ABSTRACT
In Roller Compacted Concrete massive structure, thermal effects begin just after casting of concrete and reach their critical value (maximum of temperature) after three days. Most of creep and relaxation functions proposed in the calculation codes are available for ages above 5 days. We have developed a serie of creep test under tensile loading at early ages to characterize the viscous behaviour of concrete at early ages. For roller compacted concrete, we have identified a creep function as :
£^(t,ti) = — a e - ^ ^ \ l - M ) M (11) El
where ti is age at loading, a,band c are material parameters, Aa increasing of stress, Ei young modulus at time ti
This relation is similar to those proposed by Laube (1990) and Bazant (1993)
3 Applications
3.1 Case of the Riou RCC dam The Riou dam is a gravity dam built with the RCC technique without any joint. Its dimensions are 200 meters long, 30 meters high and 18 meters large (center section). Within the national french program "BaCaRa", this construction was used to measure, to control and to observe the thermomechanical effects. Figure 1-a gives the cracks observed on the dam after six months of age. To simulate the phenomena a first three dimensional thermal computation was performed to obtain the history of temperature in the dam. In situ measurements has been dohe to adjust these imput values and to define the thermics boundary conditions. From the three dimensional mesh used for thermal computations, it is possible to extract a two dimensional mesh which has been used for mechanical computations. Computations use the Cesar-LCPC (1987) finite elements code in which the model, described above, has been introduced. Because the softening behaviour of concrete induces a localization effect, the non local damage theory is used. The damage D is calculated over a representative volume Vr. The size of Vr depends on the characteristic length Ic of the material (see Pijaudier et al 1987). The boundary conditions chosen for these computations are embedded in the foundation. The mesh uses 777 elements (Q4) and the rock is modelled as perfectly elastic. Numerical results of these computations are given for the steps of time 39 days and 187 days (figure lb and Ic). They can be compared to the real cracks observed in the dam after 6 months
Themial effects, cracking and damage in massive concrete 333
F o u n d a t i o n
Damage-.From 0.80 cracks occur
parameters values ultimate shrinkage : 138 [xm/m activation energy : 71200 J/mol damage law : At = 0.95
Ac= 1.40 Bc= 17000 P= 1.06
ultimate Young modulus: 28774 MPa maturity fonction : A = 5.60 lO" days '
d« O.QCOE+CO 0 0.167E+CO d« 0.167EfOO a 0.333E+CO ie 0.33JEfO0 c 0.5OOE+OO d« O.5O0E+O0 <3 0.667E+OO
0.667E+00 a 0.833£f00 d« 0.83JEfO0 a O.lOCE+01
Figure 1 : Cracking in a Roller Compacted Concrete dam
a - Cracks observed after 6 months, b - c - Damaged zone obtained by calculation at times 39 days and 187 days.
