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:
We are very excited to announce that we have signed with FPrimeC Solutions Inc. a partnership agreement to expand sales network and support centres for iPile™ | Pile Integrity Testing-PIT in Europe and the Middle East. Great times ahead for your concrete testing.
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.
The moisture and humidity testing system for floor covering related industries has been developed to help users avoid moisture related problems in flooring and to identify the cause of problems if they do occur.
The system involves testing the subfloor using both the Tramex CME non-destructive test and in situ Relative Humidity Hygro-i probes. The ambient conditions of the building are also measured and the moisture condition of many floor coverings can be checked.
ASTM F2659: Non-destructive testing
The CME instant test for concrete is completely non-destructive and specified by many floor-covering manufacturers around the world, in part because of its simplicity and the repeatability of the results.
The ease and speed of the CMEX allows for many tests, conforming to ASTM F2659, to be carried out over a large area in a short amount of time. Simply turn on the meter and push it onto the surface being tested and read the results on the clear display. The results are calibrated to show percentage moisture content by weight.
ASTM F2659 calls for 3 to 5 readings in the same location. If there are any variations in those readings, record the highest result.
This is repeated in at least 8 locations for the first 1000sq.ft. and 5 for every 1000sq.ft. thereafter, allowing the user to build up a moisture map of the entire slab and identify when and where to test further if testing, for example, to ASTM F2170.
ASTM F2170: Hygro-i in situ testing
The reusable Hygro-i relative humidity probe has been designed specifically for measuring the potential moisture condition of a slab or screed as per ASTM F2170.
To perform this in-situ type relative humidity test, simply drill a hole in the concrete 40% of the thickness of the slab.
Push the sleeve into the hole, insert the Hygro-i probe into the sleeve and cover with the cap. Leave for the standard test period. When taking readings, plug the interface into the Hygro-i probe and instantly read the relative humidity, temperature and dew point simultaneously on the clear display. When testing is complete the Hygro-i probes can be removed with the extraction tool and used repeatedly. The user must be mindful of all the proper safety precautions as mentioned in the ASTM standard.
Ambient Conditions: Hygro-i testing
In conjunction with the Hygro-i relative humidity probe, the CMEX also functions as a digital hygrometer. The meter displays the relative humidity, temperature, dew point and mixing ratio, allowing the user to assess the environmental ambient conditions within the building, making sure they are suitable for installing a floor covering. Using an Infrared Surface Thermometer in combination with the relative humidity results it becomes easy to identify condensation problems.
Moisture Content of Wood: Pin-type wood probe
The CMEX can also be used with the hand held pin probe, for measuring moisture in wood. This is especially useful when installing wood floor coverings.
By comparing the moisture content of wood with the ambient relative humidity conditions we can determine when the wood is in balance with the ambient conditions within the building.
Another way of identifying when wood is in balance with the building, is to discreetly take moisture content readings from wood somewhere already in service within the building. By taking this in-service moisture content reading it becomes more predictable if the wood floor covering is likely to move, to shrink or expand, or if it is in balance.
Excessive moisture in concrete floor slabs and screeds or a disequilibrium between moisture content in the building and the flooring materials can lead to major problems in many types of floor coverings.
The new FeedBack DataLogger from Tramex can help identify humidity issues, saving you from possible floor failures with anhydrite screeds. When it comes to talking about drying screeds and concrete floors in general, there seems to be an elephant in every room: ambient humidity.
Although it’s mentioned in every datasheet, handbook and national standard, ambient humidity seems to be overlooked or misunderstood by many architects, builders and flooring installers. As a rule-of-thumb, concrete slabs are expected to dry at a rate of 1mm per day (or an inch per month) and anhydrite screeds the same up to 40mm, or two days per mm when poured deeper (ie a 60mm screed will take: 40mm @ 1 day = 40 days + 20mm @ 2 days = 40 days which = 80 days in total).
‘Ideal conditions’, as stated by screed manufacturers in their guidance literature, are usually agreed on as being in the region of 20deg C and 40-60% RH. These are the optimum conditions to allow the moisture within the slab or screed to evaporate from the surface. The rate of evaporation will depend on ambient conditions. Warm, dry, flowing air will allow for faster evaporation. These conditions, while ideal, are obviously not the normal state of a building site in the UK for most of the year, except the three short months of summer, at which time doors and windows should be thrown wide open to create a good flow of dry air, lifting moisture from the material and carrying it away.
However, for the rest of the year, when temperatures are closer to 5deg C and humidity upwards of 70-80% RH, leaving doors and windows open will have the opposite effect, instead introducing more humidity into the environment and slowing the drying further. Then, with the addition of wet trades applying plaster to the walls, humidity in the air is raised even higher. At most times of the year, heaters and dehumidifiers are needed to artificially create those ‘ideal conditions’.
Building sites which aren’t artificially conditioned will maintain a high humidity level and, when temperatures drop (overnight for example), can easily reach dew point, resulting in condensation settling on the surface of the floor, thus wetting and re-wetting the screed. An obvious solution in this instance, and one which seems to be the go-to quick fix in the UK today, is the use of a damp-proof membrane (DPM). This will slow the rate of drying of the floor to a level which isn’t harmful to the floorcovering.
DPMs can be ideal for this scenario (a sort of get out of jail card) and also in the situation where an older floor which was installed many years before, is still showing high levels of moisture. Moisture in a slab or screed should continue to dry slowly over many years even with floorcoverings installed and so an older floor shouldn’t be expected to read as high as a new floor which is emitting its construction moisture.
If a reading which would be regarded as normal for a new floor, is found in an older floor, it could be an indication that there’s a breach in the damp-proof course (DPC) or even that one was never installed. Again, this can be an ideal situation for a DPM which will ensure moisture, intruding from below the slab, isn’t going to cause a failure. (Be sure, in this case, to select a DPM which is suitable for residual construction moisture as well as groundwater vapor).
Once a DPM is installed, however, it becomes even more important to monitor the ambient conditions on site leading up to the floor cover installation. This is because when the condensation point is reached, in normal circumstances as described above, most of the condensation is absorbed into the surface of the screed, whereas with a DPM in place, this condensation will sit on the surface with nowhere to go. This means even a small trace of moisture can cause problems for the adhesive. This consequence of the use of a DPM is often overlooked.
The Tramex Feedback datalogger, for example, is a suitable tool for monitoring these ambient conditions over the course of the drying stage of the floor and right up to and during the installation. Readings of ambient temperature, humidity and dew-point are recorded by the device and read from a smartphone or tablet using the Tramex Feedback app. Anhydrite screeds are sensitive to high ambient humidity conditions and readily absorb moisture from the air, slowing or blocking the drying completely. Removal of the laitance from the surface of the screed after the initial curing will allow the surface to release its moisture, whereas not sanding/abrading the surface will normally result in the laitance hardening and making it significantly more difficult to remove at a later stage.
While some anhydrite screed installers will return to site after the initial curing period and remove the laitance as part of their service and will hand a copy of the instructions over to the contractor, ensuring everyone is aware what type of screed it is and how to treat it, these highly professional screeders are the exception and unfortunately not the rule. The more common scenario sees the contractor arriving to site with no idea that this is an anhydrite screed and therefore how to treat it.
Knowing the screed is anhydrite will have important ramifications on several aspects, including choosing which type of DPM to use. DPMs designed for concrete and sand/cement screeds are usually not suitable for use with anhydrite screeds. Manufacturers are now producing DPM products for use specifically with anhydrite; however, most cannot be used when underfloor heating (UFH) systems are installed. Moisture testing of anhydrite screeds is another issue which causes confusion for contractors as these screeds do behave differently to concrete and sand/cement screeds.
The three main tests in use in the UK are the British Standard humidity box, non-destructive electronic moisture meters and the German (DIN) standard carbide method (bomb test or speedy test). The British Standard humidity box measures the ERH or equilibrium relative humidity of the screed. This is performed by affixing a specially designed sealed box to the surface of the screed with butyl tape (which, unlike silicone, doesn’t affect the RH readings) in a location of the floor which reads highest with a preliminary electronic concrete moisture meter test.
Ensure the box is out of the way of direct sunlight or drafts from open doorways. Equilibrium is achieved when the trapped air inside the box is no longer receiving humidity from or giving it to the screed. At this point a measurement should be taken. 75% RH or below is a commonly acceptable result. The length of time required for the airspace inside the box to achieve equilibrium with the slab or screed depends on its thickness and whether a screed is bonded to the sub-floor or not. In the case of anhydrite screeds, which are usually poured to a depth of between 40-60mm and are normally placed over insulation, British Standards recommend a first reading is taken after four hours, and equilibrium may be assumed when two consecutive readings taken at four-hour intervals show no change.
In practice this test method is often reported to be performed unsatisfactorily. For example, many testers will leave the box in place for too short a time and it’s rare to hear of anyone checking readings twice at four-hour intervals. The reason for taking subsequent readings four hours apart is to ensure the recorded reading is taken during a period of equilibrium. A small change in ambient temperature will have a dramatic effect on the readings, destabilizing the fine balance of equilibrium inside the box and sending the RH reading up or down depending on the temperature change.
This fluctuation in temperature can result in an unpredictable spike in RH (eg from between 72-82% as in figure 1) as the equilibrium is upset. Stability will only resume about three to four hours later. For this reason, users of this test will often take a reading in the morning which reads high and possibly another in the afternoon which reads low and wonder which is correct, causing further uncertainty. But this uncertainty can be overcome.
Verification of the humidity box test results is easier when used together with a datalogger such as the Tramex Feedback. The external probe is placed into the box and monitors the temperature and humidity readings for the entire duration of the test, thus showing clearly when equilibrium was achieved and what the correct reading was at the appropriate time.
Electronic moisture meter testing is non-destructive and provides instant readings. A purpose-built concrete moisture meter such as the Tramex CME4 and CMEX2 provides more helpful readings (of moisture by mass in concrete and cementitious screeds) than a general purpose, comparative moisture meter.
This method of testing allows the user to map a whole area, very quickly assessing the moisture condition and locating the highest reading points for further testing with more elaborate, time-consuming methods (such as the humidity box test already described) when such methods are required. When testing anhydrite screeds with an electronic moisture meter it’s essential the laitance has been removed from the surface of the screed to gain a meaningful reading. The laitance acts as a barrier or skin, trapping moisture at the surface of the screed, therefore producing a false positive reading on the instrument which is designed to take a correct measurement based on the drying curve of the slab/screed in normal drying conditions.
For the same reason it’s important ambient humidity conditions are within the normal range of between 40-60% to avoid condensation which can also lead to false positive readings. The CM test (known as the bomb test or speedy test in the UK) is the German national standard test and is required as a final certification of moisture conditions of slabs/screeds in many European states. The CM test involves removing a sample of the slab/screed with a hammer and chisel and crushing it using a mortar and pestle, then weighing the required amount and placing into an airtight chamber together with calcium carbide which, when in contact with moisture, produces acetylene gas.
The higher the concentration of moisture the more gas is produced which is read as pressure from the devices gauge. This test is ideal for certain proprietary and fast drying screeds which act by chemically binding most construction moisture and therefore cannot be tested with relative humidity or electrical impedance devices which will give high results. In theory the CM test is the most suitable for anhydrite screeds owing to the chemical nature of the test, showing only ‘free’ moisture which can cause floor failure. In practice, however, the test is easy to get wrong and requires a good deal of knowledge and skill to get exactly right.
GSSI, the world’s leading manufacturer of ground penetrating radar (GPR) equipment, announces the release of a major software update for the StructureScan™ Mini XT – the newest generation of GSSI’s popular all-in-one concrete inspection GPR system. The update expands StructureScan™ Mini XT capabilities with an increased depth range, improved Focus Mode, and a new Auto Drill feature.
The update increases StructureScan™ Mini XT’s depth range by 20% to up to 24 inches for greater visibility in survey situations involving thick structural concrete and slab on grade. Additionally, algorithm improvements enhance the StructureScan™ Mini XT’s gain at greater depths.
The improved Focus Mode uses input from the StructureScan™ Mini XT’s 2.7 GHz high-resolution antenna to resolve closely spaced and bundled targets within concrete, offering precise visualization where traditional GPR hyperbolas would condense data into a singular dot. Users can sweep between raw GPR data and the easy-to-read focused view. The new Auto Drill feature searches for potential obstructions to a planned core location. The innovative software tool uses a specialized algorithm to identify possible obstacles to drilling operations by analyzing a user-selected position and size (1/2’ to 6”) on a 3D grid.