2.3 Analysis of fracture patterns

The X-ray computed tomography (CT) was employed to examine the internal structure and fracture patterns of the concrete specimens tested in compression [4]. Using the CT radiographic inspection technique objects are reconstructed from their cross sectional images. Over the years, nondestructive evaluations using CT imaging of the microstructure of concrete [13], soil [14, 15], rock [16], and asphalt concrete [17, 18] have been largely used. Compression tested specimens have exhibited similar fracture patterns, i.e., cone at one end and vertical columns, as types 2 and 3 patterns described in the ASTM C 39 test method. Figure 3 shows tomographic images of the 3D reconstructed SCC and NC specimens acquired from the top, middle and bottom. Images reveal concrete structural damage and fracture patterns along with the aggregate fracture throughout the tested specimens. The SCC specimens did not exhibit any segregation.

Several sectional images of 100 200 mm tensile split cylindrical specimens were visually inspected to identify fractured aggregate. The analysis showed that the percentage of fractured aggregate for SCC was about 15–25% (9–15 out of 60) while for NC was around 10% (4–6 out of 60) after determining that each section facet contained around 60 coarse aggregate particles. The number of broken aggregate particles from three tested specimens was averaged to calculate the fracture

specimens. These data indicate that the lower air void content also contributed to an increase in strength of the SCC as cracks initiate more easily in the cement paste with higher air voids. Some of these air voids represent "flaws" in the cement paste at the aggregate-paste boundary possibly forming microcracks or "bond cracks" in the ITZ which ultimately lead to the failure of the concrete due to the propagation

This research verified that SCC achieved self-compactability and adequate flow under its own weight, without external vibration or compaction based on the mix proportion parameters and the materials used. The study also showed that splitting tensile and compressive strengths in SCC were higher than those of normal vibrated concrete due to the addition of chemical and mineral admixtures. Compressive strength had an average increase of over 60%, whereas the splitting tensile strength showed a 30% increase. The ratio between the splitting and compressive strength was similar to values found in the literature. Also, a better bonding between aggregate and cement paste was achieved in SCC due to the use of chemical and mineral admixtures, as the material has shown smaller aggregate-matrix interface microcracks than normal concrete. This characteristic affected both the splitting tensile and compressive strengths. Another indicator of the stronger aggregate-paste bonding was the greater percentage of the fractured aggregate in SCC (20–25%) compared to the 10% for normal concrete. Moreover, when compared to NC, the number of air voids in SCC was lower and they appeared relatively smaller and more rounded. These factors

of microcracks under localized tensile stresses.

Concrete Microstructure Characterization and Performance

DOI: http://dx.doi.org/10.5772/intechopen.90500

have contributed as well to the increase in strength of the SCC.

3. Concrete microstructure damage by projectile impact

Penetration of projectiles into concrete target was investigated in this research by using both experimental and simulation methods. The over-all objectives of the project included: (a) Building up an experimental facility to conduct the penetration test; (b) Developing of a rational constitutive model to incorporate distributed damage effects; (c) Enhancing of the model implementation by combining the Finite Element Method (FEM) and the Discrete Element Method (DEM) so that post-fracture behavior can be simulated; and (d) Developing of methods to backcalculate model constants from comparing experimental with simulation results.

2.5 Conclusions

Figure 5.

35

Gas operated facility for testing concrete samples.

Figure 3.

Tomographic images of SCC (top row) and NC (bottom row) specimens after being tested in compression (from left to right: top, middle, and bottom of specimen).

percentages. The greater number of fractured aggregate in the SCC specimens was another indication of a better bonding between aggregate and cement paste.
