ICOLD Bulletin
:
The Physical Properties of Hardened Conventional Concrete in Dams
Section 3 (Elastic properties)
As submitted for ICOLD review, march 2008 Section 3-23
3.5 SIGNIFICANCE OF ELASTIC PROPERTIES ON DAM BEHAVIOUR
The modulus of elasticity is an important concrete parameter, vital for structural
analysis and for evaluating the strain distributions and deformations in concrete dams,
especially when their design is based on elasticity considerations [3.17].
The concrete deformation that occurs immediately after the load applications depends
on the “instantaneous” elastic modulus. The subsequent development of the
deformation, over a period of time with constant load, is the result of creep. Sometimes
this last one is generally accounted for by determining a “sustained” modulus of
elasticity to be used in static analyses [3.32].
From the experimental point of view, the sustained modulus of elasticity should be
evaluated from the concrete specimens with the same testing procedure prescribed for
the instantaneous elastic modulus [3.18], except for the testing time that can attain two
years or more, under constant sustained loads. These sustained moduli are further
discussed in Section 4 (paragraph 4.5 modelling creep in structural analysis).
As already discussed in paragraph 3.2.3.5, the elasticity modulus has an appreciable
increasing trend with time, particularly in pozzolanic cement concrete, and could be
even faster than for compressive strength (Section 2). From a series of laboratory
modulus data at different ages (up to 365 days) from more than 50 Brasilian dam
concretes [3.34], it was found that, in the presence of mineral admixtures, the average
ratio between the elasticity modulus at 365 days and at 90 days is about 1.2. Single
values can even attain ratios in excess of 1.5. This fact could also have a detrimental
effect in highly hyper-static structures such as dams and especially arch dams that
have a complex relation with their foundation.
For these structures, in fact, problems could arise in transferring direct loads to the
foundation rock or in reacting to imposed deformations like thermal loads. Different
stress levels and distributions, compared to those predicted in the design phase, can
appear and cracks could develop. Thus, in order to avoid extreme situations that are
not expected, it is recommended to consider the possible increase of the concrete
elastic modulus already in the design stage. This problem should be taken into account
also in the monitoring and interpretation of the dam behaviour [3.33].