Introduction of Drop Hammer Deflectometer In pavement design and load capacity evaluation, the drop-weight deflectometer (abbreviated as FWD) is currently one of the most advanced pavement strength non-destructive testing equipment in the world. Its measurement results are relatively accurate and have a large amount of information. Compared with the traditional Beckman beam measurement deflection, it is easy to use, fast, safe, save manpower, and simulate the actual situation to apply dynamic loads. After analysis and comparison, the US Federal Highway Administration confirmed that FWD is a good dynamic load assessment device for pavement carrying capacity, and selected FWD as an important device for pavement strength assessment when implementing the SHRP plan. Shell Petroleum has formally incorporated FWD into the Shell Pavement Design Manual. This shows the important role of the drop hammer deflectometer in measuring the bearing capacity of the pavement. working principle FWD uses a computer system to control the hydraulic system to activate the drop weight device, so that a certain mass of drop weight falls freely from a certain height, the impact force acts on the bearing plate and is transmitted to the road surface, thereby applying a pulse load to the road surface, resulting in instantaneous deformation of the surface Sensors distributed at different distances from the measuring point detect the deformation of the surface of the structural layer, and the recording system transmits the signal to the computer, that is, to determine the dynamic deflection and deflection basin under dynamic load. The test data can be used to inversely calculate the modulus of the pavement structural layer, so as to scientifically evaluate the bearing capacity of the pavement. Practical application in engineering inspection ◠Construction process control detection After the construction of a certain section of highway subgrade project was completed and the inspection was completed, it was transferred to the construction of the subbase of the pavement. After the construction of the subbase was completed, the deflection of the layer was tested by FWD. The modulus of the pavement structural layer is backcalculated through the collected deflection basin data. The results show that the modulus value of the base layer is extremely uneven and most of the values ​​are smaller than the soil modulus value. According to the law of pavement structural layer design modulus decreasing from top to bottom, it is determined that there is a problem with the construction quality of this layer. Based on the test results, the construction unit can promptly order the construction unit to carry out rework, which eliminates the hidden dangers of quality and avoids adverse social impacts and greater economic losses. ◠Application in rigid pavement void identification When mud and voids are generated under the concrete slab, the use of grouting or back sealing is an effective repair method, but this should be based on accurate assessment of the voiding status, otherwise it may cause the pavement performance to decline or even accelerate damage . Therefore, it is a common problem in the maintenance management of cement concrete pavement to use reliable methods to evaluate the location and severity of the foundation vacancy under the board, and take effective measures in time to restore the normal performance of the pavement. . Adopt FWD test plate corner deflection, the applied load is set to 3, with deflection as the abscissa, and the load as the ordinate, the deflection values ​​corresponding to the three load levels are marked on the plan; draw a linear fitting line, And extend the straight line to intersect with the coordinate axis; if the support under the board is good, the extension line will pass through the origin of the coordinate, or within 0.05mm from the origin, and greater than 0.05mm will be considered as a vacancy, and the more the deviation from the origin, the more serious the vacancy situation is The theoretical basis of this method is: both theoretical calculations and on-site tests indicate that the angle deflection of the 0-loaded plate estimated by the regression curve should be 0, or close to 0, before the noticeable plate angle vacancy occurs. The advantage of this method is that it is fast and simple, does not require a deep theoretical foundation, is suitable for on-site evaluation, and has achieved good results in practical applications at home and abroad. It is a method worthy of promotion. Recommended Equipment-PRIMAX Falling Weight Deflectometer ◠Brand history Phønix → Carlbro → Grontmij → Sweco 1965 → 1999 → 2005 → 2015 Sweco is the largest consulting company in the field of construction and engineering in Europe. Grontmij A / S became a member of the Sweco Group in 2015. Grondmij Holland is a world-leading sustainable design, engineering and management consulting company, active in the fields of sustainable development planning and design, water, energy, transportation and monitoring. Holonmeier merged with the Danish Carl Bro Road Consulting Company founded in 1987. Carl Bro has extensive experience in the field of pavement technology and testing equipment. The drop-weight deflectometer produced by the company has an international reputation and has been used in more than 45 countries worldwide. In 1965, the world's first manually operated drop hammer deflectometer; In 1968, the world's first trailer-type drop hammer deflectometer; In 1981, for the first time in the world, multiple geophones were used to measure deflection basins; In 1984, the world's first vehicle-mounted drop hammer deflectometer; In 1997, the world's first light-weight drop deflection instrument; In 2005, the world's first ultra-heavy drop hammer deflectometer with a capacity of more than 300kN; Since 2007, the PRIMAX series of drop-weight deflectometers; ◠Purpose ★ Evaluate whether the bearing capacity meets the standard during the acceptance inspection ★ Road maintenance stage, predict the remaining service life of the pavement, and decide the road section ★ Evaluate the void and joint force transmission of cement concrete pavement ★ Obtain the airport pavement grade PCN value to evaluate the airport pavement strength ★ Real-time calculation of dynamic deformation modulus of pavement structure ★ Reverse calculation of rebound modulus ◠Technical specifications Load characteristics 10kN (LWD) 7 ~ 150kN (FWD) 7 ~ 250kN (HWD) 7 ~ 350kN (SHWD) Pulse waveform: half sine wave Pulse duration: 20-30ms, pulse rise time: 10-15ms Load sensor Resolution 0.1kN, accuracy 1% ± 0.1kN of full scale Carrier board 4-point bearing plate (with rubber cushion) Available in 300mm and 450mm diameters Deflection sensor Seismic velocity sensor Quantity: 1 ~ 3 (LWD), 1 ~ 18 (FWD / HWD / SHWD) Range: 1 ~ 2200 microns Accuracy: better than 0.2% ± 1 microns of full scale Sensor test beam length: 3 meters Temperature Sensor There are 3 types of road surface, air and road structure layer, and infrared road surface thermometer can be purchased Accuracy ± 1 ℃, resolution 0.1 ℃ distance sensor Accuracy is better than 0.1% of the whole process Test speed 35 seconds / point, about 65 points / hour (related to the distance between measuring points) Data Acquisition System All channels (temperature, deflection, distance, load) are collected simultaneously Resolution: 16 bits Sampling rate: 4kHz / channel, optional 10kHz / channel size and weight 4300 × 1840 × 1520mm (trailer type, vehicle type depends on vehicle size) 1180kg (without falling weight) Hand-held drop hammer deflectometer Trailer drop hammer deflectometer Vehicle-mounted drop hammer deflectometer Acquisition software interface The use of different models Freestanding Cast Iron Bathtub
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