Unbonded JPCP Overlay Design of Existing JPCP

A unbonded JPCP overlay is a rehabilitation option for existing JPCP. For this design type, a jointed plain concrete overlay is placed over an existing JPCP PCC layer (intact or fractured with an appropriate AC separation layer) or composite pavement (AC over JPCP).

Unbonded jointed plain concrete overlays over existing JPCP intact PCC slab do not require much pre-overlay repairs. This is because, the 1 to 2 inches AC separator layer placed between the overlay PCC and existing PCC (1) separates the movements in the existing and overlay PCC layers and (2) prevents distresses in the existing JPCP PCC from reflecting into the overlay. Full contact friction between the overlay PCC and AC separator layer is assumed over the design life by PMED.

General Design Inputs

A Unbonded JPCP Overlay Design of Existing JPCP is an existing JPCP pavement rehabilitated with a new, unbonded JPCP surface.

The following options allow you to establish the basic parameters of your project.

Ways to Access this Interface

Open a new pavement project. The General Information area appears at the top-left corner of the Project Tab.

Figure 648: General Information

Populating the Inputs for this Interface

Design type: Select Overlay
Pavement type: Select JPCP over JPCP (unbonded)

In addition to selecting Design and Pavement type, select values for the following basic design parameters:

Design life (years): This control allows you to select from a list the period of time in years from overlay placement where the rehabilitated pavement is expected to perform adequately without significant loss of functional and structural integrity. Pavement performance is predicted over the design life beginning from the month the rehabilitated pavement is open to traffic.
Existing construction: This control allows you to select the month and year when the construction of existing JPCP was completed.
Pavement construction: This control allows you to select the month and year when the overlay is scheduled to be placed.
Traffic opening: This control allows you to select the month and year the rehabilitated pavement is scheduled to be open to traffic. PMED predicts pavement performance beginning from this month and year.

Performance Criteria

Performance verification forms the basis for determining the adequacy of a trial design. Adequacy for PMED implies that the trial design meets all performance criteria at a user or agency selected level of reliability within the trial pavements design life or the PMED analysis period. If the trial design after analysis does produce distress/smoothness levels in excess of the user or agency threshold levels, then the trial design must be modified to produce more acceptable levels of distress and smoothness. Trial design modification includes changes to later material types and properties and JPCP design features. In general, trial design site properties such as climate, traffic, and foundation/subgrade type cannot be altered to improve design adequacy.

The distress types considered in JPCP design are transverse “slab” cracking and transverse joint faulting. In addition, PMED predicts transverse joint spalling which is used solely as an input for predicting smoothness characterized using the International Roughness Index (IRI).

Figure 649: Performance Criteria

Populating the Inputs in this Interface

This table allows you to define the limits or threshold values of distresses and smoothness that are used by PMED to define performance. The limits of distresses and smoothness must be specified at a user/agency reliability level to make them meaningful. The limits of distresses and smoothness must be seen as upper boundary levels within an acceptable range. PMED goes not require lower boundary levels for distresses as they are assumed to be zero immediately after JPCP construction. For smoothness/IRI, however, the lower boundary level is never zero and mostly ranges from 30 to 70 in/mi. As the smoothness/IRI lower boundary level cannot be assumed to be zero, PMED allows for user-specific input for this parameter, which it calls initial IRI.

This table has three columns:

Performance Criteria: This column provides a list of distresses and smoothness required by PMED to assess adequacy of unbonded concrete overlay design.
Limit: This column allows you to define the limit or threshold values of the distresses and smoothness provided.
Reliability: This column allows you to define the probability that the trial design (combination of climate, materials, design features, and so on) will perform as predicted by PMED over the design life during which it is subjected to a given level of traffic loading.

Note:
You can override the program defaults to enter project-specific distress and smoothness limits or threshold values at user/agency specified reliability values based on representing agency policies and guidelines for unbonded concrete overlay design. Refer to Chapter 8 of the AASHTO Manual of Practice for more guidance on selecting distress/smoothness limits and reliability level.

Initial IRI (in. /mile): The limit control allows you to define the level of smoothness immediately after overlay construction (expressed in terms of IRI). Initial IRI is a very important input as the time from initial construction to attaining threshold IRI value is very much dependent on the initial IRI obtained at the time of construction. Thus, the initial IRI value provided must be what is typically attained in the field. You can override the PMED default value of 63 in. /mi to reflect agency policy and guidelines.
Terminal IRI (in. /mile): The limit and reliability controls allow you to define the threshold value for IRI at the end of the design life at a user or agency specified reliability level.
Mean joint faulting (in.): The limit and reliability controls allow you to define the threshold value for transverse joint faulting at the end of the design life at a user or agency specified reliability level. Transverse joint faulting is the differential elevation across the transverse joint measured approximately 1 ft from the slab edge (longitudinal joint for a conventional lane width), or from the rightmost lane paint stripe for a widened slab. Since joint faulting varies significantly from joint to joint, the mean faulting of all transverse joints in a pavement section is the parameter predicted by the PMED.
JPCP transverse cracking (percent slabs): The limit and reliability controls allow you to define the threshold value for transverse cracking at the end of the design life at a user or agency specified reliability level. PMED predicts the combined percentage of PCC slabs with bottom-up and top-down transverse cracks that occurs mostly in the middle third of the slab. It is reported as percent slabs cracked. A description of the two main transverse cracking mechanisms are described below:
Bottom-up transverse cracking: When the truck axles are near the longitudinal edge of the slab, midway between the transverse joints, a critical tensile bending stress occurs at the bottom of the slab under the wheel load. This stress increases greatly when there is a high positive temperature gradient through the slab (the top of the slab is warmer than the bottom of the slab). Repeated loadings of heavy axles under those conditions result in fatigue damage along the bottom edge of the slab, which eventually result in a transverse crack that propagates to the surface of the pavement.
Top-down transverse cracking: Repeated loading by heavy truck tractors with certain axle spacing when the pavement is exposed to high negative temperature gradients (the top of the slab cooler than the bottom of the slab) result in fatigue damage at the top of the slab, which eventually results in a transverse or diagonal crack that is initiated on the surface of the pavement. The critical wheel loading condition for top-down cracking involves a combination of axles that loads the opposite ends of a slab simultaneously. In the presence of a high negative temperature gradient, such load combinations cause a high tensile stress at the top of the slab near the critical pavement edge. This type of loading is most often produced by the combination of steering and drive axles of truck tractors and other vehicles. Multiple trailers with relatively short trailer-to-trailer axle spacing are other common sources of critical loadings for top-down cracking.

Pavement Structure

This section explains adding additional layers to a pavement, editing the properties of those layers, and working with the pavement structure as a whole.

PCC (New JPCP) Layer

Portland cement concrete (PCC) basically consists of portland cement, water, and fine and coarse aggregates. In general, PCC properties such as early and long-term strength, elastic modulus, shrinkage, thermal expansion, durability, and so on depend on the quantities and qualities of its components. The most popular types of cement used for pavement construction is Types I, II, and III portland cements and these cement types provide adequate levels of strength and durability. Some special applications such as the need to open a pavement early for traffic may require the use of special cements to provide higher levels of properties (e.g., high-early strength cements) or the use of blended cements with aggregates susceptible to alkali-aggregate reactions. It is essential that pavement designers select the type of cement and aggregates that will obtain the best performance from the concrete and thus pavement. This choice involves the correct knowledge of the relationship between cement and aggregates properties and PCC performance.

The key PCC materials inputs required by PMED are:

Flexural strength (MR)
Elastic modulus (EPCC)
Coefficient of thermal expansion (CTE)
Ultimate shrinkage
Concrete mix properties (e.g. cement type, cement content, aggregate type, etc.)

These inputs are required for predicting pavement responses to applied loads, long-term strength and elastic modulus, and effect of climate (temperature, moisture, and humidity) on PCC expansion and contraction.

Interface

Figure 650: PCC Inputs

Methods to Populate Inputs

There are two methods for entering data for the Portland Cement Concrete layer:

Manual entry
Import from workspace library

Inputs

PCC

Thickness: This control allows you to define the thickness of the PCC layer used in the trial design. Note that this input has a significant impact on PMED prediction of future JPCP performance.
Unit weight: This control allows you to define the weight per unit volume of the PCC mix design. Typical values ranges from 140 to 160 pcf.
Poisson’s ratio: This control allows you to define the PCC’s Poisson’s ratio (defined as the ratio of PCC lateral and longitudinal strain under applied loading). Poisson’s ratio is an important input required for structural analysis. Values between 0.15 and 0.18 are typical PCC used in JPCP construction.

Figure 651: PCC Inputs

Thermal

PCC coefficient of thermal expansion (in/in/deg F x 10-6): This control allows you to define the expansion or contraction a material undergoes with change in temperature. PCC coefficient of thermal expansion (CTE) is defined as the increase in length per unit length of PCC (typically 1-in) for a unit increase in temperature (typically 1 ˚F). PCC CTE has a significant impact on critical curling stresses developed in the JPCP PCC slab and thus the development of transverse cracks. PCC CTE can be considered to be a very critical input that influences the development of transverse cracking.
PCC thermal conductivity (BTU/hr-ft-deg F): This control allows you to define the ability of the PCC material to uniformly conduct heat through its thickness when subjected to a temperature differential at the top surface and bottom of the PCC layer. PMED provides a typical value of 1.25 BTU/hr-ft-˚F.
PCC heat capacity (BTU/lb-deg F): This control allows you to define the amount of heat required to raise a unit mass of PCC material (typically 1 Ib) by a unit temperature (typically 1 ˚F). PMED provides a typical value of 0.28 BTU/hr-ft-deg F.

Figure 652: PCC Thermal

Mix

Cement type: This control allows you to select the type of cement used in the PCC mix. PMED allows you to select one of the following Portland cement types:
- Type I: Type I cements are described as general purpose cements suitable for most pavement construction situations where special properties such as high early strength, high durability, etc., are not required.
- Type II: Type II cements contain no more than 8% tricalcium aluminate (C3A) and are suitable for situations where moderate sulfate resistance is required.
- Type III: Type III cements have constituents very similar to Type I cements. However, the constituents are ground to produce a finer mix that produces higher levels of early strengths. Thus, Type II cements are generally used in JPCP construction where early strength is required.

Note:
For PCC mixes with non-conventional cement types, select the cement type provided by PMED which is closest in characteristics.

Cementitious material content (lb/yd3): This control allows you to define the total weight of cementitious material (portland cement, pozzolans, lime, fly-ash, etc.) per unit volume of PCC as per the PCC mix design. The cementitious material along with water content is used to calculate PCC ultimate shrinkage, a critical parameter in structural analysis.
Water to cement ratio: This control allows you to define the ratio of the weight of water to the weight of all cementitious materials used in the PCC mix design. This input is used to calculate the water content of the PCC mix and is used along with cementitious material content to calculate PCC ultimate shrinkage, a critical parameter in structural analysis.
Aggregate type: This control allows you to select the coarse aggregate rock type in the PCC mix. If the coarse aggregate rock type is not uniform, the predominant rock type must be selected. Coarse aggregate is typically described as the material the material retained on the No. 4 sieve. Coarse aggregate rock type is used to calculate PCC ultimate shrinkage. PMED allows you select one of the following coarse aggregate rock types:
- Quartzite: This option allows you to select quartzite as the predominant coarse aggregate rock type in the PCC mix. Quartzite is a metamorphic rock which is extremely resistant to weathering and erosion.
- Limestone: This option allows you to select limestone as the predominant coarse aggregate rock type in the PCC mix. Limestone is a sedimentary rock composed largely calcium carbonate and provides a solid base for highway pavements.
- Dolomite: This option allows you to select dolomite as the predominant coarse aggregate rock type in the PCC mix. Dolomite is a sedimentary carbonate rock composed mainly of calcium magnesium carbonate. Dolomite like limestone provides a solid base for highway pavements.
- Granite: This option allows you to select granite as the predominant coarse aggregate rock type in the PCC mix. Granite is an intrusive, felsic, igneous rock and is extremely resistant to weathering and erosion.
- Rhyolite: This option allows you to select rhyolite as the predominant coarse aggregate rock type in the PCC mix. Rhyolite is an igneous, volcanic (extrusive) rock and is extremely resistant to weathering and erosion.
- Basalt: This option allows you to select basalt as the predominant coarse aggregate rock type in the PCC mix. Basalt is an extrusive igneous rock and is extremely resistant to weathering and erosion.
- Syenite: This option allows you to select syenite as the predominant coarse aggregate rock type in the PCC mix. Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts. Syenite is extremely resistant to weathering and erosion.
- Gabbro: This option allows you to select gabbro as the predominant coarse aggregate rock type in the PCC mix. Gabbro is an intrusive mafic igneous rock, rich in iron and magnesium, poor in silica (quartz) and chemically equivalent to basalt. Gabbro is extremely resistant to weathering and erosion.
- Chert: This option allows you to select chert as the predominant coarse aggregate rock type in the PCC mix. Chert is a sedimentary rock widely used in highway pavement construction. It is moderately resistant to weathering and erosion.
PCC set temperature (deg F): This control allows you to define the PCC temperature when the freshly placed PCC slab becomes solid. PCC set temperature can be user entered or estimated by PMED using (1) average of hourly ambient temperatures for month of construction and (2) cementitious material content. The default for this control is to let PMED calculate the PCC set temperature. You can, however, override the PMED default and enter your own PCC set temperature.
- Calculated internally? This control allows you to decide whether PMED should calculate PCC set temperature or not by selecting one of the following options:
- True: Selecting this option allows PMED to calculate the PCC set temperature.
- False: This option allows you to input directly PCC set temperature.
- User-specified PCC set temperature: This control allows you define the PCC set temperature in degrees Fahrenheit (minimum 70, maximum 212).
Ultimate shrinkage (microstrain): This control allows you to define the ultimate (long-term) shrinkage strain that PCC will develop under laboratory controlled conditions of temperature and humidity (e.g., 40% relative humidity). The default for this control is to let PMED calculate PCC ultimate shrinkage internally. You can, however, override the PMED default and enter your own PCC ultimate shrinkage.
- Calculated internally? This control allows you to decide whether PMED should calculate PCC ultimate shrinkage or not by selecting one of the following options:
- True: Selecting this option allows PMED to calculate the PCC ultimate shrinkage.
- False: This option allows you to input directly PCC ultimate shrinkage.
- User-specified PCC ultimate shrinkage: This control allows you define the PCC ultimate shrinkage in microstrain (minimum 300, maximum 1000).
Reversible Shrinkage (%): Change in PCC humidity/moisture affects the volume of PCC with moisture causes PCC expansion while drying causes PCC shrinkage. Note that this is very different from the drying shrinkage in PCC due to moisture loss during PCC hardening that result in irreversible shrinkage. This control allows you to define the percent of PCC ultimate shrinkage that can be recovered due to changes in PCC humidity/moisture (minimum 0, maximum 100).
Time to develop 50% of ultimate shrinkage (days): This control allows you to define the time required in days to develop 50% of PCC ultimate shrinkage (minimum 30, maximum 50).
Curing method: This control allows you to select curing method applied after PCC placement. PMED allows you to select one of the following options:
- Wet Curing: These are curing methods that prevent PCC surface moisture loss by supplying additional water to the PCC surface. The most common practice is to place wet burlap or other natural or synthetic-fiber blankets at the PCC surface to prevent rapid evaporation of moisture. Selecting this option implies the use of the above listed methods or similar techniques for PCC curing.
- Curing Compound: These are curing methods that prevent moisture loss at the PCC surface by placing a liquid membrane-forming chemical compound (typically resins, waxes or synthetic rubbers dissolved in a solvent) that forms a near-impermeable membrane over the PCC once the solvent evaporates. Selecting this option implies the use of the above listed methods or similar techniques for PCC curing.

Figure 653: PCC Inputs

Strength

PMED requires both PCC modulus of rupture and elastic modulus.

PCC strength input level: This control allows you to select modulus of rupture and elastic modulus input level. PMED allows for the following options:
- Level 1: This option allows you to directly input the 7-, 14-, 28-, and 90-day PCC elastic modulus and modulus of rupture. For this option, the 20-yr and 28-day PCC elastic modulus and modulus of rupture ratios are also required.
- Level 2: This option allows you to directly input the 7-, 14-, 28-, and 90-day PCC compressive strength. For this option, the 20-yr and 28-day PCC compressive strength ratio is also required.
- Level 3: This option allows you to directly input any of the following combination of inputs:
  - 28-day PCC modulus of rupture only.
  - 28-day PCC compressive strength only.
  - 28-day PCC modulus of rupture and 28-day PCC elastic modulus.
  - 28-day PCC compressive strength and 28-day PCC elastic modulus.
Level 1 Modulus of Rupture/Elastic Modulus Table: This table allows you to define PCC modulus of rupture and PCC elastic modulus at specified ages as follows:
- PCC Age: This column lists the ages for which PCC modulus of rupture and PCC elastic modulus are required. The ages specified by PMED cannot be changed.
- Modulus of Rupture (psi): This column allows you to define PCC modulus of rupture.
- Elastic Modulus (psi): This column allows you to define PCC elastic modulus.

Figure 654: PCC Strength 1

Level 2 Compressive Strength Table: This table allows you to define PCC compressive strength at specified ages as follows:
- PCC Age: This column lists the ages for which PCC modulus of rupture and PCC elastic modulus are required. The ages specified by PMED cannot be changed.
- Compressive strength (psi): This column allows you to define PCC compressive strength.

Figure 655: PCC Strength 2

Level 3 Modulus of Rupture, Compressive Strength, and Elastic Modulus Table: This control allows you to define your preferred combination of PCC strength and elastic modulus inputs at Level 3. PMED allows for the following combination of inputs by checking the appropriate radio button and check box:
- 28-day PCC modulus of rupture only.
- 28-day PCC compressive strength only.
- 28-day PCC modulus of rupture and 28-day PCC elastic modulus.
- 28-day PCC compressive strength and 28-day PCC elastic modulus.

Figure 656: PCC Strength 3

Note:
PMED uses transform equations to internally compute PCC modulus of rupture and PCC elastic modulus using the Level 2 and 3 PCC strength inputs. Regardless of the level of input, early age modulus and strength along with 20-yr to 28-day ratios are used to estimate monthly values throughout the pavements design life.