As only a single specimen of each mix at a specific age as tested no characteristic values or confidence intervals could be established for the specimens tested in this study. This made it problematical to establish trends and draw clear conclusions, especially given the heterogonous nature of the material tested. The large confidence intervals shown in Figure 2.11 indicating the tensile strength and strain capacity of early age concrete specimens from the study conducted by Branch, et al. (2002) and the large degree of scatter observed in the results of the study conducted by Doa, et al. (2009) convey this point strongly.
From the 44 tensile tests used for the analysis of early age concrete properties, there was only one specimen for which its Young’s modulus and strain capacity could not be reliably determined. Three specimens produced significant outliers in terms of Young’s modulus, according to the definition described in the previous section, as well as strain capacity. Taking into account the heterogeneous nature of the early age concrete specimens, together with all the factors that can influence test results, this is believed to be acceptable. This
section described these outliers along with the typical linear elastic behaviour of early age concrete.
Figure 7.3 shows the initial ascending portions of the stress-strain curves of the 4h, 5h and 6h MR test specimens. The 4h and 5h test specimens produced virtually straight lines up till the peak stress was reached. In contrast, the 6h specimen produced an extremely steep initial section, reaching an initial peak stress of 11.77 kPa at 18.75 μ. Throughout this initial section of the 6h MR test the strain measured by means of the insitu concrete measurements resulted in great variability with initial strain fluctuation between negative and positive values. After flatting out, the stress-stain curve reached in ultimate peak stress of 12.17 kPa at a strain of 265.5 μ, representing its tensile strength. More typical and expected linear behaviour for 6h test specimen is indicated by the straight black line in Figure 7.3. This irregular behaviour was thought to be a combination of a multifunction in the physical isitu concrete measurement system as well as the LVDT’s that were responsible for measuring the displacement over the specimen gauge length. For this reason, the strain capacity, Young’s modulus, and consequently also the characteristic length of the 6h MR test specimen, could not be determined.
Figure 7.3: Irregular behaviour of 6h MR test specimen
It should also be noted that the linear region of the stress-strain curve often extended past the 50% of peak stress limit as predicted by Gutsch (2002). The 5h MR specimen is a good
0 2 4 6 8 10 12 14 -50 50 150 250 350 450 550 650 750 Str e ss [kPa] Strain [μ] 4h 5h 6h Expected linear behaviour
example of this as it displayed a linear relationship between stress and strain virtually up to the point where its tensile strength was reached, as shown in Figure 7.3.
The 8h MSR test specimen also proved to be an outlier, as shown in Figure 7.4. This test specimen was extremely stiff and produced a very steep initial linear section and an unusually long plastic region leading up to its tensile strength.
Figure 7.4: Irregular behaviour of 8h MSR test specimen
Using the method described earlier in this section of determining the Young’s modulus of a test specimen after the initial setting time, it was determined to be 293.19 MPa. This is significantly higher than the Young’s modulus of any other MSR specimen, which can be expected seeing that this was the oldest specimen tested, and displayed the highest tensile strength. However, the trend of a linear relationship between Young’s modulus and tensile strength was not adhered to by this specimen, as can be seen for Figure 7.17. Consequently this test specimen is a clear outlier.
Figure 7.5 shows the irregular stress-strain behaviour of the 5h MF1.8 test specimen. As with the 8h MSR test specimen, this specimen produced a very steep initial linear section and an unusually long plastic region leading up to the point of its tensile strength. As a result, the trend of a linear relationship between Young’s modulus and tensile strength was not adhered to by this specimen, and it is also considered an outlier in terms of Young’s modulus and strain capacity. 0 5 10 15 20 25 0 100 200 300 400 500 600 700 Str e ss [kPa] Strain [μ] 6h 7h 8h Initial linear section
Figure 7.5: Irregular behaviour of 5h MF1.8 test specimen
The last significant outlier with regards to the Young’s modulus and strain capacity of early age concrete was the 6h MF0.6 test specimen, as shown in Figure 7.6. In contrast to all the other outliers, this specimen produced a significantly lower than expected Young’s modulus, as well as a considerably higher strain capacity. The region marked by the blue circle indicates the presence of slippage in the tensile application system, as a slight drop in the applied tensile stress was observed without failure occurring. Another exception observed for this test specimen is the small linear region ranging from 0 to 3.7 kPa. Although the interval used to determine the Young’s modulus did not fall completely within the approximate linear region, it did not include the region of slippage and is still believed to be valid for the chosen definition of Young’s modulus although it might not provide the most accurate representation of the material’s linear elastic behaviour. Another irregularity observed in this test is the large flat region of the stress-strain curve from 1200 to 1800 μ just prior to the point of its tensile strength. The result is a significantly higher strain capacity than any other specimens, including all mixes, after the initial setting time has occurred. This did not agree with the trend, as the strain capacity usually decreased to a minimum at an age of around 5 – 6 hours.
The ascending stress–strain tensile properties of early age concrete for all 44 test specimens is presented in Table E.1 in Appendix E.
0 2 4 6 8 10 12 14 16 18 0 100 200 300 400 500 600 700 800 900 Str e ss [kPa] Strain [μ] 4h 5h 6h Initial linear section
Figure 7.6: Irregular behaviour of 6h MF0.6 test specimen