Compressive creep test for WPC

A compressive creep test was performed to determine the deformation-time response as well as the creep behaviour for WPC. The deformation-time curve is used to predict the creep-strength and -modulus of the WPC. It is also performed to determine whether creep is a major concern for the use of WPC as a load-bearing element.

7.5.1. Test methodology

The test-setup is performed according to ASTM D2990 (2009). This code specifies the following requirements:

 Temperature, where the test is performed, shall be maintained constant with a tolerance of ± 2°C, since the creep test is sensitive to temperature changes.

 The creep test is also sensitive to the change of relative humidity in the test and it shall be maintained constant with a tolerance of ±5%.

 A minimum of two specimens should be tested.

 The deformation of the specimen is measured by the following time schedule: 0,1,6,12, and 30min;

1,2,5,20,50,100,200,500,700,1000 hours.

 The test specimens should be preconditioned for at least 48 hours prior to the test, to ensure that the specimens are in moisture and temperature equilibrium.

 The total applied load shall be applied (at start of test) and removed (at end of test) rapid and smoothly within 1 to 5 seconds.

Eight specimens with a dimension of 22x 50x150mm were used for this experiment. The applied load is 30%

of the compressive yield strength. Thus, a creep load of 8.15MPa is applied.

The test-setup is shown in Figure 7.18. A steel disk is placed on top of a ball-joint to ensure that the ball-joint remains stable at the top of the specimens. The bolt-joint ensures that the load is applied vertically to the top of the specimens. Specimens were placed on top of each other and were separated by a steel plate. A load-cell was used to ensure the correct constant load of 8.97kN was applied. Bolts were used to apply the load.

Four bolts were tightened on threaded rod for each pair of specimens. The creep load was applied within 5 seconds.

Figure 7.18: Compressive creep of WPC test-setup

The specimens were sanded with sandpaper to ensure clean surfaces where the creep targets were attached.

These targets are shown in Figure 7.19. The targets are used to measure the creep and were attached 100mm apart. The creep was measured by using a MarCator 1075R extensometer (also known as a digital indicator)

over a distance of 100mm. This creep measuring device is shown in Figure 7.20. The device is sensitive to temperature changes and is therefore kept in the same controlled room as the creep specimens. The device is zeroed at 100mm before measurements, for each of the measuring times, were taken.

Figure 7.19: Creep targets

Figure 7.20: MarCator 1075R (Creep measuring device)

7.5.2. Results

Eight specimens were tested for creep and the strain-time responses of these test specimens are shown in Figure 7.21 and Figure 7.22. From this figure it can be seen that, Specimens 2 and 5 exhibit a greater creep strain compared to the other specimens. This can be due to Specimens 2 and 5 rotating about their vertical axes, which occurred during the experiment. This resulted in the same compressive force being transferred over a smaller surface area, which caused higher compressive stresses in the specimens. The temperature and

humidity fluctuated during the creep test, as shown in Figure 7.23. From Figure 7.21 it is clear that the large fluctuation in humidity (Figure 7.23) had an insignificant effect on the WPC.

Figure 7.21: Creep strain-time responses for WPC test specimens (at 30% compressive yield stress)

Figure 7.22: Creep strain-time responses for WPC test specimens (at 30% compressive yield stress) represented on a logarithmic scale

The average strains of test specimens in Figure 7.21 and Figure 7.22 were calculated to determine a single creep strain-time response for WPC, which is shown in Figure 7.24 . In this figure, the initial elastic as well as plastic strain is shown. The initial elastic strain was calculated from the results obtained from the direct

0

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

Creep Strain (%)

compression test (Section 7.2). Primarily creep shows an increase in the strain which occurs at a decreasing rate whereas the secondary creep illustrates an approximately constant strain over time. Refer to Section 4.1.5 for the typical strain-time curve and the description thereof. Tertiary creep might not yet occur, due to the relatively short time period. However, for the time period tested, it seems that the creep strain remains constant over an extended period for 30% of the yield compressive stress. The strain, at the 1000 hour time period, is approximately constant at 0.41%. This indicates that the walling system, which is the critical element when compressive creep is considered, will displace about 11mm (0.41% multiplied with the length of walling unit, 2700mm), if no tertiary creep or unloading occurs.

Figure 7.23: Temperature and humidity fluctuation during compressive creep test

The average creep-modulus to time response for WPC was calculated by taking the averages of the creep modulus for each specimen. The creep modulus is determined by dividing the constant stress (8.15MPa) by the strain at each measured time. This response is shown in Figure 7.25. From this curve, it is clear that with time the stiffness of the material to sustain a load decreases. However, the curve flattens out after about 20 hours, after which the modulus remains approximately constant over an extended time.

0 10 20 30 40 50 60 70 80 90

0 10000 20000 30000 40000 50000 60000

Temperate (°C) and humidity (%)

Time duration during creep test (min)

Humidity Temperture

Figure 7.24: Average creep strain-time response of WPC

Figure 7.25: Average creep modulus-time response for WPC (30% compressive yield stress) 0

500 1000 1500 2000 2500 3000

0 1000 2000 3000 4000 5000 6000

Creep modulus (MPa)

Time (minutes) 0

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

0 10000 20000 30000 40000 50000 60000

Strain (%)

Time (minutes)

Primary creep Secondary creep

Initial elastic strain

Initial plastic strain