To verify whether the threshold was similar for all cement
contents, more test data on samples were plotted in Fig. 12. Specimen
with different cement contents, mixing with fiber bundles of
40 mm or with polypropylene fibers of 6 mm, were cured for 28
days and tested to show the relationship between their strength
versus fiber contents. When the fiber content was 2%, the relationship
between strength versus fiber contents is approximately
linear. The fiber bonds and tensile strength provided by fibers were
the main source of improvement for specimens with low cement
contents. However, for the specimens with cement content of 4%
and 6%, it appears that the cementation effect became the dominant
source of improvement and that excessive fibers do not
further assist in increasing the strength of improved clay. The
strength of both types of specimens reached a maximum point in
the fiber content range tested. For the improved clay with a cement
content of 4%, the threshold is around the fiber content of 1.5%. The
threshold is found to be between fiber content 0.5% and 1% at a
cement content of 6%.