Structure of Concrete: Understanding the Three-Phase System for Enhanced Performance

To truly master concrete design and construction, it's essential to look beyond its outward appearance and delve into its microscopic world. The structure of concrete, defined by the shape, size, amount, and distribution of its various components, is complex yet fascinating. Far from being a homogeneous material, concrete is best understood as a three-phase system, each phase playing a distinct and crucial role in its overall performance.

This blog post will break down these three fundamental phases: the aggregate phase, the binding medium (hydrated cement paste) phase, and the often-overlooked but vital transition zone. By understanding the intricacies of each, we can appreciate how these elements interact to produce the versatile and widely used construction material we rely on.

Concrete as a Three-Phase System

Concrete is primarily composed of the following three interdependent phases:

1. Aggregate Phase: Consisting of aggregates of varying sizes.
2. Binding Medium Phase: The cement paste or mortar phase.
3. Transition Zone Phase: An interfacial zone that exists between the aggregate and the binding medium phase.

Structure of the Aggregate Phase

The aggregate phase forms the bulk of the concrete mix, typically occupying about 60-70% of the total volume. While its chemical composition is less critical, its physical characteristics significantly influence key concrete properties such as unit weight, elastic modulus, and dimensional stability. Aggregate is generally considered the strongest phase of concrete.

The shape and size of aggregate greatly affect concrete strength. Larger aggregates, for instance, can lead to a higher tendency for water films to accumulate very next to their surface, weakening the cement paste-aggregate transition zone. This phenomenon is known as internal bleeding.

• Bleeding: This is a phenomenon in freshly placed concrete where free water in the concrete mix rises upward to the surface due to the settlement of heavier solid particles by gravity.
• Internal Bleeding: Sometimes, bleeding water does not reach the concrete surface but gets trapped on the underside of coarse aggregate or reinforcement bars. This trapped water forms voids and weakens the bond at these interfaces.

Structure of the Hydrated Cement Paste (HCP) Phase

The binding medium phase, specifically the hydrated cement paste (HCP), is arguably the most significant phase of concrete. It profoundly influences the overall behavior of hardened concrete, affecting critical properties like strength, durability, creep, shrinkage, and elastic characteristics.

Key Components of Hydrated Cement Paste:

• Calcium Silicate Hydrates (C-S-H):
  • Generally known as C-S-H gel or tobermorite gel.
  • This is the primary component responsible for cement's cementitious property.
  • It occupies the major volume of solid in HCP (about 50-60%).
  • Its strength primarily derives from van der Waals forces.
  • The exact structure of C-S-H is complex and still being researched.
• Calcium Hydroxide (Ca(OH)$_2$):
  • Also known as portlandite.
  • Constitutes about 20-25% of the total volume of HCP.
  • It has a lower strength contribution compared to C-S-H.
  • Tends to form large crystals with a distinctive hexagonal prism morphology.
• Calcium Sulpho-aluminates:
  • These include Ettringite and Monosulphate.
  • Constitute about 15-20% of the total solid volume of HCP.
  • The presence of Monosulphate hydrates can make concrete susceptible to sulfate attack.
• Unhydrated Clinker Grains:
  • The amount depends on the size of cement particles and the degree of hydration.
  • Smaller particles hydrate faster, while larger particles decrease in size over time.
  • Some unhydrated clinker grains may persist in the HCP microstructure even after long hydration periods.

Voids in Hydrated Cement Paste:

HCP inherently possesses a certain degree of porosity due to various types of voids:

• Interlayer Space in C-S-H:
  • Accounts for about 28% porosity in solid C-S-H.
  • These spaces vary from approximately 5Å to 25Å.
  • They are generally too small to significantly impact concrete strength and durability.
• Capillary Voids:
  • These are spaces not filled by the solid components of HCP, irregular in shape.
  • Sizes can vary from 10μm to 50μm, and sometimes beyond.
  • Their dimensions and quantity largely depend on the water-cement (w/c) ratio and the degree of hydration.
  • Voids larger than 50μm are considered macropores and are detrimental to strength and permeability.
  • Voids smaller than 50μm are called micropores and are more responsible for shrinkage and creep.
• Air Voids:
  • Generally spherical in shape.
  • Entrained air voids are typically 50μm to 200μm in size, intentionally introduced to improve freeze-thaw resistance.
  • Entrapped air voids are larger, sometimes up to 3mm, and are unintentionally present.
  • Large air voids can adversely affect concrete strength and impermeability.

Strength of Hydrated Cement Paste (HCP):

• Strength in solid HCP products is attributed to van der Waals forces of attraction.
• The small crystals of C-S-H possess enormous surface area and adhesive capability, allowing them to strongly adhere to each other and to other solids like calcium hydroxide, unhydrated cement clinker grains, and aggregates.
• The strength of HCP is inversely proportional to its porosity.
• The quantity of voids in HCP depends on the extent of mixing and the degree of hydration.

Dimensional Stability of HCP (Effects of Relative Humidity, Stress, and Degree of Saturation):

• As the degree of saturation increases, HCP becomes less stable.
• When directly exposed to environmental humidity, HCP begins to lose water and shrinks.
• When Relative Humidity (RH) drops below 100%, free water in larger cavities (greater than 50nm) starts to escape, causing slight shrinkage.
• Factors responsible for drying shrinkage are also largely responsible for creep of HCP.
• Creep strain can occur even at 100% relative humidity.

Durability of Hydrated Cement Paste (HCP):

• HCP is alkaline, making it susceptible to degradation when exposed to acids or acidic water.
• Permeability is a prime factor influencing durability.
• Both strength and permeability are closely related to the capillary porosity or the solid-to-space ratio within the HCP.

Transition Zone Phase

The transition zone (also known as the interfacial transition zone, or ITZ) is a thin layer, typically ranging from 10 to 50μm, that forms between the aggregate particles and the hydrated cement paste. This zone is critical because its formation can lead to poor bonding between HCP and aggregate particles, partly due to the formation of thin water films around the aggregate during mixing.

Due to a higher water-cement ratio near the aggregate surface, the transition zone is generally more porous than the bulk cement paste. This increased porosity means the ITZ is often the weakest zone in concrete, despite occupying a relatively small volume. Micro-cracking frequently forms within the transition zone even before any external load is applied. Consequently, cement paste or mortar, provided they have the same w/c ratio and are tested under identical conditions, will almost always be stronger than concrete.

The permeability of concrete is also higher than that of pure cement paste. Interestingly, while the individual constituents of concrete (aggregate and HCP) may exhibit linear behavior under load, concrete itself often shows non-linear behavior, largely influenced by the characteristics of this vital transition zone.

Conclusion

Understanding concrete as a three-phase system—comprising the robust aggregate phase, the strength-governing hydrated cement paste, and the crucial, often vulnerable, transition zone—is fundamental for any structural engineer. The intricate interactions within and between these phases dictate concrete's performance in terms of strength, durability, dimensional stability, and overall behavior under stress. By appreciating these microscopic details, we can better design, produce, and utilize concrete to build resilient and long-lasting structures.

Frequently Asked Questions

What are the three main phases of concrete?

The three main phases of concrete are the aggregate phase, the binding medium (hydrated cement paste) phase, and the transition zone phase.

What percentage of concrete volume is typically occupied by the aggregate phase?

The aggregate phase typically occupies about 60-70% of the total volume of concrete mix.

Why are the physical characteristics of aggregate more important than its chemical composition?

Physical characteristics like shape and size are more important because they directly influence concrete’s unit weight, elastic modulus, and dimensional stability, which are based on physical properties.

What is internal bleeding in concrete?

Internal bleeding occurs when bleeding water, instead of rising to the surface, gets trapped on the underside of coarse aggregate or reinforcement bars, leading to a weakened bond in the transition zone.

Why is the hydrated cement paste (HCP) phase considered the most significant?

HCP is most significant because it influences many strength and durability-governing properties of hardened concrete, including strength, durability, creep, shrinkage, and elastic properties.

What is C-S-H gel, and why is it important?

C-S-H gel (Calcium Silicate Hydrates) is the main product of cement hydration. It’s crucial because it’s primarily responsible for the cementitious property that binds concrete components together and contributes significantly to its strength.

How does porosity affect the strength of hydrated cement paste?

The strength of HCP is inversely proportional to its porosity. Higher porosity (more voids) generally leads to lower strength.

What are capillary voids, and what influences their size and quantity?

Capillary voids are irregular spaces in HCP not filled by solid components. Their dimensions and quantity largely depend on the water-cement ratio (w/c) and the degree of hydration.

What is the significance of the transition zone in concrete?

The transition zone is the interface between aggregate and HCP. It’s often the weakest zone in concrete due to higher porosity, leading to poor bonding and micro-cracking, significantly influencing overall concrete strength and durability.

How does the water-cement ratio affect the transition zone?

A higher water-cement ratio tends to increase the porosity of the transition zone, making it weaker and more permeable.

Why is concrete generally weaker than its cement paste or mortar, given the same w/c ratio?

Concrete is typically weaker because of the presence of the transition zone around aggregates, which is inherently more porous and weaker than the bulk cement paste or mortar itself.

What is the role of calcium hydroxide in HCP?

Calcium hydroxide (Ca(OH)$_2$) contributes to the alkalinity of HCP but offers less strength contribution compared to C-S-H gel. It tends to form large, hexagonal crystals.

How do air voids impact concrete performance?

Both entrained and entrapped air voids can adversely affect concrete strength and impermeability, though entrained air is intentionally added for freeze-thaw resistance in controlled amounts.

What is the relationship between drying shrinkage and creep in HCP?

The factors or mechanisms responsible for drying shrinkage are also largely responsible for creep in HCP. Both relate to the movement and loss of water within the porous structure.

Why is concrete’s permeability higher than cement paste’s?

Concrete’s permeability is higher because the transition zone, a permeable layer around aggregates, provides continuous pathways for moisture ingress, which are not present in bulk cement paste.