Different rubber aggregates lead to changes in the effect of stress conditions on the mechanical behavior of concrete, and studies on the triaxial properties of self-compacting rubber concrete (SCRC) are rare. In this study, 35 cylindrical specimens taking lateral stress and rubber type as variables were prepared to study the fresh properties and mechanical behaviors of SCRC under triaxial compression, where the rubber contains two types, i.e., 380 μm rubber powder and 1–4 mm rubber particles, and four contents, i.e., 10%, 20% and 30%. The test results demonstrated that SCRC exhibited a typical oblique shear failure mode under triaxial compression and had a more moderate descending branch compared with self-compacting concrete (SCC). The presence of lateral stress can significantly improve the compression properties, including initial elastic modulus, peak stress and peak strain, with an improvement range of 3%–73% for peak stress. While rubber aggregates mainly targeted the deformation abilities and toughness for improvement, and the peak strain improvement ranges were 0.1–3.1 times and 0.1–1.0 times for SCRC containing rubber powder and SCRC containing rubber particles, respectively, relative to SCC. At a high lateral stress of at least 12 MPa, the loss of strength due to the addition of rubber can be controlled within 10%, in which case the content of rubber powder and rubber particles was recommended to be at most 20% and 30%, respectively. Based on the Mohr-Coulomb theory, the failure criteria of SCRC with different rubber types were established. For analysis and design purposes, an empirical model was proposed to predict the stress-strain behavior under triaxial compression, considering the influence of different rubber content and lateral stress. The results obtained in this study can provide a valuable reference for the design and application of self-compacting rubberized concrete in practical projects, especially those involving three-way compression states and requiring high-quality deformation and energy dissipation.
Rapid industrialization has caused many environmental problems [
Relevant studies have shown that rubberized concrete has some performance advantages. Khaloo et al. [
In order to improve the compactness, scholars suggested combining rubber with self-compacting concrete (SCC) [
Unfortunately, studies on the mechanical properties of rubberized concrete have mainly focused on uniaxial compression properties and simple rubber concrete materials [
To this end, conventional triaxial tests were conducted on SCRC to investigate its mechanical properties more comprehensively by analyzing the interaction effects of lateral stress, rubber type and content, and to establish triaxial compression failure criterion and stress-strain constitutive model considering variables.
The ordinary Portland cement with a compressive strength of 42.5 MPa was used to prepare the mixture as per the Chinese standard “Common Portland Cement” (GB 175-2007) [
Cementitious materials | Apparent density (kg/m3) | Chemical properties (%) | |||||||
---|---|---|---|---|---|---|---|---|---|
CaO | SiO2 | Al2O3 | Fe2O3 | MgO | SO3 | K2O | Na2O | ||
Cement | 3100 | 61.73 | 23.86 | 6.53 | 3.91 | 3.62 | 2.12 | 0.38 | 0.27 |
FA | 2350 | 4.01 | 53.97 | 31.15 | 4.16 | 1.01 | 2.2 | 2.04 | 0.89 |
Aggregates | Particle dimension | Fineness modulus | Apparent density (kg/m3) | Water absorption (%) |
---|---|---|---|---|
Coarse aggregate | 5–20 mm | – | 2750 | 0.92 |
Fine aggregate | Max 4.75 mm | 2.6 | 2650 | 1.5 |
Rubber powder | 380 μm | – | 1168 | 1.7 |
Rubber particle | 1–4 mm | – | 1095 | 1.55 |
The purpose of this paper was to study the effect of rubber aggregates on the performance of SCRC under the condition that natural sand was replaced by equal volume. Therefore, the proportions of cementitious, water, Plasticizer and crushed stone were kept constant except for rubber and natural sand as per the Chinese standard “Technical specification for application of self-compacting concrete” (JGJ/T 283-2012) [
Spec. No. | Cement | Fly ash | Water | Crushed stone | Natural sand | Rubber | Plasticizer |
---|---|---|---|---|---|---|---|
SCC | 350 | 150 | 205 | 940 | 800 | 0 | 2.7 |
SCRC-A10 | 350 | 150 | 205 | 940 | 720 | 35 | 2.7 |
SCRC-A20 | 350 | 150 | 205 | 940 | 640 | 70 | 2.7 |
SCRC-A30 | 350 | 150 | 205 | 940 | 560 | 105 | 2.7 |
SCRC-B10 | 350 | 150 | 205 | 940 | 720 | 33 | 2.7 |
SCRC-B20 | 350 | 150 | 205 | 940 | 640 | 66 | 2.7 |
SCRC-B30 | 350 | 150 | 205 | 940 | 560 | 99 | 2.7 |
Note: SCRC-A denotes SCRC with rubber powders; SCRC-B denotes SCRC with rubber particles; the numbers 10, 20, 30 refer to the rubber content.
In order to evaluate the uniformity and workability of SCRC, each series of concrete mixtures was sampled three times for slump flow (
where
For each series of concrete mixtures, five cylindrical specimens with dimensions of Ф100 mm × 200 mm were cast for triaxial compression tests. After setting for 24 h, the specimens were demoulded and then water cured for 7 days. The specimens were then placed in a curing chamber at a temperature of 20 ± 2°C and a relative humidity of more than 95% for 28 days. Before testing, the ends of the specimens were smoothed and made parallel, and then the surfaces were oiled to reduce the friction between the specimens and the rubber sleeve as well as the restraining effect of the ends.
A 1500-kN RMT-201 rock testing device was used to carry out a triaxial compression test as per the Chinese standard “Standard for test methods of engineering rock mass” (GB/T 50266-2013) [
During the above specimen preparation, an additional specimen was reserved for scanning electron microscopy (SEM) observation for each series of SCRC. Likewise, one sample for each series of SCRC that had been damaged was withdrawn for observation. Cubes with the size of 10 mm × 10 mm × 10 mm were cut from the middle of the specimen, soaked and cleaned with alcohol solution and dried in a vacuum desiccator for 24 h. Then the surface was polished using sandpaper and coated with a layer of platinum.
As shown in
As shown in
In this experiment, the authors found that lateral stress is a vital factor in influencing the failure modes in comparison with rubber types and content. So, typical failure modes of SCC and SCRC under different lateral stress are shown in
The compressive stress-strain (
The results showed that the compressive behaviors, e.g., initial elastic modulus, peak stress, and peak strain, were improved significantly with the increase of the lateral confining stress, which limited the transverse deformation and crack expansion. In particular, the presence of rubber made the improvement of compression performance by lateral stress more significant, as will be analyzed in detail later. In addition, SCRC-A possessed a milder descent phase or even no descent phase compared to SCRC-B. The reason is that the rubber aggregates act like discontinuous energy-consuming matrices during the loading process, while the number of rubber powder per unit volume of the specimen is more than that of the rubber particles. This leads to better energy dissipation ability of SCRC-A and retards its damage.
where
An important conclusion that can be drawn is that the strength loss becomes more and more severe as the rubber content increases. The reason for this is the lower strength of rubber and the adhesion between rubber and cement paste [
Another important conclusion is that rubber powder caused more severe strength loss than rubber particles at the same content. This is because the specific surface area (i.e., the ratio of area to volume) of rubber powder is larger compared with the rubber particles, and it has worse adhesion with the cement paste. In that case, the force transfer performance between the rubber powder and cement paste becomes weaker, and the rubber powder even shows overall peeling, which can be seen in
A further novel finding is that at a lateral stress of 12 MPa, the strength losses were 3%, 7%, 11% for SCRC-A and 2%, 5%, 8% for SCRC-B when the rubber content was 10%, 20%, 30%, respectively. Therefore, a practical suggestion is given that the replacement rate limits for rubber powder and rubber particles can be taken as 20% and 30%, respectively, only when the lateral stress reaches 12 MPa.
Given the trend line of the relationship between peak stress and lateral stress, a linear equation was fitted using Origin software, following the least square method, as shown in
Consistent with the method in
where
Toughness reflects the ability of a material to absorb energy and plastically deform without fracturing. According to the method proposed in the research [
where
The results showed that the toughness of SCRC was generally better than that of SCC, which was attributed to the enhancement effect of the rubber on the post-peak deformation capacity. The rubber type had little effect on the toughness, while the presence of lateral stress made the toughness advantage of SCRC over SCC more prominent and most pronounced at a lateral stress of 6 MPa. Therefore, as a recommendation, triaxial compressive SCRC with a lateral stress of 6 MPa can be used preferentially to improve the energy absorption capacity of the structure.
The failure and strength of materials under multiaxial stresses is a common concern in engineering science. As one of the classical strength theories, Mohr-Coulomb failure theory is often used to describe the response of brittle materials or materials with much higher compressive strength than tensile strength under combined shear and normal stresses. Mohr-Coulomb failure theory states that a material will fail when the shear stress in the oblique section exceeds the shear strength. Considering that SCRC exhibited oblique shear damage under triaxial stresses, and that the presence of rubber severely weakened the cohesion between aggregates, the Mohr-Coulomb theory was chosen as the failure criterion for SCRC.
According to the Mohr-Coulomb theory, strength failure envelopes were made for a series of Mohr circles, as shown in
Parameters | Variance, |
|||
---|---|---|---|---|
Spec. No. | ||||
SCC | 5.945 | 2.718 | 0.748 | 0.97 |
SCRC-A10 | 5.430 | 1.688 | 0.882 | 0.96 |
SCRC-A20 | 5.135 | 0.934 | 1.064 | 0.97 |
SCRC-A30 | 2.260 | 1.463 | 0.944 | 0.95 |
SCRC-B10 | 5.749 | 2.270 | 0.796 | 0.98 |
SCRC-B20 | 5.617 | 1.906 | 0.843 | 0.98 |
SCRC-B30 | 4.957 | 1.692 | 0.878 | 0.97 |
where
To facilitate engineering applications,
where
The stress-strain relationship is a macroscopic reflection of the mechanical behavior, and many scholars have proposed different constitutive models for concrete theoretically and experimentally. Hognestad et al. [
where
Parameters | COD, |
COD, |
||
---|---|---|---|---|
Spec. No. | ||||
SCC | 0.61 | 0.98 | 13.65 | 0.92 |
SCRC-A10 | 0.65 | 0.99 | 8.28 | 0.99 |
SCRC-A20 | 0.72 | 0.99 | 5.62 | 0.94 |
SCRC-A30 | 0.94 | 0.99 | 9.00 | 0.99 |
SCRC-B10 | 0.66 | 0.99 | 12.8 | 0.93 |
SCRC-B20 | 0.73 | 0.99 | 11.62 | 0.96 |
SCRC-B30 | 0.91 | 0.99 | 10.82 | 0.99 |
Spec. No. | SCC | SCRC-A | SCRC-B |
---|---|---|---|
Lateral stress | |||
0 MPa < |
9.08 | 0.35 | 9.08 – 78.70 |
3 MPa < |
2.24 | 2.24 – 15.02 |
|
6 MPa < |
1.77 | 0.47 | |
12 MPa < |
1.12 |
The conventional triaxial compression tests, together with fluidity test and microstructural observations, revealed the effect of rubber type and rubber content on the triaxial compression performance of SCRC. The following conclusions can be drawn: The rubber aggregates increased air voids and micro-cracks in the concrete mixture. The flowability of SCRC decreased with increasing rubber content but was little affected by the rubber type. Under uniaxial compression, the specimens SCRC exhibited longitudinal splitting failure. Under triaxial compression, the specimens SCRC showed oblique shear failure. Compared to ordinary SCC, SCRC had a more moderate descending branch and superior deformation capacity. The presence of lateral stress significantly improved the compressive properties of SCRC, e.g., initial elastic modulus, peak strength and peak strain, with an enhancement range of 3%–73% for peak stress. Although the addition of rubber reduced the compressive strength of SCRC to some extent, it can substantially improve the deformation capacity and toughness, and made the lateral stress more effective in improving the compressive performance, with the improvement range of 0.1–3.1 times for peak strain. The effect of rubber powder was more significant than that of rubber particles. Considering the compressive performance and deformation capacity, it was recommended that when SCRC is applied to the design of structures with lateral stress not less than 12 MPa, the content limits of rubber powder and rubber particles are 20% and 30%, respectively, with the latter being the preferred option. Mohr-coulomb theory was used as the failure criterion of SCRC with different rubber types. Guo model, which considered the effects of variables, can reveal the triaxial stress-strain behavior of SCRC well.
The results obtained in this study can provide a valuable reference for the design and application of SCRC in practical projects, especially those involving three-way compression states and requiring high-quality deformation and energy dissipation. In view of the preliminary investigation of the triaxial performance of SCRC and the variation pattern of triaxial constitutive model parameters with variables, the sample size will be expanded later to further investigate the dual effects of lateral stress and rubber content on the constitutive model, especially for SCRC containing rubber powder.
Self-compacting rubberized concrete
Self-compacting concrete
Self-compacting rubberized concrete containing rubber powder
Self-compacting rubberized concrete containing rubber particles
Control parameters in Guo model
Control parameters in Guo model
Cohesion in Mohr-Coulomb theory
Maximum circular spreading diameter of concrete in slump flow test
Diameter of concrete circle at an angle perpendicular to
Peak stress in the uniaxial compression test
Rubber content
Slump flow
Area enclosed by the stress-strain curve before the peak
Area enclosed by the stress-strain curve before the stress drops to 80% of the peak
Time required for concrete to reach a diameter of 500 mm
Compressive stress
Peak stress in the triaxial compression test
Lateral stress
Compressive strain
Peak strain in the triaxial compression test
Peak strain in the uniaxial compression test
Ratio of the peak stress of SCRC to SCC
Ratio of the peak strain of SCRC to SCC
Fitting parameter in Mohr-Coulomb theory
Fitting parameter in Mohr-Coulomb theory