ICE ABRASION ON MARINE CONCRETE STRUCTURES
Doctoral thesis
Permanent lenke
http://hdl.handle.net/11250/2374733Utgivelsesdato
2015Metadata
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Sammendrag
A significant amount of the world’s undiscovered oil- and gas resources are located North of
the Arctic Circle. In the light of this, there is currently an increasing interest in developing
necessary infrastructure to support sustainable development of these resources. Also increased
availability to the Arctic due to reduced amounts of ice for longer periods of the year will ease
the access for ship transport and increase the seasonal window for construction activities in
these areas. Towards the end of the last millennium, the exploration of oil and gas in the
North Sea was the main driving force behind development in concrete technology for use in
large offshore concrete structures. Knowledge and expertise derived from this period has
benefited in other sectors of the concrete industry as well, such as costal bridges and quay
structures where durability is important. Going forward, the ability of concrete structures to
withstand exposure from sea ice and low temperatures is decisive and calls for increased
knowledge and new technology.
Ice abrasion is a severe degradation mechanism responsible for reduced service life in marine
concrete structures exposed to sea ice. The phenomenon is caused by ice-concrete friction
forces and results in gradual loss of concrete cover. In extreme cases, ice abrasion has brought
about complete deterioration of the reinforcement cover of marine concrete structures, which
represents a potential threat regarding the safety against load-carrying failure. Current
regulations with regard to material selection and design of concrete structures exposed to ice
abrasion are defined in ISO 19903:2006 and ISO 19906:2010, where most of the regulations
are given as functional descriptions. The interpretation of the regulations is therefore mainly
based on considerations without sufficient documentation, which calls for reliable and
research-based estimation methods.
As a first approach, an investigation was performed in order to develop equations for rough
estimation of expected abrasion rates based on the experimental results in the current study.
The equations were derived on the basis of a regression analyses where the measured ice
abrasion rates from the experiments were taken as the response parameter and the three
explanatory variables in the experiments; compressive strength, ice pressure and ice
temperature, as predictors. Based on simplified assumptions, a direct application of the
equations provided fairly good approximations of maximum abrasion depths when
benchmarked against a marine concrete structure with known abrasion depths. The best prediction was achieved for the parts of the structure with orientation parallel to the
dominating ice drift directions. For the faces with lower ice exposure, the equations underpredicted
the associated abrasion depths significantly. The ice-concrete coefficient of friction
is reported to increase with decreasing drift speed and cause higher abrasion rates. As our
equations did not include the speed of ice as predictor, they were not able to account for this
effect.
The distribution of ice abrasion on structures is highly dependent on the direction of the ice
drift. In previous field studies dealing with concrete abrasion due to ice drift, reliable
information about site specific ice conditions is sparse. On this basis, results from in-situ
measurements of three severely damaged concrete lighthouses due to long term sea ice
exposure are presented and discussed. As one of the inspected lighthouses showed particular
severe damage, this structure formed the main basis of the investigation. The focus was on
local ice conditions with main emphasise on the amount of ice drift per direction and its
influence on distribution of abrasion depths along the perimeter of the lighthouse. The
investigation revealed that the largest abrasion depths were observed on the faces oriented
parallel with the dominating ice drift direction(s). Further, the rate of abrasion increased
significantly with decreasing annual ice drift. This trend was most pronounced for the faces
with the lowest ice exposure. The investigation revealed that abrasion depths on faces
oriented diametrically opposite to each other increased linearly with the amount of ice drift
along these faces.
In previous experimental- and field investigations, ice abrasion rates were traditionally
reported as average values, often without quantification of the uncertainty in the results. In
order to allow a probabilistic approach in the results analysis, an appropriate probability
distribution function to represent abrasion rates is needed. Based on our experimental results
we performed goodness of fit tests and found that the abrasion rates were well represented by
a 3-parameter Weibull distribution. The distribution was statistically significant based on a
confidence level of 95%.
An ice abrasion test rig was developed where a vertical oriented fresh-water ice cylinder was
sliding in a repetitive back and forth motion on the surface of the concrete specimens. Ice
pressure and concrete compressive strength were the most important parameters governing ice
abrasion rates. Abrasion rates increased with increasing ice pressure. For some experiments the abrasion rates showed a quadratic increase with the ice pressure. As a result, it is
important to seek to reduce the ice loads for concrete structures exposed to abrasion. This can
be achieved by giving the structure a sloped cross section in the ice exposed zone, which
causes the ice to fail in bending which is associated with lower loads as compared to failure in
crushing. The results were ambiguous with regard to the effect of ice temperature on abrasion
and no clear conclusion was reached based on the conducted experiments. We suspect that
this was related to a specific feature of the experimental set-up and recommend that further
studies investigate this closer.
Ice abrasion on concrete is a complex process which may be investigated on several scales,
pending on the aim of the research. In order to seek a fundamental understanding of the
governing tribology processes involved, it would also be of interest to study concrete ice
abrasion at nano scale.