• any experience on quality control of unbound pavement layers? Equipment and requirements?
• does the pavement design give out bearing capacity values for each layer (megapascals)? Are these used for quality control?

LWD – lightweight deflectometer (life wasting device?)

Three main generations of LWD – finnish, german and danish. Common principle – load released from height creates short impulse of pressure to load plate, stress under the plate causes deformation of measurable surface from which the deformation module is calculated. Serie of 6…8 drops and usually average deflection of last 3 drops is taken as value for E-modulus calculation. For some purposes, deflection value itself can be used, but for compaction quality, the relation between final E-modulus or deflection to initial can also be used (how much the device was able to compact the materials)

Finnish: Loadman by AL Engineering (Anssi Lampinen) and Inspector by Englo LLC (Toomas Sõmera) are very similar, with some technical differences. Main characteristic – closed monoblock construction, additional load plates (can be changed) and rather high stress level which clearly exceeds possible stress levels of real life, during testing process; rigid rubber buffer. Negative: before new drop, device has to be turned 180 degrees and reposition to surface may be inaccurate. Positive: simple operation, suitability for compaction control – however, during testing material will be hypercompacted through extremely high stress level (depending on load plate diameter, up to 1,5 MPa).

German: Zorn, HMP and Terratest (earlier models) are almost identical (named also German Dynamic Plate test). Main characteristic – separate rod&weight (10 kg) and plate block with integral sensor (buffer – steel springs) – fixed level low stress model. Factory-calibrated to produce 100 kPa stress under 300 mm plate. Optional load block with 15 kg. Relatively easy to compare between different fixed-conditions devices (PLT), but not with the design values which have to be calculation based (multi-layer structures).

Danish: Keros, Prima 100, Dynatest LWD 3031 and 3032, 3ipe, Terratest 9000 – based on Keros design modified by different producers. Main characteristic – variable weight (5..20 kg), variable height of falling weight and switchable (on Dynatest and Terratest) or replacable load plate, load cell to measure kN level, deflection cell, optional two additional sensors to measure deflection bowl. Sensor type same as FWD devices. Wide range of stress level to match real construction conditions, as well as for research. Problems: related to Bluetooth connection. In particular, for Dynatest – reliability of connectors of geophone cables to main unit. Cabling of central sensor and load cell should have more permanent connection (only additional sensors should have external connectors). Advantage: due to high accuracy of deflection cell, can measure also on bound layers (however, valuable here is comparison of deflections, not the calculated E-modulus); extremely low stress possible for weaker subground measurements. Possibility to measure material modulus directly on the compacted Proctor-mold (for research).

Possible use areas:

• bearing capacity of unbound layer (at agreed stress level – fixed in regulation or depending on layer depth which has to be fixed in design)
• bearing capacity of cold-stabilized layer (Italian experience, first measurement within 4 hr, second on 24 hr)
  
• bearing capacity of lower grade roads (assessment of need to restrict loads)
• quality of rebuilding after pipe/cable works under pavement (difference of deflection in transition area) – and use of profilometer for quality control within warranty period
• Measurement of E-modulus of materials in Proctor-mold (scientific base – US doctoral works and ASTM draft standard)