TECHNOLOGY OF MAKING AIRFIELD PAVEMENTS IN RELATION TO THE EXISTING REQUIREMENTS IN TERMS OF TEXTURE

: The assessment of the texture of airfield pavements, in addition to assessing their roughness, is a component of the broader concept of anti-skid properties, which significantly affect the safety of air operations. The roughness of the pavement directly affects the adhesion of the aircraft tire to the pavement, and the texture additionally affects the effectiveness of dynamic drainage of water from the pavement. The current standard requirements in terms of roughness and texture of airfield pavements do not distinguish the technology used. In the paper, the authors focused on the analysis of the results in terms of the texture of airfield pavements and presented the resulting conclusions in relation to whether the technology of its implementation has an impact on the texture parameter.


Introduction
Airfield pavement is a determined and specially prepared surface of airfield functional element (AFE) that serves a certain function in air operations.The safety of aircraft operations on airfield pavements depends mainly on the load bearing capacity of these aircraft and other operational parameters such as evenness and anti-skid properties.When discussing the anti-skid properties of airfield pavements, other aspects apart from the friction coefficient determining its roughness should also be taken into account.Not to mention that a vital element of friction characteristics of the contact area between tyre and surface is a surface texture [4].
Surface roughness directly affects the adhesion of the aircraft tyre to the surface; and texture impacts the effective dynamic water removal from the surface.Drainage characteristics of the movement area, more precisely a quick removal of water from the runway surface is a major safety factor, which is taken into consideration already at the design stage and later during the construction and maintenance of the movement area.Removing water from airfield pavement is a basic requirement and is used to reduce the thickness of the water layer on the surface.The proper drainage is ensured mainly by applying surface with the appropriate longitudinal and transverse inclination (natural water removal).Dynamic water removal is achieved by the pavement texture [19].
The basic type of artificial airfield pavements include rigid (Rig. 1. flexible) made of cement concrete, but asphalt concrete airfield pavements (Rig.2. susceptible) and complex pavements (classified between flexible and rigid -Fig.3).[3] are also observed.In the case of airfield pavement made of cement concrete, when the texturing assembly finishes its runway paving job, grooving operation is commenced.The most important and final method of paving the top surface layer directly connected with its texture is usually to drag the burlap behind a slip form paver [13].

Fig. 4. Texturing of the surface from cement concrete
The situation looks different in the case of susceptible pavements.Embedding structural layers of the cement concrete airfield pavement should be executed with spreaders.After that, the layer is thickened to the required compaction index using heavy rubber and steel rollers.In this case, texturing of the upper layer of the pavement is not done.Moreover, the texture is achieved by obtaining the required physical and mechanical properties of the mineral-asphalt mixture (including compaction index) and aggregate graining of the overlay layer [12].Under special conditions, it is allowed to provide the abrasive layer of airfield pavement from cement concrete with the appropriate texture, improving its roughness and facilitating surface water removal.The possible application of grooving requires to take into account (in the project) the greater thickness of the abrasive layer than the one recommended in the standard that does not include such a procedure.

Texture of airfield pavements -requirements
In practice, it is aimed to ensure good anti-skid properties of airfield pavements, which indicates that they fulfil the required criteria of friction coefficients and texture depth.
New airfield pavements have to meet specific depth requirements.They were included in documents issued by global aviation organizations, such as EASA (European Aviation Safety Authority), ICAO (International Civil Aviation Organization) and FAA (Federal Aviation Administration), which are described in Table 1.The applicable aviation documents set out requirements for new airfield pavements and are focused mainly on the runway surface.Depth measurement of macrotexture with the volumetric [14] or profilometric method (measurement of the mean profile depth MPD / dMPD) according to PN-EN ISO 13473-1: 2019 Characteristics of the pavement texture using surface profiles.Part 1: Determining the average profile depth [15].MPD value can be converted into the estimated texture depth ETD.To this end, a conversion equation is used  = 0.2 + 0.8 .
Unfortunately, the applicable reference documents regarding texture of airfield pavements are believed to raise many doubts.This leads to the question of the required minimum value for new pavements -1.0 mm or 1.14 mm, and also how to treat a record permitting value below 1.0 mm.It should also be decided whether the paving technology has any impact on texture.

Test methodology and results
The answer to the above question was arrived at while performing a broader scope of tests, by which, apart from the existing measurement method regarding texture (profilometric method), an innovative assessment method of anti-skid properties of airfield pavements was applied.What is more, this method enables a dynamic and simultaneous measurement of the friction coefficient and texture depth.
Tests were conducted using a measurement system based on ASFT (Airport Surface Friction Tester) on T-10 trailer, equipped additionally with 2D/3D profile laser scanner with high frequency (taking texture measurement in the wheel track of the friction tester vehicle).Compared to the currently used measurement profilometric method, the suggested measurement method of texture depth enables to take measurement both in the parallel and perpendicular direction to the direction of movement of the friction tester as a part of assessing the anti-skid properties of airfield pavements.This method is not limited by point measurement, i.e. measurement is not only carried out as a function of the length of the measurement section but also as a function of its width (corresponding to the width of contact between the wheel's tyre of the friction tester and the surface).The measuring system for the adopted method of assessing the anti-skid properties includes (Fig. 6): friction measurement module -friction coefficient measurement; texture depth measurement module-measurement of CMPTD (Continuous mean profile depth and texture coefficient).

Fig. 6. Diagram of a measuring system for the assessment of the anti-skid properties
The full scope of tests comprised measurement by calculating speeds of 65km/h and 95km/h on cement concrete and asphalt pavements and pavements of all ages (new pavements 'N', pavements under operation 'E' and pavements planned for maintenance due to minimum, boundary coefficient friction values 'S').As a part of the manuscript under consideration, the results obtained during tests performed on new pavements made from cement and asphalt concrete were analyzed by measuring speeds of 65km/h and 95km/h.Pavements for the given age ranges were adopted based on their real operation period (age), for "N" up to 3 years, for "E" from 3 up to 20 years and for "S" above 20 years.
It is apparent from the field tests that for each of the given measuring conditions (e.g.measuring speed 65km/h, asphalt concrete pavement), 6 measuring sections were determined.Each of them was 100m long.The sections for specific measuring conditions were located on one airport functional element (AFL), e.g.taxiway of the selected, tested aircraft.Individual sections were treated as a single sample, but 6 sections for the given measuring conditions: as a population of samples.The measurement of CMPTD and µ (developed measuring system) was conducted on a continuous basis across the whole length of the measuring section with the frequency of readout every 0.2 m for CMPTD and every 10 m for µ.Due to this, further analysis was made based on 10 m sections for specific measuring sections.Moreover, point profilometric measurements were taken on every measuring section using the ELATextur device (for 20 m, 40 m and 60 m of the measuring section).A diagram of measurements according to the adopted methodology is illustrated in Fig. 7.   Additionally, the obtained results were presented also in the visual form.

Summary
The topic addressed by the authors is important for safety of aircraft operations.It aims to ensure good anti-skid properties of airfield pavements, which the given (required) values should define; in this case, it is friction coefficient and texture depth.The obtained results indicate that by estimating the texture of airfield pavements, it is necessary to distinguish whether the estimation refers to cement concrete or asphalt concrete pavement.
By the same token, they indicate that it is unlikely to achieve the average texture depth MTD (≈ ETD) amounting currently up to 1.00 mm required for new airfield pavements.
The confirmation of this is coefficients of pavement's texture, which were obtained both in the case of using a point measurement method (MPD and ETD), and the suggested new measurement method.(CMPTD).They are respectively higher for asphalt concrete pavement and respectively lower for cement concrete pavements.Therefore, the authors believe that the introduction of separate requirements in this scope in reference to the airfield pavement technology should be considered.
The authors plan further works on the texture of airfield pavements.The effects of conducted tests and analyses in this field will provide a supplement to the existing manuscript [18] [16] [2] [17] [22] and will be an incentive for a more intense discussion with other authors working in this area, whose results were presented in the following articles [5] [9] [10] [21] [11].

Table 1 Requirements regarding texture depth for new airfield pavements
Engineering Sciences Data Unit) can be used.It was devised for the runway based on the texture information illustrated in Table 2[8].It is assumed that a texture depth of 0.25 mm should ensure the safety of air operations.