Rapid bottom sealing construction method for steel hanging box cofferdam in strong wave sea area

Abstract

The invention relates to a rapid bottom sealing construction method for a steel hanging box cofferdam in a strong wave sea area, which comprises the following specific steps of: measuring and adjusting a construction drawing of the steel hanging box cofferdam after a steel casing of a pile foundation is driven; steel hanging box cofferdams are assembled on steel corbels of the steel casings, measurement is conducted again after cofferdam bottom plates are assembled, and bucket-shaped necking structures corresponding to the steel casings are machined according to measured data; after the steel hanging box cofferdam is assembled, the whole cofferdam is lifted, and a bucket-shaped necking structure is installed at the bottom of a cofferdam bottom keel; steel corbels are cut off, the whole steel hanging box cofferdam is put down to the position, close to the height affected by waves, of the bucket-shaped necking structure, the bucket-shaped necking structure is filled with sand bags, and the sand bags are further fixed through annular plates and stiffening rib plates; opening a communicating hole of the steel hanging box cofferdam, and continuously lowering to a designed position; and bottom sealing concrete is poured, and bottom sealing operation is completed. According to the method, the problem that the bottom sealing operation of the steel hanging box cofferdam in the strong wave area can only wait for low-tide and flat-tide periods is innovatively solved, and the construction speed is greatly increased.

Description

Technical Field

[0001] The present invention relates to the technical field of construction of cofferdams for pile caps, and particularly to a rapid bottom sealing construction method for a steel suspension box cofferdam in a strong wave sea area. Background Art

[0002] It is difficult to seal the bottom of the cofferdam in a strong wave area. The impact of the waves on the bottom plate of the cofferdam causes large vibrations, and the cast-in-place bottom concrete is easily eroded by the sea waves, resulting in the failure of bottom sealing. Therefore, for the steel suspension box cofferdam in a strong wave sea area, plugging holes and bottom sealing operations are mostly carried out when there is no water at low tide level. Even so, under the impact of strong waves, plugging materials, bottom concrete, etc. may also be overturned. For long-line bridges in strong wave sea areas, the construction period is tight. If the steel suspension boxes all wait for low and flat tides to be lowered and bottom sealed, it may delay the construction period and cause greater losses.

[0003] Therefore, how to safely and efficiently seal the bottom of the steel suspension box cofferdam at high tide level is crucial for the cofferdam construction of long-line bridges in strong wave areas. Summary of the Invention

[0004] The present invention aims to solve the deficiencies of the prior art and provides a rapid bottom sealing construction method for a steel suspension box cofferdam in a strong wave sea area.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A rapid bottom sealing construction method for a steel suspension box cofferdam in a strong wave sea area, the specific steps are as follows:

[0007] S1. After the steel casing of the pile foundation is driven, measure the plane position and inclination angle of the steel casing. According to the measurement results, readjust the construction drawing of the steel suspension box cofferdam so that the minimum distance between the outer wall of each steel casing and the inner edge of the surrounding cofferdam bottom keel is controlled within 20 - 25 cm;

[0008] S2. Assemble the steel suspension box cofferdam on the steel bracket of the steel casing. After the assembly of the cofferdam bottom plate is completed, measure the distance between each steel casing and the inner edge of the surrounding cofferdam bottom keel again. According to the measured data, process the corresponding bucket-shaped reduced opening structure for each steel casing;

[0009] The bucket-shaped reduced opening structure includes a reduced opening bottom ring plate, a reduced opening side wall ring plate, and a reduced opening top plate; the distance between the inner wall of the reduced opening bottom ring plate and the outer wall of the steel casing is 5 - 8 cm; the reduced opening side wall ring plate is vertical and the bottom of the reduced opening side wall ring plate is fixed on the reduced opening bottom ring plate. The top of the reduced opening side wall ring plate is welded to the bottom of the cofferdam bottom keel and the inner wall of the reduced opening side wall ring plate is arranged in an inscribed circle state with the inner edge of the cofferdam bottom keel. The gap position between the top of the reduced opening side wall ring plate and the cofferdam bottom keel is filled and welded with a reduced opening top plate;

[0010] S3. After the steel suspension box cofferdam is assembled, lift it as a whole and install the bucket-shaped reduced opening structure at the bottom of the cofferdam bottom keel of the steel suspension box cofferdam;

[0011] S4. Cut off the steel brackets on the outer wall of the steel casing. Lower the overall steel suspension box cofferdam to a position close to the maximum wave height influence height of the bucket-shaped necking structure. Fill the bucket-shaped necking structure with several sandbags, and further fix the sandbags using an annular plate and several stiffening rib plates.

[0012] S5. Open the connecting holes of the steel suspension box cofferdam, and continue to lower the steel suspension box cofferdam below the sea surface until the design position.

[0013] S6. Pour the bottom-sealing concrete to complete the bottom-sealing operation.

[0014] The bottom-sealing concrete of the steel suspension box cofferdam is poured in compartments. The area of a single compartment does not exceed six steel casings, and an experimental compartment covering only a single steel casing is set. When the pouring effect of the bottom-sealing concrete in the experimental compartment is good, then pour the bottom-sealing concrete of the remaining compartments synchronously.

[0015] In step S2, several necking stiffening plates are evenly distributed around the circumference on the side of the necking sidewall ring plate facing away from the steel casing. The necking stiffening plates connect the necking bottom ring plate, the necking sidewall ring plate, and the necking top plate.

[0016] In step S2, the elevation of the steel bracket is ensured not to be submerged by the high tide level.

[0017] In step S4, the sandbags are made of polypropylene or polyester fiber woven bags. The diameter of the sandbags is 15 - 20 cm, and the sandbags are filled densely in the bucket-shaped necking structure.

[0018] In step S4, the outer side of the annular plate is fixedly connected to the inner side of the necking top plate. The distance between the inner side of the annular plate and the outer wall of the steel casing is 2 - 3 cm. The annular plate is fabricated and installed in blocks on-site and finally forms a ring.

[0019] In step S4, the stiffening rib plates are evenly welded circumferentially between the top of the annular plate and the top of the necking top plate.

[0020] In step S5, during the lowering process of the steel suspension box cofferdam, pay attention to the impact on the sandbags at the bucket-shaped necking structure. If there is damage or displacement, stop lowering, re-adjust the position of the sandbags, and further increase the number of stiffening rib plates to reinforce and compact the sandbags, and then continue to lower until the design elevation position.

[0021] In step S6, check whether there is any erosion or displacement of the sandbags inside the steel suspension box cofferdam. If not, weld the tension and compression rods. The bottom of the tension and compression rods is welded to the bottom keel of the cofferdam and the top is obliquely welded to the outer wall of the steel casing. After the tension and compression rods are successfully connected to the steel casing, remove the lifting bars at the top of the steel casing. The steel suspension box cofferdam is converted from the state of being suspended by multiple points to being supported by multiple tension and compression rods welded to the steel casing to complete the system conversion.

[0022] In step S6, the concrete for the bottom seal is quick-setting concrete.

[0023] The beneficial effects of the present invention are as follows: The present invention innovatively solves the problem that the bottom sealing operation of the steel suspension box cofferdam in a strong wave area can only be carried out during low tide or slack tide. For projects with a large number of sea platforms and tight construction periods, the construction speed can be significantly increased; while reducing a large amount of high-altitude waiting operation time, the construction safety is also improved. Brief Description of the Drawings

[0024] Figure 1 It is a schematic diagram of step S3 in a specific embodiment of the present invention;

[0025] Figure 2 It is a detailed view of the bucket-shaped necking structure in the present invention;

[0026] Figure 3 It is Figure 2 the sectional view taken along line A-A in

[0027] Figure 4 It is Figure 2 the sectional view taken along line B-B in

[0028] Figure 5 It is a three-dimensional view of the bucket-shaped necking structure in the present invention;

[0029] Figure 6 It is a schematic diagram of step S4 in a specific embodiment of the present invention;

[0030] Figure 7 It is a three-dimensional view of the annular plate and the stiffening rib plate in the present invention;

[0031] Figure 8 It is the bottom seal compartment diagram of step S6 in a specific embodiment of the present invention;

[0032] In the figure: 1 - steel casing; 2 - bottom keel of the cofferdam; 3 - steel bracket; 4 - bottom plate of the cofferdam; 5 - bucket-shaped necking structure; 6 - sandbag; 7 - annular plate; 8 - tension and compression rod; 9 - stiffening rib plate; 10 - experimental compartment;

[0033] 51 - necking bottom ring plate; 52 - necking side wall ring plate; 53 - necking top plate; 54 - necking stiffening plate;

[0034] The following will describe in detail with reference to the embodiments of the present invention and the accompanying drawings. Detailed Embodiments

[0035] The principles and features of the present invention will be described below in conjunction with the accompanying drawings. The embodiments cited are only used to explain the present invention and are not intended to limit the scope of the present invention. In the following paragraphs, the present invention will be described more specifically by way of example with reference to the accompanying drawings. The advantages and features of the present invention will be clearer according to the following description. It should be noted that the accompanying drawings are all in a very simplified form and use non-precise scales, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0038] A rapid bottom sealing construction method for a steel suspension box cofferdam in a strong wave sea area, the specific steps are as follows:

[0039] S1. After the steel casing 1 of the pile foundation is driven, measure the plane position and inclination angle of the steel casing 1. According to the measurement results, readjust the construction drawing of the steel suspension box cofferdam so that the minimum distance between the outer wall of each steel casing 1 and the inner edge of the surrounding cofferdam bottom keel 2 is controlled within 20 - 25 cm.

[0040] S2. Assemble the steel suspension box cofferdam on the steel corbel 3 of the steel casing 1. After the cofferdam bottom plate 4 is assembled, measure the distance between each steel casing 1 and the inner edge of the surrounding cofferdam bottom keel 2 again. According to the measured data, process the bucket-shaped reduced opening structure 5 corresponding to each steel casing 1.

[0041] The bucket-shaped reduced opening structure 5 should completely cover the cofferdam bottom keel 2 and leave a spacing with the corresponding steel casing 1. The specific structure is as follows:

[0042] The bucket-shaped reduced opening structure 5 is as Figure 2 、 Figure 3 、 Figure 4 、 Figure 5 shown, and includes a reduced opening bottom ring plate 51, a reduced opening side wall ring plate 52 and a reduced opening top plate 53.

[0043] The distance between the inner wall of the reduced opening bottom ring plate 51 and the outer wall of the steel casing 1 is 5 - 8 cm. For example, 7 cm can be adopted to facilitate the later plugging.

[0044] The flared side wall ring plate 52 is vertical, which is convenient for blanking processing and on-site installation. The bottom of the flared side wall ring plate 52 is fixed on the flared bottom ring plate 51. The top of the flared side wall ring plate 52 is welded to the bottom of the cofferdam bottom keel 2, and the inner wall of the flared side wall ring plate 52 and the inner edge of the cofferdam bottom keel 2 are arranged in an inscribed circle state, which is convenient for the smooth pouring of the blocking concrete.

[0045] At the gap position between the top of the flared side wall ring plate 52 and the cofferdam bottom keel 2, a flared top plate 53 is filled and welded. The edge of the flared top plate 53 is fixedly connected to the cofferdam bottom keel 2 by a weld.

[0046] A number of flared stiffening plates 54 are evenly distributed around the circumference of the side of the flared side wall ring plate 52 facing away from the steel casing 1. The flared stiffening plates 54 connect the flared bottom ring plate 51, the flared side wall ring plate 52, and the flared top plate 53. The flared stiffening plates 54 play a role in strengthening the connection of the flared bottom ring plate 51, the flared side wall ring plate 52, and the flared top plate 53 and improving the overall stiffness of the bucket-shaped flared structure 5.

[0047] The elevation of the steel corbel 3 is ensured not to be submerged by the high tide level and not to be disturbed by the waves at high tide.

[0048] S3. After the steel suspension box cofferdam is assembled, using a PLC control system, the steel suspension box cofferdam is synchronously lifted at the tops of multiple steel casings 1. As Figure 1 shown, a bucket-shaped flared structure 5 is installed at the bottom of the cofferdam bottom keel 2 of the steel suspension box cofferdam.

[0049] S4. Cut off the steel corbels 3 on the outer wall of the steel casing 1. The whole steel suspension box cofferdam is lowered to a position close to the maximum wave height influence height of the bucket-shaped flared structure 5. A number of sand bags 6 are used to fill the bucket-shaped flared structure 5, and the sand bags 6 are further fixed by the annular plate 7 and a number of stiffening rib plates 9. As Figure 6 、 Figure 7 shown, to prevent the sand bags 6 from being washed away and displaced by the sea waves after the steel suspension box cofferdam is lowered in place.

[0050] The sand bags 6 are made of polypropylene or polyester fiber woven bags to prevent the sand bags 6 from being eroded and damaged by the sea waves, which may lead to the failure of the blocking. The diameter of the sand bags 6 is 15 - 20 cm. The sand bags 6 are filled densely in the bucket-shaped flared structure 5.

[0051] For the convenience of installation, the annular plate 7 is processed on-site, fabricated and installed in blocks, and finally formed into a ring. Specifically, the annular plate 7 corresponding to a single steel casing 1 is fabricated in four blocks on-site and formed into a ring after being installed in blocks.

[0052] The outer side of the annular plate 7 is fixedly connected to the inner side of the necking top plate 53, and there is a certain distance reserved between the inner side of the annular plate 7 and the outer wall of the steel casing 1; the reserved distance between the inner side of the annular plate 7 and the outer wall of the steel casing 1 should be determined according to the remaining situation of the welding materials on the outer wall of the steel casing 1. Preferably, the distance between the inner side of the annular plate 7 and the outer wall of the steel casing 1 is 2 - 3 cm, so that the reserved distance can smoothly pass through the welding residues on the outer wall of the steel casing 1 during the lowering process of the steel suspension box cofferdam, ensuring the smooth lowering of the steel suspension box cofferdam.

[0053] The stiffening rib plates 9 are evenly welded circumferentially between the top of the annular plate 7 and the top of the necking top plate 53. The stiffening rib plates 9 play a role in increasing the stiffness of the annular plate 7, thereby resisting the impact of sea waves, so as to prevent the sand bags 6 from being washed away and displaced under the action of sea wave surges.

[0054] S5. Open the communication hole of the steel suspension box cofferdam, and continue to lower the steel suspension box cofferdam below the sea surface until the design position.

[0055] During the lowering process of the steel suspension box cofferdam, always pay attention to the impact situation of the sand bags 6 at the bucket-shaped necking structure 5. If the phenomena of being washed away or displaced occur, stop the lowering, re-adjust the position of the sand bags 6, and further increase the number of stiffening rib plates 9 to reinforce and compact the sand bags 6, and then continue to lower until the design elevation position.

[0056] Adopt an intuitive eye observation method to monitor the impact situation of the sand bags 6 at the bucket-shaped necking structure 5. Specifically:

[0057] If the sand bags 6 are eroded and ruptured by sea waves, the filled sand grains will overflow. Under the upward impact of sea waves, sand grains can be observed emerging in the water inside the steel suspension box cofferdam; in addition, the sand bags 6 may also be impacted by sea waves into shredded cloth strips and even be washed into the steel suspension box cofferdam from the bucket-shaped necking structure 5; in both cases, it can be observed with the naked eye whether the sand bags 6 are eroded and damaged.

[0058] Of course, it is allowed that the sand bags 6 have only slight misalignment and displacement inside the bucket-shaped necking structure 5, or the sand bags 6 are punched out with small holes and there is slight leakage of sand grains. As long as multiple sand bags 6 can fill the space of the bucket-shaped necking structure 5 and there is no large-area loss of sand grains inside the bucket-shaped necking structure 5 causing voids, it can play a role in alleviating the impact of sea waves and facilitating the smooth pouring of the bottom seal.

[0059] S6. Pour the bottom seal concrete to complete the bottom seal operation.

[0060] Check whether the sandbags 6 inside the steel suspension box cofferdam are washed and displaced. If not, weld the tension and compression bars 8. The bottom of the tension and compression bars 8 is welded to the bottom keel 2 of the cofferdam, and the top is obliquely welded to the outer wall of the steel casing 1. After the tension and compression bars 8 are successfully connected to the steel casing 1, remove the lifting bars at the top of the steel casing 1. The steel suspension box cofferdam is converted from the state of being suspended by multiple points to being supported by welding multiple tension and compression bars 8 and the steel casing 1, completing the system conversion.

[0061] Pour the underwater concrete for sealing the bottom of the steel suspension box cofferdam in compartments. The underwater concrete for sealing the bottom is quick-setting concrete. The area of a single compartment does not exceed six steel casings 1, and an experimental compartment 10 covering only a single steel casing 1 is set. As Figure 8 shown, when the pouring effect of the underwater concrete for sealing the bottom of the experimental compartment 10 is good, then pour the underwater concrete for sealing the bottom of the remaining compartments synchronously.

[0062] The working principle of the present invention is as follows:

[0063] Firstly, adopt the water bucket-shaped necking structure 5 to reduce the distance between the steel casing 1 and the bottom keel 2 of the cofferdam, reducing the difficulty of necking processing, on-site installation, and underwater sealing.

[0064] Secondly, at a height not affected by waves, fill the gap between the water bucket-shaped necking structure 5 and the steel casing 1 with several sandbags 6, and cover the top of the sandbags 6 with an annular plate 7. The annular plate 7 further reduces the gap between the steel casing 1 and the steel suspension box cofferdam; the multiple layers of sandbags 6 directly bear the impact of the sea waves, playing a buffering and energy-consuming role.

[0065] Finally, after the steel suspension box cofferdam is lowered in place, due to the space between the water bucket-shaped necking structure 5, the annular plate 7 and the steel casing 1 being filled with dense sandbags 6, only a small amount of concrete enters this space when pouring the underwater concrete for sealing the bottom. And under the support and wave-dissipating action of the sandbags 6, the underwater concrete for sealing the bottom is not easily washed away, providing a certain time for the concrete to set and harden, thereby greatly increasing the probability of successful underwater sealing.

[0066] At the same time, when calculating the anti-floating stability of the steel suspension box cofferdam, the bonding force between the small amount of concrete poured into the necking and the steel casing 1 is not considered, and more use is made of the anti-buoyancy provided by the tension and compression bars 8 and the shear connection keys.

[0067] In the present invention, the problem that the underwater sealing operation of the steel suspension box cofferdam in strong wave areas can only be carried out during low tide and slack tide is innovatively solved. For projects with a large number of sea platforms and tight construction periods, the construction speed can be greatly increased; while reducing a large amount of high-altitude waiting operation time, the construction safety is also improved.

[0068] The present invention has been described by way of example in conjunction with the accompanying drawings. Obviously, the specific implementation of the present invention is not limited by the above methods. As long as various improvements are made by adopting the method concept and technical solution of the present invention, or directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area, characterized in that: The specific steps are: S1. After the steel casing (1) of the pile foundation is installed, the plane position and inclination of the steel casing (1) are measured. According to the measurement results, the construction drawing of the steel hanging box cofferdam is readjusted so that the minimum distance between the outer wall of each steel casing (1) and the inner side of the surrounding cofferdam bottom keel (2) is controlled to be 20-25 cm; S2, assembling the steel box cofferdam on the steel bracket (3) of the steel casing (1), and after the cofferdam bottom plate (4) is assembled, measuring the distance between each steel casing (1) and the inner edge of the surrounding cofferdam bottom keel (2) again, and processing the bucket-shaped shrinkage structure (5) corresponding to each steel casing (1) according to the measured data; The water barrel-shaped necking structure (5) comprises a necking bottom ring plate (51), a necking side wall ring plate (52) and a necking top plate (53); the distance between the inner wall of the necking bottom ring plate (51) and the outer wall of the steel casing (1) is 5-8 cm; the necking side wall ring plate (52) is vertical and the bottom of the necking side wall ring plate (52) is fixed on the necking bottom ring plate (51); the top of the necking side wall ring plate (52) is welded to the bottom of the cofferdam bottom keel (2) and the inner wall of the necking side wall ring plate (52) and the inner edge of the cofferdam bottom keel (2) are arranged in an inscribed circle state; the gap between the top of the necking side wall ring plate (52) and the cofferdam bottom keel (2) is filled with the necking top plate (53) by welding; S3. After the steel box cofferdam is assembled, it is lifted as a whole, and a bucket-shaped shrinkage structure (5) is installed at the bottom of the cofferdam bottom keel (2) of the steel box cofferdam; S4, cutting off the steel bracket (3) on the outer wall of the steel casing (1), lowering the steel box cofferdam as a whole to a position close to the maximum wave height impact height of the bucket-shaped contraction structure (5), filling the bucket-shaped contraction structure (5) with a plurality of sand bags (6), and further fixing the sand bags (6) with an annular plate (7) and a plurality of stiffening ribs (9); S5. Open the connecting hole of the steel box cofferdam and continue to lower the steel box cofferdam below the sea surface until it reaches the designed position; S6, pouring bottom sealing concrete to complete the bottom sealing operation; The bottom concrete of the steel hanging box cofferdam is poured in compartments, and the area of ​​a single compartment does not exceed six steel casings (1). A test compartment (10) covering only a single steel casing (1) is provided. When the pouring effect of the bottom concrete of the test compartment (10) is good, the bottom concrete of the remaining compartments is poured synchronously.

2. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 1 is characterized in that: In step S2, a plurality of shrinkage stiffening plates (54) are evenly distributed on the circumference of one side of the shrinkage side wall ring plate (52) facing away from the steel casing (1), and the shrinkage stiffening plates (54) are connected to the shrinkage bottom ring plate (51), the shrinkage side wall ring plate (52), and the shrinkage top plate (53).

3. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 2 is characterized in that: In step S2, the elevation of the steel corbel (3) is ensured not to be submerged by the high tide level.

4. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 3 is characterized in that: In step S4, the sandbag (6) is made of a polypropylene or polyester fiber woven bag, the diameter of the sandbag (6) is 15-20 cm, and the sandbag (6) is densely filled in the bucket-shaped necking structure (5).

5. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 4 is characterized in that: In step S4, the outer side of the annular plate (7) is fixedly connected to the inner side of the necked top plate (53), and the distance between the inner side of the annular plate (7) and the outer wall of the steel casing (1) is 2-3 cm. The annular plate (7) is manufactured and installed in blocks on site, and finally forms a ring shape.

6. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 5 is characterized in that: In step S4, the stiffening rib plate (9) is uniformly welded around the circumference between the top of the annular plate (7) and the top of the necked top plate (53).

7. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 6 is characterized in that: In step S5, during the lowering of the steel box cofferdam, attention is paid to the impact of the sandbags (6) at the bucket-shaped necking structure (5). If the sandbags (6) are damaged or displaced, the lowering is stopped, the positions of the sandbags (6) are readjusted, and the number of stiffening ribs (9) is further increased to reinforce and compact the sandbags (6), and the lowering is continued until the sandbags (6) are at the designed elevation.

8. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 7 is characterized in that: In step S6, it is checked whether the sandbags (6) in the steel box cofferdam are eroded and displaced. If not, the tension and compression rods (8) are welded. The bottom of the tension and compression rods (8) are welded to the bottom keel (2) of the cofferdam and the top is welded obliquely to the outer wall of the steel casing (1). After the tension and compression rods (8) are successfully connected to the steel casing (1), the suspension rods on the top of the steel casing (1) are removed. The steel box cofferdam is transformed from a state of being suspended at multiple points to being supported by welding multiple tension and compression rods (8) and the steel casing (1), completing the system conversion.

9. The method for rapid bottom sealing of a steel box cofferdam in a strong wave sea area according to claim 8 is characterized in that: In step S6, the bottom sealing concrete is quick-setting concrete.

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