The objective of defining an appropriate procedure to pre-crack SHCC for subsequent durability testing is addressed here based on the comparative test results.
As noted in the previous section, large variability in crack width was reported for uniaxial tensile test specimens of small gauge cross sections. Based on the results, a cross section of least dimension of 30–40 mm is recommended. This thickness recommendation is also in agreement with a typical depth of a SHCC class of applications such as overlay repairs, whereby the actual conditions are simulated appropriately.
Subsequent or simultaneous durability test procedures may also dictate the specimen dimensions. Specimen diameters for permeability tests are generally in the range of 50–100 mm, while capillary absorption specimens typically have a height of at least 60 mm (e.g. ). To allow such testing, a larger cross-section dimension in the range of 60–100 is recommended.
The gauge length ranging from 80 to 120 mm in these comparative tests appeared to have allowed multiple crack formation. In only one set of results, i.e. from L3, a larger cross section (30 mm × 30 mm) was accompanied by the minimum gauge length of 80 mm. Nevertheless low variability in crack data was found, although cracks predominantly formed in the central part, as seen in Fig. 2 for L3. In the L1 specimens (see Fig. 2), the 120 mm gauge length allowed a longer central portion to form saturated multiple cracking (zones II and III in Fig. 2). Thus, durability test samples taken from a larger length may be more representative for the durability of actual field SHCC where uniformly spaced cracks may form. A specimen with a longer central part of uniform section may also allow taking two samples from each dumbbell specimen. Thus, a gauge length of 120 mm is recommended.
From the reported results, it is not possible to distinguish the specimen size and test boundary conditions as sources of variability in crack data. Nevertheless, the data presented for larger specimens had rotationally fixed boundaries at each end, while rotationally semi-free ends were used with the smaller specimens. Carefully applied fixed–fixed boundaries should be preferred in order to activate most of the material’s strain capacity, see Sect. 4. However, if it is impossible to accommodate geometrical imperfections in specimens and adapters, a set of rotationally fixed-free boundaries is recommended, i.e. fixed at one end, and free at the other end.
For crack width characterization, either DIP or DIC is recommended, but a resolution of at most 10 μm must be used to avoid significant errors. A useful presentation of crack width data is shown in Fig. 6, in the form of crack width histograms. This representation is believed to allow eventual linking of crack distributions with deterioration resistance. In addition, average crack widths per set, standard deviation, maximum crack widths as well as average crack spacing must be reported, although all these values might be derived from the histogram data with reasonable accuracy. The durability of cracked SHCC as dependent on the crack pattern is a major subject of investigation for RILEM Technical Committee 240-FDS.
An alternative way of presenting crack width distributions is the so-called crack width polygon . The format of such curves resembles the one of grain size distribution curves and allows to present the crack width distribution independent on crack density or spacing. It is also possible to characterize the crack width distribution by a single numerical value based on the crack width polygon.
It has to be considered that only the average value of the crack spacing can be derived from the observed number of cracks since SHCC crack patterns do not necessarily comprise equally spaced cracks. Determination and evaluation of crack spacing distributions will be subject of further investigations since the crack spacing is expected to have a significant influence on the capillary absorption of cracked SHCC.