LICENTIATE THESIS - Luleå tekniska universitet

CONCRETE STRENGTH DEVELOPMENT IN SWEDISH
BRIDGES
by Håkan Thun, Lennart Elfgren and Ulf Ohlsson
ABSTRACT
In the paper the development of tensile and compression strength is presented for old
concrete railway trough bridges. The compression strength has usually increased 50 to
100% over the years for these trough bridges. However the same could not be said for
the tensile strength. When the results from drilled out cores were compared for
differrent structural parts, i.e. the slab and the longitudinal beams, it turned out for one
bridge that the compressive strength was approximately 15% higher in the longitudinal
beam than in the slab. This indicates that there is a difference in strength between the
side beams and the slab. Until this difference is verified with further studies, a reduction
of the concrete strength with 15% has been proposed when the tests are done in the
side beam and the analysis concerns the slab.
In the paper a rather well-known method, the Capo-test, is used to determine the insitu
concrete compression strength in old railway trough bridges. The Capo-test has
been compared with the results from drilled cores, which is the common way to
determine the in-situ strength of a concrete structure. The study indicates that the
Capo-test can be used on objects with old age, but with some restrictions, especially
regarding the aggregate size. Results from tests performed on the Lautajokki Bridge in
Northern Sweden show that the Capo-test might in some cases overestimate the
compressive strength. An improved formula is presented for evaluation of the
compression strength from Capo tests. It gives a better correlation than the general one
and the one proposed by Rockström & Molin (1989) for old structures, but it might
give results on the unsafe side and further studies are necessary.
Keywords: strength development, Capo-test, drilled cores, concrete, bridges
RESEARCH SIGNIFICANCE
During the last decades it has become more and more important to assess, maintain and
strengthen structures like bridges, dams and buildings due to increased age of many
structures and the high costs to construct new infrastructure. Therefore it is of great
interest to find methods to evaluate the concrete strength. In this paper a rather wellknown
method, the Capo-test, is used to determine the in-situ concrete compression
strength in old railway trough bridges. Up to this date the method has primarily been
used to estimate the strength of the cover-layer of new structures. The Capo-test has
been compared with the results from drilled cores, which is the common way to
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