Concrete Technology Overview
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Questions and Answers

How does the type of cement influence the durability of concrete?

The type of cement affects the rate of hardening and overall durability by determining the chemical composition and hydration properties that influence strength.

What role does water play in both the fresh and hardened states of concrete?

In the fresh state, water impacts workability and cohesion, while in the hardened state, it is involved in hydration, contributing to dimensional changes like shrinkage and creep.

Describe how aggregate properties influence concrete performance.

The grading, shape, texture, type, and quality of aggregate affect the elastic properties of both fresh and hardened concrete, impacting strength and durability.

Explain how porosity affects the physical properties of concrete.

<p>Porosity in both fresh and hardened states influences workability, cohesion, as well as strength, stiffness, and density, particularly affecting resistance to freeze-thaw cycling.</p> Signup and view all the answers

What is the purpose of admixtures in concrete, and how do they modify its properties?

<p>Admixtures are small quantities added to cement to alter the properties of fresh and hardened concrete, improving workability, strength, or durability.</p> Signup and view all the answers

What are the advantages of using up to 70% cement replacement with PFA or GGBS in concrete?

<p>It reduces heat of hydration, lessens restrained thermal stress, and improves sulphate resistance.</p> Signup and view all the answers

How does microsilica addition affect workability in concrete mixtures?

<p>Microsilica increases workability for the same water content and enhances mobility for a given slump.</p> Signup and view all the answers

What problem arises from the use of PFA and GGBS additions in cement blends regarding setting times?

<p>They increase setting times by 1 to 4 hours compared to standard blends.</p> Signup and view all the answers

What is the effect of microsilica on the risk of plastic shrinkage cracks in concrete?

<p>Microsilica increases the tendency to develop plastic shrinkage cracks due to reduced bleed.</p> Signup and view all the answers

What is a significant disadvantage of using microsilica in concrete?

<p>It is typically higher in price compared to standard cement.</p> Signup and view all the answers

How does the addition of microsilica influence chloride ion diffusion in concrete?

<p>Microsilica may have questionable resistance to chloride ion diffusion, requiring increased cover depth.</p> Signup and view all the answers

What does the specification BS EN 197-145 cover in relation to cement?

<p>It defines and specifies 27 distinct common cement products and their constituents.</p> Signup and view all the answers

What is a consequence of reduced alkali-silica reaction in concrete with microsilica?

<p>It leads to improved durability by minimizing expansive reactions.</p> Signup and view all the answers

What was the significant advancement John Smeaton made in the production of Portland cement?

<p>He added clay to limestone before heating, creating a hydraulic cement.</p> Signup and view all the answers

Describe the main components of Portland cement clinker as defined in 2002.

<p>It consists of at least two-thirds calcium silicates with the remainder being aluminum and iron clinker phases and other compounds.</p> Signup and view all the answers

What was the purpose of lightweight aggregates in concrete construction?

<p>They were introduced to reduce the self-weight of arch and dome constructions.</p> Signup and view all the answers

Who patented reinforced concrete and what was it initially used for?

<p>William Wilkinson patented reinforced concrete, which was initially used for manufacturing boats and plant tubs.</p> Signup and view all the answers

What is the pozzolanic reaction and its significance in cement blends?

<p>It involves the reaction of Portland cement with reactive silica materials for improved properties.</p> Signup and view all the answers

Explain the composite nature of concrete.

<p>Concrete is a composite of coarse aggregate bound by cement-based mortar made of fine aggregate and cement paste.</p> Signup and view all the answers

What was the significance of the main London sewage system in the context of new cements?

<p>It marked a significant application of the 'new' cements in large-scale civil engineering projects.</p> Signup and view all the answers

What was the role of asbestos-cement when it became commercially available?

<p>Asbestos-cement provided increased resistance to environmental factors and structural durability.</p> Signup and view all the answers

What is GGBS and how does it differ from PFA in terms of cementitious properties?

<p>GGBS is a semi-cementitious material that can set on its own, unlike PFA which is not cementitious on its own.</p> Signup and view all the answers

What is the typical blending percentage of GGBS with Portland cement and why is it important?

<p>The typical blending percentage of GGBS with Portland cement is between 10-90%. This blending releases hydroxide and sulfate ions that accelerate strength gain.</p> Signup and view all the answers

What are the initial set time requirements for cement strength classes 42.5 and 52.5?

<p>The initial set time should be greater than 60 minutes for strength class 42.5 and greater than 45 minutes for strength class 52.5.</p> Signup and view all the answers

Describe the origin and characteristics of microsilica.

<p>Microsilica is a by-product from the reduction of high purity quartz in submerged electric arc furnaces and has a very small particle size of 0.1 micron.</p> Signup and view all the answers

What is the relative density of microsilica compared to Portland cement?

<p>The relative density of microsilica is 2.2, whereas Portland cement has a relative density of 3.1.</p> Signup and view all the answers

How does the particle size of microsilica contribute to its performance in cement blends?

<p>The ultra-fine particle size of microsilica provides nucleation sites for Ca(OH)2 and CSH hydrates, enhancing the reactivity.</p> Signup and view all the answers

What advantages does the addition of PFA and GGBS offer in concrete mixtures?

<p>The addition of PFA and GGBS can increase the volume of fine powder, leading to greater cohesiveness and reduced bleeding.</p> Signup and view all the answers

Explain the role of amorphous silicon in the effectiveness of microsilica as a pozzolanic material.

<p>The high proportion of amorphous silicon in microsilica enhances its reactivity and ability to react with Ca(OH)2 to form additional CSH.</p> Signup and view all the answers

What is the average production cost range of aggregates per tonne?

<p>The average production cost range of aggregates is £5-10 per tonne.</p> Signup and view all the answers

What is the approximate range of tonnes per annum for land-based sand and gravel?

<p>The approximate range for land-based sand and gravel is 68-80 x 10^6 tonnes per annum.</p> Signup and view all the answers

What standard covers aggregates for concrete in the UK?

<p>The standard that covers aggregates for concrete in the UK is BS EN 12620: 2002.</p> Signup and view all the answers

Why are admixtures added to concrete during mixing?

<p>Admixtures are added to modify one or more properties of the fresh or hardened concrete.</p> Signup and view all the answers

What are workability aids and why are they used in concrete?

<p>Workability aids, also known as water-reducing admixtures, are used to improve the workability of concrete.</p> Signup and view all the answers

What is the primary issue that transportation costs create for the aggregate supply chain?

<p>Transportation costs can significantly increase the final price of aggregates, affecting overall supply.</p> Signup and view all the answers

List two physical and mechanical requirements for aggregates according to BS EN 12620.

<p>Durability (thermal and weathering) characteristics and chemical requirements are two of the requirements.</p> Signup and view all the answers

What is the role of trial mixes in the evaluation of admixtures?

<p>Trial mixes are used to evaluate admixtures with similar cements and aggregates to ensure compatibility.</p> Signup and view all the answers

What characterizes a concrete mix with low workability?

<p>A low workability mix requires a large amount of external work or vibration to achieve full compaction.</p> Signup and view all the answers

How does the presence of macro-voids affect the compressive strength of hardened concrete?

<p>The presence of macro-voids significantly reduces the compressive strength of hardened concrete.</p> Signup and view all the answers

Explain the relationship between water content and workability in concrete.

<p>Workability generally increases with an increase in water content in the concrete mix.</p> Signup and view all the answers

What is the significance of achieving full compaction in concrete?

<p>Achieving full compaction is important to ensure maximum potential strength and reduce voids in the hardened concrete.</p> Signup and view all the answers

How does cement content affect the workability of concrete for a given water-to-cement (W/C) ratio?

<p>For a given W/C ratio, an increase in cement content results in increased workability.</p> Signup and view all the answers

What trade-off must be considered when compacting concrete with vibration?

<p>The trade-off involves balancing the time and effort required to vibrate the concrete against the need for full compaction.</p> Signup and view all the answers

What effect does increased internal friction have on the workability of concrete?

<p>Increased internal friction between individual particles reduces workability, making compaction more difficult.</p> Signup and view all the answers

Why is there no single measurement technique to quantify workability in concrete?

<p>There is no single measurement technique because workability is influenced by various factors that cannot easily be quantified.</p> Signup and view all the answers

Flashcards

Water in Concrete

The amount of water affects the workability, cohesion, and setting time of concrete. It also plays a role in shrinkage and creep in the hardened state.

Cement Paste

A thixotropic material that binds the concrete together. In its fresh state, it allows for flow (like a pudding) and in its hardened state, it becomes a strong, durable material.

Aggregate in Concrete

The inert material in concrete that contributes to strength and stability. Its size, shape, and texture affect the concrete's properties.

Porosity in Concrete

The amount of air voids, capillary pores, and gel pores in the hardened concrete, influenced by the cement paste hydration. It affects strength, stiffness, and freeze-thaw resistance.

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Admixtures in Concrete

Small quantities of chemicals added to modify the properties of fresh and hardened concrete. They can improve workability, strength, or durability.

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Pozzolanic Reaction

A reaction involving silica compounds (e.g., volcanic ash, fly ash) that strengthens cement by forming calcium silicate hydrates, a crucial element in modern cement blends.

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Hydraulic Cement

A type of cement produced by heating limestone and clay, resulting in a material capable of hardening underwater. It revolutionized construction, enabling more durable structures like the Eddystone Lighthouse.

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Clinker

A crucial component of Portland cement, formed from calcium silicates, comprising at least two-thirds of its mass.

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Reinforced Concrete

A concrete type reinforced with steel, significantly increasing its tensile strength and allowing for more complex shapes.

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Concrete

A construction material that combines coarse aggregate with cement-based mortar. This mortar, in turn, consists of fine aggregate bound by cement paste.

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Prestressed Concrete

A process involving applying tension to concrete before it hardens, creating a material with enhanced compressive strength and reduced cracking.

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Asbestos-cement

A construction material made by combining cement and asbestos fibers, known for its fire resistance and durability.

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Portland Cement

A type of cement widely used in modern construction, named after the Isle of Portland where it was first produced. It consists of a mixture of calcium silicates and other compounds, offering high strength and versatility.

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What is GGBS?

A finely ground powder created by drying and grinding a specific material. It's partially cementitious, meaning it can set on its own, but is often blended with Portland cement to accelerate strength gain.

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What is PFA?

A dust that remains after burning coal, often used as a partial replacement for Portland cement in concrete mixtures.

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What are the key components of GGBS?

GGBS contains a high percentage of silica dioxide (SiO2) and calcium oxide (CaO).

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What are the key components of PFA?

PFA has a high proportion of silica dioxide (SiO2) and aluminum oxide (Al2O3).

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What is microsilica?

Microsilica, also known as silica fume, is a by-product of silicon and ferrosilicon production, characterized by a very fine particle size and high silica content.

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How does microsilica influence cement hardening?

Microsilica acts as a nucleation site for the formation of calcium hydroxide (Ca(OH)2) and calcium silicate hydrate (CSH), which are essential for cement hardening.

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What is initial set?

The transition of concrete from a semi-fluid to a rigid state, after which compaction is no longer possible without causing damage to the concrete.

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What is relative density?

The density of a material relative to the density of water.

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Longer setting times with PFA or GGBS

The use of fly ash (PFA) or ground granulated blast-furnace slag (GGBS) in concrete can lead to longer setting times (1-4 hours longer).

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Longer curing times with PFA or GGBS

Concrete with PFA or GGBS requires longer curing times to limit carbonation, but the final carbonation depth remains similar to regular concrete.

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Winter concreting considerations with PFA or GGBS

Concrete with PFA or GGBS needs more care during winter concreting due to reduced heat generation during the setting process.

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Slower strength gain with PFA or GGBS

Concrete with PFA or GGBS shows a slower rate of strength gain, but can achieve higher final strengths if curing is continued for an extended period.

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Challenges with microsilica

Using microsilica in concrete can lead to mixing, handling, and dispersing problems that require more expertise and care.

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Microsilica cost

Microsilica is more expensive than regular cement.

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Plastic shrinkage cracks with microsilica

Microsilica can increase the tendency for plastic shrinkage cracks to develop in concrete due to a reduced bleed.

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Strength regression with microsilica

At very high concrete strengths, there are concerns about strength regression when using microsilica.

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What are admixtures?

Materials added to concrete during mixing to change its properties (fresh or hardened).

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What do workability aids do?

These admixtures make concrete easier to work with.

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What are water reducing admixtures?

They reduce the amount of water needed to make concrete flow well.

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What are 'flocs' in concrete?

Large clumps of cement particles that form when water is added.

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What is BS EN 12620:2002?

The standard that covers aggregates used in concrete.

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What are aggregates?

Materials that are used in concrete but don't chemically react.

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What is particle density?

The weight of aggregates per cubic meter.

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What are the main limitations for using aggregates?

The main source of limitations for using aggregate.

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Workability of Concrete

The ease with which air trapped in fresh concrete can be removed. It's influenced by the internal work needed to overcome friction between particles for full compaction.

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High Workability Concrete

High workability concrete requires less effort to compact. This means it's easier to move and shape during construction.

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Low Workability Concrete

Low workability concrete requires more effort to compact, meaning it's more difficult to move and shape during construction.

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How Voids Affect Strength

The strength of concrete decreases if there are large air voids or gaps. Full compaction helps create a strong concrete.

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Water Content and Workability

Adding more water generally makes concrete easier to work with because it reduces friction between particles.

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Cement Content and Workability

Increasing the amount of cement (for the same amount of water) increases the workability of concrete.

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Study Notes

Concrete Technology

  • Concrete uses burnt rocks (like limestone or gypsum) as inorganic bonding agents.
  • Ancient civilizations (Egyptians, Mycenaean, Minoan) used lime mortars extensively.
  • Lime mortars involve heating calcium carbonate rocks to form calcium oxide (quicklime).
  • Quicklime reacts with water to create slaked lime (hydrated calcium hydroxide).
  • Slaked lime binds sand and aggregates to form mortar which hardens through CO2 reaction.
  • This process is slow, best at 50-70% relative humidity. Cannot harden underwater.
  • Roman concrete (50BC-500AD) used finely ground reactive silica (pozzolana).
  • Roman concrete was a breakthrough for large marine structures like the Colosseum and Pantheon.
  • Innovations included air entrainment using blood, waterproof polymer coatings from figs, and lightweight aggregates.
  • Saxon concrete (500AD-1100AD), e.g., Reading Abbey (1121AD)
  • John Smeaton (1756) invented Portland cement adding clay to limestone before heating.
  • James Parker (1796) patented 'Roman' cements (30% lime, 25% silica, 30% carbonic acid).
  • Joseph Aspdin (1824) patented the process for Portland cement production, often used today.

Basic reactions of lime mortar technology

  • Key reaction: Limestone or chalk heated to 1000°C produces calcium oxide (quicklime).
  • Quicklime reacts with water to form calcium hydroxide (slaked lime).
  • Calcium hydroxide reacts with atmospheric carbon dioxide to form calcium carbonate, which is the hardening process.
  • The CO2 reaction process is exothermic (releases heat).

Concrete as a composite material

  • Concrete is a multi-level composite:

  • Coarse aggregate (gravel, rock.)

  • Fine aggregate (sand.)

  • Cement paste

  • Cement paste is Portland cement mixed with water and other cement replacements.

  • The particles within concrete range in size from centimeters to angstroms.

  • The grading and proportioning of constituents (aggregate, cement, and water) are critical to concrete properties.

Portland cement

  • Portland cement clinker consists primarily of two thirds calcium silicates and remainders are aluminum and iron containing phases with other compounds.
  • The ratio of CaO to SiO2 should be at least 2.0
  • Magnesium oxide (MgO) content should not exceed 5.0 % by mass

Properties required for concrete

  • Fresh state: Cohesive, Workable, ease of placing, ease of compacting.
  • Hardened state: strength; compression, tension, etc.; dimensional stability; durability.

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Explore the fascinating history and science of concrete technology. This quiz covers ancient uses of lime mortars, Roman advancements in concrete, and key innovations that shaped the construction industry. Delve into the evolution of materials and techniques that made iconic structures possible.

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