The failure characteristics of recycled concrete containing brick aggregates are still indistinct, especially how the angular aggregates effect the crack propagation. Based on the concept of modeled concrete, the development of cracks in concrete containing the natural aggregate and brick aggregate under a compression loading was studied. The strain distribution was analyzed with the Digital Image Correlation (DIC). The modeled aggregates include circular and squared ones, and the squared modeled aggregates were placed in different orientations, including 0°, 22.5° and 45°. The results show that when the aggregate is placed at 45°, the upper and lower vertices of the aggregate lead to the highest critical strain concentration, therefore, cracks are easy to propagate from these areas and the strength of the corresponding modeled concrete is the lowest. When the modeled natural aggregate is placed at the orientation of 0°, the strain concentration first appears at the interface on both lateral sides of the aggregate. The brick aggregate has a lower elastic modulus and strength than the surrounding mortar. As a result, cracks always propagate through the brick aggregate, which is the primary reason for the low strength of the corresponding concrete.

Global population growth, construction of infrastructures and house building activities create enormous amounts of the construction and demolished waste (C&D waste). In recent years, China’s urbanization process is very fast, and the annual production of C&D waste reaching over 18 billion tons [

It is not easy to understand or predict the mechanical properties of recycled concrete because of its multi-phase and complex microstructures. The modeled concrete was designed to understand the relationship between the aggregate phase and hardened mortar phase, along with the crack development in the process of stress failure [

Zheng et al. [

The previous research on the modeled recycled concrete simplified the aggregate to a circle shape [

Marble stone was used as the modeled natural aggregate and sintered clay bricks were used as the modeled recycled aggregate. Circular aggregates from stone and bricks with a diameter of 35 mm and a thickness of 20 mm were prepared to make plate specimens of 20 mm thickness. At the same time, squared aggregates of 35 mm side length and with a thickness of 20 mm were also prepared. The mortar poured surrounding the aggregate had a mix proportion as cement: water: sand = 1:0.6:3. The Chinese standard P.O. 42.5 cement was used as the cementing agent. The aggregates were placed at center of the plate molds and the cement mortar was poured around them. The modeled concrete is a plate made of the aggregate and cement mortar, with the overall dimension of 100 mm × 100 mm × 20 mm. For the squared aggregates, three different orientations of 0°, 22.5° and 45° were adopted.

In order to accurately grasp the physical and mechanical properties of each phase of materials, prismatic specimens of 40 mm × 40 mm × 80 mm were prepared from each material, i.e., marble stone, bricks and the mortar that is used for making the modeled concrete. The compressive strength, elastic modulus and Poisson’s ratio of the prismatic specimens were tested on an electronic universal testing machine. The mechanical properties are shown in ^{3}, the densities of the mortar and the brick are 2180 kg/m^{3} and 1810 kg/m^{3}, respectively. The elastic modulus of the natural stone material is significantly higher than the other materials. Comparing the compressive strength of the materials, it is found that it follows the same trend as the elastic modulus. However, the Poisson’s ratios of them are relatively similar to each other.

Material | Density (kg/m^{3}) |
Elastic modulus (GPa) | Compressive strength (MPa) | Poisson’s ratio |
---|---|---|---|---|

Natural aggregate | 2560 | 65.0 | 170.1 | 0.160 |

Mortar | 2180 | 16.3 | 18.5 | 0.170 |

Brick aggregate | 1810 | 2.5 | 5.4 | 0.177 |

Digital image correlation (DIC) technology is a non-contact and non-destructive testing method. It directly provides full-field displacements to sub-pixel accuracy and full-field strains by comparing the digital images of a test object surface acquired before and after deformation. By using the randomly arranged speckles on the surface of the specimen and then comparing the change of the relative position of these speckles while the specimen is under loading process, the displacement field on the surface of the specimen can be calculated. The strain distribution field can be further analyzed to obtain the process of strain concentration and crack propagation.

Dark speckles were made on the front surface of the test specimen and the size of the speckles was about 0.7 mm, covering the entire area to be measured. An industrial camera (model JHSM300f) was used for images capturing with a pixel of 2048 × 1536. The whole surface of the specimen is in the visible range, which is 150 mm × 120 mm. Therefore, the resolution can reach 0.08 mm/pixel. From the above analysis, the size of the speckle is suitable for calculating the strain distribution field according to related researches [

The prepared plate specimen was placed on an electronic universal testing machine for compression loading. Two sheets of Teflon were placed on the upper and lower surfaces of the test specimen, and lubricating oil was applied between the two layers to reduce friction. Before the formal loading, the specimens were preloaded to 5 kN and unloaded to 0 kN three times to dismiss uneven compression. The loading rate of preloading was 0.1 mm/min. The formal loading includes two processes: Force loading and displacement loading. The force loading rate is 2 kN/min. When the load reaches 20 kN, it was changed to displacement loading. The loading rate is 0.05 mm/min. The whole loading test setup is shown in

The compressive strength of each specimen was obtained from dividing the observed peak load by the area of the cross section (100 mm × 20 mm). For calculating the secant modulus, the rising section of the loading curve was adopted at 20% and 70% of the peak load. Both the compressive strength and the secant modulus of the specimens are shown in

From

The specimens containing the circular aggregate show lower compressive strengths compared to those of squared aggregates, except the orientation of 45°. There should be two reasons to explain this. First, the circular aggregate has a diameter 35 mm, which is the same as the side length of the squared aggregate. Therefore, the area of the circular aggregate is smaller than the squared one, which results in more mortar in the specimen of circular aggregates. Second, the ITZ may play a more essential role in the specimen containing circular aggregates because of the smooth and warped surface. In contrast, the surface of the squared aggregate has a certain orientation as it is placed.

For the secant modulus of the modeled concrete, similar trend is found as the strength which is presented in

The variation of the secant modulus shows that it is related to the aggregate shapes and orientations. According to the different phases in concrete, each phase can be considered as a series of springs, which are connected in different positions. When the orientation of the aggregate are changed, the connection relationship of the springs changes, which leads to a change in the secant modulus of the concrete performance.

Compressive strength (MPa) | Secant modulus (GPa) | Failure modes | |
---|---|---|---|

CNA | 20.4 | 25.7 | Cracks along interface |

SNA-0° | 21.4 | 27.6 | Cracks along lateral interface |

SNA-22.5° | 23.7 | 29.4 | Cracks along lateral interface |

SNA-45° | 19.4 | 25.2 | Cracks on the angles of aggregate |

CBA | 18.5 | 24.2 | Cracks penetrating aggregate |

SBA-0° | 20.9 | 25.3 | Cracks penetrating aggregate and interface |

SBA-22.5° | 21.4 | 25.0 | Cracks penetrating aggregate |

SBA-45° | 18.3 | 24.4 | Cracks penetrating aggregate and interface |

In the following section, the strain of the horizontal direction (ε_{x}) of the specimens was recorded and analyzed. For each specimen, three figures are provided: First at 20% of the peak force (F_{p}), second at 80% of the peak force and third at 100% of the peak force. At each load level, two figures are illustrated, see

From

When the load reached 80% of the specimen’s strength (

As soon as the specimen reaches its peak force, it can be seen from the graph of strain concentration (

According to the strain curve along L1, it can be concluded that the strain concentration happens on both lateral sides of the aggregate. The strain values are 0.0006 and 0.0018, respectively, on the left and right lateral interfaces, and there should be micro cracks. For the L2 curve at the lower part of the specimen, the maximum strain value is about 0.0013. According to these curves, we can see the quantitative degree of the strain concentration around the aggregate, which is helpful to understand the influence of the aggregate on its surrounding strain concentration.

When the squared aggregate is used and its sides are parallel to the sides of the mortar specimen, this contributes to an even strain distribution and less stress concentration around the aggregate, see

As 80% of the peak force is reached, the crack at the left end of curve L2 develops further, and the strain value reaches to 0.007. At the same time, a small strain concentration appears along the left side of aggregate, and its value is about 0.0015, which can be seen from the curve L1 in

When it reaches the peak load, for

For the specimens containing a squared natural aggregate placed in an inclined direction of 22.5°, as shown in

With the increase of load, the upper tip of the aggregate cause obvious strain concentration and evolves into a crack. The strain concentration is along the inclined surface of the aggregate, as shown in

Before reaching the peak load, the crack continues to develop downward along the right surface of the aggregate, and then to develop vertically after crossing the lower tip of the aggregate. This crack develops from the top of the specimen and gradually extends to the bottom of the specimen, see

When the modeled squared natural aggregate is aligned at an angle of 45°, the strain distribution figures and curves are illustrated in

When the load increases to 80% of the peak load, the strain concentration in

As the load continues to increase to failure, see

When brick aggregate is introduced in the specimen, a larger variation of the strain distribution has been observed. The strength and elastic modulus of the brick aggregate are less than that of the hardened mortar. This differs completely from ordinary concrete and it results in completely different failure modes and cracks propagating laws. In the following part, the modeled concrete made of circular brick aggregate and squared brick aggregate with three different orientations is tested and the development of cracking is also analyzed to summarize the influence of brick aggregate on cracking performance.

According to

When the load increases to 80% of the peak load, an obvious crack can be seen in the upper right corner of the specimen. It can also be clearly seen from the curve in

As the load continues to increase, the crack at the upper right corner of the specimen propagates downward and gradually penetrates the interface on the right side of the aggregate, see

For the modeled squared brick aggregate concrete, according to the strain contour at 20% peak load (see

When the load increases to 80% of the peak load, the cracks are also initiated from the interior of the brick aggregate, as shown in

With the continuous increase of the load, before the failure, there are cracks on the upper and lower surfaces of the specimen in the mortar area, see

For the specimen that the squared brick aggregate is aligned at 22.5°, when the compressive load is 20% of the peak load, the strain of X-direction fluctuates between ± 0.00015, see

As the load reached to 80% of the peak force, the cracks propagates from the lower left and right corners of the specimen (see

The load continues to increase, the cracks in the mortar become longer and wider, and the strain value increases to 0.016. The maximum strain concentration internal the brick aggregate increases to 0.006, as shown in

When the squared brick aggregate is assigned at an angle of 45°, the aggregate is still the weakest area in the specimen. The strain concentration can be seen inside of the aggregate as 80% peak load is reached. There is also an obvious strain concentration starting from the bottom corner of the specimen in

As the load reaches its peak value, multiple cracks emerge on the left top corner and other parts of the specimen, which can be attributed to the difference in the material properties of the brick aggregate and the mortar.

In summary, comparing the natural aggregate and brick aggregate modeled concrete, it is obvious that brick aggregate is the weak spot in concrete. Internal fractures always occur inside the brick aggregate before the mortar cracks. In addition, there are obvious strain concentrations at the interface area around the natural aggregate, especially when the sharp angle of the aggregate is facing the stress direction. This should be caused by the great difference of elastic modulus between the aggregate and the mortar. However, the phenomenon of strain concentration at the interface area between the brick aggregate and mortar is not distinct. This may result from two reasons, first, the difference of elastic modulus between brick aggregate and mortar is relatively small, second, the damage develops inside the aggregate prior to the interface area.

The modeled concrete containing stone and brick aggregates with different orientations were studied in this investigation, based upon the crack propagation and damage characteristics during compressive loading, the following conclusions may be drawn:

Applying modeled aggregate to recycled concrete, not only can suffice the understanding of crack development process but also the strain concentration of the specimen can be obtained by using DIC.

The mechanical test shows that the elastic modulus of brick aggregate is 2.5 MPa, which is only 1/6 of that of the hardened mortar. This attributes that the modeled concrete containing brick aggregate has a lower elastic modulus as to the corresponding modeled natural concrete.

The elastic modulus of the natural aggregate in the modeled concrete is about 4 times of that of the hardened mortar, and the strength of the natural aggregate is much higher than the mortar. Therefore, the strain concentration often occurs in the interface, and the cracks pass through the interface.

When the modeled aggregate is placed at 45°, there are obvious strain concentrations at the top and bottom tips of the aggregate, and the strain concentration is higher than any other cases. And also, the strength of the specimen in this case is the lowest compared to others.

According to the strain distribution field obtained with DIC, it can be seen that the failure of the brick aggregate modeled concrete starts from the brick aggregate itself. The brick aggregate cracks earlier than the surrounding mortar, which is the main reason for the low strength of brick aggregate concrete.

There may be some drawbacks in the application of two-dimensional DIC in this test, because the out-of-plane displacement of the specimen is regarded as the expansion or contraction in the plane after the captured figures being analyzed with DIC. This results in the distortion of the strain value obtained from the analysis, but the strain distribution can still be used to judge the position and propagating trend of the crack.

According to the influence of modeled aggregate orientations on elastic modulus, strength and failure process of concrete, it is found that the strain concentration is easily caused by aggregate tips or sharp edges. Moreover, the aggregate should not have a large flat surface, which is easy to cause splitting failure. Therefore, the method of removing the tip of aggregates and reducing the flaky aggregate is highly recommended to improve the quality of recycled aggregates.

The research based on the modeled concrete can clearly show its damage evolution process, and it is still necessary to carry out more extensive research based on the changes of parameters such as different types of aggregates and aggregate shapes and numbers, so as to better grasp the damage characteristics of recycled concrete.