Whitetopping: Whitetopping History Thickness Advantages Design 2. Ultra-Thin Whitetopping History

1. CONCRETE APPLICATIONS I CIMT 210 Whitetopping: Whitetopping History Thickness Advantages Design 2. Ultra-Thin Whitetopping History Bond and thickness Joint Spacing Construction Overlays_of_Asphalt_Pavement

2. 1. Whitetopping • History Paved surfaces for roadways, both portland cement concrete and asphalt, must withstand very harsh conditions: Wheel loads from moving vehicles The pavement must endure environmental effects. Climatic changes Moisture variations Freezing and thawing temperature

3. 1. Whitetopping • History Whitetopping is a portland cement concrete overlay on existing asphalt concrete pavement. It can be used as a road surface course where traditional paving materials have failed due to rutting or general deterioration. There are three types of whitetopping: Conventional (thickness greater than 8 inches) Thin (thicknesses over 4 but less than 8 inches.) Ultra-thin – (2 to 4 inches) Ultra-thin whitetopping (UTW) is a bonded, fiber reinforced concrete overlay.

4. 1. Whitetopping • History Whitetopping used in the United States since 1918 and routinely used since mid-1970's. Method of rehabilitating asphalt pavements Was developed in the early 1990's. Thicker bonded concrete overlays of existing asphalt pavements have been built on three major highways in the U.S.

5. 1. Whitetopping B. Thickness Unbonded, Classical WhitetoppingUntil 1991, most whitetopping projects in the U.S. did not purposely seek bond between the interface of the concrete overlay and the underlying asphalt surface. Rather, the existing asphalt serves as base for the new concrete overlay. Defined as: "A concrete overlay, usually of thickness of 100 mm(4 inches) or more” Unbonded PCC overlays, often called "classical whitetopping" Bonded PCC overlays, often called "thin composite whitetopping"

6. 1. Whitetopping C. Advantages Applicable where the depths of potholes are less than 50 mm (2 inches).  If rut or pothole depths are deeper, the potholes are filled or the surface is milled.  All three types of rigid pavement (JPCP, JRCP and CRCP) have been successfully used as Classical whitetopping (McGhee, 1994). The chief advantage of classical whitetopping is that it requires minimal surface preparation Minimum overlay thicknesses tend to be in the 125 - 175 mm (5 - 7 inch) range, which is quite thick and possibly unsuitable in situations where a specific elevation must be maintained such as in curbed areas or under bridges. 

7. 1. Whitetopping D. Design The design procedure contained in the 1993 AASHTO Guide is virtually identical to the AASHTO empirical design for new rigid pavements with one exception: The effective modulus of subgrade reaction (k) is determined based on the existing flexible pavement resilent modulus.  Although perfectly acceptable, this method gives little credit to the existing pavement's remaining strength.

8. 1. Whitetopping D. Design The design procedure contained in the 1993 AASHTO Guide is virtually identical to the AASHTO empirical design for new rigid pavements with one exception: The effective modulus of subgrade reaction (k) is determined based on the existing flexible pavement resilent modulus.  Although perfectly acceptable, this method gives little credit to the existing pavement's remaining strength.

9. 2. Ultra-Thin Whitetopping • History Ultra-Thin Whitetopping "UTW"Emerges Term was needed to differentiate this new technology from classical whitetopping. Three features differentiated the whitetopping of asphalt: (1.) The concrete overlay was substantially thinner (2.) Bond between the concrete overlay and the underlying asphalt created composite action. (3.) Short joint spacing significantly improved overlay performance.

10. 2. Ultra-Thin Whitetopping A. History “Ultra-thin whitetopping." requires bonding a relatively thin layer of concrete to the underlying asphalt Over 200 ultra-thin whitetopping sections have been built, primarily on low-volume roadways.

11. 2. Ultra-Thin Whitetopping B. Bond and Thickness The concrete overlay and the underlying asphalt act as a composite section rather than two independent layers. Significantly reduces the load-induced stresses The concrete overlay can be significantly thinner for the same loading as compared to a no bond to the underlying asphalt.

12. 2. Ultra-Thin Whitetopping B. Thickness UTW defined as: "A concrete overlay 50 mm to 100 mm thick with closely spaced joints bonded to an existing asphalt pavement."

13. 2. Ultra-Thin Whitetopping C. Joint Spacing Joints are typically design much closer than for typical new-construction rigid pavement. The closer joint spacing, on the order of 1 - 4 m (3.3 - 13.1 ft.), does the following : Reduces the moment arm of the applied wheel load and minimizes the stresses due to bending. Reduces the curling and warping stresses by reducing the size of the slab that can curl or warp. Because of the short joint spacing, the overlaid PCC slabs transfer load to the underlying flexible pavement by deflecting downward as a unit rather than bending  

14. 2. Ultra-Thin Whitetopping C. Joint Spacing Figure : Shorter joint spacing reduces slab-bending. Wheel loads cause bending in concrete pavements with conventional joint spacing. In UTW, shorter joint spacing casues more transfer of wheel loads to the underlying asphalt through deflection.

15. 2. Ultra-Thin Whitetopping D. Construction • Constructing UTW Overlays Proper construction • of ultra-thin whitetopping consists of four • fundamental steps: • Prepare the asphalt surface by milling and • cleaning, or water or abrasive blasting. • 2. Place, finish, texture, and cure using • conventional techniques and materials. • 3. Saw joints to prevent cracking. • 4. Open to traffic. A clean surface is required • for proper bond.

16. 2. Ultra-Thin Whitetopping D. Construction Milling the surface followed by cleaning improves bond because it exposes more of the aggregate of the asphalt pavement. The milling creates a rough surface the also enhances the bond between the two layers. If milling is not done, water or abrasive blasting should be used to clean the asphalt surface.

17. 2. Ultra-Thin Whitetopping D. Construction When water blasting is used, the surface must be allowed to air dry before the concrete is placed. Once a surface is cleaned it is important to keep it clean until the concrete overlay is placed. Dust, dirt and debris that falls or blows onto the asphalt surface must be removed. If the surface is cleaned on the day prior to paving, air cleaning may be required on the day of paving to remove dirt and dust.

18. 2. Ultra-Thin Whitetopping D. Construction If traffic is allowed on the milled surface, the surface must be recleaned prior to paving. Paving UTW is no different from paving any other concrete pavement. Conventional slip-form and fixed-form pavers, as well as small equipment - such as vibrating screeds –

19. 2. Ultra-Thin Whitetopping D. Construction Typical concrete finishing and texturing procedures are appropriate for ultra-thin whitetopping. Proper curing is critical to avoiding shrinkage cracking in the concrete overlay and to prevent debonding between the asphalt and concrete.

20. 2. Ultra-Thin Whitetopping D. Construction Because the overlay is a thin concrete slab, it has high surface area to volume ratio and can lose water rapidly due to evaporation. Curing compound should be applied at twice the normal rate. Care must be used during application in order to avoid spraying curing compound on a prepared asphalt surface, which will decrease bonding. Joints should be sawed with lightweight saws as early as possible to control cracking.

21. 2. Ultra-Thin Whitetopping D. Construction Saw-cut depth should be 1/4 - 1/3 of overlay thickness. Typically, the joints are not sealed. They have performed well without sealant because the short joint spacing minimizes joint movement. Performance to date shows no benefit from sealant use.