1. Introduction

Although the technology of producing concrete is relatively simple the microstructure of the resultant product is highly complex. Concrete microstructure is defined as the microscopical detailing of the concrete components from its macrostructure. To better understand the mechanisms that intrinsically control durability of Portland cement concrete (PCC) it is necessary to define and understand those factors affecting concrete microstructure. Typically, the microstructure is a consequence of both the concrete formulation and the processes taking place during mixing, placing and curing. Past studies have already established that deterioration originates in the concrete at the micro- or sub-microscopic level, i.e., in its microstructure [1, 2].

Several current studies on concrete failure indicated that an often major cause of variability of the properties and performance of the hardened concrete was the inadequate dispersion of cement paste in the fresh concrete [3]. In this way, harmful impurities that permeate or diffuse throughout the hardened concrete may initiate its deterioration due to a variation in the ability of the concrete to restrict their transport. Moreover, the cement particles tend to irregularly coagulate and cluster in the mix leading to uneven regions of dense and high porosity hardened paste because of poor dispersion and inhomogeneity during mixing and placing. This holds true even for high density microstructures resulting when using of low

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 microstructure and influence on performance [3].

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

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

> Fine Agg. (kg)

0.3 3 5.5 16.3 21.1 340 0 0.4 4 5.5 16.3 21.1 100 15 0.45 4.5 5.5 16.3 21.1 80 25 0.5 5 5.5 16.3 21.1 50 50 0.6 6 5.5 16.3 21.1 20 100

Coarse Agg. (kg)

HRWR (ml)

VMA (ml)

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.

conduct slump tests without incorporating any mineral admixtures.

Cement (kg)

Concrete Microstructure Characterization and Performance

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

with the testing equipment.

Water (kg)

Self-consolidating concrete mix design.

W/C ratio

Table 1.

Figure 1.

31

U-box schematic for testing SCC.

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 structural analysis.
