Материал: Russian Journal of Building Construction and Architecture

Russian journal of building construction and architecture

The condition for achieving the maximum effect of compaction of an asphalt concrete mix is the expression

(1) where К < 1 is the coefficient determining the rational maximum contact pressure under the mill of the roller that changes depending on the initial density of a compacted material.

The coefficient К is commonly recommended to equal 0.8…0.9. However the maximum effect of compaction is not always energy-saving and leads in a considerable increase in the mass and power of rollers.

Rational contact pressures that cause an insignificant growth in the deformation of a layer as

Between the criterion of a force impact q/R (here q is a linear pressure of the mill of the roller; R is the radius of the mill) and contact pressures occurring under the mill of the roller allows rational value of the latter to be identified using q/R according to (1). Therefore it is crucial to determine rationalpressureunderthemilloftherollertochoosecompactionmethodsinhighwayconstruction.

1. Related experimental and theoretical studies. This problem has been addressed experimentally in a laboratory setting during compaction of layers of sandy and fine asphalt concrete mixes on a special setup that is generally illustrated in Fig. 1. The setup was a special basin 4 fixed on non-deformed stands and heated from below with heating elements 8. Below and from the sides the basins were fitted with the deformation readers that were variable potentiometers СПЗ-23а. The signal was transmitted from the readers onto the light oscillograph K12-22.

The study was performed using sandy and fine mixes. Their temperature was 80…120 оС depending on the initial density of a compacted material according to [7]. The initial density ranged from 0.9…1.0 of the standard density by adding some extra asphalt concrete mix using a roller.

The sequence was as follows. Heated up to the required temperature, the asphalt concrete mix was made even using an even layer with the thickness of 5 сm and compacted by adding some extra layer till the corresponding density was attained. A smooth roller 1 (Fig. 2) was used with the radius R = 17.5 сm and width В = 33 сm. By means of attachable metal disks 2 a linear pressure was controlled so that the criterion of a force impact q/R ranged from 0.01 to 0.1 МPа.

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The area of a compacted band was split in two with one part exposed to a vacuum following each run of the roller using a vacuum chamber 3. Exhaustion of the latter was 5…10 kPа. Each experiment was conducted with the constant q/R. The number of runs on each area was 8.

Fig. 1. General view of the laboratory setup

Oscillograph К12-22

Fig. 2. Principal scheme of the laboratory setup for determining rational contact

pressures under the mill of the roller: 1 is the mill; 2 are attachable metal disks; 3 is the vacuum chamber;

4 is the basin; 5 is the spinner; 6 is the deformation reader; 7, 9 is the thermoisolators; 8 are the heating elements; 10 is the later; I is a vacuum layer of an asphalt concrete mix; II is a non-vacuum layer of an asphalt concrete mix

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Russian journal of building construction and architecture

As a result of the studies rational values of the parameter q/R were determined that caused an insignificant deformation growth as it goes up measured by deformation readers of the layer. Rational values of the parameter q/R depending on the initial density of a compacted layer of an asphalt concrete mix are given in the graphs in Fig. 3.

MPa

Fig. 3. Rational value of the parameter q/R (а) and contact pressures (b) depending on the initial density of a layer of an asphalt concrete mix:

1 is a non-vacuumed layer in the asphalt concrete mix; 2 is a vacuumed layer of the asphalt concrete mix

Note that for a small initial density and a high temperature of a compacted layer it was impossible to use of large values of the parameter q/R due to failure of a layer accompanied by a strong pressure on the mix coming from under the mill.

Knowing the time of the impact of the mill on a compacted layer as well as irreversible deformation of a layer that were determined by means of the oscillograph and accepting the triangular loading law and a corresponding law of the development of the irreversible deformation as well as the data of the rheological characteristics of the layers of the asphalt concrete mix contact pressures under the mill of the roller corresponding with the optimal value of the parameter q/R was identified using a software method employing a special calculation algorithm [14].

Fig. 3b presents the optimal contact pressures in relative units depending on the initial density. The strength limit of the layer of the asphalt concrete mix р is given by [12].

As seen from Fig. 3b, rational values of relative contact pressures during compaction of vacuumed layers of asphalt concrete mixes are 1.5…2.0 times lower than of non-vacuumed layers.

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This is due to, first of all, an extremely high strength limit of vacuumed layers of an asphalt concrete mix compared to non-vacuumed ones [12] depending on the current density, temperature and thickness of a layer, which is owing to a better quality of the structural performance [4]. Secondly, this is a reduction in internal residual stresses in a compacted layer [5] and thus a deformation modulus of a layer as it is being vacuumed, which requires smaller contact loads from the operating bodies of compactors.

Mathematical processing of the experimental data considering (1) allowed the following analytical expressions to be obtained:

–– for non-vacuumed mixes:

where k is the coefficient of excessive contact pressures for non-vacuumed mixes in relation to contact pressures for vacuumed mixes.

2. Testing the experimental data. In order to test the experimental data obtained in the laboratory setting, studies to evaluate rational values of the parameter q/R during compaction accompanied by industrial vacuumization using road rollers ДУ-54 and ДУ-47А [20] as well as an experimental vacuum-type roller with the mass of 4 tons [7].

Compaction tools were chosen so that the major parameter of rollers, i.e. a linear pressure, ranged from 7 to 35 kN/m. These linear pressures of the rollers q can be employed for the compaction temperatures of the mix 100…140 оС and are typical of lightand mediumweight rollers. The complex parameter of the compaction tools ranged within 23…58 kN/m2. The studies were conducted during laying the upper layer of the highway Strelna-Kipen- Gatchina using a sandy mix.

The temperature of the mix did not change significantly throughout the experiment and was 120…125 °С. The thickness of a compacted layer was 5 сm. The number of the runs of the roller on the same spot ranged from 4 to 8 with the exhaustion in the vacuum chamber 5…10 kPа. The results of the studies obtained by means of compaction accompanied by vaccumization were compared with those obtained using no vacuumization.

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Russian journal of building construction and architecture

The effect of the parameter q/R was evaluated based on the final criteria, i.e. the compaction and water-saturation coefficients. The results of the studies are presented in Fig. 4 and Table [20].

Cc

а)

MPa

Cc

b)

MPa

Fig. 4. Dependencies of changes in the coefficient of compaction (1) and water saturation (2) on the parameter q/R of static smooth rollers:

а) number of runs n = 4; b) number of runs n = 8;

—compaction accompanied with vacuumization;

—compaction not accompanied with vacuumization

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