In order to quantitatively study the applicability of needle/cone penetration experiment for asphalt-rubber (AR), the dynamic model of needle/cone in penetration process was established based on the
Currently, nearly 1.5 billion waste tires are discarded around the world each year and the number is still increasing sharply [
Needle/cone penetration is an important index to evaluate the AR binder’s consistency, which represents the soft (hard) degree of the binder. However, there are some differences between these two evaluation indexes. The needle penetration experiment is carried out using test needle, which is mainly reflecting the shearing effect of the needle tip on sample, and few AR binders can be tested. For cone penetration, the cone tip has the shearing effect on sample which is similar to the needle penetration, while the cone body also has compressive stress on the sample, and more AR binders can be tested. Choosing the appropriate test method can reduce the variability of the experimental results and also reduce the test times and cost. Moreover, the real quality of AR binders can be reflected. The needle penetration at 25°C following SBS modified asphalt’ evaluation system is mostly used in AR local technical standards in China [
There have been some studies on needle/cone penetration experiments. Huang and Pauli et al. carried out experiments using eight different AR binders, and the needle penetration experimental results were highly discrete [
Based on the modification mechanism of coarse/fine CR, this paper emphatically analyzed the needle/cone penetration experimental process for the AR binder with coarse CR which shows obvious solid-liquid two-phase feature macroscopically. Starting with the mechanical analysis of needle/cone in penetration process, the needle/cone’s dynamic model was established based on viscoelasticity constitutive relationship, and the differential equations of penetration depth varying with time were obtained. According to statistical principle, the probability of needle/cone contacting CR particles in penetration process was calculated and the difference of testing mechanism between them was explored. Finally, the needle/cone penetration experiments on AR binders with different particle sizes were carried out to verify the correctness of theoretical analysis. This paper can provide theoretical support for the superiority of cone penetration to evaluate the AR binder with coarse CR.
The asphalt in this research was a pure asphalt binder with 90 penetration grades received from SK, Shell, and East Sea, respectively. Ambient grinding CR of #20, #30, #40, #60, and #80 mesh produced by Shaanxi Expressway, Hunan HeDeLi, and Xian ZhongXuan was used to prepare AR binders. The main technical parameters are shown in Tables
Main technical indexes of base asphalt.
Experiment item | SK90# | Shell 90# | East Sea 90# |
---|---|---|---|
Density (25°C)/g⋅cm−3 | 1.031 | 1.032 | 1.034 |
Needle penetration experiment (25°C)/0.1 mm | 85 | 87 | 85 |
Force-ductility (15 | >100 | >100 | >100 |
Softening point (°C) | 46.0 | 45.5 | 46.0 |
Main technical indexes of CR.
Experiment item | Shaanxi Expressway | Hunan HeDeLi | Xian ZhongXuan |
---|---|---|---|
Density/ (g⋅cm−3) | 1.17 | 1.19 | 1.18 |
0.75 | 0.81 | 0.78 | |
0 | 0.01 | 0 | |
0.01 | 0 | 0.02 | |
Ash (%) | 6.00 | 5.02 | 7.20 |
Acetone extractives (%) | 6.18 | 6.13 | 3.86 |
Carbon black (%) | 28 | 28 | 29 |
Rubber hydrocarbon (%) | 61 | 57 | 57 |
The AR binders were prepared by wet process. The #20, #30, #40, #60, and #80 mesh CR (20% by the weight of asphalt) from Shaanxi Expressway, Hunan HeDeLi, and Xian ZhongXuan were mixed with base asphalt from SK, Shell, and East Sea, respectively, to prepare AR binders. The AR binders were obtained by adding CR to the base asphalt, which was melted at 80°C–90°C in an oven previously. Manually stir for 5 min to predistribute CR in base asphalt, and then blend by using a high-speed mixer (at about 1000 rpm) with 180°C (±5°C) for 45 min following ASTM D6114-19 [
In order to explore the distribution of fine CR in base asphalt, the AR binders (prepared by East Sea asphalt and Xian ZhongXuan CR) with #40, #60, and #80 mesh CR were observed using FEI Quanta FEG 250 FESEM following GB/T 16594-08 [
Figure
AR binders with different CR particle sizes. (a) AR with #20 CR, (b) AR with #30 CR, (c) AR with #40 CR, (d) AR with #60 CR, and (e) AR with #80 CR.
SEM images of AR binders with different CR particle sizes. (a) AR with #40 CR, (b) AR with #60 CR, and (c) AR with #80 CR.
Aiming at the AR binder with coarse CR, this section discusses the quantitative impact of needle/cone contacting CR particles on penetration depth in experimental process to compare the applicability of needle/cone penetration through establishing a dynamic model.
Figure
The needle/cone penetration experimental results of AR binders with #20 CR.
The penetration depth is considered to be closely related to the moment of needle contacting CR particles. In other words, the final penetration depth is shallow when needle contacts CR particles in the early stage of penetration process; the final penetration depth is deep when needle contacts CR particles in the later stage of penetration process. Therefore, the maximum depth 5.22 mm means that the needle did not contact CR particles at all; while the minimum depth 4.03 mm means that the needle contacted CR particles in the early stage of penetration process; middle range is from 4.03 mm to 5.22 mm, which means that needle contacted CR particles at some point in the middle.
The geometric structure of needle/cone [
Needle.
Cone.
Stage 1 (
Mechanical analysis of stage 1.
The infinitesimal axial resistance of the needle is
The axial resistance is
Stage 2 (
Mechanical analysis of stage 2.
As shown in Figure
Displacement
The cone analysis process is similar to that of the needle except its geometric parameters are changed. In penetration process, needle/cone may contact CR particles. Because CR particles absorb the light component of asphalt in reaction process, it is no longer a pure elastomer [
According to mechanical analysis in Section
According to the analysis in Section
Viscoelastic parameters (the
0.468 | 0.468 | 0.0468 | 0.0468 | 0.69 | 0.69 | 0.069 | 0.069 |
Needle penetration process. (a) Needle penetration process. (b) Cone penetration process.
Table
Final penetration depth of needle/cone contacting CR particles at different moments.
The moment of contacting CR particles (s) | No | 0.5 s | 1 s | 1.5 s | 2 s |
---|---|---|---|---|---|
Final penetration depth of needle (mm) | 5.22 | 4.03 | 4.15 | 4.27 | 4.40 |
Final penetration depth of cone (mm) | 3.93 | 3.04 | 3.14 | 3.21 | 3.31 |
On the one hand, the influence of coarse CR particles on the penetration experimental results is reflected in the difference of final penetration depth between the needle/cone contacting CR particles or not. On the other hand, the influence is related to the probability of needle/cone contacting CR particles. Hence, it is necessary to analyze the probability of needle/cone contacting CR particles in experiment process.
According to experimental results in Section
Parameters of probability calculation.
Needle penetration depth | 5.22 mm |
Cone penetration depth | 4.32 mm |
Half-angle tip of needle Φ1 | 4.5° |
Half-angle tip of cone Φ2 | 15° |
#20 mesh CR diameter | 0.83 mm |
Sample dish diameter | 55 mm |
Sample dish depth | 35 mm |
Density of asphalt | 1.031 g·cm−3 |
Density of CR | 1.17 g·cm−3 |
The volume of the needle inserting into sample dish is
The volume of each CR particle is
The volume of sample dish is
The volume of total CR particles is
In order to accurately calculate the probability of the needle contacting CR particles, each CR particle volume is regarded as a minimum unit and other volumes are divided according to the minimum unit. The total number of units is
The number of units of needle penetration volume is
The number of units of all CR particles volume is
According to (
From (
Similar to the analysis in Section
The number of units of cone penetration volume is
According to (
From (
As shown in Figure
Experimental penetration process. (a) Needle penetration process. (b) Cone penetration process.
As shown in Figure
Standard deviations of the needle/cone penetration experimental results at 25°C for AR binders.
For the AR binders with coarse CR (#40 mesh or lower), needle penetration experimental results are impacted easily by obvious CR particles. With the increase of the CR mesh number, the desulfurization and degradation of CR particles in reaction process are gradually intensified and the solid-liquid two-phase property of the AR binder is weakened. Hence, the stability of needle penetration experimental results is gradually improved. However, cone penetration experiment has a good applicability for the AR binder with both coarse and fine CR. The cone penetration experiment has a significant advantage for evaluating the AR binder with coarse CR (#40 mesh or lower), which also verifies the results of SEM observation and theoretical analysis.
Aiming at large variability may appear in experimental results of AR needle penetration, based on viscoelastic model, probability analysis, and experiments, the applicability of needle/cone penetration experiment was analyzed. It is concluded that the cone penetration has a significant advantage in evaluating the consistency of the AR binder with coarse CR particles. The key findings are as follows: Obvious CR particles still exist in the AR binder with coarse CR after physical and chemical reactions of swelling and desulfurization degradation, which show distinct solid-liquid two-phase property. With the increase of the CR mesh number, CR’s specific surface area increases gradually and its desulfurization and degradation reaction occupy more weight. Hence, there are no obvious CR particles under SEM, which may not impact the penetration experimental results. Compared with needle penetration, cone penetration experimental results may be more stable to test the AR binder with coarse CR. The cause is that the needle is easy to contact CR particles or not in experimental process, and CR particles have a relatively obvious influence on final penetration depth. However, the cone contacts CR particles with a high probability (97.28%) in experimental process and CR particles have little impact on penetration depth. At the same time, cone experiment is mainly based on the compressive property of larger contact area to evaluate the AR binder’s consistency, which is more suitable for evaluating the AR binder with obvious elastic property. The dispersion of cone penetration experimental results is far less than that of needle penetration experiments for the AR binder with coarse CR (#40 mesh or lower). The standard deviations of needle penetration experimental results decrease gradually with the increase of the CR mesh number but are still slightly higher than that of cone penetration experiments.
All experiments data, models, and equations during the study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this article.
The authors would like to acknowledge the financial support of the Natural Science Basic Research Plan in Shaanxi Province of China (no. 2018JM5049).