CN114105540A

CN114105540A - Recycled concrete and preparation method thereof

Info

Publication number: CN114105540A
Application number: CN202111511167.0A
Authority: CN
Inventors: 张田源; 韩浩
Original assignee: Wuhan Changhua Huixin Building Materials Co ltd
Current assignee: Wuhan Xingchenghai Cement Products Co ltd
Priority date: 2021-12-11
Filing date: 2021-12-11
Publication date: 2022-03-01
Anticipated expiration: 2041-12-11
Also published as: CN114105540B

Abstract

The application relates to the field of building materials, and particularly discloses recycled concrete and a preparation method thereof, wherein the recycled concrete comprises the following components in parts by weight: 280 parts of cement 230-containing materials, 300 parts of reclaimed materials 200-containing materials, 20-60 parts of plant ash, 10-15 parts of waste fibers, 10-20 parts of lignin, 5-15 parts of liquid paraffin, 8-30 parts of water glass, 5-12 parts of a water reducing agent, 210 parts of water 120-containing materials and 15-25 parts of a molecular sieve; the reclaimed material is a mixture of broken masonry, waste concrete blocks and industrial waste residues. The recycled concrete prepared by the application has stronger anti-permeability performance, and simultaneously has higher compressive strength, so that the recycled concrete is not easy to crack when being subjected to pressure.

Description

Recycled concrete and preparation method thereof

Technical Field

The application relates to the field of building materials, in particular to recycled concrete and a preparation method thereof.

Background

With the acceleration of basic facilities such as buildings, roads, railways, bridges and the like and the urbanization construction in China, the rapid development of urban reconstruction and building industry, the service life of some old buildings, roads and urban basic facilities needs to be removed when due, so that more and more building wastes are generated, wherein most of the building wastes are concrete. The traditional treatment method of the construction waste is stacking or landfill, which not only occupies a large amount of cultivated land, but also causes environmental pollution.

In addition, in the production process of concrete, as ore mining becomes serious and ore resources are gradually exhausted, the supply of aggregate in concrete becomes more difficult. Therefore, the application of the waste concrete to the preparation of concrete is gradually welcomed, and the recycled concrete is produced by transporting, wherein the recycled concrete refers to new concrete which is prepared by crushing, cleaning and grading waste concrete blocks, mixing the crushed, cleaned and graded waste concrete blocks with grades according to a certain proportion, partially or completely replacing natural aggregates (mainly coarse aggregates) such as sand stones and the like, and then adding cement, water and the like for mixing.

However, because the waste concrete needs to be subjected to a large external force in the crushing process, a large number of fine cracks are easy to appear in the aggregate, so that the porosity of the recycled aggregate is increased, the compressive strength of the recycled concrete is reduced, the concrete is easy to crack, the durability of the recycled concrete is weakened, and the service life of the recycled concrete is further influenced.

Therefore, there is still a need for recycled concrete with high compressive strength, high crack resistance and high durability.

Disclosure of Invention

In order to solve the problems of low compressive strength, poor cracking resistance and poor durability of recycled concrete, the first object of the present application is to provide recycled concrete.

The second purpose of the invention is to provide a preparation method of recycled concrete, which has the advantages of simple preparation method and easy operation.

In order to achieve the first object, the invention provides the following technical scheme:

the recycled concrete comprises the following components in parts by weight: 280 parts of cement 230-containing materials, 300 parts of reclaimed materials 200-containing materials, 20-60 parts of plant ash, 10-15 parts of waste fibers, 10-20 parts of lignin, 5-15 parts of liquid paraffin, 8-30 parts of water glass, 5-12 parts of a water reducing agent, 210 parts of water 120-containing materials and 15-25 parts of a molecular sieve; the reclaimed material is a mixture of broken masonry, waste concrete blocks and industrial waste residues.

By adopting the technical scheme, the strength, the compactness and the durability of the recycled concrete can be enhanced by mixing the plant ash and the cement, the plant ash is used as a filler, the water permeable resistance of the recycled concrete is improved, and the penetration capability of the recycled concrete is reduced by matching with the liquid paraffin; the waste fibers have the characteristics of low elastic modulus, good dispersibility, good flexibility and high tensile strength, and can be well distributed in a recycled concrete matrix when being doped into the recycled concrete matrix to form a three-dimensional disorder supporting net, the molecular sieve, the waste fibers and the water glass are mutually cooperated, the molecular sieve can not only fill the pores of recycled materials, but also be attached to the waste fibers, the specific surface area of the waste fibers is increased, and the compactness of the recycled concrete is favorably improved; the sodium silicate can enhance the cohesiveness of the molecular sieve and the waste fibers, so that the impermeability of the recycled concrete is enhanced, the compressive strength of the recycled concrete is favorably enhanced, and the recycled concrete is not easy to crack under pressure.

In addition, the waste fibers and the molecular sieve can effectively inhibit the generation of cracks in the concrete plasticity period and the hardening initial stage, reduce the number of the cracks, delay the crack propagation rate and the length and width of a crack opening, further effectively relieve the phenomenon of stress concentration inside the recycled concrete, and reduce the probability of cracking caused by stress of the recycled concrete.

The compressive strength of recycled concrete can not only be strengthened to the interpolation of lignin, and the water consumption when reducible concrete stirring in addition improves concrete workability, mobility and anti permeability, improves concrete strength and closely knit nature, shortens the setting time, improves compressive strength, and liquid paraffin and molecular sieve can make recycled concrete have certain mobility in addition, realize certain elastic deformation simultaneously, and then make recycled concrete be difficult to the fracture when receiving pressure more.

Preferably, the weight ratio of the molecular sieve, the waste fiber and the water glass is 1.5-2.2:1: 1-2.5.

By adopting the technical scheme, the mixture ratio among the molecular sieve, the waste fiber and the water glass is in a certain range, the compactness, the compressive strength and the durability of the recycled concrete can be further improved, the compressive strength of the recycled concrete is further favorably enhanced, and the recycled concrete is not easy to crack when being subjected to pressure.

The molecular sieve structurally comprises a plurality of pore passages with uniform pore diameters and holes which are arranged in order, has strong adsorption capacity and can be attached to waste fibers, the waste fibers have good dispersibility and tensile strength, the molecular sieve can be well distributed in the recycled concrete to form a three-dimensional supporting net, and the water glass can enhance the cohesiveness between the molecular sieve and the waste fibers and further improve the compactness and compressive strength of the recycled concrete.

Preferably, the reclaimed material is pretreated by the following steps: the method comprises the following steps of fully mixing a mixture of broken masonry, waste concrete blocks and industrial waste residues, drying at 60-80 ℃, crushing into powder, uniformly mixing the powder, plant straws, a dispersing agent and water, and drying at 80-100 ℃ for later use.

By adopting the technical scheme, the mixture of the broken masonry, the waste concrete blocks and the industrial waste residues is crushed, the dried powder has good uniformity, and the plant straws, the dispersing agent and the powder are mixed, so that the viscosity and the compactness of the mixture can be enhanced, the porosity of the mixture is further reduced, the strength of the mixture is further increased, and the anti-segregation effect of the concrete is further improved; in addition, the plant straw has small heat transfer coefficient and good heat preservation and insulation performance, and the mixing of the plant straw and the recycled concrete fills the pores among the recycled concrete particles, thereby increasing the strength of the recycled concrete.

Preferably, the weight ratio of the powder, the plant straws and the dispersing agent is 8-18:6-15: 1.

Through adopting above-mentioned technical scheme, the mixture of powder and plant straw helps improving the compactedness of powder, and the addition of dispersant helps improving the dispersibility of plant straw, makes the plant straw dispersion even, further improves the cohesiveness of powder mixture, has a large amount of holes in the recycled concrete, and the preliminary treatment of reclaimed material further reduces the hole of recycled concrete, helps reducing the probability that recycled concrete ftractures.

Preferably, the dispersant is hydroxypropyl methyl cellulose, the density is 1.39g/cm3, the 100-mesh passing rate is more than 98.5 percent, the carbonization temperature is 280-300 ℃, and the viscosity is more than 15W.

By adopting the technical scheme, the hydroxypropyl methyl cellulose is used as the dispersing agent, so that the uniformity of the mixture can be improved, the subsequently prepared recycled concrete has good compatibility, and the strength of the hardened recycled concrete is enhanced.

Preferably, the water reducing agent is one of a liquid polycarboxylic acid high-efficiency water reducing agent, an aliphatic (hydroxy) sulfonate high-efficiency water reducing agent and a naphthalene high-efficiency water reducing agent.

By adopting the technical scheme, the liquid polycarboxylate superplasticizer has good compatibility with cement, and can improve the strength of recycled concrete; the aliphatic (hydroxy) sulfonate high-efficiency water reducing agent has good adaptability and strong dispersing capacity, can obviously improve the strength of concrete, prevent the shrinkage of the concrete and further prevent the cracking of the concrete caused by the shrinkage; the naphthalene-based superplasticizer has a strong dispersing effect on cement particles, effectively prevents the coagulation of the cement particles, and can maintain the plasticity of concrete.

Preferably, the waste fibers are selected from waste carpets, the waste carpets are artificially cut, the length of the waste carpets is 19mm, and the chemical component of the waste carpets is polypropylene.

By adopting the technical scheme, the waste carpet contains a large amount of waste fibers, the waste carpet is reasonably recycled, resources are fully utilized, the waste of resources is reduced, and the waste fibers are used for preparing recycled concrete, so that the compactness of the concrete is enhanced, and the tensile strength and the durability of the concrete are further enhanced.

In order to achieve the second object, the invention provides the following technical scheme: a preparation method of recycled concrete comprises the following steps:

s1, uniformly mixing the waste fibers, the water glass and the molecular sieve according to the proportion, and stirring at the rotating speed of 300-600r/min for 15-30min to obtain a mixture A;

s1, mixing the mixture A obtained in the step S1 with the rest components, and stirring at the temperature of 80-100 ℃, the stirring speed of 500-1000r/min and the stirring time of 30-60min to obtain the recycled concrete.

By adopting the technical scheme, the waste fibers, the water glass and the molecular sieve are mixed firstly, so that the waste fibers can be uniformly mixed, and then the waste fibers are mixed with other residual raw materials at a certain temperature, which is beneficial to improving the mixing uniformity of the recycled concrete, further beneficial to improving the viscosity of the concrete and improving the compactness, the flowability and the crack resistance of the concrete.

In summary, the present application has the following beneficial effects:

1. the strength, the compactness and the durability of the recycled concrete can be enhanced by mixing the plant ash and the cement, so that the water permeable resistance of the recycled concrete is improved, and the permeability of the recycled concrete is reduced by matching with the liquid paraffin; the waste fibers have the characteristics of low elastic modulus, good dispersibility, good flexibility and high tensile strength, and can be well distributed in a recycled concrete matrix when being doped into the recycled concrete matrix to form a three-dimensional disorder supporting net, the molecular sieve, the waste fibers and the water glass are mutually cooperated, the molecular sieve can not only fill the pores of recycled materials, but also be attached to the waste fibers, the specific surface area of the waste fibers is increased, and the compactness of the recycled concrete is favorably improved; the sodium silicate can enhance the cohesiveness of the molecular sieve and the waste fibers, so that the impermeability of the recycled concrete is enhanced, the compressive strength of the recycled concrete is favorably enhanced, and the recycled concrete is not easy to crack under pressure.

2. The cooperation of abandonment fibre and molecular sieve in this application can effectually restrain the concrete plasticity phase and the production of hardening initial stage crack, reduces the quantity of crack, delays the length and the width of crack propagation rate and crack mouth, and then effectively alleviates the phenomenon that recycled concrete internal stress concentrates, reduces the probability that recycled concrete atress produced and splits.

3. The application not only can strengthen the compressive strength of recycled concrete by adding the lignin, but also can reduce the water consumption during concrete stirring, improve the workability of concrete, the mobility and the impermeability, improve the strength and compactness of concrete, shorten the setting time, improve the compressive strength, in addition, the liquid paraffin and the molecular sieve can make the recycled concrete have certain mobility, realize certain elastic deformation simultaneously, and then make the recycled concrete be difficult to the fracture when receiving pressure.

Detailed Description

The present application will be described in further detail with reference to examples.

The raw materials used in the examples are all commercially available, and the dispersant is hydroxypropyl methylcellulose with densityIs 1.39g/cm³The 100-mesh passing rate is more than 98.5 percent, the carbonization temperature is 280-300 ℃, and the viscosity is 20W; the water reducing agent is a liquid polycarboxylic acid high-efficiency water reducing agent; the waste fiber is selected from waste carpets, the waste carpets are artificially cut, the length of the waste carpets is 19mm, and the chemical component of the waste carpets is polypropylene.

Pretreatment of reclaimed materials

Preparation example 1

Fully mixing 10 parts by weight of mixture of broken masonry, waste concrete blocks and industrial waste residues, drying at 70 ℃, crushing into powder, uniformly mixing the powder, plant straws, a dispersing agent and water, and drying at 90 ℃ for later use, wherein the weight ratio of the powder, the plant straws and the dispersing agent is 13:11: 1.

Preparation example 2

Fully mixing 10 parts by weight of mixture of broken masonry, waste concrete blocks and industrial waste residues, drying at 60 ℃, crushing into powder, uniformly mixing the powder, plant straws, a dispersing agent and water, and drying at 100 ℃ for later use, wherein the weight ratio of the powder, the plant straws and the dispersing agent is 8: 15: 1.

Preparation example 3

Fully mixing 10 parts by weight of mixture of broken masonry, waste concrete blocks and industrial waste residues, drying at 80 ℃, crushing into powder, uniformly mixing the powder, plant straws, a dispersing agent and water, and drying at 80 ℃ for later use, wherein the weight ratio of the powder, the plant straws and the dispersing agent is 18:6: 1.

Preparation example 4

Fully mixing 10 parts by weight of mixture of broken masonry, waste concrete blocks and industrial waste residues, drying at 70 ℃, crushing into powder, uniformly mixing the powder, plant straws, a dispersing agent and water, and drying at 90 ℃ for later use, wherein the weight ratio of the powder, the plant straws and the dispersing agent is 20:3: 1.

Preparation example 5

Fully mixing 10 parts by weight of mixture of broken masonry, waste concrete blocks and industrial waste residues, drying at 70 ℃, and crushing into powder for later use.

Examples

Example 1

A preparation method of recycled concrete comprises the following steps:

s1, uniformly mixing the waste fibers, the water glass and the molecular sieve according to the proportion, and stirring at the rotating speed of 500r/min for 20min to obtain a mixture A;

and S1, mixing the mixture A obtained in the step S1 with the rest components, and stirring at the temperature of 90 ℃ at the stirring speed of 800r/min for 45min to obtain the recycled concrete.

The recycled concrete comprises the following components in parts by weight: 260 parts of cement, 250 parts of reclaimed materials, 40 parts of plant ash, 12 parts of waste fibers, 15 parts of lignin, 10 parts of liquid paraffin, 20 parts of water glass, 8 parts of a water reducing agent, 180 parts of water and 20 parts of a molecular sieve; the regrind was selected from preparation 1.

Example 2

A recycled concrete, which is different from example 1 in that the recycled material is selected from preparation example 2.

Example 3

A recycled concrete, which is different from example 1 in that the recycled material is selected from preparation example 3.

Example 4

A recycled concrete, which is different from example 1 in that the recycled material is selected from preparation example 4.

Example 5

A recycled concrete, which is different from example 1 in that the recycled material is selected from preparation example 5.

Example 6

A recycled concrete, which is different from the recycled concrete in example 1 in that the recycled concrete comprises the following components in parts by weight: 230 parts of cement, 300 parts of reclaimed materials, 20 parts of plant ash, 15 parts of waste fibers, 20 parts of lignin, 15 parts of liquid paraffin, 30 parts of water glass, 5 parts of a water reducing agent, 120 parts of water and 25 parts of a molecular sieve.

Example 7

A recycled concrete, which is different from the recycled concrete in example 1 in that the recycled concrete comprises the following components in parts by weight: 280 parts of cement, 200 parts of reclaimed materials, 60 parts of plant ash, 10 parts of waste fibers, 10 parts of lignin, 5 parts of liquid paraffin, 8 parts of water glass, 12 parts of a water reducing agent, 210 parts of water and 15 parts of a molecular sieve.

Example 8

A recycled concrete, which is different from example 1 in that a method for preparing recycled concrete comprises the following steps:

s1, uniformly mixing the waste fibers, the water glass and the molecular sieve according to the proportion, and stirring at the rotating speed of 600r/min for 15-30min to obtain a mixture A;

and S1, mixing the mixture A obtained in the step S1 with the rest components, and stirring at the temperature of 100 ℃ at the stirring speed of 500r/min for 60min to obtain the recycled concrete.

Example 9

The difference between the recycled concrete and the recycled concrete in example 1 is that the weight ratio of the molecular sieve to the waste fiber to the water glass is 1.5:1: 2.5.

Example 10

The difference between the recycled concrete and the recycled concrete in example 1 is that the weight ratio of the molecular sieve to the waste fiber to the water glass is 2.2:1: 1.

Example 11

The difference between the recycled concrete and the recycled concrete in example 1 is that the weight ratio of the molecular sieve to the waste fiber to the water glass is 1.2:1: 3.

Comparative example

Comparative example 1

A recycled concrete, which is different from the recycled concrete in example 1 in that the recycled concrete comprises the following components in parts by weight: 300 parts of cement, 180 parts of reclaimed materials, 70 parts of plant ash, 8 parts of waste fibers, 22 parts of lignin, 3 parts of liquid paraffin, 5 parts of water glass, 15 parts of a water reducing agent, 250 parts of water and 10 parts of a molecular sieve.

Comparative example 2

A recycled concrete, which is different from the example 1 in that no recycled material is added to the recycled concrete component.

Comparative example 3

A recycled concrete, which is different from the recycled concrete of example 1 in that waste fibers are not added to the recycled concrete component.

Comparative example 4

A recycled concrete, which differs from example 1 in that no water glass is added to the recycled concrete components.

Comparative example 5

A recycled concrete, which differs from example 1 in that no molecular sieve is added to the recycled concrete component.

Performance test

The compressive strength (MPa) of the recycled concrete obtained in examples 1 to 11 and comparative examples 1 to 5 was measured according to the compressive strength test in GB/T50081-2002 "standard for testing mechanical properties of ordinary concrete", the impermeability of the concrete specimen at a curing period of 28d was measured according to the water penetration height method in GB/T50082-2009 "standard for testing long-term properties and durability of ordinary concrete", and the tensile strength (MPa) of the recycled concrete was measured according to the cleavage tensile strength test in GB/T50081-2002 "standard for testing mechanical properties of ordinary concrete", and the results are shown in table 1 below.

TABLE 1 results of performance test of recycled concrete obtained in examples 1 to 11 and comparative examples 1 to 5

Crack resistance test

The recycled concrete prepared in example 1 and comparative examples 1 to 5 was tested for crack resistance by the ring-type constrained cracking method, which was as follows: after pouring, compacting and leveling, each group of concrete slab test pieces are moved into a constant temperature and humidity drying chamber and placed for 120 hours in the environment with the wind speed (5 +/-0.5) m/s, the temperature (20 +/-2) DEG C and the relative humidity (60 +/-5%). And recording the number of cracks every 8h, measuring the width and the length of each crack, and taking the measurement result of 24h after the recycled concrete test piece is stirred and added with water as the recycled concrete anti-cracking index, wherein the detection result is shown in the following table 2.

TABLE 2 test results of crack resistance of recycled concrete obtained in examples 1 to 11 and comparative examples 1 to 5

As can be seen from the data in Table 1, the recycled concrete prepared in examples 1 to 11 has higher tensile strength, and the test on the compressive strength of 7d and 28d shows that the recycled concrete prepared in examples 1 to 11 has better compressive strength and lower water penetration height, and the test on the crack resistance of the recycled concrete prepared in example 1 shows that the recycled concrete prepared in example 1 has good crack resistance.

Comparative example 1, by adjusting the content of the components in the recycled concrete, it can be seen that the tensile strength of comparative example 1 is much lower than that of example 1, and the compressive strength tests of 7d and 28d on the recycled concrete in comparative example 1 show that the compressive strength is lower than that of example 1, which indicates that the content change of the components in the recycled concrete affects the properties of the recycled concrete; meanwhile, the recycled concrete prepared in the comparative example 1 is subjected to crack resistance tests, and the crack resistance of the recycled concrete prepared in the comparative example 1 is found to be poor, 1 crack appears after the recycled concrete is placed for 120 hours, so that the change of the component content influences the crack resistance of the recycled concrete.

In the comparative example 2, no recycled material is added, and the data in the table 1 show that the compressive strength, the tensile strength and the water seepage height of the recycled concrete are both poor, so that the addition of the recycled material seriously influences various properties of the recycled concrete; meanwhile, the recycled concrete prepared in the comparative example 2 is subjected to crack resistance tests, and the crack resistance of the recycled concrete prepared in the comparative example 2 is found to be poor, 5 cracks appear after the recycled concrete is placed for 120 hours, so that the crack resistance of the recycled concrete is seriously influenced by the addition of the recycled material.

In the comparative example 3, the waste fibers are not added, and the data in table 1 show that the compressive strength, the tensile strength and the water seepage height of the recycled concrete are poor compared with those of the example 1, so that the addition of the waste fibers influences various properties of the recycled concrete; meanwhile, the recycled concrete prepared in the comparative example 3 is subjected to crack resistance test, and the crack resistance of the recycled concrete prepared in the comparative example 3 is found to be poor, 2 cracks appear after the recycled concrete is placed for 120 hours, so that the crack resistance of the recycled concrete is seriously influenced by the addition of the waste fibers.

In comparative example 4, water glass is not added, and the data in table 1 show that the compressive strength, the tensile strength and the water seepage height of the recycled concrete are poor compared with those of example 1, so that the addition of the water glass influences various properties of the recycled concrete; meanwhile, the recycled concrete prepared in the comparative example 4 is subjected to crack resistance test, and the crack resistance of the recycled concrete prepared in the comparative example 4 is found to be poor, 3 cracks appear after the recycled concrete is placed for 120 hours, so that the crack resistance of the recycled concrete is seriously influenced by the addition of the water glass.

Compared with the example 1, the compression strength, the tensile strength and the water seepage height of the recycled concrete are both poor, so that the addition of the molecular sieve influences various properties of the recycled concrete; meanwhile, the recycled concrete prepared in the comparative example 5 is subjected to crack resistance test, and the crack resistance of the recycled concrete prepared in the comparative example 5 is found to be poor, 4 cracks appear after the recycled concrete is placed for 120 hours, so that the crack resistance of the recycled concrete is seriously influenced by the addition of the molecular sieve.

The present embodiment is only for explaining the present application, and it is not limited to the present application, and those skilled in the art can make modifications of the present embodiment without inventive contribution as needed after reading the present specification, but all of them are protected by patent law within the scope of the claims of the present application.

Claims

1. The recycled concrete is characterized by comprising the following components in parts by weight: 280 parts of cement 230-containing materials, 300 parts of reclaimed materials 200-containing materials, 20-60 parts of plant ash, 10-15 parts of waste fibers, 10-20 parts of lignin, 5-15 parts of liquid paraffin, 8-30 parts of water glass, 5-12 parts of a water reducing agent, 210 parts of water 120-containing materials and 15-25 parts of a molecular sieve; the reclaimed material is a mixture of broken masonry, waste concrete blocks and industrial waste residues.

2. A recycled concrete as claimed in claim 1, wherein: the weight ratio of the molecular sieve, the waste fiber and the water glass is 1.5-2.2:1: 1-2.5.

3. A recycled concrete as claimed in claim 1, wherein: the reclaimed materials are pretreated as follows: the method comprises the following steps of fully mixing a mixture of broken masonry, waste concrete blocks and industrial waste residues, drying at 60-80 ℃, crushing into powder, uniformly mixing the powder, plant straws, a dispersing agent and water, and drying at 80-100 ℃ for later use.

4. A recycled concrete as claimed in claim 3, wherein: the weight ratio of the powder, the plant straws and the dispersing agent is 8-18:6-15: 1.

5. A recycled concrete as claimed in claim 3, wherein: the dispersant is hydroxypropyl methyl cellulose with the density of 1.39g/cm3, the 100-mesh passing rate of more than 98.5 percent, the carbonization temperature of 280 ℃ and 300 ℃, and the viscosity of more than 15W.

6. A recycled concrete as claimed in claim 1, wherein: the water reducing agent is one of a liquid polycarboxylic acid high-efficiency water reducing agent, an aliphatic (hydroxy) sulfonate high-efficiency water reducing agent and a naphthalene high-efficiency water reducing agent.

7. A recycled concrete as claimed in claim 1, wherein: the waste fibers are selected from waste carpets, the waste carpets are artificially cut, the length of the waste carpets is 19mm, and the chemical components are polypropylene.

8. A method for the preparation of recycled concrete according to any one of claims 1 to 7, characterized in that it comprises the following steps: