Most pavements are constructed to fulfil a certain design life from an asset management point of view. Holistically, asset management is a process of efficiently sustaining, improving and running assets using engineering, economic and business principles. Road assets should be managed with the consideration of impacts on society, environment and the economy. In an Australian context, alignment of road assets to the ISO 55000 series is evident as one of the core areas of the Austroads Guide to Asset Management of 2018. In-situ non-destructive testing for road surface strength and bearing capacity can be conducted using a Falling Weight Deflectometer (FWD) whereas in the laboratory the California Bearing Ratio (CBR) test is utilised. The article will assess FWD and CBR techniques whilst focusing on specifications for pavement evaluation and treatment design. FWD can be a great quality control and assurance tool in asset management whereas soaked CBR is important to understand the implications of saturation and its effects on different types of soil.  

What, Why, Where and When!

Key to the FWD technique is assessment of the test itself, why it is used and possible applications in the field where it can be used. Imperative would be to even look at the different types available and what are the differences when it comes to applications in the field. In general, an FWD is a dynamic loading and hydraulic driven device that is designed to simulate heavy vehicle loading while it measures the vertical deflection of the road enabling calculation of rebound modulus of the road surface. These measurements can be utilized in various projects to develop deflection bowls and eventually determine the bearing capacity and residual life of the pavement structure. 

From an asset management point of view, FWD results strongly impact continuous maintenance and rehabilitation needs. FWD testing can also be used on newly constructed pavements to measure if the design requirements were achieved which can be a critical quality assurance tool at the completion of a project. In essence the collected data can be utilized in future deterioration modelling for asset management systems and in the process allowing for the most sustainable rehabilitation designs to be selected for the pavement. Waste reduction can be achieved through the in-situ non-destructive testing of the properties of existing materials. The accurate and timeliness measurement of pavement deflections permits determination of individual layer modulus which can be used as a predictive tool for pavement life cycle valuation. Surface modulus plots obtained from FWD data show linearity or non-linearity of the subgrade, stiffness of the layer, cracking whilst highlighting layer moduli computations and errors. 

The development and advantages of FWD

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The history of FWD devices is important to understand how far the technology has grown over time, hence we will have a look at the traditional light falling weight deflectometer (LFWD) relative to the fast falling weight deflectometer (FastFWD). LWFD tests consists inducing a short dynamic pulse which is caused by the impact of the weight falling from a given height down along the guiderail onto the shock absorber. The pulse is passed through a circular, load-plate to the surface of the ground and results in deformation of the soil under the plate which is measured as a deflection from the electronic measuring system. Its use dates back to the late 70s and early 80s work conducted by Weingart. In last five years, the more recent FastFWD has been developed with benefits of testing at a much greater loading rates subject to test spacing and number of drops per test location amongst other factors. Depending on the project budget testing can be done every 50 m on a network level project whereas in smaller projects testing can be done every 10 m to match scope to budget.


California Bearing Ratio (CBR)

The CBR load penetration test was developed at the California State Highway Department in the 1930s after nine years prior experience of inhouse research consistently showed a correlation amongst CBR values, thickness and plastic deformation of soil. In the laboratory, the CBR value is measured directly on soil samples obtained from site whereas in-situ an estimate of the CBR value can be obtained indirectly using a Dynamic Cone Penetrometer (DCP). The in-situ value of CBR obtained from DCP represents unsoaked CBR values rather than soaked CBR values in which the latter is a strict requirement for pavement design. In the interest of time, cost and ease of performing the test, DCP is used as a supplementary tool to laboratory. Procedurally the CBR soil sample is compacted in a standard mould followed by penetration of the sample on both sides using a plunger at a specific penetration rate. The force required is divided by that required to penetrate a standard crushed rock, the percentage being the CBR. 

In pavement design, CBR values are utilized to design the thickness of the pavement to be constructed on top of the subgrade. CBR enables evaluation of the mechanical strength and bearing capacity of soil being used as a foundation subgrade, subbase or base course during design and analysis of pavements. Some scholars consider CBR as an index of material competence that is influenced by both strength and stiffness but not necessarily a true measurement of neither but rather a common means of comparison. The relationship amongst values of subgrade and CBR values are very soil type dependent and bearing capacity of soil is a direct measure of the resistance of the soil to lateral displacement hence it used as an index of strength and stiffness of subgrade soil. Soaking CBR samples for pavement design purposes also comes with its own challenges as moisture distribution along the specimen may not be uniform which could result in non-homogeneity of saturation leading to variation of soil strength with depth. A solution to the problem has been inverting and testing both the top and base of the soil sample. 

Comparing each method

FWD testing may not replace soaked CBR testing but it offers more data points allowing for options in choice of appropriate design CBR. In the past seven years FWD now offers the ability to estimate subgrade CBR through measuring deflection 450mm away from the load plate at each FWD test point without the need for back analysis or modelling calculations. Good correlations have been struck between laboratory soaked CBR test results and FWD test points. Using FWD the area that needs repair can actually be targeted whereas in the conventional design the entire pavement would have to undergo reconstruction. The recommended approach would be to reduce first by evaluating the existing pavement and secondly recycling any materials that can be used then as the last option reconstruct if there is need to. The first two options reduce costs for the asset owner and are enough motivation to use FWD as a quality assurance check into specifications. 

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