3.3.1 FWD testing
The purpose of FWD testing is to measure the response of a pavement structure by imitating the dynamic loading induced by a heavy vehicle. The FWD apparatus loads the pavement structure with a falling weight onto rubber buffers. The buffers are situated on a loading plate with a diameter of 300 mm. A photograph of FWD loading plates and measuring geophones is shown in Figure 3.12. The loading plate is placed directly on the pavement surface. The weight is dropped 3 times at each test location and the response of the third drop recorded (Lynch,2013).
During FWD testing the following parameters are measured:
1. Peak load and loading rate applied to the pavement structure
3. Air temperature
4. The surface temperature of the pavement
The load applied to the pavement surface is controlled by the mass of the falling weight as well as the height from which it is dropped. This load is measured as it should replicate the load applied to the pavement structure by a heavy vehicle vehicle. The load is typically representative of one wheel of a standard axle passing over it. In South Africa a standard axle is an 80 kN axle, and therefore the peak load of a FWD drop simulating a standard axle should be 40 kN.
Figure 3.12: FWD load plate and geophones (Lynch,2013)
The loading plate of the FWD has a diameter of 300 mm, resulting in a calculated pressure of 566 kPa at peak loading. The actual peak stress varies slightly due to surface properties of the pavement, the load not being exactly 40 kN and random variability (Lynch, 2013). The rate of loading is controlled by rubber buffers positioned between the load plate and the falling weight.
The deflection of the pavement surface during application of the load is measured using geophones. These geophones are spaced at intervals from the centre of the loading plate as illustrated in Figure
3.13(units in mm). The measurements at the different geophones are indicated as Dxxx, where ’D’
By measuring the deflection bowl caused by the loading, with the magnitude and geometry of the stress applied to the pavement surface known, the stiffnesses of the pavement layers can be determined through the method of back-calculation. The method of back calculation is discussed in Section 3.4.
Figure 3.13: Illustration of FWD geophone positions from the centre of the loading plate (Lynch,
2013)
Other, more fundamental, parameters were used to indicate the structural condition of the pavement layers include:
1. The maximum deflection (D0or Dmax) was measured as the maximum deflection at the point
of loading. This parameter provides an indication of the overall condition of the pavement structure.
2. The Base Layer Index (BLI) was calculated from the deflection bowl measurements as shown in Equation3.1. The BLI provides valuable information about the condition of the surfacing and base layers.
BLI = D0− D300 (3.1)
3. The Middle Layer Index (MLI) was calculated from the deflection bowl measurements as shown in Equation3.2. The MLI index provides an indication of the condition of the subbase and the upper selected subgrade.
M LI = D300− D600 (3.2)
4. The Lower Layer Index (LLI) was calculated from the defection bowl measurements as shown in Equation 3.3. The LLI gives an indication of the condition of the lower selected subgrade
and the subgrade.
LLI = D600− D900 (3.3)
Air and surface temperature are usually measured at the time of testing for each station where a FWD test is carried out. The temperature measurements are used when tests are carried out on bituminous materials like asphalt or surface seals. The properties of bituminous materials, specifically resilient modulus, are significantly affected by temperature and loading rate (Lynch,
2013).
3.3.2 LTPP FWDs
FWD tests were performed on the LTPP sections at different points in time. A large amount of FWD test data was available for these pavements, therefore, only a summary of the results is included in this study. The FWD test results available for each of the LTPP pavements is discussed in AppendixB.
The FWD data was analysed to obtain the layer indices as well as the development of these layer indices over time. The LTPP FWD tests were performed using a 40 kN loading plate with a diameter of 300 mm. In some cases the pressure applied by the loading plate was measured and used during back calculations. Where the pressure of the loading was not measured it was assumed to be 566 kPa. The results of the LTPP FWD tests are discussed in Section4.2.1.
3.3.3 N7 FWDs
FWD tests were performed on the BSM section of the N7 highway in 1994, 2000, 2004, 2010 and 2016 in order to monitor the long term pavement performance of bitumen stabilised materials. FWD tests were performed every 200 meters in the slow lanes of the North and Southbound carriageways. A timeline of the FWD testing and rehabilitation is shown in Figure 3.14.
Figure 3.14: Timeline of N7 testing and rehabilitation
The FWD tests performed on the N7 used an applied load of 40 kN on a loading plate with a diameter of 300 mm. The pressure beneath the loading plate was calculated as 566 kPa. The air
and surface temperatures were measured at each test location at the time of testing. The FWD tests performed in 1994, 2000, 2004 and 2010 made use of 7 geophones to measure the pavement deflection, while the tests performed in 2016 made use of 9 geophones. Rut measurements, ride quality, visual inspection and texture analyses were made along with the FWD measurements.
The results of the N7 FWD tests were analysed and used to divide the road into uniform sections. The division of uniform sections is discussed in Section 3.4.3. The results of the FWD back calculations are discussed in Section4.3.2.
The FWD test results from different years were used to thoroughly analyse the bitumen stabilised sections of the N7 between Cape Town and Melkbosstrand (5.8 - 18 km). The main focus was to determine the effective long term stiffness (and performance) of the bitumen stabilised base layers in pavement structures over a given number of years and to determine the trend in resilient modulus development of this layer over time.
Limitations in the N7 investigation were present due to FWD data only being available for certain years. The Western Cape’s road network is only investigated every 4 to 6 years with FWD tests, resulting in considerable time intervals between test results. The result of this is a less accurate determination of the trends in resilient modulus of the pavement structure and specifically the BSM layer.
In order to make an accurate determination of the time lapse between construction and the onset of BSM resilient modulus increase, annual FWD data from 2002 to 2017 would have to be analysed. This is not possible as aforementioned annual data is not available. The foamed section of the N7 was constructed in 2002 with FWD tests being performed in 2004, 2010 and 2016. The FWD results from 1994 and 2000 were also analysed and interpreted to compare the average stiffnesses before and after rehabilitation.