Validation of Measurement Techniques and the Determination of Data required for Thermal Modeling of Moist Masonry Walls

Transcription

1 Validation of Measurement Techniques and the Determination of Data required for Thermal Modeling of Moist Masonry Walls R.G.Williams National Physical Laboratory R.P.Tye Consultant ABSTRACT: A comprehensive programme of measurements involving twelve organizations has been carried out as part of a project partially funded by the European Community under the Competitive and Sustainable Growth Programme ( ), relating to the thermal and moisture performance of masonry walls and the measurement uncertainty associated with those measurements. The basic thermal and moisture transport properties of several types of masonry materials and mortars have been obtained, an extended series of measurements have been made using different types of hot box measurement apparatus of the thermal performance of wall systems fabricated from these materials in the dry and carefully controlled moisture content conditions. To assist in the final analysis, a series of measurements of idealized wall systems with sheets of polymethylmethacrylate replacing the mortar was also included. Over 200 hot box tests were carried out for three main reasons. Firstly to quantify the repeatability, reproducibility and uncertainty limits of guarded and calibrated hot box measurements according to ISO Secondly, to establish and verify the efficiency and possible equivalency of the more recently developed EN1934 procedure that utilises a heat flow meter attached to the surface of the test element. Thirdly, to validate a mathematical model used to derive the U-value of walls built of hollow blocks and mortar, with specific moisture contents, from the thermal properties of its components in the dry state. The results are presented and discussed with respect to the equivalency requirements and to the verification of performance of each type of hot box. In addition, comparisons are made of the effects of moisture on performance between tests made on the composite system. Finally, the overall data were analysed in order to validate the mathematical model of complex moist masonry structures and to produce a software tool to facilitate the determination of thermal properties using a procedure based on EN BACKGROUND AND OBJECTIVES During the past ten years there has been a concerted effort within the European Union to develop mandatory s for specifying, testing and applying commonly used materials for and within the building envelope. In particular the draft pren 1745 Masonry and masonry products: Methods for determining design thermal values, produced by CEN/TC125, details methods for determining design values for thermal conductivity and thermal resistance of masonry and masonry products. These design values for masonry and composite masonry walls refer to moisture content under service conditions which may be taken from tabulated design values derived from measurements on materials and calculations using numerical methods. For composite masonry systems the experimental determination of the dependence of thermal performance on moisture content, by measurements at several moisture content levels, is complicated, time-consuming and expensive. At present, the design value required at the national level is derived from measured values in different ways in different countries. Thus, a harmonized conversion procedure is required in order to obtain thermal design values of hollow block masonry systems found under service conditions (defined according to nationally varying climatic conditions) from measured values of the components, preferably from a single determination in the dry state. The desire for ized test methods to facilitate an open market has prompted the production of a number of s for measurements of the

2 thermal resistance of building materials (e.g. walls) in the dry and moist state. These s (produced by Working Group 8 of CEN/TC89) include: ISO 8990:1996 The classical hot-box for the guarded hot-box (GHB) and calibrated hot box (CHB). EN 1934:1995 A hot-box measurement procedure that employs a heat flow meter (HFM) fixed to the test element (HFM/HB) based mainly on the experience of German, Austrian and Swiss laboratories The latter has the advantage of being able to deal with a wide range of thickness of masonry structures in a modest sized hot-box. However, the repeatability and reproducibility of all of these different measurement procedures was required in order that the requirements of the Construction Products Directive 89/106/EC (attestation of conformity), in connection with the basic document Nr. 6 (energy conservation and heat insulation) and the guidance paper 1, which deals with classes and performance levels, can be met. To attain the overall goal, a comprehensive threeyear project was initiated in 1998 involving twelve organisations undertaking measurements on five materials. The properties measured by selected organisations were thermal conductivity, thermal conductance and U-value, water vapour resistance and hygroscopic sorption. The project had six objectives: 1. Determination of the relevant thermal and moisture properties of five basic constituent materials at three different moisture content levels for use in subsequent tasks. 2. Validation of EN Determination of the uncertainty, repeatability and reproducibility requirements as defined in ISO Measurement of the thermal resistance of walls fabricated using three different masonry products and two different mortars. 5. Measurement of the thermal resistance of a range of ideal wall systems by replacing the mortar with glass sheets to enable final analysis to be verified. 6. Using the measured data to develop a model and conversion procedure which enables the thermal properties of masonry wall systems to be derived from the component properties in the dry state. The present paper describes and discusses the work carried out in order to satisfy objectives 2 to 4. A comprehensive description of the whole project is given in the final project report [1]. A paper by Williams [2] gives a summary of the whole project. The results of the programme of thermal conductivity and moisture properties of the individual materials has been published in a paper by Salmon et al [3]. These contain full details of the materials; the individual participants; the relevant s; these details will not be repeated here. A paper describing the mathematical modelling has been written by Sandberg [4]. SCOPE To accomplish the task of producing data for the individual objectives, nine of the twelve organisations participated in some of the two hundred hot-box measurements that were required. The materials were autoclaved aerated concrete (), hollow concrete block (CB) hollow brick block (BB) and two mortars of different density. Table 1 contains a summary of the test plan, developed to establish the following: Repeatability Six laboratories repeated measurements on walls made of, without mortar, five times. This was done using both GHB/CHB and HFM/HB methods Reproducibility Six different wall systems were measured in the dry state by either four or six laboratories, using both measurement techniques. The majority of the measurements of the material were with 200 mm thick blocks but additional measurements were also made on 100 mm and 300 mm thick blocks. Thermal conductance Individual measurement (and repeats) by the nine laboratories on composite walls, fabricated from these different masonry materials and the two mortar types at three moisture conditions, namely dry and conditioned at 50% RH and 90% RH respectively Validation of HFM Hot Box method by comparison of the results obtained on the above measurement programme using the different hot box types. An important feature of the programme was the establishment of strict protocols for conditioning of the materials and systems, for instrumentation of the wall systems and the establishment of equilibrium times. Brief details of each are as follows: Conditioning: The unmortared walls and blocks were dried at 105º C to constant mass. The mortared walls were first left for 28 days for the mortar to fully cure. Then they were dried to constant mass, first at 70º C and then at 105º C (to re

3 Table 1 Hot box measurement schedule Test number Material description Thickness (mm) Density (kg/m3) Joint material Moisture content Apparatus type TEST IDENTIFIER A B C D E F 13 Aerated concrete none dry HFM MA Aerated concrete none dry HFM MA Aerated concrete none dry HFM MA Aerated concrete none dry HFM MA Aerated concrete none dry HFM IBP NPL MA39 FIW UI TGM 6 18 Aerated concrete none 50% Moist HFM IBP MA Aerated concrete none 90% Moist HFM IBP MA39 2 Total tests 20 Aerated concrete none dry HFM FIW IBP NPL Ul MA39 TGM 6 21 Aerated concrete none dry HFM FIW IBP NPL Ul MA39 TGM 6 22 Aerated concrete none dry HFM FIW IBP NPL Ul MA39 TGM 6 23 Aerated concrete none dry HFM FIW IBP NPL Ul MA39 TGM 6 24 Aerated concrete none dry HFM FIW IBP NPL Ul MA39 TGM 6 25 Aerated concrete none 50% Moist HFM FIW IBP NPL Ul MA39 TGM 6 26 Aerated concrete none 90% Moist HFM FIW IBP NPL Ul MA39 TGM 6 27 Aerated concrete none dry HFM Ul 1 28 Aerated concrete none dry HFM Ul 1 29 Aerated concrete none dry HFM Ul 1 30 Aerated concrete none dry HFM Ul 1 31 Aerated concrete none dry HFM FIW Ul TGM EMPA NPL IBP 6 32 Aerated concrete none 50% Moist HFM FIW Ul 2 33 Aerated concrete none 90% Moist HFM FIW Ul 2 34 Aerated concrete none dry GHB IBP NPL SP VTT FIW EMPA 6 35 Aerated concrete none 50% Moist GHB IBP SP 2 36 Aerated concrete none 90% Moist GHB IBP SP 2 37 Aerated concrete none dry GHB EMPA FIW IBP NPL SP VTT 6 38 Aerated concrete none dry GHB EMPA FIW IBP NPL SP VTT 6 39 Aerated concrete none dry GHB EMPA FIW IBP NPL SP VTT 6 40 Aerated concrete none dry GHB EMPA FIW IBP NPL SP VTT 6 41 Aerated concrete none dry GHB EMPA FIW IBP NPL SP VTT 6 42 Aerated concrete none 50% Moist GHB EMPA FIW IBP NPL SP VTT 6 43 Aerated concrete none 90% Moist GHB EMPA FIW IBP NPL SP VTT 6 44 Aerated concrete mortar 1 dry GHB EMPA FIW IBP NPL SP VTT 6 45 Aerated concrete mortar 1 50% Moist GHB NPL VTT 2 46 Aerated concrete mortar 1 90% Moist GHB NPL VTT 2 47 Aerated concrete mortar 1 dry HFM EMPA FIW VTT IBP NPL MA Aerated concrete mortar 1 50% Moist HFM EMPA VTT 2 49 Aerated concrete mortar 1 90% Moist HFM EMPA VTT 2 50 Brick - Hollow block mortar 1 dry HFM UI VTT NPL MA Brick - Hollow block mortar 1 50% Moist HFM UI NPL 2 52 Brick - Hollow block mortar 1 90% Moist HFM UI NPL 2 53 Brick - Hollow block mortar 2 dry HFM UI IBP VTT TGM 4 54 Brick - Hollow block mortar 2 50% Moist HFM UI VTT TGM 2 55 Brick - Hollow block mortar 2 90% Moist HFM UI VTT TGM 2 56 Brick - Hollow block morter 1 dry GHB NPL VTT SP EMPA 4 57 Brick - Hollow block morter 1 50% Moist GHB NPL SP 2 58 Brick - Hollow block morter 1 90% Moist GHB NPL SP 2 59 Brick - Hollow block mortar 2 dry GHB IBP SP EMPA VTT 4 60 Brick - Hollow block mortar 2 50% Moist GHB IBP EMPA 2 61 Brick - Hollow block mortar 2 90% Moist GHB IBP EMPA 2 62 Concrete - Hollow block mortar 1 dry HFM IBP TGM UI 4 63 Concrete - Hollow block mortar 1 50% Moist HFM IBP TGM UI 2 64 Concrete - Hollow block mortar 1 90% Moist HFM IBP TGM UI 2 65 Concrete - Hollow block mortar 2 dry HFM EMPA UI TGM IBP 4 66 Concrete - Hollow block mortar 2 50% Moist HFM EMPA TGM 2 67 Concrete - Hollow block mortar 2 90% Moist HFM EMPA TGM 2 68 Concrete - Hollow block mortar 1 dry GHB IBP SP EMPA 4 69 Concrete - Hollow block mortar 1 50% Moist GHB IBP SP 2 70 Concrete - Hollow block mortar 1 90% Moist GHB IBP SP 2 71 Concrete - Hollow block mortar 2 dry GHB SP EMPA IBP 4 72 Concrete - Hollow block mortar 2 50% Moist GHB SP EMPA 2 73 Concrete - Hollow block mortar 2 90% Moist GHB SP EMPA 2

4 duce cracking). It had been shown (described in detail in reference 1) that due to carbonisation, specimens thicker than 90 mm required a longer time to attain constant mass. Based on that information, the mass stability criterion for blocks less than 90 mm thick was specified as follows: Constant mass is reached when the slope of the time/mass curve is less than 0.03% of original mass per 24 hours. This slope shall be determined at least twice, requiring at least 3 measurements. Both of the slopes shall meet the given criterion. For blocks thicker than 90 mm the acceptance criterion was specified as follows: Constant mass is reached when the slope of the time/mass curve is less than 0.01% of original mass per 24 hours. This slope shall be determined at least twice, requiring at least 3 measurements. Both of the slopes shall meet the given criterion. The accuracy for the mass measurements shall be measured to an uncertainty of no greater than 50% of the mass change over the period chosen to determine the slope of the moisture curve. To determine the weight of the walls, either the whole wall or a witness specimen (defined as a block of the same thickness as those used for the wall with the edges sealed and placed in close proximity to the wall during the conditioning) shall be weighed. The two moisture conditions were to condition in air with a relative humidity of either 50% or 90% at a temperature of 23º C. The moisture content (% mass) shall be obtained for each conditioning treatment by comparing the mass of the conditioned wall or witness sample with that of the fully dried wall or witness sample. A final important requirement concerned the mixing of the mortar, where water content was found to be critical. Participants were required to mix the mortar to a firm uniform consistency to support the heavy blocks on a 12 mm thick mortar layer. The actual amount of water used was to be recorded. Hot box measurement protocol: After drying the walls until a constant mass was achieved they were allowed to cool in a dry environment for given periods of time before testing, depending on the block thickness (1, 2 and 5 days for 100, 200 and 300 mm thickness respectively). This produced a repeatable starting condition from which equilibrium times were measured. It was decided that the same time to equilibrium would be allowed for the moist walls as determined for the dry walls. In all cases, after drying or conditioning the walls were covered with a polymer sheet to maintain the wall in the dry and conditioned states during the hot box measurements. Thermocouples were mounted on the walls as specified in EN1934 or EN The instrumented walls were then covered in plastic sheet. The sheet and air gaps will have a negligible effect on the measured thermal conductance. Figure 1 illustrates a typical minimum thermocouple placement for a 1.5 m x 1.5 m mortared wall, measured in a either a GHB or CHB. Nine thermocouples were placed at least 100 mm from any mortar joint but maintaining them as close as possible at the centre of squares and equal areas. Three were placed on the mortar joints, two on different horizontal joints and one on a vertical joint. However, as the example in Figure 1 shows, the area of the blocks is approximately 18 times that of the mortar but the ratios of thermocouples measuring those areas is only 3 to 1. Therefore a simple area weighting method was used to determine the mean surface temperatures. Figure 1 Thermocouple layout for GHB and CHB measureme t/cs 100 mm from mortar joints - and as close to being in the centre of squares-ofequal-area as possible 2 t/cs on horizontal mortar joints & 1 t/cs on vertical mortar joint X O All dimensions in mms Equilibrium times were defined as the time to attain equilibrium for the same dry wall plus 4 hours. For moisture containing walls the equilibrium time is then assumed to be the same as that for the dry form of the same block type, thickness and whether or not it has mortar joints. Measurement conditions were as follows. Mean specimen temperature 10 ºC to 15 ºC, a temperature difference 20 ºC ± 1 ºC. Cold air speed 1 to 2 m/s. The relative humidities in the hot and cold boxes were recorded where possible. The procedure for carrying out the Repeatability measurements was as follows. After each measure- X X X X O X X X O X

5 ment, the wall was returned to room temperature, the apparatus was opened and the plastic sheets and thermocouples removed. These latter two were then both replaced, the apparatus closed and the next test started. RESULTS AND DISCUSSION All results are presented with the individual organisations identified by a number in order to preserve anonymity. Table 2 contains the individual values obtained on the systems. They were used to derive the repeatability and reproducibility statistics. The collected results for the average thermal conductance and moisture content measurements are shown in Table 3 and 4 respectively. When the data provided in Table 2 were analysed statistically (summarized in Table 5) using the procedures specified in ISO , it was found that for the 200 mm material the repeatability of the HFM hot box was approximately 0.8 % and of the classical guarded and calibrated hot boxes was 0.6%. However, the repeatability of the HFM hot box for the thicker and thinner blocks was about 1.7%. The reproducibility was approximately 3.9% and 2.6% for the HFM hot box and guarded and calibrated boxes respectively. Comparison of the results obtained on the 200 mm material with no mortar showed that the mean values obtained by the guarded/calibrated system were approximately 2.2% higher than those obtained by the HFM hot box. For the 100 mm and 300 mm thick systems the differences increased to 3.6% and 5.8% respectively. In all cases the thermal conductance values measured with the HFM hot box apparatus were lower than those of the GHB and CHB apparatus. The current measurements have not been analysed to evaluate differences between guarded and calibrated hot boxes. Overall the statistical analysis indicates that the measurement uncertainty of the guarded and calibrated systems is approximately ± 5.2% and for the HFM hot box it is ±7.8%. It has been shown that the differences between the two measurement approaches is within the measurement uncertainty of the two methods and overall the procedures specified in EN 1934 for the HFM hot-box method produce values in reasonably good agreement with those obtained by the GHB and CHB methods. This is confirmed by comparison of the measured values of the thermal conductance obtained with the two types of hot boxes and the thermal conductance calculated from the thermal conductivity values for the material [1], [2]. The results are shown in Table 6. The results of the measurements on the various mortared hollow block wall systems containing moisture showed more scatter than for those on the dry and moisture containing walls made from. In a number of cases this scatter amounted to approximately 20%. In general the scatter of the measurements on the systems containing moisture was 10% or less and the results could be examined to evaluate the effects of moisture on performance. This was carried out by normalising the results to a dry density value and a typical curve for one system is shown in Figure 2. While scatter is still evident, the general trend from each of the measurements is similar and the slope of the curve indicates an effect of the order of 3.5% change in thermal conductance for each 1% increase by weight in moisture content. For the other material systems which included mortared joints, the overall apparent change in thermal conductance was significantly less than 3.5% for each 1% moisture. The above results confirm the rather surprising findings obtained when the individual materials were examined [3]. Based on critical analysis of many masonry systems by Valore [5,6] the indications have been that, as a minimum, a 1% increase by weight of moisture produces an increase of 5% in both the thermal conductivity and thermal conductance of masonry. The present results on materials and systems would indicate that the effect is less and for certain materials less than the 4% increase stated in the recently published EN ISO 1046:2000. The present results also highlight how important the mortar is to the overall thermal properties of the walls. The study on the two individual masonry materials showed [2] that the method of mixing influenced the amount of moisture used. This was identified as the likeiest cause of variations in the thermal conductivity of about 25 to 30% and corresponding large differences in water vapour transmission values. SUMMARY A controlled measurement programme has been carried out using three different types of hot box apparatus, on selected masonry systems both in the in the

6 Table 2 Individual measurements on dry with no mortar. Thermal conductance (W/m 2 K) - normalised to 500 kg/m 3 Laboratory HFM, dry, no mortar GHB / CHB, dry, no mortar 100 mm 200 mm 300 mm 100 mm 200 mm 300 mm

7 Table 3 Results of hot box measurements of the material by different laboratories. Material Condition Hot box AVERAGE CONDUCTANCE MEASURED BY THE DIFFERENT LABORATORIES type type W/m².K W/m².K W/m².K W/m².K W/m².K W/m².K W/m².K W/m².K W/m².K Test Lab 1 Lab 2 Lab 4 Lab 6 Lab 11 Lab 10 Lab 5 Lab 3 Lab mm Dried at 105 ºC G & C/HB to mm Dried at 105 ºC HFM/HB mm 23 o C & 50% RH G & C/HB mm 23 o C & 50% RH HFM/HB mm 23 o C & 90% RH G & C/HB mm 23 o C & 90% RH HFM/HB Test Lab 1 Lab 2 Lab 4 Lab 6 Lab 11 Lab 10 Lab 5 Lab 3 Lab 7 37 to mm Dried at 105 ºC G & C/HB to mm Dried at 105 ºC HFM/HB mm 23 o C & 50% RH G & C/HB mm 23 o C & 50% RH HFM/HB mm 23 o C & 90% RH G & C/HB mm 23 o C & 90% RH HFM/HB Test Lab 1 Lab 2 Lab 4 Lab 6 Lab 11 Lab 10 Lab 5 Lab 3 Lab mm Dried at 105 ºC G & C/HB to mm Dried at 105 ºC HFM/HB mm 23 o C & 50% RH G & C/HB mm 23 o C & 50% RH HFM/HB mm 23 o C & 90% RH G & C/HB mm 23 o C & 90% RH HFM/HB Lab 1 Lab 2 Lab 4 Lab 6 Lab 11 Lab 10 Lab 5 Lab 3 Lab mm + Mortar 1 Dried at 105 ºC G & C/HB mm + Mortar 1 Dried at 105 ºC HFM/HB mm + Mortar 1 23 o C & 50% RH G & C/HB mm + Mortar 1 23 o C & 50% RH HFM/HB mm + Mortar 1 23 o C & 90% RH G & C/HB mm + Mortar 1 23 o C & 90% RH HFM/HB Key G & C/HB Guarded and Calibrated Hot Box HFM/HB Heat Flow Meter Hot Box dry state and containing known amounts of moisture. The study was part of an overall investigation to provide the European Union with the tools to meet a Construction Products Directive for such products. Measurements were carried out and analysed using appropriate international or European Standard methods. The results indicated that the measurement uncertainty of the HFM hot box method as specified in EN 1934 was approximately ±8 % and for the calibrated and guarded hot box apparatus it was approximately ± 5%. The values obtained by the HFM apparatus were always slightly lower than from the calibrated and guarded hot boxes. However, the overall results confirmed the findings of the study on the individual materials which showed that the effects of moisture on thermal conductance were less than had been shown previously. The current results were also a valuable input to the development of a model and conversion procedure [1],[4] to enable thermal properties to be calculated from thermal conductivity values of the components in the dry state. REFERENCES 1. R Williams, Final Report Contract SMT4- CT (DG 12 - HIAS) : Determination of the thermal resistance of walls (masonry) in dry and moist state and a conversion procedure to get the appropriate design value. (Project funded by the European Community under the Competitive and Sustainable Growth Programme ( ) 2. R Williams, Overview of a project to determine the thermal resistance of masonry walls in dry and moist states and a conversion procedure to determine the appropriate design

8 value. Proceedings of the 6 th Symposium Building Physics in the Nordic Countries. 3. D. R. Salmon, R. G. Williams and R. P. Tye, (2002), Thermal Conductivity and Moisture Measurements on Masonry Materials Insulation Materials; Testing and Application: Vol 4 ASTM STP 1426 Eds, AC Desjalous and RR Zarr, American Society of Testing and Materials. 6. R. C. Valore Jr, (1980) Calibration of U- values of Hollow Concrete Masonry American Concrete Institute. Concrete International Design and Construction pp P. I. Sandberg,. (2002), Thermal Modelling of moist masonry walls Presented at 11 th Symposium for Building Physics, Dresden September R.C. Valore Jr, (1956) Insulatory Concretes, J. Am Concrete Inst, 28(5) pp Table 4 Moisture content of the conditioned walls 100 mm 200 mm 300 mm 200 mm No Mortar No Mortar No Mortar Mortar 1 50% RH 90% RH 50% RH 90% RH 50% RH 90% RH 50% RH 90% RH mass% mass% mass% mass% mass% mass% mass% mass% Lab Lab Lab Lab Lab Lab Lab Lab 9 Lab Lab Average St. Dev% Hollow Concrete Hollow Brick Hollow Concrete Hollow Brick Mortar 1 Mortar 1 Mortar 2 Mortar 2 50% RH 90% RH 50% RH 90% RH 50% RH 90% RH 50% RH 90% RH mass% mass% mass% mass% mass% mass% mass% mass% Lab Lab Lab 3 Lab Lab 5 Lab Lab Lab 9 Lab 10 Lab Average St. Dev%

9 Table 5 Statistical analysis of measured thermal conductance data for dry material + s Laboratory 100 mm HFM, dry, no mortar 200 mm 300 mm 100 mm GHB / CHB, dry, no mortar 200 mm 300 mm Apparatus HFM/HB G & C / HB - thermal conductance (normalised to a density of 500 kg/m 3 ) [W/m 2 K] thickness of specimen (mm) mean value general mean repeatability reproducibility Table 6 Comparison of measured and calculated values of thermal conductance of dry ACC block system Thermal conductance ( W/m 2.K) Measured Calculated % difference from values values calculated values HFM/HB G & C/HB HFM/HB G & C/HB

10 Figure 2 Thermal conductance at different moisture content for 200 mm mm - No mortar Conductance values normalised to a density of 500 kg/m³ Thermal conductance vs Moisture content - By lab Thermal conductance (W/m².K) Example of ± 5 % error bar Lab1/hfm Lab1/ghb Lab2/hfm Lab2/ghb Lab3/hfm Lab3/ghb Lab4/ghb Lab5/ghb Lab6/hfm Lab7/ghb Lab10/hfm Lab11/hfm Moisture content (Mass %)

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