13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Fig. 5b shows an example of stress-crack opening relationships at different ages for DTUIF. Furthermore, it shows the fracture energy given by the area under the curves is increasing with age. The development of modulus of elasticity E seen in Fig. 6 shows some scatter and furthermore, a drop in E is seen for DTUII mix. With the exception of the Contec mixes, the mean static elastic modulus of the concretes Ecm, reached about 60GPa after 28 days. The static elastic modulus of the Contec mixes was at 40GPa, thus significantly lower. This seems to be caused by discontinues grain size distribution curve and by a disadvantageous pore size distribution of the hardened concrete. The static elastic modulus of the DTU mixes developed significantly faster than Contec mixes. 10 100 1000 0 10 20 30 40 50 60 70 Age [hours] E c [GPa] Contec DTUI DTUII 10 100 1000 0 10 20 30 40 50 60 70 Age [hours] E c [GPa] Contec-F DTUIF DTUIIF Figure 6. Development in modulus of elasticity. Fig. 7 shows the development in the tensile strength, ft. The 28 days tensile strength of all HPC mixes was high around 6 MPa. The tensile strength of the DTU mixes made with silica fume, fly ash and cement with rapid hardening was observed to develop faster. After three days it was at roughly 5 MPa - well above the strength of the Contec mixes, 3 MPa. Tensile strength development of Contec mixes tended to increase slower than DTU mixes. There was no obvious change of tensile strength caused by adding fibers to HPC matrix. In general results of development of elastic modulus E and tensile strength ft follow the literature, see e.g. [4]. Their results also indicate a drop in the modulus of elasticity at 28 days. 10 100 1000 0 1 2 3 4 5 6 7 8 Age [hours] f t [MPa] 10 100 1000 0 1 2 3 4 5 6 7 8 f t [MPa] Age [hours] Contec-F DTUIF DTUIIF Contec DTUI DTUII Figure 7. Development in tensile strength. Development in fracture energy with age is shown in Fig. 8. The development of this property is in agreement with observation found in literature see e.g. [1,4]. Same as at these papers the fracture energy is found increase with age.
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