8.2 Guidelines for Thin Whitetopping (TWT)

The following guidelines provide the recommendations for TWT design thickness, joint spacing, ACP support layer, and better bond.
  • Design Life of 5 to 10 yr. is recommended for TWT.
  • Normally 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 thinner slabs require shorter joint spacing, often encroaching on the wheel path.
  • Designs for > 7 in. TWT were not included because our standard concrete pavement designs should be used along with load transfer devices.
  • Contraction joint spacing is set at 6-ft. intervals to prevent edge loading and reduce the costs for saw cutting. All sawed TWT panels shall be square except as necessary in pavement width transitions.
  • Use engineering judgment to allow the contractor to meet plan requirements when using the ride specification at intersections or curb and gutter sections.
  • Recommend ≥ 4 in. asphalt concrete pavement (ACP) support layer (including ASB and level-up).
  • Uniform support will improve performance.
  • Recommend milling the existing ACP to provide a better bond and remove rutting ≥ 1/2 in.
  • Opinions differ on the need for fibers to control shrinkage cracking.
  • Saw cuts must be made as soon as possible, without delay.
  • Saw-cut depths must be adjusted accordingly in thickened end sections.
  • At bridge approaches, an approach slab is recommended with an expansion joint as shown in existing standards.
  • The department has yet to establish when expansion joints are needed for wide pavements.
  • The use of steel fibers is not recommended at locations where de-icing salts may be used.
  • Apply governing Special Specification, Thin Whitetopping (Concrete Overlay).
  • Use Thin Whitetopping Details, TWT-04 Standard Sheet.
  • Use Table 8-8 as a general guide for determining proper TWT thickness. More research will be conducted to improve the design procedure.
Table 8-8: Thin Whitetopping Thickness (in.) Design
Trucks per Day per Lane
Design Life (yr.)
5
6
7
8
9
10
≤ 200
4
4
4
4
4
4
250
4
4
4
4
5
5
300
4
4
4
5
5
5
350
4
4
5
5
5
5
400
4
5
5
5
5
5
450
5
5
5
5
5
6
500
5
5
5
5
6
6
600
5
5
5
6
6
6
700
5
5
6
6
7
7
800
5
6
6
7
7
7
900
6
6
6
7
7
n/a
1,000
6
6
7
7
n/a
n/a
The traffic data (in terms of ADT, Percent Directional Distribution, and Percent Trucks) are obtained from the traffic analysis report provided by TPP.
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 (assume 50% split, unless observation proves otherwise)
  • 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.
Example:
  • 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
  • For 255 trucks per day per lane and 10-yr. design, the required TWT thickness is 5 in.