AIRPORT PAVEMENTS
Actions due to traffic
Traffic
Road Pavements - number of passes of a standard shaft (80KN or 130 kN).
Airport pavements - numbers of trains passages leading to a set of aircraft design.
Aircraft Project
This is not necessarily heavier but one which requires a greater thickness of pavement, that is, one in which the whole PMD (Maximum Take-off Weight) x n. No matches x type landing gear is more condition.
Weight differences between aircraft and aircraft design
$$\text{Log}\left( R_{1} \right) = Log\left( R_{2} \right)*\sqrt{\frac{W_{2}}{W_{1}}}$$
R1 - equivalent number of annual aircraft operations project
R2 - Number of aircraft operations in question
W1 - Wheel load the aircraft design
W2 - Wheel load the aircraft in question
In this procedure it is considered that 95% of the weight of the aircraft is borne by trains, main and inside the aircraft.
ACN-PCN classification
The ACN-PCN system of rating airport pavements is designated by the International Civil Aviation Organization (ICAO) as the only approved method for reporting strength.
The Pavement Classification Number (PCN)
The PCN expresses the bearing capacity of a pavement for unlimited operation.
The PCN is provided by airport authorities.
The PCN is qualified by:
PCN Numeric Value |
Pavement Type | Subgrade Strength Category |
Allowable Tire Pressure | Method of PCN Determination |
|---|---|---|---|---|
| 31 | R – Rigid F – Flexible |
A – High B – Medium C – Low D – Ultra low |
W – no limit X – to 1,5 MPa (217 psi) Y – to 1,0 MPa (145 psi) Z – to 0,5 MPa (73 psi) |
T – Technical U – Using aircraft |
The Aircraft Classification Number (ACN)
The ACN expresses the relative effect of an aircraft on a pavement for a specified standard subgrade.
ACN depends on aircraft parameters and runway parameters:
Aircraft parameters: weight, C.G. position, landing gear geometry (twin gear or bogie), tire pressure
Runway parameters: pavement type, subgrade category.
Unrestricted operation if ACN is less or equal to PCN
Overload operation is possible depending on country and airport policy.
ICAO rule for overload operations:
–10% PCN margin for flexible pavements
–5% PCN margin for rigid pavements
–Annual number of overload movements less than 5% of total annual movements
The ACN is qualified by:
| Subgrade Category | Designation | Pavement type | Characteristic Subgrade Strength | Range of Subgrade Strengths | Soil Classification |
|---|---|---|---|---|---|
| Unified Classification | |||||
| High | A | Rigid Flexible |
150 MN/m2/m CBR 15% |
k ≥ 120 MN/m2/m CBR ≥ 13% |
GW, GP, GM |
| Medium | B | Rigid Flexible |
80 MN/m2/m CBR 10% |
60 < k < 120 MN/m2/m 8 < CBR < 13% |
GC, SW, SM, SP |
| Low | C | Rigid Flexible |
40 MN/m2/m CBR 6% |
k 25 < k < 60 MN/m2/m 4 < CBR < 8% |
SC, ML, CL, OL |
| Ultra Low |
D | Rigid Flexible |
20 MN/m2/m CBR 3% |
k ≤ 25 MN/m2/m CBR ≤ 4% |
OM , CH, MH |
Standard Subgrade Strength Designations and Subgrade Strength based on Soil Classification
Pavement types
Non-Reinforced Grass Strips
The non-reinforced grass strips which may be applied in most climatic conditions during all year are suitable only for the lightest types of general aviation aeroplanes. Their suitability depends on appropriate composition and good drainage characteristics of the subsurface. In an ideal case, such an aerodrome would be located on flat land, on a natural layer of gravel covered with approximately a 20 cm layer of topsoil. The surface should be covered with a grass carpet. The type of grass should be chosen so that the roots are sufficiently dense to create a thick and strong carpet in order to reinforce the soil surface. The majority of grasses used are slow growing and therefore do not require frequent mowing. It is unusual to find such an ideal interplay of all factors.
The problem is an appropriate water supply. In spring, after a period of melting snow,
and in autumn, after longer-lasting rains, the surface may be sodden and its bearing strength reduced.Even if artificial drainage is provided, it is hard to justify the use of non-reinforced grass
aerodromes for a business activity because it is difficult to provide the required regularity of operation.
In summer, in a period of dry weather and with an intensive operation, the grass carpet will be damaged, sometimes even destroying the roots.
Reinforced Pavements with Hard Surface
In an aerodrome intended for a year-round regular operation of aeroplanes with mass greater than 2 000 kg in European climatic conditions, the use of a hard surface is essential.
It is appropriate to use cement-concrete rigid pavements for some movement areas while, in other cases, asphalt-flexible pavements are useful. In general, the types of construction do not differ from constructions used in road building except in the required thickness, which is considerably greater in the aerodrome pavements designed for transport airplanes than in the roads as a consequence of larger point loads.
The construction of a reinforced pavement consists of subgrade, sub-base, bearing course and wearing course.
Subgrade
The area chosen for an aerodrome should have soil with suitable mechanical properties. The earth in the subgrade should have a sufficient bearing strength but at the same time it must be impermeable and its volume should not be affected by frost or changes in humidity. A soil analysis determines which kinds of soil need to be removed from the aerodrome pavement subgrade, which may be improved by adding different soil, and which soil and minerals in the aerodrome vicinity may be used in its construction.
The choice of type of pavement construction has a considerable effect on the bearing strength of the subgrade. In simple terms, the greater the bearing strength
of the subgrade, the relatively thinner and cheaper may be the entire construction
of the pavement. The type of pavement is affected also by the availability of suitable building material in the aerodrome vicinity. The choice of type of pavement is decided by an appraisal of life cycle costs, taking into account not only the costs
of the pavement construction but also maintenance, and reconstruction of the runway system in a monitored time horizon, including an assessment of the losses due to closing the aerodrome during its repairs and reconstruction.
Sub-base
The finished subgrade, which may be protected by a geotextile, is covered by a layer of gravel or crushed stone. That layer is supposed to fulfil a draining and filtration function. It drains the condensation created by temperature variations from the pavement construction and also catches any capillary water. The water is led away by means of catch-drains to collectors.
Bearing Course
The role of a bearing course is to receive and distribute the pressures from the aeroplane undercarriage to an appropriately large area of sub-base and subgrade. The bearing course usually has several layers. The thickness and composition of individual stratums depend on the subgrade bearing strength and on the construction of the wearing course. In order that the pavement may be designed economically, the upper layer should always have greater bearing strength than the layer under it. As a bearing course, it is possible to use clay or argilliferous stabilisation, cement stabilisation (gravel with fine fractions admixed with 8 to 10% of cement), macadam or gravel.
Flexible (Asphalt) Pavements
The relatively lower bearing strength of asphalt pavements is due to the different manner of transmitting the load. With asphalt pavements, the load is transmitted by an interaction of individual material particles under the effect of physical bonds of asphalt. The bearing strength is limited to the load that causes a permanent deformation of the flexible asphalt layer.
The upper part of the asphalt pavement is usually composed of two asphalt layers which have different functions. A supporting asphalt layer containing coarse gravel fractions is put in place on the bearing course. Its role is to transmit the load on the bearing layers. Its thickness depends on the required resultant bearing strength, or when reconstruction is carried out, also on the condition and profile of the pavement underneath. Depending on the total thickness, which is usually within the range from 10 to 40 cm, the asphalt layer may be spread several times by a
finisher in order that the required compactness can be obtained.
The upper wearing layer contains finer fractions of quality aggregate. The function of the wearing layer is to resist friction forces that are created by braking on landing, or during a rejected take-off and turning of the aeroplane. The pavement surface roughness is designed to ensure appropriate braking action. In order that the wearing layer can resist these forces, it should be at least 4 cm thick. Its second function is to create an impermeable surface. It must perfectly seal the entire construction of the pavement. If water penetrates into the subgrade, it will gradually erode, lose bearing strength, and subsequently lead to rupture of the bearing courses. The wearing course also transmits the load to the layers underneath.
Besides other things, it is important to monitor the temperature of the asphalt composition during the construction. Maintenance of the prescribed temperature is a precondition of achieving the required compactness of the strata and the bonding of the strips. It is often at badly treated joints that ruptures appear.
When considering the bearing strength of the asphalt pavement, one of the decisive factors is the overall thickness ‘h’ of the pavement including the bearing course.
Rigid (Cement-concrete) Pavements
The main advantage of rigid pavements is their higher bearing strength which is derived, among other things, from different transmission of the load. The upper layer, a cement-concrete plate, rests on a semi-flexible subgrade. The rigidity of the cement-concrete plate depends on the quality of the mixture.
Another advantage is a longer design life of the cement-concrete plate. The design life of a rigid pavement on an appropriate subgrade and with proper maintenance may be 20 to 30 years.
A cement-concrete plate is usually made from plain concrete 20 to 30 cm thick. The thickness of the plate is limited by the possibilities of regular compacting of the concrete mixture and by the fact that, with an increasing thickness of the plate, the magnitude of the internal stress increases due to temperature variations causing differential expansion of the upper and the lower parts of the plate.
The resultant characteristics of the cement-concrete plate, when evaluating the construction of the pavement in relation to the load characteristics of the aeroplane undercarriage, is expressed by the radius of its relative rigidity ‘l’ by the relation:
$l = \sqrt[4]{\frac{D}{k}} = \sqrt[4]{\frac{E*h^{3}}{12*\left( 1 - \mu \right)*k}}$ [m]
where:
D bending rigidity of the plate D = $\frac{E*h^{3}}{12*\left( 1 - \mu \right)*k}$
E Young’s modulus of concrete elasticity in tension and compression [N*m-2]
h thickness of the cement-concrete plate [m]
μ Poisson constant, usually μ = 0.15 [non-dimensional]
k modulus of cubic compressibility or ‘modulus of subgrade reaction’ [N*m-3].
Combined Pavements
Reconstruction of a rigid pavement is performed by laying a new cover of several layers of asphalt.
Block Paving
For aprons, and parking areas in particular, the use of special paving has become more common. Rectangular paving bricks pressed from high-strength concrete of dimensions 10 cm x 20 cm by 8 cm thick are most common (other shapes are used also). The blocks are laid into coarse-grained and sharp sand. Finer sand is vibrated into the joints by which the blocks are fixed one to another. By mutual ‘interlocking’ of the blocks, the pavement transmits vertical loads on a considerably greater area.
The main advantages of the block paving are as follows:
- they may be laid even in winter in sub-zero temperatures
- they may be used by aircraft immediately on completion of laying
- maintenance of pavements is speedy and simple.
Major anomalies.
Anomalies in flexible pavements
Cracking
Longitudinal and Transversal cracks
The longitudinal slots disposed longitudinally on the pavement and the transverse slits are arranged transversely on the pavement.
Longitudinal cracks :
Construction joints badly executed, retraction of asphalt due to their hardening or reduced temperature, reflection cracks existing layers that support the asphalt.
Transverse Cracks :
Shrinkage of mixtures bituminous due to their hardening, or at low temperatures, and reflection existing cracks in the layers that support the bituminous mixtures.
Cracks like "Crocodile Skin"
A series of slits intersecting to form a standard aspect of crocodile skin. These begin at the base of asphalt (or base stabilized), where the traction tensions, due to the application of load the pavement are maximal. After its initiation occurs to spread appearing on the surface parallel to each other. With the repetition of traffic loads due to bind the slits forming a mesh similar to alligator skin (parts with the longer side below 0.6 m).
Cracks like "crocodile" are considered very serious structural defects. Failure by fatigue of the bituminous layers subjected to repeated loads due to traffic.
Cracks Reflection
Generally orthogonal slits, which appear in the bituminous mixtures applied on a rigid pavement. These cracks appear in the area of the joints of the slabs by making such a mirror, the surface of the asphalt pavement.
Cycles due to expansion / contraction of the concrete due to temperature variations and due to traffic which induces shear stress. The rigid layers have a different thermal expansion of flexible, which causes it to crack.
Block Cracks
Cracks in the block are crisscrossing cracks that divide the pavement into pieces / rectangular blocks.
Reasons are daily cycles of temperature, shrinkage and hardening of bituminous mixtures thereof over the useful life of the pavement.
Cracks Slip
Cracks in Crescent or half-moon, grouped in series. The tips are oriented against the direction of traffic. These cracks arise when there is a low resistance of asphalt or when the bond between the wear layer and the lower layers is weak.
Braking and maneuvering of airplanes causing the pavement, waving surface and consequent deformation.
Distortions.
This anomaly occurs generally due to a weakening of the foundation, which, through the action of air traffic, suffers a reduction lagged. A distortion can be accompanied by cracking of the surface layer of bituminous pavement. As a result of distortion and regular longitudinal and transverse pavement is reduced and the risk of aquaplaning is increased, as may occur accumulation of water therein.
Types of distortions:
-undulation
Series regular elevations and depressions, transverse to the direction of traffic, with spacing between peaks less than or equal to 1.5 m.
Undulation is attributable by action traffic instability combined with or linked layers or in the base of the pavement.
-depressions
In localized areas or depressions on the entire surface. Are easily located by water lakes formed after a downpour in the pavement. They may also be located through stains resulting from previous accumulations of water. When dry, cause depressions in the pavement roughness. When these are met by water can cause hydroplaning.
Depressions are attributable by nesting or decrease of the strength of materials, or inadequate compaction during construction.
-ruts
Ruts are depressions the pavement more specifically in the area where pass the wheels of the aircraft. Beside the depressions may occur blistering of the pavement. In some cases only if you view the ruts after a rainstorm. The formation of ruts result of permanent deformation of the pavement layers and the foundation of it. Causes: Consolidation or lateral movement of materials to the pavement loading due to traffic.
-lateral slip
Transverse displacement of material of the pavement, causing cracking, dips and rises in a localized area.
Causes: Bituminous Mixture unstable, with large horizontal pressures that can be induced by traffic or the expansion of an adjacent rigid pavement (joints, when filled by incompressible materials, preventing the free expansion of the slabs, "pushing" these adjacent the flexible pavement).
-blistering
A blistering is characterized by an elevation in the road surface. The lifting may occur either sharply over a small area, as may occur in a large area with a gradual development. The blistering sometimes can be together with surface cracking.
Main causes: Expansiveness the foundation soil due to the action of water or ice.
Breakdowns.
The area gaps resulting from loss of aggregates connected by bitumen asphalt mixtures.
Causes: Increased stiffness of the mix pavement due to age or lack of bonding therein, abrasion compounded from the traffic jam.
Weaknesses of friction.
There are several possible causes for the lack of friction in airport pavements. In general, a wet asphalt pavement has a friction coefficient significantly lower than the same dry pavement. The pavements may become slippery, either by forming a thin film of water on very smooth surfaces, either by formation of thicker films that provide hydroplaning of aircraft traveling at high speed. The causes for the existence of flat surfaces can be both polishing aggregates such as forming a continuous film of bitumen surface. A pavement with low friction coefficient may also result from surface contamination thereof by spills of oils or fuels.
The main shortcomings of friction:
- exudation,
- polishing of aggregates,
- erosion due to engine exhaust jet,
- pour fuel.
Technical Cuts.
Any repair the pavement. A patch, regardless of how well it is performed, is always considered an anomaly. It is causes by patches of the pavement due to the recovery of abnormalities or cuts technicians. The problem existing in such cases may be ill or compaction of the layers connected. This may be reflected in fact, discontinuities arise in the pavement.
Anomalies in rigid pavements
Cracking and Fractures.
The cracks and fractures are the most common anomalies and more easily observed on rigid pavements may or may not extend to the full depth thereof. Apart from the tensions produced by traffic, the tensions caused by buckling or by the contraction of the slabs are the cause of cracks and fractures. Apart from the tensions produced by traffic, the tensions caused by buckling or by the contraction of the slabs are responsible for the anomalies mentioned above. When the presence of improperly spaced joints or impaired healing of cement concrete pavement, generate excessive movements excessive shrinkage before it reaches the resistance to which has been sized. Thus, cracks can be generated and cracks even before opening to traffic.
The different variants of cracking and fractures:
- canto party,
- cracks longitudinal, transverse and diagonal,
- cracks durability shaped "D",
- cracks due to shrinkage of concrete,
- widespread cracking; scales,
- corner unbundled,
- board disaggregated,
- slab shattered.
Settlements, Technical Cortes, Descaling and Joints.
The difference in dimensions between adjacent points of the pavement (lifting of slabs) is an anomaly that increases the risk of FOD on the aircraft, including cutting and burst tire. When checking the burst tire of an airplane safety of the crew and passengers is seriously called into question.
The most frequent anomalies:
- elevation of all or part of the slab,
- settlement of slabs, or scaling,
- small patches, technical or cuts,
- large patches, technical or cuts,
- descaling,
- ejection water and fine elements,
- loss / damage of the sealing element joints.
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