Flexural Creep Test
Three pre-cracked beams were loaded with a sustained load at various percentages starting with 50 % of the load reported at 1.75 mm bending deflection. The chronological order of testing is shown in Fig. 1. The limited number of rigs was the reason why different load levels were performed using one specimen. The creep rig is designed similarly to the testing machine used for the flexural test. However, the load is kept at a constant level by simple leverage (Fig. 3). Lubricants were used to minimise friction at the supports. Later on roller bearings were installed at one support.
The test results combine both the creep of concrete in compression and the creep of the fibres in tension. There is also creep deflection due to fibre pull-out over time. The beams have to be kept at constant humidity. Therefore, they were protected from drying out by aluminium sheets. The creep rigs are situated in the basement of the OTH lab with quite uniform climate conditions, but unfortunately not that uniform as in a climate chamber.
Creep load is applied using a balance (Fig. 4). When increasing the load, the necessary dead weight and the leverage arm length has to be calculated. The creep deformation versus time is registered for all specimens in given time intervals. For most samples, the test started with a load level of 50 % of the load at 1.75 mm deflection and was increased after about 90 days to a load level of 60 %, some at later age even to 70 and 80 %.
Mass lost during testing and fibre counting in cross section after the tests were done in a last step for proper interpretation of results.
Fig. 3 The simple, but sufficient stiff test-rig used for these tests
Fig. 4 Applying the exact creep load. Note Supporting rollers in this equipment not sufficiently moveable, instead lubricants are used