Impact of static and dynamic loads of vehicles on pavement

The present paper substantiates urgency of studies on early deterioration of pavement layers and on reasons entailing it. The aim was to investigate static and dynamic loads imposed on a road surface by vehicles in order to detect signs of early deterioration of pavement and understand its reasons. Certain tasks were set: to define diagnostic capacity for performing test works on road surface with the use of static and dynamic loading equipment; to carry out an elastic modulus analysis on the basis of static and dynamic loading tests performed; to assess strength properties of flexible pavement. The paper describes methods for determining strength properties of a road surface with the use of special static and dynamic loading equipment. The authors provide data on diagnostic capacity of tests of a pavement performed with the use of special testing equipment. The paper gives a list of potentials, advantages and fundamental features of the following units of equipment: apparatus for plate bearing tests, Dina-3M, UDN-NK, Dynatest apparatus. The main parameters of equipment for dynamic loading tests are provided. The results of both dynamic and static loading tests performed in Saint Petersburg and the Leningrad region are given. Deflection bowls and elastic modulus were determined. Coefficients of the dynamic elastic modulus being reduced to the static one were calculated. Strength properties of flexible pavement were assessed.


Introduction
Interaction between a vehicle and a road surface is one of the key factors of road safety. Currently, early deterioration of pavement layers and its reasons are relevant for studying. Within an operation period, road surface bears load from moving vehicles and is subjected to weather and climate impacts; these factors cause stress-strain state arising in layers. Static loads act longer than dynamic ones, so deformations under static loads reach their peak. Dynamic loads have short-term nature, an interval of their intensity depends on transportation and operation conditions (traffic intensity, vehicle class, axle loads, speed, total weight of a vehicle). The following authors carried out researches dedicated to this topic: Ph.D., professor E.V. Uglova, Ph.D. A.N. Titaturian, G.N. Kiryukhina. The authors of the present paper have assessed the state of flexible pavement on four road sections. The study helped to obtain values of variation coefficients for the examined road sections and to deduce that strength of the rigid structure is heterogeneous. The authors proved the need to diagnose road surfaces and determined the parameters of traffic flows with the use of a wide range of equipment for testing and diagnosing roads produced by the international company Dynatest [1,2]. Scientists E.R. Domke, S.A. Zhestkova obtained the results of studies on issues of reducing a number of traffic congestions and controlling traffic capacity of highways with taking into account characteristics of roads [3]. The issue of influence of the state of pavement surface on road safety was studied by such scientists as Paweł Rajczyk and Marcin Knapiński [4,5]. Motion of a vehicle causes vertical and horizontal stresses in a pavement that are decaying with depth due to material and resistance parameters of road layers. All impacts including temperature, moisture and drying initiate stresses and deformations of tension, shear, compression and torsion in a pavement, which consequently reduce its strength characteristics (Figures 1,2).  Strength characteristics of a pavement depend on a "deflection bowl", which means the maximum permissible deflection value, as well as on the amount of load applied during a

Materials and methods
The authors applied methods of comparative analysis, theoretical, empirical and mathematical research methods for comparison, processing of data and for diagnostic studies. Dynamic and static loading methods for experimental research were used. The study involved the use of automated measuring, analytical and control engineering software and equipment: deflection indicator with software for performing field surveys, apparatus for a static plate bearing test, loading plate for performing static test, dynamic loading unit Dynatest used for evaluating the elastic modulus.

Discussion
Strength parameters of a pavement can be determined through the use of static and dynamic loading equipment. This method lies in assessing the bearing capacity of a pavement and determining an indicator of a common elastic modulus. The apparatus for plate bearing test consists of a loading plate (metal disk) with a cylindrical pier in the center and a loading device having a hydraulic pump and a counterweight of a vehicle (Figures 3.4).  Process of testing is notable for obtaining measurements with a plate bearing test apparatus and determining a load plate settlement depending on an applied or removed load, according to which a graph is plotted containing a polynomial curve of general deformation during primary loading, unloading and a polynomial curve of elastic deformation during secondary loading (Road industrial methodical document ODM 218.5.007-2016 "Methodical recommendations for determining the elastic modulus with a static stamp").
Dynamic loading apparatuses are widely applied for assessing the elastic modulus (E). They can be used as trailers attached to mobile laboratories or to other vehicles. This equipment is used in Finland, Denmark, Sweden and the USA. Netherlands, Sweden and Denmark use dynamic loading apparatuses produced by Phoenix, Kuab. Dynatest apparatus is widely applied in Russia [6,7]. Testing facilities used in foreign countries, such as Dynatest 8000 FWD (performs measurements by analysis of falling weight deflection); 8000 VMD (deflection indicator installed in a van or minibus); 8081 HWD (performs measurements of elastic vibrations under the influence of a large weight load) satisfy the required conditions: the amplitude and duration of loading are identical to the impact of a heavy moving wheel, deformation measurements are highly accurate. General technical characteristics and the main components of the apparatus as well as the test results are presented in figures 5, 6. The use of automated software and equipment for measures, analytics and control -a deflection indicator with software -allows determining the deformation rate caused by a controlled loading on-site and with such accuracy, which is not available for other existing methods. In Russia, the diagnostics and assessment of roads' state, as well as determination of the elastic modulus (E) were carried out using Dina-3M and UDN-NK, which impose dynamic loads, and a long base lever-type deflection indicator imposing static loads [8]. The result of comparative analysis of the parameters of the dynamic loading apparatuses is presented in table 1. The analysis of technical characteristics of the equipment chosen for comparison enabled determining the elastic modulus. The advantages of Dynatest apparatus are a high accuracy of simulating a load from a moving car wheel imposed on a pavement and a short time of determination of the following parameters: state of a pavement structure; layer-bylayer determination of the elastic modulus of the whole structure and particular layers; bearing capacity of a pavement and estimated residual life of a top layer; modeling of "weak" road sections that increas chances for traffic collisions [6,7,9,10].
The main strengths of the dynamic loading apparatus Dynatest are the following: during the correlation process, the impact on values of both peak loads and elastic vibrations coming from possible undesirable high frequency components is minimized in the load

Results
Dynamic and static loading tests were performed in St. Petersburg and the Leningrad region [6,7,10], taking into account Road industrial methodical document ODM 218.5.007-2016. "Methodological recommendations for determining the elastic modulus with the use of an apparatus for static plate bearing test".
The obtained data on elastic deflection of a pavement are presented on the graph ( Figure  7). The temperature of the upper layers during testing was 19.
where X1 and X2 are empirical coefficients of regression dependence; la is the actual deflection of a structure on a distinctive section that corresponds to the permissible percentage of deformed surface of a pavement (estimated reliability of a pavement), under static application of a design load; lD is the same under short-term loading.
Taking into account formula 1 and the obtained test results, the output data are obtained for defining the coefficient of reducing the dynamic elastic modulus (E) to the static one, taking into account Road industrial methodical document ODM 218.3.072 -2016 "Methodological recommendations for determining the residual life of the bearing capacity of non-rigid pavements and elastic moduli of structural layers (table 2). According to calculation results, the ratio of dynamic modules to static modulus obtained on gravel and gravel-sand layers is 1, on lightened asphalt concrete pavement is 1.6, on permanent pavement is almost 2.

Conclusions
Diagnostic assessment of static and dynamic loads' impact on a pavement have proved the necessity to use modern equipment, which is selected proceeding from such aspects as safety of technological measurement processes, convenience, swiftness of information receiving and processing, ability to analyze the entire spectrum of parameters under study. The deflection under static loading is greater than under dynamic one, since the impact of the first is significantly longer. Under a short-term dynamic loading, there is a greater value of the elastic modulus that characterizes the ability to undergo elastic deformations under loads imposed.