Aggregates & Recycling

Question 1

Explain why aggregates are added to concrete mix

Answer 1

Cost-cement £60/t, Agg £10/t
Reduces shrinkage
Reduces heat of hydration
Abrasion & fire resistance-harder more thermally stable

Question 2

How do aggregates reduce shrinkage

Answer 2

Aggregates don’t shrink, hcp does, diluting overall shrinkage and reducing cracking around structure

Question 3

How do aggregates reduce heat of hydration

Answer 3

agg acts as heat sink, absorbs heat of hydration, preventing concrete from becoming too hot

Question 4

How do aggregates affect design

Answer 4

Agg determine density, strength & stiffness. Important for calculating self weight and deflections.
Wrong aggregates produce durability problems-aggregate alkali reaction

Question 5

Give sizes of fine aggregate and coarse aggregate and why are two different sizes used

Answer 5

Fine- 63 micrometers-4mm
Coarse- >4mm, max determined by rebar spacing
Most cost effective way of achieving low voids (and hence cement) content

Question 6

Explain how the crushed shape of aggregates affects strength and workability

Answer 6

Rough surfaces mean better bond & interlocking=higher (15%) strength concrete
Angular shape=lower workability, more difficult for particles to move past each other

Question 7

Explain how the uncrushed shape of aggregates affects strength and workability

Answer 7

Round smooth shape means poor packing and interlocking=lower strength
Rounded shape=better workability, particles move past each other easily

Question 8

Explain how density of aggregates affects concrete

Answer 8

Higher density-greater self weight of concrete-greater dead load

Question 9

Explain how porosity of aggregates affects durability of concrete

Answer 9

More porous aggregate-greater porosity of concrete-increased permeability-outside agents penetrate concrete more easily-degrade steel reinforcements-decreases durability

Question 10

State the 4 different states of water content in aggregates and how they can affect concrete

Answer 10

Oven dry, air dry, saturated surface dry, wet
Dry porous aggregates absorb water, affects w/c ratio, workability + curing problems
Wet porous aggregates add water, decrease strength and durability

Question 11

Why is saturated surface dry aggregate preferred

Answer 11

Aggregate itself is saturated with water but surface is dry. No net addition or subtraction of water to concrete mix from aggregates.

Question 12

Why are aggregates that contain active silica avoided

Answer 12

Aggregates containing silica not stable in pH>13, the pH of concrete

Question 13

List 3 contaminants that are avoided in aggregates

Answer 13

Sulphates-cause expansion
Clays-increase water demand
Chlorides-cause reinforcement corrosion

Question 14

Why must the coefficient of expansion of an aggregate be the same as HCP

Answer 14

An aggregate that expands and contracts markedly will cause cracking

Question 15

List 3 types of sources of aggregates

Answer 15

Natural-gravel deposits or crushed rocks
Artificial-waste materials: sludges, mining residues, steel slag
Recycled concrete

Question 16

Why recycle aggregates

Answer 16

Avoid landfill charges & aggregate levy
Aggregates not always available
Reduces transportation

Question 17

Explain how the performance of recycled aggregates is different from standard

Answer 17

Reduces strength-ITZ increase in size

Lower workability-increased surface and presence of adhered mortar leads to increased water demand

Question 18

Explain the environmental issues related to recycled concrete

Answer 18

Increase in water demand and weaker ITZ requires increase in water content to maintain workability and then increase in cement content to maintain w/c ratio. Increasing carbon footprint, lower transportation distance may cancel out.