Design Procedure AASHTO

8/6/2019 Design Procedure AASHTO

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DESIGN PROCEDURE of AASHTO

Step 1: Existing pavement design.

(1) Existing slab thickness

(2) Type of load transfer (mechanical devices, aggregate interlock, CRCP)(3) Type of shoulder (tied, PCC, other)

Step 2: Traffic analysis.

(1) Past cumulative 18-kip ESALs in the design lane (N p, for use in the remaining life methodof D eff determination only

(2) Predicted future 18-kip ESALs in the design lane over the design period (N r )

Step 4: Deflection testing (strongly recommended).

When designing an unbonded overlay for existing JPCP, JRCP, or CRCP, follow theguidelines given below for deflection testing and determination of the effective static k-valueWhen designing an unbounded overlay for existing AC/PCC, follow the guidelines given inSection 5 7, Step 4, for deflection testing and determination of the effective static k-valueMeasure slab deflection basins in the outer wheel path along the project at an intervalsufficient to adequately assess conditions Intervals of 100 to 1,000 feet are typical Measuredeflections with sensors located at 0, 12, 24, and 36 inches from the centre of load A heavy-load deflection device (e g , Falling Weight Deflectometer) and a load magnitude of 9,000

pounds are recommended ASTM D 4694 and D 4695 provide additional guidance ondeflection testing For each slab tested, backcalculate the effective k value using Figure 5.10

or a backcalculation procedure

The AREA of each deflection basin is computed from the following equation.

Where

d0= deflection in centre of loading plate, inches

di = deflections at 12, 24, and 36 inches from plate centre, inches

AREA will typically range from 29 to 32 for sound concrete.

(1) Effective dynamic k-value Enter Figure 5.10 with d 0 and AREA to determine theeffective dynamic k-value beneath each slab for a circular load radius of 5.9 inches andmagnitude of 9,000 pounds NOTE that for loads within 2,000 pounds more or less,deflections may be scaled linearly to 9,000-pound deflections.


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If a single overlay thickness is being designed for a uniform section, compute the meaneffective dynamic k-value of the slabs tested in the uniform section.

(2) Effective static k-value

Effective static k-value = Effective dynamic k-value/2

Step 5: Determination of required slab thickness for future traffic (D f ).

The elastic modulus, modulus of rupture, and load transfer inputs to determine Dr for unbonded PCC overlays of PCC and AC/PCC pavements are representative of the new PCCoverlay to be placed rather than of the existing slab This is emphasized because it is the

properties of the overlay slab (i.e., elastic modulus, modulus of rupture, and load transfer),which will control the performance of the unbonded overlay.

1) Effective static k-value beneath the existing pavement Determine from one of thefollowing method.

a) B ackcalculate the effective dynamic k value from deflection basins as described inStep-4 Divide the effective dynamic k-value by 2 to obtain the effective static k-value.

b) Conduct plate load tests (ASTM D 1196) after slab removal at a few sites. Thisalternative is very costly and time-consuming and not often used.

2) Design PSI loss.

The serviceability of a pavement is defined as its ability to serve the type of traffic(automobiles and trucks) which use the facility The primary measure of serviceability is thePresent Serviceability Index (PSI), which ranges from 0 (impossible road) to 5 (perfect road)The basic design philosophy of this Guide is the serviceability-performance concept, which

provides a means of designing a pavement based on a specific total traffic volume and aminimum level of serviceability desired at the end of the performance period.

Selection of the lowest allowable PSI or terminal serviceability index (P t) is based on thelowest index that will be tolerated before rehabilitation, resurfacing, or reconstruction

becomes necessary. An index of 2.5 or higher is suggested for design of major highways and2.0 for highways with lesser traffic volumes.

For relatively minor highways where economics dictate that the initial capital outlay be keptat a minimum, it is suggested that this be accomplished by reducing the design period or thetotal traffic volume, rather than by designing for a terminal serviceability less than 2 0 Sincethe time at which a given pavement structure reaches its terminal serviceability depends ontraffic volume and the original or initial serviceability (P 0), some consideration must also begiven to the selection of P 0 (It should be recognized that the P 0 values observed at theAASHO Road Test were 4.2 for flexible pavements and 4.5 for rigid pavements ).PSIimmediately after overlay (P1) minus PSI at time of next rehabilitation (P2).


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Once P 0 and P t are established, the following equation should be applied to define the totalchange in serviceability index.

Where,

P0 (Original or Initial serviceability)

Pt (Lowest or Terminal serviceability index)

3) J, load transfer factor for joint design of the unbonded PCC overlay.

The load transfer coefficient, J, is a factor used in rigid pavement design to account for theability of a concrete pavement structure to transfer (distribute)

load across discontinuities, such as joints or cracks Load transfer devices, aggregateinterlock, and the presence of tied concrete shoulders all have an effect on this value

Generally, the J-value for a given set of conditions ( e.g. , jointed concrete pavement with tiedshoulders) increases as traffic loads increase since aggregate load transfer decreases with loadrepetitions.

The value of J recommended for a plain jointed pavement (JCP) or jointed reinforcedconcrete pavement (JRCP) with some type of load transfer device (such as dowel bars) at the

joints is 3.2 ("protected corner" condition at the AASHO Road Test) This value is indicativeof the load transfer of jointed pavements without tied concrete shoulders.

For jointed pavements without load transfer devices at the joints, a J-value of 3.8 to 4.4 isrecommended (This basically accounts for the higher bending stresses that develop in

undowelled pavements, but also includes some consideration of the increased potential for faulting). If the concrete has a high thermal. Coefficient, then the value of J should beincreased On the other hand, if few heavy trucks are anticipated such as a low-volume road,the J-value may be lowered since the loss of aggregate interlock will be less.

The value of J recommended for continuously reinforced concrete pavements (CRCP)without tied concrete shoulders is between 2 9 to 3 2, depending on the capability of aggregate interlock (at future transverse cracks) to transfer load In the past, a commonly usedJ-value for CRCP was 3 2, but with better design for crack width control each agency shoulddevelop criteria based on local aggregates and temperature ranges.

One of the major advantages of using tied PCC shoulders (or widened outside lanes) is thereduction of slab stress and increased service life they provide To account for this,significantly lower J-values may be used for the design of both jointed and continuous

pavements For continuously reinforced concrete pavements with tied concrete shoulders (theminimum bar size and maximum tie bar spacing should be the same as that for tie bars

between lanes), the range of J is between 2.3 and 2.9, with a recommended value of 2.6 Thisvalue is considerably lower than that for the design of concrete pavements without tiedshoulders because of the significantly increased load distribution capability of concrete


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pavements with tied shoulders For jointed concrete pavements with dowels and tiedshoulders, the value of J should be between 2.5 and 3.1 based on the agency's experience Thelower J-value for tied shoulders assumes traffic is not permitted to run on the shoulder.

4) PCC modulus of rupture of unbonded PCC overlay.

The modulus of rupture (flexural strength) of Portland cement concrete is required only for the design of a rigid pavement The modulus of rupture required by the design procedure isthe mean value determined after 28 days using third-point loading (AASHTO T 97, ASTM C78) If standard agency practice dictates the use of centre-point loading, then a correlationshould be made between the two tests.

B ecause of the treatment of reliability in this Guide, it is strongly recommended that thenormal construction specification for modulus of rupture (flexural strength) not be used asinput, since it represents a value below which only a small percent of the distribution may lieIf it is desirable to use the construction specification, then some adjustment should beapplied, based on the standard deviation of modulus of rupture and the percent (PS) of thestrength distribution that normally falls below the specification:

Where,

S'c = estimated mean value for PCC modulus of rupture (psi),

Sc = construction specification on concrete modulus of rupture (psi),

SD s = estimated standard deviation of concrete modulus of rupture (psi),

z = standard normal variate

=0.841, for PS = 20 percent,

= 1.037, for PS = 15 percent,

= 1.282, for PS = 10 percent,

= 1.645, for PS = 5 percent, and

= 2.327, for PS = 1 percent

5) Elastic modulus of unbonded PCC overlays :

Although there are many types of material properties and laboratory test procedures for assessing the strength of pavement structural materials, one has been adopted as a basis for design in this Guide If, however, the user should have a better understanding of the "layer coefficients" that have traditionally been used in the original AASHTO flexible pavementdesign procedure, it is not essential that the elastic moduli of these materials be characterizedIn general, layer coefficients derived from test roads or satellite sections are preferred.


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Elastic modulus is a fundamental engineering property of any paving or roadbed material For those material types which are subject to significant permanent deformation under load, this

property may not reflect the material's behaviour under load Thus, resilient modulus refers tothe material's stress-strain behaviour under normal pavement loading conditions The strengthof the material is important in addition to stiffness, and future mechanistic-based procedures

may reflect strength as well as stiffness in the materials characterization procedures Inaddition, stabilized base materials may be subject to cracking under certain conditions and thestiffness may not be an indicator for this distress type It is important to note, that, althoughresilient modulus can apply to any type of material, the notation MR as used in this Guideapplies only to the roadbed soil Different notations are used to express the moduli for sub-

base (E SB ), base (E B S), asphalt concrete (E AC), and Portland cement concrete (E c).

The procedure for estimating the resilient modulus of a particular pavement material dependson its type Relatively low stiffness materials, such as natural soils, unbound granular layers,and even stabilized layers and asphalt concrete, should be tested using the resilient modulustest methods (AASHTO T 274) Although the testing apparatus for each of these types of materials is basically the same, there are some differences, such as the need for triaxialconfinement for unbound materials.

Alternatively, the bound or higher stiffness materials, such as stabilized bases and asphaltconcrete, may be tested using the repeated-load indirect tensile test (ASTM D 4123).This teststill relies on the use of electronic gauges to measure small movements of the sample under load, but is less complex and easier to run than the triaxial resilient modulus test.

B ecause of the small displacements and brittle nature of the stiffest pavement materials, i e,Portland cement concrete and those base materials stabilized with a high cement content, it isdifficult to measure the modulus using the indirect tensile apparatus Thus, it is recommendedthat the elastic modulus of such high-stiffness materials be determined according to the

procedure described in ASTM C 469.

The elastic modulus for any type of material may also be estimated using correlationsdeveloped by the state's department of transportation or by some other reputable agency Thefollowing is a correlation recommended by the American Concrete Institute (4) for normalweight Portland cement concrete.

Where,

Ec = PCC elastic modulus (in psi)

f'c = PCC compressive strength (in psi) as determined using AASHTO T 22, T 140, or ASTM

C 39

6) Loss of support (LS) for unbonded PCC overlay:


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This factor, LS, is included in the design of rigid pavements to account for the potential lossof support arising from sub-base erosion and/or differential vertical soil movements It istreated in the actual design procedure by diminishing the effective or composite k-value

based on the size of the void that may develop beneath the slab Table 2 7 provides somesuggested ranges of LS depending on the type of material (specifically its stiffness or elastic

modulus) Obviously, if various types of base or sub-base are to be considered for design, thenthe corresponding values of LS should be determine for each type.

The LS factor should also be considered in terms of differential vertical soil movements thatmay result in voids beneath the pavement Thus, even though a non-erosive sub-base is used,a void may still develop, thus reducing pavement life Generally, for active swelling clays or excessive frost heave, LS values of 2.0 to 3.0 may be considered Each agency's experience inthis area should, however, be the key element in the selection of an appropriate LS valueExamination of the effect of LS on reducing the effective k-value of the roadbed ,soil mayalso be helpful in selecting an appropriate value.

For unbonded PCC overlay. LS=0

7) Overlay design reliability, R (percent)

The selection of an appropriate level of reliability for the design of a particular facilitydepends primarily upon the projected level of usage and the consequences (risk) associatedwith constructing an initially thinner pavement structure If a facility is heavily trafficked, itmay be undesirable to have to close or even restrict its usage at future dates because of thehigher levels of distress, maintenance, and rehabilitation associated with an inadequate initialthickness On the other hand, a thin initial pavement (along with the heavier maintenance andrehabilitation levels) may be acceptable, if the projected level of usage is such that fewer

conflicts can be expected. Reliability Identifying by the present value or equivalent uniformannul cost, total cost, Future cost, initial cost and Various Functional Classifications

Suggested Levels of Reliability For Various Functional Classifications

Functional Classifications

Recommended Level of Reliability

Urban RuralInterstate and Others Freeways 85 - 99.9 80 - 99.9

Principal Arterials 80 - 99 75 - 95Collectors 80 - 95 75 - 95

Local 50 - 80 50 - 80

8) Overall standard deviation (S 0) for rigid pavement:

The estimated overall standard deviations for the case where the variance of projected futuretraffic is considered (along with the other variances associated with the revised pavement

performance models) are 0.39 for rigid pavements and 0.49 for flexible pavements.


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The estimated overall standard deviations for the case when the variance of projected futuretraffic is not considered (and the other variances associated with the revised pavement

performance models are 0.34 for rigid pavements and 0.44 for flexible pavements).

9) Drainage coefficient (C d):

The treatment for the expected level of drainage for a rigid pavement is through the use of adrainage coefficient, Cd, in the performance equation (It has an effect similar to that of theload transfer coefficient, J ) As a basis for comparison, the value for Cd for conditions at theAASHO Road Test is 1.0

Table 2.5 provides the recommended Cd values, depending on the quality of drainage and the percent of time during the year the pavement structure would normally be exposed tomoisture levels approaching saturation. As before, the latter is dependent on the averageyearly rainfall and the prevailing drainage conditions.

Recommended Values of Drainage Coefficient, CdPavement Design

Quality of DrainagePercent of Time Pavement Structure is Exposed

to Moisture Levels Approaching Saturation

Less Than 1 % 1 - 5 % 5 - 25 % Greater Than 25 %

Excellent 1.25 - 1.20 1.20 - 1.15 1.15 - 1.10 1.1

Good 1.20 - 1.15 1.15 - 1.10 1.10 - 1.00 1

Fair 1.15 - 1.10 1.10 - 1.00 1.00 - 0.90 0.9

Poor 1.10 - 1.00 1.00 - 0.90 0.90 - 0.80 0.8

Very Poor 1.00 - 0.90 0.90 - 0.80 0.80 - 0.70 0.7

Deff From Remaining Life For PCC Pavements:

The remaining life of the pavement is given by the following equation:

Where,

RL = remaining life, percent

NP = total traffic to date, ESALs

N1.5 = total traffic to pavement "failure," ESALs

N 1.5 may be estimated using the new pavement design equations or nomographs in Part II To be consistent with the AASHO Road Test and the development of these equations, a "failure"PSI equal to 1.5 and a reliability of 50 percent are recommended.


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Deff determined from the following equation:

Where,

CF = condition factor determined from Figure 5.2D = thickness of the existing slab, inches

(NOTE: maximum D for use in unbonded concrete overlay design is 10 incheseven if the existing D is greater than 10 inches)

Step 5: Determination of Overlay Thickness.

The thickness of unbonded PCC overlay is computed as follows:

Where

Dol = Required thickness of unbonded PCC overlay, inches

Df = Slab thickness determined, inches

Deff = Effective thickness of existing slab determined, inches

The thickness of overlay determined from the above relationship should be reasonable whenthe overlay is required to correct a structural deficiency.

Step 6: Joints

Transverse and longitudinal joints must be provided in the same manner as for new pavementconstruction, except for the following joint spacing guidelines for JPCP overlays Due to theunusually stiff support beneath the slab, it is advisable to limit joint spacing to the followingto control thermal gradient curling stress :

Maximum joint spacing (feet) = 1.75 * Slab thickness (inches)

Example: slab thickness = 8 inches

joint spacing = 8 * 1.75 = 14 feet

Design Procedure AASHTO

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