improving access to the frost dilatometry...

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14TH CANAD IAN CONFERENCE ON BU I LD ING S C I ENCE AND T E CHNOLOGY

IMPROVING ACCESS TO THE FROST DILATOMETRY

METHODOLOGY FOR ASSESSING BRICK

MASONRY FREEZE THAW DEGRADATION RISK

R. Van Straaten

ABSTRACT

The application of frost dilatometry has a significant advantage relative to available ASTM methods in

providing application-specific risk assessments. The main application considered in this paper is the

interior retrofit thermal insulation of heavy brick masonry buildings. For such projects, the cost of testing

and analysis present an obstacle to widespread application of frost dilatometry. The possibility of reducing

costs by using archived material property data for analysis is evaluated through a review of testing results

from 202 bricks from various sample lots. The range of material property values is large between lots and

no correlation with vintage or visible brick characteristics was found. Hence, such a wide range of material

property values would need to be used in analysis that limited insight would be anticipated. The potential

to estimate critical saturation from more basic testing was also evaluated. Correlations are too limited to

avoid the necessity of testing. The frost dilatometry methodology was examined seeking efficiencies. A

number of optimizations were made including sample slice size reduction, improvements in measurement

techniques, a simplified packaging procedure, and optimization of freeze-thaw cycle routines. The sum of

these improvements has resulted in cost and time of testing decreasing by about a half. To ensure that

opportunities for thermal insulation of masonry buildings aren’t missed due to limited assessments, further

improvements to the overall process should continue to be investigated. Growth of the archive database,

more analysis, and experimentation with further changes throughout the assessment process may result in

further efficiencies.

INTRODUCTION

Mensinga et al. (2010) describe the frost dilatometry process in detail and propose it as a more accurate

method of predicting freeze-thaw resistance, compared to commonly used methods such as the cold

soak/boil ratio or saturation coefficient (ASTM 2012) and rapid freeze-thaw method (ASTM 2008). The

metric used in frost dilatometry is the critical degree of saturation (Fagerlund 1977); masonry can

experience unlimited freeze-thaw cycling below this critical moisture content level without damage, while

above it, damage will occur quickly. Unlike previous methods, which use simple pass/fail criteria (e.g.

“suitable for exposure” versus “unsuitable for exposure”), this metric results in a limit state design process

for assessing the retrofit risk of the assembly. In practice, the measured critical saturation is used as a risk

threshold in hygrothermal simulations; local climate conditions, building exposures, and enclosure design

are used as inputs to determine the durability risk associated with the retrofit of interior insulation.

This approach has been used on several projects (Wilkinson et. al. 2009, Wytrykowska 2012, Ueno 2013a,

Ueno 2013b). However, because it involves the use of specialized equipment and is relatively labour-

intensive, its application has been mostly limited to high-value and/or historically significant buildings.

Engineers may consider use of the default material properties provided in the simulation software database

or literature to avoid this time and monetary cost. Another option would be to derive critical saturation from

more basic measurements than frost dilatometry. Alternatively, the testing methodology can be examined

to find efficiencies. This paper details an investigation into these strategies to make the approach more

accessible for a wider range of projects.

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USE OF REPRESENTATIVE BRICK MATERIAL PROPERTIES

The use of representative material properties rather than sample measurements is consistent with typical

engineering practice. Budgets and time are often limited, and the use of published data in calculations often

provides sufficient accuracy in various applications. If the range of input variable values are known

sensitivity analysis can further be applied.

In terms of masonry testing, Hughes and Bargh (1982) attempted to establish correlation between brick

type, building locations, and ‘degree of weathering’. The authors had previous success applying such an

approach to limestone but were unsuccessful with brickwork. Laefer et. al. (2004) encountered similar

challenges. The authors suggest that these difficulties were due to a lack of uniformity in brick

manufacturing processes.

Building Science Laboratories has provided testing of bricks from a number of 1830-1950’s buildings,

mostly in northeastern North America. The material testing results from 202 bricks from various sample

lots (where each lot contained bricks of similar vintage and type from the same building) were compiled

to see if correlations could be found. The bricks have almost all been tested for dry density and liquid water

uptake or A-value as shown in Figure 1 and Figure 2. Three new bricks and the WUFI material North

American database (Fraunhofer IBP. 2013) value for “old brick” have been included. Test results for free

water saturation, vacuum saturation, and critical saturation are included in Figure 3, Figure 4 and Figure 5.

Where these properties have not been measured, but 24-hour cold soak and 5-hour boil measurements

have, these alternate values have been included in the plots. The full dataset is available at

buildingsciencelabs.com.

It is noted that the types of tests selected for different projects has generally evolved over time and vary

based on client demand. For instance, the demand for 24-hour cold soak and 5-hour boil testing has been

declining. As well, the precision of critical saturation measurement has improved, resulting in less

conservative reported values.

Some patterns can be observed within buildings. For the 1930’s New York project, the fill bricks have a

lower density and higher A-Value, but this doesn’t hold for other buildings where interior bricks are similar

to face bricks (see Table 1). This is surprising since one can often observe that fill and interior bricks were

selected as inferior quality (e.g. larger visible pores) than face bricks. However, the ranges of all material

properties found in our testing are quite large (see Table 1). It can be seen in the plots that some projects

show little variation in material properties while others show very wide variation.


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FIGURE 1: BRICK ARCHIVE DRY DENSITY, SAMPLE LOT SIZE, VINTAGE, AND REGION

FIGURE 2: BRICK ARCHIVE LIQUID WATER UPTAKE, VINTAGE, AND REGION


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FIGURE 3: BRICK ARCHIVE FREE WATER SATURATION AND 24HR COLD SOAK

FIGURE 4: BRICK ARCHIVE VACUUM SATURATION AND 5HR BOIL


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A comparison of the dataset to the database of brick properties found in the WUFI v5.2 database (2013) is

valuable because the software is in common use for enclosure analysis and practitioners will often use

WUFI values when measurements are not available. Comparison data is summarized in Table 1. It does not

include critical saturation values which are used for post-processing WUFI outputs. The range of values in

the WUFI database captures nearly all the values for older bricks in our archive. Use of the entire range for

sensitivity analysis in assessment may be sufficient but it is clear that use of a single value, such as the

North American value for “old brick”, would not reflect the wide range in actual brick properties.

Unfortunately the range is so large that the analysis results will likely have a similar large range providing

limited insight.

DERIVING CRITICAL SATURATION FROM OTHER MATERIAL PROPERTIES

The ratio of 24 hr cold soak to 5 hr boil moisture contents (C/B Ratio) are plotted against the ratio of free

water saturation and critical saturation in Figure 6. The plot shows limited correlation. Plots of other

measurements from the same dataset provided by Ueno et al. (2013b) show even less correlation. Hence,

it appears that critical saturation cannot be drawn from more basic material properties. As a result of this

observation and the large material properties variations found in some sample lots, we recommend testing

of multiple samples for projects at this time.

FIGURE 5: BRICK ARCHIVE CRITICAL FREEZE THAW SATURATION

TABLE 1: BRICK MATERIAL PROPERTY SUMMARY FROM LAB TESTING ARCHIVE AND WUFI


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IMPROVEMENTS TO THE FROST DILATOMETRY METHODOLOGY

Previous work (Mensinga et al. 2010; Schumacher 2011) has described the basic methodology for using

frost dilatometry to determine the critical degree of saturation and freeze-thaw resistance of masonry

materials. To summarize, the current approach involves the following major steps:

1. Cut sample “slices” from whole bricks for testing,

2. Saturate each slice and then dry to desired range of moisture contents,

3. Wrap each slice in plastic to maintain moisture content,

4. Allow the moisture content to equalize throughout each slice,

5. Measure the pre-test length of each slice (and re-wrap),

6. Subject each slice to several freeze-thaw cycles, and

7. Re-measure each slice length to determine dilation (growth).

8. This testing methodology has been examined to find efficiencies.

REDUCED SAMPLE SIZES

The sample slices used in Building Science Laboratories’ critical saturation testing have typically been 50

x 89 x 10 mm. 10 mm is the thinnest that slices can be cut without excessive slice breakage. The 89 mm

length fits the commonly available 3”-4” (76-102 mm) micrometer which is long enough that strain in

sample above critical saturation levels is measurable after a few freeze-thaw cycles. It is also the longest

length that allows for cutting and squaring a number of slices from a typical brick. The width was based

on typical brick sizes after the top and bottom were cut to square. It was identified that the slice width could

be reduced to 25 mm without affecting the process. A photo of the old and new samples is provided in

Figure 7. This reduction allowed twice as many slices to fit into the controlled temperature bath for freeze-

thaw cycling, thus doubling the throughput and reducing overall testing time requirements. It also allowed

twice as many slice samples to be taken from sample bricks, which was useful in some cases due to limited

sample availability.

FIGURE 6: FREE WATER SATURATION / CRITICAL SATURATION RATIO VS C/B RATIO


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SIMPLIFIED SLICE PACKAGING PROCEDURE

In earlier Building Science Laboratories tests, samples were wrapped with a thin, flexible plastic wrap,

taped with a sheathing tape, and placed in zip lock bags. The intent was for the flexible wrap to provide a

surface for the sheathing tape; the sheathing tape was intended to seal the slice, inhibiting drying

throughout the process. For the current study, an exercise was conducted where sheathing tape was omitted

and the moisture content was carefully measured through each step of testing. The amount of drying

through the plastic wrap was negligible and the process was simplified, saving manual labour.

FIGURE 7: NEW (TOP LEFT) AND OLD (BOTTOM LEFT) BRICK SLICE SAMPLES.

MEASUREMENT JIG (RIGHT)

SAMPLE LENGTH MEASUREMENT

Determining critical saturation through the frost dilatometry method involves measuring small increases in

brick slice length, often in the range of 200 to 5000 microstrain. One microstrain is one part per million,

so 1000 microstrain would be a dilation of 0.1%. The ability to measure these small strains accurately and

precisely sets both the uncertainty of the overall measurement and the lower limit of resolving sample

dilation.

Metal targets were used by Mensinga et al. (2010) for the point of contact with the micrometer for these

measurements. However, installing targets and waiting for their epoxy to set is a time consuming process.

To maintain a high degree of precision while eliminating the metal targets, a jig was constructed that

ensured slices were measured at the same location repeatedly. A test was conducted comparing slice lengths

obtained with metal targets but no jig to lengths obtained using the jig but omitting the metal targets. The

results show significantly better measurement repeatability using the jig alone (see Table 2). The

measurement variability is approaching the ±0.00005” precision of the micrometer (equivalent to about 15

microstrain). Hence, the use of jig without the metal targets was adopted.

TABLE 2: REPEATED SLICE LENGTH MEASUREMENTS


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FREEZE-THAW CYCLE TIME IMPROVEMENT

The time required for freeze-thaw cycling of sample slices is a fixed requirement within projects and often

occupies more than 80% of the testing and analysis schedule. For small laboratories with a single chilled

bath it also is a resource constraint. Samples were typically cooled from 20°C to -15°C over a

2-hour period, held for 1 hour, thawed over a 2-hour period to 20°C, and held for 1 hour before being

repeated.

It is generally accepted that increasing cooling rate increases freeze thaw damage (Lindmark 1996) and has

been shown not to significantly affect critical saturation measurements (Fagerlund 1992). Furthermore, it

was hypothesized that slices do not have to be heated all the way to 20°C to thaw and that less than 1 hour

was needed to ensure that the sample was completely thawed or chilled to -15°C.

In order to ensure that the target temperature was being reached at the brick sample core, a thermistor was

installed within the core of a sample slice (Figure 9). To measure the temperature accurately at the core, a

hole was drilled in the end of the slice reaching the center. The hole was drilled along the longest axis to

minimize thermal conductivity effects of the thermistor wire and cable. The sample core temperature

during freeze-thaw cycling was then measured in the middle of a full load of samples in the chilled bath.

FIGURE 9: SLICE CORE TEMPERATURE WITH INTIAL AND MODIFIED CYCLING ROUTINE

(LEFT) ANDTEMPERATURE PROBE (RIGHT)

The core temperature of the sample slice was found to closely follow the bath temperature as shown in

Figure 9. A slight jog in the temperature occurs when the temperature crosses 0°C. This is due to the latent

heat capacity of the sample moisture. This jog is not evident in the modified cycling routine; this is likely

due to secondary factors, such as (a) dryer sample used for test, (b) fewer slices in the chilled bath, which

increases the bath’s effective cooling power, and (c) the actual chilled bath coolant had been changed to a

non-toxic coolant. The measurements demonstrate that by increasing the cooling rate, decreasing the target

thaw (upper) temperature, and decreasing the hold time from an hour to 30 minutes, the cycling time can

be reduced by almost half.

CONCLUSIONS

Within the limited number of brick material property test results in our database, correlation could not be

found between material properties and region, manufacturing method, or manufacturer. Correlations may

eventually be found as the archive grows in the future and more analysis is provided. In one case, interior

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bricks were shown to have a lower density and higher A-Value, but this does not hold for other buildings

where they are similar to the face bricks. Some sample lots of older bricks have a tight variance in material

properties while others show a broad range. Where WUFI users apply values found in the WUFI database,

it appears they would need to apply the range of all the bricks in the database and not just the value given

for old North American bricks.

The ASTM C216 (2012) Severe Weather classification requirements (maximum values for averages of 5

new sample bricks) are also included in Figure 4 and Figure 6. Many of the old bricks in the datset would

not meet these requirements. Figure 6 further shows a number of samples in the lower right quadrant which

exceed the C/B threshold but have critical saturation levels below the free water saturation moisture

content. Fagerlund used the critical saturation to free water saturation ratio as a proxy for freeze-thaw

durability, meaning that extreme field exposure is necessary for bricks with low ratios to experience freeze-

thaw degradation. In-depth analysis (Wytrykowska 2012, Ueno 2013a, Ueno 2013b) has found cases where

brick moisture contents can be maintained well below critical saturation levels by implementing a

reasonable level of rain water control. If ASTM C276 were used alone these projects would be considered

non-suitable for thermal insulation retrofits, missing the opportunity for thermal comfort and energy

savings. This demonstrates one of the strengths of using the frost dilatometry approach.

C/B ratios show limited correlation to the ratios of free water saturation and critical saturation, but the

variance is large. Plots of other measurements from the same dataset provided by Ueno et al. (2013b) show

even less correlation. Hence, it appears that critical saturation cannot be drawn from more basic material

properties. As a result of this observation and the large material properties variations found in some sample

lots, we recommend testing of multiple samples for projects at this time.

Developments have been made to reduce testing time and cost requirements. Sample sizes have been

reduced, packaging procedures simplified, metal set pins eliminated (by constructing a jig to allow speedy

and repeatable measurements), and freeze-thaw cycling times reduced. These optimizations have

considerably reduced time and cost requirements without sacrificing testing precision and accuracy.

Together, these improvements have decreased cost and time requirements by about one half.

To ensure that opportunities for thermal insulation of masonry buildings aren’t missed due to limited

assessments, further improvements to the overall process should continue to be investigated. Growth of the

archive database, more analysis, and experimentation with further changes throughout the assessment

process may result in further efficiencies.

ACKNOWLEDGMENTS

We would like to thank the various consulting companies which have contributed to our knowledge and

the material property dataset by utilizing our testing services within their projects. Ones that can be named

because the project details have been previously published are Halsall Engineering and Building Science

Corporation (BSC). We would particularly like to thank Dave DeRose and Len Guilium from Halsall and

Kohta Ueno from BSC who have all contributed to our shared knowledge through project discussions and

publications.


REFERENCES

ASTM. 2012. ASTM standard C 216-12, Standard specification for facing brick. American Society forTesting and Materials, West Conshohocken, PA.ASTM. 2008. ASTM standard C 666-03, Standard test methods for resistance of concrete to rapidfreezing and thawing. American Society for Testing and Materials, West Conshohocken, PA. Fagerlund, G. 1977. The critical degree of saturation method of assessing the freeze/thaw resistance ofconcrete. Materials and Structures, 58(10): 217–229.Fagerlund, G. 1992. Effect of the freezing rate on the frost resistance of concrete. Nordic ConcreteResearch, Publ. 11, Oslo (taken from Laugesen et al. 1996).Fraunhofer IBP. 2013. WUFI (Version 5.2) [Software]. Available from www.wufi.de.Laugesen, P., Geiker, M., Jorgen, E., Pederson, N., Thorgersen, F. 1996. HETEK, Method for test of theFrost Resistance of high Performance Concrete: State of the Art. Denmark, Ministry of Transportation,Road Directorate, Report No. 55. Copenhagen.Hughes, R., Bargh, B. 1982. The weathering of brick: causes, assessment and measurement. Report ofthe Joint Agreement between the U.S. Geological Survey and the Illinois State Geological Survey,Champaign, IL.Künzel, H. 1995. Simultaneous heat and moisture transport in building components: one and two-dimensional calculation using simple parameters. Ph.D. thesis, Fraunhofer Institute for BuildingPhysics, Holzkirchen, Germany.Laefer, D., Boggs, J., and Cooper, N. 2004. Engineering properties of historic brick – variabilityconsiderations as a function of stationary versus nonstationary kiln type. Journal of the AmericanInstitute of Conservation of Historic and Artistic Works, 4(3): 255-272.Lindmark, S. 1996. A hypothesis on the mechanism of surface scaling due to combined salt and frostattack. Report TVBM-7104, RILEM TC 117 meeting, Helsinki, Finland. Mensinga, P., Straube, J., and C. Schumacher. 2010. Assessing the freeze-thaw resistance of clay brickfor interior insulation retrofit projects. In Proceedings of the Performance of the Exterior Envelopes ofWhole Buildings XI. December 2010. ASHRAE, Florida. Schumacher, C.J. 2011. Assessing the freeze-thaw resistance of clay brick for interior insulation retrofitprojects. Interior Insulation Retrofit of Mass Masonry Wall Assemblies Workshop, Westford, MA, 30 July2011. Straube, J.F., and Burnett, E.F.P. 2005. Building science for building enclosure design. Building SciencePress, Somerville, MA.Ueno, K., Kerrigan, P., Wytrykowska, H., and Van Straaten, R. 2013a. Retrofit of a multifamily massmasonry building in New England. Building America Report, U.S. Department of Energy, Golden, CO. Ueno, K., Straube, J., Van Straaten, R. 2013b. Field monitoring and simulation of a historic massmasonry building retrofitted with interior insulation. In Proceedings of the Performance of the ExteriorEnvelopes of Whole Buildings XII. December 2013. ASHRAE, Florida. Wilkinson, J., D. DeRose, J.F. Straube, and B. Sullivan. 2009. Measuring the impact of interiorinsulation on solid masonry walls in a cold climate. In Proceedings of Canadian Building Science &Technology Conference, 6-8 May 2009. National Building Envelope Council, Montreal, QC, pp. 97–110.Wytrykowska, H., Ueno, K., & Van Straaten, R. 2012. Byggmeister test home: cold climate multifamilymasonry building condition assessment and retrofit analysis. Building America Report, U.S. Departmentof Energy, Golden, CO.


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