Buyers Guide

Using strains within the elastic limit of the materials a sinusoidal load is applied to a beam in either the two point or four point bending test configuration. The determination of complex modulus takes into account the energy dissipated as heat during the test.

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Bending or flexural tests are the most widely used methods for assessing the fatigue resistance of asphaltic materials. Four point bending is included in both CEN and AASHTO specifications. The specimen is a prismatic beam, which is subjected to sinusoidal loading in the controlled strain mode. In Europe the principal alternative is two-point bending using trapezoidal specimens, a test which was developed at LCPC in France.

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A static axial stress is applied to a cylindrical specimen for a fixed period of time during which axial strain is continuously monitored. The axial stress is then removed and both the permanent and recovered strain determined. Investigations have shown that dynamic creep correlates better with in-service pavement rutting measurements than static creep. Static creep is not included in the latest European standard and does not feature in current American standards.

A dynamic axial stress is applied to a specimen for a specified number of load cycles while axial strain is monitored. It has been found that the results of this type of test correlate better than static creep with inservice pavement rutting. Correlation can be enhanced with the application of confining stress.

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A sinusoidally varying axial stress is applied, at a range of frequencies, to a specimen measuring 100mm in Ø by 150mm high. The specimen is cored from a 150mm Ø sample prepared by gyratory compaction. Recoverable (dynamic) and permanent strains are measured using on-specimen transducers and the phase lag between stress and strain is determined for the various test conditions providing information on the visco-elastic properties of the material. Dynamic modulus tests form part of the Simple Performance suite of tests and are detailed in AASHTO TP62.

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A repeated load is applied along the vertical Ø of a cored or laboratory moulded specimen at various frequencies and magnitudes. The resultant horizontal (indirect) deformations are measured and used to provide a measure of stiffness. In Europe the test is mainly used as a rapid method of quality control but it can also be used for a variety of other purposes including failure investigation. Similar tests to measure resilient modulus were detailed in ASTM and AASHTO standards, but they have now been withdrawn.

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A cored or laboratory compacted slab of hot mix asphalt is subjected to repeated passes of a loaded wheel while the resultant rut depth is monitored. Wheel tracking is popular because it is more simulative than other laboratory methods of assessing resistance to permanent deformation and results have been shown to correlate well with in-service pavement rutting. There are two widely used wheel tracking test methods in Europe: one was developed at LCPC in France and the other is based upon the method developed at the TRL in the United Kingdom. Less widely used, the Hamburg wheel tracker tests specimens submerged in water and provides a measure of durability as well as resistance to rutting. There is no national standard for wheel tracking in the United States.

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In order to produce materials in the laboratory with properties which most closely simulate those in the pavement, gyratory compactors and roller compactors are used. In gyratory compaction a 150 or 100mm cylindrical sample is subjected to a vertical compressive stress while the mould is subjected to a gyratory motion, at a specified angle to the vertical, that generates interparticle movement within the material. A slightly larger angle of compaction is specified in the USA and other non-European countries. Roller compaction is considered to be the laboratory method that produces materials which are most simulative of those laid in the highway. The most common type uses a steel faced roller to compact 305mm square slabs. A vibrating roller is an option used to increase compactive effort. The roller compactor developed at the LCPC in France uses single or twin pneumatic tyres that compact the mixture as it is incrementally pushed up inside the mould. The use of pneumatic tyres generates a kneading action and ensures that the aggregate is not damaged during compaction.

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Generally performed in compression for convenience, strength tests measure the maximum stress a specimen can sustain in the controlled rate of deformation mode. In the Marshall stability test the specimen is loaded in a pseudo-diametral mode. In the Duriez test a cylindrical specimen is loaded axially and compressive strength is calculated as the ratio of maximum load to the initial cross-sectional area of the specimen.

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The mechanical properties of unbound materials are normally assessed using the triaxial test. A cored or laboratory prepared specimen is subjected to repeated haversine loading, the variations in axial stress and axial deformation are recorded and the resilient modulus is calculated. The test is normally performed using specified sequences of axial and confining stress combinations. Difficulties in preparing representative specimens can limit the use of the test to research establishments. Recently attempts have been made to find more simple, robust and user-friendly alternatives. The Springbox and K-Mould are notable examples. At present NTEC and Cooper Technology are conducting a two year investigation into unbound materials testing with the aim of developing a lower-cost system that can be used on a routine basis.

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