There are many ways of evaluating pavement structural capacity or adequacy and it is very common to perform deflection measurements with non-destructive testing equipment (COST Action 325, 1997). Once again, there is usually no knowledge of the pore pressure or pore suctions in the soil or pavement layer being assessed, so a total stress interpretative framework is necessary even though an effective stress framework would be more desirable.
There are several reasons for deflection measurements to be carried out: for quality assurance, to evaluate the bearing capacity of the unbound material of the granular base, sub-base and subgrade layers, to identify weak parts of the road, to investigate reinforcement requirements, to establish priorities for road strengthening and for research purposes.
Deflection measurements can be performed in various ways, using:
i) static deflection measurement equipment;
ii) automated beam deflection measurement equipment;
iii) dynamic deflection measurement equipment;
iv) deflection instruments with a harmonic load; and
v) deflection measurement equipment with an impulse load.
i) Static devices include: static and dynamic plate loading (bearing) tests and Benkelman beam.
Plate bearing test. This involves measurement of the deflection caused by a known static load applied through a hydraulic jack on the pavement layer surface by a circular plate. Circular plates are of specified diameters -300,450 and 600 mm with loads up to 60 kN. It is a slow and laborious test and needs a heavy vehicle acting as stationary reaction frame against which the hydraulic jack reacts. The plate test device may be mounted inside a van or at the rear of a lorry.
Dynamic plate bearing test. This is performed by lifting and dropping a known load from a known height onto the circular plate and measuring deflections. In this way, the dynamic effect of vehicles to the pavement is simulated.
The two preceding plate bearing tests are generally only used for measurement of bearing capacity of unbound layers.
Benkelman beam. This device consists of two main parts: a stand and a beam (Fig. 10.4). One end of the beam rests on the road surface but the other one is connected to a dial test indicator. The beam is suspended on
Fig. 10.4 Benkelman beam: (1) dual tyres of a loaded vehicle, (2) tip of the beam, (3) ball bearing, (4) adjustable support legs, (5) dial test indicator, (6) stand (Tehnicne specifikacije za ceste, TSC 06.630, 2002). Reproduced by permission of Direkcija Republike Slovenije za Ceste
the stand at two thirds of its length from the end being in contact with the road surface. In this way, when this end moves in one direction, the other end moves in the opposite direction half of that movement and it will be measured by dial gauge. The tip of the beam is placed between the dual tyres of a loaded vehicle. The vertical movement of the surface is then recorded as the truck moves slowly away from the loading area as this results in the rebound of the deflection that was caused by the application of the loaded vehicle. The rebound deflection/deflection ratio depends on the condition of the underlying road layers. For example, high water content caused by thawing can result in a relatively low ratio. If the deflection bowl is large, as in the case of weak subgrades, and the supporting legs are placed in the deflection area, this may produce inaccurate measurements.
ii) Automated beam deflection measurement equipment is represented by the Lacroix Deflectograph. It was developed in order to carry out measurements more quickly than by the time-consuming Benkelman beam. The measurement beam is automatically displaced along the measurement direction, while a measurement vehicle proceeds at slow speed. Equipment includes a T-shaped frame towed between the axles of lorry, enabling deflection to be measured in both wheel paths simultaneously (Fig. 10.5).
The measurement starts when the rear wheels of lorry are at a certain distance behind the tip of measuring beam and ends when the wheels reach a certain distance in front of it. Then the entire T-frame is moved forward a specified distance in the direction of movement. The axial load can vary from 80 kN to 130 kN.
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Fig. 10.5 Deflectograph Lacroix equipment: (1) measuring beam, (2) T frame, (3) starting position of tip of measuring beam, (4) end position of tip of measuring beam (Tehnicne specifikacije za ceste, TSC 06.630, 2002). Reproduced by permission of Direkcija Republike Slovenije za Ceste |
iii) Curviameter records the surface deflection under a dynamic load. The vertical displacement is determined in the right-hand wheel path by means of velocity sensors when the loaded twin-wheel rear axle of the measuring lorry passes over. Like the Lacroix deflectograph, loads can vary from 80 kN to 130 kN. The High Speed Deflectograph also belongs to this category. At a driving speed of 80-90 km/h, laser Doppler sensors measure the movement of the pavement surface under a 10-tonne axle.
iv) Deflection measurement by harmonic load can be performed by equipment that can exert a sinusoidal vibration in a road pavement. A well-known example of this type of equipment is the Dynaflect, in which the power is transferred to the road by means of crank at a specified frequency via two steel wheels. The deflections are measured by five geophones spaced at various offsets from the centre of load.
v) Bearing capacity assessment. The most common device to measure bearing capacity of roads is the Falling Weight Deflectometer (FWD). In a FWD test an impulse loading is applied to the pavement which is similar in magnitude and duration to that of a single heavy moving wheel load. The vertical response of the pavement system is measured on the surface with deflection sensors at different distances from the loading. Usually only the peak values are registered, see Fig. 10.6.
Frequently six or seven sensors are used but the spacing between the sensors can vary. Typical values are given in Table 10.1.
The deflections can be used to back-calculate the layer moduli of the structure. This requires information on the number of layers in the pavement structure and
their thicknesses. This is an inverse process where a single, correct solution does not exist. A number of software solutions exist to perform such back-analysis.
The deflections can also be used to estimate some simple parameters which are related to the condition of the pavement, some of the more common ones are:
SCI = D0 — D2 (10.1)
BDI = D2 — D4 (10.2)
BCI = D4 — D5 (10.3)
1 N-1
AREA = (Di — + Di )(n — n_0] (10.4)
D0
where SCI = the Surface Curvature Index, BDI = the Base Damage Index, BCI = the Base Curvature Index and AREA = the area of the deflection basin (all assuming radii as per the light pavement in Table 10.1).
A falling weight deflectometer is mounted on a trailer or in a van, so it needs a stable surface. On soft ground or in trenches the smaller portable light weight FWDs (Fig. 10.7) can be used. These equipments measure the central deflection, resulting in a surface modulus.
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Table 10.1 Some typical set-ups for FWD testing Radial position of falling weight deflectometer sensors
Data in bottom row taken from Highways Agency (1999). Deflection Dj is measured at distance rt from centre of the impact of the load. |
