Keywords: pavement core sample; Marshall test; Necessity
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Asphalt pavement is the main type of highway pavement in China. In recent years, with the rapid growth of traffic volume and the continuous improvement of axle load, there are more and more quality problems of asphalt pavement, and the service life of some sections is far from the design life, that is, various pavement diseases appear, which seriously reduce the use function of pavement and cause adverse social impact. By looking up the detection data in the construction process, it is found that the detection data in the construction process meet the design requirements. So what is the reason for the damage of asphalt pavement? There are many reasons for pavement damage and different ways, but it is undoubtedly very important to correctly judge and identify the reasons for eliminating the hidden dangers of asphalt pavement quality accidents, improving and improving the quality of asphalt pavement and prolonging the service life of asphalt pavement. Marshall stability, flow value and density are the most important control indexes of asphalt pavement at present. Strictly controlling the detection of these indexes is of great significance for improving the high temperature stability, plastic deformation resistance, compactness and porosity of asphalt pavement. At the same time, only by truly and objectively reflecting the Marshall stability, flow value and density of asphalt pavement can the quality of asphalt pavement be correctly evaluated.
1 relevant provisions of current test methods in China.
China's current Technical Specification for Highway Asphalt Pavement Construction (JTJ032-94) stipulates that Marshall stability, flow value, density and void ratio of hot-mixed asphalt pavement should be tested by sampling and molding at the mixing station. Through a large number of experiments, it is proved that the Marshall stability of asphalt mixture is closely positively correlated with the compactness of the specimen and negatively correlated with the void ratio in a straight line under the condition of certain raw materials and mixture ratio. Because voidage is a different expression of density, density has a vital influence on Marshall stability. According to the test results, the density of indoor molded specimens depends not only on the mixture ratio of materials, but also on the compaction times (compaction work) and compaction temperature to a great extent. The more compaction times, the greater the compaction work per unit volume of asphalt mixture, and the easier the specimen is to be compacted. Similarly, due to the characteristics of asphalt material itself, in a certain range, the higher the temperature, the lower its viscosity and the worse its ability to resist plastic deformation. Under the action of external force, the specimen is more easily compacted. Therefore, the test results obtained by this method can not truly reflect the actual quality of asphalt pavement.
2. The necessity of Marshall test with field core samples.
The indexes such as void ratio, asphalt saturation, asphalt volume percentage and mineral aggregate void ratio calculated by Marshall test, density test and indoor molding specimen cannot represent the real quality of pavement. It is not difficult to analyze that the main factors affecting the quality of asphalt pavement are the quality of raw materials, the mixture ratio of materials (mineral aggregate gradation) and the compaction density of asphalt mixture. If the samples sampled by the mixing station are used to represent the real situation of the pavement, at least the following two necessary conditions must be met: (1) The quality and mixture ratio of raw materials are consistent with the materials and mixture ratio actually used on the pavement; (2) The compactness of the specimen is consistent with that of the pavement. However, in actual construction, due to the segregation of the mixture during transportation and paving, the mixture ratio of pavement materials will change locally. What's more, the compaction work in the indoor test is the standard compaction work stipulated in the test regulations, and the compaction work depends on the tonnage of the roller, the rolling times and the combination of compaction machinery, so the compaction work is not necessarily equal to the pavement compaction work, and the construction rolling temperature is difficult to be consistent with the indoor test temperature, or even quite different. From the above reasons, it is not difficult to see that the Marshall stability, flow value, density and other indicators obtained from the sampling and molding specimens of the mixing station are difficult to truly represent the quality of the asphalt pavement on site. If the core sample drilled by the coring machine is used as the test piece, the stability, flow value, density and other indicators can be detected, which can better represent the real quality of asphalt pavement.
3. The feasibility of Marshall test with field core sample instead of mixing station molding sample.
T07 10-2000 in the Test Code for Asphalt and Asphalt Mixture in Highway Engineering JTJ052-2000 stipulates the Marshall test method of asphalt pavement core drilling, but it is also pointed out in the description of the provisions: "Because the drill bit diameters are often 100mm and 150mm, the diameter of the specimen must be less than/kloc. This shows that the Marshall test of asphalt pavement core sample can not be used as the basis for testing whether asphalt pavement is qualified, mainly because the diameter of core sample is too small, which is inconsistent with the curvature of Marshall tester, and it is easy to produce test errors. If it is limited to this, it is entirely possible for us to change the coincidence degree between the core sample and Marshall specimen indenter, thus eliminating the test error caused by it. 3. 1 The existing indenter was reformed to improve the consistency between the core sample and indenter. According to the current test regulations, the Marshall specimen indenter is made into a fixed curvature inner diameter, the standard indenter is 10 1.6 mm, and the Marshall specimen indenter is152.4 mm. Taking the standard Marshall specimen indenter as an example, if the core sample diameter is 99.8mm, The difference with the standard specimen diameter 10 1.6 mm is 65433 ... In actual operation, in order to eliminate the deviation of 1.8mm, two gaskets with the same length and width as the indenter are cut with a 0.9mm thick metal plate, and the gaskets are bent into the same curvature as the indenter. During the test, two gaskets were placed in the upper and lower indenters respectively, so that the curvature inner diameter of the indenter was reduced by 65438±0.8mm, which could eliminate the above test deviation. It should be pointed out that the deviation between the diameter of the core sample and the inner diameter of the indenter is not a constant due to the influence of the machining accuracy of the inner diameter of the bit, the eccentricity of the drilling rig and the firmness of the drilling rig. When selecting the gasket thickness, the drilled core sample diameter can be counted, and half of the difference between the upper limit value (maximum value) of the core sample diameter and the specified value of Marshall specimen can be taken as the thickness value of the selected gasket. If the selected bit is a standard bit, the difference between the diameters of each sample is generally not great and can be ignored.
3.2 Processing special drill bits. For standard Marshall specimens, we now commonly use drills with inner diameters of 100mm and150 mm. When the drilling rig is fixed, the difference between the diameter of the core sample and the inner diameter of the drill bit will generally not exceed 0.5 mm Take a 1‰ drill bit as an example. If the inner diameter of the drill bit is 100Inm, Generally, the diameter of the core sample can reach 99.5 ~ 100 mm, which is obviously insufficient for the standard diameter of Marshall specimen10/.6 mm. According to the stress situation of Marshall test, if the fitting degree between the specimen and the indenter is not enough, it will cause stress concentration, which will lead to premature failure and rheological state of the specimen, thus leading to the decrease of Marshall stability. However, we can find that the error between the inner diameter of the drill bit and the diameter of the drill piece is not large, which makes it possible to set the standard inner diameter of the drill bit. In fact, the rigs used now are not special rigs. They are used to drill core samples of asphalt pavement, semi-rigid pavement base material and cement concrete. In these applications, only asphalt pavement has strict requirements on the core sample diameter. Therefore, while meeting the requirements of asphalt pavement coring, it does not affect the use of other drilling chips. In this way, the inner diameter of the drill bit can be processed into 10 1.6 mm to meet the requirements of Marshall test for asphalt pavement coring. 10 1.6 mm and 152.4mm bits for asphalt pavement need to be specially designed and manufactured even if they are not matched with other bits.
4 example analysis
In recent years, we have carried out a large number of Marshall tests on the drilled asphalt concrete core samples by using the method of padding gaskets in the head of Marshall tester, and the test results have good regularity and feasibility. Comparing the test results of the target mixture ratio of Marshall specimens formed in the laboratory with the Marshall test results of core samples drilled by engineering entities and related test items, it is not difficult to find that porosity, density and stability are closely related, which can also prove the regularity of Marshall test results of core samples of engineering entities and the stability of core sampling test methods. At the same time, it also reminds us to strive for the smallest possible void ratio and the largest possible density, so as to achieve greater pavement stability and improve road service quality. As long as the core samples drilled by the engineering entity coincide with Marshall indenter to a certain extent, the test results can reach or approach the test results of indoor molding specimens under the same material and mixture ratio. Of course, due to the small diameter of the core sample, even if the core sample is in good agreement with the indenter, its test results will still be affected to some extent, but this effect is not significant.
5 conclusion
(1) In order to make the diameter of the drilled core sample consistent with the diameter of the standard Marshall specimen, it is suggested to make101.6 0.2 mm and152.4 0.2 mm special biters for asphalt concrete, and bring them into the management of special test instruments. (2) It is suggested that the Marshall test of pavement solid drilling should be evaluated by mathematical statistics with each sub-project as the unit. (3) Practice has proved that there is an increasing functional relationship between Marshall stability and specimen density, that is, the greater the specimen density, the greater its Marshall stability. Because there is a percentage problem of compactness in asphalt pavement, there is also a percentage problem of Marshall stability of engineering entities. It is suggested that the lower confidence limit of Marshall stability of pavement core drilling should be specified on the basis of Marshall stability of standard mixture ratio (compactness is 100%). (4) Because the project entity density can't reach the target mixture ratio density in the construction stage, the corresponding void ratio will have the same difference. In order to distinguish it from the designed void ratio, it is suggested that the construction allowable void ratio index should be set for the pavement solid core specimen. (5) In order to pursue higher solid stability of pavement engineering and smaller plastic deformation, it is suggested to improve the compaction standards of asphalt pavements at all levels, such as 96% of the maximum Marshall test value of secondary highways and 97% or 98% of high and first-class highways. Through the engineering example, it is proved that under the same conditions of materials and mixture ratio, the Marshall test of drilling core sampling method has a good correlation with the Marshall test of indoor molding specimen, which can accurately and objectively evaluate the substantive information of pavement quality and provide a feasible detection method for all construction parties to truly master and understand the quality of asphalt pavement.