Chapter VII Structure of Hardened Concrete...Silica fume and particle packing Advanced Concrete...

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CIV415 CONCRETE TECHNOLOGY Chapter VII Structure of Hardened Concrete Assist.Prof.Dr. Mert Yücel YARDIMCI Spring, 2014/2015 Advanced Concrete Technology - Zongjun Li 1

Transcript of Chapter VII Structure of Hardened Concrete...Silica fume and particle packing Advanced Concrete...

CIV415 CONCRETE TECHNOLOGY

Chapter VII

Structure of Hardened Concrete

Assist.Prof.Dr. Mert Yücel YARDIMCI Spring, 2014/2015

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Structure Levels

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Understanding the behavior of cement-based systems, including traditional concrete, requires that one first understand its structure, especially its structure in the nanometer and micrometer scales. The objectives of this chapter are to understand the multiscale nature of concrete, to develop an appreciation of the structure of ordinary Portland cement concrete, and to outline an approach to microstructural engineering—methods of modifying the structure.

Structure Levels

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Structure Levels

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Structure Levels

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The technique used for examination in this level is backscatter electron detector scanning microscopy. The specimen is dried and potted in a low-viscosity epoxy preparation, which is then sliced with a diamond saw to get a plane surface. The slice is polished before the SEM test. Different gray levels in the figure are due to different electron backscatter coefficients of different chemical compositions. The brightest ones are unhydrated cement particles and the darkest areas are pores. Cement particles, capillary voids, and unhydrated cement particles, and the interface between coarse aggregates and matrix can be seen.

at 400x

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The high-magnification SEM level corresponds to the micrometer scale. The structure of concrete at this level is basically examined in an SEM using a secondary electron detector. The fracture surface has to be used in the secondary mode of an SEM. the aggregation of C–S–H, the plate-shaped CH The needle-shaped AFt (ettringite) can be clearly distinguished.

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At a very early hydration age, the ettringite (AFt) phase starts to form with consumption of sulfate, which is detected in the sample at 0.73 h.

Structure Levels

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STRUCTURE OF CONCRETE IN NANOMETER SCALE: C–S–H STRUCTURE

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The structure of calcium silicate hydrate (C–S–H) gel is considered at the nanometer scale.

Since C–S–H is the most important hydration product in cement-based materials—it takes up approximately 50–70% of the fully hydrated cement paste.

TRANSITION ZONE IN CONCRETE

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In the presence of an aggregate, the structure of the hydrated cement paste in the vicinity of large aggregate particles is usually different from the structure of the bulk paste or mortar in the system.

Herdened concrete is considered as three-phase material: • Aggregate • Bulk cement paste • Interfacial Transition Zone (ITZ)

between aggregate and the paste

• ITZ is a thin shell, typically 10 to 50 μm thick around the large aggregate.

• The ITZ is generally weaker than either of the two other phases of concrete.

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publish.illinois.edu

Significance of the transition zone

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The volume fraction of the transition zone in concrete is usually only a few percent, but its influence on concrete properties is far more than such a percentage. It is a fact that many concrete macroscopic properties are sourced in the transition zone.

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It has been discovered that at a given cement content, water/cement ratio, and age of hydration, both HCP and cement mortar are always stronger than the corresponding concrete. Also, the strength of concrete goes down as the coarse aggregate size is increased. The porous nature of the transition zone in concrete is responsible for the reduction of compressive strength as compared to the corresponding HCP and mortar

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The higher permeability coefficient of concrete can be attributed to its transition zone. Since the transition zone is more porous than the aggregate and bulk hydrated cement paste (HCP), water can more easily flow through transition zone, which results in a high permeability.

Influence of the transition zone on properties of concrete

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The transition zone is generally considered the weakest link of the concrete chain.

Influence of the transition zone on properties of concrete

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The transition zone is generally considered the weakest link of the concrete chain. It has a strength-limiting effect in concrete. Because of the presence of the transition zone, concrete fails at a considerably lower stress level than either of the two main components, as demonstrated in Figure 4-14.

Since it does not require high energy levels to extend the cracks already existing in the transition zone, even at 40 to 70% of the ultimate strength, higher incremental strains are obtained per unit of applied stress.

Influence of the transition zone on properties of concrete

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The structure of the transition zone, especially the volume of voids and microcracks present, has a great influence on the stiffness or the elastic modulus of concrete. In a composite material, the transition zone serves as a bridge between the two components: the bulk matrix and the coarse aggregate particles. Even when the individual components are of high stiffness, the stiffness of the composite may be low because of the broken bridge (i.e., voids and microcracks in the transition zone) that hinder stress transfer, as well as larger deformation occurrences due to the porous nature of the interface.

Influence of the transition zone on properties of concrete

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The characteristics of the transition zone also influence the durability of concrete. Prestressed and reinforced concrete elements often fail due to corrosion of the embedded steel. The rate of steel corrosion is greatly influenced by the permeability of concrete. The existence of relatively large numbers of pores and microcracks in the transition zone at the interface with steel and coarse aggregate make concrete more permeable than the corresponding hydrated cement paste or mortar. Subsequently, oxygen and moisture can penetrate into concrete more easily and lead to corrosion of the steel in the concrete.

MICROSTRUCTURAL ENGINEERING

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Developing microstructural systems with defined and prerequisite properties for concrete is called microstructural engineering.

The term “engineering” here, as elsewhere, first requires carefully designed actions with clear objectives in producing a better material.

Second, it needs a proper measurement system to measure the changes in the structure as feedback to be used to deliberately improve previous actions; hence, stronger, less permeable, and more durable cement-based materials can be finally produced.

Superplasticizer and dispersion in cement systems

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Adding water to normal cement produces a flocculated system.

If a sufficient dose of a commercial superplasticizer is added to another cement paste, otherwise identical with the first one, after dilution, it will be seen that no flocs are evident, and the cement grains settle individually.

Silica fume and particle packing

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Cement size varies from 2 to 80 μm, with a mean value of 18 to 20 μm. It appears that cement particles cannot form a dense packing because the water-filled pockets are roughly the same size as the cement particles that exist throughout the mass, as shown in Figure 4-17.

Silica fume and particle packing

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An admixture of much finer particles is needed to pack into the water-filled pockets among the cement grains. Silica fume (or microsilica) provides such particles.

The mean particle size of commercial silica fume is typically less than 0.1 μm, two magnitudes smaller than cement particles. A denser packing may be ensured when microsilica is added to ordinary cement paste.

Microsilica is an active and effective pozzolan; that is, it reacts readily with the calcium hydroxide produced by the cement around it to generate additional calcium silica hydrate.

Silica fume and particle packing

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Together with metalkaolin with an average particle size of 2 μm and GGBS of 8 μm, adding silica fume into a cement system can lead to an optimal size distribution and denser packing, as shown in Figure 4-17.

Transition zone improvement

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The methodology presented earlier using silica fume to modify the structure of concrete is also effective in improving the structure of the transition zone due to its packing and pozzolanic reaction effect. Bentur and Cohen (1987) prepared two sets of mortar, one was plain mortar and the other silica fume-bearing mortar.

The image analysis on the SEM photos of the transition zone of different pastes showed that there were clear distinctions between normal paste and silica fume-modified paste in the porosity distribution along the line from the surface of the aggregate, as shown in Figure 4-19.