Greg White

APSDS is a layered elastic tool for aircraft pavement thickness determination developed and distributed by Mincad Systems and based on the sister software Circly. As aircraft pavement thickness determination remains an empirical science, mechanistic-empirical design tools such as APSDS require calibration to full scale pavement performance, via the S77-1 curve. APSDS provides the unique advantage over other tools that it models all the aircraft in all their wandering positions, negating the need for designers to use pass to cover ratios and acknowledging that different aircraft have their wheels located at difference distances from the aircraft centerline. APSDS requires a range of input parameters to be entered, including subgrade modulus, aircraft types, masses and passes and a pavement structure. A pavement thickness is then returned which has 50% design reliability. Greater levels of reliability are obtained by conservative selection of input values. Whilst most input parameters have a linear influence on pavement thickness, subgrade modulus changes have a greater influence at lower values and less influence at higher values. When selecting input values, designers should concentrate their efforts on subgrade modulus and aircraft mass as these have the greatest influence on the required pavement thickness. Presumptive or standard values are generally acceptable for the less influential parameters. S77-1 pavement thicknesses are of a standard composition with only the subbase thickness varying. Non-standard pavement structures are determined using the principle of material equivalence and the FAA provides range of material equivalence factors, of which the mid-range values are most commonly used. APSDS allows direct modelling of non-standard pavement structures. By comparing different APSDS pavements of equal structural capacity, implied material equivalences can be calculated. These APSDS implied material equivalences lie at the lower end of the ranges published by FAA. In order to obtain consistence between APSDS and the FAA guidance, the following material equivalence values are recommended: Asphalt for Crushed Rock. 1.3; Crushed Rock for Uncrushed Gravel. 1.2; Asphalt for Uncrushed Gravel. 1.6. Proof rolling regimes remain an important part of the design and construction of flexible aircraft pavements. Historically, designers relied on Bousinesq's equation and the assumption of point loads on semi-finite homogenous materials to determine proof rolling regimes using stress as the indicator of damage. The ability of APSDS to generate stress, strain and deflection at any depth and any location across the pavement allows these historical assumptions to be tested. As the design of a proof rolling regime is one of comparing damage indicators modelled under aircraft loads to those under heavy roller loads, the historical simplifications are generally valid for practical design scenarios. Where project specific data is required, APSDS can readily calculate stresses induced by proof rollers and aircraft at any location and depth for comparison. APSDS is a leading tool for flexible aircraft pavement thickness determination due to its flexibility, transparency and being free from bias. However, the following possible areas for improvement are considered worthy of future research and development: Improvements to the user interface; Ability to model aircraft masses as frequency distributions; Ability to copy stress with depth data to Excelâ„¢ spreadsheets; Ability to perform parametric runs; Inclusion of a reliability based design module.

Insitu cementitious stabilisation is an economical, environmentally sustainableand socially advantageous means of rehabilitating pavements. With the recentavailability of a wide range of binders and advanced construction equipment,the characterisation of cementitiously stabilised pavement materials hasbecome the focus of further advancement of this technology.Australian practice has moved towards the use of Indirect Diametric Tensile(IDT) methods for the characterisation of these materials. A draft protocol forthe IDT test has been prepared and specifies samples to be compacted bygyratory compactor. This procedure provides for both monotonic and repeatedload testing, which aims to measure the material's strength, modulus andfatigue life.A range of host materials, including a new crushed rock and a reclaimedexisting pavement base course, were assessed when stabilised with a GeneralPurpose cement binder as well as with a slag-lime blended binder. Materialswere assess for their inherent material properties, Unconfined CompressionStrength (UCS), Unconfined Compression modulus, IDT strength and modulusunder both monotonic and repeated load.A number of amendments and refinements to the testing protocol wererecommended. These included the use of minimum binder contents to ensurethe binder was uniformly distributed and to promote heavy binding of thematerials to ensure they behaved elastically. It was also recommended thatsamples be gyratory compacted to a pre-determined sample height to allow aconstant density to be achieved.The variability of the test results was examined. UCS results were found to becomparatively as variable as other researchers had reported. IDT strengthresults contained a similar level of variability, which was considered to beacceptable. Modulus results, both monotonic and repeated load, were found tobe five to ten times more variable than strength results, which is a generallyaccepted trend for modulus testing.Under repeated loading, some challenges with the test protocol wereencountered. The primary challenge was obtaining reliable and repeatablediametrical displacement data for modulus calculation. This was partiallyovercome by the insertion of smooth spacers to prevent the Linear VoltageDisplacement Transformer (LVDTs) becoming caught on the sample sides.The achievement of reliable and repeatable IDT modulus results throughimproved displacement measurements should be the focus of future researchefforts in this area.