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DESIGNING AND CONSTRUCTION OF ROADS, SUBWAYS, AIRFIELDS, BRIDGES AND TRANSPORT TUNNELS
UDC 625.717
Military Air Engineering University (Voronezh) Ph. D. in Engineering, Assoc. Prof., Head of 32 department of Engineering Airfield Maintenance A. N. Popov Ph. D. student of 32 department of Engineering Airfield Maintenance I. G. Shashkov
Voronezh State University of Architecture and Civil Engineering D. Sc. in Physics and Mathematics, Prof. of Dept. of Building Machinery and Engineering Mechanics
A. V. Kozlov
Russia, Voronezh, tel.: 8-919-243-32-17; e-mail: [email protected]
A. N. Popov, I. G. Shashkov, A. V. Kozlov
A TECHNIQUE OF ESTIMATION OF TECHNICAL CONDITION OF RIGID AIRFIELD PAVEMENTS IN THE CONTEXT OF RISK THEORY
Problem statement. To provide for safe takeoff and landing of modern aviation complexes, special attention is given to technical condition of artificial pavements of runways which can be serviceable or faulty, efficient or limiting. Available standard methods of an expeditious estimation of an operational-technical condition of airfield pavements are based on general principles of defect graduation and of definition of integrated total generalized indicator of pavement condition and often yield the results contradicting each other, which complicate making decision in relation to operation.
Results and conclusions. The classification of linear constructions of airfields by responsibility level is proposed. Theoretical basics and practical recommendations on estimation of a technical condition of rigid airfield pavements by permissible level of are formulated with respect to level of risk with the use of principals of reliability theory and of risk theory. The recommendations proposed rest on new principles of technical regulation established by Federal Law N 184-FZ “On technical regulation”.
Keywords: airfield pavement, reliability, risk, runway.
Introduction
Airfield pavements in modern aerodromes are complex engineering structures that have to
meet so many high requirements imposed on them, among them particularly operational ones.
The advancing level of technical operation involves first of all more rational organizational patterns that would assist in creating a maintenance system allowing for timely and efficient rehabilitation with minimum cost and labour.
Critical to the technical operation of aerodrome structures are the following:
- adherence to the operational code during design process;
- creating a pavement control system and related structures at different stages of their operation.
There are two methods in which paving is maintained - by searching and eliminating distresses and by carrying out planned inspection and test activities. The first method is disadvantageous in the sense that it puts limits to its actual use. First, as aerodrome pavement is routinely and normally used, a likelihood of identifying distresses is numerically small especially when such massive structures as a runway are in operation, thus resulting in a significant drop in the efficiency achieved by searching and eliminating distresses at this point. Furthermore, in practice, technical operation is organized with no sufficient information available or none at all. This being the case, searching and eliminating distresses allows for assumptions to be made. A likelihood of identifying pavement failures and their timely removal depends on whether these turn out true or false.
The second issue that is detrimental to the efficiency of the operational system in question is that searching and eliminating distresses involves a series of events where in order for any technical measures to be taken, there first should be an ongoing defect to be removed and only then can they act on its elimination. Since there should be some time delays between these steps for proper preparations to be in place, searching and eliminating defects allows for a time delay in their removal. In some cases, this may not meet the safety requirements for airport runways.
The third issue offsetting the efficiency of this method of the technical operation is that it does not enable scheduling of repair and maintenance works as there is not a whole lot of variety of pavements resulting in all of their present applications to a certain point under the same conditions show the same likelihood of failure.
The above suggests that the major system of operating airport pavements that ensures its frequency and aircraft safety is a system of inspection and test activities. This system means works
associated with restoring operational characteristics of runways are conducted not after defects occur but so that to prevent them doing so. At the core of this united system of repair and maintenance works on airport facilities is a system of timely scheduled inspection and test activities.
The major challenge this system poses especially given its poor funding is that it provides no comprehensive methods of analyzing the outcome of a study that would potentially enable not only to assess the pavement condition but also to make predictions as to changes that might happen and therefore sensibly allocate funding.
1. Analyzing current methods of the assessment of the technical condition of aerodrome pavements
The assessment of the technical condition of aerodrome pavements of state aviation in accordance with the regulatory documents [8, 9] includes the qualitative and quantitative assessment. The qualitative assessment is performed to determine whether a pavement is fit to use in terms of its load-bearing capacity in a specified type of aircraft by comparing aircraft classification numbers ACN and a load-bearing capacity of PCN with the same subgrade strength [9].
A pavement classification number should not be lower than an aircraft classification number operating on this very pavement [3, 9]:
K • ACN < PCN, (1)
where K is a coefficient considering the intensity of air traffic flows.
If the condition (1) is not met, it is necessary that the aircraft mass limits are introduced and intensity of take-offs and landings is decreased.
The method ACN—PCN found a range of applications and is adopted as a regulatory document in the Russian Federation [9, 10].
The quantitative assessment defines the operational suitability of pavements based on the analysis of the nature and number of distresses [9]. The criteria describing the condition of the pavement surface are the parameters describing the defects and wears (the width of exposure, area, etc.) revealed during inspection. These are also quantitative indices of the technical condition of a pavement surface and deterioration rate that reflect a number of ongoing defects and wears and intensity of their manifestation.
In practice, a whole range of Russian and foreign methods of operational assessment of the technical condition of rigid aerodrome pavements are put into use [4, 5, 8, 12]:
- of signal assessment Sk (FGUP GPI and NII GA “Aeroproject”);
- determining the pavement condition index Ik (Ministry of Aviation Industry);
- assessment of the technical and operational pavement performance using
N. V. Sviridov’s method;
- determining the pavement integrity index MI (26 CSRI MR RF);
- a standard method of determining an aerodrome pavement condition index PCI (USA);
- determining a complex index KK (26 CSRI MR RF).
The index Sk is computed based on cracks occurring on the pavement plates, spalling and slabbing and is calculated using the formula
S, = 5.00 - 100(0.10W„ + 0.05 Nmr +0.03N,), (2)
N0
where NCK, Nmp, NM is a respective number of the surface plates experiencing cracks, spalling and slabbing; N0 is a total number of the plates on the pavement surface.
The method of signal assessment Sk is to be in compliance with the requirements set for an airport performance [5].
The index Ik makes provisions for a close relation between “a weight” of defects and areas of the pavement under our estimation as well as for a range of an influence major structural indices have on the pavement condition index. The pavement is subdivided into sections for which we calculate individual condition indices:
100 4 7
h = -S-IWI V>M, (3)
Sk j=1 i=1
where Sk is the area of the k-th section; Wj is a factor weight: W1 is a service life cycle, W2 is an adhesion coefficient, W3 is an evenness, W4 is clogging; Vik is a percentage of the plates with the failures of the i-th type obtained as a result of the survey of the k-th section; bi is a
weight of the i-th failure type; atj is the assessment of the influence of the i-th failure type on
the j-th factor.
The weight of the i-th failure type bi and effect assessment aij are defined by the method of expert poll.
N. V. Sviridov came up with the method of the assessment of technical and operational condition of pavements according to which a number of plates subjected to a certain type of distress is divided into a total number of the plates in this area and as a result a density of distress is obtained which is then multiplied by the weight of distress. Thus an average distress weight is obtained for each its type and their sum also yields a total average distress weight whose value further guides the judgment as to what condition a pavement is currently in.
The general disadvantage underlying the methods of signal assessment Sk of the pavement condition index Ik and N. V. Sviridov’s method is the assessment of the technical pavement condition according to a number of distressed plates with no respect to the value of distresses (height, area, etc.). A typical feature of the pavement integrity index MI is that it gives the consideration to a number of distresses and the effect ‘a weight’ of distress has on the flying safety. The pavement integrity index is determined using the following formula:
MI = , (4)
i=l n
where n, nz is a total number of plates and distresses of the i-th type; az is the weight of distresses of the z'-th type; k is a number of the identified distress types. If distresses and failures of different types occur in one plate, when determining the index MI.
Abroad they use a US-developed method ASTMD5340-93 of quantitative and qualitative assessment of the aerodrome pavement condition which is routinely used to determine the aerodrome pavement condition index (PCI). This method is based on the same approach as the Russian methods are, i. e. on visual identification of distresses in pavements, classification of these distresses according to their weight and their severity, determining the integral assessment of the pavement condition with regard to the density of distress propagation in the pavement area. The index of the aerodrome pavement condition PCI was determined using the formula
PCI = 100 - MaxCDV, (5)
where MaxCDV is the greatest value of the altered reduced value.
In order to define the value of failures in rigid pavements, this method utilizes the already determined weight functions for each distress. The functions are presented as graphs with varying degrees of distress (low, average, high) and its volume considered.
Similar to the index PCI in its essence and the algorithm of the assessment of the pavement
condition is the complex index KK that is defined as described in [4]:
KK = 100 - (KAPA + KEPE + KBPB + KrPr )100, (6)
where PA, Pe, Pb, Pr are weight coefficients for the pavement areas, KA, KE, KB, Kr are the
values of the quality indices of the pavement area.
Depending on the value of the complex index, it is recommended that operational maintenance, routine or major repairs be in place. A peculiar thing about the K is that it takes the quality of the surface into special account.
Despite the common principles, each of these methods are particular in its own individual way and are fundamentally different:
1. A varying approach to the algorithm of visual observation:
- the method of determining the index PCI and the complex index KK implies that the elements of the airfield are divided into areas which are in turn are divided into samples;
- N. V. Sviridov’s method, the method of determining the pavement condition index Ik and the method of determining the pavement integrity index MI: the elements of the pavement are divided into areas (sections);
- the method of signal assessment Sk: the pavement is assessed element-wise.
2. A varying set of distresses of artificial pavements:
- according to the method of determining KK, 18 types of distresses are taken into account;
- according to the method PCI — 15;
- according to the method MI — 9;
- according to N. V. Sviridov’s method — 12;
- according to the method Ik — 12;
- according to the method of signal assessment Sk — 3.
3. Varying indices and ambiguity of the category of the technical condition of aerodrome pavement.
Table 1 presents the resulting assessments of the condition of a part of the artificial runway obtained using various methods.
Table 1
Example of the pavement condition assessment
Calculation method Index Value Pavement condition
Using the pavement condition index PCI 9.3 Poor
Using the complex index K 58.0 Good
Using the method of signal assessment SK 4.0 Fit to operate
Using the pavement integrity index MI 3.1 Restricted operation
Using N. V. Sviridov’s method 1.2 Satisfactory
The analysis shows that the conclusions made as to the pavement condition are at odds with one another, which obviously poses extra challenges on making operation-related decisions.
4. A common disadvantage of all the methods considered is that the pavement condition can only be assessed at the moment of its monitoring which makes it impossible to forecast changes in its technical condition.
2. Theoretical principles and practical guidelines on the assessment of the reliability of the pavement areas using the major regulations and the risk theory
“Technical Regulation’ law was enacted in 2002 that established the risk management as a characteristic shared by all structures. Its authors propose the assessment of the aerodrome pavement condition in terms of a reliability level accepted and a degree of risk based on the new principles of technical regulation and of the reliability theory. The subjects of the technical regulation in [15] were buildings and structures of any purpose (as well as networks of engineering and technical provision and systems of engineering and technical provision) as well as processes associated with buildings and structures and project designs (including examination), construction, installation, remedy works, operation and recycling (demolition).
The principles provided in [15] hold for all life cycles of a building or a structure. The document in question sets out minimum necessary requirements for buildings and structures as
well as for processes associated with buildings and structures and project designs (including examination), construction, installation, remedy works, operation and recycling (demolition). Following the identification process, a building or a structure is referred to an appropriate liability level which is a characteristic of a building or a structure defined according to the scope of economical, social and environmental consequences that may follow their deterioration:
- I — high. These are buildings and structures in compliance to the Urban Design Code of the Russian Federation deemed overly hazardous, technically complex or unique objects;
- II — normal: buildings and structures, except for buildings and structures of the high and low liability levels;
- III — low: these are buildings and structures for a temporary (seasonal) use as well as buildings and structures for secondary use involved in the construction or reconstruction of a building or a structure or those located on lands granted for individual housing construction.
The values of the reliability coefficient as related to buildings and structures of all liability levels are given in Table 2.
Aerodrome pavements which following the construction are thus linear construction systems with building above ground levels consisting of load-bearing structures are referred to structures with the high liability level whose failure is highly likely to have a heavy economic, social and environmental impact.
According to the regulations [8], aerodrome pavements are divided into groups of areas based on the effect of the aircraft load and load-bearing capacity (Fig. 1).
Hence the fundamental principles [15] can apply to the pavement classification according to a liability level:
- high — areas E (runways areas adjacent to the edges);
- normal — areas A, E (aircraft ramps), B and r;
- low — areas E (terminal and junction helipads).
The overall feature of aerodrome pavement is its reliability. The reliability of aerodrome pavement is knowingly a figure that describes the system’s ability to deliver unfailing performance thus ensuring a safe take-off, landing and taxiing. Depending on the specification of a system and its operation conditions, the regulations and guidelines defines the following in-
dicators of a system’s reliability. They are unfailing performance, durability, maintenance and repairability, a longer service life and variability.
To provide an unbiased assessment of the operational suitability of aerodrome pavements, it would make sense to introduce another indicator of operational durability that describes its ability to retain its working capacity till some boundary condition.
Table 2
Classification of linear aerodrome structures according to the liability levels with consideration to [15]
According to [15] Considering the structure [17] According to the guidelines [2] According to [16]
A structure liability level s t G ei ic M-l e O o lbiail le s a e era ya £ c a o a u o r A Aerodrome element Acceptable reliability values p acceptable s ir re el uirl bla r « V CS ^ C w ra ei % « « e o c Intolerable risk Coefficient of variation of the quality of aerodrome pavement A degree of risk and damage
I >1.1 E Areas of the runway adjacent to the edges 0.9 0.1 > 0.05 <0.1 Low
II >1.0 B Middle of a runway 0.85 0.15 > 0.05 <0.15 Average
A Edges of a runway 0.8 0.2 > 0.05
Main helipad 0.8 0.2 > 005
r Edges of the middle of a runway except those adjacent to the junction helipads 0.8 0.2 > 0.05
E Aircraft parking 0.7 0.3 > 0.05
III >0.8 E Junction helipads 0.7 0.3 > 0.1 <0.2 High
Terminal helipads 0.6 0.4 > 0.1
Fig. 1. Groups of aerodrome pavement areas:
А — main helipads; runway edges; E — runway areas adjacent to its edges; auxiliary and junction helipads; bridge structures, ramps and similar structures used for aircraft parking; B — middle of the runway; r — width edges of the middle of the runway except those adjacent to the junction helipads
These all comply with the requirements for the load-bearing capacity but fail to provide flying safety. It is recommended in [16] that tolerable risk be used as a measure of a level of flying safety required. Risk is a probability of a damage to life or health of people, property of individuals and corporations, municipal and state property, environment, life or health of animals and plants with respect to damage done.
The reasons causing disruptions in flying safety are distress and damage of aerodrome pavements that are to be inspected and monitored within the framework of the risk theory. Operational durability thus defines a life cycle of the aerodrome pavement T with certain damage of a tolerable risk level. Their total number can be found based on the conditions of flying safety and is established using the acceptable reliability level Pacceptabie (see Table 2).
With respect to the analysis performed we propose a classification of linear aerodrome structures (see Table 2) that is not at odds with the current legislation of the Russian Federation considering:
- liability levels and reliability coefficients according to the requirements [15];
- minimum reliability levels for aerodrome pavements consistent with the surveys presented in [2];
- intolerable risk and coefficient of variation of aerodrome quality according to the requirements [16].
Let us look at a pavement as a system of N0 plates. related to the system, a plate failure is a
damage to it. In order to disrupt the performance of the entire pavement, a runway for exam-
ple, it is necessary that there is a number of distresses that overall lead to an event making further flying operation impossible.
The system reliability equals a probability of its unfailing performance which is calculated using the known formula [2]:
N -Y N
P(t) = lim 0 Y Ot , (7)
At--> 0 NO
N0--
where N0 is a total number of plates; Not is a number of failed plates in a calculating time interval; t is time.
A more simple formula can be used in practical calculations:
NN
P(t)= NT = ^V, (8)
N0 NH + NOt
where NH is a number of intact plates.
Obviously, just as there can be no absolutely reliable technical systems, there cannot be any pavements like that either. In the process of the operation of a pavement with N0 plates, by the time t there is usually NH intact plates and Not failed ones.
Hence
No = NH+Not
is a constant value.
Assumingly, the failed plates are not substituted. A number of failed plates is
Nh = No-Not„
then the expression of reliability can be written as follows:
N - N N
P (t) = No Not = i - Nol , (9)
NN
O O
N
D(t) =
N
(l0)
where D(t) is a pavement damage defined with a ratio of a number of the failed (distresses) plates to a total number of plates on the runway area.
Reliability can also be as follows
P (t) = l - D (x).
(ll)
Fig. 2 gives a graph of the reliability function that illustrates a probable behaviour of a pavement during its operation.
Fig. 2. Reliability and failure of the pavement
We should not forget, though, that during the pavement operation there may come a moment when its reliability is less than Pacceptabie, i. e. there are distressed plates with distresses with the values unacceptable according to the safe flying conditions based on [8, 9].
Hence the expression (11) for the pavement area can be written as follows:
P(t) = 1 - D(x) = 1 > P^. (12)
N 0
The formula (12) allows one to determine the reliability of areas of aerodrome pavements which are classified in Fig. 1.
A degree of a plate’s distress is defined by distresses and their severity. How accurate the assessments of the operational condition are depends on a set of distresses available to be ana-
lyzed. In the above methods of the assessment of the technical condition of a pavement surface, a number of the distresses considered varies from 3 (the method of signal assessment using the index Sk) to 18 (the method of determining the complex quality index KK), which in turn are divided into groups depending on a degree and effect they have on the operational condition. Beyond any doubt, an infinite increase in a number of distresses allows for a more accurate assessment of the technical condition of aerodrome pavements.
However, this approach results in increasing labour costs incurred in the inspection and maintenance of pavement and complexity of mathematical analysis. Other than that, the effect certain distresses have on operational durability (flying safety) is not significant and thus can be left out of consideration. At the same time, minimizing a number of the parameters (n^0) is also unacceptable, since the result obtained does not reflect the actual condition of aerodrome pavements. The paper [11] defines a set of distresses that exert a direct influence on flying safety. They are namely spalling of the edges of the plates, deep raveling, rutting, potholes and depressions, reinforcement stripping, cracking of the edge of the plates, failure of patching material.
Therefore, in order to calculate the reliability of a pavement area, it is necessary to determine a number of distressed plates Not.
A degree of the development of these distresses of a certain plate is estimated from the perspective of the risk theory:
P(t) = 1 - r, (13)
where r is a degree of risk.
A degree of risk of disrupting flying safety due to distresses occurring in aerodrome pavements is evaluated using the ratio [14]
r = 0,5 - O
f:
(14)
where Ha is the actual value of distresses; is a root-mean-square deviation of the actual value of distresses; Hmax is a maximum value of distresses when a probability of non-desired
effects is 50 %; amx is a root-mean-square deviation of the maximum value of distresses; ®(u) is a Laplace function.
Following the results of the statistical calculations, the indices H® and o* are defined based on a sufficient number of measurements of the values of distresses occurring in aerodrome pavements.
The coefficient of the variation of the actual value of a distress C^ is evaluated using the following formula:
CH = j*. (15)
The maximum value of a distress Hmax and its root-mean-square deviation amx is determined using the following:
a = CHmax • H , (16)
max V max ’ V /
where CV m“ is a coefficient of the variation of the maximum value of a distress determined using
CH~ = CH = ^-, (17)
H*
Нmax — 2Ндоп —
Н2оп +
25 (СН- ) -1 (Ніп - 25а2оп)- Н„
--------------, (18)
25 (СНт“ ) -1
where Hacceptabu is a maximum permissible value of a distress; adon is a root-mean-square deviation of a maximum permissible value of a distress.
A root-mean-square deviation of the value of a distress is determined using the following formula:
a = CH^on • h (19)
udon ^V 11 don'
In order to assess the reliability of the pavement in areas A, E, B or r, it is necessary that risks ri of the i-th distress on a certain plate is estimated and the obtained value of the reliability for this area is compared to Pacceptabie using Table 1:
P (t) = 1 - r > P^,. (20)
If the condition (12) is not met, a plate is deemed distressed.
The value of pavement reliability is generally determined using the ratio
P = 1 -^ > PM. (21)
No
Conclusions
1. Linear structures of aerodromes are classified according to a liability degree in compliance to the Federal Law № 384-03 ‘Technical Safety Regulations for Buildings and Structures’.
2. It is proposed that the index of operational durability is used as the indicator of operational and technical condition for time interval of continuous operation of aerodrome pavements T with certain damages of tolerated risks. Their total number may be found in pavements based on flying safety conditions.
3. Theoretical principles of the assessment of reliability of areas of aerodrome pavements using the fundamental principles of the reliability and risk theories.
References
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15. The Federal Law N 384-03 “Technical Regulations on Safety of Buildings and Constructions” (Moscow, 2009) [in Russian].
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