Drying Shrinkage of Test Specimens

As already presented in Table 3.1, the drying shrinkage was measured on a 100 mm × 100 mm × 500 mm in conformity with the size of the specimens subjected to uniaxial tensile creep. The phenomenon of drying shrinkage in itself is known to be a complex phenomenon because of its sensitivity to many factors. The investigation of the drying shrinkage strain was carried out to distinguish between creep and drying shrinkage.

4.4.1 Results

The results of the drying shrinkage measured over a gauge length of 285 mm as described in Section 3.3.4 is presented in Figure 4.8 as shrinkage-time relationship over a period of 8 months. The time is reported in days commencing from the zero point which started at 28 days after curing by complete immersion in water.

0 100 200 300 400 500 600 700 0 40 80 120 160 200 240 Sh ri nk ag e st ra in ( ×1 0 -6) Time (days) Specimen 1 Specimen 2 Average 100 mm × 100 mm × 500 mm specimens

90 The shrinkage deformation was calculated as the average displacement of the LVDT reading over the gauge length. Since this strain is subtracted from the actual creep deformation of specimens subjected to sustained uniaxial tensile loadings, the strain is converted to shrinkage displacement, δsh

(mm) as:

+(ℎ ='-. /#0 '-&._{1-2. .$ℎ} × .-2. .$ℎ % $ℎ ') ()& (4.4)

The drying shrinkage measured up to 120 days before an anomaly in the reading shows an average deformation of 440 µstrain. The anomaly is due to significant changes in the temperature/humidity of the climate controlled room where the test was carried out. The average shrinkage-time relationship tends to be approaching equilibrium with the test environment after about three months (150 days).

4.4.2 Discussion

The shrinkage response of the unsealed specimens shows a nonlinear rapid increase at the onset of the test up to 3 months. This agrees with the postulation that the drying shrinkage of Portland cement materials majorly occurs within the first 3 months and then continues at a decreasing rate (Neville, 2012). An anomaly in the strain-time relationship is observed at two instances when the humidity and temperature in the climate room became unstable due to the malfunctioning of the control unit. However, as soon as this was rectified, the shrinkage deformation can be observed to again assume its initial trend. The rapid increase in the shrinkage within the first three months indicates an increase in the loss of water from the specimen to the environment (evaporation) and begins to decrease as equilibrium is approached.

The effect of the macro synthetic fibres on the drying shrinkage of normal concrete was studied by comparing experimental results with the prediction model of the final draft of Model Code 2010. Figure 4.9 shows that the macro fibres increase the drying shrinkage of concrete. In FRC, macro fibres are known to create more voids in concrete as reported in this research. Since drying shrinkage is reported to depend on the size and type of voids in concrete (Pelisser et al., 2010), these voids could

91 have caused increase in the drying shrinkage of macro fibre concrete. However, on the overall, it can be concluded that the fibres have no significant effect on reducing the drying shrinkage of concrete. It is however acknowledged that the shrinkage was not compared to that of the same concrete without fibres. Some authors have also presented this view (Amin et al., 2014; Mesbah & Buyle-Bodin, 1999).

0 100 200 300 400 500 600 0 40 80 120 160 200 240

Sh

rin

ka

ge

s

tra

in

(

×

10

)

Figure 4.9: Time-dependent development of drying shrinkage

4.5 Summary

Experimental tests to study the behaviour of macro synthetic FRC subjected to compression, uniaxial tension, uniaxial tensile creep and drying shrinkage (macro level investigation) have been carried out.

The compressive strength test carried out for plain concrete and macro synthetic FRC at a fibre content of 9.1 kg/m3 _{has shown that the addition of macro synthetic fibres to plain concrete lowers its}

strength by about 8 %. It is supposed that due to the balling tendency of fibres, increase air content could be trapped leading to reduced density of the FRC and invariably, the compressive strength.

Macro synthetic FRC specimens have also been investigated in uniaxial tensile test. Strain softening response is reported with significant CMOD before the fibres were fully engaged. The significant CMOD could be related to the significant difference between the elastic modulus of the fibre to the concrete matrix. However, significant toughness with a slight pseudo-hardening behaviour

92 is observed in the post crack region. Macro synthetic fibres are known to be flexible in nature. Inclined fibres at the cracked plane tend to eventually become aligned to the direction of the applied load with increase in the crack opening displacement, hence producing the pseudo-hardening response.

Specimens subjected to uniaxial tensile creep at varying stress level from low to high have shown significant creep even at a low stress level of 30 % average residual tensile strength. Tensile failure of specimens after 10 days and less than 1 day were also recorded at 60 % and 70 % post crack strength respectively. Increase in creep loads have generally led to increase in the time-dependent CMOD. Some variability found in the results have been attributed to the fibre counts on the cracked plane; the more the fibre counts, the lesser the creep response.

The drying shrinkage of macro synthetic FRC has shown to be close to the typical response of concrete modelled using Model Code 2010.