High performance lightweight concrete

Abstract

The application discloses a kind of high-performance lightweight concrete, it is related to concrete field, it is prepared by including the following weight parts of component: fine aggregate 60-90 parts;Lightweight coarse aggregate 180-220 parts;Cement 140-160 parts;Water 30-40 parts;Water reducing agent 3-5 parts;Modified bamboo fiber 0.7-1.2 parts;The preparation method of the modified bamboo fiber is as follows: S1, prepare into mixed aqueous solution with hydroquinone, triethylene tetramine, obtain modified liquid;S2, bamboo fiber is added into modified liquid, stirs 1-4h under the condition of 23-28 ℃, filters and collects solid substance, washes after drying, obtains coated bamboo fiber;S3, coated bamboo fiber is mixed with aqueous solution of silane coupling agent, stirs 2-4h under the condition of 45-55 ℃, filters and collects solid substance, washes after drying, obtains modified bamboo fiber.The application has the effect of improving the compressive strength, splitting tensile strength and thermal insulation performance of lightweight concrete.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of concrete, in particular to a high-performance lightweight concrete. BACKGROUND

[0002] Concrete is generally a composite material made of cement as a gel material, coarse aggregate, fine aggregate and water, and is widely used in engineering construction.

[0003] At present, with the development of urbanization, the use of concrete, especially lightweight concrete, has greatly increased. Lightweight concrete generally replaces high-density coarse aggregate such as gravel with low-density coarse aggregate such as ceramsite and foaming material, and different lightweight coarse aggregate can be selected according to the required performance of the concrete, such as anti-seismic and thermal insulation, to obtain high-performance lightweight concrete.

[0004] However, due to the large difference in density between lightweight coarse aggregate and other components such as fine aggregate and cement, segregation is prone to occur during the mixing of lightweight concrete, resulting in low strength of the prepared lightweight concrete and difficulty in meeting the performance requirements. SUMMARY

[0005] In order to solve the problem that lightweight concrete is prone to segregation during mixing, resulting in low strength of the lightweight concrete and difficulty in meeting the performance requirements, the present application provides a high-performance lightweight concrete.

[0006] The high-performance lightweight concrete provided by the present application adopts the following technical solution:

[0007] A high-performance lightweight concrete is prepared from the following components by weight:

[0008] Fine aggregate 60-90 parts;

[0009] Lightweight coarse aggregate 180-220 parts;

[0010] Cement 140-160 parts;

[0011] Water 30-40 parts;

[0012] Water reducing agent 3-5 parts;

[0013] Modified bamboo fiber 0.7-1.2 parts;

[0014] The preparation method of the modified bamboo fiber is as follows:

[0015] S1, prepare a mixed aqueous solution of hydroquinone and triethylenetetramine to obtain a modification liquid;

[0016] S2, add bamboo fiber to the modification liquid, stir at 23-28°C for 1-4h, filter and collect the solid material, wash and dry to obtain coated bamboo fiber;

[0017] S3, mixing the envelope bamboo fiber with the aqueous solution of silane coupling agent, stirring at 45-55℃ for 2-4h, filtering and collecting the solid substance, washing and drying to obtain the modified bamboo fiber.

[0018] By using the above technical scheme, the cement is used as a gel material to bond the fine aggregate and the lightweight coarse aggregate, and the water reducing agent is used to reduce the amount of mixing water, and on the one hand, the random distribution of the modified bamboo fiber in the lightweight concrete has a reinforcing effect on the lightweight concrete, and on the other hand, the hydroquinone and triethylene tetramine modified bamboo fiber is easier to disperse in the lightweight concrete, forming a fiber network to hinder the floating of the lightweight coarse aggregate, so as to avoid the segregation of the lightweight concrete as much as possible, so that the lightweight coarse aggregate is uniformly distributed, the heat dissipation channel in the concrete is reduced, and the thermal conductivity of the concrete is reduced. Under the joint action of hydroquinone, triethylene tetramine and silane coupling agent, the connection effect of the lightweight coarse aggregate and the modified bamboo fiber is improved, and the connection strength of the modified bamboo fiber and other components is improved. Under the comprehensive action, the compressive strength, splitting tensile strength and thermal insulation performance of the lightweight concrete are improved.

[0019] Optionally, in the step S1, the molar ratio of hydroquinone to triethylene tetramine is 1:0.8-1.2.

[0020] Preferably, in the step S1, the molar ratio of hydroquinone to triethylene tetramine is 1:0.9-1.1.

[0021] By using the above technical scheme, the hydroquinone and triethylene tetramine are reacted on the surface of the bamboo fiber to form a film, and by controlling the molar ratio of hydroquinone to triethylene tetramine, the distribution area and thickness of hydroquinone and triethylene tetramine on the surface of the bamboo fiber are controlled, and the dispersion effect of the bamboo fiber in the concrete is improved.

[0022] Optionally, in the step S2, the average length of the bamboo fiber is 15-30mm.

[0023] Preferably, in the step S2, the average length of the bamboo fiber is 20mm.

[0024] Optionally, in the step S2, the average diameter of the bamboo fiber is 0.2-0.3mm.

[0025] Preferably, in the step S2, the average diameter of the bamboo fiber is 0.25mm.

[0026] Optionally, in the step S2, the aspect ratio of the bamboo fiber is 75-100.

[0027] Preferably, the aspect ratio of the bamboo fiber is 80.

[0028] By adopting the technical scheme, the agglomeration degree of the bamboo fibers in the concrete is reduced by selecting the bamboo fibers, adjusting the aspect ratio of the bamboo fibers, and modifying the bamboo fibers together by using hydroquinone, triethylenetetramine, and silane coupling agent, so that the modified bamboo fibers are dispersed to form a reticular structure to hinder the segregation of the lightweight coarse aggregate, and the length and diameter of the bamboo fibers are adjusted to improve the reinforcing effect of the modified bamboo fibers in the lightweight concrete, and the compressive strength, splitting tensile strength, and thermal insulation performance of the lightweight concrete are improved.

[0029] Optionally, the lightweight coarse aggregate is selected from one or both of ceramic particles and EPS particles.

[0030] Optionally, the lightweight coarse aggregate includes ceramic particles and EPS particles in a weight ratio of 20-40:1.

[0031] Optionally, the particle size of the ceramic particles is 5-10 mm.

[0032] Optionally, the particle size of the EPS particles is 1-8 mm.

[0033] Preferably, the particle size of the EPS particles is 3-5 mm.

[0034] Optionally, the water reducing agent is selected from one or more of polycarboxylic acid type water reducing agent, naphthalene type water reducing agent, aminosulfonate type water reducing agent, aliphatic type water reducing agent, and lignin ring acid salt.

[0035] By adopting the technical scheme, the particle size of the ceramic particles and the particle size of the EPS particles are controlled to fill the gaps between the ceramic particles and the EPS particles, further reducing the generation of heat dissipation channels in the lightweight concrete, and the sizes of the ceramic particles, the EPS particles, and the modified bamboo fibers are adapted to each other, greatly improving the effect of the fiber net formed by the modified bamboo fibers on hindering the upward floating of the ceramic particles and the EPS particles, and under the combined action, the compressive strength, splitting tensile strength, and thermal insulation performance of the lightweight concrete are improved.

[0036] In summary, the present application includes at least one of the following beneficial technical effects:

[0037] 1. Cement as a gel material binds fine aggregate and lightweight coarse aggregate, and uses water reducing agent to reduce the amount of mixing water, and on the one hand, the random distribution of modified bamboo fiber in lightweight concrete has a reinforcing effect on lightweight concrete, and on the other hand, the modified bamboo fiber modified by hydroquinone and triethylene tetramine is easier to disperse in lightweight concrete, forming a fiber network to hinder the floating of lightweight coarse aggregate, and as much as possible to avoid the segregation of lightweight concrete, so that the lightweight coarse aggregate is uniformly distributed, reducing the heat dissipation channel inside the concrete, and reducing the thermal conductivity of the concrete. Under the joint action of hydroquinone, triethylene tetramine and silane coupling agent, the connection effect of lightweight coarse aggregate and modified bamboo fiber is also improved, and the connection strength of modified bamboo fiber and other components is also improved. Under the comprehensive action, the compressive strength, splitting tensile strength and thermal insulation performance of lightweight concrete are improved;

[0038] 2. By selecting bamboo fiber, adjusting the aspect ratio of bamboo fiber, and combining hydroquinone, triethylene tetramine and silane coupling agent to modify bamboo fiber, the agglomeration degree of bamboo fiber in concrete is reduced, so that the modified bamboo fiber is dispersed to form a network to hinder the segregation of lightweight coarse aggregate, and by adjusting the length and diameter of the bamboo fiber, the reinforcing effect of the modified bamboo fiber in the lightweight concrete is improved;

[0039] 3. By controlling the particle size of the ceramsite and the particle size of the EPS particles, the EPS particles fill the gap between the ceramsite, further reducing the generation of heat dissipation channels in the lightweight concrete, and in addition, the size of the ceramsite, EPS particles and modified bamboo fiber are mutually adapted, greatly improving the effect of the fiber network formed by the modified bamboo fiber to hinder the floating of the ceramsite and the EPS particles. Under the comprehensive action, the compressive strength, splitting tensile strength and thermal insulation performance of lightweight concrete are improved. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 is a flow chart of the preparation method of the modified bamboo fiber in this application; DETAILED DESCRIPTION

[0041] The application will be further described in detail below in conjunction with the examples. The following examples are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. In the following examples, the specific conditions are not specified, and the methods used are conventional methods known in the art unless otherwise specified. Unless otherwise specified, the professional and scientific terms used in this text have the same meaning as familiar to those skilled in the art. In addition, any method or material similar or equivalent to the described content can also be applied in the present application.

[0042] The raw materials used in the examples are all commercially available, the cement is ordinary portland cement, the fine aggregate is river sand, the water reducing agent is polycarboxylic acid water reducing agent, and the EPS particles are foamed polystyrene particles with a density of 20 kg / m 3 .

[0043] Examples 1-10

[0044] Examples 1-10 provide a high-performance lightweight concrete, the composition and proportion of which are shown in Table 1.

[0045] With reference to Figure 1 , the preparation method of the modified bamboo fiber in Examples 1-10 is as follows:

[0046] S1, 1 mol of hydroquinone, 1 mol of triethylene tetramine and 10 L of water are mixed and stirred uniformly to obtain a modification liquid;

[0047] S2, 1 kg of bamboo fiber with an average length of 20 mm and an average diameter of 0.25 mm is added to the modification liquid, stirred at 25°C for 2 h, the solid material is filtered and collected, and after water washing, it is spread and dried to obtain coated bamboo fiber;

[0048] S3, the coated bamboo fiber is placed in a 1.5 vol% silane coupling agent aqueous solution, stirred at 50°C for 3 h, the solid material is filtered and collected, and after water washing, it is dried at 50°C to obtain modified bamboo fiber.

[0049] In Examples 1-10, the silane coupling agent is KH560, and the ceramsite is shale ceramsite with a particle size of 5-10 mm; in Examples 8-10, the particle size of the EPS particles is 3-5 mm.

[0050] Table 1: Composition and proportion of high-performance lightweight concrete

[0051]

[0052] Example 11

[0053] Example 11 provides a high-performance lightweight concrete, and the difference between Example 11 and Example 9 is that the particle size of the EPS particles in Example 11 is 1-2 mm.

[0054] Example 12

[0055] Example 12 provides a high-performance lightweight concrete, and the difference between Example 12 and Example 9 is that the particle size of the EPS particles in Example 12 is 6-8 mm.

[0056] Example 13

[0057] Example 13 provides a high-performance lightweight concrete, wherein the difference between Example 13 and Example 9 is that the average length of the bamboo fibers used in the preparation of the modified bamboo fibers in Example 13 is 15 mm, and the average diameter of the bamboo fibers is 0.2 mm.

[0058] Example 14

[0059] Example 14 provides a high-performance lightweight concrete, wherein the difference between Example 14 and Example 9 is that the average length of the bamboo fibers used in the preparation of the modified bamboo fibers in Example 14 is 30 mm, and the average diameter of the bamboo fibers is 0.3 mm.

[0060] Example 15

[0061] Example 15 provides a high-performance lightweight concrete, wherein the difference between Example 15 and Example 9 is that in the preparation of the modified bamboo fibers in Example 15, step S1 is as follows: 1 mol of hydroquinone, 0.8 mol of triethylene tetramine, and 10 L of water are mixed and stirred uniformly to obtain a modification liquid.

[0062] Example 16

[0063] Example 16 provides a high-performance lightweight concrete, wherein the difference between Example 16 and Example 9 is that in the preparation of the modified bamboo fibers in Example 16, step S1 is as follows: 1 mol of hydroquinone, 1.2 mol of triethylene tetramine, and 10 L of water are mixed and stirred uniformly to obtain a modification liquid.

[0064] Comparative Example 1

[0065] Comparative Example 1 provides a high-performance lightweight concrete, wherein the difference between Comparative Example 1 and Example 2 is that polypropylene fibers are used to replace the bamboo fibers in step S2 in the preparation of the modified bamboo fibers.

[0066] Comparative Example 2

[0067] Comparative Example 2 provides a high-performance lightweight concrete, wherein the difference between Comparative Example 2 and Example 2 is that bamboo fibers are used to replace the modified bamboo fibers.

[0068] Comparative Example 3

[0069] Comparative Example 3 provides a high-performance lightweight concrete, wherein the difference between Comparative Example 3 and Example 2 is that the modified bamboo fibers are prepared as follows:

[0070] The bamboo fibers are placed in a 1.5 vol% silane coupling agent aqueous solution, stirred at 50°C for 3 h, the solid material is collected by filtration, washed with water, and dried at 50°C to obtain the modified bamboo fibers.

[0071] Comparative Example 4

[0072] Comparative Example 4 provides a high-performance lightweight concrete. The difference between Comparative Example 4 and Example 2 is that the preparation method of the modified bamboo fiber in Comparative Example 4 is as follows:

[0073] Take 1 mol hydroquinone, 1 mol triethylenetetramine and 10 L of water and mix them evenly to obtain a modified solution; add 1 kg of bamboo fiber with an average length of 20 mm and an average diameter of 0.25 mm to the modified solution, stir at 25 °C for 2 h, filter and collect the solid material, wash with water and spread out to air dry to obtain modified bamboo fiber.

[0074] Testing and Inspection

[0075] Samples were prepared according to the composition and proportion of high-performance lightweight concrete in Examples 1-16 and Comparative Examples 1-4. The preparation method is as follows: cement, lightweight coarse aggregate, modified bamboo fiber and fine aggregate were weighed according to the proportion and added to a concrete mixer for mixing. Then, water-reducing agent and water were weighed according to the proportion. After the water-reducing agent and water were mixed evenly, they were added to the concrete mixer and mixed again to obtain high-performance lightweight concrete.

[0076] The following tests were performed on the samples prepared according to Examples 1-16 and Comparative Examples 1-4:

[0077] (1) The 28-day compressive strength (MPa) and 28-day splitting tensile strength of each specimen were tested in accordance with GBT 50081-2019.

[0078] (2) The thermal conductivity (W / (m·K) of each sample was tested in accordance with GB / T 10295-2008.

[0079] The test data are shown in Table 2.

[0080] Table 2: Compressive strength, splitting tensile strength, and thermal conductivity of high-performance lightweight concrete

[0081]

[0082]

[0083] The following detailed description of this application is based on the experimental data provided in Table 1-2.

[0084] Examples 1-3 investigated the effects of fine aggregate, cement, water, and water-reducing agent on the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete. The high-performance lightweight concrete prepared in Example 2 exhibited slightly higher 28-day compressive strength and 28-day splitting tensile strength than the high-performance lightweight concrete prepared in Examples 1 and 3. The thermal conductivity of the high-performance lightweight concrete prepared in Example 2 was slightly lower than that of the high-performance lightweight concrete prepared in Examples 1 and 3. This indicates that although the high-performance lightweight concrete prepared in Example 2 has higher compressive strength, higher splitting tensile strength, and better thermal insulation performance, the effects of fine aggregate, cement, water, and water-reducing agent are relatively small.

[0085] Using Example 2 as a control, Examples 4 and 5 investigated the effects of lightweight coarse aggregate on the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete. The high-performance lightweight concrete prepared in Example 2 exhibited higher 28-day compressive and splitting tensile strengths than the high-performance lightweight concrete prepared in Examples 4 and 5. Furthermore, the high-performance lightweight concrete prepared in Example 2 had lower thermal conductivity than the high-performance lightweight concrete prepared in Examples 4 and 5. This indicates that the content of lightweight coarse aggregate affects the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete.

[0086] Using Example 2 as a control, Examples 6 and 7 investigated the effects of modified bamboo fiber on the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete. The high-performance lightweight concrete prepared in Example 2 exhibited higher 28-day compressive and splitting tensile strengths than those prepared in Examples 6 and 7. Furthermore, the high-performance lightweight concrete prepared in Example 2 had a lower thermal conductivity than those prepared in Examples 6 and 7. This indicates that the content of modified bamboo fiber affects the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete.

[0087] Using Example 2 as a control, Examples 8-10 investigated the effects of the composition and proportion of lightweight coarse aggregate on the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete. The 28-day compressive strength and 28-day splitting tensile strength of the high-performance lightweight concrete prepared using Examples 8-10 were greater than those of the high-performance lightweight concrete prepared using Example 2. The thermal conductivity of the high-performance lightweight concrete prepared using Examples 8-10 was lower than that of the high-performance lightweight concrete prepared using Example 2. This indicates that using a composite of ceramsite and EPS particles as lightweight coarse aggregate improved the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete.

[0088] Furthermore, the 28-day compressive strength and 28-day splitting tensile strength of the high-performance lightweight concrete prepared in Example 9 are greater than those of the high-performance lightweight concrete prepared in Examples 8 and 10. The thermal conductivity of the high-performance lightweight concrete prepared in Example 9 is less than that of the high-performance lightweight concrete prepared in Examples 8 and 10. This indicates that the ratio of expanded clay aggregate to EPS particles affects the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete.

[0089] Using Example 9 as a control, Examples 11 and 12 investigated the effect of EPS particle size on the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete. The high-performance lightweight concrete prepared in Example 9 exhibited higher 28-day compressive strength and 28-day splitting tensile strength than the high-performance lightweight concrete prepared in Examples 11 and 12. Furthermore, the high-performance lightweight concrete prepared in Example 9 had lower thermal conductivity than the high-performance lightweight concrete prepared in Examples 11 and 12. This indicates that high-performance lightweight concrete prepared with EPS particle size of 3-5 mm exhibits better compressive strength, splitting tensile strength, and thermal insulation performance.

[0090] Using Example 9 as a control, Examples 13 and 14 investigated the effect of the aspect ratio of the bamboo fibers used in preparing the modified bamboo fibers on the compressive strength, splitting tensile strength, and thermal insulation performance of the resulting high-performance lightweight concrete. The high-performance lightweight concrete prepared using Example 9 exhibited higher 28-day compressive strength and 28-day splitting tensile strength than the high-performance lightweight concrete prepared using Examples 13 and 14. Furthermore, the high-performance lightweight concrete prepared using Example 9 had a lower thermal conductivity than the high-performance lightweight concrete prepared using Examples 13 and 14. This indicates that the high-performance lightweight concrete prepared using bamboo fibers with an aspect ratio of 80 exhibits better compressive strength, splitting tensile strength, and thermal insulation performance.

[0091] Using Example 9 as a control, Examples 15 and 16 investigated the effect of the molar ratio of hydroquinone to triethylenetetramine on the compressive strength, splitting tensile strength, and thermal insulation performance of the high-performance lightweight concrete obtained during bamboo fiber modification. The high-performance lightweight concrete obtained in Example 9 exhibited higher 28-day compressive and splitting tensile strengths than those obtained in Examples 15 and 16. Furthermore, the high-performance lightweight concrete obtained in Example 9 had a lower thermal conductivity than those obtained in Examples 15 and 16. This indicates that the molar ratio of hydroquinone to triethylenetetramine significantly affects the compressive strength, splitting tensile strength, and thermal insulation performance of the obtained high-performance lightweight concrete.

[0092] Using Example 2 as a control, Comparative Example 1 investigated the effect of the type of modified fiber on the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete. The 28-day compressive strength and 28-day splitting tensile strength of the sample prepared in Example 2 were significantly greater than those of the sample prepared in Comparative Example 1. Furthermore, the thermal conductivity of the sample prepared in Example 2 was significantly lower than that of the sample prepared in Comparative Example 1. This demonstrates that the selection of bamboo fiber is crucial to the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete.

[0093] Using Example 2 as a control, Comparative Examples 2-4 investigated the effects of unmodified bamboo fiber and modified bamboo fiber on the compressive strength, splitting tensile strength, and thermal insulation performance of the prepared high-performance lightweight concrete. The 28-day compressive strength and 28-day splitting tensile strength of the sample prepared in Example 2 were significantly greater than those of the samples prepared in Comparative Examples 2-4. Furthermore, the thermal conductivity of the sample prepared in Example 2 was significantly lower than that of the samples prepared in Comparative Examples 2-4, indicating that the modified liquid and the silane coupling agent aqueous solution have a synergistic effect in improving the compressive strength, splitting tensile strength, and thermal insulation performance of the high-performance lightweight concrete.

[0094] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A high-performance lightweight concrete, characterized in that: It is prepared from the following components in parts by weight: 60-90 parts fine aggregate; 180-220 parts of lightweight coarse aggregate; 140-160 parts cement; 30-40 parts water; 3-5 parts water-reducing agent; 0.7-1.2 parts of modified bamboo fiber; The modified bamboo fiber is prepared as follows: S1. Hydroquinone and triethylenetetramine are prepared into a mixed aqueous solution to obtain the modified solution; S2. Add bamboo fiber to the modified solution, stir at 23-28℃ for 1-4 hours, filter and collect the solid material, wash and dry to obtain coated bamboo fiber. S3. Mix the coated bamboo fiber with an aqueous solution of silane coupling agent, stir at 45-55℃ for 2-4 hours, filter and collect the solid material, wash and dry to obtain modified bamboo fiber.

2. The high-performance lightweight concrete according to claim 1, characterized in that: In step S1, the molar ratio of hydroquinone and triethylenetetramine is 1:0.8-1.

2.

3. The high-performance lightweight concrete according to claim 1, characterized in that: In step S2, the average length of the bamboo fiber is 15-30 mm.

4. The high-performance lightweight concrete according to claim 1, characterized in that: In step S2, the average diameter of the bamboo fiber is 0.2-0.3 mm.

5. The high-performance lightweight concrete according to claim 1, characterized in that: In step S2, the aspect ratio of the bamboo fiber is 75-100.

6. The high-performance lightweight concrete according to claim 1, characterized in that: The lightweight coarse aggregate is selected from one or both of ceramsite and EPS particles.

7. The high-performance lightweight concrete according to claim 6, characterized in that: The lightweight coarse aggregate includes expanded clay and EPS particles in a weight ratio of 20-40:

1.

8. The high-performance lightweight concrete according to claim 7, characterized in that: The particle size of the ceramsite is 5-10 mm.

9. The high-performance lightweight concrete according to claim 7, characterized in that: The EPS particles have a particle size of 1-8 mm.

10. The high-performance lightweight concrete according to claim 1, characterized in that: The water-reducing agent is selected from one or more of polycarboxylate water-reducing agents, naphthalene water-reducing agents, aminosulfonate water-reducing agents, aliphatic water-reducing agents, and lignin sulfonates.

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