continuosly reinforced concrete pavement design for airport

Continuously Reinforced Concrete Pavement Design for Airport

1. INTRODUCTION Rigid pavements can be constructed with no transverse joints, if adequa adequate te reinfo reinforci rcing ng steel steel is provid provided. ed. Contin Continuou uously sly reinfo reinforce rced d concre concrete te 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. steel. Thus if high percenta percentage ge of steel are used, used, the crack interval interval is very small. small. Even though the crack interval interval on CRCP

is very low, the cracks cracks

requires very little or no maintenance maintenance and do not needs to be sealed sealed as often as cracks on pavements containing lesser amounts of reinforcement. A prop proper erly ly desi design gned ed CRCP CRCP typi typica call lly y deve develo lope pess regu regula larl rly y spaced spaced,, hair hair line transve transverse rse cracks cracks at 3 to 10 ft

(1 to 3m) interval intervals. s. 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 main advantag advantagee of CRCP is elimin eliminati ation on of

transv transvers ersee

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 airc aircra raft ft CRCP CRCP is part partic icul ular arly ly just justif ifie ied d .Thi .Thiss type type of desi design gn offe offers rs high high

C.O.E.& T.,Akola


potential, particularly particularly in areas where high-quality base materials are scarce. scarce. Continous reinforcement lends additional structural capacity to the pavement. Altho lthoug ugh h the the use use of CRCP CRCP is widesp desprread ead in highw ighway ay applications, its use for the airport has been relatively limited. The largest airp airpor ortt appl applic icat atio ion n of CRCP CRCP pres presen entt is at an U.S. U.S. Air Air forc forcef efac acil ilit ity y in palma palmada dale, le, calif. calif. Other Other CRCP CRCP applic applicati ations ons includ includee 0' Hare Hare intern internati ationa onall Airport and midway Airport, Chicago. In India the CRCP is not provided till now any where for air ports.

C.O.E.& T.,Akola


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 airp airpor ortt use use indi indica cate tess that that redu reduce ced d thic thickn knes esss are are not not adeq adequa uate te.. CRCP 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


3. MATERIALS Materials used in the construction of CRCP should conform to accepted standards as outlined in this chapter. 3.1-REINFORCEMENT: 3.1-REINFORCEMEN T: -

For the rein. reqd. for pavement deformed steel reinforcing bars are to be used. used. Reinfo Reinforce rcemen mentt should should be speci specifie fied d on the basis basis of yield yield strength. strength. The recommended recommended yield yield strength of longitudinal longitudinal reinforce reinforcement ment is 60,000 Psi (414 Mpa) and that of transverse transverse reinforcem reinforcement ent is 40,000 Psi (276Mpa). The deformed bars should conform to ASTM A615,A617 or A706. 3.2-CONCRETE:-

Paving Paving quality quality concrete concrete should should be specified specified for CRCP for Airports. Airports. Concrete Concrete should should be specifie specified d in terms of the flexural strength strength and tested in accordance with ASTM C78. Flexural strength is specified since the primary action of loaded concrete concrete pavement pavement slab is flexure, flexure, and failure failure is caused caused by action of flexure. flexure. Wide Wide variat variation ionss are encoun encounter tered ed in co-rel co-relati ating ng flexur flexuree and and compre compressi ssive ve strength, thus it is imperactial to specify a comp. strength for design. A 90 day flexural strength often is is used for design, 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


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 strength between between 28 & 90 days .An Airport Airport pavement pavement normally normally requires requires considerable associated work such as marking, lighting etc .prior to opening to traffi traffic. c. Concrete Concrete flexur flexural al strengt strength h 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


C.O.E.& T.,Akola


4. PAVEMENT THICKNESS DESIGN Several different different airport airport pavement thickness design procedures 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 .Design curves are available available for the said 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 (Kvalue) 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


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 Kvalue value should should be assign assigned ed to the the top of the layer layer immed immediat iately ely below below the 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 construction operations and reduces reduces the susceptibility of the foundation or weakening from moisture effects. 4.1.3 DESIGN LOAD:-

Airport Airport traffic traffic usually usually is comprised comprised of a mixture mixture of several 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 weight of the design design aircraft as load parameter. Aircraft transmits 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


4.1.4.TRAFFIC VOLUME:-

The The stru struct ctur ural al desi design gn of CRCP CRCP requ requir ires es cons consid ider erat atio ion n of freque frequency ncy of traffic traffic

as well well as magnit magnitude ude of loads .The .The design design method method

presented in this method accomodates five different traffic levels expressed in terms of annual departures .The design curves assume a 20-years life. Desi Design gn for for othe otherr than than a 20-y 20-yea ears rs life life can can be deve develo lope ped d by calculating the total no. of departures that will accumulate over the desired design life. life. The thickness given by the accompanying accompanying curves can be related related to the total total no of departu departures res

that that will occur occur over a 20-yea 20-years rs period period i.e.

thickness versus annual departures multiplied by 20-years.

Using these

data a relationship between between thickness and total accumulated accumulated depatutres can can be established that can be used to determine thickness requirements for design lives other than 20-years.

C.O.E.& T.,Akola


5. REINFORCEMENT DESIGN The The desi design gn of the the rein reinfo forc rcem emen entt for for CRCP CRCP is crit critic ical al for for providing a satisfactory pavement. Rein. design procedures should prevent overstressing of steel while providing providing optimum crack spacing spacing and width. The design of longitudinal rein must satisfy the three conditions discussed discussed in in section section 5.1,5.2,5 5.1,5.2,5.3. .3. The maximum maximum rein. rein. determ determined ined by any 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 design equati equation on is used to compute compute longitudin longitudinal al rein .The equation was developed developed emperically emperically from experience experience on CRCP for highway application, application, the CRCP design equation equation is Ps = (1.3 - 0.2F) 0.2F) (fr/fs) x 100 ........(1) Where,

Ps = the reqd. % or L-rein. F = the friction friction factor. 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


fsfs- As reco recomm mman ande ded d by pack packar ard d x trey treybi big, g, Mcco Mccoll llou ough gh x Huds Hudson on,, the 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 used..

Even Eventt direc directt tensi tensile le stre streng ngth th data data are not avai availa labl ble, e, it may be

reasonably assumed at 2/3 or fiexural fiexural strength. strength.

The recommanded 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 bottom of slab and top or subbase. subbase. The friction friction factor factor indicates indicates the force 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 C.O.E.& T.,Akola

0.9


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 The

L-re L-rein in must must be capa capabl blee or with withst stan andi ding ng the the forc forces es

generated generated by the expansion expansion and contracti contraction on of pavement pavement due to temp. temp. changes. changes. The The following following formul formulaa developed developed by Mccollou Mccollough gh & Ledbetter Ledbetter is 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

=

Max Maxm. seasoan soanll temp. emp. diff diffre rent ntiial for for pavem avemen entt.

5.3 STRENGTH RATIO:-

The third consideration consideration in selecting selecting the amount amount of longitudinal longitudinal rein. is the ratio of cone. tensile strength to specified minimum yield strength of steel. steel. The tensile stresses in cone. cone. and steel are equal in uncracked CRCP after a crack forms in CRCP the tensile stresses are carried solely by rein. This redist redistrib ributi ution on of tensil tensilee stres stresses ses after after crack cracking ing requir requires es consid considera eratio tion n 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


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

Ps = rein percentage. Ft = Tensile strength strength of cone. Psi fy = Minimum yield strength of steel steel Psi

C.O.E.& T.,Akola


5.4 TRANSVERSE REIN. :-

Tranverse rein is recommanded for CRCP airport pavements to control longitudinal cracks that sometimes forms due to shrinkage and load loadin ing. g. It also also aids aids in cons constr truc ucti tion on by supp suppor orti ting ng and and main mainta tain inin ing g longitudinal rein spacing. The formula developed by Treybig ,Mccol lough and Hudson to calculate amount of T-rein is C.O.E.& T.,Akola


Ps = Ws x Fx 50/Fs ...............(4) Where,

Ps = the reqd. % of T-rein. Ws = Width of paving slab, Ft. F = Friction Friction factor 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 factors causing causing different different types of stresses stresses in slab it will develope develope cracks cracks at regular regular interv intervals, als, which which are are held held tightly tightly closed closed by the the reinforcement. The peformance of CRCP is is highly dependent on crack crack width crack spacing and the stress in rein. at cracks Mccollough and Noble have developed developed limiting limiting criteri criteriaa

for these these factors based based on the performanc performancee of

CRCP for highways in the state state of Texas. 5.5.1 CRACK WIDTH :SPALLING: - Observations of inservice CRCP highway located in the state

of

Texas Texas

show show a correl correlati ation on betwee between n cra crack ck width width and spalli spalling. ng.

The

maximum crack width recommanded in CRCP to avoid spalling is 0.042 in (1.07m (1.07mm) m)

Note Note

C.O.E.& T.,Akola

that that

crack crack width width is tempe temperat rature ure

depend dependent ent

and


recommended (in CRCP to avoid spalling) crack width of 0.042 in (1.02mm) is maximum value. Water infiltration: - the infiltration of water into a CRCP through cracks can

affect affect the perfo performa rmance nce of CRCP CRCP by causin causing g founda foundatio tion n erosio erosion n and for corr corros osio ion n of the the rein reinfo forc rcem emen ent. t. Crac Crack k widt widths hs grea greate terr than than 0.025 0.025 inch inch (0.63mm) are quite permeable and allow substantial quantities of water to infiltrate the pavement. How ever as mentioned earlier crack width is temp. dependent and crack crack widths widths greater greater than than 0.025 in (0.63mm) (0.63mm) will probably probably not occur occur simultaniously with every occasion of significant surface water. 5.5.2 CRACK SPACING :-

Spalling:- Limiting crack spacing to no more than 8Ft. 8Ft. (2.6m) should with a 90% confidenc confidencee level level restrict restrict the incidence incidence of spalled spalled cracks cracks to less than 40% limiting crack. Spacing to no more than 6Ft.(2m) restrict the incidence of spalled cracks to less less than 30% however however the the confidence confidence level level also drops to 84%. A lower limit limit of crack spacing spacing is required to achieve full full bond between steel and cone. cone. Theoretical calculations calculations show show that full bond can be achieved at a minimum minimum crack spacing on the the order of 3Ft. (1m) A lower limit on crack crack spacing is also required required to ensure slab continuity. continuity. Theoretical Theoretical analysis analysis show crack spacing on the order of 4Ft.(1.3m) is required for slab continuity.

C.O.E.& T.,Akola


6. PAVEMENT JOINTING C.O.E.& T.,Akola


Normal Normally ly two types types of constr construct uction ion joints joints are necess necessary ary for CRCP. Because pavements are constructed in multiple lanes, a longitudinal constructions joint is required between between lanes. lanes. A transverse transverse construction construction joint 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 wide paving lanes lanes are constructe constructed. d. Transvers Transversee rein carried carried out through through weakened weakened plane joints joints to provide provide continuit continuity y and aggregate aggregate interlock across the joint.

7. TERMINAL TREATEMENTS

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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 abut abutss othe otherr pave paveme ment ntss of stru struct ctur ures es,, prov provis isio ions ns must must be made made for for end end movements movements.. Failure Failure to do so may result result in damage to the CRCP adjecent adjecent pav pavem emen entt

of

abut abutti ting ng

stru struct ctur ure. e.

Trey Treybi big, g,

Mcco Mccoll llou ough gh

and and

Hids Hidson on

recommanded end movement must be restrained accomodated through the use of anchoragelugs of wide flange beam joints resp. The details details of wide flange flange beam joint joint are shown shown in fig. and is the type of joint recommanded for this condition. In these instances CRCP slab length should be limited limited to about 1000 Ft. Ft. (305m). (305m). This This limiting length may result result in end movement movement of @3/4m. @3/4m. (20mm) assuming assuming seasonal seasonal temp. temp. variation of 100 0 F (38 0 C)

8. DESIGN EXAMPLE

C.O.E.& T.,Akola


An example of the design for CRCP for an airport is given in the following. Assu Assume me a CRCP CRCP is to be desi design gned ed for for 75Ft 75Ft wide wide prim primar ary y 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 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 (380 C) 8.1 SLAB THICKNESS:-

Enter Enter the design design curve curve for DC 10-10 10-10 aircraft aircraft (fig- ) with with the par param amet eter erss assu assume med d abov abovee and and read read the the pave paveme ment nt thic thickn knes esss of 12.2 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


A) The longitudinal reinforcement would be designed as described in section5. 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 withstand the forces forces generated generated by seasonal 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


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 final

design design a 12.5 12.5 in (120mm (120mm)) thick thick conc conc.. slab. slab. The

CRCP design equation controls the L-rein percentage percentage and the value of 0.84% is selected for design using using fig. 8 rein bars spaced at 7.5m (190mm) on centre 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


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


9. CONVERSIONS The unit of different quantities used in report are different from SI units units so so to convert convert them in SI unit following following conversio conversion n factors factors can be be used. 1)

1inch

= 25.4mm

2)

10 Ft

= 3.05m

3)

1 in2

= 645.16mm2

4)

1 Psi

= 6.89 kpa.

5)

1 Rsi

= 6.89 Mpa.

6)

1 Pci

= 0.272 MN/m3

7)

l lbs

= 0.454 Kg.

10. REFERENCES l. Airport planning and designing By S. K. Khanna & M. G. Arora 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


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.

Thee foll Th follow owin ing g form formul ula a de deve velo lope ped d to comp comput utee th thee 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 te tem mp. differential tial for pavement.

C.O.E.& T.,Akola


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


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 sub-base Fs = Allowable working stress Psi.

C.O.E.& T.,Akola

continuosly reinforced concrete pavement design for airport

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