32
2 Dynamic Compressive Mechanical Properties of UHPCC
compressive behavior of hybrid (steel fiber blended with 2% polyvinyl alcohol fiber)
fiber reinforced ultra high toughness cementitious composites (UHTCC) at the strain
rate ranging from 113.8 s
−1 to 192.1 s
−1 . It was concluded that, increasing the volume
fraction of steel fibers had obvious strengthening effect on the dynamic compressive
strength, while the above effect was weakened with increasing the strain rate. The
energy absorption capacity significantly increased with increasing the strain rate and
steel fiber content.
Generally, the existing experimental assessment for the effects of steel fiber
content and type on the dynamic compressive mechanical properties of steel fiber
reinforced concrete, particularly for UHPCC, is far from sufficient. And the variation range of strain rate for SHPB test in the existing studies is not wide enough.
At present, the impact test is performed by using a 50-mm-diameter SHPB device
at the strain rate ranging from 17.6 s
−1 to 328.4 s
−1 , in which two typical steel
fibers (micro-straight and hooked) and three volume fractions (0%, 1.0% and 2.0%)
are considered. The dynamic compressive stress–strain curves under different strain
rates are derived, and the effects of steel fiber content and type as well as the strain
rate on the dynamic compressive properties of UHPCC are assessed. Furthermore,
the dynamic increase factor (DIF) for the dynamic compressive strength, as well as
the visco-elastic damage model of UHPCC are discussed.
2.2 Specimen Preparation
For the preparation of UHPCC, a forced single-axis mixer with a capacity of 60L and
a constant mixing speed (47 rpm) was used. To achieve good workability, particle
distribution and packing density, mixing procedure for the present UHPCC was
controlled rigorously. Firstly, since small particles tended to agglomerate and it was
easier to break these chunks when the particles were dry. The cementitious materials (cement, silica fume, ultra-fine mineral admixture) and sand were put together
simultaneously and dry-mixed uniformly for 3 min. Secondly, the water pre-mixed
with super-plasticizer was then added gradually and mixed for 3 ~ 5 min. Finally,
steel fibers were dispersed carefully by hand into the mixture and mixed for another
3 ~ 5 min in order to guarantee the fibers well distributed throughout the matrix. The
shear action of steel fibers helped to destroy any remaining agglomerates in the fresh
mixture, thus improving the workability of the composites.
After the prepared matrix was poured into the moulds, the freshly cast specimens
were covered with plastic sheets to prevent moisture losing and kept at room temperature for 24 h. Then, they were demoulded and cured in a standard curing room with
the temperature of 17.7 ~ 21.7 °C and relative humidity of 98.4% for another 28d.
After that, the specimens were taken out of the standard curing room and stored at
room temperature until the time of tests. For the present two steel fibers types and
three volume fractions, 120 cylindrical specimens were cast for the dynamic SHPB
test. As shown in Fig. 2.1a, the specimen size of 48mm × 35 mm was selected by
considering the diameter of the SHPB device (50 mm) and the maximal fiber length
2 Dynamic Compressive Mechanical Properties of UHPCC
compressive behavior of hybrid (steel fiber blended with 2% polyvinyl alcohol fiber)
fiber reinforced ultra high toughness cementitious composites (UHTCC) at the strain
rate ranging from 113.8 s
−1 to 192.1 s
−1 . It was concluded that, increasing the volume
fraction of steel fibers had obvious strengthening effect on the dynamic compressive
strength, while the above effect was weakened with increasing the strain rate. The
energy absorption capacity significantly increased with increasing the strain rate and
steel fiber content.
Generally, the existing experimental assessment for the effects of steel fiber
content and type on the dynamic compressive mechanical properties of steel fiber
reinforced concrete, particularly for UHPCC, is far from sufficient. And the variation range of strain rate for SHPB test in the existing studies is not wide enough.
At present, the impact test is performed by using a 50-mm-diameter SHPB device
at the strain rate ranging from 17.6 s
−1 to 328.4 s
−1 , in which two typical steel
fibers (micro-straight and hooked) and three volume fractions (0%, 1.0% and 2.0%)
are considered. The dynamic compressive stress–strain curves under different strain
rates are derived, and the effects of steel fiber content and type as well as the strain
rate on the dynamic compressive properties of UHPCC are assessed. Furthermore,
the dynamic increase factor (DIF) for the dynamic compressive strength, as well as
the visco-elastic damage model of UHPCC are discussed.
2.2 Specimen Preparation
For the preparation of UHPCC, a forced single-axis mixer with a capacity of 60L and
a constant mixing speed (47 rpm) was used. To achieve good workability, particle
distribution and packing density, mixing procedure for the present UHPCC was
controlled rigorously. Firstly, since small particles tended to agglomerate and it was
easier to break these chunks when the particles were dry. The cementitious materials (cement, silica fume, ultra-fine mineral admixture) and sand were put together
simultaneously and dry-mixed uniformly for 3 min. Secondly, the water pre-mixed
with super-plasticizer was then added gradually and mixed for 3 ~ 5 min. Finally,
steel fibers were dispersed carefully by hand into the mixture and mixed for another
3 ~ 5 min in order to guarantee the fibers well distributed throughout the matrix. The
shear action of steel fibers helped to destroy any remaining agglomerates in the fresh
mixture, thus improving the workability of the composites.
After the prepared matrix was poured into the moulds, the freshly cast specimens
were covered with plastic sheets to prevent moisture losing and kept at room temperature for 24 h. Then, they were demoulded and cured in a standard curing room with
the temperature of 17.7 ~ 21.7 °C and relative humidity of 98.4% for another 28d.
After that, the specimens were taken out of the standard curing room and stored at
room temperature until the time of tests. For the present two steel fibers types and
three volume fractions, 120 cylindrical specimens were cast for the dynamic SHPB
test. As shown in Fig. 2.1a, the specimen size of 48mm × 35 mm was selected by
considering the diameter of the SHPB device (50 mm) and the maximal fiber length