3649

Russian Journal of Building Construction and Architecture

DOI10.25987/VSTU.2019.44.4.005

UDC625.7/.8

Ye. V. Uglova 1, A. N. Tiraturyan 2, O. A. Shiloh 3

PREDICTION OF FAILURE FATIGUE ACCUMULATION IN ASPHALT

CONCRETE LAYERS OF FLEXIBLE PAVEMENTS

Don State Technical University 1, 2, 3

Russia, Rostov-on-Don

1 D. Sc. in Engineering, Professor, Head of the Dept. of Highways, e-mail: Uglova.ev@yandex.ru

2 PhD in Engineering, Assoc. Prof. of the Dept. of Highways, tel. +7-951-820-03-03, e-mail: tiraturjan@list.ru 3 Lecturer of the Dept. of Highways, tel. +7-961-320-28-81, e-mail: shilooa@mail.ru

Statement of the problem. One of the most common and serious defects occurring on the road surface is a network of fatigue cracks. Predicting the development and distribution of this type of defect requires the design and calibration of a method for calculating the accumulation of fatigue damage in a package of asphalt concrete layers.

Results. The analysis of existing methods for predicting fatigue failure of non-rigid pavement used in domestic and foreign practice was performed, main directions of improving this approach and developed a software package for the implementation of the suggested method were outlined.

Conclusions. For the first time, a software package for predicting fatigue failure in the layers of asphalt concrete has been developed, which allows one to take into account the degradation of its structural properties as well as the basic laws of the distribution of transport load, and climatic factors throughout the year.

Keywords: pavement, elastic modulus, fatigue failure, crack grid, mathematical model, stress-strain.

Introduction. Over the last few years, the legal foundation of the road industry of the Russian Federation has undergone considerable changes related to an extended list of road construction materials employed in construction as well as introduction of new current methods of selecting ashalt concrete compositions based on the Superpave method (the СПАС as a domestic equivalent) [1]. An attempt has been made at improving the construction founda-

© Uglova Ye. V., Tiraturyan A. N., Shiloh O. A., 2019

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tions and calculating non-rigid roadway pavements resulting in the guideline (Industrial Road Standards) ОДН 218.046-01 that has been in force since 2000 that was later revisited into a preliminary national standard (PNS) ПНСТ 265-2018 “Highway Roads for General Use. Designing Non-Rigid Roadway Pavements”.

Analysis of the document reveals that despite the addition of a few new calculations, e. g., regarding bending calculations of monolith foundation layers (non-rigid) as well as a new kind of shear resistance of subbase and poorly connected layers of pavement base with a static load impact, there were not any major changes. The new PNS still relies on the standard mechanism of three-criteria calculation using limit states based nomograms designed using solutions for statistical loading of non-rigid pavement surfaces [5].

In the period from 2013 to 2015 leading road industry specialists of the Russian Federation decided to tackle the current legislation for designing non-rigid roadway pavements. In “Solutions” of the measures taken it was noted that the current guidelines for calculating non-rigid pavements (IRS ОДН 218.046-01) were outdated in the way they specified changes in the traffic flow parameters, road construction materials and technologies as well as more rigid requirements imposed by the country’s social and economic growth as well as highway routes themselves. “Program for Implementing the Concept of Improving the Guidelines for Designing Roadway Pavements” was suggested that would allow a transition to a modern level of roadway pavement design and thus longer maintenance-free cycles.

The following were the key sections of the program:

––modeling the stress-strain of roadway pavements based on modern precise solutions for layer systems (instead of nomograms developed as early as in the 1970––1980s and requiring a multi-layer road structure to be reduced to a two-layer system);

––introducing the criteria for predicting accumulating residual deformations and fatigue failures during operation to enable the parameters of a designed pavement to be connected with its state following a specified life cycle;

––thermal resistance calculations for an asphalt concrete pavement;

––establishing three levels of determining calculation characteristics of construction layers of roadway pavements depending on the importance of an object, contractor’s requirements and designer’s capacity using the experiment data, empirical dependencies, values from the tables;

––creating a climate database for a construction area of highways and modeling the water and heat mode of the subbase and roadway pavement during operation.

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Russian Journal of Building Construction and Architecture

Testing grounds designed by Rosavtodrom and the state-owned company Avtodor spanning all the climate zones were supposed to be the key element of improving roadway pavements. Using the results of experiments, newly developed methods of calculating roadway pavements as well as calculation characteristics of construction layer materials are tested and innovative technology and materials are evaluated.

A similar approach has been employed for a long time by European countries as well, in the USA in particular [6, 7, 13, 16] that have transitioned to mechanical and empirical methodology of designing non-rigid pavements that involved precise solutions of the elasticity and viscoelasticity theory for predicting cracking and rutting as well as empirical elements describing properties of certain road construction materials. Therefore methods and comprehensive measures for predicting accumulating failures in asphalt concrete layers will be addressed in the paper.

1. Method of predicting fatigue failures of non-rigid pavements. The suggested method of predicting of fatigue cracking is based on the hypothesis of linear summing of fatigue failures of Palmgren-Miner [2, 3] using which the parameter D, which is the proportion of accumulated failures over the operation period of Т years, is determined. As part of the solution for predicting fatigue failures, this criterion can be written as follows [6, 8, 9]:

where nt is an actual number of loads over the t-th year of operation; N( t )is an acceptable number of loads for cyclic deformations.

The value is split monthly:

where Npi is a total calculation number of calculated loads to the point on the structure surface as of the i-th month of the t-th year determined based on changes of traffic intensity over the years and average monthly distribution of traffic flow; N fi is an acceptable number of loads for cyclic deformations on the structure surface over the i-th months of the t-th year.

The index N pi ,which is traditional in the Russian Federation, is identified in accordance with the PNS (ПНСТ) 265-2018 for calculating a total number of calculated loads.

An acceptable number of loads N( t ) for a specified deformation mode of an asphalt concrete pavement is calculated using the formula by the National Center for Asphalt Technology [12]:

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that are assumed to be 1.2, –3.291, –0.854 respectively; f is a stretching deformation in the lower boundary of a package of asphalt concrete layers; Eнижi is the elasticity modulus of the lower layer of asphalt concrete calculated given the temperature mode of the i-th month, MPa. Stretching deformations in the lower boundary of a package of asphalt concrete layers are determined based on an accurate solution of the elasticity theory for a multi-layer semi-space under a calculated load. The calculation was performed considering the temperature and actual humidity over the months of each year over the entire calculated life cycle.

One of the most important tasks in predicting fatigue failures is to take into account a reduction in the load-carrying and distributing capacity of a package of asphalt concrete layers over the calculated life cycle of pavements as fatigue cracks develop. According to [18––20], the following dependence can be used:

where Ei is the rigidity modulus of an asphalt concrete layer over the time period t, МPа;

Ei 1 is the rigidity modulus of an asphalt concrete layer in the previous time period t–1,

МPа; акт is an actual stretching deformation in the lower boundary of a layer, m/m;

пред is a level of acceptable deformations employed in laboratory tests for innovative materials used for paving asphalt concrete layers; Vбит is the content of a bitumen binder in an as-phalt concrete layer, %; Npi is a total number of loads over the i-year of the month; n is an index which is 5,6; kтемп is the temperature coefficient; F is a caliber coefficient determined as a result of laboratory tests for innovative materials used for paving asphalt concrete layers.

A structural scheme for this predicting method is in Fig. 1. It was implemented as a mathematical model and software package Pavement Life Cycle. In order to analyze the viability of the results of predicting fatigue failure in the layers of non-rigid pavements the following options of road structures were proposed (Table).

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Russian Journal of Building Construction and Architecture

Сonstruction of roadway pavement:

–– thickness of a layer, material; Construction area –– subgrade

Fig. 1. Structural scheme of the calculation of roadway pavement for accumulating fatigue failures

The final values of the proportion of accumulated fatigue failures can be equaled to the area of a strip on a highway pavement covered with a network of fatigue cracks. A complex set of

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natural experiments to determine this dependence was conducted in the USA and documented [12––14, 17] (Fig. 2).

Table

Options of structures of non-rigid roadway pavements

Proportion of accumulated failures D

Fig. 2. Ratio of the proportions of accumulated failures and area of a network of fatigue cracks

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Russian Journal of Building Construction and Architecture

All of the structures in Table are equally strong with the total elasticity modulus Еобщ = 665 МPа, which allows them to be analyzed their resistance to fatigue durability immediately from the viewpoint of the properties of construction layers. The values of the elasticity modulus of asphalt concrete layers of non-rigid pavements were determined immediately during the natural experiments on a tool for measuring fatigue durability with a four-point loading system of an asphalt concrete slab. The results of determining the rigidity modulus for asphalt concrete used in the modeled structures are provided in Fig. 3. As the graph suggests, the obtained results are considerably different from the standard design values of the elasticity modulus of asphalt concrete which are specified by the IRS (ОДН) 218.046-01 and PSN (ПНСТ) 265-2018, which proves it necessary to determine the elasticity modulus of asphalt concrete immediately in laboratory settings.

Fig. 3. Results of determining the rigidity modulus of asphalt concrete

2. Results of predicting accumulating fatigue failures. The results of predicting accumulating fatigue failures for each of the investigated road structures are presented in Fig. 4––6. As seen from the graphs, the least proportion of the accumulated failures and predicted area of fatigue failures (15 % of the rolling area) corresponds with the roadway pavement № 1 with a reinforced foundation layer. For roadway pavements with less rigid foundations (i. e. for the structures № 2, 3) the area of the rolling area covered with a network of fatigue cracks will correspond with 37 an 29 % respectively. It should be noted that a predicted area of the rolling area covered with the network of fatigue cracks are significantly affected with the thickness of a package of asphalt concrete layers, which is proved by the best resistance to fatigue failures of the structure № 3.

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Fig. 4. Result of predicting accumulating fatigue failures and area of the network of fatigue cracks along the strips for the roadway pavement № 1

Fig. 5. Result of predicting accumulating fatigue failures and area of the network of fatigue cracks along the strips for the roadway pavement № 2

Fig. 6. Result of predicting accumulating fatigue failures and area of the network of fatigue cracks along the strips for the roadway pavement № 3

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Russian Journal of Building Construction and Architecture

The proposed method of predicting fatigue failures in asphalt concrete layers can be regarded as an extra criterion for developing and interpreting optimal structures of roadway pavements and their protection by the authorities. An unsurpassed advantage of the approach is precise solutions of the main components of the stress-strain of roadway pavements based on a mathematical model of a multi-layer semi-space as well as a body of data on actual rigidity modulus of modern asphalt concrete used in construction of modern roadway pavements. The main direction of improving this method is to develop mechanisms of modeling their life cycle given a series of measures for safeguarding them, i.e. paving a wear layer and protective layers.

Conclusions

1.For the first time in this country a method of predicting fatigue failure in asphalt concrete layers has been developed and implemented as a software package based on a mathematical model of a multi-layer semi-space and allowing one to determine accumulated failures in accordance with the hypothesis of Palmgren-Miner and the percentage of strips of highways covered with a network of fatigue cracks.

2.The elasticity modulus of modern types of asphalt concrete was determined in the laboratory setting using a tool for four-point loading for different operating temperatures. The resulting values are used for calculating asphalt concrete resistance to fatigue failures.

3.As part of testing calculations it was found that a type of a foundation layer and the thickness of a package of asphalt concrete layers have the greatest effect on the rate of accumulating fatigue failures. A rigid foundation allows a two-time reduction in the rate of fatigue failures compared to a non-reinforced one. However, it should be noted that a rigid foundation layer might cause other types of defects such as reflective and thermal cracks.

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