The complete formula of the reological model, combining a Maxwell and a Kelvin component, can be

With M as notation for the parameters of the Maxwell element and K for the material characteristics related to the Kelvin element. The parameters are calculated from monotonic compression tests and short-term creep tests.

As already mentioned, the test programme included tests on masonry wallets (29 x 19 x 85 cm) with three types of typical mortar compositions: cement mortar, hydraulic lime mortar and a hybrid, cement-lime mortar. And an extension of the test programme towards air hardening lime mortar has been planned.

On all masonry types, three kinds of tests have been performed (A more detailed discussion of the results can be found in Verstrynge 2008):

- Monotonic compression tests, under a deformation controlled environment;

- Short-term creep tests (also called accelerated creep tests or step-by-step tests), at which stress is increased in steps of 5% and in between kept constant for a duration of 2 hours;

- Long-term creep tests, during which masonry wallets are loaded at different constant stress levels.

The long-term tests were set up in order to assess the strain rate of the different mortar types at different stress levels. These tests are still ongoing. A second goal of these tests is to assess the simulation of long-term behaviour of masonry, by means of the proposed model and the model parameters which are derived from short-term tests. The model parameters were calculated as an average for each mortar type. At least three test specimens were made for each mortar type and each type of test, which results in a total of 27 specimens. The resulting model parameters are summarized in Table 1.

The compressive strength of the masonry (fc) is obtained from monotonic compression tests. The strength of the cement mortar was remarkably low, possibly due to bad curing conditions and absorption of the water out of the mortar by the bricks, leading to a weaker mortar.

The Poisson's ratio (u) is calculated from the compression tests, without considering the last part of the tests, where dilatation occurs and the Poisson's ratio suddenly increases. This dilatant behaviour of the

Figure 5. Viscous damage parameter (DV) in function of stress level from short-term creep tests on lime and cement mortar wallets. Stress is formulated dimensionless.

Table 1. Overview of the parameters used during the simulations with the rheological model. Presentation of results: average value (± standard deviation).

Table 1. Overview of the parameters used during the simulations with the rheological model. Presentation of results: average value (± standard deviation).

Parameters of

rheological |

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