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Ultra-Thin Whitetopping or UTW is a reversal of normal highway maintenance practice. Instead of Hot Mixed Asphalt (HMA) being placed on top of failed concrete, concrete is placed on failed HMA, more specifically, Fiber Reinforced Concrete (FRC). The Federal Highway Administration has approved UTW and has participated in a number of demonstration projects sponsored by various states. AASHTO (American Association of State Highway and Transportation Officials) has also participated in the research and development of the technology along with the Departments of Transportation (DOTs) of a number of the states. Several state DOTs have adopted this technology along with county highway agencies and city street departments.

UTW is a high early-strength concrete that is placed to a depth of 2.5-6.0 inches on milled HMA. The thickness of the UTW is dictated by the amount of unsound HMA that is milled from the existing pavement.

Why is UTW better than HMA?

By definition, HMA is a flexible pavement. The problem with HMA is its inability to resist deformation under vehicle braking. This results in pushing, rutting and shoving of the HMA, which can create unsafe conditions. In highly trafficked intersections, replacing or overlaying the HMA is required every two-to-three years when replaced with HMA, whereas UTW will survive approximately 10 years.

Synthetic fibers improve the UTW technology!

Fibers improve durability. In the late 1990s, the FRC industry studied the benefits of what is now called microsynthetic fibers at dosage levels beyond 1.0 to 1.5 pcy. One of the applications identified was where concrete would outlast HMA at high Average Daily Traffic (ADT) intersections and/or high truck-trafficked roadways. The typical point of use for UTW is either high traffic intersections or roadways subjected to a high volume of truck traffic. The primary focus of the industry’s testing of FRC was to extend the life of the pavement system beyond what the HMA could provide, thus emphasizing long-term durability. Fibers reduce volume change. It was found that fibrillated polypropylene fiber added to concrete would reduce the amount of volume change associated with temperature and moisture variations. Also, the fibers’ effect on drying shrinkage cracking was evident as reported in a paper written by Dr. Ronald F. Zollo as well as work conducted by others.

Fibers increase fatigue strength.
More importantly, the contribution of the fibrillated polypropylene fibers to the fatigue strength of the concrete was proven in an ACI paper written by Dr. V. Ramakrishnan and Mr. Robert C. Zellers, PE/PLS, ABC Polymer’s Director of Engineering Services. This peer-reviewed paper showed that the synthetic fibers extended the usable life of the concrete by increasing the number of cycles to failure; but more importantly, the actual load per cycle was increased versus the performance properties of the plain concrete.

Fibers reduce repair time.
Another aspect of the intersection repair process involves reducing the time to a minimum. The new technology inherent in UTW uses an accelerated construction site timetable that limits the time on intersection projects to 72-to-96 hours. The fibers in FRC are a contributor to this quick turnaround time, making UTW very competitive with HMA overlays in terms of the time that traffic will be inconvenienced.

How to make UTW using FRC:

• Mix in three pounds per cubic yard of an approved microsynthetic fiber (typically fibrillated polypropylene fibers).

Products meeting the above requirements:
ABC Polymer Industries FiberForce 300™ (1500 denier fibrillated polypropylene fiber).

• Construction Process
  1. Mill the HMA down to sound material, followed by any additional maintenance to ensure the remaining HMA is sound. Thoroughly clean the milled surface so as to guarantee the best mechanical bond possible between the HMA and the UTW. Place and finish the UTW with a broomed or tined texture.

2. Cut the joints with a wet saw, which is a major departure from the normal cutting process. Instead of 12′ x 12′ slab sections, UTW slab sections are 3′ x 3′ or 4′ x 4′. The reason for this change is that a rigid pavement is being placed on a flexible pavement. The pavement can then be reopened to traffic when the target early strength has been attained.
3. This technology may require the fiber supplier to participate in obtaining approvals from the appropriate agencies and to adjust mix designs to accommodate the extra surface area contributed by the fibers. ABC Polymer’s Engineering Department can respond to any requests regarding approvals or mix designs.

Contacting Bobby Zellers, Director, Engineering Services, at RZellers@ABCFibers.com will bring you up to speed on this technology and provide you with the support needed to succeed with fiber reinforced UTW.