In the restoration industry, the level of understanding of moisture in building materials is generally very high. However, there is a tendency to treat concrete with suspicion and it is not uncommon for waivers to be used to reduce liability for the drying contractor. This is understandable considering there are many unknown variables in concrete, partly due to it being the only building material fabricated on site. Adding to the confusion about moisture in concrete is a mistaken assumption that measuring the vapor in concrete equates to a measurement of the total moisture content in the concrete. This assumption has contributed to many mistakes being made in the flooring and restoration industries. A better understanding of the meaning of measuring both the moisture and the vapor will go a long way to removing much of the confusion and allow for better decision making.

The Importance of Temperature and Equilibrium Equilibrium Relative Humidity (ERH) measurement, as per the ASTM F2170 standard, is a way of measuring the vapor in an airspace in equilibrium with the moisture in concrete as a percentage of the maximum vapor which the airspace can hold. For the results to be meaningful, it is important that the air is in equilibrium with the moisture in the concrete, which is why it is referred to as ERH not RH. However, this does not mean ERH measurement is a measurement of the moisture content. Relative Humidity is used rather than Absolute Humidity because the temperature of the air has a dramatic effect on the amount of vapor the air is capable of holding, making the absolute humidity measurements such as Grains Per Pound meaningless for this purpose.

To explain this simply, the Equilibrium Relative Humidity and the Humidity Saturation Point will both be affected in the same way by the changing temperature. As such, the ERH remains meaningful at different temperatures whereas the absolute humidity levels change. This highlights the need for the humidity to be in equilibrium with the moisture in the concrete, again explaining why it is referred to as ERH and why the temperature must remain stable during the measurement phase. Variables when ERH testing ERH measurement per ASTM F2170 gives useful information about the condition of the concrete but due to variables including the unknown amount of un-hydrated cement present and the unknown level of air permeability of the concrete, it cannot be considered an accurate measurement of the moisture content. To understand this more clearly, the vapor saturation level needs to be considered. It is often assumed that vapor will always saturate at 100% RH but this isn’t necessarily the case.

Take a salt calibration check for example: there is humidity saturation identified by the presence of liquid water but the RH is at 75% due to the presence of salts. Combining Non-Destructive Impedance and ERH Testing Adds Value Measuring the impedance of the concrete is also commonly used to identify the moisture content of the concrete. ASTM F2659 was written to standardize the methodology for carrying out this test. This is a practical site test, which is popular due to its ease and speed of use. However, there are variables which make the moisture content readings less meaningful, especially if we don’t know the vapor level within the concrete. For example, different ambient conditions will alter the Equilibrium Moisture Content (EMC) and different aggregates can absorb and adsorb moisture differently, also affecting the EMC. Therefore, a combination of measuring both ERH and Moisture Content is advisable. The combined approach can be compared to looking down a rifle and lining up two iron sights, instead of one. Ideally, an ERH of 85% will line up to a moisture content of 4% and 90% ERH will line up to 4.5% EMC but when they do not line up there is much information which can be gained about the concrete which would otherwise be unknown if relying on one of these test methods alone.

Many things can be learned about the concrete by combining different test methods, including identifying concrete which is highly alkaline which can have a low ERH to MC level, or identifying concrete with a high Vapor to MC level, which indicates a good quality concrete. Concrete needs a minimum RH of 85% in order for hydration to continue to take place, this hydration will continue to reduce the MC while, ideally, the RH remains at 85% or above until all the hydration is complete. Consider the Ambient Environment and Age of the Concrete Further testing of the ambient conditions is important to improve the meaningfulness of these results, as there are conditions from the environment which can also affect the relationship between the MC and ERH. The age of the concrete also needs to be taken into consideration as most moisture or RH specifications are made for new concrete where construction water is still present. Older concrete, where the moisture should be in equilibrium with the average ambient conditions, is also easier to understand with a combination of testing. Achieving Faster, More Efficient and Reliable Testing With meaningful and simple training, a technician is able to make much more informed decisions based on the data from combined tests, thus removing many of the unknowns as to the condition of the concrete.

Although more testing is carried out, the combined testing should actually be more efficient and much faster. Quick tests such as impedance are used to create a moisture map and more difficult testing such as ERH testing should be used as much as necessary to confirm the vapor condition of the slab, rather than as a method of moisture mapping. This is over time-consuming and doesn’t add to the value of the data. There is little doubt that there is a lot of room for improvement when it comes to testing concrete for moisture that can lead to potential flooring issues. The current methodology has many an expert scratching their head trying to understand the rationale for it. The view of this author is that the combination of ERH and Impedance measurement along with ambient measurements is enough to answer most questions about the condition of the concrete. However it is not the only way that this can be done. There are different methods developed in different parts of the world including tests which combine both humidity and moisture testing and there are several ways to achieve more meaningful information. The first step in improving one’s knowledge is to move away from the idea that one test competes with another, - a doctor would not for example refuse to use an X-Ray because an MRI is better. He or she would use the most practical means available to achieve the information needed to treat a patient, and that’s just pragmatic and good practice.

By Andrew Rynhart

​Metal has an infinite dielectric constant – GPR cannot pass through metal because it’s a perfect reflector for GPR energy – and so it will be shown onscreen as the brightest possible positive dielectric change.

​Whereas an air-filled PVC pipe, through which the radar energy moves easily, will demonstrate a negative change, since the dielectric constant of air is less than that of ground materials. The data image below shows a water-filled PVC pipe that clearly identifies the top and bottom of the pipe.

​The same is true when identifying underground voids. Below is an example of a well-defined void underneath a reinforced concrete slab with an asphalt overlay.

​In this new blog we present a new chapter from GSSI Academy. It is a basic introduction to some of the key concepts of ground penetrating radar. To begin, we discuss the importance of the dielectric constant and methods to determine the right dielectric. Just as Ground Penetrating Radar is also known as GPR, Georadar, and ground probing radar, the term dielectric constant can also be known as velocity and medium type, depending on specific GPR manufacturers.

GPR Theory: Understanding the Dielectric Constant:

GPR works by transmitting and receiving a high frequency electromagnetic wave through the ground, whether it’s soil, concrete, gravel, or other material. Radar waves travel at different velocities depending on what material it’s traveling through; anything ‘different’ that it interacts with will produce a reflection, to be received by the GPR device. Upon receiving the reflection, a GPR device will take note of the amount of time it takes for the signal to return and the strength of that reflection. The system will take these two pieces of information and convert them into a depth reading; in order to do so, they must be programmed with what’s known as the dielectric constant. This constant describes the speed at which electromagnetic waves move through a particular material.

The Importance of the Dielectric Constant:

The dielectric constant is critically important to getting accurate depth readings with GPR systems. Dielectric constants, also known as relative dielectric permittivity, are measured on a scale of 1 to 81, where 1 is the dielectric constant for air (through which radar waves travel most quickly) and 81 the constant for water (through which radar waves travel most slowly). Metallic objects exist outside the scale, since radar waves cannot penetrate them at all; they are described as having an infinite dielectric constant.

In order to convert the variable that is produced by a radar reading – time – into the desired product of the reading – depth – GPR systems must be accurately programmed with the correct dielectric constant for the ground material in question. This enables GPR systems to produce meaningful depth readings, instead of timed reflection readings; these time reflection readings are transformed in an equation with the proper dielectric constant. As a result, depth readings from GPR systems are only as accurate as the dielectric constant with which they are programmed for each particular ground material. There are three key methods for determining an accurate dielectric constant, each with their own benefits and drawbacks.

Methods of Determining the Dielectric Constant:

Published Reference

One way of determining the dielectric constant is by utilizing a published reference, available from GSSI in manuals and products documentation, as well as online. Using a published reference is the quickest and easiest way to obtain a dielectric constant because it doesn’t require field analysis if ground material type is known. It’s not the most accurate, though, because published references are averages and not site specific.

Hyperbola Fitting

A more accurate way to determine the constant can be with utilizing a method known as migration, or hyperbola fitting. Hyperbola fitting relies on data gathered from pipe-like targets in the ground; individual GPR units are capable of calculating dielectric constants for different soils simply by fitting a hyperbola tool to hyperbola data gathered in real-time from one of these pipe-like targets. This is the most consistent way to determine the dielectric constant in the field.

Load testing is a common practice among bridge engineers for the assessment of bridge safety and serviceability. Diagnostic load testing is one type of load test methods that can be used either as a means for estimating the load carrying capacity of an in-service bridge or as an acceptance test before the bridge is put into service [Olaszek et al, 2014]. This article provides a quick review to Diagnostic Load Testing of Bridges and its application to bridge condition assessment.

Diagnostic Load Testing

Bridge Load Testing-Diagnostic Load Tests can be used to evaluate specific response characteristics of the bridge such as lateral load distribution and secondary stiffening effects and to validate the load rating analytical models.

Diagnostic load testing usually involves installation of a variety of sensors, including:

strain gauges, acceleration sensors, displacement sensors and highly sensitive rotation devices.

Sensors are normally installed on structural components. The test is generally performed with controlled load situations. Applied loads are typically limited to legal load levels or load levels that are known to be safe for a particular structure (historical data).

​The main goal of the diagnostic load testing of bridges is to identify structural response (deformations, strains, etc.) for a given load condition. The measured responses are compared with those that are derived from the analysis of the theoretical models, for the same loading condition. Head to head comparison is then used as the basis for validating the theoretical model and defining how accurately the model simulates actual load paths [Santini Bell et al, 2013; Shahsavari et al, 2019].

The field portion of diagnostic load tests can be executed with low-cost because they can be done quickly and with minimal impact on traffic. The load testing vehicle is typically a legally loaded dump-truck and the instrumentation is installed in temporary fashion which can usually be installed in a day with a small crew. Actual tests are conducted with brief road closures or moving blockades to minimize conflicts with the traveling public.

Diagnostic Load Testing for Structural Model Calibration

The ability to calibrate a structural model is the primary concept behind diagnostic load testing. The basis of comparison can be static and/or dynamic structural responses generated by a known loading condition. Static measurements are often global responses such as midspan displacement and girder rotation at an abutment or local member cross-section responses such as axial or flexural deformation obtained from strain measurements. Dynamic measurements usually consist of acceleration, which are further processed to generate structural mode shapes and natural frequencies.

The calibration of the Finite Element (FE) models using the monitoring data helps reduce possible errors induced by simplifications or inaccurate assumptions made in model development. Wrong assumptions are mostly due to insufficient knowledge about structural details, materials properties, inevitable simplifications of the details or ignorance of the non-structural components, and boundary conditions. In the model calibration procedure, prior to refining the wrong assumptions made in the model developments, it is essential to determine the important parameters causing a deviant response in the model. The refinement attempts are required to change the recognized influential parameters and minimize the errors until the desired accuracy is achieved.

Performing load tests with moving loads can be very efficient and minimize impact on traffic, but it requires that truck position be monitored and recorded along with all the structural responses. The key part of diagnostic load testing is an apples-to-apples comparison of data. This requires a convenient method for extracting field data for specific truck positions corresponding to the simulated load cases from the model. The procedure is based on direct comparison of responses for many sensor locations and many different load cases. The load test results are a series of response histories as a function of load position.

Relative Services to Bridge Condition Assessment

When a bridge experiences an unexpected occurrence of accidental events, such as truck accident, a major concern for bridge managers is effective and informed operational decision-making with respect to the remaining capacity of the bridge. Given the load rating reduction estimated by the bridge analytical model, an integrated decision-making protocol combining different approaches will be beneficial to bridge managers for making decisions on the damaged (pre-repair) state of the structure with respect to different damage scenarios. A calibrated structural model helps establish a baseline to determine the structural system response for performance prediction and investigate the bridge decreased load carrying capacity under different damage scenarios [Shahsavari et al, 2020].

Monitoring a bridge’s structural response has the potential to:

detect the presence of structural changes for condition assessment,

inform the bridge manger to assist in daily operational decision-making,

validate the structural design assumptions,

refine a structural model of the bridge to be used for performance prediction [Read More].

Concluding Remarks

Unlike conventional load test practices which have the potential to induce damage into bridge components (e.g., PS/C girders) and reduce its serviceability, diagnostic load testing of bridges appears to be promising for determining the bridge maximum safe load capacity through analytical investigations. The structural models calibrated through controlled load tests would be beneficial for operational decisions such as those relating to maintenance scheduling and overweight vehicle permitting.

Diagnostic load testing would measure structural performance at the applied test loads but not provide any indication as to how the bridge would behave at higher loads or what the effective factor of safety would be for legal post loads. Creating a calibrated structural model that can predict the impact of operational and environmental variations on bridge performance provides support for better management strategies such as fatigue performance prediction, load rating deterioration and real-time condition assessment.

To obtain a realistic capacity, an alternative approach would be a combination of diagnostic and traditional load testing procedures in such a way that loading a bridge up to its serviceability limit without inducing any damage to the structure or cause any reduction to service life. The use of Structural Health Monitoring (SHM) sensors being deployed at critical locations may be warranted to examine bridge performance over time. The measured parameters include, but not limited to, changes in stress in critical regions, changes in deflection, identification of crack activity, etc.

​Moisture testing devices are an integral tool for the restoration industry. Though a team of professionals can employ every piece of equipment on a job site, unless the damaged or dampened building materials are free from moisture to a required standard, the job will not be satisfactory for the long term.

Tramex moisture meters make it possible to identify and track the source and extent of moisture problems without destroying the materials being tested. We design our products to fit your needs because we know working conditions on the job site, such as tight spaces, difficult to reach places, corners, curves, and textures, are challenging. These areas can only be correctly measured with the precise moisture meter.

We also know, the sheer variety of materials required to be tested demands using the right restoration moisture meter. How do you know you have the moisture meter for the restoration project? Here are a couple of things to consider:

How quickly you can get a measurement

The type of scale(s) on the meter

The depth moisture can be detected

How do you determine the best moisture meter for your next restoration project? Here's a quick overview.

The Moisture Encounter ME5 provides an instant measurement and evaluation for a wide range of building materials

The Concrete Moisture Encounter CME5 is a non-destructive meter for measuring moisture content instantly in concrete slabs 

A digital version of the CME4 handheld the CMEX II provides instant and precise measurements in concrete and other floorings (incorporating the optional Hygro-i plug-in ports transforms it into an exemplary all-in-one instrument)

The MRH III is a handheld digital moisture meter calibrated for most building materials (the optional plug-in Hygro-i2 makes it suitable for water damage restoration, flooring, checking indoor air quality, inspectors and pest companies)

A mobile and non-destructive impedance device, the Dec Scanner is excellent for surveying instant moisture of roofing and waterproofing, checking for water leaks, and integrity tests

For a pocket-sized meter, the Skipper Plus is non-destructive and is a comprehensive, safe method for detecting excess moisture in boat hulls and fittings

The handheld PTM 2.0 digital, pin-type meter takes exact measurements in wood, drywall products, and comparative WME (Wood Moisture Equivalent) values in wood by-products as well as more than 500 wood species 

The options of different moisture measurement meters from TRAMEX make it a versatile tool for restoration experts. It is precisely what professionals need to shore up the exactness required in many contracts.

With in-built quality you can trust, you can count on an investment in TRAMEX by having ownership of a quality product to bring you serviceability for years to come.

​Because of the versatility of the GEM-2 instrument, our customers are constantly finding new ways to use it. One application is the measurement of sea ice thickness in the polar regions. Both airborne and ground-based EMI (ElectroMagnetic Induction) have been used before to estimate sea ice thickness. Because of its moderate length (<2 metres) the GEM-2 can easily be mounted in a kayak or sled for towing. 

​There many parallels between ice measurements and a traditional ground conductivity survey. The simplest assumption for data analysis is a layered model, with the seawater conductivity much greater than that of the sea ice, snow and air above it. (> 2,000 mS/m vs. < 100 mS/m). But the picture is more complicated when ice-platelet clusters form under the ice. These are clumps of crystalline ice which float in the seawater on the underside of the ice layer; their density and depth, and hence bulk conductivity, are highly variable. They will cause errors in ice thickness measurements if not taken into account. 

Scientists from the Alfred Wegener Institute of the Helmholtz Centre for Polar and Marine Research in Germany have developed calibration and deployment methods for characterizing sea ice shelves in the Antarctic. In particular, they have been able to exploit the GEM-2's advanced capabilities to perform more complex characterizations than simply measuring ice thickness.

Scientists conducted expeditions to various locations with and without platelet formations. They used the GEM-2 to record response at several frequencies, for different heights and correlated these results with physical measurements at that location, the 'ground truth'. They found that having independent measurements for both in-phase and quadrature at each frequency reduced the ambiguity of the recorded signal, leading to more accurate calibrations. Also they found the higher frequencies (63 kHz and 93 kHz) most useful in characterizing the platelet layer. These types of measurements are not available from more primitive single-frequency/fixed frequency instruments.

Many free-air calibrations were carried out to estimate any long-term drift and temperature dependency of the GEM-2; temperatures ranged between -10 C and -24 C . They did not observe a strong temperature dependence of the zero-level offsets for any frequency, and the instrument performed well.  The cold affected the capacity of GEM-2 batteries;  this was solved by keeping a spare battery inside someone's coat while the other battery was in use. The GEM-2 comes with a spare battery, and a battery change takes less than a minute, so near-continuous surveying is easy.

The GEM-2 itself is made of industrial-grade components rated from -40 C to +85 C, so operating temperatures are not normally an issue. 

This post is based partly on the paper "Towards an estimation of sub-sea-ice platelet-layer volume with multi-frequency electromagnetic induction sounding" by Priska A. Hunkeler, Stefan Hendricks, Mario Hoppmann, Stephan Paul And Rüdiger Gerdes. The paper is at https://epic.awi.de/id/eprint/36936/

Other photos courtesy of Alfred Wegener Institute ( www.awi.de )

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When drying concrete after water intrusions it is important to monitor and measure the moisture content of the concrete in two phases: First during the drying phase; and again after the drying is complete. This allows the restorer to establish valuable knowledge of how the drying process is progressing and, once the drying process is complete and the concrete has been brought back to pre-loss conditions, decide what mitigation, if any, will be required before reinstalling a floor covering. In order to monitor and measure the moisture content of the concrete in a meaningful way the restorer needs to understand the different test methods, the meaning of their results and how they relate to the restoration industry compared to the flooring industry for which these test methods have been developed. This is an important point to keep in mind because the testing methods prescribed were designed for the flooring industry to test the drying process of newly poured concrete and establish when it is dry enough to receive a floor coating or covering. This is different to the restoration industry as the goal here is to dry the concrete back to pre loss conditions. Equilibrium Relative Humidity as per ASTM F2170, Calcium Chloride vapor emission testing as per ASTM F1869 and non-destructive Electrical Impedance measurement as per ASTM F2659 are the most commonly specified tests for measuring the moisture in concrete in the United States. ​

​F2170 and F1869 are both considered quantitative tests whereas F2659 is considered qualitative and while the differences are about how the tests are perceived rather than what they are actually measuring, this will mean that most flooring manufacturers will require that either F2170 or F1869 are carried out before a floor can be installed. In fact, both F2170 and F1869 both measure the water vapor, not the moisture content, in concrete and as such they are very impractical tests to carry out during the drying phase as they require the building to be in service condition for at least 48 hours before the testing can begin. This is due to the fact that changing environmental conditions will affect the relationship between the water vapor and the water. Most tests which have been developed for the flooring industry are designed to measure the construction moisture within the concrete and not the moisture from intrusion or other external sources.

As such it is important to be able to distinguish between the different sources when inspecting concrete after water damage, and the best way to do this is to look at the results from a variety of test methods. The restorer needs to be able to identify background moisture which could either be from construction moisture still in the slab, or moisture which is still entering a slab from beneath if there is an insufficient sub floor sealer, or from above if there are dew point issues or leaks in the building. Without this understanding it is common for restorers to attempt to dry against nature and waste a lot of energy drying moisture which will possibly return after the drying has been completed. This is especially important if the moisture is coming from beneath the concrete due to the absence of a sealer.

By using an electrical impedance device, which gives instant and repeatable results, to map and monitor the moisture during the drying phase, a restorer is able to focus on the changes in readings rather than the readings themselves. At the beginning of the drying process the moisture content readings will be high and should rapidly reduce as the drying progresses. As the drying progresses the difference between readings over time will decrease and this will help determine when the drying is complete. The mapping of these readings will also indicate where to test further once the building is back in an in-service condition, using either Relative Humidity testing as per ASTM F2170 or Vapor Emission testing as per ASTM F1869. If the impedance device used indicates a percentage moisture content value (MC%) of the slab then this information can be very useful when further testing with Equilibrium Relative Humidity testing.

For example. Relative Humidity testing per F2170, when used as a stand-alone test, is prone to giving false positive readings and possibly false negative readings due to the quality of the concrete;- false positive readings due to the concrete having less air movement when it is of a high quality; and false negative readings due to uncured materials such as salts lowering the equilibrium relative humidity. If the results of the non-destructive impedance test and the Relative Humidity test do not concur then it is possible to do further simple testing which can complete the information needed. The combination of ambient testing, surface concrete temperature testing, in situ RH testing and non-destructive impedance testing will cover the majority of the testing needs, with calcium chloride testing only used when the results of the others do not concur. The use of multiple, yet simple testing methods allows for a complete picture of the moisture condition of the slab. The approach of marketing one test method over another has, in my view, done a disservice to the industry. The combination of tests helps draw a much better and more complete picture, as long as it is clearly understood what exactly is being measured with each test method and how they can be affected by the different ambient conditions that arise during the entire restoration process.