The HVS Laser Profilometer is used to measure the deformation of the surface of the test section, commonly referred to as rutting. The profilometer consists of an aluminium frame that spans the cross sectional width of the test section resting on legs on either side of the test section.

A measuring head containing the laser head is attached to the beam of the frame via a motorised carriage and moves along the length of the beam taking readings every 10 mm over a total length of 2.56 m. Transverse profiles of the test section are taken at specified points along the test section (usually 16 points per test section). This profile data can be processed to determine the average rut depth and correlate permanent deformation of the test section with the MDD data. Three-dimensional plots of the surface deformation of each section are generated for comparison during various stages of APT. (top of page)

The RSD is essentially an electronic version of the Benkelman Beam. It is used to measure the surface deflection bowl under HVS loading. The RSD consists of a T-shaped beam that has a measuring point at the sharp end of the beam and two reference feet at the other side of the beam. An LVDT is located in the beam at a position 1/6^th the length of the beam from the T-intersection.

The measuring point of the RSD is positioned between the two tyres of a dual tyre configuration. The tyres are then moved away from the reference feet of the RSD towards and beyond the measuring point. The position of the tyres is logged at regular (preselected) intervals. The pavement is deflected and the LVDT records the movement of the pavement surface relative to the reference feet. These data are logged and the elastic surface deflection bowl generated from the data. Measurement accuracy is approximately 10 microns. Data collection is automated and thus the RSD can be utilised to capture the entire deflection basin, consisting of 256 data points, as the dual wheel traverses the RSD at creep speed.

Due to the nature of the testing and the speed of measurement it is necessary to halt HVS operation during RSD measurements. The RSD is a versatile pavement monitoring device, which can be used to measure deflection bowls separate from the HVS. (top of page)

The MDD is used to measure the in situ elastic and/or permanent deformation in the various pavement layers of a test section. The plastic deformation data have been used to develop transfer functions relating load repetitions to plastic strain in road building materials. The elastic deflection data can be used to determine the effective elastic moduli of each pavement layer.

The primary component of the patented MDD system is a series of Linear Variable Displacement Transducer (LVDT) modules, which are mounted in a road in a 39 mm diameter hole drilled in the test section. Up to six LVDT modules can be mounted at various depths, not closer than 150 mm apart, in each prepared hole.



Once the hole has been drilled in the test section, an anchor rod is anchored into the subgrade approximately 3 m below the pavement surface. The reference rod is connected to the anchor rod by means of a ball snap connector to allow for the removal and re-use of the LVDT modules. A lining membrane is installed, prior to the installation of the LVDT modules.

Anchoring of the LVDT modules to the soil is accomplished by a number of small steel balls that are forced out of a sleeve against the membrane at the desired depths. The LVDT modules measure the displacement of the soil relative to the reference rod to an accuracy of ± 10 microns. The top of the MDD hole is sealed with a cap, which contains a connector to the data acquisition system. (top of page)

Two transducer types are utilised for the in situ measurement of stresses and strains in the HVS test sections:

 

• The Soil Pressure Transducer (SOPT), and;
• The Pavement Strain Transducer (PAST) 

The SOPT is a precision gauge for the measurement of vertical stresses in the unbound layers of the pavement. The entire transducer is made from titanium, providing maximum resistance to galvanic and mechanical deterioration. Each SOPT unit is epoxy and sand coated to ensure good performance in all types of unbound pavement construction materials. 

The PAST is used for measuring the horizontal strains at the bottom of the upper bound pavement layer (bottom of asphalt layer in most instances). They can be installed directly onto the base layer, in either the longitudinal or transverse direction, prior to surfacing of the test section. 

Both the SOPT and PAST series of gauges have a design life in excess of 36 months of APT (up to 100 million normal load repetitions for the PAST III strain gauge type). The transducers are automated and connected to the HVS data acquisition system allowing for continuous monitoring of stresses and strains at various locations within the pavement during HVS trafficking.

These instruments are used infrequently in HVS testing in South Africa. They are however utilised in HVS APT internationally, particularly during APT of concrete pavement structures and thick (>50 mm) asphalt overlays.

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The CAM allows for the monitoring of load associated movement and progression of cracks in a pavement. An accurate measurement of the magnitude of cracks and rate of crack increase (increase in width and associated movement) assists in the selection of the appropriate surfacing to accommodate such movement.

The small size of the CAM allows for its installation between the paths of a dual wheel HVS loading configuration. The CAM was specifically developed for measuring the crack activity with the dual tyre configuration. Although crack activity can be measured next to a single tyre, this is not the standard procedure. The CAM body needs to be glued to the one side of the crack and the vertical plate to the other side of the crack. The captured data are used in the simulation of cracking of materials in laboratory analysis, in order to evaluate the materials ability to retard crack reflection.

The CAM is capable of measuring both the relative horizontal and vertical movement across a crack traversed by the wheel. The CAM can be used during standard operation of the HVS but only for channelised trafficking of the test section. (top of page)

The SIM Mark III consists of an array of instrumented steel pins that are connected to a rigid base plate. The pins measure the tyre-pavement contact stress in 5 directions as a tyre moves over the SIM. SIM Mark III is used at typical weighbridge stations and is used to measure the 3D contact stresses of all the tyres of real world trucks at a relatively slow speed of 5 km/h. (top of page)

Thermocouples are temperature sensors installed in the HVS test section to monitor the temperature at various depths within the pavement structure during APT. Thermocouples can be installed at various depths within the pavement structure. In most instances thermocouples are installed on the surface of the test section. This data is then processed and used in combination with rut measurement data to establish the affect of pavement temperature on deformation of the bound surface layer. Thermocouples can be monitored during standard operation of the HVS. (top of page)

In addition to the instrument monitoring of test sections, visual investigations of the test sections are made daily to identify crack initiation in the bound surfacing of the section. Once cracking is visible, all cracks are marked and photos are taken of the site during the halt in HVS operation for instrument measuring (discussed above). These photos are digitised to provide an accurate account of crack growth on the test section. This process is particularly important for the investigation of reflection cracking in asphalt overlays. (top of page)

Once trafficking of the test section has been completed the section is trenched (usually transverse) in order to investigate any visible signs of distress in the pavement structure. Trenching plays an important role in identifying pavement failure mechanisms, such as pumping and rutting of the underlying pavement layers. The interface between each layer is clearly marked and photos are taken of these interfaces for comparative analysis.

Measurements of layer thickness are taken for direct comparison with design and to ensure that correct layer thickness is used during forward-calculation with MDD data. Layer thickness is also vital for correct analysis of materials stiffness when utilising deflection based back-calculation programs.

Samples are removed during the trenching operation for laboratory testing. Laboratory tests include standard tests, such as moisture content, density and bitumen content, as well as specialised tests such as Repeated Simple Shear Test under Constant Height (RSST-CH) and fatigue beam testing. (top of page)

The accurate capturing of pavement responses, environmental changes and traffic loading are vital to the success and value of HVS testing. The specifically developed data acquisition hardware and software allows high accuracy combined with the ability to measure permanent deformations of up to 150 mm. The HVS DAS records data measured by the RSD, the CAM, the MDD, the HVS Laser Profilomter and the SOPT and PAST gauges. This centralised data capturing and recording process facilitates intensive analysis of data. The raw data are processed using the standard HVS data processing software and then stored in the HVS database. (top of page)

The data recorded over the past 24 years of HVS testing are contained in a 4 Gbyte database at CSIR Built Environment. In addition to information recorded related to the HVS tests conducted, such as number of load repetitions at specified loads, the data include plastic deformation and elastic deflection basins at various levels within the test pavement structure. The deflection bowls each contain 256 measuring points. Information such as crack movements, pavement structure, temperature during testing, traffic category and location of test is also recorded and stored in the database. (top of page)

Dynamic Cone Penetrometer (DCP)

The Dynamic Cone Penetrometer (DCP) is used to evaluate in-situ strength of pavement base, subbase, and subgrade materials. The Dynamic Cone Penetrometer (DCP) technology for road pavements originated in the early 1970s at the Transvaal Provincial Administration, Roads Branch (now GDPTRW). Since then, DCP technology has improved tremendously owing to the full-scale structural evaluation of pavements with the Heavy Vehicle Simulator (HVS).

The theory behind the DCP is that the resistance to penetration of a steel cone through the material is an indication of the in-situ strength of the material. The DCP consists of a steel cone on the end of a long steel rod. The cone is driven into the material by an 8 kg sliding hammer falling a distance of 575 mm onto an anvil attached to the penetrometer rod. The test requires to personnel, one to lift and drop the DCP hammer and another to measure and record the depth of penetration. The DCP is used in conjunction with HVS testing to ascertain the thickness of pavement layers, for instrumentation purposes. DCP data can also be interpreted using a computerized system developed by the CSIR, providing a structural evaluation of the pavement.
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