continuosly reinforced concrete pavement design for airport
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Continuously Reinforced Concrete Pavement Design for Airport
1. INTRODUCTION
Rigid pavements can be constructed with no transverse joints, if
adequate reinforcing steel is provided. Continuously reinforced concrete
pavements are defined as those with no transverse joints and with relatively
heavy amount temperature steel to ensure holding the cracks tightly closed.
In continuously rein. slabs, cracks will develop as a result of
several factors .The spacing of cracks varies inversely with percentage of
steel. Thus if high percentage of steel are used, the crack interval is very
small. Even though the crack interval on CRCP is very low, the cracks
requires very little or no maintenance and do not needs to be sealed as often
as cracks on pavements containing lesser amounts of reinforcement.
A properly designed CRCP typically developes regularly
spaced, hair line transverse cracks at 3 to 10 ft (1 to 3m) intervals. The
resultant pavement is composed of series of short slabs held tightly together
by longitudinal rein. A high degree of shear transfer across the cracks is
assured because the cracks are held tightly closed.
The main advantage of CRCP is elimination of transverse
joints which are costly to construct and maintain. CRCP usually provides a
very smooth riding surf ace. Also in channelized traffic areas for heavy jet
aircraft CRCP is particularly justified .This type of design offers high
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Continuously Reinforced Concrete Pavement Design for Airport
potential, particularly in areas where high-quality base materials are scarce.
Continous reinforcement lends additional structural capacity to the pavement.
Although the use of CRCP is widespread in highway
applications, its use for the airport has been relatively limited. The largest
airport application of CRCP present is at an U.S. Air forcefacility in
palmadale, calif. Other CRCP applications include 0' Hare international
Airport and midway Airport, Chicago. In India the CRCP is not provided till
now any where for air ports.
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Continuously Reinforced Concrete Pavement Design for Airport
2. PURPOSE
The purpose of this report is to present a design procedure for
CRCP for airports. The design procedure consist of:
(a) determining CRCP thickness.
(b) determining longitudinal rein.
(c) determining transverse rein. &
(d) determining terminal treatments.
The thickness design procedure is based on the stipulation that
the same slab thickness be used for CRCP as would be determined for plain
jointed concrete pavement. The performance of earlier CRCP designed for
airport use indicates that reduced thickness are not adequate. CRCP
performance at airports has been quite good where the thickness of the CRCP
was comparable to thickness of plain jointed concreted pavements.
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Continuously Reinforced Concrete Pavement Design for Airport
3. MATERIALS
Materials used in the construction of CRCP should conform to
accepted standards as outlined in this chapter.
3.1-REINFORCEMENT: -
For the rein. reqd. for pavement deformed steel reinforcing bars
are to be used. Reinforcement should be specified on the basis of yield
strength. The recommended yield strength of longitudinal reinforcement is
60,000 Psi (414 Mpa) and that of transverse reinforcement is 40,000 Psi
(276Mpa). The deformed bars should conform to ASTM A615,A617 or A706.
3.2-CONCRETE:-
Paving quality concrete should be specified for CRCP for
Airports. Concrete should be specified in terms of the flexural strength and
tested in accordance with ASTM C78.
Flexural strength is specified since the primary action of loaded
concrete pavement slab is flexure, and failure is caused by action of flexure.
Wide variations are encountered in co-relating flexure and compressive
strength, thus it is imperactial to specify a comp. strength for design.
A 90 day flexural strength often is used for design, however the
specified age selected depends on the individual project and anticipated start
of traffic- Mix proportions may be based on an earlier age such as 14 or 28
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Continuously Reinforced Concrete Pavement Design for Airport
days, to avoid long curing times for laboratory specimens .A general thumb
rule often used is that concrete usually will achieve 10% increase in flexural
strength between 28 & 90 days .An Airport pavement normally requires
considerable associated work such as marking, lighting etc .prior to opening to
traffic. Concrete flexural strength on the order of 600 to 750 Psi (4.1 to
5.2Mpa) at 90 days and typically are used for design purpose.
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Continuously Reinforced Concrete Pavement Design for Airport
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Continuously Reinforced Concrete Pavement Design for Airport
4. PAVEMENT THICKNESS DESIGN
Several different airport pavement thickness design procedures
are available .All yields reasonable results, although some small differences in
thickness will be observed due to different basic assumptions and operational
requirements.
4.1.EXAMPLE METHOD:-
The Federal Aviation Administration (FAA) thickness design
method is used in this report .Design curves are available for the said method
for different aircrafts with different gear conditions. These design curves were
extracted directly from FAA advisory circular 150/5320-6C.
Use of these design curves requires input of concrete flexural
strength, gross weight of design aircraft, modulus of subgrade reaction (K-
value) and annual departure level. Each of the design parameter is discussed in
the following.
4.1.1 CONCRETE FLEXURAL STRENGTH:-
As mentioned previously, concrete strength is determined by
flexural testing in accordance with ASTM C78. Normally the 90-day strength
is used for design, however different age may be necessary depending upon
the particular situation.
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Continuously Reinforced Concrete Pavement Design for Airport
4.1.2 MODULUS OF SUBGRADE REACTION (K-VALUE)
A modulus of subgrade reaction (K-value) is a measure of the
stiffness of foundation supporting the concrete pavement .The designed K-
value should be assigned to the top of the layer immediately below the
concrete pavement. The K-value is indicated in units of lb/in3(MN/m3) and
ideally is measured by a plate-loading test.
A stabilised subbase provides the uniform support needed for all
weather conditions, minimises the effect of frost action, provides a stable
working platform for construction operations and reduces the susceptibility of
the foundation or weakening from moisture effects.
4.1.3 DESIGN LOAD:-
Airport traffic usually is comprised of a mixture of several
aircraft having different gear types, wheel loads and wheel spacings. Most
airport pavement design are based on a single design aircraft.
The thickness design method presented in this report uses the
gross weight of the design aircraft as load parameter. Aircraft transmits load
to pavement through their landing gear assemblies. Since it is impossible to
predict precisely what percentages of load will be supported by the nose gear
and main gears, the FAA used the following simplifying assumptions. The
nose gear assembly is assumed to carry 5% of gross weight of aircraft and the
main landing gears supports remaining 95% of gross weight.
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Continuously Reinforced Concrete Pavement Design for Airport
4.1.4.TRAFFIC VOLUME:-
The structural design of CRCP requires consideration of
frequency of traffic as well as magnitude of loads .The design method
presented in this method accomodates five different traffic levels expressed
in terms of annual departures .The design curves assume a 20-years life.
Design for other than a 20-years life can be developed by
calculating the total no. of departures that will accumulate over the desired
design life. The thickness given by the accompanying curves can be related
to the total no of departures that will occur over a 20-years period i.e.
thickness versus annual departures multiplied by 20-years. Using these
data a relationship between thickness and total accumulated depatutres can be
established that can be used to determine thickness requirements for design
lives other than 20-years.
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Continuously Reinforced Concrete Pavement Design for Airport
5. REINFORCEMENT DESIGN
The design of the reinforcement for CRCP is critical for
providing a satisfactory pavement. Rein. design procedures should prevent
overstressing of steel while providing optimum crack spacing and width.
The design of longitudinal rein must satisfy the three conditions
discussed in section 5.1,5.2,5.3. The maximum rein. determined by any of
three following requirements should be selected as the design value. In no case
the longitudinal rein. percentage be less than 0.5% of slab area.
5.1 CRCP DESIGN EQUATION
THE CRCP design equation is used to compute longitudinal
rein .The equation was developed emperically from experience on CRCP for
highway application, the CRCP design equation is Ps = (1.3 - 0.2F) (fr/fs) x
100 ........(1)
Where, Ps = the reqd. % or L-rein.
F = the friction factor.
fr = the tensile strength of cone. Psi.
fs = the allowable working stress for steel Psi.
Suggested values for the input parameters are discussed in the
following.
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Continuously Reinforced Concrete Pavement Design for Airport
fs- As recommanded by packard x treybig, Mccollough x Hudson, the
suggested working stress for steel is 75% of specified minimum yield strength.
fr- should direct tensile strength data be available measured values should be
used. Event direct tensile strength data are not available, it may be
reasonably assumed at 2/3 or fiexural strength. The recommanded value of
2/3 represnts a reasonable average.
F- The friction factor for the subbase is represneted by a single numerical
value that is a gross approximation of a very complex interaction between the
bottom of slab and top or subbase. The friction factor indicates the force
required to slide a slab over the subbase in terms of weight of slab.
Treybig Mccollough and Hudson recommanded the following friction factors
for reindesign.
SUB-BASE TYPE FRICTION FACTOR
Surface treatment 2.2
Lime stabilization 1.8
Asphat stabilization 1.8
Cement stabilization 1.8
River gravel 1.5
Crushed stone 1.5
Sand stone 1.2
Natural subgrade 0.9
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Continuously Reinforced Concrete Pavement Design for Airport
Based on these reports, the friction factor suggested for design is
1.8 for stabilized sub-based which are preferred for CRCP.A Nomograph
solving the CRCP design equation for L-rein is shown in fig.
2. REIN. FOR TEMP. EFFECTS: -
The L-rein must be capable or withstanding the forces
generated by the expansion and contraction of pavement due to temp.
changes. The following formula developed by Mccollough & Ledbetter is
suggested to compute the temp. reinforcement requirements. Ps = 50ft /(Fs-
195T) ....... ..(2)
Where, Ps = percentage rein.
ft = tensile strength of cone. Psi
fs = working stress for steel. Psi
T = Maxm. seasoanl temp. diffrential for pavement.
5.3 STRENGTH RATIO:-
The third consideration in selecting the amount of longitudinal
rein. is the ratio of cone. tensile strength to specified minimum yield strength
of steel. The tensile stresses in cone. and steel are equal in uncracked CRCP
after a crack forms in CRCP the tensile stresses are carried solely by rein. This
redistribution of tensile stresses after cracking requires consideration in
design. As recommended by Treybig & Hudson it can be found out by the
equation developed to accommodate the redistribution of tensile stresses.
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Continuously Reinforced Concrete Pavement Design for Airport
Ps = Ft/Fy x l00......... ..(3)
where, Ps = rein percentage.
Ft = Tensile strength of cone. Psi
fy = Minimum yield strength of steel Psi
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Continuously Reinforced Concrete Pavement Design for Airport
5.4 TRANSVERSE REIN. :-
Tranverse rein is recommanded for CRCP airport pavements
to control longitudinal cracks that sometimes forms due to shrinkage and
loading. It also aids in construction by supporting and maintaining
longitudinal rein spacing. The formula developed by Treybig ,Mccol lough
and Hudson to calculate amount of T-rein is
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Continuously Reinforced Concrete Pavement Design for Airport
Ps = Ws x Fx 50/Fs ...............(4)
Where, Ps = the reqd. % of T-rein.
Ws = Width of paving slab, Ft.
F = Friction factor for sub-base
Fs = Allowable working stress Psi.
The width of slab in equation (4) refers to the width of pavement
that is tied together, not paving lane width.
A nomograph solving the formula for trnasverse rein is shown in fig(l)
5.5 CRACKS:- As the transverse joints in CRCP are eliminatd due to the
loading and another factors causing different types of stresses in slab it will
develope cracks at regular intervals, which are held tightly closed by the
reinforcement. The peformance of CRCP is highly dependent on crack width
crack spacing and the stress in rein. at cracks Mccollough and Noble have
developed limiting criteria for these factors based on the performance of
CRCP for highways in the state of Texas.
5.5.1 CRACK WIDTH :-
SPALLING: - Observations of inservice CRCP highway located in the state
of Texas show a correlation between crack width and spalling. The
maximum crack width recommanded in CRCP to avoid spalling is 0.042 in
(1.07mm) Note that crack width is temperature dependent and
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Continuously Reinforced Concrete Pavement Design for Airport
6. PAVEMENT JOINTING
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Continuously Reinforced Concrete Pavement Design for Airport
Normally two types of construction joints are necessary for
CRCP. Because pavements are constructed in multiple lanes, a longitudinal
constructions joint is required between lanes. A transverse construction joint
must be provided where paving ends and begins. Another type of L-joint
known as weakened plane joint may be required to control warping stresses
when very wide paving lanes are constructed. Transverse rein carried out
through weakened plane joints to provide continuity and aggregate interlock
across the joint.
7. TERMINAL TREATEMENTS
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Continuously Reinforced Concrete Pavement Design for Airport
Since it is possible to construct long slabs of CRCP with no
transverse joints rather large thermally induced end movements should be
anticipated. Wherever end movements may a problem, such where the CRCP
abuts other pavements of structures, provisions must be made for end
movements. Failure to do so may result in damage to the CRCP adjecent
pavement of abutting structure. Treybig, Mccollough and Hidson
recommanded end movement must be restrained accomodated through the use
of anchoragelugs of wide flange beam joints resp.
The details of wide flange beam joint are shown in fig. and is
the type of joint recommanded for this condition. In these instances CRCP
slab length should be limited to about 1000 Ft. (305m). This limiting length
may result in end movement of @3/4m. (20mm) assuming seasonal temp.
variation of 100 0 F (38 0 C)
8. DESIGN EXAMPLE
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Continuously Reinforced Concrete Pavement Design for Airport
An example of the design for CRCP for an airport is given in the
following.
Assume a CRCP is to be designed for 75Ft wide primary
taxiway to meet the following conditions:
-- design aircraft DC 10-10 with a gross weight of 40,0000 lb(182000kg)
-- Foundation modulus 400 lb/m3 (logMN/m3).
-- Concrete fiexural strength 600 Psi (4.2mpa)
-- Annual departures 3000.
-- Minimum spefied yield strength of steel .
1) Longitudinal = 60,000 Psi(414Mpa)
2) Transverse = 40,000 Psi(276Mpa)
-- Paving lane width 25Ft (7.6m) all longitudinal construction joints tied.
-- Cement stabilised subbase - Assumed friction factor = 1.8.
-- Seasoanl temp. differential l00 Ft (38 0 C)
8.1 SLAB THICKNESS:-
Enter the design curve for DC 10-10 aircraft (fig- ) with the
parameters assumed above and read the pavement thickness of 12.2 in
(310mm). This thickness would rounded upto the next half inch to 12.5 in
(320mm).
8.2 Rein. design:-
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Continuously Reinforced Concrete Pavement Design for Airport
A) The longitudinal reinforcement would be designed as described in section-
5.
8.2.1 CRCP DESIGN EQUATION:-
Working stress = 75% x 60,000
= 45,000 Psi (310Mpa)
Friction Factor = 1.8
Tensile strength of conc. = 2/3 x 600 = 400 Psi (2.8mpa)
Solving the CRCP equation (1) with the assumed input parameters yields.
Ps= (1.3 - 0.2 x 1.8) X 400/45000 X 100
Ps= 0.84%
8.2.2 TEMPERATURE:-
The rein reqd. to withstand the forces generated by seasonal
temp. changes is computed using equation (2) given in section 5.2 which
yields. Ps = 50 X 400/(45000 - 195 X 100)
= 0.78%
8.2.3 STRENGTH RATIO:-
The strength ratio between concrete and steel is computed by the
procedure given in s/c5.3.
Ps = (400/60,000) x 100
= 0.67%
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Continuously Reinforced Concrete Pavement Design for Airport
B)TRANSVERSE REINFORCEMENT:-
The transverse reinforcement is determined using equation (4)
from s/c 5.4 Ps = 75x 1.8 x 50/30,000
= 0.23%
8.3 FINAL DESIGN:-
The final design a 12.5 in (120mm) thick conc. slab. The
CRCP design equation controls the L-rein percentage and the value of 0.84%
is selected for design using fig. 8 rein bars spaced at 7.5m (190mm) on centre
are used for the longitudinal reinforecement. The transverse reinforcement
reqd. is 0.23% which can be met by using 4 bars on 7 in (17 7mm) centres.
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Continuously Reinforced Concrete Pavement Design for Airport
CONCLUSION
Though construction cost of this pavment is high , this give
durability, life, low maintenances. If taken into number of year consideration
this pavment is good. It also works for takeoff and landing of high fuel jet.
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Continuously Reinforced Concrete Pavement Design for Airport
9. CONVERSIONS
The unit of different quantities used in report are different from
SI units so to convert them in SI unit following conversion factors can be
used.
1) 1inch = 25.4mm
2) 10 Ft = 3.05m
3) 1 in 2 = 645.16mm 2
4) 1 Psi = 6.89 kpa.
5) 1 Rsi = 6.89 Mpa.
6) 1 Pci = 0.272 MN/m 3
7) l lbs = 0.454 Kg.
10. REFERENCES
l. Airport planning and designing
By S. K. Khanna & M. G. Arora
2. Airport Engineering
By Venketeppa Rao.
3. Principles of Pavement design.
By Yoder
4. Design of Highway Pavements (Including Airport Pavements)
By S. K. Sharma.
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Continuously Reinforced Concrete Pavement Design for Airport
The CRCP design equation is
Ps = (1.3 - 0.2F) (fr/fs) x 100
Where, Ps = the reqd. % or L-rein.
F = the friction factor.
fr = the tensile strength of cone. Psi.
fs = the allowable working stress for steel Psi.
The following formula developed to compute the
temp. reinforcement requirements.Ps = 50ft /(Fs-195T)
Where, Ps = percentage rein.
ft = tensile strength of cone. Psi
fs = working stress for steel. Psi
T = Max. seasonal temp. differential
for pavement.
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Continuously Reinforced Concrete Pavement Design for Airport
Ps = Ft/Fy x l00
where, Ps = rein percentage.
Ft = Tensile strength of cone. Psi
fy = Minimum yield strength of steel
Psi
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Continuously Reinforced Concrete Pavement Design for Airport
Ps = Ws x Fx 50/Fs
Where, Ps = the reqd. % of T-rein.
Ws = Width of paving slab, Ft.
F = Friction factor for sub-base
Fs = Allowable working stress Psi.
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