2.2 Evaluating the bond between coarse aggregate and cement paste

The SCC was extensively researched in the past decades to find ways to enhance its performance [8, 9]. The general focus of those efforts was the improvement of the interface properties between paste and aggregates. That improvement has led to the manufacturing and use of higher strength and enhanced durability concretes [10, 11]. The physical interface between aggregate and cement paste and the aggregate fracture patterns were investigated in the second phase of this study. First, the ITZ cracking behavior for the two types of concrete was quantified using the SEM capable of acquiring micro-scale level images. Secondly, a visualization of the coarse aggregate distribution in the specimens and their fracture pattern were performed by employing the X-ray Computed Tomography (CT) system [2]. In addition, a comparison of the number of air voids in the NC and SCC was conducted on six rectangular samples with the dimensions of 70 70 12 mm. The samples were cut from concrete cylinders at 0.3, 0.45, and 0.6 water-cement ratios and analyzed under a digital stereo-zoom microscope.

water-cement ratios of 0.3, 0.4, and 0.6 after 60 days of curing. The SEM was used to capture pictures of each w/c ratio sample at different ITZ locations. Smaller crack widths within their physical interface were observed from the acquired SEM photomicrographs for SCC samples compared the NC samples as showed in Figure 2. This finding indicated that a better aggregate-paste bonding was achieved in the

The use of silica fume led to a lower porosity and less growth of calcium hydroxide in the ITZ resulting in an increase in tensile and compressive strength of the SCC when compared to NC. Typically, silica fume has a "filler effect" in the concrete structure that reduces internal bleeding in the fresh concrete while enhancing the aggregate-paste bond strength [12]. It also renders its structure more homogeneous due to a reduction of the large pores in the ITZ. The split tensile strength tests revealed a larger number of broken aggregate particles in SCC than in NC when the fractured surfaces of the concrete samples were inspected. Furthermore, the strength of the ITZ was also enhanced by the addition of silica fume, as some of the aggregate fracture occurred at the physical interface instead within the

Table 2 shows the specimens' cracks widths from the images acquired at each water/cement ratio. The SEM image analysis indicated that a tendency for the cracks to shrink existed for lower w/cm ratios. However, a certain relationship between the interface crack widths and the water-cement ratios for either type of concrete was not found. Establishing a quantitative relationship between the crack

w/c ratio NC width range (µm) SCC width range (µm) 0.3 0.62–1.75 0.01–0.75 0.4 0.63–5.63 0.01–1.12 0.6 1.23–3.38 0.01–0.61

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

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

width and strength, if any, has to be further investigated.

Interfaces crack widths for normal and self-consolidating concretes.

2.3 Analysis of fracture patterns

SCC specimens did not exhibit any segregation.

transition zone for SCC compared to NC.

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

Concrete Microstructure Characterization and Performance

transition zone.

Table 2.

33

To determine the characteristics of the interfaces between aggregates and paste for both SCC and NC, small samples of 25.5 mm diameter and 4 mm thick were obtained from untested cylinders of both types of concretes at three different

### Concrete Microstructure Characterization and Performance DOI: http://dx.doi.org/10.5772/intechopen.90500

water-cement ratios of 0.3, 0.4, and 0.6 after 60 days of curing. The SEM was used to capture pictures of each w/c ratio sample at different ITZ locations. Smaller crack widths within their physical interface were observed from the acquired SEM photomicrographs for SCC samples compared the NC samples as showed in Figure 2. This finding indicated that a better aggregate-paste bonding was achieved in the transition zone for SCC compared to NC.

The use of silica fume led to a lower porosity and less growth of calcium hydroxide in the ITZ resulting in an increase in tensile and compressive strength of the SCC when compared to NC. Typically, silica fume has a "filler effect" in the concrete structure that reduces internal bleeding in the fresh concrete while enhancing the aggregate-paste bond strength [12]. It also renders its structure more homogeneous due to a reduction of the large pores in the ITZ. The split tensile strength tests revealed a larger number of broken aggregate particles in SCC than in NC when the fractured surfaces of the concrete samples were inspected. Furthermore, the strength of the ITZ was also enhanced by the addition of silica fume, as some of the aggregate fracture occurred at the physical interface instead within the transition zone.

Table 2 shows the specimens' cracks widths from the images acquired at each water/cement ratio. The SEM image analysis indicated that a tendency for the cracks to shrink existed for lower w/cm ratios. However, a certain relationship between the interface crack widths and the water-cement ratios for either type of concrete was not found. Establishing a quantitative relationship between the crack width and strength, if any, has to be further investigated.


Table 2.

2.2 Evaluating the bond between coarse aggregate and cement paste

under a digital stereo-zoom microscope.

Compressive Strength of Concrete

Figure 2.

32

The SCC was extensively researched in the past decades to find ways to enhance its performance [8, 9]. The general focus of those efforts was the improvement of the interface properties between paste and aggregates. That improvement has led to the manufacturing and use of higher strength and enhanced durability concretes [10, 11]. The physical interface between aggregate and cement paste and the aggregate fracture patterns were investigated in the second phase of this study. First, the ITZ cracking behavior for the two types of concrete was quantified using the SEM capable of acquiring micro-scale level images. Secondly, a visualization of the coarse aggregate distribution in the specimens and their fracture pattern were performed by employing the X-ray Computed Tomography (CT) system [2]. In addition, a comparison of the number of air voids in the NC and SCC was conducted on six rectangular samples with the dimensions of 70 70 12 mm. The samples were cut from concrete cylinders at 0.3, 0.45, and 0.6 water-cement ratios and analyzed

To determine the characteristics of the interfaces between aggregates and paste for both SCC and NC, small samples of 25.5 mm diameter and 4 mm thick were obtained from untested cylinders of both types of concretes at three different

Photomicrographs of (a) normal concrete and (b) self-consolidating concrete physical interfaces (w/c = 0.40).

Interfaces crack widths for normal and self-consolidating concretes.
