Towards a more representative assessment of frost damage to porous building materials

Building and Environment 164 (2019) 106343

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Towards a more representative assessment of frost damage to porous building materials

T

Chi Feng∗, Staf Roels, Hans Janssen KU Leuven, Department of Civil Engineering, Building Physics Section, 3001, Leuven, Belgium

ARTICLE INFO

ABSTRACT

Keywords: Ceramic brick Frost damage Freeze-thaw cycle Frost temperature Moisture content

Current frost resistance tests rely on freeze-thaw cycling at very low frost temperatures and very high moisture contents, conditions that not readily reflect the spectrum of milder frost conditions met at actual exposure. This analysis performs freeze-thaw testing on four types of ceramic brick at frost temperatures from −2 °C to −20 °C and moisture saturation degrees from 0.1 to 1.0. The outcomes prove that both the frost temperature and the moisture content affect the extent of frost damage. The extension of frost test conditions proves to be more representative, and the application of frost damage isopleths is therefore suggested.

1. Introduction Frost damage has since long been recognized as one of the most prevalent and substantial causes of building material decay [1]. Resultantly, an extensive volume of research activity has been performed to understand the essential principles of frost damage to building materials as well as to develop testing procedures for the evaluation of the frost resistance of building materials. The latter efforts have resulted in a wide spectrum of direct and indirect frost resistance tests. In general, the direct tests all involve freeze-thaw cycling of building material samples: samples are conditioned to a certain moisture content and subjected to alternating freezing and thawing episodes by imposing certain frost and thaw temperatures. The resulting frost damage is then assessed with visual inspection, dilatation measurements or mechanical property testing. The indirect tests, on the other hand, employ correlations between results of direct freeze-thaw tests and associated physical properties of the building material, such as moisture saturation and sorptivity, or pore size distribution parameters. Freeze-thaw cycling is therefore, directly and indirectly, the central element in both types of frost resistance tests, and commonly very low frost temperatures and very high moisture contents are imposed over a large number of cycles. It should be noted though that the complete lack of uniformity between the different available protocols [2] and the resultant damage patterns being different from those of actual exposure [1] are oftencited grounds for concern on the dependability of such freeze-thawcycling-based evaluations. An even greater weakness of the freeze-thaw cycling is their exclusive application of very low frost temperatures and very high

∗

moisture contents. Such tests can be deemed binary: if the material successfully endures such extreme conditions, it will most likely also perform well under the milder conditions of actual exposure; if not successful, it is considered unfit. While such binary assessment might be a good compromise with respect to the efficiency of the evaluation procedure and the conservative nature of its outcomes, it may remain wholly distinct from the material's performances under actual exposure. Illustratively, Fig. 1 repeats [3]'s measurements of exterior surface temperatures on a clay brick wythe during the winter of 2012–2013 in St. Albert (Canada). The daily minimum temperatures range from 10 °C to −25 °C, but the latter is only reached exceptionally. All in all, most of the frost exposure occurs at frost temperatures between 0 °C and −10 °C and not at the very low frost temperatures commonly imposed in freeze-thaw testing. A similar reasoning can be made in relation to moisture content: the moisture levels at the brick wythe surface may occasionally go up to the levels imposed in freeze-thaw tests but are generally more modest. The simultaneity of very low frost temperature and very high moisture content is accordingly rarer, and it has to be said that the actual exposure of building materials to frost conditions should be characterized as significantly milder than the levels used in the common freeze-thaw testing. A first step to bridge the gap between the extreme test conditions and the milder actual conditions is hence to move beyond the usual binary testing and perform freeze-thaw testing at a spectrum of frost temperatures and moisture contents. Such freeze-thaw testing over a spectrum of frost temperatures and moisture contents has particular relevance for the performance evaluation of post insulation of cavity walls or the application of interior insulation for thermal retrofits of historic buildings, at present a hot

Corresponding author. E-mail address: [email protected] (C. Feng).

https://doi.org/10.1016/j.buildenv.2019.106343 Received 14 May 2019; Received in revised form 2 August 2019; Accepted 11 August 2019 Available online 12 August 2019 0360-1323/ © 2019 Elsevier Ltd. All rights reserved.

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evaluation approach via isopleths is made. 2. Materials and methods In this section, the materials used in this study are presented by their hygric properties, pore structure parameters, and expected frost resistance. Subsequently, the freeze-thaw cycling protocol and the frost damage quantification are explained. 2.1. Investigated building materials As a typical facade material, ceramic bricks are chosen as target materials in this study. Specifically, four types of ceramic bricks are used. Brick A is a frost-resistant facade brick, while Brick B is frostsensitive and therefore intended for interior purposes; both are commercially available. Additionally, a third manufacturer produced two brick types C and D specifically for this project, with firing temperatures of 925 and 970 °C (below the normal firing temperatures) in an attempt to obtain frost-sensitive bricks [14,15]. Before testing, all raw bricks are reduced to samples of 17 × 10 × 5 cm3, by cutting away 0.5–1 cm of material at all sides with a mechanical saw, to avoid any disturbance by surface roughness.

Fig. 1. Measurements of exterior surface temperature on a clay brick wythe in St. Albert (Canada) [3].

topic with respect to the energy efficiency of the built environment [4–9]. Such thermal upgrades lower the temperatures and raise the moisture contents near the exterior surface of the wall to a certain degree [10–13], and the crucial question with respect to potential frost damage is how these moderately changed conditions – typically not reaching down or up to the extreme conditions in freeze-thaw tests – affect the frequency and intensity of frost damage. To that aim, concrete information on the impact of the actual frost temperature and moisture content on frost damage is crucial. In this article, we present such investigation on the impact of frost temperature and moisture content on the frost damage to ceramic bricks, where freeze-thaw testing over a spectrum of milder conditions takes the central position. While this study cannot solve all discrepancy between the unique frost condition in freeze-thaw testing and the spectrum of frost conditions in actual exposure, it contributes to a better understanding of freeze-thaw testing and forms a meaningful step in the right direction. The reader should note that the principal target of this study is hence not an absolute frost resistance evaluation of the ceramic bricks involved, but rather a relative judgment of how milder frost conditions influence the intensity of frost damage when compared to the usually extreme frost conditions imposed in traditional freeze-thaw testing. Below the Materials and methods section presents the ceramic bricks studied in this investigation, as well as the freeze-thaw cycling protocols and the damage quantification method applied. Four ceramic brick types are evaluated and freeze-thaw testing at combinations of six frost temperatures and seven moisture contents is performed, with the resulting damage gauged with active acoustic emission. The ensuing Results and discussion section gathers all results and puts these in perspective. Introductorily, the influence of the number of freeze-thaw cycles is investigated, to then come to the impact of milder frost conditions on the frost damage, where it is shown that the levels of frost temperature and moisture content indeed have a strong influence. Based on these outcomes a first attempt at an improved frost damage

2.1.1. Hygric properties and pore structure Table 1 summarizes crucial properties of these four types of bricks: the bulk density (ρbulk, kg·m−3), open porosity (φ, -) and saturated moisture content (wsat, kg·m3), from the vacuum saturation test [16], as well as capillary absorption coefficient (Acap, kg·m−2s−0.5) and capillary moisture content (wcap, kg·m−3), from the capillary absorption test [17]. Clearly, Brick A has a similar bulk density and open porosity as the others, but differs strongly in its absorption coefficient, implying a larger pore size. Fig. 2 depicts the pore volume distributions (fv(r), m3·m−3log10(m)−1) obtained with mercury intrusion porosimetry [18], confirming that larger pore size of Brick A. The hygric properties and pore structure parameters are used below to assess the frost resistance of the four bricks, based on two indirect approaches. Several authors equally emphasize the relation between the mineral composition and the frost resistance of a material [19–22], which often runs via the pore structure and the mechanical resistance of the materials, but remains complex. Given that this study does not target an absolute evaluation of frost resistance but rather a relative comparison, no mineralogy examination has been performed for the employed materials. 2.1.2. Frost resistance The hygric properties and pore structure parameters listed above can be used to come to the expected frost resistance of the four brick materials used in this study, via indirect tests. The well-known index Fc (−) from Maage [23] relates the frost resistance to the pore structure parameters for bricks:

Fc = 0.0032/ PV + 2.4 P3 = 0.0032

bulk /

(1)

+ 2.4 P3 3

−1

where PV is the pore volume per unit mass (m ·kg ), and P3 the pore volume fraction for pores with diameter greater than 3 μm (%, provided in Table 2). The coefficients in Eq. (1) differ from the originals due to unit conversion. With an Fc value greater than 70 the material can be deemed frost-resistant, for an Fc value smaller than 55 the material is

Table 1 Fundamental properties of ceramic bricks. Brick type

ρbulk (kg·m−3)

A B C D

1839 1704 1750 1778

a

(8)a (5) (8) (20)

φ (−) 0.314 0.354 0.335 0.321

(0.001) (0.004) (0.002) (0.003)

Data in parenthesis indicate the standard deviations of multiple samples. 2

wsat (kg·m3)

Acap (kg·m−2s−0.5)

wcap (kg·m−3)

306.7 347.5 327.9 312.9

0.506 0.143 0.145 0.165

177.8 243.0 246.1 225.2

(0.5) (5.3) (3.7) (4.1)

(0.025) (0.015) (0.007) (0.002)

(8.1) (15.0) (8.7) (3.0)

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least half a day, while CEN/TS 772-22 limits the cycle to a few hours. Additionally, EN 12371 applies a period of full immersion of the samples as initial moisture conditioning, CEN/TS 772–22 on the other hand adds cyclic spraying of the exposure surface as further continuous moisture conditioning. Finally, EN 12371 does not define a preset number of freeze-thaw cycles but tests until failure, CEN/TS 772–22 contrarily prescribes 100 cycles, while other authors go as low as 6 cycles [25]. Despite these many inconsistencies, freeze-thaw tests are still widely accepted as the standard method to assess the frost resistance of building materials, and the methodology is hence also applied for this investigation. In this study, freeze-thaw tests on brick samples are performed at six frost temperatures (T, °C) and seven moisture contents. The selected frost temperatures are −2, −4, −6, −8, −14 and −20 °C. For a better comparison of the different bricks and the different moisture contents, we normalize the latter to saturation degrees (S, -), defined as the proportion of the actual moisture content (w, kg·m−3) to the vacuum saturated moisture content [28]:

Fig. 2. Pore volume distributions of ceramic bricks. Table 2 The frost resistance evaluation of different bricks based on the Fc and Gc indexes. Brick type

A B C D

Fc index

Gc index Fc value

Judgment

Gc value

Judgment

0.915 0.138 0.048 0.097

220 33 11 23

Frost-resistant Frost-sensitive Frost-sensitive Frost-sensitive

−3.4 −0.7 0.3 −0.4

Frost-resistant Frost-sensitive Frost-sensitive Frost-sensitive

The selected saturation degrees are 0.10, 0.25, 0.40, 0.55, 0.70–0.75, 0.85 and 1.0. After sample cutting and drying, the samples are preconditioned to the respective moisture contents by either direct water absorption or vacuum saturation and subsequent drying, and are wrapped with plastic films to avoid evaporation. Before measurement, samples are left untouched for at least two days. Given the strong liquid transport capability of the materials (Table 1), a uniform moisture distribution can be expected after the conditioning. During the test, they are placed vertically in the climate chamber on top of an insulation board (Figs. 3 and 4). It should be noted that moisture loss is inevitable during freeze-thaw cycling, especially for samples with high saturation degrees. However, the moisture loss should be minimal when compared to samples’ overall moisture content, and samples of the same initial moisture content should experience similar moisture loss, making the comparison possible. It should also be noted that, once the freeze-thaw cycling starts, a homogeneous moisture distribution is no longer guaranteed because of moisture transport. Thus the moisture content should be interpreted as the average value for the whole sample. The execution of the actual freeze-thaw cycling is based on a prior Belgian standard for freeze-thaw testing NBN B05-203 [29], and the resulting temperature evolution is illustrated in Fig. 5 a). The freezing process starts at an ambient air temperature of 15 °C and cools down to 0 °C within 2 h. After that, the targeted frost temperature is approached at a rate of −4 °C/h and maintained for 8–14 h. For the thawing process, the ambient temperature rises rapidly to 15 °C in about 1 h and

considered frost-sensitive, between 55 and 70 is the transition zone. In Belgium the Gc (−) factor is also widely applied to ceramic and natural building materials [24]:

Gc =

14.53

0.309 (100

60 Acap / wsat ) + 0.203 (100 wcap/wsat )

(3)

S = w / wsat

P3

(2)

where the coefficients 100 and 60 are present for unit conversion purposes. If the Gc factor is smaller than −2.5 the material is deemed frost-resistant, otherwise potential frost damage must be considered. Inserting the data in Table 1 and the P3 values in Table 2 into Eqs. (1) and (2) allows obtaining the Fc and Gc values for all four types of bricks. Table 2 summarizes the results. As is clearly revealed, Brick A should be classified as frost-resistant while Brick B, C and D should be considered as frost-sensitive, according to both the Fc and the Gc indexes. The Fc and Gc values make use of respectively the pore structure parameters and the hygric properties, but both result in concurring predictions of the expected frost resistance of the four brick materials tested. Given the aim of studying frost damage at milder frost conditions, the dominant presence of frost-sensitive bricks was a criterion during the brick selection. 2.2. Freeze-thaw testing 2.2.1. Freeze-thaw cycling protocol Given that frost damage to building materials can be attributed to various physical mechanisms [1] – volumetric expansion of ice, formation of ice lenses, hydraulic pressure upon freezing – many protocols for freeze-thaw testing are available [25]. While they all share the concept of a freeze-thaw cycle, there are multiple differences between the various methods, with relation to the uni- or omnidirectional frost ingress, to the imposed levels and rates of change of temperature, to the applied moisture conditions and their implementation, to the number of freeze-thaw cycles to be employed, etc. A review in Ref. [2] is concluded with the remark that “there is no ‘right’ freezing and thawing resistance test method. All of them have been made for their own purpose …“. Illustratively, the European standard for natural stone EN 12371 [26] applies an omnidirectional test, while its counterpart for clay masonry units CEN/TS 772–22 [27] uses a unidirectional test. Similarly, the frost temperature in EN 12371 is about −10 °C versus −15 °C in CEN/TS 772–22. Moreover, a full cycle in EN 12731 is at

Fig. 3. Wrapped brick samples. 3

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contents show that the core temperatures inside the samples follow the imposed climate chamber temperatures without significant delay: minimally 8 h are spent at the imposed frost temperature and at least 4 h at the imposed thaw temperature. As exemplified by Fig. 5 b), the core temperatures for saturated samples during a trial cycle with −20 °C frost temperature show a short delay of around 2 h for the freezing and thawing process, respectively. The 24-h freeze-thaw cycle and the climate chamber conditions evidently provide sufficient exposure to and penetration of the imposed frost and thaw conditions. In what follows hence, the imposed chamber temperatures are used as the correct indicator of the obtained sample temperatures. Following [25], ten freeze-thaw cycles are employed. While certain choices in this freeze-thaw protocol can certainly be discussed, the significant disparity in the currently standardized freeze-thaw protocols is to be noted. The intent of this study is moreover a comparative investigation of bricks and frost conditions, wherein the application of a consistent protocol is the most important. Testing four bricks at six frost temperatures and seven moisture contents with ten freeze-thaw cycles each, if done to the full extent, requires 1680 freeze-thaw cycles of 24 h. To reduce the time and effort required, not all combinations are executed. The campaign starts with the most extreme frost conditions and continues then with the milder conditions. Tests at milder conditions are however only performed if significant damage is observed at the neighboring more extreme conditions. For example, the tests with frost temperature −20 °C and saturation degree 0.1 do not yield any frost damage to neither of the four bricks. The tests at more mild frost temperatures are hence not performed for this saturation degree. Table 3 summarizes the applied test conditions for all brick types. Clearly, an extensive number of test conditions still exists. Given the limited volume of the climate chamber used, only a single sample of each type of brick is hence tested at each test condition. While this choice can be discussed, it is inferred that it provides more information than obtained by testing at fewer frost conditions with multiple samples per condition. It will be shown below that the obtained damage surfaces are relatively smooth, and that reliable outcomes are hence obtained. It will also be shown that these damage surfaces are not linear with frost temperature or moisture content and that a denser scanning is therefore preferable.

Fig. 4. Brick samples in the climate chamber.

2.2.2. Frost damage quantification There are different methods available to quantify frost damage. During trial measurements the dilatometric method [30,31] was applied, but it failed to provide reliable results in most of our cases. Consequently, in the actual study the resulting frost damage is quantified via the evolution of the Young's modulus, which is obtained with the active acoustic emission method, also called the ultrasonic method because an ultrasonic pulse is applied [1,32]. In a simple scheme, the ultrasonic pulse is emitted by one transducer and collected by another. The time needed for the pulse to traverse the sample is measured as t0 and t (s), before and after freeze-thaw cycling respectively. Different damage indexes can then be calculated based on t0 and t. Some scholars advocate the relative ultrasonic pulse transmission time ( ) as the frost damage index [33,34]:

Fig. 5. The freeze-thaw cycle.

remains at that level for 6 h. The variable interval at the actual frost temperature is motivated by achieving a 24-h cycle in all measurements. Prior trials with different frost temperatures and moisture Table 3 Test conditions for different brick types. Brick type

T = −2 °C

−4 °C

−6 °C

−8 °C

−14 °C

−20 °C

S = 0.10 0.25 0.40 0.55 0.70–0.75 0.85 1.0

– B B A, A, A, A,

– B B A, A, A, A,

– B B A, A, A, A,

– B B A, A, A, A,

– B A, A, A, A, A,

A, A, A, A, A, A, A,

B, B, B, B,

C, C, C, C,

D D D D

B, B, B, B,

C, C, C, C,

D D D D

B, B, B, B,

4

C, C, C, C,

D D D D

B, B, B, B,

C, C, C, C,

D D D D

B, B, B, B, B,

C, C, C, C, C,

D D D D D

B, B, B, B, B, B, B,

C, C, C, C, C, C, C,

D D D D D D D

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(4)

= t 0/ t

3. Results and discussion

The relative dynamic elasticity modulus (P) is equally supported by many researchers [32–34], as well as the ASTM C666 standard [35]:

P = E / E0 = ( t 0 / t ) 2 =

This section introductorily examines the impact of the number of freeze-thaw cycles, to then go into the impact of milder frost conditions on the extent of frost damage. Because of their substantial impact, a novel frost damage quantification approach via isopleths is finally suggested.

(5)

2

where E0 and E (Pa) are the elasticity modulus (Young's modulus) before and after freeze-thaw cycling. Ω, a derivative of P, was proposed by Løland [36] and accepted by others [28]:

=1

P=1

(t 0 / t ) 2 = 1

2

3.1. Impact of freeze-thaw cycle number Given that only ten freeze-thaw cycles have been used, far below most standards’ prescriptions, the impact of the freeze-thaw cycle number on the frost damage is investigated first (Fig. 6). This reveals that the freeze-thaw cycle number has a systematic effect: generally a larger number of cycles gives more frost damage, as quantified through Ω. This is an expected reasonable trend, confirmed by other studies [1,37]. Fig. 6 also demonstrates that the impact of the first five cycles may be the most important, as the damage increase from five to ten cycles is mostly insignificant, with some exceptions most probably caused by experimental uncertainties. Some other studies also provide support to this assumption [38,39]. This observation corroborates that the application of just ten freeze-thaw cycles suffices for a good comparative evaluation of frost damage over a spectrum of frost conditions. It should be noted that freeze-thaw testing at six frost temperatures is performed, while Fig. 6 shows results for −4 °C, −8 °C and −20 °C solely, representing mild, moderate and harsh conditions respectively. Moreover, the moisture saturation degrees in Fig. 6 only start from S = 0.55 rather than their lower limit of 0.10. This is partly because tests combining high frost temperatures and low moisture contents are not always performed on all four brick types, and partly because higher moisture contents tend to cause greater damage, where uncertainties interfere less and tendencies appear more clearly. The conclusions drawn for the selected set of results displayed in Fig. 6 are though also valid for other frost temperatures and moisture contents. In what follows, only the results after ten cycles are shown.

(6)

Clearly, , P and Ω are mathematically related, so they should have a similar index performance, as confirmed by experiments [32]. For that reason, only Ω is retained here. For getting a better view on the evolution of the frost damage, ultrasonic measurements are performed before freeze-thaw cycling and after both five and ten cycles. For the ultrasonic evaluations, a C373 N high-performance ultrasonic tester (produced by MATEST®) emitting a pulse with a frequency of 55 kHz is used to measure the time needed for the pulse to travel across the sample. Since the tester gives more reliable results for a larger time span, the measurements are along the longitudinal direction, always at the same sensor locations. This is also the reason not using a smaller sample size. In the appendix, we demonstrate the inherent variations of the travel time (with the ultrasonic pulse transmission velocity) in different samples of the same material, reaching up to multiple 10's of percent and thus similar to the impact of freeze-thaw damage. To obtain reliable outcomes hence, it is highly important to compare travel time on a sample-by-sample basis, before, during and after freeze-thaw cycling, instead of testing distinct groups of samples before, during and after. It should finally be mentioned as well that trial measurements prove that the plastic films wrapped around samples have a negligible impact on the ultrasonic evaluations, confirmed by comparing the ultrasonic pulse transmission time with and without films. Thus they are kept throughout the whole study.

Fig. 6. Impact of freeze-thaw cycle number on Ω 5

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3.2. Impact of milder frost conditions

while Brick B remains far more sensitive to frost damage at moderate frost temperatures and saturation degrees. For Bricks C and D, all in all a larger frost resistance is seen, given the overall lower Ω values. They again behave differently though, with a more pronounced plateau for brick D at the lower frost temperatures and higher saturation degrees. To conclude, when evaluating potential frost damage of thermal upgrades of existing walls, it is evident that a dependable assessment at actual exposure – the only exposure that is actually relevant – cannot suffice with information at a single extreme frost condition only, but that instead a damage surface over a spectrum of frost temperatures and moisture contents is required. Fig. 8 illustrates these trends from a different perspective, since larger Scri values indicate better frost resistance, when all other conditions are equal. Fig. 8 demonstrates that Scri depends on the frost temperature, rather than the commonly adopted single value, and it is therefore reasonable to express Scri in function of the frost temperature. In addition, it should be noted that the moisture saturation degrees at the capillary moisture content (Scap) are 0.74 and 0.72 for Brick C and D respectively, both below their Scri at the lowest frost temperature. This indicates that the moisture content levels in these two brick types would hardly ever exceed Scri in common facade applications. They can therefore be assumed as almost completely frost-resistant as long as the frost temperature does not go below −20 °C. Based on the same reasoning Brick A, whose Scap value is 0.56, would generally be frost-resistant, if the frost temperatures stay above −17 °C. For Brick B with an Scap value 0.69, it is difficult to endure a mild frost temperature of −6 °C after reaching capillary moisture content. It should be noted that the responses of the four bricks to the most extreme frost conditions are not in line with the expected frost resistances, estimated via pore structure parameters and hygric properties

Subsequently, the frost resistance of the four ceramic bricks and the impact of the frost temperatures and moisture contents are presented. These also serve to identify their critical saturation degrees (Scri, -), the saturation degree below which no detectable damage will occur even if the material goes through a very large number of freeze-thaw cycles [25,28,40–42]. Since the EN 12371 standard [26] prescribes a relative change of the dynamic elasticity modulus by 30% as the threshold for frost damage, Ω = 0.3 is used as the critical value in this study. These results are illustrated in Figs. 7 and 8. It is mentioned earlier that only a single sample is used for each set of frost condition, thus the experimental uncertainties are somewhat more on the foreground, yielding some occasional irregularities (especially for Brick C and D). However, such irregularities are generally less than 0.05 in terms of Ω, thus the resulting damage surfaces are reasonably smooth and sufficiently accurate to represent the overall trends. A first glance at Fig. 7 reveals that both the frost temperature and the moisture content are important contributing factors to frost damage: generally speaking, at equal moisture contents a lower frost temperature leads to more damage and at equal frost temperatures a higher moisture content incurs more damage, except for some minor deviations (possibly due to experimental uncertainties). This is also confirmed by Fig. 6. Resultantly, the most extreme condition (T = −20 °C and S = 1) yields the largest Ω values, typically obtained in standard freeze-thaw testing by imposing very low frost temperatures and very high moisture contents. The damage surfaces in Fig. 7 however also demonstrate that this sole extreme frost condition is not representative for the milder conditions typically met in actual exposure, and that the deviation differs from material to material. For Brick A, a

Fig. 7. Impact of temperature and moisture content on Ω

sharp decline in the frost damage is observed when the frost temperature goes above - 8 °C or when the saturation degree goes below 1,

in section 2.1.2. For Brick B, C and D, the situation is especially interesting: they have similar basic porosity and moisture properties and 6

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Fig. 8. Scri for different ceramic bricks (by interpolation of Fig. 7).

therefore calculated resistance indexes (Tables 1 and 2), but their measured damages are conspicuously different (Fig. 7). This indicates that the frost damage predicted based solely on porosity and/or moisture properties is not always reliable. Actually, it is well-known that these indirect methods are not failureproof, and that the outcomes of freeze-thaw testing depend strongly on the imposed protocol [1,2]. Thus the predicted and measured frost damage should always be evaluated with caution. Given that this study emphasizes the comparative analysis of milder frost conditions, rather than the absolute frost resistance, no efforts are devoted to explaining this deviating finding. Ultimately, the frost damage surfaces for bricks C and D are still sufficiently illustrative for the primary purpose of this study, which is to illustrate the variety in how milder frost conditions impact the frost damage to ceramic brick.

Fig. 9. Mould growth isopleths and the imaginary temperature and saturation variations of a brick wall before/after interior insulation [45].

3.3. Frost damage estimation via isopleths Frost damage has since long been recognized as one of the most prevalent and substantial causes of building material decay [1]. It is hence surprising that there is a total lack of representative frost damage prediction models, that may assess how certain materials will withstand certain exposures. On the one hand, there are the direct and indirect frost resistance approaches, which however exclusively focus on the material, as the exposure is restricted to a single extreme frost condition. On the other hand, exposure-based damage indicators are available, e.g. Time-of-Frost, Amount-of-Frozen-Water and Number of Freeze-Thaw Cycles [43], which though only relate to the exposure while the properties of the materials involved are not accounted for. Neither of these two approaches would allow assessing the impact of internal insulation on the potential frost damage, for example, implying that there is still much room for improvement with respect to potential frost damage estimation. As an initial step in that improvement process, the use of frost damage isopleths is put forward here. Within the building physics research field, isopleths are a common approach when evaluating mould growth [44]. Fig. 9 reproduces such a mould isopleth application from Ref. [45]: the mould growth velocity is depicted in function of temperature and relative humidity, and measurements of both are superimposed. In Fig. 9, the two signals represent measurements at two locations, but they may also indicate simulations before and after a thermal retrofit, for example. Comparison of the two signals in function of the mould growth velocities indicates that B will lead to much more mould growth than A (in Ref. [45], the final mould growth for B is 10 times larger than for A). While it could be stated that these isopleths do not take the material into account, this is actually not true because the example in Fig. 9 employs an isopleth obtained for a specific material, and other isopleths exist for other materials. The frost damage surfaces shown in Fig. 7 can equally be translated into such frost damage isopleths, see Fig. 10. In that figure, Brick A is

Fig. 10. Frost damage isopleths of Brick A and the imaginary temperature and saturation variations of a brick masonry wall before/after interior insulation.

assumed the brick present in a masonry facade, and its 3-D damage surface is transformed into a 2-D contour plot for simplicity. The superimposed hourly temperature and moisture saturation signals represent virtual data for the exterior surface of a brick masonry wall before and after interior insulation, simulated by the hygrothermal code Delphin® and limited here to the winter season. The frost damage isopleth clearly illustrates that without interior insulation most data points stay in the range Ω < 0.3, implying only minimal frost damage will take place. On the contrary, with interior insulation many data points move to Ω values of 0.3–0.5, hence increasing the potential frost damage considerably. One could proceed from here with an accumulation of the different data points and their corresponding frost damage level, to come to the overall potential frost damage risk. However, given that the impact of the number of cycles is not linear (Fig. 6), far more research on the buildup of frost damage over successive frost conditions is required first. Nevertheless, the frost damage isopleth approach as introduced in Fig. 10 forms a substantial (but admittedly initial) improvement over the current frost damage estimation approaches, as it merges the exposure and the material. 4. Conclusions This paper studies the impact of frost temperature and moisture content on the frost damage to four types of ceramic bricks. Freezethaw testing at frost temperatures ranging from −2 °C to −20 °C and at moisture saturation degrees going from 0.1 to 1.0 is performed. Ultrasonic measurements are carried out to assess the resulting frost 7

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damage with the decrease of Young's modules as the index. It has been clearly demonstrated that the spectrum approach to freeze-thaw testing carries much more information on the frost resistance of building materials with respect to actual exposure, and hence outperforms a traditional binary freeze-thaw test. The following conclusions have been drawn:

c) Damage isopleths are put forward to more reliably assess the potential frost damage when applying materials under actual exposure. Acknowledgements This project is supported by EU H2020 project “RIBuild - Robust Internal Thermal Insulation of Historic Buildings” (project No. 637268). We thank Vandersanden, Wienerberger and Vogelensangh, for providing the bricks used in this study. We express our gratitude to Prof. Els Verstrynge, Patricia Elsen and Bernd Salaets from KU Leuven, for supporting the experiments. We acknowledge student Mathias Van Gorp for his contribution to the measurements.

a) Both frost temperature and moisture content are significant contributing factors to frost resistance: lower frost temperatures and/or higher moisture contents tend to cause more severe frost damage; b) Resultantly, the critical saturation degree is not a constant, but a function of the frost temperature; Appendix. Material inhomogeneity and the ultrasonic test

In the course of this research, multiple measurements of the travel time of ultrasonic waves through the brick samples have been made. Such travel time can be easily transformed into the ultrasonic pulse transmission velocity (V, m·s−1), which is closely related to a material's mechanical properties [46–49]. Fig.A1 illustrates the velocities obtained for intact samples at various moisture contents (expressed as saturation degree, S). Since there is only limited literature available on this relation [50–52], we assume that our results may be appreciated by other researchers.

0

Fig. A1. The ultrasonic pulse transmission velocity in bricks at different moisture contents.

It is evident that sizeable variations of the V values of duplicate samples at the same moisture content exists, particularly for Brick A and B, which must stem from discrepancies in the moisture conditioning and/or the mechanical properties. However, the moisture conditioning via oven drying (S = 0), capillary absorption (S = 0.7–0.75) and vacuum saturation (S = 1) are all known for their consistency, giving only minimal variations in the obtained moisture content. That consistency is also obtained for all other saturation degrees, given that the moisture conditioning there consists of supplying a preset amount of water to the sample. Resultantly, these large variations in V most possibly come from the natural variability in the mechanical properties of the samples, similar to the cases for dry samples [53,54]. One explanation is that there are tiny cracks and fissures in samples. While these cracks and fissures do not readily affect the moisture conditioning – as expected from the basic principles – the ultrasonic wave is much more sensitive to their existence. The magnitude of this material inhomogeneity error is similar to that of the impact of frost damage, implying that the quantification of frost damage can be easily obscured. To overcome this difficulty, the solution is to measure respective samples before and after testing, and to employ the relative change of V (or the travel time t) as damage indicator, rather than using an averaged V (or t) value as a common basis. As is clearly reflected in Fig. 7, this protocol produces very regular and reasonable results.

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