Uniaxial Tensile Creep Test

After the determination of the average uniaxial tensile strength, the creep test was then pursued. Specimens for the creep test had been cracked using the same procedure and crosshead displacement rate for the uniaxial tensile test. The specimens were cracked to an average crack width of 0.5 mm, unloaded and transferred to the creep frame where they were subjected to creep stresses of 30, 40, 50, 60 and 70 % of the average residual tensile strength. The creep frame was designed to accommodate two specimens in series as shown in Fig. 3.

Prior to subjecting the specimens to the creep loads using a lever arm system, the creep frames were calibrated using a 50 kN tensile load cell to determine the precise loads to be supported (including self-weight of lever arms and suspension cables) at each stress level. Table 3 shows the details of the various stress levels tested.

The result of the tensile creep measured in terms of CMOD for 8 months was measured using a 10 mm LVDT (two for each specimen) mounted in the same fashion as specimens for the uniaxial tensile strength test. It should be noted that unlike the uniaxial tensile strength test specimens where a gauge length of 120 mm was used, specimens tested for creep had a gauge length of 70 mm. This was due to

Fig. 3 Tensile creep frames showing specimens under sustained loadings [8]

Table 3 Applied stress levels and number of specimens tested

Load levels as % of the post-crack strength

30 %

40 %

50 %

60 %

70 %

Actual applied stress (MPa)

0.234

0.312

0.39

0.468

0.546

Equivalent creep load (N)

1498

1997

2496

2995

3494

Number of specimens

4

4

4

2

2

the smaller LVDTs used for the creep test. All tests were carried out in a controlled climate room where temperature and relative humidity were maintained for the test period at 23 ± 1 °C and 65 ± 5 % respectively.

The displacement reading obtained from the creep test will include material creep and drying shrinkage. The specimens were tested unsealed, therefore basic creep, drying creep, crack widening and drying shrinkage occurred at the same time. The drying shrinkage could be subtracted as it was measured separately while the material creep is insignificant compared to the crack widening.

 
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