Method for simply evaluating printability of pavement concrete in laboratory

Abstract

The invention relates to a method for simply evaluating pavement concrete printability in a laboratory, and belongs to the technical field of concrete 3D printing. Shooting the top surface of the sample, carrying out gray binarization processing, calculating the area ratio of a gap region, and further evaluating the flatness of the top surface; the thickness of the batten-shaped concrete sample in the initial state is measured, the distance between the collapse position of the batten-shaped concrete sample after 15 min and the side edge in the initial state is recorded, the relative collapse edge ratio is calculated, and then the collapse edge conditions on the two sides are evaluated; the shape change of the concrete is observed, the initial plastic strength is measured, and then the shape preserving capacity is evaluated; and judging whether the concrete in the mix proportion has printability or not. According to the method, comprehensive evaluation is carried out through the top surface flatness, the edge collapse conditions on the two sides and the shape preserving capacity, and the printability of the pavement concrete can be efficiently and visually evaluated.

Description

Technical Field

[0001] The invention relates to a method for simply evaluating the printability of pavement concrete in a laboratory, and belongs to the technical field of concrete 3D printing. Background Art

[0002] At present, the construction of cement concrete pavement in my country mostly adopts three-roller pavers and small paving machines for paving, which has problems such as long construction period, large consumption of support templates, and great construction difficulty. 3D printing technology does not require mechanical processing and molds, and can directly generate components. With good design concepts and printing processes, this technology can greatly improve production efficiency. Applying 3D printing technology to cement concrete pavement can effectively solve the above construction problems.

[0003] However, the 3D printing technology of pavement concrete has very high requirements on the workability of concrete. It must not only be dense enough without vibration, but also be self-stabilizing without formwork support. However, there is currently no unified understanding and standard on whether pavement concrete is printable.

[0004] If the printability of 3D printed concrete can be evaluated reliably and efficiently, it will be of great significance to the development of 3D printing technology in pavement engineering. Therefore, it is very necessary to propose a simple laboratory method to evaluate the printability of pavement concrete. Summary of the invention

[0005] In view of the above problems, the present invention provides a simple laboratory method for evaluating the printability of pavement concrete. The method can evaluate the printability of pavement concrete efficiently and intuitively by making a comprehensive evaluation based on the top surface flatness, the collapse condition of the two sides and the shape retention ability.

[0006] The technical solution of the present invention is as follows:

[0007] A simple laboratory method for evaluating the printability of pavement concrete includes the following steps:

[0008] (1) Mix concrete and print out a 1m long strip-shaped concrete specimen;

[0009] (2) Use a camera with horizontal calibration to photograph the top surface of the strip-shaped concrete specimen, crop the photographed image to leave only the concrete part, and ensure that there is no other picture in the image. Then use MATLAB to perform grayscale binarization on the image, divide the pixels in the image into void area and dense area according to the grayscale value, count the void area area, calculate the void area ratio, and then evaluate the top surface flatness;

[0010] (3) Measure the thickness h of the strip-shaped concrete specimen in the initial state, and record the distance Δd15min between the collapse position of the strip-shaped concrete specimen 15 minutes later and the side edge in the initial state, calculate the relative collapse ratio, and then evaluate the collapse conditions on both sides;

[0011] The sample thickness h can be controlled by adjusting the height of the inclined plate in the printing device according to the specific requirements of the test, but due to the fluidity of fresh concrete, the sample thickness h is usually 1 to 3 cm lower than the height of the tail end of the inclined plate 7;

[0012] (4) Observe the shape change of concrete, measure the initial plastic strength, and then evaluate the shape retention ability;

[0013] (5) When at least one of the top surface flatness, the collapsed edge conditions on both sides, and the shape retention ability reaches excellent, the concrete with this mix ratio is judged to be printable; when the top surface flatness, the collapsed edge conditions on both sides, and the shape retention ability are all good or below, the concrete with this mix ratio is judged to be not printable.

[0014] Preferably, in step (1), a strip-shaped concrete sample with a length of 1 m is spread by a 3D printing device, and the thickness of the strip-shaped concrete sample is 9 to 15 cm.

[0015] Preferably, in step (1), the 3D printing device comprises a base plate, a paving device and a power traction device, wherein the base plate is rectangular, and a convex strip is provided on each of two sides thereof, and a track is provided on the convex strip;

[0016] The paving device is a box structure, with upper and lower ends opened, the front end closed, and a printing outlet provided at the rear end. Rollers are provided on both sides of the bottom of the paving device, and the rollers can move along the track under the traction of the power traction device;

[0017] The paving device comprises a material platform, a vertical material baffle plate and an inclined panel connected in sequence. A top plate is fixedly arranged on the top of the box body at the upper part of the space formed by the front wall of the box body and the inclined panel of the paving device. The material platform is used to fill the newly mixed concrete. During 3D printing, the newly mixed concrete fills the space formed by the top plate, the bottom plate, the vertical material baffle plate, the front wall of the box body, the two side walls of the box body and the inclined panel.

[0018] The top plate is a V-shaped top plate, which is low in the middle and high on both sides; a horizontal plate is arranged behind the inclined plate, and the inclined plate, the horizontal plate, the two side walls of the box body and the bottom plate together form a printing outlet; the box body of the paving device is made of organic glass, and the material table, the vertical material baffle plate, the inclined plate and the horizontal plate are all fixedly connected by bolts on the side walls of the box body;

[0019] The inclined panel has an inclination angle of 6° to 10°, and is higher in the front and lower in the back;

[0020] An extended template is extended to the rear of both sides of the box body.

[0021] Preferably, the dimensions of the box are 65 mm long × 50 mm wide × 55 mm high, the length of the bottom plate is greater than 2 m, and the width of the bottom plate is greater than the width of the box;

[0022] A crossbeam is arranged on both sides of the box body, and the roller is arranged at the bottom of the crossbeam;

[0023] A vertical baffle is provided on the bottom plate as a starting point for paving;

[0024] The power traction device is driven by an electric motor.

[0025] The use process of the present invention is:

[0026] (1) Fix the base plate with tracks to the ground and apply lubricating oil on the upper surface of the base plate and the track surface;

[0027] (2) Place the paving device on one end of the base plate, align the vertical plate with the front end of the horizontal plate, and make the rollers on both sides fit the track;

[0028] (3) Start mixing concrete. The volume of the mixed concrete is generally not less than 60L;

[0029] (4) Transfer the mixed concrete to the material table and start loading from the position of the vertical baffle until the space area consisting of the V-shaped top plate, bottom plate, vertical baffle plate, front wall of the box, two side walls of the box and inclined panel is filled;

[0030] (5) Use the power traction device to drive the paving device at a uniform and slow speed, continuously replenish concrete on the material platform and fill the loss of concrete under the V-shaped top plate. During the paving process, ensure that the height of the concrete filling into the front end of the vertical baffle is constant, and ensure that the concrete on the bottom plate is subjected to continuous and constant pressure to form a dense shape. The paving speed should be controlled at 2.0-2.5cm / s;

[0031] (6) After the concrete has completely slid off the material table and has been squeezed out of the inclined plate, the paving device is continuously pulled out of the base plate. The extruded concrete must not be disturbed during the process. In order to avoid the effects of water evaporation and partial hydration of cement, the total time from concrete mixing to paving should be controlled within 20 minutes.

[0032] During the printing process, the thickness of the concrete is related to the height of the inclined plate and the horizontal plate. The height of the combination of the inclined plate and the horizontal plate is adjustable, and the width of the concrete is controlled by the distance between the two side plates of the paving device.

[0033] Preferably, in step (2), if the shooting environment is too dark or too bright, the exposure compensation needs to be increased or decreased. Sufficient light and uniform brightness are required during shooting, and the concrete surface needs to be evenly focused to ensure that the collected pictures are clear and the picture resolution is not less than 2k.

[0034] Preferably, in step (2), during the grayscale binarization processing, the pixel points with a grayscale of 0 to 127 are set to 0, which is a gap area; the pixel points with a grayscale of 128 to 255 are set to 1, which is a dense area; 0 and 1 represent black and white respectively, and then the area of ​​the gap area is counted, and the area ratio of the gap area is calculated by formula (1):

[0035]

[0036] Preferably, when the area ratio of the gap area is (0, 5%], the top surface flatness is excellent; when the area ratio of the gap area is (5%, 15%], the top surface flatness is good; when the area ratio of the gap area is (15%, 30%], the top surface flatness is medium; when the area ratio of the gap area is (30%, 100%], the top surface flatness is poor, as shown in Table 1:

[0037] Table 1 Top surface flatness evaluation table

[0038] Top surface flatness grade excellent good middle Difference Void area percentage (0,5] (5,15] (15,30] (30,100)

[0039] Preferably, in step (3), the relative edge collapse ratio δ is calculated using formula (2):

[0040]

[0041] When δ is (0,15%], the edge collapse condition on both sides is excellent; when δ is (15%,25%], the edge collapse condition on both sides is good; when δ is (25,35%], the edge collapse condition on both sides is medium; when δ is (35,100%], the edge collapse condition on both sides is poor, as shown in the table:

[0042] Table 2 Relative edge collapse ratio evaluation table

[0043]

[0044] Preferably, in step (4), the test method of the initial plastic strength is as follows:

[0045] Put the freshly mixed concrete into a cylinder with an inner diameter of 20cm and a height of 10cm. After filling, immediately move the cylinder vertically upward to obtain cylindrical concrete. Observe the shape change of the concrete and judge whether the concrete collapses within 1min. Collapse means that the concrete surface cracks, scatters or loses more than 1cm in height. If so, the shape retention ability is extremely poor; if not, it is necessary to further calculate the initial plastic strength.

[0046] The calculation process of initial plastic strength is:

[0047] Place the cylinder with a fixed bracket just above the cylindrical concrete, close to the upper surface. Do not disturb the concrete. Add fine sand evenly into the cylinder at a rate of 50 g / s until the concrete collapses. Record the mass m of the fine sand used and calculate the initial plastic strength σ using formula (3):

[0048]

[0049] Where, σ is the initial plastic strength, unit is ×10 -3 MPa; m is the mass of fine sand; r is the radius of the cylinder; g is taken as 9.8;

[0050] When σ is [4,∞), the conformal ability is excellent; when σ is [2,4), the conformal ability is good; when σ is [0.5,2), the conformal ability is medium; when σ is [0,0.5), the conformal ability is poor, as shown in the table:

[0051] Table 3 Conformity evaluation table

[0052]

[0053] Preferably, in step (5), when the top surface flatness, the collapse of the two sides and the shape retention ability are all excellent, the concrete under this mix ratio is judged to have excellent printability. In some pavement projects with lower construction quality requirements, the requirements for the printability of the material can be appropriately lowered, that is, when at least one of the three indicators reaches excellent under specific engineering requirements, the concrete under this mix ratio can also be judged to meet the requirements of 3D printing concrete material properties; when the three indicators are all good or below, the concrete under this mix ratio does not meet the requirements of 3D printing concrete material properties, and the mix ratio needs to be improved and re-evaluated.

[0054] After mixing concrete of a certain target mix ratio, the present invention prints out a strip-shaped concrete specimen through a 3D printing device, and evaluates the extrudability of the concrete under the mix ratio by observing the surface conditions, including the flatness of the top surface and the collapse of the edges on both sides. In addition, the shape-keeping ability of the concrete before setting is evaluated through a simple initial plastic strength test method, thereby reflecting the constructability of the concrete under the designed mix ratio. Combining the evaluation methods of extrudability and constructability, it is finally intuitively judged whether the concrete under the designed mix ratio is printable.

[0055] For any details not provided in the present invention, please refer to the prior art.

[0056] The beneficial effects of the present invention are:

[0057] 1. The present invention obtains strip-shaped concrete specimens through a 3D printing device, and proposes three evaluation methods and evaluation indicators for judging the printability of pavement concrete, which can intuitively, reliably and efficiently evaluate the printability of pavement concrete.

[0058] 2. The 3D printing device provided by the present invention is small in size, suitable for laboratory research, and can be spread evenly and maintain a stable shape of newly printed concrete, which can make up for the shortcomings of existing test devices for 3D printing concrete. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The drawings in the specification, which constitute a part of the present application, are used to provide further understanding of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0060] Figure 1 It is a schematic diagram of the overall structure of the 3D printing device of the present invention;

[0061] Figure 2 are the pictures in step (2), wherein (a) is a cropped picture of the top surface of the strip-shaped concrete sample; (b) is a picture after grayscale binarization processing;

[0062] Figure 3 It is a schematic diagram of the measurement when evaluating the collapse conditions on both sides;

[0063] Figure 4 Schematic diagram of the device used to calculate the initial plastic strength;

[0064] In the figure, 1-paving device, 2-bottom plate, 3-vertical baffle, 4-roller, 5-material table, 6-vertical material baffle, 7-inclined plate, 8-horizontal plate, 9-top plate, 10-extended formwork, 11-power traction device, 12-convex strip, 13-cross beam. DETAILED DESCRIPTION

[0065] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the implementation of this specification, but are not limited to this. Anything not fully described in the present invention shall be based on the conventional technology in the art.

[0066] Example 1

[0067] A simple laboratory method for evaluating the printability of pavement concrete includes the following steps:

[0068] (1) Mix concrete and print out a 1m long strip-shaped concrete specimen;

[0069] (2) Use a camera with horizontal calibration to photograph the top surface of the strip-shaped concrete specimen, crop the photographed image to leave only the concrete part, and ensure that there is no other picture in the image. Then use MATLAB to perform grayscale binarization on the image, divide the pixels in the image into void area and dense area according to the grayscale value, count the void area area, calculate the void area ratio, and then evaluate the top surface flatness;

[0070] (3) Measure the thickness h of the strip concrete specimen in the initial state, and record the distance Δd15min between the collapse position of the strip concrete specimen after 15 minutes and the side edge in the initial state, such as Figure 3 As shown, the relative edge collapse ratio is calculated to evaluate the edge collapse conditions on both sides;

[0071] The sample thickness h can be controlled by adjusting the height of the inclined plate in the printing device according to the specific requirements of the test, but due to the fluidity of fresh concrete, the sample thickness h is usually 1 to 3 cm lower than the height of the tail end of the inclined plate 7;

[0072] (4) Observe the shape change of concrete, measure the initial plastic strength, and then evaluate the shape retention ability;

[0073] (5) When at least one of the top surface flatness, the collapsed edge conditions on both sides, and the shape retention ability reaches excellent, the concrete with this mix ratio is judged to be printable; when the top surface flatness, the collapsed edge conditions on both sides, and the shape retention ability are all good or below, the concrete with this mix ratio is judged to be not printable.

[0074] Example 2

[0075] A simple laboratory method for evaluating the printability of pavement concrete, as described in Example 1, in step (1), Figure 1 As shown, a strip-shaped concrete sample with a length of 1 m is spread by a 3D printing device, and the thickness of the strip-shaped concrete sample is 9 to 15 cm.

[0076] In step (1), a base plate 2, a paving device 1 and a power traction device 11 are included. The base plate 2 is rectangular, and a convex strip 12 is provided on each side thereof, which can serve as a track foundation, and a track is provided on the convex strip 12;

[0077] The paving device 1 is a box structure, with upper and lower ends opened, the front end closed, and a printing outlet provided at the rear end. Rollers 4 are provided on both sides of the bottom of the paving device 1. The rollers 4 can move along the track under the traction of the power traction device 11. The setting of the rollers can reduce the friction between the box and the bottom plate during the paving process. After combining with the power traction device, the continuous control of the paving speed can be easily achieved, and paving along the track can avoid deviation of the printing track. In the present invention, the movement direction of the paving device is taken as the front;

[0078] The paving device 1 includes a material table 5, a vertical material baffle plate 6 and an inclined panel plate 7 which are connected in sequence. At the upper part of the space formed by the front wall of the box and the inclined panel plate of the paving device, a top plate 9 is fixedly arranged on the top of the box. The material table 5 is used to fill the newly mixed concrete. During 3D printing, the newly mixed concrete fills the space formed by the top plate 9, the bottom plate 2, the vertical material baffle plate 6, the front wall of the box, the two side walls of the box and the inclined panel plate 7.

[0079] The top plate 9 is a V-shaped top plate, which is low in the middle and high on both sides.

[0080] A horizontal plate 8 is arranged behind the inclined plate 7, and the inclined plate 7, the horizontal plate 8, the two side walls of the box and the bottom plate together form a printing outlet. During 3D printing, the newly mixed concrete can be loaded on the material table 5 and slide down to the bottom plate 2 along the vertical baffle plate 6 until it fills the space limited by the top plate 9, the bottom plate 2, the vertical baffle plate 6, the front wall of the box, the two side walls of the box and the inclined plate 7. The main purpose of setting the V-shaped top plate is to increase the pressure on the concrete on both sides by increasing the concrete stacking height, so as to avoid the problem that the concrete on the bottom plate is not smoothed due to too little pressure on both sides; when the printing device is moved along the top plate, the concrete is smoothed. Figure 1 During the process of moving forward in the middle direction, the test personnel manually added concrete on the material table to fill the height loss of concrete under the V-shaped top plate, ensuring that the concrete was fed continuously and always under constant pressure during the printing process. The concrete falling on the bottom plate is continuously subjected to the force from the inclined plate, horizontal plate and the two side plates of the box body on the surface, as well as the gravity of the concrete material. The main function of the inclined plate and the horizontal plate is to squeeze and smooth the concrete surface during the paving process. The inclined plate has an inclination angle of 6°, with a high front and a low back state. The combination of the inclined plate and the horizontal plate can achieve a good smoothing effect.

[0081] The box of the paving device is made of plexiglass, which helps to observe the real-time appearance of concrete during the paving process. The material table, vertical baffle plate, inclined plate and horizontal plate can be bonded by acrylic glue. After being assembled together, the material table, vertical baffle plate, inclined plate and horizontal plate are fixed and connected by bolts on the side wall of the box. The material table, vertical baffle plate, inclined plate and horizontal plate can be drilled at different heights on both sides of the box and bolted to achieve the adjustment of the angle and height of the internal plate according to the actual test requirements.

[0082] An extended template 10 is extended from the rear of both sides of the box, the main purpose of which is to provide a transition from a three-way stress state to a state without template support for the concrete at the beginning of paving, reducing the deformation of the concrete due to stress changes and helping to maintain the stability of the shape of the printed concrete. The printing process continues from the addition of materials until the concrete is completely extruded from the printing device.

[0083] The box size is 65mm long × 50mm wide × 55mm high. Usually, the length of the paved sample is not less than 1m, so the length of the bottom plate 2 should be greater than 2m, and the width of the bottom plate 2 should be greater than the width of the box, preferably 5 to 10cm wider, to ensure that the paved concrete is not affected by the two sides of the bottom plate.

[0084] A crossbeam 13 is arranged on both sides of the box body, and a roller 4 is arranged at the bottom of the crossbeam.

[0085] A vertical baffle 3 is provided on the bottom plate 2 as the starting point for paving, and the initially filled concrete is subjected to three-dimensional stress under the joint action of the vertical baffle, the two side plates of the box body and the inclined plate. At this time, the concrete reaches a dense state under the action of three-dimensional stress.

[0086] The power traction device 11 is driven by a motor to achieve uniform paving at a constant speed.

[0087] Example 3

[0088] A simple laboratory method for evaluating the printability of pavement concrete, as described in Example 2, in step (2), if the shooting environment is too dark or too bright, it is necessary to increase or decrease the exposure compensation, and sufficient light and uniform brightness are required during shooting, and the concrete surface needs to be evenly focused to ensure that the collected pictures are clear and the picture resolution is not less than 2k.

[0089] Preferably, in step (2), when grayscale binarization is performed, the pixel points with grayscale in the range of 0 to 127 are set to 0, which is a gap area; the pixel points with grayscale in the range of 128 to 255 are set to 1, which is a dense area; 0 and 1 represent black and white, respectively. Figure 2 As shown, the area of ​​the gap area is then counted, and the area ratio of the gap area is calculated by formula (1):

[0090]

[0091] Preferably, when the area ratio of the gap area is (0, 5%], the top surface flatness is excellent; when the area ratio of the gap area is (5%, 15%], the top surface flatness is good; when the area ratio of the gap area is (15%, 30%], the top surface flatness is medium; when the area ratio of the gap area is (30%, 100%], the top surface flatness is poor, as shown in Table 1:

[0092] Table 1 Top surface flatness evaluation table

[0093] Top surface flatness grade excellent good middle Difference Void area percentage (0,5] (5,15] (15,30] (30,100)

[0094] Example 4

[0095] A simple laboratory method for evaluating the printability of pavement concrete, as described in Example 3, in step (3), the relative collapse ratio δ is calculated using formula (2):

[0096]

[0097] When δ is (0,15%], the edge collapse condition on both sides is excellent; when δ is (15%,25%], the edge collapse condition on both sides is good; when δ is (25,35%], the edge collapse condition on both sides is medium; when δ is (35,100%], the edge collapse condition on both sides is poor, as shown in the table:

[0098] Table 2 Relative edge collapse ratio evaluation table

[0099]

[0100] Example 5

[0101] A simple laboratory method for evaluating the printability of pavement concrete, as described in Example 4, in step (4), the test method for the initial plastic strength is as follows:

[0102] Put the freshly mixed concrete into a cylinder with an inner diameter of 20cm and a height of 10cm. After filling, immediately move the cylinder vertically upward to obtain cylindrical concrete. Observe the shape change of the concrete and judge whether the concrete collapses within 1min. Collapse means that the concrete surface cracks, scatters or loses more than 1cm in height. If so, the shape retention ability is extremely poor; if not, it is necessary to further calculate the initial plastic strength.

[0103] The calculation process of initial plastic strength is:

[0104] Use a fixed bracket to place the cylinder directly above the cylindrical concrete, close to the upper surface, without disturbing the concrete. Figure 4 As shown in the figure, fine sand is evenly added into the cylinder at a speed of 50 g / s until the concrete collapses. The mass m of fine sand used is recorded, and the initial plastic strength σ is calculated by formula (3):

[0105]

[0106] Where, σ is the initial plastic strength, unit is ×10 -3 MPa; m is the mass of fine sand; r is the radius of the cylinder; g is taken as 9.8;

[0107] When σ is [4,∞), the conformal ability is excellent; when σ is [2,4), the conformal ability is good; when σ is [0.5,2), the conformal ability is medium; when σ is [0,0.5), the conformal ability is poor, as shown in the table:

[0108] Table 3 Conformity evaluation table

[0109]

[0110] Example 6

[0111] A simple laboratory method for evaluating the printability of pavement concrete, as described in Example 5, in step (5), when the top surface flatness, the collapse of the two sides and the shape retention ability are all excellent, the concrete under the mix ratio is judged to have excellent printability. In some pavement projects with lower construction quality requirements, the requirements for the printability of the material can be appropriately lowered, that is, when at least one of the three indicators reaches excellent under specific engineering requirements, it can also be judged that the concrete under the mix ratio meets the requirements of 3D printing concrete material properties; when the three indicators are all good or below, the concrete under the mix ratio does not meet the requirements of 3D printing concrete material properties, and the mix ratio needs to be improved and re-evaluated.

[0112] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A simple laboratory method for evaluating the printability of pavement concrete, characterized in that: The steps include: (1) Mix concrete and print out a 1m long strip-shaped concrete specimen; (2) Use a camera with horizontal calibration to photograph the top surface of the strip-shaped concrete specimen, crop the photographed image to leave only the concrete part, and then use MATLAB to perform grayscale binarization on the image. Use the grayscale value to divide the pixels in the image into void areas and dense areas, count the void area area, calculate the void area ratio, and then evaluate the top surface flatness; (3) Measure the thickness h of the strip-shaped concrete specimen in the initial state, and record the distance Δd15min between the collapse position of the strip-shaped concrete specimen 15 minutes later and the side edge in the initial state, calculate the relative collapse ratio, and then evaluate the collapse conditions on both sides; (4) Observe the shape change of concrete, measure the initial plastic strength, and then evaluate the shape retention ability; (5) When at least one of the top surface flatness, the side edge collapse and the shape retention ability reaches the excellent level, the concrete under this mix ratio is judged to be printable; When the top surface flatness, the collapse of the two sides and the shape retention ability are all good or below, the concrete with this mix ratio is judged to be not printable.

2. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 1, characterized in that: In step (1), a strip-shaped concrete sample with a length of 1 m is spread by a 3D printing device, and the thickness of the strip-shaped concrete sample is 9 to 15 cm.

3. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 1, characterized in that: In step (1), the 3D printing device includes a base plate, a paving device and a power traction device, wherein the base plate is rectangular and has a convex strip on each of its two sides, and a track is provided on the convex strip; The paving device is a box structure, with upper and lower ends opened, the front end closed, and a printing outlet provided at the rear end. Rollers are provided on both sides of the bottom of the paving device, and the rollers can move along the track under the traction of the power traction device; The paving device comprises a material platform, a vertical material baffle plate and an inclined panel connected in sequence. A top plate is fixedly arranged on the top of the box body at the upper part of the space formed by the front wall of the box body and the inclined panel of the paving device. The material platform is used to fill the newly mixed concrete. During 3D printing, the newly mixed concrete fills the space formed by the top plate, the bottom plate, the vertical material baffle plate, the front wall of the box body, the two side walls of the box body and the inclined panel. The top plate is a V-shaped top plate, which is low in the middle and high on both sides; a horizontal plate is arranged behind the inclined plate, and the inclined plate, the horizontal plate, the two side walls of the box body and the bottom plate together form a printing outlet; the box body of the paving device is made of organic glass, and the material table, the vertical material baffle plate, the inclined plate and the horizontal plate are all fixedly connected by bolts on the side walls of the box body; The inclined panel has an inclination angle of 6° to 10°, and is higher in the front and lower in the back; An extended template is extended to the rear of both sides of the box body.

4. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 3, characterized in that: The dimensions of the box are 65mm long × 50mm wide × 55mm high, the length of the bottom plate is greater than 2m, and the width of the bottom plate is greater than the width of the box; A crossbeam is arranged on both sides of the box body, and the roller is arranged at the bottom of the crossbeam; A vertical baffle is provided on the bottom plate as a starting point for paving; The power traction device is driven by an electric motor.

5. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 1, characterized in that: In step (2), the shooting requires sufficient light and uniform brightness, and the image resolution is not less than 2k.

6. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 1, characterized in that: In step (2), during the grayscale binarization process, the pixel points with a grayscale between 0 and 127 are set to 0, which is a gap area; the pixel points with a grayscale between 128 and 255 are set to 1, which is a dense area; 7. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 6, characterized in that: When the area ratio of the void area is (0,5%], the top surface flatness is excellent; when the area ratio of the void area is (5%,15%], the top surface flatness is good; when the area ratio of the void area is (15%,30%], the top surface flatness is medium; when the area ratio of the void area is (30%,100%], the top surface flatness is poor.

8. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 7, characterized in that: In step (3), the relative edge collapse ratio δ is calculated using formula (2): When δ is (0,15%], the edge collapse condition on both sides is excellent; when δ is (15%,25%], the edge collapse condition on both sides is good; when δ is (25,35%], the edge collapse condition on both sides is medium; when δ is (35,100%], the edge collapse condition on both sides is poor.

9. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 8, characterized in that: In step (4), the test method of initial plastic strength is as follows: Put the freshly mixed concrete into a cylinder with an inner diameter of 20cm and a height of 10cm. After filling, immediately move the cylinder vertically upward to obtain cylindrical concrete. Observe the shape change of the concrete and judge whether the concrete collapses within 1min. Collapse means that the concrete surface cracks, scatters or loses more than 1cm in height. If so, the shape retention ability is extremely poor; if not, it is necessary to further calculate the initial plastic strength. The calculation process of initial plastic strength is: The cylinder is placed on a fixed bracket just above the cylindrical concrete, close to the upper surface. Fine sand is evenly added into the cylinder at a rate of 50 g / s until the concrete collapses. The mass m of fine sand used is recorded, and the initial plastic strength σ is calculated using formula (3): Where, σ is the initial plastic strength, unit is ×10 -3 MPa; m is the mass of fine sand; r is the radius of the cylinder; g is taken as 9.8; When σ is [4,∞), the conformal ability is excellent; when σ is [2,4), the conformal ability is good; when σ is [0.5,2), the conformal ability is medium; when σ is [0,0.5), the conformal ability is poor.

10. The method for simply evaluating the printability of pavement concrete in a laboratory according to claim 9, characterized in that: In step (5), when the top surface flatness, the collapse of the edges on both sides and the shape retention ability are all excellent, it is judged that the concrete under this mix ratio has excellent printability.

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