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Concrete Overlay Design

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The design of concrete overlays is similar to that of traditional concrete pavements with two exceptions: the existing pavement structure and the requirement to satisfy bonding criteria (for bonded concrete overlays). The first step to design of concrete overlays is existing pavement characterization, followed by the selection of a thickness design procedure. Special considerations for each overlay type are also discussed. Miscellaneous design details for concrete overlays are on a separate page.

Existing Pavement Characterization

Figure 1. Overlay Design Factors

The condition of the existing pavement structure is one of the main factors affecting the design of concrete overlays. Effectively characterizing the existing pavement condition is crucial to not only the design of concrete overlays but also to the selection of the type of concrete overlay. As indicated by Figure 1[1], existing pavement characterization is an integral design factor.

At a minimum, it is recommended that the existing pavement be verified with cores or historical records.[1] Field verification of historical records is important as historical records often do not represent the in-place pavement. In addition to field verification of historical records, the existing condition of the pavement should also be evaluated as it will influence the type of overlay and identify any repairs required prior to the overlay. Characterization of the existing pavement is discussed in the following sections.

Surface Considerations

The existing pavement thickness and condition will dictate whether the concrete overlay will be bonded or unbonded. For a bonded overlay of asphalt to be practical, a minimum asphalt thickness (in fair condition) of 3 inches (75 millimeters) should remain in-place after any milling and/or pre-overlay repairs. Improper or insufficient evaluation of existing pavement thickness can result in improper assumptions regarding structural contribution of the existing pavement.

Evaluation of the existing pavement condition is required to determine the type and extent of any pre-overlay repairs that will be required. Significant base failures may be present if the existing pavement is in poor condition with many localized failures. For this situation, an unbonded overlay, which is less sensitive to the underlying pavement condition, may be more cost effective than a bonded overlay. Random cracks in the existing pavement do not necessarily lead to a reduction in service life of the overlay.[1]

Structural Considerations

Whether the overlay is bonded or unbonded, the structural condition of the existing pavement should be assessed. A common method for structural evaluation is deflection testing, particularly with the falling weight deflectometer. Coring is recommended for all overlay projects and is useful for determining pavement layer thickness, presence of asphalt stripping, and/or petrographic examination for material characterization. The condition of the asphalt layer in a composite pavement is a good indicator of the underlying concrete pavement condition.

In addition to the structural condition of the existing pavement, the degree of support beneath the structural layers should also be defined. The modulus of subgrade reaction (k-value) is the parameter commonly used to quantify support beneath a concrete pavement. Typically the k-value is defined immediately beneath the concrete pavement; however, this is not always the case for overlays. Overlays typically account for structural contributions from the existing pavement (asphalt, concrete, or composite) and therefore the k-value is defined for those materials beneath the existing pavement.

Traffic Characterization

The quantification of traffic over the design life of the pavement is required for all thickness design procedures. Traffic is commonly described by equivalent single axle loads (ESALs) or load spectra (a distribution of traffic loading). The average annual daily traffic and the percentage of trucks are the minimum traffic parameters required for pavement design. When discrete segments of the project have special traffic generators, it may be advisable to adjust the thickness design for those discrete segments. In addition to quantifying the existing traffic, reasonable growth factors should be incorporated into the traffic count. See Pavement Thickness Design Factors for more information regarding traffic characterization.

Material Properties

Concrete strength is a key design input, particularly flexural strength. The designer should verify that the design strength is achievable with locally available materials. The modulus of elasticity of the concrete is another material property input for most design programs. The modulus of elasticity is a critical parameter for bonded overlays whereas it is of secondary importance for unbonded overlays. The use of macro fibers provides enhanced toughness and ductility of the concrete which may result in reduced thicknesses for bonded overlays.

Another material property that is mainly used in mechanistic-based design procedures is the coefficient of thermal expansion (CTE). The CTE is a controlling factor in selecting slab dimensions and is more critical for bonded overlays than unbonded overlays. The CTE of the bonded overlay aggregates should be similar or less than that of the existing concrete pavement to ensure similar thermal movements thereby reducing stress on the bond between the two layers.

Climatic Factors

Similar to the design of traditional concrete pavements, climate affects the durability and stresses of concrete overlays. The significant difference is the relative thickness; bonded overlays are more susceptible to unfavorable weather conditions that can affect the concrete's ability to prevent freezing, excessive heat buildup, or retain moisture. The designer should be cognizant of the increased susceptibility to climatic factors and select materials accordingly. Joints and load-transfer systems should be designed to minimize stresses from curling and/or warping as well as seasonal temperature changes.

Thickness Design Selection

Several thickness design procedures are available for design of bonded overlays or unbonded overlays. The Guide to the Design of Concrete Overlays Using Existing Methodologies[2] provides detailed guidance on the use of the Bonded Concrete Overlay on Asphalt Thickness Designer[3], Guide for Design of Pavement Structures 4th edition[4], and Mechanistic-Empirical Design Guide - A Manual of Practice [5]. The designer should take into consideration the condition of the existing pavement at the time of construction, which could be years from the initial design and commencement of design work. Further degradation of the existing pavement condition should be expected and reflected in the design analysis.

Design Methodologies Background

Table 1 is a summary of the most common design methodologies for concrete overlays, a list of which can be found below. Summarized in Table 1 are typical design inputs and pertinent information for the various design procedures. Two critical aspects to concrete overlay design are (1) how the procedure treats the bond between existing pavement and the concrete overlay and (2) whether the procedure considers the existing pavement as contributing significant structural capacity to the overlay or contributing to the quality of the pavement foundation. Table 1 is intended to help designers select the appropriate design methodology for each project. The Guide to Concrete Overlays Third Edition[1] provides a detailed summary of each design methodology and its intended application.

  1. Bonded Concrete Overlay on Asphalt (BCOA) Thickness Designer [3]
  2. BCOA ME [6]
  3. Guide for Design of Pavement Structures. 4th edition. Implemented with WinPas [4]
  4. Mechanistic-Empirical Design Guide - A Manual of Practice [5]
  5. StreetPave [7]
  6. OptiPave V2.0 [8]
  7. Flowable Fibrous Concrete for Thin Pavement Inlays [9]
  8. Illinois DOT's spreadsheet for bonded concrete inlay/overlay of asphalt design [10]


Table 1. Summary of Current Overlay Design Software (from Reference [1]).

Overlay Type Typical Design and Software Parameters
Traffic (Millions of ESALs) Typical Concrete Slab Thickness Maximum Joint Spacing (ft) Range of Condition of Existing Pavement Macro-Fibers Option (in software) Transverse Joint Dowel Bars *Mainline Longitudinal Tie Bars Recommended Design Procedure
Bonded Concrete Overlay of Asphalt Pavement Up to 15 3 - 6 in. 1.5 times thickness (in.) Fair to Good Yes No No 1[3], 2[6], 8[10]
Bonded Concrete Overlay of Concrete Pavement Up to 15 3 - 6 in. Match existing cracks and joints and cut intermediate joints Fair to Good Yes No No 3[4], 4[5], 5[7]
Bonded Concrete Overlay of Composite Pavement Up to 15 3 - 6 in. 1.5 times thickness (in.) Fair to Good Yes No No 1[3], 2[6], 8[10]
Thin Fibrous Overlays of Asphalt Pavements Up to 15 2 - 3 in. 4 - 6 ft. Fair to Good Yes No No 7[9]
Unbonded Concrete Overlay of Asphalt Pavement Up to 100 4 - 11 in. Slab < 6 in. - 1.5 times thickness (in.)

Slab ≥ 6 in. - 2.0 times thickness (in.) Slab > 7 in. - 15 ft.

Deteriorated to Fair Yes For slabs > 7 in. T ≥ 6 in. - use agency standards 3[4], 4[5], 5[7]
Unbonded Concrete Overlay of Concrete Pavement Up to 100 4 - 11 in. Slab < 5 in. - 6 x 6 ft. panels

Slab 5 - 7 in. - 2.0 times thickness (in.) Slab > 7 in. - 15 ft.

Deteriorated to Fair Yes For slabs > 7 in. T ≥ 6 in. - use agency standards 3[4], 4[5], 5[7]
Unbonded Concrete Overlay of Composite Pavement Up to 100 4 - 11 in. Slab < 6 in. - 1.5 times thickness (in.)

Slab ≥ 6 in. - 2.0 times thickness (in.) Slab > 7 in. - 15 ft.

Deteriorated to Fair Yes For slabs > 7 in. T ≥ 6 in. - use agency standards 3[4], 4[5], 5[7]
Unbonded Short-jointed Concrete Slabs Up to 100 > 3 in. 4 - 8 ft. Poor to Fair Yes For slabs > 7 in. For ≥ 3.5 in. slabs at tied concrete shoulders or for T ≥ 6 in. - use agency standards 6[8]

Design Considerations for Bonded Overlay Systems

Bonded concrete overlays rely on the integrity of the underlying pavement and the bond with the overlay. The bond reduces tensile stresses in the overlay by transferring horizontal shear stresses to the underlying pavement. Design considerations for bonded concrete overlays are dependent upon the underlying pavement type: asphalt, composite, or concrete.

Bonded Overlays of Asphalt and Composite Pavements - Design Considerations

Three modes of failure have been identified for bonded concrete overlays of asphalt or composite pavements: corner breaks, failure of the bond plane, or failure of the underlying asphalt. Therefore, bonded concrete overlay design methods must account for: concrete corner stresses, bond plane stresses, and asphalt strains. These stresses and strains are then compared to pertinent fatigue models and the governing failure mode is determined.

Designers can optimize the joint spacing of the bonded concrete overlay by understanding how bond plane stresses and corner stresses interact. Joint spacing is always 6 feet or less in either direction. Joints must be cut as quickly as possible to minimize the development of curling stresses. Curling stresses lead to delamination at the edge and can cause failure of the bond plane.

Bonded Overlays of Concrete Pavements - Design Considerations

The first step in design of a bonded overlay on concrete is determining the required thickness of a new concrete pavement. The equivalent pavement thickness is then calculated considering the remaining life of the existing pavement. The difference between the new and equivalent pavement thicknesses is the required overlay thickness. Bonded concrete overlays can typically provide a minimum service life of 15 years when properly constructed. The common failure method is delamination at the bond plane followed by fatigue failure.

The bond between the new and existing pavement is critical. Bond strength is typically not an issue given proper construction means and methods. However, temperature differentials in excess of 30F within the first few days can result in delamination failures. Also crucial to maintenance of the bond is minimizing curling and warping stresses. These can be minimized by thorough curing and minimizing relative humidity and temperature differentials between the two layers.

The jointing of the bonded concrete overlay must match that of the underlying pavement. Transverse joints should be cut full depth and be at least as wide as the crack/joint in the underlying pavement, see figure below. Longitudinal joints should be cut to the mid-depth of the overlay, or greater. Steel reinforcement and dowels are not commonly used in bonded concrete overlays.

Figure 2. Jointing of Bonded Concrete Overlay of Concrete Pavement

Design Considerations for Unbonded Overlay Systems

Unbonded concrete overlays are essentially designed as new concrete pavements. The underlying pavement is considered as a base course. These overlays have design lives in the range of 20-30 years.

Load transfer of unbonded concrete overlays is typically greater than that of conventional concrete pavements. The underlying pavement provides additional load transfer. This additional load transfer should be considered in addition to the intended load transfer system of the overlay.

Unbonded Overlays of Concrete Pavements - Design Considerations

Figure 3. "Keying" (top) and elimination of "keying" with asphalt separation layer (bottom)

The first design consideration is the suitability of the existing concrete pavement as a base layer. There are three conditions which result in the existing concrete pavement being unsuitable:

  • Subgrade and/or subbase problems, e.g. unstable
  • Widespread materials-related distress, especially those causing movement of the pavement
  • Movement of the entire pavement has resulted in severe deterioration

An interlayer (i.e. separation layer) is a key component to unbonded overlays of concrete pavements. The interlayer separates the two concrete pavements and acts as a stress relief layer. Additionally, the interlayer serves to minimize reflective cracking, allowing the two pavements to move independently, and preventing reflective faulting (i.e. "keying") as shown in the figure to the right.

Thin asphalt layers, 1 inch, are commonly used as separation layers. The asphalt layer should not be used as a leveling course to correct grade; grade corrections should be done with the overlay. Thick asphalt layers are discouraged as no structural contribution from the asphalt separation layer is considered. Base asphalt mixtures are recommended to provide additional friction during concrete placement. If the overlay is poorly drained, the asphalt separation layer can scour (strip). In this scenario, a porous asphalt mixture is recommended to reduce the pore pressure and increase stability.

Another potential separation layer is a geotextile. Observations from MnROAD testing of various nonwoven geotextiles indicates an effect of geotextile thickness on physical and audible properties of concrete overlays. Table 2 provides typical values and guidance for specifying geotextiles as separation layers. Table 3 provides a summary of geotextile material properties. There are two major considerations with regards to the use of geotextiles as separation layers: (1) suitability of the existing pavement and (2) the material properties, particularly color. If the existing pavement exhibits significant faulting, or faulting is expected to occur, a geotextile will not provide adequate cushion between the existing pavement and the overlay; "keying" will likely occur. The color of the geotextile can help maintain concrete placement temperatures; black will absorb heat for cool-weather placements and white will reflect heat for hot-weather placements.

Figure 4. Restraint at joint intersection: round dowels (left) and plate dowels (right)

Expansion joints of the existing pavement must be matched in the overlay to prevent blowups. Joint spacings are reduced as a result of the high stiffness of the underlying concrete pavement. Joints are not commonly doweled for thicknesses less than 7 inches. However, before dowels are used, the designer should consider shorter joint spacings with the use of fibers. The use of dowel bars in thin overlays (< 7 inches) creates constructibility issues. Smaller diameter dowels are required which results in an increased bearing stress, sometimes leading to premature socketing. Plate dowels have been introduced to provide adequate clearance between the dowel and paving machine for thin concrete pavement applications. Additionally, plate dowels allow independent movement of adjacent panels with less shrinkage restraint than round dowels, allowing them to be placed closer to joint intersections (see figure to the right).


Table 2. Interlayer Fabric (from Reference [1]).

≤ 4-in. overlay - consider 13 oz/yd2, typical thickness 130 mils ≥ 5-in. overlay - consider 15 oz/yd2, typical thickness 170 mils Weather resistance > 60%
2 kPa - 120-150 mils 2 kPa - 155-185 mils Test tensile strength after 500 hours of UV accelerator
20 kPa - 80-110 mils 20 kPa - 110-140 mils Tensile strength after test must be at least 60% of initial strength
200 kPa - 20-50 mils 200 kPa - 40-70 mils

Table 3. Geotextile Separation Layer Material Properties (from Reference [1]).

Property Requirements Test Procedure
Geotextile Type Nonwoven, needle-punched, no thermal treatment to include calendaringa EN 13249, Annex F (Certification)
Color Uniform/nominally same color fibers (Visual Inspection)
Mass per unit area ≥ 450 g/m2 (13.3 oz/yd2)b

≥ 500 g/m2 (14.7 oz/yd2)
≤ 550 g/m2 (16.2 oz/yd2)

ISO 9864 (ASTM D 5261)
Thickness under load (pressure) [a] At 2 kPa (0.29 psi): ≥ 3.0 mm (0.12 in.)

[b] At 20 kPa (2.9 psi): ≥ 2.5 mm (0.10 in.)
[c] At 200 kPa (29 psi): ≥ 0.10 mm (0.04 in.)

ISO 9863-1 (ASTM D 5199)
Wide-width tensile strength ≥ 10 kN/m (685 lb/ft) ISO 10319 (ASTM D 4595)
Wide-width maximum elongation ≤ 130 percent ISO 10319 (ASTM D 4595)
Water permeability in normal direction under load (pressure) ≥ 1 x 10-4 m/s (3.3 x 10-4 ft/s) at 20 kPa (2.9 psi) DIN 60500-4 (modified ASTM D5493)
In-plane water permeability (transmissivity) under load (pressure) [a] ≥ 5 x 10-4 m/s (1.6 x 10-4 ft/s) at 20 kPa (2.9 psi)

[b] ≥ 2 x 10-4 m/s (6.6 x 10-4 ft/s) at 200 kPa (29 psi)

ISO 12958 (ASTM D6574)b or

ISO 12958 (modified ASTM D4716)

Weather resistance Retained strength ≥ 60 percent EN 12224 (ASTM D 4355 @ 500 hrs exposure for grey, white, or black material only)
Alkali resistance ≥ 96 percent polyproplene/polyethylene EN 13249, Annex B (Certification)

a Calendering is a process that passes the geotextile through one or more heated rollers during the manufacturing process. The surface of the geotextile is modified during this process. Calendering may reduce the absorption properties of the geotextile on the calendered side.
b Added to the Materials Specifications for overlays.

Unbonded Overlays of Asphalt Pavements - Design Considerations

Unbonded concrete overlays can be placed directly on rutted or crowned asphalt. The thickest concrete coincides with the locations of the highest loads on the pavement which caused rutting of the asphalt. If the rutting is deep, the sawed joint depth should be adjusted accordingly.

The variable concrete thickness results in a challenge when payment is on a square-yard basis. Therefore, contract documents should specify payment for placement and materials separately. Payment for placement should be on a square-yard basis. Payment for materials should be on a cubic-yard basis. Separate payment methods limits the risk to contractors and additional costs to the owner.

Unbonded Overlays of Composite Pavements - Design Considerations

Unbonded concrete overlays of composite pavements are similar to those of concrete pavements, with one difference. Composite pavements, by definition, already contain an asphalt layer. The design consideration is whether the existing asphalt surface is suitable as a separation layer. If not, the asphalt should be milled and resurfaced with the required asphalt layer.

Related Pages

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 "Guide to Concrete Overlays - Sustainable Solutions for Resurfacing and Rehabilitating Existing Pavements (Third Edition)," National Concrete Pavement Technology Center, ACPA Publication TB021.03P, Ames, IA, 2014, http://www.cptechcenter.org/technical-library/documents/Overlays_3rd_edition.pdf.
  2. "Guide to the Design of Concrete Overlays Using Existing Methodologies", National Concrete Pavement Technology Center, 2012, http://www.cptechcenter.org/technical-library/documents/Overlays_Design_Guide_508.pdf
  3. 3.0 3.1 3.2 3.3 "BCOA Thickness Designer", American Concrete Pavement Association, Accessed 5 January 2017, http://apps.acpa.org/applibrary/BCOA/
  4. 4.0 4.1 4.2 4.3 4.4 4.5 "Guide for Design of Pavement Structures. 4th ed.", American Association of State Highway and Transportation Officials, 1993.
  5. 5.0 5.1 5.2 5.3 5.4 5.5 "AASHTOWare Pavement ME Design", American Association of State Highway and Transportation Officials, Accessed 5 January 2017, http://www.aashtoware.org/Pavement/Pages/default.aspx
  6. 6.0 6.1 6.2 "BCOA-ME Design Guide", Vandenbossche, J.M., Accessed 5 January 2017, http://www.engineering.pitt.edu/Sub-Sites/Faculty-Subsites/J_Vandenbossche/BCOA-ME/BCOA-ME-Design-Guide/
  7. 7.0 7.1 7.2 7.3 7.4 "StreetPave - Structural Design Software for Street and Road Concrete Pavements", American Concrete Pavement Association, Accessed 5 January 2017, http://www.acpa.org/streetpave/
  8. 8.0 8.1 "OptiPave2", TCPavements, Accessed 5 January 2017, http://www.tcpavements.cl/esp/software
  9. 9.0 9.1 "Flowable Fibrous Concrete for Thin Concrete Inlays", Bordelon, A. and J. Roesler, 2011, http://ascelibrary.org/doi/pdf/10.1061/41167%28398%2984
  10. 10.0 10.1 10.2 "Design and Concrete materials Requirements for Ultra-Thin Whitetopping", Roesler, J., Bordelon, A., Ioannides, A., Beyer, M., and Wang, D. University of Illinois at Urbana-Champaign, 2008.