2. Research methodology

The first part of this section addresses the development of microstructure and its control of performance with a focus on the normal (NC) and self-consolidating concrete (SCC). The second part details the damage caused by steel projectiles of different sizes impacting large concrete specimens at various speeds.

### 2.1 Concrete microstructure characterization and performance comparison

This research study had four objectives: (1) To investigate the bonding between coarse aggregates and cement paste; (2) To evaluate fresh SCC test methods – slump flow and U-tube; (3) To compare NC and SCC splitting tensile strength and compressive strength; and (4) To visualize the distribution of coarse aggregate in NC and SCC concrete specimens. The SEM imaging was employed to investigate the bonding between the cement paste and coarse aggregate of the two types of concrete by studying their interfacial transition zone (ITZ). The ITZ, which is mostly comprised of calcium hydroxide, is a narrow and fuzzy cement paste area surrounding the aggregate particles. Being a very porous region due to a high water content, the ITZ tends to increase with the aggregate size. Moreover, due to its weaker structure compared to the bulk paste in the concrete this transition zone directly affects the concrete properties, especially its strength and stiffness. The aggregate distribution throughout the specimens and the fracture patterns of the NC and SCC specimens tested for compressive strength were visualized using the

X-ray tomography imaging system. The standard compressive strength test method depicts typical fracture patterns indicative of the mix strength or existing problems with the testing equipment.

Additional information on materials and admixtures used for preparing the specimens can be found in Druta et al. [4]. Table 1 presents the mix proportions for casting the SCC specimens. The type I Portland cement was replaced by blast furnace slag (25%), fly ash (15%), and silica fume (5%). Similar mix proportions and equal amount batches were prepared for casting cylindrical NC samples and conduct slump tests without incorporating any mineral admixtures.


Table 1.

water/cement (w/c) ratios. The development of a dense homogeneous microstructure is also affected by the pattern packing of the cement particles and aggregate. Thus, the microstructure developed during the mixing, setting and hardening process will directly affect the resulting properties and performance of concrete. Microstructural development is also controlled by a combination of uniform dispersion of cement particles, mineral admixtures and aggregates along with cement hydration. Moreover, developments occurring at a micro- or sub-microscopic level in the concrete matrix influence aspects such as early or retarded setting, drying shrinkage, permeability, frost damage, excessive bleeding, and/or inadequate strength. Understanding the concrete behavior at these small scale levels is the initial and most important step toward achieving the means to control its micro-

Additionally, the examination of concrete microstructure either in transmitted or reflected energy is a valuable tool in the study of concrete microstructure. These methods can be used in inspecting various concrete specimens to determine potential defects or material characteristics. For example, water/cement ratios can be determined by comparing paste porosity with a series of carefully prepared reference samples. In this chapter, the physical interface between aggregate and cement paste and the aggregate fracture were studied. The Scanning Electron Microscope (SEM) capable of acquiring micro-scale level images was used to quantify the ITZ cracking for normal and self-consolidating concrete. The X-ray Computed Tomography (CT) system was employed to visualize the coarse aggregate distribution in the concrete specimens and their fracture pattern. SEM is a device that creates highly magnified 2-D images of structures to analyze their various components interactions and potential flaws using accelerated electrons. The X-ray CT technique uses high energy beams which penetrate samples of different thicknesses then stacks up the acquired images to reconstruct a 3-D model of the scanned sample for

The first part of this section addresses the development of microstructure and its control of performance with a focus on the normal (NC) and self-consolidating concrete (SCC). The second part details the damage caused by steel projectiles of

This research study had four objectives: (1) To investigate the bonding between

2.1 Concrete microstructure characterization and performance comparison

coarse aggregates and cement paste; (2) To evaluate fresh SCC test methods – slump flow and U-tube; (3) To compare NC and SCC splitting tensile strength and compressive strength; and (4) To visualize the distribution of coarse aggregate in NC and SCC concrete specimens. The SEM imaging was employed to investigate the bonding between the cement paste and coarse aggregate of the two types of concrete by studying their interfacial transition zone (ITZ). The ITZ, which is mostly comprised of calcium hydroxide, is a narrow and fuzzy cement paste area surrounding the aggregate particles. Being a very porous region due to a high water content, the ITZ tends to increase with the aggregate size. Moreover, due to its weaker structure compared to the bulk paste in the concrete this transition zone directly affects the concrete properties, especially its strength and stiffness. The aggregate distribution throughout the specimens and the fracture patterns of the NC and SCC specimens tested for compressive strength were visualized using the

different sizes impacting large concrete specimens at various speeds.

structure and influence on performance [3].

Compressive Strength of Concrete

structural analysis.

30

2. Research methodology

Self-consolidating concrete mix design.

Splitting tensile and compressive strength tests were performed on both types of concrete, whereas U-box and slump flow tests were conducted to evaluate the filling ability and the self-compactability of the SCC, respectively [5, 6]. The slump flow test, currently used for fresh SCC, is a good indicator of concrete consistency and ability to self-consolidate [6, 7]. For SCC, instead of measuring the drop in height of the fresh concrete the average of two perpendicular diameters (R1 and R2) of the spread concrete is determined. A good self-consolidation is achieved for a spread of 600 mm or larger, up to 800 mm, within a time period of 60 s [4]. Figure 1 illustrates the filling ability apparatus that comprises an approximately 700 mm tall tube with a round bottom divided into two equal sections by a middle wall incorporating a sliding gate. Once the left section is filled with concrete the sliding gate is lifted allowing the fresh concrete to flow freely in the right section through a specially designed grate provided with closely spaced rebars. The height levels of the concrete in both sections are measured, then followed by a height difference calculation. An adequate SCC filling and compactability is achieved if a difference of 30 mm or less between H1 and H2 is recorded.

Figure 1. U-box schematic for testing SCC.
