ICOLD Bulletin
:
The Physical Properties of Hardened Conventional Concrete in Dams
Section 3 (Elastic properties)
As submitted for ICOLD review, march 2008 Section 3-12
3.2.4 Estimation of the elastic modulus
The elastic modulus of a concrete is conventionally measured using standardised tests
directly based on concrete samples subjected to uniaxial loading (paragraph 3.2.4.1).
The sizes of these samples should be at least three times the maximum size of the
aggregate used in concrete. However, from an experimental point of view, this is not
always easy: in fact the maximum size aggregate usually ranges from 80 to 200 mm
and large specimens, for example cylinders of 45 x90 cm, should be required [3.16].
Due to the practical difficulties in performing such tests, dam concrete is usually wet-
screened, removing aggregates larger than 40 mm, and using standard cylinders of
15x30 cm (2.2.1 in Section 2).
However this procedure can result in a incorrect estimation of the elasticity values. A
method has been recently proposed [3.17] in order to predict the modulus of the real
size aggregate concrete from the experimental results obtained form the wet-screened
concrete specimens (paragraph 3.2.4.2). It is based on the application of the simple
elastic models presented in the paragraph 3.2.3.1.
Furthermore empirical approaches can be used to estimate the elastic modulus, less
complicated and time-consuming compared to the experimental determination. A lot of
empirical relationships have been proposed and recommended by national and
international standards, estimating the elastic modulus directly from the concrete
compressive strength (paragraph 3.2.4.3).
3.2.4.1 Experimental determination in laboratory
Dam concrete is designed for a compressive stress state, therefore the static modulus
of elasticity is typically measured with concrete in compression. For example, the ASTM
C469 [3.18] provides the static Young’s Modulus and Poisson’s Ratio using a
compressive test procedure.
It stipulates the use of chord modulus with a lower point at strain of 50
µε
and the upper
point corresponding to 40 per cent of the compressive strength at the time of loading.
The lower and upper chord modulus points are chosen to avoid seating effects and also
the modulus is obtained in the “elastic” range of the stress-strain curve. ASTM C 469
uses 6 by 12 inch (15 by 30 cm) cylinders.
For drilled core specimens, only diamond drilled cores with length-to-diameter ratios of
greater than 1.5 are used. The strains along the axis of compression should be
measured with two or more gage lines such that eccentric loading and non-uniform
response can be monitored. The gage length for strain measurement is an important
consideration. ASTM C469 specifies that the gage shall not be less than three times
the maximum size of aggregate or more than 2/3 the height of the specimen. The
preferred gage length is typically about one half the height of the specimen.
Before loading the specimen it is important to follow a specific centering sequence,
accurately placing the test specimen in the centre of the loading platens of the testing
machine and monitoring the strains values provided by the gauges. If these strains
eccessively differ by more than 20% from their mean value. It is recommended to re-
centre the test specimen and repeat the test.