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 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

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.

C.O.E.& T.,Akola

Continuously Reinforced Concrete Pavement Design for Airport

C.O.E.& T.,Akola

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 (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.

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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 Kvalue 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

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. C.O.E.& T.,Akola

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 C.O.E.& T.,Akola

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

C.O.E.& T.,Akola

that

crack width is temperature

dependent

and

Continuously Reinforced Concrete Pavement Design for Airport

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 the performance of CRCP by causing foundation erosion and for corrosion of the reinforcement. Crack widths greater than 0.025 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 widths greater than 0.025 in (0.63mm) will probably not occur simultaniously with every occasion of significant surface water. 5.5.2 CRACK SPACING :Spalling:- Limiting crack spacing to no more than 8Ft. (2.6m) should with a 90% confidence level restrict the incidence of spalled cracks to less than 40% limiting crack. Spacing to no more than 6Ft.(2m) restrict the incidence of spalled cracks to less than 30% however the confidence level also drops to 84%. A lower limit of crack spacing is required to achieve full bond between steel and cone. Theoretical calculations show that full bond can be achieved at a minimum crack spacing on the order of 3Ft. (1m) A lower limit on crack spacing is also required to ensure slab continuity. Theoretical analysis show crack spacing on the order of 4Ft.(1.3m) is required for slab continuity.

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Continuously Reinforced Concrete Pavement Design for Airport

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

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 (380 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 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 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 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 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

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.

C.O.E.& T.,Akola

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

C.O.E.& T.,Akola

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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