Permanent deformation, or rutting, is a load and temperature-related distress. There are two parts to the formation of a rut: 1) volumetric change, and 2) shear flow. Volumetric change happens when the HMA densifies under traffic loads. Once the HMA achieves a certain densification (usually assumed to be about 4% air voids), further deformation occurs at the result of shear flow due to traffic loads. Both are accentuated by warm weather. A number of laboratory test methods have been tried over the years to predict the in-service rutting characteristics of HMA. This section covers the most commonly used methods.
The US Corps of Engineers gyratory compactor was the first one to include sufficient instrumentation to evaluate the shearing characteristics of the mix at compaction temperatures. Loaded wheel testers, sometimes called torture testers, have been gaining acceptance for pass-fail evaluations of mixtures at in-service temperatures. The simple shear tester (SST) developed under the original Strategic Highway Research Program (SHRP) research is also used but requires large, expensive equipment (at least at this time). This test is primarily limited to research evaluations. Uniaxial or triaxial testing is one of the oldest methods of assessing the permanent deformation behavior of HMA. While the other test methods have been used over the years because they are usually simpler to use (except the SST) and require less expensive or less complicated for the technician, these methods are beginning to be used more. Preliminary recommendations for a simple performance test are based on variations of these methods.
The next few slides define several of the terms commonly associated with permanent deformation testing.
The first three loaded wheel testers listed use either laboratory compacted gyratory samples or slabs of HMA (lab or field). The last two testers fall more into the accelerated loading facility category than a lab test. That is, they are used more on actually pavements rather than typical laboratory-sized samples.
The current Asphalt Pavement Analyzer (APA) is based on the original Georgia Loaded Wheel Tester (GLWT). This unit has an environmental chamber that can be set to test the samples at the anticipated maximum pavement temperature. Either HMA slabs or gyratory compacted samples are placed in the chamber; either 3 slabs or 6 gyratory samples can be tested at one time. The load is applied by loading a pressurized rubber hose that lays across the samples with a steel wheel roller at a preselected axial pressure. Loading continues for a predetermined number of cycles; vertical deformation in the sample is measured at various points during the loading so that the rutting potential of the mix can be evaluated.
This slide shows 4 of the 6 gyratory samples (the other two are off the screen to the left). The hoses can be seen in this picture as well as the pressure lines for controlling the hose pressure (green hoses in front). The steel wheels are in the up position and can be seen at the top of the photo. Plastic molds are used to provide lateral support for the samples. The sample height is usually limited to 75 mm although special molds are available that can use standard height gyratory samples (i.e., 115 mm)
Either manual or electronically monitored rut depths in the “wheel path” are measured. If the manual method is used, a minimum measurement at the start of the test and at the end of the test, usually between 6,000 and 10,000 cycles are taken. Measurements at 0, 1,000, 2000, 4,000, and 8,000 or 10,000 cycles are common. This provides information on how fast the rut is progressing after the initial densification. For example, Georgia DOT specifies a maximum rut depth of 5 mm after 8,000 cycles on high quality HMA.
Some states use APA results to set pass-fail limits on the maximum rutting allowed after 8,000 passes. This slide shows an example of how the data could be used.
This slide gives the student an idea of the range of testing variables that can be used in the APA test. It also compares the parameters originally used in the Georgia Loaded Wheel Tester with those more commonly used in the APA. Note that the only significant changes are that the APA can test samples in either a wet or dry condition and that the upper end of the test temperature has been increased slightly.
The primary stage represents rutting associated with volumetric change. The secondary stage, the rate of deformation is a constant. The tertiary flow stage, the rate of deformation continues to increase and is associated with the start of non-linear behavior (i.e., sample damage) . At low stress levels, the rate of deformation in the secondary stage can approach 0 as the total strain reaches some terminal value. One of the limitations in using just the APA rut depth at 8,000 cycles is that the deformation could be in either the secondary or tertiary flow stage. The concept that one mix is beginning to fail in shear flow while the other is not may not be readily apparent from just the evaluation of a single number. Tertiary flow is generally considered to occur at a constant volume.
Another commonly used loaded wheel tester is the Hamburg. This unit can test two slabs at one time (one slab under each moving wheel loaded by the hanging weights). This unit is more popular in Europe than in the United States.
The PurWheel, developed at Purdue University, is a variation of the Hamburg. This test can also be performed wet or dry.
This test uses a rubber tired roller to apply the loads. Note the tire tread is visible in this figure. The slab is contained in a plaster of paris coated to provide horizontal support.
This slide provides a comparison of typical testing parameters used with both the Hamburg and PurWheel loaded wheel testers. The major difference is the type of wheel used to apply the loading.
The loaded wheel testing used at LCPC uses a circular HMA with varying structures. Up to 4 different mixes and/or structures can be tested (1/4 of the circle for each) The carriage has four loaded tires and the whole mechanism rotates around the center. More information can be found on the LCPC web site: www.lcpc.fr/LCPC/English/Presentation.
Unlike the previous test methods and devices, the US Corps of Engineers gyratory compactor evaluates the HMA at compaction temperatures rather than typical in-service temperatures. This means that this unit will evaluate HMA properties at may be more associated with the aggregate properties rather than combinations of asphalt binders and aggregates.
This photo shows the original type of unit used for this type of sample compaction and testing during compaction. It is a large floor model with dial gauges and strip chart recorders (not shown) that need to be read and interpreted.
The new version makes use of the electronic measurement, data acquisition and software advancements. A portable computer has replaced the need to manually read and/or interpret data. This particular unit is housed in a trailer for use on construction sites.
The simple shear test was originally developed during the SHRP research and is essentially a variation of the traditional direct shear box testing used in soils testing.
The SST equipment is a dual actuator closed loop testing unit equipped with an environmental chamber. One actuator is used to control the axial load needed to maintain the height of the sample constant. The second actuator is used to move the bottom platen attached to the sample back and forth horizontally. This applies the dynamic shear loading on the sample. A gyratory compacted sample is cut on both the top and bottom so that the faces are parallel. The loading platens are then epoxied to these cut faces and the two LVDTs for measuring the height of the sample are attached to the platens at 180 o . The sample is then put into the chamber (next slide) and the LVDT wires plugged into the test equipment.
This figure was obtained from the MTS web site and depicts the essential components associated with standard triaxial testing of asphalt concrete. In this example the deformation of the full height of the sample is measured. It is also common, although more complicated to set up, to instrument only the center 1/3 of the sample for vertical deformation measurements.
HMACharacterization Permanent Deformation 4Terms• Uniaxial– Vertical axial load only – no confiningpressure• Triaxial– Both vertical and horizontal pressure used• Horizontal pressure used to simulatefield conditions of lateral support
HMACharacterization Permanent Deformation 5Terms• Creep– Static load applied over some time interval• Usually 1 hour or more• Vertical deformation measured• Creep recovery• Load removed and rebound of materialmeasured
HMACharacterization Permanent Deformation 6Terms• Repeated loading– Step load followed by a rest period• Load from 0.05 to 3 seconds• Rest–From 0.6 to 1 minute– Typical = 0.1 second load & 0.9 sec rest
HMACharacterization Permanent Deformation 7Terms• Dynamic loading– Cyclic loading• Can be all compression or include stressreversal–Tension to compression cycles»Simulates stress reversal seen inpavement as tire passes a point
HMACharacterization Permanent Deformation 20US Corps of EngineersGyratory• Originated at the US Corps of EngineersWaterways Experiment Station• Generally applicable to dense-graded mixes– May not be strictly applicable to mixes thatare not sensitive to bitumen content• Can get Mohr’s circle information
HMACharacterization Permanent Deformation 22US Corps Gyratory (New)
HMACharacterization Permanent Deformation 23Simple Shear Tester (SST)• Simple Shear Tests– Permits controlled rotation of principal axesof stress and strain– Assumes shear deformation rather thandensification is the primary ruttingmechanism9