Lisø PhD Dissertation Manuscript - NTNU

optimal extensibility and flexural strength of the masonry. (Both too much and too little
suction may be unfavourable.) The suction rate may also serve as an indicator of the bricks
frost resistance, by reflecting the volume of capillary pores and its ability to absorb water.
5. Methodology and results
The main purpose of the ongoing work has been to characterize the risk of frost decay or
damage on a porous, mineral material exposed to a given climate, by combining
knowledge on relevant material parameters and climate parameters. Or, in other words: To
establish a quantitative connection between the number of freezing point crossings, the
amount of precipitation in the form of rain and durability of different porous building
materials. In the most general form a freezing and thawing exposure index (FTI) can be
expressed as:
FTI = Function of (climate properties, material properties) (1)
In this paper we will focus on the climate properties of such an index, using
weather data from the reference 30-year period 1961-1990 (only weather stations with
series of observational data of 10 years or more are used). The presented data are readily
available for most observing stations in Norway. Data from thirteen observing stations in
different parts of the country and with different climates are considered here as examples
to illustrate the methodology (see Table 2, LAST PAGE). The selected locations are
Kristiansand (observing station at Kjevik), Bergen, Stavanger, Lyngdal, Oslo, Røros,
Ålesund, Ørland, Trondheim (observing station at Værnes), Bodø, Tromsø, Karasjok and
Fruholmen (all shown on the map of Norway in Fig. 5), representing a wide range of
climates.
Oslo, Lyngdal, Røros and Karasjok have a continental type of climate with
relatively low annual precipitation, maximum precipitation in summer and a large annual
temperature cycle. Karasjok and Røros have especially cold winters and low winter
precipitation. Kristiansand, Stavanger, Bergen, Ålesund, Bodø, Tromsø and Fruholmen
have maritime climate, high precipitation all over the year with a spring minimum, and a
less distinct annual temperature cycle. The climate in Tromsø and Fruholmen is cooler and
the cold periods during the winter dominate. The climate of Trondheim and Ørland is more
or less in between continental and maritime, and cold winter periods are frequently broken
by moist and warm air masses from the Norwegian Sea.
A potential high-risk frost decay area would have both a high number of freezing
point crossings and large amounts of precipitation in the form of rain ahead of freezing
events. An index should therefore express both of these elements of climate. The synoptic
data used are daily air temperatures as measured three times a day (0600, 1200 and 1800
UTC), daily maximum and minimum air temperatures, and precipitation as measured 0600
UTC (covering the last 24 hours). To divide the precipitation amounts into snow and rain,
the observed information of the present and past weather, noted three times a day, is used.
All freezing events are considered, and air temperature transitions from values above 0°C
to values below 0°C are counted to produce the index. Both measurements at the synoptic
times (0600, 1200, and 1800 UTC) and daily maximum and minimum air temperatures are
used to accomplish this. Precipitation in the form of rainfall for days where a freezing
event has occurred and the prior 48 hours (2 days), 72 hours (3 days) and 96 hours (4 days)
is counted. These monthly 2-day, 3-day and 4-day spells of rainfall are accumulated for
one year.
Averaging the calculated annual climate indices over the 30-year reference period
1961-1990 resulted in a ranking of the relative potential of a climate to promote
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