Árbók VFÍ/TFÍ - 01.06.2003, Side 217

HITASTIGSMÆLINGAR í KÁPU-
STEYPU í BORGARFJARÐARBRÚ
VÍSBENDING UM ENDINGU STEYPU
Gísli Guðmundsson lauk prófi í jarðfræði frá Háskóla íslands 1983. M.Sc. 1988 og Ph.D. 1992 í jarðefnafræði frá Arizona
State University.Tempe, Arizona. Hann starfaði sem jarðfræðingur hjá Orkustofnun 1983-1984, aðstoðarmaður í rann-
sóknum hjá Arizona State University, Department of Geology, 1984-1990, sérfæðingur í rannsóknum hjá Sawyer
Research Products, Inc., Ohio, 1986-1987 og félagi í rannsóknum hjá University of Bristol, Department of Geology,
Bristol, U.K. 1990-1992. Gísli hefur starfað sem sérfræðingur við Rannsóknastofnun byggingariðnaðarins frá júní 1992.
Abstract
Concrete durability is a critical issue for concrete in aggressive environments.The goal of this research project is to under-
stand the nature of some of the variables which control the aggressiveness of the environment.
Temperature sensors were cast in concrete, a bridge pier, located in the tidal zone, which was repaired with cover-crete in
1999.
The temperature data reveals the presence of numerous micro-freeze/thaw cycles occurring during relatively long cold
spells.The cause of these freeze/thaw cycles are the tidal currents, which during high-tide heat the concrete up just above
freezing and during low tide, the air temperature cools the concrete down to a maximum of about -10 °C. Each cycle is 12
hrs. long, or two per 24 hrs, if the air temperature remains relatively cold. From the collected data, freezing conditions are
reached in the concrete down to 10 to 15 cm depth below the surface, but the temperature changes very rapidly from the
surface into the concrete, Therefore, the zone in the concrete where relatively low temperatures are reached is relatively
thin.
The severity of these micro-cycles compared to more normal freeze/thaw cycles is not known, but it can be assumed to be
relatively low due to the relatively little difference between freezing and thawing temperatures. On the other hand it is
most certain that these micro-cycles are very common in the tidal zone. During a normal winter these micro-cycles could
easily be as many as 150 to 200.
The measured temperatures in the real structure have some resemblance to standardized freeze/thaw test conditions,spe-
cially at, or close, to the surface. In general, it can be safely stated that the temperature conditions in a freeze/thaw test are
far from the measured conditions in the structure, both with respect to the minimum and specially the maximum tem-
perature.
Inngangur
Tildrög þessa verkefnis eru steypuskemmdir sem komið hafa fram í stöplum
Borgarfjarðarbrúar. Yfirborðsflögnun á sér stað í steypunni. Flögnunin er mest í þeim
hlutum stöplanna sem eru við lægstu fjörumörk. Flögnunin minnkar þegar ofar dregur
og hún er engin fyrir neðan lægstu fjörumörk. Steypan sem verður fyrir skemmdum er
sprungin við yfirborðið, sprungurnar liggja um sementsefjuna samsíða yfirborðinu og
valda því að sementsefjan flagnar smám saman af. Sökum þess að sprungurnar liggja
ekki í gegnum fylliefnin, þá sitja þau eftir í steypunni og standa úr út sementsefjunni uns
efjan sem heldur þeim föstum er farin, sjá mynd 1-1. Þar sem flögnunin er mest er bendi-
járn komið fram í yfirborð steypunnar. Bendijárnið virðist því vera óvarið gegn tæringu.
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