For any kind of asphalt pavement, the degree of compaction is the most important construction quality management project in construction technology, and it is also an important index of pavement quality evaluation. The definition formula given in Code for Field Inspection of Highway Subgrade and Pavement is:
K=ρt/ρo× 100%
Where k-the compaction degree of a measured part of asphalt pavement is100%;
ρ t-actual density of asphalt mixture core sample specimen, g/cm3;
ρo- Standard density of asphalt mixture, g/cm3.
The appendix of Technical Specification for Continuous Construction of Highway Asphalt Pavement clearly stipulates that the standard density of asphalt mixture shall be based on Marshall density of sampling test in asphalt mixing station. For coarse-grained asphalt concrete and asphalt macadam, it is reasonable to use the drilling density in the test section as the standard density.
A, asphalt mixture standard density detection
There are inevitable human factors in every link of Marshall test in laboratory sampling, and these effects are obvious for Marshall density. Due to the above reasons, it is difficult to find the problem of unqualified compaction degree with Marshall density as the standard density in actual testing.
1. 1 density of pavement core sample in test section
We know that the test section should be paved before the formal paving, and its main purposes are: ① to determine the standard mixture ratio used in production; (2) determining the loose laying coefficient; Pet-name ruby determine rolling method and rolling times. As long as the above parameters are determined, the production of asphalt mixture can proceed normally. When determining the above parameters, the degree of compaction is also one of the evaluation indexes. Of course, if the factors such as asphalt-aggregate ratio, gradation and construction conditions do not change in the actual construction process, it is also feasible to take the density of the test section as the standard density.
1.2 partial temperature
According to experience, in the process of indoor Marshall specimen, the density of mixture specimen will increase with the increase of molding temperature, while the porosity will decrease; On the contrary, lowering the temperature will lead to a decrease in density and an increase in porosity. In engineering practice, the void ratio of indoor Marshall specimen is an important index to measure asphalt mixture. When doing Marshall test, we found that although the asphalt content changed up and down by 5 claws, the change range reached 2%, the stability and flow value met the requirements, and sometimes the stability and density did not even peak. In the end, there is only one index to determine the asphalt content. The same is true in the production process. In most cases, only the porosity of Marshall test will exceed the requirements. Therefore, many construction units artificially change the compaction temperature or ignore the temperature control in order to meet the void ratio requirements of Marshall specimen during molding.
1.3 sampling contingency
When the laboratory takes samples for Marshall test, it usually takes one group in the morning and one group in the afternoon to get the Marshall density of that day. The normal production capacity is 240 ne// hour, and only two groups are taken every day, so the Marshall density sampling on that day is accidental.
There are inevitable human factors in every link of Marshall test in laboratory sampling, and these effects are obvious for Marshall density. Due to the above reasons, it is difficult to find the problem of unqualified compaction degree with Marshall density as the standard density in actual testing.
1.4 Maximum theoretical density
The maximum theoretical density can be obtained by calculation method, vacuum method or solvent method. Solvent method and vacuum method can best reflect the actual situation of drilling and coring, but neither method can keep the core sample, and the test itself is more complicated. The calculation method is the simplest and easiest method to control the quality in the construction process.
In SMA production practice, we have tried to use the maximum theoretical density as the standard density. The formula for calculating porosity is vv = (1 ~ P solid /P principle) x 100% compactness k = PS/PO x 100%. When the theoretical density is taken as the standard density, Ps=Po solid, and the principle of P=P can be deduced as follows: w=( 1-O).
Second, the comparison between Marshall density and maximum theoretical density
At present, the volume design method of Supmpave mixture is gradually popularized in the United States. By 2000, Marshall method and Super-pave mixture volume design method were completely abandoned, that is, the maximum theoretical density was used as the compaction standard. At present, the maximum theoretical density is 92%, and there is a trend to increase the maximum theoretical density to 93% as the compaction standard.
Table 1 Comparison Table of Qualified Rate of Two Compaction Standards
It can be seen from the table 1 that although the compaction degree of the L pavement in the test section reaches 96% of the standard of the Ministry of Communications, none of them is unqualified, but if measured by 92% of the maximum theoretical density, 67% of the density is unqualified; If measured at 93% of the maximum theoretical density, 100% is unqualified.
Test section 4 in table 1 also shows that 96% of Marshall density of 20% sections is still unqualified. If the maximum theoretical density is 93%, 80% of the road sections are unqualified.
The compaction standard in the United States tends to increase from 92% to 93% of the maximum theoretical density. According to this standard, the density of pavement is even less.
Third, the actual density of asphalt pavement
The current code stipulates the above three standard densities, so it is necessary to study which density is the standard density and which one to choose.
This can be chosen according to the practice of the construction unit, with the qualification rate as the standard, because Marshall test is done every day.
Yes, Marshall standard density exists every day, so it is used more. If the maximum theoretical density is taken as the standard density, the measured maximum relative density of ordinary asphalt mixture must be taken as the compaction standard of that day among colleagues who make Marshall specimens every day. For the standard requirements of the compaction degree of ordinary asphalt mixture, the laboratory quasi-density is 96%, the maximum theoretical density is 92%, and the test section density is 98%.
Four. conclusion
It is suggested that the maximum theoretical density should be adopted in the compaction test of asphalt pavement in highway construction in the future, which can effectively control the compaction degree and its porosity index. If the density is really insufficient, it is necessary to resort to the most theoretical density as the standard.
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