ULTRA THIN WHITE TOPPING

 

The significant increase in the number of automobiles observed in the recent years has created a need not only for the construction of new highways but also for the maintenance and rehabilitation of existing highway networks. Pavements are prone to damage due to the repeated wheel loads as well as temperature and other environmental effects.

Paved surfaces for roadways, both Portland cement concrete and asphalt, must withstand very harsh conditions. Wheel loads from moving vehicles cause strains, stresses and deflections in the paved surface and underlying roadbed. These loads are repetitive in nature and over an extended period of time will cause fatigue damage to the pavement. Additionally, the pavement must endure environmental effects. Climatic changes cause temperature variations through the depth of the pavement, inducing internal stresses.

Ultra-thin whitetopping is a relatively new rehabilitation technology applying a 50 to 100 mm (2- to 4-in) thick concrete overlay on top of existing asphalt pavement. Since the first experimental section of UTW was installed in 1991 on an access road to a landfill in Louisville, Kentucky, more than 170 UTW projects have been constructed across the United States [1]. The prediction of UTW overlay load-carrying performance is somewhat uncertain, however, because of its nature as a composite system. In order to help state highway agencies and contractors better design and apply the UTW, well-controlled pavement response and performance data is needed to improve and refine the existing UTW design procedures. As part of this research effort, in the spring of 1998, FHWA and ACPA entered into a cooperative agreement to conduct Accelerated Load Facility (ALF) tests of ultra-thin whitetopping. Eight full-scale lanes of UTW were placed over existing HMA pavements that were in various stages of rutting distress after extensive testing with the ALFs for a Superpave validation study [2]. The asphalt pavements had been built with seven different HMA mixtures which displayed a broad range of stiffness. The experiment employed various combinations of thickness, joint spacing, and fiber reinforcement, and HMA base type. Full-scale loading of the UTW sections began in May 1998 and was completed in November 1999. Pavement response and performance data were collected during the testing period. One type of response data was the pavement deflections, monitored during the testing.

 

 

 

 

 

 

                            

 

 

 

 

UTW is a technique which involves placement of a thinner (than normal) thickness ranging from 2 to 4 inches with closely spaced joints and bonded to an existing asphalt pavement. The application of UTW has been targeted to rehabilitate deteriorated asphalt pavements with fatigue and/or rutting distress. However the actual depth of UTW depends on the grade of concrete used, intensity of traffic, thickness of existing asphalt pavement after milling etc.

   Bonding allows the concrete overlay and the under laying asphalt to act as a composite section, reducing critical stresses caused by wheel loads and temperature changes.

 

 

DESIGN PROCEDURE

 

 

Following UTW design procedure is adopted:

 

  1. Based on traffic data number of equivalent axle loads are obtained. The elastic modulus and thickness of the existing asphalt pavement are also obtained.
  2. Allowable tensile stress in asphaltic concrete has been calculated.
  3. Thickness for UTW is assumed. Maximum tensile stress in allowable compression due to UTW for both bound and unbound conditions are found out.
  4. Maximum tensile stress in allowable compression is compared with the allowable stress
  5. Maximum tensile stress in UTW due to both axle load and temperature differentials has been calculated.
  6. Stress ratio in UTW is obtained and Maximum allowable load repetitions are determined.
  7. If the UTW fatigue criterion indicates a small number of ESAL’s, than UTW thickness increased and repeat the steps 4 to 6.

 

                    

 

CONSTRUCTION PROCEDURE

                                         

 

Surface Preparation

A clean surface is required for proper bond. 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 that 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. 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. If traffic is allowed on the milled surface, the surface must be re-cleaned prior to paving.

 

Placing

After surface preparation, there must be enough asphalt remaining to form a sufficient composite section that can carry the load. There must be enough asphalt to minimize concrete tensile stresses, and enough concrete to minimize asphalt strains. Based on U.S. experiences to date, it is recommended that the minimum asphalt thickness after milling exceed 3 inches. Paving UTW isn’t any different than paving any other concrete pavement. Conventional slip-form and fixed-form pavers, as well as small equipment - such as vibrating screeds - have all been used successfully, without major modifications.

 

 

 

 

 

 

 

 

 

Finishing and Texturing

Typical concrete finishing and texturing procedures are appropriate for ultra-thin whitetopping. Texturing of the finished UTW pavement surface is required to provide adequate surface friction of the roadway. Surface friction is provided by carpet drag or brooming, which also reduces noise.

 

 

 

 

 

 

 

Curing

Proper curing is critical to avoiding shrinkage cracking in the concrete overlay and to prevent de-bonding between the asphalt and concrete. 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 UTW is similar to curing new PCC pavements. It requires curing the entire pavement surface and edges as soon as surface conditions permit after the finishing operations using either blanket or membrane methods. The most common practice is to spray liquid, membrane curing compound.

 

Joint Sawing and Sealing

Sawing is critical to avoid random cracking in whitetopping. Partial-depth saw cutting operations should commence immediately after the concrete has gained enough strength to prevent raveling and spalling of the joint

Joints should be sawed with lightweight saws as early as possible to control cracking. Saw-cut depth should be ¼ - 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.

 

 

 

 

 

 

 

 

PERFORMANCE

The performance of a UTW system is determined by the following factors.

a) Sufficient bond between the UTW overlay and the Hot Mix Asphalt (HMA) base. The milling of the HMA prior to constructing the overlay improves the bond between the UTW overlay and the existing HMA pavement. This is because the milling process exposes the aggregate of the HMA mix, which in turn establishes an interlocking effect between the milled HMA surface and the UTW overlay thereby improving the bond between them. The tensile load stresses in the concrete are reduced as solid bond at the concrete-asphalt interface creates a composite action and pushes the neutral axis away from the concrete layer (refer to Figure 2.1). As the neutral axis moves further down, the tensile stresses in asphalt also decrease, as the HMA layer is closer to the neutral axis.

 

 

 

 

 

 

 

 

b) Shorter joint spacing or small panels. Smaller joint spacing helps to reduce the stresses generated by bending as well as curling and warping effects on the pavement due to temperature and moisture gradients. Traditionally, concrete pavements are designed to absorb energy by bending are thus are constructed thick enough to resist the bending stresses. Short joint spacing allows the load to be carried with lower deflection and the joint movement is minimized. With UTW, short joint spacings are used so that the energy is absorbed by deflection rather than by bending (refer to Figure 2.2). Typically the joint spacings for a

UTW system are somewhere between 0.6 m and 1.5 m (2’ to 6’).

 

 

 

 

 

 

 

 

 

 

c) Sufficient thickness of the remaining asphalt. After surface preparation, (i.e., the milling of the asphalt surface) there should be enough asphalt remaining to form a sufficient composite section that can carry the load. There should be enough structural stability in the asphalt surface for the composite action in the UTW to develop. In addition, the thicker asphalt section carries more loads and helps the neutral axis to shift further down, thereby causing composite action. Thicker asphalt section increases the overall thickness of the pavement thereby decreasing the tensile stresses in concrete

                                                                                                                         

 

 

Factors Affecting Utw Performance

a) Concrete Mixture Proportions

b) Typical Construction Procedures

c) Traffic loading

d) Pavement Monitoring

 

 

REPAIR OF UTW

 

When to repair

    Generally, repairs should be considered when:

  • Panels are broken into four or pieces
  • Surface irregularities or settlement affect ride quality
  • Loose or missing concrete is evident

Repair procedures

  The repair of UTW pavement involves locating and replacing deteriorated panels. Because UTW joint spacing usually creates small panels, the removal and replacement of concrete is very simple.

There are six steps involved in repair of UTW

  1. Identify the panels to be removed
  2. Saw cut the perimeter to full depth of concrete
  3. Remove the concrete panels and any deteriorated asphalt
  4. Prepare the patch area
  5. Place, finish, and cure the new concrete
  6. Saw joints and open to traffic

 

Identify and isolate slabs to be removed

The first step in the repair process for UTW is to determine which panels need to be replaced. The panels are usually identified with bright colored paint .cracks in panels do not require repair, but if the UTW overlays breaks up begins moving or dislodging, repairs are warranted. The entire panel containing the distress should be removed and replaced.

 

Saw cut panels to bottom of concrete overlay

The distressed panel should be cut at all joint lines using diamond or abrasive-bladed saws to the bottom of the concrete overlay. Care should be taken not to damage the underlying asphalt layer. Full-depth sawing to the bottom of the concrete overlay results in clean cuts, easy removal of the deteriorated concrete, and less joint spalling.

 

Remove panels

There are two ways to remove the deteriorated concrete:

  • Break up

·         Full panel removal

To break up the concrete, jackhammers are used. Care must be taken not to damage the adjacent panels with any equipment during removal process.

 

 

Prepare patch area

Before concrete placement, the asphalt surface should be cleaned by air blasting with clean, compressed air. Sand or shot blasting is required if air blasting will not remove foreign material from the surface. The asphalt surface must be kept clean prior to the placement of new concrete.

 

Place new concrete

The placement of new concrete into the patch areas of the UTW pavement should follow 6steps:

  • Place the concrete into repair areas
  • Consolidate the concrete using a hand held vibrator
  • Finish with a straight edge or vibratory screed to meet the existing grade
  • Texture the concrete surface to match the surrounding panels
  • Apply curing compound immediately after the bleed water sheen disappears
  • Cover with insulating blankets if the ambient temperature falls below about 5 degree centigrade within 24 hours of placement.

Saw joints and open to traffic

Saw joints as soon as possible without reveling the concrete to match the width and location of existing and adjacent joints. The repaired pavement can be open to traffic once the concrete has reached the specified opening strength, maturity or curing time.

 

 

 

 

ADVANTAGES AND DISADVANTAGES OF UTW

 

Advantages

  • Economical cost – moderately less than that of conventional asphalt surfacing.
  • High standard of surface texture giving good skid resistance and reduced water spray.
  • Reduced noise levels compared to dense-graded asphalt and sprayed seals.
  • Good ride qualities.
  • Flexible and tolerant to surface deflections.
  • Assists waterproofing of underlying surface.
  • Thin layer reduces need for cold planing, etc. to match adjoining surfaces.
  • No loose aggregate such as in a spray seal

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DISADVANTAGES

  • Higher cost than sprayed seal.
  • Effective treatment may require preliminary regulation and SAMI treatments.
  • Low shear resistance may make it unsuitable in areas of high shear forces.

 

 

CONCLUSIONS

  • UTW has emerged as a fast growing technology for pavement rehabilitation.
  • UTW is a fast, competitive remedy for rutted asphalt pavements offering the performance and durability of concrete.
  • UTW overlays are bonded to existing asphalt to create a composite section, resulting in improved performance and durability at a lower cost.
  • UTW develops the required strength for opening to traffic in as little as 24 hours.

 

Due to the above reasons UTW can be treated as an optimal overlay material than conventional concrete pavements

 

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