Effect of waste crab shell powder on matrix asphalt

2.1 Materials

The asphalt that was chosen is 90# matrix asphalt, which is made in China at Panjin Asphalt Plant. The matrix asphalt’s characteristics are listed in Table 1 according to the Chinese technical specification (JTG E20-2011).

A selection of WS powder was made in Hohhot, China’s Inner Mongolia Autonomous Region. After being cleaned with tap water, it was dried for 12 h at 100°C in an oven. Following grinding with a grinder, particles larger than 0.15 mm were removed by sieving. Particles smaller than 0.15 mm in size were retained. The sample is shown in Figure 1(a).

Figure 1

Research methodology flowchart. (a) Sample of WS powder, (b) asphalt high-speed shear machine, (c) penetrometer, (d) ductility machine, (e) softening point tester, (f) rotary shear rheometer, (g) low-temperature BBR, (h) differential scanner, (i) infrared spectrum scanner, (j) SEM, and (k) AFM.

2.2 Preparation of the modified asphalts

A high-speed shear shown in Figure 1(b) was used in the laboratory to make WS powder modified asphalt. The base asphalt was heated to 150°C in an oven, and then 5, 10, 15, and 20% (mass percentage) of WS powder modifier was added to the base asphalt. The mixture was then mixed at 150°C for 60 min at a speed of 3,000 rpm. This constituted the preparation procedure.

2.3 Test methods

The experimental flow chart of this study is shown in Figure 1. According to the national standard (JTG E20-2011), needle penetration, softening point, and ductility tests were performed on asphalt binders to assess the physical qualities of asphalt treated using WS powder. The above devices are respectively shown in Figure 1(c) and (e). Temperatures for the needle penetration test were 15, 25, and 30°C; for the ductility test, the temperature was 15°C. Take the logarithm of the penetration values tested under temperature conditions of 15, 25, and 30°C, with y = lg P and x = T. Perform a linear regression of the y = a + bx univariate equation to obtain the penetration temperature index A _lgPen. Determine the penetration index (PI) of asphalt according to formula (1), and record as PI.

where lg P is the logarithm of penetration values measured under different temperature conditions; T is the test temperature (°C); K is a constant term of regression equation a; A _lgPen is the coefficient b of the regression equation.

Under the AASHTO T315 standard shown in Figure 1(f), the high temperature rutting resistance of WS meal modified asphalt was investigated using a DSR (Gemini II ADS), which measured parameters including complex modulus, loss modulus, phase angle δ, and |G*|/sin δ. With temperature increments of 2°C per min, high temperature scanning tests were conducted in the 46–70°C range. We used samples that had a 1 mm gap and a diameter of 25 mm. Additionally, the frequency was consistently maintained at 10 rad·s⁻¹.

In frequency scan tests, the mechanical response of temperature with loading frequency was measured at 46, 52, 58, 64, and 70°C. WS meal modified bitumen was subjected to dynamic shear loads at modest strain levels (1%), with loading frequencies ranging from 0.1 to 100 rad·s⁻¹.

MSCR test evaluates the stress effects and elastic recovery of repetitive asphalt loading and unloading. As a result, the MSCR test offers a more thorough analysis of its deformation properties under dynamic loading. The evaluation index is unrecoverable creep compliance. For the MSCR test, the test temperature is 58°C.

BBR test was carried out in compliance with ASTM D6648-2008 (R2016) guidelines shown in Figure 1(g). The tests were carried out from −34 to −16°C in order to determine the low-temperature rheological properties of the bitumen treated with crab shell powder. To test the creep stiffness and creep rate of the beams throughout loading times ranging from 8 to 240 s, a continuous load of 980 mN was applied.

An apparatus from TA Instruments, USA, model Q20 shown in Figure 1(h) was used to conduct the DSC investigation. Since the samples’ precision did not exceed 51 mg, their weight was ascertained using a balance (1/10,000 g). The samples were first heated to room temperature in the DSC, then chilled to −20°C at a rate of 10°C·min⁻¹ and held for 1 min, and finally heated to 225°C at a rate of 10°C·min⁻¹ while being exposed to purge gas for 2 min. There was a last DSC curve. The glass transition temperature (T _g) was calculated from the DSC test curve, and the glass transition process was examined using the DSC data.

The functional groups of modified bitumen can be characterized through the use of FTIR testing [38]. Using NEXUS 870FT FIRT shown in Figure 1(i), spectrums of modified bitumen made from WS meal at several doses were gathered. With a resolution of 4 cm⁻¹, the test was conducted over a scanning region of 500–4,000 cm⁻¹.

Tests using AFM with a 15 µm × 15 µm scanning area were conducted at 25°C. A Quanta 200 SEM shown in Figure 1(k) and made by FEI, USA was used to analyze the microstructural features of the bitumen affected by WSs. Using Gwyddion software, the AFM pictures were quantitatively analyzed, and the roughness value was chosen as a quantitative index.

The surface morphology of WS powder and the adhesion of crab shell to asphalt were tested by using the United States FEI QUANTA 650 FEG SEM, as shown in Figure 1(j). In order to observe the adhesion interface between the asphalt and the crab shell, a small amount of asphalt was dropped on the unground WS and the sample was placed in the oven at 135°C for 2 h, so that the asphalt and the WS were fully integrated.

3.1 Conventional physical performance and softening point difference experiment

The three basic indexes of needle penetration, ductility, and softening point were used to evaluate the change rule of the performance of WSA with different doping amounts. The storage stability of crab shell powder modified asphalt was evaluated by referring to the softening point difference test in ASTM D5976. The heat storage time was 48 h. Softening point difference is negatively correlated with the uniformity of modified asphalt. The smaller the softening point difference is, the less easily the modified asphalt is separated. The basic indexes of WSA are shown in Figure 2.

Figure 2

Physical properties of WSA with different contents. (a) Penetration; (b) softening point, ductility, PI; and (c) softening point difference.

The needle penetration of WSA at 15, 25, and 30°C is shown in Figure 2(a). When doping is increased at 15°C, needle penetration decreases relatively slowly; when temperature rises, on the other hand, needle penetration decreases more quickly, with 30°C showing the quickest rate of reduction. The needle penetration of WSA fell by 13.03, 22.80, and 18.37% at 15, 25, and 30°C, respectively, when the WS content was 20%. As a result, WS powder enhanced 90# asphalt’s consistency. The softening point of WSA rises as the WS content does, as seen in Figure 2(b). When compared to 90# asphalt, the 20% content WSA has a 17.85% greater softening point at 55.12°C. The better the asphalt’s ability to sense temperature, the higher the PI value. The PI value increases linearly with the quantity of mixing, as shown in Figure 2(b), suggesting that WS benefits 90# asphalt. It should be noted that only the ductility data fit curve is curved, and that the rate of ductility decline is rather rapid in the early stages before tending to level off at 15% dosage. It is evident that WS decreases 90# asphalt’s low-temperature performance and has a larger impact on the ductility of the asphalt. This could be as a result of the WS particles influencing the way asphalt deforms during tensile contraction, which has a somewhat detrimental effect on asphalt viscosity. In conclusion, WS significantly enhanced 90# asphalt’s temperature-sensitive and high-temperature performance while reducing its low-temperature performance.

It can be seen from Figure 2(c) that the softening point difference of the modified asphalt with 5, 10, 15, and 20% crab shell powder is 0.5, 0.8, 1.6, and 2.6°C, respectively, and the softening point difference of the modified asphalt with crab shell powder increases with the addition amount. When the content of crab shell powder is 20%, the softening point difference reaches 2.6°C, which exceeds the specified standard of modified asphalt (2.5°C). This is because the more the amount of crab shell powder is added, the more serious the accumulation phenomenon of crab shell powder in asphalt, resulting in segregation. Therefore, the content of crab shell powder should not exceed 20% as far as possible.

3.2 Temperature sweep

The Test Procedure for Asphalt and Bituminous Mixtures in Highway Engineering (JTG E20-2011) was adhered to for both sample preparation and testing. The goal of this experiment is to investigate the high-temperature performance of WSA. The angular frequency parameter is set to 10 rad·s⁻¹, with a test applied strain of 1%, and a temperature scanning range of 46–70°C. Figure 3 illustrates how temperature affects WSA’s dynamic mechanical reaction.

Figure 3

Rheological properties of WSA at different temperatures; (a) Complex modulus; (b) loss modulus; (c) phase angle; and (d) rutting sensitivity.

In general, as temperature rises, matrix asphalt stiffness steadily decreases. Research has demonstrated that the complex modulus and loss modulus, respectively, are indicative of the stiffness and viscosity of matrix asphalt [39]. The viscoelasticity of the asphalt binder diminishes with increasing temperature, as observed in Figures 3(a) and (b), suggesting that the addition of WS does not alter the viscoelasticity law of the base bitumen. As the amount of WS incorporation grew, so did the base bitumen’s complex modulus and loss modulus.

The asphalt binder’s percentage of viscous and elastic components is characterized by the phase angle δ. This binder typically demonstrates coexisting viscous and elastic viscoelastic qualities [40]. Additionally, the asphalt binding material is more elastic when the phase angle δ is smaller. Figure 3(c) illustrates that δ increases with temperature in all asphalt binders. As the WS content increases at constant temperature, the phase angle δ decreases, making the modified bitumen more elastic.

The rutting factor (G*/sin δ) is a superior indicator of the asphalt binder’s resistance to permanent deformation, according to research from the strategic highway research program [41]. Furthermore, the researchers concluded that the binder’s temperature sensitivity is accurately characterized by the slope of the logarithm of the rutting factor vs the test temperature. WSA with 20% WS doping exhibited the highest rutting resistance across all temperature circumstances, as illustrated in Figure 3(d). Matrix bitumen exhibited the largest slope value, indicating the highest temperature sensitive. This suggests that the high-temperature rutting resistance of WSA is significantly enhanced by any addition of WS.

The physical characteristics and microstructure of WSA are likely intimately linked to the aforementioned observations. To put it another way, WS just serves as filler and spatial site resistance particles in WSA, which raises the asphalt’s viscosity by strengthening the molecules’ resistance to thermal movement. Because of WS’s strong mechanical strength, the asphalt binder’s elasticity ratio is improved, lowering the WSA loss factor.

3.3 Frequency sweep

Figure 4 displays the results of frequency scanning of WSA with varying quantities of blending in order to assess the fluctuation of matrix asphalt with loading frequency following WS blending.

Figure 4

Dynamic shear modulus of WSA based on frequency sweep. (a) 46°C; (b) 52°C; (c) 58°C; (d) 64°C; and (e) 70°C.

The primary indicator of traffic load duration on asphalt pavement is frequency; the higher the frequency, the shorter the loading time. When the frequency is low and the loading time is lengthy, the higher deformation of the asphalt binder owing to shear stress results in a reduced complex modulus. The complex modulus rises and the shear stress-induced deformation reduces with increasing frequency. According to Figure 4, the complex modulus of WSA with varying dose rises as loading frequency increases, suggesting that asphalt’s high temperature properties also rise as loading frequency increases. Each asphalt binder’s complex modulus value drops as temperature rises. When WS powder was added to the control asphalt binder at the same temperature, the asphalt binder’s complex modulus rose. This suggests that the deformation capacity of matrix asphalt to withstand traffic loads at high temperatures can be enhanced by addition of WS powder. Consequently, incorporating WS powder can enhance matrix asphalt’s performance at elevated temperatures.

3.4 Results and analysis of the BBR test

The creep modulus S and creep rate m of asphalt treated with varying doses of crab shell powder were tested using the asphalt bending beam rheological test to assess the asphalt’s resilience to cracking at low temperatures. S stands for the asphalt binder’s constant load resistance measure, which indicates the material’s capacity to withstand long-term deformation. M represents the change in the asphalt binder’s stiffness as a function of load, which shows how the material’s stiffness changes over time and how well stress relaxation works. The smaller the value of S, the better the low-temperature flexibility of asphalt; the larger the value of m, the better the stress relaxation ability and crack resistance of asphalt. The creep modulus S and creep rate m of asphalt with different dosages of WSA are shown in Figure 5.

Figure 5

Morphology of WSA at different dosages.

The BBR test measures the extent to which the asphalt binder creeps at low temperatures under steady load conditions. As illustrated in Figure 5, the S and m values of various dosages of modified asphalt at −16, −22, −28, and −34°C show that creep strength increases with decreasing temperature, while creep rate decreases with decreasing temperature. This demonstrates that as the temperature drops, the resistance of asphalt with varying dosages to low-temperature deformation decreases as well. This occurs because, at low temperatures, asphalt is in a glassy state where its molecular chains are nearly fixed and cannot be readily reoriented or changed.

In comparison to matrix asphalt, WSA’s modulus of strength progressively increased and its m-value progressively decreased at the same temperature. At 15% dosage of WS powder, the lowest m-value and highest modulus of strength were observed. Additionally, it was discovered that the modulus of strength and m-value of WSA followed a linear trend after being linearly fitted. As the dose of WS powder increases, the slope of the modulus of strength gradually rises, reaching its maximum value at a 15% dosage. That is to say, as the dosage increases, the stiffness modulus and low-temperature performance of WSA decrease at an accelerated rate. It is evident that the WS meal somewhat reduces asphalt’s resistance to cracking at low temperatures. The extremely rough surface of the crushed WS meal may cause this effect by obstructing the interaction between asphalt molecules.

3.5 Results and analysis of the MSCR test

WS powder modified asphalt was subjected to MSCR tests, with a test temperature of 58°C, in order to examine the irrecoverable creep flexibility (J _nr) and creep recovery (R) of WSA under high temperature conditions. While the high stress level (3.2 kPa) more closely approximates the behavior of asphalt under heavy traffic, the low stress level (0.1 kPa) better captures the elasticity and permanent deformation resistance of WSA. Figures 6 and 7 present the MSCR test results for WSA.

Figure 6

Creep recovery curves of WSA at 0.1 kPa stress level.

Figure 7

Creep recovery curves of WSA at 3.2 kPa stress level.

The creep and recovery findings for asphalt treated with varying amounts of discarded crab shell powder at 0.1 and 3.2 kPa are presented in Figures 6 and 7. The strain of all WSA specimens at 3.2 kPa is clearly more significant than the strain at 0.1 kPa, indicating that deeper rutting deformation in asphalt pavement corresponds to higher traffic loads. However, regardless of the shear stress level, the strain of virgin asphalt was higher than that of WSA, suggesting that WSA can considerably lower the strain of virgin binder.

Figure 8 displays the asphalt binder’s unrecoverable creep flexibility (J _nr) and rate of return (R). R is able to describe the elastic content of the binder, while J _nr is able to assess the rutting deformation. Better elasticity and resilience to rutting are found in binders with low J _nr and high R. As the stress in Figure 8 is increased from 0.1 kPa to 3.2 kPa, J _nr increases and R decreases, suggesting that the low stress levels contribute in the asphalt pavement’s permanent deformation resistance.

Figure 8

J _nr and R of WSA.

Meanwhile, it was discovered that virgin asphalt’s R was increased by WS powder. Compared to the matrix asphalt, the R of the 5, 10, 15, and 20% dosages increased by 3.4, 7.1, 24.6, and 21.4%, respectively, under 0.1 kPa loading stress. The R of various WSA doses rose by 5.8, 5.3, 21.5, and 19.5%, respectively, at a loading stress of 3.2 kPa as compared to matrix asphalt. Stated differently, the creep recovery of WSA outperforms that of the virgin binder. The J _nr of WSA is less than that of virgin asphalt, as Figure 8 illustrates. The J _nr of various WSA doses at 0.1 kPa were 3.1, 15.8, 22.2, and 35.9% less than that of virgin asphalt, respectively. In the meantime, at 3.2 kPa, the J _nr of various WSA doses was 4.0, 13.4, 19.1, and 32.2% lower than that of virgin binder. These findings demonstrate that, at high temperatures, discarded crab shell powder can improve matrix asphalt’s resistance to permanent deformation.

3.6 Results and analysis of the DSC test

DSC tests were conducted for various dosages of WSA in order to investigate the glass transition temperature (T _g) of asphalt amended with WS powder. Figure 9 shows the DSC curves for matrix asphalt and modified asphalt with varying amounts of WS powder. The glass transition temperature is determined by taking the midpoint of the first-order heat flow temperature curve.

T _g is recognized as the transition temperature at which the macromolecular segments of an amorphous polymer shift between highly elastic and glassy states. This temperature is the point at which the polymer shows elasticity; below it, brittleness and brittle fracture are observed [42]. Consequently, an asphalt’s performance at low temperatures improves with a lower T _g value.

The glass transition temperatures of WS powder modified asphalt with a dose of 5, 10, 15, and 20% are −4.17, −4.56, −3.52, and −3.09°C, respectively, as can be seen from Figure 9. The glass transition temperature of matrix asphalt is −6.48°C. The glass transition temperatures of the asphalt modified by WS meal were found to be higher than those of the matrix asphalt. This suggests that adding WS meal to asphalt can raise its glass transition temperature and cause a reduction in its low-temperature performance.

Figure 9

DSC curves of WS modified asphalt. (a) 0%; (b) 5%; (c) 10%; (d) 15% and (e) 20%.

It is important to note that, similar to the DSC test results, the WSA ductility test results indicated a decline in ductility values relative to the matrix bitumen. The asphalt has higher molecular stability and cross-linking, which results in a decrease in T _g. In conclusion, the WS meal marginally decreased the matrix asphalt’s performance at low temperatures.

3.7 Functional group characterization

An investigation into the modification method of WS powder modified bitumen was conducted using Fourier infrared spectroscopy test. The location of the distinctive peaks was used to analyze the WS powder’s chemical reaction mechanism and identify any alterations to its functional groups. Figure 10 displays the test results for modified asphalt with varying dosages of WS powder and matrix asphalt.

Figure 10

IR spectra of matrix asphalt and WSA.

The same peak types and positions are seen in the IR spectral test results of matrix asphalt and WSA without dosing, as shown in Figure 10. However, there is some fluctuation in the peak value, which is related to variations in the dosage of WS meal. The presence of hydroxyl groups and N–H bonds is linked to the transmittance drop at 3,475 m⁻¹. The stretching vibrations of CH₂ and CH₃, which correlate to the alkyl functional groups of the bitumen, are responsible for the two significant absorption peaks at 2,928 and 2,856 cm⁻¹, respectively. The C–H bonds in the bitumen matrix have an impact on the transmittance in the 2,900–2,400 cm⁻¹ region. Furthermore, the C–N bond that the chitin created is linked to the decreased transmittance in the 1,462 cm⁻¹ region. The most important characteristic of the C–H out-of-plane bending vibration in the 1,4-disubstituted aromatic ring is the ability to couple the backbone vibrations of CH₃ in the aromatic ring at 2,928 and 2,856 cm⁻¹ to the absorbance near 773 cm⁻¹. This indicates that the bitumen CH₃ is in the alkyl group in the asphalt. The WS powder and the matrix asphalt did not appear to undergo any discernible chemical reaction that produced new chemical components; instead, the transformation that occurred was primarily physical, as revealed by the results of IR spectroscopy.

3.8 Microscopic morphology in WSA

The use of Atomic force microscopy to test and analyze the changes in the micro-morphology of different dosages of WS powder modified asphalt is necessary because the micro-morphological structure of asphalt is complex and changes in the microstructure will have a greater impact on the macroscopic properties of asphalt. Figure 11 displays the two- and three-dimensional atomic force diagrams of WS powder modified asphalt in different dosages.

Figure 11

Morphology of WSA with different dosages.

The term “honeycomb structure” refers to an oval-shaped stripe pattern that resembles a bee’s tail. The influence of the honeycomb structure is mostly related to the asphaltene in the asphalt since the polarity of asphalt is mostly caused by heterocyclic atoms in the asphalt. The “honeycomb structure” of asphalt modified with varying dosages of WS powder is evident from the two-dimensional atomic force diagram in Figure 11. As the dosage of WS powder is increased, the “honeycomb structure” per unit area exhibits a decreasing trend and becomes more evenly distributed on the asphalt’s surface, reflecting the asphalt’s homogeneous polarity, which is primarily due to the heterocyclic atoms in the asphalt. The asphalt surface exhibits improved compatibility, reflecting the WS powder and matrix asphalt without any exclusion features. Additionally, the microstructure of asphalt changed to varying degrees as WS powder doping increased, significantly impacting on the material’s surface roughness and related modulus. There is a good correlation between asphalt adhesion and roughness, and the greater the roughness, the worse the adhesion of asphalt [43]. The nanoscale spatial structure of the asphalt surface is more clearly depicted in Figure 11’s three-dimensional atomic force diagram. The asphalt surface is covered in many bumps and depressions, and its folds progressively increase as dosage increases. This could be the result of hydrogen bonding between the many hydroxyl groups in the chitin found in crab shell meal and the hydroxyl groups in the phenolic and carboxylic acids in the asphalt, which strengthens the bonds between the macromolecules in the asphalt and gives it a more wrinkled appearance. This would imply that the WS powder improves the high-temperature stability of the matrix asphalt. In summary, the AFM results demonstrate that the WS powder enhances the asphalt’s performance at high temperatures and exhibits good compatibility with the matrix asphalt. This is in line with the findings of the frequency and temperature scans.

3.9 Adhesion of asphalt to WSs

In order to study the interaction between the asphalt and the crab shell, the surface morphology of the WS powder and the adhesion between the crab shell and the asphalt were tested by using the United States FEI QUANTA 650 FEG SEM. Figure 12 shows the SEM image of WS powder and the interaction interface between crab shell and asphalt.

Figure 12

SEM images of WS powder particles and adhesion of waste crab shell to asphalt. (a and b) WS microscopic morphology; (c) adhesion morphology of asphalt and crab shell surface; and (d) adhesion between asphalt and crab shell fracture surface.

Figure 12(a) and (b) are the SEM images of the WS powder after grinding, in which it can be seen that the surface of the WS powder is uneven and very rough, with an irregular layered structure in general, which is conducive to the adhesion between the asphalt and the WS powder. In order to observe the adhesion interface between the asphalt and the crab shell, a small amount of asphalt was dropped on the unground WS and the sample was placed in the oven at 135°C for 2 h, so that the asphalt and the WS were fully integrated. Figure 12(c) and (d) are SEM images of the interface between asphalt and WSs. In Figure 12(c), the irregular stripes on the left are the asphalt adhering to the WS, and the bare crab shell is on the right. It can be seen from the figure that the asphalt and the crab shell are closely fused, and the transition from the asphalt area to the crab shell area is smooth and natural. The difference between Figure 12(c) and (d) is that the crab shell fracture surface is on the right and the asphalt adhesion surface is on the left. The crab shell fracture surface is extremely rough, while the area on the left that is adhered by the asphalt is relatively smooth. Therefore, SEM shows that the surface of the crab shell powder after grinding is very rough, which is conducive to the adhesion of asphalt. At the same time, both the smooth area and the broken area of the WS adhere closely to the asphalt, thus changing the performance of asphalt.