A NEW APPROACH TO CRACK CONTROL FOR REINFORCED CONCRETE - An investigation of crack widhts close to the reinforcement and the correlation to service life

Kristian Tammo

Forskningsoutput: AvhandlingDoktorsavhandling (sammanläggning)

Sammanfattning

One of the most important factors governing durability of concrete structures is
reinforcement corrosion, and to prevent degradation associated with it, it is desirable
to use large concrete covers. However, if the concrete cover is increased, the majority
of design methods will predict that the crack widths at the concrete surface increase
significantly. This can be contradictory, since the same design methods both specify
smallest concrete cover and require crack width limitations to prevent reinforcement
corrosion.
To fulfil crack control demands current European structural codes imply that the
engineer tend to use as small covers as possible, use small diameter reinforcement
bars with narrow spacing and place excessive amounts of crack reinforcement in
sections with high bending moments. All these measures are disadvantageous with
respect to durability, which is best achieved by increasing the concrete cover and
providing space for effective casting of the concrete. The structural codes focus on
limitation of crack widths at the concrete surface, which is well defined and
measurable. However, in view of durability the crack width close to the bars should
be more decisive for the durability and service life of the structure. The width of the
cracks close to the bar can be expected to control the total amount of free oxygen,
moisture and other corrosive agents transported via the crack to the reinforcement.
This thesis examines the crack width close to the bar surface and the correlation to
parameters such as steel stress, concrete cover, steel diameter and concrete quality.
The test results show that the crack widths on the concrete surface are of the order
twice the crack width close to the bar surface and that the crack width near the
reinforcement bar is much less affected by a change in concrete cover than the crack
width at the concrete surface.
The test results also showed that the crack width close to the bar is affected only to a
limited extent by bar diameter and concrete quality and that steel stress is the
dominating parameter. During the tests it was noticed that small inclined cracks
developed at the bar interface and in the vicinity of the crack so that the concrete near
the bar was separated from the rest of the concrete and more or less followed the bar.
This mechanism has similarities with pull-out failures, but is limited by the
continuous reinforcement. The formation of the small inclined cracks induced a cone
shaped concrete piece, which is the main reason for the different development of
crack widths close to the reinforcement and at the concrete surface.
Non-linear FE-calculations have also been performed, showing similar relations
between the crack width close to the reinforcement bar and the crack width at the
concrete surface, as the experimental tests described above. The FE-analysis
confirmed that the crack width close to the reinforcement bar was almost unaffected
by the thickness of the concrete cover. In the results from the FE-calculations the
formation of the cone shaped concrete piece during the tensile loading was described
in more detail.
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To evaluate the total effect of crack widths and concrete cover on the corrosion risk,
cracked beams exposed to chlorides have also been tested. The main conclusion from
this test series was that large concrete covers protect the reinforcement better than
small concrete covers from chloride induced corrosion for exposure from below, even
if the crack widths are larger at the concrete surface. It was also shown that the
corrosion risk increases for chloride exposure from above and for high steel stresses.
Originalspråkengelska
KvalifikationDoktor
Tilldelande institution
  • Avdelningen för Konstruktionsteknik
Handledare
  • Thelandersson, Sven, handledare
Tilldelningsdatum2009 mars 27
Förlag
StatusPublished - 2009

Bibliografisk information

Defence details

Date: 2009-03-27
Time: 13:15
Place: Lecture hall C, V-building, John Ericsson väg 1, Lund University Faculty of Engineering

External reviewer(s)

Name: Emborg, Mats
Title: Professor
Affiliation: Institutionen för Väg- och Vattenbyggnad, Luleå Tekniska Universitet

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Ämnesklassifikation (UKÄ)

  • Husbyggnad

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