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
In this article, we will review an interesting category of ultrasonic test methods for concrete inspection and testing: Ultrasonic Pulse-Echo (UPE) is widely used for the inspection of concrete elements. The method has proven to be extremely useful in determining the thickness of concrete elements with one side access (i.e. tunnel linings, trunk sewer linings, abutment walls), detect sub-surface defects such as voids, honeycombing, and delamination, and to verify Location of grouting defects in tendon ducts.
Ultrasonic Pulse Echo
Ultrasonic Pulse Echo is a non-destructive testing (NDT) method for scanning sub-surface targets in concrete elements. UPE methods use acoustic stress waves to study the properties of sub-surface layers, and locate defects by identifying any anomaly of acoustical impedance that is different from concrete. The test method was developed to address practical limitations of the general Ultrasonic Pulse Velocity test, such as the need to access both sides of the concrete element.
The ACI 228.2R Section 3.2.2 provides a comprehensive review on the evolution of ultrasonic pulse echo method, and instruments over the past few decades. While traditional UPE instruments were capable of providing A-Scans and B-Scans, modern Ultrasonic Pulse Echo Tomography devices are capable of providing real-time B-Scans that would enable engineers to see sub-surface targets with further clarity. Mobile-based Applications, along with Artificial Intelligence and Modern signal processing techniques have brought superior speed and clarity, with ease of use.
How Does Ultrasonic Pulse Echo Work?
As we discussed earlier, UPE uses stress waves. The principle concept behind the test is measuring the transit time of ultrasonic wave in concrete. A modern UPE instrument consists of an array of piezoelectric transducers that are capable of exciting concrete surface through short-burst high amplitude pulse-high voltage and high current- (see Strategic Highway Research Program-SHPR2, TRB, 2013). As the pulse propagates within the concrete, it gets reflected and refracted at the interface of voids, or other internal targets. Any anomaly in acoustical impedance leads The emitted impulse and the reflected stress waves are monitored at the receiving transducer. The signals are analyzed to calculate the wave travel time.
According to the SHRP2, “Based on the transit time or velocity, this technique can also be used to indirectly detect the presence of internal flaws, such as cracking, voids, delamination or horizontal cracking, or other damages.”
Applications of UPE Methods
Ultrasonic Pulse Echo methods are widely used in concrete inspection and testing. The following section describes the main applications and Use Cases:
1. Estimate Thickness of Concrete Elements
Ultrasonic Pulse Echo is widely used by engineers to assess the thickness of concrete elements. This is specially important in concrete elements with one-side access (Single Side Access), such as Tunnel linings: Thickness measurement is critical in the QC process for tunnel linings. It is also an important parameter for structural evaluation purpose.
Trunk Sewers: In trunk sewers, UPE can help engineers estimate the thickness of existing lining. This becomes extremely challenging because intrusive methods involving hot work with core drilling is not a safe nor cost-effective solution. Moreover, there is always the risk of coring in shallow sections with high hydro static pressure.
Concrete Tanks: Testing concrete tanks that are used in industrial chemical processes is often challenging. Maintenance managers of such facilities often have very short downtime windows, and permission to get inside the tank is not always practical (unless during essential maintenance cycles). UPE enables thickness measurement and quality assessment from exterior face.
2. Grouting Defects in Tendon Ducts
Along with Ground Penetrating Radar (GPR) and Impact-Echo, UPE can provide critical information about voids and defects that might have happened during grouting process of tendon ducts in post-tensioned concrete elements.
3. Locate Sub-Surface Defects
UPE tomography can be used to assess certain defects in concrete elements. UPE can pinpoint the following defects:
Delamination: UPE methods can be used to assess the location and extent of delamination in concrete bridge decks, parking garage slabs, and concrete tanks. Honeycombing: UPE is a great tool in the Quality Control and Quality Assurance of new construction. UPE can be used to localize honeycombs in concrete. Detailed Bridge Condition Survey - Delamination of concrete in bridge decks. Honeycomb concrete - UPE Scan. Honeycomb area during construction
4. Quality Control and Quality Assurance
UPE can used as in-direct method to assess the overall quality of concrete. Through the measurement of pulse velocity, engineers can evaluate the quality of concrete materials after construction.
5. Evaluation of Fiber Reinforced Concrete
While GPR has certain pratical limitations in evaluation Fiber Reinforced Concrete (FRC) elements, UPE methods provide a reliable alternative in thickness measurement and quality control of elements. This makes them an interesting alternative in inspection and testing of concrete linings in tunnels.
Limitations of UPE and Practical Considerations
Like all other NDT methods, UPE comes with its practical challenges for certain field conditions.
Close Spacing of Test Points: In order to generate reliable and precise maps of sub-surface defects, engineers need to use close spacing between test points. This can make the test time-consuming for large test areas. A practical solution is to use another method such as GPR for rapid screening, and use UPE for high-resolution imaging of defects.
Coupling Issues: The quality of acoustic signals depend heavily on the coupling of the transducer and concrete surface. This cab be quite challenging for rough surfaces. Modern devices have tried to address the issue with spring supported mechanism at the base of transducers to allow for maneuvering around the rough areas.
Undetected Defects: Certain defects might remain undetected. This is specially true for very shallow flaws or when operators work with low frequencies.
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.
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.
Obtaining as much information about the roof as possible in advance is invaluable. A set of roof drawings or plans make moisture mapping much easier and a knowledge of the construction will make the job of calibration much faster.
If a set of drawings or roof plan is not available, prepare a plan and report sheets for each section being surveyed or, better still, use the Tramex Moisture Mapping App. To perform a non-destructive flat roof moisture inspection to ASTM D7954 with the Dec Scanner, first ensure that the surface is free of debris and is dry from rain or dew. Aggregates may be left in place but should be dry and of uniform thickness.
Once calibration and range selection is complete, proceed by moving the Dec Scanner along the imaginary gridlines in a continuous, systematic manner. The Dec Scanner is designed to cover the width of an average roll of roofing felt, making it simple to follow a systematic row-by-row methodology. Mark areas of concern onto the roof plan/Tramex Moisture Mapping App as well as directly onto the roof surface if required. Marking the surface directly can be helpful for finding the precise location for core samples later. Areas where the roof has non-uniform composition or thickness, such as areas which have been recovered or seams, should be tested and noted separately as they may provide different results. By continuously checking this way, a complete picture of the moisture condition can be built up quickly. An area of up to 100,000 sqft can be reasonably covered in one day.
Selected suspect wet areas should be confirmed by core sampling using gravimetric analysis in accordance with ASTM C1616. It is permitted to check core samples immediately after extraction with a pin-type resistance meter such as the Tramex PTM. An identified area of high moisture may also be checked with extended insulated resistance pins before core sampling, by first puncturing the surface of the roofing membrane with the Tramex Hole Punch and then with the Tramex hand held resistance probe – together with 7" or 15" insulated pins – insert the pins into the insulation for a further relative reading. These readings should be recorded on the report sheets to correlate with gravimetric measurements at the verification stage.
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.
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:
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Concrete piles and drilled shafts are an important category of foundations. Despite their relatively high cost, they become necessary when we want to transfer the loads of a a heavy superstructure (bridge, high rise building, etc.) to the lower layers of soil. Quality control and quality assurance has been a popular, yet challenging task for geotechnical engineers, inspectors, and piling contractors, mainly because these elements are generally buried under ground, with only pile head being accessible most of the time. Different intrusive and non-intrusive methods have been developed over the past decades to help engineers with easy, reliable and cost-effective evaluation of quality in these elements. Pile Integrity test is referred to a group of nondestructive tests that aim to provide quantitative data on physical dimensions of pile elements, their continuity, and last but not least, consistency of the pile material.
Pile integrity test (PIT), or as ASTM D5882 refers to it as "low strain impact integrity testing of deep foundation is a widely used nondestructive test method for the evaluation of pile quality, integrity and to help estimate the unknown length of existing piles and foundations. Pile integrity test can be either used for for forensic evaluations on existing piles, or quality assurance in the new construction. The integrity test is applicable to driven concrete piles and cast-in-place piles. The following image provides a visual summary of major integrity issues in deep foundations.
Low strain impact integrity testing provides acceleration or velocity and force (optional) data on slender structural elements (ASTM D5882). Sonic Echo (SE) and Impulse Response (IR) are employed for the integrity test on deep foundation and piles. The test results can be used for evaluation of the pile cross-sectional area and length, the pile integrity and continuity, as well as consistency of the pile material; It is noted that this evaluation practice is approximate. The PIT method works best for column type foundations, such as piles and drilled shafts. The method provides a rapid and simple tool for evaluation of a number of piles in a single working day.
How to Perform PIT?
Surface preparation is the first thing to do when performing a pile integrity test. Any type of contamination should be removed (using a grinder) to reach to solid and sound concrete surface. The pile head surface should be accessible, above water, and clean of loose concrete, soil or other foreign materials resulting from construction. This step is so vital, because then connection between the sensor and concrete should be solid (firm contact). The acceleration sensor should be placed on concrete firmly. To do so, a couplant material should be used to attach the acceleration sensor the pile head. An impactor (usually a hand-held hammer) is used for impacting pile head; the impact should be applied axially with the pile. Motion transducer should be capable of detecting and recording the reflected echos over the pile top. Acceleration, velocity, or displacement transducers can be used for this purpose. At the minimum, acceleration transducer should have an Analog to Digital Converter with 12 bit resolution; and a Sample Frequency of at least 25 KHz. The location of the sensor should be selected away from the edges of the pile. 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.
The distance between the impact location and the sensor should be no larger than 300 mm. Several impacts are applied to the top of the pile. The reflected echos are then recorded for each individual impact. As an alternative, the average can be determined and used. As mentioned earlier, acceleration transducer can be used for the purpose of this test. In this case, the apparatus shall provide signal conditioning and integrate acceleration to obtain velocity. The apparatus shall balance the velocity signal to zero between impact events.
What Information Does Pile Integrity Test Provide?
The Pile Integrity Test (PIT) provides information about:
+ Evaluate integrity and consistency of pile material (concrete, timber);
+ Evaluate pile cross-sectional area and length;
Limitations of Pile Integrity TestThe PIT provide an indication of soundness of concrete; however, the test has certain limitations:
+ PIT can not be used over pile caps.
+ It does not provide information regarding the pile bearing capacity.
+ Test should be undertaken by persons experienced in the method and capable of interpreting the results with specific regard to piling.
+ This test is not effective in piles with highly variable cross sections
+ It is not effective in evaluating sections of piles below cracks that crosses the entire cross sectional area of the pile.