Optionally, the mass ratio of the slurry to the base aggregate is (1-2): 50, for example 1:50, 1.5:50 or 2:50.

The action mechanism of the artificial defect aggregate is explained by two ways of causing the defect action of the artificial defect substance, wherein one way is low in strength or loose and porous, so that microcrack generation or crack penetration of the whole defect substance, such as expanded shale, porous ceramsite and the like, is facilitated, and the other way is that the defect substance has weaker interface binding force with a cement matrix, so that microcrack generation or crack expansion, such as rubber particles, plastic pellets and the like, is facilitated. In the process of reducing concrete cracking, the quantity of the introduced artificial defects is important, too few defects can not effectively reduce the cracking difficulty of the concrete, and too many defect substances can greatly weaken the compressive strength. The former is an effective part of the defects, and it is difficult to reduce the amount of the defects, while the latter is an effective part only at the interface with the cement matrix, but the defects themselves are largely inactive, so that the lower use efficiency of the defects also limits the reduction of the amount of the defects. In summary, the core idea of the present invention is formed by removing the part which does not exert the defect effect while retaining the effective part-weak interface of the second defect, and the inventor creatively adopts the technical scheme of applying the artificial defect substance to the surface of the aggregate. The aggregate is one of the raw materials of the concrete, is uniformly distributed in the cement matrix, has sufficient quantity, only weakens the interfacial binding force between the aggregate and the matrix, and can effectively weaken the cracking strength of the concrete, but under the condition of being pressed, the aggregate can still effectively play the role of a force transmission framework, so that the whole compressive strength is weakened less, and the doping quantity of defect substances can be greatly reduced while the defect effect is not influenced.

In a second aspect, an embodiment of the present invention provides a method for preparing the artificial defect aggregate, including the steps of:

S1, stirring and mixing carbon nano tube CNT with the mass ratio of (0-3) to 7 with acrylic slurry;

s2, adding the basic aggregate into the mixture of the S1, and stirring until the surface of the basic aggregate is coated with sizing agent;

and S3, drying the coated aggregate obtained in the step S2.

Optionally, the preparation method further comprises the step of sieving the dried coated aggregate in the step S3 with a 20-200 mesh sieve, and collecting undersize to obtain the artificial defect aggregate.

Optionally, the mass ratio of the carbon nanotubes to the acrylic slurry is (0-3): 7, for example, 0:7, 0.43:7, 1:7, 2:7 or 3:7.

Optionally, the mass ratio of the carbon nano tube to the acrylic slurry is (1-2): 7.

Optionally, S3 is dried by adopting an oven, the drying treatment temperature is 75-85 ℃ and the drying treatment time is 20-25h, and the aggregate is turned every 10-15min before drying for 1h, and then the drying is continued for 23h.

Optionally, stirring in the S1 at a speed of 1000-2000 r/min for 10-20 min.

Optionally, the mass ratio of the mixture to the base aggregate in S2 is (1-2): 50, for example 1:50, 1.5:50 or 2:50.

Optionally, the step S2 of uniformly coating the sizing agent on the surface of the aggregate is to judge according to the color of the aggregate, wherein the color of the aggregate is changed from white to black, namely the surface of the aggregate is uniformly coated with the sizing agent.

Optionally, stirring in the step S2 and drying in the step S3 are performed by adopting spraying equipment, wherein the drying temperature is 80-150 ℃ and the drying time is 0.5-1 h.

In a third aspect, embodiments of the present invention provide an artificial defect aggregate or an application of the artificial defect aggregate prepared by the preparation method in preparing concrete.

In a fourth aspect, embodiments of the present invention provide a concrete comprising an artificial defective aggregate.

In a fifth aspect, an embodiment of the present invention provides a method for preparing concrete, wherein a cement material and an artificial defect aggregate according to the first aspect of the present invention are mixed and stirred to obtain a mixture, then a water reducing agent and water are mixed, 1/2 to 3/4 of the volume of the mixture of the water reducing agent and water is added to the mixture, stirring is performed, then fibers are added to the mixture, and the remaining mixture of the water reducing agent and water is added, and finally stirring is performed to obtain the concrete.

Optionally, the water reducer is a polycarboxylate water reducer, and the fiber can adopt polyvinyl alcohol PVA or polyethylene PE.

(III) beneficial effects

The invention has the following beneficial technical effects:

(1) The invention provides an artificial defect aggregate, a preparation method and application thereof, and the prepared concrete can weaken the remarkable reduction of compressive strength when the tensile property stability is improved by introducing the artificial defect aggregate.

(2) Less introduction of artifacts

In the invention, the artificial defect substance (acrylic slurry and CNT) which plays a role of defect is coated on the surface of the aggregate to form a thinner defect layer, and the required quantity is smaller than that of directly doping the defect substance.

(3) Size control of artifacts

The artificial defect is prepared by adopting the method of coating the aggregate with the acrylic slurry, and the size of the whole artificial defect can be directly controlled by the size of the aggregate, so that an additional crushing process is not required.

(4) The artificial defect distribution is more uniform

According to the invention, the defect substances are coated on the surface of the aggregate, and in the process of preparing the concrete, the aggregate can be well distributed through a standard stirring flow, and at the moment, the uniform distribution of the defect substances is indirectly realized.

(5) Controllable artificial defect attribute

According to the invention, the attribute and the surface state of the defect layer of the aggregate can be changed by adjusting the content ratio of the acrylic slurry to the CNT, so that the defect degree of the defect aggregate can be adjusted. The CNT is hydrophobic and its hydrophobic nature makes it incompatible with cement hydration products, which determines the degree of interfacial bonding of the aggregate to the cement matrix.

Drawings

FIG. 1 is an electronic scan of an artificial defect aggregate of a control group of the present invention;

FIG. 2 is an electronic scan of an artificial defect aggregate according to example 1 of the present invention;

FIG. 3 is an electronic scan of an artificial defect aggregate according to example 2 of the present invention;

fig. 4 is an electronically scanned view of an artificial defect aggregate according to example 3 of the present invention.

Detailed Description

The invention will be better explained by the following detailed description of the embodiments with reference to the drawings.

Example 1

The preparation method of the artificial defect aggregate comprises the following steps:

S1, placing CNT powder and acrylic slurry in a mass ratio of 1:4 in a container, and stirring at a speed of 1000r/min for 10min;

S2, adding quartz sand into the mixture obtained in the step S1, wherein the mass ratio of the mixture to the quartz sand is 1:50, and the particle size of the quartz sand is 70-140 meshes;

S3, taking the quartz sand aggregate coated in the S2, drying at 80 ℃, turning the aggregate every 10min in the first 1h, and then continuing to dry for 23h;

S4, taking the coated quartz sand dried in the S3, sieving with a 50-mesh sieve, and collecting undersize materials to obtain the artificial defect aggregate;

The preparation method for obtaining the concrete by adopting the artificial defect aggregate comprises the following steps:

mixing cement, fly ash, silica fume and artificial defect aggregate, and stirring for 5min;

Mixing the water reducer with water, adding 3/4 of the water reducer into the mixture, and stirring for 5min;

slowly adding PE fiber, and adding the mixture of the water reducer and water which is left 1/4 of the water reducer and water for a plurality of times in the process;

finally, rapidly stirring for 5min to obtain concrete slurry;

And (5) molding and curing the concrete slurry to obtain the concrete.

The artificial defect aggregate is prepared into concrete, and the proportion is shown in table 1:

table 1 mix proportion design of concrete (kg/m³)

Example 2

The preparation method of the artificial defect aggregate comprises the following steps:

S1, placing acrylic slurry in a container;

S2, adding quartz sand into the S1, wherein the mass ratio of the acrylic slurry to the quartz sand is 1:50, and the particle size of the quartz sand is 70-140 meshes;

s3, taking the quartz sand aggregate coated in the S2, drying at 80 ℃, turning the aggregate every 10min in the first 1h, and then continuing to dry for 23h;

S4, taking the coated quartz sand dried in the S3, sieving with a 50-mesh sieve, and collecting undersize materials to obtain the artificial defect aggregate;

the preparation method of the concrete obtained by adopting the artificial defect aggregate is the same as that of example 1

Example 3

The preparation method of the artificial defect aggregate comprises the following steps:

S1, placing CNT powder and acrylic slurry into a container according to a mass ratio of 3:17, and stirring at a speed of 1000r/min for 10min;

S2, adding quartz sand into the mixture obtained in the step S1, wherein the mass ratio of the mixture to the quartz sand is 1:50, and the particle size of the quartz sand is 70-140 meshes;

S3, taking the aggregate coated in the S2, drying at 80 ℃, turning the aggregate every 10min in the first 1h, and then continuing to dry for 23h;

S4, taking the coated quartz sand dried in the S3, sieving with a 50-mesh sieve, and collecting undersize materials to obtain the artificial defect aggregate;

the preparation method of the concrete obtained by adopting the artificial defect aggregate is the same as that of example 1

Example 4

The preparation method of the artificial defect aggregate comprises the following steps:

s1, placing CNT powder and acrylic slurry in a mass ratio of 1:4 into a container, and stirring at a speed of 1000r/min for 10min;

s2, adding quartz sand into the mixture obtained in the step S1, wherein the mass ratio of the mixture to the quartz sand is 1.5:50, and the particle size of the quartz sand is 70-140 meshes;

S3, taking the aggregate coated in the S2, drying at 80 ℃, turning the aggregate every 10min in the first 1h, and then continuing to dry for 23h;

S4, taking the coated quartz sand dried in the S3, sieving with a 50-mesh sieve, and collecting undersize materials to obtain the artificial defect aggregate;

the preparation method of the concrete obtained by adopting the artificial defect aggregate is the same as that of example 1

Example 5

The preparation method of the artificial defect aggregate comprises the following steps:

s1, placing CNT powder and acrylic slurry in a mass ratio of 1:4 into a container, and stirring at a speed of 1000r/min for 10min;

S2, taking the mixture in the step S1, adding river sand, wherein the mass ratio of the mixture to the river sand is 1:50, and the particle size of the river sand is 70-140 meshes;

S3, taking the aggregate coated in the S2, drying at 80 ℃, turning the aggregate every 10min in the first 1h, and then continuing to dry for 23h;

S4, taking the dried coated river sand in the S3, sieving with a 50-mesh sieve, and collecting undersize materials to obtain the artificial defect aggregate;

the preparation method of the concrete obtained by adopting the artificial defect aggregate is the same as that of example 1

Example 6

The preparation method of the artificial defect aggregate comprises the following steps:

s1, placing CNT powder and acrylic slurry in a mass ratio of 1:4 into a container, and stirring at a speed of 1000r/min for 10min;

s2, adding slag into the mixture obtained in the step S1, wherein the mass ratio of the mixture to the slag is 1:50, and the grain diameter of the slag is 70-140 meshes;

S3, taking the aggregate coated in the S2, drying at 80 ℃, turning the aggregate every 10min in the first 1h, and then continuing to dry for 23h;

s4, taking the dried coating slag in the S3, sieving with a 50-mesh sieve, and collecting undersize materials to obtain the artificial defect aggregate;

the preparation method of the concrete obtained by adopting the artificial defect aggregate is the same as that of example 1

Example 7

The preparation method of the artificial defect aggregate comprises the following steps:

s1, placing CNT powder and acrylic slurry in a mass ratio of 1:4 into a container, and stirring at a speed of 1000r/min for 10min;

S2, taking the mixture in the step S1, adding ceramsite, wherein the mass ratio of the mixture to the ceramsite is 1:50, and the particle size of the ceramsite is 70-140 meshes;

S3, taking the aggregate coated in the S2, drying at 80 ℃, turning the aggregate every 10min in the first 1h, and then continuing to dry for 23h;

S4, taking the dried coated ceramsite in the S3, sieving with a 50-mesh sieve, and collecting undersize materials to obtain the artificial defect aggregate;

the preparation method of the concrete obtained by adopting the artificial defect aggregate is the same as that of example 1

Comparative example 1

Aggregate uncoated slurry

Mixing cement, fly ash, silica fume and aggregate, and stirring for 5min;

Mixing the water reducer with water, adding 3/4 of the water reducer into the mixture, and stirring for 5min;

slowly adding PE fiber, and adding the mixture of the water reducer and water which is left 1/4 of the water reducer and water for a plurality of times in the process;

finally, rapidly stirring for 5min to obtain concrete slurry;

And (5) molding and curing the concrete slurry to obtain the concrete.

The artificial defective aggregate of examples 1 to 3 and the untreated aggregate of comparative example were prepared to obtain concrete, respectively, were steamed at 75℃for 3 days, and were subjected to compressive strength and tensile test after being left for 7 days, and the test results are shown in Table 2.

TABLE 2 mechanical Properties of the concretes prepared in examples 1-3 and the blank

Comparative example 2

Coarse river sand was used for the artificial defect aggregate, and the same as in comparative example 1 was used.

Comparative example 3

Air glass microspheres are used for the artificial defect aggregate, and the same as in comparative example 1 is used.

Comparative example 4

The artificial defect aggregate adopts PE pellets, and the other materials are the same as in comparative example 1.

Comparative example 5

The artificial defect aggregate used Super Absorbent Polymer (SAP) and was otherwise as in comparative example 1.

Comparative example 6

The artificial defect aggregate was rubber particles, and the same as in comparative example 1.

Comparative example 7

The artificial defect aggregate was an oligomer aggregate (GPA) and was otherwise as in comparative example 1.

TABLE 3 Deltadelta/Deltasigma values for concretes prepared with artificial defects for comparative examples 2-7

Here, the ratio of the ultimate strain increase rate to the compressive strength increase rate is used to measure the efficiency of the artificial defect, i.e., Δδ/Δσ, wherein, it is to be noted that the negative sign indicates that the artificial defect may decrease the compressive strength of the concrete, and that the larger the absolute value of the value is, the more ultimate strain is brought about by the loss of the compressive strength per unit, that is, the higher the efficiency of the artificial defect is, and that the positive sign indicates that the artificial defect may increase the compressive strength of the concrete.

Table 2 summarizes the efficiency values for the artificial defect aggregates prepared in examples 1-3. As is clear from Table 2, the artificial defective aggregate represented by example 3 is excellent in effect, and can improve the compressive strength of concrete as well as the ultimate strain of concrete, and the artificial defective aggregate prepared by example 2 is higher in efficiency than that of example 1. The overall efficiency of the artificial defect aggregates of examples 1-3 of the present invention was better than the efficiency values of the artificial defects of comparative examples 2-7.

To demonstrate the differences in the artificial defect aggregates prepared in examples 1-3, three coated aggregates were microscopically characterized. Fig. 1 to 4 show scanning electron microscope images of the coated aggregate of the comparative examples and examples 1 to 3 of the present invention. FIGS. 1 to 4 show the state of the aggregate surface at three CNT/polymer ratios, and the more the amount of CNTs exposed on the aggregate surface increases with the increase of the CNT content, the worse the interface bonding force with the matrix, and the stronger the defect property. As can be seen from the figure, the difference in CNT content resulted in a large difference in morphology of the aggregate surface. In example 1, the CNT content was highest, and it was seen that the surface of the aggregate was almost covered with CNT, and the CNT was hydrophobic, which resulted in some hydrophobicity of the surface of the aggregate, thus decreasing compatibility of the aggregate with the cement matrix, and exerting the effect of the defective layer, whereas in example 2, no CNT was added, so that the surface of the aggregate was all acrylic paste, while the acrylic paste was hydrophilic, although it promoted hydration of the cement at the surface thereof, but the elastic modulus thereof was lower, and also exerted the effect of the defective layer, in example 3, the surface of the aggregate was provided with both CNT and acrylic paste, which resulted in the surface of the aggregate forming a partially hydrophilic and partially hydrophobic structure, and the presence of CNT also compensated for the defect that the elastic modulus of the acrylic paste was smaller, which resulted in some extent in the artificial defective aggregate of example 3, but due to the presence of the coating, the binding force between the aggregate and the matrix was still smaller than that of the binding force with the matrix in the blank group, which resulted in the aggregate of example 3 had the effect of interfacial binding force, thus increased.

It should be noted that the above embodiments are merely for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that the technical solution described in the above embodiments may be modified or some or all of the technical features may be equivalently replaced, and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the scope of the technical solution of the embodiments of the present invention.

Claims (11)

Translated fromChinese

1.一种人为缺陷骨料，其特征在于，由基础骨料及其表面覆盖的浆料经干燥所形成；所述基础骨料为矿渣、河沙、石英砂、陶粒、废玻璃中的至少一种；所述浆料由丙烯酸浆料与碳纳米管粉末拌合制成。1. An artificial defective aggregate, characterized in that it is formed by drying a base aggregate and a slurry covering its surface; the base aggregate is at least one of slag, river sand, quartz sand, ceramsite, and waste glass; and the slurry is made by mixing acrylic slurry and carbon nanotube powder.2.根据权利要求1所述的人为缺陷骨料，其特征在于：所述碳纳米管CNT与丙烯酸浆料的质量比为(1～2):7。2. The artificial defect aggregate according to claim 1, characterized in that the mass ratio of the carbon nanotubes CNT to the acrylic slurry is (1-2):7.3.根据权利要求1所述的人为缺陷骨料，其特征在于：所述浆料与基础骨料的质量比为(1～2):50。3. The artificial defect aggregate according to claim 1 is characterized in that the mass ratio of the slurry to the base aggregate is (1-2):50.4.一种根据权利要求1-3中任一项所述的人为缺陷骨料的制备方法，其特征在于，包括如下步骤：4. A method for preparing an artificially defective aggregate according to any one of claims 1 to 3, characterized in that it comprises the following steps:S1，将质量比为(0～3):7的碳纳米管CNT与丙烯酸浆料粉末搅拌混合；S1, stirring and mixing carbon nanotubes CNT and acrylic acid slurry powder in a mass ratio of (0-3):7;S2，将基础骨料加入S1的混合物中，搅拌，直至基础骨料表面涂敷上浆料；S2, adding the base aggregate to the mixture of S1 and stirring until the surface of the base aggregate is coated with the slurry;S3，将S2中得到的涂敷骨料进行干燥处理。S3, drying the coated aggregate obtained in S2.5.根据权利要求4所述的人为缺陷骨料的制备方法，其特征在于：所述制备方法还包括步骤，将S3中烘干的涂敷骨料，过20～200目筛，收集筛下物得到人为缺陷骨料。5. The method for preparing artificial defective aggregate according to claim 4, characterized in that: the method further comprises the step of passing the dried coated aggregate in S3 through a 20-200 mesh sieve, and collecting the sieve undersize to obtain the artificial defective aggregate.6.根据权利要求4所述的人为缺陷骨料的制备方法，其特征在于：所述碳纳米管与丙烯酸浆料的质量比为(1～2):7。6. The method for preparing artificial defect aggregate according to claim 4, characterized in that the mass ratio of the carbon nanotubes to the acrylic acid slurry is (1-2):7.7.根据权利要求4所述的人为缺陷骨料的制备方法，其特征在于：S2中混合物与基础骨料的质量比为(1～2):50。7. The method for preparing artificial defective aggregate according to claim 4, characterized in that the mass ratio of the mixture to the basic aggregate in S2 is (1-2):50.8.根据权利要求4所述的人为缺陷骨料的制备方法，其特征在于：S3中干燥处理的温度为75～85℃，时间为20-25h。8. The method for preparing artificial defective aggregate according to claim 4, characterized in that the drying treatment in S3 is carried out at a temperature of 75-85°C and for a time of 20-25 hours.9.权利要求1-3任一项所述的人为缺陷骨料或权利要求4-8任一项所述的制备方法所制备的人为缺陷骨料在制备混凝土中的应用。9. Use of the artificial defect aggregate according to any one of claims 1 to 3 or the artificial defect aggregate prepared by the preparation method according to any one of claims 4 to 8 in preparing concrete.10.一种混凝土，其特征在于，其包含权利要求1-3任一项所述的人为缺陷骨料。10. A concrete, characterized in that it comprises the artificial defective aggregate according to any one of claims 1 to 3.11.一种混凝土的制备方法，其特征在于，将胶凝材料和权利要求1-3任一项所述的人为缺陷骨料进行混合，搅拌得到混合料；之后将减水剂与水混合，取减水剂和水混合物体积的1/2～3/4加入到混合料中，搅拌；之后向混合料加入纤维，并加入剩下的减水剂和水的混合物；最后搅拌得到混凝土。11. A method for preparing concrete, characterized in that a cementitious material and the artificial defect aggregate according to any one of claims 1 to 3 are mixed and stirred to obtain a mixture; then a water reducer is mixed with water, and 1/2 to 3/4 of the volume of the mixture of the water reducer and water is added to the mixture and stirred; then fibers are added to the mixture, and the remaining mixture of the water reducer and water is added; and finally, the mixture is stirred to obtain concrete.