It is very important to detect the compaction degree of highway subgrade. In order to get accurate test data, it is necessary to formulate a reasonable method, and every detail is crucial. Zhong Da Consulting will explain the testing method of highway subgrade compactness for you.

With the improvement of social requirements for highway engineering quality, the management level, quality monitoring system, supervision means and mechanized construction level of highway construction projects are also improved. Compaction quality of subgrade and pavement is one of the most important internal indexes of road engineering construction quality management. Only by fully compacting the structural layer of subgrade and pavement can the strength, stiffness and smoothness of subgrade and pavement be guaranteed, and the service life of subgrade and pavement engineering can be guaranteed and extended. The compaction quality of highway subgrade is mainly evaluated by specific testing methods and data. Whether these quality inspection methods and data are scientific, true and effective directly affects the accuracy of subgrade quality evaluation. On-site compaction quality is expressed by compaction degree. For subgrade soil and pavement base, the degree of compaction refers to the ratio of the actual dry density on site to the maximum dry density obtained from the actual test. For asphalt pavement, the degree of compaction refers to the ratio of the density actually achieved on site to the indoor standard density.

1, determination method of standard density (maximum dry density) and optimum water content

The so-called compactness refers to the ratio of the dry bulk density of compacted soil to the standard dry density of soil. In the process of compaction, the relative magnitude of attraction and repulsion between soil particles determines the structure of compacted soil. When the water content of soil sample is small, the attraction between particles is large, and under certain external compaction, the attraction can not be effectively overcome, so the soil particles move relatively, so the compaction effect is poor. After increasing the water content, the bound water film gradually thickens and the gravity decreases. Under the same action conditions, soil particles are easy to move and compact, and the compaction effect is better. When the water content increases to a certain extent, free water has appeared in the pores, and the expansion of the bound water film is no longer significant, so the reduction of gravity is not very significant. At the same time, with the increase of water content, the effect of free water filling in pores to prevent soil particles from moving is gradually obvious, and the compaction effect is reduced at this time. Therefore, the compaction quality of subgrade can be effectively judged by detecting the dry density of soil.

Due to the different structural levels of road materials and other factors, the determination methods of indoor standard density are also diversified, and some methods need to be further improved in practice. The maximum dry density refers to the maximum dry density on the standard compaction curve (hump curve), and the corresponding water content is the optimal water content.

1. 1 Method for determining maximum sub-density and optimum water content of subgrade soil

According to the different load stress of subgrade, the requirements for subgrade compactness are also different. The higher the highway grade, the higher the requirement for subgrade strength and the higher the requirement for subgrade compactness. Compaction standards for subgrade of expressway and Class I highway are: subgrade 0 ~ 80 cm is not less than 95%, embankment 80 cm ~ 150 cm is not less than 93%, and embankment 150 cm is not less than 90%. For zero-fill-zero excavation, the groove 0 ~ 30 cm below the road bottom should not be less than 95%. In special arid areas with annual average rainfall less than 150mm and low groundwater level (equivalent to areas with humidity coefficient ≤0.25), the compaction standard can be reduced by 2% ~ 3%. When the average rainfall in that year exceeds 2 000 mm, the humidity coefficient is >: 2. When the water content of natural soil exceeds 5% of the optimal water content, it should be stabilized before compaction.

Both shaking table method and surface vibration compactor method use vibration to measure the maximum dry density of soil. The former is more complicated in test equipment and operation, while the latter is relatively simple, which is closer to the actual situation of vibration compaction. Therefore, for sand, eggs, boulders, rockfill and other cohesionless free drainage, the surface vibration compactor method is first recommended.

1.2 Method for determining maximum dry density and optimum water content of pavement base mixture

Theoretical calculation method is a scientific method to determine the maximum dry density and optimum water content.

1.2. 1 lime soil and lime-fly ash stabilized granular materials

According to the maximum dry density ρ 1 of binder and the relative density γ of aggregate measured in laboratory test, the determined mass ratio of binder and aggregate is converted into volume ratio V 1∶V2, and the maximum dry density of mixture can be calculated.

The optimum water content w0 of lime soil and lime-fly ash stabilized aggregate is the weighted value of the optimum water content w 1 of binder and the water content w2 of saturated coating of aggregate. The water content of saturated coating refers to the water content of aggregate when it is taken out after soaking in water and the surface is not wiped. Except for aggregates with extremely high water absorption, this value can be 3% for gravel and 4% for crushed stone.

1.2.2 cement stabilized granular materials

The maximum dry density ρ0 of this kind of material is related to the maximum dry density ρG of aggregate and the quality of hardened cement. After mixing cement with water, it is dried in an oven at 105℃. By weighing the cement quality before the test and the hardened cement quality after drying, the water increment of cement hydration can be obtained. Because cement contains hydrated water, the optimum water content of cement stabilized granular materials cannot be determined correctly by oven method. According to the contrast test, the optimum water content w0 of cement stabilized granular materials consists of the hydration heat of cement, the saturated water content of aggregate and the water needed for mixing cement (water cement ratio is 0.5).

1.3 determination method of standard density of asphalt mixture

The standard density of asphalt mixture shall be based on Marshall density or test section density of sampling test in asphalt mixing plant. The specific methods are as follows: (1) Water weight method is suitable for dense type I asphalt concrete specimens, but not for asphalt mixture specimens with super absorbent aggregate; Surface drying method is suitable for type I or type II asphalt concrete specimens with rough surface, but not for asphalt mixture specimens with water absorption greater than 2%. Wax sealing method is suitable for type I or type II asphalt concrete specimens and asphalt macadam mixture specimens with water absorption greater than 2%. When the density cannot be determined by water specific gravity method or surface drying method, wax sealing method should be adopted. Volume method, which is suitable for asphalt macadam mixture with large void ratio and open graded asphalt mixture specimen with large void permeability. When carrying out density test, the test method should be properly selected according to the characteristics of the mixture itself.

2. On-site density testing method

At present, the commonly used on-site compaction measurement methods include ring knife method, sand filling method, nuclear densimeter method and so on.

2. 1 ring knife method

This method is mainly used to determine the density of cohesive soil without aggregate. Instruments and equipment include: ring knife (inner diameter 6 cm~8 cm, height 2 cm~3 cm, wall thickness 1.5 mm ~ 2 mm), balance (sensitivity 0. 1g), soil-repairing knife, wire saw, vaseline, etc. The test method is as follows:

(1) Coat a layer of vaseline on the inner wall of the ring knife in advance, and put the edge of the ring knife down on the soil at the set detection position.

(2) using a soil cutter or a wire saw to cut the soil sample into a soil sample slightly larger than the diameter of the ring cutter, and then vertically pressurizing the ring cutter until the soil sample extends out of the upper part of the ring cutter; Cut off the residual soil at both ends to make it flush with the knife edge surface of the ring, and measure the water content with residual soil samples.

(3) Wipe the outer wall of the ring cutter, and weigh its mass to the accuracy of 0.1g..

(4) result arrangement: calculate the dry density of unearthed samples, and then get the compaction coefficient K. ..

2.2 Sand filling method

The sand filling method should meet the requirements: when the maximum particle size of aggregate is less than 15mm and the thickness of test layer is less than 150 mm, a small sand filling pipe with a diameter of φ 100mm should be used for the test; When the aggregate particle size is equal to or greater than 15 mm, but not greater than 40 mm, and the measured layer thickness is greater than 150 mm, but not greater than 200 mm, a large sand filling pipe with a diameter of φ 150mm should be used for testing ... The required instruments and equipment include: sand filling bucket (inner diameter 100 mm) Total height is 360 mm), metal graduated tank, bottom plate, bench scale (weight 10 kg ~ 15 kg, sensitivity is 5 g), sand measurement (particle size is 0.25 mm~0.50 mm, weight is 20 kg~40 kg). The test method is as follows:

(1) When testing a bid section, the density of sand used shall be calibrated.

(2) At the test point of compaction coefficient, select a piece of 40 cm×40 cm flat land and place the pad horizontally on the test point.

(3) Drill a trial hole with a diameter of 100 mm along the middle hole of the bottom plate, and the depth of the trial hole is equal to the thickness of the rolled layer. All the excavated soil materials are put into plastic bags with known quality to obtain the sample quality.

(4) Take representative soil samples from the sampling for water content test.

(5) Install the cylindrical sand cylinder on the bottom plate, make the lower opening of the cylindrical sand cylinder align with the middle hole and the trial hole of the bottom plate, turn on the switch of the cylindrical sand cylinder, and inject the measured sand into the trial hole. The measured sand weight injected into the test hole can be obtained by weighing the weight change of sand in the measuring cylinder sand cylinder, and then the volume of the test hole can be obtained.

(6) After the test, the sand in the test hole is taken out for later use; If the measured sand humidity changes obviously or is mixed with impurities, it needs to be dried and sieved again.

(7) result arrangement: calculate the dry density of unearthed samples, and then get the compaction coefficient K. ..

2.3 nuclear humidity densitometer method

When this method is used to measure the surface compaction density of asphalt mixture, the surface is measured by scattering method, and the measured layer thickness of asphalt mixture surface should not be greater than the maximum thickness determined according to the performance of the instrument. When used to determine the compaction density and water content of soil foundation or base material, it should be determined by direct transmission method after drilling, and the thickness of the measured layer should not be greater than 20 cm. The required instruments and equipment include: nuclear density hygrometer, fine sand (0. 1.5 mm ~ 0.3 mm), balance or bench scale, brush, etc. Test methods and precautions are as follows:

(1) Determine the position and preheat the instrument. Determine the test position by random sampling, but the minimum distance from the road edge or other objects shall not be less than 30cm. The distance between the nuclear instrument and other radiation sources shall not be less than 10 m, and the instrument shall be preheated according to the specified time. If the scattering method is used, the nuclear instrument should be placed in the test position smoothly; If the direct transmission method is adopted, put down the radioactive source rod and insert it into the pre-drilled hole.

(2) Open the instrument and read the data. When the instrument is turned on, the tester leaves the instrument for 2m and measures according to the selected measuring time. When the measuring time is up, the tester reads the displayed value and turns it off quickly.

(3) Precautions for use safety: ① When the instrument is working, all personnel should retreat to a place 2m away from the instrument; (2) When the instrument is not in use, the handle should be placed in a safe position, and the instrument should be placed in a special instrument box where it meets the nuclear radiation safety regulations; (3) The instrument shall be kept and used by specialized personnel who have passed the examination of relevant departments.

Compaction quality of subgrade and pavement is one of the most important indexes of road treatment. In order to ensure the strength of subgrade and pavement, the structural layer of subgrade and pavement must be fully compacted. For asphalt roads, by measuring the maximum dry density, optimum water content and standard density of asphalt mixture of subgrade soil and pavement base material, according to the actual needs of road characteristics, the ring knife method, sand filling method and nuclear humidity density meter method are flexibly used to carry out on-site density detection, so as to obtain accurate detection data, accurately evaluate the compaction quality of road subgrade and ensure the safety of road use.

Detailed information of "Highway Subgrade Compaction Testing Method" was consulted and designed in Zhong Da, and all the relevant construction materials you want are available.

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