Calcium Carbide Process
The calcium carbide method or calcium carbide method (CM measurement) is a fast and, in many cases, sufficiently accurate field method for moisture measurement. It can be used when all-round contact between the calcium carbide and the material sample can be established.
method
In this process, the moist sample is weighed and then placed in a pressure vessel containing steel balls and a glass ampoule with calcium carbide. Everything is crushed and mixed in the sealed container. The water contained forms acetylene with calcium carbide according to the following equation :
The amount of acetylene is determined by measuring the pressure increase using a manometer and is the measure of the amount of water previously present. The water content is determined with reference to the sample mass. This method has proven itself for checking the water-cement value of fresh concrete . Measurement errors of ± 1–3% of the moisture present in the sample are to be expected.
Possible errors are weighing errors of a sample that is too small and an imperfect reaction due to insufficient contact of the interior of particles that are too large with the calcium carbide. During the previous crushing, moisture can also evaporate from the material or be absorbed from the air. The amount of sample material depends on the grain size (2–5 mm). For example, 50 g sample material is required for cement screed and 100 g for anhydrite screed .
The taking of samples to about the level of base course take place. During the measurement process, due to the strong temperature dependency of the tank pressure, attention must be paid to the constant temperature. The measuring times are relatively long and can be up to 20 minutes for food. The measuring method is easy to handle, but requires a relatively high effort for sample preparation and measurement.
criticism
The CM measurement as a measuring method for determining the residual moisture of a screed has come under increasing criticism in recent years, which is related on the one hand to the sources of error inherent in the process and on the other hand to the recommended limit values for the so-called readiness for covering that are used today Screed mixes (CEM II cement instead of CEM I cement, changed water / cement ratio) are no longer valid.
A study by the Swiss Federal Materials Testing Office was published as early as 1997, which came to the conclusion: “In summary, it can be stated that the CM method ... neither provided usable results on the cement mortar samples examined, nor was it used for building practice should]".
A study published in March 2012 by the Technical Commission for Building Adhesives (TKB) and the University of Siegen came to the conclusion that the CM limit value of 2% (for cement screeds) does not reliably separate screeds that are ready for covering from screeds that are not ready for covering. With this limit value, wet screeds are also rated as dry.
In March 2016, leaflet TKB 16 of the Technical Commission for Building Adhesives (TKB) from the Adhesives Industry Association and the Central Association of Parquet and Floor Technology (ZVPF) , Federal Association for Color Design of Building Protection , Association of the German Parquet Industry eV (vdp) , Association of multilayer modular floor coverings eV ( MMFA) , Central Association of Space and Equipment (ZVR) , Association of Manufacturers of Resilient Floor Coverings eV (FEB) and Federal Association of Sworn Experts for Space and Equipment eV recognized the CM measurement as a recognized rule of technology. This means that a screed can also be checked for readiness for covering using a CM measurement.
An alternative to the CM method is to measure the relative humidity (KRL) of the screed. The material moisture of the screed develops a partial vapor pressure in the surrounding air, which is measured as relative humidity. For this purpose, a screed surface is sealed by means of a film that is taped off on all sides, or the screed material is insulated in a film bag. In the isolated air space, the material moisture creates a relative humidity equilibrium. Based on an assumed indoor air humidity of 50%, for example, the humidity or the degree of dryness of the sample or the test surface is evaluated. If an air humidity equilibrium that is almost the same as assumed develops in the isolated air space, it is assumed that the material moisture equilibrium has been reached and the screed is described as dry.
swell
- T. Brokamp, R. Trettin: Readiness for covering and moisture (TKB report 1; PDF; 4.2 MB), March 2012
- K. Heim / K. Moser: Comparison of different measurement methods for water content measurements on cement and Anhydrite-bound screed test specimens using direct and indirect measuring methods. in: 9. Feuchtetag 1997 Modern methods and devices for material moisture measurement, Weimar 1997
- K. Kupfer: Material moisture measurement: Basics, measurement methods, applications, standards. Expert Verlag, 1997.
- C.Hübner: Development of high-frequency measuring methods for determining soil and snow moisture . Scientific reports, FZKA 6329, Research Center Karlsruhe, 1999.
Individual evidence
- ↑ a b DIN 18560-4: 2012-06.
- ↑ K. Heim / K. Moser: Comparison of different measurement methods for water content measurements on cement and Anhydrite-bound screed test specimens using direct and indirect measuring methods. in: 9. Feuchtetag 1997 Modern methods and devices for material moisture measurement, Weimar 1997
- ↑ T. Brokamp, R. Trettin: Readiness for covering and moisture (TKB report 1; PDF; 4.2 MB), March 2012.
- ↑ Recognized rules of technology for CM measurement. (pdf) TKB leaflet 16th Industry Association for Adhesives, March 2016, accessed on December 31, 2019 .
- ↑ W.Pitt: De facto further than CM . In: floor wall ceiling . No. 5 . Holzmann Medien GmbH & Co.K, 2018, p. 44-45 .