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Ultra-Thin Whitetopping (UTW) Project
Ultra-Thin Whitetopping (UTW) is a rehabilitation technique in which a 50- to 100-mm thick layer of hydraulic cement concrete is placed over a milled surface of rutted and/or cracked asphalt concrete pavement. In 1998, to help State and local highway agencies make decisions about using UTW for various applications, the Federal Highway Administration (FHWA) and the American Concrete Pavement Association (ACPA) entered into a cooperative agreement to research UTW. The agreement resulted in a research project involving the evaluation of eight 14.6-m long test lanes of UTW placed over asphalt concrete pavements in the FHWA's Pavement Test Facility in McLean, Virginia.
The purpose of this study is to validate design equations and models recommended in the ACPA's design methods for UTW and to document the performance of UTW. More specifically, the objectives are to: (1) evaluate UTW performance under controlled wheel-loads and temperature; (2) study the effects of design features on UTW performance; (3) measure pavement responses to develop mechanistic models; and (4) verify and develop models to predict load-carrying capacity.
The study is specifically aimed to address the effects of overlay thickness, joint spacing, and fiber reinforcement on UTW performance. The experiment matrix is shown in Table 1.
The overlay thickness and joint spacings are typical values currently employed in UTW designs. Joints are sawed longitudinally and transversely with the same spacing. The fiber concrete is a conventional portland cement concrete with fibrilated polypropylene fiber added to increase resistance to plastic shrinkage cracking and to provide additional tensile strength.
The UTW was placed over 200-mm thick AC pavements that were in various stages of rutting distress after extensive testing with Accelerated Loading Facility (ALF) pavement testing machines at different times during the previous 5 years. The pavement sections had been built with seven different AC mixtures as part of an experiment to validate the Superpave performance grading asphalt binder system. Prior to placement of the UTW overlays, the AC in each section was milled to either of two depths (80 or 115 mm) to remove the surface ruts and to provide for a final pavement thickness, after overlaying, of 200 mm.
Thus four lanes met the definition of thin whitetopping, i.e. greater than 100 mm.
Loading of the Test Sections
Test sections were loaded by two ALF pavement testing machines. The FHWA ALF machines apply real-world truck loading in an outdoor, but controlled pavement temperature environment. Loads can be varied between 44 and 100 kN on a pair of dual 11R22.5 radial ply truck tires (which represent one half of a single-axle load). Actual loadings during the UTW experiment were either 44 or 54 kN; at these levels, the ALF applied 1.52 or 3.62 80-kN equivalent single axle loads (ESAL) per pass, respectively, to a typical test section (Lanes 11 and 12). The speed of the load was a constant 17 km/h (4.7 m/sec), with an average load rate of 35,000 repetitions per week.
Since the ALF's radiant heaters tried to maintain the temperature at the top of the asphalt concrete a constant 27°C, the slabs experienced thermal gradients similar to those measured at MNROAD. In fact, there were diurnal cycles every two to three hours.
The concrete mixture used for the UTW contained 363 kg/m3 of cementitious material, 50 percent of which was Type I cement per AASHTO M-85 and 50 percent of which was Grade 120 slag per AASHTO M-302. The aggregate was a number 7 crushed stone with a 12.5 mm top size per ASTM C-33. The mixture included a mid-range water reducer in aiming to achieve a design slump of 178 mm while maintaining a water-cementitious material ratio of 0.45. The target entrained air content was 4.5 to 7.5 percent, while target strengths were 27,600 kPa in compression and 4,500 kPa in flexure. A 7-day moist cure was specified. Fibrilated polypropylene fiber (1.78 kg/m3) was used in half of the sections. For some reason, the concrete producer withheld some water from the second batch of concrete, i.e. the plain mixture. This is likely to be the reason the plain concrete had 20% higher strength and stiffness parameters.
Results of field tests for slump, unit weight, and air content and of 28-day laboratory tests are shown in Table 2.
The test sections were instrumented to allow measurement of pavement deflections and strains at various locations in the concrete slabs and the overlaid AC layer (figure 1). Fifteen (15) to 18 strain gauges were installed in each lane near the top and bottom of the UTW and on top of the AC. Interior and joint deflections were monitored with linear voltage displacement transducers (LVDT's).
Data Collection and Analysis
Data was collected for key parameters in the UTW design procedure, including the layer moduli, PCC flexural strengths, bond strengths, and the percent of fatigue life consumed for the existing AC pavements. The design factorial provided pavement performance data to test the accuracy of the UTW design equations. Fatigue cracking, faulting, and roughness data will be used to calibrate models. Pavement response data (deflections and strains) have been compared to theoretical calculated values. It has been discovered that critical edge strains occur at the tops of slabs. It was also discovered that strains at the top edge increase with traffic while those at the bottom decrease a proportional value, so the neutral axis remains constant. The most significant findings for design relate to the impact of joint and asphalt concrete stiffness on response. Pavement response data is also being used to study the effects of load transfer and bond strengths on UTW performance.
Loading of the test sections began in May, 1998, and phase I was completed in November, 1999. Loading of individual sections varied between 7 and 30 weeks, i.e. from 266,000 to 1,072,630 wheel loads. The total ESAL's was over 19.5 million.
In April of 2000 cracked slabs on lanes 6 and 10 were replaced with fastrack and an additional 400,000 loads were applied to each of the lanes over the next three months. A final report is under preparation by FHWA staff on the contents of the database as well as recommendations for design.