Try for free
DiscoverMobile appPricingFor schoolsHelpLogin
← Back to flashcardsQuick Flashcards

Aggregates Lecture Flashcards

Learn the fundamentals of concrete aggregates, including types, properties, grading, moisture conditions, and their impact on concrete strength and durability.

29 cards~10 min
Aggregates Lecture Flashcards — Qwi
1 / 29

All flashcards in this set

1What are concrete aggregates?
Answer

Materials like sand, gravel, crushed stone, slag, or recycled concrete used in construction

They are the mineral components that make up most of the concrete volume and affect its properties.
2Aggregates constitute less than 50% by volume of concrete.
Answer

False

They generally constitute 60 to 80 % by volume of the whole.
3Difference between aggregates and cement in cost?
Answer

Aggregates cheaper | Cement more expensive

4What are the three main classifications of concrete aggregates by weight?
Answer

Heavyweight, normal-weight, and lightweight aggregates

These categories are based on the density of the aggregate material.
5Heavyweight aggregates produce concrete with a density range of {{4000 – 8500 kg/m³}}.
Answer

4000 – 8500 kg/m³

6Lightweight aggregates are typically made from volcanic rocks such as pumice.
Answer

True

7What sieve size must fine aggregates pass through?
Answer

4.75 mm sieve

Fine aggregates consist of particles that mostly pass this sieve size.
8Coarse aggregates are particles greater than {{0.19 inch (4.75 mm)}}.
Answer

0.19 inch (4.75 mm)

9Size range of coarse aggregate vs fine aggregate?
Answer

Coarse: >4.75 mm | Fine: ≤4.75 mm

10Which shape characteristic measures how nearly equal the three principal axes of a particle are?
Answer

Sphericity

It quantifies how close a particle is to being a perfect sphere.
11Roundness describes the sharpness of particle edges.
Answer

False

Roundness describes the outline of the particle; angularity measures edge sharpness.
12Effect of angular vs rounded particles on concrete strength?
Answer

Angular increase strength | Rounded decrease strength

13How does a rougher surface texture affect the bond with cement paste?
Answer

It enhances bond, leading to higher compressive strength

Rough surfaces provide better mechanical interlock with the cement matrix.
14Surface texture classification depends on the degree of {{polished or dull}}.
Answer

polished or dull

15Smooth aggregate surfaces improve compressive strength more than rough surfaces.
Answer

False

Rough surfaces provide stronger bond and higher compressive strength.
16How is the specific gravity of an aggregate defined?
Answer

Mass of aggregate in air divided by mass of equal volume of water

It is a dimensionless ratio indicating relative density.
17Most normal-weight aggregates have an absolute SG between {{2.4 and 2.9}}.
Answer

2.4 and 2.9

18Difference between absolute SG and apparent SG?
Answer

Absolute: solid volume only | Apparent: includes external pores

19What does SSD stand for in aggregate moisture condition?
Answer

Saturated surface dry

All pores are filled with water while the surface remains dry.
20In the bone-dry (OD) condition, all pores are filled with water.
Answer

False

Bone-dry means all moisture is removed; pores are empty.
21Normal-weight aggregate absorption usually ranges from {{0.5 to 4%}} by weight.
Answer

0.5 to 4%

22What does grading refer to in aggregates?
Answer

Particle size distribution

It describes how aggregate sizes are spread across a range.
23Typical fineness modulus of aggregate ranges between {{2.0 and 4.0}}.
Answer

2.0 and 4.0

24Characteristics of WGA vs PGA grading?
Answer

WGA: evenly spans sizes, S-shaped curve | PGA: uniform sizes, uniformly graded curve

25What are the two forms of alkali-aggregate reactivity?
Answer

Alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR)

Both can cause expansion and cracking in concrete.
26ASR occurs only in cold, dry environments.
Answer

False

It especially occurs in warm‑moist environments.
27Name two deleterious substances that can affect concrete durability.
Answer

Coal (organic impurity) and clay lumps

Both can cause deterioration, increased water demand, or popouts.
28Materials finer than No. 200 sieve improve bonding without increasing water demand.
Answer

False

Finer materials affect bonding and increase water requirement.
29Deleterious substances can cause {{popouts}} in hardened concrete.
Answer

popouts

Introduction to Concrete Aggregates

Concrete aggregates are the mineral components that make up most of the concrete volume. They include sand, gravel, crushed stone, slag, or recycled concrete. These raw materials are mixed with cement and water to form the concrete matrix.

Proportion and Cost

Aggregates constitute the majority of concrete by volume, typically 60 to 80 %. This high proportion makes aggregates much cheaper than cement, which is the more expensive component in large‑scale concrete production.

Classification by Weight

Aggregates are classified into three main weight categories based on density:

  • Heavyweight aggregates – density 4000 – 8500 kg/m³
  • Normal‑weight aggregates – density around 2400 – 2900 kg/m³
  • Lightweight aggregates – often made from volcanic rocks such as pumice

Size Classification

Size determines whether an aggregate is considered fine or coarse.

Fine Aggregates

Fine aggregates pass through a 4.75 mm (No. 200) sieve. They consist mainly of sand particles and are used to fill voids between coarse particles.

Coarse Aggregates

Coarse aggregates are particles larger than 4.75 mm (0.19 inch). Their size range extends upward from this lower limit, while fine aggregates are limited to sizes ≤4.75 mm.

Shape and Surface Texture

Two key shape characteristics affect concrete performance:

  • Sphericity – measures how nearly equal the three principal axes of a particle are; a perfect sphere has maximum sphericity.
  • Roundness – describes the outline of the particle. It is not the same as angularity, which measures the sharpness of edges.

Roundness is often confused with angularity; roundness does not describe edge sharpness. Angular particles increase concrete strength because their irregular surfaces provide better mechanical interlock with the cement paste. Conversely, rounded particles tend to reduce strength.

Surface texture also influences bond strength. Rougher textures enhance the bond with cement paste, leading to higher compressive strength, whereas smoother (polished or dull) surfaces provide weaker interlock.

Specific Gravity and Absorption

The specific gravity of an aggregate is the ratio of the mass of the aggregate in air to the mass of an equal volume of water. Normal‑weight aggregates typically have an absolute specific gravity between 2.4 and 2.9.

Two types of specific gravity are distinguished:

  • Absolute specific gravity – considers only the solid volume of the aggregate.
  • Apparent specific gravity – includes the external pores of the aggregate.

Aggregate absorption, the amount of water the aggregate can take up, usually ranges from 0.5 % to 4 % by weight for normal‑weight aggregates.

Moisture Conditions

Moisture condition of aggregates is critical for mix design:

  • Saturated surface dry (SSD) – all pores are filled with water, but the surface remains dry. In this condition, the aggregate does not contribute additional water to the mix.
  • Bone‑dry (OD) – all moisture is removed; pores are empty. This condition does not add water to the concrete.

Grading and Fineness Modulus

Grading refers to the particle size distribution of aggregates. Proper grading ensures a well‑packed mix with minimal voids.

The fineness modulus (FM) is a single number that represents the average size of the aggregate particles. Typical FM values for concrete aggregates range between 2.0 and 4.0.

Two common grading types are:

  • Well‑graded aggregate (WGA) – particles evenly span a range of sizes, producing an S‑shaped curve on a gradation chart.
  • Poorly‑graded aggregate (PGA) – particles cluster around a single size, resulting in a uniformly graded curve.

Alkali‑Aggregate Reactivity

Aggregates can participate in two forms of alkali‑aggregate reactions, both of which may cause expansion and cracking in concrete:

  • Alkali‑silica reaction (ASR)
  • Alkali‑carbonate reaction (ACR)

ASR is especially prevalent in warm, moist environments; it does not occur only in cold, dry conditions.

Deleterious Substances and Their Effects

Certain impurities in aggregates are harmful to concrete durability. Examples include:

  • Coal (organic impurity)
  • Clay lumps

These substances can cause popouts in hardened concrete, increase water demand, and lead to deterioration.

Materials finer than the No. 200 sieve do improve bonding, but they also increase water demand, contrary to the notion that they improve bonding without affecting water content.

Summary

Understanding the type, size, shape, texture, specific gravity, absorption, moisture condition, grading, and chemical reactivity of aggregates is essential for designing durable, high‑strength concrete. Proper selection and handling of aggregates ensure optimal performance and longevity of concrete structures.