Thin Whitetopping (TWT) is a 4 to 7 in. thick concrete overlay bonded to an existing ACP to create a composite section. TWT is typically constructed at intersections where rutting and shoving in asphalt pavement continue to cause problems. TWT may also be used at access or exit ramps to interstate highways, entire sections of urban roadways, low-volume rural roads, bus lanes, and parking areas. TWT can provide better serviceability, longer service life, lower life-cycle cost, and improved safety over ACP subject to heavy trucks, especially in accelerating/decelerating, turning movement, or slow traffic environments.
This rehabilitation technique purposely seeks to bond the concrete overlay to the existing asphalt. The composite action significantly reduces the load-induced stresses in the concrete overlay. Therefore, the concrete overlay can be significantly thinner for the same loading as compared to a whitetopping section with no bond to the underlying asphalt. TWT will significantly reduce traffic delays and total maintenance effort accompanying the frequent maintenance of an asphalt surface.
Guidelines for Thin Whitetopping (TWT)
TxDOT has significant experience with TWT and has developed the Preliminary Guidelines for Thin Whitetopping (TWT) June, 2004. It can be used as a general guide for construction and proper TWT thickness design. More research will be conducted to improve the design procedure for optimal design life, thickness and size of the concrete panels through Life Cycle Cost analysis.
General guidelines for construction and thickness design are:
- Design Life of 5 – 10 years is recommended for TWT.
- Typically used at intersections where rutting and shoving in asphaltic pavement continue to cause problems.
- Designs for < 4 in. TWT were not included, because the initial cost difference between 4 in. slabs and those with less than 4 in. is negligible and following the experience of others.
- Designs for > 7 in. TWT were not included, because standard concrete pavement designs should be used to include load transfer devices.
- Contraction joints spacings are set at 6 ft to prevent edge loading and reduce the costs of saw cutting. All sawed TWT panels shall be square (length = width) except as necessary in pavement width transitions.
- When using the ride specifications at intersections or curb & gutter sections, engineering judgment must be used to allow the contractor to meet Plan requirements.
- Recommend ≥ 4 in. ACP support layer (including ASB and Levelup).
- Uniform support will improve performance.
- Recommend milling the existing ACP to facilitate a better bond and remove rutting ≥ ½ in.
- Opinions differ on the need for fibers to control shrinkage cracking.
- Saw cuts must be made as soon as possible, without delay.
- The saw cut depths must be adjusted accordingly in thickened sections.
- At bridge approaches, an approach slab is recommended with an expansion joint.
- Guidelines have yet to be established when expansion joints are needed for wide pavements.
- The use of steel fibers is not recommended at locations where deicing salts may be used.
The following TWT Thickness Design table can be used as a general guide for determining proper TWT thickness. More research should be conducted to refine the design procedure.
Thin Whitetopping Thickness (in) Design Table
|Trucks per Day per Lane||Design Life (Years)|
|Less than = 200||4||4||4||4||4||4|
The traffic data (in terms of ADT, Percent Directional Distribution, and Percent Trucks) are obtained through agency traffic planning authorities.
Trucks per Day per Lane = ADT * PDD * PT* LDF , where
- ADT- Average Daily Traffic for the first year of design period
- PDD- Percent Directional Distribution of Traffic
- PT- Percent Trucks
- LDF- Lane Distribution Factor, use 1.0 for 4 lanes or less, 0.7 for 6 lanes,
- and 0.6 for 8 lanes or more. The lane number is for both directions.
Design Life = 10 years, ADT = 13,500, PDD = 63%, PT = 3.0%, and
4 lanes roadway, LDF =1.0
Trucks per Day per Lane = 13,500 * 0.63 * 0.03 * 1.0 = 255
Read TWT Catalog, for 255 trucks per day per lane and 10 years design, the required TWT thickness is 5 in.
Original article content and pictures contributed by TxDOT.