Calibration of Light and Medium Weight Deflectometers

Calibration System “CS01” for Light and Medium Weight Deflectometers, presented by Dr. Bernhard Korsch, head of the calibration office and Frank G. Schulz, CEO of TERRATEST®. Both of them proudly show the admission documents just received from the German Federal Highway Research Institute (BASt). Only four of seven applicants have obtained the authorization for calibrating also Medium Weight Deflectometers.

Legal Basis: Acc. to Engineering Code for Soil and Rock in Road Construction TP VF-StB Part B8.3, Light Weight Deflectometers for dynamic load plate test have to be recalibrated once within 12 months. Regular calibration ensures precise measurement on the site and validity of measuring results when analyzing compaction quality of the soil acc. to ZTV E and ZTV A. Results originating from instruments not calibrated should not be used as compaction proof. The validity of the calibration becomes obvious by the test label attached to the components of the device.

Calibration of the New Medium Weight Models: Seven offices admitted for Calibration of Light Weight Deflectometers acc. to technical test regulation TP BF-StB Part B8.4 (2016), have applied to the German Federal Highway Research Institute (Bundesanstalt für Straßenwesen BASt in Bergisch Gladbach) for supplementary authorization of calibrating the new Medium Weight Deflectometer, but only four of the applicants obtained approval.

“We are very proud of having instantaneously met the stringent requirements of the BASt for Medium Weight Deflectometers”, declares Dr. Bernhard Korsch, head of the calibration office of TERRATEST® Berlin. “We had to transform the calibration system worked out in-house, with respect to the parameters of Medium Weight instruments. It was a particular challenge to adapt the software, due to the new settlement measuring ranges of the Light Weight Deflectometer and the Medium Weight Deflectometer – and we obtained the success we have earned. The development of the device itself and a corresponding calibration system was a quite complex task.”

In the beginning of October 2016, seven calibration offices were examined by the BASt, but the admission was granted only to three of them (apart from TERRATEST®). The Medium Weight Deflectometer, acc. to directive TP Stone-Road Construction, Part 8.2.1, disposes of the double impact force compared to a Light Weight model. This value is achieved by a falling weight with a mass of 15 kg and corresponding disk springs. This configuration is particularly useful for assessment of bearing capacity for layers without binders acc. to ZTV SoB-StB (anti-freezing and gravel-bearing layers, surfacing without binders). The same is valid for other applications with comparable bearing capacity, e.g. for qualified ground improvement in traffic routes construction.

Until now, TERRATEST besides the light weight models with usual impact force produces a version with force factor 1.5, used in North Rhine – Westphalia (Western Germany). In this region, a corresponding testing directive is in force. Increased force was put into practice by means of a falling weight with a mass of 15 kg and corresponding disk springs (as mentioned above). This instrument type was supplied with a so-called factory calibration based upon the requirements of TP BF-StB Part B8.3.

TERRATEST® from now on offers a low cost possibility of modifying load assemblies of these instruments, to configure a Medium Weight Deflectometer acc. to TP Stone-StB Part 8.2.1. Calibration takes place acc. to TP BF-StB part B8.4. At the same time, the measuring electronics are completed by new firmware, thus simplifying switching between Light Weight and Medium Weight applications. For this reason, on the construction site the exchange between different load assemblies required is easy to realize. It is sufficient to select command “MFG” in the menu of the TERRATEST® measuring electronics, to exchange the load assembly and then to start measurement. Simplified handling is a clear benefit of the arrangement. The advantage of a retrofit is obvious: Also former models this way can be used in conformity with ZTV SoB-StB. Of course it is possible to equip all existing Light Weight deflectometers with a Medium Falling Weight. “This enables our clients to examine also layers with comparatively high bearing capacity acc. to ZTV SoB-StB, without the need of purchasing new instrumentation”, announces Frank G. Schulz, CEO of TERATEST®. Until now, Light Weight Deflectometers could only be used for a maximum Ev₂ value of 120 MN/m². In between, numerous clients profit from the available option and convert their 15 kg load assembly to the new configuration with double impact force.”

Under laboratory conditions, light or medium weight deflectometers are calibrated on a rigid basis consisting of a concrete cube with dimensions (LxWxH) = 80 x 80 x 80 cm. The procedure itself is composed of two distinct measures:

1)    First the force of the load assembly is calibrated on the basis of a special load cell. The result must amount to 7070 (light weight) resp. 14140 N (medium weight models), at a tolerance of 1%. The force value can be modified by varying the height of fall, by means of the unlatching device installed at the top. The pulse time of impact is defined to 17 ms + 1.5 ms resp. + 1.0 ms for medium weight models, to be adjusted by the initial stress of disk springs. After precise setting of height of fall and initial stress of disk springs, three measuring sequences with 10 impacts each are carried out. Calibration is acceptable in the case of deviations for force and pulse time < 1% between distinct impacts, runaways are not tolerated.

2)    The procedure for the load assembly is now complete. The next step is calibrating the settlement measuring assembly in three different ranges: Soft, medium and hard. For that purpose, the load plate is clamped on a steel plate (thickness: 30 mm, diameter: 340 mm, mass: 30 kg), intended to symbolize the half-space below the load plate.

a)    Preparation: Below the steel plate, different polypropylene mats (for the calibrating ranges mentioned above) will have to be inserted in turn. For the hard mat, a settlement of 0.3 mm is expected, corresponding to soil relatively well compacted with an Ev_D value of 75 MN/m², usual in road construction. The soft mat with a settlement of 1.6 mm reflects properties of soft uncompressed, e.g. naturally grown soil with an Ev_D value of 14 MN/m². This defines the calibrated range of the light weight deflectometer, on the basis of the mats prescribed by BASt, to the zone between 15 and 75 MN/m².

For the Medium Weight configuration on the other hand, another set of mats has to be used, corresponding to a settlement of 0.3, 0.5 and 1.2 mm. As the stress below the load plate in the course of the impact achieves the double value compared to the trial described above, the calibrated range comprises the zone between 30 < Ev_D < 150 MN/m². This range corresponds to Ev₂ values even above 200 MN/m², so that reliable measurement on layers with particularly high rigidity is ensured.

b)    Procedure: As to the maximum settlement, the method consists in comparing the result of the TERRATEST® sensor to the results of three distance sensors. With the mats in place, these sensors are placed onto the load plate, intended to record its displacement in the course of the impact – this delivers an accurate reference value for maximum settlement. As this measurement represents a dynamical process at an extremely short pulse time (17 ms), accurate determination of maximum settlement by an acceleration meter is by no means trivial. “We are really impressed by the measuring accuracy of our instruments”, confirms Sven Krone, test coordinator for calibration of Light and Medium Weight Deflectometers at TERRATEST®. “The results are extremely repeatable; our devices are able to capture a nearly identical result during each single impact. Deviations are restricted to an absolute minimum. The scattering of the results for 10 impacts amounted to 3 µm. Relate this to the thickness of human hair of 50 µm!”

c)    Result: The average of maximum settlement determined by the three distance sensors is compared with the maximum settlement measured by the TERRATEST® sensor. The calibration is accepted only, if the deviation within all of the three ranges is sufficiently low (in the µ zone). Results of the TERRATEST® sensor and values issuing from the three distance sensors are mutually converted by means of a calibrating factor. After completion, reliable measurement on construction sites is ensured. All TERRATEST® instruments feature an internal counter, informing the user in time, before the end of cycle is reached. Calibration is then carried out at the factory service; a simple phone call is enough. The measure in all will take approximately one week.

“This way, the widespread acceptance of dynamical load plate test for light weight deflectometers can be expected for Medium Weight models as well. The new calibration directive is a great progress, as it enlarges the practical applicability of the measuring principle also to layers with an extremely high rigidity”, declares Frank G. Schulz. “The alternative of a static load plate test with a counter weight, e.g. a loaded truck, consumes too much time and cost, and is often nearly impossible to perform, especially in pipeline construction or places difficult to reach. For this reason, the demand for our instruments is continuously increasing, also originating from public institutions like communities or electricity, gas and water suppliers. We are sure that this development in future will hold on.”

TERRATEST® at BAUMA 2016

The Light Weight Deflectometer for the dynamic load plate test was in extremely high demand at BAUMA 2016 in Munich.

During the trade fair, TERRATEST® was able to secure orders from international customers from England, Iceland, New Zealand, Poland, Israel, The Netherlands and Italy. From 11th - 17th April, over 3,400 exhibitors from almost 60 countries enticed around 580,000 visitors from 200 countries to experience the world's largest exhibition at the trade fair grounds in Riem, Munich. TERRATEST® exhibited the Light Weight Deflectometer from Berlin for the third consecutive time at BAUMA. TERRATEST® CEO, Frank G. Schulz, commented: “BAUMA was a roaring success for our Light Weight Deflectometer, we didn't anticipate such huge demand for our Light Weight Deflectometers.” He continued: “Furthermore, we had interesting talks with potential distributors, which is hugely promising. This will enable us to market the TERRATEST® Light Weight Deflectometer even more successfully internationally. From the very beginning, BAUMA has played an important role for our company.” The new BAUMA-models, “TERRATEST 4000 USB” and “TERRATEST 5000 BLU” with Bluetooth technology, were presented at the trade fair. At the upcoming BAUMA 2019, TERRATEST® will be represented once again with its own stand.

Eurovia Vinci Uses Light Weight Deflectometer 5000 BLU by TERRATEST

Eurovia (VINCI) is a globally known industry leader in urban development and transport infrastructure construction. The company's industrial work involves the production of materials and aggregates for rail and road infrastructure, and this enables them to manage and control their supply chain. Moreover, they are known for other services like expert consulting, design coordination, and infrastructure maintenance as part of their public and private partnership projects. Eurovia uses only the highest-quality equipment to deliver the best service to its clients. The light weight deflectometer 5000 BLU by TERRATEST is one of the most innovative products that they use for compaction testing in construction sites.

Some wired devices could get in the way of proper and quick testing in certain construction sites. In this case, a wireless light weight deflectometer proves to be more practical. Eurovia relies on TERRATEST's Bluetooth-enabled LWD because it can perform and complete dynamic load plate tests in about two minutes without a loading vehicle and a laboratory. The good thing ab

Eurovia Vinci recommends checking the natural soil using a light weight deflectometer before beginning construction. This way, you can avoid problems associated with poor soil compaction and bearing capacity and perform the necessary measures to improve the soil quality. You can use the light weight deflectometer 5000 BLU by TERRATEST throughout the construction site, especially in hard to reach places like backfills and ditches.

Eurovia Vinci is able to optimize its compaction works and help its clients save money daily with a Bluetooth light weight deflectometer. The intelligent device comes with voice navigation and an app for the ultimate wireless operation. Measurements are transferred via Bluetooth to the measuring computer, tablet, or smartphone from the load plate. This can be beneficial to protect the integrity of measuring computers, which may malfunction when placed directly to mechanics or exposed to vibrations and extreme conditions in construction sites.

How Light Weight Deflectometer is used for Road Testing

Testing soil properties is critical to any construction project. In fact, it is one of the most important initial steps to take during the early conceptualization stages of a build up. This is because soil conditions and properties can have significant effects in the basic structure and the future state of the ground surface. Typical soil testing is usually performed in a specialist laboratory, where a series of procedures are undertaken in order to determine the soil's chloride, sulphate, pH, and carbonate levels, its organic limit, clay sedimentation, and other properties. Determining and ascertaining soil density, strength, contamination, and compaction are all vital at the beginning stages of the construction process.

Despite its crucial role in the design and construction process, soil testing is very commonly overlooked, causing much delay, and sometimes, even cancelation of projects because of the unsuitability or the instability of the soil and ground surface at the site. These kinds of situations can very well be rectified if soil testing was done during the initial stages of the project.

Fortunately today, there are different types of soil testing devices that can be used right onsite to ensure immediate measurement and determination of soil quality and suitability. Modern devices like light weight deflectometers are designed for easy, fast, and convenient compaction testing. LWDs allow for dynamic load plate testing by making falling weight deflection testing possible on different testing surfaces, including rather soft subbases and subgrades, where heavy weight devices can't be used. Newer models even feature advanced inclusions like Bluetooth technology for easy data transmission, high-performance rechargeable batteries, a GPS system, in-built printer, and even a USB drive and text input function. Even more advanced models feature PC integration via enhanced software that allow for easier data management, transfer, documentation, reporting, and analysis, for on-the-go testing.

Benefits of Using Portable Light Weight Deflectometer for Construction Quality Control

Traditional compaction and soil quality control methods can be costly over time. In addition to price considerations, they can also require significant time investment during field testing, as well as extensive training when it comes to operation and safety. Older devices that engineers and designers used to test and monitor compaction would only measure one of two things: modulus (stiffness) or density. Newer devices, such as portable light weight deflectometers can measure both and more.

The stiffness or modulus of a soil or pavement is a qualitative term that indicates the surface's general resistance to deformation. Modulus is interchangeable with a soil's elastic modulus, resilient modulus, and modulus of subgrade reaction as it determines displacements of and strains to the subgrade as the load is introduced. Also during construction, great emphasis is placed on meeting a specified dry density for the compacted fill. Water content and dry density measurements are convenient control parameters for engineering properties. This is why it is important to have instruments that allow for accurate measurement of these values so as to ensure efficient quality assurance.

The many benefits of using portable light weight deflectometers for quality control in construction projects include:

• Relatively more manageable initial cost
• Easy operation by even inexperienced operators (no extensive training required)
• Fast and easy set-up (without calibration and manual set-ups)
• Portability and applicability when it comes to testing otherwise impossible to reach surfaces with traditional devices/methods
• Data integration 

Portable light weight deflectometers have the capability to measure density and the stiffness modulus of different types of soils and subgrades. They allow for the same accurate results as other mechanistically based methods and techniques so you can select the appropriate type of construction or rehabilitation method to be implemented on any given pavement, based on considerable economic significance.

Benefits of Using Mobile Light Weight Deflectometer

Mobile, lightweight deflectometers are among the latest and most advanced soil testing devices out in the market today. While traditional methods are preferred for their established effectiveness and wide acceptance, modern instruments also offer great things to be praised, especially when it comes to the technological advancements they offer over traditional methods. Dated instruments may have been the standard for the longest time, but there is also a lot to be said about their modern counterparts, which make work a lot easier and manageable for users. Portable light weight deflectometers, for instance, provide many notable updates to their heavy weight counterparts, making them more practical and even more suitable for certain construction, engineering, and design projects.

Light weight deflectometers use the same science behind conventional falling weight deflectometers, thus offering the same accuracy, if not a greater degree of precision when it comes to the results they return. These devices offer the great advantage of portability, making them more ideal for use in areas or locations that are simply impossible to reach via traditional falling weight devices and other measuring equipment. The sheer size and weight of other types of soil modulus measurement devices restricts their usefulness in many different types of project locations and applications.

Light weight deflectometers also don't require tedious set-up and calibration processes, which can take up a great deal of time. They don't require time and energy consuming safety tests and other manual set-up processes, which can lead to inaccurate results. Instead, these devices streamline the entire testing process by eliminating these pre-testing rituals, while still ensuring the accuracy of results. Light weight deflectometers are portable in design, which makes them suitable for single-person operation, unlike other devices that require a minimum of two people to operate. Finally, light weight deflectometers are not extremely technical to use, eliminating the need for long learning curves for users.