To obtain the failure evolution law, a pullout test model of the anchor system is proposed based on the digital image correlation (DIC) measurements. By the study of the displacement field, the strain field, and the force transfer law of the anchor system under the pulling load, the failure law of the anchor system is revealed. The results show that (1) the failure mode and the ultimate bearing capacity of the anchor system are related to the thickness of the anchor agent; (2) in the anchor system, the pulling force is gradually transferred from the loading end to the free end along the steel bar, and the greater the thickness of the anchoring agent, the deeper the transfer range; (3) during the loading, the deformation of the anchoring system is mainly concentrated at the interface between the anchoring agent and the concrete and expands to the depth along the steel bar; and (4) the failure evolution rate of the anchorage system is related to the loading stage. The failure evolution of the anchor system can be divided into the elastic phase, the plastic phase, and the deformation rebound phase.
The need to reduce on-site construction time in civil infrastructure has led to an important increase in the use of precast concrete elements (PCEs). PCEs provide structures with higher material quality, better durability, reduced environmental impact, and increased work zone safety. The connection of these elements is typically done using field-cast nonshrink cementitious grouts [
Bond in cementitious materials is a topic that has been extensively researched in the past decades [
Studies of grout interface for rebar show that bonding forces is made up of three components: chemical adhesion, friction, and mechanical interlock. The adhesive strength between the interface is negligible [
However, despite a relatively large body of knowledge, there is still a lack of a good understanding of the failure evolution law, which has prevented the development of rational anchorage design procedures. This is due mainly to the fact that all kinds of transducer embedded in concrete can only be used to monitor the particular deformation at a certain region and it is difficult to directly observe the internal failure process of the anchor system. If the damage and crack arose at the other region that has not been monitored by an embedded transducer, it would fail to get any valuable data. However, the deformation in these regions is important because it might cause the entire failure eventually. It is difficult to determine the possible weak regions for concrete by theoretical analysis and calculation beforehand due to many holes and microcracks exist in concrete. Sometimes concrete might break in the region that was considered as low stress previously because damage in this region would result in the redistribution of stress [
The full-field measurement method provides a solution to the above problems which is more adapted than point-based techniques to provide 2D displacement and strain field within a given zone in the specimen. Among others, Digital Image Correlation (DIC) is probably the most widely adopted noncontact measuring technique in the material testing and structure monitoring domain. DIC is an optical technique to detect and quantify changes between a series of digital images. DIC-based approaches are used as noncontact measurement techniques to analyze 2D and 3D displacement fields. The early development of these techniques in the mechanics’ research domain dates to the 80s [
Therefore, based on the advantages of DIC, it is realized a comprehensive observation and investigation of the failure evolution law of the anchorage system during the loading process. In this paper, based on DIC measurements, a pullout test model of the anchor system is proposed. Through the pullout test, the loading transmission route of the reinforcing bar, the displacement field, and the strain field of the anchor system can be observed during the whole loading process. The deformation evolution characteristics of the interface in the anchor system are analyzed and the fail evolution law is explored.
DIC method is mainly used to measure the deformation field of a material or structure surface under external load or other factors. It has the advantages of full-field measurement, noncontact, relatively simple optical path, and adjustable measurement horizon. Ruocci et al. [
Illustrative of the principle of DIC in-plane displacements. (a) Before deformation
Considering tensile and shear effects with very small Δ
Substituting equations (
Generally, for an arbitrary point
In the case of a very small
The distance before and after deformation can be obtained as follows:
Therefore, the strain in all direction can be defined as
Figure
Figure
Conventional model in the pullout test.
Proposed model in the pullout test.
Many factors affect the bonding performance between two materials, the anchoring agent thickness being one of them. The bars are anchored in holes with diameters of 40, 50, 75, and 160 mm corresponding to the anchoring agent thickness of 8, 13, 25, and 93 mm, respectively. The details of the specimen parameters are shown in Table
Specimen parameters.
Number | Rebar diameter (mm) | Hole diameter (mm) | Anchoring agent thickness (mm) |
---|---|---|---|
SJ-40 | 25 | 40 | 8 |
SJ-50 | 25 | 50 | 13 |
SJ-75 | 25 | 75 | 25 |
SJ-160 | 25 | 160 | 93 |
An ordinary Portland cement P·O42.5 with a Blaine fineness of 382 m2/kg and a density of 3070 kg/m3 was used to prepare the concrete. The fine aggregate (FA) was an ordinary river sand with an apparent specific gravity of 2.59. The coarse aggregate (CA) consisted of dolomitic limestone with an apparent specific gravity of 2.85. The concrete mixture was developed to perform similarly to a prefabricated concrete element in terms of strength. Therefore, the concrete was designed with a water-to-cement ratio of 0.35 by mass, cement : FA : CA ratio of 1 : 1.7 : 2.5 (by mass), and a targeted 28 d compressive strength of 55 MPa.
The available commercially nonshrink cementitious grout was used in the test. The grout was supplied in a bag containing the solid fraction that was mixed with a certain amount of water following the manufacture’s recommendations. The grout had a water-to-solid ratio of 0.16 by mass and produces a 28 d compressive strength of 62 MPa.
The specimen manufacture process includes 4 steps. First, corresponding to the specimens, the PVC pipes with different diameters were embedded in the mold as the reserved holes. Then, the concrete was vibrated in the mold, as shown in Figure
Specimen manufacture process. (a) Casting concrete matrix. (b) Removing the mold. (c) Embedding the rebar. (d) Spraying speckle.
The test system including an XY-350 hydraulic pullout instrument and a reaction rack was used to apply load at the specimen (as illustrated in Figure
Loading system in the test.
In the test, the foil-type resistance strain gauge and one CCD industrial camera were used to collect the deformation data of the reinforcing bar and the target surface, respectively. The strain gauges were placed at 5 measuring points (MP 1#–5#) from the loading end to the free end along the upper edge of the reinforcement. Due to the symmetry, only left half surface is analyzed. The resolution of the collected speckle image was 1600 pixels × 1200 pixels, and the object surface resolution was 0.24 mm/pixel. The location of the strain gauge, coordinated system, and analyzed area is shown in Figure
Analyzed region and strain gauge position.
The specimens mainly exhibited two failure modes including splitting failure (tensile cracks in the concrete parallel to the reinforcing bar), shown in Figure
Specimen failure. (a) SJ-40. (b) SJ-50. (c) SJ-75. (d) SJ-160.
For SJ-40, now that there was not sufficient bond strength between the bar and the concrete due to the thin thickness of the grouting agent, it was mainly splitting failure. As the load increases, tensile cracks first appeared at the interface between the bar and the grout near the loading end and developed to the free end. Finally, the cracks penetrated the entire specimen. The bar and the grout were completely debonded, and the specimen was destroyed (as shown in Figure
With the increase of the grout thickness, the failure modes of SJ-50 and SJ-75 gradually transformed into the cracking at the interface between the grout and the concrete, until to form the shallow angle concrete cone in SJ-160, as shown in Figure
Table
Pulling force when the specimen is broken.
Number | Rebar diameter (mm) | Anchoring agent thickness (mm) | Ultimate anchoring capacity (kN) |
---|---|---|---|
SJ-40 | 25 | 8 | 49.00 |
SJ-50 | 25 | 13 | 69.04 |
SJ-75 | 25 | 25 | 76.34 |
SJ-160 | 25 | 93 | 96.08 |
Due to the similar law in specimens, SJ-160 is taken as the typical representative for analysis. Figure
Curve of pulling force and strain.
From the above analysis, it can be found that the pullout force is transmitted from the loading end to the free end along the axial direction of the bar and the strain decreases with the increase of the distance to the loading point. At the same time, the evolution trend of the force-strain curves is similar.
Figure
Curve of axial force difference between the two adjacent MPs with the pulling force.
Based on the DIC technology, the deformation field map of the analyzed region is given including the displacement field and the strain field.
The displacement field is calculated from equation (
Horizontal displacement field of SJ-160. (a) 36 kN. (b) 62 kN. (c) 75 kN. (d) 96 kN.
Figure
Vertical displacement field of SJ-160. (a) 36 kN. (b) 62 kN. (c) 75 kN. (d) 96 kN.
Based on the displacement field, the principal strain is calculated by
Taken SJ-160 as an example, the
When the loading reached 75 kN, the influence range of
Based on the principal strain field, the maximum shear strain can be further calculated by
The
As shown in Figure
From the above analysis, it can be found that (1) under the pulling force, the deformation of the anchoring agent expands from the loading end to the deep, and the deformation is continuously reduced and (2) due to the large thickness of the anchoring agent, during the entire loading stage, the maximum deformation value is always located at the interface between the bar and the anchoring agent.
By analyzing the evolution characteristics of the relative displacement between the anchoring agent and the concrete, the transmission law of the force in the anchoring system is analyzed.
The relative displacement of the anchoring agent and the concrete is calculated as follows. The 5 mm × 5 mm calculation window of the pixel point is selected in the anchoring agent and the concrete of the specimen, which center is 5 mm away from the interface, as shown in Figure
Calculation window of the displacement.
Evolution curve of the relative displacement.
From Figure
In the present work, a DIC-based method is proposed to analyze the anchorage system. The method is applied to measure the displacement field and the strain field of the anchorage system. The failure evolution law of the bonding interface in precast concrete structure is studied by the pullout test. The following can be concluded: The failure mode and ultimate bearing capacity of the anchor system is related with the thickness of the anchor agent. In the anchor system, the pulling force is gradually transferred from the loading end to the free end along the steel bar. And the greater the thickness of the anchoring agent, the deeper the transfer range. During the loading, the deformation of the anchoring system is mainly concentrated at the interface between the anchoring agent and the concrete and expands to the depth along the steel bar. The failure evolution rate of the anchorage system is related to the loading stage. The failure evolution of the anchor system can be divided into the elastic phase, the plastic phase, and the deformation rebound phase.
The experimental data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was financially supported by the National Natural Science Foundation of China (grant nos. 51408009 and 51608010), Fundamental Research Funds for the Beijing’s Universities (110052971921/062), and Yuyou Talent Project of North China University of Technology (grant no. XN012/044) for this study.