The problem of road surface quality in our country is extremely acute. Therefore, it is very important when accepting the work of construction services to qualitatively and correctly carry out tests of reinforced concrete. And according to the results of these works, a decision should already be made on the commissioning of road infrastructure facilities. In this article, it will be about the features and rules (GOST) of testing asphalt concrete.
Key Points
To check the compliance of the road surface with the accepted standards, it is necessary to make special samples, the shape and geometric dimensions of which are rigidly set. This will avoid the occurrence of measurement errors. In this case, the material is subjected to significant pressure in order to seal. However, in some cases, tests of asphalt concrete are performed on the material without pressure hardening. After all, the physical and mechanical properties of the mixture are established for all its states, and if the material does not meet the standards, then the finished coating will not be able to provide either reliable grip of vehicle tires or safety in various weather conditions.
In some cases, it is necessary to use not specially made samples, but core cut out of real road surface. Tests of asphalt concrete in this case will allow to fix the mismatch of the characteristics of the declared and established requirements.
Some features of the manufacture of a mixture for samples
Testing of asphalt concrete should be carried out only on properly manufactured samples. For the manufacture of the mixture, an electric mixer is used, equipped with heating elements to maintain the required process temperatures.
Before being placed in the mixer, all components are necessarily dried and heated to certain temperatures. Depending on the type of mixture, the material can be heated from 80 to 170 degrees Celsius.
The binder is mixed with minerals before being placed in the device. This work is done manually by the installation operator. Mixing the mixture well with your hands is simply impossible, so after mixing with a spatula, the resulting substance is loaded into a special laboratory mixer. The time required for uniform mixing of all components of the mixture can vary significantly depending on the binder and components used (from three to six minutes).
Tests for determining the composition of pavement samples
This test allows you to fairly accurately determine the percentage of minerals and binders in the samples (samples) of the road surface.
The mineral content is determined using the so-called extraction method.
For work, you will need accurate electronic scales, a special extraction nozzle, an oven, a refrigerator, a porcelain crucible, solvents and a sufficient amount of cotton wool.
In preparation for this test, it is necessary to dry the samples very well. To do this, they are twisted in three, and preferably four layers of filter paper and placed in a drying cabinet for a certain time.
A glass vessel filled with solvent is heated to the boiling point of the contents. Since the solvent is a highly flammable substance, heating must be carried out in a sand bath to ensure safety. When a hot solvent is applied to the sample, it removes and removes the binder of asphalt concrete. The procedure is repeated until the solvent ceases to change color. It remains only to weigh the minerals and calculate their mass fraction.
Hydrostatic weighing method
This method of testing the coating is one of the most common due to ease of implementation, low cost and performance. In accordance with GOST, testing of asphalt concrete by hydrostatic weighing can be carried out both on cores cut from a real coating and on samples specially made in laboratory conditions.
Studies are conducted to determine the density of asphalt concrete taking into account pores throughout the volume of the sample. The fact is that their number and size cannot be accurately and quickly determined by any diagnostic methods. But density is one of the most important indicators regulated by GOST and industry standards.
Thin holes must be drilled in all samples. Then a thread is threaded into these holes and air weighing is performed. Accuracy of mass indicators up to three decimal places is required, therefore it is recommended to use modern electronic scales with high accuracy. Then the samples need to be weighed in water. However, before the procedure, they must be kept in the liquid for 30 minutes so that they are saturated with water. Further, the process can proceed according to two scenarios: weighing the impregnated samples in air or in water. Depending on the technology adopted, the calculation technique will vary.
This method has been known for a very long time, but is still actively used by leading laboratories around the world to this day.
Calculation of the density of reinforcing minerals in the coating composition
When testing asphalt concrete, the laboratory must carry out a set of measures to determine the specific gravity of minerals in the mixture. This technique is calculated, but, despite the lack of experimental data, it plays a very important role in assessing the quality of pavings of various types and consistencies.
The calculations are based on reference data on the density of other characteristics of each of the minerals that make up the mixture. When choosing the density indicators of the individual components of the mixture should be guided exclusively by state standards in this area (GOST). If you take data from other sources, this will lead to an erroneous conclusion and the adoption of the wrong decision by the management and contractors of construction or research works. Of course, mass fractions of the components will also be taken into account.
Is it possible to determine the density by calculation?
Laboratory testing of asphalt requires expensive equipment. And not every organization can afford the purchase of such equipment. Therefore, in some cases it is allowed to determine the values of some quantities by the calculation method. Perhaps this method will not give accuracy to a few decimal places, but it still allows you to determine the degree of quality of the coating.
So, to determine the total density of asphalt, you can use the simplest formula. The main thing is to know the density indicators of the binder, as well as the proportions and composition of the mineral sealant.
A pycnometric method for determining the density of asphalt. What is its essence?
This method is quite applicable, because it is regulated by GOST. The method of testing asphalt requires grinding samples (cores) of the coating to a certain size. Further, using high-precision electronic scales, it is necessary to obtain two samples weighing 100 g. The error in this case should not exceed one hundredth of a gram.
The resulting mixture is placed in a glass flask with known characteristics (mass, weight, volume, and so on). The flask is filled with water about one third. The resulting mixture must be thoroughly mixed by shaking in the hands, after which a number of manipulations are performed.
Why and how is the swelling test carried out?
Such testing of asphalt concrete samples as swelling is also carried out without fail. If this indicator is exceeded, this will not only affect the life of the automotive transport infrastructure, but also pose a threat to human life and health.
The principle is based on comparing the geometry of the material before and after saturation with moisture. Such experiments will require a drying oven.
The indicator is calculated using a simple formula.
The same sample is alternately weighed first in air and then in water. After which the sample is in a liquid for some time, it is saturated. After several hours, this sample is re-weighed in air and in water. The data obtained is substituted into the formula.
Water resistance test
This test is carried out on samples after a very long time in water. More precisely, during this test, the strength indices of dry samples are compared with the characteristics of cores that were in a water bath for at least 15 days.
For testing, you will need a vacuum dryer, a mercury laboratory thermometer and a powerful hydraulic press.
How to determine the ability of a material to absorb water?
The test report of asphalt concrete without fail requires the results of experiments to determine the water-saturation ability of the road surface. This is a very important indicator. The amount of absorbed liquid significantly depends not only on the material itself, but also on the conditions for saturation (primarily temperature and pressure).
This test method requires a high-precision balance in the laboratory, a vacuum oven, a mercury thermometer and a glass bulb of sufficient volume.
The principle is based on determining the change in mass of a sample before and after saturations. Knowing the density of water, as well as the mass of a dry sample, it is very easy and simple to determine this indicator.
Test Method for Asphalt Concrete for Compressive Load Resistance
The compression resistance index is one of the most important. It is taking into account its value that the operating modes of roads, the maximum axle load of vehicles, and so on are established.
The essence of the test is that the sample is subjected to compression in a powerful press until a destructive process begins.
The prepared pavement sample is placed on the press plate. The top plate is brought to the surface of the sample at a distance of 1-2 millimeters. Only after these actions can the hydraulic drive be turned on. Metal plates absorb heat well, which can affect the purity of the experiment. To minimize the error, it is recommended to warm the press plates to the set temperatures. However, such an opportunity is not always available. You can put a piece of paper on the stove. This measure will also minimize the heat loss of asphalt concrete.
Preparatory work for pressure resistance tests
First of all, it is necessary to prepare samples. Depending on the goals set, these can be either cores from a ready-made pavement or material made in laboratory conditions for research.
Before proceeding directly to the compression tests, it is necessary to withstand samples at a certain temperature (50, 20 or 0 degrees Celsius). Exposure times may vary. So, it is enough to withstand cold coating samples for one hour. Hot coating (we are talking about manufacturing technology) must be kept in the heating device for at least two hours. If it is necessary to withstand samples at zero temperature, then they are placed in ice water.
Necessary equipment and apparatus for testing resistance to compressive loads
It is necessary to have a powerful (about 100 kN) hydraulic press in the arsenal with the ability to adjust forces with a small pitch.
Since tests of cast asphalt concrete must be carried out under different temperature conditions, it is imperative that a mercury thermometer is always available. Mercury is a hazardous substance. Therefore, the presence of such devices requires obtaining permits, training and retraining of laboratory workers according to the rules of operation and maintenance of mercury devices for various purposes.
During the test, you will also need special thermostatic tanks with a volume of at least eight liters.