Axle loads

This meeting was attended by representatives of various groups who agreed to cooperate in the construction of test sections on US 69 northwest of Lufkin. Two new lanes were being constructed to convert the two-lane facility into a four-lane highway. Traffic volume averages 6000 vehicles per day, 15% of these are 18-KIP axle-load trucks. SNPA per-... 

Assumption 3 was used because of the absence of an appropriate fatigue design criterion for the sulfur-asphalt pavement material. The fatigue life was evaluated on the ability of the four pavement systems to withstand an 18 kip axle load for 106 applications. 

Discussion of Results. The results of the relative fatigue life analysis of the four pavement sections are given in Table X. The analysis indicates that the 12-in. asphaltic concrete pavement (System 1) and the 1%-in. sulfur-asphalt over 10-in. asphaltic concrete base (System 4) were not able to withstand the 106 passes of an 18-kip axle load. For System 4, the iy2-in. S-A-S surfacing was adequate, but analysis indicated that... 

The all-sulfur asphalt pavement (System 2) and System 3 with 1%-in. A/C surfacing over 10-in. S-A-S base were sufficiently adequate to satisfy the design criteria. For all pavement systems analyzed, subgrade failure was not predicted in 106 passes of an 18-kip axle load. Shell (20) criteria were used for subgrade failure criteria. 

Axle load Weight existing on a motor vehicle s axle. An axle weight hmit on a highway refers to the maximum weight aUowed on the truck s heaviest axle. 

Also, an additional shift is proposed for an extraordinary high number of traffic loads. These are locations where design lane traffic is expected to be >10 x 10 equivalent single axle loads (ESAL). An ESAL is defined as one 80 kN dual tyre axle (Asphalt Institute SP-1 2003). 

Note ESAL, equivalent standard axle load. 

The target bitumen content should be lower than the maximum permissible bitumen determined by using the creep coefficient procedure, when the mixture is to be used in sections with more than 3000 ESAL (equivalent standard axle load) per day. For details on the procedure used to determine the creep coefficient, see Section 6.3.3.3. 

Layers of pavements under the effect of axial loads are subjected to repeated tension. The magnitude of the tensile strain developed in pavements under a certain load mainly depends on the stiffness of the layers. For flexible pavements and after detailed measurements in situ, it was found (Pell 1967) that the magnitude of the tensile strain ranges between 30 and 200 microstrain, for a standard axle load (8 tonnes). This tensile strain generates conditions for fatigue of asphalt layers to occur, which appears as cracking. 

In the following paragraphs, a detailed description will be given on how the traffic volume is converted to ESALs, giving first a detailed account to vehicular loads, vehicle classification and ways of measuring axle loads. Reference to the tyre parameters, such as contact... 

Comparing the effect of overloaded vehicles to the number of ESALs, it was found that an increase of permissible axle load by 20% resulted in an increase of ESALs by 27.5% (Tsohos and Nikolaides 1989). 

The measurement of every vehicle axle load (weight) in its simplest form is carried out by placing the vehicle axle on an appropriate device and weighing it. This procedure is slow and time-consuming. It also requires the participation of police or other authorised personnel to stop the vehicles for inspection. Additionally, with this method, it is impossible to record a sufficient number of axial loads over a period for the sample to be representative with the aim of drawing results and enforcing regulations. The enforcement of fines to every overloaded vehicle is also an impossible task. 

The WIM system is a set of mounted sensor(s) and electronics with software that measures dynamic vehicle tyre forces and vehicle presence of a moving vehicle with respect to time and provides data for calculating wheel or axle loads and gross weights, as well as other parameters such as speed, axle spacing, silhouettes, and so on (Jacob et al. 2002). 

Apart from calculating the axial, tyre (wheel) and vehicle weight, WIM systems provide data such as vehicle speed, vehicle classification, axle spacing, wheelbase distance (front-most to rear-most axle), sequential vehicle record number, lane and direction of travel, date and time of passage, ambient temperature record or conversion to equivalent single-axle loads (ESALs). 

THE CONCEPT OF EQUIVALENT STANDARD AXLE LOADING - EQUIVALENCY FACTORS FOR FLEXIBLE PAVEMENTS... 

When multiplied by the number of axles of a given axle weight, these equivalency factors provide the number of standard axle load applications that will have an equivalent effect on the performance of the pavement structure. This number of standard axle load applications is expressed as equivalent standard axle loads or loading (ESAL). 

Table 12.3 Equivalency factors for converting axle loads to ESALs, for pt = 2.5 and SN = 5...

Axle load Equivalency factor per type of Axle (Oij) ... 

From various experimental studies considering different structural distresses, axle configurations and standard axle loads, different values of coefficient 7 were derived (see Table 12.4). 

With respect to standard single axle load, the values used vary among countries. Most of them seem to adopt the value of 80 kN, while others adopt the value of 100 or 130 kN. 

Considering the above plus the fact that pavements are constructed with a greater variety of materials, perhaps there is a need for a new extensive study to re-establish axle load equivalency factors. 

After extensive WIM studies, some countries, apart from using equivalency factors per axle load, have also developed standard axle equivalency factors per CV category or group of CVs. These equivalency factors are known as equivalent track factors. 

Source Nikolaides A., Conversion of traffic on national road network into standard axle loads. Proceedings of 2nd National Conference Bituminous Mixtures and Povements .Thessaloniki, Greece, 1996. 

The higher value of coefficient y confirms the predominant role of axle load magnitude and, hence, heavy loads or overloads in rigid pavements. This in turn signifies that rigid pavements are relatively insensitive to load repetitions. 

However, in the Australian pavement design methodology for rigid pavements (Austroads 2012), the axle loads determined as in flexible pavement (using the equivalency law) are multiplied by a load safety factor (LSE). The ESP is related to project reliability, which varied from 80% to 97.5%. Eor a reliability of 95%, the LSE for an unreinforced slab is 1.3, while that for a doweled or continuously reinforced slab is 1.25. 

NTC. 2005. Review of Heavy Vehicle Axle Load Data Information Data. National Transport Commission. Melbourne NTC. 

The analytical methodologies enable to design flexible pavements consisting of, theoretically, any number of different asphalt and base/sub-base layers. Layer thickness determination may be carried out using any configuration of axle loading and at any environmental temperature some methodologies have the ability to examine partial bonded or un-bonded interfaces apart from the fully bonded interfaces. The determination of the thickness of the layers is carried out by the use of appropriate software developed. 

The semi-analytical methodologies provide limited number of solutions in terms of type of asphalt and base/sub-base layers and use only standard axle loading configuration and... 

In most pavement design methodologies for roads and highways, the traffic load is expressed in terms of equivalent standard axle load (ESAL). The bearing capacity or strength of the subgrade material is expressed in terms of CBR or resilient modulus, Mr, whereas that of the unbound or hydraulically bound layers is expressed in terms of modulus of elasticity and that of the asphalt layers is expressed in terms of stiffness modulus. 

The objective of this step is to produce a graph of serviceability loss versus time. The serviceability loss owing to environmental factors is added to that resulting from the cumulative axle loads (see Figure 13.7). In general, if this loss is significant, measures are taken... 

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