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Section 7: Bonded and Unbonded Concrete Overlays

Bonded concrete overlay (BCO) consists of a 2- to 8-inch thick concrete layer placed on top of the existing concrete pavement with operations conducted to ensure full bond between new and old concrete layers. BCO is one of the most cost effective ways of enhancing structural capacity of under-designed pavements. It increases structural capacity of the pavement system by reducing deflections.

TxDOT maintains many miles of thin PCC pavement. Although PCC pavement has served more traffic than it was originally designed for, most sections are in reasonably good condition. BCO can be a cost-effective method of rehabilitation to extend PCC pavement service life even further. The use of BCO on PCC pavement is based on the fundamental design assumption in BCO; that is, the old and new concrete layers behave as a monolithic layer. Providing full bond is of the utmost importance. During construction, specific steps are taken to enhance and ensure the full bond between old and new concrete. This will be discussed further in Chapter 10, Rigid Pavement Rehabilitation.

Bonded concrete overlays are not recommended for construction over jointed concrete pavements. CRCP-bonded concrete overlays have been constructed and have performed successfully in several districts but have not been used widely throughout the state. For any district considering bonded concrete overlay that has no experience in this design, obtain training presentations materials from CST-M&P, Rigid Pavement & Concrete Materials Branch is recommended.

The 1993 AASHTO Guide for Design of Pavment Structures is recommended for “Unbonded Concrete Overlay” thickness design. The designer can use the automated procedure AASHTO DARWin® program and select the “Overlay Design” Module and “Bonded PCC Overlay of PCC Pavement.”

Overlay Thickness Design

Dol = Df - Deff

Where:

Dol = required slab thickness of overlay, in.

Df = slab thickness to carry future traffic, in.

Deff = thickness of existing slab, in.

The slab thickness Df to carry future traffic is determined for the design traffic, as if it were built as a new pavement on the prepared base.

The design engineer will determine the design life and request the future traffic from TP&P. The modulus of subgrade reaction of 300 pci is recommended for new pavement structures with stabilized bases. However, some existing pavements might not have stabilized bases, and the k-value should be evaluated using FWD or DCP. Other input values will follow the same recommendations as listed at Approved Design Method.

Determination of Effective Slab Thickness by Condition Survey Method

Deff = Fic * Fdur * Ffat * D

Where:

Fic = joints and cracks adjustment factor

Fdur = durability adjustment factor

Ffat = fatigue adjustment factor

D = thickness of existing slab, in.

For BCO design, the condition of existing pavement is one of the most important factors. If the pavement condtion is deteriorated in the form of punchouts and deteriorated cracks, BCO may not be a good alternative.

The items that should be surveyed are:

  1. the number of punchouts per mile
  2. the number of deteriorated transverse cracks/joints per mile
  3. the number of existing and new repairs prior to the overlay per mile
  4. presence of D-cracking or ASR cracking and
  5. evidence of pumping of fines or water

Punchouts are the only structural distresses in CRCP; the number of punchouts per mile is a good indication of the structural condition of the existing pavement. If there are more than 10 punchouts per mile, then the pavement is in poor structural condition and may not be a good candidate for BCO.

The number of deteriorated transverse cracks per mile is the next item to be surveyed. Even though some transvers cracks may appear to be deteriorated, quite often then are in good condition. In Texas, it is rare to observe deteriorated transverse cracks, except for spalled cracks. Most spalled cracks are not necessarily structurally deficient. Based on the research findings, it is recommended that only transverse cracks much wider than normal, all along the transverse crack throughout the lane width should be counted as “deteriorated transverse cracks.”

Number of patches per mile should be recorded. Durability related problems should be noted and their severity recorded. Overall, it has been quite rare to observe durability related problems in CRCP in Texas. However, if D-cracking or ASR-related cracks are observed, their severity should be evaluated and recorded. In Texas’ old CRCP, base was not stabilized and pumping has been observed. Evidence of pumping should be recorded.

Joints and Cracks Adjustment Factor, Fjc

The Joints and Cracks Adjustment Factor, Fjc, accounts for the extra loss in the present serviceability index (PSI) caused by deteriorated reflection cracks in the overlay. Deteriorated reflection cracks develop due to unrepaired deteriorated joints, cracks, and other discontinuities in the existing slab prior to the overlay.

A deteriorated joint or slab will rapidly reflect through an overlay and contribute to loss of serviceability. Therefore, full-depth repair is recommended on all deteriorated reflection cracks and any other major discontinuities in the existing pavement prior to overlay. The target Fjc is 1.00.

If it is not possible to repair all the deteriorated areas, use the total number of unrepaired deteriorated joints, cracks, punchouts, and other discontinuities per mile in the design lane to determine the Fjc from Figure 8-3.

Joints and Cracks Adjustment Factor for
Bonded Overlays, Fjc (click in image to see full-size image) Anchor: #CEGCEBGEgrtop

Figure 8-3. Joints and Cracks Adjustment Factor for Bonded Overlays, Fjc

Source: AASHTO Guide for Design of Pavement Structures (1993)

Durability Adjustment Factor, Fdur

The Durability Adjustment Factor, Fdur, adjusts for an extra loss in PSI of the overlay when the existing slab has durability problems, such as D-cracking or reactive aggregate distress. Fdur is determined using historical records and condition survey data.

Durability Adjustment Factor, Fdur

Historical Records and Condition Survey Data

1.00

No evidence or history of PCC durability problems

0.96—0.99

Pavement is known to have PCC durability problems, but there is no visible spalling.

0.88—0.95

Cracking and spalling exist (in these conditions, a bonded PCC overlay is not recommended)


Fatigue Damage Adjustment Factor, Ffat

The Fatigue Damage Adjustment Factor, Ffat, adjusts for past fatigue damage in the slab. It is determined by observing the extent of punchouts (CRCP) that may be caused primarily by repeated loading.

Fatigue Damage Adjustment Factor, Ffat

Historical Records and Condition Survey Data

0.97—1.00

Few transverse cracks/punchouts exist (none caused by D-cracking or reactive aggregate distress), < 4 punchouts per mile

0.94—0.96

A significant number of transverse cracks/punchouts exist (none caused by D-cracking or reactive aggregate distress), 4 to 12 punchouts per mile

0.90—0.93

A large number of transverse cracks/punchouts exist (none caused by D-cracking or reactive aggregate distress), >12 punchouts per mile

BCO is not recommended


Steel Design

Steel should be placed at a depth that provides a minimum concrete cover of 3 in. When BCO thickness is 3 in. or less, reinforcement in the form of longitudinal steel is not needed. In fact, fibers have been successfully used.

The design engineer will determine the steel bar size and numbers of longitudinal bars, tiebars, and transverse bars. The performance of CRCP depends on the amount of longitudinal reinforcement, and BCO with 4 in. or thicker without longitudinal reinforcement will effectively reduce the amount of longitudinal steel in the combined slab (existing plus BCO layers), which could increase steel stresses and make transverse crack widths larger. See the recommended steel percentage and vertical location in the table below.

BCO Thickness

Longitudinal Steel (%)

Vertical Location

Fibers

<=3 in.

No

N/A

Yes

<= 5 in.

0.6%

Bottom of BCO

No

> 5 in.

0.6%

Middle of BCO

No



Unbonded Concrete Overlays

Unbonded concrete overlay consists of a concrete layer (5 in. or greater) on top of an existing concrete with a “separation interlayer” to separate new overlay and existing concrete. An unbonded overlay is a feasible rehabilitation alternative for PCC pavement for practically all conditions. These types of rehabilitation methods are most cost-effective when the existing pavement is badly deteriorated due to the reduced amount of repairs made to the existing pavement.

Unbonded CRCP concrete overlays may be used over CRCP, jointed concrete pavement (JCP), or jointed reinforced concrete pavement (JRCP). Unbonded CRCP overlay uses the same design procedure as new CRCP pavements. This use of unbonded CRCP overlay can be credited for contributing to the structural capacity of the existing concrete pavement and results in a thinner concrete pavement design than required for CRCP constructed on a new location.

The 1993 AASHTO Guide for Design of Pavement Structures is recommended for unbonded concrete overlay thickness design. The designer can use the AASHTO DARWin® program and select the “Overlay Design” Module and “Unbonded PCC Overlay of PCC Pavement.”

Overlay Thickness Design

Dol = (Df2 - Deff2)1/2

Where:

Dol - required slab thickness of overlay, in.

Df = slab thickness to carry future traffic, in.

Deff = effective thickness of existing slab, in.

Determination of Effective Slab Thickness by Condition Survey Method

Deff = Fjcu * D

Where:

D = existing slab thickness, in. (Use 10 in. when existing D >10 in.)

Fjcu = joints and cracks adjustment factor

Fjcu adjusts for PSI loss due to unrepaired joints, cracks, and existing expansion joints, exceptionally wide joints (>1 in.), or AC full-depth patches.

Use the total number of unrepaired deteriorated joints, cracks, punchouts, and other discontinuities per mile in the design lane to determine the Fjcu from Figure 8-4.

Joints and Cracks Adjustment Factor for
Unbonded Overlays, Fjcu (click in image to see full-size image) Anchor: #CEGIJGCHgrtop

Figure 8-4. Joints and Cracks Adjustment Factor for Unbonded Overlays, Fjcu

Source: AASHTO Guide for Design of Pavement Structures (1993)

Steel Design

The steel placements are the same as new CRCP when unbonded overlay is 6 in. or thicker. When unbonded overlay is less than 6 in., use longitudinal reinforcement at about 0.6% of concrete cross-sectional area. The design engineer will determine the steel bar size and quantities for longitudinal bars, tiebars, and transverse bars and consult with the Rigid Pavement & Concrete Materials Branch of CST-M&P.

More information about bonded and unbonded concrete overlays is detailed in Bonded Concrete Overlay and Unbonded Concrete Overlay of Chapter 10, Rigid Pavement Rehabilitation.


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