continuosly reinforced concrete pavement design for airport

8/14/2019 continuosly reinforced concrete pavement design for airport

1/27

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

C.O.E.& T.,Akola


2/27


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.

C.O.E.& T.,Akola


3/27


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.

C.O.E.& T.,Akola


4/27


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

C.O.E.& T.,Akola


5/27


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.

C.O.E.& T.,Akola


6/27


C.O.E.& T.,Akola


7/27


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.

C.O.E.& T.,Akola


8/27


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.

C.O.E.& T.,Akola


9/27


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.

C.O.E.& T.,Akola


10/27


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.

C.O.E.& T.,Akola


11/27


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

C.O.E.& T.,Akola


12/27


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.

C.O.E.& T.,Akola


13/27


Ps = Ft/Fy x l00......... ..(3)

where, Ps = rein percentage.

Ft = Tensile strength of cone. Psi

fy = Minimum yield strength of steel Psi

C.O.E.& T.,Akola


14/27


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

C.O.E.& T.,Akola


15/27


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

C.O.E.& T.,Akola


16/27


17/27


6. PAVEMENT JOINTING

C.O.E.& T.,Akola


18/27


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

C.O.E.& T.,Akola


19/27


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

C.O.E.& T.,Akola


20/27


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:-

C.O.E.& T.,Akola


21/27


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%

C.O.E.& T.,Akola


22/27


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.

C.O.E.& T.,Akola


23/27


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.

C.O.E.& T.,Akola


24/27


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.

C.O.E.& T.,Akola


25/27


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.

C.O.E.& T.,Akola


26/27


Ps = Ft/Fy x l00

where, Ps = rein percentage.

Ft = Tensile strength of cone. Psi

fy = Minimum yield strength of steel

Psi

C.O.E.& T.,Akola


27/27


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.

continuosly reinforced concrete pavement design for airport

Documents