Research on less temperature flawing resistance of steel residue asphalt composition based on SCB test

In view of the increasing application of steel residue in asphalt pavement construction in China, in order to analyze the less temperature flawing resistance of steel residue asphalt composition, the less temperature semi-circular bending test (SCB) was used to analyze the less temperature flawing resistance of steel residue asphalt composition (SSAC-13) with various residue content based on the stress peak value and stress attenuation, rupture toughness, rupture energy and critical rupture energy density. The results show that the content of steel residue has a noticeable influence on the less temperature flawing resistance of steel residue asphalt composition. Considering the less temperature flawing resistance, the recommended steel residue content is 40%.


Instruction
In the case of less temperature, the flawing of asphalt pavement will be very serious, which will affect its performance.Currently, there are many test and assessment methods for the less temperature flaw resistance of asphalt composition, and the trabecular less temperature bending test is used as the evaluation method in the project code of asphalt pavement in China.However, due to the complex stress state of asphalt pavement, less temperature bending strain cannot accurately project the less temperature flaw resistance of the composition [1].In some studies, indirect tensile (IDT) tests are directly used to project the flaw resistance of HMA at less temperatures, but there is still a big difference between the flawing resistance and the actual stress state [2,3].The principle of semi-circular bending test (SCB) is rupture mechanics.Therefore, the interior of the sample is closer to the actual stress state, and it can also more accurately reflect the rupture behavior of the asphalt composition.Coupled with the advantages of repeatability consistency and simplicity in sample preparation and testing.At present, the number of studies on evaluating the less temperature flaw resistance of asphalt composition by SCB test is increasing.[4][5][6].Saha, G. and Biligiri, K.P. [7] studied the rupture damage of asphalt composition by SCB test.Wang, H. [8] et al studied the less temperature performance of rubber improved asphalt composition by SCB test.The results show that the bending tensile strength and rupture energy density of the composition peak after adding rubber powder.In the AASHTO TP124-IL-SCB standard, rupture energy (Gf) and flexibility index (FI) are used to project the flawing resistance of asphalt composition.Based on SCB test, Ding Hao et al. [9] studied the influence of basalt fiber on the flaw resistance of asphalt composition by using rupture energy and flexibility index.Li Xiaoxu [10] measured the lesstemperature rupture properties of fiber asphalt composition by tensile test and semi-circular bending test, and also studied the influence of fiber and asphalt content on the flaw oppose of fiber asphalt composition.
With the increasing shortage of sand and gravel resources in highway construction in China in recent years, the research and application of steel residue in asphalt composition are gradually increasing.Because the optimum asphalt ratio of steel residue asphalt composition and the porosity of steel residue will higher noticeablely with the higher of steel residue content, the less temperature flaw resistance of the composition is improved.However, the steel residue has a more noticeable property of thermal expansion and cold shrinkage, and adding too much steel residue may aggravate the less temperature flaw resistance of the composition.Existing studies [11,12] show that the adding of steel residue is conducive to improving the less temperature flawing resistance of asphalt composition, but it has not been deeply and systematically studied in combination with SCB test, especially the lack of research based on rupture mechanics.So, this research studied the influence of various steel residue content on the less temperature flawing resistance of steel residue asphalt composition (SSAC-13) by taking the peak stress and stress attenuation, rupture toughness, rupture energy and critical rupture energy density as indicators based on SCB test, so as to determine the reasonable content of steel residue and guide the design and construction of steel residue asphalt composition.

Sample preparation
Based on the existing research results [13], six groups of steel residue asphalt composition (SSAC-13) with various steel residue content are composed of materials as shown in table 1, among which the properties of raw materials such as steel residue and SBS improved asphalt aggregate meet the requirements of technical specifications.Large Marshall samples (Φ=152.4mm,h=115mm) with various steel residue content were prepared, and then semi-circle test pieces were cut out for SCB test.The diameter of the test pieces was 150mm, the thickness was 20mm, the support span was 120mm, and the ratio of the support span to the diameter of the test pieces was 0.8.

Semi-circle bending test
Six kinds of asphalt compositions were tested by semicircular bending test with 4 parallel samples in each group.The measurement temperature was −10℃ and the surcharge speed was 10 mm/min.The pre-cutting flaw depths used in the test were 5 and 10 mm, respectively.In figure 1, the surcharge process of the SCB sample is shown on the left, and the appearance of the SCB sample after the test is shown on the right.The stress-displacement curve of the indenter and the displacement-time curve at the flaw can be obtained.As can shown in table 2, when steel residue content is 40%, the attenuation rate of the peak stress is 20.1% with the higher of the pre-cutting flaw depth.It was also found that the stress peak value of SSAC-13 decreased by 50.9% and 46.8% when the steel residue content was 0% and 100%, respectively.The stress attenuation rate decreases first and then highers with the higher of steel residue content, in which the SSAC-13 sample with 40% steel residue content has the highest rupture stress peak and the lessest stress attenuation rate, indicating that its flawing resistance is the best.

Rupture toughness analysis
The characterization index of stress field strength at flaw tip of flawed sample is rupture toughness.When the asphalt composition produces imperfect plastic deformation during the rupture process, the stress-strain curve is shown in figure 2. The rupture toughness K1C can be calculated from the follessing formula, and the results are shown in table 3.
Where: Pmax is the great value load, σmax is the great value stress, K1C is the rupture toughness, W is the sample height, and a is the pre-cutting flaw depth.From table 3 that with the higher of steel residue content, the rupture toughness first highers and then decreases.The rupture toughness of 40% steel residue content SSAC-13 is the highest and has good flaw resistance.In order to project the less temperature flawing resistance of SSAC-13, the rupture energy and critical rupture energy density of the composition were further analysed.

Rupture energy and rupture energy density analysis
The energy absorbed by the sample during rupture is called rupture energy.In some range, the better the flaw resistance of asphalt composition, the greater the rupture energy.The higher the J1C value of asphalt composition, the higher the energy absorbed by flaw per unit area of expansion, the better the flaw resistance.Table 4 shows the values of rupture energy J and critical rupture energy density J1C of SSAC-13 with various steel residue contents.From table 4 that with the higher of the pre-cutting flaw depth, the rupture energy gradually decreases.When the pre-cut flaw depth is 5mm and 10mm, with the higher of steel residue content, the rupture energy of SSAC-13 sample highers first and then decreases, and reaches the great value value when the steel residue content is 40 %.The rupture energy density shows the uniform pattern.

Comparison of rupture performance between SSAC-13 and other asphalt compositions
To better analyze the less temperature flaw resistance of SSAC-13, the less-temperature SCB test results of various asphalt binders and composition grades were compared and analyzed, and the data were shown in table 5.
Table 5. Comparative analysis of the results of some SCB tests*.

Researcher
Composition type asphalt pre-cutting flaw depth (mm) K1C(MPa•m 1/2 ) J1C (kJ/m 2 ) Augusto [14]   AC- From the results in table 5, the rupture toughness of SSAC-13 asphalt composition under 5mm and 10mm pre-cutting flaw depth conditions is basically similar to that of SBS improved asphalt composition with the uniform type, and the critical rupture energy density is higher than that of SBS improved asphalt composition with the uniform type.The steel residue content has a noticeable impact on anti-flawing performance, especially when the steel residue content is greater than 40%.The rupture toughness and critical rupture energy density of the composition decreased obviously, indicating that reasonable steel residue content should be paid attention to in the application process of the residue and rubber asphalt composition.Considering the anti-flawing performance at less temperature, it is recommended to use asphalt composition with 40% steel residue content for highway surface layer.

Conclusions
(1).The incorporation of steel residue will noticeablely affect the less temperature flaw resistance of steel residue asphalt composition.For SSAC-13 steel residue asphalt composition, the stress peak and stress attenuation, rupture toughness, rupture energy and critical rupture energy density obtained by semi-circular bending test all higher first and then decrease with the higher of steel residue content.The great value value is reached when the content of steel residue is 40%.(2).Compared with the less temperature flaw resistance of the uniform type of improved asphalt composition, the less temperature flaw resistance of steel residue asphalt composition is close to or better.
Considering the less temperature flawing resistance, 40% steel residue is recommended when SSAC-13 steel residue asphalt composition is applied to the surface layer of highway.

Figure 1 .
Figure 1.SCB test process and rupture sample.

3 .
SCB semi-circle bending test results and analysis3.1.Test resultsFlaw displacement-time curve and load-time curve can be obtained by SCB test, and the stressdisplacement curve of flaw can be obtained by combining the data, which can be used to calculate the rupture energy.At the uniform time, the stress peak and stress attenuation rate of samples with various pre-cutting flaw depths were obtained by the test, as shown in table 2.

Table 2 .
Stress peak value and stress decay rate of samples with various pre-sawn seam depths.

Table 3 .
Rupture toughness of SSAC-13 with various steel residue content.

Table 4 .
Rupture energy J and critical rupture energy density J1C of SSAC-13 with various steel residue contents.