1. A method for rapidly evaluating the printability of slurry in 3D printing of a cement-based material is characterized by comprising the following steps of:

(1) according to the thixotropy theory of the Bingham fluid and the combination of actual printer system parameters, the optimal yield stress range of printable slurry is obtained: tau is₁＜τ＜τ₂；

(2) By a linear relationship of yield stress τ to slump s: τ ═ 3.30 ρ (H-s), where ρ is the slurry density and H is the slump cone height, giving a slump printable range: s₁＜s＜s₂；

(3) In order to ensure the accuracy of evaluation, a jump table fluidity index d is used for evaluation, and the fluidity range can be determined according to reference values provided by documents or equipment manufacturers: d₁＜d＜d₂；

(4) After the preparation of the slurry is finished, slump tests and table jump fluidity tests are respectively carried out at more than one different time nodes to obtain a slump value s and a slump value d, normalization processing is carried out, and a slump index p is obtained_sFluidity index p_dDrawing in the same scatter diagram; wherein the content of the first and second substances,

from the scatter plot, the evaluation criteria are as follows:

(4-1)p₁as a threshold for satisfying the extrudability, p₂As a threshold to satisfy constructability; when p is_s、p_dWhile falling on p₁、p₂In between, printability is deemed to be satisfied; wherein the content of the first and second substances,

(4-2) fitting the scattergram to obtain a time-dependent loss curve of the fluidity of the slurry, wherein the gentler the curve, the longer the printable window period, the curve and p₁、p₂The optimal printable window period is defined between the intersection points.

2. The method for rapidly evaluating the paste printability in 3D printing of cement-based materials according to claim 1, wherein the optimal yield stress range of step (1) is derived by the following steps:

the above-mentioned tau₁To the rear cover pressure P_layerSaid τ being₂Is the maximum extrusion stress P of the extrusion device_extrusionWherein the maximum extrusion stress P of the extrusion device_extrusionThe rear cover pressure P is provided by the equipment provider or obtained according to the actual measurement_layerThe self-weight pressure for the subsequent printed layer being a function of time t, P_layerρ gh (t), where: ρ is the paste density, g ═ 9.8N/kg, and h is the print layer height.

3. Use of a method according to claim 1 for rapid assessment of paste printability in 3D printing of cementitious materials in 3D printing.

Background

The 3D printing process of the extruded concrete determines that the printing paste needs to meet unique rheological requirements so as to achieve good printability. The printability of the slurry refers to that the rheological parameters of the slurry are just maintained in a certain specific range, and meanwhile, the slurry has good fluidity so that the slurry can be smoothly extruded out, and has good collapse-keeping performance so that the slurry has certain molding capacity after being extruded out. Early Le et al examined the extrudability and the constructability of the paste by non-stop extrusion length and number of printable layers, respectively, and the evaluation of the constructability was based mainly on visual observation of the absence of significant deformation of the bottom layer. With the development of 3D printing technology in the building field in recent years, a method for evaluating the printability through rheological properties such as yield stress, fluidity, shear strength and the like is derived. However, since the current 3D printing system does not form a uniform standard, the printable range of various parameters obtained by those skilled in the art is different, so that the applicability of the above evaluation method is not strong. In addition, when the mix proportion is designed, the printable formula is determined by a preprinting test, and the printable range of the related evaluation index is measured reversely, not by the forward design. Therefore, a rapid, efficient and strong-universality evaluation method is urgently needed to provide guidance for forward design of 3D printing cement-based materials, reduce test cost in a mix proportion design stage, and improve work efficiency.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a method for rapidly evaluating the printability of slurry in 3D printing of a cement-based material and application thereof, and the printability of the cement-based material can be rapidly and efficiently evaluated with low cost and strong universality.

The invention is realized by the following technical scheme:

a method for rapidly evaluating the printability of slurry in 3D printing of a cement-based material comprises the following steps:

(1) according to the thixotropy theory of the Bingham fluid and the combination of actual printer system parameters, the optimal yield stress range of printable slurry is obtained: tau is₁＜τ＜τ₂；

(2) By a linear relationship of yield stress τ to slump s: τ ═ 3.30 ρ (H-s), where ρ is the slurry density and H is the slump cone height, giving a slump printable range: s₁＜s＜s₂；

(3) In order to ensure the accuracy of evaluation, a jump table fluidity index d is used for evaluation, and the fluidity range can be determinedAccording to the reference value provided by the literature or equipment manufacturer, the following steps are determined: d₁＜d＜d₂；

(4) After the preparation of the slurry is finished, slump tests and table jump fluidity tests are respectively carried out at more than one different time nodes to obtain a slump value s and a slump value d, normalization processing is carried out, and a slump index p is obtained_sFluidity index p_dDrawing in the same scatter diagram; wherein the content of the first and second substances,

from the scatter plot, the evaluation criteria are as follows:

(4-1)p₁as a threshold for satisfying the extrudability, p₂As a threshold to satisfy constructability; when p is_s、p_dWhile falling on p₁、p₂In between, printability is deemed to be satisfied; wherein the content of the first and second substances,

(4-2) fitting the scattergram to obtain a time-dependent loss curve of the fluidity of the slurry, wherein the gentler the curve, the longer the printable window period, the curve and p₁、p₂The optimal printable window period is defined between the intersection points.

Preferably, the optimal yield stress range of step (1) is derived as follows:

the above-mentioned tau₁To the rear cover pressure P_layerSaid τ being₂Is the maximum extrusion stress P of the extrusion device_extrusionWherein the maximum extrusion stress P of the extrusion device_extrusionThe rear cover pressure P is provided by the equipment provider or obtained according to the actual measurement_layerThe deadweight pressure for the subsequent printed layer being a function of time t, p_layerρ gh (t), where: ρ is the paste density, g ═ 9.8N/kg, and h is the print layer height.

The application of the method for rapidly evaluating the printability of the slurry in 3D printing of the cement-based material in the 3D printing is disclosed.

The invention has the following beneficial effects:

compared with the prior art, the method has the characteristics of high speed, high efficiency, low test cost and the like, more importantly, the method is not influenced by a printing system, has strong universality, can independently calculate the printable range of the index aiming at different printers, and provides guidance and basis for the forward design of the mix proportion. The blindness of determining the printability through field printing in the 3D printing concrete mix proportion design stage is avoided, and material resources and human resources are saved.

Drawings

FIG. 1 is a flow chart of a method for rapidly evaluating the printability of a slurry in 3D printing of a cement-based material;

FIG. 2 is a schematic diagram of a method for processing evaluation index data and evaluation criteria in embodiment 1;

FIG. 3 is a scatter diagram showing evaluation indexes of the slump test and the table jump flow test in example 2.

Detailed Description

The invention is described in further detail below with reference to specific embodiments and with reference to the following drawings.

Example 1

A method for rapidly evaluating the printability of slurry in 3D printing of cement-based materials is shown in figure 1 and comprises the following specific steps:

(1) determining upper and lower threshold values of yield stress tau for printable slurry

And obtaining the printable optimal yield stress range of the slurry according to the thixotropy theory of the Bingham fluid and the combination of actual printer system parameters.

Bingham fluid means that shear stress is linear with shear rate, but flow only begins when shear stress is greater than yield stress; the fluid has a gel structure at rest.

When the optimal yield stress range of the printable slurry is theoretically calculated, the maximum extrusion force provided by the extrusion device of the printing system and the overlaying pressure to be borne by the slurry in the deposition process, which is determined by the size of the nozzle, the discharging speed and the like, are considered.

The printability of the 3D printing material means that slurry has certain fluidity and can be smoothly extruded, and the slurry can lose fluidity after being extruded so as to bear the overlying pressure of a subsequent printing layer. Thus, the theoretical yield stress τ of the printable slurry should be less than the maximum extrusion stress P provided by the extrusion device_extrusionAnd is greater than the self-weight pressure P of the subsequent printing layer_layerI.e. the yield stress τ should satisfy:

P_layer＜τ＜P_extrusion

wherein the maximum extrusion stress P of the extrusion device_extrusionCan be provided by equipment manufacturers or obtained according to actual measurement, the larger the value is, the more favorable the discharging is, the smaller the limiting effect of the extrusion type is, and the larger the adjustable range of the mixing proportion is; the pressing force that can be provided by the feeding system of various printers varies, and generally: p_{Pump type}＞P_{Closed extrusion type}＞P_{Open spiral type}. Rear cover pressure P_layerDirectly related to the actual printing parameters, such as printing speed, nozzle diameter, etc., as a function of time t, here reduced to p_layerρ gh (t), where: ρ is the paste density, g ═ 9.8N/kg, and h is the print layer height.

The optimum yield stress range for the printable slurry can be determined from the above equation:

τ₁＜τ＜τ₂

(2) obtaining the slump range of the printable slurry according to the linear relation between the yield stress tau and the slump s

The yield stress test cost is high, and the slump test is widely applied to evaluating the workability of the slurry in engineering practice and has the characteristics of convenience in operation, low test cost and the like. Regarding the relationship between yield stress and slump, there has been intensive research, and according to the theoretical results in the literature (Chidiac S E, Habibeigi F, Chan D.Slump and slide flow for the characterization of yield stress of fresh concrete [ J ] Aci Materials Journal,2006,103(6):413 and 418.), the linear relationship between yield stress τ and slump S is: τ ═ 3.30 ρ (H-s), where ρ is the slurry density and H is the slump cone height, giving a slump printable range:

s₁＜s＜s₂

(3) in order to improve the accuracy of evaluation, the flow index d of the jumping table can be used for evaluation

The printable range of the flow d of the diving table can be determined according to reference values provided by literature or equipment manufacturers:

d₁＜d＜d₂

(4) slump test and table jump flow test

After the preparation of the slurry is completed, at 2 different time nodes t₁、t₂Respectively carrying out slump test and table jump fluidity test to obtain slump value s and fluidity value d, carrying out normalization treatment and obtaining slump index p_sFluidity index p_dPlotted in the same scatter plot as shown in fig. 2.

In the embodiment, the 2 different time nodes can be expanded into a plurality of nodes, and the fitting of the multi-point data into a curve (the fitting of the two-point data into a straight line) can more accurately reflect the loss of the slurry fluidity over time.

According to fig. 2, the evaluation criteria are as follows:

(a)p₁as a threshold for satisfying the extrudability, p₂As a threshold to satisfy constructability; when p is_s、p_dWhile falling on p₁、p₂In between, printability is considered to be satisfied.

(b) Fitting the scatter diagram can obtain a time-dependent loss curve of the fluidity of the slurry, wherein the gentler the curve is, the longer the printable window period is, the curve is connected with the p₁、p₂The optimal printable window period (Open time) is defined between the intersection points, i.e. t in FIG. 2₁～t₂。

Example 2

A method for rapidly evaluating the printability of slurry in 3D printing of cement-based materials comprises the following specific steps:

(1) determining upper and lower threshold values of yield stress tau for printable slurry

In the embodiment, a desktop type concrete (mortar) 3D printer developed by Jian, Tu and Zhen measuring company is adopted, and the extruding device is of an open spiral type, so that the extruding force provided is small, and the extruding stress P is small_extrusion500Pa is taken.

In this example, the slurry density ρ is 1.65g/cm³A20 mm circular nozzle is adopted, and the height h of a single-layer is 10 mm. Considering that the strength of the lower printing layer is continuously improved along with the progress of hydration, the self weight of the subsequent printing layer is not completely born by the yield stress of the slurry any more. Thus, P_layerCan be simplified to the dead weight pressure of 2 printing layers. The layer height h is originally a function of time t: h (t), which is simplified to a constant of 2 layers of height in this example, i.e., a single layer of 10mm, and a two-layer height h of 0.02m, according to the formula P_layerρ gh (t), we find: p is a radical of_layer＝1.65×10³×9.8×0.02＝323.4Pa。

Thus, the yield stress of the printable slurry should satisfy:

323.4Pa＜τ＜500Pa

(2) obtaining the slump range of the printable slurry according to the linear relation between the yield stress tau and the slump s

The linear relationship between yield stress τ and slump s is: τ ═ 3.30 ρ (H-s), i.e.In this example, the height H of the collapsed cylinder was 0.15 mm.

Thus, the slump range of the printable slurry is:

58.2mm＜s＜90.6mm

namely, the slump s is preferably about 74 mm.

(3) In order to improve the accuracy of evaluation, the flow index d of the jumping table can be used for evaluation

The printable range of the fiber cement-based printing material in this embodiment is taken as follows:

120mm＜d＜180mm

namely, the flow degree d of the diving table is preferably about 150 mm.

(4) Slump test and table jump flow test

After the preparation of the slurry was completed, two (or three) slump tests and a table jump flow test were performed at different time periods, respectively. In order to plot the slump value s and the fluidity value d in the same scattergram, the data were normalized.

As shown in FIG. 3, the slump s is normalized as follows:

the evaluation index p corresponding to the slump s_sLower threshold value p of₁Upper threshold value p₂Respectively as follows:

slump index p of printable slurry_sIt should satisfy: 0.78 < p_s＜1.22。

Similarly, the flow degree value of the jumping table can be normalized, and a flow degree index p of the jumping table is obtained_dThe range of (A): p is more than 0.8_d＜1.2。

For the fiber cement-based slurries of this example, 12min (t) after adding water, respectively₁)、22min(t₂) Two slump tests and two jump table flow tests were performed and the data are shown in table 1 below.

TABLE 1 EXAMPLE 2 slump and Table jump fluidity measured values and normalization processing results

According to fig. 3, the evaluation criterion is as follows:

(a) when p is_s，p_dWhile falling between the two thresholds, printability is considered to be satisfied. Slump index p at 22min in this example_sAnd the table jump fluidity index p_dAll fall outside the printable range and are considered to be unsatisfactory for extrudability and constructability.

(b) The curve obtained by scatter-point fitting in fig. 3 reflects the loss of fluidity of the slurry over time, with a more gradual curve indicating a slower loss of fluidity over time and a longer printable window. The printable window period of the slurry in the embodiment is 5-17 min after water is added.