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Asphalt Pavement Technology
Bituminous Mixtures Laboratory (BML)
Equipment French (Laboratoire Central des Ponts et Chaussees) Pavement Rutting Tester
Measures the rutting susceptibility of asphalt paving mixtures using a reciprocating, pneumatic rubber tire. Slabs of asphalt concrete can be tested at temperatures ranging from 30° C to 70 °C. Rutting susceptibility is based on pass/fail criteria. This machine is used in France to evaluate mixtures that either have no performance history or will be subjected to heavy traffic.
The French (Laboratoire Central des Ponts et Chaussees) Pavement Rutting Tester tests slabs for permanent deformation at 60°C using a smooth, reciprocating, pneumatic rubber tire inflated to 0.60 ±0.03 MPa. This tester is used in France to evaluate mixtures subjected to heavy traffic; mixtures that incorporate materials that tend to lead to rutting, such as some natural sands; and mixtures that have no performance history. It is also used for quality control purposes during construction. This tester costs $85,000 and is shown in on the cover and in figure 1.
This machine tests a slab with a length of 500 mm, a width of 180 mm, and a thickness of either 50 mm or 100 mm. Other thicknesses between 20 and 100 mm can be tested by fabricating a nonstandard-sized mold or by putting the slab on plaster of Paris or steel plates. A thickness of 50 mm is specified for mixtures that will be used in surface course layers less than or equal to 50 mm. Thin surface course layers in France are generally placed at thicknesses ranging from 30 to 40 mm. The 100-mm thickness is specified for mixtures that will be used in surface or base course layers greater than 50 mm. In France, these layers are generally placed at thicknesses ranging from 60 to 80 mm. A slab cut from a pavement can also be tested, but it must be cut to fit the mold. Cut slabs having small gaps between the slab and the mold must be secured in some way, such as with plaster of Paris. The allowable deviation in thickness for a cut slab is the average thickness ±5 mm. The mass of a 100-mm-thick slab is approximately 22 kg.
The base and sides of the mold that holds the slab are rigid steel. Slabs are compacted to at least two air-void levels in France, and often to three levels. After compaction, the slab is aged at room temperature for up to 7 days. The density of the slab is obtained after compaction, but the slab is then placed back into the mold for testing.
The machine tests two slabs simultaneously using two reciprocating tires. The wheel load on both slabs must be equal to avoid asymmetric pressures on the tire assembly. However, the two slabs do not have to be replicates and, in fact, the French recommend testing mixtures in random order to account for variabilities associated with the machine over time. Replicate slabs are tested at different times using both sides of the machine.
Hydraulic jacks underneath the slabs push them upward to create the load. The standard load is 5,000 ±50 N; the maximum load is 5,500 N. Pressure gauges on the control panel of the machine give the pressure in each jack. Each pressure gauge is calibrated in increments of 0.1 MPa using a load cell. A graph of pressure vs. load is constructed and used to apply the desired load. The weight of the mold and slab are not included in the applied load. The average pressure provided by 5,000 N was determined to be 0.59 MPa for the left tire of the Federal Highway Administration machine and 0.55 MPa for the right tire. The tires were inflated to 0.60 MPa and loaded on flat steel plates to obtain the contact area.
The French Pavement Rutting Tester uses the same type of tire as the French Plate Compactor. Each tire has a diameter of 415 mm and a width of 110 mm. The standard tire pressure is 0.60 ±0.03 MPa; the maximum pressure is 0.71 MPa. It takes approximately 0.1 s for the tires to travel from one end of a slab to the center with the speed being fastest at the center. The tires remain at a fixed elevation as they travel back and forth across the slabs. The average speed of each wheel is approximately 7 km/h; each wheel travels approximately 380 mm before reversing direction, and the device operates at approximately 67 cycles/min (134 passes/min). One cycle is defined as two passes of the tire (back and forth).
Initially, 1,000 cycles are applied at 15 to 25°C to densify the mixture and to provide a smoother surface. This requires approximately 15 min. The thickness of each slab is then calculated by averaging 15 thickness measurements taken at 15 standard positions using a gauge with a minimum accuracy of 0.1 mm. This thickness is considered the initial thickness of the slab. The slabs are then heated to the test temperature of 60 ±2°C for 12 h. A test temperature of 50 ±2°C is sometimes used in France for base courses. The test is started, and the average rut depth in each slab is measured manually at 30, 100, 300, 1,000, and 3,000 cycles when testing 50-mm slabs, and at 300, 1,000, 3,000, 10,000, and 30,000 cycles when testing 100-mm slabs. Rut depth measurements at 30, 100, and 300 cycles are included in the 100-mm slab test if it is hypothesized that the slab will fail before 3,000 cycles. The average percent rut depth based on the initial thickness of the slab is calculated.
The application of 3,000 cycles requires 1.5 h, whereas 30,000 cycles require 9 h. The tester can apply 3,000 cycles in approximately 45 min, but after each rut depth measurement, the temperature of the slab must be reestablished before the test is started again. Rut depths are measured manually, which requires the environmental chamber to be opened.
When testing 50-mm slabs, a mixture is acceptable according to the French specification if the average percent rut depths at 1,000 and 3,000 cycles are less than or equal to 10 and 20 percent, respectively. When testing 100-mm slabs, a mixture is acceptable if the average percent rut depth at 30,000 cycles is less than or equal to 10 percent. Slopes for different mixtures taken from log rut depth vs. log cycle plots can also be compared. Rut-susceptible mixtures generally have higher slopes, but there is no French specification on the slope.
The test method reportedly is not valid for mixtures with nominal maximum aggregate sizes greater than 20 mm. The slab width of 180 mm is relatively small compared to the tire width of 110 mm. A space of only 35 mm exists on each side of the slab between the tire and the steel mold. Therefore, mixtures with aggregates greater than 20 mm may be inhibited from shearing outward and upward. Aggregates larger than 20 mm may also wear the tires severely, and often cannot be compacted properly using the French Plate Compactor. The machine was developed primarily for testing surface layer mixtures. It was later used in France to test surface treatments for chip retention and elasticity. Surface treatments are often placed on stabilized soils when tested.
Disadvantages of the French Pavement Rutting Tester are that the data cannot be used in mechanistic pavement analyses and cannot be used to deter-mine the modulus of the mixture or layer coefficients used by American Association of State Highway and Transportation Officials thickness design procedures. This is due to the complex and unknown state of stress in the slab.