What is Ultra-Thin Whitetopping?


Whitetopping is a pavement rehabilitation option where a portland cement concrete (PCC) layer is placed over an existing hot mix asphalt (HMA) pavement system [1]. There are three types of whitetopping applications: conventional whitetopping has a thickness greater than 200 mm (8 in), thin whitetopping (TWT) can be 100 mm (4 in) but is less than 200 mm (8 in) thick, and ultra-thin whitetopping (UTW) has a thickness equal to or less than 100 mm (4 in).

UTW and some TWT must have a good bond to the existing HMA pavement to have successful performance as a composite pavement system. A well bonded pavement system results in a composite structure that distributes traffic and environmental loading differently than most traditional PCC or HMA pavement systems [1]. After rehabilitating the HMA with UTW composite pavement results in moduli that are comparable to structurally sound HMA pavement structures. Figure 1 shows how the stress distribution in a bonded system versus an un-bonded system can be very different. Due to the composite action from the bonded system the stresses in the top layer are significantly lower than for the un-bonded case. This composite system takes advantage of the fact that concrete has high resistance to compression. With a sound bond much of the concrete layer is in compression and because concrete is much stronger in compression than in tension, the design of the concrete layer can be thinner than in the unbounded case.

composite UTW.jpg
Figure 1: Effect of Composite Action on UTW Behavior Under Flexural Loading [1].

Because of the thinness of the UTW layer it is more likely that the concrete may crack under the plastic or drying shrinkage stresses [1]. Therefore, almost all UTW applications use fiber reinforced concrete (FRC) to control cracking due to plastic and drying shrinkage. Using FRC also reduces the permeability of the concrete layer. Early entry joint sawing also helps to reduce stresses and this early age cracking.

Where can UTW be used?


UTW is best suited for HMA pavements that have general surface deterioration and/or functional distresses. UTW is best selected for low traffic roads, parking lots, or light aircraft aprons. Figure 2 shows an UTW being placed on rural highway RT 10 in Logan County, IL [5]. UTW can be used on HMA that is approaching the end of its structural or functional life. UTW is not a good rehabilitation option for pavements where severe sub-base or subgrade distresses are evident. The stiffness of the existing HMA pavement system plays a significant role in the performance of the UTW. Therefore, some degree of engineering consideration should be utilized in the selection and design stages. If the HMA pavement has severe and extensive sub-base and subgrade issues, UTW is not a good rehabilitation option. Any distress that indicates a problem with the sub-base or subgrade, such as alligator cracking or potholes, should be analyzed to determine its impact on the UTW application. Stripping and/or raveling and potholes are not distresses that are good candidates for UTW placement either, as these distresses can cause de-bonding of the UTW to HMA pavement.

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Figure 2: UTW Placed on Route 10 in Logan County, IL [5].

The HMA pavement should also be of a sufficient thickness. Some literature recommends that after any cold milling and before UTW placement the remaining HMA layer must be at least 150 mm (3 in) thick [1, 3]. Because many installers mill the same thickness of HMA that is to be replaced with UTW it is most effective to use this application on very thick HMA structures. When the UTW rehabilitation option is selected and some areas of HMA are found to not meet this minimum thickness, the installer may choose to remove this area of HMA and install a full depth PCC pavement section. This results in a section, or possibly several sections, of full depth PCC pavement with most of the rehabilitated road remaining UTW over HMA pavement.
When used in the right setting and constructed properly the UTW can perform very well. It is advised to be sure UTW is used in the correct application, if not early failure can occur. If many early failures are realized the UTW could receive a bad reputation as an unviable solution for modern HMA pavement rehabilitation, when it was the improper application to be blamed for the early failure and not the UTW option in itself.

UTW has shown good performance in both hot and cold climates. In Iowa on the Route 21 project it was found that 2 years after pacement all the whitetopping sections performed well [1]. During a joint research project hosted by Tennessee and Georgia spanning from 1992 to 1998, eight out of nine UTW pavements only showed signs of low-severity cracking and maintained relatively high pavement condition index’s (PCIs) [1]. It was found in this study that the single pavement that did not perform well was found to have excessively distresses, thin, or no HMA supporting layer as determined through coring the affected areas. In Virginia UTW were placed on top of deteriorated HMA pavements at the Accelerated Loading Facility (ALF) in 1998 and evaluated in 2000 [1]. The UTWs that were constructed with shorter joint spacing and over HMA pavements with higher stiffness’ were found to be in good condition. The UTW that had longer joint spacing and were constructed over HMA with softer binders showed extensive damage. Therefore, UTW is found to not be a fix-all but can be added to the mix-of-fixes that an engineer might select in a modern HMA pavement rehabilitation selection.

When should UTW be used?


UTW is best used over old pavements. New or tender HMA pavements can inhibit the bond between the UTW and HMA surface. These tender mixes can also further consolidate under the load of the UTW layer and traffic, causing early failures of the concrete layer. Therefore, UTW is best for a HMA pavement that is approaching the end of its functional or structural life, but shows little indication of severe structural failure such as severe alligator or fatigue cracking, extensive potholes, etc.

Why use UTW?


Placement of the UTW is fast using “Fast Track” technology. Using “Fast Track” paving techniques to place UTW offer the long-term benefits that are characteristic of concrete pavement while maintaining minimal delays during construction [3]. An important benefit that has been reported by UTW advocates is the new pavement surface’s ability to resist fuel spillage. This benefit is utilized when considering placement in parking lots, fueling stations, and aircraft aprons [1].

Using an UTW overlay requires a sound bond between the UTW layer and the existing HMA pavement. This bond interface creates a necessary composite section and ensures that the neutral axis is lowered so that the load-related stresses are significantly reduced [1]. Short joint spacing further reduces these load-related stresses. Because the slabs are not as long they do not develop large bending moments as traditional concrete slabs do.

This joint spacing is selected through a rule of thumb that declares 12 to 18 inches of joint spacing for every 1 inch of overlay depth, [1-3]; therefore, a 3 inch overlay should be jointed into 3x3 or 4x4 foot squares. Shorter joint spacing significantly reduces the slab curling stresses and inherently reduces cracking due to these stresses. When a slab cracks, the short joint spacing keeps the distress localized; this localization ensures that the distress goes largely unnoticed by the motorist. Therefore the rehabilitation option is usually to do nothing until pieces of the UTW overlay are ejected onto the pavement surface. To fix such distresses the entire slab is replaced. Because the UTW is so thin and the joint spacing is so short the slab replacement can be a snap. In fact, the thinner the concrete layer the more economical it is to replace the entire slab than to do routine preventive maintenance. With “Fast Track” technology and the utilization of high early strength (HES) concrete, an UTW slab can be extracted, replaced, and re-opened to traffic within six hours [1]. Figure 3 depicts a section of road where UTW short joint spacing can be seen [6].

UTW joint spacing.jpg
Figure 3: Typical Joint Spacing of UTW Overlay [6].

The most common placement of UTW is in intersections where rutting and shoving are an ongoing problem. Although asphaltic concrete (AC) overlays are the least expensive and most commonly used overlay system, it is only a short term fix for the rutting and shoving that can be found in HMA intersections [3]. While full depth HMA or PCC pavement reconstruction could be a long term fix; these alternatives are expensive and cause long user delays. An UTW overlay has been proposed as a longer lasting and/or more economical alternative. The UTW overlays, when used in the proper application, have shown very good performance and are, in fact, known to perform better and last longer than AC overlays. In addition, the UTW overlays are comparative in cost than the traditional AC overlays, particularly if life cycle costs (LCC) are considered. Figure 4 illustrates an UTW over a distressed HMA pavement. Furthermore, selecting an UTW rehabilitation option can improve the durability, performance, and ride quality of the deteriorated HMA pavement surface.

UTW diagram.jpg
Figure 4: UTW over HMA Pavement with Typical Distresses [7].

Because UTW is, as named, ultra-thin, installers are able to mill the same thickness of HMA that is to be replaced with UTW to maintain the same grade as the original pavement surface. This alleviates any issues that an overhead clearance change or existing curb and gutter may present. With TWT or conventional whitetoppings the curb and gutter often must be replaced or the asphalt must be removed completely and fully replaced to maintain clearance under traffic signals or bridges.

There are also many different placement techniques. An installer can choose the best technique for the job. UTW can be placed by hand with fixed forms or by machine with fixed or slipforms. Therefore, if there is limited space, an installer can place the concrete by hand with the fixed form option to rehabilitate the pavement. If a slipform paver is used, as in Figures 2 and 5, the machine will spread, consolidate, screed, and float finish the fresh concrete in one operation.

SLIP form machine.jpg
Figure 5: Slip-form Paving Machine Placing UTW over Milled and Cleaned HMA [8].

How is UTW placed and what considerations should be made?


There are many design criteria and construction considerations that must be made to ensure the best performance of the UTW overlay. Only SOME of the most important criteria will be visited in this section of this report. Project selection is a paramount stage to ensure proper performance of the UTW. As stated above, the best pavement candidates for UTW are old HMA pavements that exhibit signs of nearing the end of the functional or structural life but that do not show extensive or severe structural failure, or sub-base or subgrade degradation.

There are numerous design criteria that must be considered [1]. The key to success in any concrete pavement is a uniform and stable support system, this includes UTW. Therefore because the HMA pavement is the support system of the UTW, any mechanism that has caused failure in the HMA pavement can also lead to failure of the UTW. Visual and FWD testing is encouraged and carried out by many who utilize UTW.

Pre-overlay repair is an important consideration. Although many HMA distresses will not reflect through to the UTW surface, some can cause other failures of the UTW [1]. If the UTW is used solely for aesthetic purposes then it is not routine to do any pre-overlay repairs. However, full-depth patching to correct localized subgrade failures has been reported by some UTW users. This patch can either be done of HMA or, a better option is to patch with a section of full depth PCC. In which case, the section of existing HMA is removed, the base stabilized, and the full depth PCC is placed at the same time as the UTW overlay. Localized potholes can be filled with unbound aggregate, cold or hot-mixed asphalt, or PCC. If a crack width is larger than the maximum aggregate size in the UTW than the cracks are recommended to be sealed. Some users recommend matching the UTW joints with longitudinal construction cracking that has formed in the HMA. If this is done then these joints are recommended to be sealed after UTW placement.

As previously stated a strong bond between the UTW and the existing HMA pavement is necessary to provide good performance of the UTW overlay. A fully bonded system is ideal, but in practice only a partially bonded system is realized [1]. It has been found that the best way to achieve this bond is through texturizing the HMA surface before placement of the UTW overlay. Although shot blasting has also been reported, cold milling has been found to be the best way to ensure maximum bonding between the HMA and UTW overlay. It is important to be sure that there is enough HMA thickness left after cold-milling to be sure that the UTW has a sufficient support base. The surface is then swept to be sure that all loose material and dirt are removed as this loose material can cause de-bonding. Air or water blasting has also been used but does not ensure a better bond than sweeping. If water residue from saw cutting is found on the milled surface extra seeping is recommended for the affected area.

High quality design materials should be used for UTW in conjunction with the “fast track” placement technology [3]. From a stress perspective, aggregate used for UTW that have a low coefficient of thermal expansion (CTE) perform the best [1]. Supplementary cementitious materials and admixtures can be used at the design engineer’s discretion. Reinforcing fibers are recommended for use in all UTW to keep the joint and crack widths small while also decreasing the permeability of the UTW.
Placement climate is very important as with any pavement placement. The air temperatures should not exceed 32°C (90°F) or fall below than 4°C (39°F). The milled HMA surface should not exceed 45°C (110°F) or be an uncomfortable temperature to an open palm. A light water fog can cool down the surface of the existing HMA and is recommended directly before placement so that water is not drawn from the concrete mixture during curing. Care should be taken to not have any standing water or puddles as these will cause de-bonding of the UTW to the HMA and could also cause failures of the concrete mixture. Water should not be added to a concrete mixture without recommendation of the engineer, as this can adversely affect the performance and durability of the pavement.

As stated above there are two placement techniques: fixed form and slipform. All forms should be rigidly supported [1]. Although a fixed forms placement technique allows for space constraints it can be labor intensive. Slipform paving machines spread, consolidate, screed, and float finish the concrete in a single operation without the use of fixed forms, but the machinery can be cumbersome and additional floating can be specified. Care should be taken to ensure proper consolidation. Although dowels can be placed in advance of a paving train for TWT, dowels are usually not used for UTW applications due to the short joint spacing. The forward motion of the paving operation is an important key to providing a smooth surface. Coordination should be made to ensure continuous paving during the placement of the UTW whenever possible.

Texturing should be done at just the right time as to not disturb the setting and curing of the concrete layer. Texturing can be done with a tining comb, burlap or turf drag, or a stiff bristled broom. Any comb, drag, or brooming should be done perpendicular to traffic to provide the best tire to pavement friction.

Curing is one of the most important considerations for concrete placement. It is important to prevent rapid water loss of the concrete pavement while the pavement sets and cures [1]. Preventing this rapid water loss is especially important with UTW because the pavement layer is thin with a large surface area compared to the volume of concrete. White liquid curing compounds are often applied twice to ensure good coverage. Plastic sheeting and or wet cotton mats or burlap can also be used to keep the moisture from evaporating too rapidly from the fresh concrete. Any mats used to keep moisture in the concrete should never be allowed to dry out during the curing process. This will exacerbate rapid water loss of the concrete due to the wicking mechanism of the mat material when dry.

Weather contingency plans should always be in place while placing concrete pavements [1]. Rain can cause excess water intrusion to the surface of fresh concrete and cause spalling or other surface defects. Wind speed, ambient air temperature, and humidity can play significant roles in premature water loss due to evaporation. A light water fog can help alleviate some of these detrimental conditions, but as with rain, puddles and standing water on the fresh surface should be avoided.

Joint sawing timing is critical to prevent early age distresses of the pavement [1]. Waiting too long to cut the joints can result in high shrinkage stresses and resultant cracking. Cutting the joints too soon can cause spalling at the joints from the cement not being hard enough to hold the aggregates in, here the saw blade will catch the aggregate and tear the entire aggregate from the surface of the pavement. To avoid these problems proper timing and “green” cutting machines are utilized. Backup saws are recommended to ensure that equipment failure does not result in late joint cutting and early age distresses. The depth of joint saws should be between one fourth and one third the thickness of the overlay. Joint sealing is not usually recommended for UTW because the short joint spacing and fiber reinforcement ensures that the joint cracks do not open as widely as traditional whitetopping or some TWT overlays. It has been recommended that the joint spacing be designed so that the wheel paths will cross over the middle of the slabs. When a longitudinal joint is made in the wheel path and directly under the tire, many corner cracks that may not otherwise propagate appear in these affected slabs.

Failure Modes and UTW Rehabilitation


The main distress types found in UTW overlays can be classified into two categories: early age and long term.

The following are early age distresses [1]:
  • Uncontrolled cracking due to late sawing, restrained shrinkage, or thermal movement.
  • Plastic shrinkage cracking.
  • Joint raveling due to early saw cutting or early opening to traffic.

The following are long term distresses [1]:
  • Corner cracking due to improper joint spacing or heavy traffic; this distress is most common on longitudinal joints found in the wheel path but can be found on adjacent slabs that are not loaded (this is the most common distress associated with UTW overlays).
  • De-bonding and delamination of the UTW from the original HMA pavement, this can be due to environmental as well as traffic loading. Stripping of the asphalt binder from the underlying layer can aggravate the de-bonding problem.
  • Longitudinal cracking, commonly found in the wheel path.
  • Transverse cracking where slabs have longer joint spacing.
  • Fractured or shattered slabs.
  • Surface wear.
  • Failure due to support layers.
  • Spalling.
  • Faulting.

Fault trees can be used to understand the failure mode of any pavement type. Figure 6 is an example of a fault tree for the UTW case [1].
fault tree UTW.png
Figure 6: Fault Tree for UTW Distresses [1].

The most common maintenance selection for UTW is to do nothing until the slab starts to show signs of possible material ejection [1]. Figure 7 shows a slab that could be considered shattered as it has many cracks. Although the slab has obviously failed, in most cases this distress will still go unnoticed by the motorist. The bonding of the UTW to the HMA prevents the pieces of the slab from being ejected onto the surface of the pavement. However, the effect of traffic is to continue to loosen the pieces and eventually pull the pieces of shattered slab onto the surface of the pavement. Figure 8 shows several definitively shattered slabs with some UTW still connected to a piece of HMA concrete that has been ejected. It is important to replace the slab before this material ejection is realized.
shattered slab UTW.jpg
Figure 7: Shattered Slab UTW Pavement [1].

shattered slabs UTW.jpg
Figure 8: Shattered Slabs with Ejected Material on UTW Pavement [1].

As mentioned previously one UTW slab can be removed, replaced, and opened to traffic within 6 hours with the utilization of “fast track” paving techniques, HES concrete, and man-power. It has been reported that 4 such slabs can be rehabilitated in this same way within 12 hours [1]. This reduces costs for both the agency and the user. Figure 9 shows a single slab replacement in progress [8]. This slab replacement rehabilitation option is more economical than any maintenance option due to the thinness and short joint spacing of the UTW. If slab shattering is extensive and severe underlying issues in the sub-base or subgrade may be to blame. In this case, it has been common practice to tear out any affected layers in the failing section, replace with new material, and finish with a full depth PCC pavement.

UTW slab replacement.jpg
Figure 9: UTW Slab Replacement [9].

State of the Art UTW


Installers in Europe are placing UTW in conjunction with steel reinforcement and sometimes mechanical bonding with anchors through to the sub-base. The claim is that the UTW “not only functions as an independent overlay regardless the type, quality and condition of the sub base but can also give an additional strength to the underlying structure due to the very high stiffness and thus load spreading capacities in combination with bonding” [10].

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Figure 10: Contec Ferroplan® UTW Placement [10].

Conclusions


UTW overlays can be used to rehabilitate some distressed asphalt pavements, but is not a cure all. UTW should be selected carefully with attention to engineering recommendations and construction specifications.

References:

  1. Thin and Ultra-Thin Whitetopping – Elasto Plastic Concrete: NCHRP Synthesis 338 http://www.elastoplastic.com/index.php?option=com_docman&task=doc_download&gid=113&Itemid=
  2. Concrete Thinker, Portland Cement Association, Ultra-Thin Whitetopping http://www.concretethinker.com/applications/Ultra-thin-Whitetopping.aspx
  3. Research Development and Technology. Missouri Department of Transportation: Research Report RI 97-043, RI 99-012. http://library.modot.mo.gov/RDT/reports/Ri99012/brief.pdf
  4. WIKIPEDIA
  5. GRT http://www.grtinc.com/utw/
  6. U.S. Department of Transportation, Federal Highway Administration: Pavements. https://www.fhwa.dot.gov/Pavement/concrete/sr04ch2.cfm
  7. http://www.docstoc.com/docs/42046722/Bonded-Concrete-Resurfacing-of-Asphalt-Pavements
  8. TRIBLOCAL Lombard. http://triblocal.com/lombard/community/stories/2011/02/lombard-public-works-department-receives-recognition-for-north-industrial-park-project/
  9. Ultra-Thin Whitetopping Repair Demonstration http://www.veengle.com/s/ultra%20whitetopping.html
  10. Ultra thin white topping http://www.contec-aps.com/business-areas/bridges-rehabilitation/ultra-thin-white-topping.html