Falling Weight Deflectometer testing

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A Falling Weight Deflectometer is a non-destructive tool used to assess the structural condition of a pavement. This is determined by collecting a series of deflection measurements on a section of pavement, the data from this can be analysed in various ways to ascertain the necessary information. 

Types of evaluation include:

- Direct deflection results

- Determination of stiffness moduli  

- Critical stress and strain calculations 

- Determination of the residual structural pavement life / service life

- Determination of overlay thickness 

The Falling Weight Deflectometer is usually a trailer mounted device that generates a load pulse by dropping a weight from a known height onto a dampened loading plate. The mass, dampeners, and  drop height can be adjusted to create the desired impact loading on the pavement. This is so the rise time (the time in m/s that the load is applied from zero to maximum) can be adjusted to match the rise time of the moving wheel load of the pavement. 

Peak vertical deflection measurements are collected at several radial distances from and including the centre of the loading plate. This data is then related to a computer which will save the data for each station. The majority of Falling Weight Deflectometer’s are able to measure the half-shape of the radius up to 2.5m from the centre plate.

Commonly measurements are taken at intervals of 20m, however this can differ depending on the requirements of the survey. The survey is usually conducted in the nearside wheel track, unless comparative data is required. Comparative data is taken from between the wheel paths, this is because it can be assumed that the pavement between the wheel paths has not been loaded. 

In the United Kingdom target loads are usually 50kN. However, this load should be increased to 75kN if the central geophone does not yield a response of 100 µm. 

Falling Weight Deflectometer’s can also be used to evaluate Ridged Jointed Pavements. The most common use for the Falling Weight Deflectometer on ridged concrete pavement is to measure the efficiency of the load transfer between two slabs. Load transfer efficiency (LTE) is how well a moving wheel load is transferred from the approach slab to the leave slab, in other terms it is to check whether to two slabs either side of a joint deflect uniformly or not. A low LTE could indicate under slab voiding. 

The data arising from the Falling Weight Deflectometer can be used to establish the structural condition of a pavement. 

Firstly, analysis on the Falling Weight Deflectometer deflection data is conducted. The deflections from the Falling Weight Deflectometer are normalised to a standard load of 50kN and deflection parameters D1, D1-D4, and D7 are plotted on a graph. 

The first plot D1 shows the overall response of the pavement. The second parameter is D1-D4, this indicates the response from the bound layers. The third parameter D7N indicates the strength of the subgrade. Low values indicate a stiff subgrade, where as high values could indicate high moisture content reducing the bearing capacity. 

Determination of stiffness moduli can be determined using back-calculation software if accurate pavement construction data is available. This can be ascertained by pavement core logs, and ideally ground penetrating data. From this, a modal of the pavement can be established in homogenous sections. The stiffness moduli arising from this analysis is a good evaluation to determine any areas of deterioration. It is also important to note that some materials are temperature sensitive, so temperature correction need to be made to make the results comparable. The temperature of the pavement will have been recorded at 100mm below the surface of the pavement during the Falling Weight Deflectometer survey. 

At Alptest we have the experience and knowledge to offer all levels of Falling Weight Deflectometer testing and analysis including elements not yet mentioned. We strive to deliver clear results that provide the information required for any levels of survey. We are committed to tailoring inventive and realistic solutions to meet expectations whilst delivering on cost, reliability, and safety.