Ourtreasure and metal detectors are characterized by easy use. So is the Rover UC, a metal detector and ground scanner disguised as a walking stick, which is ready for use immediately after unpacking. Via Android App the user adjusts settings and starts the measurement. Moreover, the measured data can be evaluated directly on the smartphone display. Thanks to its inconspicuous appearance, the metal detector Rover UC allows treasure hunting even in more frequented places.
During individual trainings, our customers familiarize themselves with the special features of the metal detectors and ground scanners. This basic training focused on typical errors when using ground scanners like the Rover UC and provided instructions, tips and tricks how to detect objects successfully and analyze the scan data properly.
With to the following quick start guide, treasure hunters are 5 steps closer to discoveries of treasure troves, burial chambers and cavities even in rough terrain.
1. Use auxiliary lines
Especially for beginners it can be very helpful to limit the measuring field beforehand by small markings such as auxiliary lines on the ground. This guarantees that the measuring paths always have the same length. Additional markings like straight lines further help the treasure hunter to walk parallel paths in the measuring field and to ensure that the tracks have the same distances. In this way the measurement data is not distorted and can be properly analyzed by the user afterwards.
2. Set the size of the measuring field
Experienced treasure hunters are usually able to estimate their measuring field quite well. If the user notices after the measurement that the scanned area is smaller or larger than previously estimated, the size of the measuring field can easily be adjusted afterwards via app. The measurement data is automatically adapted to the new information.
3. Complete your scan for best measurement results
The joy of successful treasure hunts and about detected objects is great, of course, and can be especially overwhelming as soon as conspicuous objects and anomalies are detected. Nevertheless, for correct scan results, the measurement must be completed. If the measurement with the ground scanner is interrupted at the spot of the find, the size and details of the detected structure remain uncertain – maybe it is a tunnel or a long artifact such as a weapon? In order to determine the size of the object precisely, the entire measuring field must be scanned.
4. How to analyze 3D Ground Scans
3D ground scanners like the Rover UC offer a fast and efficient analysis of the measurement data directly on the smartphone: With only one finger touch the 3D graphics on the touchscreen can be rotated and enlarged. The app allows to determine the size, position and depth of detected objects more accurately. For more detailed results, users may transfer the scan data to the PC and use the Visualizer 3D software for further analysis.
5. How to discriminate metals
Whether the detected objects are ferrous or precious metals can be clarified with further functions: Rover UC can also be used as pin pointer to retrieve previously detected objects during excavation. The magnetometer mode is used especially for the localization of ferromagnetic metals such as iron, cobalt and nickel. With this function, objects such as iron nails and screws can be distinguished from valuable artifacts.
With recent advancements in Sensor technology, Structural Health Monitoring (SHM) systems have been developed and implemented in various civil structures such as bridges, buildings, tunnels, power plants, and dams. Many advanced types of sensors, from wired to wireless sensors, have been developed to continuously monitor structural condition through real-time data collection. However, there are still a remarkable number of questions associated with the use of SHM sensors. For designing a SHM system, one of the critical missions is discovering how to determine an appropriate type of sensor that can efficiently meet the scopes of the designed sensing system. This article aims to present a brief review to different types of sensors for structural health monitoring and real-time condition assessment of structures.
Smart Sensors for Structural Health Monitoring
Structural health monitoring heavily relies on collecting accurate and high quality real-time measurements of structural element condition, communicating this information with control system, and signalling necessary warnings should an irregular pattern is ever observed. Sensors for structural health monitoring are designed to facilitate the monitoring process, and enabling maintenance engineers with decision-making tools, which will ensure the safety of the facility, and the public. A typical health monitoring system is composed of a network of sensors that measure the parameters relevant to the current state of the structure as well as its surrounding environment, such as stress, strain, vibration, inclination, humidity, and temperature.
The latest advances in research on sensor technology for structural health monitoring has been resulted in various types of SHM sensors. The following provides a brief review to the most widely used SHM sensors for structural monitoring.
1. Fiber Optic Sensors
Fiber optic sensors have been under great development in recent years. In civil engineering, these sensors can be used to measure different parameters such as strains, structural displacements, vibrations frequencies, acceleration, pressure, temperature, humidity and so on. The monitoring of the structure can be either local, concentrating on the material behavior or global, concentrating on the whole structural performance. Fiber optic sensors have been tested for different applications such as strain monitoring of concrete components in a bridge [G. T. Webb et al, 2017].
2. Accelerometer for Structural Health Monitoring
An accelerometer is an electromechanical device that measures acceleration forces. Such forces can be static, like the continuous force of gravity on structural components, or dynamic to sense motions or vibrations like when a truck crossing a bridge. The application of accelerometers extends to multiple disciplines, from smart phones to rotating machinery and civil infrastructure. In the context of structural monitoring, the accelerometers can be used for real-time monitoring the variations of structural dynamic characteristics due to damage or change in structural performance [Chih-Hsing Lin et al]. The accelerometers are manufactured in single or multi-axis models to detect magnitude and direction of the proper acceleration. Accelerometers have also a wide use in constructions where there is a need to control the dynamic behaviour of the structure, either short or long term [Ref].
Piezoelectric accelerometer: A piezoelectric accelerometer is an accelerometer that employs the piezoelectric effect of certain materials to measure dynamic changes in mechanical variables (e.g., acceleration, vibration, and mechanical shock). This type of vibration transducers offers a very wide frequency and dynamic range. Piezoelectric accelerometers are used in many different industries, environments and applications. Piezoelectric measuring devices are widely used today in the laboratory, on the production floor, and as original equipment for measuring and recording dynamic changes in mechanical variables including shock and vibration [Ref].
3. Vibrating Wire Traducers
Vibrating wire sensors are a class of sensors that are very popular in geotechnical and structural monitoring. The fundamental component of the vibrating wire sensor is a tensioned steel wire that vibrates at a resonant frequency that depends on the strain in the wire. This mechanism is used in a variety of sensor configurations to measure static strain, stress, pressure, tilt, and displacement. The use of frequency, rather than amplitude, to convey the signal means that vibrating wire sensors are relatively resistant signal degradation from electrical noise, long cable runs, and other changes in cable resistance. This has contributed to their reputation for long term stability and wide usage for monitoring structures such as dams, tunnels, mines, bridges, foundations, piles, unstable slopes, and excavations.
Vibrating Wire Strain Gauge: Vibrating wire strain gauge is an old technique with its roots in the early 20th century. A thin steel wire held in tension between two end blocks makes a vibrating wire. A transverse vibration is excited by a short pulse of an electromagnet with surrounding coil positioned near the midpoint of the wire. The frequency of the vibration varies with the tension of the wire. If the distance between the end blocks changes, the natural frequency will change as well. The coil measures both the natural frequency and its changes. This gauge measures strain in a variety of materials and it can be easily cast or embedded in concrete. The frequency signal can be transmitted over long led cables to a readout unit and monitored. These sensors are widely used in different applications like bridges, tunnels and other large structures.
Vibrating Wire Displacement Transducer: Vibrating wire displacement transducers are designed to measure displacements across joints and cracks in concrete, rock, soil and structural members. In essence, the transducer consists of a vibrating wire in series with a tension spring. Displacements are accommodated by a stretching of the tension spring, which produces a commensurate increase in wire tension. These sensors are mostly used for crack width measures for example in bridges and tunnels.
4. Linear Variable Differential Transformer (LVDT)
An LVDT (linear variable differential transformer) is an electromechanical sensor used to convert mechanical motion or vibrations, specifically rectilinear motion, into a variable electrical current, voltage or electric signals, and the reverse. One can use LVDT in the applications where displacements to be measured are ranging from a fraction of mm to few cm's. LVDT position sensors are frequently used in testing and structural monitoring applications. These sensors are ideal for recording displacements on structural members due to live loads and temperature variations.
5. Load Cells
A load cell is a type of transducer, specifically a force transducer. It converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. As the force applied to the load cell increases, the electrical signal changes proportionally. Load cells have found their applications in a variety of fields that demand accuracy and precision. These sensors are employed in many historic buildings, where various building materials such as stone and brick have been used [Ref].
6. Strain Gauges in Structural Health Monitoring
The most common type of load cell used in structural monitoring is strain gauge. A strain gauge is a device used to measure strain on an object. The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change is related to the strain by the quantity known as the Gauge factor. These sensors are widely used to monitor strain in steel structures and reinforced concrete structures [Ref].
Strain Gauge Rosette: A strain gauge rosette is one type of strain gauge composed of two or more strain gauges that are positioned closely to measure strains along different directions of the component under evaluation. Single strain gauges can only measure strain effectively in one direction, so the use of multiple strain gauges enables more measurements to be taken, providing a more precise evaluation of strain on the surface being measured [Ref]
7. Inclinometer (Tiltmeter)
An inclinometer or clinometer is an instrument used for measuring angles of slope (or tilt), elevation, or depression of an object with respect to gravity’s direction. These sensors are suited for monitoring structures, the towers of vertical lift bridges, and monitoring twist in structural elements [Ref].
8. Acoustic Emission Sensor
Acoustic Emission (AE) Sensors measure high-frequency energy signals that are generated from local sources of stress waves. Discontinuities and defects in a material produce stress waves, which then propagate to the material’s surface and are picked up by the active AE sensor. By converting these waves into electrical signals, AE is an ideal technique to effectively assess the behavior of materials under stress. These sensors are mainly used to detect the onset or growth of existing cracks in structural components [Ref].
9. Temperature Sensors
Civil engineering structures are subjected to the environmental changes and therefore it is necessary to measure the temperature that affects to some extend every physical process [Ref]. Thermocouples are one of the most widely used temperature sensors to control the temperature in certain points of the structure. Most of the large concrete structures have a lot of thermocouples installed while casting and under construction in order to have a full control over temperature changes under curing.
A precious coin treasure was recently recovered: The shiny, well-preserved silver coins rested at a depth of 2.5 m (8.2 ft). We congratulate the treasure hunter on locating the hoard and are pleased to be allowed to present this discovery with our professional metal detector eXp 6000.
Ancient silver coins found: The obverse shows the head of a man looking to the right - probably Kings Antiochos and/or Seleukos. Successful treasure hunt thanks to professional metal detector. The powerful treasure detector and ground scanner eXp 6000 locates treasures and cavities to a depth of 25 m (82 ft). Thanks to various probes, the eXp 6000 can be used for different treasure hunting tasks:
The story behind the silver coins:
The idea of coins is about 2500 years old. The currency was invented almost simultaneously in China and the Middle East. The distribution of coins from Asia Minor to Persia, Greece, the Roman Empire and into the world was driven by the flourishing long-distance trade of that time.
Historical value of the silver coins: The splendor of these ancient coins, which probably originate from the Seleucid Empire, not only impresses the discoverer of the coin treasure. On the one hand, it is an impressive discovery of a largely unknown treasure of this size. On the other hand, it is also fascinating to touch silver coins which were once taken as travel duties or from royal estates or were in circulation in exchange for goods and services within and outside the oriental empire – a piece of living history. The historical value of these 2000 year old coins now exceeds the original value as a means of payment.
Historical and geographical classification of silver coins: With his victory over the Persians, Alexander the Great extended his territory and reign to India. After his death, the Alexander Empire disintegrated into numerous empires – such as the Seleucid Kingdom, which was located in the area of the extinct ancient Persian Empire (Achaemenid Empire) in the Near East.
Map of the Macedonian Empire (334 - 323 B.C.): The Macedonian Empire was an ancient kingdom in the northern-most part of ancient Greece, bordering the kingdom of Epirus on the west and the region of Thrace to the east. For a short period of time it became the most powerful state in the ancient Near East.(Public Domain/Wikimedia Commons)The favorable location on the Silk Road favored trade within and outside the Seleukid Empire. Transport routes and ports were expanded, goods such as ceramics and metal jewelry made of silver, gold and bronze were exported to Iran and Greece, and craftsmen such as mosaic layers were hired in neighboring empires. Glass foundry and shipbuilding were also up-and-coming crafts that emerged in Syria and Phoenicia, while in Mesopotamia and Babylonia textile textile manufacturing became the focus.
Shiny, well preserved coin find: The ancient Greek coins are a fascinating piece of history.
Ancient treasure trove of coins: The silver coins seem to originate from the Seleucid kingdom around 270 to 220 BC.Is the coin treasure maybe the hidden savings of a merchant? Perhaps a trader was surprised by a storm on his journey and had an accident. Was the collection of silver coins stolen and hidden by a thief? The details remain uncertain, but it is clear: The flourishing trade inside and outside the Hellenistic empires such as the Alexander Empire and the Seleucid Empire brought numerous coins on the market and holds further treasures such as jewelry, ceramics and mosaics awaiting their discovery.
The lightweight metal detector Rover UC allows treasure hunting in rough terrain, since only a smartphone is required for the first evaluation of the measured data. Without much preparation, the metal detector is immediately ready-to-use and does not only detect metals, but also distinguish between ferromagnetic and non-ferromagnetic metals.
In this case, the Rover UC detected a cavity while treasure hunting in the Middle East: A hidden burial chamber at 2 m depth was found in an ancient temple ruin. The treasure hunter was astonished as his discovery revealed valuable objects that are not made of gold. With the 3D ground scan function, his Rover UC tracked down the underground vault and led the treasure hunter to a successful discovery. The smartphone with the Rover UC App finally became a light source to illuminate the treasure find.
The discovered glass mosaic bowl with a diameter of 14.5 cm and a height of 4.5 cm weighs about 145 g, according to the treasure hunter. At first sight, the bowl appears to be very detailed, but rather inconspicuous in color. On closer inspection, the artfully arranged glass stones stand out, revealing their full splendor only above a light source: A beautiful, green shining mosaic showing fish around a spiral-shaped center. The discoverer of this treasure find describes the art object as a Phoenician glass mosaic bowl.
The glass production was originally brought to Egypt by craftsmen from Mesopotamia. Syria and Mesopotamia became important centers of glass production in the Mediterranean region in the 9th century BC. In the Hellenistic period, i.e. in the reign of Alexander the Great, Egyptian glass works in Alexandria again acquired a leading role. From there the technique of glass processing finally reached Rome.
Early on it was possible to make open vessels such as jugs and bowls from colored, translucent glass stones. Together with the mosaic technique, complex patterns were created, as the presented treasure trove shows. By fusing glass threads together, further forms and stripes could be created.
In fact, the Phoenicians were not only sovereign in the production of beautiful glass mosaic bowls, but also in the trade with the extraordinary objects. The entire Mediterranean area was supplied from the glass manufactories in the coastal towns in today’s Syria. The utility glass from Sidon and the glass artworks from Alexandria were exported in large quantities to Rome.
In its peak between 1200 and 900 BC, Phoenicia dominated the entire Mediterranean region as far as the Atlantic and was thus the greatest trading and naval power of antiquity. Phoenicia was famous in the ancient world mainly for its textiles and dyes (purple), objects made of precious metals, ivory carvings and glassware and had a significant influence on Greek art. Impressed by the production of glass, art spread throughout the Roman Empire and brought glass objects via the Silk Road to China.
A client of OKM detectors sent us 3D images and photos of his grave chamber find in Tunisia. The discoverer did not comment on found artifacts and burial valuables such as weapons and jewelry. Without any further information on the burial site, it is uncertain for whom it was built and which epoch of Tunisia’s lively history the discovery belongs to – starting with the Phoenicians, influenced by the trading power of Carthage and the competition with the Roman Empire, through numerous battles for supremacy and religion to the French colonial domination.
Discovery of burial chamber with Rover C II
Numerous data were collected during the measurement with the Rover C II as well as during the immediate inspection of the site. The measurement data were evaluated with the OKM software Visualizer 3D and clearly show a long cavity. According to the client’s specifications, the burial chamber is located at a depth of about 9,8 to 13,2 ft (3 to 4 m). Pictures testify to the underground find and illustrate the nature of the walls of the site.
The metal detector Rover C II is not an ordinary metal detector with sound output, but a grave and cavity detector which can create excellent three-dimensional graphics of the scanned underground. The combination of geoelectrical measurement and metal detection makes this treasure hunter particularly interesting for archaeologists and cavity seekers looking for treasures, burial caves and buried artifacts.
The Rover C II has meanwhile been replaced by the advanced OKM treasure and cavity detector Rover C4. The current model Rover C4 offers:
multilingual user interface
LED illumination for night measurements
wireless data transfer
4 memory locations for measurement data
Standard probe and Super Sensor with innovative LED orbit
different modes of operation that make the Rover C4 a versatile treasure detector.
We have Rover C4 on sale and you can buy it here
It is not uncommon for concrete contractors to request the location of rebar, conduits, post-tension cables, electrical, or plumbing in order to aid in remediation risks. Additionally, savvy contractors have been using GPR technology to determine concrete slab thickness. This provides the concrete contractor with several benefits including: the proper assignment of concrete cutting and coring tools, and the ability to better quote the work required. Trained and certified GPR operators understand the benefits and limitations of the technology and how to determine the best course of action in challenging survey conditions. Yet, even the most knowledgeable operators will wonder if there’s a tip or trick that they can deploy in the field to collect better data. Here we explore the difference between suspended slab and slab-on-grade surveys, and infield processing techniques which operators can use to get the best out of their slab-on-grade data.
The detectability of the slab bottom depends on the underlying material and amount of steel within the slab. It is easier to see when a contrasting material such as water, air or metal is under the slab because they will have a stronger dielectric contrast. In Figure 1, the data is representative of an elevated concrete slab. Note the several hyperbolic reflections on the screen, this is indicative of a double rebar mat. Towards the bottom of the data image, there is a strong dielectric contrast at the concrete-to-air boundary and therefore products a clear indicator of the bottom of the slab.
In slab-on-grade situations, the bottom may be very weak or invisible if the slab rests on sand or another concrete structure (supporting beam, for instance) with similar dielectric properties. This can be challenging due to the low dielectric contrast for the concrete-to-sand boundary and intersecting hyperbolic tails from objects embedded in the slab. The former results in weak or non-existent reflections and the latter tends to mask the reflection from the bottom of the concrete interface. Figure 2 illustrates a reinforced concrete roadway with a strong reflective boundary towards the center of the roadway. This is an example of a void. The concrete-to-grade boundary is less reflective as the air-filled void.
2-D versus 3-D Data Collection: In most cases, GPR manufacturers recommend using 2D scanning for real-time locating and simple imaging services, and to use 3D data collection for complex survey sites. However, slab-on-grade surveys are an exception to this rule. It is recommended to conduct 2D scanning in these situations because the layer interface is a planer target which is more easily viewed from the side.
Depth Setting: Conducting a preliminary scan of the area will help the operator determine the appropriate GPR system settings. A common mishap is that the system is set to a depth that is less than the overall concrete depth. This inherently causes data loss at the deeper levels. Remember to always set the depth range 2-3 inches deeper than the expected slab thickness to ensure that the full slab thickness is captured.
Gain: Display Gain and System Gain can be used to brighten a weak back reflection. However, these settings should be used with caution. Display Gain and System Gain settings often influence the rest of the data. Some systems apply a correction factor to gain based on assumed material dielectric.
Migration: Migration eliminates hyperbolas by collapsing them into dots representing the actual targets. This can be helpful to make target identification more intuitive and makes the data easier to interpret. This is especially true for slab-on-grade because the tails of hyperbolic targets can sometimes intersect and hide the concrete-to-sand boundary reflection. By collapsing the hyperbolas into dots, the bottom of the slab can become more recognizable.
Cross Polarization: When detecting linear metal targets (pipes, rebar, etc.), antenna orientation relative to the target becomes important. Antenna dipoles (transmitter and receiver) are most sensitive to the metal targets that are parallel to them. In other words, if an operator is scanning across the slab with the GPR system in its normal orientation, it is sensitive to targets that are running perpendicular to the direction the operator is moving (parallel to the antenna dipoles).
Some systems can be modified to turn the antenna 90 degrees. This method is known as “cross-polarizing”. If the operator scans over a metal target that is again perpendicular to their direction of travel, the GPR system is not as sensitive to it. This “weakens” the amplitude of the metallic objects and may result in a stronger concrete bottom reflection.
In summary, GPR interpretation and survey efficiency is a skill that requires training, field experience, time, and practice. These tips are intended to help operators troubleshoot a very specific type of survey scenario. An operator may employ all of them, some of them, or only one of them in an attempt to conduct a successful survey. In some extreme cases none of the solutions may work, and only a trained operator will know what tools to use and when.
The world’s leading manufacturer of ground penetrating radar (GPR) equipment, introduces the new 200 MHz (200 HS) antenna, the first of the next-generation high-performance GS Series, designed for applications that require deeper depth penetration. The new 200 HS antenna serves as the foundation for the GS Series, which is ideal for geophysical, geotechnical, or environmental applications that require high reliability under challenging survey conditions.
The newly designed 200 HS antenna is paired with the HS Module and wirelessly connects to a Panasonic Toughpad G1 or SIR 4000 control unit. The wireless HS Module incorporates system electronics, an internal GPS, and connectivity ports in an IP-65 rated housing. The 200 HS uses GSSI’s patented HyperStacking technology, which improves signal to noise performance and increases the antenna depth penetration, nearly double the conventional GPR antenna designs, under all soil conditions. The 200 HS is FCC, RSS-220, and CE certified.
The GS Series features a modular design that allows the user to select which controller best suits their needs; the rugged SIR 4000, combined with our new WiFi Module, or the Panasonic G1. Both controller options provide different advantages to the customer. The SIR 4000 builds on the same user interface and menu options that customers are familiar with to accommodate the 200 HS. The Panasonic G1 features a GIS map mode that will display GPR data collected on the left side of the screen and a location map on the right side of the screen.
Designed alongside the 200 HS are several survey accessories to enhance the ease of use in data collection. The antenna comes with a tow handle with various grip options. Optional accessories include a GPS mount and a four-piece wheel kit that will decrease the wear of the antenna on prepared surfaces such as grass and asphalt.
Over the past decade, many countries have heavily invested in wind energy. While the quality control, routine inspection, and performance monitoring of the turbine and the blades have significantly developed over the past few years, the quality control and monitoring of the foundation elements is often overlooked. This is essential in keeping these massive towers grounded and secure.
Wind Turbine foundation can be as large as 10-15 meters (diameter), the Foundation block can be as thick as 1 to 2 meters, depending on the tower size, and soil characteristics. Due to their relatively large size, these foundations are often considered mass concrete.
This can lead into buildup of significant heat (from the cement hydration process) and develop massive temperature gradient in the foundation block. This may result in thermal contraction cracking shortly after the concrete hardens — compromising the structural integrity and durability of the foundation.
Excessive heat development and dissipation may result in thermal contraction cracking shortly after the concrete hardens — compromising the structural integrity and durability of the foundation.
Wind Turbine Foundations have sophisticated congested steel reinforcement to provide stability against dynamic loads. This will make the placement of concrete challenging, and it may result in poor quality patches in the foundation.
While the use of Self-Consolidating Concrete (SCC) and steel fibres can help overcome some of these challenges, by reducing the amount of steel bars, and proper placement. However, the quality of foundations needs to be evaluated ahead of installing the tower and the turbine.
Quality Control of Wind Turbine Foundation
Routine quality control tests such as slump test (flow test in case of SCC), air content, and strength measurement are necessary to monitor the strength development in concrete mass foundations. The process of concrete placement and curing should be carefully planned.
Any interruptions in work, or change in work order should be fully recorded. After placement, proper curing regime should be adopted to eliminate the risk of early age shrinkage cracking.
Another issue could be alkali-silica reactions. Since these foundations are normally exposed, the risk of ASR will be high should the aggregates are potentially reactive.
Non-Destructive Testing can be used to evaluate the quality of these foundations during and after hardening.
1- Temperature and Strength Monitoring
Monitoring temperature gradient in mass concrete is important in minimizing the risk of cracking after hardening. Temperature sensors (wired and wireless) can be used to collect information from different locations of the foundations. Moreover, depending on the type of concrete, this information can be translated into concrete strength using maturity method.
Maturity method provides a simple approach for evaluating the strength of cement-based materials in real-time, i.e. during construction .
2- Ultrasonic Tomography
Ultrasonic Tomography can be used to evaluate the shallow depth deficiencies in the foundations. Depending on reinforcement pattern, this technique provides a reliable and cost-effective tool to scan concrete for potential defects. The method works based on transmitting and receiving ultrasonic signal from an array of transducers; the collected signals are merged to develop 2D maps of sub-surface defects, or other anomalies.
Impact-Echo is a nondestructive test method for evaluating concrete and masonry structures. The test utilizes stress waves (sound) that is normally generated through striking concrete by an impactor (Impact), and recording the reflections and refraction from internal flaws and other boundaries (Echo).
As P- and S- waves propagate within concrete element, they get reflected by internal interfaces (concrete-crack, concrete-air, concrete-rebar) or external boundaries. The arrival of these echos on the surface induces displacement. This displacement can be measured by placing a sensitive transducer (which then converts displacement or acceleration into electrical voltage). Data is recorded by a data acquisition and data logging system. Learn More about Impact-Echo
The method can be used to identify delamination, discontinuity and major voids within the foundation blocks. In foundations with known thickness, the results can be analyzed to show the depth of defects.
4- Ground Penetrating Radar
Ground penetrating radar (GPR) is a very useful technique for nondestructive concrete imaging and scanning. GPR uses pulsed electromagnetic radiation to scan concrete. GPR consists of a transmitter antenna and a receiver antenna, and a signal processing unit. GPR emits electromagnetic pulses (radar pulses) with specific central frequency to scan the subsurface medium. The reflected waves from subsurface layers, and objects are captured by the receiver antenna.
Cracks in Wind Turbine Foundation
It is critical to repair early age cracks in wind turbine foundations. Wind turbine structures are subject to dynamic loads from the oscillation of tower, blades, and the operation of turbine. Due to this changing load, the cracks can progress in width, and depth, creating durability related issues and structural performance concerns. epoxy injecting into these cracks can help control these cracks at the early stage.
For concrete and cementitious screeds to receive a floorcovering we know that the base must be suitably “dry enough”. This is to prevent damage to the flooring material and to ensure a proper bond with adhesives. So, what is “dry enough” and how long does it take for concrete to reach this goal?
Water is a vital component in the manufacture of concrete and the concrete must be kept moist during the critical curing phase to ensure the intended concrete strength. During the curing phase, the process of Hydration chemically binds a large portion (approx. 50%) of the water with the cement paste which sets and hardens. What moisture is left after this reaction is either physically bound moisture that is trapped in the pores of the concrete, or what is known as Free Moisture.
This Free Moisture is what must be allowed to evaporate in order to reduce the moisture in the concrete to that acceptable level of “dry enough”. Although a the rule of thumb is to allow an inch per month (or mm per day), a number of factors such as slab depth will greatly influence the drying time. We will briefly look at these.
But to consider how long this will take we first need to establish what measurement can be regarded as “dry enough” and what methods of testing can be used. The simplest and most fool-proof method of testing moisture in concrete and screeds is with an impedance type moisture meter. Impedance type moisture meters commonly provide quantitative measurements (as opposed to qualitative, 0-100 reference readings) and give a very quick and helpful indication of the overall moisture content of the concrete in percentage measurements of moisture content by weight, MC%.
More and more flooring covering and adhesive manufacturers are specifying the readings with this measurement that suit their products. This makes life very easy for the flooring installer as the test is extremely fast and has a low potential for user error. A common measurement specified by many manufacturers is ≤4% for products which are not designed to be moisture tolerant. Products which have a high moisture tolerance can be specified with readings as high as 6%.
The British Standard Relative Humidity Hood test has been the most relied upon test method for many years and is specified by many UK manufacturers of flooring products. An insulated, impermeable box is affixed to a position on the surface of the slab which has been identified as the highest reading position (with an impedance meter preferably). The box is sealed with butyl tape to a clean, dust free surface and the airspace inside is allowed to equilibrate with the RH within the slab. Equilibration can take anything from 4 hours to 72 hours depending on the slab thickness.
Once equilibrium is established a reading can be taken and compared to a second reading 4 hours later (or 24 hours later in the case of a 72 hour test) and, in most cases, a slab can be considered ‘dry enough’ when a reading of 75% or less is recorded with no change from the first measurement to the second, although floor covering manufacturers specifications should also be consulted.
(For more detailed instruction see BS5325, BS8201 & BS8203).
The RH Hood test is useful in that it is non-destructive and fairly simple to perform, as long as the steps are followed correctly and the box is not disturbed during the equilibrium period. However, the potential for user error is high, in that the possibility of skewed readings due to temperature change can easily be missed by ignoring the British Standard advice for a follow up test, 4 hours or 24 hours later as mentioned. A solution to this problem may be found by using a data-logging probe.
A commonly asked question is how the RH Hood method and the Moisture Content method correlate and this is a useful point to note: In a laboratory situation where temperature and humidity are constant at say 80%RH & 20ºC , a sample of average quality concrete will eventually equalize at approx. 4% MC. In the field, however, conditions are usually anything but stable and so temperature changes can cause large swings in RH test results and a high ambient RH of over 65% can result in condensation on the surface of the slab, causing higher MC% readings. However, other factors also affect the correlation between RH% and MC%, especially the water-cement (w/c) ratio.
A sorption Isotherm chart such as the one in Figure 1, provides a helpful indication of the measurements that should be expected so that when readings are far apart from each other and do not correlate as expected, it can be a good indication that one reading could be very wrong and that further investigation is needed. As such, we can see that performing two different tests is worth much more than the sum of their parts. The in-depth Relative Humidity Sleeve Method has been included in British Standards since 2011 and is growing in popularity.
This method is similar to the RH Hood method in that a trapped airspace is allowed to equilibrate with the RH within the slab. This test is destructive, however, involving drilling a hole into the slab to a specified depth of the total thickness, placing a plastic tube (or sleeve) into the cleaned hole and sealing with a plastic cap. Once equilibrium is established a hygrometer probe is placed into the sleeve and given 30 minutes to acclimate before taking a reading.
It is vital, to ensure a proper reading, that the probe is not placed in the hole too early as the heat from the drill will disturb the equilibrium. The benefit of the RH Sleeve method is that the entrapped airspace is much smaller and therefore equilibrium is established much faster than is possible with the RH Hood method. If following probe manufacturers’ instructions, the test can be performed in a shorter period of 24 hours or less in some cases.
However, the complication with this test method is sometimes in regard to confusion over the drying goal reading values required. It has been shown that readings taken with the RH Sleeve method can be higher than the RH Hood method, commonly by between 5%-10%. Floor covering manufacturers who specify the in-depth sleeve method will often specify an upper limit of 85% instead of the 75% associated with the RH Hood. See Figure 2.
Having established our “drying goals” and ‘what is dry enough?’, we can now turn back to estimated drying times and the question of ‘how long?’. In concrete construction a large amount of water is initially used in the mix, often ca. 180 litres per cubic meter. This amounts to approx. 10-14% of the total weight of the material, depending on the water-cement (w/c) ratio. As discussed, approx. 50% of this water becomes chemically bound in the curing phase.
The w/c ratio is an important factor affecting the drying time of concrete. The lower the w/c ratio (i.e. less water & more cement) the finer the pores in the pore structure, which in turn reduces the transport velocity of moisture in the concrete and therefore produces a slower rate of drying. However, the lower water content of course also reduces the amount of water that has to evaporate, which should result in an overall shorter drying time. These and other factors are illustrated in the following charts which are interpreted from testing produced by the Swedish Concrete Association.
All drying times listed are to reach a drying goal of 4%MC (Impedance Method), 75%RH (RH Hood Method), and 85%RH (In-depth RH Sleeve Method). Drying time for upper floor, above-grade slabs (drying from both sides) with different thickness and w/c ratios (Days) in normal drying conditions: 60%RH & 18ºC:
Low strain pile integrity test is a common nondestructive test (NDT) procedure for quality control and quality assurance (QC/QA) in deep foundation construction. The test can be used to identify physical defects (voids or discontinuity, referred to as pile integrity) in piles, or determine unknown length of existing deep foundations.
Low Strain Pile Integrity Test belongs to the family of shaft head impact tests, where the response of an impact made on the head of pile head is recorded by a motion transducer (i.e. accelerometer), and used for analysis. Alternatively, engineers can use other tests such as crosshole or down-hole tests for the purpose of integrity test.
Pile Integrity Test Principle
The general principle behind the pile integrity test is relatively simple. By Assuming that the stress wave travels at the speed of C inside the pile shaft, the pile depth can be determined by measuring the time lapse, T, between striking pile head and receiving reflections on pile head.
How To Perform Low Strain Pile Integrity Test?
Pile Integrity Test (PIT) is normally performed by striking the pile head with a light hand-held hammer and recording the response of the pile using a motion transducer (i.e. accelerometer) coupled to the pile head. The hammer strike (blows) generate compressive stress wave that will travel through the pile. This wave is partly reflected from the pile toe or other anomalies within the pile in its way back to pile head. Any change in impedance (due to change in pile cross section, concrete density, or shaft-soil properties) within the pile can impact the reflecting signal.
1- When To Perform Pile Integrity Test?
The integrity testing should be performed no sooner than 7 days after casting or after concrete strength achieves at least 3/4 of its design strength, whichever occurs earlier.
2- How to Prepare The Surface?
The surface of the pile head should be prepared ahead of the test. The pile surface should be accessible, and above water. All loose concrete, soil or other foreign materials resulting from construction should be removed from pile surface. If there is any type of contamination on the surface, it should be removed (using a grinder) to reach to solid and sound concrete surface.
3- How To Couple Transducer and Pile Head?
A firm connection between the sensor’s tip and concrete surface (pile tip) is needed for successful application of the test method. A thin layer of Vaselin, or putty is normally used to make a firm connection between the sensor and the pile head.
4- What type of hammer should you use?
Low strain impact integrity testing is performed using a hand held hammer. The hammer can be as light as couple of hundred grams, to relatively heavier options. The impacts induced by smaller hammer have higher frequency content, and shorter rise time. Larger hammers on the other hand, induce higher energy. Sharp and narrow input pulses are reported to be better than wider ones. However, when the size is reduced, the frequencies contained in the impact increases; these waves attenuate faster, and are tend to decrease the ability of investigating longer piles. Hammers less than 1 Kg with a plastic impact tip are ideal for most cases. When pile diameter is larger than 1 m (1000 mm), heavier hammers will be more suitable. The hammer tip should be made of material that does not damage concrete during the impact, as this will impact the test results.
5- Striking Pile Head: Where, How, How Many?
The low strain impact should be applied to the pile head within a distance of 300 mm from the sensor. It is also important to place the transducer far from the pile edge to reduce the effect of edge. Make sure that the impact is applied axially. For inclined piles, make sure the transducer is place perpendicular to the pile surface (parallel to pile longitudinal axis), and the strike direction is parallel to this direction.
For circular (diameter less than 500 mm) and rectangular cross sections, place the sensor near the center of the pile and strike several times around the pile head (i.e. 10 impacts). Increasing the number of impacts will reduce the effect of background noise, and helps enhance repeatable parts, which will make interpretation easier. For piles with larger diameter (i.e. Diameter > 500 mm) additional locations should be considered to obtain useful integrity information about the pile.
How to interpret Pile Integrity Test Results
The motion transducer collects reflection on the pile head. The measurements can be either acceleration (accelerometer), or velocity (geophone). A typical reflection from a sound pile is displayed in the following graph.
Results can be displayed in time domain (where horizontal axis shows the arrival time of echoes on pile head). Alternatively, time stamps can be converted to depth values. Results can be presented in negative or positive formats. The first peak is usually from the surface wave triggering the motion transducer. In a sound pile, the next major peak is usually the one associated with pile toe. A minimum sampling rate of 25 kHz, and time array length of 100 ms is generally good for evaluating most piles. In the event of using an accelerometer, integration of test results are used to show the measurements in velocity format.
1. Wave Speed Adjustment
The speed of stress wave can be adjusted based on the type and condition of pile material. A number of researchers and engineers have developed correlation tables between the quality of material, and the compression wave speed in the material. For example, wave speed in sound concrete is approximately 4,000 m/s (~13,000 ft/s).
2. Low Pass Filter
Low pass filter is used to reduce the effect of high frequency reflections caused by shear wave influence at the top of the pile and steel reinforcement inside the pile.
The reflecting signal can get attenuated for several reasons. High impedance, and longer pile length can attenuate the returning signal, making it difficult to identify pile toe. In this case, an exponential amplification function is used across the pile length to amplify the low energy reflections from the pile toe and other internal anomalies. This function is applied if the reflection of the pile toe is not apparent. This function increases the amplitude of signal exponentially with time along the recorded signal. Application of this function should be handled with care, as it also amplifies background noise.
Pile Integrity Test Standards
The test has been adapted by many standards and codes around the globe. The most commonly used standard that is used for performing pile integrity test and reporting is the ASTM D5882-16, Standard Test Method for Low Strain Impact Integrity Testing of Deep Foundations, ASTM International, West Conshohocken, PA, 2016, www.astm.org