Finite element modeling and thickness design method for additional paving structure of cement concrete pavement of airport

Abstract

The invention discloses a finite element modeling and thickness design method for an additional paving structure of an airport cement concrete pavement. The thickness design method comprises the following steps: investigating and collecting pavement conditions and structural parameters of the existing cement concrete pavement, and performing thickness reduction on an old concrete pavement layer according to the existing pavement conditions; modeling a three-dimensional finite element structure of the pavement structure; dividing road surface strips and calculating the accumulated fatigue damage of the overlay layer of the airplane on each strip; determining the design thickness of the additional paving layer according to the maximum accumulated fatigue damage in each strip; checking the minimum paving thickness and the like. According to the method, the simplified three-dimensional finite element model is provided on the premise that the calculation result is converged and stable, the model calculation time is shortened, meanwhile, the influence of the existing pavement condition on the additional layer is considered, and the lateral offset of the airplane is considered in the calculation of the accumulated action times, so that the additional layer thickness design method is more scientific and reasonable.

Description

Technical Field

[0001] The invention relates to a finite element modeling and thickness design method for a rigid overlay structure of an airport cement concrete pavement, and belongs to the field of technical design of airport pavement structures. Background Art

[0002] Since cement concrete pavement is the main structural type of airport pavement in my country, rigid overlay of cement concrete pavement is one of the important methods of structural reinforcement of cement concrete pavement, especially widely used in military airports and civil-military airports.

[0003] The design of the thickness of the rigid overlay structure of the existing cement concrete pavement refers to the early experience and semi-empirical formulas of foreign countries. At the same time, in order to obtain the calculation results of the analytical solution, a lot of simplifications and assumptions were made to the structural theoretical calculation model. On the one hand, there are differences between domestic and foreign cement concrete pavements from the thickness of the structural layer to the selection of materials. It is unreasonable to completely copy the foreign experience to guide the calculation of the rigid overlay thickness; on the other hand, the calculation model with a lot of simplifications and assumptions has a large deviation from the actual project. With the rapid development of airport construction, the early experience can no longer meet the needs of modern airport construction, and a more reasonable design method is needed to guide the construction. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a finite element modeling and thickness design method for the rigid overlay structure of airport cement concrete pavement, which can provide more scientific and reasonable thickness calculation results while reducing the calculation time as much as possible, so as to solve the problems caused by the over-simplification of the existing theoretical calculation model of structural thickness and the early empirical formula, so as to better meet the needs of domestic airport construction.

[0005] In order to solve the above problems, the present invention provides a finite element modeling method for an airport cement concrete pavement overlay structure, wherein the soil base model is a dense liquid elastic foundation, an old pavement base layer is laid on the dense liquid elastic foundation, an old pavement panel is laid on the old pavement base layer, a new pavement panel is laid on the old pavement panel, the new pavement panel is used for the load of the main landing gear of the aircraft, the old pavement base layer, the old pavement panel and the new pavement panel constitute a structural layer; the old pavement panel and the new pavement panel have the same size, and the edge of the old pavement base layer exceeds the edge of the old pavement panel and the new pavement panel by 1m; and the horizontal displacement of the opposite sides of the structural layer is constrained.

[0006] Preferably, the plane size of the old road panels and the new road panels is 10m×10m, and the plane size of the old road surface base is 11m×11m.

[0007] Preferably, the old road panel and the new road panel, and the old road panel and the old road surface base layer are in smooth contact in the horizontal direction and in hard contact in the vertical direction.

[0008] Preferably, the model entity unit size of the old road panel and the new road panel is 10 cm, and the model entity unit size of the old road surface base is 20 cm. Unit size refers to the unit size of the old road surface base, old road panel and new road panel in the calculation model.

[0009] Preferably, the type of the model entity unit of the structural layer is a hexahedral non-conforming unit.

[0010] The present invention also provides a method for designing the thickness of an airport cement concrete pavement overlay structure, comprising the following steps:

[0011] Step 1): Investigate and evaluate the existing pavement conditions, calculate the pavement structure condition index (SCI) based on the survey results, and repave if the SCI is not less than 80;

[0012] Step 2): Determine the design life and forecast the aviation traffic volume within the design life, and determine the aircraft model combination and proportion under the forecast traffic volume;

[0013] Step 3): Calculate the wheel footprint size of the main landing gear of each aircraft model;

[0014] Step 4): Initially plan the thickness of the additional layer;

[0015] Step 5): Establish a calculation model through the finite element modeling of the rigid overlay structure of the airport cement concrete pavement, and take the main landing gear load action position when the maximum principal stress of the plate bottom is the maximum as the main landing gear critical load position of each model, wherein the critical load position of the single-axle two-wheel main landing gear is the middle of the longitudinal seam, the critical load position of the three-axle two-wheel main landing gear is the middle of the plate or the middle of the transverse seam, and the critical load position of the double-axle two-wheel main landing gear is the middle of the plate, the middle of the longitudinal seam or the middle of the transverse seam;

[0016] Step 6): Calculate the allowable number of actions of the main landing gear load of each model, Nei, using the following formula:

[0017]

[0018] In formula (1), the calculation formula of DF is as follows:

[0019]

[0020] In formula (2), σ i is the maximum load stress of the main landing gear of each model at the critical load position, unit: MPa; f rp is the design strength of cement concrete for new road slab, unit: MPa;

[0021] Step 7): Considering that the aircraft will produce lateral deviation during the actual taxiing process, the lateral distribution of the aircraft wheel track conforms to the normal distribution, and the calculation formula is as follows:

[0022]

[0023] In formula (3), f ix is the distribution probability density of aircraft type i at position x of the pavement cross section, the abscissa of the pavement centerline is 0; σ is related to the pavement position, 0.75 for runways and rapid exit taxiways, 0.5 for taxiways, and 0 for aprons; x is the abscissa of the calculated position, unit: meter; d i is the horizontal coordinate of the main landing gear of model i, and the complex landing gear structure is calculated separately for each main landing gear;

[0024] Step 8): Divide the two sides of the centerline of the pavement cross section into multiple strips of equal width. According to the aircraft lateral distribution law determined in step 7), calculate the distribution probability of the main landing gear of each aircraft type in each strip, and then calculate the cumulative number of times N that the main landing gear of each aircraft type acts in each strip. ij , the calculation formula is as follows:

[0025] N ij =N is ×P ij Formula (4);

[0026] In formula (4), N ij N is the cumulative number of times model i acts on strip j; is is the number of times that aircraft type i operates at the calculated pavement position within the design life; P ij is the lateral distribution probability of model i on strip j, and the calculation formula is as follows:

[0027]

[0028] In formula (5), f ix is the distribution probability density of aircraft type i at position x on the road surface cross section, D j is the abscissa of the centerline of strip j;

[0029] Step 9): Calculate the cumulative damage CDF of all models on each strip j , the calculation formula is as follows:

[0030]

[0031] In formula (6), CDF j CDF is the fatigue cumulative damage factor of the lower layer of the main landing gear load of the specified aircraft type i at strip j within the design life, I is the number of aircraft types, I = 1, 2, ..., I; ij is the fatigue damage of the main landing gear of model i on strip j, and the calculation formula is as follows:

[0032]

[0033] Step 10): Obtain the CDF of each strip of the calculated pavement within the design life j CDF of the maximum value max ;

[0034] Step 11): Determine the maximum cumulative damage CDF of the pavement surface max Whether 0.9≤CDF is satisfied max ≤1, if so, the initial proposed additional layer thickness meets the requirements, otherwise return to step 4) to re-plan the additional layer thickness until it meets the requirements;

[0035] Step 12): Based on engineering experience, check whether the calculated thickness h of the overlay layer meets the minimum overlay thickness requirement.

[0036] Preferably, in step 3), the wheel footprint size of each aircraft model includes wheel footprint length and width, which are specifically calculated and determined according to the method in MH / T5004-2010 "Civil Airport Cement Concrete Pavement Design Specification".

[0037] Preferably, in step 5), when judging the critical load position, the stress reduction caused by the load transferred to the plate should be considered, wherein when the critical load position is in the middle of the plate, the stress reduction coefficient is 0.05; when the critical load position is in the middle of the longitudinal seam or the middle of the transverse seam, the stress reduction coefficient for Class C aircraft is 0.3, and for non-Class C aircraft, the stress reduction coefficient is 0.25.

[0038] Preferably, in step 8), both sides of the center line of the road cross section are divided into 40 strips, and the width of each strip is 25 cm.

[0039] Preferably, in step 12), the minimum thickness of the additional layer is designed to be 24 cm for airports of Class C and below, and the minimum thickness of the additional layer is designed to be 26 cm for airports of Class C and above.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) The present invention provides a three-dimensional finite element modeling method for a three-layer plate structure including an existing pavement base layer, an existing cement concrete surface layer, and a cement concrete overlay surface layer. Compared with the existing simplified model based on multiple assumptions and empirical and semi-empirical overlay thickness calculation methods, the present invention is in line with the pavement structure form based on a semi-rigid base layer widely used in cement concrete pavements of airports in my country, is closer to the actual stress state of the pavement structure, and the calculation results are more scientific and reliable.

[0042] (2) The modeling method provided by the present invention is based on a large number of convergence calculation tests. Under the premise of ensuring the stability of the output results, the plane size and entity unit size of the model are reduced as much as possible, the modeling and calculation time are shortened, and both calculation accuracy and calculation efficiency are taken into account.

[0043] (3) Based on a large number of calculation tests, the present invention summarizes the critical load position of a single-axle, two-wheel main landing gear, as well as the recommended critical load position of a double-axle, two-wheel main landing gear, which greatly reduces the calculation workload of designers.

[0044] (4) The present invention takes into account the lateral distribution of the aircraft wheel tracks, provides a function of the lateral distribution of the aircraft wheel tracks that takes into account different pavement locations, and provides a method for calculating the cumulative number of times the main landing gear acts on the pavement based on the lateral distribution of the aircraft wheel tracks. Compared with the cumulative number of times statistical method in the existing specifications, it is closer to the actual situation and the result is more reasonable. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematic diagram of the structural layers modeled for finite elements;

[0046] Figure 2 Flow chart of the thickness design method for airport cement concrete pavement overlay structure. DETAILED DESCRIPTION

[0047] In order to make the present invention more clearly understood, preferred embodiments are described in detail below with reference to the accompanying drawings.

[0048] Example

[0049] This embodiment provides a three-dimensional finite element modeling method for an airport cement concrete pavement overlay structure, which specifically includes the following steps:

[0050] (1) Obtain the structural and material parameters of the pavement by consulting the airport's historical data, conducting field tests, or combining the pavement assessment report. The structural and material parameters include the existing pavement subgrade reaction modulus, the existing pavement base thickness, the existing pavement base material rebound modulus and Poisson's ratio, the existing pavement concrete surface thickness, and the existing pavement surface cement concrete material rebound modulus and Poisson's ratio. Historical data may include design drawings, construction process data, or completion data. Field test methods include core drilling, field testing, or indoor testing. The structural and material parameters of the pavement can also be obtained through the pavement assessment report. The thickness of the new overlay is planned to be 34 cm, and the rebound modulus and Poisson's ratio of the overlay cement concrete material are determined based on the design documents.

[0051] (2) A three-dimensional finite element model of a three-layer plate on a dense liquid foundation is established. The three-layer plate includes the existing semi-rigid base layer of the pavement, i.e., the old pavement base layer, the existing cement concrete surface layer, i.e., the old pavement slab, and the additional cement concrete surface layer, i.e., the new pavement slab. Figure 1As shown, the old road surface base, old road slab and new road slab constitute the structural layer. The plane size of the existing cement concrete surface layer and the additional cement concrete surface layer is set to 10m×10m, and the plane size of the base layer is set to 11m×11m. The soil base reaction modulus, structural layer thickness and material parameters are determined according to step (1).

[0052] (3) The horizontal contact between the additional cement concrete surface layer and the existing cement concrete surface layer, and between the existing cement concrete surface layer and the base layer is set to smooth, the vertical contact is set to hard contact, and the vertical stiffness scaling factor is 1.

[0053] (4) Constrain the horizontal displacement of opposite sides of the structural layer.

[0054] (5) The entity unit size of the surface layer model is set to 10 cm, the entity unit size of the base layer model is set to 20 cm, and the entity unit type of the structural layer is selected as the hexahedral non-coordinated unit.

[0055] The obtained model consists of dense liquid elastic foundation, old pavement base, old pavement panel, and new pavement panel from bottom to top. The new pavement panel is used for the aircraft main landing gear load.

[0056] This embodiment also provides a method for designing the thickness of a rigid overlay layer of an airport cement concrete pavement based on a three-dimensional finite element model. The process of the thickness design method is as follows: Figure 2 The specific steps include:

[0057] (1) Investigate and evaluate the existing pavement conditions and calculate the pavement structure condition index (SCI). Rigid overlay can only be carried out if the SCI is not less than 80.

[0058] (2) The design life is 30 years. The aviation traffic volume within the design life of the designed pavement is predicted. The predicted traffic volume should include the combination of aircraft types and the number of flights of each aircraft type. An example is shown in Table 1.

[0059] Table 1 Example of traffic volume prediction results

[0060] model Average annual takeoffs within the 30-year design life B737-800 9000 A320 8000 A321 8000 B757-200 2000 B767-300 2000 B777-200 1000

[0061] (3) Calculate the wheel footprint dimensions of each aircraft type. Simplify the wheel footprint of the main landing gear into a rectangle. The length and width of the wheel footprint are calculated and determined according to the method in the Design Specifications for Cement Concrete Pavement of Civil Airports (MH / T5004-2010).

[0062] (4) According to the aircraft model determined in step (2) and the main landing gear wheel footprint size calculated in step (3), the calculated main landing gear wheel footprint size and load are loaded onto the three-dimensional finite element model established in the previous embodiment. The loading position should be the position where the maximum principal stress at the bottom of the added layer is maximized, that is, the critical load position. When judging the critical load position, the stress reduction caused by the load transferred from the plate should be considered, wherein the stress reduction coefficient is 0.05 when the critical load position is in the middle of the plate; the stress reduction coefficient is 0.3 for Class C aircraft when the critical load position is in the middle of the longitudinal seam or the middle of the transverse seam for Class C aircraft and 0.25 for non-Class C aircraft. Specifically, the critical load position of the single-axle double-wheel main landing gear is the middle of the longitudinal seam, the critical load position of the three-axle double-wheel main landing gear is the middle of the plate or the middle of the transverse seam, and the critical load position of the double-axle double-wheel main landing gear is the middle of the plate, the middle of the longitudinal seam, or the middle of the transverse seam. In this embodiment, the main landing gear of B737-800, A320, and A321 is a single-axle double-wheel, and the main landing gear load loading position is the middle of the longitudinal seam; the main landing gear of B757-200 and B767-300 is a double-axle double-wheel, and the trial load position is the middle of the plate, the middle of the longitudinal seam, or the middle of the transverse seam. According to the trial calculation results, the maximum principal stress at the bottom of the added layer is the largest at the load position in the middle of the transverse seam, so the critical load position is determined to be the middle of the transverse seam; the main landing gear of B777-200 is a three-axle double-wheel, and the trial load position is the middle of the plate and the middle of the transverse seam. According to the trial calculation results, the maximum principal stress at the bottom of the added layer is the largest at the load position in the middle of the plate, so the critical load position is determined to be the middle of the plate.

[0063] (5) is calculated as the maximum load stress σ of the main landing gear of each model at the critical load position i ;

[0064] (6) Calculate the allowable number of actions N of the main landing gear load for each aircraft type ei , the calculation formula is as follows:

[0065]

[0066] In formula (1), the calculation formula of DF is as follows:

[0067]

[0068] In formula (2), σ i is the maximum load stress of the main landing gear of each model at the critical load position, unit: MPa; f rp is the design strength of cement concrete for new road slab, unit: MPa;

[0069] (7) Considering that the aircraft will have lateral deviation during the actual taxiing process, the lateral distribution of the aircraft wheel track conforms to the normal distribution. The standard deviation of the normal distribution is related to the pavement location, which is 0.75 for the runway and rapid exit taxiway, 0.5 for the taxiway, and 0 for the apron. In this embodiment, the pavement calculation location is the runway, the standard deviation is 0.75, and the distribution probability density calculation formula of each aircraft type on the pavement cross section is as follows.

[0070]

[0071] In formula (3), f ix is the distribution probability density of aircraft type i at position x of the pavement cross section, the horizontal coordinate of the pavement centerline is 0; σ is related to the pavement position, 0.75 for runways and rapid exit taxiways, 0.5 for taxiways, and 0 for aprons; x is the horizontal coordinate of the calculated position; d i is the horizontal coordinate of the main landing gear of model i, and the complex landing gear structure is calculated separately for each main landing gear;

[0072] (8) Divide the pavement cross section into 40 strips on each side of the centerline, with each strip being 25 cm wide. According to the aircraft lateral distribution law determined in step (7), calculate the distribution probability of the main landing gear of each aircraft type on each strip, and then calculate the cumulative number of times N that the main landing gear of each aircraft type is applied on each strip. ij , the calculation formula is as follows:

[0073] N ij =N is ×P ij Formula (4);

[0074] In formula (4), N ij N is the cumulative number of times model i acts on strip j; is is the number of times that aircraft type i operates at the calculated pavement position within the design life; P ij is the lateral distribution probability of model i on strip j, and the calculation formula is as follows:

[0075]

[0076] In formula (5), f ix is the distribution probability density of aircraft type i at position x on the road surface cross section, D j is the abscissa of the centerline of strip j;

[0077] (9) Calculate the cumulative damage CDF of all models on each strip j , the calculation formula is as follows:

[0078]

[0079] In formula (6), CDFj CDF is the fatigue cumulative damage factor of the lower layer of the main landing gear load of the specified aircraft type i at strip j within the design life, I is the number of aircraft types, I = 1, 2, ..., I; ij is the fatigue damage of the main landing gear of model i on strip j, and the calculation formula is as follows:

[0080]

[0081] (10) Calculate the CDF of each strip of the pavement within the 30-year design life j CDF of the maximum value max ;

[0082] (11) In this embodiment, the maximum cumulative damage CDF of the pavement surface is calculated max =0.87<0.9, does not satisfy 0.9≤CDF max ≤1, so the initial thickness of the additional layer is reduced to 32cm, and the above steps 2) to 11) are repeated to recalculate the CDF max =0.93, satisfying 0.9≤CDF max ≤1 requirement.

[0083] (12) The airport in this embodiment is a Class E airport, and the minimum overlay thickness is 26 cm. The calculation result meets the minimum overlay thickness requirement, so the thickness design calculation is completed.

Claims

1. A finite element modeling method for an airport cement concrete pavement overlay structure, wherein the soil foundation model is a dense liquid elastic foundation, an old pavement base is laid on the dense liquid elastic foundation, and an old pavement panel is laid on the old pavement base, characterized in that: A new pavement panel is laid on the old pavement panel, and the new pavement panel is used for the load of the main landing gear of the aircraft. The old pavement base layer, the old pavement panel and the new pavement panel constitute a structural layer; the old pavement panel and the new pavement panel have the same size, and the edge of the old pavement base layer exceeds the edge of the old pavement panel and the new pavement panel by 1m; and the horizontal displacement of the opposite sides of the structural layer is constrained.

2. The finite element modeling method for the airport cement concrete pavement overlay structure according to claim 1 is characterized in that: The plane dimensions of the old road panels and the new road panels are 10m×10m, and the plane dimensions of the old road surface base are 11m×11m.

3. The finite element modeling method for airport cement concrete pavement overlay structure according to claim 1, characterized in that: The old road panel and the new road panel, and the old road panel and the old road surface base layer are in smooth contact in the horizontal direction and in hard contact in the vertical direction.

4. The finite element modeling method for airport cement concrete pavement overlay structure according to claim 1, characterized in that: The model entity unit size of the old road panels and the new road panels is 10cm, and the model entity unit size of the old road surface base is 20cm.

5. The finite element modeling method for airport cement concrete pavement overlay structure according to claim 1, characterized in that: The type of model entity element of the structural layer is hexahedral non-conforming element.

6. A method for designing the thickness of an airport cement concrete pavement overlay structure, characterized in that: The steps include: Step 1): Investigate and evaluate the existing pavement conditions, calculate the pavement structure condition index (SCI) based on the survey results, and repave if the SCI is not less than 80; Step 2): Determine the design life and forecast the aviation traffic volume within the design life, and determine the aircraft model combination and proportion under the forecast traffic volume; Step 3): Calculate the wheel footprint size of the main landing gear of each aircraft model; Step 4): Initially plan the thickness of the additional layer; Step 5): Establish a calculation model through finite element modeling of the airport cement concrete pavement overlay structure according to any one of claims 1 to 5, and take the main landing gear load action position when the maximum principal stress of the plate bottom is the maximum as the main landing gear critical load position of each model, wherein the critical load position of the single-axle two-wheel main landing gear is the middle of the longitudinal seam, the critical load position of the three-axle two-wheel main landing gear is the middle of the plate or the middle of the transverse seam, and the critical load position of the double-axle two-wheel main landing gear is the middle of the plate, the middle of the longitudinal seam or the middle of the transverse seam; Step 6): Calculate the allowable number of actions of the main landing gear load of each model, Nei, using the following formula: In formula (1), the calculation formula of DF is as follows: In formula (2), σ i is the maximum load stress of the main landing gear of each model at the critical load position, unit: MPa; f rp is the design strength of cement concrete for new road slab, unit: MPa; Step 7): Considering that the aircraft will produce lateral deviation during the actual taxiing process, the lateral distribution of the aircraft wheel track conforms to the normal distribution, and the calculation formula is as follows: In formula (3), f ix is the distribution probability density of aircraft type i at position x of the pavement cross section, and the abscissa of the pavement centerline is 0; σ is related to the pavement position, 0.75 for runways and rapid exit taxiways, 0.5 for taxiways, and 0 for aprons; x is the abscissa of the calculated position, in meters; d i is the horizontal coordinate of the main landing gear of model i, and the complex landing gear structure is calculated separately for each main landing gear; Step 8): Divide the two sides of the centerline of the pavement cross section into multiple strips of equal width. According to the aircraft lateral distribution law determined in step 7), calculate the distribution probability of the main landing gear of each aircraft type in each strip, and then calculate the cumulative number of times N that the main landing gear of each aircraft type acts in each strip. ij , the calculation formula is as follows: N ij =N is ×P ij Formula (4); In formula (4), N ij N is the cumulative number of times model i acts on strip j; is is the number of times that aircraft type i operates at the calculated pavement position within the design life; P ij is the lateral distribution probability of model i on strip j, and the calculation formula is as follows: In formula (5), f ix is the distribution probability density of aircraft type i at position x on the road surface cross section, D j is the abscissa of the centerline of strip j; Step 9): Calculate the cumulative damage CDF of all models on each strip j , the calculation formula is as follows: In formula (6), CDF j CDF is the fatigue cumulative damage factor of the lower layer of the main landing gear load of the specified aircraft type i at strip j within the design life, I is the number of aircraft types, I = 1, 2, ..., I; ij is the fatigue damage of the main landing gear of model i on strip j, and the calculation formula is as follows: Step 10): Obtain the CDF of each strip of the calculated pavement within the design life j CDF of the maximum value max ; Step 11): Determine the maximum cumulative damage CDF of the pavement surface max Whether 0.9≤CDF is satisfied max ≤1, if so, the initial proposed overlay thickness meets the requirement, otherwise return to step 4) to re-plan the overlay thickness until it meets the requirement; Step 12): Based on engineering experience, check whether the calculated thickness h of the overlay layer meets the minimum overlay thickness requirement.

7. The airport cement concrete pavement overlay structure thickness design method as claimed in claim 6, characterized in that: In the step 3), the wheel footprint size of each aircraft model includes the wheel footprint length and width, which are specifically calculated and determined according to the method in MH / T5004-2010 "Civil Airport Cement Concrete Pavement Design Specification".

8. The airport cement concrete pavement overlay structure thickness design method as claimed in claim 6, characterized in that: In the step 5), when judging the critical load position, the stress reduction caused by the load transferred to the plate should be considered, wherein when the critical load position is in the middle of the plate, the stress reduction coefficient is 0.05; when the critical load position is in the middle of the longitudinal seam or the middle of the transverse seam, the stress reduction coefficient for Class C aircraft is 0.3, and for non-Class C aircraft, the stress reduction coefficient is 0.

25.

9. The airport cement concrete pavement overlay structure thickness design method as claimed in claim 6, characterized in that: In the step 8), the two sides of the center line of the road cross section are divided into 40 strips, and the width of each strip is 25 cm.

10. The airport cement concrete pavement overlay structure thickness design method as claimed in claim 6, characterized in that: In the step 12), the minimum thickness of the additional layer is designed to be 24 cm for airports of Class C and below, and the minimum thickness of the additional layer is designed to be 26 cm for airports of Class C and above.

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