Przewodność cieplna autoklawizowanego betonu komórkowego według PN-B-06258

Brak w Polsce norm określających właściwe metody badań przewodności cieplnej i brak właściwej aparatury spowodowały w przeszłości wprowadzenie do kilku norm 'materiałowych' powołania na aparaturę przestarzałą. Specyficzna sytuacja istnieje w odniesieniu do betonów komórkowych; według PN-B-06258 od 1965 r. do badań l można stosować dwie metody, z powołaniem na przestarzały aparat Bocka i tzw. aparat termistorowy; w praktyce CEBET od lat stosuje tylko 'aparat termistorowy'. Autorzy wykazali systematyczne błędy pomiarowe badań l według PN-B-06258, wywołane wadami konstrukcji 'aparatu termistorowego', datującymi się z początku lat sześćdziesiątych. Określono współczynnik korekcyjny w odniesieniu do wyników badań l betonów komórkowych badanych przez CEBET za pomocą 'aparatu termistorowego'.

According to PN-B-06258 thermal conductivity of autoclaved cellular concrete may be determined with use of two apparatus: Bock and 'thermistor apparatus'. Bock apparatus belongs to the wide group of GHP (Guarded Hot Plate) apparatus, covered by ISO 8302, but it has very old construction (it was produced in former GDR in 50. and 60.) and does not fulfill the requirements of ISO 8302. 'Thermistor apparatus', better known as Kondratev's - calorimetr, is a non-stationary heat conduction device, nowhere used outside CEBET. Within the inter-laboratory comparative tests, carried out by CEBET and Department of Thermal Physics of ITB in 1997, the big difference of test results for the same specimens occurred between three apparatus: Bock apparatus, 'thermistor apparatus' and GHP apparatus constructed in ITB, compatible with ISO 8302. The difference between results of tests carried out in CEBET using Bock and 'thermistor apparatus' was up to 23 %. Tests results given by Bock apparatus were higher up to 8 % and given by 'thermistor apparatus' were lower up to 14 % than results given by GHP apparatus constructed in ITB; the errors were of systematic character. The errors of Bock apparatus are due to edge losses of heat from the specimens and that property of the apparatus has been found already 35 years ago. The authors analysis of 'thermistor apparatus' has demonstrated two sources of systematic errors: too small Biot's number on the specimen's surface and neglected thermal capacity of a thermistor and that of rubber sheathing. The systematic decreasing of thermal conductivity by 'thermistor apparatus' used by CEBET results in false image of autoclaved cellular concrete sold on Polish market. The authors propose a correction factor to the results issued by CEBET based on the results of the comparative tests: between 1,06 and 1,14, depending upon density of cellular concrete.