Annular restrained shrinkage test device and early cracking assessment method for reinforced concrete

Abstract

The present invention belongs to the field of concrete disease research, and relates to an annular restraint shrinkage test device and a reinforced concrete early cracking assessment method. The annular restraint shrinkage test device includes a base plate, a prefabricated crack sheet, an inner steel ring, an outer steel ring, a strain gauge, a steel mesh and a strain collector; the outer steel ring is sleeved on the outside of the inner steel ring and is concentrically arranged on the base plate; an annular space for forming a concrete ring is formed between the outer steel ring and the inner steel ring; the steel mesh is arranged in the annular space; the prefabricated crack sheet is placed on the outer surface of the inner steel ring along the axial direction of the inner steel ring and extends into the concrete ring along the outer surface of the inner steel ring; the strain gauge is arranged on the inner surface of the inner steel ring, the steel mesh and the outer surface of the concrete ring; the strain gauge is connected to the strain collector. The present invention provides an annular restraint shrinkage test device and a reinforced concrete early cracking assessment method that can improve the probability of successful crack induction and eliminate the randomness of crack generation in the test.

Description

Technical Field

[0001] The invention belongs to the field of concrete disease research and relates to an annular restraint shrinkage test device and a reinforced concrete early cracking assessment method. Background Art

[0002] Early cracking of concrete is often accompanied in physical projects such as roads and bridges. During the construction process, steel bars are often configured to improve the crack resistance of concrete structures. Continuously reinforced concrete pavement (CRCP) is a high-grade pavement structure widely used at home and abroad, with good long-term road performance. The dominant factor of the main disease (thrust failure) of CRCP is the distribution pattern of transverse cracks. Engineering experience shows that the number of cracks in the early stage of CRCP (28 days old, not open to traffic) accounts for 70%-85% of the total number of cracks after crack stabilization, and the distribution pattern is complex and random. Poor early crack distribution pattern may lead to CRCP crack diseases, induce premature thrust failure, and seriously affect the performance and life of the pavement. Relevant scholars analyzed the influence of crack width on thrust and found that under the influence of external loads and environmental factors, the crack width of CRCP is basically linearly related to thrust, and concluded that when the coefficient of variation of crack spacing is less than 10%, the occurrence of thrust diseases can be effectively prevented and reduced.

[0003] The laying of test roads is often time-consuming and laborious. Common constraint test methods in engineering projects include the prism method, the flat plate method, and the ring method. The annular constraint shrinkage test is widely used in shrinkage tests of various types of concrete because of its advantages such as simple and easy operation of the device, ability to provide sufficient constraints, uniform stress generated by the constraints, easy promotion and standardization. The direct factor affecting the shrinkage cracking of the concrete ring is generally the thickness of the inner steel ring. Hossain et al. used inner steel rings of different thicknesses to study and found that when the concrete wall thickness is the same, the specimens with thicker steel rings crack earlier than the thinner specimens. Related studies have shown that a high temperature and low humidity curing environment can accelerate the drying shrinkage of early-age concrete, which is conducive to a more obvious cracking effect of the concrete ring. Summary of the invention

[0004] In order to solve the above technical problems existing in the background technology, the present invention provides an annular restraint shrinkage test device and a reinforced concrete early cracking assessment method which can improve the probability of successful crack induction and eliminate the randomness of crack generation in the test.

[0005] In order to achieve the above object, the present invention adopts the following technical solution:

[0006] A ring-shaped constrained shrinkage test device, characterized in that: the ring-shaped constrained shrinkage test device comprises a base plate, a prefabricated crack sheet, an inner steel ring, an outer steel ring, a strain gauge, a steel mesh and a strain collector; the outer steel ring is sleeved on the outside of the inner steel ring and is arranged on the base plate concentrically with the inner steel ring; an annular space for forming a concrete ring is formed between the outer steel ring and the inner steel ring; the steel mesh is arranged in the annular space; the prefabricated crack sheet is placed on the outer surface of the inner steel ring along the axial direction of the inner steel ring and extends into the concrete ring along the outer surface of the inner steel ring; the strain gauge is arranged on the inner surface of the inner steel ring, the steel mesh and the outer surface of the concrete ring; the strain gauge is connected to the strain collector.

[0007] Preferably, the prefabricated crack piece used in the present invention is a hard plastic slice or steel slice with a rectangular cross-section; the thickness of the prefabricated crack piece is not greater than 1 mm, and the height of the prefabricated crack piece is higher than the height of the inner steel ring and / or the outer steel ring.

[0008] Preferably, the steel mesh used in the present invention is annular steel bars; the annular steel bars include continuous steel bars and partial steel bars arranged on the continuous steel bars along the axial direction of the continuous steel bars; the partial steel bars are connected to the continuous steel bars through diagonal steel bars; the partial steel bars and the diagonal steel bars form a hexagonal steel bar structure as a whole; the strain gauges are respectively on the continuous steel bars, the diagonal steel bars and the partial steel bars; the prefabricated crack sheets are placed on the outer surface of the inner steel ring and extend to the hexagonal steel bar structure.

[0009] Preferably, the plane where the strain gauge arranged on the inner surface of the inner steel ring used in the present invention is located is perpendicular to the plane where the prefabricated crack gauge is located; the plane where the strain gauge arranged on the inner surface of the inner steel ring is located is parallel to the plane where the strain gauge arranged on the outer surface of the concrete ring is located.

[0010] Preferably, the strain gauges used in the present invention include a first strain gauge and a second strain gauge; the number of the first strain gauges and the number of the second strain gauges are both multiple; the first strain gauge is arranged on the inner surface of the inner steel ring and the steel mesh; the second strain gauge is arranged on the outer surface of the concrete ring.

[0011] Preferably, the model of the first strain gauge used in the present invention is 120-3AA; the model of the second strain gauge is 120-50AA.

[0012] Preferably, the thickness of the inner steel ring used in the present invention is greater than the thickness of the outer steel ring; the thickness of the inner steel ring is less than the thickness of the concrete ring.

[0013] Preferably, the outer steel ring used in the present invention is a split structure; a groove and a handle are provided on the bottom plate; the outer steel ring and the inner steel ring are concentric and embedded in the groove.

[0014] A reinforced concrete early cracking assessment method, characterized in that: the reinforced concrete early cracking assessment method comprises the following steps:

[0015] 1) Build the circular restraint shrinkage test apparatus as described above;

[0016] 2) forming a concrete ring on the annular restraint shrinkage test device constructed in step 1); a steel mesh is arranged inside the concrete ring along the axial direction of the concrete ring; strain gauges connected to a strain collector are arranged on the inner surface of the inner steel ring of the annular restraint shrinkage test device, the outer surface of the concrete ring, and the steel mesh inside the concrete ring;

[0017] 3) After the concrete ring obtained in step 2) is initially set, the prefabricated crack sheet is removed, demoulding is performed, and continuous drying is performed in an environment with a temperature of 35±2° C. and a relative humidity of 50±5%. The concrete ring forms active cracks under the action of the prefabricated crack sheet and passive cracks under dry conditions, and monitoring data of the strain gauge is regularly obtained through a strain acquisition instrument;

[0018] 4) calculating the stress growth coefficient and the cracking risk index based on the monitoring data obtained in step 3);

[0019] 5) Based on the calculation results obtained in step 4), the risk of early cracking of concrete is evaluated and the cracking time of concrete is predicted.

[0020] Preferably, the calculation formula of the cracking risk index in step 4) adopted in the present invention is:

[0021] Ф(t)=σ res (t) / f ts (t)

[0022] in:

[0023] The Ф(t) is the cracking risk index of concrete at time t, and the cracking risk index is an evaluation standard for the concrete's ability to resist early shrinkage cracking;

[0024] The σ res (t) is the maximum residual stress of the concrete ring at time t;

[0025] The f ts (t) is the splitting tensile strength of concrete at time t;

[0026] The t is the time when the strain gauge collects data;

[0027] The σ res The expression for (t) is:

[0028]

[0029] in:

[0030] The R oc is the outer diameter of the concrete ring;

[0031] The R os is the outer diameter of the inner steel ring;

[0032] The R is is the inner diameter of the inner steel ring;

[0033] The ε steel (t) is the strain value of the inner steel ring measured at time t;

[0034] Said ρ is the reinforcement ratio of the concrete ring;

[0035] The E s is the elastic modulus of the inner steel ring;

[0036] The E c is the elastic modulus of the concrete ring;

[0037] Preferably, the reinforcement ratio ρ of the concrete ring is expressed as:

[0038]

[0039] The R oc is the outer diameter of the concrete ring;

[0040] The R ic is the inner diameter of the concrete ring;

[0041] The n is the number of active slits;

[0042] Said l is the length of the part of the steel bar;

[0043] Said r is the radius of the steel bar;

[0044] Said h is the height of the concrete ring;

[0045] The c is the thickness of the concrete ring;

[0046] Preferably, the calculation method of the stress growth coefficient in step 4) is:

[0047] S=E s ·R ic ·t s ·|α| / (2R is ·t c ·T 0.5 )

[0048] in:

[0049] Said S is the stress growth coefficient;

[0050] The E s is the compressive elastic modulus of the inner steel ring;

[0051] The R ic is the inner diameter of the concrete ring;

[0052] The R is is the inner diameter of the inner steel ring;

[0053] The s is the thickness of the inner steel ring;

[0054] The c is the thickness of the concrete ring;

[0055] The α is the strain magnification coefficient;

[0056] Said T is the time when the concrete cracks;

[0057] Preferably, in the step 5), the early cracking risk of concrete is assessed based on the stress growth coefficient. When the stress growth coefficient S≥0.34, the cracking risk is high; when 0.17≤S≤0.34, the cracking risk is medium-high; when 0.10≤S≤0.17, the cracking risk is medium-low; when S≤0.10, the cracking risk is low; the actual cracking time of concrete is predicted based on the cracking risk index.

[0058] The beneficial effects of the present invention are:

[0059] The present invention aims at the deterioration of CRCP crack distribution, provides a circular restraint shrinkage test device and a reinforced concrete early cracking assessment method, uses a circular restraint test instead of a test road method to explore the influence of a partial continuous reinforcement method on concrete cracking, and finds that by setting a prefabricated crack sheet and pulling it out after the initial setting of the concrete, a partial continuous reinforcement method that induces active crack generation can significantly improve the early crack resistance of concrete, increase the uniformity of concrete crack distribution, and reduce the crack width, which provides a possibility for the optimization control of PRRCP cracks. The circular restraint shrinkage test device provided by the present invention is reasonably designed, can provide sufficient constraints for reinforced concrete to achieve an ideal cracking effect, and is suitable for monitoring the early cracking behavior of reinforced concrete and assessing the cracking risk. The probability of successful crack induction is greatly increased by slitting (the probability of successful active crack induction is not less than 90%), and the randomness of crack generation during concrete shrinkage is eliminated (the principle is that slitting causes a concrete notch, stress concentration occurs at the notch, and the tensile strength is preferentially reached, so that cracking occurs at the notch, and this conclusion can also be obtained by Abaqus finite element simulation), which creates a premise for the precise study of the partial continuous reinforcement method. At the same time, the reinforced concrete early cracking assessment method provided by the present invention can provide sufficient constraints for the shrinkage cracking of reinforced concrete. The device can monitor the cracking behavior of concrete and evaluate the early cracking resistance of concrete through the collected data. The main factors affecting concrete cracking in the shrinkage test are the thickness of the inner steel ring and the concrete ring. The larger the relative thickness ratio (the relative thickness ratio of the AASHTO steel ring to the concrete ring is 0.167, and there are many cases of no cracking during the test. In order to increase the constraint, ASTM increases the relative thickness ratio to 0.333. The addition of steel bars in the present invention will improve the cracking resistance of concrete, and the relative thickness ratio is increased to 0.667 to prevent concrete from cracking), the earlier the concrete cracks. In order to avoid the phenomenon of concrete not cracking after embedded steel bars, reasonable improvements were made based on the test dimensions recommended by the American Association of State Highway and Transportation Officials (AASHTO) and the American Society for Testing and Materials (ASTM) (the test specimen dimensions recommended in my country's "Guidelines for Durability Design and Construction of Concrete Structures" are: inner diameter 41.3mm, outer diameter 66.7mm (the thickness between the inner and outer surfaces of the concrete ring is 25.4mm), and height 25.4mm) to enhance the restraining effect of the inner steel ring on the shrinkage of the concrete ring, making it more suitable for the early cracking study of reinforced concrete. Active crack control is required at some of the steel bars in the partial continuous reinforcement method. In the traditional crack induction method, triangular steel is often embedded on the outer surface of the inner steel ring to cause concrete defects. Embedding a large volume of triangular steel in concrete is bound to affect the shrinkage process of concrete.The present invention provides an evaluation method that has the least influence on the stress mode of concrete shrinkage process and can improve the success probability of crack induction at the designated position: a 1mm thick prefabricated crack sheet is placed close to the inner steel ring, 2 to 3cm higher than the top of the concrete ring to facilitate the pull-out after the initial setting of the concrete, and a concrete cut with a length of 8mm and a width of 1.5mm is generated at the pull-out position (formed after the prefabricated crack sheet is removed) and stress concentration is generated, thereby achieving the purpose of cracking at the designated position. The early cracking behavior of reinforced concrete is studied based on the ring test, aiming to replace the test road with reinforced concrete rings to save manpower and material resources. During the test, strain gauges are attached to the inner steel ring and the surface of the steel bar to accurately monitor the cracking time of the concrete; strain gauges are attached to the concrete surface at the cut to obtain the development law of the width of the actively induced crack over time. Combined with the strain data collected by the collector and the measured concrete mechanical properties, the early shrinkage cracking behavior and cracking risk of concrete under the conditions of internal steel bar constraints and external constraints of the inner steel ring are analyzed and evaluated. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 This is a schematic diagram of a portion of the continuous reinforced concrete pavement taken in the test road;

[0061] Figure 2 It is a schematic diagram of the top view of the annular restraint shrinkage test device provided by the present invention;

[0062] Figure 3 It is a schematic diagram of the vertical cross-section structure of the annular restraint shrinkage test device provided by the present invention;

[0063] Figure 4 It is a schematic diagram of the circular reinforcement of the continuous reinforcement method;

[0064] Figure 5 It is a schematic diagram of the circular reinforcement of the partial continuous reinforcement method;

[0065] Figure 6 is a schematic cross-sectional view of the prefabricated crack sheet used in the present invention;

[0066] Figure 7 It is a schematic diagram of the stress measurement points of steel bars in the test road;

[0067] Figure 8 It is a schematic diagram of the steel bar strain measurement points in the annular restraint shrinkage test;

[0068] Fig. 9 It is the stress diagram of steel bars in the partially continuous reinforced concrete test road with cutouts;

[0069] Fig.10 It is the stress diagram of steel bars in the continuous reinforced concrete test road without cutting joints;

[0070] Fig.11is the steel bar strain diagram of the continuously reinforced concrete ring specimen;

[0071] Fig.12 It is the steel bar strain diagram of the partially continuously reinforced concrete ring specimen without cuts;

[0072] Fig.13 It is the steel bar strain diagram of the partially continuously reinforced concrete ring specimen with cutouts;

[0073] Fig.14 is the concrete strain diagram at the cut of four ring specimens;

[0074] Fig.15 It is the inner steel ring strain diagram of the ring specimens with different reinforcement methods;

[0075] in:

[0076] 1-base plate; 2-outer steel ring; 3-concrete ring; 4-bolt; 5-annular reinforcement; 6-inner steel ring; 7-strain gauge; 8-prefabricated crack plate; 9-handle; 10-groove. DETAILED DESCRIPTION

[0077] The present invention provides an annular restraint shrinkage test device, comprising a base plate 1, a prefabricated crack sheet 8, an inner steel ring 6, an outer steel ring 2, a strain gauge 7, a steel mesh and a strain collector; the outer steel ring 2 is sleeved on the outside of the inner steel ring 6 and is concentrically arranged on the base plate 1 with the inner steel ring 6; an annular space for forming a concrete ring 3 is formed between the outer steel ring 2 and the inner steel ring 6; the steel mesh is arranged in the annular space; the prefabricated crack sheet is placed on the outer surface of the inner steel ring 6 along the axial direction of the inner steel ring 6 and extends along the outer surface of the inner steel ring 6 toward the concrete ring 3; the strain gauge 7 is arranged on the inner surface of the inner steel ring 6, the steel mesh and the outer surface of the concrete ring 3; the strain gauge 7 is connected to the strain collector.

[0078] The prefabricated crack sheet 8 is a hard plastic slice or steel slice with a rectangular cross section; the thickness of the prefabricated crack sheet is not more than 1mm, and the height of the prefabricated crack sheet is higher than the height of the inner steel ring 6 and / or the outer steel ring 2. The prefabricated crack sheet 8 is used for active crack induction. For example, the length of the prefabricated crack sheet 8 is 100mm, the width is 10mm, and the thickness is 1mm. Since the size of the concrete ring 3 is small, it is not suitable to use a larger knife-edge crack induction device. Different from the common method of setting a triangular steel on the outer surface of the inner steel ring to cause concrete defects, the active crack induction method adopted by the present invention is: the prefabricated crack sheet 8 is arranged flush with the bottom plate 1 and close to the inner steel ring 6, and the concrete is initially set and has a certain plasticity. After that, it is pulled out to form a concrete incision similar to the road surface cutting structure, with a length of 10mm and a width of 1.5mm (when the prefabricated crack sheet is pulled out, the incision width is slightly larger than the thickness of the prefabricated crack sheet), and penetrates the concrete ring 3 in the height direction to induce the concrete ring 3 to generate active cracks. This method is similar to the cutting method of pavement structure. It not only achieves the purpose of inducing cracks at the specified location, but also greatly reduces the impact on the stress mode of the concrete ring.

[0079] The steel mesh is a circular steel bar 5; the circular steel bar 5 includes continuous steel bars and partial steel bars arranged on the continuous steel bars along the axial direction of the continuous steel bars; the partial steel bars are connected to the continuous steel bars through oblique steel bars; the partial steel bars and the oblique steel bars are a hexagonal steel bar structure as a whole, that is, a partially continuous reinforcement method; the strain gauges 7 are respectively on the continuous steel bars, the oblique steel bars and the partial steel bars; the prefabricated crack sheet is placed on the outer surface of the inner steel ring 6 and extends to the hexagonal steel bar structure. Specifically, the partially continuous reinforcement method refers to the configuration of continuous steel bars and partial steel bars at intervals in the concrete slab, the partial steel bars are connected to the continuous longitudinal steel bars into a unified whole with oblique steel bars, and active crack control is set at the partial steel bars with high reinforcement ratio, so that the concrete cracks preferentially occur at the position with higher reinforcement ratio, forming an excellent crack distribution pattern with uniform spacing between active cracks and passive cracks. Compared with cracking at other weak positions, the safety of the structure is better guaranteed. The present invention laid a partially continuous reinforced concrete pavement in Shanxi Province in September 2022 to study the early cracking behavior of concrete under this reinforcement method. Active crack control in concrete pavement with partial continuous reinforcement is achieved by cutting slits on the surface, with the slits located at the steel bars with a higher reinforcement ratio.

[0080] The plane where the strain gauge 7 arranged on the inner surface of the inner steel ring 6 is located is perpendicular to the plane where the prefabricated crack gauge is located; the plane where the strain gauge 7 arranged on the inner surface of the inner steel ring 6 is located is parallel to the plane where the strain gauge 7 arranged on the outer surface of the concrete ring 3 is located.

[0081] The strain gauge 7 includes a first strain gauge and a second strain gauge; the number of the first strain gauge and the number of the second strain gauge are both multiple; the first strain gauge is arranged on the inner surface of the inner steel ring 6 and the steel mesh; the second strain gauge is arranged on the outer surface of the concrete ring 3. The model of the first strain gauge is 120-3AA; the model of the second strain gauge is 120-50AA. That is, the cracking time of the concrete is monitored by pasting 120-3AA strain gauges on the inner surface of the steel ring and the surface of the steel bar, and the development and change of the crack width is monitored by pasting 120-50AA strain gauges on the outer surface of the concrete ring corresponding to the slit. The arrangement of the strain gauges, 4 steel ring strain gauges 7 are evenly arranged on the inner surface of the steel ring 6, and the average value is taken as the actual compressive strain of the inner steel ring; the steel bar strain gauge 7 is pasted at the specified position on the surface of the steel bar 5 to explore the strain change law of different steel bars after the concrete cracks; the concrete strain gauge 7 (the concrete strain gauge is 50AA; the steel bar and steel ring strain gauges are 3AA) is arranged on the outer surface of the concrete ring perpendicular to the slit 8 to monitor the development of the active crack width. Each strain gauge is connected to the strain collector through a wire in a 1 / 4 bridge connection mode. The cracking time and crack width change of concrete are monitored by collecting the strain changes of each measuring point and combining the development of concrete surface cracks. The position of the steel bar strain gauge 7 is pasted on some steel bars, the inside and outside of continuous steel bars, and the oblique steel bars to monitor the strain changes of each steel bar when concrete cracks.

[0082] The thickness of the inner steel ring 6 is greater than that of the outer steel ring 2; the thickness of the inner steel ring 6 is less than that of the concrete ring 3. The present invention refers to the test dimensions recommended by AASHTO and ASTM, increases the relative thickness of the inner steel ring compared to the concrete ring, and then increases the relative stiffness of the inner steel ring. On the one hand, it can improve the constraint of the inner steel ring on the reinforced concrete and shorten the cracking time of the concrete; on the other hand, it can ignore the radial moisture gradient during the shrinkage of the concrete, thereby reducing the impact on the stress calculation. Exemplarily, the outer diameter of the inner steel ring 6 is 300mm, the inner diameter is 260mm, the thickness is 20mm, and the height is 70mm, which is slightly increased compared to the size recommended by the specification (the inner diameter of the steel ring is 305mm, the thickness of the inner steel ring is 12.3-13.1mm, the wall thickness of the outer concrete ring is 75mm, and the height of the steel ring is 152mm. ASTM recommends reducing the concrete wall thickness to 37.5mm) to improve the constraint effect on the reinforced concrete ring and make the reinforced concrete obtain an ideal cracking result. Prefabricated crack sheets are arranged close to the inner steel ring to actively induce cracks on the inner side of the concrete ring. The outer steel ring 2 has an outer diameter of 380 mm, an inner diameter of 360 mm, a thickness of 10 mm, and a height of 70 mm. It consists of two detachable semicircular rings. High relative stiffness is achieved by adjusting the size of the inner steel ring and the concrete ring through theoretical derivation and formula calculation to increase the relative stiffness of the two (changing the stiffness by changing the size, and then changing the strength of the constraint), so as to increase the degree of concrete constraint and shorten the cracking time.

[0083] The outer steel ring 2 is a split structure and can be fixedly connected by bolts 4 to form an integral steel ring; a groove 10 and a handle 9 are provided on the bottom plate 1; the outer steel ring 2 and the inner steel ring 6 are concentric and embedded in the groove 10, which ensures the concentricity of the inner and outer steel rings on the one hand, and effectively prevents leakage of concrete during vibration on the other hand. At the same time, a handle is welded on the bottom plate to facilitate moving the device.

[0084] like Figure 1 As shown, longitudinal continuous steel bars and partial steel bars are arranged at intervals along the longitudinal direction of the partially continuous reinforced concrete test road, and the partial steel bars are connected to the longitudinal continuous steel bars by oblique steel bars. The transverse steel bars play a supporting role, and active crack control is set in the center of the partial steel bars. The present invention aims to use an annular restraint test instead of a test road to study the early cracking problem of reinforced concrete. The size of the concrete ring in the test (height 70mm, thickness 30mm) is approximately reduced by the size of the test road (width 0.58m, thickness 0.30m). The schematic diagram of the annular restraint device is shown in Figure 2 as well as Figure 3 In order to explore the effect of different reinforcement methods on concrete shrinkage cracking and verify that PCRCP has excellent early cracking behavior, continuous reinforcement and partial continuous reinforcement were used. In the test, the steel bars were located at the middle thickness and middle height of the concrete ring. Two types of ring steel bars were made. The schematic diagram of the ring steel bars is shown in Figure 4 as well as Figure 5 In the continuous reinforcement method, the reinforcement ratio of the section along the circumferential direction is 4.04%; in the partially continuous reinforcement method, the reinforcement ratio of the section where some steel bars are located is three times that of the section with only continuous steel bars, which are 4.04% and 1.35% respectively. The annular restraint test uses prefabricated crack pieces for similar slitting treatment, which can simulate the slitting process at the active crack control of the test road. The cross-sectional dimensions of the prefabricated crack pieces are shown in Figure 6 When concrete cracks, stress redistribution occurs in the steel bars and concrete. Steel bar stress sensors and strain gauges are arranged at the steel bar measuring points in the test road and the ring restraint test to monitor the changes in steel bar stress and strain during concrete shrinkage and cracking. Figure 7 as well as Figure 8 .

[0085] In view of the defects of the existing CRCP, such as the high initial construction cost and the random and uneven distribution of cracks, the present invention proposes a new type of rigid pavement structure - Partially Continuously Reinforced Concrete Pavement (PCRCP) based on the annular restrained shrinkage test device provided above. PCRCP refers to the configuration of continuous steel bars and partial steel bars at intervals in the concrete slab, connecting the partial steel bars with the continuous longitudinal steel bars into a unified whole with oblique steel bars, forming a hexagonal steel bar structure as a whole, and setting active crack control at the steel bars with high reinforcement ratio, so that the concrete cracks preferentially at the position with high reinforcement ratio. Compared with cracking at other weak positions, the safety of the structure is well guaranteed, and an excellent crack distribution pattern composed of active cracks and passive cracks is formed.

[0086] The present invention takes into account that drying shrinkage causes tensile stress inside concrete, which leads to shrinkage cracks in early concrete structures. Therefore, steel bars with strong tensile properties are often added in physical projects, and the steel bars and concrete form a synergistic whole. The addition of steel bars will inevitably improve the crack resistance of concrete and delay its cracking time, but it cannot completely eliminate cracks. When the tensile stress inside the concrete is greater than the ultimate tensile strength and cracks are generated, the steel bars can tighten the concrete on both sides of the crack, which is beneficial to avoid the further expansion of the width of the undesirable cracks and thus cause the occurrence of various diseases. Crack induction measures are taken at the designated location, and the internal stress of the concrete will be concentrated at this location first. The larger tensile stress causes the concrete to produce active cracks here first. After cracking, the concrete stress around the crack is released, thereby reducing the cracking probability of the surrounding concrete, and generating new passive cracks at a distance from the crack, thereby forming an excellent crack distribution pattern of active and passive synergistic cracking. The present invention studies the strain changes of the steel bars and concrete during the entire cracking process by pasting strain gauges on the inner surface of the inner steel ring, the steel bar surface, and the outer surface of the concrete ring. Combined with the knowledge of elastic mechanics, the formula is derived, the residual stress of concrete at different times is calculated, and the anti-cracking performance of concrete is evaluated. The annular specimen is unfolded along the central axis and has similar structural characteristics to the road panel. The inner steel ring replaces the pavement base to provide a stable and uniform constraint for the reinforced concrete, indicating that the annular specimen has the potential to replace the test road to study the shrinkage cracking behavior of the pavement at an early age. Based on the annular constraint test, the influence of steel bars on the early cracking behavior of concrete can be studied, and the early shrinkage cracking process of reinforced concrete can be explored from both qualitative and quantitative aspects: the early shrinkage cracking behavior of concrete is analyzed by observing the width, spacing and other related characteristics of the cracks in the concrete, and the anti-cracking performance of concrete is evaluated by combining the measured strains of the inner steel ring, steel bars and concrete and the calculated cracking indicators such as residual tensile stress, cracking risk index and stress growth coefficient. The present invention hopes to fill the relative blank of the circular ring method in the field of reinforced concrete through the cracking law of reinforced concrete ring, and provide feasible suggestions for the design and construction of CRCP and PCRCP, so that the early cracks in the pavement can have a uniform distribution shape and controllable opening and closing width as much as possible, thereby reducing early pavement diseases and improving durability.

[0087] The present invention also provides a method for evaluating early cracking of reinforced concrete, the method comprising the following steps:

[0088] 1) Build the circular restraint shrinkage test apparatus as described above;

[0089] 2) forming a concrete ring 3 on the annular restraint shrinkage test device constructed in step 1); a steel mesh is arranged inside the concrete ring 3 along the axial direction of the concrete ring 3; strain gauges 7 connected to a strain collector are arranged on the inner surface of the inner steel ring 6 of the annular restraint shrinkage test device, the outer surface of the concrete ring 3 and the steel mesh inside the concrete ring 3;

[0090] 3) placing the concrete ring 3 obtained in step 2) in a laboratory environment, removing the prefabricated crack sheet after the concrete ring is initially set, removing the outer steel ring and the bottom plate for demoulding after 1 day, and continuously drying in an environment with a temperature of 35±2° C. and a relative humidity of 50±5%, so that the concrete ring forms active cracks under the action of the prefabricated crack sheet and passive cracks under dry conditions, and at the same time, regularly obtaining monitoring data of the strain gauge 7 through a strain acquisition instrument;

[0091] 4) calculating the stress growth coefficient and the cracking risk index based on the monitoring data obtained in step 3);

[0092] 5) Based on the calculation results obtained in step 4), the risk of early cracking of concrete is evaluated and the cracking time of concrete is predicted.

[0093] The calculation method of the cracking risk index in step 4) is:

[0094] Ф(t)=σ res (t) / f ts (t)

[0095] in:

[0096] Ф(t) is the cracking risk index of concrete at time t, and is the evaluation criterion for the concrete's ability to resist early shrinkage cracking;

[0097] σ res (t) is the maximum residual stress of the concrete ring at time t;

[0098] f ts (t) is the splitting tensile strength of concrete at time t;

[0099] t is the time when the strain gauge collects data;

[0100] σ res The expression for (t) is:

[0101]

[0102] in:

[0103] R oc is the outer diameter of the concrete ring;

[0104] R os is the outer diameter of the inner steel ring;

[0105] R is is the inner diameter of the inner steel ring;

[0106] ε steel (t) is the strain value of the inner steel ring measured at time t;

[0107] Preferably, the calculation method of the stress growth coefficient in step 4) is:

[0108] S=E s ·R ic ·t s ·|α| / (2R is ·t c ·T 0.5 )

[0109] in:

[0110] S is the stress growth coefficient;

[0111] E s is the compressive elastic modulus of the inner steel ring;

[0112] R ic is the inner diameter of the concrete ring;

[0113] R is is the inner diameter of the inner steel ring;

[0114] t s is the thickness of the inner steel ring;

[0115] t c is the thickness of the concrete ring;

[0116] α is the strain enhancement coefficient, in με / d 0.5 ;

[0117] T is the time when concrete cracks, in d;

[0118] Preferably, in step 5), the early cracking risk of concrete is assessed based on the stress growth coefficient. When the stress growth coefficient S≥0.34, the cracking risk is high; when 0.17≤S≤0.34, the cracking risk is medium-high; when 0.10≤S≤0.17, the cracking risk is medium-low; when S≤0.10, the cracking risk is low; the actual cracking time of concrete is predicted based on the cracking risk index.

[0119] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0120] Partial continuous reinforced concrete test road:

[0121] In order to evaluate the effectiveness of induced cracks, two sections of partially continuous reinforced concrete pavement with and without induced crack measures were compared. Figure 7 In the test road, a steel bar strain gauge is used to measure the stress change of the steel bar. The arrangement method is to weld the two ends of the steel bar strain gauge to the steel bar. T1, T2, and T3 represent the measurement point positions, which are located at part of the steel bar, the inside of the continuous steel bar, and the outside of the continuous steel bar, respectively. For example, the steel bar strain gauge in the test road is too long to be arranged at the inclined steel bar, so only three measurement points are set. The effect of active induced cracks on steel bar stress can be seen in Fig. 9 as well as Fig.10 As shown in the figure, due to the existence of active induced cracks, PCRCP (2) is mainly subjected to stress at T1 and T2, and T3 is subjected to less stress; while PCRCP (6) is different, with more stress at T3 and less stress at T1 and T2. Fig. 9 as well as Fig.10 In the figure, 2-T1 and 6-T1 mean: 2 and 6 represent the numbers of the test road sections, T1 refers to the strain gauge measuring point arranged on some steel bars, where 2 is the slit section and 6 is the unslit section; 2-T1 represents the measured stress in some steel bars of the slit section.

[0122] When the concrete pavement slab generates internal forces under the action of environmental loads, the internal forces will accumulate at the active induced cracks, and the induced cracks will extend downward along the pre-cut position; when they extend to the steel bars, the stress is borne by the steel bars, so the overall stress of the PCRCP (2) steel bars in the cut is greater, and the strength of the steel bars can be fully utilized. Fig. 9 as well as Fig.10 ,The results show that active induced cracks significantly affect the stress distribution of steel bars in PCRCP.

[0123] Restricted Contraction Test Protocol:

[0124] Inner diameter R of inner steel ring is 260mm, outer diameter R os is 300mm, thickness is d s The inner diameter of the concrete ring is 20mm and the height is 70mm. ic 300mm, outer diameter R oc 360mm, thickness d c The diameter is 30mm, and the height h is 70mm. The prefabricated cracks are made of hard plastic slices or steel slices with a rectangular cross section, a length of 100mm, a width of 10mm, and a thickness of 1mm. The height of the prefabricated crack slices is slightly higher than the top of the concrete ring 3, so that it can be pulled out after the initial setting of the concrete. The strain gauges are pasted to the designated positions of the steel bars and the inner steel ring, and the annular restraint device is assembled. The annular steel bars are obtained by bending and welding 6mm plain round steel bars. The annular steel bars are fixed at the middle thickness and middle height of the concrete ring, and prefabricated cracks are set at some of the steel bars.

[0125] Three annular restraint specimens were designed with different reinforcement methods, including continuous reinforcement, partial continuous reinforcement, and partial continuous reinforcement with active slits, numbered ①, ②, and ③. The strain changes of the steel ring, steel bar, and concrete during the entire cracking process were monitored by pasting strain gauges on the inner surface of the inner steel ring, the surface of the steel bar, and the outer surface of the concrete ring. The cracking time of the concrete ring was determined by observing the sudden changes in the strain on the surface of the concrete ring 3 and the collector. The crack width of the concrete ring was measured using a crack width gauge. Combined with the relevant knowledge of elastic mechanics and the development and change of the early elastic modulus and splitting tensile strength of concrete with age, the early shrinkage cracking behavior and cracking risk of concrete under the conditions of internal restraint of steel bars and external restraint of inner steel ring were clearly analyzed and evaluated.

[0126] Raw materials and mechanical properties:

[0127] The raw materials used in the experiment include ordinary silicate cement P.O42.5 produced by Shanxi Taiyuan Shitou Cement Co., Ltd. (hereinafter referred to as cement, see Table 1 for details), river sand from Taiyuan, Shanxi (medium sand, fineness modulus M x =2.65, hereinafter referred to as sand, see Table 1 for details), crushed stone (particle size of 4.75-9.5 mm, hereinafter referred to as stone, see Table 1 for details) produced by Taiyuan Jiantai Stone Co., Ltd., and polycarboxylic acid high-performance water-reducing agent (hereinafter referred to as water-reducing agent, see Table 1 for details) produced by Jiangsu Subote New Materials Co., Ltd. The mix ratio of concrete is shown in Table 1, and the mechanical properties of concrete are shown in Table 2.

[0128] Table 1 Concrete mix ratio (kg / m 3 )

[0129] cement water sand stone Water reducing agent 472 223 774 871 3.78

[0130] Table 2 Concrete mechanical properties

[0131] Age 1d 3d 7d 14d Cube compressive strength / MPa 13.0 28.1 40.4 42.5 Splitting tensile strength / MPa 2.16 3.50 4.03 4.26 Elastic modulus / GPa 10.5 17.1 20.3 24.3

[0132] Restricted shrinkage test results:

[0133] During the pouring of concrete, the concrete is compacted by vibrating and tamping. After pouring, the surface of the specimen is covered with plastic wrap to prevent water loss. It is placed in the laboratory environment. Small water droplets should gather on the plastic wrap during the process. 1d After removing the outer ring and the bottom plate, the concrete strain gauge is pasted on the active cut on the outer surface of the concrete ring. After the strain gauge is pasted, it is connected to the strain collector and the computer in a 1 / 4 bridge connection to continuously monitor the strain data. During this period, it is placed in an environment with a temperature of 35±2℃ and a relative humidity of 50±5% for continuous drying. Regular observations are made every day to determine the cracking position and time of the concrete ring through the sudden change of strain and the expansion of surface cracks. The width of the cracks is observed with a magnifying glass. After multiple cracks are generated during the test, the spacing between the cracks is recorded.

[0134] During the test, the active cracks started from the prefabricated crack position and expanded to the outer surface of the concrete ring, forming through cracks. The crack information of each concrete ring 3 is shown in Table 3. The continuous reinforced concrete ring 3① with more steel bars only produced one crack and the crack was produced the latest, indicating that sufficient steel bars can significantly improve the crack resistance of concrete. The spacing between the two cracks in the partially continuously reinforced concrete ring 3② without prefabricated slits is 115°, and the widths are 0.13mm and 0.07mm respectively; the angle between the active crack and the passive crack in the partially continuously reinforced concrete ring 3③ with prefabricated cracks is 170°, and the widths are 0.10mm and 0.09mm respectively, which verifies that the partially continuous reinforcement method with prefabricated cracks has the advantage of optimizing the crack spacing and width.

[0135] Table 3 Concrete ring 3 crack information table

[0136]

[0137] Steel bar strain:

[0138] See also Fig.11 , Fig.11 The strain diagram of the continuous reinforcement ring specimen shows that the tensile strain of the upper and lower reinforcements S1 and S3 increases slowly, and the shrinkage of concrete causes the middle continuous reinforcement S2 to be compressed and the compressive strain gradually increases. The time of the sudden increase in strain is the latest, which means the longest cracking time. Fig.12 This is the steel bar strain diagram of the partially continuously reinforced concrete ring specimen without cutting. Fig.13 This is the strain diagram of the steel bars in the continuous reinforced concrete ring specimen with a slit. The comparison of the two figures shows that the strain of the steel bars in the slit specimen is greatly increased, indicating that the steel bars are subjected to greater tensile stress, exerting their strength and achieving the design purpose. Fig. 9It was found that compared with the concrete ring 3 without slits, the strain of the steel bars in the concrete ring 3 with active slits developed faster, and the maximum strain value of the steel bars reached 440.5με, which was about 6.6 times the maximum strain value of the steel bars in the specimen without slits. This is because the stress concentration of the concrete at the slits caused microcracks to occur earlier inside the concrete, causing the steel bars to bear tensile stress earlier. The results show that when the concrete ring cracks to the steel bars, the strain of the steel bars suddenly increases, and stress redistribution occurs between the steel bars and concrete. With the same reinforcement method, the strain of the steel bars in the slit specimens changes earlier and the strain of the steel bars increases significantly. The advantage of active slits is that the concrete can crack preferentially at the specified position, giving full play to the excellent tensile properties of the steel bars at the cracking position. The design idea of ​​the partial continuous reinforcement method is to set active slits at the part of the steel bars with a larger reinforcement ratio (hexagonal steel bar structure). After successful induction, cracks are preferentially generated at the position with a larger reinforcement ratio, thereby improving the safety of the structure.

[0139] Concrete strain at the cut:

[0140] like Fig.14 As shown in the figure, during the test, it was found that the strain value measured on the concrete surface at the cut of different concrete rings 3 was proportional to the active crack width measured by the crack width gauge. In the relationship, A is a constant related to the concrete material properties and mold size. The concrete strain at the 14d cut and the measured active crack width are shown in Table 4. Based on this annular restraint shrinkage test device, the change of the active crack width d can be inferred by monitoring the change of the concrete strain value at the cut.

[0141] d=Aε c (1)

[0142] Table 4 Concrete strain table and crack width table of concrete ring at 14d

[0143] 14d concrete strain at the cut joint (με) Active crack measured width (mm) Concrete Ring 1 915.4 0.10 Concrete Ring 2 921.5 0.10 Concrete Ring 3 651.8 0.08 Concrete Ring 4 507 0.06

[0144] Inner steel ring strain:

[0145] The measured variation of the inner steel ring strain with time is shown in Fig.15 ,Analysis of the strain curve shows that: the continuously reinforced concrete ring specimen 1-CRCP has the largest reinforcement ratio, and the concrete shrinkage is most restricted. At the same time, the steel ring strain value is the smallest, and the cracking time is also the latest. The first crack occurs at 9.0d, and the steel ring strain value drops suddenly.

[0146] It was observed that under the partial continuous reinforcement mode, the strain value of the inner steel ring of the ring specimen 1-PCRCP-Z with slits was smaller when cracking occurred than that of the ring specimen 1-PCRCP without slits, indicating that under the same shrinkage and drying environment, slits can weaken the interaction between the inner steel ring and the concrete ring to a certain extent, reducing the strain value of the inner steel ring at the same time. The cracking time of the ring specimen after slits was advanced by 0.8 days. This is because the stress concentration occurred at the concrete ring slit, which caused the internal tensile stress to develop to a higher level at the same time, thereby generating active cracks at the slits in advance.

[0147] Analysis of concrete crack resistance Concrete strain ε under reinforcement internal constraint conditions c The calculation formula is

[0148]

[0149] In the formula, ε sh is the strain of concrete during free shrinkage, E s is the elastic modulus of the steel ring, E c is the elastic modulus of concrete, and ρ is the reinforcement ratio of the specimen.

[0150] In some continuous reinforcement methods, the steel bars are not evenly arranged along the circumference, and the equivalent reinforcement ratio ρ is used for calculation

[0151]

[0152] In the formula, R oc is the outer diameter of the concrete ring, R ic is the inner diameter of the concrete ring, n is the number of active cuts, l is the length of the partial steel bar, r is the radius of the steel bar, h is the height of the concrete ring, t c is the thickness of the concrete ring.

[0153] Considering the influence of the inner steel ring on the restricted shrinkage of reinforced concrete, the concrete strain ε under the condition of internal steel constraint is combined with the calculation method of the maximum residual stress of concrete under the uniform shrinkage mode proposed by Hossain et al. and Moon et al., and the calculation formula of the maximum residual stress of concrete in the annular constraint test under the condition of internal steel constraint is obtained:

[0154]

[0155] In the formula, R oc is the outer diameter of the concrete ring, R os is the outer diameter of the inner steel ring, R is is the inner diameter of the inner steel ring, ε steel (t) is the strain value of the inner steel ring measured at time t. The maximum residual stress is related to the reinforcement ratio, the size and elastic modulus of the inner steel ring and concrete ring, and the strain of the inner steel ring.

[0156] The cracking risk index Ф is used as an evaluation standard for the concrete's ability to resist early shrinkage cracking. The calculation formula is as follows:

[0157] Ф(t)=σ res (t) / f ts (t) (5)

[0158] Where Ф(t) is the cracking risk index of concrete at time t, σ rea (t) is the residual stress of the concrete ring at time t, f ts (t) is the splitting tensile strength of concrete at time t, and t is the time when the strain gauge collects data.

[0159] Although the Ф value of concrete when cracking should theoretically reach 1, most test results show that concrete cracks when the Ф value does not reach 1. Affected by creep, the appearance of internal microcracks usually occurs when the tensile stress reaches 60% to 90% of the tensile strength.

[0160] According to formula (4), the cracking risk coefficients of the partially continuous reinforced concrete ring 3 with and without active slits at 3d and 7d are quantified. The cracking risk index and cracking time are shown in Table 5. As can be seen from the table, there are large differences in the cracking risk coefficients of concrete with different reinforcement methods. The cracking risk coefficient of the concrete ring with prefabricated slits is reduced by 10.4% at 3d and 8.6% at 7d compared with the specimen without slits. That is, the larger the cracking risk index, the earlier the concrete cracking time. The actual cracking time of the concrete can be predicted based on the size of the cracking index. Obviously, the actual cracking time of the concrete ring can be more accurately evaluated by the cracking risk coefficient.

[0161] Table 5 Concrete ring cracking risk factor

[0162]

[0163] The stress growth coefficient is obtained by linear regression analysis. First, the inner steel ring strain of each concrete ring 3 developing with time is plotted as the ordinate and the square root of time as the abscissa. A linear regression analysis is performed according to the following formula:

[0164] ε(t)=|α|t 0.5 +β (6)

[0165] Where ε(t) is the strain value of the inner steel ring at time t, |α| is the absolute value of the strain increase coefficient, β is the regression constant, and t is the time when the strain gauge collects data.

[0166] The stress growth coefficient can reflect the speed of tensile stress growth in the concrete ring 3. In the ASTM C1581 concrete cracking potential classification: when S ≥ 0.34, the cracking risk is high; when 0.17 ≤ S ≤ 0.34, the cracking risk is medium-high; when 0.10 ≤ S ≤ 0.17, the cracking risk is medium-low; when S ≤ 0.10, the cracking risk is low. The calculation formula of the stress growth coefficient is:

[0167] S=E s ·R ic ·t s ·|α| / (2R is ·t c ·T 0.5 ) (7)

[0168] Where: S is the stress growth coefficient; E s is the compressive elastic modulus of the inner steel ring, which is 1.5×10 5 MPa; R ic is the inner diameter of the concrete ring, which is 150 mm; R is is the inner diameter of the inner steel ring, which is 130 mm; t s is the thickness of the inner steel ring, take 20mm; t c is the thickness of the concrete ring, which is 30 mm; |α| is the absolute value of the strain increase coefficient; T is the time when the concrete cracks, d.

[0169] After calculation, the strain increase coefficient and stress growth coefficient of each concrete ring 3 are shown in Table 6. It can be seen that the addition of steel bars can significantly reduce the strain increase coefficient and stress growth coefficient and reduce the risk of cracking.

[0170] Table 6 Strain increase coefficient and stress growth coefficient of each concrete ring 3

[0171]

[0172] Based on the above tests, the annular restrained shrinkage test device and reinforced concrete early cracking assessment method proposed in this application can accurately monitor the cracking time and width of the reinforced concrete ring 3. The cracking time and risk level of the concrete ring 3 under restrained shrinkage conditions can be predicted more accurately through the cracking risk index and stress growth coefficient.

Claims

1. A ring-shaped restraint shrinkage test device, characterized in that: The annular restraint shrinkage test device comprises a base plate (1), a prefabricated crack sheet, an inner steel ring (6), an outer steel ring (2), a strain gauge (7), a steel mesh and a strain collector; the outer steel ring (2) is sleeved on the outside of the inner steel ring (6) and is arranged on the base plate (1) concentrically with the inner steel ring (6); an annular space for forming a concrete ring (3) is formed between the outer steel ring (2) and the inner steel ring (6); the steel mesh is arranged in the annular space; the prefabricated crack sheet is arranged on the outer surface of the inner steel ring (6) along the axial direction of the inner steel ring (6) and extends along the outer surface of the inner steel ring (6) into the concrete ring (3); the strain gauge (7) is arranged on the inner surface of the inner steel ring (6), the steel mesh and the outer surface of the concrete ring (3); the strain gauge (7) is connected to the strain collector; The prefabricated crack sheet is a hard plastic slice or steel slice with a rectangular cross section; the thickness of the prefabricated crack sheet is not greater than 1 mm, and the height of the prefabricated crack sheet is higher than the height of the inner steel ring (6) and / or the outer steel ring (2); The steel mesh is an annular steel bar (5); the annular steel bar (5) comprises a continuous steel bar and a portion of steel bars arranged on the continuous steel bar along the axial direction of the continuous steel bar; the portion of steel bars is connected to the continuous steel bar via an oblique steel bar; the portion of steel bars and the oblique steel bars form a hexagonal steel bar structure as a whole; the strain gauge (7) is respectively on the continuous steel bar, the oblique steel bar and the portion of steel bars; the prefabricated crack gauge is placed on the outer surface of the inner steel ring (6) and extends towards the hexagonal steel bar structure.

2. The annular restraint shrinkage test device according to claim 1, characterized in that: The plane where the strain gauge (7) arranged on the inner surface of the inner steel ring (6) is located is perpendicular to the plane where the prefabricated crack gauge is located; the plane where the strain gauge (7) arranged on the inner surface of the inner steel ring (6) is located is parallel to the plane where the strain gauge (7) arranged on the outer surface of the concrete ring (3) is located.

3. The annular restraint shrinkage test device according to claim 2, characterized in that: The strain gauge (7) comprises a first strain gauge and a second strain gauge; the number of the first strain gauges and the number of the second strain gauges are both plural; the first strain gauge is arranged on the inner surface of the inner steel ring (6) and the steel mesh; the second strain gauge is arranged on the outer surface of the concrete ring (3).

4. The annular restraint shrinkage test device according to claim 3, characterized in that: The model of the first strain gauge is 120-3AA; the model of the second strain gauge is 120-50AA.

5. The annular restraint shrinkage test device according to any one of claims 1 to 4, characterized in that: The thickness of the inner steel ring (6) is greater than the thickness of the outer steel ring (2); the thickness of the inner steel ring (6) is less than the thickness of the concrete ring (3).

6. The annular restraint shrinkage test device according to claim 5, characterized in that: The outer steel ring (2) is a split structure; a groove (10) and a handle (9) are provided on the bottom plate (1); the outer steel ring (2) and the inner steel ring (6) are concentric and embedded in the groove (10).

7. A method for assessing early cracking of reinforced concrete, characterized in that: The reinforced concrete early cracking assessment method comprises the following steps: 1) constructing a ring-shaped restraint shrinkage test device as described in any one of claims 1 to 6; 2) forming a concrete ring (3) on the annular restraint shrinkage test device constructed in step 1); a steel mesh is arranged inside the concrete ring (3) along the axial direction of the concrete ring (3); strain gauges (7) connected to a strain acquisition instrument are arranged on the inner surface of the inner steel ring (6) of the annular restraint shrinkage test device, the outer surface of the concrete ring (3) and the steel mesh inside the concrete ring (3); 3) After the concrete ring (3) obtained in step 2) is initially set, the prefabricated crack sheet is removed, demoulding is performed, and continuous drying is performed in an environment with a temperature of 35±2° C. and a relative humidity of 50±5%. The concrete ring forms active cracks under the action of the prefabricated crack sheet and passive cracks under dry conditions, and monitoring data of the strain gauge (7) is regularly obtained through a strain acquisition instrument; 4) Calculating the stress growth coefficient and the cracking risk index based on the monitoring data obtained in step 3), evaluating the early cracking risk of concrete and predicting the cracking time of concrete according to the obtained calculation results; The calculation method of the cracking risk index in step 4) is: Ф(t)=σ res (t) / f ts (t) in: The Ф(t) is the cracking risk index of concrete at time t, and the cracking risk index is an evaluation standard for the concrete's ability to resist early shrinkage cracking; The σ res (t) is the maximum residual stress of the concrete ring at time t; The f ts (t) is the splitting tensile strength of concrete at time t; The t is the time when the strain gauge collects data; The σ res The expression for (t) is: in: The R oc is the outer diameter of the concrete ring; The R os is the outer diameter of the inner steel ring; The R is is the inner diameter of the inner steel ring; The ε steel (t) is the strain value of the inner steel ring measured at time t; Said ρ is the reinforcement ratio of the concrete ring; The E s is the elastic modulus of the inner steel ring; The E c is the elastic modulus of the concrete ring; Preferably, the reinforcement ratio of the concrete ring is expressed as: The R oc is the outer diameter of the concrete ring; The R ic is the inner diameter of the concrete ring; The n is the number of active slits; Said l is the length of the part of the steel bar; Said r is the radius of the steel bar; Said h is the height of the concrete ring; The c is the thickness of the concrete ring; Preferably, the calculation method of the stress growth coefficient in step 4) is: S=E s ·R ic ·t s ·|α| / (2R is ·t c ·T 0.5 ) in: Said S is the stress growth coefficient; The E s is the compressive elastic modulus of the inner steel ring; The R ic is the inner diameter of the concrete ring; The R is is the inner diameter of the inner steel ring; The s is the thickness of the inner steel ring; The c is the thickness of the concrete ring; The α is the strain magnification coefficient; Said T is the time when the concrete cracks; The method for assessing the risk of early cracking of concrete is based on the stress growth coefficient. When the stress growth coefficient S≥0.34, the cracking risk is high; when 0.17≤S≤0.34, the cracking risk is medium-high; when 0.10≤S≤0.17, the cracking risk is medium-low; when S≤0.10, the cracking risk is low; the actual cracking time of concrete is predicted based on the cracking risk index.

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