Aggregates for Concrete

Soils and Materials 1 Lecturer – Dr Stephanie Barnett

Burnaby 1.03

Characteristics of Aggregate

• Strength • Deformation • Toughness • Hardness • Volume stability • Chemical compatibility • Shape • Texture • Porosity (absorption) • Density • Grading (particle size distribution)

Characteristics of Aggregate Affect workability of concrete • Strength • Deformation • Toughness • Hardness • Volume stability • Chemical compatibility • Shape • Texture • Porosity (absorption) • Density • Grading (particle size distribution)

Characteristics of Aggregate Affect strength of concrete

• Strength • Deformation • Toughness • Hardness • Volume stability • Chemical compatibility • Shape • Texture • Porosity (absorption) • Density • Grading (particle size distribution)

Characteristics of Aggregate Affect durability of concrete • Strength • Deformation • Toughness • Hardness • Volume stability • Chemical compatibility • Shape • Texture • Porosity (absorption) • Density • Grading (particle size distribution)

Naturally occurring aggregate

• Sand and gravel from land or sea

• Rounded by attrition

• Require washing, grading and some crushing of larger particles

Photo by Stan Zurek

Crushed aggregate

• Larger rocks can be mechanically crushed to approximate size suitable for use in concrete

• Particles are often sharp and angular

• Visibly distinct from naturally occurring sand and gravel

Where?

Description of Shape Shape Description

Rounded Fully water worn or completely shaped by attrition.

Irregular Naturally irregular or partly shaped by attrition. Rounded edges.

Flaky Thin relative to other two dimensions.

Angular Well defined edges and roughly planar faces.

Elongated Length is considerably larger than other two faces. Usually angular particles.

Flaky and elongated Length is larger than the width and width is larger than the thickness.

Description of Texture

Texture Description Examples Glassy Conchoidal fracture. Black flint

Smooth Water worn, or smooth due to fracture of laminated or fine grained rock.

Gravels Slate

Granular More or less uniform rounded grains. Sandstone

Rough Fine or medium grained rock containing no easily visible grains or crystalline constituents.

Basalt Limestone

Crystalline Easily visible crystalline constituents. Granite Honeycombed Visible pores and cavities. Pumice

Photo shows conchoidal fracture (photo by Ji-Elle)

Density

Density of the aggregate will affect density of the concrete • Normal weight aggregates have density of 2600-2700 kg/m3

• Normal concrete densities are 2200-2400 kg/m3

• Applications for lightweight and heavyweight aggregates

Lightweight aggregate

• Reduced weight of structure

• Better thermal insulation

• Reduced density is due to voids within aggregate particles

Lightweight aggregate Pumice Expanded clay

Sintered pulverised fuel ash (LYTAG®)

Pantheon, Rome

St Pancras International Station, London

Photo by Przemyslaw Sakrajda

Strength versus density relationships for lightweight aggregate concrete

Heavyweight aggregates

High density concrete is most commonly required for radiation shielding

• Naturally occurring high density aggregates – Concrete densities of 3500-4500 kg/m3

• Lead shot

– Concrete densities of ~7000 kg/m3

Porosity

• Porosity of aggregate affects amount of moisture absorbed by the aggregate

• The amount of water that can be absorbed by the aggregate and the actual amount of water in it must be taken into account

Porosity

Oven dry Air dry SSD Wet

Will absorb water Will contribute water

Aggregate Size

• Maximum aggregate size might be e.g. 10 mm – 20 mm – 40 mm

• Decided based on – Spacing of reinforcement – Required concrete cover

• Maximum aggregate size must be smaller than the reinforcement spacing and required cover

Aggregate Grading Grading refers to the distribution of different sizes of particles

Aggregate Grading

• Extremely important to the design of any concrete mix

• Most concrete mixes need particle sizes to be evenly distributed from the maximum size of coarse aggregate down to the smallest sand particles

Aggregate Grading A mixture of different particle sizes leaves less empty space than if they are all the same size

Aggregate Grading Void Content Curve50:50 20mm:10mm granite plus sand

% sand

0 20 40 60 80 100

% v

oid

s

0

10

20

30

40

Aggregate Grading Aggregate grading is determined by sieve analysis (see soils lectures)

Aggregate Grading

0%10%20%30%40%50%60%70%80%90%

100%

0.01 0.1 1 10 100

% P

assi

ng

Particle Size d (mm)

Particle Size Distribution

20 mm10 mmsand

Aggregate grading is determined by sieve analysis (see soils lectures)

• Typically specified by a minimum and maximum particle size

• Some allowance for particles outside this range

Coarse Aggregate Grading

Example: 10/25 mm Dashed lines represent acceptable limits for this particle size range

Coarse Aggregate Grading

0%10%20%30%40%50%60%70%80%90%

100%

4 8 16 32

% P

assi

ng

Particle Size d (mm)

0/4 mm Standard sets limits only for particles ≥ 4 mm Dashed lines represent acceptable limits for this particle size range

Fine Aggregate Grading

0%10%20%30%40%50%60%70%80%90%

100%

0.01 0.1 1 10

% P

assi

ng

Particle Size d (mm)

Further described by how much passes through a 0.5 mm sieve

Fine Aggregate Grading

Description % passing 0.5 mm sieve Coarse sand 5-45 Medium sand 30-70 Fine sand 55-100

Aggregates and Durability

• Toughness and hardness of aggregates is important in applications where abrasion or erosion of the concrete is a concern

• Chemical reaction between aggregates and cement and/or chemicals from the environment can cause serious damage to concrete

Kinzua Dam, Western Pennsylvania

Abrasion-erosion damage

An expansive, water absorbing gel is formed which causes cracking within and around the aggregate

Alkali silica reaction in concrete

Alkali-silica reaction in concrete

Cracks grow as more gel is formed

The Royal Devon and Exeter Hospital was the first major structure in the UK found to be suffering from “concrete cancer” only 11 years after it was built in 1974.

Alkali-silica reaction in concrete

Further Reading

British Standards Institute. (2008). BS EN 12620: Aggregates for concrete. Retrieved from https://bsol.bsigroup.com/ Neville, A.M. (2011). Properties of Concrete. (5th ed.). Harlow, Essex: Pearson, pp. 108-177.